Controllable Construction And Surface Enhanced Fluorescence Property Of Silver Nano-assembly Systems

Surface enhanced fluorescence(SEF)generated from local surface plasmon reso-nance(LSPR)of noble metal nanoparticles,is a powerful way of enhancing fluores-cence signal and improving photostability of fluorophore,which can improve the sen-sitivity and accuracy of fluorescence detection method,therefore exhibiting great ap-plication values in DNA-related detection,immunoassay,single molecule detection and resonance energy transfer technology.Noble metal nano-assembly system can make the electric field in the "hot spot" regions highly localized because of its unique coupling LSPR effect,thus greatly improving the performance of SEF.Now noble metal nano-assembly system occupies an important position in the research field of SEF spectroscopy.In the preparation of noble metal nano-assembly system,colloidal chemistry method has broken the limitation of traditional "top-down" technology such as lithography in precision and cost.Alternatively,colloidal chemistry method pro-vides an effective technical path for the design of SEF "hot spot" and the construction of new noble metal nanostructures via the "bottom-up" assembly process.So far,re-searchers have been successfully prepared metal nanoparticle monomer,and dimer,as well as irregular aggregates in sol system and applied them in the field of SEF.How-ever,for the noble metal nano-assembly system with more complex structure and morphology,the regulation of size,morphology and gap of metal nanoparticles,and the study of corresponding SEF effect are still rarely reported.In this article,Ag nanoparticles(Ag NPs)with different sizes were prepared via polyol process.Polystyrene(PS)/Ag composite microspheres,satellite-structured Ag@SiO2/Ag nanocomposite particles and PS/Ag@SiO2 composite microspheres were obtained through bottom-up hierarchical assembly of Ag NPs.On this basis,the influences of the size of Ag NPs,spacing between Ag NPs and the thickness of SiO2 shell to the optical properties and SEF performance of Ag nano-assembly system were studied.Moreover,finite difference time domain method was used to simulate and predict the SEF behavior of the Ag nano-assembly system,and thus providing re-search ideas for constructing silver Ag nano-assembly system with a higher fluores-cence enhancement factor.The main research content and conclusions of the article are as follows:(1)Preparation and characterizations of PS/Ag composite microspheresAg NPs were prepared in the ethylene glycol reaction system by liquid phase re-duction method,and then loaded onto the surface of PS microspheres to obtain PS/Ag composite microspheres by swelling-heterogeneous agglomeration process.The ef-fects of reaction conditions on the size and morphology of Ag NPs as well as the structure of PS/Ag composite microspheres were investigated.The optical properties of PS/Ag composite microspheres were also analyzed.The results show that the growth of Ag NPs follows the Oswald ripening principle,and the molecular weight and concentration of the stabilizer polyvinylpyrrolidone(PVP)have a great influence on the size of the final product.Low crosslinking degree of the PS microspheres and high concentration of Ag NPs are advantageous to the formation of Ag NPs with good uniformity and high load degree on the PS/Ag composite microspheres.High density loading of Ag NPs on PS microspheres produces plasma resonance coupling effect.(2)Preparation and characterizations of the satellite-structured Ag@SiO2/Ag nanocomposite particlesThe hydrolysis of orthosilicate(TEOS)was controlled by Stober method to form SiO2 shell on the surface of Ag NPs,and the small sized Ag NPs were loaded on the SiO2 shell by the entropy driving principle,therefore obtaining the satellite-structured Ag@SiO2/Ag composite nanoparticles with the large-sized Ag NPs as the core,SiO2 as the shell on Ag core,and the small-sized Ag NPs as satellites.The regulation method of thickness of SiO2 shell was studied,and the LSPR property,fluorescence emission intensity and fluorescence life of composite nanoparticles under different SiO2 thickness were studied.The results show that the thickness of SiO2 is a key fac-tor affecting SEF performance of Ag@SiO2/Ag composite nanoparticles.When the thickness of the SiI2 layer is 3 nm,the dye molecules out of the "hot spot" region are quenched,while the dye molecules located in the interstitial plasma have fluorescence enhancement due to the coupling LSPR effect,and the overall fluorescence enhance-ment factor is 2.31.With the increase of SiO2 thickness,the fluorescence quenching effect disappears and the fluorescence enhancement factor is further increased.When the thickness of SiO2 is 13 nm,the SEF performance of the Ag@SiO2/Ag composite nanoparticles achieves the best,and the fluorescence enhancement factor can reach 10.45.(3)Preparation and characterizations of the PS/Ag@SiO2 composite micro-spheresCore-shell Ag@SiO2 nanoparticles were successfully prepared by the combination of polyol method and stober method,and then were used as structural units to con-struct Ag@SiO2 arrays on the surface of PS microspheres,thus obtaining PS/Ag@SiO2 composite microspheres.The effects of preparation conditions on the structure,morphology and optical absorption of PS/Ag@SiO2 composite micro-spheres were studied.The stability and uniformity of PS/Ag@SiO2 composite micro-spheres were also evaluated.The relationship of structure and SEF effect of PS/Ag@SiO2 composite microspheres was discussed through the combination of ex-perimental test and computational simulation.The results show that the control of CTAB and Ag@SiO2 dosage can form dense Ag@SiO2 arrays on the surface of PS microspheres,and as-prepared PS/Ag@SiO2 composite microspheres have good ro-bustness and uniformity.The fluorescence intensity of the composite microspheres increases gradually with the increase of the content of FiTC.However,excessive FiTC can cause fluorescence self-quenching,resulting in lower fluorescence intensity.When the size of Ag core is 85 nm and the thickness of Si02 is 5 nm,PS/Ag@SiO2 composite microspheres have the best SEF performance,the fluorescence enhance-ment factor is 39.39,which is also consistent with the simulation results.(4)FDTD simulation study on the fluorescence enhancement effect of Ag nano materialsThe optical properties of Ag NPs and Ag@SiO2 nanoparticles with different typical morphologies were simulated by FDTD method,and the electric field strength simu-lation was carried out for Ag NPs@SiO2 arrays with different typical morphologies.The results show that the scattering spectrum gradually occupies a dominant position in the extinction spectrum,and the position of LSPR peak red shifts and widens as the size of spherical Ag NPs increases.The LSPR peak of the Ag nanoshell occurs red-shift due to the hybridization of the plasma.Whereas the redshift of LSPR peak for Ag nanorods is caused by the prolonged oscillation period.The increase of resonance in the Ag nanocube in multiple-mode makes the LSPR peak red shift.As the dipole moment of Ag nanotetrahedron is weakened,leading to weak scattering and weak ab-sorption of light,and thus the extinction efficiency is greatly reduced.Moreover,when the Ag NPs are coated by SiO2,the LSPR peak also occurs redshift.The field strength of the Ag nanoshell@SiO2 arrays is affected by the inner diameter and the embedded species,and the maximum fluorescence enhancement factor(EFmax)can reach 146.34.The field strength of the Ag nanorods@Si02 arrays is affected by size and long diameter ratio,EFmax can reach 2361.89.Electric field localization of Ag nanocube @SiO2 arrays and Ag nanotetrahedron @SiO2 arrays are more significant,the effective EFmax is 50.40 and 113.01,respectively.