Formation of PdPt alloy nanodots on gold nanorods: tuning oxidase-like activities via composition

A low iridium content greatly improves the peroxidase-like activity of noble metal nanozymes for sensitive colorimetric detection

The enzyme-like activity of noble metal nanomaterials has been widely demonstrated. However, as an important noble metal, iridium (Ir) and its alloy nanomaterials have been less studied, particularly regarding the effect of Ir content on enzyme-like activity. Here, we demonstrated for the first time that a low Ir content can greatly improve the peroxidase-like activity of Pt-based nanozymes. When the weight percentage of Ir was 3.45% in trimetallic PtAuIr hollow nanorods (HNRs) and 2.86% in bimetallic PtIr HNRs, their specific activity increased by approximately 70% compared to their PtAu and Pt counterparts, respectively. However, a slightly higher percentage of Ir significantly diminished the enhancement effect on their specific activity. Density functional theory (DFT) calculations show that the rate-determining step (RDS) energy barrier of the nanozyme with low Ir content is lower than that of the nanozyme with slightly higher Ir content. Furthermore, we studied the kinetic properties of the PtAuIr nanozyme using TMB as the substrate. Its Michaelis-Menten constant (Km) and Vmax were 1.756 mM and 2.152 × 10-6 M s-1, respectively. Additionally, a colorimetric detection platform based on the PtAuIr nanozyme was established and applied to detect o-phenylenediamine (OPD), with a detection limit as low as 0.076 μM. This study highlights the important role of the Ir content in Pt-based nanozymes and demonstrates that PtAuIr nanozymes have potential applications in environmental detection.

A Pt1Pd Single-Atom Alloy Nanozyme with Boosted Enzyme-Like Activity for Efficient Photo-Mediated Tumor Therapy

Single-atom nanozymes (SAzymes) are emerging natural enzyme mimics and have attracted much attention in the biomedical field. SAzymes with Metal─Nx sites designed on carbon matrixes are currently the mainstream in research. It is of great significance to further expand the types of SAzymes to enrich the nanozyme library. Single-atom alloys (SAAs) are a material in which single-atom metal sites are dispersed onto another active metal matrix, and currently, there is limited research on their enzyme-like catalytic performance. In this work, a biodegradable Pt1Pd SAA is fabricated via a simple galvanic replacement strategy, and for the first time reveals its intrinsic enzyme-like catalytic performance including catalase-, oxidase-, and peroxidase-like activities, as well as its photodynamic effect. Experimental characterizations demonstrate that the introduction of single-atom Pt sites contributes to enhancing the affinity of Pt1Pd single-atom alloy nanozyme (SAAzyme) toward substrates, thus exhibiting boosted catalytic efficiency. In vitro and in vivo experiments demonstrate that Pt1Pd SAAzyme exhibits a photo-controlled therapeutic effect, with a tumor inhibition rate of up to 100%. This work provides vital guidance for opening the research direction of SAAs in enzyme-like catalysis.

Enhanced the intrinsic oxidase-like activity of chiral carbon dots via manganese doping for selective catalytic oxidation

It is highly desirable to design and construct chemical catalysts with high activity and specificity as the alternatives of natural enzymes for industrial application. Chiral carbon dots (CDs), possessing both the intrinsic enzyme-like activity and specific recognition ability, are one of good candidates for enzyme-like catalysts. However, their catalytic activity is far from that of natural enzymes and needs to be enhanced. In this work, the modulation of the chiral structure and catalytic activity of chiral CDs with intrinsic oxidase-like activity was implemented by manganese (Mn) doping. Under the light condition, chiral CDs (l-Ser-CDs and d-Ser-CDs) derived from chiral serine (Ser) show weak catalytic activity and low selectivity toward the oxidation of L type of dopamine (l-DOPA), whereas the Mn functionalized chiral CDs (l-Mn-CDs or d-Mn-CDs) exhibit 6.9 times higher in catalytic activity and 2.9 times in selectivity ratio (SR) than Ser-CDs. Mn-CDs involve two-path catalytic process, in which the photogenerated electrons could reduce O2 to O2- as the active species and the holes would oxidize DOPA directly. Moreover, doping of Mn enables the CDs to generate more O2-. Besides, l-Mn-CDs have higher catalytic activity than that of d-Mn-CDs (+54.2 %), and the chiral Mn-CDs have stronger selective adsorption capacity towards chiral DOPA than Ser-CDs. Our work provides a new method for designing and preparing novel chiral artificial enzymes.

