Copolymers of Maleic Acid and Their Amphiphilic ... - ACS Publications

Feb 24, 2009 - Nanoparticles. Nadezhda Samoilova, Elena Kurskaya, Maria Krayukhina,* Andrey Askadsky, and. Igor Yamskov. A.N. NesmeyanoV Institute of ...
0 downloads 0 Views 983KB Size
Download PDF
J. Phys. Chem. B 2009, 113, 3395–3403

3395

Copolymers of Maleic Acid and Their Amphiphilic Derivatives as Stabilizers of Silver Nanoparticles Nadezhda Samoilova, Elena Kurskaya, Maria Krayukhina,* Andrey Askadsky, and Igor Yamskov A.N. NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow 119991, Russian Federation ReceiVed: July 28, 2008; ReVised Manuscript ReceiVed: December 22, 2008

Silver nanoparticles were prepared by reduction of the corresponding metal salt with NaBH4 in the presence of appropriate dispersing agents, namely, copolymers of maleic acid: poly(N-vinyl-2-pyrrolidone-alt-maleic acid), poly(ethylene-alt-maleic acid), poly(styrene-alt-maleic acid) or their amphyphilic derivatives. A thorough study of the whole process of silver nanoparticles production including formation of polymeric silver salt and stabilization of nanoparticles has been carried out. The degree of cooperativity of copolymer silver ions binding process and binding capacity of copolymers with respect to silver ions was calculated. To estimate the effect of silver binding on dimension of copolymers particles in solutions viscometric method and atomic force microscopy were used. The most stable and highly silver-“loaded” colloids, identified with optical absorption spectroscopy and transmission electron microscopy, were prepared at biphilic copolymers concentrations lower than their critical aggregation concentrations and copolymer/Ag+ molar ratio was equal to unity. Under optimal reduction conditions, the final morphology of Ag nanoparticles is governed by structural similarity (strict alternation of monomer units) and similar polymer chains length of all copolymers; hydrophobic ligands only assist in stabilization of nanoparticles. A simple method for evaluating the thickness of polymer protective layer based on atomic force microscopy and transmission electron microscopy data is proposed. Introduction Recently, nanoparticles (NPs) have attracted considerable attention of researchers due to their unique optical, mechanical, chemical, biological, electronic, and catalytic properties; see, for example, books and reviews.1-6 In the year 2006, the number of publications concerned with nanoscience and nanotechnology reached almost a value of 38000 (Current Content data), and the amount of information increases every year. In most cases the polymer matrix serves as a suitable scaffold for immobilization of metal NPs preventing them from aggregation by reducing their surface energy. The stabilizing effect of macromolecules depends on the polymer structure. First, this is the availability of functional groups in macromolecules to interact with metal precursor and/or resulting metal colloid, for instance, via formation of a complex or ion-pair. Second, often hydrophobic ligands in the polymer (or structural elements of polymer chain) can assist in the generation of small, stable, colloidal particles. Anionic polymers capable of forming complexes with metal species and stabilizing metal NPs, thus preventing them from agglomeration are currently under extensive study. Among them, monocarboxilic acids polymers, namely, poly(acrylic acid) and poly(methacrylic acid), occupy a special place.7-9 On the other hand the stabilization of silver NPs in the presence of dicarboxylic acid (e.g., maleic acid, MA) copolymers has not been investigated yet so thoroughly. Some copolymers of MA or their derivatives have been used as protecting agents against agglomeration of nanocrystals.10-12 Akashi et al.10 reported passivation of Au nanoparticles by poly(vinyladenin-alt-MA). Transfers of hydrophobically capped CoPt3, Au, CdSe/ZnS, or * To whom correspondence [email protected].

should

be

addressed.

E-mail:

Fe2O3 nanocrystals from organic to aqueous solutions by wrapping an amphiphilic polymer based on poly(1-tetradecenealt-MA)11 and quite recently on poly(isobutylene-alt-MA)12 were described. The main advantages of MA copolymers are as follows: • commercial availability or, at least, possibility of synthesis according to known procedures • structural feature (strict alternation of comonomer units) • ready regulation of hydrophobic-hydrophilic balance inside the family of maleic acid copolymers varying comonomers of MA • simple preparation of amphiphilic polymer derivatives • solubility in water In the present study we have used the following copolymers of maleic acid (CMA): poly(N-vinyl-2-pyrrolidone-alt-maleic acid), poly(ethylene-alt-maleic acid), poly(styrene-alt-maleic acid) or their amphiphilic derivatives containing octadecylic residues. They have been tested as the stabilizing (capping) molecules for colloidal silver particles, prepared by reduction of polymeric silver salts with NaBH4. This report focuses on (a) study of the process of CMA-silver salt formation, (b) revelation of peculiarities of nanoparticles formation in the presence of dicarboxylic acid copolymers under reduction of Ag ions with NaBH4, and (c) study of behavior and properties of polymer particles in solution after CMA-silver salt reduction. Keeping in mind potential biological applications of silver NPs, simple and reproducible conditions for production of stable nanocomposites with high NPs capacity values have been developed. Resulting polymer silver salts and colloids were investigated using optical absorption spectroscopy, viscometry, atomic force microscopy (AFM), and transmission electron microscopy (TEM).

