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Nanotechnology 'pencil sharpeners' add to researchers' nanofabrication toolbox

(Nanowerk Spotlight) Proponents of 'atomically precise manufacturing' and 'molecular manufacturing' love to talk about the mind-boggling possibilities that these technologies would offer. These visions range from the modest, such as improved materials and more efficient production methods for chemicals (already on the horizon), to the outrageous, such as molecular desktop fabs (far, far out). Articles about revolutionary nanotechnology almost always skip the hard part, i.e. the tremendous amount of research breakthroughs that are required to get from where we are today to the promised land. It's as if some flight enthusiasts in 1903, when Orville Wright took off on the first powered flight, would have debated first class cabin design of 600-seat jet airliners flying New York - Tokyo non-stop. Nanotechnology researchers today are still struggling with very basic problems such as being able to control the synthesis of nanoparticles. It might come as a bit of a shock to the nanotechnology enthusiasts among you, but even something utterly fundamental as synthesizing metal nanoparticles today is more of an empirical trial and error approach than a predictable, fully controlled process. Let's take a look at new research coming out of a Spanish university that exemplifies the tiny, yet elementary, steps researchers need to take to master the nanoscale.
"We have been working for a long time on metal nanoparticle synthesis, with the aim of understanding and manipulating their optical response, based on surface plasmon resonances" Dr. Luis M. Liz-Marzán tells Nanowerk. "Our current work focuses on the search for mechanistic insights and correlating morphological control with optical manipulation."
Liz-Marzán is a professor in the Department of Physical Chemistry at the University of Vigo in Spain. In a new paper published in Angewandte Chemie International Edition ("Chemical Sharpening of Gold Nanorods: The Rod-to-Octahedron Transition"), Liz-Marzán and his team demonstrate that gold nanoparticles can be sharpened (just like pencils) through a chemical growth process. Specifically, they synthesized such particles with various shapes and sizes through overgrowth on gold nanorods in the presence of a polymer (poly-vinylpyrrolidone, or PVP).
"We demonstrate the sequence of growth of different crystallographic faces, which provides important information regarding growth mechanisms of anisotropic nanoparticles" says Liz-Marzán. "We can easily follow the relative growth rate of different faces from the shape evolution. This additionally provides evidence of the effect of PVP on the respective surface energies, which had been argued before but with little evidence."
Structural model of gold nanorod growth
Structural model of gold nanorod growth. Left: Original nanorod with octagonal cross section and {100}, {110}, {111} facets. Center: Rod with sharp tips, square cross section, {110} and {111} facets. Right: Growth of a sharpened rod into an octahedral particle via deposition of gold atoms (dark spheres) along the {110} facets. (Image: Dr. Liz-Marzán/University of Vigo)
Metal nanoparticles with sharp apexes and edges have been proposed as excellent candidates for the fabrication of highly sensitive (bio)sensors and surface enhanced Raman spectroscopy (SERS) substrates. This is based on a huge enhancement of the electric field at such sharp sites, as a consequence of localization of the surface plasmon modes. Of course it would be very helpful if these particles could be fabricated in a controllable manner.
Liz-Marzán notes that, in general, metals (most of them with a cubic structure) tend to nucleate and grow into thermodynamically stable nanoparticles with their surfaces bound by the low-energy facets so as to minimize the total surface energy.
"Nevertheless" he says, "highly anisotropic shapes, not favorable from the perspective of thermodynamics, have been obtained by the introduction of capping agents which can alter the surface energies for the different crystallographic planes. In our work, we present evidence of tip sharpening and reshaping during the growth of gold nanorods, with a complete conversion from spherically capped cylinders into single-crystal octahedrons."
When the Spanish researchers conducted a detailed study of the growth process they found a gradual change in the morphology of the particles directly related with the faster growth of certain crystallographic facets.
"Fine-tuning of the reaction conditions allows the isolation of nanoparticle colloids with a variety of intermediate shapes, in turn leading to a variation of the relative intensities and positions of longitudinal and transverse surface plasmon modes" says Liz-Marzán.
Although the Spanish team's results are not a direct solution to a particular problem, they constitute an important step forward toward understanding metal nanoparticle growth. Once this is fully understood, synthesis will be completely predictive, rather than eminently empirical as it is today.
They do point out, though, that apart from the mechanistic value of their results, "the formation of sharp tips and edges in the obtained nanoparticles suggests that near-field enhancement at such spots can be very high, as compared to nanorods with rounded edges, and they can thus find applications in fields such as biosensing and SERS." The ultimate challenge here, of course, is to reach the single molecule detection limit.
Interestingly, the sharpening of the gold tips can also become a critical part of self-organization processes at the nanoscale because of the variation in the electric field distribution.
Just don't put that molecular desktop fab on your Christmas wish list yet...
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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