Preparation of dendrimer SAM on Au substrate and adsorption/desorption of poly-L-glutamate on the SAM

A self-assembled monolayer (SAM) of a dendrimer has been fabricated by covalent bonding of amine-terminated dendrimers to 3-mercaptopropionic acid SAM on an Au substrate, where an amide bond was formed between the amine and the carboxylic acid using amide-coupling reagents. Each reaction step was inspected by transmission surface-enhanced infrared absorption spectroscopy. Reaction was affected by solvents of the coupling reagents and concentrations of the coupling reagents and dendrimers. The pH-dependent adsorption/desorption behavior of sodium poly-L-glutamate (PGA) has been examined on the dendrimer SAM prepared. The adsorbed amount was abundant at acid pH below the pK(a) of carboxylate, while desorption from PGA-adsorbed dendrimer SAM prepared at acid pH proceeded as the pH was raised. The reaction was reversible and reproducible within 30 min under controlled pH, although the carboxylic acid species adsorbed at pH 4.2 was deprotonated to a carboxylate species after more than 30 min of desorption at pH 9.4.