by Lori Tamura
The ALS is pleased to announce that Pupa Gilbert, professor of physics at the University of Wisconsin at Madison, is the 2018 recipient of the David A. Shirley Award for Outstanding Scientific Achievement at the ALS. Members of the ALS Users’ Executive Committee (UEC) selected Gilbert “for her development of polarization-dependent imaging contrast mapping to image the orientation of carbonate nanocrystals in marine biominerals.”
“PIC mapping” uses photoemission electron microscopy (PEEM) at Beamline 11.0.1 to measure and quantitatively display in color the orientation of crystals at the nanoscale and microscale. Compared to x-ray diffraction, which uses highly penetrating hard x-rays and samples many crystal grains at once, PIC mapping is surface sensitive, using soft x-rays to probe one crystal at a time, down to 20 nm in size.
A key enabler of this technique is the ability to rotate the polarization of the illuminating x-ray beam to observe how it affects the sample’s response—an effect known as dichroism. This effect had been observed in synthetic materials, but Gilbert was the first to observe and then exploit this contrast mechanism in naturally occurring samples such as mollusk shells, sea-urchin teeth, coral skeletons, and parrotfish teeth.
“They’ve had half a billion years to adapt and fine-tune their structure to their functions,” said Gilbert of these natural materials. “There’s a history to them. And so it’s very interesting because we learn about evolution as well, not only about the materials that we’re studying in particular.”
By using color—hue and brightness—to quantify crystal orientations, Gilbert’s data beautifully display the organization of many biomineral crystals at a glance, just as they were deposited by the animal. “There’s so much information in color that you could never get with black and white imaging,” said Gilbert, who is passionate about light, color, and optics, and has co-authored a book titled Physics in the Arts. “The fact that now I can display quantitative information in color,” she said, “makes me so deeply happy. I can’t even describe how satisfied I feel.”
Gilbert has also pioneered the application of a related technique, called component mapping, to biominerals. Whereas PIC maps use color to indicate crystal orientation, component maps use color to indicate chemical composition. The local presence of up to three chemical components can be displayed by assigning a primary color—red, green, or blue—to each component. The component concentrations are measured, pixel-by-pixel, using x-ray absorption spectroscopy (XAS), and the color of each pixel ends up being a mixture of the primary colors, reflecting the mixture of components at that location.
With these tools, Gilbert has uncovered new nanoscale structural principles that could not be detected with previous techniques. For example, PIC maps of parrotfish teeth revealed remarkable nanoscale organization, explaining why they are the stiffest of all apatite-based biominerals studied to date. Component maps were instrumental in understanding the fast growth rate of corals by particle attachment rather than ion recruitment, which may be key to their ability to adapt to higher levels of ocean acidification. And PIC mapping of nacre in bivalves (modern and fossil specimens) allowed the correlation of nacre tablet thickness with the temperature of the environment in which it formed, suggesting that it might serve as a viable “paleothermometer,” recording the temperature history of Earth’s oceans.
Looking to the future, Gilbert is excited by the prospect of the higher-brightness beams that will be available as a result of the ALS Upgrade project (ALS-U). “By having higher brightness, we’re going to be able to do PIC mapping in three dimensions, using ptychography and tomography,” said Gilbert. And regarding component mapping, “Being able to do the chemistry in three dimensions means that we may be able to actually watch the biomineral as it’s being formed in biological systems, which is super exciting because at present that’s just not possible.”
The award will be presented to Gilbert at the upcoming ALS User Meeting, where she will give a keynote Shirley Award Talk, titled “Studying Biomineralization with X-Rays.” The award is named after David Shirley, a Professor of Chemistry at UC Berkeley and Director of LBNL from 1980 to 1989, whose vision was instrumental in building the ALS.