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Cite This: J. Am. Chem. Soc. 2017, 139, 14853-14856

Resonance Enhancement of Nonlinear Optical Scattering in Monolayer-Protected Gold Clusters Stefan Knoppe*,§ and Thierry Verbiest* Department of Chemistry, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

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S Supporting Information *

aperture z-scan measurements.30 Powder X-ray diffraction and electronic structure calculations indicate that clusters larger than Au144(SR)60 adopt a fcc packing structure, and their absorption spectra share great similarities.31 This is related to a transition from molecular to bulk properties (>Au144(SR)60), i.e., collective behavior of the electrons resulting in plasmonic properties. Here, we present wavelength-dependent hyper-Rayleigh scattering (HRS) measurements of gold:thiolate clusters that fall into the “transition” region. HRS allows the determination of the first hyperpolarizability βHRS, a molecular property that serves as a measure for the efficiency of the frequency-doubling process and provides information on electronic structure and molecular symmetry in solution.32,33 The coherent analogue to HRS is second-harmonic generation (SHG). Both HRS and SHG were observed in small monolayer-protected gold and silver clusters.34−40 Large two-photon absorption cross sections were observed for MPCs.30,41−46 These nonlinear optical properties render MPCs interesting candidates as contrast agents in multiphoton microscopy applications, where especially the SHG channel can provide information on local symmetry and changes thereof. Successful application of MPCs in multiphoton microscopy requires large hyperpolarizabilities and stability in solution and under irradiation with lasers at high intensity. The clusters characterized by HRS and SHG so far are “ultrasmall” and contain less than 40 metal atoms, with βHRS on the order of 10−28 esu. Typical labels for SHG microscopy applications have hyperpolarizabilities 1 order of magnitude larger.47 The second-order NLO properties of MPCs therefore need to be optimized, and an obvious strategy is studying other cluster sizes. It is not entirely clear whether the first hyperpolarizability scales with the size of the clusters or is dominated by their atomic structure that leads to different point groups (note that the first hyperpolarizability is symmetrydependent).48 Several stable clusters with more than 100 Au atoms have been reported.6,9,10,18,26−28,49−53 We determined the first hyperpolarizability of Au130(pMBA)50 (p-MBA = para-mercaptobenzoic acid), Au144(2PET)60 (2-PET = 2-phenylethanethiolate) and Au500(2PET)120 (for details see Figures S-8−S-11). The Au130(pMBA)50 cluster shows a large βHRS (2996 × 10−30 and 1394 × 10−30 esu at 1300 and 1072 nm, respectively) and Au144(2PET)60 and Au500(2-PET)120 have values of βHRS that are about 1 order of magnitude lower than for Au130(p-MBA)50. We relate this to three factors: (1) The p-MBA ligand is aromatic, leading

ABSTRACT: Monolayer-protected metal clusters (MPCs) have recently gained significant research interest, since they are promising candidates for various applications in bioimaging and catalysis. Besides this, MPCs promise to aid in understanding the evolution of the metallic state from bottom-up principles. MPCs can be prepared with atomic precision, and their nonscalable properties (indicating molecule-like behavior) have been studied with a variety of techniques both theoretically and experimentally. Here, we present spectrally resolved second-order nonlinear optical scattering experiments on thiolate-protected gold clusters (Au130(SR)50, Au144(SR)60, and Au500(SR)120). The three clusters share common resonance enhancement around 490 nm, which is ascribed to an interband transition. This indicates emerging metallike properties, and we tentatively assign the onset of metal-like behavior somewhere between 102 and 130 gold atoms.


onolayer-protected metal clusters (MPCs) have recently gained attention as ultrasmall nanoparticles with nonscalable molecular properties.1−4 MPCs form core−shell structures in which a metal core is protected by a monolayer of organic ligands.5−11 These can be described as “superatom complexes” with electronic and/or geometric shell closing,12−14 which leads to enhanced stability of the clusters against decomposition.15 MPCs are typically regarded as having molecule-like properties with discrete energy levels, and hence featured absorption spectra and distinct HOMO− LUMO gaps.16−19 It is not entirely understood at which size the molecule-like properties of MPCs transition toward collective properties, such as the evolution of band structures, metal-like relaxation behavior, or support of a localized surface plasmon resonance (LSPR). In larger gold nanoparticles, the latter can be calculated by virtue of Mie theory.20−23 From a “top-down” point of view, the free electron model predicts the onset of discrete behavior in gold nanoparticles with diameters of ca. 2 nm.24 In a “bottom-up” approach, the onset of plasmonic behavior is not entirely understood, as it requires a transition from discrete to collective behavior. The Au144(SR)60 cluster shows metallic relaxation behavior in pump−probe spectroscopy experiments, suggesting a “core localized plasmon”.19,25 Optical absorption spectra of Au329(SR)84, Au500(SR)120 and Au940(SR)160 clusters indicate plasmonic behavior.26−29 Plasmonic behavior in Au144(SR)60 is further evidenced by open© 2017 American Chemical Society

