Detecting Engineered Nanoparticles in Environmental Samples

The use of engineered nanoparticles (ENPs) in various applications and consumer products continues to increase, and these nanoparticles require thorough characterization for proper environmental risk assessment. James Ranville, a professor at the Colorado School of Mines, in Golden, Colorado, has been studying colloids and and particles in environmental processes and developing methods to collect and analyze colloids from rivers, reservoirs, mountain streams, soil solutions, and ground waters. He spoke with us about his work using field-flow fractionation–inductively coupled plasma mass spectrometry (FFF-ICP-MS) and ICP-MS for the detection of engineered nanoparticles in environmental samples.

In a 2008 paper you said that little is known about the fate and behavior of ENPs in the environment (1). Is that still true today?

In some ways we have greatly improved our understanding of ENP fate and behavior, but significant knowledge gaps remain. The fundamental colloid chemistry that dictates how an ENP will behave under different environmental conditions is fairly well known, at least under laboratory conditions. The state of the art in using materials-flow models has improved, providing better predictions of which environmental pathways and compartments are important for understanding ENP fate. However, we are yet to fully track an ENP through an actual environmental pathway, which is at least in part due to our current analytical limitations. That being said, the use of field mesocosms to combine some level of experimental control with the reality of field conditions have been quite successful in advancing our understanding of ENP environmental fate and behavior.

You have compared the advantages and limitations of single-particle ICP-MS and asymmetrical flow field-flow fraction (AF4)–ICP-MS for the analysis of engineered nanoparticles (ENPs) (2). What did you find?

Each method has its own strengths and weaknesses based on its inherent principle of operation and the current state of the instrumentation used. As we have moved from the analysis of relatively simple ENPs to more complex materials, we find that the two approaches are truly complementary. As an example, imagine a metal oxide ENP with a polymer coating or protein corona. Single particle ICP-MS provides the element-specific mass of this composite ENP as well as its number concentration. AF4 and centrifugal field-flow fractionation (FFF) provide the means to obtain fractions that are separated by the hydrodynamic size and buoyant mass respectively. Combining the data obtained by single-particle ICP-MS analysis of FFF fractions gives insight into the thickness and mass of the organic coatings and can quantify the number of metal oxide particles if the ENP is an aggregate structure.

You carried out a study to determine the smallest ENPs that can be measured by single-particle ICP-MS for 40 different metallic elements (3). What did you find?

In the study by Lee and colleagues (3), we examined the mass detection limit of large number of elements using a commercial quadrupole-based ICP-MS system. From these data, and making assumptions about particle density, composition and shape, we were able to estimate the size detection limit for ENPs that contained the elements measured. The size detection limit is a classic “signal-to-noise” issue common to all analytical methods. As ENP size decreases a point is reached at which the integrated counts associated with the ENP-generated pulse of ions are indistinguishable from the background. Thus the magnitude of the background and the sensitivity of the instrument response define the size detection limit. The size detection limits presented in the study reflect the inherent sensitivity of the quadrupole MS system used and its ability to avoid or reduce background interferences. Results would likely be different using other approaches for mass selection and detection.

What do those results tell us specifically about our current capabilities? Can we measure ENPs of the sizes that are currently being released into the environment?

We have further discussed the current capabilities and limitations of single-particle ICP-MS for size and number concentration analysis in a recent review paper by Montano and colleagues (4). Given the current limitations of the available instrumentation, not all ENPs can be measured by single particle ICP-MS. For example, most quantum dots, if well dispersed, remain below the size detection limit of the technique. In simple matrices, such as those studied by Lee and colleagues (3), the background signal generated over the 20–100 µs dwell times that are now commonly used is very low if isobaric or polyatomic interferences are not present. Thus the size detection limit is primarily based on instrument sensitivity. This determination is not so simple when measuring ENPs in complex environmental and biological matrices. Elevated background signals generated by the matrix as well as potential matrix effects on instrument sensitivity and droplet transport efficiency may likely raise the size detection limits for many ENPs. It should also be noted that the discussion of size detection is focused on individual dispersed ENPs. Aggregation of ENPs could lead to detectable signals but data interpretation becomes complicated.

What are the broader implications of those results for the analysis of ENPs in the environment?

Characterization and quantification of ENPs in environmental samples is important in monitoring current ENP exposure, validating model predictions of current and future exposures, and more fully evaluating the life cycle of nano-enabled products. Use of single-particle ICP-MS for environmental analysis is still in its early stages, and its application largely is limited to a relatively small group of researchers. Application by a broader community, which will be facilitated by improvements in instrumentation and data analysis packages, will help progress the technique for environmental systems. However, the wider adoption of the single-particle technique must be accompanied by careful development of methods and standard operating procedures that provide robust and accurate analysis of particle concentration and size. While the basic methods for calibration, determination of transport efficiency, and data analysis exist, more study on the effects of sample matrices needs to be performed to lead to a reliable methodology that can be broadly implemented.

One of the factors complicating the measurement of ENPs in the environment is the transformation of the particles. How much do we know today about the transformation of ENPs in the environment? And how well can we detect and measure those transformed particles?

Many of the major transformation processes (dissolution, homo- and heteroaggregation, Ag sulfidation, and so forth) have been quite well studied. Under laboratory conditions, a whole host of methods are available to track these transformations. But again our ability to track ENP transformations in environmental samples is challenged by their expected trace-level concentrations and the presence of naturally occurring particles. Some information regarding these transformations may be accessible by combining FFF with single-particle ICP-MS as described previously. As one example, heteroaggregation of ENPs with natural particles could lead to single-particle ICP-MS signals that do not match the buoyant mass or hydrodynamic size obtained by FFF.

What are your next steps in your work on nanoparticle analysis?

We continue to work at refining the single-particle ICP-MS and FFF techniques for application to environmental and biological matrices. We collaborate with instrument manufacturers to try to make the methods more accessible to a broader user group while also working to validate the methodology to ensure it is robust and accurate. As we move to more-complex materials, we recognize the value in being able to detect multiple elements in individual particles. This capability would allow us to “fingerprint” the nanoparticles to provide a means to identify their source (engineered versus natural) and follow their transformations. Finally, we are examining the influence of materials properties and environmental variables on the release of ENPs from nano-enabled products. This is a key process in the overall life cycle of ENPs and currently is not well understood. Further developments in nanometrology are still key to the successful evaluation of both the benefits and the implications of nanotechnology.

References

• M. Hassellöv, J.W. Readman, J.F. Ranville, and K. Tiede, Ecotoxicology17, 344–361, (2008). DOI: 10.1007/s00216-016-9676-8

• D.M. Mitrano, A. Barber, P. Westerhoff, C.P. Higgins, and J.F. Ranville, J. Anal. At. Spectrom. 27, 1131–1142 (2012). DOI: 10.1039/c2ja30021d

• S. Lee, X. Bi, R.B. Reed, J.F. Ranville, P. Herckes, and P. Westerhoff, Environ. Sci. Technol. 48, 10291–10300 (2014). DOI: 10.1021/es502422v

• M.D. Montano, J.W. Olesik, A.G. Barber, K. Challis, and J.F. Ranville, Anal. Bioanal. Chem.408, 5053–5074 (2016). DOI: 10.1007/s00216-016-9676-8