Weipeng Liu

Hierarchically nanostructured MoS2 with rich in-plane edges as a high-performance electrocatalyst for the hydrogen evolution reaction

As a low-cost and earth-abundant electrocatalyst for the hydrogen evolution reaction (HER), molybdenum sulfide (MoS2) is recognized as a two dimensional material composed of catalytically active edges and an inert basal plane. Thus, optimizing simultaneously the hierarchical nanostructure and active site density could substantially improve its HER activity but it is not an effortless strategy to do so. Herein, we report for the first time a facile and controllable synthesis for engineering hierarchically nanostructured MoS2 nanosheets with rich in-plane edges (IE-MoS2 NSs). The IE-MoS2 NSs feature HER-oriented nanostructures with a tiny size, few-layer, expanded interlayer spacing and an unconventional in-plane-edge structure where separated MoS2 nanoflakes stand on the basal plane of nanosheets. Defects (Mo species with multiple chemical states and oxygen heteroatoms) were identified and they were proposed to be responsible for stabilizing the highly active MoS2 nanoflakes, forming the unique in-plane edges. IE-MoS2 NSs exhibit surprisingly high HER activity with a low onset potential of −87 mV and a Tafel slope of 41 mV per decade, exceeding other nanostructured MoS2 materials in this work and surpass almost all documented MoS2-based electrocatalysts. The surging activity has been discussed in detail, showing that the in-plane-edge nanostructure boosts the number of catalytic sites and promotes the intrinsic activity of each site at the same time, facilitating the catalytic process for H2 evolution.

Strongly coupled MoS2 nanoflake–carbon nanotube nanocomposite as an excellent electrocatalyst for hydrogen evolution reaction

As a promising non-precious metal electrocatalyst for the hydrogen evolution reaction (HER), MoS2 suffers from impeded electrical conductivity and scarce active sites. This tricky situation can be ameliorated but not eliminated by the simple involvement of nanocarbons. Herein, a leaves-and-branch structure of strongly coupled and porous MoS2–carbon nanotube (CNT) nanocomposite was synthesized, where few-layer MoS2 nanoflakes are anchored radially and intimately on the surface of CNT. Mo–O–C has been unveiled to be the bridge between these two phases and a sandwich-like structure was proposed for the interface within the strongly coupled MoS2–CNT. This genuine nanocomposite exhibits remarkably improved electrocatalytic activity towards HER, reaching −850 mA cmgeo−2 with the expense of only 290 mV of overpotential and maintaining a low Tafel slope of 47 mV per decade even under a current density of 100 mA cmgeo−2. After detailed analysis, this surging activity has been ascribed not so much to the simple addition of the properties of MoS2 and CNT, but to a strong interfacial attachment between them which not only stabilizes tiny and edge-terminated MoS2 nanoflakes, but also constructs a three dimensional hierarchical structure to boost electron and mass transfer during the HER operation.

Multiple amplified enzyme-free electrochemical immunosensor based on G-quadruplex/hemin functionalized mesoporous silica with redox-active intercalators for microcystin-LR detection

A novel multiple amplified enzyme-free immunosensor was developed for competitive immunoassay of microcystin-LR (MC-LR). Classical electrochemical immunosensors usually employ enzymes as biocatalysts to afford amplified signals, but the proteolytic degradation and poor stability are still crucial problem. Herein, monodisperse core-shell mesoporous silica (SiO2@MSN)-functionalized DNAzyme concatamers were synthesized to load hemin and methylene blue (MB) as the mimic enzyme. Firstly, the surface of SiO2@MSN was conjugated with secondary antibody as the recognition of MC-LR antibody and with a DNA strand as the initiator. Two auxiliary DNA strands were then selected for the in-situ propagation to form a double-helix DNA through hybridization chain reaction (HCR), forming numerous DNAzymes (G-quadruplex/hemin) after the addition of hemin. Secondly, MB was inserted into the formed double-helix DNA, and also loaded in the brush-like structure of mesoporous SiO2@MSN. The molecular docking study showed that electrons can transfer more effectively with π-π stack of hemin/G-quadruplex and intercalation of MB/DNA, thus the catalytic ability of DNAzymes can be greatly improved. With the aid of MB, DNAzymes can catalyze the reduction of H2O2 to produce the electrochemical signal. This enzyme-free electrochemical immunosensor can successfully detect MC-LR in a range of 0.5ng/L and 25μg/L with a detection limit of 0.3ng/L. This stable, sensitive and selective nonenzymatic electrochemical immunoassay shows promise for applications in food and environmental monitoring.

Ultradispersed and Single-Layered MoS2 Nanoflakes Strongly Coupled with Graphene: An Optimized Structure with High Kinetics for the Hydrogen Evolution Reaction

As one of the most promising Pt alternatives for cost-effective hydrogen production, molybdenum disulfide (MoS2), although has been studied extensively to improve its electrocatalytic activity, suffers from scarce active sites, low conductivity, and lack of interaction with substrates. To this end, we anchor ultradispersed and single-layered MoS2 nanoflakes on graphene sheets via a hybrid intermediate (MoOx-cysteine-graphene oxide), which not only confines the subsequent growth of MoS2 on the graphene surface but also ensures the intimate interaction between Mo species and graphene at the initial stage. Mo-O-C bond and a possible residual MoO3-x layer are proposed to comprise the interface bridging the two inherent incompatible phases, MoS2 and graphene. This strongly coupled structure together with the highly exposed MoS2 morphology accelerates the electron injection from graphene to the active sites of MoS2, and thus the hydrogen evolution reaction (HER) can achieve an overpotential of ∼275 mV at ∼-740 mA cm-2, and a Pt-like Tafel slope of ∼35 mV dec-1. Our results shed light on the indispensable role of interfacial interaction within semiconducting material-nanocarbon composites and provide a new insight into the actual activity of MoS2 toward the HER.

