Electrochemical behavior of intramolecular cyclization reaction of catecholamines at carbon nanotube/carboxymethylcellulose electrode

Neurotransmitter catecholamines [dopamine (DA), epinephrine (EP), norepinephrine (NE), and 3,4-dihydroxyphenylalanine (DOPA)] exhibit an electron transfer-chemical reaction-electron transfer (ECE) reaction, wherein two electron transfer reactions are connected by a chemical reaction. The electrochemical behavior of catecholamines depends strongly on the kinetic property of the chemical reaction, that is, the intramolecular cyclization rate. This work provides systematic knowledge of the intramolecular cyclization reactions of catecholamines in the ECE process. We adopt a carbon nanotube (CNT)/carboxymethylcellulose (CMC) electrode because it shows an apparent ECE profile in cyclic voltammetry (CV) results, unlike other electrodes. The analysis of the electrochemical behavior in the ECE mechanism is justified by simulations using the DigiElch simulation program. The cyclization rate of various catecholamines follows a specific order: DA < NE < DOPA < EP. Because the cyclization step involves hydrogen ions, the rate depends on pH- the cyclization rate increases with a decrease in pH. The electrochemical behavior due to cyclization is verified on the basis of the CV results, particularly the change in the intensity of the cathodic peak (Peak Ic) due to the reduction of catecholamine-o-quinone. The intensity of Peak Ic is determined by (i) the type of catecholamines, (ii) pH, and (iii) the time scale of CV. This work contributes to the in-depth understanding of catecholamines and the development of novel strategies for the electrochemical sensing of catecholamines.

Automated summary

This study investigates the intramolecular cyclization reactions of catecholamines in the ECE process, revealing that the cyclization rate is pH-dependent and follows a specific order for different types of catecholamines. The electrochemical behavior is verified by analyzing the change in cathodic peak intensity due to the reduction of catecholamine-o-quinone. This work contributes to a deeper understanding of catecholamines and the development of novel strategies for electrochemical sensing.