Grafting poly(ionic liquid) brushes for anti-bacterial and anti-biofouling applications

Article abstract of DOI:10.1039/c2jm31527k

Polymer brushes provide an ideal platform for studying biofouling and screening anti-biofouling materials. In the present work, high-density poly(ionic liquid) brushes based on imidazolium salts were successfully grafted to surfaces via surface-initiated ring-opening metathesis polymerizations from catecholic initiator and their anti-microbial activity was evaluated. Very uniform poly(ionic liquid) coatings with thickness up to 80 nm were obtained on TiO₂. Various characterization techniques including infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis and transmission electron microscopy were used to characterize poly(ionic liquid) brushes modified TiO₂ nanomaterials. Subsequently, the anti-bacterial and anti-biofouling properties of poly(ionic liquid) brushes were evaluated. It was found that poly(ionic liquid) brushes can obviously resist adhesion of Chlorella spores and the counter-anions have a key impact on the anti-microbial property. The hexafluorophosphate anion poly(ionic liquid) coated TiO ₂ nanomaterials have excellent anti-bacterial properties compared to pristine TiO₂ nanoparticles against both E. coli and S. aureus.

Full text of DOI:10.1039/c2jm31527k

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PAPER
Grafting poly(ionic liquid) brushes for anti-bacterial and anti-biofouling
applications
*
Qian Ye, Tingting Gao, Fei Wan, Bo Yu, Xiaowei Pei, Feng Zhou and Qunji Xue
*
Received 12th March 2012, Accepted 30th April 2012
DOI: 10.1039/c2jm31527k
Polymer brushes provide an ideal platform for studying biofouling and screening anti-biofouling
materials. In the present work, high-density poly(ionic liquid) brushes based on imidazolium salts were
successfully grafted to surfaces via surface-initiated ring-opening metathesis polymerizations from
catecholic initiator and their anti-microbial activity was evaluated. Very uniform poly(ionic liquid)
coatings with thickness up to 80 nm were obtained on TiO2. Various characterization techniques
including infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis and
transmission electron microscopy were used to characterize poly(ionic liquid) brushes modified TiO2
nanomaterials. Subsequently, the anti-bacterial and anti-biofouling properties of poly(ionic liquid)
brushes were evaluated. It was found that poly(ionic liquid) brushes can obviously resist adhesion of
Chlorella spores and the counter-anions have a key impact on the anti-microbial property. The
hexafluorophosphate anion poly(ionic liquid) coated TiO2 nanomaterials have excellent anti-bacterial
properties compared to pristine TiO2 nanoparticles against both E. coli and S. aureus.
increasingly important to understand not only their chemical
Introduction
properties, but also their physical and biological properties and
environmental impact. As an analogue to some extent, the study
on the biological properties (e.g., antitumor, anti-cancer,
toxicity, DNA cleavage) of ionic liquids is an emerging research
field.13,14 The anti-microbial behavior is an important charac-
teristic of ILs. Recently examinations have shown imidazolium,
pyridinium and quaternary ammonium based ionic liquids have
significant anti-microbial activities.15,16 Expectedly, many ionic
liquids displayed biocidal activity against Gram-positive and
Gram-negative bacteria, fungi and algae.17,18 Interestingly, the
biological properties of ionic liquids are altered easily through
varying the cationic and anionic components.19
Biofouling, which is the undesirable growth of marine organisms
on artificial surfaces1,2 including ship hulls, aquaculture cages,
and pipelines, may cause serious problems for mariners, such as
excessive consumption of fuel, initiation of corrosion, increased
frequency of dry-docking and the introduction of invasive
species.1–4 More than 4000 species of marine organisms (micro-
organisms, plants and animals) are responsible for marine
biofouling.5 These species have different mechanisms of adhesion
owing to their different adhesive compositions, and they can
adapt to environmental changes, so the creation of effective anti-
biofouling surfaces is a very challenging task.6,7 The prevention
of the settlement of marine bacteria and diatoms over the surface
is very important since the primary settlement makes secondary
colonization by marine creatures easier.
Polymeric ionic liquids, also called poly(ionic liquid)s (PILs),
are polymers prepared by polymerization of ionic liquid mono-
mers.20 However, poly(ionic liquid)s are not liquid, and as solids
they lose the special characteristics of liquids. Since Ohno and Ito
first obtained polymerized ionic liquids (PILs) in 1998,21 they
have gained considerable attention.22 Poly(ionic liquid)s act as
a new class of functional polymer materials with unique prop-
erties combining those of molecular ionic liquids and specific
properties of polymers, such as film formation and processibility.
