A microneedle patch senses plant stress

When plants face drought, infections, and other stressors, their cells produce unstable molecules known as reactive oxygen species, including hydrogen peroxide. Snooping on a plant’s production of hydrogen peroxide could reveal changes in stress levels that cue farmers to respond. A new microneedle-based patch that attaches directly to a plant leaf is able to rapidly measure hydrogen peroxide levels with the help of a biofriendly electrode (ACS Sens. 2025, DOI: 10.1021/acssensors.4c02645).

“Hydrogen peroxide makes the world go round,” says Ron Mittler, a plant biologist at the University of Missouri who wasn’t involved in the work. Everything that happens in a plant, from its development to its responses to environmental changes, affects hydrogen peroxide production, he says. “It’s a very universal stress molecule.”

Traditional ways of measuring hydrogen peroxide levels in plants are not very sensitive or accurate, says Liang Dong, an electrical engineer at Iowa State University. These methods generally rely on collecting samples and reacting them with compounds to produce a color change; the resulting color tells researchers how much hydrogen peroxide is present. These methods can also be time consuming. “What if we have a device that can be attached to the plant for in situ measurement?” he asks. “You capture data that will be beautiful and useful.”

So Dong and his colleagues created a wearable sensor for plants. The sensor contains a roughly 1 cm² patch that sticks to plants with an array of polyurethane microneedles that are coated first with gold and then with a hydrogel. The hydrogel contains graphene oxide, which enhances the needles’ ability to grab fluids from the plant leaves, and the natural polymer chitosan, which mitigates the graphene oxide’s toxicity to the plant. The gel also contains the enzyme horseradish peroxidase, which catalyzes an electrochemical reaction that produces electrons in proportion to the amount of hydrogen peroxide. The sensor collects the current to provide a readout of hydrogen peroxide levels at that spot on the plant, says study coauthor Nawab Singh, an electrochemist at Iowa State University.

The team calibrated their sensor against standard hydrogen peroxide samples and then tested it on tobacco and soybean plants that had been inoculated with pathogenic bacteria. They used the sensor to measure hydrogen peroxide levels of these plants 12 and 24 h after inoculating them and compared these values with readings from uninoculated plants. Each measurement took about 1 min. Quantifications of hydrogen peroxide levels measured by the sensor agreed with those from a traditional color-based test.

Researchers have developed microneedle sensors for plants before, with some drawbacks. For instance, one microneedle patch designed to detect hydrogen peroxide uses colorimetric methods. This is nonideal because it requires an external device to read out measurements.

Another concern is that the microneedle array is wounding the plant, which could itself cause stress, Mittler says. And depending on the needles’ height, they will hit different layers of cells that may have different levels of reactive oxygen species, he says. Still, “being able to quickly give a readout of hydrogen peroxide is really very cool and exciting.”

Dong says that the researchers don’t see a major uptick in hydrogen peroxide when they stick the sensor onto a plant’s leaves. The approach of coating needles with a biocompatible layer is new and may reduce damage to the plant, Dong says.

The researchers hope to tweak the design to lessen harm to the plant. And they hope to make the patches—which could be used eight or nine times—even more reusable. They could also integrate other sensing capabilities, such as ones based on technologies Dong’s team has developed to sense nitrate levels and water availability, as well as add wireless data transmission.