Template-free synthesis of nanoporous gadolinium phosphonate as a magnetic resonance imaging (MRI) agent

Article abstract of DOI:10.1039/c5ra02004b

A new series of organic-inorganic hybrid porous gadolinium phosphonate (GdP) materials have been synthesized using benzene-1,3,5-triphosphonic acid as a phosphonic acid linker through a simple hydrothermal technique. The particle size and morphology are well controlled through the selection of suitable reaction conditions. The obtained materials exhibit very high specific surface area and crystalline pore walls with a triclinic structure having unit cell parameters a = 7.76 (3) ?; b = 8.60 (6) ?; c = 14.09 (7) ?; α = 100.08 (4)°; β = 84.23 (6)°; γ = 116.29 (6)°. The obtained GdP materials are further used as a magnetic resonance imaging (MRI) contrast agent with r₁ and r₂ values of 2.6 and 4.7 s^-1 mM^-1, respectively.

Full text of DOI:10.1039/c5ra02004b

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PAPER
Template-free synthesis of nanoporous gadolinium
phosphonate as a magnetic resonance imaging
(MRI) agent†
Cite this: RSC Adv., 2015, 5, 42762
Malay Pramanik,a Fa-Kuen Shieh,b Saad M. Alshehri,c Zeid Abdullah Alothman,c
de
ac
*
*
Kevin C.-W. Wu
and Yusuke Yamauchi
A new series of organic–inorganic hybrid porous gadolinium phosphonate (GdP) materials have been
synthesized using benzene-1,3,5-triphosphonic acid as a phosphonic acid linker through a simple
hydrothermal technique. The particle size and morphology are well controlled through the selection of
suitable reaction conditions. The obtained materials exhibit very high specific surface area and crystalline
pore walls with a triclinic structure having unit cell parameters a ¼ 7.76 (3) A˚; b ¼ 8.60 (6) A˚; c ¼ 14.09
(7) A˚; a ¼ 100.08 (4)ꢀ; b ¼ 84.23 (6)ꢀ; g ¼ 116.29 (6)ꢀ. The obtained GdP materials are further used as a
magnetic resonance imaging (MRI) contrast agent with r1 and r2 values of 2.6 and 4.7 sꢁ1 mMꢁ1, respectively.
Received 1st February 2015
Accepted 28th April 2015
DOI: 10.1039/c5ra02004b
www.rsc.org/advances
of materials leads to non-uniform porous structure.2 On
the other hand, the phosphonate linker with a pyramidal
Introduction
geometry has three adjacent oxygen atoms with phosphorus,
thus it can force the metal ions to assemble in the liner fashion
(with less surface area).2,7 So far, the synthesis of porous
crystalline phosphonate materials with uniform pore diameter
and high surface area has been limited to the utilization of
different structure directing agents (SDAs). However, the
removal of SDAs while remaining the original porous structure
of the materials is still a challenge.
