Unexpected enhancement of pH-stability in Au3+/Ag+ loaded H-bonded layer-by-layer thin films

In this work, a polymeric film was synthesized through a layer-by-layer (LBL) self-assembly technique using polyacrylic acid (PAA) and polyethylene oxide (PEO), resulting in the formation of a hydrogen-bonded LBL film. The formation of these films was evaluated by PMIRRAS and QCM-D. The synergy of these techniques allowed the understanding of the mechanism of formation of the film by showing the H-bonding formation and film growth. Au and Ag metal ions were successfully incorporated into the films, as corroborated by the combination of the information obtained by XRR and PMIRRAS. The films were exposed to increasing pH, showing a pronounced improvement in stability in films loaded with Au ions, extending the stability from pH 4 to 10. This behavior allows the use of this system in a wider range of applications, including the possibility of working in biological conditions. On the other hand, films loaded with Ag disintegrated at pH above 4. At acidic pH (below 3), these films released the Ag ions, which may be useful for the preparation of antibacterial stimuli-responsive nanomaterials. In both cases, the films were adequate to produce metal nanoparticles by metal loading and in situ reduction.

Neuronal cell response on aligned fibroporous electrospun mat generated from silver ion complexed ethylene vinyl alcohol copolymer

Generating electrospun mats with aligned fibers and obtaining neurite extension in the aligned fiber direction could provide hope for fabricating nerve guidance conduits or wraps through an easy method. The growing interest in generating electrospun mats with aligned fibers for tissue engineering is looking for simple methods to generate the same. Here, in this study, ethylene vinyl alcohol copolymer (EVAL) chains were complexed with silver ions (Ag⁺ ) to generate aligned fibers during the electrospinning process. The fibers thus produced were subjected to physico-chemical characterization and biological studies to ensure their properties and to examine whether suitable for neuronal cell attachment and neurite extension that may be useful in making nerve guidance conduits or wraps. The presence of silver ions and its complex formation with -OH of EVAL has been confirmed with EDX and XPS analysis respectively. The alignment of fibers was visualized from SEM analysis and confirmed using directionality analysis using Fiji-ImageJ software. Mechanical properties done with dumbbells punched out in longitudinal and transverse directions also substantiated the alignment of fibers. The results obtained from direct contact, MTT, and live/dead assay showed the cells are viable on the material. From the actin staining and immunostaining assays, it was evident that the PC12 cells could attach and extend their neurites in an aligned manner on the fibers. The maximum neurite extension was up to 200 μm in length. These properties of electrospun EVAL-Ag mat with aligned fibers indicated that it could be developed as a biocompatible nerve guidance conduit or wrap.

Polyacrylic Acid/Polyaniline-Coated Multimode Interferometer for Ammonia Detection

A coaxial optical fiber interferometer (COFI) is proposed here for ammonia sensing, which comprises two light-carrying single-mode fibers (SMF) fused to a section of no-core fiber (NCF), thus forming an optical interferometer. The outer surface of the COFI is coated with a layer of polyacrylic acid (PAA)/polyaniline (PAni) film. The refractive index (RI) of the sensitive layer varies when PAA/PAni interacts with ammonia, which leads to the resonance wavelength shift. The surface morphology and structure of the PAA/PAni composites were characterized by using a scanning electron microscope (SEM) and Fourier-transform infrared (FTIR) spectroscopy. When the sensor was exposed to an ammonia atmosphere of different concentrations at room temperature, the sensing performance of the PAA/PAni composite film was superior to that of a sensitive film formed by single-component PAA or PAni. According to the experimental results, the composite film formed by 5 wt% PAA mixed with 2 wt% PAni shows better performance when used for ammonia sensing. A maximum sensitivity of 9.8 pm/ppm was obtained under the ammonia concentration of 50 ppm. In addition, the sensor shows good performance in response time (100 s) and recovery time (180 s) and has good stability and selectivity. The proposed optical fiber ammonia sensor is adapted to monitor leakage in its production, storage, transportation, and application.

