Monodisperse Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy Core-Shell Composite Nanospheres for IBU Loading and Release

Abstract

The magnetic targeting drug delivery system is an effective way of targeting therapy. In this study, the monodisperse Fe₃O₄ nanoparticles with a particles size of about 180 nm were first prepared via a solvothermal method. Subsequently, the core-shell structure Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/polypyrrole (PPy) composite nanospheres were successfully synthesized by coating Fe₃O₄ nanoparticles with SiO₂ shell layer using the Stöber method and PPy shell by solvothermal method in turn. The as-prepared nanoparticles were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA), and Ultraviolet-Visible spectrophotometer (UV-Vis). The results indicated that the as-prepared composite nanospheres displayed a well-defined core-shell structure and monodispersity. The thicknesses of SiO₂ shell and PPy shell were ~6 nm and ~19 nm, respectively. Additionally, the as-prepared nanoparticles exhibited high saturation magnetization of 104 emu/g, 77 emu/g, and 24 emu/g, and have great potential applications in drug delivery. The drug loading and drug release of the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres to ibuprofen (IBU) under stirring and ultrasonication were investigated. Their drug loading efficiency and drug release efficiency under ultrasonication were all higher than 33% and 90%, respectively. The drug release analyses showed sustained release of IBU from nanospheres and followed the Korsmeyer-Peppas model.

1. Introduction

Ibuprofen (IBU) is the most commonly used and most frequently prescribed non-steroidal anti-inflammatory drug to relieve inflammation, fever, and pain from headache, migraine, toothache, joint pain, back pain, muscle pain, and menstrual cramps through oral administration [1,2,3,4]. However, its biological half-life is very short, resulting in frequent administration of drugs, which increases the toxic side effects of the system. Further, due to the extremely poor water solubility, oral administration can stimulate the gastrointestinal tract, produce side effects such as chest pain, headache, and vomiting, and in severe cases, can cause gastric bleeding, which limits the maximum daily dosage. To overcome these shortcomings, the drug delivery system can be used to deliver IBU directly to the infected site through parenteral route [5,6], and as the molecular size of IBU is only 0.6–1 nm, which is suitable to be connected to the nano-drug carrier [7,8,9], this makes it an ideal candidate for use in controlled release drug delivery systems.

At present, various composite nanoparticles, such as silica/mesoporous silica nanoparticles, metal-organic matrix nanocomposites, titanium dioxide nanoparticles, polymer nanofilms, and other nanomaterials [7,9,10,11,12,13,14,15,16,17,18,19,20,21], have been used for the loading and release of ibuprofen. Silica/mesoporous silica nanoparticles are non-toxic, biocompatible, biologically inert, and have strong stability even at 1000 °C [10,11,12,13,14]. A large number of hydroxyl groups on their surface can provide connection sites for drug loading to achieve drug delivery. A metal-organic framework analog to MIL-101(Cr) containing amine groups was synthesized by Silva et al. [15]. It was first found that the amino groups have a significant influence on the loading and release of IBU and nimesulide (NMS) due to the possibility of interactions between the functional groups in IBU and the NH₂ groups in the as-prepared matrix. Pawlik et al. applied a co-delivery of gentamicin and ibuprofen from nanoporous anodic titanium dioxide layers and examined the effects of crystalline structure and thickness of the nanoporous TiO₂ layer on the amount of released drugs and drug release profiles [16]. Mucoadhesive IBU-loaded chitosan films suitable for oral mucosal drug delivery were prepared, which is an alternative green process in the drug delivery system [7]. Although these nano-drug carriers can achieve high-efficiency loading of IBU and slow controlled release in vitro, they are still lacking targeting in vivo and cannot directly target the disease center that produces certain toxic side effects. Therefore, it is necessary to combine these materials with magnetic nanoparticles to obtain magnetic targeting drug carriers to directly target IBU to the disease center under the action of an external magnetic field, minimizing the toxicity and side effects of the system.

