Metal ion and anion-based "tuning" of a supramolecular metallogel

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.

Novel supramolecular luminescent metallogels containing Tb(iii) and Eu(iii) ions with benzene-1,3,5-tricarboxylic acid gelator: advancing semiconductor applications in microelectronic devices

A novel strategy was employed to create supramolecular metallogels incorporating Tb(iii) and Eu(iii) ions using benzene-1,3,5-tricarboxylic acid (TA) as a gelator in N,N-dimethylformamide (DMF). Rheological analysis demonstrated their mechanical robustness under varying stress levels and angular frequencies. FESEM imaging revealed a flake-like hierarchical network for Tb-TA and a rod-shaped architecture for Eu-TA. EDX analysis confirmed essential chemical constituents within the metallogels. FT-IR, PXRD, Raman spectroscopy, and thermogravimetric analysis assessed their gelation process and material properties, showing semiconducting characteristics, validated by optical band-gap measurements. Metal-semiconductor junction-based devices integrating Al metal with Tb(iii)- and Eu(iii)-metallogels exhibited non-linear charge transport akin to a Schottky diode, indicating potential for advanced electronic device development. Direct utilization of benzene-1,3,5-tricarboxylic acid and Tb(iii)/Eu(iii) sources underscores their suitability as semiconducting materials for device fabrication. This study explores the versatile applications of Tb-TA and Eu-TA metallogels, offering insights for material science researchers.

Scrolling in Supramolecular Gels: A Designer's Guide

Gelation by small molecules is a topic of enormous importance in catalysis, nanomaterials, drug delivery, and pharmaceutical crystallization. The mechanism by which gelators self-organize into a fibrous gel network is poorly understood. Herein, we describe the crystal structures and gelation properties of a library of bis(urea) compounds and show, via molecular dynamics simulations, how gelator aggregation progresses from a continuous pattern of supramolecular motifs to a homogeneous fiber network. Our model suggests that lamellae with asymmetric surfaces scroll into uniform unbranched fibrils, while sheets with symmetric surfaces undergo stacking to form crystals. The self-assembly of asymmetric lamellae is associated with specific molecular features, such as the presence of narrow and flexible end groups with high packing densities, and likely represents a general mechanism for the formation of small-molecule gels.

Anticounterfeiting Feature of a Writable and Self-Erasable Ni(II)-Metallogel Pad via Fluorescent "Turn-On" Detection of Cyanide

A Schiff base 5-(2-hydroxy-3-methoxybenzylidieneamino)-1-H-imidazole-4-carboxamide (HL) comprising multibinding sites has been synthesized with the aim of fabricating a supramolecular gel. The gelator HL was characterized by FT-IR, 1H & 13C NMR, and ESI-MS techniques and also formed a [Ni(L)2] complex. The gelation property of HL was investigated with various metal ions, wherein Ni(II) selectively forms a mechanically and thermally stable supramolecular metallogel (MG) in the presence of a triethylamine base in DMF-MeOH media. Characterization of MG was accomplished with different spectro-analytical techniques such as FT-IR, ESI-MS, powder-XRD, SEM, rheological investigations, UV/vis, and fluorescence. The gelator HL displays moderate emission upon addition of Ni2+ and gives "turn-off" fluorescence output by forming the complex [Ni(L)2] (MG) due to the chelation-enhanced quenching of fluorescence (CHEQ). Job plot and ESI-MS data suggested a 2:1 stoichiometry between HL and Ni(II) in MG. Further, MG exhibited highly selective and ultrasensitive "turn-on" fluorescence signaling with CN- in the background presence of several cations and anions. The limit of detection (LoD) of MG was determined to be 6.9 × 10-9 M for CN- using the fluorescence technique. Notably, MG behaves as a fluorescent writable pad material explicitly with CN- under 365 nm UV light but not under ordinary light and the fluorescent text is self-erased after 15 min. Hence, MG can be used as a metallogel pad in the presence of CN- to communicate secret messages. Overall, the present work explores the fabrication of a thermo- and mechanostable Ni(II)-metallogel (MG), which selectively and ultrasensitively detects CN- both in the solution phase and in the gel form, wherein MG behaves as a writable and self-erasable pad with anticounterfeiting features for practical applications.

