Fabrication of Ag-Cu2O/Reduced Graphene Oxide Nanocomposites as Surface-Enhanced Raman Scattering Substrates for in Situ Monitoring of Peroxidase-Like Catalytic Reaction and Biosensing

Ag-Cu2O Supported Biomass-Derived rGO for Catalyzing Suzuki-Miyaura Cross-Coupling

The search for cost-effective, efficient, and ecofriendly heterogeneous catalysts for the Suzuki-Miyaura reaction is crucial due to challenges with expensive, toxic homogeneous catalysts. This study centrally aims at crafting a pioneering green catalyst by adorning reduced graphene oxide (rGO), sourced from basil seeds (Ocimum basilicum L.), with an Ag-Cu2O composite structure. Comprehensive characterization of the Ag-Cu2O/rGO nanocomposite was conducted through FTIR, SEM, hHR-TEM, EDS, XPS, XRD, TGA, and N2 adsorption/desorption analyses. Results showed that nanosized Ag-Cu2O particles were partially integrated into rGO sheets derived from basil seeds, acting as active species for oxidative addition with aryl halides in the SMR. The catalytic efficacy of this robust nanocatalyst was assessed in Suzuki-Miyaura cross-coupling reactions, targeting the synthesis of biaryls employing various aryl halides and aryl boronic acids. The findings underscore that the Ag-Cu2O/rGO nanocatalyst manifests rapid reaction kinetics (15 min) alongside commendable yields (99%). The Ag-Cu2O/rGO demonstrates impressive recyclability, maintaining catalytic efficiency over four cycles. Utilizing it as a green substrate for metal loading highlights its potential, offering well-defined coordination sites. This approach facilitates stable heterogeneous catalyst fabrication, crucial for significant bond formations. Notable features include broad applicability, exceptional functional tolerance, scalability, and practicality. Moreover, it holds promise for automating safe processes and enabling efficient late-stage functionalization of complex molecules with moderate to high efficiency, presenting promising prospects for various applications in chemical synthesis.

NIR-II-Responsive Versatile Nanozyme Based on H2O2 Cycling and Disrupting Cellular Redox Homeostasis for Enhanced Synergistic Cancer Therapy

Disturbing cellular redox homeostasis within malignant cells, particularly improving reactive oxygen species (ROS), is one of the effective strategies for cancer therapy. The ROS generation based on nanozymes presents a promising strategy for cancer treatment. However, the therapeutic efficacy is limited due to the insufficient catalytic activity of nanozymes or their high dependence on hydrogen peroxide (H2O2) or oxygen. Herein, we reported a nanozyme (CSA) based on well-defined CuSe hollow nanocubes (CS) uniformly covered with Ag nanoparticles (AgNPs) to disturb cellular redox homeostasis and catalyze a cascade of intracellular biochemical reactions to produce ROS for the synergistic therapy of breast cancer. In this system, CSA could interact with the thioredoxin reductase (TrxR) and deplete the tumor microenvironment-activated glutathione (GSH), disrupting the cellular antioxidant defense system and augmenting ROS generation. Besides, CSA possessed high peroxidase-mimicking activity toward H2O2, leading to the generation of various ROS including hydroxyl radical (•OH), superoxide radicals (•O2-), and singlet oxygen (1O2), facilitated by the Cu(II)/Cu(I) redox and H2O2 cycling, and plentiful catalytically active metal sites. Additionally, due to the absorption and charge separation performance of AgNPs, the CSA exhibited excellent photothermal performance in the second near-infrared (NIR-II, 1064 nm) region and enhanced the photocatalytic ROS level in cancer cells. Owing to the inhibition of TrxR activity, GSH depletion, high peroxidase-mimicking activity of CSA, and abundant ROS generation, CSA displays remarkable and specific inhibition of tumor growth.

Methylmercury-sensitized "turn on" SERS-active peroxidase-like activity of carbon dots/Au NPs nanozyme for selective detection of ochratoxin A in coffee

The improvement and regulation of catalytic performance of nanozyme have long been pursued with sustained efforts. Herein, gold nanoparticles (S-CDs/AuNPs) with weak peroxidase-like (POD) activity were synthesized by Au-S bond using a sulfur doped carbon dots (S-CDs) as reducing agent and stabilizer. However, methylmercury (MeHg+) could selectively and sensitively regulate the POD-like activity of S-CDs/AuNPs. The catalytic activity of S-CDs/AuNPs was significantly activated with the addition of MeHg+, resulting in a significant enhancement of electromagnetic fields to present an obvious SERS signal. More intriguingly, the introduction of ochratoxin A (OTA) could simultaneously turn off the UV-vis absorbance signals and the surface-enhanced Raman scattering (SERS) signal. Based on these findings, a selective colorimetric-SERS dual-mode OTA detection strategy was established with gold amalgamation (Au@HgNPs) as the probe, and the low limit of detection (LOD) of OTA was 0.29 µgL-1 (Colorimetric) and 0.16 µgL-1 (SERS), respectively, with good recoveries from 95.9 to 104.0% (Colorimetric) and from 96.7 to 108.9% (SERS), respectively. The work paves a new way to design nanozyme-based colorimetric and SERS protocol for traces OTA residues analysis in foodstuff analysis.

Enhancing peroxidase-like activity of AuNPs through headspace reaction: A signal amplification strategy for colorimetric and fluorescent sensing of trace Hg2

BACKGROUND

Recently, headspace single-drop microextraction (HS-SDME) has attracted some attention for developing sensitive and selective colorimetric assays due to its excellent capability to reduce matrix interference and enrich analytes. However, the single droplet limits direct visual observation of color change and its quantitative measurement suffers from reduced optical path length. Therefore, amplifying the detection signals in both volume and intensity is an important and challenging task for improving the sensitivity, stability, and accuracy of such colorimetric analysis.

