Synthesis and characterization of Fe3O4/[email protected]
SEM and TEM images were used to characterize to study the surface morphology and particle size of the Fe3O4/CNC and Fe3O4/[email protected] (Fig. 1). The rod-like structures of CNC can interweave each other to form a web-like network, which was a good support to prevent the aggregation of Fe3O4 NPs(An et al. 2017). SEM image of Fe3O4/CNC showed that Fe3O4 NPs loading on CNC had a nearly spherical morphology with a rough surface, and particle size ranged between 100 and 250 nm(Fig. 1(a)). TEM micrograph (Fig. 1(b)) evidenced that Fe3O4 NPs were well dispersed in the CNC network after solvothermal process. In Fig. 1(c), thin tannic acid shell layers had formed around the Fe3O4/CNC cores, which were utilized as the adhesive coating on the surface of Fe3O4/CNC to guarantee a homogeneous growth of ZIF-8. As seen from Fig. 1(d) and Fig. 1(e), SEM image of Fe3O4/[email protected] confirmed that the Fe3O4/CNC was successfully coated with ZIF-8 to form Fe3O4/[email protected] complex, and ZIF-8 exhibited a cubic crystal structure. Compared with the TEM image of Fe3O4/CNC, Fe3O4/[email protected] nanocomposites exhibited a distinct core-shell structure, and the average size of ZIF-8 shell was approximately 40 nm. Under a higher magnification of TEM, continuous and uniform ZIF-8 NPs were well incorporated on the Fe3O4/CNC surface (Fig. 1(f)), demonstrating the successful synthesis of Fe3O4/[email protected] Furthermore, Energy-dispersive X-ray spectroscopy (EDS) of Fe3O4/[email protected] was shown (Fig. S1), and the presence of Fe, O, N and Zn as expected to be observed in Fe3O4/[email protected]
The FT-IR spectra were illustrated in Fig. 2(a) to contrast the functional groups on Fe3O4, ZIF-8 and Fe3O4/[email protected] surface. Concretely, for pure CNC, the peaks at 1060 and 2904 cm− 1 were caused by the C-O-C stretching of pyranose and the C-H stretching vibration, respectively(Xiong et al. 2013). The broad peak in the band of 3464 cm− 1 corresponded to the O-H stretching vibration, while the absorbance band at 1609 cm− 1 originated from O-H bending vibration(Jahan et al. 2010). In the case of Fe3O4/CNC, the peak derived from Fe-O bonds was expected at 582 cm− 1, indicating that the Fe3O4 NPs were successfully immobilized on the CNC. O-H stretching of CNC became weak due to the interactions of the hydroxyl groups and Fe3O4 NPs(Liu et al. 2015). The spectrum of Fe3O4/CNC after TA coating displayed some changes including the -OH stretching of the phenolic and methylol group that appeared at 3420 cm− 1. Additionally, the 1718 and 1080 cm− 1 vibrational bands corresponded to the C = O and C-O stretching vibrations, respectively. The peaks at 1434 and 1346 cm− 1 in the spectrum of TA belonged to the aromatic C-C and phenolic C-O stretching vibrations, respectively(Dutta and Dolui 2011). Moreover, One-layer coating TA-CNC was also obtained due to the hydrogen bonding between TA and the C6 hydroxyl group of the CNC. For the Fe3O4/[email protected], the significant observed bands around 3136 and 2928 cm− 1 were ascribed to the stretching vibration of C-H in methyl and imidazole rings. Worthwhile mentioning that the series of complex and compact observed bands in the spectra of 700–1350 cm− 1 and 1350–1500 cm− 1 can be attributed to the stretching and plane bending of imidazole ring(Zheng et al. 2014). The absorption peaks at 421 and 759 cm− 1 were ascribed to the nature of the ZIF-8 structure confirming the formation of the Zn-N bond.
