Detection of mercury ions based on mercury-induced switching of enzyme-like activity of platinum/gold nanoparticles

Nanozymes for biomedical applications: Multi-metallic systems may improve activity but at the cost of higher toxicity?

Nanozymes are nanomaterials with intrinsic enzyme-like activity with selected advantages over native enzymes such as simple synthesis, controllable activity, high stability, and low cost. These materials have been explored as surrogates to natural enzymes in biosensing, therapeutics, environmental protection, and many other fields. Among different nanozymes classes, metal- and metal oxide-based nanozymes are the most widely studied. In recent years, bi- and tri-metallic nanomaterials have emerged often showing improved nanozyme activity, some of which even possess multifunctional enzyme-like activity. Taking this concept even further, high-entropy nanomaterials, that is, complex multicomponent alloys and ceramics like oxides, may potentially enhance activity even further. However, the addition of various elements to increase catalytic activity may come at the cost of increased toxicity. Since many nanozyme compositions are currently being explored for in vivo biomedical applications, such as cancer therapeutics, toxicity considerations in relation to nanozyme application in biomedicine are of vital importance for translation. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials Diagnostic Tools > Diagnostic Nanodevices.

Development and performance of NLISA for C-reactive protein detection based on Prussian blue nanoparticle conjugates

Prussian blue nanoparticles (PBNPs), also called nanozymes, are very attractive as an alternative to horseradish peroxidase in immunoassay development due to their simple and low-cost synthesis, stability and high catalytic activity. Today, there is a method for highly effective PBNP synthesis based on the reduction of an FeCl3/K3[Fe(CN)6] mixture by hydrogen peroxide. However, there is a lack of research showcasing the use of these highly effective PBNPs for specific target detection in clinical settings, as well as a lack of comprehensive comparisons with conventional methods. To address this gap, we prepared diagnostic reagents based on highly effective PBNPs by modifying them using gelatin and attaching anti-C-reactive protein (CRP) monoclonal antibodies through cross-linking with glutaraldehyde. As a result, a solid-phase colorimetric immunoassay in a sandwich format (nanozyme-linked immunosorbent assay [NLISA]) using highly effective PBNPs as a label for CRP detection has been demonstrated for the first time. The assay demonstrated a detection limit of 21.8 pg/mL, along with acceptable selectivity, precision (CV < 25%) and accuracy (the recovery index was within acceptable limits (75-125%) for LLOQ /ULOQ range. The analytical performance of this method is on par with sensitive assays developed in the last 5 years. Notably, the results obtained from NLISA align with those from an immunofluorescence assay conducted by a certified clinical laboratory. Furthermore, this study underscores the technological challenges involved in constructing an analysis that necessitate further exploration.

A selective colorimetric and efficient removal strategy for mercury(II) in aquatic systems using mesoporous Fe3O4-loaded silver probes

Mesoporous Fe3O4-loaded silver nanocomposites (Fe3O4@Ag) were simply fabricated as bi-functional nanozymes for the catalysis-based detection and removal of Hg2+ ions. It was found that the as-prepared magnetic Fe3O4@Ag could display peroxidase-like catalysis activity that could be rationally enhanced in the presence of Hg2+ ions. To our surprise, the shell of the Ag element may decrease the catalysis of the Fe3O4 to some degree. However, the Ag particles could serve as the probes for specifically recognizing Hg2+ ions and trigger increased catalysis through the formation of Ag-Hg alloys, with a decreased signal background. A high-throughput colorimetric analytical method was thereby developed based on the Fe3O4@Ag catalysis for probing Hg2+ ions in the muscles of fish by using 96-well plates, at linear Hg2+ concentrations ranging from 0.010 to 2.5 mg kg-1. Moreover, the developed colorimetric analytical method was applied to evaluate Hg2+ levels in muscle samples of different kinds of fish. Unexpectedly, an obvious difference of Hg2+ levels in muscles of four kinds of fish was discovered, with the order of snakehead (Ophicephalus argus) > largemouth bass (Micropterus salmoides) > crucian carp (Carassius auratus) > silver carp (Hypophthalmichthys molitrix), where the carnivorous fish showed higher Hg2+ levels than the omnivorous or plant-based ones. Moreover, the as-fabricated Fe3O4@Ag adsorbents with their large specific surface area and high environmental robustness could exhibit efficient Hg2+ adsorption with capacities of up to 397.60 mg g-1. A removal efficiency of 99.40% can also be expected for Hg2+ ions from wastewater, with the magnet-aided recycling of Fe3O4@Ag adsorbents. Such an Fe3O4@Ag-based colorimetric analysis and removal strategy for Hg2+ ions should find wide applications in the fields of aquatic food safety, environmental monitoring, and clinical diagnostics of Hg-poisoning diseases.

