Oxidase-like mimic of Ag@Ag3PO4 microcubes as a smart probe for ultrasensitive and selective Hg(2+) detection

Engineering pineapple peel cellulose nanofibrils with oxidase-mimic functionalities for antibacterial and fruit preservation

In this study, an antibacterial material (CNF@CoMn-NS) with oxidase-like activity was created using ultrathin cobalt‑manganese nanosheets (CoMn-NS) with a larger specific surface area grown onto pineapple peel cellulose nanofibrils (CNF). The results showed that the CoMn-NS grew well on the CNF, and the obtained CNF@CoMn-NS exhibited good oxidase-like activity. The imidazole salt framework of the CNF@CoMn-NS contained cobalt and manganese in multiple oxidation states, enabling an active redox cycle and generating active oxygen species (ROS) such as singlet molecular oxygen atoms (1O2) and superoxide radical (·O2-), resulting in the significant inactivation of Staphylococcus aureus (74.14%) and Escherichia coli (54.87%). Importantly, the CNF@CoMn-NS did not exhibit cytotoxicity. The CNF@CoMn-NS further self-assembled into a CNF@CoMn-NS paper with flexibility, stability, and antibacterial properties, which can effectively protect the wound of two varieties of pears from decay caused by microorganisms. This study demonstrated the potential of using renewable and degradable CNF as substrate combined with artificial enzymes as a promising approach to creating antibacterial materials for food preservation and even extending to textiles and biomedical applications.

o-phenylenediamine Derived Fluorescent Carbon Quantum dots for Detection of Hg(II) in Environmental Water

With the expansion of human activities, the consequent influx of mercury (Hg) into the food chain and the environment is seriously threatening human life. Herein, nitrogen and sulfur co-doped fluorescent carbon quantum dots (yCQDs) were prepared via a hydrothermal method using o-phenylenediamine (OPD) and taurine as precursors. The morphological characteristics as well as spectral features of yCQDs indicated that the photoluminescence mechanism should be the molecular state fluorophores of 2, 3-diaminophenothiazine (oxOPD), which is the oxide of OPD. The as-synthesized yCQDs exhibited sensitive recognition of Hg2+. According to the investigation in combination of UV-Vis absorption spectra, time-resolved fluorescence spectra and quantum chemical calculations, the abundant functional groups on the surface of yCQDs allowed Hg2+ to bind with yCQDs through various interactions, and the formed complexes significantly inhibited the absorption of excitation light, resulting in the static fluorescence quenching of yCQDs. The proposed yCQDs was utilized for Hg2+ sensing with the limit of detection calculated to be 4.50 × 10- 8 M. Furthermore, the recognition ability of yCQDs for Hg2+ was estimated in tap water, lake water and bottled water, and the results indicated that yCQDs have potential applications in monitoring Hg2+.

Biosynthesis of bifunctional silver nanoparticles for catalytic reduction of organic pollutants and optical monitoring of mercury (II) ions using their oxidase-mimic activity

In this study, an aqueous extract of Sclerocarya birrea leaves was used as a reducing agent to synthesize silver nanoparticles (AgNPs). The synthesis was carried out at room temperature and was both rapid and simple. Different characterization techniques such as UV/visible spectroscopy, surface-enhanced Raman spectroscopy, X-ray diffraction, and focused ion beam scanning electron microscopy were used to confirm the formation of AgNPs. The synthesized nanoparticles exhibited catalytic activity for the reduction of 4-nitrophenol, methyl orange, methylene blue, and rhodamine 6G. The catalytic activity was monitored by measuring the UV/visible absorbance spectra of the compounds using sodium borohydride as a reducing agent and found to be high. Additionally, the particles displayed oxidase-like activity. In the presence of AgNPs, 3, 3', 5, 5'-tetramethylbenzidine (TMB) which is colorless was transformed to oxidized TMB, which is blue, using dissolved oxygen as the oxidant. In the presence of Hg2+, the oxidase-like activity was enhanced. On the basis of this observation, an assay for the analysis of Hg2+ was developed. The linear range of the calibration curve is wide (0-600 µM) and the limit of detection (LOD) is low, as small as 34.8 nM. The method is strongly selective towards Hg2+. Tap water obtained from the laboratory where these experiments were carried out was used to study the feasibility of the method in real sample analyses.

