Structure Determination of a Water-Soluble 144-Gold Atom Particle at Atomic Resolution by Aberration-Corrected Electron Microscopy Maia Azubel,*,† Ai Leen Koh,‡ Kiichirou Koyasu,§ Tatsuya Tsukuda,§ and Roger D. Kornberg† †
Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, United States Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States § Department of Chemistry, School of Science, The University of Tokyo, Tokyo 113-0033, Japan ‡
S Supporting Information *
ABSTRACT: Structure determination by transmission electron microscopy has revealed the long sought 144-gold atom particle. The structure exhibits deviations from facecentered cubic packing of the gold atoms, similar to the solution structure of another gold nanoparticle, and in contrast to a previous X-ray crystal structure. Evidence from analytical methods points to a low number of 3-mercaptobenzoic acid ligands covering the surface of the particle.
KEYWORDS: gold nanoparticles, transmission electron microscopy, 3-mercaptobenzoic acid, low-dose, thiol-protection
T
whereby essentially homogeneous particles are obtained without purification. The size of the nanoparticles is determined by the gold:ligand ratio in the reaction, and with a ratio of 1:7, 3-MBA-protected particles of the same electrophoretic mobility as the presumptive 4-MBA-protected Au144 particle were obtained. Structures of gold nanoparticles have long been sought to illuminate the principles of their formation and stability, their physical properties, and their chemical reactivity. The first atomic structure was obtained by X-ray crystallography of a 4-MBA-protected particle, which proved to contain 102 gold atoms surrounded by 44 4-MBA ligands.20 The structure established the existence of a large gold particle as a discrete chemical compound, gave insight into the basis for stability,21,22,1 and revealed a gold-ligand motif altogether different from the “standard model”.21 Other large gold particles have since been crystallized, but X-ray diffraction never extended to atomic resolution (Azubel, unpublished). In particular, the atomic structure of the presumptive Au144 particle, a major focus of gold nanoparticle research, has remained unsolved. We previously described the atomic structure determination of a 3-MBA-protected Au68
hiolate-protected gold nanoparticles are of interest for fundamental studies and for a wide range of applications.1 Evidence for large particles of discrete sizes came originally from mass spectrometry, beginning with ionization by laser desorption. The surface ligands were lost, but the gold cores could be detected. Initial analysis of a crude mixture, containing particles 1.5−3.5 nm in diameter, revealed a 29 kDa, presumptive 140-gold atom particle.2−4 This particle was characterized by optical absorption,5 electrochemistry,6−8 and NMR3,4,8,9 and was variously referred to as a Au140, Au144, Au146, or Au147 nanoparticle. With the use of electrospray ionization rather than laser desorption, surface ligands were retained, and compositions of Au144(SR)59, Au144(SR)60, and Au146(SR)59 were reported.10−12 The SR ligands were hydrophobic, and the particles were therefore organo-soluble. With the use of hydrophilic ligands, large water-soluble particles have also been obtained.13,14 Electrospray ionization of water-soluble particles proved more challenging, but recently, with 4-mercaptobenzoic acid (4-MBA) as ligand, a water-soluble Au144 particle was identified.15,16 Water-soluble gold nanoparticles have also been formed with 3-MBA rather than 4-MBA as ligand.17 The 3-MBA-protected nanoparticle is of particular interest for its high reactivity toward other thiol compounds and exceptional utility in bioconjugation.18 We have described a modification18 of the original method19 of thiolate-protected gold nanoparticle synthesis, © 2017 American Chemical Society
Received: August 24, 2017 Accepted: November 14, 2017 Published: November 14, 2017 11866
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Figure 1. Characterization of presumptive Au144(3-MBA)x particle. (a) Gel electrophoresis of Au68(3-MBA)32 (lane 1), presumptive Au144(3-MBA)x particle (lane2), and presumptive Au144(4-MBA)60 particle (lane 3). (b) UV−vis spectrum of the presumptive Au144(3-MBA)x particle.
particle by aberration-corrected electron microscopy with low electron dose and single particle reconstruction.23 We report here on the structure determination of a 3-MBA-protected Au144 particle by the same approach.
