Fabrication of a colorimetric ammonia nitrogen indicator using ube peel-derived anthocyanins in a gellan gum/carboxymethyl cellulose matrix

Water quality and food safety often hinge on a single, invisible chemical: ammonia. Found in everything from agricultural runoff to the air above spoiling meat, this compound is a potent pollutant that can harm ecosystems and signal that food is no longer safe to eat. Detecting it usually requires expensive laboratory equipment or toxic chemicals that create their own waste. Scientists have long looked for a simpler alternative, turning to nature's own color-changing pigments. These pigments, known as anthocyanins, are the same compounds that give blueberries, red cabbage, and purple sweet potatoes their vivid hues. They act as natural pH indicators, shifting color when they encounter acidic or alkaline environments. Since ammonia makes water more alkaline, these pigments can theoretically serve as a visual warning system, changing from red to blue or brown as ammonia levels rise. However, these delicate pigments are fragile; they degrade quickly when exposed to light or air, making them difficult to use in a practical, durable sensor.

A team of researchers at De La Salle University in the Philippines set out to solve this problem by creating a sturdy, porous sensor that could hold these fragile pigments in place. They chose a specific source for their pigment: the peels of the ube, or purple yam, a staple crop in the Philippines that generates significant waste during processing. Instead of discarding these peels, the team extracted the anthocyanins and embedded them into a new type of gel matrix. This matrix was a blend of two natural polymers: gellan gum, which provides structural strength, and carboxymethyl cellulose, a derivative of cellulose that helps the material absorb water. To bind these two ingredients together, the researchers used calcium chloride, a common salt that acts as a bridge between the polymer chains, creating a stable network.

The researchers faced a balancing act. They needed a material that was strong enough to hold its shape but porous enough to let water and ammonia gas flow through easily. If the material was too dense, the ammonia could not reach the pigments inside; if it was too loose, the sensor would fall apart. To find the perfect recipe, they tested various combinations of the two polymers and different concentrations of the calcium salt. They discovered that a specific mix, containing twice as much carboxymethyl cellulose as gellan gum and treated with a low concentration of calcium chloride, created the most effective structure. This specific blend absorbed the most water, swelling to nearly thirty-four times its dry weight, which indicated a highly open and accessible internal structure. This high swelling capacity was crucial because it allowed the ammonia to diffuse quickly into the sensor, triggering a color change.

Once the optimal matrix was identified, the team encapsulated the ube peel extract within it. They then tested how well this new sensor reacted to ammonia. They exposed the material to varying concentrations of ammonia gas, ranging from none to very high levels, and watched the color shift over time. The results were clear: as the concentration of ammonia increased, the sensor darkened, shifting from its original reddish hue to a deeper, brownish tone. This change happened because the ammonia, which is alkaline, reacted with the anthocyanins, altering their chemical structure and how they reflected light. The researchers found that the sensor worked best when given one and a half hours to react, at which point the relationship between the color change and the ammonia concentration was the most predictable and reliable.

Beyond its ability to detect ammonia, the new sensor proved to be mechanically sound. Tests showed that adding the pigment did not weaken the material, and the final product maintained a tensile strength similar to the matrix without the pigment. Detailed microscopic images revealed that the material had a sheet-like, porous structure, confirming that the freeze-drying process had created the necessary channels for water and gas to move through. Chemical analysis confirmed that the pigments were physically trapped within the polymer network through natural bonds, rather than being chemically altered, meaning the sensor retained the natural properties of the ube extract.

This work demonstrates a practical way to turn agricultural waste into a functional tool for environmental and food safety monitoring. By using the peels of the purple yam and a blend of simple, natural polymers, the researchers created a sensor that is both sustainable and effective. It offers a potential alternative to complex, expensive detection methods, providing a visual, low-cost way to monitor ammonia levels. The study suggests that such materials could be developed further for real-world applications, turning a common byproduct of food processing into a guardian for clean water and safe food.