Analytical evidence for the monolayer-protected cluster Au225[(S(CH2)5CH3)]75

The Brust synthesis of thiolate-protected gold clusters has been modified to produce identifiable proportions of a hexanethiolate-protected Au-225 core nanoparticle that display quantized double layer charging voltammetry consistent with a Au-225 core dimension. Transmission electron microscopy (TEM) and thermogravimetric results indicate an average nanoparticle formula of Au-225[(S(CH2)(5)CH3)](75). A simulated pulse voltammogram that accounts for the TEM nanoparticle dispersity matches reasonably well with that of the polydisperse synthetic sample containing the Au-225 component. In confirmation of the size determination, an HPLC analysis using ratiometric absorbance and electrochemical detectors gives a core radius of 1.0 nm for the Au-225 nanoparticle.