Porous Silicon Biosensors and Nanostructures
Figure 1. Image shows Teflon etch cell with Pt
wire cathode (black) and Anode (Red). ;
Porous silicon is prepared by anodic electrochemical etching of a single crystal silicon wafer using an electrolyte containing hydrofluoric acid (HF) (Fig. 1). Pore diameter, porosity and pore channel morphology are controlled by varying etch parameters including wafer doping level and type, current density, and electrolyte formulation. Pore diameters typically range between 20-150 nm (Fig. 2) but pores exceeding several microns are also possible.
Figure 2. SEM images demonstrating
a range of pore diameters.
Porous silicon layers that vary in thickness, porosity, and refractive index can be fabricated to create photonic bandgap devices. These are multilayer structures where the spatial periodicity is on the same order of the probe light frequencies (visible, NIR, IR etc). When the optical thickness (OT) of each layer is tuned to 1/4 λ, under white light illumination, constructive interference occurs such that certain frequencies are highly reflected (eg. Bragg mirrors, Microcavities). These devices (Fig. 3) can be used for optical biosensing. By functionalizing the internal surface area of the device with bio-receptors the color of the light that is reflected will change when the target (if present in the sample solution) binds. We have developed this technology for detecting immunoglobins in whole blood and drugs of abuse in urine.
After etching the porous silicon film it can be released from the silicon wafer. This film can be embedded in polymer for making novel biosensor or optical devices. The film can also be fractured using sonication into micro and nanoparticles for developing novel drug delivery scaffolds.
Figure 3. Example of porous silicon Bragg mirror optical devices
designed to reflect different frequencies of white light
Figure 4. Example of porous silicon microparticles that can
be further processed for drug delivery applications.
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