Improvement Of Optical Properties And Applications Of Metal Nanoclusters Based On Synergistic Effect

Metal nanoclusters?NCs?,an emerging nanomaterials,have been widely employed in bioimaging,chemical sensing and energy catalysis owing to their unique physical,chemical and optical properties,such as photoluminescence,quantum size effect and good biocompatibility.However,the prepared NCs at present exhibit one fluorescence emission and the wavelength is not adjustable.There are some changles of NCs such as low fluorescence intensity and poor stability,especially for the CuNCs which are easily oxidized by air.The aim of the paper is to synthesis NCs with excellent optical properties by the simple and green methods that are based on the synergistic effect between metal elements and different materials.The specific work includes the following three parts:?1?The red-AuNCs with red-fluorescent emission were prepared with?-lactoglobulin??-Lg?serving as the capping and reducing agent at 37°C.The emission wavelength blue shifted to yellow?yellow-AuAgNCs@?-Lg?when AgNO₃ solution was added to be reduced simultaneously.And emission changed to orange?orange-AuAgNCs@?-Lg-Ag⁺?with a“Ag⁺shell”when more AgNO₃ was introduced.Transmission electron microscopy?TEM?showed that the average particle sizes of NCs with different emission wavelengths were 2.08,1.90 and2.20 nm,and the quantum yields?QY?were 0.31%,1.15%and 0.43%,respectively.It indicated that the introduction of Ag element can not only adjust the emission wavelength,but also improve QY to a certain extent.In addition,yellow-AuAgNCs@?-Lg and orange-AuAgNCs@?-Lg-Ag⁺can be used for the detection of"turn on"and"turn off"of cysteine?Cys?solution,respectively.The detection limits were 166?mol/L and 80?mol/L.?2?The metal-organic frameworks?MOF-5?were introduced when the preparation of CuNCs protected by glutathione?GSH?at room temperature.The composites CuNCs@GSH/MOF-5 were produced by a facile one-pot synthesis and exhibited strong orange fluorescence.The fluorescence spectra showed that the excitation wavelength of the composites is the same as that of CuNCs@GSH,but the emission wavelength was blue shifted by 20 nm.X-ray photoelectron spectroscopy?XPS?proved that CuNCs@GSH could be successfully synthesized in MOF-5.The fluorescence intensity of CuNCs@GSH was enhanced about 35-fold owing to the confining scaffold of the MOF-5 and the stability was extended from 3 days to 3 months.Interestingly,emissions could change between blue,orange and red as they were partially reversible in different pH environments.?3?In order to accelerate the reaction rate,chitosan?CS?was introduced as a microreactor in the synthesis of CuNCs@GSH.The composites of CuNCs@GSH/CS with yellow emission were prepared at 40°C by one-step process.The fluorescence intensity of the composites was about 65 times higher than that of CuNCs@GSH,and its stability was maintained for at least 5months.Solvent evaporation in the reaction process would induce self-assembly of CuNCs@GSH and accelerated the reaction rate.The reaction could be completed in 4 hours.TEM and scanning electron microscopy?SEM?showed that CuNCs@GSH was mainly distributed on the surface of CS film,so the fluorescence intensity almost disappeared after repeated three times.The fluorescence intensity of the composites was less affected at 40°C,but decreased significantly at 80°C.