Manganese as a Catalytic Mediator for Photo-oxidation and Breaking the pH Limitation of Nanozymes

Catalytic Hybrid Lipid Nanoparticles Potentiate Circle RNA-Based Cytokine Immunotherapy

Cytokine therapeutics in cancer immunotherapy are greatly limited by their short half-time, serious toxicity, and frequent administration, which can possibly be addressed by ribonucleic acid (RNA) technology through the expression of targeting cytokines in situ. However, the intracellular translation of RNA remains restricted due to the generation of excessive reactive oxygen species (ROS) and overconsumption of adenosine triphosphate (ATP) within the transfected cells. Herein, hybrid lipid nanoparticles (Mn-LNPs) are developed by incorporating small-sized trimanganese tetraoxide nanoparticles within conventional lipid nanoparticles, showing the ability to generate oxygen, eliminate ROS, and boost intracellular ATP, thus greatly enhancing the translation efficiency. This hybrid platform is employed to encapsulate interleukin 12 (IL-12)-encoding circular RNA (Mn-LNPs@RNAIL-12) for tumor immunotherapy, exhibiting unparalleled advantages in the proliferation of cytotoxic T cells and stimulation of antitumor immunity. Moreover, the antitumor efficacy of Mn-LNPs@RNAIL-12 is further strengthened by synergizing with immune checkpoint blockade therapy to achieve durable and potent antitumor performances.

Enhancing the photo-induced oxidase-like activity of fluorescein with methyl viologen for colorimetric detection of organophosphorus pesticide

Overcoming the intrinsic low activity of most photoinduced oxidase mimics has been extremely challenging. In this work, we developed a methyl viologen (MV2+) mediated strategy to enhance the oxidase-like activity of fluorescein. The presence of MV2+ gives it a high affinity for TMB with a low Michaelis-Menten constant (Km) of 0.053 mM, which is about 2.8 times lower than that of fluorescein and with a remarkable catalytic constant (Kcat) as 0.2490 s-1, which is 3 times as high as that of fluorescein. Fluorescein diacetate (FDA) without oxidase-like activity can be hydrolyzed in situ to produce fluorescein in the presence of carboxylesterases (CaE). Based on the inhibition of CaE activity by organophosphorus pesticides (OP), a novel colorimetric signal biosensor was established with a wide linear range from 1.0 to 200 ng/mL. This work not only provides a convenient and feasible strategy for enhancing the activity of photoinduced oxidase mimics but also blazes a new pathway for the sensitive detection of OP.

Microbe-mediated synthesis of defect-rich CeO2 nanoparticles with oxidase-like activity for colorimetric detection of L-penicillamine and glutathione

To enhance production efficiency, curtail costs, and minimize environmental impact, developing simple and sustainable nanozyme synthesis methods has been the focus of relevant research. In this report, graphite-coated CeO2 nanoparticles (CeO2 NPs) with multiple defects (Ce3+ defects, oxygen vacancies and carbon defects) were synthesized via the culture filtrate of the extremely radioresistant bacterium Deinococcus wulumuqiensis R12 (D. wulumuqiensis R12). The as-prepared CeO2 NPs exhibit remarkable oxidase (OXD)-like activity, efficiently catalyzing the oxidation of the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) to form oxTMB, even in the absence of H2O2. The electron-rich bioactive substances in the supernatant were demonstrated to modulate the electronic state of the Ce atom and played a key role in the formation of multiple defects, thereby enhancing the OXD-like activity of CeO2 NPs. Based on the inhibitory effect of sulphydryl groups (-SH) on the TMB-CeO2 system, a colorimetric strategy for the detection of both L-penicillamine (L-PA) and glutathione (GSH) was devised and successfully applied in real sample analysis. The linear ranges of L-PA and GSH detection were found to be 10-500 μM and 9-200 μM with the limits of detection (LODs) at 8.53 and 5.19 μM, respectively. This work provides a straightforward, eco-friendly and nontoxic method for the synthesis and defect construction of CeO2 NPs with OXD-like activity.

Photoexcited Electro-Driven Reactive Oxygen Species Channeling for Precise Extraction of Biomarker Information from Tumor Interstitial Fluid

Direct electrochemical detection of miRNA biomarkers in tumor tissue interstitial fluid (TIF) holds great promise for adjuvant therapy for tumors in the perioperative period, yet is limited by background interference and weak signal. Herein, a wash-free and separation-free miRNA biosensor based on photoexcited electro-driven reactive oxygen channeling analysis (LEOCA) is developed to solve the high-fidelity detection in physiological samples. In the presence of miRNA, nanoacceptors (ultrasmall-size polydopamine, uPDA) are responsively assembled on the surface of nanodonors (zirconium metal-organic framework, ZrMOF) to form core-satellite aggregates. The produced lifetime-constraint singlet oxygen upon light irradiation is captured by the catechol of constrained uPDA, and the oxidized quinone is immediately electro-reduced to the catechol at transient collision process on the electrode, resulting in a cascade electron transfer and amplified current. Thereby, the nanosensor exhibits a low detection limit (1.1 fM), and high reproducibility (relative standard deviation of 2.0%). Compared with quantitative real-time polymerase chain reaction (qRT-PCR), the clinical accuracy (area under the curve value) is significantly increased from 0.75 to 0.93 in distinguishing breast cancer patients from healthy donors. This study demonstrates an inspiration on the synergy of the reactive oxygen channeling between nanodonor/nanoacceptor and the synchronous electron transfer cascade on the electrode to solve the bottleneck problem of detecting unprocessed clinical samples in a sample-in-answer-out manner.

Redox-mediated dsDNA-dye photooxidase mimic enable catalytic oxidation of 3,3',5,5'-tetramethylbenzidine by dissolved O2 at neutral pH for improved biosensing

Catalytic oxidation of 3,3',5,5'-Tetramethylbenzidine (TMB, an excellent chromogenic substrate) at neutral pH is critically important for amplified bioanalysis. Although some nanozymes exhibited the peroxidase activity at neutral pH, it is difficult to modulate their activity for homogeneous detection of biomolecules. In this work, we developed a redox-mediated dsDNA-dye photooxidase mimic that enables catalytic oxidation of TMB by dissolved O2 at neutral pH for improved biosensing. During illumination, the double-stranded DNA-SYBR Green I (dsDNA-SG) photogenerated singlet oxygen (1O2) can oxidized Mn2+ to Mn3+ that can efficiently oxidize TMB to produce a distinct blue within 4 min under neutral conditions. The catalytic oxidation of TMB can be readily modulated by the formation or dissociation of dsDNA during the sensing. After investigating a series of redox mediators, we found that only the Mn3+/Mn2+ redox mediator can lead to the oxidation of TMB at neutral pH. The maximum reaction rate of Mn2+-mediated dsDNA-SG photooxidase mimic under neutral conditions (pH 7.0) was 1.7 × 10-4 mM/s, even higher than that of horseradish peroxidase (HRP, 8.0 × 10-5 mM/s). The redox-mediated dsDNA-SG photooxidase mimic was used for detection of APE1 at pH 7.0 with over 130-fold higher sensitivity than that at 4.0, owing to the high enzymatic activity of APE1 at neutral pH. Meanwhile, we further extended this photooxidase mimic for the sensitive detection of DNA (LOD, 8 pM) and heavy metal ions at neutral pH. The redox-mediated dsDNA-dye photooxidase mimic with the ease of modulating its enzymatic activity and working at neutral pH is quite appealing for biosensing.

