Combination of paper-based thin film microextraction with smartphone-based sensing for sulfite assay in food samples

Herein, a simple, green, and novel paper-based headspace-thin film microextraction in combination with smartphone-based on-cell detection was introduced. Sulfite was selected as a model compound to study the performance of the method. The method was based on converting sulfite to SO₂ by acidification of the sample solution. A piece of cellulose paper impregnated with Fe(III), 1,10-phenanthroline, and acetate buffer (pH = 5.5) was used for headspace-thin film microextraction. The sulfur dioxide adsorbed on the thin film reduced Fe(III) to Fe(II). The red color complex formed between Fe(II) and 1,10-phenanthroline was on-cell detected by a smartphone. The effect of various parameters on the method sensitivity was investigated. The method had three individual linear ranges (0.1-1.0, 1-50, and 50-700 µg L^-1) with a detection limit of 0.04 µg L^-1. The method provided a simple and green assay for the determination of sulfite in food samples. Recoveries were higher than 94% with the relative standard deviations lower than 4.8%. Expensive and complicated facilities were not required in the method.