Electrochemically triggered assembly of films: A one-pot morphogen-driven buildup

Article abstract of DOI:10.1002/anie.201007436

Polymers that "click": A polymer film is obtained by the Cu ^I-catalyzed Sharpless click reaction between two polymers, bearing either azide or alkyne groups, both present simultaneously in a Cu^II solution (see picture). The Cu^I morphogen is generated at an electrode by applying an adequate potential. This concept can be extended to supramolecular films formed by coordination complexes. Copyright

Full text of DOI:10.1002/anie.201007436

Communications
DOI: 10.1002/anie.201007436
Film Formation
Electrochemically Triggered Assembly of Films: A One-
Pot Morphogen-Driven Buildup**
Gaulthier Rydzek, Loꢀc Jierry, Audrey Parat, Jean-Sꢁbastien Thomann, Jean-
Claude Voegel, Bernard Senger, Joseph Hemmerlꢁ, Arnaud Ponche, Benoꢂt Frisch,
Pierre Schaaf,* and Fouzia Boulmedais
Angewandte
Chemie
4374
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 4374 –4377

Self-assembly plays a central role in biology[1] and modern
chemistry.[2] Except for self-assembled monolayers,[3] self-
organized structures obtained by mixing interacting species
are mainly observed in the bulk phase. Self-assembly taking
place exclusively on surfaces and leading to films extending
over the monolayer represents a real challenge. The main
difficulty is the spontaneous and rapid interaction between
the molecules as soon as they are mixed in solution. This can
be circumvented by using different strategies, such as step-by-
step deposition[4] or simultaneously spraying of the interacting
species on the substrate.[5] In nature, biomineralization
processes overcome the propensity of the different molecules
to interact spontaneously by heterogeneous nucleation ini-
tiated by specific proteins.[6] At a higher structural level,
formation of complex tissue morphologies is driven by
morphogenetic fields.[7,8] These fields arise from the forma-
tion of gradients through production and diffusion of
morphogens. According to Potter, morphogens are specific
molecules to which cells respond in a concentration-depen-
dent manner.[8] This definition can be extended to the
formation of films on a substrate; the buildup of the film is
triggered by the presence of molecules or ions, the morph-
ogens, generated at or attracted by the substrate. A first
example of morphogenetic self-assembly was recently intro-
duced by Melosh and co-workers.[9] They described a dynamic
polymerization of actin filaments from an electrode surface
through ionic activation. This activation was obtained by
diffusion of Mg2+ ions towards the electrode under the
establishment of an electric double layer.[9]
Here, we introduce a new strategy to form films by
generation of a morphogen at the substrate in the simulta-
neous presence of all the reactants in the solution. We
illustrate our concept through the CuI-catalyzed click reac-
tion.[10] We use two complementary functionalized polymers
present simultaneously in the solution, one bearing azide
groups and the other alkyne groups. CuI ions, which play the
role of morphogens, are produced electrochemically in a
continuous way from CuII ions present in the solution by
application of a potential cycled between À350 and + 600 mV
(versus Ag/AgCl at a scan rate of 50 mVsÀ1).[11] The CuI ions
then diffuse from the surface towards the solution and
catalyze the click reaction. This leads to the continuous
buildup of a film through formation of triazole molecules
between the azide- and alkyne-bearing units at the film/
solution interface (Scheme 1).
[*] G. Rydzek, Dr. L. Jierry, Prof. P. Schaaf, Dr. F. Boulmedais
Institut Charles Sadron (UPR 22)
Centre National de la Recherche Scientifique
23 rue du Loess, 67034 Strasbourg (France)
Fax: (+33)03-88-41-40-99
E-mail: pierre.schaaf@ics-cnrs.unistra.fr
Homepage: www-ics.u-strasbg.fr
Scheme 1. One-pot morphogen-driven formation of films using electro-
chemically controlled click chemistry.
Dr. A. Parat, Dr. J.-C. Voegel, Dr. B. Senger, Dr. J. Hemmerlꢀ
Biomaterials and Tissue Engineering (UMR 977)
Institut National de la Santꢀ et de la Recherche Mꢀdicale
11 rue Humann, 67085 Strasbourg (France)
and
The validity of the concept was tested on several systems
based on functionalized polyanions (poly(acrylic acid), PAA),
polycations (poly(allylamine hydrochloride), PAH), and
neutral polymers (poly(N-hydroxypropylmethacrylamide),
PHPMA). These polymers were modified either with alkyne
Universitꢀ de Strasbourg, Facultꢀ de Chirurgie Dentaire
1 place de l’Hꢁpital, 67000 Strasbourg (France)
Dr. A. Ponche
