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Highly dispersible polymer-coated silver Nanoparticles
Surface & Coatings Technology 203 (2009) 2841–2844
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Short communication
Highly dispersible polymer-coated silver Nanoparticles Aleksey N. Vasiliev a, Eric A. Gulliver b, Johannes G. Khinast c, Richard E. Riman a,⁎ a b c
Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, 607 Taylor Rd., Piscataway, NJ 08854, United States Ferro Inc., Electronic Materials Division, 3900 South Clinton Ave., South Plainfield, NJ 07080, United States Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Rd., Piscataway, NJ 08854, United States
a r t i c l e
i n f o
Article history: Received 8 January 2009 Accepted in revised form 16 February 2009 Available online 26 February 2009 Keywords: Silver Nanoparticles Thiolation Polymer Coating Dispersibility
a b s t r a c t Highly dispersible silver nano-material was obtained by surface functionalization of silver nanoparticles with polylactic acid. The thiolation method was used for attaching carboxyl groups to the silver surface by treatment with 3-mercaptopropionic acid. The carboxyl groups were further activated by diisopropylcarbodiimide and reacted with polylactic acid. The functionalized nanoparticles contained 1.88 wt.% organic phase. While the as-received silver nanopowder exhibited poor colloidal stability in tetrahydrofuran, particle size analysis revealed that the functionalized material displayed high dispersibility in this solvent. © 2009 Published by Elsevier B.V.
1. Introduction Colloidal silver nanoparticle dispersions can be utilized in many applications such as, medicine (antibiotic materials), biochemistry (biosensors), electrochemistry (electrode materials), and optics (color filters) [1–5]. In addition, nanoscale silver particles have found applications in memory devices [6], cosmetics [7], and dental restorative materials [8]. However, large-scale production methods for silver particles and nano-particles are frequently based on aqueous processes [9]. These processes generate particles with hydrophilic surfaces that are difficult to disperse uniformly in organic media. Nanoparticles also have very large surface areas and hence, very high surface energies that drive spontaneous room temperature agglomeration and sintering via secondary recrystallization onto larger particles [10,11]. In order to overcome these problems, the surface of silver nanoparticles must be modified in order to make them more hydrophobic and to prevent agglomeration. The surface properties of such modified nanoparticles can be tailored to achieve a desired property by using different functional groups. These surface modified nanoparticles structures offer a number of applications such as conductive materials in gas sensors, reagents for DNA analysis, catalysts, etc. [12]. Such modifications can also improve the stability of the particles and their dispersibility in organic media. In order to improve the non-aqueous dispersibility of hydrophilic silver nanoparticles, their surface needs to be modified by an organic layer having a high affinity for organic media. It is known that formation of an organic self-assembled monolayer on metals can ⁎ Corresponding author. E-mail address: [email protected] (R.E. Riman). 0257-8972/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.02.019
significantly change their surface properties. These organic layers can improve the stability of nanoparticles by forming a physical or electrostatic barrier against agglomeration, change the reactivity of the surface metal atoms and form an electrically insulating film [13]. Surface modifiers can consist of chemical moieties and organic chains to increase hydrophilicity [14] or hydrophobicity [15] of the surface, as well as reactivity with other substances. One of the methods of metal-surface functionalization is thiolation by organic thiol compounds. In the case of noble metals, thiols readily form very stable metal-sulfur bonds with the surface atoms under mild conditions. The functionalization of silver surfaces (powders and films) by alkylthiols of various chain lengths was widely studied [16–19]. Other thiol compounds immobilized on silver are 3-mercaptopropionic acid [20], 2-mercaptoethanol, 2-aminoethanethiol, and 2-mercaptoethanesulfonic acid Na salt [21]. This method was also successfully applied for the modification of nanoparticles of noble metals. For example, gold nanoparticles were functionalized by ω-substituted alkanethiols, containing terminal Br, CN, vinyl and ferrocenyl groups [22] and NH2-groups [23]. Functionalization of silver nanoparticle surfaces significantly affects their characteristics. It was found that the optical properties of silver nanoparticles change after functionalization by alkanethiols. In particular, λmax of the localized surface plasmon resonance spectra linearly shifts to the red by 3 nm for every carbon atom in the alkane chain while spectral shifts as large as 40 nm are caused by only 60,000 alkanethiol molecules per nanoparticle [24]. Due to this property, surfacefunctionalized nanosilver may find applications in new chemosensor and biosensor technologies. Thiolation of nanosilver particles can also change their dispersibility [25]. Silver nanospheres functionalized by a quaternary ammonium salt form a stable colloidal solution in water. The strongly hydrophilic surface
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Fig. 1. TEM image of silver nanoparticles.
