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Apr 26, 2018 - Anticancer Drug Flutamide and the Environmental Pollutant. 4‑Nitrophenol. Subbiramaniyan Kubendhiran, Rajalakshmi Sakthivel, Shen-Min...
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Article Cite This: Anal. Chem. 2018, 90, 6283−6291

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Innovative Strategy Based on a Novel Carbon-Black−β-Cyclodextrin Nanocomposite for the Simultaneous Determination of the Anticancer Drug Flutamide and the Environmental Pollutant 4‑Nitrophenol Subbiramaniyan Kubendhiran, Rajalakshmi Sakthivel, Shen-Ming Chen,* Bhuvanenthiran Mutharani, and Tse-Wei Chen Anal. Chem. 2018.90:6283-6291. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/13/18. For personal use only.

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 106, Taiwan, ROC S Supporting Information *

ABSTRACT: In the present work, a noncovalent and eco-friendly approach was proposed to prepare a carbon-black/β-cyclodextrin (CB/β-CD) nanocomposite. CB/β-CD-nanocomposite-modified screen-printed carbon electrodes were applied for the simultaneous determination of the anticancer drug flutamide (Flut) and the environmental pollutant 4nitrophenol (4-NP). The electrochemical performance of the proposed sensor relied on the conductivity of CB, the different binding strengths of the guests (Flut and 4-NP) to the host (β-CD), and the different reduction potentials of the nitroaromatic compounds. Fascinatingly, the proposed sensor exhibited an excellent electrochemical performance with high sensitivity, selectivity, and reproducibility. The obtained wide linear ranges were 0.05−158.3 and 0.125−225.8 μM for Flut and 4-NP. The low detection limits of 0.016 and 0.040 μM with the higher sensitivities of 5.476 and 9.168 μA μM−1 cm−2 were achieved for the determination of Flut and 4NP, respectively. The practical feasibility of the proposed sensor was studied in tap-water and human-serum samples.

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malfunction, and methemoglobinemia.11,12 The other compound, 4-NP, is used as an intermediate in chemical industries for the production of pesticides, paints, plasticizers, explosives, and medicines.13 Nonetheless, 4-NP is considered an environmental pollutant because of its carcinogenic, mutagenic, and teratogenic activities against animals, humans, and plants even in low concentrations.14,15 Therefore, in this work, we have selected the anticancer drug Flut and the environmental pollutant 4-NP as model analytes for simultaneous determination. To solve the selectivity problem, fabricated nanomaterials are often used for the electrochemical sensor rather than molecularly imprinted polymers because of the risk factor.16,3 Therefore, we have utilized fabricated nanomaterials for the electrochemical sensor. Carbon nanomaterials such as graphite, graphene, and carbon nanotubes have hitherto been known to be used for the fabrication of electrochemical sensors.17−19 Owing to the tough synthesis procedure, the usage of graphene, carbon nanotubes, and fullerenes has been neglected.20 Avoiding those drawbacks, we have chosen to use the super conductive, inexpensive, and easily available nanomaterial carbon black (CB) for the simultaneous determination of

itroaromatic compounds are an important class of industrial chemicals that are used in a variety of diverse products, such as explosives, pesticides, synthetic intermediates, dyes, and drugs. In addition, the biological functions of nitroheterocyclic drugs depend on the reduction of the nitro group, the reactivity of the redox intermediates, and the stabilities of radicals.1,2 Nevertheless, because nitro-group-containing aromatic compounds have strong electron-withdrawing groups, they are poorly biodegradable in the environment.3 Hence, researchers are devoted to developing simple, inexpensive, and rapid analytical techniques to identify and quantify nitroaromatic compounds. To date, several methods have been employed for the determination of nitroaromatic compounds, including luminescence,4 fluorescence,5 chromatographic,6 and electrochemistry7 techniques. Among these, electrochemical methods have been hitherto known to be a low-cost, reliable, and highly sensitive analytical technique.8 However, selectivity is the crucial parameter that largely affects the electrochemical performance of the sensor. The main difficulty of electrochemical reduction comes from interfering effects from similar nitro-aromatic compounds.9 Flutamide (Flut) and 4-nitrophenol (4-NP) belong to the class of nitroaromatic compounds. Flut is a synthetic nonsteroidal antiandrogenic agent used in the treatment of prostate cancer.10 However, continuous usage of Flut induces health disorders, such as rectal bleeding, an inflamed prostate, blood in the urine, liver © 2018 American Chemical Society

