Biosensors - ripe results
Enzymes are nature's catalysts, speeding up the reactions of life. As large molecules, they have very specific shapes that lock certain chemicals into place, and enable them to react. Using enzymes in sensors makes good use of their ability to select a specific chemical and convert it into a known product. This enables us to measure a chemical that is hard to detect by converting it into one that is easy to discover. A simple way to detect the product of the enzyme reaction is at an electrode - this generates a current showing how much target chemical is present.
Of course, that is a very simple description of what's going on in an enzyme-based biosensor. What's more important is the difference that these biosensors can make in all sectors of industry. They turn complex laboratory analysis into quick and simple tasks that virtually anyone can perform.A tool for the food industryThe food industry takes advantage of the fact that its natural products lend themselves well to detection with biosensors. A European consortium recently developed a hand-held device that can be used to measure sugars and organic acids in fresh fruit. The ratio of sugar to acid concentration provides a measure of ripeness so, in essence, the sensor detects when fruit is ready to be picked and sold. With this device, fruit growers, retailers and processors can monitor fruit quality in storage and at the point of sale.
However, sensor system development is not simply a case of finding the right enzyme and placing it on to an electrode. The biosensor typically consists of an enzyme layer, sandwiched between two membranes, overlaid on an electrode. The inner membrane prevents unwanted molecules reaching the electrode, whilst the outer one provides an enclosed environment suitable for the enzyme and protects it from the sample liquid.
The partners had to design suitable membranes that would be both selective and efficient. They developed a number of polymeric membranes that can be applied by coating on to screen printed electrodes. Membrane thickness is carefully controlled. The strip electrodes are inserted into a small electronic device which monitors the reaction at the sensor and displays the measurement results. After each test, the operator throws away the strip and replaces it with a new one.A multidisciplinary approachDesigning and testing the sensor required a broad range of skills and equipment. One of the partners, Geest UK Produce Marketing, provided the fruit as well as knowledge about the fruit industry and carried out testing on site as a representative end-user. The other participants brought a mix of sensor and electronic engineering expertise. Sophisticated computing techniques were vital to improve signal processing and to model the diffusion and response kinetics in the multi-layer system. By definition, the project required a truly multidisciplinary approach.
By extensively testing a new hand-held meter prototype, the partners found that they could detect and measure acids (like citric acid and Vitamin C) and sugars in undiluted fruit juice. This is the first robust sensor system that can operate without any real sample preparation.
During the project, the team made contacts with potential product users such as fruit distributors. This kept their goals focused on real market needs as well as breaking new technological ground. As a result, the project team created a working prototype sensor system which is now ready to be developed into a product. SensAlyse, in the UK, is playing a key role in exploiting the product and background technology.The fruits of their labourClear markets for the device are in fresh fruit products. Different grades of oranges, for instance, could easily be labelled with confidence, providing the consumer with more choice. In fact, there are widespread applications across the food industry. With further development the sensor could be used wherever fermentation is monitored or where sugar is converted into other products. Opportunities are also presented in the clinical diagnostics market. The results of this project could find wider uses in other industries, environmental monitoring and in medicine.
These devices will no doubt play an increasingly important role in process monitoring as well. This takes time and relies upon regular samples being sent off to a laboratory for analysis. Finding the problem and then adjusting the process wastes a lot of time and resources. With improved sensor systems, most people could perform these checks, not just the scientists.
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