Bimetallic complexation for significant fluorescence enhancement of faecal pigment towards water quality testing

Systematic spectroscopic investigations revealed bimetallic complexation of faecal pigments (FP) in water through their multiple binding sites. FP-Zn(II)/Gd(III) complexes showed narrow emission (FWHM ∼ 26 nm) with maximum intensity compared to different bimetallic complexes in water. The fluorescence intensity of bimetallic Zn(II)/Gd(III) complexes of FP increased ∼3–5 fold in aqueous media compared to the FP-Zn(II) complexes, i.e. well-known as Schlesinger’s test. The optimum Zn(II): Gd(III) stoichiometry was observed at 3:2 for maximum FP fluorescence enhancement. The bimetallic FP-Zn(II)/Gd(III) complexes also exhibited higher fluorescence lifetimes and quantum yield than FP-Zn(II) complexes. The photophysical investigations demonstrated that bimetallic Zn(II)/Gd(III) complexation increased the rigidity of chromophores present in FP. Consequently, a decrease in non-radiative decay rate constants (k_nr) by ∼ 1.5 times was observed for bimetallic complexes compared to FP-Zn(II) in water. Hence, a detection limit of the nanomolar concentration range of FP could be achieved in the aqueous media. The effect of diverse interfering factors (humic acid, water hardness, pH) on the fluorescence behaviour of FP-Zn(II)/Gd(III) complexes was also investigated with an approach mimicking real water samples. Finally, a simple non-extraction analytical approach for sensitive detection of FP in water was demonstrated while also considering the fluorescence matrix interference problem.