Received: August 9, 2017 Published: October 11, 2017 14853

DOI: 10.1021/jacs.7b08338 J. Am. Chem. Soc. 2017, 139, 14853−14856


Journal of the American Chemical Society

Figure 1. Left: Dependence of the nonlinear optical spectra of Au130(p-MBA)50 in water on the wavelength of the laser (black, shortest wavelength to purple, longest wavelength). The narrow peaks are the HRS signals, and the broad background is the luminescence emission. The shape of the luminescence emission is independent of the excitation wavelength and peaks around 540 nm, indicating that three-photon-excited luminescence occurs at wavelengths close to or longer than 1070 nm. Spectra were brought to scale at 540 nm for comparison. Middle: Dependence of the HRS signal on the square of the normalized laser intensity confirms a second-order NLO process. Right: Dependence of the intensity of the luminescence background on the cube of the laser intensity. Linear correlation indicates cubic dependence on the intensity, in agreement with three-photon absorption. Corresponding plots for 1300 nm excitation are in Figure S-12.

Figure 2. Top, left to right: Visible spectra of Au102(p-MBA)44 (H2O), Au130(p-MBA)50 (H2O), Au144(SCH2CH2Ph)60 (THF), and Au500(2-PET)120 (THF). Bottom: Dependence of the HRS signal on the laser wavelength (black dots), from left to right: Au102(p-MBA)44 (H2O), Au130(p-MBA)50 (H2O), Au144(2-PET)60 (THF), and Au500(2-PET)120 (THF). The (rescaled) visible spectra are shown in red. Au130(p-MBA)50, Au144(2-PET)60, and Au500(2-PET)120 show common resonance enhancement around 490 nm; this feature was fitted (green traces). The blue traces are cumulative fits using multiple Gaussians. Such resonance enhancement is absent in Au102(p-MBA)44.

with the cube of the intensity), since other third-order processes such as four-wave mixing lead to much narrower line widths. Furthermore, the shape of the emission feature is independent of the excitation wavelength (Figure 1). Fifthorder optical nonlinearity has not previously been reported for thiolate-protected gold clusters. We suggest further studies such as measurements on the time-dependence of the emission, since third-order nonlinear scattering or four-wave mixing would lead to instantaneous responses, whereas luminescence has delayed emission due to the lifetime of the excited states. The extinction spectra of Au102(p-MBA)44, Au130(p-MBA)50, Au144(2-PET)60, and Au500(2-PET)120 are shown in Figure 2, top panel. The latter yields red dispersions typical of plasmonic gold nanoparticles that were attributed to a plasmon resonance.27 Au130(p-MBA)50 yields brown solutions with a red hue, and Au144(2-PET)60 yields brown solutions. The extinction spectrum of Au130(p-MBA)50 shows a weak peak around 450 nm, a shoulder around 500 nm, and a weak shoulder around 550 nm. In contrast, the spectrum of Au144(2-

to expansion of the delocalized system. This typically improves βHRS since more electrons are involved. (2) Au130(p-MBA)50 is soluble in water, whereas the other two clusters are soluble in apolar, organic solvents. We assume that the different dielectric constants of the environment play a role here, but further work on the solvent dependence of HRS properties of MPCs are required. Furthermore, the carboxyl group in the p-MBA ligand can strongly interact with the solvent. (3) Au130(p-MBA)50 is chiral and slightly prolate,49,53 while Au144(2-PET)60 and Au500(2-PET)120 have a spherical shape (based on electron microscopy) and their chirality is not confirmed.27,54−56 All these aspects combined might explain why the first hyperpolarizability is much larger for the water-soluble Au130(pMBA)50 cluster. Au130(p-MBA)50 also shows a broad background feature that peaks at 540 nm. The intensity of the broad feature scales with the cube of the laser intensity (Figure 1 and Figure S-12). We tentatively assign this feature to three-photon excited luminescence, which is a fifth-order NLO process (but scales 14854

DOI: 10.1021/jacs.7b08338 J. Am. Chem. Soc. 2017, 139, 14853−14856


Journal of the American Chemical Society

enhancement of third-harmonic generation signals around 480−490 nm was related to interband transitions.64 We assume that this feature is not observed in Au102(p-MBA)44, since the cluster is too small to exhibit metal-like properties.18 In summary, we demonstrate that S-HRS is an excellent technique to detect resonance enhancements in MPCs. We ascribe the observed resonance enhancement to emerging metallic behavior, and tentatively narrow the onset of metallic behavior in thiolate-protected gold clusters between 102 and 130 gold atoms. Note, however, that we suggest additional experimental efforts that will clarify the nature of the observed resonances. The Au130(p-MBA)50 cluster combines a number of properties that render it an excellent candidate as multiphoton contrast agent in biological systems. The cluster is water-soluble and very stable and displays a large first hyperpolarizability in the NIR window. Three-photon-excited luminescence should provide excellent spatial resolution, even exceeding that of twophoton fluorescence dyes. Furthermore, the small core diameter of 1.5 nm (as obtained from TEM measurements, Figure S-1) should lead to little stress within tissue.

