Density-Dependent Electrochemical Properties of Vertically Aligned Gold Nanorods

Abstract

Gold nanorods (AuNRs) with an aspect ratio of 2.33 or 3.16 were self-assembled onto 1,6-hexanedithiol-modified gold electrodes based on covalent interaction at a solution temperature of 35 degrees C. The formation of the 1,6HDT/AuNR bilayers as a function of the nanorods' adsorption time was studied by atomic force microscopy and quartz crystal microbalance, whereas their physical properties and chemical bonding were studied by contact angle and FT-IRRAS spectroscopy measurements. It was found that both types of nanorods were covalently bonded to the Au-1,6HDT-SAM modified electrodes in an end topography and with a high surface density. The electrochemical properties of the Au-1,6HDT-AuNR modified electrodes, as a function of the nanorods' adsorption time, were studied by cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy using [Fe(CN)(6)](3-/4-) as the redox probes. The highest enhancement of the electrical current in the cyclic voltammograms was recorded at the Au-1,6HDT-AuNR modified electrodes for 7 h of chemisorption of 2.33 aspect ratio rods or 15 h of chemisorption of 3.16 aspect ratio rods. The high decrease of the apparent charge-transfer resistance upon nanorod self-assembly suggests a charging of the rods by the [Fe(CN)(6)](3-/4-) in solution and electron transfer across them. Moreover, the variation of the tunneling parameter beta suggests that the electron tunneling process through the 1,6HDT molecules is more efficient at the electrodes modified with bilayers containing short rods (beta = 0.78 +/- 0.08 angstrom(-1)/per methylene unit) than at the electrodes modified with bilayers containing long rods (beta = 0.84 +/- 0.10 angstrom(-1)/per methylene unit). The self-assembly of the AuNRs in an end-bonding topography with a high surface coverage restored almost completely the electronic communication that was entirely blocked by the preceding 1,6HDT layer.