Zinc oxide thin film based nonenzymatic electrochemical sensor for the detection of trace level catechol

Abstract

In the present work, a novel zinc oxide thin film based nonenzymatic, electrochemical sensor is developed for the detection of catechol. The zinc oxide thin film electrode on FTO conducting substrate is prepared by a simple and cost effective spin coating technique. The developed sensor exhibits promising cyclic voltammetric as well as amperometric response when the concentration of the catechol in phosphate buffer (pH ∼ 7) is varied in the range of 2 to 15 μM. The interference of the sensor for the detection of catechol is compared in presence of chlorophenol and formaldehyde. The repeatability of the sensor performance towards catechol is also investigated at different time intervals. To understand the underlying mechanism of catechol sensing by the ZnO thin film, we have studied the phase, micro-structural and optical features of the electrode before and after electrochemical sensing experiments. It has been observed that the XRD pattern, morphology and optical transmittance of the electrode changes significantly after electrochemical interaction with catechol. Specifically, the 2D thin film morphology upon electrochemical interaction with catechol starts changing to a 1D nanowire like morphology which in turn influences the phase, optical transmittance as well as sensing performance. The modulation of structural, optical features and sensing performances of the developed electrode are again supported by electrochemical impedance spectroscopy.

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1. Introduction

Phenol and its analogous compounds (e.g. catechol, chlorophenol etc.) are often considered as prime organic hazards for water pollution due to their detrimental effects on animals, plants and aquatic organisms.^1–3 Generally, such phenolic compounds are released into water by various industries including but not limited to steel, metal, cement, polymer, ceramic and wood-paper sectors.^4,5 However, the genesis of phenolic compounds by some natural processes (e.g. atmospheric degradation of benzene in the presence of light) also cannot be avoided. Due to the high toxicity and harmful impact in the environment, the detection of these organics in industrial water has become very significant. Generally, spectrophotometric (e.g. fluorimetric/colorimetric detection), chromatography in conjugation with spectrophotometric procedures (e.g. liquid chromatography with ultraviolet detection (LCUV), liquid chromatography with mass spectrometry (LCMS)) and electrochemical techniques are used for the detection of phenolic compounds in water.^6–9 In the spectrophotometric based techniques, the variations of spectral properties (absorbance, emission) of the sensor due to the interaction with the analytes are recorded. These sensing procedures are sophisticated, costly and needs specialised skill for operation. On contrary, electrochemical methods are considered advantageous due to their high sensitivity, simplicity, along with portable architecture.^10,11

In electrochemical sensing application the substrates play important role for the immobilisation of the sensing materials. The anchoring of the sensing materials is significant for the collection of stable electrochemical signal. Usually, glassy carbon/metallic (e.g. Pt/Au etc.) electrodes are used for the immobilisation of the sensing materials.^12–14 For electrochemical sensing of phenolic hazards, the conventional graphite/glassy carbon based electrodes are not suitable due to their poor sensitivity. To improve further the sensitivity of graphite/glassy carbon electrodes, these are often modified using nanomaterials/biological molecules/enzymes.^15–20 The immobilization of nanomaterials/biological systems/enzymes on graphite/glassy carbon electrode with optimum anchoring ability is again a challenging task. Moreover, the biological systems/enzyme molecules are very unstable in open environment which often restricts them to be viable for commercialization.²¹ Therefore, a new cost effective, sensitive, stable, non-enzymatic working electrode system is demanding for detecting low concentration of phenolic compounds. Viewing in the same line, herein, we have developed a new zinc oxide thin film based working electrode on FTO coated conducting glass substrate and utilised them for the electrochemical detection of catechol. ZnO nanomaterials and their composite counterpart are already studied for electrochemical sensing of different analytes including catechol.^22–25 However, the electrochemical detection of catechol using ZnO thin film on FTO substrate are not very frequent in the literature. We made an attempt here to understand the electrochemical phenomenon occur during the interaction of catechol with zinc oxide electrode. The prepared electrode being non-enzymatic, cost effective and portable, it can be suitable as novel electrode for detection of trace level catechol.

