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Microwave-facilitated rapid synthesis of gold nanoclusters with tunable optical property for sensing ions and fluorescent ink DOI: 10.1039/x0xx00000x
a
a
a
Jia Zhang,* Yue Yuan, Gaolin Liang, Muhammad Nadeem Arshad, c a Sobahi, and Shu-Hong Yu*
b,c
c
Hassan A. Albar, Tariq R.
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Metal nanoclusters are composed of exactly a few to hundreds of atoms, with the core sizes ranging from subnanometer to 1 approximately 2 nm. Both theoretical modeling and experimental evidence reveal that electron energy quantization occurs in such ultrasmall size regime, resulting in discrete electronic structure and 2 molecule-like properties such as quantized charging and 3 luminescence. Until now, a number of ligands (or stabilizers) have been used to protect gold nanoclusters against aggregation and oxidation, which can be simply classified to two categories, i.e. 4-6 macromolecules (proteins, dendrimers, and synthetic polymers) 7-9 and small molecules (thiols and rationally designed peptides). And thiolate-stabilized gold nanoclusters (thiolated AuNCs for short) are widely studied, among which glutathione-capped AuNCs (GSAuNCs) is perhaps the most extensively studied system. Basically, the synthesis of GS-AuNCs is almost via the reduction of polymeric GS-Au(I) complexes, and NCs with specific gold atoms can be formed by tuning reaction parameters, including solvent, pH, 10 temperature, and reductant. While these GS-AuNCs exhibit visible to infrared luminescence, their quantum yield (Φ) are mostly low. Recently, Xie et al. improved the preparation of GS-AuNCs by the 11 use of heat in 24 h and elucidated the luminescence mechanism, but some key issues still remain opaque, such as how the clusters evolve. Moreover, 24 hours of reaction is still time-consuming. Here, we report a facile route to rapidly synthesize luminescent GS-AuNCs with Φ reaching 4.4% via the decomposition of GS-Au(I) complex by microwave irradiation. By increasing the time, the
a.
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China. Fax: 86-551-63603040; Tel: 86-551-63603040; E-mail: [email protected]; [email protected]. b. Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia. c. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia. Electronic Supplementary Information (ESI) available: Experimental details, Fig. S1S12 and Table S1-S6 as noted in the text. See DOI:10.1039/x0xx00000x
photoluminescent (PL) emission wavelengths are blue-shifted and the particle sizes increase proportionally. The influences of microwave conditions including time, power, and temperature on the reaction have been thoroughly investigated. Owing to the remarkably strong luminescence, the GS-AuNCs are tested their capability for nanomolar-level metal ions sensing and potential use as invisible fluorescent ink. To our knowledge, it is the first attempt to use AuNCs as fluorescent ink. (a)
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Luminescent glutathione-capped gold nanoclusters (GS-AuNCs) with tunable emissions are efficiently synthesized by a solutionbased microwave method.
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Received 00th January 20xx, Accepted 00th January 20xx
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Fig. 1 (a) UV-vis absorption spectra and (b) photoemission spectra of GS-AuNCs prepared within from 10 min to 3 h. In figure (a), plots have been shifted upwards for clarity. Photos of the as-prepared AuNCs solutions under (c) room light and (d) UV light (365 nm). The synthetic conditions: microwave temperature and power are 90 oC and 300 W, solution pH is 2.0, and GSH/Au molar ratio is 1.5.
The time evolution of GS-AuNCs from the dissociation of GS-Au(I) complex is observed from continual changes in the absorption spectra (Fig. 1a). As time increases, the absorption spectra of solutions exhibit a hyperchromic shift of onset from 440 nm (10 min) to 480 nm (20 min) and finally to 500 nm (30 min-3 h), but a shoulder peak at 400 nm is only clearly observed after 3 h of reaction. Neither step-like multiple bands nor surface plasmonic band around 520 nm appear in the spectra, indicating the obtained 12,13 NCs are not conventional Aun(SG)m clusters and the cores of the 14 NCs in the metallic size regime are less than 2 nm. The color of solutions turns from light yellow to bright yellow with time (Fig. 1c). Photoemission spectra (Fig. 1b) better illustrate the evolution of these NCs. Since the original GS-Au(I) complex is known having no luminescence, the main broad emission peak at 670 nm with two shoulder peaks at 710 and 810 nm appearing in the spectrum of
