2 − 1. Characterization
The morphology of the fabricated amine-CQDs was explained by transmission electron microscopy (TEM). The TEM image displays that CQDs have a spherical shape, and the mean diameter size of these nanoparticles was estimated to be roughly 7.5nm (Fig. 1a). To identify the elements of prepared amine-CQDs, EDS analysis was carried out. As shown in Fig. 1b, the elements carbon, oxygen, and nitrogen were found. The Fourier transform infrared (FT-IR) spectrum of amine-CQDs is demonstrated in Fig. 1c. The band at 3473 cm− 1 belongs to the stretching vibration of OH. Two peaks at 3364 cm− 1 and 3301 cm− 1 are ascribed to the NH asymmetric and symmetric stretching vibrations, respectively. The asymmetric and symmetric stretching vibrations of CH emerged at 2941 cm− 1 and 2873 cm− 1, respectively. Further, the absorption peak at 2154 cm− 1 corresponds to C = C vibration. The vibrations at 1594 cm− 1 and 1458 cm− 1 are attributed to C = O stretching and NH bending vibrations, respectively. The absorption band at 1363 cm− 1 is assigned to C-N vibration, and the peak at 710 cm− 1 is ascribed to the out-of-plane NH bending vibration. Furthermore, The FT-IR spectra of RhB and TMS are reported in Figure S1.
The optical properties of amine-CQDs were investigated by the spectra of UV-Vis and Excitation-Emission photoluminescence (PL). In Figure S2a, two typical absorption peaks are located at 245 nm and 340 nm, which are attributed to the π→π* transition of C = C and n→π* transition of C = O, respectively. The highest emission intensity is observed under excitation at a wavelength of 365 nm (Figure S2b). The stability of amine-CQDs was investigated with absorption and FL spectra. The photostability of amine-CQDs was assessed after two months of storage; as shown in Figure S3, no noticeable change in FL and UV-Vis intensities is observed. According to Figure S4, the FL intensity of amine-CQDs can remain stable during UV irradiation for thirteen minutes.
2–2. Optimization of experimental parameters
In order to achieve an excellent response for TMS determination, consequential factors such as pH, the ratio of fluorophores, and interaction time were optimized for the RF sensor. The pH effect on the F0/F value (F0 and F are the FL intensities ratio of the RF sensor in the absence and presence of TMS, where F0=(F442/F569)0 and F=(F442/F569)) was examined by tuning the B-R buffer from 2 to 11. As shown in Fig. 2a, the F0/F value increases by moving to more acidic pHs, while other parameters are considered constant. So, the maximum value is related to pH = 3, which is selected as the optimum value because the amine functional group on the CQDs can be protonated by catching H+ at acidic media; thus, the active sites of CQDs increase. The second parameter was the volumetric ratio of CQDs to RhB, where pH and other parameters were kept constant. By enhancing the volumetric ratio, the values of F0/F were raised, so according to Fig. 2b, the volumetric ratio of 0.2 was chosen as the optimal point because by increasing the concentration of RhB, its interaction with functional groups of CQDs enhances, so lead to reduce the active sites of CQDs, consequently bring about decrease the interaction between CQDs and TMS. The interaction time as the third parameter was optimized, while pH = 3, the volumetric ratio of 0.2, and other items were maintained constant. As illustrated in Fig. 2c, by raising time up to 15 minutes, the F0/F value goes up with a steep slope, and then it becomes approximately invariant. Fifteen minutes was the best interaction time because it takes more time for the analytes to reach the surface of the sensor; for this reason, the interaction time has increased.
2–3. Quenching mechanism
To construe the process of TMS sensing by an amine-CQDs/RhB sensor, the mechanism between amine-CQDs and RhB was examined at first. The emission of conjugated amine-CQDs is reduced compared to free CQDs due to forming a new bond between amine-CQDs and RhB; this causes the Fluorescence resonance energy transfer (FRET) phenomenon to occur, so the emission intensity somewhat decreases27. According to Figure S2a, the absorption spectra of amine-CQDs and RhB do not overlap with each other.
Photoinduced electron transfer (PET) can occur when the excited electron is transferred from CQDs (donor) to TMS (acceptor). Thus, for this reason, TMS connects to the carboxylic group of CQDs and creates a new interaction. In the following, we investigated the quenching behavior by way of the Stern-Volmer equation (Eq. 1), zeta potential analysis, and alterations of FT-IR spectrum.
