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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 2 — Feb. 1, 2013
  • pp: 157–165
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Synthesis of green-emitting Pt8 nanoclusters for biomedical imaging by pre-equilibrated Pt/PAMAM (G4-OH) and mild reduction

Shin-ichi Tanaka, Koichi Aoki, Atsushi Muratsugu, Hidekazu Ishitobi, Takashi Jin, and Yasushi Inouye  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 2, pp. 157-165 (2013)
http://dx.doi.org/10.1364/OME.3.000157


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Abstract

We synthesized green-emitting platinum (Pt) nanoclusters (excitation: 460 nm, emission: 520 nm) by reducing Pt ions from pre-equilibrated Pt/fourth-generation poly(amidoamine) dendrimers (PAMAM (G4-OH)) complexes with a mild reductant. The structural characteristics of the resulting Pt nanoclusters, Pt8L8 (L = C2H2O2S), were determined by Electrospray ionization (ESI) mass spectroscopy. These nanoclusters possess a 28% quantum yield, which is higher than those of green-emitting Au and Ag nanoclusters. We also found that Pt nanoclusters have considerably low cytotoxicity and biocompatibility, and demonstrated that they could be used for biomedical imaging. This study provides the possibility to extend the photoluminescent wavelength of Pt nanoclusters to the near infrared region, which is ideal for biological imaging applications.

© 2013 OSA

1. Introduction

2. Synthetic method

PAMAM (G4-OH) (Sigma-Aldrich) (0.25 µmol) was added to 5 mL of millipore water (18.2 MΩ) and then mixed with H2PtCl6 (Wako Pure Chemical Industries (Japan)) (0.5 M, 30 µL). The Pt ions are coordinated with the interior tertiary amines of the PAMAM dendrimers [42

42. M. R. Knecht, M. G. Weir, V. S. Myers, W. D. Pyrz, H. Ye, V. Petkov, D. J. Buttrey, A. I. Frenkel, and R. M. Crooks, “Synthesis and characterization of Pt dendrimer-encapsulated nanoparticle: effect of the template on nanoparticle formation,” Chem. Mater. 20(16), 5218–5228 (2008). [CrossRef]

,43

43. Y. Gu, P. Sanders, and H. J. Ploehn, “Quantitative analysis of Pt-PAMAM ligand exchange reactions: time and concentration effects,” Colloids Surf. A Physicochem. Eng. Asp. 356(1-3), 10–15 (2010). [CrossRef]

]. The reaction mixture was allowed to stand in the dark at 4°C to minimize Pt ion oxidization of the PAMAM dendrimers [42

42. M. R. Knecht, M. G. Weir, V. S. Myers, W. D. Pyrz, H. Ye, V. Petkov, D. J. Buttrey, A. I. Frenkel, and R. M. Crooks, “Synthesis and characterization of Pt dendrimer-encapsulated nanoparticle: effect of the template on nanoparticle formation,” Chem. Mater. 20(16), 5218–5228 (2008). [CrossRef]

,43

43. Y. Gu, P. Sanders, and H. J. Ploehn, “Quantitative analysis of Pt-PAMAM ligand exchange reactions: time and concentration effects,” Colloids Surf. A Physicochem. Eng. Asp. 356(1-3), 10–15 (2010). [CrossRef]

]. Since the ligand-to-metal charge transfer (LMCT) UV absorption at 250 nm indicates the complexation of Pt ions with PAMAM (G4-OH), we measured UV–Vis absorbance spectrum of the Pt/PAMAM (G4-OH) solution during incubation by using an UV-2450 spectrophotometer (SHIMAZDU) and quartz cell with 1 cm path length. The background spectrum was subtracted by using an identical cell filled with millipore water (18.2 MΩ).