Received: March 4, 2018 Accepted: April 26, 2018 Published: April 26, 2018 6283

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Analytical Chemistry Scheme 1. Preparation and Application of the CB/β-CD Nanocomposite

This manuscript describes the preparation of a stable aqueous suspension of CB using β-CD for the first time via a simple ultrasonication process. The presence of β-CD in the CB/β-CD composite enhances its supramolecular function and solubility. In particular, the CB/β-CD nanocomposite exhibits an improved electrochemical performance and a very high supramolecular recognition and enrichment capability for the simultaneous determination of Flut and 4-NP. Moreover, the practical applicability of the sensor was demonstrated in tapwater and human-serum samples to attain the prototype level.

Flut and 4-NP. CB is a spherically shaped carbon nanomaterial with an average size of 30−100 nm.21 Remarkably, CB has a high surface area and suitable pore-size distribution, electronic conductivity, and chemical and electrochemical stability.22 However, its poor solubility in water and most common solvents limits the manipulation, characterization, and practical application of CB.23 A traditional acid treatment improves the solubility of CB through surface modifications.24 However, it results in corrosion and environmental pollution. Thus, in this work, we have utilized the noncovalent, eco-friendly technique of using β-cyclodextrin (β-CD) as a dispersing agent. Cyclodextrins (CDs) are captivating molecules, classified as α, β, or γ-CDs on the basis of whether they contain six, seven, or eight glucose units. CDs have been used in both research and applied technologies.25 CDs are toroidal in shape with hydrophobic inner cavities and hydrophilic exterior cavities. Additionally, CDs contain two rims of hydroxyl groups; one rim comprises the primary (tail) hydroxyl groups, and the other comprises the secondary (head) hydroxyl groups.26 The amazing molecular structures of CDs are well-known for forming inclusion complexes with different inorganic, organic, and biological guest molecules by host−guest interactions.27 Furthermore, CDs exhibit high molecular selectivity and enantioselectivity, due to the supramolecular assemblies in their hydrophobic cavities.28 In particular, β-CD has a stronger binding constant with target molecules (>103) when compared with two other CDs.29 As a result, β-CD has been utilized in catalysis and for increasing solubility, molecular recognition, and environmental protection.25 These interesting properties of β-CD are used in the fabrication of different electrochemical sensors.28,30 Hence, β-CD was used for the preparation of the CB/β-CD nanocomposite and the fabrication of the modified electrodes.

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EXPERIMENTAL SECTION Preparation of the CB/β-CD Nanocomposite and Fabrication of the Modified Electrodes. The CB/β-CD nanocomposite was prepared by a simple ultrasonication method. The list of chemicals and reagents used in this work were given in the Supporting Information. CB (5 mg) was added to 5 mL of water and sonicated well for 30 min. Then, 10 mg of β-CD was mixed into the above solution and ultrasonicated for 1 h at the operating frequency of 40 kHz. Finally, a well-dispersed, black-colored, homogeneous CB/βCD solution was obtained. Then, about 5 μL of the CB/β-CD dispersed solution was pipetted out on SPCE and dried well. The as-prepared CB/β-CD-modified SPCE was used for further electrochemical studies. Scheme 1 illustrates the material preparation and application of the CB/β-CD nanocomposite. Figure S1 shows photographic images of the CB and CB/βCD dispersed solutions. It can be seen that in the CB solution, the CB particles are not dispersed well because of their poor solubility. In the case of the CB/β-CD solution, the obtained black solution is homogeneous and well-dispersed because of the interactions between the hydroxyl groups of β-CD and the polar groups of the water molecules.31 Moreover, the formation

