Distance- and Wavelength-Dependent Dielectric Function of Au Nanoparticles by Angle-Resolved Surface Plasmon Resonance Imaging

The optical properties of nanoparticles (NPs) near metal surfaces must be better understood in order to fully exploit their signal-enhancing capabilities in optical biosensors, such as in surface plasmon resonance studies. We use angle-resolved SPR imaging and a modified Maxwell-Garnett model to determine the optical properties of 10-nm-diameter gold NPs deposited on planar Au coated with various thicknesses of SiO2. We investigate how the intrinsic NP dielectric constants and extinction coefficients vary as a function of particle-to-metal-substrate distance (d(P-S)) for short distances that span the dimension of the NP (0-26 nm) and as a function of excitation wavelength (lambda(exe)) for values that span the NP absorbance spectrum. When the NPs arc deposited almost directly onto the Au substrate, the NP dielectric function shows anomalous dispersion. At distances far from the planar Au, normal dispersion is observed. The distance dependence, viewed in terms of extinction coefficients, is consistent with a red shift in the NP absorption as d(P-S) decreases. Although this shift has been predicted and observed, the distance dependence of the intrinsic NP dielectric function is not known. Indeed, NP dielectric functions, which are exquisitely sensitive to experimental conditions, are generally not available. Such fundamental NP properties are needed for quantitative applications in high-sensitivity refractive-index-based biosensors and may also he useful for testing theoretical models.