polymers
Article
Protein-Repellence PES Membranes Using
Bio-grafting of Ortho-aminophenol
Norhan Nady 1,2, *, Ahmed H. El-Shazly 2 , Hesham M. A. Soliman 3 and Sherif H. Kandil 4
1
2
3
4
*
Polymeric Research Department, Advanced Technology and New Materials Research Institute (ATNMRI),
City of Scientific Research and Technological Applications (SRTA-City), New Boarg El-Arab City 21934,
Alexandria, Egypt
Chemical and Petrochemical Engineering Department, School of Energy, Environmental and Chemical and
Petrochemical Engineering, Egypt-Japan University of Science and Technology (E-JUST),
New Boarg El-Arab City 21934, Alexandria, Egypt; elshazly_a@yahoo.com
Nanomaterials Research Department, Advanced Technology and New Materials Research
Institute (ATNMRI), City of Scientific Research and Technological Applications (SRTA-City),
New Boarg El-Arab City 21934, Alexandria, Egypt; h.soliman@mucsat.sci.eg
Department of Materials Science, Institute of Graduate Studies and Research, Alexandria University,
Alexandria 21544, Egypt; s.kandil@usa.net
Correspondence: Norhan.Nady77@yahoo.com; Tel.: +20-109-091-8521
Academic Editor: Katja Loos
Received: 3 July 2016; Accepted: 8 August 2016; Published: 15 August 2016
Abstract: Surface modification becomes an effective tool for improvement of both flux and selectivity
of membrane by reducing the adsorption of the components of the fluid used onto its surface.
A successful green modification of poly(ethersulfone) (PES) membranes using ortho-aminophenol
(2-AP) modifier and laccase enzyme biocatalyst under very flexible conditions is presented in this
paper. The modified PES membranes were evaluated using many techniques including total color
change, pure water flux, and protein repellence that were related to the gravimetric grafting yield.
In addition, static water contact angle on laminated PES layers were determined. Blank and modified
commercial membranes (surface and cross-section) and laminated PES layers (surface) were imaged
by scanning electron microscope (SEM) and scanning probe microscope (SPM) to illustrate the formed
modifying poly(2-aminophenol) layer(s). This green modification resulted in an improvement of
both membrane flux and protein repellence, up to 15.4% and 81.27%, respectively, relative to the
blank membrane.
Keywords: protein-repellence; poly(ethersulfone); green surface modification; bio-catalysis;
ortho-aminophenol
1. Introduction
Membrane fouling can be defined as the irreversible adsorption/deposition of components on
and into the surface of membrane. Reversible fouling can be easily removed by backwashing (reverse
the direction of the rinsing fluid) of the membrane; thus, reversible fouling cannot be considered as
great a problem as irreversible fouling. Fouling depends mainly on the physicochemical interactions
between the membrane material and the components in the flowing fluid. The properties of both the
components and the membrane material are of importance, because they determine whether these
components can attach/adsorb onto the membrane surface [1,2]. Membrane fouling can be caused by
various components, such as colloidal particles [3,4], minerals [5,6], antifoam [7,8], proteins [9,10], and
microorganisms [11,12]. Various remediation methods have been proposed to influence the interaction
between these foulants (i.e., the components that can be adsorbed on or onto the membrane surface) and
membranes, including adjustment of the system hydrodynamics [13–15], surface modification [16,17],
Polymers 2016, 8, 306; doi:10.3390/polym8080306
www.mdpi.com/journal/polymers
Polymers 2016, 8, 306
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and regular cleaning [18,19]. Regular cleaning can be done physically (i.e., backwashing or mechanical
vibration) [17,18] or chemically (i.e., flowing acidic, alkalin, cleaning agent, and/or surfactant) [19].
Practically, in-depth fouling is very hard to remove because the foulant will also partially block the
local flow that is needed to remove and carry away the foulant. Clearly and even more relevantly,
once a foulant is attached to the membrane, it works as an initiator for attachment of more foulants.
For example, protein adsorption can be an initial step for attachment and growth of microorganisms
(i.e., biofilm formation) [20].
The hydrophobicity of the membrane surface has been stated as the reason of membrane fouling.
Thus, increasing the hydrophilicity of different hydrophobic membrane materials has been reported
using surface modification methods, including coating, composite, blending, chemical, plasma
and UV-assistant grafting, or a combination of two or more methods [16,17]. The effectiveness
of increasing the hydrophilicity of the membrane surface for the minimize adsorption of foulants such
as protein has been explained as depending on the fact that the hydrophilic surface attracts so much
water that adsorption of proteins is reduced [17,21,22]. However, surface structure has significant
impact on membrane anti-fouling performance [23]. In this respect, both steric hindrance and the
osmotic effect of hydrated (grafted) polymer brushes contribute to resistance against membrane
fouling [24]. Besides, novel low-fouling surfaces or membranes with a high regular “patterns” have
been investigated [25]. Although the effect of the membrane roughness on its tendency for foulant
adsorption is debated [17], the change in the surface topography by applying such regular patterns
has been shown to enhance its repellence for different foulants [26,27]. The definite explanation of the
effect of these novel surface patterns is not completely understood yet. Meanwhile, the interplay by
different influencing parameters such as surface energy, interfacial mass transfer, the hydrodynamic
interactions, etc. has been proposed to induce localized turbulence and/or shear stresses that help in
reduction the foulant adsorption [28–31].
The position of side groups (i.e., amino and hydroxyl) in aminophenol strongly affect on its
polymerization and on the structure of the produced oligomers or polymers of poly(aminophenol)
(see Figure 1) [32,33]. The chemical and electrochemical polymerization of the three isomers of
aminophenol have been presented [32]. The electrochemical oxidation of 4-aminophenol (4-AP) on
different materials of electrode has been published [34–37]. In these studies, 4-benzoquinone was
the proposed product of the electrooxidation of the 4-AP in aqueous medium [23–25]. In addition,
the effect of chemically polymerized 4-AP on inhibition the growth of different bacterial strains such
as Staphylococcus aureus (gram positive) and Escherichia coli (gram negative) at different levels of the
polymer concentration has been reported [37]. In addition, the enzymatic-catalyzed polymerization of
4-AP onto poly(ethersulfone) (PES) membranes has been reported to significantly improve the protein
repellence of the modified membranes [38], whereas the chemical and electrochemical polymerization
of 3-aminophenol (3-AP) takes place via C–O and/or C–N coupling [32,39]. The electrochemical
polymerization of 3-AP in aqueous solution on SnO2 electrodes proposed that only the amino group
was oxidized while the hydroxyl group remained unchanged [39]. However, the oxidation of the 3-AP
on a platinum electrode in acid medium yielded a polymeric product that may have a crosslinked or
a linear structure analogous to polyphenol [32]. Meanwhile, the structure of the electrochemical
polymerization of 2-aminophenol (2-AP) has been investigated by different researchers [40–42].
Most studies proposed that the formed oligomers or polymers of poly(2-AP) have a ladder structure
with a repeating phenozaxine units [40,41]. In addition, a linear dimer formed by N–N coupling;
2,20 -dihydroxyazobenzene, has been identified as a polymerization product of 2-AP in non-acidic
media [42].
Polymers
2016, 8,
306
Polymers
Polymers 2016,
2016, 8,
8, 306
306
33 of
18
3 of
of 17
17
Figure
Molecular structure
structure of
of 2-aminophenol,
2-aminophenol,
Figure 1.1.
1. Molecular
Molecular
structure
of
2-aminophenol,
0 -dihydroxyazobenzene.
2,2
2,2′-dihydroxyazobenzene.
2,2′-dihydroxyazobenzene.
4-aminophenol,
4-aminophenol,
4-aminophenol,
phenoxazine,
phenoxazine,
and
and
Poly(arysulfone)
membranes
such
poly(ethersulfone)
(PES)
have
aa high
ability
to
Poly(arysulfone) membranes
membranessuch
suchasas
as
poly(ethersulfone)
(PES)
have
high
ability
to interact
interact
Poly(arysulfone)
poly(ethersulfone)
(PES)
have
a high
ability
to interact
with
with
different
molecules
such
as
organic
substances,
proteins,
live
cells,
etc.
that
result
in
membrane
with different
molecules
such
as organic
substances,
proteins,
live
cells,
thatinresult
in membrane
different
molecules
such as
organic
substances,
proteins,
live cells,
etc.
thatetc.
result
membrane
fouling.
fouling.
(or
eliminate)
fouling,
several
surface
modification
methods
have
been
fouling.
To
reduce
(or this
eliminate)
this
fouling,
several
surfacemethods
modification
methods
have[43,44].
been
To
reduceTo
(orreduce
eliminate)
fouling,this
several
surface
modification
have been
reported
reported
[43,44].
In
previous
work
[45–47],
the
modification
of
PES
membranes
using
laccase
reported
[43,44].
