How does the particle density affect the electrochemical behavior of gold nanoparticle assembly?

Gold nanoparticles assembled on conducting substrates by organic films are usually employed as new-type electrodes, which show great potential in electrochemistry. The purpose of this work is to clarify how the particle density influences the electrochemical behavior of nanoparticle assembly electrodes. In this work, gold nanoparticles (AuNPs) are immobilized on gold substrates by using, 11-amino-1-undecanethiol (AUT) as bridging molecules, and the resulting Au/AUT/AuNPs are characterized by scanning electron microscopy (SEM) and electrochemical methods. It is found that the particle density has a great influence on the cyclic voltammetric (CV) behaviors of Au/AUT/AuNP electrodes. Briefly, the higher the particle density, the more reversible the CV behavior of AuNP assembly electrodes. The particle-density-dependent CV behavior is attributed to the variations in the number of electron tunneling channels between AuNPs and the underlying gold substrates as a function of particle density. A higher particle density results in more tunneling channels and then a smaller apparent tunneling resistance, R-t(app). The R-t(app) of the Au/AUT/AuNPs reflects the tunneling probability across the insulating organic layer that immobilizes the AuNPs on the substrates. It is demonstrated that the influence of Rt(app) on the CV behavior of the Au/AUT/AuNP electrode is quite similar to that of the solution resistance. The theory of solution resistance can be successfully used to calculate Rt(app) and the particle density. The calculated particle densities are in reasonable agreement with those obtained from SEM measurements. It is also shown that the effect of tunneling on the CV behavior of Au/AUT/AuNPs can be greatly reduced by using electrodes with a high particle density and slow potential sweep. Our work provides solid evidence that the tunneling process across the bridging molecules has a great effect on the electrochemical behaviors of nanoparticle array electrodes. As a result, when assembled nanoparticle electrodes are used in electroanalytical works, the tunneling process must be taken into account, especially for the electrode with a relatively lower particle density.