Synthesis and characterization of the superparamagnetic iron oxide nanoparticles modified with cationic chitosan and coated with silica shell

Highlights

• The new, facile methodology for synthesis of silica covered SPIONs is proposed.

• The SPIONs was modified with cationic chitosan and coated with silica shell.

• Negatively charged, rounded in shape particles of ca. 330 nm were obtained.

• The product exhibits the superparamagnetic properties.

• The product properties imply its potential applications in biomedicine areas.

Abstract

Novel method for synthesis of superparamagnetic iron oxide nanoparticles (SPION) modified with a cationic chitosan (CCh) and coated with a silica shell, SPION-CCh-SiO₂ was developed. The process was carried out in two steps. In the first step the chitosan coated SPIONs were obtained by co-precipitation of Fe²⁺ and Fe³⁺ with ammonium hydroxide in aqueous solution of CCh. In the second one, the silica shell is formed on their surfaces. The formation of SPION-CCh-SiO₂ was achieved by direct decomposition of tetraethoxysilane (TEOS) adsorbed on a surface of SPION-CCh dispersed in aqueous phase under sonication and mechanical stirring at room temperature. The chemical composition and physicochemical properties of the materials were determined using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Dynamic Light Scattering (DLS) and zeta potential measurements. The morphology of the particles was evaluated by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Magnetic properties were confirmed using Atomic Force Microscopy/Magnetic Force Microscopy (AFM/MFM) and magnetization measurements. The resulting products are negatively charged, rounded in shape and exhibit the superparamagnetic properties what implies their potential applications in engineering and biomedicine areas.

Introduction

Recently, magnetite (Fe₃O₄) nanoparticles (NPs) have attracted considerable attention because of their biocompatibility and remarkable magnetic properties [1], [2], [3]. Fe₃O₄ magnetic nanoparticles have an inverse spinel structure, which exhibit superparamagnetism with high saturation magnetization. These superparamagnetic iron oxide nanoparticles (SPION) can find wide range of potential applications in various areas such as: magnetic separation, magnetic resonance imaging, cell labeling, and targeted drug delivery [4], [5], [6], [7]. Unfortunately, the practical applications of pristine SPIONs is hindered by their tendency for aggregation with formation of large clusters and rapid biodegradation when they are exposed in a biological systems. Coating the Fe₃O₄ NPs with an inert host material, or fabrication of inert host particles containing the magnetic particles are considered as possible methods which can be applied to avoid these limitations. Various inorganic materials have been reported to form stable shells on nano/microparticles [8], [9] and among them silica is one of the most popular. This is due to its desirable properties, including non-toxicity, ease of fabrication, stability in most chemical and biological environments, and what is also very important - minimal residual magnetism [10], [11], [12]. Silica improves biocompatibility and provides a negative surface charge under physiological pH, imitating most biological species. The silanol groups make the surfaces of particles lyophilic, thus enhancing the stability for their suspensions even during changes in pH or electrolyte concentration [13]. Additionally, nanostructures derived from silica-coated SPIONs could further be directed to a specific target via the antibody-antigen recognition by conjugation of antibodies to the outer silica surface [14].

Up to date, many methods have been reported for coating of SPIONs with silica, including sol–gel process, microemulsion, and solution reaction [15], [16], [17]. According to Santra et al. [18], magnetite/silica particles can be prepared by reverse-micelle microemulsion method in organic phase such as cyclohexane and heptane. Barnakov et al. performed silica coating of magnetite nanoparticles dispersed in toluene in the presence of surfactants [19]. Zhang et al. [20] used an in situ reverse microemulsion micelle synthesis method to generate silica-coated magnetite particles dispersed in an organic solvent. Woo et al. [21] employed a technique of magnetite treatment with toluene, ammonium hydroxide, and ethanol, followed by the addition of tetraethoxysilane for 1 day. The solution was then heated to 150 °C for 1 h and cooled before 3 aminopropyl-trimethoxysilane was added and again the solution was heated to 150 °C for 1 h and subsequently cooled down to room temperature. The silica-coated magnetite particles with surface propylamine functional groups were obtained as the reaction product. Wang and co-workers examinated the preparation of Fe₃O₄/silica composite microspheres via miniemulsion polymerization by injecting tetraethoxysilane, Triton X-100, and water into ferrofluid of Fe₃O₄ NPs [22] and submicron-size Fe₃O₄/silica core/shell microspheres by sol–gel method [23]. In those syntheses considerable amounts of the organic solvents and surfactants were employed which is not desired from environmental point of view.

