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Fabrication of a Novel Type of Metallized Colloids and Hollow Capsules Alexei A. Antipov,*,† Gleb B. Sukhorukov,† Yuri A. Fedutik,‡ Ju¨rgen Hartmann,† Michael Giersig,§ and Helmuth Mo¨hwald† Max-Planck Institute of Colloids and Interfaces, D-14476 Golm/Potsdam, Germany, Physico-Chemical Research Institute, Belarussian State University, 220050 Minsk, Belarus, and Hahn-Meitner Institute, Glienicker Strasse 100, D-14109 Berlin-Wannsee, Germany Received January 16, 2002. In Final Form: May 23, 2002 A novel type of micrometer-sized entity containing silver was introduced. Three different approaches were employed to modify the colloidal particles with silver. According to the first technique, the particles were first coated with layers of poly(styrenesulfonate) and silver ions followed by the reduction of silver by photoirradiation. The second technique involved coating the colloidal particles with a shell capable of reduction, and sequential addition of silver salt resulted in the reduction of metal in the shell matrix. The last approach utilized the reaction of a silver mirror to silver particles. Varying the sequence of polyelectrolyte layers and silver cover resulted in different morphology, stability, and properties of the structures. The core could be removed subsequently, giving rise to hollow silver-containing capsules. The obtained shells and capsules were characterized by TEM, AFM, and confocal laser scanning microscopy. These metallized structures have a potential to serve as superior catalysts with tailored qualities and capabilities.
Introduction The fabrication of colloids with desired properties has been the aim of many recent investigations.1,2 The modification of colloids was found to be a powerful tool for drastically changing and precisely tuning their optical, mechanical, and surface properties. One of the most effective tools for achieving this ability is to coat colloids with different species which possess the desirable properties. Recently, a major fraction of publications on this type of modification was concentrated on the layer-by-layer self-assembly (LbL) technique which is based on electrostatic adsorption of oppositely charged species.2,3 Different templates, such as latexes, inorganic crystals, protein associates, and oil droplets, could be easily covered by polyelectrolytes and proteins, inorganic particles, biological cells, or sheets, giving rise to novel entities.2-5 Another robust approach called controlled precipitation has an advantage in that uncharged molecules can be forced to settle down onto colloidal particles by gradually changing the solvent properties, thereby quickly forming a thick layer of material.6 The disadvantage of this method is that the appropriate conditions for precipitation should * To whom correspondence should be addressed. Tel: +49 331 567-9235. Fax: +49 331 567-9202. E-mail: alexei.antipov@ mpikg-golm.mpg.de. † Max-Planck Institute of Colloids and Interfaces. ‡ Belarussian State University. § Hahn-Meitner Institute. (1) For a review, see: Ford, W. T.; Badley, R. D.; Chandran, R. S.; Babu, S. H.; Hassanein, M.; Srinivasan, S.; Turk, H.; Yu, H.; Zhu, W. M. ACS Symp. Ser. 1992, 492, 422-431. (2) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mohwald, H. Colloids Surf., A 1998, 137, 253266. (3) For a review, see: Sukhorukov, G. In Studies in Interface Science; Mo¨bius, D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; Vol. 11, p 383. (4) Voigt, A.; Buske, N.; Sukhorukov, G. B.; Antipov, A. A.; Leporatti, S.; Lichtenfeld, H.; Baumler, H.; Donath, E.; Mohwald, H. J. Magn. Magn. Mater. 2001, 225, 59-66. (5) (a) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mohwald, H. J. Phys. Chem. B 2001, 105, 2281-2284. (b) Neu, B.; Voigt, A.; Mitlohner, R.; Leporatti, S.; Gao, C. Y.; Donath, E.; Kiesewetter, H.; Mohwald, H.; Meiselman, H. J.; Baumler, H. J. Microencapsulation 2001, 18, 385395.
