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Aqueous synthesis of near-infrared highly fluorescent platinum nanoclusters
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 215601 (http://iopscience.iop.org/0957-4484/26/21/215601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.117.10.200 This content was downloaded on 11/05/2015 at 10:42
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 215601 (7pp)
doi:10.1088/0957-4484/26/21/215601
Aqueous synthesis of near-infrared highly fluorescent platinum nanoclusters Jenifer García Fernández, Laura Trapiella-Alfonso, José M Costa-Fernández, Rosario Pereiro and Alfredo Sanz-Medel Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, E-33006 Oviedo, Spain E-mail: [email protected] and [email protected] Received 19 January 2015, revised 22 March 2015 Accepted for publication 8 April 2015 Published 6 May 2015 Abstract
A one-step synthesis of near infrared fluorescent platinum nanoclusters (PtNCs) in aqueous medium is described. The proposed optimized procedure for PtNC synthesis is rather simple, fast and it is based on the direct metal reduction with NaBH4. Bidentated thiol ligands (lipoic acid) were selected as nanoclusters stabilizers in water media. The structural characterization revealed attractive features of the PtNCs, including small size, high water solubility, near-infrared luminescence centered at 680 nm, long-term stability and the highest quantum yield in water reported so far (47%) for PtNCs. Moreover, their stability in different pH media and an ionic strength of 0.2 M NaCl was studied and no significant changes in fluorescence emission were detected. In brief, they offer a new type of fluorescent noble metal nanoprobe with a great potential to be applied in several fields, including biolabeling and imaging experiments. S Online supplementary data available from stacks.iop.org/NANO/26/215601/mmedia Keywords: platinum nanoclusters, aqueous synthesis, fluorescence (Some figures may appear in colour only in the online journal) with functional groups like –OCH3, –COOH, –NH2, have been used as protecting ligands for water-monodisperse metal NCs synthesis [11–14]. Taking advantage of bidentate thiolate linkers, fluorescent gold and silver NCs synthesized with dihydrolipoic acid have revealed remarkable optical properties, including relatively high and stable photoluminescence and long quantum yields (QYs) in a wide pH range, which makes them very interesting candidates for biological investigations [14]. In this way, water soluble synthesis of fluorescent metal NCs can be successfully achieved, opening the way to many possible applications of such small and biocompatible nanolabels [15, 16]. Recent advances have enabled facile synthesis of fluorescent metal NCs with tunable emission colors, establishing them as a new class of ultrasmall, biocompatible fluorophores for applications as biological labels. So far, synthetic efforts have been mainly focused on two noble metals: Au and Ag [17–19]. Although great progress has been achieved, particularly for gold and silver NCs, challenges for further advancement of fluorescent metal NCs are still ahead of us.
1. Introduction Research on nanomaterials has experienced a rapid growth over the last decades due to the unique physical and chemical properties that many materials exhibit when reduced to the nanoscale. Small noble metal nanoclusters (NCs) of Au and Ag consisting of isolated particles of a few nanometers in diameter have become promising alternatives for important current applications like sensing [1], catalysis [2], photonics [3] and biolabelling [4]. It has been demonstrated that such nanostructures exhibit interesting and valuable optoelectronic characteristics [5]. There are different synthetic routes to obtain fluorescent metal NCs [6–8]. Among them, the most common approach for synthesis of NCs to be used as fluorescent labels relies on the use of capping agents, such as thiol ligands. The strength of the metal-S bond, as well as the small hydrodynamic diameters involved and the modifiable surface properties [9, 10], guarantee high stability of the NC. In this context, dihydrolipoic acid and its polyethylene glycol derivatives 0957-4484/15/215601+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Fluorescent metal NC can be combined with other detection modalities, such as elemental mass spectrometry, allowing multiplex detection of biomarkers by labeling selected antibodies to NCs of different nature followed by their sensitive elemental detection. Availability of different nature metal NCs is needed in the development of such multiplexing assays. In this context, recent work paid attention to platinum (another noble metal) as a possible bioimaging probe [7, 20, 21]. Moreover, water-soluble platinum nanoclusters (PtNCs) could be less cytotoxic and emit a brighter fluorescence than other fluorescent nanomaterials, such as gold clusters [20]. In this context, controllable synthesis of PtNCs with high quality is of paramount importance and highly desirable. Typically, the strategies of PtNCs synthesis were developed in non-polar media. In such cases, NCs must be transferred from the organic environment into an aqueous medium to allow biological applications. This can be achieved by ligand exchange; unfortunately, an incomplete exchange of the original surface ligands in the NCs can lead to an uncontrolled aggregation of the clusters [9] and this problem constitutes the main challenge to obtain water-soluble NCs from such synthetic route. Moreover, the QY values obtained for PtNCs synthesized up to now (ranging from 4 to 28%) are low as compared to other metal NCs produced in organic media. Additionally, to our knowledge, the work published so far referring to platinum as fluorescent label reported UV or visible luminescence emissions in the range of 350–570 nm [7, 10, 18–21]. In order to carry out in vivo imaging experiments, disease diagnostics, or even image-guided therapy, photoluminescent emission should be closer to the near infrared region (NIR) because of the low background autofluorescence and less harmful biological effects found in this spectral region. Finally, as pointed out recently [22], the reported PtNCs syntheses consist of rather lengthy and tedious procedures. Considering all the above facts, new routes of watersoluble PtNCs synthesis with high water solubility and photoluminescence emissions in the NIR range are of present interest. It should be also noted that research on different novel water compatible metal NCs is highly demanded, to be used as tags for development of multiplexing applications by optical and also by mass spectrometry. In this work we propose a synthetic route based on the simple reduction of a mixture containing the lipoic acid (LA) ligand and the platinum precursor with sodium borohydride directly in aqueous medium, as an one-step route to obtain water soluble PtNCs of interesting photophysical properties.
