pKa Of Chitosan Research Articles

High internal phase Pickering emulsion solely stabilized by low-content chitosan: Insight into relations between network microstructures and rheological properties

Nano-sized and well-dispersed chitosan particles (CSPs) are prepared by adjusting pH towards pKa of chitosan (CS) aqueous solutions. The prepared CSPs are able to stabilize high internal phase Pickering emulsions (HIPPEs) with low CSP contents, at minimum of 0.2 wt% CSP stabilizing a maximum of 85 % oil phase fraction in a near-neutral environment. It is clarified by microscopy, small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS) measurements that CSPs act as Pickering particle emulsifiers to form a network immobilizing oil droplets in HIPPEs. Using power-law analysis on SAOS results, the solid-like but frequency-dependent rheological response is attributed to the physical connection among CSPs, while the overshoot of the loss modulus in LAOS test is assigned to the featured resistance of networks to the applied large deformation. The oil phase fraction, CSP concentration, temperature and ionic strength are found to be key factors for the rheological behavior and emulsion stability. This work is expected to guide the design and processing of HIPPE stabilized solely by polysaccharides.

Validity and Breakdown of Superposition Principles in the Viscoelasticity of Chitosan–Gum Arabic Complex Coacervates

Complex coacervates of chitosan and gum arabic, two naturally abundant weak polyelectrolytes, were investigated by oscillatory shear rheology at varying temperatures, salt concentrations, salt types, pH, and polyelectrolyte mass ratios. The complex shear moduli are shown to scale with temperature, salt concentration, and pH, implying that the concepts of time–temperature superposition (TTSP), time–salt superposition (TSSP), and time–pH superposition (TpHSP) are applicable. However, in the case of TSSP and TpHSP, a superposition breakdown occurs in the low-frequency regime toward high salt concentrations (>300 mM) and pH values near the pKa of chitosan (≈6.5). Since the shear moduli G′ and G″ are observed to increase and become less frequency-dependent at high salt concentration and pH, the superposition breakdown is ascribed to gelation, presumably due to the associative interaction of chitosan chains and the formation of a chitosan network.

Microwave-Assisted Incorporation of AgNP into Chitosan-Alginate Hydrogels for Antimicrobial Applications.

Herein, improving the antibacterial activity of a hydrogel system of sodium alginate (SA) and basic chitosan (CS) using sodium hydrogen carbonate by adding AgNPs was investigated. SA-coated AgNPs produced by ascorbic acid or microwave heating were evaluated for their antimicrobial activity. Unlike ascorbic acid, the microwave-assisted method produced uniform and stable SA-AgNPs with an optimal reaction time of 8 min. Transmission electron microscopy (TEM) confirmed the formation of SA-AgNPs with an average particle size of 9 ± 2 nm. Moreover, UV-vis spectroscopy confirmed the optimal conditions for SA-AgNP synthesis (0.5% SA, 50 mM AgNO3, and pH 9 at 80 °C). Fourier transform infrared (FTIR) spectroscopy confirmed that the -COO- group of SA electrostatically interacted with either the Ag+ or -NH3+ of CS. Adding glucono-δ-lactone (GDL) to the mixture of SA-AgNPs/CS resulted in a low pH (below the pKa of CS). An SA-AgNPs/CS gel was formed successfully and retained its shape. This hydrogel exhibited 25 ± 2 mm and 21 ± 1 mm inhibition zones against E. coli and B. subtilis and showed low cytotoxicity. Additionally, the SA-AgNP/CS gel showed higher mechanical strength than SA/CS gels, possibly due to the higher crosslink density. In this work, a novel antibacterial hydrogel system was synthesized via 8 min of microwave heating.

Hybrid Acrylated Chitosan and Thiolated Pectin Cross-Linked Hydrogels with Tunable Properties.

We developed and characterized a new hydrogel system based on the physical and chemical interactions of pectin partially modified with thiol groups and chitosan modified with acrylate end groups. Gelation occurred at high pectin thiol ratios, indicating that a low acrylated chitosan concentration in the hydrogel had a profound effect on the cross-linking. Turbidity, Fourier transform infrared spectroscopy, and free thiol determination analyses were performed to determine the relationships of the different bonds inside the gel. At low pH values below the pKa of chitosan, more electrostatic interactions were formed between opposite charges, but at high pH values, the Michael-type addition reaction between acrylate and thiol took place, creating harder hydrogels. Swelling experiments and Young’s modulus measurements were performed to study the structure and properties of the resultant hydrogels. The nanostructure was examined using small-angle X-ray scattering. The texture profile analysis showed a unique property of hydrogel adhesiveness. By implementing changes in the preparation procedure, we controlled the hydrogel properties. This hybrid hydrogel system can be a good candidate for a wide range of biomedical applications, such as a mucosal biomimetic surface for mucoadhesive testing.

