Electroless plated gold as a support for carbon nanotube electrodes

Abstract

A monolayer of –NH₂ terminated 3-aminopropyltriethoxysilane (APS) was self-assembled onto a p-type silicon (1 0 0) substrate. This amine terminated silane monolayer provided an electrostatic point of attachment for citrate stabilised gold colloid nanoparticles, which act as ‘seed’ particles for the electroless deposition of gold, creating an electrolessly deposited gold layer on silicon. A –NH₂ terminated cysteamine monolayer was then deposited onto the gold layer and carbon nanotubes, with high carboxylic acid functionality, were immobilised via a condensation reaction. A redox active molecule ferrocenemethanol was then chemically attached to the immobilised carbon nanotubes. These nanostructures were used as working electrodes in cyclic voltammetry to observe the oxidation and reduction of ferrocene. Important electrochemical parameters such as electrode kinetics, electron transfer rate and surface concentration of the redox active molecules were obtained, providing information on the ability of electroless plated gold surfaces to act as supports for carbon nanotube-based electrodes. This information has also provided insights into the behaviour of vertically aligned carbon nanotubes immobilised on nanoscale gold wires, which have been previously fabricated using atomic force microscopy.

Introduction

Since their discovery [1], [2], [3] carbon nanotubes have received considerable research attention due to their unique structural, mechanical and electronic properties [4], [5]. Carbon nanotubes display metallic, semiconducting and superconducting electron transport, store guest molecules within their hollow core and have the largest elastic modulus of any known material [5]. It is expected that these unique properties will see carbon nanotubes used in a variety of different fields, such as nanoelectronic devices [4], [6], electron field emission sources [7], batteries and ultra-sharp probes for scanning probe microscopy [8], [9]. One field that has shown a great deal of interest in carbon nanotubes is electrochemistry, in which nanostructuring of carbon nanotube electrodes helps to promote electron transfer reactions [10]. Furthermore, due to the carbon nanotubes’ superb chemical stability, low mass density, low resistivity and large surface area they are the ideal base material for electrochemical sensors or electrodes [5], [10], [11].

There are primarily two strategies employed to modify electrode surfaces with carbon nanotubes [5]. These are through physisorption or chemisorption via chemical reaction or electrochemical surface activation [5], [12]. An example of a physisorption strategy was reported by Cai and Chen [13] in which carbon nanotubes dispersed in cetyltrimethylammonium bromide were spread onto a glassy carbon electrode and held in place with a Nafion coating. Similarly, Zhou and coworkers [14] fabricated a modified electrode by drop coating a N,N-dimethylformamide (DMF) solution of single-walled carbon nanotubes (SWCNTs) on a gold disk electrode.

However, more recently it is the chemically attached, vertically aligned, carbon nanotube electrodes which are showing promise for future device fabrication [15], [16], [17], [18]. These systems provide direct electrical communication between the underlying electrode and redox active species with no need for redox mediators and show potential to create reagent-less sensing devices [5]. The most common method to create chemically immobilised carbon nanotube electrode arrays is by self-assembly [7], [8], [9], [19], [20], [21]. Oxidative treatment of SWCNTs results in the creation of carboxylic acid end groups that can be converted to carbodiimide leaving groups using dicyclohexylcarbodiimide, which in turn can be reacted with an amine terminated self-assembled monolayer [4].

A substantial body of work exists utilising a self-assembled alkanethiol [7], [19], [21] or silane [6] monolayer to chemically attach carbon nanotubes to gold electrodes. For example Lee and Lee [7] have used 11-mercaptoundecanoic acid to attach carbon nanotubes to a patterned gold array on polyethylene terephtalate to create flexible field emitter arrays. Liu and coworkers [19] have prepared electrochemically addressable carbon nanotube microelectrodes using an insulating amino-n-undecylmercaptan monolayer. Zeng and Huang [6] have created a multi-walled carbon nanotube/(3-mercaptopropyl)trimethoxysilane bilayer modified gold electrode for the electrochemical study and determination of fluphenazine. Finally, Gooding et al. [20] have measured the electrochemical characteristics of ferrocenemethylamine-modified single-walled carbon nanotubes immobilised to a self-assembled monolayer of mercaptoethylamine on a polished gold electrode.

Although substantial work has been performed to create electrodes on polycrystalline [21], magnetron sputtered [7], thermally evaporated [22], [23], [24] and bulk [19], [20], [25] gold surfaces, other forms of gold remain to be characterised as support substrates. Electroless gold deposition has recently received considerable research interest as a result of its ability to create coatings on small selected areas [26], [27], [28], [29]. Electroless deposition is an autocatalytic redox process, occurring only on catalytic surfaces [29], which is simple and inexpensive to perform using solution chemistry [27]. In this method Au³⁺ is reduced to bulk metal resulting in surface confined growth of the catalytic gold nanoparticles followed by an eventual coalescence to form a complete layer [27], [28]. Previously, Dong and coworkers [27] have assessed the suitability of gold films prepared by electroless plating for use as electrochemical electrodes. Using the technique of cyclic voltammetry, electroless plated gold was found to be highly stable with no observable change in voltammogram shape after several hundred cycles, suggesting it is ideal for use as an electrode material. However, the effect of changes in surface topography, important to electrochemistry [20] which occurs as a result of placing a self-assembled monolayer, or subsequently carbon nanotubes, on the surface is unclear. To date, very little work has been directed towards the characterisation of self-assembled monolayers on electroless gold. One example by Evans and coworkers [26] used infrared spectroscopy to measure self-assembled monolayer order. For a self-assembled monolayer of octadecanethiol a high level of monolayer ordering was observed compared to a di(nonacosa-10,12-diyn) disulfide layer which showed considerable levels of disorder. The differences in the ordering of these layers would be expected to influence the electrochemical behaviour if they were used as electrodes.

In our previous work [30] carbon nanotubes were immobilised on gold nanostructures electrolessly plated on lithographically defined regions of 3-aminopropyltriethoxysilane (APS) utilising a scanning probe technique known as atomic force anodisation lithography. Due to the inherent difficulty in creating an electrical connection to a gold nanostructure for electrochemical analysis, important information about the electron transfer kinetics could not be obtained. This work attempts to address this issue by using electroless deposition to create a complete gold layer on a silicon wafer, which due the increased surface area, could easily be connected. The chemical approach used remained exactly the same as that used for the nanostructures. Important electrochemical parameters such as electrode kinetics, electron transfer rate and surface concentration of redox active molecules could therefore be obtained. To our knowledge this work also provides the first electrochemical analysis of a carbon nanotube electrode chemically attached to a self-assembled monolayer on electroless plated gold.