Reactive polymer coatings that "Click".

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Bioactive Surfaces
DOI: 10.1002/anie.200600357
Reactive Polymer Coatings that “Click”**
Himabindu Nandivada, Hsien-Yeh Chen,
Lidija Bondarenko, and Joerg Lahann*
Future advances in the design of biologically active interfaces
require novel strategies for the robust and specific attachment
of biological ligands onto surfaces.[1]Herein, a new type of
biofunctional surface based on alkyne-containing vapor-
deposited polymer coatings is reported. These reactive coat-
ings are applicable to a wide range of substrates and can be
modified by subsequent spatially directed “click chemistry”.
Click chemistry represents a family of powerful and efficient
chemical reactions, which are modular, widely applicable,
relatively insensitive to solvents and pH value, while resulting
in stereoselective conversions with high to very high yields.[2]
The most widely used click reaction is the Huisgen 1,3-dipolar
cycloaddition between azides and terminal alkynes.[3]This
coupling reaction has been employed for drug discovery
applications[4]and for the target-guided synthesis of enzyme-
inhibitors.[5,6]Moreover, Huisgen 1,3-dipolar cycloadditions
of azide- and alkyne-functionalized self-assembled monolay-
ers (SAMs) have been used as a versatile tool for tailoring
surface functionalities.[7–11]Their bioorthogonality, that is, the
dependence on proximity and proper alignment of the
reactants make click reactions promising candidates for
biointerface design.[12]However, further use is currently
hampered by the availability of polymer coatings that can
undergo surface-directed dipolar cycloadditions. In the past,
chemical vapor deposition (CVD) polymerization of substi-
tuted [2,2]paracyclophanes has been instrumental in creating
a wide array of functionalized poly-(p-xylylenes) with a
diverse class of functional groups, such as amines,[13,14]
esters,[15–17]aldehydes,[18]and alcohols,[19–21]which enable the
immobilization of biomolecules.
To further extend the concept of click-chemistry-based
immobilization, we use CVD polymerization for synthesizing
alkyne-containing polymer coatings (Scheme 1). Moreover,
we demonstrate the applicability of these reactive coatings by
[*] H. Nandivada, H.-Y. Chen, Dr. L. Bondarenko, Prof. J. Lahann
Department of Chemical Engineering
University of Michigan
Ann Arbor, MI-48109 (USA)
Fax: (+1)734-764-7453
E-mail: lahann@umich.edu
[**] J.L. gratefully acknowledges support from the NSF in form of a
CAREER grant (DMR-0449462) and funding from the NSFunder the
MRI program (DMR-0420785). We thank Prof. Larson, University of
Michigan, for use of the fluorescence microscope and Prof. J. Kim,
Materials Science and Engineering, University of Michigan, for use
of the spectrofluorometer.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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conducting spatially directed Huisgen 1,3-dipolar cycloaddi-
tion on a surface.
Prior to polymer deposition using CVD polymerization,
the starting materials diethynyl[2,2]paracyclophane (2a) and
ethynyl[2,2]paracyclophane (2b) were prepared from the
commercially available [2.2]paracyclophane (1), according to
a synthesis recently reported by Hopf and co-workers.[22]
Under these conditions, pseudo-ortho and pseudo-meta
derivatives have been reported as the major isomers,[22]but
in this case they were not separated for subsequent CVD
polymerization.
First, we attempted the CVD polymerization of the
dialkyne 2a, which was expected to yield poly(diethynyl-p-
xylylene) (3a). For this purpose, 2a was sublimed at 90–1108 8C
and a reduced pressure of 0.5 mbar. The reactants were then
transported into pyrolysis chamber (6508 8C) and then into a
deposition chamber (158 8C), where the spontaneous forma-
tion of a polymer film was observed. However, the FTIR
spectrum of polymer films formed under these conditions did
not show the band for the alkyne C?H stretch around
3200 cm?1, indicating the absence of alkyne groups. Instead,
several side products were formed, which were not further
characterized, but could potentially be due to an alkyne–
vinylidine rearrangement.[23]Alteration of process conditions,
(e.g., pyrolysis temperatures below 5508 8C), resulted in
alkyne-functionalized polymers, with typical ellipsometric
thicknesses of about 50 nm (for 50 mg of the precursor
polymerized onto a 4-inch wafer). Nevertheless, these poly-
mer films showed little reactivity, underwent thermal decom-
position, and generally had quite poor stability towards
organic solvents (see Supporting Information). Thus, they
were not pursued further.