rhodamine B were purchased from Sigma-Aldrich (Milwaukee, USA). 98% LA was obtained from Acros OrganicsThermo Fisher Scientific (Geel, Belgium). NaOH and NaCl salts were purchased from Prolabo (Leuven, Belgium). The equipment necessary to carry out the PtNCs synthesis was a two-necked round-bottomed flask, a reflux system, amber vials and a magnetic stirrer. Poly-L-lysine coated glass slides from Sigma-Aldrich (Milwaukee, USA) were used in the laser confocal microscopy experiments. Ethyl-3-(dimethylaminopropyl) carbodiimide (EDC), used to immobilize the synthesized PtNCs onto the poly-L-lysine glass slide surface, was purchased from Fluka (Sigma-Aldrich, Milwaukee, USA) and stored at −18 °C into the freezer. 2.2. Synthesis of PtNCs
In the here optimized synthesis of PtNCs, aqueous stock solutions of platinum precursor (H2Cl6Pt, with a concentration of 5 mM) and aqueous solutions of 25 mM LA ligand were prepared in ultrapure water (the addition of 1 mL of 1 M NaOH solution was needed for solubilization). An aqueous NaBH4 solution with a concentration of 625 mM was freshly prepared by dissolving 24 mg of NaBH4 in 500 μL of ultrapure water. In order to carry out the synthesis, 2000 μL of 5 mM H2Cl6Pt (aq) and 2000 μL of 25 mM LA were put into an amber vial under magnetic stirring conditions. After 20 min to ensure the homogeneity of the mixture, 400 μL of 625 mM NaBH4 (aq) was incorporated, being the final precursors molar ratio 1:5:25 (Pt:ligand:reducing agent). Reaction was left to proceed during 6 h under room temperature magnetic stirring. All the process was carried out under dark conditions. Once the reaction is complete, i.e. when no changes are detected in the observed fluorescence emission spectrum, the excess of ligand is removed by ultrafiltration using a 10 KDa membrane filter (5000 rpm × 10 min × 3 cycles). The yellow colored solution containing the sought PtNCs remained on the filter and was re-suspended in slightly alkaline water. Then it was stored protected from visible light. The strong yellow color characteristic of the resulting NCs solution could be an emission limitation factor due to possible inner filter associated problems. To get rid of this effect, different dilution factors were assayed in order to obtain the maximum fluorescence emission signal. Experiments show that a final dilution factor of 1:100 (NCs:water, v/v) provides highest fluorescence intensity at 680 nm. 2.3. Optical and structural characterization of PtNCs
Optical characterizations of the synthesized PtNCs were accomplished by examining the corresponding absorbance and fluorescence spectra. Fluorescent measurements were carried out using a Cary Eclipse fluorescence spectrophotometer (Varian, USA) and optical absorption spectra were recorded with a Genesys 10S UV–vis spectrophotometer (Thermo Scientific, USA). Aiming at a proper structural characterization of the synthesized PtNCs, high-resolution transmission electron
2. Experimental section 2.1. Reagents and materials
All chemicals were of analytical grade and used as received. Deionized ultrapure water (resistivity 18.2 MΩ cm−1) was used throughout the work. Chloroplatinic acid solution 8% wt in H2O (H2Cl6Pt), sodium borohydride (NaBH4) and 2
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microscopy (HR-TEM) and energy-dispersive x-ray microanalysis (EDX) techniques were used to investigate their size, shape and chemical composition. HR-TEM images were performed on a JEOL JEM-2100F (Tokyo, Japan), offering resolutions of 2.3 Å between points and 1.0 Å between lines. That instrument is equipped with a high-resolution CCD camera (Gatan) and an EDX microanalyzer enabling the identification of the elements present in the scrutinized sample. 2.4. Confocal microscopy studies
To assess the PtNCs potential as imaging nanoprobes the obtained material was immobilized onto the surface of polyL-lysine coated glass slide by EDC chemistry. After the immobilization process the glass slide was examined by Laser confocal microscopy adjusting the measurement parameters. A confocal microscope Leica TCS-SP2-AOBS with a multilaser configuration was employed for this purpose. Thus, an objective 63x was used to focus the region of the sample to be explored, and then an argon laser emitting at 488 nm and working at 30% of power was selected for the excitation of the sample. The emission from the sample examined was collected in a wavelength range between 550 and 750 nm.
Figure 1. Fluorescence emission spectra of PtNCs synthesis by
magnetic stirring under room temperature conditions. The maximum value of the 680 nm centered peak is achieved at 6 h of reaction. Stability of the signal after 24 h makes this synthesis strategy more competitive than the one involving reflux conditions.
3 to 11) and at higher ionic strength media (0.2 M NaCl) were carried out.
2.5. QY measurement
3. Results and discussion
The QY of PtNCs in water was determined using a relative method, based on the standard rhodamine B (QY = 0.68 in ethanol), and was calculated according to the following equation:
(
3.1. Optimization of the PtNCs synthetic route
In order to optimize the PtNCs synthesis, the Pt to aqueous stabilizer ligand molar ratio constitutes a parameter to be controlled. Different Pt:LA molar ratios (1:1, 1:5 and 1:10) were eventually assayed. The optical properties of the products obtained are collected in figure S-1 of the supporting information. The highest fluorescence emission and the best signal to noise ratio (with a maximum emission wavelength centered at 680 nm) were observed for 1:5 (Pt:LA) molar ratio. Moreover, as can be seen in figure S-1 of the supplementary information, a slight shift on the emission maximum along with a broadening of the fluorescence emission spectra has been observed when changing the Pt-to-ligand ratios used in the synthesis. This is most likely a result of a change on the obtained PtNCs that occurs when varying the Pt:ligand molar ratios used during the synthesis. The effect of reaction time and temperature in the synthesis was also checked. Reaction under room temperature stirring and under reflux conditions was monitored every hour. Under reflux conditions a sudden decrease of the fluorescence emission peak was apparent after more than four hours (see figure S-2 of supporting information); it is likely that too long time under reflux gives rise to a degradation of the PtNCs. When room temperature stirring was carried out, maximum signals at 680 nm were obtained after 6 h of reaction. As can be seen in figure 1, for reaction times higher than 6 h at room temperature, the analytical emission at 680 nm kept constant with time. This room temperature strategy brings about NCs stability, as well as synthesis simplicity and security.
)2
QYPtNC = QYST ( IPtNC / IST )( AST / APtNC ) ηPtNC / ηST ,
where I, A and η are the integrated emission intensity, absorbance at excitation wavelength and refractive index of the solvent, respectively. While PtNCs are dissolved in aqueous media η was 1.333, the rhodamine B standard was dissolved in ethanol (η value of 1.361). Rhodamine B was selected as standard dye for the determination of the PtNCs QYs considering that it has a well-known QY value and it emits in a similar spectral region to the PtNCs. 2.6. Stability studies
In order to obtain a further characterization of the synthesized PtNCs, some important parameters affecting the eventual applicability of the NCs for further bioanalytical applications, such as photostability, stability under different media conditions (pH, ionic strength) and long-term storage were evaluated. Photostability of the synthesized PtNCs was tested by monitoring their photobleaching time profiles. Moreover, aliquots exposed to daylight were periodically measured along to successive days and results observed with time were collected. The effect of pH and NCs stability in a saline medium were then assessed. Thus, fluorescence intensity measurements of PtNCs at different pH values (ranging from 3
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Figure 2. Optical characterization of the synthesized PtNCs. (A) Absorbance spectra of PtNCs (black line) and blank of reaction (gray line).
