Local Refractive Index Sensing Based on Edge Gold-Coated Silver Nanoprisms

Abstract

Bulk and surface refractive index sensitivity for localized surface plasmon resonance (LSPR) sensing based on edge gold-coated silver nanoprisms (GSNPs) and gold nanospheres was investigated and compared with conventional surface plasmon resonance (SPR) sensing based on propagating surface plasmons. The hybrid GSNPs benefit from an improved stability since the gold frame protecting the unstable silver facets located at the silver nanoprisms (SNPs) edges and tips prevents truncation or rounding of their sharp tips or edges, maintaining a high refractive index sensitivity even under harsh conditions. By using layer-by-layer deposition of polyelectrolytes and protein adsorption, we found that GSNPs exhibit 4-fold higher local refractive index sensitivity in close proximity (<10 nm) to the surface compared to a flat gold film in the conventional SPR setup. Moreover, the sensitivity was 8-fold higher with GSNPs than with gold nanospheres. This shows that relatively simple plasmonic nanostructures for LSPR-based sensing can be engineered to outperform conventional SPR, which is particularly interesting in the context of detecting low molecular weight compounds where a small sensing volume, reducing bulk signals, is desired. S urface plasmon resonance (SPR) is one of the most widely used optical techniques for label-free, real-time biosensing. 1 Conventional SPR analysis is based on propagating surface plasmons which are electromagnetic charge oscillations, excited at the interface between a thin metal film and a dielectric. 2 Excitation of surface plasmons using light results in a pronounced minimum in the reflectivity spectrum, where the condition for excitation is highly dependent on the refractive index (RI) in close proximity of the metal surface. This allows for sensitive detection of molecular events occurring at the metal interface, either using angular or spectral interrogation. 3 Today, conventional SPR is routinely used in several different areas and has evolved into a mature biosensing technology. 4−6Over the past decade, so-called localized surface plasmon resonance (LSPR) has received considerable interest. 7−9 LSPR is caused by collective electron oscillations confined in metal nanostructures and can typically be excited by visible or nearinfrared light, where the resonance frequency is highly dependent on the shape, size, material, composition, surface modification, and aggregate morphology. 10−16 The resonance condition is also dependent on the polarizability of the surrounding medium. The possibility to use metal nanostructures for detection of small local refractive index changes and the possibility for integration and miniaturization has made LSPR-based sensing an attractive alternative to conventional SPR. 17 Both sensing strategies utilize the plasmonic properties of noble metals and the strong electromagnetic field created at the plasmon resonance frequency. The intensity and decay length of the electromagnetic field are considerably different for propagating surface plasmons compared to localized...

High-sensitivity integrated devices based on surface plasmon resonance for sensing applications

“…[52] (MoS 2 /graphene hybrid structures). A final comparison in terms of the FOM parameter also shows a better performance of the proposal presented in this contribution than previously reported values for this parameter [46,48,51,52].…”

“…[47] (aluminum gratings coated with gold), Ref. [48] (gold coated over silver nanoprisms), and Ref. [49] (metallic metasurfaces).…”

Polypeptide functionalized gold nanoparticles for bioanalytical applications

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