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Surface Functionalization of Porous Coordination
Nanocages Via Click Chemistry and Their Application
in Drug Delivery
Dan Zhao, Songwei Tan, Daqiang Yuan, Weigang Lu, Yohannes H. Rezenom, Hongliang
Jiang, Li-Qun Wang,* and Hong-Cai Zhou*
The discrete coordination-driven self assemblies have received
continuous attention due to their molecular architecture
esthetics and applications in recognition, catalysis, storage,
etc.[1] Among these self assemblies, one species that has
emerged recently is the porous coordination nanocages formed
between carboxylate ligands and metal clusters, which are also
known as metal-organic polyhedra (MOP).[2] Due to the robust
porous structure and versatile functionality, they have found
applications as plasticizer, gas sponge, ion channel, coatings,
and building units.[3] Presumably, the porous shell and uniform yet tunable cavity make them good candidates for drug
delivery purpose. However, almost all the coordination nanocages reported so far are hydrophobic, which greatly limits their
applications in aqueous condition. We hypothesize this problem
can be circumvented by turning these nanocages into colloids
through surface functionalization with hydrophilic polymers.
In this Communication, we report a porous coordination
nanocage covered with alkyne groups and its surface functionalization by grafting with azide-terminated polyethylene glycol
(PEG) through “click chemistry”. In addition, its drug load and
release capacity has been evaluated using an anticancer drug
5-fluorouracil as a model.
The metal-organic cuboctahedron was chosen as the prototype of nanocage in this study.[2a,2c] It is composed of 12
dicopper paddlewheel clusters and 24 isophthalate moieties,
with 8 triangular and 6 square windows that are roughly 8 and
12 Å across, respectively. The internal cavity has a diameter of
around 15 Å. The 5-position of isophthalate moieties would
be the reaction site for surface functionalization. The Cu(I)catalyzed Huisgen cycloaddition between azide and alkyne, a
so-called “click reaction”, was chosen as the synthetic tool in
D. Zhao, Dr. D. Yuan, Dr. W. Lu, Dr. Y. H. Rezenom,
Prof. H.-C. Zhou
Department of Chemistry
Texas A&M University
College Station, Texas, 77842. USA
E-mail: zhou@mail.chem.tamu.edu
S. Tan, Prof. H. Jiang, Prof. L.-Q. Wang
MOE Key Laboratory of Macromolecular Synthesis
and Functionalization
Department of Polymer Science and Engineering
Zhejiang University
Hangzhou, 310027, China
E-mail: lqwang@zju.edu.cn
DOI: 10.1002/adma.201003012
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this study due to its high yield, mild reaction condition, and
easy operation.[4] Based on the retrosynthetic analysis and the
convenience of implementation, alkyne-covered nanocage and
azide-terminated PEG are two prerequisites. The synthesis and
characterization of alkyne-covered metal-organic cuboctahedron
is not as straightforward as it seems to be. Since NMR signal
would be elusive due to the presence of paramagnetic Cu(II)
in these nanocages,[5] single crystal X-ray diffraction might be
the only characterization tool available. Therefore, obtaining a
single crystal of the nanocage would be of paramount importance for characterization. Figure 1a illustrates all the ligand
precursors we’ve tried in synthesizing this “clickable” nanocage,
which comprise isophthalate moiety capable of forming metalorganic cuboctahedron and alkyne group suitable for click
reaction. Solvothermal reaction, a process involving heating
the ligand/metal mixture solution within sealed environment
at high temperature, is often used in synthesizing these coordination nanocages.[2a] The solvothermal reaction between
5-ethynylisophthalic acid (H2ei) and copper salt ended up with
a coordination polymer, in which the terminal alkyne groups
in the ligand are coupled with each other under the catalysis
of copper.[6] In order to avoid the harsh synthetic conditions
in solvothermal reaction, we adopted a milder approach, in
which H2ei was first deprotonated by a sterically hindered
base (2,6-lutidine) and then reacted with copper salt.[2b] The
product precipitated from the solution immediately and was
insoluble in all the solvents tested (methanol, THF, DMF, etc.),
which makes the growth of single crystal through recrystallization impossible. We attribute this insolubility to the extremely
high solvation energy needed for these nanocages, which is
magnified by the hydrogen bonding interaction between terminal alkyne groups.[7] 5-((Triisopropylsilyl)ethynyl)isophthalic
acid (H2tei) was used instead in order to weaken the hydrogen
bonding interaction and increase the product’s solubility. It is
hoped that the alkyne-coved nanocage can be obtained once the
triisopropylsilyl (TIPS) protecting groups are removed. As has
been anticipated, the TIPS-covered nanocage with increased
solubility was formed using the same mild preparation procedure, whose structure was determined by single crystal X-ray
diffraction.[8] Unfortunately, this nanocage could not survive
during deprotection, which renders the second trial unsuccessful. The long alkane chain in 5-(undec-10-ynyloxy)isophthalic acid (H2uyi) helps to increase the product’s solubility, and
the terminal alkyne group makes the deprotection unnecessary.
