Complexing Eu3+/Tb3+ in a Nanoscale Postmodified Zr-MOF toward Temperature-Modulated Multispectrum Chromism

Orthogonal Postsynthetic Copolymerization of Hydrogen-Bonded Organic Frameworks into a PolyHOF Membrane

Hydrogen-bonded organic frameworks (HOFs) have shown promise in various fields; however, the construction of HOF/polymer hybrid membranes that can maintain both structural and functional integrity remains challenging. In this study, we here fabricated a new HOF (HOF-50) with reserved polymerizable allyl group via charge-assisted H-bonds between the carboxylate anion and amidinium, and subsequently copolymerized the HOF with monomers to construct a covalently bonded HOF/polymer hybrid (polyHOF) membrane. The resulting polyHOF membrane not only exhibits customizable mechanical properties and extreme stability, but also shows an exceptional ratiometric luminescent temperature-sensing function with very high sensitivity and visibility even when the lanthanide content is two orders of magnitude lower than that of the reported mixed-lanthanide metal-organic frameworks (MOFs) and lanthanide-doped covalent organic frameworks (COFs). This orthogonal postsynthesis copolymerization strategy may provide a general approach for preparing covalently connected HOF/polymer hybrid membranes for diverse applications.

Taking Advantage of a Luminescent ESIPT-Based Zr-MOF for Fluorochromic Detection of Multiple External Stimuli: Acid and Base Vapors, Mechanical Compression, and Temperature

Luminescent materials responsive to external stimuli have captivated great attention owing to their potential implementation in noninvasive photonic sensors. Luminescent metal-organic frameworks (LMOFs), a type of porous crystalline material, have emerged as one of the most promising candidates for these applications. Moreover, LMOFs constructed with organic linkers that undergo excited-state intramolecular proton-transfer (ESIPT) reactions are particularly relevant since changes in the surrounding environment induce modifications in their emission properties. Herein, an ESIPT-based LMOF, UiO-66-(OH)2, has been synthesized, spectroscopically and photodynamically characterized, and tested for detecting multiple external stimuli. First, the spectroscopic and photodynamic characterization of the organic linker (2,5-dihydroxyterephthalic acid (DHT)) and the UiO-66-(OH)2 MOF demonstrates that the emission properties are mainly governed by the enol → keto tautomerization, occurring in the organic linker via the ESIPT reaction. Afterward, the UiO-66-(OH)2 MOF proves for the first time to be a promising candidate to detect vapors of acid (HCl) and base (Et3N) toxic chemicals, changes in the mechanical compression (exercised pressure), and changes in the temperature. These results shed light on the potential of ESIPT-based LMOFs to be implemented in the development of advanced optical materials and luminescent sensors.

Modular Gating of Ion Transport by Postsynthetic Charge Transfer Complexation in a Metal-Organic Framework

Nature's design of biological ion channels that demonstrates efficient gating and selectivity brings to light a very promising model to mimic and design for achieving selective and tunable ion transport. Functionalized nanopores that permit modulation of the pore wall charges are a compelling approach to gain control over the ion transport mechanism through the pores. This makes way for employing a noncovalent supramolecular approach for attaining charge reversal of the MOF pore walls using donor-acceptor pairs that can demonstrate strong charge transfer interactions. Herein, robust Zr4+-based mesoporous MOF-808 was postsynthetically modified into an anion-selective nanochannel (MOF-808-MV) by modification with dicationic viologen-based motifs. Charge modulation and even reversal of the MOF-808-MV pore walls were then explored taking advantage of strong charge transfer interactions between the grafted dicationic viologen acceptor moieties and anionic, π-electron-rich donor guest molecules such as pyranine (PYR) and tetrathiafulvalene tetrabenzoic acid (TTF-TA). Tunability of the MOF pore charge from positive to neutral to negative was achieved via simple methodologies such as diffusion control in case of guest molecule like PYR and by pH modulation for pH-responsive guest like TTF-TA. This results in a concomitant modulation in the selectivity of the nanochannel, rendering it from anion-selective to ambipolar to cation-selective. Furthermore, as a real-time application of this ion channel, Na+ ion conductivity (σ = 3.5 × 10-5 S cm-1) was studied at ambient temperature.

Review on Uric Acid Recognition by MOFs with a Future in Machine Learning

Uric acid (UA) is produced from purine metabolism and serves as a prevalent biomarker for multiple diseases including cancer. Hyperuricemia or hypouricemia can cause multiple dysfunctions throughout the biological processes. Consequently, there is a pressing need for monitoring UA concentration in body fluid. While clinical methods are known, the availability of a point-of-care testing (PoCT) kit remains conspicuously absent. In the case of electrochemical recognition of UA, the oxidation potential of ascorbic acid closely aligns with that of UA and thus it hinders the detection process, which eventually may result in false positive signals. Several chemosensors are known in the field of supramolecular chemistry, and metal-organic frameworks (MOFs) are one of the best-performing contenders due to their robustness, stability, and versatile structures. In this review, we tried to unbox the up-to-date development of UA sensing by MOFs. We delve into the state of UA recognition by MOFs, exploring both electrochemical and fluorometric pathways and drawing comparisons with structurally similar probes like covalent organic frameworks (COFs) to understand/establish the advantages of MOFs specifically in UA sensing. In the absence of a PoCT kit, we have provided the conceptual outlook for designing a PoCT device termed a "Urimeter" via electrochemical operation. For the first time, we have proposed different methods of how UA sensing can be tied up with artificial intelligence and machine learning (AI-ML).

Recent advances in the development of europium(III) and terbium(III)-based luminescent supramolecular metallogels

In the recent past, special attention has been paid to the development of metallogels as novel luminescent materials from rationally designed gelators with lanthanide ions, especially europium (Eu(III)) and terbium (Tb(III)) metal ions. Lanthanide (Ln(III)) based metallogels possess various useful properties with an extensive range of applications in the field of advanced materials, and electronic and bio-technologies. Lanthanide ions in coordination with appropriate sensitizer ligands can reproduce metal-based optical, redox, and electronic properties in soft gel materials. The optical properties of the luminescent Ln(III) based metallogels can be tuned over the complete visible spectrum (400-750 nm) including the generation of white light by mixing both Eu(III) and Tb(III) with the ligand in various stoichiometric ratios. Additionally, the dynamic nature of the lanthanide-ligand (Ln-N) coordination bond allows the Ln(III) based metallogels to respond to various external stimuli. Luminescent self-healing supramolecular gels using organic ligands as 'hosts' and Ln(III) ions as 'guests' are also a current topic of research interest. In this review, we discuss and summarize some selected recent examples of newly developed luminescent Eu(III) and Tb(III) based supramolecular metallogels with potential applications in the fields of optoelectronic devices, stimuli responsiveness, self-healing, luminescent films, and sensors.