Recently, the interest in the poly(ionic liquid)s has increased due
to their particular properties, such as inherent conductivity,23
high thermal stability, excellent mechanical and electrochemical
properties,24 biocompatibility, and as good matrices for enzyme
immobilization.25 Much attention has thus been paid to these
poly(ionic liquid)s for their potential applications such as cata-
lytic membranes,26 ionic conductive materials, solid state poly-
electrolytes,27 catalysts,28 biosensors, high CO2 absorbing
For more than half a century, quaternary ammonium salts
have been well known to possess biocidal activity due to the
strong electrostatic interaction and lipophilic interactions with
the cell wall of microorganisms.8 After adsorption onto the
bacterial cell, quaternary ammonium salts can diffuse through
the cell wall, disrupt the cytoplasmic membrane causing leakage
potassium and other constituents, and finally cause cell death.9,10
Ionic liquids (ILs) consisting of ammonium, phosphonium,
imidazolium etc., are low-melting organic salts that have been
attracting considerable attention in the past decade.11,12 It is
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail:
zhouf@lzb.ac.cn
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resins29,30 and microwave absorbing materials.31 However, the
biological properties of poly(ionic liquid)s have not yet been
reported so far.
(40 ꢀC, 300 mTorr) to afford 60 g (71%) of a clear, colourless oil
as a mixture of exo and endo isomers.40
1H NMR (400 MHz, CDCl3) (ppm): major isomer (endo) 6.21
(dd, J ¼ 6.0, 3.2 Hz, 1H, H-1), 6.00 (dd, J ¼ 5.6, 2.8 Hz, 1H, H-2),
3.22 (dd, J ¼ 9.6, 6.8 Hz, 1H, H-8), 3.05 (t, J ¼ 9.6 Hz, 1H, H-8),
3.00 (s, 1H, H-6), 2.88 (s, 1H, H-3), 2.57–2.48 (m, 1H, H-5), 1.98–
1.92 (m, 1H, H-4exo), 1.50 (dd, J ¼ 8.4, 2.0 Hz, 1H, H-7), 1.31 (d,
J ¼ 8.4 Hz, 1H, H-7), 0.62–0.57 (m, 1H, H-4endo).
13C NMR (100 MHz, CDCl3) (ppm): 138.2 (C-1), 131.6 (C-2),
49.6 (C-7), 45.5 (C-8), 43.1 (C-5), 42.1 (C-6), 38.2 (C-3), 32.7
(C-4).
Polymer brushes are ordered polymeric films with one end of
the polymer chains being chemically attached to the substrate
surface. This imparts surfaces with a wide range of properties
including biocompatibility and bacterial resistance,32,33 while the
chemical bonding nature increases durability so allowing marine
fouling tests at relatively longer periods, and provides an ideal
model platform for screening potential anti-biofouling mate-
rials.34 Poly(ethylene glycol), poly(oligo(ethylene glycol)meth-
acrylate),
poly(2-hydroxyethyl
methacrylate),
and
poly(acrylamide) have been the most commonly studied anti-
fouling materials.35,36 Another class of antifouling material is
zwitterionic polymers with a mixture of anionic and cationic
terminal groups,35,37 such as poly(2-methacryloyloxyethyl phos-
phorylcholine), poly(sulfobetaine methacrylate) and poly-
(carboxybetaine methacrylate). Polymeric surfaces containing
alkylpyridinium or quaternary ammonium moieties have been
tested for anti-microbial applications.35 Anionic polyelectrolyte
brushes of poly(3-sulfopropylmethacrylate) can trap silver ions
and so have good anti-bacterial activity.38,39
Attaching ionic liquid moieties to polymer chains has the
potential of developing novel effective anti-biofouling materials.
To this end, we aimed to study the anti-bacterial and anti-
biofouling properties of poly(ionic liquid) brushes grafted hybrid
materials. Poly(ionic liquid) brushes were grafted on a titania
surface via surface initiated ring-opening metathesis polymeri-
zation (ROMP). We researched the anti-biofouling properties of
poly(ionic liquid)s grafting hybrids and found they significantly
resisted adhesion of Chlorella spores and had excellent anti-
bacterial properties against both E. coli and S. aureus.
2. Synthesis of ionic liquid monomer 1-norbornylmethylene-3-
methylimidazolium bromide (NM-MIm-Br). N-Methylimidazole
(4.92 g, 60 mmol) and norbornylmethylene bromide (9.35 g,
50 mmol) were placed into a 25 mL round-bottomed flask. The
ꢀ
reaction mixture was stirred for 48–60 h at 60 C under Ar to
yield a very viscous liquid, 1-norbornylmethylene 3-methyl-
imidazolium bromide. The crude reaction product was washed
with diethyl ether and hexane and the residual solvent was
evaporated under reduced pressure to give a brown viscous
liquid. The crude product was purified by silica gel column
chromatography to give a colorless viscous liquid (mixture of exo
and endo isomers, 9.82 g, yield 73%).
