Porous silicon is a unique photonic material produced by electrochemical etching of p+ single crystal silicon wafers in ethanolic hydroflouric acid. At the Center for Future we are interested in developing optical biosensing technology that leverages the photoluminescence and highly reflecting properties of mesoporous porous silicon (me-PSi) microcavities for detection of pathogenic organisms. A me-PSi microcavity sensor is an interferometric multi-layer device comprised of alternating layers of high and low index of refractive determined by the porosity of the layers. A photoluminescent active layer is sandwiched between two highly reflective dielectric Bragg mirrors. The mirrors create a stop-band that prevents photons, generated within the active layer from escaping the device except for those at the wavelengths coincident with the Fabry-Perot resonance's. Changes within 3D microstructure of the device that alter the optical path length will induce a shift in the characteristic Fabry-Perot resonance's. Molecular binding events that occur for example, between a target (antigen, c-DNA, protein) and an immobilized probe (antibody, protein or small molecule receptor) within the device will be detected by the induced shift in the optical output of the device. We are currently developing prototype devices based on a single probe chip format utilizing known binding partners with the specific goals to optimize the microstructure and surface derivation chemistries to minimize nonspecific binding from controlled target solutions. Longer-term goals are to develop rapid detect arrayed devices fully integrated with on-chip microfluidics and diagnostic control to quantitatively detect single compounds in the complex milieu of real biological samples. Our preliminary efforts to integrate porous silicon devices at the wafer level with standard IC silicon and MEMS fluidic processing hold promise for the future development of arrayed detectors that not only will be able to diagnose the presence of a pathogenic organism but also is phenotype.

Figure 1. Porous silicon microcavitiy sensors are produced by electrochemical etching of single crystal p+ silicon <100> wafers in ethanolic HF. Etch conditions are selected to produce a 3D mulitlayered microstructure consisting of nanometer sized features that yield a highly reflective interferometric device that can be tuned to operate over a wide range of wavelengths. The samples shown have been tuned to operate in the visible range at ~650nm.

Figure 2. Crossectional SEM view of a porous silicon microcavity illustrating the alternating layers of high (78%) and low (54%) porosity. Index of refraction is related to porosity, the higher the porosity the lower the index of refraction ( ). This microcavity consists of a multi-layer stack of alternating high and low index. Each layer is 1/4 wavelength ( ) thick (d), where d= /4 and is the wavelength the sensor is tuned to operate at. The multilayer stack as described comprises a Bragg mirror which is highly reflective over a wavelength range, called the stop-band. The width of the stop-band depends on the difference ( ) in refractive index between the two layers. The peroidicity of the dielectric stack is interupted by a middle layer of 1/2 thick. This assymetry causes a Fabry-Perot resonance to appear in the middle of the stop-band as illustrated in Fig. 5. Increasing the thickness of the middle layer will result in higher order resonance's as illustrated in Fig. 3.

Figure 3. The signal transduced from an optical porous silicon microcavity sensor can be monitored by following shifts in the reflection dips (blue) or the corresponding photoluminescent (PL) peaks (red). Surface layers within the 3Dmicrostructure of the device are oxidized and chemically derivitized in order to immobilize a highly specific bimolecular probe. These processes alter the porosity of microcavity and consequently the effective optical path length (index of refraction) of the device. A change in the effective index of refraction induces a red shift (decreasing porosity) in the characteristic Fabry-Perot resonance's signaling a binding event.

Images provided courtesy of the CDC public health image library (PHIL), and Out of the Frying Pan.

Figure 4. Our laboratory is currently working on direct optical detect biosensors to identify pathogenic organisms such as enterohemmoragic Escherichia coli O157 and Candida albicans. These opportunistic pathogens cause serious outbreaks of disease in humans. Exposure to just a few E. coli O157 organisms can cause chronic diarrhea, hemorrhagic colitis and hemolytic uremic syndrome, in which the red blood cells are destroyed causing kidney failure. Infection by E. coli O157 can be fatal to many particularly immuno-compromised individuals, children under 5 and the elderly. Candidiasis is an infectious condition caused by the opportunistic fungus of the genus Candida, which includes eight species of fungi most notable of which is C. albicans. Candidiasis is usually limited to the mucous membranes and skin making this a dermatolocially important disease.

Figure 5. Data illustrating the operation and sensitivity of a biosensor, operating in reflection mode, to surface layer chemical derivitization for probe immobilization. Freshly oxidized microcavities exposed to either buffered salt or aq. methanolic solutions typically exhibit a red shift of 6-7nm. Illustrated in (a), exposure of an oxidized microcavity to 1% aminosilane (aq. methanol) results in a cumulative red shift of 9-11nm which includes an additional 3-4nm red shift due to binding of the silane. In (b) the no shift is detected upon exposure to glutaraldehyde when the surface in not pretreated with silane.