The XRD patterns for the different samples were shown in Fig. 2(b). The broad and strong peaks at 14.9° and 24.5° were assigned to the (110) and (002) planes of CNC, respectively(French 2013). Besides, Fe3O4/CNC exhibited similar diffraction peaks with Fe3O4 NPs at 19.90°, 30.59°, 36.01°, 43.50°, 57.2°, and 62.8°, which corresponded to the (220), (311), (222), (400), (422), (511), and (440) lattice planes. These results further indicated the successful synthesis of Fe3O4 in the CNC. After loading of the ZIF-8, the XRD patterns showed new diffraction peaks at 2θ = 7.2° (011), 10.3° (002), 12.6° (112), 14.7° (022), 16.6° (013), 17.8° (222), 24.5° (233) and 26.7° (134) (JCPDS card No. 89-3739 for ZIF-8), highlighting the ZIF-8 was successfully coated on the Fe3O4/CNC. XRD analysis revealed the favorable crystalline structure of Fe3O4/[email protected]
N2 adsorption-desorption isotherms of Fe3O4/[email protected] was shown in Fig. 3(a)), and the isotherm of nanocomposites displayed a typical type IV isotherm. The pore width was 5.1 nm and the BJH pore volume was 0.31cm3/g, indicating a typical mesoporous structure. The BET surface area of Fe3O4/[email protected] was 463.63 m2/g, which was larger than many detection materials (Table S1). These analyses suggested that Fe3O4/[email protected] possessed a high surface area and mesoporous structure, which would probably provide more active sites and thereby improve the catalytic activity. Additionally, the hysteresis curve of sample magnetization were tested by VSM instrument and the result was shown in Fig. 3(b). The saturation magnetization (Ms) value of Fe3O4 was about 79.11 emu/g. In contrast with Fe3O4, the relatively weak magnetism Fe3O4/CNC (59.65 emu/g) and Fe3O4/[email protected] (44.01 emu/g) were mainly due to the TA coating and dielectric property of the outer shell. Nevertheless, the Fe3O4/[email protected] still held strong magnetism and responded rapidly to external magnetic field, which was conveniently separated in the post-treatment process of wastewater.
XPS analysis was further performed to investigate the element composition and the chemical states of the Fe3O4/[email protected] The XPS spectrum showed the surface composition of Fe3O4/[email protected] containing C, N, O, Fe and Zn elements as seen in Fig. 4(a). The binding energy of 285.5 eV was related to C-C in cellulose, which indicated the successful synthesis of Fe3O4/CNC(Hu et al. 2017). Additionally, the peaks at 398.8 eV in Fig. 4(a) represented N 1s, supporting the presence of the C = N–C in the ZIF-8(Yang et al. 2015). To amply reveal the microscopic interactions, the C 1s XPS spectrum was shown in Fig. 4(b) and three peaks at 284.04, 284.65 and 285.2 eV were associated with C-C, C = N and C-N, respectively(Chen et al. 2021). For O 1s in Fig. 4(c), the peak appeared at 531.91, 532.53 and 533.72 eV were in line with -OH, N-O and -COOH. One small Zn 2p peak observed at 1021.45 eV was assigned to zinc (+ 2) oxide, and the spectra of Zn 2p were in good agreement with its oxidation state. On the basis of previous work, the two characteristic peaks locating at 1021.8 and 1044.8 eV in the high resolution spectrum of Zn were due to Zn 2p3/2 and Zn 2p1/2, respectively, further indicating the presence of ZIF-8 (Fig. S2)(Yang et al. 2015). Notably, the Fe 2p spectrum showed the main peaks at around 710.7 and 724.2 eV mainly corresponded to the Fe2p3/2 and Fe2p1/2, respectively, obviously demonstrating the existence of Fe3O4 (Fig. 4(d))(Wu et al. 2019).
Peroxidase-like activity of Fe3O4/[email protected] and the detection of H2O2
The peroxidase-like activity of ZIF-8, Fe3O4/CNC and Fe3O4/[email protected] were studied at room temperature and the formation of the typical yellow product (2, 3-diaminophenazine, DPA) with a maximum adsorption at 450 nm was monitored by UV-vis absorption. In Fig. 5(a), the UV-vis absorption spectra at 450 nm were observed for ZIF-8, Fe3O4/CNC and Fe3O4/[email protected] systems and the absorption intensity of Fe3O4/[email protected] was comparatively high to ZIF-8 and Fe3O4/CNC. The inserted photographic image showed the color change of the different catalytic systems. It was observed that the mixed solution of OPD with either H2O2 or Fe3O4/[email protected] was colorless or slight yellow. Both the UV-vis absorption spectra and the naked eye detection demonstrated that the Fe3O4/[email protected] had excellent peroxidase-like catalytic ability to effectively catalyze the OPD oxidation. The reason behind this was that the interlaced web-like network structures of CNC can enhance the dispersion of Fe3O4 NPs. Besides, ZIF-8 possesses a porous structure with a large number of exposed active sites, which coated on the surface of Fe3O4 NPs to further improve the peroxidase-like activity of the composite.
