ARTICLE pubs.acs.org/JPCC
Structural and Optical Properties of Isolated Noble MetalGlutathione Complexes: Insight into the Chemistry of Liganded Nanoclusters Bruno Bellina, Isabelle Compagnon, Franck Bertorelle, Michel Broyer, Rodolphe Antoine,* and Philippe Dugourd Universite de Lyon, F-69622, Lyon, France; Universite Lyon 1, Villeurbanne, France; CNRS, UMR5579, LASIM
Lars Gell,† Alexander Kulesza,‡ Roland Mitric,‡ and Vlasta Bonacic-Koutecky †,§ †
Institut f€ur Chemie, Humboldt-Universit€at zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany Fachbereich Physik, Freie Universit€at Berlin, Arnimallee 14, 14195 Berlin, Germany § Interdisciplinary Center for Advanced Sciences and Technology (ICAST), University of Split, Mestrovicevo Setaliste bb., 2100 Split, Croatia ‡
bS Supporting Information ABSTRACT: Gold(I) and silver(I)thiolate oligomers generated under nanoparticle growth conditions have been suggested to play an important role in the growth mechanism of thiolate-protected noble metal clusters. In this work, we explore the formation of isolated noble metalglutathione complexes by complementing electrospray mass spectrometry and optical action spectroscopy with TDDFT calculations. We have isolated and recorded action spectra of [Au +GSH2H], [Ag+GSH2H], and [3Ag+2GSH4H] complexes. Competition between photofragmentation and photodetachment channels related to electron binding energies was observed. Our findings show that, although structural properties of silver and gold metalglutathione oligomers are similar, their optical properties in the UV range differ substantially. The experimental spectra were interpreted and assigned by comparison with TDDFT simulations, which allowed us to identify the key role of OmetalS subunits and characterize their optical properties depending on the choice of metal.
I. INTRODUCTION The optical and electronic properties of metal nanoparticles and clusters have drawn considerable research interest in chemistry, materials, and biology.1,2 Particular attention was paid to gold and silver clusters that exhibit nonscalable properties that differ significantly from those observed in bulk materials. Gold and silver nanoclusters of low nuclearity (with few dozen atoms and sizes less than ∼1 nm) exhibit unique molecule-like discrete electronic states and a nonzero HOMOLUMO gap.3 Their optical spectra are characterized by structure-dependent patterns with multiple peaks of different intensities, which is in contrast with optical spectra of large nanoparticles that exhibit sharp localized plasmon resonances.4 Remarkably, due to the discreteness of their electronic structures, such small clusters exhibit strong visible or near-IR emission57 that has already been exploited for numerous applications in the field of biosensing, single-molecule spectroscopy, or optoelectronics. The tremendous advances in nanochemistry permitted the synthesis of almost monodisperse thiolate-protected gold nanoclusters4,814 with remarkable structural and optical properties. r 2011 American Chemical Society
Because of their high stability and monodispersed preparation by wet chemistry techniques, liganded clusters with 25 gold atoms (Au25) have been intensively studied.1517 Their high stability is due to the presence of 18 surface-capping sulfurcontaining ligands.18 The X-ray and theoretical studies have demonstrated that these complexes should not be regarded as thiolate-protected Au25 clusters but consist of a smaller Au13 core with “staple” bridged RSAuRS units.19,20 Recent total structure determination of Au25(SR)18 (R: phenylethylthiolate) in the anionic and neutral charge states shows that the structure consists of a nearly icosahedral Au13 core surrounded by six V-shaped S AuSAuS motifs in an approximately octahedral arrangement.20 In general, gold(I) thiolate oligomers produced under gold nanoparticle growth conditions have been suggested to play an important role in the nanoparticle growth mechanism. The chainlike oligomers,
Received: July 27, 2011 Revised: October 19, 2011 Published: October 19, 2011 24549
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The Journal of Physical Chemistry C such as Au3(SR)4, have been also proposed to passivate smaller nanoclusters.21,22 Concerning the electronic structure of the thiolate liganded Au25 cluster, the highest occupied molecular orbital (HOMO) and three lowest unoccupied MOs are mainly composed of 6sp atomic orbitals of gold.23 However, a dense bandlike manifold of molecular orbitals arising from Au(5d) and S(3p) atomic levels and localized at the peripheral AuS “staple” units begins approximately 1 eV below the HOMO and extends for over 2 eV. The multiple peaks observed in the optical absorption spectra of Au25(SR)18 is, therefore, a subtle mixture of the excitations of the gold core and AuS “staple” components. Furthermore, it has been recently demonstrated theoretically that TDDFT optical absorption spectra cannot be understood as a simple linear combination of the core and AuS “staple” spectra.24 An interesting finding is that the fluorescence quantum yield of organic soluble Au25 species