HPLC of Monolayer-Protected Gold Clusters with Baseline Separation

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HPLC of Monolayer-Protected Gold Clusters with Baseline Separation Stefan Knoppe, and Pascal Vogt Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05064 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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Analytical Chemistry

HPLC of Monolayer-Protected Gold Clusters with Baseline Separation Stefan Knoppe1,2* and Pascal Vogt1 1: Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany 2: Max-Planck Institute for Intelligent Systems, Heisenbergstraße 3, 70569 Stuttgart, Germany ABSTRACT: The properties of ultrasmall metal nanoparticles (ca. 10-200 metal atoms), or monolayer-protected metal clusters (MPCs), drastically depend on their atomic structure. For systematic characterization and application, assessment of their purity is of high importance. Thus far, the gold standard for purity control of MPCs is mass spectrometry (MS). Mass spectrometry, however, cannot always detect small impurities; MS of certain clusters, e.g. ESI-TOF of Au40(SR)24 is not successful at all. We here present a simple reversed phase HPLC method for purity control of a series of small alkanethiolate-protected gold clusters. The method allows the detection of small impurities with high sensitivity. Linear correlation between alkyl chain length of Au25(SCnH2n=1)18 clusters (n = 6, 8, 10, 12) and their retention time was noticed.

Monolayer-protected noble metal clusters (MPCs) can be understood as atomically precise models of nanoparticles.1,2 Typically containing 10–500 metal atoms, they fall in the fundamentally interesting transition region between truly molecular (to the lower end) and emerging metallic behavior. Among MPCs, thiolate-protected gold clusters are outstanding as the most well-studied class. This is due to their exceptional stabilities, recent advances in synthesis,3,4 isolation,5,6 characterization7–11 and theoretical understanding12–15 of their stability and (strongly non-scalable) properties. Single crystal x-ray structure determination has become a routine method to determine the geometric structure of MPCs and forms a solid basis for correlation between structure and properties.7,16–20 However, it is difficult to grow crystals of suitable quality for single crystal XRD. Development of MPCs as building blocks in materials, and the use of MPCs in catalysis,21 sensing22 and imaging23,24 requires access to large amounts of MPCs, and it is impractical to crystallize each synthetic batch individually. Therefore, fast, sensitive and reliable analytical methods are required to determine the purity and composition of MPC samples. Typical separation methods used for MPCs include fractionated precipitation,25 ultracentrifugation,26 gel electrophoresis,5 size-exclusion chromatography (SEC),27,28 and capillary electrophoresis.29 These methods are usually used on a preparative scale and do not readily provide information on the nature of the MPCs, i.e. their sum formulae. The actual composition of the obtained MPC fraction is commonly determined by a combination of absorption spectroscopy – in certain cases such as [Au25(SR)18]-/0 or Au38(SR)24, the strong non-scalable properties of MPCs lend to highly characteristic optical properties – and mass spectrometry. However, the techniques have certain drawbacks, such as limited selectivity regarding separation, i.e. in gel electrophoresis, or limited use when detecting minor impurities, i.e. in absorption and mass spectrometry. Mass spectrometry has emerged as the method of choice to determine the purity of MPCs. Both electrospray ionization

(ESI) and matrix-assisted laser ablation/desorption ionization (MALDI) are commonly employed for these purposes,30,31 the success of the latter crucially depending on the choice of matrix.31 Mass spectrometry, however, has certain limitations. For example, no ESI mass spectrum of the Au40(SCH2CH2Ph)24 cluster32 (CH2CH2Ph: 2phenylethanethiolate, 2-PET) has been reported to date,33 while the MALDI-MS can be measured.32,34,35 It should be pointed out that the Au40(SR)24 species might form different structures, depending on the protecting ligand. The cluster was successfully crystallized with 2-methylthiophenolate as ligand (an aromatic thiolate), and the cluster has an oblate hexagonalprismatic core.36 In contrast, the Au40(2-PET)24 species (2-PET is an aliphatic thiolate) is assumed to be composed of a prolate bi-icosahedric Au26 core.37 The structure determination of the latter is elusive to date. As of yet, however, ESI mass spectra have not been reported for either of the species. It is desirable to couple a sensitive separation technique with a detection method that is able to determine different MPCs based on their properties. High-Performance Liquid Chromatography (HPLC) coupled with optical detection combines these requirements and is therefore an attractive technique that is available to many researchers. There is an extensive body of literature on HPLC separations of gold:thiolate MPCs.38 These can be classified into several categories: 1) Separation of MPCs by their core size (number of metal atoms in the cluster),39–51 2) Separation of MPCs by their charge (keeping the cluster size constant),52,53 3) Separation of doped MPCs (e.g. Au25-xAgx(SR)18 clusters),54,55 3) Separation of clusters after ligand exchange (e.g. Au25(SR)18-x(SR’)x,56–64 and 4) separation of enantiomers of intrinsically chiral MPCs (e.g. rac-Au38(SR)24 where SR is achiral).65–70 Often, HPLC is coupled with mass spectrometry (LC/MS) for direct detection and assignment.48,50,71,72 Overall, however, HPLC is rarely used for purity control of MPCs. Given that the properties of MPCs are strongly size-dependent and non-scalable, their characterization requires highly pure compounds. As demonstrated by the fact that Au40(2-PET)24 is undetectable in ESI mass spectrometry, mere characterization

