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Research Papers

# Determination of Cyanuric Acid by Electrochemical Cyclic Voltammetry Method Using CuGeO3 Nanowires as Modified Electrode MaterialsOPEN ACCESS

[+] Author and Article Information
L. Z. Pei

Key Lab of Materials Science
and Processing of Anhui Province,
School of Materials Science and Engineering,
Anhui University of Technology,
Ma'anshan,
Anhui 243002, China
e-mail: lzpei@ahut.edu.cn, lzpei1977@163.com

Y. K. Xie, Y. Q. Pei, Z. Y. Cai

Key Lab of Materials Science
and Processing of Anhui Province,
School of Materials Science and Engineering,
Anhui University of Technology,
Ma'anshan,
Anhui 243002, China

C. G. Fan

Key Lab of Materials Science
and Processing of Anhui Province,
School of Materials Science and Engineering,
Anhui University of Technology,
Ma'anshan,
Anhui 243002, China

1Corresponding author.

Manuscript received March 1, 2013; final manuscript received October 22, 2013; published online November 28, 2013. Assoc. Editor: Jung-Chih Chiao.

J. Nanotechnol. Eng. Med 4(3), 031003 (Nov 28, 2013) (5 pages) Paper No: NANO-13-1014; doi: 10.1115/1.4026024 History: Received March 01, 2013; Revised October 22, 2013

## Abstract

A simple electrochemical method for the determination of cyanuric acid (CA) has been developed based on a CuGeO3 nanowire modified glassy carbon electrode. The dense CuGeO3 nanowire film can be formed on the surface of the glassy carbon electrode. The roles of scan rate, CA concentration, and electrolytes with different pH values on the electrochemical responses of CA have also been analyzed. The intensities of two anodic peaks vary linearly with the increase of the scan rate from 25 to 200 mVs−1. The intensity of the electrochemical CV peak increases with the increase of the acidity of the electrolytes. The two anodic peak currents are linear with the CA concentration in the range of 0.005–2 mM. The linear correlation coefficient is 0.984 and 0.980 for the cyclic voltammogram peaks (cvp) cvp1 and cvp2, respectively. The detection limit is 4.3 μM and 2.1 μM for cvp1 and cvp2, respectively. The proposed electrochemical method is convenient and effective sensing of CA.

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## Introduction

Cyanuric acid (CA) belongs to the derivative compound of melamine. It is intentionally added into food ingredients to make the products contain a higher protein content due to high nitrogen content of CA [1]. CA was also found as nonprotien nitrogen in pet food. However, CA is thought to be harmful to humans, especially the toxicity increases when the CA coexists with melamine forming a hydrogen-bonded biomolecule network which is the insoluble crystal in the kidneys of cats and other species [2,3]. In 2007, numerous pet foods in the United States were recalled after dogs and cats consuming the products suffered renal failure [4,5]. As an additional adulterant, CA is also found in the pet foods. CA is not approved by the U. S. FDA as a source of nonprotein nitrogen in hog, chicken, fish or aquaculture feeds [6,7]. Therefore, CA should not be present in foods at any level. Trace amounts of CA in foodstuff with high sensitivity have been an important research issue.

CA was found in the environment, such as a degradation product of s-triazine pesticides. CA was also used as a microbicide and disinfectant in water treatment as a chlorine stabilizer [4]. A gas chromatography mass spectrometry (GC-MS) method was developed for the relatively high CA concentrations which were found in adulterated pet foods and animal feeds [4,8]. The detection limit is 77.5 μM by the GC-MS method which is unsuitable for the concentration levels expected in the contaminated feeds. He et al. [9] reported the detection of melamine, CA and melamine cyanurate using surface-enhanced Raman spectroscopy (SERS) coupled with gold nanosubstrates. The detection limit for CA, melamine, and melamine cyanurate by SERS was estimated to be 0.26 μM. Ehling et al. [10] reported the determination of the CA by high-performance liquid chromatography (HPLC) method. The method allowed for the detection of CA at about 90 μg/g in wheat flour. Chromatographic system based on zwitterionic hydrophilic interaction chromatography (ZIC-HILIC) columns was used for the detection of CA [11]. The method is applicable over the range of 0.5 to 50 μg/g. Wang et al. [12] previously described a liquid chromatography–mass spectrometry (LC-MS) method to simultaneously determine CA and melamine and validated the method in pet foods and biological matrices. The detection limits of the melamine and CA were 0.0315 μM and 0.0304 μM, respectively. However, time-consuming steps and expensive instruments limit the application of the existing methods. Therefore, it is important to explore new, convenient and effective methods for the detection of CA.

