Unraveling Dynamical and Light Effects on Functionalized Titanium Dioxide Nanoparticles for Bioconjugation

Functionalizing nanoparticles (NPs) with biological molecules is a promising modern strategy in bionanotechnology to build up smart bioinorganic devices for medical applications. Bifunctional linkers provide an interesting and ductile bioconjugation approach especially because they behave not only as anchoring and tethering agents but also as spacers between the NP and the biomolecules, which helps in maintaining their 3D structural and functional properties. In this work, by means of a wide set of density functional theory (DFT) electronic structure calculations and density functional tight binding (DFTB) molecular dynamics simulations, we provide an all-round investigation of the functionalization of realistic curved TiO2 NPs (2-3 nm size with similar to 800 atoms) with a catechol derivative, such as dopamine and DOPAC. We span from single-molecule adsorption to the full coverage regime. For the low coverage, we achieve a detailed description of the mechanisms of molecular adsorption, of the interfacial electronic charge-transfer effects, and of the processes following visible light irradiation (exciton formation, trapping, charge carrier diffusion, or recombination). We then consider a growing molecular layer on the NP and analyze the self-assembling mechanism and the effects on the electronic properties of the complex. Finally, for the maximum coverage (46 molecules per NP) we perform molecular dynamics runs at 300 K to compare the molecular configuration and electronic properties of the NP/linker complex interface before and after thermal treatment to better account for the competition between molecule/surface and molecule/molecule interactions. The use of curved NP surfaces combined with dopamine, with respect to a flat one and DOPAC, respectively, is found to be more effective for bioconjugation.