Introduction

Magnetic nanowires have recently received considerable attention as a result of their potential applications, including photonics [1], electronics [2,3,4], sensing [5, 6], and biomedicine [7, 8]. Compared with traditional materials, nanomaterials have many novel properties, such as optical [9], electrical [10], thermal, and magnetic [11] properties. Nanowires, such as iron oxide, have received significant attention as useful candidates for biosensors because of their biocompatibility and magnetic properties. In a biosensor, a probe with nanobrush statically captures the biological signals. If we can control the movement of the nanobrush, the contact rate between the probe and the determined signals can be theoretically simultaneously improved, thereby greatly improving the sensitivity of the biosensor. We assumed that superparamagnetic and flexible nanowire arrays can sway with changes in the magnetic field, thus controlling the movement of the probe, meaning that the sensor sensitivity with these nanowire arrays can be improved.

Many methods have been used to fabricate iron oxide nanowires, including hydrothermal methods [12,13,14], electrospinning [15], and sol–gel methods [16]. The length and diameter of the iron oxide nanowires prepared by these methods are hard to control. Yan [17, 18] used an electrostatic self-assembly method for the preparation of superparamagnetic iron oxide nanowires. The nanowires had diameters of around 200 nm and lengths between 1 µm and 0.5 mm, with either positive or negative charges on their surface. However, the nanowires prepared by this way were dispersed in water and it was difficult to control the length and diameter of the nanowires. On the basis of their work, we first used the method combined with electrostatic self-assembly and an AAO template to prepare ordered nanowire arrays with superparamagnetic properties, as well as tunable diameter and length.

We report a novel and effective approach for the formation of iron oxide nanowire arrays. Because of the electrostatic interaction, negatively charged iron oxide nanoparticles and positively charged polymers can be assembled into nanowires using an AAO template under a stable magnetic field. The length and diameter of the nanowire arrays were controlled by the diameter and depth of the AAO template throughout the process.

Materials and methods

Materials

Poly(dimethyldiallylammonium chloride) (PDADMAC, M w < 100,000) and polyacrylic acid (PAA, M w = 1800 and M w = 800) were procured from Sigma-Aldrich, Singapore. Aluminum (99.999% purity, 0.5 mm thickness) was procured from Beijing Technology Co., Ltd., China. AAO templates with a 400 nm pore diameter and a 30 μm thickness were procured from Shanghai Shangmu Technology Co. Ltd., China.

Fabrication

We prepared solutions to fabricate iron oxide nanowire arrays in the AAO template. Solution (I) was prepared by the mixture of a certain mass fraction of nanoparticle solution and a 1 mol L−1 NH4Cl solution. Solution (II) was prepared by the mixture of PDADMAC and a 1 mol L−1 NH4Cl solution. The PH values of these solutions mentioned above were adjusted to 7–8 by 10 wt% ammonia solution. Solution (III) was a mixture of solutions (I) and (II) under a certain charge ratio (Z). The relative amount of each component was monitored by the Z in solution (III). Z was defined as the ratio between the cationic charges borne by the polymers and the anionic charges carried by the particles at their surfaces. Then, the AAO template was immersed in solution (III) for a period of time, with the purpose of making the nanoparticles and polymer adequately filled with the AAO template. After, the sample was placed in ultra-pure water to dialysis with 0.5 T of the magnetic field for a period of time. In the whole process of dialysis, the ordered nanowire arrays were fabricated from the controlled electrostatic co-assembly of oppositely charged nanoparticles and commercially available polyelectrolytes, with the decrease in the concentration of NH4Cl in the holes of the AAO template.

Characterization

Morphological analysis was performed with field emission scanning electron microscopy (FESEM, Ultra 55, Carl Zeiss Ltd., Jena, Germany). The magnetic hysteresis loops were obtained by a vibrating sample magnetometer (VSM, BKT-4500Z, Albert days of Beijing Science and Technology Co., Ltd.).

Results and discussion

Figure 1a, d shows typical FESEM images of the AAO template prepared by a two-step anodization process with a constant direct voltage of 45 V for 5 h in C2H2O4 at 5 °C with a pore diameter of around 90 nm. Figure 1b, e shows typical FESEM images of the AAO template prepared by a hard anodization with a constant direct voltage of 120 V for 1 h in C2H2O4 at 5 °C with a pore diameter of around 190 nm. Figure 1c, f shows typical FESEM images of the commercially available AAO template with a pore diameter of around 400 nm. The iron oxide nanoparticles investigated here were synthesized according to the Massart technique [17,18,19] by alkaline coprecipitation of iron (II) and iron (III) salts, oxidation of the magnetite (Fe3O4) into maghemite (γ-Fe2O3) nanoparticles and by size sorting of subsequent phase separations [20]. To improve their stability, the nanoparticles were coated with PAA using the precipitation dispersion technique [21].

