Gel Sol Research Articles

Optimization and evaluation of acetylcholine esterase immobilization on ceramic packing using response surface methodology

In the present attempt a method for the immobilization of acetylcholine esterase (AChE) was developed. In this method, the enzyme was immobilized onto a ceramic cylinder support using a sol–gel–multiwall carbon nanotube (MWCNT) composite. Response surface methodology (RSM) was used for the design and analysis of immobilization experiments. Quadratic mathematical model equations were derived for the prediction of enzyme activity. Then the effects on enzyme activity at 30, 40 and 50 min after process initiation of varying each of two parameters over five levels were investigated. These parameters were the AChE:MWCNT ratio ( X 1), and AChE–MWCNT:sol–gel ratio ( X 2). The optimum values of X 1 and X 2 for the immobilization of AChE on ceramic packing were found to be 1.07 and 0.43, respectively. Using these optimum parameters it was shown that enzyme immobilization with MWCNTs and sol–gel was more effective than immobilization with sol–gel or graphite and sol–gel. Scanning electron microscopic (SEM) images revealed a porous surface comprised of MWCNT–AChE encapsulated in sol–gel. Furthermore, the system was highly reproducible with standard deviations after three successive assays of 1.88%, 2.11% and 2.13% at 30, 40 and 50 min after process initiation, respectively.

Thermal stability of gelatin gels: Effect of preparation conditions on the activation energy barrier to melting

Abstract 10, 20, and 40 wt.% aqueous gelatin gels were prepared under isothermal (annealing at 15, 20, and 25 °C for 15 to 120 min) and nonisothermal (cooling at 1 °C min−1) conditions. Isoconversional kinetic analysis of DSC data on gel melting (gel–sol transition) of all types of gels revealed significant variations in the activation energy throughout the process. Activation energy barrier to melting of isothermally prepared gels was in the range 160–190 kJ mol−1 and found to increase with increasing the annealing temperature that was the major effect discovered. Activation energy barrier to melting of nonisothermally prepared gels was determined to be around 120–140 kJ mol−1 and increase with increasing the concentration. Local reversibility of the gel melting was demonstrated by using temperature modulated DSC.

Hydrogels of collagen hydrolysate cross-linked with dialdehyde starch

Processing hydrogels of collagen hydrolysate (H) cross-linked with dialdehyde starch (DAS) by dipping or casting into biodegradable materials for various applications, is complicated by their marked tendency to aging. One-hour action by temperatures at 60–90 °C reduces sorbed water content in hydrogels by approx. 12%; dependence of the extent of this reduction on temperature (within the mentioned range) was not detected. Effect of thermal action on duration of their disintegration in an aqueous medium and on its pH (within limits 4.8–7.4) was not found either, neither on their gel–sol transition temperature. This supports the view that aging is caused by time-dependent increasing network density of inter-chain hydrogen cross-links. The given temperature interval is satisfactory for processing hydrogels through technologies currently used in processing synthetic plastics (compression molding, injection molding).

Biodegradable in situ gel-forming controlled drug delivery system based on thermosensitive PCL–PEG–PCL hydrogel. Part 2: Sol–gel–sol transition and drug delivery behavior

In this work, a biodegradable and injectable in situ gel-forming controlled drug delivery system based on thermosensitive poly(ε-caprolactone)–poly(ethylene glycol)–poly(ε-caprolactone) (PCEC) hydrogel was studied. The prepared PCEC hydrogel undergoes temperature-dependent sol–gel–sol transition, which is a flowing sol at ambient temperature and turns into a non-flowing gel at around physiological body temperature. Furthermore, the sol–gel phase transition mechanism was investigated using 13C-nuclear magnetic resonance imaging and a laser diffraction particle size analyzer. The in vitro release behaviors of several model drugs, including a hydrophilic small-molecule drug, a hydrophobic small-molecule drug and a macromolecular protein drug, from PCEC hydrogel were also investigated in detail. The results showed that the model drugs could be released from the PCEC hydrogel system over a sustained period. In addition, an anaesthesia assay was conducted using the tail flick latency (TFL) test to evaluate the in vivo controlled drug delivery effect of the PCEC hydrogel system. In the TFL assay, a lidocaine-loaded PCEC hydrogel produced significantly longer-lasting local anaesthetic effects compared with lidocaine aqueous solution at the same dose. Therefore, PCEC hydrogel is promising for use as an injectable local drug delivery system.

Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical properties of the silica aerogels. Strong acids such as HCl, HNO3 and H2SO4 resulted in shrunk (70–95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34–50%) aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity (%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses and N2 adsorption–desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086 g/cm3), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09 W/m K) and hydrophobic (148°) were obtained for molar ratio of Na2SiO3:H2O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20 min gelation time. The resulting aerogels exhibited the thermal stability up to around 420 °C.

Electrically induced gel–sol transition of polyvinyl alcohol/polyacrylamide semi-interpenetrated hydrogels

Polyvinyl alcohol/polyacrylamide semi-interpenetrated hydrogels were prepared via freeze–thaw process. When a 20 V of DC was applied across the gels, the gels with lower polyacrylamide content underwent a contraction or partly turned into solution, while for the gels with higher polyacrylamide concentration, a complete gel–sol transition was observed in a short time.

Highly selective iodide-responsive gel–sol state transition in supramolecular hydrogels

We demonstrate for the first time a highly selective anion-responsive reversible gel–sol state transition in a supramolecular hydrogel of Ag(I)-glutathione (GSH) coordination polymers, which allows for facile and selective visual recognition of I−via naked eyes even in a strongly colored and/or fluorescent background. Such a strategy overcomes the drawback of spectral interferences which are often encountered in conventional colorimetric and fluorimetric means. It was rationalized that I− functioned as a depolymerizing agent for the Ag(I)-GSH supramolecular hydrogels. A feasible quantitative assay for I− was established that afforded satisfactory results for simulated wastewater samples. We believe that this strategy can in principle be applicable to other species by following a smart gel–sol state transition in designed supramolecular hydrogel.

Smart hydrogels based on double responsive triblock terpolymers

In this work a new kind of “smart” hydrogel is presented, which is composed of a poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP-b-PEO-b-P(GME-co-EGE)) triblock terpolymer. The thermo-sensitive poly(glycidyl methyl ether-co-ethyl glycidyl ether) block exhibits a cloud point which is easily adjustable by the comonomer ratio. Copolymerization of GME and EGE produces nearly statistical copolymers with a weak preferential incorporation of GME, exhibiting a sharp coil-to-globule transition with almost no hysteresis. The triblock terpolymers aggregate in aqueous solution triggered by both pH and temperature. At sufficiently high concentration this stimuli-responsive behaviour leads to a reversible gel formation with gel strengths and transition points tuneable by pH, temperature, concentration, and block lengths. At pH = 7, an unique gel–sol–gel transition accompanied with a strengthening of the gel is observed upon heating using rheology. Moreover, the structure of the low temperature gel phase is investigated by means of small-angle neutron scattering (SANS).

Gelation induced reversible syneresis via structural evolution

This paper reports a large volume change in a cholesterol based molecular gel in the presence of multi-amine as the gelling solvents dissociated from the gel in a dynamic manner. The release capability can be controlled by the ratio of multi-amine. The shrinkage capability can be restored and recycled by a gel–sol transformation. The mechanism of the shrinkage was followed by SEM, confocal laser scanning microscopy and X-ray diffraction studies. A structural evolution from lamellar vesicles to a more compact hexagonal structure driven by competitive intermolecular hydrogen bonds was proved to dominate the process of the solvent release. Moreover, the process can be stopped by addition of organic acid such as trifluoroacetic acid, which favors the application of this kind of material for molecular delivery systems.

A novel pH-sensitive PEG-PPG-PEG copolymer displaying a closed-loop sol–gel–sol transition

A thermogelling PEG-PPG-PEG triblock copolymer, Pluronic F127, was modified with diamines containing different numbers of secondary amino groups in order to produce a novel pH-sensitive gelling material. The degree of end group modification was over 95% without any noticeable multi-block formation. Modified F127 displayed a closed-loop sol–gel–sol transition in 16 wt% PBS solution and the loop was significantly shifted with pH depending on the nature of end-capping diamines. Temperature- and pH-sensitive phase transition was caused by a subtle change in hydrophilicity and hydrophobicity balance derived from the pKa values of the diamine groups at the distal PEG blocks. 1H-NMR, GPC and dynamic mechanical analysis were employed for the characterization of modified F127 and its gelation behavior.

