Single-Atom Co–N4 Electrocatalyst Enabling Four-Electron Oxygen Reduction with Enhanced Hydrogen Peroxide Tolerance for Selective Sensing

Universal Covalent Grafting Strategy of an Aptamer on a Carbon Fiber Microelectrode for Selective Determination of Dopamine In Vivo

The chemical information on brain science provided by electrochemical sensors is critical for understanding brain chemistry during physiological and pathological processes. A major challenge is the selectivity of electrochemical sensors in vivo. This work developed a universal covalent grafting strategy of an aptamer on a carbon fiber microelectrode (CFE) for selective determination of dopamine in vivo. The universal strategy was proposed by oxidizing poly(tannic acid) (pTA) to form an oxidized state (pTAox) and then coupling a nucleophilic sulfhydryl molecule of the dopamine-binding mercapto-aptamer with the o-quinone moiety of pTAox based on click chemistry for the interfacial functionalization of the CFE surface. It was found that the universal strategy proposed could efficiently graft the aptamer on a glassy carbon electrode, which was verified by using electroactive 6-(ferrocenyl) hexanethiol as a redox reporter. The amperometric method using a fabricated aptasensor for the determination of dopamine was developed. The linear range of the aptasensor for the determination of dopamine was 0.2-20 μM with a sensitivity of 0.09 nA/μM and a limit of detection of 88 nM (S/N = 3). The developed method has high selectivity originating from the specific recognition of the aptamer in concert with the cation-selective action of pTA and could be easily applicable to probe dopamine dynamics in the brain. Furthermore, complex vesicle fusion modes were first observed at the animal level. This work demonstrated that the covalently grafted immobilization strategy proposed is promising and could be extended to the in vivo analysis of other neurochemicals.

Catalyst durability in electrocatalytic H2O2 production: key factors and challenges

On-demand electrocatalytic hydrogen peroxide (H2O2) production is a significant technological advancement that offers a promising alternative to the traditional anthraquinone process. This approach leverages electrocatalysts for the selective reduction of oxygen through a two-electron transfer mechanism (ORR-2e-), holding great promise for delivering a sustainable and economically efficient means of H2O2 production. However, the harsh operating conditions during the electrochemical H2O2 production lead to the degradation of both structural integrity and catalytic efficacy in these materials. Here, we systematically examine the design strategies and materials typically utilized in the electroproduction of H2O2 in acidic environments. We delve into the prevalent reactor conditions and scrutinize the factors contributing to catalyst deactivation. Additionally, we propose standardised benchmarking protocols aimed at evaluating catalyst stability under such rigorous conditions. To this end, we advocate for the adoption of three distinct accelerated stress tests to comprehensively assess catalyst performance and durability.

Precisely Engineering Asymmetric Atomic CoN4 by Electron Donating and Extracting for Oxygen Reduction Reaction

The development of nonpyrolytic catalysts featuring precisely defined active sites represents an effective strategy for investigating the fundamental relationship between the catalytic activity of oxygen reduction reaction (ORR) catalysts and their local coordination environments. In this study, we have synthesized a series of model electrocatalysts with well-defined CoN4 centers and nonplanar symmetric coordination structures. These catalysts were prepared by a sequential process involving the chelation of cobalt salts and 1,10-phenanthroline-based ligands with various substituent groups (phen(X), where X=OH, CH3, H, Br, Cl) onto covalent triazine frameworks (CTFs). By modulating the electron-donating or electron-withdrawing properties of the substituent groups on the phen-based ligands, the electron density surrounding the CoN4 centers was effectively controlled. Our results demonstrated a direct correlation between the catalytic activity of the CoN4 centers and the electron-donating ability of the substituent group on the phenanthroline ligands. Notably, the catalyst denoted as BCTF-Co-phen(OH), featuring the electron-donating OH group, exhibited the highest ORR catalytic activity. This custom-crafted catalyst achieved a remarkable half-wave potential of up to 0.80 V vs. RHE and an impressive turnover frequency (TOF) value of 47.4×10-3 Hz at 0.80 V vs. RHE in an alkaline environment.

Atomic Engineering of Single-Atom Nanozymes for Biomedical Applications

Single-atom nanozymes (SAzymes) showcase not only uniformly dispersed active sites but also meticulously engineered coordination structures. These intricate architectures bestow upon them an exceptional catalytic prowess, thereby captivating numerous minds and heralding a new era of possibilities in the biomedical landscape. Tuning the microstructure of SAzymes on the atomic scale is a key factor in designing targeted SAzymes with desirable functions. This review first discusses and summarizes three strategies for designing SAzymes and their impact on reactivity in biocatalysis. The effects of choices of carrier, different synthesis methods, coordination modulation of first/second shell, and the type and number of metal active centers on the enzyme-like catalytic activity are unraveled. Next, a first attempt is made to summarize the biological applications of SAzymes in tumor therapy, biosensing, antimicrobial, anti-inflammatory, and other biological applications from different mechanisms. Finally, how SAzymes are designed and regulated for further realization of diverse biological applications is reviewed and prospected. It is envisaged that the comprehensive review presented within this exegesis will furnish novel perspectives and profound revelations regarding the biomedical applications of SAzymes.

