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

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.

A Siderophore Mimicking Gelation Component for Capturing and Self-Separation of Fe(III) from an Aqueous Solution of Mixture of Metal Ions

The carbohydrazide-based gelation component N2,N4,N6-(1,3,5-triazine-2,4,6-triyl)tris(benzene-1,3,5-tricarbohydrazide) (CBTC) was synthesized and characterized using various spectroscopic tools. CBTC and trimesic acid (TMA) get self-assembled to form metallogel with Fe3+, specifically through various noncovalent interactions in a DMSO and H2O mixture. The self-assembly shows remarkable specificity toward Fe(III) among different transition metal salts. It is pertinent to point out that the binding specificity for Fe3+ can also be found in nature in the form of siderophores, as they are mainly involved in scavenging iron selectively from the surroundings. DFT studies have been used to investigate the possible interaction between the different components of the iron metallogel. To determine the selectivity of CBTC for iron, CBTC, along with trimesic acid, is used to interact with other metal ions, including Fe(III) ions, in a single system. The gelation components CBTC and TMA selectively bind with iron(III), which leads to the formation of metallogel and gets separated as a discrete layer, leaving the other metal ions in the solution. Therefore, CBTC and TMA together show iron-scavenging properties. This selective scavenging property is explored through FE-SEM, XPS, PXRD, IR, and ICP-AES analysis. The FE-SEM analysis shows a flower-petal-like morphology for the Fe(III) metallogel. The resemblance in the CBTC-TMA-Fe metallogel and metallogel obtained from the mixture of different metal salts is established through FE-SEM images and XPS analysis. The release of iron from the metallogel is achieved with the help of ascorbic acid, which converts Fe3+ to Fe2+. In biological systems, iron also gets released similarly from siderophores. This is the first report where the synthesized gelation component CBTC molecule is capable of scavenging out iron in the form of metallogel and self-separating from the aqueous mixture in the presence of various other metal ions.

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.

Self-Assembly of a Series of Carbazole-Based Vinyl-benzoxazole Derivatives in Gel Phases

A series of carbazole-based vinyl-benzoxazole derivatives have been synthesized in order to verify whether X-ray diffraction (XRD) simulation can give more information about intermolecular stacking in the gel phase. It was found that their gelation capabilities were strongly dependent on the length of the alkyl chain. The compounds with shorter alkyl chains have lower critical gelation concentrations (CGCs) in nonpolar alkane and alcohols with longer carbon chains. On the other hand, compounds with long alkyl chains presented small CGCs in polar methanol. Powder XRD structure solution gave more information about intermolecular stacking than the traditional way of analyzing diffraction peaks to derive approximate molecular stacking patterns. The results verified that gelators had a similar head-to-tail π-stacking between aromatic groups in gel phases although different slipping angles existed. Moreover, ordered stacking between the alkyl chains was also present.

Suberic Acid-Based Supramolecular Metallogels of Ni(II), Zn(II), and Cd(II) for Anti-Pathogenic Activity and Semiconducting Diode Fabrication

The importance of three synthesized metallogels of suberic acid distinctly with nickel, zinc, and cadmium acetate salts has been uncovered. For the creation of these soft materials, N,N'-dimethyl formamide was utilized as a source of the trapped solvent. The synthesized metallogels display intriguing viscoelasticity, and the interpretation of experimental parameters obtained from rheological results advocates the gel behavior. Microstructural analysis combined with energy-dispersive X-ray confirms the occurrence of individual gel-developing constituents as observed in different hierarchical microstructural patterns. Significant variations in microstructural arrangements with diverse extent of supramolecular non-covalent patterns inside gel networks were perceived through field emission scanning electron microscopy, atomic force microscopy, and transmission electron microscopy analyses. Fourier transform infrared and electrospray ionization-mass spectral analyses and powder X-ray diffraction analysis of metallogel samples of different gel-establishing ingredients help to investigate the possible supramolecular interactions dictating the metallogel scaffolds. Thermogravimetric analysis of xerogel samples was collected from the synthesized metallogels to understand the thermal stability. These gel materials were characterized by their potential antibacterial efficiency. The potency of metallogels against selective Gram-positive and Gram-negative bacteria was visualized via a spectrophotometer. Human pathogens like Klebsiella pneumoniae (MTCC 109), Salmonella typhi (MTCC 733), Vibrio parahaemolyticus, Bacillus cereus (MTCC 1272), Lactobacillus fermentum (NCDO 955), and Staphylococcus aureus (MTCC 96) are employed in this study. Apart from the biological significance, our metallogels demonstrate as incredible diode performance of fabricated semiconducting systems, which exhibit a considerable amount of non-linearity demonstrating a non-ohmic conduction mechanism at room temperature in dark conditions. Device fabrication was achieved from these metallogels employing the sandwich model with indium tin oxide-coated glass substrates/metallogel/Al structure.

