CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402287
Blue Luminescence of Dendrimer-Encapsulated Gold Nanoclusters Jun Myung Kim, So Hyeong Sohn, Noh Soo Han, Seung Min Park, Joohoon Kim,* and Jae Kyu Song*[a] Direct evidence for the blue luminescence of gold nanoclusters encapsulated inside hydroxyl-terminated polyamidoamine (PAMAM) dendrimers was provided by spectroscopic studies as well as by theoretical calculations. Steady-state and time-resolved spectroscopic studies showed that the luminescence of
the gold nanoclusters consisted largely of two electronic transitions. Theoretical calculations indicate that the two transitions are attributed to the different sizes of the gold nanoclusters (Au8 and Au13). The luminescence of the gold nanoclusters was clearly distinguished from that of the dendrimers.
1. Introduction Confinement of gold clusters to sizes comparable to the Fermi wavelength of electrons (ca. 0.7 nm) results in molecule-like properties of the gold clusters, including size-dependent luminescence.[1] Well-defined monodisperse templates such as dendrimers have been used to control the size of nanoclusters, which have ranged in diameter from less than 1 nm up to 5 nm, and also the size-dependent luminescence of nanoclusters.[2–4] Particularly, blue luminescent gold nanoclusters (Au8) have been synthesized by the dendrimer-based approach, which has subsequently been extended to the preparation of size-controlled Au nanoclusters with luminescence spanning from UV to near-IR.[5, 6] However, these interesting studies triggered intense discussion on the origin of such luminescence.[7–9] For example, Bard and co-workers demonstrated that the blue emission around 450 nm, which was the characteristic luminescence of Au8, originated from the oxidation of hydroxyl-terminated polyamidoamine (PAMAM) dendrimers.[7] Imae and co-workers also reported the pH-dependent blue fluorescence of amine-terminated PAMAM dendrimers.[8] Other dendrimers such as poly(propyleneimine) and poly(ethyleneimine) dendrimers, have been reported to show blue fluorescence after air treatment.[9] These results raised questions regarding the size-dependent luminescence of Au nanoclusters, suggesting that the emission originated mainly from the dendrimers.
[a] J. M. Kim,+ S. H. Sohn,+ N. S. Han, Prof. S. M. Park, Prof. J. Kim, Prof. J. K. Song Department of Chemistry Research Institute for Basic Sciences Kyung Hee University Seoul 130-701 (Korea) E-mail: [email protected] [email protected] [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402287.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
In this work, we have clarified the controversies regarding the luminescence of Au nanoclusters encapsulated in dendrimers. The Au nanoclusters were prepared by adjusting the ratio of Au to the dendrimer template to control the number of Au atoms encapsulated inside dendrimers. This approach differs from that of the previous method of preparation, which resulted in simultaneous synthesis of luminescent small nanoclusters and nonluminescent large nanoparticles.[5, 6, 10] This control of stoichiometry removed the need to separate the nonluminescent large Au aggregates,[11–15] which contrasts with previous work.[5, 6, 10] Detailed steady-state and time-resolved spectroscopic studies[16] provided direct evidence for the blue luminescence of the Au nanoclusters, which was clearly distinguished from the luminescence of the dendrimers. Theoretical calculations also suggested that the luminescence of the Au nanoclusters were attributed to Au8-dominating nanoclusters encapsulated inside dendrimers.
2. Results and Discussion The photoluminescence of the Au nanoclusters, which were encapsulated inside hydroxyl-terminated fourth generation PAMAM dendrimer (G4-OH), was examined by steady-state photoluminescence spectroscopy. The blue emission (black line in Figure 1 a) was observed at 455 nm from Au nanoclusters encapsulated in G4-OH, which are denoted as G4-OH(Au). Indeed, the blue emission (red line in Figure 1 a) was also found from G4-OH (without encapsulated Au nanocluster), as previously reported by several research groups.[7–9] However, the photoluminescence of G4-OH(Au) was distinguishable from that of G4-OH, with the emission peak of G4-OH(Au) being redshifted by 15 nm compared with that of G4-OH (440 nm). In addition, the emission intensity of G4-OH(Au) was five times higher than that of G4-OH at the same concentration (10 mm). Therefore, the increased emission intensity and the redshifted peak position suggested the presence of luminescent Au ChemPhysChem 2014, 15, 2917 – 2921
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Figure 1. a) Emission spectra of G4-OH(Au) (black line) and G4-OH (red line) in aqueous solutions. The difference spectrum (blue line) was obtained by subtracting the emission of G4-OH from that of G4-OH(Au). All the emission spectra were obtained by using an excitation wavelength of 355 nm. The sharp peak at 405 nm was due to the Raman shift of water. b) Emission spectra of aqueous solutions of all the individual reagents that were used in the preparation of G4-OH(Au). The Raman peak of water was removed and this region of the spectrum was reconstructed from Gaussian fitting process (dotted line). c) Emission spectra of mixtures of G4-OH and the reagents in aqueous solutions.
