Using magnetic nanofluid emulsion functionalised with certain macromolecules (polymers) IGCAR researchers have developed an optical probe that can in less than a second detect the presence of urea in a very broad range (0.003 to 334 grams per litre).

Researchers at the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam have developed an inexpensive, highly sensitive optical probe that can almost instantaneously detect the presence of urea in a very broad range (0.003 to 334 grams per litre). Currently available analytical approaches cannot measure urea in a large concentration range, and some methods need pre-treatment of the sample.

Knowing the concentration of urea in the blood and urine helps in diagnosing renal and liver diseases.

Developing the sensor

The two-member team led by Dr. John Philip from the Metallurgy and Materials Group used magnetic nanofluid emulsion (oil droplets finely dispersed in water) attached (functionalised) with certain macromolecules (polymers) for detecting urea. The polymer attached to oil droplets appear like a ball with long hairs in all directions and keeps the droplets well separated. It is the attached polymer that interacts with urea. The nanoemulsion also contains superparamagnetic nanoparticles that remain suspend in the oil phase. These nanoparticles make the nanoemulsions magnetically responsive.

In the presence of a magnetic field, the emulsion itself forms a one-dimensional array due to the presence of superparamagnetic nanoparticles. The oil droplets in the array remain separated with certain spacing between them. “The distance of separation between the droplets is determined by the macromolecules used for functionalisation,” says Dr. Philip.

When urea is added to the emulsion, the spacing between the droplets changes causing the droplets to come closer to each other. “There is a direct correlation between the concentration of urea present in the sample and the change in distance between the droplets,” says A.W. Zaibudeen, first author of the paper. The results were published in the journal Sensors and Actuators B: Chemical.

The urea in the sample interacts with the polymer causing it to change shape — the hair-like structure of the polymer bends or collapses. The change in shape, in turn, reduces the net repulsion between the droplets leading to a change in the spacing between the droplets. It takes less than a second for the spacing between the droplets to change when urea is added to the emulsion.

“The change in the spacing can be detected using a miniature optical spectroscopy. In principle, we can bring about a colour change in the presence of urea that is easily discernible by the naked eyes by using a proper functional molecule,” says Dr. Philip.

Calibrating the detection range

“We have calibrated the detection range of urea by using known concentration of urea. We attempted different macromolecules for functionalisation to achieve the best sensitivity and to cover a large range of urea concentration,” says Dr. Philip.

The researchers used three different polymers in order to measure urea in a large concentration range — very minute concentration range of 0.003-0.6 grams per litre which covers the normal range of urea in human serum, mid concentration range of 0.18-33.3 grams per litre, and large concentration range of 2.4-334 grams per litre.

To mimic urine and serum samples, the researchers added sodium, potassium and iron to the emulsion. “The probe was still able to detect urea but at slightly reduced sensitivity,” Dr. John says.

In the next phase, they are trying to identify appropriate polymers that will exhibit specificity to urea so that even in the presence of other ions the sensitivity is not affected.

Published in The Hindu on January 13, 2018