Monitoring a Polymer Crystal's Evolution

X-ray techniques allow scientists to examine crystals as finished products, but "you miss a lot of the action"--that is, the details of how a crystal grows and develops into its final form, said chemistry professor Chad A. Mirkin of Northwestern University. Mirkin, his collaborator Michael J. Bedzyk, and their colleagues now have found a way to follow the evolution of crystals all the way from the tiniest seed.

The method is an outgrowth of dip-pen nanolithography (DPN), a technique Mirkin's group announced six years ago. In DPN, an atomic force microscope tip is coated with molecules that are then "written" onto a surface in a controlled manner, much like a quill pen applied to paper.

Mirkin's coworker Yi Zhang was using DPN to write poly-DL-lysine hydrobromide features onto a mica surface when he discovered that the scanning process was inducing the polymer to crystallize.

"It's a fascinating process, and one that's very controllable," Mirkin reported in San Diego. Each scan of the microscope tip induces additional growth of the crystals and provides a snapshot of the crystals' evolution, he explained.

Unlike X-ray techniques, this new method "allows you to look at what happens in the early stages of growth" and at how environmental factors such as temperature and humidity can affect the growth, he said. At room temperature, for example, polylysine forms triangular prisms. But when the researchers raised the temperature by 15 °C and lowered the humidity by 15%, the polylysine crystals gradually began to change from triangular to cubic. Mirkin showed a video (39.5 MB) of this geometric transformation during his talk.

No other tool exists that allows one to initiate nucleation on the nanoscale, control the kinetics of crystal growth, and simultaneously acquire images, Mirkin told C&EN. Furthermore, the size of the smallest crystal they studied in these experiments is five orders of magnitude smaller than what could be studied using X-ray methods.

Mirkin pointed out that this tool not only provides important fundamental information about crystallization but also might lead to a practical way to identify the ideal conditions for crystallizing biopolymers and proteins that are difficult to crystallize. He envisions a "massively parallel" DPN system that could probe, in a combinatorial manner, the crystallization of hundreds of thousands of different compounds on many different substrates in a short time. Using a multipen system, Mirkin noted, "we can put about 160,000 features on a surface in about 30 minutes." So his team seems well on its way to achieving massive parallelization.

Shortly after Mirkin's ACS presentation, further details of the study were published in Science (2005, 307, 1763).