Synergistically Enhanced Electrocatalytic Performance of an N-Doped Graphene Quantum Dot-Decorated 3D MoS2-Graphene Nanohybrid for Oxygen Reduction Reaction

MoS2@MWCNTs with Rich Vacancy Defects for Effective Piezocatalytic Degradation of Norfloxacin via Innergenerated-H2O2: Enhanced Nonradical Pathway and Synergistic Mechanism with Radical Pathway

Molybdenum disulfide (MoS2)-based materials for piezocatalysis are unsatisfactory due to their low actual piezoelectric coefficient and poor electrical conductivity. Herein, 1T/3R phase MoS2 grown in situ on multiwalled carbon nanotubes (MWCNTs) was proposed. MoS2@MWCNTs exhibited the interwoven morphology of thin nanoflowers and tubes, and the piezoelectric response of MoS2@MWCNTs was 4.07 times higher than that of MoS2 via piezoresponse force microscopy (PFM) characterization. MoS2@MWCNTs exhibited superior activity with a 91% degradation rate of norfloxacin (NOR) after actually working 24 min (as for rhodamine B, reached 100% within 18 min) by pulse-mode ultrasonic vibration-triggered piezocatalysis. It was found that piezocatalysis for removing pollutants was attributed to the synergistic effect of free radicals (•OH and O2•-) and nonfree radical (1O2, key role) pathways, together with the innergenerated-H2O2 promoting the degradation rate. 1O2 can be generated by electron transfer and energy transfer pathways. The presence of oxygen vacancies (OVs) induced the transformation of O2 to 1O2 by triplet energy transfer. The fast charge transfer in MoS2@MWCNTs heterostructure and the coexistence of sulfur vacancies and OVs enhanced charge carrier separation resulting in a prominent piezoelectric effect. This work opens up new avenues for the development of efficient piezocatalysts that can be utilized for environmental purification.

Humidity activated ultra-selective room temperature gas sensor based on W doped MoS2/RGO composites for trace level ammonia detection

The lack of highly efficient, cost effective and stable ammonia gas sensors functionable at room temperature even in extreme humid environments poses significant challenge for the future generation gas sensors. The prime factors that impede the development of such next generation gas sensors are the strong interference of humidity and sluggish selectivity. Herein, we fabricated tungsten doped molybdenum disulphide/reduced graphene oxide composite by an in-situ hydrothermal method to exploit the adsorption, dissolution (solubility), ionization and transmission process of ammonia and thereby to effectuate its trace level detection even in indispensable humid environments. The protype based on 5 at.% Tungsten doped MoS2/RGO (W5) gas sensor exhibited 3.8-fold increment in its response to 50 ppm of ammonia when the relative humidity varied from 20 % to 70 % with ultra-high selectivity at room temperature. The as prepared gas sensor revealed a practical detection limit down to 1 ppm with a substantial response and rapid recovery time. Furthermore, W5 gas sensor exhibited a 42-fold increment in response to 50 ppm of ammonia relative to its pristine (MoS2/RGO) MG composite with a RH of 70 %. The proton hopping mechanism accountable for such an enormous enhancement in ammonia sensing and its potential for breath sensor are briefly annotated.

A quadruplet 3-D laser scribed graphene/MoS2, functionalised N2-doped graphene quantum dots and lignin-based Ag-nanoparticles for biosensing

Troponin I is a protein released into the human blood circulation and a commonly used biomarker due to its sensitivity and specificity in diagnosing myocardial injury. When heart injury occurs, elevated troponin Troponin I levels are released into the bloodstream. The biomarker is a strong and reliable indicator of myocardial injury in a person, with immediate treatment required. For electrochemical sensing of Troponin I, a quadruplet 3D laser-scribed graphene/molybdenum disulphide functionalised N2-doped graphene quantum dots hybrid with lignin-based Ag-nanoparticles (3D LSG/MoS2/N-GQDs/L-Ag NPs) was fabricated using a hydrothermal process as an enhanced quadruplet substrate. Hybrid MoS2 nanoflower (H3 NF) and nanosphere (H3 NS) were formed independently by varying MoS2 precursors and were grown on 3D LSG uniformly without severe stacking and restacking issues, and characterized by morphological, physical, and structural analyses with the N-GQDs and Ag NPs evenly distributed on 3D LSG/MoS2 surface by covalent bonding. The selective capture of and specific interaction with Troponin I by the biotinylated aptamer probe on the bio-electrode, resulted in an increment in the charge transfer resistance. The limit of detection, based on impedance spectroscopy, is 100 aM for both H3 NF and H3 NS hybrids, with the H3 NF hybrid biosensor having better analytical performance in terms of linearity, selectivity, repeatability, and stability.

