Synthesis, characterization and drug release studies of poly(2-hydroxyethyl methacrylate)/KIT-5 nanocomposite as an innovative organic–inorganic hybrid carrier system

Abstract

Poly(2-hydroxyethyl methacrylate)/KIT-5 as an ordered mesoporous polymer–silica nanocomposite was synthesized through in situ polymerization. The chemical and physical properties of the nanocomposite were analyzed using a series of different techniques, including XRD, BET, FT-IR, SEM and TEM analysis. Afterward, this innovative ordered mesoporous nanocomposite was utilized for the delivery and controlled release of the model drug ibuprofen. In addition, the in vitro cytotoxicity of the nanocomposite was assessed by MTT assay. Overall, the results were indicative of the potential of the nanocomposite as an excellent carrier system.

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1. Introduction

The synthesis and design of materials for safe and effective drug delivery play an extremely vital role in modern biomedical science and medicine. Preventing degradation of therapeutic compounds and controlling their delivery rate are two central material characteristics that increase drug effectiveness and diminish adverse side effects, which in turn can lead to more patient compliance and commercial value.^1,2

Versatile drug nanocarriers, including liposomes, dendrimers and polymeric nanoparticles have been utilized to approach the mentioned goals.^3–5 Actually, liposome-based therapeutics have been on the market for more than a decade, but use of liposomes in clinics poses several challenges, including the high production cost. Whilst commercial drug delivery systems (DDSs) based on polymers have been very successful systems that cover several fields, such as implantable, oral, transdermal and injectable drug reservoirs, one of the main and most specific problems of such systems is the loss of activity of several drugs before reaching the target tissue as a result of premature degradation of the active agent.^6,7

As an alternative to nanoscale drug carriers constructed from organic materials, innovative inorganic material-based systems have been greatly advanced. Ever since the discovery of MCM-41-type ordered mesoporous silica by Mobil Corporation scientists in the early 1990s, there has been large amount of research conducted on the controlled syntheses and applications of mesoporous silica.^8,9 In particular, biomedical application research on mesoporous silica nanoparticles (MSNs) has received great attention over the last few years.^10,11 For instant, MSNs have been intensively suggested for use in controlled drug/gene release and as delivery carriers due to the following unique properties:^12–18

1. Tunable particle size. The particle size of MSNs can be tuned from 50 to 300 nm permitting a facile endocytosis by living animal and plant cell without any significant cytotoxicity.

2. Stable and rigid framework. In contrast to other polymer-based drug carriers, MSNs are more resistant to heat, pH, mechanical stress and hydrolysis-induced degradation.

3. Uniform and tunable pore size. The pore size distribution of MSNs is very narrow and pore diameter can be tailored between 2 and 20 nm. These features enable one to adjust the loading of various drug molecules and study the kinetics of drug release with high precision.

4. High surface area and large pore volume. The total surface area and pore volume of mesoporous silicas are very large, which allow high loadings of drug molecules.

However, conventional pure MSNs have little control over drug release mainly because of weak interactions between drug molecules and unmodified surfaces. To overcome this problem, the surface is usually functionalized with chemical groups that are able to link to the drug molecules through stronger bonds.^19–23 Extensive efforts toward the functionalization of MSNs have been carried out using different approaches, such as post-synthesis grafting, co-condensation, and post-polymerization.²⁴ In the post-polymerization method, which has been less studied on DDSs, mesoporous polymer–inorganic hybrid materials are synthesized using in situ free radical polymerization of vinyl-based monomers confined at the mesopore surface. After polymerization of the monomers, a thin layer of polymer is formed on the inner surface of the MSNs, while the mesopores remain fully accessible. This method makes it possible to exploit the desired physical properties of the MSNs such as large pore diameter and ordered mesostructure, as well as the additional properties of the introduced functional groups.^25–27

Herein, and in continuing our previous works to develop new organic–inorganic hybrid materials, we will introduce a simple, yet efficient hybrid organic–silica carrier system based on a cage-type mesoporous silica with three-dimensional (3D) porous network (KIT-5) and 2-hydroxyethyl methacrylate as a biocompatible hydrogel.^28–30 The potential of the nanocomposite to load drug molecules was investigated using ibuprofen (IBU) as a model drug and the release pattern was studied in simulated body fluid (SBF). To evaluate the efficiency of the carrier system, a comparison was made with pure KIT-5 as a vehicle, and as anticipated, the nanocomposite showed a more controllable release pattern.