nanoclusters, which were protected and stabilized by the dendrimers. It is possible that the blue emission of G4-OH(Au) could be related to chemical reagents including impurities used in the preparation of G4-OH(Au), therefore, emission spectra were obtained from all the individual reagents. However, except with G4-OH, blue emission was barely observed from the individual chemical reagents, which ruled out emission from reagents 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org and impurities (Figure 1 b). In addition, the emission spectra from mixtures of G4-OH and reagents were obtained in the absence of Au precursor (HAuCl4), because the chemical reaction could change the emission profile of G4-OH in terms of the emission intensity and peak position. However, the emission spectra of the mixtures were nearly identical to that of G4-OH (Figure 1 c), which indicated that the chemical reaction with the reagents neither increased the emission intensity nor changed the peak position of G4-OH. Bard and co-workers studied the blue fluorescence of hydroxyl-terminated PAMAM dendrimers and concluded that the blue emission was related to oxidation of the hydroxyl-terminal groups of the dendrimers.[7] Similarly, the group of Imae investigated the blue emission of dendrimers but attributed the emission to interaction of the tertiary amino moieties with oxygen inside the dendrimers.[8, 9] In this regard, the shifted peak at 455 nm could be correlated to the oxidization of G4-OH by the Au precursor (HAuCl4), which was also known for the oxidizing reagent. However, the emission spectrum from mixtures of G4-OH and HAuCl4 (orange line in Figure 1 c) was also similar to that of untreated G4-OH, which indicated that the addition of Au precursors did not change the emission profile of G4-OH, even if a chemical reaction of G4-OH might occur with HAuCl4. Thus, the observed emission of G4-OH implied that interaction of oxygen (or air) with tertiary amino-branching sites of G4-OH was involved in the blue fluorescence (440 nm), which was not altered by the chemical reaction under the current experimental conditions. It is notable, therefore, that the blue luminescence of G4-OH(Au) was clearly distinguished from the emission of G4-OH by the increased emission intensity and the redshifted peak position, which indicated the presence of luminescent Au nanoclusters encapsulated within the dendrimers. Since the emission of G4-OH was inevitably included in the emission spectrum of G4-OH(Au), the emission of G4-OH was subtracted from that of G4-OH(Au) to obtain the emission spectrum of the encapsulated Au nanoclusters (blue line in Figure 1 a). The shape of the subtracted spectrum was similar to that of G4-OH(Au). The emission maximum at 460 nm corresponded to the well-known emission of Au8-dominating nanoclusters.[5, 6] The energy states of Au nanoclusters have often been explained by the spherical jellium approximation using the free-electron filling model as used in the alkali metal nanoclusters.[6, 17] In this model, the emission energy of Au nanoclusters was estimated as a function of the number of atoms (N) by a relation of EFermi/N1/3, where EFermi is the Fermi energy of bulk Au (5.5 eV).[10] Therefore, the emission energy of N = 8 in the model (451 nm) suggested that Au nanoclusters prepared in our study were Au8, with a spherical shape. In contrast, the full width at half maximum (FWHM) of the emission was broad, in addition to the asymmetric shape, which was similar to the previous study that showed that the emission in the blue region was broader than in other regions.[6] The spectral broadening could be ascribed to spectral overlap of emission bands, suggesting that a few electronic states were involved in the blue emission. Indeed, the observed emission spectrum was better reproduced by two Gaussian functions (Figure 2 a), rather than by a single Gaussian ChemPhysChem 2014, 15, 2917 – 2921
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Figure 2. a) The difference emission spectrum was deconvoluted by two Gaussian functions (green lines), which indicated that two electronic transitions were involved in the emission of G4-OH(Au). The abscissa scale in the spectrum was converted into energy for better comparison. The most probable electronic transitions (bars) of the optimized geometries (A1, A2, and A3) are shown to compare the calculated energies with the emission spectrum. b) Among the calculated geometries of Au8, a fused tetrahedron (A1) was the most stable; a distorted bicapped octahedron (A2) and a capped tetrahedron (A3) were higher in energies than A1. The calculated stable geometries of Au8 were nearly spherical.
function (see the Supporting Information, Figure S1), which suggested that at least two transitions were involved in the emission of G4-OH(Au). To examine these emissive states, the electronic transitions of Au8 were calculated by using the timedependent density functional theory (TD-DFT) module in the GAUSSIAN package.[18] First, stable structures of possible twodimensional and three-dimensional geometries were obtained by using analytical gradients with full optimization with LANL2MB and LANL2DZ basis sets. The frequencies of the optimized geometries were calculated to ensure that the structures represented stable points on the potential energy surface. It is noted that the optimized geometries of Au8 were nearly spherical, which supported the conclusion that the spherical jellium model could be applied to calculate the emission energy of Au8. Among the optimized geometries of Au8, a fused tetrahedron (A1) was the most stable (Figure 2 b), whereas a distorted bicapped octahedron (A2) and a capped tetrahedron (A3) were found to have higher energies with the employed basis sets. For example the energies of A2 and A3 were calculated to be 0.15 and 0.36 eV, respectively, above the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org energy of A1 at the B3LYP/LANL2MB level. The energies of A2 and A3 also lay 0.19 and 0.42 eV, respectively, above the energy of A1 at the B3LYP/LANL2DZ level. To understand the origin of the two bands of the emission spectrum in more detail, the electronic transitions were investigated for these stable structures. The most probable electronic transitions from the ground to excited states of A1, A2, and A3 were calculated to be 2.94, 3.26, and 3.01 eV, respectively, at the B3LYP/LANL2DZ level. To compare the calculated energy with the observed spectrum (Figure 2 a), the solvation effect of each isomer was calculated in aqueous solution, which was in the range of 0.04–0.19 eV. To a first approximation, it was supposed that the solvation effect in the aqueous solution would be similar to that within the dendrimer in aqueous solution, because the interaction between gold nanoclusters and dendrimer is known to be weak in dendrimer-encapsulated gold clusters.