Gadolinium containing photochromic micelles as potential magnetic resonance imaging traceable drug carriers

Article abstract of DOI:10.1111/j.1751-1097.2012.01124.x

Novel photochromic amphipathic molecules, KMR-AZn (Gd-DTPA-AZCn), composed of hydrophilic Gd-DTPA and hydrophobic alkylated azobenzene were prepared. In aqueous environment, KMR-AZn indicated self-assembly. The resulting aggregates were demonstrated to be able to include a hydrophobic drug substitute (hydrophobic fluorescent dye) into the internal core, and to release the included compound upon photoirradiation within 10 min through the influence of azobenzene photoisomerization. This micellar MRI contrast agent exhibited three- to four-fold higher r₁ relaxivity (r₁ = 14.5-16.5 mm ^-1 s^-1, 0.47 T at 40°C) than the widely applied small molecule contrast agent Gd-DTPA (Magnevistr₁ = 4.1 mm^-1 s^-1, 0.47 T at 40°C). This dual functionality of encapsulated compound release and increased MR imaging contrast indicates that KMR-AZn is a potential candidate for application as a lipid-based MRI-traceable drug carrier.

Full text of DOI:10.1111/j.1751-1097.2012.01124.x

Photochemistry and Photobiology, 2012, 88: 876–883
Gadolinium Containing Photochromic Micelles as Potential Magnetic
Resonance Imaging Traceable Drug Carriers
Yushi Heta1, Kentaro Kumaki1, Hiroki Hifumi1, Daniel Citterio1, Akihiro Tanimoto2 and Koji Suzuki*1
1Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Yokohama, Japan
2Department of Diagnostic Radiology, Keio University School of Medicine, Tokyo, Japan
Received 3 November 2011, accepted 14 February 2012, DOI: 10.1111/j.1751-1097.2012.01124.x
phototriggered changes are generally independent of the
biological condition. This makes light a widely applied
ABSTRACT
Novel photochromic amphipathic molecules, KMR-AZn
(Gd-DTPA-AZCn), composed of hydrophilic Gd-DTPA and
hydrophobic alkylated azobenzene were prepared. In aqueous
environment, KMR-AZn indicated self-assembly. The resulting
aggregates were demonstrated to be able to include a hydropho-
bic drug substitute (hydrophobic fluorescent dye) into the
internal core, and to release the included compound upon
photoirradiation within 10 min through the influence of azoben-
zene photoisomerization. This micellar MRI contrast agent
exhibited three- to four-fold higher r1 relaxivity (r1 = 14.5–
technology for external stimulation under biological condi-
tions. Therefore, drug carriers such as micelles and liposomes
incorporating photosensitive substances have gradually been
developed to improve drug release efficiency by irradiating UV
to near-infrared light (7,8). The disadvantage of limited light
penetrability can be overcome by the application of light-
guiding catheters, which are often used in photodynamic
therapy (PDT), and which offer a further means of precise
spatial control.
On the other hand, there are some reports regarding the
application of imaging probes to drug carriers (so called
‘‘image-guided drug delivery’’; 9,10). This therapeutic strategy
allows the monitoring of drug pharmacokinetics from outside
a patient’s body via the imaging probe, which is a promising
means to observe the therapeutic process, as the existence of
the imaging probe (fluorescence, luminescence, MRI and PET)
in a diseased region indicates the existence of the therapeutic
drug at the same location. In recent years, the fields of DDS,
molecular imaging and nanotechnology became more sophis-
ticated and merged intricately. Some researchers suggested
highly functionalized drug carriers, which contain a therapeutic
drug, magnetic particles for imaging, and therapeutic antibodies
for active targeting, which allow for both disease diagnosis and
therapy (11).
Magnetic resonance imaging (MRI) is a powerful noninva-
sive diagnostic technique suitable for whole body imaging,
which fulfills the demand for continuous tracking of a drug
carrier. To enhance the MRI signal intensity, gadolinium ion
(Gd3+)-based contrast agents (CAs) were developed several
decades ago (12,13). More recently, numerous lipid-based CAs
able to form liposomes and micelles have been developed (14).
