Cheaper way to detect ammonia
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.
Courtesy With: THE HINDU