Aurozyme: A Revolutionary Nanozyme in Colitis, Switching Peroxidase-Like to Catalase-Like Activity

A therapeutic strategy that could address colitis of multiple etiologies while restoring the dysbiosis of gut microbiota is attractive. Here, Aurozyme, a novel nanomedicine comprised of gold nanoparticles (AuNPs) and glycyrrhizin (GL) with a glycol chitosan coating layer, as a promising approach for colitis, is demonstrated. The unique feature of Aurozyme is the conversion of harmful peroxidase-like activity of AuNPs to beneficial catalase-like activity due to the amine-rich environment provided by the glycol chitosan. This conversion process enables Aurozyme to oxidize the hydroxyl radicals derived from AuNP, producing water and oxygen molecules. In fact, Aurozyme effectively scavenges reactive oxygen/reactive nitrogen species (ROS/RNS) and damage-associated molecular patterns (DAMPs), which can attenuate the M1 polarization of macrophage. It exhibits prolonged adhesion to the lesion site, promoting sustained anti-inflammatory effects and restoring intestinal function in colitis-challenged mice. Additionally, it increases the abundance and diversity of beneficial probiotics, which are essential for maintaining microbial homeostasis in the gut. The work highlights the transformative potential of nanozymes for the comprehensive treatment of inflammatory disease and represents an innovative switching technology of enzyme-like activity by Aurozyme.

User-friendly and ultra-stable all-inclusive gold tablets for cysteamine detection

To date, a range of nanozymes has been reported for their enzyme-mimicking catalytic activity such as solution-based sensors. However, in remote areas, the need for portable, cost-effective, and one-pot prepared sensors is obvious. In this study, we report the development of a highly stable and sensitive gold tablet-based sensor for cysteamine quantification in human serum samples. The sensor is produced in two steps: synthesis of a pullulan-stabilized gold nanoparticle solution (pAuNP-Solution) using a pullulan polymer as a reducing, stabilizing, and encapsulating agent and then, casting the pAuNP-Solution into a pullulan gold nanoparticle tablet (pAuNP-Tablet) by a pipetting method. The tablet was characterized by UV-vis, DLS, FTIR, TEM, and AFM analyses. The pAuNP-tablet exhibited a high peroxidase-mimetic activity via a TMB-H₂O₂ system. The presence of cysteamine in the system introduced two types of inhibition which were dependent on the cysteamine concentration. By determining Michaelis-Menten's kinetic parameters, we gained mechanistic insights into the catalytic inhibition process. Based on the catalytic inhibition capability of cysteamine, the limit of detection (LoD) was calculated to be 69.04 and 82.9 μM in buffer and human serum samples, respectively. Finally, real human serum samples were tested, demonstrating the applicability of the pAuNP-Tablet for real-world applications. The % R values in human serum samples were in the range of 91-105% with % RSD less than 2% for all replicas. The stability tests over 16 months revealed the ultra-stable properties of the pAuNP-Tablet. Overall, with a simple fabrication method and a novel employed technique, this study contributes to the advancement of tablet-based sensors and helps in cysteamine detection in clinical settings.

Multibranched Au-Ag-Pt Nanoparticle as a Nanozyme for the Colorimetric Assay of Hydrogen Peroxide and Glucose

Many studies have recently produced artificial enzymes with metal nanoparticles (NPs) to overcome the limitations of natural enzymes, such as low stability, high cost, and storage problems. In particular, gold NPs exhibit peroxidase-like activity and are strongly influenced by external parameters, such as pH, temperature, size, shape, and functional layer, which change the enzyme activity. Here, chitosan-capped multibranched Au-Ag-Pt NPs (CCNPs) that mimic peroxidase were synthesized using various peroxidase-mimicking strategies. The results demonstrated that enzyme activity sequentially increased because of the multibranched Au-Ag NPs coated with Pt and chitosan. The enzyme activity of the particle was evaluated through the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), which causes a color change into blue. This change was observable with the naked eye and could be used practically. The color change depended on the concentration of hydrogen peroxide (H₂O₂), and it was shown that the CCNPs could be applied to measure H₂O₂ with a limit of detection (LOD) of 0.054 mM. Furthermore, with glucose oxidase, the CCNPs can be used for glucose detection with an LOD of 0.289 mM. Also, the potential of the CCNP application in human serum was shown through the serum test. Thus, this study suggested the utilization of the multibranched Au-Ag-Pt NPs that mimic the peroxidase activity of natural enzymes and the possibility of application in various biological analyses.

