INTRODUCTION 2 BIOSENSORS

Biosensors represent a rapidly expanding field, at the present time, with an estimated 60% annual growth rate. The major impetus comes from the health-care industry. For example 6% of the western world are diabetic and would benefit from the availability of a rapid, accurate and simple biosensor for glucose. Food quality appraisal and Environmental monitoring are other important areas. The estimated world analytical market is about £12,000,000,000 per year of which 30% is in the health care area. There is clearly a vast market expansion potential as less than 0.1% of this market is currently using biosensors. Research and development in this field is wide and multidisciplinary, spanning biochemistry, bioreactor science, physical chemistry, electrochemistry, electronics and software engineering

DEFINITION OF A BIOSENSOR

A biosensor is an analytical device which converts a biological response into an electrical signal

The term 'biosensor' is often used to cover sensor devices used in order to determine the concentration of substances and other parameters of biological interest even where they do not utilise a biological system directly.

A Biosensor can be ideally defined as followed :

Biosensors are defined as analytical devices in

corporating a biological material (e.g. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, natural products etc.), a biologically derived material (e.g. recombinant antibodies, engineered proteins, aptamers etc) or a biomimic (e.g. synthetic catalysts, combinatorial ligands, imprinted polymers) intimately associated with or integrated within a physicochemical transducer or transducing microsystem, which may be optical, electrochemical, thermometric, piezoelectric,

magnetic or micromechanical.

A BIOSENSOR HAS THREE PARTS

A sensitive biological element

The biological element senses or generates the sig

nal.

It may be a biological material such as a tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, natur

al products etc.

It may be a biologically derived material (e.g. recombinant antibodies, engineered proteins, aptamers etc) or a biomimic (e.g. synthetic catalysts, combinatorial ligands, imprinted polymers)

The transducer or the detector element

A Transducer transforms the signal resultin

g from the interaction of the analyte with the biological element into another signal (i.e., transducers) that can be more easily measured and quantified.

It makes use of a physical change accompanying the reaction. This may be

1. the heat output (or absorbed) by the reaction (calorimetric biosensors),
2. changes in the distribution of charges c ausing an electrical potential to be produced (potentiometric biosensors),
3. movement of electrons produced in a redox reaction (amperometric biosensors),
4. light output during the reaction or a light absorbance difference between the reactants and products (optical biosensors), or
5. effects due to the mass of the reactants or products (piezo-electric biosensors).

Associated electronics or signal processors

Primarily responsible for the processing and display

of the results in a user-friendly way.

EXAMPLE :

The most widespread example of a commercial biosensor is the blood glucose biosensor, which uses the enzyme glucose oxidase to break blood glucose down. In doing so it first oxidizes glucose and uses two electrons to reduce the FAD (a component of the enzyme) to FADH2. This in turn is oxidized by the electrode (accepting two electrons from the electrode) in a number of steps. The resulting current is a measure of the concentration of glucose. In this case, the electrode is the transducer and the enzyme is the biologically active component.

A Schematic diagram showing the main components of a biosensor

The biocatalyst (a) converts the substrate to product. This reaction is determined by the transducer (b) which converts it to an electrical signal. The o

utput from the transducer is amplified (c), processed (d) and displayed (e).

There are three so-called 'generations' of biosensors; First generation biosensors where the normal product of the reaction diffuses to the transducer and causes the electrical response, second generation biosensors which involve specific 'mediators' between the reaction and the transducer in order to generate improved response, and third generation biosensors where the reaction itself causes the response and no product or mediator diffusion is directly involved.

The electrical signal from the transducer is often low and superimposed upon a relatively high and noisy (i.e. containing a high frequency signal component of an apparently random nature, due to electrical interference or generated within the electronic components of the transducer) baseline. The signal processing normally involves subtracting a 'reference' baseline signal, derived from a similar transducer without any biocatalytic membrane, from the sample signal, amplifying the resultant signal difference and electronically filtering (smoothing) out the unwanted signal noise. The relatively slow nature of the biosensor response considerably eases the problem of electrical noise filtration. The analogue signal produced at this stage may be output directly but is usually converted to a digital signal and passed to a microprocessor stage where the data is processed, converted to concentration units and output to a display device or data store.

A successful biosensor must possess at least some of the following beneficial features:

1. The biocatalyst must be highly specific for the purpose of the analyses, be stable under normal storage conditions and, except in the case of colorimetric enzyme strips and dipsticks, show good stability over a large number of assays (i.e. much greater than 100).
2. The reaction should be as independent of such physical parameters as stirring, pH and temperature as is manageable. This would allow the analysis of samples with minimal pre-treatment. If the reaction involves cofactors or coenzymes these should, preferably, also be co-immobilised with the enzyme.
3. The response should be accurate, precise, reproducible and linear over the useful analytical range, without dilution or concentration. It should also be free from electrical noise.
4. If the biosensor is to be used for invasive monitoring in clinical situations, the probe must be tiny and biocompatible, having no toxic or antigenic effects. If it is to be used in fermenters it should be sterilisable. This is preferably performed by autoclaving but no biosensor enzymes can presently withstand such drastic wet-heat treatment. In either case, the biosensor should not be prone to fouling or proteolysis.
5. The complete biosensor should be cheap, small, portable and capable of being used by semi-skilled operators.
6. There should be a market for the biosensor. There is clearly little purpose developing a biosensor if other factors encourage the use of traditional methods and discourage the decentralisation of laboratory testing.

APPLICATION OF BIOSENSORS

Biosensors have been applied to a wide variety of analytical problems including in medicine, drug discovery, the environment, food, process industries, security and defence.

Some examples are given below:

• Glucose monitoring in diabetes patients <-- historical market driver
• Other medical health related targets
• Environmental applications e.g. the detection of pesticides and river water contaminants
• Remote sensing of airborne bacteria e.g. in counter-bioterrorist activities
• Detection of pathogens
• Determining levels of toxic substances before and after bioremediation
• Detection and determining of organophosphate
• Routine analytical measurement of folic acid, biotin, vitamin B12 and pantothenic acid as an alternative to microbiological assay
• Determination of drug residues in food, such as antibiotics and growth promoters, particularly meat and honey.
• Drug discovery and evaluation of biological activity of new compounds.
• Detection of toxic methabolites such as mycotoxins.