Engineering of metal-organic frameworks (MOFs) for thermometry

Metal-organic frameworks (MOFs ) are excellent candidates for use in chemistry, material sciences and engineering thanks to their interesting qualitative features and potential applications. Quite interestingly, the luminescence of MOFs can be engineered by regulation of the ligand design, metal ion selection and encapsulation of guest molecules within the MOF cavity. Temperature is a very crucial physical parameter and the market share of temperature sensors is rapidly expanding with technology and medicinal advancement. Among the wide variety of available temperature sensors, recently MOFs have emerged as potential temperature sensors with the capacity to precisely measure the temperature. Lanthanide-based thermometry has advantages because of its ratiometric response ability, high quantum yield and photostability, and therefore lanthanide-based MOFs were initially focused on to construct MOF thermometers. As science and technology have gradually changed, it has been observed that with the inclusion of dye, quantum dots, etc. within the MOF cavity, it is possible to develop MOF-based thermometry. This review consolidates the recent advances of MOF-based ratiometric thermometers and their mechanism of energy transfer for determining the temperature (thermal sensitivity and temperature uncertainty). In addition, some fundamental points are also discussed, such as concepts for guiding the design of MOF ratiometric thermometers, thermometric performance and tuning the properties of MOF thermometers.

Highly Luminescent MOF and Its In Situ Fabricated Sustainable Corn Starch Gel Composite as a Fluoro-Switchable Reversible Sensor Triggered by Antibiotics and Oxo-Anions

Frequent use of antibiotics and the growth of industry lead to the pollution of several natural resources which is one of the major consequences for fatality to human health. Exploration of smart sensing materials is highly anticipated for ultrasensitive detection of those hazardous organics. The robust porous hydrogen bonded network encompassing a free-NH₂ moiety, Zn(II)-based metal-organic framework (MOF) (1), is used for the selective detection of antibiotics and toxic oxo-anions at the ppb level. The framework is able to detect the electronically dissimilar antibiotic sulfadiazine and nitrofurazone via fluorescence "turn-on" and "turn-off" processes, respectively. The antibiotic-triggered reversible fluoro-switching phenomena (fluorescence "on-off-on") are also observed by using the fluorimetric method. An extensive theoretical investigation was performed to establish the fluoro-switching response of 1, triggered by a class of antibiotics and also the sensing of oxo-anions. This investigation reveals that the interchange of the HOMO-LUMO energy levels of fluorophore and analytes is responsible for such a fluoro-switchable sensing activity. Sensor 1 showed the versatile detection ability which is reflected by the detection of a carcinogenic nitro-group-containing drug "roxarsone". In view of the sustainable environment along with quick-responsive merit of 1, an in situ MOF gel composite (1@CS; CS = corn starch) is prepared using 1 and CS due to its useful potential features such as biocompatibility, toxicologically innocuous, good flexibility, and low commercial price. The MOF composite exhibited visual detection of the above analytes as well as antibiotic-triggered reversible fluoro-switchable colorimetric "on-off-on" response. Therefore, 1@CS represents a promising smart sensing material for monitoring of the antibiotics and oxo-anions, particularly appropriate for the real-field analysis of carcinogenic drug molecule "roxarsone" in food specimens.

Highly regenerative, fast colorimetric response for organo-toxin and oxo-anions in an aqueous medium using a discrete luminescent Cd(II) complex in a heterogeneous manner with theoretical revelation

The highly luminescent complex [CdQ₂(H₂O)₂] (1) shows ultra-selectivity and high sensitivity to the explosive organo-toxin trinitrophenol (TNP). This detection is extremely fast with a high quenching constant (5.3 × 10⁴ M^-1) and a very low limit of detection (LOD) of 137 nM/59 ppb. This motivated us to detect the lethal carcinogenic arsenical drug roxarsone (ROX), which is reported here for the first time. The quenching constant and LOD for ROX using 1 were found to be 4.9 × 10⁴ M^-1 and 86 nM (or 37 ppb), respectively. Moreover, the probe also recognizes three lethal toxic oxo-anions (MnO₄^-, Cr₂O₇^2- and CrO₄^2-) with outstanding quenching constant (2.2 × 10⁴ M^-1, 1.4 × 10⁴ M^-1 and 1.1 × 10⁴ M^-1) and very low LODs (141 nM/61 ppb, 178 nM/78 ppb and 219 nM/95 ppb). Compared to the previously reported homogeneous sensing nature of the discrete complexes, our complex showed the detection of toxic pollutants in a heterogeneous manner, which results in high recyclability and hence multi-cycle sensing capability. Interestingly, 1 shows the possibility for real-time monitoring through naked eye detection by visible colorimetric changes in solid, solution and strip paper methods, i.e., triphasic detection ability. In addition, the sensor also exhibited the cross-sensing ability for these pollutants. The experimental sensing mechanism is strongly supported by the exhaustive theoretical investigation. Based on the fluorescence signal shown by each analyte, an integrated AND-OR logic gate is constructed. Furthermore, the sensing ability of 1 remains intact towards the detection of versatile real field samples including lethal carcinogenic arsenical drug roxarsone in the real food sample.

