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 its non-toxicity and equal or even superior electrochemical performance towards the heavy-metal-ion determination [1]. Meanwhile, various carbon nanomaterials such as graphene oxide, carbon nanotubes, and carbon nanofibers had been composited with bismuth to further improve the sensitivity and reproducibility of the bismuth-based electrode. In recent years, carbon dots (CDs), as a new carbon nanomaterial, have also been successfully composited with bismuth and applied in photocatalysis[2] and energy storage[3]. However, there have been less previous researches for the enhancement of electrochemical performance of bismuth-based electrodes by CDs. It is reported, CDs may have similarity properties to other carbon nanomaterials such as good electrical conductivity and a large active area, which could be used as modified electrode materials to improve the performance of electrochemical sensors [4]. Therefore, it would be of special interest to fabricate a novel bismuth-based electrode based on CDs for improving the electrode performance of Cd2+ and Pb2+ detection.In this work, CDs, Nafion, and bismuth were deposited onto the GCEs sequentially to form a novel chemical sensor for Cd2+ and Pb2+ ions rapid detection. It is hoping to develop an inexpensive, highly selective, rapid and environmental friendly sensors for Cd2+ and Pb2+ determination in the practical environment and agriculture samples. Methods CDs were prepared with a facile one-pot hydrothermal method using glucose as the carbon source. 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 modified electrode 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 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 bare GCE and Nafion/GCE both display two weak stripping peaks for Cd2+ and Pb2+, respectively. After modification on the GCE surface with CDs-Nafion film, the stripping peak currents of Cd2+ and Pb2+ significantly increased indicating the CDs may have the ability to spur the electron transfer rate of the detection system [5]. More importantly, BiF/CDs/Nafion/GCE exhibited increased 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 what was obtained 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 approximately -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 are 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 promise for application in the fields of agricultural and environmental monitoring. Table 1. Electrochemical performance of the bare GCE, Nafion/GCE, CDs/Nafion/GCE and BiF/CDs/Nafion/GCE in detecting 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 the 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).