Identification of tyrosine in the presence of tryptophan using Cd-enriched colloidal CdS nanoparticles: A fluorescence spectroscopic study

A simpler identification method of tyrosine in the presence of tryptophan using CdS nanoparticles by conventional spectroscopic technique is proposed. Effect of both sulfide-enriched CdS as well as Cd²⁺-enriched CdS on tryptophan is investigated through absorption and emission spectroscopy. Quenching of tryptophan emission obeyed Stern-Volmer relation and was found to be independent of temperature, indicating a possible static quenching. The time-resolved fluorescence decay of tryptophan was minimally affected by sulfide-enriched CdS as well as Cd²⁺-enriched CdS nanoparticles, suggesting quenching to be static. In the presence of Cd²⁺-enriched CdS nanoparticles, the emission of tryptophan in phosphate buffer shows a typical spectral broadening along with a long wavelength increase in fluorescence emission. Additionally, spectra followed a typical isoemissive point at 440 nm when tryptophan alone was there. Similarly, isoemissive point at 340 nm was observed in the case of tyrosine. However, a further red shift of isoemissive point (470 nm) in the mixture of both tyrosine and tryptophan was observed. This observation might make Cd²⁺-enriched CdS nanoparticles useful for using as marker for tyrosine in the presence of tyrptophan.