Anion-based pH responsive ionic liquids: design, synthesis, and reversible self-assembling structural changes in aqueous solution

A Novel Strategy for the Characterization of Self-Assembled Structures Using the Static Solid-State Phosphorus Nuclear Magnetic Resonance Technique

Structural characterization of assemblies in solutions is essential for understanding the relationship between the structure and material properties. In this study, we introduce a novel approach to investigate amphiphilic self-assemblies in solutions using the phospholipid molecule 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso PC) as a 31P NMR probe. The high natural abundance and gyromagnetic ratio of 31P make it one of the most sensitive nuclei in the low-frequency region, enabling efficient detection even in dilute solutions. Lyso PC can readily co-assemble with amphiphilic molecules and ions in aqueous solutions, forming various structures, such as hexagonal, lamellar, and micellar assemblies. The characteristic line shapes of these assemblies reflect the chemical environment around the probe and provide insights into the different phase states of the assemblies. This strategy offers a simple, cost-effective, and static method for obtaining structural information about various assemblies. Our work not only introduces a sensitive probe for characterizing assemblies in a solvent environment but also inspires new ideas for the development of similar spectroscopic probes.

Ionic liquids revolutionizing biomedicine: recent advances and emerging opportunities

Ionic liquids (ILs), due to their inherent structural tunability, outstanding miscibility behavior, and excellent electrochemical properties, have attracted significant research attention in the biomedical field. As the application of ILs in biomedicine is a rapidly emerging field, there is still a need for systematic analyses and summaries to further advance their development. This review presents a comprehensive survey on the utilization of ILs in the biomedical field. It specifically emphasizes the diverse structures and properties of ILs with their relevance in various biomedical applications. Subsequently, we summarize the mechanisms of ILs as potential drug candidates, exploring their effects on various organisms ranging from cell membranes to organelles, proteins, and nucleic acids. Furthermore, the application of ILs as extractants and catalysts in pharmaceutical engineering is introduced. In addition, we thoroughly review and analyze the applications of ILs in disease diagnosis and delivery systems. By offering an extensive analysis of recent research, our objective is to inspire new ideas and pathways for the design of innovative biomedical technologies based on ILs.

Ionic liquids with reversible photo-induced conductivity regulation in aqueous solution

Stimulus-responsive ionic liquids have gained significant attention for their applications in various areas. Herein, three kinds of azobenzimidazole ionic liquids with reversible photo-induced conductivity regulation were designed and synthesized. The change of electrical conductivity under UV/visible light irradiation in aqueous solution was studied, and the effect of chemical structure and concentration of ionic liquids containing azobenzene to the regulation of photoresponse conductivity were discussed. The results showed that exposing the ionic liquid aqueous solution to ultraviolet light significantly increased its conductivity. Ionic liquids with longer alkyl chains exhibited an even greater increase in conductivity, up to 75.5%. Then under the irradiation of visible light, the electrical conductivity of the solution returned to its initial value. Further exploration of the mechanism of the reversible photo-induced conductivity regulation of azobenzene ionic liquids aqueous solution indicated that this may attributed to the formation/dissociation of ionic liquids aggregates in aqueous solution induced by the isomerization of azobenzene under UV/visible light irradiation and resulted the reversible conductivity regulation. This work provides a way for the molecular designing and performance regulation of photo-responsive ionic liquid and were expected to be applied in devices with photoconductive switching and micro photocontrol properties.

Design and Properties of Natural Rosin-Based Phosphoester Functional Surfactants

As an important forestry biomass resource, rosin has a wide range of applications in medicine, adhesives, surfactants and other fields. Using natural dehydroabietic acid as a raw material, dehydroabietic acid-based phosphorus monoester (DPM) and diester (DPD) surfactants were designed and synthesized. The chemical structures and self-assembly properties were characterized by FT-IR, NMR and TEM, and the effects of pH on critical micelle concentration, γCMC, emulsifying properties, foam properties and micelle morphology were studied. The results showed that the CMC, γCMC value and aggregate morphology had certain pH responsiveness. The γCMC value under acidic conditions was smaller than γCMC under alkaline conditions, and the foaming performance and foam stability under acidic conditions were better than those under alkaline conditions. TEM micelle morphology studies have shown that DPM and DPD surfactants can self-assemble into rod-shaped and spherical micelle morphologies with a pH change in an aqueous solution. At the same pH, the foaming and emulsification properties of DPD were better than those of DPM. The best foaming and emulsification ability of DPD were 11.8 mL and 175 s, respectively. At the same time, the foaming ability of DPD is also affected by pH. DPD has excellent foaming properties in acidic conditions, but these disappeared in neutral conditions.

