| Abstract
| - This work describes the use of atomic force microscopy (AFM) to measure the size of dendrimer-stabilizedPt nanoparticles (Pt DNs) deposited from aqueous solutions onto mica surfaces. Despite considerableprevious work in this area, we do not fully understand the mechanisms by which PAMAM dendrimerstemplate the formation of Pt DNs. In particular, Pt DN sizes measured by high-resolution transmissionelectron microscopy (HRTEM) are reported to be larger than expected if one assumes that each PAMAMmolecule templates one spherical Pt nanoparticle. AFM provides a vertical height measurement thatcomplements the lateral dimension measurement from HRTEM. We show that AFM height measurementscan distinguish between “empty” PAMAM and Pt DNs. If the complexation of Pt precursor with PAMAMis prematurely terminated, AFM images and feature height distributions show evidence of arrestedprecipitation of Pt colloids. In contrast, sufficient Pt−PAMAM complexation time leads to AFM imagesand height distributions that have relatively narrow, normal distributions with mean values that increasewith the nominal Pt:PAMAM ratio. The surface density of features in AFM images suggest that these PtDNs reside on the mica surface as two-dimensional surface aggregates. These observations are consistentwith an intradendrimer templating mechanism for Pt DNs. However, we cannot determine if the mechanismobeys a fixed loading law because we do not have definitive information about Pt DN shape. A second peakin the Pt DN height distribution appears when the Pt loading exceeds about 66% of PAMAM's theoreticalcapacity for Pt. Excluding these secondary particles, the dependence of mean feature height on the Pt:PAMAM ratio follows a power-law relationship. Also considering the magnitudes of the measured meanheight values, the data suggest that Pt DNs exist as ramified, noncompact aggregates of Pt atoms interspersedwithin the PAMAM framework.
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