2. Experimental

2.1. Sample characterization

The samples were analyzed using FT-IR spectroscopy (using a Perkin Elmer 65 in KBr matrix in the range of 4000–400 cm⁻¹). The BET specific surface areas and BJH pore size distribution of the samples were determined by adsorption–desorption of nitrogen at liquid nitrogen temperature, using a series BEL SORP 18. The X-Ray powder diffraction (XRD) of the matrices were carried out on a Bruker D8 Advanced X-Ray diffractometer using nickel filtered Cu Kα radiation at 40 kV and 20 mA. Moreover, scanning electron microscope (SEM) studies were performed on a Hitachi S4160 FE-SEM. The thermal gravimetric (TGA) data were obtained by a Setaram Labsys TG (STA) in a temperature range of 30–650 °C and heating rate of 10 °C min⁻¹ in N₂ atmosphere. Transmission electron microscope (TEM) observations were performed on a Philips CM30 Electron Microscope at an accelerating voltage of 300.00 kV.

2.2. Synthesis of mesoporous silica

The large pore cage type mesoporous silica, named KIT-5 for brevity, was prepared in aqueous solution using Pluronic F-127 (EO₁₀₆PO₇₀EO₁₀₆) template as a non-ionic structure-directing agent and tetraethylorthosilicate (TEOS) as the silica precursor.

In a typical synthesis method, 2.5 g (0.198 mmol) of F-127 was dissolved in 120 g (6.60 mol) of distilled water and 5.25 g (0.05 mol) of concentrated hydrochloric acid (35 wt% HCl). To this mixture, 12 g (0.057 mol) of TEOS was added. The mixture was stirred at 45 °C for 24 h for the formation of the mesostructured product. Subsequently, the reaction mixture was heated for 24 h at 95 °C under static conditions for hydrothermal treatment. The solid product was then filtered, washed with deionized water and dried at 80 °C. Another sample was prepared by varying the hydrothermal treatment temperature from 95 to 150 °C. The samples were designated KIT-5(T) where T denotes the hydrothermal treatment temperature. Finally, the samples were calcined at 550 °C for 6 h in order to remove the template.

2.3. Preparation of poly(2-hydroxyethyl methacrylate)/KIT-5 (PHEMA/KIT-5)

2-Hydroxyethyl methacrylate (HEMA) (0.25 mL, 2.06 mmol or 0.50 mL, 4.11 mmol) and KIT-5 (0.5 g) in 7 mL of tetrahydrofuran (THF) were placed in a round bottom flask. Azobisisobutyronitrile (3% mol, 0.010 g or 0.020 g) was added and the mixture was heated to 65–70 °C for 5 h while being stirred under N₂ gas. The resulting white fine powder composite (PHEMA/KIT-5) was collected by filtration, washed several times with THF, and finally dried at 60 °C under reduced pressure. The samples were labeled PHEMA/KIT-5(95)_x and PHEMA/KIT-5(150)_x based on the utilized KIT-5 (the subscript x stands for the weight ratio of the consumed polymer and equals 1/2 or 1, which is not written here in the case of 1. It should be mentioned as well that these subscripts are theoretical ratios and the practical ratios may be less than these figures).

2.4. Cytotoxicity assay

Dimethylthiazoldiphenyltetrazolium bromide (MTT) assay was used for cell toxicity evaluation. L929 cell line was supplied from National Cell Bank of Iran (Pasteur Institute of Iran) and cultured in Roswell Park Memorial Institute medium (RPMI, Gibco) supplemented with 10% fetal bovine serum (FBS, Nanobioarray), 100 U mL⁻¹ penicillin and 100 μg mL⁻¹ streptomycin (Sigma, USA). The number of 1 × 10⁴ cells per well in 100 μlit of medium were cultured in a 96-well microtiter plate (Orange, Belgium) at 37 °C in a humidified atmosphere with 5% CO₂. After 24 h, three different concentrations of each sample in culture medium (0.2, 0.02 and 0.002 g mL⁻¹) were added to each well. The cells with the same condition and without any sample were considered as control. The cells were exposed to the samples with final concentration of 0.1, 0.01 and 0.001 g mL⁻¹ for another period of 24 h. Afterwards, the medium were discarded and 100 μL of a solution of MTT (0.5 mg mL⁻¹, Sigma) was added to each well followed by incubation for 5 h at 37 °C. The formed formazan crystals were then dissolved in isopropanol (100 μL, Sigma) for 10 minutes at 37 °C under gentle shaking. The optical density (OD) was recorded on a multiwall microplate reader (Statfax 2100, USA) at 545 nm.³¹

