Langmuir 1994,10, 4095-4102
4095
Polymerized Phospholipid Membrane Mediated Synthesis of Metal Nanoparticles Michael A. Markowitz, Gan-Moog Chow, and Alok Singh* Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio I Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C. 20375-5348 Received May 23, 1994. In Final Form: August 17, 1994@ The synthesis of Au and Co/Co(OH)znanoparticles has been accomplished with the use of polymerized vesicles composed of mixtures containing 10% or 50% (w/w) of a palladium ion bound negatively charged and a zwitterionic phosphospholipid, 1,2-bis(tricosa-l0,12-diynoyl)-sn-glycero-3-phosphohydro~ethanol, The divalent palladium ion bound to pholipid 1,2-bis(tricosa-l0,12-diynoyl)-sn-glycero-3-phosphocholine. the negatively charged phospholipids in the polymerized vesicle membrane initiated electroless metalization of gold or cobalt on the vesicle surface leading to the formation of metallic nanoparticles. The particles have been characterizedby transmission electron microscopy (TEM),high-resolution transmission electron microscopy (HRTEM),and electron diffraction. Polymerized vesicles containingcatalytic sites only on the external membrane surface were partially metalized with crystalline gold particles. By use of the internal volume of polymerized vesicles as the reaction vials, two types of unagglomerated nanoparticles (4-10 nm), Au and Co/Co(OH)z, have been synthesized. HRTEM studies have demonstrated that particle nucleation and growth occurred within the vesicles and that particle growth could be initiated by either single or multiple nucleation sites.
Introduction There is great interest in the development of novel strategies for the preparation of advanced materials suitable for applications such as catalysis, sensors, optics, ceramics, and metallurgy. Many of these strategies have been focused on the synthesis of nanoparticles. Nanoparticles, which range in size from 1to 100nm, have more surface atoms and thus larger surface energy than the same bulk material. Materials assembled from nanoparticles also have a significant amount of interfaces between the packed nanoparticles. As a result, materials composed of nanoparticles may have unique optical, dielectric, magnetic, mechanical, and transport prope r t i e ~ . ' - ~The nanoparticles should have a high degree of purity, a narrow size distribution, and chemical stability and should remain monodispersed during formation and processing into materials. The formation of well-dispersed suspensions of nanoparticles is a particularly difficult challenge since particles of this size irreversibly agglomerate in order to minimize their high interfacial energy. Conventional approaches for the synthesis of nanoscale particles include the vapor phase method, mechanical milling, and solution chemistry.l-' Both vapor phase synthesis and mechanical milling produce agglomerated nanoparticles. The solution chemistry approach has utilized surfactants to prevent agglomeration during nanoparticle synthesis by stabilizing the particles in solution.8-11 Colloidal stability can be achieved either by Abstract published inAduance ACSAbstracts, October 1,1994. (1)Hayashi, C. J. Vac.Sci. Technol. A 1987,5,1375. (2)Research Opportunities for Materials with Ultrafine Microstructures; National Academy Press: Washington, DC, 1989. (3)Ichinose, N.; Ozaki, Y.; Kashu, S.Superfine Particle Technology; Springer-Verlag: London, 1992. (4)Gleiter, H. Nanostruct. Mater. 1992,I, 1. (5)Chow, G.-M.; Markowitz, M. A.; Singh, A. J.Min. Met. Mat. 1993, 62. (6)Rao, C. N.R. Mater. Sci. Eng. 1993,B18, 1. (7)Chow, G.-M.; Gonsalves, K. In NanoStuctured Materials: Syn@
thesis, Properties and Uses; Edelstein, A. S., Cammarata, R. c. Eds.; IOP Publishing, Ltd.: England, in press. (8)van Wonterghem, J.; Morup, S.;Charles, S.;Wells, S.; Villadsen, J . Phys. Reu. Lett. 1986,55,410. (9)Bonnemann, H.;Brijoux, W.; Brinkman, R.; Jouben, T.; Koraff, B. Angew. Chem. Int. Ed. Engl. 1990,30, 1312.
