4612
Langmuir 2007, 23, 4612-4617
Synthesis of Silver Nanoparticles for Remote Opening of Polyelectrolyte Microcapsules Darya Radziuk,† Dmitry G. Shchukin,*,† Andre Skirtach,† Helmuth Mo¨hwald,† and Gleb Sukhorukov‡ Max-Planck Institute of Colloids and Interfaces, D14424 Potsdam, Germany, and Department of Materials, Queen Mary, UniVersity of London, E 14 NS London ReceiVed NoVember 24, 2006. In Final Form: December 21, 2006 Silver nanoparticles of 10, 18, and 23 nm were synthesized in aqueous medium by chemical reduction of silver nitrate in excess of sodium borohydride. Modification of polyelectrolyte shells with synthesized silver nanoparticles was performed using the layer-by-layer approach. Remote opening of the polyelectrolyte/silver capsules was performed with a CW Nd:YAG FD laser with an average incident power output up to 70 mW. Capsules with a mixture of 10 and 18 nm silver nanoparticles in its polyelectrolyte shell were ruptured after less than 7 s of laser irradiation, while microcapsules with 23 nm silver nanoparticles in the shell were broken after 11 s of laser treatment and 10 nm silver nanoparticles were broken after 26 s.
Introduction
* To whom correspondence should be addressed. Tel: +49(0)331567-9781. Fax: +49(0)331-567-9202. E-mail: Dmitry.Shchukin@ mpikg.mpg.de. † Max-Planck Institute of Colloids and Interfaces. ‡ University of London.
Silver NPs have been used for surface modification of polyelectrolyte multilayer capsules fabricated by the layer-bylayer (LbL) technique20-22 by alternatingly absorbing oppositely charged polyelectrolytes on colloidal template cores followed by core dissolution.21,23-25 Controlling the capsule permeability allows one to fill its interior with drug or essential chemicals for their delivery into biological tissues. To release the encapsulated compound, the modified polyelectrolyte capsules undergo a microwave,26 ultrasonic,27 laser light,28 and magnetic29b treatment. The presence of inorganic NPs29 and especially silver NPs26 significantly improves the efficiency of the irradiation-stimulated release of the encapsulated compound. The effects of light on biological tissue have been extensively studied in photodynamic therapy to stimulate apoptosis in the targeted cells by a laser,30 in ophthalmology for retrieval of cross-sectional morphological and functional information from superficial regions of biological tissue,31 in examining the optical properties of blood in the visible and near-infrared spectral range,32 etc.
(1) Podsiadlo, P.; Paternel, S.; Rouillard, J.-M.; Zhang, Z.; Lee, J.; Lee, J.-W.; Gulari, E.; Kotov, N. Langmuir 2005, 21, 11915-11921. (2) Prucek, R.; Kvı´tek, L.; Hrba´cˇ, J. Chem. 2004, B 43, 1-5. (3) Riboh, J.; Haes, A.; McFarland, A.; Yonzon, C.; Van Duyne, R. J. Phys. Chem. 2003, B 107, 1772-1780. (4) Malinsky, M.; Kelly, K.; Schatz, G.; Van Duyne, R. J. Am. Chem. Soc. 2001, 123, 1471-1482. (5) Zynio, S.; Samoylov, A.; Surovtseva, E.; Mirsky, V.; Shirshov. Bimetallic layers increase sensitivity of affinity sensors based on surface plasmon resonance. Sensors 2002, 2, 62-70. (6) Hulteen, J.; Treichel, D.; Smith, M.; Duval, M.; Jensen, T.; Van Duyne, R. J. Phys. Chem. 1999, B 103, 9846-9853. (7) Henry, A.; McCarley, R. J. Phys. Chem. 2001, B 105, 8755-8761. (8) Fuller, S.; Wilhelm, E.; Jacobson, J. J. Microelectromech. Syst. 2002, 11, 54-60. (9) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Nano Lett. 2002, 2, 427-430. (10) Creighton, J.; Blatchford, C.; Albrecht, M. J. Chem. Soc. Faraday Trans. 