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Ironoxide-silicananocompositesyieldedbychemicalrouteandsol–gelmethod
ArticleinJournalofSol-GelScienceandTechnology·March2016
DOI:10.1007/s10971-016-3996-1
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ORIGINAL PAPER: NANO-STRUCTURED MATERIALS (PARTICLES, FIBERS, COLLOIDS, COMPOSITES, ETC.)
Iron oxide-silica nanocomposites yielded by chemical routeand sol–gel method
E. Puscasu1 • L. Sacarescu2 • N. Lupu3 • M. Grigoras3 • G. Oanca1 •
M. Balasoiu4,5 • D. Creanga1
Received: 1 August 2015 /Accepted: 19 February 2016
� Springer Science+Business Media New York 2016
Abstract Magnetic nanoparticles yielded by chemical
route were surface modified with stabilizing agents being
further coated by sol–gel method with silica shell to be
used for various applications. Iron oxide magnetic cores
were dispersed in water by single citrate layer and,
respectively, by double oleate hydrophilic coating. Sol–gel
reaction with tetraethylorthosilicate provided further coat-
ing with silica that confers increased reactivity for ligand
coupling. Microstructural and magnetic properties were
investigated by standard methods evidencing nanometric
size, good crystallinity, and superparamagnetic behavior.
Comparative analysis evidenced similar crystallite size for
both citrate- and oleate-coated magnetic nanoparticles,
while granularity was changed after silica adding. Satura-
tion magnetization diminished less for oleate-stabilized
nanoparticles than for citrate-stabilized ones after silica
coating and moderate thermal treatment. Such prepared
magnetic nanocomposites could have possible utilization
as magnetic vectors for targeted biomolecules.
Graphical Abstract
Keywords Iron oxides � Citrate � Oleate double layer �Sol–gel coating � Superparamagnetic nanocomposites
1 Introduction
Nanotechnology development offered tremendous oppor-
tunity for various applications of nanoparticles and
nanocomposites based on their special properties from
viewpoint of both microstructural and magnetic features.
Multidisciplinary approach is imperiously needed to yield
basic nanocores, stabilize them against aggregation ten-
dency, coat them with adequate molecular shell and graft
on them biomolecules of interest when biomedical pur-
poses are intended. Magnetic nanosystems appeared as
very promising tools either for clinical diagnosis through
& D. Creangadorina.creanga@gmail.com; mdor@uaic.ro
E. Puscasu
emil.puscasu@ymail.com
L. Sacarescu
livius@icmpp.ro
N. Lupu
nicole@phys-iasi.ro
M. Balasoiu
balasoiumaria@yahoo.com
1 Physics Faculty, ‘‘Alexandru Ioan Cuza’’ University, 11
Blvd. Carol I, 700506 Iasi, Romania
2 ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Iasi,
Romania
3 National Institute of Research and Development for
Technical Physics, 47 Blvd. D. Mangeron, Iasi 700050,
Romania
4 Joint Institute for Nuclear Research, Dubna, Moscow Region,
Russian Federation 141980
5 Horia Hulubei Institute of Physics and Nuclear Engineering,
Bucharest, Romania
123
J Sol-Gel Sci Technol
DOI 10.1007/s10971-016-3996-1
http://crossmark.crossref.org/dialog/?doi=10.1007/s10971-016-3996-1&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1007/s10971-016-3996-1&domain=pdf
magnetic resonance imagistic [1–3] or for therapeutic
methods based on magnetic fields: magnetically assisted
drug delivery [4–6] and tumor therapy through hyperther-
mia with magnetic nanoparticles (MNP) and electromag-
netic fields [3, 7, 8].
To prepare magnetic nanocores various techniques were
developed, the most utilized being sol–gel technique,
thermal decomposition of iron complex combinations and
coprecipitation in alkali medium which successfully pro-
vided convenient amounts of material that can be easy
manipulated for surface modification with organic mole-
cules. Sol–gel procedure was found useful for nanoparticle
coating with silica shell, not only for iron compounds but
also for other metallic particles since silica is known to
interact easily with cations and the abundant silanol groups
at the surface of silica-coated nanoparticles allow activa-
tion with various functional groups [9].
