Methoxy group substitution on catechol ring of dopamine facilitates its polymerization and formation of surface coatings

Article abstract of DOI:10.1016/j.polymer.2017.03.061

Deposition of polydopamine on substrates is a facile and effective method of surface modification and the deposited polydopamine can reduce silver ions to form silver nanoparticles (AgNPs) for antibacterial applications. However, polydopamine deposition is a time-consuming process that usually requires 24?h to produce a dense surface coating. Since oxidation of dopamine is critical for its polymerization, we hypothesize herein that substitution of an electron-donating group on the catechol ring of dopamine can enhance its oxidation potential and subsequently accelerate its polymerization. In this work, dopamine substituted with a 5-methoxy group (OMeDA) was prepared. OMeDA polymerized faster than dopamine under similar reaction conditions, resulting in a polymer coating of 13?nm thickness on a silicon surface after 8?h, compared to the 24?h required for dopamine to form a coating of similar thickness. A polymer layer with AgNPs can be directly formed on the silicon substrate after exposure to a solution containing OMeDA and silver nitrate. After 2?h exposure, the silver content on the modified surfaces prepared with OMeDA was 187% higher than that obtained with dopamine, and the antibacterial efficacy of the former against Staphylococcus aureus was correspondingly higher than that of the latter. This study demonstrates that OMeDA with an electron-donating group in the catechol ring offers improvements over dopamine for surface modification applications.

Full text of DOI:10.1016/j.polymer.2017.03.061

Polymer 116 (2017) 5e15
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Methoxy group substitution on catechol ring of dopamine facilitates
its polymerization and formation of surface coatings
a
b
b
a, **
Jieyu Zhang , Yong Shung Cheah , Sridhar Santhanakrishnan , Koon Gee Neoh
,
b, *
Christina L.L. Chai
Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore
Department of Pharmacy, National University of Singapore, Kent Ridge, Singapore
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 November 2016
Received in revised form
Deposition of polydopamine on substrates is a facile and effective method of surface modification and
the deposited polydopamine can reduce silver ions to form silver nanoparticles (AgNPs) for antibacterial
applications. However, polydopamine deposition is a time-consuming process that usually requires 24 h
to produce a dense surface coating. Since oxidation of dopamine is critical for its polymerization, we
hypothesize herein that substitution of an electron-donating group on the catechol ring of dopamine can
enhance its oxidation potential and subsequently accelerate its polymerization. In this work, dopamine
substituted with a 5-methoxy group (OMeDA) was prepared. OMeDA polymerized faster than dopamine
under similar reaction conditions, resulting in a polymer coating of 13 nm thickness on a silicon surface
after 8 h, compared to the 24 h required for dopamine to form a coating of similar thickness. A polymer
layer with AgNPs can be directly formed on the silicon substrate after exposure to a solution containing
OMeDA and silver nitrate. After 2 h exposure, the silver content on the modified surfaces prepared with
OMeDA was 187% higher than that obtained with dopamine, and the antibacterial efficacy of the former
against Staphylococcus aureus was correspondingly higher than that of the latter. This study demon-
strates that OMeDA with an electron-donating group in the catechol ring offers improvements over
dopamine for surface modification applications.
15 March 2017
Accepted 22 March 2017
Available online 23 March 2017
Keywords:
Dopamine derivative
Surface modification
Silver nanoparticles
Antibacterial coating
©
2017 Published by Elsevier Ltd.
1. Introduction
biomedical applications.
The surface-immobilized PDA layer has an additional unique
property in that it is able to form functional coatings containing
metallic nanoparticles as PDA can readily reduce metal ions to
metal nanoparticles which will subsequently bind to the catechol
and amine groups of the PDA [3e6]. Some metallic nanoparticles
(e.g. Ag and Cu) exhibit excellent antibacterial properties [7], and
incorporation of these nanoparticles on surfaces using DA or PDA is
a facile method for the fabrication of antibacterial surfaces which
will have wide applications [8,9]. However, the preparation of
antibacterial surfaces in this manner requires a relatively long
processing time, typically 24 h to form a PDA layer of sufficient
thickness to cover the surfaces [10e12], and this significantly limits
its applications.
Surface modification confers new functionalities such as anti-
corrosive, antibacterial and antifogging properties to existing ma-
terials, and has become an important field in materials science. A
facile and effective method formulated in 2007 for modifying sur-
faces of nearly all types of organic and inorganic materials is via
immersion of the substrates in a dopamine (DA) solution to form a
polydopamine (PDA) layer. This method was inspired by observa-
tions that catechol and amine groups in the adhesive proteins
secreted by mussels are able to attach to various surfaces [1]. The
PDA layer formed by the polymerization of DA has excellent
biocompatibility [2], which makes it particularly useful in
The formation of PDA is a complex redox process and a series of
intermediates have been proposed to form during the polymeri-
zation process [13]. Although the detailed molecular mechanism is
still under debate, the oxidation of DA is critical for its polymeri-
zation [13]. Substituents on the catechol ring of DA may affect its
*
Corresponding author. Department of Pharmacy, National University of
Singapore, 18 Science Drive 4, 117543, Singapore.
** Corresponding author. Blk E5, 4 Engineering Drive 4, #02-34, 117576, Singapore.
E-mail addresses: chenkg@nus.edu.sg (K.G. Neoh), phacllc@nus.edu.sg
(
C.L.L. Chai).
http://dx.doi.org/10.1016/j.polymer.2017.03.061
032-3861/© 2017 Published by Elsevier Ltd.
