Electrochemically-deposited benzophenone moieties: Precursors for dual mode patterning of polymer brushes on conducting surfaces

Article abstract of DOI:10.1039/c0cc04046k

A new method for patterning polymer brushes on conducting surfaces via electro-deposited benzophenone moieties is reported. The dual patterning capability of the technique was demonstrated and characterized via the AFM and for the first time, an IR-imagin

Full text of DOI:10.1039/c0cc04046k

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Electrochemically-deposited benzophenone moieties: precursors for dual
mode patterning of polymer brushes on conducting surfacesw
Maria Celeste Tria, Jin Young Park and Rigoberto Advincula*
Received 24th September 2010, Accepted 14th November 2010
DOI: 10.1039/c0cc04046k
A new method for patterning polymer brushes on conducting
surfaces via electro-deposited benzophenone moieties is
reported. The dual patterning capability of the technique was
demonstrated and characterized via the AFM and for the first
time, an IR-imaging technique for a patterned brush.
on any conducting surfaces by using only a single photo-
crosslinker, which would avoid extensive synthetic efforts of
various initiators or anchoring groups specific for different
electrodes.
Scheme 1 depicts the dual patterning routes of the method
presented. The process was used on a patterned conducting
surface, in this case a patterned ITO (see ESIw for the detailed
preparation procedure), where 4-benzoylphenyl-3,5-bis(4-(9H-
carbazol-9-yl)butoxy)benzoate (CbzBP) was first selectively
electro-patterned on the conducting regions of the surface. A
polymer thin film was spin-coated and irradiated with UV
light after which it was rinsed with an appropriate solvent to
remove the unbound polymer, creating the patterns on the
conducting surface. On the other hand, a polymer thin film
was also spin-coated on a plain conducting substrate with
electrodeposited CbzBP. A photo mask was employed upon
UV irradiation of the film to selectively graft the polymer only
on the exposed areas. The unadsorbed polymers were rinsed
off to form the patterns on the surface.
Patterning of surfaces with polymeric materials has been of
high interest because of its vast applications in different
areas such as biomaterials, micro/nanofluidic systems, micro-
electromechanical devices (MEMS), and photonic crystal
materials.1 Over the past decade, research efforts were geared
toward developing strategies of fabricating patterns on
surfaces, which includes microcontact printing,2 photo-
lithography,3 and electron beam lithography.4 Conventionally,
patterned polymer brushes are produced from polymers grown
from patterned initiators that are lithographically printed on
surfaces. Several non-conventional methods have also been
presented to provide an alternative way to the traditional
technique such as capillary force lithography,5 photoetching,6
and electro-oxidation method.7 Despite the success of these
approaches in producing patterned surfaces, most of these
techniques either necessitate lengthy or complex steps or could
only be applicable to specific surfaces by self-assembled
monolayers (SAM), which require synthesis of several types
of initiators (e.g. thiol or silane initiators) depending on the
substrate being employed.
The electro-active benzophenone molecule (CbzBP) was
synthesized via coupling of 4-hydroxybenzophenone with the
To address these issues, we present a simple yet versatile
route of patterning polymer brushes by using electro-
chemically-deposited benzophenone moieties that could either
be electrochemically or photochemically patterned on the
surface. The dual patterning capability of the method relies
on the presence of both an electro-active and photoactive
functional group in the molecule of interest. Benzophenone
is well known for its photo reactivity and its ability to attach
C–H bonds in a wide range of chemical environments.8
Carbazole, on the other hand, is known to be electro-
chemically active and can be electrodeposited on a wide range
of conducting surfaces by either potentiodynamic or potentio-
static methods.9 This approach offers an advantage over other
techniques for patterning surfaces because of its applicability
Department of Chemistry and Department of Chemical and
Biomolecular Engineering, University of Houston, TX 77204-5003,
USA. E-mail: radvincula@uh.edu; Fax: +1 713-743-1755;
Tel: +1 713-743-1760
w Electronic supplementary information (ESI) available: Materials
and surface characterizations, synthesis of the electro-active photo-
crosslinker, cyclic voltammograms of the CbzBP and benzophenone,
UV-vis data for the uncrosslinked and crosslinked CBz-BP, XPS
survey spectra of the films and IR-imaging of the pattern focused at
the CQO region. See DOI: 10.1039/c0cc04046k
Scheme 1 The preparation of patterned polymer brushes using the
electrochemically-active benzophenone moiety via electropatterning
(Route 1) and photopatterning (Route 2).
c
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Chem. Commun., 2011, 47, 2393–2395 2393