(PAACꢀC, PAHCꢀC, PHPMACꢀC) or with azide (PAAN3, PAHN3
PHPMAN3) functions grafted at 5% through ethylene oxide
,
Institut de Science des Matꢀriaux de Mulhouse (LRC 7228)
Centre National de la Recherche Scientifique
15 rue Jean Starcky, BP 2488, 68057 Mulhouse (France)
ꢀ
(EO) arms. A bifunctional alkyne spacer HC C-(CH2-
ꢀ
CH2O)3-C CH (CꢀCÀEO3ÀC C) was also used. The films
ꢀ
Prof. P. Schaaf
Ecole Europꢀenne de Chimie, Polymꢂres et Matꢀriaux
Universitꢀ de Strasbourg
25 rue Becquerel, 67087 Strasbourg (France)
and
International Center for Frontier Research in Chemistry
8 allꢀe Gaspard Monge, 67083 Strasbourg (France)
formed from the following solutions were investigated:
PAACꢀC–PAAN3, PHPMACꢀC–PHPMAN3, PAHCꢀC–PAHN3
,
PAACꢀC–PHPMAN3, PHPMACꢀC–PAAN3, and CÀEO3ÀC
–
ꢀ
C
ꢀ
C
PAAN3; the concentration of all polymers was 0.5 mgmLÀ1.
The buildups were monitored by an electrochemical quartz
crystal microbalance (EC-QCM) which we used for both
applying a controlled potential on the gold electrode and
measuring the film thickness. The buildup of the film was
performed under a slight flux (0.1 mLminÀ1) to ensure a
constant concentration of reactants. The polymer formulae
and synthesis are given in the Supporting Information
together with the experimental conditions for the buildup.
Figure 1a shows a typical evolution of the signal for the
PAACꢀC–PAAN3 system. To strongly anchor the film on the
surface, the substrate was precoated with a poly(ethylene
Dr. J.-S. Thomann, Dr. B. Frisch
Laboratoire de Conception et Application de Molꢀcules Bioactives
UMR 7199, CNRS/Universitꢀ de Strasbourg, 74 route du Rhin
67400 Illkirch (France)
[**] G.R. was supported by a fellowship from the “Ministꢂre de la
Recherche et de la Technologie”. This work was supported by the
ANR (grant CLICKMULTILAYER ANR-07-BLAN-0169). We are
grateful to Prof. M. W. Hosseini for fruitful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007436.
Angew. Chem. Int. Ed. 2011, 50, 4374 –4377
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
imine) (PEI) layer. After an initial electrostatic adsorption of
peaks centered at 401 and 405 eV characteristic of azides[12]
are absent from the XPS spectrum. These results provide
strong evidence of a quasiquantitative reaction between the
two functionalized PAA polyanions.
The film morphology was also investigated by atomic
force microscopy (AFM). Figure 1b displays a typical 3D
image and the cross-section profile of a scratched PAACꢀ
–
C
PAAN3 film obtained by application of CV over 120 minutes.
Typical topography images of PAACꢀC–PAAN3 films are given
in Figure S8 in the Supporting Information. The deposited
organic PAACꢀC–PAAN3 layer covers the surface totally with a
thickness of 83 nm at pH 3.5 and is rather smooth as shown by
the surface roughness (7 nm). When this film was put into
contact with an aqueous solution at pH 9 where PAA (pKa
4.5) is fully charged,[13] the film thickness increased to 355 nm
but the surface topography and roughness remained largely
unchanged (Figures S8 and S9 in the Supporting Informa-
tion). A totally reversible swelling/deswelling process was
observed by switching the pH back and forth between 3.5 and
9, proving the film robustness and stability.
Figure 1. a) Evolution of the normalized frequency shift, measured at
15 MHz (n=3) by EC-QCM, of a PAA C–PAAN3 mixture (black line)
ꢀ
C
and nonfunctionalized PAA (gray line) 0.6 mm CuSO4 solution during
the application of a potential cycled between À350 and +600 mV
(versus Ag/AgCl, scan rate of 50 mVsÀ1). After the electrostatic
adsorption of polymers on a PEI-precoated surface, CV was applied as
indicated by the black and gray arrows. b) Typical AFM 3D image
(8 mmꢃ8 mm; contact mode) of a scratched PAA C–PAAN3 film,
ꢀ
C
obtained after 120 min, observed in the liquid state at pH 3.5 and its
cross-section profile.
We also investigated the effect of the CuI concentration
which was varied in two ways: by changing the CuII
concentration in the solution (Figure 2) and by varying the
lowest limit of the applied potential range (Figure S10 in the
Supporting Information). In both cases, reducing the produc-
tion of CuI at the electrode results in thinner films. One can
observe a reduction of the thickness increase as the buildup
progresses. This is not due to a decrease of the CuI generation
during the buildup process (Figure S11 in the Supporting
Information) but might be due to a limited diffusion of CuI
towards the film/solution interface.[11]
the PAA polymers (5 minutes), the application of cyclic
voltammetry (CV) induced a continuous increase in the
absolute value of the normalized frequency shift (ÀDfn/n).
