Methods used to characterize and quantify multi-walled carbon nanotubes (MWCNTs) from soil samples are still rarely available to date. Depending on the method used, quantification may be biased by soot ubiquitously present in sediments and soils.
It is a big challenge to differentiate between soot and MWCNTs, as they are chemically and physically similar in terms of density or thermal stability. However, shape could be a contrasting factor for the detection of MWCNTs in soil or pure soot.
In this article, a shape factor ρ, derived from AF4-MALS, is evaluated for its capabilities to detect MWCNTs in these matrices.
Detection of MWCNTs in Soot
Researchers used different shaped particles, such as MWCNTs, native soil and soot, to display the shape differentiation capabilities of AF4-MALS. In the quantification method, samples (particle powder, dry soil) were dispersed in 2% sodium deoxycholate/0.05% sodium azide, sonicated, and centrifuged at 17.500 g for 10 minutes.
The supernatant half of the volume was used as a working suspension, and 10-5 M ammonium nitrate/0.02% sodium azide was used as an AF4 carrier. Retention time calibration with different certified latex standards was used to determine the hydrodynamic radius (rh), and the PN3621 21-angle MALS detector was used to determine the radius of gyration (rg). Both of these parameters were integrated to provide a shape factor ρ = rg/rh.
As expected, the soot displayed a relatively homogeneous ρ-distribution over the peak area (Figure 1) with average values of about 0.9, as the soot dispersions contained fractal-like aggregates that deviated from a spherical shape (ρ > 0.775). When MWCNTs were added, the ρ values increased in a concentration-dependent manner. Automated electron microscopy and image analysis (see Gogos et al. 2014) can be used to confirm the results.
Figure 1. Fractograms obtained by AF4-MALS with shape factor ρ (symbols) for equal mass injections (5 µg) of a pristine long MWCNT, soot and a 1:1 mixture of both. Lines show the 92° MALS signal of pure soot (solid) and pure MWCNT (dashed).
Detection of MWCNTs in Soil
Similar to soot, native soil demonstrated ρ-distributions of approximately 0.9 (Figure. 2). Again, when MWCNTs were added, the resulting ρ values increased in a concentration-dependent manner. The resulting method detection limits for MWCNTs in soil were 1.6 to 4 mg g-1.
Figure 2. Fractograms obtained by AF4-MALS with shape factor ρ (symbols) of standard additions of a MWCNT to soil. Lines show the 92° MALS signal of native soil (solid) and soil + 16 mg g-1 MWCNT (dashed).
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Tthe generation of a shape factor ρ from rg and rh values eventually enables the differentiation of MWCNTs and soot. The increment in shape factor is based on the ratio between MWCNTs and soot, i.e. ρ is concentration dependent.
The soil particles, extracted using the present method, differ from MWCNTs in terms of ρ and help to detect MWCNTs in such complex matrices. Detection limits were still much higher than any currently predicted environmental concentration.
Gogos, A., Kaegi, R., Zenobi, R. and Bucheli, T. D., Environmental Science: Nano, 2014, 6(1), 584-594.
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