Dedicated to Professor Dumitru Oancea on the occasion on his 75th anniversary MESOPOROUS SILICA- CERIA COMPOSITES AS CARRIERS FOR DRUG DELIVERY… [621463]

ACADEMIA ROMÂNĂ
Revue Roumaine de Chimie
http://web.icf.ro/rrch/
Rev. Roum. Chim. ,
2016 , 61(6-7), 557-563

Dedicated to Professor Dumitru Oancea
on the occasion on his 75th anniversary
MESOPOROUS SILICA- CERIA COMPOSITES
AS CARRIERS FOR DRUG DELIVERY SYSTEMS
Marilena PETRESCU,a Raul-Augustin MITRAN,a,b Cristian MATEIa and Daniela BERGERa*
aUniversity “Politehnica” of Bucharest, Faculty of Applied Chemi stry and Material Science, Bucharest, Polizu no. 1-7,
Bucharest, 011061, Roumania
bRoumanian Academy, Institute of Physical-Chemistry “Ilie Murgul escu”, 202 Splaiul I ndepedentei, Buchares t, 060021, Roumania
Received November 12, 2015
The paper presents the synthesis and characterization of mesopo rous silica-ceria
composites with an ordered hexa gonal pore array characteristic for MCM-41 materials,
which were applied as carriers for oxytetracycline, an antibiotic active against a broad
range of Gram-negative and Gram -positive bacteria. The mesoporo us matrices and
antibiotic-loaded samples were c haracterized by various techniq ues: small- and wide-angle
XRD, FTIR spectroscopy, N 2 adsorption-desorption isotherms, SEM and TEM. The drug
delivery profiles were determined in saline buffer phosphate so lution, pH=5.7 by using
UV-vis spectroscopy. All drug re lease profiles exhibited a pron ounced burst effect, but
slower kinetics was noticed for oxytetracycline delivered from mesoporous silica-ceria
composites.

INTRODUCTION*
The design of functional materials for biomedical
applications has focused many research efforts. Among inorganic materials tested as vehicles in drug delivery systems, ordered mesoporous silica has
received a lot of interest,
1-5 since 2001 when MCM-
41 silica matrix was reported as carrier for ibuprophen
3 due to its outstanding properties: ordered
pore array with tunable pore dimension, high surface area and total pore volume, as well as biocompatibility. Nonporous and nonordered porous
colloidal silica materials are accepted as excipients in
pharmaceutical formulations, being listed in the Inactive Ingredients Databa se of U.S. Food and Drug
Administration.
1

* Corresponding author: [anonimizat] Recently, several studies reported interactions
between mesoporous silica nanoparticles (MSN) and biological systems, inorganic nanoparticles being transported into cells through endocytosis that occurs through either specific or nonspecific
cellular uptake depending on MSN properties
(shape, size and surface properties). Tremendous efforts have focused on the synthesis of uniform-sized and shaped dispersible MSN with tunable pore size that can accommodate enough high content of pharmaceutical active ingredient.
5,6
Ceria possesses a cubic fluorite structure that
tends to be a nonstoichiometric compound due to its dual oxidation state of cerium atoms, +4 and +3, which means that the reduction of Ce
4+ to Ce3+ is
accompanied by formation of oxygen vacancies,

558 Marilena Petrescu et al.

responsible for its catalytic and antioxidant
properties. Based on X-ray photoelectron
spectroscopy and X-ray absorption near edge spectroscopy studies one can conclude that the concentration of Ce
3+ relative to Ce4+ increases
with the particle size decrease, with a Ce3+ content
of minimum 6% in 6 nm nanoparticles and 1% in
10 nm particles.7
Nanoceria exhibits radical scavenger properties
in cell cultures as consequence of interconversion
of Ce4+ ↔ Ce3+ on nanoparticles surface acting as
superoxide dismutase and catalase mimetic.8 Based
on its ability to scavenge free radicals, nanoceria could be applied as therapeutic agent for: the decrease of retinal degeneration, the reduction of superoxide and peroxynitrite formation in ischemic cardiomyopathy, the treatment of neurological
diseases.
8-10 Andreescu et al. reported a strong
attachment of dopamine to the ceria nanoparticles surface resulting from charge transfer complexes. This process involves the oxidation of dopamine by CeO
2 to dopaquinone intermediates and the
attachment of quinonic moiety to ceria surface
lowering the free dopamine level in biological systems exposed to nanoceria.
11
As drug vehicle, ordered mesoporous silica
could be combined in a composite or hybrid
material with inorganic or organic compound, in
order to design advanced functional materials that gather the advantages of both components of the composite. Herein, we report the synthesis of mesoporous silica-ceria composites that were employed as carriers in drug delivery systems
using a model molecule, oxytetracycline, and
compared with MCM-41 silica material.