Instead, we shifted our focus to the CVD polymerization
of the monoalkyne 2b. In sharp contrast to the dialkyne-
containing polymer 3a, poly(4-ethynyl-p-xylylene-co-p-xyly-
lene) (3b) was prepared without appreciable side reactions,
even under very typical CVD conditions[20](pressure of
0.5 mbar, sublimation, pyrolysis, and substrate temperatures
of 90–1108 8C, 6808 8C, and 158 8C, respectively). Moreover, the
FTIR spectrum of 3b revealed a strong band at 3286 cm?1for
the alkyne C?H stretch and a signal at 2100 cm?1, which can
be attributed to the carbon–carbon triple bond. Evidence
from the FTIR data was reaffirmed by X-ray photoelectron
spectroscopy (XPS), which was used to quantify the surface
elemental composition of 3b. The analysis revealed that 3b
contained 1.3% oxygen, which may be due to contaminations
during CVD polymerization or subsequent sample handling.
The high resolution C1s spectrum of 3b further revealed a
symmetric and narrow peak centered at 285.6 eV with a full
width at half maximum (FWHM) of 1.13 eV. This peak is
associated exclusively with the presence of carbon that is
bound to carbon or hydrogen.[24]The C1s peak spectrum
further showed a smaller signal centered at 291.7 eV, which
can be attributed to a p–p* shake-up signal characteristic of
aromatic p electrons. Thissignalhas beenobserved forsimilar
polymer systems.[19]For polymer 3b, ellipsometry gave a
thickness of 91.81?0.03 nm (for 50 mg of precursor); 3b is
stable in aqueous solutions and organic solvents such as
acetone, ethanol, methanol, and chloroform. Probing the
adhesiveness of 3b using the scotch tape test[18]showed that
the film had good adhesion to a wide variety of substrates
such as glass, poly(dimethylsiloxane) (PDMS), silicon, and
gold.
A spectrofluorometer study of the polymer 3b showed a
characteristic excitation peak at 380 nm and a characteristic
emission peak at 450 nm. These peaks disappeared after
heating the polymer to 1508 8C for 3 h, presumably as a result
of cross-linking of the polymer (see Supporting Information).
To assess the thermal stability of polymer 3b, we compared
the FTIR spectra of samples that were stored at 208 8C, 808 8C,
1508 8C, and 2508 8C, respectively. The C?H stretch at 3283 cm?1
continuously decreased with increasing temperature and was
absent in samples stored at 2508 8C (Supporting Information).
Again, this result suggests that the polymer has limited
thermal stability, most likely arising from cross-linking of the
ethynyl groups.
To assess whether the reactive coating 3b can be used for
heterogeneous click reactions, its reactivity against azides was
studied. Specifically, the Huisgen 1,3-dipolar cycloaddition
between 3b and an azide-containing biotin-based ligand 4 in
the presence of copper(ii) sulfate and sodium ascorbate was
examined (Scheme 2). As described for solvent-based sys-
tems, this coupling reaction yielded triazoles.[25]Sodium
Scheme 1. Synthesis of alkyne-containing polymers by CVD polymeri-
zation of diethynyl[2,2]paracyclophane (2a) and 4-ethynyl-
[2,2]paracyclophane (2b).
Scheme 2. Huisgen 1,3-dipolar cycloaddition between biotin-based
azide-ligand 4 and the polymer 3b.
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ascorbate acts as a reductant, generating Cuiions in situ,
which then function as the catalyst.[25]The biotin azide 4 was
chosen as the representative ligand in this study, because
biotin forms a strong noncovalent interaction with streptavi-
din (which has been widely used for binding biotinylated
biomolecules).[15]
To ensure spatial control over the cycloaddition reaction,
a microcontact printing (mCP) approach was chosen. For this
approach to be successful, the Cuicatalyst and the reductant
have to be used independently of each other. Therefore a thin
layer of 4 and sodium ascorbate was spread onto a film of 3b
and dried using N2. Next, a patterned PDMS stamp was inked
with a solution of CuSO4and kept in contact with the
substrate for 12–18 h (Figure1). After rinsing, the patterned
substrate was incubated with an aqueous solution of rhod-
amine-labeled streptavidin. Fluorescence microscopy was
used to assess the immobilization of 4 onto polymer 3b. The
fluorescence micrographs shown in Figure 2a,c confirm
selective protein coupling in the regions where the CuSO4
solution was microcontact printed, thus demonstrating the
spatially directed binding of 4 to polymer 3b. This result
shows that the alkyne groups on the polymer surface are
reactive and can be effectively used as anchoring sites for
surface modifications. The two-step approach was found to be
superior to the concurrent microcontact printing of catalyst
and azide.
