Recent advances in functional materials for uranium monitoring through optical and electrochemical techniques

Abstract

Uranium is a naturally occurring element widely employed in nuclear energy production and various industrial applications. However, it poses substantial risks to the environment and human health due to its combined radiological and chemical toxicity, thereby making rapid, accurate, and field-deployable detection technologies critical. Conventional detection methods, including atomic absorption or fluorescence spectroscopy and inductively coupled plasma spectroscopy, are limited by their high costs, complicated operational procedures, and dependence on specialized laboratory facilities, which severely restrict their practical on-site applications. To address these limitations, significant advances have been made in recent years in the development of functional materials for uranium sensing, especially metal–organic frameworks (MOFs), covalent organic frameworks (COFs), nanomaterials, DNAzymes, and other emerging materials. These materials support versatile optical and electrochemical detection strategies, such as fluorescence, colorimetry, and electrochemical sensing, which offer high sensitivity, fast response, and considerable potential for real-time field monitoring. This review highlights recent advances in the detection mechanisms, sensing performance, and practical advantages and limitations of various sensing systems. Finally, a comparative analysis is presented to identify key challenges and provide perspectives for the future development of efficient and reliable uranium detection technologies.

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1. Introduction

Uranium, a naturally occurring radioactive element, plays a critical role in both the energy and defense sectors as a primary nuclear fuel.^1–3 As a high-energy-density energy source, uranium is indispensable for nuclear power generation, whereas its involvement in nuclear weapons testing, atomic bombings, and the deployment of depleted uranium munitions has led to extensive nuclear pollution. Major nuclear accidents, including those at Chernobyl and Fukushima, have further exacerbated environmental contamination.^4–6 The radiological and chemical toxicity of uranium presents considerable risks to ecological systems and human health. In aqueous environments, uranium mainly exists in the form of the uranyl ion (UO₂²⁺), and its excessive accumulation results in persistent environmental contamination. Radiation emitted during uranium decay can cause cellular damage and elevate cancer risks, while its chemical toxicity exerts adverse effects on vital organs, including the kidneys, liver, and bones.^7–9 The World Health Organization has established a maximum contaminant limit of 30 µg L⁻¹ for uranium in drinking water, underscoring the urgent demand for effective monitoring techniques. Accordingly, the development of rapid, highly selective detection materials for UO₂²⁺ in aqueous systems has emerged as a major research direction.^10,11

Traditional methods for detecting UO₂²⁺ in aqueous solutions, including atomic absorption spectroscopy,¹² atomic fluorescence spectroscopy,¹³ and inductively coupled plasma spectroscopy,¹⁴ are limited in their widespread application by high costs and complex operational requirements.¹⁵ In contrast, newer techniques such as fluorescence, colorimetry, and electrochemical detection provide more practical alternatives.^16–19 Fluorescence methods are particularly recognized for their high sensitivity and rapid response,²⁰ while colorimetric detection is valued for its simplicity and visual readout capability.²¹ Electrochemical detection, characterized by its low detection limits, is especially well-suited for complex environmental samples.²² In recent years, advanced materials, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), nanomaterials, and DNAzymes, have attracted significant attention for their potential in uranium detection.^23–26 MOFs and COFs stand out due to their porous structures, high specific surface areas, and tunable functional groups, which make them ideal candidates for UO₂²⁺ detection.^27,28 Nanomaterials, with their excellent optical and electrical properties, can be surface-functionalized to achieve the selective detection of UO₂²⁺.²⁵ DNAzymes, which leverage the biomolecular recognition capabilities of DNA, further enhance the selectivity of UO₂²⁺ detection.²⁹ These functional materials offer substantial advantages, rendering them highly promising for the future development of uranium detection technologies.

Over the past few years, several reviews focusing on UO₂²⁺ detection have been published. Specifically, in 2022, Liu et al.³⁰ reviewed the advances in the adsorption, detection, and synchronous application of MOF-based materials for U(VI) in aqueous solutions. In 2023, Rani et al.³¹ summarized the research progress of various nanomaterials (e.g., metal nanoparticles and carbon-based materials) in uranium detection and removal. In 2024, Sharma et al.³² analyzed the advantages and limitations of electrochemical techniques based on chemically modified electrodes for the detection of radionuclides, such as uranium and cesium. In 2025, Sun et al.³³ elaborated on the preparation and characterization of carbon dots and their composites, along with their applications, mechanisms, and challenges in the detection and adsorption of uranium and other radioactive elements. In the same year, Poulin-Ponnelle et al.³⁴ reviewed the research progress on fluorescent probes for toxic elements associated with the nuclear industry (e.g., uranium and cesium), covering multiple sensing platforms, fluorescence mechanisms, and application prospects. However, most of these reviews tend to focus on specific material categories or individual detection methods rather than providing a comprehensive overview. In view of this, the present review summarizes the recent progress in UO₂²⁺ detection by covering a wide spectrum of representative sensing materials, including MOFs,³⁵ COFs,³⁶ nanomaterials,³¹ DNAzymes,³⁷ and other functional systems. Notably, this work provides the first comprehensive cross-material and cross-technique comparative analysis of UO₂²⁺ sensing platforms. We systematically examine the sensing mechanisms, analytical performance, and structure–property relationships of various material systems, critically evaluate their respective advantages and limitations, and further discuss key challenges that hinder their practical deployment, as well as feasible pathways for future improvement. Through this integrated and forward-looking perspective, the review aims to deliver a holistic understanding of UO₂²⁺ detection and provide valuable guidance for the rational design of next-generation high-performance uranium sensing platforms.

2. Fluorescence

Fluorescence is an analytical technique based on photoluminescence, with its mechanism varying depending on the type of material used.³⁸ By tailoring the chemical structures or surface properties of fluorescent probes, these probes can be engineered to interact specifically with a target analyte, thereby inducing a distinct change in fluorescence. This allows for the detection of the target substance through the monitoring of corresponding signal variations (Fig. 1).³⁹

Fig. 1 Schematic of the fluorescence detection of UO₂²⁺ with different materials.

2.1 MOFs

MOFs are a new class of porous materials that have emerged rapidly over the past few decades. They are constructed from metal ions or clusters connected to organic ligands through coordination bonds, forming crystalline structures with periodic networks, high porosity, and large specific surface areas.⁴⁰ Due to these distinctive properties, MOFs have been widely applied in numerous fields, among which the design of fluorescent sensors based on their fluorescent characteristics has become a prominent research focus.⁴¹

2.1.1 Fluorescence mechanism of MOFs. The luminescence derived from the metal centers in MOFs is predominantly associated with lanthanide ions, such as Eu³⁺ and Tb³⁺. This luminescence originates from 4f–4f electronic transitions; since these transitions are Laporte-forbidden, the direct excitation of the lanthanide ions is inefficient.⁴² To address this limitation, organic ligands function as antennas: they absorb incident light and subsequently transfer the absorbed energy intramolecularly to the metal center. This process, commonly referred to as the “antenna effect”, promotes the lanthanide ion to an excited state. The ion then returns to its ground state through radiative decay, emitting its characteristic sharp-line fluorescence. This efficient sensitization strategy renders lanthanide ions ideal emissive centers in luminescent MOFs.⁴³

In addition to metal-centered luminescence, luminescence from ligand centers primarily arises from π → π* or n → π* electronic transitions within the π-conjugated system of the organic ligands.⁴⁴ The rigid crystalline framework of MOFs significantly suppresses non-radiative relaxation processes, such as molecular vibrations and rotations of the ligands, through spatial confinement effects, thereby substantially enhancing the fluorescence quantum yield. In certain systems, additional mechanisms, such as intermolecular charge transfer (CT) or aggregation-induced emission (AIE), are also involved. Furthermore, by tuning the energy-level alignment, electronic properties, and geometric structure between the metal and organic ligands, a variety of luminescence mechanisms can be achieved, including ligand-to-ligand charge transfer, metal-to-metal charge transfer, and metal-to-ligand or ligand-to-metal charge transfer.⁴⁵

2.1.2 Fluorescent MOFs for UO₂²⁺ detection. UO₂²⁺ exhibits a linear geometry and possesses multiple vacant coordination sites in its equatorial plane, enabling it to coordinate with oxygen or nitrogen donor atoms from the carboxyl or amino groups of MOF organic linkers.⁴⁶ This coordination interaction induces changes in the fluorescence intensity of the MOF, and UO₂²⁺ detection is therefore achieved by monitoring such fluorescence variations, including fluorescence quenching, enhancement, or ratiometric fluorescence signals.⁴⁷

Fluorescence quenching primarily operates through three mechanisms: photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), and the inner filter effect (IFE).⁴⁸ In PET, excited-state electrons from the fluorescent MOF are transferred to UO₂²⁺, creating a non-radiative relaxation pathway back to the ground state and resulting in significant fluorescence reduction.⁴⁹ FRET occurs when the emission spectrum of the MOF overlaps considerably with the absorption spectrum of UO₂²⁺, allowing non-radiative energy transfer to UO₂²⁺ and subsequent fluorescence quenching.⁵⁰ IFE arises when the fluorescent material itself or other light-absorbing species absorb either the excitation or emitted light, leading to decreased fluorescence intensity.⁵¹ Qin et al.⁵² synthesized a Zn-based MOF (HNU-50), whose fluorescence originates from the π-conjugated system of the pyromellitic acid (PMA) ligand. The uncoordinated oxygen atoms on the ligand serve as binding sites for UO₂²⁺ (Fig. 2A). A notable spectral overlap was observed between the emission spectrum of the PMA ligand and the absorption spectrum of uranyl nitrate in the 335–476 nm region. Consequently, UO₂²⁺ absorbs the emission from the PMA ligand, leading to fluorescence quenching of HNU-50 via the FRET mechanism.

