Fabrication of highly uniform gel coatings by the conversion of surface-anchored metal-organic frameworks

Article abstract of DOI:10.1021/ja409205s

We report the fabrication of 3D, highly porous, covalently bound polymer films of homogeneous thickness. These surface-bound gels combine the advantages of metal-organic framework (MOF) materials, namely, the enormous flexibility and the large size of the maximum pore structures and, in particular, the possibility to grow them epitaxially on modified substrates, with those of covalently connected gel materials, namely, the absence of metal ions in the deposited material, a robust framework consisting of covalent bonds, and, most importantly, pronounced stability under biological conditions. The conversion of a SURMOF (surface-mounted MOF) yields a surface-grafted gel. These SURGELs can be loaded with bioactive compounds and applied as bioactive coatings and provide a drug-release platform in in vitro cell culture studies.

Full text of DOI:10.1021/ja409205s

Communication
pubs.acs.org/JACS
Fabrication of Highly Uniform Gel Coatings by the Conversion of
Surface-Anchored Metal−Organic Frameworks
†
†
†
†
∥,‡
†
Manuel Tsotsalas, Jinxuan Liu, Beatrix Tettmann, Sylvain Grosjean, Artak Shahnas,
†
⊥
⊥
†
Zhengbang Wang, Carlos Azucena, Matthew Addicoat, Thomas Heine, Joerg Lahann,
†
∥,§
†
,†
Jo
̈
rg Overhage, Stefan Bras
̈
̈
e, Hartmut Gliemann, and Christof Woll*
†
‡
§
Institute of Functional Interfaces (IFG), Soft Matter Synthesis Lab, Institute for Biological Interfaces (IBG), and Institute of
Toxicology and Genetics (ITG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany
∥
Institute for Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
⊥Jacobs University Bremen, Center for Functional Nanomaterials, 28759 Bremen, Germany
*
S Supporting Information
scale with the length of the linker, one can predict that the upper
limit of pore sizes within MOFs can be extended to at least 20
nm, thus also allowing larger proteins to be accommodated as
guests in these lattices.
ABSTRACT: We report the fabrication of 3D, highly
porous, covalently bound polymer films of homogeneous
thickness. These surface-bound gels combine the advan-
tages of metal−organic framework (MOF) materials,
namely, the enormous flexibility and the large size of the
maximum pore structures and, in particular, the possibility
to grow them epitaxially on modified substrates, with those
of covalently connected gel materials, namely, the absence
of metal ions in the deposited material, a robust framework
consisting of covalent bonds, and, most importantly,
pronounced stability under biological conditions. The
conversion of a SURMOF (surface-mounted MOF) yields
a surface-grafted gel. These SURGELs can be loaded with
bioactive compounds and applied as bioactive coatings and
provide a drug-release platform in in vitro cell culture
studies.
Surface-mounted MOFs (SURMOFs) grown using liquid-
phase epitaxy (LPE) on suitably modified substrates are a
particularly interesting class of MOFs with respect to
applications in surface coatings, as required, for example, in cell
culture experiments. The earliest studies on the suitability of
SURMOFs for biological applications revealed major limitations,
including the presence of potentially cytotoxic metal ions and the
pronounced water instability of the MOF types, which are suited
2
1
for the SURMOF coating processes. Although a slow
degradation of matrix materials may be beneficial for certain
22
medical applications (e.g., in the context of drug release ), a
marked increase in the water stability of the basic compound
would be an important improvement. The same argument
applies to the prospect of using MOF coatings for biological (e.g.,
21
5
cell culture substrate) and biomedical (e.g., drug release)
applications.
1
−3
he potential of metal−organic frameworks, MOFs,
a
T
porous material constructed from organic linkers and metal
Herein, we present an alternative approach to the fabrication
23,24
or metal-oxo nodes, goes far beyond gas storage and separation,
the fields where these coordination polymers (PCPs) first found
application. This class of highly porous designer solids is of
pronounced interest also for biological and medical applica-
of metal-free, 3D-structured coatings.
The process to be
described here is based on a covalent cross-linking of MOF
2
5,26
ligands via copper-free click chemistry
and yields robust gels
with pronounced stability under biological conditions. The
surface-grafted gels, SURGELs, obtained by cross-linking the
organic struts within a SURMOF combine the advantages of the
MOF class of materials with that of a covalently connected gel:
they offer enormous variability for introducing functional groups
into the framework while featuring a pronounced stability against
water and the absence of metal ions. In addition, these SURGEL
coatings can be fabricated in the form of well-defined oriented
layers with vertical compositional gradients. Our method is based
4
−7
tions.
