Source: Advanced Manufacturing
Original Author: Rayleigh
Water quality testing technology is not only an important reference and control means for water pollution treatment and recycling, but also an important tool for evaluating treatment effectiveness.
The Earth’s water storage is abundant, estimated to be as much as 1.45 billion cubic kilometers, but the amount directly usable for human production and daily life is pitifully small and unevenly distributed across regions. Currently available freshwater resources mainly come from rivers, lakes, and some groundwater, accounting for only 0.26% of the Earth’s total water volume. The remaining water on Earth is primarily distributed in oceans and high mountain glaciers. The former is salty and bitter, with high utilization costs; the latter is mostly frozen in polar ice caps and permafrost, with 87% of freshwater resources being difficult to utilize. Currently, the total population in countries facing severe water shortages reaches 2 billion! Moreover, as the population continues to grow and climate change exacerbates the situation, this problem will become even more severe.
Image Source: Roslan Rahman/AFP/Getty
At the same time, in many developing countries including China, water pollution is also very serious, often with the co-existence of conventional pollutants and various types of toxic pollutants, posing a significant threat to ecosystems and human health, further exacerbating the water shortage conflict. To protect the ecological environment and public health, many countries attach great importance to and vigorously develop various water quality testing technologies.
Water quality testing technology is not only an important reference and control means for water pollution treatment and recycling technologies, but also an important tool for evaluating treatment effectiveness or recyclability.
Image Source: Merkushev Vasiliy/Shutterstock.com
Current water quality testing methods include offline (gas phase, liquid phase) chromatography, mass spectrometry technology, and capillary electrophoresis, etc. These technologies typically can only test for one type of substance. Therefore, if it is necessary to detect toxic substances such as organic pollutants, heavy metals, and harmful bacteria, large instruments such as atomic absorption, high-performance liquid chromatography, ultra-high-performance liquid chromatography, and inductively coupled plasma mass spectrometry must be equipped simultaneously.
These large instruments have high precision, but due to their high costs, complex structures, and large sizes, and the high operational and maintenance requirements, they are difficult to implement and promote in economically backward areas, and are also a significant burden in developed areas. Therefore, for the broad developing countries (especially rural areas), high-performance, low-cost, portable, and online water quality monitoring technologies have greater value and urgent demand.
Technologies currently under development in this area include electrochemical sensors, UV-visible absorption spectroscopy, and fiber optic evanescent wave sensing methods, among which electrochemical sensors are inexpensive but have low detection limits and weak anti-interference capabilities. UV absorption spectroscopy has a fast response but can measure a limited range of pollutant types. The fiber optic evanescent wave sensing technology (Fiber Optic Evanescent Wave Sensor, abbreviated as FOEW) has attracted much attention due to its relatively low cost, fast detection, high sensitivity, and strong selectivity.
What is an evanescent wave? When light enters a less dense medium (n) from a denser medium (nr) at an angle greater than the threshold, the refracted light beam disappears, and all light is reflected back. However, at the total reflection position, an energy field flows along the direction of the less dense medium, which is the evanescent wave (also known as a disappearing wave). When light is transmitted in the optical fiber, the evanescent wave is concentrated on the surface of the fiber core, and its intensity decays exponentially in the direction perpendicular to the interface, with a wave field depth (Dp) ranging from tens of nanometers to micrometers. When the fiber cladding is stripped away, the evanescent wave field will interact with the substance to be measured, thereby affecting the output light intensity. Its basic structure is shown in the diagram below.
Figure 1: Principle of Fiber Optic Evanescent Wave Field
Image Source: TrAC Trends in Analytical Chemistry Volume 127, June 2020, 115892
To enhance the evanescent wave signal or facilitate sampling, the fiber optic end face can also be easily bent into a U-shape or processed into a conical shape, or made into various Bragg fiber grating structures.
The fiber optic evanescent wave sensing technology is based on the interaction of the evanescent wave field with the substance to be measured to achieve detection. These interaction mechanisms include absorption, fluorescence excitation, surface plasmon resonance, and Raman scattering, etc. Most of these interactions are very weak. To further enhance sensitivity and improve selectivity, a highly selective specific substance, also known as a probe, is often implanted and fixed on the end face of the fiber optic evanescent wave field.
The probe will only react with the target substance marked that can be contacted by the detector surrounding it. Due to the shallow depth of the evanescent wave, which only exists in a thin layer near the fiber interface, it forms an optical isolation in the detection solution, allowing only the target substance within the thin layer to participate in the reaction. Non-target substances will not be “captured,” thus providing high selectivity.
For example, with a fluorescent probe, when the target substance is “captured” on the fiber surface, the fluorescent group on the target substance is excited by the evanescent wave field to produce fluorescence, and part of the fluorescence enters the fiber for conduction. The light signal is converted into an electrical signal by the signal collector, and after data processing, the target substance information can be obtained, while the probability of free non-target substances in the sample being “captured” and excited is very low. These characteristics not only greatly reduce interference but also avoid complicated sample pretreatment, significantly shortening detection time, which is very beneficial for detecting water bodies with various mixtures.