Compared to the present methods, much effort has been devoted to the electrochemical sensors owing to their simplicity, low cost, accurateness, high stability and sensitivity. Unfortunately, to date, no literatures were reported using the electrochemical determination of CA at solid electrodes. In our past research, copper germanate (CuGeO3) nanowires attracted special interest because CuGeO3 nanowires exhibited excellent electrochemical properties for potential applications in nanoscale electrochemical sensors [13-15]. In the paper, electrochemical method based on a CuGeO3 nanowire modified glassy carbon electrode (GCE) has been developed for the determination of CA by a cyclic voltammetry method. The electrochemical cyclic voltammogram (CV) peak currents are linear with the CA concentration in the range of 0.005–2 mM with the linear correlation coefficient of 0.984 and 0.980 for cvp1 and cvp2, respectively. The detection limit is 4.3 μM and 2.1 μM for cvp1 and cvp2, respectively. The roles of scan rate and electrolytes with different pH values on the electrochemical responses of CA have also been analyzed. The proposed method will have great implication potential for the determination of CA.

## Experimental Methods

###### Preparation of the CuGeO3 Nanowire Modified GCE.

The CuGeO3 nanowires were obtained by the hydrothermal deposition method which was described elsewhere [16]. The hydrothermal deposition temperature and duration time were 250 °C and 12 h, respectively. The CuGeO3 nanowire suspension was prepared by dispersing 10 mg CuGeO3 nanowires in 10 ml dimethylformamide (DMF) solvent with sufficient ultrasonication for about 1 h. Then, the uniform CuGeO3 nanowire suspension was obtained. Prior to modification, a GCE with the diameter of 3 mm was polished to a mirror using polish paper and alumina pastes of 0.5 μm and cleaned thoroughly in an ultrasonic cleaner with alcohol and water sequentially. A CuGeO3 nanowire modified GCE was prepared by coating 10 μl nanowire suspension on the surface of bare GCE and allowed to be evaporated at room temperature in air.

###### Characterization Methods.

The surface morphology of the CuGeO3 nanowire modified GCE was observed by scanning electron microscopy (SEM) (JEOL JSM-6490LV SEM) with a 15-KV accelerating voltage. CA was purchased from Sinopharm Chemical Reagent Co., Ltd. of China which was analytical grade. Electrochemical (EC) measurement was performed in a model CHI6046 electrochemical working station. The CuGeO3 nanowire modified GCE served as the work electrode. Platinum plate and saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. All potentials in the study were reported with respect to SCE. Cyclic voltammograms (CVs) were recorded from −1.0 to 1.0 V at a potential scan rate of 50 mVs−1 in the mixed solution of 0.1 M KCl and CA with different concentrations.

## Results and Discussion

###### Surface Morphology of the Modified GCE.

The surface morphology of the CuGeO3 nanowire modified GCE is shown in Fig. 1. The diameter of the CuGeO3 nanowires is 50 nm to 200 nm. The CuGeO3 nanowires can form a dense film on the surface of the GCE. Therefore, the glassy carbon substrate at the CuGeO3 nanowire modified GCE might have no effect on the electrochemical responses of the CA.

