Scanned Chemical Enhancement And Its Application In Surface-Enhanced Raman Scattering

As the explosive development of analysis and detecting technique based onsurface-enhanced Raman scattering （SERS）, the attempts to further understand theenhancement mechanism have continued and explaining this mechanism has becomean increasingly urgent problem. Both electromagnetic （EM） and chemical （CM）mechanism contribute to the huge enhancement in SERS. The former is a result ofsurface plasmon resonance in nanoparticles and independent of the molecular impact.Conversely, the latter mechanism involves a strong correlation between the Fermilevel of the substrate and the molecular energy level in a different system. Thus, it isdifficult to determine the magnitude of the CT contribution, and this mechanismremains a matter of controversy.As we all know, CM mechanism usually involves a charge-transfer between amolecule and a substrate. Herein, we proposed a CT complex system to study theSERS CM mechanism. It is benefits for understanding the effect of CT on the SERSspectrum, owing to the clearly CT model in this system. Meanwhile, we propose apH-dependent SERS enhancement experiment, which is based on the specialphotoelectrical property of the band edge position of oxide semiconductors, such asTiO₂is sensitive to the pH value. Moreover, we have study the SERS origin ofp-Aminobenzenethiol （PATP）, which is attracted considerable attention recently.Based on the experimental phenomenon, a metal ions sensing platform was developed.This thesis included the following parts:（1） SERS enhancement mechanism in charge-transfer complex system wasinvestigated. A CT complex composed of Ag/4-Mercaptophenol （MPH）/n-TiO₂wasfabricated by a self-assembly method. On the basis of the DFT calculated orbitalsdiagrams, we have identified the CT states and experimentally investigated the SERS spectra of Ag/MPH/n-TiO₂systems. It was clearly found that the developedAg/MPH/n-TiO₂system present excitation wavelengths-dependent and layernumber-dependent SERS spectra. The results indicate the following. First, when theexcitation energy is higher than the energy of the CT transition, the CT process occurs,resulting in selectively enhanced nontotally modes. Second, the CT resonance occursat higher energy in the Ag/MPH/n-TiO₂system than in the monolayer TiO₂systemowing to the blue-shift of CT states with the continuous introduction of TiO₂. Thedegree of CT was selected to study the layer number-dependent SERS spectra. Basedon the layer number-dependent SERS data, it has been inferred that the degree of CTrepresents a resonance phenomenon. The present results not only reveal the nature ofthe CT mechanism, but also provide a method to measure CT action.（2） pH-dependent SERS was investigated in Ag/MPH/7-TiO₂system. We proposea pH-dependent SERS enhancement experiment using Ag/MPH/7-TiO₂system. It isbased on the special photoelectrical property of the band edge position of oxidesemiconductors, such as TiO₂is sensitive to the pH value. It was found that increasingthe pH of the buffer solution negatively shifts the conduction band edge of TiO₂,thereby increasing the conductive band electron density at an equilibrium state. Thus,the relative band intensities of Ag/MPH/TiO₂increase in the SERS spectrum, which isattributed to the Herzberg Teller contribution that occurs via CT. This experimentdescribed here could be used to assist us in understanding the CT mechanism, therebyproviding a basic method for carrying out investigations using the SERS mechanism.Moreover, this work also opens up a novel and effective idea for dsigningbiocompatibility nanosensors for intracellular applications.（3） pH-Response mechanism of p-Aminobenzenethiol on Ag nanoparticles.p-Aminobenzenethiol （PATP） is one of the most popular molecules in the study of thechemical enhancement mechanism because of its high-quality signal and unusualperformance. The b2-type bands of PATP located at1573,1440,1391, and1142cm1have been considered as evaluation indexes of the CT contribution to SERS. However,recently questions have been raised that dispute about the origin of the b2-type bands.In these works, the pH-dependent behavior of PATP is one of the decisive factors.Herein, we have analyzed the pH-induced variations of the PATP behavior usingtwo-dimensional correlation spectroscopy. It was found that the very first step of the pH-response process involves protonation of the amine group. This protonation resultsin rearrangement of the electronic cloud of the benzene ring that subsequentlychanges the ring skeleton vibration. These results can help us to assess thepH-response mechanism and consequently understand the true origin of the b2-typeSERS bands of PATP.（4） Sensing platform based on chemical enhancement. In our experiment, wefound that the b2-type bands were selectively enhanced after the introduction of Ag⁺ions to the solution. Based on this, we present a simple and cost-effective approachto fabricate of anti-aggregated Au NPs sensing platform used for the detection ofmetal ions, which is based on PATP and bovine serum albumin （BSA）-modified AuNPs. The proposed sensing platform exhibits excellent stability even in highly ionicconditions due to its electrostatic stabilization, as well as the interaction of proteinside chains or domains. High detection rates for Ag⁺and Hg2+ions were obtained inreal water, such as tap water and seawater. Moreover, it is a universal method ofpreparing anti-aggregated sensing platform for different analyte through substitutingPATP with other native or artificial probes that selectively respond with analyte.