Vanadium Single Atoms Embedded in MoS2 Enabled Gut-Brain Axis Neurotransmitter Detection at pM Levels.

The detection of monoamine neurotransmitters is of paramount importance as the neurotransmitters are the chemical messengers regulating the gut-brain axis (GBA). It requires real-time, ultrasensitive, and selective sensing of the neurotransmitters in the gastric/intestinal fluid. However, multi-components present in the gastric/intestinal fluid make sensing challenging to achieve in terms of ultra-high sensitivity and selectivity. Herein, an approach is introduced to utilize vanadium single atom catalytic (SAC) centers in van der Waals MoS2 (V-MoS2) to selectively detect real-time serotonin (5-HT) in artificial gastric/intestinal fluid. The synergetic effect of V-SACs and the surface S-bonds on the MoS2 surface, enables an extremely wide range of 5-HT detection (from 1 pM to 100 µM), with optimum selectivity and interference resistance. By combining density functional theory calculations and scanning transmission electron microscopy, it is concluded that the V-SACs embedded in the MoS2 network create active sites that greatly facilitate the charge exchange between the material and the 5-HT molecules. This result allows the 5-HT detection in GBA studies to be more reliable, and the material tunability provides a general platform to achieve real-time and multi-component detection of other monoamine neurotransmitters in GBA such as dopamine and norepinephrine.

Biosignals in the Gut-Brain Axis Transmission: Function and Detection.

The gut-brain axis (GBA) is an important information pathway connecting the brain, the central nervous system (CNS), and the gastrointestinal (GI) tract. On the one hand, gut microbiota can influence the function brain through GBA; on the other hand, the brain can also change the structural composition of gut microbiota via GBA. It contains a myriad of biosignals, such as monoamines, inflammatory cytokines, and macro-biomolecules, as the information carriers. Highly selective, sensitive, and reliable sensing techniques are essential to resolve the specific function of individual biosignals. This review summarizes the widely reported biosignals related to GBA and their functions, and organizes the latest sensing tools to provide feasible characterization ideas for GBA-related work. In addition, these low-cost, fast-responding sensors can also be used for early identification and diagnosis of GBA-related diseases (e.g., depression). Finally, the problems and deficiencies in this field are pointed out to provide a reference for the orientation of researchers in the sensing field.

Two-dimensional covalent organic framework films prepared on various substrates through vapor induced conversion.

Covalent organic frameworks (COFs) can exhibit high specific surface area and catalytic activity, but traditional solution-based synthesis methods often lead to insoluble and infusible powders or fragile films on solution surface. Herein we report large-area -C=N- linked two-dimensional (2D) COF films with controllable thicknesses via vapor induced conversion in a chemical vapor deposition (CVD) system. The assembly process is achieved by reversible Schiff base polycondensation between PyTTA film and TPA vapor, which results in a uniform organic framework film directly on growth substrate, and is driven by π-π stacking interactions with the aid of water and acetic acid. Wafer-scale 2D COF films with different structures have been successfully synthesized by adjusting their building blocks, suggesting its generic applicability. The carrier mobility of PyTTA-TPA COF films can reach 1.89 × 10-3 cm2 V-1 s-1. When employed as catalysts in hydrogen evolution reaction (HER), they show high electrocatalytic activity compared with metal-free COFs or even some metallic catalysts. Our results represent a versatile route for the direct construction of large-area uniform 2D COF films on substrates towards multi-functional applications of 2D π-conjugated systems.

Near-Equilibrium Growth of Chemically Stable Covalent Organic Framework/Graphene Oxide Hybrid Materials for the Hydrogen Evolution Reaction.

Facile synthesis and post-processing of covalent organic frameworks (COFs) under mild synthetic conditions are highly sought after and important for widespread utilizations in catalysis and energy storage. Here we report the synthesis of the chemically stable aza-fused COFs BPT-COF and PT-COF by a liquid-phase method. The process involves the spontaneous polycondensation of vicinal diamines and vicinal diketones, and is driven by the near-equilibrium growth of COF domains at a very low monomer concentration. The method permits in situ assembly of COFs and COF-GO hybrid materials and leads to the formation of a uniform conducting film on arbitrary substrates on vacuum filtration. When used as electrocatalysts, the as-prepared membranes show a fast hydrogen evolution reaction (HER) with a low overpotential (45 mV at 10 mA cm-2 ) and a small Tafel slope (53 mV dec-1 ), which are the best among metal-free catalysts. Our results may open a new route towards the preparation of highly π-conjugated COFs for multifunctional applications.

