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Cobalt spinel oxides, which consist of tetrahedral site (AO4) and octahedral site (BO6), are a potential group of transition metal oxides (TMO) for electrocatalytic nitrate reduction reactions to ammonia (NRA). Identifying the true active site in spinel oxides is crucial to designing advanced catalysts. This work reveals that the CoO6 site is the dominant site for NRA through the site substitution strategy. The suitable electronic configuration of Co at the octahedral site leads to a stronger interaction between the Co d-orbital and the O p-orbital in O-containing intermediates, resulting in a high-efficiency nitrate-to-ammonia reduction. Furthermore, the substitution of metallic elements at the AO4 site can affect the charge density at the BO6 site via the structure of A-O-B. Thereafter, Ni and Cu are introduced to replace the tetrahedral site in spinel oxides and optimize the electronic structure of CoO6. As a result, NiCo2O4 exhibits the best activity for NRA with an outstanding yield of NH3 (15.49 mg cm-2 h-1) and FE (99.89%). This study introduces a novel paradigm for identifying the active site and proposes an approach for constructing high-efficiency electrocatalysts for NRA.

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Covalent organic frameworks (COFs) are a promising alternative toward catalysis, due to the unique framework structure and the excellent chemical stability. However, the scarcity of unsaturated metal sites and the low conductivity have constrained the advancement of these materials for catalysis of electrochemical reactions. Exploring next-generation conductive metal-covalent organic frameworks (M-COFs) with extra metal active sites is crucial for improving their catalytic activity. Herein, a novel fully-conjugated M-COFs (Co-PorBpy-Co) with two types of metal sites is proposed and achieved by solvothermal method in the presence of carbon nanotube (CNT). The electrocatalyst constructed by the Co-PorBpy-Co exhibits excellent oxygen reduction reaction (ORR) activity (E1/2 = 0.84 V vs RHE, n = 3.86), superior to most COFs-based catalysts. Theoretical result shows the CoN2 sites are extremely active for ORR, and Co-PorBpy-Co exhibits excellent conductivity for electron transfer. The Zn-air battery constructed by Co-PorBpy-Co/CNT manifests excellent power density (159.4 mW cm-2 ) and great cycling stability, surpassing that of 20 wt% Pt/C catalyst. This work not only proposes a novel design concept for electrocatalysts, but establishes a mechanism platform for single-metal atom electrocatalysis and synergistic effect.

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Woven covalent organic frameworks (COFs) possess three-dimensional (3D) frameworks with well-dispersed variable metal centers, showing great promise in heterogeneous catalysis. Until now, woven COFs have not been exploited as catalysts. Herein, COF-112 (a typical woven COF) is utilized as an ORR catalyst to reveal the role of the metal center and linkage. Through metal center variation, the optimal COF-112Co with imine linkage exhibits superior ORR activity (Eonset = 0.87 V vs. RHE, n = 3.86, and JL = 5.78 mA cm-2). Experimental and theoretical studies demonstrate the non-metallic ORR active site and confirm the influence of metal variation in COF-112. A linkage conversion strategy reveals the importance of the imine linkage on the 4e- ORR. This work reveals the structure-activity relationship of woven COFs, which will broaden the application of COFs and extend the diversity of electrocatalysts.

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With the demand for efficient hydrogen/oxygen evolution reaction (HER/OER) bifunctional electrocatalysts, defect-rich two-dimensional (2D) heterostructured materials attract increasing attention due to abundant active sites and facile mass/charge transfer. However, precise manipulation of lattice defects in a 2D heterostructured material is still a challenge. Herein, through pyrolytic sulfurization of a layered Fe-doped Ni/Mo MOF precursor, a series of defect-rich Fe-doped NiS/MoS2 ultrathin nanosheets were obtained. For 0.1Fe-NiS/MoS2, abundant lattice defects induced by Fe atoms provide more water adsorption sites, and intimate interface between NiS and MoS2 can optimize the adsorption energy of a HER/OER intermediate. As a result, both HER and OER activities are significantly enhanced. The respective overpotential is 120 mV and 297 mV for the HER and OER. Small Tafel slopes of 69.0 mV dec-1 and 54.7 mV dec-1 indicate favorable electrochemical reaction kinetics. The catalytic performance of this material can be compared with those of 20% Pt/C and RuO2 catalysts and top-rated MoS2-based materials. For overall water splitting, only 1.66 V voltage is required to deliver 10 mA cm-2. Long-term stability of 0.1Fe-NiS/MoS2 presents a prospect for its practical application.

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By integrating a target-responsive MSN-based controlled release system with a sensitization-SPR co-enhanced thionine/MoS2 QDs/Cu NWs photocathode, a highly sensitive split-type PEC aptasensing platform for AßO detection in blood is constructed. Ultralow detection limit (2.1 fM) and high selectivity show great potential in early AD diagnosis.

