Luminescent and magnetic materials with a high content of Eu3+-EDTA complexes

Article abstract of DOI:10.1039/c6dt00249h

Bifunctional optical magnetic materials with a high europium content have been prepared. Chelating groups were introduced on the Fe₃O₄ surface with organosilanes containing ethylenediaminetetraacetic acid (EDTA) derivatives, which were previously prepared via a reaction between EDTA-dianhydride and aminoalkoxysilane agents: 3-(trimethoxysilyl)propylamine (1N), N-[3(trimethoxysilyl)propyl]ethylenediamine (2N) and N¹-(3-trimethoxysilylpropyl)diethylenetriamine) (3N). The first coordination sphere of Ln-EDTA complexes present on the modified surfaces of Fe₃O₄ particles was completed by addition of β-diketonate ligands (tta: thenoyltrifluoroacetone, dbm: dibenzoylmethane, bzac: benzoylacetone and acac: acetylacetone) in order to improve their luminescence properties. The materials were characterized by powder X-ray diffraction (XRD), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and wavelength dispersive X-ray fluorescence (WDXRF) and Fourier-transform infrared (FT-IR) spectroscopy as well as by zeta potential measurements and luminescence spectroscopy. The hybrid materials exhibited intense red emission, which can be assigned to the 4f-4f transitions of the Eu³⁺ ion, indicating an efficient intramolecular ligand-to-metal energy transfer. The experimental intensity parameters (Ω₂ and Ω₄), lifetimes (τ), as well as radiative (A_rad) and non-radiative (A_nrad) decay rates of the Eu³⁺ ion were determined and discussed. The strategies used to obtain these materials may contribute to the development of several bifunctional systems for practical applications.

Full text of DOI:10.1039/c6dt00249h

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PAPER
Luminescent and magnetic materials with high content of Eu3+-
EDTA complexes
Received 00th January 20xx,
Accepted 00th January 20xx
G.P.Pires,a I. F.Costa,a H. F.Brito,b W. M. Faustinoa and E. E. S. Teotonioa
Bifunctional optical magnetic materials with high europium content have been prepared. Chelating groups were
introduced on the Fe3O4 surface with organosilanes containing ethylenediaminetetraacetic acid (EDTA) derivative, which
were previously prepered via reaction between EDTA-dianhydride and aminoalkoxysilanes agents: 3-
DOI: 10.1039/x0xx00000x
www.rsc.org/
(Trimethoxysilyl)propylamine
(1N),
N-[3(trimethoxysilyl)propyl]ethylenediamine
(2N)
and
N1-(3-
trimethoxysilylpropyl)diethylenetriamine) (3N). The first coordination sphere of the Ln-EDTA complexes present on the
modified surfaces of Fe3O4 particles was completed by addition of β-diketonate ligands (tta: thenoyltrifluoroacetonate,
dbm: dibenzoylmetane, bzac: benzoylacetone and acac: acetylacetone) in order to improve their luminescence properties.
The materials were characterized by powder X-ray diffraction (XRD), vibrating sample magnetometry (VSM), scanning
electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), wavelength
dispersive X-ray fluorescence (WDXRF) and Fourier-transform infrared spectroscopy (FT-IR) as well as by zeta potential
measurements and luminescence spectroscopy. The hybrid materials exhibited intense red emission assigned to the 4f-4f
transitions of the Eu3+ ion, indicating an efficient intramolecular ligand-to-metal energy transfer. The experimental
intensity parameters (Ω2 and Ω4), lifetimes (τ), as well as radiative (Arad) and non-radiative (Anrad) decay rates of the Eu3+ ion
were determined and discussed. The stragegies used to obtain these materials may contribute to development of several
bifunctional systems for practical applications.
a.Department of Chemistry. Federal University of Paraíba
Cidade Universitária – Campus I, 58051-900, João Pessoa – PB - Brazil
E-mail: teotonioees@quimica.ufpb.br.
b.Institute of Chemistry. University of São Paulo
Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo – SP - Brazil.
Electronic Supplementary Information (ESI) available: Magnetic properties, WDXRF
spectrum, europium analytical curve, additional SEM and TEM images, FT-IR
spectra, C, H and N elemental analysis, luminescence spectra and experimental
intensity parameters. See DOI: 10.1039/x0xx00000x
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Furthermore, the EDTA based silanes participate in the
functionalization step in order to obtain different kinds of
materials, containing higher number of coordinating groups.16
These features were explored in our design strategy to prepare
new bifunctional luminescent magnetic hybrid materials.
In this work, we report an approach to prepare
luminescent magnetic materials with high lanthanide content.