Figure 1
figure 1

SEM of AAO template with different diameters; a and d were formed by traditional two-step anodization. b and e were formed by improved HA process. c and f were purchased AAO template

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It has been well established that several factors affect the filling ratio of iron oxide nanoparticles and polymers in the AAO template. First, the concentration of iron oxide particles affected the filling ratio. The AAO templates with a pore diameter of around 190 nm were immersed in a series of solutions with different concentrations of nanoparticles at Z = 0.7. Figure 2 shows the effect of different concentrations of iron oxide nanoparticles on the filling rate. Under the condition that the concentration of nanoparticles was less than 4 wt%, nanoparticles and cationic polyelectrolytes were attached to the pore walls of the AAO template instead of assembling into nanowires. We decided that the reason why the nanoparticles and cationic polyelectrolyte polymers were easily attached to the hollow wall at lower concentrations of nanoparticles is because of the large specific surface energy of the pore wall of the AAO template. Thus, nanoparticles and PDADMAC were well filled into the pores of the AAO template and obtained a high filling ratio when the concentration of nanoparticles reached 4 wt%.

Figure 2
figure 2

SEM of nanowires assembled with different concentration nanoparticles and PDADMAC under a 0.7 charge ratio in a pore diameter of 190 nm of AAO templates: a 1 wt%, Z = 0.7. b 2 wt%, Z = 0.7. c 3 wt%, Z = 0.7. d 4 wt%, Z = 0.7

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The pore diameters of the AAO templates are found to have a strong influence on the filling ratio in Fig. 3. At the pores of the AAO template around 90 nm, the nanoparticles and PDADMAC entered the pores of AAO template but barely assembled into nanowires. The result revealed that the pore diameters of the template are smaller, and the pore spaces between nanoparticles are larger. Figure 3b, c shows the middle and nozzle of the pores of the AAO template with a pore diameter of around 190 nm, respectively. Although γ-Fe2O3 nanoparticles and PDADMAC were better filled into the AAO templates, the assembled nanowires were loose and broken, as shown in Fig. 3b. At the nozzle of the AAO template, nanoparticles and PDADMAC were grown into dense nanowires. These results demonstrate that inadequate dialysis at the nozzle of the templates can easily form nanowires while the middle of AAO templates cannot. Iron oxide nanoparticles and PDADMAC were sufficiently assembled into nanowires in the AAO template form Fig. 3d.

Figure 3
figure 3

SEM of nanowires assembled in different diameters of AAO template: a 4 wt%, Z = 0.7, D p = 90 nm. b 4 wt%, Z = 0.7, D p = 190 nm. c 4 wt%, Z = 0.7, D p = 190 nm. d 4 wt%, Z = 0.7, D p = 400 nm

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The effect of the Z on the filling ratio is shown in Fig. 4. We found that the nanoparticles and the cationic polyelectrolyte polymer easily entered the pores of the AAO template and grew into nanowires at Z = 0.35 and 0.7. However, nanoparticles and the polymer were not grown into nanowires at Z = 7. Because of the action of static between anionic charges nanoparticles and the hole wall of the AAO template, nanoparticles could not easily enter into the hole of the AAO templates. The cationic polyelectrolyte polymers were easily attached to the hole wall of the AAO template, but a low concentration of cationic polyelectrolyte polymer (Z = 7) led to nanoparticles barely entering into the hole of the AAO template. The effect of PAA with different molecular weights on the filling ratio was not significant (Fig. 5).

Fig. 4
figure 4

SEM of nanowires assembled under different charge ratio Z: a 4 wt%, Z = 0.35, D p = 400 nm. b 4 wt%, Z = 0.7, D p = 400 nm. c 4 wt%, Z = 7, D p = 400 nm

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Fig. 5
figure 5

SEM of nanowires assembled in diameter 400 nm of AAO templates with nanoparticles coated by different molecular weight PAA. a M W = 1800 g mol−1, b M W = 800 g mol−1

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The magnetic hysteresis loops (Fig. 6) obtained by the VSM show that the nanowires have superparamagnetic properties. The result shows that iron oxide nanowires retained the superparamagnetic properties of iron oxide nanoparticles. By dissolution of the samples, we can obtain a nanobrush in the AAO template. Figure 7a shows the SEM of the nanowires prepared in the AAO template with a pore diameter of around 190 nm, and the thicker pore wall was corroded by 0.3 mol L−1 phosphoric acid for 20 min. It can be seen from the diagram that we obtained a nanobrush in AAO template. Figure 7b shows the morphology of nanowires after completely etching the AAO template in 0.3 mol L−1 phosphoric acid. We found that the nanowires were more flexible than fragile. It is likely that the improved flexibility is because of the presence of polymers.

Fig. 6
figure 6

Magnetic hysteresis loops of nanowire arrays

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Fig. 7
figure 7

a SEM of partial dissolution of the AAO template in 0.3 mol L−1 phosphoric acid for 20 min. b Morphology of nanowires after removal of AAO template

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Conclusions

A novel method combined with electrostatic assembly and the AAO template was used to fabricate superparamagnetic nanowires with controllable diameter and length. When the concentration of the nanoparticles was higher, the pore size of the AAO template was larger and the Z was smaller, and nanoparticles and cationic polyelectrolyte polymers were most easily assembled in the AAO template. Simultaneously, the molecular weight of PAA wrapped on the surface of the nanoparticles had no effect on the result of the assembly process. Using 0.3 mol L−1 phosphoric acid, we can obtain completely ordered nanowire arrays. The hysteresis loops of nanowires were measured using VSM, where the coercivity and remanence are almost zero, to perform a typical superparamagnetism. Therefore, superparamagnetic and flexible γ-Fe2O3 nanowire arrays have potential applications for improving sensor sensitivity.