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solution (after 10 min) demonstrate the decomposition of the GSAu(I) complex to yield luminescent clusters. Prolonging reaction time will induce obvious hypochromic shift as well as narrowing of the main emission peak, and besides, the two shoulder peaks are suppressed but not vanish. Table S1 lists the main emission wavelength (λem) and Φ of the AuNCs prepared within from 10 min to 3 h. Fig. 1d shows the as-prepared AuNCs solutions under UV light. One can observe a color transition from faint red to intense orange. The most intense orange-emitting solution (after 3 h) shows a well-defined emission peak at 605 nm, with Φ measured to be 2.4% (using quinine sulfate as reference, 54%). Further extending the reaction, we found the highest Φ (4.4%) was achieved for NCs by 5 h of reaction (Fig. S1a). We then found the increase of microwave power shall result in slightly larger photoemission intensity (Fig. S1b). We also studied the effect of microwave temperature. Basically, elevating temperature helps to increase the PL intensity (Fig. S1c), and improves Φ from 0.32% (70 o o o C) to 0.89% (90 C) and then to 1.1% (110 C). We tried to prolong o the reaction time at 110 C to obtain a higher Φ than 4.4%, but the 11 attempt failed. As control, we followed the previous method and obtained similarly orange-emitting NCs with Φ of 1.2% after o reaction at 70 C for 24 h. The above results may suggest that the microwave-assisted method is superior to the heat-facilitated one in terms of using less time to obtain higher quantum efficiency. The o synthesis of AuNCs (by 3 h at 90 C and 300 W) is quite repeatable. For five syntheses, the relative standard deviations relative to Φ and dominant PL emission wavelength were 8.3% and 0.36%, respectively. The storage stability of the AuNCs was studied. We barely observed changes in the emission spectra after 60 days of o storage at 4 C (Fig. S2a). However, we also noted they were not stable at ambient environment, with all emissions evolving to ~605 nm (Fig. S2b). This implies the AuNCs (by 3 h) own the highest thermodynamic stability compared to those clusters formed in other shorter time windows.
Fig. 2 TEM and high-resolution TEM images of GS-AuNCs for different time of reaction: (a) 10 min; (b) 20 min; (c) 30 min; (d) 45 min; (e) 60 min; (f) 90 min; (g) 120 min; (h, i, j) 180 min. The insets in the upper right corner of (a) to (g) are the size histograms by DLS, and that of (h) contains both the size histograms by DLS and TEM. (k) The EDX pattern of the GS-AuNCs (by 3 h).
We used transmission electron microscope (TEM) and dynamic light scattering (DLS) to give absolute evidence of the generation of clusters, as shown in Fig. 2. The size of NCs (by 3 h) is apparently
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larger than that of NCs (by 10 min), but there is some difficulty to View Article Online DOI: 10.1039/C5CC03086B differentiate the size of two adjacent clusters, like NCs by 10 and 20 min of reaction. Fortunately, the DLS shows the hydrodiameters (HDs) of the AuNCs almost increase with the irradiation time (Table S2). By correlating λem with the size of clusters, we find the λem shifts inversely with size increase. This observation of anti-Jellium model provides compelling proof for the {Au(0) core/Au(I)-SG complex shell} structure of the GS-AuNCs and that the origin of luminescence should be ascribed to aggregation-induced emission 11 (AIE) of the shell. Of note is that the AuNCs do not continue to grow after elongation of reaction time past 3 h, identified by the size of clusters prepared for 5 h (Fig. S3). A high-resolution TEM image of the AuNCs (by 3 h) reveals lattice fringe separated by 0.235 nm (Fig. 2j), identical with the (111) lattice spacing of the face-centered cubic (fcc) gold. However, the NCs do not adopt the fcc structure because the diffraction angles are deviant from those for the fcc gold (Fig. S4). This echoes with the other gold 15,16 clusters. Besides, the appearance of lattice fringes featured by fcc Au does imply a kind of connection between nanoclusters and nanocrystals. The energy dispersive X-ray (EDX) spectrum consolidates the existence of Au and S in the sample. The yield of AuNCs (by 3 h) is 99% (Au atom basis). We further used matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS) to analyze the molecular formula of the AuNCs, as shown in Fig. S5. Unfortunately we do not observe meaningful m/z peaks between 1 kDa and 30 kDa in the spectra collected in the linear negative ion mode. For the spectra collected in the linear positive ion mode, some m/z peaks exist between 1 kDa and 3.5 kDa, and the peaks at larger m/z become more appreciable at a larger reaction time. Given that the clusters have relatively large sizes, the MS result clearly suggests the occurrence of fragmentation. Since the process is rather complex, we do not assign the fragmented peaks. o We used the GS-AuNCs (90 C/180 min, denoted as AuNCs-3h) in following study because of its thermodynamic stability and fairly high Φ. All the expected atoms, including Au, S, C, N and O, are revealed in X-ray photoelectron spectroscope (XPS) survey scan (Fig. S6a). The individual Au and S spectra show Au 4f7/2 peak centered at 84.3 eV and S 2p peak centered at 163.0 eV (Fig. S6b,c). Integration of Au 4f and S 2p peak intensities gives a S/Au atomic ratio of 0.83. According to the previous study, the Au 4f7/2 spectrum contains Au(I) and Au(0) components with binding energies of 84.35 and 83.8 eV. The content