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Figure 1. FESEM images of CB (A), β-CD (B), and CB/β-CD (C) and elemental-analysis results of CB/β-CD (D).

py (EDX). The obtained EDX spectrum of CB/β-CD (Figure 1D) exhibited the signals for carbon and oxygen with atomicweight percentages of 72.7 and 27.3%, respectively. The 27.3 wt % oxygen signal asserts the presence of β-CD in the prepared nanocomposite. Furthermore, elemental mapping was performed to survey the elemental distribution in the prepared CB/β-CD nanocomposite. Figure S3 shows the FESEM image of CB/β-CD, the corresponding EDS-layered image of CB/βCD, and the elemental mapping of carbon and oxygen. As shown in Figure S3C,D, the higher amount of red-colored particles exemplifies the carbon distribution, and the lower amount of green-colored particles represents the oxygen distribution. These elemental-mapping and EDX-analysis results clearly indicate the existence of the elements and their quantities (weight percents) in the CB/β-CD nanocomposite. XPS and EIS Spectra. X-ray photoelectron spectroscopy (XPS) is an important tool for the investigation of the surface oxidation state, integrity, and purity of a nano- or micromaterial. The binding energy was used to identify the chemical composition and oxidation states of the elements. Figure 2A−C exhibits the XPS-survey spectrum of CB/β-CD and the corelevel spectra of the C 1s and O 1s states. The survey spectrum of CB/β-CD has an intense signal for C 1s located at a binding energy of 285.4 eV. In addition, a small peak is located at a

of the homogeneous CB/β-CD solution and the interactions between CB and β-CD can be explained on the basis of a previous report.32,36 The van der Waals forces and hydrogenbonding interactions play major roles in the formation of the nanocomposite. The van der Waals forces occur between β-CD and CB, and the hydrogen-bonding interaction occur between the neighboring β-CD molecules. These are the major interactions that are involved in elevating the formation of the composite.32,36 This noncovalent formation was confirmed by the FTIR spectra of CB, β-CD, and CB/β-CD (Figure S2).

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RESULTS AND DISCUSSION Investigation of Surface Morphology and Elemental Composition. The surface morphology of the prepared materials was examined by field-emission scanning electron microscopy (FESEM), and the obtained images are displayed in Figure 1A−C. The FESEM image of CB (Figure 1A) shows aggregated and small spherelike structures. In the case of β-CD (Figure 1B), flakelike structures with smooth surfaces were observed. On the other hand, the β-CD was absorbed on the surface of small, spherically shaped CB and observed in the CB/β-CD nanocomposite (Figure 1C). Moreover, the elemental composition of the prepared nanocomposite was estimated quantitatively by energy-dispersive X-ray spectrosco6285

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Figure 2. XPS-survey spectrum of CB/β-CD (A) and core-level spectra of the C 1s (B) and O 1s (C) states. (D) EIS spectra of bare SPCE, CB/ SPCE, and CB/β-CD/SPCE. The inset shows the Randles equivalent-circuit model.

Figure 3. (A,B) CV responses of bare SPCE, β-CD/SPCE, CB/SPCE, and CB/β-CD/SPCE in the presence of a 0.05 M PB solution containing 100 μM Flut and 4-NP at a scan rate at 50 mV/s. (C) CVs obtained with bare SPCE, β-CD/SPCE, CB/SPCE, and CB/β-CD/SPCE for the simultaneous addition of 100 μM Flut and 4-NP to a 0.05 M PB solution. (D) CV response with CB/β-CD/SPCE in the absence of 100 μM Flut and 4-NP, the presence of 100 μM Flut or 100 μM 4-NP, or the presence of 100 μM Flut and 100 μM 4-NP in a 0.05 M PB solution.

