Optical sensor detects one molecule in 10 million

Researchers at the Optoelectronics Research Centre (ORC) in Southampton, UK, are developing an optical chemical sensor that can detect compounds with a sensitivity of one molecule in 10 million.

Based on an optical fibre that is 1000-times thinner than standard optical fibre used in telecommunications, the sensor has many advantages over other optical sensors currently used in biotechnology and chemistry applications.

"Other optical sensors currently under investigation or on the market have detection limits of around 1 in 1 million," says Dr Gilberto Brambilla, a senior research fellow at the ORC. "Our sensor is more sensitive, can easily be coupled to standard optical equipment and also features an intrinsic fluidic channel for delivery of the analyte."

The sensor, known as a coated all-coupling nanowire microcoil resonator (CANMR), is based on an ultra-thin optical fibre (400 nm diameter) wrapped around a channel through which the analyte flows. The channel is less than 1 mm in diameter and the fibre is wound around it several times. The fibre is coated with a polymer such as Teflon to protect the sensor and light is launched into one end of the fibre using standard telecoms equipment. This then travels along the fibre and around the analyte a million times before being detected at the other end of the coil.

"To make the ultra-thin optical fibre, we take a standard telecoms fibre, heat it up and stretch it," explains Dr Brambilla. "This results in a thin piece of fibre in between two tapered ends which can be used to couple light into the device with very low loss."

Because the fibre's diameter is smaller than the wavelength of the light it is transmitting, some of the light propagates outside the physical boundaries of the fibre. This is known as the evanescent field. When this evanescent field interacts with the analyte, its propagation speed changes. It is these small changes that give information about the concentration of the analyte.

"Making and handling the ultra-thin optical fibre for the sensor is extremely challenging," says Dr Brambilla. "The ORC has a long history in glass fibre manufacture and we have built on this expertise."

Notes for editors

1. For high-resolution diagrams please contact Marketing Officer, Optoelectronics Research Centre, (email: light@orc.soton.ac.uk, tel: +44 (0)23 8059 387)
2. This press released is based on work published in Optics Express (Optical microfibre coil resonator refractometric sensor, Optics Express 15, 7888-7893 (2007)). There is also an erratum presented with this paper (click here to see erratum). The work is being funded by the Engineering and Physical Sciences Research Council (EPSRC) under the standard research grant EP/C00504X/1.
3. For more information on Dr Gilberto Brambilla’s research group please visit http://www.orc.soton.ac.uk/ofnrd.html
4. A coated all-coupling nanowire microcoil resonator (CANMR) can be fabricated as follows: first, an expendable rod is initially coated with a layer of thickness d of a low-loss polymer such as Teflon; then an optical fiber nanowire is wrapped on the rod; next, the whole structure is coated with the same low-loss polymer, and finally the rod is removed. Figure 1 shows the final structure after the rod is removed: the nanowire is shown in blue, the analyte channel (in the space previously belonging to the rod) in grey and Teflon in green.
5. The Optoelectronics Research Centre at the University of Southampton is one of the largest university-based research groups entirely devoted to optoelectronics in the world, and has maintained a position at the forefront of photonics research for over four decades. Its long and well established track record in the fields of optical fibre, lasers, waveguides, devices and optoelectronic materials has fostered innovation, enterprise, cross-boundary and multi-disciplinary activities. It has built strong links with industry, research institutions and universities around the world, from informal collaborations to large-scale funding. Several spin out companies, including Southampton Photonics Inc, Stratophase Ltd, Mesophotonics Ltd and ChG Southampton Ltd, have commericalised the fruits of ORC research; while its extensive outreach programme has brought lasers and optical fibres into schools across the country.

Further information

Dr Gilberto Brambilla, Optoelectronics Research Centre, tel: +44 (0)23 8059 7673, email gb2@orc.soton.ac.uk

Marketing Officer, Optoelectronics Research Centre, tel: +44 (0)23 8059 3877, email: light@orc.soton.ac.uk