Ultraviolet Photodissociation of Selected Gold Clusters: Ultraefficient

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Letter

Ultraviolet Photodissociation of Selected Gold Clusters: UltraEfficient Unstapling and Ligand Stripping of Au (pMBA) and Au (pMBA) 25

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David M. Black, Christopher M Crittenden, Jennifer S. Brodbelt, and Robert L. Whetten J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00442 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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The Journal of Physical Chemistry Letters

Ultraviolet Photodissociation of Selected Gold Clusters: Ultra-Efficient Unstapling and Ligand Stripping of Au25(pMBA)18 and Au36(pMBA)24 David M Black1, Christopher M Crittenden2, Jennifer S Brodbelt2, Robert L Whetten1 1

Department of Physics and Astronomy, The University of Texas at San Antonio, 78249, USA

and 2Department of Chemistry, The University of Texas at Austin, Texas 78712, USA. AUTHOR INFORMATION Corresponding Author *Robert L Whetten ([email protected]) *Jennifer S Brodbelt ([email protected])

ACS Paragon Plus Environment

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ABSTRACT: Here we report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped monolayer protected cluster (MPC) ions generated by electrospray ionization. Gold clusters Au25(pMBA)18 and Au36(pMBA)24 (pMBA = para-mercaptobenzoic acid), were analyzed in both the positive and negative modes. Whereas activation methods including collisional- and electron-based methods produced relatively few fragment ions, even a single ultraviolet pulse (at λ = 193 nm) caused extensive fragmentation of the positively charged clusters. Upon photoactivation using a low number of laser pulses, the staple motifs of both clusters were cleaved and stripped of the protecting ligand portions without removal of any contained gold atoms. This striking process involved Au – S and C – S bond cleavages –– via a pathway made possible by 6.4 eV photon absorption. Monomer evaporation (neutral gold atom loss) occurred upon exposure to multiple pulses resulting in a size series of bare gold-cluster ions. All MS/MS methods produced the singly charged ring tetramer ion, [Au4(pMBA)4 + Na]+ for each cluster.

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The development and applications of monolayer-protected clusters (MPCs) has continued to advance owing to their unique and tunable properties, many of which are size-dependent. MPCs exhibit molecular characteristics and can be synthesized with specific numbers of metal atoms and ligands.

Because of the significant promise of adapting MPCs for photovoltaics1-2,

catalysts3-4, medical diagnostics5-6, therapeutics7-8, and sensors9-10, the ability to create reproducible batches of clusters containing a specific number of core atoms and protecting ligands is critical.

Deciphering the novel structures of MPC materials make them prime

candidates for examination by mass spectrometry11 and chromatography12-14. Mass spectrometry has already become an important tool (along with microscopy, crystallography, spectroscopy and centrifugation15-16) for determination and characterization of novel MPC compositions17-18. Combined mass spectrometry and X-ray crystallographic analysis has confirmed the structures of the types of MPCs examined in the present report, detailing both the core architecture and protecting ligand composition.

Au25(pMBA)18 (pMBA = para-

mercaptobenzoic acid) is composed of a 13-atom icosahedral gold core surrounded by six protecting dimeric ligand “staples” each comprised of three ligands and two gold atoms [-SRAu-SR-Au-SR-]19; Au36(pMBA)24 is composed of a 28-atom face-centered cubic gold core enclosed by four dimeric staples and 12 simple bridging thiolates [-Au-SR-Au-]20. Synthesis, crystal structures, theoretical analyses, and mass spectrometry experiments have been reported on various pMBA-protected, as well as other aqueous and organosoluble clusters 19-20. To date there have been few reports of tandem mass spectrometric (MS/MS) characterization of MPCs, i.e., procedures that are routinely used to determine molecular configuration in smalland bio-molecule structures and have been predominantly restricted to collision induced dissociation (CID) experiments21-23. Murray and coworkers21 used CID to characterize sodiated


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Au25(SC2H4Ph)18-y(S(C2H4O)5CH3)y, noting dissociation of the protecting ligand staples. Au1Lx – Au4Lx oligomer ions as well as various higher-mass ions, including [Na3Au24L16]3+, were observed upon CID.29

