High sensitivity formalin detection in aqueous solutions using plasmonic multifunctional metal insulator metal nanoring based optical refractive index sensor platform

Why cleaner water needs smarter sensors

Formalin, a watery form of formaldehyde, quietly slips into our lives through building materials, industrial waste, and even some medical and food uses. Because it is a proven human carcinogen, even small amounts in drinking or wastewater can pose serious risks over time. Traditional laboratory tests can detect formalin very accurately, but they are slow, expensive, and tied to centralized facilities. This study introduces a tiny light-based sensor that could one day sit directly in a water line or portable device, spotting formalin quickly and at very low levels, using cleverly engineered metal structures a thousand times thinner than a human hair.

A tiny ring-shaped trap for light

At the heart of the new sensor is a patterned metal structure called a nanoring, built in a “metal–insulator–metal” stack: two metallic layers with a thin transparent layer in between. The researchers design two nested rings with short perpendicular arms, all resting on a glass-like base and backed by a reflective metal layer. When infrared light shines on this landscape of rings, electrons in the metal slosh collectively at specific colors, a phenomenon known as a plasmonic resonance. These resonances are exquisitely sensitive to the surrounding liquid. If the liquid filling the tiny spaces above and around the rings changes—say, because more formalin is present—the preferred color of the resonance shifts, and that shift can be measured.

Choosing the best metals and shapes

To understand how to get the strongest signal from such a small device, the team used detailed computer simulations that solve Maxwell’s equations for light on a very fine three-dimensional grid. They tested different common plasmonic metals—gold, silver, and aluminum—for both the rings and the reflecting base layer. Silver emerged as the best all-around choice, creating sharper resonances and higher sensitivity to changes in the liquid. The researchers then tuned the geometry: the thickness of the rings, the thickness of the reflector, and the size of the nested rings and arms. They discovered that making both the rings and the back reflector about 80 nanometers thick gave an excellent compromise between strong, narrow resonances and practical device size, ensuring the sensor could be both compact and efficient.

How light reveals hidden formalin

Once optimized, the sensor was tested—again in simulation—against realistic water–formalin mixtures. Formalin slightly increases how much the liquid bends light, a property called the refractive index. The team varied this index over the range expected for typical aqueous formalin and calculated how the sensor’s reflected color changed. They found four distinct resonances in the near- to mid-infrared, each shifting linearly as the formalin level rose. One mode showed especially large color shifts, making it excellent for detecting bigger changes in contamination, while another mode produced a narrower, cleaner dip in the spectrum, ideal for teasing out trace amounts. Electric-field maps showed that in the most sensitive mode, light energy was tightly concentrated along the inner edges of the rings, exactly where it interacts most strongly with the surrounding liquid.

Small device, strong performance

To judge how useful the platform might be outside the lab, the authors compared their simulated sensor to many earlier plasmonic designs. Their device achieved higher sensitivity than most previous refractive index sensors, while also keeping the active volume small. They introduced a simple “sensitivity-to-volume” ratio to capture this balance: how much the resonance color moves per unit change in refractive index per unit device volume. The formalin sensor reached a favorable value, indicating it squeezes a lot of sensing power into a tiny footprint. The estimated detection limits suggest it could pick up very low concentrations of formalin, suitable for environmental and possibly medical monitoring.

From simulation to real-world water safety

Although the work is based on numerical experiments rather than a fabricated chip, the design uses materials and patterning methods that already exist in advanced nanofabrication, such as electron-beam lithography and atomic layer deposition. With these, uniform stacks of silver and glass-like materials could be built across large areas. The authors argue that, once realized physically and integrated with simple fluid channels, their nanoring sensor could continuously watch over water supplies or industrial effluents and be adapted to detect other hazardous chemicals or biological markers by tailoring the surface. In plain terms, this study points toward future “lab-on-a-chip” devices where a small, smart patch of metal rings and light quietly safeguards our water by flagging dangerous formalin levels in real time.

Citation: Khodaie, A., Rafighirani, Y., Heidarzadeh, H. et al. High sensitivity formalin detection in aqueous solutions using plasmonic multifunctional metal insulator metal nanoring based optical refractive index sensor platform. Sci Rep 16, 10192 (2026). https://doi.org/10.1038/s41598-026-40507-5

Keywords: formalin detection, plasmonic sensor, nanoring, water pollution, optical biosensor