Rapid Semiconducting Supramolecular Mg(II)-Metallohydrogel: Exploring Its Potential in Nonvolatile Resistive Switching Applications and Antiseptic Wound Healing Properties

An effective strategy was employed for the rapid development of a supramolecular metallohydrogel of Mg(II) ion (i.e., Mg@PEHA) using pentaethylenehexamine (PEHA) as a low-molecular-weight gelator in aqueous medium under ambient conditions. The mechanical stability of the synthesized Mg@PEHA metallohydrogel was characterized by using rheological analysis, which showed its robustness across different angular frequencies and oscillator stress levels. The metallohydrogel exhibited excellent thixotropic behavior, which signifies that Mg@PEHA has a self-healing nature. Field emission scanning electron microscopy and transmission electron microscopy images were utilized to explore the rectangular pebble-like hierarchical network of the Mg@PEHA metallohydrogel. Elemental mapping through energy-dispersive X-ray spectroscopy analysis confirmed the presence of primary chemical constituents in the metallohydrogel. Fourier transform infrared spectroscopy spectroscopy provided insights into the possible formation strategy of the metallohydrogel. In this work, Schottky diode structures in a metal-semiconductor-metal geometry based on a magnesium(II) metallohydrogel (Mg@PEHA) were constructed, and the charge transport behavior was observed. Additionally, a resistive random access memory (RRAM) device was developed using Mg@PEHA, which displayed bipolar resistive switching behavior at room temperature. The researchers investigated the switching mechanism, which involved the formation or rupture of conduction filaments, to gain insights into the resistive switching process. The RRAM device demonstrated excellent performance with a high ON/OFF ratio of approximately 100 and remarkable endurance of over 5000 switching cycles. RRAM devices exhibit good endurance, meaning they can endure a large number of read and write cycles without significant degradation in performance. RRAM devices have shown promising reliability in terms of long-term performance and stability, making them suitable for critical applications that require reliable memory solutions. Significant inhibitory activity against the drug-resistant Klebsiella pneumonia strain and its biofilm formation ability was demonstrated by Mg@PEHA. The minimum inhibitory concentration value of the metallohydrogel was determined to be 3 mg/mL when it was dissolved in 1% DMSO. To study the antibiofilm activity, an MTT assay was performed, revealing that biofilm inhibition (60%) commenced at 1 mg/mL of Mg@PEHA when dissolved in 1% DMSO. Moreover, in the mouse excisional wound model, Mg@PEHA played a crucial role in preventing postoperative wound infections and promoting wound healing.

Exploration of a wide bandgap semiconducting supramolecular Mg(II)-metallohydrogel derived from an aliphatic amine: a robust resistive switching framework for brain-inspired computing

A rapid metallohydrogelation strategy has been developed of magnesium(II)-ion using trimethylamine as a low molecular weight gelator in water medium at room temperature. The mechanical property of the synthesized metallohydrogel material is established through the rheological analysis. The nano-rose like morphological patterns of Mg(II)-metallohydrogel are characterized through field emission scanning electron microscopic study. The energy dispersive X-ray elemental mapping analysis confirms the primary gel forming elements of Mg(II)-metallohydrogel. The possible metallohydrogel formation strategy has been analyzed through FT-IR spectroscopic study. In this work, magnesium(II) metallohydrogel (Mg@TMA) based metal-semiconductor-metal structures have been developed and charge transport behaviour is studied. Here, it is confirmed that the magnesium(II) metallohydrogel (Mg@TMA) based resistive random access memory (RRAM) device is showing bipolar resistive switching behaviour at room temperature. We have also explored the mechanism of resistive switching behaviour using the formation (rupture) of conductive filaments between the metal electrodes. This RRAM devices exhibit excellent switching endurance over 10,000 switching cycles with a large ON/OFF ratio (~ 100). The easy fabrication techniques, robust resistive switching behaviour and stability of the present system makes these structures preferred candidate for applications in non-volatile memory design, neuromorphic computing, flexible electronics and optoelectronics etc.