These CAs consist of a hydrophobic chain and a hydrophilic
head group, such as the diethylenetriamine pentaacetic acid
(DTPA)-Gd complex (15). On the other hand, a lot of study
has been performed to control the release of encapsulated
substances by photoreactive amphiphiles forming micelles
(7,16) and vesicles (17–19). However, to our best knowledge,
amphiphiles combining both MRI contrast increasing func-
tionality and stimuli sensitive functionality in a single molecule
in an effort toward the development of a ‘‘multifunctional
nanodrug’’ have not been realized so far.
)1
16.5 mM s)1, 0.47 T at 40ꢀC) than the widely applied small
molecule contrast agent Gd-DTPA (Magnevistꢁ r1 = 4.1 mM)1 s)1,
0.47 T at 40ꢀC). This dual functionality of encapsulated
compound release and increased MR imaging contrast indicates
that KMR-AZn is a potential candidate for application as a
lipid-based MRI-traceable drug carrier.
INTRODUCTION
Lipid-based colloidal aggregates such as micelles and lipo-
somes have been extensively developed as drug carriers over
recent decades, with the goal to improve the pharmacokinetic
properties or bioavailability of the encapsulated drugs (1). At
the same time, stimuli responsive molecular assemblies have
attracted much attention as drug delivery systems (DDS) and
molecular imaging agents to achieve ‘‘localized treatment’’ or
‘‘localized diagnosis.’’ Such assemblies allow the selective
treatment or imaging only in locations where external energy is
applied, or where the change of a specific physiological
parameter caused by some physiological anomaly occurs.
Among the typically used external stimuli (pH [2], temperature
[3], redox [4,5] and enzyme [6]), light-sensitive drug delivery
systems possess a particularly high potential for practical
application from the viewpoint of high-spatiotemporal con-
trollability of light, and the availability of many light-sensitive
substances. One disadvantage of photocontrolled release
systems is the limited penetrability of light through human
skin and tissue. Nevertheless, light is still one of the most
promising stimuli for responsive drug delivery carriers, because
*Corresponding author email: suzuki@applc.keio.ac.jp (Koji Suzuki)
ꢂ 2012 Wiley Periodicals, Inc.
Photochemistry and Photobiology ꢂ 2012 The American Society of Photobiology 0031-8655/12
In this article, novel micellar MRI CAs are proposed,
which have a photochromic molecule incorporated into the
876

Photochemistry and Photobiology, 2012, 88 877
Nile Red in micelles was measured as a function of light irradiation
hydrophobic chain. As the hydrophobic group, an azobenzene
derivative, which undergoes photoisomerization from the trans
to the cis configuration upon ultraviolet or visible light irradi-
ation (16), was coupled to one carboxylic acid residue of a
DTPA-Gd derivative, which acts as the hydrophilic group.
Herein, we first report on the design, synthesis and the funda-
mental properties of a series of bifunctional (MRI CAs and light-
induced drug release) amphiphilic probes (KMR-AZn), which
can assume the form of simplest low-molecular weight micelles.
time. Irradiation was performed at 365
10 nm (Nikon UV-1A filter,
Tokyo, Japan), 10 mW cm)2. At the same time, absorbance was
measured in the wavelength range of 210–600 nm (after irradiation, the
solutions were diluted to a probe concentration of 0.04 mM with
phosphate buffer to prevent aggregate-induced peak distortions) to
confirm trans ⁄ cis isomerization of KMR-AZn.
