Materials Science and Engineering C 39 (2014) 56–60

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In vitro controlled release of vitamin C from Ca/Al layered double hydroxide drug delivery system Xiaorui Gao a,b,⁎, Le Chen b, Juan Xie a, Yaobing Yin a, Tao Chang a, Yancong Duan a, Nan Jiang a a b

College of Science, Hebei University of Engineering, Handan 056038, China School of Chemistry and Chemical Engineering, Changzhou University, Changzhou, 213164, China

a r t i c l e

i n f o

Article history: Received 21 September 2013 Received in revised form 6 February 2014 Accepted 17 February 2014 Available online 22 February 2014 Keywords: Layered double hydroxide Vitamin C Thermal stability Controlled release Drug delivery system

a b s t r a c t A new drug delivery system for vitamin C (VC), Ca/Al layered double hydroxide (LDH), is demonstrated in this work. VC anions were intercalated successfully in the Ca/Al LDH gallery by a coprecipitation method. The interlayer space of 9.8 Å suggests that VC anions are vertical to the LDH layers in the form of interdigitated bilayer. The loading of VC in LDH is 36.4 wt.%. The thermal stability of VC is significantly enhanced after intercalation. In vitro VC release results show that the release time of VC in a phosphate buffer at pH 7.4 was significantly extended, and the maximal percentage of VC released is 80% of the total. The Avrami–Erofe'ev equation most satisfactorily explains the release kinetics of VC, which is that the release of VC is mainly dominated by the ion-exchange reaction. © 2014 Published by Elsevier B.V.

1. Introduction Vitamin C (VC) is a water soluble vitamin and helps some of our most important body systems. For example, it can promote collagen biosynthesis, provide photoprotection, cause melanin reduction, scavenge free radical, and enhance the immunity (anti-viral effect) [1,2]. However, the solution of VC is unsettled for natural light, thermal and alkaline conditions, resulting in decomposition to biologically inactive compounds [3]. To prevent the decomposition of VC, delivering it to a specific location is very important. Layered materials as drug delivery vehicles have received much attention in recent years. Because the release of drugs in drug-intercalated layered materials is potentially controllable, these new materials have great potential as a delivery host in the pharmaceutical field [4,5]. Layered double hydroxide (LDH) is a layered solid with the general formula [M1 − n− )x/n mH2O (abbreviated as M1 − xM′x − A) where M xM′x(OH)2](A is a divalent cation, M′ is a trivalent cation, An − is an exchangeable anion, and m is the number of moles of co-intercalated water [6]. The LDH's basal layer has a positive charge due to the partial substitution of divalent cations by trivalent ones, and the interlayer space is electrically balanced by the intercalation of anions, together with water molecules [7]. Various kinds of inorganic or organic anions could be readily introduced and stabilised into the hydroxide interlayer by simple ion exchange reaction or coprecipitation [8–10].

⁎ Corresponding author at: College of Science, Hebei University of Engineering, Handan 056038, China. Tel.: +86 310 8578 760. E-mail address: [email protected] (X. Gao).

http://dx.doi.org/10.1016/j.msec.2014.02.028 0928-4931/© 2014 Published by Elsevier B.V.

Several studies [11–13] describing the intercalation and controlled release of VC in the LDHs have been reported. They focused on the Mg/Al, Zn/Al and Mg/Fe LDH systems, but few on the Ca/Al LDH system. Moreover, in most of them, the release of VC from LDHs was carried out by an ion-exchange reaction in CO2− 3 solution. In this paper, considering that Ca/Al LDH has good biocompatibility and almost does not have any acute cytotoxic effect for the human body [14], it was selected as the drug delivery system; meanwhile, VC anions were intercalated in its interlayer space by a simple coprecipitation method. In addition, the release experiment of VC from Ca/Al LDH was carried out in a phosphate buffer solution at pH 7.4 (simulated intestinal fluid) so that this material can be applied in the human body. 2. Experimental 2.1. Materials All chemicals used in this research were analytical grade reagents and used without further treatments. Deionised water used in the experiments was boiled for 30 min to remove any dissolved gases. 2.2. Preparation of LDH materials Ca2Al–NO3 was synthesised by a coprecipitation method as a reference [15]. Ca2Al–VC was prepared by the similar method. A base solution was prepared by dissolving 1.06 g of NaOH and 2.97 g of C6H7O6Na in 100 mL of deionised water. This solution was then dropped to 100 mL