In
previous
work
[45–47],
the
modification
of
PES
membranes
using
laccase
In previous work [45–47], the modification of PES membranes using laccase biocatalyst and phenolic
biocatalyst
and
phenolic
acids
has
investigated
[48].
used
enzyme
has
biocatalyst
andhas
phenolic
acids modifiers
modifiers
has been
been laccase
investigated
[48].
The
used
laccase the
enzyme
has
acids
modifiers
been investigated
[48]. The
used
enzyme
hasThe
ability
to laccase
oxidize
phenolic
ability
to
the
phenolic
acids
[48,49]
their
radical
forms.
formed
ability[48,49]
to oxidize
oxidize
the free
phenolic
acids
[48,49]
toformed
their free
free
radicalradicals
forms. The
The covalently
formed reactive
reactive
radicals
acids
to their
radical
forms.
Theto
reactive
can
react radicals
to
form
can
react
form
homopolymer
and/or
bind
onto
membrane,
mostly
their
can covalently
covalentlyand/or
react to
tobind
form
homopolymer
and/or
bindthrough
onto PES
PES
membrane,
mostly through
through
their
homopolymer
onto
PES membrane,
mostly
their
phenolic OH-groups,
as shown
phenolic
OH-groups,
as
shown
in
the
proposed
layer
structure
in
Figure
2
[50].
Furthermore,
phenolic
OH-groups,
as shownininFigure
the proposed
layer structure
Figure 2 [50]. Furthermore,
this
in
the proposed
layer structure
2 [50]. Furthermore,
this in
laccase-catalyzed
modification this
has
laccase-catalyzed
modification
has
been
used
to
modify
the
PES
membrane
using
4-aminophenol
laccase-catalyzed
modification
has
been
used
to
modify
the
PES
membrane
using
4-aminophenol
been used to modify the PES membrane using 4-aminophenol isomer and induces significant reduction
isomer
and
induces
reduction
isomer
andadsorption
induces significant
significant
reduction in
in protein
protein adsorption
adsorption [38].
[38].
in
protein
[38].
R
R22
R
R11O
O
R
R22
nn
**
O
O
S
S
O
O
PES
PES
R
R11OH
OH
O
**
O **
laccase
m
m laccase
R
R11O n
O n
O
O
R
R22
R
R1
OH
OH 1
R
R22
R
R11
R
R11O
O
O
O
S
O
S
O **
O
O
m
m
modified
modified PES
PES
R1
R1 == H
H or
or OH
OH
R2
R2 == COOH
COOH or
or CH=CH-COOH
CH=CH-COOH
Figure
2. Tentative
mechanism for
the reaction
of laccase-generated
radicals with
poly(ethersulfone)
Figure
Figure 2.
2. Tentative
Tentative mechanism
mechanism for
for the
the reaction
reaction of
of laccase-generated
laccase-generated radicals
radicals with
with poly(ethersulfone)
poly(ethersulfone)
(PES),
and
subsequent
formation
of
grafted
brushes
[50].
(PES),
(PES), and
and subsequent
subsequent formation
formation of
of grafted
grafted brushes
brushes [50].
[50].
In
2-AP
isomer
modifier
was used
to be grafted
the PES
membrane)
this
paper,
2-AP
isomer
modifier
was
used
(i.e.,
to
grafted
onto
the
In this
thispaper,
paper,
2-AP
isomer
modifier
was (i.e.,
usedmonomer
(i.e., monomer
monomer
to be
beonto
grafted
onto
the PES
PES
and
enzyme
laccase
biocatalyst
to
modify
the
PES
membrane;
5
and
15
mM
2-AP
modifier
and
membrane)
and
enzyme
laccase
biocatalyst
to
modify
the
PES
membrane;
5
and
15
mM
2-AP
membrane) and enzyme laccase biocatalyst to modify the PES membrane; 5 and 15 mM 2-AP
1 laccase
−1
0.5
U·mL−and
biocatalyst
were
used to modify
the PES
membranes
at 30,
60, and 120at
modifier
0.5
biocatalyst
were
to
the
membranes
30,
60,
−1 laccase
modifier
and
0.5 U·mL
U·mL
laccase
biocatalyst
were used
used
to modify
modify
the PES
PES
membranes
atmin
30,reaction
60, and
and
(modification)
times;
sodium acetate
(0.1
M of buffer
pH 5.5)
was
used;
blank
andand
modified
120
(modification)
times;
sodium
acetate
(0.1
M
5.5)
used;
blank
120 min
min reaction
reaction
(modification)
times;buffer
sodium
acetate
buffer
(0.1
M of
of pH
pH and
5.5) was
was
used;
and
blank
and
commercial
were
characterized
the
techniques:
the
commercial
membranes
weremembranes
characterized
using
the followingusing
techniques:
the membrane
total color
and modified
modified
commercial
membranes
were
characterized
using
the following
following
techniques:
the
membrane
total
color
change,
the
added
modifier
per
unit
area
(the
grafting
yield),
pure
water
flux,
change,
the
added
modifier
per
unit
area
(the
grafting
yield),
pure
water
flux,
static
water
contact
membrane total color change, the added modifier per unit area (the grafting yield), pure water flux,
static
contact
angle
on
PES
on
silica
dioxide
layer
angle
on laminated
on silica slides
(silicon
layer(silicon
on silicon
support),
reduction
static water
water
contact PES
anglelayer
on laminated
laminated
PES layer
layer
on dioxide
silica slides
slides
(silicon
dioxide
layer on
on silicon
silicon
support),
reduction
in
irreversible
protein
adsorption,
and
flux
reduction
due
to
the
irreversible
in
irreversible
protein
adsorption,
and
flux
reduction
due
to
the
irreversible
protein
adsorption.
support), reduction in irreversible protein adsorption, and flux reduction due to the irreversible
protein
protein adsorption.
adsorption. A
A scanning
scanning electron
electron microscope
microscope (SEM)
(SEM) was
was used
used to
to image
image both
both the
the surface
surface and
and
the
the cross-section
cross-section of
of the
the commercial
commercial blank
blank (unmodified)
(unmodified) and
and modified
modified membranes
membranes and
and also
also to
to image
image
Polymers 2016, 8, 306
4 of 18
A scanning electron microscope (SEM) was used to image both the surface and the cross-section of
the commercial blank (unmodified) and modified membranes and also to image the surface of both
the blank and the modified laminated PES layer. A scanning probe microscope (SPM) was used to
illustrate the shape of the formed modifying layer on the laminated PES layer. The general outlook of
the obtained data allowed us to conclude the effect of bio-catalyzed grafting of PES membranes (or
surfaces) by 2-AP modifier on antifouling property of PES membrane or surface. Part of this work
has been presented at the 2014 World congress on Advances in Civil, Environmental and Materials
Research (ACEM14), Busan, Korea [51].
2. Materials and Methods
2.1. Materials
The following materials were purchased from Sigma-Aldrich (Saint Louis, MO, USA):
Ortho-aminophenol (2-AP, ≥99.5%), dichloromethane ACS (DCM, 99.9%), catechol (>97%), sodium
acetate (≥99%), and acetic acid (99.9%). Poly(ethersulfone) (PES) polymer was gifted from BASF
(Ludwigshafen, Germany). One hundred fifty-millimeter silicon wafers with 2.5 nm native oxide top
layer were obtained from wafer Net Inc. (San Jose, CA, USA). Commercial PES membranes were
purchased from Sartorius (Göttingen, Germany) (0.2 µm pore size, symmetric, 50 mm diameter, 150 µm
thickness, > 28 mL·min−1 ·cm−2 water flow rate at ∆P = 1 bar). Laccase (>10.4 U·mg−1 ) from Trametes
versicolor was obtained from Fluka (Gillingham, England). MilliQ water was used to prepare fresh
solutions before each experiment.
2.2. Enzyme Activity (Assay)
The activity of laccase enzyme has been determined as was described in previous work [52] using
catechol [53]. The amount of enzyme required to oxidize 1 µmol of catechol per min at 25 ± 1 ◦ C was
used as a unit of laccase activity.
2.3. Model Membrane Preparation
Silicon strips coated by a thin layer of silicon dioxide were used as support of pure PES layers
(surfaces) as was described in previous work with slight modification [50]. The square (1 × 1 cm2 )
silicon dioxide strips were used to laminate PES layer on it using spin-coating of 0.5% w/w polymer
solution (PES) in DCM solvent. Coating time and speed were 30 s and 2500 rpm, respectively.
2.4. Membrane (Surface) Modification
The membranes were laid on the bottom of glass beakers including 40 mL sodium acetate buffer
(0.1 M) containing both the 2-AP modifier and the laccase enzyme. Air was bubbled (as O2 source and
for gentle mixing) inside the reaction medium. Reaction times of 30, 60, or 120 min (modification) were
used. The membranes were washed by strong spray flushing of deionized water, followed by dipping
and shacking in freshly boiled deionized water for three times. The modified membranes were kept in
desiccators for 48 h before using.
