Selective colorimetric sensing of sub-nanomolar Hg2+ based on its significantly enhancing peroxidase mimics of silver/copper nanoclusters

Synergism between copper and silver nanoclusters induces fascinating structural modifications, properties, and applications

Among the group 11 transition metal elements, Cu and Ag are widely studied due to their cost effectiveness and easy availability. However, the synergism between copper and silver is also very promising, exhibiting intriguing structures, properties, and applications. Nanoclusters, which are missing links between atoms and nanoparticles, are highly fluorescent due to their discrete energy levels. Their fluorescence can be efficiently tuned because of the synergistic behaviour of copper and silver. Furthermore, their fluorescence can be selectively altered in the presence of various analytes and sensing platforms, as reported by various groups. Moreover, copper clusters can be utilized for sensing silver while silver nanoclusters can be utilized for sensing ionic copper due to the strong interaction between copper and silver. Furthermore, DFT studies have been performed to understand the structural modification due to CuAg synergism. A concise summary of the synergism between copper and silver can open a new window of research for young scientists venturing into the field of environmental nanoscience.

Colorimetric detection of Hg2+ based on the enhanced oxidase-mimic activity of CuO/Au@Cu3(BTC)2 triggered by Hg2

It is imperative to develop a rapid detection method for Hg2+ due to its harm to human health and the ecological environment. In this research, CuO/Au@Cu3(BTC)2 was synthesized through reducing HAuCl4 by CuxO@Cu3(BTC)2, which was obtained by reducing Cu3(BTC)2 with hydrazine hydrate. The oxidase-mimic activity of CuO/Au@Cu3(BTC)2 can be enhanced by Hg2+ through forming a Au-Hg alloy. Therefore, a colorimetric method was designed for Hg2+ detection with a linear relationship in the 0.05-25 μM range and a limit of detection of 9.7 nM. This strategy exhibited a strong selectivity to Hg2+ and was applied in a real water sample with reliable recoveries. This work provides a possibility for the rapid detection of Hg2+.

Challenges and Opportunities of Using Fluorescent Metal Nanocluster-Based Colorimetric Assays in Medicine

Development of rapid colorimetric methods based on novel optical-active metal nanomaterials has provided methods for the detection of ions, biomarkers, cancers, etc. Fluorescent metal nanoclusters (FMNCs) have gained a lot of attention due to their unique physical, chemical, and optical properties providing numerous applications from rapid and sensitive detection to cellular imaging. However, because of very small color changes, their colorimetric applications for developing rapid tests based on the naked eye or simple UV-vis absorption spectrophotometry are still limited. FMNCs with peroxidase-like activity have significant potential in a wide variety of applications, especially for point-of-care diagnostics. In this review, the effect of using various capping agents and metals for the preparation of nanoclusters in their colorimetric sensing properties is explored, and the synthesis and detection mechanisms and the recent advances in their application for ultrasensitive chemical and biological analysis regarding human health are highlighted. Finally, the challenges that remain as well as the future perspectives are briefly discussed. Overcoming these limitations will allow us to expand the nanocluster's application for colorimetric diagnostic purposes in medical practice.

Multifunctional DNA nanoprobe for tumor-targeted synergistic therapy by integrating chemodynamic therapy with gene silencing

