Nanosecond Laser-Assisted Fabrication of Colloidal Gold and Silver Nanoparticles and Their Conjugation with S-Ovalbumin

Abstract

In the present work, we have investigated the functionalization of protein S-ovalbumin with laser-generated gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) using spectroscopic techniques. Biocompatible gold and silver nanoparticles were fabricated by nanosecond pulsed laser ablation (PLA) of metal target in water and directly conjugated to protein S-ovalbumin. The localized surface plasmon resonance (LSPR) of gold and silver nanoparticles is sensitive to particle size, shape, and surrounding medium refractive index. UV–visible absorption spectrum of both AuNPs and AgNPs showed red shift in LSPR after protein addition. The effect of laser fluence on the size of AuNPs and nanoparticle-protein conjugation in the size range 6–32 nm has been systematically studied. The presence of NPs resulted in broadening of S-ovalbumin absorption peak around 278 nm. Raman spectra of S-ovalbumin-NP assembly demonstrated breaking of disulfide bond and appearance of peak around 290 and 206 cm−1 corresponding to Au–S and Ag–S bond formation, respectively. Further, increase in the ratio of the 1350/1320-cm−1 doublet was observed, which indicates that hydrophobicity for tryptophan residues in the protein-NP assembly has increased. Fluorescence spectroscopy showed quenching in emission intensity of S-ovalbumin in the presence of AuNPs and AgNPs. The number of binding sites between AuNPs and S-ovalbumin has been found to be 1.4 with binding constant 4.5 × 109 M−1. The calculated values of binding constant and number of binding sites for AgNPs and S-ovalbumin were 2.4 × 109 M−1and 0.96, respectively.