Formation and Structure of Nanotubes in Imidazolium-Based Ionic Liquid Aqueous Solution

Self-assembled structures have attracted much attention for their potential applications in biological and electrochemical studies. Understanding the aggregation mechanism is necessary for utilizing the structures and improving the properties. In this study, the tubular cluster aggregations formed by the 1-dodecyl-3-methylimidazolium salicylate ([C₁₂mim][Sal]) have been studied by molecular dynamics simulations. The rod-like and funnel-shaped structures were observed during the simulations, and finally, the nanotube structure enclosed by a bilayer membrane was formed. For the first time, the point cloud fitting method was used to obtain the axis equation of the tubular cluster. Based on the equation, the structure of tubular clusters was analyzed in detail. The imidazolium ring and anions were distributed at the ionic liquid-water interface, while the dodecyl groups were buried in the nanotube membrane away from the water. Electrostatic interactions between cations and anions played a dominant role in stabilizing the structure of the nanotube. The tubular cluster size, membrane thickness, and permeability of water molecules through the membrane of the cluster were also calculated. Furthermore, the orientation analysis revealed that multitudinous aggregation structures could be formed by the long alkyl chain in aqueous solution, which might be beneficial for the strengthening and separating processes.

Concentration- and Temperature-Responsive Reversible Transition in Amide-Functionalized Surface-Active Ionic Liquids: Micelles to Vesicles to Organogel

A ubiquitous example of DNA and proteins inspires the scientific community to design synthetic systems that can construct various self-assembled complex nano-objects for high-end physiological functions. To gain insight into judiciously designed artificial amphiphilic structures that through self-assembling form various morphological architectures within a single system, herein, we have studied self-aggregation of amide-functionalized surface-active ionic liquids (AFSAILs) with different head groups in the DMSO/water mixed system. The AFSAIL forms stimuli-responsive reversible micelle and vesicle configurations that coexist with three-dimensional (3D) network structures, the organogel in the DMSO/water mixed system. The self-assembly driving forces, self-organization patterns, network morphologies, and mechanical properties of these network structures have been investigated. With the proven biodegradability and biocompatibility, one can envisage these AFSAILs as the molecules with a new dimension of versatility.

Insight into the formation and permeability of ionic liquid unilamellar vesicles by molecular dynamics simulation

Unilamellar vesicles in solution could open up new horizons for reaction and material delivery, but the formation mechanism especially for the permeability of the small molecule through the vesicle membrane is still unknown. In this study, the formation and permeability of the unilamellar vesicles formed by the ionic liquid 1-dodecyl-3-methylimidazolium salicylate ([C12mim][Sal]) have been investigated by molecular dynamics simulation. Starting from a random distribution of ionic liquids, the entire process of vesicle formation could be observed on a nanosecond time scale, during which planar and cup-like structures are formed at the intermediate stage. Energy analysis reveals that the electrostatic interactions between cations and anions play a dominant role in forming and stabilizing the vesicle. Radial density distribution functions indicate that the final stable vesicle is a spherical bilayer structure. Besides, it was found that the structure of vesicles is maintained with the increase of temperature, while the water molecules in the vesicles could be completely exchanged quickly. These results suggest that vesicles may be beneficial for the enrichment or release of molecules.