Fig. 2 (A) Mechanism of HNU-50 fluorescence detection of uranyl ions. Reproduced with permission from ref. 52. Copyright 2020, the American Chemical Society. (B) Schematic of the preparation of Eu-DATP and its application for UO₂²⁺ analysis. Reproduced with permission from ref. 55. Copyright 2024, the American Chemical Society.

In contrast to fluorescence quenching, fluorescence enhancement is primarily achieved through two distinct mechanisms: inhibition of non-radiative energy transfer and sensitization of the intrinsic luminescence of uranyl ions. For MOFs with lanthanide-centered emission, coordination of UO₂²⁺ to the MOF can induce structural rigidification, which significantly reduces non-radiative energy loss and enhances the efficiency of the “antenna effect”, thereby markedly boosting the characteristic fluorescence of the lanthanide ions. Alternatively, when UO₂²⁺ coordinates with the organic ligands in the MOF, energy transfer from the ligands to the uranyl ion may occur, which sensitizes and activates the intrinsic emission of UO₂²⁺ itself. Both of these mechanisms can be effectively utilized to construct fluorescence-enhanced sensors for UO₂²⁺ detection.

Ratiometric sensing for UO₂²⁺ detection utilizes the ratio of emission intensities at two distinct wavelengths from a fluorescent probe as the response signal.⁵³ This relative signal change enables the sensitive and accurate quantification of UO₂²⁺. Although UO₂²⁺ exhibits weak intrinsic photoluminescence, its coordination with organic ligands can significantly enhance its green emission at 509 nm via the “antenna effect”, wherein the ligand transfers energy to UO₂²⁺. Concurrently, this coordination process may alter the fluorescence emitted from the metal nodes or dopants within the MOF. By monitoring changes in the ratio of these dual emission signals, ratiometric sensing achieves highly precise and sensitive UO₂²⁺ detection.⁵⁴ For example, Tong et al.⁵⁵ developed a Eu-MOF with pore environments capable of simultaneously activating the intrinsic fluorescence of UO₂²⁺ and quenching the emission of Eu³⁺, thereby constructing a dual-signal, inversely varying ratiometric fluorescent sensor (Fig. 2B). In this system, the organic ligand 2,5-diaminoterephthalic acid (DATP) absorbs light and transfers energy to Eu³⁺ via the antenna effect, resulting in its characteristic strong red emission. When UO₂²⁺ is captured and immobilized by the free amino groups within the MOF channels, the energy absorbed by DATP is distributed to both Eu³⁺ and UO₂²⁺. This leads to reduced energy transfer to Eu³⁺, which attenuates its red emission, while the enhanced energy transfer to UO₂²⁺ intensifies its intrinsic green fluorescence. By calculating the intensity ratio at these two emission wavelengths, a concentration-dependent signal for UO₂²⁺ is established. This built-in self-referencing mechanism renders the ratiometric approach more reliable and resistant to interference than single-intensity-based detection.

2.2 COFs

Similar to MOFs, COFs are crystalline materials composed of light elements connected by covalent bonds, forming two- or three-dimensional topologies.⁵⁶ They are primarily constructed from aromatic organic ligands, such as phenyl, furan, and bipyridine units, which form extended covalent networks. Compared with MOFs, COFs are composed entirely of nonmetallic light elements, which endow them with superior thermal and chemical stability.⁵⁷ Additionally, COFs possess high specific surface areas, porous structures, strong fluorescence, and tunable chemical functionalities, making them versatile materials for a wide range of applications.⁵⁸

2.2.1 Fluorescence mechanism of COFs. The fluorescence of COFs primarily originates from the photophysical properties of their organic building blocks and the unique effects induced upon framework formation. The emission mechanisms can be categorized into three main types: (1) intrinsic fluorescence of the building units. Most fluorescent COFs contain large π-conjugated aromatic moieties, such as pyrrole, bipyridine, triphenylbenzene, and carbazole, which exhibit strong fluorescence due to high S₀ → S₁ transition probabilities and efficient π* → π radiative relaxation. (2) Framework-induced fluorescence enhancement and modulation. The rigid covalent backbone restricts intramolecular rotations and vibrations, thereby suppressing non-radiative decay pathways and substantially enhancing emission intensity. Moreover, the close and ordered packing of emissive units within the framework can further modulate excited-state lifetimes and emission wavelengths. (3) Aggregation-induced emission effect. The incorporation of AIE-active units (e.g., acridone or tetraphenylethylene) results in fluorescence quenching in isolated molecules due to intramolecular rotations; in contrast, confinement within the COF structure restricts these motions, leading to strong fluorescence upon aggregation.⁵⁹

2.2.2 Fluorescent COFs for UO₂²⁺ detection. Fluorescent COFs designed with specific functional groups, such as hydroxyl, carboxyl, imine, and pyridine, can coordinate with UO₂²⁺ ions through coordination bonds.⁶⁰ Upon binding, UO₂²⁺ alters the local electronic environment, triggering photoinduced electron or energy transfer that results in fluorescence quenching.⁶¹ Cui et al.⁶² developed an sp²-carbon-conjugated fluorescent COF (TFPT-BTAN-AO) containing triazine and cyano groups with excellent π-conjugation, which enables efficient light absorption and emission. The coordination of UO₂²⁺ with the amidoxime groups of the COF facilitates electron transfer from the COF to UO₂²⁺, leading to significant fluorescence quenching with a detection limit of 6.7 nM (Fig. 3). Similarly, Zhen et al.⁶³ synthesized a novel olefin-linked COF (GC-TFPB-AO), featuring a π-conjugated system that produces stable fluorescence. UO₂²⁺ coordinates with the amidoxime, pyridine, and hydroxyl groups in the COF, inducing electron transfer from the organic ligands to UO₂²⁺ and effectively quenching the COF's fluorescence.

Fig. 3 Schematic of TFPT-BTAN-AO regeneration detection and extraction of UO₂²⁺. Reproduced with permission from ref. 62. Copyright 2020, Springer.

2.3 Nanomaterials

In addition to MOFs and COFs, nanomaterials have emerged as promising candidates for the detection and remediation of radioactive waste, owing to their unique physicochemical properties, including an exceptionally high surface area, ultrasmall size, quantum effects, and intrinsic fluorescence.⁶⁴ In recent years, their application in fluorescence-based detection of UO₂²⁺ has grown significantly, with metal nanoclusters and quantum dots as the primary examples.

2.3.1 Fluorescence mechanism of nanomaterials. Metal nanoclusters are nanoscale aggregates composed of several to hundreds of metal atoms (e.g., gold, silver, or copper) and typically have a size of less than 2 nm.⁶⁵ They occupy a transitional state between discrete molecules and bulk solids. Owing to quantum confinement effects, nanoclusters exhibit discrete electronic energy levels, which allow electrons to undergo radiative transitions between these levels and return to the ground state.⁶⁶

Another key type of nanomaterial for fluorescence-based UO₂²⁺ detection is quantum dots (QDs), which are semiconductor nanocrystals typically ranging in size from 2 to 10 nm. Their ultrasmall dimensions confine electrons and holes within the nanoscale domain, giving rise to quantum confinement effects that generate discrete and tunable electronic energy levels. Fluorescence emission from QDs arises from radiative transitions between these quantized energy states.⁶⁷

2.3.2 Fluorescent nanomaterials for UO₂²⁺ detection. Gold nanoparticles (AuNPs) typically exhibit weak fluorescence and are mainly characterized by their surface plasmon resonance (SPR) absorption bands.⁶⁸ By leveraging the complementary spectral overlap between the absorption spectrum of AuNPs and the emission spectrum of gold nanoclusters (AuNCs), ratiometric fluorescence sensors based on the inner filter effect can be developed. For instance, Nan et al.⁶⁹ employed blue-emitting carbon dots (B-CDs) as an internal reference, whose fluorescence remains unaffected by changes in the UO₂²⁺ concentration. Upon interaction with UO₂²⁺, the uranyl ion coordinates with the P=O and P–O groups on the AuNP surface, triggering the aggregation of AuNPs. This aggregation event causes a redshift and the broadening of the AuNP SPR absorption peak. Initially, the limited spectral overlap between AuNP absorption and AuNC emission results in a weak IFE, leading to strong red fluorescence from the AuNCs. Following AuNP aggregation, the enhanced spectral overlap induces significant IFE-mediated quenching of AuNC fluorescence, resulting in a pronounced decrease in the red emission intensity (Fig. 4). Thus, the ratio of the fluorescence intensity of red-emitting AuNCs to that of blue-emitting B-CDs serves as a reliable readout for quantifying the UO₂²⁺ concentration.

