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Laser Desorption and Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of 29-Kda Au:SR Cluster Compounds

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Laser Desorption and Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of 29-Kda Au:SR Cluster Compounds Anal. Chem. 2004, 76, 6187-6196 Laser Desorption and Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of 29-kDa Au:SR Cluster Compounds T. Gregory Schaaff* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6131 Positive and negative ions generated by laser-based for electron microscopy2,3 or bright fluorescent microscopy4-6 ionization methods from three gold:thiolate cluster com- probes. Since the optical and electronic properties of cluster and pounds are mass analyzed by time-of-flight mass spec- nanocrystal compounds are inexorably linked to the size of the trometry. The three compounds have similar inorganic inorganic core, many future applications rely on the ability to core masses (∼29 kDa, ∼145 Au atoms) but different synthesis and isolate compounds with either a narrow core size n-alkanethiolate ligands associated with each cluster distribution or those that are molecularly pure (i.e., one single compound (Au:SR, R ) butane, hexane, dodecane). structure). Giant (nanometer-scale) cluster compounds have been 7 Irradiation of neat films (laser desorption/ionization) and isolated for selected systems recently (e.g., Pd145 metallic clusters 8 films generated by dilution of the cluster compounds in and semiconductor Ag2S cluster compounds ). However, due to an organic acid matrix (matrix-assisted laser desorption/ their inherent compositional and structural complexity, these types ionization) with a nitrogen laser (337 nm) produced of compounds represent a significant challenge for routine distinct ion abundances that are relevant to different analytical chemical techniques, even those developed for other structural aspects of the cluster compound. Laser de- macromolecular systems. sorption/ionization of neat Au:SR compound films pro- Because chemical properties are derived from the organic duces ions consistent with the inorganic core mass (i.e., ligands, one of the unique aspects of the Au:SR compounds devoid of original hydrocarbon content). Matrix-assisted is the ability to isolate and accumulate cluster compounds with distinct inorganic core sizes through fractionation,9-11 laser desorption/ionization produces either ions with m/z chromatography,11-13 supercritical extraction,14 and electrophoretic values consistent with the core mass of the cluster methods.15 As a result of such separations, it was possible to map compounds or ions with m/z values consistent with the the evolution of optical properties from bulklike (broad plasmon approximate molecular weight of the cluster compounds, resonance excitation)10 to molecular-like (discrete electronic depending on ionization conditions. The ion abundances, transitions)11,15 metallic electronic structure. In addition, other and ionization conditions under which they are detected, interesting properties have been discovered for these cluster provide insight into desorption/ionization processes for these unique cluster compounds as well as other analytes (2) Hainfeld, J. F.; Furuya, F. R. J. Histochem. Cytochem. 1992, 40, 177-184. typically studied by matrix-assisted laser desorption/ (3) Powell, R. D.; Halsey, C. M. R.; Hainfeld, J. F. Microsc. Res. Technol. 1998, 42,2-12. ionization. (4) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759-1762. (5) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, Gold:thiolate (Au:SR) cluster compounds (or monolayer pro- 281, 2013-2016. tected cluster compounds) constitute a special subset of metallic (6) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. nanostructures that have been the subject of numerous studies (7) Tran, N. T.; Powell, D. R.; Dahl, L. F. Angew. Chem., Int. Ed. 2000, 39, 4121-4125. 1 in recent years. Conceptually, these compounds consist of a dense (8) Wang, X. J.; Langetepe, T.; Persau, C.; Kang, B. S.; Sheldrick, G. M.; Fenske, metallic gold core surrounded by a shell of thiolate ligands (See D. Angew. Chem., Int. Ed. 2002, 41, 3818-3822. (9) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Chart 1.). The measured optical properties are dominated by the Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. electronic structure associated with metallic bonding within the Mater. 1996, 8, 428-433. inorganic core, while the gross chemical properties are derived (10) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706-3712. with the organic (or biologic) ligands attached to that core. Similar (11) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; metallic and semiconductor cluster compounds are finding ap- Cullen, W. G.; First, P. N.; GutierrezWing, C.; Ascensio, J.; JoseYacaman, - plications in biologic imaging, such as strongly scattering centers M. J. J. Phys. Chem. B 1997, 101, 7885 7891. (12) Song, Y.; Jimenez, V.; McKinney, C.; Donkers, R.; Murray, R. W. Anal. Chem. 2003, 75, 5088-5096. * Phone: (865) 574-2297. Fax: (865) 574-9771. E-mail: [email protected]. (13) Jimenez, V. L.; Leopold, M. C.; Mazzitelli, C.; Jorgenson, J. W.; Murray, R. Current address: BWXT Y-12, P.O. Box 2009, MS 8189, Oak Ridge, TN 37821- W. Anal. Chem. 2003, 75, 199-206. 