2.5. Membrane Color Change
The CIELAB coordinates for the modified real membranes were measured with SP62 Sphere
Spectrophotometer (Lansing, MI, USA) as was described in previous work [50–55]. ∆E* (membrane
total color change) was determined relative to the blank commercial membrane that was used as a
standard (compare mode was used). The smallest aperture size (8 mm diameter) was used. The average
of three readings, which were taken from three different places on each sample, was calculated.
The membranes were dried for more than 48 h before the membrane color change was determined.
Polymers 2016, 8, 306
5 of 18
2.6. Pure Water Flux
Water flux was measured as was described in previous work [48] using a Millipore stirred
ultrafiltration cell with 13.4 cm2 active transport area. Dead-end operation mode was used.
2.7. The Added Modifier per Unit Membrane Surface Area (Grafting Yield)
The added modifier was calculated gravimetrically from the difference in weight of the membrane
before and after modification and was related to the unit area of membrane surface as was described
in previous work [49,50]. The membranes were kept for 48 h in glass-covered dishes in desiccator to
remove any moisture.
2.8. Protein Repellence
The adsorption of Bovine serum Albumin (BSA) on the blank and the modified commercial
membranes was evaluated as was described in previous research [49,50]. Briefly, the membranes
were shaken in 50 mL BSA solution (1 g·L−1 ) at 25 ◦ C for 24 h. Adsorbed BSA was determined from
the difference between the original BSA concentration and the residual BSA concentration in the
shaken solution using the pre-prepared standard curve and UV-spectrophotometer (Kyoto, Japan) at
280 nm wavelength.
2.9. Scanning Electron Microscope (SEM)
The surface and the cross-section of both blank and modified commercial membranes were
scanned using a Jeol Jsm 6360LA Scanning Electron Microscope (SEM, Tokyo, Japan) as was described
in previous work [49]. For cross-section imaging, the membranes were frozen by immersion in liquid
nitrogen and then were fractured and were coated by Au. Magnifications of 5000× and 15,000× were
used. The laminated PES layers or surfaces were imaged at a voltage of 10–20 KV after coating with
Au. Magnifications of 15,000× and 30,000× were used.
2.10. Scanning Probe Microscope (SPM)
Blank and modified laminated PES layers were prepared on square strips (1 × 1 cm2 ) and were
imaged using Scanning Probe Microscope (SPM, Shimadzu, Kyoto, Japan). Non-contact mode was
used to image 5 × 5 µm2 strips.
2.11. Static Water Contact Angle
The static water contact angle on the blank and the modified laminated PES layers was determined
using a Krüss DSA 100 apparatus (Hamburg, Germany). Drops of demineralized water (7 µL) were
deposited on different spots of each laminated PES layer. Four different spots on each layer of two
different independently modified slides were measured and were averaged. All the tested membranes
were kept in desiccators for 48 h before measuring. MilliQ water was used in all the experiments.
2.12. Membrane Tensile Strength
Samples were cut in a dumbbell shape with dimensions as was described in previous work [49].
Constant crosshead speed of 6 mm/min was used. Two samples from each membrane were tested.
3. Results and Discussion
In this study, 2-AP monomer was used to modify PES membranes using laccase-catalyzed grafting
method. The laccase was used to catalyze conversion of the modifier 2-AP into its free radical form that
is required for the grafting onto the PES membrane. The formed free radicals can react with each other
to form homopolymers of poly(2-AP) and/or to graft on/onto membrane surface [49]. The modified
membranes were intensively washed using boiled water until no absorption was determined by
spectrophotometer. Figure 3 shows the effect of the grafting yield on the total color change of the
Polymers 2016, 8, 306
6 of 18
membrane (∆E*). Different modification conditions were used as following: 5 and 15 mM 2-AP
modifier concentrations, 25 and 40 ◦ C reaction (modification) temperatures, and 30, 60, and 120 min
reaction (modification) times. The concentration of enzyme was 0.5 U·mL−1 dissolved in 0.1 M
sodium acetate buffer, which was used to keep the pH of the reaction medium at 5.5. As seen in
Figure 3, the total color change of the membrane (∆E*) was increased gradually with the increases of
the grafting yield up to 66.21 µg·cm−2 (the total color change of the membrane was 26.2 when using
15 mM 2-AP, 40 ◦ C reaction temperature, and 120 min reaction time). The increases of each studied
parameter e.g., modifier concentration, reaction time, etc. resulted in an increase in the grafting yield.
Similar to the previous study using 4-AP modifier [38], different changes of the membrane color at
25.5 µg·cm−2 grating yield were determined (At 40 ◦ C: (a) 5 and (b) 15 mM 2-AP modifier with 120
and 60 min reaction times, respectively; also at 25 ◦ C: (c)15 mM 2-AP modifier at 60 min reaction
time). However, the case of using 2-AP modifier differs from the case of using 4-AP modifier [38]
regarding protein repellence. The three modified membranes using 2-AP modifier did not show as
good a protein repellence as those using 4-AP modifier. Previous studies [17,39] emphasize the effect
of both surface hydrophilicity and structure/shape of the formed modifying layer on the foulant
repellence of the modified surface [49]. Regarding the static water contact angle as an indicator for
the surface hydrophilicity, these three modified membranes using 2-AP modifier have not shown a
noticeable change in the static water contact angle values, around 77◦ ((a) 76.4◦ ± 1.5◦ , (b) 74.1◦ ± 1◦ ,
and (c) 80.4◦ ± 3◦ , respectively; the blank PES surface has 75.9◦ ± 2◦ static water contact angle).
Regarding the shape of the modified surfaces, (as shown in SEM images in the following discussion),
the produced modifying surfaces are similar in shape and may also be similar in structure; more
investigation on the produced structure are on going, whereas the membrane color increased more
strongly with modification at 40 ◦ C, most probably due to formation of dense layers that are more
saturated in color [56] (i.e., have higher extinction coefficients). Color saturation (∆C*, not measured
in this study) is a characteristic parameter indicating the intensity of the color: the color with a high
saturation will appear more intense than the same color with less saturation. It seems that the three
modification conditions produce the same grafting yield, static water contact angle, and the same
shape and/or structure because they show almost the same protein repellence (i.e., the error limits have
been considered) (see irreversible protein adsorption evaluation results in the following discussion).
However, the total change in the color of the three membranes may be a result of change in their color
saturation [54,56]. In addition, at grafting yield equal or greater than 25 µg·cm−2 , there is no clear
relation between the grafting yield and the total color change (∆E*) of the membranes. This may be
interpreted by the fact that the increasing of the added modifier is accompanied with a difference
in the color saturation, as previously illustrated (color depth, not measured in this work) [57–59],
instead of the total color change of membranes. This may be attributed to the formation of extra
layers instead of the increases in the grafting density (increase the length of the grafted oligomers
rather than increases the number of the grafted oligomer) [56–59]. It is very clear that most of the
tested membranes carry not more than 25.5 µg·cm−2 grafting yield (about 2.55 mg·m−2 ). Assuming
the mass of one molecule of 2-AP is 18.1 × 10−19 mg and occupies about 1 nm2 of space, a dense
monolayer would thus approximately yield a coverage of 1.8 mg·m−2 . If we assume formation of
phenoxazine [40,41], a dense monolayer would thus approximately yield a coverage of 3 mg·m−2 .
This is in full agreement with assuming the increasing in the grafting yield more than 25.5 µg·cm−2
will result in increasing the length of the grafted oligomers and/or adsorption rather than increasing
the number of the grafted oligomer per unit area (grafting density) [59]. In addition, this rough
calculations propose formation of one or two layers of modifying layer on the membrane as maximum,
which can be the reason for avoiding the harmful effect on the membranes’ pores and consequently on
the membranes’ original flux.
Polymers 2016, 8, 306
Polymers 2016, 8, 306
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7 of 17
28
25.5 µg·cm-2
24
20
16
∆E* 12
8
4
0
0
10
20
30
40
Grafting Yield
5 mM @ 25ᵒ C
15 mM @ 25ᵒ C
50
60
70
(µg·cm-2)
5 mM @ 40 ᵒC
15 mM @ 40 ᵒC
Figure 3.
(∆E*)
with
grafting
yield;
common
reaction
condition
is pH
3. Change
Changeofofthe
themembrane
membranecolor
color
(∆E*)
with
grafting
yield;
common
reaction
condition
is
−1 −
1
◦
enzyme
(laccase).
