A method for synthesizing chiral gold nanoparticles through L- or D-cysteine functionalization is presented. Extensive characterization, including scanning electron microscopy, X-ray photoelectron, and circular dichroism spectroscopies, confirms their morphology and optical activity. These nanoparticles exhibit stability and represent a promise for selectivity for enantiomer and biomarker detection, with significant implications for theranostics and environmental sensing.

Chirality, the property of objects being nonsuperimposable on their mirror image, is critical in natural phenomena and technological applications, particularly molecular interactions and biological processes. The achievement of nanoscale chirality has led to the development of gold nanoparticles (AuNPs) with transformative potential in sensing and biosensing, which may find applications in theranostics and drug-contaminated water treatment. Herein, a wet synthesis approach for chiral AuNPs using L- or D-cysteine, a chiral amino acid, is presented. The synthesis begins with small gold spheres, then functionalized with L- or D-cysteine, which binds selectively to kink sites, inducing a twisted shape. These functionalized AuNPs inherit chiral properties, enabling selective interactions with biomolecules, enhancing sensitivity and specificity for detecting enantiomers and biomarkers. Microscopy reveals the twisted shape, dynamic light scattering confirms stability over months, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy validate successful cysteine functionalization, and finally, circular dichroism spectroscopy confirms optical activity in correspondence to both the molecular absorption and the nanoparticle surface plasmon resonance. These optically active AuNPs demonstrate significant promise for molecular diagnostics and environmental sensing, offering new frontiers in biomedical and environmental applications.