Graphitic carbon nitride (g-C₃N₄) nanomaterials for foliar applications and soil–water remediation in agriculture: a review

Abstract

Considering the serious issues of global warming and food security, novel technologies are required in agriculture to enhance productivity from the available arable land. Abiotic stresses, including salinity, drought, and pollution, have led to considerable agricultural losses. Nanotechnology offers potential solutions for cultivating crops under adverse conditions. This review examines the implications of metal-free graphitic carbon nitride (g-C₃N₄) nanomaterials in agriculture and their interaction with the plant systems. g-C₃N₄ is a rich source of carbon (C) and nitrogen (N) and has diverse applications. Elements like C and N are essential for plants to complete their life cycle. Additionally, they play key roles in the physicochemical and microbial dynamics of the soil. The fluorescence property of g-C₃N₄ has been reported to enhance photosynthesis in plants through artificial light harvesting. Photosynthesis influences carbohydrate synthesis and produces more biomass at harvest time. Additionally, g-C₃N₄ activates antioxidant enzymes under plant stress conditions. Owing to the intrinsic properties of g-C₃N₄, it has been identified as a potential candidate for pollutant degradation and associated stress mitigation. The biocompatibility, accumulation in the plant system, and industrial scalability of g-C₃N₄ are discussed in this review. Aligning with Sustainable Development Goals (SDG, 2030) 2, 3, 6, 12, and 13, this review explores the potential of g-C₃N₄ for yield enhancement in agriculture. Additionally, this review serves as a reference to encourage the use of biocompatible g-C₃N₄ in agroecosystems.

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Environmental significance

The increasing challenges of climate change and food insecurity necessitate sustainable strategies to improve crop productivity under environmental stress. This review highlights the potential of metal-free graphitic carbon nitride (g-C₃N₄) nanosheets as an eco-friendly nanomaterial in agriculture. Their biocompatibility, enrichment in essential elements (carbon and nitrogen), and ability to enhance photosynthesis, stress tolerance, and soil health underline their promise in reducing the reliance on agrochemicals. Furthermore, their unique photocatalytic and optoelectronic properties enable the light-driven degradation of contaminants and promote plant growth under stress conditions. These findings emphasise the transformative potential of nanotechnology in promoting eco-friendly, efficient, and scalable agricultural interventions. The strategies discussed here can be generalised to broader agro-environmental systems, supporting global efforts toward climate-resilient and low-impact food production and mitigating pollutant-related stress.

1. Introduction

In recent times, sustainable agriculture has emerged as a central global priority in response to the shortcomings of contemporary agriculture.¹ The challenges in agriculture include climate fluctuation, soil degradation, biodiversity extinction and soaring demand for food.² Globally, the population is projected to increase at a rate of 1.06% annually, rising from approximately 7.9 billion in 2022 to an anticipated 9.73 billion by 2064.³ This significant demographic shift necessitates an augmentation in agricultural production.⁴ The modern agricultural system must feed the growing population while maintaining ecological stability and sustainable productivity.⁵ Since the green revolution, farming practices have been characterised by intensive chemical use, resulting in greenhouse gas emissions, soil nutrient imbalance, declining soil organic matter, hazards to living organisms and water contamination.^6,7 Similarly, chemical fertilisers play a crucial role in adequate food production to support the growing global population.

Nevertheless, a substantial amount of applied nutrients is non-utilised due to limited efficiency in nutrient use, with more than half of the nutrients being transformed into unavailable forms.⁸ These losses contribute to environmental hazards through precipitation, adsorption, leaching and volatilisation.⁹ Collectively, these circumstances degrade the soil and water quality with long-term negative impacts on environmental sustainability. On the other hand, food production is the largest human use of land, accounting for 34% use of the available terrestrial land surface on Earth, excluding glaciers.⁷ Under current consumption trends and policies, food production contributes approximately 21% of global greenhouse gas emissions,¹⁰ accounts for 70% of freshwater consumption¹¹ and contribute to 80% of deforestation.¹² These environmental variabilities highlight a paradigm shift toward sustainable agricultural models that maximise efficacy while reducing ecological footprints. Agricultural sustainability depends on maintaining soil nutrient cycles and harmonious interactions between plants, microbes and the environment.^13,14

Recent advancements in nanotechnology reveal its potential to address these pressing challenges in agriculture.^5,8,15 Nanotechnology has firmly established its presence in numerous sectors, including cosmetics, biomedicine, electronics, pharmaceuticals and agriculture.⁶ Innovations in nanotechnology are critical in enhancing agricultural production through improved plant breeding, the targeted delivery of plant protection agents, and deployment of biosensors for precision agriculture.¹⁶ The range of applications is extensive, encompassing the direct deployment of nanomaterials, nutrient biofortification, enhanced plant health, increased photosynthetic rate and higher nutrient cycling.^5,17 Nanoscale materials enhance crop resilience to combat biotic and abiotic stress caused by several factors.¹⁸ Nanotechnology is providing solutions to address the limitations of contemporary agricultural activities.^19,20 It is noteworthy that agriculture is directly connected to human health. Therefore, minimising the application of chemicals is essential to achieve environmentally sustainable agrarian production.^21,22 Nanotechnology has an enormous application scope in agriculture, enhancing crop growth and productivity.²³

Nanoparticles (NPs), sized between 1 to 100 nm, feature a high surface-to-volume ratio, leading to reduced chemical use in agriculture.²⁴ Materials such as zinc, zinc oxide, titanium dioxide, copper, copper oxide, cobalt, cesium, selenium, silver, iron oxide, calcium carbonate, silicon, silicon dioxide, manganese carbonate, magnesium carbonate, and carbon NPs are widely studied for their impact on plant growth.^25,26 Surface-functionalized catalytic metal NPs mimic the actions of enzyme in photosynthetic processes, especially H₂ evolution, O₂ evolution, and CO₂ reduction.²⁷ Conversely, metal NPs resist degradation and harm to living organisms and the environment.^28,29 For example, treating rice crop with ferric oxide (Fe₃O₄) at 500 mg L⁻¹ negatively influences its growth due to the high concentration required.³⁰ In addition to plants, soil species, such as microorganisms, nematodes, and earthworms, are also more vulnerable to Fe₃O₄ NPs at various levels.³¹

Apart from metal and metal oxide NPs, carbon-based nanomaterials, such as carbon dots (CD), carbon nanotubes (CNT), graphene sheets, and fullerenes, offer numerous benefits in agriculture.³² Carbon-based nanomaterials are highly rated for their biocompatibility with living organisms.³³ Research findings explored the potential effect of CNTs³⁴ and CDs³⁵ on plant growth. Carbon-based nanomaterials enhance crop productivity by improving nutrient use efficiency and mitigating both biotic and abiotic stress factors.³⁶ A comparative study was conducted to evaluate three distinct carbon nanomaterials: 1) CD, 2) fullerene, and 3) graphene nanoplatelets at varying concentrations to assess their effectiveness in mitigating drought stress in bell pepper plants. This study found that all three carbon nanomaterials significantly enhanced the morpho-physiological and biochemical responses under water stress conditions compared to the control group. Among the three carbon nanomaterials, 200 mg L⁻¹ of graphene nanoplatelets increased the number of flowers and fruits.³⁷ For instance, under drought conditions, applying nitrogen-doped fluorescent carbon dots as a foliar spray significantly boosts the root length, root tip count, and fresh and dry weights of maize plants. This leads to a cumulative yield increase of 30% in maize.³⁸ Findings indicate that C₆₀ fullerene is resistant to mineralisation and remains in the soil environment for at least 1–2 years. C₆₀ fullerene exhibits high lipophilicity, suggesting the potential for biomagnification.³⁹ The long-term persistence of these nanomaterials poses risks to human health and may lead to unpredictable deformities in living organisms.⁴⁰ Similarly, Sharma et al. reported that engineered carbon nanomaterials exhibit a wide range of toxicity in living organisms.³²

Alternatively, g-C₃N₄ offers advantages such as metal-free composition, cost-effective material production, and broader environmental applicability.⁴¹ In contrast to the high-cost fabrication techniques associated with CNTs and fullerenes, g-C₃N₄ can be synthesised via simple thermal polymerisation with low-cost precursors.⁴² Notably, the production of CNTs requires complex techniques, including chemical vapour deposition, arc discharge, and laser ablation.^43,44 Furthermore, these methods employ high temperatures ranging from 550 °C to 1400 °C.⁴⁵ The absence of intrinsic photocatalytic properties in carbon nanomaterials limits their light absorption and poor charge separation capacities. Often, surface modification with metal oxides and heteroatoms can enhance their catalytic activity.⁴⁶ In contrast, layered structures and tri-s-triazine composition ensure visible light-driven g-C₃N₄-mediated photocatalysis. The versatility of g-C₃N₄ is further highlighted by its tunable dimensions (0D, 1D, 2D, and 3D), as well as doping and heterojunction formation with other nanomaterials.^47,48 These features position g-C₃N₄ as an alternative to other carbon nanomaterials.⁴⁹ Fig. 1 displays the Scopus data on the number of scientific documents focused on “g-C₃N₄” research across various fields from 2011 to 29th November, 2025, demonstrating a gradual increase over the years, accompanied by a constant rate after 2021.

Fig. 1 Number of reports in the literature on g-C₃N₄-related studies across various fields from 2011 to 2025.

g-C₃N₄ demonstrates deliverable outcomes in drug delivery, H₂O₂ production, N₂ fixation, and the development of nanosensors.⁴⁷ Due to its superior physiochemical properties, g-C₃N₄ is regarded as the “Holy Grail” and a promising next-generation photocatalyst.^48,50,51 At the same time, g-C₃N₄ is widely used in various fields but is less commonly employed in agriculture. In the search for metal-free, eco-friendly nanomaterials, g-C₃N₄ is an ideal candidate for achieving sustainability in agriculture.^52,53 NPs exhibit excellent photocatalytic properties owing to their photoluminescence property.⁵⁴ The photocatalytic properties of nanomaterials has been widely investigated to promote pollutant degradation using visible light from solar radiation. This phenomenon can be crucial for increasing the photosynthetic efficiency in plants through artificial light harvesting. Thus, graphene-based nanomaterials have been attracting interest among researchers due to their stability under various conditions.⁵⁵ The N content in g-C₃N₄ endows it with unique characteristics, including complexing ability, semiconducting properties, stability under a wide pH range (pH 1–14), and impressive thermal stability up to 600 °C.⁵⁶

Determining the sustainability of g-C₃N₄ requires comparable benchmarked impacts with other existing photocatalysts. Globally, the photocatalyst market has been projected to attain a value of $5.07 billion by 2026, demonstrating an annual growth rate of 10.5% since 2020. Concurrently, titanium dioxide (TiO₂) dominates the photocatalyst industry, accounting for 98% of total applications across various sectors, including consumer products and construction.⁵² A predominant advantage of g-C₃N₄ as a sustainable photocatalyst over other photocatalysts is the use of non-metal precursors for its synthesis. Most comparable photocatalysts to g-C₃N₄ are produced from metal precursors, which contributes to environmental toxicity.^57,58

The traditional photocatalysts, such as TiO₂ ⁵⁹ and Fe₂O₃,⁶⁰ highly rely on extrinsic modifications like doping and adding nanostructures for their activation and to sustain their photocatalytic efficiency. The defect-related active sites in g-C₃N₄ nanostructures display reduced recombination and superior carrier lifetime, enabling high photocatalytic efficiency. Especially, the active role of nitrogen-based defects enhances charge localisation and molecular adsorption.⁶¹ Aquino de Carvalho and Gilbertson emphasised that g-C₃N₄ activated with a visible light LED source offered 52% energy saving in its use phase compared to TiO₂.⁵² Thus, the multitude of benefits exhibited by g-C₃N₄, including broad light utilisation, non-metallic nature, and biocompatibility, surpass that of comparable materials.

Although TiO₂ is comparable with g-C₃N₄, TiO₂ NPs are associated with toxicity concerns and are regarded as unsuitable for the enhancement of plant growth, particularly considering their long-term impacts on the ecosystem and food safety concerns. The direct application of TiO₂ NPs to plants at higher concentrations poses a toxicity risk and can potentially affect the food web adversely.^62,63 Notably, g-C₃N₄ is a nitrogen-rich source, presenting a viable alternative to metal-based photocatalysts. Carbon nitride was discovered in the early 19th century. Subsequent investigations have predicted the presence of carbon nitride in five distinct phases, namely, C₃N₄ (g-C₃N₄), α-C₃N₄, β-C₃N₄, cubic C₃N₄, and pseudocubic C₃N₄.^52,64,65 Among these phases, g-C₃N₄ has relatively higher chemical and thermal stability.⁶⁶ Additionally, 2D g-C₃N₄ nanosheets can stack into layers owing to the presence of weak van der Waals forces, forming a nanostructure with enhanced catalytic properties.⁶⁷

The features of g-C₃N₄ nanomaterials such as chemical stability, eco-friendliness, modifiable electronic properties, and photocatalytic properties make them ideal options for applications in the environmental and energy-related sectors. The hetero-aggregation of biomolecules with g-C₃N₄ provides a way to explore a broader array of applications.⁶⁸ However, only a few studies have examined the use of g-C₃N₄ in promoting crop production and agricultural intervention.⁶⁹ Fig. 2 depicts the potential application of g-C₃N₄ in the agroecosystem. Accordingly, this review delves into the potential and prospects of novel g-C₃N₄ applications in agriculture. The following section describes the synthesis routes, available precursors, production process optimisation and stability of g-C₃N₄ materials.

Fig. 2 Various applications of g-C₃N₄ in agriculture.

2. Synthesis and properties of g-C₃N₄

g-C₃N₄ is an excellent nanomaterial widely applied in various research fields,⁵² including the health sector, drug delivery, wound healing, artificial enzymes, and bioimaging.⁷⁰ Thus far, nitrogen-rich urea,⁷¹ thiourea,⁷² cyanamide,⁶⁷ dicyandiamide,⁷³ and melamine,^55,74 have been reported as precursors for the synthesis of g-C₃N₄.^75,76 Generally, bulk g-C₃N₄ is prepared through various methods, including thermal polymerisation,⁷⁷ microwave-assisted synthesis,⁷⁸ direct pyrolysis,⁷⁹ hydrothermal/solvothermal synthesis,^80,81 vapour phase deposition,^82,83 and template-assisted synthesis.^84,85 Together, these synthesis methods underscore distinct advantages and unique characteristics that define the utility of each approach. Notably, thermal pyrolysis has been the predominant approach employed in reported studies for producing bulk g-C₃N₄.⁸⁶ Fig. 3 illustrates the precursors for synthesising g-C₃N₄ through thermal pyrolysis and the temperature they require. The merits of g-C₃N₄ firmly establish it as an outstanding material in photocatalytic applications. The synthesis parameters, including precursor type, temperature, time and heating rate, define the structure, morphology, and optoelectrical properties of g-C₃N₄.^78,87,88 Collectively, its photocatalytic characteristics are directly influenced by its porosity, surface area, crystallinity, composition, photoluminescence, and functional groups.^89,90 In this case, the procedure for the synthesis of g-C₃N₄ often determines its characteristics. For example, Safari et al. synthesised g-C₃N₄ using urea as the precursor at varying temperatures ranging from 400 °C to 600 °C.⁹¹ The photocatalyst produced at a temperature of 450 °C exhibited superior photoreduction activity⁹¹ and complete material disappearance at temperatures higher than 600 °C, indicating the decomposition of its precursor.⁹² The material obtained at 600 °C demonstrated an almost stoichiometric C/N ratio, attaining a value of 0.72.⁹² Capobianco et al. reported that polymerisation was complete around 575 °C, and beyond 600 °C, disruption of the structure occurs.⁹³ Wang et al. revealed that g-C₃N₄ prepared using melamine at 550 °C for 4 h was obtained in 31.5-times higher yield than g-C₃N₄ prepared using urea precursor under the same conditions. When the temperature was reduced to 500 °C and the calcination time was set to 1 h, the yield increased by up to 4.8%.⁹⁴ Moreover, the grafting of functional groups on the surface of g-C₃N₄ provides more active sites and functional defects. The commonly available functional groups in g-C₃N₄ are hydroxyl, aromatic, amino, urea, carboxyl, and cyano groups. The inherent defects of g-C₃N₄, when tailored through functional group modification, enable excellent performance and extended applicability in multiple fields.⁹⁵ Overall, the findings on the synthesis of g-C₃N₄ highlight how different parameters translate to varying outcomes.^96,97 Fabricating g-C₃N₄ with the desired dimensions (0D, 1D, 2D and 3D) is a key factor governing its performance across designated applications. Its size affects its active surface area, optical absorption, transition route, porosity and e⁻/h⁺ trasnsport.⁹⁸ Many researchers are focusing on modifying pristine g-C₃N₄ to enhance its photocatalytic efficiency through dimensional tuning, formation of g-C₃N₄-based heterojunction nanomaterials or elemental doping.^99,100 Distinctive approaches have been used to prepare 0D, 1D, 2D, and 3D nanomaterials. An extensive assessment of fabricating techniques to control their dimensions encourages their effective utilisation in renewable fields.¹⁰¹ Table 1 lists the dimensions of g-C₃N₄ along with its key properties and applications. To increase its photocatalytic efficiency, g-C₃N₄ has been synthesised in tailored forms such as quantum dots, nanosheets, and mesoporous structures, which are designed to increase its surface area and enrich its active sites.