Reaction
at
25
°C
using
5
mM
5.5
(0.1
M
sodium
acetate
buffer),
and
0.5
U·mL
pH 5.5 (0.1 M sodium acetate buffer), and 0.5 U·mL enzyme (laccase). Reaction at 25 C using 5 mM
◦ C using
2-AP (blue diamond) and 15 mM 2-AP (red diamond), and reaction at 40 °C
using 55 mM
mM 2-AP
2-AP (green
(green
circle) and
and15
15mM
mM2-AP
2-AP
(purple
circle).
Resection
times
30,and
60,120
andmin
120were
mintested
were and
tested
the
(purple
circle).
Resection
times
30, 60,
the and
grafting
grafting
was increased
with increasing
reaction
time.
Theline
solid
a guide
for
yield wasyield
increased
with increasing
the reactionthe
time.
The solid
black
is ablack
guideline
for is
general
trend.
general trend.
In general, adding a hydrophilic compound that carries carboxylic, hydroxylic, or aminic groups
In general, adding a hydrophilic compound that carries carboxylic, hydroxylic, or aminic
will result in enhancing the hydrophilicity of the modified surface [60,61]. This phenomenon will be
groups will result in enhancing the hydrophilicity of the modified surface [60,61]. This phenomenon
pronounced only in cases where these groups are kept free for bonding to water molecules [62]. As seen
will be pronounced only in cases where these groups are kept free for bonding to water molecules
in Figure 4, pure water flux of the modified membranes was increased up to 15.4% relative to the
[62]. As seen in Figure 4, pure water flux of the modified membranes was increased up to 15.4%
blank membrane. This unexpected increase in the membrane flux may be interpreted to the formation
relative to the blank membrane. This unexpected increase in the membrane flux may be interpreted
of homogeneous layer without clogging the pores (see SEM images in the following discussion).
to the formation of homogeneous layer without clogging the pores (see SEM images in the following
In addition, the pore size of the membrane used in this study is relatively wide (0.2 µm), which can
discussion). In addition, the pore size of the membrane used in this study is relatively wide (0.2 µm),
be considered a reason for diminishing any negative effect of modification on the original flux of the
which can be considered a reason for diminishing any negative effect of modification on the original
membrane. Meanwhile, the highest pure water flux reduction is not at the highest grafting yield but it
flux of the membrane. Meanwhile, the highest pure water flux reduction is not at the highest
was found to be 1.4% for the membrane modified using 15 mM 2-AP modifier, 120 min reaction time,
grafting◦ yield but it was found to be 1.4% for the membrane modified using 15◦ mM 2-AP modifier,
and 25 C reaction temperature. Generally, the PES membranes modified at 25 C show comparable
120 min reaction time, and 25 °C reaction temperature.
Generally, the PES membranes modified at
pure water flux to those membranes modified at 40 ◦ C when using low modifier concentration (2-AP
25 °C show comparable pure water flux to those membranes modified at 40 °C when using low
concentration). This is a logical result considering the limitation of low concentration (5 mM) to extend
modifier concentration (2-AP concentration). This is a logical result considering the limitation of low
the reaction. However, when using 15 mM 2-AP concentration, there are plenty of monomers can be
concentration (5 mM) to extend the reaction. However, when using 15 mM 2-AP concentration,◦ there
reacted with each other (homopolymer) as well as bonding onto the membrane surface. At 25 C, the
are plenty of monomers can be reacted with each other (homopolymer) as well as bonding onto the
speed (i.e., the speed of colored homopolymers formation was noticed by the naked eyes) of formation
membrane surface. At 25 °C, the speed (i.e., the speed of colored homopolymers formation was
a free radical is comparable to the rate of bonding this free radical onto the membrane surface and/or
noticed by the naked eyes) of formation a free radical is comparable to the rate of bonding this free
react with other free radicals inside the reaction medium (homopolymer formation) [63–65]. The overall
radical onto the membrane surface and/or react with other free radicals inside the reaction medium
speed of the reaction may be slow but the possibility of grafting is probably high [62,63], whereas,
(homopolymer
formation) [63–65]. The overall speed of the reaction may be slow but the possibility
at 40 ◦ C, the speed of free radical formation is high and presence of plenty of free radicals cause the
of grafting is probably high [62,63], whereas, at 40 °C, the speed of free radical formation is high and
acceleration of the reaction of monomers with each other (the homopolymer formation inside the
presence of plenty of free radicals cause the acceleration of the reaction of monomers with each other
reaction medium was noticeable by naked eye and the homopolymer formation process is faster at
(the◦ homopolymer
formation inside the reaction medium was noticeable by naked eye and the
40 C than at 25 ◦ C reaction temperature). The fast homopolymer formation may result in reducing
homopolymer formation process is faster at 40 °C than at 25 °C reaction temperature). The fast
the possibility of grafting on the membrane surface and increases the chance of adsorption/grafting
homopolymer formation may result in reducing the possibility of grafting on the membrane surface
of ready-formed oligomers on the membrane surface [61]. The general observation is that the added
and increases the chance of adsorption/grafting of ready-formed oligomers
on the membrane surface
2-AP modifier may be mainly grafted on the membrane surface at 25 ◦ C reaction temperature, whereas
[61]. The general observation is that the added 2-AP modifier◦ may be mainly grafted on the
it may be mainly adsorbed on and/or onto membrane surface at 40 C reaction temperature especially
membrane surface at 25 °C reaction temperature, whereas it may be mainly adsorbed on and/or onto
membrane surface at 40 °C reaction temperature especially in case of using high concentration of the
modifier (15 mM 2-AP) [49,53]. This expected high reaction speed at 40 °C has been assisted by
increase the solubility of the modifier at this temperature [61].
Polymers 2016, 8, 306
8 of 18
in
case of
using
Polymers
2016,
8, 306high concentration of the modifier (15 mM 2-AP) [49,53]. This expected high reaction
8 of 17
speed at 40 ◦ C has been assisted by increase the solubility of the modifier at this temperature [61].
On the other hand, the increasing
increasing in the
the reaction
reaction temperature
temperature accelerates
accelerates the overall reaction
and the added 2-AP modifier per unit area of the
the membranes
membranes was doubled with the increase of the
number of the added polar groups on the membrane surface and, consequently, the water flux was
increased. In addition, (as shown in SEM of the membrane cross section modified at 40 ◦°C
C reaction
temperature), the added modifier may cause sticking of some fibers and creation more open cavities
inside the membranes. Theses created open cavities can improve the pure water flux than the flux of
modified at
at 25
25 ◦°C
the membranes modified
C [53].
Flux (m3·m-2·h-1 )
18
16
14
Blank
12
10
0
10
5 mM @ 25ᵒ C
20
30
40
50
Grafting Yield (µg·cm-2)
15 mM @ 25 ᵒ C
5 mM @ 40 ᵒC
60
70
15 mM @ 40 ᵒC
4. The effect of the
the grafting
grafting yield
yield on
on the
the pure
pure water
water flux
flux of
of the
the membrane.
membrane. Common
Common reaction
Figure 4.
at 25
25 ◦°C
condition is
is pH
pH 5.5
5.5 (0.1
(0.1 M
Msodium
sodiumacetate
acetatebuffer),
buffer),and
and0.5
0.5UU·mL
condition
·mL−−11 enzyme (laccase). Reaction at
C
reaction at
at 40
40 ◦°C
using 5 mM 2-AP (blue diamond) and 15 mM 2-AP (red diamond), and reaction
C using 5 mM
2-AP (green circle) and 15 mM 2-AP
2-AP (purple
(purple circle).
circle). Resection times 30, 60, and 120 min were tested
tested
increasing the reaction
reaction time.
time. The solid and dashed lines are
and the grafting yield was increased with increasing
trend for
for the
the studied
studied conditions.
conditions.
guides for general trend
In addition,
addition,we
wecan
canobserve
observesimilar
similartrend
trend
each
studied
condition:
water
flux
In
forfor
each
studied
condition:
firstfirst
the the
purepure
water
flux was
was increased
and was
thendecreased
was decreased
with increasing
the grafting
yield (although
of the membrane
increased
and then
with increasing
the grafting
yield (although
of the membrane
fluxes
fluxes
are
still
higher
than
the
flux
of
the
blank
membrane).
This
trend
may
be
attributed
to
are still higher than the flux of the blank membrane). This trend may be attributed to the decreasesthe
in
decreases
inof
the
number
open (free)
groups
have
the ability
to react
with
the water
the
number
open
(free)ofgroups
that have
the that
ability
to react
with the
water
molecules
andmolecules
facilitate
andwater
facilitate
water
fluxthe
[53,66].
Thus, the
low
grafting
yield may
show
better
flux and
better
the
flux the
[53,66].
Thus,
low grafting
yield
may
show better
flux and
better
protein
repellence
protein
repellence
than
the
high
grafting
yield
[49].
Using
wide
pore
size
(0.2
µm)
membranes
than the high grafting yield [49]. Using wide pore size (0.2 µm) membranes minimized the effect of
minimized the
of modification
onflux.
the membranes
original
flux.
