Colorimetric determination of Hg(II) based on a visually detectable signal amplification induced by a Cu@Au-Hg trimetallic amalgam with peroxidase-like activity

Rapid and highly selective colorimetric detection of mercury(II) ions in water sources based on a ribavirin functionalized AuNP sensor

Solvated mercuric ions (Hg²⁺), a toxic and harmful water pollutant, can easily accumulate in organisms and cause serious damage to the kidney, liver, and central nervous system. To realize rapid and efficient detection of mercury (II) ions in water sources, a kind of new colorimetric sensor of gold nanoparticles (AuNPs) functionalized with ribavirin (Rib-AuNPs) was proposed and characterized by TEM, DLS, XRD, and UV-vis in this work. The color of the Rib-AuNP solution rapidly changed from wine-red to gray-blue with the addition of Hg²⁺ based on the aggregation mechanism. The limits of detection (LODs) are 0.20 μM by the naked eye and 3.64 nM by UV-vis spectroscopy with a fine linear relationship in the range of 0-0.25 μM (R² = 0.9834) and 0.25-0.80 μM (R² = 0.9893) of Hg²⁺, indicating that the detection system of Rib-AuNPs could be applied to analyze Hg²⁺ with excellent selectivity and anti-interference in real water samples.

Colorimetric and ratiometric fluorescent dual-mode sensitive detection of Hg2+ based on UiO-66-NH2@Au composite

A colorimetric and ratiometric fluorescent dual-mode assay is constructed for sensitive and specific Hg²⁺ sensing based on UiO-66-NH₂ and Au composite (UiO-66-NH₂@Au). The addition of Hg²⁺ stimulates the peroxidase-like activity of UiO-66-NH₂@Au by the formation of Au-Hg amalgam, promoting the oxidizing of chromogenic substrate OPD to DAP with the aid of H₂O₂, which lead to the change of colorimetric and fluorescent signals. The absorbance of the sensing system at 450 nm is linear positive correlation with Hg²⁺ concentration of 30-1400 nM and the color of the solution under visible light shaded from light yellow to dark yellow. With the increase of Hg²⁺ concentration, the fluorescence signal at 570 nm (DAP) increased whereas that at 455 nm (intrinsic fluorescence of UiO-66-NH₂) decreased due to inner filter effect (IFE), the fluorescence intensity ratio (F₄₅₅/F₅₇₀) decreasing linearly with Log [Hg²⁺] over the range 60-1700 nM; the fluorescence emission of sensing system under UV excitation changed from blue to yellow, which can easily be discerned visually. This assay was successfully applied to the determination of Hg²⁺ in tap water and river water. The results indicate that the colorimetric and ratiometric fluorescent dual-mode assay based on UiO-66-NH₂@Au realized visual determination of Hg²⁺ rapidly and reliably, revealed application prospect in Hg²⁺ monitoring.

To Love and to Kill: Accurate and Selective Colorimetry for Both Chloride and Mercury Ions Regulated by Electro-Synthesized Oxidase-like SnTe Nanobelts

Herein, SnTe nanobelts (NBs) with efficient oxidase-mimetic activity were synthesized by the simple electrochemical exfoliation method. A specific inhibition effect of Cl^- on the enzymatic behavior of the pure SnTe NBs was discovered, which was accordingly used for establishing a highly feasible, sensitive, selective, and stable Cl^- colorimetric assay. The detection concentration range was 50 nM to 1 mM, and the lowest detection limit was 20 nM for Cl^-. In addition, a signal on-off-on route based on the SnTe NB nanozyme was designed to realize the reliable and specific detection of Hg²⁺. Therein, the SnTe NBs were grafted with gold nanoparticles to form a hybrid of SnTe/Au, resulting in the depression of the oxidase-like activity, which can then be recovered in the presence of the Hg²⁺ due to the formation of a gold amalgam. Especially, it was found that the high concentration of Cl^- over 3 mM could again exert suppression influence toward the enzymatic activity of the SnTe/Au-Hg system. Based on the to-love-and-to-kill interaction between Cl^- and Hg²⁺, the detection range for Cl^- can be extended to 40 to 250 mM. In return, the assays of Cl^- could avoid in advance its interference toward the accurate Hg²⁺ assays. We systematically clarified the oxidase-like catalytic mechanism of the SnTe-derived nanozyme systems. The as-proposed colorimetry can be successfully applied in practical samples including the sweat, human serum, or seawater/tap water, relating to cystic fibrosis, hyper-/hypochloremia, or environmental control, respectively.