synthesizing the particles in the presence of surfactants or by dispersing synthesized particles in surfactants. The choice of surfactant always depends on the type of materials to be synthesized or dispersed. One recent approach to making nanoparticles is to utilize the confined environment of vesicles or micelles as reaction cages.12-19 In addition, Ni, CdS, and ZnS have been deposited on the exterior surfaces of vesicles.20,21 As demonstrated in these reports, vesicles coated on the exterior surface tend to agglomerate while particles formed within vesicles remained dispersed and had a narrow size distribution. However, the issues of purity and yield and properties of the synthesized nanoparticles have not been extensively addressed. In addition, the stability of the membranes to the chemical and physical stress that occurs during the synthesis and the processing of the nanoparticles into parts for applications needs to be thoroughly investigated. In addition to potential stability problems, a drawback of using nonpolymerized vesicles as reaction cages is that only anions can preferentially diffuse through the membrane thus limiting the type of reaction chemistry that can be done. For example, unpolymerized vesicle membranes comprised of phosphatidylcholines and mixtures (10)Matijevic, E. Annu. Rev. Mater. Sci. 1986,15, 483. (11)Rosen, M. J. InSurfuctants and Interfacial Technologies;Rosen, M. J., Ed.; Marcel Dekker: New York, 1987;pp 251-270. (12)Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P . J.; Brus, L. E. J. Am. Chem. SOC.1990,112,1327. (13)Gobe, M.; Kon-No, K.; Kandori, K.; Kitahara, A. J. Colloid Interface. Sci. 1983,93,293. (14)Barnickel, P.;Wokaun, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992,148,80. (15)Mann, S.;Williams, R. J. P. J. Chem. SOC.,Dalton Trans. l98S, 311. (16)Mann, S.;Hannington, J. P.; Williams, R. J. P . Nature 1986, 324,565. (17)Bhandarkar, S.;Yaacob, I . ; Bose, A. J . Colloid. Interface. Sci. 1990,135,531. (18)Bhandarkar, S.;Yaacob,I.; Bose,A. Mater. Res. Soc. Symp. Proc.
. -180.637. -1- m- -, - - , - - .. (19)Liu, H.; Graf€, G. L.; Hyde, H.; Sarikaya,M.; Aksay, I . A. Mater. Res. SOC.Symp. Proc. 1991,218,115. (20)Ferrar, W. T.;O'Brien, D. F.; Warshawsky, A.; Voycheck, C. L. J . Am. Chem. SOC.1988,110,288. (21)Heywood, B. R.; Fendler, J. H.; Mann, S. J. Colloid. Interface. Sci. 1990,138,295.
This article not subject to U.S. Copyright. Published 1994 by the American Chemical Society
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4096 Langmuir, Vol. 10, No. 11, 1994
of phosphatidylcholines and phosphatidylglycerols have been reported to be relatively impermeable to Mn2+.22In addition, unpolymerized membranes comprised of anionic phospholipids have been demonstrated to be unstable to the asymmetric addition of high concentrations of bivalent ions.23Selectivetransport of cations across unpolymerized phospholipid membranes has been accomplished by utilizing cation carriers such as cryptands or by incorporating ionophores such as valinomycin in the membrane.24-29 Alternatively, cation permeation through vesicles might be enhanced by polymerization of the membrane since cations as well as anions may pass through the free volume in polymer matrices.30 Cation transport through channels formed in membranes as a result of polymerization would be nonselective and therefore a polymerized vesicle could be utilized for a variety of reactions involving different cations. In addition, the ability ofboth cations and anions to move through the channels may also help in maintaining a counterion balance necessary for efficient particle synthesis. Furthermore, the polymerized membranes of the vesicle would be made more stable than unpolymerized membranes to chemical (organic solvents), physical (thermal), and mechanical (lyophilization)stress thereby rendering them more amenable not only to the synthesis of nanoparticles but to subsequent processing steps as ~ e l l . Recently, ~ ~ 3 ~ ~ Markowitz et al. reported the electroless metal plating of tubules formed from a negatively charged diacetylenic phospholipid using surface bound palladium ions as the catalysts for m e t a l i ~ a t i o n .This ~ ~ approach is extended to the formation of nanoscale metallic particles. Furthermore, polymerized vesicles from mixtures of zwitterionic and palladium ion bound negatively charged polymerizable phospholipids could provide insights concerning the nucleation process at the membrane surface. The goal of this research is to explore the utility of the polymerized membrane approach for synthesizing a variety of nanoparticles. In order to accomplish this goal, vesicles were formed which had tailored surfaces which would enable control over the nature and density of nucleation sites. Mixtures of a negatively charged phospholipid, 1,2-bis(tricosa-l0,12-diynoyl)-sn-glycero-3-phosphohydroxyethanol (11,and a zwitterionic phospholipid, 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (2), were used to form the vesicles (Figure 1).In this paper, we report the synthesis ofAu and Co nanoparticles within the vesicles as well as the deposition of Au on the exterior vesicle surface.
