[ technology opportunity 2004-019]

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Water Contaminant Sensor for Monitoring PAHs, BTEX, PCBs and
Heavy Metals
2004-019

Queen’s University researchers in collaboration with Avensys Inc. have developed a novel optical sensor array suited to a variety of applications in environmental monitoring. The combination of chemically selective coatings with fibreoptic sensors will result in a monitoring system that is more robust, cheaper and faster than any existing technology. The technology was recently featured in IEEE Lasers and Electrooptics (LEOS) magazine (right).

Description:
The sensor head is a 20mm fibreoptic refractive index sensor. The technology combines the inherent ruggedness and low cost of fibreoptic data transmission with the sensitivity of a so-called long-period grating (LPG). Acting as an optical filter for some wavelengths of the transmitted light, the LPG will redirect light at these wavelengths from the core of the fibre to the cladding. Light in the cladding is not efficiently contained, and is quickly lost. The amount and wavelengths of the light lost is affected by the refractive index of the medium immediately outside the cladding, which may be water, a polymer jacket, or – as in this case - a sensitized material. If the refractive index of this medium is very different from the refractive index of the fibreoptic material, the wavelengths of the filtered light are unchanged. However, if a contaminant is accumulated outside the cladding and thereby increases the refractive index, light at different wavelengths is lost. It is this principle that allows the long-period grating to be used as a chemical sensor for water contaminants. The significant advantage of this detection method is that all contaminants increase the refractive index of water, and so a universal detector is possible.

Two techniques are employed to dramatically enhance sensitivity and allow chemically selective detection. Sensitivity is enhanced by splicing the ends of the modified fibre into a loop, which traps light inside the loop. The light is thereby forced to pass through the sensor area hundreds of times, amplifying the contaminant-induced optical loss, and greatly increasing the detection capability (Optical Loop Ring-Down technique). This platform has already been used to detect changes in refractive index in solution on the order of delta n <10 exp.-4.

Further amplification of the refractive index change as well as chemical selectivity can be accomplished by applying specially designed coatings to the surface of the fibre. In the case of organic contaminants such as BTX, PAHs or PCBs dissolved in water, hydrophobic coatings are employed. The organic contaminant, which is also hydrophobic, will be concentrated 100- to 1000-fold in the coating, resulting in a dramatic change of refractive index which is observed by the detector. In the case of metal contaminants, specific ligands known to form chemical bonds to the metal of interest are attached inside the pores of a highly porous coating. Binding of the metal to the ligand will then result in a change in refractive index. Again, the coatings have the advantage that they will increase the concentration of the analyte by up to eight orders of magnitude, thus increasing sensitivity as well as allowing for chemical speciation.

Fig. 1, above: The LPG steers light from the core of the fibre to the cladding. As the refractive index of the environment becomes close to the refractive index of the cladding, the LPG becomes very lossy for some wavelengths of light and becomes a very sensitive refractive index sensor.

Fig. 2: The LPG detector is incorporated into the fibre loop. The light is trapped in the loop and forced through the LPG several hundreds of times. The contaminant changes the optical loss of the LPG and this loss is amplified with every pass. The concentration is then inferred from the rate at which the light in the loop decays - the ring-down time.

Potential applications:
The device is ideally suited to continuous monitoring of sites (e.g. landfills, industrial plants, mines and tailing sites) contaminated with a mix of organic and inorganic pollutants, including PAHs, PCBs, mercury, copper, chromium and lead. Continuous monitoring is advantageous since spills or leaks can be detected instantly, and the resulting clean-up involves the remediation of significantly smaller areas. Although this technique will not permit the differentiation between very closely related contaminants such as toluene and benzene, it will act as an early warning system, informing users that a contaminant of a certain class has been detected, and thus prompting more detailed examination. This will dramatically decrease monitoring cost since operators need to be deployed only when a problem is detected.

Key Benefits:
The device is sensitive to a broad range of chemical contaminants from many chemical families, and it is inexpensive and portable. Material cost is less than $500. Telemetry system may be coupled to the device for remote detection.

State of development:
A Precarn-funded project in collaboration with Avensys Inc. has resulted in several prototypes and proof-of-concept testing. The research group is seeking additional development partners and funds to advance the project to market readiness. International patent applications are pending on four patent families covering various aspects of the device. US Patent No. 6,842,548 covering the Optical Loop Ring-Down technique has issued.

Contact:
Anne Vivian-Scott
Director, Commercial Development
Phone: (613) 533-2342
E-mail: avivianscott@parteqinnovations.com
Ref Tech ID 2004-019