3D Long-Range Triplet Migration in a Water-Stable Metal-Organic Framework for Upconversion-Based Ultralow-Power in Vivo Imaging

Article abstract of DOI:10.1021/jacs.8b01613

Triplet-triplet annihilation upconversion (TTA-UC) has gained increasing attention because it allows for harvesting of low-energy photons in the solar spectrum with high efficiency in relevant applications including solar cells and bioimaging. However, the utilization of conventional TTA-UC systems for low-power bioapplications is significantly hampered by their general incompatibility and low efficiency in aqueous media. Herein we report a metal-organic framework (MOF) as a biocompatible nanoplatform for TTA-UC to realize low-power in vivo imaging. Our MOF consists of a porphyrinic sensitizer in an anthracene-based Zr-MOF as a TTA-UC platform. In particular, closely aligned chromophores in the MOF facilitate a long-range 3D triplet diffusion of 1.6 μm allowing efficient energy migration in water. The tunable ratio between sensitizer and annihilator by our synthetic method also allows an optimization of the system for maximized TTA-UC efficiency in water at a very low excitation power density. Consequently, the low-power imaging of lymph node in a live mouse was successfully demonstrated with an excellent signal-to-noise ratio (SNR > 30 at 5 mW cm^-2).

Full text of DOI:10.1021/jacs.8b01613

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3D long-range triplet migration in a water-stable metal-organic
framework for upconversion based ultralow-power in vivo imaging
Jihye Park, Ming Xu, Fuyou Li, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01613 • Publication Date (Web): 10 Apr 2018
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3D long-range triplet migration in a water-stable metal–
organic framework for upconversion based ultralow-power in
vivo imaging
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Jihye Park1,†, §, Ming Xu2,†, Fuyou Li2,*, and Hong-Cai Zhou1,*
1Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA.
2Department of Chemistry & State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shang-
hai 200433, P. R. China.
ABSTRACT: Triplet-triplet annihilation upconversion (TTA-UC) has gained increasing attention because it allows for harvest-
ing of low energy photons in the solar spectrum with high efficiency in relevant applications including solar cells and bioimag-
ing. However, the utilization of conventional TTA-UC systems for low-power bio-applications is significantly hampered by their
general incompatibility and low efficiency in aqueous media. Herein we report a metal–organic framework (MOF) as a biocom-
patible nanoplatform for TTA-UC to realize low power in vivo imaging. Our MOF consists of a porphyrinic sensitizer in an an-
thracene-based Zr-MOF as a TTA-UC platform. In particular, closely aligned chromophores in the MOF facilitate a long-range
3D triplet diffusion of 1.6 µm allowing efficient energy migration in water. The tunable ratio between sensitizer and annihilator
by our synthetic method also allows an optimization of the system for maximized TTA-UC efficiency in water at a very low exci-
tation power density. Consequently, the low-power imaging of lymph node in a live mouse was successfully demonstrated with
an excellent signal-to-noise ratio (SNR > 30 at 5 mW cm-2)
.
INTRODUCTION
the upconverted emission is then observed in higher en-
ergy than the initial input. Since the TTET and TTA pro-
cesses heavily rely on triplet diffusion, the energy migra-
tion facilitated by high mobility of triplet excitons plays
an important role for the overall TTA-UC performance.7-9
Typically, in non-viscous organic solvents where chromo-
phores can freely diffuse to promote the energy migration,
a reasonable TTA-UC efficiency can be achieved with the
excitation power as low as the solar irradiance.10-11 Howev-
er, the incompatibility of organic solvents with biological
samples prompted the search for TTA-UC systems in
aqueous media. For instance, incorporation of TTA-UC
dyads has been formulated in amphiphilic polymers or
micelles to enable their upconversion luminescence in
aqueous media.12-14 However, the trade off in these ap-
proaches is often a slowdown of triplet diffusion rate due
to the limited mobility of the dissolved chromophores in
such matrices. Meanwhile, the annihilator and sensitizer
in these systems usually adopt a random orientation and
an uncontrolled distance between one another, resulting
in insufficient intermolecular excitonic couplings. These
intrinsic disadvantages inevitably impair the efficient en-
ergy transfer, thus greatly lowering the TTA-UC efficiency.
