Luminescent lanthanide metal-organic frameworks for chemical sensing and toxic anion detection

Prototype lanthanide metal-organic frameworks (LnMOFs), Ln(BTC) (Ln = Eu and Tb; BTC = benzene-1,3,5-tricarboxylate), have been considered as luminescent sensors for detecting toxic anions, while their neutral pore structures have limited the entrance and encapsulation of anions to produce highly anion-responsive photoluminescence (PL). To facilitate anions to enter the pore space of Ln(BTC), a one-pot synthesis method was proposed in which BTC was partially replaced with its structural analogue L·BF₄ (H₃L·BF₄ = 2,4,6-tricarboxy-1-methylpyridinium tetrafluoroborate) which consists of an anion affinity site of cationic methylpyridinium. Compared to the original Ln(BTC), the co-doped cationic framework Eu_0.05Tb_0.95-BTC_0.9L_0.1 is highly sensitive for detecting different toxic anions by tuning the energy absorption of organic chromophores, the energy transfer efficiency to Ln³⁺ ions and the energy allocation between different Ln³⁺ ions in the PL spectra. We demonstrated that the Eu_0.05Tb_0.95-BTC_0.9L_0.1 PL sensor has the capability of decoding various toxic anions with a clearly differentiable and unique emission intensity ratio of ⁵D₄ → ⁷F₅ (Tb³⁺, 545 nm) to ⁵D₀ → ⁷F₂ (Eu³⁺, 618 nm) transitions (I_Tb/I_Eu). Compared to Ln(BTC), the co-doped Eu_0.05Tb_0.95-BTC_0.9L_0.1 presents self-calibrating, high distinguishable and stable PL signals for detecting toxic anions.