[2] In addition, the Stokes shift was taken into account, which depended on the isomer geometries and was in the range of 0.10–0.16 eV.[19] Overall, the calculated absorption energies were shifted to take into account the solvation effect and the Stokes shift together. Interestingly, the emission energies of A1 and A3 were consistent with one of the deconvoluted bands (2.69 eV), whereas the emission energy of A2 did not match well. Presumably, the population of A3 was not significant because of the higher energy of A3 than A1 (by 0.36 and 0.42 eV calculated at the B3LYP/LANL2MB and B3LYP/LANL2DZ levels, respectively), which implied that A3 did not contribute much to the emission spectrum. Therefore, the results of the calculation suggest that one of the electronic transitions (2.69 eV) was from the most stable isomer of Au8 (A1). The remaining band centered at 2.33 eV might be correlated to another excited state of A1 because a few orbitals, other than the frontier orbitals HOMO and LUMO, were found as excited states.[20, 21] However, the oscillator strengths of other electronic transitions were calculated to be relatively weak, which ruled out the possibility of another electronic transition of A1 for the remaining emission band. Possibly, this lowenergy band could be attributed to vibrational progression of the most probable electronic transition. If this was the case, the lifetimes of the two emissive bands would be identical. Alternatively, the low-energy band might arise from different sized Au nanoclusters, which would result in different lifetimes of the two bands. Thus, because the origin of the emissive states could be distinguished by the lifetimes, the time profiles of the blue emission were investigated. The time profiles of G4-OH(Au) emission at different wavelengths were similar (Figure 3 a), except for the profile of at 420 nm. The exceptional profile at this wavelength could be explained by the inevitable inclusion of G4-OH in the emission of G4-OH(Au). In other words, the time profiles of G4-OH (Figure 3 b) were similar to that of 420 nm in G4-OH(Au), which indicated that the latter was perturbed by the former to some extent. The decay profiles of G4-OH were most reasonably fitted by a double-exponential model, IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ, where A1 and A2 are relative magnitudes and t1 and t2 are lifetimes. The lifetimes of G4-OH (0.7 and 4.5 ns) were a little shorter than
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www.chemphyschem.org that of 5.5 ns (A2 = 0.39), when the emission intensity was integrated over 1 ms (time window of the steady-state emission spectrum). In this regard, the emission band at 2.33 eV was concluded to originate from Au13 with a lifetime of 1.5 ns, whereas the emission of Au8 was at 2.69 eV with a lifetime of 5.5 ns, although these are not well-separated due to the broad spectral nature of emission (Figure 2 a). In addition, the longer lifetime (5.5 ns) emission was matched with the reported value for Au8 in G4-OH (6.7 and 7.5 ns),[6, 22] which was assigned as the transition between the sp and d bands.[5] The short lifetime (1.5 ns) of Au13 was also noted (3.4 and 5.2 ns),[6, 23] which followed the generally observed trend that the lifetime of nanocluster emission decreased with increased cluster sizes. Nevertheless, the contribution of Au8 and Au13 to the time profiles of the observed emission should be different at different wavelengths. Indeed, the emission time profile at longer wavelength indicated a slightly shorter decay than that at shorter wavelength (see the Supporting Information, Figure S2) because the emission center of Au13 with a shorter lifetime was in the longer wavelength regime. In this regard, the time-resolved emission profiles supported the conclusion that the coexistence of Au8 and Au13 was responsible for the broad emission observed in the blue regime. Significantly, the lifetimes of G4-OH(Au) were clearly different from those of G4-OH, which indicates strongly that the blue luminescence originates mainly from the Au nanoclusters encapsulated within G4-OH.
3. Conclusions Figure 3. a) Time profiles of G4-OH(Au) were obtained by using the time-correlated single-photon counting technique. The decay profiles were fitted by the double-exponential model with two lifetimes (1.5 and 5.5 ns). The fitting curves were superimposed for better comparison. b) Selected time profiles of G4-OH. The decay profiles were fitted by the double-exponential model with two lifetimes (0.7 and 4.5 ns). Intensities were normalized for better comparison.
those of G4-NH2 (1.7 and 7.5 ns),[8] which might be attributed to the difference in the terminal groups (OH and NH2). Similarly, the decay profiles of G4-OH(Au) emission were best fitted by the double-exponential model with lifetimes of 1.5 and 5.5 ns and with relative magnitudes of 0.61 and 0.39, respectively. Accordingly, the possibility of vibrational progress for the low-energy band could be ruled out because the lifetimes of the two components were not identical. Therefore, the time profiles suggested that the low-energy band originated from different sized Au nanoclusters. Indeed, the lowenergy band (2.33 eV) was consistent with the emission energy of Au13 (2.34 eV) in the spherical jellium model, which indicated that another nanocluster prepared in our study was Au13 with a spherical shape. Indeed, Au13 was also a stable sized nanocluster for G4-OH(Au),[6] the spherical shape of which could be assumed to be based on an icosahedron. The short lifetime (1.5 ns) could be correlated to Au13, whereas the longer lifetime (5.5 ns) was linked to Au8, because the emission lifetime of nanoclusters is known to decrease with increased cluster size.[6, 21] Indeed, the overall intensity of the emission with time constant of 1.5 ns (A1 = 0.61) was half 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
We have investigated the blue luminescence of Au nanoclusters encapsulated inside dendrimers. By controlling the ratio of Au to G4-OH, systems were generated that contained only luminescent small nanoclusters. Detailed studies of the steadystate and time-resolved spectroscopy allowed the blue luminescence to be attributed to Au8-dominating nanoclusters, with the lower energy band being associated with Au13. The luminescence of the Au nanoclusters encapsulated inside G4-OH was also clearly distinguished from the emission of G4-OH. These results resolve the controversies regarding the luminescence of dendrimer-encapsulated Au nanoclusters. Such clusters are expected to be useful in a range of analytical applications including cellular imaging, immunoassays, and chemical sensing.
