Minimum Gelation Concentration Research Articles

Hydrophobic End-Modulated Amino-Acid-Based Neutral Hydrogelators: Structure-Specific Inclusion of Carbon Nanomaterials.

Hydrophobic end-modulated l-phenylalanine-containing triethylene glycol monomethyl ether tagged neutral hydrogelators (1-4) are developed. Investigations determine the gelators' structure-dependent inclusion of carbon nanomaterials (CNMs) in the self-assembled fibrillar network (SAFIN). The gelators (1, 3, and 4) can immobilize water and aqueous buffer (pH 3-7) with a minimum gelator concentration of 10-15 mg mL(-1). The hydrophobic parts of the gelators are varied from a long chain (C-16) to an extended aromatic pyrenyl moiety, and their abilities to integrate 1 D and 2 D allotropes of carbon (i.e., single-walled carbon nanotubes (SWNTs) and graphene oxide (GO), respectively) within the gel are investigated. Gelator 1, containing a long alkyl chain (C-16), can include SWNTs, whereas the pyrene-containing 4 can include both SWNTs and GO. Gelator 3 fails to incorporate SWNTs or GO owing to its slow rate of gelation and possibly a mismatch between the aggregated structure and CNMs. The involvement of various forces in self-aggregated gelation and physicochemical changes occurring through CNM inclusion are examined by spectroscopic and microscopic techniques. The distinctive pattern of self-assembly of gelators 1 and 4 through J- and H-type aggregation might facilitate the structure-specific CNM inclusion. Inclusion of SWNTs/GO within the hydrogel matrix results in a reinforcement in mechanical stiffness of the composites compared with that of the native hydrogels.

Organo-gelation using Microwave Heating Source: The Case of a Polyurethane Model Compound

Organo-gels with a series of biscarbamates as gelators were prepared using microwave (MW) heating source as well as conventional heating. Biscarbamates with alkyl side chain lengths varying from C5 to C18 were used, with six solvents having dipole moments ranging from 0.07 to 4.3 D. The minimum gelation concentration and the time required for dissolution were significantly reduced with MW heating with benzonitrile, compared to the conventional heating for all the side chain lengths of the biscarbamates. While such reduction was pronounced with benzonitrile and dimethyl sulfoxide with large dipole moments, the time required for dissolution increased with MW heating (compared to the conventional) in the case of xylene and toluene with small dipole moments. Thus, MW is effective with solvents possessing large dipole moments. Although the gels consist of fibers using both methods, an inherent orientation of these fibers was seen with MW heating.

Substituent interference on supramolecular assembly in urea gelators: synthesis, structure prediction and NMR.

Eighteen N-aryl-N'-alkyl urea gelators were synthesised in order to understand the effect of head substituents on gelation performance. Minimum gelation concentration values obtained from gel formation studies were used to rank the compounds and revealed the remarkable performance of 4-methoxyphenyl urea gelator 15 in comparison to 4-nitrophenyl analogue 14, which could not be simply ascribed to substituent effects on the hydrogen bonding capabilities of the urea protons. Crystal structure prediction calculations indicated alternative low energy hydrogen bonding arrangements between the nitro group and urea protons in gelator 14, which were supported experimentally by NMR spectroscopy. As a consequence, it was possible to relate the observed differences to interference of the head substituents with the urea tape motif, disrupting the order of supramolecular packing. The combination of unbiased structure prediction calculations with NMR is proposed as a powerful approach to investigate the supramolecular arrangement in gel fibres and help understand the relationships between molecular structure and gel formation.

Microfluidic Assisted Self‐Assembly of pH‐Sensitive Low‐Molecular Weight Hydrogelators Close to the Minimum Gelation Concentration

