3.1. FTIR and DSC analysis of the S QD/CTSCD nanocomposites
Fig.2A showed the infrared spectra of the S QDs, CTSCD, and S QD/CTSCD nanocomposites. The peak at 1640 cm−1 was a typical S QDs [Shan,2019], and the peak at 896 cm−1 was β-(1, 4)-glycosidic bonds of CS. The peak at 1042 cm−1 indicated the presence of the a-(1,4)-glycosidic bond of β-CD. The peak at 1640 cm−1 corresponded to the S QDs, indicated that S QDs was successfully introduced to CTSCD. The peaks at 1560–1640 cm−1 and 2100 cm−1 were belonging to amino (–NH2), alicyclic amine I (C=O), and cyclic amine II (N–H) groups. The absorption peak of the S QD/CTSCD nanocomposite was reduced due to the reaction with the amino group.
The results of DSC analysis of CTSCD (a) and S QD/CTSCD nanocomposites (b) were shown in Fig. 2B. Curve a showed an obvious endothermic peak at 97.25℃attributed to the crystalline water that evaporated in the CTSCD. The melting peak of CS was approximately 225℃ due to the internal H bond, whereas the melting peak of CTSCD was approximately 228℃, which was more stable than that of the CS monomer [Djerahov,2016Teng, 2020. Rao, 2020, Lin,2020, Li, 2020]. From the curve b in Fig.2B, an obvious endothermic peak was found at 100℃. The hydrophobicity and thermal stability of CTSCD were evidently improved by the S QDs. The exothermic peak at 259.48℃ was the degradation peak of S QD/CTSCD nanocomposite chain skeleton, which was due to the weakening of hydrogen bond interaction and the destruction of crystal structure regulation by S QDs. The two lines in Fig.2B showed that with increased degradation temperature of the chain skeleton, the thermal stability of S QD/CTSCD nanocomposites were improved.
3.2. Optical property and TEM of the S QD/CTSCD nanocomposites
The optical properties of S QDs/CTSCD were studied on the basis of the UV absorption spectra and fluorescence spectra at room temperature. Fig.3A showed a 298 nm centered absorbance band in the UV absorption spectrum. The ultraviolet absorption of S QDs, which was modified by the CTSCD polymer, was stronger than that of S QDs. Therefore, as shown in Fig.3A, the solution of S QD/CTSCD nanocomposites was light yellow (A) in sunlight but bright blue when irradiated at 365nm UV light (B), indicating the blue fluorescence properties of S QD/CTSCD nanocomposites. When excited at 295 nm, the fluorescence spectrum showed excellent emission peak at approximately 411 nm, which was shown in Fig.3B.
The TEM micrographs (Fig. 4A) showed the good dispersion of S QDs. The particles mostly have a regular spherical shape with approximately 2-3 nm in size. A typical amorphous structure was observed with no visible lattice. These results showed that the synthetic S QDs had excellent nanoparticle properties for metal-ion sensors. The atomic force microscopy image shows the shape and height of the S QDs. The average height was 2.8 nm (Fig. 4B). The TEM image showed the good dispersion of SQD/CTSCD (Figs.4B and 4C) with a relatively uniform size distribution. The average diameter of the S QDs/CTSCD was 2.5 nm.
3.3. XPS of S QD/CTSCD nanocomposites
The composition, surface group, and structure of S QDs/CTSCD were studied by XPS. The four peaks of the nanocomposites at 284.30, 398.8, 532.04, and 165.9 eV were C1s, N1s, O1s, and S2p in the Fig.5A, respectively. The C1s spectrum (Fig.5B) had two peaks at 282.73 and 285.9 eV, which could be attributed to C–C and C–OH. Two peaks were found at 165.75 and 165.5eV in the S 2p spectra (Fig. 5C) attributed to S in SO2−(2p2/3), SO2−(2p1/2), SO3−(2p2/3), and SO3−(2p1/2) bands [Prasannan,2013]. The three peaks of 395.9, 396.05, and 397.1eV in the N1s spectrum (Fig.5D) were from the C–N–C and C–N groups, respectively. The O1s spectrum (Fig.5E) had two peaks at 532.7 and 531.9eV, which were due to the C-OH/C–O–C and C=O bands [Park,2015;Teng,2020;Wu,2020]. This finding indicated that S atom was added to the CTSCD prepared in the present study. These results were consistent with those of FTIR.
3.4 Effect of pH on the fluorescence of S QD/CTSCD nanocomposites
Fig.6 showed the effect of pH (4–10) on the fluorescence intensity of the S QD/CTSCD aqueous solution. In the experiments, 0.1 M HCl and NaOH were added to the solution to obtain the desired pH. In the strong acid environment(pH≤4), the amino groups on chitosan will protonate [Gao,2020;Yang,2020], which would destroys the weak self-assembly between the polymer and sulfur quantum dots. The sulfur quantum dots fall from the polymer chain, the fluorescence intensity increased significantly (pH=4). With increased pH from 4 to 10, fluorescence intensity decreased by approximately 7%. This negligible change might be the result of a small change in the quantum confinement due to the functionalization of the S QDs/CTSCD. S QD/CTSCD samples did not change their maximum fluorescence value and the same spectral shape was changed. Under these experimental conditions, the S QD/CTSCD samples were monodispersed and the fluorescence emission was stable [Dager,2019].