Reaction Mechanisms and Kinetics of Nanozymes: Insights from Theory and Computation

"Nanozymes" usually refers to inorganic nanomaterials with enzyme-like catalytic activities. The research into nanozymes is one of the hot topics on the horizon of interdisciplinary science involving materials, chemistry, and biology. Although great progress has been made in the design, synthesis, characterization, and application of nanozymes, the study of the underlying microscopic mechanisms and kinetics is still not straightforward. Density functional theory (DFT) calculations compute the potential energy surfaces along the reaction coordinates for chemical reactions, which can give atomistic-level insights into the micro-mechanisms and kinetics for nanozymes. Therefore, DFT calculations have been playing an increasingly important role in exploring the mechanisms and kinetics for nanozymes in the past years. The calculations either predict the microscopic details for the catalytic processes to complement the experiments or further develop theoretical models to depict the physicochemical rules. In this review, the corresponding research progress is summarized. Particularly, the review focuses on the computational studies that closely interplay with the experiments. The relevant experimental results without DFT calculations will be also briefly discussed to offer a historic overview of how the computations promote the understanding of the microscopic mechanisms and kinetics of nanozymes.

One-Pot Synthesis of MnOx-SiO2 Porous Composites as Nanozymes with ROS-Scavenging Properties

The development of nanomaterials that mimic the activity of enzymes is a topic of interest, for the decomposition of reactive oxygen species (ROS). We report the preparation of a novel nanocomposite of MnO_x needles covered with SiO₂ porous material. The material was prepared in one pot with a two-step procedure. The material was characterized by EDX, SEM, TEM, XRD, nitrogen adsorption-desorption isotherms, and XPS. The synthesis protocol took advantage of the atrane method, favoring the nucleation and initial growth of manganese oxide needles that remained embedded and homogeneously dispersed in a mesoporous silica matrix. The final composite had a high concentration of Mn (Si/Mn molar ratio of ca. 1). The nanozyme presented bimodal porosity: intraparticle and interparticle association with the surfactant micelles and the gaps between silica particles and MnO_x needles, respectively. The porosity favored the migration of the reagent to the surface of the catalytic MnO_x. The nanozyme showed very efficient SOD and catalase activities, thus improving other materials previously described. The kinetics were studied in detail, and the reaction mechanisms were proposed. It was shown that silica does not play an innocent role in the case of catalase activity, increasing the reaction rate.

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.

Mesoporous Core-Shell Pd@Pt Nanospheres as Oxidase Mimics with Superhigh Catalytic Efficiency at Room Temperature

Mesoporous Pt-Pd bimetallic core-shell nanospheres (mPd@Pt NSs) with palladium-rich cores and platinum-rich shells were synthesized via a simple, two-step, wet chemical strategy mediated by nitrogen-doped carbon dots. The BET surface area of mPd@Pt NSs was found to be 210.4 m²·g^-1, which is significantly higher than the currently reported unsupported Pt-based nanomaterials. Because of the large active surface area, the as-prepared mPd@Pt NSs show superhigh oxidase activity and exhibit excellent oxidase-like catalytic efficiency with a catalytic constant (K_cat) as high as 2.1 × 10³ s^-1 at room temperature, which is of the same order of magnitude as the natural horseradish peroxidase (HRP) (K_cat = 4.3 × 10³ s^-1) at 37 °C and five-fold greater than the reported K_cat values of oxidase-like nanozyme obtained at 30 °C.

Synthesis of Finely Controllable Sizes of Au Nanoparticles on a Silica Template and Their Nanozyme Properties

The precise synthesis of fine-sized nanoparticles is critical for realizing the advantages of nanoparticles for various applications. We developed a technique for preparing finely controllable sizes of gold nanoparticles (Au NPs) on a silica template, using the seed-mediated growth and interval dropping methods. These Au NPs, embedded on silica nanospheres (SiO₂@Au NPs), possess peroxidase-like activity as nanozymes and have several advantages over other nanoparticle-based nanozymes. We confirmed their peroxidase activity; in addition, factors affecting the activity were investigated by varying the reaction conditions, such as concentrations of tetramethyl benzidine and H₂O₂, pH, particle amount, reaction time, and termination time. We found that SiO₂@Au NPs are highly stable under long-term storage and reusable for five cycles. Our study, therefore, provides a novel method for controlling the properties of nanoparticles and for developing nanoparticle-based nanozymes.

