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Biomimetic synthesized bimodal nanoporous silica: Bimodal mesostructure formation and application for ibuprofen delivery
Materials Science and Engineering C 58 (2016) 1105–1111
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Biomimetic synthesized bimodal nanoporous silica: Bimodal mesostructure formation and application for ibuprofen delivery Jing Li, Lu Xu ⁎⁎, Nan Zheng, Hongyu Wang, Fangzheng Lu, Sanming Li ⁎ School of Pharmacy, Shenyang Pharmaceutical University, Wenhua RD 103, 110016, China
a r t i c l e
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Article history: Received 15 July 2015 Received in revised form 15 September 2015 Accepted 25 September 2015 Available online 28 September 2015 Keywords: Biomimetic synthesis Bimodal nanoporous silica Ibuprofen delivery Dynamic self-assembly
a b s t r a c t The present paper innovatively reports bimodal nanoporous silica synthesized using biomimetic method (BBNS) with synthesized polymer (C16-L-serine) as template. Formation mechanism of B-BNS was deeply studied and exploration of its application as carrier of poorly water-soluble drug ibuprofen (IBU) was conducted. The bimodal nanopores and curved mesoscopic channels of B-BNS were achieved due to the dynamic self-assembly of C16-L-serine induced by silane coupling agent (3-aminopropyltriethoxysilane, APTES) and silica source (tetraethoxysilane, TEOS). Characterization results confirmed the successful synthesis of B-BNS, and particularly, nitrogen adsorption/desorption measurement demonstrated that B-BNS was meso–meso porous silica material. In application, B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. After being loaded, IBU presented amorphous phase because nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU. In vitro release result revealed that B-BNS controlled IBU release with two release phases based on bimodal nanopores and improved dissolution in simulated gastric fluid due to crystalline conversion of IBU. It is convincible that biomimetic method provides novel theory and insight for synthesizing bimodal nanoporous silica, and unique functionalities of B-BNS as drug carrier can undoubtedly promote the application of bimodal nanoporous silica and development of pharmaceutical science. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The elaborate morphologies and gorgeous surface patterns of the external skeletons in biosilicifying organisms, including diatoms (microalga), radiolarian, sponges and plants (e.g. horsetail and rice leaves), are fascinating and have been focused great attention [1,2]. Biosilicification of nanoporous (1–100 nm) silica inspired by diatoms and radiolarian has completely changed the way of designing novel nanoporous silica [3,4]. For example, nanoporous silica synthesized through organic– inorganic liquid-crystal co-assembly method would shed light on the formation of biosilicification, as many macromolecules in biological organisms display similar characteristics of liquid-crystal self-assemblies [5]. The definition of biosilicification can be comprehended as the movement of silicic acid from environments to intracellular or systemic compartments in which it is accumulated for subsequent deposition as amorphous hydrated silica. It has been reported that various types of additives, including polypeptides, polysaccharides, peptides, polyamines, enzymes, proteins, and synthesized polymers containing amine groups can be used to achieve biomimetic synthesized nanoporous silica [3,4]. In application, biomimetic synthesized nanoporous silica plays important roles in catalysis, adsorption, separation, and biomedical systems [6–8]. ⁎ First corresponding author: Sanming Li, Wenhua RD 103, 110016 Shenyang, China. ⁎⁎ Second corresponding author: Lu Xu, Wenhua RD 103, 110016 Shenyang, China. E-mail addresses: [email protected] (L. Xu), [email protected] (S. Li).
http://dx.doi.org/10.1016/j.msec.2015.09.091 0928-4931/© 2015 Elsevier B.V. All rights reserved.
Hierarchically nanoporous silica with well-defined morphologies has been of growing interest, and a variety of templating approaches have been proposed to synthesize hierarchically nanoporous silica in the past decades. Bimodal nanoporous silica, including micro–meso, meso–meso, meso–macro, can be prepared using soft templates (such as colloidal particles, polymers, emulsion droplets, and surfactants) with abilities to create different sizes of nanopores [9–11]. The employment of different types of mixing templates failed to synthesize nanoporous silica with controlled well-ordered mesostructure and well-defined morphology. In contrast, dynamic templates generated from complex organic matrix can improve controllability of bimodal nanoporous silica inspired by biomimetic formation mechanism of time-dependent cooperative organization [11]. Though dynamic templates originating from organic mesomorphous complexes of polyelectrolyte (poly(acrylic acid), PAA; polystyrene-b-poly(acrylic acid), PSbPAA) and cationic surfactant (hexadecylpyridinium chloride, CPC; cetyltrimethyl ammonium bromide, CTAB) have been successfully applied to synthesize bimodal nanoporous silica [9–11], bimodal nanoporous silica synthesized using biomimetic method has been rarely reported. Nanoporous silica has wide application in biomedical fields due to the following advantages: (1) large surface area and pore volume provide great potential for drug adsorption and loading; (2) mesoporous (2 to 50 nm) structure and an adjustable pore size enable better control of drug loading and release; (3) surface can be easily modified to
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improve functionalities; and (4) in vivo biosafety evaluations of cytotoxicity, biodegradation, biodistribution and excretion [12]. Nowadays, the low solubility of active pharmaceutical ingredients is an increasing problem in pharmaceutical industry and therefore several different approaches have been designed to overcome this obstacle, and nanoporous silica represents a potential category among these techniques [13,14]. Poorly water-soluble ibuprofen (IBU) is a well-known non-steroidal anti-inflammatory drug and has been widely used for the treatment of inflammation, pain, or rheumatism. It has a short biological half-life (2 h), which makes it a suitable candidate for sustained or controlled drug delivery [15]. Therefore, the achievement of sustained or controlled drug delivery of IBU with improved dissolution is of great significance. Innovatively, the present paper reports bimodal nanoporous silica synthesized using biomimetic method (B-BNS) with synthesized polymer containing amine groups (C16- L-serine) as template. The formation mechanism was deeply studied through discussing interfacial interaction of surfactant template (C16-L-serine), silane coupling agent (3-aminopropyltriethoxysilane, APTES) and silica source (Tetraethoxysilane, TEOS). B-BNS was systemically characterized using Fourier transform infrared spectrometer (FTIR), transmission electron microscope (TEM), X-ray diffraction (XRD) and nitrogen adsorption/desorption measurement. IBU was used as model drug and loaded into B-BNS with solvent dispersion method. Functionalities of B-BNS for encapsulating IBU were intensively explored in two aspects, which were crystalline conversion effect and in vitro release behavior.