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choline were synthesized accordingto literature pro~edures.3~-36 Pd(NH3)4C12 catalyst solution was prepared as previously de~cribed.3~ The hydrodynamic radius of the particles formed was determined by light scattering (Coulter Submicron Particle Analyzer, N4MD) and the extent of monomer reaction a f b r exposure to W radiation was determined by phosphate analysis of the thin layer chromatographed vesicles using established literature procedures.3' Typically, the W irradiated samples were applied to thin layer chromatography plates (Silica gel H, 250 um, Analtech) and the plates were developed in 65254 CHC13/CH30H/HzO(v/v/v)and visualized by iodine vapor. Silica gel under spots corresponding to polymer and monomer species were scraped from the plate and transferred to test tubes. Likewise, each sample was spotted on a TLC plate which was predeveloped with 65:25:4CHC13/CH30H/HzO (v/v/v) and the plate was exposedin 12. The silica gel under the spot was removed from the plate and put into a test tube. For both the chromatographed and nonchromatographed samples, an equal amount of silica gel was removed. Then, 200 ,uL of an ethanolic solution of Mg(N03)26HzO (10% w/w) was added and the samples were kept at 110 "C for 12 h. Each tube was then heated under a direct flame until the evolution of brown fumes ceased. After cooling, 500,uL of 0.1M HC1 was added to each test tube and the tubes were placed in a water bath (100 "C) for 15 min and then cooled to room temperature. Then, 1.7mL of a 1:6 mixture of 10% ascorbic acid in water (w/w) and 0.5% (NH&Mo,024 in 1.0 Materials and Methods N HzS04 (w/w)was added to each test tube. The test tubes were 1,2-Bis(tricosa-l0,12-diynoyl)-sn-glycero-3-phosphohydroxyethanol and l,2-bis~tricosa-lO,12-diynoyl~-sn-glycero-3-phospho- heated at 45 "C for 20 min and then cooled to room temperature. The absorbance was measured at 820 nm. Control experiments performed with a reagent blank showed no detectablephosphorus. (22) Michaelson, D. M.; Horwitz, A. F.; Klein, M. P. Biochemistry 1973,12, 2637. A residual amount of phosphorus was found in the unspotted (23)Papahadjopoulos, D.; Ohki,S. In Liquid Crystals and Ordered silica gel and this quantity was subtracted from the phosphorus Fluids: Johnson. J . F.. Porter. R. S.., Eds.:, Plenum Press: New York. content of each spot. The relative amount of polymer and 1970;p 13. monomer for each sample was determined by dividing the value (24)Ohki, S.Biochim. Biophys. Acta 1972,25, 57. for each chromatographed spot by the value for the corresponding (25) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988,27, 1. unchromatographed spot. (26)Kirch, M.; Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1975,14, 555. Samples, mounted on carbon- or formvar-coated copper grids, (27) Lehn, J.-M. In Physical Chemistry of Transmembrane Ion were examined by transmission electron microscopy (TEM)using Motions; Spach, G., Ed.; Elsevier: New York, 1983;p 181. a J'EOL JEMSOOCX microscope operated at 200 kV. High(28)Moore, C . ; Pressman, B. C. Biochem. Biophys. Res. Commun. resolution transmission electron microscopy (HRTEM) was 1964,15,562. performed by using a Hitachi 9000 UHR microscope operated at (29)Pressman, B. C.; Harris, E. J.; Jagger, W. S.; Johnson, J. H. Proc. Natl. Acad. Sei. U.S.A. l9f37,58, 1949. 300 kV. Crystallographic information was obtained by using (30)Goosey, M. T.In Polymer Permeability; Comyn, J.,Ed.; Elsevier ,
