Cr3+/COO- Complexation Induced Aggregation of Gelatin in Dilute Solution

Characterization of the effect of chromium salts on tropocollagen molecules and molecular aggregates

Though the capability of chromium treatment to improve the stability and mechanical properties of collagen fibrils is well-known, the influence of different chromium salts on collagen molecules (tropocollagen) is not well characterized. In this study, the effect of Cr³⁺ treatment on the conformation and hydrodynamic properties of collagen was studied using atomic force microscopy (AFM) and dynamic light scattering (DLS). Statistical analysis of contours of adsorbed tropocollagen molecules using the two-dimensional worm-like chain model revealed a reduction of the persistence length (i.e., the increase of flexibility) from ≈72 nm in water to ≈56-57 nm in chromium (III) salt solutions. DLS studies demonstrated an increase of the hydrodynamic radius from ≈140 nm in water to ≈190 nm in chromium (III) salt solutions, which is associated with protein aggregation. The kinetics of collagen aggregation was shown to be ionic strength dependent. Collagen molecules treated with three different chromium (III) salts demonstrated similar properties such as flexibility, aggregation kinetics, and susceptibility to enzymatic cleavage. The observed effects are explained by a model that considers the formation of chromium-associated intra- and intermolecular crosslinks. The obtained results provide novel insights into the effect of chromium salts on the conformation and properties of tropocollagen molecules.

Systematic Evaluation of a Novel Self-Healing Poly(acrylamide-co-vinyl acetate)/Alginate Polymer Gel for Fluid Flow Control in High Temperature and High Salinity Reservoirs

Preferential fluid flow often occurs when water and CO₂ is injected into mature oilfields, significantly reducing their injection efficiency. Particle gels have been evaluated and applied to control the short circulation problems. This study systematically investigated a novel poly(acrylamide-co-vinyl acetate)/alginate-based interpenetrated gel system (Alg-IPNG) which is designed to control the preferential fluid flow problems in high-temperature reservoirs. Chromium acetate was incorporated into the gel system to provide the delayed crosslinking feature of the particle gels. The alginate polymer system can also take advantage of the Ca²⁺ ions in the formation water, which exist in most reservoirs, to reinforce its strength by capturing the Ca²⁺ to form Ca–alginate bonds. In this paper, various characterizations for the Alg-IPNGs before and after the self-healing process were introduced: (1) the elastic modulus is set at up to 1890 Pa, and (2) the water uptake ratio is set at up to 20. In addition, we also discuss their possible self-healing and reinforcement mechanisms. In particular, the self-healing starting time of the Alg-IPNG particles are modified between 38 to 60 h, which is related to the water uptake ratio, Ca²⁺ concentration, and temperature. The reinforced Alg-IPNG gel has an enhanced thermal stability (180 days) at the temperature up to 110 °C.

Colorimetric detection of Cr3+ based on gold nanoparticles functionalized with 4-mercaptobenzoic acid

Herein, we report the construction of a colorimetric probe used to detecting Cr3+ ions in aqueous solution based on functionalized gold nanoparticles. We investigated 4-mercaptobenzoic acid, 4-nitrobenzenethiol, and a mixture of 4-mercaptobenzoic acid and 4-nitrobenzenethiol as ligands for Cr3+ ions to functionalize the gold nanoparticles, respectively. The results showed that the three probes were all aggregated in the presence of Cr3+ ions, which induces a color change from ruby to violet. Moreover, gold nanoparticles modified with 4-mercaptobenzoic acid exhibit a higher response toward Cr3+ than the two other probes, which can be detected by the naked eye and UV-vis absorption spectroscopy. The detection time was rapid (within 25 min). A linear relationship was obtained from 20 to 25 μM between the ratio of the absorbance observed at 635 nm and 520 nm (A635 nm/A520 nm) with the limit of detection was 5 × 10−6 M. This method exhibited excellent selectivity for Cr3+ ions over other tested heavy metal ions, anions, and organic molecules in the absence of another shielding reagent of metal ion. The system was successfully utilized to detect Cr3+ ions in simulated samples.

