Environmental applications of single-atom nanozymes: recent developments and future directions

Abstract

Single-atom nanozymes (SANs) are a new class of artificial enzymes that feature isolated metal atoms anchored to tailored supports. This precise atomic dispersion provides maximum active site utilization while preventing aggregation and leaching, offering enhanced stability and recyclability over traditional nanozymes. This review systematically examines recent advances in the development of SANs for environmental applications, focusing on both pollutant detection and remediation. The review begins with a critical evaluation of synthesis methods of SANs, including pyrolysis, wet chemical, atomic layer deposition, and coordination templating approaches using both carbonaceous and non-carbon supports. Subsequent sections examine the structure–activity relationships governing SANs and describe the factors that modulate their enzyme-like behaviors. Particular emphasis is placed on diverse catalytic properties, with attention to activity-tuning strategies such as heteroatom doping, microenvironmental engineering, and spin-state modulation. Next, the review assesses SANs-based detection platforms, such as colorimetric, fluorescence, electrochemical, and photoelectrochemical systems for monitoring and remediating diverse pollutants. This review critically evaluates current limitations of SANs and outlines strategies for their advancement. Density functional theory (DFT) emerges as indispensable for rational design, enabling computation-guided active site engineering and elucidation of structure–activity relationships. Structurally tunable active centers provide unique opportunities to construct dual-, tri-, and multi-enzyme systems with enhanced catalytic versatility. Future research directions include tandem or bimetallic SANs for multielectron redox catalysis and integrated microdevices combining real-time detection with catalytic mineralization. Particular emphasis is placed on applications where SANs surpass conventional nanozymes or enzymes, thereby establishing design principles for next-generation, multifunctional catalytic platforms.

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Arumugam Selva Sharma

Dr. Arumugam Selva Sharma received his Ph.D. degree from Bharathiar University, India, in 2016. He conducted postdoctoral research at several institutions, including Pondicherry University, the Sree Chitra Tirunal Institute for Medical Sciences and Technology in India, and Jiangsu University in China, from 2017 to 2023. In 2023, he was appointed as an Assistant Professor at Gachon University in South Korea. His research focuses on developing diagnostic platforms that integrate PCR, LAMP, and nanoparticle-based approaches for colorimetric and fluorometric detection of disease biomarkers and pathogens.

Nae Yoon Lee

Professor Nae Yoon Lee received her Ph.D. degree from the University of Tokyo, Japan in 2004 and worked as a postdoctoral fellow at Korea Advanced Institute of Science and Technology (KAIST) and as a research professor at Ewha Womans University in South Korea from 2004 to 2007. She has been a Professor at the Department of BioNano Technology in Gachon University since 2007. Her main research field is concerned with the development of lab-on-a-chip systems particularly focusing on microdevice fabrication and sealing, surface modification, and developing portable diagnostic devices for point-of-care testing.

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Environmental significance

Single-atom nanozymes (SANs) have emerged as a novel class of catalysts in which isolated metal atoms are anchored onto support matrices. This structural configuration enables SANs to surpass the catalytic efficiencies of natural enzymes in various applications including environmental monitoring and remediation. In this review, we provide an in-depth survey of various strategies for synthesizing SANs and discuss a range of SANs-based sensing technologies, including optical and electrochemical platforms. Furthermore, we evaluate their effectiveness in the detection and remediation of diverse environmental pollutants, including organic compounds (e.g., phenol, volatile amines, and hydroquinone), pesticides, drug residues, dyes, microbial hazards, and heavy metal ions. The review concludes by outlining future research directions, emphasizing the development of integrated SANs platforms that combine ultrasensitive detection with catalytic degradation, thereby enabling comprehensive “all-in-one” solutions for environmental pollutant management.

Introduction

Natural enzymes exhibit exceptional catalytic performance due to their highly evolved active sites, conformational flexibility, and substrate-specific microenvironments. Their catalytic efficiencies often approach diffusion-limited rates, with precise selectivity and biocompatibility under physiological conditions. However, their inherent fragility such as sensitivity to pH, temperature, and proteolytic degradation combined with high production and purification costs, limits their practical applicability outside biological systems.^1,2 In contrast, conventional nanozymes, comprising metal oxides, noble metals, or carbon-based nanomaterials, have emerged as alternatives to their natural enzymatic counterparts. Nanozymes present significant advantages over natural enzymes, including robust structural integrity, cost-effective synthesis, and tunable activity even under harsh chemical conditions. However, their practical efficacy and broader application are hindered by several challenges, namely, inherent structural heterogeneity, low active-site density, and an incomplete understanding of the underlying catalytic mechanisms.^3–5 Moreover, the absence of well-defined active centers allows these nanozymes to act on a broad range of substrates. Nevertheless, this versatility is often accompanied by reduced catalytic specificity and relatively modest turnover efficiency.⁶

Qiao et al. demonstrated that a Pt₁/FeO_x single-atom catalyst effectively catalyzed CO oxidation, marking a key advancement in connecting single-atom heterogeneous catalysis with enzyme-like behavior; they also reported that atomically dispersed Pt could achieve turnover frequencies comparable to those of Pt nanoparticles.⁷ Applying this single-atom model to nanozymes has subsequently resolved persistent challenges associated with metal/metal oxide-based nanozymes, such as indistinct structure–activity correlations and limited substrate selectivity caused by multiple surface terminations. Consequently, single-atom nanozymes (SANs) have emerged as a transformative class of catalysts in which isolated metal atoms are anchored onto matrices such as M–N–C architectures or graphitic carbon nitride.⁸ SANs overcome many of the limitations of natural enzymes and nanozymes as the atom-by-atom dispersion maximizes metal utilization, imposes a well-defined coordination sphere, and facilitates precise electronic modulation.⁹ SANs have demonstrated enzyme-mimicking activities including peroxidase, oxidase, and catalase comparable to or exceeding those of both conventional nanozymes and, in some cases, natural enzymes.^1,10 For instance, Fe₃P SANs have shown turnover numbers approaching that of horseradish peroxidase (HRP), along with enhanced durability and recyclability.¹ Moreover, the well-defined electronic structures of SANs create a direct correlation between coordination geometries and favorable kinetic parameters, an advantage intrinsic to natural enzymes but rarely achieved with conventional nanozymes.^11,12

In recent years, the need for artificial enzymes that exhibit exceptional catalytic specificity has become particularly pronounced in the fields of biosensing and environmental monitoring. For example, colorimetric, chemiluminescence, fluorescence, and electrochemical assays rely heavily on natural redox enzymes, such as horseradish peroxidase or laccase, to generate measurable signals.¹³ However, natural redox enzymes denature outside of narrow temperature and pH windows, rendering them unreliable in resource-limited settings. By contrast, SANs can catalyze the oxidation of chromogenic substrates (e.g., 3,3′,5,5′-tetramethylbenzidine (TMB), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), or luminol) with a turnover several thousand times higher than that provided by metal-oxide nanozymes, enabling sub-micromolar detection of hydrogen peroxide (H₂O₂) and mycotoxins on nitrocellulose membranes or paper without signal drift over weeks of ambient storage.^14,15 Furthermore, SANs typically exhibit greater stability than conventional nanozymes or natural enzymes, particularly under adverse conditions such as elevated temperatures or high salinity. However, the stability of SANs is highly dependent on the specific metal–support system and the coordination environment of the metal atom. All these unique characteristics make SANs well-suited for biological, environmental and marine monitoring applications.^16–19 Moreover, previous studies have demonstrated that atomically dispersed active sites provide nanozymes with excellent catalytic efficacy, which, together with the pronounced environmental stability associated with SANs, is critical for applications in biosensing, environmental monitoring, and food research.^20–22

Global industrialization, fertilizer-intensive agriculture, and unchecked urbanization have broadly introduced phenolic carcinogens, persistent dyes, antibiotic residues, and heavy metals into air, water, and soil systems. Traditional remediation techniques, such as chlorination, activated sludge, and photocatalysis, often require considerable chemical or energy inputs and generate secondary pollutants. By contrast, SANs activate benign oxidants, such as molecular oxygen, H₂O₂, and persulfate, to generate reactive oxygen species that mineralize contaminants under ambient conditions. In addition to these catalytic degradation capabilities, optical and electrochemical sensors incorporating SANs can measure the heavy metals, organophosphate pesticides, and endocrine-disrupting compounds on flexible platforms at parts-per-billion (ppb) concentrations. When these sensors are embedded within “sense-and-destroy” cartridges, the same single-atom framework can not only quantify the presence of a contaminant but also catalytically eliminate it, thereby integrating detection and remediation into a single operational cycle.^2,23

Although previous reviews have addressed different aspects of SANs, including synthesis methodologies, surface functionalization, catalytic mechanisms, and biosensing applications, ongoing research continues to reveal new opportunities for the use of SANs in environmental monitoring and remediation.^{2,12,14,17,23–29} This review provides an overview of the latest advances in SANs technology, emphasizing optical and electrochemical platforms tailored for pollutant detection and degradation. This review accordingly begins with an in-depth survey of the various strategies for synthesizing SANs with different carbon and non-carbon-based supports as well as the assorted modification strategies used to regulate SAN activity. Subsequently, various SANs-based sensing technologies, including optical and electrochemical platforms, are discussed, and then their effectiveness in the detection and remediation of organic pollutants (phenol, volatile amines, and hydroquinone), pesticides, drug residues, dyes, and heavy metal ions (Scheme 1) is evaluated. Finally, the review concludes by outlining future research directions such as the development of density functional theory (DFT)-assisted synthesis, tandem or bimetallic SANs capable of sequential redox catalysis, and integrated SANs platforms that combine ultrasensitive detection with catalytic degradation to enable “all-in-one” solutions for comprehensive environmental pollutant management.

Scheme 1 Overview of SANs in environmental monitoring and remediation applications.

Literature search and inclusion criteria

Peer-reviewed journal articles on single-atom nanozymes were retrieved from the Web of Science (https://www.webofscience.com/wos/) and CAS SciFinder (https://scifinder-n.cas.org/) databases for publications available prior to August 1, 2025. Keyword searches were conducted using different combinations of words, including “single atom nanozymes”, “single atom nanozymes AND environmental applications”, “single atom nanozymes AND colorimetric”, “single atom nanozymes AND electrochemical”, and “single atom nanozymes AND fluorescence”. A total of 1429 publications were initially identified. After individual screening to retain only original research articles that specifically focused on the electrochemical and optical detection of environmental pollutants, 80 articles were selected for the review article.

SANs synthesis strategies

The method used to synthesize SANs plays a crucial role in determining their catalytic performance because SANs feature metal centers dispersed at the atomic scale and anchored within well-defined coordination environments. This section accordingly begins with a critical overview of key strategies for synthesizing SANs. Next, various post-synthesis modifications, including ligand exchange, defect passivation, and surface encapsulation, used to fine-tune the catalytic activity and improve the operational stability of SANs, are explored.

Bottom-up synthesis

Wet chemical synthesis. Wet chemical synthesis is the most widely used bottom-up approach for creating SANs. Under this approach, soluble metal precursors are chemically reduced or thermally pyrolyzed in the presence of defect-engineered supports providing lattice vacancies, heteroatom dopants, or extended π-conjugated domains that act as anchoring sites to capture and stabilize freshly formed metal adatoms. Kim et al. immobilized Fe–N₄ motifs within reduced graphene oxide (Fe–N-rGO) to produce SANs that exhibited peroxidase (POD)-like kinetics and applied them to detect H₂O₂ down to concentrations of 10 nM.³⁰ In another report, Wu et al. anchored single Fe atoms on N-doped carbon nanotubes, confirming atomic dispersion by extended X-ray absorption fine structure (EXAFS) and achieving four-fold higher POD-like activity than commercial Fe₃O₄ nanozymes.³¹ Notably, although wet chemical synthesis protocols offer a low-temperature, scalable approach, the resulting metal loadings seldom exceed ∼2 wt%. Moreover, the presence of residual capping ligands often blocks the catalytically active centers in wet chemical synthesized SANs.