seems to be parallel with the ligand’s capability of donating electron density to the metal core via the SAu bond (i.e., charge donation capability of the ligand).25 In particular, the glutathione ligand leads to an enhanced fluorescence that was observed experimentally. This finding has been attributed to the electron-rich groups present in glutathione (e.g., carboxylic and amino groups), which can interact with the gold cluster surface.25 Among gold clusters stabilized by various types of thiolates, glutathione-capped gold clusters are perhaps the most extensively studied system. Although largely applied to gold, chemical routes for stabilization of nanoclusters using suitable ligands were recently extended to silver. In fact, Bigioni and colleagues reported the synthesis of glutathione-stabilized magic-number silver cluster compounds.26 They suggested that the Ag:SG clusters may have fundamentally different structural and optical properties, possibly due to differences in Au and Agsulfur chemistry. In particular, the absorption spectra of the Ag:SG clusters differ substantially from those of Au:SG clusters. To fully understand the optical absorption of these systems, the optical properties of both the “metal core” and metal-liganded shells must be considered. Recently, TDDFT optical absorption spectra of the Ag2(SH)3 motifs were calculated, demonstrating that optical absorption of these motifs lies entirely in the ultraviolet region above 3 eV.24 To understand the differences observed between Au:SG and Ag:SG clusters, we address and rationalize here the structural and optical properties of Au(I) and Ag(I)thiolate complexes. For this purpose, isolated noble metalglutathione complexes were produced by electrospray mass spectrometry. The optical properties of [Au+GSH2H], [Ag+GSH2H], and [3Ag+ 2GSH4H] have been investigated experimentally by action spectroscopy. The comparison of experimental findings with DFT and TDDFT calculations allowed us to reveal the structural and electronic properties of these complexes.
II. METHODS a. Experimental. Glutathione GSH (γ-L-glutamyl-L-cysteinylglycine, C10H17N3O6S), HAuCl4, and AgNO3 were purchased from Sigma Aldrich. The silver complexes were prepared by mixing a solution of 50 μM AgNO3 with 50 μM glutathione in distilled water. For gold complexes, tetrachloroaurate salt with Au(III) ions was previously reduced to Au(I) using ascorbic acid. Au(I) solution (50 μM) was mixed with 50 μM glutathione in distilled water. Ammoniac (2%) was added to both solutions. An alternative electrolyte solution was prepared by mixing a solution of 3 mM
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silver nitrate salt in CH3OH and a solution of 1.1 mM GSH in water. The solution, which was initially clear, becomes cloudy and slightly yellow after a few hours. The solution was then diluted to 100 μM. Solutions were electrosprayed in the negative mode. Photodissociation measurements were performed using a modified linear quadrupole ion trap coupled to a tunable optical parametric oscillator (OPO) laser.27,28 The laser light was injected on the axis of the trap, and the trapped ions were irradiated for 500 ms (five laser shots). Mass spectra obtained after laser irradiation were recorded. After photoexcitation, as discussed below, part of the ion signal is depleted due to electron detachment from the parent ion, which results in a neutral ion that is not detected. Additional ionic fragments are observed in the mass spectrum. We recorded alternatively mass spectra with and without laser irradiation; 50 spectra with and 50 spectra without laser were recorded and summed at each wavelength. The yield of fragmentation (σ) was measured as a function of the laser wavelength (σ = ln(I0/I)/j), where j is the laser fluence and I0 and I are the intensities of the parent signal without and with laser irradiation, respectively. b. Calculations. Density functional theory (DFT) has been used to determine the structural properties of the glutathione silver and glutathionegold complexes employing the hybrid B3LYP29,30 functional and the relativistic effective core potential (RECP) of the Stuttgart group for silver and gold atoms.31 The TZVP atomic orbital basis set was used for silver and gold,32 whereas for all other atoms, the 6-311++G(d,p) basis set was employed.33 An extensive search for structures was performed using simulated annealing coupled to molecular dynamics (MD) simulations in the frame of the semiempirical AM134 method, starting from different deprotonation/complexation patterns. Parameters for Ag and Au atoms in the AM1 procedure were taken from refs 35 and 36. The found structures were subsequently reoptimized at the DFT level using gradient minimization techniques, and stationary points were characterized by calculating the vibrational frequencies. Partial charges have been determined by natural bond orbital analysis (NBO). The absorption spectra were computed using the long-range corrected version of B3LYP using the Coulomb-attenuating method (CAM-B3LYP),37 which provides a correct description of charge-transfer transitions.
