Synthesis and stability of BODIPY-based fluorescent polymer brushes at different pHs

Article abstract of DOI:10.1002/pola.27426

We report a simple strategy for the grafting of poly(methacrylic acid) [poly(MAA)] brushes from silicon substrate by surface-initiated RAFT polymerization and the subsequent coupling of BODIPY to these brushes to render them fluorescent. The poly(MAA) brushes were first generated by functionalization of hydrogen-terminated silicon substrate with methyl-10-undecenoate which both leads to the formation of an organic layer covalently linked to the surface via Sic bonds without detectable reaction of the carboxylate groups and couples to the polymerization initiator, followed by surface-initiated RAFT polymerization of tert-butyl methacrylate from these substrate-bound initiator centers, and finally conversion of tert-butyl groups to carboxylic acid groups. The poly(MAA) brushes were then made fluorescent by grafting a BODIPY derivative via an ester linkage. The stability of the BODIPY-based fluorescent polymer brushes in buffer solutions at pH 6.0 to 12.0 with added salt was investigated by ellipsometry, fluorescence microscopy, grazing angle-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy. The results of these measurements indicated that the organic molecule-initiator bond (ester linkage) is unstable and can be hydrolyzed resulting in detaching of the immobilized polymer from the silicon substrate.

Full text of DOI:10.1002/pola.27426

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Synthesis and Stability of BODIPY-Based Fluorescent Polymer Brushes
at Different pHs
Dilek Cimen, Talya Tugana Kursun, Tuncer Caykara
Department of Chemistry, Faculty of Science, Gazi University, 06500, Besevler, Ankara, Turkey
Correspondence to: T. Caykara (E-mail: caykara@gazi.edu.tr)
Received 3 June 2014; accepted 23 September 2014; published online 11 October 2014
DOI: 10.1002/pola.27426
ABSTRACT: We report a simple strategy for the grafting of pol-
y(methacrylic acid) [poly(MAA)] brushes from silicon substrate
by surface-initiated RAFT polymerization and the subsequent
coupling of BODIPY to these brushes to render them fluores-
cent. The poly(MAA) brushes were first generated by function-
alization of hydrogen-terminated silicon substrate with methyl-
10-undecenoate which both leads to the formation of an
organic layer covalently linked to the surface via SiAC bonds
without detectable reaction of the carboxylate groups and cou-
ples to the polymerization initiator, followed by surface-
initiated RAFT polymerization of tert-butyl methacrylate from
these substrate-bound initiator centers, and finally conversion
of tert-butyl groups to carboxylic acid groups. The poly(MAA)
brushes were then made fluorescent by grafting a BODIPY
derivative via an ester linkage. The stability of the BODIPY-
based fluorescent polymer brushes in buffer solutions at pH
6.0 to 12.0 with added salt was investigated by ellipsometry,
fluorescence microscopy, grazing angle-Fourier transform
infrared spectroscopy, X-ray photoelectron spectroscopy. The
results of these measurements indicated that the organic
molecule-initiator bond (ester linkage) is unstable and can be
hydrolyzed resulting in detaching of the immobilized polymer
C
V
from the silicon substrate.
2014 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2014, 52, 3586–3596
KEYWORDS: BODIPY; fluorescent; fluorescent polymer brushes;
functionalization of polymers; RAFT; stability; surface-initiated
RAFT polymerization
immobilized on the surface and the initiation mechanism is
more well-defined. Covalent attachment of initiators directly
on the silicon surface via SiAC bonds, in the absence of the
silanol groups (SiAOH), has also attached a great deal of
attention owing to the stability of the resulting SiAC
bonds.22,23 Previous studies have shown that an initiator can
be linked covalently to hydrogen-terminated silicon surface
(SiAH) via UV radiation,24,25 thermal activation26 or electro-
chemical reaction.27 The substrate-organic molecule linkage
(SiAC) is strong enough for both supporting the reaction
conditions necessary for living radical polymerization meth-
ods and subsequent polymer brush formation and character-
ization. Recently, Giasson and coworkers28 reported that
poly(acrylic acid) brushes built on mica by tethering polysty-
rene-poly(acrylic acid) diblock copolymers in a polystyrene
layer covalently linked to OH-activated mica surfaces resist
to cleavage at pH 5.5 with added NaCl for several days.
They29 also investigated effects pH and salt on the detaching
resist to cleavage of covalently attached fluorescent poly(a-
crylic acid) brushes grafted to silicon substrate. However,
there is still uncertainty regarding the robustness of the
substrate-polymer bond (SiAC or SiAOAC) and whether it is
resistant to the extreme pHs with added salt that are
INTRODUCTION Functionalization of silicon substrates is cen-
tral to many areas of research, ranging from microelectronics
to biomaterials.1,2 Functional polymer brushes on silicon
substrate are desired for providing control of reversible sur-
face properties with external stimuli (e.g., solvent, tempera-
ture, pH, and ionic strength). Different surface polymer
conformations including mushroom and brush are possible
with end-anchored functional chains by varying grafting den-
sity and the media.3 The control of conformational character-
istics have allowed for advances in areas such as surface
friction modulation, antifouling,4–6 lubricity,7–9 switchable
wettability, and autophobicity,10 to name but a few. These
polymer brushes are synthesized by living polymerization
methods such as nitroxide-mediated polymerization,11,12
atom transfer radical polymerization,13–15 single electron
transfer living radical polymerization,16 and reversible
addition-fragmentation chain transfer (RAFT)17–19 polymer-
ization with an initiator chemically linked to silicon sub-
strate. However, most of the studies are centered on the
synthesis of functional polymer brushes by surface-initiated
RAFT polymerization on silicon substrate.20,21 The polymer-
ization from the initiator covalently attached to the surface
is more attractive since a high density of initiators can be
C
V
2014 Wiley Periodicals, Inc.
3586
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
required for controlling of the configuration of functional
polymer brushes.
were added at once. After 30 min of stirring, iodoethane
(1 mmol) was added to the yellow mixture. The color of the
resultant reaction mixture changed from yellow to orange.