Due to the high complexity, diversity and heterogeneity of tumor occurrence and development, multi-mode synergistic therapy is more effective than single treatment modes to improve the antitumor efficacy. Also, multifunctional probes are crucial to realize synergistic therapy. Herein, a multifunctional DNA tetrahedron nanoprobe was ingeniously designed to simultaneously achieve chemodynamic therapy (CDT) and gene silencing for synergistic antitumor. The multifunctional DNA tetrahedron nanoprobe, DNA tetrahedron-silver nanocluster-antagomir-21 (D-sgc8-DTNS-AgNCs-Anta-21), integrated a CDT reagent (DNA-AgNCs) and miRNA-21 inhibitor (Anta-21) with a specific recognition probe (aptamer). After targeted entry in cancer cells, D-sgc8-DTNS-AgNCs-Anta-21 silenced endogenous miRNA-21 by Anta-21 and produced highly toxic ˙OH by reacting with H2O2, which induced apoptosis in the tumor cells. The targeted recognition of aptamers led to the concentration-dependent death of HeLa cells. On the contrary, the cell survival rate of normal cells was basically unaffected with an increase in the concentration of D-sgc8-DTNS-AgNCs-Anta-21. Therefore, the diverse functions, biocompatibility and programmability of DNA provide a useful and easy way to assemble multifunctional probes for synergistic therapy.

Morphology and Enzyme-Mimicking Activity of Copper Nanoassemblies Regulated by Peptide: Mechanism, Ultrasensitive Assaying of Trypsin, and Screening of Trypsin Inhibitors

To regulate nanostructure synthesis is of crucial importance for developing various applications, including catalysis, bioanalysis, and optical devices. Herein, the morphology and peroxidase (POD)-mimicking activity of peptide-templated copper nanoassemblies (Cu NAs) are regulable with peptide types. The Cu NAs templated with peptide containing single cysteine are uniform nanoclusters with strong POD-like activity. However, the Cu NAs templated with peptide containing two cysteines are fusiform-like with very weak POD-like activity. Unexpectedly, the POD-like activity of Cu NAs templated with peptide containing two cysteines with lysine between the cysteines is significantly enhanced when trypsin is incubated, which is unchanged for the Cu NAs templated with peptide containing two cysteines without lysine between the cysteines. The remarkably enhanced POD-mimicking activity originates from trypsin specifically shearing the peptide bond on the lysine, thereby allowing the aggregated Cu NAs to unravel into individual nanoclusters. Therefore, a robust colorimetric sensing platform was constructed for sensitive and selective detection of trypsin, which showed a linear concentration range of 3-1000 nM and a detection limit of 0.82 nM (S/N = 3). More interestingly, featured by trypsin inhibitor restraining trypsin activity, it enabled us to screen trypsin inhibitors as well. Subsequently, the developed assay was applied to detect trypsin in serum samples with good accuracy and reproducibility. Thus, this strategy shows great potential application in the clinic for diagnosis of trypsin-indicating diseases as well as the screening of trypsin inhibitor-based anti-cancer drugs.

A turn-on fluorescent probe with high selectivity for Hg2+ and its applications in living cells

A novel fluorescent probe L with a rhodamine B lactam structure modified with 3-methyl-2-thiophenecarboxaldehyde has been prepared based on the thiophilicity of Hg²⁺. The probe L exhibits a unique response with an “off–on-type” mode towards Hg²⁺ among other biologically relevant metal cations. The limit of detection (LOD) for probe L is 1.5 ppb. In addition, in the presence of Hg²⁺, the probe L shows a colorimetric response from colorless to pink. The recognition behavior of probe L towards Hg²⁺ has been investigated by ¹H NMR titration experiments, Job's plot, and MS and IR analyses. As a result, the ligation between the probe and Hg²⁺ leads to the scission of the spirolactam bond of free L and the restoration of its conjugated structure, which can give rise to the fluorescence enhancement of the probe L. Besides, it also can be used as a sensitivity probe in living cells for Hg²⁺ sensing, which can meet various needs in genetic and environmental samples.

A novel colorimetric fluorescent probe with low detection limits has been designed and synthesized, which shows an unique response for Hg²⁺ in living cells.

