Stimuli responsive ionogels for sensing applications-an overview

Unveiling the Rational Development of Stimuli-Responsive Silk Fibroin-Based Ionogel Formulations

We present an approach for the rational development of stimuli-responsive ionogels which can be formulated for precise control of multiple unique ionogel features and fill niche pharmaceutical applications. Ionogels are captivating materials, exhibiting self-healing characteristics, tunable mechanical and structural properties, high thermal stability, and electroconductivity. However, the majority of ionogels developed require complex chemistry, exhibit high viscosity, poor biocompatibility, and low biodegradability. In our work, we overcome these limitations. We employ a facile production process and strategically integrate silk fibroin, the biocompatible ionic liquids (ILs) choline acetate ([Cho][OAc]), choline dihydrogen phosphate ([Cho][DHP]), and choline chloride ([Cho][Cl]), traditional pharmaceutical excipients, and the model antiepileptic drug phenobarbital. In the absence of ILs, we failed to observe gel formation; yet in the presence of ILs, thermoresponsive ionogels formed. Systems were assessed via visual tests, transmission electron microscopy, confocal reflection microscopy, dynamic light scattering, zeta potential and rheology measurements. We formed diverse ionogels of strengths ranging between 18 and 642 Pa. Under 25 °C storage, formulations containing polyvinylpyrrolidone (PVP) showed an ionogel formation period ranging over 14 days, increasing in the order of [Cho][DHP], [Cho][OAc], and [Cho][Cl]. Formulations lacking PVP showed an ionogel formation period ranging over 32 days, increasing in the order of [Cho][OAc], [Cho][DHP] and [Cho][Cl]. By heating from 25 to 60 °C, immediately following preparation, thermoresponsive ionogels formed below 41 °C in the absence of PVP. Based on our experimental results and density functional theory calculations, we attribute ionogel formation to macromolecular crowding and confinement effects, further enhanced upon PVP inclusion. Holistically, applying our rational development strategy enables the production of ionogels of tunable physicochemical and rheological properties, enhanced drug solubility, and structural and energetic stability. We believe our rational development approach will advance the design of biomaterials and smart platforms for diverse drug delivery applications.

Intercalated chitosan-ionic liquid ionogel in SnO nanoplate: band gap narrow and adsorption-photodegradation process

Ionogels are a category of hybrid material containing ionic liquid stabilized by polymeric network. These composites have some applications in solid-state, energy storage devices and environmental studies. In this research, chitosan (CS), ethyl pyridinium iodide ionic liquid (IL), and ionogel (IG) consisting of chitosan and ionic liquid were used in the preparation of a SnO nanoplate (SnO-IL, SnO-CS and SnO-IG). For the preparation of the ethyl pyridinium iodide, a mixture of pyridine and iodoethane (1: 2 molar ratio) was refluxed for 24 hours. The ionogel was formed using ethyl pyridinium iodide ionic liquid in chitosan that was dissolved in acetic acid (1 % v/v). By increasing NH₃∙H₂O, the pH of the ionogel reached 7-8. Then, the resultant IG was mixed with SnO in an ultrasonic bath for 1 h. The microstructure of the ionogel was involved as assembled unit via π-π, electrostatic and hydrogen bonding interactions to be three-dimensional networks. The intercalated ionic liquid and chitosan influenced the stability of the SnO nanoplates and improved band gap values. When chitosan was contained as the interlayer space of the SnO nanostructure, the resulting biocomposite formed a well-ordered flower-like SnO structure. These hybrid material structures were characterized by FT-IR, XRD, SEM, TGA, DSC, BET, and DRS techniques. The changes in the band gap values for photocatalysis applications were investigated. In the case of SnO, SnO-IL, SnO-CS, and SnO-IG, the band gap energy was 3.9, 3.6, 3.2, and 2.8 eV, respectively. The dye removal efficiency of SnO-IG was 98.5, 98.8, 97.9, and 98.4 % via the second-order kinetic model for Reactive Red 141, Reactive Red 195, Reactive Red 198, and Reactive Yellow 18, respectively. The maximum adsorption capacity of SnO-IG was 540.5, 584.7, 1501.5, and 1100.1 mg/g for Red 141, Red 195, Red 198, and Yellow 18 dyes, respectively. Also, an acceptable result (96.47 % dye removal) was obtained with the prepared SnO-IG biocomposite for dye removal from textile wastewater.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Chronopotentiometric Evaluation of Ionization Degree and Dissociation Constant of Imidazolium-Based Ionic Liquid [C6Meim][NTf2] in Polymeric Plasticized Membranes