Aqueous systems of a surface active ionic liquid having an aromatic anion: phase behavior, exfoliation of graphene flakes and its hydrogelation

Surface active ionic liquid (SAIL) induced hydrogelation, in the absence of additives, is important considering the properties of soft-hydrogels that can be utilized in different applications. The present study is concerned with the phase behavior and hydrogelation of a SAIL, 1-hexadecyl-3-methylimidazolium p-toluenesulfonate, [C16mim][PTS]. The obtained information about the phase behavior along with the surfactant like behavior of the SAIL was exploited for effective exfoliation of graphene-flakes from graphite in aqueous medium that remain stable for at least one month. Thus the obtained dispersion of graphene-flakes was subsequently hydrogelated exploiting the observations made from the phase behavior of the SAIL, via entanglement of long worm-like micelles of the SAIL formed at higher concentration. The obtained graphene-flake based hydrogels were found to be equally stable as compared to the blank hydrogel as well as against centrifugation. The low melting point of hydrogel facilitates the extraction of graphene-flakes from the hydrogel matrix by heating and diluting the gel and there is no sign of agglomeration in the extracted graphene-flakes even if the extraction is carried out after a period of three months. The present work is an exemplary study on exfoliation, hydrogelation and extraction of graphene-flakes from a hydrogel, when required, using a SAIL and is expected to provide a new platform for utilization of SAILs for efficient graphene exfoliation and subsequent preparation of functional materials.

Smart Collagen Hydrogels Based on 1-Ethyl-3-methylimidazolium Acetate and Microbial Transglutaminase for Potential Applications in Tissue Engineering and Cancer Therapy

For the first time, collagen-based hydrogels were fabricated in the presence of a biocompatible ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM] [Ac]), by a simple biopolymer cross-linking process facilitated by the strong catalytic hydrolysis of microbial transglutaminase (MTGase). Phosphate buffer solution (PBS)-encapsulated human-like collagen (HLC) or fish bone collagen (FBC) for the composite hydrogels was simply prepared by the codissolution of biopolymers in [EMIM] [Ac] or, in the absence of the ionic liquid, by the dispersion of MTGase in the biopolymer solution, leading to the formation of MTGase-aided hydrogels (Gel1 and Gel4) and [EMIM] [Ac]/MTGase-aided hydrogels (Gel2, Gel3, and Gel5). The effects of different contents of [EMIM] [Ac] and collagens of different origins (HLC and FBC) during fabrication on a range of structural and material characteristics, including the synthesis mechanism, three-dimensional structure, swelling behavior, mechanical strength, enzymatic hydrolysis rate, cytotoxicity, fibroblast cell proliferation rate, in vitro inhibition of cancer cells and cell adhesion, and in vivo histocompatibility, were investigated. Surprisingly, fabrication with [EMIM] [Ac] had significant effects on the structure and properties of the collagen/MTGase-based hydrogels. In other words, [EMIM] [Ac] changed the underlying mechanism responsible for the advantageous properties of the hydrogels by changing the three-dimensional structure of HLC or FBC, which improved their effects on fibroblast proliferation (3T3-L1 and L929 cells) and their in vitro inhibition of cancer cells (HepG2 and MKN45 cells). The use of the ionic liquid also imbued the hydrogels with degradation resistance and anti-inflammatory properties after subcutaneous injection into mice (in vivo). The catalytic hydrolysis by MTGase and the [EMIM] [Ac] content were the major factors that influenced the properties of the collagen. This result suggests the potential application of ionic liquid-enzymatic hydrolysis in the fabrication of collagen hydrogels in circumstances where the control of the properties by an ionic liquid is desirable. Therefore, [EMIM] [Ac] could be a promising solvent for the development of collagen into smart biomaterials with controlled biodegradation rates that can meet the needs of specific potential applications, such as tissue engineering and cancer therapy.

Added-Value Surfactants

Surfactants are ubiquitous in cellular membranes, detergents or as emulsification agents. Due to their amphiphilic properties, they cannot only mediate between two domains of very different solvent compatibility like water and organic but also show fascinating self-assembly features resulting in micelles, vesicles, or lyotropic liquid crystals. The current review article highlights some approaches towards the next generation surfactants, for example, those with catalytically active heads. Furthermore, it is shown that amphiphilic properties can be obtained beyond the classical hydrophobic-hydrophilic interplay, for instance with surfactants containing one molecular block with a special shape. Whereas, classical surfactants are static, researchers have become more interested in species that are able to change their properties depending on external triggers. The article discusses examples for surfactants sensitive to chemical (e.g., pH value) or physical triggers (temperature, electric and magnetic fields).

