Chitosan-based composite coatings of upconverting NaYF4: Yb, Tm nanoparticles and their energy transfer with Rhodamine 6G dye molecules

Upconverting nanoparticles (UCNPs) emit visible and UV photons under near-infrared excitation, and can potentially be utilized in areas such as sensing, photodynamic cancer therapy or photovoltaic devices. In various applications, e.g. optical sensors, it is advantageous to immobilize UCNPs on a substrate using an embedding material. In this work, a novel composite coating design is presented, using chitosan biopolymer as a matrix to incorporate UCNPs, which is also shown to be a suitable coating system for studying energy transfer processes between UCNPs and organic molecules in aqueous media. NaYF 4 : 20% Yb, 0.5% Tm UCNPs are prepared via solvothermal method and incorporated in chitosan biopolymer to prepare thin, transparent composite coatings on glass surfaces by a facile spin-coating process. It is observed that the presence of UCNPs provides a rough surface, and the surface of the particles protruding from the coating is covered by a thin chitosan layer. The emission intensity of the UCNPs remains high after immobilization in the chitosan layer, the expected upconversion emission peaks are observed in coating form. Energy transfer between UCNPs and Rhodamine 6G fluorescent dye molecules is confirmed to occur, both in UCNP aqueous suspensions and in the composite coatings. The chitosan layer can be permeated by aqueous solutions, enabling energy transfer between the embedded UCNPs within the coating and an organic species dissolved in aqueous medium. The UCNP@chitosan composite coating design presented in this work is suitable for various future applications, mainly thin film based sensors for detecting biomolecules or metal ions.