Detection of Heavy Metal Ions Using a Long-Life Electrochemical Sensor Containing Conducting Polymer-Graphene Oxide Composite
Authors: Park Min-Ouk , Yoon Jang-Hee , Won Mi-Sook , Shim Yoon-Bo 
Published: February 2020

Exposure to high levels of trace heavy metal ions (HMIs, e.g., Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II)) poses a variety of health and environmental problems, because they tend to accumulate in the body and quality water, are toxic, and have a low rate of clearance. Therefore, it is critical to minimize the exposure of these contaminants to human and this can be ensured through effective water quality monitoring. Conductive polymers (CPs) have broad applications in various fields. We have effectively utilized various types of CPs for different electrocatalytic applications, such as polymer dyes for solar cell and sensor substrates [1-2]. To enhance the performance of the CPs, composition of graphene has been receiving much attention for sensor applications, due to their excellent conductivity because of π-π stacking and synergetic effects with other materials. Especially, one advantage of using a functionalized CP, aminopyrimidine terthiophene having selective metal-complexing property can be used to overcome the limitations of conventional stripping voltammetry that preconcentrates metal ions by applying a potential for a while. Thus, in the present work, a fast transient technique chronocoulometry (CC) is introduced for metal ions sensor.
The synthesized monomer(PATT) was reacted to the GO suspension, which was then sonicated to obtain a homogeneous mixture of PATT and GO suspension. An optimized ratio of PATT and GO was used to obtain a homogeneous solution without signicant agglomeration of the GO with the PATT. Electrochemical polymerization was achieved in an acetonitrile solution containing a mixture of PATT monomer and GO. The schematic presentation of the sensor probe fabrication is presented in Figure 1. SWASV requires a long analysis time due to slow scan rate, whereas, CC is advantageous in achieveing quick analysis of HMIs without a preconcentration process. In addition, being an integrated technique, CC shows less kinetic effect while applying potential and if in a low cost technique due to the simple design of electronics and algorithm as compared to SWASV. Although it cannot detect each HMIs in one scan, the CC can monitor the total amount of metal ions in one scan and also each metal species can be detected using the step by step scan. Therefore, the proposed sensor probe using CC method can be applicable in the real time point of care analysis.
The dynamic ranges of CC and SWASV were between 1.0 ppb and 10.0 ppm and between 1.0 ppb and 10.0 ppm, respectively, Here, the detection limits for Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) were 0.2 (±0.08), 0.8 (±0.20), 0.6 (±0.15), 0.5 (±0.12), and 0.1 (±0.05) ppb for the CC method without preconcentration, and 0.9 (±0.20), 0.6 (±0.15), 0.4 (±0.10), 1.0 (±0.25), and 0.7 (±0.15) ppb, respectively, for the SWASV method with 300 s of deposition time (n = 3). The reliability of the method for point of care analysis was evaluated by analyzing different water samples. The stability test was performed in a 1.0 ppm HMIs solution under the optimized pH and deposition time for a period of 3 months. The sensor lost only 1.7% of the initial response after approximately 45 continuous measurements. HMIs can selectively coordinate with nitrogen atom (-NH) of the pPATT ligand due to the complexation between metal ions and nitrogen compounds. In addition, negatively charged oxygenated groups of GO also interact with the HMIs. The proposed sensor was applied for detecting HMIs in real water (tap and local surface (Pusan National University, South Korea)), and the presence of metal ions was analyzed using the CC and SWASV methods. The observed values of Zn(II), Cd(II), Pb(II), Cu(II), and Hg(II) ions were 0.5 (± 0.1), 12.0 (± 0.4), 5.2 (± 0.3), 1.4 (± 0.2), and 0.9 (± 0.2) ppb using the CC method and 0.4 (± 0.1), 11.8 (± 0.5), 5.0 (± 0.2), 1.5 (± 0.6), and 10.7 (± 0.1) ppb, respectively, from SWASV method.
[1] Chandra, P.; Shim, Y.-B.; et al., Chem. Comm., 2012, 49, 1900-1902.
[2] Choi, S.-M.; Shim, Y.-B.; et al,. Anal. Chim. Acta., 2015, 892, 77-87.

Figure 1