Novel Method To Investigate the Interaction Force between Etoposide

Article pubs.acs.org/JPCC

Novel Method To Investigate the Interaction Force between Etoposide and APTES-Functionalized Fe3O4@nSiO2@mSiO2 Nanocarrier for Drug Loading and Release Processes Weiwei Zhao, Bin Cui,* Hongxia Peng, Hongjin Qiu, and Yaoyu Wang Key Laboratory of Synthetic and Natural Functional Molecule Chemistry (Ministry of Education), Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, 1 Xuefu Avenue, Chang’an District Xi’an, Shaanxi 710127, P.R.China ABSTRACT: We report a 3-aminopropyltriethoxysilane (APTES) functionalized magnetic core−shell structure drug delivery system which is composed of a nonmesporous silica (nSiO2) coated Fe3O4 nanoparticle as the core and mesoporous silica (mSiO2) as the shell (designated Fe3O4@nSiO2@mSiO2APTES). These spheres have superparamagnetism, high magnetization (39.1 emu/g), a large surface area (222.35 m2/g), uniform accessible mesopores (2.5 nm), and abundant amino groups on the mesoporous shell. We investigated the thermodynamic and kinetic properties for the drug loading and release processes using microcalorimetry for the first time. The drug loading process was exothermal, and the release process was endothermal. A series of thermodynamics parameters ΔH, ΔS, and ΔG for drug loading and release processes was calculated. For drug loading process, the critical value was determined, the kinetics equation dα/dt = 10−3.66(1 − a)1.01, and rate constant k = 10−3.66 s−1 was also obtained. The results show that the interaction between the drug molecule and the nanocarrier is a hydrogen-bond interaction which was derived from the experimental values of the molar enthalpies (ΔH < 0) and molar entropy (ΔS < 0). This method therefore promise to provide a theoretical basis for the interaction between the drug molecule and the nanocarrier, so as to guide an externally controlled drug-delivery system in cancer therapy.

1. INTRODUCTION

Controlled delivery of drugs with nanocarriers can overcome problems caused by conventional free drugs, including poor solubility, limited stability, rapid clearing, and lack of selectivity in particular.16−18 The efficacy of a drug strongly depends on the method employed for its delivery. Different stimuli have been investigated to fabricate responsive systems, including internal control and external control release, such as pH,19 temperature,20 radiation,21 enzymes,22 and other chemical species.23 For the delivery system to be effective, favorable interactions between the drug and the surfactant molecules are essential requirements.24 Traditional monitoring methods (infrared, UV−vis, elemental analysis, 1HNMR, thermogravimetry (TG), and drug release in vitro experiment, etc.)25,26 are used to investigate the reaction force of drug loading/releasing process. These methods are not only complicated to operate but also have some limitations to provide theoretical thermodynamic and kinetic data for scientific research. This is because the reaction force between the carrier and the drug is weak, not a chemical bond. However, microcalorimetry can monitor the weak change in the heat in real time, that is, it can detect the heat change of drug loading/release processes to

In recent years, Fe3O4-based core−shell nanocarriers for targeted drugs and controlled release have attracted great interest and become an international hotspot in research.1−4 In order to load drug effectively, the drug carrier will be often required to achieve nanoscale or have a high specific surface area. In recent years, the burgeoning interest in mesoporous silica nanoparticles (MSN) has been greatly spurred since they exhibit more flexible and robust properties, including excellent chemical stability, easy modification, nontoxicity, and prominent biocompatibility.5−8 Moreover, the large surface area and pore volume for MSN ensure facile transporting as well as high loading of various guest molecules.9,10 Deng et al. reported mesoporous silica spheres as a shell for removal of microcystios.9 Xu et al. used mesoporous silica load anticancer drugs ibuprofen.10 Surface modification of nanoparticles can give some of its unique properties. For example, it has unique catalyst properties using amino acids modification11 and using β-CD,12 DNA,13 polymer,14 and 3-aminopropyltriethoxysilane (APTES), etc.,15 in drug delivery system. APTES can provide a rich NH2 group on the surface of nanocarriers and form hydrogen bonds with drugs. Anticancer drugs in common use are organic such as etoposide (VP16), adriamycin (DOX), camptothecin, paclitaxel, etc., containing a −OH and NH2 reactive group or with a positive charge. © XXXX American Chemical Society

