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

Background: 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. Methods: 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). Results: 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. Conclusions: 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.