Enhancing colorimetric sensing efficiency of dipicolinic and phthalic acid through hydrogen bonding toward metal ions in aqueous medium

Abstract

Non-covalent organic heterosynthons show enhanced properties in contrast to their individual coformers, rendering them promising for various applications. Specifically, hydrogen-bonded heterosynthons have shown effective coordination behavior with diverse metal ions, hinting at their potential for rapid optical sensing. Their straightforward synthesis and the interactions between the coformers make them good candidates for detection purposes. While organic acid derivatives are well known for their effectiveness as potential sensing agents toward different water pollutants, there remains a significant gap in the literature regarding the efficiency of heterosynthons linked via hydrogen bonding in colorimetric sensing applications, particularly for metal ions in aqueous environmental matrices. Therefore, in this study, five hydrogen-bonded heterosynthons, phthalic acid/2-amino-3-methylpyridine 1, phthalic acid/imidazole 2, dipicolinic acid/pyrazole 3, dipicolinic acid/imidazole 4, and dipicolinic acid/2-amino-3-methylpyridine 5 of two distinct dicarboxylic acids were synthesized using slow evaporation method and characterized using FT-IR, UV–Vis, ¹H-NMR, and PXRD spectroscopic techniques. Physical mixtures with different molar ratios using the same coformers were prepared through a grinding method. Binary phase diagrams were constructed, and FT-IR analysis was performed to propose the type of heterosynthons being formed. The colorimetric detection efficiency of the synthesized heterosynthons toward a wide range of metal ions was examined and compared to that of their individual acids. Interestingly, all heterosynthons demonstrated enhanced colorimetric sensing performance toward metal ions. However, dipicolinic acid/pyrazole 3 and dipicolinic acid/2-amino-3-methylpyridine 5 did not show any significant color change in the presence of Ni(II) in contrast to the pure acid. This could be enhanced by improving the structural conjugation of the prepared heterosynthons and accordingly intensifying the obtained color and enhancing the naked-eye colorimetric detection results.

Graphical abstract