Selective and Sensitive Sensing of Hydrogen Peroxide by a Boronic Acid Functionalized Metal-Organic Framework and Its Application in Live-Cell Imaging

A new boronic acid functionalized Zr(IV) metal-organic framework having the capability of sensing H₂O₂ in live cells is reported. The Zr-MOF bears a UiO-66 structure and contains 2-boronobenzene-1,4-dicarboxylic acid (BDC-B(OH)₂) as a framework linker. The activated Zr-UiO-66-B(OH)₂ compound (called 1') is highly selective for the fluorogenic detection of H₂O₂ in HEPES buffer at pH 7.4, even in the presence of interfering ROS (ROS = reactive oxygen species) and other biologically relevant analytes. The fluorescent probe was found to display extraordinary sensitivity for H₂O₂ (detection limit 0.015 μM) in HEPES buffer, which represents a lower value in comparison to those of the MOF probes documented so far for sensing H₂O₂ using other analytical methods. Taking advantage of its high selectivity and sensitivity for H₂O₂ in HEPES buffer, the probe was successfully employed for the imaging of intracellular H₂O₂. Imaging studies with MDAMB-231 cells revealed the emergence of bright blue fluorescence after loading with probe 1' and subsequent treatment with H₂O₂ solution.

Flexible Metal-Organic Framework-Based Mixed-Matrix Membranes: A New Platform for H2 S Sensors

Metal-organic framework (MOF)-polymer mixed-matrix membranes (MMMs) have shown great potential and superior performance in gas separations. However, their sensing application has not been fully established yet. Herein, a rare example of using flexible MOF-based MMMs as a fluorescent turn-on sensor for the detection of hydrogen sulfide (H₂ S) is reported. These MOF-based MMMs are readily prepared by mixing a highly stable aluminum-based nano-MOF (Al-MIL-53-NO₂ ) into poly(vinylidene fluoride) with high loadings up to 70%. Unlike the intrinsic fragility and poor processability of pure-MOF membranes, these MMMs exhibit desirable flexibility and processability that are more suitable for practical sensing applications. The uniform distribution of Al-MIL-53-NO₂ particles combined with the permanent pores of MOFs enable these MMMs to show good water permeation flux and consequently have a full contact between the analyte and MOFs. The developed MMM sensor (70% MOF loading) thus shows a highly remarkable detection selectivity and sensitivity for H₂ S with an exceptionally low detection limit around 92.31 × 10^-9 m, three orders of magnitude lower than the reported powder-form MOFs. This work demonstrates that it is feasible to develop flexible luminescent MOF-based MMMs as a novel platform for chemical sensing applications.

A CuO-functionalized NMOF probe with a tunable excitation wavelength for selective detection and imaging of H2S in living cells

Recently, fluorescent nanoscale metal-organic frameworks (NMOFs) have been proven to be useful probes for the detection and imaging of active biomolecules in living cells. However, the excitation wavelengths of these NMOF fluorescence probes are mostly in the ultraviolet region, which unavoidably results in reduced cell activity, limited tissue penetration depth and inevitable biological background interference. Herein, to solve this problem, a CuO functionalized NMOF probe with a tunable excitation wavelength based on Förster resonance energy transfer (FRET) for selective detection and imaging of the third important gaseous signaling molecule hydrogen sulfide (H2S) in living cells as an example is presented. In the energy transfer system, NMOF confines the luminophore organic dye thiazole orange within its intrinsic porous matrix as the energy donor, in which the excitation wavelength of the NMOF can be tuned simply from UV to Vis through the choice of dye molecules, and the H2S-responding site copper oxide nanoparticle (CuO NP) is the acceptor. After the surface functionalization of CuO NPs onto the NMOF, the fluorescence of the NMOF can be efficiently quenched based on the FRET. When H2S appeared, the fluorescence of the nanoprobe is recovered due to the interruption of FRET. This facile yet powerful strategy not only provides an instantaneous fluorescence probe for selective H2S detection in living cells but also offers a valuable approach for using porous NMOFs to sense other biological species.

Selective Detection of H2S by Copper Complex Embedded in Vesicles through Metal Indicator Displacement Approach

A new approach for the detection of hydrogen sulfide (H₂S) was constructed within vesicles comprising phospholipids and amphiphilic copper complex as receptor. 1,2-Distearoyl- sn-glycero-3-phosphocholine (DSPC) vesicles with embedded metal complex receptor (1.Cu) sites have been prepared. The vesicles selectively respond to H₂S in a buffered solution and show colorimetric as well as spectral transformation. Other analytes such as reactive sulfur species, reactive nitrogen species, biological phosphates, and other anions failed to induce changes. The H₂S detection is established through a metal indicator displacement (MIDA) process, where Eosin-Y (EY) was employed as an indicator. Fluorescence, UV-vis spectroscopy, and the naked eye as the signal readout studies confirm the high selectivity, sensitivity, and lower detection limit of the vesicular receptor. The application of vesicular receptors for real sample analysis was also confirmed by fluorescence live cell imaging.