Cu Nanoparticle-Based Solution and Paper Strips for Colorimetric and Visual Detection of Heavy Metal Ions

The intrinsic toxicity of heavy metal ions to human health or other species calls for the need to develop an analytical tool for the easy and rapid detection of these ions based on inexpensive and stable nanomaterials. This article describes the potential utility of stable Cu nanoparticles (CuNPs) in the detection of toxic metal ions by solution and paper strip-based methods. For this, first, a dodecyl sulfate ion-stabilized CuNP (DS-CuNP) colloid was synthesized by a chemical reduction method. This was followed by treating the dispersion with heavy metal ions and monitoring the spectral change by spectrophotometric and colorimetric techniques. Among a host of metal ions, Hg²⁺, Cd²⁺, and Pb²⁺ have been found to significantly affect the surface plasmon resonance band of CuNPs by concomitantly altering the color of its solution. Notably, the brownish color of CuNP solution changed readily to milky white in the presence of Hg²⁺. Furthermore, the fabricated brownish-yellow test paper strips containing DS-CuNPs transformed to a prominent white color in the presence of a few drops of Hg²⁺ solution. This change in color of the paper strips could be visually detected by the naked eye. The experiments involving the detection of the various ions were carried out by optimizing the experimental conditions qualitatively as well as quantitatively. The limit of detection of the analytes (metal ions) has been found to be 10 μM. Routine analytical techniques like UV-vis spectroscopy, dynamic light scattering, transmission electron microscopy, and Fourier transform infrared spectroscopy formed part of the experiments.

Gold nanoparticles as adaptogens increazing the freezing tolerance of wheat seedlings

The intensive development of nanotechnology led to the widespread application of various nanoparticles and nanomaterials. As a result, nanoparticles enter the environment and accumulate in ecosystems and living organisms. The consequences of possible impact of nanoparticles on living organisms are not obvious. Experimental data indicate that nanoparticles have both toxic and stimulating effects on organisms. In this study, we demonstrated for the first time that gold nanoparticles can act as adaptogens increasing plant freezing tolerance. Priming winter wheat (Triticum aestivum L., var. Moskovskaya 39, Poaceae) seeds for 1 day in solutions of gold nanoparticles (15-nm diameter, concentrations of 5, 10, 20, and 50 µg/ml) led to an increase in freezing tolerance of 7-day-old wheat seedlings. A relationship between an increase in wheat freezing tolerance and changes in some important indicators for its formation-growth intensity, the activity of the photosynthetic apparatus and oxidative processes, and the accumulation of soluble sugars in seedlings-was established. Assumptions on possible mechanisms of gold nanoparticles effects on plant freezing tolerance are discussed.

Nanoparticles in the diagnosis and treatment of vascular aging and related diseases

Aging-induced alternations of vasculature structures, phenotypes, and functions are key in the occurrence and development of vascular aging-related diseases. Multiple molecular and cellular events, such as oxidative stress, mitochondrial dysfunction, vascular inflammation, cellular senescence, and epigenetic alterations are highly associated with vascular aging physiopathology. Advances in nanoparticles and nanotechnology, which can realize sensitive diagnostic modalities, efficient medical treatment, and better prognosis as well as less adverse effects on non-target tissues, provide an amazing window in the field of vascular aging and related diseases. Throughout this review, we presented current knowledge on classification of nanoparticles and the relationship between vascular aging and related diseases. Importantly, we comprehensively summarized the potential of nanoparticles-based diagnostic and therapeutic techniques in vascular aging and related diseases, including cardiovascular diseases, cerebrovascular diseases, as well as chronic kidney diseases, and discussed the advantages and limitations of their clinical applications.

Synthesis of Gold-Platinum Core-Shell Nanoparticles Assembled on a Silica Template and Their Peroxidase Nanozyme Properties

Bimetallic nanoparticles are important materials for synthesizing multifunctional nanozymes. A technique for preparing gold-platinum nanoparticles (NPs) on a silica core template (SiO₂@Au@Pt) using seed-mediated growth is reported in this study. The SiO₂@Au@Pt exhibits peroxidase-like nanozyme activity has several advantages over gold assembled silica core templates (SiO₂@Au@Au), such as stability and catalytic performance. The maximum reaction velocity (V_max) and the Michaelis–Menten constants (K_m) were and 2.1 × 10⁻¹⁰ M⁻¹∙s⁻¹ and 417 µM, respectively. Factors affecting the peroxidase activity, including the quantity of NPs, solution pH, reaction time, and concentration of tetramethyl benzidine, are also investigated in this study. The optimization of SiO₂@Au@Pt NPs for H₂O₂ detection obtained in 0.5 mM TMB; using 5 µg SiO₂@Au@Pt, at pH 4.0 for 15 min incubation. H₂O₂ can be detected in the dynamic liner range of 1.0 to 100 mM with the detection limit of 1.0 mM. This study presents a novel method for controlling the properties of bimetallic NPs assembled on a silica template and increases the understanding of the activity and potential applications of highly efficient multifunctional NP-based nanozymes.

Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

Nanomaterial-based artificial enzymes (or nanozymes) have attracted great attention in the past few years owing to their capability not only to mimic functionality but also to overcome the inherent drawbacks of the natural enzymes. Numerous advantages of nanozymes such as diverse enzyme-mimicking activities, low cost, high stability, robustness, unique surface chemistry, and ease of surface tunability and biocompatibility have allowed their integration in a wide range of biosensing applications. Several metal, metal oxide, metal-organic framework-based nanozymes have been exploited for the development of biosensing systems, which present the potential for point-of-care analysis. To highlight recent progress in the field, in this review, more than 260 research articles are discussed systematically with suitable recent examples, elucidating the role of nanozymes to reinforce, miniaturize, and improve the performance of point-of-care diagnostics addressing the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to the end user) criteria formulated by World Health Organization. The review reveals that many biosensing strategies such as electrochemical, colorimetric, fluorescent, and immunological sensors required to achieve the ASSURED standards can be implemented by using enzyme-mimicking activities of nanomaterials as signal producing components. However, basic system functionality is still lacking. Since the enzyme-mimicking properties of the nanomaterials are dictated by their size, shape, composition, surface charge, surface chemistry as well as external parameters such as pH or temperature, these factors play a crucial role in the design and function of nanozyme-based point-of-care diagnostics. Therefore, it requires a deliberate exertion to integrate various parameters for truly ASSURED solutions to be realized. This review also discusses possible limitations and research gaps to provide readers a brief scenario of the emerging role of nanozymes in state-of-the-art POC diagnosis system development for futuristic biosensing applications.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

L-Cysteine as an Irreversible Inhibitor of the Peroxidase-Mimic Catalytic Activity of 2-Dimensional Ni-Based Nanozymes

The ability to modulate the catalytic activity of inorganic nanozymes is of high interest. In particular, understanding the interactions of inhibitor molecules with nanozymes can bring them one step closer to the natural enzymes and has thus started to attract intense interest. To date, a few reversible inhibitors of the nanozyme activity have been reported. However, there are no reports of irreversible inhibitor molecules that can permanently inhibit the activity of nanozymes. In the current work, we show the ability of L-cysteine to act as an irreversible inhibitor to permanently block the nanozyme activity of 2-dimensional (2D) NiO nanosheets. Determination of the steady state kinetic parameters allowed us to obtain mechanistic insights into the catalytic inhibition process. Further, based on the irreversible catalytic inhibition capability of L-cysteine, we demonstrate a highly specific sensor for the detection of this biologically important molecule.

Co-reactant-free self-enhanced solid-state electrochemiluminescence platform based on polyluminol-gold nanocomposite for signal-on detection of mercury ion

Development of a self-enhanced solid-state ECL platform creates a straightforward experimental design for the fabrication of point-of-care applications. Herein, we develop a promising method for self-enhanced solid-state ECL platform of polyluminol gold nanocomposite on glassy carbon electrode [(PL-Au)_nano/GCE] via simple one-step electrochemical deposition process without involving any additional co-reactants. The presence of gold nanoparticles (AuNPs) augments the electron transfer kinetics of PL (polyluminol) and enhances the solid-state ECL intensity and promotes label-free, excellent sensitivity, and selectivity to detect Hg²⁺ in physiological pH through signal-on mode. Unlike pristine PL/GCE, electrochemically co-deposited AuNPs in the (PL-Au)_nano/GCE composite, enable the co-reactant accelerator by improving the catalytic activity of PL towards oxygen reduction reaction (ORR) yielding in-situ ROS (co-reactant) generation. Further, the ECL intensity of (PL-Au)_nano/GCE composite, gradually increases with each addition of Hg²⁺ ion. This is because of the formation of an amalgamation of Au-Hg on (PL-Au)_nano/GCE composite surface which further accelerates the yield of in-situ ROS and enhances the intensity of ECL. Whereas no ECL signals changes were observed for PL/GCE composite. The proposed self-enhanced solid-state ECL platform is selectively sensing the Hg²⁺ ion in the linear range of 0.3–200 nM with a detection limit of 0.1 nM. The demonstrated (PL-Au)_nano/GCE platform might pave new avenues for further studies in the solid-state ECL platform which could be more useful in on-site monitoring of clinical bioassay and immunosensors.

