The new, temperature-insensitive fiber optic biosensor uses bound bacteriophages to capture E. coli in a sample on the fiber surface, and reads the bacteria’s presence in the sample through spectral changes in light passing through the fiber. [Image: Dandapat et al. Opt. Lett., doi: 10.1364/OL.41.004198]
Fiber optic biosensors hold significant promise for improving the detection of microbes such as E. coli, a pathogen that has been responsible for a number of deadly episodes of food and water contamination. But there’s been a catch: the fiber’s sensitivity to temperature changes means that these sensors often can operate only in narrow temperature bands, which limits their usefulness in field settings.
Now, a research team from Canada and India has developed a new, compact fiber sensor that’s virtually insensitive to temperature changes. And, by coating the fiber with bacteria-snagging viruses, the researchers have come up with a device that can detect E. coli in contaminated water across a 40 °C temperature variation—and in 15 to 20 minutes, rather than the hours to days required using most commonly used techniques (Opt. Lett., doi: 10.1364/OL.41.004198).
Combining two gratings
Fiber optic biosensors work by measuring spectral shifts that arise from the change in optical properties, particularly refractive index, due to the presence of bacteria or other microorganisms in biochemical samples. But such sensors can run aground on the phenomenon of “temperature cross-sensitivity.” Because the refractive index of both the biochemical sample and the optical waveguide can change with temperature variations, it’s often difficult to calibrate the system for temperature shifts, and to tease out which refractive-index changes are bacteria-related signal and which are temperature-induced noise.
To overcome that issue, the research team, from the University of Quebec, Outaouais, Canada, and the Indian Institute of Technology, Kanpur, built on earlier work, in which they had overcome temperature sensitivity by concatenating two long-period fiber gratings (LPFGs) in the sensor, separated by a space. By tuning the space between the two gratings, they were able to cancel out the effects of temperature on the two gratings with the temperature-induced phase changes in the inter-grating space, thereby resulting in a sensor that was insensitive to temperature shifts at resonance wavelengths.
Making it compact
The problem with that earlier approach, however, was that the requirement of an inter-grating space as large as 20 cm meant that the fiber sensor needed to be too long for a practical, field-ready device. In the new sensor, the team has gotten around that problem by combining two LPFGs without a space between them—but by choosing the two gratings so that they selectively excite modes in the fiber cladding with opposite dispersion characteristics. The scheme effectively compensates for the temperature-induced phase changes in the fiber.
The result is a device that is a mere 3.6 cm in length—easily small enough for practical field biosensing applications—and that has a temperature sensitivity of only around 1.25 pm per degree Celsius across a temperature range of 40 degrees. And the team writes that the temperature sensitivity could be further reduced by tweaking the lengths of the LPFGs.
Rapid response
To make a biosensor out of the new fiber device, the researchers used a variety of chemical techniques to give the fiber surface an affinity to covalently bond with a specific bacteriophage—a type of virus that itself is designed to latch onto particular species of bacteria. After coating the surface with the phages, the scientists then exposed the virus-laden sensor to various concentrations of E. coli–infected water.
The E. coli snagged by the phages resulted in distinct spectral shifts in the sensor that were easily read, and that were robust across a range of temperatures. The results were also clear in only 15 to 20 minutes—considerably faster than conventional approaches, which require amplification of the bacteria’s genetic material over hours to days using complex, sophisticated equipment.
The research team is now collaborating with a Canadian firm, Security and Protection International, toward commercialization of the new sensor.