Extraordinary sensitivity for H2S and Fe(iii) sensing in aqueous medium by Al-MIL-53-N3 metal-organic framework: in vitro and in vivo applications of H2S sensing

An Al(iii) metal-organic framework (MOF) called Al-MIL-53-N₃ (1) was synthesized under solvothermal reaction conditions using Al(NO₃)₃·9H₂O and H₂BDC-N₃ (H₂BDC-N₃ = 2-azido-1,4-benzenedicarboxylic acid) ligand in a DMF/water (DMF = N,N-dimethylformamide) mixture. Phase purity was checked by performing X-ray powder diffraction, infrared spectroscopy and thermogravimetric analysis. Thermogravimetric analysis suggests that 1 is highly stable up to 300 °C under air atmosphere. The activated 1 (called 1') showed a very fast fluorescence response to H₂S (turn-on) and Fe(iii) ions (turn-off) in an aqueous medium with excellent sensitivity and selectivity even in the presence of other potentially intrusive analytes. In the presence of H₂S, the conversion of the azide moiety to amine is responsible for the fluorescence turn-on properties. On the other hand, the partial replacement of framework Al(iii) ions by Fe(iii) can be assigned for the selective detection behavior to Fe(iii) ions. The detection limits (90.47 nM for H₂S and 0.03 μM for Fe(iii) ions in water) of 1' are lower than those of the formerly reported MOF type of fluorescent sensors. The 1'-loaded J774A.1 macrophage cells are healthy and respond to intracellular H₂S to exhibit strong blue fluorescence, confirming its suitability to detect H₂S inside the cells. In addition, 1' can detect H₂S in human blood plasma (HBP) and sulfide ions in real water samples. These features make 1' a very promising candidate for the on-site sensing of Fe(iii) ions and the detection of intracellular and extracellular H₂S.

Rapid and highly sensitive detection of extracellular and intracellular H2S by an azide-functionalized Al(iii)-based metal–organic framework

Fast and highly sensitive sensing of both extracellular and intracellular H₂S by an Al(iii)-based CAU-10-N₃MOF is presented.

Tuning the stability of bimetallic Ce(iv)/Zr(iv)-based MOFs with UiO-66 and MOF-808 structures

A series of solid solutions of bimetallic Ce/Zr-UiO-66 and -MOF-808 compounds with a varying ratio of Ce⁴⁺ to Zr⁴⁺ were obtained under mild reaction conditions within 15 min. Samples with Ce ≤20 at% exhibit an enhanced thermal stability, better resistance against acids and smaller particle sizes.

Metal–organic frameworks: functional luminescent and photonic materials for sensing applications

This review summarizes the diverse routes to derive sensing applications from suitably functionalized and crystal-engineered metal–organic framework (MOF) materials, either by fluorometric responses, or based on photonic crystal-based signal transduction.

Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide Based on Metal-Organic Frameworks

The sensing of hydrogen sulfide (H₂S) has become a long-time challenging task. In this work, we developed a general strategy for sensing of H₂S utilizing postsynthetic modification of a nano metal-organic frameworks (MOF) UiO-66-(COOH)₂ with Eu³⁺ and Cu²⁺ ions. The nano MOF Eu³⁺/Cu²⁺@UiO-66-(COOH)₂ displays the characteristic Eu³⁺ sharp emissions and the broad ligand-centered (LC) emission simultaneously. Because H₂S can strongly increase the fluorescence of Eu³⁺ and quench the broad LC emission through its superior affinity for Cu²⁺ ions, the MOF Eu³⁺/Cu²⁺@UiO-66-(COOH)₂ exhibits highly sensitive turn-on sensing of H₂S over other environmentally and biologically relevant species under physiological conditions. Furthermore, this approach for fluorescent turn-on sensing of H₂S is expected to extend to other water-stable MOFs containing uncoordinated -COOH.

Synthesis and structure of Zr(iv)- and Ce(iv)-based CAU-24 with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene

Eight-fold connection of hexanuclear clusters containing Zr(iv) or Ce(iv) through rigid, rectangular tetracarboxylate ions yields new MOFs with scu topology.