10.1021/jp806683m CCC: $40.75  2009 American Chemical Society Published on Web 02/24/2009

3396 J. Phys. Chem. B, Vol. 113, No. 11, 2009 CHART 1

Experimental Section Materials. Poly(ethylene-alt-MA) with an average molecular weight M ) 25000 (Chart 1, 1a), poly(styrene-alt-MA) M ) 50000 (Chart 1, 3a) were purchased, correspondingly, from Monsanto (USA) and Sterlitamak chemical plant (Russia). Poly(N-vinyl-2-pyrrolidone-alt-MA) M ) 40000 (Scheme 1, 2a) was prepared following a procedure described previously.13 The synthesis of amphiphilic polymer derivatives (Chart 1, 1b, 2b, 3b) starting from MA copolymers and octadecylamine under mild conditions (N,N-DMF/pyridine solution, 2 h, room temperature) was reported in our earlier publication.14 The product was precipitated by acetone or CHCl3, filtered off, dialyzed, and lyophilized. Other chemicals were of analytical grade and used without further purification. Methods. The critical concentration of aggregation of CMA and their amphiphilic derivatives was determined by method of plate balance (Vilgelmi method) and refractometry as described in our previous publication.14 The silver hydrosols were prepared by borohydride reduction of silver salt in the presence of CMA or their derivatives. AgNO3, NaBH4, and polymer-stabilizers were dissolved in deionized water separately. Aqueous solutions of CMA were prepared by weighting of the solid copolymers of MA (for preparation of 1a, 2a, 3a polymers, Chart 1) or amphiphilic polymer derivatives (1b, 2b, 3b; Chart 1), dissolving them in water at room temperature and adding of 1 M solution of NaOH for pH adjustment. In such a manner copolymers of maleic anhydride hydrolyzed to copolymers of maleic acid (1a, 2a, 3a; Chart 1). To prepare the polymer silver salts, both solutions were mixed with deionized water to make the desired concentrations. Then the solutions were vigorously stirred during and after (0.5 h) addition of 10-fold molar excess of aqueous NaBH4 at room temperature. Here, the molar concentration of CMA refers to the monomer (MA) units. UV-visible absorption spectra were obtained with UVIKON922 (BRD) spectrophotometer. The sample solution was diluted by deionized water and then put at quartz cell (1-cm optical path length) for measurement. The reduced viscosities of initial CMA, amphiphilic CMA, and the dispersed solid nanocomposite solutions were measured

Samoilova et al. using an Ubbelode capillary viscometer at 25 °C in glycineNaOH buffer solution (0.1 M, pH 9). To plot the silver-CMA binding isotherms (25 °C, pH 6.0-6.2 and 7.2-7.5; 0.05 M KNO3, CMA concentration 0.001-0.005 M, Ag+/CMA ratio 0.2-2.0) the dialysis method was used, and the free silver concentration was measured in external volume (CMA was in interior, membrane confined volume) by using the Folgard method or UV absorbance data at 230 nm. Atomic force microscopy (AFM) topographic images of surfaces of CMA films, which were built on the mica slides (drop of 0.0002 M CMA solution was evaporated), were obtained in tapping mode with a FemtoScan atomic force microscope (Advanced Technologies Center, Russia). Commercially available Si cantilevers (NSG 11 S) with a spring constant of 11.5 N/m were used. The images were taken at a typical scan rate of 0.5-1.0 Hz and processed using special FemtoScan online software. Transmission electron microscopy (TEM) micrographs were performed with LEO 912 AB (Omega, Karl Zeiss; BRD) microscope operated at an accelerating voltage of 100 kV. For TEM observations, a drop of colloid solution was placed onto Formvar-coated copper grids and then evaporated. The particle size distribution was obtained from a count of 100-300 individual particles. Results and Discussion The preparation of CMA-stabilized silver NPs obviously consists of two stages, namely, the formation of polymeric silver salt and subsequent reduction of the polymer salt with NaBH4. We have attempted to reveal the influence of the specific nature of dicarboxylic acid copolymers on the overall process of silver NPs formation and to optimize the production of stable and highly silver-“loaded” colloids. All CMA used have similar macromolecular chains length and biphilic nature (except 2a, Chart 1), that is, contain both hydrophilic and hydrophobic units and may form micelle-like aggregate in aqueous solutions; the critical concentrations of aggregation (CCA) of such polymers were determined in our preliminary publication14 (see Table 1). The solutions of CMA were obtained before of silver salt formation using polymer concentrations lower or nearly equal to CCA. “Weak” associates were formed in the solution in that case (CMA, Scheme 1). In the case of copolymer 2a, 2-pyrrolidone units as well as carboxyl groups of MA residues may participate in associate’s formation due to hydrogen bonds. The first step of the silver hydrosol creation is the formation of polymer-silver salt. At the conditions used (pH 6-9) carboxyl groups of MA units may donate protons so that an electrostatic attraction occurs between metal ions and acid residues. In this case an “unwrapping” of polymer chains took place and partially disintegrated associates were obtained (CMA-Ag+, Scheme 1) because of a weakening of the intraand intermolecular hydrogen bonding of macromolecular chains. The evidence for the proposed structure is provided by the increase of CCA of CMA in the presence of silver ions; for example, it rises from 0.015 to 0.020 M for copolymer 1a and from 0.011 to 0.019 M for copolymer 1b. This structure was also supported by viscosimetric and AFM data as well as by the values of dissociation constants of polyacids and CMA/ Ag+ salts (sees below). Binding capacity value of CMA with respect to silver ions and cooperativity of polymer-silver ions binding process were assessed on the ground of the connecting isotherm curve of