Experimental section
1H NMR (400 MHz, CDCl3), (ppm): major isomer (endo)
10.24 (s, 1H, H-11), 7.63 (s, 1H, H-9), 7.41 (s, 1H, H-10), 6.23
(dd, J ¼ 6.0, 3.2 Hz, 1H, H-1), 6.01 (dd, J ¼ 6.0, 2.8 Hz, 1H, H-2),
4.10 (s, 1H, H-12), 3.96–3.86 (m, 2H, H-8), 2.82 (s, 1H, H-6), 2.70
(s, 1H, H-3), 2.68–2.60 (m, 1H, H-5), 1.95–1.89 (m, 1H, H-4exo),
1.43 (dd, J ¼ 8.4, 2.0 Hz, 1H, H-7), 1.25 (d, J ¼ 8.4 Hz, 1H, H-7),
0.64–0.60 (m, 1H, H-4endo).
13C NMR (100 MHz, CDCl3), (ppm): 139.1 (C-1), 137.1
(C-11), 131.2 (C-2), 123.7 (C-9), 122.0 (C-10), 53.9 (C-8), 49.4 (C-
7), 44.0 (C-5), 42.3 (C-6), 39.8 (C-3), 36.7 (C-12), 29.9 (C-4).
HRMS (ESI+): m/z calc. for C12H17N2, M+: 189.1386, found
189.1384.
Materials
3-Hydroxytyramine hydrochloride (99%), 5-norbornene-2-
carboxylic acid (97%) and Grubbs 2nd generation catalyst
(benzylidene[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]-
dichloro(tricyclohexylphosphine)ruthenium) were used as
received from Sigma-Aldrich. Dicyclohexylcarbodiimide (DCC,
98%) and 4-dimethylaminopyridine (DMAP, 99%) were used as
received from the Chemical Reagent Co. of J&K Chemical Ltd
(Beijing, China). Dicyclopentadiene (98%) and N-methyl-
imidazole (99%) were used as received from Chemical Reagent
Company of Shanghai (Shanghai, China). Dichloromethane
(China) was purified by distillation over CaH2, and pyridine
(China) was purified by distillation over NaOH. Ultrapure water
used in all experiments was obtained from a NANOpure Infinity
system from Barnstead/Thermolyne Corporation. Other
reagents were used as received.
3. Synthesis of ionic liquid monomer 1-norbornylmethylene-3-
methylimidazolium hexafluorophosphate (NM-MIm-PF6). The
target compound was prepared via anion exchange for 6 h at
room temperature. A dry round-bottomed flask (50 mL) was
charged with 2.68 g NM-MIm-Br (10 mol), 1.96 g (12 mmol)
NH4PF6 and 30 mL of water. Then the reaction mixture was
stirred at room temperature for 6 h. The reaction solution was
extracted with CH2Cl2 (3 ꢁ 100 mL). The combined organic
extracts were dried over anhydrous MgSO4, and the solvent was
evaporated under reduced pressure to give a viscous liquid. The
crude product was purified by silica gel column chromatography
1. Synthesis of norbornylmethylene bromide (NM-Br). Dicy-
clopentadiene (30 mL, 224 mmol), allyl bromide (46 mL,
537 mmol) and hydroquinone (150 mg) were heated to 170 ꢀC in
a Teflon-valve sealed bomb for 20 h. After cooling, excess allyl
bromide was removed by heating the mixture to 100 ꢀC. The
remaining oil was purified by distillation at reduced pressure
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to give a viscous liquid (3.05 g, yield 95%) of NM-MIm-PF6
(mixture of exo and endo isomers).
spores. A uniform Ti sample was included in the assay and served
as a control for direct comparison. Just before the experiment,
each well of the samples was filled with deionized water and was
transferred to artificial seawater for 1 h without exposure to air.
The Chlorella spores inoculum was adjusted to 2.0 ꢁ 106 spores
mLꢂ1. Chlorella spores settled on the different substrates surfaces
were grown for 3 days in an illuminated incubator at room
temperature. After 3 days growth, the substrates were water-
rinsed (three times). Ten images of each of the three replicate
samples were photographed to quantify the number of attached
Chlorella spores.42
1H NMR (400 MHz, acetone), (ppm): major isomer (endo)
9.00 (s, 1H, H-11), 7.77 (s, 1H, H-9), 7.71 (s, 1H, H-10), 6.29 (dd,
J ¼ 6.0, 3.2 Hz, 1H, H-1), 6.14 (dd, J ¼ 6.0, 2.8 Hz, 1H, H-2),
4.06 (s, 1H, H-12), 4.04–3.97 (m, 2H, H-8), 2.86 (s, 1H, H-6), 2.79
(s, 1H, H-3), 2.77–2.74 (m, 1H, H-5), 1.98–1.92 (m, 1H, H-4exo),
1.44 (dd, J ¼ 8.4, 2.0 Hz, 1H, H-7), 1.32 (d, J ¼ 8.4 Hz, 1H, H-7),
0.74–0.70 (m, 1H, H-4endo).
13C NMR (100 MHz, acetone), (ppm): 139.6 (C-1), 137.4
(C-11), 132.4 (C-2), 125.0 (C-9), 123.6 (C-10), 54.4 (C-8), 50.1
(C-7), 44.9 (C-5), 43.4 (C-6), 40.7 (C-3), 36.8 (C-12), 31.2 (C-4).