Surface plasmon resonance biosensor chips integrated with MoS2-MoO3 hybrid microflowers for rapid CFP-10 tuberculosis detection

This study reports on the modification of surface plasmon resonance (SPR) chips with molybdenum disulfide-molybdenum trioxide (MoS2-MoO3) microflowers to detect the tuberculosis (TB) markers of CFP-10. The MoS2-MoO3 microflowers were prepared by hydrothermal methods with variations in the pH and amount of trisodium citrate (Na3Ct), which were projected to influence the shape and size of microflower particles. The analysis shows that optimum MoS2-MoO3 hybrid microflowers were obtained at neutral pH using 0.5 g Na3Ct. The modified SPR biosensor exhibits a ten times higher response than the bare Au. Moreover, increasing MoS2-MoO3 thickness results in a higher detection response, sensitivity, and a smaller limit of detection (LOD). Using the optimized material composition, the Au/MoS2-MoO3-integrated SPR sensor can demonstrate sensitivity and LOD of 1.005 and 3.45 ng mL-1, respectively. This biosensor also has good selectivity, stability, and reproducibility based on cross-sensitivity characterization with other analytes and repeated measurements on several chips with different storing times and fabrication batch. Therefore, this proposed SPR biosensor possesses high potential to be further developed and applied as a detection technology for CFP-10 in monitoring and diagnosing TB.

From Pristine to Heteroatom-Doped Graphene Quantum Dots: An Essential Review and Prospects for Future Research

Graphene quantum dots (GQDs) are carbon-based zero-dimensional materials that have received considerable scientific interest due to their exceptional optical, electrical, and optoelectrical properties. Their unique electronic band structures, influenced by quantum confinement and edge effects, differentiate the physical and optical characteristics of GQDs from other carbon nanostructures. Additionally, GQDs can be synthesized using various top-down and bottom-up approaches, distinguishing them from other carbon nanomaterials. This review discusses recent advancements in GQD research, focusing on their synthesis and functionalization for potential applications. Particularly, various methods for synthesizing functionalized GQDs using different doping routes are comprehensively reviewed. Based on previous reports, current challenges and future directions for GQDs research are discussed in detail herein.

3D Porous MoS2-Decorated Reduced Graphene Oxide Aerogel as a Heterogeneous Catalyst for Reductive Transformation Reactions

The MoS2-based reduced graphene oxide aerogel (MoS2-rGOA)-assisted organic transformation reactions are presented. MoS2-rGOA is used as a heterogeneous catalyst for the reduction of benzene derivatives such as benzaldehyde, nitrobenzene, and benzonitrile to benzyl alcohol, aniline, and benzamide and their derivatives, respectively, in green solvents (water/methanol) and green reducing agents (hydrazine hydrate having N2 and H2 as byproducts). The mechanistic features of the reduction pathway, substrate scope, and the best suitable conditions by varying the temperature, solvent, reducing agent, catalyst loading, time, etc. are optimized. All of the synthesized products are obtained in quantitative yield with purity and well characterized based on nuclear magnetic resonance analysis. Further, it is also observed that our catalyst is efficiently recyclable and works well checked up to 5 cycles.

Synergistically functionalized molybdenum disulfide-reduced graphene oxide nanohybrid based ultrasensitive electrochemical immunosensor for real sample analysis of COVID-19

