In situ insights into the nanoscale deposition of 5,6-dihydroxyindole-based coatings and the implications on the underwater adhesion mechanism of polydopamine coatings

Article abstract of DOI:10.1039/c8ra04472d

The biomimetic coating polydopamine (PDA) has emerged as a promising coating material for various applications. However, the mechanism of PDA deposition onto surfaces is not fully understood, and the coating components of PDA and its relation to the putative intermediate 5,6-dihydroxyindole (DHI) are still controversial. This investigation discloses the deposition mechanisms of dopamine (DA)-based coatings and DHI-based coatings onto silicon surfaces by monitoring the nanoscale deposition of both coatings in situ using high-precision ellipsometry. We posit that the rapid and instantaneous nano-deposition of PDA coatings onto silicon surface in the initial stages critically involves the oxidation of DHI and/or its related oligomers. Our studies also show that the slow conversion of DA to DHI in PDA solution and the coupling between DA and DHI-derived precursors could be crucial for subsequent PDA coating growth. These findings elucidate the critical role of DHI, acting as an 'initiator' and a 'cross linker', in the PDA coating formation. Overall, our study provides important information on the early stage nano-deposition behavior in the construction of PDA coatings and DHI-based coatings.

Full text of DOI:10.1039/c8ra04472d

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PAPER
In situ insights into the nanoscale deposition of 5,6-
dihydroxyindole-based coatings and the
implications on the underwater adhesion
mechanism of polydopamine coatings†
Cite this: RSC Adv., 2018, 8, 27695
a
b
b
b
*
Qinghua Lyu, Hongyan Song, Nikolai L. Yakovlev, Wui Siew Tan
a
*
and Christina L. L. Chai
The biomimetic coating polydopamine (PDA) has emerged as a promising coating material for various
applications. However, the mechanism of PDA deposition onto surfaces is not fully understood, and the
coating components of PDA and its relation to the putative intermediate 5,6-dihydroxyindole (DHI) are
still controversial. This investigation discloses the deposition mechanisms of dopamine (DA)-based
coatings and DHI-based coatings onto silicon surfaces by monitoring the nanoscale deposition of both
coatings in situ using high-precision ellipsometry. We posit that the rapid and instantaneous nano-
deposition of PDA coatings onto silicon surface in the initial stages critically involves the oxidation of DHI
and/or its related oligomers. Our studies also show that the slow conversion of DA to DHI in PDA
solution and the coupling between DA and DHI-derived precursors could be crucial for subsequent PDA
coating growth. These findings elucidate the critical role of DHI, acting as an ‘initiator’ and a ‘cross
linker’, in the PDA coating formation. Overall, our study provides important information on the early
stage nano-deposition behavior in the construction of PDA coatings and DHI-based coatings.
Received 25th May 2018
Accepted 27th July 2018
DOI: 10.1039/c8ra04472d
rsc.li/rsc-advances
Despite the promising applications of PDA lms in various
elds, several important issues remain unresolved.21 One of
Introduction
these issues pertains to the deposition mechanisms of PDA
coatings, which remains unclear. Currently, limited techniques
are available to investigate the complex underwater self-
assembly of PDA coatings onto surfaces. In addition, despite
the numerous reports on the proposed structures of PDA, the
structure and the constituent building block(s) of PDA are still
controversial. Different models, including the ‘open-chain pol-
ycatechol quinone’ model; 5,6-dihydroxyindoline/its quinone
supramolecular structure; covalent polymers derived from
dopamine (DA) and its oxidized cyclization product dihydrox-
yindole (DHI); and single DHI-based eumelanin model etc.,
have been proposed.22–28 While a number of intermediates have
been proposed to be involved in PDA coating formation, the
common understanding is that DHI is the key precursor
involved in the subsequent oxidative polymerization to form
coatings, in a manner that is similar to the biosynthesis of
melanin.29,30 Hong et al. reported the isolation of a physical
DHI/(DA)2 non-covalent complex from DA–phosphate buffer
solution,23 while Vecchia and Liebscher et al. demonstrated the
presence of DHI-based units in PDA oligomers using a chemical
degradation approach and high resolution mass spectrometry
respectively.25,26 However, the most recent report by Aleri et al.
