Recent advances in metal sulfide nanozymes for sensing

Abstract

Metal sulfide nanozymes have emerged as a highly promising class of catalytic nanomaterials for sensing applications due to their tunable activity, operational stability, and facile synthesis. Their sulfur-rich frameworks can promote electron transfer and the generation of reactive oxygen species, thereby enhancing signal amplification in colorimetric, electrochemical, and other sensing platforms. These attributes have enabled the sensitive detection of a broad range of analytes, including metal ions, pesticides, biomolecules, hydrogen peroxide, phenolics, and dyes, and have supported their integration into portable and point-of-care diagnostic devices. Despite rapid progress, the field remains fragmented by several unresolved challenges, including an incomplete understanding of structure–activity relationships, limited standardization of catalytic activity evaluation, and insufficient validation of selectivity in complex biological matrices. This review critically examines recent advances in metal sulfide nanozymes, focusing on their preparation strategies, catalytic mechanisms, and sensing applications. Unlike existing nanozyme reviews, this article specifically emphasizes how compositional modulation, defect engineering, heterostructure construction, and surface functionalization govern sensing performance in metal sulfide systems. By identifying major limitations and outlining future directions for mechanistic study, this review aims to provide a more focused framework for advancing metal sulfide nanozymes toward practical biomedical and environmental applications.

Metal sulfide nanozymes, due to their sulfur-rich framework, promote electron transfer and the generation of reactive oxygen species, thereby enhancing signal amplification in sensing platforms for various analytes.

1. Introduction

Nanozymes, an emerging class of nanomaterials, possess unique enzyme-like catalytic activities and have been applied in various fields, including sensing, bioimaging, and therapeutic agents.^1–4 They serve as a substitute for protein-based biocatalysts.^5,6 Scrimin et al. first proposed the idea of “nanozymes” in 2004 when they demonstrated phosphodiesterase-like action by coordinating functionalized gold nanoparticles with Zn²⁺ ions.⁷ Gao et al. in 2007 demonstrated that inorganic materials such as iron oxide (Fe₃O₄) nanoparticles could mimic the catalytic function of horseradish peroxidase.⁸ This concept was further refined in 2013 (ref. 9) and redefined in 2021 (ref. 10) to embrace nanomaterials that catalyze the conversion of enzyme substrates into products, adhering to enzymatic kinetics, such as Michaelis–Menten, under physiologically relevant conditions. Their appeal lies in practical advantages such as low cost, high stability, facile synthesis, and tunable surface chemistry, which often overcome the fragility and high production costs associated with natural enzymes.^11,12 However, despite these strengths, the catalytic mechanisms of many nanozymes remain less well understood than those of natural enzymes, and their activity can vary substantially with composition, morphology, defect density, and surface state.¹³

According to recent reports, these nanozymes can be divided into six groups. The first group consists of metallic oxides, e.g., Co₃O₄, CeO₂, CoFe₂O₄, CuO, V₂O₅, MnOOH, and titania nanosheets.^14,15 The second group consists of metal sulfides, namely CuS and Fe₃S₄.¹⁶ The third group covers precious metal nanoparticles, such as gold (Au NPs) and silver (Ag NPs).¹⁷ The fourth group includes carbon-based nanomaterials, e.g., fullerene, graphene, and carbon nanotubes.^18–21 The fifth group is devoted to metal–organic frameworks^22–25 which are special with respect to the structure. Lastly, the sixth group is made up of composites combining two or more materials to achieve superior performance and functionalities.^26,27 These nanozymes illustrate the diverse catalytic paradigms and practical trade-offs across materials.

Among various nanozymes, metal sulfide nanozymes (e.g., CuS, Fe₃S₄, NiS, and CoS₂) have attracted considerable attention owing to their unique semiconductor characteristics, redox-active metal centers, and defect-rich surfaces that facilitate multi-electron redox reactions, radical generation, and electron transfer.^28–30 Compared with many reported metal oxides, metal sulfides often exhibit more flexible redox behavior and greater catalytic tunability; compared with noble metals, they are generally more cost-effective and easier to tailor structurally; and compared with carbon-based materials, they more readily provide intrinsic catalytic centers rather than relying primarily on surface functionalization. These features make metal sulfides especially attractive for signal transduction in colorimetric, electrochemical, and fluorometric sensing platforms.³¹ Additionally, sulfur's lower electronegativity allows greater flexibility in electron density, leading to effective binding with substrates and a lower energy barrier for the catalyzed reaction. Metal sulfides withstand varying pH and temperature conditions and maintain high catalytic efficiency under complex biological conditions, enhancing their versatility and durability. In many natural enzymes, such as hydrogenases and nitrogenases, iron–sulfur clusters serve as cofactors. Therefore, metal sulfides are structurally and chemically well-suited to serve as enzyme mimics, compatible with electron transfer in biological systems.

Metal sulfide nanozymes, owing to their synthetic versatility and operational practicality, are a highly attractive class of nanozymes. These may be prepared via hydrothermal, solvothermal, thermal decomposition, and green-synthesis methods, with high control over morphology, phase composition, and defect density.^32,33 These nanozymes can be used in natural sample matrices in which their natural counterparts frequently fail, can be tuned in composition and decorated with noble metals, and their thermal and mechanical properties can be tuned to provide stability at high temperatures and over a broad pH range to enhance selectivity and turnover.^34,35 The systematic design of nanozyme sensors toward different targets has been enabled by the engineering knobs.^36,37

In recent reports, metal sulfide nanozymes have been shown to be excellent universal nanozymes for detecting various analytes, including metal ions, small biomolecules, pesticides, synthetic dyes, and phenolic compounds, using colorimetric, fluorometric, and electrochemical sensing methods.^38,39 Rational hybridization methodologies, including layered nanosheet architectures, heterostructure design, and noble-metal incorporation, broaden the substrate range and enhance catalytic efficiency, enabling rapid, cost-effective assays with excellent stability and ultralow detection limits. For instance, Liu et al.⁴⁰ prepared a protein-templated, sponge-like casein-CuS nanocomposite through a one-step method by employing amphiphilic casein as a template, showing excellent peroxidase-like activity for glucose sensing in a cascade in conjunction with glucose oxidase. The sensor had a broad linear dynamic range from 0.083 to 750 µM, with a low limit of detection of 5 nM. Wang et al.⁴¹ synthesized a NiCoFeS/rGO composite associated with high peroxidase activity, which was a very sensitive and rapid colorimetric sensor for the detection of carbofuran. In addition, Zhang et al.⁴² prepared CuS nanoclusters with versatile enzyme-like activities, including peroxidase, oxidase, catalase, superoxide dismutase, and ascorbic acid oxidase for acid phosphatase sensing. The resulting fluorescent biosensor platform achieved a linear detection range of 0.05–25 U L⁻¹ with a detection limit as low as 0.01 U L⁻¹, demonstrating the potential of multienzymatic metal sulfide nanozymes for advanced bioanalytical applications.⁴²

Despite this growing interest, the literature on metal sulfide nanozymes remains fragmented. Most reviews discuss nanozymes broadly, or emphasize only one subclass such as metal oxides, noble metals, or carbon-based materials, without critically examining why metal sulfides are particularly suitable for sensing applications. In addition, existing summaries often catalog individual examples rather than analyzing how structural parameters such as composition, crystal phase, heterostructure construction, defect engineering, and noble-metal coupling influence catalytic performance and analytical reliability. A second limitation is that many reports focus on assay performance alone, while providing limited discussion of practical issues such as selectivity in complex matrices, reproducibility, activity standardization, reusability, and environmental stability.

This review specifically focuses on metal sulfide nanozymes as a distinct and rapidly evolving catalytic platform for sensing and biosensing. It aims to critically evaluate their preparation strategies, structure–activity relationships, and catalytic mechanisms, with emphasis on how material design governs analytical performance. In contrast to prior general nanozyme reviews, this article highlights the comparative advantages and limitations of metal sulfides relative to other nanozyme classes, identifies remaining mechanistic and translational gaps, and discusses how rational engineering can improve sensitivity, selectivity, and deployability. By synthesizing recent progress and unresolved challenges, this review seeks to provide a clearer framework for designing next-generation metal sulfide nanozyme sensors for laboratory and field applications.

2. Synthesis of metal sulfide nanozymes

Metal sulfide nanoparticles are synthesized through various methods (Fig. 1). Tailored to achieve controlled size, morphology, and functionality. Chemical methods typically rely on the breakdown of precursors to yield monodisperse, crystalline nanoparticles with adjustable optical properties, such as CdS, which displays excitonic fluorescence.⁴³ Green chemistry routes are becoming popular, among them a formamide-mediated route assembling elemental sulfur to form CdS, CuS, ZnS, etc, provides an eco-friendly method with a high rate of production.^44,45 Bacterial-mediated synthesis of nanoparticles is an alternative method in the field of green synthesis that avoids the use of toxic chemicals and provides a sustainable solution, which makes it suitable for application in producing low-cost nanoparticles such as CdS and ZnS.⁴⁶ These synthetic methods and the enzyme-mimetic activity of certain metal sulfides are summarized in Table 1. Sophisticated methods, for example, pulsed laser ablation in liquids, allow a one-pot production of CdS, CuS, and SnS nanostructures for use in the field of optics and catalysis.⁴⁷ Reaction temperature, precursor concentration, and sulfur source also affect nanoparticle properties and can be tailored for different applications such as energy storage, solar cells, and optoelectronics. Core/shell structures (e.g. ZnS/CdS)⁴⁸ and doped variants (e.g., Co-doped ZnS)⁴⁹ further enhance quantum efficiency and magnetic properties.

Fig. 1. Method for the synthesis of various metal sulfide nanozymes.

Table 1. Metal sulfide nanocomposites with varying synthesis methods and their enzyme mimetic activity.