of the functionalized particles has a cationic nature and is highly positively charged while the counteranion (Br−) remains in bulk solution. In contrast to hydrophilic nanoparticles functionalized by charged molecules, functionalization by hydrophobic long-chain organic molecules should strongly increase the dispersibility of silver in nonionic organic solvents. Such dispersions of the thiolated nanoparticles are stabilized by the presence of an organic layer on the surface preventing interaction between neighboring nanoparticles leading to their agglomeration. Stable colloidal solutions of silver in organic solvents are very important in preparation of silver–polymer composites containing well-dispersed silver nanoparticles [26–28]. The objective of this work is development of a modification method for improving the dispersibility of silver nanoparticles in organic media by modifying their surfaces to make them for applications such as dispersions in silver-containing materials. Obtained results can be useful for preparation of silver-polymer composites. 2. Experimental methods Silver nanopowder of highly uniform particle size (d = 150 nm, SBET = 3.61 m2/g)(as reported by a manufacturer specification sheet) was obtained from Ferro Corporation (South Plainfield, NJ) (Fig. 1). The as-received product contained silver (85%) and a stabilizing agent in water. 3-Mercaptopropionic acid (MPA), 4-dimethylaminopyridine (DMAP), and diisopropylcarbodiimide (DIPC) were purchased from Sigma Aldrich (St. Louis, MO). Polylactic acid (PLA) was synthesized in accordance with a procedure described earlier [29]. Before functionalization, the as-received silver (10 g) was first washed with deionized water (5 × 100 mL) and cleaned with
“piranha” solution (mixture of H2SO4 and H2O2 3:1) at temperature 0–10 °C for 1 h (CAUTION: “piranha” solution may detonate on contact with organic material) [30]. Subsequently, the silver was washed by deionized water (5 × 100 mL), suspended in 100 mL of water, sonicated for 5 min, then a solution containing 0.05 g of MPA and 0.5 g of NaOH in 10 mL of water was slowly added during stirring. The reaction mixture was sonicated again for 5 min, stirred for 1 h, and the solid product was filtered, washed by deionized water and dried on air. The carboxylated silver (3 g) was suspended in 50 mL of methylene chloride (CH2Cl2), and a solution of 0.03 g of DIPC and 0.1 g of DMAP in 50 mL of CH2Cl2 was added. After stirring for 1 h at room temperature a solution of 0.5 g of PLA in 50 mL of CH2Cl2 was added. The resulting solution was stirred overnight at room temperature. Subsequently, the polymer-modified silver was filtered, washed by CH2Cl2, 10% acetic acid, and dried in air. The loading of organic substances was determined by elemental analysis on C, H, and S (provided by Robertson Microlit Laboratories, Inc., Madison, NJ). X-ray diffraction spectra (XRD) were obtained on a Kristalloflex D500 diffractometer (Siemens Analytical X-ray Instruments, Madison, WI) with Ni-filtered CuKα radiation over the 2θ range 10–80° with a step of 0.02°. Particle size and morphology were determined by transmission electron microscopy (TEM) using a JEOL 100 CX microscope (JEOL Co., Tokyo, Japan) at 80 kV beam current. Particle diameter and the particle size distribution were determined by dynamic light scattering (DLS) on ZetaPALS (Brookhaven Instrument Corp., Holtsville, NY). For the measurement we prepared a suspension of the silver in water or tetrahydrofuran (THF) with concentration about 1 mg/mL. Before measurement the suspension was sonicated for 5 min (W-385, Heat Systems-Ultrasonics, Inc., Farmingdale, NY). The degree of agglomeration in the suspension was estimated by calculating the average agglomeration number (AAN). AAN is the average number of primary particles contained in an agglomerate and was estimated using the expression as follows [31]:
AAN =
dDLS N
!3
analysis dImage N
ð1 − eÞ
where ε is the estimated fractional porosity of the agglomerate (~ 0.4); dDLS is the number average particle size estimated from the DLS N analysis particle size data, and dImage is the number average particle size N estimated from the image analysis of the TEM micrographs of silver particles. 3. Results and discussion During the reaction with silver, the MPA formed self-assembled monolayers on the surface of silver nanoparticles (Fig. 2). The amount of MPA used for the surface functionalization was calculated considering the surface area of silver nanoparticles and the amount of
Fig. 2. Functionalization of carboxyl-functionalized silver by PLA.