Colorimetric determination of cysteine based on inhibition of GSH-Au/Pt NCs as peroxidase mimic

In this work, we reported a facile and highly sensitive strategy for colorimetric detection of cysteine (Cys) based on the inhibition of catalytic activity of bimetallic nanoclusters induced by Cys. Glutathione-modified gold-platinum nanoclusters (GSH-Au/Pt NCs) with different Au/Pt molar ratios were prepared via one-pot approach and utilized as peroxidase mimics to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂. It has been found that Cys could inhibit the peroxidase-like activity of GSH-Au/Pt NCs efficiently, which leads to a decrease of the absorption intensity of the system at 652 nm with a fading of the blue color. These findings provide a worthy method for visualization and quantitative detection of Cys with different concentrations in the range from 0.5 to 30 μM, and the detection limit is 0.154 μM. Moreover, this method displays a promising application in colorimetric analysis of Cys in urine samples.

A colorimetric sensor for detecting thiourea based on inhibiting peroxidase-like activity of gold-platinum nanoparticles

In this study, gold-platinum nanoparticles (Au@PtNPs) with peroxidase-like activity were synthesized. In the absence of thiourea (TU), the Au@PtNPs can catalyze the decomposition of hydrogen peroxide, and oxidize 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB, colorless) into oxidized 3,3',5,5'-tetramethylbenzidine dihydrochloride (oxTMB, blue). The peroxidase-like activity of the Au@PtNPs is inhibited in the presence of TU, and TMB cannot be oxidized to oxTMB effectively, and no blue color could be observed. Based on this finding, a novel colorimetric sensor for detecting TU is proposed. The absorbance response curve showed a good linearity for the concentration of TU in the range of 10 nmol L^-1 to 10 μmol L^-1 with a correlation coefficient of R² = 0.999, and the limit of detection is 9.57 nmol L^-1. The colorimetric sensor possesses excellent selectivity, anti-interference ability, and application value in actual samples.

A review on metal nanozyme-based sensing of heavy metal ions: Challenges and future perspectives

Large scale mining, manufacturing industries, exploitation of underground water, depletion of groundwater level, and uncontrolled discharge of industrial wastes have caused severe heavy metal ion pollution to the environment throughout the world. Therefore, the rapid detection of such toxic metal ions is inevitable. However, conventional methods require sophisticated instruments and skilled manpower and are difficult to operate in on-field conditions. Recently, metal nanozyme-based assays have been found to have the potential as an alternative to conventional methods due to their portability, simplicity, and high sensitivity to detect metal ion concentration to as low as parts per trillion (ppt). Metal nanozyme-based systems for heavy metal ions enable rapid and cheap screening on the spot with a very simple instrument such as a UV-vis absorption spectrophotometer and therefore, are convenient for use in field operations, especially in remote parts of the world. The sensing mechanism of a nanozyme-based sensor is highly dependent on its surface properties and specific interactions with particular metal ion species. Such method often encounters selectivity issues, unlike natural enzyme-based assays. Therefore, in this review, we mainly focus our discussion on different types of target recognition and inhibition/enhancement mechanisms, and their responses toward the catalytic activity in the sensing of target metal ions, design strategies, challenges, and future perspectives.

Green light-emitting BSA-conjugated dye supported silica nanoparticles for bio-imaging applications

BSA conjugated with amine functionalised silica nanoparticles (BSA@DSFN) proved to be an ideal material for long life fluorescent probe for cellular imaging application.

The Mimic Enzyme Properties of Au@PtNRs and the Detection for Ascorbic Acid Based on Their Catalytic Properties

Being superior to natural enzymes, nanoenzymes are drawing a great deal of attention in the field of biosensing. Herein, we developed an ultrasensitive, stable and selective colorimetric assay having dual functionalities of Au-tipped Pt nanorods (NRs). The optical and catalytic properties of Au-tipped Pt NRs were monitored using a spectrophotometer and the chromogenic substrate 3, 3′, 5, 5′-tetramethylbenzidine (TMB) in the presence of H2O2, respectively. We found that Au-tipped Pt NRs exhibited excellent peroxidase-like activity, which decomposed hydrogen peroxide (H2O2) into oxygen (O2). The produced O2 oxidized the chromogenic substrate into a blue color product. The oxidation rate of the chromogenic substrate could be monitored using a spectrophotometer at 652 nm. Notably, the peroxidase-like activity of Au-tipped Pt NRs decreased in the presence of ascorbic acid (AA). The produced O2 preferentially reacted with AA, generating ascorbyl radicals (AA·) instead of oxidizing TMB, and thereby decreased the oxidation rate of TMB. Based on this inhibitory property, a selective colorimetric assay was developed using Au-tipped Pt NRs for the detection of AA. This work offers a novel detection method for AA.

