3.1 Structural characterization
The morphology of UiO-66-NH2 and UiO-66-NH2@BDC was evaluated by SEM, as showed in Fig. S1 and Fig. 1, no obvious changes can be observed between UiO-66-NH2 and UiO-66-NH2@BDC, illustrating that the post modification process keep morphology of UiO-66-NH2 intact. PXRD was further performed to explore crystal structure changes before and after post modification. As illustrated in Fig. 2A, two obvious sharp peaks at 7.4° and 8.5° corresponding to the (111) and (200) crystal planes of UiO-66-NH2, respectively, were also observed in UiO-66-NH2@BDC. The peak at 16.9° corresponding to BDC was also found in UiO-66-NH2@BDC, demonstrating that the small-molecule BDC attached to UiO-66-NH2 successfully and the crystal configuration of UiO-66-NH2 remains unchanged in the course of the post modification process [28].FT-IR was also performed and showed in Fig. S2, the characteristic peak of stretching vibration of -NH2 in UiO-66-NH2 appear at 3454 and 3356 cm-1. After post modification, the amino characteristic peak disappeared, giving rise to the amide characteristic peaks at 3062 and 1290 cm-1 which further confirmed the PXRD results. Thus, it was confirmed that BDC was connected to UiO-66-NH2 via acid and amine condensation. XPS was used to determine the composition of the UiO-66-NH2 and UiO-66-NH2@BDC materials (Fig. 2B). UiO-66-NH2@BDC exhibited four peaks at 531.1, 398.9, 285.6, and 183.3 eV attributable to O 1s, N 1s, C 1s, and Zr 3d. The high-resolution C 1s spectral peaks at 289.0, 285.2, and 284.6 eV arise from C-C=O, C=O, and C=C groups [29-30], respectively (Fig. S3). For UiO-66-NH2@BDC, the high-resolution spectrum of O 1s (Fig. S4) showed peaks at 533.3, 531.9, and 530.6 eV arising from O-C=O, C=O, and Zr-O-Zr [31-32], respectively. Two peaks at 185.3 and 182.9 eV can be found in high-resolution Zr spectrogram (Fig. S5) which were considered as Zr 3d3/2 and Zr 3d5/2, respectively [32]. The high-resolution N 1s spectra showed a peak at 400.5 eV arising from C-N (Fig. S6). The above results indicate that in the post-modification process, the basic skeleton of UiO-66 has not changed and small molecule BDC was covalently bonded to UiO-66-NH2 successfully.
3.2 Fluorescence performance of UiO-66-NH2@BDC
Fluorescence analysis was performed to explore the optical performance of the MOF probe before and after modification. UiO-66-NH2 was first used as a sensor for S2- at concentrations of 0-400 μM where the fluorescence intensity increased gradually (Fig. S7A). The lower detection limit was 4.14 μM by the linear relationship between 25-150 μM S2- (Fig. S7B). Fluorescence titration experiments were performed to illustrate the relationship between the S2- concentration and UiO-66-NH2@BDC fluorescence. With the concentration of S2- increased from 0 to 625 μM (Fig. 3A), probe fluorescence was enhanced and showed a good linear relationship between 0-200 μM (Fig. 3B). Equation I/I0=1.348+0.0031C with a correlation coefficient of R2=0.9983 was used to express the relationship between S2- concentration and probe fluorescence ratio. The 0.13 μM ultra-low detection limit can be obtained based on 3 σ/ k. Compared with UiO-66-NH2, the UiO-66-NH2@BDC probe showed higher sensitivity, indicating that the post modification may endow additional sites for S2- interaction, thus improving the detection sensitivity. In addition, we systematically compared the sensitivities of different S2- probes, showing that the novel probes exhibit great advantages in terms of response time, detection limit, and application range (Table S1).