Herein, we have proposed a straightforward and label-free electrochemical immunosensing strategy supported on a glassy carbon electrode (GCE) modified with a biocompatible and conducting biopolymer functionalized molybdenum disulfide-reduced graphene oxide (CS-MoS₂/rGO) nanohybrid to investigate the SARS-CoV-2 virus. CS-MoS₂/rGO nanohybrid-based immunosensor employs recombinant SARS-CoV-2 Spike RBD protein (rSP) that specifically identifies antibodies against the SARS-CoV-2 virus via differential pulse voltammetry (DPV). The antigen-antibody interaction diminishes the current responses of the immunosensor. The obtained results indicate that the fabricated immunosensor is extraordinarily capable of highly sensitive and specific detection of the corresponding SARS-CoV-2 antibodies with a LOD of 2.38 zg mL^-1 in phosphate buffer saline (PBS) samples over a broad linear range between 10 zg mL^-1-100 ng mL^-1. In addition, the proposed immunosensor can detect attomolar concentrations in spiked human serum samples. The performance of this immunosensor is assessed using actual serum samples from COVID-19-infected patients. The proposed immunosensor can accurately and substantially differentiate between (+) positive and (-) negative samples. As a result, the nanohybrid can provide insight into the conception of Point-of-Care Testing (POCT) platforms for cutting-edge infectious disease diagnostic methods.

Identification of Co-O-Mo Active Centers on Co-Doped MoS2 Electrocatalyst

Strategies for harmonizing the construction of an active site and the building of electron transport for a hybrid MoS₂ catalyst are crucial for its application in electrochemical reactions. In this work, an accurate and facile hydrothermal strategy was proposed to fabricate the active center of Co-O-Mo on a supported MoS₂ catalyst by forming a CoMoSO phase on the edge of MoS₂, yielding (Co-O)_x-MoS_y (x = 0, 0.3, 0.6, 1, 1.5, or 2.1). The results show that the electrochemical performances (hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical degradation) of the yielded MoS₂-based catalysts were positively correlated with the Co-O bonds, verifying the significant role of Co-O-Mo as the active center. The fabricated (Co-O)-MoS_0.9 presented an extremely low overpotential and Tafel slope in both HER and OER, and it also demonstrated excellent BPA removal in the electrochemical degradation reaction. As compared with the Co-Mo-S configuration, the configuration of Co-O-Mo not only serves as the active center but also provides a conducting channel to facilitate electron conductivity with more accessible charge transfer at the electrode/electrolyte interface, which is favorable for electrocatalytic reaction. This work offers a new perspective for the active mechanism of metallic-heteroatom-dopant electrocatalysts and further boosts research on the development of noble/non-noble hybrid electrocatalysts in the future.

A non-enzymatic, biocompatible electrochemical sensor based on N-doped graphene quantum dot-incorporated SnS2 nanosheets for in situ monitoring of hydrogen peroxide in breast cancer cells

The current study reports the design and construction of enzyme-free sensor using N-doped graphene quantum dots (N-GQDs)-decorated tin sulfide nanosheets (SnS₂) for in situ monitoring of H₂O₂ secreted by human breast cancer cells. N-GQDs nanoparticles having a size of less than 1 nm were incorporated into SnS₂ nanosheets to form an N-GQDs@SnS₂ nanocomposite using a simple hydrothermal approach. The resulting hybrid material was an excellent electrocatalyst for the reduction of H₂O₂, owing to the combined properties of highly conductive N-GQDs and SnS₂ nanosheets. The N-GQDs@SnS₂-based sensing platform demonstrated substantial sensing ability, with a detection range of 0.0125-1128 µM and a limit of detection of 0.009 µM (S/N = 3). The sensing performance of N-GQDs@SnS₂ was highly stable, selective, and reproducible. The practical application of the N-GQDs@SnS₂ sensor was successfully demonstrated by quantifying H₂O₂ in lens cleaner, human urine, and saliva samples. Finally, the N-GQDs@SnS₂ electrode was successfully applied for the real-time monitoring of H₂O₂ released from breast cancer cells and mouse fibroblasts. This study paves the way to designing efficient non-enzymatic electrochemical sensors for various biomolecule detection using a simple method.

Lemon-juice derived highly efficient S-GQD/GO composite as a photocatalyst for regeneration of coenzyme under solar light

The innovation of a highly efficient and inexpensive graphene oxide-based photocatalyst is a challenging task for selective solar chemical regeneration/coenzyme such as nicotinamide adenine dinucleotide (NADH). Herein, we have designed lemon-juice derived highly efficient S-GQD/GO composite as a photocatalyst for regeneration of NADH under solar light. The rational design of a highly efficient photocatalytic system through the orientation of S-GQD on graphene oxide as solar light harvesting photocatalyst is explored for the first time for NADH regeneration. This highly solar light active S-GQD/GO composite photocatalyst upon integration with the NAD + is used for highly regioselective regeneration of coenzyme (76.36%). The present work provides the benchmark instances of graphene oxide-based material as a photocatalyst for selective regeneration of NADH under solar light and opens a new door for green synthesis.

