Color-changing nanopigment sensor tracks pH one to ten with stable, repeatable readings

Indicator tracks the pH of spoiling shrimp. Credit: Min-Jae Kim et al.

Chemists in South Korea have created a stable color-changing sensor that displays an easily quantifiable response to the pH of its surroundings. Published in Microsystems & Nanoengineering, Dong-Hwan Kim and colleagues at Sungkyunkwan University say their approach could pave the way for more reliable, affordable pH measurements across a broad range of real-world scenarios.

From environmental monitoring to medical diagnostics, pH is a vitally important indicator of health, quality, and safety. Currently, the most widely used tools for assessing whether an environment is more acidic (lower pH) or more alkaline (higher pH) are electrochemical sensors, which measure the potential difference between a pair of electrodes when immersed in a solution.

Although highly accurate and reliable, they are also rigid, vulnerable to electrical interference, and often difficult to apply in real-world measurements—such as in the microscopic environments of cellular interiors.

Color-changing sensors

An alternative approach is to use colorimetric sensors: specially engineered dyes that vary the wavelength of visible light they absorb and reflect as pH changes, producing distinct color shifts. These dyes can be embedded into a solid matrix to create color-changing films, which can then be coated onto substrates to provide an ambient measure of the pH of their surroundings.

Yet although these sensors are far more flexible than their electrochemical counterparts, they come with their own drawbacks: including the leaching of dyes into their surroundings, and limited long-term stability. In addition, they often operate within a narrow pH range, making them less practical for applications requiring broad detection windows.

Specially engineered nanopigments

To tackle these challenges, Kim’s team took a more in-depth approach to synthesizing a colorimetric indicator. They began by engineering nanopigments from two key components. The first was a class of molecules known as sulfonephthalein dyes—widely used in analytical chemistry due to their impressive stability, low cost, easily distinguishable colors, high reactivity, and relative ease of chemical modification.

The researchers then covalently bonded these dyes to raspberry-like silica nanoparticles: porous nanostructures with extremely high surface areas—yielding six distinct pH-sensitive nanopigments.

To further improve stability and prevent dye leaching, the team embedded the nanopigments within a solid agarose/polyethylene oxide (PEO) matrix, where hydrogen bonding within the polymer network helped secure the particles in place.

Finally, to expand the pH detection range, they blended two additional nanopigments based on Bromocresol Green and Phenol Red—dyes that exhibit complementary color-changing responses to pH. By combining these within the same matrix, the researchers aimed to achieve a broader and more uniform sensing window.

Linear pH sensing

To test performance, Kim’s team coated the nanopigment-embedded films onto a variety of solid substrates. They then analyzed the color response as they varied the surrounding pH from extremely acidic to highly alkaline conditions.

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Comparison of the leaching behavior between commercial pH paper and the pH-sensitive film over a 3-min immersion test. (playback speed: four times). Credit: Microsystems & Nanoengineering (2026). DOI: 10.1038/s41378-026-01163-xAcross a wide range—from pH 1 to pH 10—the indicator displayed a clear linear relationship between color and pH: from bright yellow at low pH to dark brown at high pH. Importantly, this behavior remained consistent across repeated measurement cycles, demonstrating strong resistance to dye leaching and degradation.

The team then showcased the sensor’s stability by monitoring the freshness of shrimp. As shrimp spoil, chemical by-products accumulate and the local environment becomes increasingly alkaline.

By placing a nanopigment-embedded PEO film inside the packaging, the researchers were able to track the pH change through its color response as the shrimp were left at room temperature. The measurements revealed a marked shift over several hours, before leveling out.

Durable and accurate sensors

Based on this performance, Kim and colleagues are hopeful that their colorimetric indicator could be applied across a broad range of scenarios where electrochemical pH sensors are less dependable.

By overcoming long-standing challenges such as dye leaching, limited stability, and narrow dynamic range, their approach could ultimately provide more durable, accurate, and affordable methods for monitoring health, safety, and environmental quality.

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