Some literature reports are worth to be mentioned
regarding sol–gel reaction for nanoparticle coating with
silica protective and reactive shell. Metallic biocompatible
TiO2 nanoparticles were coated with silica layer by sol–gel
procedure resulting in surface homogenous coverage with
controllable thickness [10]. Semiconductor fluorescent
nanoparticles stabilized in the form of quantum dots
through surface modification with carboxyl groups were
coated by sol–gel reaction using tetraethylorthosilicate
(TEOS) that provided silica shell of various thickness and
preserved fluorescence properties too [11]. Superparam-
agnetic FePt nanoparticles were embedded in silica matrix
by sol–gel method to control their behavior during high-
temperature treatment [12].
In [13], the authors worked on submicron-sized mag-
netite/silica nanocomposites that were yielded starting
from 10 nm magnetite grains coprecipitated in alkali media
according to Massart method [14], being then surface
modified with carboxyethylsilanetriol—as silane coupling
agent—and then coated with silica shell through sol–gel
procedure based on TEOS; finally, 100 nm magnetite/silica
composites were obtained with the aim to allow further
attachment of biomolecules for medical purposes.
Magnetic nanopowders designed to reach target organs
during medical procedures tend to agglomerate quickly if
directly exposed to biological media so that their prepa-
ration as colloidal suspensions is needed before medical
administration.
As underlined in [9], most of applications of magnetic
particles in biomedicine and bioengineering require non-
magnetic protection to ensure stability of particle proper-
ties by avoiding agglomeration or sedimentation as well as
to endow them with particular surface modifications
required by specific applicative purposes.
The pristine magnetic nanoparticle stability has real
limitations not only because of aggregation in liquids,
especially at physiological pH, but also because of iron
oxide reactivity with blood. The reactivity of nanosized
iron oxides affect their stability during direct contact with
biological structures where such particles can be endocy-
tated and easily digested due to cell lysosomal processes
[15], the released iron ions eventually contributing to the
total cellular iron pool [16]. In [17], the authors underlined
that silica coating of iron oxide nanoparticles is benefic not
only in preventing aggregation and improving chemical
stability in liquids, but also due to the fact that the silanol-
terminated surface groups may be modified with various
coupling agents to covalently bind to specific ligands.
Silica interaction with iron oxides could occur by direct
binding or by means of intermediate stabilizer capping
ingredients.
During last years, biomedical applications of iron oxide
core-silica shell systems were reported by some authors. In
[18], the authors reported enzyme entrapping on magnetite-
silica particles, while in [19] cross-linked enzyme mole-
cules were shown to form clusters on the surface of the
magnetite-silica nanoparticles; iron-cobalt oxide-silica
nanoparticles prepared to be used for glucose oxidase
immobilization via cross-linking with glutaraldehyde were
presented in [20]; silica-coated Fe3O4 nanoparticles func-
tionalized with amino groups to bind bovine serum albu-
min were described in [21]; in [22], a review of magnetic
nanoparticle applications in protein immobilization can be
seen. As mentioned in [23], silica layer provides magnetic
nanoparticles with chemically friendly surface which is
essential for biological utilization while the silanol surface
groups could interact with various intermediate chemical
ingredients enabling the magnetic nanocomposites to react
with molecules of particular interest.
Various technological routes have been shown to be
effective in using silica for coating or embedding iron
oxide nanoparticles to improve stability in suspension.
Maghemite silanization has resulted in single or multiple
magnetic cores in silica matrix when synthesized by rapid
flame spray pyrolysis as reported in [24]. The yielding of
superparamagnetic hierarchical material involving silica
coating was described in [25], while in [26] maghemite
nanoparticle precipitation from an iron salt precursor dur-
ing the sol–gel processing of the silica matrix was
presented.