Gelatin particle-stabilized high internal phase emulsions as nutraceutical containers

In this paper, we report for the first time the use of a well-dispersed gelatin particle as a representative of natural and biocompatible materials to be an effective particle stabilizer for high internal phase emulsion (HIPE) formulation. Fairly monodispersed gelatin particles (∼200 nm) were synthesized through a two-step desolvation method and characterized by dynamic light scattering, ζ-potential measurements, scanning electron microscopy, and atomic force microscopy. Those protein latexes were then used as sole emulsifiers to fabricate stable oil-in-water Pickering HIPEs at different concentrations, pH conditions, and homogenization times. Most of the gelatin particles were irreversibly adsorbed at the oil-water interface to hinder droplet coalescence, such that Pickering HIPEs can be formed by a small amount of gelatin particles (as low as 0.5 wt % in the water phase) at pH far away from the isoelectric point of the gelatin particles. In addition, increasing homogenization time led to narrow size distribution of droplets, and high particle concentration resulted in more solidlike Pickering HIPEs. In vitro controlled-release experiments revealed that the release of the encapsulated β-carotene can be tuned by manipulating the concentration of gelatin particles in the formulation, suggesting that the stable and narrow-size-distributed gelatin-stabilized HIPEs had potential in functional food and pharmaceutical applications.

Effects of Cr3+ on the structure of collagen fiber

We have investigated the effects of Cr3+ on the hierarchical structure of pigskin collagen fibers by use of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), wide-angle X-ray diffraction (WAXD), confocal laser micro-Raman spectroscopy (CLRS), and circular dichroism (CD). Our results demonstrate that the introduction of Cr3+ leads to the formation of a cluster of 20-40 nm between collagen fibrils, while the unique axial periodic structure (D periodicity) of the fibrils does not change. As the Cr3+ concentration increases, the order of intermolecular lateral packing, crystallite structure within helical chains, and N and C telopeptide regions decrease. The present study reveals that Cr3+ only cross-links with collagen but does not disrupt its triple helical structure.

Assemblies of double hydrophilic block copolymers and oppositely charged dendrimers

The association of poly(ethylene oxide-b-methacrylic acid) and poly(amidoamine) dendrimers was examined by dynamic light scattering and small angle neutron scattering. With increasing amounts of the G4 dendrimer as the counterion, the size of the assemblies increases until it reaches a hydrodynamic radius of about 70 nm. The structure is consistent with poly(methyl methacrylate) (PMAA) chains closely aggregating with the dendrimers at low dendrimer amounts and volume-filling PMAA blocks at higher dendrimer contents. Similar behavior was observed for G4 and G2 dendrimers, while smaller G0 molecules showed an opposite dependence. The results represent an example of finite size assemblies formed by "electrostatic self-assembly" that are stable in aqueous solution and represent equilibrium structures, the structure and size of which can be tuned through the building units, loading ratio, and pH.

Preparation of well-defined core-shell particles by Cu2+-mediated graft copolymerization of methyl methacrylate from bovine serum albumin

Small well-defined core-shell poly(methyl methacrylate)-bovine serum albumin (PMMA-BSA) particles have been prepared in a direct one-step graft copolymerization of MMA from BSA at 75 degrees C in water with a trace amount of Cu2+ (5 microM). Initially, BSA generates free radicals and acts as a multifunctional macroinitiator, which leads to the formation of an amphiphilic PMMA-BSA grafting copolymer. Such formed copolymer chains act as a polymeric stabilizer to promote further emulsion polymerization of MMA inside, resulting in surfactant-free stable core-shell particles, confirmed by a transmission electron microscopic (TEM) analysis. The PMMA-BSA copolymers as well as PMMA homopolymer inside the particles were isolated by Soxhlet extraction and characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry (TG). The highest grafting efficiency was approximately 80%. Effects of the reaction temperature, the MMA/BSA ratio, and the concentrations of Cu2+ and BSA on such core-shell particle formation have been systematically studied. Due to their inert PMMA core and biocompatible BSA shell, these small polymer particles are potentially useful in biomedical applications.