Atomic layer deposition. Atomic layer deposition (ALD) operates through a self-limiting, sequential surface chemistry in which each precursor exposure deposits at most one atomic layer, thereby affording angstrom-level control over film thickness and composition. Sun et al. used ALD to deposit single isolated Pt atoms on graphene, doubling the mass activity for the oxygen-reduction reaction relative to that of conventional Pt nanoparticle catalysts (Fig. 1A).³² More recently, Zhou et al. employed ALD to graft isolated Rh atoms onto SnO₂ by pulsing tris(acetylacetonato)-rhodium and O₂ at 200–250 °C for two cycles to deposit ∼0.05 wt% Rh sub-monolayers. The resulting single-atom film exhibited excellent catalytic kinetics and gas-sensing selectivity performance.³³ Although ALD offers angstrom-scale control, the need for vacuum hardware and long cycle times can hinder large-volume environmental applications of this technology.

Fig. 1 (A) Schematic representations of the Pt ALD mechanism over graphene nanosheets, reproduced with permission from ref. 32, Copyright 2012 Springer Nature, (B) schematic diagram of the synthesis process for the Pt₁/NPC catalyst, reproduced with permission from ref. 34, Copyright 2018 American Chemical Society, (C) schematic depiction of the iced-photochemical process, reproduced with permission from ref. 35, Copyright 2017 Springer Nature, and (D) schematic representation of the Fe/NC–NaCl preparation process employing surface–molten NaCl. The molecular visualization uses gray (C), yellow (N), and pink (Fe) spheres to denote the respective atoms, reproduced with permission from ref. 41, Copyright 2021 John Wiley and Sons.

Photochemical synthesis. The photochemical synthesis strategy employs ultraviolet (UV) photons (often at subzero temperatures) to reduce adsorbed metal complexes and suppress thermal diffusion that would otherwise form clusters during the preparation of SANs. Li et al. employed a photochemical solid-phase reduction strategy to anchor atomically dispersed Pt onto N-doped porous carbon (Pt₁/NPC), obtaining a metal loading of 3.8 wt% and achieving a 24-fold increase in activity compared with that of commercial Pt/C (Fig. 1B). Spectroscopic observations indicated that this markedly enhanced electrocatalytic performance was predominantly a result of a well-defined Pt–N₄ coordination environment established by electron transfer from the isolated Pt centers to adjacent N atoms.³⁴ Wei et al. reported a photochemical strategy in which cryogenically frozen H₂PtCl₆ was irradiated to yield atomically dispersed Pt on mesoporous carbon and TiO₂, conferring a high oxidation activity relative to commercial Pt/C catalysts (Fig. 1C).³⁵ Notably, photochemical processes have attracted renewed interest for low-carbon catalyst manufacture because they are ligand-free and energy-efficient.

High-energy ball milling. High-energy ball milling has emerged as a versatile and solvent-free method for generating atomically dispersed metal sites on defect-rich supports. Gan et al. developed a scalable dry ball milling method for the large-scale synthesis (>1 kg) of single-atom Au₁/CeO₂ catalysts. The resulting Au₁/CeO₂ exhibited exceptional catalytic performance for the preferential oxidation of CO at 120 °C, achieving complete CO conversion (100%) with high selectivity and stability. Kinetic studies revealed that the turnover frequencies for H₂ oxidation over Au₁/CeO₂ at 120 °C (<0.01 s⁻¹) were approximately two orders of magnitude lower than those for CO oxidation, demonstrating a practical and scalable route for manufacturing high-performance SACs with uniform reactivity.³⁶ Extending this concept, Xu et al. developed a two-stage ball milling method in which N-doped carbon (5.0 g) and metallic Cu powder (100 mg) were co-milled in a 70 mL iron jar containing thirty 6 mm agate balls at 100 W for 6 h to produce a CuNP–NC composite. This composite (5.0 g) was subsequently subjected to a second 6 h milling cycle under identical conditions to further reduce the particle size and enrich the defect sites capable of binding single atoms. Finally, the milled material was subjected to an acid etching treatment for 1 h, washed with water, and then dried at 80 °C overnight to yield Cu-anchored N-doped carbon (Cu–NC) SANs. These SANs exhibited nearly twice the POD-like activity of CuNP–NC owing to improved H₂O₂ activation. This enhanced efficiency makes Cu–NC SANs promising candidates for applications in catalysis and oxidative reactions.³⁷

Mass-selected soft-landing synthesis. The mass-selected soft-landing synthesis strategy offers control over nuclearity and charge states when depositing metal species onto supports. Heiz et al. generated size-filtered Pt_1−n clusters (where n = 1–30) in a gas-phase source. These clusters were subsequently deposited onto a clean MgO (100) surface after momentum softening to form atomically precise, ligand-free Pt ensembles. The researchers systematically mapped the catalytic activities of these Pt_1−n ensembles in CO oxidation, revealing direct correlations between the cluster structure and reactivity.³⁸ Furthermore, Kwon et al. deposited mass-selected Pt₇ and Pt₉ clusters onto ultrathin Al₂O₃ films in an ultra-high vacuum, achieving sub-monolayer coverage with angstrom-level spatial precision. Operando X-ray absorption and DFT analyses revealed size-dependent d-band modulation, which translated into order-of-magnitude enhancements in the ethylene hydrogenation turnover frequencies of the Pt clusters relative to those of bulk Pt.³⁹

Top-down synthesis

In contrast to bottom-up synthesis methods, top-down synthesis methods systematically split bulk metals or preformed nanoparticles into discrete single-atom sites through chemical etching and electrochemical stripping. The liberated metal atoms subsequently migrate toward the heteroatom dopants or intrinsic lattice defects, where they anchor firmly. These robust metal–support interactions inhibit agglomeration and leaching, thereby imparting exceptional structural integrity and long-term catalytic durability to the resulting materials.

High-temperature pyrolysis and gas-phase migration. High-temperature pyrolysis is a powerful top-down method for generating isolated single-atom sites. Under this method, metal–organic frameworks (MOFs) or zeolitic imidazolate frameworks (ZIFs) serve as nitrogen-rich sacrificial templates for fabricating SACs. Fan et al. encapsulated Fe-ZIF-8 crystals within a SiO₂ shell, subjected this core–shell architecture to pyrolysis at 900 °C, and then removed the silica layer to expose atomically dispersed Fe sites. The transient SiO₂ barrier effectively inhibited Fe agglomeration during carbonization, yielding an SAC exhibiting four times more POD-like activity than its uncoated counterpart.⁴⁰ Wang et al. utilized molten NaCl as a spatial-confinement medium to encapsulate an Fe–Zn ZIF precursor during 900 °C pyrolysis. The NaCl melt suppressed metal sintering, producing a high-density array of Fe–N₄ sites (3.2 × 10¹⁹ sites g⁻¹) that retained >95% catalytic turnover in 0.1 M HCl, reflecting exceptional acid resistance (Fig. 1D).⁴¹ In another study, Liu et al. synthesized a self-supporting Co–N–C catalyst featuring exclusively single-atom dispersed Co sites using a sacrificial support strategy. This approach initially deposited a Co(II) bis(1,10-phenanthroline) diacetate precursor on a Mg(OH)₂ support. The resulting composite material subsequently underwent pyrolysis at 700 °C in a N₂ atmosphere for 2 h. Following thermal treatment, the sacrificial MgO template was selectively etched using nitric acid treatment, yielding the final catalyst with a well-defined atomic dispersion of Co species within the N-doped carbon matrix. This structurally defined catalyst demonstrated exceptional performance in the chemoselective hydrogenation of nitroarenes to azo compounds under mild conditions.⁴²

Among all the preparation methods discussed, pyrolysis remains the most extensively adopted technique for gram-scale synthesis.⁴³ Pyrolysis of MOFs, such as ZIF-8, allows effective confinement of metal ions during high-temperature treatment, resulting in high-density single-atom dispersion.⁴³ This method is scalable and already demonstrates gram-level output in laboratory settings with relatively high reproducibility when key parameters such as temperature, ramp rate, and inert atmosphere are strictly controlled.^8,44 However, challenges still remain such as metal atom aggregation and uneven thermal distribution which limit its scale-up toward kilogram levels.⁴⁵ Ball milling, a solvent-free and energy-efficient method, is increasingly favoured due to its eco-friendly profile. It has shown potential for kilogram-scale synthesis and offers a green and reproducible route for preparing SANs with well-dispersed atomic sites, especially when combined with post-annealing steps.^36,46 Control over mechanical parameters (e.g., milling speed and ball-to-powder ratio) is essential to achieve uniformity and reproducibility across batches.¹⁷ Wet-chemical methods offer moderate scalability and simplicity but often suffer from limited control over the coordination environment and require post-treatment steps (e.g., calcination or ligand removal), which can compromise atom dispersion and reduce batch reproducibility.¹⁷ In contrast, atomic layer deposition (ALD) provides precise atomic control and high reproducibility by sequential self-limiting surface reactions. However, the low throughput and high operational cost of ALD hinder its suitability for large-scale production of SANs unless adapted into more scalable configurations.^43,47 At present, pyrolysis and ball milling represent the most promising synthesis approaches for scalable and sustainable preparation of SANs. While ALD and wet-chemical methods offer advantages in precision, their scalability remains constrained. Continued efforts toward process optimization, precursor engineering, and green synthesis will be essential to bridge the gap between laboratory synthesis and industrial applications of SANs.

Carbon- and non-carbon-based supports

The presence of isolated active centers in a SANs depends on the physicochemical properties of the host support. Nitrogen-rich carbon support architectures, such as graphene, carbon nanotubes, graphdiyne, and MOF-derived N-doped carbon, provide exceptionally high specific surface areas. Furthermore, their abundant heteroatom coordination motifs (M–N_x) secure individual metal atoms and modulate the d-band electronic structure, thereby contributing to improved enzyme-like catalytic turnover. Choi et al. developed an S-doped zeolite-templated carbon support featuring a three-dimensional (3D) graphene nanoribbon network and high S content (17 wt%) that enabled exceptional Pt dispersion (5 wt%) as isolated atoms and clusters. Remarkably, this catalyst selectively produced H₂O₂via a two-electron oxygen reduction pathway, maintaining sustained activity without degradation, in contrast to conventional four-electron reduction to H₂O. Indeed, its unique carbon architecture and S-doping synergistically stabilized Pt species, offering a promising route for sustainable H₂O₂ production.⁴⁸

Cheng et al. designed carbon nanotube-supported SANs (CNT/Fe–N–C) featuring atomically dispersed Fe–N_x active sites and synthesized them using NaCl crystals as sacrificial templates. The π–π stacking of Fe–N_x motifs between nanotube walls further decreased the Michaelis constant for H₂O₂ activation (0.129 mM for CNT/Fe–N–C SANs vs. 0.241 mM for Fe₃O₄ nanozymes), demonstrating the acceleration of electron transfer via conductive carbon.⁴⁹ Furthermore, the use of MOFs can exploit strong metal–support interactions. For example, single Pt atoms on CeO₂ coupled their 5d orbitals with Ce 4f states, considerably enhancing radical-scavenging efficacy in neuro-injury models.⁵⁰ Likewise, Co nanodiscs anchored on phase-distorted 1 T-MoS₂ reconfigured the sulfide lattice and synergistically boosted POD-like catalysis beyond that of either component alone.⁵¹

Thus, optimal SANs designs depend on three parameters: (i) anchoring energy, which prevents sintering, (ii) electronic coupling, which tailors adsorption energetics, and (iii) transport architecture, which maximizes substrate flux.² Carbonaceous matrices excel in providing conductivity and heteroatom tunability and are well-suited for use in electrochemical biosensors as a result, whereas reducible oxides and chalcogenides provide strong metal–support interactions for the oxygen-based chemistries integral to pollutant degradation. Therefore, rational metal–support pairing is critical to achieving improved SANs performance across bioanalytical, therapeutic, and environmental applications.