According to the mechanism proposed in [22], the direct
binding of silica on magnetic nanoparticle surface involves
the base-catalyzed hydrolysis of TEOS followed by con-
densation on iron oxide particles. The OH groups’ presence
on the iron oxide surface is essential in silica attraction by
hydrogen bonds. Thus, when OH groups are already stably
associated with magnetic particle surface via previous
capping with hydrophilic surfactant molecules, silanization
is expected to occur more successfully. Based on this,
J Sol-Gel Sci Technol
123
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52434_Controlled_Synthesis_of_Magnetite-Silica_Nanocomposites_via_a_Seeded_Sol-Gel_Approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/264639920_Application_of_Iron_Magnetic_Nanoparticles_in_Protein_Immobilization?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/264639920_Application_of_Iron_Magnetic_Nanoparticles_in_Protein_Immobilization?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/7463518_Clinical_hyperthermia_of_prostate_cancer_using_magnetic_nanoparticles_Presentation_of_a_new_interstitial_technique?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/7010809_Intracranial_Thermotherapy_Using_Magnetic_Nanoparticles_Combined_with_External_Beam_Radiotherapy_Results_of_a_Feasibility_Study_on_Patients_with_Glioblastoma_Multiforme?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/257613133_Synthesis_and_characterization_of_FePt_nanoparticles_and_FePt_nanoparticleSiO2-matrix_composite_films?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/226884041_Synthesis_of_Spherical_Submicron-Sized_MagnetiteSilica_Nanocomposite_Particles?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/223036894_An_Improved_Way_to_Prepare_Superparamagnetic_Magnetite-Silica_Core-Shell_Nanoparticles_for_Possible_Biological_Application?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51566341_Maghemite-silica_nanocomposites_sol-gel_processing_enhancement_of_the_magneto-optical_response?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/260210199_Investigation_of_formation_of_silica-coated_magnetite_nanoparticles_via_sol-gel_approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/260210199_Investigation_of_formation_of_silica-coated_magnetite_nanoparticles_via_sol-gel_approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==
silanization of citrate-coated magnetic nanoparticles with
average size of about 15 nm was presented in [9] where
silica coating resulted in about 40 nm structures. Tetram-
ethyl ammonium hydroxide-coated iron oxide nanoparti-
cles of about 14.5 nm were coated with silica by sol–gel
technique in [27] and resulted in apparently very few
increased size systems as conditioned by TEM device
contrast imaging.
We have chosen to study silica coating of two kinds of
MNP-capped samples: short-chain citrate-coated MNPs
versus long-chain oleate-coated MNPs. While citrate/
MNPs in silica were studied by some research groups [9,
20], no available literature was found regarding hydrophilic
oleate/MNPs in silica coating.
2 Experimental
2.1 Synthesis technology
The ferrophase was obtained via coprecipitation method at
high temperature [14]. All chemicals used in experiments
were analytical high-purity reagents purchased from Lach-
ner, Merck, Sigma-Aldrich, being used without further
purification, while purified water (18.2 MX/cm) usedthroughout the whole experiment was obtained using
Barnstead EasyPureII water purification system.
Briefly, 100 mL aqueous solution containing 1.332 g
ferrous chloride (FeCl2�4H2O) and 100 mL aqueous solu-tion containing 3.622 g ferric chloride (FeCl3�6H2O) weremixed using intense magnetic stirring at about 80 �C. Next50 mL of 1.7 M hot NaOH solution was dropped into the
mixture of metal salts solutions, and the black powder that
precipitated was processed for other 30 min in the same
conditions in order to ensure crystal formation and growth.
The collected magnetic slurry was washed for three times
with 200 mL deionized water volumes to remove all
impurities.
Then ferrophase was mixed with 1.7 g citric acid
(C6H8O7) dissolved in 3.5 mL water under constant
mechanical stirring (1200 rpm) at 80 �C for 1 h to getMNPs colloidal suspension; repeated washing with water
was done to eliminate surfactant excess; carefully pH
adjusting was carried out aiming to ensure long-term sta-
bility (pH * 5) of iron oxide/citrate MNPs—sample P1. Insimilar conditions, but after washing the ferrophase with
slightly acidic water, 0.3 g sodium oleate (C18H33NaO2)
dissolved in 10 mL deionized water was added to get
MNPs colloidal suspension—sample S1.
In the next step, amorphous silica addition, via the
hydrolysis of a sol–gel precursor (TEOS), resulted in final
samples P2 and S2, respectively.
After coating by sol–gel method at room temperature,
according to the method described in [28], moderate ther-
mal treatment was performed.
First, in a glass beaker equipped with mechanical stirrer
(1200 rpm) consecutively reagent addition was done:
0.25 g iron oxide/citrate in suspension from P1 and,
respectively, iron oxide/oleate MNPs from S1 dispersed in
water up to 7 mL were mixed each with 35 mL 2-propanol,
0.07 g sodium hydroxide and 1 mL tetraethylorthosilicate.