0

6
J. Zhang et al. / Polymer 116 (2017) 5e15
redox properties, and polymerization of selected DA derivatives
2 4
Na HPO (273 mg, 1.92 mmol) was added to the reaction mixture
have been investigated [14e16]. For example, DA derivatives with
electron-withdrawing groups such as -NO [15] and -Cl [16] groups
2
and refluxed for 1 h. The reaction mixture was subsequently filtered
and the filtered solid was rinsed with water. The filtrate collected
was acidified with 6 N HCl until acidic. The resulting mixture was
evaporated at 60 C under reduced pressure (30 mmHg) until a
thick slurry remained. The slurry was dissolved in ethyl acetate
have lower oxidation potentials and polymerization rates as
compared to DA [16]. This suggests that the introduction of an
electron-donating group on the catechol ring of DA should increase
its susceptibility to oxidation, and the rate of polymerization and
deposition should be accelerated. It may also be possible to use the
DA derivative with an electron-donating group for incorporating
silver nanoparticles (AgNPs) for applications as antibacterial coat-
ings. Polymerization of 5-hydroxyl substituted dopamine for sur-
face modification has been reported [17], but to the best of our
knowledge, the possibility that an electron-donating group on the
catechol ring can accelerate the polymerization of DA derivatives
has not been investigated. To verify these hypotheses, the poly-
merization of DA and 5-methoxy DA (OMeDA), as well as their
applications in surface modification and AgNP deposition were
investigated and compared.
ꢀ
4
(500 mL), then dried with MgSO , filtered and evaporated. The
crude residue was purified by flash chromatography (5% MeOH/
DCM) to give compound 2 (6 g, yield of 90%) as a light brown solid
ꢀ
1
[18] with mp 131e133 C. H NMR (400 MHz, CDCl
7.14 (d, J ¼ 1.7 Hz, 1H), 7.08 (d, J ¼ 1.7 Hz, 1H), 3.97 (s, 3H). C NMR
(101 MHz, DMSO) 191.3,148.5,145.9,141.1,127.4,110.9,105.0, 56.0.
ESI-MS: m/z 167.3 for C
3
): d 9.79 (s, 1H),
13
d
-
8 9 4
H O ([M - H] , calculated m/z 167.0).
2.2.3. Synthesis of (E)-(((3-methoxy-5-(2-nitrovinyl)-1,2-
phenylene)bis(oxy)) bis(methylene))dibenzene (compound 3)
Benzyl bromide (7.73 mL, 65.0 mmol) and potassium iodide
(1.29 g, 7.78 mmol) were subsequently added to a suspension of
2 3
compound 2 (4.37 g, 26.0 mmol) and K CO (17.96 g,130.0 mmol) in
2
. Experimental section
N,N-dimethylformamide (DMF) under nitrogen. The reaction
mixture was heated at 80 C for 4 h, and then cooled to RT followed
ꢀ
2.1. Materials
2 3
by the removal of K CO by filtration. The mixture was subse-
quently extracted with 500 mL of ethyl acetate and washed with
1 N HCl and brine. The crude product was dried over anhydrous
Silicon (Si) wafers were obtained from Mitsubishi Silicon
America, USA. Tris was obtained from Vivantis Technologies,
Singapore. Deuterium oxide and Tris-d11 were purchased from
Cambridge Isotope Laboratories, USA. Staphylococcus aureus
2 4
Na SO , and the solvent was removed under reduced pressure. The
product (9.15 g, 26.3 mmol) and ammonium acetate (6.07 g,
78.7 mmol) were dissolved in acetic acid, and nitromethane
(7.09 mL, 131.3 mmol) was added to the reaction mixture. The re-
action mixture was refluxed for 5 h, cooled to RT and then poured
into water. The aqueous phase was extracted three times with
(S. aureus) 25923 was purchased from American Type Culture
Collection (Manassas, VA). All other chemicals, if not specified,
were purchased from Sigma-Aldrich.
200 mL ethyl acetate. The combined organic phases were washed
2
2
.2. Synthesis of OMeDA
with brine (200 mL), dried over anhydrous Na SO , and concen-
trated under reduced pressure. The residue was subjected to flash
2
4
.2.1. General experimental procedures
Thin layer chromatography was performed on precoated silica
chromatography (50% dichloromethane/petroleum ether) to give
ꢀ
1
compound 3 (8 g, yield of 79%) with mp 126e128 C. H NMR
(400 MHz, CDCl
7.89 (d, J ¼ 13.5 Hz, 1H), 7.48 (d, J ¼ 13.5 Hz, 1H),
7.45e7.27 (m, 10H), 6.79 (d, J ¼ 2.0 Hz, 1H), 6.75 (d, J ¼ 2.0 Hz, 1H),
gel plates (Merck, Singapore) and visualized with UV light irradi-
ation. Compounds 2, 3 and 4 (in Scheme 1a) were purified by flash
chromatography on a column using Merck silica gel 60 (230e400
mesh) or reversed-phase preparative chromatography using a
RediSep Rf Gold C18Aq column attached to a Sepacore flash X10
system. Low-resolution electrospray ionization mass spectra
3
) d
13
3
5.12 (s, 4H), 3.88 (s, 3H). C NMR (101 MHz, CDCl ) d 154.4, 153.2,
141.5, 139.4, 137.4, 136.5 (2C), 128.8 (2C), 128.6 (2C), 128.4 (2C),
®
®
128.3, 128.2, 127.5 (2C), 125.5, 108.9, 106.8, 75.4, 71.6, 56.5. HRMS:
þ
m/z 392.1500 for C23
H22NO
5
([M þ H] , calculated m/z 392.1492).