Received: December 14, 2014 Revised: February 8, 2015

A

DOI: 10.1021/jp512447s J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Schematic Illustration of the Preparation Process of Fe3O4@nSiO2@mSiO2-APTES Microspheres, and the Study of the Drug Loading and Release Processes Using the Microcalorimetric Method

(C9H23NO3Si, APTES, purity > 99.0%) were purchased from the Shanghai Chemical Reagent Factory. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS) was purchased from Tianjin Chemical Co., Ltd. of China. Etoposide (VP16, purity > 99.0%) was received from Shanghai Adamas Reagent Co., Ltd. Other chemicals were purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. Deionized water was used in all experiments. APTES-functionalized Fe3O4@nSiO2@mSiO2 microspheres were prepared according to the reported method.9,14 Briefly, 0.10 g of Fe3O4 particles was homogeneously dispersed in a mixture of ethanol (80 mL), deionized water (20 mL), and concentrated ammonia aqueous solution (1.0 mL, 28 wt %), followed by addition of TEOS (0.03 g, 0.144 mmol). After stirring at room temperature for 6 h, the Fe3O4@nSiO2 microspheres were separated and washed with ethanol and water and redispersed in a mixed solution containing CTAB (0.30 g, 0.823 mmol) and deionized water (80 mL), concentrated ammonia aqueous solution (1.00 g, 28 wt %), and ethanol (60 mL). Then 0.40 g of TEOS (1.90 mmol) was added dropwise to the dispersion with continuous stirring. After reaction for 6 h, the product was collected and separated. Finally, the purified microspheres were redispersed in 60 mL of acetone and refluxed at 80 °C for 48 h to remove the template CTAB. Fe3O4@nSiO2@mSiO2 microspheres were finally produced. For APTES-functionalized Fe3O4@nSiO2@mSiO2 nanocomposites, Fe3O4@nSiO2@mSiO2 nanocomposites (20 mg) were added to ethanol (30 mL) followed by addition of water (2 mL). Then ammonium hydroxide (25%, 2 mL) and APTES

accurately determine the reaction force types and then provide theoretical guidance for drug-controlled release. Herein, an APTES-functionalized magnetic core−shell structure drug delivery system was prepared (denoted as Fe3O4@nSiO2@mSiO2-APTES below) by using the Fe3O4@ nSiO2 nanoparticle as the core, which can achieve the purpose of targeted drug delivery, and mesoporous silica as the shell, which can load drug as much as possible. The reason that we used APTES is because it is an important silane coupling agent and a widely used grafting agent to promote interfacial behavior of magnetic iron oxide nanoparticles (MNPs)27 and provide s rich NH2 group on its surface. A natural medicine etoposide (VP16) had been selected as a model anticancer drug to evaluate the drug load and release function of the carriers. In this article, we first demonstrate microcalorimetry as a novel method to monitor the reaction between the drug molecule and the nanocarrier under physiological conditions and in vitro drug release. The preparation process of Fe3O4@nSiO2@mSiO2APTES microspheres and the study of the drug loading and release processes using the microcalorimetric method are shown in Scheme 1, so as to achieve the aim of controlling drug loading and release process effectively. Thus, this can provide a novel method to positioning, timing, and quantitation of the drug loading and release process.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. All chemical reagents used in this experiment were of analytical grade without further purification. Ferric chloride hexahydrate (FeCl3·6H2O), sodium acetate (CH3COONa, purity > 99.0%), ethylenediamine (C2H8N2, purity ≥ 98.0%), and 3-aminopropyltriethoxysilane B


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0.05 μV·mW−1 at 309.65 K. The release process was performed using a microcalorimeter of Tian-Calvet type (C80 from Setaram) in air atmosphere and then heated from 25 to 60 °C at a 0.2 °C min−1 heating rate.

