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 s hape of ethylenediamine core poly(amidoamine) dendrimers (generations 6-9)w ere explored using tapping mode atomic force microscopy (AFM) measurements. A statistical analysis of the measurements indicated a 4% variation in vol ume for repeated measurement using the same tip. Volume determinations by n umerical integration and a spherical cap estimation method were explored. T he spherical cap model was shown to overestimate dendrimer volumes by a fac tor of similar to2. As substrates were changed from mica to more hydrophobi c surfaces, AFM-measured heights and diameters approached ideal-sphere dend rimer diameters. Acidification of dendrimer samples from generations 6-9 le d to an observed 33% increase in volume, 26% increase in height, and 9% dec rease in diameter. Expansion upon acidification can be attributed to maximi zation of charge separation and increased solvent retention within the dend rimers. Single dendrimer resolution within two-dimensional clusters was enh anced using carbon nanoprobes.