Fabrication and properties of cucurbit[6]uril induced thermo-responsive supramolecular hydrogels

Novel thermo-responsive supramolecular hydrogels consisting of cucurbit[6]uril(CB[6]) and butan-1-aminium 4-methylbenzenesulfonate (BAMB) were prepared, which were based on hydrophobic interaction, π–π stacking interaction and van der Walls interaction. Furthermore, hydrogels were not only sensitive to the thermal stimulation, but also showed an interesting property of the inducing action of CB[6] threaded in [2]pseudorotaxane. In this paper, the external morphology and sol–gel transition process of hydrogels were observed by polarized microscopy observation (PMO) and digital camera. DSC results show that the gel–sol transition temperature rose with increasing the concentration of either BMAB or CB[6]. The characteristics of the xerogels were determined by 1H NMR, FT-IR, SEM and TGA. It was found that the CB[6] was threaded in the butyl amine units of BAMB, and the inducing action of CB[6] threaded in [2]pseudorotaxane was essential for the hydrogel formation. Only when the concentrations of both BAMB and CB[6] reached certain values, hydrogels with three-dimensional network structure could be formed. And the viscosity changes for increasing the temperature of the hydrogel systems with different concentration of either CB[6] or BMAB were determined by the rheometer. A possible model was proposed to explain the driving forcing of the gelation.

A water-soluble PPDO/PEG alternating multiblock copolymer: Synthesis, characterization, and its gel–sol transition behavior

Biodegradable and nontoxic alternating multiblock copolymers based on poly ( p-dioxanone) (PPDO) and poly (ethylene glycol) (PEG) were synthesized by the coupling reaction of two bifunctional prepolymers, a dihydroxyl-terminated PPDO and dicarboxylated PEG. The prepolymers and the resulting PPDO/PEG multiblock copolymers were characterized by various analytical techniques such as FT-IR, 1H NMR, GPC, DSC and TG. At high concentration levels above critical gelation concentration (CGC), the aqueous solution of copolymers formed a gel. Temperature-sensitive gel to sol transition behaviors were investigated by the test tube inverting method. Dynamic light scattering (DLS) was used to investigate the micelle of copolymers, whose association probably caused the gelation of the system. Therefore, this novel copolymer has a great potential in injectable drug-delivery system for long-term delivery of drugs.

Characterization of photocatalytic performance of silver deposited TiO 2 nanorods

Ag–TiO 2 nanorods were synthesized via gel–sol methods. Cyclic voltammetry results showed that surface modification of TiO 2 nanorods using Ag led better electrochemical activity involving fast electron transfer to electrode. Electrochemical impedance spectroscopy results revealed that the increase in aspect ratio of TiO 2 nanorods and surface modification resulted in synergetic effect of reduction of e −/h + recombination and charge transfer resistance. Consequently the use of Ag–TiO 2 nanorods as photocatalysts exhibited significant increase in photocatalytic decomposition rate of acid red 44.

Oligonucleotide Analogues with Integrated Bases and Backbones. Part 19.

AbstractThe A*[s]U(*) dinucleosides 1 and 2 form thermoreversible gels in organic solvents. The basis of the gelation is the formation of linear aggregates by base pairing following desolvation of the nucleobases. This is evidenced by the absence of gel formation by the C(6)‐deaminated analogue 3 of 1, the correlation of gelation with the anti‐conformation, as preferred for 1, and the temperature‐, concentration‐, and time‐dependent CD spectra. The gels were also characterized by the minimum gelation concentration, the gel–sol transition (melting) temperature, and rheological properties.

PH- and temperature-sensitive multiblock copolymer hydrogels composed of poly(ethylene glycol) and poly(amino urethane)

A series of novel pH- and temperature-responsive multiblock copolymers (poly(PEG/HEP urethane)) consisting of poly(ethylene glycol) (PEG) and poly(amino urethane) (PAU) were synthesized, and their physicochemical properties were studied. The amphiphilic block copolymers were synthesized from PEG, 1,4-bis(hydroxyethyl) piperazine (HEP) and 1,6-diisocyanato hexamethylene (HDI) in the presence of dibutyltin dilaurate as a catalyst. The resulting polymers were examined by FT-IR, 1H and 13C NMR spectroscopies and gel permeation chromatography (GPC). The solution properties of the copolymers were studied by turbidity measurement and fluorescence spectroscopy. The copolymers showed a pH-dependent soluble–insoluble transition in diluted aqueous solutions. The concentrated polymer solutions exhibited a thermo-induced sol–gel–sol phase transition at pH 6.8–7.4. The gel window covers the physiological conditions. After a subcutaneous injection of the multiblock copolymer solution into mice, a transparent and soft gel was formed immediately. The in vitro release of a model anticancer drug, chlorambucil, persisted over 2 weeks under physiological conditions.