Experimental Section Chemicals and Materials Hydroxyl-terminated fourth generation PAMAM dendrimers (G4OH), HAuCl4·3 H2O, NaBH4, and cellulose dialysis sacks (MW cutoff of 12 000) were purchased from Sigma–Aldrich (USA). Sodium hydroxide was obtained from Daejung, Inc. (Korea). Deionized water (18 MW·cm, Ultra370, Younglin Co., Korea) was used as solvent.
Preparation of Dendrimer-Encapsulated Au Nanoclusters Au nanoclusters were prepared by reported methods with a few modifications.[11, 13, 14] Briefly, HAuCl4 (8 equiv.) was added to an ChemPhysChem 2014, 15, 2917 – 2921
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CHEMPHYSCHEM ARTICLES aqueous hydroxyl-terminated fourth generation dendrimer solution (10 mm) to complex Au ions with the interior amines of the dendrimers. Then, a 5-fold molar excess of NaBH4 in NaOH (0.3 m) was added slowly with vigorous stirring to reduce the Au complexes to zerovalent Au nanoclusters inside the dendrimers. The Au nanocluster solution was dialyzed by using a cellulose sack for 24 h to remove impurities. The resulting Au nanocluster solution produced no significant amount of large Au aggregates even after vigorous centrifugation (13 000 rpm for 20 min) as Crooks and others reported previously.[2, 3, 11–13] The amine-terminated dendrimers were also employed to prepare the corresponding Au nanoclusters, the properties of which were compared to G4-OH(Au).
Characterization of Dendrimer-Encapsulated Au Nanoclusters To examine oxidation states of Au in the nanoclusters, X-ray photoelectron spectra were obtained. The binding energies of Au(4f) peaks (4f5/2 and 4f7/2) matched with zerovalent Au (see the Supporting Information, Figure S3), which suggested the presence of zerovalent Au species in the nanoclusters. For steady-state and time-resolved photoluminescence spectra, the Au nanocluster solution was excited by the second harmonic (355 nm) of a cavitydumped oscillator (Mira/PulseSwitch, Coherent, 1 MHz, 710 nm, 150 fs). Emission was collected by using a set of lenses, spectrally resolved using a monochromator, detected using a photomultiplier, and recorded with a time-correlated single-photon counter (TCSPC, PicoHarp, PicoQuant). The instrumental response of the entire system was 0.05 ns, which resulted in a resolution of 0.01 ns with the use of deconvolution.[15] Transmission electron microscopy (TEM) images of the Au nanoclusters were obtained at 200 kV (Tecnai G2 F30, FEI Co.). TEM measurements also showed no significant numbers of nanoparticles larger than 1 nm (see the Supporting Information, Figure S4). The Au nanoclusters were also characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS). However, it was not possible to unambiguously assign the Au nanoclusters encapsulated inside G4-OH, and they remain to be disentangled.[24]
Acknowledgements This work was supported by the Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2012R1A1A1014408, NRF-2012R1A1A2039882, NRF-20100012794), the Agency for Defense Development through Chemical and Biological Defense Research Center, the Fundamental R&D Program for Core Technology of Materials of the Ministry of Knowledge Economy of Korea, and the National Research Foundation of Korea Grant funded by the Korean Government (MEST, NRF-2009-C1AAA001-0092939).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Keywords: dendrimers · gold · luminescence · nanoparticles · time-resolved spectroscopy
[1] C. J. Lin, T. Yang, C. Lee, S. H. Huang, R. A. Sperling, M. Zanella, J. K. Li, J. Shen, H. Wang, H. Yeh, W. J. Parak, W. H. Chang, ACS Nano 2009, 3, 395 – 401. [2] R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung, Acc. Chem. Res. 2001, 34, 181 – 190. [3] L. M. Bronstein, Z. B. Shifrina, Chem. Rev. 2011, 111, 5301 – 5344. [4] V. S. Myers, M. G. Weir, E. V. Carino, D. F. Yancey, S. Pande, R. M. Crooks, Chem. Sci. 2011, 2, 1632 – 1646. [5] J. Zheng, J. T. Petty, R. M. Dickson, J. Am. Chem. Soc. 2003, 125, 7780 – 7781. [6] J. Zheng, C. Zhang, R. M. Dickson, Phys. Rev. Lett. 2004, 93, 077402. [7] W. I. Lee, Y. Bae, A. J. Bard, J. Am. Chem. Soc. 2004, 126, 8358 – 8359. [8] D. Wang, T. Imae, J. Am. Chem. Soc. 2004, 126, 13204 – 13205. [9] D. Wang, T. Imae, M. Miki, J. Colloid Interface Sci. 2007, 306, 222 – 227. [10] M. L. Tran, A. V. Zvyagin, T. Plakhotnik, Chem. Commun. 2006, 2400 – 2401. [11] T. H. Kim, H. S. Choi, B. R. Go, J. Kim, Electrochem. Commun. 2010, 12, 788 – 791. [12] H. Ju, C. M. Koo, J. Kim, Chem. Commun. 2011, 47, 12322 – 12324. [13] J. M. Kim, J. Kim, J. Kim, Chem. Commun. 2012, 48, 9233 – 9235. [14] Y.-G. Kim, S.-K. Oh, R. M. Crooks, Chem. Mater. 