Article pubs.acs.org/Langmuir

Formation and Optical Properties of Fluorescent Gold Nanoparticles Obtained by Matrix Sputtering Method with Volatile Mercaptan Molecules in the Vacuum Chamber and Consideration of Their Structures Taiki Sumi,† Shingo Motono,† Yohei Ishida,† Naoto Shirahata,‡ and Tetsu Yonezawa*,† †

Division of Material Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0808, Japan ‡ National Institute for Material Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: This paper proposes a novel methodology to synthesize highly fluorescent gold nanoparticles (NPs) with a maximum quantum yield of 16%, in the near-infrared (IR) region. This work discusses the results of using our (previously developed) matrix sputtering method to introduce mercaptan molecules, α-thioglycerol, inside the vacuum sputtering chamber, during the synthesis of metal NPs. The evaporation of α-thioglycerol inside the chamber enables to coordinate to the “nucleation stage” very small gold nanoclusters in the gas phase, thus retaining their photophysical characteristics. As observed through transmission electron microscopy, the size of the Au NPs obtained with the addition of α-thioglycerol varied from approximately 2−3 nm to approximately 5 nm. Plasmon absorption varied with the size of the resultant nanoparticles. Thus, plasmon absorption was observed at 2.4 eV in the larger NPs. However, it was not observed, and instead a new peak was found at approximately 3.4 eV, in the smaller NPs that resulted from the introduction of α-thioglycerol. The Au NPs stabilized by the αthioglycerol fluoresced at approximately 1.8 eV, and the maximum wavelength shifted toward the red, in accordance with the size of the NPs. A maximum fluorescent quantum yield of 16% was realized under the optimum conditions, and this value is extremely high compared to values previously reported on gold NPs and clusters (generally ∼1%). To our knowledge, however, Au NPs of size >2 nm usually do not show strong fluorescence. By comparison with results reported in previous literature, it was concluded that these highly fluorescent Au NPs consist of gold−mercaptan complexes. The novel method presented in this paper therefore opens a new door for the effective control of size, photophysical characteristics, and structure of metal NPs. It is hoped that this research contributes significantly to the science in this field.



INTRODUCTION Size-dependent unique properties of gold nanoparticles (Au NPs) differ from those of bulk gold, and have been extensively investigated in recent years.1−8 Applications of Au NPs have been intensively investigated in various fields. In particular, surface plasmon absorption and fluorescent properties arising from quantum size effects have expanded the applications of NPs as color pigments8 or biosensing materials.1−7 For effective applications, size control of Au NPs is quite indispensable, and many amines, mercaptans, or polymers with high affinity to gold atoms have been used as stabilizing reagents.9−12 Especially in the case of fluorescence properties, the size of the Au NPs, and the coordination structures of the mercaptan compounds with the gold atoms, directly affect their band gap and thus can control the peak position and quantum yield. Different procedures have been proposed to obtain Au NPs, such as a chemical reduction method using metal ions in solution with a reductant, or a sputtering method using bulk © 2015 American Chemical Society

metals. In the chemical reduction method, polyvinylpyrrolidone (PVP), glutathione, and alkylthiol have been used as stabilizers to suppress the coalescence of NPs during the reduction, and thus control particle sizes in the range of 3−150 nm, with small size distributions.3,13−16 Sputtering is widely used for the preparation of thin films using a simple apparatus. It is a good technique to obtain pure NPs into a liquid capturing medium, preventing contamination, and can be applied for various conductive materials. We termed such sputtering to a liquid medium as a matrix sputtering process. In this sputtering process, Ar is ionized by high voltage and then attacks the target metal and ejects the target atoms or clusters into a vacuum chamber. The resultant NPs can disperse inside a suitable capture medium such as ionic liquids or Received: January 24, 2015 Revised: March 12, 2015 Published: March 15, 2015 4323

