Small particles—about one nanometer to tens of microns—are ubiquitous in the natural and engineered worlds. They are in the air, soil, and water on Earth, as well as at the farthest reaches of the universe. One of the major buzz words over the past 10 years, nanotechnology, involves the creation of nanoscale materials with the promise of new and revolutionary properties. Given the size and abundance of small particles, however, there is relatively little understanding about their properties and chemical composition, which limits our ability to understand, predict, and control their applications and impacts in both natural and engineered systems. At the same time, information may be available that just needs to be shared across disciplinary boundaries to increase understanding of small particles.
The National Academies’ Chemical Sciences Roundtable (CSR) held a workshop on October 25-26, 2010, to explore new opportunities, challenges, and approaches to characterizing small particles and understanding their impacts. In many scientific and engineering domains, a lack of understanding about the properties and chemical composition of small particles limits our ability to understand, predict, and control their applications and impacts.
The workshop, “Challenges in Characterizing Small Particless: Exploring Particles from the Nano- to Microscales,” explored the critical importance of small particles in environmental science, materials and chemical sciences, biological science, and engineering, and the many challenges involved in characterizing materials at the nano- and microscales. The discussions on characterization included static, dynamic, experimental, computational, and theoretical characterization. The workshop also included several “research tool” presentations that highlighted new advances in characterizing small particles.
This document summarizes the presentations and discussions that took place during the workshop.1 Where possible, background references have been provided to support statements made or data described. The reader is urged to follow up with individual guest speakers and their institutions for further clarification of statements made during the workshop or to obtain additional reference materials.
Why Small Particles Matter
The workshop began with an introduction by co-chair Barbara Finlayson-Pitts, University of California, Irvine, who noted the importance of and challenges associated with characterizing small particles in the atmosphere. Steve Schwartz, Brookhaven National Laboratory, continued this theme with a more thorough discussion of the many roles that atmospheric aerosols play in determining climate. Aerosol particles act as the seeds for cloud formation, reflect sunlight away from Earth’s surface, and serve as catalytic sites for atmospheric chemical processes. He also described efforts to characterize atmospheric aerosols and model how they influence, or force, climate.
Mort Lippmann, New York University, discussed some of the health impacts caused by airborne particles, which are also known as particulate matter. In particular, he described how the size of these particles, along with the chemicals they pick up on their surfaces, relates to the negative impacts on
1About Open Discussions: All but one chapter (Chapter 4) in this document ends with a summary of discussion topics introduced by speakers and participants in the immediate session, as well as all preceding sessions.
human health that accompany prolonged exposure to particulate matter smaller than 10 microns in diameter. Michael Hochella, Virginia Polytechnic and State University, then noted that nanoparticles are everywhere in the environment and that the mass of nanoparticles that nature puts into the environment will always dwarf the mass that humans will ever produce. He discussed some of the ways in which nanoparticles enter the environment, including volcanic eruptions and salt spray, and the effects of human activities on levels of potential toxic nanoparticles found in soil and water.
With his presentation on the ways in which small particles interact with cells, Gary McDermott, University of California, San Francisco, brought this session to a close. He described methods for imaging small particles inside cells and showed how they could be used to help with the design of new therapeutic drugs to treat human disease.
Challenges in Chemical Analysis and Imaging of Small Particles
Over the course of two sessions, speakers described the challenges of characterizing small particles and some of the real-world needs for developing new analytical technologies applicable to small particles. Alla Zelenyuk, Pacific Northwest National Laboratory, discussed her team’s use of single particle laser ablation time-of-flight mass spectrometry to characterize atmospheric aerosols and to study how other atmospheric components influence the morphology and particle chemistry of atmospheric aerosols. James Smith, National Center for Atmospheric Research, addressed the phenomenon of new particle formation in the atmosphere and the recent progress that his group has made in quantifying the composition of these spontaneously generated atmospheric nanoparticles.
