Surfactant-Free Synthesis and Functionalization of Highly Fluorescent

Jun 26, 2008 - A novel route has been developed for surfactant-free synthesis of highly fluorescent gold quantum dots (GQDs) in N,N-dimethylformamide ...
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J. Phys. Chem. C 2008, 112, 10778–10783

Surfactant-Free Synthesis and Functionalization of Highly Fluorescent Gold Quantum Dots Xiaofeng Liu,†,‡ Cuihong Li,†,‡ Jialiang Xu,†,‡ Jing Lv,†,‡ Mei Zhu,†,‡ Yanbing Guo,†,‡ Shuang Cui,†,‡ Huibiao Liu,† Shu Wang,† and Yuliang Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: April 02, 2008; ReVised Manuscript ReceiVed: May 08, 2008

A novel route has been developed for surfactant-free synthesis of highly fluorescent gold quantum dots (GQDs) in N,N-dimethylformamide (DMF). The as-prepared GQDs show instinctive fluorescence and good solubility in water. The formation mechanism and functionalization of GQDs were investigated by UV-vis spectra, fluorescence excitation and emission spectra, mass spectra, and TEM observation. Ligand-dependent optical properties of functionalized GQDs were found to be dramatically different. The approach provides a facile method of functionalization of bare GQDs for further applications, such as fluorescent biolabels, energy transfer units, and light-emitting devices. Introduction Metal and semiconductor nanoclusters have become the subject of intensive research in optoelectrics, catalysts, biological sciences, detectors, and sensors.1–3 These widespread applications mainly arise from their unique size- and morphologydependent optical and electrical properties.4–7 As for semiconductor nanoclusters with well-defined energy band gaps, broader applications including fluorescent labels and energy and electron transfer hybrid nanomaterials8,9 are developed due to facile control to be comparable with the exciton Bohr radius (often ranging over several nanometers). Compared to that of semiconductor nanoparticles, however, for metal nanoclusters without a discrete band gap, confinement effects more easily occurred at the nanoscale of monometallic clusters. The quantum-size confinement can only be observed in ultrasmall nanoclusters which have smaller sizes than the Fermi wavelength (often less than 1 nm) of the conduction electron10 resulting in electronic energy states that exhibit molecular-like transitions because the density of these states is too low to reproduce bulk metallic properties.11 Such metal clusters with robust quantum effects or molecule-like properties have been investigated intensively.11b,12 In fact, the rigor of the size requirement for metal clusters has precluded them from being applied as fluorophores. The process has been slow in developing fluorescent metal clusters and current synthetic routes are limited to gas-phase synthesis or slow reduction of metal precursors in the presence of various stabilizing ligands, such as poly(amidoamine) dendrimer and thiols.13–15 More recently, Dickson and co-workers have generated a series of fluorescent gold atomic clusters containing Au5, Au13, Au23, and Au31 with a fluorescence wavelength range from UV to near-IR, synthesized by using poly(amido) amine (PAMAM) dendrimers as a surface stabilizing agent.14 Another development has been established by Nie and co-workers to generate fluorescent gold atomic clusters through a ligandinduced etching process.16 These synthetic strategies lead to atomic clusters with enhanced stability and allowed some tailoring of chemical and physical properties.17 The advantages * Corresponding author. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate University of the Chinese Academy of Sciences.