Fig. 3 Sources of g-C₃N₄ precursors and their required synthesis temperatures.

Table 1 Comparison of the dimensions and key properties of g-C₃N₄ (ref. 101)

Dimension	Morphology	Synthesis method	Key properties	Applications
0D	Quantum dots	Microwave, solid-state and acid exfoliation	Quantum confinement, strong PL	Sensing, bioimaging, H2 evolution
1D	Nanorods, nanotubes, nanowires	Soft and hard templates, chemical vapour deposition, and hydrothermal	Fast charge transport and separation	Photocatalysis, pollutant degradation
2D	Nanosheets	Post calcination/liquid exfoliation	High surface area, increased electron transport	H2 generation, sensing, degradation
3D	Nanoporous, nanospheres, nanohydrogels	Thermal decomposition, porogens, self-assembly	High porosity, light scattering, strong adsorption	CO2 reduction, ORR, OER, pollutant degradation

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Beyond its well-established energy and environmental applications, research on g-C₃N₄ has expanded to explore its effects on plant growth and metabolism. A study has examined different structural modifications of g-C₃N₄, focusing on homogeneous lamellar (layered sheets) and irregular agglomerated forms, to understand how these variations influence plant physiological processes. Moreover, its structural, optical, and photocatalytic properties exerted a pronounced effect on pollutant degradation, modulating the physiological and metabolic pathways underlying lettuce plant growth.¹⁰² The relevant results and implications are discussed in detail in the subsequent sections herein. Furthermore, a study highlights that the lamellar structure exhibits favourable XPS outcomes, with a nitrogen content of 30.76%. SEM images reveal sheet-like morphologies with uniform pores, whereas the heterogeneous structured nanoscale aggregates have irregular interfaces. The lamellar form also exhibited a uniform elemental distribution of C, N, and O. Moreover, BET analysis confirmed its higher surface area (53.4 m² g⁻¹) and pore volume (0.18 cm³ g⁻¹) compared with the heterogeneous form (35.7 m² g⁻¹ and 0.12 cm³ g⁻¹, respectively), supporting enhanced photocatalysis, plant growth, and cytotoxicity performance.¹⁰²

It has been reported that g-C₃N₄ nanosheets are the most widely used and extensively studied form among the different forms of g-C₃N₄. Based on the literature prevalence, eight manufacturing procedures have been employed to synthesise g-C₃N₄ nanosheets,⁵² including chemical exfoliation,¹⁰³ supramolecular assembly,¹⁰⁴ biological exfoliation,¹⁰⁵ boron doping with melamine,¹⁰⁶ boron doping with urea,¹⁰⁷ C-doping,¹⁰⁸ O-doping,¹⁰⁹ and thermal etching.¹¹⁰ Overall, we discuss the sustainability of g-C₃N₄ production with respect to TiO₂ production. Compared with TiO₂, g-C₃N₄ has been reported to be an alternative in the field of photocatalysts. The sustainability of g-C₃N₄ relative to TiO₂ depends strongly on its synthesis route and precursors, which are instrumental in determining its environmental impacts. A comparison of the comprehensive life cycle assessment of the synthesis methods and precursor impacts of g-C₃N₄ with two nano-TiO₂ synthesis methods and precursors has been performed. Nano-TiO₂ is typically produced via the sol–gel (titanium isopropoxide – TTIP) and radio frequency thermal plasma methods (titanium tetrabutoxide – TTBO) with the respective precursors. According to the results, it was concluded that the eight methods for the synthesis of g-C₃N₄ generate higher global warming potential (GWP) impacts than the nano-TiO₂ sol–gel method, but have a substantially lower GWP impact than the radio frequency thermal plasma method.⁵² However, this claim has been validated in terms of producing g-C₃N₄ nanosheets, not as a representation of all g-C₃N₄ nanomaterials.

3. Augmentation of photosynthesis and plant growth using g-C₃N₄

Engineered NPs demonstrate various plant growth-promoting characteristics, including enhancing photosynthetic pigments, mitigating environmental stresses through increased antioxidant enzyme activity, and improving nutrient absorption.^111,112 Photosynthesis is a vital physiological process in plants, directly affecting crop yield.¹¹¹ The complex photosynthesis reaction proposed occurs in the chloroplasts of cells, where solar optical energy is converted into chemical energy, thus producing food materials.¹¹³ During the photo reaction, light-dependent reactions (water splitting) and light-independent reactions (CO₂ fixation) produce glucose as the product (Fig. 4).⁵⁴

Fig. 4 Function and mechanism of g-C₃N₄ in plant photoreaction (photosynthesis).

In the chloroplast, light reactions occur in photosystem I (PS I) and photosystem II (PSII), accompanied by energy transformation and electron transport. Fig. 4 illustrates the key components produced during the decomposition of water, as follows: 1) adenosine triphosphate (ATP), 2) oxygen (O₂), and 3) nicotinamide adenine dinucleotide phosphate (NADPH).¹¹⁴ Improving the crop yield through artificial photosynthesis mediated by the intrinsic properties of NPs is an emerging research area. According to research findings, g-C₃N₄ was discovered to be a non-toxic material that enhances photosynthesis and physiological efficiency more than other NPs. Herein, the following sections will highlight the role played by g-C₃N₄ with different dimensions in plant physiology and its impact on the agroecosystem.

For instance, wheat growth has been influenced by different concentrations of g-C₃N₄ (0 to 50 mg mL⁻¹). This study concluded that hydroponic growth with g-C₃N₄ at concentrations of 10 mg mL⁻¹ and 20 mg mL⁻¹ improves the photosynthetic efficiency and morphological characteristics of wheat plants. Both concentrations (10 mg mL⁻¹ and 20 mg mL⁻¹) significantly increased the plant height by 11.7% and 11.45%, and increased root length by 8.29% and 2.56%, respectively. In addition, this study demonstrated that the concentration with the optimal promoting effect was 20 mg mL⁻¹, while higher concentrations inhibited overall growth.¹¹⁵ A study demonstrated that two structural forms of g-C₃N₄, a homogeneous lamellar architecture and an aggregated heterogeneous structure, exhibited a distinct influence on lettuce plant growth applied through foliar spray. At 200 mg L⁻¹, the lamellar structure significantly enhanced the growth parameters of lettuce plant height, stem diameter, and leaf area up to 30%, 25%, and 40%, respectively. Although the heterogeneous form also improved growth relative to the control, its effects were consistently less pronounced. Biomass accumulation followed a similar pattern, with shoot and root fresh weights increasing by 40% and 30%, and dry weights by 30% and 25%, respectively, in both forms. Notable increases were also observed in chlorophyll a, chlorophyll b, and carotenoids by up to 32.1%, 26.9%, and 31.5%, respectively, and these values correlated with the SPAD values. In addition, there was an enhancement in photosynthetic rate, stomatal conductance, intercellular CO₂ and transpiration rate by up to 38.7%, 32.6%, 15.8% and 30.2%, respectively. In contrast, a higher concentration of 300 mg L⁻¹ of both forms inhibited growth, likely due to oxidative stress and metabolic imbalance.¹⁰² Collectively, the findings underscore that the structural morphology of g-C₃N₄ strongly influences plant growth and development.

The chloroplasts house photoreactive components that capture incident visible light, facilitating the conversion of water and carbon dioxide into carbohydrates, which serve as essential energy sources for plants.¹¹⁶ Wang et al. reported that hydroponic treatment with g-C₃N₄ at 10 mg L⁻¹ significantly influences the physiological activity of maize plants compared to control treatments. The enhanced light-harvesting efficiency of g-C₃N₄ is correlated with an increase in the electron transfer rate (ETR), net photosynthetic rate, and carbohydrate assimilation by 44.5%, 30.0%, and 32.3%, respectively. The notable increase in the magnesium (1.1%), phosphorus (51.8%), iron (44.6%), and manganese (121.8%) levels suggests that g-C₃N₄ positively impacts the nutrient use efficiency in maize.⁶⁹ The significant rise in manganese content was associated with the 12.1% increase in chlorophyll level,⁶⁹ which is critical for photosynthesis and biomass production.¹¹⁷ Additionally, a parallel investigation revealed that exposing soil to g-C₃N₄ at 250 mg kg⁻¹ resulted in elevated levels of essential nutrients in the shoots of rice plants compared to the untreated control, which included potassium (65%), magnesium (24.4%), sulfur (40.9%), zinc (84%), and copper (136%). This study established a robust correlation between enhanced nutrient concentrations and the proliferation of beneficial bacterial species. There was a 3.30-fold increase in beneficial endophytic bacterial species, including Saccharimonadales, Streptomyces, and Thermosporothrix,¹¹⁸ suggesting that g-C₃N₄ has the potential to augment soil microbial communities.

The soil application of g-C₃N₄ at 100 mg kg⁻¹ in cadmium (Cd)-exposed soil increased the plant height, root length, shoot and root dry weight by 25.3%, 44.5%, 49.5% and 63.4%, respectively. Meanwhile, g-C₃N₄ greatly amplifies the photosynthetic productivity and the nonphotochemical quenching coefficient of soybean plants under Cd-contaminated conditions. Owing to the effect of g-C₃N₄, chlorophyll fluorescence parameters like total chlorophyll content (52.7%), ETR (56.9%), quantum yield of PSII (56.9%), and energy conversion efficiency (58.1%) significantly increased. Moreover, g-C₃N₄ helps mitigate endogenous nitric oxide loss in nodules, thereby ensuring nitrogen availability and promoting the formation of more nodules with a 70.7% increase in nodule diameter in g-C₃N₄-treated plants.¹¹⁹ In a soybean pot experiment, the g-C₃N₄-treated group significantly influenced Cd accumulation and the carbon-to-nitrogen-to-phosphorus (C/N/P) ratios in the root zone compared to the control group. There was a significant increase in nitrogen content in the g-C₃N₄-treated groups. The total nitrogen content increased by about fivefold in the g-C₃N₄-treated groups. Compared to the control and Cd-treated group, the SOM and TOC contents increased significantly by 0.2 × 10³ mg kg⁻¹ in the g-C₃N₄ and Cd + g-C₃N₄ in the soil groups. Functional genes related to C/N/P modifications are associated with the microbial abundance in the treated group. Additionally, Cd detoxification and improved soil microbiota contribute to increased shoot and root lengths in the Cd + g-C₃N₄ group by up to 51%. The g-C₃N₄-enriched soil nitrogen content alters the rhizosphere microbial community and enzymatic activities in the Cd-exposed groups, impeding Cd uptake and associated stress factors.¹²⁰ Furthermore, we corroborate the g-C₃N₄-mediated heavy metal resistance and soil nutrient dynamics in the following section.

In a recent study, g-C₃N₄ and Fe-C₃N₄ were applied to rice plants in a column experiment, enhancing the agronomic traits, nutrient dynamics, and physicochemical parameters. The application of g-C₃N₄ did not significantly affect the plant height or tiller count throughout the entire growth cycle. However, treatment of the soil with Fe-C₃N₄ at a concentration of 2 g kg⁻¹ increased the five-hundred-grain weight and the organic matter of straw compared to the non-treated control group. The soil fractions, including total nitrogen content of ∼5.5 g kg⁻¹ and soil organic matter levels up to 7.25%, have been significantly increased by g-C₃N₄ at a concentration of 2 g kg⁻¹ soil. Compared to the control group, the soil application of g-C₃N₄ and Fe-C₃N₄ at 0.5 g kg⁻¹ significantly increased the total phosphorus by 0.02 g kg⁻¹. Notably, the increased chlorophyll content and enzyme-mimicking activities due to Fe-C₃N₄ treatment at 0.5 g kg⁻¹ soil enhanced the ATPase (37%) and NADPH (28.35 U L⁻¹) activity, which was attributed to the improved light harvesting and ETR.¹²¹

Findings indicate that NPs activate antioxidant enzymes such as peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD), which scavenge excess free radicals.¹²² Under plant stress, the key antioxidant activity in maize leaves was assessed with subsequent exposure to nitrogen-doped carbon dots. The results indicate an increase in the key antioxidant activity of SOD (26.7%) and an 18.9% reduction in malondialdehyde (MDA) levels. The elevated SOD activity effectively scavenges free radicals. The decreased MDA concentration and reactive oxygen species (ROS) are mainly due to the application of nitrogen-doped CD.³⁸ MDA is a key indicator of plant lipid peroxidation and oxidative stress. The lipid peroxidation process generates MDA as a byproduct, making it a stress marker. The application of 100 ppm rGO per g-C₃N₄ nanocomposite in rice plants resulted in low MDA levels (0.29 ± 0.014 μM mg⁻¹ protein), but a slight increase in MDA was observed at 250 ppm compared to the control. The variations in MDA level underscore the importance of dose and elicit increased stress rather than plant protection.¹²³

Cheng et al. reported that the application of g-C₃N₄ at 2 g kg⁻¹ to soil resulted in an increase in POD level up to 57.58% compared to the control group. POD has a broader substrate (hydroxyl radical (˙OH), superoxide (˙O₂⁻), and other peroxidases) affinity and is involved in multiple physiological activities, including photosynthesis and stress mitigation. Enabling the plant to cope with oxidative stress and the dominance of POD level lessens SOD and CAT, but eliminates superoxide radicals and hydrogen peroxide, respectively.¹²¹ Thus, g-C₃N₄ can play a crucial role in photosynthesis and combating stress. Applying metal-based NPs at high concentration increases the ROS and reactive nitrogen species (RNS) in the plant system.¹²⁴ The ROS and RNS reactive species are produced in subcellular organelles, viz., mitochondria, peroxisomes, chloroplasts, cytoplasm, and apoplast.¹²⁵ The elevation of ROS in the plant system causes cells to activate an oxidative stress state and is considered a well-developed defence mechanism in plants.^126,127 The excess production of RNS strengthens nitrosative signalling and alters the protein, nucleic acid, and lipid-related actions in plants.¹²⁸ RNS associates with ROS to form multiple linkages and provides nitro-oxidative signalling under stress conditions.¹²⁹ However, a higher concentration may induce oxidative stress, harming plants. At 200 mg L⁻¹, two different morphologies of g-C₃N₄ enhanced the quality traits of lettuce compared to the control group, with the homogeneous lamellar structure yielding higher soluble sugar, protein, and vitamin C contents (36.7%, 24.5%, and 33.1%, respectively). In contrast, higher concentrations reduced its overall performance, likely due to ROS-induced metabolic stress. The antioxidant activity was also greater in the homogeneous structural form, elevating SOD, POD, and CAT by 138.6%, 36.2% and 53.6%, respectively.¹⁰²

Analysis of the ground fractions revealed a significant increase in total nitrogen (TN), total phosphorus (TP), and soil organic matter (SOM) upon g-C₃N₄ exposure. The higher surface area of NPs facilitates the adsorption of free nitrogen and phosphorus ions, thereby inhibiting their mobilisation into water bodies. Moreover, the application of g-C₃N₄ and Fe-C₃N₄ was found to stimulate soil enzymes at three different growth stages of rice (basal, tiller and panicle fertilisation). Enzymes, including β-glucosidase (BG), dehydrogenase (DHA), and N-acetylglucosaminidase (NAG), are crucial for carbon cycling. They accelerate the decomposition of chitin, cellulose, and glucosamine polymers, respectively, ultimately increasing SOM.¹²¹ The Fe²⁺ moiety present in Fe-C₃N₄ promotes the Fenton reaction, generating hydrogen peroxide (H₂O₂).¹³⁰ The H₂O₂ decomposition produces free radicals,¹³⁰ which can degrade organic matter, such as lignocellulosic and livestock waste.¹³¹ The Fenton-based reaction induces the decomposition of organic matter via the generation of ROS. Combining g-C₃N₄ with metal and metal oxide NPs increases soil fertility by increasing organic matter. However, applying metals and metal oxides to the ecosystem causes environmental pollution. Therefore, dose optimisation to ensure zero toxicity is necessary to explore nanocomposite materials for wider applications.

