Key Points

When a coffee drop evaporates on a tabletop, it creates a distinctive dark circle around the edge. This daily phenomenon, or the coffee-stain effect, is due to the fact that as the liquid dries, particles suspended in the liquid move outward and cluster along the edge.

Using the phenomenon, the team developed a method to detect Rhodamine B -- a fluorescent synthetic dye used in textiles and cosmetics -- in food.

The dye is toxic and causes damage to the skin, eyes, and even the respiratory system. It is also an important environmental contaminant reported to persist in water.

"Dye molecules such as Rhodamine B are banned in products such as food and cosmetics due to their toxicity, but regulators face challenges in monitoring their illegal use, including usage of small quantities in products and a lack of availability of detection equipment," said A. W. Zaibudeen, researcher at RRI.

"Once these dyes mix with food or water bodies, they may become diluted to concentrations as low as parts per trillion, making it difficult to detect them using conventional characterization techniques. Therefore, a more sensitive method of detection, such as Surface-Enhanced Raman Spectroscopy (SERS), may be required," added Yatheendran K. M, Engineer B, Soft Condensed Matter.

The team exploited the coffee-stain effect with gold nanorods, microscopic rods a few tens of nanometers in length, by depositing a water droplet containing them on a cleaned, strongly water-attracting silicon surface and allowing the water to evaporate. As the droplet evaporated, the rods were transported to their rim and left there in a ring. When a laser is directed at the stain, any Rhodamine B molecules bound to the gold rods in these areas generate much brighter optical signals than they would individually.

At low concentrations of gold nanorods, only relatively high amounts of Rhodamine B could be detected -- roughly equivalent to a drop of dye in a glass of water. As the nanorod concentration increased, the detection limit improved sharply.

With the densest ring deposits, the system could detect Rhodamine B down to one part in a trillion. Remarkably, a hundred-fold increase in nanorod concentration translated into nearly a million-fold enhancement in sensitivity.

The scientists demonstrated that it is possible to use a simple, naturally formed pattern that is produced by the same phenomenon that produces coffee rings on a table, to transform into a fantastically potent and inexpensive technique for chemical detection.

The technique can be used for a wide range of harmful substances and can be transformed into advanced technology for reducing disease and environmental harm, the team said.

- IANS