Selection and Identification of Molecular Gold Clusters at the Nano

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Letter

Selection & Identification of Molecular Gold Clusters at the Nano(gram)Scale: Reversed Phase HPLC-ESI-MS of a Mixture of Au-Peth MPCs David M. Black, Nabraj Bhattarai, Stephan B. H. Bach, and Robert L. Whetten J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01403 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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The Journal of Physical Chemistry Letters

Selection & Identification of Molecular Gold Clusters at the Nano(gram)-scale: Reversed Phase HPLC-ESI-MS of a Mixture of Au-peth MPCs David M Black‡, Nabraj Bhattarai‡, Stephan B H Bach†, Robert L Whetten‡ †

Department of Chemistry, University of Texas San Antonio, 1 UTSA Circle, San Antonio, Texas 78249, USA, and ‡Department of Physics and Astronomy, 1 UTSA Circle, San Antonio, University of Texas San Antonio, Texas 78249, USA

Recent advances in cluster synthesis make it possible to produce an enormous variety molecule-like MPCs of size, composition, shape, and surface-chemical combinations. In contrast to the significant growth in the synthetic capability to generate these materials, progress in establishing the physico-chemical basis for their observed properties has remained limited. The main reason for this has been the lack of the analytical capability to generate and measure samples of suitable high (molecular) purity; such capability is also essential to support therapeutic and diagnostic MPC development. In order for MPC products to get to market, especially those products that are medical-field related, characterization is required to identify and quantify all components present in a material mixture. Here, we show results from analysis of several synthetic mixtures of gold MPCs by non-aqueous reversed-phase chromatography coupled with mass spectrometry detection. The additional or hidden components, revealed to be present in these mixtures, provide novel insights into their comparative stability and interactions.

Gold monolayer-protected clusters (Au-MPCs) are currently the focus of intense examination across scientific disciplines due to the promise they hold for myriad endeavors ranging from labeling1, sensing2-4, imaging5-7, medicine delivery8-11, and disease therapy12-16, as well as other 1 ACS Paragon Plus Environment


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non-health field related applications. Currently, though, there exists no well-established or standardized method for qualitative and quantitative analytical characterization of these materials for health-field related safety, regulatory or IP purposes17-20. Consequently, studies that aim to establish relationships between the performance (i.e., biological activity, photoactivity, toxicological profile, etc.) of these materials and their various physical and chemical properties (size, composition, protecting ligand, isomer, etc.) may be inherently impeded. This may lead to ambiguous or erroneous conclusions being drawn from results obtained via wellintended investigations. Therefore, it is absolutely essential to understand content and purity down to single-percent levels for given material sample. Characterization of Au-MPC mixtures is an objective which is generally performed by microscopy21, crystallography, spectroscopy (NMR, IR, Raman), centrifugation22 and mass spectrometry23-24. Although each of these methods is highly useful for characterization of the major component(s) in a mixture, there are limits to the qualitative and quantitative capacity of each of these methods for determination of the identity and amount of a given molecular entity in a product mixture. Additionally, none of these methods is favored for determination of lowlevel byproducts, impurities and degradants. Gel electrophoresis is a widely used technique which has been used widely for coarse nanoparticle separations. Although, it is a relatively simple and inexpensive technique it suffers from poor separation efficiencies, long run times, and relatively extensive and time consuming sample work-up procedures to remove separated components from the gel for subsequent analysis by other techniques. Additionally, gel electrophoresis of hydrophobic compounds can be difficult. Work spanning a decade has been carried out to separate monolayer-protected gold nanoclusters using reversed-phase high performance liquid chromatography (rp-HPLC). Some of the earliest efforts were undertaken by Murray and coworkers,25-28 wherein conventional flow rate reversed-phase chromatography using C8 stationary phase, coupled with photodiode array and electrochemical detection, was shown to be successful for separation of Au-MPCs. Although this early work frequently failed to produce highly-resolved and Gaussian shaped peaks, the experiments served as the proof-of-concept which demonstrated the applicability of reversed-phase chromatography to monolayer-protected clusters of sizes ranging from 25 to more than 200 gold atoms. More recently, Negishi and coworkers have greatly extended the pioneering work,29-31 using preparatory and conventional LC combined with UV detection to demonstrate the separation of gold and alloy MPCs over a wide range of core diameter and charge, ligand structure, and coordination isomer via reversed-phase LC. Subsequently, the collected fractions could be identified by mass spectrometry, and employed in a variety of fundamental investigations of electronic and optical properties. Negishi et al have been successful at producing impressive HPLC results displaying well-resolved and symmetrically shaped peaks for highly complex Au-MPC mixtures. Various column chemistries (interactions) have also been elucidated. We demonstrate here, for the first time, that capillary high-performance liquid chromatography (cap-HPLC) combined with micro-electrospray ionization time-of-flight mass spectrometry (µESI-ToF) can be used to separate and detect mixtures of hydrophobic phenylethane thiolate (peth) protected gold clusters (Au-MPCs), in a rapid in-line fashion. Although the work describe 2 ACS Paragon Plus Environment

