Study On The Preparation Of Two- And Zero-Dimensional MoS₂ Nanomaterials And Their Applications In Biosensors

Two-dimensional (2D) nanomaterials, derived from layered bulk crystals analogous to graphite, have aroused a great deal of consideration. These developments have been spearheaded by researches of graphene, a 2D nanomaterial that is unique due to its combination of thermal, electronic, optical, and mechanical properties. With the penetration of graphene, other 2D materials with a nano-layered structure such as hexagonal boron nitride, transition metal dichalcogenides, and metal halides have also attracted a large amount of interest. Especially, MoS2 nanosheets (MoS2-NSs),2D transition metal dichalcogenides, were obtained through efficient exfoliation of three-dimensional crystals and breaking the van der Waals interactions between nanosheets. Owing to the specific 2D confinement of electron motion and absence of interlayer perturbation, MoS2-NSS possess a direct band gap and show some remarkable properties, offering new opportunities in various areas, such as sensors, single-layer transistors, integrated circuits, phototransistors, nanometer thick photovoltaics, and materials of batteries. Recently, MoS2-NSs were found to be promising materials for hydrogenation catalysis, attributed to active sites located along the edges of their layered crystal structure. To achieve the potential applications of MoS2-NSs limited by the insufficient active sites, defect-rich MoS2-NSs, oxygen-incorporated MoS2-NSs, and metallic MoS2-NSs representing novel morphologies of MoS2-NSs were presented.In this paper, MoS2-NSs were obtained from sonication-assisted exfoliation of bulk MoS2 in surfactant aqueous solutions. Meanwhile, electro-Fenton reaction was employed to produce hydroxyl radicals, for the etching of MoS2-NSs and preparation of luminescent MoS2 quantum dots. The aggregation and sedimentation behaviors of MoS2-NSs in aqueous salt solutions were studied based on its characteristic optical absorption. It was demonstrated that salt-induced aggregation of MoS2-NSs resulted in the increase of sizes in both vertical and lateral dimensions of the two-dimensional nanomaterial, and then the sedimentation behavior of aggregated layered MoS2 occurred, subsequently. Furthermore, single-stranded DNA (ssDNA) was found to be adsorbed on the surface of layered MoS2 and inhibit the coagulation of MoS2-NSs. In contrast, double-stranded DNA (dsDNA) stayed away from the surface of MoS2-NSs and thus the inhibition effect was weakened. On the basis of the characteristic absorption of MoS2-NSs, a new platform for DNA detection was developed. Meanwhile, hemin/MoS2-NSs were synthesized through the van der Waals interaction between MoS2-NSs and hemin molecules in a mixed methanol solution. The as-prepared hemin/MoS2-NSs owned a great dispersiry in aqueous solution and exhibited catalytic activity to the oxidation of TMB in the presence of H2O2. As a new support of hemin, MoS2-NSs were presented, showing their strong van der Waals force when interacted with plane-like molecules. On the basis of high catalytic activities of hemin/MoS2-NSs, a novel H2O2 sensor was fabricated.Part 1:The Prepartaion and Characterizations of Few-Layer MoS2 Nanosheets. MoS2-NSs were obtained from sonication-assisted exfoliation of bulk MoS2 in surfactant aqueous solutions. The washing process was carried out through centrifugation and re-dispersion. Furthermore, the as-exfoliated MoS2-NSs were dispersed in pure water for the preparation of aqueous solutions. The characterizations of MoS2-NSs were conducted using atomic force microscopy (AFM), Scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis absorption spectrum, X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), Raman spectrum and photoluminescence (PL) spectrum.Part 2:Electrochemically Induced Fenton Reaction of Few-Layer MoS2 Nanosheets:Efficient and Controllable Preparation of Luminescent Quantum Dots via Transition of Nanoporous Morphology. Electrochemically induced Fenton (electro-Fenton) reaction have.been used to the efficient and controllable preparation of hydroxyl radicals, leading to the generation of graphene quantum dots (GQDs) through etching of graphene oxide (GO) sheets. Furthermore, the experiment of hydroxyl radicals being reacted with few-layer MoS2-NSs, prepared from the available sonication-assisted exfoliation of MoS2 crystals, has been conducted using electro-Fenton process and the observable morphologic changes of MoS2-NSs are monitored using transmission electron microscopy, showing that the etching of MoS2-NSs induced by hydroxyl radicals results in rapid homogeneous fracturing of the sheets into small dots via a transition of nanoporous morphology, which are demonstrated to be zero-dimensional MoS2 quantum dots (MoS2-QDs) with the vertical dimensional thickness of ca.0.7 nm and plane size of ca.5 mn. The photoluminescent properties of as-generated MoS2-QDs are explored based on the quantum confinement, edge effect and their intrinsic properties. The degree of etching and the concomitant porosity can be conveniently tuned by electro-Fenton reaction time and a new morphology, nanoporous MoS2-NSs, is obtained, raising new issues concerning MoS2-NSs applications in various significant areas.Part 3:Size-Dependent Optical Absorption of Layered MoS2 and DNA Oligonucleotides Induced Dispersion Behavior for Label-Free Detection of Single-Nucleotide Polymorphism. Size-dependent optical absorption of semiconductive (2H) layered MoS2, exhibiting great discrimination abilities to single-and double-stranded DNA, is studied. In the presence of high concentration of salt, layered MoS2 trends to aggregate rapidly, leading to the increases of sizes in both vertical and lateral dimensions of the nanosheets, which results from the interplay between van der Waals attraction and electrical double-layer repulsion. Meanwhile, the aggregation behavior of layered MoS2 is remarkably inhibited by the synergistic effects of DNA oligonucleotides. Single-stranded DNA (ssDNA) can adsorb on the surface of layered MoS2, resulting in a great dispersion, even in the presence of high concentration of salt, while the dispersion behavior is weakened when ssDNA is replaced by double-stranded DNA. Whereas compared to graphene with zero band-gap energy, layered MoS2, with semiconductive properties, exhibits great characteristic optical absorption in visible wavelength region devoted to exploring the aggregation behavior of layered MoS2. Therefore, DNA oligonucleotides induced size-control of layered MoS2, contributing to the regular change of its characteristic absorption in visible region, was considered as a label-free bioassay for the detection of single-nucleotide polymorphism. Due to its easy operation and high specificity, it is expected that the proposed assay holds great promise for further applications.Part 4:Hemin Functionalized MoS2 Nanosheets:Enhanced Peroxidase-Like Catalytic Activity with a Steady State in Aqueous Solution. Hemin functionalized MoS2 nanosheets (hemin/MoS2-NSs) were firstly obtained via the van der Waals interactions between few-layer MoS2-NSs and hemin molecules. It was demonstrated that few-layer MoS2-NSs prepared from sonication induced exfoliation of bulk MoS2 crystals in aqueous surfactant solution exhibited intrinsic peroxidase-like activity to the oxidation of 3,3’,5,5’-tetramethylbenzidine (TMB) in the presence of H2O2, which was further improved by the functionalization of hemin. Significantly, MoS2-NSs were presented as a new support of hemin, and when compared to MoS2-NSs, hemin/MoS2-NSs exhibited better dispersity in aqueous solution, which was used to the development of H2O2 sensor based on the enhanced peroxidase-like activity.