Obviously,
using
much
modification
oneffect
the membranes
original
Obviously, using
much
lower
membrane
pore
size lower
could
membrane
pore
size
could
cause
clogging
of
the
membrane
pores
that
will
severely
affect
the
cause clogging of the membrane pores that will severely affect the membrane original flux.
membrane
original
flux.
The static water contact angle was measured on the blank and the modified laminated PES
static water
contact
was measured
the blank andofthe
PES
layersThe
to illustrate
the effect
of angle
the modification
on theonhydrophobicity
themodified
pure PESlaminated
layer (surface).
layers
to
illustrate
the
effect
of
the
modification
on
the
hydrophobicity
of
the
pure
PES
layer
The effect of hydrophilic additives that were included in the used commercial membrane was avoided
(surface).
The
effect of pure
hydrophilic
additives
that were
included
in the
used times
commercial
membrane
by
using the
laminated
PES layer
on the silica
support.
Different
reaction
were used
(15, 30,
◦
was
avoided
by
using
the
laminated
pure
PES
layer
on
the
silica
support.
Different
reaction
times
60, and 120 min) at reaction temperature 25 C, as shown in Figure 5.
wereBoth
usedlow
(15,reaction
30, 60, and
120
min)
at
reaction
temperature
25
°C,
as
shown
in
Figure
5.
time and low modifier concentration resulted in reduction in the static water
Both
low(blue
reaction
and low
modifier
concentration
in 30
themin
static
water
contact
angle
line time
in Figure
5), whereas
carrying
out the resulted
reaction in
forreduction
longer than
resulted
contact
anglethe
(blue
in contact
Figure angle
5), whereas
out thesurface
reaction
for longer
than surface).
30 min
in
increasing
staticline
water
(i.e., lesscarrying
hydrophilicity
relative
to the blank
resulted
in
increasing
the
static
water
contact
angle
(i.e.,
less
hydrophilicity
surface
relative
to the
This trend is in good agreement with the water flux of each studied set: first the flux was increased
blank
is in it
good
the water
fluxtime.
of each
set:the
first
the flux
at
lowsurface).
reaction This
timetrend
and then
wasagreement
decreased with
at longer
reaction
In studied
addition,
change
in
was
increased
at
low
reaction
time
and
then
it
was
decreased
at
longer
reaction
time.
In
addition,
the
the static water contact angle assists our proposal that increasing the added materials is the reason
change in the static water contact angle assists our proposal that increasing the added materials is
the reason for decreasing the water bounding sites on the modified membrane surface. Again, using
high 2-AP modifier concentration resulted in gradually increasing the static water contact angle, as
shown in Figure 5 (red line). Creation of wide cavities may result in the increasing of the water flux
Polymers 2016, 8, 306
9 of 18
for decreasing the water bounding sites on the modified membrane surface. Again, using high 2-AP
modifier
concentration
resulted in gradually increasing the static water contact angle, as shown
Polymers
2016,
8, 306
9 of in
17
Figure 5 (red line). Creation of wide cavities may result in the increasing of the water flux of the
of
the modified
membrane,
decreasing
the surface
hydrophilicity
and increasing
the water
static
modified
membrane,
despitedespite
decreasing
the surface
hydrophilicity
and increasing
the static
water
as observed
in the membranes
modified
at 40 °C.
contactcontact
angle, angle,
as observed
in the membranes
modified
at 40 ◦ C.
On the other
other hand,
hand, the
thestructure
structureofofthe
theproduced
producedoligomer
oligomerororpolymers
polymers
can
affect
properties
can
affect
thethe
properties
of
of
produced
modified
membranes.
The difference
the produced
oligomers
or polymers
at
thethe
produced
modified
membranes.
The difference
in the in
produced
oligomers
or polymers
at different
different
reaction
(modification)
conditions
still
needs
further
investigation
using
advanced
reaction (modification) conditions still needs further investigation using advanced chemical analyses
chemical
analyses
which are underway.
techniques,
which techniques,
are underway.
Contact Angle (θ)
90
85
80
75
70
65
60
0
30
60
Modification Time (min)
5 mM 2-AP @ 25º C
90
120
15 mM 2-AP @ 25º C
Figure 5. Change of the static water contact
contact angle
angle with
with the
the reaction
reaction (modification)
(modification) time.
time. Common
−
1
−1
condition isis pH
pH5.5
5.5(0.1
(0.1MMsodium
sodium
acetate
buffer),
U·mL
enzyme
(laccase).
reaction condition
acetate
buffer),
andand
0.5 0.5
U·mL
enzyme
(laccase).
Two
Two
modifier
concentrations,
mM (blue)
mM(2-aminophenol),
(red) (2-aminophenol),
were
used
25 ◦ C
modifier
concentrations,
5 mM5(blue)
and 15and
mM15
(red)
were used
at 25
°C at
reaction
reaction temperature
and60,
15,and
30, 60,
min reaction
times.
Thelines
solidare
lines
are guides
for general
temperature
and 15, 30,
120and
min120
reaction
times. The
solid
guides
for general
trend
trend
the studied
conditions.
for
thefor
studied
conditions.
The
overall performance
performance proved
amount of
bovine serum
serum albumin
albumin (BSA)
(BSA) irreversibly
irreversibly
The overall
proved that
that the
the amount
of bovine
adsorbed
onto
the
blank
membrane
as
shown
in
Figure
6,
was
decreased
with
modification.
adsorbed onto the blank membrane as shown in Figure 6, was decreased with modification.
However,
between
thethe
amount
of added
2-AP2-AP
modifier
(the
However, there
there isisno
nostraightforward
straightforwardrelationship
relationship
between
amount
of added
modifier
grafting
yield)
andand
the the
improvement
of of
protein
repellence
ofofthe
(the
grafting
yield)
improvement
protein
repellence
themodified
modifiedmembranes.
membranes.ItIt is
is well
well
observed
that
the
best
reduction
in
protein
adsorption
was
determined
with
the
membranes
observed that the best reduction in protein adsorption was determined with the membranes modified
modified
2-AP concentration
(81.27% reduction
in irreversible
protein adsorption
case
using lowusing
2-AP low
concentration
(81.27% reduction
in irreversible
protein adsorption
in case ofinusing
◦
of
using
5
mM
2-AP
and
60
min
modification
reaction
at
25
°C).
We
can
see
full
agreement
with
5 mM 2-AP and 60 min modification reaction at 25 C). We can see full agreement with the trendthe
of
trend
of the individual
modification
conditions
have
beeninstudied
in this
work (temperature,
the individual
modification
conditions
that havethat
been
studied
this work
(temperature,
modifier
modifier
concentration,
etc.) regarding
water
and protein
repellence.
Grafting
ofmodifier
2-AP modifier
concentration,
etc.) regarding
water flux
andflux
protein
repellence.
Grafting
of 2-AP
on or
on
or
onto
PES
membrane
surface
leads
to
an
improvement
in
the
protein
repellence,
the excess
onto PES membrane surface leads to an improvement in the protein repellence, but thebut
excess
of the
of
the grafting
yieldtoleads
slight decreases
of membrane
both the membrane
flux
and the
protein
grafting
yield leads
slighttodecreases
of both the
water fluxwater
and the
protein
repellence.
repellence.
addition,
the created
as[54]
passfor
rooms
[54] forwhich
the protein,
which
In addition,In
the
created wide
cavitieswide
maycavities
work asmay
passwork
rooms
the protein,
increases
the
increases
the
possibility
of
protein
inclusion
(molecules
or
aggregates)
inside
the
membrane
pores
possibility of protein inclusion (molecules or aggregates) inside the membrane pores and consequently
and
consequently
the irreversible
protein
adsorption
increased.
Thus,
low 2-AP
the irreversible
protein
adsorption was
increased.
Thus, was
keeping
both low
2-APkeeping
modifierboth
concentration
modifier
concentration
and
lower
reaction
temperature
produce
an
effective
modified
membrane
and lower reaction temperature produce an effective modified membrane that shows good protein
that
showsClearly,
good protein
repellence.
this study,showed
all thelower
modified
membranesadsorbed
showed
repellence.
in this study,
all the Clearly,
modifiedinmembranes
total irreversible
lower total
adsorbed protein than the blank membranes.
protein
thanirreversible
the blank membranes.
Polymers 2016, 8, 306
Polymers 2016, 8, 306
10 of 18
10 of 17
Irreversible Protein Adsorption
(µg·cm-2)
60
Blank
50
40
30
20
10
0
0
10
20
30
40
Grafting Yield
5 mM @ 25ᵒ C
15 mM @ 25ᵒ C
50
60
70
(µg·cm-2)
5 mM @ 40 ᵒC
15 mM @ 40 ᵒ C
Figure 6.