A.N. Vasiliev et al. / Surface & Coatings Technology 203 (2009) 2841–2844
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Scheme 1. Activation of carboxyl groups and their reaction with PLA.
thiol necessary for the formation of a monolayer. For this calculation, we used literature data for the surface density of covalently-bound thiols on silver. The density of short-chain alkanethiols on a silver surface was reported as 5.93 molecules/nm2 with a 4.41 Å nearest neighbor spacing [32]. For thiols with longer chains, the spacing is larger due to alkyl chain packing constraint, and typically exceeds 4.6 Å [33]. Corresponding data on MPA density were not found in the literature. We arbitrary chose the amount of MPA providing a little lower average surface density (about 5.4 molecules/nm2) to ensure the formation of a monolayer. The calculation of this amount was made considering surface area of silver and based on complete immobilization of the full amount of MPA used during the synthesis. It should be noted that use of excess MPA results in formation of a multilayer with very strong intermolecular hydrogen bonds, which make the removal of physically adsorbed acid very difficult. Elemental analysis of obtained functionalized material suggests 0.18 wt.% of organic phase is present. The resulting surface density of immobilized MPA was 3.1 molecules/nm2. Thus, about 60% of MPA reacted with the silver surface. In the next step the surface carboxyl groups were activated by DIPC (Scheme 1). Different substituted carbodiimides are well known by their application in condensation reactions for organic synthesis [34]. In particular, they readily react with carboxyl groups in mild conditions producing correspondent N-acylureas. Recently, this reaction was reported for the activation of carboxyl groups on silicon and carbon surfaces for further immobilization of organic molecules [35,36]. In most cases this method is used for the formation of amide bond by reaction with amines. The hydroxyl group is less reactive to the activated carboxylic group. However, earlier it was found that addition of DMAP as a catalyst significantly increases the rate of the reaction [37]. In our work, we chose DIPC instead of more commonly used dicyclohexylcarbodiimide, because the by-product of the reaction, diisopropylurea, is soluble in CH2Cl2 and can be removed from the functionalized nanosilver by filtration and washing. We attached PLA through reaction between its end hydroxyl groups and immobilized N, N′-diisopropyl-N-propionylurea followed by formation of an ester. Analysis of the product showed that it contained 1.88 wt.% of organic phase. Considering that the average molecular weight of PLA is 15,400 g/mol, we can estimate that about 1.5% of the surface carboxyl groups were converted to immobilized polymer chains. We explain
this unexpectedly low conversion by steric hindrance of the surface reaction sites by large PLA molecules. The phase stability of the silver during surface modification was confirmed by XRD. The diffractograms of parent and modified silver contained characteristic patterns corresponding to the diffraction of f.c.c. silver [38]. Thus, the surface functionalization did not affect silver structure. Image analysis gave the number average particle size of 163 nm with polydispersity of 0.02. The agglomeration behavior of bare and modified silver nanoparticles in water and THF was determined using DLS studies. The effective agglomerate/particle size, mass-weighted average diameter, for the silver dispersions in water and THF are listed in Table 1. The data clearly indicates that bare silver possessing hydrophilic surface properties has relatively good dispersibility in water, but displayed a strong tendency to agglomeration in organic media. The number average agglomerate size (dDLS N ) for the bare silver particles in THF was 428 nm, which was computed when converted from the mass-weighted diameter of 661 nm. The corresponding average agglomeration number (AAN) was found to be ~11 particles. MPA-containing silver nanoparticles had very low dispersibility in both water and THF. As it was shown in earlier reports, surface COOH groups on silver surface do not dissociate in neutral media [39,40]. These groups form intermolecular hydrogen bonds responsible for agglomeration (they act as “glue” to connect the particles). In contrast, PLA-modified silver is hydrophobic and agglomerates in aqueous media but is well dispersible in THF. Its mass-weighted average particle size measured by DLS was 159 nm with a very low polydispersity (0.01) corresponding to the primary particle size observed from TEM data (163 nm; polydispersity = 0.02). Thus, it is evident that functionalized nanoparticles do not agglomerate in THF. In Fig. 3, dispersions of bare and PLA-functionalized silver are shown 1 h after being dispersed. The bare silver completely sedimented while PLA-functionalized silver remained as a stable colloidal suspension.