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herein pertains to organo-soluble molecules, we believe that this methodology (HPLC-MS) will extend itself to water-soluble systems as well. [In a previous Letter32, we described certain technical aspects of the development project.] Preliminary evidence indicating future possibilities of its use for semi-quantification of these structures in mixtures is also discussed. The method described here consumes only nanogram (ng) quantities of sample for separation and detection of the various mixture components with good signal-to-noise ratios and exhibit near linear responses as a function of cluster concentration.

Figure 1 shows ESI-MS results on high-quality samples of larger Au-peth MPCs, obtained without the benefit of the (inline) HPLC method. Main features in the high-resolution spectra (lower trace) were assigned to various compositions, notably (144,60), (137,56) and (130,50), in various low charge-states (z). Thermochemical treatment of the sample results in a great enhancement (middle trace) of the (144,60) features, relative to the others, which is interpreted as indicating extraordinary stability. Further treatment with an agent (captaminesalts) results in minor positive (m/z)-shifts, and improved S/N ratio. Note that all these results and inferences were made without high-performance separation. Critics of these practices may point out that the intensities patterns of ESI mass-spectral features are often poor indicators of the relative abundances of the species present in a solution-phase or powder sample: (i) sensitivity of mass-analyzers (including detectors) may vary strongly with (m/z); (ii) ionformation efficiency by ESI may vary strongly especially when the mixture contains intrinsic-ion analytes (plus their counter-ions) in conjunction with inherently neutral analytes; (iii) redoxactive species, which include many but not all the Au-MPCs discussed here, may form ions by electrochemical processes occurring in the electrospray injection, but with different efficiency; and (iv) co-elution of mixed analytes, frequently in conjunction with added or adventitious electrolyte, may greatly diminish the ionization-efficiency of certain analytes, by what are known as “ion-suppression” effects. Any or all of these factors may affect drastically the quantitative or even qualitative interpretation of the mass spectra, whether they be obtained for fundamental or application studies.

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Figure 1. Typical mass spectra of (lower) a raw mixture of gold-peth MPCs, (middle) the same mixture after being subjected to an ‘etching’ thermochemical treatment, and (top) after exposure to a captamine solution. Peaks are labeled with charge state and color coded according to identity: Au137(peth)56 in blue; Au144(peth)60 in red; and the captamine-modified Au144(peth)60 in green. All chromatography discussed here was acquired by capillary-flow non-aqueous reversed-phase liquid chromatography (NARP-HPLC). Because the peth-protected gold clusters studied here are non-polar (each ligand contributing, cumulatively, to a high degree of overall hydrophobicity), NARP-HPLC, a specific sub-type of reversed-phase chromatography, is essential. NARP-HPLC makes use of stronger, less-polar solvents, and elution of sample under gradient conditions33. The Au-MPCs investigated here are only fully soluble in strong solvents such as toluene, dichloromethane, and tetrahydrofuran. Dichloromethane was selected as the stronger solvent, i.e. mobile phase B, because of its solvent strength and because its non-flammability and high volatility make it ideally compatible for in-line electrospray ionization (ESI). Capillary flow rates (~ 10 µL/min) were chosen to obtain better ESI sensitivity and decreased sample consumption, as compared with conventional flow chromatography, while at the same maintaining the reliability, quantitative capacity, and throughput that conventional flow rates provide. Although the samples themselves have no polar functionalities and are aprotic, we found mobile phase modifier, or ion-pairing agent, to be a crucial component for achieving good peak 4 ACS Paragon Plus Environment

