Aluminum-Based Nanostructures for Highly Sensitive Plasmonic Sensors
Kuang-Li Lee1*, Hsuan-Yeh Hsu1, Po-Cheng Tsai1, Meng-Lin You1, Ming-Yang Pan1, Pei-Kuen Wei1
1Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
* Presenter:Kuang-Li Lee,
Nanostructured-based surface plasmon resonance (SPR) sensing has the features of sensitive, real-time and label-free detection and is applied to many applications, such as environmental monitoring, medical diagnostics, and food safety. Compared to the commercial prism-based SPR sensors, nanostructure-based SPR sensors provide a simple way for SPR excitation and have some benefits, including small detection volume, simple measurement and ease of multiple detections. However, mass production of metallic nanostructures with low-cost and high-throughput is important for commercial applications. Conventional nanofabrication techniques use focused ion beam to mill metal films, or electron-beam lithography to make nanostructures on the EB resist. These nanofabrication techniques are expensive, and cannot be used for mass production. Many methods have been proposed to solve the mass production problem, such as optical interference lithography, thermal or UV nanoimprint lithography, nanosphere lithography, nanostencil lithography, and the template-stripping method. On the other hand, the noble metals such as Au and Ag are commonly used for the majority of the plasmonic sensors because these materials have low optical losses in the visible and near-infrared ranges. Recently, the studies of nanostructure-based aluminum sensors have attracted a lot of attention because aluminum is a more cost-effective plasmonic material. To widely use Al material for sensing, the problems of oxidation and material degradation have to be solved. These issues can be addressed by depositing a passivation dielectric film or using a passivation treatment based on oxygen plasma to produce an oxide protecting layer. However, the large imaginary part of the dielectric function for aluminum results in a broad resonance response and a longer electromagnetic field decay length, which limit its surface sensing capability. In this study, we developed a rapid hot-embossing nanoimprinting technique for rapid and low-cost fabrication of nanostructures. Each imprinting process was completed within several seconds, and no release agent or epoxy was required. Besides, we proposed the combination of Fano resonances in capped nanoslits and a thin nanodielectric top layer to improve the surface sensitivities of aluminum-based nanostructures. The thin dielectric layer changes the resonance field distribution, reduces the decay length and improves the surface sensitivity. We show that the dielectric layer can positively enhance the wavelength sensitivities of the Wood’s anomaly-dominant resonance and asymmetric Fano resonance in capped aluminum nanoslits. The maximum improvement can be reached by a factor of 3.5. Besides, there is an optimal layer thickness for the surface sensitivity because of the trade-off relationship between the refractive index sensitivity and decay length. We attribute the enhanced surface sensitivity to a reduced evanescent length, which is confirmed by the finite difference time-domain calculations. The protein-protein interaction experiments verify the high-surface sensitivity of the structures, and a limit of quantification of 1 pg/mL anti-bovine serum albumin is achieved. Such low-cost, highly sensitive aluminum-based nanostructures can benefit various sensing applications.

Keywords: Metallic nanostructures, Surface plasmon resonances, Optical sensors, Fano resonances, Biosensors