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β-cyclodextrin-cobalt ferrite nanocomposite as enhanced sensing platform for catechol determination
Colloids and Surfaces B: Biointerfaces 98 (2012) 58–62
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
-cyclodextrin-cobalt ferrite nanocomposite as enhanced sensing platform for catechol determination Jin-Tu Han, Ke-Jing Huang ∗ , Jing Li, Yan-Ming Liu ∗ , Meng Yu College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 28 April 2012 Accepted 4 May 2012 Available online 12 May 2012 Keywords: Catechol -cyclodextrin Cobalt ferrite nanocomposite Electrochemical sensor
a b s t r a c t An electrochemical sensor based on -cyclodextrin-cobalt ferrite nanocomposite was developed for the sensitive detection of catechol (CT). To construct the base of the sensor, a novel composite was initially fabricated and used as the substrate material by combining cobalt ferrite nanocomposite and -cyclodextrin via a simple sonication-induced assembly. Due to the high catechol-loading capacity on the electrode surface and the upstanding electric conductivity of cobalt ferrite nanocomposite, the electrochemical response of the fabricated sensor was greatly enhanced and displayed excellent analytical performance for catechol detection from 1 to 200 M with a low detection limit of 0.12 M (S/N = 3). Moreover, the developed electrochemical sensor exhibited good selectivity and acceptable reproducibility and could be used for the detection of catechol in water samples. © 2012 Elsevier B.V. All rights reserved.
1. Introduction (o-benzenediol, 1,2-benzenediol or 1,2Catechol dihydroxybenzene) (CT) is a natural polyphenolic compound which is ubiquitous in nature since it commonly generates in factory processes and widely exists in higher plants such as teas, vegetables, fruits, tobaccos and some traditional Chinese medicines [1,2]. CT has been widely studied due to the biological importance such as antioxidation, antivirus and affecting the activities of some enzymes [3]. However, CT is highly toxic to humans, animals, plants and aquatic life when present above certain concentration limits [4]. Therefore, the detection and quantification of CT have gained significance in environmental protection, food safety and human health. Different analytical methods have been developed for CT determination including spectrophotometry [5], liquid-phase chromatography [6], gas chromatography [7] and capillary electrophoresis [8]. However, these techniques are usually expensive, laborious and time-consuming. Thus, the design and development of quick, simple, inexpensive and effective analytical methods are of great importance in practice. Electrochemical sensors have become the predominant analytical technique for the quantitative detection of polyphenolic compound due to their high sensitivity, low cost, fast analysis and ease of miniaturization [9,10]. Currently, high sensitivity has become one of the main goals in development of electro-assay
∗ Corresponding authors. Tel.: +86 376 6390611. E-mail addresses: [email protected] (K.-J. Huang), [email protected] (Y.-M. Liu). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.05.003
methods. Application of nanomaterial in the fabrication of sensor has gained growing interest due to its intrinsic advantages, such as low cost, good thermal stability, good conductivity and large surface area [11]. Among numerous nanomaterials, cobalt (Co)-based nanomaterials has attracted a considerable interest because of its good electro-activity and low cost [12,13]. For example, the composite of Co hexacyanoferrate nanoparticles (NPs)-carbon nanotubechitosan has been used to develop glucose sensor [14]. Pt–Co NPs supported on single-walled carbon nanotubes (SWCNTs) and Co NPs integrated with SWCNTs have been applied in methanol [15] and nitrite detection [16]. Currently, nano-magnetic particles are attracting more attention due to their distinguished properties, such as surface effect, special magnetic target and good biocompatibity properties [17,18]. Among magnetic nanoparticles, Fe3 O4 nanoparticles are particularly attractive because of their biocompatibility, low toxicity and easy preparation nanosized magnetic particles [19,20], and have been used in electrochemical biosensors due to their unique properties such as large surface area, excellent conformation stability and better contact between biocatalyst and its substrate [21]. The Cox Fe3−x O4 magnetic nanoparticles based electrochemical sensor has been used to study the direct electron transfer between the redox centers of the proteins and the electrode, and it exhibited remarkable electrocatalytic activity towards H2 O2 reduction [22]. -Cyclodextrin (-CD), a cyclic oligosaccharide consisting of seven glucose units [23], has been widely used as a dispersing reagent for insoluble chemicals and nanomaterials, including carbon nanotubes [24,25] due to its excellent film-forming ability and inclusion function. When Cox Fe3−x O4 magnetic nanoparticles are
J.-T. Han et al. / Colloids and Surfaces B: Biointerfaces 98 (2012) 58–62
59
functionalized with -CD, it is likely to obtain new materials simultaneously having unique properties of Cox Fe3−x O4 and -CD, which will provide potential applications in the fields of bio-sensors, electronics, etc. In this paper, Cox Fe3−x O4 --CD was synthesized via a simple sonication-induced assembly. Subsequently, a sensitive electrochemical sensor for CT detection was fabricated based on Cox Fe3−x O4 --CD. Due to the high CT-loading capacity on the electrode surface and the excellent electric conductivity of Cox Fe3−x O4 , the electrochemical response of the fabricated sensor was greatly enhanced and achieved sensitive CT detection. Meanwhile, the proposed electrochemical sensor also exhibited good selectivity and acceptable reproducibility and could be used for the detection of CT in water samples. Therefore, the present work offers a new way to broaden the analytical applications of Cox Fe3−x O4 in environmental analysis. 2. Experimental Fig. 1. SEM images of Cox Fe3−x O4 nanoparticles.
