Enzyme mimics of Au/Ag nanoparticles for fluorescent detection of acetylcholine

Carbon-based nanozymes: design, catalytic mechanisms, and environmental applications

Carbon-based nanozymes are synthetic nanomaterials that are predominantly constituted of carbon-based materials, which mimic the catalytic properties of natural enzymes, boasting features such as tunable catalytic activity, robust regenerative capacity, and exceptional stability. Due to the impressive enzymatic performance similar to various enzymes such as peroxidase, superoxide dismutase, and oxidase, they are widely used for detecting and degrading pollutants in the environment. This paper presents an exhaustive review of the fundamental design principles, catalytic mechanisms, and prospective applications of carbon-based nanozymes in the environmental field. These studies not only serve to augment the comprehension on the intricate operational mechanism inherent in these synthetic nanostructures, but also provide essential guidelines and illuminating perspectives for advancing their development and practical applications. Future studies that are imperative to delve into the untapped potential of carbon-based nanozymes within the environmental domain was needed to be explored to fully harness their ability to deliver broader and more impactful environmental preservation and management outcomes.

Fluorescent Iron-Doped Polymer Dot Nanozyme-Based Cascade System for Dual-Mode Detection of Acetylcholinesterase Activity and Its Inhibitors

The advancement of acetylcholinesterase (AChE) activity and its inhibitor assays is crucial for clinical diagnosis, drug screening, and environmental monitoring. A nanozyme-mediated cascade reaction system could offer promising prospects for a wide range of applications in such biosensing; however, the creation of nanozyme catalysts with diverse functionalities remains a significant challenge. Herein, we have proposed a multifunctional iron-doped polymer dots (Fe-PDs) nanozyme possessing excellent fluorescence and peroxidase (POD)-mimicking activity. Notably, the Fe-PDs nanozyme is capable of catalyzing H2O2 to produce a series of reactive oxygen species, which can simultaneously quench the fluorescence of Fe-PDs and induce a chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB), enabling the dual-mode detection of H2O2 through both fluorescence turn-off and absorbance turn-on signals. Furthermore, by integrating acetylcholine (ACh) and choline oxidase (ChOx), we have developed a three-enzyme (AChE-ChOx-POD) cascade-based fluorometric and colorimetric dual-mode sensing platform for monitoring AChE activity and its inhibitors. The sensitive and convenient dual-mode sensor has achieved low limits of detection with 0.5 mU/mL (fluorometry) and 0.014 mU/mL (colorimetry) for AChE, respectively, which are superior to the traditional Ellman's assay. More significantly, this sensor can also be extended to detect the reversible and irreversible inhibitors of AChE, such as tacrine (IC50 = 23.3 nM) and carbaryl (LOD = 0.8 nM). We firmly believe that this innovative dual-mode nanozyme-involved multienzyme cascade system-based sensing strategy will stimulate further exploration and serve as a versatile and practical tool for biochemical sensing applications.

An overview of signal amplification strategies and construction methods on phage-based biosensors

Phages are a class of viruses that specifically infect host bacteria. Compared to other recognition elements, phages offer several advantages such as high specificity, easy to obtain and good environmental tolerance, etc. These advantages underscore the potential of phages as recognition elements in the construction of biosensors. Therefore, the phage-based biosensors are currently garnering widespread attention for detecting pathogens in recent years. However, the test performance such as detection limit, sensitivity and stability of exicting phage-based biosensors require enhancement. In the design of sensors, the selection of various materials and construction methods significantly influences the test performance of the sensor, and employing appropriate signal amplification strategies and construction methods to devise biosensors based on different principles is an effective strategy to enhance sensor performance. The manuscript primarily focuses on the signal amplification strategies and construction methods employed in phage-based biosensors recent ten years, and summarizes the advantages and disadvantages of different signal amplification strategies and construction methods. Meanwhile, the manuscript discusses the relationship between sensor performance and various materials and construction methods, and reviews the application progress of phage-based electrochemical biosensors in the detection of foodborne bacteria. Furthermore, the manuscript points out the present limitations and the future research direction for the field of phage-based biosensors, so as to provide the reference for developing high-performance phage-based biosensors.

The Nanozyme Revolution: Enhancing the Performance of Medical Biosensing Platforms

Nanozymes represent a class of nanosized materials that exhibit innate catalytic properties similar to biological enzymes. The unique features of these materials have positioned them as promising candidates for applications in clinical sensing devices, specifically those employed at the point-of-care. They notably have found use as a means to amplify signals in nanosensor-based platforms and thereby improve sensor detection limits. Recent developments in the understanding of the fundamental chemistries underpinning these materials have enabled the development of highly effective nanozymes capable of sensing clinically relevant biomarkers at detection limits that compete with "gold-standard" techniques. However, there remain considerable hurdles that need to be overcome before these nanozyme-based sensors can be utilized in a platform ready for clinical use. An overview of the current understandings of nanozymes for disease diagnostics and biosensing applications and the unmet challenges that must be considered prior to their translation in clinical diagnostic tests is provided.

Recent Advancements in the Formulation of Nanomaterials-Based Nanozymes, Their Catalytic Activity, and Biomedical Applications

Nanozymes are nanoparticles with intrinsic enzyme-mimicking properties that have become more prevalent because of their ability to outperform conventional enzymes by overcoming their drawbacks related to stability, cost, and storage. Nanozymes have the potential to manipulate active sites of natural enzymes, which is why they are considered promising candidates to function as enzyme mimetics. Several microscopy- and spectroscopy-based techniques have been used for the characterization of nanozymes. To date, a wide range of nanozymes, including catalase, oxidase, peroxidase, and superoxide dismutase, have been designed to effectively mimic natural enzymes. The activity of nanozymes can be controlled by regulating the structural and morphological aspects of the nanozymes. Nanozymes have multifaceted benefits, which is why they are exploited on a large scale for their application in the biomedical sector. The versatility of nanozymes aids in monitoring and treating cancer, other neurodegenerative diseases, and metabolic disorders. Due to the compelling advantages of nanozymes, significant research advancements have been made in this area. Although a wide range of nanozymes act as potent mimetics of natural enzymes, their activity and specificities are suboptimal, and there is still room for their diversification for analytical purposes. Designing diverse nanozyme systems that are sensitive to one or more substrates through specialized techniques has been the subject of an in-depth study. Hence, we believe that stimuli-responsive nanozymes may open avenues for diagnosis and treatment by fusing the catalytic activity and intrinsic nanomaterial properties of nanozyme systems.