Fig. 4 Schematic of the IFE-based ratiometric fluorescence method for UO₂²⁺ detection. Reproduced with permission from ref. 69. Copyright 2022, the Royal Society of Chemistry.

Quantum dots inherently lack affinity for UO₂²⁺ ions, necessitating surface modification with functional groups, such as carboxyl, pyridyl, or thiol, to enable selective binding. While quantum dots alone exhibit strong fluorescence, coordination with UO₂²⁺ leads to the formation of a non-emissive ground-state complex, resulting in fluorescence quenching through a static mechanism.⁷⁰ Zheng et al.⁷¹ introduced abundant oxygen- and nitrogen-containing groups onto molybdenum oxide (MoOx) quantum dot surfaces, producing dopamine-functionalized molybdenum oxide quantum dots (DA-MoOx QDs) with high affinity for UO₂²⁺. Upon coordination, these DA-MoOx QDs aggregated into larger clusters, which cause significant fluorescence quenching.

2.4 DNAzymes

DNAzymes are catalytic DNA molecules capable of specifically cleaving substrate strands upon activation by target metal ions, exhibiting high selectivity in recognition. This property makes DNAzymes ideal for the development of sensitive and selective sensors for UO₂²⁺ detection.⁷²

2.4.1 DNAzymes for UO₂²⁺ detection. DNAzymes, functional nucleic acids, typically consist of a catalytic “enzyme strand” and a complementary “substrate strand”. The enzyme strand catalyzes sequence-specific cleavage at a critical adenosine ribonucleotide site on the substrate strand upon activation by UO₂²⁺.⁷³ Leveraging this recognition mechanism, fluorescent sensors can be constructed by labeling the DNA termini or specific bases with fluorophores and quenchers to create a reversible “signal switch”. To enhance sensitivity, DNA self-assembly-based signal amplification strategies are employed. The initial UO₂²⁺-triggered cleavage releases a primer that initiates cascade hybridization reactions, assembling large DNA nanostructures. This process spatially separates fluorophores and quenchers on a large scale, converting a single molecular event into amplified fluorescence output for sensitive UO₂²⁺ detection.⁷⁴ Chen et al.⁷⁵ developed an ultrasensitive DNA network fluorescence biosensor, in which UO₂²⁺-activated DNAzyme cleavage releases trigger strands that initiate cyclic assembly of dual-hairpin probes (Hab, Hbc, and Hca), forming extensive DNA networks. This separates fluorophore (FAM) and quencher (BHQ) labels, yielding a markedly enhanced fluorescence signal with a detection limit down to 2 pM (Fig. 5). The sensor demonstrated high selectivity and precision in complex water and soil matrices. Lin et al.⁷⁶ reported a simplified DNA fluorescence sensor based on a single hairpin self-assembly chain reaction. Here, a palindrome-containing hairpin probe undergoes cyclic assembly, triggered by the DNAzyme-activated release of primer strands, forming fluorescent DNA nanowires. Compared to previous multi-hairpin DNA networks, this approach streamlines probe design and assay procedure without compromising sensitivity.

Fig. 5 Detection mechanism of the DNA network biosensor for the ultrasensitive detection of UO₂²⁺ based on the double-loop hairpin probe assembly. Reproduced with permission from ref. 75. Copyright 2024, the American Chemical Society.

2.5 Comparison of properties between different materials

Table 1 summarizes the performance of representative materials selected from different categories for fluorescence-based UO₂²⁺ detection. Different materials exhibit distinct properties, which are inherently associated with variations in their molecular and structural characteristics. Analyzing the structure–property relationship enables us to design materials with superior performance.

Table 1 Comparison of the properties of different materials for the fluorescence detection of UO₂²⁺

Material type	Material name	pH	Response time	Linear range (µM)	LOD (nM)	Ref.
MOFs	Tb-MOF	4	—	0.74–1296	3.33	80
	HNU-50	3	—	0.01–1000	12	52
	CSMCRI-21	5	20 s	1.26–1680	130	81
	Eu^3+@UiO-67-bpydc	6	—	0.01–0.1	3	54
	IRMOF-3-DPC	4	—	26.25–1260	1510	82
	Eu-DATP	—	—	0.01–100	2.7	55
COFs	TFPT-BTAN-AO	6	2 s	0.02–6	6.7	62
	TP-COF-AO	6	2 s	0–10	8.3	83
	PyTT-Tp	5	2 s	0.015–4.5	4.92	84
	TAPM-DHBD	5	2 s	0.01–3.7	4.08	28
	GC-TFPB-AO	7	—	0.05–0.5	21.25	63
Nanomaterials	AuNCs@AuNPs@B-CDs	2.5	12 min	0.2–5	8.4	69
	DA-MoOx QDs	6	5 min	0.005–1	3.85	71
	CMH-NCDs	4–5	0.5 s	0–20	8.4	85
DNAzymes	DNA network biosensor	6.6	100 min	0.00001–1	0.002	75
	DNA@SYBR green I	7.4	20 min	0.01–1	4.6	86
	DNA hairpin@39E DNAzyme	5.5	50 min	0.00005–0.02	0.017	76

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Under acidic to neutral conditions, UO₂²⁺ remains the dominant form that can undergo cation exchange and directly coordinate with most binding sites. At pH > 7, it predominantly exists as the highly stable anionic complex [UO₂(CO₃)₃]⁴⁻, which can cause materials and sensors originally designed to capture UO₂²⁺ to lose function entirely, especially those with anionic frameworks, such as MOFs and COFs.⁷⁷ As illustrated in Table 1, the optimal detection pH for most materials ranges from 3 to 7, which corresponds to the favorable speciation of UO₂²⁺. Anionic MOFs perform well under mildly acidic conditions; however, extreme acidity or alkalinity can impair detection performance and damage MOF structures, attributed to the inherent lability of coordination bonds. Many MOF ligands contain nitrogen-containing moieties (e.g., imidazole or pyridine) or oxygen-containing groups (e.g., carboxyl groups). Excessive proton concentrations compete with UO₂²⁺ for binding sites and can protonate the ligands, thereby disrupting the coordination framework. Under alkaline conditions, hydroxide ions induce hydrolysis of metal nodes (e.g., zinc or copper ions), resulting in framework collapse.⁷⁸ Compared to MOFs, COFs exhibit superior stability, due to their entirely organic frameworks and strong covalent bonds.⁷⁹ Fluorescent nanomaterials, such as metal nanoclusters and quantum dots, are susceptible to structural damage under extreme pH conditions: ligands on metal nanoclusters may undergo protonation or hydrolysis, while quantum dot nanocrystals are prone to corrosion, both of which lead to irreversible fluorescence loss. DNAzymes are particularly sensitive to extreme pH, as hydrolysis of purine bases or phosphodiester bonds disrupts their precisely folded three-dimensional structures, resulting in denaturation and the loss of both catalytic activity and specific UO₂²⁺ recognition capability.

The fluorescence response time is a key parameter that reflects the sensing efficiency of a material. As shown in Table 1, few studies on MOFs report clear fluorescence response times, whereas COF-based sensors generally exhibit rapid responses, with the majority achieving signal stabilization within 2 seconds. Both COFs and MOFs detect UO₂²⁺ through specific coordination between the uranyl ion and functional groups on their pore walls; this coordination event triggers electron transfer and energy-level transitions, which in turn alter the fluorescence properties of the materials. This rapid physicochemical interaction typically reaches equilibrium within a few seconds to one minute. The fluorescence of nanomaterials is intrinsic to their structure, and fluorescence quenching occurs promptly once UO₂²⁺ diffuses to the nanomaterial surface via collisions or weak coordination interactions. However, reported response times vary widely, ranging from 0.5 seconds to several minutes, depending on the experimental setup. For instance, UO₂²⁺-induced the aggregation of AuNPs requires a certain amount of time to form stable clusters—a process that is essential for achieving significant and stable ratiometric fluorescence detection based on the IFE. Thus, the response speed of such sensors is constrained by the kinetics of AuNP aggregation. In contrast, DNAzyme-based sensors exhibit longer response times, ranging from tens of minutes to hours, due to their multistep catalytic processes, which involve UO₂²⁺ binding, conformational changes of the DNAzyme, and catalytic activity. Unlike porous materials (e.g., MOFs and COFs) that possess large specific surface areas and multiple binding sites, a single DNAzyme molecule typically contains only a few catalytic sites, resulting in relatively lower sensing efficiency.

The detection limit is defined as the lowest concentration of UO₂²⁺ that can be reliably quantified, with lower detection limits indicating higher sensing sensitivity of the material. As shown in Table 1, MOFs exhibit linear detection ranges spanning one to five orders of magnitude, with detection limits ranging from several nanomolar to several micromolar. Their porous structure provides abundant binding sites that resist rapid saturation, thereby yielding relatively wide linear ranges; however, the upper limit of these ranges is constrained by the diffusion rate of UO₂²⁺ within the pores. The strong coordination between UO₂²⁺ and high-affinity functional groups on MOFs enables these materials to achieve low detection limits. In comparison, COFs typically exhibit linear detection ranges of one to two orders of magnitude and detection limits ranging from several to tens of nanomolar. Despite possessing more stable covalent bonds and open pore structures than MOFs, the performance of COFs is similarly limited by the diffusion of UO₂²⁺ within their pores. Nanomaterials display linear detection ranges of one to three orders of magnitude, with detection limits generally in the single-digit nanomolar range. Their nanoscale dimensions confer high fluorescence efficiency and a large specific surface area, leading to excellent sensing sensitivity; however, the limited number of surface binding sites restricts their linear ranges due to facile saturation. DNAzymes offer linear detection ranges spanning two to five orders of magnitude and detection limits ranging from picomolar to nanomolar levels. The catalytic amplification effect and high specificity of aptamer-UO₂²⁺ binding allow a single UO₂²⁺ ion to trigger significant signal changes, resulting in ultralow detection limits.