Temporal evolution of self-organization of gelatin molecules and clusters on quartz surface

Aqueous gelatin solutions when spread on hydrophilic substrates form self-organized structures where the gelatin molecules and clusters are arranged as self-similar objects giving a mass fractal dimension df=1.67 and 1.72 for solutions made with KCl and NaCl salts as estimated from atomic force microscopic studies. The dehydration driven self-organization of particles changed the df values to 1.78 and 1.81, respectively, after 24 h. This further changed to 1.83 and 1.85 after a time lapse of 10 days. The dynamics of formation of these structures are modeled through spin-exchange kinetics in the nonequilibrium steady state regime in order to understand their complex behavior. Kawasaki spin exchange dynamics has been applied to a diffusion limited aggregation type fractal object, and the growth of the domains was observed by minimizing the free energy. The fractal dimension of such a system changed from 1.70 to 1.82 which inferred the loss of fractal behavior and the generation of a more compact object. The experimentally observed temporal evolution of these complex structures could be adequately described through the results obtained from the computer simulation data.

Pyranine-induced micellization of poly(ethylene glycol)-block-poly(4-vinylpyridine) and pH-triggered release of pyranine from the complex micelles

The pyranine-induced micellization of poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG114-b-P4VP61) in aqueous solutions and pH-triggered release of pyranine from the complex micelles were studied by dynamic and static light scattering, transmission electron microscopy, 1H NMR spectroscopy, and UV-vis spectroscopy. At pH 2, the ionized pyranine can ionically cross-link the protonated P4VP block and result in well-defined spherical complex micelles with a P4VP/pyranine core surrounded by a PEG corona. The ratio of pyranine to pyridyl units can influence the structure and the properties of the resultant complex micelles. The complex micelles are stable upon dilution and heating but are sensitive to pH changes. pH-triggered release of the incorporated pyranine from the complex micelles demonstrates that the release behavior is pH-tunable and displays good controlled-release characteristics at pH approximately 4.

Thermoresponsiveness of hybrid micelles from poly(ethylene glycol)-block-poly(4-vinylpyridium) cations and SO4(2-) anions in aqueous solutions

The SO4(2-)-induced micellization of poly(ethylene glycol)-block-poly(4-vinylpyridium) (PEG110-b-P(4-VPH+)35) and the thermoresponsiveness of these hybrid micelles are studied by dynamic and static light scattering. When the concentration of H2SO4 is high enough, PEG110-b-P(4-VPH+)35 forms stable hybrid micelles with an ionic core of P(4-VPH+)35/SO4(2-) and a PEG corona at 25 degrees C. The formation of the hybrid micelles is reversible. A thermodynamic equilibrium exists between the hybrid micelles and PEG110-b-P(4-VPH+)35 unimers. The shifts of the equilibrium are mainly attributed to the variation of the electrostatic energy and entropic energy of the system. Therefore, the temperature can determine the states of the equilibrium, which means that the dissociation or the formation of the hybrid micelles can be triggered by just varying the temperature.

Salt effects on aggregation of O-carboxymethylchitosan in aqueous solution

The effects of salt with different valences (NaCl, CaCl2 and CrCl3) on the aggregation of O-carboxylmethylchitosan (OCMCS) in dilute aqueous solution were investigated using viscometry, dynamic laser light scattering (DLS) and atomic force microscopy (AFM). With increasing OCMCS concentration beyond a critical aggregation concentration (cac) of approximately 0.045 g/l, the aggregation of OCMCS appears in solution. The driving forces of the OCMCS aggregation are intermolecular hydrogen bond, hydrophobic interaction and electrostatic repulsion. The OCMCS aggregation behavior strongly depends on the valence of salt. When NaCl is added, the aggregate size increases with NaCl concentration. When CaCl2 or CrCl3 is added to a given OCMCS concentration, there exists a critical concentration each of Ca2+ and Cr3+. Before the critical concentration, the aggregates decrease in size with increasing salt concentration due to the intra-aggregate complexation; while after the critical concentration, the size of the aggregates increases with salt concentration due to the inter-aggregate complexation. Moreover, the effect of Cr3+ on the OCMCS aggregation is greater than that of Ca2+. The formation of the intra-aggregate complexation is found to be a kinetic process and the aggregate size decreases with time; the formation of the inter-aggregate complexation is also kinetic where the aggregate size increases with time. The aggregates dominated by the intra-aggregate complexation are small, compact and spherical, while the aggregates dominated by the inter-aggregate complexation show the big, compact and spherical morphology.

