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Separations

Gold Nanocluster Prospecting via Capillary LC-MS: Discovery of Three Quantized Gold Clusters in a Product Mixture of “2-nm Gold Nanoparticles” David M. Black, Geronimo Robles, Stephan B. H. Bach, and Robert L. Whetten Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00480 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Industrial & Engineering Chemistry Research

Gold Nanocluster Prospecting via Capillary LC-MS: Discovery of Three Quantized Gold Clusters in a Product Mixture of “2-nm Gold Nanoparticles” David M Black a*, Geronimo Robles a, Stephan B H Bach b, Robert L Whetten a* a

Department of Physics and bDepartment of Chemistry, University of Texas at San Antonio, San

Antonio, Texas 78249, United States

ABSTRACT

A non-aqueous reversed phase liquid chromatography – mass spectrometry (LC/MS) method has been developed for extremely hydrophobic MPCs (monolayer protected clusters), and has been applied to the efficient separation of gold - dodecanethiolate (ddt) assemblies, leading to the identification of three dodecane thiolate-protected gold clusters – Au130(ddt)50, Au137(ddt)56, and Au144(ddt)60 – as prominent components of a commercial product of nominally 2-nm (corediameter) protected gold nanoparticles obtained from nanoComposix, Inc. Various components were separated, according to hydrophobic character, using a linear gradient of methanoldichloromethane mobile phases – on a C18 HPLC column. Varying concentrations of mobilephase modifier (triethylammonium acetate) were compared for effect on chromatographic peak

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shape and cluster retention.

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Positive electrospray ionization (ESI) was used to ionize all

components in the sample. LC separation prior to inline +ESI-MS detection facilitated sample analysis via production of simplified mass spectra for each eluting cluster species and provided insight to the relative polarity of the clusters shown here. UV-Vis detection facilitated method development and allowed determination of non-ionizing, and/or polydisperse components. 1. INTRODUCTION Monolayer protected cluster (MPC) refers to a special class of macromolecule in which the surface of its compact core, typically a large, globular cluster of metal atoms, is protected by a layer of ligand groups. MPCs differs from traditional metal cluster complexes in that the ligands are those which function in self-assembled monolayers (SAMs) adsorbates on the corresponding extended metal surfaces. Hence there is no fixed upper limit on the size of MPCs.

Recent

reports demonstrate that MPCs, comprising up to several hundred gold or silver atoms and several dozen ligands / adsorbate groups, may adopt special nanostructures that are particularly stable in solid form and in solution.1 In contrast to larger metallic MPCs, referred to as "nanoparticles", the clusters exhibit distinct or quantized stoichiometries and exhibit physical-chemical properties not observed in the corresponding bulk materials2-4. Physical and chemical properties such as size and protecting ligand structure(s) may be pre-selected and engineered to impart specific optical, electronic, and chemical properties to the synthesized clusters. These characteristics make them attractive for investigations into their use in a range of applications including as novel catalysts5, molecular probes6, 7, therapeutics8, 9 and electronic device components.10 Quantized stoichiometries make them ideal for analysis by liquid chromatography and mass spectrometry.


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Various methods have been used for separation and subsequent analysis of monolayer protected cluster mixtures, including size exclusion chromatography11, chromatography,13 gel-electrophoresis14,

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, thin layer

, and high performance liquid chromatography

(HPLC).16 Of these methods, reversed-phase HPLC offers the highest separation capacity and is applicable to analysis of both aqueous and organic clusters. Murray and Tsukuda were early developers of HPLC methodology for MPC analysis and have advanced this work using preparatory and conventional flow rate HPLC combined with various detection methods.16,

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Reversed-phase LC has been demonstrated to be effective for separation of gold, silver and alloy MPCs over a wide range of core diameter and charge, ligand structure, and coordination isomer18-22. Aqueous varieties have been successfully separated by both ion-pair reversedphase18, 23 and HILIC24 chromatography. Here, we describe a capillary flow rate gradient LC-UV-MS method suitable for separation of several dodecanethiolate (ddt) protected gold clusters that were detected in a commercially available product of 2-nm gold nanoparticles. The goal of this study was to develop an capillary HPLC method, that could be routinely carried out in fewer than twenty minutes to simplify mass spectral interpretation of any cluster components detected, including Au130(ddt)50,25 Au137(ddt)57,26 Au144(ddt)60,27 and various other smaller and larger gold and ligand compositions. Dodecanethiolate-protected clusters present a particularly challenging analysis because of their highly non-polar nature, which necessitates the need for non-aqueous LC conditions, and because of modest ESI response.

