A white-light-emitting lanthanide metal–organic framework for luminescence turn-off sensing of MnO4− and turn-on sensing of folic acid and construction of a “turn-on plus” system

Abstract

A novel single-phase white-light-emitting microscale lanthanide metal–organic framework [Eu_0.06Tb_0.04Gd_0.9(HDPNC)_1.5(H₂O)(DMF)]·2H₂O (H₃DPNC = 5-(3′,5′-dicarboxylphenyl) nicotinic acid) abbreviated as Eu_0.06Tb_0.04Gd_0.9-DPNC was obtained successfully under 365 nm excitation, by adjusting the molar ratio of Ln³⁺ and different excitation wavelengths, with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.3328, 0.3296). The tri-emission lanthanide metal–organic framework (Eu_0.06Tb_0.04Gd_0.9-DPNC), acting as an effective fluorescent platform, not only exhibits notable fluorescence quenching (turn-off) toward MnO₄⁻ in water through the fluorescence resonance energy transfer (FRET) mechanism, with a linear recognition range of 0–0.2 mM and a low detection limit (LOD) of 0.0197 μM, but also shows fluorescence enhancement (turn-on) for the detection of folic acid (FA) in water through host–guest interactions, and a linear relationship in the concentration range of 0–18 μM with a LOD of 0.0883 μM. This is the first time folic acid detection is carried out based on a Ln–MOF, and is also one of the very rare examples of application of white-light-emitting MOFs for fluorescence detection. Furthermore, interestingly, another fluorescence further enhanced (named turn-on plus) system was designed and successfully obtained by adding FA to the fluorescence quenched Eu_0.06Tb_0.04Gd_0.9-DPNC@MnO₄⁻ system, in which the mechanism of further fluorescence enhancement can be ascribed to the redox reaction between MnO₄⁻ and FA under alkaline conditions and the production of another fluorescent compound. Additionally, the Boolean logic system was also constructed based on the dual-component analysis (MnO₄⁻ and FA) of Eu_0.06Tb_0.04Gd_0.9-DPNC in water.