The aggregation behavior of PEI-SWCNT could be explained by ion condensation theory (see Supporting Text 4). According to the theory, the aggregation is promoted due to the development of a layer of strongly held counterions on highly charged one-dimensional rod-like PEI-SWCNT polyelectrolytes in solution. When the surface charge of PEI-SWCNT is sufficiently high, a strongly attractive, long-range Coulomb force develops between highly charged rod-like polyelectrolyte molecules bearing similar charges. This attractive force explains the like-charge aggregation of the highly charged PEI-SWCNT (Fig. ). It is likely that the aggregation tendency of PEI-SWCNT is one of the major reasons for non-reproducible results. In addition, this condensed counterion layer has extremely small thickness when compared with the diffuse ion layer of moderately charged polyelectrolytes such as chitosan-SWCNT +38mV (Supporting Text 4, Fig. ). The relatively smaller hydrodynamic diameter of PEI-SWCNT could facilitate the passage of PEI-SWCNT through cell wall pores, whereas the larger hydrodynamic diameter of chitosan-SWCNTs could hinder the passage of chitosan-SWCNTs.