Plasma-Liquid-Induced Synthesis of Scandium-Metalloporphyrin Frameworks for Boosted Sensing and Photosensitization

Inserting metal ions into the porphyrin ring is one of the primary strategies to enhance the properties of porphyrin-based metal-organic frameworks (MOFs). However, the straightforward, rapid, and energy-efficient synthesis of porphyrin-based MOFs with high metallization for the porphyrin ring remains challenging. Herein, a solution anode glow discharge (SAGD) microplasma is presented for the one-step synthesis of scandium-metalloporphyrin frameworks (ScMPFs). The substantial number of electrons provided by the plasma-liquid interface not only accelerated the rapid nucleation and growth of MOFs but also promoted the incorporation of scandium (Sc3+), which has a small ionic radius and strong coordination ability, into the N atoms in the porphyrin ring, and enhanced the metallization of MOFs. The sufficient Sc3+ in frameworks inhibited the recombination of electron-hole pairs, resulting in boosted reactive oxygen species yield and a low fluorescence background of MOFs. Consequently, the ScMPFs are employed for the sensitive and rapid detection of F- in water with a detection limit of 0.24 µm and for efficient bacteriostasis at low doses (10 µg mL-1, 12 mW cm-2 light irradiation for 10 min).

Nanozymes as Antibacterial Agents: New Concerns in Design and Enhancement Strategies

Nanozymes exhibiting natural enzyme-mimicking catalytic activities as antibacterial agents present several advantages, including high stability, low cost, broad-spectrum antibacterial activity, ease of preparation and storage, and minimal bacterial resistance. Consequently, they have attracted significant attention in recent years. However, the rapid expansion of antimicrobial nanozyme research has resulted in pioneering reviews that do not comprehensively address emerging concerns and enhancement strategies within this field. This paper first summarizes the factors influencing the intrinsic activity of nanozymes; subsequently, we outline new research considerations for designing antibacterial nanozymes with enhanced functionality and biosafety features such as degradable, imageable, targeted, and bacterial-binding nanozymes as well as those capable of selectively targeting pathogenic bacteria while sparing normal cells and probiotics. Furthermore, we review novel enhancement strategies involving external physical stimuli (light or ultrasound), the introduction of extrinsic small molecules, and self-supplying H2O2 to enhance the activity of antibacterial nanozymes under physiological conditions characterized by low concentrations of H2O2 and O2. Additionally, we present non-redox nanozymes that operate independently of highly toxic reactive oxygen species (ROS) alongside those designed to combat less common pathogenic bacteria. Finally, we discuss current issues, challenges faced in the field, and future prospects for antibacterial nanozymes.

Cascade enzyme-mimicking with spatially separated gold-ceria for dual-mode detection of superoxide anions

Metal-semiconductor nanozyme of dumbbell Au-CeO2 with spatially separated heterostructure has cascade superoxide dismutase (SOD)-like and peroxidase (POD)-like activities for superoxide anions detection. It was synthesized by selective growth of CeO2 at the ends of Au nanorod (Au NR). Taking advantage of the excellent local surface plasmon resonance (LSPR) effect of Au NR, the spatially separated Au-CeO2 has a higher photothermal effect than the continuously growing core-shell structure of Au@CeO2. Meanwhile, the hot electrons of Au NR could transfer to CeO2 under 808 nm laser irradiation, changing the ratio of Ce3+/Ce4+ redox couples over CeO2 and facilitating H2O2 decomposition thus enhancing POD-like activity. Based on the SOD-like activity of Au-CeO2, superoxide anion (O2·-) can be transformed into hydrogen peroxide (H2O2). Dual-mode including absorbance and temperature sensing detection of O2·-, with the detection range from nM to μM i.e., 0.1-150 μM and LOD of 0.033 μM (S/N = 3) was achieved through the cascade catalysis and photothermal effect. The as-proposed method was applicable to both cancer and normal cell samples with satisfactory accuracy and recovery.

A Smartphone-Enabled Colorimetric Microneedle Sensing Platform for Rapid Detection of Ascorbic Acid in Fruits

Achieving rapid extraction and detection of analytical solutions from plant tissues to circumvent cumbersome procedures and reduce the dependence on detection instruments remains a challenge. Herein, a colorimetric microneedle integrated platform was developed for the rapid extraction and detection of plant fluids for testing purposes. Colorimetric microneedle patches (CMPs) offer a swift and effective method to swiftly extract and detect ascorbic acid (AA) within 10 min from various fruit crops like mango, nectarine, apple, pear, and kiwifruit, facilitated by a smartphone application. CMPs are constructed for the rapid and sensitive analysis of AA with good linearity amid the range 0.05-25 μM and a low limit of detection of 30 nM. The novel CMPs demonstrate significant potential as a rapid detection platform for AA in plants. CMPs offer significant advantages over traditional ultraviolet spectrophotometry, such as simplified operational procedures and accelerated extraction and detection processes. This establishes robust groundwork for conducting in situ extraction and molecular detection of diverse crops across a spectrum of application scenarios.

Dual-mode, signal-amplified DNA biosensor for label-free, reliable assay of gestational diabetes mellitus-related miRNA (miR-135a)

miR-135a is highly expressed in patients with gestational diabetes mellitus, and its target genes are also involved in insulin signaling pathway, so it is one biomarker for gestational diabetes mellitus. Herein we designed a dual-mode DNA biosensor for reliable assay of miR-135a based on the fluorescence and colorimetric signals. Several experiments were carried out to demonstrate the assay feasibility and mechanism for this dual-mode DNA biosensor. With optimum parameters, this proposed dual-mode biosensor has been realized sensitive and quantitative assay of miR-135a. For the fluorescence and colorimetric signals, the working ranges are 0.56-61 and 8.3-74 nM, while limits of detection are 0.18 and 3.7 nM respectively. This dual-mode strategy allows two independent signals for miR-135a assay, so it can verify each other to show more accurate results with good fidelity. Furthermore, there is a good selectivity in the biosensor for target miR-135a over other nucleotide variants, as well as good anti-interference ability in complex samples. This dual-mode DNA biosensor provides a new approach for miR-135a assay and miRNA expression profiling in gestational diabetes mellitus.

Wide temperature adaptive oxidase-like based on mesoporous manganese based metal-organic framework for detecting total antioxidant capacity

Overcoming the intense variation of enzymatic activity among different temperatures is very critical in catalytic medicine and catalytic biology. Here, Mn-based metal-organic framework-based wide-temperature-adaptive mesoporous artificial enzymes (Mn-TMA-MOF) were designed and synthesized. The oxidase-like Mn-TMA-MOF showed excellent catalytic activity at 0-50 °C and avoided the activity loss and instability due to temperature variation that occurred. The excellent oxidase-like properties of Mn-TMA-MOF with wide temperature adaptativeness are mainly ascribed to the mixed oxidized state (Mn3+/Mn2+) and high substrate affinity (Km = 0.034 mM) of Mn. Moreover, the mesopore-micropores two-level structure of Mn-TMA-MOF provides a large space and surface area for enzyme catalysis. Based on the stability of Mn-TMA-MOF, we developed a colorimetric sensor that can detect total antioxidant capacity in fruits with a limit of detection up to 0.59 μM.