Consequently, a template-free synthesis of porous phos-
phonate materials with both high surface area and well-ordered
crystalline structure has been highly demanded. It has been
found that many of previous phosphonate materials were
prepared from linear mono- or di-organo phosphonic acid
ligands, and in those cases the synthesized metal–organic
structures are formed as layered structures.8 Although various
strategies using non-liner and mixed phosphonate linkers have
been utilized to prepare non-layered phosphonate materials,6a,9
the successful synthesis of non-layered crystalline phosphonate
materials with high surface area is still limited.6a,10
Crystalline metal–organic solids with permanent porosity have
a great scientic utility from last two decades because of their
various applications in chemical and biological research.1
Nanopores are sustained owing to the stable structure and
strong coordinative interaction between metal ions and organic
ligands.2 Recently, metal–organic frameworks (MOFs) (or
porous coordination polymers, PCP) have been extensively
studied. The advantage of robust coordination bonds of MOFs
has offered various kinds of MOFs with different organic
linkers, thus providing well-dened crystal structure and
porosity.3,4
On one hand, phosphonate linkers are dianionic in nature
and exhibit very strong interaction towards metal ions, which
usually result in the formation of more robust solid.5 In this
case, the growth of the metal–organic network is very rapid,
and the solid products are immediately precipitated. However,
the nal products usually exhibit less ordered crystalline
structure.6 Besides, in general, such a rapid precipitation
Among all metal-phosphonate materials, Gd(III) phospho-
nate (GdP) nanomaterials have attracted much attention owing
to their potential application as a MRI contrast agent. In this
study, we selected benzene-1,3,5-triphosphonic acid, a non-
liner linker, to prepare GdP materials with high porosity as
well as high crystallinity without using any SDA (Scheme 1). By
further tuning the synthetic conditions, we also successfully
control the nano-architectures of the synthesized nanoporous
GdP materials for MRI application. Previously, a few nano-
porous Gd carboxylate nanomaterials and Gd-based hybrid
nanoparticles have been used as a potential MRI contrast
agent.11 To the best of our best knowledge, however, this is the
aInternational Center for Materials Nanoarchitectonics (MANA), National Institute for
Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail:
Yamauchi.Yusuke@nims.go.jp
bDepartment of Chemistry, National Central University, 300 Jhong-Da Road, Chung-Li
32001, Taiwan
cChemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi
Arabia
dDepartment of Chemical Engineering National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan. E-mail: kevinwu@ntu.edu.tw
eDivision of Medical Engineering Research, National Health Research Institutes, 35
Keyan Road, Zhunan, Miaoli County 350, Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra02004b
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Table 1 Synthesis conditions for gadolinium(III) phosphonate mate-
rials (GdP)
Entry
Sample name
Base to maintain pH
Promoter
1
2
3
4
GdP-H
GdP-T
GdP-A
GdP-AC
NaOH (12.5 wt%)
—
—
—
(Me)4NOH (12.5 wt%)
NH4OH (12.5 wt%)
NH4OH (12.5 wt%)
aHClO4 (0.25 ml)
a
HClO4 solution is also used along with NH4OH solution.
Scheme 1 Synthetic pathway of formation of GdP materials.
1 mmol (0.372 g) of gadolinium(III) chloride hexahydrate was
dissolved in 10 ml double distilled water. Then, the synthetic
solution was prepared by adding the mother BTPA solution (pH
ꢂ 4.5) to the aqueous solution of gadolinium chloride. The pH
of the nal gel was maintained at 5 by the same base. The mixed
solution was stirred for overnight and nally transferred to a
Teon-lined pressure vessel and kept at 180 ꢀC for 36 hours.
Aer that, the pressure vessel was cooled to room temperature
rst report of nanoporous GdP material used as a MRI contrast
agent.
Experimental section
Materials
very slowly (5 C hꢁ1). The obtained material was collected by
ꢀ
Commercially available 1,3,5-tribromobenze, 1,3-diisopro-
pylbenzene, triethyl phosphite (98%) and nickel(II) bromide and
gadolinium(III) chloride hexahydrate (96%) were purchased
from Sigma Aldrich.
centrifugation and washing with ethanol for several times.
Characterizations
SEM images were taken with a Hitachi SU-8000 scanning
microscope at an accelerating voltage of 15 kV. TEM observation
was performed using a JEM-2010 TEM system that was operated
at 200 kV. Wide-angle powder X-ray diffraction (XRD) patterns
were obtained with a Rigaku RINT 2500X diffractometer using
monochromated Cu Ka radiation (40 kV, 40 mA) at a scanning
rate of 0.1ꢀ minꢁ1. Nitrogen adsorption–desorption analyses
were performed using a Belsorp-mini II Sorption System at 77 K.
