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Article

An Electrochemical Sensor Based on Amino Magnetic Nanoparticle-Decorated Graphene for Detection of Cannabidiol

by
Yi Zhang
,
Zongyi You
,
Chunsheng Hou
,
Liangliang Liu
* and
Aiping Xiao
*
Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Changsha 410205, China
*
Authors to whom correspondence should be addressed.
Nanomaterials 2021, 11(9), 2227; https://doi.org/10.3390/nano11092227
Submission received: 30 July 2021 / Revised: 25 August 2021 / Accepted: 25 August 2021 / Published: 29 August 2021
(This article belongs to the Special Issue Graphene-Based Nanomaterials)
Browse Figures
Versions Notes

Abstract

:
For detection of cannabidiol (CBD)—an important ingredient in Cannabis sativa L.—amino magnetic nanoparticle-decorated graphene (Fe3O4-NH2-GN) was prepared in the form of nanocomposites, and then modified on a glassy carbon electrode (GCE), resulting in a novel electrochemical sensor (Fe3O4-NH2-GN/GCE). The applied Fe3O4-NH2 nanoparticles and GN exhibited typical structures and intended surface groups through characterizations via transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), vibrating sample magnetometer (VSM), and Raman spectroscopy. The Fe3O4-NH2-GN/GCE showed the maximum electrochemical signal for CBD during the comparison of fabricated components via the cyclic voltammetry method, and was systematically investigated in the composition and treatment of components, pH, scan rate, and quantitative analysis ability. Under optimal conditions, the Fe3O4-NH2-GN/GCE exhibited a good detection limit (0.04 μmol L−1) with a linear range of 0.1 μmol L−1 to 100 μmol L−1 (r2 = 0.984). In the detection of CBD in the extract of C. sativa leaves, the results of the electrochemical method using the Fe3O4-NH2-GN/GCE were in good agreement with those of the HPLC method. Based on these findings, the proposed sensor could be further developed for the portable and rapid detection of natural active compounds in the food, agricultural, and pharmaceutical fields.