1979, 2, 790-798. (11) Laserna, J.; Cabalin, L.; Montes, R. Anal. Chem. 1992, 64, 81-91. (12) Vlcˇkova´, B.; Solecka´-C ˇ erma´kova´, K.; Mateˇjka, P.; Baumruk, V. J. Mol. Struct. 1997, 408/409, 149-154. (13) Fe´lidj, N.; Aubard, J.; Le´vi, G. Phys. Status Solidi A 1999, 175, 367-372. (14) Feng, Z.; Liang, C.; Li, M.; Chen, J.; Li, C. J. Raman Spectrosc. 2001, 32, 1004-1007. (15) Etchegoin, P.; Liem, H.; Maher, R.; Cohen, L.; Brown, R.; Milton, M.; Galop, J. Chem. Phys. Lett. 2003, 367, 223-229. (16) Copland, J.; Eghtedari, M.; Popov, V.; Kotov, N.; Mamedova, N.; Motamedi, M.; Oraevsky, A. Mol. Imaging Biol. 2004, 6, 341-349. (17) Sokolov, K.; Follen, M.; Aaron, J.; Pavlova, I.; Malpica, A.; Lotan, R.; Richards-Kortum, R. Cancer Res. 2003, 63, 1999-2004. (18) Eghtedari, M.; Copland, J.; Kotov, N.; Oraevsky, A.; Motamedi, M. Lasers Surg. Med. 2004, 164, 16.
(19) Pissuwan, D.; Valenzuela, S.; Cortie, M. Trends Biotechnol. 2006, 24, 62-67. (20) Sukhorukov, G.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V.; Mo¨hwald, H. Polym. AdV. Technol. 1998, 9, 759-767. (21) Donath, E.; Sukhorukov, G.; Caruso, F.; Davis, S.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202-2205. (22) Decher, G. Science 1997, 277, 1232-1237. (23) Sukhorukov, G.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf. A 1998, 137, 253-266. (24) Antipov, A.; Sukhorukov, G.; Fedutik, Y.; Hartmann, J.; Giersig, M.; Mo¨hwald, H. Langmuir 2002, 18, 6687-6693. (25) Skirtach, A.; Antipov, A.; Shchukin, D.; Sukhorukov, G. Langmuir 2004, 20, 6988-6992. (26) Gorin, D.; Shchukin, D.; Mikhailov, A.; Ko¨hler, K.; Sergeev, S.; Portnov, S.; Taranov, I.; Kislov, V.; Sukhorukov, G. Tech. Phys. Lett. 2006, 32, 70-72. (27) Shchukin, D.; Gorin, D.; Helmuth, M. Langmuir 2006, 22, 7400-7404. (28) Antipov, A.; Sukhorukov, G.; Fedutik, Y.; Hartmann, J.; Giersig, M.; Mo¨hwald, H. Langmuir 2002, 18, 6687-6693. (29) (a) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (b) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195-6203. (c) Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187-192. (d) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941-3949. (30) Pitsillides, C. M.; Joe, E. K.; Wei, X.; Anderson, R. R.; Lin, C. P. Biophys. J. 2003, 84, 4023. (31) (a) Huang, D., Swanson, E., Lin, C., Schuman, J., Stinson, W., Chang, W., Hee, M., Flotte, T., Bouma, B., Tearney, J., Eds. Handbook of Optical Coherence Tomography; Marcel Dekker Inc.: New York, 2002. (b) Fercher, A. Optical coherence tomography. J. Biomed. Opt. 1996, 1, 157-173. (32) Gregory, C. A.; Puliafito, J. G. Fujimoto, optical coherence tomography. Science 1991, 254, 1178-1181.
