The Kinetics of Single-Walled Carbon Nanotube Aggregation in Aqueous Media Is Sensitive to Surface Charge

Abstract

Single-walled carbon nanotubes (SWCNTs) dispersed in aqueous media have many potential applications in chemistry, biology and medicine. To disperse SWCNTs into aqueous media, it is often necessary to modify the surface of SWCNTs by either covalent or noncovalent methods. As a result of this modification, the properties of SWCNTs may be profoundly influenced by the nature of the surface modification. Here, by using SWCNTs dispersed with single-stranded DNA of different lengths, we show that the kinetics of SWCNTs’ aggregation in aqueous media is strongly dependent on the status of the overall surface charge. SWCNTs with a greater number of surface charges showed faster aggregation. The difference in the rate of aggregation can differ by more than ten-fold among different conditions tested. AFM imaging of the discrete time points along the aggregation process suggests that aggregation starts with the formation of microfilaments, which can further grow to form bigger aggregates. The formation of bigger aggregates also renders it more difficult to redisperse them back into the aqueous media. The concentration of counterions required to trigger SWCNT aggregation also shows a dependence on the concentration of KCl in the aqueous solution, which supports that electrostatic interactions instead of van der Waals interactions dominate the interactions among these individually-dispersed SWCNTs in aqueous media.