(200 μL) were added to the above solution. The resulting solution was stirred for about 2 h at 80 °C. After washing and separating the product, the sandwich structures containing highly aminated superparamagnetic mesoporous composite spheres (denoted as Fe3O4@nSiO2@mSiO2-APTES) were obtained.15 2.2. Thermodynamic Process for Drug Loading/ Release Systems. The drug loading process was measured by an RD496-2000-type microcalorimeter at 309.65 K. The calorimetric constant of 309.65 ± 0.01 K was determined by the Joule effect before the experiment, which was 64.25 ± 0.050 μV·mW−1. The enthalpy of a solution of KCl in deionized water was measured to be 17.585 ± 0.045 kJ·mol−1, which is in good agreement with the value of 17.581 ± 0.039 kJ·mol−1 in ref 27. The relative deviation of the experimental result from the value in the reference is 0.03%, which indicates that the calorimetric system is accurate and reliable. The proper amounts sample of VP16 (2.5, 5.0, 10.0, 12.5, 15.0, and 20.0 mg) and 10 mg of nanocarrier Fe3O4@nSiO2@ mSiO2-APTES were dissolved in 1.50 mL of dimethyl sulfoxide (DMSO) to loading drug at 309.65 K under atmospheric pressure. The reason we choose DMSO as the solvent is that DMSO appears to be a very good solvent with little or no effect on pharmacological action.29 The enthalpy of the process was detected by a RD496-2000 calvet microcalorimeter. The curves describe the entire drug loading process. The obtained product was denoted as Fe3O4@nSiO2@mSiO2-APTES-VP16. The drug release process was measured with a Setaram C80 calorimeter equipped with a membrane mixing cell. The temperature accuracy of the calorimeter was 0.1 K. The calorimeter was calibrated by using two well-known recommended chemical reference systems: cyclohexane−hexane and methanol−water. The calibration procedure indicated for this type of equipment was utilized. The cyclohexane−hexanebinary system was used to determine the sensitivity of the calorimeter, and the methanol−water binary system was used to check the accuracy of the calibration. The average accuracy of the calorimeter was found to be 2.6%.30 In this study, the prepared sample of Fe3O4@nSiO2@mSiO2APTES-VP16 (9.5 mg) was dissolved in 1.50 mL of sodium chloride solution (0.9% w/v, similar to the normal saline of the human blood system, pH 5.8) with mild stirring, which was sealed in a C80 membrane mixing vessel (volume 8.5 mL) in air atmosphere and then heated from 25 to 60 °C at a 0.2 °C min−1 heating rate. 2.3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a XRD system (Bruker, D8 Advance) at room temperature using Cu Kα radiation (Kα = 1.54059Å). Transmission electron microscope (TEM; FEI, Tecnai G2 F20 S-TWIN) Fourier-transform infrared (FT-IR) spectra were obtained using a Tensor-27 infrared spectrophotometer (Bruker) with the KBr pellet technique. Nitrogen adsorption/desorption analysis was measured at a liquid nitrogen temperature (77 K) using a micromeritics ASAP 2010 M instrument. UV−vis adsorption spectral values were measured on a UV-1800 spectrophotometer. Magnetization measurements were performed on a vibrating-sample magnetometer (VSM, Quantum Design, MPMS-XL-7). The above measurements were performed at room temperature. The experimental loading process was performed using a RD4962000 Calvet microcalorimeter (Mianyang CAEP Thermal Analysis Instrument Co., China). The microcalorimeter was calibrated by the Joule effect, and its sensitivity was 64.25 ±

