Mussel-inspired, perfluorinated polydopamine for self-cleaning coating on various substrates

Article abstract of DOI:10.1039/c4cc02775b

We designed a perfluorinated dopamine derivative, which, upon oxidative polymerization, formed a structurally rough film of extremely low surface energy on various substrates. The static water contact angles larger than 150° and the low water sliding angl

Full text of DOI:10.1039/c4cc02775b

Collaborative research between Prof. Woo Kyung Cho
(Department of Chemistry, Chungnam National University,
Korea) and a research team of the Center for Cell-Encapsulation
Research (Korea)
As featured in:
Mussel-inspired, perfluorinated polydopamine for self-cleaning
coating on various substrates
Two inspirations: Lotus leaf-inspired, superhydrophobic coating
with self-cleaning property is achieved on various substrates
(from left: gold, glass, PDMS, PET, TiO2, Zn foil, and V foil) by
oxidative polymerization of a mussel-inspired, perfluorinated
dopamine derivative. The photos were taken at and near
Chungnam National University by Daewha Hong, the graphic was
designed by Ji Hun Park.
See Insung S. Choi,
Woo Kyung Cho et al.,
Chem. Commun., 2014, 50, 11649.
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Mussel-inspired, perfluorinated polydopamine for
self-cleaning coating on various substrates†
Cite this: Chem. Commun., 2014,
50, 11649
Daewha Hong,‡a KiEun Bae,‡a Seok-Pyo Hong,a Ji Hun Park,a Insung S. Choi*a and
Woo Kyung Cho*b
Received 15th April 2014,
Accepted 5th June 2014
DOI: 10.1039/c4cc02775b
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We designed a perfluorinated dopamine derivative, which, upon oxida- colloidal assembly, anodization, laser fabrication, and deposition of
tive polymerization, formed a structurally rough film of extremely low hierarchically structured particles.3 The other approach is to directly
surface energy on various substrates. The static water contact angles make the micro/nanosurface structures of hydrophobic materials.
larger than 1508 and the low water sliding angles less than 78 confirmed The replica molding (or templation), involving the fabrication of a
the formation of superhydrophobic, self-cleaning surfaces.
template with desired micro/nanofeatures and the replication of the
features, is one of the most popular methods. In addition, the plasma
A nearly sphere-shaped water droplet on lotus leaves, carrying dust treatment of hydrophobic polymers, such as polyethylene terephtha-
or contaminants, rolls off easily even at a slightly tilted angle of late (PET) and polytetrafluoroethylene (PTFE), has been used for the
the leaves (o101) and cleans the leaf surface.1 Inspired by this direct preparation of rough surface structures of low surface energy.4
outstanding water repellency of lotus leaves, the fabrication of lotus Although successful, these two approaches are strictly material-
leaf-mimetic surfaces—superhydrophobic ones, defined to have a dependent or require cumbersome two-step processes. It would be
static water contact angle greater than 1501—has received a great much beneficial in the generation of superhydrophobic surfaces to
deal of interest because of the potential applications in various develop a method that generates rough hydrophobic structures on
areas, such as self-cleaning surfaces, antifouling or non-sticky any substrates in a one-step process, substrate-independent super-
surfaces, or water–oil separation.2 In nature, the superhydrophobi- hydrophobic coating. A substrate-independent coating was developed
city of lotus leaves is realized by their hierarchical structures, with inspiration from mussels in nature. The mussel, a marine
composed of micropapillae with nanometer-sized particulates, organism, secrets adhesive proteins and sticks to many different types
which are covered with hydrophobic waxes. Therefore, it is crucial of surfaces.5 The 1,2-dihydroxybenzene (catechol) and amine groups
to tailor both roughness (i.e., structural heterogeneity) and surface of 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) in the adhesive proteins
energy (i.e., coatings or materials) in the fabrication of self-cleaning, were reported to be critical in the extraordinary adhesion capability
superhydrophobic surfaces.
