Synthesis of Yellow-Emitting Platinum Nanoclusters by Ligand Etching

Article pubs.acs.org/JPCC

Synthesis of Yellow-Emitting Platinum Nanoclusters by Ligand Etching Xavier Le Guével,† Vanessa Trouillet,‡ Christian Spies,§ Gregor Jung,§ and Marc Schneider*,† †

Pharmaceutical Nanotechnology, Saarland University, Saarbrücken, Germany Institute for Applied Materials, KIT-Campus North, Karlsruhe, Germany § Department of Biophysical Chemistry, Saarland University, Saarbrücken, Germany ‡

S Supporting Information *

ABSTRACT: New types of fluorescent platinum nanoclusters (λmax.= 570 nm) have been synthesized using glutathione for ligand etching. These nanoclusters are mainly in the oxidation state PtI (90%) exhibiting an intense fluorescence signal (QY∼17%) in the yellow region and lead to the formation of blue-emitting species for a long etching time process.

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blue-emitting platinum NCs stabilized by a dendrimer25 and by dimethylformamide.26 In this work, we report the synthesis of stable platinum NCs (PtGSHs) by ligand etching. Optical and spectroscopic investigations were performed during the etching process to determine the role of the temperature and the concentration of the reactants to the formation of these ultrasmall particles. Preliminary cyctotoxicity experiments in the presence of PtGSHs were also realized using A549 cells, a lung cancer cell line.

INTRODUCTION Noble metal nanoclusters (NCs; gold and silver) have garnered tremendous research interests in the past decade in the fields of catalysis,1 sensing,2 photonics,3 or biolabeling.4−6 These NCs exhibit unique properties compared to their homologues on the nanometric scale (size >2 nm) such as fluorescence,7 chirality,8 or magnetism.9 Due to their very small size (∼1 nm), free electrons are confined to the Fermi wavelength in the cluster conduction band. The strong quantum size effect expressed by a discrete size-dependence effect and the absence of the surface plasmon band leads to interesting molecule-like properties such as strong fluorescence, allowing one to obtain NCs with distinct optical features.10,11 In addition, the high photostability,12 the water solubility,13 a surface easy to functionalize,14 and the biocompatibility15 of gold and silver NCs make them promising as a new family of fluorescent contrast agents. Since Dickson et al. reported the preparation of highly fluorescent gold and silver NCs stabilized by dendrimers,7,13 many research groups extended the synthesis of fluorescent noble metal clusters in different matrices such as oligonucleotides,16 peptides,17 proteins,4,6 or polymers.18 These new labels will find many applications in the field of nanomedicine. For example, recent studies have shown the high sensitivity of fluorescent gold and silver NCs to detect DNA,19 metal ions (Hg2+, Cu2+),2,20 or pH in the cell membrane.21 Another interesting study presented the first example of in vivo fluorescence imaging in animals with ultrasmall bovine serum albumin (BSA)-stabilized Au NCs.22 Among the different routes to synthesize gold and silver NCs in soft conditions, thiol-ligand etching offers an elegant way to produce stable and monodisperse subnanometer particles.23,24 Platinum (another noble metal) particles are already developed for their catalytic activity, but it is only recently that two research groups demonstrated the molecular-like properties of © 2012 American Chemical Society

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EXPERIMENTAL METHODS Chemicals. All products were purchased from Sigma Aldrich (Germany) and used without further purification. Ultrapure water (18.2 MΩ) was used in all experiments. Synthesis of Pt NCs. Platinum NCs were synthesized following the procedure previously described by Kimura et al.27 Briefly, 5 mL of H2PtCl6 (3.38.10−3 M) was added to 0.5 mL of mercaptosuccinic acid (MSA; 2.37 × 10−2 M) under vigorous stirring. Then, 2.5 mL of NaBH4 (6.76.10−2 M) was added, and the solution was left under stirring for 1 h. The formation of small platinum nanoparticles (Pt NPs) was indicated by the change of color from pale yellow to a dark brown solution. Pt NPs were purified three times with MeOH before the etching process. For comparison, silver and gold nanoparticles (Ag NPs and Au NPs) protected with MSA were prepared using the methods already reported by Pradeep et al.23 PtGSHs were obtained by an etching process adding different amounts of glutathione (GSH) to Pt NPs at different weight ratios Pt:GSH from 1:2 to 1:20. For example, the Received: December 5, 2011 Revised: February 14, 2012 Published: February 21, 2012 6047