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.

Multifaceted Therapy of Nanocatalysts in Neurological Diseases

With the development of enzymes immobilization technology and the discover of nanozymes, catalytic therapy exhibited tremendous potential for neurological diseases therapy. In especial, since the discovery of Fe₃O₄ nanoparticles possessing intrinsic peroxidase-like activity, various nanozymes have been developed and recently started to explore for neurological diseases therapy, such as Alzheimer's disease, Parkinson's disease and stroke. By combining the catalytic activities with other properties (such as optical, thermal, electrical, and magnetic properties) of nanomaterials, the multifunctional nanozymes would not only alleviate oxidative and nitrosative stress on the basis of multienzymes-mimicking activity, but also exert positive effects on immunization, inflammation, autophagy, protein aggregation, which provides the foundation for multifaceted treatments. This review will summarize various types of nanocatalysts and further provides a valuable discussion on multifaceted treatment by nanozymes for neurological diseases, which is anticipated to provide an easily accessible guide to the key opportunities and current challenges of the nanozymes-mediated treatments for neurological diseases.

Pt Nanoparticles with High Oxidase-Like Activity and Reusability for Detection of Ascorbic Acid

Noble metal nanoenzymes such as Pt, Au, Pd, etc. exhibit magnificent activity. However, due to the scarce reserves and expensive prices of precious metals, it is essential to investigate their enzyme-like activity and explore the possibility of their reuse. In this work, the oxidase-like activity and reusability of several Pt nanoparticles with different morphologies were detected. We compared the Pt nanoparticles (NPs) with a size of about 30 nm self-assembled by 5 nm Pt nanoparticles and Pt nanoparticles (Pt-0 HCl) with a diameter of about 5 nm, and found that their Michaelis-Menten constants (Km) were close and their initial performance similar, but the Pt NPs had better reusability. This was probably attributed to the stacked structure of Pt NPs, which was conducive to the substance transport and sufficient contact. At the same time, it was found that the size, dispersion, and organic substances adsorbed on the surface of Pt nanoparticles would have a significant impact on their reusability. A colorimetric detection method was designed using the oxidase-like activity of Pt NPs to detect ascorbic acid in triplicate. The limits of detection were 131 ± 15, 144 ± 14, and 152 ± 9 nM, with little difference. This research not only showed that the morphology of the catalyst could be changed and its catalytic performance could be controlled by a simple liquid phase synthesis method, but also that it had great significance for the reuse of Pt nanoenzymes in the field of bioanalysis.

Au2Pt-PEG-Ce6 nanoformulation with dual nanozyme activities for synergistic chemodynamic therapy / phototherapy

Although synergistic therapy for tumors has displayed significant promise for effective treatment of cancer, developing a simple and effective strategy to build a multi-functional nanoplatform is still a huge challenge. By virtue of the characteristics of tumor microenvironment, such as hypoxia, slight acidity and H₂O₂ overexpression, Au₂Pt-PEG-Ce6 nanoformulation is constructed for collaborative chemodynamic/phototherapy of tumors. Specifically, the Au₂Pt nanozymes with multiple functions are synthesized in one step at room temperature. The photosensitizer chlorin e6 (Ce6) is covalently linked to Au₂Pt nanozymes for photodynamic therapy (PDT). Interestingly, the Au₂Pt nanozymes possess catalase- and peroxidase-like activities simultaneously, which not only can generate O₂ for relaxation of tumor hypoxia and enhancement of PDT efficiency but also can produce ∙OH for chemodynamic therapy (CDT). In addition, the high photothermal conversion efficiency (η = 31.5%) of Au₂Pt-PEG-Ce6 nanoformulation provides the possibility for photoacoustic (PA) and photothermal (PT) imaging guided photothermal therapy (PTT). Moreover, the presence of high-Z elements (Au and Pt) in Au₂Pt-PEG-Ce6 nanoformulation endows it with the ability to act as an X-ray computed tomography (CT) imaging contrast agent. All in all, the Au₂Pt-PEG-Ce6 exhibits great potential in multimodal imaging-guided synergistic PTT/PDT/CDT with remarkably tumor specificity and enhanced therapy.

