Spectroscopic Studies on the Interaction of Amino Acids with Gold Nanoparticles

Abstract

Gold nanoparticles are biocompatible and exhibit unique chemical, electrical as well as optical properties which vary in a size-dependent manner. They have generated an enormous interest due to their implications in various bioanalytical applications. For instance, they are used in the colorimetric determination of proteins, protein kinetics, and protein modifications. However, the stability of gold nanoparticles is an area of concern since they are extremely prone to aggregation depending upon the solution conditions such as pH, ionic strength, etc. Hence, the nanoparticles need to be surface-modified with biomolecules such as proteins for their utility in bioanalytics. Also, a detailed, systematic understanding of the gold-biomolecule interaction is required to ascertain the molecular basis of small molecule-induced aggregation. In this thesis, we have investigated the interaction of gold nanoparticles with amino acids, the building blocks of proteins, that were chosen based on their acidic or basic side-chains. In order to probe the effect of nanoparticle size during amino acid-induced aggregation, two different sizes (8 nm and 13 nm) of gold nanoparticles were synthesized by the Turkevich method and the amino acids were either used in isolation or as a combination with varying ratios. Aggregation kinetics of gold nanoparticles were monitored by changes in the absorption spectra and surface plasmon band. We observed that only cysteine induced aggregation of gold nanoparticles at pH 5 whereas all the remaining amino acids rendered the nanoparticles water-soluble and monodisperse. Varying the ratio of cysteine to that of lysine revealed that at higher cysteine concentrations, the extent of aggregation increased as a function of time. Additionally, both the extent and aggregation kinetics were faster for 13 nm gold nanoparticles compared to that for 8 nm.