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Approximately two billion people around the world live in nations with ‘high water scarcity.’ As the global population continues to grow and the climate changes, health professionals warn that the demand for safe drinking water will increase by up to 30% by the year 2050.
In addition to the lack of available water, many water supplies, particularly in developing countries, are polluted with a wide range of health hazards ranging from sewage and industrial water discharge in surface water to salt-water intrusion and mismanaged waste in groundwater.
Current Methods for Water Quality Analysis
The effective monitoring and evaluation of water supplies have not always been adequate, especially in developing nations. Several advanced analytical techniques have been used to obtain more information about the contaminants present in ground and surface water supplies. Some of these include liquid and gas chromatography, mass spectrometry (MS), capillary electrophoresis, and spectrophotometry.
These techniques provide highly sensitive and repeatable data regarding oxygen, pH, and turbidity of water samples, in addition to assessing the presence of inorganic and organic contaminants.
Despite their usefulness, many of these techniques are limited in their ease of operation, as they often require highly trained professionals. Furthermore, many of these analytical methods are expensive, time-consuming, and risk sample loss during complex pre-treatment processes.
Water Quality Sensors
The limitations associated with conventional analytical techniques for the assessment of water quality has pushed researchers to investigate other potential detection methods.
Several online sensors, such as those based on electrochemical and optical technologies, have not only provided precise data on water quality measurements but have also demonstrated several advantages as compared to other analytical techniques. Benefits include rapid response rates, low cost, portability, and the option to employ in-situ real-time detection capabilities.
What are Fiber-Optic Evanescent Wave Sensors?
The evanescent wave (EW) sensing technique arises through the combination of thin optical fibers with sensitive coatings. These can significantly enhance the sensitivity and selectivity of traditional fiber-optic sensors while simultaneously offering a lower limit of detection (LOD).The basic working principle behind fiber optic EW (FOEW) sensors involves the interaction of the sample with an evanescent field of light that traverses an optical fiber. As compared with other sensing technologies, the evanescent field has a penetration depth that ranges from ten to several hundred nanometers, eliminating any potential background that could arise from the bulk solution.
FOEW Sensors and Water Quality Analysis
Several different types of FOEW sensors have been investigated for their detection capabilities of a wide range of pollutants present in water supplies, some of which include heavy metal ions, volatile organic compounds (VOCs), and microorganisms.
Heavy metal ions
Human exposure to heavy metal ions such as lead (Pb2+), mercury (Hg2+), chromium (Cr6+), and cadmium (Cd2+) through drinking water supplies can lead to several severe health conditions ranging from cancer and kidney disease to brain damage and hypertension.
FOEW sensors are ideal for ion monitoring due to their high sensitivity capabilities, rapid response rates, small size, and immunity from electromagnetic interference.
Several biosensor developments for the detection of heavy metal ions in water have modified the surface of the unclad fiber of FOEW sensors with a functional nucleic acid (FNA).
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For the detection of Hg2+, for example, thymine (T)-rich DNA is often the FNA of choice, as these biological molecules have a high affinity for binding to Hg2+. This binding results in the formation of T-Hg2+-T complexes that reduce the fluorescence signal to ultimately allow for highly accurate Hg2+ concentration levels to be obtained. Comparatively, the biosensing material of choice for monitoring Pb2+ concentration levels is DNAzyme.
In addition to biosensors, several FOEW chemical sensors have been investigated for their substantial metal ion detection capabilities.
As compared to biosensors, which have stability limitations due to the use of biomaterials, FOEW chemical sensors are non-biological and therefore eliminate this concern.
In 2016, a tapered optical fiber coated with chitosan was found to detect Pb2+ at a sensitivity of 40.554 ab/ppm.
A 2018 study discussed the advantages of a three-layer polymer FOEW sensor for the detection of Hg2+ in aqueous solutions. With a response rate averaging at 50 seconds and a LOD of 0.1 mg/L, this sensor provided a sensitivity that was 6.5 times greater than any other conventional FOEW sensor with a core/cladding structure.
Like many other organic pollutants, human exposure to VOCs can cause widespread toxic effects.
In addition to harming human health upon ingestion, the presence of VOCs in water supplies can also lead to harmful ecological effects due to their consumption of dissolved oxygen in the water. This causes biodegradation to occur.
Several types of FOEW sensors have been investigated for their potential to detect VOCs, some of which include taper hydrophobic polymer fibers and D-shaped fibers comprised of solvatochromic dyes.
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With a maximum response time of 5 minutes, these FOEW sensors have also been shown to provide immediate and efficient monitoring of VOCs in emergencies, which would otherwise be impossible if conventional testing methods such as GCMS and LCMS were employed.
Pathogenic bacteria such as Escherichia coli, Salmonella enterica, and Listeria monocytogenes are some of the most common foodborne pathogens that can cause serious health effects ranging from gastrointestinal illnesses to even death.
In 2011, one of the earliest FOEW sensors was developed for the detection of E. coli. This U-shaped FOEW sensor was capable of detecting concentrations that were below 1000 colony-forming units (cfu)/mL by measuring EW absorbance changes at 280 nanometers (nm).
Similar detection limits of 1000 cfu/ml of E. coli were achieved in a 2018 study in which a multimode microfiber probe was used. Changes in the bacteria concentrations were obtained through this method by measuring shifts of the optical spectrum that occurred as E. coli would bind onto the microfiber surface.
References and Further Reading
Tortajada, C., & van Rensburg, P. (2019). Drink more recycled wastewater. Nature 577; 26-28. doi:10.1038/d41586-019-03913-6
Jiao, L., Zhong, N., Zhao, X., et al. (2020). Recent advances in fiber-optic evanescent wave sensors for monitoring organic and inorganic pollutant sin water. Trends in Analytical Chemistry 127. doi:10.1016/j.trac.2020.115892
Li, Y., Ma, H., Gan, L., Gong, A., et al. (2018). Selective and sensitive Escherichia coli detection based on a T4 bacteriophage immobilized multimode microfiber. Journal of Biophotonics 11(9). doi:10.1002/jbio.201800012
Ibrahim, S. A., Ridzwan, A. H., Mansoor, A., & Dambul, K. D. (2016). Tapered optical fibre coated with chitosan for lead (II) ion sensing. Electronics Letters 52; 1049-1050. doi:10./1049/el.2016.0762