###### Voltammetric Characteristics of CA at the Modified GCE.

CVs in 0.1 M KCl solution with and without CA at bare GCE and CuGeO3 nanowire modified GCE are measured in the potential ranging from −1.0 to +1.0 V (Fig. 2). The potential scan rate is 50 mVs−1. No CV peaks are observed from the CV curves at the nanowire modified GCE in 0.1 M KCl solution without CA. Therefore, no electrochemical activity is shown at the CuGeO3 nanowire modified GCE in 0.1 M KCl solution without CA. It is very interesting that two pairs of CV peaks are observed from the electrochemical CV curve in 0.1 M KCl solution with 2 mM CA. Two semi-irreversible electrochemical CV peaks are observed. The anodic CV peaks (cvp1, cvp2) are located at 0.21 V and 0.01 V, respectively. And two cathodic CV peaks (cvp1′, cvp2′) are located at 0.02 V and −0.52 V, respectively. These CV peaks can only be shown from KCl solution with CA at the CuGeO3 nanowire modified GCE. Therefore, it is believed that the electrochemical CV peaks originate from the CA. CuGeO3 nanowires have an essential role for the formation of the electrochemical CV peaks. Two pairs of CV peaks are similar to those at the CuGeO3 nanowire modified GCE in KCl solution with 2 mM tartaric acid and cysteine, respectively [13,15]. However, the potentials of the CV peaks are different. Two anodic CV peaks (cvp1, cvp2), located at 0.26 V and −0.04 V, and two cathodic peaks (cvp1′, cvp2′), located at −0.15 V and −0.35 V, were obtained at the CuGeO3 nanowire modified GCE for the analysis of cysteine, respectively [13]. Two anodic CV peaks (cvp1, cvp2) were located at 0.18 V and 0.02 V from the CV curve of 2 mM tartaric acid. Two cathodic peaks (cvp1′, cvp2′) were located at 0.05 V and −0.33 V, respectively [15]. Thus, the CA and cysteine can be distinguished via the potentials of the CV peaks by CV method. The results show that CuGeO3 nanowires have different electrochemical activity in KCl solution with CA. In order to demonstrate whether the anodic and cathodic CV peaks are semireversible electrochemical reaction process, the experiments with initial potential scan direction and different reversal potential were conducted. The potentials and intensities of the electrochemical CV peaks maintain very similar when changing the initial potential scan direction and reversing the potential scan at different positive potential. Therefore, the cvp1 and cvp2 are considered to be caused from the potential of cvp1′ and cvp2′, respectively.

Cvp1-cvp1′ obtained from the CVs of ascorbic acid at the CuGeO3 nanowire modified GCE was assigned to the oxidation and reduction process between ascorbic acid and dehydroascorbic acid [14]. Similarly, cvp1-cvp1′ obtained from CuGeO3 nanowire modified GCE in CA solution is also considered to be the electrochemical oxidation and reduction process between cyanuric acid and dehydrocyanuric acid. The adsorption and desorption process of cysteine have also been analyzed by impedance spectroscopy indicating the adsorption and desorption process of cysteine and cystine at gold electrode, respectively [17]. The electrochemical responses of tartaric acid and ascorbic acid at the CuGeO3 nanowire modified GCE also showed adsorption and desorption behavior [14,15]. Therefore, cvp2-cvp2′ obtained from the CVs of CA is considered to contribute to the adsorption and desorption process of cyanuric acid and dehydrocyanuric acid at the nanowire modified GCE.

###### Effect of the Electrolytes.

The electrochemical responses of the CA at the CuGeO3 nanowire modified GCE under various electrolytes with different pH values are shown in Fig. 3. NaOH (pH = 12), KBr (pH = 7), CH3COOH-CH3COONa (pH = 5) and H2SO4 (pH = 2) with the concentration of 0.1 M are used as the electrolytes so as to show the role of different electrolytes on the electrochemical responses of CA at the nanowire modified electrode. In 0.1 mM KBr solution with 2 mM CA, the electrochemical responses of the CA (Fig. 3(a)) are similar to those in KCl solution. But the peak potential of cvp1 shifts to more positive direction. The potential of cvp1 is +0.29 V. In addition, the slight difference on the intensities of the anodic CV peaks is also observed. No electrochemical CV peaks are observed from the CV curve of the CA in alkaline solution (Fig. 3(b)). Therefore, it is shown that the CuGeO3 nanowire modified GCE has no electrochemical activity in alkaline solution with CA. However, a broad and strong anodic peak occurs instead of cvp1 and cvp2 obtained from neutral solution when the electrochemical measurement was carried out in acidic solution (Figs. 3(c) and 3(d)). The peak potential of the strong anodic peak located between the potential of cvp1 and cvp2 which originates from the hydrogen ions and varies in different acidic solution. The intensity of the electrochemical CV peak increases obviously with the increase of the acidity of the electrolytes. It is considered that the hydrogen ions may participate in the electrochemical reaction of the CA which is similar to those of other biological molecules at the CuGeO3 nanowire modified GCE [14,15].

