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

Abstract

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 Mndoped ZnS QD to effectively detect Hg 2 + , Pb 2 + , and Cd 2 + through the quenching of QD emission and understand the mechanism of sensing. Stern-Volmer plots indicate that the quenching is static in nature for Pb 2 + , and Cd 2 + , while for Hg 2 + , 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 Pb 2 + down to 0.4 ppb. The findings will be important for future improvement of colorimetric/visual sensors based on QD emission.Heavy metal ions such as lead, mercury and cadmium pose severe potential threats to living beings as they can easily be accumulated in the body and cannot be detoxified by any chemical or biological processes. [1][2][3] Detections of these toxic metal ions are important and have received considerable interest. [4][5][6][7][8][9][10][11][12] Instrumental techniques such as inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS) are considered ideal methods to detect heavy metal ions at very low concentrations (∼ subppb). However, these methods suffer from disadvantages such as requirement of sophisticated instruments, time consump-tion, and high cost. Detecting heavy metal ions using rapid and easy methods with high sensitivity is a focal point of research.In the last two decades, there has been a significant development of several optical, [4][5][6][7] electrochemical, [8][9][10] and gel based sensors, [11,12] for sensing of heavy metal ions at trace levels (∼ 10 ppb). Sensors based on plasmonic nanomaterials show a change in color (shift in absorption wavelength) upon addition of the toxic heavy metal ions, depending upon the affinity of the heavy metal ion towards the surface capping agent on the nanoparticles. [13][14][15][16] These methods are restricted to solution state sensing and do not exhibit quick response. Therefore, there is an urge to develop visual sensors, which can effectively detect heavy metal ions rapidly (within seconds) under ambient conditions. [17] Due to their tunable and high photolumin...

Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace Heavy Metals in Water

“…The composite formed by Mn-doped ZnS QD and graphene oxide detects Pb 2+ down to 0.4 ppb. Further, the interaction between the metal ion and the passivating ligands attached to the QD also contributes partway to the specific detection combined with the quenching of QD [34]. The green approach is also present on the fast detection, which is vital since the production of nanostructures requires to minimize the use of solvent, energy, time, and the release of odd substances to the environment.…”