Fabrication of a boronic acid-surface and insitu optical detection of catecholamines

We fabricated a boronic acid-surface on a flat silica surface for optical detection of recognition of catechol structure. The boronic acid-surface was obatained by chemical reaction between 4-carboxyphenylboronic acid (CPB) and the primary amine group of 3-aminopropyltriethoxysilane (APS) modified onto the silica surface. The UV-vis spectroscopic and the X-ray photoelectron spectroscopic (XPS) measurements indicated that 90% of the primary amines of APS reacted with CPB. Recognition of the catechol structure on the CPB surface was observed with in-situ time dependent measurement using optical configuration of surface plasmon spectroscopy. Injection of phosphate buffers containing 1.0 mM dopamine (DA) and catechol, respectively, at pH 7.0 allowed increase of the reflectivity, suggesting complexation between DA or catechol and the boronic acid of CPB. On the other hand, increasing of the reflectivity (δR) in the case of tyramine, which is very similar in chemical structure to DA but has no catechol structure, was quite small. The ratio of (δR) between DA and tyramine (DA/tyramine) was experimentally estimated to be 5.5, which briefly shows that the CPB-modified surface can recognize catechol structure. The (δR) behaviors of DA differed from the pH of buffers. At pH 8.1, polymerization of DA was observed on the CPB-modified surface while no recognition at pH 6.0. The efficient pH for the 1:1 complexation between DA and boronic acid of the CPB-modified surface is thought to be around 7.0.