###### Effect of the Scan Rate.

The role of the scan rate on the electrochemical responses of the CA at the CuGeO3 nanowire modified GCE has been analyzed using the scan rate from 25 to 200 mVs−1. The CV curves of 2 mM CA in 0.1 M KCl solution at the CuGeO3 nanowire modified GCE are shown in Fig. 4. The potentials of the anodic and cathodic peaks for the CA are very similar. However, the intensities of the CV peaks increase obviously with the increase of the scan rate from 25 to 200 mVs−1. The inset in bottom-left part of Fig. 4 indicates the relationship between the scan rate and current of the anodic CV peaks. The linear relation is shown from the anodic current and scan rate in the range of 25–200 mVs−1. The result demonstrates that the kinetic of the overall electrochemical reaction process may be controlled by an adsorption process [18,19]. The role of the scan rate on the electrochemical behavior of the CA is similar to that of tartaric acid and ascorbic acid at the CuGeO3 nanowire modified GCE [14,15]. Scan rate may promote the electrochemical reaction between cyanuric acid and dehydrocyanuric acid.

###### Effect of the CA Concentration.

By measuring different CV curves of the CA with different concentrations in KCl solution using the CuGeO3 nanowire modified GCE, the linear range and detection limit are obtained. Figure 5 shows the CVs of the CA in the concentration range of 0.005–2 mM. The calibration plots of the intensities of anodic peaks and CA concentration are shown in the inset in the bottom-left part of Fig. 5. Increasing the CA concentration, the intensities of the CVs of the CA at the CuGeO3 nanowire modified GCE increase obviously. The linear range, detection limit and correlation coefficient for detecting CA using the CuGeO3 nanowire modified GCE are obtained in the concentration range of 0.005–2 mM. The data are shown in Table 1. The linear range is 0.005–2 mM with the correlation coefficient of 0.984 and 0.980 for cvp1 and cvp2, respectively. The detection limit is 4.3 μM and 2.1 μM for cvp1 and cvp2, respectively, at a signal-to-noise ratio of 3. Compared with the detection of the CA by other methods, the CuGeO3 nanowire modified GCE exhibits a low detection limit and wide linear range [4,6-9].

As a practical use, the CuGeO3 nanowire modified GCE has been used to detect CA in milk samples. The CA concentration in the milk samples is 5, 20, and 40 μM, respectively. The measured values were calculated from five separate measurements. The recoveries of CA were determined by standard addition and the corresponding results are listed in Table 2. Good results suggest that the CuGeO3 nanowire modified GCE is very reliable and sensitive for the determination of CA.

###### Stability and Reproducibility of the Modified GCE.

The stability and reproducibility of the CuGeO3 nanowire GCE have been analyzed in a continuous operation mode. Figure 6 shows the CVs of 2 mM CA with the repeat detection for the 1st and 20th time in 0.1 M KCl solution using same CuGeO3 nanowire modified GCE. The relative standard deviation (RSD) is 4.71% and 2.73% for cvp1 and cvp2, respectively. The CuGeO3 nanowires can attach firmly to the GCE showing the stability. Therefore, the stability and reproducibility of the CuGeO3 nanowire modified GCE are good. The CuGeO3 nanowire modified GCE can be used at least for two weeks with only a slightly decline in the electrochemical signal.

## Conclusion

In summary, the electrochemical detection performance for cyanuric acid using CuGeO3 nanowire modified GCE has been analyzed by cyclic voltammetry method. Two pairs of semi-irreversible electrochemical CV peaks with anodic CV peaks (cvp1, cvp2) at 0.21 V and 0.01 V, and cathodic peaks (cvp1′, cvp2′) at 0.02 V and −0.52 V, are observed, respectively. The intensity of the CV peaks increases linearly with the increase of the CA concentration and scan rate. The linear range is 0.005–2 mM and detection limit is 4.3 μM and 2.1 μM for cvp1 and cvp2, respectively. The CuGeO3 nanowires are showed to have potential application potential for the detection of CA.

## Acknowledgements

This work was supported by the Natural Science Foundation of the Education Bureau of Anhui Province of China (KJ2012Z038, KJ2011A042), Natural Science Foundation of Anhui Province of China (1308085ME72), Innovative Research Foundation of Postgraduate of Anhui University of Technology (2012021) and Postgraduate Supervisor Innovation Fund Project by Anhui University of Technology (D201101).