(
2
LRESIMS) were recorded on an Applied Biosystems MDS SCIEX API
000 mass spectrometer. High resolution mass spectra (HRMS)
2.3. Synthesis of OMeDA
were recorded on an Agilent mass spectrometer (electrospray
ionization (ESI)-time of flight). Melting points (mp) were deter-
mined using a SRS OptiMelt MPA100 mp apparatus. Nuclear mag-
netic resonance (NMR) spectra were recorded on a Bruker
Palladium on carbon (10%, 1 g) and 6 N HCl (1.0 mL) were added
to a suspension of compound 3 (1 g, 2.55 mmol) in a solution of
ethanol and tetrahydrofuran (1:1) under nitrogen in a glass vessel.
The vessel was flushed with hydrogen gas and pressurized to
345 kPa. The reaction mixture was stirred at RT for 15 min following
which the hydrogen gas was removed by bubbling nitrogen gas
through the mixture. The reaction mixture was then filtered
through celite and the filtrate was concentrated under reduced
pressure. The residue was purified by reversed-phase chromatog-
1
spectrometer (Avance III 400 MHz) at 400 MHz for H and 100 MHz
1
3
for C using chloroform-d, methanol-d
DMSO)-d as solvent. The chemical shifts are given in ppm, using
the proton solvent residue signal (CDCl 7.26, CD OD: 3.31,
SO: 2.50) as a reference in the H NMR spectra. The deute-
rium coupled signal of the solvent (CDCl 77.16, CD OD: 49.00,
CD SO: 39.52) was used as a reference in C NMR spectra. The
4
or dimethyl sulfoxide
(
6
3
:
d
3
d
1
(
3 2
CD )
3
:
d
3
d
13
(
3
)
2
raphy (95% H
250 mg, 45%) as a pale brown solid with mp 190e192 C. H NMR
(400 MHz, CD OD) spectrum:
6.41 (d, J ¼ 1.9 Hz, 1H), 6.38 (d,
2
O/methanol) to produce OMeDA (compound 4,
ꢀ
1
following abbreviations were used to describe the signals:
s ¼ singlet, d ¼ doublet, t ¼ triplet, m ¼ multiplet and q ¼ quartet.
The synthetic route to OMeDA is shown in Scheme 1a, and
described below:
3
d
1
3
J ¼ 1.9 Hz, 1H), 3.84 (s, 3H), 3.12 (t, 2H), 2.81 (t, 2H). C NMR
(101 MHz, CD
3
OD) spectrum:
d
150.0, 146.7, 134.1, 128.5, 110.3,
þ
1
05.2, 56.7, 42.3, 34.3. HRMS: m/z 184.1 for C
9
H
14NO
3
([M þ H] ,
2.2.2. Synthesis of 3,4-dihydroxy-5-methoxybenzaldehyde
calculated m/z 184.1).
(compound 2)
To a suspension of 5-bromovanillin (1, 9.2 g, 38.4 mmol) and
2.4. Polymerization of OMeDA or DA in solution
NaOH (15.4 g, 384.8 mmol) in degassed water (400 mL), was added
Cu powder (123 mg, 1.92 mmol) at room temperature (RT). The
reaction mixture was refluxed for 60 h and then cooled down to RT.
DA or OMeDA (0.3 mM) was dissolved in Tris buffer (10 mM, pH
8.5), and the reaction mixture was incubated at 37 C with shaking
ꢀ

J. Zhang et al. / Polymer 116 (2017) 5e15
7
Scheme 1. (a) Synthetic procedure and structure of OMeDA. (b) Proposed major structural units of PDA and poly(OMeDA).
ꢀ
at 150 rpm over 24 h. At different time points (0, 2, 4, 8, 16 and
4 h), the UV-Vis absorbance spectrum (200e800 nm) of the so-
(Mettler Toledo, Switzerland) at a heating rate of 10 C/min in ni-
trogen atmosphere.
2
lution was measured using a Shimadzu UV-1601 spectrophotom-
eter with Tris buffer as the reference. Tris buffer was also used for
baseline correction prior to measurement. The absorbance of the
solution at 360 nm (A360) was also continuously recorded over 4 h
using a microplate reader (Tecan GENios, Switzerland). The prog-
ress of polymerization of DA and OMeDA were also monitored us-
ing NMR spectroscopy. DA or OMeDA (10.5 mM) was dissolved in
Tris-d11 buffer (10 mM, pH 8.5) containing 1.75 mM DMSO as an
2
.5. Surface modification of Si with OMeDA and DA
2
Si wafers (0.725 mm thick) were cut into 1 ꢁ 1 cm squares, and
washed with a solution of 4.8% HF and 7% HNO in water, and then
cleaned ultrasonically in DI water and ethanol for 10 min each.
After drying in air, each substrate was placed in one well of a 24-
well plate followed by addition of 1.5 mL of 10.5 mM DA or
OMeDA in Tris buffer. The reaction was carried out for different
3
ꢀ
internal standard. The solution was incubated at 37 C with shaking
at 150 rpm over 24 h. At predetermined time points (0, 2, 4, 8, 16
ꢀ
periods of time at 37 C under continuous shaking at 150 rpm, and
1
and 24 h), H-NMR analysis of the samples was carried out on a
the substrates were then washed thoroughly with DI water and
ethanol, and dried in air. The substrates are denoted as DAxh and
OMeDAxh to indicate Si substrates incubated in DA and OMeDA
solution for x h, respectively.
Bruker Avance III spectrometer at 400 MHz.
PDA or polymer of OMeDA (poly(OMeDA)) obtained after
polymerization of the respective monomer (10.5 mM) for 24 h in
Tris buffer as mentioned above was collected by centrifuging the
reaction mixture at 21,000ꢁg for 30 min. The polymers were
resuspended in deionized (DI) water and centrifuged for 3 cycles to
remove the soluble residues, and then freeze-dried for 48 h. The
Fourier transform infrared (FT-IR) spectra of the polymers were
obtained in transmission mode using an Alpha Platinum-ATR FT-IR
spectrophotometer (Bruker, Singapore) under ambient conditions.