Breaking the pH Limitation of Nanozymes: Mechanisms, Methods, and Applications

Although nanozymes have drawn great attention over the past decade, the activities of peroxidase-like, oxidase-like, and catalase-like nanozymes are often pH dependent with elusive mechanism, which largely restricts their application. Therefore, a systematical discussion on the pH-related catalytic mechanisms of nanozymes together with the methods to overcome this limitation is in need. In this review, various nanozymes exhibiting pH-dependent catalytic activities are collected and the root causes for their pH dependence are comprehensively analyzed. Subsequently, regulatory concepts including catalytic environment reconstruction and direct catalytic activity improvement to break this pH restriction are summarized. Moreover, applications of pH-independent nanozymes in sensing, disease therapy, and pollutant degradation are overviewed. Finally, current challenges and future opportunities on the development of pH-independent nanozymes are suggested. It is anticipated that this review will promote the further design of pH-independent nanozymes and broaden their application range with higher efficiency.

Fine-Tuning Electron Transfer for Nanozyme Design

Nanozymes, with their versatile composition and structural adaptability, present distinct advantages over natural enzymes including heightened stability, customizable catalytic activity, cost-effectiveness, and simplified synthesis process, making them as promising alternatives in various applications. Recent advancements in nanozyme research have shifted focus from serendipitous discovery toward a more systematic approach, leveraging machine learning, theoretical calculations, and mechanistic explorations to engineer nanomaterial structures with tailored catalytic functions. Despite its pivotal role, electron transfer, a fundamental process in catalysis, has often been overlooked in previous reviews. This review comprehensively summarizes recent strategies for modulating electron transfer processes to fine-tune the catalytic activity and specificity of nanozymes, including electron-hole separation and carrier transfer. Furthermore, the bioapplications of these engineered nanozymes, including antimicrobial treatments, cancer therapy, and biosensing are also introduced. Ultimately, this review aims to offer invaluable insights for the design and synthesis of nanozymes with enhanced performance, thereby advancing the field of nanozyme research.

Field Detection of Uranyl in Coastal Water of China Using a Portable Device via DNA Photocleavage

The urgent need for field detection of uranium in seawater is 2-fold: to provide prompt guidance for uranium extraction and to prevent human exposure to nuclear radiation. However, current methods for this purpose are largely hindered by bulky instrumentation, high costs of developed materials, and severe matrix interferences, which limit their further application in the field. Herein, we demonstrated a portable and label-free strategy for the field detection of uranyl in seawater based on the efficient photocleavage of DNA. Further experiments confirmed the generation of ultraviolet (UV) light-induced reactive oxygen species (ROS), such as O2•- and •OH, which fragmented oligomeric DNA in the presence of uranyl and UV light. Detailed studies showed that DNA significantly enhances uranyl absorption in the UV-visible region, leading to the generation of more ROS. A fluorescence system for the selective detection of uranyl in seawater was established by immobilizing two complementary oligonucleotides with the fluorescent dye SYBR Green I. The strategy of UV-induced photocleavage offers high selectivity, excellent interference immunity, and high sensitivity for uranyl, with a detection limit of 6.8 nM. Additionally, the fluorescence can be visually detected using a 3D-printed miniaturized device integrated with a smartphone. This method has been successfully applied to the on-site detection of uranyl in seawater in 18 Chinese coastal cities and along the coast of Hainan Island within 3 min for a single sample. The sample testing and field analysis results indicate that this strategy has promising potential for real-time monitoring of trace uranyl in China's coastal waters. It is expected to be utilized for the rapid assessment of nuclear contamination and nuclear engineering construction.

Ultrasmall CuMn-His Nanozymes with Multienzyme Activity at Neutral pH: Construction of a Colorimetric Sensing Array for Biothiol Detection and Disease Identification

Biothiol assays offer vital insights into health assessment and facilitate the early detection of potential health issues, thereby enabling timely and effective interventions. In this study, we developed ultrasmall CuMn-Histidine (His) nanozymes with multiple enzymatic activities. CuMn-His enhanced peroxidase (POD)-like activity at neutral pH was achieved through hydrogen bonding and electrostatic effects. In addition, CuMn-His possesses laccase (LAC)-like and superoxide dismutase (SOD)-like activities at neutral pH. Based on three different enzyme mimetic activities of CuMn-His at neutral pH, the colorimetric sensing array without changing the buffer solution was successfully constructed. The array was successfully used for the identification of three biothiols, glutathione (GSH), cysteine (Cys), and homocysteine (Hcy). Subsequently, excellent application results were shown in complex serum and cellular level analyses. This study provides an innovative strategy for the development of ultrasmall bimetallic nanozymes with multiple enzymatic activities and the construction of colorimetric sensing arrays.

Preparation and Application of Carbon Dots Nanozymes

Carbon dot (CD) nanozymes have enzyme-like activity. Compared with natural enzymes, CD nanozymes offer several advantages, including simple preparation, easy preservation, good stability and recycling, which has made them a popular research topic in various fields. In recent years, researchers have prepared a variety of CD nanozymes for biosensing detection, medicine and tumor therapy, and many of them are based on oxidative stress regulation and reactive oxygen species clearance. Particularly to expand their potential applications, elemental doping has been utilized to enhance the catalytic capabilities and other properties of CD nanozymes. This review discusses the prevalent techniques utilized in the synthesis of CD nanozymes and presents the diverse applications of CD nanozymes based on their doping characteristics. Finally, the challenges encountered in the current utilization of CD nanozymes are presented. The latest research progress of synthesis, application and the challenges outlined in the review can help and encourage the researchers for the future research on preparation, application and other related researches of CD nanozymes.

Facile colorimetric sensor using oxidase-like activity of octahedral Ag2O particles for highly selective detection of Pb(II) in water

Pb(II) is a prevalent heavy metal ion classified as a 2B carcinogen. Excessive intake of Pb(II) in the human body can damage the central nervous system, kidneys, liver, and immune system, leading to permanent brain damage, anemia, and cancer. Colorimetry can be applied to rapidly determine Pb(II) residues, but there are still many challenges in the accuracy and sensitivity of detection. Based on the inhibitory impact of Pb(II) on the oxidase-like activity of octahedral silver oxide (Ag2O), a colorimetric sensor with smartphone-assisted analysis for the Pb(II) detection was first developed. Herein, it has been found that Pb(II) can adsorb onto the surface of octahedral Ag2O, hindering the production of O2- in the reaction system. This ultimately results in the suppression of oxidase-like activity, leading to a lighter purple appearance of the colorimetric reaction solution. The sensor exhibits a high degree of sensitivity and a limit of detection (LOD) for Pb(II) was calculated as 2.2 μg L-1. Hence, the developed colorimetric sensor with high sensitivity, excellent specificity, and high tolerance to sodium ions is hopeful to have practical applications in Pb(II) detection in environmental water samples. Moreover, the sensor will provide a novel strategy for heavy metal ion detection and other substances.