A portable dual-mode colorimetric platform for sensitive detection of Hg2+ based on NiSe2 with Hg2+-Activated oxidase-like activity

The nanozyme-based colorimetric strategy for heavy metal detection has broad application prospects nowadays. However, the inefficient recognition capabilities of nanozyme sensors for targets hinder its further application. Herein, the authors synthesize bare nickel selenide (NiSe₂) via a one-step hydrothermal reaction, in which the Se element possesses a strong binding ability with mercury (Hg). As expected, NiSe₂ exhibits oxidase-like activity in the presence of Hg²⁺, that is, Hg²⁺ can enhance the oxidase-like activity of NiSe₂. The enhanced mechanism is the accelerated electron transfer between NiSe₂-Hg²⁺ and substrate caused by the formation of Hg-Se bonds. Besides, the oxidase-like activity of NiSe₂ exhibits excellent selectivity, sensitivity and stability in response to Hg²⁺, which enables NiSe₂-Hg²⁺ to efficiently oxidize colorless TMB to blue TMB even in harsh environments. Based on this, a dual-mode colorimetric sensor integrating solution reaction and test paper is developed for the detection of Hg²⁺. In the Hg²⁺ concentration range of 10-700 nM, the colorimetric platform presents a liner response to Hg²⁺, which can reach a low LOD of 5.18 nM in solution reaction and 8.42 nM in the test paper. The proposed strategy can also be applied to real water samples with good recovery and excellent self-calibration capability.

Reversible inhibition of the oxidase-like activity of Fe single-atom nanozymes for drug detection

Mechanism research of nanozymes has always been of great interest since their emergence as outstanding mimics of friable natural enzymes. An important but rarely mentioned issue in mechanism research of nanozymology is the inhibitory effect of nanozymes. And conventional nanozymes with various active sites hinder the mechanism research, while single-atom Fe-N-C nanozymes with similar active sites to natural enzymes exhibit structural advantages. Herein, we synthesized Fe single-atom nanozymes (Fe-SANs) with ultrahigh oxidase-like activity and found that a common analgesic-antipyretic drug 4-acetamidophenol (AMP) had inhibitory effects for the oxidase-like activity of Fe-SANs. We investigated the inhibitory effects in detail and demonstrated that the inhibition type was reversible mixed-inhibition with inhibition constants (K i and ) of 0.431 mM and 0.279 mM, respectively. Furthermore, we put forward a colorimetric method for AMP detection based on nanozyme inhibition. The research on the inhibitory effects of small molecules on nanozymes expands the scope of analysis based on nanozymes and the inhibition mechanism study may offer some insight into investigating the interaction between nanozymes and inhibitors.

β-Cyclodextrin-Stabilized Biosynthesis Nanozyme for Dual Enzyme Mimicking and Fenton Reaction with a High Potential Anticancer Agent

The myth of inactivity of inorganic materials in a biological system breaks down by the discovery of nanozymes. From this time, the nanozyme has attracted huge attention for its high durability, cost-effective production, and easy storage over the natural enzyme. Moreover, the multienzyme-mimicking activity of nanozymes can regulate the level of reactive oxygen species (ROS) in an intercellular system. ROS can be generated by peroxidase (POD), oxidase (OD), and Fenton-like catalytic reaction by a nanozyme which kills the cancer cells by oxidative stress; therefore, it is important in CDT (chemo dynamic therapy). Our current study designed to investigate the enzyme mimicking behavior and anticancer ability of cerium-based nanomaterials because the cerium-based materials offer a high redox ability while maintaining nontoxicity and high stability. Our group synthesized CeZrO4 nanoparticles by a green method using β-cyclodextrin as a stabilizer and neem leaf extract as a reducing agent, exhibiting POD- and OD-like dual enzyme activities. The best enzyme catalytic activity is shown in pH = 4, indicating the high ROS generation in an acidic medium (tumor microenvironment) which is also supported by the Fenton-like behavior of CeZrO4 nanoparticles. Inspired by the high ROS generation in vitro method, we investigated the disruption of human kidney cells by this nanoparticle, successfully verified by the MTT assay. The harmful effect of ROS in a normal cell is also investigated by the in vitro MTT assay. The results suggested that the appreciable anticancer activity with minimal side effects by this synthesized nanomaterial.

