A switchable polymer brush system for antifouling and controlled detection

Article abstract of DOI:10.1039/c7cc00193b

A stimuli-responsive polymer brush system is designed to switch on and off surface functionality and prevent functional groups from fouling by grafting together two polymer brushes with precisely controlled lengths. The polymer brush with functional group

Full text of DOI:10.1039/c7cc00193b

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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Switchable Polymer Brush System for Antifouling and Controlled
Detection†‡
Serkan Demirci,*abc Selin Kinali-Demirciac and Shan Jiang*bd
www.rsc.org/
A stimuli-responsive polymer brush system is designed to switch on functional groups are “switched off” and shielded from fouling.
and off surface functionality and prevent functional groups from When the stimuli-responsive polymer brush collapses and becomes
fouling by grafting together two polymer brushes with precisely shorter than the polymer brush modified with functional groups, the
controlled lengths. The polymer brush with functional groups has functionality is “switched on”. By shielding the surface functionality
fixed length, while the other brush extends and collapses as when not in use and only exposing it when needed, the fouling can
environment changes.
be drastically reduced.
One significant challenge in biomaterial design is to combine anti-
The key to the success of this design is to precisely control the
fouling properties with the functionality at surface and interface. 1 length of the polymer brush. Polymer brush has been widely
Since many functional groups can be easily fouled when used investigated and applied in a broad range of applications including
unprotected, different chemistries have been applied to help filtration, separation, biosensor, tissue engineering, and drug
6–8
improve surface fouling performance, such as polyethylene glycol delivery.
Recent advances in polymer chemistry enabled the
(PEG), and zwitterionic polymers. 2,3 They usually form a hydration synthesis of polymer brush with well-defined architecture and
layer that helps reduce the foulant adsorption. However, in the length, such as nitroxide-mediated radical polymerization, atom-
meantime, anti-fouling rendition may prevent surface from being transfer radical polymerization (ATRP), reversible addition-
modified to the desired properties and deteriorate material fragmentation chain transfer (RAFT) polymerization, and single
performance. For example, circulation time of nanoparticles in blood electron transfer-living radical polymerization (SET-LRP). 9–13 Taking
can be prolonged by PEG modification. However, PEG is also known advantage of these different polymerization techniques, both RAFT
to reduce the cell uptake in drug formulation which may have a and SET-LRP were applied in our study to synthesize a binary mixture
negative effect in intracellular delivery efficacy. 4 Another example of polymer brushes with precisely controlled lengths.
is the Zwitterionic polymers, which can effectively prevent protein
In addition, cyclodextrins (CDs)14,15 were applied to functionalize
fouling. However, it remains challenging to modify the polymers and the polymer brush. CDs are cyclic oligosaccharides consisting of a
introduce surface functionality while keeping the antifouling (1,4)-linked glucopyranose units having a truncated cone-shaped
property. 5
molecular structure. Through the host guest supramolecular
Here we adopt a different approach and address the challenge of interaction, CD offers a versatile means to interact with a wide
surface fouling using stimuli-responsive polymer brush without variety of molecules. 16,17 These supramolecular systems have been
sacrificing the surface functionality. Two polymer brushes of utilized in a significant role in many applications ranging from
18–21
different lengths are grafted together on the surface. One polymer agriculture to biotechnology.
In addition, CD-functionalized
brush is stimuli-responsive and its thickness changes as the materials can provide high separation or purification performance
environment changes. The other polymer brush is fixed in length and and can be used for the removal of organic molecules from liquid and
modified with functional groups. When fully extended, the stimuli- vapour phase. 22–24 However, CD molecules can also be easily fouled
responsive polymer brush is longer than the polymer brush modified when exposed to contaminants. To protect CD molecules against the
with the functional groups. Under such conditions, the surface undesirable interactions, a series of poly(2-N-morpholinoethyl
methacrylate) (PMEMA) chains with different lengths were
synthesized. The modified surface was characterized by XPS, FTIR,
ellipsometry and AFM. In addition, pH- and thermo-responsive
properties were investigated by contact angle measurements. FITC-
Ada molecule was used to optimize the polymer brush system for
fouling protection and controlled detection. Aniline was used as a
contaminant to examine the fouling performance. The optimized
polymer brush system was further functionalized to detect the
hepatitis C virus.
a. Department of Chemistry, Iowa State University, Ames, IA 50011, USA. E-mail:
srkndemirci@gmail.com, sdemirci@iastate.edu.
b. Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA.
