C60 in water:: Nanocrystal formation and microbial response

Upon contact with water, under a variety of conditions, C-60 spontaneously forms a stable aggregate with nanoscale dimensions (d = 25-500 nm), termed here nano-C-60. The color, hydrophobicity, and reactivity of individual C-60 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 C-60 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-C-60 remains stable in solution at or below ionic strengths of 0.05/for months. In addition to demonstrating aggregate formation and stability over a wide range of conditions, results suggest that prokaryotic exposure to nano-C-60 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 C-60 but also that of the aggregate form generated in aqueous media.