Spectrometric determination of l-cysteine and its enantiomeric purity using silver nanoparticles

The system containing silver nanoparticles (∼45 nm) and l-cysteine at the concentration range from 10−9 to 10−2 mol L−1 was studied by absorption and electronic circular dichroism spectroscopy in the UV–vis range and surface enhanced Raman spectroscopy at different pH and ionic strength. By later one the presence of l-cysteine was clearly detectable even at 10−9 mol L−1. The analysis of circular dichroism spectra by statistical methods, namely two-step cluster analysis, principal component analysis, and partial least square regression, allowed determination of l-cysteine in the range from 10−5 to 10−3 mol L−1. Moreover this system is very sensitive to addition of d-cysteine; hence, the enantiomeric ratio can be estimated and the presence of d-cysteine in the system is clearly detectable even at 10−6 mol L−1, i.e. the content of this enantiomer is as low as 2% in the mixture.