Dass and coworkers11 employed CID for characterization of

Au25(SPhCH2CH3)18 in a quadrupole ion mobility time-of-flight mass spectrometer (Q-IM-TOF). These experiments revealed the formation of product ions corresponding to low mass oligomers, as well as ones originating from loss of the protecting staples and fragmentation of the Au13 core.11 Wysocki and coworkers used surface-induced dissociation to characterize isomers of Ag11(glutathione)7 separated in the gas phase by ion-mobility.24 Previous work in our lab has demonstrated the capability of in-source CID to induce fragmentation in gold MPCs up to 36.6 kDa23. Both clusters examined, Au130(SC8H9)50 and Au144(SC8H9)60, formed a characteristic series of fragment ions that were consistent with the accepted structures of the two MPCs.31 As shown in these previous reports, CID of MPCs resulted in limited fragmentation that impeded comprehensive MPC structural interrogation.

Ligand losses via relatively low-energy

rearrangement reactions and formation of corresponding low mass oligomer ions are generally insufficient to pinpoint structures in detail. Photodissociation (PD) is an alternative activation method based on absorption of photons for energization of ions.25-36 Irradiation using UV photons is particularly useful owing to the high energy deposition per photon, such as the 6.4 eV of energy deposited by absorption of each 193 nm photon. UVPD typically results in richer fragmentation patterns than CID methods and has been especially useful for characterization of biological molecules, including nucleic acids, oligosaccharides, lipids, peptides, and proteins.25

For these types of organic molecules,

dissociation may occur directly from the excited states, or the ions may undergo internal conversion followed by intramolecular vibrational energy re-distribution (IVR); the combination


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of pathways results in the diverse array of ions generated by UVPD.25,37-38 To the best of our knowledge, characterization of MPC ions via UV laser irradiation has, to date, been limited to electron emission and action spectroscopy studies39. New means for characterization of MPCs are crucial since an accurate m/z alone – i.e., a mass spectrum – may lead to an ambiguous or erroneous assignment of the number of ligands and core atoms in an MPC. Here, we report for the first time on the extension of photodissociation – disclosing ion activation via deep UV irradiation at 193 nm (6.4 eV) – for structural interrogation of two small MPCs, Au25(pMBA)18 and Au36(pMBA)24 (shown in the supplementary information section).

We also report

comparative results obtained by conventional CID, higher-energy collisional dissociation (HCD), and electron transfer dissociation (ETD).

Figure 1. ESI mass spectra of MPC mixture containing Au25(pMBA)18 and Au36(pMBA)24 in (a) positive mode, and (b) negative mode. Each peak is labeled by [(# Au atoms, # ligands) + Adducts]Charge State, or by [(# Au atoms, # ligands) - # of sites of deprotonation]Charge State. Upon electrospray ionization (ESI), poly-acid, highly water-soluble clusters produce negative ions by deprotonation or positive ions by cationization with NH4+ or metal ions.34 Figure 1


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shows the positive and negative mode ESI mass spectra of a synthetic product mixture containing Au25(pMBA)18, Au36(pMBA)24, as well as various other lower-level MPC byproducts. The positive mode ESI mass spectra show doubly- and triply-charged Au25(pMBA)18 and triplycharged Au36(pMBA)24, all primarily as ammonium adducts. The negative mode ESI mass spectra show doubly and triply charged Au25(pMBA)18 and triply- and quadruply charged Au36(pMBA)24. The total charge on each of the observed ions is the sum of the number of cationizing species (or number of deprotonations) plus the inherent core charge state of 1- for Au25(pMBA)18 or 0 for Au36(pMBA)24.

Figure 2. MS/MS spectra of [Au25(pMBA)18 + 4NH4]3+ produced by (a) CID (CE = 30), (b) HCD (CE = 5), and (c) ETD (reaction time = 150 ms). Each ion is labeled in the following way: [(# Au atoms, # ligands) + Adducts]Charge State. The selected precursor ions are denoted by an asterisk (*).


