Multi-walled carbon nanotubes: Size-dependent electrochemistry of phenolic compounds

The electrochemical activity of functionalized multi-walled carbon nanotubes (MWCNTs) of different sizes (D × L: 100-170 nm × 5-9 μm and 6-9 nm × 5 μm, corresponding to LD- and SD-MWCNTs, respectively) towards the electro-oxidation of hydroquinone (HQ), tert-butylhydroquinone (TBHQ), catechol (CT), and dopamine (DP) was investigated. The electrochemical response for all phenolic compounds at MWCNT-modified glassy-carbon electrodes (GCEs) presented a short decrease in the overpotential of the oxidation reactions (<100 mV); however, the values of ΔEp (peak-to-peak separation) were considerably reduced at the LD-MWCNT surface and even more at the SD-MWCNT-modified GCE in comparison with the unmodified GCE. This result indicates the higher electrocatalytic activity of SD-MWCNTs, which is in agreement with the nanotube diameter and the increased defect density, as revealed by XRD and Raman spectroscopic measurements, respectively. Additionally, a substantial increase (three-fold) in the current for the oxidation of HQ, TBHQ, and DP at the SD-MWCNT-modified GCE was obtained. Amperometric measurements also revealed the improved performance of SD-MWCNT-modified GCE (up to four-fold increase in sensitivity).