Kinetically Controlled Sequential Growth of Surface-Grafted Chiral Supramolecular Copolymers

Article abstract of DOI:10.1002/anie.201601048

We report a facile strategy to grow supramolecular copolymers on Au surfaces by successively exposing a surface-anchored monomer to solutions of oppositely charged peptide comonomers. Charge regulation on the active chain end of the polymer sufficiently slows down the kinetics of the self-assembly process to produce kinetically trapped copolymers at near-neutral pH. We thereby achieve architectural control at three levels: The β-sheet sequences direct the polymerization away from the surface, the height of the supramolecular copolymer brushes is well-controlled by the stepwise nature of the alternating copolymer growth, and 2D spatial resolution is realized by using micropatterned initiating monomers. The programmable nature of the resulting architectures renders this concept attractive for the development of customized biomaterials or chiral interfaces for optoelectronics and sensor applications.

Full text of DOI:10.1002/anie.201601048

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201601048
German Edition: DOI: 10.1002/ange.201601048
Supramolecular Polymer Brushes
Kinetically Controlled Sequential Growth of Surface-Grafted Chiral
Supramolecular Copolymers
+
+
Hendrik Frisch , Eva-Corinna Fritz , Friedrich Stricker, Lars Schmüser, Daniel Spitzer,
Tobias Weidner, Bart Jan Ravoo,* and Pol Besenius*
[
5]
Abstract: We report a facile strategy to grow supramolecular
copolymers on Au surfaces by successively exposing a surface-
anchored monomer to solutions of oppositely charged peptide
comonomers. Charge regulation on the active chain end of the
polymer sufficiently slows down the kinetics of the self-
assembly process to produce kinetically trapped copolymers
at near-neutral pH. We thereby achieve architectural control at
three levels: The b-sheet sequences direct the polymerization
away from the surface, the height of the supramolecular
copolymer brushes is well-controlled by the stepwise nature of
the alternating copolymer growth, and 2D spatial resolution is
realized by using micropatterned initiating monomers. The
programmable nature of the resulting architectures renders this
concept attractive for the development of customized bioma-
terials or chiral interfaces for optoelectronics and sensor
applications.
were used by Kim and co-workers to grow poly(pseudo-
rotaxanes) on a Au surface and by Scherman and co-
[6]
workers to reversibly link polyethylene glycol polymers on
gold. To date, ordered supramolecular polymer brushes have
not been obtained as the kinetically controlled self-assembly
[7]
of supramolecular polymers remains challenging and pre-
vents the dynamic polymerization from being confined to
surfaces. To overcome this limitation, we report a facile
concept that combines charge-regulated self-assembly and
supramolecular copolymerization techniques. This combined
approach passivates the growing chain end of the surface-
anchored polymers, slowing down the self-assembly kinetics
sufficiently to graft copolymers in a stepwise fashion perpen-
dicular to the solid substrate. Using surface plasmon reso-
nance (SPR), we show that the polymerization at near-neutral
pH is additive and can only be reversed under acidic and basic
conditions. Sum frequency generation (SFG) and IR vibra-
tional spectroscopy studies also confirm that the peptidic
hydrogen-bonding sequences direct the polymerization away
from the surface. AFM in water was used to resolve the length
of the surface-bound copolymers as well as the microstruc-
tured 2D patterns of controlled height.
I
nterfacial oligopeptides have sparked tremendous efforts in
the areas of biomimetic materials and polymer chemistry,
enabling one to tailor various surface properties, such as its
wettability, biocompatibility, cell-adhesion, or antifouling
[1]
properties. To date, however, the large majority of surface-
bound covalent polymers that emulate biological function-
ality are based on purely covalent architectures.
