3.1 Synthesis of U-CDs
As an important component of hemicellulose, xylan accounts for about 15%-35% of plant cells.[32]Xylan has good water solubility, low carbonization temperature and high carbon atom ratio, so it is an ideal carbon source for CDs synthesis.[39] Herein, taking hemicellulose main ingredients xylan as carbon source material, urea as a nitrogen dopant, then biomass CDs was synthesized by using ‘one step’ rapid hydrothermal synthesis method, namely U-CDs (Fig. 2A). Xylan surface was rich in hydroxyl group, making it a selective sensor for Cu2+ sensing. The property of U-CDs was characterized by FTIR, XRD, XPS, TEM, HRTEM and AFM, as shown in Fig. 2. Initially, as shown in Fig. 2B, the peak showed at 3340 cm− 1 in the FTIR spectrum of xylan belonged to the stretching vibration of O-H. The absorption peak near 2925 cm− 1 corresponded to the stretching vibration of the C-H bond. And C = O and C = C caused by the stretching vibration absorption peaks appeared around 1703cm− 1 and 1635 cm− 1, respectively. The peak at 1353 cm− 1 was the in-plane bending vibration of phenolic hydroxyl groups. The peak at 1250 − 1000 cm− 1 was related to the stretching vibration of C-O-H and C-O-C in xylan molecular. On the other side, FTIR method showed that U-CDs has a strong infrared absorption band near 3415 cm− 1, indicating that the chemical composition contains abundant hydroxyl groups (Fig. 2C). Besides, the peak near 3247 cm− 1 indicated that the surface of U-CDs contained a small amount of − NH2 groups. And absorption peak of 2925 cm− 1 was attributed to the stretching vibration of C − H. The wide absorption peak near 1600 cm− 1 corresponded to stretching vibration of C = C and bending vibration of N − H, and the absorption peak near 1460 cm− 1 was related to stretching vibration of C − N. The characteristic absorption peak at 1068 cm− 1 was the C − O stretching vibration absorption peak. FTIR analysis showed that U-CDs showed a clear infrared absorption peak related to N, indicating that CH₄N₂O was used as dopant nitrogen atom doping of CDs can be achieved effectively. Importantly, U-CDs showed excellent fluorescence lifetime with 1238 ns, as shown in Fig. 2D. According to the formula, the fluorescence quantum of U-CDs yield was 36%. Under sunlight, the color of U-CDs was light brown liquid. And after UV-Vis lamp irradiation (365 nm), U-CDs showed bright blue fluorescence signal, which proved that U-CDs have excellent photoluminescence effect, as shown in Fig. 2C inset. Further, XRD analysis of U-CDs showed a wide diffraction peak near 2θ = 20° (Fig. 2E), indicating that U-CDs has an amorphous carbon core structure. TEM in Fig. 2F shown that the spherical particles were monodispersed at room temperature and did not agglomerate when the U-CDs concentration was as high as 0.5 mg/mL. The size of U-CDs solution ranged from 1.50 nm to 3.0 nm, indicating that U-CDs has a wide particle size distribution range with average size about 2.40 nm. Figure 2G showed the HRTEM analysis results of U-CDs. It can be observed that U-CDs has significant lattice fringes. According to the selection diffraction results, the spacing between the graphitized carbon core layers was 0.21 nm. Further, AFM images of U-CDs showed that the height of U-CDs was mainly distributed in 3.5 nm, indicating that U-CDs was mainly composed of a single graphite layer with outstanding quantum confinement effect.[23] The main element composition of U-CDs were analyzed by XPS, as shown in Fig. 2I-K. Figure 2I showed the XPS high-resolution spectrum of C1s. The analysis showed that the main forms of C element in U-CDs were C − C/C = C, C − N, C − O, C = O which attributed to the 284.5 eV, 285.3 eV, 286.3 eV and 288.6 eV, respectively. Among them, the existence of C = C bond reflected the large number of sp2 hybrid carbon atoms in the structure of U-CDs. The N1s map in Fig. 2J showed XPS peaks of 399.4 eV and 400.8 eV, corresponding to pyrrole N and graphite N, respectively. The comparison of peak area of N1s spectra displayed that pyrrole N is the main form of N element in the structure composition of U-CDs. As an electron donor, pyrrole N can increase the electron cloud density on CDs surface, which is conducive to exciton radiation. Figure 2K showed the binding energies of O1s peaks of 532.7 eV and 531.6 eV can be attributed to C − OH/C − O−C and C = O, respectively. According to above results, indicating U-CDs has a large number of N/O functional groups.