Electrochemical Studies of Biofunctionalized MoS2 Matrix for Highly Stable Immobilization of antibodies and Detection of Lung Cancer Protein Biomarker

To address the issue of lack of stable immobilization of antibodies on the biosensing matrix for repeated cycles of measurement, chitosan (CS) bio-functionalized MoS2 is prepared to serve as a...

A highly efficient rGO grafted MoS2 nanocomposite for dye adsorption and electrochemical detection of hydroquinone in wastewater

Scheme depicting the synthesis and the fabrication of rGO–MoS2 nanocomposite-based enzymatic biosensor for estimation of hydroquinone.

The electrochemical detection of prostate specific antigen on glassy carbon electrode modified with combinations of graphene quantum dots, cobalt phthalocyanine and an aptamer

Herein, a novel aptasensor is developed for the electrochemical detection of prostate specific antigen (PSA) on electrode surfaces modified using various combinations of a Cobalt phthalocyanine (CoPc), an aptamer and graphene quantum dots (GQDs). Electrochemical impedance spectroscopy (EIS) as well as differential pulse voltammetry (DPV) are employed for the detection of PSA. In both analytical techniques, linear calibration curves were observed at a concentration range of 1.2-2.0 pM. The glassy carbon electrode where CoPc and GQDs are placed on the electrode when non-covalently linked followed by addition of the aptamer (GQDs-CoPc(ππ)-aptamer (sequential)) showed the best performance with a limit of detection (LoD) as low as 0.66 pM when using DPV. The detection limits were much lower than the dangerous levels reported for PSA in males tested for prostate cancer. This electrode showed selectivity for PSA in the presence of bovine serum albumin, glucose and L-cysteine. The aptasensor showed good stability, reproducibility and repeatability, deeming it a promising early detection device for prostate cancer.

Enhanced Photoluminescence of Multiple Two-Dimensional van der Waals Heterostructures Fabricated by Layer-by-Layer Oxidation of MoS2

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS₂ and MoO_x with enhanced photoluminescence (PL). We directly oxidized multilayer MoS₂ to a MoO_x/1 L-MoS₂ heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS₂ covered by MoO_x showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS₂ because of preserved crystallinity and compensated dedoping by MoO_x. By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoO_x/MoS₂/MoO_x/MoS₂, where the MoS₂ monolayers are separated by MoO_x. The heterostructures showed the multiplied PL intensity as the number of embedded MoS₂ layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS₂ layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.

Highly sensitive electrochemical detection of cancer biomarker based on anti-EpCAM conjugated molybdenum disulfide grafted reduced graphene oxide nanohybrid

An ultrasensitive, electrochemical biosensor has been fabricated by utilizing molybdenum disulfide (MoS₂) grafted reduced graphene oxide (MoS₂@rGO) nanohybrid as a sensing platform. Biomolecular-assisted synthetic method was adopted to synthesize MoS₂@rGO nanohybrid, where L-cys was used to reduce GO. The MoS₂@rGO nanohybrid exhibits improved electrochemical performance when it has been electrophoretically deposited onto the indium tin oxide (ITO) coated glass substrate. Further, epithelialcell adhesion moleculeantibodies (anti-EpCAM) specific to cancer biomarker has been covalently immobilized on the MoS₂@rGO/ITO electrodes for label-free detection of EpCAM. Electrochemical results confirm that anti-EpCAM/MoS₂@rGO/ITO based biosensor can detect EpCAM in the concentration range of 0.001-20 ng mL^-1 with a detection limit of 44.22 fg mL^-1 (S/N = 3). The biosensor's excellent analytical performance has been attributed to the efficient immobilization of EpCAM antibodies on the MoS₂@rGO surface, which results in high specificity for EpCAM antigen. The fabricated biosensor showed good selectivity, reproducibility, and stability. The successful detection of EpCAM antigen in spiked samples (human saliva, serum and urine) makes this platform an alternative method for early screening of cancer biomarker.