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synthesized carboxylic acid carbazole moiety that was developed
in our lab and is published elsewhere.10 The CbzBP was
initially deposited through potentiodynamic method on a
plain ITO substrate, which was done with a scan rate of
50 mV sÀ1 at the potential range between 0 to 1.3 V
(Fig. S1, ESIw). The stability of the benzophenone group
during electro-deposition is one of the main concerns for the
succeeding photo-grafting step. To verify this, a CV run was
made for pure benzophenone within this potential window.
The cyclic voltammogram showed no redox peaks between
0 to 1.3 V, signifying that the benzophenone is unaffected at
this potential region (Fig. S2, ESIw). This also indicated that
the redox peaks found in the CV diagram of CbzBP is solely
attributed to the carbazole moieties that are being cross-linked
during the electrodeposition process. The electrochemical
cross-linking of the carbazole moieties was further supported
by the UV-vis analysis before and after the electrodeposition
of the CbzBP on the ITO substrate (Fig. S3, ESIw). The
presence of the broad maxima at around 400 nm and
800 nm for the electro-generated CbzBP which are assigned
to the dicarbazyl radical cation and dicarbazyl dication,
respectively,11 signifies the successful cross-linking of the
carbazole moieties. On the other hand, these peaks are
not observed in the control system where CbzBP was just
spin-coated on the ITO surface.
after photo-grafting of the polymer showed the presence of all
the expected elements in the CbzBP and the PS (Fig. S5, ESIw).
High-resolution scans of the films before and after photo-
crosslinking of PS showed an increase in the peak intensity of
C 1s attributed to the grafting of the carbon-rich polymeric
chain (Fig. 1a) and a decrease in the N 1s intensity due to the
coverage of the CbzBP layer by the grafted PS on the surface
(Fig. 1b). ATR-IR spectrum of the grafted polystyrene showed
the expected peaks at 3034 cmÀ1, 2932 cmÀ1, 1600 cmÀ1
,
1500 cmÀ1, 1456 cmÀ1, and 850 cmÀ1 corresponding to the
aromatic C–H stretch, aliphatic C–H stretch of the polymer
backbone, 2 bands for CQC (in ring) stretches, C–C aromatic
stretch (in ring), and C–H bending and ring puckering,
respectively. All these steps were also done on Au substrate
to monitor the thicknesses of the electrodeposited CbzBP
and the final photografted polymer, which was found to be
2.93 nm and B6.5 nm, respectively.
After the confirmation of the technique on unpatterned
surfaces, the whole process was then applied to patterning
the polymer brush on conducting substrates. Fig. 2 shows the
atomic force microscopy (AFM) images of the patterned
polymer brushes fabricated using the two routes shown in
Scheme 1. As expected, the polymer only selectively attached
to the regions where the CbzBP is electropatterned for the first
route (Fig. 2a) and to the exposed areas under UV light in the
case of the second route (Fig. 2c). This selective polymer
grafting is even emphasized on the 3D image of the AFM
scans where the height is higher on areas where the PS is
expected to be grafted (Fig. 2b and d).
A thick film of polystyrene (PS) (MW 250K) was spin-coated
on the electro-generated CzBP film and was subsequently
irradiated with UV for an hour to photo-graft the polymer
on the surface. One hour of irradiation time was necessary
to reach the maximum thickness based on prior kinetic
measurements made by plotting the irradiation time versus
thickness (Fig. S4, ESIw). This result was also in corroboration
with the one reported by Frank and co-workers for PS
(MW 233K).8a The surface modification was monitored and
verified by X-ray photoelectron spectroscopy (XPS) and
attenuated total reflection-infrared (ATR-IR) spectroscopy
as shown in Fig. 1. XPS survey scans of the films before and
FT-IR imaging (Varian) with a focal plane array (FPA)
detector of the grafted PS brush was also conducted to give
information about the chemical composition of the patterns
produced. This method serves as a powerful tool for patterned
surfaces that could not be chemically characterized by AFM,
optical microscope or SEM, which are the traditional methods
for imaging patterned materials on surfaces. To our knowledge,
this is the first report for the use of this technique to chemically
Fig. 1 XPS spectra of the film before and after photo-crosslinking
