SYNTHESIS AND INVESTIGATION OF TRANSITION METAL DICHALCOGENIDES: NANOSTRUCTURES AND THEIR APPLICATIONS

Abstract

Transition-metal dichalcogenide (TMD) quantum dots (QDs) represent a rapidly advancing class of nanomaterials distinguished by tunable photoluminescence (PL), high surface reactivity, chemical stability, and biocompatibility, yet achieving water- stable QDs with strong emission remains challenging. This thesis systematically examines the hydrothermal synthesis of WS2 and MoSe2 QDs and correlates their physicochemical properties with optical behavior to develop high-performance fluorescent probes for detecting industrial, environmental, and pharmaceutical contaminants. Alkaline-synthesized WS2 QDs (pH ≈ 11) resolved the instability associated with acidic exfoliation, producing highly dispersible, blue-emitting QDs with multiexponential lifetimes and strong structural stability. These QDs were comprehensively characterized using XRD, FTIR and HR-TEM, while their optical responses were evaluated through steady-state and time-resolved spectroscopy, collectively confirming their crystalline integrity, defect-mediated emissive states and robust aqueous stability. The WS2 QDs demonstrated exceptional sensitivity toward hydrogen peroxide (0.33 nM–594 μM, LoD 1.7×10-6 M) through reversible redox cycling between W(IV) and W(VI), enabling nearly complete signal recovery across multiple cycles. Complementarily, hydrothermally prepared MoSe₂ QDs characterized by strong 260 nm absorption and blue excitation-dependent emission served as highly sensitive probes for 2,4,6-trinitrophenol (TNP), showing linear Stern–Volmer behaviour (3.3–99 nM) and an LoD of 1.43 nM dominated by the inner filter effect. Together, these findings highlight the tunability and responsiveness of TMD QDs toward oxidative and nitroaromatic pollutants. Further extending their sensing versatility, WS₂ QDs were applied for detecting heavy-metal ions and pharmaceutical contaminants. For Cr6+, the QDs exhibited strong and reversible PL quenching driven primarily by static surface complexation, supported by XPS evidence of partial oxidation of W and S atoms. The platform delivered ultralow detection limits (39.5–154 pM across different water matrices), a broad linear range (0.66–660 nM), and excellent recyclability over more than 18 redox cycles, establishing it as one of the most sensitive WS2-based Cr6+ sensors to date. WS2 QDs also enabled sensitive, label-free detection of Vitamin B12 via a static-dominated ix ANEESHA mechanism (0.33–565 nM range, 555 pM LoD), and amoxicillin via combined static– dynamic quenching supported by FTIR-verified adsorption and minor lifetime changes (0.33–607 nM, 1.24 nM LoD). Collectively, this research demonstrates that hydrothermally synthesized WS2 and MoSe2 QDs form a robust, regenerable, and environmentally benign sensing platform capable of detecting oxidants, metals, nitroaromatics, vitamins, and antibiotics. The study provides crucial insight into structure, property and function relationships in TMD QDs, and establishes their strong potential for real-sample monitoring, multimodal sensing, catalytic remediation, and emerging biophotonic applications.