Gold Nanoparticle Embedded on a Reduced Graphene Oxide/polypyrrole Nanocomposite: Voltammetric Sensing of Furazolidone and Flutamide

A new electrochemical sensor has been constructed based on the in situ preparation of gold nanoparticle embedded on reduced graphene oxide and polypyrrole nanotube (AuNP@rGO/PPyNT) composite through a nanosecond laser-induced heating technique. The as-prepared composite is used for individual as well as the simultaneous electrochemical determination of chemotherapy drug (furazolidone, FU) and anticancer drug (flutamide, FLT). The composite was analyzed by X-ray Diffraction, scanning electron microscopy/energy-dispersive X-ray analysis, transmission electron microscopy, Raman spectrometry, and X-ray photoelectron spectroscopy analysis, thus confirming the successful synthesis of this composite and its physical features. The modified AuNP@rGO/PPyNT electrode was examined through cyclic voltammetry and differential pulse voltammetry (DPV) methods in pH 7.0 for the determination of FU and FLT in individual, simultaneous, and mixed systems. The fabricated sensor showed wide linear responses (0.01-1080.11 μM and 0.01-1214.11 μM) of analytes, with the lower limits of detection of 2.3 and 2.45 nM and higher sensitivity of 53.75 and 50.06 μA μM^-1 cm^-2, respectively. Furthermore, the constructed sensor demonstrates higher stability, reproducibility, and repeatability, and is effectively applied for the analysis of FU and FLT content in the human serum sample analysis with satisfactory recovery.

Production and Properties of Molybdenum Disulfide/Graphene Oxide Hybrid Nanostructures for Catalytic Applications

Molybdenum disulfide (MoS₂) can be an excellent candidate for being combined with carbon nanomaterials to obtain new hybrid nanostructures with outstanding properties, including higher catalytic activity. The aim of the conducted research was to develop the novel production method of hybrid nanostructures formed from MoS₂ and graphene oxide (GO). The nanostructures were synthesized in different weight ratios and in two types of reactors (i.e., impinging jet and semi-batch reactors). Physicochemical analysis of the obtained materials was carried out, using various analytical techniques: particle size distribution (PSD), thermogravimetric analysis (TGA), FT-IR spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Due to the potential application of materials based on MoS₂ as the catalyst for hydrogen evolution reaction, linear sweep voltammetry (LSV) of the commercial MoS₂, synthesized MoS₂ and the obtained hybrid nanostructures was performed using a three-electrode system. The results show that the developed synthesis of hybrid MoS₂/GO nanostructures in continuous reactors is a novel and facile method for obtaining products with desired properties. The hybrid nanostructures have shown better electrochemical properties and higher onset potentials compared to MoS₂ nanoparticles. The results indicate that the addition of carbon nanomaterials during the synthesis improves the activity and stability of the MoS₂ nanoparticles.

MoS2-Carbon Inter-overlapped Structures as Effective Electrocatalysts for the Hydrogen Evolution Reaction

The ability to generate hydrogen in an economic and sustainable manner is critical to the realization of a future hydrogen economy. Electrocatalytic water splitting into molecular hydrogen using the hydrogen evolution reaction (HER) provides a viable option for hydrogen generation. Consequently, advanced non-precious metal based electrocatalysts that promote HER and reduce the overpotential are being widely researched. Here, we report on the development of MoS2-carbon inter-overlapped structures and their applicability for enhancing electrocatalytic HER. These structures were synthesized by a facile hot-injection method using ammonium tetrathiomolybdate ((NH4)2MoS4) as the precursor and oleylamine (OLA) as the solvent, followed by a carbonization step. During the synthesis protocol, OLA not only plays the role of a reacting solvent but also acts as an intercalating agent which enlarges the interlayer spacing of MoS2 to form OLA-protected monolayer MoS2. After the carbonization step, the crystallinity improves substantially, and OLA can be completely converted into carbon, thus forming an inter-overlapped superstructure, as characterized in detail using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A Tafel slope of 118 mV/dec is obtained for the monolayer MoS2-carbon superstructure, which shows a significant improvement, as compared to the 202 mV/dec observed for OLA-protected monolayer MoS2. The enhanced HER performance is attributed to the improved conductivity along the c-axis due to the presence of carbon and the abundance of active sites due to the interlayer expansion of the monolayer MoS2 by OLA.

Depositing reduced graphene oxide onto tungsten disulfide nanosheets via microwave irradiation: confirmation of four-electron transfer-assisted oxygen reduction and methanol oxidation reaction

Core–shell structured rGO@WS₂ nanostructures exhibited four electron transfer towards the ORR and remarkable methanol oxidation reaction.

