Synthesis and characterization of degradable water-soluble fluorescent polymers

Article abstract of DOI:10.1021/ma102159c

The synthesis and characterization of a water-soluble nonconjugated polymer (PMFT) that was obtained by polymerization of a fluorescence generating diol monomer with terephthalic acid was investigated. The mixture of compound 1 (1.25 g, 1.87 mmol) and com

Full text of DOI:10.1021/ma102159c

10196 Macromolecules 2010, 43, 10196–10200
DOI: 10.1021/ma102159c
MALDI-TOF and ESI spectra were recorded on Bruker
AutoFlexIII and SHIMADZU LCMS 2010 systems, respec-
tively. Elemental analysis was supplied by Flash EA1112. The
gel permeation chromatography (GPC) measurements were
achieved on a Waters 410 system against polystyrene stan-
dards using tetrahydrofuran (THF) as eluent. Florescence
spectra were performed on a Hitachi F-4500 fluorometer
equipped with a xenon lamp as excitation source at room
temperature, and the PMT voltage was 400 V. The fluores-
cence quantum yields (QYs) were determined with quinine
sulfate (0.1 M H2SO4, QY = 0.577) as reference.27 UV-vis
spectra were recorded on a Jasco V-500 spectrophotometer.
The absorbance at 490 nm for MTT assay was recorded on a
microplate reader (BioTek Synergy HT). The UV viewing
cabinet used in this study was from ShangHai GuCun Optic
Instruments Factory, model ZF-20D, equipped with two
8 W 254 nm and two 8 W 365 nm UV lights. The pH indica-
tion was carried on a Mettler Toledo Delta 320 pH indica-
tor, calibrated with standard buffer of pH = 7.00 at room
temperature.
Materials. 9,9-Bis(60-bromohexyl)-2,7-dibromofluorene
(1) was prepared according to the literature procedure.2
4-(Hydroxylmethyl)benzeneboronic acid (97%) and quinine
sulfate dehydrate (98%) were purchased from Alfa-Aesar.
Terephthaloyl chloride (>99%) was purchased from J&K
Chemical. Sodium deuteroxidesolution (40 wt% in D2O, 99þ
atom % D) was purchased from Sigma-Aldrich. PdCl2(dppf)
(98%) was obtained from Pacific ChemSource, Inc. (China),
and 1-methylimidazole (>98%) was purchased from Beijing
Ouhe Technology Co., Ltd. (China). All the reagents and
solvents were used as received except where noted otherwise.
THF was driedoverrefluxing with sodium and benzophenone
as an indicator. Pyridine was dried by refluxing with P2O5 for
24 h and distilled in a dry environment. The water used in
the experiment was filtered by a Millipore filtration system.
The buffer solutions were prepared by 0.1 M Na2HPO4 and
0.1 M NaOH and indicated by the pH indicator. Renal cell
carcinoma (A498) was purchased from cell center of Institute
of Basic Medical Sciences, Chinese Academy of Medical
Sciences (Beijing, China). Dulbecco’s modified Eagles medium
(DMEM, high glucose, without phenol red) and fetal bovine
serum were purchased from Hyclone/Thermofisher (Beijing,
China).
Synthesis of 2,7-Di(40-phenylmethanol)-9,9-bis(60-bromo-
hexanyl )fluorene (3). The mixture of compound 1 (1.25 g,
1.87 mmol) and compound 2 (760 mg, 5 mmol) in THF
(10 mL) and 2.0 M aqueous K2CO3 solution (8 mL) was
charged with argon for 30 min. Then 40 mg of PdCl2(dppf )
(dppf = 1,10-bis(diphenylphosphine)ferrocene) was added
under an argon atmosphere, and the mixture was then stirred
at 70 °C for 24 h. After cooling down to room temperature,
THF was removed in vacuum from the mixture. The crude
product was extracted with 50 mL of CHCl3 for three times.