Isolated Binary Fe-Ni Metal-Nitrogen Sites Anchored on Porous Carbon Nanosheets for Efficient Oxygen Electrocatalysis through High-Temperature Gas-Migration Strategy

Atomically dispersed dual-site catalysts can regulate multiple reaction processes and provide synergistic functions based on diverse molecules and their interfaces. However, how to synthesize and stabilize dual-site single-atom catalysts (DACs) is confronted with challenges. Herein, we report a facile high-temperature gas-migration strategy to synthesize Fe-Ni DACs on nitrogen-doped carbon nanosheets (FeNiSAs/NC). FeNiSAs/NC exhibits a high half-wave potential (0.88 V) for the oxygen reduction reaction (ORR) and a low overpotential of 410 mV at 10 mA cm-2 for the oxygen evolution reaction (OER). As an air electrode for Zn-air batteries (ZABs), it shows better performances in aqueous ZABs and excellent stability and flexibility in solid-state ZABs. The high specific surface area (1687.32 m2/g) of FeNiSAs/NC is conducive to electron transport. Density functional theory (DFT) reveals that the Fe sites are the active center, and Ni sites can significantly optimize the free energy of the oxygen-containing intermediate state on Fe sites, contributing to the improvement of ORR and the corresponding OER activities. This work can provide guidance for the rational design of DACs and understand the structure-activity relationship of SACs with multiple active sites for electrocatalytic energy conversion.

Insights into the biosynthesis of palladium nanoparticles for oxygen reduction reaction by genetically engineered bacteria of Shewanella oneidensis MR-1

Owing to the increasing need for green synthesis and environmental protection, the utilization of biological organism-derived carbons as supports for noble-metal electrocatalysts has garnered public interest. Nevertheless, the mechanism by which microorganisms generate nanometals has not been fully understood yet. In the present study, we used genetically engineered bacteria of Shewanella oneidensis MR-1 (∆SO4317, ∆SO4320, ∆SO0618 and ∆SO3745) to explore the effect of surface substances including biofilm-associated protein (bpfA), protein secreted by type I secretion systems (TISS) and type II secretion systems (T2SS), and lipopolysaccharide in microbial synthesis of metal nanoparticles. Results showed Pd/∆SO4317 (the catalyst prepared with the mutant ∆SO4317) shows better performance than other biocatalysts and commercial Pd/C, where the mass activity (MA) and specific activity (SA) of Pd/∆SO4317 are 3.1 and 2.1 times higher than those of commercial Pd/C, reaching 257.49 A g-1 and 6.85 A m-2 respectively. It has been found that the exceptional performance is attributed to the smallest particle size and the presence of abundant functional groups. Additionally, the absence of biofilms has been identified as a crucial factor in the formation of high-quality bio-Pd. Because the absence of biofilm can minimize metal agglomeration, resulting in uniform particle size dispersion. These findings provide valuable mechanical insights into the generation of biogenic metal nanoparticles and show potential industrial and environmental applications, especially in accelerating oxygen reduction reactions.

Current Advances on the Single-Atom Nanozyme and Its Bioapplications

Nanozymes, a class of nanomaterials mimicking the function of enzymes, have aroused much attention as the candidate in diverse fields with the arbitrarily tunable features owing to the diversity of crystalline nanostructures, composition, and surface configurations. However, the uncertainty of their active sites and the lower intrinsic deficiencies of nanomaterial-initiated catalysis compared with the natural enzymes promote the pursuing of alternatives by imitating the biological active centers. Single-atom nanozymes (SAzymes) maximize the atom utilization with the well-defined structure, providing an important bridge to investigate mechanism and the relationship between structure and catalytic activity. They have risen as the new burgeoning alternative to the natural enzyme from in vitro bioanalytical tool to in vivo therapy owing to the flexible atomic engineering structure. Here, focus is mainly on the three parts. First, a detailed overview of single-atom catalyst synthesis strategies including bottom-up and top-down approaches is given. Then, according to the structural feature of single-atom nanocatalysts, the influence factors such as central metal atom, coordination number, heteroatom doping, and the metal-support interaction are discussed and the representative biological applications (including antibacterial/antiviral performance, cancer therapy, and biosensing) are highlighted. In the end, the future perspective and challenge facing are demonstrated.

Enzymatic Galvanic Redox Potentiometry for In Vivo Biosensing

Redox potentiometry has emerged as a new platform for in vivo sensing, with improved neuronal compatibility and strong tolerance against sensitivity variation caused by protein fouling. Although enzymes show great possibilities in the fabrication of selective redox potentiometry, the fabrication of an enzyme electrode to output open-circuit voltage (EOC) with fast response remains challenging. Herein, we report a concept of novel enzymatic galvanic redox potentiometry (GRP) with improved time response coupling the merits of the high selectivity of enzyme electrodes with the excellent biocompatibility and reliability of GRP sensors. With a glucose biosensor as an illustration, we use flavin adenine dinucleotide-dependent glucose dehydrogenase as the recognition element and carbon black as the potential relay station to improve the response time. We find that the enzymatic GRP biosensor rapidly responds to glucose with a good linear relationship between EOC and the logarithm of glucose concentration within a range from 100 μM to 2.65 mM. The GRP biosensor shows high selectivity over O2 and coexisting neurochemicals, good reversibility, and sensitivity and can in vivo monitor glucose dynamics in rat brain. We believe that this study will pave a new platform for the in vivo potentiometric biosensing of chemical events with high reliability.

