Redox-responsive organometallic hydrogels for in situ metal nanoparticle synthesis

Recent applications of nanoparticles in organic transformations

A variation in the size of metal nanoparticles leads to a difference in their properties. As the size of metal nanoparticles decreases, the surface area increases which leads to an increase in the reactivity of metal nanoparticles. Metals like Au, Ag, Pd, and Pt have interesting properties when used in nanometric dimensions. They function efficiently in significant industrial processes as electrocatalysts and photocatalysts in various organic reactions. Recently, the green biosynthesis of nanoparticles has attracted the attention of researchers. With environmental pollution rising over the past few decades, metal nanoparticle catalysts could be the key to subdue the toxic effects. Being versatile, they can be used to degrade pollutants, develop solar cells, convert toxic nitroaromatic compounds, significantly reduce CO₂ emissions per unit of energy, and many more. Owing to their unique properties, nanoparticles have wide applications in biomedicine, for example, gold cages are promising agents for cancer diagnosis and therapy. Transition metal-oxide nanoparticles have been considered one of the best supercapacitor electrodes with high electrochemical performance. In this review, we have summarised fundamental concepts of metal nanoparticles over the last decade's main emphasis from 2010 to 2021. It focuses on the exceptional use of these nanocatalysts in various organic reactions. Additionally, we have also discussed the utility of these reactions and their crucial role in solving the problems of today. Through this article, we hope to provide the necessary framework needed to further advance the applications of metal nanoparticles as catalysts.

In Situ Preparation of Composite Redox-Active Micelles Bearing Pd Nanoparticles for the Reduction of 4-Nitrophenol

Owing to the redox activity of the poly(ferrocenylsilane)-based polymer, several noble metal nanoparticles can be successfully prepared. As reported herein, the in situ preparation of Pd nanoparticles was performed using a redox-active platform of poly(ferrocenylmethylethylthiocarboxylpropylsilane) (PFC) micelles. PFC/Pd nanocomposites (NCs) with Pd nanoparticles uniformly dispersed at the surface of PFC nanospheres were obtained. The morphology of PFC/Pd NCs was further confirmed via high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Taking advantage of Pd nanoparticles, the PFC/Pd NCs showed significant catalytic activity during the reduction process of 4-nitrophenol by sodium borohydride. Although PFC micelles themselves showed no catalytic activity, they promoted the catalytic behavior of Pd nanoparticles obviously by anchoring the Pd nanoparticles at their surface to avoid the aggregation and leaching of Pd nanoparticles. In all, PFC/Pd NCs exhibited great potential as a composite nanocatalyst. Moreover, the PFC micelle was found to be a desired platform for nanocatalysts.

Application of Redox-Responsive Hydrogels Based on 2,2,6,6-Tetramethyl-1-Piperidinyloxy Methacrylate and Oligo(Ethyleneglycol) Methacrylate in Controlled Release and Catalysis

Hydrogels have reached momentum due to their potential application in a variety of fields including their ability to deliver active molecules upon application of a specific chemical or physical stimulus and to act as easily recyclable catalysts in a green chemistry approach. In this paper, we demonstrate that the same redox-responsive hydrogels based on polymer networks containing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) stable nitroxide radicals and oligoethylene glycol methyl ether methacrylate (OEGMA) can be successfully used either for the electrochemically triggered release of aspirin or as catalysts for the oxidation of primary alcohols into aldehydes. For the first application, we take the opportunity of the positive charges present on the oxoammonium groups of oxidized TEMPO to encapsulate negatively charged aspirin molecules. The further electrochemical reduction of oxoammonium groups into nitroxide radicals triggers the release of aspirin molecules. For the second application, our hydrogels are swelled with benzylic alcohol and tert-butyl nitrite as co-catalyst and the temperature is raised to 50 °C to start the oxidation reaction. Interestingly enough, benzaldehyde is not miscible with our hydrogels and phase-separate on top of them allowing the easy recovery of the reaction product and the recyclability of the hydrogel catalyst.

Redox-Active Micelle-Based Reaction Platforms for In Situ Preparation of Noble Metal Nanocomposites with Photothermal Conversion Capability

Polyferrocenylsilane (PFS)-based polymers are an attractive family of organometallic polymers with unique redox-active properties. Herein, we report a novel amphiphilic redox-active PFS-based homopolymer, poly(ferrocenylmethylethylthiocarboxypropylsilane) (PFC), with a hydrophobic backbone chain and hydrophilic carboxylic acid side groups in each repeating unit. Self-assembly was induced by addition of water to a molecularly dispersed solution of PFC in DMSO. Spherical PFC micelles with controllable hydrodynamic diameters (60-180 nm) were obtained under various conditions. These PFC micelles could be readily endocytosed by A549 cells and HUVEC cells and show no significant cytotoxicity toward them at the concentration of 200 μg/mL. On this basis, Au nanoparticles (AuNPs) were prepared through in situ reduction of HAuCl₄ by PFC micelles as nanoreactors without requiring any other reductants. The PFC/Au nanocomposites (NCs) were found to exhibit significant photothermal behavior. Moreover, PFC micelles could also act as nanoreactors for other noble metals such as Ag, Pd, and Pt. By taking advantage of properties of the nanostructures and noble metal nanoparticles comprising these materials, the PFC micelles and PFC/noble metal NCs may have great potential in biomedical or catalytic applications.

