Ultrasensitive Sensors made from Porous Silicon

Porous Silicon Membranes

By Khalid Tantawi

A video that I produced about fabricating porous silicon:

Porous Silicon membrane structures as thin as 3 um were fabricated by Khalid Tantawi in the labs of University of Alabama in Huntsville [Porous Silicon Membrane structure for Investigation of Transmembrane Proteins by Tantawi et. al.,  Journal of Superlattices and Microstructures, 2013]   

Introduction:

Porous silicon was first discovered in 1956, but it did not gain any importance until the 1990's when two optical properties were discovered: Its fluorescence and electrolumniscence properties. 

Porous silicon membranes as thin as 3 micrometers were fabricated for the purpose of studying transmembrane proteins. The epithelial sodium channel (ENaC) proteins were studied using electrochemical impedance spectroscopy (EIS).  A portion of the experiments were published. The experimental procedure invloves developing porous silicon membranes, followed by depositing lipid bilayer membranes using the Langmuir-Blodgett and Langmuir-Schafer techniques. Finally the epithelial sodium channel (ENaC) proteins were fused into the lipid bilayer membrane.  Functionality of the device was investigated using electrochemical impedance spectroscopy (EIS).  

Figure 2. Porous Silicon made in n-type silicon with uniform pores of about 1.5 um wide and pores as deep as 15 um.  Samples were made in a HF acid solution that contains ethanol.

Advantages of Porous Silicon over other porous materials:

1. Optical Properties:

• Fluorescence: Porous silicon re-emits higher energy photons at the lower energy red light.  This property allows porous silicon to find applications in  biology in which porous silicon nanostructures are monitored while delivering a cargo in a living organ.

• Electrolumeniscence: The emission of photons when subjected to an electric current.  The obvious application of this property is in Light-Emitting Diodes (LEDs).

• Ability to change index of refraction: By changing the depth of the pores, the index of refraction changes. 

2. Biological Properties:

• Biocompatibility: Porous silicon is not attacked by the immune system when it eneters a living organ.

• Biodegradability: It may be degraded by the body to orthisilicic acid

3. Other properties:

• Electronic Integrability: This material is easily integrated into electronic devices, which are predominantly based on silicon.

• Low cost.

 Fabrication:

Porous silicon is electrochemically fabricated in an aquous or ethanoic soloution of Hydrofluric (HF) Acid  (Note: HF acid is a very toxic material, and may result in serious injuries if was not handled properly). The silicon is anodically etched (i.e. silicon should be connected to the anode of the cell, cathodic etching will not result in porous silicon).  Porous silicon samples were found to retain their structure after formation if the samples are dipped immediately in a solution that contains ethanol in it.

Parameters that govern formation of porous silicon:

The parameters are arranged by importance. for further reading see reference [1]:

1. Doping type of the silicon: This is the most important parameter as it determines the method being used and the average diameter of the pores. By knowing the doping density, you should get an idea of the current density or potential level and requirement of light illumination. 

2. Doping Density: This is the second most important factor, it affects the the final porosity of the structure, and determines the exact required current density and potential for a given HF acid concentration. 

3. Hydrofluoric Acid concentration: This parameter is coupled with the current density. 

4. Light illumination: this parameter is usually used to give an extra degree of freedom for controlling the corrent density for n-type porous silicon.

5. Concentration of wetting agents: most common wetting agent used is Ethanol. The wetting agent improves pore uniformity by allowing the hydrogen bubbles to escape the silicon surface more easily than in aquous HF solution.  

6. Temperature

7. Time

Other parameters that have been reported in literature [2-4] to affect the final structure of porous silicon are:

• Continuous or pulsed current

• Orientation of the silicon substrate (i.e. vertical or horizontal)

• Clearance between the silicon substrate (Anode) and the Cathode.

• The contact area between the silicon and the anode conductor. 

Figure 1. Scanning Electron Microscope images of different n-type Porous Silicon samples of various pore diameters. Each sample was fabricated in  different parameters from the others. Images were taken in the NMDC labs of the University of Alabama in Huntsville by Khalid Tantawi.

Figure 2.  An illustration of the electrochemical cell used to fabricate a porous silicon substrate (left). Samples of  porous silicon (right)

Selected Journal Publications in Porous Silicon Production:

1. Khalid Hasan Tantawi, Bakhrom Berdiev, Ramon Cerro and John D. Williams, "Porous Silicon Membrane structure for Investigation of Transmembrane Proteins",  Journal of Superlattices and Microstructures, Vol. 58, pp. 72-80, March 2013, .

2. Khalid Hasan Tantawi, R. Cerro, B. Berdiev, M. Martina- Diaz, F. Montez, Darayas Patel, and J. Williams, “Investigation of Transmembrane Protein Fused in Lipid Bilayer Membranes Supported on Porous Silicon”, Journal of Medical Engineering and Technology, Vol. 37, pages 28-34 (2013)

3. Khalid Hasan Tantawi, Porous Silicon Platform for Investigation of Transmembrane Proteins: An Artificial Cell Membrane, (A PhD Thesis), LAP LAMBERT Academic Publishing, 2013

 Selected Conference Presentations and Proceedings

1. Khalid Hasan Tantawi, Bakhrom Berdiev, Ramon Cerro, and John Willaims, "Investigations on Transmembrane Ion Channels Suspended over Porous Silicon Membranes", Biotech World Conference & Expo, Washington DC, May 12-16 2013

2. Khalid Hasan Tantawi, Ramon Cerro, Bakhrom Berdiev, M. Elena Diaz, Francisco Javier Montez, and John D Williams, “Investigation of Epithelial Sodium Channel Proteins Supported on Porous Silicon Using Atomic Force Microscopy”, Science and Technology Open House, Tuskegee, AL, April, 2012.

3. Khalid Hasan Tantawi, Ramon Cerro, Bakhrom Berdiev, Elena Diaz, Javier Gonzales, and John D. Williams, “Lipid Bilayer Membranes with Incorporated Protein on Porous Silicon”, 22nd National NSF EPSCoR Conference, Coeur d’Alene, Idaho, October 24-27, 2011

4. Khalid Hasan Tantawi and John D. Williams, “Biological membranes integrated on porous silicon technology", Alabama Composites Conference, Birmingham, Alabama (August, 2010).

A video was produced that shows the production of porous silicon with hole diameters 1000 times less than the width of a human hair. Produced in Nano & Micro Devices Center. University of Alabama in Huntsville by Khalid Tantawi. Porous Silicon is biocompatible, and is used in optoelectronics due to its florescence, and as a lipid bilayer membrane platform for studying transmembrane proteins.  It is also very active chemically.

By: Khalid Tantawi, PreOptiPost - Nashville and Middle Tennessee area