Fabrication and Investigation of Polyelectrolyte Capsules with Gold and Silver Nanoparticles in the Shell

I.V. Marchenko,1,2 T.V. Bukreeva,1 Yu.V. Grigoriev,1 G.S. Plotnikov2 1 Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiy pr. 59, Moscow, 119333 Russia. 2 Moscow State University, Leninskie gory, Moscow, 119992 Russia.

Abstract. Polyelectrolyte capsules with silver and gold nanoparticles in the shell composition have been obtained using calcium carbonate and polystyrene microparticles as cores. Capsules were modified with silver nanoparticles using silver mirror reaction. Gold nanoparticles were embedded in capsule shells via their adsorption from previously obtained sol. A significant difference in the structure of capsules obtained on cores of different types has been shown by atomic-force and transmission electron microscopy.

Introduction Metal nanoparticles with plasmon resonance [1, 2] have unique properties and are widely used in various

nanotechnologies, which have been intensively developed during the last decade [3]. Functionalization of the surface of metal nanoparticles with organic molecules [4], which is sometimes performed even in the stage of nanoparticle synthesis, makes possible their wide practical application. At the same time, the properties of metal nanoparticles that are not stabilized with organic ligands can be changed after preparation, for example, through deposition of various inorganic or organic materials on their surface. For practical applications, it is of interest to form composites from polymers and metal nanoparticles, in which a polymer matrix can stabilize particles, preventing their aggregation, and serve as a protective shell against the environment. Metal nanoparticles can be embedded not only in bulk materials and thin films but also in walls of polymer capsules [5–8]. Currently, polymer microcapsules are used in pharmaceutical, cosmetic, food, textile, and agricultural industries. Polyelectrolyte nano- and microcapsules, due to their monodispersity in a wide range of sizes, simplicity of permittivity control, and possibility of easily changing the shell material and choosing it in a wide range, became a promising technological object. The technique of formation of such capsules is successive adsorption of oppositely charged polyelectrolyte macromolecules on the surface of colloidal particles (cores) of various nature. When a capsule is formed, the core is removed, generally through dissolution.

Introduction of metal nanoparticles into the walls of polyelectrolyte capsules can be used for optical absorption due to plasmon resonance for temperature release of the capsule content, in particular, for address drug delivery. For release of a drug in a body absorption band of nanoparticles should be in the infrared part of the spectrum because such radiation is not adsorbed by cells. The frequency of the plasmon resonance depends on the size, aggregation and the distance between the nanoparticles. Therefore the position of the absorption band of nanoparticles can be adjusted by obtaining in the capsule shell nanoparticles with different sizes and distribution.

In this study, we propose different methods for embedding gold and silver nanoparticles in the walls of polyelectrolyte capsules and compare nanocomposite capsules obtained on the basis of different cores.

Experimental Materials

We used the following materials: sodium polystyrene sulfonate (PSS) with MM = 70000; acetaldehyde; sodium chloride; calcium chloride; sodium carbonate; polyallylamine hydrochloride (PAH) with MM = 70000; trisodium salt of ethylenediamine tetraacetic acid (EDTA) (Sigma Aldrich, Germany); tetrahydrofurane (Irea2000, Russia); ammonium hydroxide (Fluka, the United States); silver nitrate. In the work gold sol, prepared by Dr. O. Dementyeva in Institute of Physical Chemistry, was used (concentration of 1.3 ⋅ 1012 particles per ml, size 18 ± 2 nm). The water for experiments was purified using a Milli-Q Plus 185 system (Millipore, the United States).

Fabrication of polyelectrolyte capsules As cores for capsules, we used polystyrene particles 4.4 μm in diameter (concentration of COOH groups

4.23 μg equ/g) (Diafarm, St. Petersburg, Russia) and spherical calcium carbonate particles, prepared by the technique described below. Amorphous CaCO3 precipitate, which is formed (as a result of colloidal aggregation) during rapid mixing of CaCl2 and Na2CO3 solutions, is transformed into ordered spherulites of micrometer size.