Applying g-C₃N₄ greatly influences plant photosynthesis and increases crop production. Thus, using g-C₃N₄ can greatly enhance crop yields in agriculture, while ensuring environmental safety. Alternatively, pristine metal NPs also provide similar benefits to plants as g-C₃N₄. However, metals and metal oxides are associated with toxicity concerns to living organisms. Herein, the next section critically reviews the benefits of NPs on plant growth, photosynthesis, and the related hazards posed by pristine metal NPs.

4. Role of NPs in plant growth and photosynthesis mechanism

Some engineered nanomaterials promote the photosynthetic efficiency artificially in the plant system.^{69,121,132–134} For instance, silica NPs attach to PSII and split water, subsequently producing oxygen, and thereby increasing the transport of electrons to the quinone molecule, an electron acceptor.¹³⁵ Integrating fluorescent NPs into the photosynthesis machinery acts as an artificial antenna.¹³⁶ NPs convert ultraviolet light into visible light, which is essential for improving the photosynthetic efficiency in chloroplasts.¹³⁷

Alternatively, enhanced artificial photosynthesis is a highly anticipated technique mediated by photocatalytic NPs. Materials such as titanium dioxide (TiO₂)^52,138 and platinum (Pt)¹³⁹ are explored in multiple ways to determine their catalytic properties to enhance photosynthesis. However, the results contradict the anticipated output because of their limited visible light absorption and large optical bandwidth.⁶⁷

Exposure of algae (Chlorella vulgaris) to CeO₂ NPs at 10 mg L⁻¹ increased their photosynthesis performance. The exposure to CeO₂ NPs increases the photosynthesis-oriented pigments (Chl a and Chl b). Also, it upregulates photosynthesis-related genes such as psaA, atpB, petA, petB, and rbcL.¹⁴⁰ Similarly, applying iron dioxide (FeO₂) NPs in soybean under hydroponic conditions reduced Fe deficiency and increased the chlorophyll content.¹⁴¹ In another study, mung bean plants were exposed to manganese (Mn) NPs in laboratory settings. The NPs were found to be located on the chloroplast perimeter, which was corroborated by transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM) analysis. NPs show promising results, even at higher doses than conventional Mn salts. To support this, Mn NP-treated plants showed enhanced chlorophyll-binding protein (CP43) binding at PS II.¹⁴²

Apart from improving photosynthesis, NPs can enhance crop quality¹⁴³ and produce biofortified food products. Nanopriming with Fe NPs at a concentration of 150 ppm in rice seeds resulted in an increase in agronomic characteristics and Fe content in the rice seeds.¹⁴⁴ In another study, seed nanopriming with ZnO NPs at a concentration of 25 ppm mitigates drought stress and improves the agronomic profile of the rice crop.¹⁴⁵ A study observed that soil (50 mg kg⁻¹) and foliar (40 ppm) application of copper (Cu) NPs in cowpea plant significantly influences its physicochemical properties and morphological characters.¹⁴⁶ Li et al. demonstrated that the soil application of molybdenum disulfide (MoS₂) NPs in soybean plants improves their yield and boosts rhizosphere health mediated by N fixation and delayed nodule senescence.¹⁴⁷ The soil application of silver (Ag) NPs in wheat plants significantly increases the plant fresh weight, leaf area, number of grains, root length, and shoot length under heat stress conditions.¹⁴⁸ NPs can play a crucial role in activating antioxidant enzyme activities, assisting plants in avoiding abiotic stresses.^149,150 The application of silicon (Si) NPs enhanced the performance of blueberry and bamboo plants under hypoxia stress (waterlogged) and heavy metal (lead toxicity) stress conditions, respectively.^151,152

The above-mentioned results have furnished the advantages of NPs in improving plant physiology. However, attention must be paid to the negative consequences of metal accumulation in the ecosystem to avoid undesirable outcomes.¹²⁴ For instance, the antimicrobial properties of NPs have been applied in preserving cultural heritage items such as stone, paper, textiles, and wood.¹⁵³ This clearly indicates that NPs have antimicrobial capacity, which could be hazardous to beneficial microbes as well. In another study, the plant application of silver, copper, copper oxide, zinc oxide, iron-based NPs, titanium dioxide, and other rare earth elements confirmed the antibacterial and antifungal properties of NPs. Nevertheless, agricultural applications are prone to NP accumulation in the food system.¹⁵⁴ Engineered NPs are an anthropogenic cause of pollution, but less prevalent. The rise in the use of NPs in the natural biota increases their bioaccumulation and creates toxicity. The stress associated with NPs affects living organisms due to the non-availability of a specialised defence mechanism.¹⁵⁵ Research findings indicate that applying NPs to increase the photosynthetic efficacy is worthwhile in agriculture. However, focusing on environmental safety, identifying NPs without toxic implications could circumvent detrimental effects. According to the reports of in vitro and in vivo toxicity studies, NPs cause problems in two ways in living organisms: 1) exposure to NPs at lower concentrations over a long period creates chronic toxicity and 2) exposure to NPs for a shorter period at high concentrations may cause acute toxicity.¹²⁴ Table 2 summarises the toxicity of NPs on various organisms susceptible to accumulation and exposure.

Table 2 Examples of toxicity caused by metal/metal oxides and carbon-based NPs

NP	Organism	Concentration	Impact	References
ZnO	Plant-dragon head	0, 40, 80, 160, and 400 mg L^−1	Foliar application of ZnO NPs resulted in a higher ZnO level in tissues compared to conventional Zn sources	156
			The MDA and H2O2 levels are significantly higher in ZnO treatment, causing lipid peroxidation and leading to oxidative stress
ZnO	Algae-Chlorella vulgaris	0.5, 10, 20, and 25 mg L^−1	The sorption of ZnO with tire wire particles affects aquatic life	157
			The combined effect enhances the bioavailability of ZnO, reduces chlorophyll, biomass, and antioxidant enzyme activity, and increases MDA and H2O2 contents
Cu	Algae-Chlamydomonas reinhardtii	5, 10, 15, 25, 50, and 100 mg L^−1	The concentration of 25 mg L^−1 of Cu NPs exhibits severe toxicity to the unicellular organism	158
			The study confirms increased ROS levels and stress-responsive metabolites due to NP toxicity
Ag	Plant-Lycopersicon esculentum	10, 20, and 30 mg L^−1	Under hydroponic conditions, NP exposure exhibited a significantly lower growth rate (2–7 times) due to phytotoxicity	159
Ag	Plant-Camelina sativa	0.5, 1, 2, 3, and 4 g L^−1	Applying 4 g L^−1 Ag NPs drastically reduces the height and dry weight to 53% and 61%, respectively	160
			Ag NPs at 2 g L^−1 increase stress signalling and antioxidant responses
Cu	Plant-Bacopa monnieri	5, 10, 20, 30, 40, 50, 75, and 100 mg L^−1	Cu NPs induce abiotic stress due to cellular permeability	161
ZnO	Soil microbiome (archaea, fungi, bacteria, and viruses)	0, 50, and 500 g kg^−1 soil	A high dose of ZnO, 500 mg kg^−1 soil, affected the complexity and connectivity of the microbial network	162
			The NPs impede the genes involved in C, N, P, and S cycling
TiO2	Plant-Oryza sativa	20, 50, and 100 mg kg^−1	Multi-omics studies reveal that anatase and rutile forms of TiO2 NPs induce toxicity in rice	116
NiO	Rat	1, 20, and 150 mg kg^−1	NiO NPs cause antioxidant imbalance and result in increased testicular toxicity	163
CuO	Moina macrocopa	0.013 to 0.039 mg L^−1	Accumulation of NPs in the gut region of M. macrocopa severely inhibits the digestive and antioxidant enzyme activities	164
ZnO	Capsicum annum	10, 20, 50, 100, and 200 mg L^−1	Nano-priming of seeds leads to decreased plumule dry weight	165
Graphene oxide (GO)	Bacteria-5 Bacillus strains (B. megaterium, B. cereus, B. subtilis, B. mycoides, and B. marisflavi)	0–100 μg mL^−1 GO	Based on the time and dose of GO, the biochemical and cell viability were significantly affected in the bacterial strains	166
Multi-walled carbon nanotube (MWCNT)	Plant-Allium cepa	Allium cepa roots-10, 20, and 50 ppm	Higher concentration of MWCNT causes DNA damage and apoptosis in plant and mammalian cells	167
	Human-lymphocytes	Lymphocyte treatment-1, 2, 5, and 10 μg mL^−1
	Swiss albino male mice-bone marrow cells and Pbr322 plasmid DNA	Bone marrow assay-2, 5, and 10 mg per kg body weight
MWCNT	Plant-Allium cepa	5 and 10 μg mL^−1	The nanotubes induced cytotoxicity and genotoxicity in Allium cepa	168
Carbon quantum dots (CQD)	Human cells	∼0.1 mg mL^−1	High doses of CQD cause a form of apoptosis, suggesting cytotoxicity at elevated concentrations	169
CQD	Mice	0–10 μg mL^−1	CQDs, as a drug delivery vehicle, accumulated in the tumour cells in mice, and may cause toxicity	170

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5. Mechanism of action of g-C₃N₄ in the plant system

Nanomaterials enhance the photosynthetic performance by modulating the charge separation efficiency and interfacial charge transfer dynamics. Owing to the auxiliary light-harvesting property of g-C₃N₄, it acts as a potential light-harvesting component in the photosystem of plants. Electron transfer is a complex aspect of the photosynthesis machinery, which demonstrates electron flow from photon absorption to the final production of photosynthates.³ For instance, the application of fluorescent carbon dots (CD) increases carbohydrates in mung bean plants, possibly due to the influence of NPs on the photosynthesis mechanism. The photosynthetic indicators, such as ETR, chlorophyll content, and RuBisCO activity, were significantly influenced in the mung bean sprouts.¹⁷¹ Duan et al. reported that g-C₃N₄ improves the chlorophyll content and photosynthetic efficiency in wheat seedlings. The results substantiate that g-C₃N₄ at concentrations of 10 mg mL⁻¹ and 20 mg mL⁻¹ upregulated the expression of chlorophyll a/b binding protein genes in wheat leaves. Gene induction in wheat leaves enhances chlorophyll accumulation and their photosynthetic performance, accompanied by efficient photon absorption and electron transfer.¹¹⁵ Conversely, fluorescent nanomaterials absorb broader wavelengths of light, which can augment the artificial light-harvesting capacity of chloroplasts.¹⁷² The fluorescent intensity was observed to estimate the electron transfer efficiency of g-C₃N₄ and the chloroplast complex. The water-dispersed g-C₃N₄ showed better UV absorption and emitted blue fluorescence in an aqueous solution. As shown in Fig. 5(a), the fluorescence emission from g-C₃N₄ is correlated with the absorption spectrum of chloroplasts.⁶⁹ The photoluminescent properties of NPs enhance photosynthesis through photon absorption and emission. Plants receive photons from sunlight and utilise only the visible spectrum in a short wavelength range. Chlorophyll is a natural antenna that absorbs and passes the captured energy to the reaction sites.¹⁷³ Conversely, the photons absorbed by artificial antennas enable higher electron transfer between PSI and PSII. NPs act as artificial antennas, absorbing more photons, leading to greater electron transport within photosystems.^136,173,174 The photosynthesis process in higher plants occurs based on cyclic and non-cyclic electron flow. The electrons generated from water splitting at PSII move through the linear electron transport chain to NADP⁺, forming NADPH.¹⁷⁵ In this process, protons released into the thylakoid lumen contribute to the proton (H⁺) gradient in the thylakoid layer, aiding adenosine triphosphate synthase to produce ATP molecules through electron-coupled proton translocation.^175,176 In contrast, cyclic electron flow around PSI returns the electron from ferredoxin back into the cytochrome protein complex (b6/f) instead of going to NADP⁺. This process increases the proton motive force and supports additional ATP generation, thereby balancing the ATP/NADPH ratio required for carbon fixation.¹⁷⁷

Fig. 5 (a) Absorption spectra of chloroplast and g-C₃N₄, and PL of aqueous g-C₃N₄ excited at 330 nm. (b) DCPIP reduction rate in the presence of g-C₃N₄, chloroplasts and g-C₃N₄/chloroplast complex (where g-C₃N₄ at 1, 5, 10, 25, and 50 mg L⁻¹) under a light intensity of 210 W⁶⁹ (adapted from ref. 69 with permission from Elsevier, Copyright 2021).

The photosynthetic machinery features photon-harvesting complexes, which transfer energy to its reaction sites. When NPs have the appropriate design and compatibility with the reaction centre, a massive amount of energy is absorbed and transferred to produce photosynthates. Regarding interfacial charge transfer, apart from light-absorbing capacity, NPs can donate electrons directly to chloroplasts.¹⁷⁸ For example, carbon quantum dots (CQD) exhibit similar photoluminescent properties to g-C₃N₄. For instance, electron charge transfer between the CD and chloroplasts has been reported in mung bean plants.^171,178 However, the overall pathway for interfacial charge transfer between g-C₃N₄ and chloroplasts is still unclear.

Fig. 6 depicts the possible electron transfer mechanism between g-C₃N₄ and the chloroplast. When the wavelength of light excites electrons in both chloroplasts and CD, accelerated electron transfer occurs from CD to the chloroplast and from the chloroplast to the photosynthetic machinery.¹⁷¹

Fig. 6 Illustration of interfacial charge transfer between g-C₃N₄ and chloroplast³ (adapted from ref. 3 © 2023, RSC. Published under the CC BY-NC 3.0 licence).

A higher electron transfer rate from the donor (CQD) to the acceptor (chloroplast) occurs in the light-harvesting mechanism, which encompasses both the NP and the chloroplast. The massive electron flow rate between PSII and PSI enhances the total electron transfer chain, resulting in increased NADP reduction, ATP production, and O₂ splitting.^3,179,180 Fluorescent NPs can generate light-excited electron and hole pairs that do not undergo recombination.

In an in situ characterisation, synthesised g-C₃N₄ was treated with isolated chloroplasts to confirm its light-harvesting properties. g-C₃N₄ emits a strong blue fluorescence peak at 440 nm when excited at a wavelength of 313 nm. Further investigation by confocal laser scanning microscopy (CLSM) and UV absorption spectroscopy confirmed that the g-C₃N₄ was complexed with the chloroplast perimeter. As shown in Fig. 7(e), the CLSM image presents blue fluorescence from g-C₃N₄ and red fluorescence from the chloroplasts.⁶⁹ Stokes-shifted photons may lead to the conversion of non-photosynthetically active luminescence into a photosynthetically active spectrum. For instance, exposure to green-synthesised fluorescent CDs improved the light-harvesting and photosynthetic rate in lettuce plants.¹⁸¹ PSI inherently produces a fluorescence peak at a wavelength above 700 nm. Nonetheless, the PSII fluorescence peak is detected in the far-red to near-infrared region, ranging from 650–780 nm.¹⁸²

Fig. 7 (a and b) Digital camera images of g-C₃N₄ and hyaluronic acid composites under daylight and UV light (λ = 365 nm), respectively. 1) HA microgel, 2) g-C₃N₄@HA microgel, 3) ^Bg-C₃N₄@HA microgel, 4) ^Pg-C₃N₄@HA microgel, and 5) ^Sg-C₃N₄@HA microgel¹⁸⁶ ((a) and (b) have been adapted from ref. 186, © 2024 MDPI. Published under the CC BY license). CLSM depicts the distribution of g-C₃N₄ fluorescent signal in maize at λ = 405 nm. (c) CLSM image of blue fluorescence, indicating the presence of g-C₃N₄ in the root; (d) CLSM image of blue fluorescence, indicating the presence of g-C₃N₄ in the leaf; (e) high-resolution CLSM image, indicating isolated g-C₃N₄ present around the chloroplast; and (f) blue and red fluorescence in the CLSM image, indicating the presence of g-C₃N₄ and isolated chloroplast⁶⁹ (adapted from ref. 69 with permission from Elsevier, Copyright 2021) (please refer to the web version of this article to view the colour-coded elements described in the figure legend).

