Researchers at the Indian Institute of Science (IISc) have developed an inexpensive sensor made with a photoluminescent paper that enables the visual detection of even miniscule amounts of hydrogen peroxide (H2O2). The chemical is found in diverse products and biological cells. The sensor uses a sensitive photoluminescence assay for H2O2 that the researchers developed by inducing photoluminescence in terbium (Tb3+), a rare earth metal, in a hydrogel matrix. They also designed a sensitizer molecule for the hydrogel that causes the Tb3+ to emit green light under a UV lamp. When they combined the sensitizer molecule with a masking agent, the green light emission disappeared. However, when they added H2O2 to this combination, it unmasked the sensitizer molecule, causing the Tb3+ to emit green light once again. “The molecule we have designed is very specifically unmasked by hydrogen peroxide,” professor Uday Maitra said. The researchers coated and air-dried paper discs, about 0.45 cm in diameter, with the soft hydrogel solution containing the sensitizer molecule and a liquid containing H2O2. When exposed to H2O2, the discs emitted green light when placed under a UV lamp. The intensity of the emitted light was proportional to the concentration of H2O2. Green luminescence from the paper discs allowed naked-eye detection of H2O2 at the micromolar level. “You don’t need any sophisticated instruments. All you need is a simple UV light source,” researcher Arnab Dutta said. Researcher Arnab Dutta working in the lab of IISc professor Uday Maitra. Courtesy of Arnab Dutta. When the researchers used the technique to randomly test five different hand sanitizer brands, they found that only three contained 0.125% H2O2, the level mandated by the World Health Organization. One sample tested much lower than 0.125% and another was found to contain almost 0% H2O2. The results indicate that the low-cost system could be practically adopted, especially in resource-limited areas, to quantify and detect H2O2 for quality control and other applications, testing biological fluids such as blood for H2O2 or tracing peroxide-based explosives. The paper disc is also biodegradable. “Hydrogen peroxide can be detected on a larger scale using titration and other experiments, but those are cumbersome and require training. This method is easy because of its simplicity,” Maitra said. Maitra’s lab is working with various sensitizer molecules that trigger the photoluminescence of lanthanides (i.e., metallic elements such as Tb3+) in the presence of specific chemicals or compounds. The team previously developed paper-based sensors for detecting specific antioxidants in green tea, as well as sensors for various enzymes. Currently, the researchers are focused on reducing the reaction time of the H2O2 photosensor, which is longer when the concentration of H2O2 is low. Maitra said they are also working on developing a portable device that will automate the detection process, and that the group is in touch with a startup company. “We have a few prototypes made with UV LEDs and a camera, to generate the emission, take a photograph, and use an image processing app to quantify the amount of hydrogen peroxide,” he said. The research was published in ACS Sensors (www.doi.org/10.1021/acssensors.1c02322).