found that the use of DHI under classical conditions i.e. Tris
buffer pH 8.5 did not give rise to any coatings.31 Due to the
Underwater adhesion is a remarkably difficult process to effect
due to the presence of a thin hydration layer that prevents the
contact between a polymer and the surface. Yet in nature, there
are many examples of marine adhesives that are able to glue
surfaces underwater with high strength and durability. One
example is the marine mussel adhesive proteins (MAPs) that
enable mussels to adhere to surfaces (e.g. intertidal rocks) with
great tenacity.1–3 In the context of MAPs, studies have identied
that the presence of 3,4-dihydroxyphenylalanine (DOPA) and
lysine residues are critical for the adhesive properties.4–9
Inspired by this, a number of biomimetic catechol(amine)-
based materials have been developed in the past decade.10–15
Among these, polydopamine (PDA) holds the greatest promise
due to the ease of formation, almost universal substrate scope,
versatility and ease of further functionalization; these proper-
ties have garnered signicant interest in applications such as
anti-bacterial and anti-fouling coatings, nanotechnology and
biomedicine.16–20
aDepartment of Pharmacy, National University of Singapore, 18 Science Drive 4,
Singapore 117543. E-mail: phacllc@nus.edu.sg
bInstitute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way,
Singapore 138634. E-mail: wuisiew@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04472d
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complicated aggregation/organization of PDA, a denitive robust (i.e. adhered to the surface) despite treatment with water
relationship between PDA coating and its putative intermediate ow (for the effects of different concentrations and buffers on
DHI has not been demonstrated unequivocally.
deposition kinetics, see Fig. S2†). It was also observed that the
In this study, we aimed to explore the deposition mechanism deposition rate dropped rapidly with time under Tris condi-
of PDA coatings onto silicon wafers by the in situ monitoring of tions: the coatings were deposited linearly in the rst 8 min with
the nano-coating process via high-precision ellipsometry a deposition rate of 0.15 ꢀ 0.01 nm minꢁ1, while the deposition
(HPEL). This purpose-built equipment with a 0.1 nm sensitivity rate decreased to half i.e. 0.07 ꢀ 0.02 nm minꢁ1 in the following
has been successfully utilised in the study of adsorption and 8 min. Aer 20 min, the deposition rate was much slower than
desorption of single molecules.32 DA and the putative interme- the initial stage (Fig. S2†), implying that the coating growth had
diate DHI were used in this study to gain a better understanding reached steady state. These results were in line with the ndings
of the structural features needed for adhesion onto surfaces. To of Bernsmann et al. in which the growth of PDA coating on
the best of our knowledge, this is the rst reported in situ study silicon oxide was examined using quartz crystal microbalance:
on the nanoscale deposition of PDA- and DHI-based coatings a 2.46 nm-thick coating was formed in 15 min and a drop in
using HPEL. Using a combination of techniques including deposition rate in Tris buffer was also noted in their study.34
Atomic Force Microscope (AFM), Attenuated Total Reection
The fast deposition of lms in alkaline buffer is consistent
Fourier Transform Infrared (ATR-FTIR) and Nuclear Magnetic with previous reports that the formation of PDA coatings
Resonance (NMR), we were able to elucidate the deposition requires an elevated pH (mildly alkaline) to initiate the oxidative
mechanisms of both PDA and DHI-derived coatings. Our polymerization of the DA monomer.35,36 It should be noted that
studies revealed new insights into the relationship between PDA this rapid and instantaneous deposition differs from the slow
coating and its putative intermediate DHI.
self-assembly of DA at lower pH conditions (Fig. 1). Although
the presence of the catechol moiety is one important contrib-
utor to the process of underwater adhesion,37–42 some other
interactions must also be responsible for this rapid deposition
onto the silicon surface, e.g. electrostatic or p–p stacking.43,44
Others have hypothesized that the deposition is initiated by
radical species at the early stages, followed by the polymeriza-
tion of these species to coat the surfaces.34,45–47 In the next
section, we examined if the putative intermediate DHI can
initiate a fast and spontaneous deposition on surfaces.