RESULTS

In this study, a "headspace-nanoenzyme" (HS-NE) strategy was proposed that successfully addressed these challenges and enabled the colorimetric and fluorescent dual-mode detection of trace Hg2+. Atomic Hg0, generated via chemical vapor generation (CVG), underwent headspace reaction with AuNPs droplet to form Au@HgNPs, thus catalyzing the oxidation of o-phenylenediamine (OPD) in the presence of H2O2. The absorbance and fluorescence intensity of oxidized OPD were proportion to the concentration of Hg2+ in the sample solution. Due to the greatly enhanced peroxidase-like activity by Au@HgNPs, the limit of detection was as low as 0.98 nM and 0.21 nM for the colorimetric and fluorescent modes, respectively. The applicability of this assay was further demonstrated with determination of Hg2+ in real environmental and biological samples. Moreover, a convenient and cost-effective paper-based sensing platform was fabricated for rapid on-site detection of Hg2+.

SIGNIFICANCE AND NOVELTY

This novel HS-NE strategy combines HS-SDME and nanoenzyme-based sensing to achieve dual effects of eliminating matrix interference and amplifying the measurement signal, resulting in improved accuracy, enhanced stability, high sensitivity, and exceptional selectivity, with great potential for on-site determination of trace Hg2+.

Advances in the Application of Transition-Metal Composite Nanozymes in the Field of Biomedicine

Due to the limitation that natural peroxidase enzymes can only function in relatively mild environments, nanozymes have expanded the application of enzymology in the biological field by dint of their ability to maintain catalytic oxidative activity in relatively harsh environments. At the same time, the development of new and highly efficient composite nanozymes has been a challenge due to the limitations of monometallic particles in applications and the inherently poor enzyme-mimetic activity of composite nanozymes. The inherent enzyme-mimicking activity is due to Au, Ag, and Pt, along with other transition metals. Moreover, the nanomaterials exhibit excellent enzyme-mimicking activity when composited with other materials. Therefore, this paper focuses on composite nanozymes with simulated peroxidase activity that have been prepared using noble metals such as Au, Ag, and Pt and other transition metal nanoparticles in recent years. Their simulated enzymatic activity is utilized for biomedical applications such as glucose detection, cancer cell detection and tumor treatment, and antibacterial applications.

Plasmonic Ag@Cu2O core-shell nanostructures exhibiting near-infrared photothermal effect

This work was devoted to the investigation of the optical properties, structural characterization, and photothermal conversion performance of Ag@Cu2O nanostructures. The selection of anisotropic silver core, specifically Ag nanocubes, was driven by the possibility to tune LSPR across a broader range of the electromagnetic spectrum. The thickness of the Cu2O shell was intentionally changed through the variation in the Cu salt to the metal core nanoparticles ratios. The LSPRs of Ag(nanocube)@Cu2O core-shell nanoparticles can be fine-tuned to the spectral region to become resonant with the excitation wavelengths of 808 nm NIR laser. Due to the high refractive index of the deposited Cu2O, the redshifts of the plasmon band wavelength in the extinction spectra were observed. Consequently, the photothermal activities of the Ag(nanocube)@Cu2O core-shell NPs have been controlled by the shell thickness at the nanoscale. Ag@Cu2O nanoparticles with thickest shell (∼70 nm) exhibit the most efficient NIR photothermal effect under the irradiation of 808 nm laser at ambient conditions. Results of this work demonstrate that Ag@Cu2O hetero-nanostructures may be optimized and used for the efficient transformation of light into other forms of energy, specifically heat.

Research Progress of SERS Sensors Based on Hydrogen Peroxide and Related Substances

Hydrogen peroxide (H2O2) has an important role in living organisms, and its detection is of great importance in medical, chemical, and food safety applications. This review provides a comparison of different types of Surface-enhanced Raman scattering (SERS) sensors for H2O2 and related substances with respect to their detection limits, which are of interest due to high sensitivity compared to conventional sensors. According to the latest research report, this review focuses on the sensing mechanism of different sensors and summarizes the linear range, detection limits, and cellular applications of new SERS sensors, and discusses the limitations in vivo and future prospects of SERS technology for the detection of H2O2.

Advances in Surface-Enhanced and Tip-Enhanced Raman Spectroscopy, Mapping and Methods Combined with Raman Spectroscopy for the Characterization of Perspective Carbon Nanomaterials

Surface-enhanced Raman spectroscopy (SERS) is based on the effect of the plasmonic enhancement of intensity of the Raman scattering of molecules in cases when they are adsorbed on a substrate [...].

Novel ultrasensitive Raman assay method based on enzyme mimetics for ultra trace of H2O2

Enzyme mimetics have been widely applied on H₂O₂ assay, but it is still challenging and interesting to realize the sensitive detection for ultra-trace H₂O₂. Here, an ultrasensitive Raman assay method based on novel WO₃@IP6-Fe³⁺ enzyme mimetics with peroxidase-like activity was established. WO₃ microspheres (MSs) were found to have weak peroxidase-like activity, and the combination of IP6-Fe³⁺ and WO₃ can produce stronger activity. WO₃@IP6-Fe³⁺ MSs showed polyhedron-like structure, uniform size, and smooth surface. Although WO₃@IP6-Fe³⁺ enzyme mimetics have low catalytic efficiency and high absorbance background, the proposed Raman method can bypass the above problems. In Raman method, high concentration of WO₃@IP6-Fe³⁺ can be used to overcome low catalytic efficiency without high absorbance background. Moreover, 3,3',5,5'-tetramethylbenzidine oxide has prominent characteristic Raman peak at 1608 cm^-1, greatly improving the sensitivity and eliminating interference of impurities. Due to the high sensitivity and low background, Raman assay showed the ultra-low limit of detection (5.49 × 10^-15 M), which was 4-7 orders of magnitude lower than other detection methods. The ultrasensitive Raman assay not only provided the possibility for the enzyme mimetics-based detection of ultra-trace H₂O₂, but also enable the enzyme mimetics with low activity to be applied.

