Recent advances in coordination-driven polymeric gel materials: design and applications

An Encyclopedic Compendium on Chemosensing Supramolecular Metal-Organic Gels

Chemosensing, an interdisciplinary scientific domain, plays a pivotal role ranging from environmental monitoring to healthcare diagnostics and (inter)national security. Metal-organic gels (MOGs) are recognized for their stability, selectivity, and responsiveness, making them valuable for chemosensing applications. Researchers have explored the development of MOGs based on different metal ions and ligands, allowing for tailored properties and sensitivities, and have even demonstrated their applications as portable sensors such as paper-based test strips for practical use. Herein, several studies related to MOGs development and their applications in the chemosensing field via UV-visible or luminance along with electrochemical approach are presented. These papers explored MOGs as versatile materials with their use in sensing bio or environmental analytes. This review provides a foundational understanding of key concepts, methodologies, and recent advancements in this field, fostering the scientific community.

Regulating Gelation and Luminescence Behaviors of Single Pyridine-Functionalized Cyanostilbene via Metal Ions

Luminescent metal-organic gels (LMOGs) have gained much attention due to their crucial role in visual recognition and information encryption. However, it is still a challenge to simplify the design of ligands and enrich the stimuli responses in LMOGs simultaneously. Herein, although a single pyridine ligand cannot form gel alone, after coordination with metal ions, two kinds of LMOGs have been obtained with pyridine-metal complexes, where metal ions can act as cogelators and regulate luminescence of the pyridine-functionalized cyanostilbene ligand at the same time. The effects of metal types on the fluorescence emission color, the fluorescence quantum yield, the fibril network, and the assembly mode of the gel have been investigated systematically. In addition, two competitive ligands were used to regulate the fluorescence and phase transition of the gel. Finally, the logic gates and the information encryption and decryption have been successfully constructed. This kind of material is expected to be applied to fluorescence display, advanced information encryption, high-tech anticounterfeit, and so forth.

Bimetallic Organic Gel for Effective Methyl Orange Dye Adsorption

A bimetallic organic gel (MOG-Fe/Al) was synthesized through the solvothermal method. The gel state of the product obtained under optimized gel formation conditions is sufficient to carry 2 g of weight for a long time. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Brunauer-Emmett-Teller (BET) technique, and X-ray photoelectron spectroscopy (XPS) analysis confirmed the structures and morphologies of the synthesized materials. MOG-Fe/Al, with good stability, excellent durability, and wide applicability, exhibited efficient MO adsorption capacity as high as 335.88 mg/g at 25 °C. Adsorption-influencing factors including solution pH, contact time, and temperature were investigated. The adsorption performance of the bimetallic organic gel was better than that of the monometallic organic gels (MOG-Fe and MOG-Al), and its adsorption processes were in accordance with the pseudo-second-order kinetic and Langmuir isothermal models. The excellent adsorption capacity of the MOG-Fe/Al is due to its surface structure, pore volume, π-π interactions, hydrogen bonds, and electrostatic interactions.