Composite	Synthesis method	Enzyme mimic	References
Cu2−xS nanoplates and nanospheres	Solvothermal method	Peroxidase	50
NiCo2S4	Template regulated	Peroxidase	51
FeS2/Fe3S4	Solvothermal method	Peroxidase	52
FeS2-HPC	Hydrothermal method	Peroxidase and oxidase	53
NiS nanocubes	Template regulated (MOF-based)	Peroxidase	54
FeS2 functionalized porous carbon	Hydrothermal method	Peroxidase	55
MoCuSx nanozyme	Hydrothermal method	Peroxidase	56
Wool-ball-like copper sulfide	Template regulated	Oxidase and peroxidase	57
L-NiMoS2	Pulsed laser ablation	Haloperoxidase	58
FCS-FA	Template regulated	Peroxidase	59
NiSx/MoS2 NSs	Template regulated	Peroxidase	60
CoS nanospheres	Hydrothermal method	Peroxidase	61
CuS	Green synthesis	Catalase, peroxidase, superoxide dismutase, ascorbic acid oxidase	62
PBA:MoSx	Etching and deposition	Peroxidase and catalase	63
NiS nanocubes	Hydrothermal method	Peroxidase	64
CuCo2S4	Hydrothermal method	Laccase	65

2.1. Hydrothermal synthesis

The hydrothermal technique is the most widely used technique, in which temperature and pressure play vital roles. The precursors in an aqueous medium are sealed in a Teflon-lined autoclave at a specified temperature for a specified time to obtain homogeneous nanoparticles. For instance, Song et al.⁶⁶ synthesized Fe/MoS₂ nanozymes for efficient wound healing application using the hydrothermal approach. In their protocol, ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O), thiosemicarbazide, ferric chloride, and polyvinylpyrrolidone were dissolved in water and heated at 200 °C for 12 hours in a Teflon-lined autoclave. The resulting Fe/MoS₂ nanozymes exhibited a uniform morphology, high crystallinity, and robust enzyme-mimetic activities, making them promising candidates for biomedical applications, including wound healing. In another work, K. Wu et al.⁶⁷ prepared a cobalt–nickel bimetallic sulfide nanozyme on montmorillonite via the hydrothermal method. The process involved dissolving montmorillonite in ethylene glycol, separately adding cobalt and nickel solutions, and subsequently adding thiourea. The above mixture was loaded into an autoclave and heated at 180 °C for 12 h. This nanozyme exhibited high 3,3′,5,5′-tetramethylbenzidine (TMB) affinity (V_max = 68.31 × 10⁻⁸ M s⁻¹) and was thereby used for hydrogen peroxide detection.

2.2. Co-precipitation method

It is a mild chemical method for synthesizing metal sulfide nanozymes. In this method, supersaturation is created by mixing a metal salt and sulfur precursors, leading to the formation of small, stable metal sulfide nanoparticles. The procedure has been praised for its simplicity, high yield, scalability, and short reaction duration. An additional advantage is that a high-pressure reaction is not necessary, which renders the process safer and more convenient.^68,69 However, a major challenge is maintaining homogeneity in the final product composition, which could lead to variations in properties. This method is also sensitive to particle agglomeration and poor crystallinity of the final products. Moreover, the reaction lifetime can be long, the yields may be lower for some substances, and the hydrophobic surfaces of the obtained nanomaterials may promote their sedimentation.⁷⁰

2.3. Hot-injection method

The hot injection method is an effective route to synthesize metal sulfide nanozymes with size and morphology control.⁷¹ In this process, a cold metal–sulfur precursor solution is rapidly injected into a hot solution, typically a solvent and a surfactant, thereby inducing a burst of nucleation and particle growth. This method enables the fabrication of high-quality nanozymes with excellent crystallinity and optical properties for a range of applications. This approach can derive high-quality nanozymes with good crystallinity and optical properties for several applications.^72,73 For example, Zhang et al.⁷⁴ synthesized monodisperse CdS nanocrystals using the hot injection technique. In this procedure, cadmium stearate was used as the cadmium precursor and thioacetamide as the source of sulfur, while dodecylamine served as the hot solvent. The precursors were injected rapidly into the hot solution to facilitate nanocrystal growth.

2.4. Microwave-assisted synthesis

Microwave-based methods have several advantages over traditional heating methods, including significantly shorter reaction times, narrower particle-size distributions, and higher product purity.^75,76 However, they do not necessarily alleviate the aggregation effect. High-temperature chemistry synthesis, although efficient for some materials, is highly sensitive to small variations in conditions and thus yields few reproducible results. A broad range of metal sulfide nanoparticles, including copper, zinc, cadmium, manganese, and lead sulfides, can be obtained via the thermal decomposition of metal oleates, which provides a general synthetic approach for producing various metal sulfide nanoparticles.

2.5. Green synthesis approach

The green synthesis approach utilizes various eco-friendly routes to synthesize metal sulfides. For instance, sonochemical synthesis uses ultrasound to induce chemical reactions, a convenient and reproducible method for nanoparticle synthesis.⁷⁷ Ball milling is also the simplest method for synthesizing metal sulfide nanoparticles via mechanical grinding alone, without the need for any additional chemical or thermal treatment. It has been effectively employed to synthesize ZnS quantum dots and hexagonal CuS nanoparticles.

Green synthesis is also a bio-friendly technique that uses biological or natural entities (e.g., plant extracts, microbes, DNA, or NADH) as natural reducing/capping agents in nanoparticle formation. In plant-based synthesis, phytochemicals present in plant extract (like flavonoids, terpenoids, and polyphenols) are known to act as reducing as well as capping agents, which not only support the nucleation and growth of nanoparticles but also influence the size and shape of nanoparticles.⁷⁸ Biomineralization is a biosynthetic route in which active microorganisms come into contact with target metal ions. The ions are transported into the microbial cell, where nanoparticle formation is catalyzed by specific enzymes. Examples include magnetotactic bacteria for the biomineralization of Fe₃S₄ and FeS₂ nanoparticles, and sulfate-reducing bacteria for producing Fe_1−xS.⁷⁹ Green synthesis approaches are valued for their economic feasibility and environmental friendliness. The obtained nanomaterials exhibit excellent properties and contain no toxic materials. The phytochemical remnants on the surfaces of nanoparticles generated using plant extracts endowed the surfaces with functionalities that could enhance hydrophilicity, pollutant adsorption, and photocatalytic activity. These methods also generally yield less aggregation and improved nanoparticle dispersion. The ability to modulate the bandgap, thus allowing its activation by visible light, is also crucial for good photocatalysis, and this is the key feature of the above-mentioned materials. It has been reported that biosynthesized nanoparticles exhibit higher catalytic reactivity and surface area than chemically synthesized nanoparticles. Although promising, green synthesis approaches still face challenges in achieving fine control over nanoparticle morphology and composition, and their scalability may be hindered for mass production. In addition, a thorough risk evaluation is required to assess potential environmental and human health impacts associated with the manufacture, application, and disposal, as they may pose unknown risks. For instance, Mirali et al.⁸⁰ synthesized FeS₂ nanoparticles using Acanthophyllum laxiusculum root extracts as a natural surfactant and sodium thiosulfate as the sulfur source at 200 °C. The as-prepared nanoparticles showed intrinsic peroxidase-like activity, and their best activity was observed at 25 °C and pH 4. Kinetic studies showed a K_m of 0.041 mM and a maximum reaction velocity (V_max) of 11.38 × 10⁻⁸ M s⁻¹, demonstrating their strong potential as enzyme mimics.

A major challenge in green nanozyme synthesis is to balance the need for the precise tailoring of nanozyme properties with the principles of green chemistry. However, control over size and shape is substantially lower for these methods than for hydrothermal, solvothermal, and co-precipitation methods, as products are typically less crystalline, less monodisperse, and less shape-controlled. This underscores a key challenge for the future: it would be beneficial to develop novel green synthetic strategies that provide at least comparable, if not superior, control over the exact structural, morphological, or compositional attributes of metal sulfide nanozymes, comparable to that afforded by the synthetic methodologies.

2.6. Template-assisted synthesis

It is a relatively sophisticated method and has the advantage of using previously known building blocks as templates to direct the formation of the target material, thereby shaping the final products with defined shape, size, and orientation of the generated nanoparticles.⁸¹ Templates may be “hard” (e.g., mesoporous silica, polymers) or “soft” (e.g., surfactants, micelles, polymers). Hard templating is favored for the preparation of rigid mesostructures with high crystallinity, whereas soft templating is suitable for the multi-morphological and multi-physicochemical modification. Soft templating usually yields lower costs and shorter reaction times, but the resulting mesostructure may be extremely difficult to control. The selection of the chemical precursors is one of the main aspects in the synthesis of metal sulfide nanoparticles, with a direct influence on the final properties of the particles. Proficient knowledge of precursor chemistry, such as their decomposition processes and interactions with various solvents, is necessary in order to predict rational precursor-solvent combinations.⁸² This tailored approach can ensure accurate tuning of the final nanozyme features, regardless of the specific synthesis route employed. This tendency underscores a clear need for greater systematicity and holism in understanding how specific precursor combinations within a given technique behave, how they may be influenced by certain synthesis parameters, and how this behavior varies across techniques. Techniques such as response surface methodology (RSM), which have been demonstrated to be effective for optimizing metal sulfide nanozyme synthesis, will be useful in this context. This emphasis on precursor-method synergy is crucial for enabling truly rational design and fine-tuning of metal sulfide nanozyme structures for practical catalytic uses, as a break from the conventional trial-and-error empirical approach, and prediction-accelerated discovery.^81,83

3. Impact of synthesis processes on catalytic properties of metal sulfides

The inherent physical and chemical properties of metal sulfide nanozymes are strongly influenced by the method of synthesis, which in turn defines their catalytic activity, as summarized in Tables 2 and 3.

Table 2. Synthesis routes of metal sulfide nanozymes and their effects on the morphology, purity, catalytic activity, and reproducibility.