2.1. Chemicals and materials
2.2. Apparatus All the voltammetric measurements were performed on a CHI 660D electrochemical workstation (Shanghai CH Instrument, China). A three-electrode system was employed for the electrochemical detection, which was composed of a modified carbon glassy electrode (GCE) as working electrode, a Pt wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The pH measurements were made with a pH meter (MP 230, Mettler-Toledo, Greiffensee, Switzerland). The images of scanning electron microscope (SEM) were obtained at Hitachi S-4800 (Japan). 2.3. Preparation of Cox Fe3−x O4 -ˇ-CD nanoparticles The Cox Fe3−x O4 nanoparticles were prepared according to references [22,26]. In short, 5 mL of 2 mol L−1 CoCl2 ·6H2 O in HCl 7.4% solution and 40 mL of 0.5 mol L−1 FeCl3 ·6H2 O solution were preheated to 50 ◦ C. The resultant solutions were then mixed and added into a boiling solution of 200 mL of 1 mol L−1 NaOH under vigorous stirring for 30 min. After cooling to room temperature, the dark brown precipitate was separated from the dispersion medium by using a permanent magnet. Cox Fe3−x O4 was obtained after the resulting suspension was dried in an oven at 100 ◦ C for 5 h. 1 mg of Cox Fe3−x O4 was then dispersed in 4 mL of -CD dimethylformamide (DMF) solution (2 wt%) to give a 0.25 mg mL−1 black suspension with the aid of ultrasonic agitation for 2 h. 2.4. Fabrication of the modified electrodes Before the start of the electrochemical experiments and modification procedures, the GCE was first polished to a mirror-like surface with 0.3 and 0.05 m ␣-alumina slurries and then ultrasonicated for 1 min in ethanol and water successively. Next, 5 L of the prepared Cox Fe3−x O4 --CD composite was dropped on the pretreated GCE and dried in a desiccator for 24 h at room temperature.
3. Results and discussion 3.1. Characteristics The as-prepared Cox Fe3−x O4 nanoparticles were characterized with SEM (Fig. 1) and X-ray diffractometry (XRD) (Fig. 2). In Fig. 2, it can be easily seen that all diffraction peaks were consistent with previous reports, which were characteristic of Cox Fe3−x O4 [22,26]. 3.2. Electrochemical characteristics of electrochemical sensor Fig. 3 showed the cyclic voltammograms (CVs) of different electrodes in a 0.1 M PBS (pH 6.0). A pair of well defined redox peaks of CT was observed within the potential window from −0.2 to 0.7 V revealing that CT underwent a quasi-reversible redox process on the electrodes. The peak currents of CT were very small at the bare GCE (curve a). While at the -CD/GCE, the peak current increased obviously (curve b). It indicated -CD molecules have good adsorption capacity to CT. The concentration of CT on the surface of the modified electrode was increased, which resulted in pronounced peak currents enhancement. Similarly, the peak current showed a remarkable increase at Cox Fe3−x O4 /GCE due to its excellent properties (curve c), such as large surface area and the excellent electric conductivity of Cox Fe3−x O4 . From Fig. 3d, we can observe that the peak current at Cox Fe3−x O4 --CD/GCE was nearly 3.5 times higher 60 50
(311) 40
Intensity (a.u.)
-CD, CT, CoCl2 ·6H2 O, FeCl3 ·6H2 O were purchased from Sigma (St. Louis, MO, USA). Phosphate-buffered saline (PBS, 0.1 M) with various pH values was prepared with stock standard solution of Na2 HPO4 , NaH2 PO4 and 0.1 M KCl. All other reagents were used without any further purification. All the solutions were prepared with double-distilled water.
30
(440)
(220)
20
(400)
(111)
(422)
10 0 10
20
30
40
50
2 (deg) Fig. 2. XRD patterns of Cox Fe3−x O4 nanoparticles.
60
60
J.-T. Han et al. / Colloids and Surfaces B: Biointerfaces 98 (2012) 58–62
15
d
10
c
0.5
15
0.4
b 5
a
e
a
E/V
0.3
10
0.2
a
0.1
5
b
0.0
I/
0
-0.1 5
I/
-5
6
7
8
9
pH
0
-10 -15
-5
-20 -0.2
0.0
0.2
0.4
0.6
0.8
-10
E/V
-0.2
Fig. 3. Cyclic voltammograms of bare GCE (a), -CD/GCE (b), Cox Fe3−x O4 /GCE (c) and Cox Fe3−x O4 --CD/GCE (d) in pH 6.0 PBS at 100 mVs−1 .
than that at bare GCE. The significant improvement of peak current clearly demonstrated that Cox Fe3−x O4 --CD composite film acts as an efficient promoter to enhance the kinetics of the electrochemical process, which is probably caused by the synergistic effect of the Cox Fe3−x O4 and -CD. These results indicated that the Cox Fe3−x O4 -CD composite film modified electrode can enhance the detection sensitivity. The effect of scan rates vs. the peak currents of CT at the Cox Fe3−x O4 --CD/GCE was investigated (Fig. 4). The redox processes of the CT gave roughly symmetric anodic and cathodic peaks at relatively slow scan rates. When the scan rate increased, the redox potentials (Epa and Epc ) of CT shift slightly. At the same time, the redox peak currents were proportional to the scan rate in the range of 0.02–0.2 Vs−1 . The linear regression equations were ipa (A) = −4.448 − 0.051 (mV s−1 ) (R = 0.993) and ipc (A) = 1.368 + 0.034 (mV s−1 ) (R = 0.995) (inset), indicating a surface-controlled process.