Modification strategies for regulating the activity of SANs

Heteroatom-directed microenvironment engineering and spin-state modulation have emerged as potent control for regulating post-synthesis activity in SANs. Remote heteroatom doping provides powerful electronic modulation without sacrificing site isolation. Indeed, the insertion of P into Fe–N₄ motifs on carbon nanowires redistributed charge at the metal center and tripled the POD-like turnover frequency at 25 °C.⁵² By contrast, B donation reduced the activation barrier for hydroxyl radical generation, thereby accelerating chromogenic TMB oxidation.⁵³ Furthermore, F-doped Fe–N–C SANs exhibited enhanced CO₂ electroreduction performance owing to the electron-withdrawing effect of F, which stabilizes Fe³⁺ active sites.⁵⁴ Besides first-shell chemistry, tailoring the coordination number and spin state affords another degree of freedom. A five-coordinated FeN₅ site prepared by controlled ligand addition generated six times more hydroxyl radicals than the canonical Fe–N₄ analog, translating into superior tumor ablation in murine models.⁵⁵ Conversely, lowering the spin of Fe–N₄ centers to match that of natural catalase eliminated radical overproduction, enabling non-cytotoxic intracellular H₂O₂ scavenging.⁵⁶

At the support level, the insertion of single Ni atoms into S-vacancy-rich MoS₂ accelerated interfacial charge transfer and realized dopamine sensing down to 0.2 nM, two orders of magnitude below that realized by pristine dichalcogenide.⁵⁷ Under harsh conditions, thermodynamic mobility of metal atoms leads to deactivation dominated by Ostwald ripening, ligand leaching, and support corrosion. To address this challenge, current stabilization strategies focus on complementary approaches, including enhancing metal–support interactions through high-energy M–N or M–O covalent bonds (2 eV), embedding atoms within thermally robust carbon nanoframes or silica shells (stable up to 900 °C), and employing redox-active “self-healing” matrices to restore migrated atoms to their original states. Nevertheless, no single strategy has ensured continuous operation beyond 1000 h under combined acidic and saline conditions, highlighting the importance of SANs stability for practical applications.

However, recent research work has been expanding the notion of catalytically “active sites” from strictly isolated atoms to correlated atomic assemblies that balance high turnover frequencies with enhanced stability. For example, diatomic SANs comprising Fe–Se pairs embedded in a carbon matrix have simultaneously accelerated oxygen reduction and peroxide decomposition, increasing the specific power of neutral Zn–air batteries by approximately 30%.⁵⁸ Furthermore, dual-metal or trimeric constructs, such as Ni(Co)–Ru–P clusters synthesized by gradient atomic layer deposition, have achieved near-unity faradaic efficiency in H₂N₄ dehydrogenation to power self-driven H₂ generators.⁵⁹ Thus, by uniting atomic-precision synthesis, micro-environment regulation, and durability assurance, next-generation SANs have the potential to become low-cost, high-performance materials for environmental monitoring and green catalysis.

Structure–activity relationships and factors influencing the enzyme-like activities of SANs

The nanozyme and catalytic efficacy of SANs are intrinsically governed by their metal coordination environment and electronic structure. A central factor influencing catalytic efficiency in SANs is the identity and coordination structure of the metal active center.^10,60,61 Recent studies underscored that Fe–N₄, Co–N₄, and Mn–N_X active sites embedded within nitrogen-doped carbon substrates facilitate divergent reactive oxygen species (ROS) generation pathways, depending upon their electronic configuration, ligand field, and interaction with reaction mediators (H₂O₂).^1,62 Metal–nitrogen motifs such as Fe–N₄, Co–N₄, and Mo–N_x have shown distinct reactivities depending on the coordination number and electronic structure of the metal atom.^6,63 For instance, a comparative study of M–N₄ sites (M = Fe, Co, and Zn) revealed that Fe–N–C SANs exhibit superior POD-like activity, as evidenced by enhanced signals in electron paramagnetic resonance (EPR) indicating higher generation of hydroxyl radicals. DFT calculations attributed this superiority to favorable electron transfer properties and optimized adsorption energetics. Specifically, the d-band center of Fe–N–C (−1.40 eV) was closer to the Fermi level than those of Co–N–C or Zn–N–C analogues, facilitating stronger interactions with intermediates and promoting H₂O₂ activation. This underscores the critical role of the electronic structure of the metal center in determining catalytic performance.⁶⁴ The catalytic performance of SANs is driven by the surrounding ligand numbers and species. For instance, a ligand engineering strategy applied to Pt-based SANs transformed Pt–N₄ into a Pt–N₃PS structure. Although H₂O₂ binding affinity remained largely unchanged due to the similar carbon support, the altered electronic structure such as inverse Bader charge and modified charge density of the Pt–N₃PS site shifted the free-energy of the reaction pathway. This electronic redistribution resulted in a higher turnover number and greater POD-like catalytic efficiency compared to the conventional Pt–N₄ configuration.^24,65

The electronic structure and geometry of the metal center in SANs govern the energy profile of the reaction pathway. Consequently, these structural parameters directly determine the identity of the rate-determining step (RDS) by influencing the adsorption energies of key intermediates and the activation barriers for critical events such as bond cleavage and electron transfer. As a key indicator of catalytic performance, the RDS not only governs the overall reaction kinetics but also controls the selectivity of ROS generation. This structure–activity relationship is clearly demonstrated by the distinct RDS behaviors observed across different metal–N_x configurations. For conventional Fe–N₄ motifs, the RDS in POD-like catalysis typically involves the transformation of surface hydroxyl species derived from H₂O₂ dissociation, such as the 2OH → O + H₂O step, which carries a relatively high barrier (∼1.05 eV).⁶⁶ In contrast, the engineered FeN₃P coordination environment, where one nitrogen atom is replaced by phosphorus, significantly lowers this barrier. DFT calculations and XAFS analyses showed that P coordination modifies the local electronic structure, reduces electron withdrawal from the Fe center, and facilitates electron donation via a P → Fe → N pathway. As a result, the FeN₃P site achieves lower activation barriers for O and OH formation (<0.5 eV), enhanced stabilization of reactive intermediates, and a shift in the RDS from sluggish OH conversion on Fe–N₄ to a much easier O migration step on FeN₃P. This modulation of orbital overlap and charge distribution at the Fe center rationalizes the observed 10-fold increase in catalytic activity compared to Fe–N₄ analogues.¹ A study by Xu et al. showed that the design of a cobalt single-atom nanozyme can directly affect and improve the rate-determining step (RDS) in OXD-like catalysis. A concave carbon framework with many edge Co–N₄ sites was constructed, and this changes the electronic state of the cobalt center. In this way, the catalyst has stronger interaction with oxygen, which makes the adsorption and activation of O₂ easier. Theoretical calculation also confirms that this structure lowers the activation energy of the RDS, which is usually the first oxygen splitting step. Due to this decrease, the catalytic activity becomes 3.1 times higher. This finding shows a clear relationship between the structure and performance, and gives useful guidance for future nanozyme design.⁶⁷

The catalytic activity and ROS selectivity of SANs are highly sensitive to environmental pH, salinity, and radical scavengers significantly. This in turn directly influences metal valence states, substrate adsorption, and reaction pathways. For example, Fe–N–C SANs exhibit maximal POD-like activity under acidic conditions (pH ≈ 3–4), where low pH facilitates favorable Fe(III)/Fe(II) redox cycling and enhances H₂O₂ activation.⁶⁶ However, such acidic dependence limits practical applications. To address this limitation, coordination engineering has been employed. Huang et al. demonstrated that in Co-based SANs, reducing the nitrogen coordination number (e.g., Co–N₂) shifts the catalytic mechanism toward a singlet oxygen (¹O₂)-dominated non-radical pathway. This pathway maintains high efficiency over a broad pH range between 3 and 9, as it avoids pH-sensitive radical generation and instead relies on stable electronic configurations less affected by proton concentration.⁶⁸ Such structural design strategies effectively decouple catalytic performance from acidic conditions, thereby enhancing the practicality of SANs for environmental applications. The presence of halide ions (e.g., Cl⁻ and Br⁻) in saline environments introduces competitive adsorption and leads to the formation of reactive halogen species, which can further alter the dominant ROS pathway.⁶⁹ Radical scavenging experiments provide key insights into reaction mechanisms. For example, if catalytic activity continues when sodium azide (a known singlet oxygen, ¹O₂, quencher) is added, but ceases when isopropanol (a hydroxyl radical scavenger) is introduced, this strongly indicates a non-radical mechanism in which ¹O₂ is the primary reactive species.⁷⁰ Such a non-radical mechanism is frequently observed in Co–N₄ nanozymes.⁷¹

Enzyme-like properties of SANs

SANs exhibit multispecies enzymatic activity including POD-like, superoxide dismutase (SOD)-like, oxidase (OXD)-like and catalase (CAT)-like functionalities, often demonstrating multiple activities simultaneously. The following section provides a focused discussion on the major enzyme-like activities of SANs. In particular, it examines the mechanisms and applications of their POD-like, SOD-like, OXD-like and CAT-like properties. A comprehensive understanding of these properties is critical, as it provides insights into the catalytic mechanisms inherent to SANs and helps with the rational design for practical applications in fields such as biosensing, biomedicine, and environmental remediation.

Peroxidase (POD)-like activity

POD-like behaviour is the most widely studied enzymatic function of SANs. In this mechanism, H₂O₂ is decomposed into highly reactive hydroxyl radicals that oxidize organic substrates such as TMB, diazoaminobenzene (DAAB), and o-phenylenediamine (OPD). In natural peroxidases such as HRP, the catalytically active center consists of a porphyrin-bound iron cofactor, in which the Fe atom is coordinated with four nitrogen atoms on the porphyrin ring plane.^2,72 This structural motif provides the fundamental inspiration for designing Fe-based single-atom materials to emulate the enzymatic active site. For instance, Fe–N–C SANs obtained via high-temperature pyrolysis of MOFs have been successfully applied to biosensing, including the detection of butyrylcholinesterase (BChE) activity.⁷³ Similarly, Wang and co-workers developed Fe-SA/NC catalysts and employed them in a ratiometric fluorescence sensing platform for acetylcholinesterase (AChE) activity, demonstrating the adaptability of POD-like SANs to diverse biosensing applications.⁷⁴ Besides iron, other transition metals have been explored to replicate or extend POD-like activity. Recent studies have focused on molybdenum-based SANs, where systematic modulation of the coordination environment has been used to tune catalytic behavior. A representative example is the Mo–N_x–C series, synthesized by pyrolyzing Mo-ZIF-8 MOFs supported on N-doped porous carbon. These materials were evaluated through the colorimetric oxidation of TMB at 652 nm, a classical peroxidase assay. Among the series, Mo–N₃–C exhibited the highest POD-like activity. Advanced spectroscopic techniques such as X-ray absorption fine structure (XAFS) analysis revealed that the number of nitrogen ligands coordinating the Mo center played a decisive role in dictating peroxidase specificity and catalytic efficiency (Fig. 2A).⁷⁵ The atomically dispersed M–N–C motifs not only reproduce the spatial and electronic features of enzyme cofactors but also provide opportunities for rational modulation of activity through careful control of the metal type and coordination environment.

Fig. 2 (A) Schematic showing the theoretical investigation of the mechanism for the coordination number-dependent POD-like specificity of Mo–N_x–C SANs, reproduced with permission from ref. 75, Copyright 2020 Elsevier Inc. (B) Scheme showing the synthesis of Cu-SANs with SOD-like activity, reproduced with permission from ref. 76, Copyright 2025 John Wiley and Sons. (C) Schematic depicting the synthesis of Co–N–C SANs and their OXD-like activity in enabling the colorimetric detection of L-cysteine, reproduced with permission from ref. 79, Copyright 2025 Elsevier Inc. (D) Schematic showing the activity increment of Pd/CeO₂ SANs, reproduced with permission from ref. 82, Copyright 2022 American Chemical Society.