The combined organic phase was washed with brine and
dried over anhydrous MgSO4, and the solvent was removed
under vacuum. The residue was purified by silica gel column
chromatography using ethyl acetate/petroleum ether 1/4
(v/v) mixture as eluent to obtain white solids (684 mg, 52%).
1H NMR (400 MHz, CDCl3) δ: 7.78 (d, J = 7.8 Hz, 2H), 7.69
(d, J=7.5 Hz, 4H), 7.60 (d, J=7.8 Hz, 2H), 7.56 (s, 2H), 7.49
(d, J=7.5 Hz, 4H), 4.77 (s, 4H), 3.26 (t, J=6.6 Hz, 4H),
Synthesis and Characterization of Degradable
Water-Soluble Fluorescent Polymers
Hui Chong, Xinrui Duan, Qiong Yang,* Libing Liu, and
Shu Wang*
Beijing National Laboratory for Molecular Sciences, Key
Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China
Received June 22, 2010
Revised Manuscript Received November 12, 2010
Introduction. Water-soluble conjugated polymers show ex-
cellent fluorescence quenching or energy transfer efficiencies in
the presence of oppositely charged electron/energy acceptors,1-4
resulting in the amplification of transduction optical signals of
chemical or biological recognition events.5-15 In recent years,
new applications of conjugated polymers in cell imaging,
therapy, and biomedical fields have also been successfully
explored.16-18 In contrast to conventional fluorescent organic
dyes, conjugated polymers are favorable due to their better
photostabilities, higher extinction coefficients, and higher
detection sensitivities. However, most of the water-soluble
conjugated polymers that have been reported to date are
nondegradable.19 These polymers contain conjugated arylic
backbones and are hard to remove from an animal or human
body by metabolic paths. So there are potential concerns of
toxicity or carcinogenesis when they are used in vivo. On the
contrary, biodegradable polymers seldom suffer from these
problems. There is thus a need to develop degradable water-
soluble fluorescent conjugated polymers.
One promising strategy is to develop degradable polymers
with fluorescent and water-soluble characteristics. So far,
most reported degradable fluorescent polymers20-24 need
to conjugate or encapsulate organic dyes or quantum dots
to facilitate their visual detection or monitoring. Unfortu-
nately, most of these polymers exhibit cytotoxicity and poor
photostability.20,25 Previous work reported that19,26 noncon-
jugated polymers, where nonconjugated spacers connect the
conjugated chromophores, can maintain the fluorescent
properties of conjugated polymers except for tuning their
energy bands. In this paper, we describe the synthesis and
characterization of a novel water-soluble nonconjugated
polymer (PMFT) that is obtained by polymerization of a
fluorescence generating diol monomer with terephthalic
acid. The fluorescent property can be contributed from the
fluorescence generating units, and the water solubility can
be obtained by introducing a charged group in the side chain
of the polymer. Fragile ester bonds, which are easy to break
up, are used as nonconjugated spacers. The PMFT is easily
degraded under basic conditions at a fast rate. In a physio-
logical environment (pH = 7), PMFT hydrolyzes at a slow
rate. The slow degradation pattern with the enhancement of
fluorescence intensity of PMFT upon degradation shows a
potential in vivo application in biological and biomedical
fields, especially in triggering drug controlled release.
Experimental Section. Instruments. 1H NMR and 13
C
NMR spectra were recorded on a Bruker AV400 instrument.
*Corresponding authors. E-mail: yangqiong@iccas.ac.cn (Q.Y.);
wangshu@iccas.ac.cn (S.W.).
pubs.acs.org/Macromolecules
Published on Web 11/23/2010
r 2010 American Chemical Society

Communication
Macromolecules, Vol. 43, No. 24, 2010 10197
Figure 1. Normalized absorption (black dashed line) and fluorescence spectra (red line) of PMFT (a) and MF (b) in water. [PMFT] = 1.0 ꢀ10-5
M
in RUs; excitation wavelength is 333 nm.
