We
demonstrated the possibility of acquiring Raman spectra of colloidal
quantum dots (QDs) at low concentration in water with a size as small
as 2.5 nm in diameter using Raman tweezers microspectroscopy. We measured
the spectra of CdSe QDs capped with thioglycerol and with l-cysteine. This technique was applied to probe the interaction between
Co2+ and Cys–CdSe QDs whose fluorescence emission
is quenched in the presence of this metal cation. The quenching mechanism
was so far hypothetical. The Raman spectra of Cys–CdSe QDs
recorded in the absence and in the presence of Co2+ demonstrated
the binding of Co2+ cations to the carboxylate groups of
the l-cysteine ligand grafted on the surface of the 4.2 nm
CdSe QDs. The frequency of modes for the grafted ligand is changed
with respect to the free ligand in solution. Considering the vibrational
coupling between the excitonic state and the ligand, we inferred that
the binding of a metal cation to the grafted ligand modifies this
coupling, so that exciton relaxation through crystal defects is favored.
This result rationalizes the fluorescence quenching observed during
the metal cation–QD interaction.
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