3.1. Structural Characterization
Figure 1 depicts the schematic process of formation mechanism for magnetic drug carrier. In this experiment, Fe
3O
4@SiO
2 nanocomposites were prepared by hydrothermal method and their structure and crystallographic surfaces were characterized by XRD in order to further determine a better understanding of the formation of Fe
3O
4@SiO
2/rGO nanocomposites. To characterize the formation of Fe
3O
4 magnetic nanomaterials, XRD (
Figure 2) was used to determine their structure and crystallographic planes. The diffraction peaks appear at 2θ = 18.4°, 30.34°, 35.7°, 43.2°, 53.5°, 57.4°, and 62.8°, corresponding to the crystallographic planes (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1)), and (4 4 0). The XRD spectrum of Fe
3O
4 referring to the JCPDS database (File No. 19–0629) [
21], which shows the formation of Fe
3O
4 with a cubic anti–spinel structure. In addition, the formation of high–purity Fe
3O
4 can also be observed because there are no peaks of other phases in the XRD pattern. In
Figure 2, the XRD diffraction pattern of Fe
3O
4@SiO
2 shows a diffraction peak of 22.8°, which is the amorphous diffraction peak of SiO
2 [
22].
To further determine the formation of Fe
3O
4@SiO
2 composites, the characteristic peaks of the composites were determined using Fourier infrared spectroscopy. The infrared spectra of Fe
3O
4@SiO
2 (
Figure 3) were used to study its chemical bonding species, and the symmetric stretching vibration peaks of Si–O appeared at 467 cm
–1 and 796 cm
–1, the peak at 574 cm
–1 was the Fe–O vibrational absorption peak, the peak at 945 cm
–1 belonged to the bending vibration absorption peak of Si–OH, and the broad and strong absorption band at 1095 cm
–1 was the Si–O–Si. The peaks at 1625 cm
–1 and 3413 cm
–1 are the bending vibration peak and the antisymmetric stretching vibration peak of H–O–H of structured water, respectively. Among them, the absorption peak at 945 cm
–1 in
Figure 2 indicates the presence of silica hydroxyl group (Si–OH) on the surface of SiO
2 coated with Fe
3O
4, which is beneficial for the subsequent loading of anticancer drugs in combination with the mesoporous structure and large specific surface area of SiO
2.
The surface morphological characteristics of Fe
3O
4 and Fe
3O
4@SiO
2 were characterized by SEM, and the surface morphology is shown in
Figure 4. The magnetic Fe
3O
4 has a nanosphere structure with a rough and obvious granular surface. there is a certain agglomeration between the Fe
3O
4 particles, forming nanoclusters with large size and uneven distribution between the particles, which affects the magnetic iron oxide properties. In contrast, the surface of Fe
3O
4@SiO
2 (
Figure 4d) is smoother and has a uniform nanosphere structure. More importantly, the encapsulated SiO
2 can play a role in magnetic iron oxide to alleviate the agglomeration problem of nanoparticles and make the magnetic iron oxide uniformly dispersed, while increasing the specific surface area of magnetic iron oxide, which is beneficial to the loading of anti–cancer drugs.
The structure of graphene oxide is transparent muslin–like with shallow surface folds, while pure graphene and nitrogen–doped graphene still have the characteristics of light and transparent with significantly deeper surface folds and more bulges. The morphological characterization of the magnetic carrier composite formed after hydrothermal treatment with Fe
3O
4@SiO
2 is shown in
Figure 5d. The Fe
3O
4@SiO
2 nanospheres are uniformly dispersed on the surface and folds of nitrogen–doped graphene without extensive agglomeration, indicating that graphene can overcome the agglomeration problem of nanoparticles.
The TEM images in
Figure 6 show that Fe
3O
4@SiO
2 /N–rGO retains a hollow structure and a shell of exceptionally thin SiO2 after calcination treatment. HRTEM was used to further characterise the details of the hollow composites as shown in
Figure 6(c, d). Folded few–layer graphene can be clearly seen on the surface of Fe
3O
4@SiO
2 /N–rGO as shown in
Figure 6(a, b). Large Fe
3O
4@SiO
2 nanostructures are present on the surface of the carbon material.