Specic surface areas were calculated by the Multipoint BET
method at a P/P0 range of 0.05–0.3. Total pore volumes were
calculated by the Density Functional Theory (DFT) method. TG-
DTA-MS was measured using a Rigaku Thermo Mass Photo TG-
DTA-PIMS 410/S. X-ray photo-electronic spectroscopy (XPS)
spectra were recorded at room temperature by using a JPS-
9010TR (JEOL) instrument (Mg Ka X-ray source). All the binding
energies were calibrated by referencing to C 1s (285.0 eV). FT-IR
spectra were recorded using an FT-IR spectrometer (Spectrum
One, PerkinElmer L1200361) at room temperature.
Synthesis of BTPA
10 mmol (3.15 g) of 1,3,5-tribromobenzene was dissolved into
30 ml of 1,3-diisopropylbenzene by heating at 150 ꢀC under
inert atmosphere. Then the reaction mixture was cooled to
room temperature and anhydrous nickel(II) bromide (500 mg)
was added slowly to the reaction mixture under vigorous stir-
ring under inert atmosphere. The reaction mixture was heated
ꢀ
gradually to 180 C and triethyl phosphite (7.5 ml) was added
dropwise during 5–6 h. The heating was continued for next 6 h
at the same temperature under inert atmosphere. Then the
volatile components with 1,3-diisopropylbenzene were distilled
off, and a dark viscous residue was collected. The compound
hexaethyl-1,3,5-benzenetriphosphonate was puried over a
column of silica gel eluted with CHCl3 [CHCl3 : MeOH ¼ 10 : 1
(v/v)]. The colorless hexaethyl-1,3,5-benzenetriphosphonate was
obtained by the solvent evaporation. The yield is 1.95 g (around
40%). 1.5 g of the product ester was hydrolyzed by 15 ml water in
the presence of 10 ml concentrated HCl for 48 h. The hydrolyzed
solution was evaporated to dryness and the residue was dis-
solved into 15 ml distilled water and decolorized by activated
charcoal, then the ltrate was evaporate under reduced pressure
to get a white solid, BTPA. The yield was 0.90 g (around 92%).
The compound has been characterized by 1H, 13C, and 31P NMR
(Fig. S1†).
Results and discussion
The morphology of the obtained GdP materials is determined
by scanning electron microscope (SEM), as shown in Fig. 1. For
GdP-H, GdP-A, and GdP-T, non-uniform spherical particles are
mostly observed and their particle sizes are all about 100 nm.
For GdP-AC material, it is interesting to note that the material
has a ower-like morphology with ca. 100–150 nm in particle
Synthesis of GdP materials
Four types of GdP materials are prepared by using different size. Such ower morphology is generated by randomly
bases in the presence/absence of promoter (as listed in Table 1). assembling uniform akes of 3–4 nm in width (Fig. 1d). The
In a typical synthetic condition, 0.33 mmol (0.106 g) of benzene- representative TEM images for GdP-H and GdP-AC are shown in
1,3,5-triphosphonic acid (BTPA) was dissolved into 5 ml of Fig. 1e and f, respectively. A disordered nanoporous structure
double distilled water and the pH of the solution was increased can be clearly seen in the GdP-H (Fig. 1e). In contrast, the TEM
by 4.5 by NaOH, (Me)4NOH or NH4OH. In another closed vessel, image of the GdP-AC sample (Fig. 1f) shows aggregation of
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Fig. 1 SEM images of (a) GdP-H, (b) GdP-A, (c) GdP-T, and (d) GdP-AC, and TEM images of (e) GdP-H and (f) GdP-AC.
nanoakes that is in a good agreement with the SEM image materials.13 With the increase of the polarization ability of
(Fig. 1d). In the cases of GdP-H, GdP-A and GdP-T, we suggest oxyanions, the induction effect towards crystallization would
that there was no critical physical and chemical force for self- increase gradually. Among all oxyanions, chlorate (ClO4ꢁ) has
assembly. Therefore, to minimize the surface/volume ratio, the highest polarization ability owing to the higher charge/
the morphology of the materials became spheres. In contrast, radius value than other oxyanions.14 In our case, we used
ꢁ
GdP-AC synthesized with the presence of a promoter shows a HClO4 as the ClO4 source and serve as a promoter in the
ꢁ
totally different morphology, and the formation mechanism reaction medium. In the presence of ClO4 ions, the crystal-
will be discussed below.
