Chitosan-laponite nanocomposite scaffolds for wound dressing application.

The pivotal issue of skin regeneration research is the development of effective biomaterials that exhibit biological activities as fungicide and bactericide, combining simple and low cost manufacturing technologies. In this context, nanocomposite scaffolds based on chitosan (Ch)/Laponite (Lap) were produced by using different concentrations of Lap via freeze-drying process for potential application in skin regeneration. The influence of Lap concentration on the scaffold properties was evaluated. The prepared scaffolds were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), porosity, swelling capacity, and mechanical analyses. The results revealed that the scaffolds exhibited a porous architecture, besides the increase in the clay content, leads to an increase in the porosity, an improvement of mechanical strength, and a decrease of swelling capacity. In vitro tests were also carried out to evaluate the biocompatibility of the materials, such as bioadhesion, antibacterial activity, viability, and cell adhesion. Viability and cell adhesion demonstrated that all scaffolds were not cytotoxic and the fibroblast cells readily attached on the surface of the scaffolds. Thereby, the results suggested that the nanocomposite scaffolds are biomaterials potentially useful as wound dressings.

Self-assembled amphiphilic chitosan nanoparticles for quercetin delivery to breast cancer cells.

Novel drug delivery strategies are needed to meet the complex challenges associated to cancer therapy. Biocompatible pH-sensitive drug delivery nanocarriers based on amphiphilic co-polymers seem to be promising for cancer treatment. In the present study, a drug delivery system was produced by encapsulating quercetin into novel pH-sensitive self-assembled amphiphilic chitosan nanoparticles. Up to 83% of quercetin was entrapped by the nanoparticles. The particle diameter, as measured by dynamic light scattering (DLS), ranged from ∼235 to ∼312 nm for the blank and ∼490 to ∼502 nm for the loaded carriers. The results showed that the payload release is larger at acidic pH (5.0) than at physiological pH (7.4). Fitting the data to the Korsmeyer-Peppas model indicated that anomalous diffusion is the predominant release mechanism at pH 5.0, while Fickian diffusion operates at pH 7.4. The MTT assay revealed that blank nanoparticles were non-antiproliferative for the cell tested. The results further revealed that quercetin maintains its metabolism inhibition against MCF-7 cells after encapsulation. Cellular uptake experiments showed that nanoparticles accumulated on the cell surface, whereas few were internalized. Haemocompatibility test results suggest that the nanoparticles exhibit suitable blood compatibility for biological applications. Results suggest that nanoparticles might be a promising pH-sensitive drug delivery system for applications in anticancer treatment.

Synergistic effect of quercetin and pH-responsive DEAE-chitosan carriers as drug delivery system for breast cancer treatment.

Amphiphilic chitosans, which may self-assemble in aqueous solution to form nanoaggregates with different conformations depending to the environmental pH, can be used as drug transport and delivery agents, when the target pH differs from the delivery medium pH. In this study, quercetin, a bioactive flavonoid, was encapsulated in a pH-responsive system based on amphiphilic chitosan. The hydrophilic reagent 2-chloro-N,N-diethylethylamine hydrochloride (DEAE), also known to inhibit the proliferation of cancer cells, was used as a grafting agent. Drug loading experiments (DL ∼5%) showed a quercetin entrapment efficiency of 73 and 78% for the aggregates. The sizes of blank aggregates measured by dynamic light scattering (DLS) varied from 169 to 263nm and increased to ∼410nm when loaded with quercetin. The critical aggregation concentration, zeta potential and morphology of the aggregates were determined. pH had a dominant role in the release process and Fickian diffusion was the controlling factor in drug release according to the Korsmeyer-Peppas mathematical model. In vitro studies indicated that the DEAE-modified chitosan nanoaggregates showed a synergistic effect with quercetin on the control of the viability of MCF-7 cells. Therefore, DEAE-modified chitosan nanoaggregates with pH-sensibility can be used as optimized nanocarriers in cancer therapy.

Photochemical synthesis of silver nanoparticles on chitosans/montmorillonite nanocomposite films and antibacterial activity.