Nickel oxide decorated MoS2nanosheet-based non-enzymatic sensor for the selective detection of glucose

Understanding blood glucose levels in our body can be a key part in identifying and diagnosing prediabetes. Herein, nickel oxide (NiO) decorated molybdenum disulfide (MoS₂) nanosheets have been synthesized via a hydrothermal process to develop a non-enzymatic sensor for the detection of glucose. The surface morphology of the NiO/MoS₂ nanocomposite was comprehensively investigated by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. The electro-catalytic activity of the as-prepared NiO/MoS₂ nanocomposite towards glucose oxidation was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and amperometry in 0.1 M NaOH. The NiO/MoS₂ nanocomposite-based sensor showed outstanding electrocatalytic activity for the direct electro-oxidation of glucose due to it having more catalytic active sites, good conductivity, excellent electron transport and high specific surface area. Meanwhile, the NiO/MoS₂ modified glassy carbon electrode (GCE) showed a linear range of glucose detection from 0.01 to 10 mM by amperometry at 0.55 V. The effect of other common interferent molecules on the electrode response was also tested using alanine, l-cysteine, fructose, hydrogen peroxide, lactose, uric acid, dopamine and ascorbic acid. These molecules did not interfere in the detection of glucose. Moreover, this NiO/MoS₂/GCE sensor offered rapid response (2 s) and a wide linear range with a detection limit of 1.62 μM for glucose. The reproducibility, repeatability and stability of the sensor were also evaluated. The real application of the sensor was tested in a blood serum sample in the absence and presence of spiked glucose and its recovery values (96.1 to 99.8%) indicated that this method can be successfully applied to detect glucose in real samples.

This study reported that NiO/MoS₂ based nanocomposite can be used as an electrocatalytic material to detect glucose with high selectivity in a blood serum.

Glassy carbon electrodes modified with reduced graphene oxide-MoS2-poly (3, 4-ethylene dioxythiophene) nanocomposites for the non-enzymatic detection of nitrite in water and milk

The detrimental effect of (NO₂^-) on environment, a sensitive and selective detection of nitrite (NO₂^-) ions with point-to-care device is need to be fabricated. Herein, we report the non-enzymatic nitrite sensor using a novel reduced graphene oxide/molybdenum disulfide/poly (3, 4-ethylene dioxythiophene) (rGO/MoS₂/PEDOT) nanocomposite electrode. The rGO/MoS₂/PEDOT nanocomposite was synthesized using facile and cost-effective hydrothermal and polymerization approaches. The characteristics of rGO-MoS₂-PEDOT nanocomposite was investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) analyses. The rGO-MoS₂-PEDOT nanocomposite modified glassy carbon electrode (GCE) was directly used for electrocatalytic detection of nitrite ions present in the solution. TEM images show the PEDOT nanoparticles with an average size of 100-120 nm are uniformly covered on the outer face of rGO-MoS₂ nanosheets. The interaction between the PEDOT and rGO-MoS₂ is evidenced in the FTIR, XRD and XPS studies, and they produced synergistic effect, resulting enhanced electrocatalytic performance activity towards oxidation of nitrite. Under optimized conditions, the fabricated electrode exhibited remarkably good sensitivity (874.19 μA μM^-1 cm^-2), low detection limit (LOD) (0.059 μM, S/N = 3), wide linear range (0.001-1 mM) of nitrite with desirable selectivity, long-term stability and reproducibility. Furthermore, the practical feasibility of the fabricated sensor is validated by the successful detection of nitrite ion in some water and milk samples with excellent correlation. Thus, feasible easier synthesis method was adopted first time to fabricate rGO-MoS₂-PEDOT nanocomposite nitrite sensor in the milk and water samples with enhanced selectivity, sensitivity and LOD.

Recent Advances on Graphene Quantum Dots: From Chemistry and Physics to Applications

Graphene quantum dots (GQDs) that are flat 0D nanomaterials have attracted increasing interest because of their exceptional chemicophysical properties and novel applications in energy conversion and storage, electro/photo/chemical catalysis, flexible devices, sensing, display, imaging, and theranostics. The significant advances in the recent years are summarized with comparative and balanced discussion. The differences between GQDs and other nanomaterials, including their nanocarbon cousins, are emphasized, and the unique advantages of GQDs for specific applications are highlighted. The current challenges and outlook of this growing field are also discussed.

