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Department of Chemistry and Biochemistry

Hall, Mississippi, MS 38677, USA. E-mai

7300; Tel: +1 662 915 1826

† Electronic supplementary informa10.1039/c3nr03872f

‡ These authors contributed equally to th

Cite this: Nanoscale, 2013, 5, 12082

Received 26th July 2013Accepted 9th September 2013

DOI: 10.1039/c3nr03872f

www.rsc.org/nanoscale

12082 | Nanoscale, 2013, 5, 12082–120

Au103(SR)45, Au104(SR)45, Au104(SR)46 and Au105(SR)46nanoclusters†

Amala Dass,‡* Praneeth Reddy Nimmala,‡ Vijay Reddy Jupallyand Nuwan Kothalawala

High resolution ESI mass spectrometry of the “22 kDa” nanocluster

reveals the presence of amixture containingAu103(SR)45, Au104(SR)45,

Au104(SR)46, and Au105(SR)46 nanoclusters, where R ¼ –CH2CH2Ph.

MALDI TOF MS data confirm the purity of the sample and a UV-vis

spectrum shows minor features. Au102(SC6H5COOH)44, whose XRD

crystal structure was recently reported, is not observed. This is due to

ligand effects, because the 102 : 44 composition is produced using

aromatic ligands. However, the 103-, 104- and 105-atom nano-

clusters, protected by –SCH2CH2Ph and –SC6H13 ligands, are at or near

58 electron shell closing.

Gold thiolate molecules1 contain a distinct number of goldatoms and thiolate groups with potential applications in catal-ysis, biomedicine, sensors and nano-devices.2 Single crystalXRD3 and mass spectrometry are the two most powerful tools inthe area of nanoclusters for identication and assignmentpurposes. Mass spectrometry is useful for the positive identi-cation of Aun(SR)m molecules4–10 and to probe the progressof the reaction.10,11 The breakthrough in the XRD crystalstructure of Au102(SC6H4COOH)44 showed that these aremolecules and revealed the presence of staple motifs.3 Subse-quently, the crystal structures of Au25(SCH2CH2Ph)18 andAu38(SCH2CH2Ph)24 were reported.12–14 The Au102 XRD reporthas generated tremendous interest in theoretical treatments15–22

with subsequent reports23–26 on improved synthesis, character-ization and ligand exchange.27 All of the experimental work islimited to water soluble ligands and low resolution MS data,centered on mainly the HSC6H4COOH ligand. However, noexperimental work on �22 kDa (in the core mass as observed inLDI MS)28 has been reported in the organic soluble case since

, University of Mississippi, 352 Coulter

l: amal@olemiss.edu; Fax: +1 662 915

tion (ESI) available. See DOI:

is work.

85

the single crystal 102-atom XRD report. Here, we report theisolation of the “22 kDa” nanoclusters. The high resolutionelectrospray mass spectrometric characterization of the nano-cluster shows that the synthesis using a PhCH2CH2SH thiolproduces a mixture of nanoclusters – Au103(SR)45, Au104(SR)45,Au104(SR)46, and Au105(SR)46.

Au25, Au38 and Au144 are the most studied Au–SR moleculesdue to their enhanced stability, which facilitates the synthesis ofthese molecules in workable quantities. Recently an improvedsynthesis of Au102(SPhCOOH)44 was reported.23 The lack offurther experimental reports on Au102(SR)44, where R is either a–CH2CH2Ph or –C6H13 group, may be attributed to its relativeinstability on etching, when compared with Au25 and Au144.Here, we report the high resolution electrospray mass spec-trometry data of the pure “22 kDa” species protected by organicsoluble thiols with –CH2CH2Ph or –C6H13 ligands. We foundthat Au102(SCH2CH2Ph)44 molecules do not form. Single sizedspecies were not found, but a mixture of Au103(SR)45,Au104(SR)45, Au104(SR)46, and Au105(SR)46 nanoclusters29 wereformed.