We synthesized a set of complementary supramolecular
comonomers, 1, 2, and 3, based on dendritic C -symmetric
[1a,d]
In
3
[
8]
pioneering studies, Gazit and co-workers prepared vertically
aligned supramolecular nanotubes on surfaces by crystalliza-
tion or vapor deposition techniques using hydrophobic
peptide motifs (Figure 1). The incorporation of basic (K)
and acidic (E) amino acids into alternating hydrophilic–
hydrophobic oligophenylalanine sequences results in oppo-
sitely charged complementary comonomers 1 and 2. We have
previously reported on similar dendritic amphiphiles that
[
2]
diphenylalanine. More recently, the Matile group developed
sophisticated synthetic methods, referred to as self-organizing
[3]
surface-initiated polymerization, that rely on the ring open-
ing of cyclic disulfide monomers to produce synthetic photo-
[
4]
systems. Ternary supramolecular cucurbituril complexes
[
+]
[
*] H. Frisch, F. Stricker, D. Spitzer, Prof. Dr. P. Besenius
Institute of Organic Chemistry
Johannes Gutenberg-Universität Mainz
Duesbergweg 10–14, 55128 Mainz (Germany)
E-mail: besenius@uni-mainz.de
[
+]
[+]
H. Frisch, E.-C. Fritz, Prof. Dr. B. J. Ravoo
Organic Chemistry Institute
Westfälische Wilhelms-Universität Münster
Correnstrasse 40, 48149 Münster (Germany)
E-mail: b.j.ravoo@uni-muenster.de
L. Schmüser, Dr. T. Weidner
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
Figure 1. Surface-grafted supramolecular copolymerization and the
molecular structures of the complementary comonomers 1 and 2 and
the initiator 3.
1
002/anie.201601048.
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polymerize in solution to produce one-dimensional alternat-
ing supramolecular copolymers with a nanorod-like morphol-
ogy by a combination of attractive Coulomb interactions,
[9]
hydrogen bonding, and hydrophobic shielding. Further-
more, we prepared a tripodal thioether derivative of the
cationic comonomer 3 that served as a surface-anchored
supramolecular initiator for Au surface substrates.
To determine the isoelectric point of the ampholytic
supramolecular copolymer based on the comonomer pair
1
and 2, we performed pH-dependent polymerization studies
in solution. In pure monomer solutions, self-assembly into
homo-aggregates at acidic and basic pH is observed when the
charges are screened (Supporting Information, Figure S1).
The pH titration curves intersect at pH 5.2 (Figure S2).
Mixing the two monomer solutions at this pH leads to an
instant change in the circular dichroism (CD) spectrum, which
deviates strongly from the linear combination of the spectra
of 1 and 2 (Figure S3). A negative CD band appears at
Figure 2. A) Experimental setup of the SPR measurements, including
the exchange reaction of 3 (red discs) and the copolymerization of
and 2 (green and blue discs). B) SPR sensorgram of the copolymer-
ization (addition of the initiator 3 and the complementary comono-
mers 1 and 2) with subsequent polymer removal by the addition of
acid (pH 2, orange arrow) and base (pH 12, brown arrow). C) IR
spectra of surface-grafted copolymers and the copolymers formed in
solution.
1
[
9,10]
2
15 nm, indicative of the formation of parallel b-sheets.
Negative-stain transmission electron microscopy (TEM)
images further confirm the presence of nanorod-like struc-
tures (Figure S4).
[11]
We postulated that the b-sheet-directed
and charge-
[12]
regulated self-assembly of cationic and anionic comono-
mers in solution could be translated onto solid substrates,
allowing us to graft supramolecular polymers off surfaces. The
multivalent tripodal cationic monomer 3 binds strongly to Au
surfaces via multiple gold–sulfur interactions, and therefore
functions as an initiator for the copolymerization on the solid
support. SPR analysis allowed us to monitor the supramolec-
ular polymerization in real time and to resolve the comono-
mer selectivity as well as the polymerization kinetics. To avoid
non-specific binding to the bare Au surface, all SPR experi-
ments were performed on a triethylene glycol thioether (4)
functionalized self-assembled monolayer (SAM). Control
experiments showed that 1 and 2 have no affinity for surfaces
functionalized with SAMs of 4 (Figure S5). In the first step,
initiator 3 was immobilized by displacing weakly binding 4
indicates that the copolymers are indeed kinetically trapped
and simultaneously highlights the stability of the surface-
bound copolymers. To further confirm that the assemblies are
stabilized by reversible electrostatic interactions, aqueous
solutions of pH 2 and pH 12 were added to the surface.