Silver and Gold Containing Compounds of p-Block Elements As Perspective Materials for UV Plasmonics

We present a review of phase formation tendencies, methods for preparation and optical properties of alloys and compounds from the binary systems of silver or gold with metals and metalloids from the p-block of the Periodic system of elements. Reference data about the homogeneity regions in the systems of interest, together with information about the crystalline structure of existing indexed compounds in them, is proposed and statistically analyzed. General background for the synthesis of intermetallic alloys and compounds, and the tendencies for their preparation for plasmonic purposes are presented. The high plasma frequency, ω_p of p-block metals makes their alloys with silver and gold an interesting object of study, due to the possibility of ω_p variation over a wide interval in the ultraviolet (UV) spectral region with a view to finding more efficient materials for excitation of a localized surface plasmon resonance (LSPR) necessary for various applications and techniques operating in this part of the electromagnetic spectrum. Unlike the alloys between the noble metals Cu, Ag, and Au, which form continuous series of solid solutions, different areas can be observed in the phase diagrams of the Ag(Au)-p-block systems, containing solid solutions, intermetallic compounds, and heterogeneous mixtures. The ability to vary the plasma frequency of solid solutions, like the alloys between the noble metals Cu, Ag, and Au, is the reason to pay attention to the compositions of the Ag(Au-p-block systems that fall in these regions of their phase diagrams. The analysis of the published results for complex permittivity shows that the addition of small amounts of conductive p-block elements to noble metals reduces the energy gap for interband transitions and increases their plasmonic activity in the UV spectral range. The article analyzes the relationship between electrical resistivity and LSPR excitation efficiency, which shows that the intermetallic compounds from Ag(Au)-p-block systems with a well-ordered crystalline structure and good conductivity level can be more effective materials for UV plasmonics than the boundary solid solutions. Intermetallic compounds can be easily obtained in the form of bulk samples, thin films, and nanoparticles with controlled size and geometric shape. The spectral dependences of the plasmon efficiency of the intermetallic compounds, determined from their complex permittivity functions, show that they are promising materials for excitation of LSPR in the UV spectral region.

C. G. Mono- and Bimetallic Nanoparticles for Catalytic Degradation of Hazardous Organic Dyes and Antibacterial Applications

In the present work, gold (Au), silver (Ag), and copper (Cu) based mono- and bimetallic NPs are prepared using a cost-effective facile wet chemical route. The pH for the synthesis is optimized in accordance with the optical spectra and supported by the finite difference time domain simulation studies. FESEM and TEM micrographs are used to analyze the morphology of the prepared nanoparticles. TEM images of bimetallic nanoparticles (BMPs) verified their bimetallic nature. XRD studies confirmed the formation of fcc-structured mono- and bimetallic NPs. Photoluminescence studies of the as-synthesized NPs are in good agreement with the previous publications. These synthesized NPs showed enhanced catalytic activity for the reduction/degradation of 4-nitrophenol, rhodamine B, and indigo carmine dyes in the presence of sodium borohydride (NaBH4) compared to NaBH4 alone. For the reduction of 4-nitrophenol, Au, Cu, and CuAg nanoparticles exhibited good catalytic efficiency compared to others, whereas for the degradation of rhodamine B and indigo carmine dyes the catalytic efficiency is comparatively high for CuAg BMPs. Furthermore, the antibacterial assay is carried out, and Ag NPs display effective antibacterial activity against Klebsiella pneumoniae, Salmonella ser. Typhimurium, Acinetobacter baumannii, Shigella flexneri, and Pseudomonas aeruginosa.

Immobilized glucose oxidase on hierarchically porous COFs and integrated nanozymes: a cascade reaction strategy for ratiometric fluorescence sensors

Covalent organic frameworks (COFs) with uniform porosity, good stability, and desired biocompatibility can function as carriers of immobilized enzymes. However, the obstructed pores or partially obstructed pores have hindered their applicability after loading enzymes. In this study, the hierarchical COFs were prepared as an ideal support to immobilize glucose oxidase (GOD) and obtain GOD@COF. The hierarchical porosity and porous structures of COFs provided sufficient sites to immobilize GOD and increased the rate of diffusion of substrate and product. Moreover, N,Fe-doped carbon dots (N,Fe-CDs) with peroxidase-like activity were introduced to combine with GOD@COF to construct an enzyme-mediated cascade reaction, which is the basis of the sensor GOD@COF/N,Fe-CDs. The sensor has been successfully built and applied to detect glucose. The limit of detection was 0.59 μM for determining glucose with the proposed fluorescence sensor. The practicability was illustrated by detecting glucose in human serum and saliva samples with satisfactory recoveries. The proposed sensor provided a novel strategy that introduced COF-immobilized enzymes for cascade reactions in biosensing and clinical diagnosis.

