Development of paper based colorimetric method using pigment from red dragon fruit for determination of Cu and Fe

Betalain, a natural pigment found in red dragon fruit, has been proposed as a natural reagent for metal detection. In the present study, this pigment was extracted and used as a colorimetric reagent for Cu and Fe on a paper-based analytical device (PAD) and then applied to water samples. The extract was stable at 5 ± 3 °C for 16 weeks. In a solution with a pH of 4-5, the betalain extract changed color from pink to light orange (Cu) and yellow (Fe) and was selective against Na, K, Ca, Ba, Al, Mg, Zn, Hg, Ni, and Pb. The color change was caused by a metal-betalain complex with an estimated ratio of 1:2 (Cu-betalain) and 1:9 (Fe-betalain). Betalain PAD was produced under optimal conditions using Whatman CF1 paper containing 20 µL of 100 mg/mL betalain extract at pH 4-5. This process resulted in a limit of quantification of 3.133 mg/L (Cu) and 4.736 mg/L (Fe) and a limit of detection of 1.034 mg/L (Cu) and 1.563 mg/L (Fe). Based on these findings, betalain PAD made from red dragon fruit could be a viable alternative for the on-site detection of Cu and Fe in water. Ultimately, betalain PAD is a toxic waste-free, portable, fast, and prospective on-site tool for metal detection.