Estimation of Size for 1-2 nm Nanoparticles Using an HPLC Electrochemical Detector of Double Layer Charging

Reversed-phase HPLC chromatographic separations of Au monolayer protected clusters (MPCs) in the size range 1-2 nm (diameter) are detected electrochemically (ECLC) on the basis of double layer charging of the metal-like nanoparticles by the detector electrode. Discrete double-layer charging peaks are detected for the nanoparticles with resolution similar to that of the photodiode array detector. The ECLC experiment further seeks not solely to detect the nanoparticle peaks, but to estimate the nanoparticle sizes from the electrochemical responses. Hydrodynamic current-potential curves of nanoparticles eluted in different peaks were obtained by repeating chromatographic separations at different working electrode potentials; the E1/2 separations between the voltammetric waves give estimates of cluster capacitance and, thus, size. Other approaches to core size determination are discussed, including determination based upon relative values of optical absorbance and current for each eluted peak. The data show that, at least for alkanethiolate MPCs, the elution order on the HPLC column employed is small clusters first and that there are significant differences in reversed-phase chromatographic retention times for clusters with apparently very similar core sizes over the range of 1-2-nm diameter.