Self-Assembled Carbon Nanotube Electrode Arrays: Effect of Length of the Linker between Nanotubes and Electrode

The influence of the length of the carbon chain of a self-assembled monolayer (SAM) on gold electrodes on the electrochemical performance of carbon nanotube arrays attached to the SAM was explored. Four electrode constructs were assessed, all of which were modified with four different lengths (C2-C11) of amine-terminated alkanethiols. The four constructs were gold electrodes modified (1) with SAMs alone, (2) with carbon nanotubes randomly dispersed onto the SAM-modified electrodes by drop coating, (3) with vertically aligned carbon nanotubes formed by self-assembly onto the SAMs, and (4) with vertically aligned nanotubes with ferrocene attached to the nanotubes. By use of ruthenium hexaammine as a redox probe, the attachment of the carbon nanotubes to the SAM, either randomly dispersed or aligned, enabled electrochemistry to be observed at SAMs that were passivating prior to attachment of the nanotubes. The electrochemistry decayed exponentially with methylene chain length as expected but with a surprisingly low attenuation factor (beta value) for the nanotube-modified surfaces. For randomly dispersed nanotubes, the beta value was 0.27 per -CH2- (s = 0.04, n = 4), and for the vertically aligned nanotubes, 0.66 per -CH2- (s = 0.04, n = 4). A similar beta value of 0.62 per -CH2- for vertically aligned nanotubes with ferrocene attached provided good evidence that the results with ruthenium hexaammine were due to tunneling through the SAM rather than electrochemistry proceeding via defects in the SAM or the nanotubes penetrating the SAM to the underlying electrode.