Scheme 1. Synthetic Routes for Water-Soluble PMFT and MF
2.07-2.03 (m, 4H), 1.74 (s, 2H), 1.64 (dd, J = 13.9 Hz, 6.9,
4H), 1.20 (dd, J = 14.1 Hz, 7.1 Hz, 4H), 1.10-1.07 (m,
2H), 0.73 (s, 2H). 13C NMR (100 MHz, CD3OD) δ: 151.33,
140.40, 140.23, 140.12, 139.87, 136.00, 127.28, 126.76,
125.90, 120.92, 120.10, 74.50, 63.65, 33.11, 32.39, 28.61,
27.34, 23.72, 23.43, 19.71. MS (MALDI-TOF): m/z = 704.5
(M þ 2H). Anal. Calcd for C39H44Br2O2: C, 66.48; H, 6.29.
Found: C, 66.27; H, 6.45.
Synthesis of Poly(2,7-di(40-phenylmethanol )-9,9-bis(60-bro-
mohexanyl))fluorenylene Terephthalate (PFT ) (4). To a solu-
tion of compound 3 (420 mg, 0.6 mmol) in 2 mL of dry THF
was added terephthaloyl chloride (150 mg, 0.74 mmol), and
the mixture was stirred vigorously for 15 min. Then 0.4 mL of
dry pyridine was injected into the reaction solution. The
resulting solution was stirred at room temperature for 24 h.
The solution was poured into 20 mL of H2O, and the
precipitate was collected by a centrifuge. The crude product
was dissolved in 5 mL of CHCl3 and precipitated in 25 mL
of methanol and then collected by a centrifuge. The pre-
cipitation procedure was repeated three times to yield pale
white powder (200 mg, 40%). 1H NMR (400 MHz, CDCl3)
δ: 8.24 (br, 2H), 8.17 (br, 2H), 7.79 (br, 2H), 7.70 (br, 4H),
7.65-7.47 (m, 8H), 5.45 (br, 4H), 3.25 (br, 4H), 2.04 (br,
4H), 1.63 (br, 6H), 1.19 (br, 4H), 1.08 (br, 4H), 0.71 (br, 4H).
Mn = 4700, Mw=11 800, and PDI = 2.50 based on GPC
analysis.

10198 Macromolecules, Vol. 43, No. 24, 2010
Communication
Synthesis of Poly(2,7-di(40-phenylmethanol )-9,9-bis(60-(300-
methylimidazolium)hexanyl )fluorenylene Terephthalate)
(PMFT ). A solution of PFT (150 mg, 0.032 mmol) and
1-methylimidazole (2 mL, 25 mmol) in 5 mL of CH3CN and
2 mLof THFwas stirredat 65°C for 24h. Uponcooling down
to room temperature, the solvent and 1-methylimidazole were
removed under vacuum. The residue was dissolved in 3 mL
of methanol and precipitated in 20 mL of ethyl acetate to get
a crude product. The precipitation procedure was repeated
twice to obtain a yellow solid (36 mg, 20%). 1H NMR (400
MHz, DMSO-d6) δ: 8.96 (br, 2H), 8.17 (br, 4H), 8.08 (br, 2H),
7.92 (br, 2H), 7.79 (br, 6H), 7.70 (br, 2H), 7.60 (br, 8H), 5.47
(br, 4H), 3.97 (br, 2H), 3.75 (br, 6H), 2.11 (br, 4H), 1.53 (br,
6H), 1.05 (br, 4H), 0.96 (br, 4H), 0.56 (br, 4H).
