Development of Bismuth/Graphene Nanocomposite-Modified Electrodes and Their Application for Electrochemical Detection of Trace Heavy Metal Ions

Abstract

Heavy metals are considered one of the main sources of environmental pollution. Their main source is industrial activity, which directly or indirectly discharge the heavy metal species into the environment. Certain heavy metals (e.g., mercury, cadmium, chromium, and lead) are highly toxic and affect several human organs, even at trace levels. For example, mercury and lead are known to cause damage to the nervous system, cadmium causes kidney and bone disease, and excessive absorption of zinc affects multiple aspects of the immune system. Heavy metal pollution is of great concern for global sustainability. It is therefore essential to monitor heavy metals in the environment, drinking water, food, and biological fluids. Traditional analytical methods include atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled atomic emission spectrometry (ICP-AES). Although these techniques are highly sensitive and selective, they require laborious pre-treatment processes, expensive instruments, and professional personnel. In contrast, analytical electrochemical stripping techniques have attracted great interest in the detection of heavy metals. Owing to properties including short analytical time, low cost, and high sensitivity, electrochemical detection methods have been widely recognized as powerful techniques for the detection of heavy metals. In particular, anodic stripping voltammetry (ASV), a technique that consists of deposition and stripping steps, is used very frequently. Mercury electrodes have proven to be valuable tools for this technique. However, considering their toxicity and the difficulty involved in the handling of mercury, these electrodes have recently been replaced by mercury-free electrodes. Several elements have been tested for their capacity to replace mercury (e.g., bismuth, gold, silver, antimony, and carbon). In recent years, bismuth electrodes have been attracting increasing attention in ASV analysis. The remarkable stripping performance of bismuth electrodes is attributed to the ability of bismuth to form fused alloys with heavy metals. These alloys are analogous to the amalgams formed by mercury. Additionally, because of their fast electron transfer rate, unique structural characteristics, and high surface area, carbon-based nanomaterials are employed extensively as sensing materials to detect heavy metals. This dissertation aims at presenting the fabrication of bismuth-based nanocomposites that can be employed in the development of electrochemical sensors for the detection of heavy metals. Firstly, chemically activated reduced graphene (AG) was used with Nafion as a sensing material for the first time. This material was placed on a glassy carbon electrode (GCE, modified by in-situ-deposited bismuth) to fabricate an electrochemical platform for the simultaneous and individual determination of three heavy metals in solution. Next, the graphene was prepared by electrochemical deposition directly onto a GCE surface. The bismuth film was then deposited in situ on the surface of the electrochemical sensor electrode that was used for sensitive determination of trace heavy metals. Finally, an iron oxide/graphene nanocomposite was directly produced by heat treatment of a mixture of iron oleate and graphene. The GCE was modified with the as-prepared nanocomposite, and employed as an electrochemical sensing platform for sensitive analysis of heavy metal ions. The generated iron oxide nanoparticles were homogeneously embedded in the graphene layers. These act as mutual spacers in the nanocomposite, preventing restacking of the graphene, and enhancing the detection sensitivity towards heavy metals. These bismuth-based nanocomposites enable the electrochemical sensing of heavy metals with graphene- or nanoparticle-modified electrodes. The results suggest that these nanocomposites can improve the sensitivity and stability of the system.