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Mechanism of Silver Ion Reduction in Concentrated Solutions of Amphiphilic Invertible Polyesters in Nonpolar Solvent at Room Temperature A. Voronov,*,† A. Kohut,† S. Vasylyev,‡ and W. Peukert‡ Coatings and Polymeric Materials, North Dakota State UniVersity, 1735 NDSU Research Park DriVe, Fargo, North Dakota 58105, and Institute of Particle Technology, Friedrich-Alexander UniVersity Erlangen-Nuremberg, Cauerstr. 4, Erlangen, 91058 Germany ReceiVed June 6, 2008. ReVised Manuscript ReceiVed August 15, 2008 Fast formation and efficient stabilization of silver nanoparticles from [Ag(NH3)2]OH are achieved in concentrated nonpolar solutions of amphiphilic invertible polyesters based on poly(ethylene oxide) (PEO) and aliphatic dicarboxylic acids. Surface-modified silver nanoparticles able to be dispersed in both a polar and nonpolar medium are developed in the form of a ready-to-use colloidal solution with an enhanced silver concentration. The PEO fragments of polyesters form cavities (also called pseudo-crown ester structures) that can bind metal ions. The reduction of bound metal ions proceeds via oxidation of polyoxyethylene fragments. No chemical reducing agents are necessary in this approach. The polyesters act simultaneously as an efficient reducing agent and stabilizer. The main focus of the present research is to clarify the chemical mechanism of silver ion reduction in amphiphilic polyester solutions. A one-electron reduction mechanism is proposed to explain the formation of silver nanoparticles. The effect of the poly(ethylene oxide) fragment length and the polyester concentration are explored by examining several amphiphilic polyesters.
Introduction In recent years, the preparation and characterization of nanometer-sized particles have received extreme interest because of their unique properties and potential use in many significant applications.1 Due to the nanoparticles’ small size, large specific areas, and tunable physicochemical properties, they differ essentially from the materials in the bulk.2 Many different methods employed for synthesis of nanoparticles were reviewed recently including polymer-based approaches.1 Polymer-assisted or polymer template synthesis of metal nanoparticles has received considerable attention because (i) of the small concentrations of homopolymer and block copolymers capable of stabilizing nanoparticles effectively by steric stabilization, (ii) of the polymer containing appropriate functional groups serving as both reducing and stabilizing (capping) agent, (iii) of the possibility to control the size and morphology of nanoparticles by varying the polymer/metal salt ratio, and (iv) it offers to prepare novel metal-polymer nanocomposites.3,4 During the past few decades, many methods have been developed to synthesize polymer-stabilized nanoparticles. These methods include photochemical reduction, electrochemical techniques, chemical reduction, a polyol process, and radiolytic methods.5-10 Most synthetic approaches to the development of particles are successful, but scalable obtaining of small nanoparticles with a narrow particle size distribution and good stability in various dispersion media still remains as the greater goal of scientists. Agglomeration during synthesis remains as a second * To whom correspondence should be addressed. Telephone: + 1 701 231 9563. Fax: + 1 701 231 8439. E-mail:
[email protected]. † North Dakota State University. ‡ Friedrich-Alexander University Erlangen-Nuremberg. (1) Bajpai, S. K.; Murali Mohan, Y.; Vajpai, M.; Tankhiwale, R.; Thomas, Y. J. Nanosci. Nanotechnol. 2007, 7, 1–17. (2) Trindade, T.; Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13(11), 3843. (3) Thomas, V.; Namdeo, M.; Murali Mohan, Y.; Bajpai, S. K.; Bajpai, M. J. Macromol. Sci., Part A: Pure Appl. Chem. 2008, 45, 107–119. (4) Jewerajka, S. K.; Chatterjee, U. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1841.