HRMS (ESI+): m/z calc. for C12H17N2, M+: 189.1386, found
189.1384.
Characterization
1H and 13C NMR spectra were recorded on a 400 MHz spec-
trometer (Bruker AM-400) using CDCl3 as solvent. HRMS
spectra was obtained on a Bruker Daltonics micoTOF-QIImass
spectrometer. Chemical composition information of samples
were obtained by X-ray photoelectron spectroscopy (XPS); the
measurement was carried out on a PHI-5702 multi-functional
spectrometer using Al Ka radiation and the binding energies
were referenced to the C 1s line at 284.8 eV from adventitious
carbon. The morphology was investigated by transmission elec-
tron microscopy (TEM) (Hitachi Model JEM-2010). Thermal
stability was determined with a thermogravimetric analyzer
(TGA) (Netzsch, STA 449C) over a temperature range of 25–800
ꢀC at a heating rate of 10 ꢀC minꢂ1 under N2 atmosphere. FT-IR
4. Surface-initiated polymerization. The preparation and
modification of TiO2 nanowires were described in our previous
work.41 In a typical SI-ROMP, TiO2 nanowires modified by
a biomimetic anchor were immersed in a solution of Grubbs 2nd
generation catalyst (10 mg) in 5 mL dichloromethane and then
placed in a flask under Ar flow for 20 min. They were then
carefully rinsed (three times) with pure dichloromethane in order
to remove the excess catalyst. The [Ru]-functionalized samples
were immersed in 5 mL of freshly prepared solutions of NM-
MIm-PF6 (0.25 M) in dichloromethane. These polymerizations
were carried out at room temperature under Ar protection. At
various times, the poly(ionic liquid) grafted hybrids were isolated
from the polymerization solution and extensively washed with
pure dichloromethane using centrifugation. The thickness of the
poly(ionic liquid) brushes was controlled by changing the poly-
merization time. The hybrid samples were further dried under
vacuum overnight for further analysis. SIP on planar substrates
were performed in a similar way.
spectra were recorded on
a
TENSOR 27 instrument
(BRUCKER). Optical micrographs were taken on OLYMPUS
BX51.
Results and discussion
Poly(ionic liquid) brushes were grafted onto substrate surfaces
by various polymerization technologies, including electro-
polymerization,43 controlled/living radical polymerization44 and
reversible addition–fragmentation transfer polymerization.27
However, the grafting density or the thickness of poly(ionic
liquid)s is usually very limited using such techniques. In order to
increase the grafting density which would be an important factor
for a functional material, ring-opening metathesis polymeriza-
tion (ROMP) offers an effective way in polymerization of nor-
bornene monomers with side chain substituents by ionic liquid to
obtain high-density poly(ionic liquid) brushes from catecholic
initiator41 (Scheme 1). The good adhesion of the catecholic
initiator onto variable substrates45,46 allows modification of the
target substrates with chemically bonded poly(ionic liquid)
coatings.
5. Test of anti-bacterial activity. The anti-bacterial activity of
PNM-MIm-FP6 modified TiO2 nanoparticles as disperse solu-
tions were evaluated against S. aureus and E. coli, which was
investigated by suspending the bacteria in PBS and inoculating
given concentrations of the samples for each microorganism. In
these experiments, microorganisms were cultivated in sterilized
Lur-ia-Bertani (LB) broth and then incubated overnight at 37 ꢀC
with a shaking incubator. To investigate the anti-bacterial
performances of synthesized PNM-MIm-FP6-TiO2 nano-
particles under light irradiation and dark conditions, the samples
(5 mg) were dispersed in 1 mL physiological saline and then 1 mL
diluted microbial solution (1 ꢁ 107 CFUs per mL) was added,
bare TiO2 nanoparticles were made as the control. The resultant
ꢀ
solutions were inoculated at 37 C with a shaking incubator for
24 h, and half of the sample was wrapped in black tape to protect
the sample from light in order to obtain the anti-bacterial abili-
ties of the sample without light irradiation. Subsequently, 5 mL
suspensions of bacteria were spread on each LB agar plate and
The FT-IR spectrum of NM-MIm-Br monomer shows
a broad absorption peak at about 3423 cmꢂ1 in comparison to
NM-MIm-PF6 monomer due to more absorbed moisture
(Fig. 1A). The absorbed water is very difficult to remove, which is
a possible reason that NM-MIm-Br monomer could not be
polymerized in ROMP conditions (water would deactivate the
ROMP catalyst). The successful polymerization of NM-MIm-
PF6 on TiO2 nanowires was first confirmed by FT-IR spectros-
copy (Fig. 1B). Fig. 1B demonstrates the FT-IR spectrum
evolution from blank TiO2, initiator modified TiO2, to poly(ionic
liquid) brushes grafted TiO2 (2 h). Blank TiO2 shows
ꢀ
incubated for 24 h at 37 C.
