Electrochemistry of Pyrroloquinoline Quinone (PQQ) on Multi-Walled Carbon Nanotube-Modified Glassy Carbon Electrodes in Biological Buffers

Pyrroloquinoline quinone (PQQ) is an important electrocatalyst and redox cofactor for many enzymes used in bioanalytical applications. Careful selection of electrode modifications and buffer compositions is required due to PQQ's tendency to be irreversibly reduced on bare electrodes and the strong effect of pH on its electrochemistry. Multi-walled carbon nanotubes (MWCNTs) can effectively modify glassy carbon electrodes, but PQQ's behavior at these surfaces has not been investigated. While phosphate buffers have been used extensively to characterize PQQ's electrochemistry, phosphate buffers can act as chelating agents for important metal ions needed by PQQ and biomolecules. Different physiological buffers could overcome these challenges and expand the buffer options for biofuel cells. Using cyclic voltammetry, we studied PQQ in HEPES, MOPS, TRIS, and phosphate buffers at pH 7 with MWCNT-modified electrodes. While there were no significant differences in PQQ's behavior with the various buffers, the ionic composition of the non-phosphate buffers was very important. We observed that K+ and Mg2+ were the most influential ions for the reversible reduction-oxidation of PQQ. PQQ's electrochemistry was also affected by the length of and the functional groups on the MWCNTs, indicating that the electron transfer kinetics were quite sensitive to the surface chemistry of the electrodes. (C) The Author(s) 2017. Published by ECS. All rights reserved.