A quantitative study of DHI formation in PDA solution
(10 mM DA, 10 mM Tris buffer 8.5) was carried out for the rst
time. The authentic, air-sensitive DHI compound was freshly
prepared via the reported method.48 In our experiment that
mimics the coating process (minus the silicon substrate),
a 100 mL DA–Tris solution was exposed to air for a pre-
determined time following which the organics were extracted
using ethyl acetate and the concentrated extracts were analysed.
From the 1H-NMR spectra (Fig. 2), DHI was shown to be the
main intermediate present, along with the presence of
Results and discussion
The setup for in situ HPEL is shown in Fig. S1.† The general
experimental protocol can be briey described as follows: 1 mL
sample of DA or DHI was prepared and injected into the HPEL
cuvette. This was allowed to incubate for a predetermined time
to allow for deposition onto the silicon substrates. Then de-
ionized water was owed into the system in order to test the
robustness of the lm/coating formed underwater. The initial
deposition kinetics of DA on silicon wafers in different pH Tris-
buffers under static conditions were rst examined. As dis-
played in Fig. 1, DA samples at pH 5 and 7 only gave rise to
coatings with a thickness of 0.4 ꢀ 0.1 nm aer a 20 min incu-
bation. The thickness of these deposited coatings was close to
the reported self-assembly monolayer.33 However, these layers
were readily washed away during the subsequent ow of de-
ionized water, indicating that the adsorption of DA at these
pH values is physical and reversible. At pH 8.5, signicant
deposition was observed and the deposited coatings were
Fig. 1 The nano-deposition kinetics of DA-based coatings on Si wafer Fig. 2 1H NMR spectra of (a) organic extracts of PDA solution and (b)
at pH 5, 7 and 8.5 in the first 30 min (10 mM DA, 10 mM Tris buffer).
authentic DA/DHI mixture.
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unreacted DA monomer. Interestingly, these 1H-NMR signals than when a fresh DHI solution was used. This indicates that
matched the spectra of the mixture of DHI/DA, but were not in DHI and/or its low molecular weight oligomers are capable of
good agreement with the spectrum of pure DHI (Fig. S3†), coating silicon substrates rapidly, while high molecular weight
suggesting the presence of strong inter-molecular interactions oligomers or aggregates of DHI may possess weak adhesion
between DA and DHI. Hong et al. also noted a similar non- property, possibly due to their inability to stack efficiently.49,50
covalent interaction between DA and DHI in PDA solution and
We further investigated the role of oxidation in the initial
isolated a self-assembled physical complex of DHI/(DA)2 from stages of DHI coating deposition under neutral conditions (pH
DA-phosphate buffer.23 We carefully quantied the DHI formed 7). Pre-treatment of DHI–Tris solution (pH 7) with N2 degassing
in solution by 1H-NMR spectroscopy using dimethylsulfoxide resulted in a relative lower deposition rate. Similarly, the addi-
(DMSO) as an internal standard and found that the real-time tion of reducing agent Na2S2O4 into the DHI–Tris solution
concentration of DHI was ca. 4.3 mM as measured at the signicantly slowed down the deposition process. In this case,
30 min time-point. Further experiments showed that the only a 0.5 ꢀ 0.1 nm-thick coating was formed aer 30 min of
concentration of DHI at 1, 4 and 8 h were also at micromolar incubation (Fig. S4†). The above observation suggests that the
levels, ranging from 3.6 to 0.9 mM. These data demonstrated oxidation of DHI is critical in initiating rapid coatings onto the
that DHI is one main oxidized intermediate formed in the PDA silicon surface. This is in accordance with previous studies on
solution.
