Investigating Nanoparticles in the Environment with SP-ICP-MS

There is growing concern about the unknown effects that nanoparticles may have on the environment, especially in drinking water and plants. Single-particle inductively coupled plasma–mass spectrometry (SP-ICP-MS) is emerging as a useful technique for analyzing nanoparticles and their presence in environmental and biological systems. Honglan Shi, a chemistry professor at Missouri University of Science and Technology, and her research group have been using SP-ICP-MS to investigate nanoparticles in drinking water and plant uptake. She recently spoke to Spectroscopy about this work.

You have published several studies on the use ofSP-ICP-MS to detect zinc oxide in drinking water (1,2). How did the approach you used differ from previously described methods?

Before the SP-ICP-MS method was established, there were no direct methods to detect nanoparticles, whether zinc oxide or others, in drinking water. Methods such as imaging by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with X-ray diffraction (XRD) to analyze surface elements were useful when combined with size cut-offs by filtration, but these techniques could not isolate particles by element. With the SP-ICP-MS methods, nanoparticles are completely atomized in the plasma and arrive at the mass spectrometer detector as a packet of ions. The intensity of the pulse signal generated is correlated to the mass in the particle and then converted to particle sizes for that specific element or metal oxide particle of interest. In this way, general information regarding the particle sizes and size distributions for elements of interest can be determined in a variety of matrices that were otherwise impossible, or at the very least, extremely difficult to quantitate by any other technology. Furthermore, each particle is detected as a pulse signal, and the pulse signal frequency is proportional to the particle concentration in the sample-thus the particle concentration can be quantitatively measured. Dissolved elements in samples are detected as a constant signal that can be quantified with appropriate calibration.

The key advantages of this technique include high sensitivity and selectivity to detect particulate species directly in water matrices, the ability to track the particle fates directly without changing the them, and both particulate and dissolved fractions of a specific element can be quantified in a single run. If nanoparticles become a concern for regulating agencies, monitoring their presence in drinking water will be necessary. Implementation of SP-ICP-MS using existing ICP-MS systems in drinking water treatment facility labs may prove to be the best way to monitor these potentially harmful contaminants.

In those studies, you characterized and quantified titanium dioxide (TiO₂), cerium dioxide (CeO₂), silver, gold, and zinc oxide (ZnO) nanoparticle concentrations in surface water and treated drinking water (1,2). Why did you focus on those nanoparticle types? Are there are any other nanoparticle types that are found in water that you can detect using SP-ICP-MS?

Each of these particles is among the most produced nanoparticles species in the world because of their useful properties in a variety of applications including antimicrobial agents, outdoor paint coatings, sunscreen, electronics, and many more. Surface-stabilized gold and silver nanoparticles have been the primary focus of the preliminary SP-ICP-MS work because of their well-defined production processes and stable suspensions through surface stabilization. Silver nanoparticles often find use as an antimicrobial agent in athletic footwear and food packaging. Gold and silver nanoparticles are used in drug delivery and biological applications. Interest in TiO₂, CeO₂, and ZnO nanoparticles arose from their presence in the Earth’s crust as well as their numerous nanoparticle applications that may lead to environmental release. Examples include their use as pigments in surface coatings and UV absorbers in sunscreens, among many others. Because of the element-specific nature of SP-ICP-MS, it is possible to detect many other types of metallic nanoparticles, such as copper, iron oxide, and nickel.

In another example of your work (3), you explored how Ce nanoparticles affect plants by using SP-ICP-MS to detect the size and size distribution of particulate Ce, particle concentration, and dissolved cerium in the shoots of four plant species (cucumber, tomato, soybean, and pumpkin). How did the SP-ICP-MS method differ from previous methods to study plant uptake?

Characterization and quantification of nanomaterials in complex biological samples at environmentally relevant (very low) concentrations was a serious challenge prior to SP-ICP-MS technology development. The developed SP-ICP-MS method in our study is the only method that can detect the size and size distribution of particulate Ce, particle concentration, and dissolved Ce concentration in plant tissues simultaneously at these low environmental concentrations. Previous studies used imaging techniques, SEM and TEM, to image nanoparticles in tissue samples or they completely digested the tissue and used traditional ICP-MS to measure total elemental mass in the whole plant. These technologies are either not quantitative or dissolve the particles. Our method used enzymatic digestion to extract CeO₂ nanoparticles without changing their status (no dissolution), allowing us to extract the intact particles from the plant tissue. Compared with imaging technology, SP-ICP-MS can detect very low concentrations of nanoparticles, and more quantitatively, and can be used to track the transportation and transformation of nanoparticles through the plant.