Pore-Networked Soft Materials Based on Metal-Organic Polyhedra

ConspectusThe last two decades have witnessed a tremendous development of crystalline microporous adsorbents in a wide range of applications including molecular adsorption, storage and separation, purification, as well as catalysis. The main players as porous materials that have contributed to the developments are extended molecular frameworks (e.g., metal-organic frameworks, MOFs; covalent-organic frameworks, COFs) or discrete porous molecules (e.g., metal-organic cages, MOCs; porous organic cages, POCs) thanks to the high degrees of freedom in their structural designability and tunability. To overcome the processability issue originating from their powder forms after synthesis, one main strategy is to hybridize the microporous adsorbents as pore-containing fillers with solvents or polymers as processable matrices to produce porous soft materials, such as porous liquids, gels/aerogels, and mixed-matrix membranes, depending on the form of matrix used. Nevertheless, the fabrication of "ideal" hybrid materials relies on the homogeneous distribution of the pore-containing fillers within the matrices. It is still challenging to find a versatile way to solve the aggregation issues of fillers and their insufficient interaction with the matrices, which are concerned with inhibiting the translation of the distinctive properties of microporous adsorbents into the obtained hybrid soft materials.Herein, we describe a new bottom-up approach for the fabrication of "pore-networked soft materials" based on the concept of directly interconnecting the pore-containing fillers into a continuous pore network within the matrices. The advantages of the pore-networking strategy lie in two main aspects: (i) the elimination of the need to struggle with the aggregation issue of fillers due to their overall interconnection throughout the matrices; (ii) the generation of continuous pore networks that guarantee the efficient molecular mass transfer in the materials. In this Account, we summarize our state-of-the-art progress of pore-networked soft materials based on the use of MOCs, alternatively called metal-organic polyhedra (MOPs) herein, as pore units for the pore network construction. The good solubility of MOPs in organic solvents allows them to be feasibly processed in solution, wherein the coordination of MOPs with organic linkers leads to the formation of linked MOP gels featuring not only intrinsic MOP cavities but also tunable extrinsic porosities generated between linked MOPs through the control of MOP/linker structures and network connectivity. Furthermore, the matrix of the linked MOP network, here referred to as the continuous phase with respect to the entire porous MOP network, is not limited to the solvents. We anticipate that the implementation of air, liquids, and polymers as the matrices could result in different forms of pore-networked soft materials like aerogels, foams, gels, monoliths, and membranes. For instance, we demonstrate the fabrication of linked MOP aerogel and permanently porous gel with their potential applications on selective CO2 photoreduction and gas sorption, respectively. We believe that the pore-network strategies will advance the development of porous soft materials featuring unique advantages and properties beyond the current hybrid systems.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Calix[4]arene-based Supramolecular Gels for Mercury Ion Removal in Water

A calix[4]arene-based gelator 1, with lower-rim mono triazolylpyridine group, capable of spontaneous self-assembly into microspheres in different ethanol/H2 O mixtures, is synthesized. The concentration-dependent 1 H NMR spectra and X-ray single-crystal structure of 1 provided evidence for self-assembly of gelator 1 via cooperative interactions of intermolecular noncovalent forces. Furthermore, metallogels by self-assembly of 1 was found to exhibit remarkable selectivity toward Hg2+ ions. 1 H NMR spectra support that Hg2+ ion was bound to the nitrogen atoms of two coordination sites of 1, which composed of triazole and pyridine. Moreover, the results of field emission scanning electron microscopy and rheology experiments indicated that Hg2+ ions not only enhanced the gelling ability of gelator 1 in ethanol but also led to morphological change of its self-assembly through metal-ligand interactions. Finally, the in situ gelation, triggered by mixing a gelator solution of 1 in ethanol with water samples such as deionized (DI), tap, and lake water, leads to the effective removal of Hg(II) from a water sample which reduced from 400 to 1.6 ppm.

Biomimetic Approach toward Visible Light-Driven Hydrogen Generation Based on a Porphyrin-Based Coordination Polymer Gel

There has been a widespread interest in developing self-assembled porphyrin nanostructures to mimic nature's light-harvesting processes. Herein, porphyrin-based coordination polymer gel (CPG) has been developed as a "soft" photocatalyst material for hydrogen (H₂) production from water under visible light. The CPG offers a hierarchical nanofibrous network structure obtained through self-assembly of a terpyridine alkyl-amide appended porphyrin (TPY-POR)-based low molecular weight gelator with ruthenium ions (Ru^II) and produces H₂ with a rate of 5.7 mmol g^-1 h^-1 in the presence of triethylamine (TEA) as a sacrificial electron donor. Further, the [Fe₂(bdt)(CO)₆] (dbt = 1,2-benzenedithiol) cocatalyst, which can mimic the activity of iron hydrogenase, is coassembled in the CPG and shows remarkable improvement in H₂ evolution (catalytic activity; rate ∼10.6 mmol g^-1 h^-1 and turnover number ∼1287). The significant enhancement in catalytic activity was supported by several controlled experiments, including femtosecond transient absorption (TA) spectroscopy and also DFT calculation. The TA study supported the cascade electron transfer process from porphyrin core to [Ru(TPY)₂]²⁺ center, and subsequently, the electron transfers to the cocatalyst [Fe₂(bdt)(CO)₆] for H₂ production.

