Synthesis, Characterization Of SERS Active Silver Nanoparticles

Silver nanoparticles show remarkable optical properties that depend onnanoparticle size and shape. The number of surface plasmon resonance bands andeffective spectral ranges for surface-enhanced Raman scattering (SERS) has alsobeen demonstrated to be highly dependent on the morphology exhibited by silvernanostructures. The primary focus of this work was the design of silvernanoparticles with different morphologies and investigation of the SERS activityof the nanostructures. Our study is outlined as follows:1. A simple method to synthesize triangular silver nanoparticles by lightirradiationWe describe a simple method to synthesize triangular silver nanoparticles byphotoreducing the silver ions by citrate. A noteworthy difference of the presentmethod as compared with the previous photo-induced methods is that good shapecontrol over the nanoparticles can be realized in the absence of soft templates orpolymer directing agents. The formation process of the silver nanoparticles isinvestigated by UV-vis spectroscopy and transmission electron microscopy(TEM). It was found that the concentration of reactant play important roles in themorphology control of produced silver nanoparticles. As one of the application ofthese nanoparticles, they are used as surface-enhanced Raman scatteringsubstrates and 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BVPP) is used as a Ramanprobe to evaluate the enhancement ability of the triangular silver nanoparticles.2. The preparation and SERS study of triangular silver nanoparticlesself-assembled filmsWe report the self-assembly of triangular silver nanoparticles onto themonolayer of poly(diallydimethylammonium chloride) (PDDA). Attractiveelectrostatic interactions between the negatively charged nanoparticles and thepositively charged polyelectrolyte are responsible for the self-assembly. Thesurface morphology and the optical properties of the self-assembly films arestudied by atomic force microscopy (AFM) and UV-vis spectroscopy. R6G isused as a Raman probe to evaluate the enhancement ability of the silver film. It isalso found that the enhancement ability of the triangular silver nanoparticle filmis lower than that of the spherical silver nanoparticle film.3. Preparation of silver nanoparticles by photo-reduction for surface enhancedRaman scatteringA novel substrate for SERS has been developed. Based on thesurface-catalyzed reduction of Ag+ by citrate on the silver nanoparticles surfaceunder light irradiation, small silver seeds on a quartz slide can be enlarged. Theoptical properties and characteristics of the silver films have been investigated byUV-vis spectroscopy, SEM and AFM. The results indicate that the particle sizeand shape are different at different reduction time. At the first 3 hours, sometriangular and hexagonal nanoparticles formed;with the reduction proceeding,the shape of the silver particle becomes irregular and the size becomes larger. Thesilver films obtained are very suitable as SERS active substrate. The relationshipbetween SERS intensity and the reduction time has been investigated for BVPPmolecular adsorbed on the silver film. The SERS intensity reaches a maximum at8 hours reduction. The AFM measurements indicate that roughness features withan average size of 100 nm are present in the surface which yielded the strongestSERS signal. Pyridine was used as a probe molecular to investigate theenhancement factor (EF) of the silver films, According to the formalism of Tianand co-workers, the EF of the silver films is estimated to 3.4×105. The silverfilm which can remain active for more than 50 days seems to be suitable forvarious analytical applications.4. Surface-enhanced Raman activity and stability study of silver films preparedby reduction of Ag+ ions in DMFA new SERS-active silver film prepared by direct depositing silvernanoparticles on glass is reported. The spontaneous reduction of Ag+ ions inN,N-dimethylformamide (DMF) (in the absence of a protecting agent) leads tothe deposition of silver nanoparticles on clean glass surfaces in contact with thesolution. The observed optical micrograph and AFM images show thearrangement of the nanoparticles is inhomogeneous and the particles appear to beclosely packed. These films are shown to be excellent substrates for SERSmeasurements, demonstrating significant enhancement, good stability and tracedetection capability. The SERS enhancement of the silver films with differentdeposition time are compared and found that the 8-hour film gives the largestenhancement ability which enhancement factor is estimated to be about 109. It isalso found that the silver films stored for 80 days in air show no significantdegradation in their sensitivity.5. Directly synthesis of triangular and hexagonal silver nanoplates on surfaceTriangular and hexagonal silver nanoplates are synthesized directly on glasssurfaces using photoreduction of silver ions by the citrate onto surface confinedsilver seeds. The reaction is characterized by atomic force microscopy and Uv-visspectra. We find that for facile nanoplates growth, the seeds size must be smallerthan 10 nm. By varying the irradiation time, nanoplates with different sizes andshapes can be obtained. However, the growth mechanism for nanoplates prepareddirectly on surface is more or less different from that of triangular nanoplatessynthesized in solution. The growth process of nanoplates has also been inferredfrom the results.