Enhancement of the electron transfer rate in carbon nanotube flexible electrochemical sensors by surface functionalization

Thin films of single-walled carbon nanotubes (SWNTs) possess high potential for use in flexible electrochemical sensors in wearable medical devices. However, the electrochemical activity of such materials is not yet fully understood. We studied the enhancement in electrochemical activity of SWNT-based flexible electrochemical sensors. To exclude the effect of surface contaminations from the fabrication processes on the electrochemical activity, we introduced the dry process for high-quality and clean SWNT film formation and the clean device fabrication process with a protective layer. So-fabricated SWNT electrodes exhibited a low electron transfer rate. Electrochemical functionalization with an H2SO4 solution successfully enhanced the electrochemical activity of the SWNT electrode for the inner sphere probes such as [Fe(CN)6]4−/3−. This method is quite gentle and controllable, but also effective at increasing the electron transfer rate without either degrading the potential window. We found out that there was a correlation between the electron transfer rate and the amount of defects evaluated from Raman scattering spectroscopy. XPS analysis showed that the functionalization process introduced CO and CO species, suggesting that these species constituted active sites for inner sphere probes. The electrochemical functionalization was also effective for enhancing the limit of detection in dopamine detection with the flexible SWNT electrode. The limit of detection was ∼100 nM for functionalized electrode whereas it was ∼1 μM for as-fabricated electrodes.