However, the use of QDs for the detection of phosphate-containing
metabolites has not been studied extensively. For instance,
QD-based ATP probes have been investigated on the basis of direct
interactions between QDs and ATP [19,20]. Notably, most of these
probes work in a ‘‘turn-off’’ mode, and their selectivity is limited
due to the nonspecific interactions between ATP and QDs. To overcome
this problem, a quantum dot-tagged aptamer was designed
for the detection of ATP by the fluorescence resonance energy
transfer (FRET) approach reported by Chen et al. [21]. Recently, a
new strategy for the detection of ATP using an aptamer/QD biosensor
based on chemiluminescence resonance energy transfer (CRET)
was developed by Zhou et al. [22]. Furthermore, the use of ion-specific
ligands attached to the surface of nanomaterials has received
much attention in this field. Recently, Hong and coworkers reported
a conjugate of thioglycolic acid–capped CdTe quantum dots
and Eu3+ ions (TGA-CdTe QD-Eu3+) for use as a fluorescence turnon
sensor for nucleoside triphosphates (NTPs) [23]. Moreover, Li
and colleagues reported using gold nanoclusters as selective luminescent
probes for phosphate-containing metabolites. The sensing
approach was based on phosphate-containing molecules that modulate
the quenching behavior of Fe3+ toward the luminescence of
glutathione-bound gold nanoclusters (AuNCs@GSH) [24].
It is well known that phosphate anions exhibit high formation
constants with Fe3+ [25], so phosphate-containing molecules such
as PPi are expected to form stable complexes with Fe3+. Therefore,
fluorescence sensors were proposed for the detection of PPi in this
work based on Fe3+-decorated Cys-CdS QDs. The formation of a stable
Fe3+ complex will produce an efficient quencher, and therefore
the level of PPi can be determined by the fluorescence quenching
approach.