Optical characterization of porous silicon monolayers decorated with hydrogel microspheres

Abstract

The optical response of porous silicon (pSi) films, covered with a quasi-hexagonal array of hydrogel microspheres, to immersion in ethanol/water mixtures was investigated. For this study, pSi monolayers were fabricated by electrochemical etching, stabilized by thermal oxidation, and decorated with hydrogel microspheres using spin coating. Reflectance spectra of pSi samples with and without deposited hydrogel microspheres were taken at normal incidence. The employed hydrogel microspheres, composed of poly-N-isopropylacrylamide (polyNIPAM), are stimuli-responsive and change their size as well as their refractive index upon exposure to alcohol/water mixtures. Hence, distinct differences in the interference pattern of bare pSi films and pSi layers covered with polyNIPAM spheres could be observed upon their immersion in the respective solutions using reflective interferometric Fourier transform spectroscopy (RIFTS). Here, the amount of reflected light (fast Fourier transform (FFT) amplitude), which corresponds to the refractive index contrast and light scattering at the pSi film interfaces, showed distinct differences for the two fabricated samples. Whereas the FFT amplitude of the bare porous silicon film followed the changes in the refractive index of the surrounding medium, the FFT amplitude of the pSi/polyNIPAM structure depended on the swelling/shrinking of the attached hydrogel spheres and exhibited a minimum in ethanol-water mixtures with 20 wt% ethanol. At this value, the polyNIPAM microgel is collapsed to its minimum size. In contrast, the effective optical thickness, which reflects the effective refractive index of the porous layer, was not influenced by the attached hydrogel spheres.PACS81.05.Rm; 81.16.Dn; 83.80Kn; 42.79.Pw

Bottom-Up Fabrication of Hybrid Plasmonic Sensors: Gold-Capped Hydrogel Microspheres Embedded in Periodic Metal Hole Arrays

“…A plot of polyNIPAM height against ethanol content in the surrounding medium and published refractive indices of used ethanol/water mixtures are available in the Supporting Information (Figure S4 and Table S2, respectively). In accordance with published measurements, the polyNIPAM particles are collapsed at 20–30 wt % ethanol content and swollen to different extents at all other compositions of the ethanol/water mixtures . In Figure c, the gray area in the background illustrates the measured heights of the polyNIPAM particles in nm.…”

“…PolyNIPAM reacts sensitively to ethanol/water mixtures with different compositions leading to significant volume changes; i.e., polyNIPAM either swells or collapses depending on the balance between the solvent/polymer and the polymer/polymer interactions. This phenomenon is called co-nonsolvency and has been described in the literature. , To test this hypothesis, we eliminated the sensor response to refractive index changes from the data points presented in Figure b. For this purpose, a linear fit was applied to the data points.…”

“…This phenomenon is called co-nonsolvency and has been described in the literature. 32,33 To test this hypothesis, we eliminated the sensor response to refractive index changes from the data points presented in Figure 3b. For this purpose, a linear fit was applied to the data points.…”

“…In accordance with published measurements, the polyNIPAM particles are collapsed at 20−30 wt % ethanol content and swollen to different extents at all other compositions of the ethanol/water mixtures. 32 In Figure 3c, the gray area in the background illustrates the measured heights of the polyNIPAM particles in nm. A strong correlation between the height of the polyNIPAM microsphere on the surface and the deviation of the linearity in the optical response can be easily noticed.…”

Light-triggered antifouling coatings for porous silicon optical transducers

“…Moreover, studies have shown that biosensor activity could be impaired rapidly because of fouling after only several hours . To overcome these severe limitations, studies have shown that surface modification of PSi with functionalized polymers and hydrogels, such as poly(ethylene glycol), improves their resistance to nonspecific protein binding and introduces new functionalities and may pave the way for designing better biosensing devices.…”