A Novel Electrochemical Sensor Based on Carbon Dots-Nafion Composite Modified Bismuth Film Electrode for Simultaneous Determination of Cd2+ and Pb2+

Abstract

Introduction Bismuth film electrodes (BiFEs) have become a hopeful alternative to mercury drop and film electrodes due to their non-toxicity and equal or even superior electrochemical performance towards heavy-metal-ion detection [1]. Meanwhile, various carbon nanomaterials (e.g., graphene oxide, carbon nanotubes, etc.) had been composited with bismuth for improving the sensitivity and reproducibility of BiFEs. However, one primary problem with the BiFEs is that the hydrophobicity of these carbon nanomaterials may have a negative impact on the performance of the electrodes in aqueous media.It has been reported that carbon dots (CDs), as a new hydrophilic carbon nanomaterial, may have similar properties to other carbon nanomaterials, thus CDs can be used as modified electrode materials to improve the performance of electrochemical sensors [2]. CDs have not been used for the enhancement of the electrochemical performance of bismuth-based electrodes. Due to the composited feasibility of CDs and bismuth, they have been successfully applied in photocatalysis[3] and energy storage[4]. Therefore, it would be of special potential to fabricate a novel bismuth-based electrode based on CDs for improving the electrode detection performance for heavy metals.The objective of this study is to develop inexpensive, highly selective, rapid and environmental friendly sensors for Cd2+ and Pb2+ detection. In this work, CDs, Nafion, and bismuth were deposited onto the glassy carbon electrodes (GCEs) sequentially to form a novel chemical sensor for Cd2+ and Pb2+ ions rapid detection. Methods CDs were prepared with a facile one-pot hydrothermal method using glucose as the carbon source[5]. To fabricate BiF/CDs/Nafion/GCE, the bare GCEs were polished well with a diamond suspension solution for the further modified. Then, the CDs were electrodeposited on the GCE using cyclic voltammetry in H2SO4 solution containing CDs. Finally, 5 μL of 1 wt.% Nafion film was used as an adhesive to stabilize the CDs and improve the reproducibility of the modified electrode. The obtained electrochemical sensor was abbreviated as CDs/Nafion/GCE. For comparison, Nafion/GCE was prepared using a similar method. Bismuth was in situ deposited onto the CDs/Nafion/GCE from non-deaerated HAc-NaAc (pH 4.5) buffer solution in the presence of 1000 μg L-1 Bi3+ with model target metals. A conventional three-electrode system was adopted to analyze Cd2+ and Pb2+ concentrations using an Autolab PGSTAT204 workstation (Metrohm). Conclusions Fig. 1 and Table 1 present the differential pulse voltammetry (DPV) electrochemical performance of bare GCE, Nafion/GCE, CDs/Nafion/GCE and BiF/Nafion/GCE in 0.1 M acetate buffer (pH 4.5) containing 0.25 μg mL-1 Cd2+ and Pb2+. The DPV curves of both bare GCE and Nafion/GCE display two weak stripping peaks for Cd2+ and Pb2+. After modification on the GCE surface with CDs-Nafion film, the stripping peak currents of Cd2+ and Pb2+ significantly increased, indicating that the CDs may have the ability to spur the electron transfer rate of the detection system [6]. More importantly, BiF/CDs/Nafion/GCE showed a higher sensitivity for simultaneous detection of Cd2+ and Pb2+ compared with bare GCE and CDs/Nafion/GCE. The Cd2+ current obtained with BiF/CDs/Nafion/GCE was 71.02 and 4.85 times higher than with bare GCE and CDs/Nafion/GCE, respectively. The Pb2+ current obtained with BiF/CDs/Nafion/GCE was 51.85 and 6.31 times higher than those with bare GCE and CDs/Nafion/GCE, respectively.Fig. 2 and Table 2 show the simultaneous stripping analysis of Cd2+ and Pb2+ using the BiF/CDs/Nafion/GCE. The DPV curves show individual peaks at approximate -0.75 V for Cd2+ and -0.55 V for Pb2+. The separation between two peaks was approximately 200 mV, a difference that is sufficient to detect Cd2+ and Pb2+ simultaneously and selectively. The inset in Fig. 2 presents the linear relationship between the peak currents and the concentrations of Cd2+ and Pb2+ (The linear equations: Ip=18.82C+3.49 (R2=0.994) for Cd2+ and Ip=25.12C+3.55 (R2=0.993) for Pb2+). The calculated detection limits (3δ method) for Cd2+ and Pb2+ are 0.003 and 0.002 mg L-1, respectively.The low detection limits suggest that the BiF/CDs/Nafion/GCE is a feasible sensor to simultaneously detect Cd2+ and Pb2+, which has great potential for application in the fields of environmental monitoring. Table 1. Electrochemical performances of the bare GCE, Nafion/GCE, CDs/Nafion/GCE and BiF/CDs/Nafion/GCE in simultaneous detection of Cd2+ and Pb2+. Bare GCE Nafion/GCE CDs/Nafion/GCE BiF/CDs/Nafion/GCE Cd2+ I/μA 0.116 0.045 1.697 8.239 E/V -0.782 -0777 -0.797 -0.772 Pb2+ I/μA 0.192 0.103 1.577 9.957 E/V -0.540 -0.525 -0.565 -0.520 Table 2. The analytical performance of BiF/CDs/Nafion/GCE in simultaneous detection of Cd2+ and Pb2+. Analytes Linear range (mg L-1) LOD (mg L-1) R2 Simultaneous analysis Cd2+ 0.05 - 0.50 0.003 0.994 Pb2+ 0.05 - 0.50 0.002 0.993 Keywords: Carbon points; Bismuth film; Cadmium; LeadFunding: This work was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (Project no. 2018ZX07109003).