Stirring was continued in air atmosphere at 25 ꢀC for 50
min. The reaction progress was monitored by thin layer
chromatography. After completion of the reaction, the
organic materials were extracted with 45 mL of diethyl ether.
The combined organic phases were washed with water and
dried with MgSO4. The solvent was evaporated and the resi-
due was crystallized with chloroform to afford the pure
CBET (80%) as a pale-yellow product.
In this study, we report on the surface functionalization of
hydrogen-terminated silicon (SiAH) substrate with well-
defined fluorescent polymer brushes and their stability at
different pHs. tert-Butyl methacrylate (tBMA) was first graft-
polymerized on the SiAH surface via the surface-initiated
RAFT polymerization using 4-chlorobenzyl ethyl trithiocar-
bonate as the RAFT agent and surface-immobilized azo com-
pound as the initiator. Poly(tert-butyl methacrylate)
[poly(tBMA)] was converted into poly(methacrylic acid) [pol-
y(MAA)] by chemically cleaving of the tert-butyl groups. The
poly(MAA) brushes were then made fluorescent by grafting a
BODIPY derivative via an ester linkage. The stability of
BODIPY-based fluorescent poly(MAA) brushes in different
buffer solutions with added NaCl was investigated by ellips-
ometry, fluorescence microscopy, grazing angle-Fourier trans-
form infrared spectroscopy, and X-ray photoelectron
spectroscopy.
1H-NMR (d6-DMSO) d (ppm): ꢁ7.5–7.2 (m, 4H, ArH), 2.9 (q,
2H, CH2CH3), 1.2 (t, 3H, CH2CH3). FTIR (GA-FTIR) t (cm21):
ꢁ3140–2790 (s, aliphatic CAH), ꢁ1600–1450 (s, aromatic
CAH), 1080 (s, C@S).
Synthesis of 8-Hydroxymethyl-2,6-diethyl-1,3,5,7-tetramethyl
pyrromethene fluoroborate (PMOH)
To an air equilibrated solution of PM605 (50 mg, 0.13
mmol, 1 quiv.) in 110 mL of 1:1 (v:v) deionized water/tet-
rahydrofuran was added to lithium hydroxide (112 mg,
2.68 mmol, 20.2 equiv.) under stirring. The solution was
stirred at room temperature in the dark for 5 h. The reac-
tion was quenched with 10 mL of saturated aqueous solu-
tion of ammonium chloride. The resultant mixture was
extracted with dichloromethane (300 mL) and dried with
anhydrous MgSO4. It was then gravity filtrated and concen-
trated under reduced pressure to give a desired alcohol as
a pink solid (the yield of the crude mixture is 98%). The
compound was used in the next step without further purifi-
cation. GA-FTIR t (cm21): ꢁ3630–3020 (s,vb, OH), ꢁ3020–
2800 (s, CAH), 1680 (s, C@O).31
EXPERIMENTAL
Materials
Silicon wafers (n-type, with a thickness of 0.5 mm, a diame-
ter of 125 mm and polished on one side) were purchased
from Shin-etsu, (Handoutai, Japan). tert-Butyl methacrylate
(tBMA, Aldrich, 98%) was purified by distillation under
reduced pressure yielding a colorless product. A small
amount of di-tert-butyl-p-cresol was added to avoid polymer-
ization during the distillation. Ethanol (Aldrich, 96%),
dichloromethane (Aldrich, 99%), diethyl ether (Aldrich,
98%), 4,40-azobis-4-cyanopentanoic acid (ACPA; Fluka, 97%),
N,N0-dicyclohexylcarbodiimide (DCC; Aldrich, 99%), Lithium
aluminium hydride (Aldrich, 95%) were used as received.
Triethylamine (Aldrich, 99.5%) was distilled from calcium
hydride under an argon atmosphere at reduced pressure.
Tetrahydrofuran (Aldrich, 99%) was purified by distillation
from calcium hydride followed by distillation from sodium/
benzophenone kethyl. 1-Ethyl-3-(3-dimethylamino-propyl)
carbodiimide (EDAC, 98%), phosphate-buffered saline (PBS)
tablets, 4-chlorothiophenol (97%), carbon disulfide (99.9%),
iodoethane (99%), anhydrous magnesium sulfate (MgSO4;
97%), chloroform (99%), ammonium chloride (99.5%), lith-
ium hydroxide (98%), hydrofluoric acid (48 wt % in H2O,
99.99%), methyl-10-undecenoate (99%), 4-(dimethylamino)-
pyridine (DMAP, 99%), trifluoroacetic acid (99%) were pur-
chased from Sigma and used without further purification. 8-
Acetoxymethyl-2,6-diethyl-1,3,5,7-tetramethyl pyrromethene-
fluoroborate (PM605) was purchased from Exciton. Deion-
ized water (>18.3 MX.cm) was used in all experiments.
