Abstract
Sensors composed of a porous silicon monolayer covered with a film of nanostructured gold layer, which provide two optical signal transduction methods, are fabricated and thoroughly characterized concerning their sensing performance. For this purpose, silicon substrates were electrochemically etched in order to obtain porous silicon monolayers, which were subsequently immersed in gold salt solution facilitating the formation of a porous gold nanoparticle layer on top of the porous silicon. The deposition process was monitored by reflectance spectroscopy, and the appearance of a dip in the interference pattern of the porous silicon layer was observed. This dip can be assigned to the absorption of light by the deposited gold nanostructures leading to localized surface plasmon resonance. The bulk sensitivity of these sensors was determined by recording reflectance spectra in media having different refractive indices and compared to sensors exclusively based on porous silicon or gold nanostructures. A thorough analysis of resulting shifts of the different optical signals in the reflectance spectra on the wavelength scale indicated that the optical response of the porous silicon sensor is not influenced by the presence of a gold nanostructure on top. Moreover, the adsorption of thiol-terminated polystyrene to the sensor surface was solely detected by changes in the position of the dip in the reflectance spectrum, which is assigned to localized surface plasmon resonance in the gold nanostructures. The interference pattern resulting from the porous silicon layer is not shifted to longer wavelengths by the adsorption indicating the independence of the optical response of the two nanostructures, namely porous silicon and nanostructured gold layer, to refractive index changes and pointing to the successful realization of two sensors in one spot.
Highlights
Optical biosensors are composed of a biological layer and a light-interacting transducer
Porous silicon (PSi) monolayers were fabricated by anodization of boron-doped ‹100›-oriented single-crystal Si wafers with low resistivity (0.001–0.002 Ω cm−1) in ethanolic HF solution
The thickness of the PSi layer was determined by taking cross-sectional scanning electron micrograph (SEM) (Figure 1B) and was 4.34 μm ± 0.05 μm
Summary
Optical biosensors are composed of a biological layer and a light-interacting transducer. Porous silicon served as highly attractive material for preparing as well as supporting metal nanostructures and often did not significantly contribute to the optical signal transduction, which is based on localized surface plasmon resonance (Virga et al, 2012). An improved performance of optical sensors based on merged photonic/plasmonic structures in comparison to sensors consisting of only one component have been proposed These sensors can be composed of 1D porous silicon photonic crystal structures covered with a thin metal layer, which show so-called Tamm resonances in their reflection spectra (Juneau-Fecteau and Fréchette, 2018; Ahmed and Mehaney, 2019). The gold nanostructure on top of the porous silicon layer has successfully been exploited for SERS spectroscopy providing additional information on the target analyte
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