We extensively investigated in vitro and in vivo the toxicities of as-grown single-walled carbon nanohorns (SWNHs), a tubular nanocarbon containing no metal impurity. The SWNHs were found to be a nonirritant and a nondermal sensitizer through skin primary and conjunctival irritation tests and skin sensitization test. Negative mutagenic and clastogenic potentials suggest that SWNHs are not carcinogenic. The acute peroral toxicity of SWNHs was found to be quite low--the lethal dosage for rats was more than 2000 mg/kg of body weight. Intratracheal instillation tests revealed that SWNHs rarely damaged rat lung tissue for a 90-day test period, although black pigmentation due to accumulated nanohorns was observed. While further toxicological assessments, including chronic (repeated dose), reproductive, and developmental toxicity studies, are still needed, yet the present results strongly suggest that as-grown SWNHs have low acute toxicities.

Material storage in the inner spaces of nanocarbon materials is visualized for the first time. It is shown that creating paths of a specific size to the inner space (see Figure) is indispensable for material storage in carbon nanopores, and that the adsorption rate for fullerene molecules is largely dependent on the nanoscale curvature of the graphitic planes composing the nanospaces.

To increase the practical daily capacity to produce single-wall carbon nanohorns (SWNHs) with high purity by CO 2 laser ablation of graphite, we made a new three-chamber system composed of a target reservoir, a laser ablation chamber, and a collection chamber. The frequency of exchanging targets was reduced by increasing the target size (diameter, 10 cm; length, 50 cm), and the time taken to exchange targets was reduced by adding an automated exchange mechanism. Most of the SWNHs produced in the laser ablation chamber were immediately swept into the collection chamber by an Ar gas carrier. This avoided the problem of fluffy SWNHs blocking the laser beam irradiating the target and enabled the CO 2 laser to be operated continuously. The resulting SWNHs had a purity of 92-95% under the optimized conditions of laser power density (15-30 kW/cm 2 ) and target rotation speed (1-3 rpm). This increase in purity represents a remarkable improvement on the previous level of 85-90%. A practical production capacity of 1 kg/day was achieved, which is about 100 times greater than the daily capacity attained by the previous single-chamber system.