Magnetic properties of superparamagnetic nanoparticles loaded into silicon nanotubes
© Granitzer et al.; licensee Springer. 2014
Received: 19 May 2014
Accepted: 7 July 2014
Published: 21 August 2014
In this work, the magnetic properties of silicon nanotubes (SiNTs) filled with Fe3O4 nanoparticles (NPs) are investigated. SiNTs with different wall thicknesses of 10 and 70 nm and an inner diameter of approximately 50 nm are prepared and filled with superparamagnetic iron oxide nanoparticles of 4 and 10 nm in diameter. The infiltration process of the NPs into the tubes and dependence on the wall-thickness is described. Furthermore, data from magnetization measurements of the nanocomposite systems are analyzed in terms of iron oxide nanoparticle size dependence. Such biocompatible nanocomposites have potential merit in the field of magnetically guided drug delivery vehicles.
61.46.Fg; 62.23.Pq; 75.75.-c; 75.20.-g
Porous materials with their substantial surface areas are versatile structures with specific properties of value for diverse fields such as photonics, catalysis, and therapeutics . As an elemental semiconductor, porous silicon is a unique example of this type of material whose biocompatibility and biodegradability lend it great potential value to biomedical applications . The complementary morphology of hollow silicon nanotubes (SiNTs) also provides opportunities in areas such as battery technology, photovoltaics, as well as drug delivery. SiNTs are tunable in their inner diameter as well as in their wall-thicknesses . They provide a uniform structure compared to the dendritic pore growth of porous silicon in the target porous regime (30 to 90 nm pore diameter), and therefore, such structures are attractive for infiltration with nanoparticles or molecules (e.g., superparamagnetic (SPM) iron oxide nanoparticles of the form Fe3O4). In terms of possible candidates for loading, superparamagnetic Fe3O4 nanoparticles (NPs) also offer a low toxicity and thus can be applied to diverse uses in biomedicine, e.g., for hyperthermia, NMR imaging, and functionalization with anti-cancer agents .
In this work, SiNTs are infiltrated with Fe3O4 NPs to achieve a nanocomposite system which can, in the long term, be considered for use as a magnetic-assisted drug delivery vehicle. Previously, porous silicon loaded with iron oxide NPs of different sizes has been investigated with the cytocompatibility of this system showing encouraging results . The cytocompatibility of SiNTs has also been recently evaluated . In the following work, the infiltration of Fe3O4 NPs into SiNTs of different wall thicknesses is described and the fundamental magnetic properties of these composites investigated as a function of the Fe3O4-nanoparticle size.
Silicon nanotubes were fabricated by a multistep process previously described  involving deposition of silane (SiH4) on preformed ZnO nanowire array templates on F-doped tin oxide (FTO) glass or Si wafer segments, followed by sacrificial etching of the ZnO phase resulting in the desired nanotube product. Hollow nanotube inner diameter is adjustable by size selection of the initial ZnO nanowire template, while shell thickness control is achieved by concentration/duration of silicon deposition. In these experiments, SiNTs with 10-nm wall thickness are obtained at 530°C with a 5-min Si deposition time, and SiNTs with 70-nm wall thickness are obtained at 580°C with a 5-min Si deposition time. Internal nanotube diameter is dependent on ZnO nanowire diameter, which in the experiments described here, is fixed at 50 nm. The wall thickness determines the dissolution of the material in vitro and thus is of importance for controlled drug release (vide infra).
Iron oxide NPs have been prepared by a known route utilizing decomposition of an iron complex at high temperature . NPs of different sizes (4 and 10 nm) are infiltrated into SiNTs with 10- and 70-nm wall thicknesses. The infiltration process performed at room temperature is supported by a magnetic field to assure optimal filling of the nanotubes. The infiltration process has been optimized with respect to the wall-thickness of the SiNTs and the size of the NPs used.
For the case of relatively thick-walled nanotubes (70 nm), the loading of Fe3O4 NPs is readily achieved by initial removal of the SiNT film from the underlying substrate (such as FTO glass) and placing it face down on top of a Nd magnet with a piece of filter paper in between. Fe3O4 NPs (oleic acid terminated, hexane solution) at a concentration of 7 mg/mL are added dropwise, followed by rinsing the infiltrated sample with acetone several times, and allowed to air dry.
