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2008, 112, 18722–18726 Published on Web 11/06/2008
Platinum Carbonyl Clusters: Double Emitting Quantum Dots PR. Selvakannan, I. Lampre, M. Erard, and H. Remita* Laboratoire de Chimie Physique, Baˆt. 349-350, UMR 8000-CNRS, UniVersite´ Paris Sud-11, 91405 Orsay, France ReceiVed: September 10, 2008; ReVised Manuscript ReceiVed: October 23, 2008
The photophysical study of platinum carbonyl clusters shows that these clusters exhibit double emitting properties in the visible and near-infrared spectral region for single excitation wavelength. When deposited on glass, they self-assemble into long nanowires or spherical aggregates which still exhibit fluorescence properties analogous to those of the individual clusters, as shown by images taken with a fluorescent microscope at two different emission wavelengths with single excitation. Introduction Metal and semiconductor nanoparticles have emerged as a new generation of chromophores and fluorophores in the spectral region ranging from visible to near-infrared (NIR) given their unique optical properties.1-4 Semiconductor nanoparticles exhibit size-dependent fluorescence properties due to quantum confinement of electrons.5 Because of their photostability, sizedependent emission, narrow emission bandwidth, and large Stokes shift compared to currently used organic dyes, they constitute inorganic fluorophores and find applications such as fluorescence-based imaging, detection, and sensing.6,7 Spatial confinement of electrons in the case of noble metal nanoparticles such as gold and silver leads to intense surface plasmon resonance (SPR) absorption bands in the visible-NIR spectral region.8 SPR absorption bands can be tuned by varying the size and shape of the nanoparticles.9 Thus, metal nanoparticles position themselves as inorganic chromophores and have lots of possible applications such as microscopic contrast agents,10 DNA sequencing,11 cancer hyperthermia,12 and surface enhanced raman scattering (SERS).13 Recently, a class of metal clusters called metal quantum dots (QDs) have received a lot of attention due to discrete, size tunable electronic transitions from visible to NIR. Studying these systems bridges the gap between the optical properties of atoms and nanoparticles.14,15 These QDs consist of few metal atoms, and their dimensions are smaller than the Fermi wavelength of electrons.16 In this smallest size regime, the clusters have discrete electronic states and display fluorescent properties: such clusters of few gold or silver atoms have recently been studied.17 Their photoluminescence is size dependent and shifts toward higher energies with decreasing cluster size. It is challenging to develop a new generation of fluorescent probes which show multiple emissions because they are potential candidates for multicolor imaging, energy transfer quantification, and studying multiple events simultaneously, especially in complex biological systems.18 Hence, another required property is the use of visible light instead of UV light for excitation as the latter may lead to photodegradation of biological and organic * To whom correspondence should be addressed. E-mail: hynd.remita@ lcp.u-psud.fr.
10.1021/jp808033q CCC: $40.75
species. Within this context, we report the fluorescence properties of platinum clusters and their self-assembly into fluorescent nanostructures. Chini and co-workers first synthesized the trigonal prismatic [Pt3(CO)6]n2- (n ) 2-10) clusters by reduction of Pt(II) or Pt(IV) with CO in alkaline methanol.19,20 Their electronic structures were theoretically studied.21-24 These clusters were also synthesized by radiolysis, and the cluster nuclearity (n) was controlled by the irradiation dose.25,26 They display two intense optical absorption bands in the visible domain, their position being sensitive to the cluster nuclearity. With increasing n, the absorption bands show a red shift.26 Here, we have studied the fluorescent spectral and imaging properties of [Pt3(CO)6]42-. Experimental Section Platinum carbonyl clusters [Pt3(CO)6]42- and [Pt3(CO)6]32were prepared by the radiolytic reduction of platinum complexes H2PtCl6 (Aldrich) and Pt(acac)2 (STREM Chemicals) at two different concentrations 10-3 M and 2.5 × 10-3 M in strongly alkaline methanol by γ-radiolysis (Co60 source) under CO atmosphere (dose ) 4600 Gy, pressure ) 1 atm).25,26 Alkaline pH was maintained to stabilize