considerable drawback in most of the synthetic approaches. Due to van der Waals interaction, the particles tend to agglomerate, and the use of a polymeric material as steric stabilizer has been considered as one of the promising methods to develop a fairly stable dispersion of nanoparticles.11-13 To synthesize silver colloidal nanoparticles, a number of chemical methods are being used. In most of them, various reducing agents have been used, including hydrazine, ferric ions, ascorbic acid, and so forth.14-16 The size and shape of nanoparticles are sensitive to the reaction conditions and concentration of a precursor. However, these syntheses very often require materials that are not environmentally benign. The much more attractive processes do not require additives such as surfactants or reducing agents at all. Thus, a variety of water-soluble polymers, such as poly(vinyl alcohol) (PVA), poly(vinylpyrrolidone) (PVP), poly(ethylene glycol) (PEG),17 gum acacia,18,19 and cellulose-based polymers20 have received much attention because they can act as both a (5) Chou, K. S.; Ren, C. Y. Mater. Chem. Phys. 2000, 64, 241. (6) Silvert, P. V.; Urbina, R. H.; Elhisissen, K. T. J. Mater. Chem. 1997, 7, 293. (7) Marignier, J. L.; Belloni, J.; Delcourt, M. O.; Chevalier, J. P. Nature 1985, 317, 344. (8) Zhu, Y.; Qian, Y.; Zhang, M.; Chem, Z.; Lu, B.; Wang, C. Mater. Lett. 1993, 17, 314. (9) Zhu, Y.; Qian, Y.; Li, X.; Zhang, M. Chem. Commun. 1997, 1081. (10) Yin, Y.; Xu, X.; Xia, C.; Ge, X.; Zhang, Z. Chem. Commun. 1998, 941. (11) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (12) Overbeek, J. T. G. Colloidal Dispersions; Royal Society of Chemistry: London, 1981. (13) Bossel, C.; Dutta, J.; Houriet, R.; Hilborn, J.; Hofmann, H. Mater. Sci. Eng. 1995, A 204, 107. (14) Chou, K. S.; Lai, Y. S. Mater. Chem. Phys. 2004, 83, 82. (15) Wu, S. P.; Meng, S. Y. Mater. Chem. Phys. 2005, 89, 423. (16) Wei, G. D.; Deng, Y.; Nan, C. W. Chem. Phys. Lett. 2003, 367, 512. (17) Luo, C.; Zhang, Y.; Zeng, X.; Zeng, Y.; Wang, Y. J. Colloid Interface Sci. 2005, 288, 444. (18) Mucalo, M. R.; Bullen, C. R.; Maley-Harris, M.; MCIntire, T. M. J. Mater. Sci. 2002, 37, 493. (19) Yu, F.; Liu, Y.; Zhuo, R. J. Appl. Polym. Sci. 2004, 91, 2594. (20) Kwon, J.-W.; Yoon, S. H.; Lee, S. S.; Seo, K. W.; Shim, I.-W. Bull. Korean Chem. Soc. 2005, 26, 837.
10.1021/la801769v CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
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reducing agent as well as a stabilizer. Recent synthetic examples include application of hydrophilic homopolymers and amphiphilic copolymers such as poly(ethylene glycol),21 diamine-terminated poly(ethylene oxide),22 amine-functionalized third-generation poly(propyleneimine) dendrimers,23 and R-biotinyl-poly(ethylene glycol)-block-[poly(2-(N,N-dimethylamino)ethyl methacrylate)].24 Gold and silver nanoparticles were developed using amphiphilic Pluronic triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPOPEO) in aqueous25,26 or organic27,28 media. The mechanism of gold nanoparticle formation has been well studied in aqueous solutions of PEO-PPO-PEO having various poly(ethylene oxide) and poly(propylene oxide) block lengths.28 Recently, we presented the synthesis of new amphiphilic polyesters comprising hydrophilic poly(ethylene glycol) and hydrophobic aliphatic dicarboxylic acid moieties alternately linked in a macrochain.29 We have found the polyesters to be promising compounds for the stabilization of inorganic particle dispersions both in polar and nonpolar dispersion media30,31 and as templates for the synthesis of metal nanoparticles highly stable in dispersion media strongly differing by polarity.32,33 Variation of the polyester molecular characteristics (length and ratio of hydrophilic and hydrophobic units, molecular weight), solvents, and concentration has been shown to allow polyester selfassembly in the presence of selective solvents strongly differing by polarity.29,32-35 The conformational changes of the polyester macromolecules when changing the polarity of the surrounding solvent have been recently confirmed in hyper-Rayleigh scattering measurements.34 We have shown that the amphiphilic polyesters act simultaneously as an efficient reducing agent and stabilizer in the singlestep synthesis and stabilization of noble metal nanocolloids. The nanoparticles are synthesized both in polar and nonpolar solvents in the absence of an additional reducing agent or energy input.32,33 Compared to already reported single-step methods for metal nanoparticles synthesis, the presented approach demonstrates the possibility for (i) spontaneous (fast) economical formation of silver nanoparticles at room temperature, (ii) synthesis of a ready-to-use colloidal solution with an enhanced silver nanoparticle concentration, and (iii) formation of surface-modified nanoparticles able to be dispersed in both a polar and nonpolar medium. The produced nanocolloids can be easily transferred from one solvent to another without any loss of colloidal stability. We have shown that synthesized metal nanoparticle dispersions remain stable both in polar and nonpolar solvents at least for 9-12 months. (21) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (22) Iwamoto, M.; Kuroba, K.; Zaporojtschenko, V.; Hayashi, S.; Faupel, F. Eur. Phys. J. D. 2003, 24, 365. (23) Sun, X.; Jiang, X.; Dong, S.; Wang, E. Macromol. Rapid Commun. 