3. RESULTS AND DISCUSSION 3.1. Microstructure and Properties of As-Prepared Fe3O4@nSiO2@mSiO2-APTES. As-prepared Fe3O4@nSiO2@ mSiO2-APTES nanocarrier is a typical “core−shell” structure with a magnetite core (∼167 nm), a nonporous silica layer in the middle layer, and an ordered mesoporous silica phase in the outer layer as can be clearly observed (Figure 1A−C). In our

Figure 1. TEM image of (A) Fe3O4@nSiO2 particles. (B) TEM and (C) HRTEM images of Fe3O4@nSiO2@mSiO2-APTES microspheres. (D) XRD patterns of Fe3O4 particles (a) and Fe3O4@nSiO2@mSiO2APTES-VP16 microspheres (b).

synthesis system, the silica oligomers interact with the CTAB template via Coulomb forces, and both of them cooperatively assemble on the Fe3O4@silica nanocarriers’ surface, and the ordered mesostructure is formed. Notably, due to their unique perpendicular orientation, the mesopore channels of the nanocarrier are readily accessible, favoring the adsorption and release of large guest objects triggered by external stimulus. XRD patterns of the pure Fe3O4 and Fe3O4@nSiO2@mSiO2APTES-VP16 samples are displayed in Figure 1D. For Fe3O4 (Figure 1D-a), the diffraction peaks can be readily indexed to a face-centered cubic structure (Fd3m space group) of magnetite according to JCPDS card No. 19-0629. In the case of Fe3O4@ nSiO2@mSiO2-APTES-VP16 (Figure 1D-b), after coating of SiO2, a new broad peak around 23° can be assigned to the amorphous SiO2 shell of Fe3O4@nSiO2@mSiO2-APTES (PDF no. 38-0651) (marked as a clover). Successful amino functionalization of Fe3O4@SiO2@mSiO2APTES can be confirmed by IR spectra as shown in Figure 2. For samples A and B, the peak at 574 cm−1 corresponds to Fe− O stretching vibration. Peaks at 1091, 799, and 466 cm−1 can be ascribed to the stretching and deformation vibrations of SiO2. The weak and broad band around 3400 cm−1 is the typical O− H stretching mode caused by hydrogen bonds. For Fe3O4@ C


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analysis. In addition, because of the magnetic behavior of the metallic core, NMR analysis cannot be applied (field distortion). Fe3O4@nSiO2@mSiO2-APTES and Fe3O4@nSiO2@mSiO2APTES-VP16 microspheres have magnetization saturation values of 40.4 and 39.1 emu/g, respectively (Figure 3A). The magnetization saturation values of Fe3O4@nSiO2@mSiO2APTES-VP16 compared with Fe3O4@nSiO2@mSiO2-APTES have only reduced 1.3 emu/g. As a result of high magnetization, the Fe3O4@nSiO2@mSiO2-APTES-VP16 nanocarrier shows a fast response to the external magnet field and can be readily separated from the dispersion in less than 10 s (Figure 3A, inset), which makes them favorable in diverse applications. Thus, it is sufficient to achieve the goal of targeting drug delivery. Nitrogen sorption measurements were conducted to further characterize the pore parameters of the products. The liquid nitrogen adsorption/desorption isotherms of Fe3O4@nSiO2@ mSiO2-APTES are shown in Figure 3B. The loops of sample exhibit typical IV-type isotherms with H4 hysteresis according to the IUPAC classification, which is usually related to mesoporous silica channels. The BET surface area and total pore volume of Fe3O4@nSiO2@mSiO2-APTES are calculated to be 222.35 and 0.14 cm3/g, respectively. The pore size distribution curves (inset in Figure 3B) further confirm that the average pore size of Fe3O4@nSiO2@mSiO2-APTES was 2.5 nm. The N2 adsorption/desorption results suggest that the pore structure nanoparticles have mesoporous nature and suitability as drug carriers. The result reveals that the mesoporous silica layer has been coated on the surface of the outer Fe3O4@nSiO2 using CTAB as template and formed the pore structure. 3.2. Researching Drug Loading Mechanism Using Microcalorimetry. 3.2.1. Thermochemical Behaviors and Thermodynamic of Drug Loading Processes. Different amounts of sample of VP16 (2.5, 5.0, 10.0, 12.5, 15.0, and 20.0 mg) and 10.0 mg of nanocarrier Fe3O4@nSiO2@mSiO2APTES were dissolved in 1.50 mL of DMSO to loading drug at 309.65 K under atmospheric pressure. The curves describe the entire drug loading process as shown in Figure 4. The heat energy (−Q) can be calculated through intergration of the thermogram curve (Figure 4), and the molar enthalpy (ΔH) can be calculated by equation ΔH = −Q/n; the results are listed in Table 1. The heat effect (−Q) versus the amount of