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.

Recent Advances in Stimuli-Responsive Metallogels

Recently, stimuli-responsive supramolecular gels have received significant attention because their properties can be modulated through external stimuli such as heat, light, electricity, magnetic fields, mechanical stress, pH, ions, chemicals and enzymes. Among these gels, stimuli-responsive supramolecular metallogels have shown promising applications in material science because of their fascinating redox, optical, electronic and magnetic properties. In this review, research progress on stimuli-responsive supramolecular metallogels in recent years is systematically summarized. According to external stimulus sources, stimuli-responsive supramolecular metallogels, including chemical, physical and multiple stimuli-responsive metallogels, are discussed separately. Moreover, challenges, suggestions and opportunities regarding the development of novel stimuli-responsive metallogels are presented. We believe the knowledge and inspiration gained from this review will deepen the current understanding of stimuli-responsive smart metallogels and encourage more scientists to provide valuable contributions to this topic in the coming decades.

Mechanoresponsive Metal-Organic Cage-Crosslinked Polymer Hydrogels

We report the formation of metal-organic cage-crosslinked polymer hydrogels. To enable crosslinking of the cages and subsequent network formation, we used homodifunctionalized poly(ethylene glycol) (PEG) chains terminally substituted with bipyridines as ligands for the Pd₆ L₄ corners. The encapsulation of guest molecules into supramolecular self-assembled metal-organic cage-crosslinked hydrogels, as well as ultrasound-induced disassembly of the cages with release of their cargo, is presented in addition to their characterization by nuclear magnetic resonance (NMR) techniques, rheology, and comprehensive small-angle X-ray scattering (SAXS) experiments. The constrained geometries simulating external force (CoGEF) method and barriers using a force-modified potential energy surface (FMPES) suggest that the cage-opening mechanism starts with the dissociation of one pyridine ligand at around 0.5 nN. We show the efficient sonochemical activation of the hydrogels HG_{3 -6} , increasing the non-covalent guest-loading of completely unmodified drugs available for release by a factor of ten in comparison to non-crosslinked, star-shaped assemblies in solution.

A Wide Bandgap Semiconducting Magnesium Hydrogel: Moisture Harvest, Iodine Sequestration, and Resistive Switching

Water harvesting from the ubiquitous moisture is pivotal for delivering fresh water to earth's arid/semiarid regions, and sequestration of iodine from the solution is crucial for environmental safety due to its severe effect on human metabolic processes. In this context, herein, a multifunctional supramolecular metallohydrogel (Mg@TAEA) is synthesized through direct mixing of magnesium nitrate hexahydrate and the low molecular weight gelator tris(2-aminoethyl)amine. Electron microscopy reveals that Mg@TAEA is sculptured in vertically grown well-oriented micrometer-sized flakes. The porous crystalline material (52 m²/g) was found to be an efficacious host matrix for water harvesting from moisture (847 mg/g). Mg@TAEA shows effective (513 mg/g) iodine sequestration from solution and adsorption of carbon dioxide (15 mg/g). The wide bandgap semiconducting Mg@TAEA (3.6 eV) material is a potential candidate for building memory devices, and the I_on/I_off ratio of the device based on the indium tin oxide (ITO)/Mg@TAEA/Ag heterostructure was found to be ∼62. We further extended our work by analyzing the charge transport properties of the system and found space charge limited conduction (SCLC) and trap-filled SCLC to be responsible for the nonlinear transport behavior observed in the device.

