Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold

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Interface-Rich Materials and Assemblies

Sputter Deposition Towards Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography Yohei Ishida, Akihiro Morita, Tomoharu Tokunaga, and Tetsu Yonezawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00067 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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Sputter Deposition Towards Short Cationic Thiolated Fluorescent Gold Nanoclusters: Investigation of their Unique Structural and Photophysical Characteristics Using High-Performance Liquid Chromatography

Yohei Ishidaa, Akihiro Moritaa, Tomoharu Tokunagab, and Tetsu Yonezawaa,*

a

Division of Material Science and Engineering, Faculty of Engineering, Hokkaido

University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan b

Department of Quantum Engineering, Graduate School of Engineering, Nagoya

University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan E-mail: [email protected] (T.Y.)

1 ACS Paragon Plus Environment

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ABSTRACT We herein present the preparation of short, bulky cationic thiolate (thiocholine) protected fluorescent Au nanoclusters via sputter deposition over a liquid polymer matrix. The obtained Au nanoclusters showed near-infrared fluorescence and had an average core diameter of 1.7 ± 0.6 nm, which is too large compared with that of the reported fluorescent Au nanoclusters prepared via chemical means. We revealed the mechanism of formation of this unique material using single-particle electron microscopy, optical measurements, X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography fractionations. The non-crystallized image was observed via single-particle high-angle annular dark-field scanning transmission electron microscopy observations and compared with chemically synthesized crystalline Au nanoparticle with the same diameter, which demonstrated the unique structural characteristic speculated via XPS. The size fractionation and size-dependent fluorescence measurement, together with other observations, indicated that the nanoclusters most probably contained a mixture of very small fluorescent species in their aggregated form, and were derived from the sputtering process itself and not from the interaction between thiol ligands.


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INTRODUCTION Fluorescent gold nanoclusters have been used in various fields such as sensing, optoelectronics, cell labeling, and bioimaging owing to their unique optical properties and chemical stability.1-11 These nanoclusters are considered to be transitions between single noble metal atoms, which show distinct optical properties, and noble metal nanoparticles, which show characteristic surface plasmon resonance.12 The number of atoms constituting the nanoclusters tunes the fluorescence wavelength.13 The potential application of fluorescent nanoclusters includes biomedical applications, and they are hypothesized to replace organic dyes and quantum dots, particularly for sensing, imaging, therapy, and targeted drug delivery.1-2 Noble metal nanoclusters and nanoparticles are commonly synthesized using chemical reduction in the presence of stabilizing molecules such as DNA, proteins, dendrimers, phosphine, and thiol using metal salt precursors and reducing agents such as NaBH4.1,8,14-21 Among these strategies, thiol-based synthesis pioneered by Brust22 for the synthesis of plasmonic nanoparticles and extended by Whetten and Murray23-26 for the synthesis of (fluorescent) metal nanoclusters is becoming popular and is known to produce nanoclusters with various properties. However, despite its promise, the application of fluorescent metal nanoclusters is still in its infancy owing to purity- and toxicity-related issues, especially considering the harmful byproducts obtained from the use of reductants.1–2, 6 A greener synthetic approach of fluorescent nanoclusters is thus desired in this regard, leading to the development of strategies based on physical synthetic


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approaches that do not require reductants such as laser ablation27 and sputtering techniques28-34. Noble metal nanoclusters synthesized via physical means, such as the matrix sputtering techniques pioneered by our group35 (Figure 1), are rare. The basic idea of this method is the physical ejection of surface atoms through the collision of ionized gas (commonly Ar+) on a metal target and deposition of these ejected atoms/clusters onto a liquid matrix such as ionic liquids or polyethylene glycol (PEG) to capture the nanoclusters or nanoparticles. Fundamentally, a common feature of these liquid matrices is their low vapor pressure, which is a prerequisite to prevent vaporization under vacuum conditions. While many researchers reported series of plasmonic noble metal nanoparticles with diameters in the range of ca. 3–10 nm28-30, we developed a novel methodology to obtain fluorescent nanoclusters with diameters less than 3 nm by using thiol molecules as a stabilizer, inspired from chemical methods.35 In the chemical synthesis of metal nanoparticles, controlling the concentration ratio between metal ions and stabilizing reagents is a possible means of systematic size control. This idea is also applicable in the sputtering process—the strong affinity of thiols toward noble metal atoms enables the formation of fluorescent nanoclusters via sputtering deposition. We succeeded in synthesizing various fluorescent monometallic nanoclusters of Au, Ag, Cu, and bimetallic Au-Ag nanoclusters, all of which exhibited stable emission in both solution and solid form via our matrix sputtering method with the induction of cationic-, neutral-, and anionic-charged thiol ligands.35-39 Moreover, we revealed the most plausible mechanism of formation of these unique fluorescent nanoclusters, which had relatively larger diameters (ca. 1–3 nm) as compared with


