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Porous Silicon Biosensors and Nanostructures

Photo of Teflon etch cell

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.

SEM images demonstrating a range of pore diameters

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 blood1 and drugs of abuse in urine2.

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 devices3. The film can also be fractured using sonication into micro and nanoparticles for developing novel drug delivery scaffolds.

Photo of porous silicon Bragg mirror

Figure 3. Example of porous silicon Bragg mirror optical devices
designed to reflect different frequencies of white light

Photo of porous silicon microparticles

Figure 4. Example of porous silicon microparticles that can
be further processed for drug delivery applications.

Further Reading

Whole blood optical biosensor. Bonanno LM, DeLouise LA. Biosens Bioelectron. 2007 Oct 31;23(3):444-8. Epub 2007 Jul 20.
Label-free porous silicon immunosensor for broad detection of opiates in a blind clinical study and results comparison to commercial analytical chemistry techniques. Bonanno LM, Kwong TC, DeLouise LA. Anal Chem. 2010 Dec 1;82(23):9711-8. doi: 10.1021/ac101804s. Epub 2010 Nov.
Integration of a Chemical-Responsive Hydrogel into a Porous Silicon Photonic Sensor for Visual Colorimetric Readout. Bonanno LM, Delouise LA. Adv Funct Mater. 2010 Jan 19;20(4):573-578.

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