We show that the cytotoxicity of water-soluble fullerene species is a sensitive function of surface derivatization; in two different human cell lines, the lethal dose of fullerene changed over 7 orders of magnitude with relatively minor alterations in fullerene structure. In particular, an aggregated form of C60, the least derivatized of the four materials, was substantially more toxic than highly soluble derivatives such as C3, Na+ 2 - 3[C60O7 - 9(OH)12 - 15](2-3)-, and C60(OH)24. Oxidative damage to the cell membranes was observed in all cases where fullerene exposure led to cell death. We show that under ambient conditions in water fullerenes can generate superoxide anions and postulate that these oxygen radicals are responsible for membrane damage and subsequent cell death. This work demonstrates both a strategy for enhancing the toxicity of fullerenes for certain applications such as cancer therapeutics or bactericides, as well as a remediation for the possible unwarranted biological effects of pristine fullerenes.

Upon contact with water, under a variety of conditions, C60 spontaneously forms a stable aggregate with nanoscale dimensions (d = 25−500 nm), termed here “nano-C60”. The color, hydrophobicity, and reactivity of individual C60 are substantially altered in this aggregate form. Herein, we provide conclusive lines of evidence demonstrating that in solution these aggregates are crystalline in order and remain as underivatized C60 throughout the formation/stabilization process that can later be chemically reversed. Particle size can be affected by formation parameters such as rates and the pH of the water addition. Once formed, nano-C60 remains stable in solution at or below ionic strengths of 0.05 I for months. In addition to demonstrating aggregate formation and stability over a wide range of conditions, results suggest that prokaryotic exposure to nano-C60 at relatively low concentrations is inhibitory, indicated by lack of growth (≥0.4 ppm) and decreased aerobic respiration rates (4 ppm). This work demonstrates the fact that the environmental fate, distribution, and biological risk associated with this important class of engineered nanomaterials will require a model that addresses not only the properties of bulk C60 but also that of the aggregate form generated in aqueous media.

Nanocrystalline titanium dioxide (nano-TiO(2)) is an important material used in commerce today. When designed appropriately it can generate reactive species (RS) quite efficiently, particularly under ultraviolet (UV) illumination; this feature is exploited in applications ranging from self-cleaning glass to low-cost solar cells. In this study, we characterize the toxicity of this important class of nanomaterials under ambient (e.g., no significant light illumination) conditions in cell culture. Only at relatively high concentrations (100 microg/ml) of nanoscale titania did we observe cytotoxicity and inflammation; these cellular responses exhibited classic dose-response behavior, and the effects increased with time of exposure. The extent to which nanoscale titania affected cellular behavior was not dependent on sample surface area in this study; smaller nanoparticlulate materials had effects comparable to larger nanoparticle materials. What did correlate strongly to cytotoxicity, however, was the phase composition of the nanoscale titania. Anatase TiO(2), for example, was 100 times more toxic than an equivalent sample of rutile TiO(2). The most cytotoxic nanoparticle samples were also the most effective at generating reactive oxygen species; ex vivo RS species generation under UV illumination correlated well with the observed biological response. These data suggest that nano-TiO(2) samples optimized for RS production in photocatalysis are also more likely to generate damaging RS species in cell culture. The result highlights the important role that ex vivo measures of RS production can play in developing screens for cytotoxicity.