Highly selective and sensitive detection of Hg2+, Cr2O72−, and nitrobenzene/2,4-dinitrophenol in water via two fluorescent Cd-CPs

Abstract

Two novel luminescent cadmium coordination polymers (Cd-CPs), [Cd(TPA)(DIB)]_n (1) and [Cd(TPA)(BIYB)]_n (2), have been successfully designed and synthesized using Cd²⁺, deprotonated 3,3′-thiodipropionic acid (H₂TPA), and 4,4-bis(imidazol-1-ylmethyl)biphenyl (BIYB) or 1,4-di(1H-imidazol-1-yl)benzene (DIB). CP 1 has a 3D structure with a new topology. CP 2 possesses a double layer 2D structure with two types of 1D tunnel. The fluorescence property application and mechanisms of the two Cd-CPs for detecting Hg²⁺, Cr₂O₇²⁻, nitrobenzene (NB), or 2,4-dinitrophenol (2,4-DNP) have been researched. CPs 1–2 demonstrate good chemical stability, high sensitivity and selective responsive detection of Hg²⁺, Cr₂O₇²⁻, NB, or 2,4-DNP. The high quenching percentages in dilute aqueous solutions were 98.5% (Hg²⁺, 1), 96.2% (Hg²⁺, 2), 98.5% (Cr₂O₇²⁻, 1), 96.4% (Cr₂O₇²⁻, 2), 99.6% (NB, 1) and 99.2% (2,4-DNP, 2). The detection limits were 2.1 × 10⁻⁷ (Hg²⁺, 1), 1.1 × 10⁻⁶ (Cr₂O₇²⁻, 1), 2.7 × 10⁻⁷ (NB, 1), 1.9 × 10⁻⁷ (Hg²⁺, 2), 2.4 × 10⁻⁷ (Cr₂O₇²⁻, 2), and 1.3 × 10⁻⁷ (2,4-DNP, 2) and are similar to the permissible limits set by the USEPA. The mechanism of Hg²⁺ luminescence quenching involves an interaction with the free thio sites of the CPs. The Cr₂O₇²⁻, NB, and 2,4-DNP quenching mechanism involves the overlap of the absorption bands of the recognition materials with the excitation or emission bands of CPs 1–2. To the best of our knowledge, these are the best examples of CPs as probes for sensing Hg²⁺, Cr₂O₇²⁻, NB, or 2,4-DNP.