Figure 1(a)
Fig. 1 (a) Time evolution of absorbance for Pt/PAMAM (G4-OH) complexation at 250 nm. (b) UV-Vis spectra showing the complexation of Pt ions with PAMAM (G4-OH): the absorbance at 250 nm corresponds to the LMCT band. The left and right arrows show the decrease in the absorbance of PAMAM (G4-OH) at around 200 nm and that of PtCl62- at 262 nm, respectively.
illustrates the time evolution of absorbance at 250 nm. Although the LMCT band at 250 nm sharply decreased in the first 3 hours, it slowly increased thereafter, reaching its maximum in the first day (24 h). After that, the absorbance gradually decreased again. On the other hand, as shown in Fig. 1(b), the absorbance of PtCl62- at 262 nm [44

44. C. H. Gammons, “Experimental investigations of the hydrothermal geochemistry of platinum and palladium: V. equilibria between platinum metal, Pt(II), and Pt(IV) chloride complexes at 25 to 300 °C,” Geochim. Cosmochim. Acta 60(10), 1683–1694 (1996). [CrossRef]

] and that of PAMAM (G4-OH) at around 200 nm precipitously decreased in the first 3 hours, and slowly changed thereafter. In the first 3 hours, Pt(IV) ions interacted with the external tertiary amines of PAMAM (G4-OH) and were reduced to Pt(II) ions. This reaction corresponds to decrease in absorbance at 262 nm and 200 nm. When Pt(II) ions formed coordination bonds with theinternal tertiary amines of PAMAM (G4-OH), the absorbance of the LMCT band at 250 nm increased. Complexation of Pt(II) ions with PAMAM (G4-OH) reached the equilibrium after 24 hours. We believe the coordination bonds are broken when Pt(II) ions oxidize PAMAM (G4-OH). This result indicates that the number of Pt ions complexed with PAMAM dendrimers reaches the maximum approximately one day after the reaction started. Therefore, we decided that the reduction reaction started 24 hours after pre-equilibrating the Pt-PAMAM complex. We added a reductant (trisodium citrate (Wako Pure Chemical Industries (Japan)); 1 M, 300 µL) to the pre-equilibrated Pt/PAMAM (G4-OH) solution, and incubated this reaction mixture at 90°C for two weeks under continuous stirring to form nanoclusters [45

45. Blue-emitting species were produced in the same manner using a stronger reductant (NaBH4) rather than trisodium citrate. No green photoluminescence were observed from the species.

]. No precipitates were observed after incubation. Then, ultracentrifugation (Optima MAX-XP Benchtop Ultracentrifuge, Beckman Coulter, Inc.; 100,000 G) was performed for 30 min at 4°C to remove Pt colloidal nanoparticles.

3. Purification

4. Characterization

Furnace atomic absorption spectrometry (AA-6700F (SHIMAZDU)) was implemented with the 266 nm line for the fractions 2 to 4. We found only Pt to be present in fraction 3 (5.74 mg/L) and 4 (1.64 mg/L), while we did not detect Pt in fraction 2.

ESI mass spectrometry was then performed to determine the molecular weight of the chemical constituents in the fraction 4 by using LTQ XL (Thermo Fisher Scientific K.K.). The fraction 4 was dissolved in a 50% (v/v) water/methanol solution for measurement [28

28. Z. Wu, E. Lanni, W. Chen, M. E. Bier, D. Ly, and R. Jin, “High yield, large scale synthesis of thiolate-protected Ag7 clusters,” J. Am. Chem. Soc. 131(46), 16672–16674 (2009). [CrossRef] [PubMed]

]. As shown in Fig. 3
Fig. 3 ESI mass spectrum of Pt nanoclusters. The peak, m/z = 2353.22, is assigned to [Pt8L8 + 3Na + 4H]- (L = C2H2O2S), and shows Pt nanoclusters consist of eight platinum atoms.
, the main peak was detected at m/z = 2353.22, which is assigned to [Pt8L8 + 3Na + 4H]- (L = C2H2O2S). From the results, we found that the synthesized nanoclusters composed eight platinum atoms and that they were monodispersed. The fluorescent peaks (excitation: 460 nm, emission: 520 nm) observed in Fig. 2(c) and Fig. 2(d) originate from Pt nanoclusters, while the peaks (excitation: 330 nm, emission: 420 nm) observed in Fig. 2(b) and Fig. 2(c) are attributed to PAMAM (G4-OH), of which fluorescence was reported in [36

36. S.-I. Tanaka, J. Miyazaki, D. K. Tiwari, T. Jin, and Y. Inouye, “Fluorescent platinum nanoclusters: synthesis, purification, characterization, and application to bioimaging,” Angew. Chem. Int. Ed. Engl. 50(2), 431–435 (2011). [CrossRef] [PubMed]