Control of Extrinsic Porosities in Linked Metal-Organic Polyhedra Gels by Imparting Coordination-Driven Self-Assembly with Electrostatic Repulsion

The linkage of metal-organic polyhedra (MOPs) to synthesize porous soft materials is one of the promising strategies to combine processability with permanent porosity. Compared to the defined internal cavity of MOPs, it is still difficult to control the extrinsic porosities generated between crosslinked MOPs because of their random arrangements in the networks. Herein, we report a method to form linked MOP gels with controllable extrinsic porosities by introducing negative charges on the surface of MOPs that facilitates electrostatic repulsion between them. A hydrophilic rhodium-based cuboctahedral MOP (OHRhMOP) with 24 hydroxyl groups on its outer periphery can be controllably deprotonated to impart the MOP with tunable electrostatic repulsion in solution. This electrostatic repulsion between MOPs stabilizes the kinetically trapped state, in which an MOP is coordinated with various bisimidazole linkers in a monodentate fashion at a controllable linker/MOP ratio. Heating of the kinetically trapped molecules leads to the formation of gels with similar colloidal networks but different extrinsic porosities. This strategy allows us to design the molecular-level networks and the resulting porosities even in the amorphous state.

Visible-Light-Driven Photocatalytic CO2 Reduction to CO/CH4 Using a Metal-Organic "Soft" Coordination Polymer Gel

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with Ru^II ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO₂ photoreduction to CO (3.5 mmol g^-1  h^-1 ) with >99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e^- /8H⁺ photoreduction of CO₂ to CH₄ is realized with >95 % selectivity (6.7 mmol g^-1  h^-1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY)₂ ]²⁺ acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO₂ reduction to CO and CH₄ was outlined.

Innovative Microstructural Transformation upon CO2 Supercritical Conditions on Metal-Nucleobase Aerogel and Its Use as Effective Filler for HPLC Biomolecules Separation

This work contributes to enlightening the opportunities of the anisotropic scheme of non-covalent interactions present in supramolecular materials. It provides a top-down approach based on their selective disruption that herein has been employed to process a conventional microcrystalline material to a nanofibrillar porous material. The developed bulk microcrystalline material contains uracil-1-propionic acid (UPrOH) nucleobase as a molecular recognition capable building block. Its crystal structure consists of discrete [Cu(UPrO)₂ (4,4′-bipy)₂ (H₂ O)] (4,4′-bipy=4,4′-bipyridine) entities held together through a highly anisotropic scheme of non-covalent interactions in which strong hydrogen bonds involving coordinated water molecules provide 1D supramolecular chains interacting between them by weaker interactions. The sonication of this microcrystalline material and heating at 45 °C in acetic acid–methanol allows partial reversible solubilization/recrystallization processes that promote the cross-linking of particles into an interlocked platelet-like micro-particles metal–organic gel, but during CO₂ supercritical drying, the microcrystalline particles undergo a complete morphological change towards highly anisotropic nanofibers. This unprecedented top-down microstructural conversion provides a nanofibrillar material bearing the same crystal structure but with a highly increased surface area. Its usefulness has been tested for HPLC separation purposes observing the expected nucleobase complementarity-based separation.

Advances and Novel Perspectives on Colloids, Hydrogels, and Aerogels Based on Coordination Bonds with Biological Interest Ligands

This perspective article shows new advances in the synthesis of colloids, gels, and aerogels generated by combining metal ions and ligands of biological interest, such as nucleobases, nucleotides, peptides, or amino acids, among other derivatives. The characteristic dynamism of coordination bonds between metal center and biocompatible-type ligands, together with molecular recognition capability of these ligands, are crucial to form colloids and gels. These supramolecular structures are generated by forming weak van der Waals bonds such as hydrogen bonds or π–π stacking between the aromatic rings. Most gels are made up of nano-sized fibrillar networks, although their morphologies can be tuned depending on the synthetic conditions. These new materials respond to different stimuli such as pH, stirring, pressure, temperature, the presence of solvents, among others. For these reasons, they can trap and release molecules or metal ions in a controlled way allowing their application in drug delivery as antimicrobial and self-healable materials or sensors. In addition, the correct selection of the metal ion enables to build catalytic or luminescent metal–organic gels. Even recently, the use of these colloids as 3D-dimensional printable inks has been published. The elimination of the solvent trapped in the gels allows the transformation of these into metal–organic aerogels (MOAs) and metal–organic xerogels (MOXs), increasing the number of possible applications by generating new porous materials and composites useful in adsorption, conversion, and energy storage. The examples shown in this work allow us to visualize the current interest in this new type of material and their perspectives in the short-medium term. Furthermore, these investigations show that there is still a lot of work to be done, opening the door to new and interesting applications.