The synthesis and study of silver nanoparticles (NPs) are of a considerable interest in both research and technology. The reason for this interest is the highest conductivity and reflectivity of silver among all metals. Furthermore, silver has pronounced antibacterial properties.1 Optical, magnetic, catalytic, and electrochemical properties of silver NPs are strongly dependent on their size, shape, and media composition.2 This dependence can be practically exploited for development of novel biosensors, chemical sensors, electrooptical devices, and data storage media,3-9 in surface enhanced Raman spectroscopy,10-15 and in biological imaging.16-19
10.1021/la063420w CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007
Synthesis of SilVer Nanoparticles
In this way, as a light sensitive component in the vis/IR region, silver NPs embedded into the capsule wall enhance the interaction of the laser beam with the modified polyelectrolyte shell, to remotely release encapsulated compound from the capsule inside the cell and to decrease the negative effects of the laser beam on biological tissues. The silver layer was formed in situ onto the melamineformaldehyde latex particles precoated with two [poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), PSS/PAH] bilayers. The suspension of PSS/PAH melamineformaldehyde latex particles was mixed with acetaldehyde in an ultrasonic bath followed by slow dropwise addition of freshly prepared [Ag(NH3)2]NO3. Further coating with two additional PSS/PAH bilayers stabilizes the quantity of reduced silver colloids in the polyelectrolyte multilayers. This composite shell is sensitive to laser light with a wavelength of 830 nm resulting in the deformation and rupture of the shell due to the local heating of NPs. This effect allowed one to realize remote release of the encapsulated material from microcapsules inside the living tissue.33 However, according to this method, silver colloids directly reduced on the surface of the coated latex particles are polydisperse and not well-characterized. Moreover, it is difficult to estimate the exact amount of reduced silver on the capsule surface. In that way, there is no information about the distribution of silver NPs in the capsule shell. Because living tissues are sensitive to monochromatic coherent radiation, the selection of the minimal values of laser intensity and its time duration is an important issue for remote opening of the drug-loaded capsules inside tissues. In the case of controllable size (nm) and monodispersity of silver, the process of interaction between laser beam and capsule shell becomes less invasive for biological objects.33 It is important to develop a simple and time-saving procedure for the synthesis of silver NPs without organic stabilizers recharging the NP surface and influencing the multilayer structure of the capsule shell. There are two basic methods to synthesize colloidal particles: dispersion and condensation. The main idea of dispersion methods is to fragment the particles to nanometer size, for example, by mechanical grinding of an initial material (mechanical method), dispersion of the substance in liquid under the action of rapidly changing compression, and expansion of the system (ultrasonic method).34 Employing dispersion methods, it is possible to synthesize silver colloids with a size of hundreds of nanometers. Condensation methods, in turn, which are based on chemical reduction, oxidation, mutual exchange reaction, and hydrolysis, result in NPs of much lower size. The idea of this approach is in the reaction between corresponding precursor salts. The most developed condensation method producing inorganic NPs is chemical reduction of metal ions from its salt in different solvents by inorganic (sodium borohydride, hydrogen, and hydrazine) or organic (citric acid,35 ascorbic acid, formaldehyde, and reducing sugars) reducing agents. A simple and widely used method for silver formation is chemical reduction by sodium borohydride proposed by Creighton.9 Among the investigated factors influencing the resulting silver colloid are temperature,36 the presence of surfactants and nonsaturated carboxylic acids,37 and the way and velocity of (33) Skirtach, A.; Javier, A.; Kreft, O.; Ko¨hler, K.; Alberola, A.; Mo¨hwald, H.; Parak, W.; Sukhorukov, G. Angew. Chem., Int. Ed. 2006, 45, 4612-4617. (34) Boldyrev, A. Physical and Colloidal Chemistry, 2nd ed.; High School: Moscow, 1983; Vol. 408, pp 284-294. (35) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55-75. (36) Van Hyning, D.; Zukoski, C. Langmuir 1998, 14, 7034-7046. (37) Pal, T.; Sau, T. K.; Jana, N. R. J. Colloid Interface Sci. 1998, 1, 30-36.
