Functionalization, Dispersion, and Cutting of Boron Nitride Nanotubes in Water

Carbon nanotubes (CNTs) are promising as the drug carriers to cells for various biomedical applications including cancer therapy. Toxicity of CNTs was a debatable issue as previous studies were performed using long and unfunctionalized CNTs, which tend to entangle in the biological systems and lead to various lung lesions and blocking of the airways. More recent in vitro and in vivo studies indicate that high-purity single-walled CNTs are nontoxic when functionalized as hydrophilic nanomaterials. 7 Furthermore, in vitro investigation showed that water-soluble, short length (100 300 nm) CNTs exhibit longer blood circulation times and lower uptake by the reticuloendothelial system (RES). Boron nitride nanotubes (BNNTs) are structurally similar to CNTs and potentially useful for biomedical applications. In particular, BNNTs offer unique potentials for boron neutron capture therapy (BNCT) because boron has a very high neutron capturing cross section. In addition, BNNTs are proven to have extraordinary mechanical properties comparable to that of CNTs. Therefore, there are increasing interests in exploring the application of BNNTs in other areas for high-strength composites, radiation shielding in space vehicles, and nuclear reactor facilities. The key step toward all of these applications is to disperse functionalized BNNTs in organic solvents and water. However, noncovalent functionalization of BNNTs in water is merely unexplored. 17 In particular, the stability of the dispersed BNNTs in water was not emphasized and investigated prior to biological experiments. In fact, toxicity of BNNTs on human embryonic kidney (HEK293) cells was recently found, contradicted to a prior report. Because short nanotube length was reported as an essential factor for CNTs to be biological compatible without causing accumulation and toxicity in the tested animals, it is thus important to establish the mechanism behind functionalization of BNNTs in water, their dispersion stability, as well as exploring possible length shortening processes. In this Article, we show that multiwalled BNNTs can be welldissolved and dispersed in water through functionalization with methoxy-poly(ethylene glycol)-1,2-distearoyl-sn-glycero3-phosphoethanolamine-N conjugates (mPEG-DSPE, Figure S1 of the Supporting Information). This PEGylated phospholipid is biocompatible and soluble in aqueous and organic solutions. The dispersion of these mPEG-DSPE functionalized BNNTs (mPEG-DSPE/BNNTs) is stable for more than 3 months without noticeable aggregation. The functions of mPEG, DSPE, and solvents are then systematically scrutinized. Furthermore, we found that long hour ultrasonication process can shorten the initial lengths of BNNTs (>10 μm) to below 500 nm. We believe that these findings are important to implement BNNTs in future biomedical applications.