Method of synthesis	Typical conditions	Key advantages	Key disadvantages	Control over size/morphology	Control over crystallinity/purity	Scalability/reproducibility	Suitability for nanozymes
Hydrothermal synthesis	High temp./pressure, aqueous medium	High crystallinity, good dispersibility, low cost, environmentally friendly	Impurities (e.g., iron oxide), multiphase products, safety risks	Good	Good	Moderate	Excellent
Solvothermal synthesis	High temp./pressure, organic solvent	Broad applicability, precise morphological control, inherits hydrothermal advantages	High cost (organic solvents), toxicity/safety hazards	Excellent	Good	Moderate	Excellent
Co-precipitation method	Room Temp, aqueous medium	Simple operation, high yield, short reaction time, no high-pressure environment	Poor homogeneity, prone to agglomeration, low crystallinity, slow reaction rate for some, hydrophobic products	Moderate	Low	Good	Moderate
Green synthesis (plant-mediated/biomineralization)	Biological/plant extracts, mild conditions	Eco-friendly, cost-effective, enhanced surface functionalities, reduced aggregation, bandgap modulation	Challenges in precise control over morphology/composition, scalability limitations, and potential environmental risks	Variable	Variable	Challenging	Moderate (good for biological activity)
Template-assisted synthesis	Guided by template (hard/soft)	Precise morphological control, high crystallinity (hard templates), tunable properties	Multi-step process, template removal challenges, cost (hard templates), sensitive to conditions (soft templates)	High	High (hard), variable (soft)	Variable	Excellent
Microwave-assisted synthesis	Microwave irradiation	Reduced reaction time, smaller particle size distribution, higher purity	Aggregation may not be improved	Good	Good	Good	Good

Table 3. Influence of key synthesis parameters on metal sulfide nanozyme's properties and catalytic activity.

Synthesis parameter	Affected property	Observed effect	Impact on catalytic activity
Temperature	Particle size, crystallinity, reaction rate	Higher temperatures can reduce reaction time, influence particle growth, and phase formation	Affects active site formation and electron transfer efficiency
pH	Catalytic activity, surface charge, stability	Regulates H2O2 decomposition pathways, influences substrate binding	pH-dependent peroxidase/catalase mimicry. CdS activity increases at alkaline pH
Precursor concentration	Particle size, morphology, purity	Influences nucleation and growth kinetics, affects crystal structures	Controls specific surface area, active site density
Solvent type	Morphology, dispersibility, reaction environment	Organic solvents in solvothermal methods broaden applicability; aqueous in hydrothermal	Affects particle aggregation, stability, and reactivity
Capping agent	Particle size, aggregation, surface functionalization	Prevents excessive growth, introduces surface groups	Improves dispersibility, enhances pollutant adsorption, influences biocompatibility
Reaction time	Particle size, crystallinity, yield	Longer times can lead to larger particles or higher crystallinity	Affects active site development and overall catalytic efficiency
Milling speed (ball milling)	Particle size, crystallinity	Higher speeds reduce time, can yield smaller particles	Affects surface area and defect formation for activity
Doping concentration	Band gap, defect density, electronic structure	Introduces specific defects, alters electronic properties	Enhances charge separation, extends light absorption, boosts activity

3.1. Control over size and morphology

Particle size is particularly important for catalytic activity. Small particles usually possess excellent catalytic efficiency due to their large surface-to-volume ratio, which can offer more active sites exposed to the reactants.^84,85 However, a report on FeS₂ nanoparticles showed a straight relationship where the peroxidase activity was inversely proportional to the nanoparticle size.⁵² Similarly, the shape of nanozymes is an important factor in their catalytic activity. Different morphologies, such as nanosheets, nanofibers, nanotubes, nanorods, and core–shell nanoparticles, exhibit distinct catalytic activities. For example, Zhou et al. synthesized metastable copper sulfide (Cu_2−xS) nanozymes with varying sizes and compositions to tune the electronic structure and enhance enzyme-like properties. Among the prepared morphologies, the large and thin nanoplates with a stoichiometric ratio of Cu₁.₂₅S exhibited the best peroxidase-like activity, which was much higher than other nanoplate types and nanospheres due to the large difference in specific surface area, shown in Fig. 2. The studies illustrated that sulfur vacancies induced the electronic configuration of copper active sites, facilitating the cleavage of O–O bond in H₂O₂ to generate ˙OH radicals and significantly enhance peroxidase-mimetic performance.⁸⁶ Specific synthesis methods offer precise control over these parameters.

Fig. 2. Schematic illustration of CuS with different morphology. Adapted with permission from ref. 87. Copyright 2021, American Chemical Society.

Although an increase in specific surface area due to smaller particle sizes is frequently cited as a key factor for the superior catalytic activity, it is far from the only factor. The nature, availability, and activity of the catalytic sites are also important. This is clearly demonstrated by both the strong dependence on morphology and precise crystallinity and by the controlled introduction of defects. The result is that vacancy defects in fact dramatically enhance enzyme catalysis, and that exposure of certain catalytically active metal atoms or crystal planes indicates that gross maximization of overall surface area is not enough. The active-site design, not the number of active sites, is critical. Hence, future synthesis strategies for metal sulfide nanozymes should go beyond merely reducing particle size. The primary emphasis should be on “active site design,” including atomically precise control of crystal facets, the strategic introduction of specific defects (e.g., vacancies), and surface functionality at the atomic or molecular scale. This approach is expected to leverage the intrinsic activity of each catalytic site in a unique manner, potentially yielding more efficient or selective nanozymes.

3.2. Active site exposure and surface chemistry

The high surface-area-to-volume ratio of nanozymes not only enhances catalytic activity but also facilitates subsequent surface modification and bioconjugation, enabling functionalization to achieve tailored interactions with substrates and biological entities. The catalytic activity is closely related to the number of catalytically active metal atoms or specific crystal planes exposed on the NP surface. This indicates that control over crystal facet growth during synthesis is important. Surface modification, especially with phytochemicals in green synthesis, could greatly enhance hydrophilicity and pollutant adsorption for superior photocatalytic activity. Conjugation with biocompatible entities such as polyethylene glycol, silica, or dextran is necessary to improve the biocompatibility and to tailor the properties for specific use, such as in medicine. Nevertheless, it is important to highlight that some capping agents, such as hexadecyltrimethylammonium bromide, can have the dual effect of preventing nanoparticle growth and promoting the formation of undesired aggregates. The crystalline structure of iron sulfide-based nanozymes significantly affects their catalytic activities; e.g., the peroxidase-like activity of cubic FeS₂ is better than that of a tetragonal CuFeS₂ system.⁸⁸

Responsiveness of nanozymes to certain environmental signals is not an incidental feature but a treatable design variable (Fig. 3). The activity of nanozymes can be influenced by the concentrations, pH, glutathione, and H₂O₂, as well as the degree of oxygenation, and nanozymes have environmental specificity. For instance, the photocatalytic activity of CdS is drastically enhanced in alkaline pH,⁸⁹ and FeS@SRB biohybrids are rationaSlly designed for tum or-targeted therapy, indicating a customized response to the special tumor microenvironment.⁹⁰ This means that the synthesis could be designed to impart “intelligence” into the nanozyme. The aim in the synthesis of nanozymes should change from simply making catalysts with the highest possible activities under the most ideal laboratory conditions.^91,92 Instead, the attention should be focused on making the “smart” or “adaptive” nanozymes whose catalytic activity can be modulated, triggered, or controlled with high precision by certain physiological or environmental signals.^93,94 This is especially true for advanced biomedical applications such as targeted drug delivery and theranostics that require high spatiotemporal activity with minimal off-target effects, for which localized and on-demand activity is essential. In this way, the design is shifted toward the construction of dynamic, context-dependent nanozyme systems.⁹²

Fig. 3. Factors affecting the catalytic activity of metal sulfide nanozymes.

4. Sensing mechanism of metal sulfide-based nanozymes

The metal sulphide nanozymes imitate natural enzymatic pathways via electron-transfer catalysis (Fig. 4). These typically involve complex redox cycling among the metal sulphide lattice, the substrate (H₂O₂), and a reporter molecule.⁵⁵ Locating this activity precisely is critical. Computational modelling and experimental methods, such as kinetic isotope effects and electron spin resonance, are used to determine the active site and mechanism of action, i.e., whether it is sulfur vacancies or metal edges, responsible for the catalytic mechanism.

Fig. 4. Schematic representation of different enzyme-mimicking activities by metal sulfide nanozymes.

Among the nanozymes, peroxidase-like metal sulfide nanoparticles can be grouped into heme and non-heme types. They are oxidoreductases and catalyze the oxidation of a wide range of organic and inorganic compounds in the presence of H₂O₂. They work through surface phenomena, and exposed metal centers are responsible for the activation of H₂O₂ and the formation of reactive hydroxyl radicals (˙OH), which then oxidize chromogenic substrates (i.e., TMB) to a blue-colored oxidized form (oxTMB), which can be read at 652 nm.¹⁹ In Fe²⁺-based iron sulfide nanozymes such as FeS₂ and Fe₃S₄, Fe²⁺ ions interact directly with H₂O₂ by frontier orbital overlapping, with FeS₂ presenting more effective inner-orbital complexation as compared to Fe₃S₄, accompanied by greater binding energies and a lower energy barrier of activation. Sulfur-based ligands make Fe²⁺/Fe³⁺ redox cycling more facile and thus promote Fenton-like ˙OH generation (Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH).⁸⁸ Similar redox cycling by Cu²⁺ sites in cuprous oxide nanozymes and bimetallic cobalt–nickel sulfides (Co–Ni–S), which synergistically enhance catalytic activity and affinity toward substrate using Michaelis–Menten kinetics, is reported. The resulting ˙OH radicals oxidize TMB to the well-known blue oxidation product (oxTMB) with its maximum absorption at 652 nm, prompting those metal sulfide nanozymes as a promising choice for colorimetric biosensing.⁹⁵ Metal sulfide nanoparticles also display an enzyme-like activity to superoxide dismutase (SOD) via their production of superoxide radicals (O₂˙⁻) into H₂O₂ and O₂ through the reversible redox cycling activity of the metal centers. The metal in its oxidized state takes an electron from the superoxide radical, which forms O₂, and the metal is reduced; however, the reduced metal can react with another O₂˙⁻ and be reoxidized and also release H₂O₂.⁹⁶ Fan et al. demonstrated that Fe–Mn–S nanozymes synthesized via coordination precipitation effectively catalyze superoxide decomposition, owing to abundant catalytic sites and enhanced electron transfer.⁹⁷ Additionally, the catalase-like activity of metal sulfide nanozymes facilitates the conversion of H₂O₂ to water and oxygen. Their nanostructured surface and high specific surface area provide numerous active sites that rapidly transfer electrons and efficiently scavenge reactive oxygen species, similar to natural catalase enzymes.⁹⁸ Altogether, the unique multi-enzymatic mimicry performance of metal sulfide nanoparticles (oxidase, peroxidase, SOD, and catalase-like activities) originates from their engineerable surface chemistry, defect engineering, and metal-mediated redox cycles, making them promising and robust nanozyme candidates for catalytic, sensing, and biosensing applications.

