Magnetofluorescent micellar complexes of terbium(III) as potential bimodal contrast agents for magnetic resonance and optical imaging

Article abstract of DOI:10.1039/c4cc09759a

Terbium(iii) was coordinated to four novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) amphiphilic bisamide ligands. The complexes were assembled into mono-disperse micellar nano-aggregates and for the first time Tb^III has b

Full text of DOI:10.1039/c4cc09759a

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Magnetofluorescent micellar complexes of
terbium(III) as potential bimodal contrast agents
for magnetic resonance and optical imaging†
Cite this: Chem. Commun., 2015,
51, 2984
Received 6th December 2014,
Accepted 12th January 2015
Michael Harris,a Sophie Carron,a Luce Vander Elst,b Sophie Laurent,b
Robert N. Mullerb and Tatjana N. Parac-Vogt*a
DOI: 10.1039/c4cc09759a
www.rsc.org/chemcomm
Terbium(III) was coordinated to four novel 1,4,7,10-tetraazacyclo- supramolecular structures that exhibit large increase in T2 at stron-
dodecane-1,4,7,10-tetraacetic acid (DOTA) amphiphilic bisamide ger magnetic field strengths.4
ligands. The complexes were assembled into mono-disperse micellar
In order to be used as a negative bimodal MRI/OI CA, a single
nano-aggregates and for the first time TbIII has been evaluated as a lanthanide complex needs to have a large magnetic moment and
single lanthanide negative bimodal contrast agent for MRI/OI. The efficient luminescent emission in aqueous medium. Among the
complexes show characteristic TbIII emission with quantum yields lanthanide series we have anticipated that TbIII complexes should
reaching 7.3% and transverse relaxivity r2 per TbIII micelle at 500 MHz meet both requirements, but surprisingly, to the best of our
and 310 K reaches maximum values up to 60 sÀ1 micelleÀ1. The knowledge, their potential as bimodal contrast agents has
efficient T2 relaxation at high magnetic field strengths is sustained by never been investigated so far.
the increased rotational correlation time of the nano-aggregates and
high magnetic moment of the terbium ion.
Here, we report for the first time a single lanthanide
negative bimodal MR/OI CA based on TbIII. Kinetically inert
complexes are formed with two novel DOTA ligands derivatized
Current contrast agents (CAs) used in magnetic resonance imaging with p-alkyl phenyl amides which can be excited at a wavelength of
(MRI) are mostly based on complexes of GdIII with diethylene 265 nm and through sensitised emission resulting in characteristic
triaminepentaacetic acid (DTPA) or 1,4,7,10-tetraazacyclododecane- TbIII luminescence.5 The amphiphilic complexes were assembled
1,4,7,10-tetraacetic acid (DOTA) ligands. They cause a shortening of into mono-disperse micelles to increase the rotational correlation
the longitudinal relaxation time (T1) of water protons resulting in a time. Size, relaxation and luminescence efficiency of the resulting
positive contrast, but suffer from dramatic loss of efficiency at high micelles have been investigated in detail.
magnetic fields.1
The ligands were prepared using established methods.3,6 The
On the other hand, negative contrast agents cause a shortening of obtained ligands Tb–cis-DOTA-BC12PheA, Tb–cis-DOTA-BC14PheA,
the transverse relaxation time (T2) resulting in a negative contrast. Tb–trans-DOTA-BC12PheA, and Tb–trans-DOTA-BC14PheA are
Complexes of DyIII based on DTPA derivatives have been shown to represented in Fig. 1. All ligands have been characterised by
hold potential as T2 contrast agents.1a,2 Recently, sensitised emission nuclear magnetic resonance spectroscopy, mass spectrometry,
of DyIII was used to create a bimodal probe, exhibiting favourable
properties for both MRI and optical imaging.3 Negative contrast
agents can be further incorporated into micelles resulting in
a KU Leuven, Department of Chemistry, Celestijnenlaan 200F, 3001 Leuven,
Belgium. E-mail: michael.harris@chem.kuleuven.be; Fax: (+32) 16-327-992;
Tel: (+32) 16-324-010
b Department of General, Organic and Biomedical Chemistry,
NMR and Molecular Imaging Laboratory, University of Mons, 7000 Mons,
Belgium. E-mail: luce.vanderelst@umons.be; Fax: (+32) 65-373-533;
Tel: (+32) 65-373-518
Fig. 1 Schematic representation of, Tb–cis-DOTA-BC12PheA and Tb–cis-
DOTA-BC14PheA (1), Tb–trans-DOTA-BC12PheA, and Tb–trans-DOTA-
BC14PheA (2) with coordinated water molecule omitted. R stands for C12
or C14 alkyl chains.