Gold Nanozymes: From Concept to Biomedical Applications

In recent years, gold nanoparticles have demonstrated excellent enzyme-mimicking activities which resemble those of peroxidase, oxidase, catalase, superoxide dismutase or reductase. This, merged with their ease of synthesis, tunability, biocompatibility and low cost, makes them excellent candidates when compared with biological enzymes for applications in biomedicine or biochemical analyses. Herein, over 200 research papers have been systematically reviewed to present the recent progress on the fundamentals of gold nanozymes and their potential applications. The review reveals that the morphology and surface chemistry of the nanoparticles play an important role in their catalytic properties, as well as external parameters such as pH or temperature. Yet, real applications often require specific biorecognition elements to be immobilized onto the nanozymes, leading to unexpected positive or negative effects on their activity. Thus, rational design of efficient nanozymes remains a challenge of paramount importance. Different implementation paths have already been explored, including the application of peroxidase-like nanozymes for the development of clinical diagnostics or the regulation of oxidative stress within cells via their catalase and superoxide dismutase activities. The review also indicates that it is essential to understand how external parameters may boost or inhibit each of these activities, as more than one of them could coexist. Likewise, further toxicity studies are required to ensure the applicability of gold nanozymes in vivo. Current challenges and future prospects of gold nanozymes are discussed in this review, whose significance can be anticipated in a diverse range of fields beyond biomedicine, such as food safety, environmental analyses or the chemical industry.

Protein-mediated wool-ball-like copper sulfide as a multifunctional nanozyme for dual fluorescence "turn-on" sensors of cysteine and silver ions

Developing a multifunctional nanozyme based biosensor with a convenient approach and high reliability is of vital interest for multiplex detection. In this study, wool-ball-like copper sulfide (WBLCS) was obtained facilely using an amphiphilic protein. The acidic amino acid residues and the amphiphilic properties of protein molecules play cooperative roles in the fabrication of hierarchical nanostructures. Unlike copper sulfide with irregular morphologies, the single component WBLCS acts as a multifunctional nanozyme possessing both superior cysteine oxidase- and peroxidase-mimicking activity. Fascinatingly, the addition of silver ions can significantly enhance the performance of the cascade system at a very low fluorescence substrate concentration. Based on this, dual fluorescence "turn on" sensors of cysteine and silver ions with extremely high sensitivity and selectivity are developed. This is the first report to explore multiple fluorescence "turn on" sensing systems based on one single nanozyme. Hence, the present finding has significant implication towards the design of superstructured nanozymes combining different multi-functionalities at the nanoscale for sensing multiplex target molecules sensitively and selectively in practice.

Pleurotus sajor-caju-Mediated Synthesis of Silver and Gold Nanoparticles Active against Colon Cancer Cell Lines: A New Era of Herbonanoceutics

Herbal medicines are widely used worldwide and much appreciated because of their fewer side effects and the ability to fight diseases at the root cause. Active ‘phyto’ ingredients require a scientific approach and a mechanism to distribute components at the target site for better therapeutic results. Nanotechnology, on the other hand, has created new hope for cancer treatment but is still far from being proven in clinical settings. This article combines a unique approach to synthesis with the use of Pleurotus sajor-caju, followed by microwave irritation of silver and gold nanoparticles that ensures the capping of the active phyto ingredient and further enhances the effects of nanomedicine to fight colon cancer, thus opening a new era of what we call herbonanoceutics. The article also compares the characteristics and properties of silver (Au) and gold (Ag) nanoparticles synthesized by an in house developed novel microwave-assisted rapid green synthesis method. The as-prepared Ag NPs and Au NPs were compared using ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Our comparative study revealed that both assemblies display face-centred cubic structures (FCCs) and are nanocrystalline in nature. The advantage of the approach was that the sizes of gold and silver were identical in range with a similar distribution pattern. This has helped us to study the activity against colon cancer cell line (HCT-116) without incoherence since size plays a key role in the application. More specifically, morphological changes, cell viability, the production of reactive oxygen species (ROS) and the fragmentation of DNA have been further reported to assess better the results obtained with the two metals. Our results suggest that the newly adopted synthesis method may ensure the dual benefits from phyto ingredients which further enhances the effectiveness of advanced nanomedicine.

