Low-Cost Epoxy-Cast Serpentine Lab-on-Chip with Pre-loaded Methyl Red for Smartphone-Based Colorimetric Screening of Free Acidity in Edible Oils
DOI:
https://doi.org/10.64354/sdfvqj08Keywords:
lab-on-chip; epoxy microfluidics; methyl red; acid value; edible oil; food quality control; smartphone colorimetryAbstract
The acid value (AV) of edible oils serves as a critical safety indicator; however, routine measurement typically requires trained personnel, organic solvents, and centralized laboratory facilities. To address these limitations, a low-cost lab-on-chip (LOC) device was developed and evaluated, incorporating Methyl Red dried within a cast-epoxy serpentine microchannel (1.0 × 1.0 mm cross-section; ≈160 mm path length; 45 × 22 × 4.5 mm chip; material cost less than US$0.50). Sunflower oil samples were heated at 180 °C for 15, 30, and 45 minutes, yielding reference AVs of 0.232 to 2.037 mg KOH g⁻¹ as determined by AOCS Cd 3d-63 titration. When 20 µL of the oil, diluted 1:10 (v/v) in ethanol, was injected into the chip, the indicator film turned from yellow toward red. The resulting color change was photographed using a Samsung Galaxy S24 Ultra under standardized settings, and the shift in the blue channel (ΔB = B_sample − B_blank) was quantified in ImageJ. With 0.02% (w/v) Methyl Red, ΔB was linear with AV (ΔB = 16.112·AV − 3.327; R² = 0.9875), with an LOD of 0.326 and an LOQ of 1.087 mg KOH g⁻¹. A higher indicator loading (0.04% w/v) gave a sharper calibration (ΔB = 28.282·AV − 4.670; R² = 0.9972) and lowered the LOD and LOQ to 0.154 and 0.513 mg KOH g⁻¹, below the Codex limit for refined oils (0.6 mg KOH g⁻¹). Two independent samples (E and F, heated for 20 and 35 min) were not used in calibration gave recoveries of 97.2% and 100.4% with the 0.04% chips, whereas the 0.02% chips underestimated them (67.0% and 74.7%). Inter-chip precision of the 0.02% chips gave an RSD of 1.83%, and the interday precision of the 0.04% chips gave a between-day RSD of 9.0%; however, a later batch of chips gave about half the response of the calibration batch, so each batch of chips needs its own calibration. Each test takes less than 10 min and needs no liquid reagents at the point of testing.
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