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Superior to anodic photoelectrochemical (PEC) method, cathodic bioanalysis integrates merits of excellent anti-interference and high stability, representing a promising and competitive methodology in precise monitoring targets in complex matrices. However, serious consideration of photocathode is far behind the anodic one, developing high-performance photocathode for PEC biosensing is thus urgently desired. Herein, a high-performance cathodic PEC aptasensing platform for detection of amyloid-beta oligomers (AßO) was constructed by integrating CuO/g-C3N4 p-n heterojunction with MoS2 QDs@Cu NWs multifunction signal amplifier. The CuO/g-C3N4, exhibiting intense visible light-harvesting and high photoelectric conversion efficiency, was synthesized by in-situ pyrolysis of Cu-MOF and dicyandiamide. The MoS2 QDs@Cu NWs was obtained by electrostatical self-assembly, which acted not only as a sensitizer to boost PEC response, but also as a nanozyme for biocatalytic precipitation. The aptasensor was fabricated by DNA hybridization between the cDNA on photocathode and MoS2 QDs@Cu NWs-labeled aptamer. Based on "on-off-on" photocurrent response generated by multifunction signal amplification, ultrasensitive aptasensing of AßO was realized in a wider linear range from 10 fM to 0.5 µM with an ultralow detection limit of 5.79 fM. The feasibility of the sensor for AßO determination in human blood serum was demonstrated.

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Cathodic photoelectrochemical (PEC) bioassay is more resistant to reductive interferents, and development of high-performance photocathode is imperatively required in precise monitoring target in complex matrices. In this work, a plasmonic Au activated amorphous MoSx photocathode (a-MoSx/Au) was fabricated by sequential electrodeposition. Coupled with a sensitization amplification strategy induced by target-aptamer recognition, an ultrasensitive and high-affinitive signal-on cathodic PEC aptasensor for insulin detection was developed. Under optimum conditions, the sensor exhibits a wide linear range (0.1 pg/mL~100 ng/mL) and an ultralow detection limit (28 fg/mL) even lower than most sensors reported so far. Plasmonic Au activation and target-induced sensitization effect are responsible for high-performance PEC aptasensing of insulin at a-MoSx photocathode.

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The design and fabrication of high visible-light activated photoelectrode are essential to precisely detect biomolecule in biological system. Herein, an ultrasensitive photoelectrochemical (PEC) aptasensor for specific recognition of adenosine is established based on carbon dots sensitized-amorphous molybdenum sulfide (a-MoSx/CDs) photoanode and dual amplification strategy. The heterostructured photoanode achieved by sequential electrodeposition reveals significantly boosted photocurrent with good stability and repeatability under visible light illumination, giving the credit to highly activated visible light absorption, uniform coverage and good electric contact to the underlying substrate, as well as the energy-band alignment between the two components. By stepwisely immobilizing complementary DNA probe (NH2-DNA) and adenosine aptamer (Apt), followed by methylene blue (MB) binding with the guanine base on Apt, a dual amplified self-powered PEC aptasensor for adenosine detection is constructed. Based on the co-sensitization effect of CDs and MB, ultrasensitive and high-affinitive determination of adenosine is realized over the concentration range of 0.01 nM-1000 nM at 0 V (vs. SCE), with satisfactory stability and reproducibility. The detection limit is as low as 3.3 pM, demonstrating a performance even surpassing most of the sensors reported so far. The prospective application of the co-sensitized a-MoSx photoanode for ultrasensitive aptasensing is highlighted in this work.

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At present, the most effective catalytic materials are noble metals, which large-scale applications are strictly restricted by low reserve and high cost. For the development of sustainable energy, exploring cost-effective non-precious metal materials has therefore become an urgent task. As one of the promising candidates, the hybrid of transition metal sulfide and carbon-nitrogen skeleton attracts great attention due to variability, graded pore structure and high conductivity. Herein, a sustainable multifunctional electrocatalytic material has been designed and achieved by in-situ encapsulating Co9S8/Co in N-doped carbon nanotube (NCNT). Efficient multifunctional electrocatalysis towards oxygen reduction, oxygen evolution and hydrogen evolution reactions are realized. This work highlights the importance of rich Co-N and CoNC coupling centers generated by in-situ engineering transition metal sulfide into carbon-nitrogen skeleton for multifunctional catalytic conversion of sustainable energy. The results here may also be instructive for designing other complexes with perspective for catalysis, sensing, energy storage and conversion.

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The design and construction of high-performance photoelectrode with efficient visible-light absorption and fast charge transport rate are crucial to developing photoelectrochemical (PEC) sensor. Herein, a well-aligned amorphous MoSx (a-MoSx)@ZnO core-shell nanorod arrays was uniformly grown on indium tin oxide (ITO) via a facile all-electrochemical strategy. Simultaneous boosted PEC performance and stability in the visible region were obtained. Those could be attributed to the collaboration effect of fast electron transfer path within the oriented ZnO nanorod and extended visible-light absorption upon combining a-MoSx, as well as favorable energy-band alignment between the two PEC materials. By employing tobramycin-binding aptamer as recognition element, a self-powered PEC aptasensor for tobramycin was successfully fabricated for the first time. The sensor exhibited a rapid response in a wide linear range of 0.010-50 ng/mL with good stability and reproducibility. The detection limit was as low as 5.7 pg/mL. Ultrasensitive and high-affinitive determination of tobramycin in serum samples was realized at 0 V (vs.SCE) with desired accuracy and satisfactory recovery. This work reveals the promising application of all-electrodeposited a-MoSx@ZnO NR arrays-based photoanode for self-powered PEC bioassay in the tobramycin-related disease diagnosis.

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