Composition-Dependent Thermal Stability and Water-Induced Self-Healing Behavior of Smectite/Waterborne Polymer Hybrid Film

By casting an aqueous suspension containing a water-soluble polymer, polyvinylpyrrolidone, and a layered silicate, synthetic hectorite, on the solid substrate, films with varied interlayer expansion were obtained depending on the composition. The thermal stability, water resistance, water-induced self-healing behavior, and adhesion were examined to find their composition dependence, which is thought to be originated from the nanostructure variation. Polyvinylpyrrolidone was thermally stable up to 300 °C for the hybrid with the polymer/clay weight ratio of 0.36 and 260 °C for the weight ratios of 1.08 and 1.80 as shown by the changes in the appearance and structure after heat treatment. The hybrid film with the polymer/clay ratio of 0.36 maintained the film shape when it was soaked in water for 24 h. The hybrids with the polymer/clay ratios of 1.08 and 1.80 were re-dispersed/dissolved into water after the immersion, while the water resistance of the films was enhanced by the thermal treatment at 200 °C for 2 h and showed very fast water-induced self-healing.

A CO2-responsive hydrogel film for optical sensing of dissolved CO2

CO₂-monitoring plays an important role in medicine, environmental sciences, and food industries. Here, a new CO₂-responsive hydrogel film was fabricated from branched polyethyleneimine (BPEI) and partially oxidized dextran (PO-Dex) via layer-by-layer (LBL) assembly, based on the in situ Schiff base reaction between the two polymers. The swelling behaviours of the films were studied using Fabry-Perot fringes on their reflection spectra. Like ordinary hydrogels, the BPEI/PO-Dex films swell in water. In addition, they swell to a larger degree when CO₂ is introduced, due to the reaction between CO₂ and the amino groups in BPEI. The CO₂-induced swelling can be reported by the shift of the Fabry-Perot fringes on their reflection spectra; therefore, the BPEI/PO-Dex film can be used as an optical sensor for dissolved CO₂. In the new sensor, the BPEI/PO-Dex film acts as a CO₂-sensing material and Fabry-Perot cavity simultaneously. The introduction of ordered structure is no longer required. The response of the sensor to CO₂ is linear and reversible. Unlike other hydrogel-based sensors that suffer from a slow response, the new sensor can respond to CO₂ quickly, making it applicable for real-time, continuous monitoring of CO₂ levels in solution.

Antibacterial Polymeric Films Fabricated by [2+2] Cycloaddition-Retroelectrocyclization and Ag+ Ion Coordination

The [2+2] cycloaddition-retroelectrocyclization (CA-RE) between N,N-dialkylaniline-substituted alkynes and 7,7,8,8-tetracyanoquinodimethane (TCNQ) is employed to fabricate functional cross-linked polymer films containing the intramolecular charge-transfer (CT) chromophores at the cross-linking points. Polystyrene bearing N,N-dialkylaniline-substituted alkynes (P1) and TCNQ polyester (P2) are mixed in tetrahydrofuran (THF), then this solution is spray-coated onto an indium tin oxide or glass plate. Heating to 100 °C initiates the [2+2] CA-RE reaction, resulting in the formation of cross-linked polymer films. The reaction progress and completion are evaluated by monitoring the CT absorption band and cyano vibration peaks. The resulting cross-linked polymer films show reversible cathodic electrochromism between the neutral and anion radical states. In addition, they also display the visual detection behavior of protic acids and Lewis acids, such as Ag⁺ ions. Accordingly, the Ag⁺ ion-loaded polymer films are prepared, and their antibacterial activities are studied. As more Ag⁺ ions are loaded, the CT band more bathochromically shifts and more potent antibacterial activities are obtained. Therefore, the antibacterial activity of the polymer films can be visually recognized by the film colors. Furthermore, the loaded Ag⁺ ions can be released from the polymer films by application of an electrochemical potential.

