By: Nicholas Stittleburg, Bio-Medical Electronics Student, Western Technical College, La Crosse, WI
An optical biosensor is a man-made hybrid of a biological component (like an antibody receptor) that recognizes a specific protein and a physics component (transducer) that sends a light signal when a targeted protein binds to the biosensor. Embedded in a disposable strip, the sensor can detect TB infections in sputum or blood serum samples.
By amplifying specific binding events between fluorescence molecules, the biosensor can detect protein toxins, viruses, antibodies and other biomolecules. Such sensor technology is critical to defending against threats of bioterrorism and has medical applications in diagnosing respiratory diseases like tuberculosis. 
Biosensors can also be used in the detection of avian bird flu. Quick identification of avian influenza in poultry is critical in controlling outbreaks, but current detection can require several days to produce results.
However, a new biosensor developed at the Georgia Tech Research Institute (GTRI)can detect avian influenza in just minutes. In addition to being a rapid test, the biosensor is economical, field-deployable, is sensitive to different viral strains and requires no labels or reagents.
Worldwide, there are many strains of avian influenza viruses that cause varying degrees of clinical symptoms and illness. In the United States, outbreak of the disease - primarily spread by migratory aquatic birds - have plagued the poultry industry for decades with millions of dollars in losses. The only way to stop the spread of the disease is to destroy all poultry that may have been exposed to the virus.
A virulent strain of avian influenza (H5N1) has begun to threaten not only birds but humans, with more than 300 infections and 200 deaths reported to the World Health Organization since 2003. Looming is the threat of a pandemic, such as the 1918 Spanish flu that killed about 40 million people, health officals say.
The biosensor is coated with antibodies specifically designed to capture a protein located on the surface of the viral particle. For this study, the researchers evaluated the sensitivity of the three unique antibodies to detect avian influenza virus.
The sensor utilizes the interference of light waves, a concept called interferometry, to precisely determine how many virus particles attach to the sensor's surface. More specifically, light from a laser diode is coupled into an optical waveguide through a grating and travels under one sensing channel and one reference channel.
Researchers coat the sensing channel with the specific antibodies and coat the reference channel with non-specific antibodies. Having the reference channel minimizes the impact of non-specfic interactions, as well as changes in temperature, pH and mechanical motion. Non-specific binding should occur equally to both the test and reference channels and thus not affect the test results.
An elctromagnetic field associated with the light beams extends above the waveguide and is very sensitive to the changes caused by antibody-antigen interactions on the waveguide surface. When a liquid sample passes over the waveguides, binding occurs on the top of a waveguide because of viral particles attaching and causes water molecules to be displaced. This causes a change in the velocity of the light traveling through the waveguide.
At the end of the waveguide, the light beams from the sensing and reference channels are combined to create an interference pattern. The pattern of alternating dark and light vertical stripes, or fringes is imaged on a simple CCD detector. By doing a mathematical Fourier transform, the researchers determine the degree to which the fringe patterns are in or out of step with each other, known as a phase shift. This phase shift tells the amount of virus bound to the surface.
Beyond the waveguide sensor, the only additional external components required for field-testing with the GTRI's biosensor include a sample-delievery device (peristaltic pump), a data collection laptop computer and a swab taken from a potentially infected bird or human. Power is supplied by a nine volt battery and USB connection. The waveguides can be claned and reused dozens of times, decreasing the per-test cost of the chip fabrication. 
Like mentioned before biosensors are not only used to detect avian bird flu but also things such as TB and breast cancer at a very early stage. They are also used to monitor antigens to track treatment and also disease progression. Other applications include environmental monitoring, bio-threat detection, food inspection and industrial uses. Benefits of bionsensors include but are not limited to ultra-high sensitivity and specificity, a very simple use-one step with no reagents required, fast detection times, low power comsumption and light weight and finally inexpensive replaceable sensor elements. 
Since biosensors are a relatively new technology there is not a whole great deal known about them or written about them. However, in the future it is not hard to envision airports that instead of having just metal detectors when you walk in also having a system that scans the person to make sure that they are not carrying a virus or disease. Biosensors will also become more practical in hospitals to uncover such things as breast cancer and many other forms of cancer or TB.
- "Biosensor Portfolio." Licensable Technologies. Los Alamos National Laboratory. 2 May 2008 <http://www.lanl.gov/orgs/tt/pdf/techs/biosensor.pdf>.
- "New Biosensor Detects Avian Influenza Virus in Minutes, Not days." Georgia Tech Research Institute. 2 May 2008 <http://www.gtri.gatech.edu/casestudy/biosensor-detects-avian-flu-in-minutes>.
- "Optical Biosensor." Medical Science Technology. Doe medical sciences. 2 May 2008 <http://www.doemedicalsciences.org/ait/pastprojects.shtml>.