Two novel low molecular weight gelator-driven supramolecular metallogels efficient in antimicrobial activity applications

A remarkable ultrasonication technique was successfully employed to create two novel metallogels using citric acid as a low molecular weight gelator, in combination with cadmium(ii)-acetate and mercury(ii)-acetate dissolved in N,N-dimethyl formamide at room temperature and under ambient conditions. The mechanical properties of the resulting Cd(ii)- and Hg(ii)-metallogels were rigorously examined through rheological analyses, which revealed their robust mechanical stability under varying angular frequencies and shear strains. Detailed characterization of the chemical constituents within these metallogels was accomplished through EDX mapping experiments, while microstructural features were visualized using field emission scanning electron microscope (FESEM) images. Additionally, FT-IR spectroscopic analysis was employed to elucidate the metallogel formation mechanism. Significantly, the antimicrobial efficacy of these novel metallogels was assessed against a panel of bacteria, including Gram-positive strains such as Bacillus subtilis and Staphylococcus epidermidis, as well as Gram-negative species like Escherichia coli and Pseudomonas aeruginosa. The results demonstrated substantial antibacterial activity, highlighting the potential of Cd(ii) and Hg(ii)-based citric acid-mediated metallogels as effective agents against a broad spectrum of bacteria. In conclusion, this study provides a comprehensive exploration of the synthesis, characterization, and antimicrobial properties of Cd(ii) and Hg(ii)-based citric acid-mediated metallogels, shedding light on their promising applications in combating both Gram-positive and Gram-negative bacterial infections. These findings open up exciting prospects for the development of advanced materials with multifaceted industrial and biomedical uses.

A self-healable metallohydrogel for drug encapsulations and drug release

A self-healable metallohydrogel (MOG) of Mn(ii) has been prepared using a low molecular weight gelator, Na₂HL {H₃L = l-(3,5-di-tert-butyl-2-hydroxy-benzyl)amino aspartic acid}. The MOG has been characterized by MALDI TOF mass spectrometry, rheological studies, IR spectroscopy, and microscopic techniques. Non-steroidal anti-inflammatory drug (NSAID), indomethacin (IND) and anti-cancer drug gemcitabine (GEM) were encapsulated into the metallohydrogel. The GEM-loaded metallogel (MOG_GEM) shows better delivery and more adverse cytotoxicity than the drug against breast cancer cell lines MDA-MB-468 and 4T1. The anti-cancer property was evaluated with in vitro MTT cytotoxic assay, live-dead assay and cell migration assay. In vitro cytotoxicity assay against RAW 264.7 cell line with the treatment of MOG_IND shows the improved anti-inflammatory response in the case of MOG_IND compared to the drug alone.

Designer Gelators for the Crystallization of a Salt Active Pharmaceutical Ingredient-Mexiletine Hydrochloride

We report an approach to obtain drug-mimetic supramolecular gelators, which are capable of stabilizing metastable polymorphs of the pharmaceutical salt mexiletine hydrochloride, a highly polymorphic antiarrhythmic drug. Solution-phase screening led to the discovery of two new solvated solid forms of mexiletine, a type C 1,2,4-trichlorobenzene tetarto-solvate and a type D nitrobenzene solvate. Various metastable forms were crystallized within the gels under conditions which would not have been possible in solution. Despite typically crystallizing concomitantly with form 1, a pure sample of form 3 was crystallized within a gel of ethyl methyl ketone. Various type A channel solvates were crystallized from gels of toluene and ethyl acetate, in which the contents of the channels varied from those of solution-phase forms. Most strikingly, the high-temperature-stable form 2 was crystallized from a gel in 1,2-dibromoethane: the only known route to access this form at room temperature. These results exemplify the powerful stabilizing effect of drug-mimetic supramolecular gels, which can be exploited in pharmaceutical polymorph screens to access highly metastable or difficult-to-nucleate solid forms.