of Au(I) in the shell constituted 82% of the total Au in AuNCs. This suggests that the atomic ratio of S/Au in the Au(I)-SG complex in the shell of such luminescent NCs is very close to 1:1. The molar ratio of S/Au in the clusters was further analyzed by thermogravimetric analysis (TGA) data. As shown in Fig. S6d, the weight loss behavior for our AuNCs is in large difference from those for the monolayer-protected 7,15,17 AuNCs. Two repetition of analyses gave 36.1% and 37.5% major mass loss, and 38.2% and 38.5% total mass loss between 100 o and 800 C, which can be converted to S/Au atomic ratios of 0.38 and 0.39, respectively. Such significant discrepancy in the S/Au atomic ratio derived from XPS and TGA data compelled us to perform quantitative elemental analysis of the clusters. It shows the AuNCs contain 19.57% C, 2.99% H, 6.78% N, 18.91% O, and 6.65 % S (Table S3), yielding the atomic ratio of C:H:N:O:S being 7.8:14.4:2.3:5.7:1, which is close with the thiolated glutathione
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We recorded the emission intensity of the AuNCs-3h solution in the presence of various ions (Fig. S9). For metal ions, some ions were found to quench the luminescence of the NCs, with the most 2+ appreciable for divalent Cu . For anions, only sulfide decreased the PL intensity a little, but at 100 µM compared to 1 µM for metal ions. 2+ Therefore, the AuNCs can be used as a sensor to detect Cu ion. The optimization of parameters for the detection is discussed in
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Fig. 3 (a) The PL spectra of AuNCs-3h solution (A410nm = 0.006) incubated with Cu2+ ion at different concentrations and (b) the corresponding calibration curve. The excitation wavelength was 410 nm. (c, d) Photos showing fluorescence of NCs-absorbed ordinary cotton sticks under UV light (365 nm). In image (c), the most left one is the blank stick, and the other four are sticks immersed in AuNCs-3h solution (1 mg/mL) with the same procedure. In image (d), the top tips of the left and right two sticks were immersed in Cu2+ solutions of 3.2 ppm and 6.4 ppm, respectively; and all the bottom tips were immersed in Cu2+-free solution.
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detail in the ESI† (Fig. S10). Fig. 3a,b show the detection Cu Online ion. Viewof Article DOI: 10.1039/C5CC03086B By correlating the ratio of I/I0 with the ion concentration, we obtain a calibration curve that can be fitted linearly from 0 to 0.6 µM (y = 1 2 – 0.0014*x, R = 0.998). The limit of detection (LOD) was 12.5 nM (S/N = 3). Interestingly, we found the assay can be performed in other media besides water. We used ordinary cotton sticks to absorb clusters and dried them in an oven. Under UV light, in vivid contrast to the blue-emitting control stick, the treated ones showed brightly strong luminescence, just like flaming matchsticks (Fig. 3c). 2+ When we immersed one tip of the stick into solution of Cu ion for seconds and pulled out, the luminescence was greatly reduced, just like flame of matchsticks was extinguished. In comparison, the 2+ other tip that was treated in Cu -free solution remained constant in luminescence (Fig. 3d). This may provide a convenient strategy to 2+ detect surface-bound Cu ion by using cheap cotton stick as a carrier for the immobilization of the clusters.
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ChemComm Accepted Manuscript
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formula (-SG, C10H16N3O6S). Since no other element is present, the NCs thus contain 45.1% Au and, therefore, the atomic ratio of S/Au is calculated to be 0.78, consistent with the XPS result. The incompetency of TGA data in the analysis of thiolate-to-Au molar ratio for our AuNCs seems to be caused by the underestimate of the o weight loss which does not reach a plateau till 800 C; however, the exact reason is uncertain right now. The luminescent properties of the AuNCs-3h were then probed. The emission intensity decreases with the increase of excitation wavelength, but with no change in the λem (Fig. S7a). The lifetime measurement exhibits a characteristic fluorescence decay that can be fitted with a sum of two exponentials, with a short lifetime (τ1 = 3.28 µs, 87%) and a long one (τ2 = 14.4 µs, 13%) (Fig. S7b). The present all-microsecond-scale lifetime may suggest slower radiative relaxation via the triplet excited state through ligand-to-metal 3 charge transfer or ligand-to-metal-metal charge transfer. In addition, the emission intensity minimized as the temperature of AuNCs solution elevated (Fig. S7c), reminiscent of the lipoic acid18 protected AuNCs discovered recently. This suggests the utility of the AuNCs-3h in intracellular thermo-imaging only if it can be tested with minimal toxicity, internalized into cells, and also possess essential stability. It shows PL drops by continuous excitation (Fig. S7d, Fig. S8), exhibiting a somewhat unsatisfactory capability against continual irradiation. But the Φ varied less than 10 % after o two months of storage at 4 C, demonstrating potential for the clusters to be used in sensing.