] and [46

46. W. I. Lee, Y. Bae, and A. J. Bard, “Strong blue photoluminescence and ECL from OH-terminated PAMAM dendrimers in the absence of gold nanoparticles,” J. Am. Chem. Soc. 126(27), 8358–8359 (2004). [CrossRef] [PubMed]

]. We deduce from these results that fraction 4 contains Pt8L8 whereas fraction 3 contains Pt8 nanoclusters enclosed in PAMAM (G4-OH). Finally, Pt8 nanoclusters were found to have longer wavelength emissions than Pt5 nanoclusters [36

36. S.-I. Tanaka, J. Miyazaki, D. K. Tiwari, T. Jin, and Y. Inouye, “Fluorescent platinum nanoclusters: synthesis, purification, characterization, and application to bioimaging,” Angew. Chem. Int. Ed. Engl. 50(2), 431–435 (2011). [CrossRef] [PubMed]

].

Then, we evaluated the fluorescent lifetime of these Pt nanoclusters using fluorescent lifetime measurement system. Femtosecond pulses from a regenerative amplifier (Libra HE, Coherent) were led to an optical parametric amplifier (OPerA Solo, Coherent), which generates 60 fs pulses at 1 kHz repetition rate. The beam was directed to two BBO crystals to generate a third harmonic pulse with a wavelength of 460 nm. The excitation light was softly focused onto samples placed in 1 cm cuvettes. The power density of the excitation beam was set to ~5 μJ/cm2, which is sufficiently low to avoid any saturation effects. Time-resolved spectra were obtained using a photon-counting streak camera (C4780, Hamamatsu Photonics, Japan) through a 25-cm monochromator (250is, Chromex). The measured fluorescence lifetime of green-fluorescence Pt nanoclusters was 6.5 ± 0.5 ns (Fig. 4
Fig. 4 Fluorescence lifetime of Pt8 nanoclusters: the fluorescence lifetime of Pt8 nanoclusters was obtained by single exponential fitting (6.5 ± 0.5 ns).
), which was obtained by single exponential fitting. The fluorescence decay curve matches well to the single exponential decaying function, which indicates that the synthesized Pt nanoclusters have single molecular structure. Next, we evaluated the photobleaching characteristic of the Pt8 nanoclusters by observing decay in fluorescence intensity. The fluorescence intensity decay was measured and fitted by a single exponential function, F(t) = F0exp(-t/Td) (t: irradiation time, F0: fluorescence intensity at t = 0, Td: decay time which corresponds to F(Td)/F0 = 1/e). Decay time of Pt8 nanoclusters was 125 min, while that of organic fluorophore (1,1′-Dioctadecyl-3,3,3′,3′-tetramethylindocarbocyamine iodide (DiI)), which is well-known as a high resistant fluorophore to photobreaching, was 50 min. We found that the Pt8 nanoclusters were less subject to photobleaching than DiI. Furthermore, we examined the absolute quantum yield (QY) of the Pt nanoclusters with a QY measurement system (C10027, Hamamatsu Photonics, Japan). The Pt8 nanoclusters exhibited a QY of 28% in water, which betters the 18% QY of Pt5 [36

36. S.-I. Tanaka, J. Miyazaki, D. K. Tiwari, T. Jin, and Y. Inouye, “Fluorescent platinum nanoclusters: synthesis, purification, characterization, and application to bioimaging,” Angew. Chem. Int. Ed. Engl. 50(2), 431–435 (2011). [CrossRef] [PubMed]

]. This value also exceeds those of other green-emitting nanoclusters such as Au (QY = 25%, em = 510 nm) [1

1. J. Zheng, C. Zhang, and R. M. Dickson, “Highly fluorescent, water-soluble, size-tunable gold quantum dots,” Phys. Rev. Lett. 93(7), 077402 (2004). [CrossRef] [PubMed]

,2

2. J. Zheng, P. R. Nicovich, and R. M. Dickson, “Highly fluorescent noble-metal quantum dots,” Annu. Rev. Phys. Chem. 58(1), 409–431 (2007). [CrossRef] [PubMed]

] and Ag (QY = 16%, em = 520 nm) nanoclusters [23

23. C. I. Richards, S. Choi, J.-C. Hsiang, Y. Antoku, T. Vosch, A. Bongiorno, Y.-L. Tzeng, and R. M. Dickson, “Oligonucleotide-stabilized Ag nanocluster fluorophores,” J. Am. Chem. Soc. 130(15), 5038–5039 (2008). [CrossRef] [PubMed]

]. We consider that the molecular structure of Pt nanoclusters and ligands affect their electronic properties, which might result in broadening of excitation/emission spectra and high QY. In order to elucidate their luminescence mechanism in detail, we will perform molecular orbital calculation in future.

