The sensor is made of an oil-in water nanoemulsion, produced by emulsifying ferrimagnetic iron oxide nanoparticles, around 10 nm in size, suspended in octane and in the presence of an anionic surfactant of sodium dodecylsulphate in water. The nanoparticle droplets align to form a 1D array when a magnetic field of around 90 Gauss is applied, and the sensor is illuminated with a fibre-optic based light source.

When ammonia is present in the surrounding solution, the lattice periodicity of the droplet array changes thanks to the fact that ammonia penetrates into the electric double layer around the emulsion droplets. This causes the wavelength of light reflected from the sensor to be significantly shifted towards the blue end of the electromagnetic spectrum – something that can be easily monitored using a digital camera.

“We plotted the reflected Bragg peak for different ammonia concentrations, and found that it blue shifts monotonically, increasing with concentrations of ammonia,” team leader John Philip told nanotechweb.org. “We can calculate the exact concentration of ammonia present from a Bragg shift calibration curve. This is a standard procedure in which the Bragg shift for a few different known concentrations of ammonia are first measured. Since the curve follows a linear trend, any unknown concentration can then be determined.”

The device can detect concentrations as low as 1ppm of ammonia and is much easier to fabricate and use than existing ammonia sensors that rely on ion-selective electrodes, infrared gas analysers, detectors based on semiconductor films such as SnO2 ad MoO3, and optical gas sensors that exploit the reaction of ammonia vapour with either a pH-dependent dye material or film. “In these techniques, the target ammonia molecules interact with the sensing element and produce changes in the light absorbance or emission spectra of the sample that are then monitored using suitable instruments,” explains team member Vellaichamy Mahendran. “Ours is a much more simple, sensitive, faster and inexpensive set up."

Ammonia is found in explosives, fertilizers and indusial coolants, among other things, and the new optical sensor could be used to continuously monitor environments in which the chemical is present - such as rivers around industrial plants, for example, says Philip.

The team, which has published its results in Applied Physics Letters, now hopes to further improve the sensitivity of its sensor and use it to detect other toxic analytes. Another important goal is to make the sensor in a reusable thin film form, adds Philip.