Copolymers of Maleic Acid as Stabilizers

J. Phys. Chem. B, Vol. 113, No. 11, 2009 3397

SCHEME 1

CMA and Ag+ binding. The Hill coefficient (n) describing the degree of cooperativity was calculated from the Hill equation:

log(f/(1 - f )) ) n log[Ag+]f + log Kd where f ) [Ag+]bonded/[CMA], [Ag+]f is the molar concentration of unbound Ag+. All parameters were calculated for mononuclear binding process without taking the Donnan effect and the activity coefficient of interacting molecules into consideration. For example, poly(ethylene-alt-MA) 1a, whose hydrophilic properties are lower and higher than those of 2a and 3a, respectively, was characterized by the Hill coefficient n ) 0.9 ((0.1) and the degree of silver binding (per MA repeating unit) θ ) 25-30% at pH 6.0-6.2; under mild basic conditions (pH 7.2-7.5) n ) 1.1 ((0.1) and θ ) 60-65%. The dissociation constants of the polymer-silver salts were higher than those of initial polymer acids (e.g., pKd of 1a/Ag+ ) 2.4; cf. pK1 ) 3.6 and pK2 ) 6.2 for parent polymer acid 1a). These findings validate the schematic sketch of CMA and CMA-Ag+ (Scheme 1). The above results suggest that the process of CMA binding of silver ions under acid conditions is noncooperative while at mild basic pH values the cooperativity is feebly marked. Earlier, U.P. Strauss et al.15 reported that the interaction of silver ions with the copolymers of MA with ethylene or vinyl methyl ether

(degree of neutralization of polyacids was R ) 0.70 and 0.67, respectively) was specific and cooperative; the degree of binding of silver ions never exceeded the initial value of neutralization degree of the polyacid. Partially decarboxylated poly(acrylic acid)16,17 may be treated as a close analogue of the proposed polymer-stabilizers, especially of poly(ethylene-alt-MA) 1a. It was reported16 for silver binding process at basic conditions that a 57% decarboxylation of poly(acrylic acid) led to a decrease in the Hill coefficient from 2.7 to 1.8. A decrease of the Hill coefficient was also observed when pH of poly(acrylic acid)-silver ions binding reaction was changed from basic to acid. It could be proposed that under our reaction conditions (θ < 100%) one MA residue binds no more than one ion of silver and that the metal-carboxylate complex is of a two-coordinate type. This polymer-silver ion binding mechanism predetermines the mode of formation of intermediate clusters and NPs of silver in the course of reduction. In contrast to poly(acrylic acid),17 in the case of CMA, formation of “blue silver” was not observed under the reduction at the same polyacid/Ag+ ratios and concentrations;17 reduction of aqueous solutions of silver ionloaded CMA results in transparent golden-brown solutions only. “Blue silver” is the early silver ions reduction product stabilized by the polyacrylate (PA) chain. This product is attributed to a complex of charged linear silver cluster and PA:

3398 J. Phys. Chem. B, Vol. 113, No. 11, 2009

Samoilova et al.