## References

Sugita, T., Ishiwata, H., and Yoshihira, K., 1990, “Release of Formaldehyde and Melamine from Tableware made of Melamine—Formaldehyde Resin,” Food Addit. Contam., 7, pp. 21–27. [PubMed]
Brown, C. A., Jeong, K. S., Poppenga, R. H., Puschner, B., Miller, D. M., Ellis, A. E., Kang, K. I., Sum, S., Cistola, A. M., and Brown, S. A., 2007, “Outbreaks of Renal Failure Associated With Melamine and Cyanuric Acid in Dogs and Cats in 2004 and 2007,” J. Vet. Diagn. Invest., 19, pp. 525–531. [PubMed]
Perdigao, L. M. A., Champness, N. R., and Beton, P. H., 2006, “Surface Self-Assembly of the Cyanuric Acid–melamine Hydrogen Bonded Network,” Chem. Commun., 42, pp. 538–540.
Karbiwnyk, C. M., Andersen, W. C., Turnipseed, S. B., Storey, J. M., Madson, M. R., Miller, K. E., Gieseker, C. M., Miller, R. A., Rummel, N. G., and Reimschuessel, R., 2009, “Determination of Cyanuric Acid Residues in Catfish, Trout, Tilapia, Salmon and Shrimp by Liquid Chromatography-Tandem Mass Spectrometry,” Anal. Chim. Acta, 637, pp. 101–111. [PubMed]
U. S., Food and Drug Administration, 2007, “Pet Food Recall (Melamine)/Tainted Animal Feed,”
Landers, B. R., 2012, “Effects of Dietary Melamine and Cyanuric Acid in Young Pekin Ducks and Weanling Pigs,” A Thesis presented to the Faculty of the Graduate School University of Missouri-Colombia, Columbia, MO.
Hilts, C., and Pelletier, L., 2008, “Background Paper on Occurrence of Melamine in Foods and Feed. WHO Expert Meeting on Toxicological and Health Aspects of Melamine and Cyanuric Acid,” Health Canada, Ottawa, Canada.
Miao, H., Fan, S., Wu, Y. N., Zhang, L., Zhou, P. P., Li, J. G., Chen, H. J., and Zhao, Y. F., 2009, “Simultaneous Determination of Melamine, Ammelide, Ammeline, and Cyanuric Acid in Milk and Milk Products by Gas Chromatography-Tandem Mass Spectrometry,” Biomed. Environ. Sci., 22, pp. 87–94. [PubMed]
He, L. L., Liu, Y., Lin, M. S., Awika, J., Ledoux, D. R., Li, H., and Mustapha, A., 2008, “A New Approach to Measure Melamine, Cyanuric Acid, and Melamine Cyanurate Using Surface Enhanced Ramon Spectroscopy Coupled With Gold Nanosubstrates,” Sens. Instrum. Food Qual., 2, pp. 66–71.
Ehling, S., Tefera, S., and Ho, I. P., 2007, “High-Performance Liquid Chromatographic Method for the Simultaneous Detection of the Adulteration of Cereal Flours With Melamine and Related Triazine by-Products Ammeline, Ammedide, and Cyanuric Acid,” Food Addit. Contam., 24, pp. 1319–1325. [PubMed]
Heller, D. N., and Nochetto, C. B., 2008, “Simultaneous Determination and Confirmation of Melamine and Cyanuric Acid in Animal Feed by Zwitterionic Hydrophilic Interaction Chromatography and Tandem Mass Spectrometry,” Rapid Commun. Mass Spectrom., 22, pp. 3624–3632. [PubMed]
Wang, L., Henday, S. M., Liu, X. D., Tracy, M., and Schnute, W. C., 2007, “Simultaneous Determination of Melamine and Cyanuric Acid Using LC-MS With the Acclaim Mixed-mode WAX-1 Column and Mass Spectrometric Detection,” Montreux Symposium, Dionex, Sunnyvale, CA, USA.
Dong, Y. P., Pei, L. Z., Chu, X. F., Zhang, W. B., and Zhang, Q. F., 2010, “Electrochemical Behavior of Cysteine at a CuGeO3 Nanowires Modified Glassy Carbon Electrode,” Electrochim. Acta, 55, pp. 5135–5141.
Pei, L. Z., Xie, Y. K., Cai, Z. Y., Yang, Y., Pei, Y. Q., Fan, C. G., and Fu, D. G., 2012, “Electrochemical Behaviors of Ascorbic Acid at Copper Germanate Nanowire Modified Electrode,” J. Electrochem. Soc., 159(3), pp. K55–K60.
Cai, Z. Y., Pei, L. Z., Yang, Y., Pei, Y. Q., Fan, C. G., and Fu, D. G., 2012, “Electrochemical Behavior of Tartaric Acid at CuGeO3 Nanowire Modified Glassy Carbon Electrode,” J. Solid State Electrochem., 16(6), pp. 2243–2249.
Pei, L. Z., Zhao, H. S., Tan, W., Yu, H. Y., Chen, Y. W., Zhang, Q. F., and Fan, C. G., 2009, “Low Temperature Growth and Characterizations of Single Crystalline CuGeO3 Nanowires,” Cryst. Eng. Comm., 8, pp. 1696–1701.
Hager, G., and Brolo, A. G., 2009, “Protonation and Deprotonation of Cysteine and Cystine Monolayers Probed by Impedance Spectroscopy,” J. Electroanal. Chem., 625, pp. 109–116.
Sehlotho, N., and Nyokong, T., 2006, “Electrocatalytic Oxidation of Thiocyanate, L-cysteine and 2-mercaptoethanol by Self-assembled Monolayer of Cobalt Tetraethoxy Thiophene Phthalocyanine,” Electrochim. Acta, 51, pp. 4463–4470.
Davis, J., Moorcroft, M., Wilkins, S. J., Compton, R. G., and Cardosi, M. F., 2000, “Electrochemical Detection of Nitrate and Nitrite at a Copper Modified Electrode,” Analyst, 125, pp. 737–742.
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## References