Solid-state 13C NMR spectra were acquired at 100.61 MHz on a
Bruker DRX 400 spectrometer (Bruker, Singapore) equipped with a
2.6. Deposition of AgNPs on Si surfaces
The Si substrates coated with PDA or poly(OMeDA) as
mentioned above were immersed in 1.5 mL of 50 mM AgNO
aqueous solution at 37 C with shaking at 150 rpm for 2, 4, 6, 24 or
4
water and ethanol, and dried in air. This process of AgNP immo-
bilization is referred to as the “two-step method” in this study. The
obtained substrates are denoted as DAxhAgyh and OMeDAxhAgyh
which indicate the Si substrates were incubated in DA and OMeDA
3
ꢀ
8 h. After that, the substrates were washed thoroughly with DI
4
mm cross polarization-magic angle spinning (CP-MAS) probe. The
contact time was set to 2 ms. The spectra were obtained after 800
scans at a spinning speed of 5 kHz for PDA, and 11541 scans at 8 kHz
for poly(OMeDA). Thermogravimetric analysis (TGA) was con-
ducted using a DTG-60AH analyzer (Shimadzu, Japan) at a heating
þ
solution for x h followed by incubation in Ag solution for y h.
For the “one-pot method” of AgNP immobilization, each Si
substrate was immersed in 1.5 mL of freshly prepared solution
containing 10.5 mM DA or OMeDA, and 50 mM AgNO
The reaction was carried out at 37 C with shaking at 150 rpm for
ꢀ
rate of 5 C/min in nitrogen atmosphere. Differential scanning
3
in Tris buffer.
calorimetry (DSC) was carried out with a DSC822e apparatus
ꢀ

8
J. Zhang et al. / Polymer 116 (2017) 5e15
2
h. After that, the substrates were washed thoroughly with DI
2.8. Antibacterial effect of surface coating containing AgNPs
water and ethanol, and dried in air. The obtained substrates pre-
pared by the one-pot method using DA and OMeDA are denoted as
DA/Ag2h and OMeDA/Ag2h substrates, respectively.
ꢀ
S. aureus was cultured at 37 C overnight in tryptic soy broth,
and then harvested by centrifugation at 2700 rpm for 10 min. The
bacterial cells were resuspended in PBS at a concentration of
UV-Vis absorption spectroscopy was used to monitor the for-
8
mation of AgNPs in solution. OMeDA or DA (40
160 M), and NaOH (1 mM) were added in DI water and the re-
action was carried out for 2 h. The UV-Vis absorbance spectra
200e800 nm) of the resultant solution were recorded at 1 min
m
M), AgNO
3
1 ꢁ 10 colony forming unit (CFU)/mL as estimated from the
(
m
absorbance of the bacterial suspension. Absorbance at 600 nm
8
(A600) of 0.1 is equivalent to ~10 CFU/mL based on calibration from
spread plate counting. The pristine and functionalized Si substrates
were placed in a 24-well microplate, and 1 mL of bacterial sus-
(
intervals. NaOH aqueous solution was used as the reference and
also for baseline correction prior to measurement.
8
pension in PBS (1 ꢁ 10 CFU/mL) was added to each well followed
ꢀ
by incubation at 37 C for 4 h. The bacterial suspension was then
removed, and the substrates were gently washed three times with
PBS.
2.7. Surface characterization
The viability of adherent bacteria on the substrates was assessed
using a LIVE/DEAD Baclight™ bacterial viability kit (Life Technol-
ogies, USA) according to the manufacturer's instructions. Briefly,
X-ray photoelectron spectroscopy (XPS) analysis was conducted
on a Kratos AXIS Ultra DLD spectrometer (Kratos, UK) with a
monochromatized Al K X-ray source (1486.7 eV photons). The
the bacterial cells on each substrate were stained with 50 mL of the
a
combination dye provided with the kit in the dark for 15 min at RT,
and then imaged using fluorescence microscopy (Leica, DM IL LED,
USA).
The number of viable bacteria was quantified using the spread
plate method as described in the literature [19]. Briefly, each sub-
strate with adherent bacteria was immersed in 2 mL of PBS and
sonicated for 7 min followed by 30 s of vortexing, to suspend the
adherent bacteria in PBS. The bacterial suspension was then serially
binding energy was calibrated based on the C 1s peak at 284.6 eV.
The PDA or poly(OMeDA)-coated Si surfaces were gently scratched
with a scalpel, and atomic force microscopy (AFM; Bruker Dimen-
sion ICON, USA) was used to estimate the thickness of the coating
layer. The surface morphology of the substrates was observed using
field emission scanning electron microscopy (FESEM, FEI QUANTA
6
50 FEG, USA). The Ag content on the substrate was measured using
inductively coupled plasma-mass spectroscopy (ICP-MS). Each
substrate was immersed in 1 mL of 50% (v/v) nitric acid and incu-
diluted, and 100 mL of each dilution was spread onto a nutrient agar
ꢀ
ꢀ
plate followed by incubation overnight at 37 C for determination
of the number of viable bacteria.
bated at 80 C with shaking for 1 h to digest the Ag deposits. The
supernatants were collected, and the Ag concentration in the so-
lution was measured using a HP 7500A ICP-MS system (Agilent
Technologies, USA). To investigate the possible release of surface
Ag, each Ag-containing substrate was incubated in 1 mL of phos-
phate buffered saline (PBS) for 7 days, and the PBS was then
collected and analyzed by ICP-MS.
2.9. Statistical analysis
For each condition and time point, at least three samples were
used. One-way analysis of variance (ANOVA) with Tukey post-hoc
Fig. 1. UV-Vis spectra of the reaction mixture of (a) DA and (b) OMeDA polymerization in Tris buffer over 24 h (c) A360 over 4 h polymerization of DA and OMeDA in Tris buffer. (d)
Percentage of residual aromatic H during polymerization of DA and OMeDA in Tris buffer.