(B) Fluorescence spectra of PtNCs using 430 nm (line in black) and 480 nm (dotted line) as excitation wavelengths; gray line is the fluorescence spectrum from the blank of reaction.
Pt2+ centers eventually present within the NCs. In any case, owing to the limited information available [25], much effort is required to further elucidate the luminescent mechanism from these PtNCs. Aiming at a proper structural characterization of the synthesized PtNCs, HR-TEM and EDX techniques were used to investigate their size, shape and chemical composition. HR-TEM images of PtNCs obtained after 6 h of reaction are depicted in figure 3: a population of mostly isolated nanoparticles, with a spherical shape having an average value of 2.8 nm, is observed (the mean diameter size was calculated from appropriate image analysis of the spots). EDX analysis confirmed the elemental composition of the NCs (figure S-4 in the supporting information). The characteristic bands and pattern of Pt metal as well as the presence of S, O and C (from the protecting LA ligand) are apparent in the EDX spectrum, as well as an indication of the presence of Pt–S bonds.
3.2. Optical and structural characterization
Optical characterization of the synthesized PtNCs was accomplished by examining the corresponding absorbance and fluorescence spectra. The absorption UV–vis spectrum showed no characteristic absorption band in the whole visible region: just a continuous absorbance decreasing from 350 until 700 nm is noticed (see figure 2(A)). As can be observed in figure 2(B), the fluorescence emission spectra, using two different excitation wavelengths (430 and 480 nm) produced a well-defined emission peak centered at 680 nm, in both cases (the excitation spectrum measured while monitoring the emission at 680 nm is collected in figure S-3 of the supporting information). From figure 2(B) it can be seen that a rather similar sensitivity was achieved when using both excitation wavelengths. However, when exciting at 480 nm, an additional emission band appears at 575 nm (also observed for the blank sample, and therefore no corresponding to PtNCs) which could interfere further fluorimetric applications. The best excitation/emission pair for eventual PtNCs application turned out to be 430/680 nm. Thus, the selected emission wavelength for analytical purposes lies into the NIR region. This is an important finding since there is no precedent of other luminescent PtNCs with these favorable spectral features (see table 1 for comparison to other PtNCs). Usually, metal nanoparticles absorb light strongly, but they do not or only show weak luminescence. After reduction of the size of metal nanoparticles to the size of metal NCs, the properties of particles disappear and the energy bands turn into more or less discrete energy levels. Thus, the collective oscillation of electrons is obstructed and metal NCs do not give rise to surface plasmon resonance effect. However, they will follow quantum mechanical rules. Through interaction with light and then electronic transitions between the energy levels, they show bright luminescence [23]. The observed photoluminescence spectra of the PtNCs are in agreement with the well-described luminescence properties of squareplanar Pt2+ compounds [24]. Therefore, based on such findings it could be possible that the photoluminescence property of the PtNCs here synthesized originates from square-planar
3.3. Quantum yield
The QY of the synthesized PtNCs was evaluated according to a relative method, based on the standard rhodamine B, and a value of 47% was obtained. This value is significantly higher than the highest one given for PtNCs in the literature so far (see table 1 for comparison to other PtNCs). Moreover, this value competes with conventional fluorophores such as quinine sulfate (58%), cresyl violete (53%) or cyanine dyes like Cy5 (28%). 3.4. Stability tests
To attain a further characterization of the synthesized PtNCs, parameters affecting the eventual analytical applicability of the NCs, such as photostability, stability under different aqueous media conditions (pH, ionic strength) and long-term storage were evaluated. NCs photostability was first assessed. Photostability of the synthesized PtNCs was tested by monitoring their photobleaching time profiles. A rather small variation of the registered fluorescent emission at 680 nm (less than 10% attenuation) was detected after 2 h of 4
Nanotechnology 26 (2015) 215601
Table 1. Research papers available up to now concerning different PtNCs synthesis methodologies.
5
Ref.
Water solubility
Synthesis method
QY
λexc/λem
NCs size
Stability
[25] [10] [20] [21] [28] [7]
No Yes No No Yes Yes
Organic media Organic → aqueous phase transfers Organic media (2 weeks) Organic media (6 days) Chicken egg aqueous media Organic media (2 weeks)
Not provided 4.6% 18% 17% Not provided 28%
350/484 500/544 410/436 380/470 490/570 280/350 460/520
1 nm 2 nm Not provided 2 nm 2.5 nm Not provided
Up to 6 months in dark conditions Up to 6 months at 4 °C Not provided Up to 1 month without changes Not provided Not provided
J García et al
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emission was rather stable for more than three months. Fluorescence intensity measurements of PtNCs at different pH values (ranging from 3 to 11) and at a rather high ionic strength media (0.2 M NaCl) were also carried out. Results showed that PtNCs fluorescence emission was neither affected by the pH in the interval measured nor by the presence of 0.2 M NaCl (see figure S-5 of the supporting information).
3.5. Fluorescence imaging measurements
Finally, to prove the potential of the here developed PtNCs as possible fluorescent labels for imaging purposes, a blank of reaction and an aliquot of the synthesized PtNCs were immobilized onto the surface of a poly-L-lysine glass slide and were measured by laser confocal microscopy. Optical properties of functionalized colloidal fluorescent nanoparticles may be reduced, or even disappear, when they are placed in a solid media (a critical step in the development of sensing and immunosensing applications). Therefore, to determine the capabilities of the here-synthesized PtNCs for their eventual use as labels in bioimaging experiments, PtNCs were immobilized as described in a microscopy glass slide and luminescence registered using confocal imaging/fluorescence microscopy. Results proved that the fluorescent NCs maintain their optical and spectroscopic properties after immobilization in a solid surface, and therefore could be used as highly fluorescent labels in bioanalytical applications. Moreover, as it is shown in figure 4, the PtNCs luminescence can be easily distinguished from the blank of reaction.