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Japan) was used as a protective agent for Au NPs. Water was purified by Organo/ELGA Purelab system (>18 MΩ). The Au target (99.9%) was supplied by Tanaka Precious Metals (Tokyo, Japan). Pentaerythritol tetrakis(3-mercaptopropionate) with a molecular weight of 488.66 (PEMP, Sigma-Aldrich, USA) was also used. 2. Pretreatment of PEG and α-TG for Matrix Sputtering. To remove volatile substances, and especially water, PEG and α-TG were dried in vacuum at 100 °C. This was important in order to avoid potential bubble formation during sputtering that could affect the rate of sputtering and the structure of the NPs. 3. Matrix Sputtering Preparation of Au NPs. 3.1. Apparatus. For the preparation for Au NPs, an MSP-10 magnetron sputter for SEM observation (Shinkuu Device, Ibaraki, Japan) was used. A schematic illustration of this process is shown in Figure 1a. The diameter and thickness of the target were 60 mm ϕ, and 1 mm, respectively. In order to stabilize the Au NPs in the gas atmosphere, a dish containing α-TG was introduced into the chamber. The α-TG evaporated under the sputtering conditions, stabilizing the Au NPs. 3.2. Preparation of Au NPs in PEG. PEG (10.0 g) was put in a glass Petri dish with a diameter of 65 mm ϕ and was set horizontally against the sputtering target. The distance between the surface of PEG and the gold target was set at 25 mm. The pressure of the vacuum chamber was controlled between 2 and 30 Pa. Sputtering was carried out in air for 10, 20, 30, and 40 min at room temperature. The sputtering voltage was approximately 200 V, and the current was maintained at 10, 20, or 30 mA. 3.3. Preparation of Au NPs in PEG in the Presence of α-TG. α-TG (0.05 g) was collected in a plastic case with a diameter of 20 mm, and placed in the chamber at a distance of 10 mm from the edge of the Petri dish containing PEG (Figure 1b). The sputtering conditions including time, current, and distance between the surface of PEG and the target surface were the same as described in subsection 3.2. The sputtering pressure was 10−20 Pa. 4. Characterization. Immediately after the sputtering deposition, the UV−vis extinction spectra were measured using a spectral photometer (PerkinElmer, Lambda 750, 250−800 nm) with a quartz cell of 1 mm optical path. Emission and excitation spectra of the obtained Au NPs dispersion in PEG were measured using a fluorescence spectrophotometer (JASCO, FP-6600) with a quartz cell of 10 mm optical path. The filter (Sharp cut filter Y-50), which cut off emissions lower than 500 nm, was mounted in front of the detection window. Transmission electron microscopy (TEM) observations of the size and shape of Au NPs were carried out using a Hitachi H-9500 (acceleration voltage of 300 kV). TEM samples were prepared by dropping a gold dispersion of PEG onto collodion-coated copper grids. The grids were then soaked in methanol for 30 min in order to remove the excess PEG, and dried under vacuum.20 Fluorescence quantum yields were measured and recorded with a Hamamatsu Quantaurus-QY C11347-01 quantum yield analyzer with a quartz sample cell with a 10 mm optical path. The measurement was carried out on each sample, with a fixed excitation wavelength. The fluorescent wavelength was measured in the range of 200−950 nm. The resultant Au NPs dispersions were diluted five times with pure PEG, for the measurement of their quantum yield. This equipment allows for multiple reflections on the inner surface of the integrating sphere (the inner surface of the sphere was clad by highly reflective materials, typically 99% reflectivity for the range of wavelengths used in our measurements) and, when combined with a highly sensitive detector, eliminated most of the optical anisotropy. This method enabled the achievement of a highly accurate quantum yield with less than 0.01% theoretical uncertainty, and with less than 0.1% uncertainty for our experimental data.