28.4 mmol DCC with 50 ml dichloromethane at room temperature. The palmitic acid solution and L-serine methyl ester solution were mixed on 0 °C cold water bath, and then DCC solution was dropwise added into the above mixture. After stirring this system for 3 h on 0 °C cold water bath, stirring was kept overnight at room temperature. The obtained solution was filtered and successively washed using water, a saturated aqueous solution of NaCl, a saturated aqueous solution of NaHCO3, HCl (1 M), and again water. The organic layer was dried using MgSO4 and filtrated. Crude product was obtained after evaporation of organic solvent. C16-L-serine was synthesized by hydrolyzing C16-L-serine methyl ester. C16-L-serine methyl ester was dissolved using methanol under stirring on 0 °C water bath. Afterwards, 0.05 M NaOH was added and methanol was removed using reduced pressure distillation. White precipitate was separated out after adjusting pH of the obtained mixture to 2–3 by HCl (0.05 M). Finally, filtered, water washed, and dried the precipitates to get C16-L-serine. 2.3. Preparation of B-BNS
2. Materials and methods
In a typical run of preparing B-BNS, 1.1 mmol C16-L-serine was dissolved in 10 ml deionized water on 50 °C water bath, to which 10 ml NaOH (0.1 M) was added under stirring. Then cooled down the temperature of water bath to room temperature and this mixture was stirred for 1 h. Afterwards, a mixed solution consisting of 0.24 ml APTES and 1.57 ml TEOS was dropwise added to the mixture under mild stirring, and stirring was kept for 10 min after APTES and TEOS were added. Then the system was remained statically for 24 h, filtered, centrifuged, water washed and dried. Finally, the dried sample was calcined at 550 °C for 6 h with a slow heating rate to get B-BNS.
2.1. Materials
2.4. Characterization of B-BNS
N, N′-dicyclohexyl carbon imine (DCC), N-dimethyl aminopyridine (DMAP), palmitic acid and L-serine methyl ester were purchased from Chengdu Xiya Chemical Technology Co. Ltd.(Chengdu, China). Tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Aladdin (Shanghai, China). All other chemicals were commercially available and were used as purchased without any further purification. Deionized water was prepared by ion exchange.
2.4.1. FTIR FTIR (Spectrum 1000, Perkin Elmer, USA) spectra of samples (C16-Lserine methyl ester, C16-L-serine, B-BNS) were recorded from 400 to 4000 cm−1 in transmittance mode with a resolution of 1 cm−1. Samples were milled and mixed with dried KBr in an agate mortar and pestle to prepare KBr disks.
2.2. Synthesis of C16-L-serine The synthesis process of C16-L-serine was referred to the reference reported [16] and chemical reaction steps were listed in Fig. 1. Briefly, palmitic acid and DMAP (molar ratio = 10:1) were added to dichloromethane and this mixture, which was named as palmitic acid solution, was stirred on 0 °C cold water bath for 1 h. L-Serine methyl ester solution was prepared by mixing triethylamine, L-serine methyl ester and dichloromethane, followed by stirring at room temperature for 40 min. Afterwards, DCC solution was prepared by dissolving
2.4.2. TEM The mesoporous structure of B-BNS was characterized using a Tecnai G2 20 TEM instrument (FEI, the United States) operated at 200 kV. Before examination, sample was dispersed in ethanol through sonication and subsequently deposited on carbon-coated copper grids with porous carbon films. 2.4.3. Small angle XRD Small angle XRD patterns of samples were generated at 30 mA and 30 kV with a Ni filtered CuKa line as the source of radiation. Data were obtained from 0.7° to 6° (diffraction angle 2θ).
Fig. 1. Chemical reactions to synthesize C16-L-serine. A, dehydration–condensation reaction; B, ester hydrolysis reaction.
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2.4.4. Nitrogen adsorption/desorption measurement The surface area and pore volume of B-BNS were studied by determining the nitrogen adsorption and desorption using a SA3100 surface area and pore size analyzer (Beckman Coulter, USA). The specific surface area (SBET) was evaluated from nitrogen adsorption data over the relative pressure range from 0.05 to 0.2 using the Brunauer–Emmett–Teller (BET) method. Pore size distributions (PSDs) were determined from adsorption branches of isotherms using the Barrett–Joyner–Halenda (BJH) method. The total pore volume (Vt) was determined from the amount adsorbed at a relative pressure of 0.99.
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ZRD6-B dissolution tester (Shanghai Huanghai Medicament Test Instrument Factory, China). Samples with the same IBU content were respectively exposed to simulated gastric fluid (SGF, pH 1.0) and simulated intestinal fluid (SIF, pH 6.8). At predetermined time intervals, aliquots (2.5 ml) of the dissolution medium were withdrawn and then an equivalent amount of fresh medium was added to maintain a constant dissolution volume. The withdrawn dissolution medium samples were administered through 0.45 μm microporous membrane and analyzed using HPLC (concrete conditions have been stated above). 3. Results and discussion
2.5. IBU loading process 3.1. Formation mechanism of B-BNS IBU adsorption to B-BNS was carried out by soaking B-BNS in hexane solution of IBU [15]. The carrier:drug ratio in the loading solution was 1:1 (w:w). Then, the mixture was stirred for 24 h in order to achieve maximum loading in the nanopores and curved mesoscopic channels of B-BNS. The loading was performed under ambient conditions in closed containers to prevent evaporation of hexane during the loading period. Finally, the mixture was dried at 50 °C under vacuum. Drug loading content was measured by taking an accurately weighed quantity of IBU loaded B-BNS, then extracting the loaded IBU completely using methanol under ultrasound, and finally measuring IBU content with high performance liquid chromatography (HPLC) method. The HPLC used for the dosage consisted of a Shimadzu system. The column used was a Kromasil (C18) 250 × 4.6 mm. The absorbance value read at 263 nm in the UV visible detector were used for quantitative analysis of IBU on the base of a calibration curve. The mobile phase consisted of sodium acetate buffer (6.13 g sodium acetate was dissolved in 750 ml redistilled water and its pH was adjusted to 2.5 using phosphoric acid) and acetonitrile (40:60, v/v), filtered through a membrane filter (0.45 μm) and degassed by ultrasonication before use. The flow rate amounted to 1.0 ml/min and the injection volume was 20 μl. The drug loading content (%) was calculated using the following equation: Drug loading content ð%Þ ¼
Weight of IBU in B−BNS 100: Weight of B−BNS with loaded IBU
2.6. Functionalities of B-BNS as IBU carrier 2.6.1. XRD XRD is a common method to test whether a crystalline drug phase can be detected. Herein, crystalline state of B-BNS, IBU and IBU loaded B-BNS were measured using XRD (X'pert PRO, PANalytical B.V., The Netherlands). XRD patterns of samples were generated at 30 mA and 30 kV with a Ni filtered CuKa line as the source of radiation. Data were obtained from 5° to 45° (diffraction angle 2θ). 2.6.2. DSC Differential scanning calorimeter (DSC, Q1000, TA Instrument, USA) was used and samples (B-BNS, IBU and IBU loaded B-BNS) were placed in pierced aluminum pans and heated from 0 to 100 °C at a scanning rate of 10 °C/min under nitrogen protection. 2.6.3. FTIR FTIR (Spectrum 1000, Perkin Elmer, USA) spectra of samples (IBU and IBU loaded B-BNS) were recorded from 400 to 4000 cm−1 in transmittance mode with a resolution of 1 cm−1. Samples were milled and mixed with 100-fold amount of dried KBr in an agate mortar and pestle. KBr disks were prepared with a compression force of 10 tons using a 13mm-diameter round flat face punch. 2.6.4. In vitro release behavior In vitro dissolution experiment of pure IBU and IBU loaded B-BNS was carried out using USP paddle method (50 rpm, 37 °C) with a
