Figure 2 Fluorescence intensity of CDs with varied irradiation
time (A); concentrations of sodium chloride (B); concentrations of
hydrogen peroxide (C); temperature (D); organic solvents (E) and metal
ions (F).
3.4. Exploration of CDs sensing pH and the corresponding mechanism
During investigating the fluorescence stability of CDs towards pH, we
found that their fluorescence intensity showed different response on
various pH. To be specific, the fluorescence of CDs continuously and
regularly decreased in the range of pH from 2 to 7 (Figure 3A), proving
the potential of the CDs prepared here sensing pH in the acidic
environment. Accordingly, the linear relationship between the
fluorescence intensity of CDs and varying pH was established, while a
linear equation was fitted as Y= 718.37X + 164.76 with
R2 of 0.984 (Figure 3B). Meanwhile, the fluorescence
intensity of CDs solution could well recover when their pH was
repeatedly adjusted from 2 to 7 (Figure 3C), indicating their
satisfactory acid resistance.
To clarify the mechanism of CDs responding to pH, the further
experiments were designed and performed. The zeta potentials of CDs
gradually decreased with pH increasing from 7 to 2 (Figure 3D), which
revealed that there occurred the interaction between the increased
H+ and the surface groups of CDs, thus changing the
surface states of CDs and further decreasing their fluorescence.
Moreover, the UV spectra of CDs with varied pH were recorded, revealing
the blue-shift with pH increasing (Figure 3E), and the infrared spectra
of CDs also showed that the stretching vibration peaks of -OH expanded
with pH increasing (Figure 3F). Therefore, these findings demonstrated
that the decrease of pH led to the protonation of CDs, which altered
their surface states, thus resulting in the quenched fluorescence[32,33].