0.0
0.2
0.4
0.6
0.8
E/V Fig. 5. CVs of Cox Fe3−x O4 --CD/GCE in PBS with different pH values of (a–e): 5.0, 6.0, 7.0, 8.0 and 9.0; scan rate: 100 mVs−1 ; inset was the plot of potentials vs pH.
increased slowly with a rise in pH from 5.0 to 6.0. At a pH higher than 6.0, the peak currents decreased with a rise in pH from 7.0 to 9.0. It has been reported that the pKa value of CT was 9.4 [34]. When pH was between 5.0 and 6.0, the hydroxyl in CT would not ionized, which would increase the adsorption capacity of the CT. When pH increased from 6.0 to 9.0, the increased hydroxyl ion in solution might decrease the adsorption capacity of CT to -CD. So pH 6.0 was selected for the further experiment. In addition, the relationship between pH and the peak potential of CT was studied. It showed that the peak potentials shifted negatively with arise in pH from 5.0 to 9.0 for CT. The equations for anodic and cathodal peak potential with the pH for CT were Epa = −0.063 pH + 0.745 (V, R = 0.994) and Epc = −0.044 pH + 0.418 (V, R = 0.992), respectively. The slopes of these equations suggested that the electrode process was a two-proton coupled two-electron transfer [27].
3.3. Effect of pH The effect of solution pH at Cox Fe3−x O4 --CD/GCE was investigated over a range of 5.0–9.0 (Fig. 5). The peak currents of CT 9
15
6 3 0
I/
10
-3 -6
5
-9 -12
I/
-15
0
0
30
60
90
120
150
180
210
Scan rate / mVs -1
-5
3.4. Effect of the amount of Cox Fe3−x O4 -ˇ-CD The relationship between the peak currents of CT and the amount of Cox Fe3−x O4 --CD on the GCE was investigated. The peak currents of 50 M CT on the Cox Fe3−x O4 --CD/GCE increased remarkably when the amount of Cox Fe3−x O4 --CD suspension increased from 1 to 5 L. However, when the amount of Cox Fe3−x O4 --CD exceeded 5 L, the peak current decreased dramatically. The results may be attributed to the thicker film of Cox Fe3−x O4 --CD, which blocked the electrical conductivity. Consequently, a Cox Fe3−x O4 --CD suspension of 5 L was utilized to modify the GCE.
a 3.5. Effect of the accumulation time
-10
i -15 0.6
0.4
0.2
0.0
-0.2
E/V Fig. 4. CVs of Cox Fe3−x O4 --CD/GCE in PBS at different scan rates: 20, 40, 60, 80, 100, 125, 150, 175, and 200 mVs−1 ; the plot of the peak current vs scan rates (inset).
The sensitivity of the proposed method was undoubtedly improved by the accumulation time. The effect of the accumulation time was investigated in the range of 1–10 min. With the accumulation time increased from 0 to 5 min, the peak currents of CT increased gradually, owing to the increased amount of CT on the Cox Fe3−x O4 --CD/GCE. After that, no further increase was observed due to the surface saturation, therefore, 5 min was chosen as the optimal accumulation time.
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Table 2 Determination of CT in water samples (n = 4).
10
-12
-10
g
I / µA
8
Samples 6
4
I (µA)
-8
Tap water 10 Shi river water 10 Pond water 10
2 0
-6
Added (M)
50
100
150
By HPLC (M)
By this method (M)
RSD (%)
Recovery (%)
9.5 9.4 9.6
9.2 10.3 9.5
3.2 3.5 2.4
92.3 104.1 97.6
200
C / µM
a
3.8. Reproducibility and stability of Cox Fe3−x O4 -ˇ-CD/GCE -4
-2 0.0
0.2
0.4
0.6
0.8
E (V) Fig. 6. Differential pulse voltammograms of CT at Cox Fe3−x O4 --CD/GCE in 0.1 M PBS (pH 6.0). CT concentrations (a–g): 1.0 M, 5.0 M, 10.0 M, 30.0 M, 50.0 M, 100.0 M, 200.0 M.
3.9. Determination of CT in water samples
3.6. Determination of CT Differential pulse voltammetry (DPV) was used for the determination of trace amounts of CT under the optimum conditions. The DPV responses for different concentrations of CT were illustrated in Fig. 6. The resulting calibration plot was linear over the range from 1 to 200 M. The linear equation was ip (A) = −2.121 − 0.034 C (M) with a correlation coefficient of 0.991. The detection limit was 0.12 M (S/N = 3). Comparing with the method reported previously [28,29,3,30–34], the proposed method in this work had wide linear range and low detection limit. This further confirmed the synergetic effect of Cox Fe3−x O4 and -CD. The detailed specific features were shown in Table 1. It indicated that this method was preferable in the determination of trace amount of CT.
