Measurement of second-order nonlinear optical response of molecules by hyper-Rayleigh scattering (HRS) is often affected by the presence of two-photon excited fluorescence. For crystal violet we found experimentally that both HRS and two-photon absorption induced fluorescence give the same depolarization ratio for the signal. This shows that the depolarization measurement is generally not useful for distinguishing HRS from an unwanted fluorescence contribution. A theoretical explanation of the equality of the depolarization ratio for the two processes is given. Spectral study shows that signals from HRS and two-photon excited fluorescence for crystal violet are well separated to allow the measurement of β by standard HRS technique. A modified external reference method is developed to deal with the measurement of β by HRS for molecules that absorb the second harmonic light. The resulting β value for crystal violet is 1140×10−30 esu, which is considerably smaller than the literature value but still amounts to a factor of 33 compared to that of p-nitroaniline.
A simple and sensitive colorimetric gold nanoparticle probe for detection of melamine in infant formula was developed by the reduction of Au(III) salt with sodium citrate. This method is rather simple that does not involve any surface modification of the nanoparticles or multistep sample treatment. This technique is based on the fact that the optical properties of gold nanoparticles depend on distance between particles. Gold nanoparticles are aggregated in a neutral media in the presence of melamine; this causes an easily measurable change in the absorption spectrum of the particles, which can be monitored with the naked eye or UV-vis spectrophotometer. Here, the color of gold nanoparticles changed from wine red to blue in the presence of melamine and no color change was observed before melamine was introduced into the milk sample. The observed color change is the result of the coupling of the surface plasmon resonance (SPR) between particles in close proximity. This method is also sensitive and a limit of detection of 0.90 ppm of melamine was obtained with the UV-vis spectrophotometer in aqueous solution and 0.46 ppm of melamine was detected visually with the naked eye in a solution of infant formula.
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