Unique Properties of Nanowire Electrodes Enhanced Electrochemical Signals Provide Heightened Sensitivity

Many of the strategies proposed that exploit nanostructures for biosensing require the measurements of small currents, which require sophisticated electronics [15,18]. Nanowire electrodes, where nanoscale objects are used as an ensemble, generate nano- to microamps currents that are easily measured using simple and inexpensive instrumentation. Moreover, nanowire electrodes present another advantage for electrochemical biosensing; they produce amplified currents when electrocatalytic reactions are used for readout [20].

The amplification of electrocatalysis at DNA-modified nanowires reflects the importance of diffusion in this type of process [33] The efficiencies of electrocatalytic reactions depend strongly on the kinetics of cross-reactions for the two redox-active species involved, but are also very sensitive to rates of diffusion for the primary electron acceptor [33] This species must move through the film it is bound to (in this case DNA) in order for turnover to be achieved by the secondary electron acceptor. Systematic studies of electrocatalysis between Ru(NH3)|+ and Fe(CN)6~ at macro- and nanowire electrodes revealed significant enhancements in the efficiency of the reaction at the nanostructured surface [20] The different efficiencies of electro-catalysis at macroelectrodes as compared to nanowire electrodes indicate that the mobilities of ions bound to these two substrates may be different. The three-dimensional structures of the nanowires open the possibility that radial diffusion may occur, and also that the radius of curvature of the individual nanowires may lead to more efficient diffusion for ions binding in equilibrium with the DNA film. Moreover, at the nanowires, Ru(III) appears to exhibit greater diffusional mobility as compared to when bound to a macroelectrode, which may also contribute to the observed increase in electrocataly-sis. The more open structure of this film likely permits better diffusion of the primary electron acceptor, although still prohibiting the secondary electron acceptor from reaching the surface.

The net result of facile diffusion for ions along and around DNA-modified nanowires is enhanced electrocatalytic turnover of bound ions (Figure 6.5). At low concentrations of

0 0

Post a comment