Substrate Preparation and Initiator Immobilization
The silicon wafers were sliced into rectangular strips of
about 1 3 2 cm2. To remove the organic residues on the sur-
face, the silicon substrates were initially cleaned in a
“piranha” solution, which is a 3:1 mixture of concentrated
H2SO4 and H2O2 (30%) heated to ꢁ90 ꢀC for 30 min (CAU-
TION: “piranha” solution reacts violently with organic mate-
rials and must be handled with extreme care), followed by
copious rinsing with doubly distilled water. The silicon strips
were blown dry with purified nitrogen. The cleaned silicon
strips were etched for 5 min in 10% HF solution in a N2
purged steel reaction chamber covered with a quartz win-
dow, then quickly rinsed in degassed deionized water and
dried in a stream of nitrogen to obtain hydrogen-terminated
silicon (SiAH) substrate.
Twenty microliter of liquid methyl-10-undecenoate was
introduced onto the freshly prepared SiAH surface. The sur-
face was slipped into two quartz plates, and a uniform thin
liquid film of methyl-10-undecenoate formed on the SiAH
surface (Si-UDM). The surface was placed in a nitrogen
purged steel reaction chamber covered with a quartz win-
dow and irradiated with a UV light for 4 h. After irradiation,
the modified surfaces (Si-UDM) were washed with copious
Synthesis of RAFT Agent
4-Chlorobenzyl ethyl trithiocarbonate (CBET) was synthe-
sized according to the literature procedure.30 In a typical
experiment, a mixture of 4-chlorothiophenol (1.5 mmol) and
triethylamine (2 mmol) was vigorously stirred at 25 ꢀC for
15 min. Then H2O (1.0 mL) and carbon disulfide (3 mmol)
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596
3587

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
amounts of acetone to remove the residuals and dried in a
stream of nitrogen.
washed with dry dichloromethane, acetone, absolute ethanol
and deionized water and then dried under a nitrogen
stream.
To obtain the hydroxyl ended surface, 50 mL of diethyl
ether was added to a dry flask, followed by degassing with
purified nitrogen; 2.5 g of lithium aluminium hydride pow-
der was added slowly into the flask with stirring. The Si-
UDM substrates were then inserted in the solution. The
reaction mixture was left to stand at room temperature for
12 h. The methyl-10-undecenol immobilized substrates (Si-
UDA) were removed from the reaction mixture and washed
thoroughly with acetone, concentrated HCl solution, acetone,
and water, in that order. The substrates were dried with
nitrogen.
Stability Experiments
To gather information about pH stability of PMOH coupled
poly(MMA) brushes, PBS solution (0.1 M) was prepared
with deionized water and the pH was adjusted to 6 and
12 with hydrochloric acid (0.1 M). Then 0,58 g of NaCl
was added to 10 mL of buffer solutions. The pH was
measured with a Mettler Toledo pH meter with Ag/AgCl
electrode. The substrates were added to the buffer solu-
tions of the given pH and stirred for 2 h. The substrates
were removed from the mixture and washed with deion-
ized water and absolute ethanol, then dried under a nitro-
gen stream.
The Si-UDA substrates were added to a solution of 100 mg
of DMAP and 2 mL of DCC in 50 mL of dry dichlorome-
thane, followed by addition of 1.3 g of ACPA for the prepa-
ration of the Si-ACPA surface. The reaction mixture was
stirred at room-temperature overnight. The Si-ACPA sub-
strates were then removed from the reaction mixture and
repeatedly washed with ethanol and water, then dried with
nitrogen.
Instrumental Techniques
The Nuclear Magnetic Resonance (NMR) spectra were
recorded on a Bruker Ultrashield 300 MHz NMR spectrome-
ter with chemical shifts in ppm (for CDCl3, tetramethylsilane
was used as internal standard). The Grazing Angle-Fourier
Transform Infrared (GA-FTIR) spectra of the samples were
collected using a Harrick Scientific GA-FTIR attachment
coupled with a Thermo Nicolet 6700 spectrometer, collect-
ing 128 sample scans, and utilizing Nicolet’s OMNIC soft-
ware. The X-ray Photoelectron Spectroscopy (XPS)
measurements were carried out on a SPECS XPS spectrome-
ter equipped with a Mg Ka X-ray source. After peak fitting
of the C 1s spectra, all the spectra were calibrated in refer-
ence to the aliphatic C 1s component at a binding energy of
285.0 eV. The water contact angle measurements were con-
ducted at room temperature using a goniometer (DSA 100,
Surface-Initiated RAFT Polymerization of tBMA
To prepare poly(tBMA) brushes on the silicon surface, tBMA
(30 mmol), CBET (0.12 mmol), and ethyl acetate were added
in a glass reactor, which was designed to hold six initiator-
immobilized silicon substrates oriented normal to the base of
the reactor. To ensure smooth stirring and prevent damage to
the surfaces of the substrates, we isolated the magnetic stir-
ring bar at the center of device from the slides by a 1 cm high
glass O-ring. The solution was diluted to 30 mL volume with
dry ethyl acetate and degassed by purging with nitrogen for
30 min. The reaction flask was sealed under nitrogen atmos-
phere and kept at 80 ꢀC oil bath for a predetermined period of
time. After the reaction, the silicon substrate with surface-
grafted poly(tBMA) was removed from the reaction mixture
and washed thoroughly with an excess volume of ethyl acetate
to give rise to the Si-g-poly(tBMA) surface.
€
Kruss) equipped with a microliter syringe. Deionized water
(4 mL, 18 MX cm resistivity) was used as the wetting liquid.