2004, 16, 167 – 172. [15] X. Yuan, Z. Luo, Y. Yu, Q. Yao, J. Xie, Chem. Asian J. 2013, 8, 858 – 871. [16] N. S. Han, H. S. Shim, J. H. Seo, S. Y. Kim, S. M. Park, J. K. Song, J. Appl. Phys. 2010, 107, 084306. [17] H Haberland, Clusters of Atoms and Molecules, Springer, Berlin, 1994. [18] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N.Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian Inc., Wallingford CT, 2009. [19] S. Rath, S. Nozaki, D. Palagin, V. Matulis, O. Ivashkevich, S. Maki, Appl. Phys. Lett. 2010, 97, 053103. [20] T. D. Green, K. L. Knappenberger, Nanoscale 2012, 4, 4111 – 4118. [21] J. Zheng, C. Zhou, M. Yu, J. Liu, Nanoscale 2012, 4, 4073 – 4083. [22] Y. Bao, C. Zhong, D. M. Vu, J. P. Temirov, R. B. Dyer, J. S. Martinez, J. Phys. Chem. C 2007, 111, 12194 – 12198. [23] R. Zhou, M. Shi, X. Chen, M. Wang, H. Chen, Chem. Eur. J. 2009, 15, 4944 – 4951. [24] Y. Jao, M. Chen, S. Lin, Chem. Commun. 2010, 46, 2626 – 2628. Received: May 1, 2014 Published online on July 24, 2014
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Blue Luminescence of Dendrimer-Encapsulated Gold Nanoclusters Jun Myung Kim, So Hyeong Sohn, Noh Soo Han, Seung Min Park, Joohoon Kim,* and Jae Kyu Song*[a] Direct evidence for the blue luminescence of gold nanoclusters encapsulated inside hydroxyl-terminated polyamidoamine (PAMAM) dendrimers was provided by spectroscopic studies as well as by theoretical calculations. Steady-state and time-resolved spectroscopic studies showed that the luminescence of
the gold nanoclusters consisted largely of two electronic transitions. Theoretical calculations indicate that the two transitions are attributed to the different sizes of the gold nanoclusters (Au8 and Au13). The luminescence of the gold nanoclusters was clearly distinguished from that of the dendrimers.
1. Introduction Confinement of gold clusters to sizes comparable to the Fermi wavelength of electrons (ca. 0.7 nm) results in molecule-like properties of the gold clusters, including size-dependent luminescence.[1] Well-defined monodisperse templates such as dendrimers have been used to control the size of nanoclusters, which have ranged in diameter from less than 1 nm up to 5 nm, and also the size-dependent luminescence of nanoclusters.[2–4] Particularly, blue luminescent gold nanoclusters (Au8) have been synthesized by the dendrimer-based approach, which has subsequently been extended to the preparation of size-controlled Au nanoclusters with luminescence spanning from UV to near-IR.[5, 6] However, these interesting studies triggered intense discussion on the origin of such luminescence.[7–9] For example, Bard and co-workers demonstrated that the blue emission around 450 nm, which was the characteristic luminescence of Au8, originated from the oxidation of hydroxyl-terminated polyamidoamine (PAMAM) dendrimers.[7] Imae and co-workers also reported the pH-dependent blue fluorescence of amine-terminated PAMAM dendrimers.[8] Other dendrimers such as poly(propyleneimine) and poly(ethyleneimine) dendrimers, have been reported to show blue fluorescence after air treatment.[9] These results raised questions regarding the size-dependent luminescence of Au nanoclusters, suggesting that the emission originated mainly from the dendrimers.
[a] J. M. Kim,+ S. H. Sohn,+ N. S. Han, Prof. S. M. Park, Prof. J. Kim, Prof. J. K. Song Department of Chemistry Research Institute for Basic Sciences Kyung Hee University Seoul 130-701 (Korea) E-mail: [email protected] [email protected] [+] These authors contributed equally to this work. Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402287.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
In this work, we have clarified the controversies regarding the luminescence of Au nanoclusters encapsulated in dendrimers. The Au nanoclusters were prepared by adjusting the ratio of Au to the dendrimer template to control the number of Au atoms encapsulated inside dendrimers. This approach differs from that of the previous method of preparation, which resulted in simultaneous synthesis of luminescent small nanoclusters and nonluminescent large nanoparticles.[5, 6, 10] This control of stoichiometry removed the need to separate the nonluminescent large Au aggregates,[11–15] which contrasts with previous work.[5, 6, 10] Detailed steady-state and time-resolved spectroscopic studies[16] provided direct evidence for the blue luminescence of the Au nanoclusters, which was clearly distinguished from the luminescence of the dendrimers. Theoretical calculations also suggested that the luminescence of the Au nanoclusters were attributed to Au8-dominating nanoclusters encapsulated inside dendrimers.