Superoxide dismutase (SOD)-like activity

SOD-like activity represents a crucial antioxidant function of SANs, enabling the dismutation of superoxide anions into oxygen and H₂O₂. This property is commonly evaluated through colorimetric assays, such as the riboflavin–nitro blue tetrazolium (NBT) or WST systems, where the reduction of tetrazolium salts into chromogenic formazan provides a quantifiable measure of activity at 560 or 450 nm, respectively.^2,72 A broad range of SANs incorporating metals such as Mn, Pt, Au, Ni, Cu, Fe, Rh, Mo, and Co have demonstrated SOD-mimicking behavior, but copper-based systems have received particular attention owing to their structural analogy to natural Cu-only SOD enzymes. For instance, Cu–N₄ SANs were shown to effectively scavenge superoxide radicals in sepsis models, thereby interrupting reactive oxygen and nitrogen species (RONS)-mediated inflammatory cascades and improving multi-organ outcomes, underscoring their therapeutic potential in critical care (Fig. 2B).⁷⁶ Similarly, bimetallic designs such as Cu/Zn single-atom nanozymes have been developed to structurally resemble natural Cu/Zn-SOD, demonstrating selective and highly efficient scavenging of superoxide anions with catalytic activities (7820 U mg⁻¹), surpassing those of natural SOD, while avoiding unwanted POD or OXD-like reactions. This selectivity, coupled with low toxicity and stability against aggregation, highlights the advantages of single-atom approaches over conventional nanoceria systems.⁷⁷

Oxidase (OXD)-like activity

OXD-like properties are another crucial enzymatic mimicry observed in SANs. Unlike POD-like activity, which depends on H₂O₂, OXD-like catalysis involves the direct reduction of oxygen to reactive intermediates, enabling substrate oxidation in the absence of external oxidants.^2,72,78 Transition-metal–nitrogen–carbon frameworks, particularly those containing Fe, Co, or Ni centres, have demonstrated strong OXD-like activity.^2,72 The underlying mechanism often follows a ping-pong pathway, where electron transfer occurs sequentially between the substrate, the metal site, and molecular oxygen. For instance, cobalt-based Co–N–C SANs derived from ZIF-8 precursors have demonstrated remarkable OXD-like activity, catalyzing the oxidation of TMB into a chromogenic product, oxTMB, which serves as the basis of a highly sensitive and selective colorimetric platform for L-cysteine detection (Fig. 2C).⁷⁹ Similarly, rational engineering of Fe–N–C SANs with densely exposed FeN₄ sites on hierarchical porous carbon supports has significantly enhanced intrinsic OXD-like activity by modulating the electronic structure of iron centers through edge effects. These optimized Fe–N–C systems achieved superior catalytic kinetics in TMB oxidation and were successfully applied in uric acid detection.⁸⁰ In another study, boron–nitrogen co-doped Zn-based SANs (ZnBNC-SANs) were developed with finely tuned Zn–N–B coordination environments that shifted the electronic band structure to promote OXD-like catalysis. These ZnBNC-SANs formed the basis of an advanced RNA sensing platform independent of H₂O₂, integrating CRISPR/Cas13 technology with DNA extension reactions to achieve ultra-sensitive detection as low as 20 aM.⁸¹ These studies suggest that the rational design of the atomic coordination environment is a critical determinant of catalytic function. By precisely engineering Co-, Fe-, and Zn-based single-atom centers, powerful OXD-like SANs can be realized, offering broad potential for advancing fields such as biosensing, clinical diagnostics, and point-of-care testing.

Catalase (CAT)-like activity

CAT-like activity in SANs involves the disproportionation of H₂O₂ into water and molecular oxygen, thereby preventing harmful peroxide accumulation. SANs with atomically dispersed Fe–N₄ sites synthesized via high-temperature pyrolysis have demonstrated dual CAT- and SOD-like functions, effectively reducing electron spin resonance signals generated by superoxide radicals and confirming their intracellular ROS-scavenging potential.⁵⁶ In another report, Pd/CeO₂ SANs prepared through a simple aqueous method exhibited superior catalytic efficiency, with 3 to 7-fold higher CAT-like activity compared with conventional CeO₂ nanozymes, highlighting the synergistic advantages of atomic dispersion and strong metal–support interactions (Fig. 2D).⁸² Even greater enhancements have been achieved through rational coordination engineering. For instance, L-cysteine-activated Cu–MoO_x SANs (MCCP SANs) displayed a CAT-like catalytic efficiency 138 times greater than MnO₂ nanozymes and a binding affinity 14.3 times higher than natural catalase. Complementing this, graphene-supported Cl–Cu–N₄-centered SANs (Cu–N₄ClG) were designed with atomic precision to replicate the coordination environment of natural SOD enzymes. These SANs not only catalyzed the dismutation of superoxide anions into oxygen and H₂O₂ but also displayed sequential CAT-like activity to decompose H₂O₂, thereby integrating dual catalytic pathways for ROS regulation. Their nitroxide radical scavenging and photothermally enhanced activity enabled effective protection of chondrocytes and alleviation of osteoarthritis.⁸³ More recently, cascade platforms have been explored to integrate complementary enzyme-like activities. A representative example is Pt@Cu–NC, a composite system in which CAT-like Pt nanoparticles are anchored onto Cu–N–C frameworks possessing intrinsic SOD-like activity. This architecture enabled a highly efficient ROS-scavenging cascade, with apparent rate constants 20-fold higher for superoxide anions and 390-fold higher for H₂O₂ elimination compared with mixed controls. Surface PEGylation enhanced dispersibility, stability, and biocompatibility, while cellular assays confirmed superior intracellular ROS removal.⁸⁴

SANs-based optical and electrochemical techniques for environmental monitoring and remediation

Although SANs have become widely employed in biomedical applications, they have enormous untapped potential for use in environmental monitoring as well. Incorporating their unique catalytic and physicochemical properties into appropriately engineered optical or electrochemical platforms will play a vital role in realizing their full range of field applications.^25,26 In optical platforms, the SANs-catalyzed oxidation of chromogenic substrates can yield quantifiable colorimetric, fluorometric, or luminescence outputs, whereas in electrochemical platforms, SANs-driven redox events can be converted into measurable current or potential readouts. The following sections critically evaluate the latest advances in each technique, elucidate their operational strengths and inherent limitations, delineate their merits and limitations, and identify their specific advantages for pollutant detection and degradation applications.

Assay techniques

Colorimetric assays. Colorimetric-based SAN optical assays depend on a well-defined catalytic cycle in which the isolated metal centers (typically Fe, Co, Mn, or Ce coordinated as M–N_x/S_x motifs) in SANs accelerate the one- or two-electron reduction of H₂O₂ or dissolved O₂ to generate reactive oxygen species that oxidize chromogenic substrates such as TMB, OPD, or ABTS. The resulting quinone imine products exhibit strong visible absorption (e.g., oxidized (ox) TMB at 652 nm), enabling direct optical transduction.^12,26,27 In contrast to metal/metal-oxide nanozymes, every metal atom in a SANs is catalytically accessible, lowering the activation barrier for substrate oxidation and pushing the detection limits into the nanomolar to picomolar range. For example, in colorimetric “turn-on” assays, the appearance of blue oxTMB (λ ≈ 652 nm) or green oxABTS provides a direct optical indication of trace peroxides that are enzymatically or chemically generated by the target analyte. Conversely, SANs also enable “turn-off” colorimetric assays in which analytes bind to or block atomic active sites to suppress chromogenic turnover. For example, metallophilic interactions between Hg²⁺ or Ag⁺ and surface N/O ligands reduced the POD-like cycle of Ce–N–C SANs to realize selective visual detection of heavy metals.²⁷ Furthermore, the possibility of integrating dual enzyme-mimetic activities (e.g., POD-like and OXD-like) within a single SAN broadens the potential of employing different substrates and facilitates the realization of ratiometric readouts that can help to mitigate common interference in complex environmental matrices. As a result, SANs-driven colorimetric assays can provide portable, low-cost, and highly sensitive tools for on-site environmental monitoring and pollutant remediation process tracking.

Fluorescence and ratiometric assays. Fluorescence-based SANs optical assays typically exploit the strong electron or energy transfer capabilities of atomically dispersed active centers to modulate the emissive state of a fluorogenic substrate. In the most common format, a single atom (e.g., Fe, Ce, or Cu) catalyzes the selective redox conversion of a non-fluorescent precursor into a highly fluorescent product. The catalytic turnover frequency (or reaction kinetics) provides an optical signal with an intensity correlated to the concentration of the target analyte (e.g., a peroxide, metal ion, or pesticide metabolite). An alternative mechanism relies on the SANs acting as an efficient fluorescence quencher through Förster resonance energy transfer, the inner filter effect, or photo-induced electron transfer. Binding of the analyte blocks or reverses this quenching, thereby regenerating the signal in a ratiometric fashion.⁷⁴ Because the catalytic sites are atomically isolated and all metal centers are maximally accessible, high turnover frequencies and detection limits that often reach the nanogram per milliliter level can be realized without the need for external oxidants. The absence of added peroxide is particularly advantageous in environmental matrices such as surface water or agricultural runoff, where residual disinfectants could otherwise generate false positives. Moreover, the optical nature of the result permits portable, chip-level, or smartphone-assisted analyses that facilitate rapid on-site screening of trace contaminants while retaining sufficient selectivity and quantitative accuracy for regulatory monitoring.

Electrochemical assays. Electrochemical assays for the detection of environmental pollutants generally operate by transducing redox events at an electrode–solution interface into electrical signals, namely steady-state currents in amperometry or peak currents/potentials in voltammetry. In a typical format, the target analyte (or a reporter species, such as H₂O₂, generated in situ) undergoes catalytic oxidation or reduction at the electrode surface; the magnitude, onset potential, or kinetic profile of this faradaic process constitutes the analytical result.²⁵ When SANs are immobilized on the electrode, each isolated metal center (e.g., M–N_x, M–S_x, or dual-metal motif) functions as an atomically precise active site that can mediate the two-electron peroxide reduction, multi-electron oxygen reduction, or selective small-molecule oxidation with minimal overpotential. Because every metal atom is exposed and electronically coupled to a conductive scaffold, SANs deliver high turnover frequencies, enhance electron transfer kinetics, and suppress competing side reactions that would otherwise obscure the analytical signal.

These mechanisms provide several practical advantages for environmental monitoring. First, the reduced activation barriers and tailored redox potentials of SANs sites reduce operational voltages, thereby diminishing background currents and improving the limits of detection to the nanomolar or even picomolar level. Second, the chemical robustness of the surrounding carbon or oxide support endows SANs-modified electrodes with high tolerance to the wide pH ranges and dissolved-ion concentrations characteristic of natural and industrial water.²⁵ Third, the high dispersion of catalytically active atoms minimizes the use of precious metals while simultaneously mitigating fouling and prolonging sensor lifetimes during continuous deployment. Fourth, the nanoscale thickness and improved mechanical properties of SANs facilitate direct printing onto flexible substrates, enabling wearable devices that can stream real-time data. Moreover, SANs-functionalized amperometric and voltammetric assays can be readily coupled with electro-Fenton “sense-and-treat” configurations to realize simultaneous real-time monitoring and in situ remediation of environmental contaminants.⁸⁵

SANs-assisted pollutant degradation and self-reporting systems

In addition to sensing, SANs-assisted “sense-and-destroy” platforms utilize atomically dispersed metal centers (e.g., Fe–N₄, Co–N₄, and Cu–N₂) embedded in conductive carbon frameworks to realize dual-function active sites for both detection and degradation. For example, during the degradation stage, single-atom sites activate mild oxidants such as H₂O₂, peroxymonosulfate (PMS), or dissolved O₂via Fenton- or Fenton-like one-electron pathways to produce highly reactive hydroxyl or sulfate radicals that non-selectively cleave chromophores, aromatic rings, and heteroatom bonds in dyes, antibiotics, and nitrosamines even at near-neutral pH. In the self-reporting step, the same catalytic centers exhibit POD/OXD-like activity toward chromogenic probes (TMB, ABTS, and OPD) or electroactive reporters such that the instantaneous radical flux is translated into a proportional colorimetric, fluorometric, or amperometric signal. Essentially, SANs can deliver unified active sites where pollutant destruction (“kill”) and reaction progress indication (“tell”) are intrinsically coupled, enabling real-time on-site verification of remediation efficiency without external analytical instrumentation.