A "heat set" Zr-Diimide based Fibrous Metallogel: Multiresponsive Sensor, Column-based Dye Separation, and Iodine Sequestration

Sensing and monitoring hazardous contaminants in water and radioactive iodine sequestration is pivotal due to their detrimental impact on biological ecosystems. In this context, herein, a water stable zirconium-diimide based metallogel (Zr@MG) with fibrous columnar morphology is accomplished through the "heat set" method. The presence of diimide linkage with long aromatic chain manifests active luminescence properties in the linker as well as in the supramolecular framework structure. The as-synthesized Zr@MG xerogel can selectively detectCr₂O₇^2- (LOD = 0.52 ppm) and 2,4,6-trinitrophenol (TNP) (LOD = 80.2 ppb) in the aqueous medium. The Zr@MG paper strip-based detection for Cr₂O₇^2- and nitro explosive makes this metallogel reliable and an attractive luminescent sensor for practical use. Moreover, a column-based dye separation experiment was performed to show selective capture of positively charged methylene blue (MB) dye with 98 % separation efficiency from the mixture of two dyes. Also, the Zr@MG xerogel showed effective iodine sequestration from the vapor phase (232 wt%).

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.

Self-Assembling Oligo(2-oxazoline) Organogelators for the Encapsulation and Slow Release of Bioactive Volatiles

Herein, we report a class of distinctive supramolecular nanostructures in situ-generated from the cationic ring-opening polymerization of a particular 2-oxazoline monomer, i.e., 2-(N-tert-butyloxycarbonylaminomethyl)-2-oxazoline (Ox1). Driven by side-chain hydrogen bonding between neighboring molecules and van der Waals interactions, the growing oligomers of Ox1 precipitate in the form of macroscopic platelets when the degree of polymerization reaches 5-7. A similar self-assembly occurred in the block copolymerization of 2-ethyl-2-oxazoline (EtOx) or 2-pentyl-2-oxazoline (PeOx) and Ox1 as the second monomer. These polymeric aggregates were found to disassemble into rod-like nanoparticles under appropriate conditions, and to form stable organogels in some polar solvents like dimethylformamide as well as in natural liquid fragrances such as (R)-carvone, citronellal, and (R)-limonene. Scanning electron microscopy revealed that the morphology of their xerogels was solvent-dependent, mainly with a lamellar or fibrous structure. The rheology measurements confirmed the as-obtained organogels feature an obvious thixotropic character. The storage modulus was about 7-10 times higher than the loss modulus, indicating the physical crosslinking in the gel. The fragrance release profiles showed that the presented supramolecular gel system exhibits good sustained-release effect for the loaded bioactive volatiles.

Preparation of Tris-Tetrazole-Based Metallogels and Stabilization of Silver Nanoparticles: Studies on Reduction Catalysis and Self-Healing Property

An ionic multifunctional gelator molecule triethylammonium 5-(3,5-bis((1H-tetrazol-5-yl)carbamoyl)benzamido)tetrazol-1-ide G7 is synthesized and characterized by spectroscopic tools and mass spectrometry. G7 tends to form a stable organogel in a mixture of N,N-dimethylformamide/dimethylsulfoxide (DMF/DMSO) and water. Introduction of different metal perchlorate salts in a DMSO solution of G7 furnished a series of metallogels M¹G7, M²G7, M³G7, M⁴G7, M⁵G7, M⁶G7, and M⁷G7 [M¹ = Fe(III), M² = Co(II), M³ = Cu(II), M⁴ = Zn(II), M⁵ = Ag(I), M⁶ = Ni, and M⁷ = Fe(II)]. Among them, M¹G7, M³G7, M⁴G7, M⁶G7, and M⁷G7 help individually in the synthesis and stabilization of bimetallic nanocomposites containing silver nanoparticles (AgNPs). Iron(III)-containing nanocomposites M¹G7AgNPs have been utilized in the form of catalysts in the reduction reaction of nitroaromatic compounds to corresponding amines with a quantitative yield. The organogel G7 has also shown the abilities to absorb different metal ions from aqueous solutions and allow selective transition of M¹G7 from the gel state to the crystalline state. Fe(III) formed dual metallogels with Zn(II), which can be used for further applications. Furthermore, the nanocomposite M¹G7AgNP powder, in the presence of the organogel G7, gets converted into a nanostructured metallogel, which shows exclusive self-healing properties. This is the first example where a nanocomposite powder contains the dual-metal system (Fe(III) and Ag(0)) and shows a reduction catalytic property, and its nanostructured dual-metallogel form manifests the self-healing property in a fabricated metallogel.