F0/F = Ksv [Q] + 1 (Eq. 1)
Where F0 and F are the FL intensities ratio of the RF sensor in the absence and presence of TMS, respectively. [Q] represents the concentration of quencher, ‘Ksv’ is the Stern-Volmer quenching constant 28. As shown in Fig. 3a, the value of F0/F was plotted for different concentrations of quencher at three temperatures. By increasing temperature, the slope of the curve that represents Ksv increased. So the FL quenching might have been happened due to dynamic quenching 29,30. As it can be seen in Fig. 4a, by enhancing the concentration of TMS to this probe, the emission peak of RhB remains constant, while the emission intensity of CQDs reduces. So, TMS could have been detected by CQDs. To further investigate the sensing mechanism, the interaction between amine-CQDs and TMS was studied by zeta potential analysis. As shown in Figure S5, free amine-CQDs have a zeta potential of -45.8mV, but the zeta potential of them in the presence of TMS makes a dramatic shift to -33 mV, this increases in the surface charge of the amine CQDs after the adding TMS to the solution confirms that functional groups of amine-CQDs have reacted with TMS, so the FL signal reduces. Also, the interaction between amine-CQDs and TMS was examined by FT-IR spectrum. According to Fig. 3b, when TMS was incubated in amine-CQDs solution, the band intensities of NH stretching vibration reduced to 3364nm. Not only the absorption peak position of C = O has switched from 1594 nm to 1564 nm, but also the intensity has been lessened. In the FT-IR spectrum of CQDs + TMS, an absorption peak is created in the 1016 nm region, which is attributed to the C-O bond. This band is not observed in the FT-IR spectra of TMS and amine-CQDs. By reducing the intensity of the C = O peak and forming the C-O− band, it can be turned out that a new electrostatic interaction has been established between the C-O− group of amine-CQDs and the amine-group of TMS, which completes the sensing process.
2–4. Calibration and validation method
We analyzed the response of the RF probe at different concentrations of TMS under the optimal experimental conditions. By increasing the concentration of TMS to this probe, the signal intensity reduced at λ = 442nm (Fig. 4a). As shown in Fig. 4b, a good linear relationship could be observed between F0/F and the concentration of TMS in the range of 0.446–7.083 µg ml− 1. The linear regression equation is F0/F = 0.0323[Ca] + 0.9888 (Ca is the concentration of TMS), and the correlation coefficient is 0.9967 (n = 3). The detection limit (LOD) and quantitation limit (LOQ) of the proposed method for five tests were estimated to be 0.033 µg ml− 1 (3Sb/m) and 0.109 µg ml− 1(10Sb/m), respectively. The comparison between the proposed RF probe and other reported methods to determine TMS is listed in Table 1. This method has a good linear range and a lower LOD than spectrometric methods; this RF probe is cheaper, faster, and more environmentally friendly compared to chromatographic methods.
In order to evaluate the repeatability of the present recognizing system, we iterated one point by one operator at one place with the same working condition and the same measurement system. Twenty-five experiments were performed in five working days; in this way, five tests were done every day. Thus, as displayed in Fig. 5a, the relative standard deviation (RSD) for intra-day repeatability was calculated to be less than 0.88%, while the RSD for inter-day was computed 0.71%.
Table 1
The comparison of the present study with other reported approaches for TMS detection
Methods
|
LDR (µg ml− 1)
|
LOD (µg ml− 1)
|
Ref.
|
EC
|
0.133-445
|
0.044
|
32
|
RP-HPLC
|
10–50 × 10− 3
|
0.5 × 10− 3
|
33
|
LC-MS/MS
|
0.1–19.3 × 10− 3
|
-
|
34
|
UV-VIS
|
15–70
|
1.25
|
35
|
FL
|
0.75–3.50
|
0.17
|
36
|
FL
|
0.446–8.230
|
0.033
|
This work
|
2-5. Selectivity
To validate the selectivity of the CQDs/RhB ratiometric probe towards TMS under the optimal conditions, we investigated the response of this sensor to some interfering substances (Al3+, Ca+2, Zn2+, Fe3+, Cu2+, Mg2+, K+, Na+, Glucose, Sucrose, L-Cys, L-Leu, and Creatin) in the presence of TMS. As illustrated in Figure 5b, among interfering substances, except for copper and iron, all other species are less than 5% troublemakers on the sensing system. Both copper and iron have a 6.0% and 5.4% effect on the sensing process due to the formation of strong interaction with CQDs. The acceptable outcome of selectivity analysis signifies that we can employ this ratiometric sensor to determine TMS in the actual condition.
2–6. Analytical performance of the ratiometric sensor in biofluids
In order to demonstrate the practical application of the proposed method for TMS recognization in real samples, the experiments were carried out in human urea and serum (AL-Ghadir Hospital, Tehran, Iran). The recoveries of TMS in the human urea were obtained in the range of 94%-106%, whereas the RSD range was between 1.1%-5.7%. The recovery values in human serum were calculated from 92–102%, whereas RSD was acquired from 2.4–6%. The obtained results in Table 2 indicate that the ratiometric probe is a reliable procedure for analyzing TMS in biofluids.
Table 2
Results for determination of TMS in real samples
Sample
|
Spiked (µg mL− 1)
|
Found ± SD (µg mL− 1)
|
Recovery (%, n = 3)
|
RSD (%, n = 3)
|
Urea
|
0.658
|
0.702 ± 0.04
|
106.7
|
5.7
|
0.987
|
1.022 ± 0.025
|
103.5
|
2.4
|
1.425
|
1.356 ± 0.047
|
95.1
|
3.5
|
1.864
|
1.753 ± 0.054
|
94.1
|
3.1
|
2.193
|
2.210 ± 0.026
|
100.8
|
1.1
|
Serum
|
0.767
|
0.713 ± 0.025
|
92.9
|
3.5
|
0.987
|
0.953 ± 0.023
|
96.6
|
2.4
|
1.096
|
1.066 ± 0.049
|
97.2
|
4.5
|
1.864
|
1.711 ± 0.046
|
92.0
|
2.7
|
1.974
|
2.018 ± 0.121
|
102.2
|
6.0
|