Characterization of carbon nanofiber electrode arrays using electrochemical impedance spectroscopy: effect of scaling down electrode size.

We report here how the electrochemical impedance spectra change as (i) electrode size is reduced to nanometer scale and (ii) spacing between vertically aligned carbon nanofiber (VACNF) electrodes is varied. To study this, we used three types of electrodes: standard microdisks (100 microm Pt, 10 microm Au, and 7 microm glassy carbon), randomly grown (RG) VACNFs where spacing between electrodes is not fixed, and electron beam patterned VACNF nanoelectrode arrays (pNEAs) where electrode spacing is fixed at 1 microm. As the size of the microdisk electrode is reduced, the spectrum changed from a straight line to a semicircle accompanied by huge noise. Although a semicircle spectrum can directly indicate the electron transfer resistance (R(ct)) and thus is useful for biosensing applications, the noise from electrodes, particularly from those with diameters < or =10 microm, limits sensitivity. In the case of VACNFs, the electrode spacing controls the type of spectrum, that is, a straight line for RG VACNFs and a semicircle for pNEAs. In contrast to microdisks, pNEAs showed almost insignificant noise even at small perturbations (10 mV). Second, only pNEAs showed linearity as the amplitude of the sinusoidal signal was increased from 10 to 100 mV. The ability to apply large amplitudes reduces the stochastic errors, provides high stability, and improves signal-to-noise (S/N) ratio. This new class of nanoelectrochemical system using carbon pNEAs offers unique properties such as semicircle spectra that fit into simple circuits, high S/N ratio, linearity, and tailor-made spectra for specific applications by controlling electrode size, spacing, and array size.