2.5.1 Improvement in fluorescence performance. The core of fluorescence-based detection lies in the intensity and stability of the fluorescence signal, which are fundamentally dependent on the excellent luminescent properties of the sensing material.

For ligand-based luminescent MOFs, the selection of ligands with AIE properties offers distinct advantages. Such ligands exhibit weak fluorescence in their monomeric state but display significantly enhanced emission when confined within the fixed, ordered framework of MOFs. For metal-centered luminescent MOFs, the careful selection of organic ligands is critical to match their triplet energy levels with those of lanthanide ions (e.g., Eu³⁺ and Tb³⁺), thereby enabling efficient energy transfer. Alternatively, strong fluorophores, such as organic dyes or quantum dots, can be post-synthetically grafted onto MOF pores or frameworks to serve as new emission centers. For instance, Ahmad et al.⁸⁸ utilized ZIF-8 as a porous host to uniformly disperse quantum dots within its channels, which effectively suppressed aggregation, maintained high fluorescence efficiency, and consequently enhanced the photoluminescence quantum yield. Cui et al.⁸⁹ fabricated a composite of Tb-MOF and terbium alginate hydrogel (Tb-AG); this composite improved material stability and provided additional coordination sites for uranyl ions via the carboxyl oxygen atoms in Tb-AG, thereby greatly enhancing fluorescence quenching efficiency and achieving both high sensitivity and stability. COFs generally exhibit weaker fluorescence compared to MOFs, as dynamic covalent bonds facilitate nonradiative decay pathways. The introduction of lanthanide ions coordinated to COFs can act as antenna moieties, transferring absorbed energy from the organic framework to amplify the characteristic emission of lanthanides and dramatically improve the fluorescence performance. Mao et al.⁹⁰ coordinated Eu³⁺ to the nitrogen atoms in the bipyridine units within the COF, which restricted the free rotation of bipyridine and suppressed nonradiative energy dissipation caused by intramolecular rotation. This modification significantly enhanced the intrinsic fluorescence of the COF and enabled turn-on fluorescence detection of uranyl ions via resonance energy transfer. Similar to MOFs, post-synthetic modification can incorporate highly emissive molecules into COF pores, leveraging confinement effects to protect and enhance luminescence. For quantum dots, surface modification with organic molecules or polymers can improve their dispersibility and stability while reducing nonspecific interactions that induce fluorescence quenching. The introduction of heteroatom dopants (e.g., nitrogen or sulfur) modifies their electronic structure, allowing for the tuning of emission color and enhancement of fluorescence intensity. Yang et al.⁹¹ employed a ligand exchange strategy to functionalize originally hydrophobic CdSe/CdS/ZnS quantum dots, successfully converting them into water-soluble fluorescent probes with excellent dispersibility and fluorescence properties. In the design of DNAzyme-based fluorescent sensors, the selection of high-performance fluorophores or integration with fluorescent materials enables highly specific detection. Zhang et al.⁸⁷ developed a “switch-on/off” fluorescent sensing platform by integrating upconversion nanoparticles (UCNPs), AuNPs, DNAzyme, and hybridization chain reaction (HCR). Specifically, UCNPs serve as energy donors and AuNPs as energy acceptors: in the absence of UO₂²⁺, AuNPs remain dispersed and quench UCNPs' fluorescence via FRET, putting the system in an “off” state. Upon the introduction of UO₂²⁺, the target ion triggers DNAzyme hydrolysis to produce single-stranded DNA (ssDNA), which further initiates HCR for the formation of long-chain double-stranded DNA (dsDNA). The electrostatic repulsion between dsDNA and AuNPs then induces AuNP aggregation, which weakens FRET and enables the subsequent fluorescence recovery of UCNPs (i.e., an “on” state) (Fig. 6). This system synergistically combines the high specificity of DNAzyme-based recognition, the robust signal amplification of HCR, the low-background advantage of UCNPs, and the efficient quenching capability of AuNPs, thereby achieving ultrasensitive, highly selective, and rapid detection of UO₂²⁺.

Fig. 6 Upconversion fluorescence platform for the sensitive quantification of UO₂²⁺ based on specific DNAzyme-mediated signal amplification and fluorescence resonance energy transfer. Reproduced with permission from ref. 87. Copyright 2024, Springer.

In summary, single materials often struggle to simultaneously achieve high fluorescence intensity and excellent stability. The key to performance enhancement lies in the strategic integration of complementary material properties through sophisticated composite design.

3. Colorimetry

Colorimetric detection is an analytical technique based on detectable color changes in a solution. It enables direct visual readout, facilitating rapid on-site screening and straightforward semi-quantitative assessment (Fig. 7).⁹²

Fig. 7 Schematic of the colorimetric detection of UO₂²⁺ with different materials.

3.1 MOFs

The colorimetric detection of UO₂²⁺ based on MOFs relies on the catalytic reaction between MOF-enzyme composites and substrates, which induces a color change. The binding of UO₂²⁺ to the MOF-enzyme composite inhibits the enzyme's catalytic activity, preventing substrate oxidation and enabling colorimetric detection of uranyl ions.⁹³ In 2022, Zhou et al.⁹⁴ embedded hemin into two-dimensional ZIF-L nanosheets to form a biomimetic peroxidase, ZIF-L-hemin (Fig. 8). In the presence of H₂O₂, the composite catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), producing blue-colored oxidized TMB (oxTMB). Upon UO₂²⁺ introduction, coordination between U–N and the electron-rich 2-methylimidazole in ZIF-L disrupts the axial electron donation from imidazole to hemin-Fe, inhibiting the catalytic cycle. This results in a decreased oxTMB yield and a color shift from deep blue to light blue or colorless.

Fig. 8 Schematic of the synthesis procedure of ZIF-L-hemin and the structures of HRP and ZIF-L-hemin. Reproduced with permission from ref. 94. Copyright 2022, the American Chemical Society.

3.2 COFs

The colorimetric method is based on the generation of highly reactive free radicals or reactive oxygen species on the surface of COF materials under light irradiation, which catalyze the substrate's colorimetric reaction.⁹⁵ The introduction of UO₂²⁺ inhibits this catalytic process through interactions with the COFs or the substrate. Xu et al.⁹⁶ developed a visible-light-responsive sulfone-based COF (TAS-COF) that exhibits oxidase-like activity upon visible-light excitation, catalyzing the conversion of the colorless substrate TMB into blue oxTMB. The imine groups in oxTMB coordinate with UO₂²⁺ to form a colorless complex, causing the fading of the blue color in the presence of uranyl ions (Fig. 9). Thus, monitoring the significant absorbance changes of the TAS-COF-TMB system enables sensitive colorimetric detection of UO₂²⁺.

Fig. 9 Illustration of the colorimetric detection of UO₂²⁺ using visible-light-responsive TAS-COF. Reproduced with permission from ref. 96. Copyright 2022, MDPI.

3.3 Nanomaterials

AuNPs can be employed to design colorimetric sensors for UO₂²⁺ detection. When dispersed, AuNPs exhibit strong absorption at a characteristic wavelength and reflect or scatter the complementary light, resulting in a distinct color. Upon aggregation, the plasmon coupling between particles induces a red shift in the localized surface plasmon resonance (LSPR) absorption peak, causing the solution color to change due to the absorption of longer-wavelength light.⁹⁷ Since AuNPs lack inherent specificity, their surfaces are functionalized with ligands that selectively coordinate to UO₂²⁺. The binding of UO₂²⁺ to these ligand crosslinks dispersed AuNPs into larger aggregates, triggering a visible color transition. Zhang et al.⁹⁸ functionalized AuNPs with phosphate groups, imparting a negative charge and yielding a stable wine-red colloidal solution. Specific coordination between UO₂²⁺ and the phosphate groups reduces the electrostatic repulsion among the AuNPs, inducing nanoparticle aggregation. This shifts the SPR peak from 519 nm to approximately 650 nm, with the solution color changing from wine red to blue, enabling sensitive colorimetric detection of UO₂²⁺.

3.4 DNAzymes

In the presence of UO₂²⁺, DNAzymes specifically cleave their substrate and form peroxidase-mimicking DNAzyme complexes that catalyze the oxidation of chromogenic ions in solution, causing a visible color change for detection.²⁹ Zhang et al.⁹⁹ developed a colorimetric assay combining DNAzyme-modified agarose microbeads with cascade signal amplification for UO₂²⁺ detection. A bifunctional DNA probe integrates a uranyl-specific DNAzyme substrate as the recognition element and a rolling circle amplification (RCA) primer as the signal transducer, immobilized on MBs via the biotin–streptavidin interaction. Upon UO₂²⁺ addition, the DNAzyme cleaves the substrate strand, releasing the primer to initiate RCA, producing abundant guanine quadruplex (G-quadruplex) repeats. The G-quadruplexes complex with hemin to form peroxidase-like catalysts that oxidize ABTS²⁻ with H₂O₂, generating a blue-green color detectable by the naked eye or UV-vis spectroscopy (Fig. 10).