2. Results and Discussion The photoluminescence of the Au nanoclusters, which were encapsulated inside hydroxyl-terminated fourth generation PAMAM dendrimer (G4-OH), was examined by steady-state photoluminescence spectroscopy. The blue emission (black line in Figure 1 a) was observed at 455 nm from Au nanoclusters encapsulated in G4-OH, which are denoted as G4-OH(Au). Indeed, the blue emission (red line in Figure 1 a) was also found from G4-OH (without encapsulated Au nanocluster), as previously reported by several research groups.[7–9] However, the photoluminescence of G4-OH(Au) was distinguishable from that of G4-OH, with the emission peak of G4-OH(Au) being redshifted by 15 nm compared with that of G4-OH (440 nm). In addition, the emission intensity of G4-OH(Au) was five times higher than that of G4-OH at the same concentration (10 mm). Therefore, the increased emission intensity and the redshifted peak position suggested the presence of luminescent Au ChemPhysChem 2014, 15, 2917 – 2921
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Figure 1. a) Emission spectra of G4-OH(Au) (black line) and G4-OH (red line) in aqueous solutions. The difference spectrum (blue line) was obtained by subtracting the emission of G4-OH from that of G4-OH(Au). All the emission spectra were obtained by using an excitation wavelength of 355 nm. The sharp peak at 405 nm was due to the Raman shift of water. b) Emission spectra of aqueous solutions of all the individual reagents that were used in the preparation of G4-OH(Au). The Raman peak of water was removed and this region of the spectrum was reconstructed from Gaussian fitting process (dotted line). c) Emission spectra of mixtures of G4-OH and the reagents in aqueous solutions.
nanoclusters, which were protected and stabilized by the dendrimers. It is possible that the blue emission of G4-OH(Au) could be related to chemical reagents including impurities used in the preparation of G4-OH(Au), therefore, emission spectra were obtained from all the individual reagents. However, except with G4-OH, blue emission was barely observed from the individual chemical reagents, which ruled out emission from reagents 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org and impurities (Figure 1 b). In addition, the emission spectra from mixtures of G4-OH and reagents were obtained in the absence of Au precursor (HAuCl4), because the chemical reaction could change the emission profile of G4-OH in terms of the emission intensity and peak position. However, the emission spectra of the mixtures were nearly identical to that of G4-OH (Figure 1 c), which indicated that the chemical reaction with the reagents neither increased the emission intensity nor changed the peak position of G4-OH. Bard and co-workers studied the blue fluorescence of hydroxyl-terminated PAMAM dendrimers and concluded that the blue emission was related to oxidation of the hydroxyl-terminal groups of the dendrimers.[7] Similarly, the group of Imae investigated the blue emission of dendrimers but attributed the emission to interaction of the tertiary amino moieties with oxygen inside the dendrimers.[8, 9] In this regard, the shifted peak at 455 nm could be correlated to the oxidization of G4-OH by the Au precursor (HAuCl4), which was also known for the oxidizing reagent. However, the emission spectrum from mixtures of G4-OH and HAuCl4 (orange line in Figure 1 c) was also similar to that of untreated G4-OH, which indicated that the addition of Au precursors did not change the emission profile of G4-OH, even if a chemical reaction of G4-OH might occur with HAuCl4. Thus, the observed emission of G4-OH implied that interaction of oxygen (or air) with tertiary amino-branching sites of G4-OH was involved in the blue fluorescence (440 nm), which was not altered by the chemical reaction under the current experimental conditions. It is notable, therefore, that the blue luminescence of G4-OH(Au) was clearly distinguished from the emission of G4-OH by the increased emission intensity and the redshifted peak position, which indicated the presence of luminescent Au nanoclusters encapsulated within the dendrimers. Since the emission of G4-OH was inevitably included in the emission spectrum of G4-OH(Au), the emission of G4-OH was subtracted from that of G4-OH(Au) to obtain the emission spectrum of the encapsulated Au nanoclusters (blue line in Figure 1 a). The shape of the subtracted spectrum was similar to that of G4-OH(Au). The emission maximum at 460 nm corresponded to the well-known emission of Au8-dominating nanoclusters.[5, 6] The energy states of Au nanoclusters have often been explained by the spherical jellium approximation using the free-electron filling model as used in the alkali metal nanoclusters.[6, 17] In this model, the emission energy of Au nanoclusters was estimated as a function of the number of atoms (N) by a relation of EFermi/N1/3, where EFermi is the Fermi energy of bulk Au (5.5 eV).[10] Therefore, the emission energy of N = 8 in the model (451 nm) suggested that Au nanoclusters prepared in our study were Au8, with a spherical shape. In contrast, the full width at half maximum (FWHM) of the emission was broad, in addition to the asymmetric shape, which was similar to the previous study that showed that the emission in the blue region was broader than in other regions.[6] The spectral broadening could be ascribed to spectral overlap of emission bands, suggesting that a few electronic states were involved in the blue emission. Indeed, the observed emission spectrum was better reproduced by two Gaussian functions (Figure 2 a), rather than by a single Gaussian ChemPhysChem 2014, 15, 2917 – 2921
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Figure 2. a) The difference emission spectrum was deconvoluted by two Gaussian functions (green lines), which indicated that two electronic transitions were involved in the emission of G4-OH(Au). The abscissa scale in the spectrum was converted into energy for better comparison. The most probable electronic transitions (bars) of the optimized geometries (A1, A2, and A3) are shown to compare the calculated energies with the emission spectrum. b) Among the calculated geometries of Au8, a fused tetrahedron (A1) was the most stable; a distorted bicapped octahedron (A2) and a capped tetrahedron (A3) were higher in energies than A1. The calculated stable geometries of Au8 were nearly spherical.
