Wide-bandgap quantum dots with large-span fluorescence switching and two-photon emission via protonation/deprotonation†

Wide-bandgap quantum dots (QDs) have recognized as the third generation of low-dimensional semiconductors with reliable optical response, outstanding stability and biocompatibility for long-term biosensing in health care applications, especially in extreme environments. However, the applicable scenarios are limited, because optical switching over a large span from the wide bandgap (deep UV region) to the visible region is challenging due to the stable framework and the high breakdown difficulty. In this work, we adopted a facile protonation/deprotonation treatment process, which is suitable for different biogenic environments, to modulate the one-photon and two-photon fluorescence of wide-bandgap QDs. Two fluorescent centers are coordinative to control one-photon emission from deep blue (410 nm) to yellow (585 nm) emission with a wide large-span modulation, superior to most reports under different acid–base environments. The electron transition between electron-donating amine (–NH₂) groups and H⁺ (–OH) changes the degree of nonradiative transition, narrowing the breadth of the full width at half maximum (FWHM) by 34.4%. Moreover, our QDs exhibit two-photon fluorescence at 710 nm, which have never reported before for wide-bandgap nitrides. It shows pH-independent two-photon fluorescence because of the intrinsic electron–phonon coupling of the π-conjugated structure. This work introduces a simple design strategy to realize fluorescence control over a large span in wide-bandgap nanomaterials, enabling distensible applications in biological health and pH related linear or nonlinear optical fields.