E-mail: sjiang1@iastate.edu
c. Department of Chemistry, Amasya University, Amasya 05100, Turkey.
d. Division of Materials Science & Engineering, Ames National Laboratory, Ames,
Iowa 50011, USA
†
SD conceived the idea. SD and SKD designed and performed the experiments. SD
and SJ co-wrote the manuscript. All authors have given approval to the final
version of the manuscript.
‡
Electronic Supplementary Information (ESI) available: Experimental details, NMR,
HRMS, FTIR, XPS, AFM, GPC and fluorescence microscopy
.
See
DOI: 10.1039/x0xx00000x
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nm
nm
16
5
4
3
2
1
0
5
4
3
2
1
0
60
12
8
40
20
4
0
0
a
b
0
1
2
3
4
5
0
1
2
3
4
5
µm
µm
Fig. 2 AFM images of the PS (a), and PMEMA/PS (b).
expose the more hydrophobic moiety to the surface. This change is
clearly demonstrated in Fig. 1, as temperature sweeps from 25 °C to
50 °C, the contact angle changes from 35° to 57° with a transition
temperature at ~37 °C. The surface pKa value for the CD-
functionalized polymer brushes was determined as 5.3 through
contact angle measurement as shown in Fig. S18.
When extended PMEMA chains are long enough, they can
effectively shield CDs on the PS chain. However, in order to expose
the CD groups on the PS brush when PMEMA brush collapses, the
polymer chains also have to be short enough. As a result, PMEMA
chains need to be precisely controlled and fine-tuned for the purpose
of effectively turning on and off the surface functionality. The
molecular weight of the polymer brush was characterized by gel
permeation chromatography (GPC) while the dry thickness of the
polymer brush was measured by ellipsometry. According to the
previous studies25, 26, the molecular weight of the grafted polymer
chains is similar to the free polymer chains in solution. Fig. 3 shows
clearly that as the polymerization progresses, the molecular weight
keeps increasing. From the slope of the linear curve, the grafting
Scheme 1 Preparation of the CD-functionalized polymer brushes.
Scheme 1 shows the fabrication of the two polymer brushes on
the surface. Starting from the silicon wafer, PMEMA was synthesized
through RAFT polymerization while polystyrene (PS) brush was
synthesized via SET-LRP. The prepared PMEMA/PS surface was then
functionalized with tetraethylene glycol modified CD, which was
synthesized via a click reaction. For more details, pleases refer to the
electronic supplementary information (ESI). The ratio of RAFT
agent/initiator was calculated as 3.44 from XPS survey spectrum. In
addition, the molar ratio of PMEMA/PS/CD was determine as
3.13/1.09/1 from high resolution XPS spectra of C 1s.
density of PS and PMEMA/PS surfaces,
ꢀ
(chains/nm2), was
estimated to be 0.21 and 0.61 chains/nm2, respectively. The grafting
density together with the polymer Rg calculation suggest that
PMEMA/PS layer is densely packed and adopted the stretched brush
confirmation.11 In addition, the thickness of the polymer brush is
proportional to the molecular weight of the polymer chains.