Electroless Deposition of Palladium Nanoparticles on Graphdiyne Boosts Electrochemiluminescence

Modulating the electronic structure of metal nanoparticles via metal-support interaction has attracted intense interest in the field of catalytic science. However, the roles of supporting substrates in regulating the catalytic properties of electrochemiluminescence (ECL) remain elusive. Here, we find that the use of graphdiyne (GDY) as the substrate for electroless deposition of Pd nanoparticles (Pd/GDY) produces the most pronounced anodic signal enhancement in luminol-dissolved oxygen (O2) ECL system as co-reactant accelerator over other carbon-based Pd composite nanomaterials. Pd/GDY exhibits electrocatalytic activity for the reduction of O2 through a four-electron pathway at approximately -0.059 V (vs Ag/AgCl) in neutral solution forming reactive oxygen species (ROS) as intermediates. The study shows that the interaction of Pd and GDY increases the amount and stability of ROS on the Pd/GDY electrode surface and promotes the reaction of ROS and luminol anion radical to generate excited luminol, which significantly boosts the luminol anodic ECL emission. Based on quenching of luminol ECL through the consumption of ROS by antioxidants, we develop a platform for the detection of intracellular antioxidants. This study provides an avenue for the development of efficient luminol ECL systems in neutral media and expands the biological application of ECL systems.

Study on synergistic effects of 4f levels of erbium and black phosphorus/SnNb2O6 heterostructure catalysts by multiple spectroscopic analysis techniques

Lanthanide single atom modified catalysts are rarely reported because the roles of lanthanide in photocatalysis are difficult to explain clearly. Based on the construction of Er single atom modified black phosphorus/SnNb2O6 (BP/SNO) heterojunctions, the synergistic effect of 4f levels of Er and heterostructures was studied by combining steady-state, transient, and ultrafast spectral analysis techniques with DFT theoretical calculations. According to the Judd-Ofelt theory of lanthanide ions, the CO2 photoreduction test under single wavelength excitation verifies that the 4F7/2/2H11/2 → 4I15/2 emissions of Er in BPEr/SNOEr can be more easily absorbed by SNO and BP, further proving the role of the 4f levels. As a result, the CO and CH4 yields of BPEr/SNOEr-10 under visible light irradiation are 10.7 and 10.1 times higher than those of pure BP, respectively, and 3.4 and 1.5 times higher than those of SNO. The results of DFT calculations show that the Er single atoms can cause surface reconstruction, regulate the active sites of BP, and reduce the energy change value in the key steps (CO2* + H+ + e- → COOH* and COOH* → CO* + H2O). This work provides novel insights into the design of lanthanide single atom photocatalysts for CO2 reduction.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS)

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

Phosphorescence Enzyme-Mimics for Time-Resolved Sensitive Diagnostics and Environment-Adaptive Specific Catalytic Therapeutics

Enzyme mimics (EMs) with intrinsic catalysis activity have attracted enormous interest in biomedicine. However, there is a lack of environmentally adaptive EMs for sensitive diagnosis and specific catalytic therapeutics in simultaneous manners. Herein, the coordination modulation strategy is designed to synthesize silicon-based phosphorescence enzyme-mimics (SiPEMs). Specifically, the atomic-level engineered Co-N4 structure in SiPEMs enables the environment-adaptive peroxidase, oxidase, and catalase-like activities. More intriguingly, the internal Si-O networks are able to stabilize the triplet state, exhibiting long-lived phosphorescence with lifetime of 124.5 ms, suitable for millisecond-range time-resolved imaging of tumor cells and tissue in mice (with high signal-to-background ratio values of ∼60.2 for in vitro and ∼611 for in vivo). Meanwhile, the SiPEMs act as an oxidative stress amplifier, allowing the production of ·OH via cascade reactions triggered by the tumor microenvironment (∼136-fold enhancement in peroxidase catalytic efficiency); while the enzyme-mimics can scavenge the accumulation of reactive oxygen species to alleviate the oxidative damage in normal cells, they are therefore suitable for environment-adaptive catalytic treatment of cancer in specific manners. We innovate a systematic strategy to develop high-performance enzymemics, constructing a promising breakthrough for replacing traditional enzymes in cancer treatment applications.

Cyclodextrin-supported Co(OH)2 Clusters as Electrocatalysts for Efficient and Selective H2 O2 Synthesis

Co-based material catalysts have shown attractive application prospects in the 2 e- oxygen reduction reaction (ORR). However, for the industrial synthesis of H2 O2 , there is still lack of Co-based catalysts with high production yield rate. Here, novel cyclodextrin-supported Co(OH)2 cluster catalysts were prepared via a mild and facile method. The catalyst exhibited remarkable H2 O2 selectivity (94.2 % ~ 98.2 %), good stability (99 % activity retention after 35 h), and ultra-high H2 O2 production yield rate (5.58 mol gcatalyst -1 h-1 in the H-type electrolytic cell), demonstrating its promising industrial application potential. Density functional theory (DFT) reveals that the cyclodextrin-mediated Co(OH)2 electronic structure optimizes the adsorption of OOH* intermediates and significantly enhances the activation energy barrier for dissociation, leading to the high reactivity and selectivity for the 2 e- ORR. This work offers a valuable and practical strategy to design Co-based electrocatalysts for H2 O2 production.

