Preparation and characterization of carboxyl functionalization of chitosan derivative magnetic nanoparticles

Abstract

The functionalized magnetic Fe₃O₄-chitosan derivative nanoparticles have been prepared by the covalent binding of alpha-ketoglutaric acid chitosan (KCTS) onto the surface of Fe₃O₄ magnetic nanoparticles via carbodiimide activation. Transmission electron microscopy (TEM) showed that the KCTS-bound Fe₃O₄ nanoparticles were regular spheres with a mean diameter of 26 nm. X-ray diffraction (XRD) patterns indicated that the Fe₃O₄ nanoparticles were pure Fe₃O₄ with a spinel structure, and the binding of KCTS did not result in a phase change. The binding of KCTS to the Fe₃O₄ nanoparticles was also demonstrated by the measurement of thermogravimetric analysis (TGA), differential scanning calorimetry analysis (DSC) and Fourier transform infrared (FTIR) spectra. Magnetic measurement revealed that the saturated magnetization of the KCTS-bound Fe₃O₄ nanoparticles reached 24.8 emu g⁻¹ and the nanoparticles showed the characteristics of superparamagnetism.

Introduction

With the rapid development of nanotechnology, magnetic nanoparticles are currently studied widely. Superparamagnetic iron oxide (Fe₃O₄) nanoparticles have attracted researchers from various fields such as physics, medicine, biology, and materials science due to their multifunctional properties such as small size, superparamagnetism, and low toxicity, etc. [1], [2]. Several methods have been developed to synthesize magnetic Fe₃O₄ nanoparticles: (1) coprecipitation of ferrous (Fe²⁺) and ferric (Fe³⁺) aqueous solution in the presence of a base [3]; (2) thermal decomposition of an iron complex [4]; (3) by a sonochemical approach [5]. However, Fe₃O₄ nanoparticles tend to aggregate due to strong magnetic dipole–dipole attractions between particles. So, stabilizers such as surfactants, oxide or polymeric compounds (especially biocompatible polymer) with some specific functional groups have been used to modify these particles to increase the stability [6], [7], [8], [9]. Superparamagnetic Fe₃O₄ nanoparticles coated with polymers are usually composed of the magnetic cores to ensure a strong magnetic response and a polymeric shell to provide favorable functional groups and features, which have various applications in biomedicine fields for diagnostic magnetic resonance imaging [10], drug delivery systems [11], therapeutic regimes [12], cell/enzyme immobilization [13] and so on.

Chitosan, poly(1 → 4)-2-amino-2-deoxy-d-glucan, is a polyaminosaccharide with many significant biological (biodegradable, biocompatible, bioactive) and chemical properties (polycationic, hydrogel, reactive groups such as OH and NH₂). So, chitosan and its derivatives have been widely used in many biomedical fields [14]. As a special functional material, the preparations of magnetic chitosan beads have been attracting the researchers these years [15], [16]. However, as the resultant magnetic composites were either aggregated or unstable due to polymer cross-linking or physisorption, and chitosan has no suitable functional groups to bind directly onto Fe₃O₄ nanoparticles, many researchers are looking for various methods to modify chitosan and bind onto Fe₃O₄ particles. Carboxymethylated chitosan-Fe₃O₄ particles have been prepared by the carboxymethylated chitosan and then covalently bound onto Fe₃O₄ nanoparticles via carbodiimide activation [17], [18]. Chitosan-poly(acrylic acid) (CS-PAA) polymer magnetic microspheres were prepared by cationic CS coating negative charged Fe₃O₄ nanoparticles by electrostatic adsorption and subsequent polymerization of acrylic acid (AA) onto the CS-coated Fe₃O₄ cores [19]. These polymer magnetic microspheres had a high Fe₃O₄ loading content, and showed unique pH-dependent behaviors on the size and zeta potential.

More recently, in our group, a kind of chitosan derivatives (KCTS) with carboxyl group were prepared through modifying the chitosan with alpha-ketoglutaric acid, which could be covalently coupled with diverse bioactive macromolecules, enzymes and liposomes [14]. Following our previous work, in this study, we have been prepared Fe₃O₄ magnetic nanoparticles with 20–30 nm by hydrothermal method using H₂O₂ as an oxidizer. Then we successfully prepared the monodisperse KCTS-bound Fe₃O₄ nanoparticles by the covalent binding of KCTS onto Fe₃O₄ nanoparticles via carbodiimide activation. The size, structure, and magnetic properties of the resultant magnetic nanoparticles were characterized by TEM, XRD and vibrating-sample magnetometer (VSM). The binding of KCTS to the magnetic nanoparticles via carbodiimide activation was confirmed by FTIR, TGA and DSC. This information will be useful for further applications of the novel magnetic material in the immobilized enzyme system or removal of metal ions and many other industrial processes.