Silver nanoparticles (AgNPs) were synthetized on chitosans/montmorillonite nanocomposite films by photochemical method. Nanocomposites were prepared using chitosans with different molar masses and deacetylation degrees, as well as modified with diethylaminoethyl (DEAE) and dodecyl groups. AgNPs formation on the films was followed by the appearance of the plasmon band around 440nm as a function of irradiation time. TEM images revealed AgNPs with spherical morphology for all nanocomposites. For nanocomposites using modified chitosans, the AgNPs synthesis occurred quickly (1.5h) while for the others films it was above 11h. The film of modified chitosan with dodecyl and DEAE groups presented smaller and more uniform nanoparticles size along mixture of exfoliated and intercalated structures. This modified chitosan is an amphiphilic compound that can act controlling the size/shape of the AgNPs. The results of antibacterial activity suggested that all nanocomposite-AgNPs films inhibited the growth of Escherichia coli and Bacillus subtilis.

Photophysical Behavior of Isocyanine/Clay Hybrids in the Solid State †.

In the present study, we have attempted to investigate, for the first time, the photophysical behavior of 1,1'-diethyl-2,4'-cyanine (ICY)/clay mineral hybrids in the solid state. The effects promoted by ICY loading and clay type on the spectroscopic properties were studied by UV-vis diffuse reflectance spectroscopy (DR) and different fluorescence techniques. The hybrids were characterized by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). UV-vis-DR revealed the formation of ICY H-aggregates in Wyoming montmorillonite (SWy-1) and Laponite (Lap); however, J-aggregates were predominant for ICY on Arizona (SAz-1) and Barasym (SYn-1) montmorillonites. The formation of J-aggregates was favored on clays with a high layer charge density (SAz-1 and SYn-1). Increasing ICY loading leads to an increase in H-aggregates, which become predominant in all of the samples. The fluorescence spectra of ICY-Lap and ICY-SYn-1 hybrids showed two emissive bands, and they were assigned to the monomeric and J-aggregate species. The fluorescence lifetime showed consistent and distinct values for the two species. The longer fluorescence lifetime can be assigned to the ICY monomers, while the second component has a short lifetime value and may be attributed to J-aggregate emission species. Moreover, confocal fluorescence micrographs showed two different fluorescent domains; monomers (greenish domain) and J-aggregates (orange domain) can be clearly distinguished. For ICY adsorbed on SWy-1 and SAz-1, the intensities of the fluorescence spectra were very low, and it was not possible to measure the fluorescence lifetimes due to high iron content in these clays, which acts as an efficient quencher of the excited singlet state of the dye molecules. XRD and TGA curves showed that the intercalation of ICY into the interlayer regions of SWy-1, SAz-1, and SYn-1 occurred for high dye concentration only. In the case of Laponite, ICY adsorbs on the external surface of the layer. Our studies indicate that the ICY-clays, in particular, ICY-SYn-1 and ICY-Lap, are promising hybrid materials with interesting optical and photophysical properties.

Photochemical Deposition of Silver Nanoparticles on Clays and Exploring Their Antibacterial Activity.

Photochemical method was used to synthesize silver nanoparticles (AgNPs) in the presence of citrate or clay (SWy-1, SYn-1, and Laponite B) as stabilizers and Lucirin TPO as photoinitiator. During the photochemical synthesis, an appearance of the plasmon absorption band was seen around 400 nm, indicating the formation of AgNPs. X-ray diffraction results suggested that AgNPs prepared in SWy-1 were adsorbed into interlamellar space, and moreover, showed some clay exfoliation. In the case of SYn-1, AgNPs was not intercalated. For the AgNP/Lap B sample, the formation of an exfoliated structure occurred. Transmission electron microscopy revealed the spherical shape of AgNPs for all samples. The particle sizes obtained for AgNP/SWy-1, AgNP/SYn-1, and AgNP/Lap B were 2.6, 5.1, and 3.8 nm, respectively. AgNPs adsorbed on SYn-1 reveal nonuniform size and aggregation of some particles. However, AgNP/SWy-1 and AgNP/Lap B samples are more uniform and have diameters smaller than those prepared with SYn-1. This behavior is due to the ability to exfoliate these clays. The antibacterial activities of pure clays, AgNP/citrate, and AgNP/clays were investigated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). AgNPs in the presence of clays (AgNPs/SYn-1 and AgNPs/SWy-1) showed a lower survival index percentage compared to those obtained for pure clays and AgNPs. The AgNP/SWy-1 sample showed good antibacterial activity against both tested species and the lowest survival index of 3.9 and 4.3 against E. coli and S. aureus, respectively. AgNPs are located in the interlayer region of the SWy-1, which has acid sites. These acidic sites may contribute to the release of Ag(+) ions from the surface of AgNPs. On the other hand, Laponite B and AgNP/Lap B samples did not demonstrate any bactericidal activity.

