Illuminating the molecules with a laser tuned to the correct frequency makes them vibrate and absorb the light, the amount of which depends on concentration.
So-called quantum-cascade lasers are key to scaling down mid-infrared chemical-sensing tools to palm size, says Boris Mizaikoff, an associate professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology.
"These diode lasers are roughly the size of those in laser pointers or CD players. They operate at room temperature and can emit nearly across the mid-infrared band."
A photonic-band-gap hollow waveguide — basically a hollow, flexible tube — holds a gas to be sampled. Liquid-phase detectors use a planar, silver-halide waveguide to transmit the laser light. The waveguides efficiently propagate only one wavelength, making them useful for detecting specific molecules. For example, a waveguide coupled to a frequency-matched quantum-cascade laser can detect ethyl-chloride levels down to 30 ppb, on par with gas and liquid chromatography methods. The system needs only 1 ml of sample gas, compared to a few hundred milliliters for conventional multipass gas cells.
Breath diagnostics is a promising application for the technology. Many diseases have specific biomarkers present in breath that rapidly grow in concentration as the disease progresses but remain at extremely low levels, necessitating sensitive and reliable tools to detect the changes. The technology may also bring instruments capable of continuously monitoring water quality with single-digit, parts-per-billion sensitivity.