The formation mechanism of B-BNS mainly depends on the active behavior of C16-L-serine (Fig. 2). Initially, C16-L-serine forms micelle in aqueous solution because it consists of hydrophilic and hydrophobic ends on each side. After adding 10 ml NaOH (0.1 M), alkaline environment (pH is about 13) is provided for transformation of C16-L-serine between protonation and non-protonation. When APTES and TEOS are added, the positively charged ammonium ion of APTES interacts with the negatively charged head group of hydrophilic part of C16-L-serine through neutralization, which induces favorable electrostatic interactions and thus produces dynamic C16-L-serine [5]. The dynamic template can influence formation in two aspects: (1) APTES electrostatic interacts with C16-L-serine and meanwhile, the alkoxysilane sites of APTES are polymerized with TEOS to form silica framework, thus leading to unstable interface of organic/inorganic hybrid (organic: C16-Lserine; inorganic: APTES and TEOS) sphere. Therefore, a certain amount of C16-L-serine will undergo new dynamic self-assembly with the overflow of internal surfactant solution and entrance of external solution. Under such circumstances, TEOS accelerates the enlarged mesostructure formation through its hydrolysis and polycondensation, resulting in secondary larger nanoporous structure. (2) The cooperative interactions, including surfactant with APTES and APTES with TEOS, drive the helical packing of surfactant due to the stable configuration and organization of silicates around the micellar superstructures of surfactant, resulting in curved C16-L-serine and then curved mesoscopic channels of silica [17]. In summary, the functions of APTES in the formation of B-BNS can be concluded as: (1) electrostatic interacts with C16-Lserine; (2) solidifies with micelle and enhances solidification with TEOS. As for TEOS, it cannot only crosslink with APTES but also induce the new dynamic self-assembly of C16-L-serine. It was worth noticing that the synthesis of B-BNS belongs to biomimetic method because amines actively catalyze the condensation of silica precursors [18]. The herein presented formation mechanism of B-BNS revealed its interesting and fantastic formation (dynamic self-assembly) and can lay foundation for the study of B-BNS. 3.2. Characterization of B-BNS 3.2.1. FTIR The successful synthesized C16-L-serine methyl ester, C16-L-serine and B-BNS were verified with FTIR analysis (Fig. 3). The FTIR spectrogram of C16-L-serine methyl ester showed characteristic peaks of secondary amide clearly, including symmetrical stretching vibration of – NH at 3346.1 cm−1, stretching vibration of carbonyl group belonging to ester at 1743.4 cm− 1, stretching vibration of carbonyl group at 1642.7 cm−1, in-plane bending vibration of N–H at 1544.8 cm−1, and stretching vibration of C–N at 1240.1 cm−1. In addition, characteristic peak of the long carbon chain, which was rocking vibration of –CH2 at 719.8 cm−1 [19], further confirmed the dehydration-condensation reaction happened to palmitic acid and L-serine methyl ester. Under alkaline condition, C16-L-serine methyl ester can hydrolyze to produce C16-Lserine evidenced by the characteristic peaks, including symmetrical stretching vibration of –NH at 3316.4 cm− 1, stretching vibration of
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Fig. 2. Schematic illustration of the function of C16-L-serine in the process to prepare B-BNS.
carbonyl group belonging to carboxylic acid at 1734.3 cm− 1 [20], stretching vibration of carbonyl group at 1649.5 cm−1, in-plane bending vibration of N–H at 1542.3 cm− 1, stretching vibration of C–N at 1240.1 cm−1 and particular two peaks (out-of-plane bending vibration of –OH at 939.9 cm−1 and stretching vibration of –NH at 3399.9 cm−1). With the synthesis of anionic surfactant C16-L-serine as template, B-BNS was prepared evidenced by characteristic peaks of silica (including Si– O–Si bending vibration at 470.9 cm− 1 and Si–O–Si antisymmetric stretching vibration at 1102.9 cm−1). 3.2.2. TEM In order to study how pH affects the bimodal nanoporous structure formation, the compared B-BNS was synthesized under lower pH condition (pH equals to 11) by adding 10 ml HCl (0.01 M) after the addition of NaOH. According to the TEM images of compared B-BNS (Fig. 4A and B, B is the amplifying image of A), it was obvious that the obtained nanoparticles showed core-shell structure and the central core consisted of a number of disordered gasbags. The above observation indicated that dynamic level of C16-L-serine under lower pH condition was not sufficient to accomplish its mission to synthesize bimodal nanoporous silica
because no mesoscopic channels existed. It also reflected that TEOS cannot accelerate and solidify the enlarged nanoporous formation under low dynamic level of C16-L-serine but can massively condense on the surface of self-assembled sphere, thus leading to the thick shell morphology. On the contrary, when pH rose to 13, B-BNS (Fig. 4C and D, D is the amplifying image of C) can be successfully obtained based on its bimodal nanopores and curved mesoscopic channels. The nanopores that were pointed at using black arrows were the enlarged nanopores. The achievement of enlarged nanopores and curved mesoscopic channels were attributed to dynamic self-assembly function of C16-L-serine. TEM results demonstrated that in the synthesized system to prepare B-BNS, pH was extremely important to determine success. 3.2.3. Small angle XRD Fig. 5 showed small angle XRD patterns of B-BNS before and after calcination. B-BNS with template presented main peak at about 2.2– 2.3° 2θ, indicating that mesostructure was formed using C16-L-serine as template [21,22]. However, after calcination, no peak was detected for B-BNS, reflecting that a certain amount of silica framework was decomposed by calcination. The polycondensation of silica on dynamic self-assembly template was not stable to the surfactant removal. Though the collapse of B-BNS affected the overall mesostructure formation, it did not influence the existence and performance of bimodal nanoporous structure. 3.2.4. Nitrogen adsorption/desorption The nitrogen adsorption/desorption isotherm of B-BNS exhibited type IV isotherms with two distinct adsorption steps at the relative pressure of 0.45–0.85 and 0.85–0.99, respectively (Fig. 6A). The first step corresponded to nitrogen capillary condensation in the common nanopores of B-BNS, and resulted in a relatively narrow peak (in the range of 7 to 8 nm) in the pore size distribution curve (Fig. 6B). The second adsorption step, where relative pressure p/p0 = 0.85–0.99, resulted in a broad pore size distribution centered at about 30 nm, corresponding to the secondary enlarged nanopores as observed in the TEM images. The nitrogen adsorption/desorption result confirmed that B-BNS can be regarded as meso–meso porous silica material. 3.3. Functionalities of B-BNS as IBU carrier
Fig. 3. FTIR spectra of C16-L-serine methyl ester, C16-L-serine and B-BNS.