Triggered peroxidase-like activity of Au decorated carbon dots for colorimetric monitoring of Hg2+ enrichment in Chlorella vulgaris

Developing a rapid, low-cost, and multimode detection method for heavy metal ions remains a compelling goal for many applications, including food safety, environmental and biological analysis. This study investigated the influence of Hg²⁺ on the peroxidase-like activity of gold nanoparticles (GNPs) decorated on carbon dots (CDs) from lysine (denoted as GNP@CDs). A new type of Hg²⁺-triggered peroxidase-like activity of GNP@CDs was discovered, which could catalyze the oxidation of the colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue TMB. Based on the regulation of the catalytically triggered activity, a sensitive colorimetric method for the detection of Hg²⁺ was developed, with a linear range of 7-150 nM, providing a limit of detection as low as 3.7 nM. The sensor is simple and rapid, and was successfully applied to the detection of Hg²⁺ enrichment in chlorella, suggesting a promising application in biological analysis.

Cobalt-based metal organic frameworks: a highly active oxidase-mimicking nanozyme for fluorescence “turn-on” assays of biothiol

With Co-MOFs as an oxidase-mimicking nanozyme, the AR oxidized product, non-fluorescent resazurin could be reduced to fluorescent resorufin by l-cysteine, which is specifically applied for fluorescence “turn-on” detection of l-cysteine.

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.

Protein-protected metal nanoclusters: An emerging ultra-small nanozyme

Protein-protected metal nanoclusters (MNCs), typically consisting of several to a hundred metal atoms with a protein outer layer used for protecting clusters from aggregation, are excellent fluorescent labels for biomedical applications due to their extraordinary photoluminescence, facile synthesis and good biocompatibility. Interestingly, many protein-protected MNCs have also been reported to exhibit intrinsic enzyme-like activities, namely peroxidase, oxidase and catalase activities, and are consequently used for biological analysis and environmental treatment. These findings have extended the horizon of protein-protected MNCs' properties as well as their application in various fields. Furthermore, in the field of nanozymes, protein-protected MNCs have emerged as an outstanding new addition. Due to their ultra-small size (<2 nm), they usually have higher catalytic activity, more suitable size for in vivo application, better biocompatibility and photoluminescence in comparison with large size nanozymes. In this review, we will systematically introduce the significant advances in this field and critically discuss the challenges that lie ahead. Ultra-small nanozymes based on protein-protected MNCs are on the verge of attracting great interest across various disciplines and will stimulate research in the fields of nanotechnology and biology. This article is characterized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

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.

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.

TiO2 Nanowires from Wet-Corrosion Synthesis for Peptide Sequencing Using Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

In this work, TiO₂ nanowires synthesized from a wet-corrosion process were presented for peptide sequencing by photocatalytic reaction with UV radiation. For the photocatalytic decomposition of peptides, the peptide sample was dropped on a target plate containing synthesized TiO₂ nanowire zones and UV-irradiated. Subsequently, the target plate was analyzed by laser desorption/ionization time-of-flight (LDI-TOF) mass spectrometry using the synthesized TiO₂ nanowires as a solid matrix. The feasibility of peptide sequencing based on the photocatalytic reaction with the synthesized TiO₂ nanowires was demonstrated using six types of peptides GHP9 (G₁-H-P-Q-G₂-K₁-K₂-K₃-K₄, 1006.59 Da), BPA-1(K₁-S₁-L-E-N-S₂-Y-G₁-G₂-G₃-K₂-K₃-K₄, 1394.74 Da), PreS1(F₁-G-A-N₁-S-N₂-N₃-P₁-D₁-W-D₂-F₂-N₄-P₂-N₅, 1707.68 Da), HPQ peptide-1 (G-Y-H-P-Q-R-K, 884.45 Da), HPQ peptide-2 (K-R-H-P-Q-Y-G, 884.45 Da), and HPQ peptide-3 (R-Y-H-P-Q-G-K, 884.45 Da). The identification of three different peptides with the same molecular weight was also demonstrated by using the synthesized TiO₂ nanowires for their photocatalytic decomposition as well as for LDI-TOF mass spectrometry as a solid-matrix.

Protamine-gold nanoclusters as peroxidase mimics and the selective enhancement of their activity by mercury ions for highly sensitive colorimetric assay of Hg(II)

We certify that protamine-gold nanoclusters (PRT-AuNCs) synthesized by one-pot method exhibit peroxidase-like activity. The catalytic activity of PRT-AuNCs followed typical Michaelis-Menten kinetics and exhibited higher affinity to 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate compared to that of natural horseradish peroxidase. Meanwhile, we found that Hg(II) could dramatically and selectively enhance the peroxidase-like activity of PRT-AuNCs, and the enhanced mechanism by Hg(II) was demonstrated to be generation of the cationic Au species and the partly oxidized Au species (Au^δ+) by Hg²⁺-Au⁰/Au⁺ interaction. Based on this finding, quantitative determinations of Hg(II) via visual observation and absorption spectra were achieved. The proposed strategy displays high selectivity that arises from the strong aurophilic interaction of mercury towards gold. Moreover, the developed method is highly sensitive with a wide linear range and low detection limit of 1.16 nM. This strategy is not only helpful to develop effective nanomaterials-based artificial enzyme mimics but also irradiative to discover new applications of artificial mimic enzymes in bio-detection, medical diagnostics, and biotechnology. Graphical abstract Protamine-gold nanoclusters (PRT-AuNCs) synthesized by one-pot method exhibit peroxidase-like activity. Hg(II) can stimulate the peroxidase-like activity of PRT-AuNCs selectively, enhancing their ability to catalyze the chromogenic reaction of TMB by H₂O₂.

