Water-Soluble, Thermostable, Photomodulated Color-Switching Quantum Dots.

Photoswitchable probes are of great utility in fluorescence microscopy, permitting numerous determinations, including molecular localization for super-resolution, based on their modifiable emission intensity and spectra. We have coated a blue-emitting (425 nm) quantum dot (QD) with a diheteroarylethene photochrome (PCf), the closed form isomer of which has absorption and emission maxima at 440 and 520-530 nm, respectively, and thus functions as a fluorescent acceptor for the QD donor in Förster resonance energy transfer (FRET). The transition from the non-absorbing, non-fluorescent open state to the fluorescent closed state is achieved by irradiation in the near-UV and reversed by visible light. The PCf is coupled to an amphiphilic polymer that stably coats the QD, thereby creating a water-soluble color-switching QD (csQD) emitting in the blue after visible light irradiation and in the green after UV irradiation. Thus, csQDs photomodulate between two observable states, without the "off" state of previous constructs. The resulting change in the emission ratios of the QD and PCf is up to 180 %, and the csQD can undergo multiple photocycles with minimal fatigue.