PL intensity
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Compared to Cu , Ag ion would elicit a totally different change in PL spectra. Fig. 4a shows the emission spectra of AuNCs-3h + solution in the presence of Ag ion at different concentrations. The PL intensity at 605 nm decreases a bit at lower ion concentrations, but then increased at concentrations higher than 1 µM. Accompanied by this, the emission band shifts to 610 nm at 0.2 µM and finally reaches 627 nm. Correlation of emission peak shift (Δλ) Ag+ origin as well as intensity ratio (I627 /I605 ) with the concentration of + Ag ion is shown in Fig. 4b. The peak shift changes more sensitively than the intensity ratio, so the Δλ versus ion concentration is more preferable for the calibration. A linear line can be fitted from 0 to 1 2 µM (y = 14.9*x, R = 0.97), and the LOD was determined to be 75 nM (S/N = 3). Fig. 4c,d show that, in contrast to the use of intensity ratio, using peak shift as the y-axis variable avoids most of metal ions interfering with the detection, further showing the Δλ as a better choice for the calibration. We evaluated the sensor in the + detection of Ag ion in lake water and obtained agreeable results (Table S4). Some AuNCs, including GS-AuNCs, had been used as sensors for 2+ 19 + 20 Cu ion and Ag ion individually, but there was no attempt to
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detect both by a same clusters system. We analyzed the mechanism of the PL change for the two ions by optical absorption spectra and time-resolved fluorescence spectroscopy (Fig. S11). The absorption spectrum of the AuNCs solution remains constant in the presence 2+ + of Cu ion, while changes dramatically in the presence of Ag ion, indicating the interaction of clusters with silver but not with copper. Meanwhile, it shows significant changes in lifetimes of AuNCs, 2+ + decreasing for Cu ion and increasing for Ag ion (Table S5). In this 2+ sense, we can conclude that the Cu ion-induced PL quenching of + AuNCs is due to a dynamic process, and the Ag ion-induced PL enhancing due to a different mechanism. It should be noted that for the other metal ions that suppress the fluorescence of AuNCs such 2+ 2+ 2+ as Co , Ni , and Hg , the PL quenching is all due to a dynamic process (Fig. S12, Table S6).
This work is financially supported by the National Basic Research View Article Online DOI:2013CB933900), 10.1039/C5CC03086B Program of China (Grants 2014CB931800, the National Natural Science Foundation of China (Grants 21407140, 91022032, 21431006). J. Zhang appreciates the Grants from the China Postdoctoral Science Foundation (Grant 2013M531515) and the Fundamental Research Funds for the Central Universities (Grant WK2060190036), and Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (Grant 2015SRG-HSC038). This work is also funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, under grant No.(89-130-35-HiCi). The authors, therefore, acknowledge technical and financial support of KAU.
Notes and references 1 2 3 4
Fig. 5 Photos of an ordinary piece of filter paper written in invisible words by using the AuNCs-3h ink (a) under room light and (b, c) under UV light (365 nm). In the image (b), the first row shows Chinese characters meaning University of Science and Technology of China; the second row shows the acronym for University of Science and Technology of China; and the last two rows show Biomimetic and Nanochemistry Laboratory. The concentration of AuNCs in image (b) is 1 mg/mL, and concentrations in image (c) are 1, 0.1, and 0.01 mg/mL from top to bottom.
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Besides selective metal ions sensing, the AuNCs can also be used as fluorescent ink due to the property of luminescence in solid state. Fig. 5b presents some Chinese characters meaning University of Science and Technology of China and other English acronym and words written on a piece of ordinary filter paper with a pen by using the AuNCs-3h ink under UV light. Compared to the image shown in nothing under room light (Fig. 5a), intense orange color appears clearly in the writing marks despite the intrinsic blue background. As the concentration of the ink decreases, the fluorescent image becomes faint, but still perceptible, even at 0.01 mg/mL (Fig. 5c). This kind of AuNCs ink, in complementation with fluorescent carbon 21 inks, may provide opportunity for the construct of multicolor patterning for potential applications such as in optoelectronic fields and information encryption and storage. In summary, we report a microwave-facilitated strategy for rapid synthesis of highly luminescent GS-AuNCs. Through controlling the reaction time, the dominant photoemission wavelength is facilely tunable, represented by the generation of AuNCs with discrete sizes. Correlating the sizes by TEM and DLS characterization with the main photoemission peak gives an anti-Jellium model that supports the aggregation-induced emission of the Au(I)-SG complex shell as the origin of luminescence for these AuNCs. To analyze the S/Au molar ratio in the AuNCs, we suggest it is better to use XPS and elemental 2+ + analysis than TGA. Detection of Cu ion and Ag ion with a same sensor is realized in aqueous solution as well as on cotton stick by using the intensely orange-emitting AuNCs. Moreover, due to the preservation of luminescence in solid state, a kind of AuNCs ink is prepared and utilized for writing words that are invisible with naked eye observation but glow upon UV irradiation.