Meanwhile, Chandra et al. tested excitation wavelengths of 390, 420, and 442 nm in three samples, viz., CQD, plant-extracted chloroplasts, and the CQD–chloroplast mixture. The peaks were observed at different wavelengths, reportedly 440–450 nm for CQD, 650–780 nm for chloroplast, and at both ranges of 440–450 and 650–780 nm for CQD–chloroplast.¹⁸³ It was inferred that chloroplasts and PSII tend to emit the same level of electrons to a higher state at the wavelength of 650–780 nm. These equivalent values reveal insights into the near relation of the chloroplast-housed chlorophyll-embedded PSII system. The CQD–chloroplast complex is observed to have a lower peak intensity at 400–450 nm and higher at 650–780 nm. Subsequently, the CQDs are hypothesised to act as an electron donor to the chloroplast within the complex, resulting in lower fluorescence than pure CQDs but higher fluorescence than isolated chloroplasts at the respective wavelengths.^184,185 These results substantiate the functional interaction at the chloroplast and CQD interface with a spectral overlap-based charge transfer mechanism, as described in previous studies.

Upon excitation, electrons are transferred to the chloroplast, causing it to gain additional energy and exhibit enhanced fluorescence relative to its unexcited state. This mechanism confirms the efficient utilisation of electrons between the donor and acceptor. Fig. 4 reiterates the mechanism of action of g-C₃N₄ in the photosynthetic process. These phenomena reveal that g-C₃N₄ on the chloroplast surface absorbs non-photosynthetic wavelengths and emits blue fluorescence.

Fig. 7(a and b) illustrate the optical activity of doped and undoped g-C₃N₄@HA (HA – hyaluronic acid) microgels under daylight and UV light, respectively. Exposure to UV light resulted in the emission of strong fluorescence compared to the pure HA microgels at a wavelength of 365 nm.¹⁸⁶ Similarly, Fig. 7(c–f) indicate the blue fluorescence emission from g-C₃N₄ inside the plant system, including root, stem and leaf.⁶⁹ It facilitates the chloroplast to capture more light and enhance water splitting at PSII. As a result of H₂O decomposition in PSII, more electrons will be produced and flow through the protein complexes. As a result of increased electron flow, the photosynthetic efficiency in maize plants was drastically improved.^69,172 Furthermore, Kopnov et al. confirmed that 2,6-dichlorophenolindophenol (DCPIP) reduction, ferricyanide reduction assay, and photoluminescence (PL) lifetime measurement are strategies to estimate electron transfer between fluorescent NPs and the chloroplast.¹⁸⁷ Furthermore, the reduction level of DCPIP confirms an increase in the ETR between the photosystems (PSII to PSI).^69,187 During photoreaction in the chloroplast, DCPIP exhibits a higher affinity to electrons, causing a diversion of electron flow from PSII to PSI.¹⁷¹ Fig. 5(b) indicates that DCPIP reduction was faster in the g-C₃N₄ and chloroplast complex than in g-C₃N₄ and chloroplast alone, resulting in a higher ETR. In addition, 10 mg L⁻¹ g-C₃N₄ reduces DCPIP to the maximum level compared to other treatments and increases ETR up to 44.5%.⁶⁹ However, ferricyanide reduction and PL lifetime assays have not yet been explored for g-C₃N₄. Future studies incorporating both these techniques are essential to provide stronger validation and recommendations for the application of g-C₃N₄ in agricultural systems. NPs are crucial in the intricate photophosphorylation process, facilitating oxygen production in the chloroplast.¹⁸⁸

The ferricyanide assay is based on the photophosphorylation activity in the chloroplast. In the presence of light and ferricyanide, isolated chloroplasts produce oxygen and transfer electrons from PSII to reduce ferricyanide to ferrocyanide. Thus, increased ferricyanide reduction indicates higher electron production. A mixture of chloroplast/CQD resulted in higher ferricyanide reduction compared to pure chloroplast. PL lifetime measurement is another method employed to evaluate electron transfer between mixtures. CQDs alone remain excited for a long time (4.16 ns), whereas when mixed with chloroplasts, their PL lifetime is drastically reduced (0.08 ns). The shortened lifetime indicates quick and efficient electron transfer from CQDs to the chloroplast.¹⁸³ The fluorescence properties of nanomaterials can substantially influence the photosynthetic responses in plants. Analysis of two g-C₃N₄ morphologies showed that the homogeneous lamellar structure exhibited broader emission peaks at 420–440 nm upon excitation at 285–365 nm, whereas the heterogeneous form showed a narrower 420–430 nm peak upon excitation in the range of 265–365 nm. Fluorescence lifetime measurements further indicated a longer lifetime for the lamellar structure (τ = 3.5603 ± 1.0269 μs) compared to the heterogeneous form (τ = 2.4970 ± 0.1390 μs). The longer lifetime and emission peaks suggest slower charge-carrier recombination and more efficient electron–hole separation, supporting its photophysical behaviour.¹⁰² The foliar application of two different morphologies to lettuce plants results in enhanced overall growth and physiological responses, including photosynthetic efficiency and stronger antioxidant activity. Additionally, the UV-vis absorption spectra of both morphologies exhibited absorption in the visible light region,¹⁰² which indicates their better utilisation of light. These improvements strongly correspond with the findings reported by Chandra et al.,¹⁸³ where the prolonged PL lifetime in chloroplast-CQD systems enhanced photosynthesis by improving charge carrier separation. The parallel trends suggest that the longer fluorescence lifetime of the lamellar g-C₃N₄ structure may similarly contribute to the observed enhancement in lettuce performance. The morphology-dependent metabolic enhancement correlates with improved plant growth by increasing antioxidant mechanisms and carbon utilisation. Moreover, the principal component analysis of the lettuce plant revealed clear separation of both homogeneous and heterogeneous forms of g-C₃N₄ from the control group. In the homogeneous g-C₃N₄ group, the level of caffeic acid increased from 13.26 to 111.21 μg g⁻¹ FW (log₂ FC = 3.07) and neochlorogenic acid from 1.74 to 19.12 μg g⁻¹ FW (log₂ FC = 3.46), reflecting enhanced antioxidant metabolism. In the heterogeneous g-C₃N₄ group, compounds such as apigenin-7-(4″,6″-diacetylglucoside)-4′-glucoside (log₂ FC = 3.09) and cytisine B (log₂ FC = 4.61) were markedly upregulated with the enrichment of carbon metabolism and glucogenesis pathways.¹⁰² These findings substantiate that distinct morphologies of g-C₃N₄ nanomaterials drive divergent metabolic routes in lettuce, leading to pathway-specific adjustments.

NPs greatly enhance enzyme activities, such as phosphoenolpyruvate carboxylase (PEPC) and RuBisCO. These enzymes play a significant role in CO₂ fixation during light-independent reactions, affecting carbohydrate accumulation and photosynthesis.¹⁸⁹ Among the carbon nanomaterials explored in the literature,^171,190 10 mg L⁻¹ g-C₃N₄ exposure significantly influenced PEPC and RuBisCO activity by 156% and 242%, respectively. The results indicate that g-C₃N₄ exposure remarkably increased the potential quantum efficiency of PSII (F_v/F_m ratio).⁶⁹ The quantum efficiency is the photon absorption capacity to drive photochemical reactions within PSII. The quantum efficiency of PSII is expressed as [F_v/F_m = (F_m − F_o)/F_m], where F_m is the maximum fluorescence measured with an intense flash of light that closes all PSII reaction sites, exhibiting the maximum fluorescence yield. F_o is the minimum fluorescence observed after dark adaptation of the leaf. The maximum F_v/F_m ratio is between 0.75 and 0.85, which is proportional to the quantum yield of photochemistry.^191,192

The expression of the psaA and psbA genes responsible for photosynthesis was upregulated in maize plants. The improved photosynthetic activity is directly proportional to the increase in ATP (32.3%) and NADPH (96.9%).⁶⁹ A similar study reported that nitrogen-doped carbon dots upregulate the psbA gene response up to 81.7-fold and stimulate the production of the D1 protein. The D1 protein repairs the damage to the PSII system that occurs during abiotic stress. Thus, nitrogen-doped carbon dots increase the net photosynthetic rate and repair light-harvesting pathways in the plant system.³⁸ These results strongly suggest future directions for applying g-C₃N₄ to promote photosynthesis in plant systems.

6. Uptake, translocation, and tracing of g-C₃N₄

The soil or foliar application of NPs is well known to be translocated from root to shoot (xylem to aerial parts) and shoot to root (phloem to root system).¹⁹³ In foliage application, the penetration of NPs relies on the organelles, such as stomata, cuticles, and trichomes, present in the leaves.¹⁶ The stomatal opening size falls within the micrometre scale, and thus NPs enter the phloem cells. Notably, stomata occupy over 5% of the total leaf surface. However, the frequency of stomatal conductance depends on temperature coefficients like RH, CO₂ level, and light intensity.¹⁹⁴

Conversely, the root system facilitates the uptake of NPs through the epidermis. The release of negatively charged mucus or organic acids from the root surface often attracts the positively charged NPs towards the roots.¹⁹⁵ Fig. 8 depicts a schematic representation of the uptake and translocation pathways of NPs in plants. The root anatomy is arranged in the order of epidermis, cortex, endodermis, pericycle, and vascular bundle. The compaction of cortex cells and Casparian strips restricts the apoplastic (intercellular movement) absorption of NPs.¹⁹⁴

Fig. 8 Uptake and translocation pathway of NPs in the plant system.³¹⁸ Reproduced with permission from ref. 318.

The symplastic (intracellular movement) or protoplast pathway relies on the size of plasmodesmata, which is restricted to 2–20 nm in diameter.¹⁹⁶ Furthermore, Sun et al. designed a protocol for imaging NPs within the plant system, especially in root tissues, using CLSM and TEM. The fluorescently labelled Fe₃O₄ NPs are internalised inside the Arabidopsis thaliana root epidermis.¹⁹⁷ The metal and metal-doped carbon NPs with sizes ranging from 5 to 200 nm were accurately imaged within the plant root tissues (xylem catheters, epidermal cells, and cell walls).¹⁹⁷

Recent advances in microscopic techniques have eased the accurate tracking of NPs in plant tissues, including their presence in leaves, stems, and roots. Characteristics such as size, shape, and charge of NPs determine their uptake and mobility.¹⁶ Wang et al. first reported the presence of g-C₃N₄ in the root, stem, and leaf of maize plants. However, to date, no other studies have observed the in situ detection of g-C₃N₄ in the plant system. TEM images confirm that g-C₃N₄ was detected on the surface of maize root with a diameter of 56.5 nm, and its morphology was changed to a tube-like structure. In addition, the CLSM image exhibits blue fluorescence inside the root tips, validating the uptake of g-C₃N₄ by the maize plant. The fluorescence intensity was predominant in roots than in leaves, highlighting that g-C₃N₄ accumulates in roots and is translocated to other parts.⁶⁹

NPs can accumulate in plants through the shoot or root systems and be transported to different parts via vascular tissues.²⁸ The fluorescent g-C₃N₄ was tracked using CLSM equipped with 405 nm laser excitation. Surface-sterilised root tips and leaves with a length of 0.5–1 cm and a breadth of ∼1 mm were mounted on a glass slide, covered with a glass slip, and imperviously coated with nail oil. Fig. 7(c–f) confirm the uptake and translocation of g-C₃N₄ in the root, stem, and leaf veins of a maize plant, carried out in a hydroponic system. g-C₃N₄ exhibits blue fluorescent illumination at the wavelength of 405 nm, indicating its uptake and translocation in the maize plant.⁶⁹ The evidence from NP tracking and its impact on growth kinetics in plant systems suggests that g-C₃N₄ is a promising option for achieving higher agricultural productivity. These results substantiate its uptake and translocation in the plant system. However, comprehensive studies on its intracellular localisation are required to delineate its uptake and translocation routes. Heterogeneity in the size and morphology of NPs may lead to behavioural inconsistency in biological systems. NPs can be taken up by plants, particularly when their size falls within the nanoscale, ranging from 1–100 nm. This size dependency influences their effective translocation and accumulation within plants.¹⁹⁸ g-C₃N₄ is a 2D material with a size typically varying from 2–5 nm in thickness, with lateral dimensions of several hundreds of nanometres to micrometres.¹⁹⁹ To overcome this inconsistency, an alternative strategy to enhance the uptake and translocation of g-C₃N₄ in the plant system is the use of g-C₃N₄ carbon dots (g-C₃N₄/CD). Due to their tunable size and surface functionalities, they offer better interaction with the plant system.²⁰⁰ Indeed, g-C₃N₄/CDs have a precise size, promising their efficient uptake and translocation in plants.²⁰⁰ To get clarity on the synthesis and photocatalytic properties of g-C₃N₄/CDs, refer to the comprehensive study by Wang et al.²⁰¹ However, the optical and photocatalytic properties of g-C₃N₄/CDs remain untapped and require systematic evaluation within the plant system. In a comparative study on nucleic acid delivery in leaf cells of Nicotiana benthamiana, three different morphologies of g-C₃N₄, including nanosheets, nanoporous, and CDs, were applied as a foliar spray. The g-C₃N₄ morphologies were modified with exogenous nucleic acid (dsRNA). The main objective of this study was to silence the TMV coat protein gene (dsCP) through a biocompatible g-C₃N₄-based delivery vehicle. Notably, the TEM images of g-C₃N₄/CD exhibit a diameter of 2–4 nm and better penetration capacity than the nanoporous and nanosheet morphologies. Stomatal opening and closing play a significant role in the uptake and penetration of the three different morphologies. This phenomenon was compared with abscisic acid (ABA), an inducer of stomatal closing and an abundant water condition in the soil. The results disclose that a decreased stomatal aperture occurs upon ABA treatment and better stomatal opening upon water treatment. To test the uptake efficiency, dsGFP, which is an exogenous nucleic acid, was modified with g-C₃N₄/CD@dsGFP for silencing the GFP gene. The results showed that the encapsulated g-C₃N₄/CD@dsGFP-induced GFP gene silencing was weaker in the ABA treatment group. As shown in Fig. 9, the delivery efficiency of exogenous nucleic acid is regulated by stomatal opening and the endocytosis pathway.²⁰² Endocytosis is the primary entry route for membrane lipids, proteins, and extracellular molecules into plant cells.²⁰³ The uptake and translocation of NPs are highly dependent on stomatal aperture and the NP size. Similarly, g-C₃N₄/CD@dsCP successfully silenced the TMV coat protein in Nicotiana benthamiana. This is essential for TMV replication and systemic movement in plants. A single spray of g-C₃N₄/CD@dsCP conferred viral resistance for at least 5 days and was not observed to exhibit any adverse effects on plant growth and development.²⁰² Thus, we conclude that g-C₃N₄ exhibiting different morphologies can be used as a promising solution for gene editing technologies. Furthermore, to evaluate the performance of different g-C₃N₄ morphologies inside the plant system, more systematic studies have to be conducted.

Fig. 9 Schematic representing the application of g-C₃N₄ morphologies, uptake, and targeted gene delivery system²⁰² (adapted from ref. 202, © 2025, Wiley. Published under the CC BY 4.0 licence).

