Surface Enhanced Raman Scattering Of Several Micro/Nano Substrate Materials

Raman spectroscopy not only can provide a wealth of vibrational information of the target molecule, but also is indifferent to aqueous enviornment. Since surface-enhanced Raman scattering (SERS) effect was found, the applications of Raman spectroscopy attract more attention.Since substrates play key roles in SERS, the development of novel substrate materials is one of the important aspects in the field of Raman study. Many substrate materials involve metal nanoparticles (NPs), which can notablely enhance the Raman response. But during the use of SERS substrate materials, the substrate materials show a weak stability due to the oxidation or agglomeration of metal NPs. Meanwhile, the Brownian motion of NPs in solution cause a low reproducibility of SERS signals on these substrate materials. Under certain circumstances, some modifying additives were introduced to improve the stability of substrate materials in the preparation process. But these modifying components may significantly interfere with the Raman signals of the target molecules. Aiming at the solving of the above-mentioned problems, this dissertation focused on the SERS effect of several micro/nano SERS substrate materials. The major contents included the following aspects.(1) In Chapter1, the basic principles and related applications of Raman spectroscopy and SERS phenomenon were reviewed, followed by the reviewing of the preparations and applications of various SERS substrate materials and then the intrudcing of the research topic and contents of this dissertation.(2) In Chapter2, a new method was developed to prepare micro/nano SERS substrate materials without using any coupling-agent to improve the stability of substrate materials and eliminate the adverse effects that the modifying components may produce. Firstly, through in-situ reduction of silver nitrate without using any coupling-agent, a micro/nano-structured SERS substrate was prepared by coating silver on hollow buoyant silica microspheres as a lab on a bubble (LoB). The silver coated LoBs (LoBs@Ag) floated on the surface of a solution could provide a very convenient platform for the direct detection of target molecules in the solution. The LoBs@Ag substrate not only immobilized well-distributed Ag NPs on the surface LoBs, but excluded the interference of coupling agents. This yielded high-resolution SERS spectra with excellent reproducibility. Then, the Raman response of crystal violet on the LoBs@Ag substrate was investigated by means of SERS combined with density functional theory (DFT) calculations. The LoBs@Ag substrate exhibited a remarkable Raman enhancement effect for crystal violet with an enhancement factor of6.9×10and wide adaptability from dye, pesticide to bio-molecules. Hence, on the basis of this micro/nano substrate material LoBs@Ag. a rapidand sensitive SERS method was proposed for the detection of trace organic pollutants or bio-molecules.(3) As an analogue of graphite, graphitic carbon nitride (g-C3N4) is not only an excellent carrier of SERS substrates, but itself may function as a SERS substrate. Therefore, in Chapter3, the spectral properties of ultrathin g-C3N4nanosheets were investigated. Firstly, bulk micro-scale g-C3N4was prepared by polymerization of melamine, then ultrathin g-C3N4nanosheets with atomic layers of1,2, and4were synthesized in a well-crystallized form by controlling the intercalation time in a simple intercalation-exfoliation process. The electronic structures of the ultrathin nanosheets were captured in their Raman spectra that clearly evolved with the layer number of the superthin nanosheets for the first time. A clear correlation between the spectral properties and the layer number of the superthin nanosheets was clarified by combining Raman spectra and the first-principles calculations. The changes in the electronic structure of the nanosheets with different layers were investigated by analyzing the Raman vibrational modes of g-C3N4.(4) In Chapter4, the SERS effect of g-C3N4itself was evaluated, and and the mechanism of long-term stability of the micro-scale g-C3N4loading nano-Ag type SERS substrate material was investigated. Bulk micro-scale g-C3N4was firstly prepared with polymerization that was inrtroduced in the Chapter3; then g-C3N4/Ag micro/nano-structured SERS substrate material was fabricated by a self-assembly method. It was found that the g-C3N4/Ag substrate could provide a great number of hot spots and concentrated the analyte by the n-n stacking interaction between analyte molecules and g-C3N4, making a dramatic Raman enhancement. In particular, the g-C3N4/Ag substrate uniformly immobilized Ag NPs on the surface and edges of g-C3N4sheets by an interaction between Ag NPs and g-C3N4, leading to much improved long-term stability. This could be explained in terms of the electron-donor effect of g-C3N4, which was further confirmed by DFT calculations. The inherent Raman enhancing effect of g-C3N4itself also contributed to the total SERS responses. Due to multiple enhancement contributions, the g-C3N4/Ag substrate exhibited a strong Raman enhancement effect for with an enhancement factor of4.6×108(evaluated by using crystal violet as a probe). Similarly, using this micro/nano substrate material g-C3N4/Ag, it could be achieved a quick SERS detection of trace dyes, pesticides or bio-molecules.(5) Chapter5summarized the preparations, SERS actives and related applications of the above mentioned several micro/nano SERS substrate materials in the present dissertation, and prospected the potential applications of LoBs@Ag substrate in practical testing, g-C3N4in electronic devices and photoelectric conversion and other areas for future research.