In the past 15 years, numerous MOF structures have
been reported, and in particular, the attachment of functional
groups to the organic linkers and to the nodes has provided
unique possibilities to confer specific functions to these
frameworks. MOFs have had their greatest impact in the fields
8
−12
13−16
of catalysis
and sensing,
whereas applications in
The recent success in increasing
1
7,18
biomedicine are emerging.
the maximum pore diameters to values as high as 10 nm within
27
on epitaxially grown metal−organic frameworks deposited on
modified substrates using liquid-phase epitaxy (LPE). SUR-
MOFs resulting from this LPE process are crystalline and highly
metal−organic frameworks has been a major breakthrough for
19
life science applications. Pores of this size may allow larger
biomolecules, such as small proteins, to be embedded within the
framework, thus opening a new dimension for biological and
biomedical applications. A recent theoretical analysis of the
28
29
oriented and have a very low defect density. As is the case for
bulk MOF materials, SURMOFs can be grown from a large
20
stability of a particular large-pore MOF type has shown that for
this isoreticular series the framework stability is largely provided
by direct linker−linker interactions. Because these interactions
Received: September 11, 2013
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
variety of functionalized organic linkers. When using linkers
equipped with azide side groups, postsynthetic modifications
(
PSM) of the frameworks via click chemistry become possible, in
30 31
bulk MOFs as well as in SURMOFs. Here, we use linkers
functionalized with two azide side groups to cross-link the MOFs
as described by Ishiwata et al. for bulk (powder) MOF
2
6
materials.
Figure 1 shows the reaction scheme of bifunctional linker
diazido-stilbenedicarboxylic acid (DA-SBDC) and cross-linker
Figure 2. (a) Schematic representation of the cross-linking process
within the SURMOF-2 structure before the cross-linking reaction (top),
after the cross-linking (middle), and after treatment with EDTA for 30
min (bottom). (b) Corresponding X-ray diffraction (XRD) data and (c)
IRRAS spectra.
Figure 1. Copper-free click reaction between the diazido-stilbenedi-
carboxylic acid (DA-SBDC) and the cross-linker trimethylolethane
tripropiolate and the corresponding schematic representation.
direct evidence of the incorporation of the linker into the
SURMOF framework.
To remove the Cu ions from the framework of the cross-linked
SURMOF, the substrate was immersed in a solution of
ethylenediaminetetraacetic acid (EDTA). As demonstrated in
previous work, the SURMOF-2 series is stable with respect to
trimethylolethane tripropiolate using a copper-free click reaction
and the corresponding schematic representation.
21
distilled water and artificial seawater but not with respect to
solutions containing molecules with a strong coordination
affinity for metal ions. The IRRAS spectrum in Figure 2c of
pristine SURMOF shows carboxylate bands (−COO−, 1610−
When (DA-SBDC)-SURMOF-2 is immersed in a solution
containing an electron-deficient alkyne cross-linker, the sponta-
neous coupling of alkyne groups with the azido groups in the
32
−1
−1
framework takes place at room temperature. This particular
cross-linker contains three alkynyl groups. Because the
neighboring electron-withdrawing groups increase the reactivity
1550 cm for asymmetric vibration and 1420−1300 cm for
symmetric vibration). After immersion of the cross-linked
SURMOF for 30 min in EDTA solution, the carboxylate bands
mostly disappear (indicating that the carboxylate groups are no
longer coordinated to the copper ions), whereas the carbonyl
33
of the alkyne groups, the alkynyl groups react spontaneously
25
with the azide groups, even in the absence of a catalyst. In
previous work, electron-deficient alkyne moieties have found
numerous applications, including surface modification, DNA
modification, gold nanoparticle functionalization, and hydrogel
−1
CO stretching band at 1730 cm and the C−O−C stretching
band at 1250 cm− (both assigned to the carbonyl groups
contained in the cross-linker) are still present and a shoulder at
1
2
5,34−36
−1
cross-linking.
1710 cm appears that is assigned to the carbonyl groups
For the experiments carried out in this study, we prepared a
DA-SBDC)-SURMOF-2 grown on COOH-terminated organo-
contained in the free DA-SBDC linker. (See Supporting
Information Figure S1 for the IRRAS spectrum of the free DA-
SBDC linker.) As demonstrated, the cross-linked SURMOF
shows a pronounced stability against EDTA solution. In contrast,
the DA-SBDC SURMOFs without cross-linking showed rapid
degradation. After only 5 min, no material was left on the
substrate as evidenced by IRRAS (Supporting Information
Figure S2).