Fe₃O₄ magnetic nanoparticles show good application prospects in the targeting drug delivery system (TDDS) due to their high magnetism, good stability, and biocompatibility [22,23]. The solvothermal method is a commonly used method to prepare Fe₃O₄ magnetic nanoparticles. A new core-shell microsphere Fe₃O₄@MOFs/GO was synthesized by exploring the layer-by-layer self-assembly method for IBU drug loading properties [24]. However, the size of the microspheres prepared exceeds 400 nm and is not suitable for targeting drug delivery in vivo—in order for drug carriers to successfully pass through the 100–1000 nm gap between neovascularization cells such as alveoli and capillaries without causing embolism, the particle size threshold of drug carriers in tumor exosmosis should be about 400 nm, preferably below 200 nm [25]. The Fe₃O₄ microspheres with diameters of 400–800 nm obtained using the solvothermal method were more easily swallowed by the reticuloendothelial system and could not achieve drug delivery [26]. Therefore, controlling the size of Fe₃O₄ magnetic nanoparticles through the solvothermal method is still a major challenge. Polypyrrole (PPy) has excellent electrical conductivity, stability, biocompatibility, and biodegradability, and the amino groups on the skeleton can provide the connection sites for the drug and connect hydrophobic drugs through π–π stacking action [27,28,29].

In this paper, Fe₃O₄ nanospheres with a size of about 180 nm were obtained by the solvothermal method using ethylene glycol as a reductant and solvent and polyethylene glycol as a surfactant. Subsequently, the core-shell structure Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres were synthesized by combining Fe₃O₄ nanoparticles with SiO₂ and PPy. The drug controlled release behavior of these two composite nanospheres to IBU under stirring and ultrasonication was studied. Fe₃O₄ nanoparticles can endow composite nanospheres with magnetic field-mediated targeting drug delivery capability. Moreover, SiO₂ and PPy can not only prevent magnetic Fe₃O₄ core from aggregating, but also have non-toxicity, stability, biocompatibility, and biodegradability, which can provide more connected sites for IBU drug load and can be efficiently located at the tumor site to achieve targeted drug delivery.

2. Materials and Methods

2.1. Preparation of Fe₃O₄ Nanoparticles

The Fe₃O₄ nanoparticles were prepared via a solvothermal method. Typically, 6.75 g of FeCl₃·6H₂O was dissolved in ethylene glycol to obtain an orange-yellow solution. When the FeCl₃·6H₂O was completely dissolved, 1.0–1.5 g of the surfactant polyethylenegl-4000 (PEG-4000) was added, and 11–12 g of anhydrous sodium acetate was added after the dissolving of PEG-4000. After continuous stirring for 30 min, the mixed solution was transferred into a Teflon-sealed autoclave at 200 °C for 6–8 hours. After the autoclave was cooled to room temperature, the black products were washed alternately with distilled water and anhydrous ethanol several times. The obtained Fe₃O₄ nanoparticles were ultrasonically dispersed in distilled water for further reaction. Among them, ethylene glycol was used as a reductant and solvent, sodium acetate was hydrolyzed to produce hydroxyl groups to precipitate iron ions and ferrous ions, and polyethylene glycol was used as a surfactant to improve the dispersion of particles.

2.2. Preparation of Fe₃O₄/SiO₂ Composite Nanospheres

The SiO₂ shell modification was carried out on the obtained Fe₃O₄ nanoparticles by the Stöber method [30,31]. Firstly, 50 mL of absolute ethanol, 1 mL of deionized water, 2 mL of ammonium hydroxide (25%), and 300 mL of tetraethyl orthosilicate (TEOS) were mixed and reacted in a water bath at 40 °C for 10 min. The TEOS was hydrolyzed to obtain primary SiO₂ nanoparticles, and then the ultrasonically dispersed Fe₃O₄ nanoparticles were added to the above mixture. After mechanically stirring the mixture for 12 h at room temperature, the products were magnetically separated and washed several times with distilled water and absolute ethanol, respectively. Finally, the obtained Fe₃O₄/SiO₂ composite nanospheres were dried or re-dispersed in distilled water by ultrasound for further coating of the polypyrrole shell.