The morphology of the surfaces was recorded on an Atomic
Force Microscope (AFM) (Park Systems XE70 SPM Controller
LSF-100 HS). A triangular shaped Si3N4 cantilever with inte-
grated tips (Olympus) was used to acquire the images in
the noncontact mode. The normal spring constant of the
cantilever was 0.02 N/m. The force between the tip and the
sample was 0.87 nN. The ellipsometric measurements were
conducted in ambient conditions using an Ellipsometry
Hydrolysis of Poly(tBMA) to Poly(MAA)
The Si-g-poly(tBMA) substrates were treated with trifluoro-
acetic acid solution (%10 trifluoroacetic acid in dichlorome-
thane) at room temperature with stirring overnight to
obtain Si-g-poly(MAA) surfaces with hydroxyl end groups.
After ester cleavage, the substrates were washed several
times with dichloromethane, absolute ethanol and deionized
water. The substrates were then dried under a nitrogen
stream.
(model DRE, EL X20C) equipped with
a He-Ne laser
(k 5 632.8 nm) at a constant incident angle of 75ꢀ. The
average dry thickness of polymer films on silicon wafer was
determined by fitting the data with a three-layer model
[native silicon (refractive index, n 5 3.86) 1 silicon oxide
layer (n 5 1.46) 1 polymer brush (n 5 1.46)].32 The fluores-
cence images of PMOH coupled poly(MMA) brushes were
recorded with an Olympus BX51 fluorescence microscope.
The fluorescence intensity of all samples was measured
using image analysis software ImageJ. Gel Permeation Chro-
matography (GPC) analysis was performed with an Waters
HPLC system consisting of a binary pump, an autosampler, a
temperature controlled column oven and differential refrac-
tometer. The analytical separation was performed on Waters
Styragel HR4 and HR6 columns (7.8 3 300 mm) at 27 ꢀC
Attachment of PMOH to Poly(MAA) Brushes
For coupling of PMOH to poly(MAA), PMOH (56 mmol),
EDAC (32 mmol), and DMAP (49 mmol) were dissolved in
dry dichloromethane (20 mL) in a 100 mL round bottom
flask. Si-g-poly(MAA) surfaces were placed in a reactor
equipped with a magnetic stir bar. The mixture was then
stirred at room temperature for 4 h under nitrogen stream.
The substrates were removed from the mixture and then
3588
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
SCHEME 1 Schematic diagram illustrating the processes of immobilization of initiator on the SiAH surface, the surface-initiated
RAFT polymerization of tBMA from the Si-ACPA surface, converting the Si-g-poly(tBMA) into Si-g-poly(MAA), and coupling of
PMOH to Si-poly(MMA). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
with a flow rate of 1 mL min21. THF was used as the
RESULTS AND DISCUSSION
mobile phase and the system was calibrated with narrow-
Uniform, dense, and covalently bonded polymer chains on
disperse polystyrene standards (Shodex).
silicon surface can be taken place by immobilizing initiators
or RAFT agents on substrate. In this study, an azoinitiator
with have long chain length (C10) was covalently immobi-
lized to silicon surface to initiate the growth of the poly-
mer brushes in the presence of the sacrificial chain transfer
agent for RAFT polymerization. According to studies in this
area, immobilization of long-chain molecules forms
a
densely packed, crystalline or liquid-crystalline monolayer
due to strong molecular interactions (van der Waals forces)
between the long carbon chains.33,34 Longer chain length
also increases the thermodynamic stability.35 In this study,
methyl-10-undecenoate (C10, UDM) was chosen as a spacer
between silicon substrate and initiator owing to it is essen-
tial for the efficient initiation of RAFT polymerization. After
the SiAH surface was prepared by HF treatment of the sili-
con substrate, the azo initiator ACPA was immobilized on
the hydrogen-terminated silicon (SiAH) surface by the
three-step process shown in Scheme 1. First, UV-induced
coupling of UDM with the SiAH surface produces the ester-
terminated Si-UDM surface. Subsequent reduction of the Si-
UDM surface by lithium aluminium hydride forms the
hydroxyl-terminated Si-UDA surface. Esterification of the
hydroxyl terminal groups with ACPA initiator yields the Si-
ACPA surface. The formation of the SiAH surface and the
attachment of the organic molecules on this surface at each
modification step were confirmed with GA-FTIR [Fig.
1(a–d)], XPS [Fig. 2(a–c)], AFM [Fig. 3(a–c)], and water con-
tact angle measurements. Figure 1(a) shows the GA-FTIR
spectrum of the SiAH surface freshly prepared, which con-
sists of a narrow absorption centered at 2310 cm21. The
presence of UDM on the silicon surface was confirmed by
FIGURE 1 GA-FTIR spectra of (a) SiAH, (b) Si-UDM, (c) Si-UDA,
(d) Si-ACPA, (e) Si-g-poly(tBMA), and (f) Si-g-poly(MAA). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
the presence of the bands at approximately 1741 cm21
,
which corresponding to C@O stretching vibrations
[Fig. 1(b)]. XPS analysis of the UDM monolayer [Fig. 2(a)]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596
3589

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 2 O 1s, N 1s, C 1s, and S 2p core level XPS spectra of (a) Si-UDM, (b) Si-UDA (c) Si-ACPA, (d) Si-g-poly(tBMA), and (e) Si-
g-poly(MAA). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
verified the presence of C 1s and O 1s peaks curve-fitted
to components with binding energies at approximately
285.0 eV (CAH/CAC), 285.2 eV (CAO), and 287.5 eV
(C@O) for C 1s and 533.6 eV (C@O) and 534.0 eV (CAO)
for O 1s [Fig. 2(a)]. After reduction of the Si-UDM surface
by lithium aluminium hydride, the GA-FTIR spectrum
In the subsequent functionalization process, ACPA was
attached to the UDA layer via an esterification reaction. The
attachment of ACPA was apparent from the appearance of
the bands at 1740, 1800, and 3200 cm21, which are
assigned to C@O stretching (carboxylic acid), C@O stretching
(ester) and NAH stretching vibrations, respectively [Fig.