To complement the fluorescence study, patterned surfaces
were further analyzed by imaging ellipsometry (Figure 2b
and d) which revealed protein patterns, which are inline with
the corresponding fluorescence patterns. The observed thick-
ness differences of about 1–2 nm between the biotinylated
and the non-biotinylated regions after incubation with
streptavidin are comparable with the thickness of a protein
monolayer.[26]
In conclusion, the alkyne-containing polymer 3b has been
found to show remarkable reactivity towards azides through
the chemoselective Huisgen 1,3-dipolar cycloaddition. In
contrast to the dialkyne containing polymer 3a, reactive
coating 3b showed excellent adhesion and stability at
elevated temperatures and in solvents. Further work will be
directed towards the elucidation of the CVD polymerization
mechanism as well as its scope with respect to different
cycloadditions. The development of bioactive surfaces is an
important step towards advanced biomaterials and biointer-
faces. Our regioselective immobilization strategy could be
applicable in the design of biofunctional surfaces for diag-
nostics (e.g. microarrays), biosensors, and biomedical device
coatings.
Experimental Section
CVD: 2a (50 mg) was sublimedat 90–1108 8C and 0.5 mbar and carried
into the pyrolysis chamber by argon at a flow rate of 20 sccm. After
pyrolysis (at different pyrolysis temperatures Tpyr), the polymer was
Figure 1. Immobilization of azide-containing ligand on the reactive polymer
coating 3b. Subsequently the CuSO4catalyst precursor is microcontact
printed on a preadsorbed layer of biotin azide ligand 4 on the reactive
polymer 3b.
Figure 2. a),c) Fluorescence micrographs showing the binding of
TRITC-streptavidin to patterns of biotin azide, b),d) Corresponding
ellipsometric images for determining layer thickness. TRITC=tetrame-
thylrhodaminisothiocyanate, PDMS=poly(methylsiloxane).
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deposited on the substrate at 158 8C. Tpyr=6508 8C. IR (grazing angle of
858 8): n ˜ =837, 1039, 1150, 1439, 1505, 1624, 1694, 1910, 2916, 3010,
3048 cm?1. XPS signal (%; referenced to aliphatic C at 285.0 eV): C
96.9, O 3.1; Tpyr=5508 8C. IR (grazing angle of 858 8): n ˜ =839.35, 877,
922, 1036, 1261, 1439, 1623, 1913, 2100, 2848, 2921, 3013, 3052,
3285 cm?1. XPS signal (%; referenced to aliphatic C at 285.0 eV): C
93.7, O 6.3.
2b: CVD as described above. IR (grazing angle of 858 8): n ˜ =833,
894, 1158, 1251, 1411, 1454, 1493, 1513, 1605, 1699, 1900, 2102, 2859,
2926, 3015, 3286 cm?1; XPS signal (%; referenced to aliphatic Cat
285.0 eV): C 98.7, O 1.3 ; XPS signals: 285.6 eV (C1s); 291.7 eV (p–
p*).
Surface Characterization: IR spectroscopy: Nicolet 6700 spec-
trometer. XPS elemental analyses: Axis Ultra X-ray photoelectron
spectrometer (Kratos Analyticals, UK) equipped with a monochrom-
atized AlKaX-ray source.
Height analysis data were recorded using an EP3-SW imaging
ellipsometer (Nanofilm AG, Germany) at a wavelength of 532 nm.
Both, nulling (four zones) and mapping experiments were performed
at an angle of incidence of 658 8. An anisotropic Cauchy parameter-
ization model was used for curve fitting. For the mapping mode, data
was recorded by an imaging scanner with a lateral resolution of 1 mm
with a field of view of about 100 mm?500 mm.
Patterning of 4 on polymer films of 3b: Patterned PDMS stamps
were created as described elsewhere.[14]A thin layer of solution of
ligand 4 (Photoprobe biotin, Vector labs, 10 mgmL?1) and sodium
ascorbate (1 mm) in a 2:1 mixture of water and tert-butyl alcohol was
spread on 3b and dried using N2. The patterned PDMS stamp was
oxidized for 20 min using UV-ozone cleaner (Jelight) and inked with
CuSO4solution (1 mm in methanol) adopting an approach reported
by Abbott et al.[27]. The stamp was then kept in contact with the
polymer substrate for 12–18 h. After stamp removal, the patterned
substratewasincubatedwith
(50 mgmL?1in aqueous phosphate buffer PBS containing 0.02%
(v/v) Tween 20 and 0.1% (w/v) bovine serum albumin (BSA)) for 1 h.
The substrate was then repeatedly washed with the incubating buffer,
PBS and finally rinsed with deionized water. The fluorescence
micrographs were captured using a Nikon TE200 fluorescence
microscope.
rhodamine-labeledstreptavidin
Received: January 26, 2006
Published online: April 19, 2006
.
immobilization · reactive coatings
Keywords: alkynes·chemicalvapordeposition·clickchemistry·
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