Regulating the pro- and anti-oxidant capabilities of bimetallic nanozymes for the detection of Fe2+ and protection of Monascus pigments

The emerging properties of mimicking enzymes open up new horizons for nanomaterials. Regulating their enzyme-like activity and exploiting their applications are currently the hot topics for nanozymes. Among their activities, the pro-oxidant and antioxidant capabilities of nanozymes are important to determine their unique physiological functions. In this paper, we demonstrate that PtRu NPs exhibit multiple enzyme-like activities (e.g. peroxidase, oxidase, ferroxidase, catalase and SOD) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity. The PtRu bimetallic nanozymes therefore show pro-oxidant and anti-oxidant functions. It was found that the enzyme-like activities of PtRu NPs are highly dependent on the Pt/Ru molar ratio and show a similar trend in the order of activity: Pt₉₀Ru₁₀ > Pt₇₅Ru₂₅ > Pt > Pt₄₀Ru₆₀, indicating that proper alloying of Pt with Ru can enhance both pro- and anti-oxidant capabilities. By employing the ferroxidase-like activity and catalase-like activity, we verified the applications of PtRu nanozymes in the detection of Fe²⁺ ions, and tried for the first time to protect Monascus pigments (MPs) from hydrogen peroxide oxidation. These results not only provide an effective way to optimize the pro- and anti-oxidant capabilities of nanozymes, but also provide prospects for the applications of nanozymes in protecting biologically active natural products.

Oxidase-Inspired Selective 2e/4e Reduction of Oxygen on Electron-Deficient Cu

Development of low-cost and efficient (electro)catalysts with tunable 2e/4e oxygen reduction reaction (ORR) selectivity toward energy conversion, biomimetic catalysis, and biosensing has attracted growing interest. Herein, we reported that carbon nanohybrids with O- or N-coordinated Cu (Cu-OC or Cu-NC) showed superior activity for 2e and 4e electrocatalytic ORR with selectivities of 84.0% and 97.2%, respectively. Experimental evidence demonstrated that the strong electron-rich O-doped carbon in Cu-OC donated electrons to Cu²⁺, weakening the binding strength of H₂O₂ at Cu-O centers and facilitating the 2e ORR pathway for selective production of H₂O₂. However, the poor electron-donor ability of the N-doped carbon in Cu-NC made Cu-N sites more electron deficient due to the reduced electron transfer from N-doped carbon to Cu²⁺, promoting 4e ORR by enhancing adsorption of O₂ and the ORR intermediates. The high 4e ORR activity of Cu-NC rendered its potential for application in a Zn-air battery and oxidase-mimicking activity for 3,3',5,5'-tetramethylbenzidine (TMB) and ascorbic acid (AA) oxidation. The maximal velocity (V_max) of TMB and AA oxidation over Cu-NC was higher than some natural oxidases and noble-metal-based artificial enzymes. The lower activation energy for AA oxidation over Cu-NC resulted in a 263-fold higher oxidative rate than TMB, further prompting nonenzymatic sensing of AA by the competitive oxidation strategy.

Exploring the Effect of the Irradiation Time on Photosensitized Dendrimer-Based Nanoaggregates for Potential Applications in Light-Driven Water Photoreduction

Fourth generation polyamidoamine dendrimer (PAMAM, G4) modified with fluorescein units (F) at the periphery and Pt nanoparticles stabilized by L-ascorbate were prepared. These dendrimers modified with hydrophobic fluorescein were used to achieve self-assembling structures, giving rise to the formation of nanoaggregates in water. The photoactive fluorescein units were mainly used as photosensitizer units in the process of the catalytic photoreduction of water propitiated by light. Complementarily, Pt-ascorbate nanoparticles acted as the active sites to generate H2. Importantly, the study of the functional, optical, surface potential and morphological properties of the photosensitized dendrimer aggregates at different irradiation times allowed for insights to be gained into the behavior of these systems. Thus, the resultant photosensitized PAMAM-fluorescein (G4-F) nanoaggregates (NG) were conveniently applied to light-driven water photoreduction along with sodium L-ascorbate and methyl viologen as the sacrificial reagent and electron relay agent, respectively. Notably, these aggregates exhibited appropriate stability and catalytic activity over time for hydrogen production. Additionally, in order to propose a potential use of these types of systems, the in situ generated H2 was able to reduce a certain amount of methylene blue (MB). Finally, theoretical electronic analyses provided insights into the possible excited states of the fluorescein molecules that could intervene in the global mechanism of H2 generation.