What did those results indicate?

The experimental results showed that an enzymatic digestion method with macerozyme R-10 can successfully extract CeO₂ nanoparticles from different plant species without dissolution of nanoparticles. The developed SP-ICP-MS protocol can simultaneously detect particle size, size distribution, particle concentration, and the dissolved form of Ce in plant tissues. This study is the first to demonstrate the presence of dissolved Ce and particulate Ce in plant tissues exposed to CeO₂ nanoparticles. The extent of plant uptake and accumulation appears to be dependent on the plant species. More studies on plant uptake of nanoparticles are required to understand how different types of surface coatings, particle shapes, and particle size affect uptake and distribution through plants.

In another plant uptake study (4), you used an enzymatic digestion method followed by SP-ICP-MS to determine the amount of gold nanoparticles in tomato plant tissues. How did this work differ from your Ce nanoparticle research with plants?

This was the first key publication of our SP-ICP-MS method development for studying nanoparticle uptake by plants. It was in this study that we developed the novel enzymatic digestion method using macerozyme R-10 enzyme to extract nanoparticles-in this case, gold nanoparticles from tomato plants. Gold nanoparticles were selected because surface-stabilized gold nanoparticles with a narrow size distribution were commercially available. The results of the study demonstrated that tomato plants can uptake intact gold nanoparticles from the roots and transport them into plant tissue. The successful development of this extraction followed by SP-ICP-MS detection method provides a promising way to systematically study the interactions between nanoparticles and plants. The method is anticipated to be applicable for other nanoparticles and plant species (as shown in our study of Ce nanoparticle uptake in four plant species mentioned above). 

Can you tell us about your work using SP-ICP-MS to detect titanium dioxide in sunscreen (5)?

Many commercial sunscreens contain TiO₂ nanoparticles as a UV absorber rather than organic compounds. The method developed in our work can simultaneously determine primary particle size, size distribution, particle concentration (particles per unit weight sunscreen) of TiO₂ nanoparticles, and mass content (weight % in sunscreen) can also be determined by standard-addition SP-ICP-MS. Standard-addition SP-ICP-MS involves adding known concentrations of TiO₂ nanoparticle standards to the sample to determine mass content. The major advantages of this method are easy sample preparation and high-throughput analysis to avoid time-consuming and costly acid digestion-ICP-MS methods.

What are the next steps in your research on analyzing nanoparticles?

More method development and applications of SP-ICP-MS technology have been conducted by our research team and will be published soon. Our research is focused on environmental, biological, and health components of nanoparticles. Environmental work focuses on analyzing nanoparticles in various environmental compartments including surface and drinking water and environmental release and fate studies. In biological systems, the impact of nanoparticles on aquatic ecology is of interest, specifically because toxicity to environmental microcosms has not been well studied in the environmental relevant concentration range. The health impacts of nanoparticles in single cell level have been under investigation. Using SP-ICP-MS and single cell ICP-MS to evaluate the impact of nanoparticles on the cellular level is one of our current research focuses and will provide direct and specific information that can be used for nanotoxicity and drug delivery study at individual cell systems. Our ultimate goal is to establish SP-ICP-MS as a reliable technique for routine analysis of nanoparticles in environmental and biological systems.  

References

• A.R. Donovan, C.D. Adams, Y. Ma, C. Stephan, T. Eichholz, and H. Shi, Chemosphere144, 148–153 (2016).

• A.R. Donovan, C.D. Adams, Y. Ma, C. Stephan, T. Eichholz, and H. Shi, Anal. Bioanal. Chem. (2016). DOI 10.1007/s00216-016-9432-0.

• Y. Dan, X. Ma, W. Zhang, K. Liu, C. Stephan, and H. Shi, Anal. Bioanal. Chem. (2015). DOI 10.1007/s00216-016-9565-1.

• Y. Dan, W. Zhang, R. Xue, X. Ma, C. Stephan, and H. Shi, Environ. Sci. Technol. 49, 3007–3014 (2015). DOI: 10.1021/es506179e.

• Y. Dan, H. Shi, C. Stephan, and X. Liang, Microchem. J. 122, 119–126 (2015).