Mechanochemical and Conventional Synthesis of Zn(II)/Cd(II) Luminescent Coordination Polymers: Dual Sensing Probe for Selective Detection of Chromate Anions and TNP in Aqueous Phase

Isostructural
Zn­(II)/Cd­(II) mixed ligand coordination polymers (CPs) {[M­(IPA)­(L)]}n (CP1 and CP2) built from isophthalic acid (H2IPA) and 3-pyridylcarboxaldehyde nicotinoylhydrazone
(L) were prepared using versatile synthetic routes: viz.,
diffusion of precursor solutions, conventional reflux methods, and
green mechanochemical (grinding) reactions. Both robust CPs synthesized
by different routes were characterized by various analytical methods,
and their thermal and chemical stability as well as the phase purity
was established. Crystallographic studies revealed that CP1 and CP2 are isostructural frameworks and feature a
double-lined two-dimensional network composed of Zn2+/Cd2+ nodes connected through IPA and pillared by
the Schiff base ligand L with a double-walled edge. The
photoluminescent (PL) properties of CP1 and CP2 have been exploited as dual detection fluorosensors for hexavalent
chromate anions (CrO42–/Cr2O72–) and 2,4,6-trinitrophenol (TNP)
because it was observed that the emission intensity of aqueous suspensions
of CPs selectively quenches by chromate anions or TNP among large
pools of different anions or nitro compounds, respectively. Competitive
experiments in the presence of interfering anions/other nitro compounds
also revealed no major effect in the quenching efficiency, suggesting
the selective detection of hexavalent chromate anions as well as TNP
by the LCPs. The limits of detection by CP1 for CrO42–/Cr2O72– and TNP are 4 ppm/4 ppm and 28 ppb, respectively, whereas the limits
of detection by CP2 for the same analytes are 1 ppm/1
ppm and 14 ppb, respectively. A probable mechanism for the quenching
phenomena is also discussed.