the clusters after formation. The prepared samples were electrosprayed, and the mass spectra were recorded with a reflector time-of-flight using an ES-Q-TOF Tandem Mass spectrometer (Micromass). UV-visible spectra of these samples were carried out on an HP diode array HP8453 spectrophotometer. Fluorescence spectra were recorded using a SPEX fluorolog 111 spectrofluorometer operated at the resolution of 2 nm equipped by a Hamamatsu R3896 photomultiplier. The fluorescence decay curves were recorded using a time-correlated single photon counting setup with standard electronics (Ortec, Phillips & Tennelec, USA) and using a Ti:Sappphire laser as a source (MIRA 900, 10W Verdi, Coherent, Watford, UK). The laser repetition rate was reduced to 3.8 MHz using a pulse picker (SiO2 crystal, APE, Berlin, Germany). The 420 nm excitation wavelength was obtained by frequency doubling the 840 nm laser radiation with a LBO crystal. Average laser power at the sample was 200 µW. Fluorescent images of these clusters were taken with a Nikon 2008 American Chemical Society
Letters
Figure 1. UV-visible absorption spectra of the alkaline methanol solutions of platinum carbonyl clusters [Pt3(CO)6 ]n2- (n ) 3 and 4) synthesized from 2.5 × 10-3 M H2PtCl6 (spectrum 1, red curve, path length 1 mm) and 10-3 M H2PtCl6 solutions (spectrum 2, green curve, path length 2 mm) and from 2.5 × 10-3 M Pt(acac)2 (spectrum 3, blue curve, path length 2 mm), dose rate ) 2.3 kGy/h, dose ) 4600 Gy. Inset: the structures of Chini clusters and photographs of vials containing the clusters [Pt3(CO)6]n2- (n ) 3 and 4) in alkaline methanol.
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Figure 2. Emission and excitation spectra of the tetrameric platinum clusters [Pt3(CO)6]42- synthesized from H2PtCl6 in alkaline methanol: spectrum 1 (blue curve), λexc ) 380 nm; spectrum 2 (red curve), λexc ) 620 nm; spectrum 3 (black dashed curve), λem ) 740 nm. The emission of the H2PtCl6 precursor in alkaline methanol upon excitation at 380 nm is shown in spectrum 4 (gray dashed curve). The sharp peak present around 430 nm is due to Raman scattering. A bandpass filter was used to avoid the second order transmission during the excitation at 380 nm.
TE2000U inverted microscope with a conventional CCD camera (Pixel Fly, PCO). See Supporting Information for additional details. Results and Discussion Figure 1 shows the UV-visible spectra of the [Pt3(CO)6]n2(n ) 3 and 4) clusters synthesized in alkaline methanol. The disappearance of the ligand field transitions of the precursor platinum complexes indicates the total reduction of the platinum ions (see Supporting Information, Figure S1). Spectrum 1 (precursor H2PtCl6, red curve) exhibits two intense absorption bands at 360 nm (3.44 eV) and 560 nm (2.21 eV) and one weak absorption band at 500 nm (2.47 eV). Spectrum 2 (precursor H2PtCl6, green curve) presents two intense absorption bands at 390 nm (3.14 eV) and 620 nm (2 eV) and a weak absorption band at 495 nm (2.5 eV). These values match very well with the reported values for trimeric [Pt3(CO)6]32- (red) and tetrameric [Pt3(CO)6]42- (green) clusters, respectively (inset Figure 2). Spectrum 3 (blue curve) corresponds to the clusters synthesized from Pt(acac)2 and showed similar features of both trimeric and tetrameric clusters. The nuclearity of the clusters was confirmed by the mass spectra (see Supporting Information, Figure S2). [Pt3(CO)6]42- is stable under air in alkaline methanol for a few hours, while [Pt3(CO)6]32- is stable (several days) under CO or N2 atmosphere, our experimental conditions, but less stable in the presence of O2 (the oxidation of [Pt3(CO)6]n2leads to clusters of higher nuclearity [Pt3(CO)6]n+12-,26 see Supporting Information, Figure S3). It is to be noted that the syntheses were done at Pt concentration g10-3 M, but by dilution, the trimeric cluster is converted into the tetrameric cluster which remains stable in solution. Previous experimental studies and molecular orbital (MO) calculations showed that both trimeric and tetrameric clusters display three absorption bands, the first and third transitions being much more intense than the second transition.21-24 MO calculations also indicated that, in the HOMO and LUMO of these clusters, the s and d orbitals of platinum and the π* orbitals of the CO ligands interact (back bonding interactions) and also that the π* orbitals of the CO ligands interact with each other.