New Opportunities of Electrochemistry for Monitoring, Modulating, and Mimicking the Brain Signals

In vivo electrochemistry is a powerful key for unlocking the chemical consequences in neural networks of the brain. The past half-century has witnessed the technology revolutionization in this field along with innovations in electrochemical concepts, principles, methods, and devices. Present applications of electrochemical approaches have extended from measuring neurochemical concentrations to modulating and mimicking brain signals. In this Perspective, newly reported strategies for tackling long-standing challenges of in vivo electrochemical brain monitoring (i.e., basal level measurement, electroactivity dependence, in vivo stability, neuron compatibility, multiplexity, and implantable device fabrication) are highlighted. Moreover, recent progress on neuromodulation tools and neuromorphic devices in electrochemical frameworks is introduced. A glimpse of future opportunities for electrochemistry in brain research is offered at last.

Continuous On-Site H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Enabled by an Oxygen-Doped Single-Cobalt Atom Catalyst with Nitrogen Coordination

Single-Co atom catalysts are suggested as an efficient platinum metal group-free catalyst for promoting the oxygen reduction into water or hydrogen peroxide, while the relevance of the catalyst structure and selectivity is still ambiguous. Here, we propose a thermal evaporation method for modulating the chemical environment of single-Co atom catalysts and unveil the effect on the selectivity and activity. It discloses that nitrogen functional groups prefer to proceed the oxygen reduction via a 4e- pathway and notably improve the intrinsic activity, especially when being coordinated with the Co center, while oxygen doping tempts the electron delocalization around cobalt sites and decreases the binding force toward HOO* intermediates, thereby increasing the 2e- selectivity. Consequently, the well-designed oxygen-doped single-Co atom catalysts with nitrogen coordination deliver an impressive 2e- oxygen reduction performance, approaching the onset potential of 0.78 V vs RHE and selectivity of >90%. As an impressive cathode catalyst of an electrochemical flow cell, it generates H2O2 at a rate of 880 mmol gcat-1 h-1 and faradaic efficiency of 95.2%, in combination with an efficient nickel-iron oxygen evolution anode.

CoIn dual-atom catalyst for hydrogen peroxide production via oxygen reduction reaction in acid

The two-electron oxygen reduction reaction in acid is highly attractive to produce H2O2, a commodity chemical vital in various industry and household scenarios, which is still hindered by the sluggish reaction kinetics. Herein, both density function theory calculation and in-situ characterization demonstrate that in dual-atom CoIn catalyst, O-affinitive In atom triggers the favorable and stable adsorption of hydroxyl, which effectively optimizes the adsorption of OOH on neighboring Co. As a result, the oxygen reduction on Co atoms shifts to two-electron pathway for efficient H2O2 production in acid. The H2O2 partial current density reaches 1.92 mA cm-2 at 0.65 V in the rotating ring-disk electrode test, while the H2O2 production rate is as high as 9.68 mol g-1 h-1 in the three-phase flow cell. Additionally, the CoIn-N-C presents excellent stability during the long-term operation, verifying the practicability of the CoIn-N-C catalyst. This work provides inspiring insights into the rational design of active catalysts for H2O2 production and other catalytic systems.

Single-atom nanozyme-based electrochemical sensors for health and food safety monitoring

Electrochemical sensors and biosensors play an important role in many fields, including biology, clinical trials, and food industry. For health and food safety monitoring, accurate and quantitative sensing is needed to ensure that there is no significantly negative impact on human health. It is difficult for traditional sensors to meet these requirements. In recent years, single-atom nanozymes (SANs) have been successfully used in electrochemical sensors due to their high electrochemical activity, good stability, excellent selectivity and high sensitivity. Here, we first summarize the detection principle of SAN-based electrochemical sensors. Then, we review the detection performances of small molecules on SAN-based electrochemical sensors, including H₂O₂, dopamine (DA), uric acid (UA), glucose, H₂S, NO, and O₂. Subsequently, we put forward the optimization strategies to promote the development of SAN-based electrochemical sensors. Finally, the challenges and prospects of SAN-based sensors are proposed.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Single-Atom Indium Boosts Electrochemical Dopamine Sensing

A rational design of high-efficiency electrocatalysts and thus achieving sensitive electrochemical sensing remains a great challenge. In this work, single-atom indium anchored on nitrogen-doped carbon (In₁-N-C) with an In-N₄ configuration is prepared successfully through a high-temperature annealing strategy; the product can serve as an advanced electrocatalyst for sensitive electrochemical sensing of dopamine (DA). Compared with In nanoparticle catalysts, In₁-N-C exhibits high catalytic performance for DA oxidation. The theoretical calculation reveals that In₁-N-C has high adsorption energy for hydroxy groups and a low energy barrier in the process of DA oxidation compared to In nanoparticles, indicating that In₁-N-C with atomically dispersed In-N₄ sites possesses enhanced intrinsic activity. An electrochemical sensor for DA detection is established as a concept application with high sensitivity and selectivity. Furthermore, we also verify the feasibility of In₁-N-C catalysts for the simultaneous detection of uric acid, ascorbic acid, and DA. This work extends the application prospect of p-block metal single-atom catalysts in electrochemical sensing.