OO OH OH
OHO
H2N
HO
NOH
OHH H

Fig. 1 – The chemical structure of oxytetracycline.

Oxytetracycline (Fig. 1) is a broad spectrum
antibiotic with activity against various bacteria that
is still used to treat infections. It is administered by
oral route and is partially absorbed in the gastrointestinal tract.
12 Recently, oxytetracycline-
loaded MCM-41 silica and aluminosilicates proved to have good bactericidal activity against Staphylococcus aureus ATCC43300.
13 EXPERIMENTAL
1. Synthesis of mesoporous silica-ceria composites
Mesoporous silica-ceria composites containing 10% and 20%
(mol) ceria nanoparticles were prepared by sol-gel method using
tetraethyl orthosilicate (TEOS, Fluka) as silicon source and
ammonium cerium (IV) nitrate ((NH 4)2Ce(NO 3)6, Sigma-Aldrich)
as ceria precursor, in the presence of cetyl trimethylammonium bromide (CTAB, Alfa Aesar), in basic medium. To the aqueous solution of CTAB, 25% ammonia aqueous solution (Scharlau) was added and when the reaction mixture reached 40°C, TEOS was added dropwise. After precipitation, aqueous solution of ammonium cerium(IV) nitrate was added. The molar ratios,
TEOS : (NH
4)2Ce(NO 3)6 : CTAB : NH 3 were 1:0.1:0.25:3.37 for
MCM-CeO 2(1) containing 10%(mol) ceria nanoparticles and
1:0.2:0.27:3.7 for MCM-CeO 2(2) with 20%(mol) CeO 2,
respectively. The reaction mixture was ageing under vigorous magnetic stirring at 60°C/1h for a good impregnation of colloid al
silica with cerium ions and then was hydrothermally treated at
100 °C, 96 h in a teflon-lined autoclave under autogeneous
pressure in static conditions. Th e solids were filtered off, wa shed
with ethanol and warm water and dried at room temperature. In order to remove the structure directing agent (CTAB), a calcini ng
step at 550°C/5h with a heating rate of 1°C/min was performed.
For comparison, a MCM-41 silica sample was prepared
according to a reported procedure.
14 For the surfactant removal,
a two-steps procedure was carried out. Firstly, 1 g of silica sample was refluxed in 100 mL ethanol solution containing 2 g
of ammonium nitrate at 90 °C for 1h and then the solid was
filtered off, washed and dried. A second refluxing step in acid ic
alcoholic solution (9 mL HCl 37% in 100 mL ethanol) at 90°C for 1h was performed.
15 The MCM-41 sample treated in such
way was denoted MCM-41E. Also, in this study the calcined (550°C/5h) MCM-41 sample was used.
2. Drug loading and in vitro release studies
Drug loading was carried out by incipient wetness
impregnation method. To an oxytetracycline hydrochloride (Sigma) aqueous solution of 100 mg/mL concentration, 100 mg inorganic carrier was added, gently stirred and dried under vacuum at room temperature, in d ark conditions, for 8 h. The
resulted drug-loaded samples were labeled oxy(content%wt)@support.
In vitro drug delivery experiments were performed in
saline phosphate buffer solution (PBS, pH 5.5) at 37 °C, under
magnetic stirring (150 rpm). An amount of oxytetracycline-loaded sample containing 12.5 mg drug was suspended into 90 mL PBS. Aliquots were periodi cally withdrawn, properly
diluted and analyzed by UV-VIS spectroscopy (Ocean Optics USB 4000 spectrometer).
Both inorganic supports and oxytetracycline-loaded
s a m p l e s w e r e c h a r a c t e r i z e d b y F T I R s p e c t r a r e c o r d e d i n t h e
wavenumber range of 4000-200 cm
-1 using CsI pellet
technique (Bruker Tensor 27 spectrometer), small- and wide-angle XRD (Rigaku Minifl ex II diffractometer), N
2
adsorption/desorption isotherms performed at 77 K (Quantachrome Autosorb iQ
2 porosimeter), TEM (FEI Tecnai
G2-F30) and SEM (Tescan Vega 3 LM).
RESULTS AND DISCUSSION
The wide-angle XRD patterns of both silica-
ceria composites evidenced the presence of fluorite phase with cubic symmetry characteristic to ceria