Vigorous stirring was carried out for 3 h at room temper-
ature in order to ensure interaction of reagents with the P1
and S1 samples.
Then, silica-coated particles were separated from the
reaction medium by centrifugation at 3500 rpm and were
repeatedly washed with water until the pH reached *6.Finally, waxy (gelatinous) magnetic materials were
dried under vacuum at 90 �C for 6 h and then wereannealed for 3 h up to 165 �C temperature to finalize ironoxide/citrate/silica composites preparation [26]—P2 sam-
ple, and, respectively, iron oxide/oleate/silica compos-
ites—sample S2 (Scheme 1).
2.2 Investigation methods
Transmission electron microscope (TEM) model Hitachi
High-Tech HT7700—with scanning transmission electron
microscopy (STEM) module and also with energy-disper-
sive X-ray analysis (EDX) module (HV of 100.0 kV, range
20 keV/130 kcps), was utilized to image and estimate
nanosystem sizing for P1, S1, P2 and S2 samples. X-ray
diffraction (XRD) analysis using Shimadzu LabX XRD-
6000 diffractometer (Cu-Ka radiation at k = 1.5406 Å)was applied for checking crystalline structure of MNPs and
calculate crystallites size. Magnetic properties analysis by
vibrating sample magnetometry (VSM) was performed
using Lake Shore VSM 7410 model at room temperature in
order to evidence magnetization capacity up to 2T and to
evaluate magnetic core diameter.
3 Results and discussion
TEM image analysis and measurement showed rather
regular geometric structures, mostly quasi-spherical—with
about 15 nm average size for iron oxide/citrate MNPs (P1
sample, Fig. 1a)—concordant with [9] and about 20 nm for
iron oxide/oleate MNPs (S1 sample) (Fig. 2a). The dif-
ferences could be discussed as follows.
As shown for example, in [29] citric acid interaction
with surface ions of magnetite or maghemite particles
consists in efficient binding via one or two carboxylate
groups that results in only monolayer citrate—even when
citric acid is added in excess. In [17], the authors sustained
J Sol-Gel Sci Technol
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https://www.researchgate.net/publication/3105400_Preparation_of_Aqueous_Magnetic_Liquids_in_Alkaline_and_Acidic_Media?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/231652434_Controlled_Synthesis_of_Magnetite-Silica_Nanocomposites_via_a_Seeded_Sol-Gel_Approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/216212877_Modifying_the_Surface_Properties_of_Superparamagnetic_Iron_Oxide_Nanoparticles_through_A_Sol-Gel_Approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/223036894_An_Improved_Way_to_Prepare_Superparamagnetic_Magnetite-Silica_Core-Shell_Nanoparticles_for_Possible_Biological_Application?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51566341_Maghemite-silica_nanocomposites_sol-gel_processing_enhancement_of_the_magneto-optical_response?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/49942838_Design_of_water-based_ferrofluids_as_contrast_agents_for_magnetic_resonance_imaging?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/260210199_Investigation_of_formation_of_silica-coated_magnetite_nanoparticles_via_sol-gel_approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/260210199_Investigation_of_formation_of_silica-coated_magnetite_nanoparticles_via_sol-gel_approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/260210199_Investigation_of_formation_of_silica-coated_magnetite_nanoparticles_via_sol-gel_approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/225867869_Magnetite_Nanoparticles_Stabilized_Under_Physiological_Conditions_for_BiomedicalApplication?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==
the efficacy of such MNP capping technique as a useful
intermediate step in further improving MNP coating with
silica shell, due to the fact that citrate binding ensures
rather stable and uniform distribution of OH from carboxyl
groups, allowing better interaction with silica. In [20], the
utilization of magnetite nanoparticles capped with citrate
ions as seeds for silica coating by sol–gel procedure was
also reported. This way single MNP cores in silica shell
were yielded together with some clusters of MNP cores in
silica matrix—what we probably obtained also in our
samples besides dominant single core MNPs coated in
silica shell (Fig. 1b).