Correlation of the spin state and catalytic property of M-N4 single-atom catalysts in oxygen reduction reactions

The rational design of high-performance catalysts for oxygen reduction reactions (ORRs) is of great importance for large-scale applications in the field of proton-exchange membrane fuel cells and the green synthesis of H₂O₂. The effect of spin states of paramagnetic metal ions on the selectivity of ORRs is significant for single-atom catalysts (SACs). In this work, via spin-polarization density functional theory (DFT) calculations, we systematically investigated the popular paramagnetic metal-nitrogen graphene (M-N₄-C, M = Mn, Fe, and Co) SACs to mainly focus on the correlation of spin states and catalytic performance (e.g. activity and selectivity). Both thermodynamically and kinetically, it was found that Co-N₄-C (S = 1/2) has excellent 2e^- oxygen reduction performance (hydrogen peroxide production) with an ultralow overpotential of 0.03 V, and the hydrogenation of OOH* is the rate-determining step (RDS) with an energy barrier of 1.20 eV. The 4e^- ORR tends to occur along the OOH dissociation pathway (O* + OH*) on Co-N₄-C (S = 3/2), in which OOH* decomposition is the RDS with an energy barrier of 1.01 eV. It is proved that the spin magnetic moment is the key factor to regulate the ORR property via multi-angle electronic analysis. The spin states of catalysts play a crucial role in the activity and selectivity of ORRs mainly by manipulating the bond strength between OOH and catalysts. This will provide new insights for the rational design of ORR catalysts with magnetic metals.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

High Durability of Fe-N-C Single-Atom Catalysts with Carbon Vacancies toward the Oxygen Reduction Reaction in Alkaline Media

Single-atom catalysts (SACs) have attracted extensive interest to catalyze the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the development of SACs with high selectivity and long-term stability is a great challenge. In this work, carbon vacancy modified Fe-N-C SACs (Fe_H -N-C) are practically designed and synthesized through microenvironment modulation, achieving high-efficient utilization of active sites and optimization of electronic structures. The Fe_H -N-C catalyst exhibits a half-wave potential (E_1/2 ) of 0.91 V and sufficient durability of 100 000 voltage cycles with 29 mV E_1/2 loss. Density functional theory (DFT) calculations confirm that the vacancies around metal-N₄ sites can reduce the adsorption free energy of OH*, and hinder the dissolution of metal center, significantly enhancing the ORR kinetics and stability. Accordingly, Fe_H -N-C SACs presented a high-power density and long-term stability over 1200 h in rechargeable zinc-air batteries (ZABs). This work will not only guide for developing highly active and stable SACs through rational modulation of metal-N₄ sites, but also provide an insight into the optimization of the electronic structure to boost electrocatalytical performances.

NBOH Site-Activated Graphene Quantum Dots for Boosting Electrochemical Hydrogen Peroxide Production

Carbon materials are considered promising 2/4 e^- oxygen reduction reaction (ORR) electrocatalysts for synthesizing H₂ O₂ /H₂ O via regulating heteroatom dopants and functionalization. Here, various doped and functionalized graphene quantum dots (GQDs) are designed to reveal the crucial active sites of carbon materials for ORR to produce H₂ O₂ . Density functional theory (DFT) calculations predict that the edge structure involving edge N, B dopant pairs and further OH functionalization to the B (NBOH) is an active center for 2e^- ORR. To verify the above predication, GQDs with an enriched density of NBOH (NBO-GQDs) are designed and synthesized by the hydrothermal reaction of NH₂ edge-functionalized GQDs with H₃ BO₃ forming six-member heterocycle containing the NBOH structure. When dispersed on conductive carbon substrates, the NBO-GQDs show H₂ O₂ selectivity of over 90% at 0.7 -0.8 V versus reversible hydrogen electrode in the alkaline solution in a rotating ring-disk electrode setup. The selectivity retains 90% of the initial value after 12 h stability test. In a flow cell setup, the H₂ O₂ production rate is up to 709 mmol g_catalyst ^-1  h^-1 , superior to most reported carbon- and metal-based electrocatalysts. This work provides molecular insight into the design and formulation of highly efficient carbon-based catalysts for sustainable H₂ O₂ production.

Tuning Carbon Defect in Copper Single-Atom Catalysts for Efficient Oxygen Reduction

Defect chemistry in carbon matrix shows great potential for promoting the oxygen reduction reaction (ORR) of metal single-atom catalysts. Herein, a modified pyrolysis strategy is proposed to tune carbon defects in copper single-atom catalysts (Cu-SACs) to fully understand their positive effect on the ORR activity. The optimized Cu-SACs with controllable carbon defect degree and enhanced active specific surface area can exhibit improved ORR activity with a half-wave potential of 0.897 V_RHE , ultrahigh limiting current density of 6.5 mA cm^-2 , and superior turnover frequency of 2.23 e site^-1 s^-1 . The assembled Zn-air batteries based on Cu-SACs can also show well-retained reversibility and voltage platform over 1100 h charge/discharge period. Density functional theory calculations reveal that suitable carbon defects can redistribute charge density of Cu-N4 active sites to weaken the O-O bond in adsorbed OOH* intermediate and thus reduce its dissociation energy. This discovery offers a universal strategy for fabricating superior single-atom catalysts with high-efficiency active sites toward energy-directed applications.