Mesoporous silica-ceria composites 559

nanoparticles, besides th e amorphous silica phase
(Fig. 2A). The small-angle XRD results proved the formation of an ordered hexagonal pore array, both composite samples exhibiting the Bragg reflections
characteristic to MCM-41 materials (Fig. 2B), higher
intensity can be observed for the composite with lower content of ceria nanoparticles. The MCM-41
samples exhibited a highly-ordered 1D pore
framework as small-angle XRD proved (Fig. 2B). No significant differences between small-angle XRD data of MCM-41 and MCM-41E (presented
elsewhere
13) were noticed.
In the FTIR spectra of inorganic mesoporous
matrices one can notice the characteristic bands of silica and ceria: a large intense asymmetric and
medium intense symmetric stretching vibrations at
1085 cm
-1 and 800 cm-1 respectively, assigned to
Si-O-Si bonds, at 960 cm-1 and 460 cm-1 indicating
the presence of silanol groups and Si-O deformation bands overla pped with very intense
Ce-O band (Fig. 3).
The TEM investigation of MCM-CeO 2(1)
composite showed an ordered pore array for silica
phase, in agreement with small-angle XRD results, which presented spherical particles, while ceria phase was formed as very small nanocrystals with
an average size of 5 nm that tended to be
distributed in bundles (Fig. 4A). The SEM analysis of MCM-CeO
2(2) composite evidenced the
formation of silica spherical particles with a
diameter of 250-300 nm and small ceria nanoparticles on silica surface (Fig. 4B). The MCM-41 sample calcined at 550°C had bigger
spherical particles than the composite samples with
the diameter ranging between 400-600 nm (Fig. 4C), while the MCM-41E sample exhibited spherical particles associated in rods, preserving
the morphology of surfactant micelles (Fig. 4D) as
SEM investigation proved.

A B
Fig. 2 – XRD patterns of silica-ceria composites calcined at 55 0°C/5h: wide angle (A) and small angle in comparison with MCM- 41 (B).

Fig. 3 – FTIR spectra of silica-ceria composites in comparison with CeO 2 and MCM-41.

560 Marilena Petrescu et al.

Fig. 4 – TEM image of MCM-CeO 2(1) (A) and SEM images of: MCM-CeO 2(2) (B), MCM-41 sample c alcined at 550°C (C)
and MCM-41E sample (D).

Fig. 5 – N 2 adsorption-desorption isotherm s of synthesized supports and re presentative drug-loaded samples (A) and the
corresponding pore size distributions (B).

The nitrogen adsorption-desorption isotherms
of silica-ceria composites exhibited relative high porosity, being of type IV, characteristic for mesoporous materials (Fig. 5A). The specific
surface area values ( S
BET) were computed using
Brunauer-Emmett-Teller method in the 0.05-0.30 relative pressure range and the pore size distribution curves were determined by Barrett-Joyner-Halenda model from desorption branch of
isotherms; all samples presented unimodal pore size distribution curves (Fig. 5B). The textural
parameters (the specific surface area, S
BET, total
pore volume, Vp and the average pore diameter,
dBJH) determined from N 2 adsorption-desorption
isotherms of carriers were gathered in Table I. The
porosity of the MCM-CeO 2(2) is lower than the
other supports, having a fraction of macropores, explained by the presence of ceria nanoparticles on the silica particles surface (Fig. 5A and 5B). In
comparison with MCM-41 sample, with the