Self-Assembly of Amphiphiles into Vesicles and Fibrils: Investigation of Structure and Dynamics Using Spectroscopy and Microscopy Techniques

Amphiphiles are a class of molecules which are known to assemble into a variety of nanostructures. The understanding and applications of self-assembled systems are based on what has been learned from biology. Among the vast number of self-assemblies, in this article, we have described the formation, characterization, and dynamics of two important biologically inspired assemblies: vesicles and fibrils. Vesicles, which can be classified into several categories depending on the sizes and components, are of great interest due to their potential applications in drug delivery and as nanoscale reactors. The structure and dynamics of vesicles can also mimic the complex geometry of the cell membrane. On the other hand, the self-assembly of proteins, peptides, and even single amino acids leads to a number of degenerative disorders. Thus, a complete understanding of these self-assembled systems is necessary. In this article, we discuss recent work on vesicular aggregates composed of phospholipids, fatty acids, and ionic as well as nonionic surfactants and single amino acid-based fibrils such as phenylalanine and tyrosine. Beside the characterization, we also emphasize the excited-state dynamics inside the aggregates for a proper understanding of the organization, reactivity, and heterogeneity of the aggregates.

Chemical Tuning of Zwitterionic Ionic Liquids for Variable Thermophysical Behaviours, Nanostructured Aggregates and Dual-Stimuli Responsiveness

The design and synthesis of a series of zwitterionic ionic liquids (ZILs) to understand the structure-property relationship towards an increase of the thermal stability, a variation of the glass transition temperature, the shape-tuning of nanostructured aggregates and the tuning of the stimuli responsiveness are demonstrated. The substitution reaction of imidazole with various aliphatic and aromatic bromides followed by the reaction of the corresponding substituted imidazoles with bromoalkyl carboxylic acids of varying spacer length produces the ZILs. In aqueous solution, a ZIL molecule either exist in its ionic liquid (substituted imidazolium bromide) form or its zwitterionic (substituted imidazolium alkyl carboxylate) form with an isoelectric point (pI) depending on the pH value of the solution. Upon changing the pH to near or above the pI, the aqueous ZIL solution undergoes transition from a transparent to a turbid phase due to the formation of insoluble hierarchical nanostructured aggregates of various morphologies, such as spheres, tripods, tetrapods, fern-like, flower-like, dendrites etc. depending on the pH of the solution and the nature of the alkyl/vinyl/aryl substituents. Upon heating the solution a phase transition occurs from turbid to transparent, exhibiting a distinct reversible upper critical solution temperature (UCST)-type cloud point (T_cp ). It is observed that the cloud point varies with the nature of the substituent, an increase of the concentration of the ZIL as well as with changes of the pH of the solution.

Unveiling the Aggregation Behavior of Doxorubicin Hydrochloride in Aqueous Solution of 1-Octyl-3-methylimidazolium Chloride and the Effect of Bile Salt on These Aggregates: A Microscopic Study

In this article, we have unveiled the aggregation behavior of a potent chemotherapeutic drug, doxorubicin hydrochloride (Dox) in a well-known imidazolium based surface active ionic liquid (SAIL), 1-octyl-3-methylimidazolium chloride (C₈mimCl). The aggregates formed by Dox in C₈mimCl have been characterized using dynamic light scattering (DLS), fluorescence lifetime imaging microscopy (FLIM), high-resolution transmission electron microscopy (HR-TEM), analytical transmission electron microscopy (analytical TEM), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and Fourier-transform infrared spectroscopy (FTIR) measurements. It is found that Dox forms large spherical aggregates in the presence of C₈mimCl SAIL. We have also explored the driving force behind this aggregation behavior of Dox in C₈mimCl. Furthermore, it is observed that in the presence of a common bile salt, sodium cholate (NaCh), Dox/C₈mimCl spherical aggregates disrupt to form rodlike fibrillar aggregates. Therefore, formation of spherical aggregates and also its disruption into rodlike fibrillar aggregates have been performed, and this is expected to open a new scope for the design of a new generation smart drug delivery system where the drug itself aggregates to form the delivery system.