The nanocluster is synthesized and puried as follows:HAuCl4 (1.17 g in 20 mL water) was mixed with 2 g of tetraoc-tylammonium bromide in 125 mL of CH2Cl2. Aer phasetransfer and removal of the aqueous layer, 1.06 mL phenyl-ethane thiol was added and stirred for 30 min. Then 1.14 gNaBH4 dissolved in 20 mL cold water was added and the reac-tion continued for 30 min. The mixture was washed with water,followed by a methanol wash to remove excess thiol. Furtherpurication to obtain the “22 kDa” nanoclusters was performedby a solvent fractionation that was batch dependent, usingrepeated crystallizations30 with toluene and CH3CN solventmixtures, as reported previously.1,4,28,30

Fig. 1 shows the MALDI MS spectrum of the puried “22kDa” nanoclusters. The expanded spectrum shows the pres-ence of multiple species mirroring the ESI results. Calibrationof the high mass region in a MALDI spectrum can be a chal-lenge due to the lack of appropriate calibration standards ofsufficient resolution to make accurate mass measurements.

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 MALDI-TOF mass spectrum (positive linear mode) of the “22 kDa”nanoclusters using a DCTB9 matrix. Inset shows the expansion of the peaksshowing the presence of multiple peaks.

Fig. 3 Powder XRD of the “22 kDa” cluster containing a mixture of Au103(SR)45,Au104(SR)45, Au104(SR)46, and Au105(SR)46 nanoclusters in comparison withAu67(SR)35. The diffraction features in both the cases match, suggesting a Marksdecahedral structure of the “22 kDa” clusters.

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Hence, no assignments were attempted of the MALDI data. Themultiply charged peaks observed in ESI makes it a more suit-able technique for high resolution studies of larger species.31

MALDI and ESI runs of different synthetic batches of the “22kDa” clusters studied over a 5 year period, 2009–2013, byvarious investigators, show slightly varying distributions of the103-, 104-, and 105- nanoclusters. However the major species inall of the studies are among the ones reported here, namely the103-, 104-, and 105- species, and Au102(SCH2CH2Ph)44 was notobserved.

High resolution ESI MS data of the “22 kDa” nanoclusters inthe presence of metal acetates32 are shown in Fig. 2. Two sets ofpeaks corresponding to the 2+ and 3+ charge states are observedas shown in Fig. 2A. Analysis of the 3+ peaks, shown in Fig. 2B,reveals the identity of the clusters as Au103(SR)45, Au104(SR)45,Au104(SR)46, and Au105(SR)46. No negative ions were observed inthe ESI MS analysis.

Fig. 2 ESI mass spectra of the “22 kDa” nanoclusters in a 50 : 50 toluene : CH3CN mwith KOAc and (B) expansion of the 3+ peaks showing the presence of Au103(SR)45peaks using rubidium, potassium and sodium acetate salts respectively, confirms theand two asterisks represent nanoclusters with one and two cationic adducts respectnot detected.

This journal is ª The Royal Society of Chemistry 2013

The four peaks, (103,45), (104,45), (104,46) and (105,46),could be due to one single species, but appear as four peaks inthe mass spectra due to different adducts or fragmentation.For example, Tsukuda's work showed that due to differentcharges states, Au25(SR)18 can show three peaks due to adductformation, corresponding to Au25(SR)18

+, [Au25(SR)18.TOA]+,

[Au25(SR)18. 2TOA]+.33 To rule out the possibility of adductformation, we studied the set of four peaks, by intentionallyadding alkali metal salts to promote adduct formation. We didthis systematically by using Na, K and Rb ions, so that the shiin mass can be followed.32 Caesium ions were intentionallyavoided due to the similarity in the mass of caesium (132.9 Da)and the SCH2CH2Ph ligand (137.2 Da). Fig. 2 Rb, K and Naspectra show the analysis of the 2+ peaks in the presence ofrubidium, potassium and sodium acetate salts. These peaksconrm the assignments made in Fig. 2B. This intentional

ixture with the addition of metal acetates. (A) ESI spectrum in the full mass range, Au104(SR)45, Au104(SR)46, Au105(SR)46. The (Rb) (K) and (Na) expansion of the 2+presence of the same set of peaks as in the 3+ region. The peaks marked by one

ively, where the mass difference corresponds to the cations. Notably Au102(SR)44 is

