Analysis on Electrochemical Property of Gold Electrode Modification for Water Quality Examination
Authors: Han Yun-Ying , Lee Pin-Syuan , Xie Qiuzhe , Chiang Hwey-Yu , Lin Chih-Ting 
Published: July 2021

Water plays an indispensable role in manufacturing industry. However, discharge of polluted water containing heavy metal ions often appears in streams and causes serious damage to creatures and environment. Therefore, water-quality monitoring is essential to ensure both developments and safeties. To address this need, different approaches of metal ion detection include spectrophotometry, conductivity test, membrane electrode method and atomic emission spectroscopy. Nevertheless, the accuracy and efficiency of these approaches could still be improved. For these reasons, this study aims to develop an innovative water quality examination technique by using electrochemical impedance spectroscopy (EIS) measurements. By improving the sensitivity and reproducibility of the developed approach, this technique could contribute to metal ion detection.
In this work, the main approach to detect the target ion, Cu2+, is surface modification for electrochemical impedance analysis method. To accomplish the development, first, the relationship between ion concentration and the impedance of solution was analyzed. CuCl2 solutions with a concentration between 10-2 to 10-6 M were used as samples for measurement. The experiment, in the Faradaic Mode, was carried out at a starting voltage of 0.34 V. The amplitude was 200 mV, with frequencies varied from 1 MHz to 1 Hz. The effect of temperature variations between 40 to 140 degree Celsius was also discussed. Then, the electrodes were respectively immersed in solutions which are 11-MUA (24 hours), 3-MPA (6 hours) and 2-thiobarbituric acid molecules (24 hours) to construct different exterior structures. Finally, histidine is connected to the electrode for capturing and recognizing copper ions [1].
With the modified electrode, the measured ion concentration was changed to 10-3 to 10-9 M. The same experimental parameters as mentioned above were used to measure Ca2+, Mg2+, Na+ and Cu2+, respectively. Also, the copper ion selectivity of each type of monolayers was evaluated. The diameter of the semicircle extrapolated in the Nyquist diagram represents the charge transfer resistance (Rct ) of the solution. In Faradaic mode, Rct should have a negative relationship with ion concentration [2]. By adopting the method proposed by Edward & Craig (2013), the copper ions can be identified by the Rct value obtained in this study.
According to experimental results illustrated in Fig. 1, the diameter of the impedance diagram becomes smaller as the concentration increases, meaning a decrease of the Rct. Besides, as the temperature increases, Rct decreases gradually (Fig. 2). On the other hand, there is no significant difference between the trend of Rct when using electrodes with different modification materials (Fig. 3). The detection limit of the bare electrode or modified with planar structured thiolates falls on 10-6 M. However, the detection limit of the electrode with 11-MUA or 3-MPA is approximately reduced to 10-8 M (Fig. 4). Compared to other ions, Rct of Ca2+ and Mg2+ in specific concentration performs similarly when using 11-MUA modified electrode. The correlation coefficient of the calibration relationship between Na+ concentration and the resistance are lower than those in Cu2+ detection (Fig. 5). Based on the above results, it is illustrated that MUA modified electrode performs the best ion selectivity in Cu2+ detection. Finally, for the MPA electrode, the quadratic calibration function between ion concentration and Rct of Cu2+ can also be established as shown in Fig. 6. Based on this experimental result, MPA modified electrode also has high selectivity to Cu2+.
In the study, it has been confirmed that the lower ion concentration results a higher impedance of the solution. Besides, the negative logarithm of the concentration (-log[Cu2+], pCu2+) is positively related to Rct in a quadratic form. Also, as the temperature increases, the ion migration rate becomes faster, and the Rct value gets lower; hence, the environmental temperature was controlled to be 25 degree Celsius during the experiments. By modifying 11-MUA and 3-MPA on the electrode, the measurement limit has been significantly reduced, verified by a smoother fitting trend. In conclusion, the proposed 11-MUA and 3-MPA modified approaches have a higher copper ion selectivity and concentration measurement accuracy. In other words, an efficient and sensitive ion detection approach becomes available for water quality examination by utilizing the method proposed in the study.
[1] Perrin D. D. (1959). Histidine-Copper (II) Complexes.
[2] Edward P, and Craig E (2013). Electrochemical impedance spectroscopy: an overview of bioanalytical applications, Analytical Methods, 2013(5), 1098–1115.

Figure 1