Applications of SANs in environmental analysis

Metal ion pollutants

Trace metal ions play critical roles as cofactors in numerous physiological processes; however, their excessive accumulation can lead to adverse effects ranging from neurotoxicity to carcinogenesis.¹⁷ Industrial runoff, mining activities, and farm-chemical effluent have significantly polluted soils and aquatic systems with hazardous metal ions, especially Cr, Hg, As, Cd, and Pb.^17,27 Therefore, monitoring these toxic metal ions in environmental samples requires a rapid response, sub-nanomolar sensitivity, and low operational cost.²⁷ These requirements can be fulfilled by SANs-based sensors, as each isolated metal atom on the nanozyme surface functions as an active site, endowing the sensing platform with high selectivity and sensitivity.

Mao et al. immobilized isolated Fe atoms on single-layer N-doped graphene (SA-Fe/NG), reporting that chelation of 8-hydroxy-quinoline inhibited the POD-like oxidation of TMB, but competitive binding by Cr(VI) restored the blue oxTMB signal, yielding a linear detection range from 30 nM to 3 μM with a limit of detection (LOD) of 3 nM.⁸⁶ Compared with conventional nanozymes, the realization of 100% Fe utilization shortened the response time of SA-Fe/NG to below 2 min and enabled accurate determinations of Cr(VI) contents in tap- and fish-water extracts. Furthermore, Li et al. doped single Fe atoms into an N/S-coordinated carbon framework (Fe–N/S–C). In the presence of glutathione, the OXD-like pathway was blocked, and the subsequent addition of Hg²⁺ formed an Hg–S complex, releasing the catalytic sites and reviving the chromogenic reaction. A smartphone-assisted chip based on this “off/on” logic quantified Hg contents ranging from 1 nM to 10 μM with an LOD of 0.17 nM (Fig. 3A).⁸⁷

Fig. 3 (A) Fe–N/S–C SANs-based colorimetric sensor for ultrasensitive and multimodal mercury(II) ion detection, reproduced with permission from ref. 87, Copyright 2022, Elsevier B.V., (B) schematic illustration of the time-resolved Fe³⁺/Cr⁶⁺ detection sensor utilizing the OXD-like activity of Ce–N–C SANs, reproduced with permission from ref. 89, Copyright 2015 RSC Publishing, and (C) real-time mercury ion sensing using a solution-gated graphene transistor functionalized with SANs, reproduced with permission from ref. 91, Copyright 2020 American Chemical Society.

Notably, single-atom Ce–N–C exhibited intrinsic phosphatase-like activity that was specifically inhibited by Al³⁺; a ratiometric fluorimetric probe tracking the Ce³⁺/Ce⁴⁺ cycle delivered an LOD of 22.98 ng mL⁻¹ across 5–25 μg mL⁻¹ accordingly, outperforming the drinking-water limit of 100 mg kg⁻¹.⁸⁸ Song et al. developed Ce–N–C SANs that exhibited OXD-like activity, reporting that Fe³⁺ and Cr⁶⁺ amplified the activity of Ce–N–C by accelerating Ce³⁺/Ce⁴⁺ electron transfer. Based on this mechanism, they constructed a time-resolved sensor to detect Fe³⁺ and Cr⁶⁺ with LODs of 34.72 ng mL⁻¹ (within 0.25–1.5 μg mL⁻¹) and 93.65 ng mL⁻¹ (within 0.5–5 μg mL⁻¹), respectively, and recoveries of 88.66–113.24% and 85.49–111.22%, respectively. Additionally, the Ce–N–C OXD-like sensor enabled rapid quantification within 30 s (Fe³⁺) and 60 s (Cr⁶⁺) (Fig. 3B).⁸⁹ A recent study reported the synthesis of single-atom Ir-doped carbon dot nanozymes (Ir-CD SANs) via in situ pyrolysis. Maximizing the availability of Ir atoms enhanced the POD-like activity (178.81 U mg⁻¹) and aqueous stability (>30 d) of the Ir-CD SANs. This improvement enabled the sensitive colorimetric detection of Hg²⁺ within a linear range of 0.01–10 μM at an LOD of 4.4 nM.⁹⁰

Although user-friendly, optical readouts may suffer from turbidity or background color interference; electrochemical transduction circumvents these issues and permits onsite microdevice integration. Yao et al. fabricated an Fe–N–C SANs functionalized solution-gated graphene transistor for detection of Hg²⁺. The N atoms in the Fe–N–C SANs selectively chelate Hg²⁺, while the catalytic site in SANs acts as a signal amplifier. This design enables continuous tracking of Hg²⁺ with a LOD of 1 nM in drinking water, without requiring sample pre-treatment (Fig. 3C).⁹¹ These early demonstrations suggest an approaching era of integrated SANs devices, including dip-in strips, catalytic paper, and 3D printed flow cells. Such devices can provide on-the-spot readouts and then self-neutralize toxic metal ions without causing secondary contamination. However, several key engineering challenges, such as preventing the agglomeration of isolated sites under radical attack, maintaining electron transport in thick catalytic layers, and designing macroporous supports to avoid pressure drops in continuous-flow reactors, remain to be addressed.

Organic pollutants

Organic pollutants, such as phenolic compounds, hydroquinone, and harmful gases, including volatile amines, formaldehyde, benzene, and toluene, are common products of industrial activities.¹⁷ These compounds can react readily and build up in living organisms, posing serious threats to the environment and human health. Even at the sub-micromolar or sub-parts-per-million (ppm) levels, exposure to phenols induces oxidative stress in aquatic biota and compromise endocrine function in higher organisms, and the inhalation of volatile organics is linked to neurodegeneration and lung carcinogenesis.¹⁷ Hydroquinone, a diphenolic isomer, has been widely used as a chemical precursor and synthetic intermediate in cosmetics, dyes, and pesticides. However, it is classified as a persistent organic pollutant in environmental systems owing to its high toxicity and poor biodegradability.⁹² Therefore, the rapid detection of such organic pollutants in water and air could provide the data necessary to inform sound regulations and help deliver early alerts through environmental monitoring. Chu et al. converted an Fe-guanidine coordination lattice into nitrogen-doped graphene nanosheets hosting atomically dispersed Fe–N₄ sites (Fe-CNG).⁹² The unique bidentate O/N coordination endowed each Fe center with an electron configuration favorable for the activation of dissolved O₂, producing both superoxide radicals and singlet oxygen under mildly acidic conditions. When applied in a TMB-based colorimetric assay, Fe-CNG converted a clear TMB solution into a blue product. This reaction was inhibited by the presence of hydroquinone or vitamin C, which caused the blue color to fade; thus, the extent of color change provided a direct measure of hydroquinone or vitamin C levels in the sample. The Fe-CNG SANs exhibited a linear response to hydroquinone in concentrations of 0.10–40 μM with an LOD of 0.025 μM and yielded quantitative recoveries of 96.7–104.3% for spiked hydroquinone in lake and tap water samples.⁹²

Liu et al. developed hierarchically porous Fe/Mn–N–C diatomic nanozymes with a dodecahedral architecture by pyrolyzing Fe/Mn@ZIF-8 precursors capable of both sensitive optical detection and catalytic remediation in aqueous environments. The incorporation of Mn(II) acetylacetonate and ferrocene enriched the carbon matrix with Fe–N_x and Mn–N_x active centers, and the additional C–N functionalities increased the POD- and SOD-like activities. This catalytic property enabled highly selective and colorimetric detection of hydroquinone concentrations of 0–200 μM with an LOD of 0.216 μM. Together with H₂O₂, these nanozymes provided near-quantitative hydroquinone degradation and retained their efficiency over multiple reuse cycles (Fig. 4A).⁹³

Fig. 4 (A) Synthetic route for Fe/Mn co-doped N–C SANs and integrated platform demonstrating hydroquinone quantification through colorimetric analysis coupled with oxidative degradation, reproduced with permission from ref. 93, Copyright 2023, Elsevier B.V., (B) preparation scheme for Fe₁@CN-20 SANs and their applications in simultaneous detection and degradation of 2,4-DP and adrenaline pollutants, reproduced with permission from ref. 94, Copyright 2022, Elsevier B.V., and (C) portable colorimetric–fluorescence dual-signal biosensor for visual detection of VA, reproduced with permission from ref. 96, Copyright 2023, Elsevier B.V.

Lin et al. realized laccase-like reactivity using Fe₁@CN-20, a catalyst in which single Fe atoms are axially N-coordinated within a carbon matrix. In addition to successfully completing colorimetric assays, Fe₁@CN-20 detected and degraded a series of compounds, including 2,4-dichlorophenol, 4-chlorophenol, 2,6-dimethoxyphenol, catechol, phenol, and adrenaline (Fig. 4B). Indeed, this material quantified phenol concentrations as low as 2.6 μM within 0.0–50 μM and completely mineralized 2,6-dimethoxyphenol in ambient air, demonstrating the “detect-and-destroy” capability associated with many SANs.⁹⁴ More recently, Xia et al. dispersed Mn atoms on graphyne to create a colorimetric sensor array capable of discriminating five bisphenol analogs with LODs of 0.28–0.44 μM.⁹⁵ In parallel, Song et al. developed a portable dual-signal platform coupling Fe–N–C SANs with carbon quantum dots to monitor volatile amines generated during meat spoilage.⁹⁶ The atomically dispersed Fe–N₄ centers imparted a high POD-like activity (40.22 U mg⁻¹) that catalyzed the oxidation of colorless TMB to its blue form. In this platform, the oxidized TMB was strongly absorbed in the emission window of the carbon quantum dots, quenching their fluorescence through an inner filter effect. When volatile amines evolved from deteriorating meat, they chemically reduced the oxidized TMB back to its colorless form, thereby restoring the fluorescence of the carbon quantum dots while fading the blue color (Fig. 4C). This colorimetric–fluorescence dual-signal platform enabled the colorimetric detection of NH₃, exhibiting a linear trend within 0.5–50 ppm with an LOD of 0.984 ppm, as well as a fluorescence assay with an LOD of 0.0838 ppm.⁹⁶

Wang et al. developed a photo-reduction method to synthesize single-atom Ru anchored on 3D porous graphene (Ru-SA/GF) that provided high metal exposure. Under identical conditions (2,4-dichlorophenol (2,4-DP), 50 mM, 50 μL; 4-aminoantipyrine (4-AP), 98 mM, 50 μL; PBS, 0.1 M, pH 6.8, 350 μL), Ru-SA/GF SANs exhibited biomimetic laccase-like activity comparable to natural laccase in catalytic mechanism, stability, and substrate specificity. The unique architecture of 3D porous graphene coupled with the maximally exposed Ru active sites enabled enhanced electrocatalytic oxidation of hydroquinone, catechol, and resorcinol, which correspond to distinct voltammetric peaks; this engineered Ru-SA/GF system achieved high simultaneous detection sensitivities for these three phenolic compounds with LODs of 0.017, 0.023, and 0.038 μM, respectively. Furthermore, the nanozymes exhibited excellent environmental remediation potential through efficient degradation of phenolic pollutants, establishing their dual functionality for sensing and wastewater treatment applications.⁸⁵Table 1 lists the summary of SANs-based detection/degradation platforms for metal ions and organic pollutants.

