Metal-enhanced Fluorescence From Near-Infrared Ag₂Se Quantum Dots

Quantum dots(QDs)have gained considerable attention due to their excellent optical properties such as high brightness,high stability,broad excitation spectra,tunable and narrow emission spectra.In particular,in the near-infrared(NIR)region(650-1450),fluorescence could penetrate deep into the body and be less absorbed and scattered by the blood and tissues,so NIR fluorescent QDs is favorable for in vivo imaging with high signal-to-noise ratios.However,NIR fluorescent QDs especially silver chalcogenide(Ag2X,X=S,Se,Te)QDs generally have relatively weak fluorescence and poor stability,which limits their further applications.Therefore,it is urgent to investigate NIR fluorescent nanomaterials with high brightness,excellent photostability and good biocompatibility.Surface plasmons on metallic nanostructures have been found to modify favorably the brightness and photostability of the nearby QDs,which is termed as metal-enhanced fluorescence(MEF),Hence,amplification of light from NIR QDs by coupling to metallic nanostructures is a highly desirable strategy for dramatically improving detection sensitivity,and can open up new routes for high-performance diagnostic devices and photonics.Herein,we have focused on the enhancement of fluorescence from Ag2Se QDs.At first,the ultrasmall near-infrared fluorescent Ag2Se QDs were prepared by quasi-biosynthesis and the fluorescence spectrum exhibited a NIR fluorescence peak at 730 nm.Then,based on MEF,a NIR fluorescent nanocomposite(AuNF@SiO2-Ag2Se core-shell nanoparticles)was designed.Gold nanoflowers(AuNFs)were synthesized by using 2-[4-(2-Hydroxyethyl)-1-piperazinyl]ethanesulfonic acid(HEPES),as a reducing and particle stabilizing agent.Next,AuNFs were coated with silica shell by modified Stober method,and the thickness of silica shell can vary from 5 nm to 30 nm.Then Ag2Se QDs were covalently linked with AuNF@SiO2.Finally,2-folds fluorescence enhancement of Ag2Se QDs was achieved with the optimal distance of 25.9 nm.Besides,the mechanism of fluorescence enhancement was explored.In order to improve the load efficiency of Ag2Se QDs,a three-layer core-shell nanostructure(AuNF@SiO2@Ag2Se)consisting of a gold nanoflower core,a silica spacer and an outer layer of Ag2Se-doped silica was prepared.The ultrabright AuNF@SiO2@Ag2Se nanocomposites possess good stability and exhibit low cytotoxicity,implementing the fluorescence-converging of Ag2Se QDs.Moreover,immunomagnetic nanobeads(IMNs)and streptavidin-conjugated AuNF@SiO2@Ag2Se nanocomposites were simultaneously employed to capture the target virus and selectively binding to the available biotin-sites on biotinylated antibody.A rapid quantitative assay based on fluorescence sandwich immunoassay was proposed for the detection of EV71 virions.