AFM and hydrodynamic diameter (d) measurements showed a progressive aggregation with a subsequent increase of d upon the increasing concentration of plasmid DNA. The PEI-SWCNT:DNA ratio of 3:1 showed a minimal increase of d after adding DNA, whereas ratios as low as 1:1 showed significant aggregation with an increase of d (d = 300 nm, Fig. ), and thus those preparations could not be infiltrated into plant tissues because of precipitation out of the solution (Fig. ).