Sensitive Electrochemical Sensor for Glycoprotein Detection Using a Self-Serviced-Track 3D DNA Walker and Catalytic Hairpin Assembly Enzyme-Free Signal Amplification

Approaches for the detection of targets in the cellular microenvironment have been extensively developed. However, developing a method with sensitive and accurate analysis for noninvasive cancer diagnosis has remained challenging until now. Here, we reported a sensitive and universal electrochemical platform that integrates a self-serviced-track 3D DNA walker and catalytic hairpin assembly (CHA) triggering G-Quadruplex/Hemin DNAzyme assembly signal amplification. In the presence of a target, the aptamer recognition initiated the 3D DNA walker on the cell surface autonomous running and releasing DNA (C) from the triple helix. The released DNA C as the target-triggered CHA moiety, and then G-quadruplex/hemin, was formed on the surface of electrode. Eventually, a large amount of G-quadruplex/hemin was formed on the sensor surface to generate an amplified electrochemical signal. Using N-acetylgalactosamine as a model, benefiting from the high selectivity and sensitivity of the self-serviced-track 3D DNA walker and the CHA, this designed method showed a detection limit of 39 cell/mL and 2.16 nM N-acetylgalactosamine. Furthermore, this detection strategy was enzyme free and exhibited highly sensitive, accurate, and universal detection of a variety of targets by using the corresponding DNA aptamer in clinical sample analysis, showing potential for early and prognostic diagnostic application.

Stretchable Oxygen-Tolerant Sensor Based on a Single-Atom Fe-N4 Electrocatalyst for Observing the Role of Oxidative Stress in Hypertension

Oxidative stress and related oxidative damage have a causal relation with the pathogenesis of hypertension. Therefore, it is crucial to determine the mechanism of oxidative stress in hypertension by applying mechanical forces on cells to simulate hypertension while monitoring the release of reactive oxygen species (ROS) from cells under an oxidative stress environment. However, cellular level research has rarely been explored because monitoring the ROS released by cells is still challenging owing to the interference of O₂. In this study, an Fe single-atom-site catalyst anchored on N-doped carbon-based materials (Fe SASC/N-C) was synthesized, which exhibits excellent electrocatalytic activity for the reduction of hydrogen peroxide (H₂O₂) at a peak potential of +0.1 V and can effectively avoid the interference of O₂. Furthermore, we constructed a flexible and stretchable electrochemical sensor based on the Fe SASC/N-C catalyst to study the release of cellular H₂O₂ under simulated hypoxic and hypertension conditions. Density functional theory calculations show that the highest transition state energy barrier from the oxygen reduction reaction (ORR), i.e., O₂ to H₂O, is 0.38 eV. In comparison, the H₂O₂ reduction reaction (HPRR) can be completed only by overcoming a lower energy barrier of 0.24 eV, endowing the HPRR to be more favorable on Fe SASC/N-C compared with the ORR. This study provided a reliable electrochemical platform for real-time investigation of H₂O₂-related underlying mechanisms of the hypertension process.

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁ -N₄ ) transforms to O*-(Se₁ -C₂ ) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm^-2 in Zn-air battery.

Elucidating Electrocatalytic Oxygen Reduction Kinetics via Intermediates by Time-Dependent Electrochemiluminescence

Facile evaluation of oxygen reduction reaction (ORR) kinetics for electrocatalysts is critical for sustainable fuel-cell development and industrial H₂ O₂ production. Despite great success in ORR studies using mainstream strategies, such as the membrane electrode assembly, rotation electrodes, and advanced surface-sensitive spectroscopy, the time and spatial distribution of reactive oxygen species (ROS) intermediates in the diffusion layer remain unknown. Using time-dependent electrochemiluminescence (Td-ECL), we report an intermediate-oriented method for ORR kinetics analysis. Owing to multiple ultrasensitive stoichiometric reactions between ROS and the ECL emitter, except for electron transfer numbers and rate constants, the potential-dependent time and spatial distribution of ROS were successfully obtained for the first time. Such exclusively uncovered information would guide the development of electrocatalysts for fuel cells and H₂ O₂ production with maximized activity and durability.

Inter-Metal Interaction with a Threshold Effect in NiCu Dual-Atom Catalysts for CO2 Electroreduction

Dual-atom catalysts (DACs) have become an emerging platform to provide more flexible active sites for electrocatalytic reactions with multi-electron/proton transfer, such as the CO₂  reduction reaction (CRR). However, the introduction of asymmetric dual-atom sites causes complexity in structure, leaving an incomprehensive understanding of the inter-metal interaction and catalytic mechanism. Taking NiCu DACs as an example, herein, a more rational structural model is proposed, and the distance-dependent inter-metal interaction is investigated by combining theoretical simulations and experiments, including density functional theory computation, aberration-corrected transmission electron microscopy, synchrotron-based X-ray absorption fine structure, and Monte Carlo experiments. A distance threshold around 5.3 Å between adjacent NiN₄  and CuN₄  moieties is revealed to trigger effective electronic regulation and boost CRR performance on both selectivity and activity. A universal macro-descriptor rigorously correlating the inter-metal distance and intrinsic material features (e.g., metal loading and thickness) is established to guide the rational design and synthesis of advanced DACs. This study highlights the significance of identifying the inter-metal interaction in DACs, and helps bridge the gap between theoretical study and experimental synthesis of atomically dispersed catalysts with highly correlated active sites.

An Electrochemical Nanosensor for Monitoring the Dynamics of Intracellular H2 O2 Upon NADH Treatment

Reactive oxygen species (ROS)-based therapeutic strategies play an important role in cancer treatment. However, in situ, real-time and quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein we report one selective hydrogen peroxide (H₂ O₂ ) electrochemical nanosensor, which is prepared by electrodeposition of Prussian blue (PB) and polyethylenedioxythiophene (PEDOT) onto carbon fiber nanoelectrode. With the nanosensor, we find that the level of intracellular H₂ O₂ increases with NADH treatment and that increase is dose-dependent to the concentration of NADH. High-dose of NADH (above 10 mM) can induce cell death and intratumoral injection of NADH is validated for inhibiting tumor growth in mice. This study highlights the potential of electrochemical nanosensor for tracking and understanding the role of H₂ O₂ in screening new anticancer drug.