It is known that the peroxidase-like catalytic activity of Fe3O4/[email protected] nanocomposites depends on the pH values, temperature and catalyst dosage. Therefore, the catalytic activity of Fe3O4/[email protected] was investigated in detail through modulating these factors. In Fig. S3(a), with the catalyst dosage increased from 2 to 12 mg, the catalytic activity first rose and then gradually fell, exhibiting the optimum catalyst dosage at 8 mg. Moreover, the pH and temperature were important factors affecting the performance of peroxidase catalysts, and hence various pH from 2.0 to 6.0 and temperature from 20 to 60 ℃ were studied (Fig. S3(b) and (c)). The catalytic activity achieved a higher level in acidic solutions (pH 3.5–4.5) than in strong acidic or alkaline solutions. The maximum catalytic activity was obtained in a solution at pH 4.0 similar to HRP(Qiao et al. 2014). In addition, Fe3O4/[email protected] exhibited quite commendable catalytic activity even the temperature at 55 ℃, indicating the catalytic possessed a good temperature resistance.
Under the optimal conditions, the Fe3O4/[email protected] was used for the sensitive assay of H2O2 by colorimetry. The UV-vis spectra of the different concentration of H2O2 were displayed in Fig. 5(b), which observed the absorbance increased gradually with the increasing of H2O2 concentration at 450 nm. Figure 5(c) showed a good linear relationship (R2 = 0.9954) between absorbance value and H2O2 concentration in the range from 2 to 200 µM with a limit of detection (LOD) of 2.24×10− 7 M (S/N = 3). Correspondingly, a clear color change from colorless to dark yellow was obviously differentiated by the naked eyes shown in Fig. 5(d). These results showed that the Fe3O4/[email protected] has great potential in the quantitative analysis of H2O2 detection.
Detection of phenol and optimization of reaction conditions
As expected, a colorimetric detection method of phenol had been established, which was proven a high-throughput analytical method with fast and visual readout advantages. When phenol is present, it can be oxidized by the Fe3O4/[email protected] forming the pink-colored complexes in the presence of H2O2, exhibiting a characteristic absorption peak at 525 nm(Wu et al. 2020). As shown in Fig. 6 (a), the reaction system including Fe3O4/[email protected] and H2O2 (curve e) appeared obvious pink compared to that of the control experiment (curve a and b). Furthermore, in comparison with experimental system consisted of either Fe3O4/[email protected] or H2O2 (curve d and e), both of them existed in the system could enhance the color reaction and generate a strong absorption peak at 525 nm. These results indicated that Fe3O4/[email protected] possessed a higher catalytic activity in the presence of H2O2 than that of Fe3O4/CNC and pure ZIF-8.
Various experimental parameters were examined and optimized as follows to find the suitable conditions for phenol detection. Since the reaction used 4-AAP as a chromogenic agent in the presence of an oxidant agent to produce a colored compound, changes in 4-AAP and H2O2 concentrations directly affected the efficiency of reaction. The influence of the 4-AAP concentration was studied in the range of 2.0–10.0 mM (Fig. S4 (a)), and the maximum absorbance was obtained with the 4-AAP concentration in 8.0 mM. As shown in Fig. S4 (b), an improvement in absorbance intensity was observed with the H2O2 concentrations increasing from 0.01 to 0.1 M and decreased slowly after that. Therefore, H2O2 concentration of 0.1 M was chosen for subsequent experiments. For temperature factor, the maximum catalytic activity for Fe3O4/[email protected] revealed that 50°C was the optimal incubation temperature Fig. S4 (c).
Figure 6 (b) demonstrated that when the phenol concentration was 0-200 µM, as the phenol concentration increased, the absorbance value gradually increased. A linear response was obtained between the absorbance and the phenol concentration in the range of 2-200 µM with the coefficient of correlation (R2) equal to 0.9908 (Fig. 6 (c)). The LOD of the designed phenol analysis platform was 3.16×10− 7 M (S/N = 3). Comparing the detection ranges and LODs of different materials for detecting phenol, it indicated that the Fe3O4/[email protected] material has certain advantages in phenol detection (Table S2).