1. Introduction

Cannabis sativa L. (C. sativa) is an annual dioecious herb belonging to the Cannabinaceae family, which is cultivated worldwide, and was one of the original crops in China [1]. C. sativa can be simply divided into industrial hemp and marijuana—generally distinguished by the content of Δ9-Tetrahydrocannabinol (Δ9-THC) in the plant; it is considered to be industrial hemp when the content of Δ9-THC is lower than 0.3% (w/w), and otherwise is referred to as marijuana. Based on the existing legal requirements in China, the cultivation of marijuana is banned; all varieties of C. sativa planted in China belong to the industrial hemp category [2]. In recent years, the medicinal usage of cannabidiol (CBD) has received unprecedented attention in the pharmaceutical and cosmetics industries. Accordingly, as the natural extraction source of CBD, C. sativa has ushered in a new round of development [3]. As an isomeride of Δ9-THC, CBD is non-psychoactive and exhibits good pharmacological effects in treating chronic pain, anxiety, inflammation, depression, and many other symptoms [4,5]. Today, the quantitative analysis methods of cannabinoids—including CBD—are mainly chromatographic methods, such as high-performance liquid chromatography (HPLC), gas chromatography (GC), and mass spectrometry [6,7,8].
As a kind of ultrasensitive detection method, electrochemical sensors or biosensors are mainly reported for the detection of Δ9-THC, since Δ9-THC is a typical psychoactive drug and is strictly regulated [9]. However, electrochemical sensors developed for the detection of CBD are rare. Since the interest in (and market for) CBD and related products are growing, the detection of CBD has also become more important [10]. A convenient and rapid strategy for the detection of CBD could meet many needs in various scenarios outside of the laboratory. Through customization in the size and composition of sensors, in combination with the design of a small and portable workstation, a preliminary and rapid detection of CBD in plants could be completed in the field, which could save a lot of work for farmers or researchers [11]. Therefore, the effort to develop electrochemical sensors for CBD using novel nanomaterials is worthwhile.
As a typical two-dimensional nanomaterial, graphene (GN) has displayed many properties, including large specific surface area, high chemical stability, and excellent electrochemical properties [12]. It is widely used to modify electrodes in order to achieve better results in applications of supercapacitors, potassium-ion batteries, and detectors for biomarkers, metabolites, viruses, etc. [13,14,15,16]. The beneficial effects of modification of electrochemical sensors have been proven, including route simplicity, high efficiency, good performance, and low cost [17]. Magnetic nanoparticles such as iron oxide also show characteristics such as low toxicity, ease of functionalization, high adsorption ability, and magnetic responsivity [18]. The introduction of magnetic nanoparticles in electrochemical sensors could facilitate of electron transfer and signal amplification. The combination of GN and magnetic nanoparticles, resulting in Fe3O4/GN nanocomposites, has been utilized in the construction of various electrochemical sensors [19]. The Fe3O4/GN nanocomposites have been applied with satisfactory performance in the electrochemical detection of arsenic ions, lobetyolin, dopamine, glucose, prostate-specific antigen, hepatitis C virus, etc. [20,21,22,23,24,25].
In this study, many materials were tested for the modification of electrodes in order to obtain higher signals in the electrochemical detection of CBD. Amino-group-modified Fe3O4 nanoparticles (Fe3O4-NH2) were finally confirmed for the modification of a glassy carbon electrode (GCE) together with GN in the form of nanocomposites. After the characterizations of the materials, GN and Fe3O4-NH2 were mixed as nanocomposites and modified on the GCE (Fe3O4-NH2-GN/GCE) to develop a novel electrochemical sensor for the highly selective and sensitive detection of CBD (Figure 1). The composition and fabrication sequences of the modifiers were investigated and optimized. Under the optimal fabrication and analytical conditions, the proposed Fe3O4-NH2-GN/GCE demonstrated enhanced electrochemical signals, good linearity, and satisfactory anti-interference ability for CBD. The CBD content of C. sativa leaf extract was detected using the proposed Fe3O4-NH2-GN/GCE, and the results were compared with those of the conventional HPLC method. Hence, it can be expected that the Fe3O4-NH2-GN/GCE has extensive potential applications in the detection of CBD and other natural ingredients.

2. Materials and Methods

2.1. Reagents and Apparatus

Detailed information about the reagents and instrumentations can be found in the Supplementary Material.

2.2. Preparation of Fe3O4-NH2 Nanoparticles

Fe3O4 nanoparticles were prepared according to our previous report [26]. Typically, 1.35 g of ferric chloride, 3.60 g of sodium acetate, and 1.00 g of PEG 6000 were mixed in 50 mL of ethylene glycol. The mixture was stirred under ultrasonication for 30 min and poured into a Teflon-lined stainless steel autoclave (100 mL). The autoclave was put into a drying oven at 180 °C for 6 h. After reaction, the black products were poured out and washed with water and ethanol three times each.
The obtained Fe3O4 nanoparticles were then dispersed in 250 mL of ethanol and ultrasonicated for 30 min [27]. After that, the materials were poured into a round-bottomed flask, and 2 mL of 3-aminopropyltriethoxysilane was dripped slowly into the Fe3O4 nanoparticle dispersion under mechanical agitation. The reaction was performed at room temperature for 6 h. Finally, the Fe3O4-NH2 nanoparticles were washed with ethanol three times and stored in ethanol at 4 °C.