Mesoporous silica-ceria composites 561

average pore diameter, dBJH, of 2.82 nm, the silica-
ceria composites have slightly lower pore sizes
(2.67 nm) (Table I).
The incipient wetness impregnation procedure
was chosen to obtain oxytetracycline-loaded
samples because it is a simple and reliable method,
especially for highly soluble drugs with a low chemical stability. Oxytetracycline was loaded onto the silica-ceria composites, as well as on
pristine MCM-41 samples taking into account the
inorganic support porosity.
The presence of oxytetracycline molecules loaded
onto the inorganic supports was indicated by FTIR
spectroscopy. In the FTIR spectra of drug-loaded samples it can be observed the vibration bands that belong to the support, besi des the ones of the drug
located in the range of: 2850-2950 cm
-1 attributed to
υas,s(CH)), 1590-1650 cm-1 ascribed to the
deformation of amide moieties and 1310-1410 cm-1
assigned to the phenol groups (F ig. 6). The recorded N 2 adsorption-desorption
isotherms for oxytetracycline-loaded materials
showed a lower porosity (lower values of specific surface area and mesopores volume) than of the corresponding supports (Fig. 5A), the textural parameters of the oxytetracycline-based samples being listed in Table I. Only a slight decrease of
the average pore size was observed that means
weak interactions between drug molecules and inorganic matrix were established (Fig. 5B).
In the wide-angle XRD patterns of the
oxytetracycline-loaded samples (Fig. 7), only the
Bragg reflection of fluorite phase can be noticed,
demonstrating that the antibiotic molecules were adsorbed into the carrier mesopores in amorphous state. The lack of the drug diffraction peaks suggested that no crystalline oxytetracycline was present in all drug-loaded supports, as it was
expected.

Fig. 6 – FTIR spectra of oxytetracycline and oxytetracycline-lo aded supports.

Fig. 7 – Wide-angle XRD patterns of oxytetracycline and loaded supports.

562 Marilena Petrescu et al.

Table 1
Textural properties of carri ers and drug-loaded samples
Samples S
BET (m2/g) Vp
(cm3/g) d
BJH
(nm) Period of drug total release
(min.)
MCM-CeO 2(1) 683 0.67 2.66
MCM-CeO 2(2) 512 1.14 2.67
MCM-41E 819 0.73 2.81 MCM-41 1045 1.06 2.82
oxy20%@MCM-CeO
2(1) 265 0.05 ( dp< 10 nm) 2.64 150
oxy20%@MCM-CeO 2(2) 173 0.04 ( dp< 10 nm) 2.52 150
oxy25%@MCM-41E 144 0.12 ( dp< 10 nm) 2.81 95
oxy33%@MCM-41 163 0.10 ( dp< 10 nm) 2.80 60

Fig. 8 – The oxytetracycline cumulative release profiles from s ilica-ceria composites in comp arison with MCM-41 samples.

In vitro oxytetracycline release experiments were
performed in saline phosphate buffer solution at pH
5.7 as simulated body fluid. Oxytetracycline, very
soluble in water, has low photochemical and
chemical stability, especially in basic solutions. From all studied carriers, the antibiotic molecules were completely delivered within a period between 60 to 150 min. (Table I), all release profiles exhibiting a pronounced burst effect (Fig. 8).
The drug delivery from silica-ceria composites
was slightly slower than f rom MCM-41. The higher
content of silanol groups in MCM-41E slows down the antibiotic delivery kinetics in comparison with the corresponding calcined MCM-41 carrier. Recently, we reported a similar behavior for doxycycline that had a slower release kinetics from MCM-CeO
2(1)
than MCM-41 silica matrix.16 CONCLUSIONS
We successfully obtained silica-ceria
composites having an ordered hexagonal pore array characteristic to MCM-41 materials and relatively high porosity. The composite materials were used as carriers for oxytetracycline and
compared with MCM-41-type silica support.
The antibiotic molecules were loaded into the
mesopores of inorganic material in amorphous
state. The drug delivery kinetics was slower from silica-ceria composites than from MCM-41, from all studied vehicles, oxytetracycline being
completely recovered in 150 min. for silica-ceria
composites and in 60 min. from MCM-41. Regarding the silica matrix, the uncalcied sample led to a slower drug release than the calcined
MCM-41 support.

Mesoporous silica-ceria composites 563

Acknowledgement: The Roumanian pr oject PCCA no.
131/2012 is gratefully acknowledge d. The work of M.P. has
been funded by the Sectoral Operational Programme Human
Resources Development 2007-2013 of the Ministry of
European Funds through the Financial Agreement POSDRU
187/1.5/S/155420.
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