Pre-Adsorbed H-Mediated Electrochemiluminescence.

In conventional electrochemiluminescence (ECL) systems, the presence of the competitive cathodic hydrogen evolution reaction (HER) in aqueous electrolytes is typically considered to be a side reaction, leading to a reduced ECL efficiency and stability due to H2 generation and aggregation at the electrode surface. However, the significant role of adsorbed hydrogen (H*) as a key intermediate, formed during the Volmer reaction in the HER process, has been largely overlooked. In this study, employing the luminol-H2O2 system as a model, we for the first time demonstrate a novel H*-mediated coreactant activation mechanism, which remarkably enhances the ECL intensity. H* facilitates cleavage of the O-O bond in H2O2, selectively generating highly reactive hydroxyl radicals for efficient ECL reactions. Experimental investigations and theoretical calculations demonstrate that this H*-mediated mechanism achieves superior coreactant activation compared to the conventional direct electron transfer pathway, which unveils a new pathway for coreactant activation in the ECL systems.

Amino-Ligand-Coordinated Dicopper Active Sites Enable Catechol Oxidase-Like Activity for Chiral Recognition and Catalysis.

Developing highly active and selective advanced nanozymes for enzyme-mimicking catalysis remains a long-standing challenge for basic research and practical applications. Herein, we grafted a chiral histidine- (His-) coordinated copper core onto Zr-based metal-organic framework (MOF) basic backbones to structurally mirror the bimetal active site of natural catechol oxidase. Such a biomimetic fabricated process affords MOF-His-Cu with catechol oxidase-like activity, which can catalyze dehydrogenation and oxidation of o-diphenols and then transfer electrons to O2 to generate H2O2 by the cyclic conversion of Cu(II) and Cu(I). Specifically, the elaborate incorporation of chiral His arms results in higher catalytic selectivity over the chiral catechol substrates than natural enzyme. Density functional theory calculations reveal that the binding energy and potential steric effect in active site-substrate interactions account for the high stereoselectivity. This work demonstrates efficient and selective enzyme-mimicking catalytic processes and deepens the understanding of the catalytic mechanism of nanozymes.

Horseradish Peroxidase Immobilized in Metal Aerogels Boosts Electron Transfer and an Interfacial Reaction for Photoelectrochemical Immunoassay.

Photoelectrochemical (PEC) enzymatic biosensors have attracted widespread attention for their specificity and sensitivity, but the charge migration between an enzyme and a semiconductor remains uncertain. In this work, horseradish peroxidase (HRP) was successfully immobilized on ionic liquid-functioned Cu@Cu2O (IL-Cu@Cu2O) aerogels to boost charge transfer and an interfacial redox reaction. The photogenerated electrons flow from the conduction band of Cu2O to HRP under the assistance of Cu and are subsequently captured by [Fe(CN)6]3- in the electrolyte, which boosts the PEC response. The improved interfacial catalytic ability after the immobilization of HRP is proved by the enhanced redox ability under light irradiation. Benefiting from the excellent PEC activity and catalysis reaction of IL-Cu@Cu2O@HRP, an immunoassay platform was constructed for sensing prostate-specific antigens, which presents a wide detection range and a low limit of detection. An in-depth understanding of the direct electronic communication between a photoactive material and an enzyme for boosted charge transfer and interfacial catalysis provides a new view for the design of advanced PEC sensing platforms.

Nanozymatic Biofuel Cell-Enabled Self-Powered Sensing System for a Sensitive Immunoassay.

Self-powered sensing system (SPSS) integrating the enzymatic biofuel cell and biosensing platform has attracted tremendous interest. However, natural enzymes suffer from the intrinsic drawbacks of enzymes and enzymatic proteins. Nanozymes with enzyme-like activities are the ideal alternatives to enzymes, and it is greatly challenging to explore high-performance nanozymatic biofuel cell for SPSS. Herein, the advanced nanozymatic biofuel cell-enabled SPSS is developed for the sensitive detection of the prostate-specific antigen (PSA), where Ir single atoms supported by nitrogen-doped carbon and Au nanozymes serve as the cathode and anode, respectively. Based on the excellent electrochemical activity and stability, the resultant nanozymatic biofuel cell exhibits a higher power output and open-circuit potential than the Pt/C-based counterpart, which is beneficial for the application of SPSS. As a proof of concept, the nanozymatic biofuel cell-enabled SPSS shows a wide detection range of 0.2-500 ng mL-1 with a detection limit of 62 pg mL-1 for PSA, which provides new insight into broadening the application scenarios of nanozymes.