Synthesis of 2,7-Di(40-phenylmethanol)-9,9-bis(60-(300-methyl-
imidazolium)hexanyl )fluorene (MF). A solution of compound 3
(50 mg, 0.07 mmol) and 1-methylimidazole (1 mL, 12.5 mmol)
in 10 mL of CH3CN was stirred at 70 °C for 48 h. Upon cooling
down to room temperature, the solvent and 1-methylimidazole
were removed under vacuum. The residue was washed by ethyl
acetate for three times to afford a yellow solid (42 mg, 80%). 1H
NMR (400 MHz, DMSO-d6) δ: 8.94 (s, 2H), 7.90 (d, J = 7.7,
2H), 7.79-7.64(m, 8H), 7.59 (s, 4H), 7.43(d, J=7.5, 4H), 5.27
(t, J= 5.2, 2H), 4.57 (d, J =4.9, 4H), 3.97(t, J = 6.4, 4H), 3.75
(br, 6H), 2.11 (br, 4H), 1.52 (br, 4H), 1.05 (d, J = 5.4, 4H), 0.95
(br, 4H), 0.55 (br, 4H). MS (ESI): m/z = 354.1 (M-2Br).
PMFT Degradation Experiments. (1) NMR experiment:
The PMFT (2 mg) was filled into one NMR tube followed by
injection of 550 μL of DMSO-d6 solvent, and the 1H NMR
spectrum was recorded. Then 50 μL of 40 wt % NaOD
solution was added to the PMFT solution. After the solution
1
was incubated at room temperature for 24 h, the H NMR
spectra were recorded again. (2) Fluorescence experiment:
To determine change in emission intensity of PMFT upon
adding NaOH solution, the emission intensity at 384 nm was
recorded as a function of recording time. Six buffer solutions
with various pH (10.00, 11.00, 11.30, 11.70, 12.00, 13.00)
were made by 100 mM Na2HPO4 and 100 mM NaOH and
indicated by pH indicator. To 1980 μL of the buffer solution
in the cuvette was quickly added 20 μL of PMFT solution
in water (1 mM) under vigorous stirring, and the spectra
measurement began simultaneously at room temperature up
to 1800 s with the fixed excitation wavelength of 333 nm. The
procedure was repeated three times. A degradation experi-
ment at physiological pH (pH=7) was conducted. 10 μL of
the PMFT stock solution was injected into 990 μL of double
distilled water in the cuvette (pH=7) followed by incubation
at 37 °C. The cuvette cap was sealed tightly to prevent water
evaporation. The fluorescence spectrum was recorded every
24 h, until the emission intensity reached a plateau.
Figure 2. 1H NMR spectra of monomer MF and PMFT before and
after treatment with NaOD in DMSO-d6.
Figure 3. (a) Emission intensity ofPMFT versus incubation time in buffer solutionswith various pHvalues. (b) Relative emission intensity ofPMFTat
384 nm as a function ofpH values. Inset: fluorescence photos of PMFT under UV light after treatment using buffer solutions with various pH values for
10 min. (c) Relative emission intensity of PMFT at 384 nm as a function of incubation time in phosphate buffer at 37 °C (100 mM, pH = 7). [PMFT] =
1.0 ꢀ 10-5 M in RUs. The excitation wavelength is 333 nm.

Communication
Macromolecules, Vol. 43, No. 24, 2010 10199
Figure 4. (a) Photostability of PMFT under the irradiation of 365 nm UV light. (b) Cell viability as a function of PMFT concentration. In this
experiment, A498 cells were treated with PMFT for 24 h.
Photostability. 20 μL of PMFT stock solution was added
into 1980 μL of double distilled water in a quartz cuvette. The
cuvette was mixed sufficiently, and the fluorescence spec-
trum was recorded. Then the cuvette was placed in a UV
viewing cabinet. The distance between the UV light and the
cuvette was 11.5 cm and the 365 nm light as irradiation
source. The total irradiation time was 5 min, and every 30 s
the cuvette was taken out to record a fluorescence spectrum.