As a consequence, an undesirably high excitation power
was necessary for bioimaging (~ 0.1 W cm-2), which lowers
the practicality of TTA-UC technique with increased risk
of organ damage.12-14 Therefore, development of a water
stable TTA-UC system with a controlled orientation of the
chromophores for efficient energy migration is highly
desired for the low-power bioimaging.
As a unique imaging technique where low photon en-
ergy is employed, photon upconversion (UC) processes
have gained growing attention.1 In particular, the upcon-
version process is well suited for biological imaging be-
cause the utilization of low photon energy can prevent
tissue damages and in vivo autofluorescence from anti-
Stokes shift.2 The most widely applied technology to date,
rare-earth metal based upconversion nanophosphors (RE-
UCNPs), have been widely employed to obtain high con-
trast images with high a signal-to-noise ratio (SNR).3-4
However, RE-UCNPs often suffer from low absorption
cross-section and poor upconversion luminescence quan-
tum efficiency, which in turn undesirably requires a high
power input (~ few W cm-2).5 Therefore, an upconversion
system that can be operated at a low power density is
highly desired, particularly for biological specimens. In
this regard, triplet-triplet annihilation based upconver-
sion (TTA-UC) has emerged as an ideal alternative that
can accommodate a low excitation power density, tunable
photophysical properties on the molecular level, and high
upconversion quantum efficiency.6
TTA-UC system typically comprises a pair of chromo-
phores (dyad): sensitizer and annihilator. Upon photoex-
citation of a sensitizer, a singlet state sensitizer undergoes
intersystem crossing (ISC), giving rise to its triplet state.
Then triplet-triplet annihilation between two excited tri-
plet annihilators generated by triplet-triplet energy trans-
fer (TTET, from sensitizer to annihilator) eventually pop-
ulates as one excited singlet annihilator. Subsequently,
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Recently, metal–organic frameworks (MOFs) have cap-
Page 2 of 9
solvents and a diffusion length on the micrometer scale.
Because our synthetic strategy allows a fine-tuning of the
ratios between the sensitizer and annihilator,20 the system
was optimized for the highest TTA-UC efficiency in water
measured at solar irradiance. Consequently, the TTA-UC
MOF capable of working at ultralow excitation power was
successfully employed for real-time lymph node imaging
with a remarkable signal-to-noise ratio in vivo.
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tured widespread research interests in biological applica-
tions because of their synthetic tunability and improved
chemical stability in aqueous media.15 Furthermore, MOFs
have been explored as a promising energy transfer (ET)
platform that can realize a cooperative molecular-level
function due to their highly accessible and spatially dis-
crete linkers in the framework.16-19 Apart from the well-
defined arrangement of chromophores, the luminescence
quenching from aggregation can also be prevented when
such emissive dyes are used as the strut of MOFs. There-
fore, water stable MOFs can serve as an ideal TTA-UC
platform to facilitate efficient energy migration under
physiological conditions. Herein we report the first exam-
ple of a biocompatible TTA-UC MOF that demonstrates a
low-power in vivo imaging. With an in situ secondary
functionality incorporation, a sensitizer, Pd(II)-meso-
tetrakis(4-carboxyphenyl)porphyrin (Pd-TCPP) was in-
corporated into a water stable Zr-MOF, constructed from
an annihilator linker, 4,4’-(9,10-anthracenediyl)dibenzoic
acid (DCDPA). The efficient triplet diffusion was success-
fully demonstrated in water, exhibiting a diffusion con-
stant comparable to that of molecular systems in organic
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RESULTS AND DISCUSSION
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Design of the water-stable TTA-UC MOFs. When in-
corporating cooperative functionalities in a MOF, such as
a TTA-UC dyad, a pillar-layered structure with M2 pad-
dlewheel (M = Zn, Cu) can be easily considered through
the mixed-linker strategy (Supporting Information
Scheme S2).21 In pillar-layered structures, one chromo-
phore forms 2D layers while another chromophore con-
nects the 2D layers as pillars. However, M2 paddlewheels-
based MOFs are typically unstable in aqueous media due
to lability of their coordination bonds. Thus, a successful
MOF for TTA-UC must consider the chemical stability of
the MOFs to utilize the structural features of MOFs with-
out degradation.