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by techniques such as absorption spectroscopy, gel electrophoresis,73 mass spectrometry or transmission electron

microscopy72 cannot always unambiguously establish the purity of a compound.

Figure 1. Top, left: Raw chromatogram of Au25(DDT)18 clusters eluted with MeOH:THF 35:65 at a flow rate of 1.5 mL/min. Top, right: Chromatograms of Au25(DDT)18 eluted with MeOH:THF 35:65 at different flow rates (0.8 – 2.0 mL/min). The chromatograms were corrected for the net retention times. Bottom: Chromatograms of Au38(DDT)24 (left) and Au40(DDT)24 clusters at different flow rates. The clusters were eluted with MeOH:THF 35:65. Note that multiple compounds elute in the case of Au40(DDT)24.

We here demonstrate the separation of small thiolateprotected gold clusters using reversed-phase HPLC with baseline separation. Coupling the HPLC with a photodiode array detector allows to record absorption spectra on-line, leading to convenient assignment of MPCs with known optical properties. Our method is particularly useful in purity control of small clusters that have distinct absorption features within a few minutes, requiring far less sophisticated and comparably affordable instrumentation as compared to mass spectrometry and providing a much higher sensitivity that can even reveal impurities otherwise difficult to detect. Baseline separation is the ‘holy grail’ of any chromatographic technique, and to our knowledge has not been achieved to date by HPLC of MPCs with different sizes. We here focus on the separation of ‘ultrasmall’ MPCs with less than 100 gold atoms, protected by 1-alkanethiolates. METHODS Synthesis, isolation and characterization of the MPCs are described in the Supporting Information. Analytical data (mass spectra, absorption spectra) are shown in Figures S-1 - S-3. All HPLC experiments were conducted on a Shimadzu

Prominence HPLC system, equipped with a CBM-20A controller, DGU-20A degassing unit, two LC-20AD pumps, SIL-20A autosampler and a SPD-M20A photodiode array detector. The latter recorded full absorption spectra between 190 and 800 nm at a sampling rate of 1.5625 Hz and a slit width of 1.2 nm. The measurement cell temperature was set to 20 °C. All chromatograms shown here were extracted at a detection wavelength of 254 nm. Full on-line absorption spectra were corrected with a baseline spectrum recorded at a retention time where no sample eluted. An analytical C18reversed phase silica column (Thermo Scientific Hypersil GOLD, 250 x 4.6 mm, 5 μm particle size). A mixture of methanol and tetrahydrofuran was used as mobile phase. Unless otherwise stated, the flow rate was 1.5 mL/min, and the injection volume was 10 μL. The effect of flow rate and injection volume was investigated as well. For all separations, a step-wise gradient elution method was used. A typical method was as follows: Injection at 0.01 min into 100 % methanol, between 1.10 and 1.30 min, the mobile phase was changed to MeOH:THF 35:65. Between 15.00 and 15.20 min, the mobile phase was changed back to pure methanol, and


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Analytical Chemistry elution was continued until 20.00 min. A graphical representation of the step-wise gradient is shown in Figure S4. In some experiments, switching back to 100 % methanol

was delayed until 35.00 min and the chromatogram was recorded until 40.0 min. Over the course of the elution, the needle was rinsed four times with THF in order to prepare