III. RESULTS AND DISCUSSION a. Mass Spectrometry of Noble MetalGlutathione Complexes. Glutathionesilver and glutathionegold complexes
with different stochiometries have been produced and detected using ESI-MS in a negative mode for which the mass spectra are displayed in Figure 1. In the case of silver, a large number of species were observed, with [GSH+Ag2H] (m/z 412) being the most abundant complex. Other complexes of the general form [nGSH+(n+1)Ag(n+2)H] (2 < n < 4) are also present in the m/z range available in our instrument. All complexes exhibit several hydrogen abstractions, with the number of hydrogen atoms lost directly connected to the number of silver atoms present in the complex. The hydrogen abstraction is related to the nature of the bonding between the metal and sulfur or oxygen atoms of glutathione (as discussed below). For solutions that became cloudy and slightly yellow, indicating the formation of larger metal nanoclusters, dianion complexes of the general form [nGSH+(2n+m)Ag(2n+m+1)H]2 (with n = 6 and 0 < m 220 nm), for peptide anions.47 The electron photodetachment is thought to proceed via a two-step mechanism: (1) electronic excitation, followed by (2) a crossing with an autoionizing state, leading to a detachment of the electron. For peptides containing a tryptophan chromophore, usually deprotonation occurs on the carboxylic groups for which the binding energy is rather low (∼3.5 eV) and electronic excitation in the UV range around 260 nm (4.76 eV) is mainly due to ππ* excitations of the aromatic rings. In the case of metalglutathione complexes, both the nature of excitations are different and the VDE’s are higher (∼4.7 eV), as shown by theory, and thus the competition between electron detachment and UV photofragmentation is possible. The reason for higher VDE values in the metal complexes in comparison with free peptides is that the free
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electron pairs are additionally stabilized by the interaction with positively charged metal ions. For both silver and gold complexes, in the red part of the spectrum, fragmentation is more important than electron detachment, whereas the reverse is observed in the blue part of the spectrum (see branching ratios in Figure 2b,d). The electron detachment becomes the main channel for λ < 250 nm, in particular, for the silverglutathione complex. The fact that electron detachment becomes predominant for λ < 250 nm may be due to a higher probability to cross with an autoionizing state after an initial electronic excitation. Interestingly, the nature of fragment ions is different between the two complexes. For [Ag+GSH2H], the main fragments correspond to the loss of H2O (18 Da), CO2 (44 Da), Ag (107 Da), and a 129 Da fragment. The loss of water is a typical negative ion fragmentation channel within side chains of glutamic acid residues from the (MH) ions in small peptides.42 The loss of 129 Da corresponds to the cleavage of the GSH peptide bond between the Glu and Cys residue, as already observed for glutathione-stabilized gold clusters.41 The [Au +GSH2H] complex dissociates mainly by losses of H2O, of 44/46 Da (CO2/HCOOH) and also of the 129 Da fragment, but no metal atom loss has been detected. The same fragments (except electron loss) are also formed by collision activation (not shown), but the relative intensities of the fragments are different. The fact that Au loss is not observed for goldglutathione complexes (while Ag loss is observed for silverglutathione complexes) may be due to different binding properties in both complexes and, in particular, a stronger binding energy for AuS than for AgS, as also predicted for clusters.48 In fact, the calculated gas-phase formation energies according to eq 1 for [Au+GSH2H] is lower by 1.7 eV than in the case for [Ag+GSH2H] (+18.5 and +20.2 eV, respectively), indicating a much stronger goldsulfur bonding compared with the silver one. GSH þ Mþ f ½M þ GSH2H þ 2Hþ ðM ¼ Ag, AuÞ
ð1Þ
The combination of electron photodetachment and fragmentation leads to a total parent ion depletion yield as a function of the laser wavelength, which is different between [Au+GSH2H] and [Ag+GSH2H]. The optical absorption of these complexes lies below 300 nm. This is in agreement with previous theoretical works on Au2(SH)3 and Ag2(SH)3 motifs24 and cyclic thiolated gold clusters.49 While a defined band at 260 nm and the onset of a second band below 240 nm are observed for [Ag+GSH2H], only a monotonic increase in the total yield appears for [Au+GSH2H], without superimposed structures. The calculated absorption spectra for both complexes, shown in Figure 3, are in good agreement with the measured action spectra. The two lowest important excited states in both silver and gold species can be classified as transitions involving the nonbonding orbital on the S atom or the σ-bonding orbital between the S atom and the metal. For example, the transition involving the nonbonding orbital on sulfur in the optical absorption of [Ag+GSH2H] is located at 295 nm and has a low intensity. The first intense peak at 259 nm originates from transitions involving excitations from the metalsulfur σbonding orbital dominantly to metal localized orbitals with some contribution of the long-range charge transfer to CH and NH antibonding orbitals of the peptide chain (cf. Figure 3a). Therefore, this transition may be regarded as mainly localized within the OAgS unit. For [Au+GSH2H], the oscillator 24552
