(208b) Surface Electric Potential of Macroions Between the Limits of Molecular Ions and Charged Nanocolloids

The surface electric potential of macroions in the size range from 1-10 nm in aqueous solutions is critical to understand the supramolecular assembly involving biomacromoles, charged nanoparticles and nanoclusters and their resulting material properties. However, the electric potential of these macroions could not be accurately determined because their sizes fall between the limits of small ions and charged nanoparticles, while analytically solving the non-linear Possion-Boltzmann equation remains a grand challenge to date. In this work, we investigate Polyamidoamine (PAMAM) dendrimers with stoichiometric amine functionalized terminal groups as model macroions. We employ a a single molecule fluorescence technique, fluorescence correlation spectroscopy (FCS) and UV-vis spectroscopy to quantitatively measure the local proton concentration, which is the local co-ion concentration in vicinity to the PAMAM dendrimes of 2.2-6.7 nm in diameter. By changing the ionic strength of aqueous solution, we quantitatively determine the proton concentration gradient. The distance dependent local pH can be simply analyzed to obtain the surface electric potential of the POSS macroion without the necessity to solve the non-linear Possion-Boltzmann equation.