Nanoscale, 2013, 5, 12082–12085 | 12083

Table 1 Predicted core and staple arrangements of the experimentally observed nanoclusters

Cluster Aucore(short staple)x(long staple)y Observed charge states (corresponding electron counts)16,35

Au102(SR)44 Au79(Au(SR)2)19(Au2(SR)3)2 58 (from ref. 3 and 16)Au103(SR)45 Au79(Au(SR)2)18(Au2(SR)3)3 3+ (55), 2+ (56), 1+ (57), 0 (58)Au104(SR)45 Au80(Au(SR)2)18(Au2(SR)3)3 3+ (56), 2+ (57), 1+ (58), 0 (59)Au104(SR)46 Au79(Au(SR)2)17(Au2(SR)3)4 3+ (55), 2+ (56), 1+ (57), 0 (58)Au105(SR)46 Au80(Au(SR)2)17(Au2(SR)3)4 3+ (56), 2+ (57), 1+ (58), 0 (59)

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metal salt adduction, including the now popular caesiumacetate addition to promote the ionization of the nanoclustervia [Nanocluster $ Cs]+ adduct formation was originally pio-neered by Murray et al.32

In Fig. 2 Rb, K and Na spectra, the molecular peaks denotedby the dotted lines are present at the same mass, as in the caseof no salt addition. For each molecular ion, the cationic adductscontaining one and two cations, marked by one or two asterisks,respectively, are also seen. The mass difference of these adductscorresponds to the mass of the Na, K and Rb ions as seen by theincreasing mass difference. A Waters Q-TOF SYNAPT systemwas used for the ESI MS and the TOF calibration was performedusing CsI. A calibration check was performed using the1+ ions of Au25(SCH2CH2Ph)18 and the 3+ ions ofAu144(SCH2CH2Ph)60. In the ionization conditions used in thiswork, there was no fragmentation for the Au25(SCH2CH2Ph)18and Au144(SCH2CH2Ph)60 analysis. Hence, we conclude that thevarious clusters observed in this work – Au103(SR)45, Au104(SR)45,Au104(SR)46, and Au105(SR)46 – are present in solution and are nota product of fragmentation in themass spectrometer. Condencein these mass assignments is of high quality is for the followingreasons: (1) data from the two unique ionization methods,MALDI and ESI support the results; (2) the assignment of the ionsof 2+ and 3+ charge states agree with each other; (3) threedifferent metal adducts, namely Rb, K and Na, yield systematicand expected mass shis from the molecular ions; (4) theinstrument was calibrated with well-known clusters (Au25 andAu144) and fragmentation was ruled out; (5) the results wererepeated by multiple investigators over a period of 5 years.

We note that Au102(SCH2CH2Ph)44 is not observed in thisreport. This could be due to the differences in the ligand, since p-mercaptobenzoic acid is aromatic, acidic, and hydrophilic whencompared with –SCH2CH2Ph. We showed that using an aromaticligand, –SPh, in the synthesis leads to the formation of the36-atom species.34 The use of –SCH2CH2Ph or –SC6H13 ligandsleads to a 38-atom species. So clearly, using an aromatic ligandleads to a different core-size in some cases when compared to–SCH2CH2Ph or –SC6H13 ligands. Instead of the Au102 core pro-tected by an aromatic ligand, here we observe a mixture con-taining Au103(SR)45, Au104(SR)45, Au104(SR)46, and Au105(SR)46clusters. There is literature precedence36,37 for a mixture ofnanoclusters of related sizes in the form of Au38(SR)24/Au40(SR)24[ref. 36] and Au144(SR)60/Au146(SR)59 [ref. 37].

Zeng and coworkers'17 analysis and new perspective of theAu102 structure resulted in the construction of a new molecule,Au104(SR)46 that is more symmetric than the Au102(SR)44 mole-cule. Though Au104(SR)46 is experimentally observed in Fig. 2,chemical analysis by Reimers et al.20 shows that it is highly

12084 | Nanoscale, 2013, 5, 12082–12085

unlikely that a dithiol can exist on the Au core. Han et al. studiedother sizes such as Au102(SR)44�2x and Au102�2x(SR)44, where x¼1, 2.19 The number of Au atoms and ligands were not variedsimultaneously.