2.3. Fabrication of the Fe3O4-NH2-GN/GCE

Before modification, the GCE was polished using alumina powders (0.05 μm) and cleaned via ultrasonication for 10 min. The surface of the GCE was dried with nitrogen gas and stored for further use. Next, 12.0 mg of Fe3O4-NH2 nanoparticles and 12.0 mg of GN were mixed in 2.0 mL of water and ultrasonicated for 10 min to form a homogeneous solution, which was marked as Fe3O4-NH2-GN nanocomposites. Then, 10 μL of the Fe3O4-NH2-GN suspension (6.0 mg/mL in water) was carefully dropped on the surface of GCE and air-dried to form an active layer on the surface of the electrode. The modified electrode was referred to as Fe3O4-NH2-GN/GCE.
For comparison, 10 μL of Fe3O4-NH2 nanoparticles and GN (6.0 mg/mL in water) were fabricated on the GCE in the same procedures and conditions, which were designated as Fe3O4-NH2/GCE and GN/GCE, respectively. For confirmation of the fabrication sequence, three kinds of sequences were compared. Fe3O4-NH2 nanoparticles were firstly dropped on the surface of GCE; GN was then dropped on the surface when the nanomaterials were dried, the result of which was designated as GN/Fe3O4-NH2/GCE. GN was firstly modified on the bare GCE, and then Fe3O4-NH2 nanoparticles were modified, which was designated as Fe3O4-NH2/GN/GCE. These two electrodes were compared with Fe3O4-NH2-GN/GCE for their electrochemical response under the same conditions.

2.4. Preparation of Real Sample

Dry C. sativa leaves were ground, passed through a 40-mesh sieve, and placed in an oven at 105 °C for 10 h. After these treatments, 0.5 g of C. sativa leaves was immersed in 50 mL of anhydrous methanol solution. The mixture was extracted for 20 min using an ultrasonic extractor at a power of 200 W (KQ5200DV, Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China). After extraction, the mixture was centrifuged at 4000 r/min for 5 min (TD5, Yingtai Instrument Co., Ltd., Changsha, China). Next, 2 mL of the upper transparent solution was diluted to 20 mL with phosphate buffer solution (PBs, 10 mmol L−1, pH 5.0) and filtered with 0.45 μm filter before analyses via HPLC and using the proposed sensor.

2.5. Determination of CBD by HPLC

For comparison of the detection results, the HPLC method was applied in the detection of samples as well. An isocratic elution program consisting of 0.1% acetic acid and 75% acetonitrile was applied for 30 min at 25 °C. The flow rate was set to 0.8 mL/min. The chromatogram was observed at 220 nm. The injection volume of the sample was 10 μL. The CBD content in the C. sativa leaf extract was calculated using the standard curve obtained by the measurement standards.

2.6. Electrochemical Measurements

The electrochemical measurements were performed using the three-electrode system in CBD solution, using PBs (10 mmol L−1, pH 5.0, containing 10% methanol) as a solvent and supporting electrolyte. Cyclic voltammetry (CV) was used for the measurement, with a scan rate of 0.05 V s−1 and a potential range from 0 V to 0.8 V. Electrochemical impedance spectroscopy (EIS) was applied to characterize the sensor conductivity in the solution containing 5.0 mmol L−1 of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 mol L−1 of potassium chloride. The amplitude was 0.005 V with a frequency range of 0.1 to 105 Hz. All experiments were carried out in three duplicates at 25 ± 2 °C.

3. Results and Discussion

3.1. Characterizations of Nanomaterials

3.1.1. TEM and SEM

The morphologies of Fe3O4 nanoparticles, GN, and Fe3O4-NH2-GN were investigated via TEM (Figure 2). The round sphere of Fe3O4 nanoparticles, as well as the silk-like and wrinkled structures of GN, could be easily observed in the corresponding images (Figure 2a,b) [28]. The Fe3O4 nanoparticles showed sizes of about 430 nm, with good dispersion. After mixing, the Fe3O4-NH2-GN nanocomposites retained the characteristics of Fe3O4 nanoparticles and GN. It can be seen in the TEM image that Fe3O4 nanoparticles were dispersed on the GN sheets (Figure 2c) [29].
An SEM image of the Fe3O4-NH2-GN nanocomposites on the electrode surface was also provided in order to confirm the morphology and structure (Figure 2d), and showed the modified surface of the electrode. Though there was a kind of agglomeration in the nanocomposites, the existence of Fe3O4 nanoparticles (round spheres) on the GN could be confirmed. The irregular surface of the modified electrode might be one of the reasons for the improved electrochemical response.