Metal Cation Triggered Peptide Hydrogels and Their Application in Food Freshness Monitoring and Dye Adsorption

Metal-ligand interactions have emerged as an important tool to trigger and modulate self-assembly, and to tune the properties of the final supramolecular materials. Herein, we report the metal-cation induced self-assembly of a pyrene-peptide conjugate to form hydrogels. The peptide has been rationally designed to favor the formation of β-sheet 1D assemblies and metal coordination through the Glu side chains. We studied in detail the self-assembly process in the presence of H+, Li+, Na+, K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Co2+, Fe3+, and Cr3+ and found that the morphology and mechanical properties of the hydrogels are ion-dependent. Moreover, thanks to the presence of the metal, new applications could be explored. Cu2+ metallogels could be used for amine sensing and meat freshness monitoring, while Zn2+ metallogels showed good selectivity for cationic dye adsorption and separation.

Interpenetrating polysaccharide-based hydrogel: A dynamically responsive versatile medium for precisely controlled synthesis of nanometals

Herein, we reported an interpenetrating polysaccharide-based hydrogel in which carboxymethyl chitosan (CMC) chains were physically dispersed throughout the thermoplastic elastomer gel network has been developed as a versatile platform for precisely controlled synthesis of nanometals. Results indicated the interpenetrated CMC chains could serve as multifunctional fillers for metal ions adsorption and stabilization while the thermally reconfigurable agarose (Agar) gel medium provides three-dimensional semi-solid framework for entrapping and recollecting of the fabricated nanometals. Specifically, the CMC chains were found to strongly coordinate with silver ions as a dynamically responsive metal-biopolymer complex within the bulk gel network as confirmed by the enhanced mechanical properties and regulated shape memory performances. Moreover, by varying CMC concentrations and coupling with a layer-stacking approach, multiple biochemical gradients could be facilely generated for in-situ synthesis of silver nanoparticles, achieving a narrow size of ~7 nm, confined sphere-shape and high concentrations. The monodispersed nanometals are confirmed to be highly active (e.g., considerable catalytic performance), and which could be easily recycled from the bulk gel system via a heating treatment. Thus, this work would provide a generic methodology for the multifunctional metallogel assembly and great possibility for controllable and largescale synthesis of noble nanometals toward biomedical applications.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Light-induced assembly and disassembly of polymers with Pd n L2n -type network junctions

Polymers containing Pd n L2n complexes as network junctions were obtained by reaction of poly(ethylene glycol)-linked N-donor ligands with Pd2+. The addition of a metastable state photoacid renders the networks light sensitive, and gel-sol transitions can be achieved by irradiation with light. The inverse process, a light-induced sol-gel transition, was realized by using a molecularly defined Pd complex as an acid-sensitive reservoir for Pd2+. Upon irradiation, Pd2+ ions are released, allowing the formation of an acid-resistant polymer network. Both the gel-sol and the sol-gel transitions are reversed in the dark.

Spatiotemporal Control of Supramolecular Polymerization and Gelation of Metal-Organic Polyhedra