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Figures 2a-c show the MS/MS spectra for positively charged [Au25(pMBA)18 + 4NH4]3+ (m/z 2584 ± 10) generated upon CID (CE = 30), HCD (CE = 5), and ETD (reaction time = 150 ms), respectively. (Note that the +10 m/z ion isolation range meant that sodium adducts were also included, thus explaining the observation of sodium adducts in some of the resulting fragment ions.) The CID spectrum in Figure 2a predominantly displays losses of 1, 2, or 3 pMBA moieties plus adducted ammonium or sodium ions from the precursor cluster ion. The small tetrameric product [Au4(pMBA)4 + Na]+ is observed at m/z 1422.95. Figure 2b shows the MS/MS spectrum from HCD of the same [Au25(pMBA)18 + 4NH4]3+ cluster. The HCD spectrum is qualitatively similar to the CID spectrum in Figure 2a, with the exception that there are fewer doubly charged product ions observed in the higher m/z range. As with CID, the predominant losses upon HCD are consistent with combinations of mercaptobenzoic functionalities plus adducted sodium or ammonium ions. The [Au4(pMBA)4 + Na]+ ring structure is again observed upon HCD.

ETD of the MPC, shown in Figure 2c, resulted in only three product ions

corresponding to losses of one or two pMBA molecules with NH4+ or loss of one NH4+ by itself. The CID, HCD, and ETD spectra are remarkably simple which makes them easy to interpret but offers little diagnostic detail. In stark contrast to the MS/MS results discussed above, UVPD of the positively charged [Au25(pMBA)18 + 4NH4]3+ cluster resulted in rich spectra with product ions spanning nearly the entire m/z range, as shown in Figure 3. The changes in the abundances and distributions of fragment ions as the number of laser pulses was varied from one to ten indicate that both the clusters and their product ions exhibit high fragmentation efficiencies stemming from their significant photoabsorption cross-sections at 193 nm40-42. The richness of the UVPD spectra for


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Figure 3. UVPD mass spectra of [Au25(pMBA)18 + 4NH4]3+ acquired using a different number of laser pulses (1.4 mJ per pulse) (a) 1 pulse, (b) 2 pulses, (c) 5 pulses, and (d) 10 pulses. Ligand containing product ions are labeled in the following way: [(# Au atoms, # ligands, Sx)]Charge State where x = # of sulfur atoms. Product ions that do not contain ligands are labeled: [(Aun, Sx)] where n and x represent the numbers of gold and sulfur atoms present in the ion, respectively. The selected precursor ions are denoted by an asterisk (*).


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the cationized MPC clusters is striking, especially in comparison to the simple MS/MS spectra generated by CID, HCD, and ETD. In Figure 3a UVPD of [Au25(pMBA)18 + 4NH4]3+ results in two series of ions that are each consistent with consecutive ligand losses. In the first and most prominent series, all gold atoms are retained and all but one of the protecting ligands are lost, forming fragment ions of the type [Au25(pMBA)1S8]2+ to [Au25(pMBA)1S10]2+, where S = sulfur atom retained owing to sulfur – carbon bond cleavage and PhCO2H loss. For the second series, one or more protecting ligands are similarly lost, denoted by asterisks in Figure 3a, with each fragment in the series retaining a single sodium adduct ion. Interestingly, all fragment ions in each of the two series maintain an even number of valence-shell electrons (i.e., even-electron fragment ions), possibly owing to the stability such electronic configurations provide. Certain fragment ion compositions may be favored as a result of this effect. Proposed compositional assignments and valence-shell electron count for selected products in the UVPD mass spectra shown in Figures 3a,b are provided in Table 1. Also observed in the UVPD mass spectra is the singly charged [Au4(pMBA)4 + Na]+ ion noted previously upon CID and HCD. As the number of laser pulses is doubled from one to two, shown in Figure 3b, the relative abundances of the product ions that contain three or more ligands diminish, and the abundances of the two main series of product ions, [Au25(pMBA)1S8]2+ to [Au25(pMBA)1S10]2+ and [Au24(pMBA)2S7]2+ to [Au24(pMBA)2S9]2+, increase. This trend of systematic ligand stripping with increasing laser pulses represents a convergence toward nearly bare gold clusters. Charge state reduction, via loss of an ammonium or sodium adduct, also occurs in parallel with ligand loss, resulting in the corresponding doubly charged product ions. The fragmentation pattern in the one- and two-pulse UVPD spectra begins with three losses of an ammonium salt species, [(R – S)- (NH4)+], where (R –) = PhCO2H. This is


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followed by a series involving Au – S and C – S bond cleavages – a pathway accessed by 6.4 eV photon absorption – to produce losses consistent with benzoic acid thio-ether moieties (R – S – R).