Table 1 Particle size and polydispersity of bare and modified silver nanoparticles in water and THF obtained using DLS methods. Solvent Bare silver
Water THF
Functionalized silver
Particle massweighted diameter, nm
Polydispersity Particle massweighted diameter, nm
Polydispersity
189 661
0.20 0.16
0.23 0.01
344 159
Fig. 3. Dispersion of functionalized (left) and bare (right) silver in THF after 1 h.
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Also we tried to evaluate the dispersibility of functionalized silver nanoparticles in chloroform (CHCl3). However, any attempts to obtain their suspension in this solvent were unsuccessful due to rapid agglomeration and sedimentation of silver. Detailed studies of PLA behavior in CHCl3 were not conducted before, however, formation of large bulky aggregates of the polymer in CHCl3 has been previously reported for poly(lactic-co-glycolic acid) [41]. We believe that poor solvation of PLA in CHCl3 may be responsible for silver agglomeration in this solvent. In summary, PLA was covalently attached to silver nanoparticle surfaces. The bare commercial silver had a hydrophilic surface and its particles agglomerated in organic solvents. After functionalization, its surface became hydrophobic and was easily dispersed in tetrahydrofuran. The solvent choice significantly affects the dispersibility of these functionalized silver nanoparticles. Acknowledgement The authors would like to acknowledge the support of Johnson & Johnson for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Kobayashi, Kagaku Kogyo 52 (2001) 209. Y.-W. Yang, J.-L. Liu, Gongye Cuihua 11 (2003) 7. M. Tomonari, Kogyo Zairyo 53 (2005) 42. S. Dong, C. Tang, Guijinshu 26 (2005) 53. A.J.G. Zarbin, M.M. Oliveira, M.C. Schnitzler, E.G. Castro, G.G. do Couto, W.G. Menezes, A. Tiboni, E. Nossol, C. Almeida Filho, Metals Mater. Processes 17 (2005) 249. Y. Yang, J. Ouyang, Pat. WO 2006001923. J.W. Kim, J.S. Lee, S.J. Kim, Y.J. Kim, J. Kim, S.H. Han, I.S. Chang, Pat. WO 2005077329. W.D. Yang, Pat. KR 2006109775. Z.S. Pillai, P.V. Kamat, J. Phys. Chem. B. 108 (2004) 945. B. Guenther, A. Kumpmann, H.D. Kunze, Scr. Met. Mat. 27 (1992) 833. J. Li, Y. Lin, B. Zhao, J. Nanoparticle Res. 4 (2002) 345.
[12] M. Brust, C.J. Kiely, Coll. Surf. A: Phys.-Chem. Eng. Asp. 202 (2002) 175. [13] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. [14] A.C. Templeton, S. Chen, S.M. Gross, R.W. Murray, Langmuir 15 (1999) 66. [15] Y. Xia, X.-M. Zhao, G.M. Whitesides, Microelectron. Eng. 32 (1996) 255. [16] N.J. Brewer, T.T. Foster, G.J. Leggett, M.R. Alexander, E. McAlpine, J. Phys. Chem.: B 108 (2004) 4723. [17] W. Li, J.A. Virtanen, R.M. Penner, J. Phys. Chem. 98 (1994) 11751. [18] C.A. Bauer, F. Stellacci, J.W. Perry, Top. Catal. 47 (2008) 32. [19] A. Ulman, Chem. Rev. 96 (1996) 1533. [20] A. Kudelski, A. Michota, J. Bokowska, J. Raman Spectrosc. 