Mesoporous Silica Nanoparticles as a System for Ciprofloxacin Drug Delivery; Kinetic of Adsorption and Releasing

Mesoporous silica (MPS) nanoparticle was prepared as carriers for drug delivery systems by sol–gel method from sodium silicate as inexpensive precursor of silica and Cocamidopropyl betaine (CABP) as template. The silica particles were characterized by SEM, TEM, AFM, XRD, and N2adsorption–desorption isotherms. The results show that the MPS particle in the nanorange (40-80 nm ) with average diameter equal to 62.15 nm has rods particle morphology, specific surface area is 1096.122 m/g, pore volume 0.900 cm/g, with average pore diameter 2.902 nm, which can serve as efficient carriers for drugs. The adsorption kinetic of Ciprofloxacin (CIP) drug was studied and the data were analyzed and found to match well with pseudofirst order kinetic model. The CIP drug-loaded mesoporous silica (CIP-mSiO2) nanoparticles has capacity of about 16.3 mg drug/ mg mSiO2 were achieved, and capable of releasing 26% and 98.6% of their drug content after 90 min in water and PBS solution(pH,7.4) respectively. In-vitro controlled release studies of CIP in Simulated Body Fluid were carried out under stirring conditions. A study on release kinetics and mechanism using Koresmeyer-Pepps model, first order kinetic, and kopcha model shows that the Korsmeyer-Peppas and Kopcha models, both conform more closely to the release data.


Introduction
Among the materials which may have widespread potential as drug carriers such as colloidal systems, liposomes, micro emulsion, etc. (1)(2)(3)(4), mesoporous silica (MPS) have some engaging properties, for example large pore volume and surface area, narrow pore size range, chemically inert and allowing easier functionalization of their surface (5,6) which make them an attractive drug carrier and its release.
In 2001, MPS was first reported as a drug delivery system and in which they loaded ibuprofen drug into the mesoporous of MPS which exhibited high drug loading capacity and sustained drug release (7). An amphiphilic molecules modified with amino acid were used as drug model and the loading on MPS containing a cage and cylindrical pore. The controlled release from its carrier has been also studied (8). A novel mesoporous silica nanoparticles as a carrier for Ibuprofen drug was synthesized and the release kinetics was evaluated. The results show that the synthesized carrier exhibited high loading and a very good release rate (9).
The feasibility of loading rifampin as a drug model into prepared mesoporous silica nanoparticles was determined using methanol, water, and dimethyl sulfoxide solvents in adsorption experiments to load rifampin within the mesoporous. The loading results show that methanol was the best solvent, providing a drug loading efficiency of 52 % and capable of releasing 95% after 24 h using buffer phosphate saline BPS (pH=7.4) (10). The loading and release of two anticancer drugs 5-fluorouracil and 7hydroxycoumarin from MCM-48 nanoparticles were investigated and the results show that loading capacities of 5-fluorouracil and 7-hydroxycoumarin onto the nanoparticles of about 24 and 14 % were achieved, respectively (11). MPS nanoparticles have diameters in the range of a few hundred nanometers and two pore structures were synthesized , loaded with doxorubicin drug, and the release into a buffer solution was studied to determine the pore structure and type of MPS nanoparticles effect (12,13). The aim of this study is to synthesize MPS silica nanoparticles with a large surface area, high pore volume and regular distribution of pore sizes and to characterize the prepared mSiO 2 particles using different techniques. Ciprofloxacin (CIP) drug was selected as a model drug to study the kinetics of loading and of releasing in water and buffer solution.

Materials and methods
Sodium silicate (14% NaOH, 27% SiO 2 w/w) as silica precursor and Cocamidopropyl betaine (CABP) as template were obtained from the state company of vegetable oils -Iraq. Ciprofloxacin (CIP), as an antibiotic, molar mass 367.8 g/mol, λ max at 277nm is purchased from DSM with purity 98%. Figure 1 shows its structure, The prepared mesoporous silica was characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (SEM; Oxford instruments model SEM: S-3200N). The isotherms of N 2 adsorptiondesorption at 77 K were determined using Autosorb-1 Quantachrome Instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The particle size and particle size distributions were analyzed using Atomic Force Microscopy (AFM) SPMAA 3000, Advanced Angestrum Inc., USA. The XRD patterns were obtained with a Rigaku diffractometer using Cu Kα (λ = 0.154 nm) radiation.

Preparation of Mesoporous Silica
Firstly, 12 g of (CAPB) were dissolved in 150 mL of distilled water and 17mL of H 2 SO 4 (1M) were added. Then, 3.5g of sodium silicate dissolved in 150 ml of distilled water were added drop by drop to the mixture from burette for 3 hours. The formed white precipitate was separated by filtration and washed with water after aging at 80 °C for one day. After drying at 80 °C , the calcination was performed at 600 °C for 4 hours to remove the surfactant.

Adsorption Kinetic Procedure
The kinetic study of CIP adsorption on mSiO 2 adsorbent was performed by mixing the amount of adsorbents (0.05 g) with 100 mL of CIP (20 mg/L) solution in 250 mL flask. The shaking was performed using thermostatic shaker bath at the temperature 289 K. At various time intervals, a sample was pipetted and the absorbance at maximum wavelength 277 nm was measured to determine the concentration. The amount of drug adsorbed was determined by the equation: Where q e is the equilibrium adsorption capacity of CIP adsorbed on unit mass of the adsorbent (mg /g), C 0 is the initial concentration of CIP drug (mg L -1 ), C e the CIP equilibrium concentration respectively, W (g) is the weight of adsorbent, and (v) is the volume of CIP solution.

