Biosensors, the next gold standard for water quality testing?
It is something we use daily, and when traces of it are detected on other planets it causes widespread excitement. Water is at the centre of all life as we know it and makes up the majority of the planet we call home. Yet clean water is a different story. Even in this age of innovation, when self-driving cars and holidays to space are on the horizon, access to clean water for all is a goal, not a reality. The World Health Organisation (WHO) reported that in 2017 2.2 billion people did not have access to safely managed drinking water, and that contamination of drinking water is approximated to cause 485,000 diarrhoeal deaths annually. The importance of clean water is highlighted even more so in these current times during the COVID-19 pandemic when the importance of regular and thorough hand washing is driven home.
The WHO highlights the importance of testing drinking water, and determined that fluoride and arsenic are some of the most important contaminants to test for. Traditional testing methods include mass spectrometry and polymerase chain reaction. These techniques need to be executed by trained operators and cannot be completed on-site. Moreover, they are time-consuming and require expensive equipment.
A potential alternative can be found in biosensors. These come in many shapes and forms, although the two most commonly described types are electrochemical and optical biosensors, referring to the transduction method. Biosensors are made up of two main components: a receptor and transducer. The receptor is specific to one particle or organism, for example a contaminant such as E. coli. Once the target has bound to the receptor, a change is elicited and a signal generated that a transducer turns into a signal that we can read. It can tell us whether a set threshold has been passed or not, or even provide a quantitative value. This design allows biosensors generally to be faster, simpler and portable, so that testing can be executed on-site and not only by trained technicians. They are also highly sensitive and more affordable.
With these advantages in mind, it will come as no surprise that biosensors have not gone unnoticed. While there are some commercial biosensors available, it is reported that many remain as descriptions in literature. This is not to say there is no market for them; this slow commercialisation is attributed to a number of requirements deemed necessary for this to take place. These include achieving the lowest possible limits of detection, no need to add reagents, portability, an assay time of seconds or minutes and the ability to identify multiple analytes using one device. Another proposed complication to their commercialisation is the interdisciplinarity of their fabrication. With 69 different biosensors described in literature solely for environmental monitoring in 2017, and with an estimated 200 to 500 companies working on biosensors across all fields in the same year, it seems it is only a matter of time before biosensors become widely commercially available.
While biosensors appear to have a promising outlook, the stability and durability of the receptors need to be improved. In line with these challenges, other technologies are being researched to be combined with biosensors, for instance molecularly imprinted polymers (MIPs) and nanomaterials. MIPs can be thought of as a mould in which the target molecule has been imprinted, forming cavities of a specific shape. They tend to be more robust and have been nicknamed ‘plastic antibodies’. Nanomaterials are, as the name suggests, constructed by particles of nanoscale size. What makes them useful is characteristics such as conducting electricity well, adsorbing strongly and the ability for their surfaces to be modified by attaching functional chemical groups.
Aside from biosensors, there are also other technologies that attempt to replace the current methods for monitoring water quality. A research group at MIT, for instance, has developed a cheap device to measure the presence of heavy metals, using resin beads to attract and capture heavy metal ions. Like biosensors, the test can be done on-site and is affordable, however this test currently only measures metals. Another approach described by Højris et al. involves an optical sensor that determines, based on shape and light diffraction, whether particles in water are bacteria or abiotic. Testing time is just 10 minutes, maintenance is low and if changes are detected that may indicate pollution, the device could be auto-triggered to track the occurrence. Once again, however, the sensor currently detects only bacteria.
Water quality testing is not the only field that is benefitting from the development of biosensors. They are being researched for their use in medical applications, pollution measurements and food safety amongst others. The first biosensor was actually described as early as 1962 by Clark and Lyons, to measure blood glucose levels. Since then the biosensor field has developed considerably, expanding into other areas including water quality, that are in need of improved testing devices and involving other technologies to overcome limitations. Testing, of course, is only an initial step – it does not clean water for us. But it can tell us where there is an issue of contamination, so that we can work towards a reality in which everyone has access to clean water, as it should be.
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