Figure 3 (A) Fluorescence intensity of CDs solutions in the
presence of different pH; (B) Fluorescence spectra of CDs with varied pH
ranging from 2 to 7; (C) Linear relationship between fluorescence
intensity of CDs and pH; (D) Zeta potentials of CDs, (E) UV-vis spectra
of CDs and (F) FTIR patterns of CDs with different pH from 2 to 7.
3.5. Establishing the assay strategy of tigecycline by CDs
Based on the favorable fluorescence of CDs prepared here, we addressed
that whether they could serve as the fluorescent probe towards the
potential targets. As shown in Figure 4A, the fluorescence of CDs was
quenched when tigecycline was introduced, proving that these CDs showed
the potential for sensing tigecycline. Meanwhile, the fluorescence
intensity of CDs gradually decreased with the concentration of
tigecycline increasing from 7 nM to 200 μM (Figure 4B), and the linear
equation was fitted as F0/F-1 =
3.0642CTGC - 0.0422 with R2 of 0.992, while the
detection limit was calculated as 3.56 nM (Figure 4C). These findings
suggested that we successfully built up a strategy of detecting
tigecycline by CDs described here.
3.6. Optimizing the conditions of CDs detecting tigecycline
To acquire the optimal detection conditions of tigecycline by CDs, the
further experiments were designed and performed. As shown in Figure S3A
and S3B, the incubation temperature and time scarcely showed the effect
on the detection processes, thus 10 minutes of incubation at room
temperature were selected as the optimized conditions of CDs assaying
tigecycline.
3.7. Anti-interference and
selectivity for CDs detecting tigecycline
To investigate the anti-interference ability of the proposed assay,
tigecycline were initially added into the paralleled solutions of CDs,
and then the substances that might exist in the surrounding environment,
including Na+, K+,
Ba2+, Ca2+, Mg2+,
SO42-, glucose, cysteine, arginine,
histidine, lysine, urea and uric acid, were respectively introduced. As
shown in Figure 4D, the fluorescence intensity of CDs and tigecycline in
the absence or presence of these substances were recorded, and the
results described that their fluorescence intensity were hardly affected
by other substances, revealing the generally satisfactory
anti-interference. In addition, the selectivity of CDs detecting
tigecycline was explored by examining the effect of the similar or other
classes of antibiotics on their fluorescence intensity. The results
showed that the fluorescence of CDs only responded to tigecycline with a
concentration of 200 μM rather than any other antibiotic introduced
(Figure 4E), demonstrating the favorable selectivity during sensing
tigecycline by CDs prepared here.
3.8. Detection of tigecycline in the lake water sample
To evaluate the practicability of CDs detecting tigecycline, the lake
water served as the actual samples by the established assay. In
particular, the lake water was supplemented with tigecycline with
different concentrations, and the concentrations of tigecycline in the
lake water were separately calculated according to the proposed
equation. Again, the recoveries of tigecycline were examined as 96.0%
and 108.0% (Table S1), demonstrating that the fluorescent method we
raised here exhibited the satisfactory practicality.
3.9. Detection mechanism of
tigecycline by CDs
Additionally, we also explored the specific mechanism of CDs sensing
tigecycline. Particularly, the UV spectrum of tigecycline did not
overlap with that of CDs (Figure 4F), thus excluding the internal
filtration effect. Then, the UV spectra of TGC, CDs and CDs with TGC
were obtained, and the absorption band of CDs with TGC differed from
that of TGC or CDs only, revealing that there appeared the new substance
through the interaction between CDs and tigecycline (Figure 4G).
Meanwhile, there appeared no obvious variation for the fluorescence
lifetime of CDs with tigecycline compared with that of CDs only (Figure
4H). Taken together, these findings demonstrated that the quenched
fluorescence of CDs was mainly associated with the static quenching. In
addition, the broadening of the absorption band in the range of
2800-3750 cm-1 was observed after introducing
tigecycline into CDs compared with that of tigecycline, suggesting that
there may form the new substance between CDs and tigecycline (Figure
4I), which was consistent with our previous findings. Therefore, the
static quenching was mainly responsible for their reduced fluorescence
intensity of CDs [34,35].