the electrochemical self-assembly of DHI-based lms.51,52 Based
The deposition of DHI on silicon surfaces was then studied on these ndings, the deposition mechanism of DHI can be
to examine whether this intermediate can form coatings on its interpreted as the initial attachment of small oxidized species,
own. DHI solutions were prepared under conditions similar to followed by the polymerization of these deposited species. To
that used for PDA coatings (10 mM precursor, Tris buffer 8.5). further support this notion, the durability of DHI coatings in
Once DHI sample was injected, a fast and instantaneous the initial stage (1–10 min in this context) was further exam-
deposition was noted; aer a few minutes, however, the beam ined. The initial deposits (1.3 ꢀ 0.1 nm) formed at 1 min were
intensity dropped signicantly as the solution darkened with robust under water ow conditions, while treatment with
time, and subsequent deposition kinetics could not be moni- aqueous Na2S2O4 showed an instantaneous decrease in coating
tored (Fig. 3, indicated as asterisk). When fresh DHI–Tris thickness to ca. 0.5 ꢀ 0.2 nm (Fig. 4a). Similar reducing treat-
samples (10 mM) at pH 5 and 7 were used, rapid depositions ment to the coating obtained aer above 4 min deposition time
onto surfaces were observed at both the pH values. The thick- did not show any signicant effect on the stability of the coat-
ness of coatings formed at pH 7 and pH 5 aer 30 min were 7.3 ings (Fig. 4b), suggesting that these deposited species may have
ꢀ 0.2 nm and 5.7 ꢀ 0.2 nm respectively. These coatings were
robust enough to endure the treatment with water-ow. The
successful depositions of these coatings onto the silicon
substrate were veried by further AFM analyses (discussed
further below). In addition, rapid drop in deposition rates with
time at both pH 5 and 7 were observed. The decrease of depo-
sition rates may be explained by the saturation of binding sites
on the silicon surfaces and/or competing self-aggregation
pathway of DHI in solutions. The latter was supported by the
observation that aged DHI solution (10 mM, pH 7, exposed to
air for 10 min) indeed led to a slower deposition of coatings on
a clean silicon substrate (Fig. 3, indicated as red dashed line)
Fig.
3 The nano-deposition kinetics of DHI-based coatings at Fig. 4 The stability of DHI-based nano-coatings in the initial (a) and
different pHs in the first 30 min.
later (b) stages following treatment with reducing agent.
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already formed irreversible linkages e.g. through covalent Arzillo et al. on the buildup of eumelanin that DHI can rapidly
bonding and the coating is not compromised by the treatment adsorb onto the surfaces of DHI-eumelanin particles.57
with reducing agents.
Our observations strongly suggest that the deposition process
Further AFM studies veried that coatings were formed on of DHI-based coatings is dominated by monomeric species of
the silicon substrates in the deposition study (Fig. 3, 10 mM DHI, low molecular weight oligomers and/or small nano-
DHI, Tris buffer pH 5, 7 or 8.5, 30 min). However, a smaller aggregates, while larger aggregates and particles cannot deposit
amount of DHI-coating was formed at pH 8.5 and the surface onto silicon surfaces but competitively suppress the deposition
was not fully coated (Fig. 5a), while at low pH values the process via the sequestration of these species, leading to a self-
substrates were well coated and uniform disk-shaped topogra- limiting thickness of coatings. As the competing self-
phies were observed (Fig. 5b and S5†) which were consistent aggregation of DHI/its oligomers in solution is unavoidable, it
with the observations in recent DHI-based coating studies using can be speculated that neither the use of small nor large amounts
spin-coating,53,54 supporting the notion of stacking interactions of DHI can yield substantially thicker coatings. Small quantities
(presumably p–p) leading to larger aggregates.55–58 We tenta- of DHI cannot provide enough DHI-related species as feeds for
tively rationalized that the poor DHI-coating obtained at alka- sustaining coating growth, while the use of larger amounts of
line pH maybe the result of rapid oxidation and self-aggregation DHI may lead to rapid coating growth in the early stages, but the
effects in solutions.
