Size-Dependent Catalytic Activity and Dynamics of Gold Nanoparticles at the Single-Molecule Level

Multivariate Bayesian Optimization of CoO Nanoparticles for CO2 Hydrogenation Catalysis

The hydrogenation of CO2 holds promise for transforming the production of renewable fuels and chemicals. However, the challenge lies in developing robust and selective catalysts for this process. Transition metal oxide catalysts, particularly cobalt oxide, have shown potential for CO2 hydrogenation, with performance heavily reliant on crystal phase and morphology. Achieving precise control over these catalyst attributes through colloidal nanoparticle synthesis could pave the way for catalyst and process advancement. Yet, navigating the complexities of colloidal nanoparticle syntheses, governed by numerous input variables, poses a significant challenge in systematically controlling resultant catalyst features. We present a multivariate Bayesian optimization, coupled with a data-driven classifier, to map the synthetic design space for colloidal CoO nanoparticles and simultaneously optimize them for multiple catalytically relevant features within a target crystalline phase. The optimized experimental conditions yielded small, phase-pure rock salt CoO nanoparticles of uniform size and shape. These optimized nanoparticles were then supported on SiO2 and assessed for thermocatalytic CO2 hydrogenation against larger, polydisperse CoO nanoparticles on SiO2 and a conventionally prepared catalyst. The optimized CoO/SiO2 catalyst consistently exhibited higher activity and CH4 selectivity (ca. 98%) across various pretreatment reduction temperatures as compared to the other catalysts. This remarkable performance was attributed to particle stability and consistent H* surface coverage, even after undergoing the highest temperature reduction, achieving a more stable catalytic species that resists sintering and carbon occlusion.

Effect of Gold Nanoparticle Size on Regulated Catalytic Activity of Temperature-Responsive Polymer-Gold Nanoparticle Hybrid Microgels

Gold nanoparticles (AuNPs) possess attractive electronic, optical, and catalytic properties, enabling many potential applications. Poly(N-isopropyl acrylamide) (PNIPAAm) is a temperature-responsive polymer that changes its hydrophilicity upon a slight temperature change, and combining PNIPAAm with AuNPs allows us to modulate the properties of AuNPs by temperature. In a previous study, we proposed a simpler method for designing PNIPAAm-AuNP hybrid microgels, which used an AuNP monomer with polymerizable groups. The size of AuNPs is the most important factor influencing their catalytic performance, and numerous studies have emphasized the importance of controlling the size of AuNPs by adjusting their stabilizer concentration. This paper focuses on the effect of AuNP size on the catalytic activity of PNIPAAm-AuNP hybrid microgels prepared via the copolymerization of N-isopropyl acrylamide and AuNP monomers with different AuNP sizes. To quantitatively evaluate the catalytic activity of the hybrid microgels, we monitored the reduction of 4-nitrophenol to 4-aminophenol using the hybrid microgels with various AuNP sizes. While the hybrid microgels with an AuNP size of 13.0 nm exhibited the highest reaction rate and the apparent reaction rate constant (kapp) of 24.2 × 10-3 s-1, those of 35.9 nm exhibited a small kapp of 1.3 × 10-3 s-1. Thus, the catalytic activity of the PNIPAAm-AuNP hybrid microgel was strongly influenced by the AuNP size. The hybrid microgels with various AuNP sizes enabled the reversibly temperature-responsive on-off regulation of the reduction reaction.

Effect of Particle Heterogeneity in Catalytic Copper-Containing Single-Chain Polymeric Nanoparticles Revealed by Single-Particle Kinetics

Single-chain polymeric nanoparticles (SCPNs) have been extensively explored as a synthetic alternative to enzymes for catalytic applications. However, the inherent structural heterogeneity of SCPNs, arising from the dispersity of the polymer backbone and stochastic incorporation of different monomers as well as catalytic moieties, is expected to lead to variations in catalytic activity between individual particles. To understand the effect of structural heterogeneities on the catalytic performance of SCPNs, techniques are required that permit researchers to directly monitor SCPN activity at the single-polymer level. In this study, we introduce the use of single-molecule fluorescence microscopy to study the kinetics of Cu(I)-containing SCPNs towards depropargylation reactions. We developed Cu(I)-containing SCPNs that exhibit fast kinetics towards depropargylation and Cu-catalyzed azide-alkyne click reactions, making them suitable for single-particle kinetic studies. SCPNs were then immobilized on the surface of glass coverslips and the catalytic reactions were monitored at a single-particle level using total internal reflection fluorescence (TIRF) microscopy. Our studies revealed the interparticle turnover dispersity for Cu(I)-catalyzed depropargylations. In the future, our approach can be extended to different polymer designs which can give insights into the intrinsic heterogeneity of SCPN catalysis and can further aid in the rational development of SCPN-based catalysts.

Single-Molecule Catalysis in the Palladium Cross-Coupling Reaction Cycle

Single-molecule (SM) methods are applied to study various types of catalytic processes in chemical and biochemical reactions. In this study, the Suzuki-Miyaura cross-coupling reaction forming a fluorescent product is investigated within the SM approximation. Stochastic analysis of emission intermittency in selected nanoscopic spots allows us to determine the single-molecule turnover frequency (SM-TOF) of the Pd catalyst in a specific probe reaction. We generate and analyze simulated intermittency time traces of a single catalyst surrounded by reactant molecules to assess the reliability of the method applied to real intermittency time trace data from hundreds of nanoscopic fluorescence spots. The results demonstrate that the proposed method can be used to evaluate the average SM-TOF of Pd in a cross-coupling reaction.

