Macroporous p-Type Silicon Fabry−Perot Layers. Fabrication, Characterization, and Applications in Biosensing

Abstract

We present in this paper that porous silicon can be used as a large surface area matrix as well as the transducer of biomolecular interactions. We report the fabrication of heavily doped p-type porous silicon with pore diameters that can be tuned, depending on the etching condition, from approximately 5 to 1200 nm. The structure and porosity of the matrixes were characterized by scanning force microscopy (SFM) and scanning electron microscopy (SEM), Brunnauer-Emmett-Teller nitrogen adsorption isotherms, and reflectance interference spectroscopy. The thin porous silicon layers are transparent to the visible region of the reflectance spectra due to their high porosity (80-90%) and are smooth enough to produce Fabry-Perot fringe patterns upon white light illumination. Porous silicon matrixes were modified by ozone oxidation, functionalized in the presence of (2-pyridyldithiopropionamidobutyl)dimethylmethoxysilane, reduced to unmask the sulfhydryl functionalities, and coupled to biotin through a disulfide-bond-forming reaction. Such functionalized matrixes display considerable stability against oxidation and corrosion in aqueous media and were used to evaluate the utility of porous silicon in biosensing. The streptavidin-biotin interactions on the surface of porous silicon could be monitored by the changes in the effective optical thickness calculated from the observed shifts in the Fabry-Perot fringe pattern caused by the change in the refractive index of the medium upon protein-ligand binding. Porous silicon thus combines the properties of a mechanically and chemically stable high surface area matrix with the function of an optical transducer and as such may find utility in the fabrication of biosensor devices.

Preparation of two-directional gradient surfaces for the analysis of cell-surface interactions

“…The non-linear nature of the gradient could be attributed to the non-linear decrease in current density across the substrate. Previous studies have observed similar results 24,25 . Several HF/ethanol ratios were also investigated and it was found that increasing the concentration of HF led to a steeper decline in the pore size along the gradient.…”

Biosensing Using Porous Silicon Double-Layer Interferometers: Reflective Interferometric Fourier Transform Spectroscopy

“…These results are in accordance with earlier work, in which an approximately exponential dependence of the pore diameter on the current density was found for highly-doped p-type samples. 4 The porous Si double-layer, produced by applying the "single-layer 1" etching conditions followed immediately by the "single-layer 2" conditions, possesses a combination of the two single layers one on top of the other. The SEM image (Fig.…”

“…2 represent the index contrast at each of the interfaces a, b, or c (see Fig. 1): (4) Where n air , n 1 , n 2 , and n Si represent the refractive index of air (or of the solution), layer 1, layer 2, and bulk Si, respectively. The quantities n 1 and n 2 represent the total refractive index of the layer and everything it contains (silicon, SiO 2 , solution, air, biomolecule, etc.).…”

Vapor-phase silanization of oxidized porous silicon for stabilizing composition and photoluminescence

“…11 The electronic or optical properties of porous silicon can also be used as the transducer of biomolecular interactions, thus qualifying its utility in biosensor applications. 12,13 The devices fabricated with PSi have shown promise, owing to the ease of fabricating one-dimensional photonic crystals. 14 A significant benefit of biosensing with PSi photonic crystals is the labelfree transduction by optical interference ͑change in refractive index n within the film͒.…”

“…15,16 Unfortunately, the instability of the underlying surface to aqueous environments has hindered the wide-scale employment of PSi for biological applications. 17,18 Recent interest in PSi was focused on its use as a delivery vehicle for therapeutic agents such as ibuprofen 19 and insulin. 20 Central to the use of PSi as an effective drug delivery system is the need to find balance between passivation of the highly reactive native Si-H x surface species to minimize damage to the loaded molecule and control the release of the drug by erosion of the PSi matrix.…”