Different aminoalkylalkoxysilanes were employed to
synthesize the chelating agents and then to functionalize
magnetite particles, providing distinct hybrid materials. The
high content of Ln3+ ions was a consequence of a great number
of coordinating sites introduced by functionalization steps. The
materials show considerable magnetization, allowing the
magnetic manipulation with a regular magnet. Usually, the
Introduction
Bifunctional luminescent magnetic (LM) hybrid materials are
remarkable and attractive because they may combine both
optical and magnetic properties, which allow easy
manipulation and chemical functionalities, playing a key role in
different tasks and applications. For instance, LM nanoparticles
can be used as probes in vivo assay,1,2 as reusable catalysts,
that might be separated by magnetic manipulation3 and in
metal ions4 or organic molecules5 pre-concentration procedure
for quantitative analysis.
Nowadays, special attention has been given to the surface
modification in order to adsorb selectively transition metals
and trivalent lanthanide ions (Ln3+).6 For this purpose,
magnetite surface has been functionalized with EDTA groups,
leading to potential materials for the adsorption and
separation of rare-earth ions from aqueous solutions.7
Moreover, magnetite particles have been combined to the
Fe3O4 acts
a
luminescence quencher. However, this
disadvantage was overcome by functionalizing the magnetite
with organosilanes containing EDTA derivative and
coordinating
experimental intensity parameters (Ωλ), radiative (Arad) and
non-radiative rates (Anrad
for the Eu3+ materials were
determined. The photonic features such as the experimental
emission quantum efficiency ( ) and the emission lifetimes (
β
-diketonate ligands to the Eu3+ ion. The
Gd3+-imprinted chitosan/carbon nanotubes to prepare
a
)
retrievable hybrid materials capable of selective adsorption of
the Gd3+ion.8 In this case, luminescent magnetic microspheres
have been prepared by adsorption of pre-encapsulated Eu3+-β-
diketonate complexes to the magnetite surface.9 Besides,
magnetic mesoporous silica nanospheres have been used as a
support for Eu3+ tetrakis(β-diketonate) complexes containing
ionic liquid compounds as linkers.10 In different approach,
luminescent Eu3+ and Tb3+ β-diketonate complexes have been
prepared directly on the magnetite surface functionalized with
calixarene through a one-pot method, yielding magnetic and
red-green emitting nanomarkers.11
η
τ)
for these hybrid materials are also obtained and discussed.
Experimental
Materials
The following reagents were purchased from Sigma-Aldrich:
Iron(III) chloride hexahydrate (FeCl3 6H2O, ACS reagent, 97 %)
iron(II) chloride tetrahydrate (FeCl2 4H2O, Puriss. p.a., ≥ 99 %),
⋅
⋅
The principal goal of the development of luminescent
hybrid materials based on Ln3+ complexes is to achieve
excellent luminescent properties.12 Efficient antennae, such as
carboxylates, heteroaromatics and β-diketonates ligands
should be employed to ensure a strong absorption energy by
the ligands, leading to the efficient ligand-metal energy
transfer. Moreover, some ligands can also protect the emitting
ions from interactions with high-energy oscillator groups
(mainly OH and NH), preventing luminescence quenching. For
the hybrid materials, it is also important to consider the
content of emitting Ln3+centers in the luminescence
intensities.
sodium hydroxide (NaOH, ACS reagent, ≥ 97.0%), silver nitrate
(AgNO3, ACS reagent, ≥ 99.0 %), hydrochloric acid (HCl, ACS, 97
%),
hydrogen
peroxide
(H2O2,
30
%),
ethylenediaminetetraacetic acid (EDTA, ACS, 99.4 %), acetic
anhydride (Ac2O, ACS, ≥ 98.0 %), polyvinylpyrrolidone (PVP,
average molecular weight 40,000), tetraethoxysilane (TEOS,
reagent grade, 98 %), pyridine (ACS reagent, ≥ 99.0 %)
ammonium hydroxide solution (NH4OH, ACS reagent, 28.0-30.0
%
NH3
H2NCH2CH2CH2Si(OCH3)3,
[3(trimethoxysilyl)propyl]ethylenediamine
basis),
3-(Trimethoxysilyl)propylamine
(1N,
N-
97 %),
(2N,
N1-(3-
(3N,
H2NCH2CH2NHCH2CH2CH2Si(OCH3)3,
97
%),
It is well known that EDTA derivatives are versatile
chelating agents. Usually, they act as hexadentate ligands,
which allow anchoring covalently metal complexes on different
material surfaces, avoiding leaching of the metal ions.13-15
trimethoxysilylpropyl)diethylenetriamine
H2NCH2CH2HNCH2CH2NHCH2CH2CH2Si(OCH3)3, technical grade),
2-thenoyltrifluoroacetone (tta, 99 %), benzoylacetone (bzac,
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99 %), acetylacetone (acac, > 99 %), dibenzoylmetane (dbm, 98 The
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DOI: 10.1039/C6DT00249H
ARTICLE
reaction
between
EDTA-DA
and
the
%), europium (III) oxide (Eu2O3, 99.9 %) and gadolinium (III) aminoalkylmethoxysilanes was performed as described by
oxide (Gd2O3, 99.9 %). Ethanol (P.A.) was obtained from Tedia. Patel16 with some modification. EDTA-DA (5.06 g, 1.97 x 10-2
All chemicals were used without further purification. Pyridine mol) was dissolved in anhydrous pyridine (60 mL) under inert
was stored over 3 Å molecular sieve. Water was deionized to atmosphere using standard Schlenk line techniques. The