J. Zhang et al. / Polymer 116 (2017) 5e15
9
test was used to evaluate the data and the results are reported as
mean ± standard deviation. Statistical significance was accepted at
P < 0.05.
poly(OMeDA) since the aromatic rings are preserved during poly-
merization [13]. The FT-IR results show that relative to the band at
ꢂ1
ꢂ1
1606 cm , the bands at 1354 and 1456 cm were enhanced, and
ꢂ1
the band at 1508 cm was reduced for poly(OMeDA) compared to
3
. Results and discussion
PDA, which suggests that more indole-like moieties are present in
poly(OMeDA) than in PDA [24].
3
.1. Synthesis of OMeDA
In the 13C NMR spectrum of PDA (Fig. 2b), signals at 30e50 ppm
were assigned to aminoethyl moieties in the uncyclized units (M1
and M2) and carbon atoms of the saturated five-membered ring in
M3 and M4 [25]. The catechol carbon (¼C-OH) atoms in M1, M4, M5
(Scheme 1b) are observed at ~143 ppm [23]. Signals in the
110e130 ppm region were ascribed to the benzo carbon atoms of
the aromatic ring [23]. The signals at 30e50 ppm in the C spec-
trum of poly(OMeDA) are different from that of PDA. The peak at
~55 ppm was assigned to the carbon atom of the -OMe group [26],
and this indicates that -OMe groups are still present in the struc-
tural units of poly(OMeDA). Since the FT-IR results (Fig. 2a) suggest
less uncyclized moieties in poly(OMeDA) than in PDA and primary
amino signals in the N 1s core-level XPS spectrum of poly(OMeDA)
(Fig. 3c) were also nearly absent, it is likely that there are minimal
uncyclized moieties in the poly(OMeDA). Therefore, the indole-like
moieties (M7-10) which contain -OMe group in the catechol ring
are proposed as the major structural units of poly(OMeDA) (Scheme
1b).
To synthesize OMeDA 4, commercially available 5-
bromovanillin (1) was first subjected to copper catalyzed hydrox-
ylation to yield dihydroxy benzaldehyde 2 (Scheme 1a). O-Benzy-
lation, followed by Henry reaction gave the nitro vinyl benzene
derivative 3 with an overall yield of 79% over two steps. Subsequent
treatment with palladium on carbon in the presence of hydrogen
gas gave the desired 5-methoxydopamine 4 with a moderate yield
of 45%.
13
3.2. Comparison of polymerization of OMeDA and DA in solution
The progress of the polymerization reaction of DA and OMeDA
in solution was monitored using UV-Vis spectroscopy. Fig. 1a shows
that as polymerization of DA progressed, the absorption at 360 nm
increased presumably due to the formation of 5,6-dihydroxyindole,
an intermediate in the oxidative polymerization reaction, which
arises from intramolecular cyclization onto the quinone resulting
from the initial oxidation step [20]. For OMeDA, an absorption peak
at the same wavelength was also observed (Fig. 1b), suggesting that
a compound/s possessing a similar structure/s was obtained. For
both DA and OMeDA, the rate of increase in the absorption at
For both PDA and poly(OMeDA), the TGA data (Fig. 2b) shows
3
A
60 nm slowed after 4 h of reaction. In a subsequent experiment,
360 was continuously monitored using a microplate reader over
the initial 4 h of polymerization. Fig. 1c shows that A360 of OMeDA
increased at a faster rate than that of DA in the first 70 min of the
polymerization reaction, suggesting a faster polymerization rate for
OMeDA.
These observations were also confirmed from 1H NMR spec-
troscopy used to monitor the progress of polymerization. In a
typical experiment, DA or OMeDA was dissolved in deuterated Tris
1
buffer containing DMSO as an internal standard, and the H NMR
spectra were recorded at different time points for the disappear-
ance of the aromatic hydrogens signals due to DA or OMeDA
respectively. Fig. 1d shows that during the polymerization of
OMeDA and DA, the loss of aromatic signals attributable to OMeDA
was faster than that of DA suggesting that OMeDA polymerized
faster than DA. The faster rate of polymerization of OMeDA
compared to DA may be attributed to the presence of the electron-
donating -OMe group on the catechol ring which enhances its
oxidation potential and facilitates the formation of the quinone
intermediate [14].
To investigate the structural difference between PDA and pol-
y(OMeDA), the polymers were analyzed using FT-IR absorption
spectroscopy, solid-state NMR, TGA and DSC. As shown in Fig. 2a,
ꢂ1
the absorption bands at 1081, 1207 and 1283 cm were attributed
to C-O asymmetric stretching and bending vibrations [21]. The
ꢂ1
ꢂ1
bands at 1354 cm and 1456 cm were assigned to the stretching
vibrations of C-N-C and -C¼N of indole rings [22]. The bands at
ꢂ1
1
508 and 1606 cm were ascribed to the scissoring vibration of N-
H, and C¼C resonance vibration in the aromatic ring, respectively
[21]. These results support the proposed structure of PDA reported
in the literature, which contain the structural units with primary
amino groups that do not undergo intramolecular cyclization (M1
and M2 in Scheme 1b) and indole-like moieties (indole and dihy-
droindoles) upon oxidation during polymerization (M3-6 in
Fig. 2. (a) FT-IR spectra, (b) 13C solid-state NMR spectra, (c) TGA, and (d) DSC scans of
PDA and poly(OMeDA). The arrows in (d) indicate endothermic bands due to loss of
ꢂ1
Scheme 1b) [23]. The band at 1606 cm can serve as an internal
reference for comparison of the FT-IR spectra of PDA and
water and polymer decomposition.

