Quantitative and reversible lectin-induced association of gold nanoparticles modified with alpha-lactosyl-omega-mercapto-poly(ethylene glycol)

Gold nanoparticles (1-10 nm size range) were prepared with an appreciably n arrow size distribution by in situ reduction of HAuCl4 in the presence of h eterobifunctional poly(ethylene glycol) (PEG) derivatives containing both m ercapto and acetal groups (alpha -acetal-omega -mercapto-PEG). The alpha -a cetal-PEG layers formed on gold nanoparticles impart appreciable stability to the nanoparticles in aqueous solutions with elevated ionic strength and also in serum-containing medium. The PEG acetal terminal group was converte d to aldehyde by gentle acid treatment. followed by the reaction with p-ami nophenyl-beta -D-lactopyranoside (Lac) in the presence of (CH3)(2)NHBH3. La c-conjugated gold nanoparticles exhibited selective aggregation when expose d to Recinus communis agglutinin (RCA(120)), a bivalent lectin specifically recognizing the beta -D-galactose residue, inducing significant changes in the absorption spectrum with concomitant visible color change from pinkish -red to purple. Aggregation of the Lac-functionalized gold nanoparticles by the RCA(120) lectin was reversible, recovering the original dispersed phas e and color by addition of excess galactose. Further, the degree of aggrega tion was proportional to lectin concentration. allowing the system to be ut ilized to quantitate lectin concentration with nearly the same sensitivity as ELISA. This simple, yet highly effective, derivatization of gold nanopar ticles with heterobifunctional PEG provides a convenient method to construc t various colloidal sensor systems currently applied in bioassays and biore cognition.