Functionalizing TiO₂ Nanoparticles with Fluorescent Cyanine Dye for Photodynamic Therapy and Bioimaging: A DFT and TDDFT Study

In the field of nanomedicine, significant attention is directed toward near-infrared (NIR) light-responsive inorganic nanosystems, primarily for their applications in photodynamic therapy and fluorescence bioimaging. The crucial role of the NIR range lies in enabling optimal tissue penetration, which is essential for both irradiating and detecting nanoparticles deep within the human body. In this study, we employed density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to explore the structural and electronic properties of cyanine-functionalized TiO2 spherical nanoparticles (NPs) with a realistic diameter of 2.2 nm. We revealed that different adsorption configurations of cyanine (VG20-C-1) on the TiO2 NP surface exhibit distinct features in the optical spectra. These cyanine dyes, serving as bifunctional linkers with two carboxylic end groups, can adsorb in either a side-on mode (binding with both end groups) or an end-on mode (binding only one end group). In end-on adsorption structures, low-energy excitations are exclusive to dye-to-dye electronic transitions, while side-on structures exhibit electron charge transfer excitations from the dye to the TiO2 NP at low energy. This thorough analysis provides a rational foundation for designing cyanine-functionalized TiO2 nanosystems with optimal optical characteristics tailored for specific nanomedical applications such as photodynamic therapy or fluorescence bioimaging.