dx.doi.org/10.1021/jp211672t | J. Phys. Chem. C 2012, 116, 6047−6051

The Journal of Physical Chemistry C

Article

Figure 1. (a) Excitation (dashed line) and emission (solid line) spectra of the yellow-emitting NCs PtGSH after subtraction of GSH autofluorescence. (b) PtGSH under UV irradiation (λ = 366 nm). (c) Absorbance profile of platinum particles (Pt NPs), platinum clusters (PtGSH), and the etching ligand GSH. (d) Microscopy image (TEM) of PtGSH.

pulsed diode laser (LDH-P-C-470, Picoquant, Germany) emitting at 470 nm. The laser was driven by an oscillator module (PDL 808-MC Sepia, Picoquant, Germany) at a repetition rate of 2.5 MHz. Fluorescence was detected in 90° geometry after spectral filtering (HQ 585 ± 25, AHF Analysentechnik, Germany) by an avalanche photodiode (PDM 100ct SPAD, Micron Photon Devices, Italy). Electronic signals were recorded by a stand-alone TCSPC module (PicoHarp300, Picoquant, Germany), and the readout was synchronized by software (SymPhoTime, Picoquant, Germany). Analysis of the data was done by the software FluoFit (Picoquant, Germany). For all measurements, χ2 < 1.2 could be obtained by deconvolution fitting according to a biexponential decay. Absorbance spectra were recorded on a Lambda 35 scan UV−visible spectrophotometer (Perkin-Elmer) in transmission mode. The size and morphology of the PtGSHs were analyzed by dynamic light scattering (DLS; Zetasizer NS, Malvern). Transmission electron microscopy (TEM) and elemental analysis (EDX) were carried out with a JEOL model JEM 2010 instrument (JEOL GmbH, Eching, Germany) operated at an accelerating voltage of 120 kV.

sample Pt:GSH = 1:10 was prepared by adding 2 mg of Pt NPs and 20 mg of GSH in an Eppendorf tube. The powder dispersed instantaneously in 2 mL of water and gave a brownish color. Theoretically, the proportion of platinum in Pt NPs is estimated to be around 20 wt %, and the weight ratio Pt:GSH = 1:10 corresponds to a molar ratio of Pt:GSHmol = 1:31. Only values as weight ratios are reported in the discussion to avoid any confusion. Solutions were stirred in an Eppendorf thermomixer at 65 °C for different etching times from 1 to 14 days and then centrifuged (2000 rpm for 5 min) to remove the Pt NPs sedimented at the bottom of the Eppendorf tube. Following this, samples were purified by centrifugal precipitation with methanol and resuspended in deionized water. The synthesis of AuGSH and AgGSH obtained from Au NPs and Ag NPs, respectively, followed the same protocol as for PtGSH. The preparation of Au NPs and Ag NPs has been already described elsewhere.17,23 GSH without the presence of the metallic nanoparticles was also obtained under the same conditions and used as a control. Instrumentation. Fluorescence measurements were performed using a TECAN instrument, and lifetime measurements were measured by time-correlated single photon counting under magic-angle conditions. The excitation source was a 6048



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Furthermore, the quantum yield determined by comparison to Rhodamine 6G is evaluated to 17%, which is in the same order as the blue-emitting platinum NCs prepared by Tanaka et al. using dendrimers as a template.25 The UV−vis spectrum of PtGSH depicted in Figure 1c presents a shoulder at 380 nm, which could be attributed to the interband (sp−d) transition of the platinum−thiolate complex and a rather weak band located at 600 nm (see inset), which may correspond to the intraband (sp−sp) transition. The surface plasmon of Pt NPs located at 340 nm for platinum particles with size between 10 and 20 nm is not detected for PtGSHs, suggesting sizes of NCs less than 2 nm. In fact, electron microscopy (TEM) and EDX measurement (Figure S3) of PtGSHs confirmed the presence of small platinum particles, here called NCs, with size below 2 nm (Figure 1d). Compared to gold and silver NCs prepared under the same conditions, PtGSHs exhibit slower kinetics of etching and do not show any near-infrared (NIR) fluorescence emission (λem. >700 nm) attributed to large clusters.17,30,31 However, PtGSH fluoresces at two different wavelengths: 455 and 570 nm upon UV excitation (λexc.= 360 nm) (Figure S4). Figure 2a shows the

