Photoluminescence in atomically precise ligand protected metal nanoclusters : elucidating mechanism and applications in sensing and bio-imaging

The thesis, entitled as “Photoluminescence in Atomically Precise Ligand Protected Metal Nanoclusters: Elucidating Mechanism and Applications in Sensing and Bio-Imaging” investigates, through the example of atomically precise metal nanoclusters, the photophysical properties, especially photoluminescence, and the correlation between their structures and properties, a fundamental prerequisite for their potential bio-applications. Positioned between organometallic complexes and plasmonic nanoparticles, atomically precise metal nanoclusters exhibit discrete electronic structures, ultrasmall size, clear structure, and tunable optical properties, in particular photoluminescence, both in linear and nonlinear optical regimes, making them attractive candidates for applications bioimaging and phototherapy. The main objective of this work is to understand how composition, ligand environment, and biomolecular protection influence both radiative (luminescence) and non-radiative photophysical processes (light-to-heat conversion and singlet oxygen generation), and how these properties can be exploited for optical and biomedical applications. The objectives and results are as follows: Chapter 1 introduces atomically precise ligand-protected metal nanoclusters and present a literature review on their synthesis, structural characterization, photophysical properties, and their bioapplications. Particular attention is devoted to introduce radiative and non-radiative photophysical processes explored in the next chapters. Chapter 2 describes the experimental techniques and optical setups employed throughout this work, including mass spectrometry, steady-state and time-resolved spectroscopy, transient absorption, nonlinear optical characterization, multiphoton microscopy, and photothermal and photodynamic measurements. Chapter 3 investigates the photoluminescence mechanisms of protein-directed metal nanoclusters. The relationship between structure and luminescence is explored in different protein-metal systems, indicating the possible luminescence origin. Chapter 4 focuses on two-photon excited photoluminescence modulation in atomically precise nanoclusters. Through ligand engineering approaches, the relationship between ligand-shell rigidity and nonlinear optical response is established. Chapter 5 explores non-radiative photophysical processes, including photothermal conversion and singlet oxygen generation in atomically precise metal nanoclusters. These studies reveal how size, composition, and ligand effects govern energy dissipation pathways. Finally, Chapter 6 demonstrates the bioapplication potential of metal nanoclusters through reactive oxygen species-mediated antibacterial Ag nanoclusters and BSA-protected cadmium nanoclusters for two-photon intracellular pH imaging. Overall, this thesis establishes strong structure–property relationships linking atomically precise structures, photophysical processes, nonlinear optical properties, and biofunctionalities in metal nanoclusters. Beyond fundamental understanding, this work highlights the potential of atomically precise metal nanoclusters as multifunctional optical nanomaterials for future applications in imaging, sensing, nonlinear optics, and phototherapy.