Unveiling the Actual Catalytic Sites in Nanozyme-Catalyzed Oxidation of o-Phenylenediamine

Nanozymes have offered remarkable advantages over natural enzymes and found widespread applications including biosensors, immunoassays, nanomedicines, and environmental remediation. Oxidation of o-phenylenediamine (OPD) by nanozymes has been listed as a standard protocol for determining nanozyme activities. Given the complexity of OPD oxidation processes, however, the mechanism of nanozyme-catalyzed oxidation of OPD remains elusive. In this report, mechanistic studies of nanozyme-catalyzed oxidation of OPD are performed and a distinguishably different mechanism from that of natural enzymes is found. A combination of Fourier transform infrared spectroscopy, nuclear magnetic resonance, electrospray ionization mass spectrometry, and electron microscopic studies provides compelling evidence that polymerization of OPD occurs on the surface of several different nanozymes. The unexpected polymerization causes a dense coating layer of poly(o-phenylenediamine) (POPD) on nanozymes renders the intrinsic properties of nanozymes. Therefore, this fundamental discovery raise serious concerns using OPD-based colorimetric method for determining nanozyme activities. Without examining the surface change of nanozymes after catalytic reactions, the use of OPD-based colorimetric method for determining nanozyme activities is strongly discouraged. Furthermore, POPD is discovered as a new oxidase mimic, and this new mechanism also provides a general and robust method to coat nanomaterials with POPD polymers of enzyme-mimicking properties.

Development of enzyme-free single-step immunoassays for glycocholic acid based on palladium nanoparticle-mediated signal generation

Palladium nanoparticles (PdNPs) are composed mainly of inert noble metals, and their outstanding properties have attracted wide attention. PdNPs are not only capable of mimicking the oxidase-like characteristics of natural bio-enzymes, but they also present a clear black band in the test zone. In this work, the synthesized PdNPs promoted a transformation of colorless tetramethylbenzidine (TMB) to a blue oxidation product of TMB, providing a K_m value of 0.09 mM for TMB, and revealing the good catalytic performance of the synthesized PdNPs. For both signal generation and amplification, PdNPs effectively replaced natural bio-enzymes as a new labeling tag. Thus, the PdNP-based enzyme-free single-step immunoassays were successfully developed for efficient and sensitive detection of glycocholic acid (GCA). Under optimal conditions, a noticeable linear relationship was identified by the enzyme-linked immunosorbent assay (ELISA) over a range of 8-2390 ng/mL, while the visual limit of detection (vLOD) in the constructed lateral flow immunoassay (LFA) was 10 ng/mL for GCA. The recovery rate in spiked urine samples obtained by the ELISA ranged from 84.2 to 117.9%, which was consistent with the results in LFA. The present work demonstrates the potential of PdNPs as labeling matrices in enzyme-free single-step immunoassays.

Anderson polyoxometalates with intrinsic oxidase-mimic activity for "turn on" fluorescence sensing of dopamine

Anderson-type polyoxometalate containing Fe³⁺ and Mo⁶⁺, (NH₄)₃[H₆Fe(III)Mo₆O₂₄] (FeMo₆), was found to work as an oxidase-mimicking nanoenzyme for the first time, exhibiting the ability of catalytic oxidation of o-phenylenediamine (OPD), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3',5,5'-tetramethylbenzidine (TMB), which features easy synthesis, low cost, simple operation, and low consumption. Attributed to the nature of FeMo₆ and Fenton-like effect, a novel sensor based on two consecutive "turn on" fluorescence was developed for detecting dopamine (DA) by employing the FeMo₆-OPD system, and the linear range was from 1 to 100 μM with the detection limit 0.0227 μM (3σ/s). Moreover, to increase oxidase-mimic activity of FeMo₆, reduced graphene oxide (rGO) loading FeMo₆ composites (FeMo₆@rGO (n), n = 5%, 10%, 15%) was fabricated, and results show that oxidase-like activities of FeMo₆@rGO (n) are dependent on the mass ratio of FeMo₆/rGO, and FeMo₆@rGO (10%) exhibits the highest oxidase-mimic activity and the fastest respond time (4 min) among all reported oxidase mimic of DA to date. Graphical abstract Anderson-type Mo-POMs FeMo₆ was found to work as an oxidase-mimicking nanoenzyme for the first time and was used to detect DA for two consecutive "turn on" fluorescence sensor modes.