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Microwave-facilitated rapid synthesis of gold nanoclusters with tunable optical property for sensing ions and fluorescent ink DOI: 10.1039/x0xx00000x
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Jia Zhang,* Yue Yuan, Gaolin Liang, Muhammad Nadeem Arshad, c a Sobahi, and Shu-Hong Yu*
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Hassan A. Albar, Tariq R.
www.rsc.org/
Metal nanoclusters are composed of exactly a few to hundreds of atoms, with the core sizes ranging from subnanometer to 1 approximately 2 nm. Both theoretical modeling and experimental evidence reveal that electron energy quantization occurs in such ultrasmall size regime, resulting in discrete electronic structure and 2 molecule-like properties such as quantized charging and 3 luminescence. Until now, a number of ligands (or stabilizers) have been used to protect gold nanoclusters against aggregation and oxidation, which can be simply classified to two categories, i.e. 4-6 macromolecules (proteins, dendrimers, and synthetic polymers) 7-9 and small molecules (thiols and rationally designed peptides). And thiolate-stabilized gold nanoclusters (thiolated AuNCs for short) are widely studied, among which glutathione-capped AuNCs (GSAuNCs) is perhaps the most extensively studied system. Basically, the synthesis of GS-AuNCs is almost via the reduction of polymeric GS-Au(I) complexes, and NCs with specific gold atoms can be formed by tuning reaction parameters, including solvent, pH, 10 temperature, and reductant. While these GS-AuNCs exhibit visible to infrared luminescence, their quantum yield (Φ) are mostly low. Recently, Xie et al. improved the preparation of GS-AuNCs by the 11 use of heat in 24 h and elucidated the luminescence mechanism, but some key issues still remain opaque, such as how the clusters evolve. Moreover, 24 hours of reaction is still time-consuming. Here, we report a facile route to rapidly synthesize luminescent GS-AuNCs with Φ reaching 4.4% via the decomposition of GS-Au(I) complex by microwave irradiation. By increasing the time, the
a.
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China. Fax: 86-551-63603040; Tel: 86-551-63603040; E-mail: [email protected]; [email protected]. b. Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia. c. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia. Electronic Supplementary Information (ESI) available: Experimental details, Fig. S1S12 and Table S1-S6 as noted in the text. See DOI:10.1039/x0xx00000x
photoluminescent (PL) emission wavelengths are blue-shifted and the particle sizes increase proportionally. The influences of microwave conditions including time, power, and temperature on the reaction have been thoroughly investigated. Owing to the remarkably strong luminescence, the GS-AuNCs are tested their capability for nanomolar-level metal ions sensing and potential use as invisible fluorescent ink. To our knowledge, it is the first attempt to use AuNCs as fluorescent ink. (a)
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Luminescent glutathione-capped gold nanoclusters (GS-AuNCs) with tunable emissions are efficiently synthesized by a solutionbased microwave method.
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Received 00th January 20xx, Accepted 00th January 20xx
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Fig. 1 (a) UV-vis absorption spectra and (b) photoemission spectra of GS-AuNCs prepared within from 10 min to 3 h. In figure (a), plots have been shifted upwards for clarity. Photos of the as-prepared AuNCs solutions under (c) room light and (d) UV light (365 nm). The synthetic conditions: microwave temperature and power are 90 oC and 300 W, solution pH is 2.0, and GSH/Au molar ratio is 1.5.
The time evolution of GS-AuNCs from the dissociation of GS-Au(I) complex is observed from continual changes in the absorption spectra (Fig. 1a). As time increases, the absorption spectra of solutions exhibit a hyperchromic shift of onset from 440 nm (10 min) to 480 nm (20 min) and finally to 500 nm (30 min-3 h), but a shoulder peak at 400 nm is only clearly observed after 3 h of reaction. Neither step-like multiple bands nor surface plasmonic band around 520 nm appear in the spectra, indicating the obtained 12,13 NCs are not conventional Aun(SG)m clusters and the cores of the 14 NCs in the metallic size regime are less than 2 nm. The color of solutions turns from light yellow to bright yellow with time (Fig. 1c). Photoemission spectra (Fig. 1b) better illustrate the evolution of these NCs. Since the original GS-Au(I) complex is known having no luminescence, the main broad emission peak at 670 nm with two shoulder peaks at 710 and 810 nm appearing in the spectrum of