1(d)]. The core level XPS spectra of the ACPA overlayer con-
sist of O 1s, N 1s and C 1s peaks curve fitted to components
with binding energies at ꢁ539.9 eV (CAO) and 533.0 eV
(C@O) for O 1s, 400.5 eV (NAN) and 401.3 eV (NAC) for N
1s and 286.2 eV (O@CAO), 287.8 eV (C@O), 286.0 eV
(CAN), and 285.0 eV (CAH/CAC) for C 1s [Fig. 2(c)]. The
thickness of the Si-UDM, Si-UDA, and Si-ACPA layers was
appeared
a strong absorption band at around 3000–
3500 cm21 associated with the stretching vibrations of
AOH group of the UDA. The core level XPS spectra of UDA
consists of C 1s and O 1s peaks curve-fitted to components
with binding energies at ꢁ285.0 eV (CAH /CAC) and
286.1 eV (CAO) for C 1s and 533.5 eV (CAO) for O 1s
[Fig. 2(b)].
3590
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 3 2D AFM images in ambient conditions, surface cross-section analysis, and photographs of 4 mL water droplets (top) on
(a) Si-UDM, (b) Si-UDA (c) Si-ACPA, (d) Si-g-poly(tBMA), and (e) Si-g-poly(MAA). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
measured by ellipsometry to be 1.2 6 0.3, 1.1 6 0.5, and
3.5 6 0.4 nm, respectively. The surface morphologies of the
silicon substrate after the functionalization processes are
shown in Figure 3(a–c). The Si-UDM, Si-UDA, and Si-ACPA
surfaces are rather uniform and smooth with root-mean-
square (rms) roughness values of ꢁ0.66, 0.47, and 0.53 nm,
respectively. However, the attachment of the azo-initiator
induced a significant change in surface wettability was char-
acterized by a dramatic increase in the static water contact
angle from 63.8 6 0.3ꢀ (Si-UDA) to 76.1 6 0.5ꢀ (Si-ACPA), as
shown in the inset images in Figure 3.
poly(tBMA) exists as a distinctive and uniform overlayer
with a rms value of about 1.16 nm.
The molecular weights of the free polymer determined by
1H-NMR and GPC were Mn;NMR 5 9800 and Mn;GPC 5 9100 g
mol21, respectively. The grafting density r of the poly(tBMA)
was calculated from the dry polymer thickness h (42.1 nm),
using r5hqNA10221=Mn;NMR where q is the density of
poly(tBMA) (1.022 g cm23), NA is Avogadro’s number (6.02
3 1023 mol21) and Mn;NMR is the molecular weight of the
grafted polymer chains, which is assumed to be similar to
that of the free polymer in solution.36 We use this assump-
tion because it was not possible to determine the molecular
weight of the very small amount of generated grafted chains.
It has been shown that the molecular weight of the grafted
polymer grown by RAFT polymerization can be similar to or
slightly larger than the molecular weight of free polymer in
solution.37,38 High grafting density of 0.44 chains/nm2 was
Surface-initiated RAFT polymerization of tBMA was carried
out in the presence of free RAFT agent and the initiator-
immobilized substrate to produce free poly(tBMA) and the
corresponding polymer brushes on the substrates simultane-
ously (Scheme 1). The drastic increase in the water contact
angle (95.0 6 0.3ꢀ) is consistent with the presence of hydro-
phobic poly(tBMA) chains from RAFT polymerization. As is
also revealed by the AFM image in Figure 3(d), the grafted
found. In this estimation, average distance between grafting
1=2
points
D
(D5ð4=prÞ ),36 was found to be 1.60 nm.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596
3591

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 Fluorescence microscopy images of (a) Si-g-poly(MAA), (b) Si-ACPA, and (c) Si-g-poly(tBMA) surfaces subjected to
same reaction conditions for coupling of PMOH. (d) Fluorescence intensity is quantitatively detected by measuring the fluores-
cence signals of PMOH attached to Si-g-poly(MAA) surface. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Meanwhile, the theoretical degree of polymerization (DPn) of
poly(tBMA) chains can be estimated to be ffi 61 on the basis
of monomer conversion (96%). This generally agrees with
the actual DPn of grafted poly(tBMA) obtained from GPC (ffi
80) and NMR (ffi 76) analysis and in the GPC trace of the
To further characterize the poly(tBMA) brushes, the samples
were analyzed using GA-FTIR and XPS. The presence of pol-
y(tBMA) was confirmed by the prominent aliphatic CAH and
C@O bands recorded in the 2800–3000 cm21 and
1800 cm21 regions of the GA-FTIR spectrum, respectively
[Fig. 1(e)]. The core level XPS spectra of poly(tBMA) consist
of O 1s, N 1s, and C 1s peaks curve fitted to components
with binding energies at ꢁ534.0 eV (CAO) and 533.5 eV
(C@O) for O 1s, 400.9 eV (N-C) for N 1s and 288.6 eV
(C@O), 287.3 eV (CAS), 285.4 eV (CAN), 286.3 eV (C@S),
and 285.0 eV (CAH/CAC) for C 1s [Fig. 2(d)]. Apart from
these peaks, there appear two signals at about 163.0 eV
(SAC) and 165.2 eV (S@C) assignable to S 2p in the core
level spectra of the poly(tBMA) brushes [Fig. 2(d)].