3.7. Interferences The possible interferences of some species in the water were tested with a mixed CT solution (50 M). 500-fold Mg2+ , NH4 + , K+ , Na+ , NO3 − , Cl− , SO4 2− , 100-fold Ca2+ , Fe3+ , SO3 2− , Br− , glucose, ascorbic acid and uric acid did not interfere with the determination (signal change below 5%). 50-fold concentrations of hydroquinone, resorcinol and phenol were also investigated. Although there were peaks of hydroquinone, resorcinol and phenol appearing in the cyclic voltammograms, the peak potentials of them were separated from that of CT in the range of −0.2–0.7 V. It indicated the influences of hydroquinone, resorcinol and phenol were negligible (signal change below 5%).
Table 1 Comparison of different electrochemical sensors for the determination of CT. Modified electrode Palygorskite/CPE Polypyrrole-CNT-Tyr/GCE Al-doped silica/CPE Penicillamine/GCE Tyr/GCE MWCNT-Polypyrrole-HRP-Au MWCNT/GCE Penicillamine/GCE Cox Fe3−x O4 --CD/GCE
The Cox Fe3−x O4 --CD/GCE was used to determine 50 M CT for six times by DPV. The relative standard deviation (RSD) of the cathodic peak current was 1.86% for CT. The fabrication reproducibility of the sensor was investigated. Five electrodes were prepared independently under the same conditions, and the RSD was 4.46% for determining 50 M of CT, revealing the good stability and reproducibility of Cox Fe3−x O4 --CD/GCE. Additionally, the stability of Cox Fe3−x O4 --CD/GCE was also evaluated. The results showed that the peak currents remained 92.4% of its initial value after the modified electrode was kept at 4 ◦ C for 20 days.
Linear range (M) 5–100 3–50 0.5–50 25–175 60–800 1.6–8 20–1200 0.5–900 1–200
Detection limit (M)
Reference
0.57 0.67 0.1 0.6 6 0.93 10 0.6 0.12
[28] [29] [3] [30] [31] [32] [33] [34] This work
The Cox Fe3−x O4 --CD/GCE was used to determine CT in several water samples by the standard addition method, and the results were listed in Table 2. Each sample was determined in triplication, and the RSD was below 5.0%. The same samples were also determined by HPLC [35] to confirm the CT content. The results obtained by two methods were in good agreement. In addition, the recoveries were in the range of 92.3–104.1%. 4. Conclusions In this paper, a simple and sensitive electrochemical analytical method for the determination of CT was developed by employing the Cox Fe3−x O4 --CD modified GCE. A well-defined peak and the significant increase of peak current were observed at the Cox Fe3−x O4 --CD/GCE, due to the high CT-loading capacity on the electrode surface and the excellent electric conductivity of Cox Fe3−x O4 . The Cox Fe3−x O4 --CD modified electrode also presented lower detection limit and wider linear range. The simplicity in fabrication procedures, ease of the detection step and good reproducibility of the proposed method opened up an increasing possibility for the multicomponent analysis in environmental control, the chemical industry and pharmaceutical detection. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21075106), the Program for Science and Technology Innovation Talents in Universities of Henan Province (No. 2010HASTIT025) and Excellent Youth Foundation of He’nan Scientific Committee (No. 104100510020). References [1] [2] [3] [4] [5] [6] [7]
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[9] D. Zhao, X. Zhang, L. Feng, L. Jia, S. Wang, Colloids Surf. B 74 (2009) 317. [10] D. Zhang, Y. Peng, H. Qi, Q. Gao, C. Zhang, Sens. Actuators B 136 (2009) 113. [11] J. Wang, W.Y. Meng, X.F. Zheng, S.L. Liu, G.X. Li, Biosens. Bioelectron. 24 (2009) 1598. [12] E. Villagra, F. Bedioui, T. Nyokong, J.C. Canales, M. Sancy, M.A. Paez, J. Costamagna, J.H. Zagal, Electrochim. Acta 53 (2008) 4883. [13] J. Arguello, H.A. Magosso, R. Landers, Y. Gushikem, J. Electroanal. Chem. 617 (2008) 45. [14] M.H. Yang, J.H. Jiang, Y.H. Yang, X.H. Chen, G.L. Shen, R.Q. Yu, Biosens. Bioelectron. 21 (2006) 1791. [15] J.F. Shen, Y.Z. Hu, C. Li, C. Qin, M.X. Ye, Electrochim. Acta 53 (2008) 7276. [16] A.S. Adekunlea, J. Pillaya, K.I. Ozoemena, Electrochim. Acta 55 (2010) 4319. [17] S. Laurent, D. Forge, M. Port, Chem. Rev. 108 (2008) 2064. [18] D. Zhang, J. Zhao, G. Li, Protein Pept. Lett. 15 (2008) 764. [19] D.F. Cao, N.F. Hu, Biophys. Chem. 121 (2006) 209. [20] R. Rawal, S. Chawla, C.S. Pundir, Biosens. Bioelectron. 31 (2012) 144. [21] N. Jaffrezic-Renault, C. Martelet, Y. Chevolot, J.P. Cloarec, Sensors 7 (2007) 589.