18 MΩ (Milli-Q, Millipore).
aminoalkylmethoxysilane containing one (1N), two (2N) or
three (3N) amino groups (3.94 x 10-2 mol) was added into the
Synthesis of ethylenediaminetetraacetic acid dianhydride EDTA-DA solution. The resulting solution containing the
(EDTA-DA) reactants was stirred for 24 h, under dynamic nitrogen
EDTA dianhydride (EDTA-DA) was synthesized according to the atmosphere at room temperature. After that, Fe3O4 (2.0 g,
method described in references 17 and 18 as following: To a 8.65 x 10-3 mol) was added to the reaction mixture, followed
suspension of ethylenediaminetetraacetic acid (EDTA, 36.0 g, by the addition of concentrated aqueous ammonium
0.12 mol) in pyridine (62 mL) under inert atmosphere (N2) was hydroxide solution (13.85 mL).The mixture was also stirred for
added acetic anhydride (48 mL). The resulting mixture was 24 h at room temperature. The obtained particles were
stirred at 65°C for 24 h. Then, it was cooled and filtered under magnetically decanted and washed with ethanol and deionized
inert atmosphere. The pale yellow solid was washed with water thoroughly to remove non-magnetic products. Finally,
acetic anhydride (300 mL) and diethyl ether (300 mL) and dried the resulting material was dried under reduced pressure.
under reduced pressure (yield 91 %, 27.9 g); mp: 192-194°C;
FT-IR stretch from dianhydride carbonyl moiety:
1810 cm-1, s; s(C=O) = 1740 cm-1, s.
ν
as(C=O) = Ln3+ adsorption on functionalized SiO2|Fe3O4
EDTA-functionalized magnetic particles (1.0 g of Fe3O4|SiO2|xN
where x = 1, 2 or 3) were suspended in water (30 mL). To this
suspension was added EuCl3.6H2O (1.05g, 2.87 x 10-3 mol) or
ν
Synthesis of LnCl3.nH2O compounds
Eu2O3 (5 g, 1.42 x 10-2 mol) or Gd2O3 (5g, 1.38 x 10-2 mol) was GdCl3.6H2O (1.06 g, 2.87 x 10-3 mol) previously dissolved in 10
suspended in water (100 mL). To this suspension was dropwise mL of water. The mixture was then stirred for 24 h, at room
added concentrated hydrogen chloride under magnetic stirring temperature. Thereafter, the material was magnetically
until almost total oxide consumption. Then, the resulting decanted and washed with deionized water (200 mL) and
solution was filtered (Millipore membrane, 0.22 µm) and ethanol (100 mL). Finally, the material was dried under
heated in order to obtain a fine white powder.
reduced pressure at room temperature. The resulting products
were named as Fe3O4|SiO2|xNEu and Fe3O4|SiO2|xNGd (where
x = 1, 2 or 3).
Synthesis of the Fe3O4 particles
Magnetite nanoparticles were prepared by the chemical co-
precipitation method.19,20 FeCl3.6H2O (11.34 g, 4.20 x 10-2 mol)
and FeCl2.4H2O (4.17 g, 2.10 x 10-2 mol) were mixed in
degassed water (280 mL). The solution was then brought to a
reaction temperature of 70 °C. After that, NaOH solution (2.5
mol L-1) was added dropwise until a pH of 11 was achieved.
The solution containing the reactants was stirred for 2 h under
dynamic nitrogen atmosphere. The black solid was separated
from the mixture using a permanent magnet (Nd2Fe14B) and
washed with water until no more Cl- ions could be qualitatively
detected by silver nitrate test. Finally, the product (4.3 g of
Fe3O4) was washed with 50 mL of ethanol (yield 98 %).