Syntheses and characterization of amphiphilic quaternary ammonium chitosan derivatives.

Amphiphilic chitosan derivatives possess improved physico-chemical properties and could be used as carriers in drug delivery systems. The aim of this study was to investigate the behaviour of an amphiphilic system involving (5-pentyl) trimethylammonium and dodecyl aldehyde-modified chitosan. Amphiphilic chitosan derivatives were synthesized and characterized by (1)HNMR and ATR-FTIR spectroscopy. Self-assembled aggregates formed in aqueous solution have hydrophobic cores that were characterized by fluorescence spectroscopy using pyrene as probe and dynamic light scattering (DLS). The critical aggregation concentration of the aggregates in water varied from 0.004 to 0.037g/L and the average size distribution was in the 230-500nm range. The ζ-potential (+15.5 to +44.8mV) confirmed that the surfaces of the aggregates were positively charged and stable in physiological-like environments. TEM images suggest that the aggregates have a spherical shape, showing good agreement with DLS results. These results suggest that the synthesized copolymers have the capability of being used as carriers for hydrophobic drugs.

Photochemistry of tetraphenyldiboroxane and its use as photopolymerization coinitiator.

2-Hydroxyethyl methacrylate (HEMA) was photopolymerized in the presence of Safranine (SfH(+)) and tetraphenyldiboroxane (TPhB). Polymerization results are correlated with the photochemistry of TPhB and its ability to aggregate forming hydrophobic domains (critical aggregation concentration, cac, 1.2 × 10(-4) M). Polymerization was not observed when the TPhB concentration was below the cac, indicating that the polymerization is initiated in the hydrophobic environment. The quenching of the triplet state of SfH(+) by TPhB and the generation of the semireduced species of SfH(+) suggests an electron transfer from the boron compound to the excited dye, and that the resulting boron-centered radical initiates the polymerization process.

Polymerization kinetics and reactivity of alternative initiators systems for use in light-activated dental resins.

OBJECTIVES: The purpose of this study was to evaluate the reactivity and polymerization kinetics behavior of a model dental adhesive resin with water-soluble initiator systems. METHODS: A monomer blend based on Bis-GMA, TEGDMA and HEMA was used as a model dental adhesive resin, which was polymerized using a thioxanthone type (QTX) as a photoinitiator. Binary and ternary photoinitiator systems were formulated using 1mol% of each initiator. The co-initiators used in this study were ethyl 4-dimethylaminobenzoate (EDAB), diphenyliodonium hexafluorophosphate (DPIHFP), 1,3-diethyl-2-thiobarbituric acid (BARB), p-toluenesulfinic acid and sodium salt hydrate (SULF). Absorption spectra of the initiators were measured using a UV-Vis spectrophotometer, and the photon absorption energy (PAE) was calculated. The binary system camphorquinone (CQ)/amine was used as a reference group (control). Twelve groups were tested in triplicate. Fourier-transform infrared spectroscopy (FTIR) was used to investigate the polymerization reaction during the photoactivation period to obtain the degree of conversion (DC) and maximum polymerization rate (R(p)(max)) profile of the model resin. RESULTS: In the analyzed absorption profiles, the absorption spectrum of QTX is almost entirely localized in the UV region, whereas that of CQ is in the visible range. With respect to binary systems, CQ+EDAB exhibited higher DC and R(p)(max) values. In formulations that contained ternary initiator systems, the group CQ+QTX+EDAB was the only one of the investigated experimental groups that exhibited an R(p)(max) value greater than that of CQ+EDAB. The groups QTX+EDAB+DPIHFP and QTX+DPIHFP+SULF exhibited values similar to those of CQ+EDAB with respect to the final DC; however, they also exhibited lower reactivity. SIGNIFICANCE: Water-soluble initiator systems should be considered as alternatives to the widely used CQ/amine system in dentin adhesive formulations.