Visible Light Induced Nano-Photocatalysis Trimetallic Cu0.5Zn0.5-Fe: Synthesis, Characterization and Application as Alcohols Oxidation Catalyst

Here, we report a visible light-induced-trimetallic catalyst (Cu0.5Zn0.5Fe2O4) prepared through green synthesis using Tilia plant extract. These nanomaterials were characterized for structural and morphological studies using powder x-ray diffraction (P-XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The spinel crystalline material was ~34 nm. In benign reaction conditions, the prepared photocatalyst oxidized various benzylic alcohols with excellent yield and selectivity toward aldehyde with 99% and 98%; respectively. Aromatic and aliphatic alcohols (such as furfuryl alcohol and 1-octanol) were photo-catalytically oxidized using Cu0.5Zn0.5Fe2O4, LED light, H2O2 as oxidant, 2 h reaction time and ambient temperature. The advantages of the catalyst were found in terms of reduced catalyst loading, activating catalyst using visible light in mild conditions, high conversion of the starting material and the recyclability up to 5 times without loss of the selectivity. Thus, our study offers a potential pathway for the photocatalytic nanomaterial, which will contribute to the advancement of photocatalysis studies.

Near-Infrared Fluorescence Probe for Specific Detection of Acetylcholinesterase and Imaging in Live Cells and Zebrafish

Acetylcholinesterase (AChE) is a pivotal enzyme that is closely related with multiple neurological diseases, such as brain disorders or alterations in the neurotransmission and cancer. The development of convenient methods for imaging AChE activity in biological samples is very important to understand its mechanisms and functions in a living system. Herein, a fluorescent probe exhibiting emission in the near-infrared (NIR) region is developed to detect AChE and visualize biological AChE activities. This probe exhibits a quick response time, reasonable detection limit, and a large Stokes shift accompanied by the NIR emission. The probe has much better reactivity toward AChE than butyrylcholinesterase, which is one of the significant interfering substances. The outstanding specificity of the probe is proved by cellular imaging AChE activity and successful mapping in different regions of zebrafish. Such an effective probe can greatly contribute to ongoing efforts to design emission probes that have distinct properties to assay AChE in biological systems.

Modulating the catalytic activity of gold nanoparticles using amine-terminated ligands

Nanozymes have broad applications in theranostics and point-of-care tests. To enhance the catalytic activity of nanozymes, the conventional strategy is doping metals to form highly active nanoalloys. However, high-quality and stable nanoalloys are hard to synthesize. Ligand modification is a powerful strategy to achieve chemoselectivity or bioactivity by changing the surface chemistry. Here, we explore different ligands to enhance the catalytic activity of nanozymes, e.g., gold nanoparticles (AuNPs). We systematically studied the impacts on the enzymatic activity of AuNPs by ligand engineering of surface chemistry (charge, group, and surface distance). Our work established critical guidelines for surface modification of nanozymes. The amine group favors higher activity of AuNPs than other groups. The flexible amine-rich ligand enhances the catalytic activity of AuNPs in contrast to other ligands and unmodified AuNPs. Using a proof-of-concept model, we screened many candidate ligands to obtain polyamine-AuNPs, which have strongly enhanced peroxidase-like activity and 100 times enhanced sensitivity compared to unmodified AuNPs. The strategy of enhancing the catalytic activity of AuNPs using ligands will facilitate the catalysis-related applications of nanozymes in biology and diagnostics.

Surface ligand engineering can precisely modulate the catalytic activity of nanozymes from inactive to highly active.

Detection of Acetylcholine at Nanoscale NPOE/Water Liquid/Liquid Interface Electrodes

The interface between two immiscible electrolyte solutions (ITIES) has become a very powerful analytical platform for sensing a diverse range of chemicals (e.g., metal ions and neurotransmitters) with the advantage of being able to detect non-redox electroactive species. The ITIES is formed between organic and aqueous phases. Organic solvent identity is crucial to the detection characteristics of the ITIES [half-wave transfer potential (E_1/2), potential window range, limit of detection, transfer coefficient (α), standard heterogeneous ion-transfer rate constant (k⁰), etc.]. Here, we demonstrated, for the first time at the nanoscale, the detection characteristics of the NPOE/water ITIES. Linear detection of the diffusion-limited current at different concentrations of acetylcholine (ACh) was demonstrated with cyclic voltammetry (CV) and i-t amperometry. The E_1/2 of ACh transfer at the NPOE/water nanoITIES was -0.342 ± 0.009 V versus the E_1/2 of tetrabutylammonium (TBA⁺). The limit of detection of ACh at the NPOE/water nanoITIES was 37.1 ± 1.5 μM for an electrode with a radius of ∼127 nm. We also determined the ion-transfer kinetics parameters, α and k⁰, of TBA⁺ at the NPOE/water nanoITIES by fitting theoretical cyclic voltammograms to experimental voltammograms. This work lays the basis for future cellular studies using ACh detection at the nanoscale and for studies to detect other analytes. The NPOE/water ITIES offers a potential window distinct from that of the 1,2-dichloroethane (DCE)/water ITIES. This unique potential window would offer the ability to detect analytes that are not easily detected at the DCE/water ITIES.

A Smartphone Optical Device for Point-of-Care Testing of Glucose and Cholesterol Using Ag NPs/UiO-66-NH2-Based Ratiometric Fluorescent Probe

Point-of-care testing (POCT) with the advantages of simplicity, rapidity, portability, and low-cost is of great importance to improve healthcare, especially in resource-limited settings and home healthcare settings. Moreover, it is a great challenge to quantitative POCT of multiplexed biomarkers within a single accessible assay but provides enhanced diagnostic accuracy and improved diagnostic efficiency. Herein, a smartphone optical device has been designed for POCT of glucose and cholesterol in metabolic syndrome patients using a ratiometric fluorescent sensor. The sensing system of Ag NPs/UiO-66-NH₂ and o-phenylenediamine presents a dual-emission response to H₂O₂ (the main product of glucose and cholesterol catalyzed by glucose oxidase and cholesterol oxidase) on account of the inner filter effect, resulting in an increase in the response of the fluorescence intensity ratio (F_555 nm/F_425 nm) accompanied by a distinguishable color transition from blue to yellow green. After compositing probes with a flexible substrate, the obtained test strip can be integrated with a smartphone-based portable platform to read RGB values for accurate testing of glucose and cholesterol with both detection limits of 10 μmol L^-1, which are hundreds of times lower than their concentrations in human serum. With the advantages of low-cost, ease of operation, and broad adaptability, this smartphone optical device holds great potential for portable detection of numerous targets in personalized healthcare and clinical diagnosis.

