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Understanding and Controlled Growth of Silver Nanoparticles Using Oxidized N-Methyl-pyrrolidone as a Reducing Agent Sea-Ho Jeon,†,‡ Ping Xu,†,§ Nathan H. Mack,† Long Y. Chiang,‡ Leif Brown,† and Hsing-Lin Wang*,† Chemistry DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry, UniVersity of Massachussetts, Lowell, Massachusetts 01854, and Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China ReceiVed: August 11, 2009; ReVised Manuscript ReceiVed: NoVember 17, 2009
We report a facile synthesis of silver nanoparticles (AgNPs) by using a new reducing agent, pretreated N-methylpyrrolidone (NMP*). The resulting AgNPs are characterized by using UV-vis, TEM, and X-ray spectroscopy. These AgNPs exhibit strong surface enhanced Raman scattering response on addition of 4-mercaptobenzoic acid. A possible redox mechanism involving silver ion and NMP* was proposed. The oxidized species resulting from thermally treated NMP/O2 were analyzed by nuclear magnetic resonance and gas chromatography techniques, and it was determined that 5-hydroxy-N-methyl-2-pyrrolidone played the role of reducing agent. The facile synthesis of functional metal nanoparticles via an environmentally friendly procedure with control in particle size, and understanding of the reaction mechanisms pave the ways to further developing metal nanoparticles for chemical and biological detections. Introduction Metal nanoparticles (MNPs) are technologically important materials due to their unique optical, electronic, magnetic, and catalytic properties that have significant application in display, microelectronics, data storage, drug delivery, bioimaging, and biosensing.1–5 Because of the unique and often tunable properties of nanostructured silver, the synthesis of silver nanoparticles (AgNPs) with well-defined shapes, structures, and properties has been an area of intense interest for the past decade.6–8 Several organic molecules are noted for their ability to reduce silver ions to silver metal in solution, including ethylene glycol in polyol,9,10 ascorbic acid,11 sodium citrate,12 and hydroquinone.13 Most of the above synthetic approaches allow shape and size control of silver nanoparticles in a relatively easy manner, by varying certain experimental parameters such as temperature and concentration. It was also found that poly(vinyl pyrrolidone) (PVP), which was used as both a steric stabilizer and a reducing agent, could be used to kinetically control the growth of silver triangular nanoplates.14 It has been suggested that the hydroxy end group on the PVP chains is responsible for reducing the silver ions to form metal nanoparticles. This hypothesis is based on results from NMR spectroscopy, yet direct evidence of this hypothesis remains lacking. To our understanding, it is quite possible that the pendant pyrrolidone group in PVP can also be involved in the process of reducing silver ions to form silver metals. Despite the lack of a detailed mechanistic understanding, the synthesis of metal nanoparticles using PVP appears attractive due to its inherent simplicity. N-Methyl-pyrrolidone (NMP), with its chemical structure similar to the pendant group of PVP, is known to be a chemically stable, polar solvent, with industrial scale usage in petrochemical processing, polymerization, surface coating and the plastics industry.15–19 * To whom correspondence should be addressed. Phone: 505-6679944. Fax: 505-6670440. E-mail:
[email protected]. † Los Alamos National Laboratory. ‡ University of Massachussetts. § Harbin Institute of Technology.
In order to further understand the role that the pendant pyrrolidone group played in the reduction of silver ions by PVP, NMP was introduced into our system to study whether it has any effect on the preparation of metal nanoparticles. In this work, we have demonstrated preparation of activated NMP (denoted as NMP*) by boiling purified NMP with H2O and O2. The NMP* is an effective reducing agent that allows rapid reduction of metal ion (Ag+) to zerovalent silver metal in a very short time period. These AgNPs can be prepared at room temperature with further control in particle size by adding steric stabilizer such as PVP.14 Our synthetic method is environmentally friendly as NMP is considered nonhazardous in industrial usage. The reaction mechanisms of reducing metal ions by NMP* are carefully studied by gas chromatography (GC) and NMR. Furthermore, these as-synthesized Ag nanoparticles exhibit surface-enhanced Raman scattering (SERS) activity using 4-mercaptobenzoic acid as a SERS dye. The facile synthesis of functional metal nanoparticles via an environmentally friendly procedure with control in particle size, and understanding of the reaction mechanisms pave the ways to further developing metal nanoparticles for chemical and biological detections. Experimental Section Reagents. Anhydrous N-methyl-2-pyrrolidone (NMP, 99.5% Aldrich), silver nitrate (99.9999% Aldrich), 4-mercaptobenzoic acid (MBA, 90%, Aldrich), and poly(vinyl pyrrolidone) (PVP, MW ) 40 000, Sp2 Scientific polymer products, Inc.) were used as received. Preparation of NMP*. A total of 100 mL of NMP and 5 mL of deionized water were placed in a 250 mL one-neck flask. The mixture was purged with argon for 10 min and then refluxed for 2 h at 160 °C while purging with oxygen gas to yield a yellowish clear solution. While cooling to room temperature, the solution was again purged with argon gas, this time for 30 min, to remove the remaining oxygen gas. The solution (NMP*) was kept in a tightly capped amber glass vial.