polyethylene glycol (PEG) during the sputtering equipment.17−19 Since the viscosity of the capture medium changes with temperature, larger NPs are obtained under higher temperatures.18 Gold atoms and very small clusters, which are ejected from the gold target, can aggregate not only in a gas phase but also inside, or at the interface of, the capture medium. The growth mechanism for these NPs has not been clearly established, and thus a new strategy is required to control the size of NPs obtained using this matrix sputtering process. We have reported the use of 6-mercaptohexyltrimethylammonium bromide (6-MTAB), pentaerythritol tetrakis(3-mercaptopropionate) (PEMP), and pentaerythritol ethoxylate (PEEL) as capture media for the synthesis of Au NPs by a sputtering method.20,21 In these cases, the sizes of Au NPs obtained using 6-MTAB and PEMP (1.3 nm) were smaller than the ones obtained using PEEL (2.1 nm). These phenomena can be explained by the coordination of mercapto groups of 6MTAB and PEMP to Au NP surfaces that can prevent the aggregation and growth of Au NPs inside or at the interface of the capture medium. From these results, it is expected that we can prepare Au NPs with varying single digit nanometer sizes, by changing the capture medium, temperature, and other typical sputtering conditions such as current and sputtering time. The Au NPs obtained using 6-MTAB and PEMP fluoresced, whereas these obtained using PEEL did not. Fluorescent Au NPs reported so far typically have very small particle sizes, less than 2 nm in diameter.1−5 In this paper, we propose a new strategy to control the size and property of Au NPs by introducing a volatile stabilizer, αthioglycerol (α-TG), into the reaction chamber, along with a capture medium of polyethylene glycol, as shown in Figure 1.

Figure 1. Schematic illustrations of the sputtering equipment (a) without α-TG in the chamber and (b) with α-TG in the chamber.

In the sputtering process, gold atoms or clusters ejected by the ionized Ar aggregate to Au NPs in a gas or a liquid phase. It was our aim to coordinate the evaporated α-TG with the Au NPs in the gas phase and thus suppress the coalescence of the Au NPs in the “nucleation stage.” The effect of α-TG on the size, absorption, and fluorescence properties of Au NPs prepared by a sputtering method was investigated. The structure and fluorescence properties of the Au NPs obtained in this study were compared with results reported in literature.





RESULTS AND DISCUSSION 1. Preparation of Au NPs in PEG by the Sputtering Method. In this study, we used a simple sputtering process with a magnetron sputtering apparatus. The mechanism of this process is due to a discharge of magnetron which electric field and magnetic field are perpendicularly located. Air was

EXPERIMENTAL SECTION

1. Materials. Polyethylene glycol (PEG), with an average molecular weight of 600 and a degree of polymerization of 10−17, was purchased from Wako (Japan). α-Thioglycerol (α-TG, TCI, 4324

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during the process, even under various current conditions and sputtering time periods. Figure 4 shows representative TEM images and size distribution histograms of Au NPs obtained by (a) 10 mA

introduced at 2.0 Pa, in order to obtain a homogeneous discharge. This value, 2.0 Pa, was determined by the fact that the plasma is not stable under too low a pressure during the sputtering (such as 0.5 Pa). It should be noted that a thin film of Au was not deposited on the liquid PEG in this experiment, while it is usually deposited on solid surfaces for the purpose of SEM measurement. Figure 2 shows the UV−vis extinction spectra from the Au NP dispersions prepared in PEG with a sputtering current of 20

Figure 4. TEM images and particle size distributions of Au NPs in PEG under (a) 10 mA of sputtering current for 30 min and (b) 20 mA of sputtering current for 20 min. The average nanoparticle diameters are 5.5 ± 1.5 and 5.3 ± 1.1 nm, respectively. For the histogram, 200 Au NPs were counted in more than five TEM images. Figure 2. UV−vis extinction spectra of Au NPs in PEG prepared by 20 mA sputtering current, with various sputtering time periods.