Metal sulfide-based nanozymes serve as robust electrochemical biosensing platforms by combining heterogeneous enzyme-mimicking catalysis with efficient electron transfer at electrode interfaces.^99,100 These materials (e.g., CuS, MoS₂, WS₂, FeS) typically exhibit peroxidase- or oxidase-like activity, enabling them to catalyze the reduction of H₂O₂ or O₂via surface metal–sulfur redox couples, while the regenerated metal centers continuously shuttle electrons from the electrode to generate a measurable current.^101,102 The high surface area, abundant edge-site metal atoms, and tunable electronic structure of layered sulfides enhance both catalytic turnover and conductivity, allowing the nanozyme to act as an immobilized signal amplifier directly coupled to a biorecognition element such as an antibody, aptamer, or enzyme; changes in analyte concentration therefore modulate the nanozyme's accessibility to substrate or its inherent activity, translating into a proportional electrochemical response.¹⁰³ This architecture yields stable, sensitive, and often label-free biosensors suitable for detecting disease biomarkers, environmental pollutants, or small-molecule metabolites in complex matrices.¹⁰⁴

5. Application of metal sulfide nanozymes in sensing

Metal sulfides are an emerging class of nanomaterials that have recently transformed sensing applications in many fields. The sensing applications of metal sulfide nanozymes, such as cadmium sulfide (CdS), lead sulfide (PbS), and mercury sulfide (HgS), are attracting global attention. These materials have been shown to be highly flexible and sensitive across a range of analytes, including biomolecules, environmental pollutants, metal ions, and diagnostic biomarkers, via various sensing mechanisms.^105,106

5.1. Heavy metal ion sensing

Metal sulfide nanozymes possess strong electronic and optoelectronic properties and are thus promising candidates for sensing applications. The metal sulfides are also capable of binding with different metal ions such as Fe²⁺, Cr³⁺, Sn²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Cu²⁺, Na⁺, K⁺etc., which affect their optical or electrical properties in an observable manner. This interaction can be used to detect these harmful ions and quantify their concentration. The various sensing strategies involving metal sulfides include fluorescence, electrochemical, and colorimetric methods. These methods benefit from the high sensitivity and selectivity of metal sulfides toward specific ions. Metal sulfide-based sensors offer advantages such as low detection limits, rapid response times, and the ability to detect multiple ions simultaneously. Their nanoscale morphology enhances these properties by increasing surface area. Due to all these advantages, there has been a significant growth in interest in the use of metal sulfides in the detection of heavy metal ions.

There are various strategies for detecting metal ions using nanozymes. Metal ions such as Cr⁶⁺ and Fe²⁺ can catalyze various reactions due to their redox properties and multivalent nature. Therefore, such metal ions can act as co-reactants, activating the nanozyme to catalyze reactions that lead to color development, such as TMB oxidation for the sensing mechanism.¹⁰⁷ The modulation of the enzymatic activity of metal sulfide nanomaterials by specific metal ions may be used to develop a selective sensor for those ions. The metal ion analyte may enhance or inhibit the catalytic activity of nanozymes, resulting in a color change. Fang et al. developed a simple and effective mercury-ion nanosensor using a CuS nanozyme.¹⁰⁸ The sensing principle is based on the fact that CuS is more soluble than HgS and that mercury ions can displace copper ions on the surface of CuS nanozymes by forming HgS, leading to reduced peroxidase-like activity of CuS nanozymes. The inhibitory effect of Hg ions results in a reduced rate of TMB oxidation and hence less/no color development (Fig. 5A). This nanosensor was effective for real-sample analysis with high accuracy and reproducibility, with a recovery of 82%. In a similar approach, Das et al. developed CuS- and NiS-modified porous reduced graphene oxide as nanozymes to catalyze the oxidation of TMB to blue-colored ox-TMB in the presence of hydrogen peroxide.¹⁰⁹ The presence of Hg²⁺ ions had an inhibitory effect on the catalytic activity of this nanozyme due to the binding of Hg²⁺ ions with sulfur present on the surface of nanozymes, thus reducing its catalytic activity and the oxidation of TMB, showing the presence of mercury ions (Fig. 5B). Their research group also used CoS-modified porous reduced graphene oxide nanozyme for Hg²⁺ ion detection based on the TMB oxidation inhibitory approach of mercury ion.¹¹⁰ The mercury detection limits were 13.05, 48.73, and 14.23 nM using CuS/p-rGO, NiS/p-rGO, and CoS/p-rGO nanozymes, respectively, indicating high catalytic efficiency of CuS in comparison to other metal sulfides. Yan et al. reported metal ion-activated nanozyme activity for detecting metal ions by oxidizing TMB to produce a color for naked-eye detection. They developed Ag₂S nanoparticles modified graphene oxide (Ag₂S@GO), which acted as Hg²⁺-activated oxidase and catalyzed the oxidation of TMB, resulting in color change from colorless to blue in the presence of mercury ion.¹¹¹ Liu et al. also utilized the Hg²⁺-activated peroxide-like activity of a 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin-modified ZnS/CoS nanozyme for mercury ion detection, triggering a colorimetric signal via TMB oxidation.¹¹² The enhanced peroxidase activity is due to the generation of metal-cation vacancies on the nanocomposite surface by mercury ions.

Fig. 5. (A) Schematic illustration of Hg²⁺ detection principle. Adapted with permission from ref. 108. Copyright 2020, American Chemical Society. (B) Mechanism of peroxidase mimic activity of Transition-Metal Sulfide (TMS)/porous reduced graphene oxide (p-rGO) nanocomposites in the presence and absence of Hg(ii) ions, adapted with permission from ref. 109. Copyright 2021, American Chemical Society.

Nanozymes-based turn-off and turn-on sensors for selective metal ions have also been developed. Target metal ions can strongly bind to the active sites of the nanozyme and thus can block the nanozyme's catalytic affinity for hydrogen peroxide, which inhibits the oxidation of TMB, resulting in no color. Some target metal ions may act as co-catalysts or stabilizers after binding to the active site on the nanozyme's surface, thereby enhancing the enzyme-like reaction and intensifying the color. Recently, Mu et al.¹¹³ reported the synthesis of a novel urchin-like Co₉S₈ nanomaterial for the detection of Cu²⁺ ions. Owing to its unique morphology, the material exhibited enhanced electron-transfer capacity, enabling it to function as an efficient peroxidase mimic. Based on this property, the authors developed an off–on sensing platform for Cu²⁺ detection that demonstrated a reliable linear response over the range 0.5–10 µM and an ultralow detection limit of 0.09 µM. In this sensing approach, the oxidized form of TMB, a blue-colored TMB cation radical species, is generated by hydrogen peroxide-mediated oxidation of TMB by the Co₉S₈ nanomaterial catalyst (Fig. 6). The addition of cysteine (Cys) causes restoration of the radical through the thiol group, causing a decrease in the oxidized form of TMB (off state). When copper ions are added, rapid restoration of the absorption signal of oxidized TMB is observed as the Cys thiol group binds to copper ions, preventing the thiol group from interacting with the oxidized radical species of TMB (on state). In another study, Swaiden et al. devised a facile approach to synthesize Cus-BSA nanocomposite, which serves as a peroxidase mimic for the colorimetric detection of Cr(vi) ions in water with a detection limit of 50 nM.¹¹⁴ Although metal sulfide nanozymes are excellent candidates for various sensing applications, further enhancing their sensing operational variability has been pursued through functionalization strategies, including the addition of dopants, the creation of heterojunctions, and the formation of nanocomposites. The list of metal sulfide nanozymes used for various metal ions detection is presented in Table 4, along with the limit of detection (LOD) and the sensing mode.

Fig. 6. (a) Representation of sensing of Cu²⁺ using Co₉S₈ catalyzed oxidation reaction of TMB, (b and c) the reactions indicating the on and off state of the sensing process. Adapted with permission from ref. 113. Copyright 2018, Elsevier.

Table 4. List of various metal ions detected using metal sulfide-based sensors, along with the limit of detection (LOD) and mode of sensing.

Sensing of metal ion	Metal sulphide nanozymes	LOD	Mode of sensing	References
Ag+	CdS-rGO sheet	12.35 µM	Fluorescence	115
Cr3+	CdS NPs	16 nM	Fluorescence	116
	CdS polyindole	470 nM	Fluorescence	117
Cr4+	Fe3O4/MoS2/GCE	0.5 µM	Amperometry	118
Cd2+	MoS2 NSs	72 nM	Optical	119
	Ag2S QD	546 nM	Fluorescence	120
	ZnS-schiff base	64.56 µM	Fluorescence	121
Fe2+	ZnS-schiff base	1024 µM	Fluorescence	121
	CdS polyindole	240 nM	Fluorescence	117
Sn2+	CdS polyindole	280 nM	Fluorescence	117
Co2+	Au/Chitosan-GO/CdS QDs	0.2 µM	Surface Plasmon Resonance (SPR)	122
Cu2+	CdS/Au/GQDs	2.27 nM	Photo-electrochemical	123 and 124
	G-CdS-G@ITO	2.0 nM	Photo-electrochemical	125
	GO/CdS-C cloth	0.05 µM	Photo-electrochemical	126
Zn2+	Ag2S QD	760 nM	Fluorescence	120
Hg2+	Aptamer/Au/MoS2-MWCNT/GCE	0.05 nM	Electrochemical	127
	Sep/FeS2-MCPE	4.12 nM	Square wave anodic stripping voltammetry (SWASV)	128
Pb2+	MoS2GR/MCH/HP/Au Pt MCNTs/GCE	0.015 nM	Electrochemical	129

5.2. Pesticide sensing

Pesticides are a class of chemicals that are frequently used to reduce crop pests and boost agricultural yields. The widespread use of pesticides poses a significant risk to both aquatic ecosystems and human health. Additionally, the fact that pesticides are washed off and enter other water bodies, eventually entering the food chain, makes them a significant concern. Pesticide sensing relies on the catalytic activity of nanozymes, which leads to measurable redox outputs, such as color changes, electrical current, or voltage changes, in the presence of pesticides. Pesticides can inhibit or activate the nanozyme, causing a noticeable, measurable change that is linearly related to pesticide concentration. Metal sulfide nanozymes catalyze the oxidation of a substrate, such as TMB, to the blue-colored ox-TMB. This oxidation step is influenced by the presence of pesticides or their breakdown products, which interact with the active sites of nanozymes, leading to a color change. Han and his research group developed a Pt–VS2–MXene nanocomposite as a peroxidase-mimic nanozyme that can catalyze the oxidation of TMB to blue ox-TMB using hydrogen peroxide (Fig. 7).¹³⁰ This nanozyme works synergistically: Pt nanoparticles activate hydrogen peroxide, VS₂ nanosheets stimulate ROS generation, and MXene provides a conductive support for effective electron transport. Chlorpyrifos pesticide inhibits this catalytic step by consuming ROS and thereby blocking the oxidation of TMB.