be deduced in a case-specific manner. Unfortunately, only a few investigations have employed the potential of the chemical shell modification, although this approach might be very simple and flexible in some cases when adsorption of the desired species is hardly achievable.7 The next major step in the shell evolution was made when the core material was removed by dissolution or calcination, giving rise to empty capsules with novel properties.2-7 The possibility of encapsulating or entrapping different materials inside the capsule was shown to be a powerful tool for producing microreactors.8-10 Different catalytic reactions may proceed inside the capsule interior, such as the enzymatic decomposition of urea by the encapsulated urease8 or the hydrolysis of N-benzoyl-L-tyrosine ethyl ester by entrapped R-chymotrypsin.9 Many organic reactions involve the presence of catalysts, which are mainly different metals such as silver, copper, palladium, and platinum. Encapsulation of these metals inside the capsule or in the capsule wall would lead to the creation of novel catalyst systems. This intriguing topic deserves specific attention in various branches of chemistry, including above all organic and polymer synthesis. Recently, much work was done on the fabrication of encapsulated and immobilized metal nanoparticles. There were a few approaches which were used, including (1) metal particle encapsulated into alkanethiolate monolayers,11 (2) particles encapsulated in dendrimers,12 (3) metal oxide-metal composites,13 and (4) (6) (a) Dudnik, V.; Sukhorukov, G. B.; Radtchenko, I. L.; Mohwald, H. Macromolecules 2001, 34, 2329-2334. (b) Radtchenko, I. L.; Sukhorukov, G. B.; Leporatti, S.; Khomutov, G. B.; Donath, E.; Mohwald, H. J. Adv. Mater. 2001, 13, 1684-1687. (7) Dahne, L.; Leporatti, S.; Donath, E.; Mohwald, H. J. Am. Chem. Soc. 2001, 123, 5431-5436. (8) Lvov, Y.; Antipov, A.; Mamedov, A.; Mo¨hwald, H.; Sukhorukov, G. B. Nano Lett. 2001, 1, 125-128. (9) Tiourina, O. P.; Antipov, A. A.; Sukhorukov, G. B.; Larionova, N. L.; Lvov, Y.; Mo¨hwald, H. Macromol. Biosci. 2001, 1, 209-214. (10) Lvov, Y.; Caruso, F. Anal. Chem. 2001, 73, 4212-4217. (11) (a) Maye, M. M.; Lou, Y.; Zhong, C.-J. Langmuir 2000, 16, 75207523. (b) Li, H.; Luk, Y.-Y.; Mrksich, M. Langmuir 1999, 15, 49574959. (12) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181-190.
10.1021/la020052x CCC: $22.00 © 2002 American Chemical Society Published on Web 07/27/2002
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nanoparticles immobilized onto the latex surface.14 Here, we report the creation of different metal-polymeric composites which offer us a wide range of materials that can be used and whose structure and morphology can be finely tuned. The first major approach consists of the direct adsorption or chemisorption of metallic particles from the solution onto the surface,15 but in this case, the layered structure should be formed to produce films of desired thickness or particles of different sizes must be used, which would lead to the increased roughness of the film. The second approach is the reduction of metallic ions to form a metal coverage, for example, in the entrapment of silver ions by polyelectrolyte multilayers followed by hydrogen treatment.16 This catalytic system is expected to have several advantages. (1) Very active and, probably, selective catalytic materials can be obtained due to their morphology and large surface areas. (2) Formation of composite coverage of a few different metals, or metals together with specific polymers, serving as catalysts, would lead to several-in-one catalytic systems, capable of promoting a number of reactions simultaneously. The presence of a polymer matrix could lead to the selectivity in catalyzed reactions due to the steric, enantiomeric, or electrostatic influence. (3) A polymeric matrix could serve as a protective envelope, shielding the catalyst from probable environmental hazards. (4) Furthermore, this polymeric coverage could stabilize particles from aggregation which is often a problem in catalytic systems. (5) The solubility of this catalyst in different solvents can be easily tuned by changing the outermost layer; a hydrophobic layer would enhance oil solubility, while a hydrophilic layer would enhance water solubility. Simultaneously, it is easy to regulate the affinity of the capsule for the surface by the attachment of specific chemical