Highly fluorescent carbon nanoparticles called carbon dots (CDs) have been the focus of intense research due to their simple chemical synthesis, nontoxic nature, and broad application potential including optoelectronics, photocatalysis, biomedicine, and energy-related technologies. Although a detailed elucidation of the mechanism of their photoluminescence (PL) remains an unmet challenge, the CDs exhibit robust, reproducible, and environment-sensitive PL signals, enabling us to monitor selected chemical phenomena including phase transitions or detection of ultralow concentrations of molecular species in solution. Herein, we report the PL turn-off/on behavior of aqueous CDs allowing the reversible monitoring of the water-ice phase transition. The bright PL attributable to molecular fluorophores present on the CD surface was quenched by changing the liquid aqueous environment to solid phase (ice). Based on light-induced electron paramagnetic resonance (LEPR) measurements and density functional theory (DFT) calculations, the proposed kinetic model assuming the presence of charge-separated trap states rationalized the observed sensitivity of PL lifetimes to the environment. Importantly, the PL quenching induced by freezing could be suppressed by adding a small amount of alcohols. This was attributed to a high tendency of alcohol to increase its concentration at the CD/solvent interface, as revealed by all-atom molecular dynamics simulations. Based on this behavior, a fluorescence "turn-on" alcohol sensor for exhaled breath condensate (EBC) analysis has been developed. This provided an easy method to detect alcohols among other common interferents in EBC with a low detection limit (100 ppm), which has a potential to become an inexpensive and noninvasive clinically useful diagnostic tool for early stage lung cancer screening.

Department of Experimental Physics Faculty of Science Palacký University Olomouc 17 listopadu 12 Olomouc 786 41 Czech Republic

Department of Materials Science and Engineering and Center for Functional Photonics City University of Hong Kong 83 Tat Chee Avenue Hong Kong S A R China

Department of Physical Chemistry Faculty of Science Palacký University Olomouc 17 listopadu 12 Olomouc 771 46 Czech Republic

Nanotechnology Centre VŠB Technical University of Ostrava 17 listopadu 2172 15 Ostrava Poruba 708 00 Czech Republic

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Šlechtitelů 27 Olomouc 783 71 Czech Republic

Regional Centre of Advanced Technologies and Materials Faculty of Science Palacký University Olomouc Šlechtitelů 27 Olomouc 783 71 Czech Republic

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