Longitudinal relaxation time measurement. The longitudinal relax-
ation times of the CAs of the KMR-series and Magnevistꢁ (Nihon
Schering K.K.) were measured in PBS buffer using a 0.47 T NMS 120
Minispec NMR ANALYZER (Bruker Optics K.K., Ibaraki, Japan)
and Minispec mq20 NMR ANALYZER (Bruker Optics K.K.,
Ibaraki, Japan) at 40ꢀC and the standard inversion-recovery proce-
dure. The r1 values were determined from the slope of the plot of the
inverse of T1 versus the experimentally measured millimolar Gd3+
concentrations (concentration of the CAs) as follows:
MATERIALS AND METHODS
All chemicals and solvents were purchased from Tokyo Kasei (Tokyo,
Japan), Wako Pure Chemical Industries (Osaka, Japan), or Sigma–
Aldrich Chemical Co. (St. Louis, MO). Magnevistꢁ was obtained from
Nihon Schering K.K. (Osaka, Japan). All materials were used without
further purification. Deionized water was obtained from a Waters
Milli-Q system (Bedford, MA). The synthesis of the KMR-AZn
compounds is described in the Supporting Information. The gadolin-
ium ion concentration in CA solutions was determined using an
inductively coupled plasma atomic emission spectrometer (ICP-AES)
ICPS-8000 from Shimadzu Co. (Kyoto, Japan).
Optical measurements. Absorption spectra were recorded on a
Hitachi U-2001 double beam spectrophotometer (Hitachi Co. Ltd.,
Tokyo, Japan). Fluorescence emission spectra were recorded on a
Fluorolog-3 photometer (Model FL-3-11; Horiba Jobin Yvon Co.,
Kyoto, Japan) at 25ꢀC.
CMC studies. To a KMR-AZn solution at 0.01–10 mM (solvent:
10 mM phosphate buffer at pH 7.4), an excess amount of Nile Red was
added. The mixture was sonicated for at least 30 min, and then,
remaining insoluble dye was removed by filtration through a 0.45 lm
pore membrane (Ultrafree-CL HV 0.45 lm, Millipore). Fluorescence
measurements were performed at an excitation wavelength of 550 nm
and emission wavelength of 560–750 nm. The emission at 638 nm
(kmax) was plotted against the logarithmic probe concentration. The
critical micelle concentration (CMC) was determined from the sharp
inflection point in the plot (Figs. S1–S3).
General procedure for light-induced dye release studies. As the light
source to photoisomerize KMR-AZn, a spot light source with a
mercury lamp (LIGHTINGCURE LC6 L8868; Hamamatsu Photon-
ics K.K., Japan) was adopted. Solutions of KMR-AZn or sodium
dodecylsulfate at 2 mM in 10 mM phosphate buffer at pH 7.4 were
prepared and Nile Red was added as described elsewhere. The
solutions were transferred into a quartz cell and the fluorescence of
ðð1=T1Þobs ꢀ ð1=T1ÞdÞ
r1 ¼
;
½Mꢁ
where [M] is the millimolar concentration of the CA, (1 ⁄ T1)d is the
inverse of the relaxation time of the diamagnetic solvent (pH buffered
aqueous solution in this study) in the absence of a paramagnetic spe-
cies and (1 ⁄ T1)obs is the inverse of the observed solvent relaxation time
in the presence of a paramagnetic species (CA of the KMR-series or
Magnevistꢁ [Nihon Schering K.K.] in this study).
Magnetic resonance imaging. The MR images were obtained using a
SIGNA EXCITE XI 1.5 T (GE Healthcare, Tokyo, Japan). All
samples were dissolved in 10 mM phosphate buffer at pH 7.4 and kept
in plastic tubes and placed in an agar gel plate at room temperature.
The measurement conditions were as follows: T1-weighted sequence:
FSPGR, ET = 1, TR ⁄ TE = 200.0 ⁄ 1.60 ms, matrix 256 · 256,
FOV = 20.0 · 20.0 cm, slice thickness = 5.0 mm.
RESULTS AND DISCUSSION
Micelle formation of KMR-AZn
The compounds of the KMR-AZn series were synthesized
according to Scheme 1. First, the formation of micelles in
aqueous medium from the newly synthesized molecules was
investigated. The formation of micellar structures by the
synthesized CAs was followed by measuring the emission
spectra of the lipophilic fluorescent probe Nile Red (4,7).
