Shell-Isolated Ag Nanoparticles Constructed Hotspot Controllable SERS Substrate For Quantitative Analysis

Raman spectroscopy can provide fingerprint spectra information of molecular vibration and rotation model.But because of the tiny Raman scattering cross section of the molecular,the weak Raman scattering response has hindered the development of the Raman for a long time.Until 1974,the appearance of Surface Enhanced Raman Scattering(SERS)brought the dawn of practical application of Raman spectroscopy.However,only coin metals like Au,Ag,Cu with nano-scale roughness have great Raman enhancement,that extremely limited the application of SERS technology because of bad universality and morphology to the material surface.Shell-isolated nanoparticles enhanced Raman spectroscopy(SHINERS)was developed by our group in 2010.SHINERS can be applied on any kind of substrate,further promoted the application of Raman spectroscopy in many fields such as catalysis,electrochemistry,environmental chemistry,analytical chemistry,biomedical and others.But quantitative analysis is always a huge challenge for SERS.The electromagnetic(EM)enhancement and chemical enhancement(CE)combined together lead to the SERS enhancement.The electromagnetic(EM)mechanism describes the enhanced local electromagnetic fields due to resonant excitation of plasma oscillations(plasmons)in the metallic nanostructure.The enhancement region located between nanoparticles called "hotspot".The energy and Raman cross-section of molecular absorbed on metal surface were changed,lead to arise a chemical enhancement.To achieve the application of SERS in the field of quantitative analysis,how to construct a uniform SERS substrate is a key step in solving SERS quantitative analysis.For this purpose,following works had been done to constructs uniform SERS substrate for quantitative analysis:1.Ag nanoparticles with great SPR properties,is a fascinating substrate for SERS.However,the SERS enhancement of Ag nanoparticles usually reduced by the oxidation of Ag metal and short storage,which extremely limited its SERS substrate application.SiO2 shell isolated Ag nanoparticles can improve this problem.SiO2 shell prevent Ag from being oxide,guarantee the enhancement and extend the storage time.In order to find the best match between nano size of the nanoparticles and excitation 638 nm and 785 nm laser,50 nm,100 nm,120 nm and 150 nm shell isolated Ag nanoparticles were synthesized.4-Mercaptopyridine was used as probing molecular to find out the relationship between shell thickness and SERS enhancement.The result showed the enhancement reduced with the increase of the thickness of the shell for both 638 nm and 785 nm exciting laser.And the reducing speed of 1-4 nm shell isolate nanoparticles was much more rapidly than that with 4-9 nm shell.Furthermore,100 nm Ag nanoparticles have the best enhancement under 638 nm laser,and 120 nm Ag nanoparticles for 785 nm laser.But there is only little different of enhancement between 100 nm and 120 nm Ag nanoparticles under 638 nm laser,the same for 120 nm and 150 nm Ag NPs under 785 nm laser.Furthermore,for synthesis of the shell isolated particles,it becomes harder with the increasing of the nanoparticle's curvature.As we know the smaller of the sphere the bigger of the curvature,so 120 nm Ag NPs and 150 nm Ag NPs were selected for 638 nm laser and 785 nm laser.2.We have solved the Ag oxidation problem by coating Ag nanoparticles with SiO2 shell.But the SiO2 shell prevent the analyte to absorb onto the metal surface that weaken the nanoparticles SERS enhancement.So,we use Ag nanoparticles coasted by SiO2 shell with pinhole to construct the SERS substrate.SiO2 shell with pinhole prevent the Ag metal oxidation to some extent,and let the molecular absorb on the metal surface.Bare Ag NPs 2D monolayer film lost SERS enhancement after 7days,but Ag SHINs' enhancements was almost no attenuation.Melamine can be quantitative detected in the concentration range from 1 ppb to 5 ppm,and its correlation coefficient is 0.994.This Ag SHINs 2D monolayer film was also used to detect melamine in milk.The results are consistent with the results of HPLC.3.Three-dimensional SERS substrate has multi-layer structure,much more density of the "hotspot".Ag SHINs sol dried on the hydrophobic Si wafer,and a kind of 3D SERS substrate with uniform "hotspot" and high SERS enhancement could be obtained.SiO2 shell protect the Ag metal,and increase the storage time.The hydrophobic Si wafer make the dried Ag SHINs to construct a uniform,reproduceable 3D "hotspot" structure,which can be applied on both SHG and SERS.This SERS substrate has high enhancement,stability and easy to fabricate.This 3D SERS substrate was used to quantitatively detect 4-mercaptobenzoic acid(4-MBA)in the wide concentration range from 10-8 M to 10-5 M.It is expected to be applied to the quantitative analysis in real systems.4.As we know,in order to obtain high enhanced Raman signal,molecular often need to absorb on the metal surface and the thinner gap between nanoparticles the better.On the contrary,in order to ensure the lifetime of the silver nanoparticles,the silica shell is indeed as thick as possible.By constructing the Ag SHINs-Au satellite structure,this contradiction is subtly resolved.This kind satellite structure was assembled by Ag SHINs and tiny Au nanoparticles.Au is much more stable and the SiO2 shell prevent contact between Au and Ag nanoparticles,thus there were no charge transfer process,and "hotspot" exited between Au and Ag NPs.Exposed gold nanoparticles can also help adsorb molecules of the analyte and increase the concentration of molecules in the "hotspot".Monodispersed and coupled status of Ag SHINs-Au satellite structure were also studied to reveal the Raman enhancement.Both the experimental results and the theoretical calculations prove that the enhancement ability of the Ag SHlNs-Au satellite structure is much stronger than that of the Ag SHINs in both monodispersed and coupled status,further expanded the application range of the SHINs.