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of a mixed salt solution prepared by dissolving 2.72 g of Ca(NO3)2 ∙ 4H2O and 1.88 g of Al(NO3)3 ∙ 9H2O in a flask under N2 flow. The flask was sealed, covered with aluminium foil and stirred for 120 h at 15 °C. Due to the unstable nature of VC, it was vital to protect the reaction mixture from air, light and excessive heat. Carrying out the reaction at 15 °C inevitably limited the rate at which ion-exchange could take place, and therefore it was required to run the reaction for the extended time period of five days. The product was filtered, washed with deionised water and a small amount of acetone to facilitate drying, and then dried in vacuum. 2.3. In vitro release studies 100 mg of the Ca2Al–VC was immersed in 50 mL of a phosphate buffer at pH 7.4 (simulated intestinal fluid) and stirred slowly (50 rpm) at 37 °C up to 300 min. 3.5 mL aliquots were taken at regular intervals and analysed by UV spectroscopy at 265 nm, corresponding to the typical absorption peak of VC [16]. For every aliquot removed, 3.5 mL fresh buffer was added. The amount of VC obtained at every interval during 300 min was quantified using the UV–vis spectrophotometer. The total amount of released VC is calculated as follows: mn = 50Cn ∙ Mvc + 3.5Cn − 1 ∙ Mvc, in which mn is the mass of released VC after taking 3.5 mL aliquots n times, Cn and Cn − 1 are molar concentrations of VC tested by a UV–vis spectrophotometer after taking 3.5 mL aliquots n times and n − 1 times, respectively, Mvc is the molar mass of VC. The solid product after releasing VC from Ca2Al–VC for 300 min was collected for further analysis of structure and composition. 2.4. Characterization Carbon, hydrogen and nitrogen (CHN) elemental analyses were carried out by a Carlo Erba EA1108 elemental analyser, and metal analysis by Fisons Horizon inductively coupled plasma optical emission spectroscopy (ICP-OES). X-ray powder diffraction (XRD) patterns were recorded using a PANalytical X'pert Pro diffractometer, equipped with a solid state X'Celerator detector. The diffractometer was operated with Cu Kα radiation (α1 = 1.5406 Å, α2 = 1.5433 Å, weighted average λ = 1.5418 Å) in reflection mode, at 40 kV and 40 mA. All the FTIR spectra were recorded on a Bio-Rad FTS 6000 FTIR spectrometer equipped with a DuraSamplIR II diamond accessory in the range of 450–4000 cm−1; 100 scans at 4 cm−1 resolution were collected. The absorption in the range 2350–1850 cm−1 is from the DuraSamplIR II diamond surface. Thermogravimetric analysis data were collected using a Rhoemetric Scientific STA-1500H: 20 mg of sample was heated in a corundum crucible between room temperature and 700 °C at a rate of 10 °C ∙ min−1 under a flowing stream of argon. UV–vis absorption spectroscopy was used to determine the content of vitamin C on a T60U PG Instruments UV–vis spectrophotometer using wavelength scan mode and quartz cuvettes.

Fig. 1. XRD patterns of (a) Ca2Al–NO3 and (b) Ca2Al–VC.

the VC anion have been determined to be 4.9 Å and 21.6 Å2, respectively [20]. We speculated that VC anions are vertical to the LDH layers in the form of interdigitated bilayer (the terminal hydroxyl group hydrogen bonded to the basal layer); meanwhile, some water molecules are also present in the interlayers. The structure scheme of Ca2Al–VC is shown in Fig. 2. According to inductively coupled plasma (ICP) data (Ca: 20.5 wt.%, Al: 6.83 wt.%), the molar ratio of Ca and Al is calculated to be 2:1. Combining CHN elemental analysis data (C: 15.0 wt.%, H: 3.83 wt.%, N: 0.64 wt.%), the chemical formula of Ca2Al–VC is Ca2 Al(OH)6[(C6H7O6)0.82(NO3)0.18] ∙ 1.7H2O. The loading amount of intercalated VC is 82% (36.4 wt.%), which is higher than those from some previous reports [11,13]. The possible reason is that the protection of light and longer reaction time in the synthesis process delay the decomposition of VC. 3.2. FTIR characterization Fig. 3 shows the FTIR spectra used to verify the intercalation of VC into the LDH host. The broad peak at around 3420 cm−1 is characteristic of all LDHs and arises from the ν(OH) absorptions of the hydroxide layers and co-intercalated water molecules. Absorptions occurring below 1000 cm−1 are associated with metal–oxygen vibrations within the hydroxide layers. In the spectrum of solid NaVC (Fig. 3a), the

