Colorimetric and Fluorometric Paper-Based Assay for Cu²⁺ Detection Based on Green Synthesis of 2-Aminoterephthalic Acid-Derived Pigments

In this study, we developed a simple and economical method for the green synthesis of Cu²⁺ sensors based on betaxanthin pigments. Aminoisophthalic acid-betaxanthin was synthesized by coupling 2-aminoisophthalic acid and betalamic acid produced from DOPA-extradiol-4,5-dioxygenase in situ and in vitro. The resulting 2-aminoterephthalic acid-betaxanthin (2-AIPA-BX) presented a satisfying fluorescence quantum yield in water and a high degree of selectivity for Cu²⁺ over interfering metal ions. The bioproduction process of 2-AIPA-BX was scaled up from test tubes to 1 L-flasks, indicating the robustness and reproducibility of this method. Additionally, we successfully incorporated 2-AIPA-BX into paper-based analytical devices to facilitate simple, inexpensive, and portable setup with lower sample consumption for onsite monitoring of environmental and biological samples.