sputtering current for 30 min and (b) 20 mA sputtering current for 20 min. Size distributions were quite similar, and the average Au NP diameters were 5.5 ± 1.5 and 5.3 ± 1.1 nm, respectively. The size and distribution of Au NPs prepared under various sputtering currents and time periods are summarized in Table 1. Sputtering of Au NPs into PEG generated Au NPs of almost similar size, with diameters of approximately 5 nm. The size of these Au NPs is much larger than those obtained previously, of ca. 1.3 nm, by using 6mercaptohexyltrimethylammonium bromide (6-MTAB) or pentaerythritol tetrakis(3-mercaptopropionate) (PEMP). This difference in size is likely due to the mercapto group in 6MTAP and PEMP, which can coordinate to the NP surface, and thus suppress the growth of Au NPs (see the photographs of samples, in Figure S2). In the case of PEG, PEG does not coordinate to the Au NP surface, and only the viscosity of PEG suppresses the coalescence of Au NPs on, or in, the liquid. Thus, rather large NPs are formed with PEG. These results suggest a mechanism for the aggregation and growth of Au NPs in the present experimental system. It is anticipated that the aggregation in a gas phase should be efficient, and larger NPs should be obtained under higher current conditions due to a larger amount of ejected Au atoms and clusters. However, the current results show that the sizes of the Au NPs were quite similar even under different current conditions (from 10 to 30 mA). Thus, it appears that the collision of Au atoms and clusters in a liquid phase is the main reason for the growth of Au NPs to approximately 5 nm in the present system. Furthermore, by covering the Au NPs with stabilizers in a gaseous phase, the final size of Au NPs should be smaller due to the suppression of coalescence. These considerations led us to use a volatile mercaptan stabilizer, αthioglycerol (α-TG), in the sputtering chamber, to control the size of the Au NPs. 2. Preparation of Au NPs in PEG by the Sputtering Method, Using α-TG as a Stabilizer. To control the size of Au NPs more precisely, α-thioglycerol (α-TG) was used as a

mA for various sputtering periods (the other experimental conditions are shown in Figure S2 in the Supporting Information). The absorption spectra were measured in a quartz cell with 1 mm optical path immediately after the sputtering preparation, without further purification or dilution. These spectra showed broad absorption peaks around 2.4 eV (520 nm), which correspond to the localized plasmon absorption of Au NPs. The absorbance of these peaks became higher with longer sputtering periods. Small changes of the width of the localized plasmon absorption peaks indicate the size variation of the obtained Au NPs as indicated in Figure 4. Figure 3 shows the relationship between the extinction at λmax of the plasmon absorptions and the sputtering current

Figure 3. Relationship between extinction at plasmon absorption peaks (λmax), judged from UV−vis extinction spectra, and sputtering current (mA).

under various sputtering periods from 10 to 40 min. The extinction increased proportionally with the sputtering period and current. Since the particle sizes were almost the same, even under various sputtering currents and periods, as will be discussed later, these phenomena indicate that the amount of Au NPs increased proportionally to the sputtering period and current. Thus, it was found that the rate of sputtering is stable 4325

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Langmuir Table 1. Average Diameters of Au NPs in PEG under Various Currents and Periods sputtering period/min sputtering current/mA

10 min

20 min

30 min

40 min

10 mA 20 mA 30 mA

4.8 ± 0.8 nm 5.4 ± 1.1 nm 5.1 ± 2.8 nm

5.2 ± 1.3 nm 5.3 ± 1.1 nm 5.4 ± 0.8 nm

5.5 ± 1.5 nm 5.0 ± 0.6 nm 5.6 ± 1.2 nm

4.4 ± 0.4 nm 5.0 ± 0.5 nm 4.7 ± 0.6 nm

tends to be smaller when the number of core Au atoms increases. Synthesis of many kinds of Au nanoclusters has been reported for Au12,18,23,25,28,32,38,67,102,144 and larger, and their band gap energies (2.1, 1.8, 1.5, and 1.2 eV for Au12,18,25,32 nanoclusters, respectively5,28−30) are considerably smaller than the band gaps in this study. From literature analysis of both experimental synthesis and theoretical DFT calculations, the most plausible candidate structure for the Au component obtained in this work is determined to be the Au6 cluster, judging from the band gap energies of 3.27 and 3.30 eV from the experimental31 and the DFT calculation,29 respectively. This consideration will be further discussed along with the excitation and emission spectra. Figure 6 shows representative TEM images and size distribution histograms for Au NPs prepared with α-TG.