3. Results and discussion 3.1. Powder X-ray diffraction and elemental analysis The XRD patterns of Ca2Al–NO3 and Ca2Al–VC are shown in Fig. 1. Ca2Al–NO3 exhibits the characteristic diffraction peaks of a well crystallised layered material, which are sharp and symmetric at lower theta angle, but weaker at higher angles. The basal spacing, d002 is 8.8 Å, which is in good agreement with a previous report [15]. In the XRD pattern of Ca2Al–VC, (002) and (004) basal reflections [17,18] shift to lower angle side with broadening of the reflections, implying that VC anions have been intercalated in LDH. Meanwhile, the intercalation decreases the crystallinity of LDH [10]. The basal spacing of Ca2Al–VC is calculated to be 14.6 Å from the d002-spacing. Subtracting the inorganic layer thickness (4.8 Å) [19], the interlayer distance is 9.8 Å. The length and cross-sectional area of

Fig. 2. Schematic illustration of the possible interlayer arrangement of VC in the Ca2Al–VC.

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Fig. 3. FTIR spectra of (a) NaVC, (b) Ca2Al–NO3 and (c) Ca2Al–VC.

absorption peaks relating to O\H and C\O bond stretches are found in the expected regions. The absorption at about 1380 cm− 1 can be assigned to the stretching vibration of NO− 3 as shown in Fig. 3b, and the absorption at 1617 cm−1 is attributed to the bending vibration of O\H bond in Ca2Al–NO3. Comparing with the FTIR spectra of Ca2Al–NO3 and NaVC, the assignment of the absorption peaks in the Ca2Al–VC spectrum can be determined. The most diagnostic absorptions are those occurring at 1362 cm− 1 and 1579 cm− 1 corresponding to vibrations within the lactone linkage (C\O\C) and of the carbonyl group (C_O), respectively [13,21]. These absorptions, whilst shifted to slightly higher wavenumbers (1389 cm−1 and 1582 cm−1, respectively) are possibly due to the weakening of intramolecular H-bonds upon intercalation. The weak bending model of H2O in the FTIR pattern of Ca2Al–VC disappeared, which may be resulted from overlap with a broad and strong peak at 1582 cm−1. Similar phenomenon also occurs in previous reports [11,12]. In addition, the absorptions at around 1185–1000 cm−1 corresponding to the C\O stretch vibrations are also present in Fig. 3c. The FTIR data likewise confirm that the VC anions have been intercalated in the interlayer spaces. 3.3. Thermogravimetric analysis Ca2Al–VC and pure VC (ascorbic acid) were analysed by thermogravimetry (TG) and differential thermal analysis (DTA) to determine the intercalated water content and the decomposition temperature of VC. The corresponding TG and DTA curves are shown in Fig. 4. Ca2Al–VC decomposes by the well-established route observed for most LDHs intercalated with organic molecules. In the first step, the co-intercalated water is lost at temperatures up to 120 °C [22], leaving a dehydrated LDH (calculated loss 7.8%, observed loss 7.6%). This is then followed by the loss of water from the hydroxide layers between 200 and 346 °C (calculated loss 13.7%, observed loss 17.3%). The third step from around 346 °C onwards is mainly the decomposition of interlayer anions. Based on the DTA data of Ca2Al–NO3 shown in Fig. 4c, beyond 420 °C, the decomposition of nitrate was observed, which is basically in accordance with the literature [23]. So, it is thought that the initial weight loss of Ca2Al–VC from 346 °C is mainly resulted from the decomposition of VC anions, and followed subsequently by a combination of the further decomposition of the guest including VC and NO− 3 , resulting in a final product that is a mixture of calcium and aluminium oxide [24], and carbon soot (calculated loss 24.3%, observed loss 20.7%). From the DTA data (in Fig. 4a and b), the decomposition of intercalated VC occurs at elevated temperatures. The strong decomposition temperatures correspond to 442 °C and 232 °C for Ca2Al–VC (a) and

Fig. 4. TG–DTA curves of (a) Ca2Al–VC, (b) pure VC (ascorbic acid) and (c) Ca2Al–NO3.