5. Application to bioimaging

6. Conclusion and discussion

We synthesized green-emitting Pt8 nanoclusters (excitation: 460 nm, emission: 520 nm) that achieved a 28% QY. A key to this method is the pre-equilibration of the Pt ions with PAMAM dendrimers, which promotes the formation of coordination bonds between the Pt ions and the tertiary amines groups of the PAMAM dendrimers and the number of Pt ions trapped in the PAMAM dendrimers increases. Since the number of the tertiary amines groups are increased by using a higher generation of dendrimer, more Pt ions can be coordinated with the higher generation of PAMAM dendrimers. Therefore, synthesis of larger Pt nanoclusters opens the door to realize longer wavelength photoluminescence with such a higher generation of dendrimer. Protein such as bovine serum albmin (BSA) [6

6. J. Xie, Y. Zheng, and J. Y. Ying, “Protein-directed synthesis of highly fluorescent gold nanoclusters,” J. Am. Chem. Soc. 131(3), 888–889 (2009). [CrossRef] [PubMed]

] and ferritins [19

19. C. Sun, H. Yang, Y. Yuan, X. Tian, L. Wang, Y. Guo, L. Xu, J. Lei, N. Gao, G. J. Anderson, X.-J. Liang, C. Chen, Y. Zhao, and G. Nie, “Controlling assembly of paired gold clusters within apoferritin nanoreactor for in vivo kidney targeting and biomedical imaging,” J. Am. Chem. Soc. 133(22), 8617–8624 (2011). [CrossRef] [PubMed]

] can be also another candidate for a molecular template to prepare larger Pt nanoclusters, because they possess a nanometer-sized cavity and have already been utilized to synthesis red- and NIR-emitting Au nanoclusters. Using this method, we expect to extend the photoluminescent wavelength of Pt nanoclusters to NIR region, which is often preferable for in vivo imaging experiments. NIR-emission is especially suitable for whole body imaging application, disease diagnostics and clinical setting, where low background autofluorescence and deep penetration depth are desirable characteristics. We will also demonstrate the feasibility of Pt nanoclusters as a probe for high resolution microscopy such as two photon excitation microscopy and stimulated emission depletion (STED) microscopy in future.

Acknowledgments

This work is supported by the Osaka University Graduate School of Frontier Biosciences Global COE program. One of the authors, Y. Inouye, gratefully acknowledges financial support by a Grant-in-Aid for Scientific Research No. 24360026 from the Ministry of Education, Culture, Sports, Science and Technology. The authors thank Dr. P. Karagiannis for checking the manuscript, Dr. M. Murakami for ESI mass spectrometry, Dr. T. Fukumoto for furnace atomic absorption spectrophotometry, and Prof. K. Namba and Dr. T. Kato for allowing us to use their ultracentrifugation system.

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49.

T. Jin, D. K. Tiwari, S.-I. Tanaka, Y. Inouye, K. Yoshizawa, and T. M. Watanabe, “Antibody-protein A conjugated quantum dots for multiplexed imaging of surface receptors in living cells,” Mol. Biosyst. 6(11), 2325–2331 (2010). [CrossRef] [PubMed]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(170.2520) Medical optics and biotechnology : Fluorescence microscopy
(170.3880) Medical optics and biotechnology : Medical and biological imaging
(160.4236) Materials : Nanomaterials

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: September 17, 2012
Revised Manuscript: November 8, 2012
Manuscript Accepted: November 8, 2012
Published: January 3, 2013

Citation
Shin-ichi Tanaka, Koichi Aoki, Atsushi Muratsugu, Hidekazu Ishitobi, Takashi Jin, and Yasushi Inouye, "Synthesis of green-emitting Pt8 nanoclusters for biomedical imaging by pre-equilibrated Pt/PAMAM (G4-OH) and mild reduction," Opt. Mater. Express 3, 157-165 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-2-157


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