thesis
Learning from the past When it comes to safety, the jury’s still out on which nanoparticle characteristics we should be measuring. But, as Andrew D. Maynard explains, there’s a rich history dating back over a hundred years on how we measure them. “I write in hopes that you might give me some direction in a tragic contamination…” It’s a line that could be straight out of a Victorian novel, but it’s actually from a recent e-mail I received. I get a small but steady stream of similar enquiries from people struggling to make sense of seemingly mysterious illnesses, and wondering if engineered nanoparticles are to blame. This particular case involved the liberal use of a powder-based fire extinguisher in the home. Usually, I would write a short but sympathetic note in response to such e-mails, and not think too much more about them. In this instance though, something caught my eye — not with the writer’s issue, but with advice they had previously received. Concerned that the fire extinguisher had left their house contaminated with harmful nanoparticles, my correspondent had contacted an ‘expert in the field’ of nanoparticle characterization, and asked what they should do. To my surprise, they were told that “there are only 2–3 electron microscopes worldwide powerful enough” to characterize the alleged offending particles. This is, of course, not true. The vast majority of electron microscopes currently in use — from workhorse scanning electron microscopes to more sophisticated transmission electron microscopes, and even portable desktop models — are capable of imaging and characterizing particles a few nanometres in diameter and below. My e-mailer’s expert was a victim of the assumption that nanoparticles are so unique, and so small, that routine characterization methodologies have yet to catch up with them. It’s a misperception I’d like to think isn’t that common in nanotechnology circles. Yet it crops up surprisingly regularly in various forms. And I suspect one of the reasons why it does, is widespread unawareness of the long history of nanometre-scale particle characterization that preceded the advent of nanotechnology. In 1888, the Scottish scientist John Aitken published a letter in the journal Nature1. In it he described an apparatus for counting airborne particles that are 482
“not only invisible, but are beyond the highest powers of the [light] microscope.” The device he designed and built was the first example of a condensation particle counter — an instrument that allows fine airborne particles to be measured by introducing them into a supersaturated vapour, and counting the droplets that subsequently form. Not content with a static, laboratorybound instrument, Aitken subsequently developed a ‘pocket’ version of his particle counter, which he proceeded to haul across Europe, taking measurements everywhere from the peaks of mountains to the top of the Eiffel Tower 2. In a testament to both the charm and value of his work, we now know for instance that, on 29 May 1889, there were 52,000 particles per cm3 air measured “at the top of the tower, immediately underneath the lantern for the electric light at an elevation of nearly 300 m.” (As an aside, reproducing Aitken’s 1889 measurement tour with one of his original instruments would provide a fascinating insight into how ambient nanoparticle concentrations have changed over the past century.)
Schematic of John Aitken’s original dust counter. Reproduced by permission of The Royal Society of Edinburgh from ref. 10.
