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Optical sensor 'swells' to spot trace gases

19 Jun 2014

MIT lab develops compact way to detect “vanishingly small” molecules.

Using microscopic polymer light resonators that expand in the presence of specific gases, researchers at MIT's Quantum Photonics Laboratory, Cambridge, MA, USA, have developed optical sensors with predicted detection levels in the parts-per-billion range, suitable for sensitive trace gas detection.

The MIT team believes it has conceived an extremely sensitive, compact way to detect “vanishingly small” amounts of target molecules. They describe their new approach in the journal Applied Physics Letters.

The researchers fabricated wavelength-scale photonic crystal cavities from PMMA polymer that swells when it comes into contact with a target gas. The polymer is infused with fluorescent dye, which emits selectively at the resonant wavelength of the cavity through a process called the Purcell effect.

At this resonance, a specific color of light reflects back and forth a few thousand times before eventually leaking out. A spectral filter detects this small color shift, which can occur at even sub-nanometer level swelling of the cavity, and in turn reveals the gas concentration.

"These polymers are often used as coatings on other materials, so they're abundant and safe. Because of their deformation in response to biochemical substances, cavity sensors made of this polymer produce a sensor with faster response and higher sensitivity," said Hannah Clevenson, a PhD student at MIT, who led the experiment in the lab of principal investigator Dirk Englund.

PMMA can be treated to interact specifically with a range of different target chemicals, making the MIT team's sensor design highly versatile. “There's a wide range of potential applications for the sensor,” said Clevenson, "from industrial sensing in large chemical plants for safety applications, to environmental sensing in the field, to homeland security applications for detecting toxic gases, to medical settings, where the polymer could be treated for specific antibodies."

The thin PMMA polymer films, which are 400nm thick, are patterned with structures that are 8-10µm long by 600nm wide and suspended in the air. In one experiment, the films were embedded on tissue paper, which allowed 80% of the sensors to be suspended over the air gaps in the paper.

Swell idea

“Surrounding the PMMA film with air is important, both because it allows the device to swell when exposed to the target gas, and because the optical properties of air allow the device to be designed to trap light travelling in the polymer film,” added Clevenson,

The current experimental sensitivity of the devices is 10 parts per million, but the team predicts that with further refinement, they could detect gases with part-per-billion concentration levels. These sensors are easily reusable since the polymer shrinks back to its original length once the targeted gas has been removed.

The Applied Physics Letters article explains, “Compact, low-cost, polymer photonic crystals with high sensitivity provide the opportunity for state-of-the-art gas sensing. We have demonstrated the implementation of these sensors and their recovery after stimulant gases are removed.

"We expect that experimental sensitivity will increase with improved fabrication processes and with improved readout techniques, such as doping with an agent that will provide stronger fluorescence or with the implementation of cavity transmission measurements. This platform could be useful in a variety of industrial, environmental, and even biological settings. These devices have the potential to be applied to a variety of sensing applications, especially with surface functionalization for specific stimulants.”

About the Author

Matthew Peach is a contributing editor to optics.org.

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