Nanotechnology‐based electrochemical biosensors for monitoring breast cancer biomarkers

Breast cancer is categorized as the most widespread cancer type among women globally. On-time diagnosis can decrease the mortality rate by making the right decision in the therapy procedure. These features lead to a reduction in medication time and socioeconomic burden. The current review article provides a comprehensive assessment for breast cancer diagnosis using nanomaterials and related technologies. Growing use of the nano/biotechnology domain in terms of electrochemical nanobiosensor designing was discussed in detail. In this regard, recent advances in nanomaterial applied for amplified biosensing methodologies were assessed for breast cancer diagnosis by focusing on the advantages and disadvantages of these approaches. We also monitored designing methods, advantages, and the necessity of suitable (nano) materials from a statistical standpoint. The main objective of this review is to classify the applicable biosensors based on breast cancer biomarkers. With numerous nano-sized platforms published for breast cancer diagnosis, this review tried to collect the most suitable methodologies for detecting biomarkers and certain breast cancer cell types.

Optical Biosensor Based on Graphene and Its Derivatives for Detecting Biomolecules

Graphene and its derivatives show great potential for biosensing due to their extraordinary optical, electrical and physical properties. In particular, graphene and its derivatives have excellent optical properties such as broadband and tunable absorption, fluorescence bursts, and strong polarization-related effects. Optical biosensors based on graphene and its derivatives make nondestructive detection of biomolecules possible. The focus of this paper is to review the preparation of graphene and its derivatives, as well as recent advances in optical biosensors based on graphene and its derivatives. The working principle of face plasmon resonance (SPR), surface-enhanced Raman spectroscopy (SERS), fluorescence resonance energy transfer (FRET) and colorimetric sensors are summarized, and the advantages and disadvantages of graphene and its derivatives applicable to various types of sensors are analyzed, and the methods of surface functionalization of graphene and its derivatives are introduced; these optical biosensors can be used for the detection of a range of biomolecules such as single cells, cellular secretions, proteins, nucleic acids, and antigen-antibodies; these new high-performance optical sensors are capable of detecting changes in surface structure and biomolecular interactions with the advantages of ultra-fast detection, high sensitivity, label-free, specific recognition, and the ability to respond in real-time. Problems in the current stage of application are discussed, as well as future prospects for graphene and its biosensors. Achieving the applicability, reusability and low cost of novel optical biosensors for a variety of complex environments and achieving scale-up production, which still faces serious challenges.

An Electrochromic Ag-Decorated WO3-x Film with Adjustable Defect States for Electrochemical Surface-Enhanced Raman Spectroscopy

Electrochemical surface-enhanced Raman scattering (EC-SERS) spectroscopy is an ultrasensitive spectro-electrochemistry technique that provides mechanistic and dynamic information on electrochemical interfaces at the molecular level. However, the plasmon-mediated photocatalysis hinders the intrinsic electrochemical behavior of molecules at electrochemical interfaces. This work aimed to develop a facile method for constructing a reliable EC-SERS substrate that can be used to study the molecular dynamics at electrochemical interfaces. Herein, a novel Ag-WO_3-x electrochromic heterostructure was synthesized for EC-SERS. Especially, the use of electrochromic WO_3-x film suppresses the influence of hot-electrons-induced catalysis while offering a reliable SERS effect. Based on this finding, the real electrochemical behavior of p-aminothiophenol (PATP) on Ag nanoparticles (NPs) surface was revealed for the first time. We are confident that metal-semiconductor electrochromic heterostructures could be developed into reliable substrates for EC-SERS analysis. Furthermore, the results obtained in this work provide new insights not only into the chemical mechanism of SERS, but also into the hot-electron transfer mechanism in metal-semiconductor heterostructures.

Glucose detection through surface-enhanced Raman spectroscopy: A review

Glucose detection is of vital importance to diabetes diagnosis and treatment. Optical approaches in glucose sensing have received much attention in recent years due to the relatively low cost, portable, and mini-invasive or non-invasive potentials. Surface enhanced Raman spectroscopy (SERS) endows the benefits of extremely high sensitivity because of enhanced signals and specificity due to the fingerprint of molecules of interest. However, the direct detection of glucose through SERS was challenging because of poor adsorption of glucose on bare metals and low cross section of glucose. In order to address these challenges, several approaches were proposed and utilized for glucose detection through SERS. This review article mainly focuses on the development of surface enhanced Raman scattering based glucose sensors in recent 10 years. The sensing mechanisms, rational design and sensing properties to glucose are reviewed. Two strategies are summarized as intrinsic sensing and extrinsic sensing. Four general categories for glucose sensing through SERS are discussed including SERS active platform, partition layer functionalized surface, boronic acid based sensors, and enzymatic reaction based biosensors. Finally, the challenges and outlook for SERS based glucose sensors are also presented.

Facile synthesis of nanocellulose-based Cu2O/Ag heterostructure as a surface-enhanced Raman scattering substrate for trace dye detection

Semiconductor metal-oxide/metal heterostructures with synergetic properties have potential applications in photocatalysis and optical sensors. Here, Cu₂O sub-micro cubes were synthesized under environmentally benign conditions using 2, 2, 6, 6-tetramethylpyperdine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils as a reducing and stabilizing agent. Then the surface of the Cu₂O cubes was decorated with silver nanoparticles (AgNPs) by a substitution reaction. The Cu₂O/Ag heterostructures within the cellulose nanofibrils (CNFs) network were employed as a promising surface-enhanced Raman scattering (SERS) assay for efficient sensing of methylene blue (MB), reaching a maximum enhancement factor (EF) of 4.0 × 10⁴. Their SERS intensities depended on the coverage density of AgNPs and the wavelength of the excitation laser. The excellent SERS performance may result from the charge transfer between Ag and Cu₂O molecules and the strong electromagnetic field at the interface. The CNF-Cu₂O/Ag substrates were capable of detecting MB dye down to 10^-8 M level with a relative standard deviation of 10-15%, demonstrating great sensitivity and reproducibility.

