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A ratiometric fluorescent probe based on a “dual-key-and-lock” design was reported for the imaging of vicinal dithiol proteins in living cells.

Lysosomal polarity is considered a key indicator of lysosomal function due to its significant impact on membrane fluidity and enzymatic reactions in lysosomes. Monitoring lysosomal polarity can gain insight into the related physiological and pathological processes and develop new diagnostic methods. However, current fluorescent probes with lysosomal polarity response suffer from narrow linear range, photobleaching and complicated preparation. Herein, a ratiometric fluorescent probe (r-bCDs) for intracellular lysosomal polarity imaging is designed and constructed by amide bond assembly of polarity-sensitive red fluorescent carbon dots (rCDs) and referenced blue fluorescent carbon dots (bCDs). r-bCDs show a much wider linear range of polarity response (orientation polarizability Δf from 0.020 to 0.315) than other probes, and the interference of uneven distribution and instrument factors can be effectively eliminated by ratiometric fluorescent sensing. Imaging of intracellular lysosomal polarity with r-bCDs is implemented to observe the polarity variation caused by the change of cell state and the difference between cancer cells and normal cells. This work provides a promising tool for studying the related physiological and pathological processes and developing new diagnostic methods.

Great efforts have been made to develop facile and efficient methods to prepare carbonaceous nanostructures with long-wavelength emission. Herein, we report a low-temperature aqueous strategy to synthesize red-emissive carbon polymer dots (R-CPDs) through the regulation of oxidative polymerization of p-phenylenediamine at 80 °C. The morphology, chemical composition and photophysical properties of the R-CPDs are characterized and analyzed in detail, thereby elucidating their photoluminescence origins from the surface state and crosslink enhanced emission effect. The resulting R-CPDs possess unique features including high pH-sensitivity within pH 4–6 and a wide-range tunable solvent-color effect (λem 528–600 nm). Moreover, the R-CPDs show high stability in physiological media with high salinity, and good resistance to photobleaching. In addition to their favorable biocompatibility, the R-CPDs are applied for monitoring the pH fluctuation in HeLa cells. This study not only provides a unique red emissive carbonaceous nanomaterial for cellular imaging and multicolor applications, but also presents a novel perspective for the construction of long-wavelength emission carbon-based nanomaterials by simple and controllable strategies.

Endogenous formaldehyde (FA) exists in many living cells and in inhomogeneous distribution in organelles. In particular, lysosomes play significant roles in FA generation and the biofunction of living cells. Herein, we developed a new ratiometric fluorescent nanoprobe, based on naphthalimide derivative (ND)-functionalized carbon dots (CDs), for monitoring endogenous FA in lysosomes. The fluorescence intensity (F535) of green-emitting ND at 535 nm serves as the response signal and the fluorescence intensity (F414) of blue-emitting CDs at 414 nm acts as the reference signal. The fluorescence intensity ratio (F535/F414) of the CD-ND probe is linearly correlated with FA concentration within the range of 1-40 μM in aqueous solution, and the detection limit (3σ/slope) is estimated to be 0.34 μM. As for practical application, this nanoprobe is utilized for the ratiometric fluorescence imaging of FA in live cells. Remarkably, this nanoprobe can specifically target and stain the lysosomes and detect exogenous and endogenous FA in HeLa cells. The new FA probe shows a superior lysosomal targeting ability with a Pearson's coefficient of 0.93, which is attributed to the macromolecular size and basic amine group functionalized surface of CD-ND.