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shape and prevention of peak tailing due to undesired secondary interactions with unbound and ionized silanols on the silica packing material. Triethylammonium acetate (TEAA) was added to each mobile phase solvent at a concentration of 0.1 mM to occupy these polar functionalities, via ion-pairing with triethylammonium, to reduce the silanol activity of the column. A relatively low concentration (i.e. 3) at the lowest concentrations (i.e.,) — namely Au25L18, Au104L45 and Au137L56 —thus had sensitivity at the sub-1.0-picomole level. Response curves were plotted for each of these three components as a function of increasing concentration in Figure 5. All three monolayer-protected clusters exhibited a linear response with coefficients of determination, R2 values greater than 0.95. Although, R2 values above 0.99 are generally required for an analytical method to be considered to produce a linear response satisfactory for method validation, there is clearly a strong correlation between signal and concentration in these data. Further investigation and method development, using certified reference standards would likely yield improved linearity results. In the case of the data shown here, the upper range of linearity may have been reached for the larger two clusters due to the appearance of a lower than proportionate increase in signal for the 100 ng/µL solution vs the 80 ng/µL solution, resulting in lower than optimal R2 values. Simpler mixtures will likely show a greater dynamic range and lower limits of quantification and detection than the one analyzed here. Because our 10 ACS Paragon Plus Environment

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synthetic mixture contained so many significant components (9), all contributing to a significant percentage of the overall injected mass, it may be reasonable to expect that the linear dynamic range may span several orders of magnitude, with detection limits in the femtomole range, for more typical sample mixture compositions which might contain one main component and various low level structural variants. Further work in this direction will require a set of standard solutions of known concentrations quantified independently, e.g. gravimetrically. The results shown here demonstrate the utility of inline LC-MS for rapid, quantitative separation, and highly specific detection of massive Au-MPCs in complex mixtures. This methodology is fast and reproducible, requiring only nanogram-scale amounts of sample, with limits approaching 10-ng per component. The LC-MS method shown here is capable of separating a mixture of nine unique nanoparticle structures ranging 7.9 kDa to 36.6 kDa. Linear response was displayed, spanning of an order of magnitude concentration variation for three of the most concentrated cluster populations present in the mixture. The peth-protected Au-MPCs analyzed here thus serve as good examples for what may be possible with non-aqueous LC-MS characterization. Polar and water-soluble monolayer-protected clusters separations may be less challenging because of their compatibility with traditional reversed-phase LC solvents; however they have been shown to be very difficult samples for the production of high-quality mass spectra36-37. One ongoing development of these HPLC-MS methods will be targeted at separation and detection of water-soluble clusters because of their utility for labeling, sensing, imaging, medicine delivery, and disease therapy. We believe that the results shown here indicate that LC-MS, and related hyphenated LC-X-MS techniques, could prove to be a powerful and necessary tool to support the hoped-for breakthroughs in nanoparticle-based research and development. These methods may be useful for improved synthetic process control and understanding as well as for the production and assessment of nanoparticle standards, which will be vital for making uniform products both at the research and production lab scale. Experimental Details Synthesis The synthesis of Au144(SR)60 nanoclusters was carried out using modified Brust’s methods reported in the literature.38 Briefly, the pure Au144(SR)60 nanocluster as well as the by-product Au130 nanocluster were obtained in two steps using a Au:SR ratio of 1:3. In the first step, polydispersed nanoclusters were obtained, and allowed to etch (aerobic heating at 80°C in excess thiolate ligands), converting the less stable into oxidation resistant form purely monodispersed Au144 nanoclusters. The properties of the pure Au130 nanoclusters are described in reference.39-40



