The PH-Dependent Excited State Dynamics And Applications Of Biomolecules-Protected Metal Nanoclusters

Biomolecule-protected luminescent metal nanoclusters are gaining attention as emerging nanomaterials because they offer the benefits of renewable raw materials,eco-friendliness,and straightforward preparation.Biomolecule-protected luminescent metal nanoclusters possess favourable features such as their tunable luminescent properties,remarkable modifiability and excellent biocompatibility,which make them promising for various applications such as fluorescence sensing,bioimaging and photocatalysis,among others.However,in practical applications,the influence of the ambient pH on the photophysical properties of metal nanoclusters largely determines their working efficiency,but the regulatory mechanisms involved have not been fully elucidated.In this thesis,the excited state dynamics and nonlinear optical properties of metal nanoclusters protected by different sizes of natural biomolecules（cysteine,glutathione and bovine serum albumin）with cysteine-metal complexes as the basic structural units have been systematically investigated by various spectroscopic techniques,and the pH-responsive mechanisms of their photophysical properties have been elucidated and their potential applications in the fields of two-photon cell imaging and photocatalysis have been further explored.The main research contents and results of the thesis are as follows:1.The pH-dependent excited state dynamics of cysteine-protected gold nanoclusters（Cys@AuNCs）has been investigated.The results showed that the pH-responsive photoluminescence of Cys@AuNCs originates from the effect of the nanoclusters aggregation state on the excited state non-radiative relaxation process.Furthermore,based on the excellent two-photon absorption capability and modifiability of Cys@AuNCs,a two-photon excited ratiometric pH fluorescent probe with was constructed.The probe exhibited～200-fold ratiometric changes in the physiological pH range,which was much higher than other reported two-photon fluorescent pH probes and more favourable for quantitative measurement of cellular pH.In addition,the unique photon-activated peroxidase property of Cys@AuNCs was identified and their pH response mechanism was investigated.pH modulates the photocatalytic activity of the nanoclusters by affecting the efficiency of photochemical reactions associated with their excited states.This part of research provides insights for the development of near-infrared excited fluorescent pH probes and photoactivated peroxidases.2.The pH response mechanism of photoluminescence of glutathione-protected gold nanoclusters（GSH@AuNCs）was investigated.It is shown that pH modifies the photoluminescence properties of GSH@AuNCs by affecting the charge transfer between the ligand and the metal and the non-radiative relaxation process of excited state.In practical applications,the regulatory mechanisms of pH and structure of nanoclusters on the photocatalytic properties were further investigated in situ using spectroscopic methods.The photocatalytic oxidation capability of GSH@AuNCs was primarily determined by the particle population in the excited triplet state,and the changes of pH and structure affected the proportion of particles population in the triplet state,which regulated the photocatalytic ability of the nanoclusters,and demonstrating that AIEE is an effective method to enhance the luminescence and photocatalytic ability of the metal nanoclusters simultaneously.Furthermore,based on the excellent two-photon absorption capability of GSH@AuNCs,its potential application in two-photon excited pH fluorescence sensing and photocatalysis was demonstrated,providing references for the design of novel near-infrared fluorescent pH probes as well as photodynamic therapeutic materials.3.The pH response mechanism of the photoluminescence of novel atomically precise cadmium nanoclusters（BSA@Cd₈NCs）protected by bovine serum albumin was investigated.In this study,fluorescent nanoclusters（BSA@Cd₈NCs）with an average of 8 Cdatoms were synthesised using BSA as a ligand and reducing agent.Experimental results showed that pH affected the Cd-S motifs on the surface of BSA@Cd₈NCs by changing the structure of BSA,which led to the quenching of the delayed fluorescence emission,thus reducing the fluorescence intensity.The two-photon absorption cross section of BSA@Cd₈NCs（～1588 GM）,which is two orders of magnitude higher than that of conventional organic dyes,was further characterised and its application in two-photon excited intracellular pH imaging was explored.This part of the research provides new perspectives for the development of novel near-infrared excited fluorescent pH probes based on low-cost non-precious metal nanoclusters.In conclusion,in this thesis,the excited state dynamics of biomolecule-protected metal nanoclusters is investigated in situ under practical application setting,deepening the understanding of the mechanism of the pH-regulated photophysical properties of metal nanoclusters.These results provide theoretical and experimental basis for the design and development of high-performance metal nanoclusters and their extended applications in bioimaging and photocatalysis.