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Nanomaterials 2021, 11, 96 4 of 19 3.2. Film Swelling Studies PVA films containing silver nitrate at 1%, 3% or 5% w/w (silver nitrate/PVA), with no curing or curing at 80 ◦C, 110 ◦C or 140 ◦C and with thicknesses of 100 μm (60 mg) or 200 μm (120 mg) were prepared as described above. These films were then stored for one week in the dark before use. Films with approximate diameters of 2 cm were then placed on moistened 0.2 μm filter discs (Millipore, Billerica, MA, USA) and weighed. The films and filters were covered with a layer of 0.5 mL of deionized water. After set time periods the filter discs and adherent PVA-silver gel were moved to a Millipore vacuum apparatus and vacuum was applied to draw all excess water from the filter over approximately 15 s. The combined PVA gel and filter were reweighed and then covered with a layer of excess deionized water (approximately 0.5 mL). The weight gain (swelling) and weight loss (dissolution) were then calculated as a percentage of the original dry film weight. 3.3. Silver Release Studies and Characterization Films (10 mg squares cut from full film, n = 4) containing 1%, 3% or 5% w/w silver nitrate that had been heated for 90 min at 140 ◦C were placed in deionized water (5 mL) and all the media was sampled at regular intervals for silver analysis by atomic absorption (AA) spectroscopy. After the media was removed at each time point, 5 mL of fresh water was added to the film. The silver content in the release media was analyzed using a Perkin Elmer AA 560 spectrometer (Waltham, MA, USA) equipped with a Perkin Elmer silver lamp. This method allows the detection of low nanogram levels of silver. Initially a silver electrode system was used but the limit of detection was 10 μg/mL and unsuitable. Water was used as the release media because any buffer salts in the media would interfere with the AA methods. In order to determine the effect of temperature, a similar study was run on films containing 5% w/w silver nitrate, which had been heat cured at either 80, 110, 125 or 140 ◦C for 90 min. In a third investigation, the effect of curing time was determined through monitoring silver release from films heat cured at 140 ◦C for 15, 30, 45 or 90 min. Each release study was run for at least two weeks and the results plotted as the calculated percent silver released as a function of time. The release media from selected PVA-silver films were subjected to particle size analysis using a Zetasizer Nano-ZS instrument (Malvern Instruments Ltd., Malvern, UK). 3.4. X-ray Diffraction of PVA-Silver Films X-ray diffraction (XRD) experiments involved use of a Bruker Apex DUO instrument (Billerica, MA, USA) equipped with a Cu Kα source (λ = 0.15418 nm) and a heating stage. In order to learn more about the PVA crosslinking process, an XRD experiment was run on a dried PVA film containing 5% w/w silver nitrate with in situ heating of the film at 140 ◦C in the XRD instrument. X-ray diffractograms were collected once the sample film had reached the set temperature and thereafter at 15-min intervals for two hours. The resulting diffractograms were then plotted as an overlay against 2θ, the Bragg diffraction angle. 3.5. Scanning Electron Microscopy (SEM) SEM images of PVA-silver films were obtained using an FEI Helios 650 Nanolab dual beam instrument (Hillsboro, OR, USA) and imaging at low voltage with secondary electrons (1 keV for true surface imaging). For the SEM experiments, the films were mounted on aluminum stubs using carbon adhesive tape. The edges of the films were coated with colloidal silver in order to make an electrical connection between the film and the aluminum stub. The film area of interest was not painted with colloidal silver but was coated with a 5 nm-thick electrically conductive layer of carbon using a Leica EM MED020 modular coating system (Leica Microsystems Inc., Concord, ON, Canada). 3.6. Antimicrobial Activity of Silver-Loaded PVA Films In order to evaluate the antimicrobial activity of PVA-silver films, we selected three film sets loaded with 1%, 3% or 5% w/w silver nitrate, each of which had been previouslyPDF Image | Hydrogel Forming Dressings Containing Silver Nanoparticles
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