Figure 1 | Structural analysis and the triplet energy migration pathways in different types of TTA-UC MOFs. a,
Chemical structures of annihilator and sensitizer to construct TTA-UC MOF. b, A schematic of the proposed TTA-UC
process in TTA-UC system of Pd-TCPP/DCDPA. c, Proposed schematic of TTA-UC system based on a conventional pillar-
layered MOF (left) and the spatial distribution of the TTA-UC chromophores allowing 2D intralayer triplet migration
(right). The cyan represents the annihilator, the orange represents the sensitizer, respectively as linkers. The light blue
cubes represent paddlewheel clusters. d, Proposed schematic of TTA-UC system based on a UiO-68 analogue (left) and
the spatial distribution of TTA-UC chromophores allowing 3D triplet migration (right). The orange sphere in UiO-68 cage
represents an accessible void space. The light blue cuboctahedra represent Zr6 clusters. (c,d) The red arrows represent
TTET from sensitizers to annihilators and the blue planes represent ET and TTA between adjacent annihilators.
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duced through in situ coordination to defective coordina-
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tion sites of Zr6 clusters [Zr6O4(OH)4(COO)12] in the MOF
(Figure 2a). With this method, the concentration of the
sensitizer can be finely tuned upon varying feed ratios in
the MOF synthesis, which yields a Pd-TCPP doped UiO-
68 analogue (Figure 2b). The high connectivity of UiO-68,
defined as 2,12-connected fcu-a net allows many sub-
networks or available coordination sites on the Zr6 cluster
in the MOF without destruction of the framework.24-27
Therefore, our synthetic strategy is particularly suited for
TTA-UC as it often requires a screening of the annihilator
to sensitizer ratio to find a maximum upconversion effi-
ciency of the system.
Based on our design, the TTA-UC in the MOF platform
is schematically represented in Figure 1b. The TTA-UC
process involves multiple steps of energy transfers com-
plying Dexter exchange mechanism, which is sensitive to
orbital overlaps (Figure 1b and Supporting Information
Scheme S1).22 Therefore, proper distance and placement
between chromophores are of crucial importance when
designing a TTA-UC system (Supporting Information Sec-
tion 1). In that regard, MOFs can be a promising candi-
date to realize a precise placement of chromophores.
However, there are two drawbacks in possible MOF de-
signs with pillar-layered structure. First, with the chosen
TTA-UC dyad, the large porphyrinic sensitizer will result
in a large distance between annihilator layers, which
would hamper an effective triplet migration between 2D
layers (Supporting Information Scheme S2). Moreover,
the ratio between sensitizer and annihilator is intrinsical-
ly determined by the structure, disabling the ratio tuning
between annihilator and sensitizer which could be an
accessible method to screen high TTA-UC efficiency.6,23
Therefore, achieving highly efficient TTA-UC in a water-
stable MOF platform for bioimaging remains a great chal-
lenge.