Effect of sonication on the particle size of montmorillonite clays.

This paper reports on the effect of sonication on SAz-1 and SWy-1 montmorillonite suspensions. Changes in the size of the particles of these materials and modifications of their properties have been investigated. The variation of the particle size has been analyzed by DLS (dynamic light scattering). In all cases the clay particles show a bimodal distribution. Sonication resulted in a decrease of the larger modal diameter, as well as a reduction of its volume percentage. Simultaneously, the proportion of the smallest particles increases. After 60 min of sonication, SAz-1 presented a very broad particle size distribution with a modal diameter of 283 nm. On the other hand, the SWy-1 sonicated for 60 min presents a bimodal distribution of particles at 140 and 454 nm. Changes in the properties of the clay suspensions due to sonication were evaluated spectroscopically from dye-clay interactions, using Methylene Blue. The acidic sites present in the interlamellar region, which are responsible for dye protonation, disappeared after sonication of the clay. The changes in the size of the scattering particles and the lack of acidic sites after sonication suggest that sonication induces delamination of the clay particles.

The photophysical determination of the minimum hydrotrope concentration of aromatic hydrotropes.

A method based on the aggregate to monomol emission ratio, I(aggr)/I(monomol), was used to determine the minimal hydrotropic concentration (MHC) of aromatic hydrotropes. The main advantage of this method is that it does not require the use of probes or other additives, which might disrupt the aggregation process. Also, it relies on spectrophotometric measurements, which are more sensitive and less arduous than others, like conductivity, light scattering and surface tension.

The initiating radical yields and the efficiency of polymerization for various dental photoinitiators excited by different light curing units.

OBJECTIVES: To evaluate the efficiency of the photopolymerization of dental resins it is necessary to know to what extent the light emitted by the light curing units is absorbed by the photoinitiators. On the other hand, the efficiency of the absorbed photons to produce species that launch the polymerization process is also of paramount importance. Therefore, the previously determined PAE (photon absorption efficiency) is used in conjunction with the polymerization quantum yields for the photoinitiators, in order to be able to compare the total process on an equivalent basis. This parameter can be used to identify the best performance for the photochemical process with specific photoinitiators. METHODS: The efficiency of LED (Ultrablue IS) and QTH (Optilux 401) lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). The extent of photopolymerization per absorbed photon was determined from the polymerization quantum yields obtained by using the photoinitiators to polymerize methyl methacrylate, and afterwards combined with the previously determined PAEs. RESULTS: Although CQ presents a rather low polymerization quantum yield, its photopolymerization efficiency is practically the highest when irradiated with the Ultrablue LED. On the other hand, Lucirin is much more efficient than the other photoinitiators when irradiated with a QTH lamp, due to its high quantum yield and the overlap between its absorption spectrum and the output of the visible lamp light. SIGNIFICANCE: Difference in photopolymerization efficiencies arise when combinations of photoinitiators are used, and when LED sources are used in preference to QTH. Mechanistic understanding is essential to optimal initiator formulation.

Molar extinction coefficients and the photon absorption efficiency of dental photoinitiators and light curing units.