Figure 2. Left: Comparison of flow rate and peak retention time of Au25(DDT)18 (black), Au38(DDT)24 (red) and Au40(DDT)24 (blue). The traces shown are exponential fits of the form tR’ = A*u-B where u is the flow rate and A, B are fitting parameters. The parameters are listed in Table S-1. Right: On-line absorption spectra of the three compounds at the peak maximum. Chromatograms of Au25(DDT)18 and Au38(DDT)24 clusters at different flow rates, eluted with MeOH:THF 35:65. In all cases, baseline separation between the peaks is observed. Right: Comparison of peak net retention times of the Au25(DDT)18/Au38(DDT)24 mixture with the pure compounds. the needle for the next injection. Typical sample concentrations were 1.0 mg/mL, the clusters were dissolved in toluene and passed over a 0.2 μm PTFE syringe filter prior to injection. RESULTS AND DISCUSSION Au25(DDT)18, Au38(DDT)24 and Au40(DDT)24 clusters were prepared and isolated according to published procedures.28,74,75 ESI-TOF mass spectra and UV-Vis-NIR absorption spectra of Au25(DDT)18 and Au38(DDT)24 are shown in Figures S-1 - S3. The analytical data are in agreement with the literature. The HPLC method used here to separate small alkanethiolateprotected gold clusters is based on a step-gradient method initially presented by Negishi and co-workers.61 An analytical C18-reversed phase silica gel column was used in all experiments. We rationalized that in order to obtain significant interactions between the clusters and the column, the prior should be stabilized by linear alkanethiolates; we thus chose 1-

dodecanethiolate (DDT) as the protecting ligand. Attempts to separate clusters protected by 2-PET were less successful (data not shown here) and likely require the use of a different solvent system or stationary phase. All analytes were dissolved in either toluene or tetrahydrofuran at concentrations of about 1 mg/ml. A mixture of methanol and THF was used as mobile phase. A typical chromatogram (Figure 1, left) shows a few features that deserve brief discussion. First, at around 2 minutes, the injection peak is observed; this corresponds to elution of toluene. At ca. 4 min, a sudden change in the baseline is seen; this is due to the change from pure methanol to 65 % THF. At 18 minutes, the baseline again changes, caused by changing the eluent back to pure methanol. We define a ‘net retention time’ tR’ as the observed retention time of the analyte tR subtracted by the retention time of the ‘solvent change’ signal (tRSC). Thus, all chromatograms shown here are essentially normalized to this solvent change signal (tR’ = 0.00 min). The chromatograms of Au25(DDT)18 at



different flow rates (between 0.8 and 2.0 ml/min) are shown in Figure 1, right. The raw chromatograms showing the absolute retention time are shown in Figure S-5, left. With increased flow rate, the (net) retention time of the cluster is shortened. The product of absolute retention time and flow rate gives the amount of eluent required to elute the compound; here, a

linear correlation is found (Figure S-5, right). Overall, a flow rate of 1.5 mL/min represents a good compromise between elution time, required amount of solvent and chromatographic separation (see below). A good linear correlation of peak area and injection

Figure 3. Effect of alkyl chain length of the protecting thiolate ligands on the elution times at different flow rates. Top left: HPLC of Au25(SDect)18; top right: Au25(SOct)18; bottom left: Au25(SHex)18; bottom right: A linear relationship between elution time and alkyl chain length is observed. Data points for Au25(DDT)18 were calculated from the fit function that was used in Figure 1.

volume was observed (Figure S-6). An injection volume of 0.5 μL was still detectable, this corresponds to about 58 picomoles Au25(DDT)18 (the injected solution had a concentration of 1 mg/mL). We also considered the influence of the intrinsic charge of Au25(DDT)18. The Au25(SR)18 system is one of the few MPCs with two accessible (stable) charge states at ambient conditions. Interestingly, Au25(SR)18 is formed by air oxidation,76 silica gel column chromatography77 or readily in the presence of halide ions,78 despite that the superatom complex model12,79 predicts a closed shell configuration (8 valence electrons) for the anion, while the neutral form is an open shell 7 electron cluster. Au25(DDT)18 was reduced by addition of tetraoctylammonium bromide. HPLC shows that Au25(DDT)18 splits into two peaks (Figure S-7, left), but we were not able to achieve separation between the peaks. The on-line absorption spectra at the two maxima are identical and resemble that of the neutral species. We assume that the

reduced cluster actually oxidizes on-column, this could cause the peak splitting (assuming that small amounts of the oxidized form were present in the sample prior to injection). For consistent results, we advise that the Au25(SR)18 cluster is fully oxidized prior to injection. Using the same method as described above, we determined the elution times for Au38(DDT)24 and Au40(DDT)24 clusters (Figure 1, bottom). Au38(DDT)24 contains some minor impurities, analysis of the peak areas, however, indicates a purity of at least 95 %. Au40(DDT)24 shows elution of multiple compounds (Figure S-8). We assign the intense peak at tR’ = 4.20 min (1.5 mL/min, tR = 8.172 min) to Au40(DDT)24, based on the good agreement of the on-line absorption spectrum and reference spectra in the literature.66,80 The absorption spectra of the other peaks are shown in Figure S-9. We did not assign the impurities, however, the on-line absorption spectrum of the peak at tR = 9.450 min (flow rate 1.5 mL/min, tR’ = 5.478 min) resembles that of Au52(SR)32 (where SR is aliphatic) as