2.3. Preparation of Fe₃O₄/SiO₂/PPy Composite Nanospheres

Polypyrrole shell layer was prepared by the hydrothermal method [32]. In general, the pyrrole monomer (distilled before reaction and stored in a brown bottle in a refrigerator at 4 °C) was added dropwise to the above Fe₃O₄/SiO₂ composite microspheres solution under stirring with the ratio of Fe₃O₄/SiO₂ composite microspheres to pyrrole monomer of 100–200 mg:300 μL. After mechanical stirring for 1 h, this mixture was transferred into a Teflon-sealed autoclave, and the ammonium persulfate solution was added drop by drop (0.1–0.2 g ammonium persulfate dissolved in 10 mL distilled water). After 8 h of reaction at 140 °C, the autoclave was cooled to room temperature, and then the precipitate was separated by a magnet and washed with distilled water and anhydrous ethanol for several times. Finally, the Fe₃O₄/SiO₂/PPy composite nanospheres were obtained by drying at room temperature.

2.4. Characterizations

The morphology and size of the samples were characterized by BDX-3300 JEOL 100CX-II transmission electron microscope (JEOL, Akishima, Tokyo). The X-ray diffraction was obtained by Bruker D8 Focus diffractometer (Bruker, Billerica, MA, USA) to analyze the crystalline phase of the products. Fourier-transform infrared spectroscopy was conducted on an FTIR-650 spectrometer to determine the composition and structure of the particles (Gangdong Sci. & Tech. Development Co., Ltd., Tianjin, China). The magnetic properties of the particles were studied by MPMS SQUID vibrating sample magnetometer (VSM, Quantum Design Co., Ltd., San Diego, CA, USA). Thermogravimetric analyses and differential thermogravimetric analyses (DTG, NETZSCH-Gerätebau GmbH, Selb, Germany) were performed using the TG 209 F3 Tarsus instrument under an air atmosphere with a heating rate of 10 °C/min from room temperature up to 1000 °C. The concentration of IBU in the supernatant was measured by an alpha-1860Plus ultraviolet-visible spectrophotometer (LSI, Shanghai, China).

2.5. Drug Loading and Drug Release

The drug loading and drug release of the prepared nanospheres for IBU were performed in 0.2 mol/L of disodium hydrogen phosphate–sodium dihydrogen phosphate buffer solution (PB) at the physiological conditions of a temperature of 37 °C and a pH of 7.4 [33,34]. The content of IBU in buffer solution was quantified using UV-Vis absorption technique at λ_max = 264 nm. A total of 10 mg of Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres were added in 25 mL of 1 mg/mL IBU-ethanol solution, respectively. After stirring for 24 h, the supernatant was immersed and washed with anhydrous ethanol, and then the absorbance of supernatant and cleaning solution was measured by UV-Vis spectrophotometer to calculate the residual unloaded IBU drug concentration, thereby indirectly obtaining the drug loading of the two nanospheres. The IBU drug loading (DL, mg) is defined as Equation (1):

$$DL=C_0{V}_0-C_1{V}_1-C_2{V}_2$$

(1)

where C₀, C₁, and C₂ are the initial concentration of the drug and the concentration of the supernatant and the cleaning solution, respectively, in mg/mL. V₀, V₁, and V₂ are the initial volume of the drug and the volume of supernatant and cleaning solution, respectively, in mL.

The drug loading efficiency (E_a, %) is calculated according to Equation (2):

$$E_a=\frac{DL}{m}\times 100$$

(2)

where m is the amount of as-prepared composite nanospheres added, in mg.

The calculation formula of encapsulation efficiency (EE, %) is as Equation (3) [2]:

$$EE=\frac{DL}{m_{\mathrm{IBU}}}\times 100$$

(3)

where m_IBU is the amount of loaded IBU, in mg.

For in vitro drug release, these two IBU-loaded composite nanospheres were put into dialysis bags separately and incubated in 25 mL of 0.2 mol/L PB buffer solution (pH = 7.4), and 5 mL of the supernatant solution was removed after mechanical stirring for 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3.5 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 24 h, and 84 h, respectively. Drug concentration in the supernatant solution was determined as before, and the precipitate was re-suspended with the equivolumetric fresh PB buffer solution with the same pH and concentration. The drug release efficiency (DR, %) is defined as Equation (4) [9]:

$$DR=\frac{m_{\mathrm{R}}}{m_{\mathrm{IBU}}}\times 100$$

(4)

where m_R is the amount of released IBU, in mg.