Very popular and widely described in literature is the silica-coating method based on the use of sodium silicate solution. Liu et al. presented the procedure for preparation of silica-coated magnetite nanoparticles by addition of an aqueous HCl solution to an aqueous mixture of the sodium silicate and magnetite nanoparticles [24]. Whereas Sun et al. [25] and Correa-Duarte et al. [26] in their papers described another, two step silica-coating method with the use of sodium silicate solution as well. They employed slow silica deposition in water from the sodium silicate solution, and as a next step the extensive growth of the silica shells through sol–gel reaction in ethanol/ammonia mixtures was carried out.

The another method for synthesis of silica-coated magnetite nanoparticles is based on Stöber synthesis and various approaches have been employed for that purpose. Chen et al. [27] proposed the way for the fabrication of monodisperse and air-stable Fe₃O₄/silica microspheres. They performed a direct decomposition of tetraethoxysilane in solution in the presence of freshly synthesized Fe₃O₄ nanoparticles. Sun et al. synthesized the Fe₃O₄/SiO₂ NPs [4] by employing a combination of the mechanical stirring and ultrasonication assisted Stöber synthetic method. The resulted product was well dispersible and exhibited enhanced magnetization saturation. Yang and co-workers also prepared the Fe₃O₄/silica composite NPs: they introduced Fe₃O₄ NPs into the Stöber process after formation of the primary silica particles [28]. Kobayashi et al. proposed a method for producing silica-coated magnetite nanoparticles in the presence of magnetite nanoparticles surface-modified with a silane coupling agent, carboxyethylsilanetriol. This modification was done to increase the affinity of the magnetite particles to form silica on their surface. The synthesis of the surface-modified particles was performed in the presence of magnetite colloid in aqueous sodium salt solution of the carboxyethylsilanetriol at 70 °C. The product was washed with water and ethanol, centrifuged and sonicated. In the third step the fabrication of the magnetite/silica composite particles was carried out according to the Stöber method with using tetraethoxysilane. They obtained silica particles containing multiple magnetite nanoparticles [29].

In our previous paper [30] we have presented the approach based on the modified Stöber process, in which spherical silica particles doped with iron oxide were synthesized via base-catalyzed one-pot process using tetraethoxysilane (TEOS) and iron(III) ethoxide (ITE) as co-precursors. Depending on the concentration of ITE in the starting composition, materials of various morphologies were obtained. Using magnetic force microscopy (MFM), magnetic susceptibility and magnetization measurements it was demonstrated that resulted materials have magnetic properties. It might be concluded that in comparison with the microemulsion method, the approach based on Stöber synthesis is simple and seem to be more environmentally friendly.

In the current paper we report on the new, facile and simple approach for the synthesis of the superparamagnetic iron oxide nanoparticles (SPION) modified with cationic chitosan (CCh) and coated with silica shell, SPION-CCh-SiO₂. We proposed the two-step synthesis, (1) preparation of the SPION with surface modified with CCh (SPION-CCh) by coprecipitation of Fe²⁺ and Fe³⁺ with NH₃·H₂O and (2) formation of SPION-CCh-SiO₂ by direct decomposition of tetraethoxysilane (TEOS) in the presence of dispersed SPION-CCh under sonication and mechanical stirring at room temperature. The cationic chitosan was used to modify the surface potential of SPION, so the base-catalyzed hydrolysis and condensation of the TEOS could occur at the surface. Chitosan is the N-deacetylated product of chitin, one of the most abundant polysaccharide in nature. Chitosan and its derivatives have been applied widely because of their non-toxicity, biocompatibility, and biodegradability. Here we utilized a cationic chitosan, N-[(2-hydroxy-3-trimethylammonium) propyl] chitosan chloride (CCh), which we have synthesized and demonstrated that it is water–soluble and shows hydroscopic property, moisture retentiveness, mucoadhesivity and permeability enhancing properties better that these characteristics for chitosan [31]. The final products (SPION-CCh-SiO₂) are negatively charged, rounded in shape and exhibit the superparamagnetic properties. The morphology of the material was visualized by TEM and SEM microscopy. The chemical composition and physicochemical properties of the materials were determined using XRD, FTIR spectroscopy, DLS and zeta potential measurements. Magnetic properties were confirmed using AFM/MFM and magnetization measurements.