Synthesis of Citrate-T20-Ser-Gold Nanoparticles and effect of heavy metal cations on its colloidal stability

Abstract

Surface functionalization plays a vital role in determining the applicability of gold nanoparticles (GNPs). One commonly used approach for formulation of functionalized GNPs involves the synthesis of Citrate-capped GNPs followed by replacement of the citrate molecules by a functional ligand. Using a similar method, we demonstrate the synthesis of L-serine functionalized Tween-20 stabilized Citrate-GNPs. Tween-20 stabilization was essential to maintain the colloidal stability of the GNPs during further functionalization. The minimum concentration of Tween-20 for effective stabilization was found to be 1.88 mg mL(-1). It was found that adsorption of Tween-20 on Citrate-GNPs improved their colloidal stability due to which it resisted NaCl-mediated destabilization well up to 1 M NaCl. Interestingly, further functionalization with L-serine reduced the GNP stability mildly, and made the dispersion more susceptible to destabilization using 1 M NaCl. The size and morphology of the GNPs were characterized using TEM and DLS analysis. UV-Visible spectroscopy and optical images were used to examine the colloidal stability of the dispersions. We further tested the interaction of GNPs with nine different bivalent heavy metal cations. L-serine functionalized GNPs were found to have remarkable specificity towards lead (Pb2+) ions. The interaction was visible as a shift in the surface plasmon resonance (SPR) peak from the original 524 nm of GNPs to 660 nm displaying aggregated particles. The GNPs displayed colorimetric sensing of Pb2+ in the concentration range up to 50 ppm, which was visible to the naked eye as gradual change in GNP colour from pink, purple, violet and blue concurrently with progressively increasing Pb2+ concentrations. The L-serine functionalized GNPs have immense potential to be developed as a cost-effective, simple and rapid assay for detection of Pb2+ ions in contaminated water samples.