Carbocycle-Based Organogelators: Influence of Chirality and Structural Features on Their Supramolecular Arrangements and Properties

The rational design and engineer of organogel-based smart materials and stimuli-responsive materials with tuned properties requires the control of the non-covalent forces driving the hierarchical self-assembly. Chirality, as well as cis/trans relative configuration, also plays a crucial role promoting the morphology and characteristics of the aggregates. Cycloalkane derivatives can provide chiral chemical platforms allowing the incorporation of functional groups and hydrophobic structural units able for a convenient molecular stacking leading to gels. Restriction of the conformational freedom imposed by the ring strain is also a contributing issue that can be modulated by the inclusion of flexible segments. In addition, donor/acceptor moieties can also be incorporated favoring the interactions with light or with charged species. This review offers a perspective on the abilities and properties of carbocycle-based organogelators starting from simple cycloalkane derivatives, which were the key to establish the basis for an effective self-assembling, to sophisticated polycyclic compounds with manifold properties and 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.

4,4′-Bipyridine-based Ni(ii)-metallogel for fabricating a photo-responsive Schottky barrier diode device

4,4′-Bipyridine-based Ni(ii)-metallogel has been implemented to execute a light-responsive semiconducting Schottky barrier diode device with advanced functionality.

Luminescent Conductive Columnar π-Gelators for Fe(II) Sensing and Bio-Imaging Applications

The high demand and scarcity of luminescent, photoconductive, and transparent gels necessitate its finding as they are potential components in photonic devices such as solar cell concentrators where optical losses via scattering and reabsorption require to be minimized. In this direction, we have reported highly transparent, blue luminescent as well as photoconductive gels exhibiting the hole mobility of 10^-3 cm²/V s at ambient temperature as investigated by the time-of-flight technique. The π-driven self-standing supergels were formed using triazole-modified phenylene-vinylene derivatives as gelators in a nonpolar solvent. Different microscopic studies revealed its entangled network of interwoven fibrilar self-assembly and anisotropic order in the gel state. Supramolecular assembly of xerogels, studied by small- and wide-angle X-ray scattering (SAXS/WAXS) suggesting their local columnar hexagonal (Col_h) superstructure, is beneficial for conducting gels. Rheological measurements direct the stiffness and robustness of the organogels. In addition, the gelators were developed as a sensing platform for the ultrasensitive detection of Fe(II) ions at ppb level. ¹H nuclear magnetic resonance (NMR) titrimetric studies revealed that the interaction of the H-atom of triazole units with Fe(II) is responsible for quenching of blue fluorescence. Also, one of the gelators was successfully applied in bio-imaging using the pollen grains of the Hibiscus rosa-sinensis plant.

Stimuli-responsive gelators from carbamoyl sugar derivatives and their responses to metal ions and tetrabutylammonium salts

Carbohydrate-based low molecular weight gelators (LMWGs) are interesting compounds with a variety of applications. In this research, a library of nineteen carbamate derivatives of N-acetyl-d-glucosamine were synthesized and characterized, and several derivatives were found to be effective LMWGs. They formed gels in pump oils as well as mixtures of water with ethanol or water with DMSO. The structures of the carbamoyl chains played an important role in the gelation properties, short chain aliphatic derivatives and phenyl carbamates formed gels in more solvents than certain aromatic and dimeric carbamates. The phenyl carbamate gelator was also selected for the encapsulation of naproxen sodium, and the drug slowly diffused from the gel to the aqueous phase as indicated by UV-vis spectroscopy. In addition, we also found that the p-methoxyl benzyl carbamate derivative showed interesting stimuli-responsive gelation properties in the presence of metal salts and tetrabutylammonium salts. The gels were characterized using optical microscopy, scanning electron microscopy, rheology and other methods. The self-assembling mechanisms of the gelators were studied using ¹H NMR spectroscopy. The preparation, characterization, and molecular assembling properties of these compounds are reported. The results obtained from this study are useful for the design of other LMWGs and the sugar derivatives can be explored for different biological applications. The formation of spontaneous ionic gels can be applicable for a plethora of applications including catalysis and environmental remediation.

