3.1. Physical characterization
SEM and EDS was used to characterize the morphology, structure, and composition of the ZIF-8, Au/PEDOT, and Au/PEDOT/ZIF-8 composites. From Fig. S2A and B, the ZIF-8 has a conventional regular dodecahedron structure. Besides, the SEM images of the Au/PEDOT composite were represent in Fig. S2C and D. As it can be obviously seen in the SEM images, the Au NPs were well dispersed on the surface of coralloid PEDOT particles. Figure 1A/A1-C/C1 shows the SEM images of Au/PEDOT/ZIF-8 under different magnification. Compared with Au/PEDOT, the structurization of Au/PEDOT/ZIF-8 composite is enhanced and the arrangement is more ordered. As shown in Fig. 1A and A1, the Au/PEDOT/ZIF-8 composite mainly shows two different morphologies, one of which exhibits the typical coralloid structure of Au/PEDOT with a much rougher surface (Fig. 1B and B1), and the other one is the Au nanorods-modified ZIF-8 particles sparsely dotted among the Au/PEDOT particles (Fig. 1C and C1). As depicted in Fig. 2A, the EDS spectrum has shown the corresponding composition information of the Au/PEDOT/ZIF-8 composite. As listed, the wt % values of C, N, O, S, Zn, and Au are 45.9%, 1.0%, 18.2%, 16.3%, 0.1%, and 18.5%, respectively. According to the results, C, O, and S elements come from PEDOT, while N and Zn elements come from ZIF-8. The much lower contents of N and Zn elements may be ascribed to the sparse distribution of ZIF-8 particles and that the surface of ZIF-8 particles is coated by Au nanorods, which can be observed from Fig. 1C/C1 and Fig. 2B-H. Mapping images further reveal the distribution area of each element. As presented in Fig. 2B-H, the elements of C, O, and S mainly distribute in areas outside the circular region of ZIF particles, and the N, Zn and Au elements are mostly concentrated inside the region of ZIF-8 particles. This distribution state is consistent with the morphology shown in Fig. 1. The interior structure of the Au/PEDOT/ZIF-8 composite was further characterized by TEM and high-resolution TEM (HRTEM) images. Figure 3A and A1 show the TEM images of partial morphology of Au/PEDOT/ZIF-8 composite, which exhibit the uniform distribution of Au NPs (Φ ≈ 0.2 nm) in the porous PEDOT network structure. From the Fig. 3A2, the Au NPs possess a well-defined lattice spacing of 0.24 nm, corresponding to the (111) plane of the Au crystal. The other part of Au/PEDOT/ZIF-8 performs a different morphology shown in Fig. 3B and B1. As observed, the Au nanorods clustered around the ZIF-8 particles, consistent with the morphology shown in Fig. 1C and C1. From the HRTEM image in Fig. 3B2, the lattice spacing of Au crystal is measured as 0.24 nm, confirming the successful preparation of Au/PEDOT/ZIF-8 composite.
The XRD was used to characterized the phase and crystal structure of the prepared composites (Fig. 4A). The peaks that appeared at 7.43°,10.45°,12.68°, 16.45°,19.45°, and 22.21° are indexed as the (110), (200), (211), (310), (321), and (330) reflection planes of ZIF-8, respectively, demonstrating the presence of ZIF-8 in Au/PEDOT/ZIF-8 composite (Xie et al., 2019). The same diffraction peaks appear in the Au/PEDOT and Au/PEDOT/ZIF-8 patterns at 38.3°, 44.5°, 64.7°, 77.7°, and 81.8°, which correspond to the (111), (200), (220), (311), and (222) planes, respectively. These reflections match the typical Au pattern very well.(Dash & Munichandraiah, 2013). According to the XRD patterns of Au/PEDOT/ZIF-8, all main peaks are consistent with Au and ZIF-8, indicating that the PEDOT support has no effect on the crystal structure of the composite.
In order to prove the coordination mode and composition of Au/PEDOT/ZIF-8 composites, the products were characterized by FTIR in Fig. 4B. For ZIF-8, the unsaturated and saturated C–H stretching vibration are associated with the peaks at 3120 and 2928 cm− 1, respectively (Yang et al., 2015). The plane bending vibration and stretching vibration of the imidazole ring should be connected by a complex spectrum in the range of 700 to 1500 cm− 1 (Gao et al., 2021). The Zn–N stretch mode is observed at the peak 422 cm− 1 (Chen et al., 2019). Regarding PEDOT, the stretching modes of thiophene ring of C-C (inter-ring stretching mode) and C = C (asymmetric stretching mode), as well as the quinoid structure, produce vibrations at about 1355 and 1518 cm− 1 (Mahatme et al., 2021). The vibrations at 1200 and 1090 cm− 1 are attributed to the C–O–C bond stretching in the methylenedioxy group (Eren et al., 2012) and the C-S peak in thiophene was at 692 cm− 1. Au-S vibrations appear in small intensity around 653 cm− 1 in the spectrum (Kuzmann et al., 2016). The FTIR spectrum of Au/PEDOT/ZIF-8 comprises all of the distinctive peaks of ZIF-8 and Au/PEDOT, revealing that ZIF-8 was successfully introduced into the composite.
