Water-soluble luminescent CdS quantum dots (QDs)
capped by polyphosphate, L-cysteine, and thioglycerol
were synthesized in aqueous solution. The ligands were
found to have a profound effect on the luminesence
response of CdS QDs to physiologically important metal
cations. Polyphosphate-capped CdS QDs were sensitive
to nearly all mono- and divalent cations, showing no ion
selectivity. Conversely, thioglycerol-capped CdS QDs were
sensitive to only copper and iron ions. Similar concentrations
of physiologically relevant cations, such as zinc,
sodium, potassium, calcium, and magnesium ions did not
affect the luminescence of thioglycerol-capped CdS QDs.
On the other hand, L-cysteine-capped CdS QDs were
sensitive to zinc ions and insensitive to other physiologically
important cations, such as copper, calcium, and
magnium ions. To demonstrate the detection capability
of these new ion probes, L-cysteine and thioglycerolcapped
CdS QDs were used to detect zinc and copper ions
in physiological buffer samples. The detection limits were
0.8 íM for zinc (II) and 0.1 íM for copper (II) ions. The
emission enhancement of the QDs by zinc (II) is attributed
to activation of surface states, whereas the effective
reduction of copper (II) to copper (I) may explain the
emission decrease of the thioglycerol-capped CdS QDs
when charged with copper ions. Unlike organic fluorescent
dyes, the thioglycerol-capped luminescent CdS QDs
discriminate between copper and zinc ions and are
therefore suitable for the analysis of copper ions in
biological samples in the presence of physiological concentrations
of zinc ions. The interference of iron ions with
zinc and copper ion detection is attributed to an inner
filter effect, which is eliminated by adding fluoride ions
to form the colorless complex FeF6
3-. To the best of our
knowledge, this is first use of luminescent semiconductor
quantum dots as selective ion probes in aqueous samples.