Sugita, T., Ishiwata, H., and Yoshihira, K., 1990, “Release of Formaldehyde and Melamine from Tableware made of Melamine—Formaldehyde Resin,” Food Addit. Contam., 7, pp. 21–27. [PubMed]
Brown, C. A., Jeong, K. S., Poppenga, R. H., Puschner, B., Miller, D. M., Ellis, A. E., Kang, K. I., Sum, S., Cistola, A. M., and Brown, S. A., 2007, “Outbreaks of Renal Failure Associated With Melamine and Cyanuric Acid in Dogs and Cats in 2004 and 2007,” J. Vet. Diagn. Invest., 19, pp. 525–531. [PubMed]
Perdigao, L. M. A., Champness, N. R., and Beton, P. H., 2006, “Surface Self-Assembly of the Cyanuric Acid–melamine Hydrogen Bonded Network,” Chem. Commun., 42, pp. 538–540.
Karbiwnyk, C. M., Andersen, W. C., Turnipseed, S. B., Storey, J. M., Madson, M. R., Miller, K. E., Gieseker, C. M., Miller, R. A., Rummel, N. G., and Reimschuessel, R., 2009, “Determination of Cyanuric Acid Residues in Catfish, Trout, Tilapia, Salmon and Shrimp by Liquid Chromatography-Tandem Mass Spectrometry,” Anal. Chim. Acta, 637, pp. 101–111. [PubMed]
U. S., Food and Drug Administration, 2007, “Pet Food Recall (Melamine)/Tainted Animal Feed,”
Landers, B. R., 2012, “Effects of Dietary Melamine and Cyanuric Acid in Young Pekin Ducks and Weanling Pigs,” A Thesis presented to the Faculty of the Graduate School University of Missouri-Colombia, Columbia, MO.
Hilts, C., and Pelletier, L., 2008, “Background Paper on Occurrence of Melamine in Foods and Feed. WHO Expert Meeting on Toxicological and Health Aspects of Melamine and Cyanuric Acid,” Health Canada, Ottawa, Canada.
Miao, H., Fan, S., Wu, Y. N., Zhang, L., Zhou, P. P., Li, J. G., Chen, H. J., and Zhao, Y. F., 2009, “Simultaneous Determination of Melamine, Ammelide, Ammeline, and Cyanuric Acid in Milk and Milk Products by Gas Chromatography-Tandem Mass Spectrometry,” Biomed. Environ. Sci., 22, pp. 87–94. [PubMed]
He, L. L., Liu, Y., Lin, M. S., Awika, J., Ledoux, D. R., Li, H., and Mustapha, A., 2008, “A New Approach to Measure Melamine, Cyanuric Acid, and Melamine Cyanurate Using Surface Enhanced Ramon Spectroscopy Coupled With Gold Nanosubstrates,” Sens. Instrum. Food Qual., 2, pp. 66–71.
Ehling, S., Tefera, S., and Ho, I. P., 2007, “High-Performance Liquid Chromatographic Method for the Simultaneous Detection of the Adulteration of Cereal Flours With Melamine and Related Triazine by-Products Ammeline, Ammedide, and Cyanuric Acid,” Food Addit. Contam., 24, pp. 1319–1325. [PubMed]
Heller, D. N., and Nochetto, C. B., 2008, “Simultaneous Determination and Confirmation of Melamine and Cyanuric Acid in Animal Feed by Zwitterionic Hydrophilic Interaction Chromatography and Tandem Mass Spectrometry,” Rapid Commun. Mass Spectrom., 22, pp. 3624–3632. [PubMed]
Wang, L., Henday, S. M., Liu, X. D., Tracy, M., and Schnute, W. C., 2007, “Simultaneous Determination of Melamine and Cyanuric Acid Using LC-MS With the Acclaim Mixed-mode WAX-1 Column and Mass Spectrometric Detection,” Montreux Symposium, Dionex, Sunnyvale, CA, USA.
Dong, Y. P., Pei, L. Z., Chu, X. F., Zhang, W. B., and Zhang, Q. F., 2010, “Electrochemical Behavior of Cysteine at a CuGeO3 Nanowires Modified Glassy Carbon Electrode,” Electrochim. Acta, 55, pp. 5135–5141.
Pei, L. Z., Xie, Y. K., Cai, Z. Y., Yang, Y., Pei, Y. Q., Fan, C. G., and Fu, D. G., 2012, “Electrochemical Behaviors of Ascorbic Acid at Copper Germanate Nanowire Modified Electrode,” J. Electrochem. Soc., 159(3), pp. K55–K60.
Cai, Z. Y., Pei, L. Z., Yang, Y., Pei, Y. Q., Fan, C. G., and Fu, D. G., 2012, “Electrochemical Behavior of Tartaric Acid at CuGeO3 Nanowire Modified Glassy Carbon Electrode,” J. Solid State Electrochem., 16(6), pp. 2243–2249.
Pei, L. Z., Zhao, H. S., Tan, W., Yu, H. Y., Chen, Y. W., Zhang, Q. F., and Fan, C. G., 2009, “Low Temperature Growth and Characterizations of Single Crystalline CuGeO3 Nanowires,” Cryst. Eng. Comm., 8, pp. 1696–1701.
Hager, G., and Brolo, A. G., 2009, “Protonation and Deprotonation of Cysteine and Cystine Monolayers Probed by Impedance Spectroscopy,” J. Electroanal. Chem., 625, pp. 109–116.
Sehlotho, N., and Nyokong, T., 2006, “Electrocatalytic Oxidation of Thiocyanate, L-cysteine and 2-mercaptoethanol by Self-assembled Monolayer of Cobalt Tetraethoxy Thiophene Phthalocyanine,” Electrochim. Acta, 51, pp. 4463–4470.
Davis, J., Moorcroft, M., Wilkins, S. J., Compton, R. G., and Cardosi, M. F., 2000, “Electrochemical Detection of Nitrate and Nitrite at a Copper Modified Electrode,” Analyst, 125, pp. 737–742.