† Electronic supplementary information (ESI) available: Experimental details;
characterization of ligands and complexes; ESI-MS spectra of the complexes;
dynamic light scattering profiles of the micelle solutions. See DOI: 10.1039/
c4cc09759a
2984 | Chem. Commun., 2015, 51, 2984--2986
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Table 1 Photophysical data for the TbIII complexes in water (pH 7.4, 1 wt%)
at 298 K
and IR spectroscopy (ESI†). The four ligands were coordinated to
TbIII in pyridine according to a known procedure.7 The absence
of free lanthanide ions is verified by the addition of an arsenazoIII
indicator solution.8 The complex formation has been established
by mass spectrometry, IR spectroscopy, and total reflection X-ray
fluorescence (ESI†).
The lanthanide complexes consist of a hydrophilic centre
and two hydrophobic tails with a phenyl and a chain of 12 or
14 carbon atoms. The amphiphilic nature permits them to be
incorporated into micelles, creating slowly tumbling supramolecular
structures and limiting local motions of the lanthanide complexes.
Micelles are formed by mixing one equivalent of the Ln complex
of interest with twelve equivalents of phospholipid (DPPC) and
6.5 equivalents of surfactant (Tween 80s). Photon correlation
spectroscopy measurements showed mono-disperse size distribution
profiles (ESI†). This has allowed completely transparent micelle
solutions with particle sizes of approx. 10 nm regardless of
orientation or length of the alkyl chains.
Due to the p - p* transitions of the ligands, all terbium
complexes display well-defined absorption bands (ESI†). A ligand
centred band in the range 220–300 nm with a maximum between
240–260 nm is observed in the absorption spectrum (ESI†) of the
supramolecular structures corresponding to the ligand electronic
transitions. It was observed that the different alkyl chain lengths
and orientations show slightly shifted spectrum maxima which
are most likely caused by the varying aggregation of the ligand
into the phospholipid micelles. However, for the samples there
is no broadening or shift of the excitation spectrums (ESI†) and a
wavelength of 265 nm remains the most efficient for energy
transfer to the TbIII ions.
The emission spectra of the micelles display sharp emission
bands attributed to the 5D4 - 7FJ ( J = 6–3) transition of the TbIII
ion upon excitation at 265 nm (Fig. 2). The efficient energy
transfer from ligand to lanthanide is maintained since little
ligand-centred emission is detected.
a
a
b
tH O [ms]
tD O [ms]
qH O
QLTb [%]
2
2
2
cis-DOTA-BC12
cis-DOTA-BC14
trans-DOTA-BC12
trans-DOTA-BC14
1.6
1.8
0.8
0.9
2.2
2.6
1.2
1.2
0.5
0.6
1.5
1.4
7.1
7.3
4.9
4.2
a
Average of three measurements which vary by no more than 0.1 ms.
Estimated relative errors QTLb Æ 10%. Quantum yields relative to
b
rhodamine 101 in EtOH.
to determine the number of coordinated water molecules q with an
accuracy of Æ0.2 to 0.3:
qTb(H2O) = 5.0(Dkobs À 0.06)
(1)
in which Dkobs = 1/tH O À 1/tD O is expressed in msÀ1. After
2
2
calculation, q values are obtained (Table 1). These values show
that the orientation of the ‘arms’ of the ligands affects the
amide coordination to the TbIII ion with the trans-orientation
allowing coordination of more than 1 water molecule. The non-
ionic surfactant Tween 80s at the periphery of the assembled
structures is able to form hydrogen bonds with H2O and acts as a
competitor in lanthanide coordination resulting in a value of less
than 1 for cis-orientation and less than 2 for trans-orientation. Less
non-radiative deactivation due to O–H vibrations will take part,
leading to longer luminescence lifetimes and as a consequence a
reduced q number found by the phenomenological eqn (1).10
TbIII luminescence quantum yields are determined by excitation
at 265 nm into the ligand levels (Table 1). The variation in the
quantum yields appears to correspond to the varying values of q
between the same orientations and as expected increased water
coordination provides more quenching of luminescence. The
quantum yields reaching 7.3% are a significant increase in
comparison to DyIII quantum yield which is around 1% for
similar micellar complexes.3
Luminescence lifetimes in both H2O and D2O have been
obtained after a mono-exponential fit of the luminescence
decays, which indicates the presence of only one-luminescent
lanthanide species in the micellar structures. Within the uncertainty
of the luminescence method, the following phenomenological
eqn (1) for TbIII–polyaminocarboxylate systems9 has been employed
Fig. 3 depicts the T2 enhancement at 20, 60, 300, and 500 MHz
by the complexes. At the proton Larmor frequency of 20 MHz,
the transverse relaxivities for the complexes (r2 = 0.25 to
0.38 mMÀ1
s
À1) are slightly higher in comparison to the
Fig. 2 Corrected and normalized emission spectrum of TbIII complexes in
Fig. 3 Proton transverse relaxivity of the micelles versus proton Larmor
water (pH 7.4, 1 wt%, 298 K).
frequency at 310 K. The lines represent the fitted data.