Hollow spheres of iron oxide as an “enzyme-mimic”: preparation, characterization and application as biosensors

Nanostructured hollow spheres of iron oxide are demonstrated as “nanozymes” for the dual mode (spectrophotometric and electrochemical) detection of hydrogen peroxide & cholesterol biomarkers and a novel electrochemical sensing mechanism is proposed.

Colorimetry /SERS dual-sensor of H2O2 constructed via TMB-Fe3O4@ AuNPs

Hydrogen peroxide (H₂O₂) detection with high sensitivity plays an important role in biomedical research and food engineering. By combining colorimetry and surface enhanced Raman spectroscopy (SERS), we synthetize a novel H₂O₂ dual-sensor constructed via TMB-Fe₃O₄@AuNPs. In the presence of H₂O₂, the peroxide model enzyme might catalyze the oxidation of 3,3',5,5'- tetramethylbenzidine (TMB) as blue charge transfer complex (CTC) for colorimetry, and then facilitate the sensitivity improvement of SERS detection. The achieved results show that in colorimetry, the linear range is from 40 μM to 5.5 mM with the detection limit of 11.1 μM; in SERS detection, the linear range is from 2 nM to 1 μM with the detection limit of 0.275 nM. Clearly, this mutual reference strategy improves both the detection limit of colorimetry and the sensitivity of SERS detection. Moreover, this colorimetry/SERS dual-sensor constructed via TMB-Fe₃O₄@AuNPs is successfully applied to the H₂O₂ detection in plasma and milk, indicating the excellent performance and flexibility.

One-Pot Facile Synthesis of Noble Metal Nanoparticles Supported on rGO with Enhanced Catalytic Performance for 4-Nitrophenol Reduction

In this study, reduced graphene oxide (rGO)-supported noble metal (gold, silver, and platinum) nanoparticle catalysts were prepared via the one-pot facile co-reduction technique. Various measurement techniques were used to investigate the structures and properties of the catalysts. The relative intensity ratios of I_D/I_G in rGO/Au, rGO/Ag, rGO/Pt, and GO were 1.106, 1.078, 1.047, and 0.863, respectively. The results showed the formation of rGO and that noble metal nanoparticles were decorated on rGO. Furthermore, the catalytic activities of the designed nanocomposites were investigated via 4-nitrophenol. The catalysts were used in 4-nitrophenol reduction. The catalytic performance of the catalysts was evaluated using the apparent rate constant k values. The k value of rGO/Au was 0.618 min^-1, which was higher than those of rGO/Ag (0.55 min^-1) and rGO/Pt (0.038 min^-1). The result proved that the rGO/Au catalyst exhibited a higher catalytic performance than the rGO/Ag catalyst and the rGO/Pt catalyst. The results provide a facile method for the synthesis of rGO-supported nanomaterials in catalysis.

Significantly enhanced photothermal catalytic hydrogen evolution over Cu2O-rGO/TiO2 composite with full spectrum solar light

Reduced graphene oxide (rGO) has conspicuous photothermal characteristics in photothermal applications. Thus in our previous work, we used reduced graphene oxide (rGO) supported titanium dioxide (TiO₂) nanocomposite (rGO/TiO₂) to absorb the ultraviolet and infrared light in the photothermal hydrogen evolution process. In order to make use of the full spectrum solar energy into other clear energy, the visible light should be also considered in following research. Herein, we report a cuprous oxide (Cu₂O) decorated reduced graphene oxide (rGO) supported titanium dioxide (TiO₂) (Cu₂O-rGO/TiO₂) catalysts, which can absorb full spectrum solar light in an innovative way. The Cu₂O-rGO/TiO₂ catalyst is synthesized through a one-step hydrothermal method. The rates of hydrogen evolution are 17800 μmol·g^-1h^-1 under photothermal condition (90°C), 3800 μmol·g^-1h^-1 under photocatalysis condition only (25°C) and 0 μmol·g^-1h^-1 under thermal catalysis condition only. The result of photothermal catalytic hydrogen evolution rate is about 4.7 times that of the sum of the photocatalytic and thermal reactions. The photothermal synergetic effect promotes the photo-generated electron-holes separation through the rGO due to the temperature rising, and accelerates the reaction rates on the catalyst surface in hydrogen evolution process simultaneously. This work could provide us a new promising way for the conversion of full spectrum solar energy to hydrogen energy.

Bimetallic Metal-Organic Frameworks: Enhanced Peroxidase-like Activities for the Self-Activated Cascade Reaction

Metal-organic frameworks (MOFs) are significant useful molecular materials as a result of their high surface area and flexible catalytic activities by tuning the metal centers and ligands. MOFs have attracted great attention as efficient nanozymes recently; however, it is still difficult to understand polymetallic MOFs for enzymatic catalysis because of their complicated structure and interactions. Herein, bimetallic NiFe₂ MOF octahedra were well prepared and exhibited enhanced peroxidase-like activities. The synergistic effect of Fe and Ni atoms was systematically investigated by electrochemistry, X-ray photoelectron spectrometry, (XPS) and in situ Raman techniques. The electrons tend to transfer from Ni²⁺ to Fe³⁺ in NiFe₂ MOFs, and the resulting Fe²⁺ is ready to decompose H₂O₂ and generate ^·OH by a Fenton-like reaction. After integration with glucose oxidase (GOx), which can downgrade the pH value and generate H₂O₂ by oxidation of glucose, a self-activated cascade reagent is therefore established for efficiently inducing cell death. The changes of cell morphology, DNA, and protein are also successfully recorded during the cell death process by Raman spectroscopy and fluorescence imaging.