Catalytic Mechanisms of Nanozymes and Their Applications in Biomedicine

The research on nanozymes has increased dramatically in recent years and a new interdiscipline, nanozymology, has emerged. A variety of nanomaterials have been designed to mimic the characteristics of natural enzymes, which connects an important bridge between nanotechnology and biological science. Unlike natural enzymes, the nanoscale properties of nanozymes endow them with the potential to regulate their enzymatic-like activity from different perspectives. The mechanisms behind those methods are intriguing. In this Review, we introduce these mechanisms from the aspects of surface chemistry, surface modification, molecular imprinting, and hybridization and then focus attention on some specific catalytic mechanisms of several representative nanozymes. The applications of nanozymes ranging from bioassay, imaging, to disease therapy are also discussed in detail to prove the fact that the inherent physicochemical properties of nanomaterials not only make nanozymes the analogues of biological enzymes, but also endow them with incomparable advantages and broad prospects in biomedical fields. Finally, four characteristics and some challenges of nanozymes are summarized.

In Situ Generation of Prussian Blue with Potassium Ferrocyanide to Improve the Sensitivity of Chemiluminescence Immunoassay Using Magnetic Nanoparticles as Label

Using magnetic nanoparticles (MNPs) as a label in immunoassay (IA) possesses advantages such as high specific surface area, simple modification process. However, the catalytic activity of MNPs is low, which limits their applications in IA. The present study found it interesting that potassium ferrocyanide reacts with MNPs, leading to the in situ generation of Prussian blue. The produced Prussian blue shows high catalytic activity on a luminol chemiluminescent (CL) reaction. Therefore, a simple and sensitive immunoassay for rabbit IgG (rIgG) as model analyte using MNPs as label was developed. The CL intensity had a linear increase with the concentration of rIgG that ranged from 0.625 to 20 ng mL^-1. The limit of detection was calculated to be 0.59 ng mL^-1. In addition, the applicability of this method was evaluated using the standard addition method. The recovery ranged from 80.0% to 115.0%. What's more, the proposed CLIA method based on in situ generation of Prussian blue with MNPs was also applied to the detection of carcinoembryonic antigen (CEA) and hepatitis B virus (HBV)-related sequence-specific DNA. The LOD for the detection of CEA and sequence-specific DNA was estimated to be 0.28 ng mL^-1 and 0.044 pmol, respectively.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Nanomaterials Exhibiting Enzyme-Like Properties (Nanozymes): Current Advances and Future Perspectives

Biological enzymes are macromolecular catalysts that catalyze the biochemical reactions of the natural systems. Although each enzyme performs a particular function, however, holds several drawbacks, which limits its utilization in broad-spectrum applications. Natural enzymes require strict physiological conditions for performing catalytic functions. Their limited stability in harsh environmental conditions, the high cost of synthesis, isolation, and purification are some of the significant drawbacks. Therefore, as an alternative to natural enzymes, recently several strategies have been developed including the synthesis of molecules, complexes, and nanoparticles mimicking their intrinsic catalytic properties. Nanoparticles exhibiting the properties of an enzyme are termed as “nanozymes.” Nanozymes offer several advantages over natural enzymes, therefore, a rapid expansion of the development of artificial biocatalysts. These advantages include simple methods of synthesis, low cost, high stability, robust catalytic performance, and smooth surface modification of nanomaterials. In this context, nanozymes are tremendously being explored to establish a wide range of applications in biosensing, immunoassays, disease diagnosis and therapy, theranostics, cell/tissue growth, protection from oxidative stress, and removal of pollutants. Considering the importance of nanozymes, this article has been designed to comprehensively discuss the different enzyme-like properties, such as peroxidase, catalase, superoxide dismutase, and oxidase, exhibited by various nanoparticles.

Fluoride capped V6O13–reduced graphene oxide nanocomposites: high activity oxidase mimetics and mechanism investigation

A novel high activity of the oxidase-like nanozyme of V₆O₁₃–rGO NCs, whose activity can be significantly enhanced by fluoride capping through surface chemistry.