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MARCHENKO ET AL.: POLYELECTROLYTE CAPSULES WITH Au AND Ag NANOPARTICLES

Varying the process conditions (concentration of reagents, temperature, and intensity and duration of mixing the reaction mixture), one can obtain microspherulites with an average diameter from 2 to 15 μm and sufficiently small size spread. In this study we obtained spherulites with an average diameter of 4.5 μm. To prepare polyelectrolyte shells, we used the technique described in [9]. 2 ml of PAH solution (concentration 3 mg/ ml ) in a 0.5-M NaCl solution were added to 0.015 g of cores. The suspension obtained was stirred for 15 min using a minishaker and centrifugated for 1 min with an acceleration of 330 g, after which the substrate was taken off, and the particles were triple washed with water (deposition by centrifugation, 330 g, 1 min). Then the same procedure was performed using the PSS solution with a concentration of 3 mg/ml in a 0.5-M NaCl solution. Furthermore, using the technique of alternate adsorption of oppositely charged macromolecules on colloidal particles, we obtained a shell composed of 8–16 polyelectrolyte layers. To prevent aggregation of particles during deposition of the first two layers, the suspension was subjected to ultrasonic mixing (test tubes with the suspension were placed for 10 s in an ultrasonic bath, 35 kHz). Hollow polyelectrolyte shells—permeable capsules— were prepared by removing CaCO3 cores with addition of the EDTA salt [10]. This procedure leads to removal of calcium from a capsule due to the formation of a stable complex of this metal with EDTA. Polystyrene cores were dissolved in tetrahydrofurane.

In this study, we used two methods for embedding metal nanoparticles in the polyelectrolyte shell of capsules: reduction of metal ions on the capsule shell and adsorption of nanoparticles from a previously obtained sol. Gold nanoparticles adsorbed on the capsule shell due to their electrostatic interacrion with oppositely charged polyelectrolyte molecules. After adsorption of 9 polyelectrolyte layers (PAH/PSS)4PAH on their surface, these particles were placed for 15 min in a colloidal solution of spherical gold nanoparticles, which was stirred by a minishaker (1000 µl of gold sol per ~108 microparticles).

Using reduction of ions adsorbed on the polyelectrolyte capsule shell, we obtained silver nanoparticles. In this case silver mirror reaction was used. The second approach is based on the chemical reduction of metal during acetaldehyde oxidation:

AgNO3 + NH4OH = AgOH + NH4NO3

AgOH + 2NH4OH = [Ag(NH3)2]OH + 2H2O

2[Ag(NH3)2]OH + CH3CHO = 2Ag + CH3COONH4 + 3NH3 + H2O

Methods of investigation Microparticles were visualized using a light optical microscope (a component of a Leica TCS SP confocal

system (Germany)), equipped with a 100* immersion objective having a numerical aperture of 1.4. The size and shape of silver nanoparticles in capsule shells, as well as gold nanoparticles in synthesized hydrosols, were determined using Phillips EM300 (the Netherlands) and Phillips CM12 (FEI, the United States) transmission electron microscopes. The distribution of gold nanoparticles in a polyelectrolyte matrix was studied by semicontact atomic force microscopy (AFM). The surface image of a capsule shell in air was obtained on a Solver BIO scanning probe microscope (NT-MDT, Russia).

Results and Discussion The systems obtained using calcium carbonate and polystyrene particles as capsule cores, with reduction of

