PNNL

Patrick El-Khoury and Grant Johnson wrote invited reviews for the emerging investigator issue of Analyst Enlarge Image.

Congratulations to Dr. Grant Johnson and Dr. Patrick El-Khoury at Pacific Northwest National Laboratory on being invited to write articles for the emerging investigator issue of Analyst, published by the Royal Society of Chemistry. They joined 47 other early career researchers in sharing the analytical techniques they've devised to make discoveries about the world around us. Their articles appear in the journal's June 21 issue.

Johnson and El-Khoury are both former Linus Pauling Distinguished Postdoctoral Fellows at PNNL. Pauling Fellows conduct research of their own design while working with well-known mentors. Johnson became a fellow in 2010; El-Khoury, 2013. Both are now full-time PNNL scientists, and their analytical techniques are answering questions about clusters and nanoparticles that could benefit chemical manufacturing, pollution abatement, energy storage, and health care.

How to design ligands to make gold clusters with predetermined properties: In the 1970s, scientists began making tiny clusters of gold atoms. Interest grew as people discovered the size-tunable properties of these clusters and their potential applications in catalysis and energy conversion. Because each metal atom counts (a cluster with 6 gold atoms behaves very differently from one with 7), scientists want to protect the metal surface and do so using carbon-based molecules that keep the surface passive and avoid aggregating. Although these molecules, called ligands, have been used for years, scientists still have many questions about how they work.

Johnson reviewed three instrumental approaches he's used at PNNL to answer questions about ligands. These are commercially available electrospray ionization mass spectrometry and two custom-designed instruments he helped build: surface-induced dissociation and soft landing. In the article, he shows how combining these approaches offer detailed measurements that can't be made using other instruments. This information may be used to direct the design of ligands to produce clusters with specific properties for a given application.

The inaugural Pauling Fellow, Johnson is well known in the scientific community. He earned a bachelor's degree in chemistry from the University of Delaware Honors Program. He worked with the U.S. Army before heading off to earn a doctoral degree at Pennsylvania State University. During graduate school, he received a fellowship to study in Germany and another to attend the meeting of Nobel Laureates in chemistry.

How to interrogate trapped light waves: Both electrons and photons, tiny packets of light, can excite light waves trapped on the surface of metal nanoparticles in the form of surface plasmons. Surface plasmons are notoriously difficult to study, because of the length and time scales over which they manifest. But visualizing plasmons, and understanding how they interact with molecules could help answer important fundamental questions in the area of ultrasensitive spectroscopy.

In his review, El-Khoury and co-authors show how combining tip-enhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, and correlated scanning transmission electron microscopy-electron energy loss spectroscopy allows examining plasmons with nanometer spatial resolution, femtosecond temporal resolution, and high energy resolution.

"Altogether, these techniques may be used to visualize surface plasmons at the ultimate limits of space and time" said El-Khoury.

Throughout his tenure at PNNL, El-Khoury has developed new ways to look at plasmons as well as molecule-plasmon interactions. He built a tip-enhanced Raman nanoscope and a multimodal hyperspectral optical microscope, and he co-authored some 25 journal articles with several of his PNNL colleagues, including Johnson. Before joining PNNL, he completed postdoctoral training at the University of California, Irvine. He earned a Ph.D. in photochemical sciences from Bowling Green State University and a bachelor's degree in chemistry from the American University of Beirut.

Both researchers gratefully acknowledge their mentors. Johnson was mentored by Dr. Julia Laskin; El-Khoury by Dr. Wayne Hess. They also acknowledge the Pauling Fellowship at PNNL and DOE's Office of Science which provided funding and access to cutting-edge instruments at DOE's EMSL, a national scientific user facility, and elsewhere. "This work wouldn't have been possible without their support," said Johnson.

More information?

Johnson GE and J Laskin. 2016. "Understanding Ligand Effects in Gold Clusters Using Mass Spectrometry." Analyst 141:3573-3589. DOI: 10.1039/c6an00263c

El-Khoury PZ, P Abellan, Y Gong, FS Hage, J Cottom, AG Joly, R Brydson, QM Ramasse, and WP Hess. 2016. "Visualizing Surface Plasmons with Photons, Photoelectrons, and Electrons." Analyst 141:3562-3572. DOI: 10.1039/C6AN00308G