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 similar to2. 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.