Viability% = 100 − toxicity%

2.5. Drug loading

The model drug ibuprofen loading process was carried out on the calcined samples KIT-5(95) and KIT-5(150) and the modified nanocomposites PHEMA/KIT-5(95), PHEMA/KIT-5(95)_1/2, PHEMA/KIT-5(150) and PHEMA/KIT-5(150)_1/2. Typically, solvents which have a high solubilizing capacity for poorly soluble drugs, are used for this purpose. Additionally and importantly, the vapor pressures of such solvents are high such that residual solvent can be readily removed. Therefore, n-hexane was chosen for loading of IBU on the carriers. To this purpose, the powders (400 mg) were soaked into a solution of ibuprofen in n-hexane (20 mg mL⁻¹) for 24 hours at room temperature while being stirred (200 rpm). In addition to elevating the loading time, the weight ratio between mesoporous matrix and drug were changed by increasing the volume of loading solution, in order to obtain larger amount of drug inside the mesoporous matrices.

2.6. Drug release studies

The in vitro assessment of ibuprofen delivery from the mesoporous matrices was carried out by immersing the loaded mesopores (50 mg) in 40 mL of a simulated body fluid, designated as SBF, which is similar in ionic composition to the human plasma at 37 °C.³² The processes to evaluate the released drug were performed without stirring of the SBF solution. UV spectrometry at 262 nm was the analytical method used for screening the amount of ibuprofen delivered as a function of time.

3. Results and discussion

3.1. Catalyst characterization

Fig. 1 represents the powder XRD patterns of the mesoporous silicas, KIT-5(95), KIT-5(150) and IBU/KIT-5(150) as well as PHEMA/KIT-5(95) and PHEMA/KIT-5(150)_1/2 composites. The strong reflection in the region 2θ = 0.7° to 1° observed for the pure-silica samples could be indexed to 111 family of planes which is indicative of the cubic space group Fm3m.³³ PHEMA/KIT-5(95) and PHEMA/KIT-5(150)_1/2 samples also exhibit the same patterns indicating that the structure of the purely siliceous matrices are well preserved even after immobilization with PHEMA. Nevertheless, the reduction found in the intensity of the characteristic reflection peaks of the composites, especially in the case of PHEMA/KIT-5(150)_1/2, may be attributed to the symmetry destroyed by the hybridization of KIT-5, which is typical of the ordered mesoporous silica hybridized with organic groups.²⁷ In addition, composites contain much less KIT-5 because of the dilution of the siliceous material by PHEMA.

Fig. 1 The powder XRD patterns of KIT-5(150), PHEMA/KIT-5(150)_1/2, KIT-5(95), PHEMA/KIT-5(95) and IBU/KIT-5(150).

The length of the cubic cell a₀ and the diameter of the cage D_m are calculated using the formulae and D_m = a₀(6ε_me/πv)^1/3 (ε_me is the volume fraction of a regular cavity and v is the number of cavities present in the unit cell which is equal to 4 for Fm3m space group) respectively^33,34 (Table 1). The greater cubic cell length, cage diameter and other related parameters of KIT-5(150) compare to KIT-5(95) which are also suggested by the slight shift of its characteristic reflection peak to a lower angle, arise from the higher hydrothermal treatment temperature.^33,34 Furthermore, the decrease of d₁₁₁ spacing (or the pore center distance a₀) in polymeric composites results from decreasing of the cage size after hybridization process which may be attributed to polymer chains distribution in pores of the KIT-5 (Table 1).