2. Experimental

Zinc oxide thin film electrode is deposited on FTO coated conducting glass substrate (1 cm × 3 cm) by spin coating the transparent sol of Zn-acetate precursor. To prepare the sol first requisite amount of zinc acetate dihydrate is dissolved in 50 ml 2-methoxy ethanol at room temperature. Triethanolamine is added into the sol as stabilizing agent. The prepared sol is then deposited on FTO substrate using a spin coating unit (SCU-2008C, Apex Instruments, India) at 3000 rpm speed. The as deposited film is dried each time at 100 °C for 30 min and finally (after 10 repeated coatings) annealed at ∼200 °C for 2 h. During coating Teflon tape is used as mask to deposit the film at desired effective surface (∼2 cm²) of FTO substrate. The electrical connection from the uncoated area of the electrode is taken out by tightly holding a brush wire. The schematic fabrication procedures of the electrode as well as the digital image of the developed electrodes are presented in Fig. 1. Electrochemical experiments e.g. cyclic voltammetry (CV), amperometry and electrochemical impedance spectroscopy (EIS) are carried out in a single compartment, three-electrode cell using electrochemical workstation (SP150, Biologic instruments, France).

Fig. 1 Schematic fabrication procedure of the electrode and digital image of the developed electrodes.

Pt wire and standard Ag/AgCl electrodes are used as counter and reference electrodes respectively. Phosphate buffer solution is used as electrochemical reaction medium. The performance of the electrode is studied in different pH of catechol. The interference experiments are carried out in presence of chlorophenol, formaldehyde and catechol.

The phase formation behaviour, morphology and optical transmittance of the thin ZnO electrode are studied using grazing incidence X-ray diffractometer (X-PERT PRO Panalytical, Netherland), field emission scanning electron microscope (Σigma HD, Zeiss, Germany) and UV-vis spectrophotometer (UV-3600, Shimadzu, Japan).

3. Results and discussions

To study the electrochemical interaction of the ZnO thin film electrode with catechol, CV is carried out in the potential range −0.5 to 0.5 V. The CV plots of the developed electrodes for the detection of catechol at different pH are shown in Fig. 2. As envisaged from the figure, clear reduction peak for the electrode is observed at pH 7. At acidic and basic reaction condition, the distinguishable redox peaks are not identified. The acidic and basic environment may interact with the basic ZnO and acidic catechol respectively and may degenerate their activity.

Fig. 2 CV plots of developed sensor for the detection of 10 μM catechol at different pH. Inset shows magnified view of CV plots at pH 4 and 9.

Fig. 3(a) shows the CV curves of the developed ZnO electrode in phosphate buffer medium (pH ∼ 7) in presence and absence of catechol. In the absence of catechol, only a low background current without any considerable peak is observed, after adding catechol to the buffer, prominent reduction peak in the range of ∼−0.27 V arises in CV curves. Upon increasing the concentration of catechol, the cathodic current increases linearly (shown in the inset Fig. 3(a)). Here it is proposed that cathodic peak current is achieved due to the oxidation of catechol on ZnO surface. The ZnO surface acts as catalyst for the oxidation of catechol to o-quinone. The similar catalytic activities of ZnO for the oxidation of organic materials are reported elsewhere.^26–29 To confirm the catalytic activity we have performed the similar cyclic voltammetric studies using bare FTO coated glass substrate. Low current modulation is observed during the interaction of FTO substrates with different concentration of catechol.

Fig. 3 (a) Cyclic voltammetric curve of ZnO thin film electrode with 8 consecutive cycle recorded without and with different concentration (upto 14 μM) of catechol at pH 7. (b) Amperometric response at ZnO thin film electrode upon successive additions of 2 μM catechol.

The amperometric studies are also conducted to verify the interaction of catechol with the developed electrode. Fig. 3(b) shows amperometric response of the ZnO electrode at −0.27 V (vs. SCE) when 0.2 (μM) catechol is added successively into the buffer media. In figure, the upward arrows (↑) denote the injection point of catechol. As reflected from the figure distinct change in response is observed after immediate addition of catechol indicating the strong electrochemical interaction between the ZnO thin film electrodes with catechol.