Langmuir, Vol. 23, No. 8, 2007 4613
stirring.38 The Creighton procedure can produce silver NPs from several to 20 nm. Particles with a size more than 20 nm can be synthesized by the Schneider approach.39 The main idea of the presented paper is to fabricate silver NPs with controllable sizes for further embedding into the polyelectrolyte layers of the capsule shell and remote opening of the capsules by laser irradiation. For synthesis of silver NPs, the Creighton method was chosen. Variation of the volume or molar ratios of silver nitrate to sodium borohydride in aqueous media was employed as a control parameter to produce silver NPs of desired sizes. Experimental Section Materials. Silver nitrate (99.8%), sodium borohydride (98%), PSS (70 kDa), PAH (70 kDa), and sodium chloride were purchased from Aldrich (Germany). Polystyrene (PS) cores were produced by Microparticles GmbH (Germany). The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ cm-1. Preparation of Silver NPs. Silver NPs were synthesized by chemical reduction of silver nitrate using sodium borohydride as a reducing agent in aqueous solution without organic stabilizers. The process was carried out in 0.5 and 1 L flasks prewashed in concentrated nitric acid. The remains of the acid were removed from the glass walls by abundant amounts of deionized water. A 1 mM concentration of AgNO3 solution (room temperature) was mixed with fresh, ice-cold sodium borohydride solution of different concentrations under vigorous stirring. In the first 20 s, the mixture turned bright yellow. After complete injection (less than 2 min) of silver nitrate solution, stirring was stopped immediately and the final color of the mixture changed to dark yellow-bright brownish. The reaction mixture was kept in darkness to avoid the influence of daylight. The synthesis was conducted by varying volumes or molar concentrations of the reagents. A 1 mM concentration of AgNO3 and 1 mM NaBH4 were taken in amounts according to the ratios 1:1, 1:2, 1:3, and 1:6 and 2:1, 3:1, and 6:1. For a 1:3 ratio, the concentration of sodium borohydride was varied from 2 to 8 mM. Fabrication of Multilayer Microcapsules Coated with Silver NPs. Modification of polyelectrolyte shells with silver NPs was performed using the LbL approach.22,23,40 A 200 µL amount of PS particles was mixed with 500 µL of 2 mg/mL PAH in 0.5 M NaCl. Fifteen minutes of incubation was followed by triple washing with water and centrifugation. Particles coated with one PAH layer reversed their charge from negative to positive and could then be coated by PSS by adding 500 µL of 2 mg/mL PSS in 0.5 M NaCl in a similar manner. The adsorption cycle was repeated until four (PAH/PSS) bilayers were formed. Then, the next PSS layer was changed to negatively charged silver NPs incubating PS-(PAH/ PSS)4 microparticles in as-prepared silver colloid till saturation adsorption was achieved. Afterward, the initial PAH/PSS coating procedure was continued. Hollow capsules with (PAH/PSS)4 + PAH/ silver NPs/PSS + (PAH/PSS)2 structure were obtained after dissolution of PS cores in 0.1 M tetrahydrofuran (THF) and washing by water. Remote Opening of the Polyelectrolyte Capsules. Remote opening of the polyelectrolyte/silver capsules was performed using a homemade laser setup equipped with a CW Nd:YAG FD laser with an average incident power up to 70 mW. The collimated laser beam was focused onto the sample through a microscope objective, 100× magnification. The solution containing the capsules was deposited onto a microscope slide under the microscope objective. The sample was positioned under the laser beam by a micrometer resolution XYZ stage. Illumination was made in (38) Kapoor, S. Langmuir 1998, 14, 1021-1025. (39) Schneider, S.; Halbig, P.; Graen, H.; Nickel, U. Photochem. Photobiol. 1994, 60, 605-610. (40) Sukhorukov, G. In Studies in Interface Science; Mo¨bius, D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; Vol. 11, pp 383-384.
4614 Langmuir, Vol. 23, No. 8, 2007
Radziuk et al.
Figure 1. UV-vis absorption spectra of silver NPs fabricated in triple volumes of 2 and 3 mM sodium borohydride (1 mM AgNO3:2 mM NaBH4; molar ratios of 1:2 and 1:3) (a) and 4, 5, 6, 7, and 8 mM sodium borohydride (molar ratios of 1:4, 1:5, 1:6, 1:7, and 1:8) (b). TEM image of 10 and 18 nm silver NPs (molar ratio of 1:3, triple volume of sodium borohydride) (c). TEM image of 33 nm silver NPs (molar ratio of 1:8, triple volume of sodium borohydride) (d).
transmission mode using a 150 W light source, and the images were recorded by a charge-coupled device camera connected to a computer. The laser intensity was measured by a Newport-1830C power meter. Characterization. To characterize the size of silver NPs, a Zeiss EM 912 Omega transmission electron microscope (TEM), a Varian CARY 50 Conc UV-vis spectrophotometer, and a high-performance particle sizer (Malvern Instruments) for dynamic light-scattering (DLS) measurements of particle size distribution were employed. The ζ-potential of silver solutions was measured using a zeta sizer (Malvern Instruments). All measurements were carried out at room temperature. The original structure, surface, and inner composition of polymeric microcapsules with embedded silver NPs were investigated via TEM or a Gemini Leo 1550 scanning electron microscope (SEM).