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.

Manipulating Assemblies in Metallosupramolecular Gels, Driven by Isomeric Ligands, Metal Coordination, and Adaptive Binary Gelator Systems

Metallosupramolecular gel (MSG) is a unique combination of metal-ligand coordination chemistry and supramolecular gel chemistry with extraordinary adaptivity and softness. Such materials find broad uses in industry, pharmaceutical and biomedical sectors, and in technology generation among many others. Pyridyl-appended bis(urea) gelator systems have been extensively studied as potential MSG-forming materials in the presence of various metal ions. The previous molecular engineering approaches depicted competitive intermolecular and intramolecular binding modes involving urea and pyridyl groups and further fine-tuned by the presence of various molecular spacers. In those studies, formation of intermolecular hydrogen bonding among urea moieties to form urea tape was found to be the key factor in one-dimensional assembly and gel formation. In the present study, we show how two isomeric pyridyl-appended bis(urea) ligands can be designed appropriately to essentially eliminate the interference of competitive factors, leaving the intermolecular urea assembly practically unaffected even in the presence of metal ions. We found that one of the two ligands (L2) and the mixed ligand (L1 + L2) assemblies formed gel in the presence and absence of various metal ions. A metal ion with a linear coordination geometry significantly strengthened the gels. Moreover, an inherently weak L1 + L2 assembly appears to be more adaptive in accommodating larger metal ions especially with nonlinear coordination geometry preferences. Small-angle neutron scattering and rheological, spectroscopic, and morphological characterizations, collectively, capture a detailed interplay among ligand assembly, metal-ligand coordination, and adaptivity, driven by the pure versus mixed ligand assemblies. The knowledge gathered from the present study would be highly beneficial in engineering the metallosupramolecular polymeric assemblies toward their functional applications.

A multistimulus-responsive self-healable supramolecular copper(ii)-metallogel derived from l-(+) tartaric acid: an efficient Schottky barrier diode

A low molecular weight gelator l-(+) tartaric acid- based self-healing supramolecular Cu(ii)-metallogel offers an electronic device of Schottky barrier diode at room temperature.

Tuning Rheological Behaviors of Supramolecular Aqueous Gels via Charge Transfer Interactions

Rheological properties are critical for determining real applications of supramolecular gels in various fields. Correspondingly, the modulation of gel rheology will be very important for meeting real requirements. In this aspect, a few strategies were applied to tune the rheological behaviors of supramolecular gels, but some specific interactions like charge transfer (CT) interactions were less explored at the molecular level. Herein, we report a pyrene-containing derivative of diphenylalanine as a donor gelator and naphthalenediimide or 3,5-dinitrobenzene as matching acceptor molecules. It was found that the viscoelastic properties and strength of the original gel could be tuned through addition of different acceptor molecules to the original gel with changing the ratios of the selected acceptor molecules. As a result, storage modulus was continuously adjusted over a wide range from 190,000 to 50,000 Pa by CT interactions. Furthermore, the mechanism of the CT-induced change in rheological properties was understood and clarified through relevant techniques (e.g., UV-Vis, fluorescence, and FT-IR spectroscopy and TEM). The findings in this work would provide a novel strategy to modulate the rheological properties of supramolecular gels for adaption to broader fields of real 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.

