Supplemental Information to:
A Electrochemical Sensor for Melamine Detection Based on Copper-Melamine Complex Using OMC Modified Glassy Carbon Electrode
Zhuo Guo*1, Yang-ting Zhao1, Ya-hui Li2, Tong Bao1, Tian-shuai Sun1, Dong-di Li1, Xian-ke Luo1and Hong-tao Fan3
1. Department of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
2. HarbinInstituteofTechnology, ShenzhenGraduateSchool, Shenzhen 518055，PeoplesRChina
3. Department of Applied Chemistry,Shenyang University of Chemical Technology, Shenyang 110142, China
Corresponding author:
Optimization of experimental variables
Effect of the electrolyte solutions
Voltammetric behaviors of 40µM Mel in neutral or weak alkaline electrolyte solutions, such as phosphate buffer solution, borate buffer solution and Tris-HCl buffer solution, have been investigated using CV at the OMC/GCE sensor. Experimental results (Fig.S1) have shown that in borate buffer solution, Cu-Mel exhibits well-defined electrochemical behaviors (Guo et al., 2014). Therefore borate buffer solution was chosen as the supporting electrolyte.
Fig.S1. Effect of the 0.1 M phosphate buffer solution (a), Tris-HCl buffer solution (b) and borate buffer solution (c) on the performance of OMC/GCE for 6.0 µM Mel containing 20% methanol (v/v). Scan rate: 50 mV/s.
Effect of pH
The influence of pH of borate buffer on current response of Mel at the OMC/GCE was investigated (Fig.S2) because pH of the buffer strongly affects the formation of Cu-Mel complex, thus influencing the condition of electrochemical analysis.The effect of pH was determined using Mel (6.0 µM) at the OMC/GCE (firstly dipping into CuCl2 solution for 180 s) in the pH range from 7.0 to 10.0. With the increase of pH value, the peak potential shifted negatively, indicating that protons were involved in the electrode reaction. The oxidation peak currents of Cu-Mel slightly increased following the increase of pH from 7.0-9.0. However, at pH over 9.0, the peak currents decreased, which is presumably because strong alkaline conditions would be detrimental for the dissolution of Cu2+ salts. The Cu-Mel exhibited the largest current response at pH 9.0, which was used as the optimum pH condition for the following experiments.
Fig.S2. Effect of the concentration of pH on the performance of OMC/GCE for 6.0 µM Mel in 0.1 M borate buffer containing 20% methanol (v/v).Scan rate:50 mV/s
Effect of Cu2+ accumulation time
Dependence of oxidation peak current of Cu-Mel on accumulation time of Cu2+ (40µM) was tested at this sensor (Fig. S3). The oxidation peak current of 6.0 µM Mel increases quickly from 0 to 150 s and slowly from 150 to 180 s. At an accumulation time of 180 s, peak current of Cu-Mel achieve the highest value. The calibration curve of Cu-Mel gradually tends to be plateau when further increasing the accumulation time, indicating that adsorption of Cu2+ on the sensor is saturated. So, the accumulation time of Cu2+ was selected as 180 s for the experiment.
Fig. S3. Effect of Cu2+ accumulation time on the performance of OMC/GCE for 6.0µM Mel in 0.1 M borate buffer containing 20% methanol(v/v). Scan rate:50 mV/s
Effect of Cu2+ concentration
The electrochemical behaviour of the sensor was found to be dependent on the concentration of Cu2+ employed. This is because the ability to bind Mel onto the coating film depends on the number of Cu2+ adsorbed on the OMC surface. To examine the effect of Cu2+ concentration on response to the sensor, a Mel solution with constant concentration was used, and the CV of this Mel solution was recorded with various Cu2+ concentration in a range of 10 to 100 µM by cycling the potential between -0.5 V to 1.5 V. From the results shown in Fig. S4, it was observed that the response of OMC/GCE to Mel increased with the increase of Cu2+ concentration up to 40 µM. The response then decreased when further increasing Cu2+ concentration. Higher concentration of Cu2+ might lead to thicken the film. At the same time, some of OMC pores would be occupied by Cu2+, which decreased the electro-catalysis performance of OMC. Therefore, 40 µM of CuCl2 was selected as the optimum concentration in this work.
Fig. S4. Effect of the concentration of Cu2+ on the performance of OMC/GCE for 6.0µM Mel in 0.1 M borate buffer containing 20% methanol(v/v). Scan rate:50 mV/s
References
Guo, Z., X.-f. Xu, J. Li, Y.-w. Liu, J. Zhang, and C. Yang. 2014. Ordered mesoporous carbon as electrode modification material for selective and sensitive electrochemical sensing of melamine. Sensors and Actuators B: Chemical 200:101-108.