Table 1 Physicochemical properties of mesoporous silicas KIT-5(95), KIT-5(150), PHEMA/KIT-5(95) and PHEMA/KIT-5(150)_1/2 obtained from XRD analysis

Sample	d111 (nm)	a0 (nm)	Dm (nm)	2θ (°)
KIT-5(95)	9.57	16.6	7.32	0.922
KIT-5(150)	12.02	20.8	9.18	0.734
PHEMA/KIT-5(95)	9.40	16.28	—	0.938
PHEMA/KIT-5(150)1/2	11.3	19.57	—	0.78

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Nitrogen adsorption–desorption measurements were used to further characterize the mesoporous silicas and the nanocomposites as shown in Fig. 2 and 3. The BET specific surface areas and pore sizes of the host KIT-5(150), IBU/KIT-5(150) and PHEMA/KIT-5(150) were calculated using Brunauer–Emmentt–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Both samples show a type IV adsorption–desorption isotherm with an H2 hysteresis loop, signifying the large uniform cage-type pores (Fig. 2). Besides, the corresponding BJH adsorption pore size distributions shown in Fig. 3 demonstrate the high structural ordering with a narrow pore size distribution of the samples.

Fig. 2 N₂ adsorption–desorption isotherms of mesoporous silica KIT-5(150), PHEMA/KIT-5(150)_1/2 and IBU/KIT-5(150).

Fig. 3 Pore size distributions of KIT-5(150), PHEMA/KIT-5(150)_1/2 and IBU/KIT-5(150).

BET surface area, pore size and pore volume of KIT-5(95), KIT-5(150), PHEMA/KIT-5(95) and PHEMA/KIT-5(150)_1/2 samples have been summarized in Table 2. A comparison between the two pure mesoporous silicas reveals that KIT-5(150) possesses larger pore volume and pore size but smaller BET surface area relative to KIT-5(95). This observation, arisen from variation in hydrothermal treatment temperature, is not only in agreement with the XRD results but also with other studies.³⁴ It is also noticeable that in comparison to KIT-5(150), PHEMA/KIT-5(150)_1/2 features less BET surface area and pore volume which are acceptable consequences of immobilization the polymeric chains inside the pores. Despite the reduction of surface area and pore volume of PHEMA/KIT-5(150)_1/2, there seems to be no significant change in the pore size of the resultant composite. This fact could be attributed to the occurrence of two simultaneous but opposing incidents. To state more precisely, the presence of PHEMA chains inside the channels, reduces the pore size of mesoporous material as expected. In the meanwhile, the physical pressure on the channels wall induced by polymeric strands can cause an increase in the pore diameter. However, it is wise to advert that the exact similarity of the numbers here is accidental (the same trends could be obviously seen for KIT-5(95) and PHEMA/KIT-5(95) from Table 2).^28,29

Table 2 Physicochemical properties of mesoporous silica KIT-5(95), KIT-5(150), PHEMA/KIT-5(95) and PHEMA/KIT-5(150)_1/2 obtained from N₂ adsorption analysis

Sample	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
KIT-5(95)	1090	0.71	2.62
KIT-5(150)	517	0.79	4.03
PHEMA/KIT-5(95)	596	0.40	2.59
PHEMA/KIT-5(150)1/2	393	0.64	4.03

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Fig. 4 presents the FT-IR spectra of KIT-5(150) and PHEMA/KIT-5(150)_1/2 (spectra a and b). The spectra of pure KIT-5 (150) and the hybrid sample exhibit the typical vibrations of asymmetric and symmetric stretching in addition to the rocking of Si–O–Si at approximately around 1090, 820 and 470 cm⁻¹ which are characteristic of mesoporous silica. Moreover, the intense band at 1727 cm⁻¹ clearly shows the presence of PHEMA in the nanocomposite sample (Fig. 4b). This new band at the hybrid sample spectrum is attributed to the carbonyl bond of PHEMA.³⁵ Furthermore, the aliphatic C–H stretching peaks at around 2800–3000 emphasize the existence of PHEMA in the hybrid sample. It is worthy to note that all the mentioned evidences signify the successful synthesis of the PHEMA/KIT-5(150) composite.

Fig. 4 FT-IR spectra of (a) mesoporous silica KIT-5(150), (b) PHEMA/KIT-5(150)_1/2, (c) IBU and (d) IBU/KIT-5(150).