3.1.2. XRD

The XRD patterns of Fe3O4 nanoparticles, GN, and Fe3O4-NH2-GN were analyzed, and are shown in Figure 3a. The pattern of Fe3O4 nanoparticles exhibited typical peaks at 30.3°, 35.7°, 43.6°, 57.4°, and 62.9, which were attributed to the indices (220), (311), (400), (511), and (440) of the Fe3O4 crystal, respectively [30]. Meanwhile, in the pattern of Fe3O4-NH2-GN, the related peaks became much weaker, which might be a result of the coating of GN and the modification of the amino groups [26]. Additionally, another peak at ~26° could be observed, belonging to the characteristic reflection of the existence of GN [31]. The XRD results confirmed the existence of Fe3O4 nanoparticles and GN in the Fe3O4-NH2-GN nanocomposites.

3.1.3. Raman

Figure 3b illustrates the Raman spectra of the GN and Fe3O4-NH2-GN nanocomposites. Both GN and Fe3O4-NH2-GN showed two peaks at around 1350 cm−1 and 1570 cm−1, which were designated as D band and G band; they represented the disordered sp3 carbon structure (D band) and the sp2 ordered crystalline structure (G band) of GN [32]. After the combination of the two nanomaterials, the intensities of the peaks reduced significantly, which might be a result of the introduction of Fe3O4-NH2 nanoparticles. However, the intensity ratio of the D to G peaks was maintained, showing that the structure of GN was not affected.

3.2. Electrochemical Characteristics

The electrochemical behavior of various modified electrodes in 100 μmol L−1 of CBD were compared via the CV method. As shown in Figure 4a, the electrochemical response of CBD on bare GCE was only 0.728 μA (black line). After the respective modifications with Fe3O4 nanoparticles and GN to the GCE, small oxidation peaks at around 0.5 V could be observed on the Fe3O4/GCE (blue line) and the GN/GCE (red line), which might be due to the electron transfer properties and the good conductivity of Fe3O4 nanomaterials and GN [33]. When Fe3O4-GN suspensions were used to modify the GCE, resulting in the Fe3O4-GN/GCE, an apparent increase in peak current could be observed (5.659 μA, green line). The advantages of GN and Fe3O4 nanoparticles were combined and enhanced. Moreover, when the Fe3O4 nanoparticles were functionalized by amino groups, the resulting modified electrode (Fe3O4-NH2-GN/GCE) showed the highest response among these electrodes (8.978 μA, Pink line). In order to confirm the effect of amino groups on Fe3O4 nanoparticles, Fe3O4-nanoparticle- and Fe3O4-NH2-nanoparticle-modified electrodes (Fe3O4/GCE and Fe3O4-NH2/GCE) were compared. As a result, the peak current of Fe3O4-NH2/GCE was slightly higher than that of Fe3O4/GCE (1.366 to 1.08, not shown). A possible reason for this increase might be that the amino groups on the surface could attract more target molecules. As far as we know, there has been no previous report regarding the electrochemical oxidation mechanism of CBD. By referring to reported works on the oxidation of Δ9-THC, the oxidation process of CBD could be assumed to be a phenol-type oxidation mechanism [34,35].
In order to optimize the effects of the modifiers, the fabrication sequence of modified sensors was investigated. Through the comparison of GN/Fe3O4-NH2/GCE, Fe3O4-NH2/GN/GCE, and Fe3O4-NH2-GN/GCE, the peak currents of each sensor were obtained, as shown in Table 1. Apparently, the Fe3O4-NH2-GN/GCE showed the best response among these sensors, meaning that the modifiers should first be mixed, and then dropped directly on the surface of the electrode. Based on this finding, different preparation methods of Fe3O4-NH2-GN suspensions were tried (see the ESM). Three kinds of Fe3O4-NH2-GN nanocomposites were compared, and their corresponding peak currents are also shown in Table 1. Although the Fe3O4 nanoparticles were directly prepared in the presence of GN via ultrasonication and solvothermal methods, the electrochemical properties obtained were not as good as via the physical mix method. Hence, the Fe3O4-NH2-GN suspension was confirmed as the optimal material in this research.
The Nyquist plots from the EIS test reflect the conductivity of the electrodes (Figure 4b). The inset of Figure 4b shows a general equivalent circuit containing the solution resistance (Rs), the electron transfer resistance (Ret), the Warburg element (W), and the charge of the constant phase element (Cd) [36]. The value of Ret was calculated by fitting the experimental data to the model circuit. As shown in Figure 4b, the Nyquist plot of bare GCE showed a semicircle, with an Ret of 1287 Ω. When the GCE was modified with Fe3O4 nanoparticles and GN, the Ret of Fe3O4/GCE and GN/GCE reduced to 141.4 Ω and 28.61 Ω, respectively. Finally, the Ret of Fe3O4-NH2-GN/GCE was only 13.73 Ω, which was similar to that of Fe3O4-GN/GCE (16.19 Ω). The decreases in resistance could be attributed to the outstanding electric conductivity of GN and magnetic nanoparticles [37]. Consequently, the Fe3O4-NH2-GN/GCE was confirmed as the optimal modified sensor, by reason of its optimal response and conductivity in electrochemical detection.