Figure 3. Structural characterization of PtNCs. HR-TEM images of
PtNCs obtained after 6 h of reaction; an average value of 2.8 nm was obtained as mean diameter size after the image analysis. Cubic crystalline structure is well defined in the image of the upper corner. Moreover, due to the fact of each ring corresponds to scattering from sets of hkl planes with the same 2θ angle, planes {111}, {200} and {220} can be observed in the bottom right image.
continuous exposition of a PtNCs colloidal solution aliquot to UV light (figure S-5 in the supporting information). Thus, they are higher resistant to UV light than, for example, silver NCs that are strongly degraded at a higher rate [26]. Additionally, the bleaching time of conventional Alexa Fluor 488 organic molecules is less that some few seconds, much shorter than the time of the PtNCs under similar conditions, indicating that PtNCs are at least 1000-fold more photostable over the organic fluorophores [27]. Moreover, for a proper comparison, photostability of the standard fluorophore rhodamine B has been tested employing the same spectrofluorimeter used for characterization of the PtNCs. Under the optimized experimental conditions a progressive drift on the signal of rhodamine B was observed (a 15% reduction of the original luminescence value was registered after 90 min of continuous illumination, see figure S-6 in the supporting information). Moreover, aliquots exposed to daylight were periodically measured along successive days (figure S-5 in the supporting information). Results showed that photoluminescence from PtNCs was unaltered along the first exposition day, while a gradual decrease on the luminescence is apparent after several days of exposition to daylight (day 2: 87%, day 3: 79% of initial signal). This does not represent a significant limitation in the final expected applications of PtNCs: if they are stored in the dark, the emission intensity remains unaltered. Long-term stability has been also evaluated for aliquots of the colloidal solutions stored in the dark (figure S-5 in the supporting information). Results demonstrated that the
4. Conclusions In conclusion, a novel strategy for synthesizing water-soluble PtNCs based on the direct metal reduction with NaBH4 in the presence of bidentated thiol ligands has been developed. This route (a one-step synthesis, carried out directly in water media, just by mixing appropriate amounts of the NCs precursors and stirring the mixture at room temperature) offers a simpler approach for PtNCs synthesis than those previously reported [7, 10, 20, 21, 25, 28], see table 1, and avoids environmentally undesirable reagents (e.g. organic solvents). The here reported synthesis process enables the production of PtNCs exhibiting NIR fluorescence for the first time. These new nanostructures can become useful photoluminescent labels, as the QY achieved (47%) is the highest registered in the literature for PtNCs so far. On the other hand, pH, saline and stability studies indicate that the PtNCs here synthesized could be advantageously employed as photoluminescent nanoprobes in many fields due to its aqueous solubility and stability. In brief, we believe that the water-soluble PtNCs synthesized in this work are highly competitive, exhibiting advantageous optoelectronic properties which open the way to novel bianalytical applications. 6
Nanotechnology 26 (2015) 215601
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Figure 4. Confocal microscopy images using an argon laser at 488 nm for the excitation. (A) Fluorescent image from blank of reaction. (B) Fluorescent image from PtNCs.
Acknowledgments
[12] Fernández-Ujados M, Trapiella-Alfonso L, Costa J M, Pereiro R and Sanz-Medel A 2013 Nanotechnology 24 495601 [13] Shang L, Azadfar N, Stockmar F, Send W, Trouillet V, Bruns M, Gerthsen D and Nienhaus G U 2011 Small 7 2614 [14] Le Guével X, Tagi O, Rodríguez C E, Trouillet V, Pernia Leale M and Hildebrandt N 2014 Nanoscale 6 8091 [15] Shang L, Dörlich R M, Brandholt S, Schneider R, Trouillet V, Bruns M, Gerthsen D and Nienhaus G U 2011 Nanoscale 3 2009 [16] Shang L, Dongb S and Nienhaus G U 2011 Nano Today 6 401 [17] Kumar S, Bolan M D and Bigioni T P 2010 J. Am. Chem. Soc. 132 13141 [18] Zheng J, Zhou C, Yu M and Liu J 2012 Nanoscale 4 4073 [19] Muhammed M A H, Ramesh S, Sinha S S, Pal S K and Pradeep T 2008 Nano Res. 1 333 [20] Tanaka S, Miyazaki J, Tiwari D K, Jin T and Inouye Y 2011 Angew. Chem. Int. Ed. 50 431 [21] Le Guével X, Trouillet V, Spies C, Jung G and Schneider M 2012 Phys. Chem. C 116 6047 [22] Le Guével X 2014 IEEE J. Sel. Top. Quantum Electron. 20 6801312 [23] Zhanga L and Wang E 2014 Nano Today 9 132 [24] Trikalitis P N, Rangan K K and Kanatzidis M G 2002 J. Am. Chem. Soc. 124 2604 [25] Kawasaki H, Yamamoto H, Fujimori H, Arakawa R, Inada M and Iwasaki Y 2010 Chem. Commun. 46 3759 [26] Trapiella-Alfonso L, Menéndez-Miranda M, Costa-Fernández J M, Pereiro R and Sanz-Medel A 2014 Mater. Res. Express 1 015039 [27] Zhang J, Fu Y, Conroy C V, Tang Z, Li G, Zhao R Y and Wang G 2012 J. Phys. Chem. C 116 26561 [28] Min L, Yang D P, Wang X, Lu J and Cui D 2013 Nano Res. Lett. 8 182
The authors want to thank to Dr Zakariae Amghouz from the Unit of Electron Microscopy of the University of Oviedo for the HR-TEM measurements. Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO-13-CTQ2013-49032-C2-1-R), Spanish Ministry of Education and Science (CTQ2010-16636 and MAT201020921-C02-01) and the European FEDER program is also gratefully acknowledged.