Aitken’s portable particle counter continued to be used widely for the next 50 years (ref. 3), and paved the way to its modern counterparts — including the portable condensation particle counters that now form the basis for routine workplace nanoparticle concentration measurements4. And the condensation counting technique he invented is still used as the basis for measuring nanoparticle size distributions through techniques such as differential mobility analysis. Aitken’s dust counter may have been one of the first instruments to allow airborne nanoparticles to be quantified, but it was shortly followed by the electron microscope — which remains to this day the gold standard for individual nanoparticle characterization. The first electron microscope was built by Ernst Ruska in the early 1930s5, and was a far cry from today’s highly versatile instruments. Nevertheless, the technology rapidly led to the production of increasingly powerful microscopes, which were capable of resolving and analysing particles well into the nanometre range. My own work from the early 1990s, building on these developments, led to techniques for collecting ambient particles for electron microscope analysis that were just a few nanometres in diameter 6, and characterizing their spatial composition using a combination of scanning transmission electron microscopy and electron energyloss spectroscopy 7. Whichever way you look at it, collecting and measuring nanoparticles is not something we’ve just recently discovered how to do. Yet, while we have been able to characterize nanoparticles for decades, there has been considerable uncertainty over which measurements are important from a health perspective, and this perhaps drives some of the misconceptions around current capabilities. In 2006, I was co-author on an article calling for the development of instruments to assess exposure to engineered nanomaterials in air and water 8. Our concern at the time was that, while imaging and analysing nanometre-scale particles is not particularly difficult, it was not clear which particle
NATURE NANOTECHNOLOGY | VOL 10 | JUNE 2015 | www.nature.com/naturenanotechnology
. d e v r e s e R s t h g i R l l A . d e t i m i L s r e h s i l b u P n a l l i m c a M 5 1 0 2 ©
thesis attributes are suitable indicators of potential health impact, and how exposure to particles with these attributes could be monitored cheaply and effectively. Nine years on, there’s still a gap between our understanding of how some nanoparticles may affect health, and our ability to monitor exposures in ways that help ensure safe use. Yet, an inability to routinely monitor exposures with respect to specific particle attributes is far from being synonymous with not having the tools to characterize engineered nanoparticles. And I wonder whether this disconnect isn’t at least partially exacerbated by unawareness of a rich, if a little dusty, literature on nanoparticle characterization techniques. A few years ago, while teaching a science communication class, I asked my students to write an article based on a paper of their choice that was published several decades ago. There was an awkward silence. Until someone chimed in “can we trust papers more than a few years old?” Admittedly, the student was in a programme where the state of agreed on understanding is somewhat ‘fluid’. I was still taken aback though. I find myself having a comparable response when early research relevant to nanoparticle characterization is similarly discounted. Aitken’s work may have preceded modern nanotechnology by a century and more, and early research on electron microscopy occurred decades before ‘nano’ was trendy. Yet this doesn’t mean that this early work was irrelevant. To the contrary, it eloquently demonstrates
e-mailer’s health issues were associated with nanoparticle exposures. But it would be relatively simple to measure and characterize exposures using off-the-shelf equipment that builds on a legacy of research going back a hundred years. The harder challenge is working out what we should be measuring — what material and particle attributes are most closely associated with health risks, and at what exposure levels? Research is still needed here, as the materials we design and engineer become increasingly sophisticated9. But as we do move forward, it’s surprising how rich the measurement technology toolbox at our disposal is — if only we take the time to learn from the past. ❐ Andrew D. Maynard is at the Risk Science Center, University of Michigan School of Public Health, 1415 Washington Heights, Ann Arbor, Michigan 48109, USA. e-mail: [email protected] References Schematic of John Aitken’s pocket dust counter. Reproduced by permission of The Royal Society of Edinburgh from ref. 11.
that seemingly novel challenges don’t always demand novel solutions, and sometimes, the key to moving forward safely, is to look back at what’s already known. In the case of the dry powder extinguisher, I would be surprised if my
Aitken, J. Nature 37, 187–206 (1888). Aitken, J. Proc. R. Soc. Edinburgh 17, 193–254 (1891). McMurray, P. H. Aerosol Sci. Technol. 33, 297–322 (2000). National Institute for Occupational Safety and Health (NIOSH) Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials, NIOSH publication no. 2009–125 (NIOSH, 2009). 5. Ruska, E. Z. Phys. 87, 90–128 (1934). 6. Maynard, A. D. Aerosol Sci. Technol. 23, 521–533 (1995). 7. Maynard, A. D. J. Aerosol Sci. 26, 757–777 (1995). 8. Maynard, A. D. et al. Nature 444, 267–269 (2006). 9. Maynard, A. D., Warheit, D. & Philbert, M. A. Tox. Sci. 120, S109–S129 (2011). 10. Aitken, J. Trans. R. Soc. Edinburgh 35, 1–19 (1889). 11. Aitken, J. Proc. R. Soc. Edinburgh 18, 39–52 (1892). 1. 2. 3. 4.