linity of the nal GdP material (i.e. GdP-AC) was increased
Wide-angle X-ray diffraction (XRD) patterns of the hugely. Besides, the resulting morphology was also changed
obtained GdP materials are shown in Fig. 2a. All the four from spherical to ower-like.
materials have similar XRD patterns. Though the peak posi-
To further characterize the framework structures, the FT-IR
tions in the XRD pattern for each sample are the same, the spectrum of GdP-H was measured between 4000 cmꢁ1 to 400
crystallinity of GdP-AC is much higher over the other three cmꢁ1 (Fig. S2†). The strong absorption peak at 556 cmꢁ1 can be
samples. The crystal structure of the newly synthesized GdP assigned to the Gd–O stretching vibration. The set of bands
materials does not match with any standard JCPDS literature centered at 986 cmꢁ1 and 1085 cmꢁ1 can be assigned to the
prole. To solve this issue, the crystal structure of GdP-H tetrahedral stretching vibration of –CPO3 group. The peaks at
material was determined by MAUD program (Fig. 2b),12 and 1402 cmꢁ1 and 1520 cmꢁ1 are the characteristic vibration bands
the diffraction pattern of the GdP-H material can be desig- of benzene ring in the GdP materials. The peak at 1635 cmꢁ1 is
nated as a triclinic crystal phase with the unit cell parameters probably due to the bending vibration of water molecules
as follows: a ¼ 7.76 (3); b ¼ 8.60 (6); c ¼ 14.09 (7); a ¼ 100.08 adsorbed in the surface of the material. One small peak around
(4); b ¼ 84.23 (6); g ¼ 116.29 (6).
2351 cmꢁ1 is due to the CO2 present in the environment of the
Various oxyanions (e.g. chlorate, phosphate, and arsenate) experimental condition. The broad band centered at 3408 cmꢁ1
have been served as a promoter to increase the crystallinity of is probably due to the defected P–OH group in the material and
various microporous materials, particularly zeolite-type free water molecule attached to the surface of the porous
Fig. 2 (a) Wide-angle X-ray diffraction patterns of GdP-H, GdP-T, GdP-A and GdP-AC, respectively, and (b) experimental XRD pattern of GdP-H
(black line), computed XRD pattern (red line), and the difference between the experimental and the computed XRD patterns (blue line). The
crystal parameters of GdP, including lattice parameters and R factors, are also shown.
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Fig. 3 N2 adsorption (filled circles)–desorption (empty circles) isotherms of (a) GdP-H, GdP-T, GdP-A, and GdP-AC, (b) pore size distribution
curves, and (c) summary of surface area, pore volume, and pore diameter of the GdP materials.
