In their search for molecules with certain characteristics, chemical engineers have produced millions of new, increasingly complex synthetic materials by altering molecules' chemical structures
Chem

 

Taking their cues from nature, chemical engineers have recently tested a new method for achieving the molecular properties they seek: by changing the geometry of the surface to which molecules are bound.

“For years, we’ve been making molecules to solve problems – each one more synthetically complicated than the last – but we still haven’t come close to achieving what nature can do with much simpler chemistry,'” said Bartosz A. Grzybowski, Kenneth Burgess professor of chemical and biological engineering and chemistry at the McCormick School of Engineering and Applied Science at Northwestern University, Evanston, Illinois.

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“Nature’s most complex component of life, the protein, is made from only 21 simple amino acids. This research explores the idea that it’s not the molecule you have that’s important, it is how it interacts with its environment,” he explained.

Using this idea, the researchers developed a technique in which a single type of molecule is placed on nanoparticles with two different regions of curvature. Although the molecules are atomically identical, they demonstrate unique chemical properties depending on the region of curvature to which they are bound.

A paper describing the research, ‘Geometric Curvature Controls the Chemical Patchiness and Self-Assembly of Nanoparticles’, was published last month in Nature Nanotechnology.

When organic molecules are tethered onto non-spherical nanoparticles, their chemical properties depend on the particles’ local curvature and shape. Based on this observation, the researchers showed that it was possible to engineer chemical patchiness across the surface of a non-spherical nanoparticle using a single chemical species. “In particular, when acidic ligands are used, regions of the particle surface with different curvature become charged at different pH values of the surrounding solution,” they explained.

“This interplay between particle shape and local electrostatics allows for fine control over nanoscale self-assembly leading to structures with varying degrees of complexity,” they continued. “These structures range from particle cross-stacks to open-lattice crystals, the latter with pore sizes on the order of tens of nanometres, that is, at the lower synthetic limits of metallic mesoporous materials.”

AFFECTS OF CURVATURE

The engineers began by affixing molecules of a carboxylic acid at various points on several gold nanoparticles, some as small as five nanometers in diameter. Each nanoparticle possessed a different geometry. On nanoparticles exhibiting a greater curvature, the molecules were naturally spaced father apart; on nanoparticles with more gradual curvature, they were closer together.

The differences in curvature influences the distance between the molecules, making it possible for the researchers to induce so-called ‘patchiness’ on cylindrical- and dumbbell-shaped nanoparticles. Essentially, the molecules can ‘feel’ each other through repulsive electrostatic interactions and, as the carboxylic acids are depronated, the difficulty in adding more charges onto the nanoparticles is controlled by how crowded the molecules are.

These ‘patchy’ nanoparticles can interact and self-assemble directionally, mimicking chemical molecular bonds – and, the researchers found, altering when the charge of these attached molecules changes.

“Changing molecular properties by altering environments instead of molecular structure could free scientists to accomplish more with a smaller library of already existing molecules, and could offer alternatives to chemical processes that often require toxic chemicals,” said David Walker, a graduate student in McCormick’s Department of Chemical and Biological Engineering and the paper’s first author.

The curvature phenomenon is specific to the nano-scale, where most of the chemistry in biological systems is performed, and begins to fail for nanoparticles above 10 nanometers in diameter, the researchers said. “Larger particles have curvatures that are just too subtle for the molecules to feel the effect — similar to how humans might perceive the earth to be flat, even though we now know better,” Walker said.

The researchers are currently working to extend the work to other classes of molecules that could be beneficial for catalysis and energy purposes.

Other authors of the paper are Igal Szeifer, Christina Enroth-Cugell professor of biomedical engineering and professor of chemistry, chemical and biological engineering, and professor of medicine; graduate student Emily Leitsch; and postdoctoral researcher Rikkert Nap, all of Northwestern University.

Source: Northwestern University (2013, August 29)

http://www.engineersjournal.ie/wp-content/uploads/2013/09/Nanoscape-1024x683.jpghttp://www.engineersjournal.ie/wp-content/uploads/2013/09/Nanoscape-300x300.jpgDavid O'RiordanChemchemical,nano,research
  Taking their cues from nature, chemical engineers have recently tested a new method for achieving the molecular properties they seek: by changing the geometry of the surface to which molecules are bound. 'For years, we’ve been making molecules to solve problems – each one more synthetically complicated than the last...