The Raman spectra of the materials can also be used to characterize the layers, defects and crystal structure of rGO. the Raman spectrum of the Fe
3O
4@SiO
2/N–rGO composite (
Figure 7) shows several well–characterized peaks. The D peak at 1348.7 cm
–1 is caused by the reduced symmetry of carbon atom arrangement due to the finite size effect or defects of SP
3 carbon. the G peak at 1591.4 cm
–1 reflects the regular arrangement of SP
2 carbon and is the main characteristic peak of graphene carbon, reflecting the regular arrangement of SP
2 carbon. The degree of disorder in the crystal structure is reflected in the relative intensity of the D–band. The relative intensity of the D–band reflects the degree of disorder in the crystal structure. The D/G strength ratio of the magnetic carrier composite material is 1.18. As can be seen from the figure, the relative intensity of the D peak in the Raman pattern of graphite is significantly lower than that of Fe
3O
4@SiO
2/N–rGO, which also proves that the crystalline structure of graphite is more ordered, which also indicates that it has not been oxidized at this time.
We can find that the D peak of Fe3O4@SiO2/N–rGO is higher than that of graphite, and a strong D peak indicates that there are many defects, and more functional groups on the surface are conducive to drug loading. This also indicates that the Fe3O4@SiO2/N–rGO has more oxygen–containing functional groups, which is conducive to the next step of drug loading.
Zeta potential, also known as surface potential, is a characterization of the number of charges carried on the surface of the particles and can effectively characterize the stability of the material dispersion system. Initially, the absolute value of the potential of magnetic iron oxide is small, indicating that it has a high inter–particle attraction and tends to inter–particle agglomeration, which is prone to the typical agglomeration problem of nanoparticles when analyzed with SEM morphology (
Figure 4a); during the subsequent surface modification process, the potential of Fe
3O
4@SiO
2 nanospheres increases significantly, the electrostatic repulsion between particles increases, and the system is more physically stable. Fe
3O
4 @SiO
2/N–rGO has a slightly decreased potential value but still maintains the system stability.
3.1. Performance of Drug Carrier
For nanomagnetic targeted drug delivery carriers, they need to have to be well dispersed in the physiological environment and able to respond to external magnetic fields for targeted drug delivery.
Figure 9 shows a typical magnetization loop indicating that Fe
3O
4@SiO
2/N–rGO has superparamagnetic properties. The saturation magnetization (Ms) of Fe
3O
4 is 98.3 emu g
–1, which decreases with silicon layer cladding and graphene loading to 81.9 and 40.9 emu g
–1 for Fe
3O
4@SiO
2 and Fe
3O
4@SiO
2/N–rGO. However, the magnetic properties of Fe
3O
4@SiO
2/N–rGO, although decreased, still responded when an external magnetic field was applied, as shown in
Figure 9 (inset). Initially, it shows the good dispersion of Fe
3O
4@SiO
2/N–rGO in an aqueous solution in the absence of a external magnetic field. When a permanent magnet is placed, the magnetic composite is immediately attracted to the wall of the sample vial next to the external magnetic field.
Figure 8.
Zeta potential of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2/N–rGO.
Figure 8.
Zeta potential of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2/N–rGO.
Figure 9.
VSM of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2/N–rGO
Figure 9.
VSM of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2/N–rGO
The loading of the drug on the magnetic carrier was studied by electrochemical cyclic voltammetry. The graph (
Figure 10) shows the changes in the electrochemical signal of the carriers before and after drug loading. The drug is loaded on the carriers in two ways: one is loaded onto the silicon layer surface of magnetic nanospheres by electrostatic adsorption; the other is loaded onto nitrogen–doped graphene by hydrogen bonding interactions and π–π stacking. Fe
3O
4@SiO
2/N–rGO–EPI shows a higher current response, indicating that the drug can be loaded onto the graphene surface. Graphene has excellent electrical conductivity with the presence of off–domain electrons on the surface, and when the drug is loaded onto graphene via hydrogen bonding or π–π stacking, an effective electron transfer between the drug and graphene takes place, which in turn enhances the electrical conductivity. If the loading is only onto the silicon layer but not onto the graphene surface, no enhancement of the current response is observed and electron transfer is inhibited. Studying the drug loading will facilitate the subsequent drug loading and the study of pH sensitivity during drug release.