Results and Discussion Silver NPs (23 ( 3 nm) were formed by the reduction of 1 mM silver nitrate (4 mL) with ice-cold 7 mM sodium borohydride (12 mL) at ambient conditions. The UV-vis absorption spectrum of the obtained silver NPs is shown in Figure 1b. There is an absorption peak near 400 nm, corresponding to the characteristic
wavelength of silver absorbance.2-4 The narrow absorption peak indicate a high level of monodispersity of the silver NPs. The size of silver NPs was changed by varying the volume ratio between silver nitrate and sodium borohydride solution. First, the excess of silver nitrate was varied at 2:1, 3:1, and 6:1 (1 mM silver nitrate to 1 mM sodium borohydride). During the first 10 s, the silver solution became light yellow, brown, and, finally, gray. All samples had a negative ζ-potential at around -20 mV. The size of silver NPs prepared at excess of silver nitrate was in the range of 55 ( 5 nm for all three samples. The sediment of silver NPs appeared in all silver solutions in 2 weeks. The excess of sodium borohydride was varied at volume ratios of 1:2, 1:3, and 1:6 (1 mM silver nitrate to 1 mM sodium borohydride). After the addition of silver nitrate into sodium borohydride solution, the color of the final solution changed to light yellow at 1:2, dark yellow at 1:3, and light brown at 1:6. The ζ-potential of silver NPs prepared at the above-mentioned volume ratios was negative, and its value was in the range from -30 to -40 mV. DLS showed a size of silver NPs in the range
Synthesis of SilVer Nanoparticles
Langmuir, Vol. 23, No. 8, 2007 4615
Figure 2. Size of resulting NPs prepared at various volume and molar ratios of the precursor materials.
Figure 3. Fabrication of polyelectrolyte/silver microcapsules by the LbL technique. PS particles (a) were coated with composition of poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH)4 (b). Preformed silver NPs were adsorbed onto the (PAH/ PSS)4 + PAH multilayer (c). Final (PAH/PSS)4 + PAH/Ag/PSS + (PAH/PSS)2 polyelectrolyte capsules after removal of PS cores in 0.1 M THF (d).
of 15 ( 3 nm for all three samples. In addition, no sediment of silver particles was noticed after 2 weeks of aging. When silver nitrate was added to aqueous sodium borohydride solution, silver solsssmall clusters of reduced atomic silvers were initially produced. The silver solution usually becomes slightly yellow as the synthesis of silver NPs with the size of less than 10 nm takes place. It turns into dark yellow and light brown, when the concentration of silver colloidal particles is too high, interactions between the stabilizer chain increase, and flocculation takes place. A gray color of silver solution reveals silver aggregates and their further sedimentation in the solution. It is known that the borohydride undergoes a side reaction with water producing borate anions. The borohydrides (BH4-) and borates (BO33-) can temporarily stabilize silver sols by adsorption onto the surface and provide a substantial electrostatic barrier to aggregation.41,42 Monodisperse and stable for at least several weeks, silver NPs of small size might be formed with a more concentrated solution of sodium borohydride. According to the above-mentioned suggestions, the molar ratio of silver nitrate to sodium borohydride at a fixed volume ratio of 1:3 was varied as the next parameter. The molar concentration of silver nitrate remained constant (1 mM), and that of sodium borohydride was varied from 1 to 8 mM. First, 2 mM sodium borohydride was added to silver nitrate. The UV-vis absorption spectrum (Figure 1a) has two peaks. The main peak is near 400 (41) Komat, P.; Flumiani, M.; Hartland, G. J. Phys. Chem. 1998, B 102, 3123. (42) Bell, W.; Myrick, M. J. Colloid Interface Sci. 2001, 242, 300-305.