Metallic oxide nanomaterials act as antioxidant nanozymes in higher plants: Trends, meta-analysis, and prospect

Improving plant resistance against various environmental stresses is crucial to gain higher agricultural productivity for meeting future food demands of the fast-growing global population. Nanozymes, nanomaterials (NMs) with enzyme-like activity, have shown the potential to defend environmental stresses via scavenging reactive oxygen species (ROS) and augmenting the inherent antioxidant functions of plants. However, several studies confirmed that NMs could cause oxidative damage triggered by excessive ROS. In this study, the conversion mechanism between antioxidant and oxidant activities of metallic oxidative nanozymes was systematically reviewed and evaluated using meta-analysis approach. Moreover, our work attempts to seek the optimal dose and physicochemical property of antioxidant-functionalized NMs and put forward future research directions. The meta-analysis results indicated that NMs at a low dose (below 20 ppm) exhibited antioxidant activity which could scavenge ROS and alleviate their deleterious impacts. Conversely, their oxidant activity was activated at the exposure dose above 200 ppm which might induce ROS overproduction and lead to oxidative stress. Further, root exposure tends to stimulate the oxidant activity of NMs, and the NMs modification is highly promising for improving their bioavailability. A SWOT analysis was conducted to evaluate the strengths, weaknesses, opportunities, and threats of agro-applied nanozymes. Therefore, the rational design and development of nanozymes for better antioxidant potential will be beneficial to their applications in agriculture.

Disruption of brain conductivity and permittivity and neurotransmitters induced by citrate-coated silver nanoparticles in male rats

As one of the most exonerative, competitive, and abundant nanoparticles in curative uses, silver nanoparticles (AgNPs) play a growing important role in developing global neurodegeneration. Herein, we inspected the neurotoxic and histopathological effects of the oral dose of 26.9 nm citrate-coated AgNPs (100 and 1000 mg/kgbw, 28 days) on the brain conductivity and permittivity combined with neurotransmitter assays. While male mice in the control group were given deionized water. In terms of biophysical levels, the brain electric conductivity and relative permittivity were significantly decreased in the 26.9 nm citrate-coated AgNP treated groups versus the controls. Besides, 26.9 nm citrate-coated AgNP treatment resulted in a significant deficiency in the concentrations of brain acetylcholine esterase, dopamine, and serotonin. Total brain contents of silver ion significantly increased in a dose-dependent manner. Further, light and electron microscopy revealed a progressive disruption in the lamellar pattern of the myelinated axons of the nerve fibers, in addition to the accumulation of nanosilver in lysosomes and swollen mitochondria in axoplasm. In conclusion, 26.9 nm citrate-coated AgNPs are capable of gaining access to the brain of mice and causing electric conductivity and relative permittivity damage along with a high degree of cellular toxicity in the brain tissue. Therefore, the present study highlights, for the first time, the adverse effects of the citrate-coated AgNPs to the brain of mice and raises the concern of their probable neurotoxic impacts which is helpful for conclusive interpretation of future behavioral and potential neurodegeneration-based aspects. It would be of interest to investigate citrate-coated AgNPs mediated axonal relevant-signal transduction levels in future studies.

Preparation, Functionalization, Modification, and Applications of Nanostructured Gold: A Critical Review

Gold nanoparticles (Au NPs) play a significant role in science and technology because of their unique size, shape, properties and broad range of potential applications. This review focuses on the various approaches employed for the synthesis, modification and functionalization of nanostructured Au. The potential catalytic applications and their enhancement upon modification of Au nanostructures have also been discussed in detail. The present analysis also offers brief summaries of the major Au nanomaterials synthetic procedures, such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, sonochemical deposition, electrochemical deposition, microwave and laser pyrolysis. Among the various strategies used for improving the catalytic performance of nanostructured Au, the modification and functionalization of nanostructured Au produced better results. Therefore, various synthesis, modification and functionalization methods employed for better catalytic outcomes of nanostructured Au have been summarized in this review.

Gold/Silver Hybrid Nanoparticles with Enduring Inhibition of Coronavirus Multiplication through Multisite Mechanisms

As a large enveloped RNA virus, coronavirus is of considerable medical and veterinary significance, and anticoronavirus treatment is challenging due to its biodiversity and rapid variability. In this study, Au@Ag nanorods (Au@AgNRs) were successfully synthesized by coating AuNRs with silver and were shown for the first time to have activity against the replication of porcine epidemic diarrhea virus (PEDV). Viral titer analysis demonstrated that Au@AgNRs could inhibit PEDV infection by 4 orders of magnitude at 12 h post-infection, which was verified by viral protein expression analysis. The potential mechanism of action showed that Au@AgNRs could inhibit the entry of PEDV and decrease the mitochondrial membrane potential and caspase-3 activity. Additionally, we demonstrated that a large amount of virus proliferation can cause the generation of reactive oxygen species in cells, and the released Ag⁺ and exposed AuNRs by Au@AgNRs after the stimulation of reactive oxygen species has superior antiviral activity to ensure long-term inhibition of the PEDV replication cycle. The integrated results support that Au@AgNRs can serve as a potential therapeutic strategy to prevent the replication of coronavirus.