References [1] Mua X, Qi L, Dong P, Qiao J, Hou J, Nie Z and Ma H 2013 Biosens. Bioelectron. 49 249 [2] Jia X, Yang X, Li J, Li D and Wang E 2014 Chem. Commun. 50 237 [3] Guo S and Wang E 2011 Nano Today 6 240 [4] Le Guével X, Daum N and Schneider M 2011 Nanotechnology 22 275103 [5] Yuan X, Setyawati M I, Tan A S, Nam Ong C, Tai Leong D and Xie J 2013 NPG Asia Mater. 5 e39 [6] Liu J M, Chen J T and Yan X P 2013 Anal. Chem. 85 3238 [7] Tanaka S, Aoki K, Muratsugu A, Ishitobi H, Jin T and Inouye Y 2013 Opt. Mater. Express 3 157 [8] Schaeffer N et al 2008 Chem. Commun. 34 3986 [9] Schmid G (ed) Nanoparticles: From Theory to Application 2nd edn [10] Yuan X, Luo Z, Zhang Q, Zhang X, Zheng Y, Lee J Y and Xie J 2011 ACS Nano 5 8800 [11] Muhammed M A H, Aldeek F, Palui G, Trapiella-Alfonso L and Mattoussi H 2012 ACS Nano 6 8950
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Aqueous synthesis of near-infrared highly fluorescent platinum nanoclusters
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 215601 (http://iopscience.iop.org/0957-4484/26/21/215601) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 134.117.10.200 This content was downloaded on 11/05/2015 at 10:42
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 215601 (7pp)
doi:10.1088/0957-4484/26/21/215601
Aqueous synthesis of near-infrared highly fluorescent platinum nanoclusters Jenifer García Fernández, Laura Trapiella-Alfonso, José M Costa-Fernández, Rosario Pereiro and Alfredo Sanz-Medel Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, E-33006 Oviedo, Spain E-mail: [email protected] and [email protected] Received 19 January 2015, revised 22 March 2015 Accepted for publication 8 April 2015 Published 6 May 2015 Abstract
A one-step synthesis of near infrared fluorescent platinum nanoclusters (PtNCs) in aqueous medium is described. The proposed optimized procedure for PtNC synthesis is rather simple, fast and it is based on the direct metal reduction with NaBH4. Bidentated thiol ligands (lipoic acid) were selected as nanoclusters stabilizers in water media. The structural characterization revealed attractive features of the PtNCs, including small size, high water solubility, near-infrared luminescence centered at 680 nm, long-term stability and the highest quantum yield in water reported so far (47%) for PtNCs. Moreover, their stability in different pH media and an ionic strength of 0.2 M NaCl was studied and no significant changes in fluorescence emission were detected. In brief, they offer a new type of fluorescent noble metal nanoprobe with a great potential to be applied in several fields, including biolabeling and imaging experiments. S Online supplementary data available from stacks.iop.org/NANO/26/215601/mmedia Keywords: platinum nanoclusters, aqueous synthesis, fluorescence (Some figures may appear in colour only in the online journal) with functional groups like –OCH3, –COOH, –NH2, have been used as protecting ligands for water-monodisperse metal NCs synthesis [11–14]. Taking advantage of bidentate thiolate linkers, fluorescent gold and silver NCs synthesized with dihydrolipoic acid have revealed remarkable optical properties, including relatively high and stable photoluminescence and long quantum yields (QYs) in a wide pH range, which makes them very interesting candidates for biological investigations [14]. In this way, water soluble synthesis of fluorescent metal NCs can be successfully achieved, opening the way to many possible applications of such small and biocompatible nanolabels [15, 16]. Recent advances have enabled facile synthesis of fluorescent metal NCs with tunable emission colors, establishing them as a new class of ultrasmall, biocompatible fluorophores for applications as biological labels. So far, synthetic efforts have been mainly focused on two noble metals: Au and Ag [17–19]. Although great progress has been achieved, particularly for gold and silver NCs, challenges for further advancement of fluorescent metal NCs are still ahead of us.
1. Introduction Research on nanomaterials has experienced a rapid growth over the last decades due to the unique physical and chemical properties that many materials exhibit when reduced to the nanoscale. Small noble metal nanoclusters (NCs) of Au and Ag consisting of isolated particles of a few nanometers in diameter have become promising alternatives for important current applications like sensing [1], catalysis [2], photonics [3] and biolabelling [4]. It has been demonstrated that such nanostructures exhibit interesting and valuable optoelectronic characteristics [5]. There are different synthetic routes to obtain fluorescent metal NCs [6–8]. Among them, the most common approach for synthesis of NCs to be used as fluorescent labels relies on the use of capping agents, such as thiol ligands. The strength of the metal-S bond, as well as the small hydrodynamic diameters involved and the modifiable surface properties [9, 10], guarantee high stability of the NC. In this context, dihydrolipoic acid and its polyethylene glycol derivatives 0957-4484/15/215601+07$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Fluorescent metal NC can be combined with other detection modalities, such as elemental mass spectrometry, allowing multiplex detection of biomarkers by labeling selected antibodies to NCs of different nature followed by their sensitive elemental detection. Availability of different nature metal NCs is needed in the development of such multiplexing assays. In this context, recent work paid attention to platinum (another noble metal) as a possible bioimaging probe [7, 20, 21]. Moreover, water-soluble platinum nanoclusters (PtNCs) could be less cytotoxic and emit a brighter fluorescence than other fluorescent nanomaterials, such as gold clusters [20]. In this context, controllable synthesis of PtNCs with high quality is of paramount importance and highly desirable. Typically, the strategies of PtNCs synthesis were developed in non-polar media. In such cases, NCs must be transferred from the organic environment into an aqueous medium to allow biological applications. This can be achieved by ligand exchange; unfortunately, an incomplete exchange of the original surface ligands in the NCs can lead to an uncontrolled aggregation of the clusters [9] and this problem constitutes the main challenge to obtain water-soluble NCs from such synthetic route. Moreover, the QY values obtained for PtNCs synthesized up to now (ranging from 4 to 28%) are low as compared to other metal NCs produced in organic media. Additionally, to our knowledge, the work published so far referring to platinum as fluorescent label reported UV or visible luminescence emissions in the range of 350–570 nm [7, 10, 18–21]. In order to carry out in vivo imaging experiments, disease diagnostics, or even image-guided therapy, photoluminescent emission should be closer to the near infrared region (NIR) because of the low background autofluorescence and less harmful biological effects found in this spectral region. Finally, as pointed out recently [22], the reported PtNCs syntheses consist of rather lengthy and tedious procedures. Considering all the above facts, new routes of watersoluble PtNCs synthesis with high water solubility and photoluminescence emissions in the NIR range are of present interest. It should be also noted that research on different novel water compatible metal NCs is highly demanded, to be used as tags for development of multiplexing applications by optical and also by mass spectrometry. In this work we propose a synthetic route based on the simple reduction of a mixture containing the lipoic acid (LA) ligand and the platinum precursor with sodium borohydride directly in aqueous medium, as an one-step route to obtain water soluble PtNCs of interesting photophysical properties.