One-Shot Dual-Detection-Based Single-Molecule Super-Resolution Imaging Method for Real-Time Observation of Spatiotemporal Catalytic Activity Variations on the Plasmonic Gold Nanoparticle Surface

Understanding the relationship between the surface properties of a single plasmonic nanoparticle and its catalytic performance is critical for developing highly efficient nanocatalysts. In this study, a one-shot dual-detection-based single-molecule super-resolution imaging method in the evanescent field was developed to observe real-time spatiotemporal catalytic activity on a single plasmonic gold nanoparticle (AuNP) surface. The scattering intensity of AuNPs and the fluorescence of resorufin molecules produced on the AuNP surface were obtained simultaneously to investigate the relationship between nanoparticles and catalytic reactions at a single-molecule level. Chemisorbed adsorbates (i.e., catalytic product and resorufin) changed the electron density of individual AuNPs throughout the catalytic cycle, resulting in the fluctuation of the scattering intensity of individual AuNPs, which was attributed to the electron transfer between reactant resazurin molecules and AuNPs. The increase in the electron density of individual AuNPs affected the catalytic reaction rate. Furthermore, sequential mapping of individual catalytic events at the subdiffraction limit resolution was completed for real-time surface dynamics and spatiotemporal activity variations on the single AuNP surface. The developed method can aid in understanding surface-property-dependent catalytic kinetics and facilitate the development of nanoparticle-based heterogeneous catalysts at subdiffraction limit resolution.

Berberine mediated fluorescent gold nanoclusters in biomimetic erythrocyte ghosts as a nanocarrier for enhanced photodynamic treatment

Photodynamic therapy (PDT) is a well-established cancer treatment method that employs light to generate reactive oxygen species (ROS) causing oxidative damage to cancer cells. Nevertheless, PDT encounters challenges due to its oxygen-dependent nature, which makes it less effective in hypoxic tumor environments. To address this issue, we have developed a novel nanocomposite known as AuNC@BBR@Ghost. This nanocomposite combines the advantageous features of erythrocyte ghost membranes, the photoresponsive properties of gold nanoclusters (AuNC) and the anticancer characteristics of Berberine (BBR) for cancer treatment. Our synthesized AuNC efficiently produce ROS, with a 25% increase in efficiency when exposed to near-infrared (NIR) irradiation. By harnessing the oxygen-carrying capacity of erythrocyte ghost cells, AuNC@BBR@Ghost demonstrates a significant improvement in ROS generation, achieving an 80% efficiency. Furthermore, the AuNC exhibit tunable emission wavelengths due to their excellent fluorescent properties. In normoxic conditions, treatment of A549 lung carcinoma cells with AuNC@BBR@Ghost followed by exposure to 808 nm NIR irradiation results in a notable increase in intracellular ROS levels, accelerating cell death. In hypoxic conditions, when A549 cells were treated with AuNC@BBR@Ghost, the erythrocyte ghost acted as an oxygen supplement due to the residual hemoglobin, alleviating hypoxia and enhancing the nanocomposite's sensitivity to PDT treatment. Thus, the AuNC@BBR@Ghost nanocomposite achieves an improved effect by combining the advantageous properties of its individual components, resulting in enhanced ROS generation and adaptability to hypoxic conditions. This innovative approach successfully overcomes PDT's limitations, making AuNC@BBR@Ghost a promising nanotheranostic agent with significant potential for advanced cancer therapy.

Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

Spatial Distributions of Single-Molecule Reactivity in Plasmonic Catalysis

Plasmonic catalysts have the potential to accelerate and control chemical reactions with light by exploiting localized surface plasmon resonances. However, the mechanisms governing plasmonic catalysis are not simple to decouple. Several plasmon-derived phenomena, such as electromagnetic field enhancements, temperature, or the generation of charge carriers, can affect the reactivity of the system. These effects are convoluted with the inherent (nonplasmonic) catalytic properties of the metal surface. Disentangling these coexisting effects is challenging but is the key to rationally controlling reaction pathways and enhancing reaction rates. This study utilizes super-resolution fluorescence microscopy to examine the mechanisms of plasmonic catalysis at the single-particle level. The reduction reaction of resazurin to resorufin in the presence of Au nanorods coated with a porous silica shell is investigated in situ. This allows the determination of reaction rates with a single-molecule sensitivity and subparticle resolution. By variation of the irradiation wavelength, it is possible to examine two different regimes: photoexcitation of the reactant molecules and photoexcitation of the nanoparticle's plasmon resonance. In addition, the measured spatial distribution of reactivity allows differentiation between superficial and far-field effects. Our results indicate that the reduction of resazurin can occur through more than one reaction pathway, being most efficient when the reactant is photoexcited and is in contact with the Au surface. In addition, it was found that the spatial distribution of enhancements varies, depending on the underlying mechanism. These findings contribute to the fundamental understanding of plasmonic catalysis and the rational design of future plasmonic nanocatalysts.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

Nanostructure Tuning of Gold Nanoparticles Films via Click Sintering

Colloidal metal nanoparticles dispersions are commonly used to create functional printed electronic devices and they typically require time-, energy- and equipment-consuming post-treatments to improve their electrical and mechanical properties. Traditional methods, e.g. thermal, UV/IR, and microwave treatments, limit the substrate options and may require expensive equipment, not available in all the laboratories. Moreover, these processes also cause the collapse of the film (nano)pores and interstices, limiting or impeding its nanostructuration. Finding a simple approach to obtain complex nanostructured materials with minimal post-treatments remains a challenge. In this study, a new sintering method for gold nanoparticle inks that called as "click sintering" has been reported. The method uses a catalytic reaction to enhance and tune the nanostructuration of the film while sintering the metallic nanoparticles, without requiring any cumbersome post-treatment. This results in a conductive and electroactive nanoporous thin film, whose properties can be tuned by the conditions of the reaction, i.e., concentration of the reagent and time. Therefore, this study presents a novel and innovative one-step approach to simultaneously sinter gold nanoparticles films and create functional nanostructures, directly and easily, introducing a new concept of real-time treatment with possible applications in the fields of flexible electronics, biosensing, energy, and catalysis.

