Occupational Exposure to Carbon Nanotubes and Nanofibers

1 IntroductionMany nanomaterial-based products are now commerciallyavailable. These include nanoscale powders,solutions, and suspensions of nanoscale materials,as well as composite materials and devicesincorporating nanomaterials. The International Organizationfor Standardization (ISO) has developednomenclature and terminology for nanomaterials[ISO/TS 2008]. According to ISO 27687:2008, anano-object is material with one, two, or three externaldimensions in the size range from approximately1–100 nanometers (nm). Sub-categories ofa nano-object are (1) nanoplate, a nano-object withone external dimension at the nanoscale (i.e., 1–100nm); (2) nanofiber, a nano-object with two externaldimensions at the nanoscale, with a nanotubedefined as a hollow nanofiber and a nanorod as asolid nanofiber; and (3) nanoparticle, a nano-objectwith all three external dimensions at the nanoscale.Nano-objects are commonly incorporated in a largermatrix or substrate called a nanomaterial. Thistaxonomy differs slightly from that suggested by thetitle of this CIB “Occupational Exposure to CarbonNanotubes and Nanofibers.” By this title, NIOSH isnot suggesting an alternative taxonomy, but ratheridentifying the nano-objects (nanoscale carbon fiberand tube structures) that have been evaluatedto date in toxicology and workplace exposure measurementstudies.Carbon nanotubes (CNT) are nanoscale cylindersof carbon (essentially consisting of seamlessly“rolled” sheets of graphene) that can be producedwith very large aspect ratios. There is no single typeof carbon nanotube. They may differ in shape, dimension,physical characteristics, surface coatings,chemical composition, or surface functionalization.This includes “raw” CNT, which contain residualmetal catalysts vs. “purified” CNT, from whichmost of the metal catalysts have been removed.Single-walled carbon nanotubes (SWCNT) consistof a single rolled graphene sheet and have a typicaldiameter of approximately 1–2 nm. Multi-walledcarbon nanotubes (MWCNT) consist of manysingle-walled tubes stacked one inside the otherwith diameters in the range of 2–100 nm, dependingon the number of encapsulated tubes formingthe CNT structure. SWCNT and MWCNT canvary in length, with some being up to many tens ofmicrometers long [Thostenson et al. 2001]. Carbonnanofibers (CNF), which are structurally similar toMWCNT, have typical diameters approximately 40to 200 nm [Ku et al. [2006]. CNF have lengths rangingfrom tens of micrometers to several centimeters,average aspect ratios (length to diameter ratio)of > 100, and they display various morphologies,including cupped or stacked graphene structures.The primary characteristic that distinguishes CNFfrom CNT resides in graphene plane alignment. Ifthe graphene plane and fiber axis do not align, thestructure is defined as CNF, but when parallel, thestructure is considered a CNT [ISO/TS 2008].The synthesis of CNT and CNF requires a carbonsource and an energy source [Sanchez et al. 2009].CNT and CNF are synthesized by several distinctmethods, including chemical vapor deposition(CVD), arc discharge, laser ablation, and highpressureCO conversion (HiPco). Depending onmaterial and method of synthesis, a metal catalystmaybe used to increase yield and sample homogeneity,and to reduce the synthesis temperature.The diameter of the fibers depends on the dimensionsof the metal nanoparticle used as a catalyst;the shape, symmetry, dimensions, growth rate, andcrystallinity of the materials are influenced by theselection of the catalyst, carbon source, temperature,and time of the reaction. Different amountsof residual catalyst often exist following synthesis;consequently, post-synthesis treatments are usedto increase the purity of the product. The mostNIOSH CIB 65 • Carbon Nanotubes and Nanofibers1