SummaryThe fibrillation and subsequent gelation of low molecular hydrogelators is usually triggered by external stimuli. Generally, strong acids are employed to trigger the self‐assembly mechanism in pH‐responsive supramolecular systems. However, the generation and design of novel stable gels with performing mechanical properties is a challenging task as a result of the uneven self‐assembled networks formed. Here, we report the study of the self‐assembly process of a low molecular weight hydrogelator (LMWG) in the proximity of its minimum gelation concentration (MGC = 0.3 mg/ml). At such high dilution, the generation of homogeneous gels with good mechanical properties by turning pH by strong acids is a demanding task as a result of the lack of monodisperse 1D self‐assembled rod‐like aggregates. A microfluidic device is employed here to gradually and homogeneously change the pH. We show that self‐assembly of LMWG in well‐defined structures can be enhanced by using the diffusive mixing occurring in the microfluidic reactor channel. For very short mixing times, aggregates with 2 nm cross‐section are found in the region adjacent to the focused LMWG solution in contact with the low pH buffer solution streams, where the pH reaches values below the pKa of the LMWG and triggers the supramolecular self‐assembly. For longer mixing times, aggregates grow in size and occupy homogeneously the micro channel. The results presented here show that better controlled self‐assembly can be achieved using microfluidic mixing devices and early stages of self‐assembly can be efficiently studied by coupling synchrotron SAXS with microfluidics.

Biobased Fat Mimicking Molecular Structuring Agents for Medium-Chain Triglycerides (MCTs) and Other Edible Oils.

To develop sustainable value-added materials from biomass, novel small-molecule sugar ester gelators were synthesized using biocatalysis. The facile one-step regiospecific coupling of the pro-antioxidant raspberry ketone glucoside and unsaturated or saturated long- and medium-chain fatty acids provides a simple approach to tailor the structure and self-assembly of the amphiphilic product. These low molecular weight molecules demonstrated the ability to self-assemble in a variety of solvents and exhibited supergelation, with a minimum gelation concentration of 0.25 wt %, in numerous organic solvents, as well as in a range of natural edible oils, specifically a relatively unstudied group of liquids: natural medium-chain triglyceride oils, notably coconut oil. Spectroscopic analysis details the gelator structure as well as the intermolecular noncovalent interactions, which allow for gelation. X-ray diffraction studies indicate fatty acid chain packing of gelators is similar to that of natural fats, signifying the crystalline nature may lead to desirable textural properties and mouthfeel.

Synthesis of a Low-Molecular-Weight Fluorescent Ambidextrous Gelator: Development of Graphene- and Graphene-Oxide-Included Gel Nanocomposites.

Low-molecular-weight fluorescent supramolecular ambidextrous gelators have drawn enormous attention owing to their ever-increasing number of potential applications. In this study, we synthesized a low-molecular-weight fluorescent ambidextrous gelator consisting of hydrophobic pyrenebutyric acid moieties at the N termini of a central l-lysine, and an ethyleneoxy unit coupled with 1-(3-aminopropyl)imidazole via a succinic acid linker as a hydrophilic moiety at the C terminus. This compound showed efficient hydrogelation and organogelation, having minimum gelation concentrations of 0.5 % (w/v) in a DMSO/water mixture (1:4 v/v) and 0.2 % (w/v) in nitrobenzene. The presence of the pyrene moieties in the structure rendered the ambidextrous gelator fluorescent. Significant amounts of both graphene oxide and reduced graphene oxide were included within these gels to form a stable soft nanohybrid. Hydrogen bonding, π-π stacking and van der Waals interactions are the key factors for self-assembly leading to gelation. The complementary interactions between the π-electronic surface of 2D graphene sheet and the aromatic pyrene moieties of the gelator play key roles in the inclusion of the nanomaterials into the gels. The inclusion of carbon nanomaterials within the gels resulted in a notable improvement in the stiffness of the soft nanocomposites compared to native gels. The native gels and soft nanocomposites have shear thinning properties and are thixotropic in nature.

Phenylboronic Acid Appended Pyrene-Based Low-Molecular-Weight Injectable Hydrogel: Glucose-Stimulated Insulin Release.