volatile stabilizer in the gas phase, as illustrated in Figure 1b. The thiol group of α-TG has a good affinity to gold atoms, due to the formation of a coordination bond between Au0 and the lone pair of electrons of sulfur, or a covalent bond of AuSR.9,10,12 It is expected that the steric hindrance by the coordination of α-TG inhibits the growth of Au NPs. Figure 5 shows the extinction spectra of Au NPs obtained at 10 mA in the presence of α-TG (the spectra obtained under

Figure 5. UV−vis extinction spectra of Au NPs prepared in the presence of α-TG, under 10 mA sputtering current.

other values of current are shown in Figure S3). Compared to Figure 2, the extinction spectra did not show any plasmon absorption peaks around 2.4 eV (520 nm). Surface plasmon absorption is generated by the vibration of the group of free electrons in the surface region of NPs, resulting in an absorption peak corresponding to the vibration frequency. Therefore, the generation of plasmon absorption needs a certain particle size, usually over ca. 2 nm in diameter for Au.23−26 When the number of atoms in each particle decreases, the energy band gap becomes wider, in accordance with the quantum size effect, and the particle becomes nonmetallic. Plasmon absorption is thus not observable for such small particles (usually smaller than 2 nm).26 Judging from the absorption spectra in Figure 5, our Au NPs prepared in the presence of α-TG are very small (see the photographs of yellow colored PEG solutions in Figure S1). On the other hand, new absorption shoulders were observed around 3.4 eV (360 nm). We consider these peaks to result from the absorption of very small Au clusters27,28 instead of the plasmon absorption of relatively large Au NPs. The reason why our Au NPs capped with α-TG did not show plasmon absorption even though their diameter should be large enough to show it will be discussed later, along with the TEM and fluorescence results. Judging from Figure 5, the band gap of obtained Au NPs is around 3.2−3.4 eV. From many studies on the synthesis of pure Au nanoclusters, it has been found that the band gap energy

Figure 6. TEM image and particle size distribution of Au NPs protected by α-TG in PEG: (a) sputtering current of 10 mA, sputtering period of 20 min, and (b) sputtering current of 20 mA, sputtering period of 30 min. For the histograms, 200 Au NPs were counted in more than five TEM images. Blue lines show Gaussian fittings ((a) 2.2 nm of average diameter and 0.5 of standard deviation, (b) 3.0 nm and 0.5).

Figure 6a shows the Au NPs resulting from a sputtering current of 20 mA for 20 min, and Figure 6b shows those resulting from a current of 20 mA for 30 min. The particle sizes ranged mainly from 1 to 4 nm, and the average size increased with sputtering current and time period. Compared to the sizes of Au NPs prepared in the absence of α-TG, smaller NPs were obtained. Presumably this is due to the effective coordination of volatile α-TG, resulting in the effective suppression of the coalescence of NPs. The results demonstrate that the stabilization of NPs in a gas phase, within the sputtering process, is possible. Figure 7 shows the relationship between the sputtering time and the average size of Au NPs obtained in the presence of αTG. The size of the NPs was almost the same (approximately 5 nm) as that in the absence of α-TG, even under different sputtering currents and sputtering time periods (Table 1). However, in the presence of α-TG, the nanoparticle sizes increased with increasing current intensity and increasing 4326

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maxima were different. Interestingly, the peak maxima shifted toward the red, as the size of NPs increased. Since the band gap of NPs usually depends on the number of atoms constituting the NP (quantum size effect),25 the red-shift in fluorescence corresponds to their sizes, as summarized in Table 2. The Table 2. Sputtering Current, Sputtering Period, Particle Size from TEM, Fluorescence Maxima, and Quantum Yield of Au NPs in PEG Prepared in the Presence of α-TG