groups. In this work, we have made the first step toward the creation of this catalytic system with the formation of metallic shells and capsules. We have demonstrated the feasibility of three different approaches. The silver was used as a model to demonstrate the possibility of fabricating metallized capsules. Experimental Section Poly(sodium 4-styrenesulfonate) (PSS, ∼70 kDa), poly(allylamine hydrochloride) (PAH, 50-60 kDa), aniline, 4-nitrophenol, sodium borohydride, sodium hydroxide, silver nitrate, and horseradish peroxidase were purchased from the Sigma-Aldrich Co. (USA). Hydrogen peroxide, all buffer components, and salts were purchased from Roth (Karlsruhe, Germany). All chemicals and reagents were used without further purification. Melamine formaldehyde latexes (MF) 4.7 and 5.6 µm in diameter were obtained from Microparticles GmbH (Berlin, Germany). All solutions were prepared prior to use with the exception of salt solutions and buffers, which were prepared each week. The water used in all experiments was prepared in a 3-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ cm. PSS-doped polyaniline (PAniPSS) was prepared according to Liu et al.17 Aniline (130 mg, 50 mM) and PSS (206 mg, 50 mM monomer units) were dissolved in 20 mL of pH 4.0 citric buffer; 20 mg of horseradish peroxidase was added with stirring, and (13) (a) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650. (b) Haruta, M. Catal. Today 1997, 36, 153-166. (14) Barnickel, P.; Wokaun, A. Mol. Phys. 1989, 67, 1355-1372. (15) (a) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. (b) Caruso, F.; Spasova, M.; Saigueirino-Maceira, V.; Liz-Marzan, L. M. Adv. Mater. 2001, 13, 1090. (16) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354-1359. (17) Liu, W.; Anagnostopoulos, A.; Bruno, F. F.; Senecal, K.; Kumar, J.; Tripathy, S.; Samuelson, L. Synth. Met. 1999, 101, 738-741.
Antipov et al. after complete dissolution, 50 µL of hydrogen peroxide was added. The solution obtained was dialyzed and lyophilized. The synthesized polymer had good solubility in water and was used for adsorption experiments at a concentration of 10 mg/mL. PSS-PAH shell fabrication was done as previously reported.3,18 The first approach used to reduce metallic ions employed the reductive ability of the capsule wall. For faster and more efficient reduction, special species capable of being easily oxidized should be chosen as the wall constituents. Polyaniline (PAni) is known for its ability to reduce silver ions and was chosen for the present experiments. PAniPSS-PAH 4-bilayer shells ((PAniPSS-PAH)4) were fabricated by the sequential adsorption of PAniPSS and PAH in 0.5 M NaCl onto MF particles with intermediate 3-fold washing with water and centrifugation at 300g. When empty capsules were needed, MF cores were dissolved in 0.1 M HCl followed by washing with water. The second approach involved gathering the shell from cations of metal and polyanions, followed by reduction with some external substance, such as sodium borohydride or light irradiation. Selfassembly of PSS with silver cations (PSS-Ag) was conducted according to the previous report6 employing 5 mg/mL PSS and 18 mg/mL AgNO3 solutions in water. A solution of 50 µL of 468 nm polystyrene latex or 37 µL of 2.6 µm MF latex in 10 mL of water was mixed with 1.5 or 0.75 mL of AgNO3 solution, and 0.73 or 1.5 mL of PSS solution was added dropwise into the mixture under vigorous stirring. The complex was allowed to adsorb for 1 h and was purified from excess AgNO3 by a repetitive centrifugation cycle until the supernatant did not give a positive reaction with chloride ions. The last and also the quickest, simple, and elegant way to produce metallized shells consists of electroless (i.e. chemical reduction) deposition of metals onto the colloidal surface from solution employing reactions, such as the Tollens probe (better known as the silver mirror reaction).14,19,20 This has the great advantage of fast formation of thick metallic layers but has the limitation of the nature of the metal; for each particular case, the appropriate conditions of the on-surface reduction need to be found. The Tollens reaction on the surface of latexes was performed according to Barnickel and Wokaun.14 Two milliliters of a 1.5% suspension of latexes (bare or covered with PEM precursor) was mixed with 40 µL of acetaldehyde, and the mixture was put into an ultrasonic bath followed by the