Ln3+β-diketonate complexes on the materialsurfaces
The ethanolic solution of the diketonate ligand (1.43 x 10-3
mol) tta, bzac, acac or dbm was added to the suspension of the
magnetic material containing Eu3+ or Gd3+ (0.5 g). The reaction
was maintained for 6 h under stirring. Finally, the material was
washed with ethanol (50 mL) and dried under reduced
pressure.
Characterization of the materials
The X-ray diffraction (XRD) patterns of the materials were
recorded with a Lab-X XRD-6000 diffractometer (Shimadzu),
using Cu-Ka radiation (λ = 0.154056 nm). Measurements of
magnetization curves (at 25 °C) were carried out using a
VersaLab Vibrating Sample Magnetometer (VSM, Quantum
Design) under a test field range of -10000 to 10000 Oe.
Scanning electron microscopy (SEM) images were obtained
Fe3O4 coating by SiO2
PVP polymer (5.0 g) was dissolved in distilled water (33.3 mL,
1.85 mol), then it was added to the suspension of Fe3O4 (4.3 g,
1.86 x 10-2 mol in 200 mL of ethanol). Thereafter, the
temperature was raised to 60 °C and TEOS (2.67 mL, 1.20 x 10-2
mol) was added to the resulting mixture, followed by addition
of concentrated NH4OH aqueous solution (3.67 mL). The
mixture was maintained at 60 °C for 8 h, under dynamic
nitrogen atmosphere. The magnetic particles were
magnetically decanted, washed with water (100 mL) and
ethanol (200 mL), and dried under reduced pressure at room
temperature.
with
a Quanta 450 FEG (FEI) in order to characterize
morphological and textural aspects of the materials. The shape
and nanostructural features were determined by transmission
electron microscopy (TEM) using a JEM-1200(JEOL) operating
at 120 kV. Thermogravimetric analysis (TGA) was performed
on a DTG-60H thermal analyzer system (Shimadzu) at the
heating rate of 10 °C min-1 in a N2 flow of 50 mL min-1. The
quantitative analysis of C, H, and N of the materials were
performed by using a Perkin–Elmer model 240 microanalyzer.
Europium content was quantitatively determined by
EDTA functionalization of the silica-coated Fe3O4
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wavelength dispersive X-ray fluorescence using a XRF-1800
spectrometer (Shimadzu), measuring the magnitude of Eu L
α
spectral line using LiF (200) as analysing crystal, a Rh anode X-
ray tube (40 kV, 95 mA) and a scintillation counter. For this
purpose, analytical curve for Eu was obtained from synthetic
standards prepared by adding different amounts of Eu2O3 in
the Fe3O4|SiO2|1N matrix. Fourier-transform infrared (FT-IR)
spectra were recorded on
a
IR Tracer-100 FT-IR
spectrophotometer (Shimadzu) using KBr pellet technique in
the range of 400-4000 cm-1. Zeta potentials of the magnetic
particles were obtained with a Zetasizer Nano ZSP (Malvern
Instruments) and the measurements were performed in water
(Milli-Q, Millipore) without addition of buffers at an initial pH
of 6.3 ± 0.1 at 25 °C using a cell drive voltage of 120 V.
Photoluminescence spectra of the SiO2|Fe3O4|xNLn3+ materials
were recorded at room and liquid nitrogen temperatures using
a Fluorolog-3 spectrofluorometer (Horiba Jobin-Yvon).
Results and discussion
Functionalized magnetite particles
The X-ray diffraction (XRD) patterns of Fe3O4 (Fig. 1) show the
characteristic narrow diffraction peaks of magnetite relative
intensities match well with the standard database peaks of
reference pattern (JCPDS file No. 19-0629), confirming the
cubic inverse spinel structure of the magnetite. On the other
hand, the Fe3O4|SiO2|xN functionalized materials present
broad reflections in the 2θ interval from 20 to 28° assigned to
the siloxane lattice on the surface of magnetic particles. In
addition, the characteristic diffraction peaks of magnetite
were also recognized in diffraction patterns of all materials.
Fig. 1 XRD patterns of Fe3O4, Fe3O4|SiO2 and functionalized
materials.
The magnetic properties of the materials were measured
using a vibrating sample magnetometer (VSM) at room
temperature under an applied field range of –10000 to 10000
Oe. The field-dependent magnetic measurements recorded for
Fe3O4, Fe3O4|SiO2 and functionalized materials (Fig. 2) present
nearly zero values of coercivity
(
Hc)
and remanent
magnetization σr), suggesting that the materials exhibit
(
superparamagnetic behavior.21-23 It is important to report that
the coercivity values are consistent with the low
magnetocrystalline anisotropy intrinsic of the magnetite. The
Hc valueof Fe3O4 has not varied significantly after coating and
functionalization steps (Table S1 in the ESI), indicating that no
considerable chemical change has occurred in the magnetic
core.23 Moreover, the low values of σr suggest that the
materials cannot retain magnetization before and after
exposure to an external magnetic field. This behavior
contributes to reduce the particle aggregation due to magnetic
dipole interaction. The most remarkable difference in the σs
values of saturation magnetization may be attributed to the
layer of nonmagnetic material (i.e., SiO2 and organic groups).