Investigation of C\u2013N Formation Between Catechols and Chitosan for the Formation of a Strong, Novel Adhesive Mimicking Mussel Adhesion

Mussel-inspired catechol-containing polymers provide a promising basis for developing strong biogenic adhesives. In order to develop such an adhesive, chitosan was functionalized with protocatechuic acid (PCA) via a Michael-type addition. Kinetic models for the C–N bond formation between primary amines and catechols via Michael-type addition suggest that the reaction is promoted by a low pKa value of the primary amine used and a neutral or mildly acidic reaction pH. Since the pKa of chitosan is below 7, spontaneous reactions with catechols were expected, as was confirmed using FT-IR and UV–Vis spectroscopy where spontaneous reactions occurred at pH 5 and 6.5. FT-IR spectroscopy further validated the kinetic model by showing that the spontaneous reaction between chitosan and PCA did not occur at a pH of 1. While laccases accelerated the C–N bond formation between chitosan and catechols, the formation of either PCA-chitosan agglomerates or DHC-chitosan hydrogels was promoted. For a PCA-functionalized chitosan an adhesion strength of 4.56 ± 0.54 MPa (on sand blasted aluminum surfaces) was achieved. Since the functionalization and curing process uses naturally occurring substances exclusively and is free of toxic chemicals, a novel and sustainable bio-adhesive was developed.Graphic

Chitosan-stabilized emulsion gels via pH-induced droplet flocculation

The formation and stabilization of Pickering-type emulsion gels at low oil fractions (φ) remains a challenge. This work reports on the fabrication of emulsion gels solely stabilized by chitosan at φ values as low as 0.2 via pH adjustment and storage time. We show that slight adjustments in pH in a narrow window from 6.4 to 7.0 (near the pKa of chitosan) can effectively tune the gelation properties of chitosan through control of surface charge and floc size. Upon storage of 2 or more days, oil droplets coated with these aggregated chitosan particles flocculate to form and strengthen self-supporting emulsion gels. The increase in pH not only promotes the formation of networked chitosan particles in the continuous phase but also leads to an increase in inter-droplet interactions, resulting in an enhancement in emulsion gel strength. Increasing φ from 0.2 to 0.75 results in more rapid emulsion gel formation with a higher elastic modulus. The present findings demonstrate the effectiveness of pH-tailored chitosan particles in generating and stabilizing O/W emulsion gels with tunable microstructure and rheological properties.

Removal of Chromium by Biosorption Method (Chitosan)

Discharge of metal containing effluents into water has been a cause of major concern. Traditional treatment methods are proving to be ineffective and expensive. Chitosan was studied as a potential biosorbent due to its positive charge and relatively low cost. The study involves evaluating the metal binding performance of chitosan in a Polymer Enhanced Diafiltration (PEDF) system which uses an ultra filtration membrane to retain the chitosan which, in turn, binds the metal, thereby preventing passage into the permeate stream. Conditions for binding such as pH, concentration of polymer and chromium were studied. Optimal performance was obtained when the system was operated at pH values lower than the pKa of chitosan i.e. 6.3. Using 6 g/L chitosan at pH 4.0, chromium concentration was reduced to less than 1mg/L from a feed concentration of 20 mg/L. Equilibrium dialysis experiments were done to study the kinetics of binding and the uptake of metal per gram of polymer. Rheological measurements demonstrated that in the presence of 1-100 mM chromate, chitosan was found to be slightly shear thickening at low concentrations such as 4 g/L and 6 g/L whereas it was slightly shear thinning at higher concentrations like 12 g/L and 20 g/L This suggests that neutralization of chromium anions is due to the interaction of multiple chitosan molecules. This result is consistent with the relatively stiff nature of the polysaccharide. Overall, this study suggests that some modification of the native polymer would be required to improve uptake and make it an industrially workable process.