Switching off the attractive Coulomb interactions leads to
disassembly of the copolymers, and loss of material is
observed after the alternating addition of acid and base
(Figure 2B, orange and brown arrows). These observations
demonstrate that we can achieve kinetically trapped poly-
meric states at near-neutral pH values, but that the structures
remain stimuli-responsive and can be disassembled by
changing the pH.
To investigate the molecular order and secondary struc-
ture of the supramolecular copolymers grafted from the Au
surface, FT-IR spectroscopy experiments were carried out.
We focused on the amide I and II absorption bands, which are
(
1
Figure 2A), causing an increase in the refractive index of
.4 mRIU. Partial displacement of 4 ensures SPR conditions
under which each monomer addition occurs in a non-mass-
limited process, which is necessary to investigate the sequen-
tial addition and polymer growth mechanisms (Figure 2B).
After immobilization of 3, solutions of monomers 1 or 2 were
successively flushed over the surface at identical time
intervals. Each binding event resulted in an instantaneous
stepwise signal increase of 1.2 Æ 0.3 mRIU, and the plateau
observed within 6 min is indicative of the saturation of the
active chain ends of the surface-bound copolymers. Impor-
tantly, the observed plateau does not decrease when the
functionalized surface is washed with buffer, even when the
washing time is extended to one hour. Summarizing these
findings, a stepwise elongation process has been observed by
SPR, and the amount of available binding sites on the growing
supramolecular copolymer chain end remains constant for up
to 15 iterative additions of each monomer (Figure 2B, blue
and green arrows).
[13]
characteristic for hydrogen-bonded secondary structures.
The surface-grafted copolymers show two bands at n˜ =
À1
À1
1638 cm
(amide I; C=O stretching) and n˜ = 1550 cm
(amide II; CN stretching, NH bending) that increase in
intensity with increasing degrees of polymerization (DP =
10, 20, and 30). These bands compare very well with those
observed for copolymers consisting of 1 and 2 that were self-
assembled in solution and then deposited on a Au surface ( n˜ =
À1
1641 and 1548 cm , Figures 2C, S6). This result indicates that
the secondary structure achieved by our grafting from
approach is identical to the one achieved by conventional
solution self-assembly of 1 and 2, and has a very high b-sheet
content. The FT-IR investigations thereby support the con-
clusions drawn from the CD spectra, and, in combination with
the SPR experiments, provide strong evidence that the rather
bulky and rigid phenylalanine-based anionic and cationic
monomers form ordered copolymers that grow perpendicular
to the Au surface.
Critically, upon washing the surface-bound copolymers
with buffer, we did not observe any depolymerization, which
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To demonstrate that our stepwise copolymerization
approach can be used to graft tailor-made materials on Au,
we grew brushes of supramolecular polymers on microstruc-
tured surfaces. Microcontact printing was first applied to
obtain patterned surfaces of tetraethylene glycol thiol (5)
SAMs (Figure 3A). In the next step, the obtained micro-
structured gaps on the Au surface were functionalized with 3
supramolecular copolymer brush and demonstrates the power
of this approach.