of all of these methods for preparing fluorescent gold clusters are good control over the cluster sizes and stabilities. However, disadvantages remain that impurities are introduced by the use of surfactant and incomplete reduction in the presence of the hyperbranched ligands. The latter may circumvent partly on the gold clusters that restrict the ligand-exchanging efficiency. Still, the design and synthesis of nanoparticles with well-defined surface properties is a significant and ongoing challenge. The formation of gold, silver, and other metallic nanostructures with various morphologies has been previously demonstrated,18 based on the use of N,Ndimethylformamide (DMF) as a solvent as well as reducing agent. However, considering that DMF is able to act as a mild reducing agent at high temperature, a special focus has been given to the fabrication of GQDs in such a system for elucidating the roles of reducing agents and additives, with respect to their impact on the sizes and the quantum confinement. Herein, a novel pathway for preparing highly fluorescent GQDs is presented, which combines both advantages that are a facile, surfactant-free synthesis, high variability in the introduction of functionalized ligands, and good control over the cluster stability and physical properties. It was found that a dilute solution of HAuCl4 in DMF can be progressively reduced at high temperature to form gold atomic clusters. In this reaction, no further stabilizing surfactant is needed. DMF is expected to be a weak reducing agent as well as stabilizing ligand. The resulting weakly protected gold atomic clusters are very water-soluble and can directly be stabilized and functionalized by the reaction of a variety of ligands. Detailed synthesis and characterization techniques are described in the Experimental Section. These GQDs are expected to show new ligand-based properties and high degrees of surface functionalities. Experimental Section Materials and Methods. Chemicals and solvents were reagent grade and all were commercially available. The chemicals were used as received without further purification. Solvents were distilled prior to use throughout all experiments. The water used throughout all experiments was purified with Milli-Q equipment. The dialytic memberane were purchased

10.1021/jp8028227 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

Synthesis of Highly Fluorescent Gold Quantum Dots from Shanghai Gree Bird Science & Technology Development Co., Ltd. and used without pretreatment. Thioctic acid and S-acetyl thiocholine bromide were purchased from ACROS. 11Mercaptoundecanoic acid and poly(amido) amine dendrimer (Generation 2, 20 wt % solution in methanol) were purchased from Aldrich. HAuCl4 · 4H2O and other chemicals were purchased from Beijing Chemical Co. and used without further purification. Solvents were distilled prior to use throughout all experiments. The ultrapure water used in all experiments was purified with Milli-Q equipment. Preparation of Bare GQDs. A solution of 100 µL of 0.1 M aqueous HAuCl4 in 10 mL of DMF was stirred at room temperature for 10 min before being reacted at 140 °C for 4 h under vigorous stirring. After cooling to room temperature, the mixture was centrifuged three times at 12 000 rpm to remove any larger gold nanoparticles. It thus afforded a light brown and clear solution. After evaporating off the access solvent under vacuum, the residue was passed through a silica gel column (200-300 mesh) with methanol as eluent. The gold atomic cluster solutions were then collected and evaporated to dryness, then redissolved in a small amount of methanol as stock solution for further modification. The concentration of the obtained GQDs solutions was calculated as 1.42 × 10-3 M (see the Supporting Information for details). Purification and Functionalization of As-Prepared Bare GQDs with Various Ligands. (A) For TA, MUA, DT, DA, PVP, TPP, and PAMAM functionalized GQDs: To a stirred 3 mL of stock solution of GQDs in methanol was added 100 µLof the corresponding ligand in methanol (4.3 × 10-2 M) dropwise. The mixture was stirred and maintained in the dark for more than 24 h for completion of the ligand capping process. After removal of the solvent, the residue was redissolved in 3 mL of methanol. (B) For TCB functionalized GQDs: To a stirred 1 mL solution of 10 mg of S-acetylthiocholine bromide in methanol was added 0.5 mL of HCl (2 M aqueous) dropwise. The mixture was stirred and maintained in the dark at room temperature overnight. The acetyl group was expected to be deprotected to give free thiol as thiocholine bromide (TCB). Then, 3 mL of stock solution was added via syringe. The mixture was stirred further for more than 24 h. After removal of the solvent, the residue was redissolved in a small amount of water and passed through a dialytic memberane with a molecular weight cutoff of 2 kD. The TCB functionalized GQDs was then resuspended in deioned water. Photophysical Measurements. UV-vis absorption spectra were measured at ambient condition by a JASCO V-570 spectrophotometer. Fluorescence excitation and emission spectra were obtained on a JASCO FP-6600 fluorimeter. Transmission Electron Microscope (TEM). TEM images were taken with an electron microscopy (JEM-2011, JEOL) operated at 200 kV. The TEM specimens were prepared by dropping relative GQDs solutions on carbon coated copper grids (400 mesh). Mass Spectroscopy (MS). The electrospray ionization (ESI) mass spectrum was recorded on an APEX II FT-ICRMS spectrometer. Matrix-assisted laser desorption/ionization, using a time-of-flight technique (MALDI-TOF), mass spectra were measured with a Bruker Biflex III MALDI-TOF spectrometer. Results and Discussion Synthesis and Characterization of Bare GQDs. As they are widely used to prepare metallic and semiconductor nanostructures, surfactants were utilized to stabilize and solubilize