A thixotropic supramolecular metallogel with a 2D sheet morphology: iodine sequestration and column based dye separation

Sequestration of hazardous radioactive iodine and dye separation to reduce industrial waste through reutilization is pivotal for environmental safety. In this regard, herein, the synthesis of a new waterborne ultrasensitive supramolecular metallogel (Mg@DEOA) with a 2D sheet morphology is accomplished through direct mixing of a low molecular weight gelator diethanolamine and magnesium nitrate hexahydrate. This porous metallogel (180 m2 g-1) exhibits thixotropic properties and is injectable. The material was found to be an effective (587 mg g-1) host matrix for iodine sequestration from solution. Moreover, the Mg@DEOA xerogel was used to efficiently remove rhodamine B from a mixture of dyes with high separation factors through a xerogel packed column and as an adsorbent material for water-soluble dyes and CO. This column based application demonstrated by the metallogel could be useful for practical industrial dye-separation.

The mechanism of the excited-state proton transfer of Salicylaldehyde azine and 2,2'-[1,4-Phenylenebis{(E)- nitrilomethylidyne}] bisphenol: Via single or double proton transfer

The Salicylaldehyde azine (H₂SA) and 2,2'-[1,4-Phenylenebis{(E)-nitrilomethylidyne}] bisphenol (H₂SPA) with double proton transfer characteristics were synthesized recently (Phys. Chem. Chem. Phys., 2018, 20, 23,762). However, the detailed theoretical interpretation of proton transfer (PT) mechanism is inadequate. In the present work, density functional theory (DFT) and time-density functional theory (TDDFT) are employed to study the proton transfer mechanism of H₂SA and H₂SPA in detail. Bond parameters, infrared (IR) spectra and frontier molecular orbitals (FMOs) calculated by PBE0/TZVP method indicate the strength of hydrogen bond is enhanced in S₁ state, which can be visualized by the reduced density gradient (RDG) analysis. The potential energy surfaces (PESs) of H₂SA and H₂SPA are also constructed. The small barriers indicate that both the single proton transfer and double proton transfer of H₂SA and H₂SPA are more likely to occur in the S₁ state. In addition, the properties of H₂SA and H₂SPA after chelation with Li⁺ have also been theoretically characterized. According to the calculated fluorescence spectra of compounds (H₂SA-Li⁺ and H₂SPA-Li⁺), it was found that only the planar structure of H₂SA-Li⁺ can form metallogel, which verified the experimental results.

Metallogels: Availability, Applicability, and Advanceability

Introducing metal components into gel matrices provides an effective strategy to develop soft materials with advantageous properties such as: optical activity, conductivity, magnetic response activity, self-healing activity, catalytic activity, etc. In this context, a thorough overview of application-oriented metallogels is provided. Considering that many well-established metallogels start from serendipitous discoveries, insights into the structure-gelation relationship will offer a profound impact on the development of metallogels. Initially, design strategies for discovering new metallogels are discussed, then the advanced applications of metallogels are summarized. Finally, perspectives regarding the design of metallogels, the potential applications of metallogels and their derivative materials are briefly proposed.

Supramolecular birefringent metallogels formed by trinuclear copper(ii) complexes with myo-inositol and bipyridyl ligands

Birefringent metallohydrogels have been prepared by a fibrous supramolecular assembly resulting from the trinuclear copper(ii) complexes of myo-inositol (ins) and 2,2′-bipyridine (bipy), [Cu₃(ins)(bipy)₃]X₃, with formate, acetate and propionate as anions, X.

Heterometallic Coordination Polymer Gels Supported by 2,4,6-Tris(pyrazol-1-yl)-1,3,5-triazine

