Professor Willem Mulder

A group of scientists in Spain, Argentina and at the UWI decided to look at the properties of metals that are used as electrodes in scientific experiments. Some of these experiments include monitoring of composition of chemicals in solution and monitoring the change in composition of a solution. These solutions could be blood, water or other liquids. The range of these applications can be greatly extended by coating these electrodes with a very thin film consisting of chain-like organic molecules that involve compounds that respond to the composition of the fluid.

Publication by Willem Mulder et al
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WHAT IS BIOMIMETICS?

The modified electrodes have also found wide application in biological studies as they can be used to mimic the transport of charged particles across biological cell membranes. This particular application goes by the name biomimetics.

NEW METHODS OF MAKING THESE ELECTRODES

The researchers developed a novel method of forming densely packed organic films consisting of hydrocarbon chain molecules with a sulphur atom at one end of each molecule, through which the molecules can attach themselves to the electrode. At the other end is a group of atoms with variable composition which is in direct contact with an electrolyte solution. It is the properties of this second group of atoms that determine the characteristics of the coated electrode. Organic thin films of this nature are commonly referred to as “self-assembled monolayers” (SAMs), and there thickness is equal to the length of one molecule, typically in the order of nanometers (1 nm = 0.000000001 meter). The method of forming these films is based on a process in which the organic molecules are deposited on to gold from a vapour phase which produces a layer at a density not previously attained.

UWI’S CONTRIBUTION

A model system was investigated for testing certain theoretical predictions, based on a mathematical description that was developed at UWI, about the dependence of the voltage difference between the modified electrode and a standard reference electrode on environmental parameters such as temperature, salt concentration and the acidity or alkalinity (pH) of the solution. The surface film consisted of acetic acid, the main ingredient of vinegar, anchored to the surface via hydrocarbon chains. In solution, a fraction of the acidic groups loses its protons, the positively charged building blocks of atomic nuclei, leaving behind a negative charge on the film. In addition, there is usually a net charge present on the metal itself due to an excess or deficit of electrons, the elementary negative charges. If the combined charge is negative, positively charged salt ions from solution will accumulate near the surface while the negatively charged ones will be repelled, thus creating a diffuse charge or “electrical double layer” in the solution next to the film which is the source of the measured voltage. The interface between the electrode and the electrolyte solution, which typically contains salt and acids or alkalis, has the capacity to store electric charge. This capacity depends on voltage in a manner that depends on the composition of the film.

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NEW METHODOLOGY

A simple technique was developed to determine the voltage at which no charge is present on the metal, the so-called “potential of zero charge”. A jet of electrolyte solution was splashed against the electrode that was held at fixed potential, thus closing the cell circuit and allowing the detection of weak currents as the surface capacitor was charged. The voltage was then tuned until this charging current could no longer be detected.

In sufficiently alkaline solutions, when most protons are stripped from the acidic groups, the surface charge can reach values that would not be attainable at bare electrode surfaces without causing oxidation of the electrode or reduction of water. The resulting electric field in the solution close to the film may become extremely strong, to the point where the water molecules become fully aligned with that field. This phenomenon is known as “dielectric saturation”.

HOW TO MAKE SENSE OF THE MEASUREMENTS

The mathematical model for these modified electrodes that was developed at UWI accounts for this dielectric saturation, an effect that is usually not considered important, but which was shown to contribute significantly to the measured voltage.

Analysis of our data in light of the model established the fact that the affinity of the acidic groups that are anchored to a metal surface does not depend on the hydrocarbon chain length, and is not significantly different from that of acetic acid, something that until recently was still a matter of debate.

The fundamental understanding of surface structure and properties furnished by the theory will provide a useful conceptual and quantitative framework to tailor surface properties from pre-designed molecular building blocks, as well as a meaningful and reliable interpretation of electrochemical data (currents, voltages) obtained with modified electrodes.

NEW METHODOLOGY

Thus, the electrochemical method has been established as reliable and useful for the study of surface phenomena, and provides important fundamental information, that can be used, for example, in the development of optoelectronic devices and in biosensor technology, e.g. the use of SAMs as platforms for the immobilization of biomolecules such as enzymes, or the detection of immune responses by so- called antigen-specific T-cells, which emit protons when activated, by converting chemical stimuli into electrical currents.