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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. [email protected]; [email protected]

E. Puscasu

[email protected]

L. Sacarescu

[email protected]

N. Lupu

[email protected]

M. Balasoiu

[email protected]

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,


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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


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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)


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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


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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


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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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iron oxide-silica nanocomposites yielded by chemical route ......barnstead easypureii water...

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