Microbial Biosensors ? - Past, Present and Future

Biosensors are analytical devices used to measure biological information that converts a bodily response into an electrical signal. Biosensors consist of three major parts, the sensitive biological element (tissue, microorganisms, enzymes etc.), the transducer, and the detector element which works physicochemically. The major component of a biosensor is the transducer, which uses the physical changes of a reaction to produce an effect. Such physical changes could be thermal output, electrical potential change, redox reaction, electromagnetic radiation etc. The triggered electrical output from the transducer can then be amplified, processed, displayed and analyzed. Biosensors are a rapidly expanding field of study with an estimated annual growth rate of 60%, with the majority of the growth coming from the health-care industry.

The biosensor concept can be traced back to Professor Leland C Clark Junior, who invented the oxygen electrode in 1956. He wanted to expand the range of analytes that we could measure in the body. His first experiment involved entrapping glucose oxidase enzyme in an oxygen electrode using a dialysis membrane. The observed decrease in oxygen concentration was proportional to glucose concentration. This is the first of many variations of the basic biosensor design that emerged out of Dr Clark's concept. Through the next several decades, the biosensor technology took off radically as a variety of new devices was discovered including enzymes, nucleic acids, and cell receptors. In looking at the historical development of this technology, 1980's was certainly the inventive decade, with commercialization being the theme in the 1990's.

Biosensors' primary functions in terms of research and commercial applications are identifying target molecules, identifying the availability of a suitable biological recognition element, and the potential for disposable detection systems to replace sensitive lab techniques. Examples of these functions include monitoring health related targets, detecting pesticides, detecting pathogens, and determining the level of toxicity in an organism or in the environment. The most widespread and commercialized example is the blood glucose biosensor, which uses an enzyme to break down blood glucose. Once broken, the biosensor transfers an electron to an electrode which is converted into a measure of blood glucose concentration. This process is especially important to diabetics to monitor glucose levels in their bodies. However, many biosensors are still not commercialized and most are still single analyte devices. In recent years, the development of biosensors for environmental and clinical applications has gained much interest from researchers, due to the need for fast and cost effective methods for analysis in both environmental and clinical situations. The advantages of using biosensors are analyte specificity, ease of operation, fast analysis time, and minimal sample preparation. The future of this technology will likely head in the direction of innovating the current processes, and finding ways to make this technology more commercially viable. Table 1 shows some of the most commonly used biological recognition components and transducers in biosensor research.

There are several main categories of biosensors. Piezoelectric biosensors and optical biosensors are based on the concept of surface plasmon resonance, where surface plasmons are