Abstract
Optical sensors are prepared by reduction of gold ions using freshly etched hydride-terminated porous silicon, and their ability to specifically detect binding between protein A/rabbit IgG and asialofetuin/Erythrina cristagalli lectin is studied. The fabrication process is simple, fast, and reproducible, and does not require complicated lab equipment. The resulting nanostructured gold layer on silicon shows an optical response in the visible range based on the excitation of localized surface plasmon resonance. Variations in the refractive index of the surrounding medium result in a color change of the sensor which can be observed by the naked eye. By monitoring the spectral position of the localized surface plasmon resonance using reflectance spectroscopy, a bulk sensitivity of 296 nm ± 3 nm/RIU is determined. Furthermore, selectivity to target analytes is conferred to the sensor through functionalization of its surface with appropriate capture probes. For this purpose, biomolecules are deposited either by physical adsorption or by covalent coupling. Both strategies are successfully tested, i.e., the optical response of the sensor is dependent on the concentration of respective target analyte in the solution facilitating the determination of equilibrium dissociation constants for protein A/rabbit IgG as well as asialofetuin/Erythrina cristagalli lectin which are in accordance with reported values in literature. These results demonstrate the potential of the developed optical sensor for cost-efficient biosensor applications.Graphical abstract
Highlights
The optical phenomenon of surface plasmon resonance (SPR) has been exploited for sensor applications for decades and is based on a collective oscillation of the electron gas in certain materials including gold and silver [1]
The gold nanostructures were exclusively located on the top surface of the porous silicon layer and no gold could be observed in the pores by inspecting cross-sectional scanning electron micrograph (SEM)
A simple and inexpensive optical biosensor capable of monitoring biomolecular interactions was fabricated by the spontaneous galvanic displacement reaction of Au3+ cations on freshly etched porous silicon covered with hydrogen groups
Summary
The optical phenomenon of surface plasmon resonance (SPR) has been exploited for sensor applications for decades and is based on a collective oscillation of the electron gas in certain materials including gold and silver [1]. Galvanic displacement occurs when a “template” (metal or semiconductor) comes into contact with a more noble cation (where nobility is related to the standard redox potential). In this case, it is thermodynamically favorable for the more noble cation to “steal” electrons from the less noble template. Similar surfaces may be fabricated by spontaneous galvanic displacement of gold cations on hydrogenated silicon. In this case, the presence of HF was not required. The realization of LSPR sensors by galvanic displacement reactions has barely been reported until now, even though substrates for surfaceenhanced Raman spectroscopy (SERS) [20,21,22] or surfaceenhanced infrared spectroscopy (SEIRAS) [23, 24] were often prepared in this way
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