Aluminum film over nanosphere surface for Deep Ultraviolet plasmonic nanosensors

Deep ultraviolet (DUV) plasmonic nanosensors are emerging as potential candidates to detect biomolecules with enhanced sensitivity, as these molecules strongly absorb light in the DUV region. In the present work, we fabricated aluminium film over nanospheres surface (AlFON) and successfully demonstrated, for the first time, an AlFON based DUV plasmonic nanosensor for detecting volatile organic compounds (VOCs) such as ethanol, isopropyl alcohol (IPA) and toluene, and bovine serum albumin (BSA) protein. The AlFON was fabricated by thermally evaporating aluminium thin film onto a two-dimensional photonic crystal (2DPC). The 2DPC was prepared by drop casting colloidal polystyrene spheres (PSS) on quartz substrates that self-assembled to form a monolayer of hexagonally close packed PSS with periodic interstices. The optical reflection spectrum of 70 nm AlFON exhibited a sharp minimum at 289 nm in the DUV wavelength range which corresponds to strong absorption in V-shaped nanocavities due to excitation of localized cavity plasmons. The reflection minimum - or LSPR wavelength - is sensitive to the changes in the refractive index of the surrounding medium. The LSPR mode of the V-shaped nanocavities in 70 nm AlFON was exploited to sense VOCs and BSA. The 70 nm AlFON exhibited 244 nm/RIU sensitivity in detecting VOCs and was capable of sensing 1 mM concentration of BSA in water. The AlFON is physically stable and has a self-limiting Al₂O₃ oxide layer of less than 5 nm thickness as elucidated from x-ray photoelectron spectroscopy measurements. The effect of higher aluminum film thickness, i.e. 100 nm and 120 nm on the sensitivity of AlFON also was studied. The performance of AlFON based DUV plasmonic nanosensors is better than the previously reported Al nanostructure based nanosensors and can still be enhanced by optimizing the structure of 2DPC and AlFON.