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

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] redox couple. The apparent heterogeneous electron transfer constant, ket, has progressively increased with the surface density of AuNSs bonded to the electrodes from 0.65 × 10 cm s (8 h), 1.47 × 10 cm s (16 h), 3.95 × 10 cm s (24 h) to an excellent 85.0 × 10 cm s (32 h). Electrochemical charging of nanostars was confirmed, for the first time, by 79 times increase of double layer capacitance, Cdl, 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 (16 h) to 0.50 Å (32 h). The AuNSs were chemically stable toward [Fe(CN)6] showing no change in shape after electrochemical measurements.