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  • Nanocomposite films with a GNP DNA filler concentration

    2021-04-12

    Nanocomposite films with a GNP-DNA filler concentration of 20 wt%, 30 wt%, and 40 wt% were investigated by electrical impedance spectroscopy before and after UV-C irradiation (Fig. 2). EIS data were fitted to a RC-circuit model and surface resistivity values were calculated from the fitted resistance values. Each value in Fig. 2 was calculated averaging the results from ten EIS measurements for each sample. The relative standard deviations are in the range of 15–20%. Such high values of deviations were a consequence of the inherent variability of the electrical measurements on a nanocomposite surface with partially homogeneous filler dispersion and flexible polymer matrix. Nevertheless, results from the EIS analysis revealed that a significant decrease of surface resistivity is achieved when increasing the content of GNP-DNA to the value of 40 wt%. After exposure to UV-C radiation for 8 days, all GNP-DNA/PDMS surfaces showed an increase of their surface electrical resistivity (Fig. 2), and therefore a decrease of surface conductivity, which is reported in Table 1. The decrease of surface conductivity after radiation exposure was less evident in the 20% GNP-DNA/PDMS samples (-15%) than in the nanocomposites at higher concentrations of GNP-DNA. The largest variation of surface conductivity values after the 8-day UV-C exposure was observed for the 30% GNP-DNA/PDMS samples (-38%). These changes in the surface electrical properties are due to the lower degree of interconnections among the conductive carbon nanoparticles following irradiation by UV-C. Indeed, in previous work, we have shown that UV-C radiation causes significant changes in the morphology of exposed graphene nanoplatelets with a clear smoothing of the particle edges after irradiation, and a consequent reduction of the electrical conductivity of the network [37]. In this work, the 30% GNP-DNA/PDMS samples are more electrically conductive than the nanocomposites with less carbon fillers (20% GNP-DNA/PDMS), due to the higher degree of interconnections between the fillers, and they are also those showing a larger irradiation effect at the level of the conductive carbon network. On the other hand, upon increasing the GNP-DNA filler content to 40 wt%, an additional effect was involved in the response of the nanocomposite sensors. In this case, the UV-C interaction with the DNA biological component became predominant over the UV (R)-baclofen sale by the graphene network, thus limiting the decrease of electrical conductivity. This effect was elucidated by the DSC results, which indicated that the largest variations at the level of the DNA molecule due to the UV-induced crosslinking was maximum for the samples with 40 wt% of GNP-DNA filler (see discussion below and Table 3). 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 (R)-baclofen sale 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 acid 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.