7. g-C₃N₄-mediated pollutant remediation in agroecosystems

Soil and water are primary sources for crop production in agriculture. In recent decades, the accumulation of hazardous materials in the soil and water bodies has impeded sustainable food production.²⁰⁴ To address pollution concerns, g-C₃N₄ has been effectively used to remove organic dyes,²⁰⁵ pharmaceuticals,²⁰⁶ heavy metals,²⁰⁷ and for desalination.¹¹⁸ This section describes the various pollutant degradation mediated by g-C₃N₄. Although the nanomaterial (g-C₃N₄) is the same, the mechanisms will differ for soil and water atmospheres, respectively.²⁰⁸ Firstly, g-C₃N₄ is an ecologically sound semiconductor composed of carbon and nitrogen, which are abundant on Earth. Its excellent properties include metal-free and nontoxic nature, exceptional electronic band alignment, feasible synthesis procedure, and thermal and chemical stability.²⁰⁹ Recent studies have confirmed the role of defect-active sites in the g-C₃N₄ moiety, which promises superior photocatalytic performance compared to other materials. Density functional theory (DFT) is a quantum mechanical modelling method that unveils how carbon and nitrogen vacancies in g-C₃N₄ alter its electronic structure.^210,211 The defects narrow the band gap energy from approximately 2.80 eV to 2.67 eV.²¹¹ The intrinsic electronic feature and tunable defect surface chemistry enhance the charge separation efficiency and reactivity.²¹² The band gap reduction drives the utilisation of broader wavelengths of light. Furthermore, intra-structural modifications play a crucial role in reducing charge recombination through charge localisation and better molecular adsorption.²¹³ DFT simulations predict that the lone-pair-rich nitrogen atoms (pyridinic N)²¹⁴ and triazine ring²¹⁵ defects are chemically active. In addition, DFT calculations predict the adsorption of the reaction intermediates (O₂, H₂O₂, etc.) at the N vacancies and edge sites. The projected density states indicate that the N 2p orbitals significantly absorb light in the valence band, promoting charge transfer.²¹⁰ Conversely, the DFT values for the Ti⁴⁺ centres in titanium dioxide (TiO₂) in the anatase phase are coordinatively unsaturated due to the presence of oxygen vacancies. However, oxygen vacancies not only enhance the visible light activity but also act as a centre for electron–hole recombination. Owing to its wide band gap of ∼3.2 eV, anatase-TiO₂ only absorbs UV light, making it unsuitable for visible light degradation.²¹⁶ In similar research, DFT simulations reveal that in ferric oxide (Fe₂O₃), the Fe³⁺ electron occupies 3d orbitals, creating a sluggish surface. This results in poor electrical conductivity and limits its photoelectrochemical performance. Additionally, a high exciton binding energy is required to separate electrons and holes. A high binding energy reduces the charge separation and carrier mobility.²¹⁷ TiO₂ and Fe₂O₃ rely on defect engineering or band gap engineering for enhancing the active sites.^59,60 Overall, we reiterate that g-C₃N₄ offers exceptional structural stability compared to existing photocatalytic materials, making it a suitable choice for sustainable photocatalysis. Fig. 10(c) illustrates a schematic representation of g-C₃N₄ as a visible light photocatalyst with a band gap of 2.70 eV, promising effective pollutant degradation in water bodies.²⁰⁸

Fig. 10 (a and b) Charge separation mechanism and transport of photogenerated electron–hole pairs at N-doped g-C₃N₄, respectively. Reproduced with permission from ref. 208 (adapted from ref. 208 with permission from Elsevier, Copyright 2021). (c) Advantages of g-C₃N₄ in photocatalysis. Reproduced with permission from ref. 209 (adapted from ref. 209 with permission from Elsevier, CC-BY-NY licence). (d) Decontamination of ATZ and Pb in paddy soil using alkalised biochar and g-C₃N₄ composite²³⁴ (adapted from ref. 234 with permission from Elsevier, Copyright 2023).

Spectroscopic insights underscore the intrinsic advantage of g-C₃N₄ for solar-driven applications, making it as a promising candidate for environmental remediation. The use of time-resolved fluorescence decay curves helps in understanding the charge carrier dynamics of nanomaterials.^218–220 The photoluminescence decay curve of g-C₃N₄ is biexponential, given that it is comprised of short-lived and long-lived components. In g-C₃N₄, its longer lifetime (20–40 ns) is attributed to trap-assisted recombination near N-rich defect sites or surface heterogeneities.^218,219 The interfacial electron movement in the 2D π-conjugated and planar structure extends the lifetime of excited electrons and holes without recombination.²²¹ The decay curve of other photocatalysts achieves only sub-nanosecond and picosecond lifetimes, which indicates electron recombination and poor charge retention. To match the performance of g-C₃N₄, materials such as TiO₂ ²²² and Fe₂O₃ ²²³ require external modifications, like elemental/impurity doping and forming heterojunctions with other semiconductors. Pristine Fe₂O₃ suffers from inherent low electrical conductivity, which severely limits its photocatalytic ability.⁶⁰ In some cases, the pristine g-C₃N₄ poses limitations, including inadequate utilisation of visible light and recombination of photogenerated electrons and holes (e⁻ and h⁺). Modifying the structure of g-C₃N₄ through band gap engineering can enhance its photocatalytic performance, as shown in Fig. 10(a and b).²²⁴

Methods such as impurity doping with foreign atoms,²²⁵ exfoliation of the 2D sheet into layers,²²⁵ and heterojunction designs,²²⁶ enhance the semiconducting property of g-C₃N₄. A study reported that the lamellar g-C₃N₄ structure exhibited superior photocatalytic activity, degrading 92% of methylene blue in 3 h compared with 76% for the aggregated structure. This enhancement was linked to its higher crystallinity, longer fluorescence lifetime, and narrower band gap (2.75 eV vs. 3.13 eV), which together promote efficient charge separation, reduced recombination, and improved light utilisation.¹⁰² Thus, although both forms are chemically identical, the morphological shift significantly alters the photocatalytic performance.

For example, fabricating a heterojunction structure of ZnO and strontium with g-C₃N₄ (ZnO:Sr/g-C₃N₄) composite resulted in the reduced band gap of 1.9 eV rather than 3.5 eV of pure ZnO. In addition, this nanocomposite exhibits comparatively higher photocatalytic properties, degrading cationic and anionic dyes by up to 95–99%.²²⁷ For instance, the synergetic effect of the rGO/Fe₂O₃/g-C₃N₄ nanocomposite enables rapid charge transfer in its nanochannels, degrading ciprofloxacin and tetracycline drugs with visible light intervention.²⁰⁶ In another study, the heterojunction of g-C₃N₄/FeTiO₃/rGO promotes rapid photocatalytic charge separation and reduces the redox potential. rGO plays a crucial role in facilitating electron transfer without the recombination of e⁻ and h⁺. The reactive species, such as superoxide radicals (˙O₂⁻) and hydroxyl radicals (˙OH), were generated from the valence and conduction bands, respectively. These oxidising species can transform hazardous organic pollutants to CO₂, H₂O, and less toxic inorganic ions.²²⁸ Recently, the emergence of two-dimensional (2D) transition metal carbide and nitride materials has been recognised as a promising substitute for noble metal cocatalysts. These materials are termed MXenes, which exhibit higher conductivity, better hydrophilicity, plasmonic effect, more active sites, and a modifiable work function. MXenes are identified as potential cocatalysts with promising interfacial contact and charge extraction properties.²²⁹ Although graphene-based 2D nanomaterials have received more attention in the field of materials science, their applicability is limited due to the weak van der Waals bonding between their layered structure. Thus, to overcome this shortfall, fabricating complex layered structures containing more than one element is required to obtain compositional variables with tunable specific properties. MXenes have been identified as ideal alternatives to existing 2D nanomaterials due to their complex chemical properties. MXenes are established with the formula of M_n+1AX_n, where ‘M’ is an early transitional metal, n is 1, 2, or 3, ‘A’ refers to A-group elements (group 13 and 14), and X is carbides and/or nitrides, forming a single laminated structure.²³⁰ The combination of MXenes with narrow-bandgap semiconductors, such as metal oxides, sulfides, III–V, and II–VI group compounds, facilitates the development of a Z-scheme or type-II heterojunction photocatalysis system.²³¹ For example, a novel nanocomposite ternary heterojunction material was synthesised to improve the solar energy conservation efficiency. This study focused on utilising synergistic ternary composites by integrating 2D niobium carbide (Nb₂C) MXene with g-C₃N₄ and lead halide perovskite (CsPbBr₃) quantum dot interlayers. The addition of 2D Nb₂C to the ternary heterojunction (g-C₃N₄/Nb₂C/CsPbBr₃) resulted in the maximum reduction of CO₂ to CO with a production rate of 53.07 μmol g⁻¹ h⁻¹. The activity of the ternary composite surpasses the rates of pure g-C₃N₄, CsPbBr₃ QDs, and the binary composite g-C₃N₄/CsPbBr₃ by approximately 8.4, 10, and 2 times, respectively. The simulations and theoretical results validate that the presence of Nb₂C MXene promotes electron transfer, thereby enhancing the photocatalytic activity.²³²

Conversely, nanomaterials exhibiting quantum confinement due to their size possess unusual electronic and optical properties, leading to the phenomenon of quantum dots (QDs). QDs are known to be zero-dimensional (0D) and a maximum size of 10 nm. Since the identification of g-C₃N₄ QDs in 2014, numerous applications have been unveiled in various domains, including photocatalysis, biomedicine, optoelectronics, and sensing. Compared to other carbon-based QDs, such as carbon nanodots and graphene QDs, g-C₃N₄ QDs display higher photoluminescence quantum yields. Quantum confinement leads to distinct properties from bulk structures, including a high surface area, a non-zero band gap, enhanced dispersibility due to the significant edge effect, an amphiphilic nature, and highly adjustable physical and chemical properties.²³³ This notable breakthrough on g-C₃N₄ QDs opens a broader array of applications. Rather than applying pure or pristine nanoparticles, insights into the incorporation of MXene and QD heterojunction structures can enhance the photocatalytic efficiency of g-C₃N₄. For example, Nie et al. developed MXene QDs (MQDs), decorated with a p–n heterojunction photocatalyst by introducing two different semiconductors, viz., g-C₃N₄ and bismuth oxyiodide (BiOI). The synergistic effect of the ternary g-C₃N₄/MQDs/BiOI photocatalyst achieves higher removal of nitric oxide through deep oxidation of 42.23% and produces fewer nitrite intermediaries than individual and binary materials. In addition, the performance of the ternary nanocomposite significantly influences the photoreduction of CO₂ to CO, with a production rate of 57.8 μmol g⁻¹ h⁻¹ and a CH₄ production rate of 3.6 μmol g⁻¹ h⁻¹.²²⁹ Here, we reiterate that the nanocomposite enhances the activity of the photocatalytic system's by reducing electron recombination and facilitating the faster migration of photogenerated electrons to the active sites.

The application of an alkalized nano-biochar (BC) and g-C₃N₄ (BCNaOH/g-C₃N₄) composite significantly removed lead (Pb) and atrazine (ATZ) from the rice ecosystem. The ATZ present in overburden water was degraded through photo-responsive electron generation and Pb sorption from contaminated paddy soil reached 94.70% and 47.75%, respectively.²³⁴ Fig. 10(d) depicts the mechanism of action of nanomaterials on Pb sorption and photodecomposition of ATZ molecules. In modern agricultural practices, ATZ has been extensively used as a herbicide worldwide to control weed growth. However, despite its economic benefits, it presents hazardous concerns due to its long persistence in the soil ecosystem, lasting up to 57 weeks and exceeding the permissible limit in water bodies, which need to be addressed.²³⁵ In Table 3, we present the utilisation of g-C₃N₄ for pollutant degradation under various circumstances. Cd-contaminated soils impede crop development and present a hazard to the ecosystem.¹²⁰ In this case, g-C₃N₄ can potentially remediate contaminated soils and transform Cd into immobile forms, improving soil fertility. In a greenhouse study, the addition of g-C₃N₄ to soil under Cd-contaminated conditions reduced the oxidative stress on soybean plants and converted Cd into low-toxic forms. Compared to non-treated plants, the application of g-C₃N₄ increased the dry weight of soybean plants and reduced the Cd content in shoots and roots by 35.7% and 54.1%, respectively.¹¹⁹ Hastuti et al. demonstrated the composite application of rGO/g-C₃N₄ in hydroponic rice seedling growth under Pb-contaminated conditions. Co-exposure to the nanocomposite enhanced the seedling mass by alleviating Pb toxicity and fostered notable improvement in root and leaf length. rGO/g-C₃N₄ possesses a high surface area and active sites that adsorb Pb and convert it to an unavailable form. Therefore, the optimal doses combat heavy metal stress, whereas excessive doses or additional stressors may trigger oxidative stress and inhibit plant growth. The MDA content with the co-treatment of Pb and rGO/g-C₃N₄ yielded 0.48 ± 0.027 μM mg⁻¹ protein, which was higher than Pb treatment alone (0.48 ± 0.027 μM mg⁻¹).¹²³ Furthermore, the co-transport of g-C₃N₄ and adsorbed pollutants in water and soil must be studied to prevent water and soil contamination. The upcoming section deliberately discusses the g-C₃N₄-involved soil dynamics and microbial augmentation.

Table 3 Recent advances and applications of g-C₃N₄ in environmental remediation

Composite materials	Application	Quantitative efficiency	References
Note: CN – carbon nitride, GCN – graphitic carbon nitride (synonymous to g-C3N4), and pCN – polymeric carbon nitride have similar base composition, structural motif, and precursors.
Biosensor application
Si/g-C3N4	The composites act as biosensors to detect prochloraz and 2,4,6-trichlorophenol, which are widely used chemicals in the agroecosystem	pH = 7 to 8 enhances electrochemiluminescence sensing	236
ZIF6/CN	CN is the base framework of g-C3N4, while carbonyl has been functionalised by adding 2-thiouracil. The nanocomposite is a highly sensitive biosensor with more active sites to detect glyphosate contamination in environmental water samples	Detection limit 0.04 pg L^−1, linear range from 10^−12 to 10^−5 g L^−1	237
Cu/Fe/g-C3N4	The nanocomposites are used as biosensors under greenhouse conditions	1. Gas sensing-90%	238
		2. Humidity sensing-82%
		3. Soil moisture sensing-72% (black soil) and 59% (red garden soil)
Fe3O4@MoC MFs/g-C3N4	The nanosensor successfully detected the parathion pesticide in food samples	1. Lowest detection limit-7.8 nM	239
		2. Sensitivity-2.16 μA μM^−1 cm^−2
Water treatment and disinfection
Ag/C3N4–NiSe	The NPs efficiently degrade the RhB dye and inhibit the enzymes of harmful Staphylococcus aureus bacteria	1. Catalytic degradation-69.58%	205
		2. Antimicrobial activity-(1.75 and 3.90 mm) low and (3.35 and 4.55 mm) high concentration
AgNPs/g-C3N4	The photocatalytic degradation of 2,4-D herbicide was better under visible light irradiation than when treated with Ag NPs alone (>420 nm)	3% AgNPs/g-C3N4 degrade 2,4-D completely in 240 min with an absorption peak at 283 nm	240
PdNPs@g-C3N4-BLE	In addition to catalytic properties, the nanoparticles exhibit antimicrobial properties	1. 72–98% in Suzuki–Miyaura cross-coupling (12 reuse)	55
		2. 68–98% in cyanation reactions (6 reuse)
ZnO/CN	Nanocomposites remove cyanobacterial algal blooms by neutralising the charge of algal cells (negative) and NPs (positive)	Within 10 min at a dose of 4.0 g L^−1 (ZnO/CN – 2 : 1), it removes 93% of algal cells	241
CdO/NiFe2O4/g-C3N4	Photocatalytic nanocomposites efficiently degrade the crystal violet dye and diclofenac sodium drug	86% and 84% higher degradation of diclofenac sodium and crystal violet, when compared to pure NPs (CdO and NiFe2O4).	242
Ag@GCN	The photodegradation of rose Bengal and the composite application enhanced the removal of xylenol orange dye in wastewater	1.5 mM concentration nanocomposite degrades 76% of rose Bengal and 15% to 36% of xylenol orange dye	243
Ag@AgCl/g-C3N4	The nanomaterial degrades the new coccine food colourant under the visible light spectrum	95.7% of degradation in 60 min	244
Pbi-Zno-g-C3N4	The nanocomposite degrades persistent atrazine, slowly releases P fertiliser from biochar, and improves microbial growth on g-C3N4 treatment	85.3% degradation after 260 min	245
Biomedical applications
g-C3N4/CS/PAA/CUR	The nanocomposites are a carrier for curcumin drugs to inhibit cancer cells	Drug release (96 h) – ∼98% and 69% in acidic and neutral pH, respectively	246
pCN/CNTs	In biomedical applications, the synthesized NPs promise wound healing and sterilizing pathogens	pCN/CNTs (5 : 1) at a concentration of ∼10^6 CFU mL^−1 to (0–2 CFU mL^−1) inactivated pathogens within 2 h	247

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8. g-C₃N₄ improved the soil dynamics and microbial interaction

Recent findings underscore the potential of g-C₃N₄ in modulating soil microbial communities and nutrient dynamics in plants. In cadmium (Cd)-contaminated soils, exposure to g-C₃N₄ significantly reduced the Cd accumulation in soybean plants. Notably, soil treatment with g-C₃N₄ at a concentration of 0.8% by weight led to a 49.7% reduction in Cd content in soybean roots, a phenomenon strongly associated with elevated nitrogen levels in the rhizosphere.²⁰⁷ Nitrogen (N) application promotes the physiological activities and reduces pollutant accumulation by inducing a dilution effect in the plant system.²⁴⁸ Researchers identified that different forms of N impart varying results in the accumulation and uptake of heavy metals in plants. It should be noted that several studies indicate that nitrate (NO₃⁻) supply enhanced the uptake and accumulation of Zn and Cd in crops, including rice, wheat, cabbage, and tomato.²⁴⁸