The chemical states of each element in Au/PEDOT and Au/PEDOT/ZIF-8 was characterized by XPS. Figure 5A shows the comparison of the XPS survey spectra of above composites. As exhibited, the peaks of C 1s, O 1s, S 2p, and Au 4f are present in the spectra of both Au/PEDOT and Au/PEDOT/ZIF-8, indicating the formation of PEDOT and Au metal. Besides, a weak peak of N 1s at 401.9 eV is also observed in the spectrum of Au/PEDOT/ZIF-8, which originates from ZIF-8. However, the Zn peak is not observed and this may be ascribed to the low content of ZIF-8 in the Au/PEDOT/ZIF-8 composite. As seen in Fig. 5B, three peaks of the C-C, C-S, and C-O bonds is showed up at 284.13, 285.39, and 287.41 eV, demonstrating the presence of PEDOT, respectively, result from the deconvolution of the asymmetric C 1s photoemission of Au/PEDOT. By contrast, Au/PEDOT/ZIF-8 has higher C-C, C-S and C-O bond energy than Au/PEDOT, and a novel C-N bond developed at 285.77eV. meaning that ZIF-8 has been doped in Au/PEDOT successfully (Yuan et al., 2021). Figure 5C displays the O 1s core-level spectra of Au/PEDOT and Au/PEDOT/ZIF-8. The prominent peak at 532.35 eV in the O 1s spectra is associated with the C-O-C bond in PEDOT. The peak of the C-O-C bond appears at the same binding energy for the O 1s of Au/PEDOT/ZIF-8. In Fig. 5D, the S 2p spectra of Au/PEODT have characteristic peaks of spin-split double S 2p at 163.10 (S 2p3/2) and 164.32 eV (S 2p1/2), which are derived from the thiophene ring in PEDOT. By contrast, the two S 2p peaks of Au/PEDOT/ZIF-8 are offset by the higher binding energy. The past literature states that the first contribution at a higher binding energy can be attributed to the bridging sulfur functions, while the second contribution at a lower binding energy is related to the gold-bound thiolate group, wherein two of the three sulfur atoms of compound are bridged and the third is bonded to the gold substrate (Lach et al., 2014; Salazar et al., 2016). As a result, the presence of the Au-S bond can be proved. Figure 5E shows the Au 4f spectra two composites. The Au 4f7/2 and Au 4f5/2 peaks at 83.09 and 86.83 eV for Au/PEDOT and 83.90 and 87.61 eV for Au/PEDOT/ZIF-8 indicate the presence of Au0 state in the two composites (Liu et al., 2016; Neeli et al., 2014). In addition, the Au 4f peaks of Au/PEDOT/ZIF-8 move slightly towards higher binding energy as compared with Au/PEDOT, which is attributed to the electronic interaction between Au NPs and ZIF-8. Figure 5F shows the N 1s spectrum of Au/PEDOT/ZIF-8. The N 1s spectrum has two peaks, located at 399.88 and 401.06 eV, respectively. The former is N-Zn coordination related to the C = N of ZIF-8 and the latter is attributed to C-N from 2-methyl-imidazole (Liu et al., 2017). From the above results, it can be proved that the presence of the N 1s peak which from ZIF-8 has been successfully doped into Au/PEDOT/ZIF-8 composite.
3.2. Electrochemical activity of Au/PEDOT/ZIF-8
CV and electrochemical impedance spectroscopy (EIS) were employed to confirm the electrochemical activity of various modified electrodes in 5.0 mM [Fe(CN)6] 3−/4− solution containing 0.1 M KCl. From Fig. S3A, all the CV curves (CVs) of the electrodes show the redox peaks, which corresponds to the ferricyanide ion oxidation and reduction. It can be clearly seen that the CV of Au/PEDOT/GCE shows the highest redox peak current density. By contrast, the lower redox peak current of Au/PEDOT/ZIF-8/GCE than Au/PEDOT/GCE is might be owing to the weak electrocatalytic ability and low electrical conductivity of ZIF-8.