ROMP Synthesis of Side-Chain Ferrocene-Containing Polyelectrolyte and Its Redox-Responsive Hydrogels Showing Dramatically Improved Swelling with β-Cyclodextrin

A new side-chain ferrocene (Fc)-containing polyelectrolyte has been synthesized by controlled ring-opening metathesis polymerization of a water-soluble Fc-containing norbornene-based quaternary ammonium salt, as well as the corresponding covalently cross-linked polyelectrolyte hydrogel. In order to provide Fc-containing supramolecular polyelectrolyte hydrogels whose swelling property is largely improved by host-guest interaction, a covalently cross-linked polyelectrolyte hydrogel is soaked into the β-CD aqueous solution to form β-CD@Fc supramolecular polyelectrolyte hydrogel, or alternatively the quaternary ammonium salt supramolecular monomer is first formed, then copolymerized with a crosslinking agent to fabricate the supramolecular hydrogel with better water absorption ability. All the Fc-containing hydrogels exhibited good redox-responsiveness with swelling-shrinking behaviors by chemically reversibly adjusting the disassembly/assembly of β-CD@Fc inclusion complexes. This is the first example of side-chain Fc-containing polycationic supramolecular hydrogels possessing swelling-shrinking properties based on the splitting/combining of β-CD and Fc units, and potential applications are expected as controlled drug delivery and actuators.

A Reductive Supramolecular Hydrogel: A Platform for Facile Fabrication of Diverse Metal-Nanoparticle-Decorated Conductive Networks with Spatiotemporal Control

Nanoparticle-hydrogel hybrid composites have a great potential for a broad range of applications, such as antibacterial materials, stimuli-responsive materials and catalysis. A novel supramolecular hydrogel system was developed using an amino acid based amphiphile containing a terminal pyrrole moiety as a gelator. The pyrrole moiety could serve as both reducing agent and monomer, and a variety of metal precursors penetrating into the hydrogel could be reduced in situ into metal nanoparticles while the pyrrole moieties preorganized in hydrogel were oxidized into polyprroles, affording metal-nanoparticle-decorated covalent conductive networks. This strategy allows the facile fabrication of diverse mono- or multimetallic nanoparticle-polymer networks from one hydrogel by a simple reaction-diffusion approach. More importantly, besides homogeneous composites, unique multisegment heterogeneous systems with spatiotemporal control were also easily accessible. Furthermore, based on the same multifunctional molecule, the fabrication of hierarchically pore-structured metal-nanoparticles-polymers as well as metal-nanoparticles-carbon structures was also realized.

Functional supramolecular gels based on pillar[n]arene macrocycles

Supramolecular gels constructed from low-molecular-weight gelators via noncovalent interactions have received increasing attention. The rapid development of stimuli-responsive supramolecular gels with attractive properties is highly desirable to meet the ever-growing demand of materials science and chemistry. The inherent reversible and dynamic nature of noncovalent interactions in supramolecular gels endows the materials with sensing, processing, and actuating functions in response to specific environmental changes and offers them great potential in flexible biomaterials and intelligent devices. In particular, pillar[n]arenes with symmetrical pillar-shaped architectures have been recognized as an emerging class of synthetic macrocycles after crown ethers, cyclodextrins, calixarenes, and cucurbiturils, and proven to be excellent candidates for the fabrication of functional supramolecular gels due to their many advantages including facile synthesis, diverse functionalization, and appealing host-guest properties. This review provides a comprehensive overview of recent progress in supramolecular gels involving pillar[n]arenes and their derivatives as synthetic macrocyclic arenes, from the viewpoints of the synthetic approach, controllable assembly, stimuli-responsiveness, and functions. Perspectives of this burgeoning field of research are also given at the end.

Self-healing and shape memory metallopolymers: state-of-the-art and future perspectives

Recent achievements and problems associated with the use of metallopolymers as self-healing and shape memory materials are presented and evaluated.

