Experimental and Theoretical Study of Biosurfactants Functionalized Gold Nanoparticles for Mixture Detection and Chiral Recognition of Tryptophan by UV-Vis Spectroscopy

Tryptophan (Trp) is an essential amino acid and plays important roles in biological processes. The detection of Trp is very important for its biological and chemical study. Moreover, Trp is a chiral compound; due to its importance in biological processes, researchers have been long committed to the chiral recognition and sensing of Trp enantiomers. Two biosurfactants, sodium cholate and sodium deoxycholate, were used for the preparation of functionalized gold nanoparticles (AuNPs) which were characterized by transmission electron microscope and potentiometer. UV-Vis spectra of functionalized gold nanoparticle solutions with different concentrations of Trp, tyrosine, phenylalanine, D-Trp, and L-Trp were analyzed. Then, the discrimination mechanism was further investigated, and the promotion mechanism of biosurfactants was studied by density functional theory (DFT). Trp could induce the aggregation of unmodified AuNPs in 2 h, while phenylalanine and tyrosine could not. Adding biosurfactants promoted the aggregation process, and D- Trp rather than LTrp was found to be responsible for the aggregation. Therefore, there were interaction differences not only between Trp, phenylalanine, and tyrosine but also between Trp enantiomers. UV-vis spectroscopy could be applied for the direct detection of Trp in mixtures as well as the chiral recognition of Trp enantiomers. DFT calculations proved that the interactions of D-Trp with biosurfactants were the strongest, which contributes to the promotion of aggregation.