In-situ surface-enhanced Raman scattering based on MTi20 nanoflowers: Monitoring and degradation of contaminants

Real-time and in-situ monitoring of chemical reactions has attracted great attention in many fields. In this work, we in-situ monitored the photodegradation reaction process of methylene blue (MB) by Surface enhanced Raman scattering (SERS) technique. An effective and versatile SERS platform assembled from MoS₂ nanoflowers (NFs) and TiO₂ nanoparticles (NPs) was prepared successfully. The optimized MoS₂/TiO₂ substrate (MTi₂₀) exhibits not only an ultra-high SERS response but also the excellent catalytic degradation performance to the contaminant MB, which provided a new material for real-time and in-situ monitoring the photodegradation process. Experiments prove that the detection limit is as low as 10^-13 M, and degradation rate is as high as 97.2% in 180 s, respectively. And the activity of the substrate kept in the air for 90 days is almost unchanged. Furthermore, as a practical SERS substrate, MTi₂₀ can also detect trace amounts of other harmful substances including malachite green (MG), bisphenol A (BPA) and endosulfan. Thus, this study come up with a new orientation at the real-time and in-situ monitoring of photocatalytic reaction and may be applied in environmental monitoring and food security fields in the future.

Charge transfer channels of silver @ cuprous oxide heterostructure core-shell nanoparticles strengthen high photocatalytic antibacterial activity

Marine biological fouling has always been a hot research topic. In this study, silver @ cuprous oxide (Ag@Cu₂O) core-shell nanoparticles were synthesized via in-situ synthesis method and developed an outstanding antibacterial activity. The bacteriostasis efficiency of Ag@Cu₂O reached to 99% and 98% against Staphylococcus aureus and Pseudomonas aeruginosa, respectively. The minimum inhibitory concentration of Ag@Cu₂O decreased from 113.6 μg/mL to 56.8 μg/mL compared with Cu₂O. Ag@Cu₂O had better antibacterial activity than Cu₂O with lower content of Cu₂O and was more environment friendly. The heterostructure formed at the interface between Ag and Cu₂O promoted the separation and diffusion of photogenerated electron-hole pairs through the charge transfer channel and promoted the generation of reactive oxygen species. The outstanding antibacterial activity of Ag@Cu₂O was strongly depended on the generation of the reactive oxygen species. Density functional theory and finite element method calculations demonstrated that the structure of core-shell improved photocatalytic efficiency. Additionally, synergetic effect of released Ag⁺ and Cu²⁺ also enhanced the bacteriostasis rate and the long-term antifouling performance in 60 days. Hence, the synthesized core-shell Ag@Cu₂O can be applied as novel antifoulants in the marine field.

Designing biopolymer-based artificial peroxidase for oxidative removal of dibenzothiophene from a model diesel fuel

Naturally occurring peroxidases are important for living organisms and have manifold utility in industries. However, lack of stability in harsh reaction conditions hinders wide applicability of such enzymes. Thus, suitable alternative is vital which can endure severe reaction conditions. As a substitute of natural peroxidase, herein, biopolymer-based polyelectrolyte complexes (PECs) coordinated with Feⁿ⁺ is proposed as macromolecular peroxidase mimicking systems. Three PECs were engineered via complexation of protonated chitosan and alginate with Fe²⁺ (Fe²⁺-PEC), Fe³⁺ (Fe³⁺-PEC), and Fe₃O₄ (Fe₃O₄-PEC), respectively. Computational study showed the Fe³⁺-PEC was highly stable with abundant electrostatic and intramolecular hydrogen bonding interactions. The versatility of the Fe-PECs as artificial peroxidase biocatalysts was probed by two types of peroxidase assays - ABTS oxidation in buffer systems (pH 4.0 and 7.0) and pyrogallol oxidation in organic solvents (acetonitrile, ethyl acetate and toluene). Overall, Fe³⁺-PEC showed remarkably high peroxidase activity both in aqueous buffers and in organic solvents, whereas, Fe₃O₄-PEC showed least catalytic activity. Finally, as a proof of concept, the ability of the biocatalyst to carry out deep oxidative desulphurization was demonstrated envisaging removal of dibenzothiophene from model fossil fuel in a sustainable way.

Self-Circulation Oxygen-Hydrogen Peroxide-Oxygen System for Ultrasensitive Cathode Photoelectrochemical Bioassay Using a Stacked Sealed Paper Device

In this work, a self-circulation oxygen-hydrogen peroxide-oxygen (O₂-H₂O₂-O₂) system with photogenerated electrons as fuel and highly active hemin monomers as operators was engineered for ultrasensitive cathode photoelectrochemical bioassay of microRNA-141 (miRNA-141) using a stacked sealed paper device. During the circulation, the photogenerated electrons from BiVO₄/Cu₂O photosensitive structures assembled on a reduced graphene oxide paper electrode first reduced the electron acceptors (dissolved O₂) to H₂O₂, which was then catalytically decomposed by hemin monomers to generate O₂ again. The regenerated O₂ continued to be reduced, which made O₂ and H₂O₂ stuck in the infinite loop of O₂-H₂O₂-O₂ accompanied by the fast consumption of photogenerated electrons, generating an amplified photocurrent signal. When a target existed, a duplex-specific nuclease-induced target recycling reaction with dual trigger DNA probes as the output was performed to initiate the assembly of bridge-like DNA nanostructures, which endowed the self-circulation system with dual destruction functions as follows. (i) Reduced fuel supply: the assembled DNA bridges acting as a negatively charged barrier prevented the photogenerated electrons from participating in the O₂ reduction to H₂O₂. (ii) Incapacitation of operators: DNA bridging induced the dimerization of hemin monomers linked on the DNA hairpins to catalytically inactive hemin dimers, leading to the abortive regeneration of O₂. These destruction functions resulted in the circulation interruption and a remarkably decreased photocurrent signal. Thus, the developed cathode photoelectrochemical biosensing platform achieved ultrasensitive miRNA-141 detection with a linear range of 0.25 fM to 1 nM and a detection limit of 83 aM, and it also exhibited high accuracy, selectivity, and practicability.