By implanting different fluorescent probes on the fiber optic end face, different types of pollutants can be detected. Interestingly, different probes can be implanted at different positions along the fiber optic, allowing one fiber to “string together” different functional probes to detect multiple pollutants, as shown in the diagram below.
Figure 2: Schematic Diagram of Multi-Probe Series Structure
Image Source: TrAC Trends in Analytical Chemistry Volume 127, June 2020, 115892
Water Quality Heavy Metal Ion Detection
Heavy metal ions such as mercury, lead, chromium, and cadmium in drinking water are well-known health hazards and are the culprits of various cancers and brain damage. FOEW has significant advantages in detecting heavy metal ion pollutants and is one of the research hotspots both domestically and internationally.
The diagram below shows an example of mercury ion detection, where the light source is a common LED or semiconductor laser, and the detector is a mini spectrometer. The fiber optic is a multimode branched fiber (Figure 3b). A modified quantum dot probe is implanted on the fiber optic end face (Figure 3a), and stimulating light is introduced into the fiber. If there are no mercury ions nearby, quantum dot fluorescence can be detected at the detection end.
Figure 3: Schematic Diagram of FOEW Detecting Heavy Metal Ions
Image Source: TrAC Trends in Analytical Chemistry Volume 127, June 2020, 115892
If mercury ions “pass by” the probe, they will be quickly “captured” by the probe, forming a complex under the action of ionic bonds, causing the fluorescence to gradually quench. If mercury ions directly “collide” with the quantum dot material, they will directly transfer electrons in the quantum dot conduction band, preventing electrons from recombining with holes, leading to rapid fluorescence quenching. Both of these actions will weaken the fluorescence intensity of the probe; the higher the concentration of mercury ions, the weaker the fluorescence intensity. Thus, by detecting the change in fluorescence signal before and after, qualitative and quantitative analysis of mercury ions can be performed, with a detection limit of 1nM and a response time reduced to 3 minutes. If the specific probe is replaced with thymine-rich DNA material, the sensitivity and selectivity will be higher. If a DNAzyme core material is used, the detection limit for lead ions (Pb2+) in water can also reach 1nM, with a response time of only 5 minutes, while keeping the hardware essentially unchanged.
Water Quality VOCs Detection
Volatile organic compounds (VOCs) in water bodies are ubiquitous and pose serious environmental impacts. Their composition is complex and present in very small amounts, many of which have carcinogenic, teratogenic, and mutagenic properties, some even exhibiting genetic toxicity, leading to abnormal “feminization” development, posing serious threats to environmental safety and human survival.
Currently, people still rely on expensive gas chromatography and mass spectrometry technologies for VOCs detection. The advancement of FOEW technology brings new hope. Currently, FOEW sensors made from conical hydrophobic polymer fibers and D-shaped fibers containing fluorescent dyes can detect trace VOC components in water within 5 minutes, with a detection limit of 10 ppb, which is something traditional gas chromatography-mass spectrometry (GCMS) and liquid chromatography-mass spectrometry (LCMS) cannot achieve.
The table below summarizes the detection limits and response times that this technology can currently achieve.
Table 1: FOEW Sensor Detection of VOCs
Water Quality Pathogen Microorganism Detection
Pathogenic microorganisms in water bodies, such as Escherichia coli, Salmonella, and Listeria, are the most common pathogenic bacteria, which exist in large quantities in surface drinking water sources and can cause various gastrointestinal diseases, with severe cases even leading to death.
Figure 4: FOEW Detecting Escherichia coli
Image Source: TrAC Trends in Analytical Chemistry Volume 127, June 2020, 115892
FOEW technology has made significant progress in detecting pathogenic microorganisms. As early as 2011, a 280nm light absorption-type FOEW sensor had achieved a detection limit of 1000 cfu/mL for Escherichia coli. In 2018, this technology was further improved (as shown above) by preparing special optical fiber cones and sub-wavelength fiber cores to excite and transmit multiple evanescent wave fields in the fiber. When E. coli falls within the sensing range of the evanescent wave field, the spectral peak will shift slightly. The more E. coli there are, the greater the shift. By detecting the degree of spectral shift, the number of bacteria can be quickly assessed. This method has high sensitivity, strong anti-interference capability, is simple to operate, and is cost-effective, with great practical prospects.
Optical fiber technology has advantages such as resistance to electromagnetic interference, fast response, small size, ease of installation, and relatively low cost. Chemical probes can effectively improve selectivity and sensitivity. FOEW sensing technology is a perfect combination of optical fiber technology and chemical probes, greatly enhancing the applicability of this technology in environmental water detection. In the future, FOEW sensors will continue to develop towards miniaturization, integration, and automation, with enormous application prospects in water monitoring.
References
[1] Jiao, L., Zhong, N., Zhao, X., et al. (2020). Recent advances in fiberoptic Evanescent wave sensors for monitoring organic and inorganic pollutants in water. Trends in Analytical Chemistry 127. doi: 10.1016/j.trac.2020.115892
[2] 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
[3] 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 52Íž 1049-1050. doi:10./1049/el.2016.0762
END
The reproduced content only represents the author’s views
It does not represent the position of the Semiconductor Institute of the Chinese Academy of Sciences
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