## Figures

Fig. 1

Surface morphology of the CuGeO3 nanowire modified GCE

Fig. 2

CVs of the CuGeO3 nanowire modified GCE in 0.1 M KCl solution in absence (a) and presence (b) of 2 mM CA, scan rate, 50 mVs−1

Fig. 3

CVs of the CuGeO3 nanowire modified GCE in 2 mM CA and different electrolytes. Scan rate, 50 mVs−1. (a) KBr, (b) NaOH, (c) H2SO4, and (d) CH3COONa-CH3COOH.

Fig. 4

CVs of the CuGeO3 nanowire modified GCE in 0.1 M KCl and 2 mM CA solution using different scan rates. The inset in the bottom-left part is the calibration plots of the intensities of anodic peaks against the scan rate.

Fig. 5

CVs of the CA with different concentrations at the CuGeO3 nanowire modified GCE. KCl, 0.1 M, scan rate, 50 mVs−1. The inset in the bottom-left part is the calibration plots of the intensities of anodic peaks against the CA concentrations.

Fig. 6

CVs of the CuGeO3 nanowire modified GCE in 0.1 M KCl solution with 2 mM CA recycling for the 1st and 20th time. Scan rate, 50 mVs−1.

## Tables

Table 1 Analytical data of the CA
aWhere Ip and C represent the peak current (μA) and the concentration of the CA (mM).
bThe detection limit of the CA was analyzed using a signal-to-noise ratio of 3 (S/N = 3).
Table 2 Electrochemical determination of CA using CuGeO3 nanowire modified GCE in milk

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