Although the excitation of Nile Red at 550 nm in water
(a), (b)
N
OH
AcHN
NH2
AcHN
N
(c)
N
O
n
AcHN
N
(d)
N
O
O
n
n
H2N
N
N
COOH
N
N
H
N
N
N
(e)
(f)
HOOC
HOOC
N
N
O
HOOC
COO-
O
H
N
n
N
-OOC
-OOC
N
N
Gd3+
O
-OOC
Scheme 1. Synthesis of KMR-AZ4 (n = 1), KMR-AZ6 (n = 2) and KMR-AZ8 (n = 3). Reagents and conditions: (a) HCl, NaNO2; (b) PhOH,
NaOH; (c) 1-bromoalkane, K2CO3; (d) HCl; (e) DTPA bis-anhydride and (f) GdCl3Æ6H2O.

878 Yushi Heta et al.
results in a relatively low fluorescence, the emission intensity
drastically increases in a hydrophobic environment such as
elsewhere, with KMR-AZn concentrations above CMC. These
solutions were irradiated with a halogen light source at 365 nm
at 10 mW cm)2, and both the fluorescence emission spectra of
Nile Red and the absorption spectra of KMR-AZn in the UV–
VIS region were measured as a function of irradiation time.
Figure 2a,b,c show that the fluorescence of Nile Red in KMR-
AZn gradually decreases during photoirradiation, indicating
the dye release from inside the hydrophobic environment of
the KMR-AZn micelles into the aqueous medium. A compar-
ison experiment performed with micelles formed from sodium
dodecyl sulfate, which is not able to undergo photoisomeriza-
tion, showed no decrease in emission for as long as 30 min of
UV irradiation (see Fig. S2). In addition, the UV–VIS spectra
recorded during photoirradiation (Fig. 3a,b,c), showed a sharp
decrease of the p-p* band and a weak increase of the n-p*
band, which are attributed to the trans and cis isomer
respectively, due to azobenzene photoisomerization for all
three types of probes.
the interior of
a micelle. Fig. 1a,b,c show the relative
fluorescence intensity of Nile Red, which was solubilized into
phosphate buffer solutions in the presence of various con-
centrations of KMR-AZ4, 6 and 8 at pH 7.4. With the
increase of the concentration of KMR-AZn, a sharp increase
in fluorescence emission was observed, indicating the forma-
tion of micelles. Fig. 1d shows plots of the normalized
fluorescence intensity of Nile Red at 638 nm (maximum
emission) in dependence of the concentration of KMR-AZn.
The inflection points correspond to the critical micellar
concentration (CMC) of KMR-AZ4, 6 and 8. The calculated
results are summarized in Table 1 (see Fig. S1 for detailed
information concerning CMC plots [4]). From these results, it
was confirmed that the CMC values of KMR-AZn depend on
the alkyl chain length similar to what is known for common
surfactants (20).
The colloidal stability of the micellar systems was investi-
gated by dialysis experiments. For this purpose, the fluorescence
intensities of Nile Red and the absorption spectra of azoben-
zene were recorded after dialyzing micellar solutions with or
without UV irradiation under similar experimental conditions
as described elsewhere. Solutions of 1 mM KMR-AZn containing
Photoresponsive behavior of micelles
In a next step, the photosensitivity of KMR-AZn micelles
toward UV irradiation was examined. For this purpose, Nile
Red encapsulating micellar solutions were prepared as mentioned
1.2
1.2
[KMR-AZ4]/mM
(a)
(b)
[KMR-AZ6]/mM
6
4
2
1
0.5
0.2
0.1
3
2
1
0.5
0.2
0.1
0.05
0.02
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
560
610
660
710
560
610
660
710
Wavelength /nm
Wavelength /nm
1.2
1
1.2
1
[KMR-AZ8]/mM
3
(c)
(d)
KMR-AZ4
KMR-AZ6
KMR-AZ8
2
1
0.5
0.2
0.1
0.05
0.02
0.01
0.005
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
0.001
0.01
0.1
1
10
560
610
660
710
[KMR-AZn] /mM
Wavelength /nm
Figure 1. Emission spectra of Nile Red in 10 mM of phosphate buffer at pH 7.4 of (a) KMR-AZ4, (b) KMR-AZ6 and (c) KMR-AZ8 at various
concentrations. Excited at 550 nm. (d) Emission intensity at 638 nm versus the concentration (mM) of the KMR-AZn series.

