Chemical Sensors Based on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) as chemical sensors have developed rapidly in recent years. There have been many papers concerning this field and interest is still growing. The reason is that the specific merits of MOFs can be utilized to enhance sensitivity and selectivity by various energy/charge transfers occurring among different ligands, ligand, and metal centers, such as from ligands to metal centers or metal centers to ligands, as well as from MOF skeletons to guest species. This review intends to provide an update on recent progress in various applications of different MOF-based sensors on the basis of their luminescent and electrochemical responses towards small molecules, gas molecules, ions (cations and anions), pH, humidity, temperature, and biomolecules. MOF-based sensors function by utilizing different mechanisms, including luminescent responses of "turn-on" and "turn-off", as well as electrochemical responses.

An ideal detector composed of a 3D Gd-based coordination polymer for DNA and Hg2+ ion

A 3D Gd-based CP was employed as an effective fluorescent sensing platform for DNA and Hg²⁺ ion detection with sensitivity and selectivity, due to its ability to highly quench fluorescence and its different affinities toward ssDNA and dsDNA.

Outstanding drug loading capacity by water stable microporous MOF: a potential drug carrier

A robust, highly water stable, microporous MOF, [Zn₈(O)₂(CDDB)₆(DMF)₄(H₂O)] {where CDDB = 4,4′-(9-H carbazole-3,6-diyl)dibenzoic acid}, was synthesized based on an open N–H site by a solvothermal process and exhibited an outstanding loading capacity and satisfactory release capability for 5-fluorouracil, with a negligible cytotoxicity effect.

Highly efficient electrochemical reduction of CO2 to CH4 in an ionic liquid using a metal-organic framework cathode

Highly efficient electrochemical reduction of CO2 to CH4 is of great importance, but is challenging. Herein, Zn-1,3,5-benzenetricarboxylic acid metal-organic frameworks (Zn-BTC MOFs) deposited on carbon paper (CP) were used as cathodes in electrochemical reduction of CO2 using ionic liquids (ILs) as the electrolytes, which was the first work on combination of a MOF electrode and an pure IL electrolyte in the electrochemical reduction of CO2. It was found that the efficiency of the reaction depended strongly on the morphology of the Zn-MOFs. Compared with the commonly used metal electrodes, the electrochemical reaction showed much higher selectivity to CH4 and current density, and the overpotentials for CH4 is much lower. The excellent combination of the MOF cathodes and ILs opens a way for reduction of CO2 to CH4 effectively.

Biocompatible polymer–metal–organic framework composite patches for cutaneous administration of cosmetic molecules

Caffeine-containing polymer–nanoMOF patches as promising cutaneous formulations.

Topological modulation of the porous structure of a coordination polymer constructed from a flexible building block via framework–guest interaction during self-assembly

The porous structure of a Ln(iii) coordination polymer with a flexible ligand was modulated by the guest solvent.

Metal–organic framework MIL-101 supported bimetallic Pd–Cu nanocrystals as efficient catalysts for chromium reduction and conversion of carbon dioxide at room temperature

A series of bimetallic Pd–Cu nanocrystals supported on the zeolite-type metal–organic framework MIL-101 and their application in the reduction of Cr(vi) to Cr(iii) using formic acid and the conversion of terminal alkynes into propiolic acids with CO₂ are reported.

Layer-structured coordination polymers based on 5-(1H-tetrazol-5-yl)isophthalic acid: structure, sensitization of lanthanide(iii) cations and small-molecule sensing

Compound 1 can serve as a host for encapsulation of Ln³⁺ ions.