[22] W.Y. Yang, X. Zhou, N. Zheng, X.J. Li, Z.B. Yuan, Electrochim. Acta 56 (2011) 6588. [23] M. Chen, G.W. Diao, Talanta 80 (2009) 815. [24] K. Liu, H. Fu, Y. Xie, L. Zhang, K. Pan, W. Zhou, J. Phys. Chem. C 112 (2008) 951. [25] Y. Liu, S.Z. Kang, H.Y. Zhang, Microchem. J. 70 (2001) 115. [26] M. Pita, J.M. Abad, C. Vaz-Dominguez, C. Briones, E. Mateo-Marti, J. Angel, M.D.P. Martin-Gago, V.M. Morales, J. Fernandez, Colloid Interface Sci. 321 (2008) 484. [27] L. Han, X. Zhang, Electroanalysis 21 (2009) 124. [28] Y. Kong, X.H. Chen, W.C. Wang, Z.D. Chen, Anal. Chim. Acta 688 (2011) 203. [29] S.K. Ozoner, M. Yalvac, E. Erhan, Curr. Appl. Phys. 10 (2010) 323. [30] L. Wang, P.F. Huang, J.Y. Bai, H.J. Wang, L.Y. Zhang, Y.Q. Zhao, Microchim. Acta 158 (2007) 151. [31] S. Tembe, S. Inamdar, S. Haram, M. Karve, S.F. D’Souza, J. Biotechnol. 128 (2007) 80. [32] S. Korkut, B. Keskinler, E. Erhan, Talanta 76 (2008) 1147. [33] Z. Xu, X. Chen, X.H. Qu, S.J. Dong, Electroanalysis 16 (2004) 684. [34] J. Peng, Z.N. Gao, Anal. Bioanal. Chem. 384 (2006) 1525. [35] Z. Li, L. Wang, G.Y. Yang, W. Liu, G.L. Shi, C.Q. Jiang, Chinese J. Anal. Chem. 29 (2001) 1409.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
-cyclodextrin-cobalt ferrite nanocomposite as enhanced sensing platform for catechol determination Jin-Tu Han, Ke-Jing Huang ∗ , Jing Li, Yan-Ming Liu ∗ , Meng Yu College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 28 April 2012 Accepted 4 May 2012 Available online 12 May 2012 Keywords: Catechol -cyclodextrin Cobalt ferrite nanocomposite Electrochemical sensor
a b s t r a c t An electrochemical sensor based on -cyclodextrin-cobalt ferrite nanocomposite was developed for the sensitive detection of catechol (CT). To construct the base of the sensor, a novel composite was initially fabricated and used as the substrate material by combining cobalt ferrite nanocomposite and -cyclodextrin via a simple sonication-induced assembly. Due to the high catechol-loading capacity on the electrode surface and the upstanding electric conductivity of cobalt ferrite nanocomposite, the electrochemical response of the fabricated sensor was greatly enhanced and displayed excellent analytical performance for catechol detection from 1 to 200 M with a low detection limit of 0.12 M (S/N = 3). Moreover, the developed electrochemical sensor exhibited good selectivity and acceptable reproducibility and could be used for the detection of catechol in water samples. © 2012 Elsevier B.V. All rights reserved.
1. Introduction (o-benzenediol, 1,2-benzenediol or 1,2Catechol dihydroxybenzene) (CT) is a natural polyphenolic compound which is ubiquitous in nature since it commonly generates in factory processes and widely exists in higher plants such as teas, vegetables, fruits, tobaccos and some traditional Chinese medicines [1,2]. CT has been widely studied due to the biological importance such as antioxidation, antivirus and affecting the activities of some enzymes [3]. However, CT is highly toxic to humans, animals, plants and aquatic life when present above certain concentration limits [4]. Therefore, the detection and quantification of CT have gained significance in environmental protection, food safety and human health. Different analytical methods have been developed for CT determination including spectrophotometry [5], liquid-phase chromatography [6], gas chromatography [7] and capillary electrophoresis [8]. However, these techniques are usually expensive, laborious and time-consuming. Thus, the design and development of quick, simple, inexpensive and effective analytical methods are of great importance in practice. Electrochemical sensors have become the predominant analytical technique for the quantitative detection of polyphenolic compound due to their high sensitivity, low cost, fast analysis and ease of miniaturization [9,10]. Currently, high sensitivity has become one of the main goals in development of electro-assay
∗ Corresponding authors. Tel.: +86 376 6390611. E-mail addresses: [email protected] (K.-J. Huang), [email protected] (Y.-M. Liu). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.05.003
methods. Application of nanomaterial in the fabrication of sensor has gained growing interest due to its intrinsic advantages, such as low cost, good thermal stability, good conductivity and large surface area [11]. Among numerous nanomaterials, cobalt (Co)-based nanomaterials has attracted a considerable interest because of its good electro-activity and low cost [12,13]. For example, the composite of Co hexacyanoferrate nanoparticles (NPs)-carbon nanotubechitosan has been used to develop glucose sensor [14]. Pt–Co NPs supported on single-walled carbon nanotubes (SWCNTs) and Co NPs integrated with SWCNTs have been applied in methanol [15] and nitrite detection [16]. Currently, nano-magnetic particles are attracting more attention due to their distinguished properties, such as surface effect, special magnetic target and good biocompatibity properties [17,18]. Among magnetic nanoparticles, Fe3 O4 nanoparticles are particularly attractive because of their