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Figure 1. Optical absorption spectra of as-prepared bare GQDs in methanol (red curve), original AuCl4- ions in water (green curve), and fluorescence emission spectrum of as-prepared bare GQDs in methanol (blue curve).

metal nanoclusters in both polar and nonpolar solvents.19 In our experiment, the fluorescent GQDs were synthesized by progressively reducing tetrachloroaurate ion in DMF at high temperature (140 °C). In this system, the tetrachloroaurate ion was progressively reduced to Au(I) then to Au(0), which was determined by optical measurements. GQDs growth utilizes a weak reducing agent (DMF) as well as solvent at high temperature without any surfactants. During heating, spectacular color changes can be observed, ranging from light yellow to light brown through colorless, which is mainly due to the formation of Au(I) in the middle of the reducing process. After 4 h of reaction, the solution was cooled to room temperature and centrifuged three times at 12 000 rpm to remove any large gold nanoparticles, which afforded a clear and light brown solution. After removal of the solvent, the residue was passed through a silica gel column (200-300 mesh) with methanol as eluent. UV-vis absorption spectra show that the as-obtained fluorescent gold atomic clusters exhibit a distinct absorption peak at 291 nm, which is in sharp contrast to that of initial tetrachloroaurate ion solution (324 nm). Under UV light irradiation, the supernatant solution shows strong blue fluorescence (442 nm), indicating the presence of fluorescent gold clusters (Figure 1). In contrast, neither the initial solution containing tetrachloroaurate ions nor the gold nanoparticles formed along with the GQDs show fluorescence (data not shown). This thus indicates the fluorescence arises only from intrinsic gold atomic clusters, although it is not absolute evidence for the presence of fluorescent gold clusters. Time-Dependence Formation of GQDs. To further elucidate the formation mechanism of GQDs, the reaction was performed at 140 °C for different times while monitoring the UV-vis absorption and fluorescence emission spectra. While heating, the absorption peak at 324 nm decreased, which indicates that tetrachloroaurate ions were partly reduced to Au(I) by DMF. At this stage, no fluorescent signals can be detected. However, the reaction was performed further for longer times, while a new absorption peak at 291 nm appears with sharp increasing extinction coefficiency as shown in Figure 2A, which was mainly due to the ligand-to-metal electronic transition between DMF and formed gold clusters. Fluorescence emission spectra show that the fluorescence of the species increases along with reaction time. After 4 h of reaction, the fluorescence reaches a maximum. While heating for longer time, the fluorescence does not change but exhibits a slight decrease as shown in Figure 2B. Thus, this indicates the completion of the reducing process, which was also convinced by further reduction by sodium borohydride. Figure 2C exhibits a typical plot of fluorescent

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Figure 2. UV-vis absorption spectra (A), fluorescence excitation (dashed curve) and emission spectra (solid curve) (B) at different reaction times of 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 1 h, 2 h, 3 h, 4 h, and 7 h (indicated by arrow). (C) Plot of fluorescence emission intensities of as-prepared GQDs versus reaction times. Inset shows photograph of the corresponding samples after being reduced by 1 equiv of NaBH4.

