For the first time, scientists have created a material with a gradient of gold nanoparticles on a silica covered silicon surface using a molecular template.
The material, which was developed at North Carolina State University (NCSU) and tested at the National Synchrotron Light Source (NSLS) at the US Department of Energy’s Brookhaven National Laboratory, is said to provide the first evidence that nanoparticles can form a gradient of decreasing concentration along a surface. A description of the material appears as the cover story in the July 23 issue of Langmuir.
‘This material promises to be the first in a series with many applications in electronics, chemistry, and the life sciences,’ said Rajendra Bhat, a Ph.D. student from North Carolina State University (NCSU) and the lead author of the study.
To build the material, the scientists first prepared a thin layer of organosilanes, sticky molecules with a head and a tail, on a rectangular surface of silica. The head glues to the surface, while the tail sticks out, acting like a hook waiting for a gold nanoparticle to attach to it, explained Jan Genzer, a chemical engineering professor at NCSU and leader of the NCSU team.
The molecules, emitted vertically in the form of a vapour by a source close to one side of the surface, slowly fell on it with decreasing concentration as the distance from the source increased. This created a gradient to serve as a molecular template.
The next step was to dip the material in a solution containing the gold nanoparticles, each of which was coated with a negatively charged chemical. In the solution, the tails of the organosilane molecules took on a positive charge, so the negatively charged gold particles attached to the oppositely charged tails underneath.
To visualise the gradient of gold particles, Bhat and his colleagues used an atomic force microscope, in which a tiny needle moves along the surface, following its bumps and valleys to reveal its topography.
To look at the gradient of the organosilane molecules, the scientists used near-edge x-ray absorption fine structure (NEXAFS). In NEXAFS, extremely intense x-ray light is sent toward the material, and the electrons emitted by the material and collected with a sensitive detector provide information about the concentration of the organosilane molecules on the surface.
‘We needed to confirm that both the gold particles and the sticky groups followed the same underlying gradient template,’ Bhat said. ‘The results from both techniques were expected to coincide if the particles were attaching to the underlying layer of sticky molecules. Our results show exactly that.’
‘The distinguishing feature of our approach is that the particles follow a pre-designed chemical template provided by the organosilane sticky groups,’ said Genzer. ‘The ability to manipulate the underlying template allows us to prepare gradient structures of nanoparticles with varying characteristics.’
The main advantage of the gradient structure is that large numbers of structures can be combined on a single substrate and used for high-throughput processing.
It might, for example, save time for chemists testing clusters of nanoparticles used as catalysts – chemicals actively sought by the chemical industry to create new, less polluting sources of energy.
Clusters made of different numbers of nanoparticles could be put on a single surface, and scientists could test this surface only once in a chemical reaction, instead of having to run each cluster separately through the reaction.
The material could also be used as a sensor to detect species that have specific affinities for nanoparticles, or as a filter to select particles of given sizes.