Ultrasensitive Biosensors Using Enhanced Fano Resonances in Capped Gold Nanoslit Arrays

Abstract

Nanostructure-based sensors are capable of sensitive and label-free detection for biomedical applications. However, plasmonic sensors capable of highly sensitive detection with high-throughput and low-cost fabrication techniques are desirable. We show that capped gold nanoslit arrays made by thermal-embossing nanoimprint method on a polymer film can produce extremely sharp asymmetric resonances for a transverse magnetic-polarized wave. An ultrasmall linewidth is formed due to the enhanced Fano coupling between the cavity resonance mode in nanoslits and surface plasmon resonance mode on periodic metallic surface. With an optimal slit length and width, the full width at half-maximum bandwidth of the Fano mode is only 3.68 nm. The wavelength sensitivity is 926 nm/RIU for 60-nm-width and 1,000-nm-period nanoslits. The figure of merit is up to 252. The obtained value is higher than the theoretically estimated upper limits of the prism-coupling SPR sensors and the previously reported record high figure-of-merit in array sensors. In addition, the structure has an ultrahigh intensity sensitivity up to 48,117%/RIU.

Plasmonic Sensing and Modulation Based on Fano Resonances

“…These Fano sensors have considerable applications in the detections of the concentration of the bulk gas, liquid, and macromolecules layers. In recent years, the Fano sensors based on the SPP modes mainly focused on the periodic metallic array structures because of the easy fabrications and convenient experimental detections . In the large‐area uniform gold nanoslit arrays ( Figure a), the sharp Fano resonance was achieved, as shown in Figure b .…”

High-efficiency refractive index sensor based on the metallic nanoslit arrays with gain-assisted materials

“…Evidently, the equation shows the SPP's resonant wavelength is sensitive to the variation of the nanoslit period and environmental RI, which indicated that the nanoslit array can be used as SPP based sensors by properly tuning the period of P. Furthermore, as we all know that the CMs mainly affected by the metal film depth [22,34], so we calculated the influences of metal film depth on the transmission spectra as shown in Fig. 3(b).…”

“…The resonant peaks of λ 1 and λ 3 have slight red shift and the full width at half maximum (FWHM) become narrower with increasing the nanoslit periods, while the resonant peak of λ 2 has an obvious red shift and its FWHM has been broadened slightly with increasing nanoslit periods. The larger red-shift of the λ 2 is due to the array period directly impacts the SPP mode according to the following equation [22]:…”

“…However, the EOT-based sensors usually provide much lower sensitivity than the conventional SPR technique, so enhancing their performance is a significant focus of current research on plasmonics [16,17]. Many researchers have developed and optimized some EOT-based sensors with different strategies, such as us-ing suspended metal films [18], changing the shape of nano-patterns [19][20][21], employing more complex structures [22,23]. However, in all of these studies, they suffer from a large intrinsic Ohmic loss in metal materials, which hinders further improvements of the performances (spectra intensity, quality factor, signal-to-noise ratio) of the EOT-based sensors.…”

Recent advances in nanoplasmonic biosensors: applications and lab-on-a-chip integration

“…Nanostencil lithography (NSL) based on shadow-masked patterning of the nanostructure [28], nanoimprint lithography that creates nanopatterns by the mechanical deformation of imprint resist [29], and interference lithography where an interference pattern is recorded in a photoresist material [30] are processes that can achieve market significance by offering simple, scalable, and cost-effective fabrication methods and have also the possibility of using flexible substrates. Nanoplasmonic structures such as hybrid nanocavities [31], nanopillars [32], or nanoslits [33] are examples of recently fabricated nanostructures using some of these processes. They show extremely high sensitivity performance (8066, 1010, and 926 nm RIU -1 with figures of merit of 179, 108, and 252, respectively) that even exceeds the theoretically predicted upper limit for conventional SPR sensors, leading to LSPR wavelength shifts large enough to produce color differences noticeable by the naked eye at very small refractive index changes.…”