TABLE 1: Reaction Experimental Conditions and Optical Properties of Samples Synthesized by Reduction of Aqueous Solutions of Silver Ions in the Presence of CMA sample symbol

in accordance with Chart 1

[CMA] (M)a

CCA (M)b

A B C D E F G H I J K L M N

1a 1a 1b 1b 1b 2a 2a 2a 2b 2b 2b 2b 3a 3b

0.010 0.007 0.007 0.030 0.002 0.011 0.011 0.011 0.011 0.011 0.004 0.004 0.004 0.004

0.015 0.015 0.011 0.011 0.011

pH of initial so lution of CMA

[CMA]/[Ag+]/ [NaBH4] (M)c

λmax (nm)d 1 day/2 months

∆λ1/2 (nm)e 1 day/2 months

7 7 7 9 9 9 7 9 7 9 9 7 7 9

1/0.5/5 1/1/10 1/1/10 1/1/10 1/1/10 1/0.5/5 1/1/10 1/3/30 1/1/10 1/0.5/5 1/2/20 1/3/30 1/1/10 1/1/10

400 415/425 410/430 f 415/430 f 402/430 f 440 f 425/430 f 435 f 423/430 f 440 f 435 f 435 f 423 425/430

135 100/140 95/115 98/120 110/120 165 120/130 170 115/120 160 140 150 130 130/135

0.010 0.010 0.010 0.010 0.016

a

Molar concentration of polymer repeating units. b Critical concentration of aggregation (molar concentration). c Molar ratio of repeating units of CMA to silver ions and reducing agent. d Plasmon band location after 24 h or 2 months. e Width at half-height after 24 h or 2 months. f Additional shoulder at 340-345 nm.

Figure 1. Evolution of UV-visible absorption spectra during reaction of sample 1b (C, Table 1) prepared with [CMA] ) 0.007 M and CMA/ AgNO3/NaBH4 molar ratio 1/1/10 (reaction solution was diluted 20-fold) and spectrum of purified and dried sample 1b (1 day) after aging of solid sample during 11 months (redissolved solid). m+ Agm+n PA- (m g 8; n e 4).18,7 The complex exhibits a specific absorption band at about 600 nm. In the case of CMA, the formation of silver clusters conjugated with polymer chains apparently did not occur. In other words residues of MA comonomers (ethylene, styrene, or N-vinyl-2-pyrrolidone) could act as “breakers” in the process of formation of such linear polymer-bonded cluster. Summing up, the early stage of silver nucleation apparently occurs with participation of polymer-bonded silver ions within isolated nucleation centers (nanoreactors). Then, the growth of silver particles within CMA micelles leads to formation of compact, sterically stabilized polymeric globules (CMA-Ag°, Scheme 1). Figure 1 shows, as example, the characteristic optical spectrum of silver colloid obtained in the presence of amphiphilic poly(ethylene-alt-MA) 1b (C, Table 1) and the temporal evolution of the spectrum. The absorption band centered at 400-430 nm was observed owing to the surface plasmon resonance of silver NPs. A noticeable red shift of the

plasmon band was detected after 2 months (and, especially, after 11 months) of storage of reaction solution in comparison with the freshly prepared sample (kept for a day only) and a solution prepared from solid sample (to obtain a solid sample, freshly prepared sample was purified, dried, and then stored for 11 months). The concentration of CMA and the [CMA]/[Ag+]/[NaBH4] ratio (expressed as the molar concentration of the metal ion, NaBH4, and polymer chain units), as well as CCA, initial pH value of CMA solution, plasmon band location and width of plasmon band at half-height after 24 h or 2 months of storage of reaction solution are summarized in Table 1. The concentration of CMA usually used was from 0.003 to 0.011 M and stabilizer-to-metal and to NaBH4 ratio was 1:1:10; as the result stable highly silver-loaded colloidal systems were obtained. It should be noted that the stability of NPs-containing colloidal solutions was decreased when Ag+/CMA ratio was less than 0.1-0.5. Evidence for this comes from the change in absorption spectrum of solutions: red shift of plasmon band and an increase

Copolymers of Maleic Acid as Stabilizers

J. Phys. Chem. B, Vol. 113, No. 11, 2009 3399

Figure 2. Dependence of reduced viscosity of CMA (1a, 1b, 2b) and CMA/Ag°, prepared under conditions in Table 1 (1a/Ag° (B), 2 months; 1b/Ag° (C), 2 months; 2b/Ag° (I), 2 months) on polymer or polymer/NPs concentration (0.1 M Gly-NaOH buffer solution, pH 9, 25 °C).