I
Applied Science Publishers: New York, 1985;pp 325-327. (31)Juliano, R. L.; Regen, S. L.; Singh, M.; Hsu, M. J.; Singh, A. Biotechnology 1983, 1,882. (32)Singh, A.;Schnur, J. M. In Phospholipid Handbook; Cevc, G., Ed.; Marcel Dekker: New York, 1993;pp 233-291. (33)Markowitz, M. A.;Baral, S.; Brandow, S.; Singh. A. Thin Solid Films 1993,224, 242.
(34)Singh, A. J. Lipid Res. 1990, 31, 1522. (35)Singh, A.;Schnur,J. M. Synth. Commun. 1988, 16, 847. (36)Singh, A.;Markowitz, M. A.; Tsao, Li-I. Synth. Commun. 1992, 22, 2293. (37)Regen, S. L.; Kirszenstejn, P.; Singh, A. Macromolecules, 1983, 16, 335.
Synthesis of Metal Nanoparticles .
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Mole Fraction 1 Mole Fraction 1 Figure 2. Surface pressure-area isotherms (20"C) of mixtures of 1 and 2 (a) in the absence of Pd(NH3)4C12(pH 5.51, and (b) in the presence of 1mM Pd(NH&C12 (pH 9.5).In both cases, mole fraction of 2 = 1(a),0.8 (b), 0.6 (c),0.4(d),0.2 (e),and 0 (0.Mean molecular area plots of mixtures of 1 and 2 (c) in the absence of Pd(NH3)&12 (surface pressure = 15mN/m), and (d) in the presence of Pd(NH&C12 (surface pressure = 24 mN/m). the selected area electron diffractiontechnique as well as HFtTEM lattice imaging.
Langmuir Film Studies The lipids were dissolved separately in CHCl3. Lipid mixtures were made from the two stock solutions. Monolayers of the pure lipids and of lipid mixtures were spread with a Hamilton syringe on subphases of water and 1 mM aqueous Pd(NH3)4C12(23 "C)and compressed 10 min after s reading. Monolayers were compressed a t a rate of 14 (!2/molecule)/min and were reproducible to within 1A2/ molecule. Compression data were obtained by using a film balance equipped with a thermostated trough and a computerized data acquisition system. The surface pressure was measured using a Wilhelmy plate sensing device (NIMA).
Vesicle Preparation A thin film of the lipid mixture was hydrated at 70 "C in water 30 min in the presence or absence of Pd(NH3)4Cl2. The total concentration oflipid in each sample ranged from 2 to 20 mg/mL. In those samples in which the palladium salt was present during vesicle formation, the concentration of Pd(NH&C12 was equimolar to 1. The mixture was vortexed and then sonicated (Branson sonifier, Model 450) a t 60 "C for 5 min. The dispersion
was then allowed to cool to room temperature and then polymerized. Polymerization of the vesicles was accomplished by exposure to W radiation (254 nm, Rayonet Photochemical Reactor) at 20 "C for 15 min. Then, for those vesicles which were to be externally metalized, an aliquot of aqueous Pd(NH&C12 solution was added to those samples prepared in the absence of the palladium salt. After 5 min, the dispersions were dialyzed against water to remove buffer salt and excess Pd(NH&C12. For vesicles in which metal particles were to be formed internally, an amount of EDTA (tetrasodium salt) equimolar to the amount of Pd(NH3I4C12present was added and then the vesicles were then immediately gel filtered.