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were explored by CV in 0.05 M PB solutions at scan rates of 50 mV/s. Figure 3A shows the cyclic voltammograms (CVs) of bare SPCE, β-CD/SPCE, CB/SPCE, and CB/β-CD/SPCE for the electrochemical reduction of 100 μM Flut. The bare SPCE, β-CD/SPCE, and CB/SPCE show reduction peaks at −0.63, −0.62, and −0.61 V, for which the lowest reduction-peak currents are 9.52, 6.43, and 10.13 μA, respectively. Additionally, CB/β-CD/SPCE exhibits a well-defined sharp reduction peak (R1) at −0.57 V with an enhanced peak-current value of 42.45 μA. Furthermore, during the reverse scan, CB/β-CD/SPCE exhibits a cathodic peak (R2) at −0.05 V and an oxidation peak (O1) at 0.03 V. Figure 3B depicts the CVs of bare SPCE, βCD/SPCE, CB/SPCE, and CB/β-CD/SPCE for the electrochemical reduction of 100 μM 4-NP. As can be seen, bare SPCE, β-CD/SPCE, and CB/SPCE demonstrate negatively shifted cathodic peaks (R1) at −0.84, −0.81, and −0.78 V, with lower peak-current values of about 8.75, 5.97, and 11.40 μA. Interestingly, CB/β-CD/SPCE displays a positively shifted cathodic peak at −0.69 V, with an improved peak-current response of about 40.44 μA. Additionally, a cathodic peak (R2) at −0.10 V and an anodic peak (O1) at 0.12 V are observed in the reverse scan. Generally, the electrochemical-reduction mechanism of a nitroaromatic compound largely depends on the NO2 group, which is reduced to hydroxylamine (R1), and the reversible (O1/R2) conversion of hydroxylamine to a nitroso compound. Herein, the electroreduction of Flut and 4NP also exhibits the same mechanism with CB/β-CD-modified SPCE. However, bare SPCE, β-CD/SPCE, and CB/SPCE show small responses for the R2 and O1 peaks due to the poor catalytic activity, poor electrode surface area, and poor conductivity of β-CD.36 On the other hand, CB/β-CD/SPCE provides more contact area for the adsorption, and the host− guest interaction between β-CD and the nitroaromatic compounds enhanced the electrocatalytic activity of the proposed modified electrode.34,35 Moreover, β-CD has a supramolecular recognition capability, and it forms inclusion complexes with the analytes.36 Figure 3C,D exhibits the CV curves for the simultaneous detection of Flut and 4-NP under the same experimental conditions. As shown in Figure 3C, bare SPCE, β-CD/SPCE, and CB/ SPCE exhibited inferior electrocatalytic performances with broad and unresolved cathodic peaks. On the other hand, CB/ β-CD/SPCE exhibited good peak-to-peak separation with welldefined peak shapes. Moreover, the peak-current responses of Flut and 4-NP did not change in the simultaneous detection of 100 μM Flut and 100 μM 4-NP. Also, there were no changes in the peak potentials, which confirmed the negligible interference between Flut and 4-NP (Figure 3D). The extraordinary electrocatalytic performances of the proposed sensor materials can be explained by the synergistic effects of CB, which has a high surface area and high conductivity, and β-CD, which forms host−guest inclusion complexes with the analyte molecules. In addition, the electron-withdrawing and -donating groups of the nitroaromatic compounds play vital roles in the reduction potentials of the analytes. The electron-donating groups activate the ring, and the electron-withdrawing groups deactivate the ring. Flut has an electron-withdrawing group, and 4-NP has an electron-donating group.37 Because of the presence of the electron-withdrawing groups, NO2 becomes electron deficient and ready to receive the electrons from the electrode surface; it also decreases the activation energy of the NO2 group.7 Hence, the cathodic peak for the electroreduction of Flut arises in the lowest overpotential at 0.57 V. On the other