A facile strategy for synthesis of porous Cu2O nanospheres and application as nanozymes in colorimetric biosensing

Due to the unique advantages, developing a rapid, simple and economical synthetic strategy for porous nanomaterials is of great interest. In this work, for the first time, using sodium hypochlorite as a green oxidant, urea was oxidized to CO2 as a carbon source to prepare the fine-particle crosslinked Cu-precursors, which could be further reduced by sodium ascorbate into pure Cu2O nanospheres (NPs) with a porous morphology at room temperature. Interestingly, our study reveals that introduction of an appropriate amount of MgCl2 into the raw materials can tune the pore sizes and surface area, but has no influence on the phase purity of the resulting Cu2O NPs. Significantly, all the synthesized Cu2O NPs exhibited intrinsic peroxidase-like activity with higher affinity towards both 3,3,5,5-tetramethylbenzidine (TMB) and H2O2 than horseradish peroxidase (HRP) due to the highly porous morphology and the electrostatic attraction towards TMB. The colorimetric detection of glucose based on the resulting porous Cu2O NPs presented a limit of detection (LOD) of 2.19 μM with a broad linear range from 1-1000 μM, much better than many recently reported composite-based nanozymes. Meanwhile, this nanozyme system was utilized to detect l-cysteine, exhibiting a LOD value as low as 0.81 μM within a linear range from 0 to 10 μM. More interesting, this sensing system shows high sensitivity and excellent selectivity in determining glucose and l-cysteine, which is suitable for detecting serum samples with reliable results. Therefore, the present study not only develops a simple strategy to prepare Cu2O NPs with controllable porous structure, but also indicates its promising applications in bioscience and disease diagnosis.

Electroactive Cu2O nanocubes engineered electrochemical sensor for H2S detection

An electrochemical sensor was proposed for the detection of hydrogen sulfide (H₂S) at room temperature, by using electroactive Cu₂O nanocubes (NCs) as an electrochemical beacon. Electroactive Cu₂O NCs were synthesized on the surface of reduced graphene oxide (rGO)/Fe₃O₄ nanosheets (NSs) due to the good electronic conductivity and well-responded magnetic responses. The fabricated rGO/Fe₃O₄/Cu₂O NSs not only showed electrochemical oxidization peak at -0.1 V from Cu₂O NCs, and could be served as sensitive electrochemical beacon for the simple modification on magnetic electrodes in the applications. The unique redox reaction between Cu₂O NCs and H₂S enabled the transformation of Cu₂O NCs to Cu₉S₈ NCs, resulting in decreased electroxidation responses at -0.1 V. The constructed electrochemical platform had a limit of detection (LOD) of 230 pM and a detection range of 500 pM-100 μM. The simple and cheap electrochemical sensor developed in this paper showed potential application for H₂S detection.

Metal-semiconductor heterostructures for surface-enhanced Raman scattering: synergistic contribution of plasmons and charge transfer

After 45 years of its first observation, surface-enhanced Raman spectroscopy (SERS) has become an ultrasensitive tool applied in chemical analysis, materials science, and biomedical research. SERS-active nanomaterials, such as noble metals, transition metals, and semiconductors, have undergone extensive development. The hybridization of semiconductors with plasmonic metal nanomaterials is highly effective in boosting light harvesting and conversion, which enables the rapid growth of metal-semiconductor hybrid nanostructures in SERS-based research fields. With the combination of the unique photoelectric properties and giant SERS signals attributed to the synergistic contribution of plasmons and change transfer (CT), metal-semiconductor heterostructures allow diverse and novel applications of SERS in CT investigations for the rational design of photovoltaic devices and ultrasensitive chemical or biological sensing. In this review, we specifically discuss SERS-active metal-semiconductor heterostructures including their building blocks, enhancement mechanisms, and applications. Moreover, we highlight the current challenges and opportunities for future research in this field based on our recent studies and other related research.

Multienzymes activity of metals and metal oxide nanomaterials: applications from biotechnology to medicine and environmental engineering

With the rapid advancement and progress of nanotechnology, nanomaterials with enzyme-like catalytic activity have fascinated the remarkable attention of researchers, due to their low cost, high operational stability, adjustable catalytic activity, and ease of recycling and reuse. Nanozymes can catalyze the same reactions as performed by enzymes in nature. In contrast the intrinsic shortcomings of natural enzymes such as high manufacturing cost, low operational stability, production complexity, harsh catalytic conditions and difficulties of recycling, did not limit their wide applications. The broad interest in enzymatic nanomaterial relies on their outstanding properties such as stability, high activity, and rigidity to harsh environments, long-term storage and easy preparation, which make them a convenient substitute instead of the native enzyme. These abilities make the nanozymes suitable for multiple applications in sensing and imaging, tissue engineering, environmental protection, satisfactory tumor diagnostic and therapeutic, because of distinguished properties compared with other artificial enzymes such as high biocompatibility, low toxicity, size dependent catalytic activities, large surface area for further bioconjugation or modification and also smart response to external stimuli. This review summarizes and highlights latest progress in applications of metal and metal oxide nanomaterials with enzyme/multienzyme mimicking activities. We cover the applications of sensing, cancer therapy, water treatment and anti-bacterial efficacy. We also put forward the current challenges and prospects in this research area, hoping to extension of this emerging field. In addition to therapeutic potential of nanozymes for disease prevention, their practical effects in diagnostics, to monitor the presence of SARS-CoV-2 and related biomarkers for future pandemics will be predicted.