Removal of Chromium by Biosorption Method (Chitosan)

Discharge of metal containing effluents into water has been a cause of major concern. Traditional treatment methods are proving to be ineffective and expensive. Chitosan was studied as a potential biosorbent due to its positive charge and relatively low cost. The study involves evaluating the metal binding performance of chitosan in a Polymer Enhanced Diafiltration (PEDF) system which uses an ultra filtration membrane to retain the chitosan which, in turn, binds the metal, thereby preventing passage into the permeate stream. Conditions for binding such as pH, concentration of polymer and chromium were studied. Optimal performance was obtained when the system was operated at pH values lower than the pKa of chitosan i.e. 6.3. Using 6 g/L chitosan at pH 4.0, chromium concentration was reduced to less than 1mg/L from a feed concentration of 20 mg/L. Equilibrium dialysis experiments were done to study the kinetics of binding and the uptake of metal per gram of polymer. Rheological measurements demonstrated that in the presence of 1-100 mM chromate, chitosan was found to be slightly shear thickening at low concentrations such as 4 g/L and 6 g/L whereas it was slightly shear thinning at higher concentrations like 12 g/L and 20 g/L This suggests that neutralization of chromium anions is due to the interaction of multiple chitosan molecules. This result is consistent with the relatively stiff nature of the polysaccharide. Overall, this study suggests that some modification of the native polymer would be required to improve uptake and make it an industrially workable process.

Gelation of Vesicles and Nanoparticles Using Water-Soluble Hydrophobically Modified Chitosan

Hydrophobically modified chitosan (hmC) is a self-assembling polymer that has attracted recent attention for many applications, including as a hemostatic agent. One limitation with chitosan and its derivatives like hmC is that these polymers are soluble in water only under acidic conditions (because the pKa of chitosan is about 6.5), which could be undesirable for biomedical applications. To circumvent this limitation, we have synthesized a derivative of a C12-tailed hmC that is soluble in water at neutral pH. This water-soluble hmC (ws-hmC) is obtained by grafting O-carboxymethyl groups onto some of the primary hydroxyls on hmC. The solubility of ws-hmC at neutral pH is shown to be the result of a net anionic character for the polymer due to ionization of the carboxymethyl groups (in comparison, hmC is cationic). We also demonstrate that ws-hmC retains the self-assembling properties of hmC. Specifically, ws-hmC is able to induce gelation at neutral pH in dispersions of anionic surfactant vesicles as well as polymethylmethacrylate latex nanoparticles. Gelation is attributed to hydrophobic interactions between the hydrophobes on ws-hmC with vesicle bilayers and nanoparticle surfaces. In each case, gelation can be reversed by the addition of α-cyclodextrin, a supramolecule with a hydrophobic cavity that sequesters the hydrophobes on the polymer.

Beta-casein and its complexes with chitosan as nanovehicles for delivery of a platinum anticancer drug

The clinical application of platinum-based anticancer drugs is greatly limited by severe toxicity. Drug-delivery systems are much sought after to improve the efficacy and applicability of these drugs. Here, we describe a new drug-delivery system comprising a novel platinum complex (bipyridine morpholine dithiocarbamate Pt(II) nitrate) within nanoparticles composed of β-casein (β-CN) and chitosan (CS). The influence of pH on the formation of a colloidally-stable nanocarrier system composed of Pt complex-loaded β-CN and chitosan nanoparticles was investigated using UV–vis spectrometry, dynamic light scattering (DLS) and scanning electron microscopy (SEM). The particles of Pt complex-loaded beta-casein–chitosan formed were stable and soluble in the pH range 5.7–6.2. Hence, the optimal pH for complex formation is between the pI of casein (5.3) and the pKa of chitosan (6.5). DLS data showed that, at both pH values of 5.7 and 6.2, the particles formed had sizes between 200 and 300nm. However, the optimum pH for particle formation was pH 5.7. At this pH, the zeta-potential values of nanoparticles were positive and the particles were stable. SEM analysis confirmed the formation of nanoparticles with good colloidal stability and an average particle size of 200nm. The cytotoxicity of both free and encapsulated Pt complex was evaluated on colorectal carcinoma HCT116 cells. The results obtained indicated that both the cytotoxicity and cellular uptake of platinum were enhanced by its entrapment in β-CN–CS nanovehicles. These findings suggest that this novel drug-delivery system enables drugs to be thermodynamically stable in aqueous solutions and is potentially useful for targeted oral-delivery applications.

“Water dispersible pH-responsive chitosan nanogels modified with biocompatible crosslinking-agents”

Chitosan nanoparticles were obtained by crosslinking with two biocompatible dicarboxylic acids: polyethylene glycol dicarboxylic acid and tartaric acid. The water-in-oil (W/O) microemulsion method yielded particle sizes around 10–15 nm in the dried state (TEM) and 200–700 nm in the swollen state (QELS) from commercial chitosan. All the synthesized nanogels showed improved water solubility and most of them were stable at physiological pH. QELS studies revealed the influence of the hydrophilic character and flexibility of the crosslinker on the swelling behaviour of the nanogels. The nanoparticles showed a pH-sensitive volume transition that was consistent with the pKa of chitosan. The collected zeta potential data corroborated the electrostatic repulsion mechanism responsible for the pH-responsive behaviour.