A pyrene-containing phenylboronic acid (PBA) functionalized low-molecular-weight hydrogelator was synthesized with the aim to develop glucose-sensitive insulin release. The gelator showed the solvent imbibing ability in aqueous buffer solutions of pH values, ranging from 8-12, whereas the sodium salt of the gelator formed a hydrogel at physiological pH 7.4 with a minimum gelation concentration (MGC) of 5 mg mL(-1) . The aggregation behavior of this thermoreversible hydrogel was studied by using microscopic and spectroscopic techniques, including transmission electron microscopy, FTIR, UV/Vis, luminescence, and CD spectroscopy. These investigations revealed that hydrogen bonding, π-π stacking, and van der Waals interactions are the key factors for the self-assembled gelation. The diol-sensitive PBA part and the pyrene unit in the gelator were judiciously used in fluorimetric sensing of minute amounts of glucose at physiological pH. The morphological change of the gel due to addition of glucose was investigated by scanning electron microscopy, which denoted the glucose-responsive swelling of the hydrogel. A rheological study indicated the loss of the rigidity of the native gel in the presence of glucose. Hence, the glucose-induced swelling of the hydrogel was exploited in the controlled release of insulin from the hydrogel. The insulin-loaded hydrogel showed thixotropic self-recovery property, which hoisted it as an injectable soft composite. Encouragingly, the gelator was found to be compatible with HeLa cells.

Supramolecular phase-selective gelation by peptides bearing side-chain azobenzenes: effect of ultrasound and potential for dye removal and oil spill remediation.

Phase selective gelation (PSG) of organic phases from their non-miscible mixtures with water was achieved using tetrapeptides bearing a side-chain azobenzene moiety. The presence of the chromophore allowed PSG at the same concentration as the minimum gelation concentration (MGC) necessary to obtain the gels in pure organic phases. Remarkably, the presence of the water phase during PSG did not impact the thermal, mechanical, and morphological properties of the corresponding organogels. In the case of miscible oil/water mixtures, the entire mixture was gelled, resulting in the formation of quasi-hydrogels. Importantly, PSG could be triggered at room temperature by ultrasound treatment of the mixture or by adding ultrasound-aided concentrated solution of the peptide in an oil-phase to a mixture of the same oil and water. Moreover, the PSG was not affected by the presence of salts or impurities existing in water from natural sources. The process could be scaled-up, and the oil phases (e.g., aromatic solvents, gasoline, diesel fuel) recovered almost quantitatively after a simple distillation process, which also allowed the recovery and reuse of the gelator. Finally, these peptidic gelators could be used to quantitatively remove toxic dyes from aqueous solutions.

Versatile, Reversible, and Reusable Gel of a Monocholesteryl Conjugated Calix[4]arene as Functional Material to Store and Release Dyes and Drugs Including Doxorubicin, Curcumin, and Tocopherol.

Gels are interesting soft materials owing to their functional properties leading to potential applications. This paper deals with the synthesis of monocholesteryl derivatized calix[4]arene (G) and its instantaneous gelation at a minimum gelator concentration of 0.6% in 1:1 v/v THF/acetonitrile. The gel shows remarkable thermoreversibility by exhibiting Tgel→sol at ∼48 °C and is demonstrated for several cycles. The gel shows an organized network of nanobundles, while that of the sol shows spherical nanoaggregates in microscopy. A bundle with ∼12 nm diameter possessing hydrophobic pockets in itself is obtained from computationally modeled gel, and hence the gel is suitable for storage and release applications. The guest-entrapped gels exhibit the same microstructures as that observed with simple gels, while fluorescence spectra and molecular mechanics suggests that the drug molecules occupy the hydrophobic pockets. All the entrapped drug molecules are released into water, suggesting a complete recovery of the trapped species. The reusability of the gel for the storage and release of the drug into water is demonstrated for four consecutive cycles, and hence the gel formed from G acts as a functional material that finds application in drug delivery.

Monoglyceride-based organogelator for broad-range oil uptake with high capacity.

Oil/water separation has been a worldwide subject because of increasing release of oil-containing wastewater as well as several marine oil spills. The phase-selective organogelators (PSOGs) are thought to offer a potential and effective implement for addressing this issue. An ideal PSOG for oil adsorption must fulfill some requirements involving effective gelation, easy synthesis, low cost, and recyclable for reuse. However, beyond those, the ability of gelation for a broad-range oil phase without selectivity is also important. However, most of the reported PSOGs have limitation in this respect thus far. In this paper, a new class of saturated 1-monoglyceride-derived organogelators can efficiently uptake almost all of the common fuel oils from water and gelate organic solvents with extremely low minimum gelation concentration (MGC). In addition, the oils in the gel and gelator molecules can be recovered quantitatively through simple vacuum distillation.