VC (b), respectively. The reason may be the strong host–guest interaction in Ca2Al–VC involving hydrogen bond and electrostatic attraction [25]. 3.4. Controlled release study The release studies “in vitro” of the three samples including pure VC, physical mixture of VC and Ca2Al–NO3, and simple Ca2Al–VC have been separately measured in a phosphate buffer at pH 7.4 for time intervals of up to 300 min. The amounts of released VC were estimated by the characteristic absorption peaks at 265 nm in the UV–vis spectra of VC, and the plots of percentage of VC released vs. time for all systems are shown in Fig. 5. As it can be seen, the curves from pure VC and the

Fig. 5. Results from release studies for three drugs tested in the phosphate buffers at pH 7.4. Empty squares: pure VC; solid circles: physical mixture (VC + Ca2Al–NO3); and solid triangles: Ca2Al–VC.

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physical mixture of VC and Ca2Al–NO3 are very similar, and an almost total release of VC is completed after 20 min. However, Ca2Al–VC significantly extends the release time of VC in the buffer. The release of VC basically attains equilibrium after 120 min and is then followed by a very slow release. In addition, the amount of VC released from physical mixture of VC and Ca2Al–NO3 is little less than that from pure VC. This is possibly resulted from a weak electrostatic interaction between VC molecules and LDH surface. The release curve of Ca2Al–VC is below those of the pure VC and the mixture. 58% of the initial amount of VC is released in the buffer after 30 min. It probably arises from the release of VC adsorbed on the surface of LDH particles and deintercalated by some phosphate buffer anions through the ion-exchange reaction. A more persistent and gradual release process occurs subsequently with a released percentage of 80% of the total after 300 min, which is higher than the previously published data [11–13,26]. The sustained release may be explained by the main ion-exchange reaction between the intercalated VC anions and phosphate buffer anions in the buffer solution. In a word, it can be concluded that Ca2Al–VC shows an obvious effect of controlled release of VC. The material shows higher availability of VC as a drug and could help increase the practical delivery activity of VC. The rate of drug diffusion or deintercalation out of the LDH matrix is controlled by the rigidity of the layers and the diffusion path length [27]. The release of VC from intercalated LDH material is not complete. The cause may be attributed to the possibility that the drug molecules are deeply embedded in the LDH host, and a complete release is very difficult due to a very strong H-bond and electrostatic interaction between VC anions and the layers. 3.5. Kinetic analysis In order to gain more insight into the kinetics of the release, we applied five models to fit these release curves. They are the Avrami– Erofe'ev equation, Elovich model, modified Freundlich equation, first order model and parabolic diffusion model [28–32]. Their mathematical forms are given in Table 1. In each model, fits of the models to the experimental release data in initial release stage (within 30 min) are shown in Fig. 6. Obviously, all models can be used to describe the release kinetic of VC except for the parabolic diffusion model. In other four models, according to the R2 values and visual inspection of these “linear” plots, it appears that the Avrami–Erofe'ev model is more appropriate for describing the release of VC. The Avrami–Erofe'ev model describes the release process in terms of the formation and expansion of nucleation sites. In order to gain more insight into the mechanisms involved in guest release, the reaction exponent (n) from the Avrami– Erofe'ev data is possibly valuable. In the Avrami–Erofe'ev model, the values of n may be determined from the slope of the fitted plots. Usually, n is close to zero or one, with zero representing instantaneous nucleation. However, the final value may also represent more than one possible mechanism, the identity of which may not be easily determined. The value of n in the phosphate buffer for Ca2Al–VC is around 0.9, which corresponds to the main ion-exchange reaction of VC with anions Table 1 Kinetic models used to analyse VC release.⁎ Model

Equation

Avrami–Erofe'ev Elovich Freundlich First-order Parabolic diffusion

ln(−ln(Ct / C0)) = n ln(kd) + nln(t − t0) 1 − Ct / C0 = aln(t − t0) + b ln(1 − Ct / C0) = ln(kd) + aln(t − t0) ln(Ct / C0) = −kd(t − t0) (1 − Ct / C0) / t = kd(t − t0)(−0.5) + a

⁎ C0 is the amount of guest VC in the VC–LDH at t = 0, Ct is the amount of guest VC in the VC–LDH at time t, and kd is the rate of release. a, b, and n are constants.