Cyanide Sensing in Water Using a Copper Metallogel through "Turn-on" Fluorescence

The development of fluorescent probes for selective detection of cyanide has gained considerable attention over the past two decades due to benefits like high selectivity as well as sensitivity, fast response, visual output, accurate quantification, and a simplified sample preparation procedure. However, the propensity of supramolecular gels toward fluorescence sensing of cyanide in aqueous medium is not well explored until now. Herein, we report the design and synthesis of a novel copper based metallogel capable of sensing cyanide in water by fluorescence "turn on". Toward this, a terpyridine attached poly(aryl ether) dendrone derivative (G1) is synthesized which forms gel and exhibits Aggregation Induced Emission (AIE). The addition and diffusion of copper ions to the gel resulted in the formation of a nonluminescent copper metallogel (CuG). The copper metallogel could selectively sense cyanide in water by a fluorescence "turn-on" signal due to the regeneration of the AIE active gel. The mechanistic pathways of the sensing have been studied, and the detection limit for sensing was found to be as low as 1.09 μM. A thin film of CuG was prepared by casting the gel and used as a test strip for the visual detection of cyanide in water.

Photoresponsive Zn2+-specific metallohydrogels coassembled from imidazole containing phenylalanine and arylazopyrazole derivatives

Stimuli-responsive supramolecular gels and metallogels have been widely explored in the past decade, but the fabrication of metallogels with reversible photoresponsive properties remains largely unexplored.

The development of two fluorescent chemosensors for the selective detection of Zn2+ and Al3+ ions in a quinoline platform by tuning the substituents in the receptor part: elucidation of the structures of the metal-bound chemosensors and biological studies

Two aminoquinoline-based chemosensors (HL1 and HL2) are reported for selective detection of Zn²⁺ and Al³⁺ ions.

Novel Approach to Generate a Self-Deliverable Ru(II)-Based Anticancer Agent in the Self-Reacting Confined Gel Space

Finding the most effective method for cancer treatment is one of the thought-provoking tasks. Drug delivery by collapsing of metallogel to the cancer cell is an appealing way out. Cancer cells have an acidic environment due to excessive accumulation of lactic acid. In this work, the novel G5 gelator with a strategically free carboxylic acid arm has been designed and fabricated and characterized by several spectroscopic and microscopic techniques. These experiments suggest the formation of an ordered supramolecular gel with clover-leaf-like morphology. Mechanical properties from rheological measurements suggest the viscoelastic nature of the gel. Furthermore, we have obtained crystals of G5 from the pure dimethyl sulfoxide solution, whereas gelation gets induced by addition of water. This G5 gelator loses its gelation capability once the carboxylate is esterified by layering with methanol, which furnished the crystals of Me-G5' (G5' = G5-H). Further, the G5 gelator is used for the formation of ruthenium metallogel. Interestingly, we obtained the monomeric species [Ru(G5')(η⁶-p-cymene)Cl] [Ru(II)G5] only in confined gel space upon addition of a [Ru₂(η⁶-p-cymene)₂Cl₄] dimer to G5. The Ru(II)G5 metallogel has an inherent anticancer property with an IC₅₀ value of 10.53 μM for the A549 cancer cell line. Treatment of the Ru(II)G5 metallogel by lactic acid for mimicking the acidic environment of the malignant cell results in collapsing of the gel by releasing the ruthenium metal ion. This released ruthenium ion binds with the lactic acid derivative making the gelator G5 free and producing a new compound Ru(II)L, which has also shown the anticancer property. The molecular docking study revealed that the released G5 could interact with a monocarboxylate transporter to disrupt the lactate transport chain, which might induce apoptosis.

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.

Palladium-induced transformation of nematic liquid crystals to robust metallogel comprising self-assembled nanowires

Transformation of nematic liquid crystals to metallogels.

An amide probe as a selective Al3+ and Fe3+ sensor inside the HeLa and a549 cell lines: Pictet–Spengler reaction for the rapid detection of tryptophan amino acid

Newly synthesized amide-based probe for the selective and specific detection of (i) Al³⁺ or Fe³⁺ ions as a cation sensor and (ii) tryptophan as an amino acid sensor.