biocompatibility, low toxicity and easy preparation nanosized magnetic particles [19,20], and have been used in electrochemical biosensors due to their unique properties such as large surface area, excellent conformation stability and better contact between biocatalyst and its substrate [21]. The Cox Fe3−x O4 magnetic nanoparticles based electrochemical sensor has been used to study the direct electron transfer between the redox centers of the proteins and the electrode, and it exhibited remarkable electrocatalytic activity towards H2 O2 reduction [22]. -Cyclodextrin (-CD), a cyclic oligosaccharide consisting of seven glucose units [23], has been widely used as a dispersing reagent for insoluble chemicals and nanomaterials, including carbon nanotubes [24,25] due to its excellent film-forming ability and inclusion function. When Cox Fe3−x O4 magnetic nanoparticles are
J.-T. Han et al. / Colloids and Surfaces B: Biointerfaces 98 (2012) 58–62
59
functionalized with -CD, it is likely to obtain new materials simultaneously having unique properties of Cox Fe3−x O4 and -CD, which will provide potential applications in the fields of bio-sensors, electronics, etc. In this paper, Cox Fe3−x O4 --CD was synthesized via a simple sonication-induced assembly. Subsequently, a sensitive electrochemical sensor for CT detection was fabricated based on Cox Fe3−x O4 --CD. Due to the high CT-loading capacity on the electrode surface and the excellent electric conductivity of Cox Fe3−x O4 , the electrochemical response of the fabricated sensor was greatly enhanced and achieved sensitive CT detection. Meanwhile, the proposed electrochemical sensor also exhibited good selectivity and acceptable reproducibility and could be used for the detection of CT in water samples. Therefore, the present work offers a new way to broaden the analytical applications of Cox Fe3−x O4 in environmental analysis. 2. Experimental Fig. 1. SEM images of Cox Fe3−x O4 nanoparticles.
2.1. Chemicals and materials
2.2. Apparatus All the voltammetric measurements were performed on a CHI 660D electrochemical workstation (Shanghai CH Instrument, China). A three-electrode system was employed for the electrochemical detection, which was composed of a modified carbon glassy electrode (GCE) as working electrode, a Pt wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The pH measurements were made with a pH meter (MP 230, Mettler-Toledo, Greiffensee, Switzerland). The images of scanning electron microscope (SEM) were obtained at Hitachi S-4800 (Japan). 2.3. Preparation of Cox Fe3−x O4 -ˇ-CD nanoparticles The Cox Fe3−x O4 nanoparticles were prepared according to references [22,26]. In short, 5 mL of 2 mol L−1 CoCl2 ·6H2 O in HCl 7.4% solution and 40 mL of 0.5 mol L−1 FeCl3 ·6H2 O solution were preheated to 50 ◦ C. The resultant solutions were then mixed and added into a boiling solution of 200 mL of 1 mol L−1 NaOH under vigorous stirring for 30 min. After cooling to room temperature, the dark brown precipitate was separated from the dispersion medium by using a permanent magnet. Cox Fe3−x O4 was obtained after the resulting suspension was dried in an oven at 100 ◦ C for 5 h. 1 mg of Cox Fe3−x O4 was then dispersed in 4 mL of -CD dimethylformamide (DMF) solution (2 wt%) to give a 0.25 mg mL−1 black suspension with the aid of ultrasonic agitation for 2 h. 2.4. Fabrication of the modified electrodes Before the start of the electrochemical experiments and modification procedures, the GCE was first polished to a mirror-like surface with 0.3 and 0.05 m ␣-alumina slurries and then ultrasonicated for 1 min in ethanol and water successively. Next, 5 L of the prepared Cox Fe3−x O4 --CD composite was dropped on the pretreated GCE and dried in a desiccator for 24 h at room temperature.
3. Results and discussion 3.1. Characteristics The as-prepared Cox Fe3−x O4 nanoparticles were characterized with SEM (Fig. 1) and X-ray diffractometry (XRD) (Fig. 2). In Fig. 2, it can be easily seen that all diffraction peaks were consistent with previous reports, which were characteristic of Cox Fe3−x O4 [22,26]. 3.2. Electrochemical characteristics of electrochemical sensor Fig. 3 showed the cyclic voltammograms (CVs) of different electrodes in a 0.1 M PBS (pH 6.0). A pair of well defined redox peaks of CT was observed within the potential window from −0.2 to 0.7 V revealing that CT underwent a quasi-reversible redox process on the electrodes. The peak currents of CT were very small at the bare GCE (curve a). While at the -CD/GCE, the peak current increased obviously (curve b). It indicated -CD molecules have good adsorption capacity to CT. The concentration of CT on the surface of the modified electrode was increased, which resulted in pronounced peak currents enhancement. Similarly, the peak current showed a remarkable increase at Cox Fe3−x O4 /GCE due to its excellent properties (curve c), such as large surface area and the excellent electric conductivity of Cox Fe3−x O4 . From Fig. 3d, we can observe that the peak current at Cox Fe3−x O4 --CD/GCE was nearly 3.5 times higher 60 50
(311) 40
Intensity (a.u.)