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In particular, due to high connectivity of the Zr6 cluster
in our MOF, the annihilators are highly populated and
aligned along all directions, providing a well-aligned 3D π
orbital path in the framework for efficient diffusion of
triplet excitons.28 For instance, each annihilator in the
framework is surrounded by eight neighboring annihila-
tors (Figures 1d and 2a), which can increase the probabil-
ity of effective TTA-UC process, compared to the possible
pillar-layered structures that can only have four neighbor-
ing anthracenes (Figure 1c and Supporting Information
Scheme S2). Meanwhile, when the sensitizer was incorpo-
rated into the framework, it is likely to be fully surround-
ed by the annihilators with radial proximity, thus further
facilitating TTET process and providing 3D pathways of
fast triplet migration (Figures 1d and 2b). Most important-
ly, since the framework is based on robust Zr(IV)-
carboxylate bonds, the aggregation of chromophores can
be prevented by retention of the framework in aqueous
media.
To address these challenges, herein we propose a new
nanoplatform to realize a 3D fast triplet migration in wa-
ter where TTA-UC dyad is incorporated into a Zr-MOF
nanoparticle through in situ insertion of functional de-
fects.20 In our design, an anthracene-based annihilator
(DCDPA) is employed as a linker to construct the Zr-
MOF (Figures 1a and 1d). Then, the secondary functionali-
ty (carboxylated tetratopic sensitizer, Pd-TCPP) is intro-
Figure 2 | Structure of and characterizations of TTA-UC MOF 5. a, In situ incorporation of Pd-TCPP into DCDPA
MOF matrix through defective coordination on Zr6 cluster. b, 3D representation of TTA-UC MOF structure with tunable
Pd-TCPP to DCDPA ratio and schematic of TTA-UC process in the system. Defective sites (orange spheres) created by
missing linkers, allowing the incorporation of Pd-TCPP. c, TEM image of TTA-UC MOF 5. Scale bar = 100 nm. d, Compar-
ison between simulated and experimental PXRD patterns of TTA-UC MOF 5. e, Colloidal and photochemical stability of
TTA-UC MOF 5 in water over one week. The UCL intensities at 440 nm were measured (λex = 532 nm).
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Synthesis and characterizations of TTA-UC MOFs. By
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morphology (Figure 2c). Powder X-ray diffraction showed
the experimental diffractions match well with that of cal-
culated UiO-68 (Figure 2d). Interestingly, although the
anthracene core is known to be highly hydrophobic, dy-
namic light scattering (DLS) confirmed that 5 can be well
dispersed and stabilized in water in the absence of addi-
tional surfactant (Supporting Information Figure S7).
These results show a good agreement with our previous
observations where the 3D porous structure of the MOF
could lead to a much decreased surface contact between
particles thus stabilizing the MOF nanoparticles.31 In ad-
dition, PXRD shows the robust Zr(IV)-carboxylate bonds
can support intactness of the framework in aqueous me-
dia over a week (Supporting Information Figure S9).
Aside from the chemical stability of our Zr-based MOF,
the aqueous suspension of 5 was also tested for the colloi-
dal stability and luminescence properties. It was con-
firmed that no obvious changes were found in hydrody-
namic size as indicated from the DLS measurements,
while only a negligible decrease in the UCL intensity was
observed perhaps due to the photo-oxidation common in
these systems (Figure 2e). These results clearly support
the unique nature of our MOF nanoplatform indeed ena-
bles a long-term stability of TTA-UC MOF in water.