Design and Characterization of Ag@Cu2O-rGO Nanocomposite for the p-Nitrophenol Reduction

In this paper, we designed Ag nanoparticles coated with a Cu2O shell, which was successfully decorated on reduced graphene oxide (rGO) via a solid-state self-reduction. The Cu2O, Ag@Cu2O, and Ag@Cu2O-rGO nanocomposites were synthesized and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–Vis, and XPS to evaluate the properties of the composites. In order to compare the chemical catalytic activity, the Cu2O, Ag@Cu2O, and Ag@Cu2O-rGO nanocomposites were employed for the catalytic reduction of p-nitrophenol (4-NP) into p-aminophenol (4-AP) in aqueous solution. The Ag@Cu2O-rGO nanocomposite exhibited excellent catalytic activity due to the intense interaction and high degree of electron transfer among Ag, Cu2O, and rGO. The rGO acted as the platform to bridge the isolated nanoparticles; furthermore, the electrons could quickly transfer from the Ag core to the Cu2O shell, which improved the chemical catalytic efficiency.

(Cu2O-Au) – Graphene - Au layered structures as efficient near Infra - Red SERS substrates

Near Infra-Red Surface Enhanced Raman Spectroscopy (NIR SERS) has gained huge attention in recent years as the conventional visible SERS suffers from overwhelming fluorescence background from the fluorophore resulting in the masking of Raman signals. In this paper, we propose a novel multi-layered SERS substrate- (Cu₂O - Au) - Graphene – Au - for efficient NIR SERS applications. The proposed structure has a monolayer of Cu₂O - Au core-shell particles on a Au substrate with 1 nm thick graphene spacer layer. Mie simulations are used to optimize the aspect ratios of core-shell particles to shift their plasmon resonances to NIR region using MieLab software. Further, Finite Difference Time Domain (FDTD) simulations using Lumerical software are used for the design of the multiparticle layered SERS substrate as MieLab software works only for single particle systems. Designed structure is shown to provide high field enhancement factor of the order of 10⁸ at an excitation of 1064 nm thus ensuring the possibility of using the proposed structure as efficient NIR SERS substrate which could probably be used for various NIR sensing applications.

Highly efficient core-shell Ag@carbon dot modified TiO2 nanofibers for photocatalytic degradation of organic pollutants and their SERS monitoring

In the present study, a novel hybrid nanomaterial composed of core-shell structured Ag@carbon dot (CD) modified TiO2 nanofibers (NFs) was successfully fabricated via a simple two-step strategy for the first time. Herein, the Ag@CDs-TiO2 NFs are demonstrated to be an efficient SERS substrate. The strong LSPR-induced electromagnetic enhancement (EM) by Ag@CDs NPs and efficient charge transfer (CT) effect between Ag@CDs and TiO2 NFs synergistically contribute to the excellent SERS enhancement. In addition, the Ag@CDs-TiO2 NFs exhibit enhanced photocatalytic activity regarding the organic pollutant degradation under visible light irradiation because of the enhanced light absorption and improved separation of photo-generated electron-hole pairs. Thus, this new nanocomposite can be used as a sensitive SERS substrate for determining the catalytic activity and reaction kinetics during the photodegradation of methylene blue (MB). Compared with UV-vis spectroscopy, the SERS technique enables more accurate monitoring of the changes of adsorption molecules and actual catalytic process on the surface of the catalyst. These results are significant for the development of metal or semiconductor-based catalysts for ensuring optoelectronic, energy and environmental applications.

Enzyme Mimic Nanomaterials and Their Biomedical Applications

Nanomaterials with enzyme-mimicking behavior (nanozymes) have attracted a lot of research interest recently. In comparison to natural enzymes, nanozymes hold many advantages, such as good stability, ease of production and surface functionalization. As the catalytic mechanism of nanozymes is gradually revealed, the application fields of nanozymes are also broadly explored. Beyond traditional colorimetric detection assays, nanozymes have been found to hold great potential in a variety of biomedical fields, such as tumor theranostics, antibacterial, antioxidation and bioorthogonal reactions. In this review, we summarize nanozymes consisting of different nanomaterials. In addition, we focus on the catalytic performance of nanozymes in biomedical applications. The prospects and challenges in the practical use of nanozymes are discussed at the end of this Minireview.

Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system

In recent years, surface-enhanced Raman scattering (SERS) of a molecule/metal–semiconductor hybrid system has attracted considerable interest and regarded as the synergetic contribution of the electromagnetic and chemical enhancements from the incorporation of noble metal into semiconductor nanomaterials. However, the underlying mechanism is still to be revealed in detail. Herein, we report an irreversible accumulated SERS behavior induced by near-infrared (NIR) light irradiating on a 4-mercaptobenzoic acid linked with silver and silver-doped titanium dioxide (4MBA/Ag/Ag-doped TiO₂) hybrid system. With increasing irradiation time, the SERS intensity of 4MBA shows an irreversible exponential increase, and the Raman signal of the Ag/Ag-doped TiO₂ substrate displays an exponential decrease. A microscopic understanding of the time-dependent SERS behavior is derived based on the microanalysis of the Ag/Ag-doped TiO₂ nanostructure and the molecular dynamics, which is attributed to three factors: (1) higher crystallinity of Ag/Ag-doped TiO₂ substrate; (2) photo-induced charge transfer; (3) charge-induced molecular reorientation.