rhodamine B were purchased from Sigma-Aldrich (Milwaukee, USA). 98% LA was obtained from Acros OrganicsThermo Fisher Scientific (Geel, Belgium). NaOH and NaCl salts were purchased from Prolabo (Leuven, Belgium). The equipment necessary to carry out the PtNCs synthesis was a two-necked round-bottomed flask, a reflux system, amber vials and a magnetic stirrer. Poly-L-lysine coated glass slides from Sigma-Aldrich (Milwaukee, USA) were used in the laser confocal microscopy experiments. Ethyl-3-(dimethylaminopropyl) carbodiimide (EDC), used to immobilize the synthesized PtNCs onto the poly-L-lysine glass slide surface, was purchased from Fluka (Sigma-Aldrich, Milwaukee, USA) and stored at −18 °C into the freezer. 2.2. Synthesis of PtNCs
In the here optimized synthesis of PtNCs, aqueous stock solutions of platinum precursor (H2Cl6Pt, with a concentration of 5 mM) and aqueous solutions of 25 mM LA ligand were prepared in ultrapure water (the addition of 1 mL of 1 M NaOH solution was needed for solubilization). An aqueous NaBH4 solution with a concentration of 625 mM was freshly prepared by dissolving 24 mg of NaBH4 in 500 μL of ultrapure water. In order to carry out the synthesis, 2000 μL of 5 mM H2Cl6Pt (aq) and 2000 μL of 25 mM LA were put into an amber vial under magnetic stirring conditions. After 20 min to ensure the homogeneity of the mixture, 400 μL of 625 mM NaBH4 (aq) was incorporated, being the final precursors molar ratio 1:5:25 (Pt:ligand:reducing agent). Reaction was left to proceed during 6 h under room temperature magnetic stirring. All the process was carried out under dark conditions. Once the reaction is complete, i.e. when no changes are detected in the observed fluorescence emission spectrum, the excess of ligand is removed by ultrafiltration using a 10 KDa membrane filter (5000 rpm × 10 min × 3 cycles). The yellow colored solution containing the sought PtNCs remained on the filter and was re-suspended in slightly alkaline water. Then it was stored protected from visible light. The strong yellow color characteristic of the resulting NCs solution could be an emission limitation factor due to possible inner filter associated problems. To get rid of this effect, different dilution factors were assayed in order to obtain the maximum fluorescence emission signal. Experiments show that a final dilution factor of 1:100 (NCs:water, v/v) provides highest fluorescence intensity at 680 nm. 2.3. Optical and structural characterization of PtNCs
Optical characterizations of the synthesized PtNCs were accomplished by examining the corresponding absorbance and fluorescence spectra. Fluorescent measurements were carried out using a Cary Eclipse fluorescence spectrophotometer (Varian, USA) and optical absorption spectra were recorded with a Genesys 10S UV–vis spectrophotometer (Thermo Scientific, USA). Aiming at a proper structural characterization of the synthesized PtNCs, high-resolution transmission electron
2. Experimental section 2.1. Reagents and materials
All chemicals were of analytical grade and used as received. Deionized ultrapure water (resistivity 18.2 MΩ cm−1) was used throughout the work. Chloroplatinic acid solution 8% wt in H2O (H2Cl6Pt), sodium borohydride (NaBH4) and 2
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microscopy (HR-TEM) and energy-dispersive x-ray microanalysis (EDX) techniques were used to investigate their size, shape and chemical composition. HR-TEM images were performed on a JEOL JEM-2100F (Tokyo, Japan), offering resolutions of 2.3 Å between points and 1.0 Å between lines. That instrument is equipped with a high-resolution CCD camera (Gatan) and an EDX microanalyzer enabling the identification of the elements present in the scrutinized sample. 2.4. Confocal microscopy studies
To assess the PtNCs potential as imaging nanoprobes the obtained material was immobilized onto the surface of polyL-lysine coated glass slide by EDC chemistry. After the immobilization process the glass slide was examined by Laser confocal microscopy adjusting the measurement parameters. A confocal microscope Leica TCS-SP2-AOBS with a multilaser configuration was employed for this purpose. Thus, an objective 63x was used to focus the region of the sample to be explored, and then an argon laser emitting at 488 nm and working at 30% of power was selected for the excitation of the sample. The emission from the sample examined was collected in a wavelength range between 550 and 750 nm.
Figure 1. Fluorescence emission spectra of PtNCs synthesis by
magnetic stirring under room temperature conditions. The maximum value of the 680 nm centered peak is achieved at 6 h of reaction. Stability of the signal after 24 h makes this synthesis strategy more competitive than the one involving reflux conditions.
3 to 11) and at higher ionic strength media (0.2 M NaCl) were carried out.
2.5. QY measurement
3. Results and discussion
The QY of PtNCs in water was determined using a relative method, based on the standard rhodamine B (QY = 0.68 in ethanol), and was calculated according to the following equation:
(
3.1. Optimization of the PtNCs synthetic route
In order to optimize the PtNCs synthesis, the Pt to aqueous stabilizer ligand molar ratio constitutes a parameter to be controlled. Different Pt:LA molar ratios (1:1, 1:5 and 1:10) were eventually assayed. The optical properties of the products obtained are collected in figure S-1 of the supporting information. The highest fluorescence emission and the best signal to noise ratio (with a maximum emission wavelength centered at 680 nm) were observed for 1:5 (Pt:LA) molar ratio. Moreover, as can be seen in figure S-1 of the supplementary information, a slight shift on the emission maximum along with a broadening of the fluorescence emission spectra has been observed when changing the Pt-to-ligand ratios used in the synthesis. This is most likely a result of a change on the obtained PtNCs that occurs when varying the Pt:ligand molar ratios used during the synthesis. The effect of reaction time and temperature in the synthesis was also checked. Reaction under room temperature stirring and under reflux conditions was monitored every hour. Under reflux conditions a sudden decrease of the fluorescence emission peak was apparent after more than four hours (see figure S-2 of supporting information); it is likely that too long time under reflux gives rise to a degradation of the PtNCs. When room temperature stirring was carried out, maximum signals at 680 nm were obtained after 6 h of reaction. As can be seen in figure 1, for reaction times higher than 6 h at room temperature, the analytical emission at 680 nm kept constant with time. This room temperature strategy brings about NCs stability, as well as synthesis simplicity and security.