Photothermal-Switched Single-Atom Nanozyme Specificity for Pretreatment and Sensing.

Various applications lead to the requirement of nanozymes with either specific activity or multiple enzyme-like activities. To this end, intelligent nanozymes with freely switching specificity abilities hold great promise to adapt to complicated and changeable practical conditions. Herein, a nitrogen-doped carbon-supported copper single-atom nanozyme (named Cu SA/NC) with switchable specificity is reported. Atomically dispersed active sites endow Cu SA/NC with specific peroxidase-like activity at room temperature. Furthermore, the intrinsic photothermal conversion ability of Cu SA/NC enables the specificity switch by additional laser irradiation, where photothermal-induced temperature elevation triggers the expression of oxidase-like and catalase-like activity of Cu SA/NC. For further applications in practice, a pretreatment-and-sensing integration kit (PSIK) is constructed, where Cu SA/NC can successively achieve sample pretreatment and sensitive detection by switching from multi-activity mode to specific-activity mode. This study sets the foundation for nanozymes with switchable specificity and broadens the application scope in point-of-care testing.

Water Activation for Boosting Electrochemiluminescence.

In conventional luminol electrochemiluminescence (ECL) systems, hydrogen peroxide and dissolved oxygen are employed as typical co-reactants to produce reactive oxygen species (ROS) for efficient ECL emission. However, the self-decomposition of hydrogen peroxide and limited solubility of oxygen in water inevitably restrict the detection accuracy and luminous efficiency of luminol ECL system. Inspired by ROS-mediated ECL mechanism, for the first time, we used cobalt-iron layered double hydroxide as co-reaction accelerator to efficiently activate water to generate ROS for enhancing luminol emission. Experimental investigations verify the formation of hydroxyl and superoxide radicals in the process of electrochemical water oxidation, which subsequently react with luminol anion radicals to trigger strong ECL signals. Finally, the detection of alkaline phosphatase has been successfully achieved with impressive sensitivity and reproducibility for practical sample analysis.

Bioinspired Single-Atom Sites Enable Efficient Oxygen Activation for Switching Anodic/Cathodic Electrochemiluminescence.

Exploring advanced co-reaction accelerators with superior oxygen reduction activity that generate rich reactive oxygen species (ROS) has attracted great attention in boosting luminol-O2 electrochemiluminescence (ECL). However, tuning accelerators for efficient and selective catalytic O2 activation to switch anodic/cathodic ECL is very challenging. Herein, we report that enzyme-inspired Fe-based single-atom catalysts with axial N/C coordination structures (FeN5 , FeN4 © SACs) can generate specific ROS for cathodic/anodic ECL conversion. Mechanistic studies reveal that FeN5 sites prefer to produce highly active hydroxyl radicals and afford direct cathodic luminescence by promoting the cleavage of O-O bonds through N-induced electron redistribution. In contrast, FeN4 © sites tend to produce superoxide radicals, resulting in inefficient anodic ECL. Benefiting from the enhanced cathodic ECL, FeN5 SAC-based immunosensor was constructed for the sensitive detection of cancer biomarkers.

Iridium Single-Atomic Site Catalysts with Superior Oxygen Reduction Reaction Activity for Sensitive Monitoring of Organophosphorus Pesticides.

Tremendous efforts have been made in developing single-atomic site catalysts (SASCs) for oxygen reduction reaction (ORR), which is regarded as a pivotal cornerstone in electrochemical energy conversion. However, SASCs for ORR have not been explored for electrochemical sensing. Herein, a template-sacrificed strategy is reported for the synthesis of atomically dispersed Ir SASCs, serving as a sensing platform to detect organophosphorus pesticides (OPs) with high sensitivity and selectivity. Owing to abundant Ir single-atom active sites, Ir SASCs show excellent ORR activity and stability in a neutral medium. It is found that the ORR activity of Ir SASCs can be inhibited by thiocholine, which is the hydrolysate of acetylthiocholine. After being integrated with acetylcholinesterase (AChE), the AChE-Ir SASC-based electrochemical sensor is established and shows a superior sensitivity, which shows a wide detection range of 0.5-500 ng mL-1 with a low detection limit of 0.17 ng mL-1 for OPs. This work exhibits a broad application prospect of ORR for sensitive detection of biomolecules.