silver ions on the polyelectrolyte capsule shell, are significantly different. CaCO3 microspherulites have a porous surface and adsorb well different types of compounds [10]. Since the polyelectrolyte shell is permeable for inorganic ions, Ag + ions adsorb not only on the surface of the oppositely charged polyelectrolyte layer but also within the polyelectrolyte shell, as well as in the bulk of pores, along with polyelectrolyte molecules. Therefore, we can suggest that, after ion reduction and core dissolution, the shell looks like a loose sponge containing a large number of silver nanoparticles. This conclusion is confirmed by the AFM data (Fig. 1). Dried capsules are oblate spheres, whose height is 170–200 nm and the diameter is roughly two times smaller than that of capsules in the solution. Upon capsule drying, metal nanoparticles are likely to become immersed in the polymer matrix. In contrast to calcium carbonate particles, polystyrene cores have a fairly smooth surface, due to which a thinner and denser polymer layer is formed during polyelectrolyte adsorption. Due to these properties of a polyelectrolyte shell, nanoparticles are formed mainly on its surface, and, apparently, this is the cause of the smaller number of nanoparticles and their larger size in comparison with silver nanoparticles on CaCO3 cores.

TEM images (Fig. 2) show that in the shell obtained on polystyrene cores smaller amount of larger nanoparticles is formed than in the case of CaCO3 cores. For a shell formed on polystyrene cores the sizes of silver particles are up to 300 nm. In the case of CaCO3 cores their size is up to 70 nm. It was revealed in [10] by the gas adsorption method that the diameter of pores in such calcium carbonate microspherulites is 20–70 nm. That is their size is limited by pore sizes.

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MARCHENKO ET AL.: POLYELECTROLYTE CAPSULES WITH Au AND Ag NANOPARTICLES

In Fig. 3 capsules with silver nanoparticles, for which the silver mirror reaction was carried out for different time, are shown. As it can be seen, when the reaction time changed from 7 minutes to 1 hour, the amount of silver nanoparticles in the shells increases.

(a) (b) Figure 1. AFM images of capsules formed on polystyrene (a) and CaCO3 (b) cores with shells containing silver nanoparticles obtained by the silver mirror reaction.

(a) (b) Figure 2. TEM images of capsules formed on polystyrene (a) and CaCO3 (b) cores with shells containing silver nanoparticles obtained by the silver mirror reaction.

7 min 25 min 60 min Figure 3. TEM images of capsules formed on polystyrene cores with silver nanoparticles obtained with different time of the silver mirror reaction.

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MARCHENKO ET AL.: POLYELECTROLYTE CAPSULES WITH Au AND Ag NANOPARTICLES

(a) (b) Figure 4. TEM images of capsules formed on polystyrene (a) and CaCO3 (b) cores with adsorbed gold nanoparticles.

According to the TEM data (Fig. 4), a larger number of gold nanoparticles are adsorbed on the shell of a capsule formed on a CaCO3 core (Fig. 4a) in comparison with a polystyrene core (Fig. 4). Apparently, as in the case of reduction of silver ions on a polyelectrolyte shell, this effect is due to the porous structure of CaCO3 cores, i.e., gold nanoparticles, along with polyelectrolyte macromolecules, can be adsorbed in pores.

Conclusions Different approaches can be used to fabricate polyelectrolyte microcapsules with silver and gold

nanoparticles in the shell. Depending on the type of the core that serves as the basis for capsule formation, the structure and properties of the shells formed can be radically different. When polyelectrolyte shells are obtained on calcium carbonate microspherulites, both reduction of silver ions and adsorption of gold nanoparticles from a previously synthesized sol lead to filling of core pores with nanoparticles. As a result, after core dissolution, the capsule shell is a loose polymer sponge with a large number of built-in metal nanoparticles. In the case of reduction of adsorbed silver ions, the nanoparticles formed in the shell are strongly polydisperse. The use of polystyrene microparticles as capsule cores, in contrast to CaCO3 cores, leads to the formation of a smaller number of larger silver nanoparticles upon reduction of silver ions adsorbed on a shell. For a shell formed on polystyrene cores the sizes of silver particles is up to 300 nm and in the case of CaCO3 cores their size is up to 70 nm. Adsorption of nanoparticles from a previously synthesized hydrosol made it possible to obtain nanocomposite polyelectrolyte capsules with monodisperse nanoparticles with uniform surface distribution. Such objects are promising for designing tools to deliver drug with remote control of capsule content release.

Acknowledgments. This study was supported byRussian Foundation for Basic Research (project 08-03-00921-a).

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