Cell Viability Assay by MTT. A498 cells were seeded in a
flat bottom 96-well tissue culture plate with Dulbecco’s
modified Eagles medium (DMEM, high glucose, without
phenol red) with 10% fetal bovine serum at 37 °C. The plate
was incubated under 5% CO2 humidified atmosphere over-
night. Cells were incubated with PMFT with different concen-
trations for 24 h at 37 °C. Then the cell cultures were discarded,
and MTT solution (0.5 mg/mL in DMEM, 100 μL/well)
was added into the wells followed by incubation at 37 °C
for another 4 h. The supernatant was removed, and 100 μL of
DMSO was added into each well and the plate was shaken for
10 min. The absorbance of formazan at 490 nm was recorded
on a microplate reader for calculating the cytotoxicity.28
Results and Discussion. The synthetic routes for water-
soluble polymer PMFT and monomer MF are outlined in
Scheme 1. Monomer 3 is prepared by condensation of com-
pound 1 with compound 2 via a Suzuki coupling reaction in
the presence of catalyst PdCl2(dppf) in 52% yield. Coupling
reaction of compound 3 with terephthaloyl chloride in dry
THF and dry pyridine at room temperature for 24 h gives
precursor polymer 4 in 40% yield. The polymer 4 has weight-
average molecular weight (Mw) of 11 800 with the polydis-
persity index (PDI = Mw/Mn) of 2.50. The polymer 4 is then
reacted with 1-methylimidazole in CH3CN at 70 °C to afford
water-soluble PMFT in 20% yield. Reaction of compound 3
with 1-methylimidazole in CH3CN affords water-soluble
monomer MF in 80% yield. The PMFT is readily dissolved
in water containing 5% DMSO (v/v) at room temperature
(e.g., 1 mg/mL). The PMFT stock solution will be diluted at
least 10 times in cell experiments; thus, the 0.5% DMSO in
the solution has no effect on cells in vitro.
for PMFT. These phenomena are in agreement with inter-
molecular aggregation of conjugated polymers. PMFT is
composed of hydrophobic backbones and hydrophilic side
chains. The amphiphilic characteristic shows its tendency
to form intermolecular aggregations in aqueous solutions.
The higher fluorescence quantum efficiency is obtained for
monomer MF (76%) rather thanpolymer PMFT(2%), which
results from the fact that the intermolecular π-π stacking
leads to fluorescence self-quenching of the polymer.29,30
Structurally, the backbone of PMFT is connected by ester
bonds between fluorescent monomer diols and terephthal-
ates. The ester bonds are easily hydrolyzed upon addition of
strong bases such as NaOH. Upon treatment with NaOH,
the monomer MF and terephthalic acid would release from
the polymer. To verify the degradable property of PMFT, the
1H NMR spectra of PMFT before and after treatment
with NaOH were measured. That of monomer MF was
also recorded for comparison. As illustrated in Figure 2,
the proton chemical shift of -CH2- in the benzyl alcohol
group locates at 4.56 ppm in monomer MF. For the polymer
PMFT, the chemical shift of corresponding proton shifts to
5.47 ppm. Upon addition of NaOD to PMFT solution in the
NMR tube, the proton chemical shift of -CH2- in benzyl
group neighboring to ester bond in the backbone returns to
4.56 ppm that corresponds to benzyl alcohol group in MF,
indicating the ester bond break in the polymer main chain.
Another evidence on PMFT degradation was given by
fluorescence spectra. Since fluorescence quantum efficiency
of monomer MF (76%) is much higher than that of polymer
PMFT (2%), the release of MF from the polymer main
chain by PMFT degradation will lead to the enhancement
of emission intensity of PMFT solution. To demonstrate this
intensity change, time scan mode emission spectra of PMFT
were recorded in buffer solutions with various pH values
(from 10 to 13). As depicted in Figure 3, the fluorescence
enhancement was observed simultaneously upon addition of
PMFT solution to NaOH solution. The enhancement rate is
in positive correlation with pH value of the buffer; that is,
high-pH environment causes PMFT degradation in a rather
quick manner. And as time elapses, the degradation process
went completely. To offer the direct vision of PMFT degra-
dation, fluorescence photos of PMFT were taken under UV
light after treatment using buffer solutions with various pH
values (12 and 13) for 10 min. As seen in the inset of
Figure 3b, the PMFT solution exhibits bright blue-green
emission color in high-pH solution after degradation. These
fluorescence results confirm the NMR experiments. To test
the ability of PMFT for in vivo use, its degradation behavior
at physiological condition (pH = 7, temperature=37 °C) was
investigated. As shown in Figure 3c, the PMFT degraded in
phosphate buffer (100 mM, pH = 7), but at a rather slow rate
PMFT is nonconjugated and is composed of fluorescent
emitting monomer; thus, the optical properties of both
PMFT and monomer MF are measured (Figure 1). The
absorption maximum of PMFT is exhibited at 333 nm, which
is similar to that of monomer MF. The similar extinction
coefficients are also obtained for PMFT (2.51 ꢀ 104 M-1
cm-1) and monomer MF (3.81 ꢀ 104 M-1 cm-1). These
results show that the maximum absorption of PMFT is
corresponding to the π-π* transition of the conjugated
chromophore. As shown in Figure 1, PMFT and MF exhibit
the same emission maxima at 384 nm in aqueous solution.