The role of metal accessibility on carbon dioxide electroreduction in atomically precise nanoclusters

Atomically precise nanoclusters (NCs) can be designed with high faradaic efficiency for the electrochemical reduction of CO2 to CO (FECO) and provide useful model systems for studying the metal-catalysed CO2 reduction reaction (CO2RR). While size-dependent trends are commonly evoked, the effect of NC size on catalytic activity is often convoluted by other factors such as changes to surface structure, ligand density, and electronic structure, which makes it challenging to establish rigorous structure-property relationships. Herein, we report a detailed investigation of a series of NCs [AunAg46-n(C[triple bond, length as m-dash]CR)24Cl4(PPh3)2, Au24Ag20(C[triple bond, length as m-dash]CR)24Cl2, and Au43(C[triple bond, length as m-dash]CR)20/Au42Ag1(C[triple bond, length as m-dash]CR)20] with similar sizes and core structures but different ligand packing densities to investigate how the number of accessible metal sites impacts CO2RR activity and selectivity. We develop a simple method to determine the number of CO2-accessible sites for a given NC then use this to probe relationships between surface accessibility and CO2RR performance for atomically precise NC catalysts. Specifically, the NCs with the highest number of accessible metal sites [Au43(C[triple bond, length as m-dash]CR)20 and Au42Ag1(C[triple bond, length as m-dash]CR)20] feature a FECO of >90% at -0.57 V vs. the reversible hydrogen electrode (RHE), while NCs with lower numbers of accessible metal sites have a reduced FECO. In addition, CO2RR studies performed on other Au-alkynyl NCs that span a wider range of sizes further support the relationship between FECO and the number of accessible metal sites, regardless of NC size. This work establishes a generalizable approach to evaluating the potential of atomically precise NCs for electrocatalysis.

How Heterogeneity Affects Cooperative Communications within Single Nanocatalysts

Catalysis remains one of the most essential methods in chemical research and industry. Recent experiments have discovered an unusual phenomenon of catalytic cooperativity, when a reaction at one active site can stimulate reactions at neighboring sites within single nanoparticles. While theoretical analysis established that the transport of charged holes is responsible for this phenomenon, it does not account for inhomogeneity in the structural and dynamic properties of single nanocatalysts. Here, we investigate the effect of heterogeneity on catalytic communications by extending a discrete-state stochastic framework to random distributions of the transition rates. Our explicit calculations of spatial and temporal properties of heterogeneous systems in comparison with homogeneous systems predict that the strength of cooperativity increases, while the communication lifetimes and distances decrease. Monte Carlo computer simulations support theoretical calculations, and microscopic arguments to explain these observations are also presented. Our theoretical analysis clarifies some important aspects of molecular mechanisms of catalytic processes.

Advances in the design of amino acid and peptide synthesized gold nanoparticles for their applications

The field of therapeutics and diagnostics is advanced by nanotechnology-based approaches including the spatial-temporal release of drugs, targeted delivery, enhanced accumulation of drugs, immunomodulation, antimicrobial action, and high-resolution bioimaging, sensors and detection. Various compositions of nanoparticles (NPs) have been developed for biomedical applications; however, gold NPs (Au NPs) have attracted tremendous attention due to their biocompatibility, easy surface functionalization and quantification. Amino acids and peptides have natural biological activities as such, their activities enhance several folds in combination with NPs. Although peptides are extensively used to produce various functionalities of Au NPs, amino acids have also gained similar interests in producing amino acid-capped Au NPs due to the availability of amine, carboxyl and thiol functional groups. Henceforth, a comprehensive review is needed to timely bridge the synthesis and the applications of amino acid and peptide-capped Au NPs. This review aims to describe the synthesis mechanism of Au NPs using amino acids and peptides along with their applications in antimicrobial, bio/chemo-sensors, bioimaging, cancer therapy, catalysis, and skin regeneration. Moreover, the mechanisms of various activities of amino acid and peptide capped-Au NPs are presented. We believe this review will motivate researchers to better understand the interactions and long-term activities of amino acid and peptide-capped Au NPs for their success in various applications.

Atomically precise nanoclusters predominantly seed gold nanoparticle syntheses

Seed-mediated synthesis strategies, in which small gold nanoparticle precursors are added to a growth solution to initiate heterogeneous nucleation, are among the most prevalent, simple, and productive methodologies for generating well-defined colloidal anisotropic nanostructures. However, the size, structure, and chemical properties of the seeds remain poorly understood, which partially explains the lack of mechanistic understanding of many particle growth reactions. Here, we identify the majority component in the seed solution as an atomically precise gold nanocluster, consisting of a 32-atom Au core with 8 halide ligands and 12 neutral ligands constituting a bound ion pair between a halide and the cationic surfactant: Au₃₂X₈[AQA⁺•X^-]₁₂ (X = Cl, Br; AQA = alkyl quaternary ammonium). Ligand exchange is dynamic and versatile, occurring on the order of minutes and allowing for the formation of 48 distinct Au₃₂ clusters with AQAX (alkyl quaternary ammonium halide) ligands. Anisotropic nanoparticle syntheses seeded with solutions enriched in Au₃₂X₈[AQA⁺•X^-]₁₂ show narrower size distributions and fewer impurity particle shapes, indicating the importance of this cluster as a precursor to the growth of well-defined nanostructures.