However, several recrystallization attempts only yield glassy
solids that deny any further characterization, which is probably
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and plentiful kinetically closed pores. In these pores, the pore
space becomes inaccessible to the adsorbate at low temperature
(e.g. 77 K), which is due to the insufficient kinetic energy of
the adsorbate that cannot overcome the potential barrier of the
pore’s aperture. As a result, the equilibrium time for the sorption process in porous materials with kinetically closed pores
is much longer than the experimental time scale and smaller
adsorbate molecules tend to have higher diffusivity and thus
higher uptake amount as well. This explains the hysteresis in
H2 isotherms and the abnormally higher uptake amount of H2
over N2.[8]
Once Cu(pi) was isolated from the solution, it suffers from
the same solubility problem again. In order to reach a smooth
reaction and high yield, the click reaction functionalization was
carried out using the in situ generated Cu(pi) solution. During
the click reaction, the widely-adopted CuSO4/sodium ascorbate catalyst system was replaced by [Cu(CH3CN)4]PF6 to prevent the attacking of the vulnerable Cu(II) paddlewheel cluster
from the reducing reagent (Figure 2a). UV absorption spectra
demonstrate an absorption band shifting of copper nitrate
(starting material) from ∼790 nm to ∼700 nm of the in situ
generated Cu(pi) solution (Figure 1e). The 700 nm absorption
band, a so-called Band (I) in binuclear copper(II) acetate, is a
strong indication of the dicopper paddlewheel cluster, which
comes from the orbitally forbidden Cu d-d transitions and/or
Figure 1. a) Chemical structure of the ligand precursors used for preparing clickable coordination nanocage; b) the clickable coordination
nanocage Cu(pi) formed between pi2- and dicopper paddlewheel cluster;
c) the crystal packing arrangement of Cu(pi) based on cubic closest
packing; d) N2 (black) and H2 (blue) sorption isotherms of Cu(pi) at 77 K
(filled: adsorption; open: desorption); e) UV absorption spectra of copper
nitrate (azure), Cu(pi) (green), Cu(pi)-PEG5k (black), Cu(pi)-PEG5k after
dialysis (red), and Cu(pi)-PEG5k⊃5-FU (blue).
due to the inefficient molecular packing caused by these long
alkane chains. The single crystal of clickable nanocage (referred
to as Cu(pi) hereafter) was finally synthesized using 5-(prop-2ynyloxy)isophthalic acid (H2pi) as the ligand precursor (Figure 1b),
in which a good balance was achieved between solubility and
efficient molecular packing. Single crystal X-ray diffraction
reveals that these discrete nanocages are held together tightly
through van der Waals force and hydrogen bonding interaction
to form a cubic close packing structure (Figure 1c).
Cu(pi) was isolated from the solution and dried under vacuum
to give deep blue powder. N2 and H2 isotherms were collected
at 77 K to check its porosity. Surprisingly, there was scarcely
any N2 uptake, but substantially higher H2 uptake with remarkable hysteresis (Figure 1d).[9] We attribute this anomalous sorption behavior to the formation of “kinetically closed pores”.[10]
Although there are wide openings in Cu(pi) based on the crystal
model (Figure S1), these discrete nanocages tend to move
around during the drying process due to the lack of a strong
holding force, leading to an amorphous structure (Figure S2)
Adv. Mater. 2011, 23, 90–93
Figure 2. a) The scheme of click reaction; b) GPC curves of Cu(pi)-PEG5k
(black) and PEG5k-N3 (blue); c) MALDI MS of Cu(pi)-PEG5k (dash lines
represent the theoretical molecular weight of Cu(pi) grafted with various PEG chains); d) D-F TEM image of Cu(pi)-PEG5k (bar = 50 nm);
e) particle size distribution of Cu(pi)-PEG5k obtained from DLS.
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metal-ligand charge-transfer interactions.[5,11] After the click
reaction, the 700 nm absorption band of the product (referred
to as Cu(pi)-PEG5k hereafter) barely changes, which indicates
the dicopper paddlewheel cluster remains intact during the
click reaction modification.
