Magnetic, luminescent and core–shell structured Fe₃O₄@YF₃:Ce³⁺,Tb³⁺ bifunctional nanocomposites

Abstract

Bifunctional magnetic–luminescent nanocomposites with Fe₃O₄ nanoparticles as the cores and YF₃:Ce³⁺,Tb³⁺ as the shells were synthesized by a facile direct precipitation method. Transmission electron microscopy (TEM) images revealed that the obtained bifunctional nanocomposites had a core–shell structure, in a spherical shape with a size ranging from 160 to 220 nm, and the shell thickness of about 25 nm. The X-ray diffraction (XRD) patterns showed that a cubic spinel structure of Fe₃O₄ core and an orthogonal phase of YF₃ shell were obtained. Photoluminescence (PL) spectra confirmed that the nanocomposites displayed a strong green light emission. Magnetic measurements indicated that the obtained bifunctional nanocomposites exhibited a stronger magnetic behavior at room temperature. Therefore, the bifunctional nanocomposites are expected to develop many potential applications in biomedical fields.

Introduction

Magnetic and luminescent materials are of great importance in the fields of chemistry, biology, and medical sciences as well as in biotechnology [1], [2], [3]. Magnetic nanoparticles have widely been studied for biomedical applications, such as magnetic biosensing, targeted drug delivery, cell separation and contrast enhancement in magnetic resonance imaging [4], [5], [6]. Fe₃O₄ was chosen in this study because of its nontoxicity and good biocompatibility. Whereas high-photostability luminescent nano-materials have attracted much attention in biological labeling [7], [8], [9], [10], [11], [12], [13]. The combination of magnetic and luminescent properties of different materials into a single nanocomposite system might greatly enhance their applications in the biomedical fields [14], [15]. Bifunctional magnetic and luminescent nanocomposites provide a new platform for both diagnostics and treatment of disease due to their enhanced functionality and multifunctional properties in contrast with their single counterparts, which can enable “find-detect-treat” in one body [16], [17]. The use of bifunctional magnetic and luminescent nanoplatforms will improve further diagnostic effectiveness and reduce side effects [18], [19], [20], [21].

Recently, many different kinds of bifunctional luminescent–magnetic nanoparticles with Fe₃O₄ nanoparticles as the core and QDs as the shell had been synthesized by several different research groups with different methods [22], [23], [24], [25]. However, QDs (generally made from heavy metal ions such as Pb²⁺ or Cd²⁺) release Cd over time by UV light, they are less promising due to aggressive biological environment in vivo studies [26], [27]. Such problems would hinder their applications on biological detections and medical diagnosis.

To solve this issue, it is important to develop new photostable and nontoxic or lower toxicity luminescent markers. Lanthanide-doped nanoparticles are very promising due to their large stokes shift, sharp emission spectra, a high luminescence quantum yield, long lifetime, multiphoton and up-conversion excitation, superior photostability, low toxicity, thus they offer tremendous potential for future medical diagnosis and therapy [28]. Fang et al. employed a facile one-pot method to fabricate the Fe₃O₄@SC[6]-LaPO₄:Ce³⁺:Tb³⁺ superparamagnetic and fluorescent nanocomposites [29]. Zhang et al. had prepared Fe₃O₄@SiO₂/Y₂O₃:Tb bifunctional nanocomposites by homogeneous precipitation method [30]. Sun et al. had synthesized Fe₃O₄/SiO₂/YVO₄:Eu³⁺ magnetic/luminescent nanocomposites by a hydrothermal method [31]. Lu et al. employed a modified Stöber method combined with a layer-by-layer assembly technique to fabricate the core–shell structured luminomagnetic microsphere composed of a Fe₃O₄ magnetic core and a continuous SiO₂ nanoshell doped with Eu(DBM)₃·2H₂O fluorescent molecules [32]. YF₃ is an important host crystal for lanthanide-doped phosphors. It provides a wide band gap (> 10 eV) and suitable Y³⁺ sites where trivalent rare earth ions can be easily substituted without additional charge compensation [33]. The terbiumion (Tb³⁺) is one of the most strongly emitting ions in lanthanide ions. It is resistant to photobleaching, nontoxic, biocompatible, monochromatic, highly luminescent, most importantly, ultrasensitive both in vitro and in vivo bioassays, therefore Tb³⁺ could serve as a more efficient biological label than traditional organic fluorescent compounds or QDs [34]. Ce³⁺ ions can absorb the energy and then effectively transfer it to Tb³⁺ ions, so it is an efficient luminescent sensitizer [35].

In this work, we select Fe₃O₄ as magnetic cores and YF₃:Ce³⁺,Tb³⁺ as luminescent shells, using a facile direct precipitation method to synthesize the Fe₃O₄@YF₃:Ce³⁺,Tb³⁺ bifunctional magnetic–luminescent nanocomposites. The synthetic strategy as schematically illustrated in Scheme 1. With the excellent magnetic properties and surface amino-functional groups of Fe₃O₄ cores, YF₃:Ce³⁺,Tb³⁺ can be successfully coated. Compared with the reference [29], [30], [31], our method is simple and the cost is low. In addition, without the coated SiO₂ layer, the as-prepared product has a good green emission and strong magnetic property.