(
thiolate surfaces exposed to an organothiolate self-assembled
monolayer (SAM). The X-ray diffraction (XRD) data recorded
for the SURMOFs (Figure 2) revealed the presence of highly
ordered crystalline films, analogous to other variants of the
SURMOF-2 reticular class of MOF materials reported in
2
0
previous work. The observations that the position and relative
intensities of the XRD peaks remained virtually unchanged after
the cross-linking process demonstrated that the positions of the
The presence of an amorphous, surface-grafted film resulting
from the removal of the Cu2+ ions from crystalline MOF was
confirmed by the IRRAS and XRD data shown in Figure 2. This
data was further supported by energy-dispersive X-ray spectros-
copy (EDX) and quartz-crystal microbalance (QCM) data
presented in the Supporting Information (Figures S3−S5).
Taken together, these analytical methods unambiguously
confirmed the presence of a homogeneous, thin gel film on the
surface after cross-linking and removal of the metal ions.
Moreover, EDX data recorded for the SURMOF-2 films after the
cross-linking and after EDTA immersion demonstrated the
31
metal ions contained in the lattice with the largest cross-section
were unaffected by the cross-linking process.
The successful coupling reaction was evidenced by the infrared
reflection−absorption spectroscopy (IRRAS) data (Figure 2c)
where the azido band at 2127 cm− from the DA-SBDC linker
was substantially decreased after the coupling reaction. (Please
note that the −CC− stretching band of the cross-linker also
appears at almost the same position; see Figure S1 for the IRRAS
spectrum of the cross-linker.) The appearance of a carbonyl C
1
−1
2+
O stretching band at 1730 cm and a C−O−C stretching band
at 1250 cm− (both assigned to the carbonyl groups contained in
the cross-linker, see Supporting Information Figure S1) provided
absence of Cu ions. As expected, the corresponding XRD data
1
did not show any diffraction signals, thus demonstrating the
complete loss of crystallinity. On the basis of these observations,
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Journal of the American Chemical Society
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we concluded that the cross-linked metal−organic framework
was converted into an X-ray amorphous polymer gel.
used as a prototype biomolecule and loaded into the SURGEL.
Subsequently, the substrate was exposed to bacterial cells gene-
modified with an arabinose-triggered GFP switch. The induction
of GFP expression proved to be highly site-specific; it occurred
only for those bacteria in direct contact with the SURGEL
substrate.
To demonstrate the generality of our approach, we prepared a
37
second, different type of SURGEL based on a layer-pillar-type
Cu-DA-SBDC-dabco MOF. XRD patterns and IRRAS spectra
recorded for the corresponding SURMOF before and after cross-
linking are shown in the Supporting Information (Figures S6 and
S7). For both SURMOF variants, the flexibility and length of the
three-valent cross-linker was critical because it allowed reactions
to take place within a single layer of the SURMOF structures and
between neighboring layers. Structural models for these
SURMOF structures after the cross-linking are shown in the
Supporting Information (Figure S8). The molecular structure
and lattice parameters of bare and cross-linked SURMOF
For these studies, two different types of gene-modified bacteria
model organisms, E. coli pJN::GFP and P. putida pJN::GFP, were
used. Both can be considered bacterial model organisms of
42,43
significant biomedical and biotechnological importance.
After gene modification, arabinose will induce GFP expression
in these microbes in a highly selective manner. The gfp gene
expression can be detected in a convenient and straightforward
fashion by measuring the fluorescence of the synthesized GFP
protein.
Figure 4 shows the fluorescence microscopy image after 24 h
of incubation of P. putida pJN::GFP adhering to the surface of
38
bilayers were optimized using a local extension to Rappe’s
39
universal force field with the General Utility Lattice Program
4
0,41
(
GULP), version 4.08.
The optimized bilayer had lattice
constants of a = b = 35.3 Å and c = 5.1 Å; the DA-pillared layered
structure had lattice constants of a = b = 35.6 Å and c = 9.1 Å. The
simulated XRD data for these structures agreed well with the
experimental findings (Supporting Information Figures S6 and
S9).