Binding site control in a layer-by-layer deposited chitosan–carbon nanoparticle film electrode

Thin chitosan–carbon nanoparticle films (ca. 2 nm average thickness increase per layer) are assembled onto tin-doped indium oxide (ITO) electrode substrates in a layer-by-layer deposition process employing carbon nanoparticles of ca. 8 nm average diameter and an aqueous solution of chitosan (poly-D-glucosamine, low molecular weight, 75–85% deacetylated). Chitosan introduces amine/ammonium functionalities which are employed for the immobilization of redox systems (i) via physisorption of indigo carmine and (ii) via chemisorption of 2-methyleneanthraquinone. The number of binding sites within the chitosan–carbon nanoparticle film is controlled by changing the thickness of the film deposit or by changing the chitosan content, which is varied by changing the chitosan concentration during layer-by-layer deposition. Voltammetric characteristics and stability of the chemisorbed and physisorbed redox systems are reported as a function of pH. The physisorbed redox system is expelled from the film at a pH consistent with the pKA of chitosan (approximately 6.5). However, the 2-methyleneanthraquinone redox system remains stable even in alkaline media and only a minor inflection in the plot of midpoint potentials vs. pH indicates the film deprotonation process at the pKA of chitosan.

A Review of the Use of Chitosan for the Removal of Particulate and Dissolved Contaminants

Chitosan has unique properties among biopolymers, especially due to the presence of primary amino groups. Chitosan has been used for the chelation of metal ions in near‐neutral solution, the complexation of anions in acidic solution (cationic properties due to amine protonation), the coagulation of negatively charged contaminants under acidic conditions, and for precipitative flocculation at pH above the pKa of chitosan. The coagulation and flocculation properties can be used to treat particulate suspensions (organic or inorganic) and also to treat dissolved organic materials (including dyes and humic acid). This paper will give an overview of the principal results obtained in the treatment of various suspensions and solutions: (a) bentonite suspensions; (b) organic suspensions; (c) anionic dye solutions; and (d) humic acid solutions. Stoichiometry and charge restabilization were determined for the coagulation of humic acid, kaolin, and organic dyes with chitosan, indicating charge neutralization as the dominant mechanism for removal. Charge patch destabilization and bridging mechanisms were inferred in other cases, based on the effects of the apparent molecular weight of the chitosan preparations and effectiveness of sub‐stoichiometric doses of chitosan. For dye solutions, results showed that color can be removed either by sorption onto solid‐state chitosan or by coagulation‐flocculation using dissolved‐state chitosan; the reactivity of amine groups was significantly increased when dissolved chitosan was used. For humic materials, chitosan can be used as a primary coagulant or as a flocculant after coagulation with alum or other inexpensive coagulants. The influence of the degree of deacetylation and the molecular weight of chitosan on its performance as coagulant/flocculant is illustrated by several examples.

Formulation pH modulates the interaction of insulin with chitosan nanoparticles

Previous studies on chitosan‐insulin nanoparticles have reported diverse encapsulation efficiency and insulin release profiles despite similar formulation and preparation procedures. This study examined the efficiency and mechanism of association of insulin with chitosan nanoparticles in the pH range of 2.3 to 6.3. Nanoparticles of 237 to 235 nm were prepared by ionotropic gelation of chitosan with tripolyphosphate counterions. Insulin was quantified by an RP‐HPLC method. The insulin association efficiency (AE) spanned a broad range from 2 to 85%, and was highly sensitive to formulation pH. Highest AE was measured at insulin loading concentrations ≥ 4.28 U/mL and pH 6.1, close to the pI of native insulin and the pKa of chitosan. This association, attributed to physical adsorption of insulin through hydrophobic interactions with chitosan, was labile, and the associated insulin rapidly and completely released by dilution of the nanoparticles in aqueous media of pH 2 to 7.4. AE obtained at pH 5.3 was less than half that measured at pH 6.1 at corresponding insulin concentration, but the association at pH 5.3 appeared to be based on stronger interactions, because the release of insulin was pH‐dependent and recovery was less than 25% even upon disintegration of the chitosan matrix. Interaction of insulin with the chitosan nanoparticles rendered the protein more susceptible to acid and enzymatic hydrolyses, the effects being more predominant in nanoparticles prepared at pH 5.3 than at pH 6.1.