Porous silicon and polymer materials for optical chemical sensors

The optical characteristics of porous materials depend on their structural properties (porosity, pore size, and pore distribution). Specifically in materials with air-filled pores, the effective refractive index is a weighted average of the refractive indices of the relevant material and air, which is thus directly related to the material's porosity. Mesoporous materials, such as porous silicon and porous polymers, have therefore been exploited to create photonic devices, from simple interference filters to exotic photonic-bandgap (PBG) structures.^1,2 In addition, the high internal surface area of porous materials provides an excellent host medium to immobilize analyte-specific recognition elements through sequestration.³ Interactions of the analyte molecules with the sequestered recognition elements often alter the effective index or other optical properties. These optical variations can be employed as a simple and straightforward transduction approach in optical sensing.

Figure 1. (a) Cross-sectional scanning-electron-microscope (SEM) image and (b) reflectance of a dry-etched porous-silicon thin film at different angles of incidence (θ_i). The differences in the reflectance spectra are caused by optical interference.

Figure 2. Experimental setup for polymer-grating fabrication. Φ: Prism incidence angle.

Figure 3. (a) Cross-sectional SEM image of a polymer grating. (b) Angular dependence of the transmittance of a polymer-grating notch filter.

Typically, porous silicon is produced by electrochemical etching in a hydrogen fluoride-based liquid etchant. A major drawback of this approach is the drying process, because the pressure gradient across the gas/liquid interface during evaporation produces high capillary stress in the porous structure that can lead to systematic cracking and destruction of the pores. Supercritical drying is often employed to avoid this problem.

Figure 4. Experimental setups for characterizing (left) porous-silicon base oxygen and (right) porous-polymer sensors.

We recently developed a dry-etching method that overcomes the significant challenges of producing porous silicon by wet-etching chemistry. Our method relies on gaseous xenon difluoride to etch and produce a thin layer of macroporous silicon on the surface of bulk silicon. This layer produces an optical interference pattern whose reflectance spectra can be observed (see Figure 1). However, only a single layer of porous silicon can be produced using this technique.

Porous polymer structures represent similar materials to porous silicon. While porous silicon provides an effective platform, there is significant interest in extending these concepts to polymeric materials because of their comparatively easy fabrication process, cost effectiveness, and mechanical flexibility. Although porous-polymer PBG materials can be obtained from porous-silicon templates by casting⁴ or micropatterning,⁵ we have chosen to produce 1D PBG structures by optical interference using holography.⁶ Holographic fabrication is flexible, because the PBG can be tuned with small equipment adjustments. In addition, the platform can be extended to produce 2D and 3D PBG structures.

To fabricate porous-polymer PBG structures through holography, we prepared a photosensitive pre-polymer syrup (PPS), a mixture of monomer, photoinitiator, and solvents. We placed the PPS between two glass slides that were attached to a glass prism with index-matching oil (see Figure 2). During recording, interference between the incident laser beam and its total internal reflection forms a holographic pattern. As the PPS is exposed to the holographic pattern, polymerization occurs faster in bright than in dark regions and creates periodic structures because of the phase separation between the two chemicals in the PPS.⁷ The spacing of the periodicity (grating spacing) is determined by the angle of incidence of the laser beam to the prism surface. Figures 3(a) and (b) show a cross-sectional scanning-electron micrograph and the angle-dependent transmission of a polymer-grating notch filter with a typical full width at half maximum of 30nm at normal incidence.

Because of the spectral selectivity of these porous materials, we can apply them to luminescence-based sensing of oxygen with ruthenium-based luminophores. We applied a 2μm-thick layer of 3-methacryloxypropyltrimethoxysilane-based sol-gel containing ruthenium(II) tris(2,2^′-bipyridyl) luminophores ([Ru(bpy)₃]²⁺) to the dry-etched porous-silicon thin film by spin coating. The sol-gel compound luminesces with a peak emission wavelength in the red range (λ_peak~600nm) when excited by blue light (λ_ex~450nm). The porous-silicon thin film was specifically designed to suppress reflection of the excitation. Moreover, the luminescence is oxygen sensitive and it quenches with increased oxygen partial pressure.^8,9 Thus, with proper calibration, measurements of the luminescence intensity yield the ambient oxygen level.

To complete the sensing system, an excitation source and a luminescence detector are required (see Figure 4). For calibration, we placed the system inside a flow chamber, where a controlled mixture of nitrogen and oxygen flowed over the sol-gel sensing element. We did not use any additional optical filters. Figure 5 shows a typical response of the sensor system with dry-etched porous silicon compared with a system of oxygen-sensitive sol-gel on unetched and unpolished silicon wafer. Using the porous-silicon thin-film filter appears to double the sensitivity of the sensing reflection filter because of the effectiveness of porous silicon in suppressing the excitation's back reflection.

Figure 5. Raw response of the oxygen-sensing sol-gel on porous-silicon substrate. I: Intensity.

Applying the porous-polymer grating to oxygen sensing is even more straightforward, because the grating itself can act as sensing element by mixing ruthenium(II) tris(4,7'-diphenyl-1-10' phenathroline) luminophore ([Ru(dpp)₃]²⁺) with the PPS. During holographic recording, we tuned the grating spacing to efficiently absorb the optical energy at the excitation wavelength while effectively transmitting the luminescence. We configured the complete sensing system in transmission geometry (see Figure 4) with the LED set to illuminate the grating at a 30° angle of incidence to efficiently excite the luminophores. Figure 6 shows the system's response. It is as sensitive as a thin-film polymer oxygen sensor that uses an external bandpass filter to prevent leaching of the excitation into the detector.¹⁰

Figure 6. Response of the oxygen (O₂)-sensing polymer grating.

In summary, we have reviewed how dry-etched silicon and porous polymer can be applied to oxygen sensing. Many open questions remain regarding their operational performance, sensitivity, lifespan, and compatibility with other recognition elements. We are currently investigating these and other features of this type of sensor that could ultimately lead to an established sensor technology.

We acknowledge financial support from Le Fonds Québécois de la Recherche sur la Nature et les Technologies, the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Photonics Innovation, and the National Institutes of Health (grant 3R21EB9506). We thank Frank Bright at the University of Buffalo and Mark Andrews at McGill University for supplying the [Ru(dpp)₃]²⁺and [Ru(bpy)₃]²⁺-based oxygen-sensing materials, respectively. The dry-etched porous silicon was prepared in McGill's Nanotools and Microfabrication Laboratory.