Two-mode sensing strategies based on tunable cobalt metal organic framework active sites to detect Hg2

Heavy metal pollution poses a major threat to human health, and developing a user-deliverable heavy metal detection strategy remains a major challenge. In this work, two-mode Hg2+ sensing platforms based on the tunable cobalt metal-organic framework (Co-MOF) active site strategy are constructed, including a colorimetric, and an electrochemical assay using a personal glucose meter (PGM) as the terminal device. Specifically, thymine (T), a single, adaptable nucleotide, is chosen to replace typical T-rich DNA aptamers. The catalytic sites of Co-MOF are tuned competitively by the specific binding of T-Hg2+-T, and different signal output platforms are developed based on the different enzyme-like activities of Co-MOF. DFT calculations are utilized to analyze the interaction mechanism between T and Co-MOF with defect structure. Notably, the two-mode sensing platforms exhibit outstanding detection performance, with LOD values as low as 0.5 nM (colorimetric) and 3.69 nM (PGM), respectively, superior to recently reported nanozyme-based Hg2+ sensors. In real samples of tap water and lake water, this approach demonstrates an effective recovery rate and outstanding selectivity. Surprisingly, the method is potentially versatile and, by exchanging out T-Hg2+-T, can also detect Ag+. This simple, portable, and user-friendly Hg2+ detection approach shows plenty of promise for application in the future.

Multienzyme-Like Nanozymes: Regulation, Rational Design, and Application

Nanomaterials with more than one enzyme-like activity are termed multienzymic nanozymes, and they have received increasing attention in recent years and hold huge potential to be applied in diverse fields, especially for biosensing and therapeutics. Compared to single enzyme-like nanozymes, multienzymic nanozymes offer various unique advantages, including synergistic effects, cascaded reactions, and environmentally responsive selectivity. Nevertheless, along with these merits, the catalytic mechanism and rational design of multienzymic nanozymes are more complicated and elusive as compared to single-enzymic nanozymes. In this review, the multienzymic nanozymes classification scheme based on the numbers/types of activities, the internal and external factors regulating the multienzymatic activities, the rational design based on chemical, biomimetic, and computer-aided strategies, and recent progress in applications attributed to the advantages of multicatalytic activities are systematically discussed. Finally, current challenges and future perspectives regarding the development and application of multienzymatic nanozymes are suggested. This review aims to deepen the understanding and inspire the research in multienzymic nanozymes to a greater extent.

Hydrolysis-Induced Cu2O Networks and the Triggered Peroxidase-Mimic Activity by Cr6+ under Neutral Conditions

The current small-scale synthesis and relatively large size of Cu2O have limited its practical applications. Herein, we developed a hydrolysis strategy to prepare phase-pure Cu2O networks composed of small granules (ca. 25 nm) on a gram scale. The preparation involves in situ hydrolyzing the Hx[CuxCl2x] complexes prereduced in N,N'-dimethylformamide (DMF). The DMF-soluble Hx[CuxCl2x] complexes are critical for the homogeneous nucleation of CuCl seeds and subsequent hydrolysis, allowing for separate control over the nucleation and growth stages to regulate the formation of Cu2O networks. The novel Cu2O networks possess numerous exposed active sites and hierarchical porosities, conferring high catalytic activity and fast mass transfer capability. The inherent peroxidase-mimic activity of Cu2O is severely inhibited under neutral conditions but can be triggered by Cr6+, enabling the colorimetric assay of Cr6+ with the assistance of the oxidation-induced color change of 3,3',5,5'-tetramethylbenzidine. Through density functional theory calculation, we confirmed that the attachment of Cr6+ on the Cu2O surface reduced the dissociation energy of H2O2, enhancing the enzyme-mimic activity. The colorimetric detection method demonstrated a sensitive and specific assay capability for Cr6+ (LOD = 0.095 μM). Our work offers a straightforward protocol for novel design of metal or metal-based nanomaterials for nanozymes or other applications.

A Multienzyme Reaction-Mediated Electrochemical Biosensor for Sensitive Detection of Organophosphorus Pesticides

Ethephon (ETH), a commonly employed growth regulator, poses potential health risks due to its residue in fruits and vegetables, leading to both acute and subchronic toxicity. However, the detection accuracy of ETH is compromised by the color effects of the samples during the detection process. In this work, a multienzyme reaction-mediated electrochemical biosensor (MRMEC) was developed for the sensitive, rapid, and color-interference-resistant determination of ETH. Nanozymes Fe3O4@Au-Pt and graphene nanocomplexes (GN-Au NPs) were prepared as catalysts and signal amplifiers for MRMEC. Acetylcholinesterase (AChE), acetylcholine (ACh), and choline oxidase (CHOx) form a cascade enzyme reaction to produce H2O2 in an electrolytic cell. Fe3O4@Au-Pt has excellent peroxidase-like activity and can catalyze the oxidation of 3,3',5,5'-tetramethvlbenzidine (TMB) in the presence of H2O2, resulting in a decrease in the characteristic peak current of TMB. Based on the inhibitory effect of ETH on AChE, the differential pulse voltammetry (DPV) current signal of TMB was used to detect ETH, offering the limit of detection (LOD) of 2.01 nmol L-1. The MRMEC method effectively analyzed ETH levels in mangoes, showing satisfactory precision (coefficient of variations, 2.88-15.97%) and recovery rate (92.18-110.72%). This biosensor holds promise for detecting various organophosphorus pesticides in food samples.

Colorimetric detection of alkaline phosphatase based on the off-on effect of light-responsive oxidase mimicking activity of covalent organic framework (Cu-TpBpy-COF) under near-neutral condition

A colorimetric strategy has been developed for the detection of alkaline phosphatase (ALP) activity based on the off-on effect of the catalytic activity of light-responsive oxidase mimics covalent organic framework (Cu-TpBpy-COF) in near-neutral condition. Cu-TpBpy-COF can effectively catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) by oxygen to form a blue oxidized product (oxTMB) with an absorption peak at 652 nm. Cu2+ is the active center of Cu-TpBpy-COF and pyrophosphate (PPi) can form a complex with Cu2+ to weaken the catalytic activity of Cu-TpBpy-COF. In the presence of ALP, PPi is hydrolyzed into orthophosphates (Pi) with low affinity to Cu2+, thus resulting in absorbance restoration. The absorbance at 652 nm is related to ALP activity in the linear range 10-150 U·L-1 with a detection limit of 7.17 U·L-1. The recoveries of ALP in serum samples are in the range 94.7~107.0% with relative standard deviations (RSD) lower than 5%. The decisive role of Cu2+ on the enhancing catalytic activities of Cu-TpBpy-COF in neutral condition was verified by TpBpy-COF and TpBD-COF as controls, in which the main difference between them is that TpBpy-COF contains pyridine nitrogen. Upon Cu2+ modification, Cu-TpBpy-COF has better catalytic activity than TpBpy-COF in a broader pH range because of the in situ generation of Cu+ under irradiation.