10
J. Zhang et al. / Polymer 116 (2017) 5e15
Fig. 3. XPS wide scan spectra of (a) pristine Si and (b) DA2h, DA16h, OMeDA2h and OMeDA4h substrates. (c) N 1s core-level spectra of DA24h and OMeDA24h substrates,
respectively.
an initial weight loss upon heating from room temperature to
modified substrate were significantly reduced after 2 h of reaction
(Fig. 3b). This result together with the increase in C 1s signal and
presence of N 1s signal indicate that the deposited poly(OMeDA)
has significantly covered the Si surface in just 2 h. After 4 h, the Si
signals were no longer discernible (Fig. 3b), indicating that the
surface was completely covered with poly(OMeDA). The shorter
period of time needed for OMeDA to form a coating on the Si sur-
face as compared to DA also suggests that OMeDA polymerized
faster than DA, which is consistent with Fig. 1.
ꢀ
100 C, which is attributed to the loss of water in the samples.
The thermal degradation of PDA and poly(OMeDA) commenced
ꢀ
at 267 and 207 C, respectively, and poly(OMeDA) underwent a
more rapid weight loss than PDA. The DSC results are shown in
ꢀ
Fig. 2c. The endothermic event at 100 C is due to evaporation of
water, consistent with the weight loss observed in the TGA scans.
ꢀ
The broad endothermic bands centered around 351 C for PDA
ꢀ
and 279 C for poly(OMeDA) is associated with the decomposi-
tion of the polymers. The earlier onset of thermal degradation of
poly(OMeDA) compared to PDA may be due to the higher elec-
tron density on the aromatic rings induced by the electron-
The surface composition of the polymers deposited on Si sur-
faces was analyzed using XPS. As shown in Fig. 3c, the N 1s peak can
be fitted with three component peaks assigned to primary (RNH
2
),
donating -OMe groups, which increases the
sion between the repeating units and decreases the polymer
stability [27,28].
p
-electron repul-
secondary (R
2
NH), and aromatic (¼RNHR) amine species [21,29,30].
The primary amine is associated with uncyclized DA/OMeDA-like
units present in the polymer, and the secondary and aromatic
amine correspond to dihydroindole and indole moieties, respec-
tively. Compared with PDA, poly(OMeDA) has a higher proportion
of aromatic together with secondary amine moieties and a much
lower proportion of primary amine functionalities (Fig. 3c). This
suggests that the majority of OMeDA monomers underwent
intramolecular cyclization during polymerization to result in more
indole-like moieties in poly(OMeDA) than in PDA, consistent with
3.3. Surface modification of Si with OMeDA and DA
Since the above results have shown that OMeDA polymerized
faster than DA in solution, OMeDA may require a shorter time than
DA to form a polymer coating on a surface. To investigate this, Si
substrates were placed in Tris buffer containing DA or OMeDA, and
the changes in surface chemical composition with time were
monitored by XPS. As shown in Fig. 3a, the pristine Si surface
exhibited strong Si 2p and Si 2s signals. The Si surface after 2 h of
exposure to DA shows a significant increase in C 1s signal and the
appearance of N 1s signal (Fig. 3b), indicating successful deposition
of PDA. However, the Si 2p and Si 2s signals remain strong sug-
gesting that the deposited polymer did not fully cover the Si sur-
face. When the reaction time was extended to 16 h, the Si surface
was fully covered with PDA as indicated by the disappearance of Si
signals in the XPS spectrum (Fig. 3b). In the case of OMeDA, the
intensities of the Si 2p and Si 2s signals on the surface of the
13
the FT-IR and solid-state C NMR spectra in Fig. 2.
To confirm that a shorter reaction time is needed for surface
modification with OMeDA compared to DA, the thickness of the
deposited polymer layer on Si was measured by AFM. As shown in
Fig. 4, the thickness of the deposited polymer layer increased with
increasing reaction time, and the deposited poly(OMeDA) layer was
thicker than PDA under similar reaction conditions. Although the
XPS results (Fig. 3a and b) indicated that most of the Si surface was
covered with poly(OMeDA) after 2 h of reaction, the thickness of
this polymer layer was too thin to be measured by AFM at this time
point. After 24 h of reaction, a PDA layer with thickness of 12 ± 3 nm

J. Zhang et al. / Polymer 116 (2017) 5e15
11
Fig. 4. (a) AFM images and surface profiles of pristine Si, DA24h, and OMeDA8h substrates. The white line in the AFM images indicates the location for surface profile measurement.
b) Thickness of PDA and poly(OMeDA) layer on Si substrates as a function of polymerization time.
(
was formed on Si, while a similar thickness of poly(OMeDA) was
achieved in only 8 h (Fig. 4a). The thickness of poly(OMeDA) on Si
was 39 ± 13 nm after 24 h, which was 225% thicker than that ob-
tained with PDA over the same reaction period (Fig. 4b).