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Analytical Chemistry reported by Zhuang et al..81 It should be noted that SEC with clusters protected by long alkyl chains is likely to be less selective than with short chains, since the longer alkyl chains will ‘cloak’ the difference in the volume of the cluster. This can in principle be overcome by repeated SEC of the desired cluster, however, the yield will drop significantly. Furthermore, we expect that the selectivity of the SEC drops significantly for larger clusters, making isolation of pure Au40(DDT)24 very difficult. However, our point is that the presented HPLC method can actually discriminate between the species; all clusters Au25(DDT)18, Au38(DDT)24 and Au40(DDT)24 have distinctively different retention times with baseline separation. A comparison between peak retention time and flow rate for the three compounds is shown in Figure 2. For all compounds studied here, the retention time decreases exponentially with the flow rate, and the data could be fitted with a function of the form tR’ = A*u-B where u is the flow rate in mL/min. The fitting parameters A and B are listed in Table S-1. The ratio of retention times between two peaks is the selectivity α of the HPLC method, and is ca. 1.6 for Au38/Au25, ca. 2.05 for Au40/Au25 and ca. 1.28 for Au40/Au38. We then analyzed mixtures of equal amounts of Au25(DDT)18 and Au38(DDT)24 to confirm the baseline separation of the two compounds. Chromatograms at different flow rates are shown in Figure 2, bottom. When comparing the peak retention times at different flow rates of the mixture with the pure compounds, virtually no difference is observed. This indicates that the method is quite robust, with α = 1.6 at all flow rates. Intentionally mixing Au25(DDT)18, Au38(DDT)24 and Au40(DDT)24 leads to some loss of selectivity, the compounds, however, can still be clearly identified (Figure S-10). Variation of the mobile phase during the separation of Au25(DD)18 and Au38(DDT)24. (MeOH:THF 60:40, 50:50, 35:65 and 20:80) shows that baseline separation is achieved only for MeOH:THF 35:65 (Figure S-11). If the THF content in the mobile phase is too low, the clusters either do not elute or only weak separation is observed in an unacceptable time range (tR’ > 20 min). If the THF content is too high, elution is very fast and the two peaks merge. We finally demonstrate the influence of the alkyl chain length of the thiolate ligands on the elution times of the Au25(SR)18 system (Figure 3). A series of Au25(SCnH2n+1)18 clusters with n = 6 (hexanethiolate, SHex), 8 (octanethiolate, SOct), 10 (decanethiolate, SDec) and 12 (DDT) was prepared and analyzed at different flow rates (using MeOH:THF 35:65). Decreasing the chain length leads to shortened retention times, which we attribute to weakened interactions between the ligands and the C18 reversed phase modifier. In Au25(SHex)18, significant tailing of the peaks is seen, which becomes more apparent at low flow rates. This is probably due to relatively good solubility of Au25(SHex)18 in methanol, hindering its efficient precipitation at injection. The analytes were injected from toluene into pure methanol, and the residual solubility of Au25(SHex)18 may lead to smearing of the precipitation, which in turn leads to peak tailing, especially at low flow rates. A linear relationship between retention time and alkyl chain length was observed for all flow rates (1.0 – 2.0 mL/min). CONCLUSIONS In summary, we present a fast (< 20 min), reliable and sensitive HPLC method to separate small gold clusters protected by apolar alkanethiolate ligands with baseline

separation. Measurement of on-line absorption spectra allows for assignment of cluster identity in real time. Small impurities are detectable in the chromatograms, since MPCs have large molar extinction coefficients. This method can be applied for purity control of MPCs, analysis of purification processes, and, with use of a fraction collector, for isolation of MPCs that cannot be purified otherwise (i.e. Au40(DDT)24). A linear relationship between alkyl chain length and retention time was observed, which might be of importance for future development of separation methods for MPCs of different sizes stabilized by alkanethiolates. Development of an analogous method for clusters protected by the frequently used 2-phenylethanethiolate is ongoing and will be reported elsewhere.

ASSOCIATED CONTENT Supporting Information Detailed synthetic procedures, ESI-TOF mass spectra, absorption spectra, additional HPLC data. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * e-Mail: [email protected]

ORCID Stefan Knoppe: 0000-0002-3687-4485 Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to the University of Stuttgart and the Max-Planck Society for financial support. S.K. also acknowledges support by the Fonds der Chemischen Industrie. We thank Joachim Trinkner (University of Stuttgart, Institute for Organic Chemistry) for measurement of the mass spectra.

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HPLC of Monolayer-Protected Gold Clusters with Baseline Separation

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