Figure 2. FT-IR spectra of Fe3O4@nSiO2@mSiO2 (A), Fe3O4@ nSiO2@mSiO2-APTES (B), Fe3O4@nSiO2@mSiO2-APTES-VP16 (C), and VP16 (D).

SiO2@mSiO2-APTES, an obvious peak at 1630 cm−1 is the characteristic absorption peak originated from the symmetrical −NH3+ bending. A weak peak at 2940 cm−1 is the characteristic absorption peak of −CH2−; this is because the silanol on the surface of mesoporous SiO2 and the triethoxy of APTES has a reaction to form −Si−O−Si−CH2CH2CH2−NH2, which further verified the aminopropyl of APTES has connected to the silica shell. Thus, the mesoporous silica and APTES were connected by covalent bonds.31 The FT-IR spectrum of Fe3O4@nSiO2@mSiO2-APTESVP16 (Figure 2C) shows a strong peak at 1483 cm−1, which corresponds to the characteristic peak of the stretching vibrations of CC in the backbone of the aromatic phenyl ring.32 A band that can be assigned to CO (1765 cm−1) is apparent but shows a slight decrease in absorption compared with pure VP16 (Figure 2D). Moreover, there are clear VP16 absorption bands at 1223 and 1105 cm−1, which correspond to C−H vibration. The band at 1484 cm−1 can be assigned to absorption by the quaternary carbon atom,33 which confirms successful incorporation of VP16 into the surface of Fe3O4@ nSiO2@mSiO2-APTES nanoparticles. We detected no additional peaks for new chemical bonds, which indicate that no chemical reaction occurred between Fe3O4@nSiO2@mSiO2APTES and VP16. The VP16 drug molecules therefore appear to have been linked to APTES. From the structure of APTES and VP16 it can be seen that no chemical bond forms between them, the interaction force between them is only intermolecular forces or a hydrogen bond, so there is no need for NMR

Figure 3. (A) Magnetic hysteresis loops of (a) Fe3O4@nSiO2@mSiO2-APTES and (b) Fe3O4@nSiO2@mSiO2-APTES-VP16. (B) N2 adsorption/ desorption isotherm of Fe3O4@nSiO2@mSiO2-APTES nanocomposites. (Inset in B) Pore size distribution curve obtained from the adsorption data. D


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drug) is the critical value. This may be associated with the pore size and porosity of nanocarriers, nanoparticle size, and the interaction between drug molecule and APTES. Therefore, the released heat value (−Q) is nearly steady at the macro level when the amount of VP16 is more than 10.0 mg. To the critical value (10.0 mg) the released heat (−Q) and the molar enthalpy (ΔH) of the loading process are −262.6 mJ and −15.46 kJ· mol−1, respectively, which can provide a theoretical basis for clinical research. On the basis of these experimental data and calculated results the thermodynamic parameters and kinetic parameters of the drug loading processes were obtained through eq 6, which was deduced from eqs 1−534 ΔG⧧ = −RT ln k⧧

k=

Figure 4. Heating rate (dH/dt) vs time of the drug loading processes of Fe3O4@nSiO2@mSiO2-APTES and different amounts of VP16 (2.5, 5.0, 10.0, 12.5, 15.0, and 20.0 mg).