The stability of the Au–SRmolecules is due in part to electronshell closing16,35 and geometry.38,39 The stability of Au102(SR)44 isattributed to its 58 free valence electrons.40 Au103(SR)45,Au104(SR)45, Au104(SR)46, and Au105(SR)46 clusters are all presentin 2+ and 3+ charge states as seen in Fig. 2A and B. In Fig. 2 Rb,when a rubidium acetate salt was intentionally added to thesolution, there are three clear peaks corresponding to [NC]2+,[NC/Rb]2+ and [NC/2Rb]2+, where NC corresponds toAu103(SR)45, Au104(SR)45, Au104(SR)46, and Au105(SR)46 clusters.This indicates charge states corresponding to 2+, 1+ and 0 foreach of the 103-, 104- and 105-atom nanoclusters. These resultsclearly indicate that the title nanoclusters are present inmultiple charge states. These charge states correspond to 55, 56,57, 58 and 59 electron counts, suggesting that the 58 e� count isnot a preferred electronic shell closing. However it is notable thatthese electron counts are near to the 58 electron shell closing.Larger nanoclusters such as Au144 are known to exist in severaldifferent charge states.

The various charge states and electron counts suggests thatthe electron shell closing–stability relationship is overstated.Instead, the stability of the nanoclusters is due to a combina-tion of many factors including, geometry, electron shell closing,and [–SR–(Au–SR)n–] type staple motifs.38

Powder X-ray diffraction (XRD) analysis was performed tostudy the atomic structure. Fig. 3 shows the powder XRD patternof the sample in comparison with Au67(SR)35.30 The diffractionpattern resembles that of Au67 which was predicted to have aMarks decahedral core.30 The single crystal XRD structure ofAu102(SC6H4–COOH)44, shows a Au79 Marks decahedral core.Therefore, we conclude that the mixture of Au103(SR)45,Au104(SR)45, Au104(SR)46, and Au105(SR)46 nanoclusters alsopossess a similar Marks decahedral geometry. Theoretical calcu-lations41,42 and growth of single crystals of the samples wouldfacilitate the understanding of the structures of these species.

The predicted inner core atoms and the number of [–SR–Au–SR–] and [–SR–Au–SR–Au–SR–] type staples for the experimen-tally observed nanoclusters are shown in Table 1. The (103,45)and (104,46) clusters, which are isoelectronic with (102,44), arepredicted to have a short staple converted to a long staple withan addition of one Au and one ligand. However, the (104,45) and(105,46) clusters are predicted to have a Au80 core. Theoreticalmodelling will reveal whether these predictions are reasonable.

We gratefully acknowledge support from NSF 0903787, andthe University of Mississippi Startup Fund. We thank Charles

This journal is ª The Royal Society of Chemistry 2013

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Hussey, Glenn Hopkins, and the Office of Research and Spon-sored Programs for the Bruker Autoex MALDI TOF and WatersQ-TOF SYNAPT instrumentation; Laurence A. Angel, SaitanyaK. Bharadwaj and Lance T. Majors for preliminary experiments;Robert L. Whetten, Hannu Hakkinen, Xiao Cheng Zeng, JeffreyR. Reimers, T. Keith Hollis for sharing optimized structures andhelpful discussions.

Notes and references

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5 J. B. Tracy, G. Kalyuzhny, M. C. Crowe, R. Balasubramanian,J. P. Choi and R. W. Murray, J. Am. Chem. Soc., 2007, 129,6706–6707.

6 A. Dass, J. Am. Chem. Soc., 2009, 131, 11666–11667.7 H. Qian and R. Jin, Nano Lett., 2009, 9, 4083–4087.8 K. Watanabe, Y. Negishi, Y. Niihori, T. Iwasa andK. Nobusada, Phys. Chem. Chem. Phys., 2010, 12, 6219–6225.

9 A. Dass, A. Stevenson, G. R. Dubay, J. B. Tracy andR. W. Murray, J. Am. Chem. Soc., 2008, 130, 5940–5946.

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