Synthesis, crystal structure and characterization of a three-dimensional CdII coordination polymer constructed from 2,5-bis(1H-1,2,4-triazol-1-yl)terephthalate

Bifunctional organic ligands are very popular for the design of coordination polymers because they allow the formation of a great diversity of structures. In the title coordination polymer, the new bifunctional inversion-symmetric ligand 2,5-bis(1H-1,2,4-triazol-1-yl)terephthalic acid (abbreviated as H2bttpa) links CdII cations, giving rise to the three-dimensional CdII coordination polymer catena-poly[diaqua[μ4-2,5-bis(1H-1,2,4-triazol-1-yl)terephthalato-κ4 O 1:O 4:N 4:N 4′]cadmium(II)], [Cd(C12H6N6O4)(H2O)2] n or [Cd(bttpa)(H2O)2] n . The asymmetric unit consists of half a CdII cation, half a bttpa2− ligand and one coordinated water molecule. The CdII cation is located on a twofold axis and is hexacoordinated in a distorted octahedral environment of four O and two N atoms. Four different bttpa2− ligands contribute to this coordination, with two carboxylate O atoms in trans positions and two triazole N atoms in cis positions. Two aqua ligands in cis positions complete the coordination sphere. The fully deprotonated bttpa2− ligand sits about a crystallographic centre of inversion and links two CdII cations to form a chain in a μ2-terephthalato-κ2 O 1:O 4 bridge. This chain extends in the other two directions via the triazole heterocycles, producing a three-dimensional framework. O—H...O hydrogen bonds and weak C—H...N interactions stabilize the three-dimensional crystal structure. The FT–IR spectrum, X-ray powder pattern, thermogravimetric behaviour and solid-state photoluminescence of the title polymer have been investigated. The photoluminescence is enhanced and red-shifted with respect to the uncoordinated ligand.

A three-dimensional supramolecular framework based on the interlinkage of an interpenetrating diamondoid coordination network

In the development of coordination-driven crystalline materials, O- and N-atom donors from carboxylate and pyridyl-based ligands are widely used classes of multidentate bridging ligands composed of several terminal coordinating groups linked by either rigid or flexible spacers. The rigidity of the ligands can play a vital role in the determination of the structures formed. A new Cd^II supramolecular compound, namely poly[μ-adipato-κ²O¹:O⁴-μ-adipato-κ⁴O¹,O^1':O⁴,O^4'-diaquabis[μ-1,4-bis(pyridin-4-yl)-1,3-butadiene-κ²N:N']dicadmium(II)], [Cd₂(C₆H₈O₄)₂(C₁₄H₁₂N₂)₂(H₂O)₂]_n, (I), has been synthesized by the self-assembly of Cd(NO₃)₂·4H₂O, adipic acid (hexane-1,6-dioic acid; H₂adp) and the dipyridyl ligand 1,4-bis(pyridin-4-yl)buta-1,3-diene (1,4-bpbd) under hydrothermal conditions. Single-crystal X-ray diffraction analysis reveals that each Cd^II centre is located in a distorted octahedral coordination environment, coordinated by one water O atom, three carboxylate O atoms from two different adp^2- ligands and two N atoms from two different 1,4-bpbd ligands. The Cd(H₂O) units are interconnected by the μ₂,κ²-adp^2-, μ₂,κ⁴-adp^2- and 1,4-bpbd ligands, which lie across centres of inversion, to give a 6⁶-dia network. Large cavities within a single diamondoid network permit the mutual threefold interpenetration of crystallographically equivalent frameworks. Hydrogen-bonding interactions between the coordinated water molecules and adp^2- carboxylate O atoms anchor the interpenetrating networks into a unique three-dimensional supramolecular structure. Topologically, taking the coordinated water molecules and Cd^II centres as nodes, the whole architecture can be simplified as a binodal (3,7)-connected supramolecular framework. The identity of (I) was further characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis and powder X-ray diffraction. The solid-state photoluminescence properties of (I) were also investigated.