Detection of Uptake and Translocation of Multi-Walled Carbon Nanotubes in Lettuce.

Increasing production and use of carbon nanotubes (CNTs) will increase their accumulation in soil environments. If agricultural crops take up CNTs from soils, consequent human dietary exposure to CNTs could be a threat to public health. This study was designed to develop a method for detection of CNTs in agricultural plants and analyze the uptake of CNTs by hydroponically grown plants. Tissue digestion coupled with Raman spectroscopy and programmed thermal analysis (PTA) was used to detect pristine- (p-) and carboxyl functionalized (c-) multi-walled CNTs (MWCNTs) in leaf, stem, and root of lettuce (Lactuca sativa L.). Five different digestion reagents, i.e., sulfuric acid, hydrochloric acid, nitric acid, hydrogen peroxide, and ammonium hydroxide were compared. The fingerprint D- and G- bands of p- and c-MWCNTs were detected for lettuce tissues spiked with MWCNTs, after digestion with all the reagents. Based on the intensity ratio of D- to G-band (I_D/I_G) and interferences of plant materials, digestion with nitric acid was best suited for detection of MWCNTs in plant tissues. For lettuce treated with 20 mg/L p- or c-MWCNTs in hydroponic solutions, CNTs were detected in the leaf, stem and root tissues using the nitric acid digestion coupled with Raman analysis. In addition, digestion coupled with PTA was used to further confirm the presence of CNTs in plant tissues. Our results showed that acid digestion coupling with Raman spectroscopy or PTA can be used to detect CNTs in plant tissues, and MWCNTs can be taken up by lettuce and translocated to its leaves. These findings potentially have broad implications on the detection of CNTs in natural media and analysis of their human exposure through diet.