−Q (mJ)

ΔH (kJ mol−1)

2.5 5.0 10.0 12.5 15.0 20.0

0.004 0.008 0.017 0.021 0.025 0.034

−181.1 −199.7 −262.6 −254.1 −245.5 −249.6

−42.64 −23.50 −15.46 −11.96 −9.63 −7.35

(2)

⎡ RT ⎤ ΔG⧧θ = RT ln⎢ ⎣ Nhk ⎥⎦

(3)

RT −ΔG⧧θ / RT e Nh RT (T ΔS⧧θ −ΔH⧧θ)/ RT = e Nh RT ΔS⧧θ / R −ΔH⧧θ / RT = e e Nh

k=

Table 1. Drug Loading Enthalpy of Fe3O4@nSiO2@mSiO2APTES-VP16 in the Loading Processesa n (mmol)

RT ⧧ k Nh

and then

substance relationships of VP16 in the loading processes are shown in Figure 5.

m (mg)

(1)

(4)

From the above formula it can be obtained ln

m is the mass of drug VP16, n is the amount of substance, −Q is the heat effect produced during the drug loading processes, and ΔH is molar enthalpy during the processes.

a

⎛ ΔS θ k ⎞ ΔHmθ k = ⎜⎜ m + ln B ⎟⎟ − T h⎠ RT ⎝ R

(5)

Equation 5 can be changed into the follow expression ln

k ΔS ΔH = − kBT R RT

(6)

Substituting k = 10−3.66 s−1, kB = 1.38 × 10−23 J K−1, h = 6.626 × 10−23 J s−1, R = 8.314 J mol−1 K−1, ΔH (see Table 1), T = 309.65 K into eq 6, ΔS can be obtained; then putting ΔH and ΔS into the following formula (eq 7), ΔG can be obtained. The data calculated are listed in Table 2. (7)

ΔG = ΔH − T ΔS

From the calculated data listed in Table 2 we can see that during the whole drug loading processes ΔH ranges from −42.64 to −7.35 kJ mol−1, which are all negative, that is, the combination process between the drug molecule and the nanocarrier is an exothermic reaction. We know that the main reaction force can be determined for different combination

Figure 5. Relationship between the heat effect (−Q) and the amount of VP16 (n).

Table 2. Thermodynamic Parameters of the Drug Loading Processes at 309.65 K

We can see from Figure 4 that the six experimental processes of different sample loading amount are all exothermic processes. From Table 1 and Figure 5 we can see that the released heat energy (−Q) is increased from −181.1 to −262.6 mJ with the increase of the amount of drug VP16 when it is less than 10.0 mg. However, when it is more than 10.0 mg, the released heat energy (−Q) is about 250 mJ (from −254.1 to −249.6 mJ), that is, its combination with carrier has been basically saturated. In other words, 10.0 mg (the amount of E

m (mg)

ΔH (kJ mol−1)

ΔS (J mol−1 K−1)

ΔG (kJ mol−1)

2.5 5.0 10.0 12.5 15.0 20.0

−42.64 −23.50 −15.46 −11.96 −9.63 −7.35

−452.98 −391.19 −365.20 −353.92 −346.40 −339.01

97.63 97.63 97.63 97.63 97.63 97.63


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number. By substituting the experimental data into the kinetic eq 8 we obtain a linear relationship between ln[1 − (Ht/H0)] and ln[1/H0(dH/dt)] as shown in Figure 6. The slope represents the reaction order n, and the intercept represents ln k. The results are listed in Table 4.

systems according to the combination thermodynamic parameters of different temperatures (see Table 3). According Table 3. Criterion of the Main Acting Force34 criterion

main acting force van der Waals force (−ΔH in 0.4−4.0 kJ mol−1) hydrogen-bond interaction (−ΔH in 5−30 kJ mol−1) electrostatic force

