01 Ω cm) Si (100) in a 15%

hydrofluoric acid solution. The number of periods of the multilayer and the depth of the step and gradient refractive index layers were check details determined Microbiology inhibitor based on transfer matrix and rigorous coupled wave analysis (RCWA) simulations as explained in the ‘Results and discussion’ section. The BSW/BSSW multilayer contains periods of alternating high (H) and low (L) refractive index layers with the first layer being truncated as shown in the cross-sectional scanning electron microscope (SEM) image in Figure 1a. Etch parameters for each H layer of the step and gradient index profiles are described in Figure 1b,c, respectively, where the top number is the current density in mA/cm2 and the bottom number

is the etching duration in seconds. All L layers are etched with a 48 mA/cm2 current density for 22 s. The samples are then placed in a 1.5 mM l−1 potassium hydroxide in ethanol solution for 5 min and oxidized for 5 min at 500°C in air. Gratings of pitch 1,820 and 1,650 nm are patterned onto the gradient and step index BSW/BSSW structures, respectively, via electron beam lithography on a 250-nm-thick ZEP 520A photoresist. The indices and thicknesses shown in Figure 1b,c were determined after fabrication through SEM images and by matching measured angular reflectance spectra with RCWA simulations. Figure 1 SEM image and etch parameters of PSi BSW/BSSW sensor. (a) SEM cross-sectional image of PSi S63845 chemical structure BSW/BSSW sensor. Refractive index profiles of (b) step

and (c) gradient index BSW/BSSW sensors where the numbers shown above each layer represent the etch current (mA/cm2) and etch time (s), respectively. The field intensity of the BSW mode (red) and 1st BSSW modes (blue) are shown within the corresponding layers of the sensor. Latex nanosphere functionalization Size-selective out molecular detection was demonstrated using a prototypical small chemical molecule, APTES (size ≈ 0.8 nm), and large, 60-nm carboxyl latex nanospheres. A 4% APTES solution was prepared in methanol and water, and an aliquot was placed on the PSi sample for 10 min. The sample was subsequently immersed in methanol for 10 min to rinse away excess APTES molecules not attached to the PSi and then thermally annealed for 10 min at 150°C. The sample was then rinsed with methanol to remove any remaining unbound APTES molecules. A 4% w/v solution of carboxyl terminated latex nanospheres (Invitrogen™, Thermo Fisher Scientific, Carlsbad, CA, USA) was placed on the BSW/BSSW sensor for 1 min followed by a thorough methanol and deionized (DI) water rinsing. Attachment and quantification of the small and large species were determined by monitoring the angle-resolved reflectance spectrum in between molecular attachments. The attachment of the nanospheres was additionally verified by SEM imaging as shown in Figure 2a. No spheres were observed to penetrate the porous matrix in cross-sectional images (not shown).