OT-Luna Blog-CouNT Assay to Quantitatively Measure and Detect Carbon Nanotubes

July 13, 2016

Zhiguo Zhou, Ph.D.
Director of Luna Nanomaterials Group

https://lunainc.com/counttm-assay-quantitatively-measure-detect-carbon-nanotubes/

The enormous potential of carbon nanotubes (CNT) and their successful incorporation in commercial products including bulk composite materials and thin films have dramatically increased their production worldwide to more than 10,000 metric tons per year. As with many new materials, toxicology concerns are being raised with CNTs regarding their interactions with humans and any safety hazards and environmental health risks CNTs may pose. CNT exposure levels (inhalation and respiration) and occupational health for workers in large scale manufacturing and subsequent handling facilities are of particular concern. Bagging, maintenance of CNT reactors, and powder conditioning were found to be associated with higher exposure levels in the production area, whereas increased exposure levels in the R&D area were related to handling of multi-walled carbon nanotube (MWCNT) powder according to one 2016 publication in The Annals of Occupational Hygiene titled “Occupational Exposure to Multi-Walled Carbon Nanotubes During Commercial Production Synthesis and Handling”.

Also, multiple studies have shown that carbon nanotubes can be released into the environment from polymer-MWCNT composites. At the Nanotech 2016 Conference, scientists and engineers from government agencies including NIST, EPA and FDA, and companies and universities held a special session “Nanoparticle Release During the Life Cycle of Consumer Products & Nanocomposites” to discuss this important issue. For example, epoxy-MWCNT nanocomposites that are increasingly used in many large-volume industries are often exposed to severe mechanical and environmental stresses, such that, during a polymer-MWCNT composite’s life cycle, MWCNT incorporated in the matrix will be eventually released into the environment. One particular release mechanism is degradation of the matrix by the weathering environments. According to a NIST Protocol for Accelerating Laboratory Weathering and Measurements of Degradation of Polymer-MWCNT Composites, after 43 days, a dense CNT layer is formed on the composite surface due to degradation of the polymer matrix, exposing MWCNTs to the environment (Figure 1).

Figure 1: SEM images of epoxy-0.72 % MWCNT composite exposed in the SPHERE (NIST SPHERE accelerated laboratory weathering device) at 50°C and 75 % RH, showing the formation of a dense CNT layer on the nanocomposite surface after 43 days – http://dx.doi.org/10.6028/NIST.SP.1200-15.

However, understanding the effects of CNTs is complicated by the large diversity of structures, sizes, and surface functionality of carbon nanotubes. For the same reason, sensitive and accurate detection and quantification of multiple carbon nanotubes species in solution, water, air and other medium has been impossible. To date, the best analytical tool is the current NIOSH Method 5040 (i.e. thermal carbon analysis) which can only measure Total Carbon (TC) or Elemental Carbon (EC) for air samples that are collected on a filter. Electron microscope is often used to verify CNTs. To address this analytical need and enable the community to better understand the effects of carbon nanotubes in and on the environment, Luna developed an easy to use, quantitative CouNT^TM assay to detect and measure carbon nanotubes.

• The CouNT^TM assay can detect a variety of CNTs (single walled and multi-walled with or without surface functionalization).
• The CouNT^TM assay can distinguish CNTs from carbon black, amorphous carbon, graphite or carbon fibers. The assay is specific to CNTs, thus only report the content of CNTs in an unknown sample that may contain other carbonaceous materials.
• The limit of detection and sensitivity for CouNT^TM assay is in the range of 3.0-5.0 ppb.
• The CouNT^TM assay has a broad dynamic range, can detect up to μg-mg/mL.
• The assay can be used with commonly available laboratory instruments or fieldable devices.
• The assay turnaround time is less than 30 minutes.
• CouNT^TM is easy to use, requiring minimal training for those already doing analytical type testing.
• The CouNT^TM assay can work with original CNT samples in solution (organic or aqueous), dry powder, air particulates captured on filters or any other forms.

Figure 2 shows the linear response of the signal to increasing concentrations (in the low ppb range) of CNTs, and the good sensitivity of the CouNT^TM assay. Figure 3 shows that the CouNT^TM assay is able to selectively detect carbon nanotubes in the presence of carbon black or graphite, two common substances that coexist with carbon nanotubes. The high assay selectivity arises from the uniquely designed CNT affinity reagent.

Figure 2: Detection sensitivity of the CouNTTM assay for carbon nanotubes.

Figure 3: The CouNTTM assay is able to selectively detect carbon nanotubes in the presence of carbon black and graphite.

Carbon nanotubes are used in a variety of commercial products, and more products containing CNTs are in development. It is vital to measure the content of CNTs in workplace by CNT manufacturers and processing factories, by federal and local regulatory agencies including environmental protection and health monitoring, and by the R&D community.

If you are interested in and want to learn more about Luna’s CouNT^TM assay technology, please contact us for details.