Studying the adsorption mechanisms of nanoplastics on covalent organic frameworks via molecular dynamics simulations.

Covalent organic frameworks (COFs) with well-defined supramolecular structures and high surface-area-to-volume ratio have received extensive attention on their adsorption of contaminants from micro- to nano-size. Here, we studied the adsorption mechanisms of three typical nanoplastics (NP), including polyethylene (PE), nylon-6 (PA 6), and polyethylene terephthalate (PET) on chemically stable COFs (TpPa-X, X = H, CH3, OH, NO2 and F) by molecular dynamics simulations. Depending on molecular structure and surface composition, two distinct interactions-electrostatic interaction and van der Waals (vdW) interaction-are identified to be responsible for the adsorption of different NP pollutants on TpPa-X. The vdW interaction is dominant during the adsorption process, while polar groups in polymers and COFs can enhance the adsorption because of the electrostatic interaction. Compared with other functional COFs, we found that TpPa-OH shows the strongest adsorption with the NP pollutants employed in this study. This work reveals the COF-polymer adsorption behavior and properties at atomic scale, which is crucial to the development of promising COF materials to deal with NP pollution.

Two-Dimensional Metal-Organic Framework Film for Realizing Optoelectronic Synaptic Plasticity.

2D metal-organic framework (MOF) film as the active layer show promising application prospects in various fields including sensors, catalysis, and electronic devices. However, exploring the application of 2D MOF film in the field of artificial synapses has not been implemented yet. In this work, we fabricated a novel 2D MOF film (Cu-THPP, THPP=5,10,15,20-Tetrakis(4-hydroxyphenyl)-21H,23H-porphine), and further used it as an active layer to explore the application in the simulation of human brain synapses. It shows excellent light-stimulated synaptic plasticity properties, and exhibits the foundation function of synapses such as long-term plasticity (LTP), short-term plasticity (STP), and the conversion of STP to LTP. Most critically, the MOF based artificial synaptic device exhibits an excellent stability in atmosphere. This work opens the door for the application of 2D MOF film in the simulation of human brain synapses.

Face-to-Face Growth of Wafer-Scale 2D Semiconducting MOF Films on Dielectric Substrates.

The preparation of large-area 2D conductive metal-organic framework (MOF) films remains highly desirable but challenging. Here, inspired by the capillary phenomenon, a face-to-face confinement growth method to grow conductive 2D Cu2 (TCPP) (TCPP = meso-tetra(4-carboxyphenyl)porphine) MOF films on dielectric substrates is developed. Trace amounts of solutions containing low-concentration Cu2+ and TCPP are pumped cyclically into a micropore interface to produce this growth. The crystal structures are confirmed with various characterization techniques, which include high-resolution atomic force microscopy and cryogenic transmission electron microscopy (Cryo-TEM). The Cu2 (TCPP) MOF film exhibit an electrical conductivity of ≈0.007 S cm-1 , which is approximately four orders of magnitude higher than other carboxylic-acid-based MOF materials (10-6 S cm-1 ). Other wafer-scale conductive MOF films such as M3 (HHTP)2 (M = Cu, Co, and Ni; HHTP = 2,3,6,7,10,11-triphenylenehexol) can be produced utilizing this strategy and suggests this method has widescale applicability potential.

Electrochemical Synthesis of Large Area Two-Dimensional Metal-Organic Framework Films on Copper Anodes.

Owing to their excellent physical and electrical properties, metal-organic framework (MOF) materials with well-defined supramolecular structures have received extensive research attention. However, the fabrication of large-area two-dimensional (2D) MOF films is still a significant challenge. Herein, we propose a novel electrochemical (EC) synthesis method for the preparation of large-area Cu3 (HHTP)2 MOF film on single-crystal Cu (100) anode. The surface reaction was achieved via charge-induced molecular assembly. The synthesized MOF film exhibited a high crystalline quality with an electrical conductivity of approximately 0.087 S cm-1 , which was around 1000 times larger than the previously reported values for the same material prepared by the interface method. In addition, Cu2 (MTCP), Cu3 (BTPA)2 , and Cu3 (TPTC)2 MOF films were synthesized on Cu foil with the same strategy, which confirmed the universality of the proposed method. This controllable EC method can be effectively applied to the industrial-scale production of 2D MOF films on Cu foil.