Nanowire brushes – the electrodes of the future

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Scientists from the Electrochemistry Group of the Jagiellonian University have been working successfully for many years on the development of fast and cheap methods of fabricating a new type of electrode, based on the structures of metallic or polymeric nanowires.

Surprisingly, 1 cm2 of the surface of such an electrode contains over 10 billion single wires with diameters ranging from ten to several hundred nanometers (1 nanometer is one-billionth of a meter).

How to obtain a nanobrush?

Contrary to what might be expected, creating this type of material is not complicated at all. The first stage consists of subjecting a small piece of aluminum foil to controlled electrolysis, so-called anodization. As a result of this process, a layer of oxide (Al2O3) emerges on the surface of the metal, and its structure contains uniformly placed channels of a size ranging from several tens to several hundreds of nanometers. The sizes of the channels and the distance between them as well as the thickness of the layer of oxide obtained may be very precisely controlled by means of appropriate selection of electrolysis parameters.

As a result, a kind of nanoporous membrane is obtained, which in a further stage act as templates for the fabrication of nanowire structures. The next task is to fill the channels with suitable material – usually metal or polymer. In the final stage, the redundant layer of aluminum oxide is removed, resulting in a uniformly arranged structure of nanowires that resembles a brush.

In recent years, numerous electrodes of this type have been efficiently created at the Electrochemistry Group (Faculty of Chemistry at the Jagiellonian University), including both metallic ones (e.g., silver, gold, copper and antimony) and those based on nanowires made from conductive polymers (e.g., polypyrrole). Current research focuses on practical applications of nanostructured electrodes and proves that they are quite varied.

SEM images of nanostructured electrodes:
silver (A) and copper (B). The diameter
of a single wire is approximately 80 nm

Blood tests, water treatment and new batteries

Electrodes based on silver nanowires are successfully applied as electrochemical sensors used to determine levels of hydrogen peroxide (H2O2). Precise determination of even low amounts of this substance is essential from the medical point of view, as an increased level of hydrogen peroxide in the blood is observed in the course of certain illnesses, such as Parkinson's disease. Unfortunately, human blood is a complex system, as it is composed of numerous chemical compounds that could potentially interfere with the determination of the substance analyzed. However, it turned out that the application of an electrode in the form of a brush consisting of silver nanowires allows for the precise determination of the level of hydrogen peroxide regardless of the presence of vitamin C, uric acid or glucose in the analyzed sample. Recently, research has also been initiated on the application of nanostructured gold electrodes as sensors of adrenaline – a very important hormone, whose activity, manifested among others in the form of an increased heart rate, is felt by everybody who has ever been in a stressful situation.

Electrolysis – processes occurring as a result of the influence of electric current passing through a system consisting of electrodes and placed in a solution that conducts electricity. One of the types of electrolysis is electrocatalysis, which consists of the acceleration of the said processes through a strong interaction between the substances participating in the reaction with the surface.

The most important advantage of electrodes based on nanowire brushes is that their surface comes into direct contact with the electrolyte, which is significantly larger than that of "regular", non-nanostructured electrodes. This advantage is used, among others, in electrocatalysis. For example, silver electrodes are analyzed in the aspect of their application for the purposes of removing harmful chemical compounds from water. These compounds, so-called chlorinated hydrocarbons, penetrate into groundwater as a result of the application of pesticides and it is not at all easy to eliminate them. The application of an electrode in the form of a nanowire brush significantly accelerates the removal of these harmful substances.

Another area of application of nanowire brushes (mainly tin and antimony) refers to lithium-ion batteries, which are commonly used to supply power to most electronic devices: mobile phones, cameras and pacemakers. It proves that this type of electrode offers a much higher capacity than commonly used graphite electrodes. It is estimated that the application of material in the form of nanowires will enable the working time of devices powered by such batteries to be extended even several times over. Unfortunately, before these electrodes can be mass-produced, a series of practical problems connected with the rapid wear of the electrode material, which results in lowering the battery capacity, have to be solved. Work on developing an optimal structure is still in progress.

Electrodes made from polypyrrole nanowires are also very interesting. Polypyrrole is a long-chain organic compound (polymer), which, after appropriate modification, is characterized by conductivity similar to those of metals. At the Electrochemistry Group, an electrode was created based on polypyrrole nanowires modified by organic salts, of a diameter of approximately 80 nm, which was successfully used as a pH sensor. The examples presented herein only make up a portion of the issues currently being researched at the Electrochemistry Group. There are plenty of ideas for further research, including studies on the practical applications of electrodes in the form of nanowire brushes.

Research team from the Jagiellonian University: Professor Marian Jaskuła; Grzegorz Sulka, PhD; Leszek Zaraska, PhD; Joanna Kapusta-Kołodziej, MSc; Agnieszka Brzózka, PhD; Katarzyna Hnida, MSc Eng.; Magdalena Jarosz, MSc; Elżbieta Kurowska, MSc; Ewa Wierzbicka, MSc; Karolina Syrek, MSc; Anna Brudzisz, MSc