Quantification of Carbon Nanotubes in Environmental Matrices: Current Capabilities, Case Studies, and Future Prospects

Abstract

Carbon nanotubes (CNTs) have numerous exciting potential applications and some that have reached commercialization. As such, quantitative measurements of CNTs in key environmental matrices (water, soil, sediment, and biological tissues) are needed to address concerns about their potential environmental and human health risks and to inform application development. However, standard methods for CNT quantification are not yet available. We systematically and critically review each component of the current methods for CNT quantification including CNT extraction approaches, potential biases, limits of detection, and potential for standardization. This review reveals that many of the techniques with the lowest detection limits require uncommon equipment or expertise, and thus, they are not frequently accessible. Additionally, changes to the CNTs (e.g., agglomeration) after environmental release and matrix effects can cause biases for many of the techniques, and biasing factors vary amongst the techniques. Five case studies are provided to illustrate how to use this information to inform responses to real-world scenarios such as monitoring potential CNT discharge into a river or ecotoxicity testing by a testing laboratory. Overall, substantial progress has been made in improving CNT quantification during the past ten years, but additional work is needed for standardization, development of extraction techniques from complex matrices, and multi-method comparisons of standard samples to reveal the comparability of techniques.

Increasing evidence indicates low bioaccumulation of carbon nanotubes

“…CNTs have been detected and quantified in environmental matrices and organisms using a broad range of analytical techniques, including fluorescence spectroscopy, Raman spectroscopy, electron microscopy, elemental analysis of the metallic impurities in the CNTs, thermal methods, and radiolabeling 9, 40, 50, 51 . Qualitative measurements (e.g., electron microscopy) do not determine the mass or concentration of CNTs but instead only determine their presence or absence, and thus the preferred methods for measuring biodistribution of CNTs are quantitative ones that determines the mass of CNT in organs.…”

“…A modeled average CNT surface water concentration in Europe was estimated to be 0.0035 ng/L 39 , a concentration below the detection limit of all currently available analytical methods 40 . However, greater concentrations could be present in the environment at release locations.…”

Separation of Bacteria, Protozoa and Carbon Nanotubes by Density Gradient Centrifugation

“…The quantification herein of MWCNTs was afforded by incorporating 14 C as a label into the MWCNT structure, and using liquid scintillation counting of 14 C as the MWCNT tracer. Alternatively, when unlabeled CNTs are employed in such studies, other methods with similar limit of detection as LSC could be used for quantification of CNTs, e.g., inductively coupled plasma-mass spectrometry (ICP-MS), single-particle ICP-MS or inductively coupled plasma optical emission spectroscopy (ICP-OES) using metal catalyst impurities as proxies to quantify CNTs, Raman spectroscopy, and, in the case of SWCNTs, near-infrared fluorescence spectroscopy [49]. A sucrose gradient centrifugation procedure was well suited for separating bacteria from MWCNTs and their agglomerates.…”

Bioaccumulation of Multiwall Carbon Nanotubes in Tetrahymena thermophila by Direct Feeding or Trophic Transfer

“…The use of a sensitive detection method – accelerator mass spectrometry (AMS) - allowed for tracing 14 C-MWCNTs in the biological matrices at low (sub µg/kg) levels; this is the lowest detection level obtained to date for CNT quantification in tissues to our knowledge. 19, 20 Since MWCNTs were not expected to biodegrade under the experimental laboratory conditions of this study, quantification of 14 C could be used to trace MWCNTs in biota. Two environmentally relevant scenarios of CNT transfer to ciliates were compared at the same MWCNT doses: (i) MWCNT uptake via bactivory of MWCNT-encrusted bacteria, and (ii) grazing on medium-dispersed MWCNTs.…”

“…19 To overcome this challenge, we used 14 C-labeled MWCNTs ( 14 C-MWCNTs) to study their accumulation and trophic transfer in a microbial food chain of prey, the bacterium Pseudomonas aeruginosa, and predator, the protozoan Tetrahymena thermophila . The use of a sensitive detection method – accelerator mass spectrometry (AMS) - allowed for tracing 14 C-MWCNTs in the biological matrices at low (sub µg/kg) levels; this is the lowest detection level obtained to date for CNT quantification in tissues to our knowledge.…”