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solution (after 10 min) demonstrate the decomposition of the GSAu(I) complex to yield luminescent clusters. Prolonging reaction time will induce obvious hypochromic shift as well as narrowing of the main emission peak, and besides, the two shoulder peaks are suppressed but not vanish. Table S1 lists the main emission wavelength (λem) and Φ of the AuNCs prepared within from 10 min to 3 h. Fig. 1d shows the as-prepared AuNCs solutions under UV light. One can observe a color transition from faint red to intense orange. The most intense orange-emitting solution (after 3 h) shows a well-defined emission peak at 605 nm, with Φ measured to be 2.4% (using quinine sulfate as reference, 54%). Further extending the reaction, we found the highest Φ (4.4%) was achieved for NCs by 5 h of reaction (Fig. S1a). We then found the increase of microwave power shall result in slightly larger photoemission intensity (Fig. S1b). We also studied the effect of microwave temperature. Basically, elevating temperature helps to increase the PL intensity (Fig. S1c), and improves Φ from 0.32% (70 o o o C) to 0.89% (90 C) and then to 1.1% (110 C). We tried to prolong o the reaction time at 110 C to obtain a higher Φ than 4.4%, but the 11 attempt failed. As control, we followed the previous method and obtained similarly orange-emitting NCs with Φ of 1.2% after o reaction at 70 C for 24 h. The above results may suggest that the microwave-assisted method is superior to the heat-facilitated one in terms of using less time to obtain higher quantum efficiency. The o synthesis of AuNCs (by 3 h at 90 C and 300 W) is quite repeatable. For five syntheses, the relative standard deviations relative to Φ and dominant PL emission wavelength were 8.3% and 0.36%, respectively. The storage stability of the AuNCs was studied. We barely observed changes in the emission spectra after 60 days of o storage at 4 C (Fig. S2a). However, we also noted they were not stable at ambient environment, with all emissions evolving to ~605 nm (Fig. S2b). This implies the AuNCs (by 3 h) own the highest thermodynamic stability compared to those clusters formed in other shorter time windows.
Fig. 2 TEM and high-resolution TEM images of GS-AuNCs for different time of reaction: (a) 10 min; (b) 20 min; (c) 30 min; (d) 45 min; (e) 60 min; (f) 90 min; (g) 120 min; (h, i, j) 180 min. The insets in the upper right corner of (a) to (g) are the size histograms by DLS, and that of (h) contains both the size histograms by DLS and TEM. (k) The EDX pattern of the GS-AuNCs (by 3 h).
We used transmission electron microscope (TEM) and dynamic light scattering (DLS) to give absolute evidence of the generation of clusters, as shown in Fig. 2. The size of NCs (by 3 h) is apparently
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larger than that of NCs (by 10 min), but there is some difficulty to View Article Online DOI: 10.1039/C5CC03086B differentiate the size of two adjacent clusters, like NCs by 10 and 20 min of reaction. Fortunately, the DLS shows the hydrodiameters (HDs) of the AuNCs almost increase with the irradiation time (Table S2). By correlating λem with the size of clusters, we find the λem shifts inversely with size increase. This observation of anti-Jellium model provides compelling proof for the {Au(0) core/Au(I)-SG complex shell} structure of the GS-AuNCs and that the origin of luminescence should be ascribed to aggregation-induced emission 11 (AIE) of the shell. Of note is that the AuNCs do not continue to grow after elongation of reaction time past 3 h, identified by the size of clusters prepared for 5 h (Fig. S3). A high-resolution TEM image of the AuNCs (by 3 h) reveals lattice fringe separated by 0.235 nm (Fig. 2j), identical with the (111) lattice spacing of the face-centered cubic (fcc) gold. However, the NCs do not adopt the fcc structure because the diffraction angles are deviant from those for the fcc gold (Fig. S4). This echoes with the other gold 15,16 clusters. Besides, the appearance of lattice fringes featured by fcc Au does imply a kind of connection between nanoclusters and nanocrystals. The energy dispersive X-ray (EDX) spectrum consolidates the existence of Au and S in the sample. The yield of AuNCs (by 3 h) is 99% (Au atom basis). We further used matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS) to analyze the molecular formula of the AuNCs, as shown in Fig. S5. Unfortunately we do not observe meaningful m/z peaks between 1 kDa and 30 kDa in the spectra collected in the linear negative ion mode. For the spectra collected in the linear positive ion mode, some m/z peaks exist between 1 kDa and 3.5 kDa, and the peaks at larger m/z become more appreciable at a larger reaction time. Given that the clusters have relatively large sizes, the MS result clearly suggests the occurrence of fragmentation. Since the process is rather complex, we do not assign the fragmented peaks. o We used the GS-AuNCs (90 C/180 min, denoted as AuNCs-3h) in following study because of its thermodynamic stability and fairly high Φ. All the expected atoms, including Au, S, C, N and O, are revealed in X-ray photoelectron spectroscope (XPS) survey scan (Fig. S6a). The individual Au and S spectra show Au 4f7/2 peak centered at 84.3 eV and S 2p peak centered at 163.0 eV (Fig. S6b,c). Integration of Au 4f and S 2p peak intensities gives a S/Au atomic ratio of 0.83. According to the previous study, the Au 4f7/2 spectrum contains Au(I) and