binding energy of 534.6 eV for O 1s, which can be attributed to the existence of oxygen and the presence of β-CD in the nanocomposite. From the observed results, we have confirmed that there are no impurities present in the CB/β-CD composite. The electron-transfer-resistance properties of the different modified electrodes were evaluated by electrochemicalimpedance spectroscopy (EIS). Figure 2D depicts the Nyquist plots of bare SPCE, CB/SPCE, and CB/β-CD/SPCE in a 0.1 M KCl solution containing 5 mM K3[Fe(CN)6] and 5 mM K4[Fe(CN)6]. The inset shows the Randles equivalent-circuit model, where Rs, Cdl, Ret, and Zw represent the solution resistance, double-layer capacitance, electron-transfer resistance, and Warburg diffusion process, respectively. The electrontransfer resistances of the different modified electrodes were measured directly from the diameters of the semicircles. As can be seen from Figure 2D, bare SPCE and CB/SPCE exhibit the higher Ret values of about 780 and 339 Ω with the larger semicircles. On the other hand, CB/β-CD/SPCE shows a small circle with an Ret value of 94 Ω. These results suggest that CB/ β-CD has an increased rate of electron transfer from the electrolyte solution to the electrode surface. Hence, the CB/βCD-modified SPCE shows a lower Ret value. This proves the excellent conductivity of our proposed modified electrode. Electrochemistry of the Different Electrocatalysts. The electrocatalytic behaviors of the different electrocatalysts were assessed by cyclic voltammetry (CV) using [Fe(CN)6]−3/4 as a redox probe. Figure S4 displays the cyclic voltammograms of bare SPCE, CB/SPCE, and CB/β-CD/SPCE in 0.1 M KCl containing 5 mM [Fe(CN)6]−3/4 at a scan rate of 50 mV/s. As shown in Figure S4A, a pair of redox peaks with higher peak-topeak-separation values (ΔEp) about 487 mV was observed for bare SPCE. After the modification with CB and CB/β-CD, the ΔEp value decreased to 242 and 212 mV, respectively. On the other hand, cathodic-to-anodic-peak-current ratios (Ipc/Ipa) of bare SPCE, CB, and CB/β-CD of about 0.40, 0.65, and 0.76 were obtained. Fascinatingly, CB/β-CD revealed good reversible behavior with higher redox-peak currents and smaller peak separations. In addition, the electroactive surface areas of the modified electrodes were calculated using the Randles− Sevcik equation.33 Thereby, we conducted scan-rate studies by differentiating the scan rates from 20 to 200 mV/s, and the obtained CV image was displayed in Figure S4B. The Randles− Sevcik equation is as follows: i p = (2.69 × 105)n3/2AD1/2Cυ1/2

(1)

in which ip is the anodic- or cathodic-peak current, n is the number of transferred electrons, A is the electroactive area (cm2), D is the diffusion coefficient of [Fe(CN)6]−3/4 (cm2 s−1), C is the concentration of the [Fe(CN)6]−3/4 (mol cm−3), and υ1/2 is the square root of the scan rate (V s−1). For the calculations, we chose the slopes of Ipa versus υ1/2 (Figure S4B, inset). The electrochemical active surface areas were calculated to be about 0.053, 0.093, and 0.150 cm2 for bare SPCE, CB/ SPCE, and CB/β-CD/SPCE. In the comparison, CB/β-CD/ SPCE exhibited the lowest ΔEp value, highest Ipc/Ipa ratio, and high surface area. These results suggested that CB/β-CD could be a better electrocatalytic material for the fabrication of an electrochemical sensor. Electrochemical Behaviors of Flut and 4-NP with the Different Modified Electrodes. The electrochemical behaviors of Flut and 4-NP with the different modified electrodes 6287

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Figure 4. (A) CVs obtained in the analysis of different scan rates (20−300 mV/s) with CB/β-CD/SPCE for the simultaneous addition of 100 μM Flut and 100 μM 4-NP in a 0.05 M PB solution. (B) Linear plots of scan rates vs the cathodic-peak currents of Flut and 4-NP. (C) CVs obtained for the simultaneous addition of 100 μM Flut and 100 μM 4-NP to solutions of different pHs (3−11) with CB/β-CD/SPCE. (D) Calibration plots of different pH values vs the cathodic-peak currents of Flut and 4-NP.