In order to study the drug release behavior under ultrasound, the two IBU-loaded composite nanospheres were put in 25 mL of 0.2 mol/L, pH = 7.4 PB buffer solution. Under ultrasound, 5 mL of supernatant was removed after 5 min, 10 min, 15 min, 20 min, 30 min, 40 min, 50 min, 60 min, and 65 min, respectively, and replaced with the equivolumetric fresh pH = 7.4 PB buffer solution. The absorbance of the supernatant was measured, and the drug release curves of the two drug-loaded nanoparticles under ultrasound were obtained.

To describe the release profile, the common exponential equation of the Korsmeyer-Peppas model was used. The Korsmeyer-Peppas model [35,36,37] derived a simple mathematical relationship which described the drug release from the above two systems, and can be defined as Equation (5):

$$W=\frac{M_t}{M_{\infty }}=k{t}^n$$

(5)

where W is the fraction of drug released at time t, M_t is the cumulative released amount at time t, M_∞ is the total released amount, t is the time, k is the kinetic constant, and n is the release exponent.

3. Results and Discussion

Figure 1 gives the TEM images of the synthesized Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂ core-shell composite nanospheres, and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres. It can be seen that the Fe₃O₄ had spherical nanoparticles with an average size of about 180 nm and was monodispersed (Figure 1a,d). From Figure 1b,e, it is found that the SiO₂ layer had deposited onto the surface of Fe₃O₄ nanoparticles and there was an interface which can be clearly distinguished between the inner Fe₃O₄ magnetic core and the outer SiO₂ shell. The thickness of the SiO₂ shell was about 6 nm, which can enhance the stability and biocompatibility of Fe₃O₄ nanoparticles. According to Figure 1c,f, the PPy also had uniformly deposited on the surface of Fe₃O₄/SiO₂ composite nanospheres and formed a PPy shell of about 19 nm. After the coating process of SiO₂ and PPy, the average diameters of the Fe₃O₄/SiO₂ composite nanospheres and the Fe₃O₄/SiO₂/PPy composite nanospheres increased to around 192 nm and 230 nm, respectively. It can be clearly observed that both the obtained Fe₃O₄/SiO₂ composite nanospheres and Fe₃O₄/SiO₂/PPy composite nanospheres had obvious core-shell structures and were monodispersed.

The XRD patterns of Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂ core-shell composite nanospheres, and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres are illustrated in Figure 2. Six characteristic diffraction peaks at 2θ = 30.53°, 35.86°, 43.45°, 53.78°, 57.34°, and 62.96°, corresponding to (220), (311), (400), (422), (511), and (440) diffraction planes, respectively, were observed for all samples, which are consistent with the database of magnetite diffraction card (JCPDS 19-0629) [38,39]. This indicates that the magnetite crystalline phase remained in the two composite nanospheres. A weak broad peak at 2θ = 22° in Figure 2b was assigned to the amorphous SiO₂, indicating the formation of Fe₃O₄/SiO₂ core-shell composite nanospheres [40]. Due to the contribution of PPy, the strength of the broad peak between 21° and 28° in Figure 2c increased compared with Figure 2b, indicating that the Fe₃O₄/SiO₂/PPy core-shell composite nanospheres were synthesized [41].