Laser-induced formation of Au/Pt nanorods with peroxidase mimicking and SERS enhancement properties for application to the colorimetric determination of H2O2

Platinum nanoparticles (PtNPs) were uniformly grown on the surface of gold nanorods (AuNRs) by a laser irradiation procedure. Transmission electron microscopy confirmed that the PtNPs are uniformly grown on the surface of the AuNRs. The formation of PtNPs on the AuNRs leads to a red-shift of the absorption maximum from 734 nm to 766 nm. In addition, the efficiency of surface enhanced Raman scattering (SERS) is increased, but the photothermal conversion efficiency is decreased compared to pure AuNRs. The result indicates that electron transfer occurs between gold and platinum. The peroxidase mimicking effect of PtNPs, AuNRs and Au/Pt NRs by catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB; a quinone) in the presence of H₂O₂. The catalytic activity of Au/Pt NRs is higher than that of sole AuNRs or PtNPs by factors of 4.2 and 2.1, respectively. Thus, Au/Pt NRs have been used for the detection of peroxide and the limit of detection is 0.04 μM. This work provides an approach to integrate the peroxidase mimicking effect with SERS enhancement for potential application in detection. Graphical abstract A schematic diagram for the laser-induced growth of Au/Pt NRs and the colorimetric determination of hydrogen peroxide concentration with their peroxidase mimicking properties. The limit of detection is 0.04 μM based on the use of Au/Pt NRs as a catalyst.

Theoretical Insights into 1D Transition-Metal Nanoalloys Grown on the NiAl(110) Surface

Metallic nanoalloys are essential because of the synergistic effects rather than the merely additive effects of the metal components. Nanoscience is currently able to produce one-atom-thick linear atomic chains (LACs), and the NiAl(110) surface is a well-tested template used to build them. We report the first study based on ab initio density functional theory methods of one-dimensional transition-metal (TM) nanoalloys (i.e., LACs) grown on the NiAl(110) surface. This is a comprehensive and detailed computational study of the effect of alloying groups 10 and 11 metals (Pd, Pt, Cu, Ag, and Au) in LACs supported on the NiAl(110) surfaces to elucidate the structural, energetic, and electronic properties. From the TM series studied here, Pt appears to be an energy-stabilization species; meanwhile, Ag has a contrasting behavior. The work function changes because the alloying in LACs was satisfactorily explained from the explicit surface dipole moment calculations using an ab initio calculation-based approach, which captured the electron density redistribution upon building the LAC.

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.

Switching Peroxidase-Mimic Activity of Protein Stabilized Platinum Nanozymes by Sulfide Ions: Substrate Dependence, Mechanism, and Detection

In the present work, we use β-casein as a model protein to prepare a smart β-casein stabilized Pt nanoparticle (CM-PtNP) with peroxidase mimicking activity and systematically investigate sulfide-mediated switching effect and mechanism of CM-PtNP nanozyme's activity. Sulfide-mediated activity switching effect depends heavily on the physicochemical properties of nanozymes and the identity of substrate. On one hand, the binding of sulfide to a Pt nanozyme surface leads to the transform from Pt²⁺ to Pt⁰, resulting in more active sites and the activity "switching on"; on the other hand, the binding of sulfide ions via Pt-S interaction blocks the active sites, resulting in the activity "switching off". For substrates 3,3',5,5'-tetramethylbenzidine and 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt, the two factors play different decisive roles since the interaction of substrate molecules with nanozyme allows their different distributions on nanozyme surfaces. By virtue of this specific response, excellent sulfide colorimetric sensors with different limits of detection were developed based on CM-PtNP with different substrates. This is the first report about a fundamental understanding of how substrates influence the anion-mediated activity switching effect by illuminating the nature of anion-nanozyme interaction and nanozyme-substrate interaction. This may be useful to rationally predict the environment factors on the activities of the nanozyme and to design an effective signal amplification based on target-induced nanozyme deactivation/activation.

Hierarchical CNFs/MnCo2O4.5 nanofibers as a highly active oxidase mimetic and its application in biosensing

Recently, much attention has been paid on the nanomaterial-based artificial enzymes due to their tunable catalytic activity, high stability and low cost compared to the natural enzymes. Different from the peroxidase mimics which have been studied for several decades, nanomaterials with oxidase-like property are burgeoning in the recent years. In this paper, hierarchical carbon nanofibers (CNFs)/MnCo₂O_4.5 nanofibers as efficient oxidase mimics are reported. The products are synthesized by an electrospinning technique and an electrochemcial deposition process in which the CNFs are used as the working electrode where MnCo₂O_4.5 nanosheets deposit on. The resulting binary metal oxide-based nanocomposites exhibit a good oxidase-like activity toward the oxidations of 3,3',5,5'tetramethylbenzi-dine (TMB), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium (ABTS) salt and o-phenylenediamine (OPD) without exogenous addition of H₂O₂. The system of CNFs/MnCo₂O_4.5-TMB can be used as a candidate to detect sulfite and ascorbic acid via a colorimetric method with a high sensitivity. This work provides the efficient utilization and potential applications of binary metal oxide-based nanocomposites with oxidase activities in biosensors and other biotechnologies.