Ultra-Low Content Bismuth-Anchored Gold Aerogels with Plasmon Property for Enhanced Nonenzymatic Electrochemical Glucose Sensing.

Effective glucose surveillance provides a strong guarantee for the high-quality development of human health. Au nanomaterials possess compelling applications in nonenzymatic electrochemical glucose biosensors owing to superior catalytic performances and intriguing biocompatibility properties. However, it has been a grand challenge to accurately control the architecture and composition of Au nanomaterials to optimize their optical, electronic, and magnetic properties for further improving the performance of electrocatalytic sensing. Herein, ultra-low content Bi-anchored Au aerogels are synthesized via a one-step reduction strategy. Benefiting from the unique structure of aerogels as well as the synergistic effect between Au and Bi, the optimized Au200Bi aerogels greatly boost the activity of glucose oxidation compared with Au aerogels. Under plasmon resonance excitation, bimetallic Au200Bi aerogels with wider photics-dependent properties further show plasmon-promoted glucose electro-oxidation activity, which is derived from the photothermal and photoelectric effects caused by the local surface plasmon resonance. Thanks to the enhanced performance, a nonenzymatic electrochemical glucose biosensor is constructed to detect glucose with high sensitivity. This plasmon-promoted electrocatalytic activity through the synergetic strategy of bimetallic aerogels has potential applications in various research fields.

Hierarchically Porous S/N Codoped Carbon Nanozymes with Enhanced Peroxidase-like Activity for Total Antioxidant Capacity Biosensing.

Design of highly active carbon nanozymes and further establishment of ultrasensitive biosensors remain a challenge. Herein, hierarchically porous carbon nanozymes with sulfur (S)/nitrogen (N) codoping (SNC) were developed. Compared with N-doped carbon (NC) nanozymes, SNC nanozymes have a smaller Michaelis-Menten constant and higher specific activities, demonstrating that the S-doping in SNC nanozymes could not only enhance their affinity toward substrates but also improve their catalytic performance. These results may be caused by the synergistic effect of heteroatoms (S and N). Because of the good enzyme-like activity, the proposed SNC nanozymes were exploited to the colorimetric detection of the total antioxidant capacity (TAC) using ascorbic acid as a typical model with a limit of detection of 0.08 mM. Because of its high sensitivity and selectivity and encouraging performance, the detection method presented practical feasibility for the TAC assay in commercial beverages. This work paves a way to design the highly active carbon nanozymes and expand their applications in the construction of high-performance biosensors.

Cascade Reaction System Integrating Single-Atom Nanozymes with Abundant Cu Sites for Enhanced Biosensing.

Single-atom nanozymes (SAzymes), as novel nanozymes with atomically dispersed active sites, are of great importance in the development of nanozymes for their high catalytic activities, the maximum utilization efficiency of metal atoms, and the simple model of active sites. Herein, the peroxidase-like SAzymes with high-concentration Cu sites on carbon nanosheets (Cu-N-C) were synthesized through a salt-template strategy. With the densely distributed active Cu atoms (∼5.1 wt %), the Cu-N-C SAzymes exhibit remarkable activity to mimic natural peroxidase. Integrating Cu-N-C SAzymes with natural acetylcholinesterase and choline oxidase, three-enzyme-based cascade reaction system was constructed for the colorimetric detection of acetylcholine and organophosphorus pesticides. This work not only provides a strategy to synthesize SAzymes with abundant active sites but also gives some new insights for robust nanozyme biosensing systems.

Single-Atom Iron Boosts Electrochemiluminescence.

The traditional luminol-H2 O2 electrochemiluminescence (ECL) sensing platform suffers from self-decomposition of H2 O2 at room temperature, hampering its application for quantitative analysis. In this work, for the first time we employ iron single-atom catalysts (Fe-N-C SACs) as an advanced co-reactant accelerator to directly reduce the dissolved oxygen (O2 ) to reactive oxygen species (ROS). Owing to the unique electronic structure and catalytic activity of Fe-N-C SACs, large amounts of ROS are efficiently produced, which then react with the luminol anion radical and significantly amplify the luminol ECL emission. Under the optimum conditions, a Fe-N-C SACs-luminol ECL sensor for antioxidant capacity measurement was developed with a good linear range from 0.8 µm to 1.0 mm of Trolox.