of the mussel.6 2-(3,4-Dihydroxyphenyl)ethylamine (dopamine) has
During the last decade, there have been innumerable reports widely been used for functional coatings of various organic/inorganic
on the fabrication of superhydrophobic surfaces. The reported substrates, electrodes, and even living cells.6–8 For example,
methods can be classified into two main categories. The most widely we coated a superhydrophobic anodized aluminum oxide (AAO)
adopted approach is the two-step, fabrication-coating process: a surface with relatively hydrophilic polydopamine, a polymerized form
rough surface structure is fabricated, followed by coating with a of dopamine, to reverse the water wettability to hydrophilicity as well
material of low surface energy. Rough micro/nanostructures as to show universal coating characteristics of polydopamine.9
have been generated by various methods including lithographic Recently, the dopamine motif was utilized for the nanostructured
approaches, bioinspired silicification, layer-by-layer deposition, coating onto glass, polyester fiber, and carbon nanotubes.10 In this
work, we designed a perfluorinated DOPA ( f-DOPA) and successfully
achieved a substrate-independent, superhydrophobic coating that did
not require any fabrication steps. The in situ superhydrophobic, self-
cleaning coating was applied to various substrates, such as gold, glass,
polydimethylsiloxane (PDMS), PET, vanadium foil (V foil), zinc foil
(Zn foil), and titanium dioxide (TiO2). The manipulation of the water
flow was also made possible by this coating approach.
a Center for Cell-Encapsulation Research and Molecular-Level Interface Research
Center, Department of Chemistry, KAIST, Daejeon 305-701, Korea.
E-mail: ischoi@kaist.ac.kr
b Department of Chemistry, Chungnam National University, Daejeon 305-764,
Korea. E-mail: wkcho@cnu.ac.kr
† Electronic supplementary information (ESI) available: Detailed experimental
procedures for the synthesis of f-DOPA, coating methods, and surface character-
ization. See DOI: 10.1039/c4cc02775b
In the oxidative dopamine polymerization, the catechol group
is oxidized to o-quinone. The o-quinone moiety is suggested to
‡ These authors contributed equally to this work.
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after 2 h. The peak at 330 nm indicated the oxidation of the catechol
moiety in f-DOPA to o-quinone.15 In addition, the formation of
dopaminechrome was evidenced by a weak, broad peak at around
480 nm. The UV-Vis spectra, therefore, confirmed that the reac-
tion conditions employed were suitable for f-DOPA coating.
We coated various substrates with polymerized f-DOPA, such as
gold, glass, PDMS, PET, V foil, Zn foil, and TiO2. To the acetonitrile
solution of f-DOPA (4 mg mLꢀ1) containing a substrate an aqueous
solution of NaIO4 (100 mM) at the final ratio of 15 : 2 (v/v) was
added. After 12 h, the substrate was washed with acetonitrile, and
the coating process was repeated with a fresh f-DOPA solution.
After coating, all the substrates became brownish or dark-colored,
indicating the formation of f-DOPA films, except for the V foil that
was black before coating (Fig. 2a). X-ray photoelectron spectro-
scopy (XPS) analysis also confirmed the successful coating of all
the substrates tested. For example, the characteristic peaks of
f-DOPA at 688.0 (F 1s) and 290.7 eV (C 1s) were observed after
coating on gold (Fig. 2b), and the surface elemental ratio (C/F) was
1.03, which was nearly consistent with that of f-DOPA (1.12). In
addition, Au peaks at 83.6 (Au 4f7/2) and 87.3 eV (Au 4f5/2
)
disappeared after f-DOPA coating, indicating the formation of
thick f-DOPA films with a thickness of 10 nm or more (Fig. 2c).6
The intensity of the characteristic XPS peak(s) for glass, PDMS,
V foil, Zn foil or TiO2 also decreased significantly after coating
(Fig. S1, ESI†). In addition to the incorporation of fluorine, the
f-DOPA films were structurally heterogeneous (i.e., rough),
fulfilling the basic characteristics of superhydrophobic surfaces.
