Synthesis of Dual-Emitting CdZnSe/Mn:ZnS Quantum Dots for Sensing the pH Change in Live Cells

Intracellular pH is an important regulator of cell function, and its subtle changes may greatly affect cell activities and cause diseases. Reliable imaging of intracellular pH remains a huge challenge. Dual-emitting Mn²⁺-doped quantum dots (QDs) can be directly used as a ratiometric fluorescent probe without further modification, but they displayed low performance in terms of photoluminescence, stability, and intensity ratio regulation. Here, we report intrinsic dual-emitting CdZnSe/Mn:ZnS QDs with high photoluminescence efficiency, good stability, and biocompatibility. The emission intensity ratio was selectively regulated by Mn²⁺ doping. Because of aggregation-induced quenching of QDs, the exciton emission of CdZnSe/Mn:ZnS QDs (471 nm) was sensitive to pH, while the Mn²⁺-doped emission (606 nm) was passive to pH, which was probably due to little self-quenching in Mn²⁺-doped emission caused by weak Mn-Mn coupling interaction. Dual-emitting CdZnSe/Mn:ZnS QDs exhibited excellent pH-responsiveness in the range of pH 4.0 to 12.0 and were used for pH imaging in live HeLa cells. When the pH value of HeLa cells changed from 5.0 to 9.0, the emission changed from red to blue. Furthermore, these dual-emitting CdZnSe/Mn:ZnS QDs can provide a versatile platform for biosensors and biological imaging.