Detection and Quantification of Single-walled Carbon Nanotubes in Environmental and Biological Samples for Evaluation of Fate, Transport and Bioaccumulation
Single-walled carbon nanotubes (SWCNT) are unique, anthropogenic allotropes of nanoparticulate black carbon. As numerous industrial and commercial uses of SWCNT result the heavy expansion of production of this material, the release of SWCNT is likely to occur, increasing their level in air, water and soil. SWCNTs have been shown to cause adverse impact in organisms from direct exposure through ingestion or inhalation. In addition to direct exposure, SWCNT can also induce toxicity to organisms by indirect exposure such as adsorption of hydrophobic contaminants (HOCs). One unique property of SWCNT is the quantized nature of their electronic structure, which is dependent on the chiral wrapping angle of the sp2 hybridized graphene sheet that comprises the wall of each SWNT species. Using probe HOCs – one planar polycyclic aromatic hydrocarbon (PAH)14 C-naphthalene and one halogenated aromatic 14 C-hexachlorobenzene and purified conductive and semiconductive SWCNT species, my first study aimed at assessing the role of SWCNT electronic structure on HOC sorption. Despite their differences in electronic structures, the results indicated that overall the electronic structure does not influence the adsorption of HOCs. However, due to the large specific surface area, SWCNT have a general high affinity for HOCs. Upon release of SWCNT into aquatic environment, SWCNT have the potential to affect the distribution of organic contaminants by acting as strong sorbent.
A significant barrier to studying toxicity of SWCNT to animal models is the lack of in vivo techniques to track and quantify SWCNT for assessing their distribution, transport and bioaccumulation. The fluorescence resulting from the unique band gap of each species of semiconductive SWCNT allows the detection and quantification of a bulky SWCNT sample using near infrared fluorescence spectroscopy (NIRF). NIRF is highly sensitive to detect SWCNT in biological tissues due to the low fluorescence in the near infrared region from biological samples. Two exposure routes were investigated using NIRF: ingestion from dietary track using fathead minnow (FHM) fish model in an aquatic environment and inhalation through lung using mouse model. The SWCNT extraction conditions were optimized and validated using spike recovery experiments. SWCNT were extracted from fish tissues, intestine, and liver using ultrasonic extraction in 2% sodium deoxycholate1extraction. Proteinase K digestion was needed for dissolving mouse lung prior to SDC extraction. The quantification results showed that while SWCNT readily passed through fish dietary track with minimal partition into the lumen tissue and caused no acute toxicity; SWCNT was less mobile in respiratory system and was responsible for the lung-term pulmonary disease induced.
The fate, transport and bioaccumulation of SWCNT are essential information for risk assessment and making environmental regulations for nanomaterials. Currently the lack of standardized sensitive characterization and quantitative analytical methods for SWCNT determination at the current levels in the environment is one major barrier for evaluation of their real impact to the environment. NIRF is sensitive for environmental samples. However, this technique is not sensitive to all types of SWCNT. Metal catalysts are widely used in synthetic production of SWCNTs, leading to total metal content ranging from 5 - 30%. The metal: metal ratios and metal: carbon ratios of SWCNT are very distinctive from many geological materials. A metal fingerprinting approach was developed by monitoring the metal type and metal: metal ratios, along with elemental carbon content. SWCNT can be principally quantified using inductive coupled plasma mass spectrometry (ICP-MS). Metal content, metal: metal ratios, elemental carbon and metal: carbon ratios were analyzed for two aerosol matrices, the urban dust NIST SRM 1649b and aerosol collected at Duke University using three types of SWCNT: SG65 SWCNT, SG65i SWCNT and P2 SWCNT. Results demonstrated that the metal finger approach worked well with all aerosol matrices with detection limits near ng m-3. It worked best with elements that were less abundant in the background such as Co and Y. This method offers a robust and economic approach for application to occupational spaces for monitoring possible SWCNT release.
Applying a similar approach in sediment presents a significant challenge as background metals present in sediment complicates such analyses. To overcome these challenges, we have applied density gradient ultracentrifuge (DGU) to isolate and separate SWCNT in sediment extracts prior to both NIRF and ICP-MS analysis. Several types of SWCNTs (arc discharge, CoMoCat, and HiPCO) were spiked and subsequently extracted from estuarine sediments. SWCNTs were separated into different bands after DGU, primarily into two distinct horizons (one showed near infrared fluorescence, while the other did not). Two techniques,near-infrared spectroscopy (NIRF) and ICP-MS, were applied for quantitation of SWCNTs in these bands. Results indicate excellent separation of SWCNT from interferences in sediments. We have also discovered an apparent disconnect between the metal catalyst particles and SWCNT during density gradient ultracentrifuge separation. It is clear that the SWCNT (within the NIRF band) is not physically associated with metal catalyst. This result was further confirmed using single-particle ICP-MS. Although DGU separation seems to be an outstanding method for isolating SWCNT from aquatic sediment for analysis, our current findings indicate that metal fingerprints derived from residual catalyst may not be a good tracer for SWCNT occurrence and fate in marine sediments, as the associated metal catalyst particles in SWCNT preparations might be transported in different ways relative to the SWCNT.
Overall, my research explored several analytical techniques to detect and quantify SWCNTs at their relevant concentration in various environmental matrices. These techniques will provide essential information for evaluating the environmental impact based on SWCNTs fate, transport and bioaccumulation in the environment.
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