Table 1 Summary of SANs-based detection/degradation platforms for metal ions and organic pollutants

Metal ions
S. No.	Material	Mechanism (enzyme mimic)	Signal output	Target	Linear range	Real sample	Limit of detection	Reference
1	Fe/NG SANs	POD-like	Colorimetric	Cr^6+	30 nM–3 μM	Tap water and tuna	3 nM	86
2	Fe–N/S–C SANs	POD-like	Colorimetric	Hg^2+	1 nM–10 μM	Tap water, lake water and seawater	0.17 nM	87
3	Ce–N–C SANs	Phosphatase-like	Fluorescence	Al^3+	5–25 μg mL^−1	Tofu, cake, and egg roll	22.89 ng mL^−1	88
4	Ce–N–C SANs	OXD-like	Colorimetric	Fe^3+/Cr^6+	0.25–1.5 μg mL^−1 (Fe^3+) and 0.5–5 μg mL^−1 (Cr^6+)	Drinking water, tap water and wheat	34.72 ng mL^−1 (Fe^3+) and 93.65 ng mL^−1 (Cr^6+)	89
5	Ir-CD SANs	POD-like	Colorimetric	Hg^2+	0.01–10 μm	Tap water, lake water and seawater	4.4 nM	90
6	Fe–N–C SANs	—	Transistor	Hg^2+	30 nM–3 μM	Drinking water	1 nM	91

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Organic pollutants
S. No.	Material	Mechanism (enzyme mimic)	Signal output	Target	Linear range	Real sample	Limit of detection	Reference
1	Fe-SANs	OXD and laccase-like	Colorimetric	Hydroquinone	5–150 μM	Lake and tap water	0.025 μM	92
2	Fe–Mn–N SANs	POD- and SOD-like	Colorimetric and degradation	Hydroquinone	0–100 μM	—	0.216 μM	93
3	Fe1@CN-20 SANs	Laccase-like	Colorimetric and degradation	Phenol	160–532 μM	—	2.6 μM	94
4	Graphyne-Mn SANs	POD-like	Colorimetric	Five bis-phenol analogues	1–1000 μM	Fruit cans	0.28–0.44 μM	95
5	Fe–N–C SANs	POD-like	Colorimetric/fluorescence	Volatile amine	0.5–50.0 ppm (colorimetric) and 0.5–50.0 ppm (fluorescence)	Livestock meat	0.98 and 0.083 ppm	96
6	Ru-SA/GF SANs	Laccase-like	Electrochemical	Hydroquinone, catechol, and resorcinol	0.1–80.1 μM; 80.1–380.1 μM (hydroquinone, catechol) and 0.1–800.1 μM (resorcinol)	Tap and river water	0.017, 0.023, and 0.038 μM	85

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Pesticides

Pesticide residues originating from intensive agriculture and agro-industrial runoff have become pervasive environmental contaminants. Many synthetic insecticides, herbicides, and fungicides possess high chemical stabilities, enabling them to persist in soil and water and bioaccumulate along the food chain.^2,17,26,97 Consequently, the rapid detection of pesticide residues in complex environmental samples is indispensable for early warning surveillance. Wang et al. developed a colorimetric assay that combines SANs with a natural enzyme to quantify organophosphate pesticide (OP) residues. They produced Fe-based SANs (Fe–N/C) through one-step pyrolysis of alkaline lignin, yielding atomically dispersed Fe centers coordinated by pyrrolic N. These centers catalyze H₂O₂, generating hydroxyl radicals that oxidize TMB to its blue chromophore, thereby providing a measurable UV-visible light signal. Furthermore, the researchers included AChE in the system to impart pesticide selectivity, as it hydrolyzes acetylthiocholine (ATCh) to thiocholine (TCh), which suppresses the TMB color change. This detection mechanism relies on the chlorpyrifos-mediated inhibition of AChE, which enabled the Fe–N/C nanozyme to restore TMB oxidation, resulting in a measurable change in absorbance. When quantified through these absorbance variations, the chlorpyrifos concentration demonstrated a linear response within 0.05–10.0 μg mL⁻¹ at an LOD of 2.11 ng mL⁻¹.⁹⁸

In another study, researchers developed an Fe–N–C catalyst by combining isolated Fe atoms with small Fe clusters on N-doped porous carbon (FeAC/FeSA-NC) obtained by pyrolyzing an Fe(II)–phenanthroline complex embedded in a ZIF-8 template to guide uniform metal dispersion. The resulting nanozymes exhibited strong OXD-like activity, converting dissolved oxygen into superoxide radicals. A high catalytic rate was observed for these SANs; DFT calculations indicated that this occurred because neighboring single atoms and clusters lowered the reaction energy barrier. Therefore, the researchers applied the FeAC/FeSA-NC SANs as a ratiometric fluorescent probe to detect AChE activity and OPs. Notably, the thiol products of AChE inhibited these SANs, whereas OPs blocked AChE and restored the signal. The resulting assay exhibited a linear response from 0.005 to 50 ng mL⁻¹ with an LOD of 1.9 pg mL⁻¹.⁹⁹

Lyu et al. developed Fe-based SANs for herbicide detection by carbonizing hemin inside a ZIF-8 framework. The isolated Fe–N sites mimicked the active centers of natural peroxidase and endowed the material with strong POD-like activity. The researchers developed a point-of-care test for a target herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), using Fe-SANs as a label in a lateral flow immunoassay strip. The strip exhibited a linear response from 1 to 250 ng mL⁻¹ with an LOD of 0.82 ng mL⁻¹. It also discriminated between 2,4-D and similar phenoxy herbicides. Tests on human urine confirmed that the device can quickly and accurately measure 2,4-D exposure, highlighting its promise for the portable and sensitive monitoring of herbicide contamination (Fig. 5A).¹⁰⁰

Fig. 5 (A) Schematics showing Fe-SASC synthesis, antibody conjugation (Fe-SASC-Ab_2,4-D) and the Fe-SASC-based lateral-flow immunoassay strip for 2,4-D detection, reproduced with permission from ref. 100, Copyright 2022, AAAS, (B) a scheme showing the assembly and detection process of the SA-Fe SANs dual-mode biosensor, reproduced with permission from ref. 101, Copyright 2024, Elsevier B.V, and (C) Ni–NPC SANs platform for carbamate pesticide remediation and smartphone visualization, reproduced with permission from ref. 103, Copyright 2024, American Chemical Society.

Other researchers devised a dual-mode, broad-spectrum aptamer sensor combining colorimetric and electrochemical transduction to monitor OPs in vegetable matrices. The core catalyst comprised a single-atom Fe nanozyme (SA-Fe-NZ) obtained by carbonizing Fe-doped ZIF-8. The atomically dispersed Fe centers in the SANs exhibited robust POD-like activity that oxidized colorless TMB to its blue product. Upon binding to its OP target, the aptamer–OP complex inhibited the SA-Fe-NZ, suppressing catalytic turnover and producing an OP concentration-dependent decrease in the colorimetric signal. Simultaneously, the SA-Fe-NZ amplified the electrochemical signal, enabling a parallel voltammetric readout (Fig. 5B). This integrated sensor exhibited a wide dynamic range (10⁻¹³–10⁻² M), an extremely low LOD of 3.55 fM, and high selectivity and satisfactory recoveries in real vegetable samples.¹⁰¹

Furthermore, a recent study developed a smartphone-assisted colorimetric paper sensor for the on-site determination of carbofuran (CBF) residues by harnessing the high POD-like activity of Ni–N–C SANs synthesized from Ni(OH)₂ nanoplates. The sensing mechanism relied upon the hydrolysis of ATCh to TCh by AChE, which in turn suppressed the nanozyme-catalyzed oxidation of colorless TMB to blue oxTMB. The presence of CBF inhibited AChE activity, decreasing TCh production. This, in turn, restored the Ni–N–C catalytic cycle, yielding a concentration-dependent blue color that could be quantitatively evaluated using a smartphone camera. The paper sensor exhibited a linear response from 10 to 500 ng mL⁻¹ with an LOD of 8.79 ng mL⁻¹ and demonstrated recovery rates of 81.1–125.3% when applied to Chinese cabbage, conventional cabbage, and lettuce.¹⁰²

Electrochemical sensors employing SANs have also been developed for pesticide detection and remediation. Xu et al. dispersed single Ni atoms within a nitrogen-doped porous-carbon matrix (Ni–NPC) to create uniformly accessible Ni–N₄ catalytic sites. Under a mild potential window, this nanozyme-catalyzed peroxymonosulfate (PMS) activation generated sulfate and hydroxyl radicals that rapidly oxidized the carbamate pesticide carbaryl (Fig. 5C). Chronoamperometry indicated an anodic current increase of ∼200 μA cm⁻², reflecting the in situ electro-oxidative conversion of the analyte, and high-performance liquid chromatography confirmed 90% degradation within 60 min. Furthermore, observing the electrode with a smartphone camera allowed real-time RGB evaluation of colorimetric indicators, enabling on-site quantification without requiring laboratory infrastructure. This integrated “sense-and-destroy” strategy enables ultrasensitive electrochemical detection with simultaneous catalytic remediation in portable pesticide-monitoring platforms.¹⁰³Table 2 gives the summary of SANs-based detection assays for pesticides.

Table 2 Summary of SANs-based detection assays for pesticides

Pesticides
S. No.	Material	Mechanism (enzyme mimic)	Signal output	Target	Linear range	Real sample	Limit of detection	Reference
1	Fe–N/C SANs	POD-like	Colorimetric	Chlorpyrifos	0.05–10.0 μg mL^−1	Soil samples	2.11 ng mL^−1	98
2	Fe–N–C SANs	OXD-like	Fluorescence	Organophosphorus pesticides	0.005–50 ng mL^−1	Tap water	1.9 pg mL^−1	99
3	Fe-SAC	POD-like	Lateral flow assay	2,4-Dichlorophenoxyacetic acid	1–250 ng mL^−1	Real human urine samples	0.82 ng mL^−1	100
4	Fe-SANs	POD-like	Colorimetric and electrochemical	Organophosphorus pesticides	10^−13–10^−2 M	Vegetable samples	7.54 and 3.55 fM	101
5	Ni–N–C SANs	POD-like	Colorimetric	Carbofuran	10–500 ng mL^−1	Vegetable samples	8.79 ng mL^−1	102
6	Ni–NPC SANs	POD-like	Colorimetric	Carbamate	5–100 ng mL^−1	Lake water and pakchoi	1.5 ng mL^−1	103

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Drug residues

The widespread use of antibiotics and other medicines has resulted in the entry of these drug residues into water, soil, and food chains worldwide.⁹⁷ Many of these compounds and their deterioration products readily gain or lose electrons, making them excellent targets for redox-based detection. As a result, SANs with POD-, OXD-, or SOD-like catalytic properties are now widely used to sense antibiotic pollutants and break them down in real time.

For example, researchers developed SANs by anchoring isolated Pt atoms onto carbon nitride nanorods (SA-Pt/g-C₃N₄–K) to realize the colorimetric detection of oxytetracycline (OTC). In this colorimetric assay, the POD-like Pt=O sites on SA-Pt/g-C₃N₄–K generated hydroxyl radicals that oxidized colorless TMB to blue oxTMB. By using an OTC-specific DNA aptamer to temporarily block the catalytic pocket, color appeared only when OTC displaced the aptamer (a “turn-on” effect). This assay exhibited a linear response from 0.06 to 0.90 mg L⁻¹ for OTC with an LOD of 10.3 μg L⁻¹. In addition, the SANs-mediated radical production resulted in strong antibacterial activity.¹⁰⁴

Furthermore, researchers have fabricated Co-based SANs via template-assisted pyrolysis to realize atomically dispersed Co centers in a Co–N₅ coordination environment. Steady-state kinetics revealed pronounced POD-like behavior, and the catalyst generated substantial quantities of reactive oxygen species that amplified both colorimetric and chemiluminescence signals. Using these unique properties, Huang et al. developed a multimodal lateral-flow immunoassay for tetracycline (TC) that combined naked-eye color detection, catalytic signal enhancement, and quantitative chemiluminescence data to achieve LODs of 0.091, 0.062, and 0.056 ng mL⁻¹ for the three detection modes (Fig. 6A).¹⁰⁵

Fig. 6 (A) Schematic illustration showing the cobalt SANs-enhanced multimodal lateral flow immunoassay for on-site tetracycline detection, reproduced with permission from ref. 105, Copyright 2025, American Chemical Society and (B) schematic diagram showing the Ru SANs-enhanced PEC aptasensor for ultrasensitive CAP detection, reproduced with permission from ref. 106, Copyright 2025, Elsevier B.V.