Fig. 2 shows the AFM images of the polymer brush grafted on the
silicon wafer. In Fig. 2a, the image indicates that PS forms islands on
the substrate due to the relatively low grafting density. It is also
speculated that under such conditions PS may adopt mushroom
Fmorphology. After grafting with PMEMA, the grafting density
Also shown in Scheme 1, the obtained surface can respond to
both pH and temperature changes. When pH is higher than pKa, the
PMEMA becomes charged, and electrostatic repulsion helps extend
the polymer chains. Also because of the charge, the surface is
relatively hydrophilic. On the other hand, when temperature
decreases, PMEMA tends to form more hydrogen bonding within the
polymer chains, which will lead to the collapse of polymer brush and
Fig. 3 Evolution of the ellipsometric thickness h (nm) of the PS and PMEMA/PS
Fig. 1 Temperature-dependence of water contact angles of PMEMA/PS brush
and derivative curve of d/dT versus T (pH=7).
ꢂ
chains as a function of the ꢁꢃ,ꢄꢅꢆ
.
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the results, polymerization time for MEMA of 8 h is selected for
further experiment. Under this condition, average molecular weight
1.0
0.8
24
40
℃
℃
ꢁꢃ,ꢄꢅꢆ equals to 30,900 g/mol (ꢁꢃ,ꢇꢈꢉ = 31,600 g/mol), and PDI is
1.11.
0.6
0.4
With the optimized polymer brushes, the fouling of the
PMEMA/PS-CD wafers were tested using aniline vapour as the
fouling source. The detection capability afterwards was further
measured by the adsorption of phenolphthalein in aqueous solution.
First, PMEMA/PS-CD wafers were exposed to 5 mL aniline in a sealed
desiccator. The exposure time varied between 1 and 28 days at both
24 C and 40 C. After the exposure, the silicon wafers were removed
from desiccator and immersed for 12 h in phenolphthalein solution
(0.1 M), which was adjusted to pH 7.4 by addition of PBS buffer
solution. The depletion of phenolphthalein from solution was then
determined by UV-vis spectrophotometry at 562 nm.
As shown in Fig. 5, during the exposure to aniline vapour, if CD is
well protected or shielded by PMEMA (24 C and pH < pKa), the
fouling is considerably less, which will lead to high adsorption and
low C/C0 value in the subsequent phenolphthalein solution. On the
other hand, when CD is unprotected (40 C and pH > pKa), the fouling
is much higher and the subsequent adsorption of phenolphthalein is
lower, which will lead to high C/C0 value. Results shown in Fig. 6
clearly demonstrate that polymer brush system can effectively lower
the fouling of the surface functional groups when the protection is
switched on during the fouling experiment.
The reusability of PMEMA/PS-CD surface was investigated by
measuring the adsorption of FITC-Ada through immersion and
sonication cycles. As shown in Fig. 6, FITC-Ada desorbed after
sonication cleaning at 45 C. The immersion/sonication cycle was
repeated up to five times, and the switchable polymer brush surface
can still be reused after cleaning.
To further demonstrate the concept, the PMEMA/PS-CD was
applied to detect HCV virus. Before the detection, the surface can be
stored at 25 C where PMEMA chains are extended to protect the CD
molecules from fouling. The first step in activating the surface for
HCV detection is to modify the CD molecules under the right
conditions. As shown in Scheme 2, CD-functionalized polymer
brushes were first immersed in the solution of Ada grafted anti-HCV
molecules at 40 C for 24 h (PBS, pH=7.4), where PMEMA chains are
collapsed and CD is exposed. The same polymer brushes optimized
0.2
0.0
1
2
4
8
16
Polymerization time of MEMA (h)
Fig. 4 Fluorescence intensity of CD-functionalized PMEMA/PS surfaces with a
ꢂ
ꢂ
fixed PS, ꢁꢃ,ꢄꢅꢆ of 6,300 g/mol (ꢁꢃ,ꢇꢈꢉ = 6,150 g/mol) and various
polymerization time of MEMA.
becomes much denser. The polymer chains may now adopt extended
brush geometry.
