Binding of Ricinus communis agglutinin to a galactose-carrying polymer brush on a colloidal gold monolayer

A polymer with many pendent galactose residues was prepared by atom-transfer radical polymerization (ATRP) of galactose-carrying vinyl monomer, 2-lactobionamidoethyl methacrylate (LAMA), with a disulfide-carrying ATRP initiator, 2-(2′-bromoisobutyroyl)ethyl disulfide (DT-Br). The galactose-carrying polymer obtained (DT-PLAMA) was accumulated as a polymer brush via Au–S bond on a colloidal gold monolayer deposited on a cover glass. For comparison, a disulfide which carried one galactose residue at both ends (2-lactobionamidoethyl disulfide, Cys-Lac) was accumulated as a self-assembled monolayer (SAM) on the colloidal gold monolayer, too. The association and dissociation processes of galactose residues on the colloidal gold with a lectin, Ricinus communis agglutinin (RCA120), were observed by the increase and decrease in absorbance at 550 nm corresponding to localized surface plasmon resonance (LSPR) phenomena. The Cys-Lac SAM-carrying glass chip showed a strong non-specific adsorption of the lectin, whereas the DT-PLAMA brush-carrying one reversibly associated with the lectin, indicating reusability of the latter device. The apparent association constant of the lectin with the galactose residues in the DT-PLAMA brush was much larger than the association constant for free galactose, and the detection limit of RCA120 by the glycopolymer brush-modified device was satisfactorily low. Furthermore, a microscopic observation clearly indicated that the DT-PLAMA brush could reversibly associate with a HepG2 cell having galactose receptors, though these processes could not be observed spectrophotometrically due to a gigantic size of the cell.