slow dropwise addition of 200, 100, or 50 µL of freshly prepared 5% [Ag(NH3)2]NO3. After 60 min, particles were washed by 5 centrifugation cycles. UV-vis spectrometry was employed to study the rate of 4-nitrophenol reduction. For this procedure, 100 µL of the capsule probe, containing 0.63 mg of silver, was mixed with 3 mL of 0.4 M NaBH4 and 0.05 M NaOH water solution and used for the baseline correction. After the addition of 25 µL of 2.4 × 10-3 M 4-NP water solution, spectra were obtained with 8 min intervals. The reaction was allowed to continue under constant stirring, until no change in the absorbance at 400 nm was registered for 1 h. Confocal images were obtained by means of a confocal laser scanning microscope (CLSM) (TCS Leica) with an excitation wavelength of 488 nm. To visualize the nonfluorescent capsules in fluorescent mode, FITC-labeled dextran or fluorescein (1 mg/ mL) was added. Scanning force microscopy (SFM) images were obtained with a Digital Instruments Nanoscope IIIa in tapping mode. A 360 W ultrasonic bath from Bandelin GmbH was used to resuspend the particles during shell assembly and for the Tollens reaction experiments. Transmission electron microscopy (TEM) images were obtained on a Phillips CM12 transmission electron microscope fitted with an energy-dispersive X-ray analyzer. For the measurements, the samples were prepared by deposition onto carbon-coated copper grids. (18) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202-2205. (19) (a) Tamai, H.; Hamamoto, S.; Nishiyama, F.; Yasuda, H. J. Colloid Interface Sci. 1995, 171, 250-253. (b) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726-728. (20) Mayer, A. B. R.; Mark, J. E. J. Polym. Sci., Polym. Phys. Ed. 1997, 35, 1207-1216.
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Figure 1. Fluorescent confocal micrograph of (PAni-PAH)4 empty capsules. Fluorescence comes from FITC-labeled dextran introduced into the system for visualization.
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Figure 2. Photoreduction of Ag+ on the surface of the (PAniPSS-PAH)4 shells. Inset: (PAniPSS-PAH)4 capsules after the addition of AgNO3, and photoirradiation of one in the bottom. The scale bar corresponds to 10 µm.
Results and Discussion 1. Reduction of Silver by the (PAniPSS-PAH)4 Capsule Wall. Polyaniline doped with PSS is a stable, negatively charged water-soluble polymer, which easily forms water-insoluble complexes with polycations such as PAH. This concept was employed to produce shells made of these two polymers. To prepare empty capsules, the MF core was removed by acid treatment followed by a thorough washing procedure. Confocal images of the obtained capsules (Figure 1) indicate that very uniform and continuous capsules were formed. A slight deviation in the capsule diameter occurs due to the different distances of capsules from the focal plane of the picture. The used PAniPSS originally exists in the emeraldin form21 and can be oxidized by various agents (even mild oxidizers) to form nigranilin. This reaction was employed to reduce silver cations from the solution22 upon addition of silver nitrate. For this procedure, the capsule suspension was mixed with a 10 mg/mL AgNO3 solution. Figure 2 (inset) shows that the silver reduction in the wall of the “marked” capsule was accelerated by the continuous scanning of the capsule by the confocal microscope during 10 s. Large silver particles were observed in the wall of the irradiated capsule, while no change occurred to the nonirradiated ones. Also, reduction of the silver was conducted for the corecontaining shells (Figure 2). The square area on the image was exposed to a 10 s irradiation to accelerate the reduction process. The drastic change in the surface color indicates that the reduction took place inside the prescanned area. Reduction of the silver cations was also successfully carried out without laser irradiation. Two days of silver treatment in the dark led to the complete coverage of the shells and capsules with silver; therefore, the picture (not shown) was very similar to that of Figure 2. Optically, black shells and capsules contained particles of silver in the wall, while practically no free silver particles were found in the solution. (21) For a review, see: Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277-324. (22) Zhang, A. Q.; Cui, C. Q.; Lee, J. Y.; Loh, F. C. J. Electrochem. Soc. 1995, 142, 1097-1104.