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typical fluorescent nanoclusters prepared via chemical means.35,40 All the Au nanoclusters (1.0–3.3 nm) obtained via our matrix sputtering process showed near-infrared (NIR) fluorescence in the range of 657–778 nm; however, the difference between their particle diameters should be sufficiently large to exhibit more drastic spectral changes in their emission. Since the fluorescence wavelength of the obtained nanoclusters can be tuned by changing the concentration of thiol ligands during the synthesis, the fluorescent core is not only the surface Au–thiol species but also the nanocluster cores certainly contribute.35-39 A detailed isolation study using high-performance liquid chromatography (HPLC) for Au nanoclusters stabilized with 11-mercaptoundecanoic acid (MUA) indicated that the nanoclusters most probably contained a mixture of very small fluorescent nanoclusters or multinuclear complexes in their secondary aggregated form and not as single crystals.40 However, it is still unknown whether the driving force for the formation of their aggregates is the multiple hydrogen bonds between MUA ligands or the sputtering process itself. We hypothesize that a short, bulky cationic thiol ligand, thiocholine (TC), which does not have any strong interaction required for the formation of secondary aggregations, provide an answer to this question. In this paper, we report the synthesis of fluorescent Au nanoclusters stabilized using

TC

ligands

via

the

sputtering

method.

Information

about

cationic-thiolate-protected nanoclusters41-46 is scarce because the electrostatic repulsion between cationic ligands on the surface of the nanoclusters hinders the formation of small nanoclusters. Our sputtering technique, however, enables the synthesis of this


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material owing to a unique mechanism via sputtering deposition where small cluster species grow in both gas and liquid phases, in contrast to the common chemical means.42 With detailed investigations using electron microscopy, optical measurements, X-ray photoelectron spectroscopy (XPS), and reversed-phase HPLC (RP-HPLC), we discuss the origin of the unique structural and photophysical characteristics of these nanoclusters.

EXPERIMENTAL SECTION Materials PEG with molecular weight of 600, chloroform, acetonitrile, methanol, and tetrahydrofuran (THF) were purchased from Junsei. HAuCl4, acetylthiocholine iodide, NaBH4, and potassium hexafluorophosphate (KPF6) were obtained from Aldrich. NaBH4 was obtained from Kanto. All the chemicals were used as received. Thiocholine bromide was synthesized via the hydrolysis of acetylthiocholine iodide as reported in the previous paper42 and the counter ion was exchanged with PF6– using KPF6. TC+・ PF6– is abbreviated as TC henceforth. The purity of the obtained TC was verified using proton nuclear magnetic resonance (1H-NMR). Deionized water (> 18.2 MΩ prepared with Organo/ELGA Purelab system) was used in the present study. Au target (99.9%) was supplied by Tanka Precious Metals (Japan).

Synthesis of fluorescent Au nanoclusters via sputtering method


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A schematic illustration of the experimental set-up is shown in Figure 1. To remove volatile substances such as water, PEG and TC were dried under vacuum (rotary pump) at 100 °C for 2 h. Owing to this pre-treatment, we did not observe bubbles (which hinder stable and continuous sputtering) in the liquid matrix inside the vacuum chamber. Subsequently, 10 g of PEG and 56 mg of TC (corresponding to 2.5 × 10−2 M) were placed in a glass petri dish with a diameter of 6.5 cm and horizontally set against the sputtering target. TC was completely dissolved in PEG under these conditions. The sputtering of Au was conducted with a current of 30 mA under Ar atmosphere at a pressure of 2.0 Pa for 20 min at 30 ºC under stirring at 100 rpm. The distance between the surfaces of PEG and Au target was set as 50 mm.

