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

Abstract: Gold nanostars of ~70 nm tip to tip distances were synthesized by a seed mediated method and covalently self-assembled on 1,5-pentanedithiol modified electrodes. Electron transfer kinetics at the AuNS/dithiol modified electrodes were studied as a function of AuNS surface density which was varied by increasing their self-assembly time from 8 h, 16 h, 24 h to 32 h. Excellent electrocatalytic properties of AuNSs were observed toward electrochemistry of [Fe(CN)₆]^4−/3− redox couple. The apparent heterogeneous electron transfer constant, k_et, has progressively increased with the surface density of AuNSs bonded to the electrodes from 0.65 × 10⁻⁵ cm s⁻¹ (8 h), 1.47 × 10⁻⁵ cm s⁻¹ (16 h), 3.95 × 10⁻⁵ cm s⁻¹ (24 h) to an excellent 85.0 × 10⁻⁵ cm s⁻¹ (32 h). Electrochemical charging of nanostars was confirmed, for the first time, by 79 times increase of double layer capacitance, C_dl, from 0.34 µF (8 h) to 27 µF (32 h). The electrochemical charging of AuNSs had also a strong influence on the electron tunneling process through the 1,5PDT molecules being more efficient at dense layers of AuNSs. The tunneling parameter, β, has decreased from 1.13 Å⁻¹ (16 h) to 0.50 Å⁻¹ (32 h). The AuNSs were chemically stable toward [Fe(CN)₆]^4−/3− showing no change in shape after electrochemical measurements.