Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal-organic frameworks (MOFs), namely {[Ln(DMTP-DC)(1.5)(H2O)(3)]center dot DMF}(n) [H2DMTP-DC = 2',5'-dimethoxytriphenyl-4,4 ''-dicarboxylic acid; Ln(III) = Eu-III (1), Gd-III (2), Tb-III (3), and Dy-III (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1-4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H2O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln(III) ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 mu M for arginine and 9.31 mu M for lysine. All 1-4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1-4 at 55 degrees C and 100% RH are 9.94 x 10(-5), 1.62 x 10(-4), 1.71 x 10(-4), and 2.67 x 10(-4) S.cm(-1), respectively. The value of s increases with the decrease in ionic radius, indicating that the radius of the Ln(III) ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (-Delta S-m) of 18.86 J kg(-1) K-1 for Delta H = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1-4 are potentially multifunctional MOF materials.

Automated summary

In this study, four isostructural lanthanide metal-organic frameworks (MOFs) with rich H-bonding were synthesized and characterized. One of the MOFs can be used as a fluorescence sensor for arginine and lysine in aqueous solution, and another MOF exhibits a significant magnetocaloric effect.