What if your morning coffee spill could help detect harmful toxins? Scientists at the Raman Research Institute (RRI) in Bengaluru have found an innovative way to detect toxic substances at ultra-low concentrations. They achieved this by harnessing the “coffee-stain effect,” the phenomenon that makes a dried coffee drop form a dark ring around its edge.
A team of researchers from RRI, Bengaluru focused on Rhodamine B, a fluorescent synthetic dye widely used in textiles and cosmetics. While common in industry, the dye is toxic to humans and can persist in water, posing environmental and health risks.
To detect Rhodamine B, scientists used gold nanorods suspended in water. Droplets of this mixture were placed on a specially prepared silicon surface and left to evaporate naturally.
As the droplets dried, the nanorods moved to the edges, forming distinct ring-shaped deposits. The density of these rings played a key role in amplifying light signals.
Dense, compact rings created hundreds of “hot spots” where light was significantly intensified. This amplification allowed even tiny amounts of Rhodamine B molecules to generate strong optical signals, making detection possible at extremely low concentrations.
The results were striking. Low concentrations of nanorods could detect only relatively high amounts of the dye, but as the nanorod density increased, the detection limit improved dramatically. With the densest ring formations, scientists could detect Rhodamine B at concentrations as low as one part in a trillion. A hundred-fold increase in nanorod concentration led to nearly a million-fold enhancement in sensitivity, highlighting the enormous potential of this method.
“This approach is particularly important because dye molecules like Rhodamine B are banned in food and cosmetics due to their toxicity, yet regulators face challenges in monitoring their illegal use, especially in trace amounts,” explained A. W. Zaibudeen, a researcher at RRI.
Yatheendran K. M, an engineer involved in the study, noted that once dyes enter food or water, they can be diluted to parts-per-trillion levels, making detection nearly impossible with conventional techniques.
The method relies on Surface-Enhanced Raman Spectroscopy (SERS) combined with the coffee-stain effect, creating a low-cost, simple, and highly sensitive detection technique. Professor Ranjini Bandyopadhyay from RRI emphasized that even handheld Raman spectrometers could be employed for detection, making it practical for real-world monitoring.
Beyond Rhodamine B, this innovative approach can be applied to detect a wide range of harmful substances, offering a promising tool for improving environmental safety and public health.