Gold Nanoparticles AuNP Decorated on Fused Graphene-like Materials for Application in a Hydrogen Generation

The search for a sustainable, alternative fuel source to replace fossil fuels has led to an increased interest in hydrogen fuel. This combustible gas is not only clean-burning but can readily be produced via the hydrolysis of sodium borohydride. The main drawback of this reaction is that the reaction occurs relatively slowly and requires a catalyst to improve efficiency. This study explored a novel composite material made by combining gold nanoparticles and fused graphene-like materials (AuFGLM) as a catalyst for generating hydrogen via sodium borohydride. The novel fused graphene-like material (FGLM) was made with a sustainable dextrose solution and by using a pressure-processing method. Imaging techniques showed that FGLM appears to be an effective support template for nanoparticles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy were used to characterize and determine the size, shape, and structure of nanoparticles and composites. The TEM study characterized the fused carbon backbone as it began to take on a rounder shape. The TEM images also revealed that the average diameter of the gold nanoparticle was roughly 23 nm. The FTIR study confirmed O-H, C-C, and C=O as functional groups in the materials. The EDS analysis showed that the composite contained approximately 6.3% gold by weight. The crystal structures of FGLM and AuFGLM were identified via P-XRD analysis. Various reaction conditions were used to test the catalytic ability of AuFGLM, including various solution pHs, temperatures, and doses of NaBH₄. It was observed that optimal reaction conditions included high temperature, an acidic solution pH, and a higher dose of NaBH₄. The activation energy of the reaction was determined to be 45.5 kJ mol^-1, and it was found that the catalyst could be used multiple times in a row with an increased volume of hydrogen produced in ensuing trials. The activation energy of this novel catalyst is competitive compared to similar catalysts and its ability to produce hydrogen over multiple uses makes the material an exciting choice for catalyzing the hydrolysis of NaBH₄ for use as a hydrogen fuel source.

Research on Electric Field-Induced Catalysis Using Single-Molecule Electrical Measurement

The role of catalysis in controlling chemical reactions is crucial. As an important external stimulus regulatory tool, electric field (EF) catalysis enables further possibilities for chemical reaction regulation. To date, the regulation mechanism of electric fields and electrons on chemical reactions has been modeled. The electric field at the single-molecule electronic scale provides a powerful theoretical weapon to explore the dynamics of individual chemical reactions. The combination of electric fields and single-molecule electronic techniques not only uncovers new principles but also results in the regulation of chemical reactions at the single-molecule scale. This perspective focuses on the recent electric field-catalyzed, single-molecule chemical reactions and assembly, and highlights promising outlooks for future work in single-molecule catalysis.

Urethane functions can reduce metal salts under hydrothermal conditions: synthesis of noble metal nanoparticles on flexible sponges applied in semi-automated organic reduction

We report an additive-free one-pot hydrothermal synthesis of Au, Ag, Pd, and alloy AuPd nanoparticles (NPs) anchored on commercial polyurethane (PU) foams. While unable to reduce the precursor metal salts at room temperature, PU is able to serve as a reducing agent under hydrothermal conditions. The resulting NP@PU sponge materials perform comparably to reported state-of-the-art reduction catalysts, and are additionally very well suited for use in semi-automated synthesis: the NP anchoring is strong enough and the support flexible enough to be used as a 'catalytic sponge' that can be manipulated with a robotic arm, i.e., be repeatedly dipped into and drawn out of solutions, wrung out, and re-soaked.

Electrophoretic Deposition of Hybrid Calcium Alginate-Gold Nanoparticle Hydrogel Films via Catalyzed Electrooxidation of Hydroquinone

The electrophoretic deposition (EPD) of hybrid alginate (Alg)-Au nanoparticle (NP) films results from the localized pH drop at the electrode surface due to oxidation of hydroquinone (HQ) catalyzed by 4 and 15 nm diameter citrate-coated gold NPs (cit-Au NPs). The localized pH drop at the electrode leads to neutralization of both Alg and cit, leading to EPD of both Alg and cit-Au NPs simultaneously. Post-treatment of the film with Ca²⁺ solution leads to hybrid Ca-Alg-Au NP hydrogel films. The EPD of Alg in the presence of 4 nm cit-Au NPs occurs at ∼0.8 V (vs Ag/AgCl) as compared to ∼1.0 V in the presence of 15 nm cit-Au NPs and ∼1.4 V in the absence of cit-Au NPs. This is due to the higher catalytic activity of 4 nm cit-Au NPs compared to 15 nm cit-Au NPs for the oxidation of HQ. UV-vis spectra of Ca-Alg-Au NP hydrogel films show absorbance features for both Ca-Alg and Au NPs entrapped within the hydrogel. As the concentration of Au NPs in the EPD solution increases, the Ca-Alg absorbance and localized surface plasmon resonance (LSPR) peak of the Au NPs increases, confirming the role of the Au NPs as a catalyst for EPD of Alg. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the Ca-Alg-Au NP hydrogel films show characteristic peaks for Ca-Alg and protonated alginic acid groups. The hydrogel thickness is greater with cit-Au NPs compared to without cit-Au NPs at constant EPD potential and time. Forming Ca-Alg and hybrid Ca-Alg-Au NP hydrogel films at low potentials has potential applications in electrochemical and optical sensor development, catalysis, and biological studies.

Amalgamation of DNAzymes and Nanozymes in a Coronazyme

Artificial enzymes such as nanozymes and DNAzymes are economical and stable alternatives to natural enzymes. By coating Au nanoparticles (AuNPs) with a DNA corona (AuNP@DNA), we amalgamated nanozymes and DNAzymes into a new artificial enzyme with catalytic efficiency 5 times higher than AuNP nanozymes, 10 times higher than other nanozymes, and significantly greater than most of the DNAzymes on the same oxidation reaction. The AuNP@DNA demonstrates excellent specificity as its reactivity on a reduction reaction does not change with respect to pristine AuNP. Single-molecule fluorescence and force spectroscopies and density functional theory (DFT) simulations indicate a long-range oxidation reaction initiated by radical production on the AuNP surface, followed by radical transport to the DNA corona, where the binding and turnover of substrates take place. The AuNP@DNA is named coronazyme because of its natural enzyme mimicking capability through the well-orchestrated structures and synergetic functions. By incorporating different nanocores and corona materials beyond DNAs, we anticipate that the coronazymes represent generic enzyme mimics to carry out versatile reactions in harsh environments.