The authors report that near-infrared light induces an irreversible accumulated Surface-enhanced Raman scattering (SERS) behavior of a molecule/metal–semiconductor hybrid system. They investigate the underlying mechanism and show that it is attributed to crystallinity, charge transfer and reorientation.

Graphene and Graphene Oxide Applications for SERS Sensing and Imaging

Surface Enhanced Raman Spectroscopy (SERS) has a long history as an ultrasensitive platform for the detection of biological species from small aromatic molecules to complex biological systems as circulating tumor cells. Thanks to unique properties of graphene, the range of SERS applications has largely expanded. Graphene is efficient fluorescence quencher improving quality of Raman spectra. It contributes also to the SERS enhancement factor through the chemical mechanism. In turn, the chemical flexibility of Reduced Graphene Oxide (RGO) enables tunable adsorption of molecules or cells on SERS active surfaces. Graphene oxide composites with SERS active nanoparticles have been also applied for Raman imaging of cells. This review presents a survey of SERS assays employing graphene or RGO emphasizing the improvement of SERS enhancement brought by graphene or RGO. The structure and physical properties of graphene and RGO will be discussed too.

SERS monitoring of the Fenton degradation reaction based on microfluidic droplets and alginate microparticles

Surface-enhanced Raman scattering (SERS) spectroscopy as a powerful tool has been used to explore different catalysis degradation reactions, whereas some drawbacks caused by ferric ions still exist in the current SERS monitoring of the Fenton reaction process. In this work, microfluidic droplet- and alginate microparticle-based methods were, respectively, applied to realize SERS monitoring of the Fenton degradation process in a relatively stable environment, which benefited from reduction of the loss of ferrous ions and the aggregation of the SERS substrate. As expected, the spectroscopic evidence at the molecular level directly revealed the degradation mechanism of rhodamine dyes, showing that the chemical bonds between xanthene and carboxybenzene broke continuously during the reaction. Afterward, the degradation mechanism determined by SERS was verified via mass spectrometry detection, which confirmed the validity of the SERS-based method. More broadly, the microfluidic droplet- and microparticle-based methods are potentially applicable for SERS monitoring of more Fenton degradation reactions.

CuO@Cu/Ag/MWNTs/sponge electrode-enhanced pollutant removal in dielectric barrier discharge (DBD) reactor

In this study, a sponge modified by multi-walled carbon nanotubes (MWNTs) was used as sheet support for the adsorption of CuO@Cu and Ag nanowires to prepare a CuO@Cu/Ag/MWNTs/sponge electrode. Similar to their use in a dielectric barrier discharge (DBD) reactor, the MWNTs changed the conductivity and water absorptivity of the modified electrode, whereas the CuO@Cu and Ag nanowires significantly enhanced the tip effect to increase discharge. The optimal ratio of the Ag:CuO@Cu nanowires was 5:3 at a total adsorbed concentration of 0.8 g L^-1. Compared with CuO@Cu and Ag nanowires were separately adsorbed on the MWNTs/sponge, and the CuO@Cu/Ag/MWNTs/sponges recorded higher current response, lower discharge inception voltage, and higher removal efficiency of phenol and 2,4,5-trichlorobiphenyl (PCB29) through their degradation. The removal efficiency reached 100% within 30 min of the reaction for the degradation of phenol and 65.1% within 60 min of the reaction for the degradation of PCB29 at an input voltage of 30 V. These results show that the CuO@Cu/Ag/MWNTs/sponge structure has significant potential for use in the DBD reactor to improve the discharge efficiency of the system and reduce energy consumption, and can be further extended to other types of plasma reactors.

Catalysis of Organic Pollutants Abatement Based on Pt-Decorated Ag@Cu2O Heterostructures

Pt-decorated Ag@Cu2O heterostructures were successfully synthesized using a simple and convenient method. The Pt nanoparticle density on the Ag@Cu2O can be controlled by changing the concentration of the Pt precursor. The synthesized Ag@Cu2O-Pt nanoparticles exhibited excellent catalytic performance, which was greatly affected by changes in the Ag@Cu2O-Pt structure. To optimize the material's properties, the synthesized Ag@Cu2O-Pt nanoparticles were used to catalyze toxic pollutants and methyl orange (MO), and nontoxic products were obtained by catalytic reduction. The Pt-decorated Ag@Cu2O nanoparticles showed excellent catalytic activity, which significantly decreased the pollutant concentration when the nanoparticles were used for catalytic reduction. The redistribution of charge transfer is the nanoparticles' main contribution to the catalytic degradation of an organic pollutant. This Pt-decorated Ag@Cu2O material has unique optical and structural characteristics that make it suitable for photocatalysis, local surface plasmon resonance, and peroxide catalysis.

Glucose Oxidase-Integrated Metal-Organic Framework Hybrids as Biomimetic Cascade Nanozymes for Ultrasensitive Glucose Biosensing

Nanozyme/natural enzyme hybrid plays a vital role in biosensing, therapy, and catalysis owing to the integrated advantages in the selectivity of natural enzymes and controllable catalytic activity of nanozymes. Herein, Fe-MIL-88B-NH₂ [(Fe-metal-organic framework (MOF)] with remarkable peroxidase-like activity, ultrahigh stability, and high biocompatibility was utilized for immobilization of glucose oxidase (GOx) via an amidation coupling reaction. On the basis of the excellent selectivity and catalytic activity of Fe-MOF-GOx, a cascade catalysis was performed for the colorimetric detection of glucose. The integrated Fe-MOF-GOx not only exhibited higher stability and reusability than their mixtures including Fe-MOF and free GOx system but also possessed a wide linear range (1-500 μM), with a low detection limit of 0.487 μM for glucose detection.

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.