Colorimetric determination of Hg2+ based on the mercury-stimulated oxidase mimetic activity of Ag3PO4 microcubes

Four kinds of Ag₃PO₄ materials were prepared by controlling the experimental conditions, which were developed as oxidase mimics. Experimental results showed that different synthesis methods led to distinct crystal structures, morphologies, and surface properties, which contributed to diverse oxidase-like activities of Ag₃PO₄ materials. Among them, Ag₃PO₄ microcubes (APMCs) can efficiently catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine in the presence of dissolved oxygen to form a blue-colored oxide, presenting the best intrinsic oxidase mimetic ability. The higher-energy [110] facets with more oxygen vacancies exposed and more active sites coupled with more negative charge and larger specific surface area of APMCs contributed to its enhanced oxidase mimetic performance. Besides, mercury ions were proved to remarkably and selectively stimulate the oxidase-like ability of APMCs owing to the formation of Ag-Hg amalgam on its surface. Based on the stimulating effect of Hg²⁺ towards APMCs, a simple and rapid method for colorimetric determination of Hg²⁺ was thus established via the significant signal amplification and megascopic color variation. Under the optimal conditions, the sensing system showed a good linear relationship ranging from 0.1 to 7.0 μM and a detection limit of 20 nM for Hg²⁺, exhibiting high selectivity and good colour stability. Moreover, the colorimetric method was successfully applied to determine Hg²⁺ in real water samples. Considering these advantages, the developed colorimetric sensing system is expected to hold bright prospects for trace determination of Hg²⁺ in biological, environmental, and food samples. Graphical abstract The preparation process of Ag₃PO₄ materials and Hg²⁺-stimulated enhanced oxidase-like ability of Ag₃PO₄ microcubes in catalyzing the oxidation of TMB to generate a typical blue color, which can be applied in rapid and ultrasensitive detection of Hg²⁺ visually.

A differential photoelectrochemical method for glucose determination based on alkali-soaked zeolite imidazole framework-67 as both glucose oxidase and peroxidase mimics

A differential photoelectrochemical (PEC) method for glucose determination is reported using a nanocomposite with double mimic enzymes of glucose oxidase (GOx) and peroxidase. The nanocomposite was prepared by soaking zeolite imidazole framework-67 (ZIF-67) in 0.1 M NaOH solution at room temperature for 30 min, abbreviated as CoxOyHz@ZIF-67. The Michaelis-Menten constant of CoxOyHz@ZIF-67 to H2O2 and glucose is 121 μM and 3.95 mM, respectively. Using the photoelectrode of CoxOyHz@ZIF-67/TiO2 nanotubes (NTs), glucose was oxidized firstly by dissolved oxygen to generate H2O2 under the catalysis of CoxOyHz film as the mimics of GOx. The product of H2O2 enhanced the photocurrent of TiO2 NTs under the catalysis of ZIF-67 as the mimics of peroxidase. The molecular sieve effect of ZIF-67 frameworks reduces the interferences from molecules with size larger than the apertures in ZIF-67. Under the excitation of a 150 W xenon lamp with full spectrum, the photocurrent was measured in a two-electrode system without external additional potential. By using the photocurrent difference between two photocells, i.e CoxOyHz@ZIF-67/TiO2 NTs and Pt electrode, ZIF-67/TiO2 NTs and Pt electrode, as the signal, the selectivity for glucose determination is improved further. The differential PEC method was applied to the determination of glucose with a linear range 0.1 μM~1 mM and a detection limit of 0.03 μM. Graphical abstract.

Nanozyme-based sensing platforms for detection of toxic mercury ions: An alternative approach to conventional methods

Mercury (Hg) is known as a poisonous heavy metal which stimulates a wide range of adverse effects on the human health. Therefore, development of some feasible, practical and highly sensitive platforms would be desirable in determination of Hg²⁺ level as low as nmol L^-1 or pmol L^-1. Different approaches such as ICP-MS, AAS/AES, and nanomaterial-based nanobiosensors have been manipulated for determination of Hg²⁺ level. However, these approaches suffer from expensive instruments and complicated sample preparation. Recently, nanozymes have been assembled to address some disadvantages of conventional methods in the detection of Hg²⁺. Along with the outstanding progress in nanotechnology and computational approaches, pronounced improvement has been attained in the field of nanozymes, recently. To accentuate these progresses, this review presents an overview on the different reports of Hg²⁺-induced toxicity on the different tissues followed by various conventional approaches validated for the determination of Hg²⁺ level. Afterwards, different types of nanozymes like AuNPs, PtNPs for quantitative detection of Hg²⁺ were surveyed. Finally, the current challenges and the future directions were explored to alleviate the limitation of nanozyme-based platforms with potential engineering in detection of heavy metals, namely Hg²⁺. The current overview can provide outstanding information to develop nano-based platforms for improvement of LOD and LOQ of analytical methods in sensitive detection of Hg²⁺ and other heavy metals.