36 (2005) 709. [21] J.Y. Gui, D.A. Stern, D.G. Frank, F. Lu, D.C. Zapien, A.T. Hubbard, Langmuir 7 (1991) 955. [22] M.J. Hostetler, S.J. Green, J.J. Stokes, R.W. Murray, J. Am. Chem. Soc. 118 (1996) 4212. [23] D.V. Left, L. Brandt, J.R. Heath, Langmuir 12 (1996) 4723. [24] M.D. Malinsky, K.L. Kelly, G.C. Schatz, R.P. Van Duyne, J. Am. Chem. Soc. 123 (2001) 1471. [25] T. Yonezawa, S. Onoue, N. Kimizuka, Langmuir 16 (2000) 5218. [26] J.-W. Kim, J.-E. Lee, S.-J. Kim, J.-S. Lee, J.-H. Ryu, J. Kim, S.-H. Han, I.-S. Chang, K.-D. Suh, Polymer 45 (2004) 4741. [27] G.-B. Choi, J.-H. Lee, WO/2005/085339, Publ. 15.09.2005. [28] A. Gedanken, N. Perkas, S. Dubinsky, S. Gazit, R. Rotem, WO/2007/032001, Publ. 22.03.2007. [29] F. Akutsu, M. Inoki, H. Uei, M. Sueyoshi, Y. Kasashima, K. Naruchi, Y. Yamaguchi, Y. Sunahara, Polym. J. 30 (1998) 421. [30] C.S. Weisbecker, M.V. Merritt, G.M. Whitesides, Langmuir 12 (1996) 3763. [31] V.A. Hackley, Guide to the Nomenclature of Particle Dispersion Technology for Ceramic Systems, NIST Special Publication 945, US Government Printing Office, Washington, 2000. [32] M. Kawasaki, H. Nagayama, Surf. Sci. 549 (2004) 237. [33] P. Fenter, P. Eisenberger, J. Li, N. Camillone III, S. Bernasek, G. Scoles, T.A. Ramanarayanan, K.S. Liang, Langmuir 7 (1991) 2013. [34] F. Kurzer, K. Douraghi-Zadeh, Chem. Rev. 67 (1967) 107. [35] M. Ligaj, J. Jasnowska, W.G. Musial, M. Filipiak, Electrochim. Acta 51 (2006) 5193. [36] R. Voicu, R. Boukherroub, V. Bartzoka, T. Ward, J.T.C. Wojtyk, D.D.M. Wayner, Langmuir 20 (2004) 11713. [37] B. Neises, W. Steglich, Angew. Chem. 90 (1978) 556. [38] JCPDS Silver, File 04-0783 [39] A. Kudelski, Surf. Sci. 502–503 (2002) 219. [40] J.L. Castro, M.R. Lopez-Ramirez, S.P. Centeno, J.C. Otero, Biopolymers 74 (2004) 141. [41] E.A. Sander, A.M. Alb, E.A. Nauman, W.F. Reed, K.C. Dee, J. Biomed. Mater. Res., Part A 70A (2004) 506.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Short communication
Highly dispersible polymer-coated silver Nanoparticles Aleksey N. Vasiliev a, Eric A. Gulliver b, Johannes G. Khinast c, Richard E. Riman a,⁎ a b c
Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, 607 Taylor Rd., Piscataway, NJ 08854, United States Ferro Inc., Electronic Materials Division, 3900 South Clinton Ave., South Plainfield, NJ 07080, United States Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Rd., Piscataway, NJ 08854, United States
a r t i c l e
i n f o
Article history: Received 8 January 2009 Accepted in revised form 16 February 2009 Available online 26 February 2009 Keywords: Silver Nanoparticles Thiolation Polymer Coating Dispersibility
a b s t r a c t Highly dispersible silver nano-material was obtained by surface functionalization of silver nanoparticles with polylactic acid. The thiolation method was used for attaching carboxyl groups to the silver surface by treatment with 3-mercaptopropionic acid. The carboxyl groups were further activated by diisopropylcarbodiimide and reacted with polylactic acid. The functionalized nanoparticles contained 1.88 wt.% organic phase. While the as-received silver nanopowder exhibited poor colloidal stability in tetrahydrofuran, particle size analysis revealed that the functionalized material displayed high dispersibility in this solvent. © 2009 Published by Elsevier B.V.
