water was added
into the Cd2+ solution through a syringe pump at a flow rate of
0.5 mL/min. The reaction mixture was refluxed while boiling
under nitrogen atmosphere for 10 h to obtain bright yellow CdS
QDs. The CdS QDs colloid solution was condensed to ∼20 mL
by solvent evaporation and then dialyzed against deionized water
with a membrane (MWCO 1000) for 4 h. Dialysis was used to
purify the polyphosphate-capped CdS QDs rather then precipitation
and subsequent washings that are frequently used to purify
quantum dots due to difficulties in redispersing precipitated
polyphosphate-capped CdS QDs in aqueous solution following
precipitation. The dialyzed CdS QDs were diluted in a 100-mL
0.05 M Tris-HCl buffer solution at pH 7.2. The concentration of
particles was calculated on the basis of an estimated molar
absorption coefficient of 2 × 104 M-1 cm-1.
30 The final concentration
of polyphosphate-capped CdS QDs was ∼150 µM. This
solution was used as a stock solution. Aliquot solutions were used
for ion response measurements.