Ionic liquids (ILs) have a wide variety of applications in modern electrochemistry due to their unique electrolytic properties. In particular, they are promising candidates as dopants for polymeric membranes in potentiometric sensors and liquid-junction free reference electrodes. However, the effective use of ILs requires a comprehensive understanding of their electrolytic behavior in the polymeric phase. We report here the exploration of the electrolytic and diffusion properties of IL 1-hexyl-3-methyl-1H-imidazol-3-ium bis[(trifluoromethyl)sulfonyl]amide ([C6Meim][NTf2]) in a poly(vinyl chloride) matrix. Chronopotentiometry is utilized to determine the concentration of charge carriers, ionic diffusion coefficients and apparent dissociation constant of [C6Meim][NTf2] in PVC membranes plasticized with a mixture of [C6Meim][NTf2] and bis(2-ethylhexyl) sebacate (DOS) over a wide range of IL concentrations. The diffusion properties of [C6Meim][NTf2] are confirmed by NMR-diffusometry. The non-monotonic electrolytic behavior of the IL in PVC-DOS matrix is described for the first time. A maximum ionization degree and diffusion coefficient is observed at 30 wt.% of IL in the plasticizing mixture. Thus, it is shown that by varying the flexible parameter of the IL to plasticizer ratio in the polymeric phase one can tune the electrolytic and transport properties of sensing PVC membranes.

Thiol-ene ionogels based on polymerizable imidazolium ionic liquid

In this work, we report the synthesis of polymerizable ionic liquids (PILs) and the synthesis of ionogels by thiol-ene photopolymerization. A series of gemini imidazolium-based bis(trifluoromethylsulfonyl)imide polymerizable ionic liquids with...

A Transparent, Highly Stretchable, Solvent-Resistant, Recyclable Multifunctional Ionogel with Underwater Self-Healing and Adhesion for Reliable Strain Sensors

Ionogels have gained increasing attentions as a flexible conductive material. However, it remains a big challenge to integrate multiple functions into one gel that can be widely applied in various complex scenes. Herein, a kind of multifunctional ionogels with a combination of desirable properties, including transparency, high stretchability, solvent and temperature resistance, recyclability, high conductivity, underwater self-healing ability, and underwater adhesiveness is reported. The ionogels are prepared via one-step photoinitiated polymerization of 2,2,2-trifluoroethyl acrylate and acrylamide in a hydrophobic ionic liquid. The abundant noncovalent interactions including hydrogen bonding and ion-dipole interactions endow the ionogels with excellent mechanical strength, resilience, and rapid self-healing capability at room temperature, while the fluorine-rich polymeric matrix brings in high tolerance against water and various organic solvents, as well as tough underwater adhesion on different substrates. Wearable strain sensors based on the ionogels can sensitively detect and differentiate large body motions, such as bending of limbs, walking and jumping, as well as subtle muscle movements, such as pronunciation and pulse. It is believed that the designed ionogels will show great promises in wearable devices and ionotronics.

Dynamic Anti-Icing Surfaces (DAIS)

Remarkable progress has been made in surface icephobicity in the recent years. The mainstream standpoint of the reported antiicing surfaces yet only considers the ice-substrate interface and its adjacent regions being of static nature. In reality, the local structures and the overall properties of ice-substrate interfaces evolve with time, temperature and various external stimuli. Understanding the dynamic properties of the icing interface is crucial for shedding new light on the design of new anti-icing surfaces to meet challenges of harsh conditions including extremely low temperature and/or long working time. This article surveys the state-of-the-art anti-icing surfaces and dissects their dynamic changes of the chemical/physical states at icing interface. According to the focused critical ice-substrate contacting locations, namely the most important ice-substrate interface and the adjacent regions in the substrate and in the ice, the available anti-icing surfaces are for the first time re-assessed by taking the dynamic evolution into account. Subsequently, the recent works in the preparation of dynamic anti-icing surfaces (DAIS) that consider time-evolving properties, with their potentials in practical applications, and the challenges confronted are summarized and discussed, aiming for providing a thorough review of the promising concept of DAIS for guiding the future icephobic materials designs.