3.5. Selectivity of the sensor
Selectivity was an important parameter for assessing the fluorescence property of S QD/CTSCD nanocomposites. The effect of various metal ions, such as Fe3+, Ca2+, Pb2+, Na+, Cr3+, Bi3+, Zn2+, Mg2+, Ba2+, Cd2+, Ag+, Sr2+, Hg2+, and Ni2+ which concentration was 1.0×10–5mol/L of fluorescence was examined. As shown in Figure 7 (A), the fluorescence intensity of S QD/CTSCD nanocomposites was quenched to a remarkable extent in the presence of silver ions, whereas the fluorescence intensity in other metal ions (alkali, alkali soil, and transition-metal ions) had negligible change or remain unchanged in the presence of a single cation at 1.0×10−5mol/L. Thus, the sensor based on S QDs/CTSCD had good selectivity to silver ions.
3.6. Quenching of the S QD/CTSCD nanocomposite to silver ions.
Under optimum conditions, the sensitivity of the S QD/CTSCD nanocomposite to silver ions was investigated. In a typical operation, S QD/CTSCD solution (0.01mg/mL) was dispersed in water and mixed with different amounts of silver ions. After the samples were incubated at room temperature for 20 mins, the fluorescence emission spectra (λem=411nm) were recorded and performed in triplicate. The fluorescence intensity was calibrated by changing the silver ions concentration. F0 and F were the fluorescence intensities of S QDs/CTSCD in the absence and presence of silver ions at different concentrations, respectively.
Fig.7B showed that the fluorescence intensity of the S QD/CTSCD nanocomposites gradually decreased with increased concentration of silver ions increased because S–Ag+–S bond formed between silver ions and S QDs/CTSCD. Based on these phenomena, a favorable linear correlation (R2=0.9992) existed between I and silver ions concentration in the range of 1.0×10−5-5.5×10−5mol/L (Fig.7C), and the detection limit for silver ions was approximately 66.7 nM. The effectiveness of silver ions fluorescence detection with S QDs/CTSCD as fluorescence probe was verified and provided a platform. The selectivity of S QDs/CTSCD to silver ions might be due to the synergistic effect of S and O functional groups on S QDs/CTSCD [Lu,2014]. Where F0 (F0 was [C]=0), F was the fluorescence intensity before and after adding silver ions, KSV was the quenching effect coefficient of the sensing material, [C] was the molar concentration of the analyte (C).This quenching effect could be rationalized by the Stern–Volmer equation: I0/I=1+0.0029C(R2=0.9966). The concentration of silver ions was in the range of 1.0×10−5 – 5.5×10−5 mol/L (Fig.7D). The results also showed that the S QD/ CTSCD nanocomposite was a more sensitive silver ions fluorescence sensor than others [Gao, 2020; Dager, 2019; Jiang, 2015; Lu,2014; Yang, 2020 ]. The sensitivity of the sensor was higher than that of other methods reported in literature [Lv, 2014; Gao, 2015; Wang, 2007;Bian, 2017]. As shown in Table1, the detection limit and the analytical concentration range of different technologies. Furthermore, it could be used to directly detect silver ions in the actual sample.
3.7 Sensor mechanism
The structure of S QDs/CTSCD should have a molecular-recognition capability to detect a certain type of substrate and provided the necessary spatial-structure arrangement for binding sites and functional groups. The functionalization of S QDs by the CTSCD structure was determined by the ability of a receptor with a heteroatom (S from S QDs, N from NH2) with lone pairs to bind silver ions. The hydration radius of silver ions could exactly match the distance on the S QDs/CTSCD nanocomposites. S QDs functionalized by the crosslinking CTSCD supramolecular structures as a component of self-assembly structure was promising because their structure contains potential coordination centers of metal cations (bridged sulfur atoms and negative charge NH2 groups). The bridged S QDs in polymer structure formed S–Ag+–S bonds, which combined with colloidal particles with noncovalent bonds, thereby forming the dendritic fractal structure of aggregates based on silver ions. The experimental results showed that ACQ has occurred.
3.8 Effect of coexisting metal ions
To further investigate the selectivity of S QDs/CTSCD nanocomposites to silver ions, the competitive experiments were carried out by measuring the fluorescence intensity of S QDs/CTSCD nanocomposites in the presence of silver ions and additional metal ions. The effect of some co-existing cations on the detection of silver ions was shown in Figure 8. Fig.8 (B) showed the fluorescence response of the sensing system to silver ions in the presence of other metal ions. The emission peak at 411 nm was almost no changed of most coexisting metal ions except Ni2+.It could be concluded that most coexisting metal ions did not interfere with the binding of the silver ions to the S QD/CTSCD nanocomposites except Ni2+ ions that had a weak effect on detection. Therefore, the selective binding of silver ions could be carried out in the presence of most competitive and coexisting metal ions.
3.9. Detecting silver ions in environmental sample
Environmental sample from the Xianyang Lake, Gudu Canal, and Nanhu Lake was collected. The standard Ag+ solution was added to the water samples and analyzed via standard addition method. The results showed that the recovery of silver ions in the samples were 98.44-110.76% (Table 2).The results showed that the designed sensor was reliable and practical for detecting silver ions in different environmental water samples.