Synthesis of Au nanoparticles via chemical reduction method We synthesized TC-protected Au nanoparticles via chemical means in order to understand the unique property of Au nanoclusters obtained via sputtering. Briefly, 0.19 mL of HAuCl4 aqueous solution (2.0 × 10–2 M) was mixed with 3 mg of TC (3.8 × 10-6 mol) in 2 mL of THF/water solution (50/50 vol%). Subsequently, 50 µL of NaBH4 (0.1 M) was gradually dropped under stirring at room temperature for 3 h. The obtained Au nanoparticles were collected as a solid after reprecipitation with THF.

Analysis The extinction spectra of the Au nanoclusters were measured using a UV–vis spectrophotometer (Shimadzu, UV-1800) in a quartz cell with an optical path of 1 mm


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immediately after the sputtering deposition without dilution and purification. The fluorescence and excitation spectra of the obtained Au nanocluster dispersion in PEG were measured using a fluorescence spectrophotometer (JASCO, FP-6600) with a quartz cell of optical path 10 mm after dilution 10 times with acetonitrile (vol / vol). Emission lifetime was recorded on a picosecond fluorescence lifetime measurement system (C11200, Hamamatsu) equipped with picosecond light pulser (C10196), spectrograph (C11119–02), and streak scope (C10627). The excitation wavelength was at 350 nm, and detected at 650–800 nm. An optical cut filter (< 440 nm) was attached to the front of the detection window for steady-state and time-resolved emission measurements. Transmission electron microscopy (TEM) was employed to observe the size and shape of Au nanoclusters using JEM-2000FX (JEOL, acceleration voltage of 200 kV). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM; JEOL, JEM-ARM200, acceleration voltage of 200 kV) was used for the analysis of a single nanocluster/nanoparticle. TEM and STEM samples were prepared by dropping the sample (PEG dispersion) onto collodion-coated copper grids. The grids were thereafter soaked in methanol for 30 min to remove the excess PEG and TC and subsequently dried under vacuum. XPS was detected using JEOL JPS-9200 equipped with a monochromatic Mg Kα source operating at 100 W under ultrahigh vacuum (~ 1.0 × 10–7 Pa) conditions. The binding energies were referenced to the C1s binding energy of the adventitious carbon contamination. For XPS measurements, the Au nanoclusters were purified via reprecipitation using chloroform and methanol and the formed solid was collected via centrifugation at 3000 rpm for 20 min. As the


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fluorescence spectrum did not change during the purification, we concluded that we successfully purified the Au nanoclusters and removed the excess PEG and TC using this procedure. The purified Au nanoclusters were fractionated using HPLC (Jasco, LC-NetII/ADC system). An RP-HPLC column (Shimadzu, Shim-pack VP-ODS, 150 × 4.6 mm, particle size of 5 µm) was used with the HPLC apparatus. The dispersion of Au nanoclusters in acetonitrile (20 µL, concentration of 50.0 g L−1) was injected to the HPLC system. The mobile phase used was 100% acetonitrile at a flow rate of 0.1 mL min–1 (Jasco, PU-4180) at a constant temperature of 40 °C (Jasco, CO-4060). The chromatogram was collected by measuring the absorption at 500 nm (Jasco, UV-4075).

Ar+ Au atoms /clusters

TC in PEG (2.5 × 10-2 M) N HS

PF6–

TC

Figure 1. Schematic illustration of the matrix sputtering method and chemical structure of TC

RESULTS AND DISCUSSION Characterization of Au nanoclusters synthesized via the matrix sputtering method