IBU with small molecular volume size (1.0 × 0.5 nm2) can be loaded into B-BNS. As comparison, the reported IBU loading content in MCM-
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Fig. 4. TEM images of B-BNS synthesized under pH 11 (A and B, B is the amplifying image of A) and pH 13 (C and D, D is the amplifying image of C).
41 with the same drug loading method was about 25% by weight, and it can be increased up to 30 to 36% with amino modified MCM-41 as carrier [23]. IBU loading content of B-BNS was 36.16%, which approached to that of amino modified MCM-41, revealing that B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. 3.3.1. XRD and DSC The XRD patterns were recorded to determine whether a crystalline phase could be detected. B-BNS was amorphous single phase material due to the presence of a broad band between 15° and 30° 2θ [22,24] (Fig. 7A, B-BNS). The diffraction pattern of IBU was highly crystalline in nature as indicated by the numerous peaks. However, after being loaded into B-BNS, no crystalline IBU was detected in XRD pattern,
indicating that IBU was loaded into B-BNS in amorphous state [25–27]. It was assumed that the nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement, and so giving rise to the disordered amorphous state. DSC thermogram of B-BNS (Fig. 7B) was almost a smooth line, which indicated that the synthesized B-BNS was amorphous materials. DSC thermogram of IBU showed a strong endothermic peak at 77 °C, reflecting the melting of IBU. After being loaded into B-BNS, DSC thermogram of IBU loaded B-BNS did not exhibit any peaks, suggesting that nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement [25–27], which was in agreement with XRD result. 3.3.2. FTIR The FTIR spectra of the two samples before and after loading IBU were shown in Fig. 8. The characteristic absorption peaks of carboxyl group at 1719.7 cm−1 and the quaternary carbon atom of benzene ring at 1508.5 and 1453.7 cm−1 were obviously shown in the spectrum of IBU [28]. After loading IBU into B-BNS, peaks of carboxyl group and benzene ring decreased significantly, suggesting that IBU was successfully incorporated into B-BNS and carboxyl group of IBU was involved in hydrogen bonding with the silanol groups on the silica surface [29]. Overall, the FTIR result suggested that B-BNS can hide most peaks of IBU after loading IBU into nanoporous space and curved mesoscopic channels [20].
Fig. 5. Small angle XRD patterns of B-BNS and B-BNS with template.
3.3.3. In vitro release behavior It is known that IBU is absorbed mainly in the stomach and proximal intestine where the pKa value is ~4.4. Since the pH in the human body changes from 1 to 2 in the stomach body to 5–7 in the antrum, the release experiments were performed at pH values of 1.0 (SGF) and 6.8 (SIF). The release profiles were shown in Fig. 9. It can be seen (Fig. 9 A, IBU; B, IBU) that the IBU molecules exhibited the faster release in
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Fig. 6. Nitrogen adsorption/desorption isotherm (A) and pore size distribution curve (B) of B-BNS.
basic rather than acidic medium (22.4% in SGF and about 100% in SIF). At SIF (pH N pKa), most of IBU was ionized and less was remained in the stationary phase. On the contrary, IBU molecules at SGF were more stable and not ionized (pH b pKa) compared to SIF due to the abundance of protons under the acidic condition. Therefore, the higher solubility of IBU at SIF leaded to faster release [30]. In vitro release of IBU loaded BBNS (28.7% in SGF and about 86.6% in SIF) was also consisted with the above principle. The functionality of B-BNS for in vitro IBU release was obvious. In SGF medium, IBU loaded B-BNS displayed faster dissolution rate than that of IBU, from cumulative release of 22.4% to 28.7%. The reason was owing to the conversion of crystalline state of IBU to amorphous phase to dramatically increase apparent solubility due to higher energy state of amorphous phase [31]. As for SIF medium, IBU loaded B-BNS showed obviously controlled dissolution for 12 h with a burst release. The drug release from nanopores of B-BNS involved two processes: (1) the solvent diffused into the nanopores to dissolve the drug; (2) the solvated drug diffused to the out part of the nanopores then finally dissolved in release medium. The burst release (0–5 min) effect was due to the release of IBU molecules that physically adsorbed on the outer surface of B-BNS. When the release process started, these molecules located on the outer surface were the first to migrate to the media. In the first phase of controlled release (see the inserted in vitro release image in Fig. 9B), IBU diffused from enlarged nanopores and the outer curved mesoscopic channels. In this term, the presence of enlarged nanopores can facilitate drug release due to its larger pore
Fig. 7. A, XRD patterns of B-BNS, IBU and IBU loaded B-BNS; B, DSC thermograms of B-BNS, IBU and IBU loaded B-BNS.
diameter (centered at about 30 nm) and minimize burst release effect because of its space hindrance. In the second phase, the solvent took more time to diffuse into the small nanopores and internal curved mesoscopic channels, thus slowing down drug release for several hours. Based on above results, it can be concluded that B-BNS controlled IBU release with two release phases and improved dissolution in SGF.
Fig. 8. FTIR spectra of IBU and IBU loaded B-BNS. The chemical structure of IBU was inserted under that spectrum of IBU.
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improve dissolution in SGF because of crystalline conversion ability. It is convincible that biomimetic method provides novel theory and insight for synthesizing bimodal nanoporous silica. Nevertheless, B-BNS with the ability to improve dissolution of poorly water-soluble drug in a controlled manner highlights its unique functionality as poorly water-soluble drug carrier, which can undoubtedly promote the application of bimodal nanoporous silica and development of pharmaceutical science. Acknowledgments This work was supported by the Science and Technology Research Project Funds from Liaoning Education Department of China (No.L2014398) and National Natural Science Foundation of China (No.81473161). References
Fig. 9. In vitro release profiles of A, IBU and IBU loaded B-BNS release profiles in SGF; B, IBU and IBU loaded B-BNS release profiles in SIF.