The repeatability performance of the electrode (10 times coated ZnO thin films on FTO substrate) is evaluated by performing CV tests in presence of 10 μM catechol at 24 h interval. After each test the electrode is washed with distilled water and stored in the plastic box under vacuum desiccation. It is found that the electrodes perform satisfactorily upto three CV tests (in presence of 10 μM catechol) executed at 24 h interval. However, the sensor can be recovered further by coating ZnO thin film on the used conducting glass substrates. For newly developed as well as recovered sensors, the CV patterns in presence of 10 μM catechol are shown in Fig. 4(a) and (b) respectively. As shown in the figure, the recovered sensor is performing similar to the newly developed sensing electrode. It is important here to mention that the developed sensing electrode cannot run enormous electrochemical cycle by recovering it through simple process. In market, there are various electrochemical sensors which are used only once. Unlike those single time usable sensors, the present sensing electrode can be used 3–4 times. Moreover, the sensing electrode can be further prepared on the used conducting glass substrate cost effectively through wet chemical route.

Fig. 4 CV plots of (a) newly prepared and (b) recovered sensor for the detection of 10 μM catechol.

The interference experiments are performed in presence of formaldehyde, chlorophenol and catechol. The CV pattern of the sensor in presence of 10 μM catechol, chlorophenol and formaldehyde is presented in Fig. 5. As shown in the figure, the electrode is producing highest redox peak current at a distinctive redox potential in presence of catechol.

Fig. 5 Cyclic voltammetric curve of ZnO thin film electrode for detection of 10 μM catechol, chlorophenol and formaldehyde.

To understand the electrochemical interaction we have performed the EIS studies. Fig. 6 shows the Nyquist plots of EIS spectra for the developed ZnO electrode during the interaction with different concentration of catechol. For clarity the enlarged view of the EIS plots are shown in the inset figure. The plots shown in figure resemble the Nyquist plots for different working electrodes reported elsewhere.^19,30 Here, the semi-circles obtained at high and low frequencies correspond to the charge transduction phenomenon at FTO/ZnO and ZnO/catechol interface respectively. As reflected from the figure the charge transduction phenomenon at FTO/ZnO is not altered much due to the variation of catechol concentration. However, charge transportation resistance (corresponds to the diameter of semi-circles of Nyquist plots) at ZnO/catechol interface decreases drastically with the increase in concentration. Such drastic change in charge transduction resistance at high catechol concentration indicates rapid interaction of catechol on ZnO surface.

Fig. 6 Impedance spectroscopy of ZnO thin film electrode with variation of catechol concentration. Inset shows the enlarged view of the EIS plots.

To further analyse the interaction of catechol with ZnO surface, we have studied the XRD pattern, morphological and optical features of the electrode before and after electrochemical experiments. Fig. 7 shows the XRD pattern of blank FTO as well as ZnO coated FTO before and after electrochemical experiments.

Fig. 7 The XRD pattern of blank FTO as well as ZnO coated FTO before and after electrochemical experiments.

The characteristic peak positions (in 2θ range 26–38 degree) for ZnO and FTO superimpose in the XRD pattern which often makes difficulties to discriminate them. The poor crystallinity of ZnO film achieved due to their annealing at low temperature (∼200 °C) also may not reveal the distinguishable X-ray diffraction peaks. However, the deposition of ZnO film on FTO substrate can be identified from the variation of XRD peak intensities (in 2θ range 26–38 degree). The absolute intensity (A.I.) and normalized intensity (N.I.) of XRD peaks for FTO, FTO + ZnO (before and after electrochemical interaction) in 2θ range 26–38 degree are summarized in Table 1.

Table 1 Absolute and normalised intensity of characteristic XRD peaks for ZnO thin film before and after electrochemical sensing of catechol

Peak position (2°)	FTO		FTO + ZnO (before sensing)		FTO + ZnO (after sensing)
	A.I.	N.I.	A.I.	N.I.	A.I.	N.I.
26.5	123.15	1	411.8	1	276.57	1
33.74	104.15	0.845	377.8	0.917	230.57	0.833
37.78	101.15	0.821	396.8	0.963	261.57	0.945

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As compared to the most intense peaks at 26.5 degree, the normalized intensities of the peaks at 33.7 and 37.7 degree change after deposition of ZnO thin film. Critically investigating the XRD patterns, it is also observed that the intensities of the peaks for (FTO + ZnO) also alter after electrochemical interaction with catechol. Precisely, after electrochemical sensing by ZnO film, the normalized intensity of XRD peak at 37.78 increases compared to the intensity of peak at 33.74. Assuming the XRD peaks at 37.78 and 33.74 correspond to (002) and (100) plane of ZnO, it suggests the preferential growth of ZnO along c-axis after electrochemical interaction. Viewing the change in XRD pattern, we have presumed that such c-axial growth of ZnO may result to the formation of 1D nano-structure (e.g. nanowire). To assess our presumption we have investigated the morphology (shown in Fig. 8) of the ZnO film before and after electrochemical treatment.

