Competitive metal–ligand binding between CdTe quantum dots and EDTA for free Ca2+ determination

• EDTA induced photoluminescent quenching of TGA–CdTe QDs.
• EDTA acts as a ligand-detaching agent inducing the surface depassivation of QDs.
• EDTA forms a complex with free Ca2+ ions reducing the QDs surface depassivation.
• PL quenching of QDs was restrained in the presence of Ca2+
• The reaction schemes were implemented in MPFS for the selective Ca2+ determination.

In this work, a fluorometric approach for the selective determination of calcium by using CdTe nanocrystals as chemosensors, was developed. The quantum dots interacted not with the metal, but with a ligand that also bonded the metal. The fluorescence response was modulated by the extension of the competitive metal–ligand binding, and therefore the amount of free ligand. CdTe quantum dots (QDs) with different capping layers were evaluated, as the QDs surface chemistry and capping nature affected recognition, thus the magnitude of the ensuing fluorescence quenching. The developed procedure was automated by using a multipumping flow system.Upon optimization, thioglycolic acid (TGA) and EDTA were selected as capping and ligand, respectively, providing a linear working range for calcium concentrations between 0.80–3.20 mg L−1, and a detection limit of 0.66 mg L−1. A quenching mechanism relying on nanocrystal destabilization upon detachment of surface Cd by the ligand was proposed.

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