Down to the wire

Researchers from Southampton University are building a sensor sensitive enough to analyse solutions so weak that they contain only one molecule of target compound in 10 million of solvent

The laws of physics are strange at nano-scale. Take optical fibres, for example. Normal-size fibres act like pipes, with the light flowing inside the confines of the strands of glass. But make the fibre smaller — below the wavelength of the light — and the light will leak out of the sides of the strand while still following its path.

Using this odd property, researchers from Southampton University are building a sensor sensitive enough to analyse solutions so weak that they contain only one molecule of target compound in 10 million of solvent. Ten times more sensitive than anything else available, the sensors are likely to find applications in forensics and security-related sectors.

The sensitivity of the technique could allow the detection of small amounts of explosives or toxins and will also allow forensics experts to test extremely small samples gathered from crime scenes.

Gilberto Brambilla’s team at the university’s optoelectronics research centre makes the nanowires from standard optical fibres and stretches them carefully to reduce the diameter almost a thousand-fold. The sensor will use fibres around 400nm in diameter, although Brambilla said his stretching technique can produce diameters as low as 50nm.

‘We take a fibre, clamp it, apply a bit of tension, and then use a flame moving beneath it,’ explained Brambilla.

‘The flame softens the fibre so that when you stretch it, the diameter decreases. We have three stages, and everything is computer controlled; we devised the programming ourselves and we also built the stages.’

Working at such small dimensions is fraught with difficulty, because of the fragility of the fibres. ‘You need to heat it up, but not too much, and you need special clamps so that the fibres don’t slip,’ said Brambilla.

The flame itself causes problems, with the shear stress caused by the gas flow posing the risk of breaking the fibre. ‘Also, when you go to very small dimensions, the physical properties of the fibre change,’ said Brambilla. ‘It stretches at a lower temperature, so you have to control the heat very carefully; it can break because it’s become too soft.’ The stretching stages are isolated from vibration and enclosed within an acrylic box to block out unwanted air currents.

Slow and steady is the watchword. ‘The flame is carried across several times. You can reduce the diameter only by 2-3 per cent; any more than that, and it becomes inaccurate. So if you want precise control of the diameter of the nanowire, you have to do several passes, reducing the diameter by a few per cent per pass. The flame comes underneath the fibre typically hundreds of times to get the dimensions we need.’

Brambilla said he can make fibres from a millimetre in length up to 110mm, but only a few millimetres are needed for the sensor. The fibres are encased inside fluoropolymer resin insulation then wrapped twice around a transparent 1mm diameter tube. ‘Because of the geometry, there are certain wavelengths of light which experience resonance inside the fibre — it becomes trapped inside,’ said Brambilla. ‘It goes from the first winding into the second, then is coupled down to the first, then into the second and is coupled back, and it makes this trip around a million times.’

The analysis technique uses this resonance phenomenon and the property of evanescence, where the light propagates outside the fibre. The sensor is set up with a broadband source of light, producing many wavelengths at one end and a spectral analyser at the other. Some wavelengths become trapped in the fibre windings, producing characteristic dips in the spectrum of light emerging from the fibre.

The fraction of the light that flows outside the fibre enters the transparent tube, through which the solution being studied is flowing. This refracts the light, causing a shift in the wavelength of the transmitted light.

‘If you look at the transmission of your broad range of wavelengths, the dips shift when the refractive index of the analyte changes,’ said Brambilla. ‘So by measuring the shift in the dips you know the refractive index of the analyte, and that is directly related to the concentration of the analyte.’

The team has made samples of small-diameter nano-wires and simulated the action of the sensor. Over the next year, it hopes to assemble and test a working prototype of the sensor. ‘We are not at the stage for commercial exploitation just yet,’ stressed Brambilla.