Plasmon resonances of GZO core-Ag shell nanospheres, nanorods, and nanodisks for biosensing and biomedical applications in near-infrared biological windows I and II.

Abstract

There is currently a great deal of interest in realizing localized surface plasmon resonances (LSPRs) in two distinct windows in the near-infrared (NIR) spectrum for in vivo biosensing and medical applications, the biological window (BW) I and II (BW I, 700-900 nm; BW II, 1000-1700 nm). This study aims to demonstrate that LSPRs of Ga-doped ZnO (GZO) core-silver (Ag) shell structures exhibit promising features for biological applications in the NIR BW I and II. Here, we study three different shapes for nanoshells: the core-shell nanosphere, nanorod, and nanodisk. In the calculation of the optical response of these nanoshells, an effective medium approach is first used to reduce the dielectric function of a nanoshell to that of an equivalent homogenous NP with an effective dielectric function. Then, the LSPR spectra of nanoshells are calculated using the modified long-wavelength approximation (MLWA), which corrects the polarizability of the equivalent NP as obtained by Gans theory. Through numerical investigations, we examine the impacts of the core and shell sizes of the proposed nanoshells as well as the medium refractive index on the position and line width of the plasmon resonance peaks. It is shown that the plasmon resonances of the three proposed nanoshells exhibit astonishing resonance tunability in the NIR region by varying their geometrical parameters. Specifically, the improved spectrum characteristics and tunability of its plasmon resonances make the GZO-Ag nanosphere a more viable platform for NIR applications than the spherical metal colloid. Furthermore, we demonstrate that the sensitivity and figure of merit (FOM) of the plasmon resonances may be significantly increased by using GZO-Ag nanorods and nanodisks in place of GZO-Ag nanospheres. It is found that the optical properties of the transverse plasmon resonance of the GZO-Ag nanodisk are superior to all plasmon resonances produced by the GZO-Ag nanorods and GZO-Ag nanospheres in terms of sensitivity and FOM. The FOM of the transverse plasmon mode of the GZO-Ag nanodisk is almost two orders of magnitude higher than that of the longitudinal and transverse plasmon modes of the GZO-Ag nanorod in BW I and BW II. And it is 1.5 and 2 times higher than the plasmon resonance FOM of GZO-Ag nanospheres in BW I and BW II, respectively.