Metal Particle Formation Metal particle deposition or formation of vesicles was accomplished with a Au or Co plating bath. The gold plating bath contained chloroauric acid and sodium hypophosphite as major components while the Co bath contained CoC12 and (CH3hN.BH3 as the major compon e n t ~ The . ~ vesicular ~ ~ ~ ~dispersions (2 mL) were diluted with an equal volume of the plating bath. In the case of external metal deposition, plating was allowed to continue for 30 min to 2 h. The dispersions were then dialyzed against water ( 5 L) for 3 h to remove excess plating bath. (38)Singh,A.; Markowitz, M. A.; Bard, S. US.Pat. Pend., 1994.
4098 Langmuir, Vol.10,No.11, 1994
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In the case of metal particle formation inside polymerized vesicles, plating was allowed to continue for 6 h.
Results The hydrodynamic radii of the vesicles were determined by light scattering studies after the vesicles prepared from mixtures of 1 and 2 had been exposed to UV radiation (254 nm). The diameters of vesicles containing 10%of 1 (w/w)in the absence (pH 5.5)and presence ofPd(NH3)&12 (pH 9.5) did not vary greatly. In both cases, bimodal populations of vesicles were observed. In the case of the vesicles formed in the absence of palladium ion, the populations had diameters of 34.1 f 15 and 101 f38 nm while the diameters of vesicles formed in the presence of palladium ion were 31 f 13 and 114 f40 nm. Increasing the fraction of 1 to 50% (w/w) also did not have much effect on the diameters of vesicles formed in the presence of the palladium ion. As with the other preparations, a bimodal population of vesicles was formed which had diameters of 37.5 f 13 and 109 f 41 nm. Phosphate analysisof the thin layer chromatographed (65254 CHC13/ CH30H/H20(v/v/v))vesicles after they were exposed to W radiation revealed differences in the amount of phospholipid monomer which had reacted for the different lipid mixtures. For vesicles containing 10%1 and formed in the absence of Pd(NH3)&12 (water, pH 5 . 9 , 67 f 5% of the lipid monomers reacted. In the presence of Pd(NH3)4C12 at pH 9.5, a smaller amount of phospholipid monomer reacted (33 f 6%). The amount of reacted monomer in vesicles formed in the presence of Pd(NH3)4C12 (pH 9.5) was higher (50 f4%)for vesicles containing 50% 1. The differences in the amount of reaction of monomer are probably due to both ion and pH effects. Langmuir film studies of the phospholipid mixtures in the presence and absence of Pd(NH3)&12 (1mM palladium salt, pH 9.5,20 "C) indicate that the differences in degree of monomer reaction for the different mixtures are the result of differences in the alignment of the acyl chains arising from the small differences in the packing of the phosphocholineand phosphohydroxyethanol headgroups between the mixtures (parts a and b of Figure 2). Mean molecular area plots of the two sets of mixtures demonstrated that the mixtures were miscible in all proportions. Small positive deviationsfrom ideal mixing behavior were observed for the monolayers of mixtures formed in the absence of Pd(NH3)&12 which may be due to slight repulsions between the negatively charged headgroup of 1 and the zwitterionic headgroup of 2 (Figure 2c). These repulsions may arise from steric interactions or differences in charge. Conversely, small negative deviations from ideal mixing behavior were observed for the monolayers of mixtures formed in the presence of Pd(NH3)4C12(Figure 2d). It is possible that the binding of the palladium ion with the negatively charged headgroup of 1 has decreased the adverse electronic interactions with the headgroup of 2 leading to a tight packing of the phospholipids in the mixtures. Transmission electron micrographs of partially polymerized vesicles containing 10% (w/w) of 1 which were metalized with gold are shown in Figure 3a. The vesicles were formed in the absence of Pd(NH3)&12 and stabilized by exposure to UV radiation. The palladium ion was added and excess ions which did not bind to the negatively charged phospholipidsin the vesicle surface were removed by dialysis against water. The electronmicrographs reveal partially metalized unagglomerated and agglomerated vesicles. Electron diffraction demonstrated that the deposited metal consisted of fine gold crystalline particles (Figure 3b). The amount of intact and isolated partially
vesicle containing 10% 1 with exterior surface partially metalized with gold. (b) Electron diffraction of the sample.