Biocompatible Microelectrode for In Vivo Sensing with Improved Performance

In vivo sensing based on implantable microelectrodes has been widely used to monitor neurochemicals due to its high spatial and temporal resolution and engineering interface designability, which has become a powerful drive to decode the mysteries of degenerative diseases and regulate neural activity. Over the past few decades, with the development of a variety of advanced materials and technologies, encouraging progress has been made in quantifying various neurochemical transients. However, because of the complex chemical atmosphere including thousands of small and large biomolecules and the inherent low mechanical property of brain tissue, the design of a compatible microelectrode for the in vivo electrochemical tracking of neurochemicals with high selectivity and stability still faces great challenges. This Perspective presents a brief account of recent representative progress in the rational regulation of the microelectrode interface to resolve the questions of selectivity and sensitive decrease resulting from antiprotein adsorption, and how to decrease the mechanical mismatch of an implanted electrode with that of brain tissue. Possible future research directions on further addressing the above key issues and a more biocompatible microelectrode for in vivo long-time electrochemical analysis are also discussed.

Multi-Spatiotemporal Probing of Neurochemical Events by Advanced Electrochemical Sensing Methods

Neurochemical events involving biosignals of different time and space dimensionalities constitute the complex basis of neurological functions and diseases. In view of this fact, electrochemical measurements enabling real-time quantification of neurochemicals at multiple levels of spatiotemporal resolution can provide informative clues to decode the molecular networks bridging vesicles and brains. This Minireview focuses on how scientific questions regarding the properties of single vesicles, neurotransmitter release kinetics, interstitial neurochemical dynamics, and multisignal interconnections in vivo have driven the design of electrochemical nano/microsensors, sensing interface engineering, and signal/data processing. An outlook for the future frontline in this realm will also be provided.

Atomic Dispersion of Zn2+ on N-Doped Carbon Materials: From Non-Activity to High Activity for Catalyzing Luminol-H2O2 Chemiluminescence

Fe and Co single-atom catalysts (SACs) have been widely explored in many fields, while Zn SACs are still in their infancy stage. Herein, we unexpectedly found that atomically dispersed Zn²⁺ on N-doped carbon material (Zn-N-C) exhibited high catalytic activity on luminol-H₂O₂ chemiluminescence (CL) reaction. The Zn-N-C SACs were readily prepared through simple pyrolyzation of the cheap precursors (dopamine and ZnCl₂). The mechanism of Zn SAC-catalyzed CL reaction of luminol-H₂O₂ was investigated in detail. The activity of Zn SACs originated from the Zn-N sites in the Zn-N-C structure. The monoatomic dispersion makes Zn²⁺ catalytic performance change from no activity to high activity in luminol-H₂O₂ CL reaction. This study demonstrated the particularity of the monatomic metal catalyst over the conventional metal ion. This work provides the unprecedented perspective for design of new metal SACs in CL reaction.

Edge-hosted Atomic Co-N4 Sites on Hierarchical Porous Carbon for Highly Selective Two-electron Oxygen Reduction Reaction

Not only high efficiency but also high selectivity of the electrocatalysts is crucial for high-performance, low-cost, and sustainable energy storage applications. Herein, we systematically investigate the edge effect of carbon-supported single-atom catalysts (SACs) on oxygen reduction reaction (ORR) pathways (two-electron (2 e^- ) or four-electron (4 e^- )) and conclude that the 2 e^- -ORR proceeding over the edge-hosted atomic Co-N₄ sites is more favorable than the basal-plane-hosted ones. As such, we have successfully synthesized and tuned Co-SACs with different edge-to-bulk ratios. The as-prepared edge-rich Co-N/HPC catalyst exhibits excellent 2 e^- -ORR performance with a remarkable selectivity of ≈95 % in a wide potential range. Furthermore, we also find that oxygen functional groups could saturate the graphitic carbon edges under the ORR operation and further promote electrocatalytic performance. These findings on the structure-property relationship in SACs offer a promising direction for large-scale and low-cost electrochemical H₂ O₂ production via the 2 e^- -ORR.

Engineering Atomic Single Metal-FeN4Cl Sites with Enhanced Oxygen-Reduction Activity for High-Performance Proton Exchange Membrane Fuel Cells

Fe-N-C single-atomic metal site catalysts (SACs) have garnered tremendous interest in the oxygen reduction reaction (ORR) to substitute Pt-based catalysts in proton exchange membrane fuel cells. Nowadays, efforts have been devoted to modulating the electronic structure of metal single-atomic sites for enhancing the catalytic activities of Fe-N-C SACs, like doping heteroatoms to modulate the electronic structure of the Fe-N_x active center. However, most strategies use uncontrolled long-range interactions with heteroatoms on the Fe-N_x substrate, and thus the effect may not precisely control near-range coordinated interactions. Herein, the chlorine (Cl) is used to adjust the Fe-N_x active center via a near-range coordinated interaction. The synthesized FeN₄Cl SAC likely contains the FeN₄Cl active sites in the carbon matrix. The additional Fe-Cl coordination improves the instrinsic ORR activity compared with normal FeN_x SAC, evidenced by density functional theory calculations, the measured ORR half-wave potential (E_1/2, 0.818 V), and excellent membrane electrode assembly performance.

