There are three generations to amperometric biosensors; the first generation were not specific enough for the information required. They had a lot of problems with interference from other products and molecules, due to the dependence of oxygen reactions with glucose. This generation of biosensors used lower voltages research conducted found that if the electrodes were coated with peroxide the rate of reaction would increase and there would be less interference (Drew. P., 2010). The second generation of amperometric biosensors worked by using a transduction current and the use of a mediators, this avoid the use of oxygen. The sensor used shuttle electrons which would provide the current (Habermüller et al., 1999). Second generation of these biosensors were also found be effective in the use of testing for glucose levels in patients with diabetes (Drew. P., 2010). However, there problems found with this generation too, such as, not being able to use them repetitively, there seemed to be a loss of mediator when it was in contact with the solution. This required more research to see if the combination of mediators would be more useful. Despite this problem they were more favourable than the first generation of biosensors as they could operate at an even lower voltage, this eliminated interference. The final generation of biosensors are the third generation, this work by the direct transfer of electrons and enzymes known as "wire-enzymes" (Habermüller et al., 1999; Drew. P., 2010). Amperometric biosensors were mainly used for glucose testing with patients with diabetes; they work by the production of a current between two electrodes and the reaction of oxygen with glucose providing the level of glucose present in the blood (Chaplin. M., 2004).However there have been studies carried out to see if these biosensors can be used for other methods such as detection of protease, contamination and pesticides in water and in health care with viruses and toxicity (Habermüller et al., 1999). There is constant research being carried out, as this biosensor could be the future to testing.
Amperometric biosensors the future
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Recent studies show that nearly all biochemical molecules can be found by amperometric biosensors using a number of techniques such as, enzyme-catalysed electro-oxidation or by the use of bio-affinity reactions which will help assist the electro-oxidation (Heller, 1996). Research on amperometric biosensors conducted have proved that they could be used for other things other than glucose testing, which may make it possible that the amperometric biosensors may have better uses in other areas of research.
The main research in 1996 was concentrated on reactions with antibodies and antigens for finding DNA sequencing and hybridisation (Heller, 1996). Other evidence has shown that amperometric biosensors have also shown to be successful in the food industry and also in the measurement of lactate although there are many publication on the uses of amperometric biosensors the best application still causes much dispute, but it is clear it is an alternative analytical method compared to High Performance Liquid Chromatography (HPLC) (Kissinger 1997, Gomes et al., 2004). They are the preferred alternative as they are more cost effective, and provide rapid response with simplicity in use and pre-treatment(Gomes et al., 2004). In other areas of research such as health care found that early amperometric biosensors were used to find smaller molecules and analytes such as urea, lactase and glucose but due to continuous research it has been found that these biosensors can now detect larger molecules. This discovery means that larger protein molecules can now be identified and the binding of enzymes through bio-receptors can be measured. Measurement of this binding is done by the electrochemical transduction that appears across the surface (Caygill et al., 2010).
There have been many studies that have been carried out recently proving that the amperometric biosensors can be used for analysis of other products and not just glucose (Heller, 1996). There was an experiment conducted in 1998 has proved that amperometric biosensors could be used for other applications such as nitro-bacteria (SandeÂn et al., 1998). A study conducted in 1998 by SandeAn et al., used amperometric sensors to test for Nitro-bacteria, using an enzyme-linked immunoassay. The reaction occurred on the electrode which was coated with carbon that had been in contact with the assay and the enzyme. The mechanism of the instrument is related to competitive reactions where the primary monoclonal antibody competed against the secondary anti-body which in this case was labelled with alkaline phosphatase (SandeÂn et al., 1998). An electroactive product was generated when the antibodies reacted with the substrate; this reaction was detected using the amperometric device (SandeÂn et al., 1998). The results from this investigation found that amperometric biosensor gave a shorter detection time and was also more sensitive compared to other immunoassays that have been used in the past (SandeÂn et al., 1998).