material. On the basis of the FT-IR analysis, we propose the spin orbit coupling. The corresponding energy levels are Gd
presence of BTPA and Gd inside the porous framework of GdP- 3d3/2, Gd 3d5/2, Gd 4d3/2, and Gd 4d5/2 with the peaks at
H.15
1220.60, 1188.12, 149.68, and 143.28 eV, respectively (Fig. 4b
The architectural porosity of the synthesized GdP materials and c). The line shape and the peak positions are consistent
was determined by N2 adsorption–desorption isotherms with the previously reported gadolinium phosphate mate-
(Fig. 3a). The corresponding BET specic surface area, pore rials.17 The energy level of P 2p also splits into doublet (2p1/2
volume and pore diameter of the materials are listed in Fig. 3c. and 2p3/2 at 143.51 and 133.71 eV, respectively) (Fig. 4d). The
The adsorption–desorption isotherms of the materials show high-resolution XPS spectrum for C 1s was also measured
the typical type-II with small hysteresis loops. There is a (Fig. 4e) and the peak is located at 285.02 eV. The elemental
pronounced stage of monolayer adsorption on the materials compositions (C : P : Gd) are estimated to be 2.09 : 1.00 : 1.01,
owing to the presence of appreciable amount of micropores in which is very close to the elemental ratio of the calculated
the materials. When the relative pressure (P/P0) was over 0.01, molecular formula of C6P3O9Gd2.9$nH2O. Here the charge of
multilayer adsorption took place and amount of adsorption one Gd3+ is full-lled by three ^P–Oꢁ bonds. So, to maintain
increased gradually as the relative pressure increased, sug- the proper charge-balance structure and co-ordination
gesting the presence of relevant amount of mesopores in the number of Gd (tetrahedral/octahedral) and P (tetrahedral),
materials. Because of the presence of signicant amount of the macromolecular assembly is formed to produce the porous
mesopores in the materials at higher relative pressure (P/P0 > architecture (inset of Scheme 1).
0.5), the capillary condensation and evaporation did not take
The thermal stability of the synthesized porous GdP mate-
place at same relative pressure, thereby observing the hyster- rials was investigated by TG-DTA analysis under the continuous
esis loops.16 The corresponding pore size distribution curves ow of air with a heating rate of 5 ꢀC minꢁ1. All the four samples
were calculated by Density Functional Theory (DFT) (Fig. 3b). show almost the same TG-DTA proles. The TG plot of the GdP-
The GdP-H and GdP-AC samples exhibited the highest and H shows two distinct weight loss of the material (Fig. S5†). The
lowest surface area (366 and 155 m2 gꢁ1, respectively) among rst weight loss (16.6 wt%) up to 550 K is due to the removal of
all the samples. The low surface area and pore volume of GdP- surface-adsorbed water molecules in the porous framework of
AC were due to the dense packing of the p-electron rich the material. The second weight loss in the range 620 K to 1000
benzene rings.
K is due to the burning of C–C and C–P bonds in the material.
X-ray photoelectron spectroscopy (XPS) is a powerful tool to From the TG-DTA curve, it can be concluded that the synthe-
determine not only the electronic structure but also the sized materials have good thermal stability.
chemical composition and local structure of samples. From
The free/un-coordinated phosphonic acid is the prime
the XPS spectrum of GdP-H (Fig. 4a), we can conrm the source of acidity of this type of materials. We investigated the
presence of elements carbon, gadolinium, oxygen and phos- acidic behavior of GdP-H material in water and PBS pH 7 buffer
phorus in the material. The EDX measurement of GdP-H and solution. In case of water, the solution become a little acidic (pH
GdP-AC also proves the presence and homogeneous distribu- ꢂ 6) aer treating with GdP-H material, while in case of PBS pH
tion of C, P, Gd and O throughout the specimens (Fig. S3 and 7 buffer solution the pH of the solution do not change at all
S4†). The high-resolution XPS spectra of Gd 3d and Gd 4d state aer treating with GdP-H material. From this result, we can
reveals that the two energy states split into doublets due to the predict that the GdP-H material has little free/un-coordinated
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Fig. 4 XPS data of GdP-H (a) survey spectrum, (b) Gd 3d, (c) Gd 4d, (d) P 2p, and (e) C 1s.