Biocompatible small peptide super-hydrogelators bearing carbazole functionalities.

For the first time we have introduced carbazole capping groups onto two short peptide sequences, namely a diphenylalanine dipeptide and a glycine-diphenylalanine tripeptide, giving compounds 1 and 2, respectively. Both molecules form hydrogels at low concentrations, as low as 0.03% (w/v) for 1- well within the range of supergelators. Both gelators are composed of small ca. 2 nm thick molecular fibres, as elucidated by AFM and are non-cytotoxic towards HeLa cells at concentrations which are at or above their minimum gelation concentration.

Combinatorial Library Approach to Realize 2-Aminothiazole-Based Two-Component Hydrogelator: A Structure–Property Correlation

Crystal engineering and supramolecular synthons approach are applied to synthesize a series of 2-aminothiazole (and its methyl derivatives) salts/cocrystals with various dicarboxylic acids. On the basis of combinatorial library approach, 24 new salts/cocrystals of 2-aminothiazole and its methyl derivatives with various dicarboxylic acid (aliphatic unsaturated and saturated backbone) were synthesized and characterized. All the synthesized salts were subjected to gelation test in various solvents (polar and nonpolar). Interestingly, one of the salts/cocrystals, i.e., B3A6 (5-methyl-2-aminothiazolium hydrogen decandioate) was found to be capable of immobilizing water at slightly higher minimum gelator concentration (MGC). A structure–property correlation between various cocrystals/salts based on single crystal X-structure of 11 compounds was undertaken. The gelation property of 2-aminothiazole-based gelling agent was found to be governed by the position of a methyl group on the thiazole ring, a length of the...

D-amino acids modulate the cellular response of enzymatic-instructed supramolecular nanofibers of small peptides.

Peptides made of d-amino acids, as the enantiomer of corresponding l-peptides, are able to resist proteolysis. It is, however, unclear or much less explored whether or how d-amino acids affect the cellular response of supramolecular nanofibers formed by enzyme-triggered self-assembly of d-peptides. In this work, we choose a cell compatible molecule, Nap-l-Phe-l-Phe-l-pTyr (LLL-1P), and systematically replace the l-amino acids in this tripeptidic precursor or its hydrogelator by the corresponding d-amino acid(s). The replacement of even one d-amino acid in this tripeptidic precursor increases its proteolytic resistance. The results of static light scattering and TEM images show the formation of nanostructures upon the addition of alkaline phosphatase, even at concentrations below the minimum gelation concentration (mgc). All these isomers are able to form ordered nanostructures and exhibit different morphologies. According to the cell viability assay on these stereochemical isomers, cells exhibit drastically different responses to the enantiomeric precursors, but almost same responses to the enantiomeric hydrogelators. Furthermore, the different cellular responses of LLL-1P and DDD-1P largely originate from the ecto-phosphatases catalyzed self-assembly of DDD-1 on the surface of cells. Therefore, this report not only illustrates a new way for tailoring the properties of supramolecular assemblies, but also provides new insights to answering the fundamental question of how mammalian cells respond to enzymatic formation of nanoscale supramolecular assemblies (e.g., nanofibers) of d-peptides.

In Situ Synthesised Silver Nanoparticle‐Infused L‐Lysine‐Based Injectable Hydrogel: Development of a Biocompatible, Antibacterial, Soft Nanocomposite

AbstractThe synthesis of L‐lysine‐based amphiphilic hydrogelator consisting of a naphthalene moiety at the N termini and an ethyleneoxy unit with free primary amine at the C terminus is reported. The amphiphile showed good gelation ability with minimum gelation concentration 0.6 % w/v in binary mixtures of dimethyl sulfoxide/phosphate buffer (1:4 v/v, pH 7.4). The hydrogel was characterised by spectroscopic and microscopic studies to delineate the role of non‐covalent interactions in self‐assembly gelation. Subsequently, Ag nanoparticles were synthesised within the hydrogel by in situ photo‐reduction of AgNO3 under sunlight, in which the gelators act as reducing/stabilising agents. The nanocomposites were characterised by transmission electron microscopy, UV/Vis and X‐ray diffraction spectroscopy and thermogravimetric analysis. Rheology of the soft nanocomposite showed significant mechanical strength and thixotropic self‐recovery properties, which made the composite suitable for use as a syringe‐injectable hydrogel. These soft nanocomposites exhibited excellent antibacterial activity against both Gram‐positive and Gram‐negative bacteria. They showed low haemolytic activity and high biocompatibility to mammalian (Chinese hamster ovarian) cells. In addition, agar–gelatin film infused with these nanocomposites allowed normal growth of mammalian cells on its surface while being lethal towards bacteria.

