Analysis of catechol, 4-methylcatechol and dopamine electrochemical reactions on different substrate materials and pH conditions

The electrochemical behavior of several structurally related catecholamine molecules has been investigated on different electrode materials with cyclic voltammetry, infrared spectroscopy and scanning tunnelling microscopy. Emphasis was on the identification of subsequent chemical processes that follow the main electron transfer step and complicate the interpretation of the mechanism, including the polymerization reaction and fouling of the electrode surface. Among the materials investigated, gold was found out to be the most active for the oxidation of catechol, 4-methylcatechol and dopamine. At the same time, it was also the electrode least sensitive to fouling after voltammetric cycling, exhibiting the highest reversibility. The effect of pH was also investigated. Increase in pH enhanced the processes of quinone hydroxylation and polymerization. Spectroscopic measurements allowed detection of both solution and adsorbed species participating in the oxidation and polymerization processes. Finally, STM results showed the formation of polydopamine granules on gold surfaces, which grew in size as the number of cycles increased. Measured height of the granules, less than 0.5 nm, suggests a flat orientation of the molecules conforming the polymer.The authors thankfully acknowledge financial support from the UA-FPU 2013-5796 grant (University of Alicante, Spain), the Ministerio de Economía, Industria y Competitividad, Spain (CTQ2016-76221-P), the European Framework Program-Marie Curie Actions under the ELECTRONANOMAT Project (Grant No. PIRSES-GA-2012-318990) and Academy of Finland, Finland (Project Number #285526)