the need for an exogenous reducing agent [1]. The reduced Ag binds
to the catechol and amine groups in PDA, and serves as seed for
growth of nanoparticles [31]. The higher amount of Ag on the
OMeDA8h substrate than on the DA24h substrate when the reac-
tion time is less than 6 h indicates that poly(OMeDA) exhibits a
þ
higher reduction ability than PDA toward Ag . This is attributed to
3.4. Deposition of AgNPs on Si surfaces using two-step method
the electron-donating -OMe group in poly(OMeDA), which en-
þ
hances its ability to reduce Ag . When the deposition time was
The PDA and poly(OMeDA) deposited on Si surfaces were used
extended to 24 h, the amounts of Ag deposited on the DA24h and
OMeDA8h substrates were similar (Fig. 5f). This is likely due to the
similarity in total reactive sites on the DA24h and OMeDA8h sub-
strates since similar amounts of polymers were deposited on the
surfaces as indicated by the similar thickness of the polymer layers
to immobilize AgNPs for potential antibacterial applications. The
DA24h and OMeDA8h substrates obtained by incubation of Si
substrates in DA and OMeDA solution for 24 and 8 h, respectively,
þ
were chosen for comparison for their ability to reduce Ag from
solution and trap the formed AgNPs due to their similarity in
coating thickness. The FESEM images of the DA24hAg2h and
OMeDA8hAg2h substrates obtained by incubating the DA24h and
(
Fig. 4b). These reactive sites in the PDA and poly(OMeDA) layers for
þ
þ
Ag reduction were likely depleted by the excess Ag in the reac-
tion solution after 24 h since the Ag content on these substrates did
not increase further when the deposition time was extended to
3
OMeDA8h substrates in AgNO solution for 2 h showed the pres-
ence of AgNPs on both substrates (Fig. 5). However, the size of the
particles on the OMeDA8hAg2h substrate (Fig. 5e) was larger than
those on the DA24hAg2h substrate (Fig. 5d). Quantitative ICP-MS
analysis showed that the amount of Ag deposited on the OMe-
DA8h substrate was higher than on the DA24h substrate when the
4
8 h.
The possible release of Ag from the DA24hAg2h and OMeDA8-
hAg2h substrates was investigated by incubating these substrates
in PBS. The results show that only 1e2% of the Ag on the substrates
was released after incubation in PBS for 7 days. This indicates that
the AgNPs are strongly bound to the catechol groups in PDA and
poly(OMeDA) on the Si surface [31].
þ
incubation time in the Ag solution was shorter than 6 h (Fig. 5f). It
is well-known that PDA exhibits inherent reductive capacity that
results in Ag deposition upon exposure to its salt solutions without

12
J. Zhang et al. / Polymer 116 (2017) 5e15
Fig. 5. (aee) FESEM images of the surface of (a) pristine Si, (b) DA24h, (c) OMeDA8h, (d) DA24hAg2h, and (e) OMeDA8hAg2h substrates. Scale bar ¼ 200 nm. (f) Silver content on
the surfaces of DA24h and OMeDA8h substrates after incubation in AgNO
results of DA24h over the same incubation period.
3
solution for different periods of time. * indicates significant difference (P < 0.05) compared with the
3.5. Deposition of AgNPs on Si surfaces using one-pot method
reaction with OMeDA was 187% higher than that with DA after 2 h
of reaction (Fig. 6c). Comparison of the results in Figs. 6c and 5f
þ
The two-step method for depositing AgNPs on Si requires a long
shows that with the same time period for Ag reduction (2 h), the
amount of Ag on the DA/Ag2h and OMeDA/Ag2h substrates (from
one-pot method) was much higher than that on the DA24hAg2h
and OMeDA8hAg2h substrates (from two-step method). The
release of the surface Ag on the substrates prepared by the one-pot
method was tested, and the results show that only 1% of the surface
Ag was released from the DA/Ag2h and OMeDA/Ag2h substrates
after 7 days of incubation in PBS.
processing time since more than 10 h was needed to prepare the
OMeDA8hAg2h substrate, and more than 26 h to prepare the
DA24hAg2h substrate. To shorten the processing time, the possi-
bility of a one-pot method was investigated. When a Si substrate is
þ
þ
incubated in a solution containing Ag and DA or OMeDA, Ag is
expected to be reduced by DA or OMeDA to form AgNPs, and
simultaneously, the oxidized monomers will polymerize and de-
posit on the Si surface together with the AgNPs. To test this hy-
þ
pothesis, the reduction of Ag by DA and OMeDA in NaOH solution
3
.6. Antibacterial properties of AgNPs deposited on Si surface
and the resultant AgNP formation was monitored using the
absorbance at 410 nm which is the wavelength of surface plasmon
absorption of AgNPs [32]. When OMeDA was used, AgNPs formed in
greater quantity than DA under the same reaction condition as
shown by the differences in surface plasmon absorption in Fig. 6a.
These results are consistent with the UV-Vis absorption spectra in
Fig. 1 which indicate that OMeDA polymerized faster than DA due
to its higher oxidation potential.
Bacterial adhesion on a surface is the initial step for biofilm
formation and subsequent infections. Therefore, inhibition of initial
bacterial adhesion is critical for prevention of infections. In this
study, S. aureus was selected as the target bacterium because it is
the most prevalent causative pathogen of infections [33]. Different
substrates with deposited AgNPs prepared using the two-step
method (the DA24hAg2h and OMeDA8hAg2h substrates) or one-
pot method (the DA/Ag2h and OMeDA/Ag2h substrates) were
subjected to bacterial assay for comparison of their antibacterial
property.
The one-pot Ag deposition method was subsequently carried
out by incubating Si substrates in the reaction medium containing
þ
DA or OMeDA and Ag . As shown in Fig. 6b, when OMeDA was used,
the Si surface was more densely covered with AgNPs (the OMeDA/
Ag2h substrate) compared to when DA was used (the DA/Ag2h
substrate) after 2 h of reaction. The amount of Ag deposited after
After incubation in S. aureus suspension in PBS for 4 h, the
number of viable bacterial cells on the substrates without Ag (the
DA24h, OMeDA8h, DA2h and OMeDA2h substrates) was not

J. Zhang et al. / Polymer 116 (2017) 5e15
13
Fig. 6. (a) Absorbance of reaction mixture containing OMeDA or DA (40
mM), AgNO
3
(
160 M), and NaOH (1 mM) at 410 nm. (b) SEM images of DA/Ag2h and OMeDA/Ag2h
m
substrates. Scale bar ¼ 200 nm. (c) Quantification of amount of Ag deposited on the
surfaces shown in (b). * denotes significant difference (P < 0.05) compared with the
result obtained with the DA/Ag2 substrate.