Application of Solubility Parameters in 1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol Organogel in Binary Organic Mixtures

The gelation behavior of 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) in binary solvents has been systematically investigated. DMDBS is soluble in DMSO and insoluble in toluene (apolar) or 1-propanol (polar). When DMSO is added to a poor solvent at a certain volume fraction, DMDBS forms an organogel in the mixed solvent. With increasing DMSO content, the minimum gelation concentration increases and the gel-to-sol transition temperature decreases in both systems. However, compared with those in toluene-DMSO mixtures, the gelation ability and thermal stability are better in 1-propanol-DMSO mixtures. Scanning electron microscopy images reveal that the gelators aggregate to form three-dimensional networks. X-ray diffraction shows that the gel has a lamellar structure, which is different from the structure of the precipitate. Fourier transform infrared results reveal H-bonding is the main driving force for self-aggregation and indicate that stronger H-bonding interactions exist between gelators in 1-propanol-DMSO mixtures in contrast with toluene-DMSO mixtures. Attempts have been taken to correlate solvent parameters to gelation behavior in binary solvents. A Teas plot exhibits distinctly different solvent zones in the studied mixed solvents. The polar parameter (δp) indicates a narrow favorable domain for gel formation in the range of 1.64-7.99 MPa(1/2) for some apolar solvent-DMSO mixtures. The hydrogen-bonding parameter (δh) predicts that gelation occurs for values of 14.00-16.50 MPa(1/2) for some polar solvent-DMSO mixtures. The result may have potential applications in predicting the gelation behavior of 1,3:2,4-di-O-benzylidene-d-sorbitol derivatives in mixed solvents.

Pyrene-based fluorescent supramolecular hydrogel: scaffold for energy transfer.

The self-assembled gelation of an amino-acid-based low molecular weight gelator having a pyrene moiety at the N terminus and a bis-ethyleneoxy unit linked with succinic acid at the C terminus is reported. This amphiphile is capable of gelating binary mixtures (1/3 v/v) of CH3CN/water, DMSO/water, and DMF/water, and the minimum gelation concentration (MGC) varied from 0.2 to 0.3% w/v. The sodium salt of the amphiphile efficiently gelates water with an MGC of 1.5% w/v. The participation of different noncovalent interactions in supramolecular gelation by formation of fibrillar networks was investigated by spectroscopic and microscopic methods. High mechanical strength of the supramolecular gels is indicated by storage moduli on the order of 10(3) Pa. The hydrogel was utilized for energy transfer, whereby inclusion of only 0.00075% w/v of acridine orange resulted in about 50% quenching of the fluorescence intensity of the gel through fluorescence resonance energy transfer.

Remarkable shape-sustaining, load-bearing, and self-healing properties displayed by a supramolecular gel derived from a bis-pyridyl-bis-amide of L-phenyl alanine.

A series of bis-amides decorated with pyridyl and phenyl moieties derived from L-amino acids having an innocent side chain (L-alanine and L-phenyl alanine) were synthesized as potential low-molecular-weight gelators (LMWGs). Both protic and aprotic solvents were found to be gelled by most of the bis-amides with moderate to excellent gelation efficiency (minimum gelator concentration=0.32-4.0 wt.% and gel-sol dissociation temperature T(gel)=52-110 °C). The gels were characterized by rheology, DSC, SEM, TEM, and temperature-variable (1)H NMR measurements. pH-dependent gelation studies revealed that the pyridyl moieties took part in gelation. Structure-property correlation was attempted using single-crystal X-ray and powder X-ray diffraction data. Remarkably, one of the bis-pyridyl bis-amide gelators, namely 3,3-Phe (3-pyridyl bis-amide of L-phenylalanine) displayed outstanding shape-sustaining, load-bearing, and self-healing properties.