Application of Nanozymes in Environmental Monitoring, Management, and Protection

Nanozymes are nanomaterials with enzyme-like activity, possessing the unique properties of nanomaterials and natural enzyme-like catalytic functions. Nanozymes are catalytically active, stable, tunable, recyclable, and versatile. Therefore, increasing attention has been paid in the fields of environmental science and life sciences. In this review, we focused on the most recent applications of nanozymes for environmental monitoring, environmental management, and environmental protection. We firstly introduce the tuning catalytic activity of nanozymes according to some crucial factors such as size and shape, composition and doping, and surface coating. Then, the application of nanozymes in environmental fields are introduced in detail. Nanozymes can not only be used to detect inorganic ions, molecules, organics, and foodborne pathogenic bacteria but are also involved in the degradation of phenolic compounds, dyes, and antibiotics. The capability of nanozymes was also reported for assisting air purification, constructing biofuel cells, and application in marine antibacterial fouling removal. Finally, the current challenges and future trends of nanozymes toward environmental fields are proposed and discussed.

Dynamics of chemical reactions on single nanocatalysts with heterogeneous active sites

Modern chemical science and industries critically depend on the application of various catalytic methods. However, the underlying molecular mechanisms of these processes still remain not fully understood. Recent experimental advances that produced highly-efficient nanoparticle catalysts allowed researchers to obtain more quantitative descriptions, opening the way to clarify the microscopic picture of catalysis. Stimulated by these developments, we present a minimal theoretical model that investigates the effect of heterogeneity in catalytic processes at the single-particle level. Using a discrete-state stochastic framework that accounts for the most relevant chemical transitions, we explicitly evaluated the dynamics of chemical reactions on single heterogeneous nanocatalysts with different types of active sites. It is found that the degree of stochastic noise in nanoparticle catalytic systems depends on several factors that include the heterogeneity of catalytic efficiencies of active sites and distinctions between chemical mechanisms on different active sites. The proposed theoretical approach provides a single-molecule view of heterogeneous catalysis and also suggests possible quantitative routes to clarify some important molecular details of nanocatalysts.

In situ quantitative single-molecule study of site-specific photocatalytic activity and dynamics on ultrathin g-C3N4 nanosheets

Graphitic carbon nitride (g-C₃N₄) has attracted extensive research attention in recent years due to its unique layered structure, facile synthetic route, visible-light-responsive nature, and excellent photocatalytic performance. However, an insightful investigation of site-specific catalytic activities and kinetics on g-C₃N₄ is still warranted. Here, we fabricated ultrathin g-C₃N₄ nanosheets through thermal exfoliation. The optimized sample exhibits a high specific surface area of 307.35 m² g^-1 and a remarkable H₂ generation activity of 2008 μmol h^-1 g^-1 with an apparent quantum efficiency of 4.62% at λ = 420 nm. Single-molecule fluorescence microscopy was applied for the first time to spatially resolve the reaction heterogeneities with nanometer precision (∼10 nm). The catalytic kinetics (i.e., reactant adsorption, conversion, and product dissociation) and temporal activity fluctuations were in situ quantified at individual structural features (i.e., wrinkles, edges, and basal planes) of g-C₃N₄. It was found that the wrinkle and edge exhibited superior photocatalytic activity due to the intrinsic band modulation, which are 20 times and 14.8 times that of the basal plane, respectively. Moreover, due to the steric effect, the basal plane showed the highest adsorption constant and the lowest direct dissociation constant. Density functional theory (DFT) simulations unveiled the adsorption energies of reactant and product molecules on each structure of g-C₃N₄, which support our experimental results. Such investigation would shed more light on the fundamental understanding of site-specific catalytic dynamics on g-C₃N₄, which benefits the rational design of 2D layered materials for efficient solar-to-chemical energy conversion.

Toxic Ag+ detection based on Au@Ag core shell nanostructure formation using Tannic acid assisted synthesis of Pullulan stabilized gold nanoparticles

Herein, a sensitive colorimetric detection strategy is proposed for Ag⁺ detection based on the use of environmentally friendly synthesis of gold nanoparticles (AuNPs), at room temperature, using (tannic acid, TA), as the reductant and pullulan (PUL) as stabilizing agent. The colloidal solution (TA/PUL-AuNPs), at the optimal synthesis conditions, showed maximum absorbance at 529 nm with a berry red color. TEM and FESEM validated that the particles are spherical and monodispersed, while other characterization results elucidated the role of pullulan in the nano-synthesis. Ag⁺ addition to the probe (TA/PUL-AuNPs), pH 11, resulted in naked-eye color changes, owing to Au@Ag core shell nanostructure formation. Further, the added Ag⁺ is reduced to AgNPs, on the surface of the TA/PUL-AuNPs probe. A hypsochromic shift in the absorption maximum, from 529 to 409 nm was observed, while (A_Ag+-A_bl)@409 nm exhibited linearity with Ag⁺ concentrations, from 0.100 to 150 µM. The estimated limit of detection was 30.8 nM, which is far lower than the acceptable limit of 0.930 µM from the regulatory agency. The TA/PUL-AuNPs probe was further tested for Ag⁺ detection in lake water samples, and it displayed satisfactory detection performances for real sample applications.

Toward a new definition of surface energy for late transition metals

Surface energy is a top-importance stability descriptor of transition metal-based catalysts. Here, we combined density functional theory (DFT) calculations and a tiling scheme measuring surface areas of metal structures to develop a simple computational model predicting the average surface energy of metal structures independently of their shape. The metals considered are W, Ru, Co, Ir, Ni, Pd, Pt, Cu, Ag and Au. Lorentzian trends derived from the DFT data proved effective at predicting the surface energy of metallic surfaces but not for metal clusters. We used machine-learning protocols to build an algorithm that improves the Lorentzian trend's accuracy and is able to predict the surface energies of metal surfaces of any crystal structure, i.e., face-centred cubic, hexagonal close-packed, and body-centred cubic, but also of nanostructures and sub-nanometer clusters. The machine-learning neural network takes easy-to-compute geometric features to predict metallic moieties surface energies with a mean absolute error of 0.091 J m^-2 and an R² score of 0.97.