Researchers found that the influence of the nitrate transporter gene (NRT1.1) mediated nitrate uptake or sensing, and it conferred resistance to lead (Pb²⁺) uptake in Arabidopsis plants. The co-supply of NO₃⁻ and NH₄⁺ ensures the induction of NRT1.1, which influences the uptake of Pb²⁺. The sole NO₃⁻ supply results in the consumption of H⁺ ions by roots, causing alkalization in the rhizosphere, and impeding the uptake and bioavailability of Pb²⁺.²⁴⁹ In a contrasting result, ammonium (NH₄⁺) releases more H⁺ ions, leading to lower pH in the rhizosphere, causing solubility and enhancing Cd uptake by plants.²⁵⁰ Here, we conclude that a change in the soil rhizosphere pH associated with N supply provides varying outcomes in heavy metal accumulation in the plant system. The availability of rhizosphere N in the forms of NO₃⁻ and NH₄⁺ increased by 41.9% and 26.1% in the Cd + g-C₃N₄-treated groups. Conversely, soybean roots absorb more NH₄⁺ as a defence mechanism under Cd stress conditions.²⁰⁷ There could be two reasons for NH₄⁺ mediating stress mitigation in soybean plant, viz., 1) cell membrane depolarisation in the root cells, leading NH₄⁺ influx into the cytoplasm, which reduces Cd uptake and 2) higher NH₄⁺ content in Oryza sativa L roots inhibits pectin methylesterase activity, causing a negative charge in the cell wall, which significantly reduces Cd uptake in the roots.^249,250 Notable nutrient enrichment was negatively correlated with Cd accumulation in the plant system, highlighting the protective mechanism mediated by the nutrient-ion-metal interactions.²⁰⁷

Thus, this indicates two critical findings: 1) the enhancement of N content in the soil rhizosphere has substantial potential to reduce pollutant bioavailability and 2) g-C₃N₄ could supply available N in the form of NO₃⁻ and NH₄⁺ for plant growth and development. A comparative study on tetracycline (TC) remediation in pig farm soil and riverbed soil was demonstrated; g-C₃N₄ treatment significantly removed TC in both soil groups compared to the nontreated group. According to the treatment design, samples with different concentrations of TC, g-C₃N₄, and co-application were placed in a homeothermic incubator at the optimum temperature of 30 °C and 2000 lux illumination, with a light–dark period of 12:12 h for 30 days. However, the TC residue in the pig farm soil remains higher than in the riverbed soil. A noticeable shift occurred in the microbial community of riverbed soil, particularly in Acidobacteriota, Actinobacteriota, and Desulfobacterota. The microbiota of pig farm soil remained stable due to their stable microbial network. g-C₃N₄ significantly minimises the tetracycline-resistant genes and other antibiotic-resistant genes by restricting resistant bacterial species. Overall, g-C₃N₄ influences bacterial metabolism associated with the removal of resistant genes.²⁵¹ This study concluded that g-C₃N₄ is an eco-friendly photocatalyst and has no adverse effect on the soil microbiota.

From a microbial ecology perspective, g-C₃N₄ influenced the composition and function of soil microbial communities. The alterations in rhizosphere functional genes reflect the forms of N content in the soil. The available N content was estimated by adopting three microbial-driven N transformations, including nitrification, denitrification, and N fixation.²⁵² The relative abundance of N-fixing bacteria increased significantly, supported by the upregulation of the nitrogenase iron protein (nifH gene), which encodes the biological nitrogen fixation. Concurrently, g-C₃N₄-treatment resulted in the suppression of the nitrifying genes ammonium mono-oxygenase subunit (amoA) and nitrite oxidoreductase subunit A (nxrA) over the control group. Taxa such as Flavisolibacter, Niastella, Pirellula, and Opitutus were enriched under g-C₃N₄ exposure, as confirmed with analysis based on Spearman correlation. The genus is positively correlated (p < 0.05) with the nifH gene, which promotes the N-fixing and retention process, especially ammoniacal-N (NH₄⁺-N) in the roots of the soybean plant.²⁰⁷ At the molecular level, g-C₃N₄ treatment downregulated the expression of transporter genes such as OsIRT1 (rice iron-regulated transporter 1) and OsNIP1;1 (rice nodulin 26-like intrinsic proteins). However, in the case of metal exposure, both genes were upregulated in the rice plant.²⁵³

An enhanced N content in the rhizosphere region of the soybean plant leads to a larger microbial population, particularly in Cd-exposed groups. Compared to the control group, there is an increase in microbial classes, such as Nitrososphaeria (70%) and Actinobacteria (226%), particularly in the genera Sphingomonas (24%) and Bradyrhizobium (122%), associated with Cd uptake resistance.¹²⁰ The strong correlation between N enrichment and rhizosphere microbial abundance indicates that g-C₃N₄ treatment can augment plant growth by shifting microbial communities. In addition, g-C₃N₄ treatments influenced the activity of the urease enzyme (UE) compared to the control group. These alterations created a more favourable nitrogen-rich soil microenvironment. The application of g-C₃N₄ in agricultural soil greatly influences the N cycling in the rhizosphere, promotes N-fixation, and reduces nitrification. We elucidate this phenomenon, which provides insights into N sources for plant growth and development. Notably, the higher accumulation of readily available NH₄⁺ can minimise energy consumption through the breakdown of carbohydrates owing to nitrate reduction activity.¹²¹

Beyond soybeans, the application of g-C₃N₄ has addressed Cd and arsenate (As) toxicity, as demonstrated in rice cultivation. When applied at a concentration of 250 mg kg⁻¹ in metal-contaminated soils, g-C₃N₄ significantly improved plant biomass and reduced Cd and As accumulation in plant tissue. A key mechanism underlying this effect is that the alteration of endophytic bacterial communities in rice roots can restrict metal toxicity by conferring metalloid resistance and participating in bioremediation. In particular, the suppression of the Deinococcus (72.3%) population plays a crucial role in reducing As(V) to the more mobile and toxic As(III), contributing to limiting the As uptake in plants.¹¹⁸ Building on these findings, we propose two refined directions to tap into the application of g-C₃N₄ in the agricultural field. Firstly, designing pH-responsive g-C₃N₄ nanomaterials to reduce the risk of solubility and bioavailability of heavy metals. Additionally, pH buffering in the soil rhizosphere with NH₄⁺ and NO₃⁻ induces metal passivation. Secondly, harnessing a bio-nano hybrid system in the context of stress tolerance, nutrient cycling and pollutant resistance. Based on literature evidence, g-C₃N₄ intervention reshapes the soil microbiota, upregulates nitrogen-fixing genes, and downregulates metal transporter genes. This dual strategy of pH responsiveness and tailored microbial consortia makes g-C₃N₄ a versatile tool for agricultural applicability. This integrated approach enhances the relevance of g-C₃N₄ in this field, promoting crop resilience by improving soil fertility. Taken together, these findings demonstrate that g-C₃N₄ not only reduces the bioavailability of heavy metals but also validates that they are a potential option for mediating soil–plant–micro–metal interactions at the transcriptional and community level. Beyond their photocatalytic functions, g-C₃N₄ acts as an excellent nutritional source, supplying N in multiple bioavailable forms and enhancing soil properties by modulating the microbial dynamics. The multidimensionality of C₃N₄, such as nutrient enrichment and microbial regulation, positions it as an asset in sustainable agriculture. To date, studies have not flagged any toxicity concerns associated with g-C₃N₄. However, to ensure its safe and effective integration in the vast agricultural horizons, it is imperative to understand its long-term behaviour and environmental impact in open agricultural fields. Expanding research focusing on the forefront could unlock substantial opportunities for deploying g-C₃N₄ applications under real-world field conditions.

9. Toxicity and biocompatibility of g-C₃N₄

Studies have highlighted that carbon-based nanomaterials are naturally present in food items. Standard food preparation techniques, such as heating, roasting, and grilling, can manipulate carbon-based nanomaterials. Consequently, human exposure to carbon materials through dietary intake is frequent, and to date, has not been associated with a significant health risk.³ Numerous studies have investigated the biocompatibility of g-C₃N₄ to validate its potential for ecosystem sustainability. These studies rely heavily on cytotoxicity, genotoxicity, antimicrobial, and biomedical tests involving living systems, which are essential for thoroughly assessing the safety of nanomaterials.

Freshwater zebrafish, an aquatic model organism with more than 1000 genes homologous to humans,^3,254 has become a valuable tool in toxicity studies. Its features, transparent embryos, small size, rapid development, and well-characterised genetic background, making it ideal as a model organism for toxicity assessments.²⁵⁵

For instance, an investigation revealed the efficacy of a ternary nanocomposite (g-C₃N₄/Nb₂O₅/Ag) in degrading environmental pollutants, norfloxacin (NRF) and methylene blue (MB). Upon treatment with the nanocomposite under polluted conditions (NRF and MB) at pH 9, zebrafish embryos and larvae exhibited high mortality rates (80–90%) within 48 h post-fertilisation. Along with that, severe malformations appeared in the eye, tail, brain, and spine, with overall growth retardation. In contrast, 72 h post-fertilisation, water treated with the nanocomposite exhibited efficient photocatalytic pollutant degradation, achieving 83% and 92% for NRF and MB, respectively, and no toxic effects on zebrafish embryos.²⁵⁶ This demonstrates that g-C₃N₄-based nanocomposites offer a sustainable approach to mitigate environmental degradation.

To assess cell viability and cytotoxicity effects, scientists widely use assays, most notably the MTT assay, a calorimetric test that quantifies cell viability.²⁵⁷ Dong et al. investigated the photocatalytic degradation potential of g-C₃N₄ on the paraquat (PQ) herbicide, accompanied by toxicity studies using human lung (A549) cells, zebrafish models, and mice tissue analysis (lung and stomach). The A549 cells were employed in an MTT assay, where treatment with 10 mg L⁻¹ of g-C₃N₄ increased the cell viability from 54% to 91.4% in PQ-damaged cells. In addition, the PQ group and the g-C₃N₄ + PQ group were observed with SEM. The results demonstrate that the PQ group showed a malformed cell morphology. In the g-C₃N₄ + PQ group, the NPs shield the cells from PQ, and there was no significant difference in morphology with the control group. Furthermore, the IC₅₀ of PQ and g-C₃N₄ at 24 h was observed to be 400 μm L⁻¹ and 1250 mg L⁻¹. In the survival analysis, the zebrafish were exposed to PQ and g-C₃N₄ at 100 mg L⁻¹ and 500 mg L⁻¹, respectively. Accordingly, the g-C₃N₄ has no significant impact on zebra fish survival rate. After 7 days, PQ detoxification with g-C₃N₄ increased the survival by 70% compared with 30% in the PQ group. This study confirmed that g-C₃N₄ significantly reduced PQ accumulation and its associated toxicity across all models. Real-time fluorescence imaging of the paraquat distribution in mice and zebrafish, as shown in Fig. 11(a and b), supports this finding. Furthermore, pristine g-C₃N₄ did not cause significant apoptosis or mortality in either zebrafish or mice.²⁵⁸

Fig. 11 (a and b) Animal images recorded for g-C₃N₄ (HPCN) accumulation in zebrafish and mice, respectively²⁵⁸ (adapted from ref. 258 with permission from Elsevier, Copyright 2023). (c) Photograph of S. aureus-infested wound healing in laboratory mice treated with polymeric carbon nitride (pCN)²⁴⁷ (adapted from ref. 247, © 2023, RSC. Published under the CC BY-NC 3.0 licence).

Cytotoxicity assessment using HepG2 cells (human liver cancer cells) revealed distinct responses between the two g-C₃N₄ morphologies. The homogeneous lamellar form at 300 mg L⁻¹ exhibited markedly lower toxicity, maintaining approximately 80% cell viability, whereas the heterogeneous structure resulted in only 20% viability. The higher viability of the homogeneous lamellar form is attributed to its structural uniformity, while the heterogeneous form contains more defects and reactive sites within its nanostructure.¹⁰² These findings reveal contrasting perspectives on the suitability of g-C₃N₄ materials for biomedical applications, demonstrating that morphological variations in g-C₃N₄ can lead to markedly different biological responses and must be carefully evaluated under varying conditions.

In terms of antibacterial properties, a study investigated the incubation of bacterial strains, along with an rGO/g-C₃N₄/FeTiO₃ nanocomposite at concentrations of 50 μg mL⁻¹ and 75 μg mL⁻¹ for 24 h at 37 °C. The results indicated that the g-C₃N₄-based nanocomposite exhibited higher inhibition of both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacterial species. Furthermore, the nanocomposite at different dosages (6.25, 12.5, 25, 50, and 100 mg L⁻¹) were tested on zebrafish embryos. Post 48 h exposure to the nanocomposite, the LC₅₀ value was determined as 20.79 μm L⁻¹. After 48 h of exposure to different doses, higher concentrations cause embryonic toxicity, including decreased egg hatching rate, overall mortality and other abnormalities.²²⁸ In a similar study, the minimum inhibitory concentration (MIC) method was adopted to evaluate the antimicrobial effects of a nanocomposite (g-C₃N₄/Ca/Aloe vera) against the same Gram-positive and Gram-negative bacteria. Likewise, the nanocomposite hade an MIC value for both S. aureus and E. coli strains of 1 mg mL⁻¹. The reason for this may be that upon exposure to light, g-C₃N₄ generates ROS, which disrupts the bacterial membranes and inhibits their growth.²⁵⁹ One of the critical advantages of g-C₃N₄ lies in its simple chemical composition, consisting solely of carbon and nitrogen atoms, which might be metabolizable by living organisms.

Sen and Perveen used g-C₃N₄ for biomedical applications in their respective studies, especially for drug release and bioimaging of cancer cells.^260,261 In fabricating a drug delivery system, g-C₃N₄ was used as a carrier for cisplatin, an anticancer drug. The complex has an adsorption energy (E_ad) of −1.25 eV. The N–H non-bonding interactions ensure better interaction between the carrier and the drug. Further, the complex exhibits a higher dipole moment (12.79 Å) than g-C₃N₄, which improves its solubility in polar solvents. These weak interactions between the carrier and drug enables drug release from the carrier matrix.²⁶¹

Alipournazari et al. confirmed the successful delivery of doxorubicin (DOX) drug from the pH-sensitive starch/PVA/g-C₃N₄ hydrogel for inhibiting the proliferation of breast cancer cells (MCF-7). The nanocarrier achieved a loading efficiency of 44.75% and entrapment efficiency of 88%. According to the results, it was concluded that the cells treated with starch/PVA/g-C₃N₄ have higher viability (94.8%) than the control group (93.9%). Contrastingly, the cells treated with starch/PVA/g-C₃N₄/DOX showed 36.4% survival. In addition, the results demonstrate that the DOX release kinetics is faster at acidic pH (5.4), which is pertinent to tumour cells versus healthy tissues having a pH 7.4.²⁶²

Due to its low biological toxicity and high surface area, g-C₃N₄ is widely utilised in biomedical research, particularly as a carrier matrix in targeted drug delivery systems for cancer treatment. It can transport therapeutic agents without mobilising the active molecule. However, limitations in the shape and structure of g-C₃N₄ prevent its direct use in the biomedical field, as it lacks features such as slow-release capability and specific drug-molecule interaction. Therefore, integrating g-C₃N₄ with nano-hybrid structures has become promising for overcoming these shortcomings, thereby enhancing its utility in biomedical applications.

