Single-Electron Charging of Thioctic Acid Monolayer-Protected Gold Clusters

Abstract

There is great interest in the use of Monolayer-Protected Gold Clusters (AuMPCs) as nanoscale capacitors in aqueous media for nanobiotechnological applications, such as bioelectrocatalysts, biofuel cells, and biosensors. However, AuMPCs exhibiting subattofarad double-layer capacitance at room temperature, and the resolution of single-electron charging, has been mainly obtained in an organic medium with nonfunctional capping ligands. We report here the synthesis of Thioctic Acid Monolayer-Protected Au Clusters (TA-AuMPCs) showing electrochemical single electron quantized capacitance charging in organic and aqueous solutions and when immobilized onto different self-assembled monolayer-modified gold electrodes. The presence of functional carboxylic groups opens a simple strategy for interfacing a nanoparticle assembly to biomolecules for their use as electron donors or acceptors in biological electron transfer reactions.

Figure 1

Absorption and derivative spectra of a solution of TA-AuMPCs in EtOH (A) and in Milli-Q water (B) obtained by procedure (i) or (ii), respectively.

Figure 2

Differential pulse voltammograms (DPV) in organic media (CH₂Cl₂ + 0.1 M NBu₄PF₆ as supporting electrolyte) and plots of formal potentials of the quantized capacitance charging versus the valence states of: (a, b) C6-AuMPCs and TA-AuMPCs in solution; (c) TA AuMPCs immobilized on a 1,9-nonanedithiol self-assembled monolayer modified gold electrode. C_MPC is calculated from the ΔV according to C_MPC = e/slope of right plots, where e is the electron charge. QRE stands for Quasi Reference Electrode.

Figure 3

Differential pulse voltammograms (DPV) in aqueous media (NH₄PF₆) and the dependence of formal potentials of the quantized capacitance charging on the valence state of TA-AuMPCs immobilized on a: (a) 1,9-nonanedithiol; (b) 1,6-hexanedithiol self-assembled monolayer modified gold electrode.

Figure 4

Differential pulse voltammograms (DPVs) in aqueous media using 0.1 M NH₄PF₆ as the supporting electrolyte and plots of formal potentials of the quantized capacitance charging versus their valence states of TA-AuMPCs immobilized on a: (a) 1,9-nonanedithiol; (b) biphenyl-4,4′-dithiol self-assembled monolayer modified gold electrode.