10.1021/jp907757u 2010 American Chemical Society Published on Web 12/15/2009
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Figure 2. Time dependent (every 10 min start at 1 min) UV-vis spectra of AgNPs growth with a concentration of 1 mM AgNO3 in NMP*.
Results and Discussion
Figure 1. (a) Time-dependent UV-vis spectra of AgNPs produced reaction of AgNO3 with aged NMP and (b) TEM image of the AgNPs obtained after a 6 h reaction time (scale bare is 20 nm) (c) High resolution TEM of Ag nanoparticles with [111] lattice planes (scale bar is 2 nm). For the reaction, 0.1 mL of 1 M AgNO3 was added to 10 mL of aged NMP. Stock NMP was used as a blank and the solution was analyzed without dilution.
Preparation of the AgNPs. A total of 10 mL of NMP* was put into a 20 mL vial and to it was added 0.1 mL of aqueous 0.1 M AgNO3. The vial was capped and shaken for 1 min, after which it was kept in the dark to exclude light from playing a role in catalyzing the formation of AgNPs. Characterization. The composition of the NMP* was examined by using Varian cp-3800 Gas Chromatography (GCFID). A Cp-WAX column was used with the injector temperature set to 250 °C, oven temperature at 240 °C, and detector temperature at 300 °C. The morphologies of the produced AgNPs were measured using a JEOL 3000F high resolution transmission electron microscope (HR-TEM). All AgNP solutions were characterized by UV-vis (Varian Cary 300) from 300 to 800 nm with a scan rate of 80 nm/min. For SERS analysis, the AgNP solution was transferred onto a silicon wafer and kept in a 50 °C oven for 12 h. After drying, the AgNP was carefully washed with ethanol 3 times. Each sample was immersed in a 5% ethanolic solution of MBA for 1 h, then rinsed with fresh ethanol three times and allowed to air-dry. SERS spectra were recorded using 785 nm excitation focused onto the sample through a 0.5 NA microscope objective. The scattered Raman signal was collected in a backscattering configuration through the objective, filtered, and then dispersed onto a liquid nitrogen cooled CCD camera through a single grating spectrometer. For the liquid SERS experiment, 1 mL of AgNP solution was mixed with 0.1 mL of 5% ethanol solution of MBA. A DeltaNu Advantage 200A Raman spectrophotometer with excitation wavelength of 532 nm was used to record the SERS response.
In view of the fact that PVP has shown promises in synthesizing MNPs, we try to use NMP for controllable synthesis of MNPs as NMP and PVP have the same heterocyclic rings. When we add silver nitrate into a NMP without any reducing agent, we observed an unexpected color change of the solution, from colorless to lightly yellow and finally to dark brown. Upon further analysis, it was found that silver ions were reduced by NMP, and the color was resulting from the formation of AgNPs. Upon addition of silver nitrate into a pure, anhydrous NMP through distillation, no AgNPs were produced. This result suggests that certain species in the aged NMP are responsible for playing the role of reducing agent(s). We decided to carry out control experiments by adding an aged NMP (SigmaAldrich), which had been kept approximately 2 years inside a cabinet, to a silver nitrate solution in a tightly capped vial and kept protected from light at ambient temperature. In 1 h, the solution turned yellowish brown and both UV-vis and TEM analyses in Figure 1 revealed the formation of AgNPs. The UV-vis spectrum shows an absorption peak (λmax ) 427 nm) clearly visible within 1 h, consistent with the plasmonic absorption of AgNP.1,8,9 The absorption intensity increases with reaction time, suggesting continuing formation of AgNP. Figure 1b shows the TEM image of the AgNPs collected after a reaction time of 6 h, with particle sizes ranging from 4 to 14 nm. From the high resolution TEM (HR-TEM) image in Figure 1c, one can see that the AgNPs are mostly single crystalline and the crystal lattice plane has a 2.42 Å spacing, which is in agreement with the lattice spacing of Ag [111] crystal plane. When we attempted to repeat the AgNP synthesis using newly received anhydrous 99.5% NMP, we were surprised to find that no AgNPs were formed, and the mixture of NMP and silver nitrate did not show any change after a week of attempted reaction. These results suggest that the reduction of silver ions was not caused by NMP but rather by NMP that had been previously oxidized in the presence of water and/or oxygen. To validate this hypothesis, we intentionally oxidized NMP by heating it up to 160 °C in the presence of deionized water and oxygen for several hours. The color of the solution turned from colorless to light yellow over the course of 2 h, and it then slowly became turbid dark brown. This oxidized NMP solution is denoted as NMP*. Systematic
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Figure 3. (a) UV-vis spectra of AgNPs prepared with 1 mM AgNO3 and 3 mL NMP* kept protected from light for 4 h. (i) AgNP solution from the reaction. (ii) Precipitates, which were removed from solution (i) by centrifugation at 5000 rpm (rcf ) 14.1) for 10 min and redispersed in 3 mL of anhydrous NMP. (iii) Supernatant. NMP* was used as reference for all of the UV spectra. (b) TEM image of precipitates (scale bar is 50 nm) and (c) TEM image of supernatant (scale bar is 5 nm).