metalized vesicles observed was low probably due to agglomeration of the partially metalized vesicles. In order to form unagglomerated metal nanoparticles, the partially polymerized vesicles were used as reaction vials for the nanoparticle synthesis. Vesicles formed in the presence of Pd(NH3)&12 and containing 10% or 50% of 1 were stabilized by exposure to UV radiation. Palladium ions were removed from the external vesicle surface by addition of EDTA (tetrasodium salt) followed immediately by gel filtration. After addition of the plating bath, the resulting solution was allowed to stand for 6-24 h. No differences in the degree of vesicle metalization occurred over this time period. In Figure 4a, transmission electron micrographs of gold nanoparticles formed within the larger polymerized vesicles containing 10% of 1 are shown. The particle size ranged from 40 to 100nm. Figure 4b shows the electron diffraction rings arising from a mixture of gold and impurities which were probably due to unreacted precursors. Bright field imaging of particles
Synthesis of Metal Nanoparticles
Langmuir, Vol. 10,No. 11, 1994 4099
Figure 4. (a) Transmission electron micrograph of polymerized vesicles (mole fraction 1 = 0.1)in which gold particles have been deposited on the interior vesicle membrane surfaces. (b) Electron diffraction of the sample.
Figure 5. (a, b) Transmission electron micrographs of gold particles deposited on the interior membrane surfaces of polymerized vesicles (mole fraction 1 = 0.5). ( c ) Electron diffraction of the sample.
formed inside small diameter vesicles indicated that nanoscale particles were formed inside the membrane of partially polymerized vesicles containing 50% of 1 (w/w) (Figure 5a,b). The particles were found to be in the range of 4-15 nm. The intensity of the electron diffraction rings of the nanoscale particles inside the membrane was faint (Figure 5c). The diffraction ring was identified to be Au and unlike the vesicles comprised of 10% of 1,impurities were not observed. A HRTEM micrograph of the same sample showed the lattice fringes which correspond to Au(ll1) (Figure 6). It is noted that from HRTEM, nucleation of gold occurred both from single nucleation and multiple nucleation sites for different vesicles. In the former case, this resulted in the formation of single crystals. In addition, HRTEM revealed that particle nucleation and growth distorts the shape of the vesicle. Transmission electron micrographs of a mixture of cobalt and cobalt hydroxide particles produced inside
vesicles containing 50% of 1 (w/w) after bath sonication for 2.5 h (temperature increased from 23 "C to 50 "C during this time period)are shown in Figure 7a,b. The membrane was more visible in this case, probably due to the staining by cobalt mixtures. The deposited metal particle appears to have grown into the vesicle membrane as well as out from it. Selected area electron diffraction pattern from an area containing a high density of particles showed a polycrystalline pattern (Figure 7c). Analysis of the diffraction rings suggested the formation of a mixture of cobalt and cobalt hydroxides (Co(OH)2). Selected area electron diffraction performed on individual vesicles showed many vesicles contained single crystals. HRTEM lattice imaging on these crystals showed some vesicles with materials deposited on the inner surface of the membrane, while the center remained hollow (Figure 8a). HRTEM investigation also showed the disordered fringes surrounding the whole inner surface of the membrane,
4100 Langmuir, Vol.10,No. 11, 1994
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Figure 6. High-resolution transmission electron micrograph of gold particles deposited on interior membrane surfaces of polymerized vesicles (mole fraction 1 = 0.5).
which suggested the presence of multiple nucleation sites in some vesicles (Figure 8b).