The EIS is an effective method to study the interface characteristics of surface modified electrodes (Eugenii & Itamar, 2003). As presented in Fig. S3B, the largest semicircle domain with the highest charge-transfer resistance (Rct) of 2292.1 Ω is the Au/ZIF-8/GCE, and the bare GCE possesses relatively smaller semicircle domain with a Rct value as 581.3 Ω. When PEDOT was introduced to the composite, the Rct of Au/PEDOT/GCE and Au/PEDOT/ZIF-8/GCE decreased to 203.4 and 213.6 Ω. Obviously, the Rct value of Au/PEDOT/ZIF-8/GCE is higher than that of Au/PEDOT/GCE, owing to the poor conductivity and catalytic performance of ZIF-8.
3.3. Electrochemical sensing properties of Au/PEDOT/ZIF-8 toward CA and SY detection
The electrochemical behaviors of each modified electrons were investigated using DPV in PBS (0.1 M, pH 7.5) containing 50 µM CA and 25 µM SY. As Fig. 6 and the inset shown, the oxidation peaks of CA and SY can be seen at 0.14 and 0.61 V for all the DPV of electrodes. In contrast, the DPV of Au/PEDOT/ZIF-8/GCE shows the highest oxidation peak currents for CA and SY oxidation, which demonstrated the exceptional electrochemical sensing activity for Au/PEDOT/ZIF-8 towards CA and SY detection. The reason can be explained as the well distribution of Au NPs in the composite and the synergistic effects between Au NPs, PEDOT, and ZIF-8.
In most cases, the pH value of the solution had a significant impact on the electrochemical reaction. DPV investigated the effect of solution pH on the electrocatalytic oxidation of 50 M CA and 25 M SY in the mixed solution at the Au/PEDOT/ZIF-8/GCE in the pH range of 5.5 to 8.0. The oxidation peak current reaches its maximum at pH = 7.5, as shown in Fig. S4A and B. As a result, PBS with a pH of 7.5 was chosen as the test solution. As displayed in Fig. S4C, the same trend of the oxidation peak potential of CA and SY can be observed. By increase the pH of the solution, the oxidation peak potentials of CA and SY both move to negative potential almost linearly, demonstrating that the protons are involved in the electrode reaction process. The relationship of potentials (Ep) versus pH can obtained the equations: Ep (V) = 0.5969 − 0.0615 pH (R2 = 0.9984) for CA and Ep (V) = 0.8288 − 0.0287 pH (R2 = 0.9953) for SY. Based on the relationship between potentials (Ep) and pH for CA, the absolute value of the slope was near to the theoretical value of 0.059 V pH− 1, suggesting that the ratio of protons to transferred electrons was approximately equal (Qiu et al., 2016). However, according to the relationship between potentials (Ep) and pH for SY, the absolute value of the slope is 0.029 V pH− 1 that half of the 0.059 V pH− 1, which suggests that the electrooxidation of the target is carried out under the mechanism of 2e−/1H+. However, a slope value of 0.032 V and 0.030 V pH− 1 has also been reported by (Penthesilia-Amalia et al., 2019; Wang & Zhao, 2015), indicating that the whole electrode process depends on the composition and properties of the electrocatalytic surface.
Accumulation time can increase the loading capacity of CA and SY on the Au/PEDOT/ZIF-8/GCE, causing the electrochemical signals to be amplified. As shown in Fig. S5A, the accumulation time was discussed by DPV used Au/PEDOT/ZIF-8/GCE at different accumulation time for 50 µM CA and 25 µM SY in PBS (pH = 7.5). Fig. S5B illustrates the relationship of the oxidation peak currents versus the accumulation time. The oxidation peak current increases significantly when the accumulation time is extended from 30 s to 75 s. However, when the accumulation time is further increased from 75 s to 105 s, both the oxidation peak currents of CA and SY increase slowly, suggesting that the adsorbing capacity of CA and SY tend to a limiting value at the surface of Au/PEDOT/ZIF-8. As a result, 75 s accumulation time is selected for the higher sensitivity and working efficiency.