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These results fuelled other research to be carried out and in 2008 a study was conducted to detect the bacteria Escherichia coli more commonly known as E. coli, which is a foodborne bacterium (Lin et al., 2008). In this experimental procedure an amperometric immuno-sensing strip was used, the experiment used an enzyme linked immunoassay which was put together like a sandwich with double antibodies (Chemburu et al., 2005). The electrodes used were coated with carbon which were linked to the E.coli. Horseradish Peroxide (HRP) and Ferroncenedicarboxylate acid with the aluminium nanoparticles (AuNP) were used as the mediators and the substrates, and these were used to detect the bacteria pathogen. The use of amperometric biosensors in this study meant that there were many benefits as it was adaptable and portable (Lin et al., 2008). It was also made more sensitive with the use of HPR and in the introduction of AuNP which helped to amplify the detection of the bacteria E.coli (Chemburu et al., 2005). The advantage of using double antibodies that were specific helped eliminate the contamination and detection of other bacteria that were not relevant (Lin et al., 2008). Changes applied to the amperometric biosensor meant that there was rapid detection and there is certainly use of the amperometric biosensor in the recognition of bacteria.
There has also been a more recent study that has been conducted this year, 2010, using amperometric biosensors in the detection of human pathogens (Caygill et al., 2010). The biosensors measured the current potential present in the electrochemical of the cell, the measurement of the voltage was then detected indicating whether or not the analyte is binding. The secondary anti body that is used, will undergo a redox reaction and the current produced from this reaction is detected (Yu et al., 2005). This type of combination is known as Enzyme-linked Immunosorbent Assay (ELISA). The combination of these antibodies on the amperometric biosensor has already proved successful in a number of pathogen detections such as Epstein-Barr virus, HSV and HBV viruses (Ding et al., 2008) measuring the DNA produced by the virus and the current it supplied for detection (Albers et al.,2003) . The reason behind the success could be because the amperometric biosensors are stable, easy to use, disposable, economical viable and also efficient due to the low volume of sample required making it practical (Caygill et al 2010).
These results are showing that the future of amperometric biosensors is expanding beyond glucose testing and more towards the field of bacteria identification and detection for contaminations in medical research.
Other studies that have been conducted have also looked at different areas of medical research where the amperometric biosensor could be used and be effect. A study that took place in 2006 looked at use of these sensors in the detecting a protease such as, trypsin. The biosensor was designed to detect low levels of trypsin in the diagnosis of pancreatic diseases (Ionescu et al., 2006). In the experiment polymer-enzyme electrode (Cosnier., 1997) and an increase in the amperometric current was used, this was able to detect the activity of the protease more effective and sensitive. As this allowed the trypsin to be digested more easily with the coating of gelatine on the sensor. The results showed that this was the way forward for a more sensitive and rapid detection of the protease and the relation to pancreatic diseases. It was found that steric hindrance supressed the biosensor but as there was a polymer-enzyme electrode used this was able to reactivate the digestion that takes place with trypsin. The sensors proved to be extremely prompt and were easy to use (Ionescu et al., 2006).
Amperometric biosensors are useful for many biological and medical applications other than glucose sensing, the future of these sensors are heading in this field of research. This could be because the use of enzymes and mediators with the sensors proves to be successful and also due to the easy of adaptability and improvements in detection. These advantages mean that amperometric biosensors are the way forward for rapid testing in chronically ill patients and that self-testing of these problems may become more apparent (Pemberton et al., 2010).
The future of amperometric biosensors shows that there is an increase in the requirement of miniature biological testing. A study was conducting in adapting these biosensors using the combination of amperometric biosensors with Polymer-carbon nanotube-enzyme composites coating (Joshi et al, 2005), due to the redox reaction that takes place in the amperometric biosensors to see improvements in testing. Development of these biosensors will be ideal as they would be small and ultra-sensitive, with shorter detection times and would be easily adapted (Iijima, 2002). In the study conducted the amperometric biosensors were fused together with a thin single layer of carbon nanotubes which were amended with the enzyme to form redox polymer hydrogels (Joshi et al, 2005). Doing this to the amperometric biosensor meant that the oxidation was increased and a reduction in peaks and it also produced a current which was easier to detect because the carbon produced an electrostatic current (Ebbesen et al., 1996; Joshi et al, 2005). From this study it shows that the amperometric biosensors are most efficient and provide enhanced results when combined with other forms of analysis such as the use of nanoparticles and coating of carbon. This information implies that future and development of amperometric biosensors could be shared with nanoparticles.