in ∆λ1/2 were observed (samples A, F, J; Table 1). The same was true for “slow” nonspecific reduction of CMA-silver salts in water solutions (at room temperature and light): a dark precipitate was formed within a few days or weeks. Both for the “sluggish” reduction, and for the “fast” reduction at low reagents concentration and Ag+/CMA ratios, this phenomenon may possibly be attributed to the retarding effect in the reaction kinetic of polymer/Ag° complex formation. In other words, unstable intermediate silver clusters lost positive charge and electrostatic bonding with polymer template, so that uncontrolled growth and aggregation of silver particles were observed. Stability of NPs depends on number of polymer chain contacts with the surface of growing metal particle. When a few tiny uncharged particles appear in a system, CMA cannot form a protective cover rapidly enough. Apparently, at first the generation of particles with suitable surface areas is required to obtain stable polymer-metal complex. And for each polymer-stabilizer there should be both minimal and maximal reagent concentrations and metal ion/polymer ratios at which metal particles thus obtained become unstable. The preparation of stable NPs depends both on reaction conditions and on the polymerstabilizer characteristics (chain flexibility, hydrophobicity, etc.) and on macromolecular chain length. Our data apparently indicate that increasing Ag+/CMA ratios and concentrations of reagents above the conventional ranges results in changing relative rates of the all steps involved in the formation of NPs or even in a total change of reaction mechanism. L. Suber and co-workers19 obtained silver particles by reduction of a silver salt with ascorbic acid in the presence of sodium salt of naphthalene sulfonate-formaldehyde copolymer. They found that the silver particles size depends strongly on the concentration of silver ions and metal/polymer ratio. At lower concentrations and silver/polymer ratios, larger particles (1-2 µm) were generated, most likely due to a slower nucleation rate. At higher concentrations and metal/particle ratios, faster nucleation produced nanosize particles. G. Carotenuto and L. Nicolais20 reported an increase in the size of gold nanoparticles for lower poly(N-vinyl-2-pyrrolidone)/AuCl4 ratios, and P. Y. Silvert et al.21 found a decrease in size and polydispersity of silver colloids by increasing poly(N-vinyl-2-pyrrolidone)/metal ratio.

Investigation of rheological characteristics of the CMA solutions revealed that behavior and properties of the macromolecules in the presence of silver ions or silver NPs changed drastically. As an example of the relevant evidence, data of viscosity measurement on water solutions of poly(ethylene-altMA) 1a, its amphiphilic derivative 1b, amphiphilic polymer derivative of poly(N-vinyl-2-pyrrolidone-alt-MA) 2b, and the same copolymers, bearing nanosilver are presented in Figure 2. The intrinsic viscosity and Haggins constant obviously depend on polymer and solvent properties. Haggins constant (111.3 dl/ g) was maximal for hydrophobic polymer 1b, decreased almost by three times for 1a, and became minimal for NPs-contained polymers, for which the quality of the used solvent became the best. The intrinsic viscosity of polymer/Ag° solution was similar to that of the parent polymer-stabilizer. All polymers used in this work showed a similar character of the interaction of CMA/ Ag° particles with solvent and close particle size (strictly speaking particle hydrodynamic volumes). For example, for polymer systems 1a/Ag° and 1b/Ag° (amphiphilic polymerstabilizer was used), prepared under the same conditions, Haggins constants were 1.32 and 1.18, respectively. Consequently, as compared with the parent copolymers, CMA/Ag° particles had a small and unchanged size; these particles practically neither swell nor disintegrate under dilution of the initial solution. The behavior of the particles was similar to that of isolated small hydrophilic molecules. Under CMA/Ag+ reduction in water medium silver intramicellar CMA/Ag° complex was obtained. The hydrophobic parts of the polymer chains were gathered round NPs and closely packed polymer/ metal particles were formed (CMA-Ag°, Scheme 1). Such conformation changes of polymeric chains led to the alteration of polymer solutions viscosity. In the case of polymer particles 2a/Ag°, 2-pyrrolidone residues contribute to stabilization of NPs as in case of poly(N-vinyl-2-pyrrolidone) polymer-stabilizer. The TEM micrographs and bar diagrams characterizing the disperse composition of Ag° in sols are shown in Figure 3. The shape of the silver NPs is close to spherical. To get the detailed data, the NPs size (diameter) distribution was determined by image analysis using at least 100 counts. The average NPs size and the size ranges of 1b/Ag° (E, 24 h, Table 1) were similar to those of 2b/Ag° (I, 2 months, Table 1). The average size of

3400 J. Phys. Chem. B, Vol. 113, No. 11, 2009

Samoilova et al.