PET)60 shows several features in the visible range, notably a shoulder at ca. 500 nm. While the three compounds display distinctly different absorption spectra, they all have a feature around 500 nm in common. LSPRs are sensitive to changes in the refractive index of the surrounding, and slight changes were observed for Au130(p-MBA)50 and Au500(2-PET)120 as the refractive index of the solvent was increased, but the spectrum of Au144(2-PET)60 is virtually unaffected (Figures S-6 and S-7). We conclude that linear absorption spectroscopy is not sensitive enough to detect emerging plasmonic behavior of clusters in the transition region. In contrast, wavelength-dependence of the HRS intensity might provide such insight (“spectral HRS”, S-HRS). It is wellknown that larger gold nanoparticles show resonance enhancement of HRS and SHG at the LSPR wavelength.57−59 The enhancement is due to local field effects, and the intensity at the second-harmonic frequency 2ω is proportional to |L(2ω)|2*| L2(ω)|2, where L(ω) is a local field enhancement factor and the local field becomes Eloc(ω) = L(ω)E(ω).60 In other words, the local field generated by a localized plasmon is expected to amplify the nonlinear scattering response. We observe similar wavelength dependence when comparing Au130(p-MBA)50, Au144(2-PET)60, and Au500(2-PET)120 in S-HRS. All three clusters show resonance enhancement at ca. 485−490 nm. This feature is absent in Au102(p-MBA)44. In all four clusters an increase toward longer wavelengths is observed. Such behavior was observed earlier for plasmonic gold nanoparticles and was related to the formation of dimers in solution,61 but we refrain from concluding that this behavior in our clusters is indicative of plasmon enhancement. The wavelength region where the resonance enhancement is observed in S-HRS (ca. 490 nm) is too short to be attributed to plasmonic behavior. As the size of gold nanoparticles decreases, the LSPR is blue-shifted. However, when the free carrier density is decreased, red-shifts are expected.62 Thiolateprotected gold clusters of the general formula [Aum(SR)n]z have decreased electron density, since the ligands are electronwithdrawing. If each thiolate localizes one electron from the gold atoms, the electron density ρe = (m − n − z/m) is calculated as 0.62, 0.58, and 0.76 for Au130(p-MBA)50, Au144(2PET)60, and Au500(2-PET)120, respectively. ρe approaches 1 in larger nanoparticles, as the surface-to-volume ratio decreases and the influence of the ligands becomes less pronounced. Thus, in the clusters discussed here, a localized plasmon resonance would be expected to be significantly red-shifted (with respect to the typical 520 nm LSPR in gold nanospheres). This anticipated red-shift is also in agreement with modified Mie theory,63 that accounts for the refractive index of the thiolates and their shielding from the surrounding. In a model calculation on Au103(p-MBA)50, we obtain a LSPR wavelength of about 540 nm. In light of these considerations, it is unlikely that the observed features around 500 nm in the extinction spectra and around 490 nm in S-HRS can be attributed to a localized plasmon resonance. This does not necessarily exclude the existence of a plasmon, but it also does not back up this idea. The interband transitions in gold take place at energies >2.4 eV (∼515 nm), and this value matches the position of the features observed in the S-HRS experiments. Au144(2-PET)60 shows metal-like relaxation dynamics after visible light excitation,19 and both Au500(2-PET)120 and Au130(p-MBA)50 have higher electron densities; thus, it seems reasonable that the three clusters share common resonance enhancement due to an interband transition. In larger gold nanospheres,


S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08338. Synthesis and characterization of the clusters, details on absorption spectroscopy, and hyper-Rayleigh scattering experiments, including Figures S-1−S-12 and Tables S1−S-3 (PDF)


Corresponding Authors

*[email protected] *[email protected] ORCID

Stefan Knoppe: 0000-0002-3687-4485 Present Address §

S.K.: Institute for Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge Tanja Lahtinen and Hannu Häkkinen (University of Jyväskylä, Finland) for providing the Au130(p-MBA)50 clusters and Kuo-juei Hu (KU Leuven) for recording the TEM images. This work was supported by the KU Leuven and the “Funds for Scientific Research−Flanders” (FWO). S.K. is grateful to the FWO for a postdoctoral fellowship.


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DOI: 10.1021/jacs.7b08338 J. Am. Chem. Soc. 2017, 139, 14853−14856


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DOI: 10.1021/jacs.7b08338 J. Am. Chem. Soc. 2017, 139, 14853−14856