Discussion A number of strategies exist for the stabilization of vesicle^.^^^^^ One of these strategies involves the covalent linking of lipids containingpolymerizable groups. Vesicle stabilization may be achieved by partial polymerization through the formation of small polymer domains in the membrane. As a result, channels of free volume are formed in the membrane through which cations are able to diffuse. Thus polymerized vesicles are able to be used as reaction vials for a wide range of reactions. The vesicle membrane surface can be tailored through headgroup modification of the constituent surfactants. By doing the reactions inside the polymerized vesicle, potentially new phases as well as unagglomerated particles could be formed. Also, particle size, distribution, and chemical homogeneity can potentially be controlled. Since the reactants diffuse slowly through the membrane, the progress and rate of the reaction can be controlled which may have an effect on the properties of the crystals that are formed. The results demonstrate that polymerized vesicles formed from mixtures of zwitterionic and negatively charged diacetylenic phospholipids can assist in the formation of unagglomerated nanoparticles by allowing the necessary reagents to diffuse through the mem(39) Ringsdorf, H.; Schlarb, B.; Venzmer,J. Angew. Chem., Int. Ed. Engl. 1988,27,113.
brane into the aqueous core and reacting with a catalyst which has been bound to the internal membrane surface. Vesicle membranes formed from mixtures of a palladium ion bound negatively charged lipid and a zwitterionic lipid were partially metalized while vesicles comprised solely ofthe zwitterionic lipid were not metalized. This indicates that the initial nucleation event occurs at membrane sites which have palladium ion bound to a negatively charged phospholipid headgroup. That more of the vesicle surface was metalized than can be accounted for by the amount of negatively charged phospholipid present in the vesicles indicates that the metal grows over the surface once nucleation has occurred. In addition, the Langmuir film studies demonstrated that 1 and 2 were miscible in all proportions and, therefore, the nucleation sites were evenly distributed throughout the vesicle membranes. By directing the metalization process to take place within the partially polymerized vesicles, unagglomerated gold and cobaltkobalt hydroxide particles were synthesized. As expected, bath sonication of the vesicles resulted in the formation of cobalt nanoparticles at a relatively fast rate due to agitation of the plating bath. A possible suggestionof the observed particle size distribution is that nucleation of these particles inside the membrane did not occur at the same time. HRTEM of vesicles containing the nanoparticles confirmed that particle nucleation and growth occurred within the vesicles and that both single and multiple nucleation events can occur within the vesicle. This is perhaps due to (1)possible inhomogeneity in the degree of polymerization, polymer size, and polymer
Langmuir, Vol. 10, No. 11, 1994 4101
Synthesis of Metal Nanoparticles
Figure 7. (a, b) Transmission electron micrographs of Co/Co(OH)2 particles formed inside polymerized vesicles (mole fraction 1 = 0.5). (c) Electron diffraction of the sample. '-a
h
. .
Figure 8. (a, b) High-resolution transmission electron micrographs of Co/Co(OH)2 particles deposited on interior membrane of polymerized vesicle (mole fraction 1 = 0.5).
