Size-Dependent Electrochemical Properties of Gold Nanorods

Abstract

Electrochemical properties of Au electrodes sequentially modified by self-assembled 1,6 hexanedithiol (1,6HDT) and gold nanorods (AuNRs) are investigated by cyclic voltammetry, square-wave voltammetry, and electrochemical impedance spectroscopy, using [Fe(CN)6]3−/4− as redox probes. The nanorods stabilized by cetyltrimetylammonium bromide (CTAB) with aspect ratios of 2.20, 2.80, and 3.77 were grown by a seed-mediated procedure and chemically bonded to the 1,6HDT-coated electrodes by a place exchange reaction at a 27 °C solution temperature. Topographic tapping mode atomic force microscopy measurements revealed an end bonding of the 2.20 aspect ratio rods and a side surface bonding of the 2.80 and 2.77 aspect ratio rods. Analysis of the electrochemical responses as a function of the sizes and surface orientations of the rods revealed that the electron transfer is faster at electrodes modified with smaller and vertically aligned nanorods than those modified with larger and randomly attached nanorods (side surface bonding). A progressive increase in the charge transfer resistance RCT from bilayers composed of 1,6HDT and 2.20 aspect ratio rods to bilayers composed of 1,6HDT and rods of 2.80 or 3.77 aspect ratios was described by a tunneling parameter of β = 1.07 per thiol chain unit. This behavior suggests that the electron transfer kinetics is controlled by coherent electron tunneling across the 1,6HDT monolayer. In addition, the several orders of magnitude changes of the apparent charge transfer resistance upon nanorod adsorption suggest a charging of the rods by the redox probes in solution and electron transfer across them. It is concluded that the electron transfer proceeds via a three-step process: charging of the rods by the redox probes in solution, electron transport across the rods, and electron tunneling across the 1,6HDT−SAM toward the underlying Au substrates.