The FT-IR analysis is further employed to characterize the functional groups and structures of the Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂ core-shell composite nanospheres, and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres, and the results are presented in Figure 3. The characteristic peaks of Fe–O stretching vibration for the three particles were all observed around 570 cm⁻¹ [42,43]. Three peaks at about 464 cm⁻¹, 786 cm⁻¹, and 1100 cm⁻¹ in Figure 3b,c were attributed to the tensile vibration of Si–O, Si–O–Si, and Si–OH bonds, respectively, indicating the presence of SiO₂ in the Fe₃O₄/SiO₂ core-shell composite nanospheres [44,45,46]. The peak in Figure 3c was shifted from 786 cm⁻¹ in Figure 3b to 787 cm⁻¹ due to the contribution of C–H out-plane ring bend at 790 cm⁻¹ in polypyrrole. The peaks at 850 cm⁻¹, 925 cm⁻¹, and 1176 cm⁻¹ in Figure 3c were assigned to the C–H in-plane/out-plane deformation vibration of PPy [42,47]. Bands at 1050 cm⁻¹ and 1319 cm⁻¹ observed in Figure 3c can be attributed to C–H in-plane vibration and C–N stretching vibration of PPy, respectively [48]. The characteristic peaks at 1602 cm⁻¹ and 3430 cm⁻¹ observed in all samples corresponded to the stretching vibration of the O–H bond, which indicates that the adsorbed OH⁻ groups exist in the samples [44]. Compared with Fe₃O₄ nanoparticles and Fe₃O₄/SiO₂ composite nanospheres, there were sharper characteristic peaks at 1602 cm⁻¹ and 3430 cm⁻¹ in Fe₃O₄/SiO₂/PPy composite nanospheres due to the C=C and N–H stretching vibration absorption bands of polypyrrole at these two peaks [1], indicating the formation of Fe₃O₄/SiO₂/PPy composite nanospheres.

The diagram of magnetization vs. magnetic field at room temperature for Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂ core-shell composite nanospheres, and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres are depicted in Figure 4. The Fe₃O₄ nanoparticles exhibited a high saturation magnetization of 104 emu/g as shown in Figure 4a. After coated with the SiO₂ shell, the saturation magnetization of Fe₃O₄/SiO₂ core-shell composite decreased to 77 emu/g [49], indicating that the SiO₂ layer was successfully connected to the magnetic core. The saturation magnetization of Fe₃O₄/SiO₂/PPy core-shell composite nanospheres was further reduced to 24 emu/g, which indicates the successful modification of PPy shell on the SiO₂ shell [50]. The saturation magnetization of the latter had a significant decrease, further confirming that the thickness of the PPy shell was thicker than that of the SiO₂ layer, which is consistent with TEM results that the SiO₂ layer was 6 nm and the PPy shell was 19 nm. Even if the saturation magnetization of the final Fe₃O₄/SiO₂/PPy core-shell composite nanospheres decreases, the magnetic nanospheres can still be triggered to release targeting drugs under the external magnetic field. At the same time, the magnetic nanospheres can be stratified immediately after ultrasonic dispersion and are suitable for drug release research under ultrasound triggering. The values of remnant magnetization of Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂, and Fe₃O₄/SiO₂/PPy composite nanospheres are 4.10 emu/g, 4.09 emu/g, and 1.50 emu/g, respectively, and their coercivities are all zero Oe. It can be seen that all the prepared samples have good magnetic properties, indicating that they apply to targeted drug/gene delivery under the action of an external magnetic field.

The thermal stability of Fe₃O₄ nanoparticles, Fe₃O₄/SiO₂ core-shell composite nanospheres, and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres was further studied by thermogravimetric analysis and differential thermogravimetric analysis, and their TGA and DTG curves are illustrated in Figure 5. For Fe₃O₄ nanoparticles, there are four stages of mass loss and two weight gain steps as shown in Figure 5a. The first stage of about 3% mass loss from room temperature to 110 °C is due to the evaporation of adsorbed water, corresponding to the endothermic peak at about 100 °C. The weight loss during 230–700 °C can be attributed to the decomposition of crystalline water and the adhered hydroxyl from a small amount of ethylene glycol solvent, polyethylene glycol surfactant, and adsorbed hydroxyl at the second stage [51]. At the second and the third endothermic peaks at 266 °C and 338 °C, they are assigned to the loss of the crystal water, structure water within Fe₃O₄ crystal, and the loss of the adhered hydroxyl. The sharp endothermic peak at 266 °C corresponds to the formation of γ-Fe₂O₃ with spinel phase from Fe₃O₄. The weight loss at 700–1000 °C may be ascribed to the phase transition of Fe₃O₄ nanoparticles [52]. Two obvious endothermic peaks at 710 °C and 789 °C resulting from the disappearance of spinel crystal were observed because of the change from phase γ-Fe₂O₃ to α-Fe₂O₃ phase (Curie temperature of α-Fe₂O₃ T_c = 747 °C). For Fe₃O₄/SiO₂ core-shell composite nanospheres (Figure 5b), only about 3.4% of the weight loss was caused, indicating that the thermally stable SiO₂ shell was successfully deposited onto the surface of magnetic core [53], and the loss mainly between room temperature and 160 °C, corresponding to the endothermic peak at 100 °C, might be owing to the decomposition of the adsorbed water and ethanol. The endothermic peak at 710 °C may be the volatile removal of high temperatures. From Figure 5c, 5% weight loss in the Fe₃O₄/SiO₂/PPy composite nanospheres during room temperature and 300 °C was associated with the evaporation of absorbed water and removal of pyrrole monomer, in keeping with the two endothermic peaks at 98 °C and 290 °C. There was a weak peak at 400–500 °C and a sharp peak at 638 °C due to the destruction and decomposition of PPy chains, which indicates that PPy chains can only be destroyed at a higher temperature and have good stability. There is no mass loss at around 400 °C, indicating that there were no polypyrrole chains that were degraded. The higher mass loss in Figure 5c than Fe₃O₄/SiO₂ composite nanospheres may be related to the thermal instability of the coated PPy shell on the SiO₂ layer [54]. These results indicate that the SiO₂ shell layer and the PPy shell are present in Fe₃O₄/SiO₂/PPy composite nanospheres.

