Synthesis and surface modification of metallic nanoparticles: Sensitivity, influence of shape, concentration and excitation wavelength on chemical detection by SERS

Surface-Enhanced Raman Spectroscopy (SERS) is a powerful spectroscopic technique that can be used for detecting analytes at very low concentrations. In SERS, the intensity of vibrational signatures can be enhanced by factors from 106 to 1014 or even higher under well-controlled conditions. SERS is emerging as an important technique for the detection of biological and energetic materials. It is a vibrational spectroscopy technique, such as FTIR, in which the vibrational spectrum displays a fingerprint of the chemical composition of each chemical or biological agent together with high detection capability. As its name indicates, SERS requires a supporting substrate. The most commonly used SERS substrates in aqueous sols are zero valence silver, gold or copper nanoparticles. These nanoparticles can have different sizes or shapes such as spheres or rods. They can be prepared by electron beam, nanosphere lithographic or wet chemistry approaches.;The research presented here focused on the synthesis of small silver nanospheres (SNS), gold nanospheroids (GNS), and gold nanorods (GNR) for chemical SERS detection. GNR were synthesized using a seed--CTAB approach. These nanostuctures were stable up to a month after preparation with a small shift of the longitudinal plasmon (LP) around 623 nm. SERS activity decreased over time for all the characteristic bands of the probe molecule 4-aminobenzenethiol (4-ABT): 390, 1077 and 1596 cm-1 and the activity was closely dependent of the laser wavelength. They were SERS active at 633 nm and 785 nm. However, when laser excitation wavelength (lambdaexc) was near the transverse plasmon (∼ 520 nm) they were not SERS active.;The sensitivity for of 4-ABT was 1x10-10 M that correspond to 4.1 x109 molecules under laser spot (90 mum x 50 mum). The SERS activity of GNR was compared with that of GNS with the result that the prepared GNS were better SERS substrates than GNR. These results proved that SERS experiment is a surface depending process where the surface chemistry, shape, crystallographic planes (morphology effect) and surface area influence the spectra obtained under dilution studies. The surface enhancement factors (SEF) were 3.7x107 and 4.1x105 for GNS and GNR, respectively. The SERS enhancement was a combination of electromagnetic and chemical effect. The result presented here shows that good reproducibility expressed as relative standard deviation (1.5 % for GNR and 10% for GNS) was achieved.;GNR were aggregated as films in cyclohexane/water interface using ethanol as an inducer. The experimental setup was very efficient for the preparation of the aggregated nanorods as films. The aggregated GNR also showed a strong SERS effect for nitro explosives (3,5-dintro-4-methylbenzoic acid and 2,4,6-TNT) without alkaline hydrolysis degradation is proposed with sensitivity of 5.0 pg.;An alternative method for silver NS surface modification is proposed by changing the colloid's pH. They were extremely active for detection of RNA and DNA components (nitrogenous bases, nucleosides and nucleotides). Different pH values let obtain SERS signals in the range of 0.1 muM for the studied biomolecules. The optimum pH values were 7.0 for adenine and dAMP, 9.0 for AMP, 5.0 for adenosine, and 11.0 for deoxyadenosine.