3.3. Optimization of Electrochemical Conditions

3.3.1. Effect of Composition of Fe3O4-NH2-GN

In order to obtain the optimal mixture composition, the ratios of GN and Fe3O4-NH2 nanoparticles (1:0.5, 1:1, 1:1.25, 1:1.5, 1:2.0 and 1:2.5, w:w) were investigated, and are shown in Figure 5a. The concentration of GN was set at 2.0 mg mL−1, and the concentrations of Fe3O4-NH2 nanoparticles were verified according to the ratios. These illustrated results suggested that the electrochemical signals of CBD were the highest when the ratio was 1:1. When the ratio of Fe3O4-NH2 nanoparticles was higher than 1.0, the response gradually became weaker. Therefore, the material ratio of GN and Fe3O4-NH2 nanoparticles was set to 1:1 as the optimal composition for the fabrication of the electrode.

3.3.2. Effect of Ultrasonication Time of Fe3O4-NH2-GN

To obtain a stable dispersion, various ultrasonication times of the Fe3O4-NH2-GN suspension were tested, from 1 min to 30 min (1, 5, 10, 20, and 30 min). Then, the materials were used for the fabrication of electrodes. The changes in peak currents using the corresponding modified electrodes are plotted in Figure 5b. It can be seen that the electrochemical response was the highest when the material was treated for 10 min. However, longer ultrasonication time did not make the response better. Hence, the ultrasonication time of Fe3O4-NH2-GN suspension was confirmed at 10 min.

3.3.3. Effect of Concentration of Fe3O4-NH2-GN

The effect of concentration of Fe3O4-NH2-GN suspension was measured from 1.0 mg mL−1 to 8.0 mg mL−1, and the modification volume was fixed at 10.0 μL. As the previous experiment indicated, the ratio of GN and Fe3O4-NH2 nanoparticles was set at 1:1 (w:w). It can be seen in Figure 5c that the peak current increased as the concentration increased from 1.0 mg mL−1 to 6.0 mg mL−1. However, when the concentrations were more than 6.0 mg mL−1, this trend stopped, and the response began to gradually drop, which was similar to the results of a previous report [38]. Then, the concentration of Fe3O4-NH2-GN suspension was optimized as 6.0 mg mL−1.