We used SFG to glean information about the order within
the assemblies. SFG is a surface-specific vibrational spectros-
copy method where the intensity of the signal is proportional
to the order and density of the interfacial species. The
selection rules of SFG dictate that only ordered species at
interfaces are visible. Disordered ligands or molecules in
[14]
[
17]
solution near the surface are not detected. This allowed us
to record SFG spectra in situ at gold surfaces coated with
initiator 3 while they were successively exposed to solutions of
comonomer 1 and 2. We focused on the C=O glutamic acid
side chain modes to determine the order within the assemblies
because the b-sheet amide resonances observed by IR
[
18]
(
Figure 2C) are SFG inactive. The SFG spectra showed
À1
a pronounced feature near n˜ = 1725 cm , which was assigned
to ordered C=O moieties of the glutamic acid side chain
within 1 (Figure 4A). The polarity of the peak indicates the
[19]
Figure 4. A) SFG spectra showing the C=O side-chain bands for differ-
ent degrees of polymerization (DP). B) Amplitudes of the C=O modes.
C) Absolute SFG amplitudes as a function of the number of 1.
Figure 3. A) Structured polymerization for AFM analysis. B) AFM topo-
graphic images of different degrees of polymerization (DP=10, 20,
3
0) and the corresponding height profiles as a function of the polymer
length.
net orientation of all C=O groups within the stack. The peaks
started out as positive peaks for the trimeric species (DP = 3)
and then became negative. Figure 4B summarizes the SFG
amplitudes as a function of the DP. The reason for the positive
polarity for DP = 3 is likely the bipolar charge distribution
within the trimer. Figure 4C summarizes the absolute ampli-
tudes of the SFG signal for the number n of 1 within the
stacks. A linear fit of the SFG amplitude yields a slope of 1.5/
n. As the SFG signal depends on both the number and order
of the glutamic acid C=O groups, a slope of > 1/n indicates
to obtain line-shaped patterns of the surface-anchored
supramolecular initiator. Note that 3 was anchored directly
onto the Au surface as a free base to ensure dense packing,
[15]
which is crucial to obtain brush-like polymer structures.
The grafting of copolymers by the successive addition of
anionic 1 and cationic 2 was investigated by AFM. We
performed these experiments in phosphate buffer (20 mm,
pH 5.2) to image the material in its hydrated and native
[
16]
[17,20]
state.
As the monomers have no affinity towards the
that the overall order within the assemblies is increasing.
ethylene glycol SAM, the height of the grafted brush and the
length of the copolymer could be determined relative to the
SAM. Topographic images of different degrees of polymer-
ization (DP = 10, 20, and 30) as well as different line patterns
were recorded (Figure 3B, S7). A direct correlation between
the degree of polymerization and the grafted copolymer
height was observed, and a linear regression revealed an
increase in the polymer length of 0.68 Æ 0.05 nm per mono-
mer. This value agrees with our previous molecular dynamics
simulations of copolymers based on alanine derivatives of
It is also interesting to note that the overall C=O signal
increases after each addition of the lysine-containing co-
monomer 2. This result demonstrates that the order within the
surface-confined supramolecular stacks is promoted by inter-
actions between 1 and 2.
In an analogous approach to solid-phase synthesis, we
have herein reported the supramolecular alternating copoly-
merization of oppositely charged peptide-based monomers to
graft chiral polymers off Au surfaces. Superficially, this
strategy is not dissimilar to the layer-by-layer (LBL) depo-
[21]
1
0
and 2 in water, which suggested comonomer distances of
.47 nm. The high fidelity and selectivity of our stepwise
sition of oppositely charged polyelectrolytes into multilay-
[9a]
[7c,22]
ered films.
Our strategy confines the dynamic polymer-
grafting strategy allows us to adjust the height of the ordered
244 www.angewandte.org
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chain end of the supramolecular polymer slows down the
kinetics during the self-assembly process and thus leads to
kinetically trapped assemblies at near-neutral pH. These
assemblies can only be disrupted by screening the charges at
either high or low pH. Our approach allows us to gain
architectural control at three levels:
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for a Liebig (P.B.) and a doctoral (H.F.) fellowship. This work
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