Figure 5. (A) DPV responses obtained with CB/β-CD/SPCE for the successive addition of Flut (0.05−400 μM) in the presence of 50 μM 4-NP. (B) Calibration plot of the cathodic-peak currents vs the Flut concentrations. (C) DPV responses of CB/β-CD/SPCE for the successive addition of 4-NP (0.125−225.8 μM) in the presence of 50 μM Flut. (D) Calibration plot of the cathodic-peak currents vs the 4-NP concentrations.

hand, 4-NP has an electron-donating group, which activates the ring; thus, the electron density is increased at the NO2 group.

Therefore, the electroreduction of 4-NP appears with a higher overpotential at 0.69 V. Furthermore, the different binding 6288

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Figure 6. (A) DPV response with CB/β-CD/SPCE for the simultaneous addition of Flut and 4-NP from 5 to 65 μM in a 0.05 M PB solution. (B) Calibration plot for the cathodic-peak currents of Flut and 4-NP vs the concentrations of Flut and 4-NP.

Simultaneous Determination of Flut and 4-NP. DPV is a sensitive and suitable voltammetric technique for the simultaneous determination of nitroaromatic compounds. Hence, we have utilized the DPV for the simultaneous determination of Flut and 4-NP in a 0.05 M PB solution at pH 7. Figure 5A shows the DPV responses for the different concentrations (0.05−400 μM) of Flut in the presence of 50 μM 4-NP with CB/β-CD/SPCE. It can be seen that the cathodic-peak intensity at −0.51 V increases as the concentration of Flut increases. In addition, the cathodic peak at −0.67 V appears as a result of the addition of 4-NP, and its peakcurrent response increases little as the concentration of Flut increases. The obtained peak-current responses were plotted against the Flut concentrations, and this image was displayed in Figure 5B. As can be seen, the peak-current responses have a linear relationship with the different concentrations, with a linear-regression equation of I = −0.136x − 7.866 (R2 = 0.998). Moreover, a wide linearity from 0.05 to 158.3 μM, a limit of detection (LOD) of about 0.016 μM, and a high sensitivity of 5.476 μA μM−1 cm−2 were observed for the determination of Flut. The electrochemical determination of 4-NP in the presence of 50 μM Flut was recorded by DPV, and the obtained image was presented in Figure 5C. As shown in Figure 5C, a cathodic peak at −0.67 V was observed for the electroreduction of 4-NP, and Flut exhibited a cathodic peak at −0.51 V. Furthermore, the peak-current responses at −0.67 V increased when as the concentration of 4-NP increased from 0.125 to 225.8 μM. The linear relationship between the peakcurrent responses and different concentrations is displayed in Figure 5D. A linear-regression equation of I = −0.229x − 4.808 with a correlation coefficient of R2 = 0.994 was obtained for the determination of 4-NP. Interestingly, CB/β-CD-modified SPCE exhibited a long linear range from 0.125 to 225.8 μM, an appreciable LOD of about 0.040 μM, and a high sensitivity of 9.168 μA μM−1 cm−2. In addition, the obtained analytical-parameter values for the determination of Flut and 4-NP were compared with previous reports and tabulated in Tables S1 and S2. In the comparison, our proposed sensor material exhibited better performance than the other modified electrodes. Simultaneous additions of Flut and 4-NP were performed in the same experimental conditions. In these experiments, equal concentrations of Flut and 4-NP were added from 5 to 65 μM, and the obtained DPV responses were displayed in Figure 6A. Fascinatingly, well-resolved and sharp cathodic peaks were observed, and the peak-current responses increased when the concentrations of the analytes