Photochemistry and Photobiology, 2012, 88 879
Table 1. CMC values and longitudinal relaxivity of synthesized
compounds.
the fluorescence intensity of Nile Red decreased to 90% of the
original intensity after 10 h of dialysis. Upon further dialysis for
a total of 3 days, it decreased to almost half the original value
even without UV irradiation (see Figs. S3 and S4). For the
sample subjected to 10 min UV irradiation, the fluorescence
intensity decreased to 30% of the original intensity and reached
almost zero after 3 days of dialysis. Given the fact that the
literature reports a plasma half-life of several hours for micelles
(21), we concluded that KMR-AZ6 and 8 have sufficient
stability and phototriggered release functionality. On the other
hand, KMR-AZ4 has very low micelle stability. Rapid leakage
of Nile Red was observed even without UV irradiation (data not
shown). This is most probably due to the high-CMC value,
rendering the micelles easily disruptable by dilution.
)1
r1† ⁄ mM s)1
Entry
Compound
CMC* ⁄ mM
Monomer
Aggregates
1
2
3
4
KMR-AZ4
KMR-AZ6
KMR-AZ8
Magnevistꢁ
0.70
0.17
0.051
–
9.20
–
–
4.07
14.5
15.3
16.5
–
CMC, critical micelle concentration; *calculated from inflection points
of Nile Red fluorescence at 638 nm (see Supporting Information);
†measured at 40ꢀC, 0.47 T.
The dialysis experiment results have demonstrated that Nile
Red is effectively released from within the micelles upon UV
irradiation. In the case of a UV-induced disruption of the
micellar structures, a decrease of the KMR-AZn concentration
upon dialysis is expected. To confirm this hypothesis, the
azobenzene absorbance of dialyzed KMR-AZ6 and 8 micelle
solutions at the isosbestic points were monitored (Fig. 4c,d).
These results showed a similar trend to the fluorescence
intensities in Fig. 4a,b. The decrease of absorbance of KMR-
AZ6 and 8 was accelerated by UV irradiation, indicating micelle
Nile Red were dialyzed against an over 100-fold volume of
10 mM PBS buffer solution in the dark immediately after
preparation or after being subjected to UV irradiation for
10 min, and the fluorescence emission of Nile Red was recorded
(Fig. 4a,b). In the case of KMR-AZ8 in the absence of UV
irradiation, the fluorescence intensity showed little decrease
over 1 week of dialysis, indicating a high-colloidal stability.
When the same sample was subjected to UV irradiation for
10 min, the fluorescence intensity decreased to 80% of the
original intensity after 1 week of dialysis. In the case of AZ6,
1.2
1.2
(a)
(b)
Irradiation time/ min
Irradiation time/ min
0
1
2
5
10
15
1
1
0.8
0.6
0.4
0.2
0
0
0.25
1
2
5
0.8
0.6
0.4
0.2
0
10
15
560
610
660
710
560
610
660
710
Wavelength /nm
Wavelength /nm
1.2
1
1.2
1
(c)
(d)
Irradiation time/ min
0
2
5
KMR-AZ4
KMR-AZ6
KMR-AZ8
SDS
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
10
20
30
0
10
20
30
40
560
610
660
710
Irradiation time /min
Wavelength /nm
Figure 2. Emission spectra of Nile Red in 10 mM of phosphate buffer at pH 7.4 of (a) KMR-AZ4, (b) KMR-AZ6 and (c) KMR-AZ8 at 2 mM
during photoirradiation. (d) Emission intensity of Nile Red in the presence of KMR-AZn at 638 nm as a function of UV (365 nm, 10 mW cm)2
irradiation time.