Figure 3. Fluorescence decays of platinum clusters [Pt3(CO)6]42synthesized from H2PtCl6 (black line) and Pt(acac)2 (red line) after excitation at 420 nm and collecting the emission around 460 nm. The IRF is presented by the blue line. The data were fitted with exponential decays, but the fit lines are not shown for clarity.
Figure 2 shows the emission spectrum of the tetrameric clusters synthesized from H2PtCl6 at two different excitation wavelengths. The emission spectrum of the trimeric clusters was not carried out, as they became tetrameric clusters on dilution. When the tetrameric clusters are excited at 620 nm, they exhibit a fluorescence band around 740 nm (red curve, spectrum 2, Figure 2). Upon excitation at 380 nm, the emission spectrum (blue curve, spectrum 1, Figure 2) shows a sharp peak at 430 nm and two bands centered around 470 and 740 nm, respectively, leading to effective stokes shifts of 85 and 345 nm from the absorption band maximum. These two emission bands originate only from the clusters as the emission spectrum of the chloride precursor in alkaline methanol displays only the sharp peak at 430 nm due to Raman scattering (gray curve, spectrum 4, Figure 2). This dual fluorescence with two well-
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Figure 4. Images of spherical aggregates of [Pt3(CO)6]42- clusters made from H2PtCl6. (A) Transmission mode image, (B-D) false-color luminescence images: (B) excitation around 440 nm and emission around 480 nm, (C) excitation around 440 nm and emission above 600 nm, (D) excitation around 570 nm and emission above 600 nm.
Figure 5. Images of the 1D assembly of [Pt3(CO)6]42- made from Pt(acac)2. (A) Transmission mode image; (B-C) false-color luminescence images: (B) excitation around 440 nm and emission around 480 nm, (C) excitation around 500 nm and emission above 530 nm. (D) TEM images of the linear aggregates in low magnification. Inset: high magnification of one such linear aggregate.
resolved intense emission bands for a single excitation is relatively new, as organic dyes and quantum dots exhibit in general only one emission. This is an ideal requirement for multicolor imaging. The double emission could originate from the stacking properties of the triangular platinum units, one transition being in the triangular unit plane and the other one in the stacking direction. The excitation spectrum corresponding to the emission at 740 nm (black curve, spectrum 3, Figure 2) shows features similar to the absorption spectrum and confirms that the 740 nm
emission originates from both absorption bands but with different contributions. The intensity of the 380 nm band is smaller than that of the 620 nm band contrary to the absorption spectrum, which clearly indicates the existence of other relaxation paths from the highest excited states (in particular, emission around 470 nm). The emission quantum yield of these clusters upon excitation at 380 nm, calculated with respect to the quinine standard and taking into account both emission bands, was evaluated to be 3 × 10-3. However, upon excitation at 620 nm (the second emission band, spectrum 2) the emission
Letters quantum yield of these clusters is much higher, and it was calculated to be 2.9 × 10-2. The fluorescent spectra of the clusters made from Pt(acac)2 exhibit similar features and are presented in the Supporting Information. The fluorescence decays of the tetrameric clusters (synthesized from both H2PtCl6 and Pt(acac)2) recorded at 460 ( 6 nm upon excitation at 420 nm are presented in Figure 3. Platinum clusters made from both precursors exhibited similar decay patterns. The decays display two main components, a fast part identical to the instrumental response function (IRF) and attributed to scattered and Raman light (also seen in Figure 2), and a slow contribution with an average lifetime of 5.1 ns. There was no photobleaching