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varying the feed ratio of Pd-TCPP in the solvothermal
synthesis of the MOF, DCDPA-based TTA-UC MOF na-
noparticles with different Pd-TCPP concentrations were
synthesized (namely, TTA-UC MOFs 1 – 9). The synthe-
sized TTA-UC MOFs show good dispersibility and colloi-
dal stability in water for long-term storage (Figure 3a in-
set). Absorption spectra of TTA-UC MOFs dispersed in
DMSO clearly reveal that the presence of both Pd-TCPP
(at 417 and 523 nm) and DCDPA (at 356, 375, and 396 nm)
with their characteristic bands (Supporting Information
Figures S3–S5). Moreover, UV-Vis spectra, normalized to
a constant concentration of DCDPA, confirmed that mo-
lar ratio of Pd-TCPP in DCDPA-based MOF nanoparticles
is indeed tunable upon varying the feed ratio of Pd-TCPP
(Figure 3a). In parallel, energy dispersive X-ray spectros-
copy (EDS) and inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES) were carried out to confirm
a quantitative trend of the varied Pd-TCPP contents in
the TTA-UC MOFs. Due to the highly ordered and crys-
talline structure of TTA-UC MOF nanoparticles, the in-
serted Pd-TCPP contents can be calculated by comparing
the elemental counts of Zr(IV) and Pd(II) of the MOF
samples. Because the framework consists of DCDPA as
the strut, each DCDPA linker corresponds to one Zr atom
in an ideal fcu net. Although the insertion of Pd-TCPP on
the Zr-cluster would slightly reduce the DCDPA to Zr
ratio, the results were sufficient to see the trend due to
the low content of Pd-TCPP compared to DCDPA.24,29
According to the EDS and ICP-AES results, the ratio of
Pd-TCPP to DCDPA in TTA-UC MOFs (samples 1 – 9)
decreases upon decreasing the dosing amount of Pd-
TCPP in the synthesis (Supporting Information Tables S1
and S3), which shows an agreement with the trend in UV-
Vis results. Therefore, the collective results confirm the
controlled ratios of the annihilator and sensitizer in our
TTA-UC MOF system.
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Next, incident laser power dependence on UCL was
studied in an aqueous suspension of sample 5 (Supporting
Information Figure S10). Figure 3c shows a quadratic-to-
linear dependence of TTA-UC MOF 5 (slopes = 1.90 and
0.91, respectively) between the UCL intensity and the ex-
citation power density, which is an indicative feature of
the typical TTA-UC system. The threshold of excitation
power density (Ith, revealed as an intersection of these two
slopes in Figure 3c) was determined to be as low as 2.5
mW cm-2 that is close to the level of solar irradiance at
532 ± 10 nm, suggesting a saturation of TTA-UC quantum
yield at low-power excitation.11,32 Then, UCL quantum
efficiency (ΦUCL) of TTA-UC MOF 5 was measured in aer-
TTA-UC luminescence (UCL) spectra of TTA-UC MOFs
1 – 9 were then obtained in water (Figure 3b) to screen the
best performing TTA-UC. UCL intensity of the samples
with the extremely low contents of Pd-TCPP (samples 7 –
9) is attenuated presumably due to the insufficient TTET
contribution. While increasing Pd-TCPP in the MOFs, it
led to a much improved UCL intensity as shown in 5.
However, further increase of Pd-TCPP rather resulted in a
decreasing trend in UCL intensity (samples 1 – 4). Such
decrement can be attributed to the inner-filter effect (re-
absorption), caused by partial spectral overlap between
strong absorption from Soret band of Pd-TCPP (λabs = 417
nm) and the UCL emission of DCDPA, resulting in a de-
creased UCL intensity as well as an apparent red-shift of
UCL peaks in these samples (Figure 3b).30 These results
indeed show the advantage of our TTA-UC platform that
allows the fine-tuning of the annihilator/sensitizer ratio
for screening the best TTA-UC performer.