Note that the specific structural arrangement of these neutral species is unknown.

Intriguingly, as the

m/z 3785 3697 3695 3610 3561 3473 3424 3352 3336 3286 3214 3198 3149 3077 3061 2956 2940 2924 2819 2803 2787 2698 2682 2644 2628

Product ion [Au25(pMBA)17 [Au25(pMBA)16 [Au25(pMBA)16 [Au25(pMBA)15]2+ [Au25(pMBA)14S [Au25(pMBA)13S]2+ [Au25(pMBA)12S2 [Au25(pMBA)11S3]2+ [Au25(pMBA)11S2]2+ [Au25(pMBA)10S3 [Au25(pMBA)9S4]2+ [Au25(pMBA)9S3]2+ [Au25(pMBA)8S4 [Au25(pMBA)7S5]2+ [Au25(pMBA)7S4]2+ [Au25(pMBA)5S7]2+ [Au25(pMBA)5S6]2+ [Au25(pMBA)5S5]2+ [Au25(pMBA)3S8]2+ [Au25(pMBA)3S7]2+ [Au25(pMBA)3S6]2+ [Au25(pMBA)1S10]2+ [Au25(pMBA)1S9]2+ [Au24(pMBA)2S8]2+ [Au24(pMBA)2S7]2+

+ + + + +

+

+

Valance 8 8 8 8 8 8 8 6 8 8 6 8 8 6 8 4 6 8 4 6 8 2 4 4 6

Table 1. Proposed product ion assignments and electron shell count for selected peaks observed in Figures 2a,b. Electron count is determined according to the formula, ne = NAu - NSR - 2NS + NNa - z where ne is the number of valence shell electrons, NAu is the number of gold atoms in the cluster, NSR is the number of thiolate ligands, NS is the number of free sulfur atoms, NNa is the number of sodium adduct ions, and z is the charge state of the ion.


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protecting ligand staples are cleaved and ligand sub-units are stripped from the MPC precursor complexes, gold atoms associated with each staple relocate and/or condense to the core of the cluster. These processes are illustrated in Scheme 1a,b and occur by means of a mechanism not previously reported for these structures. A competing process is also noted (also shown in Scheme 1b), whereby – R groups are eliminated, either as two monomeric (– R) units or one dimeric species (R – R). The benzoic acid-type losses arise from S – C bond cleavage and occur with lower frequency relative to the analogous thio-ether losses. These competing pathways produce a striking fragment ion series extending from intact MPC to nearly bare gold cluster. Each member of the ion series is comprised of a distribution of individual peaks corresponding to nearly the same atomic composition – differing only in sulfur atom count. These distributions broaden slightly as the competing pathways extend toward formation of nearly bare gold clusters. This unusual re-organization process seems to be a unique hallmark of UVPD. From a practical perspective, this result shows that UVPD is an effective way to selectively remove the protecting ligands from the MPCs, thus permitting elucidation of a ligand count. Figures 3c,d show the UVPD spectra as the number of laser pulses is increased to 5 and 10, respectively. As the number of laser pulses increases, the product ion series arising from ligand losses is diminished and a second, equally fascinating, product ion series appears. The ions in this new series are consistent with singly-charged, bare clusters ranging in size from Au4+ to Au20+, as confirmed based on accurate mass measurements and isotopic patterns. Compositional assignments for selected product ions are listed in Table S1. This type of cascading cluster ion series has been observed previously upon photodissociation of bare cluster ions produced by a Smalley-type laser ablation source43-44, and also by laser desorption ionization of alkanethiolate coated gold nanocrystals45, but has not been reported by MS/MS characterization of MPCs.