-
free polymer was monomodel and had narrow D value
(1.67), which are characteristics of well-defined polymers via
surface-initiated RAFT. The radius of gyration (Rg, nm) of
poly(tBMA) was found to be 0.85 nm by using the equation:
Rg5bðDPn=6Þ1=2where DPn is the degree of polymerization
and b, is the segment length [assumed to be 0.25 nm for pol-
y(tBMA) chains].39 An indication of having polymeric
brushes prepared in the presence of free RAFT agent can be
when the interchain distances are less than the radius of
gyration of the corresponding free polymer chains.40 We
have achieved high grafting density for poly(tBMA) and the
interchain grafting distances are less than the radius of gyra-
tion of these free poly(tBMA) chains in solution (D/
2Rg 5 0.94), indicating that the chains are indeed in a
stretched brush-like conformation.
Removal of the tert-butyl groups was carried out by reflux-
ing the poly(tBMA) brushes in TFA/DCM,41 resulting in
decrease in the water contact angle to 63.7 6 0.3ꢀ owing to
the hydrophilic nature of poly(MAA) chains. The rms value
of the Si-g-poly(MAA) surface is only about 0.80 nm [Fig.
FIGURE 5 2D AFM images in ambient conditions, surface cross-section analysis, and photographs of 4 mL water droplets (top) on
Si-g-poly(MAA)-PMOH surfaces: Before (a) and after exposing the substrate to buffer solutions at pH 6.0 (b) and pH 12.0 (c). [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
3592
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 6 F 1s, O 1s, N 1s, C 1s, B 1s core level XPS spectra of Si-g-poly(MAA)-PMOH surfaces: Before (a) and after exposing the
substrate to buffer solutions at pH 6.0 (b) and pH 12.0 (c). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
3(e)]. The broad band at 2800–3400 cm21 in the GA-FTIR
spectra [Fig. 1(f)] is attributed to the -OH group formed dur-
ing the poly(tBMA) conversion. The presence of a major
CAOH peak component at 287.5 eV in the C 1s core-level
spectrum [Fig. 2(e)] is also consistent with the presence of
carboxylic acid groups on the Si-g-poly(MAA) surface.
While GA-FTIR and XPS proved useful in confirming that the
conversion of poly(tBMA) into poly(MAA) took place, they
do not provide sufficient information about thickness of the
polymers on the silicon substrate. The ellipsometry was
used to measure the dry thickness of both the poly(tBMA)
and poly(MAA) samples, which were polymerized for 10 h
FIGURE 7 Fluorescence microscopy images of Si-g-poly(MAA)-PMOH after exposing to buffer solutions at pH 6.0 (a) and pH 12.0
(b). Evolution of fluorescence intensity as a function pH 6.0 and pH 12.0 (c). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596
3593

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 8 GA-FTIR spectra of Si-g-poly(MAA)-PMOH surfaces:
Before (a) and after exposing the substrate to buffer solutions
at pH 6.0 (b) and pH 12.0 (c). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
and hydrolysis in HCl/dioxane bath for 10 h. However, the
thickness of both poly(tBMA) and poly(MAA) was almost the
same value (ꢁ42 nm), indicating that there is no detaching
under these hydrolysis conditions.
After removal of the tert-butyl deprotecting groups, the BOD-
IPY derivative PMOH was coupled to poly(MMA) chains with
EDAC to afford the fluorescent polymer, Si-g-poly(MAA)-
PMOH (Scheme 1). The Si-g-poly(MAA)-PMOH surfaces were
then washed via Soxhlet extraction in dichloromethane for
10 h to remove any potentially physisorbed PMOH. After the
covalent attachment of the PMOH to the poly(MMA), presence
of fluorescence signals in the fluorescence image of the Si-g-
poly(MAA)-PMOH surface (Fig. 4a) proved the covalent attach-
ment of the PMOH to the poly(MMA) chains. On the other
hand, no fluorescence signal was imaged the Si-ACPA (Fig. 4b)
and Si-g-poly(tBMA) (Fig. 4c) surfaces subjected to same
reaction conditions. The water contact angle of the Si-g-
poly(MAA)-PMOH surface increased from 63.7 6 0.3ꢀ to 80.1
6 0.4ꢀ owing to the hydrophobic nature of PMOH. The rms
value of the Si-g-poly(MAA)-PMOH surface is about 1.77 nm
(Fig. 5). The drastic increase in rms value was probably caused
by the uneven distribution of the collapsed poly(MMA) graft
chains with PMOH on the surface. The covalent attachment of
the PMOH to the poly(MMA) is further ascertained from the
surface composition analysis by XPS. The presence of F 1s sig-
nal at about 685 eV and B 1s signal at 195 eV in the core-level
spectra (Fig. 6) is consistent with the presence of PMOH on the
Si-g-poly(MAA)-PMOH surface.