A comprehensive exploration of the latest innovations for advancements in enhancing selectivity of nanozymes for theranostic nanoplatforms

Nanozymes have captured significant attention as a versatile and promising alternative to natural enzymes in catalytic applications, with wide-ranging implications for both diagnosis and therapy. However, the limited selectivity exhibited by many nanozymes presents challenges to their efficacy in diagnosis and raises concerns regarding their impact on the progression of disease treatments. In this article, we explore the latest innovations aimed at enhancing the selectivity of nanozymes, thereby expanding their applications in theranostic nanoplatforms. We place paramount importance on the critical development of highly selective nanozymes and present innovative strategies that have yielded remarkable outcomes in augmenting selectivities. The strategies encompass enhancements in analyte selectivity by incorporating recognition units, refining activity selectivity through the meticulous control of structural and elemental composition, integrating synergistic materials, fabricating selective nanomaterials, and comprehensively fine-tuning selectivity via approaches such as surface modification, cascade nanozyme systems, and manipulation of external stimuli. Additionally, we propose optimized approaches to propel the further advancement of these tailored nanozymes while considering the limitations associated with existing techniques. Our ultimate objective is to present a comprehensive solution that effectively addresses the limitations attributed to non-selective nanozymes, thus unlocking the full potential of these catalytic systems in the realm of theranostics.

Au@C/Pt core@shell/satellite supra-nanostructures: plasmonic antenna-reactor hybrid nanocatalysts

Integration of plasmonic nanoantennas with catalytically active reactors in deliberately designed hybrid supra-nanostructures creates a dual-functional materials platform, based upon which precise modulation of catalytic reaction kinetics becomes accomplishable through optical excitations of plasmon resonances. Here, we have developed a multistep synthetic approach that enables us to assemble colloidal Au@C/Pt core@shell/satellite supra-nanostructures, in which the Au core functions as a light-harvesting plasmonic nanoantenna, the Pt satellites act as catalytically active reactors, and the C shell serves as a nanoscale dielectric spacer separating the reactors from the antenna, respectively. By adjusting several synthetic parameters, the size of the Au core, the thickness of the C shell, and the surface coverage of Pt satellites can all be tuned independently. Choosing Pt-catalyzed cascade oxidation of 3,3',5,5'-tetramethylbenzidine in an aerobic aqueous environment as a model reaction, we have systematically studied the detailed kinetic features of the catalytic reactions both in the dark and under visible light illumination over a broad range of reaction conditions, which sheds light on the interplay between plasmonic and catalytic effects in these antenna-reactor nanohybrids. The plasmonic antenna effect can be effectively harnessed to kinetically modulate multiple crucial steps during the cascade reactions, benefiting from plasmon-enhanced interband electronic transitions in the Pt satellites and plasmon-enhanced intramolecular electronic excitations in chromogenic intermediate species. In addition to the plasmonic antenna effect, photothermal transduction derived from plasmonic excitations can also provide significant contributions to the kinetic enhancements under visible light illumination. The knowledge gained from this work serves as important guiding principles for rational design and structural optimization of plasmonic antenna-reactor hybrid nanomaterials, endowing us with enhanced capabilities to kinetically modulate targeted catalytic/photocatalytic molecule-transforming processes through light illumination.

Engineering a gold nanoparticles-carbon dots nanocomposite with pH-flexibility for monitoring hydrogen peroxide released from living cells

Constructing nanozymes with satisfactory catalytic efficiency under physiological conditions is still in great demand for facilitating the advancement of biocatalysts. We herein present a gold nanoparticles-carbon dots nanocomposite (Au-CDs) as an efficient photo-activated nanozyme for monitoring H2O2 released from living cells. The integration of CDs with AuNPs remarkably accelerates the catalytic activity at neutral pH via engaging Mn3+ ions as the mediators. Meanwhile, the reserved cyclodextrin cavities also enhance the adsorption capacity towards chromogenic substrates through host-guest interactions. Moreover, taking advantage of the inhibitory effect of H2O2 on the photo-oxidation ability of the Au-CDs nanocomposite, the Au-CDs based colorimetric method was able to realize in situ assessment of the hydrogen peroxide (H2O2) released from living cells. This method paves a new way to establish a promising biosensing platform for unraveling biological events.

DNA-Encoded Bidirectional Regulation of the Peroxidase Activity of Pt Nanozymes for Bioanalysis

Rational regulation of nanozyme activity can promote biochemical sensing by expanding sensing strategies and improving sensing performance, but the design of effective regulatory strategies remains a challenge. Herein, a rapid DNA-encoded strategy was developed for the efficient regulation of Pt nanozyme activity. Interestingly, we found that the catalytic activity of Pt nanozymes was sequence-dependent, and its peroxidase activity was significantly enhanced only in the presence of T-rich sequences. Thus, different DNA sequences realized bidirectional regulation of Pt nanozyme peroxidase activity. Furthermore, the DNA-encoded strategy can effectively enhance the stability of Pt nanozymes at high temperatures, freezing, and long-term storage. Meanwhile, a series of studies demonstrated that the presence of DNA influenced the reduction degree of H₂PtCl₆ precursors, which in turn affected the peroxidase activity of Pt nanozymes. As a proof of application, the sensor array based on the Pt nanozyme system showed superior performance in the accurate discrimination of antioxidants. This study obtained the regulation rules of DNA on Pt nanozymes, which provided theoretical guidance for the development of new sensing platforms and new ideas for the regulation of other nanozyme activities.

Phase-Controlled Ruthenium Nanocrystals on Colloidal Polydopamine Supports and Their Catalytic Behaviors in Aerobic Oxidation Reactions

The past decade has witnessed rapidly growing interest in noble metal nanostructures adopting unconventional metastable crystal phases. In the case of Ru, chemically synthesized nanocrystals typically form thermodynamically favored hexagonal close-packed (hcp) crystal lattices, whereas it remains significantly more challenging to synthesize Ru nanocrystals in the metastable face-centered cubic (fcc) phase. In this work, we have synthesized polydopamine (PDA)-supported hcp and fcc Ru nanocrystals in a phase-selective manner through one-pot thermal reduction of appropriate Ru(III) precursors in a polyol solvent. Benefiting from the unique surface-adhesion function of PDA, we have been able to grow phase-controlled sub-5 nm Ru nanocrystals directly on colloidal PDA supports without prefunctionalizing the particle surfaces with any molecular linkers or surface-capping ligands. Success in phase-controlled synthesis of capping ligand-free Ru nanocrystals dispersed on the same support material enables us to systematically compare the intrinsic mass-specific and surface-specific activities of fcc and hcp Ru nanocatalysts toward the aerobic oxidation of a chromogenic molecular substrate, 3,3',5,5'-tetramethylbenzidine (TMB), under a broad range of reaction conditions. We use UV-vis absorption spectroscopy to monitor the conversion of the reactant molecules into the one-electron and two-electron oxidation products in real time during Ru-catalyzed oxidation of TMB, which is found to be a mechanistically complex molecule-transforming process involving multiple elementary steps. The apparent reaction rates and detailed kinetic features are observed to be not only intimately related to the crystalline structures of the Ru nanocatalysts but also profoundly influenced by several other critical factors, such as the pH of the reaction medium, the initial concentration of TMB, Ru coverage on the PDA supports, and degree of nanoparticle aggregation.