In [29], the oleate double layer formation around iron
oxide nanoparticles in aqueous medium was described,
which confers hydrophilicity and still higher stability of
magnetic particles in acidic media. First layer of oleate ions
interacts with iron ions at the level of carboxylate groups of
long hydrophobic chain; second oleate layer is assembled
Scheme 1 MNP synthesis,stabilization and coating
Fig. 1 a Iron oxide/citrate MNPs before silica coating (P1). b Iron oxide/citrate MNPs after silica coating (P2). c STEM image of iron oxide/citrate MNPs in silica coating (P2). d EDX mapping of iron oxide/citrate/silica nanocomposites (P2)
J Sol-Gel Sci Technol
123
https://www.researchgate.net/publication/231652434_Controlled_Synthesis_of_Magnetite-Silica_Nanocomposites_via_a_Seeded_Sol-Gel_Approach?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/225867869_Magnetite_Nanoparticles_Stabilized_Under_Physiological_Conditions_for_BiomedicalApplication?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==
through tight hydrophobic interactions with fatty acid
chains of first layer [30], while the carboxyl groups remain
exposed to the aqueous suspension conferring hydrophilic-
ity to entire coated particles; thus, silica binding is thought
to have increased efficiency leading also to stable
suspensions.
In previous work [31], we reported the yielding of oleate-
coated MNP suspensions by similar technological approach
and stability analysis carried out by dynamic light scattering.
Zeta potential was found around -60 mV, which corre-
sponds to stable colloidal suspension according to theoretical
threshold of -30/? 30 mV [32]; other authors reported
10 nm citric acid-coated magnetite nanoparticles having
zeta potential of about -43 mV for pH of 5 (in [33]) or
citrate-coated magnetite particles in silica clusters having
zeta potential at the limit of stability [34]. It could be men-
tioned also that when oleate ions are supplied in hydrocarbon
reaction media from oleic acid source [35], they ensure
single layer coating of iron oxide particles and excellent
stabilization in hydrophobic environment– which is suit-
able for technical applications but not equally for biomedical
purposes.
After silica coating larger systems, up to 40 nm
(Fig. 1b—P2 sample and Fig. 2b—S2 sample) could be
observed—which is similar with the data reported in [9].
Some particle overlapping could be the consequence of the
fact that TEM measurements were performed on dried
particles that couldn’t be impeded to agglomerate. We may
say that TEM images show similar dispersion degree of
dominant small MNPs after and before silica coating.
STEM imaging alternatively was carried out—Fig. 1c
for P2 and Fig. 2c for S2. Good dispersion of metallic
cores surrounded by silica was evidenced by STEM pic-
tures, as shown also with TEM before sol–gel coating
procedure (Figs. 1a and 2a).
Final dispersion of the nanocomposites is going to be
adjusted when ligand binding or biomolecule grafting will
be carried out in order to complete the sample for the
biomedical application.
It is probable also that not only single iron oxide cores
resulted in silica coatings but also some magnetic cores
groups could be embedded in the same silica aggregate as
reported for example in [24]; this was concluded also in
[26] where maghemite-silica nanocomposites were yielded
Fig. 2 a Iron oxide/oleate MNPs before silica coating (S1). b Iron oxide/oleate MNPs after silica coating (S2). c STEM image of iron oxide/oleate MNPs in silica coating (S2). d EDX mapping of iron oxide/oleate/silica nanocomposites (S2)
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by direct sol–gel process. According to [29], thinner citrate
layer could be suspected not to cover entirely magnetic
particles surface so that these ones could eventually asso-
ciate and lead to multiple core systems in silica or frequent
clusters of single iron oxide-silica systems; the size of such
citrate monolayer-iron oxide-silica structures could reach
the same average value as in the case of double oleate layer
where more compact coverage is expected and significantly
less particle association before silica adding occurs. In
[36], the authors also found single or multiple citrate
capped magnetic cores embedded in silica coating which
probably exists in our samples too.
EDX mapping is presented in Figs. 1d and 2d. In
Fig. 1d, the results of EDX investigation of P2—iron
oxide/citrate/silica nanocomposites, is presented with nor-
malized values for Fe (green curve with maximum at 100
units). It is evident that Si distribution (red line) on the
direction chosen for exemplification across one MNP
agglomeration reached lower levels than Fe (green line).
Neighbor peaks of the recorded green curves are distanced
with 15–25 nm in the case of P2 (Fig. 1d). Relatively
parallel Si red curve was recorded indicating that the
scanned structures are formed from MNP cores individu-
ally coated with silica shell; only at the edges of the ana-
lyzed linear segment across MNP group Si amount is
occasionally higher than that of Fe. It is not excluded that
the maxima of Si amounts are not precisely centered on the
Fe maxima, some shifts between the two recordings being
noticed.
