In situ synthesis of gold nanoparticles using tryptophan enantiomers for determination of enantiomeric composition

Abstract

This paper reports a simple one-step colorimetric method for distinguishing l- and d-tryptophan based on the in situ formation of gold nanoparticles (AuNPs) in the presence of Cu2+. The tryptophan enantiomers act as both reducing and stabilizing agents, generating different nanoparticle aggregation behaviors and visible color changes. l-Tryptophan causes strong aggregation and a purple color, while d-tryptophan yields dispersed particles and a light pink color, enabling visual and spectrophotometric enantiomeric discrimination. At each fixed total tryptophan concentration, the absorbance at 570 nm increased with increasing d-Trp mole fraction, allowing the enantiomeric composition to be estimated. Separate linear relationships between absorbance and d-Trp mole fraction were obtained at total tryptophan concentrations ranging from 104 µM to 3.3 mM. TEM analysis showed predominantly discrete particles for d-tryptophan and extensively associated particles for l-tryptophan, with mean primary-particle sizes of approximately 43 and 47 nm, respectively. These optical and morphological observations establish distinct enantiomer-dependent endpoint colloidal states but do not, by themselves, identify their chiroptical origin. The integrated synthesis-detection approach provides a rapid and inexpensive proof-of-concept for estimating enantiomeric composition in compositionally defined d-Trp/l-Trp mixtures. Selectivity in the presence of other amino acids, reducing substances, chelating agents, or complex matrix components was not evaluated in the present study.