Solid Multiresponsive Materials Based on Nitrospiropyran-Doped Ionogels

The application of molecular switches for the fabrication of multistimuli-responsive chromic materials and devices still remains a challenge because of the restrictions imposed by the supporting solid matrices where these compounds must be incorporated: they often critically affect the chromic response as well as limit the type and nature of external stimuli that can be applied. In this work, we propose the use of ionogels to overcome these constraints, as they provide a soft, fluidic, transparent, thermally stable, and ionic-conductive environment where molecular switches preserve their solution-like properties and can be exposed to a number of different stimuli. By exploiting this strategy, we herein pioneer the preparation of nitrospiropyran-based materials using a single solid platform that exhibit optimal photo-, halo-, thermo-, and electrochromic switching behaviors.

Hydrogen Bonding Strength Determines Water Diffusivity in Polymer Ionogels

Polymeric ionogels, cross-linked gels swollen by ionic liquids (ILs), are useful vehicles for the release and storage of molecular solutes in separation, delivery, and other applications. Although rapid solute diffusion is often critical for performance, it remains challenging to predict diffusivities across multidimensional composition spaces. Recently, we showed that water (a neutral solute) diffuses through alkyl-methylimidazolium halide ILs by hopping between hydrogen bonding sites on relatively immobile cations. Here, we expand on this activated hopping mechanism in two significant ways. First, we demonstrate that water diffuses through poly(ethylene glycol)diacrylate ionogels via the same mechanism at a reduced rate. Second, we hypothesize that the activation energy barrier can be determined from relatively simple ¹H NMR chemical shift measurements of the proton responsible for H-bonding. This relationship enables water's diffusivity in ionogels of this class to be predicted quantitatively, requiring only (1) the composition-dependent diffusivity and Arrhenius behavior of a single IL and (2) ¹H NMR spectra of the ionogels of interest. High-throughput microfluidic Fabry-Perot interferometry measurements verify prediction accuracy across a broad formulation space (four ILs, 0 ≤ x_H2O ≤ 0.7, 0 ≤ ϕ_PEGDA ≤ 0.66). The predictive model may expedite IL-material screening; moreover, it intimates a powerful connection between solute mobility and hydrogen bonding and suggests targets for rational design.

Polymeric Complex-Based Transparent and Healable Ionogels with High Mechanical Strength and Ionic Conductivity as Reliable Strain Sensors

Transparent and healable ionogels with very high mechanical strength, ionic conductivity, and resilience were fabricated for use as strain sensors with satisfactory reliability. The ionogels were fabricated by casting an aqueous solution of poly(vinyl alcohol) (PVA)-poly(vinylpyrrolidone) (PVP) complexes and 1-ethyl-3-methylimidazolium dicyanamide ([EMIm][DCA]), followed by evaporation of water at room temperature. The use of [EMIm][DCA] endowed the resulting ionogels with ionic conductivity at room temperature as high as 19.7 mS cm^-1. Owing to the synergy between the abundant number of hydrogen bonds between PVA and PVP and the crystallized PVA segments that served as nanofillers, the resulting ionogels had good mechanical properties with a tensile stress of 7.7 MPa, a strain of 821%, and good resilience. In addition, the resulting ionogels showed rapid and repeatable sensing signals over a wide strain range (0.1-400%). This enabled them to detect both vigorous muscle movements, such as walking and jumping, and subtle muscle movements, such as pulse. Moreover, owing to the reversibility of hydrogen bonds, physically damaged mechanical properties, conductivity, and sensing ability of the ionogels could be conveniently healed with the assistance of water.

