A PHBA-functionalized organic-inorganic hybrid polyoxometalate as a luminescent probe for selectively sensing chromium and calcium in aqueous solution

Abstract

Detection of trace metal ions in drinking as well as irrigation water is crucial in understanding their roles among living humans and animals. Herein, an organic PHBA-functionalized polyoxometalate (POM) derivative [N(CH3)4]3K2Eu(PHBA)(H2O)2(α-PW11O39)]·7H2O (1) (PHBA = p-hydroxybenzoic acid) was successfully synthesized which acts as a luminescent probe for selective sensing of chromium and calcium in aqueous solution. The aqueous solution photoluminescence properties of 1 were carefully investigated, indicating PHBA ligand can sensitize the luminescence of Eu3+ ion in 1. The time-resolved emission spectroscopy was also performed to authenticate the energy transfer from π-π* transition of PHBA group to Eu3+ ion. The sensing properties of 1 were performed on Cd2+, Cr3+, Ba2+, K+, Li+, Pb2+, Zn2+, Al3+, Ag+, Hg2+, Co2+, Sr2+, Ca2+, Na+, Mg2+ ions in aqueous solution, and the photoluminescence intensity for Cr3+ ion display a quenching phenomenon, whereas, Ca2+ ion displays an enhancement signal in their photophysical behavior. The detection limits for detecting Cr3+ and Ca2+ ion were 1.423 μM and 0.676 mM, respectively. In the detection process of Cr3+ ion, the values of quenching rate constant (Kq) were varied from 4.34 × 106 L (mol s)−1 to 4.29 × 109 L (mol s)−1, which were lower than the value of the maximum scatter collision quenching constant (2.0 × 1010 L (mol s)−1). The results reveal that the quenching mechanism between 1 and Cr3+ ion can be attributed to dynamic collision quenching mechanism. Furthermore, the active sites for sensing Cr3+ and Ca2+ ions also have been investigated, determining the different interaction mechanisms exist in detection of Cr3+ and Ca2+ ions in solution.