)

880 Yushi Heta et al.
1.2
1
1.2
(b)
(a)
Irradiation time/ min
Irradiation time/ min
0
1
0.8
0.6
0.4
0.2
0
0
0.25
1
2
5
1
2
5
10
15
0.8
0.6
0.4
0.2
0
10
15
220 280 340 400 460 520 580
220 280 340 400 460 520 580
Wavelength /nm
Wavelength /nm
(d)
low number of surfactant
1.2
(c)
Irradiation time/ min
fast
0
2
5
10
20
30
1
0.8
0.6
0.4
0.2
0
slow
high number of surfactant
220 280 340 400 460 520 580
Wavelength /nm
Figure 3. UV–Vis spectra of (a) KMR-AZ4, (b) KMR-AZ6 and (c) KMR-AZ8 at 0.04 mM in 10 mM phosphate buffer at pH 7.4 as a function of
UV irradiation time at 365 nm (irradiation was performed at probe concentrations of 2 mM, refer to the Materials and Methods section). (d)
Schematic representation of the differences in the response time.
disruption by UV irradiation. Therefore, it was concluded that
the encapsulated Nile Red was released from the micelles due to
a disruption of micelles upon azobenzene photoisomerization.
Photoisomerization of the azobenzene moieties produces
two major changes in properties. One is a stereochemical
structural change. The other is a change of hydrophobicity of
the azoboenzene moieties. The structural change affects the
macrostructure of micelles because the cis isomers are bulkier
than trans isomers. It has been reported that the isomerization
of azobenzene covalently attached to polymers or assembled as
a monolayer revealed substantial macrostructural changes, for
example destruction of multilayer polymer composition and
decrease of monolayer flatness (22,23). Furthermore, hydro-
phobicity of azobenzene drastically changes by photoisomer-
ization. The trans azobenzene has no dipole moment, whereas
the dipole moment of the nonplanar cis isomers is 3.0 D (24).
This polarity change of azobenzene moieties affects polymer
solubility or wettability of polymer surfaces (25,26). Thus, the
isomerization of azobenzene is expected to affect stabilities and
formation of micelles. Specifically, trans to cis conformation
changes are assumed to decrease the colloidal stability.
Overall, the above-mentioned two major property changes
can be regarded as very plausible causes for the disruption of
micelles. Interestingly, the photoresponse time increased upon
alkyl length elongation, which is especially observed for
KMR-AZ8. It is well known that compounds with low-CMC
values form densely packed micellar aggregates from a
relatively large number of surfactant molecules (27). The trans
to cis isomerization rate of azobenzenes is strongly dependent
on the packing density due to the lack of free volume for the
conformational change. The photoisomerization of azoben-
zenes causes a large shape change between the rodlike trans
isomers and the bent cis isomers. This phenomenon has been
reported for Langmuir–Blodgett films (23,28,29). Thus, the
increasing photoresponse time can be explained by the
robustness of the hydrophobic aggregates due to the interac-
tion between azobenzene residues in more compact micelles
formed from surfactants with lower CMC values (Fig. 3d; 30).
Magnetic resonance imaging
To demonstrate the MRI contrast ability of the hydrophilic
Gd complex, the longitudinal relaxation times (T1) of KMR-
AZn and of clinically used Magnevistꢁ (Nihon Schering K.K.)
were measured at various probe concentrations (0.15–1.4 mM)
in 10 mM phosphate buffer (pH 7.4) at 0.47 T and 40ꢀC. The
r1 values were determined from the slope of the plot of the
inverse of T1 versus the millimolar concentrations of the CAs.
The obtained data reveals that KMR-AZn has about three-
fold higher relaxivity than Magnevistꢁ (Nihon Schering K.K.)

Photochemistry and Photobiology, 2012, 88 881
Figure 4. Emission spectra of Nile Red in 10 mM of phosphate buffer at pH 7.4 of (a) KMR-AZ8 and (b) KMR-AZ6 before and after dialysis
without and with UV irradiation. UV–Vis spectra of (c) KMR-AZ8 and (d) KMR-AZ6 before and after dialysis without and with UV irradiation.