Proteins in Ionic Liquids: Reactions, Applications, and Futures

Biopolymer processing and handling is greatly facilitated by the use of ionic liquids, given the increased solubility, and in some cases, structural stability imparted to these molecules. Focussing on proteins, we highlight here not just the key drivers behind protein-ionic liquid interactions that facilitate these functionalities, but address relevant current and potential applications of protein-ionic liquid interactions, including areas of future interest.

Anthracene Based AIE Active Probe for Colorimetric and Fluorimetric Detection of Cu2+ Ions

A novel aggregation induced emission (AIE) active anthracene based dihydroquinazolinone derivative (probe 1) has been synthesized and characterized by means of spectroscopic methods. The photophysical properties of this probe have been investigated in solvents of different polarity display that fluorescence states are of intramolecular charge transfer (ICT) character. Probe 1 show clear AIE behavior in water/THF mixture on reaching water fraction 95%. The AIE behavior of probe 1 have been exploited for the detection of metal ions in aqueous solution which reveals high selectivity and sensitivity towards Cu2+ ions by colorimetrically and function as a chemosensor in a remarkable turn-off fluorescence manner. Further, the experimental results were investigated by computational means by optimizing the ground state geometries of probe 1 and probe 1-Cu complex using density functional theory (DFT) at B3LYP/6-31G∗∗ and B3LYP/6-31G∗∗(LANL2DZ) levels of theory. Intra-molecular charge transfer was observed in probe 1 while ligand to metal charge transfer (LMCT) for probe 1-Cu complex.

Hydrogel Actuators and Sensors for Biomedical Soft Robots: Brief Overview with Impending Challenges

There are numerous developments taking place in the field of biorobotics, and one such recent breakthrough is the implementation of soft robots-a pathway to mimic nature's organic parts for research purposes and in minimally invasive surgeries as a result of their shape-morphing and adaptable features. Hydrogels (biocompatible, biodegradable materials that are used in designing soft robots and sensor integration), have come into demand because of their beneficial properties, such as high water content, flexibility, and multi-faceted advantages particularly in targeted drug delivery, surgery and biorobotics. We illustrate in this review article the different types of biomedical sensors and actuators for which a hydrogel acts as an active primary material, and we elucidate their limitations and the future scope of this material in the nexus of similar biomedical avenues.

Stimuli responsive ion gels based on polysaccharides and other polymers prepared using ionic liquids and deep eutectic solvents

Ion gels and self-healing gels prepared using ionic liquids (ILs) and deep eutectic solvents (DESs) have been largely investigated in the past years due to their remarkable applications in different research areas. Herewith we provide an overview on the ILs and DESs used for the preparation of ion gels, highlight the preparation and physicochemical characteristics of stimuli responsive gel materials based on co-polymers and biopolymers, with special emphasis on polysaccharides and discuss their applications. Overall, this review summarizes the fundamentals and advances in ion gels with switchable properties prepared using ILs or DESs, as well as their potential applications in electrochemistry, in sensing devices and as drug delivery vehicles.

Folic acid supramolecular ionogels

Herein, we report on folic acid (FA, a low molecular weight gelator) thermoreversible supramolecular organo (in 1 : 1 (v/v) water–DMSO binary solvent), and ionogels reported in 1-ethyl-3-methyl imidazolium chloride, [C2mim][Cl], and 1-octyl-3-methyl imidazolium chloride, [C8mim][Cl], solutions with 0.1 ≤ [IL] ≤ 5% (w/v).

Molecular Self-Assembly of Cyclic Dipeptide Derivatives and Their Applications

Cyclic dipeptides (CDPs) are heterocyclic 2,5-diketopiperazines with exceptional structural rigidity, enzymatic stability, and biological activity, exhibiting a substantial tendency to take part in intermolecular interactions. Strong intermolecular interactions driven by unique hydrogen bonding patterns render CDPs with a high propensity to undergo molecular self-assembly. In this Review, the aim is to provide a comprehensive summary of design strategies used to engineer the molecular self-assembly of CDPs into functional nano- and micro-architectures and molecular gels with potential applications in biomedical and materials engineering fields.