Complexes of type [M(tpt)₂]X₂ (M²⁺ = Fe²⁺, Co²⁺, Ni²⁺; tpt = 2,4,6-tri{pyrazol-1-yl}-1,3,5-triazine; X^- = BF₄ ^- or ClO₄ ^-) crystallize in a cubic lattice, with the metal ion and ligand conformation showing unusual symmetry-imposed disorder. Addition of 1 equiv AgX to the corresponding preformed [M(tpt)₂]X₂ salt in concentrated MeNO₂ solution affords thixotropic gels. Gelation was not observed in analogous reactions using [Mn(tpt)₂][ClO₄]₂, or from reactions in other, more donating solvents. Scanning electron microscopy (SEM) images from dilute solutions of the reagents confirmed the fibrous microstructure of the gels and their homogeneous elemental composition. However, energy-dispersive X-ray data show a reduced Fe/Ag ratio compared to the Co/Ag and Ni/Ag gels, where a 1:1 ratio of metals is evident. More concentrated gels decomposed to silver nanoparticles during SEM sample preparation. Mass spectrometry and ¹H NMR indicate that silver induces partial ligand displacement reactions in [Fe(tpt)₂]²⁺ and [Co(tpt)₂]²⁺, but not in [Ni(tpt)₂]²⁺. Hence, the strength of the gels, which follows the order M = Mn (no gel) < Fe < Co < Ni, correlates with the stability of octahedral [M(tpt)₂]²⁺ under gelation conditions. Iron(II) complexes of the related ligands 2,4,6-tri{pyrazol-1-yl}pyridine (tpp) and 2,4,6-tri{pyrazol-1-yl}pyrimidine (tpym) did not undergo gelation with silver salts under the above conditions. The unique properties of tpt as a gelator in this work may reflect the crystallographically observed ability of metal-coordinated tpt to chelate to exogenous silver ions, through its pendant pyrazolyl group and triazinyl N donors. In contrast, the pendant azolyl substituents in silver complexes of the nongelators tpp and tpym only bind exogenous silver in monodentate fashion.

Discotic Organic Gelators in Ion Sensing, Metallogel Formation, and Bioinspired Catalysis

Two organogelators G2 and G3 with a carboxamide group have been synthesized and characterized with different spectroscopic tools. Dimethylformamide or dimethyl sulfoxide solutions of both the compounds upon the addition of a minute quantity of water show the tendency to form gels. Supramolecular self-assembly for gel formation paves the way for aggregation-induced emission enhancement (AIEE) phenomena for both the gelator molecules. Introduction of metal ions in organogels strengthens the gel property without much affecting the fluorescence behavior. However, the introduction of Ag⁺, Fe²⁺, and Fe³⁺ ions in the G2 organogel separately results in total quenching of AIEE, making it possible to sense that particular cation in the gel state. The G3 organogel shows a similar behavior with the Fe²⁺ ion. Remarkably, other metallogels such as Ni(II)G2 and Co(II)G2 can sense sulfide ion and Cu(II)G2 can sense iodide ion by switching off the fluorescence even in multianalyte conditions. Furthermore, the copper-based metallogel Cu(II)G2 can be utilized as a catalyst and reaction medium for aerobic oxidation of catechol to quinone. To the best of our knowledge, this is the first attempt known so far to utilize a metallogel material for bioinspired catalysis such as catechol oxidation.

Toxic Metal Sequestration Exploiting a Unprecedented Low-Molecular-Weight Hydrogel-to-Metallogel Transformation

We report herein the development of a unique low-molecular-weight gelator-induced technique for environmental remediation. The motive of this work is wastewater purification using a gel-based toxic heavy metal sequestration. The essence of this technique was to bring two different functionalities, one capable of multiple coordination and another with gel-forming ability, arranged in tandem within a single ligand molecule. Naturally, the success of the approach depends on whether the two tandem-arrayed functionalities are indeed working in tandem. Our results show that the ligand molecule is an excellent example of concomitant hydrogelator and metallogelator. The most interesting aspects of this study involve the toxic metal sequestration of Pb, Cd, and Hg which was further studied in detail with spectroscopic, microscopic, and diffraction techniques. We also report here a rare property of pure organic hydrogel-to-metallogel transformation which could open up a new avenue on wastewater purification. In essence, the hydrogels can be envisaged as a unique class of metal-free zeolite analogue for environmental remediation not by just absorbance but through absorbance cum coordination, which are further corroborated by the inductively coupled plasma-optical emission spectroscopy results.

