Abstract
Introduction
Heavy metal pollution seriously affects human health. Mercury is one of the most hazardous pollution, it has been accumulated in the environment of the food chain, industrial waste and so on [1]. The adverse effects of the mercury on the enzymes in the living body result in strong toxicity, such as blindness, paralysis, nerve damage and seriously threatening the human health and animal life safety [2].
Since the mercury ion has the above hazards, the World Health Organization (WHO) and the Environmental Protection Agency (EPA) provide that the concentration of the mercury ions in the drinking water cannot exceed 2 ppb [3], so it is of great significance to study highly sensitive sensors to detect Hg2+ [4].
Electrochemical detection methods are cost-effective and miniaturizable with high sensitivity, selectivity, short analysis time, and low power consumption, so they are very attractive for trace analysis of heavy metals, making electrochemical method being emerged as a preferable detection means.
Recent trends in development of various nanomaterials modification electrode have increasingly due to their improvement of the analytical performance. SnO2 CQWs is a kind of semiconductor nanocrystals that are synthesized and dispersed in the solution. It can be deposited onto various working electrode at room temperature due to its excellent solution processability. In addition, its excellent crystallinity and high surface-to-volume ratio ensure a large number of active sites for the solid-ion interaction. On the other hand, Reduced graphene oxide (rGO) have been utilized for electrochemical sensor development, attribute to its high electrical conductivity and versatile surface modification. In order to enhance the electrochemical performance, it is extremely attractive to develop heavy metal ion detection sensors based on the synergistic effect of rGO and SnO2 by using the good charge transfer of SnO2/rGO nanocomposites interface and the excellent transfer capacity of rGO.
In this work, we believe that the benefits of SnO2/rGO nanocomposites for mercury ion detection can be improved by controlled material synthesis. We used a one-step colloidal synthesis scheme in which the morphology of SnO2/rGO nanocomposites was controlled by adjusting the concentration of rGO. The synthesized SnO2/rGO nanocomposites can be directly modified on the working electrode without further sintering due to its excellent solution processing properties. The SnO2/rGO nanocomposites modified electrode for trace mercury ion detection was studied by ASV method. It has a high sensitivity and a wide range of mercury ion detection linear ranges.
Method
For the synthesis of rGO, the graphene oxide (GO) synthesized from wormlike exfoliated graphite powder, were thermally reduced at 300 °C for 30 min in an argon atmosphere, resulting in the rGO nanosheets. In the one-step colloidal synthesis, typically, 0.6 g SnCl4·5H2O was dissolved into 20 mL of oleic acid (OA) and 2.5 mL of oleylamine to form a transparent solution. Then, 1.0-3.0 mg of rGO and 10 mL of ethanol were added with mild stirring. The mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave to react at 180oC for 3-8 h and then transferred to a cold water bath for cooling to room temperature. The product was rinsed with toluene and finally dispersed in ethanol at a concentration of 20 mg mL-1.
The bare sheet of screen printing electrode was prepared by printing gold paste on Al2O3 ceramic sheet and heat treatment at 600oC × 0.5 h. Subsequently, the electrode was rinsed with acetone solution, absolute ethanol in an ultrasonic bath for 15 min each wash. The SnO2/rGO nanocomposites was dripped on the working electrode of screen printing electrode (SPE), and the SnO2/rGO nanocomposites modified electrode was prepared after drying naturally.
The dissolution current of the modified electrode in different concentrations of Hg2+ solution was detected by ASV method. The ASV method was conducted in 0.1 M phosphate buffer solution (PBS) (Ph=6.0) in the range from -0.15 V to +0.4 V with optimized parameters: accumulation time: 120 s, deposition potential: -0.15 V.
Conclusions
In this paper, SnO2/rGO nanocomposites modified electrode was used to detect mercury ions. SnO2/rGO nanocomposites was synthesized via a simple solvothermal process and decorated on the working electrode. In addition to the excellent mercury ion adsorption of SnO2 quantum wires, the superior sensing performance of SnO2/rGO nanocomposites can be attributed to the enhanced electron transport resulting from the favorable charge transfer of SnO2/rGO nanocomposites interfaces and the superb transport capability of rGO. Determination of Hg2+ was performed in buffer solution (PH=6.0) using anodic stripping voltammetry (ASV). During deposition, the mercury ions in the solution were concentrated on the electrode surface and reduced to mercury metal. After the enrichment for a period of time, the working electrode voltage was scanned from the negative direction to the positive direction, so that the metal mercury that has been enriched on the electrode was oxidized and dissolved back into the solution, producing a dissolution current.The results showed that the sensor has much higher sensitivity and wider detect range.
References
[1] M Korbas,S R. Blechinger,P H, Krone. PNAS.30 (2008)12108.
[2] Ma F, Sun M, Sens Actuators B Chem. 209 (2015)377–383.
[3] Guidelines for drinking-water quality. WHO. (2004).
[4] Q Chen,X j Wu,D z, Wang. analyst. 136 (2011)2572.
Figure 1