Guest-dependent bond flexibility in UiO-66, a "stable" MOF

Size-Dependent Spin Crossover and Bond Flexibility in Metal-Organic Framework Nanoparticles

Size reduction offers a synthetic route to tunable phase change behavior. Preparing materials as nanoparticles causes drastic modulations to critical temperatures (Tc), hysteresis widths, and the "sharpness" of first-order versus second-order phase transitions. A microscopic picture of the chemistry underlying this size dependence in phenomena ranging from melting to superconductivity remains debated. As a case study with broad implications, we report that size-dependent spin crossover (SCO) in nanocrystals of the metal-organic framework (MOF) Fe(1,2,3-triazolate)2 arises from metal-linker bonds becoming more labile in smaller particles. In comparison to the bulk material, differential scanning calorimetry indicates a ∼ 30-40% reduction in Tc and ΔH in the smallest particles. Variable-temperature vibrational spectroscopy reveals a diminished long-range structural cooperativity, while X-ray diffraction evidence an over 3-fold increase in the thermal expansion coefficients. This "phonon softening" provides a molecular mechanism for designing size-dependent behavior in framework materials and for understanding phase changes in general.

Dynamic metal-linker bonds in metal-organic frameworks

Metal-linker bonds serve as the "glue" that binds metal ions to multitopic organic ligands in the porous materials known as metal-organic frameworks (MOFs). Despite ample evidence of bond lability in molecular and polymeric coordination compounds, the metal-linker bonds of MOFs were long assumed to be rigid and static. Given the importance of ligand fields in determining the behaviour of metal species, labile bonding in MOFs would help explain outstanding questions about MOF behaviour, while providing a design tool for controlling dynamic and stimuli-responsive optoelectronic, magnetic, catalytic, and mechanical phenomena. Here, we present emerging evidence that MOF metal-linker bonds exist in dynamic equilibria between weakly and tightly bond conformations, and that these equilibria respond to guest-host chemistry, drive phase change behavior, and exhibit size-dependence in MOF nanoparticles.

Determination of trace fluoroquinolones in honey and milk based on cyclodextrin modified magnetic metal-organic frameworks solid phase extraction coupled with ultra-high performance liquid chromatography

Long-term intake of animal-derived foods with excessive fluoroquinolones (FQs) will cause damage to human health, so it is critical to establish a feasible approach for sensitive and rapid monitoring of FQs residues. In this study, a new cyclodextrin modified magnetic metal-organic frameworks (Fe3O4@UiO-66-CD) was successfully synthesized by amidation reaction and applied to magnetic solid phase extraction (MSPE) for FQs analysis. The adsorption behavior of Fe3O4@UiO-66-CD was consistent with the pseudo-second-order kinetics and Freundlich isothermal adsorption model, which indicated that the designed material had various interactions on FQs, such as host-guest interaction and π-π interaction. The parameters of MSPE were optimized and the determination method of norfloxacin, enrofloxacin, lomefloxacin and gatifloxacin was established by using MSPE combined with ultra-high performance liquid chromatography (UHPLC) and fluorescence detector (FLD). The method validation results displayed that the detection limits were 0.02-0.09 ng/mL, and the RSDs of intra-day and inter-day precision were less than 4.1 and 6.4 %, respectively. In the target FQs analysis of real honey and milk samples, the recoveries at different fortified concentrations were in the ranges of 88.4 % to 108.6 % with RSD ≤ 5.7 %. The results showed that the proposed method was sensitive, accurate and reliable for the determination of trace FQs in animal-derived foods.

Ultrafast Water H-Bond Rearrangement in a Metal-Organic Framework Probed by Femtosecond Time-Resolved Infrared Spectroscopy

We investigated the water H-bond network and its dynamics in Ni₂Cl₂BTDD, a prototypical MOF for atmospheric water harvesting, using linear and ultrafast IR spectroscopy. Utilizing isotopic labeling and infrared spectroscopy, we found that water forms an extensive H-bonding network in Ni₂Cl₂BTDD. Further investigation with ultrafast spectroscopy revealed that water can reorient in a confined cone up to ∼50° within 1.3 ps. This large angle reorientation indicates H-bond rearrangement, similar to bulk water. Thus, although the water H-bond network is confined in Ni₂Cl₂BTDD, different from other confined systems, H-bond rearrangement is not hindered. The picosecond H-bond rearrangement in Ni₂Cl₂BTDD corroborates its reversibility with minimal hysteresis in water sorption.