This study aimed to answer the question as whether crystal defects at the surface of soluble capped CdSe nanocrystals (or quantum dots, QDs) in water colloidal suspension are involved in the mechanism of fluorescence quenching induced by metal cations. Nanocrystals of CdSe were synthesized by an aqueous protocol, varying the ratio between the CdSe precursors and the grafted ligand mercaptosuccinic acid (MSA). Changing the MSA/CdSe ratio during synthesis impacts the crystal nucleation growth, which plays an important role in surface construction of CdSe QDs and changes the surface state. In this way, we could modulate the crystal surface defects of CdSe, as verified by analysis of the individual bands which constitute the emission spectra and are associated with different relaxation processes. We found that the various tested metal cations, which interact in solution with the MSA ligand grafted on the QDs, quench their fluorescence differently, depending on the MSA/CdSe ratio used in synthesis. The crystal defects modulate the excitonic relaxation in CdSe and we demonstrated here that the surface defects intervene in the quenching of QDs induced by the binding of cations. (C) 2020 Elsevier Inc. All rights reserved.