Formation of iron oxide/Pd hybrid nanostructures with enhanced peroxidase-like activity and catalytic reduction of 4-nitrophenol

Iron oxide/Pd hybrid nanostructures with controllable Pd loading from 0.05 to 1.0 (calculated as Pd/Fe molar ratio) have been synthesized by chemical reduction of Pd²⁺ on iron oxide particles. The combination of iron oxide and Pd exhibits enhanced peroxidase-like activity and catalytic activity toward reduction of 4-nitrophenol. The catalytic enhancements were found to be dependent on the Pd loading amount as well as the synergistic effect between iron oxide and Pd. These results suggest that iron oxide with unique surface chemical state can be an active supporter and suggest an effective way to design superior hybrid nanostructures for catalytic applications.

Graphene Quantum Dot/Silver Nanoparticle Hybrids with Oxidase Activities for Antibacterial Application

We report the first attempt of using graphene quantum dot-Ag nanoparticles (GQD/AgNP hybrids) as oxidase mimics and antibacterial agents. Unlike previous silver- and graphene-based materials, the GQD/AgNP hybrids exhibit a high oxidase-like catalytic activity and possess favorable stability in neutral medium within the range from room temperature to 60 °C. In accordance with their prominent enzyme activities, the GQD/AgNP hybrids show excellent antibacterial properties against Gram-negative and Gram-positive bacteria as well as drug resistant bacteria, with an ultralow minimal inhibitory concentration (2-4 μg/mL) against 1 × 10⁷ to 1 × 10⁸ μg/mL Escherichia coli and Staphylococcus aureus. In the presence of the GQD/AgNP hybrids, the fluorescence behavior after the introduction of 2', 7'-dichlorofluorescin diacetate demonstrated a possible role of reactive oxidative species in the GQD/AgNP hybrid-mediated antibacterial therapeutic effect. Furthermore, TEM and SEM imaging identified concomitant disruption of the bacterial cell membrane and loss of barrier function during the sterilization process. Therefore, the GQD/AgNP hybrids exhibit vast potentials for serving as highly effective, broad-spectrum antibacterial agent for sterilization use without the need of additional stimulation by laser irradiation (photosensitization) or the provision of H₂O₂, facilitating their relative ease of use and cost-effectiveness.

Oxygen-tuned nanozyme polymerization for the preparation of hydrogels with printable and antibacterial properties

Nanozymes merge nanotechnology with biology and provide a lower cost and higher stability options, compared to that of natural enzymes. However, nanozyme catalyzed polymerization under physiological conditions is still a big challenge due to heavy oxygen inhibition. In this study, the simple glucose oxidase system can effectively adjust oxygen concentration and generate hydrogen peroxide, which assists in the realization of nanozyme-catalyzed polymerization. The nanozyme based hydrogel is printable due to its mild preparation with gradually increased viscosity. The antibacterial performance is ascribed to the in situ generated hydroxyl radical via the reaction of the bound nanozyme and glucose.

Microbial synthesis of bimetallic PdPt nanoparticles for catalytic reduction of 4-nitrophenol

Bimetallic nanoparticles are generally believed to have improved catalytic activity and stability due to geometric and electronic changes. In this work, biogenic-Pd (bio-Pd), biogenic-Pt (bio-Pt), and biogenic-PdPt (bio-PdPt) nanoparticles were synthesized by Shewanella oneidensis MR-1 in the absence or presence of quinone. Compared with direct microbial reduction process, the addition of anthraquinone-2,6-disulfonate (AQDS) could promote the reduction efficiency of Pd(II) or/and Pt(IV) and result in decrease of particles size. All kinds of nanoparticles could catalyze 4-nitrophenol reduction by NaBH₄ and their catalytic activities took the following order: bio-PdPt (AQDS) ∼ bio-PdPt > bio-Pd (AQDS) > bio-Pd > bio-Pt (AQDS) ∼ bio-Pt. Moreover, the bio-PdPt (AQDS) nanoparticles could be reused for 6 cycles. We believe that this simple and efficient biosynthesis approach for synthesizing bimetallic bio-PdPt nanocatalysts is important for preparing active and stable catalysts.