In his presentation, Ralph Nuzzo, University of Illinois, Urbana-Champaign, discussed the rich toolbox that is available for characterizing heterogeneous catalysts at the atomic level. This information is leading to a better understanding of how these particles assemble and how particle structure impacts particle properties. Continuing on the catalysis theme, Abhaya Datye, University of New Mexico, stressed the importance of nanoscale characterization for understanding how the interactions between catalytic nanoparticles and bulk-scale supports affect catalytic activity, information that could lead to the development of improved industrial heterogeneous catalysts.
Lee Silverman, DuPont Central Research and Development Laboratory, continued this industrial theme by discussing the tools needed to characterize composite polymeric materials containing nanoscale particle additives. Silverman noted that the information generated using these tools can play a role in the development of new materials that will not endanger the environment or human health. Yi Qiao, 3M Corporate Research Process Laboratory, discussed some of the challenges facing those who need to characterize nanoparticle dispersions used in industrial applications. He noted that to meet the needs of a manufacturing environment, a measurement technique must be fast enough to provide feedback on a meaningful timeframe, have few restrictions concerning sample preparation in terms of nanoparticle concentration and purity, and be able to distinguish “good” from “bad” so that a line operator can make necessary adjustments to the manufacturing process in real time. He then described two techniques that meet these requirements.
Rhonda Stroud, Naval Research Laboratory, concluded this session by describing how the techniques used to characterize terrestrial particles can be applied to the characterization of the 40,000 tons of extraterrestrial dust that enter the planet’s upper atmosphere annually.
Challenges in Analyzing Nanoparticles in Complex, Real-World Mixtures
In this session, three speakers addressed some of the challenges in measuring and predicting the properties and behavior of complex nanoparticle formulations and discussed the often surprising findings that come from studying nanomaterials as they occur in the real world. James Lister, Purdue University, talked about particles in the micron and submicron size ranges that are used in industrial applications, with a specific focus on the delivery forms for these particles. In particular, he addressed some of the methods for characterizing the processes used to create delivery forms such as catalyst pellets and drug tablets. He also described how these characterization tools can provide insights into how manufacturing processes impact the final properties of these delivery forms, such as dissolubility and chemical stability.
Pedro Alvarez, Rice University, then discussed his work on characterizing how nanoparticles interact with microorganisms in the environment. He explained that bacteria, the foundation of all ecosystems, provide a convenient model for studying the potential toxicity of engineered nanoparticles. Vicki Grassian concluded the session by describing methods used to understand the transformations and surface chemistry of mineral dust, a major component of atmospheric aerosols. Using these methods, she and her colleagues have been able to show how mineral dust can catalyze a wide range of chemical reactions occurring in the atmosphere.
Modeling and Simulation of Small Particles
In the workshop’s final scientific session, Angela Violi, University of Michigan, and Douglas Tobias, University of California, Irvine, described the use of several computational
approaches to modeling nanoparticles to provide insights into how structure and function are related. Violi spoke about a hybrid modeling technique that works at an atomistic level to follow the growth of particles produced during the incomplete combustion of different hydrocarbon-based fuels. She also described a computational model of the cell membrane that can follow how carbonaceous particles, such as those produced during combustion, impact the natural flow of lipids in the cell membrane.
Tobias then spoke about an atomistic computational model that is providing insights into the interactions between a particle’s reactive surface and compounds impinging on that surface. He also described a coarse-grained model for studying the atmospheric chemistry of sea salt particles that improves the prediction of the geographical distribution of ozone production in a polluted urban environment.
This document was prepared by rapporteurs Tina Masciangioli and Joe Alper for the Chemical Sciences Roundtable as a factual summary of what occurred during the workshop. In accordance with the CSR’s policies, the summary does not attempt to establish any conclusions or recommendations about needs and future directions, focusing instead on issues identified by the speakers and workshop participants.
This summary is organized according to the four main sessions. Overview presentations highlighted the critical importance of small particles in environmental science, materials and chemical sciences, biological science, and engineering. Technical sessions focused more on the research tools used to characterize small particles, such as sampling, nucleation and growth, and chemical imaging. Poster abstracts are provided in Appendix B.