RESULTS Gold nanoparticles prepared with a gold:3-MBA ratio of 1:7 are not only similar in electrophoretic mobility to particles with the proposed formula Au144(4-MBA)60 (Figure 1a) but also exhibit a shoulder in the UV−vis spectrum at 520 nm (Figure 1b), indicative of plasmon resonance. This feature was reported for other presumptive Au144 particles,11,15 although as expected,24 the use of a different thiol affected the shape of the curve. Aberration-corrected electron microscopy revealed highly homogeneous particles, suitable for single particle analysis and 3-D reconstruction (Figure 2). As in the case of the 3-MBAprotected Au68 particle,23 the presumptive Au144 particle was very sensitive to radiation damage (Supplementary Movie 1), requiring a low dose strategy for data collection. The low dose approach minimizes radiation damage by limiting the number electrons used to record the image and by focusing on an area adjacent to the field of particles to be imaged. In order to resolve individual atoms, the range of acceptable defocus values was limited to 1.5−2.5 nm, and since defocus values were extrapolated from adjacent areas, this approach was only effective when the electron microscope grids were truly flat (images from bent areas did not reveal individual atoms and were rejected). A set of about 4300 particles was subjected to reference-free 2D classification and 3D ab initio reconstruction. The initial 3D structure was iteratively refined with the use of a projectionmatching approach. In the resulting distribution of views, some classes were more highly populated (Figure 3a, taller bars), perhaps due to preferential orientation, but the overall coverage of the Euler space was satisfactory. There was, moreover, good
Figure 2. Aberration-corrected transmission electron micrograph of the presumptive Au144(3-MBA)x particle.
agreement between back projections of the reconstruction, class averages, and raw data for each view (Figure 3b). Fourier transforms showed maxima at 2.4 Å (Figure 3c), attesting to atomic resolution of the reconstruction (see below). The electron density map contoured at the level shown (Figure 4) displayed 144 well-resolved peaks with spacings of 2.75−3.09 Å, consistent with gold−gold bond lengths of 2.7− 3.0 Å. The number of peaks was definitive; additional peaks observed upon lowering the contour level could not represent additional gold atoms, because they were located much too 11867
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Figure 3. Three-dimensional reconstruction of presumptive Au144(3-MBA)x from images of single particles. (a) Distribution of 2D projections of images in Euler space. The height of each point or bar is proportional to the number of particles assigned to that projection. (b) Representative Euler space points of the reconstruction. Left panel, back projection; middle panel, class average; right panel, raw data. (c) A cross-section of the 3D reconstruction (left panel) and its Fourier transform (right panel). Red arrow indicates spots at 2.4 Å resolution.
the values expected for 50 or 60 ligands (Table 1). Of course the number 40 is an estimate, and we cannot rule out the possibility of slight variation of the number from particle to particle.