Sabzini et al. prepared a hybrid nanoformulation (chitosan/halloysite/g-C₃N₄) loaded with quercetin (QC) drug using a water-in-oil-in-water emulsion, which acts as a slow-release anticancer drug delivery system. The nanovehicle enhanced the entrapment effectiveness of QC by 86%. The targeted pH-responsive sustained release of QC was confirmed by 96 h extended release under pH 5.4, and holding the antitumor agent within the nanovehicle under neutral conditions (pH 7.4) limits its side effects by avoiding burst release. Furthermore, subjecting MCF-7 cells for 24 h to chitosan/halloysite/g-C₃N₄/QC resulted in the lowest cell viability and lower vitality than the cells treated with free QC.²⁶³

Khamene and coworkers fabricated a nanocomposite comprising polyvinyl alcohol, bacterial cellulose, calcium, aloe vera fibres, and g-C₃N₄ in a single structure. In this study, each material was selected based on its wound-healing properties. The composite dressing materials were tested against diabetic wounds in an animal model (mice). They significantly influenced tissue regeneration and accelerated wound healing, achieving a 95% healing rate in 3 weeks, compared with the 57% healing capacity of the curcumin/lignin dressing.²⁵⁹ Fig. 11(c) illustrates the wound healing property of g-C₃N₄, a similar carbon nitride group having the same composition as g-C₃N₄. The possible mechanism of the synergistic effect of g-C₃N₄ on photocatalytic wound healing involves its light-induced generation of ROS, which helps to sterilise the wound by killing bacteria.²⁶⁴ Mild ROS levels modulate inflammation by activating signalling pathways. This balanced inflammatory response supports tissue reconstruction. Additionally, ROS promotes the proliferation of fibroblasts and the production of collagen, thereby aiding in regeneration and wound closure.²⁵⁹ Furthermore, studies suggest that g-C₃N₄ exhibits good biocompatibility and does not cause any adverse effects in humans.²⁶⁵

Moreover, an in silico study on g-C₃N₃, a carbon nitride analogue, utilised molecular dynamics (MD) simulations to assess its interaction with model lipid membranes. This study concluded that g-C₃N does not penetrate the cytoplasm unless subjected to external stimuli. Due to its low likelihood of causing membrane damage, g-C₃N₃ is predicted to be biocompatible, similar to g-C₃N₄.²⁶⁶ For clarity and reference, we include the biocompatibility studies on g-C₃N₄ (Table S1). Collectively, these research findings strongly suggest that g-C₃N₄ exhibits minimal or no toxic effects and holds considerable promise for eco-friendly and biomedical applications.

10. Persistence, migration and physicochemical transformation of g-C₃N₄ in the ecosystem

The anthropogenic application of nanomaterials to soil and water bodies is intricately linked to the ecosystem. However, their deliberate use in agriculture and environmental systems necessitates a thorough understanding of their biocompatibility and potential ecological impacts. The key concern in applying nanomaterials in the agroecosystem is their potential for bioaccumulation. NPs can readily accumulate in plant tissues, infiltrate the food web, and exert toxic effects on both terrestrial and aquatic organisms. The accumulation of NPs in the food chain raises high toxicity concerns for the ecosystem.²⁶⁷

Although g-C₃N₄ is often considered a biocompatible material, its long-term effects, including degradation byproducts and bioaccumulation, are poorly understood. Current evidence from acute ecotoxicity tests in zebrafish, microbial toxicity and animal models is insufficient for widespread agricultural implications. The transformation of g-C₃N₄ under hydrolysis, oxidation, or microbial breakdown may fragment the polymer into low-molecular N-containing species in soil-water media. The breakdown of g-C₃N₄ under environmental conditions may lead to an increase in N species, such as ammonia and nitrate, in the soil and water.^120,207 Tackling nitrate pollution in the ecosystem is becoming difficult these days. The degradation products could alter soil properties and lead to chronic ecotoxicity, such as eutrophication. Nitrate (NO₃) is an inorganic source of N ions, which is prone to leaching and bioaccumulation. Anthropogenic activities lead to nitrate pollution, destroy water quality, and cause health complications in living organisms, especially blue baby syndrome in human infants.^268,269

In addition, the soil column experiment is used to determine the mobility, transformation, and soil interaction of C₃N₄ through leachate collection.^{121,270–272} Cheng and coworkers conducted a soil column experiment simulating a realistic rice agroecosystem for four months from planting to harvest. g-C₃N₄ and Fe–C₃N₄ were added to the soil, each at concentrations of 0.5 g kg⁻¹ and 2 g kg⁻¹, which influence the chemical oxygen demand (COD) and NH₄⁺ and NO₃⁻ release in the surface water. After each fertilisation with NPK at various growth stages, g-C₃N₄ and Fe-C₃N₄ exposure decreased the COD and NH₄⁺ and NO₃⁻ levels compared to the control group. Overall, the g-C₃N₄ and Fe-C₃N₄ treatments generate oxygen species in soil, which is related to their catalytic-oxidative properties, making them beneficial for pollution reduction in agriculture.¹²¹ This study highlights that the addition of g-C₃N₄ and its composite reduced N species, but treated soils increased the total nitrogen and phosphorus levels. If unregulated, their elevated levels can contribute to eutrophication, plant nutrient imbalance, and microbial disruption, which can have long-term environmental consequences.

If prioritising the application of g-C₃N₄, plenty of issues remain unaddressed regarding safety aspects. Also, the studies have not described the migration and transformation behaviour of g-C₃N₄ in the soil ecosystem. These validations are highly anticipated to determine the environmental hazards and food chain accumulation posed by the application of g-C₃N₄. The multifunctional capabilities of g-C₃N₄ include photocatalytic O₂ evolution, H₂ evolution, water splitting, H₂O₂ production, CO₂ reduction, N₂ fixation, electrochemical sensing, pollutant removal and organic synthesis.^273–276 All these applications are involved in ecosystem sustainability, which comprises soil and water systems. With the extensive use and increased production of g-C₃N₄, it is inevitably released into the environment. Although the biocompatibility of g-C₃N₄ has garnered attention, a holistic evaluation of its transformation and migration behaviour within soil matrices is yet to be investigated. Recent environmental studies mainly focused on the degradation or removal of pollutants using g-C₃N₄. Nonetheless, many disinfection studies using g-C₃N₄ have shown antimicrobial properties in wastewater treatment.^277–279 The persistence of g-C₃N₄ after targeted execution towards environmental remediation threatens nontargeted organisms present in the natural environment. In short, addressing one problem should not create another. Generally, carbon materials such as biochar and graphene oxide exhibit better dispersion in the aqueous phase due to the presence of hydrophilic functional groups on their surface. Often, the transport and fate of the particles in porous media are decided by their physicochemical properties, the media, hydraulic flow, soil colloids and solution chemistry.^280,281

In the case of g-C₃N₄, it is hydrophobic in nature, but its triazine and tri-s-triazine structural building blocks are rich in hydrophilic amine groups (–NH₂, –NH–, and =N–). Rather than being a true solution, the amino groups in g-C₃N₄ exhibit better dispersibility and adsorption capacity in aqueous and porous media.^282,283 The deposition and migration of carbon nanomaterials, including g-C₃N₄, nano biochar and graphene oxide, were tested in packed columns that resemble the actual mechanism in the soil system. Studies have concluded that the presence of heavy metals and organic pollutants inhibited the migration of g-C₃N₄ in the porous media. Simultaneously, g-C₃N₄ acts as a transporter of adsorbed pollutants in a small fraction, discharging out of the saturated column media.^271,284,285 Furthermore, the promotion or inhibition of deposition in a saturated medium is influenced by the soil components, such as limestone,²⁸⁶ humus,²⁸⁷ clay minerals²⁸⁴ and hydrated iron oxides.²⁸⁸ We speculate that the dispersibility of g-C₃N₄ and its mobility under different conditions could cause accumulation in the tropic levels.

For example, Dong and Jian demonstrated the migration behaviour of g-C₃N₄ under various conditions, including water flow rate, pH, multivalent cation solution, ionic strength (IS), humic acid incorporation, and particle size of quartz sand media.^271,272 The mobility of graphene oxide in a porous medium was high under low IS (1 mM), but highly inhibited under higher IS (10 mM).²⁸⁹ At the same time, humic acid is rich in negative charges, and its higher levels improve the electronegativity for both g-C₃N₄ and sand media. As a result of the high electrostatic repulsion and negative zeta potential of g-C₃N₄ and sand media, increased mobility was observed in the saturated media.^286,290,291 Based on the above-mentioned results, Dong et al. investigated the transport and migration of g-C₃N₄ under the combined effect of IS (0.1, 1 and 10 mM) and other factors, including quartz sand grain size (0.70–0.85, 0.35–0.45 and 0.15–0.20 mm), humic acid concentration (0, 1 and 5 mg), and solution pH (4.0, 6.0, and 8.5). In this study, the one-site kinetic model was used to simulate the retention and transport of g-C₃N₄ in porous media, with very well-fitted breakthrough curves. Overall, the mobility of g-C₃N₄ in porous media was enhanced by an increase in sand grain size, solution pH, and humic acid concentration. Following the same trend, lower IS levels result in higher mobility of g-C₃N₄ in porous media.²⁷²

A similar study demonstrated the transport behaviour of g-C₃N₄ in quartz sand porous media in a packed column experiment under various factors, including flow rates (0.5, 1.0 and 2.0 mL min⁻¹), IS (0.1, 0.01, 0.001, 0.0001 M KCl, pH = 7), pH (4, 7 and 9 using 0.001 M NaOH or 0.001 M HCl) and multivalent cations (KCl–K⁺, CaCl₂–Ca²⁺, MgCl₂–Mg²⁺, and AlCl₃–Al³⁺, IS = 0.0001 M, pH = 7). It differs from the previous study, which integrates factors like flow rates and multivalent cations. The breakthrough curves are constructed as a function of the injection concentration ratios to the effluent rate. Based on the mass balance calculation, the recoveries of g-C₃N₄ at 0.1, 0.01, 0.001, 0.0001 M IS and DI water are 2.36%, 1.96%, 14.52%, 60.46% and 88.46%, respectively. g-C₃N₄ was retained in the sand column with a maximum amount at IS greater than 0.01 M. The nitrogen lone pair electrons on the surface of g-C₃N₄ easily bonded with the Si–OH group on the quartz sand surface. This phenomenon was confirmed by the FTIR spectrum, with the peak at around 970 cm⁻¹ corresponding to the stretching vibration of Si–OH. As the electric double layer of the g-C₃N₄ colloid was compressed with an increase in IS, there is a high probability of forming hydrogen bonds between g-C₃N₄ and the media. Conversely, the first layer of g-C₃N₄ was hydrogen-bonded to the surface of quartz sand, creating new adsorption sites through π–π conjugation. The reduced recovery of g-C₃N₄ under high IS conditions is attributed to the enhanced self-adsorption of g-C₃N₄. The migration efficiency was greatly inhibited under acidic conditions (6.99%), followed by basic (88.46%) and alkaline (93.14%) conditions. Lowering the pH results in excess H⁺ ions, with a less negative zeta potential, weakening the electrostatic repulsion and colloidal stability, and thereby promoting stronger adsorption and aggregation. The flow rates (0.5 to 2 mL min⁻¹) have a weak effect on transporting g-C₃N₄ in the porous media, yielding recoveries of 86.63%, 88.46% and 91.61%.²⁷¹ According to the Schulze–Hardy rule, cations possess higher colloidal coagulation properties. Subsequently, compared to monovalent cations, multivalent cations significantly hinder the mobility of NPs in porous media.²⁹² Consistent with this trend, the g-C₃N₄ retention effect in porous media follows the order of Al³⁺ (98.49%) > Ca²⁺ (69.67%) > Mg²⁺ (65.09%) > K⁺ (39.54%). The multivalent cation crosslinks with g-C₃N₄, especially Al³⁺ ions, forming flocculation precipitation at pH > 4.3 to produce aluminium hydroxide. Thus, AlCl₃ treatment at 0.1 mM exhibits the strongest aggregation and deposition of g-C₃N₄ in the soil porous media.²⁷¹ In the case of divalent cations (Ca²⁺ > Mg²⁺) with similar charge, the flocculation effect is closely related to the hydrated shell thickness of metal cations.²⁹³

Adding more complexity by incorporating heavy metals and diverse soil components to the experimental setup provided insights into the transport behaviour of g-C₃N₄. Zhou et al. demonstrated the impact of water flow rate, ionic strength, and presence of heavy metal (Pb) on the migration behaviour of g-C₃N₄. The above-mentioned factors were tested in complex soil components, including quartz sand, montmorillonite and humic acid-coated quartz sand in a packed column experiment. The results indicated that the water flow rate (2 mL min⁻¹) and IS (0.0001 mol L⁻¹) substantially increased the recovery of g-C₃N₄ by up to 79.81%, 73.01% and 71.74%. In contrast, a lower flow rate (0.5 ml min⁻¹) and higher IS greatly inhibited the recovery of g-C₃N₄ in all media. The incorporation of Pb modulates the transport behaviour, decreasing the g-C₃N₄ recovery in quartz sand, montmorillonite, and humic acid-coated sand media by up to 0.34%, 5.86% and 15.34%, respectively, compared to the media not contaminated with Pb. The C and N functional groups and π-conjugated structures present in g-C₃N₄ mediate Pb adsorption onto g-C₃N₄. The Pb adsorption data for g-C₃N₄ fitted the Langmuir model, indicating monolayer adsorption on the surface rather than interaction between adsorbates, with a maximum capacity of 61.86 mg g⁻¹. A cotransport experiment on quartz sand, montmorillonite and humic acid-coated sand revealed that the recovery of g-C₃N₄ was substantially reduced from 79.81%, 73.01% and 71.74% to 17.57%, 21.72%, and 21.47%, respectively, in the case of mixed liquid of Pb and g-C₃N₄. The possible mechanisms for the reduction in the recovery of g-C₃N₄ could be: 1) Pb adsorption in the media decreases the zeta potential and reduces the electrostatic repulsion between g-C₃N₄ and media, 2) the Pb and g-C₃N₄ mixed solution increases the ionic strength, enabling easier agglomeration within the media, and 3) the initially deposited g-C₃N₄ in the media provides new sites for free Pb adsorption.²⁷⁰

According to the above-mentioned experimental results, there is a high chance of transporting g-C₃N₄ from surface soil to deeper soil and underground water. Varying physicochemical factors should be considered for evaluating the potential transport behaviour of g-C₃N₄ under soil conditions. In addition, the immobile stable heavy metal compounds were enhanced in cotransport with g-C₃N₄. Linking the above-mentioned findings together on the reported transport behaviour, more factors that are prevalent under natural conditions still need to be tested. In addition, the reported results were obtained under controlled circumstances, whereas the natural ecosystem possesses diverse factors, which include 1) soil structure, 2) texture, 3) porosity, 4) permeability, 5) organic matter, 6) pH, 7) cation exchange capacity, and 8) biological activity. Certainly, it is inevitable to balance the reports on g-C₃N₄ transport under controlled circumstances. The application of g-C₃N₄ in the agroecosystem focuses on plant growth and development through varying interventions such as direct plant physiological development, altering soil properties by augmenting microbes and metal passivation. We present the discussion not based on the fate of g-C₃N₄ in relation to its transport behaviour. It is case-specific that applying dosages and permissible limits has been less explored in real-time applications. Under natural conditions, g-C₃N₄ can be degraded or broken down with exposure to light, temperature and microorganisms. This situation should be considered in the transport behaviour of g-C₃N₄ at different levels, where it may not persist or accumulate at any trophic levels. The outcomes on the biocompatibility of g-C₃N₄ with various organisms, as well as a cohort study on persistent or breakdown byproducts, could strengthen its utility. Concurrently, regardless of the breakdown process, the degraded products must not compromise the integrity of the ecosystem.

To address this, we propose X-ray absorption spectroscopy (XAS) analysis for the chemical modification of g-C₃N₄.²⁹⁴ XAS has the potential to unravel the degradation routes upon exposure to light and soil circumstances. Alongside its application, structural transformations occur, such as the cleavage of its C–N bonds and the formation of N-rich intermediates, including imide, nitrile, and amine groups. The structural changes are sensitively detected via N and C K-edge, which refers to the energy required to eject an electron from the core K-shell of an atom. It indicates a shift in π* and σ* conjugated systems, as reflected in the hybridisation and bonding environments.²⁹⁵ The imide, nitrile, and amine derivatives, namely phthalimide, acrylonitrile, and nitrosamines, are reported to be highly carcinogenic and mutagenic to living organisms. The release of these contaminants may have long-term negative impacts by persisting in soil and water, causing hazards to multiple trophic levels.^296–298

The fate of g-C₃N₄ in the soil ecosystem is site-specific, which depends on the soil properties. Among the nanomaterials with similar properties that exist in the present era, there has been a rise in their negative impact on society. Thus, the multidimensional properties of g-C₃N₄ have positive outcomes that play a significant role in improving the ecosystem. Rather than researchers focusing only on applications, proportionate consideration should be given to its degradation, biocompatibility, and fate. It is essential to establish a framework for addressing the limitations to ensure clarity and applicability.