The Fe3O4, Fe3O4|SiO2 and functionalized materials present the
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σs values of 83.6, 55.9 and 15 emu g-1, respectively. This result
can be attributed to the success of the particles coating and
functionalization steps, resulting in
a lower content of
magnetite per gram of the hybrid material.
Fig.
2 Variation of magnetization of Fe3O4, Fe3O4|SiO2 and
functionalized materials with the applied field at room temperature.
SEM images show no significantly change in the
morphological characteristics of the material after silica
coating (Fig. S2 in the ESI). The Fe3O4 and Fe3O4|SiO2 materials
present
particles. On the other hand, EDTA functionalized materials
exhibit different morphologies for all employed
a granular structure formed by submicrometer
Fig. 3 Representative images of Fe3O4|SiO2|1N, Fe3O4|SiO2|2N
and Fe3O4|SiO2|3N synthesized with aminoalkylalkoxysilane
containing one, two or three amino groups, respectively.
aminoalkylalkoxysilanes as compared with non-functionalized
systems (Fig. 3). Besides, the SEM micrographs of the
Fe3O4|SiO2|1N, Fe3O4|SiO2|2N and Fe3O4|SiO2|3N materials
present large agglomerates and different shapes, showing
both rough and smooth textures. This morphological alteration
is owing to the considerable amount of chelating agents on the
magnetic matrix.
The thermal stability of Fe3O4, Fe3O4|SiO2 and
functionalized materials were evaluated by thermogravimetric
analysis (Fig. 4). The TGA curves of the Fe3O4 and Fe3O4|SiO2
systems show the smallest mass loss due to the non-
functionalized surface. In the case of Fe3O4, it is observed only
mass loss that may be attributed to the adsorbed or trapped
solvent molecules (water or ethanol), while for Fe3O4|SiO2 it is
also possible to observe mass loss owing to the residual
ethoxide groups. The TGA curves of the functionalized
materials exhibited similar weight loss trend. These materials
undergo the first mass loss below 120 °C due to the loss of
adsorbed or trapped solvent molecules (water or ethanol). The
mass loss of approximately 31 % in the temperature interval
from 160 to 550 °C may be related to the decomposition of the
organic moieties. The total mass loss of the functionalized
materials varied from 60 to 73 %, suggesting similar grafting
percentages of aminoalkylalkoxysilane onto Fe3O4|SiO2
surface.
TEM micrographs indicate that the magnetite particles
were successfully coated with tetraethoxysilane (TEOS) by the
sol-gel process (Fig.S2 in the ESI). As can be observed in Fig. 3,
TEM micrographs show similar nanostructural characteristics
for the Fe3O4|SiO2|1N, Fe3O4|SiO2|2N and Fe3O4|SiO2|3N
materials. The functionalization process produced a material
where magnetite nanoparticles are embedded in the SiO2
instead of the functional layer follow the shape of the
Fe3O4|SiO2 particles. For Fe3O4|SiO2|1N, Fe3O4|SiO2|2N and
Fe3O4|SiO2|3N
materials
(Scheme
1a-c),
the
aminoalkylalkoxysilane groups may also bind to another
alkoxyde group, increasing the extension of the siloxane
network.
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observed 6.9 ± 0.3 and 5.7 ± 0.4, which presented negative
zeta potential values of -32 ± 6 and -35 ± 5 mV, respectively
(Fig. 5), indicating negatively charged surfaces. It is noteworthy
that the pH values are slightly above their isoelectric point
(6.0-6.8 for magnetite26 and 3.2-3.8 for silica27), which are
coherent with the nature of these negatively charged surfaces.
For functionalized materials, the equilibrium pH values may be
associated with chemical characteristics of the particles
surfaces that depend on both the reaction medium and the
used
Fe3O4|SiO2|1N material without secondary amino groups, the
synthesis process (basic pH) produces predominantly
aminoalkylmethoxysilane
compound.
For
the
a
carboxylate (COO-) surface, leading to the zeta potential (31.9
mV) and pH (6.9) values. Nevertheless, the Fe3O4|SiO2|2N and
Fe3O4|SiO2|3N materials present secondary amino groups
+
Fig.