Au(0) components with binding energies of 84.35 and 83.8 eV. The content of Au(I) in the shell constituted 82% of the total Au in AuNCs. This suggests that the atomic ratio of S/Au in the Au(I)-SG complex in the shell of such luminescent NCs is very close to 1:1. The molar ratio of S/Au in the clusters was further analyzed by thermogravimetric analysis (TGA) data. As shown in Fig. S6d, the weight loss behavior for our AuNCs is in large difference from those for the monolayer-protected 7,15,17 AuNCs. Two repetition of analyses gave 36.1% and 37.5% major mass loss, and 38.2% and 38.5% total mass loss between 100 o and 800 C, which can be converted to S/Au atomic ratios of 0.38 and 0.39, respectively. Such significant discrepancy in the S/Au atomic ratio derived from XPS and TGA data compelled us to perform quantitative elemental analysis of the clusters. It shows the AuNCs contain 19.57% C, 2.99% H, 6.78% N, 18.91% O, and 6.65 % S (Table S3), yielding the atomic ratio of C:H:N:O:S being 7.8:14.4:2.3:5.7:1, which is close with the thiolated glutathione
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We recorded the emission intensity of the AuNCs-3h solution in the presence of various ions (Fig. S9). For metal ions, some ions were found to quench the luminescence of the NCs, with the most 2+ appreciable for divalent Cu . For anions, only sulfide decreased the PL intensity a little, but at 100 µM compared to 1 µM for metal ions. 2+ Therefore, the AuNCs can be used as a sensor to detect Cu ion. The optimization of parameters for the detection is discussed in
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Fig. 3 (a) The PL spectra of AuNCs-3h solution (A410nm = 0.006) incubated with Cu2+ ion at different concentrations and (b) the corresponding calibration curve. The excitation wavelength was 410 nm. (c, d) Photos showing fluorescence of NCs-absorbed ordinary cotton sticks under UV light (365 nm). In image (c), the most left one is the blank stick, and the other four are sticks immersed in AuNCs-3h solution (1 mg/mL) with the same procedure. In image (d), the top tips of the left and right two sticks were immersed in Cu2+ solutions of 3.2 ppm and 6.4 ppm, respectively; and all the bottom tips were immersed in Cu2+-free solution.
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detail in the ESI† (Fig. S10). Fig. 3a,b show the detection Cu Online ion. Viewof Article DOI: 10.1039/C5CC03086B By correlating the ratio of I/I0 with the ion concentration, we obtain a calibration curve that can be fitted linearly from 0 to 0.6 µM (y = 1 2 – 0.0014*x, R = 0.998). The limit of detection (LOD) was 12.5 nM (S/N = 3). Interestingly, we found the assay can be performed in other media besides water. We used ordinary cotton sticks to absorb clusters and dried them in an oven. Under UV light, in vivid contrast to the blue-emitting control stick, the treated ones showed brightly strong luminescence, just like flaming matchsticks (Fig. 3c). 2+ When we immersed one tip of the stick into solution of Cu ion for seconds and pulled out, the luminescence was greatly reduced, just like flame of matchsticks was extinguished. In comparison, the 2+ other tip that was treated in Cu -free solution remained constant in luminescence (Fig. 3d). This may provide a convenient strategy to 2+ detect surface-bound Cu ion by using cheap cotton stick as a carrier for the immobilization of the clusters.
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formula (-SG, C10H16N3O6S). Since no other element is present, the NCs thus contain 45.1% Au and, therefore, the atomic ratio of S/Au is calculated to be 0.78, consistent with the XPS result. The incompetency of TGA data in the analysis of thiolate-to-Au molar ratio for our AuNCs seems to be caused by the underestimate of the o weight loss which does not reach a plateau till 800 C; however, the exact reason is uncertain right now. The luminescent properties of the AuNCs-3h were then probed. The emission intensity decreases with the increase of excitation wavelength, but with no change in the λem (Fig. S7a). The lifetime measurement exhibits a characteristic fluorescence decay that can be fitted with a sum of two exponentials, with a short lifetime (τ1 = 3.28 µs, 87%) and a long one (τ2 = 14.4 µs, 13%) (Fig. S7b). The present all-microsecond-scale lifetime may suggest slower radiative relaxation via the triplet excited state through ligand-to-metal 3 charge transfer or ligand-to-metal-metal charge transfer. In addition, the emission intensity minimized as the temperature of AuNCs solution elevated (Fig. S7c), reminiscent of the lipoic acid18 protected AuNCs discovered recently. This suggests the utility of the AuNCs-3h in intracellular thermo-imaging only if it can be tested with minimal toxicity, internalized into cells, and also possess essential stability. It shows PL drops by continuous excitation (Fig. S7d, Fig. S8), exhibiting a somewhat unsatisfactory capability against continual irradiation. But the Φ varied less than 10 % after o two months of storage at 4 C, demonstrating potential for the clusters to be used in sensing.
