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Printer toner, which accounts for a large fraction of electrical and electronic wastes, poses serious environmental and health risks but cannot be efficiently recycled because of its high chemical durability and complexity. Specifically, only 20–30% of waste toner is recycled, while much of the remainder ends up in landfills and thus contributes to air/soil pollution and respiratory disease development. Herein, we report the one-pot hydrothermal processing of waste toner into well-dispersed and uniform-sized carbon dots featuring abundant surface defects (e.g., radicals and dangling bonds), strongly interacting with UV (200–400 nm) light, and exhibiting intense photoluminescence in both UV (300 nm) and visible (400–500 nm) ranges. In particular, ethanolamine is used to chemically disintegrate the carbon black cores of toner particles into nanoparticles and simultaneously passivate the nanoparticle surface. According to the results of time-correlated single photon counting spectroscopy measurements, UV- and visible-region photoluminescence originates from amorphous carbon structures in the core and oxygen/nitrogen-containing functional groups on the surface, respectively. Finally, in view of their excellent visible-region optical properties and the ability to generate reactive oxygen species under laser irradiation, the waste toner–derived carbon dots are demonstrated to be promising agents for bioimaging and photodynamic therapy, respectively. • The challenge of recycling printer toner, a significant component of electronic waste, is addressed. • One-pot hydrothermal method to recycle waste toner into waste toner-derived carbon dots (wt-CDs) is proposed. • wt-CDs interact with UV light (200–400 nm) and exhibit photoluminescence (PL) in UV (300 nm) and visible (400–500 nm) ranges. • The UV and visible PL originate from the amorphous C core and the surface O/N-containing functional groups, respectively. • wt-CDs, capable of generating singlet oxygen under laser irradiation, show potential as photodynamic therapy agents. [ABSTRACT FROM AUTHOR]