Colorimetric sensing of chlorpyrifos through negative feedback inhibition of the catalytic activity of silver phosphate oxygenase nanozymes

Intensive use of organophosphate chlorpyrifos pesticides in farming has become a serious issue due to their harmful effects on living beings. Most fruits, vegetables and soil contain chlorpyrifos, and it cannot be rinsed out completely by water washing. Therefore, a selective and sensitive detection of chlorpyrifos is significant. In the present study, the intriguing oxidase-mimicking activity of Ag₃PO₄ nanoparticles (NPs) is explored for the fast and selective detection of chlorpyrifos pesticides. Ag₃PO₄ NPs exhibit several advantages, such as great catalytic efficiency, high stability, monodispersity and reusability, over other expensive nanozymes via a facile one-step sensing. The size, shape, crystal planes and diffraction patterns of the Ag₃PO₄ NPs were observed via FESEM and HR-TEM. The surface properties and oxidation states were analyzed via XPS technique. Ag₃PO₄ NPs possess intrinsic excellent oxidase-mimicking properties against 3,3′,5,5′-tetramethylbezidyne (TMB). When chlorpyrifos and Ag₃PO₄ NP nanozymes come in proper orientation proximity, chlorpyrifos is oxidized. The oxidized chlorpyrifos produces sulfide ions and chlorpyrifos oxon. The produced sulfide ions in the reaction system interact with Ag₃PO₄ NPs and inhibit their catalytic activity by feedback inhibition. Indeed, neither any catalytic site is left to oxidize TMB nor any blue colour appears. Thus, this feedback inhibition phenomenon senses chlorpyrifos pesticides. The calculated limit of detection (LOD) for the standard chlorpyrifos is ∼9.97 ppm, and the efficacy of the Ag₃PO₄ NPs calculated in terms of the K_m value was found to be 0.15 mM. A real sample analysis was carried out by the standard addition method with two soil samples collected from Pethapur and Chiloda villages.

Ag₃PO₄ oxygenase nanozymatic activity towards chlorpyrifos sensing.

Portable Colorimetric Detection of Mercury(II) Based on a Non-Noble Metal Nanozyme with Tunable Activity

The nanozyme-based strategy is currently one of the frontiers in the detection of toxic heavy metal ions. However, the utilization of noble metal free nanozymes to construct an economically and environmentally sustainable methodology remains largely unknown. Here, chitosan-functionalized molybdenum(IV) selenide nanosheets (CS-MoSe₂ NS), greenly synthesized by an ionic liquid-assisted grinding method, were exploited for the colorimetric sensing of mercury ions (Hg²⁺). The sensing principle was based on the activating effect of Hg²⁺ on CS-MoSe₂ NS nanozyme activities, triggered by the in situ reduction of chitosan-captured Hg²⁺ ions on a MoSe₂ NS surface. Using 3,3',5,5'-tetramethylbenzidine (TMB) as a colorimetric indicator, the concentrations of activator-like Hg²⁺ ions could be quantitatively and selectively monitored, reaching a limit of detection of 3.5 nM with the ultraviolet-visible spectrophotometer. In addition, the integration system of CS-MoSe₂ NS with a smartphone achieved a portable detection limit as low as 8.4 nM Hg²⁺ within 15 min and showed high specificity and anti-interfering ability over other ions and great practicability in real water and serum samples. The eco-friendly properties of such sensing system were also confirmed. This work emphasizes the rational portable assembly of biocompatible nanozymes like CS-MoSe₂ NS for the field detection of Hg²⁺ in food, biological, and environmental samples.

A novel and ultrasensitive yellow to taupe brown colorimetric sensing and removal method for Hg(ii) based on the thermosensitive poly(N-isopropyl acrylamide) stabilized silver nanoparticles

AgNP/PNIPAm was developed as a dual-functional colorimetric probe and removal system for Hg²⁺.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Functional nanomaterials with unique enzyme-like characteristics for sensing applications

We highlight the recent developments in functional nanomaterials with unique enzyme-like characteristics for sensing applications.