of these clusters, even during illumination for hours at high laser power (200 µW). That shows the high photostability of the clusters necessary for fluorescent applications. Figure 4 presents the microscopy images of the [Pt3(CO)6]42clusters synthesized from H2PtCl6. The transmission images (Figure 4A) show that these clusters assemble into spherical aggregates when deposited on glass. As the clusters in solution exhibit two well-resolved emission bands for single excitation, the deposited clusters were imaged upon excitation at 440 nm, by collecting the emission in two different spectral regions, around 480 nm (Figure 4B) and above 600 nm (Figure 4C). As these clusters also present a second intense absorption band leading to the red emission, they were also imaged by exciting at 570 nm and collecting the emission light above 600 nm as shown in Figure 4D. These results witness that the aggregated clusters keep the absorption and emission properties of the isolated clusters in solution and can be imaged in two different wavelength ranges for single wavelength excitation. While the aggregates of the clusters synthesized from H2PtCl6 are spherical when deposited on glass as shown previously, the clusters made from Pt(acac)2 were found to gather into linear assemblies forming long wires a few microns in length as displayed in Figure 5A (fluorescence transmission image) and D (transmission electron microscopic image). The fluorescent images obtained by collecting the emission light around 480 nm reveal that the linear superstructures are fluorescent (Figure 5B). In contrast, the images recorded for emission light above 530 nm show only luminescent dots (Figure 5C) on these linear structures, which suggest the presence of low-energy traps from which the red emission originates. Assembling individual quantum dots to make luminescent nanowires for fluorescence based applications is still a challenge in the field of inorganic fluorophores. This linear assembly into wires formed during the solvent evaporation may be facilitated by the acetylacetonate ligands. High resolution TEM images (inset in Figure 5D) clearly show that the wires are made up of very small clusters. We have already reported that [Pt3(CO)6]n2- clusters, when deposited on flat supports, self-assemble into nanowires.27 The platinum carbonyl clusters were also used as precursors for the fabrication of conductive submicrometric wires by softlithography.28 Moreover, the formation of infinite platinum chains based on stacked Pt3(CO)6 units was predicted by Hoffmann and co-workers.22 Longoni and co-workers studied the self-assembly of [Pt3(CO)6]n2- clusters into infinite semicontinuous or continuous conductor wires upon cristallization.29,30 They pointed out the effect of the counterion on such assemblies.30 A 1D spontaneous organization of nanoparticles into luminescent nanowires was observed in the case of fluorescent CdTe quantum dots;31 to our knowledge, it is the first time that self-assembly of fluorescent metal clusters into nanowires is reported.
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18725 In summary, we have shown the luminescence properties of platinum carbonyl clusters [Pt3(CO)6]42-. These clusters present two emission bands for single visible excitation, which is relatively new in the field of fluorescent metal quantum dots. The effective Stokes shifts are very high. Visible excitation is an advantage for an imaging agent, when samples are photodegradable under UV light. Despite a low fluorescence quantum yield, the emission properties of these clusters render them particularly attractive as a new class of fluorophores, which can complement the existing QD-based fluorescent imaging and detection. These clusters self-assemble into either spherical aggregates or long nanowires, which both exhibit fluorescence properties. The study of the