Controlling the Sensing Volume of Metal Nanosphere Molecular Sensors
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Controlling the Sensing Volume of Metal Nanosphere Molecular Sensors Molly M. Miller and Anne A. Lazarides Mechanical Engineering Materials Science, Duke University, Durham, NC, 27708 INTRODUCTION Noble metal nanoparticles and nanoshells support surface plasmons at optical frequencies. These resonances, known as localized surface plasmons (LSPs), are sensitive to the dielectric properties of the environment and, in particular, to the refractive index of the material close to the surface of the particle. This sensitivity can be exploited in molecular detection systems that use nanoparticles functionalized with receptors to (a) bind target molecules and (b) optically transduce the resulting change in the dielectric environment. Optimization of an optical nanoparticle sensor involves tailoring the particle to the target so as to maximize the sensitivity of spectroscopic features to the dielectric variation associated with binding of target molecules to the particle surface. The dependence of nanoparticle plasmon band positions and intensities on the refractive index of the medium has been studied both with theory and experiment [1, 2, 3, 4, 5]. The localized nature of nanoparticle sensitivity previously has been observed and investigated for both nanoplates [6] and nanospheres [7, 8]. Here, we use simulations of nanoparticle UV-vis spectra to examine the localization of the environmental sensitivity of spherically symmetric nanostructures. Using accurate electrodynamic theory, we demonstrate how metal nanospheres and nanoshells can be designed to possess sensing volumes that match the region exterior to the particle into which target molecules assemble. Specifically, we consider Au nanoparticles and Au/Au2S core/shell particles in an aqueous environment (refractive index, n = 1.33) modulated by dielectric surface shells of variable thickness. The dielectric shells are models of biomolecule (n = 1.39) or polyelectrolyte (n = 1.525) surface layers. The former are representative of the target materials in biomolecule sensing systems; the latter are relevant because of their use in experimental calibration of sensing volume.[9] We show that by controlling the size and structure of nanospheres it is possible to tune their sensing volumes over a wide range of useful molecular scale sensing thicknesses. METHODS Mie theory provides an exact electrodynamic description of the interaction of electromagnetic radiation with spherical particles and spherically symmetric layered particles [10, 11]. The theory is exact and accurate insofar as the dielectric functions used to describe the material properly describe the polarization properties of the particle. The Mie solution consists of scattering coefficients, ai or bi, for each mode (electric dipole, magnetic dipole, electric quadrupole, etc) from which fields, cross sections, and efficiencies can be calculated as a function of wavelength. For a given particle geometry, extinction efficiencies, Qext, defined as single particle extinction cross sections normalized by geometri
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