A sensitive surface-enhanced resonance Raman scattering sensor with bifunctional negatively charged gold nanoparticles for the determination of Cr(VI)

Hexavalent chromium (Cr(VI)) pollution in the water system has seriously endangered human health and the environment. Herein, we propose a rapid, simple and sensitive surface-enhanced resonance Raman scattering (SERRS) sensor with the bifunctional negatively charged gold nanoparticles ((-)AuNPs) which employ as not only the oxidoreductase-like nanozyme but also the substrate to determine Cr(VI). (-)AuNPs effectively promoted the conversion of 3,3',5,5'-tetramethylbenzidine (TMB) into the blue product of 3,3',5,5'-tetramethylbenzidine diamine (oxTMB) in the presence of Cr(VI) and generated a strong SERRS signal at 1611 cm^-1. According to this principle, the Raman intensity difference at 1611 cm^-1 exhibited a satisfactory linear relationship with the logarithm of the Cr(VI) concentration from 10^-5 to 10^-9 M with a low limit of detection (LOD) of 0.4 nM. In addition, the possible SERRS enhancement mechanism, selectivity and reproducibility were also investigated. What's more, the SERRS platform was successfully applied in the complicated water samples, which was anticipated to become a promising analytical method for monitoring of Cr(VI) in the environment.

Hierarchical porous MoS2 particles: excellent multi-enzyme-like activities, mechanism and its sensitive phenol sensing based on inhibition of sulfite oxidase mimics

It is important to exploit highly efficient methods for detecting pollutants selectively and sensitively. Artificial enzymes are promising to replace natural enzymes with diverse functions for sustainable developments and various applications. However, it remains the challenge to develop novel mimic enzymes or multi-enzyme mimics for pollutant detection. Herein we report hierarchical porous MoS₂ particles prepared by a simple hydrothermal method, which demonstrated excellent sulfite oxidase (SuO_x)-, nicotinamide adenine dinucleotide (NADH) oxidase- and superoxide dismutase-mimicking activities. In addition, the catalytic conditions for SuO_x-like and NADH oxidase-like activities of MoS₂ were optimized. The catalytic mechanism of the NADH oxidase mimics is that O₂ involves in the oxidation of NADH, to generate O₂^.- intermediate and finally turn to H₂O₂, while SuO_x mimics comes from that MoS₂ particles can effectively catalyze sulfite to reduce [Fe(CN)₆]^3-. Based on the excellent SuO_x-like activity of MoS₂ particles, while phenol can inhibit the oxidation of sulfite, a phenol colorimetric sensor was explored with the dynamic range of 2-1000 μM and the limit of detection of 0.72 μM, applicable to detect phenol in effluents. Therefore, MoS₂ particles with the SuO_x-like, NADH oxidase-like and SOD-like activities has broad application prospects in environmental monitoring and bio-analysis.

Weak Interaction-Tailored Catalytic Interface of Ultrasmall Gold Nanoclusters as Enzyme Mimics for Enhanced Colorimetric Biosensing

Gold nanoclusters (AuNCs) represent an emerging type of engineered nanomaterials with intrinsic enzymatic activity for both chemical and biological applications, but the catalytic activity of most reported AuNCs remains rather limited. Herein, we report a new, efficient strategy of promoting the peroxidase-mimic activity of AuNCs by tailoring their catalytic interfaces via small molecule-mediated weak interactions. Inspired by the presence of imidazole structures in many biocatalytic centers, we screened a series of imidazole-containing small molecules to evaluate their impact on the enzymatic activity of AuNCs. Through monitoring the absorbance change of 3,3',5,5'-tetramethylbenzidine, 1-methyl-2-imidazolecarboxaldehyde (MCA) was identified to possess the most significant effect on enhancing the peroxidase-mimic activity of glutathione-stabilized AuNCs (GSH-AuNCs) among all the examined molecules. Interestingly, the enhancement effect of MCA on the catalytic activity of these AuNCs was found to be highly reversible and can be switched on/off by simply adding MCA/dialysis treatment. Molecular dynamics simulations and further experimental analysis confirmed that these MCA molecules were adsorbed on the surface of GSH-AuNCs through weak non-covalent interactions. The underlying mechanism analysis suggested that the presence of MCA can efficiently promote the production of •OH in the GSH-AuNC system. As a proof of example, we then demonstrated that the presence of MCA can greatly increase the bioanalytical performance of AuNC-based peroxidase mimics, as evidenced by a 65-fold lower LOD for glucose detection of AuNCs@MCA than that using AuNCs only. Finally, the present system has been successfully applied for sensing the blood glucose level of both healthy people and diabetics with promising results.