Simultaneous visual detection of multiple heavy metal ions by a high-throughput fluorescent probe

To develop simultaneous and in-situ detection techniques towards Cr(VI) and Mn(II), Eu/Tb@CDs with white fluorescence were prepared by a one-step hydrothermal method. With the increase of Cr(VI), all fluorescence channels of Eu/Tb@CDs exhibited obvious quenching, and the detection limit (LOD) was 0.10 μM. In the presence of Mn(II), only the fluorescence from Tb and Eu was quenched, while the fluorescence of CDs was not effected. The LOD for Mn(II) was 0.16 μM. More importantly, in the actual water samples where Cr(VI) and Mn(II) coexist, Eu/Tb@CDs can realize their rapid and simultaneous detection by simple spectral calculation. The selective and competitive experiments have also confirmed that the detection of Cr(VI) and Mn(II) was not interfered by common pollutants in groundwater. It is undeniable that the simultaneous detection of multiple targets by one probe not only greatly improves the detection efficiency, but also has important significance for the field monitoring of water quality parameters.

Light-activated carbon dot nanozyme with scandium for a highly efficient and pH-universal bio-nanozyme cascade colorimetric assay

Nanozyme-based colorimetric assays have attracted much attention due to their cost-effectiveness, high stability, and sensitivity. In particular, the catalytic cascade imparted by the biological enzyme is highly selective. However, developing an efficient, one-pot, and pH-universal bio-nanozyme cascade remains challenging. Considering the tunable activity of the photo-activated nanozyme, we herein demonstrated a pH-universal colorimetric assay based on the Sc³⁺-boosted photocatalytic oxidation of carbon dots (C-dots). As a strong Lewis acid, Sc³⁺ shows ultra-fast complexation with OH^- over a broad pH range and dramatically decreases the pH of the buffer solutions. In addition to regulating the pH, Sc³⁺ also binds to the C-dots to produce a persistent and strongly oxidizing intermediate based on photo-induced electron transfer. The proposed Sc³⁺-boosted photocatalytic system was successfully used in a cascade colorimetric assay with biological enzymes for assessing their activity as well as the detection of enzyme inhibitors at neutral and alkaline pH. Instead of designing new nanozymes for catalytic cascades, this work suggests that introducing promoters can be a convenient strategy in practical applications.

Coordinating Etching Inspired Synthesis of Fe(OH)3 Nanocages as Mimetic Peroxidase for Fluorescent and Colorimetric Self-Tuning Detection of Ochratoxin A

The development of multifunctional biomimetic nanozymes with high catalytic activity and sensitive response is rapidly advancing. The hollow nanostructures, including metal hydroxides, metal-organic frameworks, and metallic oxides, possess excellent loading capacity and a high surface area-to-mass ratio. This characteristic allows for the exposure of more active sites and reaction channels, resulting in enhanced catalytic activity of nanozymes. In this work, based on the coordinating etching principle, a facile template-assisted strategy for synthesizing Fe(OH)₃ nanocages by using Cu₂O nanocubes as the precursors was proposed. The unique three-dimensional structure of Fe(OH)₃ nanocages endows it with excellent catalytic activity. Herein, in the light of Fe(OH)₃-induced biomimetic nanozyme catalyzed reactions, a self-tuning dual-mode fluorescence and colorimetric immunoassay was successfully constructed for ochratoxin A (OTA) detection. For the colorimetric signal, 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) can be oxidized by Fe(OH)₃ nanocages to form a color response that can be preliminarily identified by the human eye. For the fluorescence signal, the fluorescence intensity of 4-chloro-1-naphthol (4-CN) can be quantitatively quenched by the valence transition of Ferric ion in Fe(OH)₃ nanocages. Due to the significant self-calibration, the performance of the self-tuning strategy for OTA detection was substantially enhanced. Under the optimized conditions, the developed dual-mode platform accomplishes a wide range of 1 ng/L to 5 μg/L with a detection limit of 0.68 ng/L (S/N = 3). This work not only develops a facile strategy for the synthesis of highly active peroxidase-like nanozyme but also achieves promising sensing platform for OTA detection in actual samples.

RhRu Alloy-Anchored MXene Nanozyme for Synergistic Osteosarcoma Therapy

Noble metal nanozymes hold promise in cancer therapy due to adjustable enzyme-like activities, unique physicochemical properties, etc. But catalytic activities of monometallic nanozyme are confined. In this study, 2D titanium carbide (Ti₃ C₂ T_x )-supported RhRu alloy nanoclusters (RhRu/Ti₃ C₂ T_x ) are prepared by a hydrothermal method and utilized for synergistic therapy of chemodynamic therapy (CDT), photodynamic therapy (PDT), and photothermal therapy (PTT) on osteosarcoma. The nanoclusters are small in size (3.6 nm), uniform in distribution, and have excellent catalase (CAT) and peroxidase (POD)-like activities. Density functional theory calculations show that there is a significant electron transfer interaction between RhRu and Ti₃ C₂ T_x , which has strong adsorption to H₂ O₂ and is beneficial to enhance the enzyme-like activity. Furthermore, RhRu/Ti₃ C₂ T_x nanozyme acts as both PTT agent for converting light into heat, and photosensitizer for catalyzing O₂ to ¹ O₂ . With the NIR-reinforced POD- and CAT-like activity, excellent photothermal and photodynamic performance, the synergistic CDT/PDT/PTT effect of RhRu/Ti₃ C₂ T_x on osteosarcoma is verified by in vitro and in vivo experiments. This study is expected to provide a new research direction for the treatment of osteosarcoma and other tumors.

Colorimetric sensor array based on Au2Pt nanozymes for antioxidant nutrition quality evaluation in food

Total antioxidant capacity (TAC) has become an important index to evaluate the food quality. Effective antioxidant detection has been the research hotspot of scientists. In this work, a novel three-channel colorimetric sensor array founded on Au₂Pt bimetallic nanozymes for the discrimination of antioxidants in food was constructed. Benefiting from the unique bimetallic doping structure, Au₂Pt nanospheres exhibited the excellent peroxidase-like activity with K_m of 0.044 mM and V_max of 19.37 × 10^-8 M s^-1 toward TMB. The density functional theory (DFT) calculation revealed that Pt atom in the doping system was active sites and there was no energy barrier in catalytic reaction which made Au₂Pt nanospheres had excellent catalytic activity. Accordingly, a multifunctional colorimetric sensor array was constructed based on Au₂Pt bimetallic nanozymes for rapid and sensitive detection of five antioxidants. Based on the different reduction ability of antioxidants, oxidized TMB could be reduced in different degrees. In the presence of H₂O₂, the colorimetric sensor array could generate differential colorimetric signals (fingerprints) by using TMB as the chromogenic substrate, which could be accurately discriminated through linear discriminant analysis (LDA) with a detection limit of <0.2 μM. The sensor array was able to the evaluate TAC in three actual samples (milk, green tea and orange juice). Furthermore, we prepared a rapid detection strip to meet the needs of practical application, making a positive contribution to food quality evaluation.