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In the first step, tetractylammonium bromide (TOAB, 0.052 M, 10 mL toluene) was mixed with HAuCl4.3H2O (0.09 M, 5mL H2O) in a 25 mL round-bottom flask and vigorously stirred for 1 hour until the phase transfer of Au(III) from aqueous phase changed to Au(III) toluene phase. The clear two phase between toluene (deep red) and aqueous (colorless) indicated the complete phase transfer. The aqueous part was removed and the deep red toluene solution was cooled at 0 ºC for 30 minutes. Then 186 μL of PhCH2CH2SH was added to the stirred solution of Au(III) and continued stirring for 1 hour. The color changed from deep red to yellow and finally colorless. The PhCH2CH2SH capped Au(III) particles were reduced by adding NaBH4 (0.171 gm, 5 mL in water) at once. The quick addition of NaBH4 to the Au(III) solution changed the color to black indicating the formation of the nanoclusters. The reaction was allowed to proceed for 24 hours so that all the excess NaBH4 will be decomposed. The aqueous phase was removed and the black residue with toluene was dried using rotary evaporator. The sample obtained after rotovapping was washed with ethanol and allowed to precipitate in a refrigerator. The washed precipitate was dried in atmosphere and ready for characterization and for etching in the next second step. In the second step, 0.03 g of the sample obtained from the first step was dissolved in 1.5 mL toluene using a 5 mL vial. 1 mL of PhCH2CH2SH was added and the solution continuously stirred for 24 hours at 80 ºC. Then the solution was transferred to a new 25 mL vial, excess methanol was added into it and the particles allowed to precipitate in a refrigerator for 15 hours. The methanol was removed and the precipitate was dissolved in CH2Cl2 (DCM). Only the desired clusters Au144 and Au130 will be soluble in CH2Cl2 (brown color) thus separating it from the insoluble byproduct. It was further recrystallized for electron microscopy and electrospray characterization.

Instrumentation Capillary-flow liquid chromatography (LC) experiments were performed on an Eksigent nanoLC 2D system which was coupled with a Bruker micrOTOF time-of-flight mass spectrometer (MS). Separation was carried out with an Ace 300Å phenyl HPLC column (0.5 mm x 150 mm, 3 µm particle size) maintained at ambient laboratory temperature. Mobile phase consisted of 0.1 mM triethylammonium acetate (TEAA) in methanol (mobile phase A) and 0.1 mM TEAA in dichloromethane (mobile phase B). The linear gradient method used for analysis was as follows: 50% MP B at 0 min increased to 100% mobile phase B over 10 min. 100% Mobile phase B was then maintained for 2 min followed by re-equilibration to initial method conditions for 3 min. The flow rate used for all experiments was 15 microliters per minute (µL/min). Injections (2.5 µL) were carried out manually via a six-port Rheodyne injection valve. Samples were dissolved in 100% toluene diluent. Prior to LC-MS work, the mass spectrometer source and analyzer regions were tuned for enhanced transmission and detection of high-mass cluster ions by direct infusion of a mixture of cesium triiodide. For the data shown here, mass spectrometer acquisition settings were selected to acquire data from m/z 100-40,000. 15,000 spectra were summed per spectrum 12 ACS Paragon Plus Environment

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acquired. Nebulizer pressure was set to 1.5 bar, and sheath gas was set to 3.0 L/min. The electrospray emitter potential was held at +4500 V. Capillary exit and skimmer voltage settings were 200 V and 0 V respectively. Lens 1 pre-pulse storage and transfer times were 45 µs and 318 µs, respectively. MCP detector voltage was increased to 2400 V for improved detection of high m/z clusters. Acknowledgements The authors would like to acknowledge support from the National Institute on Minority Health and Health Disparities (G12MD007591); an NSF PREM Grant #DMR 0934218; ‘‘Oxide and Metal Nanoparticles – The Interface Between Life Sciences and Physical Sciences”; a WelchFoundation Grant (AX-1857); and the use of samples prepared by Mr. James Morgan and encouragement and advice from Prof. Miguel Yacaman, Dr. Wendell P. Griffith, Dr. Marcos M. Alvarez (all of University of Texas at San Antonio), Prof. Borries Demeler (University of Texas Health Science Center San Antonio) and Prof Yuichi Negishi (Tokyo University of Science). Contributions DMB designed the project, carried out the experiments and analysis, and co-wrote the manuscript. NB carried out the experiments to synthesize the monolayer-protected cluster mixtures analyzed for this manuscript. RLW participated in the analysis and interpretations, and co-wrote the manuscript. All these co-authors (DMB, RLW, SBHB) participated in the conception and initiation of the project, and have reviewed and approved the final manuscript. Additional Information The authors declare no competing financial interests. References

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Selection and Identification of Molecular Gold Clusters at the Nano

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