CuxO nanorods with excellent regenerable NADH peroxidase mimics and its application for selective and sensitive fluorimetric ethanol sensing

Cu_xO nanorods with excellent NADH peroxidase mimics were synthesized by a simple hydrothermal method. The catalytic oxidation of NADH to NAD cofactor strictly follows the enzymatic kinetics with high catalytic rate and strong affinity. The catalytic mechanism of Cu_xO NRs was that in the presence of hydrogen peroxide, the catalytic oxidizing NADH to NAD ⁺ involving with O₂^.-.anion production, making it realistic to mutually convert between coenzymes. Considering that the mutual transformation of NADH/NAD cofactors plays an important role in biological function, combination of Cu_xO NRs with alcohol dehydrogenase, a highly selective method for fluorimetric detection of ethanol was established. The as-proposed sensing platform is capable of dectecting alcohol with the limit of detection of 26.7 μM (S/N = 3) and applied in practical sample with satisfied accuracy and recovery. The as-developed regenerable NADH peroxidase mimics would also cast lights in biocatalysis, synthetic biology and bioenergy.

Synthesis of Copper Nanocluster and Its Application in Pollutant Analysis

Copper nanoclusters (Cu NCs) with their inherent optical and chemical advantages have gained increasing attention as a kind of novel material that possesses great potential, primarily in the use of contaminants sensing and bio-imaging. With a focus on environmental safety, this article comprehensively reviews the recent advances of Cu NCs in the application of various contaminants, including pesticide residues, heavy metal ions, sulfide ions and nitroaromatics. The common preparation methods and sensing mechanisms are summarized. The typical high-quality sensing probes based on Cu NCs towards various target contaminants are presented; additionally, the challenges and future perspectives in the development and application of Cu NCs in monitoring and analyzing environmental pollutants are discussed.

Copper sulfide nanoclusters with multi-enzyme-like activities and its application in acid phosphatase sensing based on enzymatic cascade reaction

Nanozymes are artificial enzymes, which can substitute natural enzymes for wide range of catalysis-based applications. However, it is challenging to explore novel mimic enzymes or multi-enzyme mimics. Herein we report the facile preparation of uniform CuS nanoclusters (NCs), which possessed outstanding tetra-enzyme mimetic catalytic activities, including peroxidase (POD)-mimics, catalase (CAT)-mimics, ascorbic acid oxidase (AAO)-mimics and superoxide dismutase (SOD)-mimics. The catalytic mechanism of POD-like was coming from the oxygen vacancies of CuS. Furthermore, the steady-state kinetics of POD-, CAT- and AAO mimics of CuS NCs were systematically explored. On basis of the enzymatic cascade reaction that acid phosphatase (ACP) involved in a weak acidic environment, in the presence of O-phenylenediamine, quinoxaline fluorescent substance will be generated. Thus, a fluorescent biosensor platform was proposed for detection of ACP with the linear range of 0.05-25 U L^-1 and limit of detection of 0.01 U L^-1. The as-proposed method was applicable to real serum sample detection accurately and reproducibly. Considering the simple preparation, good stability, excellent multi-enzyme activities and controllable experimental operation, CuS NCs would provide a basis for expanding to other biocatalytic and biomedical applications.

Facile one-pot synthesis of Mn3O4 nanorods and their analytical application

One-pot synthesis of Mn3O4 nanorods in aqueous solution at room temperature without using templates and surfactants was achieved for the first time, opening a new route for preparing various metal nanorods for detecting H2O2-related targets.