Fig. 5 (a) Relaxivity intensity plots of T1 and T2 signals in xanthan gum containing of GdP-H nanoparticles with various concentrations. (b)
Comparison of the obtained r1 and r2 values with other materials.
phosphonic acids. The stability of the GdP-H material has also reported previously.19,20 Here we have also compared our result
examined by water treatment for 24 hours. Even aer water with previously reported various Mn3O4 materials (Fig. 5b). And
treatment, the wide angle XRD pattern and the surface area of it has been found that our materials have good/comparable
the GdP-H material remain unchanged, indicating that the activity with the previously reported Mn3O4 materials.18
material does not dissociate in normal atmospheric condition Although it has been reported that the Gd3+ locating near the
(Fig. S6†).
surface of the samples would exhibit higher reexivity, in this
The obtained GdP materials can be used as a magnetic study the ease and reliable synthesis process could easily
resonance imaging (MRI) contrast agent. A gradient system produce Gd3+-containing nanoparticles with diameter of about
mounted on the table of a 9.4 T magnet was used to generate 100 nm, thereby creating more Gd3+ on the surface and thus
images with high resolution.18 The GdP-H sample with various showing great potential for efficient intracellular MRI contrast
concentrations of Gd (2.7–8.0 mM) were mixed in 0.3% xanthan agents in the future. However, it has been well known that as a
gum. The corresponding r1 ad r2 values were calculated based MRI contrast agent, surface coating is sometimes necessary for
on the inverse relaxation time (1/T1) and (1/T2) versus Gd Gd-containing nanoparticles because of the high toxicity of Gd.
concentrations in the gum.19,20 The r1 and r2 values were 2.6 sꢁ1 Therefore, surface functionalization with polymers that exhibit
mMꢁ1 and 4.7 sꢁ1 mMꢁ1, respectively. The obtained r1 and r2 high biocompatibility and cell-targeting ability will be the future
relaxivity are almost the same as those Gd3+-containing samples work.
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M. Smith, E. Bittner, B. Bockrath and J. K. Johnson, J. Am.
Chem. Soc., 2004, 126, 1308–1309.
Conclusion
5 (a) G. Alberti, F. Marmottini, S. M. Mascarps and R. Vivani,
Angew. Chem., Int. Ed., 1994, 33, 1594–1597; (b)
M. D. M. Gpmez-Alcqntara, A. Cabeza, L. Moreno-Real,
M. A. G. Aranda and A. Cleareld, Microporous Mesoporous
Mater., 2006, 88, 293–303.
We have reported a simple and template-free hydrothermal
method for the synthesis of crystalline, nanoporous GdP
nanoparticles with different morphologies by using benzene-
1,3,5-triphosphonic acid (BTPA) as organic scaffold. The GdP
nanomaterials have very high surface area with triclinic crystal
phase. Our newly developed GdP nanomaterials have been
successfully used as a magnetic resonance imaging (MRI)
6 (a) M. Vasylyev and R. Neumann, Chem. Mater., 2006, 18,
2781–2783; (b) N. A. Khan, I. J. Kang, H. Y. Seok and
S. H. Jhung, Chem. Eng. J., 2011, 166, 1152–1157.
7 (a) J. Liang and G. K. H. Shimizu, Inorg. Chem., 2007, 46,
10449–10451; (b) C. Serre, J. A. Groves, P. Lightfoot,
A. M. Z. Slawin, P. A. Wright, N. Stock, T. Bein, M. Haouas,
F. Taulelle and G. Ferey, Chem. Mater., 2006, 18, 1451–1457.
8 (a) G. Cao, H. G. Hong and T. E. Mallouk, Acc. Chem. Res.,
1992, 25, 420–427; (b) P. Barber, H. Houghton,
S. Balasubramanian, Y. K. Anguchamy, H. J. Ploehn and
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9 (a) G. Alherti, U. Costantino, F. Marmottini, R. Vivani and
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contrast agent with r1 and r2 values of 2.6 and 4.7 sꢁ1 mMꢁ1
,
respectively. We believe that a very facile synthesis of Gd-
containing nanomaterials, with large internal surface area
and remarkable efficiency as a MRI contrast agent, would open
new avenues for in vivo study in near future.
Acknowledgements
The authors acknowledge the nancial support of this work by
King Saud University through National Plan for Science and
Technology grant (NPST ADV 2120-02).
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