-CD, CT, CoCl2 ·6H2 O, FeCl3 ·6H2 O were purchased from Sigma (St. Louis, MO, USA). Phosphate-buffered saline (PBS, 0.1 M) with various pH values was prepared with stock standard solution of Na2 HPO4 , NaH2 PO4 and 0.1 M KCl. All other reagents were used without any further purification. All the solutions were prepared with double-distilled water.
30
(440)
(220)
20
(400)
(111)
(422)
10 0 10
20
30
40
50
2 (deg) Fig. 2. XRD patterns of Cox Fe3−x O4 nanoparticles.
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15
d
10
c
0.5
15
0.4
b 5
a
e
a
E/V
0.3
10
0.2
a
0.1
5
b
0.0
I/
0
-0.1 5
I/
-5
6
7
8
9
pH
0
-10 -15
-5
-20 -0.2
0.0
0.2
0.4
0.6
0.8
-10
E/V
-0.2
Fig. 3. Cyclic voltammograms of bare GCE (a), -CD/GCE (b), Cox Fe3−x O4 /GCE (c) and Cox Fe3−x O4 --CD/GCE (d) in pH 6.0 PBS at 100 mVs−1 .
than that at bare GCE. The significant improvement of peak current clearly demonstrated that Cox Fe3−x O4 --CD composite film acts as an efficient promoter to enhance the kinetics of the electrochemical process, which is probably caused by the synergistic effect of the Cox Fe3−x O4 and -CD. These results indicated that the Cox Fe3−x O4 -CD composite film modified electrode can enhance the detection sensitivity. The effect of scan rates vs. the peak currents of CT at the Cox Fe3−x O4 --CD/GCE was investigated (Fig. 4). The redox processes of the CT gave roughly symmetric anodic and cathodic peaks at relatively slow scan rates. When the scan rate increased, the redox potentials (Epa and Epc ) of CT shift slightly. At the same time, the redox peak currents were proportional to the scan rate in the range of 0.02–0.2 Vs−1 . The linear regression equations were ipa (A) = −4.448 − 0.051 (mV s−1 ) (R = 0.993) and ipc (A) = 1.368 + 0.034 (mV s−1 ) (R = 0.995) (inset), indicating a surface-controlled process.
0.0
0.2
0.4
0.6
0.8
E/V Fig. 5. CVs of Cox Fe3−x O4 --CD/GCE in PBS with different pH values of (a–e): 5.0, 6.0, 7.0, 8.0 and 9.0; scan rate: 100 mVs−1 ; inset was the plot of potentials vs pH.
increased slowly with a rise in pH from 5.0 to 6.0. At a pH higher than 6.0, the peak currents decreased with a rise in pH from 7.0 to 9.0. It has been reported that the pKa value of CT was 9.4 [34]. When pH was between 5.0 and 6.0, the hydroxyl in CT would not ionized, which would increase the adsorption capacity of the CT. When pH increased from 6.0 to 9.0, the increased hydroxyl ion in solution might decrease the adsorption capacity of CT to -CD. So pH 6.0 was selected for the further experiment. In addition, the relationship between pH and the peak potential of CT was studied. It showed that the peak potentials shifted negatively with arise in pH from 5.0 to 9.0 for CT. The equations for anodic and cathodal peak potential with the pH for CT were Epa = −0.063 pH + 0.745 (V, R = 0.994) and Epc = −0.044 pH + 0.418 (V, R = 0.992), respectively. The slopes of these equations suggested that the electrode process was a two-proton coupled two-electron transfer [27].
3.3. Effect of pH The effect of solution pH at Cox Fe3−x O4 --CD/GCE was investigated over a range of 5.0–9.0 (Fig. 5). The peak currents of CT 9
15
6 3 0
I/
10
-3 -6
5
-9 -12
I/
-15
0
0
30
60
90
120
150
180
210
Scan rate / mVs -1
-5
3.4. Effect of the amount of Cox Fe3−x O4 -ˇ-CD The relationship between the peak currents of CT and the amount of Cox Fe3−x O4 --CD on the GCE was investigated. The peak currents of 50 M CT on the Cox Fe3−x O4 --CD/GCE increased remarkably when the amount of Cox Fe3−x O4 --CD suspension increased from 1 to 5 L. However, when the amount of Cox Fe3−x O4 --CD exceeded 5 L, the peak current decreased dramatically. The results may be attributed to the thicker film of Cox Fe3−x O4 --CD, which blocked the electrical conductivity. Consequently, a Cox Fe3−x O4 --CD suspension of 5 L was utilized to modify the GCE.
a 3.5. Effect of the accumulation time
-10
i -15 0.6
0.4
0.2
0.0
-0.2
E/V Fig. 4. CVs of Cox Fe3−x O4 --CD/GCE in PBS at different scan rates: 20, 40, 60, 80, 100, 125, 150, 175, and 200 mVs−1 ; the plot of the peak current vs scan rates (inset).
