W. Gao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 218–223
219
chromophore can transfer energy to a proximal acceptor chromo-
phore (typically <10 nm) through long-range nonradiative
Shimadzu TU-1901 spectrophotometer and
a
Shimadzu
a
RF-5301PC spectrophotometer, respectively. Electrical conductiv-
ity was determined by dual display potentiostat. The lifetime of
P-2 was obtained with FLS920 combined fluorescence lifetime
and steady state spectrometer. Thermal gravimetric analysis
(TGA) was performed with Simultaneous Thermal Analysis-STA
409EP under N2 atmosphere. All optical measurements were per-
formed at room temperature unless otherwise stated.
dipole–dipole coupling. It has been extensively and intensively
studied as a powerful analytical technique to interrogate changes
in molecular conformation, association and the assembly or disas-
sembly of biomolecular machinery [1–5] and therefore provide po-
tential can fulfill requirements of chemical and biosensors sensing.
However, implementation of fluorescent probes in functional de-
vices maintaining the sensitivity is still very challenging. Thus,
extensive efforts are currently being done in the search of im-
proved materials for fluorescent sensing [6].
Conjugated polymers (CPs) contain a large number of absorbing
units, and the transfer of excitation energy along the whole back-
bone of the CP to the chromophore reporter results in the amplifi-
cation of fluorescence signals [7–14]. Fluorescent CPs have been
employed in different sensing applications, and particularly as sen-
sitive probes for the detection of biomolecules [15–18]. Typically,
CPs chemosensors interacted electrostatically with other charged
species, and displayed an extraordinarily high sensitivity to fluores-
cence quenchers [19]. However, most reported CPs exhibited rather
poor solubility in aqueous media, probably due to their low charge
Synthesis
Monomer synthesis
Monomers preparation of
2,7-dibromo-9,9-bis(30-(N,N-dimethyl-
amino)propyl)-fluorene (1) (Scheme 1). To a stirred mixture of 2,7-
dibromofluorene (4 g, 12 mmol) and 60 mL of dimethyl sulfoxide
(DMSO) under nitrogen were added tetrabutyl ammonium bro-
mide (80 mg) and 8 mL of a 50 wt.% aqueous solution of sodium
hydroxide. 20 mL DMSO solution of 3-dimethylaminopropyl-
chloride hydrochloride (5 g, 32 mmol) was added dropwise to the
mixture. The reaction mixture was stirred at room temperature
for 6 h and then diluted with 50 mL of water, to dissolve all salts.
The product was extracted with ether (3 ꢀ 100 mL) and the com-
bined organic layer was washed with 10% NaOH (aq)
(2 ꢀ 100 mL), water (3 ꢀ 100 mL) and brine (1 ꢀ 100 mL). The
solution was dried over MgSO4, filtered, and stripped of solvent
by vacuum evaporation to yield a crude solid. The crude solid
was recrystallized from MeOH/H2O to afford 1 [25]. (2.93 g,
48.2% yield) as white crystals. IR (cmꢁ1): 653 (C–Br), 1104 (C–N),
1607 (fluorene ring C@C), 2820 (methyl C–H), 3100 (fluorene ring
C–H). 1H NMR (300 MHz, DMSO-d6): ä 7.82–7.80 (d, 2H, fluorene
ring), 7.69 (s, 2H, fluorene ring), 7.56–7.53 (d, 2H, fluorene ring),
2.04–2.00 (t, 4H, –CH2N), 1.92–1.88 (m, 16H, –NCH3, –CH2–),
0.60–0.52 (m, 4H, –CH2–). Elemental Anal. Calcd. for C23H30Br2N2:
C, 55.89; H, 6.12; N, 5.67. Found: C, 55.76; H, 6.12; N, 5.60.
densities which compete with the aromatic
p–p stacking of the
hydrophobic backbones. Herein we have synthesized a novel
water-soluble fluorescent CP poly[(9,9-bis(30-((N,N-dimethyl-
amino)N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-p-div
inylbenzene)]dibromide (P-2). P-2 showed good structure flexile.