Quaternary-Ammonium-Modulated Surface-Enhanced Raman Spectroscopy Effect: Discovery, Mechanism, and Application for Highly Sensitive In Vitro Sensing of Acetylcholine

Quaternary ammonium (QA) plays multiple roles in biological functions, whose dysregulation may result in multiple diseases. However, how to efficiently detect QA-based materials such as acetylcholine (ACh) still remains a great challenge, especially in complex biological environments. Here, a new effect [called quaternary-ammonium-modulated surface-enhanced Raman spectroscopy (QAM-SERS) effect] is discovered, showing that the existence of QA will modulate the intensity of SERS signals in a concentration-dependent manner. When the QAM-SERS effect is used, a new method is easily developed for in vitro detection of ACh with an extremely high sensitivity and an ultrawide dynamic range. Particularly, the linear dynamic range can be freely tuned to adapt for various physiological samples. As a proof-of-concept experiment, the time-dependent secretion of ACh from PC12 cells was successfully monitored using the QAM-SERS method, which were under either the stimulation of potassium ions or the incubation of drugs. The discovery of the QAM-SERS effect provides an easy and universal strategy for detecting ACh as well as other QA-contained molecules, which can also inspire new insights into the roles that QA could play in biology and chemistry.

Smartphone-based enzyme-free fluorescence sensing of organophosphate DDVP

A novel fluorescence strategy based on the outstanding catalytic capability of CuS nanoparticles (CuS_NPs) has been developed for highly sensitive and specific determination of o,o-dimethyl-o-2,2-dichlorovinyl phosphate (DDVP) under enzyme-free and hydrogen peroxide (H₂O₂)-free conditions. In the presence of DDVP, CuS_NPs can catalyze non-fluorescence substratum of Amplex red (AR) into resorufin, which exhibits fluorescence emission at 584 nm under excitation at 540 nm. The sensing system exhibits outstanding specificity and only responds to DDVP and no other organophosphorus pesticides (OPs). A wide linear range is obtained from 0.0001 to 0.1 μg/mL, and the limit of detection (LOD) is 0.1 ng/mL. Furthermore, paper-based test strips have been constructed for visual detection of DDVP under ultraviolet light irradiation. By integrating a smartphone installed with Color Picker APP, point-of-care detection with quantitative determination is realized, demonstrating substantial potential applications of the as-developed assay for in situ detection. Graphical abstract.

Highly Selective and Sensitive Detection of Biogenic Defense Phytohormone Salicylic Acid in Living Cells and Plants Using a Novel and Viable Rhodamine-Functionalized Fluorescent Probe

Detecting plant-derived signal molecules using fluorescent probes is a key topic and a huge challenge for scientists. Salicylic acid (SA), a vital plant-derived defense hormone, can activate global transcriptional reprogramming to systemically express a network of prominent pathogenesis-related proteins against invasive microorganisms. This strategy is called systemic acquired resistance (SAR). Therefore, monitoring the dynamic fluctuations of SA in subcellular microenvironments can advance our understanding of different physiological and pathological functions during the SA-induced SAR mechanism, thus benefiting the discovery and development of novel immune activators that contribute to crop protection. Here, detection of signaling molecule SA in plant callus tissues was first reported and conducted by a simple non-fluorescent rhodamine-tagged architecture bearing a flexible 2-amino-N,N-dimethylacetamide pattern. This study can markedly advance and promote the usage of fluorescent SA probes for distinguishing SA in the plant kingdom.

A differential photoelectrochemical method for glucose determination based on alkali-soaked zeolite imidazole framework-67 as both glucose oxidase and peroxidase mimics

A differential photoelectrochemical (PEC) method for glucose determination is reported using a nanocomposite with double mimic enzymes of glucose oxidase (GOx) and peroxidase. The nanocomposite was prepared by soaking zeolite imidazole framework-67 (ZIF-67) in 0.1 M NaOH solution at room temperature for 30 min, abbreviated as CoxOyHz@ZIF-67. The Michaelis-Menten constant of CoxOyHz@ZIF-67 to H2O2 and glucose is 121 μM and 3.95 mM, respectively. Using the photoelectrode of CoxOyHz@ZIF-67/TiO2 nanotubes (NTs), glucose was oxidized firstly by dissolved oxygen to generate H2O2 under the catalysis of CoxOyHz film as the mimics of GOx. The product of H2O2 enhanced the photocurrent of TiO2 NTs under the catalysis of ZIF-67 as the mimics of peroxidase. The molecular sieve effect of ZIF-67 frameworks reduces the interferences from molecules with size larger than the apertures in ZIF-67. Under the excitation of a 150 W xenon lamp with full spectrum, the photocurrent was measured in a two-electrode system without external additional potential. By using the photocurrent difference between two photocells, i.e CoxOyHz@ZIF-67/TiO2 NTs and Pt electrode, ZIF-67/TiO2 NTs and Pt electrode, as the signal, the selectivity for glucose determination is improved further. The differential PEC method was applied to the determination of glucose with a linear range 0.1 μM~1 mM and a detection limit of 0.03 μM. Graphical abstract.

Nanozymes: A New Disease Imaging Strategy

Nanozymes are nanomaterials with intrinsic enzyme-like properties. They can specifically catalyze substrates of natural enzymes under physiological condition with similar catalytic mechanism and kinetics. Compared to natural enzymes, nanozymes exhibit the unique advantages including high catalytic activity, low cost, high stability, easy mass production, and tunable activity. In addition, as a new type of artificial enzymes, nanozymes not only have the enzyme-like catalytic activity, but also exhibit the unique physicochemical properties of nanomaterials, such as photothermal properties, superparamagnetism, and fluorescence, etc. By combining the unique physicochemical properties and enzyme-like catalytic activities, nanozymes have been widely developed for in vitro detection and in vivo disease monitoring and treatment. Here we mainly summarized the applications of nanozymes for disease imaging and detection to explore their potential application in disease diagnosis and precision medicine.