6. Test of anti-biofouling activity. Chlorella sp. were obtained
from Institute of Hydrobiology (Chinese Academy of Sciences)
and cultured in artificial seawater at room temperature for
attachment experiments. Three replicates of poly(ionic liquid)s-
modified Ti samples were evaluated for settlement of Chlorella
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Scheme 1 Grafting poly(ionic liquid) brushes onto TiO2 surface.
characteristic of ammonium nitrogen (N+) of the poly(ionic
liquid) coating.26 Another obvious characteristic of the presence
of poly(NM-MIm-PF6) brushes is the large intensity F 1s peak at
686.8 eV (P–F).
TGA was carried out in order to evaluate the change of the
organic portion during polymer grafting at different polymeri-
zation times. As seen in Fig. 3, very similar two-step degradation
profiles for PNM-MIm-PF6-TiO2 nanowire samples were
observed. For instance, a typical poly(ionic liquid) sample
(PNM-MIm-PF6-TiO2 nanowires (2 h)) exhibited two thermal
decomposition processes at about 250 and 400 ꢀC, which shows
that the grafted poly(NM-MIm-PF6) has good temperature
stability. TGA of TiO2 nanowires showed 13.0% weight loss
Fig. 1 (A) FT-IR spectra of (a) NM-MIm-Br and (b) NM-MIm-PF6;
(B) FT-IR spectra of (a) TiO2 nanowires, (b) initiator-modified TiO2
nanowires, and (c) PNM-MIm-PF6-TiO2 nanowires (2 h).
ꢀ
between 100 and 600 C. Upon modification with ROMP initi-
a featureless spectrum except for the wide absorption peaks of
absorbed water above 3000 cmꢂ1. Curve c of Fig. 1B shows the
adsorption bands of poly(NM-MIm-PF6) at 3200–3090 cmꢂ1 and
3000–2850 cmꢂ1, which can be attributed to C–H stretching
vibration mode of the imidazolium ring and that of the alkyl
chains and characteristic vibration bands at 1575, 1460 and
1170 cmꢂ1, corresponding to the imidazolium ring cation.47 The
strong absorption peak at 841 cmꢂ1 is attributed to the absor-
bance of P–F stretching modes of PF6 anions. These absorption
bands are almost the same as those of the monomer (NM-MIm-
PF6) (Fig. 1Ab), indicating that poly(NM-MIm-PF6) has been
successfully functionalized onto TiO2 nanowires.
ator, the composite showed 14.7% weight loss, so that 1.7 wt% of
the composite was due to the modified ROMP initiator. TGA
provides evidence for the systematic increase in the amount of
poly(ionic liquid) with the polymerization time. According to
TGA results from PNM-MIm-PF6-TiO2 nanowires samples at
different polymerization times, the residual mass percentages at
600 ꢀC are 80.8% (15 min), 75.6% (30 min), 63.7% (1 h)
and 53.8% (2 h), indicating the grafting amount of poly(NM-
MIm-PF6) is 3.4% (15 min), 8.8% (30 min), 20.9% (1 h) and
30.8% (2 h).
The presence of poly(ionic liquid) layer was further observed
by TEM characterization. Non-grafted TiO2 nanowires were
observed as aggregates of size 20 to >100 nm. In contrast, PNM-
MIm-PF6 modified TiO2 nanowires had good dispersity, and no
obvious aggregation was observed. Fig. 4 shows high resolution
transmission electron microscopic (HRTEM) images of the
surface morphology of bare TiO2 nanowires and PNM-MIm-
PF6 coated TiO2 nanowires. From Fig. 4A, we can see relatively
clear crystalline orientation of TiO2 nanowires. In fact, all the
TiO2 nanowires surfaces were homogeneously coated with a very
uniform PNM-MIm-PF6 coating layer and the interface
boundary is very clear (Fig. 4B). The polymer layer has a lighter
color than the bulk TiO2 nanowires, and the thickness of
poly(ionic liquid) is increased to 80 nm (Fig. 4B) after 2 h
polymerization. The catecholic group can form a bidentate
binding between TiO2 and catechol and the binding energies have
XPS spectroscopy was employed to monitor the surface
chemical composition change of each reaction step. Fig. 1A
displays the XPS full survey spectra of TiO2 nanowires, initiator-
modified TiO2 nanowires and PNM-MIm-PF6-TiO2 nanowires
(2 h). Successful anchoring of initiator onto TiO2 nanowires was
ascertained by XPS.41 After 2 h SI-ROMP of the monomer NM-
MIm-PF6, poly(ionic liquid) brushes grafting was verified by
a substantial increase of N signal, the presence of strong F and P
signals, and the evident decrease of Ti signal due to attenuation
by the polymer layer. The N 1s XPS peaks of the sample before
and after poly(NM-MIm-PF6) grafting are shown in Fig. 2Ba
and b, revealing additional evidence for the formation of poly-
(ionic liquid) brushes on the TiO2 nanowires. The N 1s XPS
spectrum of the TiO2 initiator modified nanowires displays one
peak with a binding energy at 399.9 eV, which is attributable to
O]C–N species in the catecholic initiator. After the ROMP
reaction the N 1s peak appeared at 401.9 eV, which is
recently been estimated to be 25–30 kcal molꢂ1 48
, which suggests
that grafted poly(ionic liquid) brushes can be strongly bound to
the TiO2 nanowires surface.