size-dependent emission of these clusters as well as their self-assembly into superstructures is under work. Theoretical calculations are also in progress to investigate the origin of the observed dual emission and to try to understand the role of the counterions and the ligands in the self-assembly process. Acknowledgment. The authors thank Philippe Maitre and Sylve`re Durand, Laboratoire de Chimie Physique, for the mass spectra and Patricia Beaunier, Laboratoire de Re´activite´ de Surface, Universite´ Paris VI, for TEM observations. Pierre Archirel, LCP, Universite´ Paris Sud-11, is acknowledged for helpful discussions. C’Nano-Ile de France and the PRES Univers Sud Paris are acknowledged for financial support. Supporting Information Available: Instrumental details, the mass spectra of the clusters, the absorption spectra of the platinum clusters and their precursors, and the emission spectra of [Pt3(CO)6 ]42- synthesized from Pt(acac)2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical properties of metal clusters; Springer: Berlin, 1995. (2) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (3) Wilcoxon, J. P.; Martin, J. E.; Parsapour, F.; Wiedenman, B.; Kelley, D. F. J. Chem. Phys. 1998, 21, 1998. (4) Sun, Y.; Xia, Y. The Analyst 2003, 128, 686. (5) (a) Nirmal, M.; Brus, L. E. Acc. Chem. Res. 1999, 32, 407. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. ReV. Phys. Chem. 1990, 41, 477. (6) (a) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (b) Chan, W. C. W.; Nie, S Science 1998, 281, 2016. (7) Tekle, C.; van Deurs, B.; Sandvig, K.; Iversen, T.-G. Nano Lett. 2008, 8, 1858. (8) Liz-Marzan, L. M. Mater. Today 2004, 7, 26. (9) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 7238. (10) Conjusteau, A.; Ermilov, S. A.; Lapotko, D.; Hongwei, L.; Hafner, J.; Eghtedari, M.; Motamedi, M.; Kotov, N.; Oraevsky, A. A. Progress Biomed. Opt. Imaging 2006, 7, 9. (11) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547. (12) Loo, C.; Lin, A.; Hirsch, L.; Lee, M.-H.; Barton, J.; Halas, N. J.; West, J.; Drezek, R. Technol. Cancer Res. Treatment 2004, 3, 33. (13) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (14) (a) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103. (b) Huang, T.; Murray, R. W. J. Phys. Chem. B 2001, 105, 12498. (c) Link, S.; Beeby, A.; FitzGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2002, 106, 3410. (15) Zheng, J.; Nicoich, P. R.; Dickson, R. M. Annu. ReV. Phys. Chem. 2007, 58, 409. (16) Zheng, J.; Zhang, C.; Dickson, R. M. Phys. ReV. Lett. 2004, 93, 077402-1. (17) (a) Zheng, J.; Petty, J. T.; Dickson, R. M. J. Am. Chem. Soc. 2003, 125, 7780. (b) Zheng, J.; Dickson, R. M. J. Am. Chem. Soc. 2002, 124, 13982. (c) Duan, H.; Nie, S. J. Am. Chem. Soc. 2007, 129, 2412. (d) Schaeffer, N.; Tan, B.; Dickinson, C.; Rosseinsky, M. J.; Laromaine, A.; McComb, D. W.; Stevens, M. M.; Wang, Y.; Petit, L.; Barentin, C.; Spiller, D. G.; Cooper, A. I.; Le´vy, R. Chem. Commun. 2008, 3986.
18726 J. Phys. Chem. C, Vol. 112, No. 48, 2008 (18) (a) De Cremer, G.; Antoku, Y.; Roeffaers, M. B. J.; Sliwa, M.; Van Noyen, J.; Smout, S.; Hofkens, J.; De Vos, D. E.; Sels, B. F.; Vosch, T. Angew. Chem., Int. Ed. 2008, 47, 2813. (b) Chen, W.; Joly, A. G.; Roark, J. Phys. ReV. B 2002, 65, 245404. (19) Calabrese, J. C.; Dahl, L. F.; Chini, P.; Longoni, G.; Martinengo, S. J. Am. Chem. Soc. 1974, 96, 2614. (20) Longoni, G.; Chini, P. J. Am. Chem. Soc. 1976, 98, 7225. (21) Chang, K. W.; Wooley, R. G. J. Phys. C: Solid State Phys. 1979, 12, 2745. (22) Underwood, D. J.; Hoffman, R.; Tatsumi, K.; Nakamura, A.; Yamamoto, Y. J. Am. Chem. Soc. 1985, 107, 5968. (23) Mealli, C. J. Am. Chem. Soc. 1985, 107, 2245. (24) (a) Woolley, R. G. Chem. Phys. Lett. 1988, 143, 145. (b) Roginski, R. T.; Shapley, J. R.; Drickamer, H. G.; d’Aniello, M. J., Jr Chem. Phys. Lett. 1987, 135, 525.
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