Tapping Mode Atomic Force Microscopy Investigation of Poly(amidoamine) Dendrimers:  Effects of Substrate and pH on Dendrimer Deformation

Substrate effects, volume reproducibility, and pH effects on the size and shape of ethylenediamine core poly(amidoamine) dendrimers (generations 6−9) were explored using tapping mode atomic force microscopy (AFM) measurements. A statistical analysis of the measurements indicated a 4% variation in volume for repeated measurement using the same tip. Volume determinations by numerical integration and a spherical cap estimation method were explored. The spherical cap model was shown to overestimate dendrimer volumes by a factor of ∼2. As substrates were changed from mica to more hydrophobic surfaces, AFM-measured heights and diameters approached ideal-sphere dendrimer diameters. Acidification of dendrimer samples from generations 6−9 led to an observed 33% increase in volume, 26% increase in height, and 9% decrease in diameter. Expansion upon acidification can be attributed to maximization of charge separation and increased solvent retention within the dendrimers. Single dendrimer resolution within two-dimensional clusters was enhanced using carbon nanoprobes.