Preparation Of Two New Surface - Enhanced Raman Scattering (SERS) Substrates And Their Enhancement Effects

Surface enhanced Raman scattering (SERS) technique overcomes the disadvantages of conventional Raman spectroscopy and has been regarded as one of the most promising analytical techniques owing to its high sensitivity and selectivity. Since SERS-active substrates have a great influence on the intensity of SERS signals, many efforts have been made to fabricate more effective substrates. The development of SERS-active substrates is of great significance, which could not only promote the study of SERS theory, but also widen the application of SERS technique. At present, quite a lot of SERS substrates have been reported, but it is still necessary to further improve the sensitivity, stability and reproducibility of SERS substrates. Moreover, many fabrication methods of SERS substrates are costly, time-cosuming or require specialized instruments, which limit the practical application of SERS. In this paper, we fabricated two kinds of highly sensitive and stable SERS substrates by simple and cost-effective methods, which is capable for the detection of crystal violet (CV) at low concentrations. Our study is outlined as follows:1. The fabrication and SERS performance of PVA-GNPs substrate. We use polyvinyl alcohol (PVA), gold nanoparticles (GNPs) and micron-sized NaCl particles (pore-forming agent) et al. to prepared3D porous PVA dried gel with GNPs embedded in the network (PVA-GNPs) by a series of processes including gelation, dialysis and freeze-drying. The structure and morphology of obtained PVA-GNPs substrate were characterized by SEM, TEM and UV-Vis. To evaluate the SERS performance of PVA-GNPs substrate, CV was used as the probe molecule. It turns out that PVA-GNPs substrate has ultrahigh sensitivity since the detection limit of CV reaches as low as10-12M. And the substrate is highly stable because PVA-GNPs still have SERS activity after being stored in air for3months or even10months. Additionally, the effect of3D porous structure, pore-forming agent and GNPs concentration on SERS performance were investigated. The results indicate that PVA-GNPs films and PVA-GNPs gel without pore-forming agent have much higher detection limits of CV (10-7M and10-8M, respectively) and the increase of GNPs concentration will not lower the detection limit of CV. Therefore we believe that3D porous structure (especially the micro-pores caused by pore-forming agent) is responsible for the ultrahigh SERS sensitivity. As for the good stability, we attribute it to the fact that most of the GNPs are embedded in the network of the gel. 2. The fabrication and SERS performance of AuNPs/AgNPs decorated silica spheres. Four kinds of core-shell SERS substrates (HSS-AuNPs, PS@SiO2-AuNPs, HSS-AgNPs, PS@SiO2-AgNPs) can be made with hollow silica spheres (HSS) or solid silica spheres (PS@SiO2) as the core and citrate-stabilized gold nanoparticles (AuNPs) or silver nanoparticles (AgNPs) as the shell in the present of (3-Mercaptopropyl) trimethoxysilane. Their morphology and structure were carefully characterized by SEM and TEM. And CV was used as the probe to investigate the SERS performance of these four substrates. We found that the detection limits of CV on HSS-AuNPs, PS@SiO2-AuNPs, HSS-AgNPs and PS@SiO2-AgNPs are10-10M,10-9M,10-11M and10-11M, respectively. The high sensitivity of these substrates should be mainly related to the existence of lots of "hot spots" in which enormous electromagnetic enhancements often arise. Comparing the effect of the hollow structure and solid structure on SERS performance, we have discovered that the Raman intensities of hollow systems are always slightly higher. We suspect that hollow structure should have enhanced light interaction and be more flexible to form more "hot spots", leading to the better SERS signals. And comparing the SERS activity of AuNPs and AgNPs, we found that the detection limit of CV in AgNPs system is1-2magnitudes lower than that in AuNPs systems. It is quite reasonable since silver has better SERS performance than gold and the average diameter of AgNPs is larger than AuNPs in the present study. The reproducibility and stability of these four SERS substrates were also investigated. Most of RSD (Relative Standard Deviation) values of signal intensities of major SERS peaks are observed to range between0.1and0.3, revealing a good reproducibility of Raman signals. And all of the substrates can achieve distinctive SERS signals of CV only with a little decrease of the Raman intensity after being stored in the ambient environment for3months. Thus these substrates are relatively stable.