1,2- O-(1-Ethylpropylidene)-α- d-glucofuranose, a low molecular mass organogelator: benzene gel formation and their thermal stabilities

Fourier transform infrared spectroscopy has demonstrated that hydrogen bonds are responsible for the aggregation of 1,2- O-(1-ethylpropylidene)-α- d-glucofuranose molecules in solid and benzene gels. The thermal stability of the studied gel is described by its melting temperature T GS and thermal parameters. The deduced value of the gel–sol transition enthalpy is 50 kJ/mol. Viscosity is the major factor which influences the dynamics of the benzene molecules in the gels, as proved by the relaxation measurements.

Influence of Particle Nucleation in Pressure Sensitive Adhesive Properties: Miniemulsion versus Emulsion Polymerization

AbstractMiniemulsion polymerization is a promising approach to produce and tailor pressure sensitive adhesives (PSAs). In this paper, a systematic comparison of the adhesive properties of latexes produced by miniemulsion and conventional emulsion polymerization is presented. Specifically, the influence of the total surfactant concentration, chain transfer agent concentration and chemical composition on the final adhesive properties of the polymer 2‐ethyl hexyl acrylate/methyl methacrylate/acrylic acid was discerned using a 23 factorial design for each polymerization method. In addition to the adhesive properties (i.e., loop tack, peel strength and shear strength), molecular weight distribution, particle size distribution (PSD) and glass transition temperature were analyzed. The results show that under the conditions used in this work, it is possible to produce PSAs using miniemulsion polymerization, a process wherein monomer droplet nucleation is the dominant particle nucleation mechanism. The use of a miniemulsion polymerization process, as opposed to the conventional emulsion technique, produced several differences such as larger particles sizes and narrower molecular weight distributions. Focusing on the PSA films that exhibited adhesive rather than cohesive failure, the PSA films generated via miniemulsion polymerization displayed higher values of loop tack and peel strength compared to those produced via conventional emulsion polymerization. Shear strength results were strongly dependent on the amount of gel content and sol molecular weight for both cases.

Synthesis and characterization of PEG-PCL-PEG thermosensitive hydrogel

In this work, a series of biodegradable triblock poly(ethylene glycol)-poly(ɛ-caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) copolymers were successfully synthesized by ring-opening copolymerization, and were characterized by 1H NMR, FT-IR, GPC, and DSC. Aqueous solutions of PECE copolymers underwent thermosensitive sol–gel–sol transition as temperature increases when the concentration was above corresponding critical gel concentration (CGC). Sol–gel–sol phase transition diagrams were recorded using test tube inverting method, which depended on hydrophilic/hydrophobic balance in macromolecular structure, as well as some other factors, including topology of triblock copolymers and solution composition of the hydrogel. As a result, the sol–gel–sol transition temperature range could be varied, which might be very useful for its application as injectable drug delivery systems. The in vivo gel formation and degradation behavior was conducted by injecting aqueous PECE solution into KunMing mice subcutaneously. In vitro degradation behavior, in vitro drug release behavior, and cytotoxicity were also investigated in this paper. Therefore, owing to great thermosensitivity and biodegradability of these copolymers, PECE hydrogel is believed to be promising for in situ gel-forming controlled drug delivery system.

Rheological properties of Cedrela odorata gum exudate aqueous dispersions

Rheological studies of Cedrela odorata gum aqueous dispersions demonstrated viscoelastic properties. Mechanical spectra derived from small amplitude oscillatory shear (SAOS) results support the onset of gel-like structure at given conditions (10% w/v, 20 °C). The increase in the storage and loss moduli, along with the drop in their frequency dependence, observed at higher gum concentrations, indicates a slower relaxation mechanism, which may be related to the increasing number and structural complexity of transient junction zones among macromolecules. The transition from gel-like to sol-like structure, at higher temperature, was detected from SAOS results. The flow behaviour of the aqueous gum dispersions under steady shear is highly non-Newtonian and can be modelled by the Sisko equation. The Cox–Merz rule was found to fail at critical gum concentration for the onset of gel-like behaviour and also at a temperature slightly above the gel–sol transition. C. odorata gum aqueous dispersions may have interesting applications as stabilisers of emulsions and suspensions on account of their rheological behaviour.

Mutual Responsive Hydrazide‐Based Low‐Molecular‐Mass Organic Gelators: Probing Gelation on the Molecular Level

Organic gelator: Diacetyl- and dinaphthyl-terminated hydrazide-based oligomers displayed gelation ability in apolar solvents due to hydrogen bonding and/or π–π stacking interactions, which promoted aggregation along one dimension. Their gels were solvent polarity sensitive. Because of complementary hydrogen-bonding sites, when the ratio of the two gelators was 1:1 in apolar solvents, a multipoint directed supramolecular substitution reaction led to gel–sol transition (shown here).