Under optimal experimental conditions, in-line LC-MS

methods have been shown to be able to provide high quality chromatography for both aqueous and organic clusters, while eliminating the need to collect fractions.28-30 Analysis of this sample


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provided information about the composition of the mixture, which is not obtainable via directinfusion mass spectrometry analysis alone.

2. EXPERIMENTAL SECTION 2.1 Sample preparation Commercial samples were obtained from nanoComposix (San Diego, CA) as “2-nm diameter dodecanethiol-stabilized gold nanospheres NanoXact, Dried, Dodecanethiol, 1-mg” (Lot # KJW2006-ECP1506, SKU: AUDD2-1MG). Transmission electron micrographs, size distribution histogram and UV-Visible optical spectrum (all obtained from nanoComposix) are reproduced in the SI section. From the TEM size-distribution histogram, the (core) diameter was estimated as 2.2 ± 0.3 nm, which corresponds roughly to a mean size of 300 atoms, or core mass near 60-kDa. Samples were dissolved in neat dichloromethane at a concentration of approximately 0.5 mg/mL for injection & subsequent analysis. 2.2 HPLC-UV-MS method conditions Liquid chromatography (LC) experiments were performed on an Eksigent nanoLC 2D system coupled to a Bruker micrOTOF time-of-flight mass spectrometer (MS). All separations were carried out using an Ace 300Å C18 HPLC column (0.5 mm x 150 mm, 3 µm particle size) (Advanced Chromatography Technologies Limited, Aberdeen, UK) maintained at ambient laboratory temperature. Mobile phases were prepared with either 0 mM, 25 mM, 50 mM triethylammonium acetate (TEAA) in methanol (mobile phase A) and neat dichloromethane (mobile phase B). All solvents for direct infusion and LC-MS were obtained from Fisher


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Scientific (Fairlawn, NJ). The linear gradient methods used for analyses shown here varied from starting conditions of 50 – 70% MP B to 100% mobile phase B over twenty minutes. In each case, the gradient was a followed by a five-minute hold at 100% MP B followed by reequilibration to initial method conditions for fifteen minutes. A fifteen microliter per minute (µL/min) flow rate was used for all experiments. Injections – 5.0 µL – were carried out by an Eksigent AS-1 autosampler configured with a 20-µL sample loop. The method described here consumes ~ 2.5 µg of sample per injection for separation and detection of the various mixture components with good signal-to-noise ratios. Mass spectrometer acquisition settings were selected to acquire data from m/z 100 - 50,000. 10,000 spectra were summed per spectrum acquired. Nebulizer pressure was set to 1.5 bar, and nitrogen sheath gas was set to a flow rate of 2.0 L/min. The endplate offset and capillary potentials were held at -500 V and -4500 V. Capillary exit and skimmer voltage settings were +225 V and +33 V. Lens 1 pre-pulse storage and transfer times were 50 µs and 300 µs, respectively. MCP detector voltage was increased to 2350 V (from 2100 V standard) for improved detection of high m/z clusters. UV-Vis detection and real-time 300 nm - 650 nm spectral characterization was acquired with an UltiMate™ UV Detector (LC Packings) equipped with a Micro LC (10-100 µL/min) flow cell (UZ-M10, P/N 16011).

3. RESULTS AND DISCUSSION Due to the highly non-polar nature of these clusters – requiring the presence of toluene, dichloromethane, or tetrahydrofuran for good solution solubility – separations were carried out using non-aqueous reversed-phase (NARP) HPLC31 conditions. Non-polar solvents such as


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these are required for elution of these components from the C18 stationary phase used for these separations.

For this work, methanol was chosen for use as mobile phase A, and

dichloromethane was selected as the stronger solvent – i.e. mobile phase B. This solvent combination has been shown to perform well for NARP-LC separations of phenylethanethiolate protected clusters29, and performed equally well, here, for separations of alkane protected clusters. Additionally, the hydrophobic interaction between the aliphatic hydrocarbon portions of the stationary phase and the protecting alkyl ligands of the gold clusters offers an ideal mode of separation for the surface chemistry of these clusters.

