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  • In general graphene nanoplatelets have many useful features


    In general, graphene nanoplatelets have many useful features for the design of multifunctional composite materials, including excellent electrical and thermal conductivities, considerable weight-saving lightness, and high mechanical strength [26]. Moreover, they are characterized by a high specific surface area, which allows for several types of functionalization using both covalent and non-covalent approaches [27], [28]. However, the Idoxuridine mg of graphene nanomaterials with a polymer matrix is not a trivial task, and the overall functional properties of the materials are highly affected by the amount and homogeneity of the filler dispersion [29], [30], [31]. In this study, we investigate the fabrication process of graphene-based nanocomposite films containing UV-sensitive GNP-DNA fillers integrated with a flexible PDMS matrix, and we characterize their sensing properties under UV-C irradiation. Besides having specific UV sensitivity, DNA also acts as efficient solubilizing agent for the graphene nanomaterial. In fact, graphene nanoplatelets can be functionalized with DNA molecules by non-covalent interactions, such as base-graphene π stacking and interactions between the DNA amines and the carboxyl groups of the nanoplatelets [32], [33], [34]. The dispersion of the GNP-DNA filler in the PDMS matrix was improved using isopropyl alcohol (IPA), which enabled the preparation of nanocomposite samples with sufficiently homogenous properties. On the other hand, PDMS is well known for its chemical inertia, biocompatibility and flexibility, which make it an excellent candidate for wearable sensors. More relevantly, PDMS exhibits high optical transparency to UV radiation, and is one of few polymers with good transmittance in the UV-C band (up to 240 nm) [35]. For these reasons, PDMS was chosen as polymer matrix for embedding the GNP-DNA fillers and allowing for characterization of their response upon exposure of the nanocomposites to UV-C radiation. A full investigation of the GNP-DNA/PDMS surfaces before and after UV-C irradiation, using several complementary techniques, is presented in order to understand the mechanisms of material-radiation interactions that cause changes of the nanocomposite properties.
    Experimental methodology
    Results and discussion UV-sensitive nanocomposite films were prepared by integration of GNP-DNA fillers, dispersed in isopropyl alcohol (IPA), with a PDMS matrix made of Sylgard 184. Different filler concentrations from 20 wt% to 40 wt% were considered. The use of IPA was found to be beneficial, among several organic solvents that were investigated, in order to obtain pre-polymer mixtures that were sufficiently homogeneous at high filler fractions and that could be successfully cured in oven at 75 °C for 24 h. Higher curing temperatures were not selected to avoid the formation of porosity in the nanocomposite structure at temperatures near the boiling point of IPA. Sylgard 184 PDMS was selected as matrix for the nanocomposites due to its biocompatibility and flexible nature, allowing for the fabrication of conformable and wearable sensors, and because it is transparent to UV radiation also in the UV-C region (200–280 nm). PDMS has a transmittance value of about 50% at 254 nm, which is the wavelength at which we performed the UV-C irradiation tests, as compared to other flexible polymers, such as polystyrene and styrene elastomers, for which the UV transmittance drops to values around 10% at wavelengths below 280 nm [35]. A sufficient degree of UV-C transparency by the polymer matrix is necessary to test the interaction of UV radiation with the embedded GNP-DNA fillers without having relevant shielding effects coming from the matrix. Fig. 1 shows optical microscopy images of the nanocomposite surfaces at increasing concentration of GNP-DNA filler, indicating a clear improvement of the surface homogeneity at higher loading. Larger aggregates are present in the nanocomposite with 20 wt% of GNP-DNA with brighter and darker regions alternating in the polymer matrix (Fig. 1a). On the other hand, a smoother composite morphology with smaller white-reflecting regions was obtained at 40 wt% of GNP-DNA (Fig. 1d). The relatively homogeneous filler distribution on the macroscale has effects on the surface electrical properties of the nanocomposite films.