Solvent refractive index and core charge influences on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters

This report describes the effect of solvent refractive index and core charge on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters (MPCs). Solution spectra of dodecanethiolate MPCs (5.2 nm average diameter) reveal an 8-nm shift in plasmon band position as the solvent refractive index is varied from n(d)(20) = 1.33 to 1.55. The spectral shift agrees with predictions of Mie theory when the alkanethiolate monolayer is accounted for in the calculations. Electronic charging of the MPC gold core by electrolysis, from its rest potential (-0.16 V) to +0.82 V vs a Ag quasi-reference electrode, causes a 9-nm (516 to 525 nm) shift in the surface plasmon band position. The shift in surface plasmon band position with core charge is compared to the predictions of the concentric sphere model fur MPC capacitance and Mic theory, as a first step toward probing effects of quantized core charging on this important optical property.