BIOSENSORS AND DETECTION OF BIOWARFARE AGENTS
Biosensors are devices for detection and measurement of reactions that rely on a biological mechanism, often an enzyme reaction adapted to generate an electrical signal. Biosensors have been traditionally used in clinical diagnosis and in food and environmental analysis. By far the biggest use has been the clinical monitoring of glucose levels using the enzyme glucose oxidase. In particular, knowing the concentration of glucose is critical to proper care of diabetics. Glucose oxidase is unusually stable, a major reason for its widespread use. The enzyme catalyzes the following reaction:
glucose +O 2 → - gluconolactone +H 2 O 2
The glucose biosensor consists of a thin layer of glucose oxidase attached to the bottom of an oxygen electrode ( Fig. 23.14 ). The electrode detects oxygen released by the enzyme reaction. The current generated provides a measure of the glucose concentration. A potential of about 0.6 volts is applied between the central positive platinum electrode and the surrounding negative silver/silver chloride electrode. The electrolyte solution is saturated potassium chloride. The negative electrode (cathode) is covered by a thin Teflon membrane, which allows oxygen to diffuse through but keeps out other molecules that might react. The electrode reactions are:
Platinum cathode ( electrons consumed ) O2 4H ++ 4 e − → 2 H 2 O
Ag / AgCl anode ( electrons released ) 4Ag 4Cl − → 4 AgCl +4 e −
There is growing interest today in using biosensors to detect toxins, viruses, and perhaps other possible biowarfare agents. In particular a handheld device giving a rapid response would be highly useful. Several proposals exist that would use specific antibodies or antibody fragments as detectors for biowarfare agents. Each B cell carries antibodies specific for one antigen and one proposal is to use whole B cells in the biosensor. When an antigen binds to the antibody on the surface of a B cell,
it triggers a signal cascade. Engineered B cells have been made that express aequorin , a light-emitting protein from the luminescent jellyfish Aequorea victoria . Aequorin emits blue light when triggered by calcium ions ( Fig. 23.15 ). Living jellyfish actually produce flashes of blue light, which are transduced to green by the famous green fluorescent protein (GFP).
In the biosensor, when a B cell detected a disease agent (or any targeted microbe), the cell would release calcium ions due to activation of a signal cascade ( Fig. 23.16 ). This in turn triggers light emission by aequorin. The light emitted is detected by a sensitive chargecoupled device (CCD) detector. This approach is in its developmental stages and will allow detection of 5 to 10 particles of a pathogenic agent such as a virus or bacterium. Multiple patches of around 10,000 B cells specific to different pathogens would be assembled in array fashion onto the same chip.
Another scheme, under development by the Ambri Corporation of Australia, uses antibody fragments mounted on an artificial biological membrane, which is attached to a solid support covered by a gold electrode layer. Channels for sodium ions are incorporated into the membrane. When the ion channels are open, sodium ions flow across the membraneand a current is generated in the gold electrode. The ion channels consist of two modules,each spanning half the membrane. When top and bottom modules are united, the ion channel is open. When the top module is pulled away, the ion channel cannot operate. Binding of biowarfare agents by the antibody fragments separates the two halves of the channels, which in turn affects the electrical signal ( Fig. 23.17 ).
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