simultaneous rapid formation of large aggregates/particles will
It was also found that at pH 8.5 and 7, no signicant increase affect the subsequent coating growth. Therefore, an ideal coating
in the amount of coating was observed aer 1 h and 4 h protocol for DHI-based components would be a continual supply
respectively. At pH 8.5, slightly more amorphous granules with of fresh DHI in small quantities as feed, lest a signicant amount
20–40 nm thickness were noted but the surfaces were not well of aggregates/particles are formed in solution. Interestingly, the
coated (Fig. S6†), while at pH 7 the maximal thickness of DHI formation of PDA coatings meets this prerequisite as the DHI
coatings at 15 ꢀ 2 nm was observed aer a long deposition time generated in situ is used as feed at low concentrations, as veried
(4 h). It should be noted that the thickness of DHI-based coat- in our quantitative studies. The slow conversion of DA to DHI in
ings obtained at pH 7 (10 mM DHI) was still less than that of PDA solutions and the strong non-covalent interactions between
PDA coatings prepared under the same concentrations (ca. DA and DHI could be important factors in retarding the rapid
40 nm, 10 mM DA, Tris pH 8.5). These results show that DHI- self-polymerization/aggregation of DHI at alkaline pHs.59 These
related species are able to adhere efficiently and robustly to notions can explain the observed thickness difference between
silicon surfaces but competing self-aggregation in solutions can PDA coatings and DHI coatings and also the observed self-
signicantly affect the deposition process. This proposed limiting thickness of PDA coatings reported in our studies
competing process shares similarities with the ndings of (Fig. S2†) and other studies.34,60,61
Fig. 5 AFM images of DHI-coatings after 30 min at (a) pH 8.5; (b) pH 7.
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Fig. 6 AFM images of (a) DHI-coating (pH 7, 24 h, ca. 18–23 nm) and (b) PDA coatings (pH 8.5, 24 h, ca. 40 nm) on quartz substrates; (c) ATR-FTIR
spectra of DHI-based coating film and PDA coating film on quartz slides.
A question that arises here is whether PDA coating forma- DA sample. In contrast, these peaks were not observed to be
tion is mainly based on DHI-derived components. To charac- signicant in the sample derived from DHI only. Assuming
terise DHI-based coatings and PDA coatings by ATR-FTIR, dip- that the components in solution mirror that of the coatings,
coating studies with DA and DHI precursors, respectively, were these observations suggest that DA and DHI/its oligomers
carried out on quartz substrates. Consistent with our deposi- can form covalent adducts,23 which may rationalize the
tion studies on silicon substrate, no signicant DHI-based observed difference between PDA coating and DHI-based
coating was formed at pH 8.5, while at pH 7 a rough DHI- coating. As many studies have reported the necessity of
coating lm with a thickness 18–23 nm was obtained aer
a free amine structural element in the formation of
24 h (Fig. S7†). AFM analyses on the DHI coating and PDA catecholic-based coating materials,64–67 the presence of
coating revealed that the morphology of DHI coatings was conjugation adducts derived from both DA and DHI is likely
different to that from PDA lms, as shown in Fig. 6a and b. to be the major determinants of the adhesion/cohesion
Further characterization of the DHI-based lm and PDA lm properties of PDA lms. The precise structures of DA/DHI
based on ATR-FTIR analyses showed that the ngerprint conjugates remain to be further identied.
region from 1880–780 cmꢁ1 of PDA lms was different to that
of DHI-based lm (Fig. 6c). The latter has higher intensity
bands around 1180 cmꢁ1 that can be assigned to the stretch-
ing of C–N, C–C, C–H bending vibrations and indole ring
vibrations and also a higher intensity band centered at
856 cmꢁ1 which is possibly due to the bending N–H vibrations,
suggesting that the constituents of PDA coatings are not
identical to coatings derived from DHI. Collectively, this study
and reported studies by Aleri et al.,31 did not support the
notion that the primary building blocks of PDA coatings are
DHI-only based oligomers, e.g. trimer complex in Ding's
study27 or porphyrin-type DHI-tetramer in Chen's modeling
study.62 As the classical PDA solution comprises a large
amount of DA precursor, micromolar-quantities of DHI and
also other possible intermediates present at low quantities
(not detectable by 1H-NMR spectroscopy), it is possible that
the highly reactive DHI/its oligomers can couple with DA to
form the main adhesive/cohesive units of the PDA
coating.23,25,63
Conclusion
In summary, this investigation provides in situ insights into
the nanoscale deposition of DHI-based coatings using HPEL
and elucidates the critical roles of DHI in the formation of PDA
coatings. To our knowledge, this is the rst report of the
deposition mechanism of DHI-based coatings. It is noted that
the deposition of DHI-based nano-coating is oxidation-
dependent and pH-dependent. Our studies also show that
competing self-aggregation of DHI oligomers can signicantly
inhibit the coating process, especially at alkaline pHs. It can
be rationalized that DHI is able to play a dual role to initiate
the coating process as well as to act as a ‘cross-linker’ in the
formation of PDA coating lms. Specically, we propose that
fast nano-deposition of PDA coatings onto the silicon
substrate in the initial stages of the coating process critically
involves the oxidation of DHI and/or its related oligomers.