Supramolecular metallogels constructed from carboxylate gelators

Serendipity still plays a role in gel discovery as the prediction of gel formation is difficult. This work explores the role of ligand, metal salt, solvent, and temperature in the formation of a low molecular mass carboxylate iron(iii) system. The influence of each component is discussed. The gels obtained were characterised using thermal analysis, Fourier transform-infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, elemental analysis, microwave plasma-atomic emission spectroscopy, and inductively coupled plasma optical emission spectrometry. The response to external stimuli, including dye and gas sorption was examined.

Rheoreversible Metallogels Derived from Coordination Polymers

A series of mixed-ligand-based Cd^II /Co^II coordination polymers (CPs) that were derived from two bis(pyridyl)-bis(amide) ligands, 4,4'-oxybis(N-(pyridin-3-yl)benzamide) (LP) and 4,4'-oxybis(N-(pyridin-4-yl)benzamide) (LP1), and a variety of dicarboxylates isophthalates, terephthalates, 1,2-carboxytranscinamates, and 1,3- and 1,4-phenylene dicarboxylates were synthesized based on a rationale that they would occlude solvate guests inside their crystal lattice, thereby rendering these CPs suitable as metallogelators. The CPs were characterized by using single-crystal X-ray diffraction, elemental analysis, powder X-ray diffraction (PXRD), FTIR spectroscopy, and thermogravimetric analysis (TGA). Structural analyses revealed that the majority of the CPs were lattice-occluded molecular solids, which provided us with an opportunity to study their gelation behavior. We observed that, out of eight CPs that were tested, seven were able to produce metallogels. A thorough study of the rheological behavior of the metallogels was performed and CPG1, CPG2, CPG4, and CPG5 were found to exhibit rheoreversible behavior, which was further confirmed by rheological experiments. Interestingly, ligand LP was found to form an aqueous gel, which was exploited to produce silver nanoparticles.

A controllable etching supramolecular hydrogel based on metal ions

We report an interesting gel-etching system in which a gel can be etched with corresponding etching rates and color changes when different types of etching molecules are added to the gel. This gel system may have wide potential application in molecular recognition, pre-packaged, as well as other functional materials.

A mechano-responsive supramolecular metal-organic framework (supraMOF) gel material rich in ZIF-8 nanoplates

We report an unconventional ZIF-8 based supramolecular MOF (supraMOF) material, which is straightforwardly synthesised by leveraging the high concentration reaction (HCR) approach. Akin to traditional low-molecular-weight gels (LMWG), we show that mechano-stimulus responsive behaviour can be achieved through a reversible and fast sol-gel conversion mechanism. Remarkably, the supraMOF gel consists of 2D nanoplates of ZIF-8 with a reduced crystal symmetry.

Li+-Induced fluorescent metallogel: a case of ESIPT-CHEF and ICT phenomenon

A fluorescent metallogel has been synthesized from non-fluorescent ingredients viz. the smallest possible LMW aromatic symmetrical gelator 1 and LiOH, whereas KOH produces a non-fluorescent solution, and regioisomer 2 with LiOH shows an ICT assisted fluorescent precipitate rather than a metallogel.

Fluorescent Supramolecular Polymeric Materials

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly-disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π-π stacking interactions, metal-coordination, van der Waals interactions and host-guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Metal ‘turn-off’, anion ‘turn-on’ gelation cascade in pyridinylmethyl ureas

Pyridinylmethyl urea gels are weakened or destroyed by addition of transition metal ions but gelation is recovered by adding bromide giving rise to a gel–sol–gel chemically triggered cascade.

Amino acid-based amphiphilic hydrogels: metal ion induced tuning of mechanical and thermal stability

A phenylalanine based gelator was found to form a hydrogel in phosphate buffer solution. Its mechanical properties are influenced by a range of metal ions.