It is also possible that these nanocages decompose into fragments during the click reaction without the dicopper paddle
cluster being altered. The good solubility of Cu(pi)-PEG5k
prompts us to check whether the nanocage remained intact
using solution-based characterization tools. The nanocage core
has a molecular weight of 6761.04 without solvent being incorporated. Since there are maximal 24 click reaction sites, the
theoretical molecular weight of a single Cu(pi)-PEG5k molecule
should be 6761.04 + 5000∗n (1 ≤ n ≤ 24). Gel permeation
chromatography (GPC) was used to detect its molecular weight
and distribution. As seen in Figure 2b, except for the unreacted
azide-terminated PEG (PEG5k-N3) peak (peak 1, peak molecular
weight: 6538, polydispersity index: 1.04), three new peaks were
easily identified (peak 2, peak molecular weight: 12636; peak 3,
peak molecular weight: 18079; peak 4, peak molecular weight:
26568. Polydispersity indexes are all less than 1.04). Due to the
dendritic shape of Cu(pi)-PEG5k molecule, it is hard to draw
precise molecular weight information from the GPC method,
which is based on random coil model. However, these peaks
with higher molecular weight and narrow molecular weight
distribution do shed light on the presumed graft product.
Compared to GPC, mass spectrometry (MS) can give direct
molecular weight information. In order to prevent the Cu(pi)
core from decomposing, a milder ionization method (matrix
assisted laser desorption ionization, MALDI) was used. From
the MALDI MS data (Figure 2c), we can clearly see the Cu(pi)
core that has been grafted with one, two, three, and four PEG
chains. GPC and MS data strongly support the successful surface functionalization of the nanocage via click reaction.
An image from the dark-field transmission electron microscopy (D-F TEM) (Figure 2d) shows nanoparticles with diameters of around 20 nm, which is consistent with the particle
size distribution data obtained through dynamic light scattering (DLS) (Figure 2e). Energy dispersive spectroscopy (EDS)
analysis of these nanoparticles reveals high copper content
(Figure S3), which is a strong indication of the Cu(pi) nanocage
core. Attempts in getting higher resolution images of these
nanoparticles were unsuccessful due to the easy decomposition
of these nanoparticles under intensified electron beam. Those
dispersed nanoparticles also indicate successful PEG grafting,
otherwise an intensive agglomeration would be expected due to
the strong interaction between each Cu(pi) molecule. However,
these nanoparticles are much larger than an expected single
Cu(pi)-PEG5k molecule (the nanocage core has a diameter of
only around 3 nm), which is due to the intermolecular aggregation caused by insufficient grafting.
It has been well documented that the dicopper paddlewheel
cluster is unstable in aqueous condition due to its lability to
undergo ligand exchange with water.[12] In order to test its water
stability, Cu(pi)-PEG5k was dissolved in water and dialyzed
against water for 24 hours. After being lyophilized, the sample
was redissolved in methanol and UV absorption spectrum
was taken again. As can be seen from Figure 1e, the 700 nm
absorption band in Cu(pi)-PEG5k after dialysis barely changes,
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Figure 3. a) Chemical structure of 5-fluorouracil (5-FU); b) the release
of 5-FU from control (square) and Cu(pi)-PEG5k (circle) (data points are
mean values of duplicates).
indicating the intactness of dicopper paddlewheel cluster and
accordingly the stability of Cu(pi)-PEG5k in water. This high
water stability may come from the outer polymeric shell and
intermolecular aggregation, which protect the dicopper paddlewheel clusters by preventing water molecules from accessing
them.
Given its proven composition and water stability, Cu(pi)PEG5k was used as a carrier for drug release experiment.
5-Fluorouracil (5-FU) is a widely-used anticancer drug
(Figure 3a).[13] It was selected as a drug model in this study due
to its size, which is small enough to be loaded into the cavity
of Cu(pi) core. Loading 5-FU into Cu(pi)-PEG5k was based on
the solubility difference of 5-FU between chloroform and methanol. Based on the UV absorption spectrum (Figure 1e), Cu(pi)PEG5k was not affected by the 5-FU loading. The loading
content was determined to be 4.38 wt%. Pure PEG5k-N3 was
also subjected to the same drug loading procedure to distinguish the polymeric corona’s contribution to the loading content. It turns out that the drug loading content in PEG5K-N3 is
only 0.82 wt%, indicating a much higher drug loading capacity
in the Cu(pi) core.