In drug delivery systems, the drug loading and release of drug carriers are the key factors for their use as drug carriers. Fe
3O
4@SiO
2/N–rGO magnetic nanocomposites are suitable as magnetic targeting drug carriers in biomedical fields. The drug loaded on the carrier in this study is EPI, which causes myelosuppression, induces heart failure, and many other conditions, and thus needs to be strictly controlled in small doses for treatment. The loading efficiency and drug loading of the magnetic carrier loaded with EPI are shown in
Figure 11a. The lower the concentration of EPI, The higher the Fe
3O
4@SiO
2/N–rGO load efficiency, The load capacity of Fe
3O
4@SiO
2/N–rGO is 47.1%, which is much greater than Fe
3O
4@SiO
2–rGO, and the loading capacity is increased with the increase of the initial anti–cancer drug concentration. The silicon layer encapsulated on the outside of the magnetic iron oxide has a mesoporous structure and contains silicon hydroxyl groups on its surface. The graphene substrate also has a large number of functional groups such as hydroxyl and carboxyl groups, which can bind to the anticancer drug through π–π stacking and hydrogen bonding interactions. In addition, the surface of graphene is not smooth, and this folded structure has more spatial advantages than graphene oxide in the three–dimensional structure. With the increase of EPI concentration, a large number of drug molecules with hydroxyl groups and carbon six–membered rings will more fully form intermolecular forces with the functional groups on the carrier surface, and its loading will be larger. On the contrary, the loading efficiency decreases, and the loading of the drug at the relatively easy binding sites on the surface will prevent further loading.
Drug loading has been one of the challenges in the field of drug delivery systems. The lack of targeting of anticancer drugs by direct injection and the high burden on the patient’s body as they flow through the bloodstream after injection. In addition, anti–cancer drugs are expensive, and increasing the effective drug delivery rate can largely alleviate the financial pressure on patients’ families. In the case of externally applied magnetic field, Fe3O4@SiO2/N–rGO nanocomposites possess a magnetic field response mechanism and are able to use surface modification to load a large amount of drugs and direct them to a designated location for drug release. This drug carrier can reduce the drug dose and the drug delivery rate will be greatly improved, which has a great potential as a drug carrier for cancer treatment.
To investigate the environmental conditions and extent of drug release, the release profiles of magnetic drug carriers were plotted in different pH liquid environments. In this study, it was found that the carriers have a pH response mechanism and the drug release in an acidic environment is more different from a neutral liquid environment.
EPI of Fe
3O
4@SiO
2/N–rGO is released in phosphate buffer solutions with pH=7.2 and pH=4.3. The pH value of healthy cells in the human body is about 7.2. Emami et al.[
23] explored the effect of CS/PVP/ α– Fe
2O
3/Dox at pH=5.4 and pH=7.4 nanoparticles on the release curve of doxorubicin. At pH=5.4, nanocomposites release more drugs. From the curves in the
Figure 10a, the release rate of the anticancer drug EPI was faster in 4 h, and then tended to level off with the increase of release time, and the amount of drug released from the carrier was significantly higher under acidic conditions. The amino and hydroxyl groups contained in the drug and the functional groups of the carrier formed intermolecular forces through hydrogen bonding and π–π stacking. When the drug–loaded carriers are in an acidic environment, the hydrogen bonds formed between the functional groups will be broken faster and the intermolecular forces will be weakened. In addition, a large number of hydrogenions in the solution react with amino groups, and the protonation of amino groups will weaken the combination of drug molecules and carriers, so that the magnetic carrier can achieve the purpose of drug release.
Finally, an external magnetic field was applied to the magnetic drug carrier during the release process. At 39 °C, the EV secretion of breast cancer increased with the increase of temperature[
24], so the solution temperature increased by 2 ℃, which was conducive to killing the virus, indicating that the magnetic carrier had a certain impact on magnetothermic therapy[
25]. The heat transfer caused by temperature rise has a positive impact on drug release from the carrier.
Table 1 shows that the performance of Fe
3O
4@SiO
2/N–rGO compared with magnetic drug carriers reported in other literatures. In the research of targeted drug delivery, the drugs involved as drug delivery objects are 5–FU, paclitaxel, and DOX. Compared with these drugs, Epirubicin (EPI) is a new generation of anthracycline anticancer drugs, and its toxic side effects have been reduced. EPI is one of the most widely used anthracycline antitumor chemotherapy drugs in clinical practice. Currently, EPI is widely used in domestic clinical practice, but there are few researches on targeted delivery and lack of scientific data. It has been pointed out that although the toxic side effects of EPI have been reduced, it may still lead to severe bone marrow suppression and certain cardiotoxicity. Therefore, it is necessary to deliver the drug to the patient site through magnetic targeting drug carriers to improve the effective drug delivery rate and reduce the damage to the human body.