Interfacial recognition of sugars by boronic acid-carrying self-assembled monolayer

Disulfides that carried two boronic acid groups at their ends were prepared by coupling of 3-amino-phenylboronic acid with dithiodialiphatic acid (DTAA; number of methylene groups: 1, 2, and 3) using a water-soluble carbodiimide. The compounds obtained formed a self-assembled monolayer (SAIM) on both silver and gold colloids and on gold electrodes as proven by surface-enhanced Raman spectroscopy (SERS) and cyclic voltammetry (CV). Small sugars (fructose, glucose, mannose, and galactose) and polysaccharide (mannan) were bound to the boronic acid SAM on the gold colloid as detected by the changes in absorption of the colloid at 522 nm. The affinity of small sugars for the phenylboronic acid group was in the order fructose >> mannose > galactose > glucose, and these results could be attributed to the regioselective binding of the phenylboronic acid group in the SAM to hydroxyl groups of the sugars. The binding of sugars or sugar-carrying protein (ovalbumin) to the surface-modified gold electrode was also shown by the increase in potential difference in the cyclic voltammograms. The effect of the distance between the sulfur atom and the phenylboronic acid group in the SAM on the recognition of sugars is also discussed.