Figure 3. Fluorescent confocal micrograph of (PSS-Ag)50 shells and transmission (inset) of derived empty capsules. PSS is rhodamine-labeled. The scale bar corresponds to 10 µm.
A significant advantage of this type of catalyst is that the shell wall contains a reducing agent, in this case polyaniline, which may eliminate the problem of catalyst oxidation during synthesis which is often an issue in catalysis. 2. Reduction of the Silver Constituent of the Capsule Wall. To follow the second approach of making a silver shell, 5.6 µm MF and, in parallel, 470 nm polystyrene (PS) latex particles were covered with 50 PSSAg bilayers (a theoretical estimation with an assumption that all PSS is precipitated). PSS was rhodamine-labeled according to Dahne et al. to visualize the shell formation. Capsules were obtained after the MF core was dissolved in HCl. Figure 3 depicts a fluorescent image of the shells obtained and a transmission image of hollow capsules (inset); the absence of fluorescence in the bulk indicates that all PSS was precipitated onto the surface of the
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Figure 5. TEM micrograph of the (PSS-Ag)50 empty capsules. Reduction was done prior to (A) or after (B) dissolution. Figure 4. TEM micrograph of the (PSS-Ag)50 photoreduced shell. The scale bar corresponds to 50 nm.
latexes. To reduce the silver, the samples were exposed to white light with a high intensity for 5 h. The suspension became dark within a few minutes due to the growth of silver particles. Optical microscopy imaging showed the absence of silver particles in the bulk solution; only the investigated objects (capsules or shells) changed their color from transparent to dark brown. The structure of the silver-containing shell around the PS latex core is seen on transmission electron micrographs (Figure 4) with numerous silver particles ranging in size from 10 to 30 nm which are embedded in a polymeric matrix of PSS. Due to the low contrast resolution between core material and metallic particles during TEM imaging, clearer transmission micrographs could only be obtained from empty capsules. To produce these capsules, two approaches were employed. In the first, silver was reduced before core dissolution. In the second, core removal was followed by silver reduction. In both cases, capsules were stable in the suspension on storage at room temperature for 3 months. Typical TEM images of these two capsule types (panels A and B of Figure 5, respectively) itemize the fine wall structure. If the core was removed after the reduction, no change could occur in the capsule wall upon HCl treatment. However, in the case where shells were first treated with acid and silver chloride was formed in the capsule wall, subsequent light irradiation caused reduction of AgCl instead of the original PSS-Ag complex. This resulted in different morphologies of the capsule walls and a larger size of AgCl-derived silver particles (20-70 versus 7-30 nm). Metal particles were strongly bound to the matrix of the polymer which enveloped each crystal. This last fact might be useful for the catalytic reactions where the influence of the polymer structure affects the route of the reaction (production of different spatial isomers). Upon being dried, silver crystals were not desorbed unless a strong vapor pressure developed during the desiccation of small volumes. The density of the particles was quite large, and they covered almost half of the capsule surface. In principle, when the reduction is photo induced, the size of the reduced area depends only on the wavelength of the light used. However, if it is laser-induced, then the reduction would be confined to a few hundred nanometers.
Figure 6. Silver nanopainting. Each single silver dot is reduced by a laser beam forming the desired picture.