analysis of how NMP* impacts the formation of Ag nanoparticles was determined by a series of experiments with different heating time. The preparation of NMP* with a two hour heating time generates the greatest UV-vis absorbance of AgNPs (the 200-300 nm absorption band of NMP* does not overlap with that of AgNPs), which suggests the maximum amount of AgNPs. This NMP* was used as a stock solution, as well as the reducing agent for making AgNPs. UV-vis spectra, used to monitor the growth of AgNPs, indicate that the reduction of silver ions occurs immediately after mixing of the silver nitrate with NMP*. It is important to note that the kinetics of the reduction by NMP* are drastically different from that of the aged NMP. As shown in Figure 2, when a 10 times diluted silver nitrate solution (1.0 mM) is mixed with NMP*, an absorption peak at 434 nm attributed to the plasmon resonance of AgNP could be detected just after 1 min, and the intensity (1.75 au) is 5 times larger than that found when using the aged NMP for a reaction of 6 h (0.35 au). It is even faster than reduction by PVP in ethanol or dimethylformamide (DMF).8,20 This result suggests that NMP* has a reaction kinetics that is much faster than that of PVP. At 11 min, an absorption peak with even higher intensity (2.7 au) was located at 440 nm. It is interesting to find that with further reaction there is a blue shift of the λmax and a slight decrease of the absorbance intensity. This is consistent with a previous report in which
Figure 4. 1H NMR spectra of original NMP (a), the oxidized NMP* (b),13C NMR spectra of NMP (c), and NMP* (d), obtained in CDCl3 at 400.13 and 100.61 MHz, respectively.
Liz-Marza´n et al., found a similar effect in the reduction of silver ions by DMF.8 Meanwhile, the absorption shoulder
Oxidized NMP as a Reducing Agent SCHEME 1: Proposed Oxidation Reaction of N-Methyl-pyrrolidone by Deionized Water and O2 Gas at 160 °Ca
a i. 12 h of refluxing with the conditions yielded 9.1% of compound 3 via forming peroxide. ii. 10 mL of NMP* solution was mixed with 0.01 mL of 1 M AgNO3 at room temperature and kept in a dark place.