FITC-Ada molecule was used as a target molecule, which can
interact with CD and label the polymer brush. The optimum initial
concentration of FITC-Ada and adsorption time were determined as
25 M and 24 h, respectively (Fig. S22). The substrate was carefully
conditioned and rinsed before and after the immersion in a solution
of FITC-Ada. The fluorescence intensity of the substrate corresponds
to the extent of the interaction of CD with FITC-Ada. In our polymer
brush system, high intensity means CD is fully exposed to and
interacted with FITC-Ada, while low intensity means CD is shielded
from FITC-Ada. The same procedure was repeated in both 25 °C and
40 °C in a series of experiments to determine the optimized chain
length combination of the polymer brushes. In this experiment, the
length of PS was fixed, while the length of the PMEMA was gradually
increased as the polymerization time increases.
As shown in Fig. 4, when PMEMA length is relatively short
(polymerization time < 4 h), even at 25 °C when PMEMA chains are
extended, they cannot effectively shield the PS chains and CD
molecules. As a result, the fluorescent intensities at both 25 °C and
40 °C are high. As PMEMA chain length increases (polymerization
time ~8-16 h), the fluorescent intensity gradually decreases at 25 °C,
which suggests that PMEMA start to shield CD molecules on the PS
chains. When PMEMA length is long enough, the fluorescent
intensity at 25 °C stays low (polymerization time at 8 h). On the other
hand, when PMEMA length is too long (polymerization time at 16 h),
the fluorescent intensity at 40 °C is also low, which suggests that
PMEMA is even longer than PS after PMEMA is collapsed. Based on
pH < pKa, pH > pKa
0.8
24 C, 40 C, pH > pKa, pH < pKa
1.0
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
9
1
2
3
4
5
6
7
8
10
Cycles (time of usage)
1
7
14
21
28
Fig. 6 The reusability of the CD-functionalized PMEMA/PS surfaces. 1 – surface
before immersion in FITC-Ada; 2 – surface after immersion; 3 – surface after
sonication rinse at 45 C; 4 – surface re-immersed in FITC-Ada; 5-10 – surfaces
repeated with sonication rinse and immersion procedure.
Time of Pollutant Exposure (days)
Fig. 5 The C/C0 as a function of exposure time, where C0 and C are
concentration of phenolphthalein solution before and after the adsorption.
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self-assembly, and controlled release drug delivery system.
This work was supported in part by The Scientific and
Technological Research Council of Turkey - TUBITAK (Project #
112T868) and the start-up funds provided by Iowa State University.
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Scheme 2 Illustration of host-guest interaction between target molecule and
PMEMA/PS-CD used for HCV detection.
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7 shows the
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environmental stimuli. CD-functionalized binary polymer brush
system (PMEMA/PS) was successfully prepared via a combination of
SET-LRP, RAFT polymerization, and click chemistry. FTIR, XPS,
ellipsometry and AFM further confirmed the surface structure and
morphology change after the modification. Static water contact
angle measurements indicated the wetting-dewetting transition of
PMEMA/PS surface with a transition temperature of 37 C. PMEMA
and PS chain length was optimized to protect and de-protect CD
molecules. The fouling behaviour of PMEMA/PS surface was tested
using aniline vapour. The optimized surface was finally functionalized
for HCV detection. These results demonstrate that fouling and the
undesirable interactions of the CD molecules can be prevented by
switching on the PMEMA brush protection. This provides an effective
way to store the functional groups when not in use, and prolong the
shelf life of the functional surface. In addition, this new strategy
offers alternative ideas of combining anti-fouling property and
surface functionality without compromising the performances. The
same polymer brush design can be further applied to many other
systems, including functionalized nanoparticles, stimuli-responsive
AdaAnti-HCV
H CV
F ITC Anti-HCV
Fig. 7 Fluorescence microscopy images and fluorescence intensities for each
of the three steps in the preparation of the PMEMA/PS-CD for detecting HCV.
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