To better study the electrochemical performance of this sensor, the active surface area of each electrode and the electron transfer coefficient of Au/PEDOT/ZIF-8/GCE were calculated based on the Randles-Sevcik equation (Eq. (1) in Supplementary Materials ) (Bali Prasad et al., 2017) and Fig. S6 and S7. The average electroactive surface area of Au/PEDOT/ZIF-8/GCE was calculated as 0.142 cm2, which is 1.54 times the size of the bare electrode area (0.092 cm2). The above results indicate that the Au/PEDOT/ZIF-8 composite has the largest active surface area, which will help to improve the CA and SY oxidation peak current. According to the Eqs. (2) and (3), the α values of CA were calculated to be 0.595, and the values of n for CA were calculated to be 2.94 (approximately equal to 3), indicating that the electrochemical oxidization reaction on Au/PEDOT/ZIF-8/GCE is a three-electron transfer process for CA. Based on Eq. (4) and α = 0.5 (Laviron, 1979), the value of n for SY was calculated as 2.17 (approximately equal to 2). Besides, Fig. S7C and F show the linear fitting curves of the oxidation peak currents versus the scan rate of CA and SY, respectively, indicating that the process is an adsorption diffusion process for CA and SY. Furthermore, the redox mechanism of CA and SY is depicted at Fig. S8 based on the above results. The supporting material provides the calculations and extensive descriptions about the electrochemical parameters.
DPV was used to detect the relationship of the oxidation peak current intensity (Ip) versus cCA and cSY under the optimized experimental conditions. Changing one concentration of substance and the concentration of another substance is constant, CA or SY in the mixture is determined respectively. For the DPV depicted in Fig. 7A, fixed the concentration of SY at 20 µM, and the concentration of another substance CA was added up from 0.3 µM to 50 µM. As exhibited in Fig. 7B, the linear relationship of the oxidation peak current intensity (Ip) versus cCA can be obtained as Ip1 = -1.2439 + 4.7987cCA (R2 = 0.9930) in 0.3 ~ 1.0 µM and Ip2 = 1.2485 + 2.4426cCA (R2 = 0.9976) in 1.0 ~ 50 µM. Furthermore, Fig. 7C shows the DPVs of Au/PEDOT/ZIF-8/GCE recorded with 1 µM CA and 3 ~ 25 µM SY. As Fig. 7D shown, a linear relationship of the oxidation peak current intensity (Ip) versus cSY is also obtained with an equation of Ip = − 1.7008 + 0.6414cSY (R2 = 0.9962) in 3 ~ 25 µM. The above results suggest that CA and SY can be quantitatively analyzed by Au/PEDOT/ZIF-8/GCE in the presence of each other.
As shown in Fig. 8A, the DPV diagram is obtained on Au/PEDOT/ZIF-8/GCE in PBS (pH = 7.5) by increasing the concentrations of CA and SY simultaneously. In these DPVs, two oxidation peaks at 0.14V (CA) and 0.61 V (SY) are observed, and the oxidation peak currents of CA and SY increase gradually with the CA and SY concentration increased. DPV also clearly showed the relationship of peak current vs. CA and SY concentration. Figure 8B, B1 and C show the relationships of the oxidation peak current intensity (Ip) versus concentration of CA (cCA) and SY (cSY). As Fig. 8B and B1 exhibited, there are two linear relationships between Ip and cCA, which are obtained as Ip = 1.8444 + 2.3778 cCA (R2 = 0.9975) in 1.0 ~ 50 µM (Fig. 8B) and Ip = − 1.4772 + 5.9127 cCA (R2 = 0.9986) in 0.3 ~ 1.0 µM (Fig. 8B1), the LOD and sensitivity are calculated as 0.1 µM (S/N = 3) and 33.79 mA mM− 1 cm− 2, respectively. From Fig. 8C, a linear relationship of SY oxidation peak current (Ip) versus cSY can be expressed as Ip = − 1.7356 + 0.6623 cSY (R2 = 0.9964) in 3 ~ 25 µM, the LOD and sensitivity are calculated as 1 µM (S/N = 3) and 4.52 mA mM− 1 cm− 2. The above results indicate that the perfect analytical performance to Au/PEDOT/ZIF-8/GCE. Table S1 compares the CA and SY detecting characteristics on Au/PEDOT/ZIF-8/GCE with other reported literature to further evaluate the sensing performance of Au/PEDOT/ZIF-8/GCE. By comparison, the Au/PEDOT/ZIF-8 obtained in this work may be a prospective sensing platform to simultaneously detect the CA and SY with highly sensitivity.