This is followed by the coupling of DHI/its oligomers with DA-
related species to provide the adhesive/cohesive units for PDA
lm formation. Our data also supports the notion that the DA
monomer provides a critical structural element i.e. the free
primary amine.68 However we acknowledge that PDA coatings
may possess heterogeneity and our studies do not preclude
other parallel pathways to PDA coatings. The results of our
research provide critical information on the deposition and
formation of PDA coatings, which could be used to improve
the properties of PDA and DHI-based coatings, as well as
provide new insights for the development of catecholic coating
materials.
To support the view of conjugation between DA and DHI/
its oligomers, ESI- and APCI-Mass analyses was carried out on
the following samples: (i) mixture of DA/DHI sample (10 mM
DA, 1 mM DHI, Tris pH 8.5), (ii) DA–Tris sample (10 mM, Tris
pH 8.5) and (iii) DHI–Tris sample (1 mM DHI, Tris pH 8.5).
The main peak patterns observed from the DA/DHI sample (t
¼ 30 min) from mass spectral analysis were consistent with
that of DA sample (4 h) in our study and other reported PDA
studies (Fig. S8†).25,26 Higher intensity signals of common
peaks e.g. 390, 402, 420, 432 and 450 m/z, were found in the
spectra of DA/DHI sample (30 min) as compared to that of the
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time, the substrates were rinsed with water and ethanol, and
dried with a stream of nitrogen gas. These samples were
subsequently characterized as described below.
Experimental
In situ ellipsometry study on deposition kinetics of DA and
DHI precursors
Ellipsometry is a technique which measures the polarization of
light reected off the sample surface (Fig. S1†). It derives
sensitivity from measuring the phase shi D between normal
(p) and in-plane (s) components of polarization vector. For
layers much thinner than wavelength, d << l, D is proportional
to the thickness. Precision ellipsometry (PREL) uses modulation
of polarization, making it much more sensitive. In the present
setup, the light from laser pointer, l ¼ 650 nm, passes through
linear polarizer onto the substrate. Adsorbed molecules or
ultrathin layers change reected polarization to elliptic; the
retarder converts ellipticity D into rotation g, which is measured
using polarization modulator. Thus, g is proportional to D,
hence proportional to the thickness. For organic molecules on
silicon, 1 nm corresponds to 2 milliradians of polarization
rotation. This coefficient was used to obtain thickness scale on
all gures. With compact modulator made in A*STAR's Institute
of Materials Research and Engineering, this system measures
polarization rotation down to microradians. This translates to
sensitivity of 0.1 nm of attached layer thickness.
Characterization of coatings
AFM characterization was carried out using a Nanowizard III
instrument (JPK Instruments AG, Berlin, Germany) equipped
with NanoWizard head and controller. The triangular shaped
silicon nitride cantilevers (Nano World, PNP-TR) were used
throughout the scanning and the spring constant was cali-
brated using the thermal noise method, in the range of 0.07–
0.09 N mꢁ1. The experiments were performed in air, letting the
system equilibrate for 30–60 min. To carry out the measure-
ment, the coated samples were xed on glass slides before each
measurement. The quantitative imaging mode (QITM) was
performed for imaging. The QITM is a force spectroscopy based
imaging mode that enables the user to have the full control over
the tip-sample force at each pixel of the image so that the lateral
forces can be greatly minimized, making nondestructive
imaging straightforward. All images were processed using the
JPK data processing soware (Version 6.1.22). Attenuated total
reection (ATR)-Fourier transform infrared spectroscopy (FTIR)
analysis was carried out on a Bruker Alpha spectrometer
equipped with (RT) DLaTGS detector (Bruker, USA) and a plat-
inum ATR accessory with a 2 mm diameter single reection
diamond crystal. A clean silicon wafer and a quartz slide were
used as references respectively.
