2D-printed colorimetric paper-sensor for monitoring fish and milk freshness during the shelf life

A novel colorimetric paper-sensor (CPS) in this research was fabricated by functionalizing filter paper with mulberry anthocyanin to monitor the freshness of packaged carp and milk. A 2D printer was applied to distribute uniformly mulberry anthocyanin onto the filter paper to form 10 × 10 mm² squares, whose colors had a response to ambient pH values. The obtained CPS demonstrated satisfactory stability and reliable color-rendering capacity at pH 2–12. The correlation between color variation and sample freshness was quantitatively evaluated. The application of CPS in detecting the freshness of carp and milk at 25 °C was further studied, whose results evidenced that CPS had a significant color variation when ΔE was greater than 5. Additionally, its color turned from red-violet to gray when TVB-N of carp was greater than 20 mg/mL, implying that there was a strong positive correlation between TVB-N and color variation (R = 0.515). Finally, quantitative analysis about color was conducted by the color recognition software of smartphone. The results indicated when S value (the newly defined color calculation method) was less than 50 or R value was greater than 120, milk and fish were rotten. Consequently, 2D printer could be a promising tool to fabricate intelligent packaging materials enabling monitor visually freshness.