We make use of capillary

chromatography because of the reduced solvent consumption/requirements as compared to conventional flow chromatography, and reduced time requirements compared with nano-flow liquid chromatography. Figures 1a, b display separate LC-UV and LC-MS chromatograms acquired from analysis of the commercial mixture. (A direct infusion mass spectrum, acquired from this sample with no prior separation or fractionization, is shown in Figure S1 for reference). The separation shown in Figure 1 was achieved using a twenty-minute linear gradient, whereby mobile phase B was varied from 55% to 100% of the mobile phase composition. Figure 1b consists of several overlapping extracted-ion chromatogram traces, each of which show ion signal intensity as a function of retention time for one of the major components in the mixture. At least three unique cluster components could be baseline separated and detected. In the analysis shown of the complex Au-MPC mixture, all major components were retained greater than twice the retention time of the solvent front – approximately two minutes – and all eluted within the twenty-minute gradient period. Differences between the peak widths, shapes and retention times observed in


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Figure 1a vs 1b are caused by the different tubing lengths (and inner diameters) that were used to span the column-to-detector space (LC-UV vs LC-MS). As has been report upon previously, NARP LC of hydrophobic cluster mixtures separates components in order of increasing hydrophobicity. In order of retention time, the three main components in this commercial mixture eluted as follows (Figure 1): Au130(ddt)50 at RT 11.5 minutes, followed by Au137(ddt)56 at RT 12.4 min, and Au144(ddt)60 at RT 13.0 min. Each of these three cluster species are similar in core diameter to within ~ 2 % (i.e., < 0.1 nm). The protecting dodecane thiolate ligand layer of these clusters – an 11-methylene chain terminated by a methyl group – contributes an additional ~ 3+ nm to the hydrodynamic radius so that each is approximately 5 nm total diameter.

Because each of these three clusters is similar in

hydrodynamic radius to within less than one angstrom, the physical-chemical property which permits separation via RP-LC is likely the number of protecting ligands which relates to overall the hydrophobic character of the cluster. Matching time-of-flight mass spectra for the numbered EIC peaks in Figure 1b are shown in Figures 2a-c. In each of these three mass spectra, a charge-state envelope, encompassing the singly- to triply-charged parent ions (m/z, where z = 1+, 2+, 3+), is evident. These are used to identify the respective molecular masses (m) and so assign the components as Au130(ddt)50 (Figure 2a), Au137(ddt)56 (Figure 2b), and Au144(ddt)60 (Figure 2c) respectively, in agreement with prior MS analyses of highly purified samples. Several minor components are also observed in the mass spectrum shown in Figure 2a including: Au129(ddt)54, Au131(ddt)54, Au132(ddt)55, and Au133(ddt)55.

Although these minor components have similar stoichiometries to the main

component, Au130(ddt)50, their presence in the mass spectrum indicates that further method development work is necessary to achieve chromatographic isolation of the various components


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in the sample. Neither the TEA nor acetic-acid adducts appear to present in any mass spectra. Minimal fragmentation is observed for any of these species making identification of the known components straightforward. Besides these three main components identified positively by UV and MS spectra, there is some evidence of minor components, both smaller and larger in size. The LC-UV trace shows features at shorter retention time that might be consistent with previously known compositions, such as Au104 and Au67; however, the simultaneously acquired mass spectra do not allow a positive identification of such expected species. Mass spectra, with proposed mass and compositional assignments, for the two components that track with the asterisk labeled peaks in Figure 1a, are shown in Figures 2S and 3S. An effort was also made to identify various larger components (> 40kDa) noted to elute after Au144(ddt)60; however no previously reported upon species (i.e., Au187, Au329) were noted. A series of shallower LC gradients was used to analyze the same sample and are shown in the Figures 3a-e. Generally, as the initial method conditions were varied to include a greater contribution of dichloromethane – effectively shallowing the gradient – retention times decreased in accordance with the reversed-phase mechanism. Although these gradient variations did produce improved separation of the components, as the gradient was shallowed the chromatographic peaks exhibited broadening and therefore the separation of components was only minimally improved.

A slight improvement on separation of the three main cluster

components was made with the addition of TEAA buffer to mobile phase A. Concentrations of 0 mM, 25 mM and 50 mM were compared for any effect on peaks shape and retention of the main components. The results of these experiments are shown in Figures 4a-c. Presumably, any positive effects of chromatographic performance result from reduced interaction between the


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highly non-polar analyte and the stationary phase. The chromatograms shown in Figure 4 show that as the TEAA concentration was increased, peak widths decreased, and better resolution of the clusters was achieved (although mass spectrometric signal for each cluster decreased, owing to ionization suppression.) A TEAA concentration of 25 mM, Figure 4b, was found to decrease peak widths without significantly affecting the mass spectrometric sensitivity. The higher 50 mM concentration, Figure 4c, resulted even narrower chromatographic peaks but had a more adverse effect on mass spectrometric sensitivity was noted.

