In Situ Structural Elucidation and Selective Pb 2+ Ion Recognition of Polydopamine Film Formed by Controlled Electrochemical Oxidation of Dopamine.

Owing to the versatility and biocompatibility, a self-polymerized DA (in the presence of air at pH 8.5 tris buffer solution) as a polydopamine (pDA) film has been used for a variety of applications. Indeed, instability under electrified condition (serious surface-fouling) and structural ambiguity of the pDA have been found to be unresolved problems. Previously, pDA films (has hygroscopic and insoluble property) prepared by various controlled chemical oxidation methods have been examined for the structural analysis using ex situ solid-state NMR and mass spectroscopic techniques. In this work, a new in situ approach has been introduced using an electrochemical quartz crystal microbalance (EQCM) technique for the improved structural elucidation of pDA that has been formed by a controlled electrochemical oxidation of DA on a carboxylic acid functionalized multiwalled carbon nanotube-Nafion (cationic perfluoro polymer) modified electrode (f-MWCNT-Nf) system in pH 7 phosphate buffer solution. Key intermediates like 5,6-dihydroxy indole (DHI; 150.7 g mol-1 ), dopamine (154.1 g mol-1 ), Na+ , PO4 2- , and polymeric product of high molecular weight, 2475 g mol-1 , have been trapped on f-MWCNT-Nf surface via π-π (sp2 carbon of MWCNT and aromatic e-s), covalent (amide-II bonding, minimal), hydrogen, and ionic bonding and identified its molecular weights successfully. The new pDA film system showed well-defined peaks at E°' = 0.25 V and -0.350 vs Ag/AgCl corresponding to the surface-confined dopamine/dopamine quinone and DHI/5,6-indolequinone redox transitions without any surface-fouling complication. As an electroanalytical application of pDA, selective recognition of Pb2+ ion via {(pDA)-hydroquinone-Pb0 } complexation with detection limit (signal-to-noise ratio = 3) 840 part-per-trillion has been demonstrated.