2003, 24, 1024. (24) Ishii, T.; Otsuka, H.; Kataoka, K.; Nagasaki, Y. Langmuir 2004, 20, 561. (25) Sakai, T.; Alexandridis, P. Nanotechnology 2005, 16, 344. (26) Sakai, T.; Alexandridis, P. Langmuir 2005, 21, 8019. (27) Zhang, L.; Yu, J. C.; Yip, H. Y.; Li, Q.; Kwong, K. W.; Xu, A.-W.; Wong, P. K. Langmuir 2003, 19, 10372. (28) Sakai, T.; Alexandridis, P. J. Phys. Chem. B 2005, 109, 7766. (29) Voronov, A.; Kohut, A.; Peukert, W.; Voronov, S.; Gevus, O.; Tokarev, V. Langmuir 2006, 22, 1946. (30) Kohut, A.; Ranjan, S.; Voronov, A.; Peukert, W.; Tokarev, V.; Bednarska, O.; Gevus, O.; Voronov, S. Langmuir 2006, 22, 1946. (31) Kohut, A.; Voronov, A.; Peukert, W. Langmuir 2007, 23, 504. (32) Voronov, A.; Kohut, A.; Peukert, W. Langmuir 2007, 23, 360. (33) Kohut, A.; Voronov, A.; Samaryk, V.; Peukert, W. Macromol. Rapid Commun. 2007, 28, 1410. (34) Martinez-Tomalino, L.; Voronov, A.; Kohut, A.; Peukert, W. J. Phys. Chem. B 2008, 112, 6338. (35) Voronov, A.; Vasylyev, S.; Kohut, A.; Peukert, W. J. Colloid Interface Sci. 2008, 23, 379.
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In our experiments, we are targeting the relationship between the nanoparticle formation and polyester characteristics and the reaction conditions. To understand the potential of the proposed synthetic approach, we have to clarify the mechanism of silver ion reduction in concentrated solutions of amphiphilic invertible polyesters. Besides, the mechanism of gold ion reduction in PEOcontaining block copolymers (Pluronics) is very well described,28 which is not the case for silver ion reduction in PEO-based polymeric systems. There are a variety of reported experimental studies describing the possible mechanism of silver metal ion reduction in the presence of amphiphilic PEO-containing polymeric templates. We considered different possibilities based on the information reported in the literature for the reduction of metal ions by a PEO homopolymer and PEO-containing block copolymers. It is described that the hydroxyl functionality in PEO molecules could act as a reducing agent.36,37 The analysis of PEG 2000 after reaction by 1H NMR revealed the appearance of a new peak, providing the evidence that aldehyde is possibly formed in the system. However, the size and shape of the formed silver nanoparticles have been found sensitive not only to the concentration of silver salt and length of the PEO molecules but also to the reaction temperature. On the basis of the ambient light reduction strategy, a mechanism of silver nanoparticle formation was proposed in ref 27. The authors have found that the PEO-containing Pluronic triblock copolymer (P123) can induce reduction of [Ag(NH3)2]+ ions in ethanol by ambient light illumination. They reported on the formation of semiconductive Ag2O due to the interactions of [Ag(NH3)2]+ ions and oxyethylene groups. Upon illumination with ambient light, the reduction occurs on the Ag2O surface to produce metal silver atoms. Thus, P123 played multiple roles in the formation of silver nanocolloids: to induce the reduction of [Ag(NH3)2]+ ions to form Ag2O, to control the growth of silver nanoparticles through complexing with silver clusters, and to prevent agglomeration of silver nanoparticles through steric hindrance. It has been shown that the PEO fragments in PEO-containing copolymers are able to form cavities (also called pseudo-crown ester structures) that can bind metal ions.38-40 The reduction of bound metal ions can proceed via oxidation of polyoxyethylene fragments by the metal center.21 The reason for the conformation similar to that of the crown esters is the ion-dipole interaction between the metal ion and the electron pair of the polyethylene oxide fragments. The attraction strength depends on the length of the polymeric fragment.40,41 To this end, in the present work, we examined factors that contribute to the reduction of silver ions, the formation of silver nanoparticles, and their stabilization in the single-step synthesis of silver nanoparticles. The idea about the one-electron reduction mechanism of silver cations in concentrated solutions of amphiphilic invertible polyesters is given from (i) specific properties of the silver precursor Ag[(NH3)2]OH aqueous solution and (ii) specific properties of polyester macromolecules which form cavities (pseudo-crown ether structures) that can bind and reduce metal ions. Benzene solutions of amphiphilic invertible polyesters were (36) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (37) Anersson, M.; Alfredsson, V.; Kjellin, P.; Palmqvist, A. E. C. Nano Lett. 2002, 2, 1403. (38) Warshawsky, A.; Kalir, R.; Deshe, A.; Berkovitz, H.; Patchornik, A. J. Am. Chem. Soc. 1979, 101(15), 4249. (39) Adams, M. D.; Wade, P. W.; Hancock, R. D. Talanta 1990, 37(9), 875. (40) Liu, K.-J. Macromolecules 1968, 1(4), 308. (41) Yanagida, S.; Takahashi, K.; Okahara, M. Bull. Chem. Soc. Jpn. 1977, 50(6), 1386.