4. CONCLUSIONS As shown here, inline capillary LC-MS offers a sensitive means to obtain high resolution separation and detection of microgram-scale quantities of highly-nonpolar Aun(ddt)m clusters ranging 35.7 kDa to 40.4 kDa. Various other clusters – both smaller and larger - present in the mixture at relatively lower quantities were also noted to be present. Clusters were separated by reversed-phase mechanism resulting in the elution of clusters in order of decreasing polarity, corresponding to increasing size. Time-of-flight mass spectrometry was used to detect the clusters as they eluted from the LC column and to acquire high-resolution mass spectra for identification of each eluting cluster composition. This methodology is applicable to clusters varying in size and surface chemistry and is an effective means for determination of sample purity and mass spectral simplification. Reversed-phase separation of gold clusters may someday lend itself to synthetic process monitoring of MPC products and their respective conjugates or for quantification of the specific components in MPC mixtures. Future work will be aimed at analysis of mixtures containing clusters of increasingly larger core diameters as well as mixtures containing alloys, mixed monolayer protected clusters and conjugates.


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Figure 1. Liquid chromatography traces — (a) LC-UV (450 nm), (b) LC-MS — show separation of a commercial mixture of dodecane thiolate-protected clusters. A twenty-minute gradient method, varying MP B (DCM) from 55 – 100%, was used for the separation. Mass spectra that track with LC-UV peaks marked by asterisks are shown in the supplemental information section. LC-MS traces shown in Figure 1b are color-coded and identified by mass (m/z ratios) as: (grey) base-peak chromatogram, m/z 5000-50000; (blue) Au130(ddt)50, RT 11.5 min; (green) Au137(ddt)56, RT 12.4 min; and (red) Au144(ddt)60, RT 13.0 min.


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Figure 2. These mass spectra, acquired by the in-line LC-ESI-ToF-MS method, are used to identify the numbered peaks in Figure 1b. The labels indicate the charge states (z = 1+, 2+, 3+). The components are identified, by comparison of the observed masses, to those calculated for the following: (a) Au130(ddt)50 (35.7kDa) at RT 11.5 min; (b) Au137(ddt)56 (38.3 kDa) at RT 12.4 min; and (c) Au144(ddt)60 (40.4 kDa) at RT 13.0 min. These agree with formulations given in the prior literature.


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Figure 3. LC method comparison shows the effect of gradient on retention and separation of Au-ddt clusters. All gradients carried out using a 25 mM TEAA mobile phase concentration. (a) 50 – 100% MP B, (b) 55 – 100% MP B, (c) 60 – 100% MP B, (d) 65 – 100% MP B, (e) 70 – 100% MP B. The peak at ~ 2.0-min marks the solvent front (zero retention).


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Figure 4. LC method comparison showing the beneficial effect of TEAA concentration on chromatographic separation of the Au-ddt clusters. (a) 0-mM TEAA, (b) 25-mM TEAA, and (c) 50-mM. In each case the concentration of MP B was increased linearly from 55 – 100% MP B over 20 minutes.


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ASSOCIATED CONTENT AUTHOR INFORMATION Supporting Information Available: Direct infusion mass spectrum; mass spectra that track with peaks in Figure 1 denoted by asterisks; nanoComposix 2 nm redispersible gold batch analysis report. Corresponding Author *Robert L Whetten ([email protected]) *David M Black ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements The authors would like to acknowledge support from the National Institute on Minority Health and Health Disparities (G12MD007591), PREM: NSF PREM Grant #DMR 0934218; ‘‘Oxide and Metal Nanoparticles – The Interface Between Life Sciences and Physical Sciences”, and the Welch-Foundation Grant AX-1857; “Fundamental Chemical Research on Larger Molecular Noble-Metal Clusters”. DMB acknowledges helpful conversations with Dr. Steve Oldenburg (nanoComposix Inc.). Notes The authors declare no competing financial interest.


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ABBREVIATIONS MPC, monolayer-protected cluster; ddt, dodecanethiolate; LC-MS, liquid chromatography mass spectrometry; ESI-ToF-MS, electrospray ionization time-of-flight mass spectrometry; UV-Vis, ultraviolet-visible. TABLE OF CONTENTS GRAPHIC


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