Surface chemistry of gold nanoparticles for health-related applications

Functionalization of gold nanoparticles is crucial for the effective utilization of these materials in health-related applications. Health-related applications of gold nanoparticles rely on the physical and chemical reactions between molecules and gold nanoparticles. Surface chemistry can precisely control and tailor the surface properties of gold nanoparticles to meet the needs of applications. Gold nanoparticles have unique physical and chemical properties, and have been used in a broad range of applications from prophylaxis to diagnosis and treatment. The surface chemistry of gold nanoparticles plays a crucial role in all of these applications. This minireview summarizes these applications from the perspective of surface chemistry and explores how surface chemistry improves and imparts new properties to gold nanoparticles for these applications.

Nanozyme-based catalytic theranostics

Nanozymes, a type of nanomaterial with intrinsic enzyme-like activities, have emerged as a promising tool for disease theranostics. As a type of artificial enzyme mimic, nanozymes can overcome the shortcomings of natural enzymes, including high cost, low stability, and difficulty in storage when they are used in disease diagnosis. Moreover, the multi-enzymatic activity of nanozymes can regulate the level of reactive oxygen species (ROS) in various cells. For example, superoxide dismutase (SOD) and catalase (CAT) activity can be used to scavenge ROS, and peroxidase (POD) and oxidase (OXD) activity can be used to generate ROS. In this review, we summarize recent progress on the strategies and applications of nanozyme-based disease theranostics. In addition, we address the opportunities and challenges of nanozyme-based catalytic theranostics in the near future.

Highly sensitive photoelectrochemical detection of bleomycin based on Au/WS2 nanorod array as signal matrix and Ag/ZnMOF nanozyme as multifunctional amplifier

An ultrasensitive photoelectrochemical (PEC) biosensor was constructed based on gold nanoparticles (Au NPs)/tungsten sulfide nanorod array (WS₂ NA) photoelectrode as the PEC matrix and silver nanoparticles/flake-like zinc metal-organic framework (Ag/ZnMOF) nanozyme with the peroxidase mimetic enzyme property for sensitive detection of bleomycin (BLM). In particular, Au/WS₂ and Ag/ZnMOF were linked by thiolate DNA₁ and DNA₂ strand, respectively, and the Au/WS₂-Ag/ZnMOF probe was prepared via hybridization reaction between the two DNAs. The introduction of Ag/ZnMOF in the probe offers two functions: i) the steric hindrance effect can effectively impede electron transport and reduce the photocurrent; ii) Ag/ZnMOF nanozyme can also be used as mimic peroxidase to effectively catalyze 3,3-diaminobenzidine (DAB) to produce the relevant precipitation, which will further reduce photocurrent and eliminate false positive signals. When BLM exists, BLM with Fe²⁺ as irreversible cofactor can specifically recognize and cleave of the 5'-GC-3' active site of DNA₂, resulting in reduced precipitation deposited on the electrode and recovery of PEC signal. The highly sensitive PEC biosensor exhibits a the linear strategy from 0.5 nM to 500 nM with a detection limit down to 0.18 nM. Further, the unique strategy was conducted in biological samples for BLM detection with satisfactory consequence, offering available and efficient pathway for disease diagnosis.

Copper(II) complex as a turn on fluorescent sensing platform for acetylcholinesterase activity with high sensitivity

Acetylcholinesterase (AChE) is an important enzyme associated with many nervous diseases, demonstrating the great need for smarter sensing platform with improved sensitivity, selectivity and simplified operation. A "turn on" fluorometric assay is described herein for AChE activity detection, according to the specific enzyme catalyzed reaction of acetylcholine (ATCh) by AChE, which generates thiocholine (TCh) as the product. The well-designed fluorescent probe HBTP possesses ESIPT (Excited State Intramolecular Proton Transfer) nature, leading to a larger Stokes shift, which could be quenched upon coordination with Cu²⁺. The fluorescence-silent HBTP-Cu²⁺ complex could be broken by TCh generated from reaction of ATCh with AChE, giving rise to HBTP release which originates from competitive coordination of TCh with Cu²⁺. This complex probe HBTP-Cu²⁺ offers a limit detection as low as 0.02 mU mL^-1, which is lower than most reported literatures. Furthermore, both HBTP-Cu²⁺ and HBTP show little toxicity to live cells and is available in visualizing cellular AChE activity.

A Probe for Fluorescence Detection of the Acetylcholinesterase Activity Based on Molecularly Imprinted Polymers Coated Carbon Dots

This paper presents a new probe for fluorescence detection of the acetylcholinesterase (AChE) activity based on molecularly imprinted polymer (MIP) coated carbon dots (C-dots) composite. The C-dots were hydrothermally synthesized with grafted silica surface and sealed with molecularly imprinted polymers in silica pores (MIP@C-dots) in situ. Removed the original template molecules, the MIP@C-dots composite exhibits quite high selectivity for acetylthiocholine (ACh). With AChE, its substrate ACh will be hydrolyzed into thiocholine and the fluorescence signals exhibit a dramatic decrease at 465 nm, Under optimal conditions, the fluorescent probe shows sensitive responses to AChE in the range of 0.01-0.6 mU/mL. The detection limits of AChE are as low as 3 µU/mL. These experiments results validate the novel fluorescent probe based on MIP@C-dots composite, paving a new way to evaluation of AChE activity and Screening inhibitors.

Unveiling the Intrinsic Catalytic Activities of Single-Gold-Nanoparticle-Based Enzyme Mimetics

Gold nanoparticles (AuNPs) have been demonstrated to serve as effective nanomaterial-based enzyme mimetics (nanozymes) for a number of enzymatic reactions under mild conditions. The intrinsic glucose oxidase and peroxidase activities of single AuNPs and Ag-Au nanohybrids, respectively, were investigated by single NP collision electrochemical measurements. A significantly high turnover number of nanozymes was obtained from individual catalytic events compared with the results from the classical, ensemble-averaged measurements. The unusual enhancement of catalytic activity of single nanozymes is believed to originate from the high accessible surface area of monodispersed NPs and the high activities of carbon-supported NPs during single-particle collision at a carbon ultramicroelectrode. This work introduces a new method for the precise characterization of the intrinsic catalytic activities of nanozymes, giving further insights to the design of high-efficiency nanomaterial catalysts.