The effect
effect of
Figure
6. The
of the
the grafting
grafting yield
yield on
on the
the irreversible
irreversible protein
protein adsorption.
adsorption. Common
Common reaction
reaction
−1 enzyme (laccase). Reaction at 25 ◦ C
condition
is
pH
5.5
(0.1
M
sodium
acetate
buffer),
and
0.5
U
·
mL
−1
condition is pH 5.5 (0.1 M sodium acetate buffer), and 0.5 U·mL enzyme (laccase). Reaction at 25 °C
◦ C using 5 mM
using 55 mM
mM 2-AP
2-AP (blue
(blue diamond)
diamond) and
and 15
15 mM
mM 2-AP
2-AP (red
(red diamond),
using
diamond), and
and reaction
reaction at
at 40
40 °C
using 5 mM
2-AP (green
(green circle)
circle) and
and 15
15 mM
Resection times
times 30,
30, 60,
60, and
and 120
120 min
min were
were tested
2-AP
mM 2-AP
2-AP (purple
(purple circle).
circle). Resection
tested
and
the
grafting
yield
was
increased
with
increasing
the
reaction
time.
Adsorption
test
was
and the grafting yield was increased with increasing the reaction time. Adsorption testdone
was using
done
1 g·L−1 BSA,
0.1 M sodium acetate buffer (pH 7) and 25 ◦ C.
−1
using 1 g·L BSA, 0.1 M sodium acetate buffer (pH 7) and 25 °C.
The reduction in the water flux of the modified membranes due to the irreversible protein
adsorption (un-removed protein after back and forward washing) is much lower than the reduction
in the water flux of the blank membrane, as illustrated in Figure 7. The flux of the blank membrane
was reduced by 15.96% of its original flux, whereas the modified membranes showed residual fluxes
thethe
blank
membrane.
ForFor
example,
the the
modified
membrane
that
much better
better than
thanthe
theresidual
residualflux
fluxofof
blank
membrane.
example,
modified
membrane
showed
the best
protein
repellence
lost 3.1%
of itsoforiginal
flux due
modification.
Thus,
the flux
that
showed
the best
protein
repellence
lost 3.1%
its original
flux to
due
to modification.
Thus,
the
of this
membrane
afterafter
bothboth
the modification
and the
protein
adsorption
test,
flux
of modified
this modified
membrane
the modification
andirreversible
the irreversible
protein
adsorption
is still
than the
the of
blank
membrane
before before
proteinprotein
adsorption
by 11.81%
and higher
test,
is higher
still higher
thanflux
theofflux
the blank
membrane
adsorption
by 11.81%
and
than the
fluxthe
offlux
the blank
membrane
after irreversible
protein
adsorption
by 33.4%
(i.e., (i.e.,
fluxes
due
higher
than
of the blank
membrane
after irreversible
protein
adsorption
by 33.4%
fluxes
−23··m
−1 for
to irreversible
protein
adsorption
were were
11.74 11.74
and 15.62
m3 ·mm
h−−21·hfor
the the
blank
membrane
andand
the
due
to irreversible
protein
adsorption
and 15.62
blank
membrane
modified
membrane,
respectively).
the
modified
membrane,
respectively).
In addition, the residual flux of the modified membrane that acquired the highest grafting yield
−)2 )isisbetter
(66.21 µg
·cm−2
betterthan
thanthe
theflux
flux of
of the
the blank
blank membrane
membrane before
before and
and after irreversible protein
µg·cm
3
−
2
−
1
3 ·m
−2−1·hfor
−1
3
−2
−1
−2·h
m ·h·h forfor
the
blank
membrane,and
and15.44
15.44and
and14.29
14.29mm3·m
adsorption (i.e.,
(i.e., 13.97
13.97 and
and11.74
11.74m
m ··m
the
blank
membrane,
for the
modified
membrane
before
and
afterirreversible
irreversibleprotein
proteinadsorption,
adsorption,respectively).
respectively). Thus,
Thus, the
the
modified
membrane
before
and
after
residual flux of the modified membranes (after modification and the irreversible protein adsorption
test) was much higher than the flux of the blank membrane.
thethe
blank
PESPES
membrane
and the
PES membranes
for four different
The SEM
SEMimages
imagesofof
blank
membrane
andmodified
the modified
PES membranes
for four
modification
conditions
are shownare
in shown
Figure 8.
new
formed
onformed
the top on
of the
different
modification
conditions
in The
Figure
8. layer
The new
layer
the membrane
top of the
surfaces is not
easilyisnoticeable
printed on
paper
(clearly
seen
on theseen
screen
of SEM).
new layer
membrane
surfaces
not easily on
noticeable
printed
paper
(clearly
on the
screenThe
of SEM).
The
seems
thicker
at modification
temperature
of 40 ◦ C than
at temperature
25 ◦ C. In addition,
high
new
layer
seems
thicker at modification
temperature
of 40
°C than at temperature
25 °C. Inusing
addition,
using
high
modifier concentration
(15 gives
mM 2-AP)
givesfor
themore
chance
for more
clearly
layer,
modifier
concentration
(15 mM 2-AP)
the chance
clearly
formed
layer,formed
especially
at
especially
at longer
reaction (modification)
time (120 min).
longer reaction
(modification)
time (120 min).
Polymers
2016,
8,8,306
306
Polymers2016,
2016,8,
306
Polymers
11
of
18
11of
of17
17
11
Flux
Flux Reduction
Reduction %
% due
due to
to
irreversible
Protein
Adsorption
irreversible Protein Adsorption
18
18
Blank[15.96%
[15.96%reduction
reductionininflux
flux
Blank
afterirreversible
irreversibleprotein
proteinadsorption
adsorptiontest]
test]
after
12
12
66
00
00
10
10
°C
mM@@25
25°C
55mM
20
20
30
40
50
30
40
50
GraftingYield
Yield(µg·cm
(µg·cm-2-2) )
Grafting
15mM
mM@@25
25°C°C
15
mM@@40
40°C°C
55mM
60
60
70
70
15mM
mM@@40
40°C°C
15
Figure
7.7.Flux
Flux
reduction
percent
due
to
the
irreversible
protein
adsorption
at
different
grafting
yield.
Figure7.
Fluxreduction
reductionpercent
percentdue
dueto
tothe
theirreversible
irreversibleprotein
proteinadsorption
adsorptionat
atdifferent
differentgrafting
graftingyield.
yield.
Figure
−1 enzyme (laccase).
Common
reaction
condition
is
pH
5.5
(0.1
M
sodium
acetate
buffer),
and
0.5
U
·
mL
−1
−1
Common reaction
reaction condition
condition isis pH
pH 5.5
5.5 (0.1
(0.1 M
M sodium
sodium acetate
acetate buffer),
buffer), and
and 0.5
0.5 U·mL
U·mL enzyme
enzyme
Common
Reaction
25 ◦ C using
mM
2-AP
(blue
diamond)
and
15 mM
2-AP
and
reaction
at
(laccase).at
Reaction
255°C
°C
using
mM2-AP
2-AP
(bluediamond)
diamond)
and
15(red
mMdiamond),
2-AP(red
(reddiamond),
diamond),
and
(laccase).
Reaction
atat25
using
55mM
(blue
and
15
mM
2-AP
and
◦
40
C
using
5
mM
2-AP
(green
circle)
and
15
mM
2-AP
(purple
circle).
Resection
times
30,
60,
and
reactionatat40
40°C
°Cusing
using55mM
mM2-AP
2-AP(green
(greencircle)
circle)and
and15
15mM
mM2-AP
2-AP(purple
(purplecircle).
circle).Resection
Resectiontimes
times30,
30,
reaction
120
min
were
tested
and
the
grafting
yield
was
increased
with
increasing
the
reaction
time.
The
solid
60,and
and120
120min
minwere
weretested
testedand
andthe
thegrafting
graftingyield
yieldwas
wasincreased
increasedwith
withincreasing
increasingthe
thereaction
reactiontime.
time.
60,
and dashed lines are guides for general trend for the studied conditions.
Thesolid
solidand
anddashed
dashedlines
linesare
areguides
guidesfor
forgeneral
generaltrend
trendfor
forthe
thestudied
studiedconditions.
conditions.
The
Figure
The
scanning
electron
microscope
(SEM)
images
for
the
blank
poly(ethersulfone)
(PES)
Figure8.8.The
Thescanning
scanningelectron
electronmicroscope
microscope(SEM)
(SEM)images
imagesfor
forthe
theblank
blankpoly(ethersulfone)
poly(ethersulfone)(PES)
(PES)
Figure
membrane,
and
the
modified
PES
membranes:
(A,B)
modified
PES
membranes
using
5
and
15
mM
membrane,and
andthe
themodified
modifiedPES
PESmembranes:
membranes:(A,B)
(A,B)modified
modifiedPES
PESmembranes
membranesusing
using55and
and15
15mM
mM
membrane,
◦ C; and (C,D) modified PES membranes using 5 and 15 mM 2-AP,
modifier
(2-AP),
respectively
at
25
modifier(2-AP),
(2-AP),respectively
respectivelyatat25
25°C;
°C;and
and(C,D)
(C,D)modified
modifiedPES
PESmembranes
membranesusing
using5 and 15 mM 2-AP,
2-AP,
modifier
◦ C. Common reaction condition is pH 5.5 (0.1 M sodium acetate buffer), 0.5 U·mL−−11−1
respectively
atat40
40
respectivelyat
40°C.
