Laser-controlled corrosion protection with 'smart' nanomaterials
(Nanowerk Spotlight) About a year ago we reported on self-healing nanotechnology anticorrosion coatings, a novel method of multilayer anticorrosion protection including the surface pre-treatment by sonication and deposition of polyelectrolytes and inhibitors, developed by researchers at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany. The main novelty of the proposed system was the multi-level protection approach, where the protective systems – the 'smart' multilayers – will not only be a barrier to external impacts, but also respond to changes in their internal structure, and combine in the same system different damage prevention and reparation mechanisms.
Led again by Dmitry Shchukin and Helmuth Möhwald, the Max Planck team now reports the development of laser-activated nanocontainers filled with corrosion inhibitors. With this new nanomaterial, the healing ability of anticorrosion agents is remotely activated by light.
Using laser technology, the method relies on making sub-micron and nanocontainers sensitive to light by doping them with metal nanoparticles or organic dyes. Upon illumination with laser light, the absorption centers locally disrupt the shells of the containers, thus increasing the permeability of the container walls. The corrosion inhibitor stored in the container is then released, covering and healing the corrosion area.
To heal the corrosion centers in a sol gel film after their appearance, the Max Planck researchers synthesized two types of mesoporous containers – submicrometer silica and nanosized titania – with corrosion inhibitor (benzotriazole) entrapped inside the pore volume. The advantage of the submicron-size one is the higher quantity of corrosion inhibitor, which could be incorporated into free pore volume; the advantage of the nanoscale one is the smaller container size which allows their incorporation into very thin films.
Scheme of the inhibitor release after local laser irradiation explaining the healing of the defect. (Reprinted with permission from American Chemical Society)
As in the work we described previously, the team used a layer-by-layer (LbL) deposition technique to fabricate their coating. The LbL process involves the stepwise electrostatic assembly of oppositely charged species (e.g., polyelectrolytes and inhibitors or nanoparticles) on a substrate surface with nanometer-scale precision, and allows the formation of a coating with multiple functionality.
The scientists explain that this approach yields reservoirs with controllable storage/release properties because the permeability of the polyelectrolyte shell can be changed by varying pH, ionic strength of the medium, or other physical or chemical influence.
"Selective and variable permeability of polyelectrolyte containers toward large organic molecules, polymer molecules, and nanoparticles makes them an efficient transport tool for protection, delivery, and storage of active species and substances with unstable formulation," explain Shchukin and Möhwald. "The reinforcement of polyelectrolyte walls with noble metal nanoparticles imparts them sensitivity in the visible and near-infrared spectral regions and, in turn, to exert remote control over their release properties."
The laser effect on the containers works like this: the surface plasmon resonance of noble metal nanoparticles is located in the visible and near-infrared part of the spectrum and these nanoparticles serve as absorption centers for energy supplied by a laser beam. When laser light hits the container walls, the light energy is converted into heat energy. This heat absorption by the nanoparticles causes local heating that disrupts the local polymer shell and allows the loaded corrosion inhibitor to leave the containers.
The interaction of laser light with the absorption centers can be controlled by varying two parameters: the size of the nanoparticles and their concentration.
For real life applications, Shchukin and Möhwald anticipate that activation of the nanocontainers could be performed without further inspection: "The surface of the material could be periodically exposed to laser light. If a scratch or other defects occur and the surface becomes prone to corrosion, then illumination by laser light would ensure that these defects have been cured. In this regard, such coatings behave 'smartly' because they initiate triggering of release at defined surface areas including the corrosion pit; otherwise, the corrosion protection containers just stay non-activated under the coating and do not initiate any action."
Remote laser-induced release demonstrates a promising method of controlling the properties of substrate coatings while at the same time improving their performance. The ability to remotely release the loaded material is also important in delivery of chemicals and in biomedical application areas.
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