The thermal behavior of the UV
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 tepp 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.
The surface morphology of the GNP-DNA/PDMS nanocomposites with higher filler content (40 wt%) was investigated by SEM before and after UV-C exposure for 8 days (Fig. 6a and 6c). SEM images unveiled a markedly different surface morphology in the irradiated nanocomposite samples. In particular, after UV-C irradiation the nanocomposite surfaces appear smoother. To quantify the UV-induced changes at the level of surface roughness, a three-dimensional reconstruction of the profiles was performed with the MountainsMap software starting from two SEM images acquired at two different tilt angles (0° and 5°). The surface roughness values were calculated considering a minimum of ten profiles extracted across the reconstructed surfaces of the non-irradiated and irradiated 40% GNP-DNA/PDMS samples (Fig. 6b and 6c, respectively). Mean values of the surface roughness were 25.9 ± 3 µm and 15.4 ± 2.4 µm before and after UV-C irradiation, respectively, with a decrease of 40.5% that is consistent with the observed surface erosion.
Acknowledgements This work was supported by grants from the Italian Ministry of Education, Universities and Research (Rita Levi Montalcini Programme) and from Sapienza University of Rome (Research Grant RM116155064215F1) to M.G. Santonicola. The authors would like to thank Digital Surf for providing the MountainsMap software.
Introduction The triphenylmethane dyes fuchsin, malachite green, crystal violet, and methyl green are all histochemical stains which are known to bind duplex DNA (Fig. 1) . Previous studies on the interaction of these compounds with nucleic acids have revealed a non-intercalative mode of association and a preference for binding DNA over RNA . Furthermore, evidence presented by Kim and Norden  suggests that methyl green binds to the major groove of DNA. In contrast, the vast majority of nucleic acid-binding small molecules and natural products prefer to occupy the narrower minor groove, where hydrophobic and van der Waals interactions with the walls and floor of the groove are maximized . The notable exceptions to this trend are the pluramycins, aflatoxins, azinomycin, leinamycin, and neocarzinostatin i-gb . Intercalation of a planar delocalized-π system into the backbone of DNA is a consistent binding mode among all of these natural products, and with the exception of the aflatoxins, all of these substances also possess polar major-groove binding moieties. The process of intercalation dramatically alters the structure of duplex DNA far from the drug binding site ; the lengthening and rigidification of the double helix is evidenced by viscosity increases of DNA solutions containing added intercalators .