Tunable Plasmon Resonance of Gold Nanoparticles Functionalized by Electroactive Bisthienylbenzene Oligomers or Polythiophene

We investigate the effect of new redox molecular switches based on oligothiophene deposited on gold nanoparticles (AuNPs) as thin electroactive layers in the 5–80 nm thickness range. In doing so, we compare systems based on physisorbed electroactive layers (weak electronic coupling) with those based on covalently bonded layers (strong electronic coupling), and we investigate orientation and thickness effects. Two different deposition methods were used. The first is based on bithiophene electropolymerization and the second on diazonium salt electroreduction. In both cases, redox switching of the electroactive layer makes is possible to tune the plasmonic properties of the AuNPs, and the layer thickness has a strong impact on the amplitude of the localized surface plasmon resonance (LSPR) modulation. LSPR modulation upon redox switching also depends on the electronic coupling regime between the AuNP and the organic layer. Indeed, the apparent real part of the dielectric constant seen by the AuNP is larger when oligothiophenes are covalently bonded to the AuNPs. Moreover, the LSPR wavelength, in the 700–750 nm range, shifts in the opposite direction upon redox switching of the organic layers in weak or in strong electronic interaction with the AuNPs. These behaviors may be attributed to orientation effects, but also suggest that, in a strong electronic coupling regime, plasmon delocalization within the covalently grafted conducting organic material is enhanced