Steady-state kinetic assay of Fe3O4/[email protected]
Based on these optimal conditions, the Michaelis-Menten behavior of the Fe3O4/[email protected] was evaluated by steady-state kinetics analysis with H2O2 and OPD as substrates, respectively. The oxidation reaction process of Fe3O4/[email protected] followed the conventional enzymatic dynamic regulation of the Michaelis-Menten equation. In Fig. 7, the Lineweaver-Burk plots were used to calculate the steady-state kinetics fitting parameters of the Michaelis-Mentenconstant (Km) and maximum initial velocity (Vmax). As shown in Table 1, the Km value of Fe3O4/[email protected] with OPD as substrate was 0.883 mM, two times lower than that of HRP (1.80 mM). It was confirmed that Fe3O4/[email protected] had a better affinity for OPD than HRP due to the large surface area of catalysts and good dispersion of Fe3O4 NPs coated with ZIF-8. The Km value of Fe3O4/[email protected] with H2O2 was 0.171 mM, which was smaller than that of HRP (0.214 mM). It was indicated lower H2O2 concentration for Fe3O4/[email protected] could achieve maximum activity than that of HRP. Furthermore, the Fe3O4/[email protected] had a much smaller Km and higher Vmax than those of HRP(Kergaravat et al. 2012; Qiao et al. 2014), demonstrating that Fe3O4/[email protected] possessed higher activity and catalytic efficiency than that for HRP.
To gain a better understanding of the mechanism towards the reaction of phenol and 4-AAP to form a pink quinone imine, p-phthalic acid (PTA) was chosen as a fluorescence probe. Terephthalic acid can react with •OH to produce a fluorescent product 2-hydroxy terephthalic acid (HTA), which can be observed through fluorescence spectroscopy by displaying a fluorescent emission peak at 410 nm(Barreto 1994). In detail, 4 mg Fe3O4/[email protected] was firstly added to the mixture containing 100 µL 0.1 M H2O2, 2.89 mL NaAc-HAc buffer (0.2 M pH 4.0) for reaction. Then, 10 µL 5 mM terephthalic acid solution was added to the above solution. Subsequently, the mixed solution incubated for 20 min at room temperature, and the fluorescence spectrum was recorded with an excitation wavelength at 325 nm. As illustrated in Fig. 8(a), both PTA and PTA + Fe3O4/[email protected] system showed almost no fluorescence at 325 nm, while Fe3O4/[email protected] + H2O2 + PTA system had strong fluorescence intensity, confirming that Fe3O4/[email protected] can catalyze H2O2 to produce •OH during the peroxidase-like reaction. The reaction mechanism was clearly illustrated in Fig. 8(b). Fe3O4/[email protected] + H2O2 + PTA played the peroxidase-like catalytic activity and catalyzed H2O2 to produce •OH. Subsequently, the •OH with strong oxidizing capacity induce phenol to generate the quinoid radicals, which then reacted with 4-AAP in the presence of redundant •OH to form a pink quinone imine(Wu et al. 2020).
Selectivity and reusability of Fe3O4/[email protected]
Selectivity is significantly important to establish excellent colorimetric assay. Control experiments were carried out under the same condition in the presence of phenol and other coexisting substances. The changes of absorbance at 505 nm were monitored by UV-vis spectrophotometer, and the experimental results were exhibited in Fig. 9(a), almost no apparent absorbance at 525 nm along with slight color change were observed in the reaction solutions with interfering substances. However, the reaction solution revealed a remarkable color variation from colorless to pink accompanied with a strong absorbance at 525 nm.
In addition, the reusability of the synthesized Fe3O4/[email protected] was investigated by cyclic experiments. UV-vis spectrophotometer was used to record the absorbance of the reaction systems at 525 nm. After each cycle, the composite was separated from the reaction system with extra magnet, and washed by DI water and ethanol five times and dried, which was reused for the next cycle. In Fig. 9(b), the absorbance values of the reaction system had negligible change after sextic tests. Therefore, these results further confirmed the excellent selectivity and reusability of Fe3O4/[email protected] phenol assay platform, offering a feasible and promising strategy for detecting phenol simple, rapidly and sensitively in actual samples.