Water-stable Cd(II)/Zn(II) coordination polymers as recyclable luminescent sensors for detecting hippuric acid in simulated urine for indexing toluene exposure with high selectivity, sensitivity and fast response

Three novel Cd(II)/Zn(II) coordination polymers (CPs), namely [Cd(L)(BPDC)0.5H2O]·0.5H2O (1), [Zn2(L)2(BPDC)]·2H2O (2) and [Cd2(L)(BTC)H2O]·3H2O (3) (L = 4-(tetrazol-5-yl)phenyl-4,2':6',4''-terpyridine, H2BPDC = 4,4'-biphenyldicarboxylic acid, and H3BTC = 1,3,5-benzenetricarboxylic acid), have been successfully synthesized and characterized. CP 1 and CP 2 display new two-dimensional double-layered honeycomb frameworks containing uncoordinated nitrogen atoms from pyridine and tetrazol rings, which are easy to form hydrogen bonding with various analytes. CP 3 exhibits a 3D framework also with uncoordinated nitrogen atoms from pyridine and tetrazol rings. The fluorescence explorations indicate that CPs 1-3 possess strong blue luminescence and excellent chemical stability under a relatively wide range of pH conditions. It is worth noting that CPs 1-3 all can quantitatively detect hippuric acid (HA) which is a metabolite of toluene in human urine with high selectivity, sensitivity, fast response and relatively low detection limits. Moreover, the sensing mechanism of CPs 1-3 for HA can mainly be ascribed to the fluorescence resonance energy transfer (FRET). CPs 1-3 could be the ideal candidates as HA sensors in human urine in practical applications. Notably, to our best knowledge, it is for the first time that Cd(II)/Zn(II)-based luminescent sensors for detecting HA in simulated urine is reported here.