Fig. 8 FESEM images of (a) ZnO thin films prior to the electrochemical interaction with catechol; (b) exposed and non-exposed area of ZnO thin film during electrochemical sensing (c) & (d) exposed area of ZnO thin film in different magnifications.

The FESEM image shown in Fig. 8(a) represents the uniform surface morphology of ZnO thin films prior to the electrochemical interaction with catechol. Fig. 8(b) highlights the interfacial area which corresponds both electrochemically treated and untreated regions of ZnO film. As reflected from the figure, due to the electrochemical interaction of the film with catechol, the morphology of the film transforms into nanowire like 1D morphology which supports our presumption achieved through the analyses of XRD patterns. The prominent and enlarged views of the ZnO nanowires are presented in Fig. 8(c) and (d). Insight on Fig. 8(d) further reflects that the nanowire like morphologies are grown from the upper surface of the film which indicates the surface electrochemical interaction of the film with catechol.

The growth of nanowires due to the electrochemical interaction of thin film with catechol makes the film surface uneven which makes the film more opaque. The UV-vis transmittance of the film is shown in Fig. 9 which confirms that the film loses its optical transparency after electrochemical interaction with catechol. It is worthy to mention here that the film does not alter its morphological as well as optical features in buffer solution (pH ∼ 7) under the same environment since no distinguishable electrochemical interaction is recorded in CV studies shown in Fig. 3.

Fig. 9 UV-vis transmittance of the ZnO thin film electrode before and after sensing.

Based on the above findings, we have made here an attempt to understand the nature of electrochemical interaction of catechol on ZnO surface. We have proposed here that since the deposited ZnO films are poor crystalline, the loosely held oxygen in Zn–O bond initiates the electrochemical oxidation of catechol and simultaneously results improvement in its own crystallinity. Such improvements in crystallinity leads to the enhancement of intensity for XRD peak at 37.78 and also facilitate the formation of nanowire like morphology.

4. Conclusion

In summary, zinc oxide thin film based novel nonenzymatic electrochemical sensor is developed for the detection of catechol. Simple and cost effective spin coating technique is used to prepare zinc oxide thin film electrode on FTO coated conducting glass substrate. Electrochemical studies show that the electrode performs well towards the detection of catechol at pH 7. Cyclic voltammetry as well as amperometric response obtained from the developed electrode are found to vary linearly with catechol concentration in the range of 2 to 15 μM. The repeatability performance of the developed electrode is verified by performing CV tests in presence of 10 μM catechol at 24 h interval. It is observed that the electrodes perform satisfactorily upto three CV tests. The sensing performances of the electrode can be reinstate by coating ZnO thin film on used substrate through simple wet chemical based coating (spin/dip coating) technologies. From the electrochemical impedance spectroscopy, it is observed that the charge transportation resistance at ZnO/catechol interface decreases with the increase in catechol concentration. To study the underlying mechanism of catechol sensing over low temperature annealed ZnO thin film surface, the XRD pattern, morphology and optical transparency of the films are analyzed. It is found that the XRD pattern, morphology and optical transmittance of the film changes appreciably after the electrochemical interaction with catechol. Most interestingly, the thin film morphology of the film transforms into nanowire like morphology after electrochemical sensing of catechol. Based on the findings, it is proposed here that due to the poor crystalline nature, the loosely held oxygen in Zn–O bond facilitate the electrochemical oxidation of catechol and results improvement in its crystallinity.

Acknowledgements

Authors express their sincere thanks to Director, CSIR-CMERI for his kind support to carry out the work. The authors wish to acknowledge Central Research Facility, CSIR-CMERI for providing the FESEM facilities. AM is thankful to CSIR, India for supporting his fellowship. KM thanks DST, Govt. of India for providing him Inspire Faculty fellowship (Ref. DST/IFA12-CH-43) and associated research grant.