In order to measure the amount of polymer loaded inside the KIT-5(150), as well as thermal stability of the nanocomposite, thermo gravimetric analysis under N₂ atmosphere was conducted. Fig. 5 shows the TGA curve of PHEMA/KIT-5(150)_1/2. The mass loss at temperature below 100 °C (approximately 3%, w/w) is attributed to the thermo desorption of physically adsorbed water on the surface of the sample. The second weight loss in the range of 250 to 500 °C which is around 22% (w/w), results from the degradation of the polymer. By comparing the PHEMA/KIT-5(150)_1/2 and pure PHEMA TG curves,³⁵ it becomes clear that the weight loss of the confined PHEMA occurs at a higher temperature, which means that PHEMA/KIT-5(150)_1/2 has a higher thermal stability and a slower degradation rate than the naked PHEMA. Therefore once again, it could be concluded that the polymeric strands are mainly present inside the pores.

Fig. 5 TGA curve of PHEMA/KIT-5(150)_1/2.

The morphologies of the synthesized products i.e. KIT-5 host, PHEMA/KIT-5(150)_1/2 and PHEMA/KIT-5(95) were inspected by means of SEM technique (Fig. 6). All samples are agglomerates of small irregular particles. It can be observed that there is negligible difference in particle surface morphology between the KIT-5 host and the PHEMA/KIT-5 composites, implying that the polymerization takes place more in the channels and less on the outer surface of the matrix.

Fig. 6 Scanning electron microscopy (SEM) photographs of (a) mesoporous silica KIT-5, (b) PHEMA/KIT-5(150)_1/2 and (c) PHEMA/KIT-5(95).

In Fig. 7, characteristic TEM images of PHEAM/KIT-5(150)_1/2 and PHEMA/KIT-5(95) are shown. It should be noted that despite the appearance of some darker patches on the images, which are attributed to the encapsulation of PHEMA chains inside the mesochannels, the ordered cubic Fm3m mesostructure of KIT-5 is obviously retained and no damage in the periodic structure of the silicate framework is observed.

Fig. 7 TEM images of (a) and (b) PHEMA/KIT-5(150)_1/2 and (c) PHEMA/KIT-5(95).

3.2. Ibuprofen loading process

As stated before, ibuprofen loading was carried out by soaking of 400 mg of desired matrix in 20 mL of a 20 mg mL⁻¹ ibuprofen in n-hexane solution. The mixture was stirred at ambient temperature for 24 hours and eventually the amount of absorbed ibuprofen was measured by monitoring the ibuprofen concentration using Cary 100 scan UV-Vis spectrophotometer at a wavelength of 262 nm. In order to investigate the optimum conditions for drug loading, the one-factor at a time method was utilized. The time of drug loading process and the weight ratio of drug molecules to the matrix were the studied factors. The time factor had no significant impact on the loaded drug amount (27% (w/w) and 27.5% (w/w) in 24 and 48 h respectively). The results of drug molecules to the matrix ratio investigations are summarized in Table 3.

Table 3 Effect of weight ratio of drug to mesoporous matrix on the adsorption of ibuprofen in KIT-5(95)

Weight ratio of ibuprofen to mesoporous matrix (w/w)	1 : 1	2 : 1	3 : 1	4 : 1	5 : 1
Amount of loaded drug in KIT-5(95) (w/w)	27%	30%	32%	40%	38%

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3.3. Characterization of KIT-5(150) loaded with ibuprofen (IBU/KIT-5(150))

Fig. 1 represents the XRD pattern of mesoporous silica KIT-5(150) after loading with ibuprofen as well. Obviously, the resulting pattern resembles the XRD pattern of the pure siliceous mesopore, KIT-5(150) (reflection of 111 family of planes) which manifest the maintenance of the cage-type mesoporous structure. On the other hand, the intensity decrease of the characteristic reflection peak, clearly suggest the encapsulation of IBU molecules inside the pore channels (the same result which is not shown here was obtained for PHEMA/KIT-5(150)_1/2). Furthermore, the mathematical results obtained from XRD analysis, validate these conclusions (Table 4).³⁶

Table 4 Physicochemical properties of IBU/KIT-5(150) obtained from XRD and N₂ adsorption analyses

Sample	d111 (nm)	a0 (nm)	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
IBU/KIT-5(150)	11.31	19.59	351	0.58	3.53

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To further examine whether IBU molecules are mainly encapsulated into the mesostructure, nitrogen adsorption–desorption analysis was performed. Fig. 2 also shows the N₂ adsorption–desorption isotherm of KIT-5(150) loaded with IBU molecules. Apparently, this isotherm is in accordance with the type IV isotherm with an H2 hysteresis loop that signifies the mesoporous structure of IBU/KIT-5(150). In other words, this means that the mesoporous structure of KIT-5(150) is preserved even after the confinement of IBU molecules inside the pores. Additionally, as shown in Table 4, the BET surface area, pore volume and diameter reduction of IBU/KIT-5(150) relative to KIT-5(150) (Table 2) could be further used to conclude that ibuprofen molecules are mainly placed on the internal surface of KIT-5(150).³⁶ Moreover, the narrow pore size distribution of IBU/KIT-5(150) illustrated in Fig. 3, strongly suggest the even dispersion of drug molecules inside the channels.