intensity versus reaction time. After reducing samples of various preparation time with 1 equiv of NaBH4, different amounts of gold nanoparticles formed (inset in Figure 2C), which reveals that different amounts of residual Au(I) ions were reduced. In comparison, no gold nanoparticles were presented in samples of preparation time longer than 4 h after being treated with NaBH4. This can be another piece of evidence for complete formation of GQDs. A temperature-dependent experiment was also performed to elucidate the formation mechanism of fluorescent gold clusters. The results indicate that below 140 °C, no fluorescence signals were detected while a large number of gold nanoparticles formed, which mainly result from the lower reducing ability of DMF at low temperature. Thus it is explained that fluorescent gold clusters are generated only from a high and rapid reduction process. We also note that these fluorescence atomic clusters are extraordinarily stable either in solution or in the solid state, even being dispersed in acidic or basic environment. In contrast to other semiconductor nanocrystals, such fluorescent gold clusters have well-defined excitation and emission spectra, while preparation needs neither toxic precursor nor a complicated higher temperature.20 Moreover, such subnanometer fluorescent metal clusters may find utility as energy transfer pairs in biolabels while semiconductor nanocrystals are ill-suited due to their broader excitation spectra and larger fluorophore size. Luminescence from gold nanoclusters is considered to arise from transitions between the filled 5d10 band and 6sp1 conduction band of the gold atom.15b,21 As nanocluster size decreases, the gaps between discrete states in each band increase, leading to a blue shift in fluorescence relative to that from large clusters. Functionalization of As-Prepared GQDs. The as-prepared bare GQDs were expected to undergo a ligand-capping process

SCHEME 1: Chemical Structures of Different Capping Ligands

for functionalization. On the basis of this case, different ligands were used including thioctic acid (TA), 1-dodecanethiol (DT), dodecylamine (DA), thiocholine bromide (TCB), 11-mercaptoundecanoic acid (MUA), polyvinylpyrrolidone (PVP), triphenyl phosphine (TPP), and poly(amido) amine (PAMAM) dendrimer (second generation, not shown in Scheme 1). The chemical structures of capping ligands are outlined in Scheme 1. Detailed ligand-exchange procedures and characterizations are described in the Experimental Section. Briefly, to a stock solution of bare GQDs in methanol were added various ligands and the mixtures were stirred for more than 24 h in the dark. It should be noted here that S-acetylthiocholine bromide was pretreated by diluted aqueous HCl overnight to give free thiol TCB before reacting with gold clusters. Specifically, TCB capped gold clusters were very water-soluble for purification by filtration through a filter having a molecular weight cutoff of 2 kD. The TCB capped gold clusters were then redissolved in water for characterization, while other samples were dispersed in methanol.

Synthesis of Highly Fluorescent Gold Quantum Dots

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Figure 3. UV-vis spectra (A) and fluorescence emission spectra (B) of different ligands capped gold clusters. (C) Photograph of corresponding samples under sunlight.

UV-vis spectra of various samples of different ligand functionalized GQDs were shown in Figure 3A. After the ligand capping process, the functionalized GQDs exhibit different optical behaviors. Specifically, TA stabilized GQDs show enhanced absorbance corresponding to light scattering compared to that of bare GQDs. PAMAM and TPP capped GQDs present new absorption peaks at 486 and 541 nm, which may arise from the electronic transition between ligand and the gold cluster core. These results clearly confirm that the ligand-capping procedure is efficient, and show varied optical properties depending on different capping abilities of the ligands. Fluorescence emission spectra (Figure 3B) show that most of the ligands were almost unable to influence emission intensity from atomic gold clusters except for TPP and TA. For TPP-stabilized GQDs, the efficient quenching of fluorescence might arise from stronger electronic transition between TPP and GQDs. It thus shows dramatic color changes as shown in Figure 3C. For the TA-stabilized one, however, it might be because of agglomeration induced selfquenching due to the carboxyl group interaction of surfaceattached TA, which is shorter in alkyl chain length in comparison with that of MUA. The detailed mechanisms for such fluorescence quenching are not clear enough and need further investigation. It should be pointed out that some efforts have been made in recent years on sensing devices and bioassays based on fluorescent gold clusters, which are mainly achieved by ligand replacement or the ligand mixing process.3a,22 However, surfactant-free GQDs are able to be functionalized straightforwardly by the ligand capping process, which privides a clean and impervious environment compared to former researches, thus such functionalized GQDs may open a new scope for applications with excellent performance. The well-confined ligand-encapsulated structures enable analysis of functionalized gold clusters with various mass spectroscopies. Electrospray ionization (ESI) mass spectrometry data from TCB-stabilized GQDs (see the Supporting Information, Figure S1) reveal that the light-emitting species are gold atomic clusters consisting of only 11 gold atoms (Au11) along with capping ligands and water molecules. Another evidence