°C.Common
Commonreaction
reactioncondition
conditionisispH
pH5.5
5.5(0.1
(0.1M
Msodium
sodiumacetate
acetatebuffer),
buffer),0.5
0.5U·mL
U·mL
respectively
enzyme
(laccase),
and
120
min
modification
time.
Magnification
isis15,000×;
15,000
×;and
and
scale
bar
µm.
enzyme(laccase),
(laccase),and
and120
120min
minmodification
modificationtime.
time.Magnification
Magnificationis
15,000×;
andscale
scalebar
barisis
is111µm.
µm.
enzyme
Nohomopolymer
homopolymerlumps
lumpsare
areshown
shownin
inimaged
imagedPES
PESsurfaces
surfacesin
inFigure
Figure8.8.This
Thismay
maybe
beattributed
attributed
No
No
homopolymer
lumps
are
This
may
attributed
to using
using aaa modifier
modifier with
with two
two legs
legs that
that tends
tends to
to growth
growth in
in one
one dimension
dimension with
with formation
formation of
of aaa
to
to
using
modifier
with
two
legs
tends
dimension
with
formation
of
homogenous
layer
[49].
In
addition,
this
may
be
related
to
the
structure
of
formed
layer
that
may
homogenous
layer
[49].
In
addition,
this
may
be
related
to
the
structure
of
formed
layer
that
may
homogenous layer [49].
addition,
may be related to the structure of formed layer that may
consistof
ofaaarepeating
repeatingphenozaxine
phenozaxineunits
unitsas
asproposed
proposedin
inthe
theprevious
previouselectrochemical
electrochemicalpolymerization
polymerization
consist
consist
of
repeating
phenozaxine
units
as
proposed
in
the
previous
electrochemical
polymerization
of2-AP
2-AP[41,42].
[41,42].Moreover,
Moreover,the
theimaged
imagedcross-section,
cross-section,as
asshown
shownin
inFigure
Figure9C,
9C,illustrated
illustratedthat
thatthe
theeffect
effect
of
of
2-AP
[41,42].
Moreover,
shown
in
Figure
9C,
illustrated
that
the
effect
of
using
higher
2-AP
modifier
concentration
in
the
modification
process
takes
place
at
higher
of
using higher
higher2-AP
2-APmodifier
modifier
concentration
in modification
the modification
process
takesatplace
atreaction
higher
of using
concentration
in the
process
takes place
higher
reactiontemperature
temperature
(40°C).
°C).
The
inside
fibers
seem
not
onlythicker
thicker
than
those
fibersmodified
modified
25
reaction
(40
The
inside
fibers
seem
only
than
those
fibers
25
temperature
(40 ◦ C). The
inside
fibers
seem
not
onlynot
thicker
than
those
fibers
modified
at 25 ◦ Catat
but
°Cbut
but
also
they
werestacked
stacked
onto
each
other
and
newcavities
cavitiesthe
inside
themembranes
membranes
have
been
°C
also
they
were
onto
each
other
and
new
inside
the
have
been
also
they
were
stacked
onto each
other
and
new
cavities
inside
membranes
have been
created.
created.
Thecavities
formedmay
cavities
may
bethe
the
reason
forenhanced
enhancedmembrane
membrane
fluxand
and
reduced
protein
created.
The
formed
cavities
may
be
for
flux
reduced
protein
The
formed
be the
reason
forreason
enhanced
membrane
flux and reduced
protein
repellence.
repellence.
repellence.
Polymers 2016, 8, 306
Polymers 2016, 8, 306
12 of 18
12 of 17
Figure 9. The scanning electron microscope (SEM) images for the cross-section of the blank
Figure 9. The scanning electron microscope (SEM) images for the cross-section of the blank
poly(ethersulfone)(PES)
(PES)membrane,
membrane,(A)
(A)5000
5000×
poly(ethersulfone)
× and
and (D)
(D) 15,000×;
15,000×;the
themodified
modifiedPES
PESmembrane
membraneusing
using5
temperature,(B)
(B)5000
5000×
and (E)
(E) 15,000
15,000×;
PES membrane
membrane
◦ C reaction
5mM
mM2-AP
2-APand
and at
at 25
25 °C
reaction temperature,
× and
×; and
and modified
modified PES
using
15
mM
2-AP
and
at
40
°C,
(C)
5000×
and
(F)
15,000×.
Common
reaction
condition
is
pH
5.5 5.5
(0.1
◦
using 15 mM 2-AP and at 40 C, (C) 5000× and (F) 15,000×. Common reaction condition is pH
−1 enzyme (laccase), and 60 min reaction (modification) time.
M
sodium
acetate
buffer),
0.5
U·mL
−
1
(0.1 M sodium acetate buffer), 0.5 U·mL enzyme (laccase), and 60 min reaction (modification) time.
To figure out the shape of the formed top layer, SEM imaging of the laminated PES layer on
To figure out the shape of the formed top layer, SEM imaging of the laminated PES layer on
silicon dioxide slide was performed. As shown in Figure 10A, the blank PES layer appears as porous
silicon dioxide slide was performed. As shown in Figure 10A, the blank PES layer appears as porous
layer, which is analogous to the pores of the membrane. The modification resulted in oligomer
layer, which is analogous to the pores of the membrane. The modification resulted in oligomer chains
chains growth on the surface, as shown in Figure 10C. To illustrate the bound poly(2-AP), the
growth on the surface, as shown in Figure 10C. To illustrate the bound poly(2-AP), the surfaces have
surfaces have been imaged with higher magnification, Figure 10B for the blank laminated PES layer
been imaged with higher magnification, Figure 10B for the blank laminated PES layer and Figure 10D
and Figure 10D for the modified laminated PES layer. The grown tiny oligomers are noticeable all
for the modified laminated PES layer. The grown tiny oligomers are noticeable all over the surface.
over the surface. Moreover, the formed layer of tiny oligomers was shown by SPM in Figure 11.
Moreover, the formed layer of tiny oligomers was shown by SPM in Figure 11.
As shown in Figure 11, the added poly(2-AP) leads to the disappearance of the sharp edges of
As shown in Figure 11, the added poly(2-AP) leads to the disappearance of the sharp edges of the
the pores of the blank layer. This is accompanied by the reduction in the layer roughness as
pores of the blank layer. This is accompanied by the reduction in the layer roughness as illustrated
illustrated in Z value, which was reduced from 302 to 233 nm. The formed layer appears as pancake
in Z value, which was reduced from 302 to 233 nm. The formed layer appears as pancake shaped.
shaped. Although the small pores may be decreased in size, they still open and for that the effect of
Although the small pores may be decreased in size, they still open and for that the effect of modification
modification in the membrane flux was minimum. The reader should keep in mind the difference
in the membrane flux was minimum. The reader should keep in mind the difference between the used
between the used dense laminated PES layers (surfaces) and the commercial membranes to conclude
dense laminated PES layers (surfaces) and the commercial membranes to conclude that the added
that the added material on membranes has no effect on the pore size of the membranes, as shown in
material on membranes has no effect on the pore size of the membranes, as shown in Figure 8.
Figure 8.
The tensile strength of the blank and the modified PES membranes was measured to illustrate the
The tensile strength of the blank and the modified PES membranes was measured to illustrate
effect of modification on the strength of the blank PES membrane. As shown in Figure 12, very slight
the effect of modification on the strength of the blank PES membrane. As shown in Figure 12, very
reduction in the tensile strength of some modified membranes at low grafting yield was determined.
slight reduction in the tensile strength of some modified membranes at low grafting yield was
This may be related to the bounding of the first oligomer. A very slight decrease in the membrane
determined. This may be related to the bounding of the first oligomer. A very slight decrease in the
strength with modification at 40 ◦ C is noticeable. However, in general, most of the modified membranes
membrane strength with modification at 40 °C is noticeable. However, in general, most of the
showed a comparable strength for the blank membrane. Actually, slight improvement in the strength
modified membranes showed a comparable strength for the blank membrane. Actually, slight
of membranes modified at some modification conditions was determined.
improvement in the strength of membranes modified at some modification conditions was
determined.
Polymers 2016, 8, 306
13 of 18
Polymers 2016, 8, 306
13 of 17
(A)
(B)
(C)
(D)
Figure 10.