Figure 4. TEM images of polyelectrolyte capsules with 23 nm silver NPs, reduced in a triple volume of sodium borohydride at a molar ratio of 1:7.
nm. The second one is near 430 nm. If 3 mM sodium borohydride was added to fresh silver nitrate solution, the same two peaks appeared in the visible region. UV-vis spectra (Figure 1a), TEM (Figure 1c), and DLS measurements revealed the presence of silver NPs of two sizess10 ( 2 and 18 ( 3 nm. Increasing the concentration of sodium borohydride up to 8 mM results in the appearance of only one absorption peak at 400 nm (Figure 1b). On the basis of the TEM image and DLS measurement data, the size of particles was estimated to be 33 ( 5 nm (8 mM sodium borohydride). The TEM image of silver NPs reduced by 8 mM sodium borohydride is shown in Figure 1d. It is known that silver NPs have a characteristic absorption maximum at 380-430 nm, which makes it easy to be detected by UV-vis spectroscopy. The appearance of the characteristic absorbance maximum at ca. 400 nm confirms that the elemental Ag0 NPs can be readily synthesized. The UV-vis spectra, TEM, and DLS measurement data above-mentioned indicate that the particle size of silver NPs thus formed can be controlled by the
4616 Langmuir, Vol. 23, No. 8, 2007
Radziuk et al.
Figure 6. Rupture yield of microcapsules with three different types of silver NPs (10, 23, and the mixture of 10 and 18 nm) at various irradiation times.
Figure 5. Five micromolar polyelectrolyte capsules with 23 nm silver NPs, reduced in a triple volume of sodium borohydride at a molar ratio of 1:7 before laser irradiation (a), under laser irradiation from 1 to 5 s (b), and from 6 to 11 s (c).
initial amount molar ratio and that increasing the amount or molar ratio (AgNO3:NaBH4) leads to the production of monodisperse silver NPs with sizes in the range from 10 to 33 nm (Figure 2). Micrometer-sized PAH/PSS capsules modified with preformed silver NPs were produced using the LbL technique22 (Figure 3). The shell fabrication involves stepwise deposition of polyelectrolytes from dilute aqueous solutions onto monodisperse PS colloidal particles (Figure 3a). The first stage of polyelectrolyte multilayer film formation began with adsorption of the positively charged polycation (PAH) onto the negatively charged PS particles. The second stage involved the adsorption of the negatively charged polyanion (PSS) onto the positively charged (with PAH) surface of PS particles. The procedure of stepwise adsorption of oppositely charged polyelectrolytes was repeated until four bilayers of (PAH/PSS) were formed (Figure 3b). Silver NPs were adsorbed after the (PAH/PSS)4 + PAH step. The adsorption was repeated five times to form a dense saturated
layer of silver NPs onto the (PAH/PSS)4 + PAH multilayers. After silver deposition, the PSS layer was adsorbed (Figure 3c). For optimum coating of silver NPs in the polyelectrolyte shell, two more bilayers of PAH/PSS were deposited. The final (PAH/ PSS)4 + (PAH/Ag/PSS) + (PAH/PSS)2-coated particles were exposed to 0.1 M THF two times for core dissolution accompanied by triple washing with water (Figure 3d). TEM (Figure 4) analysis showed dark and rough capsule shells containing silver NPs as compared to pure polyelectrolyte ones. It can be seen in Figure 4 that the silver NPs are homogeneously distributed on the whole shell of the capsules. Microcapsules, containing three different types of silver NPs reduced by a triple amount of sodium borohydride (23 nm prepared from 1 mM silver nitrate + 7 mM sodium borohydride; 10 nm prepared from 1 mM silver nitrate + 3 mM sodium borohydride; the mixture of 10 nm and 18 nm NPs inside the polyelectrolyte shell) were irradiated by CW Nd:YAG laser (532 nm). The irradiation time of the laser beam (average incident power, 70 mW) on all samples was different for each type of capsule. The shell rupture and deformation of capsules were observed for all types of silver-containing capsules. However, no effect was observed during laser irradiation on the capsules without silver NPs in the polyelectrolyte shells, which coincides with our previous studies.25 Figure 5a shows a microcapsule (5 µm) with 23 nm silver NPs inside the polyelectrolyte shell before laser irradiation. The capsule remains intact after 5 s of irradiation (Figure 5b). In the next 6 s of irradiation, the capsule shell ruptured (Figure 5c). Because the silver NPs were uniformly distributed in the polyelectrolyte shell, capsules from the same solution were either deformed or ruptured in general after 10 ( 2 s of laser irradiation (Figure 6). Polyelectrolyte capsules of 10 µm embedded with the mixture of silver NPs (10 and 18 nm) were completely destroyed after 6 ( 1 s of laser irradiation. Five micrometer polyelectrolyte shells with 10 nm silver NPs ruptured only in the range of 22 ( 5 s of laser irradiation (Figure 6). Silver NPs act as absorption centers of incident laser irradiation inside the polyelectrolyte shells. It was reported that upon the interaction with a laser beam of constant intensity, metal NPs with larger sizes are heated faster than those of smaller sizes.43 In such a way, the degree and speed of deformation and/or rupture of polyelectrolyte shells with silver NPs inside them depend on the interaction between silver NPs themselves and their distribu(43) Skirtach, A.; Dejugnat, C.; Braun, D. Nano Lett. 2005, 5, 1371-1377.
