No abstract

We describe the development and optimization of porous silicon photonic crystal surface chemistry towards implantable optical devices. Porous silicon rugate filters were prepared to obtain a narrow linewidth reflectivity peak in the near‐infrared (700–1000 nm) with low background reflectivity elsewhere. The morphology of the mesoporous structures (pore diameter < 50 nm) was such that only small proteins could infiltrate the pores whereas larger proteins were excluded. To provide stability in biological media, we established an approach to build organic multilayers containing hexa(ethylene oxide) moieties in porous silicon. The optical changes associated with organic derivatization were monitored concurrently with FTIR characterization. Furthermore, the antifouling capability of our chemical strategy is assessed and the penetration of different sized proteins into the structure was determined. The structural stability in biological environments was evaluated by incubation in human blood plasma over time while monitoring the optical signature of the photonic crystal.

Surface modification with oligo(ethylene oxide) functionalized monolayers terminated with reactive headgroups constitutes a powerful strategy to provide specific coupling of biomolecules with simultaneous protection from nonspecific adsorption on surfaces for the preparation of biorecognition interfaces. To date, oligo(ethylene oxide) functionalized monolayer-forming molecules which can be activated for attachment of biomolecules but which can selectively form monolayers onto hydrogen terminated silicon have yet to be developed. Here, self-assembled monolayers (SAMs) containing tetra(ethylene oxide) moieties protected with tert-butyl dimethylsilyl groups were formed by thermal hydrosilylation of alkenes with single-crystal Si(111)-H. The protection group was used to avoid side reactions with the hydride terminated silicon surface. Monolayer formation was carried out using solutions of the alkene in the high-boiling-point solvent 1,3,5-triethylbenzene. The protecting group was removed under very mild acidic conditions to yield a free hydroxyl functionality, a convenient surface moiety for coupling of biological entities via carbamate bond formation. The chemical composition and structure of the monolayers before and after deprotection were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry. To demonstrate the utility of this surface for covalent modification, two reagents were compared and contrasted for their ability to activate the surface hydroxyl groups for coupling of free amines, carbonyl diimidazole (CDI), and disuccinimidyl carbonate (DSC). Analysis of XP spectra before and after activation by CDI or DSC, and after subsequent reaction with glycine, provided quantitative information on the extent of activation and overall coupling efficiencies. CDI activated surfaces gave poor coupling yields under various conditions, whereas DSC mediated activation followed by aminolysis at neutral pH was found to be an efficient method for the immobilization of amines on tetra(ethylene oxide) modified surfaces.

Significant effort is being expended on the integration of silicon electronics and optoelectronics. Here the authors describe a method to create planar gradient refractive index (GRIN) lenses in Si using porous silicon (PSi) technology. The authors’ approach allows the fabrication of a single planar lens or an array of such lenses with focal length that can be adjusted to match existing device(s) on the chip. The lenses are transparent in the near IR, including the optical communication window (1.3μm&lt;λ&lt;1.6μm). In addition to being potential components in future Si based integrated optical circuits, PSi GRIN lenses can also be used to improve the light coupling efficiency of existing Si based devices, such as sensors, detectors, and waveguides.