The scanning electron microscopy (SEM) images showed that the
substrate was coated with f-DOPA microparticles, ranging from
1.0 to 2.0 mm in diameter, which were hierarchically composed
of smaller nanoparticles (Fig. S2a, ESI†). The root-mean-square
Fig. 1 (a) Synthesis of 1H,1H,2H,2H-heptadecafluorodecyl 2-amino-3-
(3,4-dihydroxyphenyl)propanoate, f-DOPA. (b) UV-Vis spectra of the
acetonitrile solution of f-DOPA during the NaIO4-mediated oxidation.
The data obtained at 0 min indicate the absorbance from the solution
containing only f-DOPA.
be transformed into leukodopaminechrome via intramolecular roughness was measured to be 533.92 nm in the atomic force
cyclization by primary amines (i.e., 1,4-conjugate addition), microscopy (AFM) image (Fig. S2b, ESI†). We believe that the
which is oxidized further to dopaminechrome.11 The isomerized
5,6-dihydroxyindole moiety is highly involved in the polymeriza-
tion process of dopamine and its derivatives.11,12 The suggested
mechanism indicated that it was required to keep the catechol and
amine groups for proper polymerization and subsequent coating;
therefore, we attached the perfluoro group onto the carboxylic acid
in L-DOPA to increase the hydrophobicity of the resulting polymer
films (Fig. 1a). It was reported that a substituent at the benzylic
position was compatible with the polymerization, exemplified by
norepinephrine polymerization.13 Briefly, L-DOPA was coupled
with 1H,1H,2H,2H-heptadecafluoro-1-decanol via esterification
with protection/deprotection of the hydroxy and amine groups.
Prior to substrate coating, the oxidation process of f-DOPA in
solution was investigated by UV-Vis spectroscopy (Fig. 1b). The
acetonitrile stock solution of f-DOPA (4 mg mLꢀ1) was diluted to
0.05 mg mLꢀ1, and 0.5 mL of the diluted solution was used for
reliable UV-Vis analysis. The UV-Vis spectrum of f-DOPA showed
a characteristic peak at 283 nm, corresponding to the symmetry-
forbidden transition (La–Lb) of the catechol moiety in f-DOPA.14
Fig. 2 (a) Optical photographs of bare and f-DOPA-coated substrates.
The upper and lower lines show the bare and f-DOPA-coated substrates,
The peak intensity at 283 nm decreased upon the addition of the
aqueous sodium periodate (NaIO4) solution (1.25 mM; 0.13 mL).
respectively. Scale bar: 1 cm. (b) Wide-scan XPS spectrum of a f-DOPA-
As the reaction progressed, a new peak appeared as a shoulder
over the peak at 283 nm and was observed clearly at 330 nm
coated gold surface. (c) Narrow-scan XPS spectra of the bare and f-DOPA-
coated gold surfaces.
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Table 1 Static water and CH2I2 contact angles on the substrates and the
surface free energies of the substratesa
Water contact CH2I2 contact Surface free energy
Substrate angle (1)
angle (1)
(gS) (mJ mꢀ2
)
Gold
Glass
PDMS
154.5
156.8
155.6
154.5
154.7
156.3
153.6
148.6
149.1
148.6
137.9
142.1
144.4
140.2
0.279 (gDS : 0.251, gPS: 0.028)
0.256 (gDS : 0.249, gPS: 0.007)
0.274 (gDS : 0.258, gPS: 0.015)
0.929 (gDS : 0.910, gPS: 0.019)
0.581 (gDS : 0.581, gPS: 0.000)
0.458 (gDS : 0.458, gPS: 0.000)
0.706 (gDS : 0.706, gPS: 0.000)
Fig. 3 Static water contact angles of the bare and f-DOPA-coated substrates. PET
V foil
Zn foil
TiO2
coating of the polymerized f-DOPA involved the same processes
as the polydopamine coating, which was thought to result from
the presence of the catechol and amine groups,6,16 although the
adhesion strength would be lower than that of polydopamine
due to the perfluorinated group in f-DOPA.
a
Contact angles are the averaged values from three different samples.