Colorimetric determination of Hg2+ in environmental water based on the Hg2+-stimulated peroxidase mimetic activity of MoS2-Au composites

A colorimetric assay is described for sensitive determination of Hg²⁺ ions based on the MoS₂-Au composites as peroxidase mimetics, which are synthesized by microwave-assisted solvothermal method. The addition of Hg²⁺ stimulates their peroxidase-like activity, along with lower Michaelis constant toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with H₂O₂, allowing the composites for direct determination of Hg²⁺. A broad linear response is obtained ranging from 20 nM to 20 μM with a detection limit (LOD) of 5 nM. The superior peroxidase-like activity is attributed to the large surface area of MoS₂ nanosheets and the synergistic catalytic effect of MoS₂ and Au. The Hg²⁺-stimulation effect implies the strong interaction between Hg²⁺ and MoS₂-Au, where the XPS results confirm the presence of metallic Hg⁰, indicative of an Au-Hg amalgam. This colorimetric assay is successfully applied for the determination of Hg²⁺ in environmental water (tap water and Yellow River water) with excellent selectivity over interfering cations.

A colorimetric aptasensor for the antibiotics oxytetracycline and kanamycin based on the use of magnetic beads and gold nanoparticles

An aptamer based assay is presented for the determination of the antibiotics oxytetracycline (OTC) and kanamycin (KAN). Magnetic beads were applied for separation, and gold nanoparticles (AuNPs) for signal amplification. DNA aptamers against OTC and KAN were firstly designed. After specific recognition events, the aptamer sequences were released from the surface of magnetic beads and the remaining DNA probes captured horseradish peroxidase (HRP) modified AuNPs. Subsequently, 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine are catalytically oxidized by HRP, and the generated colorimetric responses can reflect the concentrations of OTC (at 370 nm) and KAN (at 450 nm), respectively. Experimental results demonstrate that the method is highly sensitive with the detection limit as low as 1 ag mL^-1 for OTC and KAN. An extremely wide linear range (over 11 orders of magnitude) is achieved. The high selectivity is attributed to the high affinity between aptamer and the substrate. The results of real sample tests also verify that the method is promising for antibiotics analysis in the applications of food monitoring and clinical diagnosis. Graphical abstract Schematic presentation of a colorimetric assay for antibiotics based on aptamer-modified magnetic beads and horseradish peroxidase modified gold nanoparticles. Colorimetric responses result from the enzymatic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and o-phenylenediamine (OPD), respectively.

Graphitic carbon nitride supported platinum nanocomposites for rapid and sensitive colorimetric detection of mercury ions

In this study, graphitic carbon nitride supported platinum nanocomposites (g-C₃N₄/PtNPs) have been synthesized for the first time by an ultrasonic-assisted chemical reduction method. By using g-C₃N₄ as the stabilizer, Pt ions could be reduced to PtNPs by NaBH₄ and uniformly deposited on the surface of g-C₃N₄. The resulting g-C₃N₄/PtNPs exhibited enhanced catalytic activity for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) compared to that of g-C₃N₄ or PtNPs alone. After interaction with Hg²⁺, the catalytic activity of g-C₃N₄/PtNPs was effectively inhibited due to the formation of HgPt amalgam. On the basis of this effect, a novel label free colorimetric sensor has been developed for sensitive detection of Hg²⁺ through the g-C₃N₄/PtNPs mediated catalytic reaction. A detection limit as low as 1.23 nM was achieved. This assay also exhibited excellent selectivity toward Hg²⁺ over other metal ions. In addition, it was successfully applied to the determination of Hg²⁺ in real water samples. In view of the advantages, such as simple operation, cost-effective, rapid response and naked-eye observation, the developed colorimetric sensor hold great potential for the detection of toxic Hg²⁺ in environmental and biological samples.

Colorimetric assay of heparin in plasma based on the inhibition of oxidase-like activity of citrate-capped platinum nanoparticles

We report citrate-capped platinum nanoparticles (Pt NPs) as oxidase mimetics for effectively catalyzing the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid), dopamine, and methylene blue in the presence of O₂. To confirm oxidase-like activity of citrate-capped Pt NPs, their activity toward oxygen reduction reaction was studied using cyclic voltammetry and rotating ring-disk electrode method. The results obtained showed that Pt NP NPs can catalyze the oxidation of organic substrates to the colored product and the reduction of oxygen to water through a four-electron exchange process. Because the aggregation of Pt NPs can inhibit their oxidase-like activity and protamine can recognize heparin, we prepared the protamine-modified Pt NPs through direct adsorption on the surface of citrate-capped Pt NPs. The electrostatic attraction between heparin and protamine-stabilized Pt NPs induced nanoparticle aggregation, inhibiting their catalytic activity. Therefore, the lowest detectable heparin concentrations through UV-vis absorption and by the naked eye were estimated to be 0.3 and 60nM, respectively. Moreover, the proposed system enabled the determination of the therapeutic heparin concentration in a single drop of blood.