energies of the analytes influence the reduction potentials of the nitroaromatic compounds. Hence, well-defined and wellresolved peaks were observed for the simultaneous detection of Flut and 4-NP. Effects of Scan Rate and pH. The effect of the scan rate on the electrochemical reduction of Flut and 4-NP was investigated by CV. Figure 4A display the CV responses with CB/β-CD/SPCE for the different-scan-rate analysis in a 0.05 M PB solution containing 100 μM Flut and 4-NP. The peakcurrent responses of Flut and 4-NP increased gradually when the scan rate increased from 20 to 300 mV/s. Figure 4B shows the calibration plot between the cathodic-peak currents (R1) of Flut (pink) and 4-NP (blue) versus the different scan rates. As can be seen, the cathodic-peak currents of Flut and 4-NP had linear relationships with the different scan rates. Moreover, the linear-regression equations Ipc = −0.266x − 13.897 (R2 = 0.994) for Flut and Ipc = −0.279x − 19.181 (R2 = 0.995) for 4NP were obtained. The obtained results reveal that the reduction of Flut and 4-NP with CB/β-CD/SPCE is an adsorption-controlled process.38,39 In order to evaluate the pH dependence of the proposed sensor, studies of different pHs (3−11) were performed using CV, and the images were presented in Figure 4C. It can be seen that the acidic pHs (3 and 5) exhibit well-resolved peaks. In the case of basic pHs (9 and 11), imperfect peak shapes with inferior electrochemical responses were observed (Figure 4D). Interestingly, welldefined sharp peaks with an enhanced peak-current response were observed at neutral pH (pH = 7). In addition, the potential shifts in the peaks (R1, R2, and O1) indicate the direct involvement of the proton in the electroreduction process. Figure S5A shows the CV curves of the analysis of the different pHs, and Figure S5B shows the calibration plots of different pHs and cathodic-peak potentials of Flut and 4-NP. The obtained slope values from the calibration plots were applied in the Nernst equation, Ep = −

( 0.0592n m )pH + b,

to calculate the m/n ratio,40 in

which m is the number of protons, and n is the number of electrons. Using that equation, the m/n ratios were calculated to be about 0.60 and 0.81 for Flut and 4-NP, respectively. These results exhibited that equal numbers of proton and equal numbers of electrons were involved in the electroreduction of Flut and 4-NP.41,42 Moreover, the electroreduction of Flut and the electroreduction of 4-NP with CB/β-CD/SPCE are fourelectron and four-proton transfer processes (Figure S6A,B).43,44 6289

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ranges of about 0.05−158.3 μM for Flut and 0.125−225.8 μM for 4-NP were obtained. On the other hand, low detection limits of 0.016 and 0.040 μM and high sensitivities of 5.476 and 9.168 μA μM−1 cm−2 were achieved for the determination of Flut and 4-NP, respectively. Moreover, the practical feasibility of the proposed sensor was studied in tap-water and humanserum samples. We believe that this work opens a new gate for the preparation of nanocomposites using inexpensive and highly conductive CB for the determination of various nitroaromatic compounds.