The stability of the ester linkage between the BODIPY deriva-
tive PMOH and poly(MAA) chain was next investigated at pH
6.0 and 12.0 with a NaCl concentration of 10 mM. Although
3594
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
the monovalent salt (NaCl) was not expected to directly pro-
mote polymer cleavage,42,43 it however can increase the
degree of dissociation of poly(MMA) chains leading to poly-
mer backbone extension with increasing density of negative
charges.43,44 The extended conformation of polyelectrolyte
chains in turn exposes the UDA-initiator bond to hydroxyl
ions, which potentially promotes cleavage process. After
exposing the Si-g-poly(MAA)-PMOH surface to buffer solu-
tions at pH 6.0 and pH 12.0 with added salt, the water con-
tact angle decreased slightly to about 78ꢀ. The alteration of
rms values (1.79 nm at pH 6.0 and 2.22 at pH 12.0) was
probably caused by the polymer degrafting (Fig. 5). However,
the drastic decrease in the thickness of the polymer layer
(30.2 nm at pH 6.0 and 24.6 nm at pH 12.0) was also attrib-
uted to the polymer degrafting from the substrate (initial
thickness 42.1 nm). By using ellipsometric thickness values
at pH 6.0 and 12.0, the D/2Rg values were found to be 0.51
and 0.57, respectively, indicating that the polymers are still
in the brush-like conformation.
in Table 1 revealed a significant decrease in the atomic compo-
sition of the carbon (from ꢁ61% to ꢁ49% from pH 6.0 to
12.0) in comparison with the carbon composition of bare Si-g-
poly(MAA)-PMOH (ꢁ77%). This is a result of polymer degraft-
ing from substrate and not from PMOH hydrolysis. This result
further suggests that the UDA-initiator bond is slightly pro-
tected against hydrolysis at pH 6.0 most probably because of
collapsed conformation of polymer chains close to the surface.
However, at pH 12.0, the end-grafted chains are ionized result-
ing in an extended brush conformation that exposes the UDA-
initiator bond making it more sensitive to hydrolysis by
hydroxyl ions.
CONCLUSIONS
The goal of this study is to make BODIPY-based fluorescent
polymer brushes grafted to silicon substrate [Si-g-poly(MMA)-
PMOH]. The Si-g-poly(MMA)-PMOH layer was formed by the
following: (1) creating of the surface-anchored polymerization
initiator, (2) performing RAFT polymerization of tBMA from the
surface, (3) converting the poly(tBMA) into poly(MAA) by
hydrolysis and (4) coupling of PMOH to poly(MAA). Degrafting
of BODIPY-based fluorescent polymer chains from silicon sub-
strate was unequivocally confirmed via the ellipsometry, fluo-
rescence microscopy, GA-FTIR, and XPS measurements. The
results of these measurements indicated that the UDA-initiator
bond is unstable and can be cleaved. Detaching of the grafted
polymer was observed after exposing the substrate to buffer
solutions at pH 6.0 and 12.0 with added NaCl. Although the
exact salt and pH induced conformation change is unknown,
the hydroxyl ions nonetheless penetrate through the poly(MAA)
layer to the substrate and hydrolyze the UDA-initiator bond.
Fluorescence microscopy measurements (Fig. 7) were also
done after incubation at pH 6.0 and 12.0 to ensure that the
decrease of fluorescence signal or fluorescence intensity was
due to polymer cleavage from the substrate. The fluores-
cence intensity difference between the bare substrate (Si-g-
poly(MAA)-PMOH) and after exposure to buffer solutions can
quantitatively be determined from the fluorescence micros-
copy measurements. The degrafting ratio, defined as the ratio
between the fluorescence intensity after and before exposure
to buffer solutions, is about 3.6 3 105 and 3.1 3 105 for the
substrate exposed to pH 6.0 and pH 12.0 solutions, respec-
tively. These results suggest that about 28% at pH 6.0 and
38% at pH 12.0 of the grafted polymer are cleaved from the
substrate. However, it should be noted that the covalent
attachment of organic molecule directly on the SiAH surface
via SiAC bonds, in the absence of the silanol groups, is resist-
ant to the extreme pH owing to the stability of the resulting
SiAC bonds.22,23 However, not only the PMOH was labeled on
the poly(MAA) chain by ester linkage, the initiator was also
bounded to the UDA layer via labile ester linkage, therefore,
the exact location where bond breaking occurs is unknown
because all ester linkages (CAO bonds) are potentially unsta-
ble and sensitive to hydrolysis.
REFERENCES AND NOTES
1 J. M. Buriak, Chem. Rev. 2002, 10, 1272–1308.
2 R. A. Wolkow, Annu. Rev. Phys. Chem. 1999, 50, 413–441.
3 J. Li, X. Chen, Y. C. Chang, Langmuir 2005, 21, 9562–9567.
4 E. De Giglio, S. Cometa, N. Cioffi, L. Torsi, L. Sabbatini, Anal.
Bioanal. Chem. 2007, 389, 2055–2063.
5 O. Hollman, T. Gutberlet, C. Czeslik, Langmuir 2007, 23,
1347–1353.
6 F. J. Xu, K. G. Neoh, E. T. Kang, Prog. Polym. Sci. 2009, 34,
719–761.
7 C. Drummond, J. Rodriguez-Hernandez, S. Lecommandoux,
P. Richetti, J. Chem. Phys. 2007, 126, 184906–184912.
Therefore, further confirmation that the observed changes in
ellipsometric thickness and fluorescence intensity arise
from polymer degrafting rather than hydrolysis of the PMOH-
poly(MMA) bond was obtained by GA-FTIR and XPS analysis.
After exposing the substrate to buffer solutions at pH 6.0 and
pH 12.0 with added salt, the intensity of CAH and C@O bands
recorded in the 2700–3030 cm21 and 1750 cm21 regions of
the GA-FTIR spectrum (Fig. 8), respectively, decreased drasti-
cally in comparison with the bands recorded for bare Si-g-pol-
y(MAA)-PMOH. We believe that the decrease of the band
intensity reflects the polymer degrafting.
