[Alkaline silver colloid for surface enhanced Raman scattering and application to detection of melamine doped milk]

Alkaline silver colloid with better stability and uniformity was obtained by adding appropriate amount of NaOH to synthesis reaction. The performance of the Ag colloid as surface enhanced Raman scattering (SERS) active substrate was evaluated by methylene blue as the probe molecules and achieved well Raman spectra. The concentration of methylene blue had no effect on the adsorptive behavior of methylene blue on alkaline silver colloid surface in comparison with normal silver colloid. The basic reason for this phenomenon is preferential adsorption of alkaline silver colloid for sulfur atoms of methylene blue so as to increase the intensity of 451 cm-1 Raman peak consistently. The amounts of methylene blue added to alkaline Ag colloid and time-evolution of Raman spectra were also investigated. Additionally, the alkaline silver colloid was prepared to be silver spot and applied to detect melamine doped milk. The relationship of the doping amount of melamine and the Raman signal intensity was obtained. The linearity relationship in the concentration range between 3 and 60 mg.L-1 with detect limit 0.28 mg.L-1 was achieved based on the intensity of 691 cm-1 Raman peak This method required only 5 microL sample size and 5 s for detection and suggested that this presented method with its advantages of speediness, briefness and lower cost has a good application foreground.