Comparison of the peroxidase activities of iron oxide nanozyme with DNAzyme and horseradish peroxidase

Peroxidase-based assays are the most extensively used in bioanalytical sensors because of their simple colorimetric readout and high sensitivity owing to enzymatic signal amplification. To improve the stability, modification, and cost of protein-based enzymes, such as horseradish peroxidase (HRP), various enzyme mimics, such as DNAzymes and nanozymes, have emerged over the last few decades. In this study, we compared the peroxidase activities of HRP, a G-quadruplex (G4)-hemin DNAzyme, and Fe₃O₄ nanozymes in terms of activity and stability under different conditions. The reactions were much slower at pH 7 than at pH 4. At pH 4, the turnover rate of HRP (375 s^-1) was faster than that of G4 DNAzyme (0.14 s^-1) and Fe₃O₄ (6.1 × 10^-4 s^-1, calculated by surface Fe concentration). When normalized to mass concentrations, the trend was the same. Through observation of the reaction for a long time of 2 h, the changes in the color and UV-vis spectra were also different for these catalysts, indicating different reaction mechanisms among these catalysts. Moreover, different buffers and nanozyme sizes were found to influence the activity of the catalysts. Fe₃O₄ showed the highest stability compared to HRP and G4 DNAzyme after a catalytic reaction or incubation with H₂O₂ for a few hours. This study helps to understand the properties of catalysts and the development of novel catalysts with enzyme-mimicking activities for application in various fields.

Sensitive acid phosphatase assay based on light-activated specific oxidase mimic activity

Acid phosphatase (ACP) is a key marker in the diagnosis of many diseases. However, exploiting a simple and sensitive sensor for the real-time quantitative analysis of ACP is still challenging. Herein, we attempted to develop a sensitive colorimetric sensing strategy for the detection of ACP based on light-activated oxidase mimic property of carbon dots (CDs). The synthesized CDs were proved to be capable of intrinsic light-activated oxidase mimic activity, which could generate reactive oxygen species to oxidize chromogenic substrate under ultraviolet light stimulation. Interestingly, this light-activated oxidase mimic behavior would be effectively suppressed by the antioxidant ascorbic acid (AA), a product from the hydrolysis of 2-phospho-L-ascorbic acid trisodium (AAP) mediated by ACP. Based on the above property, a facile and sensitive colorimetric sensing method for ACP was developed. Under the optimal conditions, the linear range for ACP 0.1-5.5 U/L, and the detection limit was 0.056 U/L. Compared with conventional nanozyme based ACP assay systems, the catalytic activity of light-activated nanozyme could be conveniently regulated by switching the light on and off, which made it easier to precisely control the extent of the reaction and ensured the accuracy of the assay. In addition, the proposed sensing system would be readout directly by the naked eye or smartphone-based RGB analysis system, and have been successfully applied to analyze diluted in diluted fetal bovine serum and urine samples spiked with ACP. All these results indicated that this approach holds good promise for future applications in clinical analysis and point-of-care (POC) biosensor platforms.

Amorphous Fe-Containing Phosphotungstates Featuring Efficient Peroxidase-like Activity at Neutral pH: Toward Portable Swabs for Pesticide Detection with Tandem Catalytic Amplification

Peroxidase-mimetic materials are intensively applied to establish multienzyme systems because of their attractive merits. However, almost all of the nanozymes explored exhibit catalytic capacity only under acidic conditions. The pH mismatch between peroxidase mimics in acidic environments and bioenzymes under neutral conditions significantly restricts the development of enzyme-nanozyme catalytic systems especially for biochemical sensing. To solve this problem, here amorphous Fe-containing phosphotungstates (Fe-PTs) featuring high peroxidase activity at neutral pH were explored to fabricate portable multienzyme biosensors for pesticide detection. The strong attraction of negatively charged Fe-PTs to positively charged substrates as well as the accelerated regeneration of Fe²⁺ by the Fe/W bimetallic redox couples was demonstrated to play important roles in endowing the material with peroxidase-like activity in physiological environments. Consequently, integrating the developed Fe-PTs with acetylcholinesterase and choline oxidase led to an enzyme-nanozyme tandem platform with good catalytic efficiency at neutral pH for organophosphorus pesticide response. Furthermore, they were immobilized onto common medical swabs to fabricate portable sensors for paraoxon detection conveniently based on smartphone sensing, showing excellent sensitivity, good anti-interference capacity, and low detection limit (0.28 ng/mL). Our contribution expands the horizon of acquiring peroxidase activity at neutral pH, and it will also open avenues to construct portable and effective biosensors for pesticides and other analytes.

Enhancing catalytic efficiency of carbon dots by modulating their Mn doping and chemical structure with metal salts

Nanozymes are emerging materials in various fields owing to their advantages over natural enzymes, such as controllable and facile synthesis, tunability in catalytic activities, cost-effectiveness, and high stability under stringent conditions. In this study, the effect of metal salts on the formation and catalytic activity of carbon dots (CDs), a promising nanozyme, is demonstrated. By introducing Mn sources that possess different counter anions, the chemical structure and composition of the CDs produced are affected, thereby influencing their enzymatic activities. The synergistic catalytic effect of the Mn and N-doped CDs (Mn&N-CDs) is induced by effective metal doping in the carbogenic domain and a high proportion of graphitic and pyridinic N. This highly enhanced catalytic effect of Mn&N-CDs allows them to respond sensitively to the interference factors of enzymatic reactions. Consequently, ascorbic acid, which is an essential nutrient for maintaining our health and is a reactive oxygen scavenger, can be successfully monitored using color change by forming oxidized 3,3',5,5'-tetramethylbenzidine with H₂O₂ and Mn&N-CDs. This study provides a basic understanding of the formation of CDs and how their catalytic properties can be controlled by the addition of different metal sources, thereby providing guidelines for the development of CDs for industrial applications.

Integrating Incompatible Nanozyme-Catalyzed Reactions for Diabetic Wound Healing

Multi-nanozymes are widely applied in disease treatment, biosensing, and other fields. However, most current multi-nanozyme systems exhibit only moderate activity since reaction microenvironments of different nanozyme are often distinct or even incompatible. Conventional assemble strategies are inapplicable for designing multi-nanozymes consisting of incompatible nanozymes. Herein, a versatile fiber-based compartmentalization strategy is developed to construct multi-nanozyme system capable of simultaneously performing incompatible reactions. In this system, the incompatible nanozymes are spatially distributed in distinct compartmentalized fibers, where different microenvironments can be tailored by controlling the doping reagent, endowing each nanozymes with the preferential microenvironments to exhibit their highest activity. As a proof of concept, pH-incompatible peroxidase-like and catalase-like catalytic reactions are tested to verify the feasibility of this strategy. By doping with benzoic acid in the desired location, the two pH-incompatible nanozymes can work simultaneously without interference. Further, it is demonstrated that the oxygen supply and antimicrobial power of the integrated platform can be applied for accelerating diabetic wound healing. It is hoped that this work provides a way to integrate incompatible nanozyme and broadens the application potential of multi-nanozymes.