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Table 2 Values of ts, tM, and DoM obtained by fitting of the 1H r2 data of
the Tb complexes at 310 K
of a water molecule in the first coordination sphere and long
rotational correlation time.12 Furthermore, it is reported that
DPPC forms micelles of 50 phospholipid molecules13 in which
the TbIII load will be equal to 4 molecules per micelle making a
per-particle expression possible which equals 60.4, 55.2, 59.2,
57.6 micelleÀ1 sÀ1 for the series respectively at 500 MHz
and 310 K.
DoM
ts [ps]
tM [ns]
[105 rad sÀ1 TÀ1
]
cis-DOTA-BC12
cis-DOTA-BC14
trans-DOTA-BC12
trans-DOTA-BC14
0.35
0.6
0.35
0.5
400
450
1000
1000
3.08
2.14
0.99
0.99
In conclusion, amphiphilic TbIII complexes built into
micelles reported in this study show favourable magnetic and
optical properties making them good potential candidates for
MRI and optical imaging. The micelles showed long lumines-
cence lifetimes in H2O at the emission wavelength of 546 nm
and high quantum yields. The complexes also exhibit high
transverse relaxivities, r2, all reaching around 15 mMÀ1 sÀ1 at
500 MHz and 310 K. In comparison with previously reported
DyIII–DTPA micelle complexes TbIII provides a large increase in
luminescence quantum yields. The excitation at 265 nm is
however a drawback that could be overcome by further optimi-
sation of the complexes by using different chromophores. This
may lead to further improvement of optical properties and,
result in establishing TbIII complexes as a novel class of
potential candidates for MR and optical imaging.
tR is fixed at 1000 ps.
longitudinal values (r1 = 0.15 to 0.31 mMÀ1 sÀ1). This increase is
due to the aggregation of the complexes into micelles. However,
at higher magnetic fields (no 4 100 MHz), a significant increase
of r2 takes place. It is known that transverse relaxivity depends
on the square of the magnetic field but despite this r2 shows a
strong reliance to the tM value when the external magnetic field
is increased. In that particular case, it is important that the
chemical shift difference between coordinated and bulk water
(DoM) remains lower compared with the water exchange to
avoid limitation by tM.11 The chemical shift of the coordinated
water molecule is proportional to the magnetic field and is the
sum of contact and pseudo-contact terms.
Fitting of the data is performed using the equations defining
the inner- and outer-sphere contributions as described by
Vander Elst et al.11 The correlation time tC modulatingÀt1he
Notes and references
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dipolar interaction, is related to tR, tS, and tM through tC
=
tRÀ1 + tSÀ1 + tMÀ1 whereas the Curie contribution is modulated
À1
À1
by tCC defined as tCC
= tR
+ tMÀ1. During the fitting
procedure, the parameters q = bound water molecule(s) as
determined by luminescence lifetimes, r = 0.31 nm, a =
0.36 nm, D = 3.3 Â 10À9 m2 sÀ1 and tR = 1 ns were fixed. The
parameters tS, tM, and DoM are extracted from the fitting and
are listed in Table 2 but they should be considered as estimates
as the fitting is only over 4 points.
The tS values lie in the range of 0.35–0.6 ps. Fitting the
proton nuclear magnetic relaxation dispersion (NMRD) profiles
of micelles seems to indicate that the water residence time for
trans-orientation is double that of cis-orientation. The slow
molecular motion leads to transverse relaxivities of 15.1, 13.8,
14.8, 14.4 mMÀ1 sÀ1 at 500 MHz and 310 K for Tb–cis-DOTA-
BC12PheA, Tb–cis-DOTA-BC14PheA, Tb–trans-DOTA-BC12PheA,
and Tb–trans-DOTA-BC14PheA respectively. Despite the varying
data of coordinated water molecules, there is little change to
the transverse relaxivity for all compounds. The compounds
performing as efficient r2 agents is enforced by the high
magnetic moment of the terbium ion (m = 9.81 mB), the presence
¨
¨
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2986 | Chem. Commun., 2015, 51, 2984--2986
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