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 seedmediated 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 R CT 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 ) 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.

Charge Transfer Within Multilayered Films of Gold Nanorods

“…This difference demonstrates a size effect of the gold nanorods, the short rods having a stronger catalytic activity toward the electrochemical process than the large rods. The k et variations are consistent with previous published work in which was demonstrated a size effect of the rods for low and high surface coverages with rods at Au-1,6HDT-AuNR bilayers modified electrodes (Chirea et al 2009(Chirea et al , 2010. In those two reports was demonstrated for the first time the dependence of the electron transfer process on the size of these anisotropic gold nanorods and on the type of surface bonding (end or side surface bonding of rods).…”

“…The different chemical structures of these two types of multilayers will determine a different electrochemical behaviour as it will demonstrate in the following paragraphs. The vertical alignment of the rods within these films is in accordance with previous observation related to the type of surface bonding (end-bonding) of the rods when self-assembled on gold electrodes at 35ºC solution temperature and using the 1,6hexanedithiol as bridge molecules (Chirea et al, 2009(Chirea et al, , 2010. The thicknesses of the multilayers were determined by ellipsometry measurements.…”

“…The Faradaic process was highly hindered by the 1,6hexanedithiol layers ( Figure 5, n= 1, 3, 5, 7). This electrochemical behaviour is caused by the insulating properties of this dithiol when self-assembled on gold electrodes (Chirea et al, 2009(Chirea et al, , 2010Chou et al, 2009). For example, the first layer self-assembled on the gold electrodes determined a disappearance of the Faradaic currents in the cyclic and square wave voltammograms ( Figure 5, n = 1, black curve) as compared to the bare gold electrode ( Figure 5, n=0, dashed black curve) illustrating a dense packing of the 1,6hexanedithiol layer on the gold electrodes.…”

“…The charge transfer through these types of molecular linkers is characterized by a decay of the tunnelling current with the length of the linker, the tunnelling coefficient being around β≈0.8Å -1 (Adams et al, 2003;Finklea, 1987Finklea, , 1996. However, recent studies demonstrate that the tunnelling decay is not the dominant property of the films when the nanomaterials are tethered at the end of the molecular bridges determining a fast electron transfer process and moreover, this fast electron transfer process is dependent on the size and the shape of the nanomaterials (Bradbury et al, 2008;Chirea et al, 2009Chirea et al, , 2010Kissling et al, 2010). A more detailed research has to be developed in order to fully understand the mechanism behind this fast electron transfer processes taking place at electrodes modified with hydrid nanostructures.…”

Electron Transfer at Gold Nanostar Assemblies: A Study of Shape Stability and Surface Density Influence

“…Nowadays, molecular nanoelectronics represent and important research field in which organic molecules and inorganic nanomaterials are combined to form electronic components [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. For this purpose, self-assembled monolayers (SAMs) of bifunctional organic thiols have been extensively used for the chemical binding of metallic nanomaterials to various substrates and studies of electron transport [1][2][3][4][5][6][7].…”