increased. Moreover, the obtained peak-current responses were plotted against the concentrations of Flut and 4-NP (Figure 6B). It can be seen that the peak currents have linear relationships, as the linear-regression equations I = −1.061x − 15.611 (R2 = 0.995) for Flut and I = −1.062x + 2.147 (R2 = 0.997) were obtained. These results reveal that CB/β-CD is an excellent electrode material for the determination of the anticancer drug Flut and toxic 4-NP. The host−guest interactions between CB/β-CD and analytes enhanced the electrocatalytic performance of the modified electrode.27 On the basis of previous reports, the higher interactions between the nitroaromatic compounds and β-CD require more energy for the reduction of nitro groups, which shifts the reduction potential to the negative side.3 Selectivity, Stability, and Reproducibility. The selectivity of the sensor was evaluated by DPV in the presence of interfering molecules and ions. Figure S7A displays the DPV responses of 10 μM Flut and 4-NP in the presence of 25 μM uric acid, glucose, catechol, 4-nitroaniline, phenol, 2,4dinitrophenol, paracetamol, and hydroquinone and 100 μM Cu2+, K+, Cl−, and Br−. Similar experimental conditions were used for the simultaneous determination of Flut and 4-NP in this experiment. Figure S7B shows the peak-current responses of the aforementioned molecules and ions. It can be seen that CB/β-CD/SPCE exhibits appreciable selectivity in the interference study. Moreover, the reproducibility of the sensor was evaluated in a 0.05 M PB solution. For this experiment, five parallel determinations were performed using five independent CB/β-CD/SPCE constructs. A relative standard deviation (RSD) of about 3.5% was observed for the reproducibility study. On the other hand, the stability of the sensor is an important parameter. Therefore, the stability of the sensor for the simultaneous determination of Flut and 4-NP was evaluated for 4 weeks. The results show that the peak-current responses of the sensor decreased by only 5% from the initial current value. These selectivity, stability, and reproducibility results demonstrate that CB/β-CD is good electrode material and is suitable for the simultaneous determination of Flut and 4-NP. Real-Sample Analysis. The practical applications of the proposed sensor were explored in tap-water and human-serum samples by a standard-addition method using DPV. Prior to the experiment, the tap-water samples were collected, and the pHs of the samples were adjusted equal that of the PB solution. The human-serum samples were diluted with a pH 7 buffer solution; these samples were Flut- and 4-NP-free samples. Therefore, known concentrations of Flut and 4-NP were spiked into the above samples, which were used for the real-sample-analysis experiment. Furthermore, the obtained recovery results are tabulated in Table S3. As can be seen, the proposed sensor material exhibits reasonable recovery results. Hence, we suggest that CB/β-CD/SPCE is feasible for real-time applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b00989. Additional experimental procedures; photographic images of the CB and CB/β-CD solutions; FTIR spectra of CB, β-CD, and CB/β-CD; FESEM and EDS-layered images of CB/β-CD with elemental mapping of carbon and oxygen; CV responses of bare SPCE, CB/SPCE, and CB/β-CD/SPCE; CVs of the different-scan-rate analyses of CB/β-CD/SPCE; CV obtained for the simultaneous addition of 100 μM Flut and 100 μM 4-NP with CB/βCD/SPCE; calibration plot of different pH values versus the cathodic-peak potentials of Flut and 4-NP; plausible electrochemical-reduction mechanism of Flut and 4-NP; DPV responses of 10 μM Flut and 4-NP in the presence of 25 μM uric acid, glucose, catechol, 4-nitroaniline, phenol, 2,4-dinitrophenol, paracetamol, or hydroquinone or 100 μM Cu2+, K+, Cl−, or Br− with CB/β-CD/SPCE; peak-current-change values for the addition of interfering compounds; analytical-performance comparisons for the determination of Flut and 4-NP; and recovery results of the real-sample analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +886227025238. Tel.: +886227017147. E-mail: [email protected]. ORCID

Shen-Ming Chen: 0000-0002-9305-8513 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Ministry of Science and Technology (MOST 106-2113-M-16 027-003), Taiwan, ROC.

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CONCLUSION In summary, the solubility of CB was successfully enhanced by a noncovalent approach using β-CD as the dispersing agent. A simple ultrasonication method was used for the preparation of the CB/β-CD nanocomposite. Then, the CB/β-CD was used to fabricate a simple, low-cost, and highly sensitive and selective electrochemical sensor for the simultaneous determination of Flut and 4-NP in aqueous solutions. The high conductivity and high surface area of CB and the host−guest inclusion complexes of the β-CD molecules enhance the electrochemical performance of the proposed sensor. Interestingly, wide linear 6290

DOI: 10.1021/acs.analchem.8b00989 Anal. Chem. 2018, 90, 6283−6291

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DOI: 10.1021/acs.analchem.8b00989 Anal. Chem. 2018, 90, 6283−6291