Ion-pair chromatographic separation of water-soluble gold monolayer-protected clusters

We demonstrate the efficacy of ion-pair chromatography for separations of samples of charged, polydisperse, water-soluble gold nanoparticles protected by monolayers of N-acetyl-L-cysteine and of tiopronin ligands. These nanoparticle mixtures have 1-2-nm-diameter Au core sizes as estimated from UV-visible spectra of the separated components. This size range encompasses the transition from bulk metal to molecular properties. The nanoparticle mixtures were resolved, the smallest nano-particles eluting first, on an octadecylsilyl (C18) column using isocratic elution with a methanol/water mobile phase containing tetrabutylammonium fluoride (Bu₄N⁺F^-) and phosphate buffer. The column retention increases with Bu₄N ⁺F^- concentration, lowered pH, and decreasing methanol volume fraction. The retention mechanism is dominated by ion-pairing in either the mobile phase or at the stationary/mobile-phase interface. Size exclusion effects, used in many previous nanoparticle separations, are insignificant.