FT-IR spectra of IBU (c) and IBU/KIT-5(150) (d) are shown in Fig. 4. The FT-IR spectrum of IBU/KIT-5(150) clearly shows the carbonyl and C–H stretching vibrations at 1712 and 2800–3000 cm⁻¹ respectively. It is worthy to note that the slight shift of the carbonyl stretching adsorption is attributed to the hydrogen bonding present between ibuprofen carbonyl groups and hydroxyl groups of KIT-5. Nevertheless, these stretching absorbances which are obviously in consonance with the IBU spectrum could be used to further confirm the presence of IBU molecules in the mesoporous matrix.

Fig. 8 illustrates the scanning electron microscopic images of KIT-5(150) loaded with IBU molecules by 6000 (a) and 15 000 (b) times of magnification. Making a comparison of these images with the one related to the pure KIT-5 (Fig. 6a), reveals the slight changes made in the particles morphology after loading with drug molecules. This observation suggests the presence of some IBU molecules on the outer surface of the matrix. It should be noted that this conclusion is in accordance with other studies.³⁷

Fig. 8 Scanning electron microscopy (SEM) photographs of IBU/KIT-5(150).

3.4. Ibuprofen loading and release investigation

After optimizing the time and drug ratio of loading process for KIT-5(95) (Table 3), the procedure was repeated for other carriers in the optimum conditions. As the results in Table 5 show, the pure siliceous mesopores, KIT-5(95) and KIT-5(150), possess nearly similar amounts of IBU. Nevertheless, the slightly more adsorbed IBU on KIT-5(95) is due to its greater BET surface area.³⁷

Table 5 Drug loading content (w/w) onto different matrices

Sample	KIT-5 (95)	KIT-5 (150)	PHEMA/KIT-5 (95)	PHEMA/KIT-5 (95)1/2	PHEMA/KIT-5 (150)	PHEMA/KIT-5 (150)1/2
Amount of loaded ibuprofen (w/w)	40%	37%	16%	19%	25%	38%

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In the case of polymeric composites, PHEMA/KIT-5(95) contains relatively lower amounts of IBU molecules. This observation arises from the occupation of the composite pore volume with polymeric chains. To approach this problem, PHEMA/KIT-5(150)_1/2 with greater free pore volume was utilized. It is wise to note that in this case, although some of the pore volume is still occupied with polymer strands, the stronger interaction between IBU molecules and PHEMA (mostly hydrogen bonds) results in a good loading capacity.

As mentioned previously, spectrophotometric analysis of the SBF supernatant of different matrices yielded information about the release profile of different mesoporous carriers. As illustrated in Fig. 9, cumulative release pattern of all samples, show an initial burst of drug release which could be due to the adsorbed IBU on the outer surface of the mesoporous matrices that is easily released in the first step to the delivery medium. Further release analysis of IBU from KIT-5(95), reveals a very slow and incomplete release of drug molecules which is acceptable due to its cage-type structure and narrow pore diameter. Because of the poor results obtained from KIT-5(95) based carriers, either in loading or release processes (16% drug loading and an incomplete and unrepeatable drug release for PHEMA/KIT-5(95)), the next experiments were mainly focused on KIT-5(150) based carriers. Studies on KIT-5(150) release profile reveals a more complete delivery of IBU compare to KIT-5(95) thanks to its greater pore diameter. On the other hand, IBU release from PHEMA/KIT-5(150)_1/2 shows a more controlled release pattern which could be very profitable in some cases.

Fig. 9 IBU release profile from different matrices (a) KIT-5(150), (b) KIT-5(95), and (c) PHEMA/KIT-5(150)_1/2.