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Table 1. Composition and Characteristics of Investigated Polyesters polyester composition (number and weight of units) molecular weight, g/mol (PDI)
polyester S3, poly(ethylene glycol) 300-alt-decanedioic acid D3, poly(ethylene glycol) 300-alt-dodecanedioic acid S6, poly(ethylene glycol) 600-alt-decanedioic acid D6, poly(ethylene glycol) 600-alt-dodecanedioic acid a
hydrophilic-lipophilic balance, HLBa
x/y
molecular weight of polyester repeating fragment
6700 (1.43)
18
6.4/8
466
5500 (1.44)
17
6.4/10
494
6400 (1.45)
21
13.2/8
766
17 700 (1.81)
19
13.2/10
794
Calculated according to ref 54.
used for the synthesis of nanoparticles. The contribution of polyester composition, length of the hydrophobic and hydrophilic polyester fragments, reduction conditions, and polyester concentration are addressed in the paper.
Experimental Section Materials. Silver nitrate (AgNO3, ACS, g99.0%, Sigma-Aldrich) was used as received. Solvents (acetone and benzene, CHROMASOLV Plus, for HPLC, g 99.9%) were purchased from SigmaAldrich and used without further purification. Sodium hydroxide and ammonia solution were used as received from Sigma-Aldrich. Polyester and Precursor Synthesis. The amphiphilic polyesters were synthesized by the polycondensation of decanedioic (sebacic) or dodecanedioic acid with poly(ethylene glycol)-300 or poly(ethylene glycol)-600 as described in our previous work.29 To prepare a solution of silver precursor, 0.2 g of sodium hydroxide in 1.0 mL of water was added to a solution of 0.12 g of silver nitrate in 1.0 mL of water to give a brown precipitate of silver oxide. The product was isolated by filtration and washed on a filter with Millipore water until the pH of the filtrate was about 8-9. The precipitate was completely dissolved in 0.3 mL of 32% ammonia solution to form a Ag[(NH3)]OH precursor solution. Synthesis of Silver Nanoparticles in Concentrated Solutions of Amphiphilic Invertible Polyesters. Concentrated polymeric solutions were developed by the dissolution of polyesters (25 or 50% w/w) in benzene. They were left to equilibrate for at least 16 h with intense stirring under room temperature. The formation of silver nanocolloids was performed by the addition of 15-30 µL of a precursor solution to 2 g of 25-50% (w/w) polyester solution in benzene. For selected samples, we compared the silver ion reduction at ambient light with a reduction in the dark. We switched off the stirring after a short stirring time and put the samples in a dark place. The rest of the samples were left on the laboratory desk under ambient light. Molecular Weight Measurements. The average molecular weights of the polymers before and after synthesis of the metal nanoparticles were determined by gel permeation chromatography (GPC) using a Waters 515 HPLC pump with an Ultrahydrogel 500 7.8 × 300 mm column (Waters) and a Waters 2410 refractive index detector. Tetrahydrofuran (THF) was used as eluent at a flow rate of 1.0 mL/min; 200 µL of a 1 mg/mL THF solution was injected for each sample. All samples were filtered before running through a 0.45 µm THFE filter (Nalgene). A molecular weight calibration curve was generated with polystyrene standards of low polydispersity (Polymer Laboratories). Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra were recorded for the samples of polyester and polyestersilver reaction mixture after synthesis by using a Varian Excalibur FTS 3100 spectrometer with a resolution of 4 cm-1. Thermogravimetric Analysis (TGA). The concentration of silver nanoparticles after synthesis was calculated from TGA data by the weight loss of a hybrid polymer-silver sample in the temperature range from 150 to 600 °C. Thermogravimetric analysis was performed on a TA Q500 instrument with a heating rate of 10 °C/min in a nitrogen flow.
Optical Absorption. The optical absorption spectra of the samples were recorded on a Cary Varian 100 UV-vis spectrophotometer over the wavelength range from 350 to 800 nm. Dispersions of about 20 µL of reaction mixture in 2 mL of benzene and acetone were prepared 15-20 min before the measurements. Wide-Angle X-ray Scattering (WAXS). WAXS curves were recorded by using a Phillips X-ray diffractometer (Cu KR radiation monochromatized by a Ni filter). A dispersion of silver nanoparticles in the polyester S6 solution was dried under reduced pressure until a constant weight, and then X-ray diffractogram patterns were recorded.