3.3. Stability, reproducibility and anti-interference ability
Stability was measured under DPV at room temperature and got the relationship between the oxidation peak current and storage days. As shown in Fig. 9A, both the two peak currents decreased gradually with the extension of storage time, and the relative standard deviation (RSD) decreased by 2.14% and 4.24% for CA and SY during the 10 days storage. This result demonstrates that the Au/PEDOT/ZIF-8 film is stable enough on GCE surface for a long-time sensing test. Furthermore, the repeatability of the Au/PEDOT/ZIF-8/GCE sensing platform was studied with the same one modified electrode for 10 times DPV testing continuously. As observed from Fig. 9B, the 10 cycles of DPV curves are almost overlapped with the RSD of the peak current responses of CA and SY as 2.48% and 2.77%, respectively, indicating the satisfying repeatability of Au/PEDOT/ZIF-8/GCE. To be a qualified sensing material, a high reproducibility is of great significance. The DPV of 10 of the same Au/PEDOT/ZIF-8/GCE were carried out in PBS (pH = 7.5) containing 50 µM CA and 25 µM SY. The peak current of the DPVs versus the number of electrodes were shown in Fig. 9C. As obtained, the RSD of these peak currents for CA and SY oxidation are 1.03% and 2.38%, respectively, demonstrating the good reproducibility of the Au/PEDOT/ZIF-8 as sensing platform for CA and SY detection. Consider the complexity and diversity of food ingredients, the anti-interference ability of one sensor is very important. To investigate the anti-interference ability of Au/PEDOT/ZIF-8/GCE towards CA and SY detection, 0.4 mM concentrations (20-fold concentration of CA and SY) of caffeine, ascorbic acid, glucose, riboflavin, tartrazine, quinoline yellow, NaCl, KCl, and ZnCl2 were added to investigate the interference of these substances for CA and SY detection by DPV, respectively. Corresponding DPVs are provided as Fig. S9 displays the peak current of CA and SY in PBS (pH = 7.5) with 20 µM CA, 20 µM SY, and 0.4 mM possible interferents. According to Fig. 9D and Fig. S9, these substances did not significantly affect the outcomes of CA and SY detection (signal change below 5%). From the above results, it can be demonstrated that the high anti-interference capacity of Au/PEDOT/ZIF-8/GCE for simultaneous detection of CA and SY.
3.4. The detection of CA and SY in tea drink by Au/PEDOT/ZIF-8/GCE
Finally, to verify the practicability, the Au/PEDOT/ZIF-8/GCE sensing platform was applied to simultaneously determine CA and SY in tea drink by the standard addition method. The tea drink was purchased from a nearby local market and 200 times diluted with 0.1 M PBS (pH = 7.5) for detection. For comparison, the HPLC method was also used for simultaneously detect the CA and SY in the tea drink. The linear fitting standard curves of the peak areas versus concentrations of CA and SY obtained by HPLC have been shown in Fig. S10. Table 1 listed the analysis results of DPV and HPLC. As listed, the recovery rates and RSD for CA detection are 95.38 ~ 97.28% (DPV)/95.16 ~ 106.96% (HPLC) and 0.95 ~ 2.83% (DPV)/0.91 ~ 1.69% (HPLC), respectively. And the recovery rates and RSD for SY detection are 95.80 ~ 98.73% (DPV)/95.2 ~ 103.7% (HPLC) and 0.95 ~ 2.99% (DPV)/0.58 ~ 1.89% (HPLC), respectively. Based on the above analysis data, it can be obtained that there is still a small gap between the detecting results of the Au/PEDOT/ZIF-8 sensing platform and the HPLC. However, the electrochemical sensor has its own unique advantages, such as the simple operation, quick detecting process and low-cost operating, which is unmatched by HPLC method. The simultaneous detection of CA and SY by the Au/PEDOT/ZIF-8 electrochemical sensing platform has a acceptable accuracy and high reliability, which performs a promising application prospect in food safety for rapid detection and field test.
Table 1
Determination results of CA and SY in tea drink sample by DPV and HPLC methods (n = 3).
Samples | Methods | Spiked (µM) | Determined (µM) | Recovery (%) | RSD (%) |
CA | SY | CA | SY | CA | SY | CA | SY |
Tea drink | DPV | 0 | 0 | 2.99 | / | / | / | 2.83 | / |
10 | 5 | 12.39 | 4.75 | 95.38 | 95.80 | 1.09 | 1.63 |
20 | 10 | 21.87 | 9.87 | 95.84 | 98.73 | 0.95 | 0.95 |
30 | 15 | 32.1 | 14.6 | 97.28 | 97.33 | 2.47 | 2.99 |
40 | 20 | 41.27 | 19.66 | 95.98 | 98.3 | 2.02 | 1.94 |
HPLC | 0 | 0 | 3.07 | / | / | / | 1.69 | / |
10 | 5 | 13.98 | 4.76 | 106.96 | 95.20 | 0.91 | 1.89 |
20 | 10 | 24.04 | 10.37 | 104.10 | 103.7 | 1.27 | 0.95 |
30 | 15 | 32.95 | 14.27 | 95.16 | 97.07 | 1.44 | 0.58 |
40 | 20 | 44.14 | 19.34 | 102.58 | 96.74 | 1.53 | 1.42 |