3.3.4. Effect of pH

The electrochemical detection using various pH values of the electrolyte (4.0, 5.0, 6.0, 7.0, 8.0, and 9.0) containing CBD as samples was performed with the Fe3O4-NH2-GN/GCE, using the CV method. The trend is shown in Figure 6a, and the peak current of CBD was the highest when the pH of the electrolyte was 5.0. There was a downward trend when the pH of the electrolyte became higher than 5.0, which is consistent with Zanardi’s research [39]. Thus, 5.0 was adopted as the optimal electrolyte pH value during the tests.
Moreover, it could be observed that there was a linear shit of the peak potential (Ep) to lower positive values as the pH increased. The linear equation between Ep and pH was expressed as Ep = −0.053 pH + 0.863 (r2 = 0.984) (Figure 6b). The slope of the equation was ~−0.053 V pH−1, similar to the theoretical Nernstian slope of 0.059 V pH−1. This parameter corresponded to an oxidation mechanism that included the exchange of an equal number of protons and electrons in the reaction [40].

3.4. The Influence of the Scan Rate

As an important parameter reflecting the performance of the electrode, the effect of different scan rates (from 5 mV s−1 to 200 mV s−1) on the electrochemical response of CBD in PBs (10 mmol L−1, pH 7.0) was evaluated on the Fe3O4-NH2-GN/GCE, using the CV method. Figure 7a shows the resulting CV curves at a variety of scan rates. It can be seen that the peak currents increased and shifted with the increasing scan rates. A good linear relationship could be obtained between scan rate and peak current, which could be expressed as: Ip = 164.84 v + 1.73 (r2 = 0.998) (Figure 7b), indicating that the oxidation of CBD was an adsorption-controlled process [41]. However, when the scan rate was increased to more than 200 mV s−1 (250 mV s−1 and 300 mV s−1), the response did not grow proportionately to the former linear trend (lower than former trend). Another linear dependence of the logarithm of the peak current (log Ip) against the logarithm of the scan rate could also be observed, which was fitted as: log Ip = 0.735 log v + 2.00 (r2 = 0.982) (Figure 7c). This trend suggests that the electrochemical reaction was controlled by both diffusion and adsorption [42].

3.5. Quantitative Analysis of CBD

In order to study the quantitative analysis ability of the fabricated Fe3O4-NH2-GN/GCE, the CV curves of CBD at different concentrations from 0.1 μmol L−1 to 100 μmol L−1 were observed in PBs (0.01 mol/L, pH 5.0). The results illustrated that the peak current increased with increasing CBD concentrations, and three sections of linear dependences could be found between the peak current and the CBD concentration during this range, with a detection limit of 0.04 μmol L−1 (S/N = 3), which is consistent with Liu’s work [43]. The plot of peak current versus CBD concentration is shown in Figure 7d. The three regression equations could be respectively expressed as: Ip1 = 1.284 C1 + 0.528 (0.1–0.974 μmol L−1, r2 = 0.984), Ip2 = 0.176 C2 + 1.607 (0.974–19.494 μmol L−1, r2 = 0.984), and Ip3 = 0.0617 C3 + 3.836 (19.494–100 μmol L−1, r2 = 0.988). It could be found in three regression equations that the slope of the peak current at low concentration was higher than at high concentration. At a lower analyte concentration, the number of active sites on the electrode was relatively higher. However, because of the occupancy of—and decrease in the number of—active sites at higher analyte concentrations, the sensitivity and the slope became lower [44]. This demonstrates that the quantitative analysis of CBD using the Fe3O4-NH2-GN/GCE was interesting and acceptable [45]. The detection abilities of the reported electrochemical sensors for CBD are listed and compared with the Fe3O4-NH2-GN/GCE in Table 2. Through the comparison, the proposed Fe3O4-NH2-GN/GCE exhibited a competitive detection capability and sensitivity for CBD.

3.6. Practicability of the Fe3O4-NH2-GN/GCE

The anti-interference ability, repeatability, and stability of the Fe3O4-NH2-GN/GCE were tested, and the results were satisfactory (see the ESM). Moreover, the detection ability of the Fe3O4-NH2-GN/GCE for CBD was evaluated in the extract of C. sativa leaves. In order to verify the results, the standard addition method was employed by spiking different amounts of CBD into samples. The results are shown in Table 3 and compared with those obtained via the HPLC method. The recoveries ranged from 99.1% to 100.4%, indicating that the determination was reliable, and there was consistency between the concentrations of CBD measured by both electrochemical and HPLC methods.