This achievement will provide significant value and inspiration for the formulation design of poorly water-soluble drugs. 4. Conclusion The as-synthesized B-BNS had bimodal nanopores and curved mesoscopic channels due to the dynamic self-assembly function of C16-L-serine induced by APTES and TEOS. According to nitrogen adsorption/desorption measurement result, B-BNS can be regarded as meso– meso porous silica material. In application, B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. Nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement, and so giving rise to the amorphous state of IBU. What is more important, B-BNS can control IBU release with two release phases based on bimodal nanopores and
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Biomimetic synthesized bimodal nanoporous silica: Bimodal mesostructure formation and application for ibuprofen delivery Jing Li, Lu Xu ⁎⁎, Nan Zheng, Hongyu Wang, Fangzheng Lu, Sanming Li ⁎ School of Pharmacy, Shenyang Pharmaceutical University, Wenhua RD 103, 110016, China
a r t i c l e
i n f o
Article history: Received 15 July 2015 Received in revised form 15 September 2015 Accepted 25 September 2015 Available online 28 September 2015 Keywords: Biomimetic synthesis Bimodal nanoporous silica Ibuprofen delivery Dynamic self-assembly
a b s t r a c t The present paper innovatively reports bimodal nanoporous silica synthesized using biomimetic method (BBNS) with synthesized polymer (C16-L-serine) as template. Formation mechanism of B-BNS was deeply studied and exploration of its application as carrier of poorly water-soluble drug ibuprofen (IBU) was conducted. The bimodal nanopores and curved mesoscopic channels of B-BNS were achieved due to the dynamic self-assembly of C16-L-serine induced by silane coupling agent (3-aminopropyltriethoxysilane, APTES) and silica source (tetraethoxysilane, TEOS). Characterization results confirmed the successful synthesis of B-BNS, and particularly, nitrogen adsorption/desorption measurement demonstrated that B-BNS was meso–meso porous silica material. In application, B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. After being loaded, IBU presented amorphous phase because nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU. In vitro release result revealed that B-BNS controlled IBU release with two release phases based on bimodal nanopores and improved dissolution in simulated gastric fluid due to crystalline conversion of IBU. It is convincible that biomimetic method provides novel theory and insight for synthesizing bimodal nanoporous silica, and unique functionalities of B-BNS as drug carrier can undoubtedly promote the application of bimodal nanoporous silica and development of pharmaceutical science. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The elaborate morphologies and gorgeous surface patterns of the external skeletons in biosilicifying organisms, including diatoms (microalga), radiolarian, sponges and plants (e.g. horsetail and rice leaves), are fascinating and have been focused great attention [1,2]. Biosilicification of nanoporous (1–100 nm) silica inspired by diatoms and radiolarian has completely changed the way of designing novel nanoporous silica [3,4]. For example, nanoporous silica synthesized through organic– inorganic liquid-crystal co-assembly method would shed light on the formation of biosilicification, as many macromolecules in biological organisms display similar characteristics of liquid-crystal self-assemblies [5]. The definition of biosilicification can be comprehended as the movement of silicic acid from environments to intracellular or systemic compartments in which it is accumulated for subsequent deposition as amorphous hydrated silica. It has been reported that various types of additives, including polypeptides, polysaccharides, peptides, polyamines, enzymes, proteins, and synthesized polymers containing amine groups can be used to achieve biomimetic synthesized nanoporous silica [3,4]. In application, biomimetic synthesized nanoporous silica plays important roles in catalysis, adsorption, separation, and biomedical systems [6–8]. ⁎ First corresponding author: Sanming Li, Wenhua RD 103, 110016 Shenyang, China. ⁎⁎ Second corresponding author: Lu Xu, Wenhua RD 103, 110016 Shenyang, China. E-mail addresses: [email protected] (L. Xu), [email protected] (S. Li).
http://dx.doi.org/10.1016/j.msec.2015.09.091 0928-4931/© 2015 Elsevier B.V. All rights reserved.
Hierarchically nanoporous silica with well-defined morphologies has been of growing interest, and a variety of templating approaches have been proposed to synthesize hierarchically nanoporous silica in the past decades. Bimodal nanoporous silica, including micro–meso, meso–meso, meso–macro, can be prepared using soft templates (such as colloidal particles, polymers, emulsion droplets, and surfactants) with abilities to create different sizes of nanopores [9–11]. The employment of different types of mixing templates failed to synthesize nanoporous silica with controlled well-ordered mesostructure and well-defined morphology. In contrast, dynamic templates generated from complex organic matrix can improve controllability of bimodal nanoporous silica inspired by biomimetic formation mechanism of time-dependent cooperative organization [11]. Though dynamic templates originating from organic mesomorphous complexes of polyelectrolyte (poly(acrylic acid), PAA; polystyrene-b-poly(acrylic acid), PSbPAA) and cationic surfactant (hexadecylpyridinium chloride, CPC; cetyltrimethyl ammonium bromide, CTAB) have been successfully applied to synthesize bimodal nanoporous silica [9–11], bimodal nanoporous silica synthesized using biomimetic method has been rarely reported. Nanoporous silica has wide application in biomedical fields due to the following advantages: (1) large surface area and pore volume provide great potential for drug adsorption and loading; (2) mesoporous (2 to 50 nm) structure and an adjustable pore size enable better control of drug loading and release; (3) surface can be easily modified to
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improve functionalities; and (4) in vivo biosafety evaluations of cytotoxicity, biodegradation, biodistribution and excretion [12]. Nowadays, the low solubility of active pharmaceutical ingredients is an increasing problem in pharmaceutical industry and therefore several different approaches have been designed to overcome this obstacle, and nanoporous silica represents a potential category among these techniques [13,14]. Poorly water-soluble ibuprofen (IBU) is a well-known non-steroidal anti-inflammatory drug and has been widely used for the treatment of inflammation, pain, or rheumatism. It has a short biological half-life (2 h), which makes it a suitable candidate for sustained or controlled drug delivery [15]. Therefore, the achievement of sustained or controlled drug delivery of IBU with improved dissolution is of great significance. Innovatively, the present paper reports bimodal nanoporous silica synthesized using biomimetic method (B-BNS) with synthesized polymer containing amine groups (C16- L-serine) as template. The formation mechanism was deeply studied through discussing interfacial interaction of surfactant template (C16-L-serine), silane coupling agent (3-aminopropyltriethoxysilane, APTES) and silica source (Tetraethoxysilane, TEOS). B-BNS was systemically characterized using Fourier transform infrared spectrometer (FTIR), transmission electron microscope (TEM), X-ray diffraction (XRD) and nitrogen adsorption/desorption measurement. IBU was used as model drug and loaded into B-BNS with solvent dispersion method. Functionalities of B-BNS for encapsulating IBU were intensively explored in two aspects, which were crystalline conversion effect and in vitro release behavior.