1. Introduction Colloidal silver nanoparticle dispersions can be utilized in many applications such as, medicine (antibiotic materials), biochemistry (biosensors), electrochemistry (electrode materials), and optics (color filters) [1–5]. In addition, nanoscale silver particles have found applications in memory devices [6], cosmetics [7], and dental restorative materials [8]. However, large-scale production methods for silver particles and nano-particles are frequently based on aqueous processes [9]. These processes generate particles with hydrophilic surfaces that are difficult to disperse uniformly in organic media. Nanoparticles also have very large surface areas and hence, very high surface energies that drive spontaneous room temperature agglomeration and sintering via secondary recrystallization onto larger particles [10,11]. In order to overcome these problems, the surface of silver nanoparticles must be modified in order to make them more hydrophobic and to prevent agglomeration. The surface properties of such modified nanoparticles can be tailored to achieve a desired property by using different functional groups. These surface modified nanoparticles structures offer a number of applications such as conductive materials in gas sensors, reagents for DNA analysis, catalysts, etc. [12]. Such modifications can also improve the stability of the particles and their dispersibility in organic media. In order to improve the non-aqueous dispersibility of hydrophilic silver nanoparticles, their surface needs to be modified by an organic layer having a high affinity for organic media. It is known that formation of an organic self-assembled monolayer on metals can ⁎ Corresponding author. E-mail address: [email protected] (R.E. Riman). 0257-8972/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2009.02.019
significantly change their surface properties. These organic layers can improve the stability of nanoparticles by forming a physical or electrostatic barrier against agglomeration, change the reactivity of the surface metal atoms and form an electrically insulating film [13]. Surface modifiers can consist of chemical moieties and organic chains to increase hydrophilicity [14] or hydrophobicity [15] of the surface, as well as reactivity with other substances. One of the methods of metal-surface functionalization is thiolation by organic thiol compounds. In the case of noble metals, thiols readily form very stable metal-sulfur bonds with the surface atoms under mild conditions. The functionalization of silver surfaces (powders and films) by alkylthiols of various chain lengths was widely studied [16–19]. Other thiol compounds immobilized on silver are 3-mercaptopropionic acid [20], 2-mercaptoethanol, 2-aminoethanethiol, and 2-mercaptoethanesulfonic acid Na salt [21]. This method was also successfully applied for the modification of nanoparticles of noble metals. For example, gold nanoparticles were functionalized by ω-substituted alkanethiols, containing terminal Br, CN, vinyl and ferrocenyl groups [22] and NH2-groups [23]. Functionalization of silver nanoparticle surfaces significantly affects their characteristics. It was found that the optical properties of silver nanoparticles change after functionalization by alkanethiols. In particular, λmax of the localized surface plasmon resonance spectra linearly shifts to the red by 3 nm for every carbon atom in the alkane chain while spectral shifts as large as 40 nm are caused by only 60,000 alkanethiol molecules per nanoparticle [24]. Due to this property, surfacefunctionalized nanosilver may find applications in new chemosensor and biosensor technologies. Thiolation of nanosilver particles can also change their dispersibility [25]. Silver nanospheres functionalized by a quaternary ammonium salt form a stable colloidal solution in water. The strongly hydrophilic surface
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Fig. 1. TEM image of silver nanoparticles.
