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  • The analysis of the surface

    2020-07-27

    The analysis of the surface wettability by static contact angle revealed a good grade of hydrophobicity of the non-irradiated nanocomposites, which are characterized by water contact angles (WCA) above 110° (Fig. 3). The WCA values strongly increase with the GNP-DNA content, reaching 134.9° ± 6.2° for the nanocomposites with 40 wt% of filler. Ten replicate measurements on different spots of the nanocomposite surfaces were performed, and the relative standard deviations were found to be below 5% (Table 2). After exposure to UV-C light for 8 days (λ = 254 nm, radiation dose 4.4 × 106 J/m2), the WCA values of all nanocomposites and that of pure PDMS decreased. In particular, the surface hydrophobicity of the nanocomposites was reduced in proportion to the GNP-DNA content, with the smallest WCA decrease (2.7%) for the 20% GNP-DNA/PDMS surfaces and the largest (11.0%) for the 40% of GNP-DNA/PDMS samples. It is noted that the WCA values following UV-C exposure were measured after few days from the irradiation tests, to ensure that potential surface recovery phenomena were completed. The decrease of surface hydrophobicity of pure PDMS under UV-C irradiation at ambient conditions is in agreement with reports in the literature [38], [39] and can be ascribed to an oxidation phenomenon, which results in the formation of carboxylic 175 moieties on the polymer chains [39], [40]. To further investigate the decrease of WCA values occurring under UV irradiation, a surface free energy analysis following the Owens-Wendt method [36] was carried out with two different testing liquids (water and diiodomethane). Results in terms of SFE and its dispersive and polar components for the GNP-DNA/PDMS nanocomposite films and for neat PDMS are reported in Table 2. For both irradiated and non-irradiated samples, the SFE decreases at increasing GNP-DNA content. This result indicates that the interactions of the nanocomposites with water and with diiodomethane are less favored at higher GNP-DNA content. The dispersive component (γd), which is due to the dispersive interactions among non-polar molecules, is predominant over the polar one (γp) for all investigated sample, but decrease from PDMS to the nanocomposites and upon increase of the GNP-DNA concentration. After UV-C irradiation, the SFE and its dispersive component show a marked increase in the nanocomposite samples, whereas it is almost unvaried for the neat PDMS, indicating that the irradiation mainly affects the GNP-DNA filler and less the polymer matrix. For the neat PDMS sample, an increase of the polar component is observed after irradiation, which is consistent with an oxidation phenomenon generating carboxylic acids on the polymer surface. The thermal behavior of the UV-sensitive nanocomposites was investigated by DSC before and after irradiation in an extended temperature range (from −40 to 250 °C). Fig. 4a shows the thermograms of the non-irradiated GNP-DNA/PDMS nanocomposites in comparison with a neat PDMS sample. The filler-loaded samples show a significant endothermal peak at about 146 °C, which is slightly shifted to higher temperatures after UV-C exposure (Fig. 4b), whereas PDMS is thermally stable over the entire temperature range considered for this study. To understand the nature of the endothermal peak, pure DNA samples were measured by DSC at the same conditions (Fig. 5). A similar shift of the endothermal peak to higher temperatures after UV-C irradiation was observed. Therefore, the presence of the peak in the nanocomposites was ascribed to denaturation of the double-stranded DNA component. The shift of the denaturation peak after irradiation can be explained with the occurrence of a UV-induced crosslinking of the DNA chains, which increases the thermal stability of the biological component. The DSC measurements were analyzed in terms of peak temperature and enthalpy change (ΔH) associated with the endothermic events (Table 3). The relation between the endothermic peak and the DNA component of the nanocomposites is also supported by the fact that the ΔH values increase when the concentration of the GNP-DNA filler, and so the amount of DNA, is larger. After UV-C irradiation, all samples containing DNA show a shift of the denaturation peak and a corresponding increase of the enthalpy change, which is an additional indication of the higher crosslinked nature of DNA and its chemical surrounding.