In-Situ Crafting of ZnFe₂O₄ Nanoparticles Impregnated within Continuous Carbon Network as Advanced Anode Materials

The ability to create a synergistic effect of nanostructure engineering and its hybridization with conductive carbonaceous material is highly desirable for attaining high-performance lithium ion batteries (LIBs). Herein, we judiciously crafted ZnFe2O4/carbon nanocomposites composed of ZnFe2O4 nanoparticles with an average size of 16 ± 5 nm encapsulated within the continuous carbon network as anode materials for LIBs. Such intriguing nanocomposites were yielded in situ via the pyrolysis-induced carbonization of polystyrene@poly(acrylic acid) (PS@PAA) core@shell nanospheres in conjunction with the formation of ZnFe2O4 nanoparticles through the thermal decomposition of ZnFe2O4 precursors incorporated within the PS@PAA nanospheres. By systematically varying the ZnFe2O4 content in the ZnFe2O4/carbon nanocomposites, the nanocomposite containing 79.3 wt % ZnFe2O4 was found to exhibit an excellent rate performance with high capacities of 1238, 1198, 1136, 1052, 926, and 521 mAh g(-1) at specific currents of 100, 200, 500, 1000, 2000, and 5000 mA g(-1), respectively. Moreover, cycling performance of the ZnFe2O4/carbon nanocomposite with 79.3 wt % ZnFe2O4 at specific currents of 200 mA g(-1) delivered an outstanding prolonged cycling stability for several hundred cycles.

pH-Degradable antioxidant nanoparticles based on hydrogen-bonded tannic acid assembly

Hydrogen-bonded polyphenol-based assemblies have attracted increasing interest for biomedical applications.

Fabrication of electrically conductive metal patterns at the surface of polymer films by microplasma-based direct writing

We describe a direct-write process for producing electrically conductive metal patterns at the surface of polymers. Thin films of poly(acrylic acid) (PAA) loaded with Ag ions are reduced by a scanning, atmospheric-pressure microplasma to form crystalline Ag features with a line width of 300 μm. Materials analysis reveals that the metallization occurs in a thin layer of ∼5 μm near the film surface, suggesting that the Ag ions diffuse to the surface. Sheet resistances of 1-10 Ω/sq are obtained independent of film thickness and Ag volume concentration, which is desirable for producing surface conductivity on polymers while minimizing metal loading.

Ultrathin hydrogel films for rapid optical biosensing

Novel biosensors have been designed by reporting an analyte-induced (de)swelling of a stimuli-responsive hydrogel (usually in a form of thin film) with a suitable optical transducer. These simple, inexpensive hydrogel biosensors are highly desirable, however, their practical applications have been hindered, largely because of their slow response. Here we show that quick response hydrogel sensors can be designed from ultrathin hydrogel films. By the adoption of layer-by-layer assembly, a simple but versatile approach, glucose-sensitive hydrogel films with thickness on submicrometer or micrometer scale, which is 2 orders of magnitude thinner than films used in ordinary hydrogel sensors, can be facilely fabricated. The hydrogel films can not only respond to the variation in glucose concentration, but also report the event via the shift of Fabry-Perot fringes using the thin film itself as Fabry-Perot cavity. The response is linear and reversible. More importantly, the response is quite fast, making it possible to be used for continuous glucose monitoring.

Direct probing of micromechanical properties of hydrogen-bonded layer-by-layer microcapsule shells with different chemical compositions

The mechanical properties of hydrogen-bonded layer-by-layer (LbL) microcapsule shells constructed from tannic acid (TA) and poly(vinylpyrrolidone) (PVPON) components have been studied in both the dry and swollen states. In the dry state, the value of the elastic modulus was measured to be within 0.6-0.7 GPa, which is lower than the typical elastic modulus for electrostatically assembled LbL shells. Threefold swelling of the LbL shells in water results in a significant reduction of the elastic modulus to values well below 1 MPa, which is typical value seen for highly compliant gel materials. The increase of the molecular weight of the PVPON component from 55 to 1300 kDa promotes chain entanglements and causes a stiffening of the LbL shells with a more than 2-fold increase in elastic modulus value. Moreover, adding a polyethylenimine prime layer to the LbL shell affects the growth of hydrogen-bonded multilayers which consequently results in dramatically stiffer, thicker, and rougher LbL shells with the elastic modulus increasing by more than an order of magnitude, up to 4.3 MPa. An alternation of the elastic properties of very compliant hydrogen-bonded shells by variation of molecular weight is a characteristic feature of weakly bonded LbL shells. Such an ability to alter the elastic modulus in a wide range is critically important for the design of highly compliant microcapsules with tunable mechanical stability, loading ability, and permeability.