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The Statesman



    Scientists have created an innovative material with a gradient of gold nano-particleson a silica covered silicon surface using a molecular template. The material,which was developed at North Carolina State University (NCSU) and tested atthe National Synchrotron Light Source (NSLS) at the U.S. Department of Energy’#146;sBrookhaven National Laboratory (BNL), provides the first evidence that nano-particles,each about one thousand times smaller than the diameter of a human hair, canform a gradient of decreasing concentration along a surface.

    ‘This material promises to be the first in a series with many applicationsin electronics, chemistry, and the life sciences,’ said Rajendra Bhat,a graduate student from North Carolina State University (NCSU) and the leadauthor of the study.

    Bhat worked with Jan Genzer, a chemical engineering professor at NCSU, andDaniel Fischer, a physicist from the U.S. Department of Commerce’#146;s NationalInstitute of Standards and Technology (NIST).

    In order to build the material, the researchers first prepared a very thinlayer of organosilanes, sticky molecules with a head and a tail, on a rectangularsurface of silica. According to Genzer, leader of the NCSU, the head glues tothe surface, while the tail sticks out, acting like a hook waiting for a goldnano-particle to attach to it.

    The molecules, emitted vertically in the form of a vapor by a source closeto one side of the surface, slowly fell on it with decreasing concentrationas the distance from the source increased, thus creating a gradient to serveas a molecular template.

    ‘The distinguishing feature of our approach is that the particles followa pre-designed chemical template provided by the organosilane sticky groups,’said Genzer. ‘The ability to manipulate the underlying template allowsus to prepare gradient structures of nano-particles with varying characteristics.’

    The next step was to dip the material in a solution containing the gold nano-particles,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 negativelycharged gold particles attached to the oppositely charged tails underneath.

    To visualize the gradient of gold particles, Bhat and his colleagues used anatomic force microscope, in which a tiny needle moves along the surface, followingits bumps and valleys to reveal its topography. To look at the gradient of theorganosilane molecules, the scientists used a technique called Near-Edge X-rayAbsorption Fine Structure (NEXAFS).

    In NEXAFS, extremely intense x-ray light is sent toward the material, and theelectrons emitted by the material and collected with a sensitive detector provideinformation about the concentration of the organosilane molecules on the surface.

    ‘We needed to confirm that both the gold particles and the sticky groupsfollowed the same underlying gradient template,’ Bhat said. ‘The resultsfrom both techniques were expected to coincide if the particles were attachingto the underlying layer of sticky molecules. Our results show exactly that.’

    According to Fischer, the gradient structure offers the advantage of havinga great number of structures that may combine on a single substrate and usedfor high-throughput processing. This may save chemists who are testing clustersof nano-particles as catalysts.

    These chemicals are actively being sought by the chemical industry to createnew, less polluting energy sources. The material could also be used as a sensorto detect species that have specific affinities for nano-particles or as a filterto select particles of given sizes.

    Bhat and his colleagues are now exploring the properties of similar materials,with different ‘sticky’ substances and nano-particles. ‘Thisresearch is so new that we are still thinking of potential applications forthese materials,’ he said.

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