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Photodissociation of bare gold cluster ions produced by laser ablation is reported to proceed by means of monomer evaporation (or dimer evaporation for smaller cluster sizes) process – depicted in Scheme 1c – whereby neutral gold atoms are lost in a step-wise fashion leading to the formation of a sequential size series of cluster fragment ions43. Indeed, the fragmentation observed in Figures 3c,d is consistent with a monomer evaporation process and may indicate that MPC precursor ions are stripped of all ligands prior to the commencement of this subsequent fragmentation pathway.

Scheme 1. Proposed fragmentation processes initiated by UV photon (193 nm) activation; (a) neutral loss of mercaptobenzoic acid ammonium salt from monolayer protected cluster ion, (b) competing neutral loss of (R – S – R) and (R – R) from monolayer protected cluster ion where q = 1 - 7, and (c) monomer evaporation from bare cluster ion.

Related ions, [Aun + (S)x]+ (where 0 ≤ x ≤ 9) – presumably formed from precursors generated by the pathways depicted in Scheme 1b – accompany each of the bare gold clusters in the series. Although unknown, these ions may undergo a sulfur monomer evaporation process – possibly


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competing with gold monomer evaporation – which leads to the formation of the bare gold cluster ion structures (i.e., these ions may be products of ligand stripping from the intact MPC and precursors to bare gold cluster ions). However, an exact elucidation of these processes and pathways requires systematic evaluation via MSn and are beyond scope of this work. The bare cluster sequence of ions is highly diagnostic, nearly permitting elucidation of a gold atom count (constrained only by the upper mass range of the instrument), and its formation is unique to UVPD in the present MS/MS study. A notable feature of this ion series is the oscillating trend of high- vs low-abundance products from one cluster size to the next such that singly-charged, oddnumbered clusters exhibit much higher abundances than neighboring even-numbered clusters. This interesting result is likely attributable to the increased stability that an even number of valence electrons, and concomitantly doubly occupied HOMO has on cluster stability and has been discussed in previous work that focused on dissociation of bare gold cluster ions46-47. Interestingly, the ring tetramer product ion, [Au4(pMBA)4]+, is observed in all UVPD spectra acquired using five or fewer laser pulses, but is absent from the ten-pulse UVPD spectrum. The disappearance of this product suggests that it is annihilated after exposure to multiple UV photons. Analogous results are shown and discussed for a larger MPC, Au36(pMBA)24, in the supplemental information section. In summary, UVPD is demonstrated as a highly effective approach for characterization of two MPCs, Au25(pMBA)18 and Au36(pMBA)24. While CID, HCD and ETD produce relatively few diagnostic fragment ions, UVPD of positively charged MPC ions yielded rich spectra that include fragment ions resulting from higher energy pathways. Two distinctive product ion series, corresponding to systematic losses of ligands and gold atoms, were identified.

UV

photoactivation using a low number of laser pulses resulted primarily in fragmentation pathways


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that led to near complete removal of the protecting monolayer while retaining the total number of gold atoms from the original MPC precursor. Using five or more pulses resulted in production of bare gold cluster ions Au4 - Au20 from Au25(pMBA)18 evolving from elimination of successive monomers.

The observed high efficiency of photodissociation suggests that the optical

absorption cross-section of the gold-thiolate clusters may be much larger than those of the biomolecules previously investigated by this 193 nm UVPD method. This is consistent with the reputation of gold clusters as extraordinarily strong absorbers of UV radiation. Future studies will be aimed at investigating the UVPD mass spectra for MPCs with other compositions and stoichiometries as well as quantitative aspects of the gold-cluster UVPD phenomenon. This methodology may extend to unknown MPC compositions for elucidation of cluster composition as well as to mixed-MPCs for characterization of ligand distribution around the core. Application of UVPD to MPCs developed for other applications may provide a powerful means to characterize novel MPC structures.