4 Thermogravimetric curves for Fe3O4, Fe3O4|SiO2 and
(NH) that may be protonated, yielding NH2 species due to the
functionalized materials.
slightly acidic pH of the deionized water (pH ∼5) used in the
washing step. When in contact with water (Milli Q, pH =
6.3±0.1) the protonated secondary amino groups ends up
participating in the chemical equilibrium involving carboxylate
groups, thereby preventing the increase in the pH observed for
samples without secondary amino groups. This behavior
reduces the extent of ionization of the carboxylic groups,
providing more positive surfaces.
FT-IR absorption spectra of the Fe3O4, Fe3O4|SiO2 and
functionalized materials are shown in Fig. S3 (ESI). The strong
absorption band centered at 596 cm-1 for Fe3O4 may be
attributed to the stretching vibrations
ν(Fe-O) in both
tetrahedral and octahedral sites, while the band at 435 cm-1 is
due to the Fe-O stretching in the octahedral Fe site.24,25 The
absorption band around 1641 cm-1 is assigned to
δ(H-O-H)
bend mode and the broad band at 3387 cm-1 is due to
stretching vibration modes of the hydroxyl ions or water
molecules coordinated to Fe ions.26 The absorption spectrum
of Fe3O4|SiO2 material shows an absorption band at 1090 cm-1
assigned to the asymmetric stretching of Si-O bonds (νas(Si-O))
from ≡Si-O-Si≡ group. Moreover, it is observed two absorption
bands at 1397 and 1643 cm-1 from νs(C=O) and νas(C=O)
stretching of the carboxylate groups, respectively. The bands
between 2800 and 3000 cm-1 are assigned to C-H stretching
vibrations ν(C-H) from -CH2 group. These results also confirm
the functionalization of the Fe3O4|SiO2 particles. A schematic
representation of the materials is presented in Scheme 1.
Fig. 5 Zeta potential (ζ) of the materials and pH of water after
particle suspension.
Covalently linked lanthanide complexes
All matrices containing Eu3+ ions present positive zeta
potential (29.4-32.2 mV) and pH values were found in the
intervals of 3.5-4.2. In these materials, the Eu3+ coordination
by carboxylate and residual non-protonated amino groups may
contribute to increase the zeta potential values, conferring
positive charge to particle surface. The lower pH values can be
Scheme 1 Schematic representation of the materials produced
and their functional groups. (a) Fe3O4|SiO2|1N, (b) Fe3O4|SiO2|2N
and (c) Fe3O4|SiO2|3N.
+
related to deprotonation of NH2 or due to the moderately
The surface charge was investigated by zeta potential
measurement at the equilibrium pH characteristic of each
sample suspended in water (Milli Q). The pH of water after
suspension of the Fe3O4 and Fe3O4|SiO2 materials were
acid lanthanide chloride solution used in the adsorption
process of the metal ion.
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A typical WDXRF spectrum of the final hybrid material
shows only some emission lines of the Fe and Eu atoms (Fig. S4
in the ESI). Table 2 shows the values for Eu3+ content
determined for the functionalized materials. The lanthanide
contents are higher to those ones found for other EDTA
modified materials.28,29 The higher values of Eu content were
found in samples based on Fe3O4|SiO2|2N and Fe3O4|SiO2|3N
matrices, showing that the absorption process are dependent
on the size of chain of the sililant agents. Besides the
carboxylate groups, Ln3+ ions may be also interact with amino
groups arising from aminoalkyl alkoxysilanes. This behavior
reflects the ability of the material to provide a large number of
possible emitting centers. Although Fe3O4|SiO2|2N and
Fe3O4|SiO2|3N materials (Table S2 in the ESI) present different
nitrogen contents, but similar Eu3+ contents were adsorbed on
these systems. These results indicate that the metal ions are
coordinated in similar way by aminocarboxylate moieties on
both materials surfaces.
Table 1 Europium content of functionalized materials.[a]
Material
Content (% m m-1)
Fig. 6 Phosphorescence spectra of Fe3O4|SiO2|3N Gd materials
containing H2O and the following β-diketonates as ligands: 2-
Fe3O4|SiO2|1N
Fe3O4|SiO2|2N
Fe3O4|SiO2|3N
9.4 ± 0.1
thenoyltrifluoroacetone
(tta),
dibenzoylmetane
(dbm),
13.4 ± 0.4
benzoylacetone (bzac) andacetylacetone (acac).
13.7 ± 0.8
It is possible to verify that all hybrid materials containing
Eu3+ ion exhibit luminescence when exposed to radiation in the
spectral range of 290 to 380 nm, as shown in Fig. S1. However,
the systems without diketonate ligands display the lowest
brightness, suggesting that the β-diketonate play the most
important role in the sensitization of the Eu3+ ion.
