Preparation Of Silica-coated Metal Nanoclusters And Their Application

A new class of metal nanomaterial has emerged based on the possibility of finding the missing link between atoms and metal nanoparticles,which are known as metal nanoclusters.The metal nanoclusters are usually composed of several to dozens of metal atoms with sizes smaller than 3 nm and thus comparable to the Fermi wavelength of electrons.Due to their special electronic structure,metal nanoclusters present a series of unique molecular-like properties such as high surface energy,ultra-small size,high specific surface area and good chemical activity.However,environmental factors such as ionic strength,p H level,and temperature have significant impacts on their fluorescence properties.Incorporation of the metal nanoclusters into silica matrix by conventional St?ber method has been developed as an effective way to improve their stability against external stimulus.Silica coated metal nanoclusters have been widely used for environmental detection,biomarkers,food analysis,pharmaceutical industry and so on.Recently,a covalent coupling approach was developed to improve the incorporation of nanoclusters into silica matrix.A silane coupling agent was used to form a precursor with metal nanoclusters,and then the precursor was incorporated into the silica particles upon the pre-hydrolyzed and co-condensation of silane with tetraethoxysilane(TEOS).The approach usually leads to the change of optical properties of nanoclusters and relatively low incorporation efficiency.Electrostatic adsorption method is to incorporate positive charged metal nanoclusters in the negative charged silica.However,direct incorporation of the thiolate or protein capped metal nanoclusters into silica matrix usually suffer the low efficiency since both nanoclusters and silica species from the hydrolysis and condensation of TEOS are negatively charged in the St?ber system,resulting in the formation of nanoclusters doped silica particles with low brightness.Here,we reported a non-covalent approach for efficient incorporation of metal nanoclusters(gold nanoclusters and copper nanoclusters)into silica particles with the assistance of a cationic polyelectrolyte,polydiallyldimethylammoniumchloride(PDDA).In this approach,the negatively charged gold nanoclusters(Au NCs)were firstly mixed with the positively charged PDDA to form PDDA–Au NC complexes.Then,the complexes were added into a prehydrolyzed St?ber system to get the Au NCs-doped silica particles.Furthermore,a silver nanoparticle was introduced as a core to improve the fluorescence efficacy of the Au NCs through metal-enhanced fluorescence.Finally,fluorescein isothiocyanate(FITC)was conjugated on the surface of Au NCs@silica particles to fabricate a ratiometric p H indicator.The details were stated as follows:1.Polyelectrolyte-assisted preparation of gold nanoclusters-doped silica particles with high incorporation efficiency and improved stability.Glutathione-capped gold nanoclusters(GSH-Au NCs)were synthesized with 2 nm of diameter and emission wavelength at 610 nm.The negatively charged GSH-Au NCs and the positively charged PDDA were mixed together to form the PDDA–Au NCs complexes through non-covalent electrostatic interaction,which had little effect on both absorption and emission properties of the original Au NCs.In the complexes,surface charge of the Au NCs was neutralized and even reversed by increasing the amount of PDDA.After being transferred into a prehydrolyzed St?ber system,the PDDA–Au NC complexes were incorporated into silica matrix effectively attributed to the suppressed electrostatic repulsion between the complexes and the silica matrix.The Au NCs were distributed homogeneously in the silica matrix with the incorporation efficiency as high as 88% by optimizing the ratio of Au NCs and PDDA.Moreover,the stability of GSH-Au NCs against p H and heavy metal ions is improved significantly due to the protection effect of PDDA and silica.This approach was also extendable to highly efficient incorporation of other negatively charged metal nanoclusters,such as bovine serum albumin-capped Cu nanoclusters,into silica matrix.Such luminescent metal cluster-doped silica particles with high incorporation efficiency and improved stability are expected to be potentially useful in the fields of biolabeling and bioimaging under complex and varying environments.2.Preparations of silver nanoparticles @ silica @ gold nanoclusters composite particle with enhanced fluorescence efficacy.In order to improve the fluorescent efficacy of Au NCs@Si O2,Ag nanoparticles were introduced into the complexes as a core to enhance the fluorescence of gold nanoclusters through metal-enhanced fluorescence.Firstly,the spherical silver nanoparticles(Ag NPs)with a uniform size of 40 nm were synthesized,and silica were coated on the surface of Ag NPs with various thickness by St?ber method.Then PDDA-Au NCs complexes were introduced to the St?ber system,resulting in Ag NPs@Si O2@Au NCs complexs.The distances between gold nanoclusters and silver nanoparticles(thickness of silica shell)played an important role in the optical property of gold nanoclusters in the complexes,mainly manifested as follows:(1)The absorption peak of silver nanoparticles in the complex had an obvious red shift with the increase of thickness of silica shell.(2)The fluorescence intensity of gold nanoclusters increased and then decreased with the increase of silica shell thickness.(3)The fluorescence lifetime of gold nanoclusters reduced and then increased with the increase of silica shell thickness,and it was shorter than their innate fluorescence lifetime.All the results showed that local electromagnetic field enhancement of silver nanoparticles plasma resulted in the increase of attenuation rate constant,and finally enhanced the fluorescence of gold nanoclusters.When the silica shell thickness was 13 nm,the fluorescence intensity of gold nanoclusters could improve 12.7 times while its fluorescence lifetime is 6.47 ?s.3.The preparation and application of gold nanoclusters @ silica @ FITC complexes.Recently,ratiometric fluorescent p H indicators have been developed to visualized detect p H value of an analyte.To fabricate a ratiometric p H indicator,both p H sensitive dyes and reference dyes were incorporated into a matrix,and the ratio in fluorescence intensity of both dyes indicated the p H value.FITC,an organic dye,were covalently conjugated to the surface of Au NCs@Si O2 particles by a silane coupling agent in the St?ber system,where FITC were p H sensitive dyes and gold nanoclusters were reference dyes.The green FITC and orange Au NCs could be synchronously excited by a single excitation wavelength since there was large overlapping region in their absorption spectra.Under excitation of a 365 nm UV lamp,Au NCs@Si O2@FITC particles showed visualized color changes from red to orange-green when p H value of the buffer solutions changed from 4 to 10.