In another study, researchers developed a SANs-based photoelectrochemical aptasensor in which atomically dispersed Ru centers catalyzed a precipitation reaction that amplified the signal, enabling ultra-trace detection of chloramphenicol (CAP). The atomically dispersed Ru centers exhibited exceptional POD-like activity, rapidly oxidizing 4-chloro-1-naphthol (4-CN) to form an insulating precipitate on the electrode surface that markedly suppressed the photocurrent. The introduction of CAP liberated the surface-bound aptamer through high-affinity binding, preventing 4-CN deposition, restoring the photocurrent, and thereby generating a sensitive “signal-on” response. Moreover, when coupled with a high-efficiency photoactive trilayer comprising Au nanoparticles, CdS quantum dots, and TiO₂, the self-powered photoelectrochemical aptasensor achieved an LOD of 4.12 pM for CAP with excellent selectivity and analytical accuracy (Fig. 6B).¹⁰⁶

Xie et al. developed a 3D hollow porous nano-architecture (Fe/Cu–NC) hosting atomically dispersed Fe–N_x and Cu–N_x sites using a salt-templated freeze-drying/pyrolysis process. The synergistic effects of the dual single-atom centers enhanced the POD-like activity of Fe/Cu–NC, leading the researchers to apply it in a chromogenic assay for isoniazid (INH) detection. In the presence of H₂O₂, this nanozyme oxidized colorless TMB to its blue-oxidized form, and the presence of INH subsequently reduced the chromophore, producing a concentration-dependent bleaching effect. The developed sensor exhibited a linear response to INH over 0.9–10 μM with an LOD of 0.3 μM.¹⁰⁷ In addition to sensing, the same Fe/Cu dual active sites efficiently activated PMS to generate reactive oxygen species that mineralized 90.4% of 10 mg L⁻¹ levofloxacin within 30 min under optimal conditions, confirming its practicality as a “detect-and-destroy” platform.¹⁰⁷ This research underscores how hetero-bimetallic single-atom coordination not only enhances catalytic turnover but also broadens the substrate scope to provide a versatile platform for simultaneous antibiotic monitoring and remediation. Table 3 summarizes various SANs-based detection/degradation platforms for drug residues.

Table 3 List of SANs-based detection/degradation platforms for drug residues and organic dyes

Drug residues
S. No.	Material	Mechanism (enzyme mimic)	Signal output	Target	Linear range	Real sample	Limit of detection	Reference
1	Pt/g-C3N4–K SANs	POD-like	Colorimetric	Oxytetracycline	60–900 μg L^−1	—	10.3 μg L^−1	104
2	Co-SANs	POD-like	Lateral flow assay	Tetracycline	0.1–25 ng mL^−1	Milk and honey samples	0.091 ng mL^−1	105
3	Ru/GF SANs	POD-like	Photoelectrochemical	Chloramphenicol	0.01–50 nM	River and honey samples	4.12 pM	106
4	Fe/Cu dual-SANs	POD-like	Colorimetric detection and degradation	Isoniazid detection and levofloxacin degradation	0.9–10 μM	Urine (isoniazid) and simulated pollutant (levofloxacin)	0.3 μM	107

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Dyes
S. No.	Material	Mechanism (enzyme mimic)	Signal output	Target	Linear range	Real sample	Limit of detection	Reference
1	ZnBNC SANs	POD-like	Colorimetric	p-Phenylenediamine	0.3–10 μM	Hair dyes and dyed hair samples	0.1 μM	109
2	Mn-SANs	POD-like	Degradation	Methylene blue	—	Tap water and lake water	—	110
3	Fe–NC SANs	POD and laccase-like	Degradation	Rhodamine B	—	Simulated complex water samples	—	111
4	Fe/Bi dual-SANs	OXD-like	Degradation	Rhodamine B	—	—	—	112
5	Fe–N–C SANs	OXD, POD-, and CAT-like	Degradation	Rhodamine B	—	—	—	113

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Organic dyes

Synthetic organic dyes, such as methylene blue (MB), rhodamine B (RhB), methyl orange, p-phenylenediamine (PPD), and crystal violet, are widely used in the textile, printing, and cosmetic industries.^17,108 However, these compounds exhibit high chemical stabilities and slow degradation processes, and their toxicity poses significant risks to aquatic ecosystems and human health even at low concentrations.¹⁷ Over the past five years, SANs-based frameworks have been widely used in the detection and degradation of such dyes.

For example, Feng et al. reported the controllable synthesis of B-doped Zn–N–C (ZnBNC) SANs exhibiting strong POD-like activity. Exploiting this activity, they established a colorimetric assay for PPD that provided high sensitivity and selectivity with a broad linear response range of 0.3–10 μM and a low LOD of 0.1 μM, suggesting its suitability as an alternative analytical approach for identifying PPD in commercial hair dye formulations and dyed hair samples.¹⁰⁹ In another study, Feng et al. stabilized isolated Mn atoms within a triple N–, P–, and S-doped carbon scaffold (Mn-SANs) using a sacrificial-template strategy.¹¹⁰ Experiments confirmed that Mn doping significantly enhanced catalytic activity, and the Mn-SANs efficiently degraded organic dyes to achieve 90% MB removal within 300 min, even with minimal nanozyme loading across broad pH ranges. Free radical quenching experiments and EPR analysis revealed that superoxide and hydroxyl radicals played crucial roles in the degradation mechanism (Fig. 7A).¹¹⁰

Fig. 7 (A) Scheme depicting enhanced peroxidase-mimicking activity in Mn-based SANs for catalytic dye decomposition, reproduced with permission from ref. 110, Copyright 2023, American Chemical Society, (B) schematic illustration of chiral Fe–N–C single-atom nanozymes exhibiting multi-enzyme activity for organic dye degradation, reproduced with permission from ref. 111, Copyright 2024, Elsevier B.V., (C) scheme depicting Fe/Bi dual SANs enabling cascade catalysis and PMS activation for efficient dye degradation, reproduced with permission from ref. 112, Copyright 2022, Elsevier B.V., and (D) atomically dispersed Fe–N–C single-atom nanozymes with hierarchical porosity for RhB dye degradation, reproduced with permission from ref. 113, Copyright 2025, Elsevier B.V.

Furthermore, Ning et al. synthesized chiral Fe–N–C SANs by incorporating chiral tripeptides (L/D-Pen–Phe–Trp) exhibiting laccase-like and POD-like activities for PMS activation and catalytic dye degradation. Molecular docking showed that the different activities of the L- and D-configured materials arose from the different binding affinities between the chiral peptide environments and target substrates. Further modification with Au produced chiral L-/D-Au@FeNC SANs that exhibited even higher catalytic and dye degradation performance owing to the improved dispersion stability in water imparted by the chiral peptides. Computational analysis indicated that the L-peptide bound more strongly to RhB, ABTS, and 2,4-dichlorophenol than the D-peptide. Consistent with this result, the L-Au@FeNC exhibited stronger enzyme-like activity than its D-counterpart and activated PMS more effectively to degrade RhB. Indeed, the L-Au@FeNC/PMS system rapidly and efficiently decomposed RhB by generating multiple reactive species (superoxide, hydroxyl, singlet oxygen, and sulfate radicals) while maintaining catalytic stability over repeated degradation cycles (Fig. 7B).¹¹¹

In another study, Chen et al. developed bimetallic SANs (FeBi–NC) derived from an Fe-doped Bi-MOF precursor. These SANs feature co-existing Fe–N₄ and Bi–N₄ dual active sites on a carbon support for AChE detection and dye degradation applications. The results of X-ray absorption near-edge structure and EXAFS analyses confirmed the atomic dispersions of both metal sites with high loadings of Fe (2.61 wt%) and Bi (8.01 wt%). The Fe/Bi–NC SANs exhibited remarkable OXD-like activity, exceeding those of Fe–NC and Bi–NC by 5.9 and 9.8 times, respectively. When coupled with AChE, the Fe/Bi–NC formed a cascade enzyme–nanozyme system that achieved ultrasensitive AChE detection with a low LOD of 1 × 10⁻⁴ mU mL⁻¹. The synergistic Fe–N₄ and Bi–N₄ sites exhibited strong PMS activation capabilities facilitated by enhanced electron transfer, generating reactive intermediates for rapid RhB dye degradation. By exploiting this dual-site catalysis, Fe/Bi–NC achieved 100% RhB removal within 5 min via PMS activation, demonstrating excellent efficiency in pollutant degradation (Fig. 7C).¹¹²

Wu et al. used Fe–N–C SANs featuring atomically dispersed Fe–N₄ sites on a 3D hierarchically ordered microporous–mesoporous–macroporous N-doped carbon matrix (3DOM Fe–N–C) for efficient RhB degradation. The 3D porous architecture enhanced mass transfer and active site accessibility, while the high metal utilization conferred superior triple-enzyme-mimicking activities (OXD-, POD-, and CAT-like). Relying on the POD-like activity, 3DOM Fe–N–C effectively degraded RhB via hydroxyl radicals generated from H₂O₂ (Fig. 7D).¹¹³Table 3 lists the summary of SANs-based detection/degradation platforms for organic dyes.

Microbial hazards

Microbial contaminants, including pathogenic bacteria, fungi, viruses, and their toxins, have escalated into a critical public health burden that accounts for an estimated seven million deaths annually.¹¹⁴ This increasing burden is largely attributed to the emergence of new antibiotic-resistant pathogenic strains. Notably, the atomic utilization and uniformity of active sites allow SANs to generate controllable bactericidal reactive oxygen/nitrogen species (ROS/RNS). Moreover, the tunable coordination environments of SANs permit switching between POD-, OXD-, Fenton-, CAT-, and haloperoxidase-like as well as Fenton and nitric-oxide releasing pathways, enabling broad-spectrum microbial inactivation while reducing reliance on conventional antibiotics.

