SYNYTHESIS AND MODIFICATIONS OF 2D NANOMATERIALS FOR SENSING APPLICATIONS

Abstract

Unique surface features and physicochemical properties possessed by 2 dimensional (2D) nanomaterials have made them attract tremendous recognition for a vast multitude of applications. 2D nanomaterials like transition metal dichalcogenides (TMDs), black phosphorus, transition metal carbides, nitrides, etc are being widely explored in the field of sensing. Particularly, the large surface area makes 2D nanomaterials ideal platforms for loading more and more entities for sensitive detection of the target analyte. Amongst various 2D nanomaterials, molybdenum disulfide (MoS2) has displayed a substantial potential for further exploration owing to its vast number of intriguing properties. To name a few, large surface area, good absorption coefficient, high heterogeneous electron transfer rate, chemical stability, and biocompatibility makes it an ideal candidate for fabricating a sensing platform. Further, the synergistic effect of MoS2 offers a strategic and rational approach, to overcome its intrinsic shortcomings and modify it in innumerable ways. In this work, we have synthesized MoS2 nanostructures using a simple and cost effective hydrothermal method with a large yield and reproducibility. For harnessing the potential of synthesized nanostructures for the welfare of society, we employed them in the fabrication of biosensors. Biosensors are analytical devices that integrate a biological element, transducer, and immobilization matrix. Different types of biological elements and transducing mechanisms can be utilized in the construction of biosensors. Based on careful consideration of various types, we finalized antibodies as the biological element and the electrochemical method as the transducing mechanism in our case. Therefore, in the present study, we have kept our focus on developing a stable and low-cost immobilization matrix, which is Ph.D. Thesis (Ritika Khatri) ix biocompatible, reproducible, and favors maximum immobilization of chosen biological elements i.e. antibodies. The biological element can be immobilized on the immobilization matrix using physical or chemical techniques. But covalent binding which is a chemical method is more favorable when testings are done in buffer solutions and repeated measurements are to be recorded. Therefore, we modified MoS2 nanostructures using reduced graphene oxide (rGO) and chitosan (CS), to anchor functional groups on the matrix for covalent attachment of antibodies on the immobilization matrix. Cancer is a major threat to the socio-economic development of society pertaining to the rising incidence of cases of different types of cancers in human beings. Amongst various types, lung cancer tops the list of mortalities caused due to cancer. The survival rate after five years of its diagnosis is quite poor, and the weak general health condition of people in developing nations makes the situation even more gruesome in countries like India. Lung cancer is silent at its initial stage and symptoms like shortness of breath and coughing are highly non-specific. Conventional techniques used for detecting lung cancer are time-consuming, expensive, cumbersome, and require trained personnel for operation. Biomarker-based detection of diseases has become quite popular these days since tumor growth directly changes the concentration of proteins. Therefore, biomarkers can serve as an efficient indicator for the rapid, sensitive, and specific diagnosis of a disease. Neuron-specific enolase (NSE) is a biomarker released in high concentration in the case of a person suffering from small cell lung cancer (SCLC) and is highly specific for SCLC. Therefore, we have used NSE as the target analyte for testing the performance of our biosensor. The findings of the research work conducted for achieving the above-mentioned objectives have been divided into 6 chapters. Chapter 1 begins with a brief introduction to nanoscience and nanotechnology, gradually followed by intriguing properties of 2D Ph.D. Thesis (Ritika Khatri) x nanomaterials, a discussion about biosensors and their components, choice of MoS2 as an immobilization matrix, based on a comprehensive literature review. The importance of rapid, specific, and sensitive detection of lung cancer and the choice of NSE as a target biomarker is also emphasized. The chapter then elucidates the motivation to pursue this work and the objectives of the present thesis work. Chapter 2 describes in detail the methods chosen for the synthesis of MoS2 and modified nanostructures and the technique used for film formation. This is succeeded by a discussion on all the characterization techniques which have been used in the present thesis work for confirmation of the formation of desired nanostructures and testing the biosensing performance of the fabricated platform. Chapter 3 examines the dependence of electrochemical performance on the morphology of synthesized MoS2 nanostructures. Chapter 4 and Chapter 5 present the results of studies related to the modification of synthesized MoS2 nanostructures using rGO and CS respectively. The two matrices are analyzed using various characterization tools and then the biosensing performance of the two platforms is also reported for detection of NSE. Chapter 6 summarizes the results of all the above chapters and concludes that the surface chemistry of modified MoS2 nanostructures plays a huge role in determining the analytical performance of the biosensor. This chapter winds up with prospects of this research work.