Design And Fabrication Of Nobe Metal Micro-nano Structures And Their SERS Properties

Surface-enhanced Raman scattering (SERS) has very high sensitivity, it can supply a wealth of information about the structure and adsorption states of matter at the molecular level and has been widely concerned in the area of analytical chemistry, biomedicine, food safety monitoring and etc. The preparation of SERS substrates has been a hot topic in the field of SERS because the surface morphology and material of the substrate determine the signal strength and reproducibility of the spectrum, which has an important effect of the pratical applications of SERS detection in daily life. The main principle to prepare SERS substrates is to improve their activitiy, stability and producibility as well as to lower the cost. Chemical methods have the advantages of speediness and simpleness. A large number of metal nanoparticles could be obtained in a short time. However, the chemical synthesis processes often involve the addition of some surfactants as templates or stabilizers to induce the growth of nanoparticles. The surfactants adsorbed on the surface of the metal nanoparticles will weaken the SERS activities and interfere with the SERS analysis. Therefore, the development of a clean and green chemical synthesis method is a challenge. Physical methods are usually used to fabricate ordered metal substrates, namely direct deposition of metal layer onto templates with regular structures. SERS substrates prepared by this method have better reproducibilites and cleaner surfaces which are more conducive to the adsorption and detection of probe molecules and will reduce the signal interference. During recent years, physical methods have got rapid development. In this paper, we use both chemical and physical methods to prepare several different noble metal SERS substrates. The main innovative results are as follows:(1) A facile, efficient and green route was proposed to fabricate Au nanodendrites by reducing chloride acid with Al foil. The obtained Au dendrites have good crystallinity and high purity. Through a series of time-dependent morphological evolution experiments, the possible growth process and mechanism were proposed. The as-prepared Au dendrites exhibit excellent surface enhanced Raman scattering enhancement ability for detecting p-aminothiophenol molecules with a detection limit of10"9M. Moreover, they also display high catalytic activity through the quick reduction of p-nitrophenol by sodium borohydride.(2) Ag nanodish arrays were designed and fabricated as novel SERS substrates. The nanodishes were composed of nanorings and a film. They have a seven times SERS enhancement ability than nanorings. The electromagnetic field distributions of the two structures were simulated by three-dimensional finite-difference time-domain method. The simulation results show the electromagnetic field mainly locates at the tips of every single ring and the junctions among adjacent nanoparticles for the nanoring array. For nanodishes, an extra strong electromagnetic field appeared in the cavity and distributed uniformly. The detection limit for thiram from the nanodish array is as low as1×10-7M, which has achieved the ultra trace detection standard. The calculated relative standard deviation of the Raman signal intensity of rhodamine6G measured from nine randomly selected points is less than15%, indicating the excellent reproducibility. The nanodishes have great potential for applications in SERS detection analysis and related devices design.(3) Bimetallic structures were obtained by deposition a thin Au film onto Ag nanoshell array to improve the stability of Ag substrate. Firstly, a series of Ag nanoshells were prepared as SERS substrates by sputtering silver films on silica sphere templates with different sizes of500,440and360nm. Then Au films with different thicknesses were deposited on the the Ag nanoshells with the best activity to obtain Au-Ag bimetallic nanoshells. The SERS detection results revealed that the Au-Ag bimetallic nanoshells have much weaker Raman enhancement than Ag nanoshells when the laser wavelength was532nm but have excellent SERS activities under the excitation light of780nm wavelength. The SERS intensities from Au-Ag bimetallic nanoshells got enhanced as the decreasing of the thicknesses of the outer Au shells. The good stability and biocompatibility of Au confirms the Au-Ag bimetallic nanoshells have enormous potential application value in SERS biosensing.(4)Three dimensional nanopillar and nanocone arrays were fabricated by inductively coupled plasma etching of silicon slices with silica spheres as mask templates. When the etching time was120,210and360s, nanopillars were obtained. When the etching time was450s, the pillars will evolve into cone structures. Silver films were deposited onto the nanopillars/cones as SERS substrates. The detection results show that the samples etched210s exhibit the best performances. In addition, the arrays possess excellent super-hydrophobic properties, which may be used to self-assemble silver nanoparticles to improve their SERS activities further.