Super-sensitive optical chemical sensor in development

Materials World magazine
1 Sep 2007
Schematic of the optical sensor

Researchers at Southampton University, UK, are developing an optical chemical sensor that they claim can detect compounds with a sensitivity of one molecule in 10 million. This is 10 times more sensitive than other sensors available, and could be used to detect minute amounts of toxins or explosives.

The coated all-coupling nanowire microcoil resonator makes use of a thin optical fibre stretched to around 400nm – 300 times thinner than a standard telecommunications fibre. The fibre is coated with teflon and wrapped around a transparent tube of the same material, through which an analyte flows. When a light is shone into one end, it travels along the nanowire and around the analyte over a million times.

As the fibre’s diameter is smaller than the wavelength of light it transmits, some of the light propagates outside the fibre’s physical boundaries. When this light interacts with the analyte, its propagation speed changes. This gives information about the analyte’s concentration.

‘Each substance has a refractive index which depends on its chemical composition, wavelength and concentration,’ explains Dr Gilberto Brambilla, Senior Research Fellow at Southampton. ‘By measuring the refractive index of a solution, you can measure its concentration.’

Solutions of known concentrations were first passed into the teflon tube, and a calibration curve relating to the refractive index of the fibre output was drawn. This calibration curve allows the concentration of several solutions moving through the tube to be derived.

‘There are sensors which can detect low concentrations (one in a trillion), but they are specific to [the target] compound only,’ says Brambilla. His team’s proposed device could be used to trace many different compounds. The team aims to produce and test a working prototype next year.