Nanozymes for Environmental Pollutant Monitoring and Remediation

Nanozymes are advanced nanomaterials which mimic natural enzymes by exhibiting enzyme-like properties. As nanozymes offer better structural stability over their respective natural enzymes, they are ideal candidates for real-time and/or remote environmental pollutant monitoring and remediation. In this review, we classify nanozymes into four types depending on their enzyme-mimicking behaviour (active metal centre mimic, functional mimic, nanocomposite or 3D structural mimic) and offer mechanistic insights into the nature of their catalytic activity. Following this, we discuss the current environmental translation of nanozymes into a powerful sensing or remediation tool through inventive nano-architectural design of nanozymes and their transduction methodologies. Here, we focus on recent developments in nanozymes for the detection of heavy metal ions, pesticides and other organic pollutants, emphasising optical methods and a few electrochemical techniques. Strategies to remediate persistent organic pollutants such as pesticides, phenols, antibiotics and textile dyes are included. We conclude with a discussion on the practical deployment of these nanozymes in terms of their effectiveness, reusability, real-time in-field application, commercial production and regulatory considerations.

Colorimetric detection of hypoxanthine in aquatic products based on the enzyme mimic of cobalt-doped carbon nitride

A colorimetric method for the rapid detection of Hx in aquatic products was established based on the peroxidase-like activity of cobalt-doped graphite phase carbon nitride (Co-doped-g-C3N4).

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.

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.

Recent developments of nanoenzyme-based colorimetric sensors for heavy metal detection and the interaction mechanism

This work highlights the application and interaction mechanism of metal nanoparticles, metal oxides, metal sulfides, graphene-based nanomaterials and G-quadruplex, etc. in nanoenzyme-based colorimetric sensors.

Developing a new colorimetric bioassay for iodide determination based on gold supported iridium peroxidase catalysts

A schematic sketch of the colorimetric bioassay for iodide determination based on gold supported iridium peroxidase catalysts.

Peroxidase-Like Activity of Smart Nanomaterials and Their Advanced Application in Colorimetric Glucose Biosensors

Diabetes is a dominating health issue with 425 million people suffering from the disease worldwide and 4 million deaths each year. To avoid further complications, the diabetic patient blood glucose level should be strictly monitored despite there being no cure for diabetes. Colorimetric biosensing has attracted significant attention because of its low cost, simplicity, and practicality. Recently, some nanomaterials have been found that possess unexpected peroxidase-like activity, and great advances have been made in fabricating colorimetric glucose biosensors based on the peroxidase-like activity of these nanomaterials using glucose oxidase. Compared with natural horseradish peroxidase, the nanomaterials exhibit flexibility in structure design and composition, and have easy separation and storage, high stability, simple preparation, and tunable catalytic activity. To highlight the significant progress in the field of nanomaterial-based peroxidase-like activity, this work discusses the various smart nanomaterials that mimic horseradish peroxidase and its mechanism and development history, and the applications in colorimetric glucose biosensors. Different approaches for tunable peroxidase-like activity of nanomaterials are summarized, such as size, morphology, and shape; surface modification and coating; and metal doping and alloy. Finally, the conclusion and challenges facing peroxidase-like activity of nanomaterials and future directions are discussed.

Colorimetric determination of lead(II) or mercury(II) based on target induced switching of the enzyme-like activity of metallothionein-stabilized copper nanoclusters

It is shown that metallothionein-stabilized copper nanoclusters (MT-CuNCs) display catalase-like activity. In the presence of either lead(II) or mercury(II), the catalase-like activity is converted to a peroxidase-like activity. On addition of Pb(II) or Hg(II), the inhibitory effect of MT-CuNCs on the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) with H₂O₂ is weakened. On the other hand, the catalytic effect of the nanoclusters on the chromogenic reaction is increased. The system MT-CuNCs-Pb(II)/Hg(II) exhibits high affinity for the substrates TMB and H₂O₂. Their catalytic behavior follows Michaelis-Menten kinetics. Based on these findings, a method was developed for visual detection (via the blue coloration formed) and spectrophotometric determination (at 450 nm) of Pb(II) and Hg(II). The linear range for Pb(II) extends from 0.7 to 96 μM, and the linear ranges for Hg(II) from 97 nM to 2.3 μM and from 3.1 μM to 15.6 μM. The detection limits are 142 nM for Pb(II) and 43.8 nM for Hg(II). Graphical abstract Metallothionein-stabilized copper nanoclusters (MT-CuNCs) display catalase-like activity. On addition of Pb(II) or Hg(II), the catalase-like activity is converted to a peroxidase-like activity. The latter catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂.

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.