ated water referenced to Ru(bpy)3Cl2 (Supporting Infor-
mation Figure S11).33 To the best of our knowledge, TTA-
UC MOF 5 shows the highest UCL quantum efficiency
(
ΦUCL = 1.28±0.07%) under excitation at 2.5 mW cm-2,
among that of the reported TTA-UC systems measured in
aqueous media.14,34-39 Such high efficiency can also be at-
tributed to efficient TTA process arising from the intimate
distance between adjacent anthracene cores (about 5.6 Å
based on C–C) (Supporting Information Scheme S2b)
aside from the structural design in our TTA-UC MOF that
has been previously mentioned. Meanwhile, the highly
ordered alignment of the immobilized annihilators in the
TTA-UC MOF offers the efficient exciton diffusion with
less rotational freedom within the framework, significant-
ly reducing the non-radiative decay pathways.40-41 There-
fore, a long UCL lifetime (τUCL) of 1.6 ms was observed in
TTA-UC MOF 5, which is even much longer than that of
the reported TTA-UC systems in organic solvents (~100 µs)
(Figure 3d).42-43 In particular, τUCL measured in aerated
water did not show a noticeable difference compared with
that in nitrogen atmosphere. This indicates that our sys-
tem is not much sensitive to oxygen, which is desirable for
Subsequently, TTA-UC MOF 5, optimized for the best
TTA-UC performance, was further characterized. Trans-
mission electron microscopy (TEM) confirmed that the
size of the TTA-UC MOF 5 is ~55 nm with octahedral
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our application. The insensitivity to oxygen was probably
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due to hydrophobic environment, generated from the
dense anthracene-based molecular alignment in the TTA-
UC MOF.36
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Figure 3 | Photophysical characterizations of TTA-UC MOFs. a, UV-Vis spectra of TTA-UC MOFs 1 – 9 in water. The
spectra were normalized at 375 nm and the decreasing trend of Pd-TCPP concentration was highlighted with an arrow
around 532 nm. Inset: Photographs of the MOF samples in water. The picture was taken 15 months after the samples were
synthesized and stored in a dark container, showing good water dispersibility. b, UCL spectra of TTA-UC MOFs 1 – 9 in
water (λex = 532 nm at 20 mW cm-2). A 530 nm short-pass filter was used to cut off the excitation laser. Inset: Integrated
UCL intensity of the MOFs samples. c, Integrated UCL intensity of the optimized TTA-UC MOF 5 as a function of 532 nm
excitation power density in water. Inset: bright field (left) and UCL images under 532 nm laser excitation (2.5 mW cm-2,
right) of TTA-UC MOF 5. A 530 nm short-pass filter was used to cut off the excitation laser. d, TTA-UC lifetime measure-
ment of TTA-UC MOF 5 in water. The samples were measured in the ambient air (black) and in N2 atmosphere (red), re-
spectively.
To further explore the mechanism, the process of ener-
gy migration was subsequently investigated. The triplet
ꢔ ꢃ ꢏ ꢊ
ꢋ ꢋ
(Eq. 4)
ꢕ
ꢋ
diffusion related parameters of TTA-UC MOF 5 in water
were theoretically determined according to the following
equations, where α is the absorption coefficient at 532 nm
excitation in wavenumber, τT is the lifetime of the accep-
tor triplet, γTT is the second-order annihilation constant
for the TTA, a0 is the annihilation distance of the triplets.
Consequently, TTA-UC MOF 5 shows a calculated tri-
plet exciton diffusion constant (DT) of 7.7 × 10-6 cm2 s-1 and
diffusion length (LT) of 1.6 ꢀm (Supporting Information
Section 9).44-45 Markedly, the triplet diffusion in water is
as fast as that in low-viscous organic solvents.10 Moreover,
the average nearest-neighboring distance between two
sensitizers was calculated to be 26.5 nm by using the sto-
chastic algorithm from the concentration of the sensitiz-
er.46 This value indicates that the long-range triplet diffu-
sion of 1.6 ꢀm in water is far enough to cover the average
distance between two sensitizers (26.5 nm) and the size of
the TTA-UC MOF 5 nanoparticle (~55 nm), which enables
the efficient triplet energy migration in sample 5. There-
ꢄ
ꢀꢁꢂ ꢃ ꢅ∅
(Eq. 1)
ꢈ
ꢉ
√
ꢆ
ꢆꢆꢇꢆ ꢆꢆ
ꢊꢋ ꢃ 2ꢊꢌꢍꢎ
(Eq. 2)
(Eq. 3)
ꢈ
ꢆꢆ
ꢏꢋ ꢃ ꢐꢑꢒ
ꢓ
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fore, the fast and long-range triplet energy migration in
for TTA-UC based biomedical applications in aqueous
environments.