3.2 Fabrication and microstructural characteristics of CPU-CDs
Herein, cotton cellulose paper with low cost and wide source was selected as the solid matrix material, and fluorescence U-CDs were used to adhered to cotton cellulose paper by electrostatic adsorption, as shown in Fig. 3A and C. As shown in Fig. 3B, under ultraviolet lamp (365 nm), the solid-state CPU-CDs displayed bright blue fluorescence signal, which proved that it has good photoluminescence effect and can be further used for metal ion detection. SEM analysis proved the porous structure of CPU-CDs, which provided potential for subsequent adsorption and detection of Cu2+. The characteristic absorption peak appeared after U-CDs impregnated with cotton cellulose paper, characteristic peak at 3415 cm− 1 was related to the -OH group. Compared to that at 3459 cm− 1 in U-CDs (Fig. 2D), this peak at 3415 cm− 1 was blue-shifted. Besides, a peak at 1600 cm− 1 was reduced in the CPU-CDs attributed to the stretching vibration of the C-N. Then, CPU-CDs detection Cu2+ was further conducted by using naked-eve view. As shown in Fig. 3D, the color of CPU-CDs was light blue under a UV-vis lamp (365 nm). In addition, with Cu2+ concentration increasing, the fluorescence of CPU-CDs gradually weakens, until the concentration of Cu2+ reached at 50 µM, the fluorescence signal appeared completely quenched. It can be seen that CPU-CDs shows effective and highly sensitive characteristics for Cu2+. In addition, within the test time of 52 h the bright blue fluorescence of the CPU-CDs did not significantly weaken. Moreover, the fluorescence of the CPU-CDs remained in quenching state after combined with 40 µM Cu2+, as shown in Fig. 3E. The above results prove that the CPU-CDs has excellent stability for Cu2+ detection.
3.3 Fluorescence behavior of CPU-CDs for detecting Cu2+
In order to reveal the fluorescence emission behavior of U-CDs, fluorescence emission spectra of U-CDs were investigated. As excitation wavelength of U-CDs increased from 300 nm to 480 nm, the maximum fluorescence emission wavelength at 435 nm of U-CDs gradually redshifted from 410 nm to 560 nm. The fluorescence emission behavior depending on the excitation wavelength may be related to the size effect (quantum limited effect) of U-CDs or different surface defect potential wells, as shown in Fig. 4A. The UV-vis spectrum of Fig. 4B showed that U-CDs has maximum ultraviolet excitation wavelength at 310 nm. The fluorescence intensity changes of CPU-CDs after mixed with different substances was investigated as shown in Fig. 4C. CPU-CDs had the maximum fluorescence intensity at 435 nm. When other analytes (K+, Ca2+, Na+, Mg2+, Zn2+, Fe2+, Ni+, Hg2+, Al3+, Fe3+, Cr3+, Ni2+, F−, CN−, ACO−, HPO42−, NO2−, SCN− and HS−) were blended with the CPU-CDs extract, the fluorescence intensity of CPU-CDs was not enhanced and quenched significantly. However, when Cu2+ were added to the CPU-CDs solution, the fluorescence intensity of the CPU-CDs was obviously quenched, which directly indicated the unique selectivity and high sensitivity of the membrane to Cu2+. To further investigated the behavior of CPU-CDs for Cu2+, different concentrations Cu2+ (0–50 µM) affect the fluorescence intensity change of CPU-CDs was investigated. With Cu2+ concentration increasing, the maximum fluorescence intensity at 435 nm decreased gradually and gradually red-shifted to 484 nm. The change of Cu2+ concentration and fluorescence intensity were linearly fitted in Fig. 4D, the curve showed a good linear relationship in the range of 5 µM to 25 µM with I484 nm/I435 nm=1.21328 − 0.01827[Cu2+] (R2 = 0.991). The detection limit of CPU-CDs for Cu2+ was calculated as 0.14 µM. The values were lower than the reported Cu2+ fluorescence sensors, indicating the good sensitively of CPU-CDs for Cu2+ (Table 1). Further proved that CPU-CDs could be used to detect Cu2+ via fluorescence spectra and quantitatively calculated. Furthermore, anti-interference ability and competition experiment of CPU-CDs for Cu2+ detection was conducted. Various ions including were cultured with CPU-CDs + Cu2+, then fluorescence intensity changed result were recorded in Fig. 4E. Above results displaying the CPU-CDs has good anti-interference ability and can select Cu2+ specifically. Besides, CPU-CDs also showed high time-stability during 8d, which proved CPU-CDs can be used for longtime for Cu2+ detection. Further, we then inferred the possible detection mechanisms of Cu2+ with CPU-CDs. Cotton cellulose paper contains a large number of hydroxyl groups, and after modifying U-CDs, the CPU-CDs surface has amino groups, amino groups can increase the trapping ability with Cu2+. As shown in Fig. 4F, the solid fluorescence platform CPU-CDs can be combined with Cu2+ through covalent binding and electrostatic adsorption, so as to realize real-time detection of Cu2+.
