Visual detection of Sudan dyes based on the plasmon resonance light scattering signals of silver nanoparticles.

A visual light scattering detection method of Sudan dyes is reported in food products based on the formation of silver nanoparticles (NPs) . Sudan dyes including I, II, III and IV have reducibility due to the nitrogen-nitrogen double bond and phenol group in their molecular structure, and a redox reaction could occur with AgNO3. Owing to the formation of silver NPs as a result of the redox reaction, color changes could be observed by eye from the red of Sudan to the brown of silver NPs, resulting in strong plasmon resonance light scattering (PRLS) signals characterized at 452 nm, which could be measured using a common spectrofluorometer. It was found that the PRLS intensities were proportional to the dye concentrations over the range of 0.2-2.4 microM Sudan I, 0.1-2.4 microM Sudan II, 0.1-2.4 microM Sudan III, and 0.2-3.0 microM Sudan IV, with the corresponding limits of determination (3 sigma) of 3.2, 3.0, 3.2, and 2.9 nM, respectively. Using hot chili as a model sample, detection could be made with the recovery of 90.8-103.3% and RSD of 4.0-4.9%, and the results are identical with that of a liquid chromatographic method approved by the European Commission. To make the PRLS method much more practical, we could visually detect the quantity of Sudan dyes based on the PRLS signals using simple devices such as a portable laser pointer (653 nm) and a light emitting diode (458 nm). Mechanism investigations show that the functional group of Sudan oxidized by AgNO3 is the phenol group, not the nitrogen-nitrogen double bond.