Oxidase-Like Fe-N-C Single-Atom Nanozymes for the Detection of Acetylcholinesterase Activity.

Single-atom catalysts (SACs) have attracted extensive attention in the catalysis field because of their remarkable catalytic activity, gratifying stability, excellent selectivity, and 100% atom utilization. With atomically dispersed metal active sites, Fe-N-C SACs can mimic oxidase by activating O2 into reactive oxygen species, O2- • radicals. Taking advantages of this property, single-atom nanozymes (SAzymes) can become a great impetus to develop novel biosensors. Herein, the performance of Fe-N-C SACs as oxidase-like nanozymes is explored. Besides, the Fe-N-C SAzymes are applied in biosensor areas to evaluate the activity of acetylcholinesterase based on the inhibition toward nanozyme activity by thiols. Moreover, this SAzymes-based biosensor is further used for monitoring the amounts of organophosphorus compounds.

Tuning electrochemical water activation over NiIr single-atom alloy aerogels for stable electrochemiluminescence.

The anodic oxygen evolution reaction is a well-acknowledged side reaction in traditional aqueous electrochemiluminescence (ECL) systems due to the generation and surface aggregation of oxygen at the electrode, which detrimentally impacts the stability and efficiency of ECL emission. However, the effect of reactive oxygen species generated during water oxidation on ECL luminophores has been largely overlooked. Taking the typical luminol emitter as an example, herein, we employed NiIr single-atom alloy aerogels possessing efficient water oxidation activity as a prototype co-reaction accelerator to elucidate the relationship between ECL behavior and water oxidation reaction kinetics for the first time. By regulating the concentration of hydroxide ions in the electrolyte, the electrochemical oxidation processes of both luminol and water are finely tuned. When the concentration of hydroxide ions in electrolyte is low, the kinetics of water oxidation is attenuated, which limits the generation of oxygen, effectively mitigates the influence of oxygen accumulation on the ECL strength, and offers a novel perspective for harnessing side reactions in ECL systems. Finally, a sensitive and stable sensor for antioxidant detection was constructed and applied to the practical sample detection.

Nickel Single-Atom Catalyst-Mediated Efficient Redox Cycle Enables Self-Checking Photoelectrochemical Biosensing with Dual Photocurrent Readouts.

Developing a self-checking photoelectrochemical biosensor with dual photocurrent signals could efficiently eliminate false-positive or false-negative signals. Herein, a novel biosensor with dual photocurrent responses was established for the detection of acetylcholinesterase activity. To achieve photocurrent polarity-switchable behavior, the iodide/tri-iodide redox couple was innovatively introduced to simultaneously consume the photoexcited electrons and holes, which circumvents the inconvenience caused by the addition of different hole- and electron-trapping agents in the electrolyte. Importantly, benefiting from the high catalytic activity, the enhanced photoelectric responsivity can be realized after decorating the counter electrode with nickel single-atom catalysts, which promotes a more efficient iodide/tri-iodide redox reaction under low applied voltages. It is envisioned that the proposed photocurrent polarity switching system offers new routes to sensitive and reliable biosensing.

Engineering the microenvironment of electron transport layers with nickle single-atom sites for boosting photoelectrochemical performance.

Advances in the rational design of semiconductor-electrocatalyst photoelectrodes provide robust driving forces for improving energy conversion and quantitative analysis, while a deep understanding of elementary processes remains underwhelming due to the multistage interfaces involved in semiconductor/electrocatalyst/electrolyte. To address this bottleneck, we have constructed carbon-supported nickel single atoms (Ni SA@C) as an original electron transport layer with catalytic sites of Ni-N4 and Ni-N2O2. This approach illustrates the combined effect of photogenerated electron extraction and the surface electron escape ability of the electrocatalyst layer in the photocathode system. Theoretical and experimental studies reveal that Ni-N4@C, with excellent oxygen reduction reaction catalytic activity, is more beneficial for alleviating surface charge accumulation and facilitating electrode-electrolyte interfacial electron-injection efficiency under a similar built-in electric field. This instructive method enables us to engineer the microenvironment of the charge transport layer for steering the interfacial charge extract and reaction kinetics, providing a great prospect for atomic scale materials to enhance photoelectrochemical performance.