Synthesis of SilVer Nanoparticles
tion on and in the surface of these shells. Silver NPs completely deposited and uniformly distributed inside the polyelectrolyte shells. Because of electrostatic repulsion (Coulomb repulsion), silver NPs with the smaller size could penetrate polyelectrolyte multilayers locating throughout the whole polyelectrolyte shell, while more massive silver NPs with larger sizes are basically located on the previous polyelectrolyte layer. Silver NPs with a larger radius are more responsible for absorption and local heating.43 Silver NPs of smaller radius due to their distribution in polyelectrolyte multilayer and interaction both with large silver NPs and with laser beam might be responsible for the distribution (penetration) of this heating within the polyelectrolyte multilayer. Thus, silver NPs of both small and large sizes have to be embedded into polyelectrolyte shells for their more effective rupture during laser treatment. In that way, the size control of silver NPs allows one to influence the speed of capsules deformation and/or rupture. The laser beam could negatively affect not only polyelectrolyte capsules but also the irradiated biological object. Polyelectrolyte capsules coated with silver NPs of desired and controllable sizes allow one to control a dose of energy necessary to remotely destroy them. In this way, a short-time operation minimizes the negative influence of laser irradiation on the biological tissues. In conclusion, silver NPs were prepared as a result of a silver nitrate reduction by sodium borohydride. Varying the volume and molar ratios of silver nitrate and its reducing agent, one can obtain silver NPs of 10, 18, and 23 nm diameters. Silver NPs
Langmuir, Vol. 23, No. 8, 2007 4617
of desired size were electrostatically adsorbed onto the polyelectrolyte shell of the capsule. We demonstrated the possibility to reduce the time of laser-induced opening of polyelectrolyte capsules by introducing the mixture of small (10 nm) and large (18 nm) silver NPs into a polyelectrolyte shell. Accordingly, silver NPs with a larger radius mostly absorb laser irradiation and are strong locally heated. Silver NPs with smaller sizes are mainly responsible for the transition of the absorbed laser energy that in cooperation with large particles leads to short-time rupture of the polyelectrolyte shell. The use of monodisperse small (10 nm) and large (23 nm) silver NPs embedded along in the polyelectrolyte shell leads to a large irradiation time necessary for capsule rupture (10 s for 23 nm and 22 s for 10 nm). Preformed silver NPs of fixed and desired sizes as shell constituents allow one to influence the parameters of laser irradiation (time and intensity) in order to minimize its negative effect on the media surrounding the polyelectrolyte capsule (e.g., biological tissue) during laser-induced remote opening of the polyelectrolyte capsules uptaken by a biological tissue. Acknowledgment. We thank Rona Pitschke for electron microscopy analysis. This work was supported by FP6 EU Project, STREP-NMP3-CT-2005-516922 “Selectnano”, FP6 EU Project “Nanocapsule”, and STREP-NMP3-CT-2006-033410 “MatSILC”. LA063420W
