Spatially-resolved bioelectrochemistry with scanning electrochemical cell microscopy: A microscale study of coenzyme Q₁₀ modified carbon electrodes

The field of bioelectrochemistry continues to develop due to diverse applications ranging from biosensors to biotechnology and biofuel cells. Accordingly, research understanding the fundamental energy conversion step(s) in these systems (namely electron transfer) will enable the construction of more efficient and better characterised (bio)electrodes. This work investigates surface-confined coenzyme Q₁₀ on glassy carbon (GC-coQ₁₀) and highly ordered pyrolytic graphite (HOPG-coQ₁₀) electrodes using scanning electrochemical cell microscopy (SECCM) in the cyclic voltammetric hopping mode regime. Although frequently studied in bulk at macroscopic electrodes, this is the first report where SECCM has been used to explore how the electrode surfaces employed influences the electrochemical response of immobilised biomolecules in a spatially resolved manner. Full SECCM scans comprised of 1600 cyclic voltammetric measurements on GC-coQ₁₀ and HOPG-coQ₁₀ electrodes demonstrate and quantify the microscopic variability in responses of coQ₁₀ on these surfaces. The HOPG-coQ₁₀ electrode shows a significantly larger distribution of key parameters compared to the GC-coQ₁₀ electrode, such as the voltammetric peak-to-peak separation (∆E_p = 0.87 ± 0.02 V and 0.426 ± 0.008 V on HOPG and GC, respectively) and peak full width at half maximum (e.g., anodic FWHM = 0.25 ± 0.02 V and 0.202 ± 0.003 V on HOPG and GC, respectively). Of note, a unimodal mid-point potential (E^mid) distribution is observed for GC-coQ₁₀ (E^mid = -0.150 ± 0.002 V vs. Ag|AgCl) whilst a bimodal distribution is seen on HOPG-coQ₁₀ (E^mid = -0.11 ± 0.01 and -0.06 ± 0.01 V vs. Ag|AgCl). This study provides a proof of concept that SECCM can be applied to electrode systems commonly used in classical bioelectrochemistry, highlighting the potential of this technique to be used in diverse application areas.