11. Scalability proposal for agricultural application

Conventional fertiliser production is an industrialised process that turns out nitrogen, phosphorus, and potassium in the forms of ammonia, phosphoric acid, and potassium chloride, respectively. Among these vital nutrients, nitrogen is an indispensable source for plants to complete their lifecycle. Thus, the nitrogen demand is rectified by applying various nitrogenous fertilisers. For instance, ammonia is produced through the Haber–Bosch process, which involves expensive machinery for fertiliser processing. The Haber–Bosch process involves a large-scale sour gas unit, an air separation unit, an acid-gas separation unit, and an ammonia production unit.^3,299 Since 1909, the invention of artificial nitrogen fixation by Fritz Haber and Carl Bosch has revolutionised large-scale fertiliser production through industrial ammonia synthesis.²⁹⁹ Here, ammonia production proceeds through a multi-step catalytic process operating at high temperatures (400 °C to 650 °C) and pressure ranging from 200 to 400 atmosphere.³⁰⁰ Within a decade after the development of ammonia production, industrial urea production was started by adopting the Bosch–Meiser process in 1922. The process involves temperatures above 180 °C and a pressure of 150 atmosphere.³⁰¹ Despite using expensive machinery and an energy-intensive process, the feasibility of fertilisers is due to the large-scale manufacturing industries. Fortunately, over 80% of global ammonia is used in the form of fertiliser-grade urea, which supplements the nitrogen needs of plants.³⁰² In addition to fertiliser, urea has been widely used in the synthesis of plastics, animal feeds, urea-formaldehyde-based resins, and adhesives.³⁰¹

It is known that g-C₃N₄ is synthesised using nitrogen-rich precursors, including melamine, urea, thiourea, cyanamide, dicyanamide, and ammonium thiocyanate.³⁰³ g-C₃N₄ has been proposed as an eco-friendly photocatalyst due to its attractive properties, including metal-free precursors, chemical stability, and less toxic to humans.²¹⁸ Among the existing precursors for the synthesis of g-C₃N₄, urea has outperformed all other sources in terms of quality³⁰⁴ and compatibility.

In a life cycle assessment study, eight synthesis methodologies were considered for g-C₃N₄ production, among which thermal etching has the lowest global warming potential (in kg CO₂ eq.) to produce 1 g of g-C₃N₄. Optimising the calcination procedure, including shorter heating periods and faster ramp rates, reduces the electricity demand. Considering sustainability, the precursor production of 1 kg each of melamine, urea, and cyanamide was compared with the benchmark TiO₂ (titanium isopropoxide and titanium tetrabutoxide) precursors. The environmental impact assessment indicates that urea production has the lowest impact, followed by melamine. In comparison, the two TiO₂ NP precursors exhibit a greater environmental impact than urea and melamine but are less harmful than cyanamide. This study highlights that urea-based g-C₃N₄ production has the lowest cumulative environmental impact and demonstrates good scalability among the existing precursors and synthesis methods.⁵² This study indicates that process optimisation and renewable energy sources are key to overcoming the scalability burdens.

In contrast to conventional nanomaterials such as TiO₂, ZnO, and Ag, which require high calcination temperature, hydrothermal reactors, chemical vapour deposition,^305–307 these multistep processes are energy-intensive and demand high input costs. For example, synthesising Ag NPs necessitates regulations for using reducing agents and stabilisers.³⁰⁷ Although the synthesis of TiO₂ typically involves multistep hydrothermal or sol–gel processes,^306,308,309 it raises concerns about scalability and toxicity. Conversely, g-C₃N₄ is a low-cost, metal-free replacement, synthesised via one-step thermal polymerisation of precursors without demanding more complex reactors or toxic reagents.

When considering cost-effectiveness, producing g-C₃N₄ with precursors such as urea and melamine is economical via a simple thermal condensation technique.³¹⁰ Numerous studies have reported the efficiency of g-C₃N₄ produced using different precursor sources. Meanwhile, in a comparative study among nitrogen-rich precursors, g-C₃N₄ produced from the urea precursor resulted in a better nanostructure, specific surface area, and electron–hole separation.³⁰⁴ Although promising, current synthesis techniques are optimised for laboratory-scale protocols, yet are highly anticipated to achieve cost-efficient industrial-scale synthesis.

Long ago, the synthesis of ammonia and urea was considered emerging technology, but rapidly scaled in response to global demand. Analogously, at present, g-C₃N₄ is limited to lab-scale production, but exhibits strong potential in various sectors, including agriculture, making it an ideal candidate for large-scale synthesis. Based on the compatibility of urea as a precursor, urea manufacturing industries can render a feasible platform for integrated g-C₃N₄ production. However, transitioning g-C₃N₄ synthesis from batch-based laboratory protocols to industrial-scale continuous processing is still in the early stage. This mass production further needs optimisation of uniformity in yield, which has not been reported in the published research findings. Additionally, there are technical challenges in governing high temperatures (∼450–600 °C)³¹¹ for controlling the uniformity in large reactors, and their impact on photocatalytic stability aspects. However, with targeted research strategies, pilot-scale validation, and interdisciplinary efforts, robust and industrial-scale models for g-C₃N₄ production will be attainable shortly. In the case of large-scale applications, techno-economic analysis can help break down the barriers to accepting the potential of g-C₃N₄ for farmers and other stakeholders.

12. Techno-economic assessment and adoption potential of farmers

A rigorous examination of g-C₃N₄ for agricultural deployment must integrate production energetics/costs with beneficial functions and regulatory guidelines for consideration.¹²⁴ Life-cycle analysis (LCA)³¹² and techno-economic analysis (TEA)³¹³ with respect to g-C₃N₄ are highly recommended for laboratory proof of concept to agricultural relevance. LCA and TEA studies are the principal determinants of cost and environmental burdens.³¹³ The two main factors involved in LCA and TEA are (i) the energy utilisation for thermal polymerisation and (ii) the environmental and monetary cost of precursor production. A comparative LCA analysis shows that producing less expensive routes for g-C₃N₄ with widely available nitrogen-rich precursors, particularly urea, tends to achieve the lowest cradle-to-gate impacts among the synthesis routes.⁵² The provided insights on electricity demand for thermal energy management through process optimisation and upstream feedstock selection are the primary contributors to improving the techno-economic profile of g-C₃N₄. For instance, thermal decomposition offers a scalable and economical pathway to produce an abundance of g-C₃N₄.¹⁰¹

In contrast, the well-established industrial process for nitrogen fertiliser production, the Haber–Bosch route for ammonia production, followed by urea synthesis, is an energy-intensive process.^314,315 However, decades-long optimisation processes, including heat integration and production scale-up, enable a significant reduction in unit production costs.³¹⁶ Recent LCA and TEA assessments of ammonia and urea industries indicate that significant production gains are closely linked to integrated utilities such as heat recovery and shared feedstock logistics. It helps in lowering the capital expenditure (CAPEX) and operating expenditure (OPEX) on a unit basis.³¹⁴ Likewise, the low cost of urea, with its scalability and simple chemical composition (C, N, O, and H), has made it the most commonly adopted precursor.⁹⁷

We propose that co-locating g-C₃N₄ production with urea production facilities can be a strategy for process heat integration, sourcing bulk urea as a precursor, and opportunities for shared logistics could reduce CAPEX and OPEX for g-C₃N₄ manufacture. Despite this, current g-C₃N₄ production methods are optimised at the bench level, and there is a scarcity in pilot reports or scale-up demonstrations. Thus, reliance on critical engineering pathways enables facilities to operate with continuous furnaces, a faster ramp rate and improved product quality.

From an agronomic and economic perspective, it is essential to evaluate benefits in terms of services delivered per hectare or per cropping season, not as a raw mass. Thus far, no field investigations, either at the pilot scale or full scale, have been conducted to evaluate the application of g-C₃N₄ in the agroecosystem context. However, g-C₃N₄ in the proposed agriculture utility offers multiple functions by improving soil microbial growth,¹²⁰ enhancing nutrient use efficiency,¹²¹ directly augmenting photosynthesis,⁶⁹ increasing seed germination, passivating heavy metals¹¹⁹ and serving as a photocatalyst to break down pollutants.^303,317 Conventional fertilisers have a foreseeable future, being a cheap source of plant nutrients available on a mass basis. When the multidimensional g-C₃N₄ can replace many inputs, the net economic balance can shift in favour of adopting g-C₃N₄ despite the higher per-kg material cost.

Pilot studies focusing on the amount of energy required to produce a unit mass of g-C₃N₄ (kilowatt-hours per kilogram) and quantifiable agronomic effectiveness (for example, the percentage of conventional input that is replaced or percentage reduction per unit). To ensure a robust TEA, future pilot studies should provide clear data on energy utilisation and the actual agronomic benefits deliverable in real farming scenarios.

Its acceptance by farmers will be contingent on a clear demonstration of economic benefits, ease of handling or application and integration into existing input supply chains. g-C₃N₄ can be applied in the form of a seed coating, seed priming, foliar spray, or soil amendment, depending on the specific endeavour pursued. Additionally, the role of extension services in demonstrating pilot field studies by accommodating public and private stakeholders can play a decisive role. Coordinated efforts among farmers, scientists, policymakers and other stakeholders could facilitate the adoption of g-C₃N₄ in real-time agriculture, thereby addressing the persisting limitations. The initial costs of adopting nanotechnology in agriculture are expensive at this stage due to the less developed infrastructure. Capacity building and awareness programs are essential for introducing innovations or practices that are less prevalent among the farming community. The government should establish participatory training with community engagement for knowledge sharing. As a whole, a holistic approach by all stakeholders can ensure long-term benefits, including higher yield and lower input costs.

13. Conclusion and future perspective

In conclusion, g-C₃N₄ is the most promising metal-free nanomaterial, providing numerous benefits for agricultural improvement. The g-C₃N₄ aids plant growth and development by augmenting artificial photosynthesis and improving the antibiotic mechanism. The photosynthetic rate increased remarkably in association with enhanced stomatal conductance, CO₂ concentration, chlorophyll content, oxygen evolution, ATP/NADPH formation, DCPIP reduction and increased activities of key enzymes such as RuBisCO and PEPC. Further, g-C₃N₄ increases the electron transfer rate through higher photon absorption from the visible light region. The above-mentioned factors collectively increase photosynthate accumulation, which ultimately promotes plant growth and yield. The metabolic pathways of g-C₃N₄ interaction in the plant system confirmed that it can mitigate abiotic stress in crops by improving antioxidant metabolism. A reduction in ROS levels is achieved by producing antioxidants such as POD, SOD, and CAT. g-C₃N₄ exhibits superior photocatalytic degradation and biocompatibility, making it an alternative to existing materials with similar applications. In terms of the soil ecosystem, g-C₃N₄ alters the soil microbial community, nutrient use efficiency and heavy metal passivation. The enhanced photocatalytic activity of g-C₃N₄ facilitates the degradation of organic pollutants and promotes the transformation of heavy metals into a biologically non-assimilable form.

In this context, several key technical bottlenecks must be addressed to accelerate future research with the practical deployment of g-C₃N₄ in crop improvement. (1) Considering the contrasting photosynthesis pathways and physiological behaviours of C₃ and C₄ plants, targeted studies across diverse crop groups are essential to understand differential responses. (2) The compatibility of g-C₃N₄ with other nanomaterials and agricultural inputs, and assurance of their nontoxic interaction with organisms, must be systematically investigated. (3) Although the g-C₃N₄ precursors are relatively inexpensive compared with other nanomaterials offering similar functionalities, their synthesis remains energy-intensive. Optimisation of scalable energy-efficient production methods is therefore critical for real-world agricultural applications. (4) All existing studies have been conducted under controlled laboratory or greenhouse conditions. Field environments exhibit heterogeneous soil, fluctuating light intensity, and uncertain weather, which pose unique challenges that will determine the practical feasibility of g-C₃N₄ application. In addition, designing field trials prioritising the above-mentioned external circumstances is needed to validate laboratory outcomes and identify real-world limitations. (5) There is limited information on degradation pathways, long-term decomposition products, and their ecological impacts in soil and water, which are still unexplored, posing uncertainty in sustainable application. Cohort studies and life cycle assessment studies focusing on the cradle-to-grave stage of the application of g-C₃N₄ in the agroecosystem need to be performed. (6) Mechanistic studies focusing on in situ characterisations, molecular docking and multi-omics (transcriptomics, proteomics, and metabolomics) to track the translocation pathway, localisation, biochemical effects and to predict and visualise the interaction of g-C₃N₄ within the plant system. (7) Formulating agriculturally relevant forms such as granules, urea-g-C₃N₄ hybrid forms, slow-release agents, and seed coatings. These co-application strategies with other inputs may enhance stakeholder interest by reducing application costs and improving ease of handling. (8) A comprehensive techno-economic analysis is also warranted to evaluate the potential cost savings achieved by reducing conventional agricultural inputs when g-C₃N₄ is applied. These assessments can offset the high input cost associated with intensive farming practices.

This review provides the first comprehensive insights into the potential of g-C₃N₄ nanomaterials in agroecosystems and outlines the scientific and industrial opportunities for their application. It offers guidance to researchers, students, agricultural practitioners and chemical industries on the emerging role of g-C₃N₄ in enhancing crop performance. While synthetic nitrogen fertilisers have remained largely irreplaceable for nearly a century, the current global push for sustainable agriculture demands innovative alternatives. Addressing the above-mentioned challenges through coordinated and interdisciplinary efforts may open a new horizon in nutrient management, ultimately enabling the responsible and effective use of g-C₃N₄ in crop production systems.

Author contributions

Conceptualisation: T. P. K., R. J., and S. M. R.; investigation: T. P. K., and S. M. R.; formal analysis: T. P. K.; writing – original draft preparation, R. J.; and supervision: T. P. K.

Conflicts of interest

There are no conflicts to declare.

Abbreviation

NPs	Nanoparticles
Fe3O4	Ferric oxide
CD	Carbon dots
CQD	Carbon quantum dot
CNT	Carbon nanotubes
TiO2	Titanium dioxide
LED	Light emitting diode
ATP	Adenosine triphosphate
O2	Oxygen
NADPH	Nicotinamide adenine dinucleotide phosphate
ETR	Electron transfer rate
Cd	Cadmium
POD	Peroxidase
CAT	Catalase
SOD	Superoxide dismutase
MDA	Malondialdehyde
ROS	Reactive oxygen species
RNS	Reactive nitrogen species
CeO2	Cerium dioxide
FeO2	Iron dioxide
TEM	Transmission electron microscopy
FESEM	Field emission scanning electron microscopy
ZnO	Zinc oxide
NiO	Nickel(II) oxide
GO	Graphene oxide
MWCNT	Multi-walled carbon nanotube
PL	Photoluminescence
CLSM	Confocal laser scanning microscopy
NO3	Nitrate
NH4	Ammonium
dsRNA	Double-stranded RNA
GFP	Green fluorescent protein
TMV	Tobacco mosaic virus
CP	Coat protein gene
ABA	Abscisic acid
DFT	Density functional theory
rGO	Reduced graphene oxide
SPAD	Soil plant analysis development
RuBisCO	Ribulose-1,5-bisphosphate carboxylase/oxygenase
SOM	Soil organic matter
TOC	Total organic carbon
XPS	X-ray photoelectron spectroscopy
BET	Brunauer–Emmett–Teller
ORR	Oxygen reduction rate
OER	Oxygen evolution rate
PEPC	Phosphoenolpyruvate carboxykinase
KCl	Potassium chloride
HCl	Hydrogen chloride
NaOH	Sodium hydroxide
LC50	Lethal concentration 50
CaCl2	Calcium chloride
MgCl2	Magnesium chloride
FTIR	Fourier transform infrared spectroscopy
Al^3+	Aluminium ion
Ca^2+	Calcium ion
Mg^2+	Magnesium ion
TEA	Techno-economic analysis
CAPEX	Capital expenditure
OPEX	Operating expenses

Data availability

No primary research results, software or code have been included, and no new data were generated or analysed as part of this review.

Supplementary information (SI): for Supplementary Table 1 (g-C₃N₄-based nanocomposites and their biomedical and antimicrobial applications. The table outlines assay models, and major biological or functional outcomes). See DOI: https://doi.org/10.1039/d5en00446b.

Acknowledgements

The author(s) declare that financial support was received for the publication of this article. This research was funded by Vellore Institute of Technology, Vellore, India. We thank VIT University for providing technical support throughout the manuscript preparation. TPK acknowledges VIT for providing VIT SEED GRANT (RGEMS) – Sanction Order No.: SG20230059 for the seed germination work.

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