Controlling the Sensing Volume of Metal Nanosphere Molecular Sensors

摘要

Using calculated UV-vis spectra of Au nanoparticles and nanoshells with dielectric overlayers, we have shown that the environmental dielectric sensitivity of the particle dipole plasmon band is confined to dielectric variations in the environment close to the particle surface. We have characterized the extent of the sensing volume of both single component particles and dielectric core/ metal shell particles and have concluded that the thickness of the sensitive region is controlled primarily by the size and structure of the nanoparticle or shell. For the smallest nanoparticles, i.e. particles that behave as electrostatic dipoles, the thickness of the dipolar sensing volume grows linearly with particle size. Larger nanoparticles, i.e. particles large enough to feel the nonuniformity of the incident field, have sensing volumes not much thicker than those of smaller particles. We have also shown that the size at which the sensing volume begins to be constrained is a function of the structure of the nanoparticle; Au nanoparticles with dielectric cores, i.e. nanoshells, display proportional growth of the sensing thickness to larger particle sizes than exhibited by solid Au nanoparticles and thus provide more extensive tunability of the sensing thickness. We understand this phenomenon to be a consequence of the longer wavelength (and hence, more spatially uniform nature of the incident fields) of the exciting light in nanoparticle systems that have lower energy resonances; in ongoing work, we are exploring this notion. Thus, while the primary factor controlling the sensing volumes of particle based dielectric sensors is particle size, resonant wavelength (which is the primary controlling factor in planar SPR detection systems), also plays a role. We have observed, as well, that in nanoparticle-based dielectric sensing systems, the extent of the sensing volume is a function of the refractive index contrast of the layer being detected. Elsewhere we explore this more fully. Within the range of dielectric variation relevant for biomolecule detection, however, variations in sensing volume with dielectric contrast are small and interfere not at all with the general property that nanoparticle SPR sensors, unlike planar SPR sensors, have sensing thicknesses that are comparable in size to target molecule monolayers. The tailorability of nanoparticle sensing thickness demonstrated herein should play a significant role in optimization of these systems for specific applications. Future work will extend quantitative studies of sensing volume to nonspherical particles. However, we expect the principles illuminated here to provide a basis for an understanding of what controls sensing volumes in nanostructures, generally.