Chemical investigations were carried out using X-ray photoelectron spectroscopy (XPS) in a K-Alpha spectrometer (ThermoFisher Scientific, East Grinstead, UK) using a microfocused, monochromated Al Kα X-ray source (200 μm spot size). The K-Alpha charge compensation system was employed during analysis, using electrons of 8 eV and lowenergy argon ions, to prevent any localized charge build-up. The kinetic energy of the electrons was measured by a 180° hemispherical energy analyzer operated in the constant analyzer energy mode (CAE) at 50 eV pass energy for elemental spectra. All spectra were referenced to the C1s peak at 285.0 eV binding energy (C−H) and controlled by means of the well-known photoelectron peaks of Cu, Ag, and Au, respectively. Data acquisition and processing using the Thermo Avantage software is described elsewhere.28 The spectra were fitted with one or more Voigt profiles (BE uncertainty ≃ 0.2 eV), and Scofield sensitivity factors were applied for quantification.29 Cytotoxicity of PtGSH. The kit (Lonza, Verviers, Belgium) for the Vialight plus assay is based upon the bioluminescent measurement of adenosine triphosphate (ATP) that is present in all metabolically active cells. A549 cells were incubated with 100 μL of PtGSH at different concentrations (0.01−1 mg/mL in Krebs−Ringer buffer (KRB), pH 7.4) for 4 h in 96 well plates. Three replicates for each concentration were prepared. Negative control with KRB buffer and positive control with 1% Triton X were set up at the same time. After incubation, fifty microliters of cell lysis reagent was added to each well and left for 10 min at room temperature to extract ATP from cells. Then, 100 μL of ATP monitoring Reagent plus was added to generate the luminescent signal. Two minutes later, luminescence was checked on a microplate reader. These experiments were carried out two times.

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RESULTS AND DISCUSSION The preparation of the present fluorescent platinum NCs (PtGSHs) is based on peptide (GSH) ligand-etching of Pt NPs by controlling the following parameters: temperature, aspect ratio Pt:GSH, and aging time. GSH also presents the advantage of acting as a reducing agent and being suitable for biological applications due its biological relevance. Indeed, the presence of carboxyl and amino groups allows an easy functionalization after the cluster formation. The procedure is straightforward by just adding GSH to 1 mL of Pt NPs (2 mg/mL) in an Eppendorf tube, controlling the stirring time and the temperature. The temperature turned out to be a crucial parameter to initiate the ligand etching of Pt NPs, and, below 50 °C, we did not observe any fluorescence emission. The optimum value to obtain the brightest PtGSH was estimated to be around 65 °C. As mentioned by others authors for the synthesis of gold NCs stabilized by GSH, the aspect ratio metal:GSH and the pH are determinants to control the size and the oxidation state of the clusters and, consequently, their optical properties. PtGSH synthesized at different weight ratios Pt:GSH from 1:2 to 1:20 showed the most intense fluorescence at λem.= 570 nm after 6 days for Pt:GSH = 1:10 (Figure S1) and keeping the pH in acidic conditions (pH < 4). At the optimized conditions (Pt:GSH = 1:10; 65 °C; 6 days), PtGSHs exhibit a yellow fluorescence with a maximum excitation/emission wavelength located at 490/570 nm (Figure 1a,b). Considering the weak autofluorescence of GSH at these wavelengths (Figure S2), PtGSHs show an intense fluorescence intensity around 100 times higher than the background.