Results and Discussion Characterization of Amphiphilic Polyesters. The amphiphilic polyesters used in the current study are polyesters obtained by the polycondensation of decanedioic (sebacic) acid and PEG-600 (S6), decanedioic acid and PEG-300 (S3), dodecanedioic acid and PEG-300 (D3), and dodecanedioic acid and PEG-600 (D6). The characteristics and the composition of polyesters are shown in Table 1. We confirmed the structure of the synthesized polyesters by FTIR spectroscopy (Figure 1 in the Supporting Information) and determined the weight-average molecular weight, Mw, and the polydispersity index by GPC. The composition of the polyesters was proved by the intensive doublet of absorption bands in the range of 1000-1270 cm-1 resulting from valence oscillations of the C-O bond. The presence of carbonyl groups is displayed as a broken intensive absorption band at 1730 cm-1. The spectra show a doublet of absorption bands in the range of 2800-3000 cm-1 (valence oscillations) and absorption bands at 1450 cm-1 (deformation oscillations) and 730 cm-1 (pendulum oscillations) that are characteristic of -(CH2)n- groups. The chemical structure of the polymers synthesized was characterized by 1H NMR spectroscopy. A typical 1H NMR spectrum is shown in Figure 2 in the Supporting Information. For the polyester S6, peaks appeared at 4.1-4.2 (m, 50H, PEG fragments) and 1.3 (m, 8H, (CH2)4), which were in agreement with those of sebacic acid and PEG. At 5.14 ppm, a triplet absorption peak that can be contributed to the methylene protons of the acylated PEG end unit was observed (t, 4H, COOCH2CH2O). The spectrum shows a triplet peak at 2.62 ppm and a multiplet peak at 1.83 ppm corresponding to the methylene groups in the R- and β-positions in relation to the carbonyl groups in sebacic acid moieties (4H, CH2CH2COO and 4H, CH2CH2COO, respectively). It has been shown that the PEO molecule in the crystalline state has a helical structure which contains seven units and two turns in the fiber identity period 19.3 Å.42 The PEO fragments in PEO-containing copolymers are able to form cavities (also (42) Tadokoro, H.; Chatani, Y.; Yoshihara, T.; Tahara, S.; Murahashi, S. Macromol.Chem. 1964, 73, 4, 109–127.
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Figure 1. UV-vis absorption spectra of silver nanoparticles prepared using 50% w/w benzene solution (A) of S6 in the dark and dispersed in acetone, (B) of S6 in the dark (solid lines) and dispersed in benzene, (C) of D6 and dispersed in acetone, and (D) of D3 and dispersed in acetone.
called pseudo-crown ester structures). We assume that poly(ethylene glycol) fragments of amphiphilic polyesters form similar pseudo-crown ester structures in concentrated solutions in benzene in experimental conditions. Effect of Reaction Conditions (Ambient Light, Polyester Composition, and Concentration) on Reduction and Nanoparticle Formation. The main point of our interest in this paper is the mechanism of silver ion reduction facilitated by concentrated solutions of amphiphilic polyesters. As it was reported recently, ambient light present in an ordinary chemical laboratory may induce reduction [Ag(NH3)2]+ ions in an organic solvent in the presence of a neutral amphiphilic PEOcontaining triblock copolymer P123.27 Since the same precursor [Ag(NH3)2]OH is used in our experimental systems, we have to consider the possible ambient light effect on the reduction of silver ions in the presence of amphiphilic polyesters. Remarkably, silver colloids were not found when Ag+ ions were used instead of [Ag(NH3)2]+ as the silver source.27 According to the authors, this is because there were no hydroxide ions in the solution. Their presence is crucial in the reduction reaction. In the absence of amphiphilic invertible polyesters, a [Ag(NH3)2]OH precursor aqueous solution in benzene remains colorless and no changes can be detected in the absorption spectra of this solution either at ambient light or after aging in the dark for a long time. This indicates that a reduction of [Ag(NH3)2]+ does not take place. We performed also another model experiment and added an aqueous solution of silver nitrate to concentrated polyester solution in benzene (at the same ratio of polymer/ silver ions as for [Ag(NH3)2]OH). The reaction mixture remained colorless, indicating that no reduction of Ag+ ions occurs when AgNO3 is used as a precursor. However, the colorless solution gradually turns yellow and then dark brown when a [Ag(NH3)2]OH precursor solution is
added to the benzene solution containing 50% w/w of amphiphilic invertible polyester. In turning to the P123 Pluronic triblock copolymer, the coloration change takes place for each of the polyester solutions under either ambient light or dark conditions. The formation of silver nanoparticles is confirmed by UV adsorption spectra revealing the characteristic absorption bands at 420-430 nm.27 We have found that the polyester reduction ability changes with changing polyester composition and hydrophilic-lipophilic balance (Figure 1). For each of the polyester solutions, the reduction of silver ions starts immediately after the silver precursor is dropped into the reaction mixture. To compare the reaction under ambient light and dark conditions, we recorded optical spectra within the initial 2 h of the reduction simultaneously for four polyesters. The increasing intensity of the absorption peak indicates increasing concentration of developed silver nanoparticles over time. Interestingly, the higher concentration of silver is observed in the experiments performed in the dark in comparison to reduction under ambient light. Moreover, the concentration of the silver nanoparticles formed in the presence of more hydrophobic polyesters (D3 and D6) has been found to be lower in comparison to the case of the reduction in the presence of hydrophilic polyester S6. We assume that, being dispersed in an organic solvent, an aqueous solution of silver precursor is transferred through the nonpolar phase into the hydrophilic polyester moieties. In this way, the precursor is trapped in the hydrophilic polymeric domains where the reduction reaction occurs. Our experimental finding indicates that domains of the polymers with longer PEG fragments and therefore with the higher hydrophilic-lipophilic balance are more “attractive” for the aqueous precursor solution.