8 M. K. Vyas, B. Nandan, K. Schneider, M. Stamm, J. Colloid
Interface Sci. 2008, 328, 58–66.
9 H. B. Zeng, Y. Tian, B. X. Zhao, M. Tirrell, J. Israelachvili,
Langmuir 2009, 25, 4954–4964.
10 T. H. Epps, D. M. De Longchamp, M. J. Fasolka, D. A.
Fischer, E. L. Jablonski, Langmuir 2007, 23, 3355–3362.
11 J. Li, X. Chen, Y. C. Chang, Langmuir 2005, 21, 9562–9567.
12 D. Cimen, T. Caykara, J. Mater. Chem. 2012, 22, 13231–
13238.
13 T. Wu, P. Gong, I. Szieifer, P. Vlcek, V. Subr, J. Genzer, Mac-
romolecules 2007, 40, 8756–8764.
Elementary composition and values of binding energies given
by XPS (Fig. 6) were summarized in Table 1. XPS data shown
14 W. H. Yu, E. T. Kang, K. G. Neoh, S. Zhu, J. Phys. Chem. B
2003, 107, 10198–10205.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596
3595

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
€
15 Y. Xu, A. Walther, A. H. E. Muller, Macromol. Rapid Com-
mun. 2010, 31, 1462–1466.
30 B. Movassagh, M. Soleiman-Beigi, Monatsh. Chem. 2008,
139, 927–930.
16 E. Turan, T. Caykara, J. Polym. Sci. Part A: Polym. Chem.
2010, 48, 3880–3887.
31 P. Oleynik, Y. Ishihara, G. Cosa, J. Am. Chem. Soc. 2007,
129, 1842–1843.
17 W. H. Yu, E. T. Kang, K. G. Neoh, Ind. Eng. Chem. Res.
2004, 43, 5194–5202.
32 K. N. Plunkett, X. Zhu, J. S. Moore, D. E. Leckband, Lang-
muir 2006, 22, 4259–4266.
18 A. Zengin, T. C¸aykara, J. Polym. Sci. Part A: Polym. Chem.
2012, 50, 4443–4450.
33 C. D. Bain, E.B. Troughton, Y. T. Tao, J. Evall, G. M.
Whitesides, R. G. Nuzzo, J. Am. Chem. Soc. 1989, 111, 321–335.
19 L. Liang, K. Entang, N. Koongee, Appl. Surf. Sci. 2008, 254,
2600–2604.
34 L. Strong, G. M. Whitesides, Langmuir 1988,4, 546–558.
35 L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides,
Chem. Rev. 2005,105, 1103–1170.
20 N. Gurbuz, S. Demirci, S. Yavuz, J. Polym. Sci. Part A:
Polym. Chem. 2011, 49, 423–431.
36 I. Luzinov, D. Julthongpiput, H. Malz, J. Pionteck, V. V.
Tsukruk, Macromolecules 2000, 33, 1043–1048.
21 S. Demirci, T. Caykara, J. Polym. Sci. Part A: Polym. Chem.
2012, 50, 2999–3007.
22 J. M. Buriak, Chem. Commun. 1999, 1051–1060.
37 K. Ohno, T. Morinaga, K. Koh, Y. Tsujii, T. Fukuda, Macro-
molecules 2005, 38, 2137–2142.
23 D. D. M. Wayner, R. A. Wolkow, J. Chem. Soc. Perkin Trans.
2002, 2, 23–34.
38 R. E. Behling, B. A. Williams, B. L. Staade, L. M. Wolf, E. W.
Cochran, Macromolecules 2009, 42, 1867–1872.
24 R. Boukherroub, D. D. M. Wayner, J. Am. Chem. Soc. 1999,
121, 11513–11515.
39 L. Chunzhao, B.C. Benicewicz, Macromolecules 2005, 38,
25 R. L. Cicero, M. R. Linford, C. E. D. Chidsey, Langmuir 2000,
16, 5688–5695.
5929–5936.
40 W. J. Brittain, S. Minko, J. Polym. Sci. Part A: Polym. Chem.
2007, 45, 3505–3512.
26 A. B. Sieval, A. L. Demirel, J. W. M. Nissink, M. R. Linford,
J. H. van der Maas, W.H. de Jeu, H. Zuilhof, E. J. R. Sudholter,
Langmuir 1998, 14, 1759–1768.
41 S. Kurosawa, H. Aizawa, Z. A. Talib, B. Atthoff, J. Hilborn,
Biosens. Bioelectron. 2004, 20, 1165–1176.
27 P. Allongue, C. H. de Villeneuve, S. Morin, R. Boukherroub,
D. D. M. Wayner, Electrochim. Acta. 2000, 45, 4591–4598.
42 K. N. Witte, S. Kim, Y. Y. Won, J. Phys. Chem. B. 2009, 113,
11076–11084.
28 O. Borozenko, R. Godin, K. L. Lau, W. Mah, G. Cosa, W. G.
Skene, S. Giasson, Macromolecules 2011, 44, 8177–8184.
43 E. B. Zhulina, T. M. Birshtein, O. V. Borisov, Macromole-
cules 1995, 28, 1491–1499.
29 B. Liberelle, X. Banquy, S. Giasson, Langmuir 2008, 24,
3280–3288.
44 R. Konradi, J. Ruhe, Macromolecules 2005, 38, 4345–4354.
3596
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 52, 3586–3596