Using Kelvin Probe force microscopy (KPFM), we observed local variation in the surface potential of the PEI-coated CNTs concomitant with local variation in the thickness of the PEI coating layer in the range 3-50 nm (75% of the height values lie within the range between 3-20 nm). In this experiment we used PEI 30 -SWCNT +61mV because the infiltration of this conjugate resulted in the best GFP expression in terms of the brightness of the GFP fluorescence signal and lower toxicity (Fig. ). Figure shows the correlation between PEI height and local workfunction of the sample in a representative area of the sample, visible in both in the 3D rendering of the work function on the topography (Fig. ) and in the line profiles of the work function and topography (Fig. ). We observed a negative correlation between the PEI height (in the range of 3-47 nm) and the work function (Fig. ), which confirms the anti-correlation shown in the lines profiles (Fig. ). This negative correlation, i.e., the decrease in the work-function with the thickness, represents the tendency to accumulate more positive charge in the thicker part of PEI coating. We also found potential (charge) fluctuations along the same PEI 30 -SWCNT (Fig. ). Because of this local variation in the surface potential, we posited that DNA could show various binding modes and strengths to the surface of PEI 30 -SWCNT functionalized with different thickness of the PEI polymer. To investigate whether the plasmid DNAs have different binding modes to PEI-SWCNT, we recorded AFM tapping mode images of composites with different PEI surface modifications and zeta potentials. DNA binds to chitosan-CNT +38mV in an extended conformation with no extrusions along the surface of the nanotubes (Fig. ), which is consistent with previous investigations. Similarly, DNA binds to PEI-coated SWCNTs at 20:1 and 30:1 mass ratios with zeta potential of +52mV and with zeta potential ranging between +50 mV and +60 mV respectively in an extended conformation fashion (Fig. ,b and S13a, Supporting Text 5). This type of condensation did not show the expression in the plant cells (Table , row 6, Fig. , and Supporting Appendix 1). By contrast, PEI-SWCNTs with larger zeta potentials (ζ > +60 mV) showed different binding modes of DNA. We found that DNA binds to PEI 20 -SWCNT +61mV , PEI 30 -SWCNT +61mV and PEI 40 -SWCNT +77mV in condensed, partially condensed, and extended fashions (Fig. , Figs. , and Supporting Appendix 2). Importantly, these binding modes coexist on these PEI-SWCNTs (Fig. ). Unlike the extended conformation of DNA binding where DNA is partially exposed to the buffer solution with no extrusions (e.g. Fig. , 5e, and Figs. ), condensed and partially condensed DNA conformations showed extrusions (Fig. DNA-2, 5h, and Fig. ). We observed GFP expression when PEI 30 -SWCNT +61mV and PEI 40 -SWCNT +77mV were complexed with plasmid DNA and infiltrated into plant leaves (Fig. ). When the PEI:SWCNT mass ratio was decreased to 0.1:1 (PEI 0.1 -SWCNT +30mV , Fig. ), the DNA extrusions increased in size concomitant with the loose binding of DNA to PEI-SWCNTs where the infiltrated leaves did not show any sign of GFP expression (Fig. ).