Electronic Tuning of Gold Nanoparticle Active Sites for Reduction Catalysis

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

Theoretical Tools to Quantify Stochastic Fluctuations in Single-Molecule Catalysis by Enzymes and Nanoparticles

Single-molecule microscopic techniques allow the counting of successive turnover events and the study of the time-dependent fluctuations of the catalytic activities of individual enzymes and different sites on a single heterogeneous nanocatalyst. It is important to establish theoretical methods to obtain the statistical measurements of such stochastic fluctuations that provide insight into the catalytic mechanism. In this review, we discuss a few theoretical frameworks for evaluating the first passage time distribution functions using a self-consistent pathway approach and chemical master equations, to establish a connection with experimental observables. The measurable probability distribution functions and their moments depend on the molecular details of the reaction and provide a way to quantify the molecular mechanisms of the reaction process. The statistical measurements of these fluctuations should provide insight into the enzymatic mechanism.

Biomedical Applications of Biosynthesized Gold Nanoparticles from Cyanobacteria: an Overview

Recently there had been a great interest in biologically synthesized nanoparticles (NPs) as potential therapeutic agents. The shortcomings of conventional non-biological synthesis methods such as generation of toxic byproducts, energy consumptions, and involved cost have shifted the attention towards green syntheses of NPs. Among noble metal NPs, gold nanoparticles (AuNPs) are the most extensively used ones, owing to the unique physicochemical properties. AuNPs have potential therapeutic applications, as those are synthesized with biomolecules as reducing and stabilizing agent(s). The green method of AuNP synthesis is simple, eco-friendly, non-toxic, and cost-effective with the use of renewable energy sources. Among all taxa, cyanobacteria have attracted considerable attention as nano-biofactories, due to cellular uptake of heavy metals from the environment. The cellular bioactive pigments, enzymes, and polysaccharides acted as reducing and coating agents during the process of biosynthesis. However, cyanobacteria-mediated AuNPs have potential biomedical applications, namely, targeted drug delivery, cancer treatment, gene therapy, antimicrobial agent, biosensors, and imaging.

Bottom-up Synthesis of Water-Soluble Gold Nanoparticles Stabilized by N-Heterocyclic Carbenes: From Structural Characterization to Applications

N-heterocyclic carbenes (NHCs) have become attractive ligands for functionalizing gold nanoparticle surfaces with applications ranging from catalysis to biomedicine. Despite their great potential, NHC stabilized gold colloids (NHC@AuNPs) are still scarcely explored and further efforts should be conducted to improve their design and functionalization. Here, the 'bottom-up' synthesis of two water-soluble gold nanoparticles (AuNP-1 and AuNP-2) stabilized by hydrophilic mono- and bidentate NHC ligands is reported together with their characterization by various spectroscopic and analytical methods. The NPs showed key differences likely to be due to the selected NHC ligand systems. Transmission electron microscopy (TEM) images showed small quasi-spherical and faceted NHC@AuNPs of similar particle size (ca. 2.3-2.6 nm) and narrow particle size distribution, but the colloids featured different ratios of Au(I)/Au(0) by X-ray photoelectron spectroscopy (XPS). Furthermore, the NHC@AuNPs were supported on titania and fully characterized. The new NPs were studied for their catalytic activity towards the reduction of nitrophenol substrates, the reduction of resazurin and for their photothermal efficiency. Initial results on their application in photothermal therapy (PTT) were obtained in human cancer cells in vitro. The aforementioned reactions represent important model reactions towards wastewater remediation, bioorthogonal transformations and cancer treatment.

Revealing the Catalytic Activities of Single-Dispersed Pd Clusters Embedded on Hollow Nanoparticles toward the Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is one of the revolutionary aspects that grab lots of attention for the production of clean energy sources, increasing the demands of revealing the intrinsic activities of HER catalysts for designing efficient candidates. Based on using only surface active sites on solid nanoparticles (SNPs), catalysts with high atom dispersion are immensely desirable to magnify their efficiency through maximum atom utilization. Herein, we employed hollow mesoporous silica nanoparticles (HMSNPs) as an insulating nanoplatform for engineering single-dispersed Pd clusters on their surface, assuring high dispersion of the Pd clusters. We then tracked their intrinsic activities using stochastic nanocollision electrochemistry (SNCEC). The insulating silica nanoplatform helped investigate the single-dispersed Pd clusters per contact point of Pd@HMSNP at the electrode surface, revealing exceptional HER performance with high maximum turnover numbers and low onset potential for initiating the HER compared to those of SNPs. Using insulating silica allowed electron transfer only from the single-dispersed Pd cluster at the contact point of Pd@HMSNP to the electrode. Moreover, the high-temporal measurement of SNCEC revealed the diversity of spike shapes based on the heterogeneity of the contact point of Pd@HMSNP, ensuring the capability of the single-entity measurements to distinguish the structure relationship behaviors of the single-dispersed Pd cluster at the electrode/solution interface and clearly clarifying the electron transfer process with complementary information. This work provides sufficient evidence for the importance of atom dispersions in designing highly efficient HER Pd catalysts.