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TTA-UC MOF greatly promotes the TTA-UC performance
in water. Such remarkable results achieved by our MOF
formulation indeed suggest that our system is beneficial
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Figure 4 | In vitro and in vivo imaging of TTA-UC MOF 5. a, in vitro and b, in vivo imaging with TTA-UC MOF 5 (λex =
532 nm at 20 mW cm-2). In vitro imaging was carried out by filling TTA-UC MOF 5 in the 384-well plates. 200 μL of TTA-
UC MOF 5 (2 mg mL-1) was subcutaneously injected to a nude mouse. The images were taken 10 min after the injection. c-
d, in vivo lymph node imaging with TTA-UC MOF 5 (λex = 532 nm at 5 mW cm-2). 100 μL of TTA-UC MOF 5 (2 mg mL-1)
was injected in footpad of forepaw. The images were taken at 5 min and 1 h after the injection, respectively. SNR = (I1-
I2)/(I2-I3) = 31, where I1, I2 and I3 refer to the mean UCL intensity of regions 1, 2, and 3, respectively.
Low-power bioimaging in vivo based on TTA-UC MOF
nanoparticles. To employ our concept to biological im-
aging studies, we first investigated the photostability of
sample 5 using kinetic scan assay under continuous laser
excitation (λex = 532 nm). Notably, TTA-UC MOF 5
showed no significant photo-degradation even under in-
tense laser irradiation (100 mW cm-2) during the meas-
urement, implying its applicability for real-time imaging
application (Supporting Information Figure S13). Prior to
in vivo imaging, cytotoxicity of 5 was also tested with
HeLa cells, which showed no significant cytotoxicity
(Supporting Information Figure S14).15,47 Then the upcon-
version performance of TTA-UC MOF 5 was tested with
our homebuilt bioimaging instrument.48 As shown in in
vitro imaging, an intense upconverted luminescence of
TTA-UC MOF 5 collected at 450 ± 25 nm (blue emission
channel) was observed upon excitation at 532 nm while
no signal was detected from the blank sample of saline
(Figure 4a). Next, the sample 5 was subcutaneously in-
jected into a nude mouse while the saline was subcutane-
ously injected to another mouse as a control (Supporting
Information Figure S15). Ten minutes after the injection,
the imaging was conducted under identical conditions.
While the control mouse showed no noticeable TTA-UC
response in vivo in the blue emission channel, the mouse
with TTA-UC MOF 5 injected exhibited strong upconver-
sion fluorescence with high imaging contrast (SNR of ~45
at 20 mW cm-2) despite the nature of moderate penetra-
tion from visible light excitation (Figure 4b). Both in vitro
and in vivo results indicate that the Zr-MOF nanoplatform
was successfully employed to demonstrate TTA-UC for
bioimaging.
We then further pursued an imaging of lymph node be-
cause the sentinel node of lymphatic system is known to
be correlated with tumor metastasis.49 Thus, the lymph
node imaging is important to the pathologic staging and
lymphadenectomy. Real-time imaging of the lymph node
was performed with the injection of TTA-UC MOF 5 into
forepaw footpad of a nude mouse. In Figure 4c, the up-
conversion response distributed around the injection
point was clearly observed right after the injection upon
irradiation at 532 nm laser (5 mW cm-2). Along with the
transfer of TTA-UC MOF 5, the sentinel node was lit up
by upconversion fluorescence in 1 h (Figure 4d). It is
worth noting that SNR of ~31 was achieved in vivo at very
low excitation power density of 5 mW cm-2. We also con-
firmed that after the imaging with 5, no histologic ab-
normality of the lymph node was observed from the he-
matoxylin and eosin (H&E) stain result (Supporting In-
formation Figure S16). Therefore, the TTA-UC MOF in-
deed demonstrates as a promising nanoplatform to enable
the feasibility of upconversion technique in practical im-
aging applications.
