A simpler, more portable method to detect ammonia continuously has now been developed

Scientists at the Smart Materials Section at the Indira Gandhi Centre for Atomic Research (IGCAR) have developed a simple technique to detect the presence of ammonia using optical sensors. The results of their work were published on March 12 in the Applied Physics Letters.

Ammonia is an important component of explosives, fertilisers, and industrial coolants. Thus, detectors of ammonia form the basis of devices used to check for pollution in the vicinity of urban settlements, such as in rivers, lakes, buildings, etc.

Existing detectors include infrared gas analysers, ion-selective electrodes, detectors based on semiconductor films, or sensors that depend on ammonia’s reaction with an acidity-sensitive dye.

However, these are difficult to fabricate and use, and are prohibitively expensive.

The IGCAR team, led by Dr. John Philip, has devised a simpler, more portable method to detect ammonia: using ferrimagnetic nanofluids as sensors that reflect bluer light when exposed to more of the colourless gas.

Change of colour

“The sensor produces visually perceptible colour changes, in the presence of ammonia, due to the changes in the lattice periodicity of 1-dimensional array of droplets,” the paper notes.

The sensor comprises an oil-in-water emulsion containing a suspension of ferrimagnetic iron oxide particles each measuring 10 nanometres wide. A thin coating of a surfactant, such as sodium dodecyl sulphate, is added around the particles to keep them from agglomerating.

The surfactant is anionic: it has a net negative charge.

When a magnetic field of 90 gauss is applied to the solution, the ferrimagnetic nanoparticles line up like a chain along the magnetic field lines, no longer moving randomly. The setup is then illuminated by a fibre-optic light source.

When there is ammonia in the surrounding environment, it disperses into the emulsion and becomes an ammonium cation, an ion with a net positive charge. Because the surfactant is anionic, the ammonium cation penetrates into its layer around the droplets.

Consequently, the droplets are pulled closer. In this condition, the wavelength of light reflected from the solution is shifted toward the blue end of the visible spectrum. This phenomenon is called a Bragg shift, and can be picked up by a digital camera.

As more ammonia disperses into the solution, the blue-shift gets stronger because the droplets are brought closer under the magnetic field’s guidance.

These sensors can detect concentrations ranging from 0 to 525 parts per million. As the emulsion can be produced using commonly available chemicals, and the setup allows for rapid detection, the sensors are a reliable way to continuously monitor ammonia levels.

Dr. Philip added, “If we go for a simple naked-eye detection using visual colour change of the nano-emulsion, the device could cost a few thousand rupees, but if we go for a Bragg peak measurement, it could be slightly more expensive, but definitely much cheaper than commercially available ones.”

At present, the sensor apparatus can operate only in room temperature and detect ions in aqueous solutions. The team’s work, hence, will focus on taking a gel- or film-based approach to overcome these problems.

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