)2
QYPtNC = QYST ( IPtNC / IST )( AST / APtNC ) ηPtNC / ηST ,
where I, A and η are the integrated emission intensity, absorbance at excitation wavelength and refractive index of the solvent, respectively. While PtNCs are dissolved in aqueous media η was 1.333, the rhodamine B standard was dissolved in ethanol (η value of 1.361). Rhodamine B was selected as standard dye for the determination of the PtNCs QYs considering that it has a well-known QY value and it emits in a similar spectral region to the PtNCs. 2.6. Stability studies
In order to obtain a further characterization of the synthesized PtNCs, some important parameters affecting the eventual applicability of the NCs for further bioanalytical applications, such as photostability, stability under different media conditions (pH, ionic strength) and long-term storage were evaluated. Photostability of the synthesized PtNCs was tested by monitoring their photobleaching time profiles. Moreover, aliquots exposed to daylight were periodically measured along to successive days and results observed with time were collected. The effect of pH and NCs stability in a saline medium were then assessed. Thus, fluorescence intensity measurements of PtNCs at different pH values (ranging from 3
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Figure 2. Optical characterization of the synthesized PtNCs. (A) Absorbance spectra of PtNCs (black line) and blank of reaction (gray line).
(B) Fluorescence spectra of PtNCs using 430 nm (line in black) and 480 nm (dotted line) as excitation wavelengths; gray line is the fluorescence spectrum from the blank of reaction.
Pt2+ centers eventually present within the NCs. In any case, owing to the limited information available [25], much effort is required to further elucidate the luminescent mechanism from these PtNCs. Aiming at a proper structural characterization of the synthesized PtNCs, HR-TEM and EDX techniques were used to investigate their size, shape and chemical composition. HR-TEM images of PtNCs obtained after 6 h of reaction are depicted in figure 3: a population of mostly isolated nanoparticles, with a spherical shape having an average value of 2.8 nm, is observed (the mean diameter size was calculated from appropriate image analysis of the spots). EDX analysis confirmed the elemental composition of the NCs (figure S-4 in the supporting information). The characteristic bands and pattern of Pt metal as well as the presence of S, O and C (from the protecting LA ligand) are apparent in the EDX spectrum, as well as an indication of the presence of Pt–S bonds.
3.2. Optical and structural characterization
Optical characterization of the synthesized PtNCs was accomplished by examining the corresponding absorbance and fluorescence spectra. The absorption UV–vis spectrum showed no characteristic absorption band in the whole visible region: just a continuous absorbance decreasing from 350 until 700 nm is noticed (see figure 2(A)). As can be observed in figure 2(B), the fluorescence emission spectra, using two different excitation wavelengths (430 and 480 nm) produced a well-defined emission peak centered at 680 nm, in both cases (the excitation spectrum measured while monitoring the emission at 680 nm is collected in figure S-3 of the supporting information). From figure 2(B) it can be seen that a rather similar sensitivity was achieved when using both excitation wavelengths. However, when exciting at 480 nm, an additional emission band appears at 575 nm (also observed for the blank sample, and therefore no corresponding to PtNCs) which could interfere further fluorimetric applications. The best excitation/emission pair for eventual PtNCs application turned out to be 430/680 nm. Thus, the selected emission wavelength for analytical purposes lies into the NIR region. This is an important finding since there is no precedent of other luminescent PtNCs with these favorable spectral features (see table 1 for comparison to other PtNCs). Usually, metal nanoparticles absorb light strongly, but they do not or only show weak luminescence. After reduction of the size of metal nanoparticles to the size of metal NCs, the properties of particles disappear and the energy bands turn into more or less discrete energy levels. Thus, the collective oscillation of electrons is obstructed and metal NCs do not give rise to surface plasmon resonance effect. However, they will follow quantum mechanical rules. Through interaction with light and then electronic transitions between the energy levels, they show bright luminescence [23]. The observed photoluminescence spectra of the PtNCs are in agreement with the well-described luminescence properties of squareplanar Pt2+ compounds [24]. Therefore, based on such findings it could be possible that the photoluminescence property of the PtNCs here synthesized originates from square-planar
3.3. Quantum yield
The QY of the synthesized PtNCs was evaluated according to a relative method, based on the standard rhodamine B, and a value of 47% was obtained. This value is significantly higher than the highest one given for PtNCs in the literature so far (see table 1 for comparison to other PtNCs). Moreover, this value competes with conventional fluorophores such as quinine sulfate (58%), cresyl violete (53%) or cyanine dyes like Cy5 (28%). 3.4. Stability tests
To attain a further characterization of the synthesized PtNCs, parameters affecting the eventual analytical applicability of the NCs, such as photostability, stability under different aqueous media conditions (pH, ionic strength) and long-term storage were evaluated. NCs photostability was first assessed. Photostability of the synthesized PtNCs was tested by monitoring their photobleaching time profiles. A rather small variation of the registered fluorescent emission at 680 nm (less than 10% attenuation) was detected after 2 h of 4
Nanotechnology 26 (2015) 215601
Table 1. Research papers available up to now concerning different PtNCs synthesis methodologies.
5
Ref.
Water solubility
Synthesis method
QY
λexc/λem
NCs size
Stability
[25] [10] [20] [21] [28] [7]
No Yes No No Yes Yes
Organic media Organic → aqueous phase transfers Organic media (2 weeks) Organic media (6 days) Chicken egg aqueous media Organic media (2 weeks)
Not provided 4.6% 18% 17% Not provided 28%
350/484 500/544 410/436 380/470 490/570 280/350 460/520
1 nm 2 nm Not provided 2 nm 2.5 nm Not provided
Up to 6 months in dark conditions Up to 6 months at 4 °C Not provided Up to 1 month without changes Not provided Not provided
J García et al
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emission was rather stable for more than three months. Fluorescence intensity measurements of PtNCs at different pH values (ranging from 3 to 11) and at a rather high ionic strength media (0.2 M NaCl) were also carried out. Results showed that PtNCs fluorescence emission was neither affected by the pH in the interval measured nor by the presence of 0.2 M NaCl (see figure S-5 of the supporting information).