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Fig. 2 (A) XPS survey spectra of TiO2 nanowires (a), initiator-modified TiO2 nanowires (b), PNM-MIm-PF6-TiO2 nanowires (2 h) (c). (B) N 1s XPS
spectra of initiator-modified TiO2 nanowires (a) and PNM-MIm-PF6-TiO2 nanowires (b), the inset is the high-resolution spectrum of the N 1s region of
PNM-MIm-PF6-TiO2 nanowires. (C) F 1s XPS spectra of PNM-MIm-PF6-TiO2 nanowires.
hydrophobic to hydrophilic forming a stable dispersion in an
aqueous solution of NaCl or NaBr. In contrast, PNM-MIm-PF6-
ꢂ
TiO2 nanowires substituted with other lipophilic anions ClO4
ꢂ
,
ꢂ
BF4 and N(SO2CF3)2 are water insoluble. The association
ꢂ
constants and hydrophobicities of ClO4ꢂ, BF4ꢂ, PF6 and
N(SO2CF3)2ꢂ are generally larger than hydrophilic Brꢂ and Clꢂ.
Stable dispersion in solution (NaCl, NaBr) is owing to charge
repulsion of the cations that counterbalance the van der Waals
interactions between TiO2 nanowires. In contrast, the dissocia-
ꢂ
ꢂ
ꢂ
tion of hydrophobic anions (ClO4
,
BF4
,
PF6 and
N(SO2CF3)2ꢂ) is unfavorable due to poor solvation of the anions
and the imidazolium cation with long alkyl chain in aqueous
solution.24 The tunable solubility in aqueous media is very useful
for expanding potential biological application in different
solutions.
Fig. 3 TGA traces of TiO2 nanowires (a), TiO2 initiator modified
nanowires (b), PNM-MIm-PF6-TiO2 nanowires (15 min) (c), PNM-
MIm-PF6-TiO2 nanowires (30 min) (d), PNM-MIm-PF6-TiO2 nanowires
(1 h) (e) and PNM-MIm-PF6-TiO2 nanowires (2 h) (f).
To create a surface with anti-bacterial properties, it is desirable
that the surface is toxic to bacteria at contact so as to hinder
attachment, colonization and growth. Gram-positive (S. aureus)
and Gram-negative (E. coli) bacteria represent two major classes
of bacteria and were used for testing of anti-bacterial properties
of poly(NM-MIm-PF6) grafted TiO2 nanoparticles. Poly(NM-
MIm-PF6) grafted TiO2 nanoparticles were sprayed with S.
aureus and inoculated overnight. LB agar was spread onto a Petri
plate and then S. aureus culture was spread onto it. It is antici-
pated that the PNM-MIm-PF6-TiO2 nanoparticles have
improved anti-bacterial properties compared to pristine TiO2
nanoparticles because of the synergic anti-bacterial activities of
nanosized quaternary ammonium group containing polymer49,50
and photodisinfective TiO2 nanoparticles.51,52 Fig. 6 illustrates
the inactivation of S. aureus on TiO2 nanoparticles and poly-
(NM-MIm-PF6) brushes modified TiO2 nanoparticles surfaces
Modification by poly(NM-MIm-PF6) brushes can greatly
change the solubility of the TiO2 nanowires in different solvents.
PNM-MIm-PF6-TiO2 nanowires show good dispersion in
acetone and precipitate from water solution. The imidazolium
group can be paired with different anions, such as Clꢂ, Brꢂ,
ClO4ꢂ, BF4 and N(SO2CF3)2ꢂ. These anions have different
ꢂ
solvation capabilities and so the solubility of the modified TiO2
nanowires can be altered. PNM-MIm-PF6-TiO2 nanowires
cannot be dispersed in water due to the strong ion pairing
ꢂ
interaction of lipophilic anions (PF6
) with imidazolium
(Fig. 5Aa). However, when hydrophobic anion-coupled TiO2
nanowires are treated with concentrated hydrophilic anion
solutions, they become water-soluble. From Fig. 5Ba and b, it is
seen that the PNM-MIm-PF6-TiO2 nanowires switched from
Fig. 4 TEM images of TiO2 nanowires (A) and PNM-MIm-PF6 modified TiO2 nanowires (2 h) (B).
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thus show enhanced photocatalytic anti-bacterial properties
compared to pristine TiO2 nanoparticles against both E. coli and
S. aureus due to the synergic anti-bacterial performances of
biocidal poly(ionic liquid) and light-sensitive active TiO2 mate-
rials. Even in the dark condition, the PNM-MIm-FP6 coating
TiO2 nanoparticles exhibited excellent anti-bacterial efficiency,
which is attributed to the high density of imidazolium groups of
poly(NM-MIm-PF6) on the nanoparticle surfaces.