28.4 mmol DCC with 50 ml dichloromethane at room temperature. The palmitic acid solution and L-serine methyl ester solution were mixed on 0 °C cold water bath, and then DCC solution was dropwise added into the above mixture. After stirring this system for 3 h on 0 °C cold water bath, stirring was kept overnight at room temperature. The obtained solution was filtered and successively washed using water, a saturated aqueous solution of NaCl, a saturated aqueous solution of NaHCO3, HCl (1 M), and again water. The organic layer was dried using MgSO4 and filtrated. Crude product was obtained after evaporation of organic solvent. C16-L-serine was synthesized by hydrolyzing C16-L-serine methyl ester. C16-L-serine methyl ester was dissolved using methanol under stirring on 0 °C water bath. Afterwards, 0.05 M NaOH was added and methanol was removed using reduced pressure distillation. White precipitate was separated out after adjusting pH of the obtained mixture to 2–3 by HCl (0.05 M). Finally, filtered, water washed, and dried the precipitates to get C16-L-serine. 2.3. Preparation of B-BNS
2. Materials and methods
In a typical run of preparing B-BNS, 1.1 mmol C16-L-serine was dissolved in 10 ml deionized water on 50 °C water bath, to which 10 ml NaOH (0.1 M) was added under stirring. Then cooled down the temperature of water bath to room temperature and this mixture was stirred for 1 h. Afterwards, a mixed solution consisting of 0.24 ml APTES and 1.57 ml TEOS was dropwise added to the mixture under mild stirring, and stirring was kept for 10 min after APTES and TEOS were added. Then the system was remained statically for 24 h, filtered, centrifuged, water washed and dried. Finally, the dried sample was calcined at 550 °C for 6 h with a slow heating rate to get B-BNS.
2.1. Materials
2.4. Characterization of B-BNS
N, N′-dicyclohexyl carbon imine (DCC), N-dimethyl aminopyridine (DMAP), palmitic acid and L-serine methyl ester were purchased from Chengdu Xiya Chemical Technology Co. Ltd.(Chengdu, China). Tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Aladdin (Shanghai, China). All other chemicals were commercially available and were used as purchased without any further purification. Deionized water was prepared by ion exchange.
2.4.1. FTIR FTIR (Spectrum 1000, Perkin Elmer, USA) spectra of samples (C16-Lserine methyl ester, C16-L-serine, B-BNS) were recorded from 400 to 4000 cm−1 in transmittance mode with a resolution of 1 cm−1. Samples were milled and mixed with dried KBr in an agate mortar and pestle to prepare KBr disks.
2.2. Synthesis of C16-L-serine The synthesis process of C16-L-serine was referred to the reference reported [16] and chemical reaction steps were listed in Fig. 1. Briefly, palmitic acid and DMAP (molar ratio = 10:1) were added to dichloromethane and this mixture, which was named as palmitic acid solution, was stirred on 0 °C cold water bath for 1 h. L-Serine methyl ester solution was prepared by mixing triethylamine, L-serine methyl ester and dichloromethane, followed by stirring at room temperature for 40 min. Afterwards, DCC solution was prepared by dissolving
2.4.2. TEM The mesoporous structure of B-BNS was characterized using a Tecnai G2 20 TEM instrument (FEI, the United States) operated at 200 kV. Before examination, sample was dispersed in ethanol through sonication and subsequently deposited on carbon-coated copper grids with porous carbon films. 2.4.3. Small angle XRD Small angle XRD patterns of samples were generated at 30 mA and 30 kV with a Ni filtered CuKa line as the source of radiation. Data were obtained from 0.7° to 6° (diffraction angle 2θ).
Fig. 1. Chemical reactions to synthesize C16-L-serine. A, dehydration–condensation reaction; B, ester hydrolysis reaction.
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2.4.4. Nitrogen adsorption/desorption measurement The surface area and pore volume of B-BNS were studied by determining the nitrogen adsorption and desorption using a SA3100 surface area and pore size analyzer (Beckman Coulter, USA). The specific surface area (SBET) was evaluated from nitrogen adsorption data over the relative pressure range from 0.05 to 0.2 using the Brunauer–Emmett–Teller (BET) method. Pore size distributions (PSDs) were determined from adsorption branches of isotherms using the Barrett–Joyner–Halenda (BJH) method. The total pore volume (Vt) was determined from the amount adsorbed at a relative pressure of 0.99.
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ZRD6-B dissolution tester (Shanghai Huanghai Medicament Test Instrument Factory, China). Samples with the same IBU content were respectively exposed to simulated gastric fluid (SGF, pH 1.0) and simulated intestinal fluid (SIF, pH 6.8). At predetermined time intervals, aliquots (2.5 ml) of the dissolution medium were withdrawn and then an equivalent amount of fresh medium was added to maintain a constant dissolution volume. The withdrawn dissolution medium samples were administered through 0.45 μm microporous membrane and analyzed using HPLC (concrete conditions have been stated above). 3. Results and discussion
2.5. IBU loading process 3.1. Formation mechanism of B-BNS IBU adsorption to B-BNS was carried out by soaking B-BNS in hexane solution of IBU [15]. The carrier:drug ratio in the loading solution was 1:1 (w:w). Then, the mixture was stirred for 24 h in order to achieve maximum loading in the nanopores and curved mesoscopic channels of B-BNS. The loading was performed under ambient conditions in closed containers to prevent evaporation of hexane during the loading period. Finally, the mixture was dried at 50 °C under vacuum. Drug loading content was measured by taking an accurately weighed quantity of IBU loaded B-BNS, then extracting the loaded IBU completely using methanol under ultrasound, and finally measuring IBU content with high performance liquid chromatography (HPLC) method. The HPLC used for the dosage consisted of a Shimadzu system. The column used was a Kromasil (C18) 250 × 4.6 mm. The absorbance value read at 263 nm in the UV visible detector were used for quantitative analysis of IBU on the base of a calibration curve. The mobile phase consisted of sodium acetate buffer (6.13 g sodium acetate was dissolved in 750 ml redistilled water and its pH was adjusted to 2.5 using phosphoric acid) and acetonitrile (40:60, v/v), filtered through a membrane filter (0.45 μm) and degassed by ultrasonication before use. The flow rate amounted to 1.0 ml/min and the injection volume was 20 μl. The drug loading content (%) was calculated using the following equation: Drug loading content ð%Þ ¼
Weight of IBU in B−BNS 100: Weight of B−BNS with loaded IBU
2.6. Functionalities of B-BNS as IBU carrier 2.6.1. XRD XRD is a common method to test whether a crystalline drug phase can be detected. Herein, crystalline state of B-BNS, IBU and IBU loaded B-BNS were measured using XRD (X'pert PRO, PANalytical B.V., The Netherlands). XRD patterns of samples were generated at 30 mA and 30 kV with a Ni filtered CuKa line as the source of radiation. Data were obtained from 5° to 45° (diffraction angle 2θ). 2.6.2. DSC Differential scanning calorimeter (DSC, Q1000, TA Instrument, USA) was used and samples (B-BNS, IBU and IBU loaded B-BNS) were placed in pierced aluminum pans and heated from 0 to 100 °C at a scanning rate of 10 °C/min under nitrogen protection. 2.6.3. FTIR FTIR (Spectrum 1000, Perkin Elmer, USA) spectra of samples (IBU and IBU loaded B-BNS) were recorded from 400 to 4000 cm−1 in transmittance mode with a resolution of 1 cm−1. Samples were milled and mixed with 100-fold amount of dried KBr in an agate mortar and pestle. KBr disks were prepared with a compression force of 10 tons using a 13mm-diameter round flat face punch. 2.6.4. In vitro release behavior In vitro dissolution experiment of pure IBU and IBU loaded B-BNS was carried out using USP paddle method (50 rpm, 37 °C) with a