Table.1 Comparison of detection limit
Name | LOD (µM) | References |
Rhodamine sensor | 0.34 µM | [40] |
AIE sensor | 0.44 µM | [41] |
ESIPT sensor | 0.57 µM | [42] |
Red CDs | 0.16 µM | [43] |
Hydroquinone CDs | 1.80 µM | [44] |
ZnSe/ZnS CDs | 0.17 µM | [45] |
Red emitting N-CDs | 45.87 µM | [46] |
Environmental Protection Agency | 20.0 µM | [47] |
Xylan CDs | 0.14 µM | This work |
3.3 Biocompatibility of U-CDs and fluorescence imaging of CPU-CDs
Since the raw materials of U-CDs were derived from biomass-based materials, they have good biocompatibility and safety. We directly took U-CDs solution and CPU-CDs extract solution as culture substrate to culture mung bean and zebrafish embryos, and observed the growth condition of mung bean, as shown in Fig. 5 (A). From Fig. 5 (B), when the mung bean was cultured with U-CDs solution, the mung bean gradually germinated, took root and gave birth to leaves. The results show that the U-CDs solution has higher safety for plants. Figure 5 (C) showed the change of mung bean root length within 12 days of the culture cycle. The results show that compared with the control group (water), mung bean also had better growth and development. Then we studied the compatibility and fluorescence imaging performance of the CPU-CDs with zebrafish embryos, as shown in Fig. 5(C). Zebrafish embryos showed bright green fluorescence when CPU-CDs were cultured together with zebrafish embryos, which proved that CPU-CDs had better permeability and imaging ability to animal tissues. We also further evaluated the potential of CPU-CDs detecting Cu2+ in zebrafish embryo, and results showed that green fluorescence of zebrafish embryos was significantly quenched after co-cultured with Cu2+, as shown in Fig. 5 (D, E). The results show that CPU-CDs was highly selective to Cu2+ in zebrafish embryo. Finally, we also demonstrated that CPU-CDs and CPU-CDs + Cu2+ were concentration-dependent in zebrafish embryos, further proving CPU-CDs has good potential for detecting Cu2+ in living animals.
3.4 Practical application of CPU-CDs
Water pollution caused by excessive discharge of heavy metal Cu2+ has attracted extensive attention from researchers in recent years.[1]Then, we explored the potential of CPU-CDs for detection Cu2+ in water samples including school lake, Xuanwu Lake and Yangtze River. For prove the detection accuracy of CPU-CDs with Cu2+, the concentrations of Cu2+ were calculated according to fluorescence titration curve of CPU-CDs with Cu2+. As shown in Table 2, the recovery rates of Cu2+ was 90%-119% in water samples. Above results revealed that CPU-CDs could quantitatively detect Cu2+ in real waterenvrionment with quantitative method.
Table.2 Recovery rates of Cu2+ in real water samples
Sample | Add (µM) | Detected (µM) | Recovery (%) |
School lake | 0 | No detected | - |
0.5 | 0.48 ± 0.092 | 96 |
1.0 | 1.01 ± 0.031 | 101 |
Xuanwu Lake | 0 | No detected | - |
0.5 | 0.52 ± 0.124 | 104 |
1.0 | 1.12 ± 0.314 | 112 |
Yangtze River | 0 | No detected | - |
0.5 | 0.45 ± 0.017 | 90 |
1.0 | 1.19 ± 0.163 | 119 |
3.5 Practical application of U-CDs
In order to realize the high-value application of xylan derived U-CDs, we further explored its application potential as fluorescent ink. Figure 6A showed the preparation method of U-CDs derived fluorescent ink. The dispersion of fluorescent ink is yellowish-white viscous liquid under sunlight. Under Uv-vis light (365 nm), it showed bright blue fluorescence, which proved the successful preparation of U-CDs ink and its potential as fluorescent ink, as shown in Fig. 6B. Further, we use the fluorescence ink to write ‘NJFU’ on cotton cellulose paper to prove its writing ability as ink. We surprised to find that after writing, the filter paper has no obvious traces, but under Uv-vis light (365 nm), the ink presented a bright ‘NJFU’ signal. The results showed that the fluorescent ink has better bonding ability and higher concealability, can be used for fluorescence anti-counterfeiting related applications. Then we studied the water resistance of fluorescent ink, and the results showed that the fluorescent ink still had a bright fluorescent signal and stability within 30 days of the test, as shown in Fig. 6C. It is worth noting that the fluorescent ink stored at room temperature for 120 day still has high concealment and strong fluorescence signal, as shown in Fig. 6D. Finally, we use fluorescent ink to prepare the two-dimensional code. Under Uv-vis light (365 nm), two-dimensional code showed obvious blue fluorescence, and, can be decoded as ‘NJFU’ by mobile phone scanning. Above results proved that fluorescent ink has good concealability, can be used in fluorescence anti-counterfeiting and other fields, as shown in Fig. 6E.