Especially the combination of porous silicon with a special class of polymers, namely hydrogels, has led to this progress [13–15]. Hydrogels are hydrophilic polymeric networks which are characterized

NVP-BEZ235 by their stimuli-responsive properties. Depending on their chemical composition and internal structure, hydrogels react sensitively to external triggers such as temperature, pH, and ionic strength, which cause abrupt volume changes in the hydrogel. This volume change is accompanied by a change in the refractive index of the hydrogel [16]. Hence, the foundation for successfully utilizing hydrogels for the fabrication of highly sensitive optical sensors is a reasonable understanding of the influence of the volume change on the thickness as well as the refractive

SIS3 order index of the hydrogel and their impact on the optical response of the sensor. We envision an optical sensor composed of a highly ordered array of hydrogel microspheres on top of a porous silicon film. This sensor will offer two different ways of optical transduction: scattering/diffraction of light resulting from the deposited array of hydrogel microspheres and interference of light rays reflected at the interfaces of the porous silicon film. In this work, we will report on the fabrication of porous silicon monolayers covered with a non-close packed array of hydrogel microspheres and their optical properties in comparison to bare porous silicon films. Methods Silicon wafers (p-type, boron doped, <100 > orientation, resistivity ≤ 0.001 Ω cm) were obtained from Siltronix Corp. (Archamps, France). Hydrofluoric acid (HF), ethanol, and H2O2 were supplied by (Merck KGaA, Darmstadt, Germany). N-isopropylacrylamide (NIPAM) and 3-aminopropyltriethoxysilane (APTES) were purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany). N,N′-methylenebisacrylamide (BIS), H2SO4, and HCl were received from Carl Roth (Karlsruhe, Germany). Potassium peroxodisulfate (KPS) was supplied by Fluka (St. Louis, MO, USA). Water was deionized to a resistance of at least 5-Fluoracil cell line 18.2 MΩ (Ultra pure water system (TKA, Niederelbert, Germany)) and then filtered through a 0.2-μm filter. Scanning electron

microscopy (SEM) images were obtained with a Zeiss Ultra 55 ‘Gemini’ scanning electron microscope (Carl Zeiss, Inc., Oberkochen, Germany) using an accelerating voltage of 3 keV and an in-lens detector. To suppress charging of the sample during imaging, the samples were coated with carbon prior to SEM analysis using a Bal-Tec MED 020 PR-171 concentration sputter coater (Bal-Tec AG, Balzers, Liechtenstein). Reflectance spectra were recorded at normal incidence using an Ocean Optics charge-coupled device (CCD) spectrometer (Ocean Optics GmbH, Ostfildern, Germany) fitted with a microscope objective lens connected to a bifurcated fiber optic cable. A tungsten halogen light source was focused on the sample surface with a spot size of approximately 2 mm2.