Colorimetric determination of mercury(II) via the inhibition by ssDNA of the oxidase-like activity of a mixed valence state cerium-based metal-organic framework

This work demonstrates the inhibition effects of single-stranded (ssDNA) on the oxidase-like activity of a mixed-valence state cerium-based metal-organic framework, denoted as MVC-MOF. The MVC-MOF was synthesized by partial oxidation of cerium(III) which leads to the presence of both Ce(III) and Ce(IV) ions. The latter endows the MVC-MOF with a typical oxidase-like activity. However, on addition of ssDNA, the catalytic activity of the MVC-MOF is inhibited because it binds the MVC-MOF and thereby shield its active sites. This prevents the access of substrates. The inhibition by ssDNA depends on its length but not its sequence. By contrast, negligible changes in the oxidase-mimicking activity are observed if double-stranded DNA (dsDNA) is added. By employing a thymine-rich ssDNA (T-ssDNA) as a model DNA, a colorimetric assay was developed for the determination of Hg(II). This ion binds to T-ssDNA and causes the formation of T-dsDNA. Hence, the oxidase-mimicking activity is compromised. By using the oxidase substrate 3,3',5,5'-tetramethylbenzidine that gives a colored product in the presence of oxygen, the assay has a linear response that covers 0.05 to 6 μM Hg(II) with a detection limit of 10.5 nM, and exhibits high selectivity over other metal ions. The assay was successfully applied to the determination of Hg(II) in environmental water samples. Graphical abstract Schematic of the inhibition effect of ssDNA on the oxidase-like activity of MVC-MOF that converts colorless TMB to oxTMB with blue color in the presence of oxygen, and its application in the construction of a colorimetric assay for Hg(II) determination.

A catalytic cleavage strategy for fluorometric determination of Hg(II) based on the use of a Mg(II)-dependent split DNAzyme and hairpins conjugated to gold nanoparticles

A catalytic cleavage strategy was developed for the fluorometric determination of Hg(II). The method is based on the use of a Mg(II)-dependent split DNAzyme. Fluorophore labeled hairpins were conjugated to gold nanoparticles (AuNPs) upon which fluorescence is quenched. Thymine-Hg(II)-thymine (T-Hg(II)-T) interaction causes the two DNA sequences to form an entire enzyme-strand DNA (E-DNA). The E-DNA bind to the hairpins on the AuNPs to form a Mg(II)-dependent DNAzyme structure. The circular cleavage of hairpins results in a signal amplification and in the recovery of fluorescence. The assay has a limit of detection (LOD) as low as 80 pM of Hg(II). This LOD is comparable to those obtained with other amplification strategies. The method was successfully applied to the determination of Hg(II) in Chinese herbs (Atractylodes macrocephala Koidz). Graphical abstract Schematic of a catalytic cleavage strategy based on Mg(II)-dependent split DNAzyme for fluorometric determination of Hg(II).

Chemiluminescence assay for detection of 2-hydroxyfluorene using the G-quadruplex DNAzyme-H2O2-luminol system

A chemiluminescence (CL) based assay is described for the determination of the environmental pollutant 2-hydroxyfluorene (2-HOFlu) which is found to inhibit the CL of a system composed of the G-quadruplex/hemin complex (a DNAzyme), H₂O₂, and luminol. The G-rich aptamer PW17 is transformed to a potassium(I)-stabilized G-quadruplex-hemin complex which displays peroxidase-like activity to catalyze the oxidation of luminol by H₂O₂ which is accompanied by strong blue CL emission. On addition of 2-HOFlu, it will participate in the G-quadruplex DNAzyme-mediated oxidation by H₂O₂. As a result, CL intensity is decreased. The difference in CL intensity (ΔI) before and after addition of 2-HOFlu serves as the signal for its quantitation. In water of pH 9.0, a linear relationship is found for the 1 nM to 1 μM concentration range, with a 0.2 nM detection limit. The assay is highly selective over other fluorene derivatives. It was successfully applied to the determination of 2-HOFlu in spiked lake water samples. The method is rapid, cost-effective and convenient. Conceivably, it has a wide scope in that it may be applied to other target pollutants for which G-quadruplexes are available. Graphical abstract A chemiluminescence (CL) assay is described for the determination of the environmental pollutant 2-hydroxyfluorene (2-HOFlu) based on the inhibition of the CL system composed of the G-quadruplex/hemin complex (a DNAzyme), H₂O₂, and luminol.

Chitosan–gold nanoparticles as peroxidase mimic and their application in glucose detection in serum

Chitosan–AuNPs possess peroxidase-like activity and can be used for the detection of glucose in serum.