Figure 2. Fluorescence emission spectra of PtGSH at different etching times upon excitation at λ = 480 nm (a) and λ = 360 nm (b). Data show the growth of a yellow-emitting species up to 6 days following by the emergence of a blue-emitting species for a long etching process.

increase of the fluorescence peak at 570 nm with an aging time up to 6 days (λexc. = 480 nm), and then the fluorescence intensity starts to decrease slowly, probably due to the continuous etching process. After the formation of PtGSH, the samples kept at room temperature do not show any dramatic decrease of the yellow fluorescence over a period of 1 month at room temperature, indicating a high stability of 6049



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the value cited above (τ2 = 2.01 ns) for Pt NCs is due to the fitting model using a fixed value of τ1 for all samples. Moreover, values (Table in Figure S6) of the shortest component τ1 indicate a higher abundance of PtGSH, suggesting a higher monodispersity compared to AuGSH and AgGSH. Thus, these results confirm the influence of the nature of the metal on the fluorescence energy transfer of clusters−clusters and clusters− ligands.32 Chemical investigation of the PtGSH sample was realized by XPS after drying the solution on a silicon wafer (Figure 4). The

PtGSH even in presence of residual free GSH. The evolution of the excitation peak at 480 nm (Figure S5) shows the same trend as the emission spectra with the etching time, confirming the presence of a single fluorescent species. The blue-green emission might be attributed to the fluorescence of decomposed products such as oxidized GSH and to the presence of smaller Pt NCs emitting at 420−470 nm previously reported by Tanaka25 and Kawasaki.26 The increase of the bluegreen fluorescence after an etching time of 6 days and the redshift from 430 to 455 nm (Figure 2b) could be related to the etching of Pt NPs and of large Pt NCs by GSH leading to the formation of small Pt NCs. In addition, the excitation band at lower wavelength is increasing constantly with the etching process and therefore confirms the formation of blue-emitting NCs. Indeed, it is known that permanent reassembly of metal− thiolate complexes occurs over time at elevated temperature. The yellow fluorescence emission band at longer wavelength detected upon UV irradiation could be due to the excitation of higher electronic state of the NCs. An energy transfer between blue-emitting PtGSH to the yellow-emitting PtGSH will be considered as unlikely, taking account of the long distance (>10 nm) between each cluster in solution. The fluorescence lifetime decay of PtGSH with the ratio Pt:GSH = 1:10 shows two short components at 1.04 ns (85%) and 2.01 ns (15%) using a global fitting upon λexc = 470 nm. In the case of PtGSH with the ratio 1:4, we observe an increase of the second lifetime component τ2 from 2.01 to 3.41 ns using the same experimental condition (Figure 3). One explanation

Figure 4. XPS spectra. (a) Pt 4f7/2 of PtGSH shows two peaks, one peak at 72.4 eV (green curve) and the other at 71.6 eV (pink curve), which are attributed to Pt(I) (90%) and Pt (0) (10%), respectively. (b) S 2p3/2 confirms the presence of platinum bound to sulfur (blue curve) with a peak at 163.3 eV.

Pt 4f7/2 peak could be deconvoluted in two distinct components: one located at 72.4 eV corresponding to the Pt−S charge transfer in the thiol-coated platinum particles,33 and another one at 71.5 eV, which could be attributed to the reduced platinum Pt(0).34 The atomic ratio between Pt(I) and Pt(0) is evaluated to 10:1. The presence of Pt−S at this binding energy could be also seen as an intermediate species of the Pt NCs core surrounded by Pt(I) on the surface. This hypothesis was previously suggested by Tsukuda et al.30 for thiolateprotected gold NCs and can be extended to this nanomaterial considering the similarity in nature of the metal and the synthesis conditions used to obtain the NCs. We also noticed the absence of a peak at 74.4 eV, indicating the absence of the oxidized precursor Pt(II)Cl42− in the nanomaterial. The binding energy of sulfur S 2p3/2 located at 163.3 eV could be assigned to the binding of the sulfur atom to the platinum taking account of the previous result obtained by Dablemont et al. on thiolated platinum NPs.33 The infrared measurement of Pt NPs, GSH, and PtGSH (Figure S7) indicates the presence of the carboxyl group at 1700 cm−1 and the slight loss of S−H stretching at 2520 cm−1 for PtGSH after purification in methanol. This confirms the presence of GSH covalently bound to Pt NCs. The absence of the characteristic features of the MSA present on Pt NPs, with −CH2− scissoring at 1355 cm−1, and the C−C stretching mode at 1050 cm−1 on the PtGSH pattern suggest the complete removal of the MSA group from the platinum clusters.23 Matrix-assisted laser desorption−ionization/time-of-flight spectroscopy (MALDI/ ToF) experiments were intended to elucidate the clusters composition. Despite the use of several matrices such as α-

