Origin of Luminescence‐Based Detection of Metal Ions by Mn–Doped ZnS Quantum Dots

The health hazards associated with heavy metal ions in water demand the development of efficient and portable sensors, for rapid onsite detection of these ions. Several research groups have developed colorimetric/visual sensors based on plasmonic nanomaterials and quantum dots (QDs). Attempts for specific detection of metal ions have been partially achieved through the interaction between the metal ion and the passivating ligands around the QD. However, the underlying mechanism is not clearly understood. Here, we have used water‐soluble Mn‐doped ZnS QD to effectively detect Hg2+, Pb2+, and Cd2+ through the quenching of QD emission and understand the mechanism of sensing. Stern‐Volmer plots indicate that the quenching is static in nature for Pb2+, and Cd2+, while for Hg2+, it is a combination of static and dynamic quenching. Overall, the metal ions bind to the QD through the passivating ligand. After excitation, the electron from the conduction band of the QD can get injected to the metal ion – which decreases the photoluminescence of the QD. The electron injection depends on the reduction potential of the metal ion, the orbital overlap and the overall stabilization energy of the metal ions bound to the QD. Hence, this method of sensing is not selective to a specific metal ion. A solid state sensor of QD‐rGO composite detects Pb2+ down to 0.4 ppb. The findings will be important for future improvement of colorimetric/visual sensors based on QD emission. Water‐soluble Mn‐doped ZnS quantum dots (QDs) are used to understand the quenching of QD emission in the presence of metal ions (M2+). The excited electron from the conduction band of the QD can get injected into the metal ion, causing quenching in the emission of the QD. The electron injection depends on the reduction potential of the metal ion, the orbital overlap, and the overall stabilization energy of the metal ions bound to the QD.