between 600 and 1200 nm became stronger with increasing reaction time, which was thought to result from the aggregation of the AgNPs into larger particles.8 Allowing the reaction to continue overnight, we observe a precipitate at the bottom of the vial. The color of the solution was dark green and the UV-vis spectrum exhibited a sharp peak at 417 nm and a shoulder between 550 and 750 nm (Figure 3a). We then separated the dark green solution into two parts (precipitate and supernatant) by centrifugation at 5000 rpm. The precipitate was redispersed in NMP, and its UV-vis spectrum revealed a broad absorption band over the entire measuring range from 350 to 750 nm. TEM image (Figure 3b) of this precipitate showed a mixture of large silver nanoplates (30-50 nm) along with some nanospheres (4-6 nm). UV-vis spectrum of the supernatant displays only one relatively sharp peak centered at 416 nm, with good agreement with the corresponding TEM image (Figure 3c), which reveals uniform distribution of AgNPs with sizes of 4-6 nm. Our results suggest that the activated NMP* is an effective reducing agent which offers an alternative route for the facile
J. Phys. Chem. C, Vol. 114, No. 1, 2010 39 synthesis of AgNPs. Hence, it is of great importance to clarify the structure of activated species NMP* in order to better understand the reaction mechanism that allows the formation of AgNPs. It has been shown that under certain conditions, NMP could be oxidized to form 5-hydroxy-N-methyl-2-pyrrolidone, and further oxidation leads to N-methyl succinimide and 2-hydroxy-N-methylsuccinimide.17,19 Another recent study demonstrated that, after NMP was delivered to humans or animals by injection or ingestion, several oxidized derivatives of NMP were detected in the urine and plasma, mostly in the form of 5-hydroxy-N-methyl-2-pyrrolidone.18 To this end, 1H and 13C NMR spectra of NMP and NMP* were obtained in order to determine the new species (NMP*) produced by our activation process (Figure 4). Comparing the NMP* with pure NMP, we find two new singlets {δ 2.96 (s, 3H), δ 2.74 (s, 4H)} in the 1H NMR, and three new peaks (δ 176.7, 27.4, and 23.8) in the 13C NMR. A careful analysis of the chemical shift values of NMR spectra concludes that these new NMR peaks are consistent with the literature values of N-methyl succinimide.19 The activated NMP* was further analyzed by GC-FID (gas chromatographflame ionization detector), which revealed one major NMP peak with 94% integration, 6.2% of N-methyl succinimide (NMR integration ratio is 90.9% to 9.1% respectively), and 0.76% of an unknown species with a longer retention time (see Supporting Information). It is therefore reasonable to assume that the species corresponding to the longest retention time in GC is the precursor of N-methyl succinimide, 5-hydroxy-N-methyl-2pyrrolidone. This extremely low concentration prevents us from obtaining the NMR of 5-hydroxy-N-methyl-2-pyrrolidone. A typical example of our reaction conditions involves the usage of 10-100 µL of silver nitrate in 10 mL of NMP* solution. Under such reaction conditions, only 0.01-0.1% of the NMP oxidized to hydroxy-N-methyl-2-pyrrolidone would be more than enough to fully convert silver ions to AgNPs. The hypothesized mechanisms of oxidizing NMP and subsequent reduction of silver ions are shown in Scheme 1. First, NMP is oxidized in the presence of heat and oxygen gas to form a peroxide species, as Drago et al. also found peroxide by iodometry titration as they oxidize NMP at an elevated temperature (75 °C) and high pressure (50 psi), in the presence of oxygen.19,21 Then peroxide transforms NMP to 5-hydroxy-
Figure 5. SERS spectra of 4-mercaptobenzoic acid (MBA) with silver NPs. (a) was measured solid state on silicon wafer (b) was measured in solution, using 5% MBA in ethanol. They were excited at 785 nm laser with 1 mW and 10 seconds integration time.
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N-methyl-2-pyrrolidone, and the secondary alcohol undergoes further oxidation via bimolecular elimination to form N-methyl succinimide.22 In the reduction of silver ions, 5-hydroxy-Nmethyl-2-pyrrolidone acts as a reducing agent, which is correspondingly oxidized to N-methyl succinimide when silver is produced. Our study demonstrates a new way of making silver nanoparticles and control particle growth via solution processing route. These nanoparticles can be separated and purified using centrifugation to yield monodispersed (4-6 nm) Ag nanoparticles (Figure 3). It is known that AgNPs with certain sizes and morphologies exhibit surface enhanced Raman scattering (SERS) properties. The SERS properties of our AgNPs in solution and on solid substrate are studied by comparing the SERS signal using a known Raman active dye, 4-mercaptobenzoic acid (MBA), which spontaneously forms a self-assembled monolayer (SAM) on the silver surface through covalent thiol linkage. The measured SERS spectra for the nanoparticles in solution and on silicon substrate are shown in Figure 