Observation and Analysis of Staircase Response of Single Palladium Nanoparticle Collision on Gold Ultramicroelectrodes

Collision (or impact) of single palladium nanoparticles (Pd NPs) on gold (Au), copper (Cu), nickel (Ni), and platinum (Pt) ultramicroelectrodes (UMEs) were investigated via electrocatalytic amplification method. Unlike the blip responses of previous Pd NP collision studies, the staircase current response was obtained with the Au UME. The current response, including collision frequency and peak magnitude, was analyzed depending on the material of the UME and the applied potential. Adsorption factors implying the interaction between the Pd NP and the UMEs are suggested based on the experimental results.

Atomistic origin of nano-silver paracrystalline structure: molecular dynamics and x-ray diffraction studies

Classical molecular dynamics (MD) and x-ray diffraction (XRD) have been used to establish the origin of the paracrystalline structure of silver nanoparticles at the atomic scale. Models based on the face-centred cubic structure have been computer generated and their atomic arrangements have been optimized by the MD with the embedded-atom model (EAM) potential and its modified version (MEAM). The simulation results are compared with the experimental XRD data in reciprocal and real spaces, i.e. the structure factor and the pair distribution function. The applied approach returns the structural models, defined by the Cartesian coordinates of the constituent atoms. It has been found that most of the structural features of Ag nanoparticles are better reproduced by the MEAM. The presence of vacancy defects in the structure of the Ag nanoparticles has been considered and the average concentration of vacancies is estimated to be 3 at.%. The average nearest-neighbour Ag-Ag distances and the coordination numbers are determined and compared with the values predicted for the bulk Ag, demonstrating a different degree of structural disorder on the surface and in the core, compared to the bulk crystalline counterpart. It has been shown that the paracrystalline structure of the Ag nanoparticles has origin in the surface disorder and the disorder generated by the presence of the vacancy defects. Both sources lead to network distortion that propagates proportionally to the square root of the interatomic distances.

Electrochemical Detection and Analysis of Various Current Responses of a Single Ag Nanoparticle Collision in an Alkaline Electrolyte Solution

A single silver (Ag) nanoparticle (NP) collision was observed and analyzed in an alkaline solution using the electrocatalytic amplification (EA) method. Previously, the observation of a single Ag NP collision was only possible through limited methods based on a self-oxidation of Ag NPs or a blocking strategy. However, it is difficult to characterize the electrocatalytic activity of Ag NPs at a single NP level using a method based on the self-oxidation of Ag NPs. When using a blocking strategy, size analysis is difficult owing to the edge effect in the current signal. The fast oxidative dissolution of Ag NPs has been a problem for observing the staircase response of a single Ag NP collision signal using the EA method. In alkaline electrolyte conditions, Ag oxides are stable, and the oxidative dissolution of Ag NPs is sluggish. Therefore, in this study, the enhanced magnitude and frequency of the current response for single Ag NP collisions were obtained using the EA method in an alkaline electrolyte solution. The peak height and frequency of single Ag NP collisions were analyzed and compared with the theoretical estimation.

Reactive Molecular Dynamics Simulations of the Depolymerization of Polyethylene Using Graphene-Oxide-Supported Platinum Nanoparticles

While plastic materials offer many benefits to society, the slow degradation and difficulty in recycling plastics raise important environmental and sustainability concerns. Traditional recycling efforts often lead to materials with inferior properties and correspondingly lower value, making them uneconomical to recycle. Recent efforts have shown promising chemical pathways for converting plastic materials into a wide range of value-added products, feedstocks or monomers. This is commonly referred to as "chemical recycling". Here, we use reactive molecular dynamics (MD) simulations to study the catalytic process of depolymerization of polyethylene (PE) using platinum (Pt) nanoparticles (NPs) in comparison to PE pyrolysis (thermal degradation). We apply a simple kinetic model to our MD results for the catalytic reaction rate as a function of temperature, from which we obtain the activation energy of the reaction, which shows the that the Pt NPs reduce the barrier for depolymerization. We further evaluate the molecular mass distribution of the reaction products to gain insight into the influence of the Pt NPs on reaction selectivity. Our results demonstrate the potential for the reactive MD method to help the design of recycling approaches for polymer materials.

Progress and perspectives in single-molecule optical spectroscopy

We review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in fluorescence, super-resolution, photothermal contrast, and interferometric scattering and briefly discuss a few of the applications. These advances have enabled the exploration of new emitters and quantum optics; the chemistry and biology of complex heterogeneous systems, nanoparticles, and plasmonics; and the detection and study of non-fluorescing and non-absorbing nano-objects. We conclude by proposing some ideas for future experiments. The field will move toward more and better signals of a broader variety of objects and toward a sharper view of the surprising complexity of the nanoscale world of single (bio-)molecules, nanoparticles, and their nano-environments.

Revealing the catalytic kinetics and dynamics of individual Pt atoms at the single-molecule level

Here, with single-molecule fluorescence microscopy, we study the catalytic behavior of individual Pt atoms at single-turnover resolution, and then reveal the unique catalytic properties of Pt single-atom catalyst and the difference in catalytic properties between individual Pt atoms and Pt nanoparticles. Further density functional theory calculation indicates that unique catalytic properties of Pt single-atom catalyst could be attributed intrinsically to the unique surface properties of Pt₁-based active sites.

Due to the importance of single-atom catalysts (SAC), here, the catalysis of Pt SAC was studied at the single-molecule single-atom level. Both static and dynamic activity heterogeneity are observed in Pt SAC. It reveals that the intrinsic catalytic activity of Pt SAC is higher than that of Pt nanoparticles (NPs), although they follow the same bimolecular competition mechanism. Significantly, Pt SAC presents no catalysis-induced surface restructuring, meaning that the dynamic activity fluctuation of Pt SAC can only be attributed to the spontaneous surface restructuring, and the catalysis process does not affect much of the structure of Pt₁-based active sites, all different from Pt NP catalysis, in which the surface restructuring and the catalysis can affect each other. Further, density functional theory (DFT) calculation indicates that the unique catalytic properties of Pt SAC or the different catalytic properties between Pt SAC and NPs could be attributed to the strong adsorptions of both reactant and product on Pt SAC, large surface energy of Pt SAC, and strong binding of Pt₁ on support. Knowledge revealed here provides fundamental insights into the catalysis of atomically dispersed catalyst.

