Skip to main content

Direct measurements of surface potential and charge of a single mineral nanoparticle in water by particle-impact nanoelectrochemistry

Final Report Summary - PARTICLE IMPACT (Direct measurements of surface potential and charge of a single mineral nanoparticle in water by particle-impact nanoelectrochemistry)

The main objective of the project was to study fundamental electrochemical properties of the mineral/water interface at the single nanoparticle level using the particle-impact chronoamperometry. This subject represented an important area of research owing to the growing interests in the special properties of mineral surfaces and because of the lack of a method that allows characterization of individual mineral nanoparticles. The technique used in this project records current responses resulting from random collision of nanoparticles, driven by Brownian motion, with a stationary electrode which is polarized to realize an electrochemical reaction on or of individual nanoparticles. This was particularly well suited for this project because probing nano-particulates in situ was the only way of gaining insights into the intrinsic reactivity of minerals. In addition to the research aspect, this project held a personal significance to the fellow in terms of the acquisition of essential independent research maturity and effective project management skills in order to pursue a University Professorship.

The first project investigated the intrinsic electrochemical properties of single hematite (α-Fe2O3) nanoparticles, in which the fundamental two-electron reduction of the mineral to ferrous ion was explored. Because this reduction reaction is strongly affected by the presence of adsorbates, such as pH buffers, and protons, the conventional analytical approach like cyclic voltammetry could not accurately investigate the intrinsic reaction kinetics and thermodynamics. During the particle impact analysis, the random collision between individual nanoparticles suspended in an aqueous solution and a stationary microelectrode was recorded by a potentiostat as transient current responses or “spikes” in a chronoamperogram. The integrated charge under a spike confirmed that an entire single nanoparticle was reduced upon impact at sufficiently negative potentials. This analysis showed the individual mineral particles could be electrochemically reduced at potential range that is in good agreement with the Pourbaix diagram of iron. Such observation could not be made using the conventional approach, in which particles were clustered and immobilized on a stationary electrode, because proton depletion markedly shifts the reduction potential. We further discovered that the first electron transfer process was the rate determining step and that ferrous ion rapidly diffused away from the particle surface. This work provided significant advances in understanding electrochemical reactivity of single hematite nanoparticles. (K. Shimizu, K. Tschulik, R.G. Compton, Chem. Sci., 2016, 7, 1408)

The previous project was extended to investigate the reversible cluster formation of nanoparticles, or agglomeration. Although it is a naturally occurring physicochemical process, investigating the dynamic agglomeration equilibria has been challenging without an appropriate in-situ analytical method. We investigated the agglomeration of hematite nanoparticles by the particle-impact chronoamperometry at pH values between 2.0 and 4.0. It was found that the size range of impacting particles did not differ significantly with pH indicating that monomeric particles, rather than clusters, were responsible for the observed collisions with the electrode. This observation confirmed that the agglomeration/dis-agglomeration process was both rapid and reversible. A shift of the agglomeration equilibria towards cluster formation was made apparent by the number of observed spikes per measurement which decreased as pH of the solution was increased. We further observed that effect of particle size on cluster formation that is consistent with the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. This agreement with a widely accepted theory for colloidal behavior further demonstrates the applicability of the particle-impact technique to study the properties of mineral nanoparticles at the individual level. (K. Shimizu, S.V. Sokolov, R.G. Compton, Colloid Inter. Sci. Commun., 2016,13, 19)

Another project that was explored, building on the previous work, was the electrochemical determination of the degree of cluster formation of rutile (TiO2) nanoparticles in aqueous solution using water soluble Alizarin Red S as a redox active tagging molecule. The mineral investigated covers several key interests as it is electrically inert, has large commercial application, is highly technologically important, and that the environmental impact has not yet fully investigated. As the presently available instruments for determination of the cluster size would require sample pre-treatment such as dilution which disrupts the fundamental nature of colloids, the particle-impact method was a suitable alternative to gain an insight into the intrinsic nature of rutile nanoparticles in the aquatic environment. Experiment was conducted in an approximated natural aquatic condition, which contained ca. 3 g L-1 of dye modified rutile nanoparticles roughly equivalent to 5 × 10-7 particle L-1 corresponding to nature values but 100 times higher in concentration than conventional light scattering techniques are capable of analyzing. An electrochemical signal was generated from the Faradaic reduction of the adsorbed dye molecule as a cluster collided with electrode. The particle-impact experiment found the degree of cluster formation of the dye modified rutile nanoparticle to be ca. 91 monomers (which is approximately equivalent to a spherical size of ca. 754 nm in diameter whereas a single monomer was ca. 167 nm in diameter). This finding was considerably different from the conventional analysis, which detected no significant cluster formation, using a much diluted sample due to the reversible nature of cluster formation. This project highlighted the complexity of the clustering process and the necessity of application of multiple characterization techniques in order to accurately characterize the system. Furthermore, it demonstrated the effectiveness of the particle-impact method for in situ nanoparticle analysis in the natural aquatic environment. (K. Shimizu, S.V. Sokolov, R.G. Compton, Phys. Chem. Chem. Phys. Accepted Manuscript).

The project was further extended to demonstrate the applicability of the particle-impact approach for a detection of polyethylene micro-particles in aqueous solution. Today, there are growing concerns for small plastic debris for they could act as absorbents for persistent organic pollutants increasing the risk of bio-accumulation as they are ingested by aquatic organisms. In current practice, debris is extracted from a sample by density separation and identified using optical/spectroscopic microscopes. However, it is not capable of detecting the small micro-plastic particles (< 10 μm) and is not an environmentally and economically sustainable approach. The particle-impact approach was able to accurately identify particle size distribution and concentration for micro-particles with sizes of 1–10 μm in situ. Furthermore, it was found that the frequency of spikes which appeared during measurement was directly proportional to the particle concentration for the size of 1-4 μm for a particle concentration range of over three magnitudes. The particle-impact could be a promising new approach to detect and characterize the micro-plastics. (K. Shimizu, S.V. Sokolov, R.G. Compton, Article in Preparation).

Several other projects were investigated through collaboration with colleagues within the host group. These included a hematite/glassy carbon bifunctional catalyst system combining chemical and electrochemical reactions for the oxygen reduction reaction (K. Shimizu, L. Sepunaru, R.G. Compton, Chem. Sci., 2016, 7, 3364) and a simple, sensitive, and rapid cyclic voltammetric detection of captopril, cysteine, and glutathione in an electrolyte solution containing copper(II) ion using a bare glassy carbon electrode (M.C.C. Areias, K. Shimizu, R.G. Compton, Analyst, 2016, 141, 2904; ibid., 5563; Electroanalysis, 2016, 7, 1524).

On the training aspects, the fellow regularly met with the scientist-in-charge, Prof. Compton whose guidance was precise, concise, and highly effective for the fellow to complete his tasks. Furthermore, regular communication with Prof. Compton was an indispensable part of the fellow’s training activity towards independent research maturity to pursue a University Professorship. Prof. Compton has created an open communication atmosphere in the host group, which was embraced by all group members and visiting scholars. This enabled the fellow to establish number of working collaborations that are important in the fellow’s future career prospects as well as networking.