5. Both spectra have typical strong signals at ∼1080 and ∼1590 cm-1 due to aromatic ring vibrations. Liquid samples exhibited relatively weak SERS signal presumably due to a low particle density and different wavelength of the excitation laser. It is important to note that the laser spot size for solid samples was on the order of a few micrometers and has a relatively shallow depth of field and, for the liquid sample, source laser passing sample tubes twice by reflector to go to detector. So it is very hard to compare those two samples directly. Nonetheless, the nanoparticles on the substrate tend to form particle aggregates with interstitials between nanoparticles that could serve as potential SERS “hot spots”. Although the aggregates are too small to be visualized by the SEM and the exact nature of these hot spots is hard to determine, our results suggest that the nanoparticle surfaces are relatively clean- without passivating layer or steric stabilizer thereby allowing SERS dye to attach onto the nanoparticle surface and be easily detected using SERS. Conclusions We have demonstrated an environmentally friendly procedure to synthesize metal nanoparticles by using oxidized NMP (NMP*) as a reducing agent. We also further demonstrate control over their particle size and disparity by separation and purification via centrifugation. Through analysis by NMR and GC, the oxidized NMP species responsible for reducing the silver nitrate into AgNPs is 5-hydroxy-N-methyl-2-pyrrolidone. The proposed reaction mechanisms clearly illustrate how oxidized NMP reduces Ag ions to zerovalent Ag. The asprepared AgNPs showed good SERS activity upon addition of 4-mercaptobenzoic acid, which attaches to the metal surface through thiol linkage. The nanoparticles on substrate has a much stronger SERS signal because the nanoparticles on the substrate tend to form aggregates with interstitials between nanoparticles that could serve as potential SERS “hot spots”. The steric
Jeon et al. stabilizer that covers the nanoparticles surface also limits the access of the SERS dye, resulting in a weaker SERS response. Acknowledgment. The authors acknowledge the financial support from Laboratory Directed Research and Development (LDRD) fund under the auspices of DOE, BES Office of Science, and the National Nanotechnology Enterprise Development Center (NNEDC). This work was performed in part at the U.S. Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos National Laboratory (Contract DEAC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000). Supporting Information Available: GC, NMP, and XRD. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736–4745. (2) Kyriacou, S. V.; Brownlow, W. J.; Xu, X. N. Biochemistry 2004, 43, 140–147. (3) Feng, X.; Ma, H.; Huang, S.; Pan, W.; Zhang, X.; Tian, F.; Gao, C.; Cheng, Y.; Luo, J. J. Phys. Chem. B 2006, 110, 12311–12317. (4) Chena, Z. P.; Peng, Z. F.; Luo, Y.; Qu, B.; Jiang, J. H.; Zhang, X. B.; Shen, G. L.; Yu, R. Q. Biosensor Bioelectron. 2007, 23, 485–491. (5) Reiss, G.; Hu¨tten, A. Nat. Mater. 2005, 4, 725–726. (6) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (7) (a) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736–4745. (b) Zhang, D.; Qi, L.; Yang, J.; Ma, J.; Cheng, H.; Huang, L. Chem. Mater. 2004, 16, 872–876. (c) Suber, L.; Sondi, I.; Matijevic, E.; Goia, D. V. J. Colloid Interface Sci. 2005, 288, 489–495. (d) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc. Faraday Trans. 1993, 89, 2537–2543. (8) Pastoriza-Santos, I.; Liz-Marza´n, L. M. Langmuir 1999, 15, 948– 951. (9) Pastoriza-santos, I.; Serra-Rodrı´guez, C.; Liz-Marza´n, L. M. J. Colloid Interface Sci. 2000, 221, 236–241. (10) (a) Ducamp-Sanguesa, C.; Herrera-Urbina, R.; Figlarz, M. J. Solid State Chem. 1992, 100, 272–280. (b) Silvert, P. Y.; Urbina, R. H.; Duvauchelle, N.; Vijayakrishnan, V.; Elhsissen, K. T. J. Mater. Chem. 1996, 6, 573–577. (11) Fukuyo, T.; Imai, H. J. Cryst. Growth. 2002, 241, 193–199. (12) Pillai, Z. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 945–951. (13) Pe´rez, M. A.; Moiraghi, R.; Coronado, E. A.; Macagno, V. A. Cryst. Growth Des. 2008, 8, 1377–1383. (14) (a) Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2006, 18, 1745–1749. (b) Wang, H.; Qiao, X.; Chen, J.; Wang, X.; Ding, S. Mater. Chem. Phys. 2005, 94, 449–453. (15) Åkesson, B. Arbete Ha¨lsa 1994, 40, 1–24. (16) Anundi, H.; Langworth, S.; Johanson, G.; Lind, M.-L.; Åkesson, B.; Friis, L.; Itkes, N.; So¨derman, E.; Jo¨nsson, B. A. G.; Edling, C. Int. Arch. Occup. EnViron. Health 2000, 73, 561–569. (17) Carerup, M. A.; Åkesson, B.; Jo¨nsson, B. A. G. J. Chromatogr. B 2001, 761, 107–113. (18) Carnerup, M. A.; Saillenfait, A. M.; Jo¨nsson, B. A. G. Food Chem. Toxicol. 2005, 43, 1441–1447. (19) Drago, R. S.; Riley, R. J. Am. Chem. Soc. 1990, 112, 215–218. (20) Kim, S. J. Ind. Eng. Chem. 2007, 4, 566–570. (21) Russell, G. A. J. Am. Chem. Soc. 1956, 78, 1047–1054. (22) Scha¨del, U.; Gruner, M.; Habicher, W. D. Tetrahedron 2002, 58, 5081–5086.
JP907757U