One-pot synthesized Cu/Au/Pt trimetallic nanoparticles with enhanced catalytic and plasmonic properties as a universal platform for biosensing and cancer theranostics

Cu/Au/Pt trimetallic nanoparticles (TMNPs) with enhanced catalytic activity and intense plasmonic absorption in the NIR-I biowindow (650-950 nm) were prepared using a fast, gentle and one-pot protocol. Based on these properties and assembly of thiolated-aptamers on Cu/Au/Pt TMNPs, a universal platform was developed for applications in biosensing and theranostics.

Sensitive biosensing of organophosphate pesticides using enzyme mimics of magnetic ZIF-8

Development of a sensitive detection method for the reliable screening of widely used organophosphorus (OP) toxins is a crucial request to control their side-effects. Herein, a novel fluorometric assay based on the acetylcholinesterase (AChE) inhibited enzymatic activity and the new peroxidase-like Fe₃O₄ nanoparticles@ZIF-8 composite (Fe₃O₄ NPs@ZIF-8) was developed for the determination of OPs. Magnetic Fe₃O₄ NPs were encapsulated into ZIF-8 and the high mimetic activity of produced composite was assessed on the oxidation of substrates. This observation was applied to the rapid detection of diazinon as a model OP compound. The sensing tool contains AChE and choline oxidase (CHO) enzymes, peroxidase colorimetric or fluorometric substrate, and Fe₃O₄ NPs@ZIF-8 as the catalyst. In the presence of mimic Fe₃O₄ NPs@ZIF-8, the generated H₂O₂ from the enzymatic reactions of acetylcholine is decomposed to hydroxyl radicals. The radicals oxidize the peroxidase substrates to generate a detectable signal. However, due to the inhibition effect of OPs on the enzymatic activity of AChE, lower H₂O₂ amounts are produced in the presence of diazinon. Using the fluorometric detection system, the generated signal is decreased proportionally by increasing diazinon concentration in the range of 0.5-500 nM. The limit of detection was obtained 0.2 nM. Consequently, the usage of high performance peroxidase-mimic Fe₃O₄ NPs@ZIF-8 provided a sensitive bio-assay with a potential to be applied as screening tool for toxic OP compounds. The developed assay was successfully applied for the determination of diazinon in water and fruit juices.

Acetylcholine-responsive cargo release using acetylcholinesterase-capped nanomaterials

Mesoporous silica nanoparticles capped with acetylcholinesterase, through boronic ester linkages, selectively release an entrapped cargo in the presence of acetylcholine.

In Situ Sensing of the Neurotransmitter Acetylcholine in a Dynamic Range of 1 nM to 1 mM

The neurotransmitter acetylcholine (ACh) plays a key role in the pathophysiology of brain disorders such as Alzheimer's disease. Understanding the dynamics of ACh concentration changes and kinetics of ACh degradation in the living brain is crucial to unravel the pathophysiology of such diseases and the rational design of therapeutics. In this work, an electrochemical sensor capable of dynamic, label-free, selective, and in situ detection of ACh in a range of 1 nM to 1 mM (with temporal resolution of less than one second) was developed. The sensor was employed for the direct detection of ACh in artificial cerebrospinal fluid and rat brain homogenate, without any prior separation steps. A potentiometric receptor-doped ion-selective electrode (ISE) with selectivity for ACh was designed by taking advantage of the positive charge of ACh. The dynamic range, limit of detection (LOD), and the selectivity of the sensor were optimized stepwise by (i) screening of hydrophobic biomimetic calixarenes to identify receptors that strongly bind to ACh based on shape-selective multitopic recognition, (ii) doping of the ISE sensing membrane with an ACh-binding hydrophobic calixarene to enable selective detection of ACh in complex matrices, (iii) utilizing a hydrophilic calixarene in the inner filling solution of the ISE to buffer the concentration of ACh and, thereby, lower the LOD of the sensor, and (iv) introducing a surface treatment step prior to the measurement by placing the sensor for ∼1 min in a solution of a hydrophilic calixarene to lower the LOD of the sensor even further.

An ultrasensitive signal-on electrochemiluminescence biosensor based on Au nanoclusters for detecting acetylthiocholine

For improving the sensitivity of the electrochemiluminescent (ECL) detection and extending the applications of luminophore, the development of coreactant accelerator is one of the important ways. In this work, Au nanoclusters (Au NCs) were chosen as the luminescent material, and thiocholine, which was in situ generated by enzymatic reaction, was found to serve as a coreactant accelerator for Au NC-S₂O₈^2- ECL system. Based on this discovery, a highly sensitive detection of acetylthiocholine (ATCl) was achieved using the acetylcholinesterase (AChE) biosensor. CeO₂ nanowires (CeO₂ NWs) were used to improve the stability of Au NCs on the glassy carbon electrode (GCE) due to the large specific surface area and good film-forming properties of CeO₂ NWs. ATCl was catalyzed by acetylcholinesterase (AChE) to produce thiocholine, which served as the coreactant accelerator to improve the ECL signal of Au NC-S₂O₈^2- system. The biosensor obtained a low detection limit of 0.17 nM. The integration of thiocholine and Au NCs would provide a new ECL platform for bioanalysis. Graphical abstract ᅟ.

