Controlled and Reversible Aggregation of Biotinylated Gold Nanoparticles with Streptavidin

Biotinylated gold nanoparticles were prepared by using a two-step surface modification procedure. First, a carboxyl-terminated alkanethiol was chemisorbed onto the surface of gold nanoparticles in the presence of a stabilizing agent. Subsequently, the carboxyl groups were reacted with (+)-biotinyl-3,6,9,-trioxaundecanediamine and 2-(2-aminoethoxy)ethanol. This procedure resulted in stable, ligand-modified gold nanoparticles. Upon interaction with streptavidin, the biotinylated gold nanoparticles aggregated by means of specific biomolecular recognition. Their aggregation was studied by optical absorption spectroscopy. Controlled aggregation of biotinylated gold nanoparticles resulted in a shift in the surface plasmon resonance peak and broadening of the absorption spectrum of the nanoparticles. The spectral changes were used to assess the extent of aggregation. Aggregation was found to be dependent on the concentrations of streptavidin, biotinylated gold nanoparticles, and the surface mole fraction of biotin groups on the nanoparticles. Maximum aggregation was observed when 24 nM streptavidin and 0.80 nM biotinylated gold nanoparticles were used. Reversal of nanoparticle aggregation was accomplished by the addition of soluble biotin to the streptavidin−nanoparticle aggregates. Kinetic analysis of the absorbance data showed that streptavidin-induced aggregation of biotinylated gold nanoparticles could be interpreted in terms of a Reaction-Limited Colloidal Aggregation (RLCA) model. This indicates that optical absorption spectroscopy can provide a quantitative measurement of the aggregation process.