Detection and Quantification of Multiwall Carbon Nanotubes in Plant Tissues
AuthorDas, Kamol K
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Increasing production and application of carbon nanotubes (CNTs) for industry and consumer products will lead to the accumulation of CNTs in soils and their potential uptake by agricultural plants. Consequent human exposure to CNTs and possible hazardous health effects have attracted substantial attention from academic community and public. To the other side, recent studies have demonstrated positive effects of CNTs on plant growth, raising great potential for the application of CNTs in agriculture. Unambiguous detection and quantification of CNTs in plant tissues are critical for evaluating CNTs uptake by agricultural plants and associated environmental effects and optimizing their agricultural application. An analytical method was developed for unambiguous detection of multiwall CNTs (MWCNTs) in lettuce (Lactuca sativa L.) tissues by coupling digestion with Raman spectroscopy. It is demonstrated that digestion with nitric acid has efficiently reduced the background plant materials by >98% and facilitated extraction of pristine MWCNTs (p-MWCNTs) and carboxyl functionalized MWCNTs (c-MWCNTs). The detection of p-MWCNTs and c-MWCNTs was confirmed by the G-band (1500-1600 cm-1) and D-band (~1350 cm-1), the two fingerprint peaks for CNTs obtained from Raman spectroscopy. This method has been applied to study the uptake of CNTs by lettuce grown with 5-20 mg/L MWCNTs in culture solution. The results showed that the uptake and translocation of CNTs in lettuce can be mediated by the surface chemistry of CNTs. To further quantify p-MWCNTs in lettuce leaf tissues, a novel method was developed by coupling programmed thermal analysis (PTA) with sequential digestion. The sequential digestion with HNO3 and H2SO4 has removed background leaf tissues by 99.98%, which greatly reduced the interferences of char. P-MWCNTs were quantified based on the elemental carbon evolved at high temperatures (580 °C and 740 °C). Linear relationship was obtained for the observed EC and p-MWCNTs-derived carbon (R2 = 0.98). The methods can quantify CNTs in the plant tissues with a detection limit of 64.9 µg/g. Finally, a method for a rapid quantification of CNTs in agricultural plants has been developed by coupling sequential digestion with ultraviolet-visible (UV-Vis) spectroscopic analysis. The sequential digestion of leaf, stem, and root tissues dramatically reduced the interference of plant tissues on the absorbance at 800 nm, and there was linear relationship between absorbance at 800 nm and aqueous concentration of CNTs. Detection limit of this method was demonstrated to be 18.8-182 µg/g of CNTs in plant tissues. The developed methods could be used to quantitatively evaluate the uptake and translocation of CNTs by agricultural plants. Potential information will be critical for evaluating the health effects of human exposure to CNTs through diet and managing the application of CNTs for agriculture.