NATURE NANOTECHNOLOGY | VOL 10 | JUNE 2015 | www.nature.com/naturenanotechnology
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. d e v r e s e R s t h g i R l l A . d e t i m i L s r e h s i l b u P n a l l i m c a M 5 1 0 2 ©
Learning from the past When it comes to safety, the jury’s still out on which nanoparticle characteristics we should be measuring. But, as Andrew D. Maynard explains, there’s a rich history dating back over a hundred years on how we measure them. “I write in hopes that you might give me some direction in a tragic contamination…” It’s a line that could be straight out of a Victorian novel, but it’s actually from a recent e-mail I received. I get a small but steady stream of similar enquiries from people struggling to make sense of seemingly mysterious illnesses, and wondering if engineered nanoparticles are to blame. This particular case involved the liberal use of a powder-based fire extinguisher in the home. Usually, I would write a short but sympathetic note in response to such e-mails, and not think too much more about them. In this instance though, something caught my eye — not with the writer’s issue, but with advice they had previously received. Concerned that the fire extinguisher had left their house contaminated with harmful nanoparticles, my correspondent had contacted an ‘expert in the field’ of nanoparticle characterization, and asked what they should do. To my surprise, they were told that “there are only 2–3 electron microscopes worldwide powerful enough” to characterize the alleged offending particles. This is, of course, not true. The vast majority of electron microscopes currently in use — from workhorse scanning electron microscopes to more sophisticated transmission electron microscopes, and even portable desktop models — are capable of imaging and characterizing particles a few nanometres in diameter and below. My e-mailer’s expert was a victim of the assumption that nanoparticles are so unique, and so small, that routine characterization methodologies have yet to catch up with them. It’s a misperception I’d like to think isn’t that common in nanotechnology circles. Yet it crops up surprisingly regularly in various forms. And I suspect one of the reasons why it does, is widespread unawareness of the long history of nanometre-scale particle characterization that preceded the advent of nanotechnology. In 1888, the Scottish scientist John Aitken published a letter in the journal Nature1. In it he described an apparatus for counting airborne particles that are 482
“not only invisible, but are beyond the highest powers of the [light] microscope.” The device he designed and built was the first example of a condensation particle counter — an instrument that allows fine airborne particles to be measured by introducing them into a supersaturated vapour, and counting the droplets that subsequently form. Not content with a static, laboratorybound instrument, Aitken subsequently developed a ‘pocket’ version of his particle counter, which he proceeded to haul across Europe, taking measurements everywhere from the peaks of mountains to the top of the Eiffel Tower 2. In a testament to both the charm and value of his work, we now know for instance that, on 29 May 1889, there were 52,000 particles per cm3 air measured “at the top of the tower, immediately underneath the lantern for the electric light at an elevation of nearly 300 m.” (As an aside, reproducing Aitken’s 1889 measurement tour with one of his original instruments would provide a fascinating insight into how ambient nanoparticle concentrations have changed over the past century.)
Schematic of John Aitken’s original dust counter. Reproduced by permission of The Royal Society of Edinburgh from ref. 10.
