Robust fluorescent calcium coordination polymers as Cu2+ sensors with high sensitivity and fast response

Abstract

A three-dimensional (3D) and highly fluorescent calcium-based coordination polymer (Ca-CP) has been synthesized and structurally characterized. Built on a strongly fluorescent (FL) chromophore ligand, [Ca(H₂tcbpe)(H₂O)₂] (1) (H₄tcbpe = 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-4-carboxylic acid))) is highly luminescent. Photoluminescence (PL) studies indicate that 1 undergoes a bathochromic shift in emission energy from blue to green color upon outgassing or under mechanic force. Notably, 1 exhibits selective FL sensing for Cu²⁺ ions with a detection limit (LOD) of 0.064 ppm, far below the U.S. WHO and EPA standard for drinking water. Detailed investigation of the sensing mechanism reveals that uncoordinated COO⁻ groups in 1 play a major role in recognizing Cu²⁺ ions. This is supported through analysis by multiple characterization methods including IR, EDS and XPS. Based on the proposed mechanism, an isostructural [Ca(H₂tcbpe-F)(H₂O)₂] Ca-CP (2) (H₄tcbpe-F = 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(3-fluoro-[1,1′-biphenyl]-4-carboxylic acid)) is synthesized and tested. Compound 2 demonstrates a ten-fold enhancement in Cu²⁺ detection sensitivity with a ppb level detection limit. The strong enhancement in the detection sensitivity results from optimized electron density around free COO⁻ groups by introducing electron withdrawing F groups onto the backbone of the organic linker. Excellent chemical stability under a wide range of pH conditions (1–14), high sensitivity and rapid fluorescence quenching response time (seconds) make these compounds ideal candidates for use as Cu²⁺ sensors.