A Series of Multifunctional Metal-Organic Frameworks Showing Excellent Luminescent Sensing, Sensitization, and Adsorbent Abilities

A series of highly luminescent-active metal-organic frameworks (MOFs) 1-3 with hierarchical pores have been rationally constructed and fully characterized. The predesigned semi-rigid hexacarboxylate ligand hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid (H6 L) has been adapted with various space-directed N donors (i.e., 2,2'-bipyridine, 4,4'-di(1H-imidazol-1-yl)-1,1'-biphenyl, and 1,3,5-tri(1H-imidazol-1-yl)benzene) from a bidentate V-shape to a tridentate Y-shape. This family of multifunctional MOF materials represents a variety of potential applications in the following aspects: first, as luminescent sensors that show a fast and sensitive detection for pollutant CrO4 (2-) and Cr2 O7 (2-) ions in aqueous media; second, as adsorbents that can rapidly remove harmful organic dyes; third, as an antenna that can effectively sensitize visible-light-emitting Tb(3+) ions. These multifunctional MOF materials combine optical-sensing, adsorption, and sensitization properties, thus are very useful in many potential applications. Furthermore, these materials have proved to be reusable.

Solvent-Induced Change of Electronic Spectra and Magnetic Susceptibility of Co(II) Coordination Polymer with 2,4,6-Tris(4-pyridyl)-1,3,5-triazine

One-dimensional coordination polymer [Co(Piv)2(4-ptz)(C2H5OH)2]n (compound 1, Piv(-) = pivalate, 4-ptz = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) was synthesized by interaction of Co(II) pivalate with 4-ptz. Desolvation of 1 led to formation of [Co(Piv)2(4-ptz)]n (compound 2), which adsorbed N2 and H2 at 78 K as a typical microporous sorbent. In contrast, absorption of methanol and ethanol by 2 at 295 K led to structural transformation probably connected with coordination of these alcohols to Co(II). Formation of 2 from 1 was accompanied by change of color of sample from orange to brown and more than 2-fold decrease of molar magnetic susceptibility (χM) in the temperature range from 2 to 300 K. Resolvation of 2 by ethanol or water resulted in restoration of spectral characteristics and χM values almost to the level of that of 1. χMT versus T curves for 1 and samples, obtained by resolvation of 2 by H2O or C2H5OH, were fitted using a model for Co(II) complex with zero-field splitting of this ion.

Assembly of three coordination polymers based on a sulfonic–carboxylic ligand showing high proton conductivity

Three MOFs with different structures all exhibit proton conduction behavior, especially for the Cu compound with a proton conductivity of 3.46 × 10⁻³ S cm⁻¹ at 368 K and 95% RH.

4-(4-Carboxyphenoxy)phthalate-based coordination polymers and their application in sensing nitrobenzene

Structural characterization of 4-(4-carboxyphenoxy)phthalate-based Cd²⁺ and Mn²⁺ coordination polymers are reported, and Cd²⁺ coordination polymer can serve as probe to sense nitrobenzene.

Highly thermostable lanthanide metal–organic frameworks exhibiting unique selectivity for nitro explosives

The solvent-free Eu-MOF with high red emission intensity and the microporous instinct exhibits high sensitivity for 2,4,6-trinitrophenol with K_sv constant 6.24 × 10⁴ M⁻¹.

Cage-like pores of a metal–organic framework for separations and encapsulation of Pd nanoparticles for efficient catalysis

A porous MOF can be applied for separation of organic compounds and drug delivery. It shows high catalytic reduction of 4-nitrophenol after embedding Pd NPs.

Guest-induced reversible structural transitions and concomitant on/off luminescence switching of an Eu(iii) metal–organic framework and its application in detecting picric acid

A flexible Eu-based MOF with strong red-light-emission exhibits selective detection of PA among the nitro compounds.

A Zn azelate MOF: combining antibacterial effect

A novel biocompatible and bioactive zinc azelate metal–organic framework (BioMIL-5) was hydrothermally synthesized with interesting long-term antibacterial properties.

A novel porous metal–organic framework from a new bis(acylhydrazone) ligand capable of reversibly adsorbing/desorbing water and small alcohol molecules

Weak coordination and π–π interactions link the metallomacrocycles into two interpenetrating 3D networks to form the material with hydrophilic pores.

New Zn2+ coordination polymers constructed from acylhydrazidate molecules: synthesis and structural characterization

Two types of hydrothermal in situ ligand reactions were employed to obtain three new acylhydrazidate-extended Zn²⁺ coordination polymers.

Luminescent metal–organic frameworks for chemical sensing and explosive detection

This review provides an update on the photoluminescence properties of LMOFs and their utility in chemical sensing and explosive detection.