Drug release experiments were carried out by dialyzing the
drug-loaded Cu(pi)-PEG5k (referred to as Cu(pi)-PEG5k⊃5-FU
hereafter) against PBS buffer solution (pH 7.4) at room temperature. Pure 5-FU was also dialyzed as a control experiment, in which close to 90% of the total drug was released
within 7 hours (Figure 3b). However, for Cu(pi)-PEG5k⊃5-FU,
around 20% of the loaded drug was released during the initial burst release (2 hours). After that, there is a much flatter
release curve up to 24 hours. The initial burst release may
come from the drug that was imbedded within the outer polymeric corona. The rest of the drug, presumably loaded within
the cavity of Cu(pi) core and/or the void between adjacent
Cu(pi) cores, was released very slowly. This slow release may
due to the slow diffusion rate of 5-FU caused by the strong
interaction between Lewis acid sites in Cu(pi) and base site
in 5-FU.
Besides this work, other work has been reported on the biomedical applications of coordination polymers (also known as
metal-organic frameworks, MOFs), especially in drug delivery
and imaging.[14] Compared to their organic polymer counterpart, the uniform yet tunable pore size and geometry in these
organic-inorganic hybrid materials give them new merits as
drug carriers, such as high drug loading capacity and tunable
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the U.S. Department of Energy (DE-FC3607GO17033) and the National Natural Science Foundation of China
(20674069). The microcrystal diffraction of Cu(pi) was carried out
with the assistance of Yu-Sheng Chen at the Advanced Photon Source
on beamline 15ID-B at ChemMatCARS Sector 15, which is principally
supported by the National Science Foundation/Department of Energy
under grant number CHE-0535644. Use of the Advanced Photon Source
was supported by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We
acknowledge Dr. Hansoo Kim in the Microscopy and Imaging Center
(MIC) and Dr. William D. James in the Elemental Analysis Laboratory
at Texas A&M University for their assistance in collecting the TEM
images and determining copper and fluorine content. We acknowledge
the use of Laboratory for Biological Mass Spectrometry at Texas A&M
University. We thank Alejandra Rivas-Cardona and Dr. Daniel Shantz for
their help in the DLS measurements. We appreciate the discussion with
Dr. Jian-Rong Li and Dr. Jie Zhou about nanocage synthesis and drug
release.
Received: August 19, 2010
Published online: October 22, 2010
Adv. Mater. 2011, 23, 90–93
COMMUNICATION
drug release behavior. However, the toxicity of metal ions (Cr3+,
Mn2+, Fe3+, Cu2+, Eu3+, Gd3+, Tb3+, etc.) incorporated within
these materials become a big concern for in vivo applications.
The cytotoxicity measurements of these hybrid materials should
be expected for further applications.
In summary, porous coordination nanocages covered with
alkyne groups were synthesized through judicious selection of
the ligand and reaction condition. The surface functionalization via click reaction with azide-terminated PEG turned them
into water-stable colloids, which showed controlled release of
an anticancer drug 5-fluorouracil. Due to the paramagnetism
of Cu(II) adopted, this PEG-grafted nanocage may have dual
functionality as contrast agent in magnetic resonance imaging
as well.[15] This work opens a window towards post-synthetic
modification of porous coordination nanocages. Further work
will focus on varying geometry and pore size in clickable nanocages and enriching functionality of the polymeric corona
to give the system more control for host-guest chemistry
applications.
[CCDC 777332 contains the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.]
[1] a) M. Yoshizawa, J. K. Klosterman, M. Fujita, Angew. Chem., Int. Ed.
2009, 48, 3418; b) B. H. Northrop, Y. R. Zheng, K. W. Chi, P. J. Stang,
Acc. Chem. Res. 2009, 42, 1554; c) M. D. Pluth, R. G. Bergman,
K. N. Raymond, Acc. Chem. Res. 2009, 42, 1650.
[2] a) M. Eddaoudi, J. Kim, J. B. Wachter, H. K. Chae, M. O’Keeffe,
O. M. Yaghi, J. Am. Chem. Soc. 2001, 123, 4368; b) H. Abourahma,
A. W. Coleman, B. Moulton, B. Rather, P. Shahgaldian,
M. J. Zaworotko, Chem. Commun. 2001, 2380; c) B. Moulton,
J. J. Lu, A. Mondal, M. J. Zaworotko, Chem. Commun. 2001, 863;
d) Y. X. Ke, D. J. Collins, H. C. Zhou, Inorg. Chem. 2005, 44, 4154;
e) H. Furukawa, J. Kim, K. E. Plass, O. M. Yaghi, J. Am. Chem. Soc.
