Short pulses from a low-intensity laser can spark crystallization on demand, allowing researchers to grow single crystals exactly where they want them in minutes rather than days, say the scientists who developed the technique (J. Am. Chem. Soc., DOI: 10.1021/ja905232m). It could be used to control the structure of semiconductor materials or make life easier for chemists who want to do diffraction experiments, they add.
"Now we have a real tool to choose when and where nucleation happens," says Andrew J. Alexander, a chemist at the University of Edinburgh, in Scotland, who led the study. "It is as if we have opened up a route to flash photography to view the nucleation process on the fly, as it happens."
The technique is based on an observation by Bruce Garetz, Allan Myerson, and their colleagues at the Polytechnic Institute of New York University. They noticed that lasers could cause supersaturated urea solutions to crystallize—a technique called nonphotochemical laser-induced nucleation (NPLIN)—but they did not attempt to control the timing or location of each nucleation event (Phys. Rev. Lett. 1996, 77, 3475).
Alexander had taken note of the 1996 study and was waiting for the right moment to build upon it. "At the time, I was doing a Ph.D. on completely different stuff, so this was filed under curious things that I had to have a go at myself one day," he says.
In the new study, Alexander's team prepared an agarose gel with supersaturated potassium chloride, covered it with a mask, and then struck it with 6-nanosecond polarized pulses from a pair of unfocused YAG lasers. Within minutes, crystals formed in the exposed areas. It was the first time that NPLIN had been used to control crystal growth with pinpoint 3-D accuracy and precise timing.
The British chemists have since managed to crystallize egg-white lysozyme, but they are not yet sure whether the nucleation trick will work consistently to help biochemists crystallize finicky proteins.
"While the reported spatial and temporal control of nucleation is very impressive, there are still several mysteries about NPLIN that need to be resolved," Garetz says.