For the thin-walled SiNT variant (approximately 10 nm), the infiltration process of Fe3O4 NPs in thin shell thickness SiNTs is accomplished by placing the SiNTs attached to the substrate (e.g., silicon wafer) also on top of a Nd magnet. The Fe3O4 NPs are added dropwise (also at a concentration of 7 mg/mL), and the infiltration process is accomplished by diffusion of the nanoparticles through the side porous wall of the SiNT. For the case of Fe3O4 nanoparticles that are 10 nm in diameter, the SiNT sidewall pore dimensions are insufficient to permit loading by diffusion through this orifice and thus the SiNT film must be removed from the substrate prior to loading of this sample.
Magnetic measurements were performed with a vibrating sample magnetometer (VSM; Quantum Design, Inc., San Diego, CA, USA). Magnetization curves of the samples have been measured up to a field of 1 T, and the temperature-dependent investigations have been carried out between T = 4 and 300 K. Scanning electron micrographs (SEM) were measured using a JEOL FE JSM-7100 F (JEOL Ltd., Akishima-shi, Japan), with transmission electron micrographs (TEM) obtained with a JEOL JEM-2100.
Results and discussion
The purpose of fabricating such a magnetic nanocomposite is its applicability in biomedicine as a magnetic-guided drug delivery vehicle. A key requirement of such a system is a low blocking temperature (TB) which is defined by the transition between superparamagnetic (SPM) behavior and the blocked state of the nanocomposite. TB has to be far below room temperature, which entails a missing magnetic remanence. So above TB, the system offers no magnetic remanence if the external field is switched off. From temperature-dependent magnetization measurements, the transition temperature between SPM behavior and blocked state has been extracted. The so-called blocking temperature TB depends strongly on the particle size of the infiltrated iron oxide NPs and on the distance between the particles within the tubes. To obtain TB of the nanotubes with different infiltrated NPs, zero field cooled/field cooled (ZFC/FC) magnetization measurements have been performed. For this purpose, the sample is first cooled down from room temperature to T = 4 K without an external magnetic field. Then, a low magnetic field of H = 500 Oe is applied and the magnetization measured up to T = 300 K and subsequently down to T = 4 K.
Summary of the various blocking temperatures, magnetic remanence, and coercivities gained by filling of SiNTs with iron oxide NPs of different sizes
10-nm shell SiNTs
70-nm shell SiNTs
70-nm shell SiNTs, remanence MR (emu)
T = 4 K
0.75 × 10-4
0.55 × 10-4
T = 300 K
0.01 × 10-4
0.01 × 10-4
70-nm shell SiNTs, coercivity HC (Oe)
T = 4 K
T = 300 K
These initial investigations suggest that the loading of SiNTs with different wall thicknesses retain a heavily suppressed blocking temperature (TB) far below room temperature, a promising result. A systematic investigation of the nanotube wall thickness on blocking temperature is currently under evaluation, but studies to date suggest that the magnetic properties can be tuned by the filling of the SiNTs independent of the nanotube wall thickness. Given our previous observation of thickness-dependent dissolution behavior for these nanotubes in aqueous media , this parameter can be paired with a target blocking temperature and selected based on the desired degradation window in vivo.
Silicon nanotubes filled with superparamagnetic iron oxide NPs were investigated with respect to a possible utilization as magnetically guided drug delivery vehicle. The magnetic properties were found to be dependent upon the NP size but relatively insensitive to the morphology of the nanostructured Si host. The blocking temperature is very low for all investigated samples which enables a closely packed filling of the nanotubes to achieve a magnetic moment as high as possible. These results are encouraging and fulfill the preconditions for applicability of these semiconducting nanotubes in biomedicine.
This work has been supported by the Robert A. Welch Foundation (Grant P-1212). The authors also thank Dr. Puerto Morales for the supply of iron oxide nanoparticles.
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