The photoluminescence sensitization process of the Eu3+
ion by β-diketonate ligands were investigated by using the
excitation spectra recorded at room temperature, under
emission monitored on the hypersensitive 5D0 7F2 transition
→
[a] Analytical curve and parameters are found in Fig. S5 in ESI.
Luminescence spectroscopy
The luminescence spectra of the (Fe3O4|SiO2|3N) hybrid
materials containing Gd3+ ion and β-diketonates were recorded
at 77 K, under excitation monitored on the singlet-singlet
(S0
→S1) ligand transitions, in order to obtain information
about electronic structure of the ligands. The broad emission
bands observed in the range of 400 to 650 nm can be
at 617 nm (Fig. 7 and see Fig. S7 in the ESI). Except for the
hydrated materials, all excitation spectra exhibit a high intense
broad absorption band in the range of 300 - 420 nm, which is
attributed to the phosphorescence intraligand (T1→S0)
transition of the β-diketonate ligands (Fig. 6). These broad
bands exhibit a red shift owing to decreasing in triplet state
energies in the following order: acac < bzac < dbm <
ttaligands.30 It is observed that the Fe3O4|SiO2|1N and
Fe3O4|SiO2|2N materials (see Fig. S6 in the ESI) present similar
spectral profiles. The broad emission band around 500 nm is
assigned to the SiO2 matrix. According to the literature,31,32 the
nature of these bands has been attributed to the defect
centers or the charge transfer process in the siloxane network,
among others.
attributed to the absorption of diketonates via S0→S1
transition followed by intramolecular energy transfer from
5
5
their T1 states to the D1 and D0 levels of the Eu3+ ion.33 This
result indicates that the -diketonate ligands act as
β
luminescence sensitizer for the Eu3+ ion in the hybrid
materials. Additionally, for the materials containing H2O, acac
or bzac as ligands, it is observed narrow absorption bands
arising from the intraconfigurational-4f6 transitions of the Eu3+
7
7
7
ion: F0
→
5D1 (525 nm), F0
→
5D2 (465 nm), F0
→
5L6 (394 nm),
7F0 5L7 (379 nm) and 7F1
→
→
5D1 (535 nm).
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the coordinating abilities of the water and/or β-diketonate
molecules. Therefore, it is expected that the first coordination
sphere might be saturated with few additional ligand
molecules. According to experimental data and selection rules
7FJ transitions, the chemical environment of the
5
for the D0
→
Eu3+ ion belongs to a symmetry group among Cn ,Cnv or Cs.34
Fig. 7 Excitation spectra of the Fe3O4|SiO2|3N Eu materials
containing H2O and the following diketonates as ligands: 2-
thenoyltrifluoroacetone
(tta),
dibenzoylmetane
(dbm),
benzoylacetone (bzac) and acetylacetone (acac).
As it can be seen, the relative intensities of intraligand
broad absorption bands (S0 S1) are higher than those for 4f-4f
→
transitions of the Eu3+ ion, confirming that the ligand-to-metal
ion energy transfer is operative. In addition, it is observed a
shift of the broad absorption bands to lower energy in the
following order (aca > bzac > dbm, tta), in agreement with the
decreasing energy of the excited singlet state.
Fig. 8 Emission spectra of the Fe3O4|SiO2|3N Eu materials containing
H2O and the following diketonates as ligands: 2-thenoyltrifluoroacetone
(tta, λexc
=
370 nm), dibenzoylmetane (dbm,
λexc= 350 nm),
benzoylacetone (bzac, λexc= 350 nm) and acetylacetone (acac, λexc= 315
nm).
Fig. 8 shows the emission spectra of the hybrid materials
(Fe3O4|SiO2|3N) containing Eu3+ ion and β-diketonate ligands,
Experimental
intensity
parameters
can
provide
quantitative information about intensity probabilities of 4f6
transitions, nature of the chemical environment and
luminescence efficiency may be determined from the optical
recorded at room temperature, under excitation on the S0→S1
transitions. These spectra exhibit narrow emission bands that
5
are assigned to the D0
→
7FJ transitions (where J = 0, 1, 2, 3, 4)
7F2
of the Eu3+ ion, displaying the hypersensitive 5D0
→
data.