Broad-specificity photoelectrochemical immunoassay for the simultaneous detection of ochratoxin A, ochratoxin B and ochratoxin C

A broad-specific photoelectrochemical (PEC) immunosensor was developed for the simultaneous detection of ochratoxin A, ochratoxin B and ochratoxin C (OTA, OTB, OTC) by using the direct growth of CdS nanorods on FTO as the photoelectrode and Au nanoflowers-modified glass carbon electrode (GCE) as the bioelectrode. The bioelectrode was used to capture antigens and then associate corresponding antibodies, followed by using SiO2@Cu2+ nanocomposites to conjugate the secondary antibody (Ab2) and a DNA strand as the initiator. After the hybridization chain reaction (HCR) and the addition of hemin, numerous DNAzymes (G-quadruplex/hemin) were produced. Due to the similar enzymatic property with horseradish peroxidase (HRP), G-quadruplex/hemin can accelerate the oxidation of 4-chloro-1-naphthol (4-CN) with H2O2 to yield the biocatalytic precipitation (BCP) on the bioelectrode. Then, the bioelectrode was further treated with moderate acid and thus Cu2+ was released, which can decrease the photocurrent of the photoelectrode by the formation of CuS. Due to the advantages of surface effect of Au nanoflowers, DNA amplification and high photoelectrocatalytic activity, the proposed broad-specificity PEC immunosensor can detect OTA, OTB and OTC with a detection limit of 0.02, 0.04 and 0.03 pg/mL, respectively. In addition, the acceptable stability and selectivity suggest its possible application in the detection of OTA, OTB and OTC in water samples.

Dual-Modal Split-Type Immunosensor for Sensitive Detection of Microcystin-LR: Enzyme-Induced Photoelectrochemistry and Colorimetry

Microcystins, the lethal cyanotoxins from Microcystis aeruginosa, can inhibit the activity of protein phosphatase and promote liver tumors. Herein, a dual-modal split-type immunosensor was constructed to detect microcystin-LR (MC-LR), based on the photocurrent change of CdS/ZnO hollow nanorod arrays (HNRs) and the blue shift of the surface plasmon resonance peak from Au nanobipyramids@Ag. By using mesoporous silica nanospheres as the carrier to immobilize secondary antibody and DNA primer, a hybridization chain reaction was adopted to capture alkaline phosphatase, while its catalytic reaction product, ascorbic acid, exhibited dual functions. The detailed mechanism was investigated, showing that ascorbic acid can not only act as the electron donor to capture the holes in CdS/ZnO-HNRs, leading to the increase photocurrent, but also as the reductant to form silver shells on Au nanobipyramids, generating multiply vivid color variations and blue shifts. Compared with the traditional photoelectrochemical immunosensor or colorimetric method for MC-LR, a more accurate and reliable result can be obtained, due to different mechanisms and independent signal transduction. Therefore, this work can not only propose a new dual-modal immunosensor for MC-LR detection but also provide innovative inspiration for constructing sensitive, accurate, and visual analysis for toxins.

A rolling circle amplification signal-enhanced immunosensor for ultrasensitive microcystin-LR detection based on a magnetic graphene-functionalized electrode

A novel competitive immunosensor for the sensitive and selective detection of MC-LR was developed. Magnetic graphene was synthesized, characterized and used as the substrate to immobilize antigens onto the electrode surface due to its surface area and easy separation. Furthermore, gold nanorods were modified with polydopamine and then functionalized with a secondary-antibody and circularization DNA template. By using rolling circle amplification, the DNA template can be replicated to generate extensive repeated DNA sequences, and then hybridized with the detection probe, and thus the signal was remarkably improved. Under the optimal conditions, the proposed immunosensor showed a good linear relationship between the current response and the concentration of MC-LR in the range of 0.01–50 μg L−1 with a detection limit of 0.007 μg L−1. The immunosensor was proved to be specific, reproducible and stable. Most importantly, the proposed biosensor was applied to detect MC-LR in a real water sample with good recovery, indicating its potential application in environmental monitoring.

A dual-signal readout enzyme-free immunosensor based on hybridization chain reaction-assisted formation of copper nanoparticles for the detection of microcystin-LR

Enzyme-based electrochemical biosensors are widely used in immunoassays, but the intrinsic disadvantages of enzymes including instability or sensitivity to temperature and pH should be considered. Herein, an enzyme-free and dual-signal readout immunoassay was established to detect microcystin-LR (MC-LR) sensitively and selectively. Firstly, the microplate was modified with gold nanoparticles-decorated-carbon nanotubes (AuNP-CNT) to immobilize sufficient antigens by the high surface area of CNT and high affinity of AuNP. Then, silver nanoparticles were decorated on gold nanorods to form corn-like AgNP/AuNR composite and then capture secondary antibody and initiator DNA strand. After hybridization chain reaction, long double helix DNA strands can be formed on AgNP/AuNR to germinate copper nanoparticles. A dual-signal readout from the current responses of both silver and copper ions was obtained by using differential pulse stripping voltammetry with the aid of acid-treatment. By using a competitive immunoreaction, MC-LR can be detected in a linear range from 0.005 μg/L to 20 μg/L with a lower detection limit of 2.8 ng/L. The reproducibility, stability and specificity were all acceptable, indicating its promising application in environment monitoring and sensitive electrochemical detection for other analytes.