In Fig. 2d, the results of EDX investigation of S2—iron
oxide/oleate/silica nanocomposites, can be seen. The
neighbor peaks on the two recordings (green curve for Fe
and red curve for Si) are distanced with 25–45 nm, while
the Si level is lower than for P2 sample (about 55 %
compared to 80 % in P2). This indicates the higher amount
of silica attaching to the iron oxide/citrate cores than to
oleate-coated ones due to the nature and electric charge of
citrate—ensuring primary electrostatic stabilization of
magnetic cores. In the case of S2, steric stabilization by
double oleate shell seems to allow smaller amount of silica
attaching.
Analysis of raw XRD recorded data—according to the
reference for XRD peak attribution, i.e., ASTM Card
11-614 [37], confirmed spinel-structured crystallites for all
samples (Fig. 3a, b). It is expected that partial conversion
to maghemite at the surface of some magnetite nanoparti-
cles occurred during open air manipulation and reaction
medium temperature which is not easy to discern from
XRD data. Oleate-surfacted MNPs covered with silica (S2
sample, Fig. 3b) evidenced distinct XRD peak at about 27
degrees suggesting structured silica presence. It is possible
that oleate-surfacted MNPs were embedded in silica matrix
with porous surface, while citrate surfacted MNPs (P2
sample, Fig. 3a) were encapsulated in thinner silica shell
with small, hardly visible peak comparable with recording
noise.
Average crystallite size, Dijk, was calculated (Table 1)
using Scherrer’s formula for the strongest peak (311):
Dijk ¼K � k
b � cos h ð1Þ
where K is a dimensionless factor which varies with the
shape of the crystallite (in this case K = 0.89), k (Å) isX-ray wavelength, b (rad) is line broadening at half of themaximum intensity and h (rad) is the Bragg angle of (ijk)peak.
The results presented in Table 1 for P1 are in agreement
with those of published in [38] where the authors reported
citrate-capped magnetite with crystallite size of 12 nm,
while for P2 our results are concordant with those
Fig. 3 a XRD recordings for P1 and P2. b XRD recordings for S1and S2
J Sol-Gel Sci Technol
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https://www.researchgate.net/publication/216358130_Effect_of_pH_citrate_treatment_and_silane-coupling_agent_concentration_on_the_magnetic_structural_and_surface_properties_of_functionalized_silica-coated_iron_oxide_nanocomposite_particles?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/225375454_Effect_of_a_SiO_2_coating_on_the_magnetic_properties_of_Fe_3_O_4_nanoparticles?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/225867869_Magnetite_Nanoparticles_Stabilized_Under_Physiological_Conditions_for_BiomedicalApplication?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/281037521_Standard_X-ray_diffraction_powder_patterns?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==
published in [30] for double oleate layer magnetite parti-
cles of about 10 nm.
Estimated values suggest that crystallite size could not
change significantly after silica coating in either case.
However, higher values for iron oxide/citrate MNPs com-
pared to iron oxide/oleate ones could result from accuracy
measurement diminution—with Scherrer’s formula—be-
cause of possible oleate excess remained in the colloidal
suspension as well as to some changes in surface atom
arrangement during silica binding/thermal treatment—in-
cluding conversion to maghemite or possible amorphous
phase that increased noise-to-signal ratio.
VSM recording evidenced relatively high magnetization
capacity for iron oxide/citrate MNPs (about 62 emu/g
saturation magnetization for P1). In [39], the authors
reported still higher magnetization (over 70 emu/g) for
citrate-capped MNPs with about 10 nm physical diameter,
while in [17] only 43 emu/g saturation magnetization for
magnetite/citrate MNPs with about 7 nm crystallites was
reported.
In P2, the saturation magnetization was reduced con-
siderably (with over 50 % compared to P1) following
interaction with silica (Table 2). This is similar with the
data published in [38] where citrate-stabilized MNPs with
73 emu/g were transformed in magnetite-citrate-silica
composites with about half saturation magnetization
(37 emu/g). Also the decrease of specific saturation mag-
netization of maghemite nanoparticles after embedding in
silica matrix was reported in [24].