Fig. 7 shows 4.0 mm ꢁ 4.0 mm tapping-mode AFM phase
Fig. 5 (A) Photographs of PNM-MIm-PF6 grafted TiO2 nanowires in
(a) water and (b) acetone. (B) The dispersion properties of PNM-MIm-
PF6-TiO2 nanowires in different electrolyte solutions: (a) 0.1 M NaCl, (b)
0.1 M NaBr, (c) 0.1 M NaClO4, (d) 0.1 M NaBF4, (e) 0.1 M
LiN(SO2CF3)2.
images of Ti, PMETAC-Cl-Ti (PMTEC
¼
poly[2-(meth-
acryloyloxy)ethyltrimethylammonium chloride]) and PNM-
MIm-PF6-Ti. Before polymer modification (Fig. 7A), small
particles, about 50–200 nm grain size, height ranges from 5 to 30
nm, are seen uniformly distributed over the whole Ti surface.
After modification with polymer brushes (PMETAC-Cl and
PNM-MIm-PF6) onto the Ti surface (Fig. 7B and C), the
roughness of the polymer brushes modified Ti surface becomes
low, which shows that polymer completely masks the Ti surface.
To research the anti-biofouling property of the poly(ionic
liquid) brushes (different counter-ion) modified titanium
surfaces, organic Chlorella spores adhesion test on bare Ti
substrates and poly(ionic liquid) (about 30 nm) immobilized on
Ti surfaces were tested under artificial seawater conditions.55
Bare Ti surface was coated with the poly(ionic liquid) brushes to
promote cell attachment by conversion from a hydrophobic to
a hydrophilic surface via ion-exchange altering wettability (other
under light irradiation and dark condition after 24 h cultivation.
Compared with the control, a marked difference was observed in
the plates containing TiO2 nanoparticles in that the growth of
bacterial colonies was prevented evidently in both light (Fig. 6C)
and dark conditions (Fig. 6D). These results indicate that the
activity against S. aureus increases in the order of C > D > A > B,
clearly indicating that poly(NM-MIm-PF6) brushes plays an
important role in inhibiting the growth of S. aureus. A similar
result was observed in the anti-bacterial test against E. coli. It has
been suggested that ionic liquid immobilized on a surface can
provide effective anti-microbial activity through a contact
mechanism. After the microbial cells are adsorbed on hybrid
materials surfaces by electrostatic or hydrophobic interaction, or
both,53 ionic liquids may penetrate into the microbial cell wall
and disrupt the cytoplasmic membrane and release potassium
and other constituents, eventually causing cell death. Moreover,
the presence of imidazolium rings in the structure of the poly-
(NM-MIm-FP6) brushes enhances the anti-bacterial activity
significantly.54 The PNM-MIm-FP6-TiO2 hybrid nanomaterials
ꢂ
samples were obtained via anion exchange of PF6 counter-
ꢂ
anions (PNM-MIm-PF6-Ti) with an excess of Clꢂ, BF4 and
N(SO2CF3)2ꢂ). All surfaces were tested with a single batch of
Chlorella spores allowing direct comparisons to bare Ti surface.
Representative light micrographs of settled spores (Fig. 8) indi-
cated some general trends regarding settlement preference on
these substrates. Numerous organic Chlorella spores were
attached to the bare surfaces of Ti after 3 days in the spores
Fig. 6 Inhibitory effects against S. aureus after 24 h of incubation with and without light irradiation condition, (A) TiO2 nanoparticles, light; (B) TiO2
nanoparticles, dark; (C) PNM-MIm-PF6-TiO2 nanoparticles, light; (D) PNM-MIm-PF6-TiO2 nanoparticles, dark. Inhibitory effects of against E. coli
after 24 h of incubation with and without light irradiation condition, (E) TiO2 nanoparticles, light; (F) TiO2 nanoparticles, dark; (G) PNM-MIm-PF6-
TiO2 nanoparticles, light; (H) PNM-MIm-PF6-TiO2 nanoparticles, dark (scale bars ¼ 20 mm).
13128 | J. Mater. Chem., 2012, 22, 13123–13131
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Fig. 7 AFM images of Ti (titanium deposited on silicon wafer) before (A) and after modification of (B) PMETAC-Cl (2 h), (C) PNM-MIm-PF6 (2 h);
(D–F) 3D images of the same z-scale (30 nm).