Figure 3. TEM micrographs and the histograms of silver particle size distribution of nanocomposites prepared under conditions in Table 1: (a) 1b/Ag° (C, 1 day), (b) 1b/Ag° (C, 2 months), (c) 1b/Ag° (D, 2 months), (d) 2b/Ag° (I, 2 months), (e) 3b/Ag° (N, 2 months).

silver NPs in the case of stabilizer 1b was 1.5 nm (standard deviation of 0.2 nm), and that of 2b/Ag°, 1.8 nm (standard deviation of 0.2 nm). After 2 months of aging of reaction solutions under room temperature and light NPs of 1b/Ag° (E, 2 months, Table 1) increased in size up to 3.5 nm (standard deviation of 0.1 nm) in contrast to those of 2b/Ag° (I, 2 months, Table 1). The size distribution of silver particles prepared at the increased (higher than CCA) initial concentration of CMA, for example, for sample 1b/Ag° (D, 2 months, Table 1) was broad (from about 1-13 nm) and bimodal. In addition to NPs with an average particle size of 2.1 nm (standard deviation of 0.1 nm), NPs of an average diameter of 8.2 nm (standard deviation of 0.9 nm) were found. Size distributions for samples 3a/Ag° and 3b/Ag° were similar to those of 1a/Ag° and 1b/ Ag° prepared under the same reaction conditions. Figure 4 shows AFM images of CMA and polymer-silver complexes; AFM data on polymer particle size distribution are summarized in Table 2. The origin of topography change of the polymer film surface could be related to size alteration of macromolecular aggregates as well as to that of their shape in solution. Shape of the most of polymer particles is close to spherical so that the average radius is a sufficient characteristic of their (mean) size. The whole array of particles was divided into chosen size ranges (Table 2). Amphyphilic CMA 1b has a more narrow size distribution of polymer particles (96% of the latter are in the range 20-100 nm range) than the initial CMA 1a, poly(ethylene-alt-MA). A broader particle size distribution of CMA in the presence of silver ions (sample 1a/Ag+) can be ascribed to dissociation of polymer associates and redistribution of polymer chains. Interestingly, the reduction of silver ions in the presence of polymer matrix yielded smaller polymer particles (1a/Ag°, 1b/Ag°, Table 2); no particles of the 100-300 nm range were detected, while a notable amount of particles ranging from 5 to 20 nm in size appeared (Table 2). Such small particles were not revealed in the parent polymer (1a and 1b) solutions. The size ranges and average radii of 1a/Ag° nanocomposite particles are comparable with those of 1b/Ag°, prepared with

the participation of amphyphilic polymer. Changes in the polymer particles size observed by AFM correlate well with viscometric data and validate the sketch in Scheme 1. As was expected, the size (diameter) of polymer particles containing NPs reasonably exceeded the size of metal particles themselves, see TEM (Figure 3a,b) and AFM (Figure 4c,e, and Table 2) data. Hence the thickness of the polymer protective layer can be calculated from AFM and TEM data comparison. For example, its values are about 3.0-3.5 nm and 12.0-12.5 nm for 1b (amphiphilic polymer stabilizer) and 1a, respectively (the smallest nanocomposite particles and silver NPs were taken into account). It should be pointed out that synthesis of NPs carried out in the presence of different CMA, initial or amphiphilic (1a, 2a, 3a or 1b, 2b, 3b), at similar reagent concentrations and CMA/ Ag+/NaBH4 ratios gave NPs with almost the same kind of morphology. Close similarity of nanoparticles size, NPs size distribution (TEM data) and plasmon band location were observed (Table 1). Nevertheless, an important point is that the use of amphiphilic CMA (1b, 2b, and 3b) allows preparation of the most stable NPs-containing colloids in a wider range of reagents ratios and concentrations. For example, the destabilization time for silver nanocomposites based on the parent and amphiphilic polymers was different. At nonoptimal conditions (low polymer concentrations0.002 M and polymer/Ag+/NaBH4 1/0.1/0.05 molar ratios) the appearance of a black sediment was observed after 3-5 days of storage for the systems 1a/Ag°, 2a/ Ag°, 3a/Ag° and after 30 days for samples 1b/Ag°, 2b/Ag°, 3b/Ag°. At chosen optimal conditions (stabilizer concentration 0.003-0.011 M and CMA/Ag+/NaBH4 ratios 1/1/10) the final morphology of Ag nanoparticles (particle size and particle size distribution) is strongly affected by the structural similarity (strict alternation of monomer units) and similar length of polymer chains of all used copolymers; hydrophobic ligands only assist in nanoparticles stabilization. Apparently, NPs are located within liophobic nuclei and the liophilic barrier consists of the solvated polymer chains (CMA-Ag°, Scheme 1).

Copolymers of Maleic Acid as Stabilizers

J. Phys. Chem. B, Vol. 113, No. 11, 2009 3401

Figure 4. AFM images of polymer particles: (a) 1a, (b) 1a/Ag+ (1/1 molar ratio), (c) 1a/Ag° (B, Table 1), (d) 1b, (e) 1b/Ag° (C, Table 1).

3402 J. Phys. Chem. B, Vol. 113, No. 11, 2009

Samoilova et al.