The sensitivity of the proposed method was undoubtedly improved by the accumulation time. The effect of the accumulation time was investigated in the range of 1–10 min. With the accumulation time increased from 0 to 5 min, the peak currents of CT increased gradually, owing to the increased amount of CT on the Cox Fe3−x O4 --CD/GCE. After that, no further increase was observed due to the surface saturation, therefore, 5 min was chosen as the optimal accumulation time.
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Table 2 Determination of CT in water samples (n = 4).
10
-12
-10
g
I / µA
8
Samples 6
4
I (µA)
-8
Tap water 10 Shi river water 10 Pond water 10
2 0
-6
Added (M)
50
100
150
By HPLC (M)
By this method (M)
RSD (%)
Recovery (%)
9.5 9.4 9.6
9.2 10.3 9.5
3.2 3.5 2.4
92.3 104.1 97.6
200
C / µM
a
3.8. Reproducibility and stability of Cox Fe3−x O4 -ˇ-CD/GCE -4
-2 0.0
0.2
0.4
0.6
0.8
E (V) Fig. 6. Differential pulse voltammograms of CT at Cox Fe3−x O4 --CD/GCE in 0.1 M PBS (pH 6.0). CT concentrations (a–g): 1.0 M, 5.0 M, 10.0 M, 30.0 M, 50.0 M, 100.0 M, 200.0 M.
3.9. Determination of CT in water samples
3.6. Determination of CT Differential pulse voltammetry (DPV) was used for the determination of trace amounts of CT under the optimum conditions. The DPV responses for different concentrations of CT were illustrated in Fig. 6. The resulting calibration plot was linear over the range from 1 to 200 M. The linear equation was ip (A) = −2.121 − 0.034 C (M) with a correlation coefficient of 0.991. The detection limit was 0.12 M (S/N = 3). Comparing with the method reported previously [28,29,3,30–34], the proposed method in this work had wide linear range and low detection limit. This further confirmed the synergetic effect of Cox Fe3−x O4 and -CD. The detailed specific features were shown in Table 1. It indicated that this method was preferable in the determination of trace amount of CT.
3.7. Interferences The possible interferences of some species in the water were tested with a mixed CT solution (50 M). 500-fold Mg2+ , NH4 + , K+ , Na+ , NO3 − , Cl− , SO4 2− , 100-fold Ca2+ , Fe3+ , SO3 2− , Br− , glucose, ascorbic acid and uric acid did not interfere with the determination (signal change below 5%). 50-fold concentrations of hydroquinone, resorcinol and phenol were also investigated. Although there were peaks of hydroquinone, resorcinol and phenol appearing in the cyclic voltammograms, the peak potentials of them were separated from that of CT in the range of −0.2–0.7 V. It indicated the influences of hydroquinone, resorcinol and phenol were negligible (signal change below 5%).
Table 1 Comparison of different electrochemical sensors for the determination of CT. Modified electrode Palygorskite/CPE Polypyrrole-CNT-Tyr/GCE Al-doped silica/CPE Penicillamine/GCE Tyr/GCE MWCNT-Polypyrrole-HRP-Au MWCNT/GCE Penicillamine/GCE Cox Fe3−x O4 --CD/GCE
The Cox Fe3−x O4 --CD/GCE was used to determine 50 M CT for six times by DPV. The relative standard deviation (RSD) of the cathodic peak current was 1.86% for CT. The fabrication reproducibility of the sensor was investigated. Five electrodes were prepared independently under the same conditions, and the RSD was 4.46% for determining 50 M of CT, revealing the good stability and reproducibility of Cox Fe3−x O4 --CD/GCE. Additionally, the stability of Cox Fe3−x O4 --CD/GCE was also evaluated. The results showed that the peak currents remained 92.4% of its initial value after the modified electrode was kept at 4 ◦ C for 20 days.
Linear range (M) 5–100 3–50 0.5–50 25–175 60–800 1.6–8 20–1200 0.5–900 1–200
Detection limit (M)
Reference
0.57 0.67 0.1 0.6 6 0.93 10 0.6 0.12
[28] [29] [3] [30] [31] [32] [33] [34] This work
The Cox Fe3−x O4 --CD/GCE was used to determine CT in several water samples by the standard addition method, and the results were listed in Table 2. Each sample was determined in triplication, and the RSD was below 5.0%. The same samples were also determined by HPLC [35] to confirm the CT content. The results obtained by two methods were in good agreement. In addition, the recoveries were in the range of 92.3–104.1%. 4. Conclusions In this paper, a simple and sensitive electrochemical analytical method for the determination of CT was developed by employing the Cox Fe3−x O4 --CD modified GCE. A well-defined peak and the significant increase of peak current were observed at the Cox Fe3−x O4 --CD/GCE, due to the high CT-loading capacity on the electrode surface and the excellent electric conductivity of Cox Fe3−x O4 . The Cox Fe3−x O4 --CD modified electrode also presented lower detection limit and wider linear range. The simplicity in fabrication procedures, ease of the detection step and good reproducibility of the proposed method opened up an increasing possibility for the multicomponent analysis in environmental control, the chemical industry and pharmaceutical detection. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21075106), the Program for Science and Technology Innovation Talents in Universities of Henan Province (No. 2010HASTIT025) and Excellent Youth Foundation of He’nan Scientific Committee (No. 104100510020). References [1] [2] [3] [4] [5] [6] [7]
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