Boron Doped Graphene Quantum Structure and MoS2 Nanohybrid as Anode Materials for Highly Reversible Lithium Storage

Herein, the boron-doped graphene quantum structure (BGQS), which contains both the advantages of 0-D graphene quantum dot and 2-D reduced graphene oxide, has been fabricated by top-down hydrothermal method and then mixed with molybdenum sulfide (MoS2) to serve as an active electrode material for the enhanced electrochemical performance of lithium ion battery. Results show that 30 wt% of BGQS/MoS2 nanohybrid delivers the superior electrochemical performance in comparison with other BGQS/MoS2 and bare components. A highly reversible capacity of 3,055 mAh g-1 at a current density of 50 mA g-1 is achieved for the initial discharge and a high reversible capacity of 1,041 mAh g-1 is obtained at 100 mA g-1 after 50 cycles. The improved electrochemical performance in BGQS/MoS2 nanohybrid is attributed to the well exfoliated MoS2 structures and the presence of BGQS, which can provide the vitally nano-dimensional contact for the enhanced electrochemical performance. Results obtained in this study clearly demonstrate that BGQS/MoS2 is a promising material for lithium ion battery and can open a pathway to fabricate novel 2-D nanosheeted nanocomposites for highly reversible Li storage application.

Chemical Vapor Deposition-Grown Nickel-Encapsulated N-Doped Carbon Nanotubes as a Highly Active Oxygen Reduction Reaction Catalyst without Direct Metal-Nitrogen Coordination

Nickel-encapsulated nitrogen-doped carbon nanotubes (Ni-TiO₂-NCNTs) are synthesized via chemical vapor deposition by thermal decomposition of acetylene with acetonitrile vapor at 700 °C on the Ni-TiO₂ matrix. TiO₂ is used as a dispersant medium for Ni nanoparticles, which assists in higher CNT growth at high temperatures. A reference catalyst is made by following the similar procedure without acetonitrile vapor, which is called a Ni-TiO₂-CNT. Acid treatment of these two catalysts dissolved Ni on the surface of CNTs-NCNTs, producing catalysts with enhanced surface area and defects. The transmission electron microscopy-energy-dispersive X-ray spectra analysis of acid-treated version of the catalysts confirmed the presence of encapsulated Ni. Oxygen reduction reaction (ORR) activity of these catalysts was analyzed in 0.1 N KOH solution. Among these, the acid-treated Ni-TiO₂-NCNT exhibited highest ORR onset potential of 0.88 V versus reversible hydrogen electrode and a current density of 3.7 mA cm^-2 at 170 μg cm^-2 of catalyst loading. The stability of the acid-treated Ni-TiO₂-NCNT is proved by cyclic voltammetry and chronoamperometry measurements which are done for 800 cycles and 100 h, respectively. Primarily N doping of CNTs is the reason behind the improved ORR activity.

Ultrathin layered MoS2 nanosheets with rich active sites for enhanced visible light photocatalytic activity

Edge-rich active sites of ultrathin layered molybdenum disulphide (MoS₂) nanosheets were synthesized by a hydrothermal method. The effect of pH on the formation of MoS₂ nanosheets and their photocatalytic response have been investigated. Structural and elemental analysis confirm the presence of S–Mo–S in the composition. Morphological analysis confirms the presence of ultrathin layered nanosheets with a sheet thickness of 10–28 nm at pH 1. The interplanar spacing of MoS₂ layers is in good agreement with the X-ray diffraction and high-resolution transmission electron microscopy results. A comparative study of the photocatalytic performance for the degradation of methylene blue (MB) and rhodamine B (RhB) by ultrathin layered MoS₂ under visible light irradiation was performed. The photocatalytic activity of the edge-rich ultrathin layered nanosheets showed a fast response time of 36 min with the degradation rate of 95.3% of MB and 41.1% of RhB. The photocatalytic degradation of MB was superior to that of RhB because of the excellent adsorption of MB than that of RhB. Photogenerated superoxide radicals were the key active species for the decomposition of organic compounds present in water, as evidenced by scavenger studies.

Edge-rich active sites of ultrathin layered molybdenum disulphide (MoS₂) nanosheets were synthesized by a hydrothermal method.