Green and Scalable Fractionation of Gold Nanoclusters by Anion Exchange Chromatography: Proof of Principle and Scale-Up

The chromatographic separation of atomically precise glutathione-stabilized gold nanocluster (AuNC) mixtures is demonstrated for purely aqueous anion exchange chromatography (AEX) and compared to the results of hydrophilic interaction liquid chromatography (HILIC), which requires mixtures of water and acetonitrile. For isocratic HILIC separations, the influence of the mobile phase composition with respect to ionic strength and acetonitrile content is investigated. Individual AuNCs are identified by their optical properties directly from chromatograms and by comprehensive electrospray ionization differential mobility mass spectrometry. Separations by AEX are performed in aqueous mobile phases and are thus considered as a green and sustainable technique for the efficient fractionation of AuNCs. Henry coefficients are determined, which describe the retention behavior of single AuNCs and thus form the basis for modeling, optimization, and scale-up of the separation. The separation by AEX is optimized by linear salt gradient experiments. The scalability of the separation is demonstrated by increasing the AuNC feed concentration up to 64 g/L. Moreover, we quantify the amounts of individual AuNCs by inductively coupled plasma–optical emission spectroscopy after separation and close the mass balance demonstrating the reversible adsorption from the stationary phase. Our AEX approach is superior to HILIC since it offers a higher resolution, provides access to larger clusters, is readily scalable, and is purely water-based. AEX is considered as a sustainable, scalable, and widely applicable technique for the isolation of all types of water-soluble single cluster species. Therefore, we anticipate applications for instance for highly specialized nanocatalysts, sensors, or in medicine.