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Compared to Cu , Ag ion would elicit a totally different change in PL spectra. Fig. 4a shows the emission spectra of AuNCs-3h + solution in the presence of Ag ion at different concentrations. The PL intensity at 605 nm decreases a bit at lower ion concentrations, but then increased at concentrations higher than 1 µM. Accompanied by this, the emission band shifts to 610 nm at 0.2 µM and finally reaches 627 nm. Correlation of emission peak shift (Δλ) Ag+ origin as well as intensity ratio (I627 /I605 ) with the concentration of + Ag ion is shown in Fig. 4b. The peak shift changes more sensitively than the intensity ratio, so the Δλ versus ion concentration is more preferable for the calibration. A linear line can be fitted from 0 to 1 2 µM (y = 14.9*x, R = 0.97), and the LOD was determined to be 75 nM (S/N = 3). Fig. 4c,d show that, in contrast to the use of intensity ratio, using peak shift as the y-axis variable avoids most of metal ions interfering with the detection, further showing the Δλ as a better choice for the calibration. We evaluated the sensor in the + detection of Ag ion in lake water and obtained agreeable results (Table S4). Some AuNCs, including GS-AuNCs, had been used as sensors for 2+ 19 + 20 Cu ion and Ag ion individually, but there was no attempt to
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detect both by a same clusters system. We analyzed the mechanism of the PL change for the two ions by optical absorption spectra and time-resolved fluorescence spectroscopy (Fig. S11). The absorption spectrum of the AuNCs solution remains constant in the presence 2+ + of Cu ion, while changes dramatically in the presence of Ag ion, indicating the interaction of clusters with silver but not with copper. Meanwhile, it shows significant changes in lifetimes of AuNCs, 2+ + decreasing for Cu ion and increasing for Ag ion (Table S5). In this 2+ sense, we can conclude that the Cu ion-induced PL quenching of + AuNCs is due to a dynamic process, and the Ag ion-induced PL enhancing due to a different mechanism. It should be noted that for the other metal ions that suppress the fluorescence of AuNCs such 2+ 2+ 2+ as Co , Ni , and Hg , the PL quenching is all due to a dynamic process (Fig. S12, Table S6).
This work is financially supported by the National Basic Research View Article Online DOI:2013CB933900), 10.1039/C5CC03086B Program of China (Grants 2014CB931800, the National Natural Science Foundation of China (Grants 21407140, 91022032, 21431006). J. Zhang appreciates the Grants from the China Postdoctoral Science Foundation (Grant 2013M531515) and the Fundamental Research Funds for the Central Universities (Grant WK2060190036), and Scientific Research Grant of Hefei Science Center of Chinese Academy of Sciences (Grant 2015SRG-HSC038). This work is also funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, under grant No.(89-130-35-HiCi). The authors, therefore, acknowledge technical and financial support of KAU.
Notes and references 1 2 3 4
Fig. 5 Photos of an ordinary piece of filter paper written in invisible words by using the AuNCs-3h ink (a) under room light and (b, c) under UV light (365 nm). In the image (b), the first row shows Chinese characters meaning University of Science and Technology of China; the second row shows the acronym for University of Science and Technology of China; and the last two rows show Biomimetic and Nanochemistry Laboratory. The concentration of AuNCs in image (b) is 1 mg/mL, and concentrations in image (c) are 1, 0.1, and 0.01 mg/mL from top to bottom.
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Besides selective metal ions sensing, the AuNCs can also be used as fluorescent ink due to the property of luminescence in solid state. Fig. 5b presents some Chinese characters meaning University of Science and Technology of China and other English acronym and words written on a piece of ordinary filter paper with a pen by using the AuNCs-3h ink under UV light. Compared to the image shown in nothing under room light (Fig. 5a), intense orange color appears clearly in the writing marks despite the intrinsic blue background. As the concentration of the ink decreases, the fluorescent image becomes faint, but still perceptible, even at 0.01 mg/mL (Fig. 5c). This kind of AuNCs ink, in complementation with fluorescent carbon 21 inks, may provide opportunity for the construct of multicolor patterning for potential applications such as in optoelectronic fields and information encryption and storage. In summary, we report a microwave-facilitated strategy for rapid synthesis of highly luminescent GS-AuNCs. Through controlling the reaction time, the dominant photoemission wavelength is facilely tunable, represented by the generation of AuNCs with discrete sizes. Correlating the sizes by TEM and DLS characterization with the main photoemission peak gives an anti-Jellium model that supports the aggregation-induced emission of the Au(I)-SG complex shell as the origin of luminescence for these AuNCs. To analyze the S/Au molar ratio in the AuNCs, we suggest it is better to use XPS and elemental 2+ + analysis than TGA. Detection of Cu ion and Ag ion with a same sensor is realized in aqueous solution as well as on cotton stick by using the intensely orange-emitting AuNCs. Moreover, due to the preservation of luminescence in solid state, a kind of AuNCs ink is prepared and utilized for writing words that are invisible with naked eye observation but glow upon UV irradiation.
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