Fig. 7. (a) A schematic illustration depicting the detection mechanism of H₂O₂. (b) A conceptual representation elucidating the detection principle of a Pt–VS₂–MXene-based sensor employed for the colorimetric quantification of CPS, demonstrating a linear correlation between the R/B ratio and CPS concentration. (c) Density functional theory (DFT) calculations of nanozymes. Adapted with permission from ref. 130. Copyright 2026. Elsevier.

A chemiluminescence biosensor for diazinon has also been developed by Abdolmohammad-Zadeh et al.¹³¹ using molybdenum disulfide modified zirconium-based metal–organic framework [MoS₂@MIP-202(Zr)], which acted like peroxidase in a NaHCO₃–H₂O₂ chemiluminescence system, and the inhibitory effect of diazinon on the activity of acetylcholinesterase helped in sensing diazinon. Chemiluminescence signal decreased on increasing the concentration of diazinon, and the limit of detection was observed to be 0.12 nM.

In another method, chromogenic substance o-phenylenediamine (OPD) was used for the detection of trichlorfon organophosphate pesticide using NiCoFeS/rGO as nanozyme.¹³² NiCoFeS/rGO nanozyme was fabricated using a simple one-pot hydrothermal method based on the finding that the sulfide-based mixed-metal spinel (NiCoFeS) supported on reduced graphene oxide (rGO) promotes electron transfer and lowers the activation barrier for H₂O₂ decomposition, generating reactive oxidizing species (principally hydroxyl radicals, ˙OH) that oxidize OPD to its yellow oxidized product (OPDox) (Fig. 8). rGO serves multiple roles as it increases electrical conductivity and charge mobility at the catalyst–solution interface, disperses and stabilizes NiCoFeS active sites to maximize accessible surface area, and may participate in electron shuttling that accelerates catalytic turnover. The net result is amplified OPDox formation in proportion to H₂O₂ availability. Leveraging its strong peroxidase-like activity, an increased OPDox signal directly correlates with OP levels, enabling highly sensitive colorimetric and electrochemical detection. Under adapted electrochemical conditions, the system achieved an ultralow limit of detection of 9.74 fg mL⁻¹ over a broad linear range (12.5 fg mL⁻¹ to 1.25 ng mL⁻¹), while also demonstrating excellent selectivity, stability, reproducibility, and practical applicability. Several metal sulfide-based sensors have been reported for detecting pesticide presence in water samples. The list of various pesticides detected using metal sulfide-based nanozymes, along with their mode of sensing and lower detection limit (LOD), is tabulated in Table 5.

Fig. 8. Sensing of organophosphate pesticides using NiCoFeS/rGO nanozyme. Adapted with permission from ref. 132. Copyright 2025, Elsevier.

Table 5. List of various pesticides detected using different metal sulfide-based nanozymes.

Sensing of pesticides	Metal sulfide nanozyme	LOD	Mode of sensing	References
Chlorpyrifos	Schiff base co-ordinated ZnS	—	—	121
Carbofuran	Gadolinium sulphide rod-embedded rGO	0.0128 µM	Electrochemical	133
Pyrimethanil	NiCo2S4/graphitized carbon nanofiber	20 nM	Electrochemical	134
Fenitrothion	MoS2–Au	2.2 ng mL−1	Electrochemical	135
Dicofol	CdS	55 ± 11 ppb	Fluorescence	136
Diethofencarb	MoS2	0.32 ng µL−1	Electrochemical	137
3-Phenoxybenzoic acid	Mn-ZnS QDs	0.117 µM	Fluorescence	138
Pentachlorophenol	GQDs-CdS	3 pg mL−1	Electrochemiluminescence	139
Phosphorothiolate	CdSe-ZnS QDs	96.7 pg mL−1	Fluorescence	140
Parathion (paraxon)	CdSe-ZnS QDs	1.0 nM	Photoluminescence	141
Glutathione	Graphene/Mn-CdS	0.01 µM	Photoelectrochemical	142
Pentachlorophenol	CdS/rGO	2000 pM	Electrochemiluminescence	143
Monocrotophos	AChE-CNTs NH2/AgNPs-N-F-MoS2/GCE	0.05 pg mL−1	Electrochemical	144

5.3. Sensing of biomolecules

Biomolecules such as dopamine, urea, and mycotoxins play significant roles in regulating various physiological functions. Minor variations in their concentration lead to imbalance and sickness. Bio-nano sensors can detect subtle variations in their concentration and can be used to diagnose various diseases. The sensing of biomolecules such as dopamine, Dengue Virus (DENV) E-protein, H₂O₂, 4-aminophenol, mycotoxins, carcinoembryonic antigen (immunoassay), cysteine, ascorbic acid, urea, prostate-specific antigen (PSA), cardiac troponin I, and glucose using metal sulfide-based nanozymes has been summarized in Table 6. Many researchers have developed different modified metal sulfides for the selective detection of biomolecules. For instance, Fu et al. synthesized iron sulfide functionalized with porous carbon having a large surface area for colorimetric and electrochemical detection of dopamine.⁵⁵ A petal-like MoS₂/FeS₂@C nanocomposite was prepared to serve as a peroxidase mimic system. In another method, Wang et al. fabricated CuS@CDs NCs via hydrothermal and co-precipitation, which exhibited enhanced peroxidase-like activity.¹⁴⁵ These specialized nanozymes served as efficient sensors for colorimetric and fluorescent detection of glutathione (GSH) and ferric ions (Fe³⁺) in systems where both targets coexist.¹⁴⁵ Impressively, the CuS@CDs nanocomposites (CuS@CDs NCs) achieved detection ranges of 5–300 µM for GSH and 5–100 µM for Fe³⁺, with extremely low limits of detection 0.0527 µM for GSH and 0.0163 µM for Fe³⁺. The detection of GSH is based on the peroxidase activity of CuS@CDs NCs, which catalyze the decomposition of hydrogen peroxide to generate a hydroxy radical, which further oxidizes TMB to a blue-colored oxTMB (Fig. 9). It was observed that its peroxidase activity was enhanced by using NIR radiation for 17 minutes. The presence of GSH reduces the blue oxTMB to TMB, leading to a decrease in color intensity and absorption peak corresponding to oxTMB. The iron ion can be detected using fluorescent quenching approach. The presence of Fe³⁺ quench the fluorescence of CuS@CDs NCs due to fluorescence resonance energy transfer, thereby providing an effective tool for iron sensing.

Table 6. List of various biomolecules/biomarkers detected using different metal sulfide-based nanozymes.