In coordination-based supramolecular materials such as metallogels, simultaneous temporal and spatial control of their assembly remains challenging. Here, we demonstrate that the combination of light with acids as stimuli allows for the spatiotemporal control over the architectures, mechanical properties, and shape of porous soft materials based on metal-organic polyhedra (MOPs). First, we show that the formation of a colloidal gel network from a preformed kinetically trapped MOP solution can be triggered upon addition of trifluoroacetic acid (TFA) and that acid concentration determines the reaction kinetics. As determined by time-resolved dynamic light scattering, UV-vis absorption, and ¹H NMR spectroscopies and rheology measurements, the consequences of the increase in acid concentration are (i) an increase in the cross-linking between MOPs; (ii) a growth in the size of the colloidal particles forming the gel network; (iii) an increase in the density of the colloidal network; and (iv) a decrease in the ductility and stiffness of the resulting gel. We then demonstrate that irradiation of a dispersed photoacid generator, pyranine, allows the spatiotemporal control of the gel formation by locally triggering the self-assembly process. Using this methodology, we show that the gel can be patterned into a desired shape. Such precise positioning of the assembled structures, combined with the stable and permanent porosity of MOPs, could allow their integration into devices for applications such as sensing, separation, catalysis, or drug release.

Photo-modulated wide-spectrum chromism in Eu3+ and Eu3+/Tb3+ photochromic coordination polymer gels: application in decoding secret information

Photo-switching emission of photochromic materials has paramount importance in the field of optoelectronics. Here, we report synthesis and characterization of a dithienylethene (DTE) based photochromic low molecular weight gelator (LMWG) and self-assembly with lanthanide (Eu3+ and Tb3+) ions to form a photochromic coordination polymer gel (pcCPG). Based on DTE ring opening and closing, the TPY-DTE gel shuttles from pale-yellow coloured TPY-DTE-O to dark blue coloured TPY-DTE-C and vice versa upon irradiating with UV and visible light, respectively, and both the photoisomers show distinct optical properties. Furthermore, integration of Eu3+ and Tb3+ lanthanides with TPY-DTE resulted in red and green emissive Eu-pcCPG (Q.Y. = 18.7% for the open state) and Tb-pcCPG (Q.Y. = 23.4% for the open state), respectively. The photoisomers of Eu-pcCPG exhibit photo-switchable spherical to fibrous reversible morphology transformation. Importantly, an excellent spectral overlap of the Eu3+ centred emission and absorption of DTE in the closed form offered photo-switchable emission properties in Eu-pcCPG based on pcFRET (energy transfer efficiency >94%). Further, owing to the high processability and photo-switchable emission, the Eu-pcCPG has been utilized as invisible security ink for protecting confidential information. Interestingly, mixed Eu3+/Tb3+ pcCPG exhibited photo-modulated multi-spectrum chromism reversibly where the colour changes from yellow, blue, and red to green and vice versa under suitable light irradiation.

Zirconium-based metal-organic framework gels for selective luminescence sensing

Metal–organic gelation represents a promising approach to fabricate functional nanomaterials. Herein a series of Zr-carboxylate gels are synthesized from rigid pyrene, porphyrin and tetraphenyl ethylene-derived tetracarboxylate linkers, namely Zr-TBAPy (H₄TBAPy = 1,3,6,8-tetrakis(4-carboxylphenyl)pyrene), Zr-TCPE (H₄TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene), and Zr-TCPP (H₄TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin). The gels are aggregated from metal–organic framework (MOF) nanoparticles. Zr-TBAPy gel consists of NU-901 nanoparticles, and Zr-TCPP gel consists of PCN-224 nanoparticles. The xerogels show high surface areas up to 1203 m² g⁻¹. MOF gel films are also anchored on the butterfly wing template to yield Zr-MOF/B composites. Zr-TBAPy and Zr-TCPE gels are luminescent for solution-phase sensing and vapour-phase sensing of volatile organic compounds, and exhibit a significant luminescence quenching effect for electron-deficient analytes. Arising from the high porosity and good dispersion of luminescent MOF gels, rapid and effective vapour-sensing of nitrobenzene and 2-nitrotoluene within 30 s has been achieved via Zr-TBAPy film or Zr-TBAPy/B.

Zr-based MOF nanomaterials are developed via a metal–organic gelation method for rapid and effective luminescence vapour-sensing.

State of the art methods and challenges of luminescent metal–organic frameworks for antibiotic detection

This review focuses on recent developments in the design and synthesis of luminescence MOFs for monitoring antibiotics.