The surface free energies, and their dispersive and polar components
are calculated with the averaged contact angles based on the Owens–
Wendt geometric mean equation.
Fig. 3 shows the static water contact angles before and after
coating, confirming that all the f-DOPA-coated substrates were
superhydrophobic. Regardless of different contact angles
before coating, the coating made the contact angle of all the
substrates to be about 1551 (Fig. 3). Interestingly, PTFE, which
exhibits the low surface energy (19.1 mJ mꢀ2) and is non-sticky,17
also became a self-cleaning, superhydrophobic surface with a static
water contact angle of 1491 after f-DOPA coating (data not shown).
The wetting properties were further investigated by the tilting-plate
method that measured the dynamic contact angles, because it was
essential in the confirmation of self-cleaning properties to investi-
gate the dynamic water contact angles and surface free energies.
The advancing (yadv) and receding (yrec) water contact angles of
f-DOPA-coated substrates were measured, and the contact angle
hysteresis (i.e., (yadv ꢀ yrec)) of each substrate was calculated
(Table S1, ESI†). For example, the gold substrate, after coating,
showed a low contact angle hysteresis of 9.91. A water droplet on
the substrate easily rolled off at a tilt angle of 5.31, which is clear
evidence of self-cleaning properties. All other f-DOPA-coated sub-
strates also showed the low contact angle hysteresis and self-
cleaning properties with low sliding angles of 2.51 to 6.71. In
addition, the surface free energy (gS) was calculated based on the
Owens–Wendt geometric mean equation that divides the surface
free energy into the dispersive (gDS ) and polar (gSP) ones.17
surface free energy and realization of superhydrophobic, self-
cleaning properties.
Interestingly, the wetting characteristic of the f-DOPA films
was changed to non-superhydrophobic by simple O2-plasma
treatment: after 1 min of treatment, the static water contact angle of
the f-DOPA-coated gold substrate was changed from 154.451 to
124.181. The spatio-selective oxidation of the film could be utilized
for manipulation of water droplets and flow. For example, when a
small square area of the film was made relatively hydrophilic by
plasma treatment, a water droplet was captured at that hydrophilic
area after fast rolling on superhydrophobic area with slight tilting.
Droplet-based microfluidic channels could be fabricated with ease,
demonstrated by a hydrophilic line on the superhydrophobic
surface (Fig. S3, ESI†).
In summary, we demonstrated a simple coating method for
generating superhydrophobic, self-cleaning surfaces by using a
perfluorinated dopamine derivative (f-DOPA) as a polymerization
precursor. f-DOPA was coated on various substrates via oxidative
polymerization, forming a rough structure of low surface free
energy (0.2–0.9 mJ mꢀ2) without any additional fabrication.
Although the f-DOPA coating was not transparent intrinsically
because of the chemical nature of the polydopamine moiety,
we believe that the universality of the coating method would
widen the substrate scope for certain applications of self-
cleaning, superhydrophobic surfaces.
qffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffi
ð1 þ cos yÞgL ¼ 2 gDS gLD þ 2 gSPgLP
This work was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT & Future Planning
(MSIP) (NRF-2012R1A3A2026403 and NRF-2013R1A1A1059642).
where y is the measured contact angle of a liquid on the surface,
and gL is the surface tension of the liquid.
The surface free energy (gS; gS = gDS + gSP) of each surface was
determined by measuring the contact angles with water and
diiodomethane (CH2I2) (Table 1). The surface free energy of the
f-DOPA-coated gold surface was calculated to be 0.279 mJ mꢀ2
,
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