Nanoscale Silicon Microcavity Optical Sensors for Biological Applications

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Nanoscale Silicon Microcavity Optical Sensors for Biological Applications Selena Chan, Scott R. Horner,* Benjamin L. Miller,* and Philippe M. Fauchet** University of Rochester, Center for Future Health, Rochester, NY 14627, U.S.A. * and Department of Chemistry, Rochester, NY 14627, U.S.A. ** and Department of Electrical and Computer Engineering, Rochester, NY 14627, U.S.A.

ABSTRACT The large surface area of porous silicon provides numerous sites for many potential species to attach, which makes it an ideal host for sensing applications. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a full width at half maximum value of 3 nm. The position of these peaks is extremely sensitive to small changes in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We demonstrate the usefulness of this microcavity resonator structure as a DNA optical biosensor which displays appropriate sensitivity, selectivity, and response speed. A probing strand of DNA is initially immobilized in the porous silicon matrix, and then subsequently exposed to its sensing complementary DNA strand. Red-shifts in the luminescence spectra are observed and detected for various DNA concentrations. The spectral shifts confirm successful recognition and binding of DNA molecules within the porous structure. Detailed device fabrication procedures and the results of extensive testing will be presented. The detection scheme has also been extended to include the detection of viral DNA, proteins, and potentially bacteria. This work will lead to the development of a “smart bandage”, where the detection of bacteria or viruses can be diagnosed and an antibiotic treatment can be recommended.

INTRODUCTION The most interesting property of porous silicon is its room temperature visible luminescence [1]. The characteristic luminescence spectrum of a single porous silicon layer has a FWHM value of approximately 150 nm centered at a wavelength of 750 nm. This relatively broad-band emission can be narrowed by confining the luminescence between two distributed Bragg reflectors [2,3,4]. This microcavity resonating structure narrows the luminescence in the forward direction to a single peak with a FWHM value of < 10 nm [5]. When the cavity thickness increases, the number of luminescence peaks also increases [6]. The porosity of the cavity is kept relatively low (< 70%) which allows the formation of thick films without stress related instabilities. For a microcavity resonator, the FWHM values for each of the photoluminescence peaks can be as narrow as 3 nm and any slight shift in the peak positions is readily observable. This makes this type of resonator an ideal structure for sensitive optical sensing.

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