Cardanol-derived cationic surfactants enabling the superior antibacterial activity of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) are potential antibacterial material, and their antibacterial activity in aqueous solutions depends on efficient surfactants to create strong interactions between well-dispersed SWCNTs and bacterial cells. Here, we designed and synthesized a new family of cationic surfactants by introducing different positively charged hydrophilic heads, i.e. -(CH₂)₆N⁺(CH₃)₃Br^-, -(CH₂)₂N⁺(CH₃)₃Br^- and -(CH₂)₂N⁺PyridineBr^-, to cardanol obtained from cashew nut shell liquid. These surfactants can efficiently disperse SWCNTs in aqueous solutions because benzene rings and olefin chains in cardanol enable their strong π-stacking on SWCNTs. A much higher fraction of SWCNTs can be dispersed individually compared to the commonly used surfactant, dodecylbenzene-sulfonate sodium (SDBS). SWCNTs dispersed in the cardanol-derived surfactants demonstrate significantly improved antibacterial activity. At the concentration of 0.5 wt%, their minimum inhibitory concentration is 0.33 and 0.02 μg ml^-1 against E. coli and S. aureus, respectively, which is only 0.8%-1.5% of that of SDBS-dispersed SWCNTs. The strong antibacterial activity can be attributed to both better dispersion of SWCNTs and positive charges introduced by hydrophilic heads, which are attracted to negatively charged bacterial cell surfaces. These cardanol-derived surfactants are promising as sustainable surfactants for enabling various SWCNT applications.