Self-assembly of gold nanoparticles for the voltammetric sensing of epinephrine

Abstract

A novel method for fabricating gold nanoparticles combining the dithiothreitol (DTT) and dodecanethiol (DDT) mixed self-assembled approach is proposed. The mixed self-assembled monolayers were first formed by the assembly of DTT and DDT from solution onto gold electrode. When these thiol-rich surfaces are exposed to Au colloid, the sulfurs form strong bonds to gold nanoparticles, anchoring the clusters to the electrode surface and a new nano-Au surface was obtained. In order to evaluate the electrochemical characteristics of nano-Au electrode, $\text{Fe}(\text{CN}{)}_6^{3-}$ was chosen as molecule probe and cyclic voltammetry (CV) was used. In addition, the functional nano-Au electrode was applied to the voltammetric sensing of epinephrine (EP). The nano-Au electrode is demonstrated to promote the electrochemical response of EP by cyclic voltammetry. The electrode reaction of EP is significantly improved at the nano-Au electrode which results in a large increase in the voltammetric peak current with a detection limit of 6.0 × 10⁻⁸ M and the diffusion coefficient D is as 7.40 × 10⁻⁵ cm² s⁻¹. The nano-Au electrode shows high electrocatalytic activity and excellent sensitivity property.

Introduction

The application of nanotechnology in electrochemistry has generated a frenzy of excitement [1], [2], [3]. One of the reasons for the considerable current interest in nanoparticles is because such materials frequently display unusual physical (structural, electronic, magnetic and optical) and chemical (catalytic) properties. In recent years, gold nanoparticles (GNPs) have been extensively studied in electrochemistry for its special physico-chemical characteristics [4], [5], [6], [7], [8], [9], [10]. The use of GNPs superstructures for the creation of electrochemical devices is an extremely promising prospect. The superstructures give rise to porous, high-surface-area electrodes, where the local microenvironment can be controlled by the crosslinking elements and may lead to specific and selective interactions with substrates [11]. Various methodologies have been used for the tailoring of GNPs on electrode surfaces, which include the anchoring by electrostatic interaction, covalent linkage, electrochemical deposition, etc., [12], [13], [14], [15]. Electrostatic assembly utilizes the negatively charged citrate surface of the particles, while covalent attachment relies on the reaction of the Au surface with thiols or disulfides to form Au⁰S bonds [11]. The GNPs can be immobilized as an organized mono- or multilayer on solid support with the help of short/long chain molecules having suitable functional groups like NH₂, SH at both ends [16]. The fabrication of sensors based on self-assembly GNPs nanostructure is of recent technological interest [17], [18], [19], [20]. Arrays of GNPs have been utilized for electrochemical sensors as they exhibit excellent catalytic activity towards various reactions [21]. In these, the GNPs function as an “electron antennae”, efficiently funneling electrons between the electrode and electrolyte [12], [22].

Herein a novel fabrication procedure for self-assembly GNPs nanostructure based on mixed self-assembled monolayers (SAMs) was investigated. Mixed self-assembled monolayers composed of alkanethiols of different chain lengths offer a controllable route for constructing moieties containing different chemical functionalities [23], [24]. Mixed SAMs are model systems for studies of adsorption, wetting, and heterogeneous electron transfer and have led to important applications in the development of biosensors [25] such as heterogeneous DNA/RNA sensors [26] and preparation of biologically important surfaces [27]. The shorter bisthiol were preferred in order to increase the rate of electron transfer and can provide superior conductive properties when used in electroanalysis. Working on the principle, dithiothreitol (DTT) was used as the crosslinking agent to fabricate GNPs on the surface of the electrode. STM measurements indicate that the structure of DTT monolayers on Au(1 1 1) is disordered and the majority of DTT molecules bind to the gold surface via two AuS bonds [28]. As DTT bears two SH groups, there must be some free SH groups left among the DTT molecules chemically adsorbed at gold [29]. In the presence of dodecanethiol (DDT), some AuRSAu of the DTT layer would turn into HSRSAu and the mixed self-assembled monolayers can provide a more compact and order platform to fabricate GNPs on the electrode surface. The film formed by this technique has the advantages of high organization and uniformity, which could provide a desirable microenvironment to assemble GNPs and facilitate the concentration of the analyte from the bulk solution to the electrode surface. All of these features of the GNPs superstructures together with their porosity suggest that GNPs-functionalized gold electrode could acts as sensitive and rapid sensing devices. The proposed GNPs modified electrode has been applied to investigate the electrochemical behavior of epinephrine (EP) and the electrode is demonstrated to promote the electrochemical response of EP by cyclic voltammetry (CV).