Surface-enhanced raman scattering (SERS) detection of low concentrations of tryptophan amino acid in silver colloid

The surface-enhanced Raman scattering (SERS) spectrum of L-tryptophan has been studied in the concentration range 1.4 × 10(-8) to 5 × 10(-4) M. A borohydride-reduced silver colloid was employed as the nanoparticle enhancing agent and different electrolytes have been tested for activation of the colloid. The optimum conditions have been determined for achieving high sensitivity of detection. The experimental procedure developed, which includes the use of a composite electrolyte (a mixture of NaHCO(3) and NaCl) for colloid activation, results in very high enhancement of the Raman signal (up to 10(8)). This gives the possibility of studying SERS spectra of L-tryptophan at concentrations as low as 10(-8) M, which is several orders of magnitude lower than previously reported in the literature. The observed SERS spectra were very reproducible and were detectable 2 minutes after mixing, reaching maximum strength approximately 15 minutes after mixing. The spectral characteristics were stable over the entire period of observation. We have found that SERS spectra of tryptophan in silver colloid differ in several features at low (below ～10(-5) M) and at high (above ～10(-4) M) concentrations. The most important difference is the absence of the peak near 1000 cm(-1) at low concentrations, which is usually assigned to the indole ring breathing mode. The observed spectra allow us to suggest that at low concentrations Trp molecules bind to the surface through the indole ring, which remains flat on the surface. This is in contrast to the previously reported observation of SERS spectra from Trp performed at concentration levels above 10(-5) M.     

Preparation and characterization of multilayered ZrO<Subscript>2</Subscript> coatings on silicon carbide fibers for SiC/SiC composites

We have studied the surface morphology, phase composition, and oxidation resistance of multilayered tetragonal zirconia coatings produced on silicon carbide fibers by a sol-gel process and measured the tensile strength of individual fibers as a function of the number of layers in the coating. SiC-fiber-reinforced silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their fracture surfaces have been examined. Using microindentation, we have determined the critical fiber-matrix debonding stress. The results demonstrate that the ZrO₂ coating on the fibers has the form of uniform, weakly bonded layers. The presence of a multilayered ZrO₂ interphase alters the fracture behavior of the SiC/SiC composites. The fiber debond stress in the composites markedly decreases with an increase in the number of layers in the interphase.