Fig. 10 Schematic of the DNAzyme colorimetric detection of UO₂²⁺. Reproduced with permission from ref. 99. Copyright 2022, the Royal Society of Chemistry.

3.5 Comparison of properties between different materials

For the currently reported colorimetric detection of UO₂²⁺ using MOFs, COFs, and DNAzymes, the underlying principle is uniformly based on enzyme-catalyzed oxidation or inhibition events that modulate substrate conversion, thereby inducing a color change. The final colorimetric signal is derived from the oxidized product of the substrate, whose chemical structure and absorption maximum are typically governed by the pH condition. Colorimetric detection is usually performed under fixed pH conditions, as pH affects not only the structural stability of the materials but also the catalytic activity of enzymes and the stability of substrates involved in the colorimetric reaction. Therefore, maintaining an optimal pH to ensure maximum catalytic activity and the formation of a stable, intensely colored product is crucial for achieving high-sensitivity detection.

As shown in Table 2, MOF-based colorimetric sensors exhibit the fastest response, whereas DNAzyme-based assays require the longest response time. This striking difference in response kinetics can be fundamentally attributed to the distinct nature of their catalytic systems. Although MOF, COF, and DNAzyme-based methods all rely on enzyme-mimetic catalyses, the catalytic activity of MOFs and COFs originates from the materials themselves, which act as abiotic nanozymes rather than fragile biological enzymes. Unlike natural enzymes that require strict conformational maintenance to retain activity, these porous framework materials possess inherent, structure-dependent catalytic active sites (e.g., metal nodes and functionalized organic linkers) that enable direct, unimpeded substrate access and rapid redox reactions. Consequently, their catalytic efficiency and reaction rate are generally more stable than those of biological enzymes, leading to significantly faster colorimetric responses. In contrast, DNAzyme-based colorimetric detection involves a multistep biological catalytic process, including UO₂²⁺-dependent substrate cleavage, G-quadruplex formation, and subsequent peroxidase-mimetic catalysis for substrate oxidation. Each step in this cascade requires sufficient time for molecular recognition, conformational rearrangement, and catalytic turnover, which inherently prolongs the overall reaction time and results in the slowest response among the studied methods. Notably, AuNP-based colorimetric sensors do not rely on enzymatic catalysis; instead, they leverage LSPR changes induced by physical and chemical interactions, enabling rapid color transitions driven by direct nanoparticle aggregation without catalytic lag.

Table 2 Comparison of the properties of different materials for the colorimetric detection of UO₂²⁺

Material type	Material name	pH	Response time	Linear range (µM)	LOD (nM)	Ref.
MOFs	ZIF-L-hemin	7	3 min	0.25–40	79	94
COFs	TAS-COF	3.5	10 min	0.25–25	70	96
	JUC-505-AO	5.6	25 min	0–20	148	17
Nanomaterials	VPA-AuNPs	—	9 min	0.5–10	1070	98
DNAzymes	Bifunctional DNA probe	5.5	40 min	0.001–0.05	0.48	99

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Regarding detection sensitivity, the DNAzyme-based method achieves an exceptionally low limit of detection (LOD), far outperforming MOF-, COF-, and AuNP-based sensors, while AuNP-based sensors exhibit the highest LOD. This performance gap arises directly from the fundamental differences in their sensing mechanisms. The DNAzyme-based colorimetric method for UO₂²⁺ detection capitalizes on its high specificity to UO₂²⁺, which enables precise, signal-amplified catalytic turnover even at ultratrace analyte concentrations. Each UO₂²⁺ ion can trigger the cleavage of multiple DNAzyme substrates, initiating a cascade of G-quadruplex formation and peroxidase-mimetic catalysis that amplifies the colorimetric signal exponentially, thus enabling detection at sub-nanomolar levels. In contrast, the use of AuNPs results in a relatively higher detection limit. This is because the underlying mechanism requires a sufficient concentration of UO₂²⁺ to neutralize the abundant negative charge on the AuNP surface, thereby overcoming electrostatic repulsion and triggering nanoparticle aggregation, which is accompanied by a subsequent color change. At low UO₂²⁺ concentrations, only limited aggregation occurs, producing a color change too weak to be reliably detected by either the naked eye or analytical instruments. Additionally, AuNP aggregation is a non-catalytic, stoichiometric process with no signal amplification, further limiting its sensitivity compared to catalytically amplified methods.

For MOF and COF-based sensors, their LOD values fall between those of DNAzyme- and AuNP-based sensors, which can be explained by their intermediate catalytic amplification and binding affinity. As porous framework nanozymes, MOFs and COFs offer moderate catalytic activity and UO₂²⁺ adsorption capacity, enabling signal amplification through catalytic substrate oxidation; however, they lack the extreme specificity and exponential amplification capability of DNAzyme systems. This balance results in sensitivity that is higher than those of AuNP-based aggregation sensors but lower than those of DNAzyme-based assays, aligning with the trend observed in Table 2.

3.5.1 Improvement in colorimetric detection performance. Currently, reports on the colorimetric detection of UO₂²⁺ remain limited. Colorimetric sensors based on MOFs, COFs, and nanomaterials rely on enzyme–substrate catalytic reactions to generate changes in a single signal unit, lacking effective signal amplification mechanisms; this limitation hinders the detection of ultra-low UO₂²⁺ concentrations. Additionally, the intrinsic color and turbidity of the solution, as well as the presence of other colored substances in the sample, can directly interfere with absorbance measurements, thereby compromising the accuracy and reliability of the detection results.

The sensitivity of UO₂²⁺ detection can be improved by enhancing the intrinsic selectivity of the sensing material. Materials, such as MOFs, COFs, and nanomaterials, can be modified through post-synthetic incorporation of specific functional groups, including amidoxime or phosphate groups, which exhibit strong coordination affinity toward UO₂²⁺. These optimized materials can be integrated with smartphone platforms, yielding more objective and quantifiable results compared to visual observation and enabling a transition from qualitative to semiquantitative analysis. This integrated approach facilitates rapid, on-site, and equipment-free quantitative detection, significantly improving the practicality and accessibility of UO₂²⁺ monitoring. For instance, Lei et al.¹⁰⁰ developed a convenient and portable smartphone-assisted colorimetric method for UO₂²⁺ detection. Rifampicin and Br-PADAP were used as chromogenic agents, which undergo coordination reactions with UO₂²⁺ separately, resulting in distinct color changes of the solutions: from orange to dark red in the rifampicin system and from yellow to light pink in the Br-PADAP system. As illustrated in Fig. 11, a custom colorimetric viewfinder was employed to standardize the smartphone's shooting conditions; digital images of the reacted solutions were then imported into the built-in “Detect” application to extract the S value (saturation) of HSV and the L value (lightness) of HSL. Leveraging the linear relationships between these color parameters and the UO₂²⁺ concentration, on-site rapid estimation of the concentration range was achieved. Meanwhile, combined with the characteristic peak detection using a UV-vis spectrophotometer at 375 nm (rifampicin system) and 562 nm (Br-PADAP system), precise quantification with a low limit of detection of 0.89 µM was realized. This platform integrates a dual-mode of “on-site preliminary screening and laboratory precise determination”, which not only addresses the timeliness requirement of on-site emergency detection but also ensures the accuracy of laboratory analysis, achieving the unification of “portability” and “precision”.

Fig. 11 Schematic of the chromogenic reactions for the detection of UO₂²⁺. Reproduced with permission from ref. 100. Copyright 2023, the Royal Society of Chemistry.

4. Electrochemical detection

Electrochemical detection is a sensitive and selective detection method for trace UO₂²⁺, which involves measuring the current signal generated by the redox reaction of UO₂²⁺ on the electrode surface, facilitated by high-sensitivity pulse voltammetry and a functional electrode-modified interface (Fig. 12).³²

Fig. 12 Schematic of the electrochemical detection of UO₂²⁺ with different materials.

4.1 MOFs

MOFs provide well-defined pores and high specific surface areas, and, upon carbonization, they form conductive three-dimensional networks.¹⁰¹ The resulting MOF-derived porous carbon materials possess negatively charged surfaces that strongly attract the positively charged UO₂²⁺ ions in solution, enabling efficient enrichment of UO₂²⁺ at the electrode interface. This facilitates the electrochemical reduction of UO₂²⁺ on the electrode surface, producing measurable reduction currents for uranium detection.¹⁰² In 2020, Cao et al.¹⁰³ used UiO-66-NH₂ as a precursor to embed PtRu bimetallic nanoparticles onto MOF-derived porous carbon to create a composite for electrochemical UO₂²⁺ detection (Fig. 13). The embedded PtRu nanoparticles serve as key catalytic centers, significantly lowering the overpotential required for UO₂²⁺ reduction and accelerating electron transfer. The porous carbon provides a large surface area and rapid electron- and ion-transport channels, greatly amplifying the current signal. The synergistic interaction between PtRu nanoparticles and MOF-derived porous carbon enhances the overall electrochemical performance.

