Fig. 2

a Fluorescence stability of Ln-doped QDs incubated (up to 48 h) in solutions mimicking distinct physiological environments. RS Ringer’s solution, EE endosomal environment, NIE neutral intracellular environment. b 2D fluorescence emission–excitation spectral maps of Ln-doped QDs obtained upon incubation (48 h) in various solutions mimicking physiological conditions (plasma, endosomes and cytoplasm). Spectra illustrate environment-dependent optima for excitation and emission. Red spots highlight maximum performance of fluorescence, blue spots—no or negligible fluorescence recorded