8189. (14) Clarke, N. Z.; Waters, C.; Johnson, K. A.; Satherley, J.; Schiffrin, D. J. (1) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, Langmuir 2001, 17, 6048-6050. 33,27-36. (15) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630-2641. 10.1021/ac0353482 CCC: $27.50 © 2004 American Chemical Society Analytical Chemistry, Vol. 76, No. 21, November 1, 2004 6187 Published on Web 10/06/2004 Chart 1 compounds, which also show strong size dependencies or From these studies, three to four molecular-like ions were quantum size effects (e.g., electrochemical charging of the cluster observed for a class of gold-phosphane cluster compounds, which core,16,17 chiroptical effects in gold clusters with biologically had previously been thought to contain a single component. derived ligands,15,18 and solid-state molecular crystal structure19). Recently, one such compound has been separated in high purity Laser desorption/ionization mass spectrometry has been by low-pressure chromatographic techniques and analyzed by highly efficient for analyzing the core mass of the cluster matrix-assisted laser desorption/ionization mass spectrometry.23 compounds to provide both rapid monitoring of size separation In addition, other metallic-phosphane cluster compounds have techniques and optimization of reaction parameters to produce recently been studied by electrospray ionization mass spectrom- specific cluster compounds in high yield and the qualitative etry.24 Despite many refinements in the methods for preparation observations regarding further reactions of the cluster com- and isolation (or separation) of the gold-thiolato metal cluster pounds.20,21 Owing to the central role of laser desorption/ionization compound materials, they have never been conclusively demon- (LDI)-MS, many reports have differentiated the separated cluster strated to exist as molecularly defined substances (e.g., single- compounds by their respective core masses. For example, 29- crystal structure determination). However, previous studies allude kDa Au:SC4 refers to a gold cluster compound that, upon to a molecular system composed of cluster species with nearly irradiation with UV irradiation, produces a group of ions centered identical composition and molecular weight. Clearly, an advanced at m/z 29 000 and has an associated ligand shell composed of mass spectrometry investigation, utilizing ultrasensitive and ul- butanethiolate. While LDI-MS mass spectrometry has served a trasoft (gentle) ionization methods, should enable a direct, rapid central role in isolating Au:SR cluster compounds, implementation determination of the distribution of assemblies present after of low-fragmentation ionization methods (e.g., electrospray ioniza- synthesis and isolation. tion and matrix-assisted laser desorption/ionization, MALDI) has This report presents (among other things) progress toward only achieved sporadic success. this goal and illuminates some of the mechanisms and challenges While gold:thiolate cluster compounds have received increased inherent to the (MA)LDI processes, as applied to such materials. attention in recent years due to the ease of preparation, the first Results from both LDI and MALDI mass spectrometry are analysis of gold cluster compounds by mass spectrometry was presented for three gold cluster compounds with the same Au performed on gold:phosphane cluster compounds by McNeal and core mass: 29-kDa Au:SC4, Au:SC6, and Au:SC12. The abundance co-workers using 252Cf-plasma desorption mass spectrometry.22 of structurally relevant ions under both LDI and MALDI conditions was dependent on both the irradiance delivered (on a single-shot (16) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; basis) and the total power delivered to the sample. Unlike many Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101. other macromolecular structures, structurally relevant high m/z (17) Chen, S. W.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996-10000. ions were produced from Au:SR cluster compounds under many (18) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. ionization conditions (i.e., desorption from neat films or from L. J. Phys. Chem. B 1998, 102, 10643-10646. (19) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; clusters diluted in organic matrixes). The ability to detect these Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. ions under many ionization conditions provides insight into likely (20) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266,91-98. (23) Gutierrez, E.; Powell, R. D.; Furuya, F. R.; Hainfeld, J. F.; Schaaff, T. G.; (21) Schaaff, T.
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