3.5. Fluorescence imaging measurements
Finally, to prove the potential of the here developed PtNCs as possible fluorescent labels for imaging purposes, a blank of reaction and an aliquot of the synthesized PtNCs were immobilized onto the surface of a poly-L-lysine glass slide and were measured by laser confocal microscopy. Optical properties of functionalized colloidal fluorescent nanoparticles may be reduced, or even disappear, when they are placed in a solid media (a critical step in the development of sensing and immunosensing applications). Therefore, to determine the capabilities of the here-synthesized PtNCs for their eventual use as labels in bioimaging experiments, PtNCs were immobilized as described in a microscopy glass slide and luminescence registered using confocal imaging/fluorescence microscopy. Results proved that the fluorescent NCs maintain their optical and spectroscopic properties after immobilization in a solid surface, and therefore could be used as highly fluorescent labels in bioanalytical applications. Moreover, as it is shown in figure 4, the PtNCs luminescence can be easily distinguished from the blank of reaction.
Figure 3. Structural characterization of PtNCs. HR-TEM images of
PtNCs obtained after 6 h of reaction; an average value of 2.8 nm was obtained as mean diameter size after the image analysis. Cubic crystalline structure is well defined in the image of the upper corner. Moreover, due to the fact of each ring corresponds to scattering from sets of hkl planes with the same 2θ angle, planes {111}, {200} and {220} can be observed in the bottom right image.
continuous exposition of a PtNCs colloidal solution aliquot to UV light (figure S-5 in the supporting information). Thus, they are higher resistant to UV light than, for example, silver NCs that are strongly degraded at a higher rate [26]. Additionally, the bleaching time of conventional Alexa Fluor 488 organic molecules is less that some few seconds, much shorter than the time of the PtNCs under similar conditions, indicating that PtNCs are at least 1000-fold more photostable over the organic fluorophores [27]. Moreover, for a proper comparison, photostability of the standard fluorophore rhodamine B has been tested employing the same spectrofluorimeter used for characterization of the PtNCs. Under the optimized experimental conditions a progressive drift on the signal of rhodamine B was observed (a 15% reduction of the original luminescence value was registered after 90 min of continuous illumination, see figure S-6 in the supporting information). Moreover, aliquots exposed to daylight were periodically measured along successive days (figure S-5 in the supporting information). Results showed that photoluminescence from PtNCs was unaltered along the first exposition day, while a gradual decrease on the luminescence is apparent after several days of exposition to daylight (day 2: 87%, day 3: 79% of initial signal). This does not represent a significant limitation in the final expected applications of PtNCs: if they are stored in the dark, the emission intensity remains unaltered. Long-term stability has been also evaluated for aliquots of the colloidal solutions stored in the dark (figure S-5 in the supporting information). Results demonstrated that the
4. Conclusions In conclusion, a novel strategy for synthesizing water-soluble PtNCs based on the direct metal reduction with NaBH4 in the presence of bidentated thiol ligands has been developed. This route (a one-step synthesis, carried out directly in water media, just by mixing appropriate amounts of the NCs precursors and stirring the mixture at room temperature) offers a simpler approach for PtNCs synthesis than those previously reported [7, 10, 20, 21, 25, 28], see table 1, and avoids environmentally undesirable reagents (e.g. organic solvents). The here reported synthesis process enables the production of PtNCs exhibiting NIR fluorescence for the first time. These new nanostructures can become useful photoluminescent labels, as the QY achieved (47%) is the highest registered in the literature for PtNCs so far. On the other hand, pH, saline and stability studies indicate that the PtNCs here synthesized could be advantageously employed as photoluminescent nanoprobes in many fields due to its aqueous solubility and stability. In brief, we believe that the water-soluble PtNCs synthesized in this work are highly competitive, exhibiting advantageous optoelectronic properties which open the way to novel bianalytical applications. 6
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Figure 4. Confocal microscopy images using an argon laser at 488 nm for the excitation. (A) Fluorescent image from blank of reaction. (B) Fluorescent image from PtNCs.
Acknowledgments
[12] Fernández-Ujados M, Trapiella-Alfonso L, Costa J M, Pereiro R and Sanz-Medel A 2013 Nanotechnology 24 495601 [13] Shang L, Azadfar N, Stockmar F, Send W, Trouillet V, Bruns M, Gerthsen D and Nienhaus G U 2011 Small 7 2614 [14] Le Guével X, Tagi O, Rodríguez C E, Trouillet V, Pernia Leale M and Hildebrandt N 2014 Nanoscale 6 8091 [15] Shang L, Dörlich R M, Brandholt S, Schneider R, Trouillet V, Bruns M, Gerthsen D and Nienhaus G U 2011 Nanoscale 3 2009 [16] Shang L, Dongb S and Nienhaus G U 2011 Nano Today 6 401 [17] Kumar S, Bolan M D and Bigioni T P 2010 J. Am. Chem. Soc. 132 13141 [18] Zheng J, Zhou C, Yu M and Liu J 2012 Nanoscale 4 4073 [19] Muhammed M A H, Ramesh S, Sinha S S, Pal S K and Pradeep T 2008 Nano Res. 1 333 [20] Tanaka S, Miyazaki J, Tiwari D K, Jin T and Inouye Y 2011 Angew. Chem. Int. Ed. 50 431 [21] Le Guével X, Trouillet V, Spies C, Jung G and Schneider M 2012 Phys. Chem. C 116 6047 [22] Le Guével X 2014 IEEE J. Sel. Top. Quantum Electron. 20 6801312 [23] Zhanga L and Wang E 2014 Nano Today 9 132 [24] Trikalitis P N, Rangan K K and Kanatzidis M G 2002 J. Am. Chem. Soc. 124 2604 [25] Kawasaki H, Yamamoto H, Fujimori H, Arakawa R, Inada M and Iwasaki Y 2010 Chem. Commun. 46 3759 [26] Trapiella-Alfonso L, Menéndez-Miranda M, Costa-Fernández J M, Pereiro R and Sanz-Medel A 2014 Mater. Res. Express 1 015039 [27] Zhang J, Fu Y, Conroy C V, Tang Z, Li G, Zhao R Y and Wang G 2012 J. Phys. Chem. C 116 26561 [28] Min L, Yang D P, Wang X, Lu J and Cui D 2013 Nano Res. Lett. 8 182
The authors want to thank to Dr Zakariae Amghouz from the Unit of Electron Microscopy of the University of Oviedo for the HR-TEM measurements. Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO-13-CTQ2013-49032-C2-1-R), Spanish Ministry of Education and Science (CTQ2010-16636 and MAT201020921-C02-01) and the European FEDER program is also gratefully acknowledged.
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