Highly Stable and Selective Sensing of Hydrogen Sulfide in Living Mouse Brain with NiN4 Single-Atom Catalyst-Based Galvanic Redox Potentiometry

Hydrogen sulfide (H₂S) is recognized as a gasotransmitter and multifunctional signaling molecule in the central nervous system. Despite its essential neurofunctions, the chemical dynamics of H₂S during physiological and pathological processes remains poorly understood, emphasizing the significance of H₂S sensor development. However, the broadly utilized electrochemical H₂S sensors suffer from low stability and sensitivity loss in vivo due to sulfur poisoning-caused electrode passivation. Herein, we report a high-performance H₂S sensor that combines single-atom catalyst strategy and galvanic redox potentiometry to overcome the issue. Atomically dispersed NiN₄ active sites on the sensing interface promote electrochemical H₂S oxidation at an extremely low potential to drive spontaneous bipolarization of a single carbon fiber. Bias-free potentiometric sensing at open-circuit condition minimizes sulfur accumulation on the electrode surface, thus significantly enhancing the stability and sensitivity. The resulting sensor displays high selectivity to H₂S against physiological interferents and enables real-time accurate quantification of H₂S-releasing behavior in the living mouse brain.

Identification of the Highly Active Co-N4 Coordination Motif for Selective Oxygen Reduction to Hydrogen Peroxide

Electrosynthesis of hydrogen peroxide (H₂O₂) through oxygen reduction reaction (ORR) is an environment-friendly and sustainable route for obtaining a fundamental product in the chemical industry. Co–N₄ single-atom catalysts (SAC) have sparkled attention for being highly active in both 2e^– ORR, leading to H₂O₂ and 4e^– ORR, in which H₂O is the main product. However, there is still a lack of fundamental insights into the structure–function relationship between CoN₄ and the ORR mechanism over this family of catalysts. Here, by combining theoretical simulation and experiments, we unveil that pyrrole-type CoN₄ (Co–N SAC_Dp) is mainly responsible for the 2e^– ORR, while pyridine-type CoN₄ catalyzes the 4e^– ORR. Indeed, Co–N SAC_Dp exhibits a remarkable H₂O₂ selectivity of 94% and a superb H₂O₂ yield of 2032 mg for 90 h in a flow cell, outperforming most reported catalysts in acid media. Theoretical analysis and experimental investigations confirm that Co–N SAC_Dp—with weakening O₂/HOO* interaction—boosts the H₂O₂ production.

Tuning the Site-to-Site Interaction in Ru-M (M = Co, Fe, Ni) Diatomic Electrocatalysts to Climb up the Volcano Plot of Oxygen Electroreduction

The modulating of the geometric and electronic structures of metal-N-C atomic catalysts for improving their performance in catalyzing oxygen reduction reactions (ORRs) is highly desirable yet challenging. We herein report a delicate "encapsulation-substitution" strategy for the synthesis of paired metal sites in N-doped carbon. With the regulation of the d-orbital energy level, a significant increment in oxygen electroreduction activity was demonstrated in Ru-Co diatomic catalyst (DAC) compared with other diatomic (Ru-Fe and Ru-Ni) and single-atomic counterparts. The Ru-Co DAC efficiently reduces oxygen with a halfwave potential of 0.895 V vs RHE and a turnover frequency of 2.424 s^-1 at 0.7 V, establishing optimal thermodynamic and kinetic behaviors in the triple-phase reaction under practical conditions. Moreover, the Ru-Co DAC electrode displays bifunctional activity in a gas diffusion Zn-air battery with a small voltage gap of 0.603 V, outperforming the commercial Pt/C|RuO₂ catalyst. Our findings provide a clear understanding of site-to-site interaction on ORR and a benchmark evaluation of atomic catalysts with correlations of diatomic structure, energy level, and overall catalytic performance at the subnanometer level.

Mass Production of Pt Single-Atom-Decorated Bismuth Sulfide for n-Type Environmentally Friendly Thermoelectrics

Single-atom materials are widely explored in catalysis, batteries, sensors, etc. However, limited by mass production and centimeter-scale assembly, they are rarely studied in thermoelectrics. Herein, we demonstrate a solvothermal synthesis assisted by a syringe-pump method to yield Bi₂S₃-supported Pt single-atom materials (Bi₂S₃-Pt₁) at a 10 g scale. Different from Pt_n clusters, Pt₁ single atoms can increase carrier concentration at a high doping efficiency and provide a unique atomic environment to enhance carrier mobility, and an enlarged effective mass leads to an enhanced Seebeck coefficient. As a result, a high power factor (348 μW m^-1 K^-2) is achieved at 823 K. Benefiting from the scattering of phonons by Pt₁ atomic sites, a minimum thermal conductivity of 0.37 W m^-1 K^-1 is achieved. Consequently, the Bi₂S₃-0.5 wt % Pt₁ realizes a record-high zT of ∼0.75 at 823 K, being among the best in the state-of-the-art n-type environmentally friendly metal sulfides. The enhancement of the carrier mobility and suppression of the thermal conduction by the unique Pt₁ single atoms will inspire various fields, as exemplified by electronic devices and thermal management.