[Gd3+] /mM
20
16
12
8
0.8
0.4
0.1 0.01
(b)
(a)
KMR-AZ4
KMR-AZ6
KMR-AZ8
Magnevist
KMR-AZ6
KMR-AZ8
Magnevist
Control
4
0
0.0
0.5
[Gd3+] /mM
1.0
1.5
Figure 5. (a) Inverse of longitudinal relaxation times (T1) of solutions of KMR-AZn and Magnevistꢁ were plotted as a function of Gd3+
concentration. The longitudinal relaxivities of KMR-AZn and Magnevistꢁ were determined from the slopes. (b) T1-weighted MR images of various
concentrations of CAs and control. All samples were adjusted in 10 mM phosphate buffer solution at pH 7.4. The T1 values of each sample are
displayed in millisecond units below each sample.
as shown in Fig. 5a and in Table 1. The higher relaxivity of
micellar CAs is due to restriction of molecular movement
compared with monomeric Gd chelates (15). Tumbling of Gd
chelates induces locally fluctuating magnetic fields. When the
tumbling rate is close to the Larmor frequency (proton
precession rate), the relaxation of water protons occurs
effectively. Monomeric Gd chelates like Magnevistꢁ (Nihon
Schering K.K.) have tumbling rates much higher than the

882 Yushi Heta et al.
Larmor frequency. Thus, the relaxivity of contrast agents
increases with increasing molecular weight and limited molec-
ular movement as a result of tumbling rate slowing and
approach to the Larmor frequency. In addition, the relaxivity
increase as a function of the alkyl chain length can also be
explained by the increased number of surfactant molecules in
the micellar aggregates, resulting in restriction of molecular
movements (15,27). The slight change of the slope of KMR-
AZ4 observed in Fig. 5a further supports the formation of
micelles. The distinguished intersection point at ca 0.53 mM of
Gd3+ separates a monomeric low relaxivity form from a
micellar high-relaxivity form. For KMR-AZ6 and 8, the
inflection point was not observed because of the difficulty of
relaxivity measurements in the range of the low CMC.
We also examined the photoresponse of the relaxivity
values. After UV irradiation, the relaxivities of KMR-AZ6 and
8 decreased and became 12.9 and 14.7 mM s)1, compared with
15.3 and 16.5 mM s)1 before UV irradiation. This experiment
indicated that the molecular movements after irradiation are
faster than those before irradiation, further supporting the fact
of photoresponse-triggered disruption of micelles. Figs. 5b and
S5 show the efficacy as contrast agents by phantom imaging
(T1-weighted) of KMR-AZ6, 8 and Magnevistꢁ (Nihon
Schering K.K.) at four different concentrations (0.8, 0.4, 0.2,
0.1 and 0.01 mM) in phosphate buffer solution at 1.5 T. The
images of KMR-AZn gradually brighten with higher concen-
trations similar to Magnevistꢁ (Nihon Schering K.K.) and
reflect the results of the measurement of the longitudinal
relaxivity.
Figure S4. Absorbance intensity at the isosbestic points of
KMR-AZn with ⁄ without Nile Red before and after dialysis
with ⁄ without UV irradiation.
Figure S5. Color version of Fig. 5b.
Please note: Wiley-Blackwell is not responsible for the
content or functionality of any supporting information
supplied by the authors. Any queries (other than missing
material) should be directed to the corresponding author for
the article.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
Figure S1. CMC plot for KMR-AZn (fluorescence intensity
of Nile Red at 638 nm). The intersection of the two straight
lines indicates the critical micelle concentration.
Figure S2. Emission spectra of Nile Red in 10 mM of
phosphate buffer at pH 7.4 in the presence of 2 mM of sodium
dodecyl sulfate (SDS).
Figure S3. UV–Vis spectra of KMR-AZn in 10 mM of PBS
buffer with ⁄ without Nile Red before and after dialysis
with ⁄ without UV irradiation.
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