Because of the flexibility of the molecular chain, P-2 has a good pro-
cessability. In addition, P-2 (also other conjugated polyelectrolytes)
features charged side groups and subsequently good water solubil-
ity, which together rendered its two obvious advantages: (1) more
suitable for biosensory systems in aqueous solution; (2) further en-
hanced sensitivity to oppositely charged quenchers. Moreover its
strong fluorescence will be beneficial to the application of P-2.
Protein is not only the final executant of life functions but also
the key to understand the physiology, pathology and pharmacol-
ogy in a biological system and is also very important in disease
diagnostics. In terms of the determination of proteins, reported
methods include polarographic [20], FT-IR spectroscopy [21],
quartz crystal microgravimetry sensor [22], fluorimetric determi-
nation [23] and flow injection chemiluminescence (FI-CL) [24].
Though some of the methods could be very accurate, they need
expensive equipments and complicated sample pretreatment.
Some methods have limited practical use due to low selectivity.
In this study, P-2 has been successfully applied to the determina-
tion of BSA, indicating its potential application in clinical studies.
Monomers preparation of p-divinylbenzene (4) (Scheme 2). To the
solution of 2.0 mL (21.82 mmol) 1,4-dimethylbenzene in CCl4
(60 mL), 5.8 g (26 mmol) of NBS and 0.10 g (1.03 mmol) of benzoyl
mixture peroxide were added, then stirred at 80 °C for 6 h. A crude
product was obtained after heat filter and washed with absolute
ethyl alcohol. Compound 1 was obtained as a white acicular crys-
talline solid (4.94 g, 85.8% yield) from the crude product by recrys-
tallization with absolute ethyl alcohol [26].
A mixture of 2 (4.94 g, 18.5 mmol) and triphenylphosphine
(9.0 g, 34.32 mmol) in 60 mL DMF was refluxed at 160 °C for 10 h
in a nitrogen atmosphere. The precipitate was filtered and washed
with ether [27], and then transferred into a 100 mL three-necked
flask for compound 4 synthesis.
Experimental procedures
A mixture of CH2Cl2 (30 mL) and 40% HCHO aqueous solution
(12 mL) was added into the previous flask. The solution was cooled
down to ꢁ15 °C and stirred vigorously. Ten percent of aqueous
NaOH (20 mL) was added dropwise over 1 h with constant stirring
under N2. The mixture was stirred at room temperature overnight,
and then 100 mL water was added to the solution. The solution
was extracted three times with CH2Cl2 (20 mL). The combined or-
ganic layers were washed with saturated brine twice and dried
over anhydrous MgSO4. The solvent was removed to dryness under
reduced pressure [28]. The pure product was obtained as white
acicular crystalline solid in 58.9% yield (1.42 g) by the recrystalliza-
tion of 50% ethanol twice. IR (cmꢁ1): 833 (benzene 1,4 substituted),
1607 (aromatic ring C@C), 1620 (alkene C@C), 3009 (alkene C–H)
1H NMR (300 MHz, CDCl3): d 5.43 (d, 2H, J = 10.9 Hz), 5.92 (d, 2H,
J = 17.6 Hz), 6.80 (dd, 2H, J = 17.6, 10.9 Hz), 7.59 (d, 4H,
J = 8.3 Hz), 8.12 (d, 4H, J = 8.3 Hz); Elemental Anal. Calcd. for
C8H10: C, 90.57; H, 9.43. Found: C, 90.42; H, 9.52.
Materials
All manipulations involving air-sensitive reagents were per-
formed under an atmosphere of dry nitrogen. All reagents, unless
otherwise specified, were obtained from Alfa Aesar, Sinopharm
Chemical Reagent Co. Ltd, and Tian Jin Da Mao Chemical Reagent
Factory (Tian Jin, China) and used as received. All the solvents used
were further purified before use.
Instrument
Nuclear magnetic resonance (NMR) spectra were recorded on a
INOVA 300 MHz spectrometer with tetramethylsilane as the inter-
nal standard. The elemental analysis was performed on a PERKIN
ELMER 2400 II elemental analyzer. UV–vis absorption and photolu-
minescence (PL) emission spectra were measured using
a