of the PS for (a) C 1s and (b) N 1s. (c) ATR-IR spectrum of the
photo-grafted PS film.
Fig. 2 2D and 3D AFM images of (a, b) electropatterning route and
(c, d) photopatterning route of the grafted PS brush.
c
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to pattern PS brushes using two different routes, namely
electropatterning and photopatterning as observed in the
AFM and IR-imaging analysis. The presented method is
promising in that it could extend the scope on other conducting
electrodes that are viable for semiconductor processing set-ups
and the biomedical device field.
The authors acknowledge funding from NSF DMR-
10-06776, ARRA-CBET-0854979, CHE-10-41300, Texas
NHARP 01846, and Robert A. Welch Foundation, E-1551.
Technical support from Agilent Technologies and Optrel is
also acknowledged. The authors also thank Ms Hanae
Ohtsuka of Tokyo University of Agriculture and Technology
for help in the preparation of some films.
Notes and references
1 (a) F. J. Xu, K. G. Neoh and E. T. Kang, Prog. Polym. Sci., 2009,
34, 719–761; (b) Z. Nie and E. Kumacheva, Nat. Mater., 2008, 7,
277–290; (c) M. Geissler and Y. Xia, Adv. Mater., 2004, 16,
1249–1269.
2 (a) T. Chen, D. P. Chang and S. Zauscher, Small, 2010, 6,
1504–1508; (b) D. M. Jones, J. R. Smith, W. T. S. Huck and
C. Alexander, Adv. Mater., 2002, 14, 1130–1134.
Fig. 3 2D and 3D IR-imaging frames for the (a, b) electropatterned
and (c, d) photopatterned PS brush. Scale bar is equivalent to 30 mm.
3 (a) R. Konradi and J. Ruhe, Langmuir, 2006, 22, 8571–8575;
(b) O. Prucker, M. Schimmel, G. Tovar, W. Knoll and J. Ruhe,
Adv. Mater., 1998, 10, 1073–1077; (c) F. Zhou, L. Jiang, W. Liu
and Q. Xue, Macromol. Rapid Commun., 2004, 25, 1979–1983.
4 (a) A. Rastogi, M. Paik, M. Tanaka and C. Ober, ACS Nano, 2010,
4, 771–780; (b) S. J. Ahn, M. Kaholek, W. Lee, B. LaMattina,
T. LaBean and S. Zauscher, Adv. Mater., 2004, 16, 2141–2145;
(c) B. Brough, K. L. Christman, T. S. Wong, C. M. Kolodziej,
J. G. Forbes, K. Wang, H. D. Maynard and C. M. Ho, Soft
Matter, 2007, 3, 541–546.
5 (a) Y. Liu, V. Klep and I. Luzinov, J. Am. Chem. Soc., 2006, 128,
8106–8107; (b) K. Y. Suh, M. C. Park and P. Kim, Adv. Funct.
Mater., 2009, 19, 2699–2712.
6 M. Husemann, M. Morrison, D. Benoit, J. Frommer, C. M. Mate,
W. Hinsberg, J. L. Hedrick and C. J. Hawker, J. Am. Chem. Soc.,
2000, 122, 1844–1845.
7 C. R. Becer, C. Haensch, S. Hoeppener and U. S. Schubert, Small,
2007, 3, 220–225.
8 (a) O. Prucker, C. Naumann, J. Ruhe, W. Knoll and C. Frank,
J. Am. Chem. Soc., 1999, 121, 8766–8770; (b) K. Horie, H. Ando
and I. Mita, Macromolecules, 1987, 20, 54–58.
9 (a) P. Taranekar, A. Baba, T. Fulghum and R. Advincula,
Macromolecules, 2005, 38, 3679–3687; (b) A. S. Sarac, M. Ates,
E. A. Parlak and E. F. Turcu, J. Electrochem. Soc., 2007, 154,
D283–D291; (c) C. Xia, R. Advincula, A. Baba and W. Knoll,
Chem. Mater., 2004, 16, 2852–2856.
map out patterned brushes on surfaces. Fig. 3 shows the IR
image of the patterned films on the surface using the two
routes presented. The images are focused at the aromatic C–H
stretch frequency of the film (3034 cmÀ1), which is the
signature functional group of the PS. The green color intensity
is where the high concentration of this frequency is present,
signifying the selective grafting of PS on that region, depending
on the patterning method employed (i.e. on the boundaries for
the electropatterned surface and on the squares for the photo-
patterned area). These patterns were not observed upon
scanning on other frequencies for functionalities that are not
present in the PS brush (e.g. 1680–1800 cmÀ1 for CQO
stretch, see Fig. S6 (ESIw)), which further proved the power
of IR-imaging to chemically map the functionalities of the
grafted brush on the surface.
In conclusion, a new, simple and versatile method of
patterning brushes on conducting surfaces via the use of
electro-deposited photoactive moiety was demonstrated. The
patterning was made suitable for conducting surfaces such as
ITO and Au with the use of only one electro-active
photocrosslinker, which avoids synthesis of different photo-
crosslinkers with functionalities specific for each substrate.
The bifunctionality of the CbzBP also made it possible
10 C. Kaewtong, G. Jiang, M. J. Felipe, B. Pulpoka and
R. Advincula, ACS Nano, 2008, 2, 1533–1542.
11 J. F. Ambrose and R. F. Nelson, J. Electrochem. Soc., 1968, 115,
1159–1164.
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