The formation mechanism of B-BNS mainly depends on the active behavior of C16-L-serine (Fig. 2). Initially, C16-L-serine forms micelle in aqueous solution because it consists of hydrophilic and hydrophobic ends on each side. After adding 10 ml NaOH (0.1 M), alkaline environment (pH is about 13) is provided for transformation of C16-L-serine between protonation and non-protonation. When APTES and TEOS are added, the positively charged ammonium ion of APTES interacts with the negatively charged head group of hydrophilic part of C16-L-serine through neutralization, which induces favorable electrostatic interactions and thus produces dynamic C16-L-serine [5]. The dynamic template can influence formation in two aspects: (1) APTES electrostatic interacts with C16-L-serine and meanwhile, the alkoxysilane sites of APTES are polymerized with TEOS to form silica framework, thus leading to unstable interface of organic/inorganic hybrid (organic: C16-Lserine; inorganic: APTES and TEOS) sphere. Therefore, a certain amount of C16-L-serine will undergo new dynamic self-assembly with the overflow of internal surfactant solution and entrance of external solution. Under such circumstances, TEOS accelerates the enlarged mesostructure formation through its hydrolysis and polycondensation, resulting in secondary larger nanoporous structure. (2) The cooperative interactions, including surfactant with APTES and APTES with TEOS, drive the helical packing of surfactant due to the stable configuration and organization of silicates around the micellar superstructures of surfactant, resulting in curved C16-L-serine and then curved mesoscopic channels of silica [17]. In summary, the functions of APTES in the formation of B-BNS can be concluded as: (1) electrostatic interacts with C16-Lserine; (2) solidifies with micelle and enhances solidification with TEOS. As for TEOS, it cannot only crosslink with APTES but also induce the new dynamic self-assembly of C16-L-serine. It was worth noticing that the synthesis of B-BNS belongs to biomimetic method because amines actively catalyze the condensation of silica precursors [18]. The herein presented formation mechanism of B-BNS revealed its interesting and fantastic formation (dynamic self-assembly) and can lay foundation for the study of B-BNS. 3.2. Characterization of B-BNS 3.2.1. FTIR The successful synthesized C16-L-serine methyl ester, C16-L-serine and B-BNS were verified with FTIR analysis (Fig. 3). The FTIR spectrogram of C16-L-serine methyl ester showed characteristic peaks of secondary amide clearly, including symmetrical stretching vibration of – NH at 3346.1 cm−1, stretching vibration of carbonyl group belonging to ester at 1743.4 cm− 1, stretching vibration of carbonyl group at 1642.7 cm−1, in-plane bending vibration of N–H at 1544.8 cm−1, and stretching vibration of C–N at 1240.1 cm−1. In addition, characteristic peak of the long carbon chain, which was rocking vibration of –CH2 at 719.8 cm−1 [19], further confirmed the dehydration-condensation reaction happened to palmitic acid and L-serine methyl ester. Under alkaline condition, C16-L-serine methyl ester can hydrolyze to produce C16-Lserine evidenced by the characteristic peaks, including symmetrical stretching vibration of –NH at 3316.4 cm− 1, stretching vibration of
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Fig. 2. Schematic illustration of the function of C16-L-serine in the process to prepare B-BNS.
carbonyl group belonging to carboxylic acid at 1734.3 cm− 1 [20], stretching vibration of carbonyl group at 1649.5 cm−1, in-plane bending vibration of N–H at 1542.3 cm− 1, stretching vibration of C–N at 1240.1 cm−1 and particular two peaks (out-of-plane bending vibration of –OH at 939.9 cm−1 and stretching vibration of –NH at 3399.9 cm−1). With the synthesis of anionic surfactant C16-L-serine as template, B-BNS was prepared evidenced by characteristic peaks of silica (including Si– O–Si bending vibration at 470.9 cm− 1 and Si–O–Si antisymmetric stretching vibration at 1102.9 cm−1). 3.2.2. TEM In order to study how pH affects the bimodal nanoporous structure formation, the compared B-BNS was synthesized under lower pH condition (pH equals to 11) by adding 10 ml HCl (0.01 M) after the addition of NaOH. According to the TEM images of compared B-BNS (Fig. 4A and B, B is the amplifying image of A), it was obvious that the obtained nanoparticles showed core-shell structure and the central core consisted of a number of disordered gasbags. The above observation indicated that dynamic level of C16-L-serine under lower pH condition was not sufficient to accomplish its mission to synthesize bimodal nanoporous silica
because no mesoscopic channels existed. It also reflected that TEOS cannot accelerate and solidify the enlarged nanoporous formation under low dynamic level of C16-L-serine but can massively condense on the surface of self-assembled sphere, thus leading to the thick shell morphology. On the contrary, when pH rose to 13, B-BNS (Fig. 4C and D, D is the amplifying image of C) can be successfully obtained based on its bimodal nanopores and curved mesoscopic channels. The nanopores that were pointed at using black arrows were the enlarged nanopores. The achievement of enlarged nanopores and curved mesoscopic channels were attributed to dynamic self-assembly function of C16-L-serine. TEM results demonstrated that in the synthesized system to prepare B-BNS, pH was extremely important to determine success. 3.2.3. Small angle XRD Fig. 5 showed small angle XRD patterns of B-BNS before and after calcination. B-BNS with template presented main peak at about 2.2– 2.3° 2θ, indicating that mesostructure was formed using C16-L-serine as template [21,22]. However, after calcination, no peak was detected for B-BNS, reflecting that a certain amount of silica framework was decomposed by calcination. The polycondensation of silica on dynamic self-assembly template was not stable to the surfactant removal. Though the collapse of B-BNS affected the overall mesostructure formation, it did not influence the existence and performance of bimodal nanoporous structure. 3.2.4. Nitrogen adsorption/desorption The nitrogen adsorption/desorption isotherm of B-BNS exhibited type IV isotherms with two distinct adsorption steps at the relative pressure of 0.45–0.85 and 0.85–0.99, respectively (Fig. 6A). The first step corresponded to nitrogen capillary condensation in the common nanopores of B-BNS, and resulted in a relatively narrow peak (in the range of 7 to 8 nm) in the pore size distribution curve (Fig. 6B). The second adsorption step, where relative pressure p/p0 = 0.85–0.99, resulted in a broad pore size distribution centered at about 30 nm, corresponding to the secondary enlarged nanopores as observed in the TEM images. The nitrogen adsorption/desorption result confirmed that B-BNS can be regarded as meso–meso porous silica material. 3.3. Functionalities of B-BNS as IBU carrier
Fig. 3. FTIR spectra of C16-L-serine methyl ester, C16-L-serine and B-BNS.