Ferrocenyl metal–organic framework hollow microspheres for in situ loading palladium nanoparticles as a heterogeneous catalyst

Zn–Fc MOF hollow microspheres were prepared for the in situ reduction of Pd²⁺ into Pd nanoparticles as a highly efficient heterogeneous catalyst.

Mechano-switchable, luminescent gels derived from salts of a long-chained, fatty-acid gelator

Stimulus-responsive molecular gel systems, based on metal salts of a luminescent gelator, 9,10-dioxooctadecanoic acid (DODA), are reported. These salts are structurally the simplest metallo-gelators of which we are aware that exhibit controllable mechano-responsive and luminescent properties. Aggregation is more favored by the metal salts than for DODA itself. However, gelation ability differs dramatically depending on the metal ion: whereas the salts with zinc(ii) and calcium(ii) are inefficient gelators, those with nickel(ii) and copper(ii) can gelate various aromatic liquids, alkanes, and long-chained alcohols. Unlike the DODA gels, no aggregation-induced shift in the positions of the emission spectra of the metal salts could be observed as the sols were transformed to their gel phases. Gels of both nickel(ii) and copper(ii) salts in benzonitrile are among the few known examples with crystalline networks and exhibiting thixotropic behavior. However, there are significant differences in their abilities to recover the initial viscoelastic properties. Structural data for the solid and gel states lead us to conclude that differences among the gelating abilities can be attributed principally to the specific nature of interactions of the salts at their head groups. They appear to control the mechanical and emissive properties of the gels as well as whether the initial aggregation of the salts in the sol phases will support the growth of 1D objects that are capable of maintaining strong contacts, leading to 3D networks and gel formation. Overall, the results provide a facile strategy for the design of luminescent materials with controllable mechano-responsiveness by modifying the metal ions within fibrillar assemblies.

Poly(ferrocenylsilane) electrolytes as a gold nanoparticle foundry: "two-in-one" redox synthesis and electrosteric stabilization, and sensing applications

Gold nanoparticles (AuNPs) coated with responsive polymers gained considerable interest due to their controllable size, good stability, and fast environmental response suitable for biological applications and sensing. Here we report on a simple and efficient method for the synthesis of stable and redox responsive AuNPs using organometallic polyelectrolytes in aqueous solutions of HAuCl₄. In the redox reaction, positively or negatively charged poly(ferrocenylsilanes) (PFS⁺/PFS^-) served as reducing agents, and also as stabilizing polymers. Due to their unique tunable electrostatic and electrosteric protection, AuNPs coated with PFS^-, (PFS⁺)@AuNPs, possess high redox sensitivity, with reversible, repetitive, sustainable color switching between the assembled (purple color) and disassembled (red color) states as evidenced by UV-Vis absorption and TEM measurements. Feasibility studies reported here indicate that the particles described can be applied as a colorimetric probe for the detection of redox molecules, e.g. vitamin C, in a controlled and facile manner.

Recent Advances in Metal-Containing Polymer Hydrogels

Metal-containing polymer hydrogels have attracted increasing interest in recent years due to their outstanding properties such as biocompatibility, recoverability, self-healing, and/or redox activity. In this short review, methods for the preparation of metal-containing polymer hydrogels are introduced and an overview of these hydrogels with various functionalities is given. It is hoped that this short update can stimulate innovative ideas to promote the research of metal-containing hydrogels in the communities.

Stimuli responsive self-healing polymers: gels, elastomers and membranes

The development of responsive polymers with self-healing properties has expanded significantly which allow for the fabrication of complex materials in a highly controllable manner, for diverse uses in biomaterials science, electronics, sensors and actuators and coating technologies.

A star-shaped amphiphilic block copolymer with dual responses: synthesis, crystallization, self-assembly, redox and LCST–UCST thermoresponsive transition

The micelles/aggregates that were self-assembled from a star-shaped copolymer presented redox-responsive behaviour and LCST–UCST thermoresponsive transition.

Application of ferrocene-resorcinarene in silver nanoparticle synthesis

An amphiphilic resorcinarene with ferrocene groups at the lower rim has been applied as both reductant and stabilizer in the synthesis of colloidal silver nanoparticles.

Polyferrocenylsilanes: synthesis, properties, and applications

This comprehensive review covers polyferrocenylsilanes (PFSs), a well-established, readily accessible class of main chain organosilicon metallopolymer. The focus is on the recent advances involving PFS homopolymers and block copolymers and the article covers the synthesis, properties, and applications of these fascinating materials.

Coumarin-based supramolecular gelator: a case of selective detection of F− and HP2O73−

Coumarin-based small molecular gelator 1 forms stable, fluorescent gel from CHCl₃–petroleum ether (1 : 1, v/v). The gel is selectively disintegrated in the presence of F⁻ and hydrogen pyrophosphate.