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Other applications of the amperometric biosensor have been recently found in the testing of contamination in water and food, such as pesticides which has caused many issues in humans and the environment for many years (Drew., 2010). This idea of testing for contamination using biosensors is a novel study which has just recently been carried out in 2010 (Firdoz et al., 2010). In this investigation the biosensors were used for the exposure of carbaryl based on carbon nanotubes with acetylcholine esterase (Firdoz et al., 2010). A multilayer coating was formed when there was an increased exposure to Ultra Violet light, this helped to monitor the activity of the acetylcholine esterase, the results found that the sensor showed excellent sensitivity and was able to easily measure the carbaryl pesticides that were present due to the stability of the biosensor (Drew., 2010 ;Firdoz et al., 2010). Coating the electrodes formed an increase in conductivity on the surface and also increased adsorption of the pesticides in a reduced time frame compared to other techniques. Research from Firdoz et al., (2010) has confirmed that there is definite use of the amperometric biosensors for the detection of pesticides in rapidly improving monitoring and also for the use in other enzyme assisted reactions which are problematic. Where research could take hours or days the introduction of these biosensors means that there could be almost immediate responses (Drew., 2010).
In 2009 research was carried out on clothing through direct screen-printing onto the textile substrate with use of amperometric biosensors (Yang et al., 2010). The study was conducted to observe the fabrication of thick film amperometric biosensors on clothing structures; the aim was to measure the distortion of the clothing in performance for the electro-chemicals that are present in these materials (Yang et al., 2010). The results from this study found that there is potential of textile-based screen printed amperometric sensors in sports or military applications (Yang et al., 2010). These biosensors would enable specific requirements to be met in terms of ink composition for the textile substrate interaction (Yang et al., 2010). It was thought this would be a good analysis technique as electro-chemicals sensors have been used before to test these materials, it was seen that amperometric biosensors work by detecting signals so would have been suited for the on-body physiological monitoring on a small scale because it was done on a larger scale.
There have also been studies conducted that show that the biosensors could be used for analysis of other products that are no biological molecules such as examination of red wine and the polyphenolic compound present (Gomes et al.,2004).
The future of amperometric biosensors seems to be very bright. There is much research being conducted which has shown that there are many uses of this sensor. The research conducted from 1998 to present has shown there are many applications of the biosensor, such as detection of pesticides, biological molecules such as bacteria and protein, detection of pathogens in food, detecting diseases such as pancreatic diseases and also has started to be used in clothing production and testing of drinks (Drew., 2010). The details shown from each study that has been conducted all seem to come to the same assumption that the amperometric biosensor is a great form of analysis and is the way forward to testing.
The amperometric biosensors are improving detection in many applications as they have a fast detection time, are easily adaptable because they can be combined with other techniques which in affect providing greater sensitivity then the sensor already possess. These biosensors are much smaller and use lower volumes of samples; this is making it a more desirable technique to use.
However although each experiment that has been conducted have edged towards the favour of using the amperometric biosensor, there could be many problems related to it, as it is classed as an invasive technique and can only be used once. Also there is the question would patients like to carry out these tests themselves as not all news could be good as these biosensors are portable and the ease of use could mean more home testing. The rapid turnaround time in results and the increased sensitivity could mean that this could be the way forward in healthcare.
Amperometric biosensors are the future of testing because they are proving to be popular in all types of research such as the environment and healthcare. This biosensor has a number of uses and seems to be more efficient when combined with other novel techniques such as nanoparticles and with older techniques such as mediators and substrates using as competitive inhibitors using a current as the main source of detection to find the desired information.