ΔH < 0, ΔS < 0 ΔH < 0 or ΔH ≈ 0, ΔS > 0 ΔH > 0, ΔS > 0

hydrophobic interactions

to the thermodynamic parameters, the main acting force between nanocarriers and VP16 was determined. In this context Ross and Subramanian reported that when ΔH < 0 or ΔH ≈ 0, ΔS > 0, the electrostatic force demonstrates the interaction, when ΔH < 0, ΔS < 0, van der Waals interactions or hydrogen bonds demonstrate the reaction, and when ΔH > 0, ΔS > 0, hydrophobic interactions demonstrate the binding process.35 By applying this analysis to the drug loading system, we determine that the van der Waals force or hydrogen bond was the most important factor contributing to the observed negative ΔH and ΔS (see Table 2) and hence to the stability of drug loading process. The energy for hydrogen-bond interaction is less than 200 kJ mol−1, generally for 5−30 kJ mol−1. However, the extent of van der Waals force is commonly 0.4−4.0 kJ mol−1. Thus, the interaction force between the drug molecule and the nanocarrier is a hydrogen-bond interaction based on the data listed in Tables 2 and 3. The hydrogen-bond interaction is weak, so it is very easy to break. The negative value of the entropy of activation (ΔS) indicates that the drug loading process in DMSO gets a more ordered system. In the binding system of drug loading, the Gibbs free energy change ΔG listed in Table 2 is a very important parameter, which reflects the binding degree and stability of the formed adduct.36 The values of ΔG are found to be positive, indicating that the drug loading process in DMSO is stable at 309.65 K by the isothermal equation. In other words, it is a stable system in thermodynamical fields for the drug loading process in DMSO. 3.2.2. Kinetics of the Drug Loading Process. We choose the optimal loading amount of VP16 (10.0 mg) to discuss the kinetics of the drug loading process. Equations 8 and 9 are chosen as the model function describing the drug loading processes37 dα = kf (α) dt

Figure 6. Plot of ln[1 − (Ht/H0)] vs ln[1/H0(dH/dt)] for various masses of VP16 dissolved in DMSO at 309.65 K.

Table 4. Reaction Order n and ln k of Drug Loading Processes

f (a) = (1 − a)

(9)

Combining eqs 8 and 9 yields dα = k(1 − a)n dt

(10)

Substituting a = Ht/H0 into eq 10 we get ⎡ ⎡ 1 ⎛ dH ⎞ ⎤ ⎛ Ht ⎞ ⎤ ⎟ ⎥ = ln k + n ln⎢1 − ⎜ ln⎢ ⎜ ⎟⎥ ⎢⎣ ⎝ H0 ⎠i ⎥⎦ ⎣ H0 ⎝ dt ⎠i ⎦

n

ln k (s−1)

r

2.5 5.0 10.0 12.5 15.0 20.0 average

1.19 0.71 1.01 0.92 1.04 1.09 0.99 ± 0.16

−8.43 −8.31 −8.43 −7.97 −8.22 −8.51 −8.31 ± 0.19

0.9952 0.0072 0.9939 0.9945 0.9966 0.9936

From Table 4 we can see that the average data of reaction order n of drug loading processes is 0.99 ± 0.16, which is similar to a quasi-first-order reaction. On the basis of the data for the critical mass of drug VP16 (10.0 mg) we can get the kinetics equation of the drug loading process which is dα/dt = 10−3.66(1 − a)1.01. This indicates that the reaction between drug molecules and nanocarriers is a simple reaction, which is occurs easily at low temperature, that is, it is easier to do drug loading using the nanocarriers. 3.3. Microcalorimetry Information in the Drug Release Process. The thermodynamic process of drug release is depicted in Figure 7. Figure 7a describes the entire process from 25 to 60 °C, and Figure 7b is the partially enlarged view. We can see that the drug release process is an endothermic reaction from 25 to 60 °C. Also, there are some obvious small endothermic peaks. From the above analysis of the drug loading process we know that the combination process between the drug molecule and the nanocarrier is an exothermic process and the main forces are hydrogen-bond interactions. Thus, when the drug molecule was released from the carrier it must absorb heat from the environment to destroy the hydrogen-bond interaction. An interesting phenomenon is also shown from the partially enlarged view (see Figure 7b). There are four obvious small endothermic peaks at 40−55 °C (40.77, 45.91, 50.27, and 53.99 °C). It can be seen from these data that the temperature is just right to avoid the body’s normal temperature, that is, the

