Formation of Aqueous Suspensions of Fullerenes

Colloidal suspensions of C-60, C-70, and a derivative of C-60, PCBM ([6,6]-Phenyl C-61-butyric acid methyl ester) were produced by extended mixing in water. We examined the contribution of background solution chemistry (pH, ionic strength) on the formation of colloidal suspensions in terms of mass, aggregate size, and zeta potential. These parameters were also compared between free-settling and filtered treatments. Results indicated that all three fullerenes were highly negatively charged in aqueous systems, that it took a finite time to reach kinetically stable suspensions, and that suspension formation was pH and ionic strength dependent. With isoelectric points approaching zero, the suspensions were generally stable at pH > 3. The results indicate that it is critical to state the condition under which the formation of aqueous fullerene suspensions occurs when employing such suspensions to evaluate environmental toxicity or fate and transport of fullerenes.