Boronic Acid-Functionalized Oxide-Free Silicon Surfaces for the Electrochemical Sensing of Dopamine

Boronic acid monolayers covalently bound to hydrogen-terminated Si(111) surfaces have been prepared from the UV-directed hydrosilylation reaction of 4-vinylbenzeneboronic acid. X-ray photoelectron spectroscopy (XPS) analysis of the modified surface revealed characteristic peaks from the attached organic molecule with the expected molecular composition and without the oxidation of underlying silicon. From XPS data, the surface coverage was estimated to be ca. 0.34 ± 0.04 ethylbenzene boronic acid chain per surface silicon atom (i.e., (4.4 ± 0.5) × 10^-10 mol cm^-2), which is consistent with a densely packed monolayer. The electrochemical impedance spectroscopy measurements performed at pH 7.4 in the presence of the Fe(CN)₆^3-/Fe(CN)₆^4- reporter couple showed specific dopamine-induced changes as a result of the binding of the guest molecule to the immobilized boronate species. The charge-transfer resistance (R_ct) was found to decrease from 4.9 MΩ to 14 kΩ upon increasing the dopamine concentration in the range of 10 μM-1 mM. Furthermore, the presence of the interfering ascorbic acid until a concentration of 10 mM did not significantly change the electrochemical response of the functionalized surface. Comparative electrochemical data obtained at the reference ethylbenzene monolayer provided clear evidence that the immobilized boronic acid units were responsible for the observed changes.