Study of dopamine reactivity on platinum single crystalelectrode surfaces

Dopamine is the biological molecule responsible, among other functions, of the heart beat and bloodpressure regulation. Its loss, in the human body, can result in serious diseases such as Parkinson's,schizophrenia or depression. Structurally, this molecule belongs to the group of catecholamines, togetherwith epinephrine (adrenaline) and norepinephrine (noradrenaline). The hydroquinone moiety of themolecule can be easily oxidized to quinone, rendering the electrochemical methods a convenientapproach for the development of dopamine biosensors. The reactivity of similar aromatic molecules,such as catechol and hydroquinone, at well-ordered platinum surfaces, has recently been investigatedin our group. In this paper, we extend these studies to the structurally related molecule dopamine. Thestudy has been performed in neutral pH, since this is closer to the natural conditions for these moleculesin biological media. Cyclic voltammetry and in situ infra-red spectroscopy have been combined to extractinformation about the behavior of this molecule on well-defined platinum surfaces. Dopamine appears tobe electrochemically active and reveals interesting adsorption phenomena at low potentials (0.15- _0.25 Vvs RHE), sensitive to the single crystal orientation. The adsorption of dopamine on these surfaces is verystrong, taking place at much lower potentials than the electron transfer from solution species. Specifically,the voltammetry of Pt(1 1 1) and Pt(1 0 0) in dopamine solutions shows an oxidation peak at potentialsclose to the onset of hydrogen evolution, which is related to the desorption of hydrogen and the adsorp-tion of dopamine. On the other hand, adsorption on Pt(1 1 0) is irreversible and the surface appearstotally blocked. Spectroscopic results indicate that dopamine is adsorbed flat on the surface. At poten-tials higher than 0.6 V vs RHE the three basal planes show a common redox process. The initial formationof the quinone moiety is followed by a chemical step resulting in the formation of 5,6-dihydroxyindolinequinone as final product. This oxidation process has also been investigated by vibrational spectroscopy.