Sensing of biomolecule	Metal sulphide nanozyme	LOD	Mode of sensing	References
Dopamine	CdS/Au/GQDs	0.0078 mM	Photo-electrochemical	123 and 124
	PbS	—	Electrochemical	148
	Hollow CuS nanocubes	1.67 µM	Colorimetric	149
Glucose	CuO-G-CdS	0.5 µM	Photo-electrochemical	150
	CoOx-G-CdS	0.5 µM	Photo-electrochemical	151
	CuS-NWAs	17.5 nM	Electrochemical	152
	CuO-G@CdS	0.4 µM	Photo-electrochemical	153
	CuS/CoS	1.71 µM	Electrochemical	154
	CuS/MoS2	1.52 µM	Electrochemical	155
	Ag/MoS2@Co3S4	0.08 µM	Electrochemical	156
	Porous Co3S4	0.17 µM	Electrochemical	157
	CoOx-rGO/CdS	0.87 µM	Photo-electrocatalytic	158
	CuS/S-rGO	0.032 µM	Electrochemical	159
	Cu9S5/C/MoSx	0.12 µM	Electrochemical	160
	Ni-Ni3S2/NiMoO4	55 nM	Electrochemical	161
	Ni-MoS2/rGO	2.7 µM	Electrochemical	162
	NiCo2S4 microflowers	8.24 µM	Colorimetric	163
	H-Ni(OH)2@CuS	3.3 µM	Electrochemical	164
	Ni3S2@NCNT	0.14 µM	Electrochemical	165
	Ni7S6/NiO	0.3 µM	Electrochemical	166
	Ni3S2NWs/PEDOT-rGO HFs	0.48 µM	Electrochemical	167
	FeS2@CN	0.30 µM	Colorimetric	168
	NiO NS-MoS2	3.53 µM	Electrochemical	169
	PtS2 nanosheets	0.20 µM	Colorimetric	170
	PtS2@HA-DA microspheres	29.95 µM	Colorimetric
	V-CuS nanowires	0.13 µM	Electrochemical	171
	Fe3S4	0.1 µM	Colorimetric	172
	WS2 quantum dots	4.2 nM	Chemiluminescence	173
	NC-NiS@NS-NiS	8.3 nM	Electrochemical	174
	Co 3 S 4 /CuCo 2 O 4	2.1 µM	Electrochemical	175
	MoS2 nanosheets	1.2 µM	Colorimetric	176
	CuS NCs@Ni1Co2 LDHs	0.18 µM	Electrochemical	177
	MoS2–MoO3/Ni	0.2 µM	Electrochemical	178
	3D V2O5-MoS2/rGO nanocomposites	3.99 µM	Colorimetric	179
	Chitosan coated FeS2	1.9 µM	Colorimetric	180
	Fe1−xS	37 µM	Colorimetric	181
	PbS/SiO2/AuNPs	0.46 µM	Photoelectrochemical	182
	Fe-MoS2	9.8 µM	Colorimetric	183
	CuS@Cu2MoS4 nanocubes	3.1 µM	Electrochemical	184
	CuS/NSC	2.72 µM	Electrochemical	185
	CoS@Co-MOF	0.11 µM	Electrochemical	186
H2O2	NiCo2S4 microflowers	5.19 µM	Colorimetric	163
	FeS2@CN	0.15 µM	Colorimetric	168
	PtS2 nanosheets	0.33 µM	Colorimetric	170
	PtS2@HA-DA microspheres	1.64 µM	Colorimetric
	WS2 quantum dots	2.4 nM	Chemiluminescence	173
	MoS2 nanosheets	1.5 µM	Colorimetric	176
	3D V2O5-MoS2/rGO nanocomposites	12.40 µM	Colorimetric	179
	Porous 2D FeS2 nanosheet	7.60 nM	Colorimetric	187
	Zn2SnS4 nanoflower	1.79 nM	Electrochemical	188
	Fe0.8Ni0.2S2	0.71 µM	Colorimetric	189
	Fe0.8Ni0.2S2/ZIF-67	0.039 µM	Colorimetric
	NiCo2S4 (PVP) NPs	8.00 µM	Colorimetric	190
Prostate specific antigen (PSA)	Phthalocyanine-graphene-CdS	0.63 pg mL−1	Photo-electrochemical	191
Carcinoembryonic antigen	CdS: Mn/TiO2-AgNCs-G	1.0 pg mL−1	Photo-electrochemical	192
Cholesterol	MXene-Ti3C2-CuS nanocomposites	1.9 µM	Colorimetric	193
	Chox/MoS2-AuNPs/GCE	0.26 µM	Amperometry	194
Cysteine	rGO/Mn-CdS	0.02 µM	Photo-electrochemical	195
Choline	MoS2 nanosheets	0.4 µM	Colorimetric	196
Ascorbic acid	CdTe/CdS QDs	70 nM	Electrochemiluminescence	197
Urea	NiS/GO-GCE	3.79 µM	Electrochemical	198
Folic acid	MoS2-rGO/GCE	10 nM	Electrochemical	199
Lactate	GCE/MoS2/Lox	0.17 µM	Electrochemical	200
ATP	MSO/WS2/SH-DNA/Au electrode	1.5 nM	Electrochemical	201
Guanine	COFS/NH2-rG/MoS2/GCE	0.51 µM	Electrochemical	202
Adenine		0.44 µM
Parathyroid hormone	MoS2–Au–PI	—	Electrochemical	203
Triiodothyronine		—
Thyroxine		—
Vitamin B	Ce-SnS2-GCE	—	Electrochemical	204
Vitamin B2	WS2-MIPs/GO-SWCNTs/GCE	0.7 nM	Electrochemical	205
Serotonin	rGO-Ag2Se/GCE	29.6 nM	Electrochemical	206
Valproic acid	DNPs/MoS2/GCE	0.27 µM	Electrochemical	207
l-cysteine	MoS2/PDDA-MC/GCE	0.090 µM	Amperometry	208
Uric acid	MoS2 nanosheets	0.3 µM	Colorimetric	209
Cardiac troponin I	(NiS-GQDs)-CdS-Zn2SnO4	0.3 pg L−1	Photo-electrochemical	210
Glutathione	WS2 nanosheets	61 pM	Colorimetric	211
Dengue virus (DENV) E-protein	CdS-amino GO	0.001 nM	Surface plasmon resonance	212
Chikungunya virus DNA	MoS2 nanosheets	3.4 nM	—	213

Fig. 9. Colorimetric detection of GSH by photothermal-enhanced peroxidase-like activity of CuS@CDs NCs and fluorescent detection of Fe³⁺ by fluorescent quenching of CuS@CDs NCs. Adapted with permission from ref. 145. Copyright 2024, Elsevier.

Metal sulfide nanomaterials have also been developed as multifunctional enzyme-mimic systems for sensing applications. Hollow copper sulfide nanocubes (h-CuS NCs), synthesized via a template method, have been developed as multifunctional nanozymes for dopamine detection. These nanocubes exhibit peroxidase-like catalytic activity, enabling them to catalyze hydrogen peroxide (H₂O₂) to produce highly reactive hydroxyl radicals (OH˙), which oxidize the colorless TMB substrate into a blue oxidized product (oxTMB).¹⁴⁶ The presence of dopamine leads to fading of the blue color as it deactivates, thereby enabling a flexible colorimetric detection platform. The system shows good linearity over the range of 2–150 µM for dopamine with a lower detection limit of 1.67 µM. In addition, an h-CuS NCs-based modified electrode can be used for enzyme-free electrocatalytic sensing of glucose, exhibiting good linearity for analyte detection from 0.1 mM to 10 mM. Zheng et al.¹⁴⁷ prepared a MoS₂ decorated N-doped carbon nanotubes (NCNTs@MoS₂), which exhibits a superior synergistic effect, and thus the intrinsic peroxidase-mimic activity is enhanced dramatically. Exploiting the superior catalytic activity of these hybrid nanotubes, a simple colorimetric assay has been demonstrated for sensitive detection of H₂O₂ with a detection limit of 0.14 mmol L⁻¹ and ascorbic acid with a limit of detection of 0.12 mmol L⁻¹. Using colorimetric and electrochemical detection approaches, many metal sulfide-based nanozymes have been developed to detect various biomolecules (Table 6).

5.3.1. Sensing of glucose and H₂O₂

In sensing of biomolecules, glucose and hydrogen peroxide are the most widely studied analytes (Table 6). Glucose sensing is an important area of research with valuable applications in healthcare, biology, and materials science. H₂O₂ is a key ROS involved in various biological activities, including immunity, metabolism, and cell signalling. However, elevated concentrations are associated with oxidative stress, which is a biomarker for serious conditions like diabetes, Parkinson's, Alzheimer's disease, and cancer. To diagnose and treat diseases, rapid and precise monitoring of H₂O₂ in biological fluids (e.g., urine, blood, and exhaled air) is crucial. Non-enzymatic nanozymes circumvent the limitations of natural enzymes as they have intrinsic enzyme-mimicking activity, such as peroxidase-like activity. Since H₂O₂ is an outcome of glucose oxidation, facilitated by glucose oxidase, the relationship between H₂O₂ and glucose is interesting. Therefore, it is common to use appropriate methodologies to measure glucose and H₂O₂ concurrently in both clinical and scientific research. Metal sulfide nanozymes are essential for glucose and H₂O₂ sensing, as they mimic peroxidase-like activity by catalyzing the decomposition of H₂O₂ (generated as a signalling intermediate during glucose oxidation), thereby providing a quantifiable colorimetric or electrochemical signal.

Huang et al.¹⁶³ synthesized NiCo₂S₄ microflowers, acquiring strong electron transfer capacity due to the presence of sulfur that reduces the band and increases the lattice parameter as compared to the corresponding nickel–cobalt oxide, resulting in immensely effective peroxidase-like catalytic activity. TMB, OPD and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) were utilized as substrates for peroxidase activity by NiCo₂S₄. The oxidation of glucose by glucose oxidase generated hydrogen peroxide, which oxidized TMB in the presence of NiCo₂S₄ catalyst to produce blue colored oxTMB, which can be monitored to detect glucose and hydrogen peroxide (Fig. 10). The antioxidant capacity of tannic acid, ascorbic acid, and gallic acid was also studied by their tendency to inhibit the NiCo₂S₄ catalyzed oxidation of TMB. This colorimetric assay enabled indirect detection of glucose and direct detection of hydrogen peroxide, achieving low detection limits of 8.24 µM for glucose and 5.19 µM for H₂O₂, with broad linear ranges of 20–1000 µM for glucose and 20–200 µM for H₂O₂. The production of ˙OH is often involved in the catalytic mechanism of peroxidase-like activity. To demonstrate this, the ˙OH produced from hydrogen peroxide in the presence of NiCo₂S₄ was examined using two probes, methylene blue (MB) and benzoic acid (BA). The fluorescent product, which displays a distinctive fluorescence around 405 nm, was formed when BA molecules combine with ˙OH generated by H₂O₂. Additionally, the absorbance at 664 nm dropped, and the distinctive blue color of the methylene blue (MB) vanished following its reaction with ˙OH.

Fig. 10. Schematic illustration of NiCo₂S₄ with peroxidase-like activity. Adapted with permission from ref. 163. Copyright 2021, American Chemical Society.

Later on, Fan et al.¹⁶⁸ synthesized FeS₂@CN nanocomposites under different temperatures (300–600 °C) and Fe@CN-to-S ratios (1 : 3–6) by using NH₂-MIL-88 as template. Elevated enzyme-mimetic activity was detected in FeS₂@CN relative to its antecedents (Fe@CN and NH₂-MIL-88), caused by numerous imperfections on the porous carbon surface and the unsaturated sites of S and N doping. A significant decrease in the value of the Michaelis–Menten constant (K_m) was observed for FeS₂@CN in comparison to that of conventional horseradish peroxidase (HRP). The FeS₂@CN nanozyme's catalytic mechanism incorporates the adsorption of H₂O₂ molecules on the surface of the nanozyme by means of several interactions to activate and produce ˙OH through integration with Fe²⁺. Due to the transfer or exchange of a partial electron, the ˙OH produced on the surface of the catalyst stays stable, and oxidizes TMB to produce blue oxTMB. Glucose may react quickly with oxygen in the presence of glucose oxidase to produce H₂O₂ and gluconic acid. To investigate its feasibility in real-world applications, the FeS₂@CN-based nanozymic sensor was used to measure glucose levels in human serum samples.

