A hand-held surface plasmon resonance biosensor

The detection and identification of biological molecules can be very difficult because there are so many molecules and they are all made from very similar elements. This precludes many techniques based on spectroscopy.

However, the human body is very good at detecting foreign molecules using proteins called antibodies. When you become infected with a virus or bacteria, your body recognises that the molecules that make up the virus are not the same as in your body. It manufactures antibody proteins which are designed specifically to bind to the foreign molecules. Once bound to the antibody there are various mechanisms for removing them. Having created the antibodies against the foreign body you are then immune to further infection by them.

The antibodies can be used as the chemical-sensitive layer in a biosensor. If the antibody is good, it will bind to one type of biological molecule, and no others. A liquid solution containing unknown molecules is put in contact with the antibody surface. If the target molecule is present, then it will bind to the antibody. If it is not present, then nothing happens. All that is needed is some way of detecting the binding of the target molecule to the antibody - this is where surface plasmon resonance comes in.

A surface plasmon resonance (SPR) is excited when a light beam generates a surface plasmon in a thin metal film. This occurs at a specific angle for a given wavelength of light. If the refractive index on top of the metal film changes, then the angle at which the resonance occurs also changes. The SPR biosensor has the metal film coated with the antibody. When the target molecule binds to the antibody it displaces some of the liquid solution and changes the refractive index. This is picked up by a change in the angle at which the surface plasmon resonance is excited.

My research group at CSIRO developed a hand-held SPR biosensor that could detect avian influenza and other biological molecules [1]. This self-contained unit is battery powered and contains all the optics and electronics to sense biological molecules. It is capable of detecting a refractive index change of 2 parts in 1 million!

[1] B. N. Feltis, B. A. Sexton, F. L. Glenn, M. J. Best, M. Wilkins, T. J. Davis: "A hand-held surface plasmon resonance biosensor for the detection of ricin and other biological agents" Biosensors & Bioelectronics 23, 1131-1136 (2008)

Copyright Tim Davis 2012