arises from matrix-assisted laser desorption/ionization, using time-of-flight technique (MALDI-TOF) mass spectrometry data of TA capped gold clusters, which gives peaks that mainly correspond to cluster fragment ions (see the Supporting Information, Figure S2). TPP-capped gold clusters were also examined by the MALDI-TOF technique, which shows two main peaks corresponding to two relative cluster fragment ions consist of 2 and 3 gold atoms, respectively (see the Supporting Information, Figure S3). Effect of Ligand Concentration. Ligand concentration effects on the formation of gold atomic clusters are investigated by using a typical ligand PVP. Under the same preparation condition outlined above in the presence of various amounts of PVP, differently fluorescent gold atomic clusters were obtained with the solution colors ranging from colorless to light brown (Figure 4A), which exhibit discrete excitation and emission spectra as shown in Figure 4B with quantum yields ranging from 5% to 8% as shown in Table 1, using quinine sulfate in 0.1 mol · L-1 of H2SO4 (aq) as standard.23 This thus suggested a ligand-concentration-dependent behavior in GQDs preparation. By fitting the plot in Figure 4C, we found that the GQDs emission wavelengths are dramatically related to the PVP concentration, but there is nearly no change higher than 5 × 10-2 M. Panels D and E of Figure 4 show photos of larger gold nanoparitlces and two relative samples under sunlight and UV 365 nm irradiation. The different emission colors achieved indicate the controllability of the optical and quantum confinement in the preparation. It should also be noted that the amount of PVP was a crucial factor to modulate the emission wavelengths for each sample. However, there are nearly no changes in the absorption wavelengths of samples prepared at different concentrations of PVP, while the fluorescence emission wavelength dramatically changes relative to PVP concentration. Considering such results, we suppose that the ligand-concentration-effect is considered to be a cross-linking effect. That is, PVP provided a cross-linking environment for gold cluster agglomeration. With increasing concentration of PVP, the agglomeration was enhanced, which presented a red-shift in both

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Figure 4. Comparison of the optical properties of fluorescent GQDs of different samples. (A) UV-vis spectra of PVP capped GQDs obtained at various concentrations of PVP (blue curve: 4.5 × 10-3 M; green curve: 1.8 × 10-2 M; red curve: 5.4 × 10-2 M) recorded in DMF, and contrasted PVP coated gold nanoparticles in DMF (purple curve). (B) Comparison of fluorescence excitation (dashed curve) and emission (solid curve) spectra of the as-prepared GQDs obtained at various concentrations of PVP (samples are the same as UV-vis spectra). (C) Plot of fluorescence emission wavelength of different GQDs samples versus concentration of PVP ligand. (D) Color photograph of PVP-gold nanoparticles (left) and two samples of as-prepared PVP capped GQDs (right two). (E) Corresponding photograph of the gold nanoparticles and two samples of GQDs under UV lamp (365 nm) irradiation.