It is noted that a new broad peak from 450 to 550 nm appears

10200 Macromolecules, Vol. 43, No. 24, 2010
Communication
than when under strong base conditions. The hydrolysis
proceeded in the time range from 0 to 19 days. The slow
degradation pattern with enhancement of fluorescence inten-
sity of PMFT shows a potential in vivo application in biolog-
ical field, especially in drug controlled release.
Acknowledgment. The authors are grateful for the National
Natural Science Foundation of China (Nos. 20725308, 21033010,
and 90913014).
References and Notes
The photostability of PMFT was studied under UV irra-
diation with 365 nm as light source. As depicted in Figure 4a,
although the fluorescence intensity of PMFT at 384 nm
decreased with the irradiation, the fluorescence remained
40% upon continuously irradiating for 4 min. Thus, the fairly
good photostability of PMFT makes it possible for fluores-
cence imaging use. Because of the importance for in vivo use,
the biocompatibility of PMFT was studied by a typical MTT
assay method.28 As shown in Figure 4b, the PMFT exhibits
little cytotoxicity as the concentration increases from 12.5 to
50 μM. Even at high PMFT concentration (100 μM), the cell
viability still remains about 60%. The degradation products
of PMFT are the monomer MF and terephthalic acid, where
terephthalic acid shows little cytotoxicity; thus, the cytotox-
icity of MF was also studied. Even at high MF concentra-
tion (50 μM), the cell viability still remains above 60%, so we
believe that both polymer and degradable products have little
cytotocixity in a certain range of concentration. In compar-
ison with the cytotoxicity of quantum dots (QDs)25 and
photobleaching properties of organic dyes, the PMFT has
both acceptable cytotoxicity and photostability.
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Conclusions. A new water-soluble degradable polyester
consisting of fluorescent 2,7-di(40-phenylmethanol)-9,9-bis-
(60-(300-methylimidazolium)hexanyl)fluorene chromophore
and degradable ester spacer in backbone (PMFT) has been
designed and successfully synthesized via facile operations.
The fluorescence of the PMFT is from a fluorescent phenyl-
fluorenyl moiety, and its water solubility is obtained by
introducing charged groups in the side chain of the polymer.
The PMFT is easily degraded in basic conditions, which was
1
confirmed by H NMR spectra and fluorescence measure-
ment. Degradation of PMFT was verified by 1H NMR
spectroscopy through following the proton chemical shift
of the benzyl -CH2- group adjacent to the ester moiety in
PMFT (5.47 ppm) to 4.56 ppm for the methylene proton
resonance in the MF degration product. Since fluorescence
quantum efficiency of monomer MF (76%) is much higher
than that of polymer PMFT (2%), the release of MF from
the polymer main chain by PMFT degradation leads to
the enhancement of emission intensity of PMFT solution. In
physiological environment (pH = 7), PMFT hydrolyzes at
a rather slow rate than when under strong basic conditions
(10 min). Because of its fluorescent and degradable properties,
PMFT may have various in vivo applications in biological and
biomedical fields such as drug delivery and cell imaging.
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