Influence of Complex-Formation Equilibria on the Temporal Persistence of Cysteinate-Functionalized CdSe Nanocrystals in Water

We have characterized the persistence and degradation of magic-sized CdSe nanocrystals (NCs) after their removal from the original reaction mixture and dispersion into basic aqueous solutions. Such studies are important given the myriad potential applications of semiconductor NCs and ongoing efforts to characterize the properties and reactivity of monodisperse suspensions of intact NCs. Correlated challenges are to elucidate the mechanisms by which NCs degrade and to establish conditions under which NCs persist. Our CdSe NCs degraded after dilution into aqueous NaOH, resulting in red-shifted excitonic absorption bands and eventual flocculation. Dilution of NCs into basic aqueous solutions of cysteinate resulted in degradation via a different mechanism with an absence of flocculation; kinetics varied with concentration of cysteinate. The chemical fate of NCs after dilution into basic aqueous solutions containing both Cd<sup>2+</sup> and cysteinate varied with the cysteinate-to-Cd<sup>2+</sup> molar ratio, which determined the relative solute mole fractions of various Cd<sup>2+</sup>–cysteinate complexes. CdSe NCs persisted on long time scales only when dispersed in solutions containing [Cd(cysteinate)<sub>3</sub>]<sup>4–</sup>. We present equilibria to account for the observed spectral changes after dilution of CdSe into various basic media. Cadmium(II)–cysteinate complex-formation equilibria influenced the temporal persistence of the NCs; the pathway through which CdSe NCs degraded depended on the concentration of free, uncoordinated cysteinate. Our findings indicate that solution-phase chemistry can determine whether NCs remain intact upon removal from their original reaction mixtures.