distribution within the membranes of different vesicles, (2) variation of amount of reactants (ions and reducing agents) diffusing across the membrane, (3)variation of the coverage of catalytic Pd for reduction, i.e. upon reduction of Pd ions, the subsequent surfacecharacteristics of different vesicles were different. The distortion of the vesicle membranes observed by HRTEM may be due to
the release of energy arising from the nucleation and crystallization process dissipating through the vesicle membrane as heat. Since the Tm'sof the monomers of 1 and 2 are 50.7 and 43.0"C,respectively, this could result in the fluidization of the partially polymerized membranes. In addition to stabilizing vesicles, polymerization provides channels for diffusion through the membrane
4102 Langmuir, Vol. 10, No. 11, 1994 due to the formation of randomly produced polymer domains. Non-cross-linked polymerized vesicle membranes consist of many individual polymer chains and therefore contain breaks in the polymeric network which are the transport paths through the membrane.4O The density of these polymer "breaks" or "boundaries" depends on vesicle diameter and polymer s i ~ e . ~Light l - ~ scattering demonstrated that the sizes of the vesicles formed from each diacetylenicphospholipid mixture were quite similar. Also, a recent study has shown that the size of the linear polymers formed after exposure of vesicles comprised of diacetylenic phosphocholines to UV radiation under conditions similar to those used in these experiments is dependent on vesicle diameter.@ Based solely on the similarity in the hydrodynamic radii of the vesicles, it would therefore be reasonable to expect that the polymerized vesicles formed from each phospholipid mixture contain similar numbers of polymer breaks. However, phosphate analysis of the thin layer chromatographed vesicles which had been exposed to UV light under identical conditions demonstrated that not all of the phospholipid monomers reacted and that the extent of monomer reaction depended on ratio of lipids 1 and 2. The surface pressure-area curves for monolayers of the different mixtures demonstrate that packing of the phospholipids is dependent on ratio of the two phospholipids in the mixture and that the alignment of the diacetylenic groups differ for each mixture. Because of the steric requirements necessary for the efficient polymerization of diacetylenic phospholipids, even small differences in the alignment of the acyl chain diacetylenes would result in significant differences in the ability of the diacetylenes to react. These differences would have a significant impact on the extent of monomer reaction and on the size of the linear polymers formed and, conse(40) Stefely, J.; Markowitz, M. A.; Regen, S. L. J.Am. Chem. Soc. 1988,110, 7463. (41) Samuel, N. K. P.; Singh, M.; Yamaguchi, K.; Regen, S. L. J.Am. Chem. SOC.1986,107,42. (42) Dorn, K.; Patton, E. V.; Klingbiel, R. T.; O'Brien, D. F. Makromol. Chem., Rapid Commun. 1983,4, 513. (43) Bolikal, D.; Regen, S. L. Macromolecules 1984, 17, 1287.
(44)Peek, B. M.; Callahan, J. H.; Namboodiri, K; Singh, A.; Gaber, B. P. Macromolecules 1994,27, 292.
Markowitz et al. quently, on the number of polymer breaks present in the vesicle membrane.45 Because diacetylenes react topotacticly, the polymerization of diacetylenic phosphocholine vesicles produces a high number of short chain polymers. Taken together, these two facts suggest that the partially polymerized vesicles composed of mixtures of diacetylenic phospholipids 1 and 2 are stabilized by a number of short chain polymers resulting in a high number of polymer breaks within the membrane. In polymerized vesicles, these breaks are the main channels for transport through the membrane.40 Recently, the increased permeability of polymerized vesicles formed from mixtures of diacetylenic phosphocholines and short-chain saturated phosphocholines over that of the corresponding unpolymerized vesicles has been reported.46 The synthesis of metal nanoparticles within the partially polymerized vesicles using electroless metalization chemistry indicates that these breaks act as free volume through which cations as well as anions may pass and thus greatly increase the range of reactions which could be used to prepare nanoparticles.
Conclusion Unagglomerated metal nanoparticles were produced using the phospholipid membrane surfaces as templates. Metal particles were grown on both the external and internal surfaces of the vesicle membranes formed from mixtures of diacetylenic phospholipids with zwitterionic and palladium ion-bound negatively charged headgroups. The reactants, metal cations and reducing agent, were allowed to diffuse across the polymerized vesicle membrane and react to form the nanoparticles. Particles were formed as the result of both multiple and single nucleation events. Acknowledgment. We thank M. Twigg for access to NRL's Hitachi H-9000 UHR HRTEM microscope. We gratefully acknowledge the Office of Naval Research for funding this research through an NRL Core program on Synthetic Membranes. (45) Patel, G. N.; Chance, R. R.; Witt, J. D. J.Chem. Phys. 1979,70, 4387.
(46)Markowitz, M. A.; Chang, E. L.; Singh, A. InPolymeric Materials in Diagnostics and Biosensors; Usmani, A., Ed.; American Chemical Society: Washington, DC, 1994; pp 264-274.