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The Journal of Physical Chemistry C
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Figure 5. Lowest-energy structure of [3Ag+2GSH4H]. The silver silver, silveroxygen, and silversulfur distances and NBO partial charges are also given.
Figure 4. (a) Calculated absorption spectrum of [3Ag+2GSH4H]. The lowest-energy structure is shown. The red curve shows the experimental total depletion yield. The theoretical spectrum (black line) has been broadened by a Lorentzian function with a width of 0.20 eV. (b) Leading excitations and involved orbitals in the dominant transition at 235 nm.
strength is widely spread between 250 and 300 nm with a higher number of low-intensity transitions than for [Ag+GSH2H]. The first peak with significant oscillator strength is observed at 282 nm and originates from the transition between the nonbonding orbital on sulfur and diffuse orbitals spread over the peptide chain. The transition, which is characterized by excitation from the σ-bonding orbital between the S atom and the metal is observed at 229 nm. In contrast to silver, the transition is blue shifted by 30 nm (0.63 eV) due to the lower energy of the σ-bonding orbital (Ag: E(HOMO1) = 4.67 eV; Au: E(HOMO1) = 5.13 eV). Moreover, for gold, the virtual orbitals involved in the excitations have considerably less metallocalized character, resulting in a net charge transfer to the peptide, rather than excitation within the OmetalS subunit (cf. Figure 3b). The photoinduced cleavage of the OmetalS unit leading to the loss of a neutral metal atom is enhanced by the localization of excitations within this subunit, which is much more pronounced in the dominant transitions of the silver than for gold species. In summary, the calculations show that the differences in interaction between silver and glutathione versus gold and glutathione are reflected not only in the differences in the binding energy but also in the nature of excitations, thus indicating their different chemistry. c. Optical Properties of Larger Complexes: Case of [3Ag +2GSH4H]. Optical properties of the [3Ag+2GSH4H] complex were explored experimentally by action spectroscopy and compared to the calculated TDDFT absorption spectra for the lowest-energy structure shown in Figure 4. The optical absorption of the complex lies below 300 nm. Again, UV
irradiation induces both fragmentation of and electron photodetachment from the precursor ions, as already observed for [Ag+GSH2H] and [Au+GSH2H] species. In the red part of the spectrum, fragmentation is more pronounced than electron detachment, whereas the reverse is observed in the blue part of the spectrum (see branching ratios in Figure S6 in the Supporting Information). The structures of other higher-energy isomers are given in Figure S7 (Supporting Information). The calculated absorption spectrum for [3Ag+2GSH4H], shown in Figure 4a, is in good agreement with the measured total depletion yield spectrum; in particular, the broad band at 240 nm is well-reproduced. The lowest-energy structure of the [3Ag+2GSH4H] complex contains a silver trimer stabilized by the two sulfur atoms of glutathione, forming a W-shaped structure. Additional bonds between two silver atoms and the two carboxylic groups of the glutamic acid residue are also present (cf. Figures 4a and 5). This “W” shape is expected to be specific to glutathione as compared to other thiolate ligands that do not have a carboxylate group. The analysis of the electronic transition reveals that the excitation occurs mainly within the SAgS motif, as displayed for the transition at 235 nm (see Figure 4b). “Cluster”-like excitations have not been found theoretically in this complex, in contrast to previously studied silver trimer cations bound to peptides, which exhibited strong intracluster transitions.50 The analysis of charge distribution (see Figure 5) reveals that the silver subunit is strongly positively charged and no delocalized s-electron density is present in the silver atoms. Such delocalized s electrons are responsible for intense absorption in bare Ag3+ and Ag3+ complexed with peptides.50 These results show that the presence of multiple closely connected metal atoms is not sufficient for strong optical absorption. Thus, if strong optical absorption is a desired property, electron-donating ligands might be used or external chemical reduction should be performed in order to localize electron density within the cluster.
