that Fe3+ can provide strong coordination with phosphate-containing
compounds [25]. Therefore, Fe3+ was chosen as the conjugate
ion for the fabrication of PPi sensors.
The effect of Fe3+ on the fluorescence intensity of Cys-CdS QDs
was examined. The emission spectra of Cys-CdS QDs in the presence
of various concentrations of Fe3+ in 0.1 M Tris–HCl buffer
pH 7.5 are shown in Fig. 2. The fluorescence intensity of Cys-CdS
QDs was significantly quenched by the addition of Fe3+. The degree
of the fluorescence quenching increased linearly with increasing
Fe3+ concentration in the range of 0.02–0.80 mM. At the same time,
no change in the shape of the emission spectra was observed. The
fluorescence quenching by Fe3+ may undergo an electron transfer
process from the excited electrons in the conduction band of
Cys-CdS QDs to Fe3+. Therefore, the probability of electron–hole
recombination was reduced. This result confirms that Fe3+ can
interact well with the surface of Cys-CdS QDs and potentially be
used as a decorative species for a new sensor.
The detection of PPi by Cys-CdS QDs decorated with Fe3+
Generally, the fluorescence intensity of QDs is quite sensitive to
the nature of the surface. The presence of several species on the
surface of QDs can influence the efficiency of the electron–hole
recombination process, leading to a change in the fluorescence
intensity [41]. According to the results discussed in the previous
section, the fluorescence intensity of Cys-CdS QDs was initially
modulated by Fe3+ in the sensor fabrication step. From this point
of view, we hypothesized that the complex between Fe3+ and phosphate-containing
compounds has a better quenching efficiency
than Fe3+ alone. Therefore, the electron transfer between Cys-CdS
QDs and the quencher can be increased, which decreases the observed
fluorescence intensity.
To verify this hypothesis, the influence of PPi on the fluorescence
intensity of unmodified Cys-CdS QDs and [Cys-CdS QDs]-
Fe3+ systems was examined. As shown in Fig. 3A, the fluorescence
intensity initially modulated by Fe3+ can be gradually decreased by
increasing the PPi concentration. Meanwhile, the shape of the
emission spectra did not change with increasing PPi concentration.
In a parallel experiment, we explored the effect of PPi on the fluorescence
intensity of the unmodified Cys-CdS QDs, and the results
are provided in Fig. 3B. It was found that PPi has a very small effect
to the fluorescence intensity of the unmodified Cys-CdS QDs (at pH
7.5 and the same PPi concentration level) compared with the [CysCdS
QDs]-Fe3+ as a sensor probe. This implies that Fe3+ plays an
important role in inducing the quenching efficiency upon the addition
of PPi