Mechanism Of Carboxyl-stabilized Metal Nanoclusters And Biological Application

Metal nanoclusters(MNCs)are a class of luminescent molecule-level aggregates with size between 0.2 and 2-3 nm,which corresponds to the range from few atoms to a couple of hundred atoms.Compared with bulk metals,MNCs with size approaches to the Fermi wavelength(-0.5 nm for gold and silver),the band structure becomes discontinuous and is broken into discrete energy levels.Therefore these MNCs are said to have molecule-like properties and exhibit specific absorption and photoluminescence characteristics,bridge the gap between single metal atoms and bulk metal nanoparticles.Quantum-confinement effect makes their emission wavelength could be adjusted from visible to near-infrared.Compared with traditional luminescent materials(such as organic dye and semiconductor),MNCs exhibit excellent luminescence properties(such as good photostability,large Stokes' shift and good biocompatibility).Consequently,they have been widely used in bioimaging,sensing,biolabeling,biology and medical science.Studies on the MNCs mainly focus on the design of efficient synthesis strategy and practical bioapplication.Until now,investigations about the luminescence mechanism of MNCs are still very limited,but it is imperative to clarify the photoemission mechanism for the improvement of luminous efficacy and their applications.The existing understanding of the luminescencemechanism of MNCs excessively emphasizes the role of the metal core,while the electronic and chemical structures of the ligands on the clusters' surface are underestimated in most cases.In this study,Carboxyl-Stabilized Metal Nanoclusters with different luminescence properties were successfully synthesized using photo/chemical reduction method.The structure and properties of these MNCs were characterized by UV-Vis absorption spectrum,fluorescence spectrum,X-ray photoelectron spectroscopy(XPS),high-resolution transmission electron microscopy(HRTEM),time correlated single photo counting(TCSPC),Fourier Transform infrared spectroscopy(FTIR)and dynamic light scattering(DLS).The effects of ligands and metal ions on the luminescence properties of clusters were investigated and the luminescence mechanism of MNCs was delicately discussed.The main research contents include:1.PMAA-Ag NCs were successfully synthesized using Poly(methacrylic acid)(PMAA)with carboxylate ligand as the template under photoreduction.Molar ratio of reactants,pH and photoreduction time were regulated to preparation MNCs with different photo properties.The effects of surface ligands and metal ions on the luminescence were elaborately discussed and ligand to metal charge transfer(LMMCT)was proposed to explain the origin of luminescence.2.PMVEM-Ag NCs,MEA-Ag NCs and GSH-Ag NCs were prepared using Poly(methyl vinyl ether-alt-maleic acid)(PMVEMA),mercapto-ethylamine(MEA)and glutathione(GSH)as the protective agents under Ligand-Induced Etching method.This study mainly focuses on the effect of electron and chemical structure of the surface ligands.An aggregation-induced emission luminescent model was proposed.3.DPA-Ag NCs,H2MSA-Ag NCs and MEA-Ag NCs were synthesized using D-Penicillamine(DPA),Mercaptosuccinic acid(H2MSA)and mercapto-ethylamine(MEA)as the etching agent under Ligand-Induced Etching method.Metal-centered Luminescence and ligand dependent luminescence were simultaneously observed in as synthesized luminescent metal nanocluster.The mechanism of pH stimulated Aggregation-induced emission was discussed and excited state proton transfer(ESPT)process was observed in the luminescence system.4.An effective dual-emission luminescent Ag NCs-based probe has been constructed for rapid and selective detection of Hcy based on the specific association between thiol group and Ag+ which could cause the quenching of luminescence.The detection limit can reach to 1 ?M.