IV. CONCLUSION We have explored the formation and optical properties of isolated noble metalglutathione complexes by a combination of action spectroscopy and time-dependent density functional calculations. The optical absorption of [Au+GSH2H], [Ag+GSH2H], and [3Ag+2GSH4H] complexes is characterized by a wavelength-dependent competition between electron photodetachment and photoinduced fragmentation. Interestingly, although structural properties of silver and gold glutathione complexes are similar, fragmentation channels and optical absorption patterns are different owing to differences 24553
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The Journal of Physical Chemistry C in Au and Agsulfur chemistry, which are mainly expressed by different bonding strengths and electron localization in ground and excited states. In the [3Ag+2GSH4H] complex, a strongly positive silver trimer unit has been identified, which is stabilized by the two sulfur atoms of glutathione, forming a W-shaped motif. The TDDFT study of the optical absorption of this species has revealed that the electronic excitations responsible for intense transitions occur mainly within the SAgS unit and not within the cluster, which have been found to be responsible for strong absorption and emission in silver clusterbiomolecule hybrids.50 Our joint experimental and theoretical study demonstrates that the optical properties of ligand-protected noble metal clusters can be disentangled by a bottom-up approach.
’ ASSOCIATED CONTENT
bS
Supporting Information. Glutathionesilver complexes observed by ESI-MS in negative mode with colored nonfresh solutions; lowest-energy structures for [Ag+GSH2H] and [Au +GSH2H], obtained from DFT calculations; higher-energy isomers for [Ag+GSH2H] and [Au+GSH2H]; UV photofragmentation spectra of the [GSH+Ag2H] and [GSH +Au2H]; total depletion yield as a function of the wavelength for [3Ag+2GSH4H] complex ions; and higher-energy isomers for [3Ag+2GSH4H]. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to the ANR-08-BLAN-0110-01. We thank the Hubert Curien Program for bilateral funding (Procope no. 50068937). R.M. and A.K. acknowledge the Deutsche Forschungsgemeinschaft, Emmy Noether program (MI-1236) for financial support and the NCBA International French Croatia laboratory. ’ REFERENCES (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (3) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (4) Schaaff, T. G.; Shafifullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutierrez-Wing, C.; Ascensio, J.; Jose-Yacaman, M. J. J. Phys. Chem. B 1997, 101, 7885. (5) Bigioni, T. P.; Whetten, R. L.; Dag, O. J. Phys. Chem. B 2000, 104, 6983. (6) Parker, J. F.; Fields-Zinna, C. A.; Murray, R. W. Acc. Chem. Res. 2010, 43, 1289. (7) Shibu, E. S.; Muhammed, M. A. H.; Tsukuda, T.; Pradeep, T. J. Phys. Chem. C 2008, 112, 12168. (8) Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, 8608. (9) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 1945. (10) Gies, A. P.; Hercules, D. M.; Gerdon, A. E.; Cliffel, D. E. J. Am. Chem. Soc. 2007, 129, 1095. (11) Hussain, I.; Graham, S.; Wang, Z.; Tan, B.; Sherrington, D. C.; Rannard, S. P.; Cooper, A. I.; Brust, M. J. Am. Chem. Soc. 2005, 127, 16398.
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