As it can be deduced from Fig. 10, IBU molecules contain hydroxyl and carbonyl groups through which, they can be adsorbed to the silanol groups of KIT-5 by hydrogen bonds. However, these interactions are relatively week due to the hydrophobic characteristics of IBU (benzene ring and alkyl groups) which is in contrast to hydrophilic characteristics of KIT-5.

Fig. 10 Molecular structure of Silica mesopore KIT-5, PHEMA and IBU molecule.

In the case of composite (PHEMA/KIT-5), although HEMA polymeric chains are adsorbed to the silanol groups by the same hydrogen bonds, this adsorption is much stronger than the previous one and polymeric chains are not leachable since there are much more hydrogen bonds as a consequence of more functional groups. It is important to note that in this case, not all the functional groups of PHEMA are adsorbed on the silica surface, therefore, there are still free functional groups to interact to IBU molecules through hydrogen bonds (these hydrogen bonds are also stronger than the hydrogen bonds between IBU and KIT-5 since both IBU and the polymer have hydrophobic characteristics). In addition, dipol–dipol interactions between polar parts as well as numerous van der Waals forces between nonpolar chains of polymers and nonpolar parts of ibuprofen molecules, make the adsorption of IBU on the composite much greater than its adsorption on the unmodified KIT-5 (in the latter, there is definitely no van der Waals force).

In the release process of the IBU/unmodified KIT-5 in water, it is in fact the stronger hydrogen bonds between IBU and water molecules which bring about the release process and the only decelerating factor might be the pore size of KIT-5.

However, the release process from the nanocomposite (PHEMA/KIT-5) is a different story. Since HEMA is a hydrogel, it collapses in non-aqueous environment as a result of intramolecular hydrogen bonds. Consequently, IBU molecules are trapped inside the chains and have difficulty releasing to the medium. However, in an aqueous environment, these intramolecular hydrogen bonds are replaced by intermolecular hydrogen bonds (with water molecules) and result in polymer expansion, allowing the IBU molecules to release. This release process is slower and more controllable compare to the release process from KIT-5 thanks to the mentioned stronger interactions.

It is also wise to mention that it is in fact the hydrophilic characteristics of polymer chains which bring about the release of drug molecules into the delivery medium. In other words, on the one hand, the expansion of internal hydrophilic polymer chains in aqueous environments would commence the release process, but on the other hand the limited expansion of internal chains, due to the rigid structure of KIT-5, makes the release process slow and controllable (Fig. 11).

Fig. 11 Schematic release process of IBU molecules from PHEMA/KIT-5(150)_1/2 in an aqueous environment.

3.5. Cytotoxicity

To evaluate KIT-5(150) and PHEMA/KIT-5(150)_1/2 being potentially applied as effective drug carriers for therapeutics, MTT assay was carried out. Fig. 12 shows the cytotoxicity activity of the samples in different concentrations. The concentrations of 0.1 and 0.01 g mL⁻¹ were in the range of toxic level (less than 80 percent of control) while the concentration of 0.001 g mL⁻¹ was in the range of non-toxic for both samples. In fact, the results of cytotoxicity tests showed that KIT-5 and PHEMA/KIT-5 are non-toxic materials at low concentrations as anticipated.^21,37 In this regard, it should be mentioned that application of the presented carriers may be more appropriate for potent drugs which have very high pharmacological activity, so can be useful in very low concentrations.

Fig. 12 Cytotoxicity study (MTT assay) of mesoporous matrices KIT-5(150) and PHEMA/KIT-5(150)_1/2 at different concentration.

4. Conclusion

In this study, an innovative polymer–silica hybrid carrier, PHEMA/KIT-5(150)_1/2 was prepared by a simple method, while the mesostructure of the mesoporous silica was completely retained. IBU was applied as a model drug to evaluate the loading and release properties. Relative to other corresponding carriers investigated in this article, PHEMA/KIT-5(150)_1/2 exhibit superior capability in loading and controlled releasing of the model drug. Moreover, in in vitro cell assays, the blank carriers showed almost no cytotoxicity at 0.001 g mL⁻¹ concentration. Overall, this investigation demonstrates that the novel polymer–silica composite, PHEMA/KIT-5(150)_1/2 has excellent potential as a sustained release carrier, especially in the case of polar therapeutics.

Acknowledgements

We gratefully acknowledge Dr E. Mirmomtaz and Dr M. Rafienia for their help and advice during the project. In addition, the authors wish to thank RAHA Pharmaceutical Company.

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