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TC-protected Au nanoclusters were prepared via the sputtering method outlined in the Experimental Section. The obtained Au nanoclusters were first characterized using absorption spectroscopy (Figure 2a). The Au nanoclusters did not show any characteristic absorption peaks such as plasmon resonance at 520 nm as shown in Figure 2a. The featureless extinction profile indicated that the diameter of the obtained Au nanoclusters was significantly less than 3 nm. This extinction characteristic is almost similar to that of our previous Au nanoclusters protected with various neutral, anionic, and cationic thiol ligands.35,38-40,46 We used TEM observations to verify this result. Figure 2b shows a representative TEM image and size-distribution histogram of the obtained Au nanoclusters. The average size of the Au nanoclusters was 1.7 ± 0.6 nm, which is consistent with the speculation4 obtained using the featureless extinction profile. The size distribution was relatively wider than in our previous systems owing to the lower concentration of thiol used in this experiment (2.5 × 10−2 M).40 Figure 2c shows the fluorescence spectrum of Au nanoclusters excited at 350 nm (the excitation spectrum is shown in Figure S1 in the Supporting Information). As evident from the red emission color in the sample image under UV light (Figure 2d), the emission of the obtained Au nanoclusters was in the NIR region (λmax = 755 nm). Our previous study on the series of Au nanoclusters also demonstrated NIR fluorescence in the range of 657– 778 nm, which is related to their particle diameters (1.0–3.3 nm) in general; the nanoclusters with smaller diameter tend to show higher (shorter wavelength) fluorescence.35 Excited lifetime of the obtained Au nanoclusters was recorded by time-resolved emission measurement system (Figure S2). The average excited lifetime


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of the Au nanoclusters was roughly 1 µs. The magnitude of this value is similar to those reported for polymeric Au(I)–S nanoclusters or Au multinuclear complexes.47–49 We further analyzed the obtained Au nanoclusters using XPS after purification via the repeated reprecipitation method as outlined in the Experimental Section. It is known that the XPS peaks of 4f5/2 and 4f7/2 tend to shift to higher energy regions as the size of the nanoclusters (i.e., number of Au atoms constituting the nanocluster) decreases.50 As shown in Figure 3, we observed two peaks at 88.5 and 84.9 eV corresponding to 4f5/2 and 4f7/2, respectively, and they were located at higher energy compared with bulk gold (87.4 and 84.0 for 4f5/2 and 4f7/2, respectively). These values are consistent with those reported for thiolated Au38 nanoclusters, which have a diameter of 1.1 nm, as determined using TEM.50 Moreover, the XPS of the chemically synthesized thiolated Au nanoparticles with a similar diameter (1.6 nm) was reported to be located at 84.3 eV for 4f7/2,51 which is at a lower energy compared with our Au nanoclusters synthesized via sputtering with a diameter of 1.7 nm. These results demonstrate the unique structure of sputtered Au nanoclusters and the difference between

physically

(sputtering)

and

chemically

nanoclusters/nanoparticles.


synthesized

Au

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(a)

(b)

2

35 30 25

Ratio / %

Extinction

1

20 15 10 5

0 300

400 500 Wavelength / nm

600

0

20 nm

0.0 1.0 2.0 3.0 4.0 5.0 Diameter / nm

(c)

(d)

2500 2000

Intensity / a.u.

1500 1000

500 0

600

700 800 Wavelength / nm

Figure 2. (a) Extinction spectrum, (b) representative TEM image and size-distribution histogram, (c) fluorescence spectrum excited at 350 nm, (d) sample pictures under daylight (left) and UV light (right) of the obtained Au nanoclusters. For emission measurements, the sample (PEG dispersion) was diluted 10 times with acetonitrile (vol / vol).

1400 1200

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1000

800 600 400 200

82

84

86 88 90 Binding Energy / eV

Figure 3. XPS of the purified Au nanoclusters in Au4f region


92

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Single-particle

scanning

transmission

electron

microscopy

for

physically

synthesized Au nanoclusters and chemically synthesized Au nanoparticles To understand the unique structural characteristic of the Au nanoclusters obtained via sputtering, we conducted a single-particle HAADF-STEM. The advantage of HAADF-STEM method is the realization of atomic-number-based chemical contrast, which is beneficial for heavy atoms such as Au in the present study. For a comparison with the sputtered Au nanoclusters, we also attempted to chemically synthesize Au nanoparticles with similar diameters via common chemical reduction means. With the reduction of HAuCl4 using NaBH4 in the presence of TC (see the detailed procedure outlined in the Experimental Section), we obtained non-plasmonic Au nanoparticles with the diameter of 1.5 ± 0.2 nm. The nanoparticles obtained were not fluorescent under UV-light irradiation. Their extinction spectrum, representative TEM image, and size distribution histogram are shown in Figure S3 in the Supporting Information. Figure 4 shows the representative HAADF-STEM images of the single particles

