We demonstrate a versatile fabrication technique utilizing colloidal lithography to make highly symmetric thin-walled nanorings of different materials. The approach has been extended to the fabrication of concentric multi-walled rings of several different material layers. Hybridized localized surface plasmon resonances (LSPRs) present in the near infrared for single and concentric gold nanorings, have been investigated with extinction spectroscopy and compared to theory. A particular focus has been on the potential application of gold nanorings as an ultrasensitive biosensor platform.
The rings are shown to have high sensitivity to changes in the surrounding dielectric environment, as bulk or thin film layers. The "bulk" refractive index sensitivities of gold nanorings (diameters 75-150nm) are substantially (>5 times) larger than those of nanodisks with similar diameters. The highest so far reported sensitivity, 880nm/RIU, was obtained for nanorings. Nanoring LSPRs are significantly more sensitivity to bulk refractive index changes than those of nanodisks at the same spectral position, demonstrating a clear shape dependence. However, theoretical modelling indicates that the shape dependence correlates to a systematic difference in the influence of the dielectric substrate. A shape independent bulk refractive index sensitivity has previously been predicted by theoretical calculations for solution phase structures with LSPR peak position <800nm[1]. The spectral sensitivity of nanoring LSPRs to thin films of alkanethiols was ~5.9 nm/CH2, the highest such reported sensitivity. As a proof of concept we demonstrate the real-time label-free monitoring of protein binding via molecular recognition using the biotin-neutravidin model system.
Hybridization of the dipolar plasmon resonances of the two individual rings in concentric gold-dielectric-gold nanorings was clearly observed in the experimental spectra. Altering the strength of the coupling between the ring plasmons, by either changing the thickness of the dielectric spacer or the energy of the outer gold ring LSPR, lead to significant spectral changes.
1. Miller, M. M.; Lazarides, A. A. Journal of Physical Chemistry B 2005, 109, (46), 21556-21565. |