Interface and core relaxation dynamics of IL molecules in nanopores of ordered mesoporous MCM-41: a dielectric spectroscopy study

The dynamics of ionic liquid molecules confined in ordered porous MCM-41 has been investigated by dielectric measurements and two types of confined states of IL were found; pore wall surface adsorbed IL and core of pore filled IL.

Modular microfluidic valve structures based on reversible thermoresponsive ionogel actuators

This paper reports for the first time the use of a cross-linked poly(N-isopropylacrylamide) ionogel encapsulating the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulphate as a thermoresponsive and modular microfluidic valve. The ionogel presents superior actuation behaviour to its equivalent hydrogel. Ionogel swelling and shrinking mechanisms and kinetics are investigated as well as the performance of the ionogel when integrated as a valve in a microfluidic device. The modular microfluidic valve demonstrates fully a reversible on-off behaviour without failure for up to eight actuation cycles and a pressure resistance of 1100 mbar.

Thermoresponsive poly(ionic liquid) hydrogels

A new series of LCST ILs have been copolymerised with crosslinkers of varying length to afford the first ever thermoresponsive poly(ionic liquid)-based hydrogels. These hydrogels exhibit surprisingly broad LCST and volume transition behaviour compared to standard thermoresponsive gels and linear ILs.

Crystallization kinetics behavior of ionic liquid [EMIM][BF4] confined in mesoporous silica matrices

Confinement of ionic liquid (IL) in a porous silica matrix has been found to reduce (i) the dimensionality of crystallization of IL from 3D to 1D and (ii) crystallization rate.

Monodisperse polymeric ionic liquid microgel beads with multiple chemically switchable functionalities

We present simple, inexpensive microfluidics-based fabrication of highly monodisperse poly(ionic liquid) microgel beads with a multitude of functionalities that can be chemically switched in facile fashion by anion exchange and further enhanced by molecular inclusion. Specifically, we show how the exquisite control over bead size and shape enables extremely precise, quantitative measurements of anion- and solvent-induced volume transitions in these materials, a crucial feature driving several important applications. Next, by exchanging diverse anions into the synthesized microgel beads, we demonstrate stimuli responsiveness and a multitude of novel functionalities including redox response, controlled release of chemical payloads, magnetization, toxic metal removal from water, and robust, reversible pH sensing. These chemically switchable stimulus-responsive beads are envisioned to open up a vast array of potential applications in portable and preparative chemical analysis, separations and spatially addressed sensing.

Intelligent Membranes: Dream or Reality?

Intelligent materials are claimed to overcome current drawbacks associated with the attainment of high standards of life, health, security and defense. Membrane-based sensors represent a category of smart systems capable of providing a large number of benefits to different markets of textiles, biomedicine, environment, chemistry, agriculture, architecture, transport and energy. Intelligent membranes can be characterized by superior sensitivity, broader dynamic range and highly sophisticated mechanisms of autorecovery. These prerogatives are regarded as the result of multi-compartment arrays, where complementary functions can be accommodated and well-integrated. Based on the mechanism of “sense to act”, stimuli-responsive membranes adapt themselves to surrounding environments, producing desired effects such as smart regulation of transport, wetting, transcription, hydrodynamics, separation, and chemical or energy conversion. Hopefully, the design of new smart devices easier to manufacture and assemble can be realized through the integration of sensing membranes with wireless networks, looking at the ambitious challenge to establish long-distance communications. Thus, the transfer of signals to collecting systems could allow continuous and real-time monitoring of data, events and/or processes.

Optical sensing system based on wireless paired emitter detector diode device and ionogels for lab-on-a-disc water quality analysis

This work describes the first use of a wireless paired emitter detector diode device (PEDD) as an optical sensor for water quality monitoring in a lab-on-a-disc device. The microfluidic platform, based on an ionogel sensing area combined with a low-cost optical sensor, is applied for quantitative pH and qualitative turbidity monitoring of water samples at point-of-need. The autonomous capabilities of the PEDD system, combined with the portability and wireless communication of the full device, provide the flexibility needed for on-site water testing. Water samples from local fresh and brackish sources were successfully analysed using the device, showing very good correlation with standard bench-top systems.