4. Conclusions

In this study, an electrochemical sensor (Fe3O4-NH2-GN/GCE) was fabricated for the detection of CBD. The applied materials and fabrication conditions were compared and optimized via various characterizations and evaluations. The performance of the Fe3O4-NH2-GN/GCE was investigated for aspects including pH, scan rate, anti-interference ability, repeatability, and stability. As a result, the proposed Fe3O4-NH2-GN/GCE showed an improved electrochemical response compared to a bare GCE. It displayed quantitative analysis ability for CBD, with a linear range of 0.1 μmol L−1 to 100 μmol L−1. The practicability test also showed that the result was in good agreement with that of the HPLC method in the detection of CBD in real samples. Based on these findings, the Fe3O4-NH2-GN/GCE could be further utilized for the detection of active compounds in natural extracts.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/nano11092227/s1, Figure S1: (a) Ip ratios of Fe3O4-NH2-GN/GCE in CBD solution containing various interfering substances. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range at 0–0.8 V. Scan rate at 0.05 V s−1, Figure S2: Repeatability of Fe3O4-NH2-GN/GCE in CBD solution. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range at 0–0.8 V. Scan rate at 0.05 V s−1, Figure S3: Stability of Fe3O4-NH2-GN/GCE in CBD solution. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range at 0–0.8 V. Scan rate at 0.05 V s−1.

Author Contributions

Conceptualization, L.L.; data curation, Y.Z.; formal analysis, Z.Y. and C.H.; methodology, A.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

This work was supported by the National Agricultural Science and Technology Innovation Project.

Conflicts of Interest

The authors declare no conflict of interest.