close (between pairs of the 144 atoms) or much too far (remote from the 144 atoms) to form gold−gold bonds of 2.7− 3.0 Å. Most of the 144 gold atoms conformed with facecentered cubic (fcc) packing (Figure 5a), giving rise to lattice planes with spacing of 2.3−2.55 Å, and thus the Fourier transform peaks at 2.4 Å (Figure 3c). There were, however, deviations from fcc packing on the surface (Figure 5b), conferring curvature on the particle. Ligands were not revealed in the electron density map, but the number of ligands per Au144 particle could be estimated by electrospray mass spectrometry, by X-ray photoelectron spectrometry (XPS), and by thermogravimetric analysis (TGA). Despite the difficulty of mass spectrometry of large watersoluble gold particles, results were obtained more consistent with 40 ligands than with 30, 50, or 60 ligands per particle (Supplementary Figure 1). The accuracy of XPS and TGA was previously investigated by application to a gold nanoparticle of known composition, Au102(4-MBA)44.25 XPS is most reliable for estimation of Au and S, because C and O values may be affected by contamination during sample preparation. XPS underestimated the percentage of Au in the Au102(4-MBA)44 particle by 3% (Table 1). The value obtained for the Au144 particle (81.3% Au) was a similar underestimate for 40 ligands and differed to a much greater extent from the values expected for 50 or 60 ligands (Table 1). TGA underestimated the percentage weight loss upon heating of the Au102(4-MBA)44 particle by 3% (Table 1). The value obtained for the Au144 nanoparticle (15.8% weight loss) deviated from that expected for 40 ligands to a similar extent and differed much more from
DISCUSSION Beyond confirming the existence of the long sought 144-gold atom particle, our findings are notable in several regards. Despite general conformity with fcc packing of gold atoms, there was local variation and clear deviation from a regular shape of the particle (Figure 5b). These geometrical anomalies are inconsistent with symmetry of the particle associated with stability due to the filling of electronic shells. They may explain why crystals of the Au144 particle failed to diffract to high resolution, in contrast with the Au102 particle, with decahedral symmetry, which formed crystals diffracting to 1.1 Å resolution. Electron microscopy and single particle analysis is not only useful in the absence of well diffracting crystals but also important for characterizing particles in solution in regard to both structure and homogeneity. Thus, crystals of Au102(4MBA)44 were originally formed from a scarce contaminant in a gold nanoparticle preparation.20 The structure of most of the nanoparticles in that preparation is unknown. By contrast, the structures of the Au68 and Au144 particles determined by electron microscopy must be representative of the majority, and the derivation of a high-resolution structure from an average of single particles testifies to the uniformity of the population. Finally, the evidence from mass spectrometry, XPS, and TGA for only about 40 ligands rather than the expected number of 11868
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ACS Nano Table 1. X-ray Photoelectron Scattering and Thermogravimetric Analysis XPS (% Au) Au144(3MBA)x Au144(3-MBA)60 Au144(3-MBA)50 Au144(3-MBA)40 Au102(4-MBA)44 Au102(4-MBA)44 TGA (% wt. loss) Au144(3MBA)x Au144(3-MBA)60 Au144(3-MBA)50 Au144(3-MBA)40 Au102(4-MBA)44 Au102(4-MBA)44
measured
expected (%)
difference from expected
81.3 81.3 81.3
70.6 74.2 78.3
10.7 7.1 3.0
72.9 measured
69.9 expected (%)
3.0 difference from expected
15.8 15.8 15.8
24.6 21.4 17.9
−8.8 −5.6 −2.1
22.3
25.2
−2.9
turned from yellow to colorless (final concentration of about 100 mM NaOH). The mixture was equilibrated overnight on a rocking platform at room temperature. Methanol and water were added to obtain a solution of 2.5 mM 3-MBA, 0.36 mM HAuCl4, and 27% (v/v) methanol. A freshly prepared solution of 150 mM NaBH4 was added to a final concentration of 2 mM NaHB4. The reaction was allowed to proceed for 4.5 h on a rocking platform at room temperature and was stopped by precipitation of the product with 100 mM NaCl and 2 volumes of methanol, followed by centrifugation for 10 min at 5000 rpm. The pellet was washed with 75% methanol, dried in air overnight, and dissolved in water. Electrophoresis was performed with a 10% glycerol, 12% polyacrylamide gel in Tris-borate-EDTA buffer for 30 min at 150 V. UV−vis Absorption Spectrum. The spectrum was recorded at room temperature with an Ultrospec 2100 pro UV−vis spectrophotometer. Mass Spectrometry. ESI mass spectra of presumptive Au144(3MBA)x were collected using an FT-ICR mass spectrometer (SolariX, Bruker