Cu Aerogels with Sustainable Cu(I)/Cu(II) Redox Cycles for Sensitive Nonenzymatic Glucose Sensing.

Developing functional nanomaterials for nonenzymatic glucose electrochemical sensing platforms is vital and challenging from the perspective of pathology and physiology. Accurate identification of active sites and thorough investigation of catalytic mechanisms are critical prerequisites for the design of advanced catalysts for electrochemical sensing. Herein, Cu aerogels are synthesized as a model system for sensitive nonenzymatic glucose sensing. The resultant Cu aerogels exhibit good catalytic activity for glucose electrooxidation with high sensitivity and a low detection limit. Significantly, in situ electrochemical investigations and Raman characterizations reveal the catalytic mechanism of Cu-based nonenzymatic glucose sensing. During the electrocatalytic oxidation of glucose, Cu(I) is electrochemically oxidized to generate Cu(II), and the resultant Cu(II) is spontaneously reduced back to Cu(I) by glucose, achieving the sustained Cu(I)/Cu(II) redox cycles. This study provides profound insights into the catalytic mechanism for nonenzymatic glucose sensing, which provides great potential guidance for a rational design of advanced catalysts in the future.

Atomically dispersed Ru3 site catalysts for electrochemical sensing of small molecules.

Rational design and construction of electrochemical sensing platforms with high sensitivity and selectivity is one of the challenges in practical application. Although single-atom catalysts (SACs) have attracted extensive attention, atomically dispersed metal catalysts (ADCs) with multi-atom sites can further compensate for the deficiencies of SACs, which have gradually been a research hotspot in recent years. Herein, atomically dispersed Ru3 site catalyst (Ru3/NC) is employed to catalyze small biomolecule oxidation, which exhibits much superior electrocatalytic ability of uric acid (UA) to Ru single-atom catalyst (Ru1/NC). What's more, theoretical calculations reveal that the enhanced performance is mainly derived from the dominant electronic structure of ADCs with multi-atom sites compared to SACs, leading to the more favorable adsorption of hydroxy anion groups, which can serve as one part of the active moiety and "promoter" to achieve the fast oxidation of small biomolecules. Our findings provide a new paradigm for designing promising catalysts to realize highly sensitive and selective small biomolecule detection and explore the catalytic mechanisms of small biomolecules at the atomic scale.

Amorphous metal-organic frameworks on PtCu hydrogels: Enzyme immobilization platform with boosted activity and stability for sensitive biosensing.

Cell-free enzymatic catalysis (CFEC) is emerging biotechnology that simulates biological transformations without living cells. However, the high cost of separation and preparation of the enzyme has hindered the practical application of the CFEC. Enzyme immobilization technologies using solid supports to stabilize enzymes have been regarded as an efficient strategy to address this issue. Nevertheless, the activity and stability of the immobilized enzymes are still crucial challenges for working in vitro. Herein, an enzyme immobilization platform is developed by using PtCu hydrogels coated with amorphous metallic-organic frameworks (MOFs) as multifunctional carriers to encapsulate horseradish peroxidase (HRP). Specifically, PtCu hydrogels acting as a "reservoir of metal ions" can interact with the immobilized enzyme and facilitate electron transfer, leading to the boosted enzyme catalytic performances. Furthermore, amorphous MOFs on the surface of PtCu hydrogels serve as an "armor" to protect the internal enzymes from various perturbation environments. The resultant enzyme immobilization platform (PtCu@HRP@ZIF-8) not only shows an approximately 2.4-fold enhanced activity compared with free enzyme but also exhibits improved stability against harsh conditions. The PtCu@HRP@ZIF-8-based biosensor is constructed for sensitive sensing of organophosphorus pesticides (OPs). The proposed biosensor exhibits a favorable linear relationship with the concentration of paraoxon-ethyl from 6 to 800 ng/mL, with a low detection limit of 1.8 ng/mL. This work reveals the promising potential of our proposed enzyme immobilization platform in practical applications.