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Figure 2. UV-vis absorption spectra of silver nanoparticles prepared using 2 g of (A) 25% w/w benzene solution of S6 and 15 µL of silver precursor, (B) 50% w/w benzene solution of S6 and 30 µL of silver precursor, (C) 25% w/w benzene solution of S3 and 15 µL of silver precursor, and (D) 50% w/w benzene solution of S3 and 30 µL of silver precursor. All samples were dispersed in acetone after the synthesis.
The size of the Ag nanoparticles is limited by the number of [Ag(NH3)2]+ ions in water solution microdrops stabilized by the polymer (polyesters) in benzene. The low concentration of the water phase in our system allows it to reach a high stability of microdrops as well as silver nanoparticles in dispersion. Therefore, the reduction reaction proceeds only in isolated microdrops and silver nanoparticles are strongly separated. The higher concentration of silver obtained under dark conditions may be explained in the following way. As we have shown before,32,33 as soon as [Ag(NH3)2]+ ions are added into the polyester solution, they interact with poly(oxyethylene) groups. This interaction disrupts the bonds of Ag-N and leads to the formation of Ag2O in the presence of hydroxide ions.43 Upon illumination with ambient light in the presence of low oxidizing organic substances, silver ions reduced to produce silver atoms on the surface of Ag2O.27 Thus, we believe that two mechanisms of silver ion reduction are present in samples under ambient light. They are (i) reduction due to the presence of amphiphilic polyesters and (ii) reduction due to daylight illumination. However, only nanoparticles formed at polyester macromolecules remain stable in the nonpolar solution. In turn, the possible reduction upon illumination reduces the total concentration of well-dispersed silver nanoparticles. However, this finding is interesting and shows that the hydrophilic-lipophilic balance of polymer macromolecules is an important parameter for the transport of a precursor to the reducing fragments, formation of nanoparticles, and stabilization. As we mentioned before, the reduction in concentrated polyester solutions starts immediately after the addition of the (43) Huang, Z. Y.; Mills, G.; Hajek, B. J. Phys. Chem. 1993, 97, 11542.
precursor to the reaction solution. To examine the effect of polyester concentration and length of poly(ethylene oxide) fragments, for some selected syntheses, we recorded optical spectra after 5, 15, and 30 min and 2 h of the reaction. For UV-vis measurements, the developed silver colloidal solutions were dispersed (1:100) in benzene and acetone. As expected, silver colloids formed a stable dispersion both in polar acetone and nonpolar benzene. One can see from the increase of absorbance intensity in Figure 2 that the reduction of [Ag(NH3)2]+ ions and formation of silver nanoparticles is facilitated by increasing polyester concentration and length of the hydrophilic poly(ethylene glycol) moieties. We assume that this is likely because of a higher silver ion reduction activity caused by longer PEG fragments. It appears that an increase of polyester concentration resulting in an increase of poly(ethylene oxide) fragments concentration enhances silver nanoparticle formation. To check whether the macromolecules undergo chemical changes during the reduction reaction, we recorded FTIR spectra before and after the selected syntheses. Polyester spectra for S6 and S3 were virtually identical (Figure 3). However, spectral changes were found in our model experiment when we examined the benzene solution of PEG-300 before and after the addition of various amounts of a silver precursor (Figure 4). The samples were prepared in a manner similar to that for the conventional synthesis in concentrated polyester solutions. We observed a new band after the silver precursor was added to the solution of PEG-300. Remarkably, the intensity of the broad band at 1737 cm-1 indicating the appearance of CdO groups in carbonyl compounds significantly increases with an increasing amount of
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Figure 5. 1H NMR spectra of the polyester S6 (A) before the reduction reaction and (B) after the synthesis of silver nanoparticles. Figure 3. FTIR spectra of polyesters (A) S6 and (B) S3 (1 ) before the synthesis of silver nanoparticles, 2 ) after the synthesis).