Figure 7. Average diameters of Au NPs protected by α-TG in PEG.

sputtering time periods. The changes in size relate to the probability of relative collision for α-TG in a gas phase. With regard to the sputtering current, a higher current generates a larger amount of Au atoms and clusters and the coordination of α-TG should be inefficient compared to that at a lower current. Thus, the generation of larger NPs is reasonable under a higher sputtering current. With regard to the sputtering time period, we conclude that the difference in Au NP size relates to the amount of evaporated α-TG. In this experiment, we used 0.05 g of α-TG, and almost all the α-TG evaporated after sputtering for 40 min. Therefore, the number of α-TG molecules should vary with the sputtering period; that is, the concentration of αTG in the gas phase becomes lower with increasing sputtering time, influencing the NP size. Fluorescence studies were conducted on the Au NPs stabilized by α-TG. The absence of plasmon absorption, and the very small particle sizes, enabled the fluorescence measurements. Au NPs prepared in the absence of α-TG did not fluoresce. The fluorescence spectra of Au NPs resulting from a 10 mA sputtering current are shown in Figure 8 (see the fluorescence spectra of the Au NPs obtained under other sputtering current conditions, in Figure S4). The excitation wavelength was set at the peak maximum of 3D-fluorescence spectra in Figure S5. All samples fluoresced, and the peak

sputtering current/mA

sputtering period/min

particle size/nm

fluorescence maximum/nm

quantum yield/%

10 10 10 20 20 20 30 30 30

20 30 40 20 30 40 20 30 40

2.4 2.6 3.0 2.7 2.8 3.2 2.8 3.0 3.3

705 714 749 767 730 737 739 778 763

4.6 7.5 5.0 8.3 8.5 9.0 16.1 8.9 4.1

reason why these peaks have longer wavelengths compared to those for the Au NPs previously reported20−23 is that the size of our Au NPs (2−3 nm) are larger than those characterized (1.5−2 nm) in previous studies. The excitation spectra of the Au NPs, observed at their fluorescence maxima, are also shown in Figure 8. All samples showed similar excitation spectral shapes, and exhibited obvious red-shifts, similar to the fluorescence spectra. Fluorescence quantum yields were measured by an absolute method, and ranged from 4.1% to 16.1%. The maximum value of 16.1% is extraordinary high compared to previous values reported for Au NPs (∼1% in general).2−6,32 However, these values did not suggest any tendency between the quantum yields and characteristics of NPs such as size, absorption and fluorescence maxima. Typically, Au NPs with ca. 3 nm of diameter prepared by a chemical reduction method do not fluoresce. Only the very small Au clusters such as Au8, Au23, and so forth fluoresce with a small Stokes shift. However, in the present matrix sputtering system, Au NPs with a 3 nm diameter showed near IR fluorescence with a relatively large Stokes shift and an extremely large emission quantum yield. This diameter is significantly larger than that reported previously for fluorescent Au clusters. Thus, we theorize that the obtained Au NPs are not single crystals, and consist of small Au components such as multinuclear Au complexes. Intrared emission from sulfur-Au6, -Au10, -Au12, and -Au25 multinuclear complexes have been reported with relatively large Stokes shifts and large emission quantum yields.31,33−35 Moreover, the extinction and fluorescence properties of our Au NPs are quite similar to those of a Au6 complex (excitation spectrum from 300 to 420 nm and fluorescence spectrum from 600 to 850 nm) reported in literature.32 This consideration also supports the discussion of band gap in Figure 5. It should be noted that the structure of the protecting ligand also affects the fluorescence property of Au clusters; for example, polar thiol ligands enhance fluorescence, while nonpolar ligands significantly decrease fluorescence, even in the same core Au clusters.35 Thus, we conclude that the obtained Au NPs with a 3 nm diameter (measured by TEM) are not a monocomponent, but instead are formed by the aggregation of very small, multinuclear Au components. This is the most plausible reason

Figure 8. Fluorescence and excitation spectra of Au NPs protected by α-TG in PEG subjected to 10 mA sputtering current. The excited wavelength was set at the peak maximum of 3D-fluorescence spectra in Figure S5. The excitation spectra were measured at λmax of the fluorescence. 4327

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Langmuir for the extra-high fluorescence quantum yield observed in our Au NPs. Future research aims to isolate pure Au NPs from PEG solution to determine the exact chemical structure of NPs and also to study their material chemistry using high-resolution STEM. These investigations, and further discussion of the mechanism of the highly efficient, near IR fluorescence will be reported in a future article.