Fig. 13 Schematic of the synthesis of Pt₁Ru₂-PC. Reproduced with permission from ref. 103. Copyright 2020, Springer.

4.2 COFs

COFs can be employed for electrochemiluminescence (ECL) detection of UO₂²⁺, a technique that integrates electrochemical reactions with optical signal measurement. By rational design and the selection of electron donor and acceptor units within COFs, efficient intramolecular charge transfer (ICT) can be achieved, exciting electrons in the COF to produce luminescence. Specific binding of UO₂²⁺ to functional groups in the COF alters its electronic structure, thereby modulating the ECL intensity.¹⁰⁴ Cui et al.¹⁰⁵ developed an alkene-linked COF, TBTN-COF, and synthesized a BCBA-TBTN-AO complex by converting cyano groups in the COF into amidoxime groups, enabling selective UO₂²⁺ coordination. Although the monomers are non-luminescent, the ICT within the framework induces strong ECL, where TBTN serves as the electron acceptor and BCBA and DAFB as electron donors, facilitating effective charge transfer. Under applied potential, the electrons in the COF are excited to form radical anions, which emit light upon collision with superoxide anions, generating a robust ECL signal (Fig. 14). The coordination of UO₂²⁺ with BCBA-TBTN-AO transfers electrons from the donor units to TBTN, enhancing the ECL response and establishing a highly selective “turn-on” ECL sensing system with ultralow detection limits.

Fig. 14 Schematic of the COF-based electrochemiluminescence detection of UO₂²⁺. Reproduced with permission from ref. 105. Copyright 2021, the American Chemical Society.

4.3 Nanomaterials

Nanomaterials, featuring a high specific surface area, abundant active sites, and excellent conductivity, serve as carriers to immobilize numerous recognition elements and establish efficient electron-transfer channels with electrodes, enabling highly sensitive detection of low-concentration uranyl ions.¹⁰⁶ Han et al.¹⁰⁷ modified a glassy carbon electrode (GCE) with AuNPs for uranium detection using differential pulse adsorptive stripping voltammetry. Distinct redox peaks were observed on all electrodes. After the AuNP modification, the oxidation peak current increased from 454 µA to 579 µA, corresponding to a 27.53% enhancement, which clearly demonstrates the superior electron-transfer capability of the AuNPs/GCE. In this method, UO₂²⁺ is first reduced and deposited onto the electrode at negative potentials. A subsequent positive potential scan then oxidizes and strips the deposited uranium, generating an oxidation peak current corresponding to the UO₂²⁺ concentration. The high surface area of AuNPs significantly increases the active electrode area, which amplifies the stripping signal and enhances the detection sensitivity.

4.4 DNAzymes

DNA itself is non-conductive; thus, to realize electrochemical detection, electroactive molecules are covalently attached at specific positions on the DNA strand, along with UO₂²⁺ binding aptamers.¹⁰⁸ When UO₂²⁺ binds to the aptamer, it induces conformational changes in the DNA structure. This conformational shift affects the electron-transfer efficiency between the electroactive molecule and the electrode, resulting in measurable current changes.¹⁰⁹

Jett et al.¹¹⁰ reported a DNA-based electrochemical biosensor for detecting UO₂²⁺ in water. The sensor employs a modified uranyl-binding aptamer embedded within an oligonucleotide scaffold, forming a reusable, conformation-switching electrochemical DNA biosensor (Fig. 15A). Upon UO₂²⁺ binding, the DNAzyme activity is triggered, causing cleavage at a specific site and a conformational switch from an unbound to a bound state (Fig. 15B). This structural change alters the position and environment of the methylene blue electroactive reporter attached to the DNA, thus modulating electron transfer at the electrode surface and generating a corresponding electrochemical signal change.

Fig. 15 (A) Schematic of the DNAzyme–UO₂²⁺ binding. Reproduced with permission from ref. 110. Copyright 2017, Wiley. (B) Schematic of the conformation-switching E-DNA uranyl biosensor. Reproduced with permission from ref. 110. Copyright 2017, Wiley.

4.5 Comparison of properties between different materials

As shown in Table 3, distinct variations in electrochemical detection performance are observed across different material systems for UO₂²⁺ analysis. Specifically, COFs and nanomaterials outperform MOFs and DNAzymes in terms of linear ranges and LOD values, while the latter two material categories are relatively less reported and typically exhibit inferior sensing metrics. This performance disparity stems fundamentally from the intrinsic electronic and structural properties of each material class, as well as their compatibility with electrochemical transduction mechanisms.

Table 3 Comparison of the properties of different materials for the electrochemical detection of UO₂²⁺

Material type	Material name	pH	Response time	Linear range (µM)	LOD (nM)	Ref.
MOFs	PtRu-PCs	4.6	3 min	0.168–3.528	24	103
COFs	AuNCs@COF	4	60 min	0.0005–25	0.347	111
	BCBA-TBTN-AO/GCE	5.5	150 s	0.000001–0.001	0.00036	105
Nanomaterials	AuNPs/GCE	3	600 s	0.0084–0.042	0.44	107
	AuNPs/GCE	1.6	400 s	0.010–0.20	1.26	112
	PNPs	7.4	—	0.00000037–0.00037	0.0021	113
DNAzymes	SG@dsDNA@[Fe(CN)6]^3−/4−	5.5	75 min	0.001–1	0.13	114
	Electrochemical biosensor	5	—	0.1–500	1200	110

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Most MOFs and COFs are inherently semiconductors or insulators with low electrical conductivity, which severely restricts efficient electron transfer between the electrode interface and the bulk of the material. This limitation directly translates into sluggish electron kinetics and weak signal generation, ultimately resulting in the relatively high LOD (24 nM) and narrow linear range (0.168–3.528 µM) observed for MOF-based sensors like PtRu–PCs (Table 3). In contrast, conductive nanomaterials, such as AuNPs and PNPs, possess exceptional electron mobility, large specific surface areas, and robust electrocatalytic activity. These features facilitate rapid electron transfer at the electrode surface, enabling wider linear ranges (e.g., PNPs cover 0.00000037–0.00037 µM) and lower LOD values (e.g., AuNPs/GCE at pH 3 achieves 0.44 nM). However, unmodified conductive nanomaterials lack specific recognition sites for uranyl ions, making them vulnerable to interference from coexisting ions and compromising detection selectivity—a critical drawback reflected in their limited practical applicability.

COFs bridge the gap between the low conductivity of pristine MOFs and the non-selectivity of bare nanomaterials. Their porous framework structure not only provides abundant active sites for UO₂²⁺ adsorption but also, when integrated with conductive components (e.g., AuNCs@COF), increases electron-transfer efficiency. This synergy explains the superior performance of COF-based sensors: AuNCs@COF achieves an ultra-low LOD of 0.347 nM across a wide linear range (0.0005–25 µM), while BCBA-TBTN-AO/GCE sets an impressive LOD of 0.00036 nM (0.000001–0.001 µM). These results validate that combining COFs with conductive nanomaterials addresses the conductivity limitation of pure frameworks while retaining their specific recognition capability.

For DNAzyme-based electrochemical sensors, their poor performance (e.g., SG@dsDNA@[Fe(CN)₆]³⁻/⁴⁻ has an LOD of 0.13 nM and a narrow linear range of 0.001–1 µM) arises from the intrinsic electro inactivity of DNAzymes. Unlike catalytically active nanomaterials or semiconducting frameworks, DNAzymes lack inherent electron-transfer capacity; their signal transduction relies on UO₂²⁺-induced DNA cleavage, which must be coupled with electroactive probes (e.g., [Fe(CN)₆]^3−/4−) or conductive substrates to generate a measurable electrochemical response. This multi-step coupling process introduces additional barriers to electron transfer and signal amplification, resulting in relatively higher LODs and narrower linear ranges compared with those for COF and nanomaterial-based sensors. Even the electrochemical biosensor reference shows a high LOD of 1200 nM, further highlighting the challenges of direct electrochemical detection with unmodified DNAzymes.

In summary, the performance differences in Table 3 are tightly governed by the electronic conductivity, specific recognition ability, and structural synergy of each material system. Composite structures, combining porous MOFs/COFs with conductive nanomaterials or coupling DNAzymes with electroactive transducers, have emerged as a universal strategy to overcome intrinsic limitations, enabling high sensitivity and wide linear ranges for UO₂²⁺ electrochemical detection.

