Efficient naked eye sensing of tartrate/malate based on a Zn-Xylenol orange complex in water and membrane-based test strips

Abstract

A new strategy for the real-time and naked-eye quantitative determination of tartrate and malate in aqueous solutions at physiological pH and with practical polypropylene membrane-based tests strips was developed by straightforward assembly Zn2+ ions and xylenol orange (3,3-bis [N,N-bis(carboxymethyl) aminomethyl]-o-cresolsulfonephthalein tetrasodium sodium salt, XO). The chromogenic dinuclear complex [Zn2 (XO) (H2O)x], 1 was studied in-deep as visual chemosensor for aliphatic dicarboxylates, oxyanions and nucleoside triphosphates. Complex 1 was highly sensitive and selective for visual detection of α-hydroxydicarboxylates such as tartrate and malate over non-hydroxylated dicarboxylates (oxalate, malonate, maleate, fumarate and succinate), citrate, oxyanions and nucleoside triphosphates in micromolar concentration range. Addition of tartrate and malate at pH = 7.4, gave rise to rapid color change from purple to golden with detection limit of 11 μmolL−1 for tartrate. On the basis of spectroscopic tools (UV–Vis, 1H NMR) and DFT calculations the efficient colorimetric change upon addition tartrate is attributed to a complexation process, through simultaneous coordination of the carboxylate and hydroxyl groups, to the Zn atoms with high affinity (log K = 5.08) and binding energy calculated in 8.7 kcal mol−1. The complex 1 is an order the magnitude more selective for tartrate than malate.Finally, the combination of complex 1 with polypropylene membranes negatively charged with carboxylate groups on surface generates flexible and cheap tests strips allows for the selective visual detection of tartrate in micromolar concentration range with good selectivity over other common dicarboxylates and oxyanions.