ASSOCIATED CONTENT Supporting Information Experimental methods, other MSMS spectra and proposed product ion assignments for spectra in Figure 3 are included in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Notes


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The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to acknowledge support from the National Institute on Minority Health and Health Disparities (G12MD007591), PREM: NSF PREM Grant #DMR 0934218; ‘‘Oxide and Metal Nanoparticles – The Interface Between Life Sciences and Physical Sciences”, NSF (Grant CHE1402753 to JSB) and the Welch-Foundation Grant (AX-1857 to RLW and F-1155 to JSB). The authors would like acknowledge Mr. Fangzhi Yan for use of prepared cluster samples. REFERENCES 1. Stamplecoskie, K.; Swint, A. Optimizing Molecule-Like Gold Clusters for Light Energy Conversion. J. Mater. Chem. A 2016, 4, 2075-2081. 2. Abbas, M. A.; Kim, T.-Y.; Lee, S. U.; Kang, Y. S.; Bang, J. H. Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance. J. Am. Chem. Soc. 2015, 138 (1), 390-401. 3. Li, G.; Jin, R. Atomically Precise Gold Nanoclusters as New Model Catalysts. Acc. Chem. Res. 2013, 46 (8), 1749-1758. 4. Mitsudome, T.; Yamamoto, M.; Maeno, Z.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. One-Step Synthesis of Core-Gold/Shell-Ceria Nanomaterial and Its Catalysis for Highly Selective Semihydrogenation of Alkynes. J. Am. Chem. Soc. 2015, 137 (42), 13452-13455. 5. Hainfeld, J. F.; Smilowitz, H. M.; O'Connor, M. J.; Dilmanian, F. A.; Slatkin, D. N. Gold Nanoparticle Imaging and Radiotherapy of Brain Tumors in Mice. Nanomedicine-UK 2013, 8 (10), 1601-1609. 6. Zhou, W.; Gao, X.; Liu, D.; Chen, X. Gold Nanoparticles for in Vitro Diagnostics. Chem. Rev. 2015, 115 (19), 10575-10636. 7. Saie, A. A.; Ray, M.; Mahmoudi, M.; Rotello, V. M. Engineering the NanoparticleProtein Interface for Cancer Therapeutics. In Nanotechnology-Based Precision Tools for the Detection and Treatment of Cancer, Springer: 2015; pp 245-273. 8. Ackerson, C. J.; Jadzinsky, P. D.; Sexton, J. Z.; Bushnell, D. A.; Kornberg, R. D. Synthesis and Bioconjugation of 2 and 3 Nm-Diameter Gold Nanoparticles. Bioconjugate Chem. 2010, 21 (2), 214-218. 9. Guan, G.; Cai, Y.; Liu, S.; Yu, H.; Bai, S.; Cheng, Y.; Tang, T.; Bharathi, M.; Zhang, Y. W.; Han, M. Y. High‐Level Incorporation of Silver in Gold Nanoclusters: Fluorescence Redshift Upon Interaction with Hydrogen Peroxide and Fluorescence Enhancement with Herbicide. Chem. Eur. J 2016, 22 (5), 1675-1681. 10. Chen, L.-Y.; Wang, C.-W.; Yuan, Z.; Chang, H.-T. Fluorescent Gold Nanoclusters: Recent Advances in Sensing and Imaging. Anal. Chem. 2015, 87 (1), 216-229.


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O

OH

O

OH

O

OH

S

HO

Au

O

Au

Au

S

AuMPC

S Au

Au Au S

O

S

O

Au

Au

S

AuMPC

OH

S

S Au

O

S

OH

Au

HO

Au

O

O

HO

O

HO

O

O OH

Au

Au

S

S

O

HO

HO HO O

HO

O


O OH

S

O

HO

OH

S

S Au

Au

O

Au S Au

HO

O

Au

S

O

Au

Au

Au Au S Au S Au S

HO

Au Au S

Au

O

OH

Au S

Au

HO

O

O Au

S

HO

O

Au

Au

Au

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Au S HO

193 nm

Au S

Au

Au

OH

Au S

HO

Au S Au Au

O

S

S Au S

O

Au

S

S

OH O

OH Au

O

S

O OH

OH OH

O

S Au S Au S Au Au

O

O

OH

OH OH

O

O

OH

O

HO

O

193 nm

Ultraviolet Photodissociation of Selected Gold Clusters: Ultraefficient

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