Figure 6. X-ray diffraction patterns for polyester D3 after the synthesis of silver nanoparticles. Figure 4. FTIR spectra of 40% PEG-300 benzene solution (1) before the precursor addition, 2 ) with 15 µL of precursor, 3 ) with 35 µL of precursor, 4 ) with 75 µL of precursor added to 2 g of PEG-300).
added silver precursor (Figure 4). In our opinion, the reason why we may not be able to identify similar spectral changes for the synthesis in polyester solutions is that the polyester itself displays the presence of carbonyl groups as a broken intensive absorption band at 1730 cm-1 (Figure 1 in the Supporting Information). To confirm whether the reduction proceeds without polymer chain length changes, we determined the molecular weight of polyesters S6 and S3 after the synthesis of silver nanoparticles. No essential changes in the length of polyester macromolecules have been observed. The molecular weight of S6 (6400) decreases slightly after the synthesis (6200), and the molecular weight of S3 (6700) does not change at all.
Figure 5 shows the 1H NMR spectra recorded for polyester S6 before and after the synthesis of silver nanoparticles. We observed new peaks appearing from the polyester fragments interacting with silver atoms during the reduction reaction. We calculated from the signal intensity that about 10% of amphiphilic polyester fragments interact with silver in a final colloidal solution. These data show us that developed silver nanoparticles are indeed localized in polymeric fragments after the synthesis. Figure 6 demonstrates the X-ray diffractogram pattern recorded on the dried polymer-silver hybrid nanocomposite after the synthesis. In the XRD pattern of hybrid silver-polymer nanocomposites, peaks at 38.02°, 44.22°, 64.42°, and 77.37° were observed. They are attributed to the (111), (200), (220), and (311) planes of the face-centered cubic (fcc) silver, respectively.52,53 We investigated the polyester-silver hybrid sample via thermogravimetric measurements and observed higher thermal
Mechanism of SilVer Ion Reduction
stability of the hybrid composition in comparison to the polyester (data not shown). The concentration of silver synthesized in a concentrated solution of amphiphilic polyester is about 0.244% w/w. Chemical Mechanism for Silver Ion Reduction in Concentrated Solutions of Amphiphilic Polyesters. Based on the literature and observed experimental results, we propose a mechanism for silver ion reduction in concentrated solutions of amphiphilic polyesters. The idea about the mechanism is given from (i) specific properties of the silver precursor [Ag(NH3)2]OH aqueous solution and (ii) specific properties of polyester micellar structures which form cavities (pseudo-crown ether structures) that can bind metal ions. Let us first consider specific properties of the silver precursor [Ag(NH3)2]OH. This precursor consists of a complex ion [Ag(NH3)2]+ and an anion OH- determining the precursor’s strong basic properties. According to ref 44, the silver cation interacts with two ammonia molecules and forms a linear complex ion. Since the ammonia molecules are neutral, the charge of the ion does not change, but the size (volume) of the cation increases. It leads to a weakening of the [Ag(NH3)2]OH complex and simplifies the complex dissociation and formation of the OHanion.45 In the crown ether structures, the silver cation is coordinated by the crown ether’s oxygen atoms. The ammonia molecules are detached when the complex ion [Ag(NH3)2]+ is trapped in the crown ether cavity. At the same time, the redox potential of the silver cation increases (EAg+/Ag > E[Ag(NH3)2]+/Ag).46 As we assumed before, the polyesters form pseudo-crown ether structures (cavities), bonding metal ions in organic solvents. It has been shown that crown esters form complexes with inorganic salts serving the solubility of salts in nonpolar solvents.47 Thus, when a silver precursor complex undergoes dissociation, the silver cations are trapped (solubilized) in pseudo-crown ether polyester cavities. At the same time, “bare” (nonsolvated) OHanions are formed in the reaction medium. The formation of “bare” anions results in a significant increase of their basic properties, reactivity, and nucleophility.48 We believe that the main effect of the polyester pseudo-crown ether structures involves solubilization of a precursor aqueous solution in a low polar medium, formation of polymer-metal complexes, and the following induction of “bare” nonsolvated anions.48 The interaction between the methylene group of a pseudocrown ether cavity in polyester and Ag+ leads to proton separation, reduction of silver cations, and formation of poly(ethylene oxide) macroradicals.49 This process is called the one-electron reduction mechanism and is discussed in the literature.49-51 The reaction involves (i) interaction of metal cations with mobile hydrogen atoms of the polymeric chain, resulting in (44) Kukushkin, Y. N. Chemistry of Coordination Compounds; Visshaja shkola: Moscow, 1985; p 455. (45) Nekrasov, B. V. Textbook of General Chemistry; Mir: Moscow, 1969; p 488. (46) Ohno, I.; Haruyama, S. Bull. Jpn. Inst. Met. 1981, 20, 12, 979. (47) (a) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (b) Pedersen, C. J. Org. Synth. 1972, 52, 66. (48) Hiraoka, M. Crown Compounds. Their Characteristics and Applications. Elsevier: London, 1982; p 350. (49) Berlin, A. A.; Kislenko, V. N. Prog. Polym. Sci. 1992, 17, 765. (50) Kurlyankina, V. I.; Shadrin, V. N.; Kazbekov, E. N.; Molotkov, V. A.; Bukina, M. K. Zh. Obshch. Khim. 1974, 44, 1593. (51) Kurlyankina, V. I.; Shadrin, V. N.; Kazbekov, E. N.; Molotkov, V. A. Zh. Obshch. Khim. 1978, 48, 433. (52) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (53) Swami, A.; Kumar, A.; Selvakannan, P. R.; Mandal, S.; Parischa, R.; Sastry, M. Chem. Mater. 2003, 15, 17. (54) Van Krevelen, D. W. Properties of Polymers: Correlations with Chemical Structure; Elsevier: London, 1972; p 412.