(8) n

m (mg)

i = 1, 2, ..., L (11)

In these equations, a is the conversion degree, f(a) is the kinetic function, Ht represents the heat at time t, H0 is the heat of the whole process, k is the reaction rate constant of drug loading processes, n is the reaction order, and L is the counting F


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Figure 7. Plot of heat flow vs time of the entire release process (a) and partially enlarged view (b).

4. CONCLUSIONS In this study we reported the synthesis of a multicomponent and multifunctional magnetic nanocarrier Fe3O4@nSiO2@ mSiO2-APTES which can be potentially used for targeted drug delivery. For the drug loading processes, we researched the thermochemical behavior for different amounts of drug (VP16) and then found that the combination between the drug molecule and the nanocarrier is an exothermal process. We determined 10.0 mg of drug as the critical value for loading processes which are similar to a quasi-first-order reaction. Its kinetics equation is dα/dt = 10−3.66(1 − a)1.01; the rate constant is k = 10−3.66 s−1. The thermodynamics parameters, ΔH < 0, ΔS < 0, indicate that the interaction between the drug molecule and the nanocarrier is a hydrogen-bond interaction. The value of ΔG being positive indicates that it is a stable system in thermodynamical fields for the drug loading process in DMSO. The drug release process is an endothermic reaction from 25 to 60 °C. There are also four obvious small endothermic peaks (40.77, 45.91, 50.27, and 53.99 °C), which indicates that it will not release drug at the normal human body temperature. However, the departure may not be large between the first peak at 40.77 °C and the normal body temperature. Therefore, it has a good operability to control drug release through control of the outside condition; it will not be harmful to the human body.

nanocarrier loading drugs will not release early when injected into human body. However, it will release when the temperature of the lesion sites is changed by external stimulus, so as to achieve the purpose of controlled drug delivery. Therefore, it has a good operability to control drug release through temperature. At the same time, the absorbed heat of these four peaks is different, which may be due to the interaction type between the drug molecule VP16 and the nanocarrier or the hydrogen-bond number and the intensity connecting them. There may be four interaction forces in this system: (1) −NH2 of APTES can form an intermolecular hydrogen bond with the hydrogen-bond receptor of drug molecule VP16. Because the environment of the hydrogenbond receptor is different, it is leads makes the hydrogen bond diffuicult to form, that is, it needs different energy. There are 3 hydrogen-bond donors and 13 hydrogen-bond receptors of etoposide (VP16) to form a different intensity of the hydrogen bond. The molecular structure of etoposide is shown in Chart 1. (2) The −OH of Si−OH can form an intermolecular Chart 1. Anticancer Drug Etoposide (VP16)

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AUTHOR INFORMATION

Corresponding Author

*Phone:+86 029 8153-5030. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (Grant No. 21071115) and the Innovative and Entrepreneurial Training Program for National College Students (No. 201210697010).

hydrogen-bond Si−O---H−R with the −OH of drug molecule VP16. (3) It can form the hydrogen bond between the molecules of VP16. (4) It may form two hydrogen bonds at the same time, one of which meets the structure requirements and another formed a weak hydrogen bond because of the stereohindrance effect. Due to it being difficult or easy to form the hydrogen bond, the needed energy is different. Thus, destroying the hydrogen bonds may need to absorb different energy 1 < 2 < 3 < 4. This may be the reason for the small endothermic peaks in Figure 7b. These results can provide theoretical guidance for effectively controlled drug release through external control of temperature changes.

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