Fig. 7. Number of viable adherent S. aureus on pristine and modified Si substrates with
8
or without deposited AgNPs after incubation in bacterial suspension (10 CFU/mL in
ꢀ
PBS) for 4 h at 37 C. * denotes significant difference (P < 0.05) compared with that on
the pristine Si substrate.
Fig. 8. Fluorescence microscopy images of S. aureus on pristine Si substrate, and
modified Si substrates with or without deposited AgNPs after incubation in bacterial
suspension (10 CFU/mL in PBS) for 4 h at 37 C. Viable bacterial cells were stained
green while membrane-compromised cells appeared red. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
significantly different from that on pristine Si (Fig. 7). This indicates
that neither PDA nor poly(OMeDA) has antibacterial property. The
number of viable S. aureus on the DA24hAg2h and OMeDA8hAg2h
substrates decreased by 72% and 91%, respectively, as compared to
that on pristine Si. For substrates prepared by the one-pot method,
8
ꢀ

14
J. Zhang et al. / Polymer 116 (2017) 5e15
the number of viable S. aureus on the DA/Ag2h and OMeDA/Ag2h
substrates decreased by 95% and over 2 orders of magnitude,
respectively, compared to that on pristine Si.
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containing surfaces, the bacteria on the substrates were subjected
to live/dead staining, and the results are shown in Fig. 8. There were
many live bacterial cells (stained green) but hardly any membrane-
compromised bacterial cells (stained red) on pristine Si, and a
similar observation was made for substrates without Ag (the
DA24h, OMeDA8h, DA2h and OMeDA2h substrates). On the Ag-
containing substrates (the DA24hAg2h, OMeDA8hAg2h, DA/Ag2h
and OMeDA/Ag2h substrates), the number of live bacterial cells was
much less than on pristine Si while the number of membrane-
compromised cells increased significantly. These results illustrate
the bactericidal effect of the AgNPs on the surface of the substrates.
The mechanism by which AgNPs kill bacteria is not fully under-
stood. AgNPs on the substrate can attach to bacterial surfaces and
increase the permeability of cell walls and cell membranes, thereby
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[
þ
Any AgNPs or Ag ions released from the substrates can also enter
the bacterial cells on the substrate and react with DNA and proteins,
resulting in bacterial death [35]. The higher antibacterial efficacy of
the OMeDA/Ag2h substrate compared to the other three Ag-
containing surfaces is likely due to its higher content of AgNPs
[
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. Conclusions
DA substituted with an electron-donating methoxy group
OMeDA) was shown to oxidize and polymerize more readily than
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DA. Similar to PDA, the poly(OMeDA) coating on Si surface can
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M. Wagner, A. del Campo, Antibacterial strategies from the sea: polymer-
bound Cl-catechols for prevention of biofilm formation, Adv. Mater 25
þ
reduce Ag to form AgNPs, but the reaction rate with poly(OMeDA)
is higher than that of PDA of the same thickness. AgNP-containing
coating can also be prepared using a one-pot method where the Si
(2013) 529e533.
þ
substrates were incubated in a solution containing Ag and DA or
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a plant flavonoid-inspired molecule for material-independent surface chem-
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þ
OMeDA, and reduction of Ag and polymerization of DA or OMeDA
occur simultaneously. A distinct advantage of the one-pot method
is that the processing time is much shorter than the two-step
method. The amount of AgNPs deposited in the coating via the
former (i.e. the DA/Ag2h and OMeDA/Ag2h substrates) after 2 h
reaction time was higher than that achievable using the two-step
method (i.e. the DA24hAg2h and OMeDA8hAg2h substrates)
which requires more than 10 h. The AgNP-containing coating pre-
pared using the one-pot method with OMeDA is strongly bacteri-
cidal, and most of the AgNPs on the OMeDA/Ag2h substrate are
retained on the surface even after extended incubation in aqueous
medium. Thus, this facile method is highly promising for fabri-
cating antibacterial coatings.
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zation of an indazole ligand into the self-polymerization of dopamine for
enhanced binding with metal ions, J. Mater. Chem. B 3 (2015) 7457e7465.
[21] R.A. Zangmeister, T.A. Morris, M.J. Tarlov, Characterization of polydopamine
thin films deposited at short times by autoxidation of dopamine, Langmuir 29
(2013) 8619e8628.
[
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22] L. Lv, X. Wu, M. Li, L. Zong, Y. Chen, J. You, C. Li, Modulating Zn(OH) rods by
2
marine alginate for templates of hybrid tubes with catalytic and antimicrobial
properties, ACS Sustain. Chem. Eng. 5 (2017) 862e868.
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Acknowledgments
A. Bende, S. Beck, Structure of polydopamine: a never-ending story? Langmuir
29 (2013) 10539e10548.
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In vitro investigation of enhanced hemocompatibility and endothelial cell
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Mater Interfaces 5 (2013) 1704e1714.
We thank the Faculty of Science, National University of
Singapore, SERC-A*STAR (Project no: 12212031) and National
Medical Research Council of Singapore (Grant NMRC/BnB/
[25] V. Proks, J. Brus, O. Pop-Georgievski, E. Ve
ꢂc erníkov aꢀ , W. Wisniewski, J. Kotek,
M. Urbanov ꢀa , F. Ryp ꢀa ꢂc ek, Thermal-induced transformation of polydopamine
12sep040) for financial support of this project.
structures: an efficient route for the stabilization of the polydopamine sur-
faces, Macromol. Chem. Phys. 214 (2013) 499e507.
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