Article pubs.acs.org/JPCC
Probing Histidine-Stabilized Gold Nanoclusters Product by HighPerformance Liquid Chromatography and Mass Spectrometry Yan Zhang,†,∥ Qin Hu,‡ Man Chin Paau,‡ Shunping Xie,‡,⊥ Pengfei Gao,† Wan Chan,*,§ and Martin M. F. Choi*,‡ †
School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, People’s Republic of China Department of Chemistry, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong SAR, People’s Republic of China § Department of Chemistry, Division of Environment, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: A major hurdle in assessing the biological, chemical and physical properties of current nanoparticles lies in their complex nature in terms of size, shape, and composition. As such, it is vital to develop a high-resolution analytical separation technique to fractionate these nanomaterials. Herein, we demonstrate an unprecedented chromatographic fractionation of gold nanoclusters stabilized with histidine (His-AuNCs) with core diameter smaller than 1 nm. His-AuNCs product has been successfully separated by reverse-phase high-performance liquid chromatography using a binary mixture of methanol and ammonium acetate in water and an optimal solvent elution program. The separated His-AuNCs are online-characterized by UV− vis absorption spectroscopy, and their spectral features are closely related to the number of gold (Au) atom. The absorption band shifts to the lower energy as the number of Au atom increases. The separated His-AuNCs fractions are further collected and anatomized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, capillary electrophoresis, and fluorescence spectroscopy. The mass spectrometric data unambiguously reveal that the as-synthesized His-AuNCs product is indeed a complex mixture of Au10(His)9, [Au11(His)9]−, Au11(His)10, Au12(His)9, Au12(His)11, Au12(His)12, Au13(His)9, Au13(His)11, and Au14(His)13. All separated HisAuNCs exhibit two emission bands at ca. 410 and 500 nm, demonstrating that the photoluminescence of His-AuNCs is attributed to both the Au core and the surface-attached ligands. The blue-green emission at 500 nm displays a red shift with the increase in ligands (His). This work highlights the virtues of high-resolution chromatography for understanding the identity of individual AuNCs species present in an AuNCs product, which might have been previously hidden.
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INTRODUCTION
properties may vary even with one added Au atom, it is necessary to develop AuNCs of atomically precise composition.9,10 The structural and electronic properties of AuNCs are also dependent on the nature of ligand and the interaction between gold and ligand.11−13 Since the pioneering work of Brust and coworkers in 1994,14 various methods have been developed to synthesize AuNCs/ NPs on the subnanometer scale.15,16 In the past decade, many different modified Brust−Schiffrin methods were applied to nanoscience and nanotechnology field.13,17−19 AuNCs with various core sizes have been successfully synthesized with different reducing agents and ligands by using one- or twophase Brust−Schiffrin methods.20,21 Besides the often used strong reductant sodium borohydride (NaBH4), a mild reductant, tetrakis(hydroxymethyl)phosphonium chloride, has
Gold nanoclusters (AuNCs) have become the subject of intensive research in biological, catalysis, environmental, and pharmaceutical fields.1−4 These widespread applications mainly arise from their unique size- and morphology-dependent optical and electrical properties. The quantized electronic states owing to the quantum confinement of their electrons can be observed only in ultrasmall nanoclusters (NCs) or metal clusters, which have sizes smaller than the Fermi wavelength (often 2 nm) of conduction electrons.5,6 AuNCs consisting of only tens of atoms (particle smaller than 2 nm) exhibit molecule-like behavior, including discrete electronic states and photoluminescence (PL), which is different from that of larger gold nanoparticles (AuNPs) with diameters larger than 2 nm (