Table 1. Ibuprofen (IBU) loading and release of Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres.

Sample	Fe3O4/SiO2	Fe3O4/SiO2/PPy
Concentration of IBU (mg/mL)	1	1
Drug loading (mg)	3.329	3.640
Encapsulation efficiency (%)	13.32	14.56
Drug loading efficiency (%)	33.29	36.40
Drug release efficiency (%)	91.82 (stirring for 84 h)	83.86 (stirring for 84 h)
	91.82 (ultrasound for 65 min)	90.73 (ultrasound for 65 min)

lists the drug loading, encapsulation efficiency, and drug loading efficiency of the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres for the same concentration of IBU-ethanol solution, and lists the drug release efficiency of the two IBU-loaded nanospheres under stirring and ultrasound, respectively. The Fe₃O₄/SiO₂/PPy composite nanospheres exhibited higher drug loading, encapsulation efficiency, and drug loading efficiency than that of the Fe₃O₄/SiO₂ composite nanospheres. The reason is that in Fe₃O₄/SiO₂/PPy composite nanospheres, not only SiO₂ can provide binding sites for IBU, but also the modified PPy outer layer with a large specific surface area and the amino groups on the framework of PPy can make drugs bond with nanospheres through hydrogen bonds as well as π–π stacking, which increases the drug loading capacities of the double-shell composite nanospheres.

In order to investigate the effect of drug release behavior, the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres were loaded by IBU-ethanol solution with the same concentration and then released under the same conditions. The drug release profiles of IBU from IBU-loaded composite nanospheres under stirring are graphically represented in Figure 6. It can be seen that these two composite nanospheres showed similar drug release behavior, except that the Fe₃O₄/SiO₂ core-shell composite nanospheres exhibited a slightly faster rate and a slightly higher release efficiency. With the increase of shell layer, the drug release rate slowed down, because, in the double-shell composite nanospheres, the drug release not only passes through the middle shell, but also through the outer shell, and their presence slows down the drug release rate. Similar to the study from Prabha et al., their team has synthesized and characterized Fe₃O₄-cyclodextrin (CD), Fe₃O₄-CD-polyethyleneglycol (PEG) and Fe₃O₄-CD-PEG-polyethyleneimine (PEI) nanocomposites loaded with 5-Fu. The results show that the electrostatic interaction and hydrogen bond interaction between the nanocomposites and drug molecules increased with the increase of the coatings, and then the drug encapsulation efficiency and drug loading efficiency increase. Among them, Fe₃O₄-CD-PEG-PEI nanocomposites are more suitable as the drug carriers. Similarly, with the increase of polymer coatings, drug release time increased, and slow drug release can be achieved. Therefore, the prepared Fe₃O₄-CD-PEG-PEI nanocomposites can be better used for anti-cancer drug delivery in cancer therapy [55]. From Figure 6, all of the nanospheres released about >35% of IBU in the first 5 h and >65% after 12 h. Most of the drugs adsorbed in the outer surface layer can be released, as well as some drugs bonded by hydrogen bonds that were also released continuously in 12 h. Other drugs attached to the inside of the composite nanospheres further reduce the release rate and achieve a slow drug release and meet the long-term continuous drug use of the human body. After 84 hours of drug release, the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres reached an efficient drug release rates of 91.82% and 83.86%, respectively, indicating the obtained composite nanospheres can be used as a better magnetic targeting drug delivery system.