Aitken’s portable particle counter continued to be used widely for the next 50 years (ref. 3), and paved the way to its modern counterparts — including the portable condensation particle counters that now form the basis for routine workplace nanoparticle concentration measurements4. And the condensation counting technique he invented is still used as the basis for measuring nanoparticle size distributions through techniques such as differential mobility analysis. Aitken’s dust counter may have been one of the first instruments to allow airborne nanoparticles to be quantified, but it was shortly followed by the electron microscope — which remains to this day the gold standard for individual nanoparticle characterization. The first electron microscope was built by Ernst Ruska in the early 1930s5, and was a far cry from today’s highly versatile instruments. Nevertheless, the technology rapidly led to the production of increasingly powerful microscopes, which were capable of resolving and analysing particles well into the nanometre range. My own work from the early 1990s, building on these developments, led to techniques for collecting ambient particles for electron microscope analysis that were just a few nanometres in diameter 6, and characterizing their spatial composition using a combination of scanning transmission electron microscopy and electron energyloss spectroscopy 7. Whichever way you look at it, collecting and measuring nanoparticles is not something we’ve just recently discovered how to do. Yet, while we have been able to characterize nanoparticles for decades, there has been considerable uncertainty over which measurements are important from a health perspective, and this perhaps drives some of the misconceptions around current capabilities. In 2006, I was co-author on an article calling for the development of instruments to assess exposure to engineered nanomaterials in air and water 8. Our concern at the time was that, while imaging and analysing nanometre-scale particles is not particularly difficult, it was not clear which particle
NATURE NANOTECHNOLOGY | VOL 10 | JUNE 2015 | www.nature.com/naturenanotechnology
. d e v r e s e R s t h g i R l l A . d e t i m i L s r e h s i l b u P n a l l i m c a M 5 1 0 2 ©
thesis attributes are suitable indicators of potential health impact, and how exposure to particles with these attributes could be monitored cheaply and effectively. Nine years on, there’s still a gap between our understanding of how some nanoparticles may affect health, and our ability to monitor exposures in ways that help ensure safe use. Yet, an inability to routinely monitor exposures with respect to specific particle attributes is far from being synonymous with not having the tools to characterize engineered nanoparticles. And I wonder whether this disconnect isn’t at least partially exacerbated by unawareness of a rich, if a little dusty, literature on nanoparticle characterization techniques. A few years ago, while teaching a science communication class, I asked my students to write an article based on a paper of their choice that was published several decades ago. There was an awkward silence. Until someone chimed in “can we trust papers more than a few years old?” Admittedly, the student was in a programme where the state of agreed on understanding is somewhat ‘fluid’. I was still taken aback though. I find myself having a comparable response when early research relevant to nanoparticle characterization is similarly discounted. Aitken’s work may have preceded modern nanotechnology by a century and more, and early research on electron microscopy occurred decades before ‘nano’ was trendy. Yet this doesn’t mean that this early work was irrelevant. To the contrary, it eloquently demonstrates
e-mailer’s health issues were associated with nanoparticle exposures. But it would be relatively simple to measure and characterize exposures using off-the-shelf equipment that builds on a legacy of research going back a hundred years. The harder challenge is working out what we should be measuring — what material and particle attributes are most closely associated with health risks, and at what exposure levels? Research is still needed here, as the materials we design and engineer become increasingly sophisticated9. But as we do move forward, it’s surprising how rich the measurement technology toolbox at our disposal is — if only we take the time to learn from the past. ❐ Andrew D. Maynard is at the Risk Science Center, University of Michigan School of Public Health, 1415 Washington Heights, Ann Arbor, Michigan 48109, USA. e-mail: [email protected] References Schematic of John Aitken’s pocket dust counter. Reproduced by permission of The Royal Society of Edinburgh from ref. 11.
that seemingly novel challenges don’t always demand novel solutions, and sometimes, the key to moving forward safely, is to look back at what’s already known. In the case of the dry powder extinguisher, I would be surprised if my
Aitken, J. Nature 37, 187–206 (1888). Aitken, J. Proc. R. Soc. Edinburgh 17, 193–254 (1891). McMurray, P. H. Aerosol Sci. Technol. 33, 297–322 (2000). National Institute for Occupational Safety and Health (NIOSH) Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials, NIOSH publication no. 2009–125 (NIOSH, 2009). 5. Ruska, E. Z. Phys. 87, 90–128 (1934). 6. Maynard, A. D. Aerosol Sci. Technol. 23, 521–533 (1995). 7. Maynard, A. D. J. Aerosol Sci. 26, 757–777 (1995). 8. Maynard, A. D. et al. Nature 444, 267–269 (2006). 9. Maynard, A. D., Warheit, D. & Philbert, M. A. Tox. Sci. 120, S109–S129 (2011). 10. Aitken, J. Trans. R. Soc. Edinburgh 35, 1–19 (1889). 11. Aitken, J. Proc. R. Soc. Edinburgh 18, 39–52 (1892). 1. 2. 3. 4.
NATURE NANOTECHNOLOGY | VOL 10 | JUNE 2015 | www.nature.com/naturenanotechnology
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