4.5.1 Improvement in electrochemical detection performance. MOFs carbonized at high temperatures can be converted into porous carbon materials with a high specific surface area. These carbonized materials not only retain the original porous structure of the parent MOF but also form an efficient three-dimensional conductive network, which greatly facilitates rapid electron transfer at the electrode surface. For COF materials, their overall electrical conductivity can be improved by introducing dopants or integrating conductive components to construct continuous conductive networks. Nanomaterials are commonly used as electrode modifiers to increase electron-transfer efficiency, and they are often combined with MOFs, COFs, DNAzymes, or functional groups with high affinity for UO₂²⁺ to improve detection selectivity and adsorption capacity. For example, Jiang et al.¹¹⁵ first grafted amidoxime groups onto fluorinated graphene for the electrochemical detection of UO₂²⁺. The two-dimensional layered structure of fluorinated graphene provides a large specific surface area for loading abundant amidoxime groups, which specifically bind to UO₂²⁺, thereby enhancing sensor sensitivity and uranium enrichment efficiency. Chen et al.¹¹⁶ developed a dual-signal amplification electrochemical platform by using gold nanoparticles as signal amplifiers and synthesizing silver nanoclusters via a hybridization chain reaction (HCR). The gold nanoparticles increased the loading of uranyl-specific DNAzymes on the electrode surface. In the presence of UO₂²⁺, the DNAzyme is activated and cleaves the substrate strand, triggering the HCR, which generates long double-stranded DNA (dsDNA) on the electrode surface. This dsDNA acts as a template for the in situ synthesis of abundant silver nanoclusters, leading to significant amplification of the electrochemical signal.

5. Comparison of different detection methods

As shown in Table 4, a comprehensive performance comparison reveals that each detection method possesses distinct advantages and limitations, which are systematically summarized below.

Table 4 Performance comparison of different methods for detecting UO₂²⁺

Detection method	Linear range	LOD	Selectivity	Convenience	Response time
Fluorescence	Ordinary	Low	High	Ordinary	Rapid
Colorimetry	Narrow	High	Ordinary	Excellent	Fast
Electrochemical detection	Wide	Extremely low	Ordinary	Limited	Moderate

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Fluorescence-based detection exhibits a balanced profile, with superior sensitivity and selectivity. Through sophisticated probe design (e.g., DNAzymes) and signal amplification strategies, it can achieve ultra-low LODs at the nanomolar to picomolar level. The high selectivity is inherently derived from the specific molecular recognition between uranyl ions and functional probes (e.g., G-quadruplex structures or amidoxime groups), which effectively minimizes interference from coexisting ions. Operationally, this method offers convenience as it typically requires only sample-probe mixing without complex pretreatment, enabling quantitative analysis via fluorescence intensity changes. Kinetic studies benefit from the rapid response as fluorescence quenching or enhancement occurs instantaneously upon target binding, usually within minutes, facilitating real-time monitoring. However, its primary constraint lies in the reliance on sophisticated optical instrumentation (e.g., fluorometers), which restricts its applicability for field-deployed, on-site rapid testing and confines it mainly to laboratory settings.

Colorimetric detection excels in terms of operational convenience and speed, marked as “Excellent” and “Fast” in Table 4, making it the preferred choice for naked-eye, on-site analysis. It delivers results within seconds to minutes through visible color changes. However, this convenience comes at the cost of sensitivity, characterized by a “High” LOD and a “Narrow” linear range. This performance trade-off stems from the fundamental mechanism: colorimetric signals rely on the accumulation of sufficient chromophores to produce a measurable absorbance change. At low concentrations, the signal is too weak to be detected, while at high concentrations, color saturation occurs, leading to a non-linear relationship between intensity and concentration. Furthermore, the method is highly susceptible to environmental interferences. Its “Ordinary” selectivity is attributed to the high pH sensitivity of the chromogenic reaction; even minor changes in pH or direct competition from other metal ions can alter the reaction equilibrium, resulting in false signals or background interference.

Electrochemical detection stands out with the highest sensitivity, featuring an “Extremely low” LOD and a “Wide” linear range (Table 4), making it ideal for trace-level analysis in complex matrices. This high sensitivity stems from its direct signal-transduction mechanism, in which trace UO₂²⁺ undergoes rapid redox reactions at the electrode surface and generates measurable current signals, while efficient electron transfer and interfacial signal amplification further lower the detection limit and extend the linear range. Despite these advantages, the method suffers from notable practical limitations: electrode modification typically involves complex, time-consuming assembly steps that demand skilled operation and yield unstable interfaces, resulting in “Limited” convenience. Its “Moderate” response time can be attributed to the necessary adsorption and accumulation of UO₂²⁺ prior to effective signal generation, and its reliance on costly, specialized potentiostats prevents truly portable and real-time field applications. Moreover, its “Ordinary” selectivity arises from the inherent nature of electrochemical transduction, as other redox-active metal ions can produce overlapping interfering currents or compete for active sites, making it difficult to achieve exclusive recognition of uranyl ions without highly specific functional modification.

In summary, Table 4 clearly delineates the performance spectrum: fluorescence offers high sensitivity and speed but requires lab equipment; colorimetry provides unmatched on-site convenience but sacrifices linear range and sensitivity; electrochemistry delivers ultra-sensitivity but lacks portability and simplicity. This comparison underscores the importance of tailoring method selection to specific application scenarios, whether prioritizing laboratory accuracy, field portability, or trace-level detection.

To address these performance trade-offs and achieve comprehensive detection capabilities, integrating multiple complementary techniques into a single platform has emerged as a promising strategy. This dual-modal approach not only leverages the strengths of individual methods but also mitigates their respective limitations, thereby significantly enhancing the overall reliability and practical applicability of the sensor. Chen et al.¹¹¹ developed an innovative dual-mode colorimetric and electrochemical sensor for UO₂²⁺ detection using AuNCs@COF composites (Fig. 16A and B). This material combines excellent electrocatalytic activity with optical properties, embedding nanoclusters with peroxidase-mimicking activity that catalyze substrates, inducing a color change in the presence of UO₂²⁺. Simultaneously, the composite on the electrode catalyzes electrochemical reactions with UO₂²⁺ to generate current signals. Dual measurement of electrochemical and colorimetric signals allows mutual validation, improving accuracy and reliability. Xiong et al.¹¹⁷ designed a fluorescence/colorimetric dual-mode sensing strategy based on cerium-sulfonated calix[4]arene (SC4A@Ce). This method cleverly exploits SC4A coordination with Ce³⁺ and Ce⁴⁺: Ce³⁺ fluorescence is quenched when bound to SC4A but restored upon the addition of UO₂²⁺, which displaces Ce³⁺. Meanwhile, Ce⁴⁺'s oxidase-like activity catalyzes TMB oxidation to oxTMB, changing the solution color from blue to colorless. By combining highly sensitive fluorescence with visible colorimetric analysis, this approach achieves enhanced sensitivity, accuracy, and reliability. Wu et al.¹¹⁸ developed a dual-mode colorimetric and photothermal sensor based on self-assembled TMB-CuO₂ nanosheets for UO₂²⁺ detection. In acidic environments, the nanosheets spontaneously decompose to generate Cu²⁺, H₂O₂, and TMB in situ. A Fenton reaction then oxidizes TMB to blue oxTMB, which produces both a strong colorimetric signal and an efficient photothermal effect. UO₂²⁺ selectively coordinates with the imine groups of oxTMB. This coordination disrupts the conjugated structure of oxTMB, resulting in color fading, and simultaneously suppresses its photothermal conversion efficiency. Together, these effects produce a concentration-dependent dual signal response. By integrating the substrate, catalyst, and signal units into a single nanosheet without the need for external H₂O₂, this sensor achieves highly selective detection. It enables straightforward visual screening with the naked eye and on-site quantification based on thermal signal changes.

Fig. 16 (A) Preparation of COF, AuNCs@COF, and Apt/AuNCs@COF. Reproduced with permission from ref. 111. Copyright 2025, Springer. (B) Schematic of the principles of colorimetric and electrochemical dual-mode sensor detection of UO₂²⁺. Reproduced with permission from ref. 111. Copyright 2025, Springer.

6. Summary and outlook

In summary, this review provides a comprehensive examination of recent advancements in materials research for the optical and electrochemical detection of UO₂²⁺, with particular focus on MOFs, COFs, nanomaterials, and DNA-based materials. These materials offer distinct advantages in the selective and sensitive detection of UO₂²⁺, contributing to the development of efficient monitoring platforms. Despite the remarkable breakthroughs achieved in this field, several challenges persist for the practical application of UO₂²⁺ detection.

6.1 Challenges in practical deployment

6.1.1 Complex matrix interference. Real-world uranium-contaminated samples are complex multi-component matrices, markedly different from the pure buffer solutions used in laboratory validation. Seawater, industrial wastewater, soil leachate and groundwater all contain coexisting ions, dissolved organic matter, humic acids and suspended particulates, which exert dual adverse effects on detection performance. These substances engage in non-specific binding with sensing materials, competing with UO₂²⁺ for active recognition sites, reducing detection selectivity, and inducing false positive or negative signals. They also cause physical and chemical interferences that disrupt signal transduction: dissolved organic matter absorbs excitation and emission light in fluorescence detection, exacerbating the inner filter effect, while suspended particulates induce turbidity in colorimetric assays and obscure visual and spectroscopic readouts. Electrochemical detection faces comparable vulnerabilities, as redox-active coexisting ions produce overlapping current signals at the electrode surface, and organic fouling of modified electrodes blocks active sites and impairs electron transfer. Seawater is a particularly critical matrix for marine uranium resource exploration and nuclear wastewater monitoring, and it poses an extreme challenge for detection systems. Its high salinity leads to electrostatic screening of charged recognition groups on sensing materials, severely weakening the binding affinity between sensing materials and UO₂²⁺.