Specific response is the premise that the probe can be applied to multiple environmental detections. Herein, to assessed the practical applicability of the probe UiO-66-NH2@BDC, different potential interfering substances, including Ca2+, Na+, Fe3+, K+, Mg2+, Mn2+, Ni2+, Zn2+, Cl-, CH3COO-, S2O82-, SO42-, S2O32-, SO32-, HCO3-, NO2-, Hcy, Val, Leu, Phe, Met, Tyr, Trp, and Glu were selected for selectivity testing with the UiO-66-NH2@BDC probe. As Fig. 4A illustrates, only S2- induced fluorescence enhancements of the UiO-66-NH2@BDC probe in the ion competition experiment (Fig. S9). This experiment demonstrated the UiO-66-NH2@BDC probe exhibits excellent selectivity and can be applied for S2- detection in actual samples. Real time detection capability is another indicator of probe performance and the fluorescence of UiO-66-NH2@BDC reached its peak value within 6 s and remained stable demonstrating excellent real-time detection (Fig. 4B). We also investigated the fluorescence stability of UiO-66-NH2@BDC, and observed no significant changes after one day, five days, or three weeks (Fig. 4C). In addition, PXRD was used to study the stability of UiO-66-NH2@BDC after immersion in water for 3 days and 1 week. The crystal structure remained stable (Fig. 4D), indicating high stability in water and overall excellent optical performance for the accurate detection of S2-.
3.3 Possible Mechanism
To explore the sensing mechanism between S2- and UiO-66-NH2@BDC, XRD analysis was performed (Fig. 5A). The PXRD peak pattern of UiO-66-NH2@BDC remained largely unchanged before and after S2- addition, indicating that the fluorescence turn-on effect was not caused by structural collapse. The zeta potential was further examined, showing an increase from -7.241 to -13.78 mV upon S2- addition and indicting adsorption of the analyte on UiO-66-NH2@BDC (Fig. 5B). The UV/vis absorption spectra (Fig. S10A) showed a slight red shift after S2- addition from the original peak at 287 nm and the absorption intensity significantly increased. This was ascribed to the incremental electron transfer from the organic linkers to Zr. In addition, the XPS O 1s (Fig. S10B) and N 1s (Fig. S10C) of UiO-66-NH2@BDC showed a slight change after S2- addition due to the difference between S2-, oxygen, and nitrogen in the hydrogen bonding of UiO-66-NH2@BDC [39]. The above experimental results illustrated UiO-66-NH2@BDC can achieve highly sensitive detection of S2- via hydrogen bonding.
3.4 Application
To study the detection ability of the UiO-66-NH2@BDC probe for sulfur ions in real water samples, the standard addition method was used. Tap, drinking, and industrial wastewater were used to investigate the detection capability of the newly developed UiO-66-NH2@BDC probe (Table 1). The S2- recovery ranged from 80.19 to 109.29%, while the relative standard deviation (RSD) was <0.26%. In addition, all interfering substances were added to the solution to comprehensively analyze probe feasibility for practical applications. As shown in Table S2, the recovery ranged between 103.87% and 118.30% and the RSD was <0.61% in the presence of interferents.
Table 1. Determination of S2- in water sample (n=3)
3.4 Cellular imaging
MOF-based materials have limited applications in biological fields due to their poor aqueous solubility. Post modification of MOFs have gradually solved this problem for applications in biological systems. Because of the excellent optics characteristics of the UiO-66-NH2@BDC probe, we further explored its sensing performance for S2- in cells. HeLa cells were selected and incubated with S2- (50 and 600 μM) and UiO-66-NH2@BDC. The results were showed in Fig. 6, UiO-66-NH2@BDC successfully entered the HeLa cells and achieved S2- imaging. With increasing S2- concentration, significant fluorescence enhancement was observed, similar to the fluorescence titration. These results indicate that UiO-66-NH2@BDC can be used for imaging in cellular environments.
3.5 Imaging in zebrafish
Based on cell imaging research, we further studied the application of the newly developed probes in live zebrafish to provide information for the early diagnosis of disease. Different S2- concentrations were mixed with UiO-66-NH2@BDC and subsequently incubated with zebrafish (Fig.7). Confocal microscopy revealed that the free probe exhibited no fluorescence, whereas S2- induced significant blue fluorescence that increased with increasing concentrations. These results were similar to those observed in cells, proving that the UiO-66-NH2@BDC probe can be successfully used for S2- detection in vivo.