Low-Cost Epoxy-Cast Serpentine Lab-on-Chip with Pre-loaded Methyl Red for Smartphone-Based Colorimetric Screening of Free Acidity in Edible Oils

Source: Chemical Interactions, written by Zahraa Jalil Salman (October 12, 2026). AI-generated summary by biochip.com, published October 18, 2026. Not independently reviewed.

A team at Al-Furat Al-Awsat Technical University in Iraq developed a low-cost lab-on-chip device that uses a dried methyl red indicator and a smartphone camera to screen the acidity of edible oils. The epoxy-cast chip costs less than US$0.50 to make and produces a color change as oil acidity rises. With a higher indicator loading, the device achieved a limit of detection below the Codex limit for refined oils. The platform could provide a faster, simpler option for preliminary food-quality checks outside centralized laboratories.

Turning Oil Acidity Into a Color Signal

Free acidity is commonly reported as acid value, or AV, the amount of potassium hydroxide needed to neutralize acids in one gram of oil. High AV can indicate oil deterioration, but standard testing relies on titration, trained personnel, organic solvents, and laboratory equipment.

The new chip replaces much of that setup with a narrow, winding channel cast in epoxy. The serpentine channel is 1.0 by 1.0 millimeters in cross-section and approximately 160 millimeters long, all within a chip measuring 45 by 22 by 4.5 millimeters. Dried methyl red lines the channel, acting like a built-in pH-sensitive paint that changes from yellow toward red when the prepared oil sample passes over it.

Smartphone Quantification

To test the device, the researchers heated sunflower oil at 180 degrees Celsius for 15, 30, or 45 minutes. Reference acid values measured by the American Oil Chemists' Society titration method ranged from 0.232 to 2.037 milligrams of potassium hydroxide per gram of oil.

Each test used 20 microliters of oil diluted 1:10 by volume in ethanol. After injection, the color was photographed with a Samsung Galaxy S24 Ultra under standardized settings. ImageJ software measured the change in the blue color channel, allowing the phone image to serve as a numerical readout rather than relying only on visual judgment.

Performance and Calibration

Chips containing 0.02% methyl red produced a linear relationship between the blue-channel shift and acid value, with an R-squared value of 0.9875. Their limit of detection was 0.326 milligrams of potassium hydroxide per gram, while the limit of quantification was 1.087 milligrams per gram.

Increasing the methyl red concentration to 0.04% sharpened the calibration, raising R-squared to 0.9972. It also lowered the detection and quantification limits to 0.154 and 0.513 milligrams per gram, respectively. The quantification limit was below the 0.6 milligrams per gram Codex limit cited for refined oils.

Why This Matters

Two independent oil samples, heated for 20 and 35 minutes and excluded from calibration, produced recoveries of 97.2% and 100.4% with the 0.04% chips. Tests took less than 10 minutes and required no liquid reagents at the point of testing, although the oil still had to be diluted in ethanol before injection.

The device is not yet a universal, plug-and-play test. A later chip batch produced about half the response of the calibration batch, meaning each batch requires its own calibration. Precision also varied by test condition: the 0.02% chips had an inter-chip relative standard deviation of 1.83%, while the 0.04% chips showed a between-day value of 9.0%. Those results support the chip as a low-cost screening platform, while highlighting batch consistency as an important next engineering step.

Topics: Lab-on-a-chip

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