of the functionalized particles has a cationic nature and is highly positively charged while the counteranion (Br−) remains in bulk solution. In contrast to hydrophilic nanoparticles functionalized by charged molecules, functionalization by hydrophobic long-chain organic molecules should strongly increase the dispersibility of silver in nonionic organic solvents. Such dispersions of the thiolated nanoparticles are stabilized by the presence of an organic layer on the surface preventing interaction between neighboring nanoparticles leading to their agglomeration. Stable colloidal solutions of silver in organic solvents are very important in preparation of silver–polymer composites containing well-dispersed silver nanoparticles [26–28]. The objective of this work is development of a modification method for improving the dispersibility of silver nanoparticles in organic media by modifying their surfaces to make them for applications such as dispersions in silver-containing materials. Obtained results can be useful for preparation of silver-polymer composites. 2. Experimental methods Silver nanopowder of highly uniform particle size (d = 150 nm, SBET = 3.61 m2/g)(as reported by a manufacturer specification sheet) was obtained from Ferro Corporation (South Plainfield, NJ) (Fig. 1). The as-received product contained silver (85%) and a stabilizing agent in water. 3-Mercaptopropionic acid (MPA), 4-dimethylaminopyridine (DMAP), and diisopropylcarbodiimide (DIPC) were purchased from Sigma Aldrich (St. Louis, MO). Polylactic acid (PLA) was synthesized in accordance with a procedure described earlier [29]. Before functionalization, the as-received silver (10 g) was first washed with deionized water (5 × 100 mL) and cleaned with
“piranha” solution (mixture of H2SO4 and H2O2 3:1) at temperature 0–10 °C for 1 h (CAUTION: “piranha” solution may detonate on contact with organic material) [30]. Subsequently, the silver was washed by deionized water (5 × 100 mL), suspended in 100 mL of water, sonicated for 5 min, then a solution containing 0.05 g of MPA and 0.5 g of NaOH in 10 mL of water was slowly added during stirring. The reaction mixture was sonicated again for 5 min, stirred for 1 h, and the solid product was filtered, washed by deionized water and dried on air. The carboxylated silver (3 g) was suspended in 50 mL of methylene chloride (CH2Cl2), and a solution of 0.03 g of DIPC and 0.1 g of DMAP in 50 mL of CH2Cl2 was added. After stirring for 1 h at room temperature a solution of 0.5 g of PLA in 50 mL of CH2Cl2 was added. The resulting solution was stirred overnight at room temperature. Subsequently, the polymer-modified silver was filtered, washed by CH2Cl2, 10% acetic acid, and dried in air. The loading of organic substances was determined by elemental analysis on C, H, and S (provided by Robertson Microlit Laboratories, Inc., Madison, NJ). X-ray diffraction spectra (XRD) were obtained on a Kristalloflex D500 diffractometer (Siemens Analytical X-ray Instruments, Madison, WI) with Ni-filtered CuKα radiation over the 2θ range 10–80° with a step of 0.02°. Particle size and morphology were determined by transmission electron microscopy (TEM) using a JEOL 100 CX microscope (JEOL Co., Tokyo, Japan) at 80 kV beam current. Particle diameter and the particle size distribution were determined by dynamic light scattering (DLS) on ZetaPALS (Brookhaven Instrument Corp., Holtsville, NY). For the measurement we prepared a suspension of the silver in water or tetrahydrofuran (THF) with concentration about 1 mg/mL. Before measurement the suspension was sonicated for 5 min (W-385, Heat Systems-Ultrasonics, Inc., Farmingdale, NY). The degree of agglomeration in the suspension was estimated by calculating the average agglomeration number (AAN). AAN is the average number of primary particles contained in an agglomerate and was estimated using the expression as follows [31]:
AAN =
dDLS N
!3
analysis dImage N
ð1 − eÞ
where ε is the estimated fractional porosity of the agglomerate (~ 0.4); dDLS is the number average particle size estimated from the DLS N analysis particle size data, and dImage is the number average particle size N estimated from the image analysis of the TEM micrographs of silver particles. 3. Results and discussion During the reaction with silver, the MPA formed self-assembled monolayers on the surface of silver nanoparticles (Fig. 2). The amount of MPA used for the surface functionalization was calculated considering the surface area of silver nanoparticles and the amount of
Fig. 2. Functionalization of carboxyl-functionalized silver by PLA.
A.N. Vasiliev et al. / Surface & Coatings Technology 203 (2009) 2841–2844
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Scheme 1. Activation of carboxyl groups and their reaction with PLA.