CONCLUSION
In summary, through the in situ incorporation strategy
of multiple functionalities into the Zr-MOF, a MOF-based
tunable TTA-UC system was achieved for low-power in
vivo imaging. The TTA dyad loaded water-
stable/dispersible TTA-UC MOF nanoparticles show re-
markable TTA-UC luminescence quantum efficiency as
high as 1.28% in water. In particular, a long-range triplet
diffusion of 1.6 ꢀm in water was also achieved by the well-
aligned annihilators in the MOF platform. The TTA-UC
MOF with the optimized ratio of sensitizer to annihilator
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was employed for in vivo imaging of a live mouse exhibit-
1
2
3
4
5
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ing an excellent SNR of ~45. Furthermore, the TTA-UC
MOF successfully demonstrated the lymph node imaging
of a live mouse in vivo with excellent SNR (> 30) at very
low power density (5 mW cm-2). Our findings show a
great promise that our TTA-UC MOF combined with the
fine-tuning ability can serve as an ideal host system for
TTA-UC dyad for in vivo bioimaging. We also envision
that surface modification of these MOF nanoparticles can
further improve the system enabling targeting and
theranostic functions.
ASSOCIATED CONTENT
Supporting Information
Synthesis and characterization of TTA-UC MOFs are shown
in the supporting information. The theoretical calculations
are also included. This material is available free of charge via
the Internet at http: //pubs.acs.org.
AUTHOR INFORMATION
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Corresponding Authors
zhou@chem.tamu.edu (H. C. Zhou)
fyli@fudan.edu.cn (F. Y. Li)
EXPERIMANTAL SECTION
TTA-UC MOF synthesis. In a 20 mL Pyrex vial, DCDPA
(30 mg, 0.072 mmol), zirconyl chloride octahydrate
(ZrOCl2·8H2O) (30 mg, 0.093 mmol), Pd-TCPP (see Table
S1 for varied amounts), and acetic acid (120 µL) were ul-
trasonically dissolved in 6 mL of N,N-dimethylformamide
(DMF). The reaction mixture was stirred (300 rpm) for 3 h
at 90 °C. The resulting nanoparticles were collected by
centrifugation (17000 rpm, 30 min) followed by sequential
washing with fresh DMF and dimethyl sulfoxide (DMSO)
for three times, respectively. The resulting nanoparticles
were suspended in DMSO for further characterization
and analysis unless otherwise noted where relevant.
Author Contributions
†
These authors contributed equally to this work.
Jihye Park & Ming Xu
§ Present address: Department of Chemical Engineering,
Stanford University, Stanford, CA 94305, USA
Notes
The authors declare no competing financial interest.
Acknowledgment
Upconversion quantum efficiency measurement.
H.-C.Z. acknowledges the support from the Welch Founda-
tion (A-0030). MOST of China (2015CB931801) and National
Natural Science Foundation of China (201375024 and
201375024) are acknowledged. The authors gratefully
acknowledge Dr. Dawei Feng for fruitful discussion, Dr.
Zhangwen Wei for graphics, and Mrs. Wei Yuan for assis-
tance in bioimaging.
TTA-upconversion quantum efficiency (ΦUCL) of TTA-UC
MOF samples in water was determined by relative quan-
tum yield measurement. Ru(bpy)3Cl2 in water was used as
the standard reference (absolute quantum efficiency ΦRef
= 0.040 in aerated H2O).32 A 532 nm laser was used as ex-
citation source for both of TTA-UC MOF and Ru(bpy)3Cl2.
The TTA-upconversion quantum efficiency was calculated
with
the
equation
below.5-6
ΦUCL
=
2ΦRef
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