A particular advantage of the SURMOF approach is the
potential for creating patterned substrates in a straightforward
fashion. To this end, a SURMOF was first deposited on a laterally
patterned substrate using microcontact printing. The AFM data
shown in Figure 3 revealed that the height of the corresponding
Figure 4. Schematic representation and fluorescence microscopy images
of P. putida pJN::GFP bacteria after 24 h of incubation in the presence of
SURGEL substrates. Top: bacteria settled on the SURGEL substrate
containing arabinose. Bottom: bacteria settled on an “empty” SURGEL
substrate.
arabinose-loaded SURGEL and the corresponding reference
SURGEL substrate containing no arabinose. (For these
experiments, a pristine SURGEL sample was broken in two
and only one of the two pieces was loaded with arabinose. After
simultaneous incubation, the two pieces were rejoined for image
acquisition.)
For both E. coli pJN::GFP and P. putida pJN::GFP, the
experimental results reveal that only bacterial cells adhering to
the surface of arabinose-loaded SURGEL show GFP fluores-
cence. In contrast, nonadhering bacteria in the broth supernatant
and in bacterial cells adhered to SURGEL surfaces without
previous arabinose loading showed only marginal GFP
fluorescence (fluorescence microscopy images using E. coli
pJN::GFP and the fluorescence corresponding to the bacteria
from the broth supernatant of both bacteria organisms in
Supporting Information Figures S10−S12).
Figure 3. AFM analysis of patterned SURMOF samples: (a) pristine
sample, (b) pristine sample after EDTA solution, (c) SURMOF after
cross-linking, and (d) cross-linked sample after immersion in EDTA
solution for 30 min.
SURMOF structures amounted to about 600 nm. The
conversion to the interlinked SURMOF and removal of the
metal ions using the EDTA solution as described above left the
height of the lines unchanged, as demonstrated by the height
profiles presented in Figure 3.
These surface-mounted gels, referred to as SURGELs,
prepared via cross-linking of a SURMOF precursor layer
would also allow the creation of hierarchically structured gels
with vertical composition gradients. This type of material has not
been accessible via conventional methods but may have a high
potential for advanced applications. In particular, the SURGELs
offer a promising platform for cell culture experiments.
These results demonstrate that arabinose-induced gfp gene
expression is highly localized in space and occurred only at the
water/SURGEL interface. We propose that direct contact
between the adhering cells and the gel substrate is required for
efficient arabinose transport into the bacterial cells. Thus, these
The huge potential of SURGELs as a substrate for cell culture
studies is demonstrated by studying the delivery of biomolecules
to the interior of adhering cells. Arabinose, a small sugar, was
C
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Journal of the American Chemical Society
Communication
data suggest that SURGELs loaded with bioactive molecules can
be used for the specific manipulation of adhering cells. Because
SURGELs also offer the possibility of being loaded with different
biomolecules and to introduce lateral and vertical compositional
gradients, this work opens up a number of applications in
biotechnology, including processes such as the biotransforma-
tion of surface-attached bacterial biofilms with small chemical
compounds or biomedical applications such as spatially localized
delivery of high concentrations of bioactive molecules such as
antibiotics.
In conclusion, our results demonstrate the successful
conversion of a SURMOF into a surface gel, referred to as
SURGEL, possessing pronounced stability. This new process to
fabricate SURGELs represents a versatile strategy for creating
laterally and vertically structured thin films of gel materials with
tailorable properties. Further studies to create more complex
structures including several vertically organized functional
groups are currently ongoing. We also demonstrated the
possibility of loading SURGELs with bioactive compounds.
Thus, SURGELs loaded with bioactive molecules can be used for
the specific manipulation of the cell adhered to the SURGEL
surfaces, which can be useful for biotechnological processes such
as the biotransformation of substrates or biomedical applications
including the spatial delivery of high concentrations of bioactive
molecules, including antibiotics.
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̈
̈
ASSOCIATED CONTENT
Supporting Information
■
̈
*
S
Experimental procedures, computational details, characterization
of prepared materials, and details about the biological samples.
This material is available free of charge via the Internet at http://
pubs.acs.org.
̈
(
(
(
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AUTHOR INFORMATION
Corresponding Author
christof.woell@kit.edu
■
(
(
Author Contributions
Tolan, M.; Sternemann, C.; Evers, F.; Zacher, D.; Fischer, R. A.; Wo
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The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of the
manuscript. Manuel Tsotsalas and Jinxuan Liu contributed
equally.
(
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Notes
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