Scientists at North Carolina State University have created a material that has a gradient of gold nanoparticles on a silica covered silicon substrate using a molecular template. This work provides the first evidence that nanoparticles can form a gradient of decreasing concentration on a given surface.
The first stage in preparing these materials involved depositing a very thin layer of organosilanes on the surface of the substrate. Organosilanes have a head and tail end. The head attached to the substrate, while the tail sticks out like a hook waiting to catch the gold nanopartiocles. The concentration of organosilanes was varied across the substrate by increasing the distance between the vapour source and the substrate.
The coated substrate was then dipped into a solution containing gold nanoparticles. In the solution, the organosilane tails took on a positive charge, while the gold nanoparticles were coated with a negatively charged chemical. In this way the gold nanoparticles attached to the organosilanes.
They used techniques such as atomic force microscopy (AFM) and near edge x-ray absorption fine structure (NEXAFS) to show that gold and organosilanes followed the same concentration gradient. They also showed that the particles followed a pre-designed chemical template provided by the organosilanes. This property will allow them to prepare gradient structures with varying properties.
Gradient structures have the advantage that large numbers of different structures can all be deposited onto a single substrate and simultaneously processed. This may have applications with chemists being able to test clusters of nanoparticles used catalysts.
Potential applications for these types of materials include sensors and filters as well as various uses in electronics, chemistry and life sciences.