Stimuli-Responsive Metallogels for Synthesizing Ag Nanoparticles and Sensing Hazardous Gases

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.

Photo-responsive metal-organic gels of rigid phenylene-1,3-di-substituted angular dienes with metal halides: gel-to-gel transformations triggered by [2 + 2] polymerization

Herein, the first report on gel-to-gel transformations via [2 + 2] photopolymerization in MOGs of metal halides and rigid dienes is presented. The MOGs and their xerogels show exceptional ability to undergo [2 + 2] polymerisation upon UV irradiation. Gel-to-gel transformations are very rare as the post-modification of gelators weakens the gel and transforms it to a sol. Such transformations change the molecular assemblies into gels with altered mechanical and chemical properties. These phenomena pave the way to synthesize new MOGs with improved rigidity that cannot be synthesized otherwise.

Zn(II)-Coordination Complex(s) from Chiral Bisamides of L-Phenylalanine: Nanoscale-Based Biological Applications and Metallogelation

Three 3-pyridyl-containing small organic bisamide molecules attached with innocent L-phenylalanine (PHE) side chain as building blocks and positional isomeric toluoyl terminals (PME, MME, and OME) have rationally been designed and synthesized for developing a new series of ZnII-coordination complexes. One of the unique molecular frameworks, having two hydrogen bond-equipped monodentate metal-coordinating sites and biologically potent chiral PHE moiety, was combined with ZnII halide salts under various conditions to produce the coordination complexes (CC1-CC7), thoroughly characterized by the single-crystal X-ray diffraction (SXRD) technique. Maintaining the similar component ratios of acquired CCs in 1:1 DMSO-water produced low-molecular weight metallogels (LMWGs) of PME/MME as envisaged from a rheology- and crystal engineering-based structural rationale. A structure-property correlation from the basis of PXRD of the bulk and xerogels and SXRD data of the isolated single crystals of reaction products clearly supports the crystal engineering-based design strategy based on which the metallogels are prepared. Hand-ground nanoscale ZnCl2-based coordination complex CC1 of PME was also studied for cytotoxicity (HEK-293 cell line) and anticancer activities (B16-F10 cell line) in the MTT assay.

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.

Ag-Induced metallogel based on cyclooctatetrathiophene: structural characterization and stimuli-responsive properties

Based on saddle-shaped cyclooctathiophene (COTh) as a building block, ligands 2 and 3 were synthesized bearing 3- or 4-substituted pyridyl groups as coordination groups, which showed strong gelation abilities with AgBF4 in several solvents at room temperature. This Ag+-induced metallogel exhibited outstanding stimuli-responsive properties upon addition of halogen ions, acetonitrile or H2O.

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.

Recent advances in soft functional materials: preparation, functions and applications

Synthetic materials and biomaterials with elastic moduli lower than 10 MPa are generally considered as soft materials. Research studies on soft materials have been boosted due to their intriguing features such as light-weight, low modulus, stretchability, and a diverse range of functions including sensing, actuating, insulating and transporting. They are ideal materials for applications in smart textiles, flexible devices and wearable electronics. On the other hand, benefiting from the advances in materials science and chemistry, novel soft materials with tailored properties and functions could be prepared to fulfil the specific requirements. In this review, the current progress of soft materials, ranging from materials design, preparation and application are critically summarized based on three categories, namely gels, foams and elastomers. The chemical, physical and electrical properties and the applications are elaborated. This review aims to provide a comprehensive overview of soft materials to researchers in different disciplines.

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.

Crisscrossing coordination networks: ligand doping to control the chemomechanical properties of stimuli-responsive metallogels

Metallogels respond to external stimuli by changing their mechanical properties. To gain a fine control over this phase-shifting event, we have designed and introduced intentional structural mismatches into the otherwise tightly knit metal-organic networks. Built using biphenolate-derived multidirectional/multidentate ligands, these soft materials display markedly different rheological properties depending on the level of "ligand doping", as well as the type of metal ion serving as a key structural support. A zinc metallogel optimized through this process responds to acids, both in the gas stream and liquid phase, by a rapid gel-sol transition and visually discernible colour change.

Au-Ag Nanoflower Catalysts with Clean Surfaces for Alcohol Oxidation

Shape-controlled metal nanocrystals, such as nanowires and nanoflowers, are attractive owing to their potentially novel catalytic properties and bimetallic nanocrystals composed of two distinct metals are expected to act as highly active catalysts. However, their catalytic activities are limited because of the capping agents adsorbed on the metal surfaces, which are necessary for the preparation and dispersion of these nanocrystals in solvents. Therefore, the preparation of bimetallic shape-controlled noble metal nanocrystals with clean surfaces, devoid of almost all capping agents, are expected to have high catalytic activity. Herein, we report the preparation of bimetallic Au-Ag nanoflowers using melamine as the capping agent. The bimetallic Au-Ag nanoflowers with a clean surface were subsequently obtained by a support and extraction method. The bimetallic nanoflowers with a clean surface were then used for the aerobic oxidation of 1-phenylethyl alcohol and they exhibited high rates for the formation of acetophenone compared to Au nanoflowers and spherical nanoparticles with almost the same size and Au/Ag ratio. We also show that Au-Ag nanoflowers containing only 1 % Ag (Au₉₉ -Ag₁ NFs) exhibit the highest rate of acetophenone formation among Au-Ag nanoflowers with different Au/Ag ratios owing to an increase in the electron density of the Au atoms that act as active sites for the oxidation of 1-phenylethyl alcohol.

Ultrasensitive detection of uric acid in serum of patients with gout by a new assay based on Pt@Ag nanoflowers

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed. H₂O₂ was formed by the reaction of uricase and UA and produced the hydroxyl radical (˙OH). The system was catalyzed by Pt@Ag NFs to change the color of 3,3′,5,5′-tetramethylbenzidine (TMB) from colorless to blue, and the morphology and chemical properties of Pt@Ag NFs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Under the optimized conditions, a linear relationship between the absorbance and UA concentration was in the range of 0.5–150 μM (R² = 0.995) with a limit of detection of 0.3 μM (S/N = 3). The method can be applied to detection of UA in actual samples with satisfactory results. The proposed assay was successfully applied to the detection of UA in human serum with recoveries over 96.8%. Thus, these results imply that the UA assay provides an effective tool in fast clinical analysis of gout.

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed.

Use of a bis-1,2,3-triazole gelator for the preparation of supramolecular metallogels and stabilization of gold nanoparticles

In this work, a series of functional metallogels have been prepared using a bis-1,2,3-triazole gelator prepared using the isosteric substitution method.