Small oscillations are superimposed on this continuous
increase as a result of the cyclic electrochemical formation
and destruction of a Cu0 layer.[11] When nonfunctionalized
PAA is used in the presence of CuII ions in solution, only the
small oscillations persist when CV is applied (Figure 1a).
Only the initial electrostatic adsorption of PAA polymers on
PEI is observed in the absence of CuII ions (Figure S1 in the
Supporting Information). This proves that the continuous
signal increase in the simultaneous presence of functionalized
PAA and CuII ions is due to a continuous film buildup.
Qualitatively similar evolutions were observed for the other
investigated systems (see Figures S2 to S4 in the Supporting
Information). To remove any trace of Cu ions, we brought the
film in contact with a 0.02m EDTA solution at pH 3.5. After
this treatment, CV (in 2.6 mm NaNO3 solution) showed the
disappearance of the CuII signal without any change in the
film thickness (Figure S5 in the Supporting Information). The
absence of copper traces was confirmed by X-ray photo-
electron spectroscopy (XPS) analysis (Figure S6 in the
Supporting Information).
XPS measurements confirmed the formation of triazole
units resulting from the reaction between azide and alkyne
groups. Indeed two peaks were observed, one centered at
401.9 eV characteristic of the N 1s orbital of the central
nitrogen of the triazole and one centered at 400.1 eV
corresponding to both N 1s orbitals of the two lateral nitrogen
atoms of the triazole units and to the nitrogen atoms of the
amide bonds from functionalized PAA (Figure S7 in the
Supporting Information). The ratio between the areas of
these two peaks is in agreement with an almost complete
reaction between the azide and alkyne groups (see detailed
discussion in the Supporting Information). In addition, the
Figure 2. Evolution of the thickness of the PAA C–PAAN3 films, built
ꢀ
C
*
on PEI-precoated surfaces, obtained in the presence of 0.6 ( ), 0.45
!
!
*
( ), 0.3 ( ), and 0.075 mm ( ) solutions of CuSO4 as a function of
the applied time of the potential cycling between À350 and +600 mV
(versus Ag/AgCl, scan rate of 50 mVsÀ1). The film thickness was
calculated from EC-QCM data (see the Supporting Information), by
subtracting the contribution of the electrostatic adsorption on PEI.
Film formation can also take place through passive
morphogen production. First we deposited a layer of Cu0 on
the electrode from a CuSO4 solution by applying a potential
of À350 mV. After switching off the potential and bringing the
Cu0 layer in contact with a PAACꢀC–PAAN3 solution in the
presence of a 0.6 mm CuSO4 solution, we observed a fast film
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buildup followed by a leveling off (Figure S12 in the
Supporting Information). In contrary, the direct presence of
stabilized CuI ions in the polymer solution, that is, tetrakisa-
cetonitrile CuI triflate, instead of electrochemically generated
CuI ions, did not lead to a film buildup on the substrate
(Figure S13 in the Supporting Information). The localized
generation of morphogens, that is, CuI ions, in the vicinity of
the surface is thus necessary for the film formation and its
concentration has an influence on the thickness of the
obtained film.
semipermeable membrane where the morphogen could
diffuse through; these approaches are to be developed.
Received: November 26, 2010
Revised: March 16, 2011
Published online: April 14, 2011
Keywords: click chemistry · coatings · electrochemistry ·
.
polyelectrolytes · self-assembly
This new concept of film buildup is not restricted to
copper ions and the click reaction but can also be applied to
supramolecular coordination-driven assemblies (Figure S14
in the Supporting Information). A one-pot morphogen-driven
assembly based on PAA-modified terpyridine ligands was
built by coordination complexes with RuII or FeII ions
(Figures S15 and S16 in the Supporting Information). These
morphogen ions were produced electrochemically from
surfaces where Ru0 or Fe0 was deposited before starting the
buildup of the film. This approach is conceptually different
from the electropolymerization process. Electropolymeriza-
tion is restricted to monomer incorporation and is based on
polymerization at the electrode that propagates through
polymeric chains. It is also different from layer-by-layer
multilayer films which are made from repetitive step-by-step
processes.
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systems where the morphogen can be produced electrochemi-
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