Figure 7 shows the kinetic curves of Korsmeyer-Peppas model of Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres under stirring to describe the mechanism of sustained release kinetics. Further, the kinetic model parameters, including R-square values, kinetic constant, and release exponent, are given in

Table 2. Parameters of Korsmeyer-Peppas model for IBU release under stirring.

Sample	R2	k	n
Fe3O4/SiO2	0.99998	22.36	0.3409
Fe3O4/SiO2/PPy	0.99585	19.89	0.3606

. From Figure 7, we can see that there are good linear relationships between the logarithm of cumulative release and logarithm of time in the whole process of IBU release from the two nanospheres. Table 2 shows that the release exponents of the Korsmeyer-Peppas model are 0.3409 and 0.3606, respectively, and less than 0.5, therefore indicating that the release kinetics of the sustained release system conforms to Fick diffusion mechanism [56,57].

Because both Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres can be separated quickly after ultrasonication, the study of the drug release behavior under ultrasonication of the two composite nanospheres as shown in Figure 8 can be compared with that under stirring [58]. As can be seen from Figure 8, the drug release efficiency under ultrasonication was significantly accelerated, and the drug release efficiency reached about 90% of IBU within 65 min. This is due to the fact that the drug release mechanism no longer follows the 0-level release mode under the action of ultrasound, making the drug release efficiency significantly faster than that under stirring, which is consistent with the research by Shamsipur et al. [59]. The chitosan (CS)-Fe₃O₄ nanoparticles were synthesized and connected with ciprofloxacin through a hydrogen bond. The drug loading and the controlled release behavior of ciprofloxacin loaded CS-Fe₃O₄ under ultrasound was studied. The drug loading of ciprofloxacin reached 99%. Without ultrasound, the drug was sustainably released in the first 400 min, and the release was slow after five days. When low-frequency ultrasound was used, the drug release was promoted, and 95% of the drug was released within 60 min, achieving controlled release of the drug. These results indicate that the obtained Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres can not only be used for controlled drug release under stirring, but also provide a new drug carrier for drug release under the action of ultrasound.

The sustained release kinetic curves and the kinetic model parameters of Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy core-shell composite nanospheres under ultrasonication are shown in Figure 9 and

Table 3. Parameters of Korsmeyer-Peppas model for IBU release under ultrasonication.

Sample	R2	k	n
Fe3O4/SiO2	1	2.165	0.9008
Fe3O4/SiO2/PPy	1	1.398	0.9907

. It can be seen that the two release systems had the release exponent value n = 0.9008 and 0.9907, respectively. The magnitude of release exponent indicates that the kinetic release mechanism was non-Fick diffusion [36].

4. Conclusions

In summary, we report a monodisperse Fe₃O₄ nanoparticle with a particle size of about 180 nm obtained via a solvothermal method. The facile Stöber and hydrothermal methods were utilized to prepare the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres with obvious core-shell structure and monodispersity by coating Fe₃O₄ nanoparticles with SiO₂ and PPy shell in turn. The as-prepared Fe₃O₄ nanoparticles, and the Fe₃O₄/SiO₂ and Fe₃O₄/SiO₂/PPy composite nanospheres displayed the high saturation magnetizations of 104 emu/g, 77 emu/g, and 24 emu/g, respectively, indicating the great potential applications in drug delivery. The drug loading efficiency and drug release efficiency of the two composite nanospheres were all higher than 33% and 90% under ultrasonication, respectively. The release process of nanospheres under stirring followed a Fick diffusion mechanism, while nanospheres under ultrasonication followed a non-Fick diffusion. Therefore, this study may provide new functional nanocomposites for IBU loading and release.