Citrate functionalized gold nanoparticles assisted micro extraction of L-cysteine in milk and water samples using Fourier transform infrared spectroscopy

This paper describes the sensing application of citrate functionalized gold nanoparticles (AuNPs)employing for the determination of L-cysteine in food and water samples. It is established with diffuse reflectance Fourier transform infrared (DRS-FTIR)spectroscopic analysis. The disappearance of the thiol (-SH)band in the FTIR spectra and the shift in the peaks of the amino group (NH3+)and carboxylate group (-COO-)indicated the Au-S interaction and the aggregation of the NPs. The signal intensity of L-cysteine was enhanced due to hot-spots formed by the aggregation of AuNPs producing the effective absorption of electromagnetic radiation in the IR region for molecular vibration. The relationship between AuNPs and L-cysteine was theoretically investigated by the Density Function Theory (DFT)based on LANL2DZ with the aid of the Gaussian 09 (C.01)software. Interaction between AuNPs and L-cysteine molecules resulted to a shift to higher wavelengths in the plasmon bands, further verified by transmission electron microscopes (TEM), which have indicated random aggregated particles. Further dynamic light scattering (DLS)measurements showed a relatively high degree of polydispersity confirming the aggregation of the particles. Under optimized conditions, the calibration curve showed a good linearity range from 20 to 150 ?g mL?1 with a correlation coefficient (R2)0.990. The limit of detection and quantification were 1.04 and 3.44 ?g mL?1, respectively by DRS-FTIR. This modified AuNPs sample was used successfully in milk and water samples with adequate results to determine L-cysteine.