(3) Link, S.; Beeby, A.; FizGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. Visible to Infrared Luminescence from a 28-Atom Gold Cluster. J. Phys. Chem. B 2002, 106, 3410−3415. (4) Yu, P.; Wen, X.; Toh, Y.-R.; Tang, J. Temperature-Dependent Fluorescence in Au10 Nanoclusters. J. Phys. Chem. C 2012, 116, 6567− 6571. (5) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging the Gap Between Gold(I)−Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261−5270. (6) Tlahuice-Flores, A.; Whetten, R. L.; Jose-Yacaman, M. Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters. J. Phys. Chem. C 2013, 117, 20867− 20875. (7) Shibu, E. S.; Radha, B.; Verma, P. K.; Bhyrappa, P.; Kulkarni, G. U.; Pal, S. K.; Pradeep, T. Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning. ACS Appl. Mater. Interfaces 2009, 1 (10), 2199−2210. (8) Guo, L.; Xu, Y.; Ferhan, A. R.; Chen, G.; Kim, D.-H. Oriented Gold Nanoparticle Aggregation for Colorimetric Sensors with Surprisingly High Analytical Figures of Merit. J. Am. Chem. Soc. 2013, 135, 12338−12345. (9) Hinterwirth, H.; Kappel, S.; Waitz, T.; Prohaska, T.; Lindner, W.; Lämmerhofer, M. Quantifying Thiol Ligand Density of Self-Assembled Monolayers on Gold Nanoparticles by Inductively Coupled PlasmaMass Spectrometry. ACS Nano 2013, 7, 1129−1136. (10) Kang, J. S.; Taton, T. A. Oligothiol Graft-Copolymer Coatings Stabilize Gold Nanoparticles against Harsh Experimental Conditions. Langmuir 2012, 28, 16751−16760. (11) Reilly, S. M.; Krick, T.; Dass, A. Surfactant-free Synthesis of Ultrasmall Gold Nanoclusters. J. Phys. Chem. C 2010, 114, 741−745. (12) Balasubramanian, R.; Kim, B.; Tripp, S. L.; Wang, X.; Lieberman, M.; Wei, A. Dispersion and Stability Studies of Resorcinarene-Encapsulated Gold Nanoparticles. Langmuir 2002, 18, 3676−3681. (13) Nalawade, P.; Mukherjee, T.; Kapoor, S. High-yield Synthesis of Multispiked Gold Nanoparticles: Characterization and Catalytic Reactions. Colloids Surf., A 2012, 396, 336−340. (14) Xie, Y.; Wei, Z.; Liu, C.-J.; Cui, L.; Wang, C. Morphologic Evolution of Au Nanocrystals Grown in Ionic Liquid by Plasma Reduction. J. Colloid Interface Sci. 2012, 374, 40−44. (15) Xiao, C.; Chen, S.; Zhang, L.; Zhou, S.; Wu, W. One-Pot Synthesis of Responsive Catalytic Au@PVP Hybrid Nanogels. Chem. Commun. 2012, 48, 11751−11753. (16) Selvam, T. S.; Chi, K.-M. Synthesis of Hydrophobic Gold Nanoclusters: Growth Mechanism Study, Luminescence Property and Catalytic Application. J. Nanopart. Res. 2011, 13, 1769−1780. (17) Torimoto, T.; Okazaki, K.-I.; Kiyama, T.; Hirahara, K.; Tanaka, N.; Kuwabata, S. Sputter Deposition Onto Ionic Liquids: Simple and Clean Synthesis of Highly Dispersed Ultrafine Metal Nanoparticles. Appl. Phys. Lett. 2006, 89, 243117. (18) Hatakeyama, Y.; Morita, T.; Takahashi, S.; Onishi, K.; Nishikawa, K. Synthesis of Gold Nanoparticles in Liquid Polyethylene Glycol by Sputter Deposition and Temperature Effects on their Size and Shape. J. Phys. Chem. C 2011, 115, 3279−3285. (19) Castro, H. P. S.; Wender, H.; Alencar, M. A. R. C.; Teixeira, S. R.; Dupont, J.; Hickmann, J. M. Third-Order Nonlinear Optical Response of Colloidal Gold Nanoparticles Prepared by Sputtering Deposition. J. Appl. Phys. 2013, 114, 183104. (20) Shishino, Y.; Yonezawa, T.; Kawai, K.; Nishihara, H. Molten Matrix Sputtering Synthesis of Water-Soluble Luminescent Au Nanoparticles with a Large Stokes Shift. Chem. Commun. 2010, 46, 7211−7213. (21) Shishino, Y.; Yonezawa, T.; Udagawa, S.; Hase, K.; Nishihara, H. Preparation of Optical Resins Containing Dispersed Gold Nanoparticles by the Matrix Sputtering Method. Angew. Chem., Int. Ed. 2011, 50, 703−705. (22) Zheng, J.; Zhou, C.; Yu, M.; Liu, J. Different Sized Luminescent Gold Nanoparticles. Nanoscale 2012, 4, 4073−4083.



