Bioconjugation of 32-macrocyclic polyammonium cations-functionalized gold nanoparticles with BSA.

Abstract Water-dispersed, spherical, underivatized Au-NPs with particle size less than 5 nm were synthesized from an aqueous solution of 32-macrocyclic polyammonium chloride, [32]ane- ( NH 2 + ) 8 · 8 Cl - (32-MCPAC) using sodium borohydride (NaBH 4 ) as the reducing agent. The bioconjugation of the synthesized Au-NPs at different pHs (3.6–5.6) with bovine serum albumin (BSA) protein was studied using UV–Vis, fluorescence, and Raman spectroscopy. These studies support that the Au-NPs were incorporated into the protein moiety and bound to it chemically. The binding constants ( K b ) and stoichiometries ( n ) (i.e., the number of Au-NPs bound by the proteins) of BSA protein to the Au-NPs at different pHs were determined by measuring the quenching of the fluorescence intensity of the tryptophan residues of the protein molecules after conjugation. The values for K b ( n ) were found to be 1.05 × 10 10  M −1 (1.66), 2.09 × 10 10  M −1 (2.30), and 1.86 × 10 10  M −1 (1.75) at pH 3.60, 4.60, and 5.60 for BSA–Au-NPs conjugations, respectively. The results show that BSA binds to the Au-NPs strongly at pH 4.60, which is equivalent to its isoelectric point (p I 4.6).