One of the most popular methods of covering surfaces with nanoparticles is the Layer-by-Layer method (LbL) known since 1997.
According to this method, a substrate is covered with subsequent layers of objects with opposite electric charges. This method is applied in particular to create three-dimensional structures made of polymers only or alternating layers of polymers and nanoparticles on the surface of electrodes. "From some time it has been known that many electrode reactions proceed faster, more efficiently and selectively on surfaces covered by, for instance, nanoparticles of gold or carbon. So, we decided to construct structures consisting of nanoparticles only and examine how they affect the properties of electrodes after they have been further modified by enzymes," says Prof. Marcin Opa³³o from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS).
Electrodes covered by thin layers of carbon nanoparticles could be applied to, among other things, biological fuel cells used as sources of power for medical devices placed in human body. Currently, the replacement of power sources in such devices like pacemakers requires invasive methods. For years, all over the world works have been carried out, aimed at creating a cell fuelled by a substance dissolved in blood, for example, glucose and an oxidizing agent – oxygen, which is also in blood. The task is difficult because a conductive support must be found which will allow the permanent deposition of enzyme in such a way that it would exchange electrons directly with the electrode. This was finally achieved in the IPC PAS thanks to depositing carbon nanoparticles on the electrode. "The result is surprising because enzyme usually requires additional compounds – electron shuttles dissolved in the solution. There are simply no such compounds in our experiments," says Katarzyna Szot, a PhD student from the IPC PAS. It is particularly important that the electrodes being examined work in the solutions that have similar components as blood plasma.
Carbon nanoparticles used in the experiments carried out in the IPC PAS are smaller than 10 nanometers. In the context of future applications it is important that they are cheap and easily accessible. "At first, we wanted to buy nanoparticles but it turned out that such small quantities that we needed are not traded. For example, Mercedes uses such nanoparticles on a mass scale to paint car bodies. Eventually, we received one kilogram of the material, free of charge, from Cabot Corporation,” laughs Prof. Opa³³o. The process of covering the substrate – in the experiments these are glass plates with a layer of an electric conductor – is simple and quick. The plate is immersed for one minute in the suspension of carbon nanoparticles, then it is taken out, rinsed, moved to another suspension to deposit a subsequent layer, and all the actions are repeated several times. The finished carbon layers are about 100 nanometers thick.
As carbon nanoparticles themselves are small, spaces between them are also very small, which makes the access to active surfaces located deeper in the layer more difficult. As a result the electrodes are not as efficient as they could be. In the future it will probably be possible to improve their properties due to the modification of the process of layer depositing. In the Institute of Physical Chemistry of the PAS experiments are just beginning on the application of carbon nanoparticles layers in the presence of small polystyrene balls with diameters of several hundred nanometers. After each layer has been created, scientists intend to coat it with polymer in order to strengthen the structure mechanically and then wash away the balls. The multilayer structure obtained thanks to this method would have larger pores facilitating access of oxygen, which would result in the increased reaction efficiency.
Electrochemical sensors will probably become another area where electrodes with carbon nanoparticles layers can be applied, e.g. to determine the level of dopamine in relation to ascorbic acid and uric acid. This problem is considered serious in analytical chemistry since the two last substances hinder the analysis of samples due to the overlapping of their electrochemical signals. Covering the electrodes with nanoparticles allows the signals to be separated and sensitivity to be increased.