IBU with small molecular volume size (1.0 × 0.5 nm2) can be loaded into B-BNS. As comparison, the reported IBU loading content in MCM-
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Fig. 4. TEM images of B-BNS synthesized under pH 11 (A and B, B is the amplifying image of A) and pH 13 (C and D, D is the amplifying image of C).
41 with the same drug loading method was about 25% by weight, and it can be increased up to 30 to 36% with amino modified MCM-41 as carrier [23]. IBU loading content of B-BNS was 36.16%, which approached to that of amino modified MCM-41, revealing that B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. 3.3.1. XRD and DSC The XRD patterns were recorded to determine whether a crystalline phase could be detected. B-BNS was amorphous single phase material due to the presence of a broad band between 15° and 30° 2θ [22,24] (Fig. 7A, B-BNS). The diffraction pattern of IBU was highly crystalline in nature as indicated by the numerous peaks. However, after being loaded into B-BNS, no crystalline IBU was detected in XRD pattern,
indicating that IBU was loaded into B-BNS in amorphous state [25–27]. It was assumed that the nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement, and so giving rise to the disordered amorphous state. DSC thermogram of B-BNS (Fig. 7B) was almost a smooth line, which indicated that the synthesized B-BNS was amorphous materials. DSC thermogram of IBU showed a strong endothermic peak at 77 °C, reflecting the melting of IBU. After being loaded into B-BNS, DSC thermogram of IBU loaded B-BNS did not exhibit any peaks, suggesting that nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement [25–27], which was in agreement with XRD result. 3.3.2. FTIR The FTIR spectra of the two samples before and after loading IBU were shown in Fig. 8. The characteristic absorption peaks of carboxyl group at 1719.7 cm−1 and the quaternary carbon atom of benzene ring at 1508.5 and 1453.7 cm−1 were obviously shown in the spectrum of IBU [28]. After loading IBU into B-BNS, peaks of carboxyl group and benzene ring decreased significantly, suggesting that IBU was successfully incorporated into B-BNS and carboxyl group of IBU was involved in hydrogen bonding with the silanol groups on the silica surface [29]. Overall, the FTIR result suggested that B-BNS can hide most peaks of IBU after loading IBU into nanoporous space and curved mesoscopic channels [20].
Fig. 5. Small angle XRD patterns of B-BNS and B-BNS with template.
3.3.3. In vitro release behavior It is known that IBU is absorbed mainly in the stomach and proximal intestine where the pKa value is ~4.4. Since the pH in the human body changes from 1 to 2 in the stomach body to 5–7 in the antrum, the release experiments were performed at pH values of 1.0 (SGF) and 6.8 (SIF). The release profiles were shown in Fig. 9. It can be seen (Fig. 9 A, IBU; B, IBU) that the IBU molecules exhibited the faster release in
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Fig. 6. Nitrogen adsorption/desorption isotherm (A) and pore size distribution curve (B) of B-BNS.
basic rather than acidic medium (22.4% in SGF and about 100% in SIF). At SIF (pH N pKa), most of IBU was ionized and less was remained in the stationary phase. On the contrary, IBU molecules at SGF were more stable and not ionized (pH b pKa) compared to SIF due to the abundance of protons under the acidic condition. Therefore, the higher solubility of IBU at SIF leaded to faster release [30]. In vitro release of IBU loaded BBNS (28.7% in SGF and about 86.6% in SIF) was also consisted with the above principle. The functionality of B-BNS for in vitro IBU release was obvious. In SGF medium, IBU loaded B-BNS displayed faster dissolution rate than that of IBU, from cumulative release of 22.4% to 28.7%. The reason was owing to the conversion of crystalline state of IBU to amorphous phase to dramatically increase apparent solubility due to higher energy state of amorphous phase [31]. As for SIF medium, IBU loaded B-BNS showed obviously controlled dissolution for 12 h with a burst release. The drug release from nanopores of B-BNS involved two processes: (1) the solvent diffused into the nanopores to dissolve the drug; (2) the solvated drug diffused to the out part of the nanopores then finally dissolved in release medium. The burst release (0–5 min) effect was due to the release of IBU molecules that physically adsorbed on the outer surface of B-BNS. When the release process started, these molecules located on the outer surface were the first to migrate to the media. In the first phase of controlled release (see the inserted in vitro release image in Fig. 9B), IBU diffused from enlarged nanopores and the outer curved mesoscopic channels. In this term, the presence of enlarged nanopores can facilitate drug release due to its larger pore
Fig. 7. A, XRD patterns of B-BNS, IBU and IBU loaded B-BNS; B, DSC thermograms of B-BNS, IBU and IBU loaded B-BNS.
diameter (centered at about 30 nm) and minimize burst release effect because of its space hindrance. In the second phase, the solvent took more time to diffuse into the small nanopores and internal curved mesoscopic channels, thus slowing down drug release for several hours. Based on above results, it can be concluded that B-BNS controlled IBU release with two release phases and improved dissolution in SGF.
Fig. 8. FTIR spectra of IBU and IBU loaded B-BNS. The chemical structure of IBU was inserted under that spectrum of IBU.
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improve dissolution in SGF because of crystalline conversion ability. It is convincible that biomimetic method provides novel theory and insight for synthesizing bimodal nanoporous silica. Nevertheless, B-BNS with the ability to improve dissolution of poorly water-soluble drug in a controlled manner highlights its unique functionality as poorly water-soluble drug carrier, which can undoubtedly promote the application of bimodal nanoporous silica and development of pharmaceutical science. Acknowledgments This work was supported by the Science and Technology Research Project Funds from Liaoning Education Department of China (No.L2014398) and National Natural Science Foundation of China (No.81473161). References
Fig. 9. In vitro release profiles of A, IBU and IBU loaded B-BNS release profiles in SGF; B, IBU and IBU loaded B-BNS release profiles in SIF.
This achievement will provide significant value and inspiration for the formulation design of poorly water-soluble drugs. 4. Conclusion The as-synthesized B-BNS had bimodal nanopores and curved mesoscopic channels due to the dynamic self-assembly function of C16-L-serine induced by APTES and TEOS. According to nitrogen adsorption/desorption measurement result, B-BNS can be regarded as meso– meso porous silica material. In application, B-BNS loaded IBU with high drug loading content due to its enlarged nanopores. Nanoporous space and curved mesoscopic channels of B-BNS prevented the crystallization of IBU due to the space confinement, and so giving rise to the amorphous state of IBU. What is more important, B-BNS can control IBU release with two release phases based on bimodal nanopores and
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