Thus,
photoluminescent
properties
of
the
Fe3O4|SiO2|1N, Fe3O4|SiO2|2N and Fe3O4|SiO2|3N containing
Eu3+ ion and β-diketonate ligands were quantified by
transition as the most intense one. The emission spectra for all
materials (see also Fig. S8 in the ESI) present very similar
spectral features, indicating similar symmetries for the
chemical environment around the Eu3+ ion. In the vicinity of
the metal ion, steric hindrance may play an important role in
experimental values of radiative (Arad) and non-radiative (Anrad
rates, Judd–Ofelt parameters Ωλ = 2 and 4) and emission
quantum efficiency ( ) via methodology described by Sá and
)
(
λ
η
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ARTICLE
co-workers35 reported in the supporting information. The
spectroscopic parameters are shown in Table2 (see Fig. S9 in
the ESI). The nature of ligand produces more expressive
changes in the experimental intensity parameters than the
type of aminoalkylalkoxysilane employed. Although the Anrad
values do not exhibit a regular behaviour, it is observed the
higher values for systems with water, acac and bzac, while the
lower ones are found for hybrid materials containing dbm and
tta ligands. However, the Anrad values are still higher as
Table 2 Experimental intensity parameters and average lifetime (τav) of
the hybrid materials based on Fe3O4|SiO2|1N, Fe3O4|SiO2|2N and
Fe3O4|SiO2|3N matrices, containing Eu3+ ion and β-diketonate ligands.
Arad
(s-1
Anrad
(s-1
τav
(10-3 s-1)
Atot
)
Ω2
Ω4
η
(%)
Material
)
)
(s-1
(10-20 cm2)
(10-20 cm2)
Fe3O4|SiO2|1NEu
H2O
acac
bzac
dbm
tta
438
489
597
806
669
1787
1332
1873
1433
1316
0.449
0.549
0.405
0.447
0.504
2225
1821
2470
2239
1985
7.9
9.8
10.2
10.7
9.6
20
27
24
36
34
compared with those ones for tris- and tetrakis-β-diketonate
coordination compounds, reflecting the heterogeneity of
13.7
20.1
15.6
10.5
10.6
coordinating sites of the hybrid materials.
On the other hand, Arad values present a more regular
behaviour, exhibiting an opposite trend as compared with
Fe3O4|SiO2|2NEu
Anrad. The higher values obtained for the tta and dbm systems
7F2 transition (Table 2).
5
reflect the hypersensitivity of the D0
→
H2O
acac
bzac
dbm
tta
420
1671
1941
1622
1584
1319
0.478
0.412
0.472
0.446
0.501
2091
2428
2119
2243
1996
7.6
9.5
9.9
10.7
9.4
20
20
23
29
34
The Arad values are similar to those ones obtained for some
highly luminescent Eu3+-
-diketonate complexes and higher
487
497
659
677
β
10.4
15.3
15.9
than those ones for thehybrid luminescent magnetic materials
reported in literature.36
10.9
10.5
It is important to notice that the Arad and η values follow similar
trends. In particular, η presents values ranging between 20 and
36%, reaching to the maxima values for materials with tta and dbm
ligands. This result indicates that the luminescence efficiency
depends on the ligand actuating as antenna. The high contents of
Eu-diketonate complexes covalently linked to the material surface
lead to strong bright luminescence (Fig. S1).
Fe3O4|SiO2|3NEu
H2O
acac
bzac
dbm
tta
454
1384
1639
1545
1633
1230
0.544
0.455
0.461
0.423
0.530
1838
2196
2168
2362
1878
8.1
10.9
10.6
10.6
11.2
10.0
25
25
29
31
35
557
623
729
648
10.5
12.5
15.5
13.5
The Ω2 and Ω4 parameters depend on both the local
geometry and polarizabilities of the chemical environment of
the lanthanide ion, respectively. The higher Ω2 values were
obtained for the materials with tta and dbm ligands, reflecting
the higher changes in the chemical environment of the metal
ion. On the other hand, it is possible to verify that the Ω4
parameter is less affected by the type of ligand coordinated to
the Eu3+ ion. Furthermore, the intensity parameter values are Conclusions
not significantly affected when varying the 1N, 2N and 3N
In this study, luminescent magnetic materials with high
lanthanide content have been prepared. The high Ln3+-EDTA
complex content was a direct consequence of the synthesis of
EDTA-silane precursors before the functionalization step.
Although the field-dependent magnetization was considerably
smaller for functionalized materials, the general magnetic
behavior of magnetite particles was not substantially altered
after their coating with silica and organic moieties. The hybrid
magnetic systems containing diketonate ligands exhibited high
intense luminescence due to the operative antenna effect,
while the matrices play a minor rule on the photoluminescent
properties. In addition, a comprehensive characterization of
the surface and luminescent aspects have also been
presented, showing the bifunctionality of the hybrid materials.
The experimental strategies used to prepare luminescent
magnetic materials may be applied in order to develop new
hybrid systems for applications in different areas such as
lanthanide magnetic separation, optical markers, among
others.
organosilanes.
Acknowledgements
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Desenvolvimento Científico e Tecnológico), Instituto Nacional
de Ciência e Tecnologia-Nanotecnologia para Marcadores
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