2006, 128, 8398; f) D. J. Tranchemontagne, Z. Ni, M. O’Keeffe,
O. M. Yaghi, Angew. Chem., Int. Ed. 2008, 47, 5136.
[3] a) K. Mohomed, H. Abourahma, M. J. Zaworotko, J. P. Harmon,
Chem. Commun. 2005, 3277; b) H. Furukawa, J. Kim, N. W. Ockwig,
M. O’Keeffe, O. M. Yaghi, J. Am. Chem. Soc. 2008, 130, 11650;
c) M. Jung, H. Kim, K. Baek, K. Kim, Angew. Chem., Int. Ed. 2008,
47, 5755; d) M. Tonigold, J. Hitzbleck, S. Bahnmuller, G. Langstein,
D. Volkmer, Dalton Trans. 2009, 1363; e) J. R. Li, D. J. Timmons,
H. C. Zhou, J. Am. Chem. Soc. 2009, 131, 6368.
[4] a) H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem., Int. Ed.
2001, 40, 2004; b) Y. Goto, H. Sato, S. Shinkai, K. Sada, J. Am.
Chem. Soc. 2008, 130, 14354; c) T. Gadzikwa, G. Lu, C. L. Stern,
S. R. Wilson, J. T. Hupp, S. T. Nguyen, Chem. Commun. 2008, 5493;
d) T. Gadzikwa, O. K. Farha, C. D. Malliakas, M. G. Kanatzidis,
J. T. Hupp, S. T. Nguyen, J. Am. Chem. Soc. 2009, 131, 13613.
[5] R. W. Larsen, G. J. McManus, J. J. Perry, E. Rivera-Otero,
M. J. Zaworotko, Inorg. Chem. 2007, 46, 5904.
[6] D. Zhao, D. Q. Yuan, A. Yakovenko, H. C. Zhou, Chem. Commun.
2010, 46, 4196.
[7] T. Steiner, E. B. Starikov, A. M. Amado, J. J. C. Teixeira-Dias, J. Chem.
Soc., Perkin Trans. 2 1995, 1321.
[8] D. Zhao, D. Q. Yuan, R. Krishna, J. M. van Baten, H. C. Zhou, Chem.
Commun. 2010, 46, 7352.
[9] X. B. Zhao, B. Xiao, A. J. Fletcher, K. M. Thomas, D. Bradshaw,
M. J. Rosseinsky, Science 2004, 306, 1012.
[10] T. X. Nguyen, S. K. Bhatia, J. Phys. Chem. C 2007, 111, 2212.
[11] M. L. Tonnet, S. Yamada, I. G. Ross, Trans. Faraday Soc. 1964, 60, 840.
[12] J. J. Low, A. I. Benin, P. Jakubczak, J. F. Abrahamian, S. A. Faheem,
R. R. Willis, J. Am. Chem. Soc. 2009, 131, 15834.
[13] D. B. Longley, D. P. Harkin, P. G. Johnston, Nat. Rev. Cancer 2003,
3, 330.
[14] a) P. Horcajada, C. Serre, M. Vallet-Regí, M. Sebban, F. Taulelle,
G. Férey, Angew. Chem., Int. Ed. 2006, 45, 5974; b) P. Horcajada,
C. Serre, G. Maurin, N. A. Ramsahye, F. Balas, M. Vallet-Regí,
M. Sebban, F. Taulelle, G. Férey, J. Am. Chem. Soc. 2008, 130, 6774;
c) P. Horcajada, T. Chalati, C. Serre, B. Gillet, C. Sebrie, T. Baati,
J. F. Eubank, D. Heurtaux, P. Clayette, C. Kreuz, J. S. Chang,
Y. K. Hwang, V. Marsaud, P. N. Bories, L. Cynober, S. Gil,
G. Ferey, P. Couvreur, R. Gref, Nat. Mater. 2010, 9, 172;
d) W. J. Rieter, K. M. L. Taylor, H. Y. An, W. L. Lin, W. B. Lin, J. Am.
Chem. Soc. 2006, 128, 9024; e) W. J. Rieter, K. M. L. Taylor, W. B. Lin,
J. Am. Chem. Soc. 2007, 129, 9852; f) K. M. L. Taylor, W. J. Rieter,
W. B. Lin, J. Am. Chem. Soc. 2008, 130, 14358; g) W. J. Rieter, K. M. Pott,
K. M. L. Taylor, W. B. Lin, J. Am. Chem. Soc. 2008, 130, 11584.
[15] D. D. Schwert, J. A. Davies, N. Richardson, Top. Curr. Chem. 2002,
221, 165.
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