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Nanomaterials 11 02227 g001 550
Figure 1. Diagram of Fe3O4-NH2-GN/GCE and electrochemical detection of CBD.
Figure 1. Diagram of Fe3O4-NH2-GN/GCE and electrochemical detection of CBD.
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Figure 2. TEM images of (a) Fe3O4, (b) GN, and (c) Fe3O4-NH2-GN. (d) SEM image of Fe3O4-NH2-GN.
Figure 2. TEM images of (a) Fe3O4, (b) GN, and (c) Fe3O4-NH2-GN. (d) SEM image of Fe3O4-NH2-GN.
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Figure 3. (a) XRD patterns of GN, Fe3O4, and Fe3O4-NH2-GN. (b) Raman spectra of GN and Fe3O4-NH2-GN.
Figure 3. (a) XRD patterns of GN, Fe3O4, and Fe3O4-NH2-GN. (b) Raman spectra of GN and Fe3O4-NH2-GN.
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Figure 4. (a) CV curves of CBD on bare GCE, GN/GCE, Fe3O4/GCE, Fe3O4-GN/GCE, and Fe3O4-NH2-GN/GCE. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1. (b) Nyquist plots of bare GCE, GN/GCE, Fe3O4/GCE, Fe3O4-GN/GCE, and Fe3O4-NH2-GN/GCE in 5.0 mmol L−1 of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 mol L−1 of potassium chloride. The amplitude is 0.005 V, with a frequency range of 0.1 to 105 Hz.
Figure 4. (a) CV curves of CBD on bare GCE, GN/GCE, Fe3O4/GCE, Fe3O4-GN/GCE, and Fe3O4-NH2-GN/GCE. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1. (b) Nyquist plots of bare GCE, GN/GCE, Fe3O4/GCE, Fe3O4-GN/GCE, and Fe3O4-NH2-GN/GCE in 5.0 mmol L−1 of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 mol L−1 of potassium chloride. The amplitude is 0.005 V, with a frequency range of 0.1 to 105 Hz.
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Figure 5. (a) Effect of compositions of Fe3O4-NH2-GN on Ip in CV. (b) Effect of ultrasonication time of Fe3O4-NH2-GN on Ip in CV. (c) Effect of modification volumes on peak current in CV. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1.
Figure 5. (a) Effect of compositions of Fe3O4-NH2-GN on Ip in CV. (b) Effect of ultrasonication time of Fe3O4-NH2-GN on Ip in CV. (c) Effect of modification volumes on peak current in CV. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1.
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Figure 6. (a) Effect of pH on Ip in CV. (b) Plot of peak potential (Ep) to pH values. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0−0.8 V. Scan rate of 0.05 V s−1.
Figure 6. (a) Effect of pH on Ip in CV. (b) Plot of peak potential (Ep) to pH values. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0−0.8 V. Scan rate of 0.05 V s−1.
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Figure 7. (a) CV of Fe3O4-NH2-GN/GCE in CBD solution at different scan rates. (b) The linear graph of Ip and scan rates. (c) The linear graph of log Ip and log (scan rate). (d) Plot of Ip versus concentration of CBD. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1.
Figure 7. (a) CV of Fe3O4-NH2-GN/GCE in CBD solution at different scan rates. (b) The linear graph of Ip and scan rates. (c) The linear graph of log Ip and log (scan rate). (d) Plot of Ip versus concentration of CBD. CV method: 100 μmol L−1 of CBD in 10 mmol L−1 of PBs (pH 5.0, containing 10% methanol). Potential range of 0–0.8 V. Scan rate of 0.05 V s−1.
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Table
Table 1. The Ip in different fabrication sequences and process methods of Fe3O4-NH2-GN in modified electrodes.
Table 1. The Ip in different fabrication sequences and process methods of Fe3O4-NH2-GN in modified electrodes.
Fabrication SequenceIp (μA)Process MethodIp (μA)
GN/Fe3O4-NH2/GCE1.808Ultrasonication3.352
Fe3O4-NH2/GN/GCE3.388Solvothermal4.232
Fe3O4-NH2-GN/GCE5.327Mix5.550
Table
Table 2. Comparison of different reported sensors for the electrochemical determination of CBD.
Table 2. Comparison of different reported sensors for the electrochemical determination of CBD.
ElectrodeLinear Rage (μmol L−1)LOD (μmol L−1)Ref.
GC/CB0.96–6.370.35[39]
Sonogel-Carbon-PEDOT1.59–19.10.94[34]
NACE–ED0.32–31.80.064[46]
GCE-NM-NM[47]
Fe3O4-NH2-GN/GCE0.1–100.00.04This study
GC/CB: glassy carbon/carbon black; PEDOT: poly-(3,4-ethylenedioxythiophene); NACE–ED: non-aqueous capillary electrophoresis–electrochemical detection; NM: Not mentioned.
Table
Table 3. Determination of CBD in real samples. (n = 3).
Table 3. Determination of CBD in real samples. (n = 3).
SamplesAdded (μmol L−1)Found (μmol L−1)Recovery (%)RSD (%)HPLC (μmol L−1)
Extract of C. sativa011.95-2.2312.06
1.013.00100.42.05-
5.016.8499.13.51-
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Zhang, Y.; You, Z.; Hou, C.; Liu, L.; Xiao, A. An Electrochemical Sensor Based on Amino Magnetic Nanoparticle-Decorated Graphene for Detection of Cannabidiol. Nanomaterials 2021, 11, 2227. https://doi.org/10.3390/nano11092227

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Zhang Y, You Z, Hou C, Liu L, Xiao A. An Electrochemical Sensor Based on Amino Magnetic Nanoparticle-Decorated Graphene for Detection of Cannabidiol. Nanomaterials. 2021; 11(9):2227. https://doi.org/10.3390/nano11092227

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Zhang, Yi, Zongyi You, Chunsheng Hou, Liangliang Liu, and Aiping Xiao. 2021. "An Electrochemical Sensor Based on Amino Magnetic Nanoparticle-Decorated Graphene for Detection of Cannabidiol" Nanomaterials 11, no. 9: 2227. https://doi.org/10.3390/nano11092227

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