TABLE 2: AFM Polymer Particle Size Distribution

sample 1a

1a/Ag+

1a/Ag° (B, 2 months, Table 1)

size range, nm

average particle radius, (standart deviation), nm

content, %

5-20 20-100 100-200 200-300

60 (4) 137 (7) 258 (55)

64.0 33.0 3.0

5-20 20-100 100-200 200-300

12 (5) 63 (16) 141 (14) 211 (33)

25.0 27.5 42.5 5.0

17 (0.5)

28.8

34 (2)

71.2

5-20 20-100 100-200 200-300

63 (2) 135 (28)

96.3 3.7

5-20

10 (3)

33.3

20-100 100-200 200-300

48 (9)

66.6

5-20 20-100 100-200 200-300

1b

1b/Ag° (C, 2 months, Table 1)

To study the stability of the prepared colloidal nanocomposite solutions, absorption spectra in the visible region were observed. The samples synthesized under the optimal conditions show no sign of flocculation after aging of the solutions for several months. For example, no flocculation in the sample 1b/Ag° (C, Table 1) solution was observed after a 11 months-long storage of reaction solution (Figure 1), but the spectrum of the sample exhibited a shoulder at 340 nm and a broad absorption band at longer wavelengths, characteristic of a high anisotropy. During silver reduction, if the pH of initial CMA solutions varied from 7 to 9 (at the reagents ratios and in the concentration ranges mentioned above), no substantial differences were found in the absorption spectra (Table 1) as well as in sizes and shapes of NPs. It should be noted that the final pH of the reaction solution usually rose up to a value of 9.2-9.5 imposed by the basic products of NaBH4 oxidation. But at pH 9 of the initial polymer solution the best solubility of CMA and CMA-silver salts, especially in case of polymers 1a, 1b, and 3a, 3b, was achieved. This fact has opened up the possibility of reduction of such salts in solutions at concentration ratios Ag+/CMA > 1 (samples H, K, L; Table 1). For example, the silver/CMA weight ratio in dried samples could achieve a value of 1.4 for polymer 2a or 1.3 for 2b (initial molar concentrations ratio Ag+/CMA ) 3 and pH ) 9 of the initial CMA solution). Evidently, in this case the threshold of steric stabilization was achieved, and the thickness of polymer protecting layer was at least more than the van der Waals force range (2-3 of NP average radius). However a broad particle size distribution for such highly metalloaded polymer systems was obtained. The stability of reaction solutions of this CMA/Ag° did not exceed 1-3 months of storage, but freshly prepared stabilized NPs can be purified, dried, and preserved. An addition, purified nanocomposite golden-brown powders could be obtained from colloidal reaction solutions after ultrafiltration, gel-filtration, or dialyze against deionized water and liophylization. The composition of the solid product agrees

with the initial reaction CMA/Ag+ ratio. For example, a dried silver-loaded sample obtained at CMA 2b concentration of 0.007 M and ratio CMA/Ag+/NaBH4 1/1/10 contained 29.97 or 30.30 wt % of silver (calculated or analytical data, accordingly). There were no differences between the spectrum of the sample after 1 day-long storage of reaction solution and the spectrum of the same sample after its purification, drying, 11 months-long aging, and resolution (Figure 1). Nanocomposite powders could be easily dissolved in water to form the stable colloids by simple agitation for a few minutes. This is important for convenient storage and transportation of nanocomposite materials. Conclusions Stable silver hydrosols have been obtained easily using a method based on the reduction of metal precursor in the presence of dicarboxylic (maleic) acid copolymers with NaBH4 in aqueous solutions at room temperature. These readily available copolymers are cheap and low toxic; they have been already used in medicine for, for example, the preparation of thromboresistant coatings.22 The influence of the specific nature of dicarboxylic acid copolymers on the process as a whole of silver NPs preparation and on the acquisition of stable highly silverloaded colloids was revealed. In the first step, CMA forms a polymer-silver salt complex of the two-coordinate type, and this process is noncooperative at pH < 7 and weakly cooperative under mild basic conditions. Variation of CMA concentration and CMA/Ag+ ratio allows preparation of silver NPs of different size and stability. The mechanism of silver NP formation in the presence of dicarboxylic acid copolymers differs markedly from that in the case of monocarboxylic acid polymers. The CMA concentration required for preparation of the most stable and highly metal-loaded colloids does not exceed the critical aggregation concentration of the biphilic copolymers used; at a large excess of reducing agent, the maleic acid residues/metal ion molar ratio is equal to unity. The chosen synthetic procedure yields 1.5-3.5 nm silver NPs. It was established that similar reaction conditions resulted in almost the same kind of NPs and CMA/NPs particles morphology for all polymers used; different copolymers displayed similar rheological behavior in solutions in the presence of silver NPs. The average size of polymer nanocomposite particles exceeded significantly the size of metal particles. To evaluate thickness of polymer protective layer, a simple method has been proposed; it was based on AFM and TEM data comparison. The related structure (strict alternation of monomer units) and similar length of polymer chains of all copolymers had a great influence on the morphology of Ag nanoparticles while hydrophobic ligands only assisted in NPs stabilization. Very stable, water-soluble, solid, highly metal-loaded CMA-Ag° systems could be used in practice at small amounts to improve, for example, optical and antiseptic properties of different materials or their surfaces. The preliminary investigation of antifungal activity of proposed nanocomposites proposed in the presence of plant pathogen F. Oxisporum demonstrated appropriate activity of CMA/Ag° nanocomposites even in 10ppm concentration. Elaboration of a simple and fast method for the synthesis of nanocomposites described in this work is important for realization of their large-scale production and application. Acknowledgment. We thank Dr. Sergei S.Abramchuk for help with TEM and Alexander Erofeev for help with AFM.