Fig. 6. Linear fitting of various models to the release of VC from Ca2Al–VC in a phosphate buffer at pH 7.4. Fits by (a) the Avrami–Erofe'ev model, (b) the Elovich model, (c) the first-order model, (d) the Freundlich model, and (e) the parabolic diffusion model, are shown.

in the phosphate buffer and minor anion diffusion [33]. It is likely that the presence of significant amounts of replacement anions in the phosphate buffer leads to ion exchange being the predominant release mechanism. The release of functional guests via this mechanism requires the replacement anions to move to the edges of the layers, and then force the functional guests out (hence, nucleation control). In order to certify that the VC release from Ca2Al–VC is mainly resulted from ion exchange reaction, we subsequently tested the final sample obtained after releasing VC from Ca2Al–VC for 300 min in the phosphate buffer solution by XRD and FTIR analyses. The XRD pattern is shown in Fig. 7a. Several characteristic XRD diffraction peaks by planes (002), (004), (006) and (110) of LDH appear. The basal spacing of final sample reduces to 10.5 Å, which is similar to that of HPO2− containing LDHs 4 [33,34]. This suggests that the ion-exchange reaction between VC anions and HPO2− 4 anions in the buffer takes place during the release process. The FTIR pattern (Fig. 7b) shows the broad band at ca. 1092 cm−1 due to the modes of δ(P–OH) and ν4(P–O), 849 cm−1 to ν1(P–O) and

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Fig. 7. (a) XRD patterns and (b) FTIR spectra of the samples before and after releasing VC from Ca2Al–VC in the phosphate buffer at pH 7.4 for 300 min.

778 cm−1 to ν4(P–O) [35]. Two peaks at ca. 1630 and 1372 cm−1 are attributed to the δ(P–OH) and ν(P–OH) [36]. Meanwhile, the FTIR absorption peaks of VC disappeared, indicating that most VC anions had been released from the interlayer spaces. Hence, by XRD and FTIR analyses, it can be confirmed that the release of VC from Ca2Al–VC in phosphate buffer at pH 7.4 mainly depends on ion-exchange reaction. Meanwhile, in order to obtain the residual content of VC in the final solid product, ICP and CHN elemental analysis data were obtained as follows: the weight percentages of Ca, Al, C, H and N are respectively 26.3 wt.%, 8.82 wt.%, 2.34 wt.%, 3.66 wt.% and 0 wt.%. Based on XRD and FTIR patterns of the final solid product, no corresponding characteristic peaks of CO23 − and NO− 3 appeared. So, it is concluded that its chemical formula is Ca 2Al(OH)6[(C6 H7O6 )0.10(HPO4)0.45] ∙ 2.0H2 O. Meanwhile, it is found that a very small amount of VC has not been released from the interlayer of Ca2Al–VC. The similar partial release of drug from interlayer space was also reported by some authors [25,33,37], which may be attributed to the possibility that the drug molecules were deeply embedded in the LDH host via co-precipitation. 4. Conclusions A novel drug carrier of Ca2Al–VC was prepared by a coprecipitation method. XRD, FTIR, TG–DTA, ICP and CHN elemental analysis prove that VC has been intercalated successfully in the interlayer space. VC that intercalated in LDH material has a better thermal stability in contrast to pure VC due to the strong host–guest interaction involving hydrogen bond and electrostatic attraction. The in vitro release studies show that there is no burst phenomenon at the beginning of release in the phosphate buffer at pH 7.4. The release curve attains the equilibrium after 120 min, and the maximal released percentage of VC is 80% of the total. The release mechanism shows that the release process of VC is mainly dominated by ion exchange reaction. Our results suggest that Ca/Al LDH material can be used as an excellent inorganic drug carrier for VC. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51202054, 21206026 and 81271665), the Hebei Provincial Natural Science Foundation of China (Nos. B2012402006 and B2012402011), and the Handan City Science and Technology Research and Development Project of China (No. 1221120095-4).

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