The Effect of the Buffering Capacity of the Supporting Electrolyte on the Electrochemical Oxidation of Dopamine and 4‐Methylcatechol in Aqueous and Nonaqueous Solvents

Abstract

Dopamine was electrochemically oxidized in aqueous solutions and in the organic solvents N,N-dimethyl-formamide and dimethylsulfoxide containing varying amounts of supporting electrolyte and water, to form dopamine ortho-quinone. It was found that the electrochemical oxidation mechanism in water and in organic solvents was strongly influenced by the buffering properties of the supporting electrolyte. In aqueous solutions close to pH 7, where buffers were not used, the protons released during the oxidation process were able to sufficiently change the localized pH at the electrode surface to reduce the deprotonation rate of dopamine ortho-quinone, thereby slowing the conversion into leucoaminochrome. In N,N-dimethylformamide and dimethylsulfoxide solutions, in the absence of buffers, dopamine was oxidized to dopamine ortho-quinone that survived without further reaction for several minutes at 25 °C. The voltammetric data obtained in the organic solvents were made more complicated by the presence of HCl in commercial sources of dopamine, which also underwent an oxidation process.

Antiradical Activity of Dopamine, L-DOPA, Adrenaline, and Noradrenaline in Water/Methanol and in Liposomal Systems

“… g Calculated for two-proton, two-electron reduction of L-DOPA to dopaquinone at pH 7.4 from the equation E °′ pH = E ° – 0.059 × pH, where E ° is a formal redox potential (0.745 V vs NHE at pH 0). 37 The same calculations for pH 5.5 gave an E °′ 5.5 of 0.42 V. h At pH 7.0, in agreement with values of 0.405 V 38 and 0.40 V. 39 The redox potential for nondeprotonated DA is 0.752 V 40 or 0.801 V 41 at pH 0, 0.612 V 41 at pH 3.2, and 0.56 V at pH 4.5. 42 The standard potential value ( E °) for two-electron, two-proton (−2e/–2H + ) reduction of DA quinone to DA was described as 40 E °′ = −47.93 × pH + 558.4 mV (vs Ag/AgCl, 3 M KCl) giving, after recalculation into NHE, E ° = 0.75 V at pH 0, E °′ 5.5 = 0.491 V, and E °′ 7.4 = 0.40 V. …”

“…At pH 7.0, in agreement with values of 0.405 V 38 and 0.40 V. 39 The redox potential for nondeprotonated DA is 0.752 V 40 or 0.801 V 41 at pH 0, 0.612 V 41 at pH 3.2, and 0.56 V at pH 4.5. 42 The standard potential value ( E °) for two-electron, two-proton (−2e/–2H + ) reduction of DA quinone to DA was described as 40 E °′ = −47.93 × pH + 558.4 mV (vs Ag/AgCl, 3 M KCl) giving, after recalculation into NHE, E ° = 0.75 V at pH 0, E °′ 5.5 = 0.491 V, and E °′ 7.4 = 0.40 V.…”

Non-leaching antimicrobial surfaces through polydopamine bio-inspired coating of quaternary ammonium salts or an ultrashort antimicrobial lipopeptide

“…The voltammogram showed a quasireversible electron transfer step at 0.37 V (vs. Ag/AgCl), which closely resembles that of dopamine in buffered solution. 35,36 The redox step most likely represents a two-electron, two-proton process oxidation of a polymerized dopamine film to form the corresponding reactive ortho-quinone derivative.…”

Nonenzymatic dopamine biosensor based on tannin nanocomposite

“…Furthermore, the electrochemical oxidation of DA strongly depends on the buffering properties of the supporting electrolyte. If the buffer capacity is not enough to neutralize the H + produced during the oxidation of DA, the pH at the surface of electrode will change and deprotonation and oxidation rate of DA reduces leading to a lower current response 26,28 . On the other hand, increasing the concentration of and in the electrolyte will set up a concentration gradient next to the electrode which helps to diffuse the positively charged DA toward the electrode.…”