Drug Loading
CIP was loaded inside MPS by synthetic method previously reported (14). 0.03 g of MPS was suspended in 5 mL of drug solution (concentration equal to100 mg/mL) and stirred for 24 hours. The CIP loaded MPS (CIP-mSiO 2 ) was centrifuged and the precipitate washed several times with water. Then CIP-mSiO 2 was dried at 80 °C.

In Vitro Drug Release
The prepared CIP-mSiO 2 sample was immersed in 100 mL of water or phosphate buffered saline (PBS, pH = 7.4) under slow stirring at 37.5 °C. At selected time intervals, aliquots (1 mL) were removed from the mixture solution, the amount of CIP released was estimated by the UV-Vis absorption spectra of the aliquots.

Results and Discussion:
The SEM technique was used to study morphology of mSiO 2 surface and to determine the particle size and the size distribution. It is revealed from the images that particles morphology is almost rod type. The range of the particles size is from (89.15-55.45) nm. It means that the particles size is smaller than 100 nm with relatively uniform size distribution. Figure 2b of EDX spectrum shows the presence of silicon and oxygen with zero percent of Na, which confirms that the sodium ion is completely removed from the prepared mSiO 2 by washing and no other impurities are present. Fig. 3 shows the TEM images of mSiO 2 .  The obtained isotherm is typical type -IV isotherm and has H2 type hysteresis loop which indicates the formation of mesoporous with ink bottle type pores.
The measured Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore diameter and pore volume are listed in the Table 1. The results of Table 1 show that MPS has very high surface area(S BET )and large pore volume and narrow range pore size distribution with 2.902 nm average pore size. The size of nano particle and rod type of MPS particles were confirmed by AFM technique as depicts in Fig. 5. The obtained results of particle size distribution in Fig. 5b show that it is in the range 40 -80 nm and the average diameter is 62.15 nm.

Figure 5. a) AFM image; and b) Histogram of Granularity Distribution for mSiO 2
The XRD (Fig. 6) of the prepared mesoporous silica shows a main peak at 2θ angle in the range of 1.5 -3° which reveals long range ordering of the mesoporous in the MPS, and in agreement with the literature-reported data (14,15).

Kinetics of Drug Adsorption
Lagergren-first-order (16,17), pseudosecond order (18), and intr-aparticle diffusion (19) models are applied to investigate the adsorption kinetic behavior of CIP (20 mg/L) onto MPS adsorbent. The equations of the three models are as follows: ( − ) = - Where k 1 (min -1 ), k 2 (g mg -1 min -1 ), and k D (mg g -1 min -1/2 ) are the rate constants of the pseudofirst order, pseudo-second order, and intra-particle diffusion kinetics respectively. q e and q t are amounts of CIP adsorbed on the surface of the adsorbent at equilibrium and at any time (mg g -1 ) respectively, C is constant. Initial adsorption rate (h) was calculated from the equation :-h = k 2 q e 2 . The kinetics parameters obtained from slope and intercept of the plots in Fig. 7 are shown in Table 2.  From the correlation coefficient (R 2 ) values presented in Table 2, it can be seen that the adsorption perfectly complies with pseudo first order model. Also, it can be seen from the plot of intra-particle diffusion model, the adsorption of CIP drug onto mSiO 2 adsorbent was controlled by three stages. The first linear portion is attributed to the diffusion of CIP molecules from bulk toward adsorbent. The second linear portion corresponds to intra-particle diffusion. The third linear portion is the diffusion inside small pores and then the equilibrium is established. If the data shows multilinear plots and do not pass through the origin, the rate determining step is not only intra-particle diffusion but two or more other steps are involved (20,21).

Kinetic of Drug Releasing
The amount of drug loaded in MPS samples has been calculated using the weight of CIP in the 5ml of solution, the weight of CIP in the solution after impregnation, and the weight of the MPS sample .The calculated amounts of CIP loaded in the samples was 16.3 mg drug/mg sample. The concentrations of CIP drug released into the media (water or PBS buffer) were determined using a calibration curve in water or PBS buffer at pH 7.4. Figure 8 shows the CIP drug release from mSiO 2 carrier into the two media. To study the mechanism and kinetics of the CIP drug release, the data obtained were fitted to three models as follow: The first model (equation 5) is the , where is the amount of CIP released at time in minutes, ∞ is the loaded amount of CIP in mSiO 2 particles, K-P is a kinetic constant related to host-guest pair, and is related to the host shape and drug release mechanism. The second model( equation 6) is the first order kinetic release model, where is the first order rate constant. The third model (equation 7) is Kopcha model (11,23), where A is the contribution of diffusion and B is the contribution of erosion. The fit are shown in Figs. 9 and 10 for releasing of CIP in water and PBS respectively, while the results obtained are listed in Table 3.    Table 3, that both Korsmeyer-Peppas and Kopcha models fit more closely to the obtained data and the first order kinetic release model shows the poorest fit to the data in both cases. The two fitted parameters for Korsmeyer-Peppas model are the kinetic parameter and exponential term . Since the obtained value of n in the two cases are smaller than the range (0.43 and 0.85), the particle of drug carrier is not a spherical shape and the mechanism of drug releasing is diffusion (24). In Kopcha model, If A/B ≥ 1 the diffusion is predominates, but if A/B < 1, the erosion predominates (11). The media is greater than 1(12.55 for PBS, and13.071for water medium) which indicate that the dominate mechanism is diffusion.

Conclusions:
The foregoing results of this study confirm the synthesis of mesoporous silica nanoparticle as carriers for CIP drug delivery systems by sol-gel method. The prepared sample has average diameter equal to 62.15 nm, rods particle morphology, specific surface area 1096.122 m 2 /g, pore volume 0.900 cm 3