Also in [17], the study of silica coating of magnetite/
citrate nanoparticles resulted in considerable lowering of
saturation magnetization (from 43 to about 13 emu/g)
which is interpreted also as the effect of total mass
increasing relatively to initial ferrophase amount when
silica is added. Other researchers [9] obtained magnetite/
citrate/silica composites with lower saturation
magnetization (less than 10 emu/g) and their magneto-
metric study evidenced also magnetization remarkable
diminution after silica coating (up to 2 emu/g).
According to Fig. 4a, b, saturation magnetization of
oleate-stabilized MNPs (S1) of about 48 emu/g was lower
than that for citrate-stabilized MNPs (P1); according to
[30], the relatively reduced magnetization of oleate double
layer MNPs could be partially attributed to dilution effects
caused by the presence of significant quantity of oleate.
After we have carried out the reaction with TEOS, the
sample magnetization decreased, with around 30 % (for S2
compared to S1, Fig. 4b).
It could be assumed that thinner stabilization citrate
shell allowed higher effect of conversion to maghemite
(with lower magnetic moment) than the thicker double
oleate shell (Fig. 4a, b) during thermal treatment—but
bounded silica favored capping shell preserving which is
also supposed to occur for oleate capping shell too. Dom-
inant superparamagnetic properties of prepared samples
were evidenced; very thin hysteresis loop—coercive mag-
netic field of 1.4 and 1.5 mT (for P1 and P2- Table 2) and,
respectively, of about 1.1 and *1 mT (for S1 and S2)(Table 2) were found. This fact could be assumed to affect
the precision of slope measuring around graph origin and
thus the magnetic diameter calculation precision. Magnetic
diameter (Table 2) was calculated from Langevin’s theory:
d3M ¼18 � kB � T
p � l0 �Ms � msdM
dH
� �H!0
ð2Þ
where dM is MNP largest magnetic diameter, kB is Boltz-
mann’s constant, T is the absolute temperature, Ms is sat-
uration magnetization of MNP-coated powder, l0 isvacuum magnetic permeability, ms = 0.48 9 10
6 A/m
(bulk magnetite saturation magnetization according to
[40]) and (dM/dH) is the slope in the graph origin (for H—
magnetic field intensity—near zero).
It seems that in spite of total magnetic moment lowering
during sol–gel coating and moderate thermal treatment that
could transform some magnetite particles into maghemite
ones however, magnetite particles with largest diameter
could have persisted into the analyzed samples determining
the values calculated with Eq. (2) and presented in Table 2.
It seems that granularity properties exploring by alternative
methods for colloidal suspensions—like small angle neu-
tron diffraction, need to be further applied to avoid the
Table 1 Crystallite size from XRD data
Sample 2h (�) b (rad) Dijk (nm)
P1 35.60 0.01116 12.9
P2 35.59 0.01134 12.7
S1 35.67 0.01343 10.7
S2 35.65 0.01326 10.8
Table 2 Magnetic properties ofMNPs and silica
nanocomposites
Sample Maximum magnetization at 2T (emu/g) Coercive field (mT) Magnetic diameter (nm)
P1 62.78 1.4176 9.0
P2 30.06 1.5727 10.0
S1 48.47 1.1281 9.7
S2 33.49 0.9727 9.9
J Sol-Gel Sci Technol
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relative disadvantage of TEM/STEM where particle over-
lapping could occur during fluid sample drying on the grid
supports. Such nanocomposites could have possible uti-
lization in magnetically assisted drug delivery after final
dispersion in the presence of suitable ligand or grafted
biomolecule.
4 Conclusion
Magnetic nanopowders were yielded by applying sol–gel
technique for coating with silica reactive shell the magnetic
cores previously prepared by co-precipitation method, and
stabilized in aqueous suspension with two different organic
structures: citrate and, respectively, oleate. The features of
new iron oxide/oleate/silica nanocomposites were pre-
sented in comparison with already-known iron oxide/ci-
trate/silica nanosystems with focus on the different
properties related to long-chain double oleate shell and,
respectively, short-chain single citrate shell coating.