To prove this statement, we conducted experiments similar to the bleaching of the dye under the confocal fluorescent microscope. The laser beam was pointed into the desired areas of the shell covered with PSS-Ag layers for 1 s, resulting in a photographic image (Figure 6). The size of each spot is comparable to the wavelengths of the used lasers (around 300 nm). In fact, processes similar to this can be used on planar films for microphotography processes where nanoprecision is desirable. Another field where nanosigning can be employed is the combinatorial metal labeling of different objects such as particles or cells. Not only different cover morphologiescan be obtained, but different metals can also be employed in the formation of multilayered or intermixed metallic shells. 3. Tollens Probe Reaction. The last approach used to metallize the particles refers to the experiments of Barnickel and Wokaun14 and, later, Mayer and Mark,20 who suggested using the silver mirror reaction to coat latex spheres with a layer of silver or gold. Different types of structures can be obtained by the variation of the layer sequences (Figure 7). In the case where silver is reduced onto the surface of bare latex (route
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Figure 7. Two possible methods of silver reduction. (A) The silver layer is formed on bare latex, resulting in an unstable structure after core removal (A1) which, however, can be coated with PEM prior to core removal (A2). (B) The silver layer is formed between two PEM, ensuing steady capsule formation. The color blue corresponds to silver, and the color red corresponds to PEM.
Figure 8. TEM micrographs of 2-layer (A) and sandwich-like (B) empty capsules containing 25 pg of silver per capsule.
A) after core removal, the structure will be unstable and may break into pieces (A1). This mode can be used to produce encapsulated silver particles; with the metallic structure on a latex support covered with PEM (route A2) and the core removed, the metallic particles will desorb from the PEM capsule inner wall into its interior, forming a microreactor where no polymer influence on the catalytic activity exists. Indeed, after the silvered latexes were covered with 2 PSS-PAH bilayers and the core was dissolved (which may be called two-layers form), the intensive Brownian motion of the silver nanoparticles in the capsule interior was seen by the optical microscope. Another advantage of two-layer catalysts is that if the product of the catalytic reaction has a high molecular weight (polymerization), they accumulate inside the capsule where the catalyst resides. Route B in the scheme depicts formation of a sandwichlike structure. A silver envelope is formed on precoated latexes and subsequently covered by PEM once more. Hereby, the silver is entrapped between the polymeric layers, and the framework is stable after core dissolution.
Hence, the formed structure will be similar to those obtained in the two previous methods, with the exception that the metallic cover comes in contact with PEM only by its outer surfaces due to the 3-layer pattern of the wall. TEM micrographs were taken of capsules obtained by these two approaches (Figure 8). The sizes of silver particles varied from 7 to 100 nm in both cases. Unless no appreciable difference is visible, the silver particles in sandwich-like capsules are more organized than those in the 2-layer form which contain two generations of particles, adsorbed onto the polymeric surface and desorbed inside. This last fact is indirect proof of the presence of silver nanoparticles in the free volume inside the polyelectrolyte capsules. The thickness and morphology of the obtained capsule wall are directly proportional to the amount of reduced silver (Figure 9). Estimated from the height profiles of SFM images (not shown), the metal particle size decreases from 50 to 15 ( 5 nm upon moving from 25 to 6 pg of silver per capsule. Capsules with high metal content look much rougher than those with the smallest amount, whose
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Figure 9. SFM images of empty sandwich-like capsules with 25 pg of silver per capsule (A) and 6 pg of silver per capsule (B). The maximal heights in the z direction are 750 and 250 nm, respectively.