TABLE 1: Photophysics of Different Samples of Gold Quantum Dotsa before reduction

after reduction

sample Ex. Em. fwhm QY Ex. fwhm no. (nm) (nm) (nm) (%) (nm) Em. (nm) (nm) 1 2 3

372 384 396

446 468 481

99 105 108

4.49 6.32 8.65

347 349 353

423 427 436

93 96 103

QY (%) 11.21 13.63 14.28

a Ex. ) Excitation wavelength. Em. ) Emission wavelength. fwhm ) full width at half maximum. QY ) quantum yield, determined through comparison with quinine sulfate.

fluorescence excitation and emission wavelengths. Furthermore, after encapsulation by PVP, the large coiled-like polymer cages better protect these nanoclusters from quencher in solution, or,

after being purified by continual centrifugation, there are no large gold nanoclusters to quench the atomic cluster fluorescence. It is known that subnanometer atomic clusters prepared in the absence of ligand are too small to have the continuous density of states necessary to support a plasmon characteristic of larger free-electron metal nanoparticles and are not visible under TEM observation.16,24 However, cross-linking ligands such as PVP provide a good pathway for aggregation, thus visualizing the gold clusters under TEM observation. As expected, after cross-linking in the presence of PVP ligand, the gold atomic clusters agglomerate, and are visible in TEM images as shown in Figure 5. Tunable Optical Properties. More interestingly, the PVPcapped GQDs can be chemically reduced to exhibit blue-shifted emission. After addition of 1 equiv of NaBH4 to the as-prepared

Figure 5. TEM images of as-prepared bare GQDs (A) and PVP encapsulated GQDs (B). (C) High-resolution TEM image of PVP cross-linked GQDs.

Synthesis of Highly Fluorescent Gold Quantum Dots

Figure 6. Fluorescence excitation (dashed curves) and emission (solid curves) spectra of as-prepared GQDs (samples are the same as Figure 3B) after treated with 1 equiv of NaBH4.

PVP encapsulated gold cluster solution, the fluorescence emission wavelength blue-shifted from 481, 468, 446 nm to 436, 427, 423 nm, respectively, as long as the excitation curve broadened (Figure 6). A similar phenomenon was also observed by Nie and co-workers.16 The so-called “oxidation state” of the PVP capped gold clusters is considered to undergo a reduction process, and the blue-shifted fluorescence excitation and emission mainly arise from the reduction state of gold clusters. Furthermore, in our experiment, the quantum yields of different samples of GQDs increase several fold by treatment with reducing agent (Table 1). Compared to that of bulk gold, the high QYs of ultrasmall gold clusters are probably due to the lower density of energy and electronic states present in the ultrasmall gold clusters, thus minimizing the number of internal nonradiative relaxation pathways. Higher QYs presented after reduction might result from decreased Stokes shift with respect to that of the “oxidation state”. Conclusion In conclusion, we developed a facile and novel pathway for surfactant-free synthesis of highly fluorescent GQDs, which can be further functionalized with various ligands to show tunable optical behavior. By further reduction with NaBH4, the specific GQDs shows blue-shifted emissions. Requiring neither complicated synthesis with toxic precursors nor difficult overcoating with surface passivation and solubilization chemistry, such welldefined, size-tunable discrete excitation and emission suggests that these nanomaterials may find potential utility as energy transfer units and novel light emitting devices. Furthermore, surfactant-free synthesis of GQDs provides a better platform for further functionalization with various capping ligands, which have great potential for use in sensing platforms and novel biomarkers. Acknowledgment. . This work was supported by the National Natural Science Foundation of China (2057108, 20531060) and the Major State BasicResearchDevelopmentProgram(2006CB932100,2006CB806200, 2005CB623602). Supporting Information Available: Determination of GQDs concentration and mass spectra of various ligand functionalized GQDs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Imahori, H.; Fujioto, A.; Kang, S.; Hotta, H.; Yoshida, K.; Umeyama, T.; Matano, Y.; Isoda, S. AdV. Mater. 2005, 17, 1727–1730. (b)

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