Karikalan et al.²¹⁴ adapted sonochemical approach to form a copper sulphide (CuS-covellite) and reduced graphene oxide doped with sulphur (S-rGO) nanocomposite for glucose determination. Remarkably, the covellite phase of the synthesized S-rGO/CuS gave way to the digenite phase, induced by S-rGO acting as a moderate oxidant. In comparison to bulk CuS, the oxidised CuS exhibits enhanced electrocatalytic activity. The S-rGO/CuS nanocomposite demonstrated a lower detection limit of 32 nM, along with a broad linear concentration range of 0.0001–3.88 mM and 3.88–20.17 mM. Furthermore, using actual biological samples, they confirmed the feasibility of the developed glucose sensor. Zhang et al.¹⁸⁹ fabricated Fe_0.8Ni_0.2S₂/ZIF-67 using the hydrothermal method to detect H₂O₂ by oxidising colourless TMB to blue oxTMB. The catalytic efficiency (K_cat/K_m) of the Fe_0.8Ni_0.2S₂/ZIF-67 composite was higher at 2.67 (for H₂O₂) and 3.13 (for TMB) times, and the catalytic constant (K_cat) is higher at 2.08 (for H₂O₂) and 1.98 (for TMB) times than that of Fe_0.8Ni_0.2S_2. The decrease in the value of K_m by 85 times (for H₂O₂) and the LOD by 18.2 times (for H₂O₂) was observed for the Fe_0.8Ni_0.2S₂/ZIF-67 composite compared to that of Fe_0.8Ni_0.2S₂.

Zhu et al.²¹⁵ fabricated hollow CuS nanozyme for the sensing of glucose that functions as an electrocatalyst on the electrode. Glucose is oxidized at the nanozyme–electrode interface, generating a measurable anodic current that is amplified by the intrinsic electrocatalytic activity of the CuS nanocubes, while the well-defined hollow structure ensures rapid mass transport and multiple accessible redox-active sites, thereby enhancing both sensitivity and response kinetics. In addition, the authors highlight that the h-CuS NCs also exhibit high affinity for H₂O₂ (K_m ≈ 0.94 mM) and even photothermal activity under near-infrared light, making them a prototype for multifunctional nanozyme platforms that integrate colorimetric, electrochemical, and photo-responsive signal transduction modes. In another study, another glucose sensor built on hollow core–shell CuS@Cu₂MoS₄ nanocubes where the heterojunction structure and hollow morphology together enhance electrocatalytic glucose oxidation and thus analytical performance.²¹⁶ Mechanistically, CuS nanocubes are first grown on a Cu₂O template, then the Cu₂O core is selectively etched and Cu₂MoS₄ is hydrothermally grown on the CuS surface, forming a CuS@Cu₂MoS₄ heterojunction; the intimate contact between CuS and Cu₂MoS₄ modifies the electronic structure, facilitating electron transfer and stabilizing higher-valence copper/molybdenum species that act as active sites for glucose oxidation. The hollow, porous architecture increases accessible surface area and shortens diffusion paths for glucose and electrolyte, while the coupled redox couples (e.g., Cu⁺/Cu²⁺, Mo⁴⁺/Mo⁵⁺/Mo⁶⁺) mediate the direct electrocatalytic oxidation of glucose at the modified electrode, generating a large, linear amperometric response over a wide concentration range (0.002–23 mM) with a low detection limit of 3.1 µM and good recovery in human serum samples. In effect, the CuS@Cu₂MoS₄ heterojunction functions as a nonenzymatic nanozyme layer that mimics glucose-oxidase-like catalysis through metal-sulfide electrocatalysis, eliminating the need for natural enzymes and enabling robust, high-sensitivity glucose sensing. Based on similar methodologies, several nanozyme-based glucose sensors have been developed, and the use of point-of-care testing is growing to provide a technological solution for monitoring health conditions.

5.4. Sensing of phenolic compounds and dyes

Phenolic compounds and synthetic dyes are persistent, bioaccumulative environmental pollutants released from industrial effluents, agricultural runoff, and improper waste disposal that resist natural degradation, impair water quality,²¹⁷ and pose severe risks to ecosystems and human health through oxidative stress, endocrine disruption, mutagenicity, teratogenicity, and carcinogenicity.^218,219 Consequently, there is a pressing requirement for sensitive detection and efficient degradation strategies, and metal sulfide nanozymes have emerged as a cost-effective, robust platform that combines enzyme-like catalytic activity with high stability under practical conditions, enabling both colorimetric sensing and catalytic remediation of phenolic pollutants, as summarized in Table 7. For example, Han et al.,²²⁰ synthesized spinel CuCo₂S₄ nanocomposite that exhibit strong laccase-like activity and facilitate the oxidative coupling between 4-aminoantipyrine and phenols under neutral conditions to produce a quinone imide, providing a sensitive colorimetric readout for detecting 2,4-dichlorophenol, and in a complementary approach (Fig. 11). Cai et al.²²¹ fabricated few-layer MoS₂ nanosheets decorated with uniform Pt₃Au₁ nanoparticles that show markedly augmented peroxidase-mimicking activity relative to pristine MoS₂ and enable a rapid, simple, and inexpensive colorimetric assay based on H₂O₂-driven oxidative coupling of phenol ²²² with 4-aminoantipyrine to produce a pink product for selective phenol identification. These studies demonstrate that tuned nanozyme composition and nanostructure enhance catalytic efficiency and substrate selectivity, enabling practical sensing formats and integration with downstream degradation or removal processes to reduce environmental and public health burdens.

Table 7. List of various phenolic compounds and dyes detected using different metal sulfide-based nanozymes.

Analytes	Sensing of phenolics/dyes	Metal sulphide nanomaterial	LOD	Mode of sensing	References
Phenolic compounds	4-Aminophenol	CdS-graphene-polypyrrole	0.23 nM	Photo-electrochemical	224
		MoS2/GCE	0.03 µM	Electrochemical	225
	Catechol	CdS-DNA-G	4.9 nM	Photo-electrochemical	226
	4-Methoxyphenol	NiS2-CNT NCs/GCE	30.0 pM	Electrochemical	227
	Bisphenol A	AuNPs/MoS2-NFs/IL-graphene/GCE	0.028 µM	Electrochemical	228
	Vanillin	MoS2-CNF/GCE	0.15 µM	Electrochemical	229
	Caffeic	SPE-CB-WS2/AuNP-CT	0.10 µM	Electrochemical	230
	Sinapic		0.40 µM
	Quercetin	MoS2-CNTs@GONRs/HSCD/GQDs/GCE	82 nM	Electrochemical	231
		Pd/MoS2-IL-OMC/GCE	8.0 nM	Electrochemical	232
	Eriocitrin	CoS2–MoS2 PDDA-GR/GCE	67 nM L−1	Electrochemical	233
	Luteolin	MIP/MoS2/GN-CNTs/GCE	9.0 nM	Electrochemical	232
	Propyl gallate	CoSe2@rGO/SPCE	16 nM	Electrochemical	234
	P-coumaric acids	SPE-CB-WS2/AuNP-CT	0.40 µM	Electrochemical	230
Dyes	Ponceau 4R	GCE-WS2-DNP	7.8 M	Electrochemical	235
	Tartrazine	GCE-WS2-DNP	4.5 M	Electrochemical	235
	Sunset yellow	CdS-rGO sheets	7.89 µM	Fluorescent	115

Fig. 11. Chromogenic reaction of 2,4-DP and 4-AP under the action of CuCo₂S₄ or laccase.

MoS₂-supported CuS nanozyme platform designed for the colorimetric detection and photocatalytic degradation of hydroquinone (HQ) in aqueous media.²²³ Mechanistically, the CuS nanoparticles act as the primary peroxidase-mimicking sites that catalyze H₂O₂ driven oxidation of chromogenic probes (e.g., TMB, ABTS, or o-phenylenediamine), while HQ competitively binds to these active Cu sites, quenching the colorimetric signal in a concentration-dependent manner, enabling sensitive HQ detection with a low limit of detection (LOD ≈ 3.68 µM). The MoS₂ sheets play a dual electronic role, first, they provide a two-dimensional conductive support that facilitates charge transfer through the CuS–MoS₂ heterointerface, thereby stabilizing higher-valence Cu species (Cu²⁺) and enhancing the peroxidase-like activity; second, under natural sunlight, the MoS₂–CuS system behaves as a visible-light photocatalyst, where photogenerated electrons and holes generate hydroxyl radicals (˙OH) that oxidize and degrade HQ, achieving high removal efficiency over several hours. In this way, the nanozyme platform integrates signal-amplified sensing (through MoS₂-mediated electronic coupling with CuS) and self-cleaning photocatalytic functionality, offering a single composite material that both detects and destroys phenolic pollutants in water. Electrochemical methods for sensing phenolic compounds and certain dyes have also been studied using various metal sulfide nanozymes, which rely on changes in current or voltage changes in the presence of the analyte (Table 7).

6. Current status of real-time testing using metal sulfide nanozymes

Metal sulfide nanozymes such as molybdenum disulfide, copper sulfide, and iron disulfide have emerged in recent years as highly promising alternatives to natural enzymes for point-of-care testing, combining robust catalytic activity with low cost and ease of integration into simple devices. Despite the global nanozymes market experiencing rapid commercial expansion, with projected valuations surging toward $3.4 billion by 2035, driven by massive growth in biotech and medical applications.^236–238 Metal sulfide-based nanozymes remain severely limited due to critical structural vulnerabilities. Three practical problems stand in the way. First, metal sulfides are chemically and structurally less forgiving than metal oxides or noble metals; their surfaces oxidize and degrade when exposed to air and moisture, and that loss of integrity directly reduces their catalytic performance during storage and transport.^16,17 Second, the long-term safety picture is uncertain because some metal sulfides can release metal ions or generate reactive species under biological conditions, creating real concern about leakage and systemic toxicity that regulatory agencies rightly treat as a major barrier. Third, researchers still rely on laboratory scale syntheses that are expensive and produce batches with variable size shape and surface chemistry, so reproducible large-scale manufacturing has not yet been solved. To move these materials out of the lab and into products we need a coordinated effort that targets stability, toxicity and scale.^16,17 Practical strategies include rational surface engineering to create thin protective layers that preserve active sites while blocking oxidation, thorough degradation and toxicology studies that quantify ion release and biological outcomes, and investment in green continuous manufacturing methods that produce uniform crystals with consistent catalytic behavior. Equally important are shared standards for measuring activity stability and safety so developers can compare materials objectively and engage regulators early.²³⁹ If researchers address these interrelated challenges in parallel, the strong catalytic advantages of transition metal sulfide nanozymes could finally be translated into safe affordable and reliable point of care diagnostics.