of

physically

(sputtering)

or

chemically

synthesized

Au

nanoclusters/nanoparticles with the same diameter of 1.7 nm, and their corresponding intensity line profiles for the highlighted regions. We could not quantitatively compare the difference in their intensities (brightness), as these two samples were measured separately using different STEM grids; however, the brightness of the chemically synthesized Au nanoparticle (right) appears to be higher than that of the Au nanocluster (left). The line profile of the chemically synthesized Au nanoparticles (right) showed


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intervals (bond distances) of 2.4–2.5 Å whereas a similar value was observed in Au144 (thiolate ligand)60 nanoclusters (2.47–2.49 Å) with a diameter of ca. 1.8 nm.52 The crystal structure of small Au nanoclusters (smaller than Au144(thiolate ligand)60) had a molecule-like characteristic and was different from that of the bulk Au (face-centered cubic), which was observable for the nanoclusters larger than Au187(thiolate ligand)68.53 However, the physically synthesized Au nanocluster (left image) showed a featureless intensity line profile with no specific information about bond distances. These HAADF-STEM results, combined with the optical measurements and XPS, strongly indicated that the Au nanoclusters obtained via sputtering were not single crystals and could be composed of aggregates of small fluorescent Au components.

Figure 4. HAADF-STEM images of the physically synthesized Au nanocluster (left) and the chemically synthesized Au nanoparticle (right), both protected with TC.


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Size fractionation of physically synthesized Au nanoclusters to reveal their unique fluorescence property In the previous section, HAADF-STEM results revealed the unique structural characteristics of the Au nanoclusters obtained via sputtering. To reveal their unique fluorescence property, we conducted size fractionation using an RP-HPLC. Purified Au nanocluster was dissolved in acetonitrile and fractionated using the procedure outlined in the Experimental Section. Figure 5 shows a chromatogram of the isolation of Au nanoclusters recorded by monitoring the absorbance at 500 nm. The chromatogram contained multiple peaks, indicating that the mixture was successfully separated using the RP-HPLC technique. The relative intensity of each peak did not directly reflect the relative abundance of each component in the sample as the extinction coefficient at the observed wavelength was unknown. We collected the five fractions I–V as marked in Figure 5. We did not observe peaks after the isolation at 18 min.

I

998000

II III IV V

Absorbance / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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798000 598000 398000 198000

000 0

5

10

15

20

Retention time / min

Figure 5. RP-HPLC chromatogram of Au nanoclusters detected using the absorbance at 500 nm


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Figure 6 shows the representative TEM images and size distribution histograms obtained from these images for fractions I–V. All the fractions showed a narrow core size distribution and the average diameters decreased in the order from I to VI as summarized in Table 1. The core diameters were successfully fractionated to 4.7 ± 1.0, 2.6 ± 0.4, 1.8 ± 0.3, 1.8 ± 0.3, and 1.4 ± 0.2 nm for the fractions I to V, respectively.


