Visible-light excitable thermally activated delayed fluorescence in aqueous solution from F, N-doped carbon dots confined in silica nanoparticles

Abstract Visible light has better biocompatibility and is more accessible for anti-counterfeiting compared to UV light. However, the synthesis of visible light excitable CDs-based long afterglow materials is still a challenge, especially in aqueous solution. Herein, we provide the first example of visible-light excitable thermally activated delayed fluorescence (TADF) in aqueous solution by confining the fluorine and nitrogen co-doped CDs in silica nanoparticles (F, NCDs@SiO2). The hydrogen network and covalent bonds effectively protect the triplet excitons from quenching by water oxygen, so the nanoparticles can produce long-lived TADF in aqueous solutions under the excitation of white light. Meanwhile, the large number of silanol groups on the surface of amorphous silicon endows F, NCDs@SiO2 with water solubility making them possible for time-resolved probes. We reveal that fluorine doping is essential for obtaining visible-light excitable TADF, which redshifts the excitation spectra and narrows the energy level between the singlet and triplet states through strong electron-withdrawing effect and heavy-atom effects. Our work provides a new strategy for the development of visible-light-excited TADF materials in aqueous solution and potential applications of CDs in information encryption, temperature sensor and time-resolved fluorescence bioimaging.