culture media (2.5 ꢁ 106 spores mLꢂ1). The average Chlorella
spores density was 1490 ꢃ 290 spores mmꢂ12 for bare Ti surfaces
(Fig. 8F, left column). All settlement data showed a statistically
significant reduction in spore density on poly(ionic liquid)
brushes coated Ti surfaces relative to the bare Ti surfaces
(Fig. 8F). A lower mean spores density was measured on the
PNM-MIm-Cl-Ti surfaces (760 ꢃ 143 spores mmꢂ2), this trend
was also observed for poly(ionic liquid)s with BF4 and
N(SO2CF3)2 anion (PNM-MIm-BF4-Ti surfaces (360 ꢃ 103
spores mmꢂ2), PNM-MIm-N(SO2CF3)2-Ti surfaces (844 ꢃ 239
spores mmꢂ2)). The experimental results showed that Chlorella
spores settlement density was the lowest on the PNM-MIm-PF6
modified Ti surfaces (310 ꢃ 102 spores mmꢂ2). We also studied
poly[2-(methacryloyloxy)ethyltrimethylammonium
chloride]
(PMETAC) brushes coated Ti surfaces, which were obtained via
surface-initiated atom transfer radical polymerization (SI-
ATRP) using catecholic initiator46 according to the previous
literature.56 Polymer brushes bearing ammonium moieties have
been demonstrated for anti-bacterial performance.1,6 PMETAC
brushes (about 25 nm) grafted Ti surfaces (different counter-
ions) were placed in the spores culture media and grown for 3
days in the same condition (2.5 ꢁ 106 spores mLꢂ1). To our
surprise, a large spore density was observed on PMETAC
brushes-Ti even when compared with the unmodified Ti surfaces
Fig. 8 Representative optical micrographs of settled organic Chlorella spores on unmodified Ti surfaces (A), PNM-MIm-Cl-Ti (B), PNM-MIm-BF4-Ti
(C), PNM-MIm-PF6-Ti (D), PNM-MIm-N(SO2CF3)2-Ti (E). Organic Chlorella spores settlement data on bare and poly(ionic liquid) brushes (different
counter-ions) coating Ti surfaces (F).
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Fig. 9 Representative optical micrographs of settled organic Chlorella spores on unmodified Ti surfaces (A), PMETAC-Cl-Ti (B), PMETAC-BF4-Ti
(C), PMETAC-PF6-Ti (D), PMETAC-N(SO2CF3)2-Ti (E). Organic Chlorella spores settlement data on bare and PMETAC brushes (different counter-
ions) coating Ti surfaces (F).
(Fig. 9): bare Ti surface (1490 ꢃ 290 spores mmꢂ2), PMETAC-
Cl-Ti surfaces (3272 ꢃ 437 spores mmꢂ2), PMETAC-BF4-Ti
surfaces (1839 ꢃ 223 spores mmꢂ2), PMETAC-PF6-Ti surfaces
(1231 ꢃ 228 spores mmꢂ2), and PMETAC-N(SO2CF3)2-Ti
surfaces (2982 ꢃ 119 spores mmꢂ2). These results show that
PMETAC brushes modified titanium surfaces could not prevent
Chlorella spores adhesion effectively. So we can clearly observe
that poly(NM-MIm) brushes plays an important role in the
activity of hybrid materials to efficiently inhibit the adhesion of
spores. From Fig. 8F, it is seen that the anti-adhesion of poly-
brushes. We evaluated anti-biofouling properties of poly(ionic
liquid) brushes grafting hybrid materials. Of the microorganisms
tested, Chlorella spores adhesion proved to be particularly
susceptible to the action of poly(ionic liquid), the tunability of
surface structure via anion exchange allows the anti-biofouling
properties to be tailored rationally. Moreover, the obtained
PNM-MIm-FP6-TiO2 nanomaterials have excellent anti-bacte-
rial properties compared to pristine TiO2 nanoparticles against
both E. coli and S. aureus under both light irradiation and dark
condition. The eco-friendly poly(ionic liquid)–TiO2 nano-
materials can be applied to various anti-microbial applications
ranging from light activated systems to the dark sterilization
approach.
ꢂ
(ionic liquid) incorporated with N(SO2CF3)2 was poorer than
ꢂ
that with BF4 or PF6ꢂ, this is because the lipophilic anions
ꢂ
N(SO2CF3)2 are larger and more hydrophobic and so more
effectively shield positive charges of poly(ionic liquid) (positive
charges can tightly bind the adsorbed microbial cells and prevent
their subsequent growth and proliferation, including biofilm
formation).57 The anti-biofouling of poly(ionic liquid)s
combined with BF4ꢂ or PF6ꢂ was more efficient, which might be
due partially to the hydrolytic effects of the anion (e.g., BF4ꢂ and
Acknowledgements
The work is financially supported by NSFC (21125316,
50835009), ‘‘Top Hundred Talents’’ Program of CAS and the
Key Research Program of the Chinese Academy of Sciences,
Grant no. KJZD-EW-M01.
BF6ꢂ) leading to fluoride formation in water.58,59 BF4ꢂ and PF6
ꢂ
can both undergo hydrolysis in water even at room tempera-
ture;60 released fluoride ion is an effectual inhibitor of Na+, K+
ATPase, which is located at the cell surface and thus may
interfere in the self-maintenance of the cell.61 The anti-biofouling
activity of poly(ionic liquid) incorporated with perfluorinated
anions may thus be enhanced owing to the generation of free
fluoride ion.
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