Figure 3. Lifetime decay of PtGSH prepared at two different weight ratios: Pt:GSH = 1:4 (black line) and 1:10 (red line). Samples were measured upon an excitation at 470 nm and fluorescence was collected at 585 ± 25 nm. Lifetime components for PtGSH 1−4: τ1 = 1.02 ns (85%), τ2 = 3.4 ns (15%); and for PtGSH 1−10: τ1 = 1.04 ns (85%), τ2 = 2.01 (15%).

for this trend could be attributed to the concentration of GSH surrounding each Pt NCs leading to a photoinduced electron transfer (PET) between the metal and the oxidized ligand. Another hypothesis could also be related to the distribution of Pt NCs of different sizes exhibiting distinct lifetime decay. However, the mechanism is certainly more complex and will require further investigation. Lifetime decay of PtGSH was compared to AuGSH and AgGSH prepared with the same aspect ratio 1:10 and under the same condition. PtGSH presents the same short component τ1 around 1 ns but a shorter second component τ2 at 2.66 ns instead of 3.18 and 3.41 ns for gold and silver NCs, respectively (Figure S6). The slight increase of the long component τ2 at 2.66 ns compared to 6050



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cyano-4-hydroxycinnamic acid (CHCA), indole acrylic acid (IAA) or 2,5-dihydroxylbenzoic acid (DHB), measurements recorded in positive mode failed due to the absence of the three intense platinum isotopes on the mass spectroscopy patterns. Desorbtion−ionization is a highly complex physicochemical process with possible gas fragmentation process, especially if noncovalent bound complexes are involved. This limitation still remains an issue to confirm the magic-number theories of such metal clusters. Interestingly, PtGSH presents no cytotoxicity in the concentration range 1−0.01 mg/mL using the standard vialight assay in the presence of A549 lung tumor cells for 4 h (Figure S8). Compared to platinum complexes such as cis-platin, known for their high toxicity,35 PtGSH has a bigger size and a surface protected by a tripeptide covalently bond to the metal. This might prevent reaction with enzyme and interaction with DNA.

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CONCLUSIONS In summary, we reported the synthesis of yellow-emitting platinum NCs by interfacial synthesis. Compared to gold and silver NCs protected by GSH, platinum NCs are produced with slower kinetics of etching without the formation of red-NIR emitting species. These clusters are partially bound to GSH and mainly in the oxidation state I (PtI = 90%). A longer etching process at high temperature involves the formation of blueemitting species accompanied by the degradation of the yellowemitting species. Cytotoxicity experiments as preliminary biological tests show the cell viability using a high amount of the platinum NCs (1 mg/mL). We hope this fresh data will stimulate the research on platinum clusters for fluorescence detection.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Address: Pharmaceutical Nanotechnology, Saarland University, Campus A4 1, D-66123 Saarbrücken. Phone: 0049 681 302-2438. Fax 0049 681 302-4677. E-mail: Marc.Schneider@ mx.uni-saarland.de. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.L.G. thanks the Andalusian Regional Ministry of Health, the “Andalusian Initiative for Advanced Therapies” − Fundacion Progreso y Salud, for supporting the research activity. Thanks to Chris Schild for the TEM measurement. Petra König is thanked for help with the cytotoxicity evaluation, and Leon Muijs is thanked for the CLSM imaging.

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REFERENCES

(1) Crooks, R. M.; Zhao, M. Q.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181−190. (2) Wei, H.; Wang, Z. D.; Yang, L. M.; Tian, S. L.; Hou, C. J.; Lu, Y. Analyst 2010, 135, 1406−1410. (3) Geddes, C. D.; Parfenov, A.; Gryczynski, I.; Lakowicz, J. R. J. Phys. Chem. B 2003, 107, 9989−9993. (4) Le Guevel, X.; Daum, N.; Schneider, M. Nanotechnology 2011, 22, 275103. 6051


Synthesis of Yellow-Emitting Platinum Nanoclusters by Ligand Etching

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