Daltonics Co., Ltd.). Presumptive Au144(3-MBA)x (1.0 mg/mL) in 50% (v/v) methanol/water plus 0.05% (v/v) Et3N, or water plus 1% (v/v) Et3N were electrosprayed. ESI mass spectra of CsnIn+1− cluster anions, recorded under the same condition, were used for calibration. Resolution of the mass spectra (m/Δm) was higher than 4000. TGA. Samples were dried in vacuum oven at 90 °C overnight and transferred into aluminum crucibles. TGA runs were recorded at a heating rate of 10 °C/min and temperature range of 25−600 °C on a Mettler Toledo TGA/SDTA 851 under a nitrogen flow of 20 mL/min. XPS. Dried presumptive Au144(3-MBA)x, mounted onto a doublesize tape metal holder as powder, was analyzed with PHI 5000 VersaProbe Scanning ESCA microprobe (Physical Electronics, Chanhassen, MN) equipped with an Al (Kα) X-ray radiation source (1486.6 eV, 46W) and spot size of about 200 μm. Transmission Electron Microscopy. Gold nanoparticles (2 μL of 2 mg/mL) were applied to a glow discharged, 400 mesh, ultrathin carbon film/holey carbon copper grid (Ted Pella, Inc.) for 30 s and blotted from the side to remove excess liquid. Dried samples were imaged using a low-dose strategy for searching and focusing, and images were recorded at ∼800 e−/Å2 at a magnification of 320,000 (pixel size 0.32 Å) and at defocus values of ∼1.5 nm, using a FEI (Eindhoven, The Netherlands) Titan 80-300 environmental (scanning) transmission electron microscope operating in high-vacuum mode at 300 kV and equipped with a 2K × 2K CCD camera (Gatan US1000). Three-Dimensional Reconstruction. The reconstruction was done using EMAN2 software package.26 4289 particles were extracted using a box size of 120 × 120. Taking into consideration the relatively high signal of the individual images, particles were classified into 200 classes. The resulting reference-free class averages were used to generate an initial model. The model was iteratively refined using a projection matching approach. The final electron density map was
Figure 4. Electron density map from 3D reconstruction of presumptive Au144(3-MBA)x particle. Upper panel, electron density map in blue mesh. Lower panel, electron density map with peaks identified by a peak-search routine, corresponding to locations of gold atoms, indicated by pink crosses.
60 is noteworthy. The question arises of how complete protection of the gold core and consequent stability of the particle is achieved. Answers to such questions have come from complementary analysis of the 3-MBA ligand layer by NMR and IR measurements, DFT calculations, and molecular dynamics simulations, which also predict emergent properties, such as the high reactivity of the Au144(3-MBA)40 particle toward thiol exchange.30
METHODS Gold Nanoparticle Synthesis. Solutions of 3-MBA (SigmaAldrich, 84 mM) and HAuCl4 (Sigma-Aldrich, 28 mM) in 100% methanol were freshly prepared and mixed in a 7:1 molar ratio. Water (2.5 vol) was added, and the pH was adjusted to 13 by the addition of NaOH until the pellet was completely dissolved and the solution 11869
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Figure 5. Packing arrangement of gold atoms in Au144(3-MBA)x particle. (a) Central section of the structure showing atoms packed with fcc symmetry. (b) Positions of the outermost atoms of the gold core showing deviations from fcc symmetry. Bar = 2 Å subsequently converted into a format compatible with the CCP4 software package27 in order to perform a peak search. EM maps were displayed using Coot28 and Chimera software packages.29
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ASSOCIATED CONTENT S Supporting Information *
Supplementary Movie 1 The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsnano.7b06051. Supplementary Figures 1−2; 3MBA-Au144 coordinates file (PDF) Effect of high electron dose on Au144(3-MBA)x particles (AVI)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Maia Azubel: 0000-0002-1584-2695 Kiichirou Koyasu: 0000-0002-9106-0054 Tatsuya Tsukuda: 0000-0002-0190-6379 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This research was supported by NIH RO1 AI21144 to R.D.K. and by the HFSPO to M.A. The mass spectrometric measurements were conducted under the support of ICR-JURC, Kyoto University (grant no. 2016-101). We thank H. Häkkinen and M. Pettersson for numerous discussions, essential for the preparation of the joint submission of papers. REFERENCES (1) Häkkinen, H. The Gold-Sulfur Interface at the Nanoscale. Nat. Chem. 2012, 4, 443−455. (2) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Nanocrystal Gold Molecules. Adv. Mater. 1996, 8, 428−433. (3) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. 11870
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