However, many laboratory studies have shown that these materials perform impressively and have potential for developing a real-time sensing device for real samples. For example, two-dimensional molybdenum disulfide sheets tend to dramatically amplify signals in lateral flow immunoassays for detecting tetracycline. The authors showed that larger MoS₂ nanosheets exhibited superior catalytic activity and probe-loading capacity, enabling stronger colorimetric signals and a much lower visual detection limit for tetracycline than conventional test strips, with quantitative analysis via a smartphone-based readout extending the dynamic range into the ng mL⁻¹ region. The method performs well on real milk and honey samples, with good recovery and minimal matrix interference, underscoring the advantages of MoS₂-based signal amplification for low-cost, portable screening of antibiotic residues.²⁴⁰ One notable real-world example is a wearable device by Li et al.²⁴¹ Their research group constructed a flexible, on-skin sensor using a laser-scribed graphene (LSG) electrode on polyimide, coated with a CeO₂–MoS₂ nanozyme and gold nanoparticles to catalyze hydrogen-peroxide reduction, followed by immobilization of lactate oxidase to enable selective lactate detection in sweat. The device achieves a wide linear range (0.1–50.0 mM), high sensitivity, and a low detection limit suitable for clinical-grade lactate monitoring, and it has been tested in real-time on-body during exercise, demonstrating stable performance and good agreement with standard assays.²⁴¹ Xia et al.²⁴² fabricated CuIn₂S₄ nanozyme based sweat glucose biosensor for non-invasive diabetes monitoring. The nanozyme exhibited peroxidase-like activity by catalyzing H₂O₂ decomposition to generate hydroxyl radicals, as verified by EPR spectroscopy. These radicals oxidized chromogenic substrates such as o-phenylenediamine or 4-aminoantipyrine, and the color response was analysed by smartphone-based RGB imaging to differentiate normal, pre-diabetic, and sweat glucose levels (Fig. 12).

Fig. 12. Schematic illustration of the CuIn₂S₄ nanozyme-based wearable sweat biosensor for glucose detection.

Li et al.²⁴³ constructed a photoelectrochemical enzyme-free glucose sensor by first synthesizing graphene@CdS nanowires (GP@CdS NWs) using electrostatic and hydrothermal assembly. Subsequently, they were functionalized with CuO nanoparticles via electrodeposition onto a glassy carbon electrode. The nanocomposite formed by incorporating the photosensitive material GP@CdS NWs with the electrocatalytic material CuO exhibits strong photocatalytic activity for the oxidation of glucose in an alkaline solution. A 2.45-fold increase in sensitivity was observed under visible light compared with the dark condition. The modified CuO/GP@CdS NWs electrode demonstrated several benefits, including a low detection limit, high detection sensitivity, a broad linear concentration range, and satisfactory anti-interference performance. The potential of the proposed method for real-sample analysis was demonstrated by the good recovery observed for glucose in a human serum sample. The suggested approach was used for glucose monitoring in human blood serum, employing the standard addition method to assess the viability of CuO/GP@CdS NWs for glucose detection. After diluting two blood serum samples 100 times with NaOH solution, a specific quantity of glucose was added. The recovery for glucose was observed in the range of 96.4–103.8%. This observation indicated that the suggested approach achieves high accuracy and is practically applicable for routine glucose detection in real samples.

Similarly, Zhao et al.²⁴⁴ synthesized improved photoelectrochemical glucose sensor driven by visible light. CuO NPs deposited electrochemically on modified graphene (GP)-CdS nanocomposite electrode to accomplish both photo-electrocatalytic and electrocatalytic oxidation of glucose. The glucose level in human serum was monitored using the standard addition technique to assess the viability of the CuO NPs/GP-CdS/GCE for glucose detection. Recoveries were observed in the range of 95.3–101.4%, demonstrating high accuracy and suitability for routine glucose analysis of real samples. The CuO NPs/GP-CdS composites exhibited excellent performance as a PEC glucose sensor, including a wide linear concentration range (5–2000 µM), high selectivity, high detection sensitivity (1005 µA mM⁻¹ cm⁻²), and a low detection limit (0.5 µM).

Wen et al.¹⁸⁸ synthesized uniformly dispersed bimetallic sulfide (Zn₂SnS₄) nanoflowers by using a one-pot hydrothermal method to accurately determine low H₂O₂ concentrations, where both the metal ions acted as structure and morphology directing agents for one another in the process. To evaluate the practical use of the Nafion-Zn₂SnS₄/GCE sensor for real-time monitoring of H₂O₂ release from living cells, leukemia THP-1-derived macrophages served as a representative sensing model and lipopolysaccharide (LPS) was used to trigger cellular production of H₂O₂. Gram-negative bacteria include LPS, which binds to TLR4 receptors on macrophage surfaces to activate immune cells. LPS stimulation activates the macrophages through the glycolic route, producing significant amount of H₂O₂ and lactic acid. No discernible change was observed in current upon adding LPS to a blank electrolyte solution devoid of macrophages. However, there was a sharp rise in catalytic current when LPS was added to the electrolyte solution containing macrophages, suggesting that the manufactured sensor can track H₂O₂ generated by living cells in real time. A lower catalytic current was instantly attained after catalase was added to break down H₂O₂. The accuracy of the concentration measurement of H₂O₂ produced from living cells was confirmed by validating the quantitative findings of the proposed sensor using a conventional addition method. The estimated concentration of H₂O₂ emitted from the live cells was calculated to be 5.53 µM, which is close to the amount described in the literature. The proposed sensor can detect real-time H₂O₂ release from the live-cells in a genuine complicated environment, as evidenced by the recovery, which ranged from 92.9 to 102.5%. The outcomes confirm the successful detection of extracellular H₂O₂ using the proposed sensor, indicating its potential implications in clinical diagnostics, biochemical analysis, biosensing, and cancer therapy.

To detect H₂O₂ using the colorimetric method, Huang et al.¹⁸⁷ successfully synthesized porous regular hexagonal-shaped FeS₂ nanosheets with a side length of approximately 1 µm and Fe ion as the active site. By oxidizing the TMB solution to blue at room temperature, the H₂O₂ concentration was readily determined using FeS₂ NSs for one minute. The conventional enzyme HRP (3.7 mM) had substantially higher K_m for H₂O₂ than this nanozyme (0.00342 mM). FeS₂ NSs show remarkable sensitivity in the linear range of 0.02–4.00 µM and the minimum detection limit of 7.60 nM (S/N = 3). Additionally, the nanosheets were used to readily determine H₂O₂ concentrations in various actual samples, including disinfectant, beer, and lens solution. As a fungicide, H₂O₂ is frequently found in the fluids of contact lenses. However, an excessive amount of H₂O₂ might seriously harm the cornea. Thus, the monitoring of H₂O₂ is very crucial. The contact lens solution was diluted 10 times in order to detect H₂O₂ in it. Various amounts of H₂O₂ were supplied, correspondingly. There was no H₂O₂ found in these initial lens fluids. The amount of H₂O₂ that was added to these fluids was measured. The range for the relative standard deviation (RSD) was found to be 0.86–1.32%. These findings proved that H₂O₂ in lens fluids can be effectively detected by FeS₂ NSs. Thus, recent progress in this field ensures that metal sulfide-based nanozymes will revolutionize the sensing industry and enable the development of economical, effective sensing devices for biomedical applications.

7. Challenges and future prospects

Metal sulfide nanozymes have proven highly effective for sensing and biosensing applications. Although progress in this research direction is rapid, many scientific and translational issues remain unresolved. Metal sulfide nanozymes effectively emulate the activities of peroxidases, oxidases, or catalase enzymes; however, they lack the molecular recognition specificity of natural enzymes. Their catalytic activity is typically mediated by redox reactions that generate reactive oxygen species, which can lead to cross-reactivity and interference with structurally similar molecules. The nanozymes sometimes fail to discriminate the analyte from coexisting molecules or species present in biological or environmental matrices. The reproducibility and reusability of metal sulfide nanozymes are affected by multiple factors.²⁴⁵ The leaching of metal ions, surface oxidation of sulfide to sulfate, and the structural degradation of metal sulfide nanozymes in an oxidative environment may lead to poor performance.^246–248 Heavy metal sulfides, such as PbS and CdS, may release heavy metal ions into the environment, causing toxicity in biological systems and environmental contamination.²⁴⁷ The size and morphology of metal sulfide nanozymes are known to affect their activity (Table 2); therefore, it is crucial to develop an optimized system to synthesize these nanosystems with consistent size, shape, and crystallinity. Understanding the mechanism of their action requires extensive studies.

Despite challenges associated with the use of metal sulfide-based nanozymes, they have the potential to revolutionize the sensing industry and enable the next generation of sensors. Atomic-level engineering, including doping with heteroatoms such as nitrogen and phosphorus, and precise control over sulfur vacancies, can increase catalytic turnover, boost selectivity, and reduce detection thresholds. Their nanocomposites with noble metals and carbon nanomaterials can exhibit synergistic effects, thereby enhancing electron transfer and nanozyme stability and reducing metal leaching. Metal sulfide nanozymes can also be combined with biological components, such as aptamers and antibodies, to enhance sensing specificity and selectivity. The tunable band gap of metal sulfide nanomaterials²⁴⁹ enables the development of photoelectrochemical and near-infrared-responsive sensors with high sensitivity and real-time monitoring. Introducing a bifunctional catalytic system into these nanozymes can improve the detection limit and signal amplification of sensors. These nanozymes are anticipated to enable reliable, affordable, and portable sensing technologies for environmental monitoring and biological diagnostics, driven by advances in defect engineering, hybridization techniques, and smart device integration.

8. Conclusion

Metal sulfide nanozymes have recently emerged as a promising platform for artificial enzymes, offering controlled catalytic activity, a large surface area, and excellent stability across a variety of media. Their intriguing physicochemical properties, including redox-active, semiconducting, and defect-rich surfaces, enable them to closely mimic natural enzymes such as peroxidase, oxidase, and catalase. They also contributed to the formation of several biosensing platforms for glucose, hydrogen peroxide (H₂O₂), heavy metals, and biomarkers with excellent sensitivity and selectivity. Although substantial progress has been made, several challenges remain. For example, the catalytic mechanisms of many metal sulfide nanozymes remain poorly understood, particularly the influence of surface defects and electronic structure on enzyme-mimetic activity. Moreover, aspects of biocompatibility, long-term stability, and reproducibility in the complex milieu of biological systems must also be addressed prior to clinical translation.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

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