function (see the Supporting Information, Figure S1), which suggested that at least two transitions were involved in the emission of G4-OH(Au). To examine these emissive states, the electronic transitions of Au8 were calculated by using the timedependent density functional theory (TD-DFT) module in the GAUSSIAN package.[18] First, stable structures of possible twodimensional and three-dimensional geometries were obtained by using analytical gradients with full optimization with LANL2MB and LANL2DZ basis sets. The frequencies of the optimized geometries were calculated to ensure that the structures represented stable points on the potential energy surface. It is noted that the optimized geometries of Au8 were nearly spherical, which supported the conclusion that the spherical jellium model could be applied to calculate the emission energy of Au8. Among the optimized geometries of Au8, a fused tetrahedron (A1) was the most stable (Figure 2 b), whereas a distorted bicapped octahedron (A2) and a capped tetrahedron (A3) were found to have higher energies with the employed basis sets. For example the energies of A2 and A3 were calculated to be 0.15 and 0.36 eV, respectively, above the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org energy of A1 at the B3LYP/LANL2MB level. The energies of A2 and A3 also lay 0.19 and 0.42 eV, respectively, above the energy of A1 at the B3LYP/LANL2DZ level. To understand the origin of the two bands of the emission spectrum in more detail, the electronic transitions were investigated for these stable structures. The most probable electronic transitions from the ground to excited states of A1, A2, and A3 were calculated to be 2.94, 3.26, and 3.01 eV, respectively, at the B3LYP/LANL2DZ level. To compare the calculated energy with the observed spectrum (Figure 2 a), the solvation effect of each isomer was calculated in aqueous solution, which was in the range of 0.04–0.19 eV. To a first approximation, it was supposed that the solvation effect in the aqueous solution would be similar to that within the dendrimer in aqueous solution, because the interaction between gold nanoclusters and dendrimer is known to be weak in dendrimer-encapsulated gold clusters.[2] In addition, the Stokes shift was taken into account, which depended on the isomer geometries and was in the range of 0.10–0.16 eV.[19] Overall, the calculated absorption energies were shifted to take into account the solvation effect and the Stokes shift together. Interestingly, the emission energies of A1 and A3 were consistent with one of the deconvoluted bands (2.69 eV), whereas the emission energy of A2 did not match well. Presumably, the population of A3 was not significant because of the higher energy of A3 than A1 (by 0.36 and 0.42 eV calculated at the B3LYP/LANL2MB and B3LYP/LANL2DZ levels, respectively), which implied that A3 did not contribute much to the emission spectrum. Therefore, the results of the calculation suggest that one of the electronic transitions (2.69 eV) was from the most stable isomer of Au8 (A1). The remaining band centered at 2.33 eV might be correlated to another excited state of A1 because a few orbitals, other than the frontier orbitals HOMO and LUMO, were found as excited states.[20, 21] However, the oscillator strengths of other electronic transitions were calculated to be relatively weak, which ruled out the possibility of another electronic transition of A1 for the remaining emission band. Possibly, this lowenergy band could be attributed to vibrational progression of the most probable electronic transition. If this was the case, the lifetimes of the two emissive bands would be identical. Alternatively, the low-energy band might arise from different sized Au nanoclusters, which would result in different lifetimes of the two bands. Thus, because the origin of the emissive states could be distinguished by the lifetimes, the time profiles of the blue emission were investigated. The time profiles of G4-OH(Au) emission at different wavelengths were similar (Figure 3 a), except for the profile of at 420 nm. The exceptional profile at this wavelength could be explained by the inevitable inclusion of G4-OH in the emission of G4-OH(Au). In other words, the time profiles of G4-OH (Figure 3 b) were similar to that of 420 nm in G4-OH(Au), which indicated that the latter was perturbed by the former to some extent. The decay profiles of G4-OH were most reasonably fitted by a double-exponential model, IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ, where A1 and A2 are relative magnitudes and t1 and t2 are lifetimes. The lifetimes of G4-OH (0.7 and 4.5 ns) were a little shorter than
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www.chemphyschem.org that of 5.5 ns (A2 = 0.39), when the emission intensity was integrated over 1 ms (time window of the steady-state emission spectrum). In this regard, the emission band at 2.33 eV was concluded to originate from Au13 with a lifetime of 1.5 ns, whereas the emission of Au8 was at 2.69 eV with a lifetime of 5.5 ns, although these are not well-separated due to the broad spectral nature of emission (Figure 2 a). In addition, the longer lifetime (5.5 ns) emission was matched with the reported value for Au8 in G4-OH (6.7 and 7.5 ns),[6, 22] which was assigned as the transition between the sp and d bands.[5] The short lifetime (1.5 ns) of Au13 was also noted (3.4 and 5.2 ns),[6, 23] which followed the generally observed trend that the lifetime of nanocluster emission decreased with increased cluster sizes. Nevertheless, the contribution of Au8 and Au13 to the time profiles of the observed emission should be different at different wavelengths. Indeed, the emission time profile at longer wavelength indicated a slightly shorter decay than that at shorter wavelength (see the Supporting Information, Figure S2) because the emission center of Au13 with a shorter lifetime was in the longer wavelength regime. In this regard, the time-resolved emission profiles supported the conclusion that the coexistence of Au8 and Au13 was responsible for the broad emission observed in the blue regime. Significantly, the lifetimes of G4-OH(Au) were clearly different from those of G4-OH, which indicates strongly that the blue luminescence originates mainly from the Au nanoclusters encapsulated within G4-OH.