Platinum nanoparticles in nanobiomedicine

Oxidative stress-dependent inflammatory diseases represent a major concern for the population's health worldwide. Biocompatible nanomaterials with enzymatic properties could play a crucial role in the treatment of such pathologies. In this respect, platinum nanoparticles (PtNPs) are promising candidates, showing remarkable catalytic activity, able to reduce the intracellular reactive oxygen species (ROS) levels and impair the downstream pathways leading to inflammation. This review reports a critical overview of the growing evidence revealing the anti-inflammatory ability of PtNPs and their potential applications in nanomedicine. It provides a detailed description of the wide variety of synthetic methods recently developed, with particular attention to the aspects influencing biocompatibility. Special attention has been paid to the studies describing the toxicological profile of PtNPs with an attempt to draw critical conclusions. The emerging picture suggests that the material per se is not causing cytotoxicity, while other physicochemical features related to the synthesis and surface functionalization may play a crucial role in determining the observed impairment of cellular functions. The enzymatic activity of PtNPs is also summarized, analyzing their action against ROS produced by pathological conditions within the cells. In particular, we extensively discuss the potential of these properties in nanomedicine to down-regulate inflammatory pathways or to be employed as diagnostic tools with colorimetric readout. A brief overview of other biomedical applications of nanoplatinum is also presented.

Highly active fluorogenic oxidase-mimicking NiO nanozymes

NiO nanoparticles can quickly catalyze oxidation of Amplex red to produce fluorescent products for intracellular imaging, much more efficiently than other types of tested nanozymes.

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.

Nanozyme-based bio-barcode assay for high sensitive and logic-controlled specific detection of multiple DNAs

Since HCV and HIV share a common transmission path, high sensitive detection of HIV and HCV gene is of significant importance to improve diagnosis accuracy and cure rate at early stage for HIV virus-infected patients. In our investigation, a novel nanozyme-based bio-barcode fluorescence amplified assay is successfully developed for simultaneous detection of HIV and HCV DNAs with excellent sensitivity in an enzyme-free and label-free condition. Here, bimetallic nanoparticles, PtAu_NPs, present outstanding peroxidase-like activity and act as barcode to catalyze oxidation of nonfluorescent substrate of amplex red (AR) into fluorescent resorufin generating stable and sensitive "Turn On" fluorescent output signal, which is for the first time to be integrated with bio-barcode strategy for fluorescence detection DNA. Furthermore, the provided strategy presents excellent specificity and can distinguish single-base mismatched mutant from target DNA. What interesting is that cascaded INHIBIT-OR logic gate is integrated with biosensors for the first time to distinguish individual target DNA from each other under logic function control, which presents great application in development of rapid and intelligent detection.

AuPt Alloy Nanostructures with Tunable Composition and Enzyme-like Activities for Colorimetric Detection of Bisulfide

Tuning the enzyme-like activity and studying the interaction between biologically relevant species and nano-enzymes may facilitate the applications of nanostructures in mimicking natural enzymes. In this work, AuPt alloy nanoparticles (NPs) with varying compositions were prepared through a facile method by co-reduction of Au3+ and Pt2+ in aqueous solutions. The composition could be tuned easily by adjusting the molar ratios of added Pt2+ to Au3+. It was found that both peroxidase-like and oxidase-like activity of AuPt alloy NPs were highly dependent on the alloy compositions, which thus suggesting an effective way to tailor their catalytic properties. By investigating the inhibitory effects of HS- on the enzyme-like activity of AuPt alloy NPs and natural enzyme, we have developed a method for colorimetric detection of HS- and evaluation of the inhibiting effects of inhibitors on natural and artificial enzymes. In addition, the responsive ability of this method was influenced largely by the composition: AuPt alloy NPs show much lower limit of detection for HS- than Pt NPs while Pt NPs show wider linear range than AuPt alloy NPs. This study suggests the facile way not only for synthesis of alloy nanostructures, but also for tuning their catalytic activities and for use in bioanalysis.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.