Nanometric sizes evidenced by TEM measurements
before silica coating (15 nm and respectively 20 nm for
iron oxide/citrate and respectively iron oxide/oleate MNPs)
have been increased to about 40 nm after silanization for
both types of magnetic nanopowders. Typical spinel crys-
tallites of 10–12 nm were evidenced in all samples. Satu-
ration magnetization appeared as being lower in iron oxide/
oleate MNPs (48.47 emu/g) than in iron oxide/citrate
MNPs (62.78 emu/g) since total sample mass could be
increased more in the first case when long molecular chain
arranged in double layer compared to smaller mass of
single citrate shell.
TEOS reaction resulted in diminished magnetization: in
the case of iron oxide/citrate/silica nanocomposites, satu-
ration magnetization diminished about twice like in other
authors’ report but that of iron oxide/oleate/silica
nanocomposites was diminished with only 30 % suggest-
ing that double layer MNP stabilization was more efficient
against conversion to maghemite than single citrate layer.
Dominant superparamagnetic behavior was evidenced both
before and after silica adding to magnetic nanocomposites,
with very thin coercive field.
Considering the benefits of sol–gel coating with silica
shell—known for reactive properties in biological media,
new attempts are planned to develop further the yielding of
magnetic carriers for drug delivery by ligand attachment
and suitable dispersion in the final suspension.
Acknowledgment This research was supported by JINR Grant57/04-4-1121-2015/2017.
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Fig. 4 aMagnetization curves for P1 and P2. bMagnetization curvesfor S1 and S2
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http://dx.doi.org/10.3390/molecules18077533http://dx.doi.org/10.3390/molecules18077533http://dx.doi.org/10.1016/j.jss.2011.01.060http://dx.doi.org/10.1080/02656730500158360https://www.researchgate.net/publication/230763714_Prijic_S_and_Sersa_G_Magnetic_nanoparticles_as_targeted_delivery_systems_in_oncology_Radiol_Oncol_45_1-16?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/230763714_Prijic_S_and_Sersa_G_Magnetic_nanoparticles_as_targeted_delivery_systems_in_oncology_Radiol_Oncol_45_1-16?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51029950_Mitoxantrone-Iron_Oxide_Biodistribution_in_Blood_Tumor_Spleen_and_Liver-Magnetic_Nanoparticles_in_Cancer_Treatment?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51029950_Mitoxantrone-Iron_Oxide_Biodistribution_in_Blood_Tumor_Spleen_and_Liver-Magnetic_Nanoparticles_in_Cancer_Treatment?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51029950_Mitoxantrone-Iron_Oxide_Biodistribution_in_Blood_Tumor_Spleen_and_Liver-Magnetic_Nanoparticles_in_Cancer_Treatment?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/51029950_Mitoxantrone-Iron_Oxide_Biodistribution_in_Blood_Tumor_Spleen_and_Liver-Magnetic_Nanoparticles_in_Cancer_Treatment?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/222686810_Magnetic_nanoparticles_for_drug_delivery?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/222686810_Magnetic_nanoparticles_for_drug_delivery?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/5297175_Magnetic_nanoparticles_in_MR_imaging_and_drug_delivery?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/5297175_Magnetic_nanoparticles_in_MR_imaging_and_drug_delivery?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/5297175_Magnetic_nanoparticles_in_MR_imaging_and_drug_delivery?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/41418890_Water-soluble_superparamagnetic_manganese_ferrite_nanoparticles_for_magnetic_resonance_imaging?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/41418890_Water-soluble_superparamagnetic_manganese_ferrite_nanoparticles_for_magnetic_resonance_imaging?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/41418890_Water-soluble_superparamagnetic_manganese_ferrite_nanoparticles_for_magnetic_resonance_imaging?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/41418890_Water-soluble_superparamagnetic_manganese_ferrite_nanoparticles_for_magnetic_resonance_imaging?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/242653361_Synthesis_Surface_Modification_and_Characterisation_of_Biocompatible_Magnetic_Iron_Oxide_Nanoparticles_for_Biomedical_Applications?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292ZXJQYWdlOzI5Nzc1NjM4NDtBUzozNDIwNTYxMzc0NDUzNzZAMTQ1ODU2Mzk0NjMzMA==https://www.researchgate.net/publication/242653361_Synthesis_Surface_Modification_and_Characterisation_of_Biocompatible_Magnetic_Iron_Oxide_Nanoparticles_for_Biomedical_Applications?el=1_x_8&enrichId=rgreq-157e0a2eadf0ae6c69381f2b95c7cbb2-XXX&enrichSource=Y292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