Copolymers of Maleic Acid as Stabilizers References and Notes (1) Napper, D. H.; Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (2) Schimid, G. P., Ed. Clasters and Colloids: From Theory to Application; VCH: Weinheim, Germany, 1994. (3) Pomogailo, A. D.; Rosenberg, A. S.; Ufliyand, I. E. Nanochastitsy metalloV V polymerakh (Metal Nanoparticles in Polymers); Khimiya: Moscow, 2000 (in Russian). (4) Liz-Marzan, L. M.; Kamat, P. V. Nanoscale Materials; Kluwer Academic-Plenum Publishers: Boston, MA, 2003. (5) Toshima, N. Encyclopedia of Nanoscience and Nanotechnology; Scwarz, J. A., Contescu, C., Putuera, K., Eds.; Marcel Dekker: New York, 2004. (6) Springer Handbook of Nanotechnology; Springer: Berlin; Geidelberg, Germany, 2007. (7) Ershov, B. G.; Henglein, A. J. Phys. Chem. B 1998, 102, 10663. (8) Kiryukhin, M. V.; Sergeev, B. M.; Prusov, A. N.; Sergeev, V. G. Visokomol. Soedin. Ser., B 2000, 42, 1069 (Polym. Sci. Ser. B 2000, 42, 453). (9) Mayer, A. B. R.; Mark, J. E. J. Polym. Sci., A: Polym. Chem. 1998, 35, 197. (10) Akashi, M.; Iwasaky, H.; Miyauchi, N.; Sato, T.; Susamoto, J.; Takemoto, K. J. Bioact. Compat. Polym. 1989, 4, 124. (11) Pellegrino, T.; Manna, L.; Kudera, S.; Liedl, T.; Koktysh, D.; Rogach, A. L.; Keller, S.; Radler, J.; Natile, G.; Parak, W. J. Nano Lett. 2004, 4, 703.

J. Phys. Chem. B, Vol. 113, No. 11, 2009 3403 (12) Lin, C.-A. J.; Sperlink, R. A.; Li, J. K.; Yang, T.-Y.; Li, P.-Y.; Zanella, M.; Chang, W. H.; Parak, W. J. Small 2008, 4, 334. (13) Conix, A.; Smets, G. J. Polym. Sci. 1995, 15, 221. (14) Krayukhina, M. A.; Kozibakova, S. A.; Samoilova, N. A.; Babak, V. G.; Karayeva, S. Z.; Yamskov, I. A. Russ. J. Appl. Chem. 2007, 80, 1145. (15) Strauss, U. P.; Begala, A. J. AdV. Chem. Ser. 1980, 187, 327. (16) Sergeev, B. M.; Kiryukhin, M. V.; Prusov, A. N. MendeleeV Commun. 2001, 2, 68. (17) Sergeev, B. M.; Lopatin, L. I.; Sergeev, G. B. Colloid J. 2006, 68, 761. (18) Remita, S.; Orts, J. M.; Feliu, J. M.; Mostafavi, M.; Delcourt, M. O. Chem. Phys. Lett. 1994, 218, 115. (19) Suber, L.; Sondi, I.; Matijevic, E.; Goia, D. V. J. Colloid Interface Sci. 2005, 288, 489. (20) Carotenuto, G.; Nicolas, L. Polym. Int. 2004, 53, 1009. (21) Silvert, P. Y.; Herrera Urbina, R.; Tekaia-Ellsisen, K. J. Mater. Chem. 1997, 7, 293. (22) Samoilova, N. A.; Krayukhina, M. A.; Novikova, S. P.; Babushkina, T. A.; Volkov, I. O.; Komarova, L. I.; Moukhametova, L. I.; Aisina, R. B.; Obraztsova, E. A.; Yaminsky, I. V.; Yamskov, I. A. J. Biomed. Mater. Res. 2007, 82A, 589.

JP806683M