Excitation-independent carbon dot probes for exogenous and endogenous Fe3+ sensing in living cells: Fluorescence lifetime and sensing mechanism

Ferric ions play significant roles in human bodily functions, since both overload and deficiency can cause major physiological dysfunctions. Hence, engineering of sensitive, selective sensors to detect such ions merits attention. We found that certain CDs derived from a nitrogen-containing precursor (phthalocyanine) in suitable media could provide significant advantages. Excitation-independent carbon dots (CDs) were prepared through facile one-pot hydrothermal (W-CDs) and solvothermal (E-CDs) methods. Compared to W-CDs, ethanol-mediated E-CDs provide narrower particle size distribution, higher pyrrolic and pyridinic N-content, higher quantum yield (QY(E-CDs)/QY(W-CDs) = 2.58), strong green luminescence, excellent biocompatibility and efficient two-photon absorption. The E-CDs enabled superior NIR excitable two-photon bioimaging of breast cancer cells at variable depths and fluorescence quenching (81% extinction) for 10-600 mu M Fe3+ ions. The mechanism of sensing was elucidated based on the fluorescence lifetime and the Stern-Volmer formalism. Intracellular Fe3+ ion sensing was first confirmed with E-CDs for lysed MCF-7 cells and quenching was detected in direct proportion to Fe3+ concentration (0-80 mu M test range) with an LOD of 10.8 nM. Next, MCF-7 cells supplemented with variable concentrations of Fe3+ were incubated with E-CDs for sensing intracellular Fe3+ in live cells. The E-CDs showed high sensitivity for sensing endogenous Fe3+ ions in a stress-induced cell-based model.