Salt-induced thermochromism of a conjugated polyelectrolyte

We report here the photophysical properties of a water-soluble conjugated polythiophene with cationic side-chains. When dissolved in aqueous buffer solution (PBS, phosphate buffered saline), there is ordering of the polymer chains due to the presence of the salts, in contrast to pure water, where a random-coil conformation is adopted at room temperature. The ordering leads to a pronounced colour change of the solution (the absorption maximum shifts from 400 nm to 525 nm). Combining resonance Raman spectroscopy with density functional theory computations, we show a significant backbone planarization in the ordered phase. Moreover, the ratio of ordered phase to random-coil phase in PBS solution, as well as the extent of intermolecular interactions in the ordered phase, can be tuned by varying the temperature. Femtosecond transient absorption spectroscopy reveals that the excited- state behaviour of the polyelectrolyte is strongly affected by the degree of ordering. While triplet state formation is favoured in the random-coil chains, the ordered chains show a weak yield of polarons, related to interchain interactions. The investigated polyelectrolyte has been previously used as a biological DNA sensor, based on optical transduction when the conformation of the polyelectrolyte changes during assembly with the biomolecule. Therefore, our results, by correlating the photophysical properties of the polyelectrolyte to backbone and intermolecular conformation in a biologically relevant buffer, provide a significant step forward in understanding the mechanism of the biological sensing

Structural and photophysical templating of conjugated polyelectrolytes with single-stranded DNA

A promising approach to influence and control the photophysical properties of conjugated polymers is directing their molecular conformation by templating. We explore here the templating effect of single-stranded DNA oligomers (ssDNAs) on cationic polythiophenes with the goal to uncover the intermolecular interactions that direct the polymer backbone conformation. We have comprehensively characterized the optical behavior and structure of the polythiophenes in conformationally distinct complexes depending on the sequence of nucleic bases and addressed the effect on the ultrafast excited-state relaxation. This, in combination with molecular dynamics simulations, allowed us a detailed atomistic-level understanding of the structure−property correlations. We find that electrostatic and other noncovalent interactions direct the assembly with the polymer, and we identify that optimal templating is achieved with (ideally 10−20) consecutive cytosine bases through numerous π-stacking interactions with the thiophene rings and side groups of the polymer, leading to a rigid assembly with ssDNA, with highly ordered chains and unique optical signatures. Our insights are an important step forward in an effective approach to structural templating and optoelectronic control of conjugated polymers and organic materials in general

Singlet oxygen plays a key role in the toxicity and DNA damage caused by nanometric TiO2 in human keratinocytes

Nanometric TiO2 has been reported to be cytotoxic and genotoxic in different in vitro models when activated by UV light. However, a clear picture of the species mediating the observed toxic effects is still missing. Here, a nanometric TiO2 powder has been modified at the surface to completely inhibit its photo-catalytic activity and to inhibit the generation of all reactive species except for singlet oxygen. The prepared powders have been tested for their ability to induce strand breaks in plasmid DNA and for their cytotoxicity and genotoxicity toward human keratinocyte (HaCaT) cells (100–500 mg mL1, 15 min UVA/B exposure at 216–36 mJ m2 respectively). The data reported herein indicate that the phototoxicity of TiO2 is mainly triggered by particle-derived singlet oxygen. The data presented herein contribute to the knowledge of structure–activity relationships which are needed for the design of safe nanomaterials.JRC.I.4-Nanobioscience

Inhibition of the ROS-mediated cytotoxicity and genotoxicity of nano-TiO2 toward human keratinocyte cells by iron doping

Nano-TiO2 powders are widely used in sunscreen lotions as UV filters in combination with other substances. The activation of TiO2 by UV rays leads to the release of reactive oxygen species (ROS, e.g., hydroxyl radicals and singlet oxygen) which are potentially harmful. For this reason the TiO2 particles are generally coated with inert materials (e.g., silica or alumina) that inhibit such reactivity. Alternatively, the release of ROS may be inhibited by introducing in the TiO2 lattice doping elements. In the present study we report a new modification consisting in a wet impreg-nation of TiO2 with iron salts followed by a thermal treatment that results in an inhibition of the surface reactivity. The insertion of iron ions also gradually reduces the ability of photo-activated TiO2 to cleave DNA and proteins. At the same time, a clear inhibition of cyto- and geno-toxicity toward human (HaCaT) keratinocytes was observed. The data presented herein suggest the insertion of Fe3+ ions at the surface of nano-TiO2 as a promising strategy to reduce the photo-induced toxicity of nano-TiO2 powders.JRC.I.4-Nanobioscience