For example, Dai et al. developed SANs comprising Cu atoms anchored on atomically thin carbon nitride nanodots (Cu–CNNDs) for use as an antibacterial textile coating. These nanozymes exhibited a high POD-like catalytic efficiency (up to 8.09 × 10⁵ M⁻¹ s⁻¹) comparable to that of natural enzymes. The results of experimental and theoretical analyses revealed that this catalytic activity arose from the nanoscale size effect of the C₃–N₄ support, which optimized the coordination environment of the single Cu atoms. This structural refinement ensured the complete exposure of the Cu–N₃ active sites, significantly enhancing the activation of H₂O₂ to generate hydroxyl radicals. Notably, the Cu–CNNDs demonstrated considerable antibacterial efficacy, achieving >99% bacterial inactivation. Critically, their successful integration into cotton fabrics highlights their potential for practical applications in antimicrobial textiles.¹¹⁵

A recent study synthesized sulfate-modified Fe–Cu bimetallic SANs (FeCu-SANs-SO₄²⁻) using high-temperature pyrolysis and sulfation of an eco-friendly Cu–Fe MOF precursor. The as-prepared FeCu-SANs-SO₄²⁻ demonstrated suitable OXD-like catalytic activity that was attributed to the synergistic interaction among the Fe–Cu dual active sites, a high specific surface area of 202 m² g⁻¹, and a well-defined mesoporous structure with an average pore size of 11.08 nm. Notably, the incorporation of Brønsted acidic sites enabled the nanozymes to overcome pH-dependent activity limitations while exhibiting superior thermal and long-term storage stabilities compared with those of natural enzymes. At a concentration of 0.1 mg mL⁻¹, FeCu-SANs-SO₄²⁻ achieved a bactericidal efficiency exceeding 90% against Escherichia coli, Staphylococcus aureus, Aeromonas hydrophila, and Edwardsiella tarda without requiring external stimuli. This excellent antibacterial performance was primarily attributed to the ability of the SANs to generate ROS efficiently, particularly superoxide anions and hydroxyl radicals. This induced oxidative stress by extracting electrons from essential bacterial cellular components, leading to irreversible damage (Fig. 8A).¹¹⁶

Fig. 8 (A) Schematic representation of the antibacterial mechanism in the Fe–Cu SANs system, reproduced with permission from ref. 116, Copyright 2025, American Chemical Society, (B) synthesis of Cu–g-C₃N₄/PCL nanofibers and the light-dependent dual-enzyme mimicking mechanism for antibacterial applications, reproduced with permission from ref. 117, Copyright 2025, American Chemical Society, and (C) schematics of Fe–N–C SANs synthesis, Fe–N–C/Fe–Co MNP fabrication and CRISPR-SANs biosensor operation, reproduced with permission from ref. 120, Copyright 2024, Elsevier B.V.

Wu et al. developed SANs using Cu anchored to graphitic carbon nitride nanosheets (Cu–g-C₃N₄) which exhibited OXD- and POD-like properties, enabling cascade catalytic reactions for ROS generation. The as-prepared bifunctional nanozymes exhibited broad-spectrum antibacterial efficacy against both Gram-positive and Gram-negative multidrug-resistant bacterial strains. Under visible light irradiation, Cu–g-C₃N₄ exhibited GOx-like activity, catalyzing glucose oxidation to produce H₂O₂, whereas in the dark, it functioned as a POD-like nanozyme, decomposing H₂O₂ into highly reactive hydroxyl radicals. This self-sustaining cascade reaction ensured continuous ROS generation, leading to potent antibacterial effects. Notably, Cu–g-C₃N₄ outperformed pristine g-C₃N₄ and other metal-modified variants (e.g., Cr–, K–, Fe–, and Zn–g-C₃N₄) owing to its enhanced light absorption and optimized surface adsorption properties. Researchers incorporated Cu–g-C₃N₄ into polycaprolactone (PCL) nanofibers via electrospinning to obtain Cu–g-C₃N₄/PCL nanofiber dressings for infected wound management. These dressings effectively eradicated bacterial infections and accelerated tissue regeneration in murine skin wound models (Fig. 8B).¹¹⁷

In addition to pathogenic microorganisms, toxic metabolic by-products, such as mycotoxins, are significant environmental pollutants that threaten ecological and human health. Therefore, SANs-based detection strategies have been developed for these substances as well. For example, Shen et al. engineered Fe-based SANs with an optimized Fe–N₃PS active moiety embedded in N-, P-, and S-co-doped hollow carbon nanocages (Fe–N₃PS/HC). The results of DFT calculations indicated that co-doping with P and S effectively modulated the electronic environment of the Fe center in these SANs, lowering the reaction barriers and thereby boosting their POD-like activity both kinetically and thermodynamically. This enhanced reactivity realized a three-channel nanozyme sensor array coupling Fe–N₃PS/HC with orthogonal chromogenic reactions. The resulting platform yielded analyte-specific color signatures that enabled rapid visual discrimination of five toxins: aflatoxin B1 (AFB₁), aflatoxin B2, zearalenone (ZEN), fumonisin B2 (FB₂), and deoxynivalenol (DON). A multivariate statistical analysis indicated that this array reliably differentiated these five mycotoxins as well as ten metal ions at trace levels (ng mL⁻¹ and low μM, respectively).¹¹⁸

In another study, Zhang et al. synthesized Co- and Fe-based SANs (M/N-PC, where M denotes Co or Fe) to evaluate their efficacy in mycotoxin degradation. The Co/N-PC SANs featured Co–N₄ sites alongside Co nanoclusters and achieved 99.4% patulin degradation within 60 min, outperforming Fe/N-PC (with Fe–N₅ sites alone); a DFT analysis attributed this enhancement to the synergistic effect between the Co–N₄ sites and nanoclusters, which elevated the electron density near the Fermi level, boosting catalytic activity. The degraded by-products exhibited negligible cytotoxicity, and M/N-PC demonstrated broad pH adaptability and reusability, confirming its utility for mycotoxin removal from food products (e.g., apple juice). Notably, the M/N-PC system also efficiently degraded AFB₁, DON, and ZEN by nearly 100% within 10–40 min, confirming its versatility in food safety applications.¹¹⁹

In a complementary approach, Liu et al. synthesized Fe₁₆₀–N–C SANs and Fe–Co magnetic nanoparticles (MNPs) via pyrolysis. The Fe₁₆₀–N–C SANs exhibited high POD-like activity and were employed as TMB chromogenic catalysts in a CRISPR/Cas12a-based colorimetric aptasensor for AFB₁ detection accordingly. In the absence of AFB₁, the aptamer bound to CRISPR RNA, activating the trans-cleavage activity of Cas12a, which released Fe₁₆₀–N–C SANs into the supernatant and oxidized TMB into a blue product. However, when AFB₁ was added, aptamer–target binding inhibited Cas12a activation, reducing the signal output. Critically, this study introduced an innovative CRISPR/Cas-based biosensor that integrates Fe₁₆₀–N–C SANs and MNPs, eliminating the need for target pre-amplification while enabling room-temperature detection. This system achieved a sensitivity of 1.5 × 10⁻⁷ ng μL⁻¹ over a broad linear range (10⁻⁶–1 ng μL⁻¹), surpassing the performance of conventional AFB₁ detection methods (Fig. 8C).¹²⁰

Conclusions, limitations and future prospects

The dispersion of individual metal atoms on heteroatom-doped supports in SANs maximizes active component utilization, making them ideal catalytic platforms for environmental monitoring and remediation. Although SANs showed promise, they still lack the ability to mimic the complex microenvironments that are responsible for the catalytic precision seen in natural enzymes. For example, SANs typically mimic only the primary coordination shell, usually a metal atom coordinated with nitrogen, phosphorus, or sulfur atoms, while lacking the flexible structural dynamics and allosteric regulatory features that natural enzymes use to modulate activity. Moreover, natural enzymes operate within uniquely tailored environments that feature specific hydrophobic/hydrophilic regions, local pH gradients, and detailed electrostatic interactions, all of which are difficult to reproduce using current SAN designs. These characteristics are important not only for achieving high substrate specificity but also for reducing unwanted side reactions and improving catalytic efficiency.^6,10 This fundamental limitation in mimicking enzyme-like environments restricts the broader use of SANs in reactions that demand high selectivity or function in complex biological settings. Additionally, challenges persist in scaling up atomically precise synthesis and ensuring long-term biocompatibility. While SANs offer greater precision and catalytic performance than conventional nanozymes, their design still lacks the self-regulating behavior and adaptive catalysis intrinsic to biological enzymes. Addressing this challenge will require progress in multi-shell coordination design, the use of advanced supramolecular frameworks, or the integration of protein–inorganic hybrid systems that more closely resemble the functional environments of natural enzymes.

Additionally, the precise structure–function correlations of SANs remain unclear for many systems. Although advanced characterization techniques (e.g., aberration-corrected TEM and synchrotron X-ray absorption spectroscopy) reveal coordination environments in SANs, significant challenges persist in resolving site heterogeneity and monitoring of dynamic structural evolution during catalytic reactions. Furthermore, present synthetic approaches of SANs face significant limitations, as conventional high-temperature pyrolysis and complex templating methods typically restrict metal loading to <2 wt%, while introducing batch-to-batch inconsistencies. Moreover, high-temperature processing induces structural degradation, including carbon framework collapse and bond cleavage, thereby undermining the long-term stability of M–N–C frameworks. Besides, significant challenges arise when transitioning SANs from controlled laboratory conditions to real-world applications. For example, complex environmental matrices including humic acid-rich river water, high-salinity industrial effluents, and colloidal soil extracts can deactivate catalytic sites or scavenge reactive oxygen species, substantially reducing performance. As a result, though promising “sense-and-destroy” concepts have been demonstrated, the development of fully integrated systems capable of real time detection and pollutant degradation remains limited.

To overcome current limitations, interdisciplinary approaches that make use of computational methods like DFT will be indispensable for advancing the rational design of SANs. For instance, DFT can provide atomistic insights into the electronic structure of metal centers, enabling prediction of how coordination environments influence reactivity. For advanced oxidation processes, DFT can be used to simulate the adsorption energies of oxidants (e.g., H₂O₂ and PMS) and pollutants, map charge transfer pathways, and evaluate energy barriers for ROS generation. These predictive capabilities will help design SANs with high activity and stability under the harsh oxidative conditions typical of environmental remediation. Emerging low-temperature synthesis methods, such as plasma-enhanced chemical vapor deposition (CVD) and metal–organic CVD, may enable higher single-atom loadings while maintaining structural integrity.¹²¹ Enhancing metal–support interactions through defect-engineered carbon or covalent organic frameworks could prevent active site aggregation.^122,123

The synergistic integration of multiple enzyme-like activities represents an important strategy to enhance the catalytic efficiency and functional versatility of SANs. Although conventional nanozymes are typically restricted to demonstrating one dominant catalytic function, for SANs, owing to their structurally tunable active centers, unique opportunities are provided for designing dual-, tri-, and multi-enzyme systems. For example, Fe–N₄ SANs have been reported to exhibit both POD- and CAT-like activities, enabling the scavenging of ROS during oxidative stress in cells.⁵⁶ Moreover, coupling SANs with natural enzymes represents a promising strategy to construct cascade enzyme–nanozyme systems, thereby enhancing target specificity and achieving high selectivity.¹¹² These multi-enzyme activities are valuable for cascade reactions, allowing applications in pollutant degradation, synergistic antibacterial therapies, and sensitive biosensing. Moreover, dual-active-site designs (e.g., Fe/Bi pairs) can also be used for cascade degradation processes in environmental applications. Future work must elucidate the atomic-scale mechanism of this multi-enzyme cooperation and develop generalizable synthetic strategies to reliably achieve these multifunctional properties.

For practical applications, materials must be engineered to resist environmental fouling while maintaining catalytic accessibility. Surface modifications with tailored hydrophilicity and hierarchically porous architectures, such as Fe/Mn–N–C sponges with microfluidic channels, could address this challenge. Comprehensive toxicity assessments are needed to evaluate metal leaching and reactive by-products throughout the material lifecycle. Incorporating SANs into functional materials like membranes or 3D-printed cartridges could enable continuous monitoring and treatment. Expanding target pollutants beyond conventional models to include emerging contaminants like pharmaceuticals and microplastic additives will require precise control over reactive species generation. Selective oxidation pathways must be engineered through coordination environment tuning to avoid nonspecific reactions. As highlighted by recent studies, scaling up SANs production with environmentally benign methods and establishing standardized performance metrics will be essential for real-world implementation. Through coordinated efforts in computational design, materials engineering, and environmental safety assessment, SANs can evolve from laboratory level prototypes into practical solutions for next-generation environmental monitoring and remediation. Their unique combination of atomic precision and robust performance positions them as transformative tools for addressing complex pollution challenges.

Author contributions

Arumugam Selva Sharma: conceptualization, writing – original draft, writing – review and editing, visualization. Nae Yoon Lee: writing – review and editing, supervision, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

Acknowledgements

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2023-00208684) and also by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2021-NR060117) and the Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (2020R1A6C101A184).

Notes and references

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