10. The
The scanning
scanningelectron
electronmicroscope
microscope(SEM)
(SEM)ofof
the
laminated
poly(ethersulfone)
(PES)
layer
Figure
the
laminated
poly(ethersulfone)
(PES)
layer
on
on
silicon
dioxide
slides
using
15,000×
and
30,000×
magnification:
(A,B)
the
blank
laminated
PES
silicon dioxide slides using 15,000× and 30,000× magnification: (A,B) the blank laminated PES layer
at
layer
the two magnifications;
(C,D) laminated
the modified
laminated
PESmagnifications
layer at theusing
two
the
twoatmagnifications;
and (C,D) theand
modified
PES layer
at the two
−1 enzyme
magnifications
using modification
condition:
15 mM
2-AP, pH
5.5 (0.1
M sodium
acetate
and
modification
condition:
15 mM 2-AP,
pH 5.5 (0.1
M sodium
acetate
buffer),
and 0.5
U·mLbuffer),
−1 enzyme (laccase) modified
◦
0.5
U·mL
for
60
min
at
25
°C.
(laccase) modified for 60 min at 25 C.
(A)
(B)
Figure
The scanning
scanning probe
probe microscope
laminated PES
PES layer
layer on
on silicon
silicon dioxide
dioxide slides:
slides:
Figure 11.
11. The
microscope (SPM)
(SPM) of
of the
the laminated
the
laminatedPES
PESlayer
layer(A);
(A);and
andmodified
modifiedlaminated
laminated
PES
layer
(B).
Modification
condition:
PES
layer
(B).
Modification
condition:
15
the blank laminated
−1 enzyme (laccase) modified for
−1 enzyme
15
mM
2-AP,
sodium
acetate
buffer),
and
U·mL
mM
2-AP,
pHpH
5.55.5
(0.1(0.1
MM
sodium
acetate
buffer),
and
0.50.5
U·mL
(laccase) modified for 120
120
25 ◦ C.
minmin
at 25at°C.
polymers
onon
thethe
PES
surface,
as
There are four
four possible
possiblestructures
structuresfor
forthe
theformed
formedoligomers
oligomersoror
polymers
PES
surface,
shown
in Figure
13. The
polymerization
of 2-APofhas
beenhas
determined
in previous
as
shown
in Figure
13. electrochemical
The electrochemical
polymerization
2-AP
been determined
in
work
as
an
oligomer
of
a
ladder
polymer
structure
with
repeating
phenozaxine
units
[41,42].
In
previous work as an oligomer of a ladder polymer structure with repeating phenozaxine units [41,42].
addition, a linear dimer formed by N–N coupling: 2,2′-dihydroxyazobenzene has been identified as
a polymerization product of 2-AP in non-acidic media [42]. The used pH is 5.5, for which a ladder
Polymers 2016, 8, 306
14 of 18
14
17
dimer formed by N–N coupling: 2,20 -dihydroxyazobenzene has been identified
14 of
of as
17
a polymerization product of 2-AP in non-acidic media [42]. The used pH is 5.5, for which a ladder
polymer structure
structure with
with repeating
repeating phenozaxine
phenozaxine units
most probable
probable structure
structure formed
formed on
the
polymer
units is
is the
the most
on the
membrane
surface.
Further
chemical
analyses
of
the
formed
oligomers
inside
the
reaction
medium
membrane surface. Further chemical analyses of the formed oligomers inside the reaction medium
and
the formed
formed modifying
modifying layers
layers on
on the
the membrane
membrane surfaces
surfaces are
are underway.
underway.
and of
of the
Polymers
2016,
In
addition,
a 306
linear
Polymers
2016, 8,
8,
306
Tensile Strength
Strength [MPa]
[MPa]
Tensile
66
55
44
33
Blank
Blank
22
11
00
00
10
10
20
30
40
20
30
40
-2
Grafting
Grafting Yield
Yield (µg·cm
(µg·cm-2))
55 mM
mM @
@ 25
25°°CC
50
50
60
60
55 mM
mM @
@ 40
40°°CC
15
15 mM
mM @
@ 25
25°°CC
70
70
15
15 mM
mM @
@ 40
40 °°CC
Figure
12.
the blank
blank
poly(ethersulfone)
12. Effect
Effect of
blank poly(ethersulfone)
poly(ethersulfone) (PES)
Figure 12.
of the
the grafting
grafting yield
yield on
on the
the tensile
tensile strength
strength of
of the
(PES)
−1
membrane.
reaction
condition
is
pH
5.5
(0.1
M
sodium
acetate
buffer),
and
0.5
U·mL
membrane. Common
Common
reaction
condition
is
pH
5.5
(0.1
M
sodium
acetate
buffer),
and
0.5
U·mL
Common reaction condition
5.5 (0.1 M sodium acetate buffer), and 0.5 U·mL−−11
◦25
enzyme
(laccase).
Reaction
°C
55 2-AP
mM
2-AP
(blue
and
15
(red
enzyme (laccase).
(laccase).Reaction
Reactionat at
at
°C using
using
mM (blue
2-AP diamond)
(blue diamond)
diamond)
and2-AP
15 mM
mM
2-AP
(red
enzyme
25 25
C using
5 mM
and 15 mM
(red 2-AP
diamond),
◦
diamond),
and
reaction
at
40
°C
using
5
mM
2-AP
(green
circle)
and
15
mM
2-AP
(purple
circle).
diamond),
reaction
at 540mM
°C 2-AP
using(green
5 mMcircle)
2-AP and
(green
circle)
and(purple
15 mM
2-AP Resection
(purple circle).
and
reactionand
at 40
C using
15 mM
2-AP
circle).
times
Resection
30,
60,
120
min
were
tested
and
yield
was
increased
with
increasing
Resection
times
30, were
60, and
and
120 and
min the
were
tested yield
and the
the
grafting
yield
was
increased
with
increasing
30,
60, andtimes
120 min
tested
grafting
wasgrafting
increased
with
increasing
the
reaction
time.
the
the reaction
reaction time.
time.
OH
OH
NH
NH22
22
O
O
NH
NH22
O
O
S
S
O
O
22
N
N
H
HO
O
NH
NH22
and/or
and/or
H
HO
O
O
O
N
N
O
O
O
O
NH
NH
NH
NH22
H
HO
O
O
O
N
N
NH
HO
O
NH H
O
O
S
S
N
N
H
HO
O
33
O
O
O
O
ee
afacc
urfr
SSu
ESS
PPE
11
H
HO
O
N
N
O
O
O
O22
22 H
H22O
O
2-aminophenol
2-aminophenol
H
HO
O
Laccase
Laccase
N
N
NH
NH22
O
O
S
S
44
CH
CH33
O
O
ofof
four
possible
chemical
structures
of the
(PES)
Figure
13.
Schematic
representation
four
possible
chemical
structures
of
the
Figure 13.
13. Schematic
Schematicrepresentation
representation
of
four
possible
chemical
structures
ofpoly(ethersulfone)
the poly(ethersulfone)
poly(ethersulfone)
surface
after modification
with 2-aminophenol,
containing
O-linked
and N-linked
structures.
(PES)
after
with
containing
O-linked
and
structures.
(PES) surface
surface
after modification
modification
with 2-aminophenol,
2-aminophenol,
containing
O-linked
and N-linked
N-linked
structures.
4. Conclusions
Conclusions
In this
to to
modify
thethe
PESPES
membranes
by using
laccase
biocatalyst.
This
this work,
work,2-AP
2-APwas
wasused
used
modify
membranes
by using
laccase
biocatalyst.
modification
resulted
in
a
remarkable
reduction
in
protein
adsorption,
whereas
the
water
flux
This modification resulted in a remarkable reduction in protein adsorption, whereas the water flux was
increased. This
This modification
modification does
does not
not harmfully
harmfully affect
affect the
the mechanical
mechanical properties
properties of
of the
the membranes.
membranes.
At different
different modification
modification conditions,
conditions, the formed
formed modification
modification layer seems homogenous all over the
surface without formation of big lumps of homopolymers on the membrane surfaces, as shown in
previous work using 4-hydroxybenzoic acid modifier [49]. The structure of the formed modifying
layer(s) is under investigation using other advanced chemical analytical techniques.
Polymers 2016, 8, 306
15 of 18
surface without formation of big lumps of homopolymers on the membrane surfaces, as shown in
previous work using 4-hydroxybenzoic acid modifier [49]. The structure of the formed modifying
layer(s) is under investigation using other advanced chemical analytical techniques.
Acknowledgments: This work was supported financially by the Science and Technology Development Fund
(STDF), Egypt, Grant No. 6070.
Author Contributions: Norhan Nady conceived, designed, and performed the experiments; analyzed the
data; and wrote the manuscript. Ahmed H. El-Shazly and Hesham M.A. Soliman reviewed the manuscript.
Sherif H. Kandil shared the analysis of the data and reviewed the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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