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macroradical formation, (ii) proton generation, and (iii) reduction of metal cations. It has been shown that the hydroxyl or carbonyl groups are usually formed due to the one-electron oxidation of a polymeric chain. According to reaction 1, the silver cation (oxidation agent) interacts with a mobile hydrogen of the poly(ethylene oxide) fragment (reducing agent) in the presence of “bare” anions OH-. Thus, the reduced silver, poly(ethylene oxide) macroradical, and water are formed:
In the next step (reaction 2), the Ag+ cation receives an electron from the poly(ethylene oxide) macroradical and is reduced to silver. Therefore, the macroradical is transferred into a carbcation. It interacts with the anion OH- and forms poly(ethylene oxide) fragments containing hydroxyl groups.
In the next step (reaction 3), the silver cation interacts with the mobile hydrogen of the polarized carbon on the poly(ethylene oxide) fragments. The electron joins the silver cation, forming reduced silver and a new macroradical.
In the last reaction step (reaction 4), the hydroxyl group of the macroradical generates the proton H+. Two electrons of the oxygen atom serve for the formation of a double bond (carbonyl compound) and reduced silver.
The formation of macroradicals in a one-electron reduction mechanism has been confirmed by electron spin resonance (ESR) spectroscopy.50,51 In our experimental system, the one-electron reduction mechanism is possible due to the formation of pseudocrown ether polymeric cavities and their ability to localize silver cations. Simultaneously, the nonsolvated anions become strong base properties and significantly increase nucleophilic ability. This is consistent with a reduction occurring at pH > 8 in the presence of [Ag(NH3)2]OH as a precursor.
Conclusions A novel method has been developed to prepare silver nanoparticles from [Ag(NH3)2]OH by the reduction induced in nonpolar solutions of amphiphilic invertible polyesters based on poly(ethylene oxide) and aliphatic dicarboxylic acids. The main role of the amphiphilic polyester involves solubilization of a precursor aqueous solution in a low polar medium, formation of polymer-metal complexes (here, with silver cation), and the following induction of “bare” nonsolvated anions OH-. The effect of the poly(ethylene oxide) fragment length and polyester concentration are explored by examining several amphiphilic polyesters. Being dispersed in a nonpolar solvent, an aqueous solution of the silver precursor is transferred through the nonpolar phase into the hydrophilic polymeric fragments and reduced there. Our experimental finding indicates that macromolecules of a higher hydrophilic-lipophilic balance are more “attractive” for the aqueous precursor solution.
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A possible one-electron reduction mechanism is proposed to explain the formation and stabilization of silver nanoparticles. We proposed the reduction mechanism considering (i) specific properties of the silver precursor [Ag(NH3)2]OH aqueous solution and (ii) specific properties of concentrated polyester solutions which contain cavities (pseudo-crown ether structures) that can bind and reduce metal ions. Our experimental findings indicate that the formation of silver nanoparticles in the concentrated solutions of amphiphilic invertible polyester in a nonpolar solvent at room temperature is a fast and economical process. The synthesis results in a readyto-use colloidal solution with an enhanced silver nanoparticle concentration. Due to the presence of amphiphilic polymeric
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fragments around the synthesized nanoparticles, they form stable dispersions in both a polar and nonpolar medium. Acknowledgment. We thank Dr. Peter Schulz (Department of Chemical Reaction Engineering, University of ErlangenNuremberg) for performing the NMR analyses and Dr. Angel Ugrinov (Department of Chemistry, Biochemistry and Molecular Biology, North Dakota State University) for performing the WAXS measurements. Supporting Information Available: Confirmation of the chemical structure of the polyesters by FTIR spectroscopy and NMR spectroscopy (Figures 1 and 2, respectively). This material is available free of charge via the Internet at http://pubs.acs.org. LA801769V