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I

4.7 ± 1.0 nm 30

20 nm

Ratio / %

25 20 15 10 5 0

0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 6.0 6 0.0

Diameter / nm

II

2.6 ± 0.4 nm 50

20 nm

Ratio / %

40 30 20 10 0 0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 0.0

Diameter / nm

III 1.8 ± 0.3 nm

60

20 nm

Ratio / %

50 40 30 20 10 0

0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 0.0

Diameter / nm

IV 1.8 ± 0.3 nm 70

20 nm

Ratio / %

60 50 40 30 20 10 0

V

0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 0.0

Diameter / nm

1.4 ± 0.2 nm

80

20 nm 60

Ratio / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 20 0

0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 0.0

Diameter / nm

Figure 6. TEM images and size distribution histograms of fractions I to V


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The fluorescence spectra (excited at 350 nm) for the fractions I–V obtained using RP-HPLC are shown in Figure 7. Notably, we did not observe any spectral change in these fractions according to their diameters (see the summary of their fluorescence maxima in Table 1). The chemically synthesized fluorescent Au nanoclusters in general show a size-dependent fluorescence spectral red (blue) shift as the number of atoms constituting the nanocluster increases (decreases).13 While the difference in the core diameters of fractions I–V is very large (1.4–4.7 nm), it must be sufficient to exhibit spectral changes in the fluorescence. The observed fluorescence spectra were constant. The constant fluorescence spectra after size fractionation were also observed in our previous study using size-fractionated MUA-protected Au nanoclusters with diameters ranging from 1.2 to 1.9 nm.40 These results, in addition to other observations (Figures 2–4), indicated that the nanoclusters most probably contained a mixture of very small fluorescent nanoclusters or multinuclear complexes in their secondary aggregated form. As coalescence is a diffusion-limited process, the nanoclusters showed a distribution in their diameters (Figures 2b and 6). However, we observed the constant fluorescence spectra for these fractions I–V containing differently sized Au nanoclusters (Figure 7) because they were formed by the secondary aggregation of smaller fluorescent components. As described in the Introduction Section, it is still unknown whether the driving force for the formation of aggregates is the strong interaction between the thiol ligands (such as possible multiple hydrogen bonds between MUA ligands used in our


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previous work) or the sputtering process itself. The use of short, bulky cationic TC ligand, which does not have any strong interaction required for the formation of secondary aggregations such as hydrogen bonds or van der Waals force, is generally caused by long-alkyl chains; therefore, it can be concluded that the sputtering process itself is the driving force for the formation of Au nanoclusters with unique structural and photophysical characteristics. In the unique growth process of nanoclusters via sputtering, very small atoms/clusters initially ejected from the metal-target coalesce into quasi-stable nanoclusters in the gas phase where TC ligands are absent, and further growth in the liquid phase with stabilization using TC substantializes the fluorescence of Au nanoclusters.

Figure 7. Fluorescence spectra of fractions I–V containing Au nanoclusters in acetonitrile suspension excited at 350 nm


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Table 1. Summary of the average diameters and maximum fluorescence wavelengths of fractions I–V Fraction

Average diameter / nm

Fluorescence maximum / nm

I

4.7 ± 1.0

754

II

2.3 ± 0.3

755

III

1.8 ± 0.3

756

IV

1.8 ± 0.3

755

V

1.4 ± 0.2

756

CONCLUSION In conclusion, we synthesized TC-protected fluorescent Au nanoclusters via matrix sputtering method. We revealed the mechanism for the formation of this unique material using single-particle electron microscopy, optical measurements, XPS, and RP-HPLC fractionations. The non-crystallized image was observed via single-particle HAADF-STEM observations and compared with chemically synthesized crystalline Au nanoparticle with the same diameter, which demonstrated the unique structural characteristic speculated via XPS. The size fractionations and size-dependent fluorescence measurement, together with other observations, indicated that the nanoclusters most probably contained a mixture of very small fluorescent nanoclusters or multinuclear complexes in their aggregated form, and were derived from the sputtering process itself and not from the interaction between thiol ligands. These findings enable further understanding of the formation process of fluorescent


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nanoclusters via sputtering and thus, they would be beneficial for novel greener preparation of metal nanomaterials.

ACKNOLEDGEMENT This work was financially supported from Building of Consortia for the Development of Human Resources in Science and Technology, Ministry of Education, Culture, Sports, Science and Technology, Japan (to YI), and Murata Science Foundation (to TY). We are deeply grateful to Prof. Y. Hasegawa and Dr. Y. Kitagawa (Hokkaido Univ.) for their assistance of lifetime measurements.

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Dreaden, E. C.; Alkilany, A. M.; Huang, X.; Murphy, C. J.; El-Sayed, M. A. The Golden Age: Gold Nanoparticles for Biomedicine. Chem. Soc. Rev. 2012, 41, 2740–2779.

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TOC Graphic Fluorescence


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Revised Figure 4 77x82mm (96 x 96 DPI)

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Sputter Deposition toward Short Cationic Thiolated Fluorescent Gold

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