Multiple-Responsive Hierarchical Self-Assemblies of a Smart Supramolecular Complex: Regulation of Noncovalent Interactions

We herein report a smart amphiphilic supramolecular complex ([MimA-EDA-MimA]@[DBS]₂) with stimuli-responsive self-assembly, constructed by 3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium chloride (MimACl), sodium dodecyl benzene sulfonate (SDBS), and ethylenediamine (EDA). The self-assembly of [MimA-EDA-MimA]@[DBS]₂ shows triple-sensitivities in response to pH, concentration, and salt. At a low pH, only micelles are formed, which can transform into vesicles spontaneously when the pH increases to 11.8. Vesicles can gradually fuse into vesicle clusters and elongated assemblies with increasing concentration of [MimA-EDA-MimA]@[DBS]₂. Chainlike aggregates, ringlike aggregates, or giant vesicles can be formed by adding inorganic salts (i.e., NaCl and NaNO₃), which could be derived from the membrane fusion of vesicles. The noncovalent interactions, including π-π stacking, hydrogen bonding, and electrostatic interactions, were found to be responsible for the topology evolution of assemblies. Thus, it provides an opportunity to construct smart materials through the regulation of the role of noncovalent interactions in self-assembly.

A reversible conductivity modulation of azobenzene-based ionic liquids in aqueous solutions using UV/vis light

The conductivity of azobenzene-based ionic liquids in water can be reversibly and efficiently modulated using UV/vis light irradiation.

pH-Responsive self-assembly of cationic surfactants with a star-shaped tetra-carboxylate acid and the solubilization of hydrophobic drugs

This work involved the construction of pH-responsive self-assembly systems from a pH-sensitive four-arm carboxylate acid (4EOCOOH) and either the cationic single chain surfactant dodecyl trimethyl ammonium bromide (DTAB) or the cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12). It was found that the constructed oligomeric-like structures from the mixtures of 4EOCOOH with DTAB or 12-6-12 greatly enhance the aggregation ability of the mixtures, thus improving the pH-responsivity. In particular, surfactant concentrations significantly affect the pH-responsivity at a fixed 4EOCOOH concentration. At higher surfactant concentrations, the pH-responsivity is suppressed, while at lower surfactant concentrations, the mixed aggregates gradually change from micelles to unstable large spherical aggregates or vesicles, and then to stable spherical aggregates, with decreasing pH. Moreover, the surfactant/4EOCOOH systems have different solubilization abilities for three hydrophobic drugs. For quercetin and baicalein, the systems support much better solubilization at lower pH values, while for indomethacin, the systems show better solubilization at higher pH values. In particular, compared with DTAB, 12-6-12 is more efficient in constructing pH-responsive systems, and the 12-6-12/4EOCOOH mixture shows better ability for solubilizing hydrophobic drugs. This work will be helpful in the design of high-efficiency, pH-responsive surfactant systems for solubilizing hydrophobic drugs by simply mixing pH-sensitive molecules with surfactants.

Imidazolium Salts Mimicking the Structure of Natural Lipids Exploit Remarkable Properties Forming Lamellar Phases and Giant Vesicles

Tailor-made ionic liquids based on imidazolium salts have recently attracted a large amount of attention because of their extraordinary properties and versatile functionality. An intriguing ability to interact with and stabilize membranes has already been reported for 1,3-dialkylimidazolium compounds. We now reveal further insights into the field by investigating 1,3-dimethyl-4,5-dialkylimidazolium (C_n-IMe·HI, n = 7, 11, 15) and 1,3-dibenzyl-4,5-dialkylimidazolium (C_n-IBn·HBr, n = 7, 11, 15) salts. Diverse alkyl chain lengths and headgroups differing in their steric demand were employed for the membrane interface interaction with bilayer membranes imitating the cellular plasma membrane. Membrane hydration properties and domain fluidization were analyzed by fluorescent bilayer probes in direct comparison to established model membranes in a buffered aqueous environment, which resembles the salt content and pH of the cytosol of living cells. Membrane binding and insertion was analyzed via a quartz crystal microbalance and confocal laser scanning microscopy. We show that short-chain 4,5-dialkylimidazolium salts with a bulky headgroup were able to disintegrate membranes. Long-chain imidazolium salts form bilayer membrane vesicles spontaneously and autonomously without the addition of other lipids. These 4,5-dialkylimidazolium salts are highly eligible for further biochemical engineering and drug delivery.

Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids between Micelle and Vesicle by CO2

The creation of CO2-responsive materials that undergo structural transition between micelle and vesicle is of great importance from both theoretical and practical points of view. In this work, we have developed a series of CO2-responsive single-tailed amphiphilic ionic liquids (ILs) composed of N-alkyl-N-methyldiethanolamine cation [CnMDEA](+) (n = 8, 10, 12, 14, 16, 18) and 2-pyrrolidinone [2-Pyr](-) anion. The aggregation behavior and self-assembly structures of the ILs in aqueous solution have been investigated by conductivity, surface tension, dynamic light scattering, cryogenic transmission electron microscopy, small-angle X-ray scattering, and nuclear magnetic resonance spectroscopy. For the first time, CO2 driven reversible switching of self-assembly between spherical micelle and unilamellar vesicle is found for [CnMDEA][2-Pyr] (n = 16, 18) in aqueous solutions at 20 °C and atmospheric pressure. It is shown that the mechanism behind the reversible micelle to vesicle transition involves the formation of carbamate anion from the reaction between [2-Pyr](-) and CO2.

Turn-on pH nano-fluorosensor based on imidazolium salicylaldehyde ionic liquid-labeled silica nanoparticles

A turn-on pH nano-fluorosensor based on a new probe labeled SiNPs was designed. The new probe is based on ESIPT process for Sal bearing 2-MeIm ionic liquid terminal. The pH sensing performance of the nanosensor has been investigated.

How does the surface charge of ionic surfactant and cholesterol forming vesicles control rotational and translational motion of rhodamine 6G perchlorate (R6G ClO₄)?

The rotational dynamics and translational diffusion of a hydrophilic organic molecule, rhodamine 6G perchlorate (R6G ClO4) in small unilamellar vesicles formed by two different ionic surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), with cholesterol have been investigated using fluorescence spectroscopic methods. Moreover, in this article the formation of vesicle using anionic surfactant, SDS at different cholesterol-to-surfactant molar ratio (expressed by Q value (Q = [cholesterol]/[surfactant])) has also been reported. Visual observation, dynamic light scattering (DLS) study, turbidity measurement, steady state fluorescence anisotropy (r0) measurement, and eventually microscopic images reveal the formation of small unilamellar vesicles in aqueous solution. Also, in this study, an attempt has been made to observe whether the cationic probe molecule, rhodamine 6G (R6G) experiences similar or different microenvironment in cholesterol-SDS and cholesterol-CTAB assemblies with increase in cholesterol concentration. The influence of cholesterol on rotational and translational diffusion of R6G molecules has been investigated by monitoring UV-vis absorption, fluorescence, time-resolved fluorescence anisotropy, and finally fluorescence correlation spectroscopy (FCS) measurements. In cholesterol-SDS assemblies, due to the strong electrostatic attractive interaction between the negatively charged surface of vesicle and cationic R6G molecules, the rotational and diffusion motion of R6G becomes slower. However, in cholesterol-CTAB aggregates, the enhanced hydrophobicity and electrostatic repulsion induces the migration of R6G from vesicle bilayer to aqueous phase. The experimental observations suggest that the surface charge of vesicles has a stronger influence than the hydrophobicity of the vesicle bilayer on the rotational and diffusion motion of R6G molecules.

Light induced micelle to vesicle transition in an aqueous solution of a surface active ionic liquid

A new simple surface active ionic liquid displayed reversible micelle–vesicle transition under alternative UV/vis irradiation without additives.

Light-modulated aggregation behavior of some unsubstituted cinnamate-based ionic liquids in aqueous solutions

The aggregation behavior of a new class light-responsive ionic liquids was modulated efficiently by UV light irradiation in aqueous solutions.

Micellar transitions in catanionic ionic liquid–ibuprofen aqueous mixtures; effects of composition and dilution

Interactions between 1-dodecyl-3-methylimidazolium chloride and ibuprofen molecules in aqueous solution form catanionic mixtures, with morphologies of mixed micelles dependent on solution composition.