thiol necessary for the formation of a monolayer. For this calculation, we used literature data for the surface density of covalently-bound thiols on silver. The density of short-chain alkanethiols on a silver surface was reported as 5.93 molecules/nm2 with a 4.41 Å nearest neighbor spacing [32]. For thiols with longer chains, the spacing is larger due to alkyl chain packing constraint, and typically exceeds 4.6 Å [33]. Corresponding data on MPA density were not found in the literature. We arbitrary chose the amount of MPA providing a little lower average surface density (about 5.4 molecules/nm2) to ensure the formation of a monolayer. The calculation of this amount was made considering surface area of silver and based on complete immobilization of the full amount of MPA used during the synthesis. It should be noted that use of excess MPA results in formation of a multilayer with very strong intermolecular hydrogen bonds, which make the removal of physically adsorbed acid very difficult. Elemental analysis of obtained functionalized material suggests 0.18 wt.% of organic phase is present. The resulting surface density of immobilized MPA was 3.1 molecules/nm2. Thus, about 60% of MPA reacted with the silver surface. In the next step the surface carboxyl groups were activated by DIPC (Scheme 1). Different substituted carbodiimides are well known by their application in condensation reactions for organic synthesis [34]. In particular, they readily react with carboxyl groups in mild conditions producing correspondent N-acylureas. Recently, this reaction was reported for the activation of carboxyl groups on silicon and carbon surfaces for further immobilization of organic molecules [35,36]. In most cases this method is used for the formation of amide bond by reaction with amines. The hydroxyl group is less reactive to the activated carboxylic group. However, earlier it was found that addition of DMAP as a catalyst significantly increases the rate of the reaction [37]. In our work, we chose DIPC instead of more commonly used dicyclohexylcarbodiimide, because the by-product of the reaction, diisopropylurea, is soluble in CH2Cl2 and can be removed from the functionalized nanosilver by filtration and washing. We attached PLA through reaction between its end hydroxyl groups and immobilized N, N′-diisopropyl-N-propionylurea followed by formation of an ester. Analysis of the product showed that it contained 1.88 wt.% of organic phase. Considering that the average molecular weight of PLA is 15,400 g/mol, we can estimate that about 1.5% of the surface carboxyl groups were converted to immobilized polymer chains. We explain
this unexpectedly low conversion by steric hindrance of the surface reaction sites by large PLA molecules. The phase stability of the silver during surface modification was confirmed by XRD. The diffractograms of parent and modified silver contained characteristic patterns corresponding to the diffraction of f.c.c. silver [38]. Thus, the surface functionalization did not affect silver structure. Image analysis gave the number average particle size of 163 nm with polydispersity of 0.02. The agglomeration behavior of bare and modified silver nanoparticles in water and THF was determined using DLS studies. The effective agglomerate/particle size, mass-weighted average diameter, for the silver dispersions in water and THF are listed in Table 1. The data clearly indicates that bare silver possessing hydrophilic surface properties has relatively good dispersibility in water, but displayed a strong tendency to agglomeration in organic media. The number average agglomerate size (dDLS N ) for the bare silver particles in THF was 428 nm, which was computed when converted from the mass-weighted diameter of 661 nm. The corresponding average agglomeration number (AAN) was found to be ~11 particles. MPA-containing silver nanoparticles had very low dispersibility in both water and THF. As it was shown in earlier reports, surface COOH groups on silver surface do not dissociate in neutral media [39,40]. These groups form intermolecular hydrogen bonds responsible for agglomeration (they act as “glue” to connect the particles). In contrast, PLA-modified silver is hydrophobic and agglomerates in aqueous media but is well dispersible in THF. Its mass-weighted average particle size measured by DLS was 159 nm with a very low polydispersity (0.01) corresponding to the primary particle size observed from TEM data (163 nm; polydispersity = 0.02). Thus, it is evident that functionalized nanoparticles do not agglomerate in THF. In Fig. 3, dispersions of bare and PLA-functionalized silver are shown 1 h after being dispersed. The bare silver completely sedimented while PLA-functionalized silver remained as a stable colloidal suspension.
Table 1 Particle size and polydispersity of bare and modified silver nanoparticles in water and THF obtained using DLS methods. Solvent Bare silver
Water THF
Functionalized silver
Particle massweighted diameter, nm
Polydispersity Particle massweighted diameter, nm
Polydispersity
189 661
0.20 0.16
0.23 0.01
344 159
Fig. 3. Dispersion of functionalized (left) and bare (right) silver in THF after 1 h.
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Also we tried to evaluate the dispersibility of functionalized silver nanoparticles in chloroform (CHCl3). However, any attempts to obtain their suspension in this solvent were unsuccessful due to rapid agglomeration and sedimentation of silver. Detailed studies of PLA behavior in CHCl3 were not conducted before, however, formation of large bulky aggregates of the polymer in CHCl3 has been previously reported for poly(lactic-co-glycolic acid) [41]. We believe that poor solvation of PLA in CHCl3 may be responsible for silver agglomeration in this solvent. In summary, PLA was covalently attached to silver nanoparticle surfaces. The bare commercial silver had a hydrophilic surface and its particles agglomerated in organic solvents. After functionalization, its surface became hydrophobic and was easily dispersed in tetrahydrofuran. The solvent choice significantly affects the dispersibility of these functionalized silver nanoparticles. Acknowledgement The authors would like to acknowledge the support of Johnson & Johnson for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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