Figure 10. (A) Successive UV-vis spectra of 4-NP reduction by silver-containing capsules. (B) The decrease of A400 for silvered capsule-mediated (squares) and silver solution-mediated (circles) 4-NP reduction.
morphology is very similar to that of the ordinary PEM capsules23 where well-pronounced folds and crinkles denote the high elasticity of the capsule wall. 4. Catalytic Activity. The potential of the prepared silver-containing capsules to serve as catalysts was checked by the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of sodium borohydride.24 The mechanism of this reaction involves the oxidation of Ag0 into Ag+ by 4-NP with the subsequent reduction of Ag+ by NaBH4. The progress of the reaction was studied by the UV-vis spectrum of 4-NP in the reaction mixture. 4-NP gives the peak at 400 nm, which diminishes with time as the reaction progresses with the simultaneous growth of a peak of 4-AP at 313 nm. The probe obtained by the Tollens probe reaction with 25 pg of silver per capsule was checked for its catalytic activity. Spectra were obtained with 8 min intervals, and the reaction was completed in approximately 56 min when no more absorbance was registered at 400 nm (Figure 10a). After the reaction mixture was allowed to sit for 2 h, all (23) Leporatti, S.; Voigt, A.; Mitlohner, R.; Sukhorukov, G.; Donath, E.; Mohwald, H. Langmuir 2000, 16, 4059-4063. (24) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247257.
the capsules settled down and could be easily removed from the solution. The integrity of the capsules after the reaction was confirmed by CLSM. It was surprising that the reaction could proceed under these conditions because it is a known fact that it cannot proceed in alkaline solutions with pH values higher than 12.1.24 Indeed, the rate of the reaction was checked for the freshly prepared silver solution with the same contents and surface area as silver which was prepared according to Pradhan et al.24 After the 16 min period, the reaction was stopped and provided 10% yield (Figure 10B). The explanation for the much higher activity of the capsuleentrapped silver may lie in the protective properties of the interpolyelectrolyte complex (PEC),in the acceleration of the reaction by one of the capsule components (PAH or PSS) or by PEC, or in the pH shift inside the multilayers. The addition of these polyelectrolytes to the silver solution did not lead to an increase in the reaction rate; thus, each of them alone does not affect the reaction. The PSS-PAH interpolyelectrolyte complex is unstable at these pH values; therefore, its influence on the reaction could not be checked.25 The capsule components’ influence on the rate, conversion, and mechanism of metal-catalyzed
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reactions is a promising subject and deserves further investigation. Conclusion The investigation of different methods of making silvered particles and empty capsules containing silver in their wall showed high flexibility and the opportunity to produce different types of “customized” catalytic systems. The strength of all presented methods is their simplicity and quickness. Preparation and handling are straightforward and do not require any extraordinary skills or equipment. The catalytic properties can be tuned by the polymeric surrounding of the metallic particles. The stability of these systems is also expected to be much higher in comparison to that of systems with bare metal crystals because of the protection provided by the polymeric matrix. The catalytic properties of the silver capsule system were checked by conducting a silver-mediated reaction of 4-NP reduction. The system investigated was found to possess higher activity than the pure silver solution. Empty shells with encapsulated metal nanoparticles can serve as (25) Here, it should be mentioned that despite the fact that the PSSPAH complex dissolves at pH values higher than 12 due to PAH deprotonation, capsules of the same composition are stable, probably due to the layered polymeric network.
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microreactors to selectively conduct reactions inside the small volume of the capsule. This concept can also be employed for producing polymeric products26,27 readily encapsulated inside the polyelectrolyte capsule. A metallic wall provides additional application options for making use of the conductivity in electrical sensors, microwave absorbance for temperature-switched release, and focusing of electromagnetic waves in optical cavities. We believe that this type of material coupled with recently developed polyelectrolyte capsules, especially with their permeability and mechanical properties, would encourage further investigations on the usage of different types of metals, polymeric matrixes, and latex supports. Acknowledgment. This work was supported by the BMBF as part of its Sofja Kovalevskaja program. We thank Dr. Dinesh Shenoy for the careful reading of the manuscript. LA020052X (26) Mori, T.; Kikuchi, T.; Kubo, J.; Morikawa, Y. Chem. Lett. 2001, 9, 936-937. (27) Sanchez-Cortes, S.; Francioso, O.; Garcia-Ramos, J. V.; Ciavatta, C.; Gessa, C. Colloids Surf., A 2001, 176, 177-184.