References

1. V. Pasqualini, C. Robles, S. Garzino, S. Greff, A. Bousquet-Melou and G. Bonin, Chemosphere, 2003, 52, 239–248.
2. A. S. Bassi and C. McGrath, J. Agric. Food Chem., 1999, 47, 322–326.
3. C. Carmen, M. Martinez, F. Pino, S. Kurbanoglu, L. Rivas, S. A. Ozkanb and A. Merkoçi, J. Mater. Chem. B, 2014, 2, 2233–2239.
4. A. K. Jain, V. K. Gupta and S. Jain, Environ. Sci. Technol., 2004, 38, 1195–1200.
5. B. Lochab, S. Shukla and I. K Varmab, RSC Adv., 2014, 4, 21712–21752.
6. Y. Lin, C. Chen, C. Wang, F. Pu, J. Ren and X. Qu, Chem. Commun., 2011, 47, 1181–1183.
7. A. Afkhami and H. A. Khatami, J. Anal. Chem., 2001, 56, 429–432.
8. B. L. Lee, H. Y. Ong, C. Y. Shi and C. N. Ong, J. Chromatogr., 1993, 619, 259–266.
9. Y. Jiang, L. Jia, S. Yu and C. Wang, J. Mater. Chem. A, 2014, 2, 6656–6662.
10. M. Govindhan, B. R. Adhikari and A. Chen, RSC Adv., 2014, 4, 63741–63760.
11. W. Xiong, M. Wu, L. Zhou and S. Liu, RSC Adv., 2014, 4, 32092–32099.
12. S. Lupu, C. Lete, M. Marin, N. Totir and P. C. Balaure, Electrochim. Acta, 2009, 54, 1932–1938.
13. L. Tang, Y. Zhou, G. Zeng, Z. Li, Y. Liu, Y. Zhang, G. Chen, G. Yang, X. Lei and M. Wu, Analyst, 2013, 138, 3552–3560.
14. J. He, S. Xing, R. Qiu, Z. Song and S. Zhang, Anal. Methods, 2014, 6, 1210–1218.
15. X. Lin, Y. Ni and S. Kokot, Anal. Chim. Acta, 2013, 765, 54–62.
16. J. Li and X. Lin, Sens. Actuators, B, 2007, 126, 527–535.
17. J. Zhao, D. Wu and J. Zhi, Bioelectrochemistry, 2009, 75, 44–49.
18. S. Palanisamy, S. M. Chen and R. Sarawathi, Sens. Actuators, B, 2012, 166, 372–377.
19. M. Tak, V. Gupta and M. Tomar, J. Mater. Chem. B, 2013, 1, 6392–6401.
20. X. J. Huang and Y. K. Choi, Sens. Actuators, B, 2007, 122, 659–671.
21. Y. B. Hahn, R. Ahmad and N. Tripathy, Chem. Commun., 2012, 48, 10369–10385.
22. L. Fang, K. Huang, B. Zhang, B. Liu, Y. Liu and Q. Zhang, RSC Adv., 2014, 4, 48986–48993.
23. M. Ahmad, C. Pan, Z. Luo and J. Zhu, J. Phys. Chem. C, 2010, 114, 9308–9313.
24. B. X. Gu, C. X. Xu, G. P. Zhu, S. Q. Liu, L. Y. Chen and X. S. Li, J. Phys. Chem. B, 2009, 113, 377–381.
25. M. Syed, U. Ali, N. H. Alvia, Z. Ibupotoa, O. Nura, M. Willandera and B. Danielsson, Sens. Actuators, B, 2011, 152, 241–247.
26. Y. F. Li, Z. M. Liu, Y. L. Liu, Y. H. Yang, G. L. Shen and R. Q. Yu, Anal. Biochem., 2006, 349, 33–40.
27. X. Li, C. Zhao and X. Liu, Microsystems & Nanoengineering, 2015, 1, 15014.
28. M. Wang, Y. Zhou, Y. Zhang, S. H. Hahn and E. J. Kim, CrystEngComm, 2011, 13, 6024–6026.
29. K. D. Jung, O. S. Joo and S. H. Han, Catal. Lett., 2000, 68, 49–54.
30. R. Khan, A. Kaushik, P. R. Solanki, A. A. Ansari, M. K. Pandey and B. D. Malhotra, Anal. Chim. Acta, 2008, 616, 207–213.

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