Modelling Local pH Values and Ion Distributions near Gold Nanoparticles

Abstract: Corrosion is the sum of processes by which an element, commonly a metal, wears away, deteriorates [1]–[3]. One of these processes uses the principle of electrochemistry as a basis, being the transfer of electrons between one chemical reaction and the element that undergoes corrosion [4], [5]. One example of electrochemical corrosion in everyday life is steel corrosion [6]. Iron contains electrons that can be taken up by oxygen in an acidic environment through the following reaction: O2 + 2 H2O + 4 e-  4 OH-. This reaction taking up electrons is defined as a reduction. These electrons come from iron which releases them, this process being defined as an oxidation reaction: Fe(s)  Fe2+ + 2 e-. The resulting redox reaction is maintained by the movement of electrons between two poles, often electrodes, an anode where the oxidation occurs and a cathode where the reduction occurs [7]. Iron, which was more inert is now available as an ion for interacting elsewhere in its environment. As metals have an important place in our society, so does electrochemical corrosion. Being infrastructure, medicine, food storage or transportation, corrosion must be kept under control. For instance, corrosion of metallic biomaterials can severely compromise their biocompatibility [8], [9]. Research has led to various solutions to drastically reduce corrosion caused by external environments. Metals in trace amounts also have an important impact, particularly on health. Due to their size, they can easily be absorbed by living species; this can be beneficial for health purposes for instance but, if absorbed in excess, it can also lead to a panel of risks [10]–[12]. The metal used in this project is gold. Due to its properties and sensitivity to its environment as a nanoparticle, gold has proven to be beneficial, especially in life science and modern medicine [13]–[15]. However, drawbacks behind the intensive use of gold nanoparticles (GNP) are less known and more complex to study. The variation of the pH in the bulk solution has been studied for this project. The explored assumption is that corrosion of these nanoparticles is guided by changes of the bulk pH, which is often linked to electrochemical reactions, themselves described by the Nernst equation [16]–[18]. During corrosion, the surface pH changes. This is of interest in this project and has been indirectly monitored by both the bulk pH and the surface potential. Pourbaix diagram is an example of this link between potential and pH values: it focuses on the potential of the species of interest. Depending on their sizes and whether there is a current, pH should be calculated either at the particle surface or in the bulk solution. Indeed, the current density can translate a pH difference between the two positions. As the species react at their surface, pH value there can deviate a lot from the bulk pH value. To observe pH changes, Hydrogen Oxidation Reaction (HOR) has been selected as it leads to a H+ production: H2  2 H+ + 2 e-. It has often been used for this purpose [16], [17], [19], [20]. The concept behind this project originates from another study, “Near-surface ion distribution and buffer effects during electrochemical reactions”, studying HOR on a rotating Pt disc electrode [20]. Here, researchers have studied effects that the pH, represented in the Nernst equation, has on the current density near the electrode. The previous study has been adapted for this project by now studying pH effects on the current density near spherical GNPs. Additional studies such as Oxygen Reduction Reaction (ORR) and diffusion limitation from hydrogen- and oxygen supplies have been considered in the project. The simulations have been performed on MATLAB software.