Aqueous gel formation from sodium salts of cellobiose lipids.

Cellobiose lipids (CLs) are asymmetric bolaform biosurfactants, which are produced by Cryptococcus humicola JCM 10251 and have fungicidal activity. In this study, the sodium salts of CLs (CLNa) were prepared to improve aqueous solubility of the CLs, and their surface and gelation properties in aqueous solutions were examined by surface tension, rheology, and freeze-fracture transmission electron microscopy (FF-TEM) measurements. The surface tension measurements revealed that the CLNa have high surface activity: CMC1 and γCMC1 are 0.1 mg/mL and 34.7 mN/m, respectively. It was also found that the CLNa form giant micelles above their CMC, whose average size is 116.6 ± 31.9 nm. Unlike conventional surfactants, the surface tension reduced further with an increase in concentration and the aqueous solution became viscous at the minimum gelation concentration (MGC: 5.0 mg/mL). In rheological studies, the obtained gels proved to be rather soft and their sol-gel temperature was found to be approximately 50℃. FF-TEM observation of the gels showed 3D supramolecular structures with an entangled fibrous network. Since the present CLNa aqueous gels have a degree of fungicidal activity, they could be useful for novel multifunctional soft materials applicable to the food and cosmetic industries.

Supramolecular hydrogels assembled from nonionic poly(ethylene glycol)-b-polypeptide diblocks containing OEGylated poly-l-glutamate

A poly(ethylene glycol)-b-poly(γ-(2-methoxyethoxy)esteryl-L-glutamate) (PEG-b-poly-L-EG2Glu) diblock was prepared by the ring-opening polymerization (ROP) of L-EG2Glu N-carboxyanhydride (NCA) using PEG–NH2 as the macroinitiator. When the average degree of polymerization (DP) for poly-L-EG2Glu was below 40, the PEG-b-poly-L-EG2Glu diblock could spontaneously form hydrogels in water. For the different diblocks, a thermally-induced increase in mechanical strength was observed for the PEG-b-poly-L-EG2Glu diblock between 20–45 °C. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed that continuous networks of nanoribbons were formed in these hydrogel samples. Circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) characterization suggested that the formation of sufficient β-sheet content accounted for the nanoribbon assemblies and hydrogel networks. The minimal gelation concentration and gel mechanical properties were found to strongly depend on the composition of the PEG-b-poly-L-EG2Glu diblocks.

Application of solubility parameters in a D-sorbitol-based organogel in binary organic mixtures.

The gelation behaviour of a low molecular weight gelator 2,4-(3,4-dichlorobenzylidene)-D-sorbitol (DCBS) in a binary solvent system has been studied. DCBS was soluble in pure ethanol and insoluble in pure methylcyclohexane. However, DCBS formed opaque gels in ethanol-methylcyclohexane mixtures when the methylcyclohexane content varied from 50% to 80%. Within this range, an increase in the amount of methylcyclohexane reduced the gelation time and also caused the minimum gelation concentration to decrease. Scanning electron microscopy showed that the three-dimensional network structures of the xerogels became denser (from tape-like structures to uniform fibres) when the methylcyclohexane content increased from 40% to 80%. The precipitates that formed in 90% and 100% methylcyclohexane had rod-like structures. X-ray diffraction of the xerogels showed that in the gel state, the DCBS gelator had lamellar packing, which was different from the structure of the precipitate. Fourier transform infrared spectroscopy of the xerogels showed that H-bonding was a driving force for the self-aggregation of the DCBS and it was enhanced as the methylcyclohexane content increased. To estimate the gelator-solvent interactions, the Flory-Huggins parameter was calculated for the DCBS gelator in the binary mixed solvent systems. Based on the values of the Flory-Huggins parameter, the gelation behaviours could be grouped into four domains (solution, partial gel, gel and precipitation). This is a simple method to predict the gelation behaviour of DCBS in some mixed solvents.