OBJECTIVES: The light absorption of dental photoinitiators should correlate with the spectral emission profiles of dental light curing units compared on an equivalent basis. Spectral data of dental photoinitiators and light curing units can be used to define the photon absorption efficiency (PAE) obtained by integrating the product of the absorption and emission spectra in terms of photons. This parameter can be used to identify the best performance for photochemical process with specific photoinitiators. METHODS: The efficiency of two LED and one QTH lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). Absorption and emission spectra of the photoinitiators and the LED (Ultrablue I and Ultrablue IS) and QTH (Optilux 401) LCUs were determined in the 360-550nm range. RESULTS: CQ exhibited an absorption centered in the blue region and, although the maxima of PPD, MAPO, and BAPO were in the UV-A region, their absorption extended to the visible region. Power output maxima of the LCUs were at 467 (Ultrablue I), 454 (Ultrablue IS) and 493nm (Optilux 401), and the total power densities were 170+/-1, 470+/-4 and 444+/-4mW/cm(2), respectively. SIGNIFICANCE: The use of the PAE allows a prediction of the most efficient photoinitiator/LCU systems. For similar photoinitiator concentrations, Lucirin and CQ are most efficiently photoinitiated by the QTH unit, whereas the high-power LED device is more efficient for Irgacure. PPD is photoactivated similarly by both LCUs.

A fluorescence study of the interactions between sodium alginate and surfactants.

The interactions between the polysaccharide alginate with charged ionic surfactants (anionic and cationic) in aqueous solution have been investigated using pyrene as a photophysical probe. Static fluorescence determinations have been used to obtain information about the new microenvironments arising by these interactions. Micropolarity studies using the I(1)/I(3) ratio of the vibronic bands and I(E)/I(M) ratio between the excimer and monomer emissions of pyrene shows the formation of hydrophobic domains. The interactions between the natural polyelectrolytes and the oppositely charged surfactants lead to the formation of pre-micelles at surfactant concentrations lower than the CMC of the surfactants. The aggregation process is assumed to be due to electrostatic attraction. On the other side, systems containing an anionic surfactant do not show the same behaviour at low concentrations.

A fluorescence emission study of the formation of induced premicelles in solutions of polyelectrolytes and ionic surfactants.

The interactions between PSS-co-BVE copolymers and ionic surfactants (anionic and cationic) in aqueous solution have been investigated using pyrene as a photophysical probe. Static and dynamic fluorescence determinations have been used to obtain information about the microenvironments formed between both species. Micropolarity studies using the I1/I3 ratio of the vibronic bands of pyrene and the behavior of the I(E)/I(M) ratio between the monomer and excimer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and the oppositely charged surfactants lead to the formation of induced premicelles at surfactant concentrations lower than the cmc of the surfactants. This aggregation process is assumed to be due to electrostatic attraction. At the same concentration, the excimer-to-monomer emission ratio shows its first peak. At higher surfactant concentrations, near the cmc, micelles with the same properties as those found in pure aqueous solution are formed. On the other side, systems containing an anionic surfactant do not show this behavior at low concentrations. There is no apparent dependence of the cac on the composition of the polymer, reinforcing the assumption that the electrostatic interactions induce the formation of the premicelles. The values of the cac's follow the same trend as for the cmc's, DTAC>DTAB>CTAC. The polarity of the induced premicelles, as measured by the I1/I3 ratio, also indicates that the microdomains formed by the longer chain surfactants are more hydrophobic than those of the shorter chain surfactants, as also happens with real micelles.

Influence of the layer charge and clay particle size on the interactions between the cationic dye methylene blue and clays in an aqueous suspension.

The spectroscopic behavior of the dye MB in suspensions of different clays have been used for evaluating layer charge density influence on the adsorption properties of the particles. The clays with higher charge density, like SAz-1 and SCa-3, promote a higher aggregation and do not show deaggregation at longer times, so that practically only the aggregate peak at approximately 570 nm is observed, without any change with time. This is due to, on one side, the larger particle size that decreases the surface area available for adsorption. Additionally, the clay layers will be held together more tightly, avoiding the migration of the dye to the interlamellar region. On the other hand, SWy-1, having a lower charge density, shows a completely different behavior. The dye molecules, initially adsorbed as aggregates on the outer surface of the clay, deaggregate to form monomers that migrate to the interlamellar spaces, giving rise to absorption bands at 670 and 760 nm. Experiments using Ca-exchanged SWy-1, variation of the ionic strength by addition of salt, and the use of different size fractions of the clays confirm the finding that the main factor ruling the adsorption behavior of the probe is the size of the clay particles.