Relative Study of Gold nanoparticle; A Development of Green Biosensor for Detection of L-Cysteine in Urine Sample

In this article, we present a comparative study of bio sensing applications using gold nanoparticles (AuNPs). The AuNPs were synthesized through two distinct methods: chemical reduction and a biological approach utilizing Ocimum sanctum plant extract. In the chemical reduction method, AuNPs with an average particle size of 11.39 nm exhibited a surface plasmon resonance (SPR) peak around 527 nm. In contrast, the biological method yielded larger nanoparticles, with an average size of 17.81 nm and an SPR peak around 552 nm. The shift in wavelength correlates with the increase in particle size, which is also influenced by particle aggregation. X-ray diffraction (XRD) analysis revealed a cubic crystal structure, with the (111) plane indexed at 36.75° 2θ. Transmission electron microscopy (TEM) micrograph confirmed the uniform spherical shape of the particles synthesized by the chemical reduction method, while the biological method produced spherical particles with some size variation. The size and shape of the synthesized particles were found to be influenced by factors such as the biomolecules present in the plant extract, the pH of the solution, the volume of extract, and the temperature. The synthesized nanoparticles were incubated with biomolecules (L-cysteine, L-arginine, glycine, and ascorbic acid). Among these, only the AuNP-cysteine complex exhibited a distinct spectrometric response, with additional SPR peaks observed at 650 nm and 664 nm, confirming thiol-gold binding. The minimum detectable concentration was found to be 10 μM. Compared to the chemical reduction method, AuNPs synthesized through the biological approach demonstrated weaker thiol-gold binding, though under highly nucleophilic conditions, strong binding was observed, as indicated by the new SPR peaks. Spectroscopic methods were successfully applied for the sensitive and selective detection of cysteine in urine samples, highlighting the potential of these AuNP-based biosensors in diagnostic applications.