Studying Bond Formation Between Ions and Surfaces Gets Easier
Scientists develop new method, complete successful demo, obtain surprising results
Scientists can now determine if ions form the desired bonds to a surface, thanks to a modified ion soft landing instrument. The schematic shows (1) ion funnel, (2) collisional quadrupole, (3) mass filter, (4) and (5) two lenses, (6) surface, (7) linear motion feedthrough for positioning of the surface, (8) flat mirror, (9) and (12) parabolic mirrors, (10) MIR wire grid polarizer, (11) viewports. Enlarge Image
Results: For the first time, researchers can determine if large molecules and other charged particles that softly land on a surface form the desired bonds, thanks to a new approach built by a team at Pacific Northwest National Laboratory. This method relies on in situ reflection-absorption spectroscopy to monitor the formation of a chemical bond between the ion and the surface.
Using this new approach, Dr. Julia Laskin and her colleagues made a startling discovery about the efficiency of bond formation between the ion and the surface. Using diamine, a linear molecule with nitrogen atoms at each end, as a model ion, they found that nearly every ion that hit the surface bonded to it.
"This is a very surprising result," said Laskin. "It absolutely threw us off."
Why it matters: In designing specialized sensors and other technologies, scientists want to prepare customized layers of molecules on specific surfaces. To build these layered materials, scientists must first understand the reactions involved. This new tool provides vital insights on the reactivity between ions and surfaces. Using it, scientists can study bond formation and the impact of the surface on the secondary structure and orientation of large molecules that land on it.
Methods: To study bond formation between ions and reactive surfaces, researchers previously had to expose surfaces to a beam of mass selected ions, then take the substrate outside the vacuum system and perform the analysis. But, as a general rule, researchers prefer to study reactions as they happen as exposure of surfaces to laboratory air can alter the structure of deposited molecules and introduce artifacts.
The researchers selected for their experiments a long chain-like molecule, with 12 carbon atoms in the middle and an amine group on each end. Ions of this molecule were selected using a mass filter and were sent to react with a carbon-based surface by forming an amide bond. This bond involves a carbonyl group (C=O) on the substrate and an amine group (NH2) of the ion each shedding off other atoms and bonding together. Surprisingly, the researchers found that nearly every molecule that hit the surface bonded. "We are thrilled by this finding and will explore the formation of other types of covalent bonds between ions and surfaces in our future studies", said Dr. Qichi Hu, a postdoctoral fellow working on this project.
In the past, studying bond formation between small proteins or peptides and surfaces using infrared spectrometry was difficult because the signals indicating the formation of a new amide bond between the surface and the protein were buried under strong infrared bands of the protein itself.
Now, scientists can study the reactions as they occur. The new approach allows researchers to unambiguously prove that bond formation indeed takes place and quantify the amount of amide bonds formed.
However, initial bond formation just one part of a bigger picture. Researchers can also study how the surface affects the material that land on it. For example, placing a large molecule that is happy in water on a surface that repels water will cause the molecule to reorganize. Because the conformation of a large molecules determines its function, understanding this phenomenon is essential for designing nanomaterials with controlled properties. This new technique will help answer questions about how such rearrangements occur.
The researchers developed this new approach in the Department of Energy's EMSL, a national scientific user facility at PNNL. Working in this facility allowed the team to combine different instruments and techniques to create the tool they needed.
What's next: In the future studies, the scientists will examine the effect of the substrate on the orientation and conformation of soft-landed peptides and proteins. In addition, they are continuing their work to understand the interactions between molecules and surfaces to arrive at the time when understanding leads to controlled preparation of functional materials.
Further, with this new method available at EMSL, scientists from around the world have another tool in their arsenal to study bond formation. Already, a scientist from the University of Arizona, through EMSL's rapid access program, is using the approach for his studies.
Acknowledgments: The work was done by Dr. Qichi Hu, Dr. Peng Wang, Paul Gassman, and Dr. Julia Laskin of PNNL. Dr. Wang was a postdoctoral fellow working on this research and has since finished her assignment at PNNL. The research was done in DOE's EMSL, a national scientific user facility at PNNL.
Sponsor: DOE's Office of Basic Energy Sciences funded this work.
Reference: Hu Q, P Wang, PL Gassman, and J Laskin. 2009. "In Situ Studies of Soft- and Reactive Landing of Mass-Selected Ions Using Infrared Reflection Absorption Spectroscopy." Analytical Chemistry. 81:7302-7308. DOI: 10.1021/ac901149s.