CONCLUSION In this study, we propose a novel strategy to control the growth of Au NPs by using a volatile stabilizer, α-TG, in the sputtering chamber. The size of the Au NPs obtained, measured from TEM micrographs, varied with the presence (around 2−3 nm) or absence (around 5 nm) of α-TG. Plasmon absorption was observed at 2.4 eV (520 nm) in the larger NPs; however, it was not observed and instead a new peak was found in smaller NPs, at around 3.4 eV (360 nm). The Au NPs stabilized by α-TG fluoresced in the near IR region. The fluorescence maxima shifted toward the red, according to the size of the NPs, and the photophysical characteristics of the Au NPs were similar to those of Au6 nanoclusters, from both experimental and theoretical considerations. Under optimum conditions, a very high fluorescence quantum yield of 16.1% was realized. In the future, Au nanoclusters with different cluster size distributions can be achieved by varying the concentration of α-TG and the sputtering current. This is because the α-TG concentration will affect the collision probability of Au nanoclusters in the nucleation stage, and the sputtering current will control the probability of their coalescence in the gas phase. The novel method presented in this paper therefore opens a new door for the effective control of size and fluorescence of metal NPs. It is hoped that these results will contribute widely to the body of science for metal NPs.



ASSOCIATED CONTENT

* Supporting Information S

TEM images, extinction, fluorescence, and excitation spectra, photographs of samples, and experimental setup for Au NPs prepared in the presence and absence of α-TG. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is partially supported by Grant-in-Aid for Scientific Research in Priority Area “New Polymeric Materials Based on Element-Blocks (2401)” (25102501) from MEXT and Grantin-Aid for Scientific Research (A) (24241041) from JSPS, Japan. We also thank Dr. Masaki Matsubara (Hokkaido Univ.) for his kind support on the preparation of schematic illustrations.



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DOI: 10.1021/acs.langmuir.5b00294 Langmuir 2015, 31, 4323−4329

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DOI: 10.1021/acs.langmuir.5b00294 Langmuir 2015, 31, 4323−4329

Formation and optical properties of fluorescent gold nanoparticles obtained by matrix sputtering method with volatile mercaptan molecules in the vacuum chamber and consideration of their structures.

This paper proposes a novel methodology to synthesize highly fluorescent gold nanoparticles (NPs) with a maximum quantum yield of 16%, in the near-inf...
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