Formation of hybrid micelles between poly(ethylene glycol)-block-poly(4-vinylpyridinium) cations and sulfate anions in an aqueous milieu

The SO-induced micellization of poly(ethylene glycol)--poly(4-vinylpyridinium)(PEG-b-P(4-VPH)) in aqueous solution is studied by dynamic and static light scattering, and transmission electron microscopy. The SO anions can act as cross-linkers of the P(4-VPH) blocks through the electrostatic attraction between 4-VPH and SO to induce the micellization of PEG--P(4-VPH). The resultant hybrid micelles are spherical and consist of an ion-complex core of P(4-VPH)/SO and a water-soluble PEG corona and present a density gradient between the core and corona and the core has a higher density. The SO concentrations can affect the structure of the core-corona hybrid micelles. The low SO conentration leads to a loose core while the high SO concentration leads to a compact core.

Effect of cationic size on gelation temperature and properties of gelatin hydrogels

Effect of Na+, K+ and Ca2+ on gel transition temperature (Tg) of gelatin hydrogels (5%, w/v) has been studied by oscillatory rheology in the salt concentration range I = 0.01-0.1 M, which showed increase in Tg with salt concentration with the trend for Tg showing Ca2+ > K+ > Na+. The dynamic light scattering (DLS) measurements in the sol state (T>Tg) showed two distinct relaxation modes whereas only a gel mode was observed in the gel state in all the samples which contained significant amount of heterodyne contribution. Low frequency (1.5 rad/s) isochronal storage modulus data revealed the formation of strong gel in presence of CaCl2 compared to that of NaCl and KCl situations. The slow mode relaxation and heterodyne parameter obtained from DLS data indicate the presence of larger clusters in Ca2+ gels.

Light-Scattering Study of the Aggregation of Syndiotactic Poly(methyl methacrylate) in Solution

Syndiotactic poly(methyl methacrylate (s-PMMA) may undergo aggregation in n-butyl chloride (n-BuCl) at temperatures below the theta temperature. The aggregation behavior of the s-PMMA with weight-average molecular weight M(w) =6.06 x 10(5) g mol(-1) was studied by a combination of static and dynamic laser-light-scattering experiments. A solution of concentration 1.12 x 10(-4) g mL(-1) was quenched from 50 degrees C (above the theta temperature in n-BuCl, 35 degrees C to 12 degrees C, and the aggregation process was measured over 60 h. The time dependence of M(w) the root-mean-square z-average radius of gyration < R(g) >, and the average hydrodynamic radius <R(h)> were used to monitor the growth of the aggregates, with the result M(w) approximately < R(g) > d(f) (where d(f) = 1.98 +/- 0.02), which implies the formation of a fractal aggregate. The observed fractal dimension, d(f), is close to that expected for a reaction-limited cluster aggregation for which d(f) = 2.1. In addition, atomic force microscopy was used to image the aggregates.

Anomalous self-assembly of gelatin in ethanol-water marginal solvent

Light scattering, rheology, and atomic force microscope (AFM) studies have been performed on solutions of a polyampholyte (gelatin) prepared in water-ethanol marginal solvent. At ethanol concentration approximately 45+/-2% v/v anomalous aggregation led to formation of fractal (on hydrophilic substrates; glass, quartz and silicon) aggregate of polypeptide molecules having fractal dimension d(f) in 2D=1.60+/-0.08. The time evolution morphology of these self-assembled and self-organized structures formed on hydrophilic substrates was driven by selective ethanol evaporation and was observed by an AFM. These fractal aggregates eventually transformed into near-spherical clusters with fractal corona having same fractal dimension (d(f)=1.58+/-0.05) and finally, the corona separated and regular aggregates were formed. The kinetics of aggregation on substrates could be modeled through random sequential adsorption of particles with continuum power-law size distribution. The temporal growth of aggregate hydrodynamic radius R(h)(t) and scattered intensity I(s)(t) measured in the bulk were observed to exhibit; R(h)-t(z) and I(s)-t(z)-with z=1/d(f), giving a fractal dimension d(f) in 3D approximately equal to 2.6+/-0.2, which is discussed within the framework of Smoluchowski aggregation kinetics. This growth in R(h) is accompanied by narrowing down of the particle size distribution. Solution rheology at this ethanol concentration revealed minimum thixotropy and maximum infinite shear viscosity features.