General procedure for deposition studies
(i) A clean silicon water (0.5 cm ꢂ 1.5 cm) was set inside the
cuvette of ellipsometry; when opening the pump and ow
control valve, deionized-water (DI-H2O) was owed through the
ellipsometry system; (ii) aer the reading output was stabilized
at a certain value, the ow control valve was closed and 1 mL
fresh DA or DHI samples at the specied buffers were injected
to drain the water out; (iv) the pump was then closed and the
injected samples were incubated for a predetermined time
(generally 20–30 min) to allow the coatings to deposit onto
silicon surfaces; (v) then both of the ow control valve and
pump system were opened and DI-H2O water was owed
through the cuvette for 15–20 min to wash off the injected
sample solutions and to test the underwater adhesion. In
separate experiments, Na2S2O4 (20 mM) was then injected into
the cuvette and incubated for a predetermined time (5–6 min),
followed by owing DI-H2O to test the durability of coatings
deposited on silicon. For each experiment, at least three
samples were tested.
Synthesis of 5,6-dihydroxyindole (DHI)
All reagents and solvents were purchased from Sigma Aldrich or
Alfa Aesar and were used without further purication unless
otherwise specied. NMR spectra were recorded on a Bruker
Avance III 400 MHz spectrometer at 400 MHz for 1H and at 100
MHz for 13C with methanol-d4 as solvent. The chemical shis
are given in ppm, using the proton solvent residue signal
1
(CD3OD: d 3.31) as a reference in the H NMR spectrum. The
deuterium coupled signal of the solvent was used as a reference
in 13C-NMR (CD3OD: d 49.00). The following abbreviations were
used to describe the signals: s ¼ singlet, d ¼ doublet, t ¼ triplet,
m ¼ multiplet, q ¼ quartet. DHI was prepared under nitrogen
atmosphere using the reported method.48
A mixture of
K3[Fe(CN)6] (6.6 g, 20 mmol) and NaHCO3 (2.5 g, 30 mmol) in
H2O (60 mL) was added dropwise over 5 min to a stirred solu-
tion of L-DOPA (0.99 g, 5 mmol) in 500 mL H2O. The resulting
solution was stirred at room temperature under nitrogen
atmosphere for 3 h following which 600 mg Na2S2O4 was added.
Substrates used for coating studies and general dip and coat
procedure
Silicon (Si) wafers were obtained from Mitsubishi Silicon The solution was then adjusted to pH 4 with 3 M HCl aq. and
America, USA. Quartz microscope slides (fused, 76.2 mm ꢂ extracted with ethyl acetate (250 mL ꢂ 3). The combined
25.4 mm ꢂ 1.0 mm) were purchased from Alfa Aesar. Si wafers organic phases were washed with saturated brine (100 mL ꢂ 3)
were pre-cleaned in a fresh H2O/NH4OH/H2O2 (6 : 1 : 1) solu- and were dried over Na2SO4. Evaporation of ethyl acetate to
tion at 150 ꢃC for 10 min, and then rinsed in deionized (DI) 5 mL followed by the addition of hexane (50 mL) yielded a pale
water and ethanol for 10 min each. Quartz slides substrates brown solid; the solids were then re-dissolved in ethyl acetate (5
were rst cleaned in an ultrasound water bath for 30 min, rinsed mL) followed by recrystallization from hexane (50 mL) to give
with ethanol, and then blown dry under nitrogen gas. The 287 mg (27% yield) of DHI as an off-white solid. 1H-NMR
substrates were then immersed in 20 mL of Tris buffers as (CD3OD): d (ppm) 6.209 (1H, dd, J ¼ 0.92, 3.08 Hz), 6.84 (1H,
indicated with specied precursors. Aer coating for a specied d, J ¼ 0.92 Hz), 6.946 (1H, s), 6.972 (1H, d, J ¼ 3.08 Hz); 13C-NMR
27700 | RSC Adv., 2018, 8, 27695–27702
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(CD3OD): d (ppm) 98.0, 101.4, 101.5, 122.4, 123.7, 132.1, 141.2,
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Conflicts of interest
The authors declare no conict of interest.
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Acknowledgements
This work was nancially supported by Ministry of Education
Academic Research Fund Tier 1 Grant (No. R148-000-252-114)
Singapore and supported by the Institute of Materials
Research and Engineering, Agency of Science, Technology and
Research (A*STAR), under project fund IMRE/17-1P1207.
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