A review: zinc oxide nanoparticles - friends or enemies?

Modern nanotechnology allows obtaining zinc oxide nanomaterials with unique properties that let its use in a wide range of commercial applications. Direct contact with these particles as well as their release into the environment is almost inevitable. This review aims to consider whether the toxicity of zinc oxide nanoparticles found in numerous test models is a real threat to humans and plants. Emerging reports indicated both the risks and benefits associated with the use of zinc oxide nanoparticles in a manner dependent on the concentration and a method of synthesis, as well as the tested object. The amounts needed to achieve the antibacterial activity of ZnO-NPs, and the reported amounts of these nanoparticles in consumer products are sufficient to have a negative impact on living organisms. The most sensitive to their action are human cells, and the mechanism of cytotoxicity is mainly associated with the formation of oxidative stress caused by the action of zinc ions. ZnO-NPs in small concentration can have positive affect to plants, but it poses a threat to more sensitive ones.

Encapsulation within a coordination cage modulates the reactivity of redox-active dyes

Confining molecules within well-defined nanosized spaces can profoundly alter their physicochemical characteristics. For example, the controlled aggregation of chromophores into discrete oligomers has been shown to tune their optical properties whereas encapsulation of reactive species within molecular hosts can increase their stability. The resazurin/resorufin pair has been widely used for detecting redox processes in biological settings; yet, how tight confinement affects the properties of these two dyes remains to be explored. Here, we show that a flexible Pd^II₆L₄ coordination cage can efficiently encapsulate both resorufin and resazurin in the form of dimers, dramatically modulating their optical properties. Furthermore, binding within the cage significantly decreases the reduction rate of resazurin to resorufin, and the rate of the subsequent reduction of resorufin to dihydroresorufin. During our studies, we also found that upon dilution, the Pd^II₆L₄ cage disassembles to afford Pd^II₂L₂ species, which lacks the ability to form inclusion complexes - a process that can be reversed upon the addition of the strongly binding resorufin/resazurin guests. We expect that the herein disclosed ability of a water-soluble cage to reversibly modulate the optical and chemical properties of a molecular redox probe will expand the versatility of synthetic fluorescent probes in biologically relevant environments.

Amyloid-Templated Palladium Nanoparticles for Water Purification by Electroreduction

Electrocatalysis offers great promise for water purification but is limited by low active area and high uncontrollability of electrocatalysts. To overcome these constraints, we propose hybrid bulk electrodes by synthesizing and binding a Pd nanocatalyst (nano‐Pd) to the electrodes via amyloid fibrils (AFs). The AFs template is effective for controlling the nucleation, growth, and assembly of nano‐Pd on the electrode. In addition, the three‐dimensional hierarchically porous nanostructure of AFs is beneficial for loading high‐density nano‐Pd with a large active area. The novel hybrid cathodes exhibit superior electroreduction performance for the detoxification of hexavalent chromium (Cr⁶⁺), 4‐chlorophenol, and trichloroacetic acid in wastewater and drinking water. This study provides a proof‐of‐concept design of an AFs‐templated nano‐Pd‐based hybrid electrode, which constitutes a paradigm shift in electrocatalytic water purification, and broadens the horizon of its potential engineered applications.

AuPt Bimetallic Nanozymes for Enhanced Glucose Catalytic Oxidase

Au metal nanoparticles as artificial nanozymes have attracted wide interest in biotechnology due to high stability and easy synthesis. Unfortunately, its catalytic activity is limited by the uniform surface electron distribution, fundamentally affecting the oxidation efficiency of glucose. Here, we synthesized AuPt bimetallic nanoparticles with unique surface electron structure due to the coupling effect of the two metal components, achieving improved glucose catalytic oxidase. Because of the effective work function difference between the two metals in AuPt, the electrons will transfer from Au to accumulate on Pt, simultaneously contributing to the substantial enhancement of Au-induced glucose oxidase and Pt-induced catalase performance. We systematically studied the enzyme-catalytic efficiency of AuPt with varied two metal proportions, in which Au:Pt at 3:1 showed the highest catalytic efficiency of glucose oxidase in solution. The AuPt nanoparticles were further co-cultured with cells and also showed excellent biological activity for glucose oxidase. This work demonstrates that the physicochemical properties between different metals can be exploited for engineering high-performance metal nanoparticle-based nanozymes, which opens up a new way to rationally design and optimize artificial nanozymes to mimic natural enzymes.

Varying protein architectures in 3-dimensions for scaffolding and modulating properties of catalytic gold nanoparticles

Fabrication and development of nanoscale materials with tunable structural and functional properties require a dynamic arrangement of nanoparticles on architectural templates. The function of nanoparticles not only depends on the property of the nanoparticles but also on their spatial orientations. Proteins with self-assembling properties which can be genetically engineered to varying architectural designs for scaffolds can be used to develop different orientations of nanoparticles in three dimensions. Here, we report the use of naturally self-assembling bacterial micro-compartment shell protein (PduA) assemblies in 2D and its single-point mutant variant (PduA[K26A]) in 3D architectures for the reduction and fabrication of gold nanoparticles. Interestingly, the different spatial organization of gold nanoparticles resulted in a smaller size in the 3D architect scaffold. Here, we observed a two-fold increase in catalytic activity and six-fold higher affinity toward TMB (3,3',5,5'-tetramethylbenzidine) substrate as a measure of higher peroxidase activity (nanozymatic) in the case of PduA[K26A] 3D scaffold. This approach demonstrates that the hierarchical organization of scaffold enables the fine-tuning of nanoparticle properties, thus paving the way toward the design of new nanoscale materials.