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.

Colorimetric sensing of malathion using palladium-gold bimetallic nanozyme

In this work, a simple, sensitive and selective label free colorimetric assay using palladium-gold nanorod as nanozyme is reported for malathion detection. Study investigates the peroxidase potential of the nanozyme on colorimetric substrates and explores the effect of selected organophosphates on their enzyme mimetic activity. Palladium-gold nanozyme shows excellent peroxidase mimetic activity with O-phenylenediamine in the presence of hydrogen peroxide. Its Kinetic parameters Km and kcat are better than horseradish peroxidase which makes it a superior enzyme. Nanozyme is stable over a broad temperature range (4-70°C) and shows high peroxidase activity from 2 to 6pH. The peroxidase activity of nanozyme is selectively quenched with increasing concentration of malathion and is the principle of developed assay. Assay has a lowest detection limit of 60ng/ml and shows no cross-reaction with other analogous organophosphates or metal salts. Validation on tap water samples spiked with different concentrations of malathion shows good recovery in the range of 80-106%. Assay also displays good intra and inter-assay precision which lie in the range of 2.7-6.1% and 3.2-5.9% respectively. This study demonstrated the catalytic potential of palladium-gold nanorods, which can be employed as nanozyme for developing highly sensitive detection methods.

Core-shell Cu@Au nanoparticles-based colorimetric aptasensor for the determination of lysozyme

A growing body of evidence indicates that lysozyme plays a significant role as an indicator for many diseases and a drug for treatment of infections, ulcers and to study the spatial conformation, enzyme kinetics, and molecular immunology. Therefore, highly sensitive determination of lysozyme is necessary and vital in a wide variety of fields. In this work, we put forward a simple but effective strategy for colorimetric visualization of lysozyme based on iodide-responsive Cu@Au nanoparticles (Cu@Au NPs) as well as the iodide-catalyzed H₂O₂-TMB (3,3,5,5-tetramethylbenzidine) reaction system. Colorimetric detection is applied because of its simplicity, fast response for analysis, high detection limit, low costs and practicality. In our strategy, iodide is applied for the reason that it can induce an obvious color change of the Cu@Au nanoparticles solution from gray to red, along with the change of morphologies of the Cu@Au nanoparticles from irregular to spherical. Consequently, this phenomenon results in colorimetric signal variation of the iodide-catalytic H₂O₂-TMB system. What's more, by quite simple biomolecule modification on the Cu@Au nanoparticles surface, an all-purpose colorimetric platform is established for the accurate detection of lysozyme, which could lead to the change of Cu@Au NP concentration through molecular recognition. The results show that modified Cu-Au NPs successfully achieved a simple, selective, visualized, and ultrasensitive detection of lysozyme with a linear range from 10^-7 to 10^-3M and a detection limit of 60nM.

Boosting the oxidase mimicking activity of nanoceria by fluoride capping: rivaling protein enzymes and ultrasensitive F(-) detection

Nanomaterial-based enzyme mimics (nanozymes) are currently a new forefront of chemical research. However, the application of nanozymes is limited by their low catalytic activity and low turnover numbers. Cerium dioxide nanoparticles (nanoceria) are among the few with oxidase activity. Herein, we report an interesting finding addressing their limitations. The oxidase activity of nanoceria is improved by over 100-fold by fluoride capping, making it more close to real oxidases. The turnover number reached 700 in 15 min, drastically improved from ∼15 turnovers for the naked particles. The mechanism is attributed to surface charge modulation and facilitated electron transfer by F(-) capping based on ζ-potential and free radical measurements. Ultrasensitive sensing of fluoride was achieved with a detection limit of 0.64 μM F(-) in water and in toothpastes, while no other tested anions can achieve the activity enhancement.

Platinum nanoparticles inhibit antioxidant effects of vitamin C via ascorbate oxidase-mimetic activity

Pt nanoparticles with ascorbate oxidase-mimetic activity inhibit the cytoprotective effect of vitamin C on cells challenged by H₂O₂.

PtCo bimetallic nanoparticles with high oxidase-like catalytic activity and their applications for magnetic-enhanced colorimetric biosensing

A novel magnetic-enhanced colorimetric assay was constructed based on aptamer conjugated PtCo bimetallic nanoparticles with high oxidase-like catalytic activity, high water solubility, low cell toxicity, and superparamagnetic properties.