3. Conclusions Figure 3. a) Time profiles of G4-OH(Au) were obtained by using the time-correlated single-photon counting technique. The decay profiles were fitted by the double-exponential model with two lifetimes (1.5 and 5.5 ns). The fitting curves were superimposed for better comparison. b) Selected time profiles of G4-OH. The decay profiles were fitted by the double-exponential model with two lifetimes (0.7 and 4.5 ns). Intensities were normalized for better comparison.
those of G4-NH2 (1.7 and 7.5 ns),[8] which might be attributed to the difference in the terminal groups (OH and NH2). Similarly, the decay profiles of G4-OH(Au) emission were best fitted by the double-exponential model with lifetimes of 1.5 and 5.5 ns and with relative magnitudes of 0.61 and 0.39, respectively. Accordingly, the possibility of vibrational progress for the low-energy band could be ruled out because the lifetimes of the two components were not identical. Therefore, the time profiles suggested that the low-energy band originated from different sized Au nanoclusters. Indeed, the lowenergy band (2.33 eV) was consistent with the emission energy of Au13 (2.34 eV) in the spherical jellium model, which indicated that another nanocluster prepared in our study was Au13 with a spherical shape. Indeed, Au13 was also a stable sized nanocluster for G4-OH(Au),[6] the spherical shape of which could be assumed to be based on an icosahedron. The short lifetime (1.5 ns) could be correlated to Au13, whereas the longer lifetime (5.5 ns) was linked to Au8, because the emission lifetime of nanoclusters is known to decrease with increased cluster size.[6, 21] Indeed, the overall intensity of the emission with time constant of 1.5 ns (A1 = 0.61) was half 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
We have investigated the blue luminescence of Au nanoclusters encapsulated inside dendrimers. By controlling the ratio of Au to G4-OH, systems were generated that contained only luminescent small nanoclusters. Detailed studies of the steadystate and time-resolved spectroscopy allowed the blue luminescence to be attributed to Au8-dominating nanoclusters, with the lower energy band being associated with Au13. The luminescence of the Au nanoclusters encapsulated inside G4-OH was also clearly distinguished from the emission of G4-OH. These results resolve the controversies regarding the luminescence of dendrimer-encapsulated Au nanoclusters. Such clusters are expected to be useful in a range of analytical applications including cellular imaging, immunoassays, and chemical sensing.
Experimental Section Chemicals and Materials Hydroxyl-terminated fourth generation PAMAM dendrimers (G4OH), HAuCl4·3 H2O, NaBH4, and cellulose dialysis sacks (MW cutoff of 12 000) were purchased from Sigma–Aldrich (USA). Sodium hydroxide was obtained from Daejung, Inc. (Korea). Deionized water (18 MW·cm, Ultra370, Younglin Co., Korea) was used as solvent.
Preparation of Dendrimer-Encapsulated Au Nanoclusters Au nanoclusters were prepared by reported methods with a few modifications.[11, 13, 14] Briefly, HAuCl4 (8 equiv.) was added to an ChemPhysChem 2014, 15, 2917 – 2921
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CHEMPHYSCHEM ARTICLES aqueous hydroxyl-terminated fourth generation dendrimer solution (10 mm) to complex Au ions with the interior amines of the dendrimers. Then, a 5-fold molar excess of NaBH4 in NaOH (0.3 m) was added slowly with vigorous stirring to reduce the Au complexes to zerovalent Au nanoclusters inside the dendrimers. The Au nanocluster solution was dialyzed by using a cellulose sack for 24 h to remove impurities. The resulting Au nanocluster solution produced no significant amount of large Au aggregates even after vigorous centrifugation (13 000 rpm for 20 min) as Crooks and others reported previously.[2, 3, 11–13] The amine-terminated dendrimers were also employed to prepare the corresponding Au nanoclusters, the properties of which were compared to G4-OH(Au).
Characterization of Dendrimer-Encapsulated Au Nanoclusters To examine oxidation states of Au in the nanoclusters, X-ray photoelectron spectra were obtained. The binding energies of Au(4f) peaks (4f5/2 and 4f7/2) matched with zerovalent Au (see the Supporting Information, Figure S3), which suggested the presence of zerovalent Au species in the nanoclusters. For steady-state and time-resolved photoluminescence spectra, the Au nanocluster solution was excited by the second harmonic (355 nm) of a cavitydumped oscillator (Mira/PulseSwitch, Coherent, 1 MHz, 710 nm, 150 fs). Emission was collected by using a set of lenses, spectrally resolved using a monochromator, detected using a photomultiplier, and recorded with a time-correlated single-photon counter (TCSPC, PicoHarp, PicoQuant). The instrumental response of the entire system was 0.05 ns, which resulted in a resolution of 0.01 ns with the use of deconvolution.[15] Transmission electron microscopy (TEM) images of the Au nanoclusters were obtained at 200 kV (Tecnai G2 F30, FEI Co.). TEM measurements also showed no significant numbers of nanoparticles larger than 1 nm (see the Supporting Information, Figure S4). The Au nanoclusters were also characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS). However, it was not possible to unambiguously assign the Au nanoclusters encapsulated inside G4-OH, and they remain to be disentangled.[24]
Acknowledgements This work was supported by the Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2012R1A1A1014408, NRF-2012R1A1A2039882, NRF-20100012794), the Agency for Defense Development through Chemical and Biological Defense Research Center, the Fundamental R&D Program for Core Technology of Materials of the Ministry of Knowledge Economy of Korea, and the National Research Foundation of Korea Grant funded by the Korean Government (MEST, NRF-2009-C1AAA001-0092939).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Keywords: dendrimers · gold · luminescence · nanoparticles · time-resolved spectroscopy
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