Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment

Abstract

Electrodynamic simulations of gold nanoparticle spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for nanoparticles of various shapes and nanoshells of various structures. Among single-component nanoparticles less than 130 nm in size, sensitivities of dipole resonance positions to bulk refractive index are found to depend only upon the wavelength of the resonance and the dielectric properties of the metal and the medium. Among particle plasmons that peak in the frequency range where the real part of the metal dielectric function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, lambda, to refractive index, n, is found to be a linearly increasing function of lambda, regardless of the structural features of the particle that determine lambda. Quasistatic theory is used to derive an analytical expression for the refractive index sensitivity of small particle plasmon peaks. Through this analysis, the dependence of sensitivity on band position is found to be determined by the wavelength dependence of the real part, epsilon', of the particle dielectric function, and the sensitivity results are found to extend to all particles with resonance conditions of the form, epsilon' = -2chin(2), where chi is a function of geometric parameters and other constants. The sensitivity results observed using accurate computational methods for dipolar plasmon bands of gold nanodisks, nanorods, and hollow nanoshells extend, therefore, to particles of other shapes (such as hexagonal and chopped tetrahedral), composed of other metals, and to higher-order modes. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielectric substrates and sensitivities of nanoparticles to local refractive index changes, such as those associated with biomolecule sensing.

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…When metal nanoparticles are exposed to light on resonance with their absorption wavelength, a collective oscillation of electrons in the conduction band takes place [13,14]. This creates a charge separation with respect to the lattice [2,15]. The confined conduction band electrons in the small particle volume then begin to move in phase with the radiation plane wave excitation, creating a coherent electromagnetic (EM) response which strengthens both the near field energy and the optical extinction associated with the nanoparticle surface [16,17].…”

“…Tuning of the LSPR properties can be achieved through synthesis by exploiting differences in nanoparticle size, geometry, surface morphology, aggregation, aspect ratio, and the dielectric constant of the surrounding media [15,33,[35][36][37][38][39][40][41][42][43][44][45]. Since each application relies on a specific set of conditions for optimized efficiency, the structural parameters of the nanoparticles employed for use must be tailored accordingly.…”

“…These particles also possess large surface to volume (S/V) ratios, tunable plasmon resonance, pinholes that act as hot spots to further intensify EM surface energy and, as hollow structures, are known to be more sensitive to the refractive index of their surroundings than their solid counterparts [15,66,[81][82][83][84]. Additionally, their scattering and absorption cross-sections can be adjusted through synthesis to maximize efficiency for specific applications, and since the first hyperpolarizabilities (ˇ) of hollow nanoparticles are much larger than those of solid NPs having the same size, HGNs are excellent candidates for any application involving non-linear optics [85,86].…”

“…In this case the facet energies of the hexagonal close packed (HCP) structure of the cobalt template are known to increase 0001 < 10-10 < 10-11 < 11-20 < 10-12 < 11-21 with respective energies of 131,140,149,155,156,163 (meV/Å 2 ) [109]. This means that the oxidation and generation of Kirkendall voids were initiated on the [11][12][13][14][15][16][17][18][19][20][21] plane of the cobalt lattice.…”

“…In general, solid structures have higher scattering efficiencies than hollow ones which means that there is less energy available to convert into heat in photothermal applications [15,29,158,213,214]. For example, it has been reported that silica-gold core-shell nanoparticles (150 nm diameter) had total extinctions that are comprised of only 15% absorption efficiency [215].…”