A phenomenon as simple as a coffee stain on a table may hold the key to detecting dangerous toxins in food and water at extremely low concentrations. Scientists at Raman Research Institute (RRI), supported by the Department of Science and Technology, Government of India, have developed a powerful and cost-effective technique that uses the coffee-stain effect to identify harmful dyes such as Rhodamine B at concentrations as low as one part per trillion.
When a coffee droplet dries, it leaves a dark ring around its edge. This “coffee-stain effect” occurs because particles in the liquid move outward as it evaporates, forming a dense deposit at the perimeter. RRI researchers exploited this natural process using gold nanorods — microscopic structures a few tens of nanometers long — to create highly ordered, ring-shaped deposits on silicon surfaces.
As water evaporates, the gold nanorods gather along the droplet’s edge. When densely packed, they form “hot spots” that dramatically amplify light signals. By shining a laser on these structures, scientists could detect even trace amounts of Rhodamine B bound to the nanorods, achieving sensitivity far beyond conventional detection methods.
“At low nanorod concentrations, we could only detect high amounts of Rhodamine B, equivalent to a drop of dye in a glass of water,” explained A. W. Zaibudeen, researcher at RRI. “But as the nanorod concentration increased, our sensitivity improved by nearly a million times. With dense edge deposits, we could detect dye at parts per trillion.”
Rhodamine B — a fluorescent synthetic dye used in textiles and cosmetics — is banned in food products due to its toxic effects on the skin, eyes, and respiratory system. It is also a persistent environmental pollutant, often diluted in water to levels undetectable by standard techniques.
“Detecting such small quantities is a major regulatory challenge,” said Yatheendran K. M, Engineer B, Soft Condensed Matter. “This is where highly sensitive methods like Surface-Enhanced Raman Spectroscopy (SERS) become crucial.”
The method developed at RRI is both simple and inexpensive. A single drop of liquid containing gold nanorods is enough to form the pattern naturally as it dries, concentrating nanoparticles at the edge to create ultra-sensitive detection zones. Even handheld Raman spectrometers could potentially be used to identify harmful substances using this approach.
“This technique is facile and cost-effective,” said Ranjini Bandyopadhyay, Professor – Soft Condensed Matter at RRI. “By harnessing a naturally formed pattern, we can detect picomolar quantities of toxins without expensive or complex equipment.”
Beyond detecting Rhodamine B, the technique has broad applications. It could be adapted for identifying other harmful substances in food, water, and the environment, offering a practical tool for regulators and industries to combat contamination and protect public health.