Synthesis of cadmium selenide quantum dots modified with thiourea type ligands as fluorescent probes for iodide ions

Highly luminescent and stable CdSe quantum dots (QDs) modified with 4-substituted pyridine type ligands containing thiourea groups: 1-(4-fluorobenzoyl)-3-(5-(pyridin-4-yl)-1,3,4-thiadiazol-2-yl) thiourea (PyTU); were prepared through a ligand exchange process, and were characterized by transmission electron microscopy (TEM), fluorescence spectroscopy, UV-vis spectroscopy and FT-IR spectroscopy. The synthesized nanocomposites show higher fluorescence (FL) intensity and are more stable in comparison with original QDs. The PyTU-CdSe QDs allow a highly sensitive determination of iodide via significant FL intensity quenching. Under optimal conditions, the relative FL intensity is decreased linearly with increasing iodide concentration in the range 0-50 mu M with a detection limit of 1.5 x 10(-9) M, while the FL intensity of PyTU-CdSe QDs to other anions including F-, Cl-, Br-, SCN-, HCOO-, CH3COO-, HSO3-, and C2O42- is negligible. It is found that the decreased luminescence intensity of the PyTU-CdSe QDs dependent on the concentration of iodide is best described by a Stern-Volmer equation. The possible mechanism is discussed.