Ru(bpy)32+ as a photoinduced oxidase mimic for colorimetric detection of biothiols

We have found that tris (2,2'-bipyridyl) ruthenium (II) (Ru(bpy)₃²⁺) possesses a high photo-induced oxidase-like activity and is capable of catalyzing the color reaction of 3,3',5,5'-tetramethylbenzidine (TMB) with dissolved oxygen. Ru(bpy)₃²⁺ has a catalytic constant (K_cat) that is twice as high as that of fluorescein, 170 and 275-fold higher than that of 9-mesityl-10-methyl acridine and Eosin Y, respectively. Electron spin resonance spectroscopy (ESR) and radical scavenging experiments have verified the major active radicals involved in the color reaction are •OH. A colorimetric biothiol assay has been successfully developed for the oxidase-like activity of Ru(bpy)₃²⁺ can be suppressed by sulfhydryl compounds. A linear dependence between the decrease in absorbance and the logarithm of thiol concentrations can be found ranging from 5.0 to 50 μM, with a detection limit of 1.0 μM. This work reveals a new oxidase mimic with high catalytic activity and will facilitate the utilization of this oxidase mimic in biochemical analysis.

Nanozyme's catalytic activity at neutral pH: reaction substrates and application in sensing

Nanozymes exhibit their great potential as alternatives to natural enzymes. In addition to catalytic activity, nanozymes also need to have biologically relevant catalytic reactions at physiological pH to fit in the definition of an enzyme and to achieve efficient analytical applications. Previous reviews in the nanozyme field mainly focused on the catalytic mechanisms, activity regulation, and types of catalytic reactions. In this paper, we discuss efforts made on the substrate-dependent catalytic activity of nanozymes at neutral pH. First, the discrepant catalytic activities for different substrates are compared, where the key differences are the characteristics of substrates and the adsorption of substrates by nanozymes at different pH. We then reviewed efforts to enhance reaction activity for model chromogenic substrates and strategies to engineer nanomaterials to accelerate reaction rates for other substrates at physiological pH. Finally, we also discussed methods to achieve efficient sensing applications at neutral pH using nanozymes. We believe that the nanozyme is catching up with enzymes rapidly in terms of reaction rates and reaction conditions. Designing nanozymes with specific catalysis for efficient sensing remains a challenge.

PEI-coated Prussian blue nanocubes as pH-Switchable nanozyme: Broad-pH-responsive immunoassay for illegal additive

Nanozymes are commonly used in the construction of immunosensors, yet they are generally susceptible to pH condition, which greatly hindered their practical use. To break the limitation of pH conditions, polyethyleneimine-coated Prussian blue nanocubes (PBNCs@PEI) were synthesized as the pH-switchable nanozyme, which can show peroxidase-like and catalase-like activity in acidic and alkaline condition, respectively. Besides, the modification of PEI can largely improve the catalytic activity of PBNCs. Herein, the pH-switchable catalytic property of PBNCs@PEI was used to construct the dual-mode immunosensor for the detection of illegal additive, rosiglitazone. In acidic condition, PBNCs@PEI showed excellent peroxidase-like activity, which can trigger the colorimetric reaction of Au nanostars with TMB2+/CTAB. In alkaline condition, the catalase-like activity of PBNCs@PEI prevailed, thus the decomposition of H2O2 can generate O2 to initiate the aerobic oxidation of 4-chloro-1-naphthol (4-CN), which can decrease the fluorescence intensity of 4-CN. Based on the competitive immunoassay, both the localized surface plasmon resonance wavelength shift of Au nanostars and the fluorescence intensity change of 4-CN were quantitatively related with rosiglitazone concentration, thus shedding a new light on the construction of broad-pH-responsive immunosensor. Besides, a smart device was developed to transfer the chroma value of Au nanostars into the RSG concentration, making this sensor a promising method in on-site and point-of-care detection.

Plasmonic Nanozymes: Leveraging Localized Surface Plasmon Resonance to Boost the Enzyme-Mimicking Activity of Nanomaterials

Nanozymes, a type of nanomaterials that function similarly to natural enzymes, receive extensive attention in biomedical fields. However, the widespread applications of nanozymes are greatly plagued by their unsatisfactory enzyme-mimicking activity. Localized surface plasmon resonance (LSPR), a nanoscale physical phenomenon described as the collective oscillation of surface free electrons in plasmonic nanoparticles under light irradiation, offers a robust universal paradigm to boost the catalytic performance of nanozymes. Plasmonic nanozymes (PNzymes) with elevated enzyme-mimicking activity by leveraging LSPR, emerge and provide unprecedented opportunities for biocatalysis. In this review, the physical mechanisms behind PNzymes are thoroughly revealed including near-field enhancement, hot carriers, and the photothermal effect. The rational design and applications of PNzymes in biosensing, cancer therapy, and bacterial infections elimination are systematically introduced. Current challenges and further perspectives of PNzymes are also summarized and discussed to stimulate their clinical translation. It is hoped that this review can attract more researchers to further advance the promising field of PNzymes and open up a new avenue for optimizing the enzyme-mimicking activity of nanozymes to create superior nanocatalysts for biomedical applications.

Manganese-Based Nanozymes: Preparation, Catalytic Mechanisms, and Biomedical Applications

Manganese (Mn) has attracted widespread attention due to its low-cost, nontoxicity, and valence-rich transition. Various Mn-based nanomaterials have sprung up and are employed in diverse fields, particularly Mn-based nanozymes, which combine the physicochemical properties of Mn-based nanomaterials with the catalytic activity of natural enzymes, and are attracting a surge of research, especially in the field of biomedical research. In this review, the typical preparation strategies, catalytic mechanisms, advances and perspectives of Mn-based nanozymes for biomedical applications are systematically summarized. The application of Mn-based nanozymes in tumor therapy and sensing detection, together with an overview of their mechanism of action is highlighted. Finally, the prospective directions of Mn-based nanozymes from five perspectives: innovation, activity enhancement, selectivity, biocompatibility, and application broadening are discussed.

An Atomic Insight into the Confusion on the Activity of Fe3O4 Nanoparticles as Peroxidase Mimetics and Their Comparison with Horseradish Peroxidase

Although Fe₃O₄ nanoparticles were early reported to outperform horseradish peroxidase (HRP), recent studies suggested that this material bears a very poor activity instead. Here, we resolve this disagreement by reviewing the definition of descriptors used and provide an atomic view into the origin of Fe₃O₄ nanoparticles as peroxidase mimetics. The redox between H₂O₂ and Fe(II) sites on the Fe₃O₄ surface was identified as the key step to producing OH radicals for the oxidation of colorimetric substrates. This mechanism involving free radicals is distinct from that of HRP oxidizing substrates with a radical retained on its Fe-porphyrin ring. Surprisingly, the distribution and chemical state of Fe species were found to be very different on single- and polycrystalline Fe₃O₄ nanoparticles with the latter bearing not only a higher Fe(II)/Fe(III) ratio but also a more reactive Fe(II) species at surface grain boundaries. This accounts for the unexpected gap in the catalytic constant (k_cat) observed for this material in the literature.