6.1.2 Field environmental stability issues. The structural and functional stability of sensing materials is a prerequisite for practical deployment, yet most state-of-the-art materials, including MOFs, COFs, DNAzymes and some nanomaterials, show poor stability under the harsh environmental conditions of real-world sampling sites. Porous MOFs are built on labile coordination bonds that hydrolyze or collapse under extreme pH and high ionic strength conditions, resulting in the loss of porous structure and recognition sites. COFs feature more robust covalent backbones, but their dynamic covalent bonds are prone to hydrolysis in aqueous environments during prolonged use, which impairs long-term stability. DNAzymes and nucleic acid-based sensors are highly sensitive to environmental factors; extreme pH, high temperature and nucleases in natural water bodies cause DNA denaturation and cleavage, leading to the loss of catalytic and recognition activity, which restricts their application in unprocessed natural samples. Even inorganic nanomaterials suffer from stability issues: surface ligand desorption in high-salinity solutions reduces their colloidal stability, and the oxidative corrosion of metal nanoparticles weakens their optical and electrochemical properties, giving rise to signal drift and poor reproducibility.

6.1.3 Instrumentation and operational limitations for on-site detection. On-site, real-time and portable monitoring stands as a key goal for UO₂²⁺ detection technology, yet current approaches are largely hampered by operational complexity and heavy-instrumentation dependence. The two most sensitive methods, fluorescence and electrochemical detections, rely on sophisticated, bulky and costly laboratory equipment that is not field-portable and requires controlled power supplies and operating environments. While colorimetric detection allows for naked-eye readout, its low sensitivity necessitates spectroscopic validation for quantitative analysis, which negates its portability advantage in most instances. Lab-scale detection also entails laborious sample pretreatment to mitigate matrix interference. Such processes are time-consuming and require specialized reagents and equipment, making them incompatible with rapid on-site screening. Consequently, raw sample analysis by unoptimized sensors leads to significant performance degradation due to the lack of necessary pretreatment resources in field settings. Additionally, practical operation poses notable barriers: electrochemical detection, in particular, requires trained personnel for precise electrode modification, calibration and operation, steps that are hard to implement in field settings with limited technical support, and even optical methods demand strict control of experimental parameters, a challenge without standardized, automated devices.

6.1.4 Scalability and cost limitations. Practical deployment of UO₂²⁺ detection technologies demands scalable synthesis and low production costs, yet most state-of-the-art sensing materials are fabricated via complex, low-yield lab-scale synthesis methods. These approaches are not suitable for large-scale industrial production, leading to prohibitively high material costs that hinder widespread field application. Furthermore, poor reusability renders most sensors disposable, driving up long-term monitoring costs and generating chemical waste that poses an additional environmental burden at uranium-contaminated sites. Though some materials show regeneration potential, the associated processes typically require harsh chemical reagents and extra processing steps, making them unfeasible for on-site use.

In summary, the practical deployment of UO₂²⁺ detection technologies is constrained not merely by analytical performance, but by a combination of matrix interference, material instability, instrumentation and operational barriers, performance–practicality trade-offs and scalability issues. Addressing these challenges requires a paradigm shift from lab-scale optimization of detection limits and selectivity to design optimization for real-world applications. This entails integrating material engineering, matrix adaptation and device miniaturization to develop sensing systems that are stable, portable, low-cost and robust in complex environmental conditions, a critical focus for translating the remarkable advances in functional materials outlined in this review into actionable, field-deployable uranium monitoring technologies.

6.2 Future improvement directions

6.2.1 Enhancing selectivity for UO₂²⁺. Achieving high selectivity in the presence of various interfering ions is critical for effective UO₂²⁺ detection. Functionalizing porous materials and nanomaterials with selective adsorbing groups, such as amidoxime, carboxyl, and phosphate groups, can enhance selectivity. Furthermore, tailoring the pore sizes of porous materials to fit the UO₂²⁺ ion can exploit the confinement effect, enhancing adsorption selectivity by limiting access to interfering ions. DNA-based materials, with their inherent ability for specific molecular recognition, can significantly improve selectivity through the incorporation of tailored nucleic acid sequences like DNAzymes, oligonucleotides, or aptamers.

6.2.2 Enhancing material stability. Improving the stability of detection materials in harsh environments (e.g., extreme pH, seawater, or industrial waste) remains a critical challenge. Porous materials, such as MOFs and COFs, are often prone to hydrolysis and structural collapse under extreme conditions. To improve stability, materials can be designed with more robust chemical bonds or modified by introducing hydrophobic functional groups, such as fluorides, alkyl, or aromatic groups. Furthermore, combining porous materials with other materials, such as nanomaterials, hydrogels, or polymers, can enhance structural stability and introduce new functional properties that improve overall performance.

6.2.3 Improving sensitivity and accuracy. Detection in complex environmental samples often encounters interference from multiple factors, which can affect the accuracy and sensitivity of existing materials. One promising solution is multimodal detection, which combines various sensing techniques to provide complementary data, thus improving accuracy. For example, the oxidase-like activity of COFs under light can induce colorimetric changes, while the high conductivity of nanomaterials can enable electrochemical measurements. By combining these properties in a colorimetric-electrochemical dual-mode system, the detection sensitivity and reliability can be significantly improved. Additionally, integrating the inherent fluorescence of MOFs, the specific recognition capabilities of DNA-based materials, and the superparamagnetism of iron oxide nanoparticles into a unified multimodal system could lead to more reliable and accurate detection platforms.

6.2.4 Simplifying instrumentation and operations. To surmount instrumentation and operational hurdles for on-site UO₂²⁺ monitoring, future research should focus on device miniaturization, workflow simplification and pretreatment-free sensing system development, targeting portable, automated and field-adaptable detection without reliance on bulky lab equipment or professional technical support. For high-sensitivity fluorescence and electrochemical detection, microfabrication and microfluidic technologies will enable miniaturized, battery-powered portable detectors with integrated core modules for field use; colorimetric detection will be optimized via smartphone-assisted readout systems with custom optical accessories and image-analysis algorithms to achieve on-site quantitative analysis without spectroscopic validation. Operational workflows will be streamlined by developing prefabricated modified electrodes for electrochemical detection and standardized, automated all-in-one detection kits for all technologies, reducing professional operational demands and improving the reproducibility of on-site results. Pretreatment-free sensing systems, the key to resolving laborious on-site sample processing, will be engineered by endowing sensing materials with anti-interference, anti-fouling and in situ target enrichment properties, enabling direct, high-sensitivity UO₂²⁺ detection in raw complex samples without centrifugation, filtration and other pretreatment procedures.

6.2.5 Improving scalability and reducing costs. To tackle the scalability and cost limitations restricting widespread practical deployment of UO₂²⁺ detection technologies, future research should focus on developing scalable, low-cost material synthesis approaches and designing reusable sensing systems. Complex, low-yield lab-scale synthesis methods for advanced sensing materials need optimization for industrial scalability, with the adoption of simple, high-yield fabrication processes and low-cost raw materials to cut production costs and enable large-scale manufacturing. The reusability of sensors should be enhanced via rational material engineering to construct robust, anti-fouling structures, thereby reducing their replacement frequency and lowering long-term monitoring costs. For materials with regeneration potential, mild, on-site applicable regeneration protocols should replace harsh chemical-based processes, simplifying on-site reuse and eliminating chemical waste-related environmental burdens. The design of universal sensing platforms adaptable for UO₂²⁺ detection via simple modification can also further reduce costs, promoting the large-scale practical application of low-cost, reliable uranium detection technologies.

Looking ahead, future research on UO₂²⁺ detection should focus on the development of multifunctional composite materials that integrate optical, electrochemical or multimodal sensing capabilities with enhanced selectivity, sensitivity and environmental stability. Such materials will address the core practical challenges, including complex matrix interference, poor material stability in harsh field settings, instrumentation and operational limitations, as well as scalability and cost constraints. Leveraging advances in computational chemistry and machine learning will yield valuable atomic-level insights into UO₂²⁺ recognition mechanisms and material degradation pathways, enabling the rational, predictive design of high-performance sensing materials, moving beyond empirical lab-scale optimization. More importantly, the seamless integration of such advanced materials with miniaturized, portable and field-adaptable detection devices, together with simplified, automated operational workflows and pretreatment-free sensing systems, will be pivotal to realizing real-time, on-site and reliable uranium contamination monitoring. Ultimately, the translation of these lab-scale innovations into efficient, cost-effective and robust UO₂²⁺ detection technologies will play a vital role in safeguarding the environment and human health, supporting the sustainable development of global nuclear energy and facilitating effective pollution control and environmental remediation at uranium-contaminated sites. By addressing the practical deployment challenges outlined in this review, we can advance uranium monitoring technologies that deliver long-term value for environmental protection and a more sustainable future.

Author contributions

KaiZeng Quan: writing – review & editing, writing – original draft, investigation. YuMin Wang: writing – review & editing, writing – original draft, investigation. Meng Qi: writing – original draft. Peng Xu: supervision, methodology. YaXing Wang: supervision, methodology. GuoXun Ji: writing – review & editing, investigation.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22476144).

Notes and references

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Footnote

† These authors contributed equally.

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