Ag+ -Gated Surface Chemistry of Gold Nanoparticles and Colorimetric Detection of Acetylcholinesterase

Chemical regulation of enzyme-mimic activity of nanomaterials is challenging because it requires a precise understanding of the surface chemistry and mechanism, and rationally designed applications. Herein, Ag+ -gated peroxidase activity is demonstrated by successfully modulating surface chemistry of cetyltrimethylammonium bromide-capped gold nanoparticles (CTAB-AuNPs). A surface blocking effect of long-chain molecules on surfaces of AuNPs that inhibit peroxidase activity of AuNPs is found. Ag+ ions can selectively bind on the surfaces of AuNPs and competitively destroy CTAB membrane forming Ag+ @CTAB-AuNPs complexes to result in enhanced peroxidase activity. Ag+ @CTAB-AuNPs show the highest peroxidase activity compared to similar-sized citrate-capped and ascorbic acid-capped AuNPs. Ag+ @CTAB-AuNPs can potentially develop into analyte-responsive systems and exhibit advantages in the optical sensing field. For example, the Ag+ @CTAB-AuNPs system shows an enhanced sensitivity and selectivity for acetylcholinesterase activity sensing compared to other methods.

Multifunctional nanozymes: enzyme-like catalytic activity combined with magnetism and surface plasmon resonance

Over decades, as alternatives to natural enzymes, highly-stable and low-cost artificial enzymes have been widely explored for various applications. In the field of artificial enzymes, functional nanomaterials with enzyme-like characteristics, termed as nanozymes, are currently attracting immense attention. Significant progress has been made in nanozyme research due to the exquisite control and impressive development of nanomaterials. Since nanozymes are endowed with unique properties from nanomaterials, an interesting investigation is multifunctionality, which opens up new potential applications for biomedical sensing and sustainable chemistry due to the combination of two or more distinct functions of high-performance nanozymes. To highlight the progress, in this review, we discuss two representative types of multifunctional nanozymes, including iron oxide nanomaterials with magnetic properties and metal nanomaterials with surface plasmon resonance. The applications are also covered to show the great promise of such multifunctional nanozymes. Future challenges and prospects are discussed at the end of this review.

Au-Ag and Pt-Ag bimetallic nanoparticles@halloysite nanotubes: morphological modulation, improvement of thermal stability and catalytic performance

In this study, Au-Ag and Pt-Ag bimetallic nanocages were loaded on natural halloysite nanotubes (HNTs) via galvanic exchange based on Ag@HNT. By changing the ratio of Au to Ag or Pt to Ag in exchange processes, Au-Ag@HNT and Pt-Ag@HNT with different nanostructures were generated. Both Au-Ag@HNT and Pt-Ag@HNT systems showed significantly improved efficiency as peroxidase-like catalysts in the oxidation of o-phenylenediamine compared with monometallic Au@HNT and Pt@HNT, although inert Ag is dominant in the composition of both Au-Ag and Pt-Ag nanocages. On the other hand, loading on HNTs enhanced the thermal stability for every system, whether monometallic Ag nanoparticles, bimetallic Au-Ag or Pt-Ag nanocages. Ag@HNT sustained thermal treatment at 400 °C in nitrogen with improved catalytic performance, while Au-Ag@HNT and Pt-Ag@HNT maintained or even had slightly enhanced catalytic efficiency after thermal treatment at 200 °C in nitrogen. This study demonstrated that natural halloysite nanotubes are a good support for various metallic nanoparticles, improving their catalytic efficiency and thermal stability.

An update on smart biocatalysts for industrial and biomedical applications

Recently, smart biocatalysts, where enzymes are conjugated to stimuli-responsive (smart) polymers, have gained significant attention. Based on the presence or absence of external stimuli, the polymer attached to the enzyme changes its conformation to protect the enzyme from the external environment and regulate the enzyme activity, thus acting as a molecular switch. Owing to this behaviour, smart biocatalysts can be separated easily from a reaction mixture and re-used several times. Several such smart polymer-based biocatalysts have been developed for industrial and biomedical applications. In addition, they have been used in biosensors, biometrics and nano-electronic devices. This review article covers recent advances in developing different kinds of stimuli-responsive enzyme bioconjugates, including conjugation strategies, and their applications.

A cobalt-based polyoxometalate nanozyme with high peroxidase-mimicking activity at neutral pH for one-pot colorimetric analysis of glucose

A polyoxometalate (CoPW₁₁O₃₉) with high peroxidase-mimicking activity at physiological pH enables one-pot colorimetric analysis of glucose when coupled with GOx.

Intrinsic peroxidase-like activity of rhodium nanoparticles, and their application to the colorimetric determination of hydrogen peroxide and glucose

The intrinsic peroxidase-like activity of rhodium nanoparticles (RhNPs) and their use as catalytic labels for sensitive colorimetric assays is presented. RhNPs catalyze the oxidation of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue reaction product with a maximum absorbance at 652 nm. Kinetic studies show catalysis to follow Michaelis-Menten kinetics and a "ping-pong" mechanism. The calculated kinetic parameters indicate high affinity of RhNPs for both the substrate TMB and H₂O₂. In fact, they are better than other peroxidase mimicking nanomaterials and even the natural enzyme horseradish peroxidase. On the other hand, RhNPs exhibit no reactivity towards saccharides, thiols, amino acids and ascorbic acid. Based on these findings, a sensitive and selective colorimetric method was worked out for the determination of H₂O₂ in real samples with a linear response in the 1-100 μM concentration range. By employing glucose oxidase, the glucose assay has a linear range that covers the 5 to 125 μM glucose concentration range. The detection limits are <0.75 μM for both species. The methods were applied to the determination of H₂O₂ in spiked pharmaceutical formulations, and of glucose in soft drinks and blood plasma. Figures of merit include (a) good accuracy (with errors of <6%), (b) high recoveries (96.5-103.7%), and (c) satisfactory reproducibility (<6.3%). Graphical abstract Rhodium nanoparticles catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue reaction product. The effect is exploited in photometric assays for hydrogen peroxide and glucose.