Recognition of dihydrogen phosphate ions using the cadmium complex of 2-pyridine-1H-imidazo[4,5-b]phenazine: Utilization of the mechanism of twisted intramolecular charge transfer, long wavelength emission

Article abstract of DOI:10.1039/c3nj00578j

A long wavelength emission fluorescent and colorimetric chemosensor with high selectivity for H₂PO₄^- ions was designed and synthesized according to the twisted intramolecular charge transfer (TICT) mechanism. The sensor bears a 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd ²⁺ metal complex, which showed brilliant fluorescent and colorimetric response for H₂PO₄^- anions in aqueous solution. The detection limit of the sensor towards H₂PO ₄^- is 2.8 × 10^-6 M, and other anions, including F^-, Cl^-, Br^-, I^-, AcO ^-, HSO₄^-, ClO₄^- and CN^-, had nearly no influence on the probing behavior. The test strips based on the 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd²⁺ metal complex (S₂-Cd) were fabricated, which could act as convenient and efficient H₂PO₄^- test kits.

Full text of DOI:10.1039/c3nj00578j

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Recognition of dihydrogen phosphate ions using the
cadmium complex of 2-pyridine-1H-imidazo[4,5-b]-
phenazine: utilization of the mechanism of twisted
intramolecular charge transfer, long wavelength
emission†
Cite this: NewJ.Chem., 2013,
37, 3737
BingBing Shi, YouMing Zhang,* TaiBao Wei, Peng Zhang, Qi Lin and Hong Yao
À
A long wavelength emission fluorescent and colorimetric chemosensor with high selectivity for H2PO4
ions was designed and synthesized according to the twisted intramolecular charge transfer (TICT)
mechanism. The sensor bears a 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd2+ metal complex, which
Received (in Montpellier, France)
31st May 2013,
À
showed brilliant fluorescent and colorimetric response for H2PO4 anions in aqueous solution. The
detection limit of the sensor towards H2PO4 is 2.8 Â 10À6 M, and other anions, including FÀ, ClÀ, BrÀ,
À
Accepted 19th August 2013
IÀ, AcOÀ, HSO4À, ClO4 and CNÀ, had nearly no influence on the probing behavior. The test strips based
À
DOI: 10.1039/c3nj00578j
on the 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd2+ metal complex (S2–Cd) were fabricated, which
À
www.rsc.org/njc
could act as convenient and efficient H2PO4 test kits.
the frequently used fluorescence signaling mechanisms of
photoinduced electron transfer (PET) and intramolecular
1. Introduction
Among various anions, phosphate ions are of particular interest
as they are key substrates for many biochemical reactions and
are main components of biomolecules.1 Recently, numerous
metal-based receptors, particularly Zn(II) complexes, have been
reported for fluorescent sensing of phosphate ions.2–5 However,
due to fluorescence quenching, the dependence on organic
solvents or, more specifically, the interference from other
anions, there are still major challenges to be overcome in
developing highly selective chemosensors for dihydrogen phos-
phate, and as required by biologists and medical experts, long
wavelength emission fluorescent chemosensors are welcome
since the signal could easily penetrate the body tissue to
facilitate detection and reduce damage.6–9 In contrast to the
zinc-based complexes for the recognition of the dihydrogen
phosphate, cadmium-based receptors have seldom been reported.
On Àthe other hand, various fluorescent chemosensors for
H2PO4 have been described recently owing to their high
selectivity and low background disturbance. In addition to
charge transfer (ICT), the concept of twisted intramolecular
charge transfer (TICT),10 proposed by Lippert and co-workers,
has seldom been utilized, possibly due to the difficulty in
controlling two crucial factors, the degree of the electron
transfer and the change in molecular geometry. Correspond-
ingly, the TICT-based fluorescent chemosensors are still very
scarce, although in principle, the well-designed ones should
show very good performance.
Furthermore, phenazine derivatives have been synthesized
and have been used in organic electronics for a long time,11–14
but they have seldom been used in host–guest chemistry, and
there is no report on sensing ions based on its metal complex.
Phenazines are ideal platforms for the development of cation,
anion, and neutral molecule recognition. Moreover, among the
different fluorogenic units, phenazine is very sensitive to con-
formational change.
Our research group has a longstanding interest in molecular
recognition.15–18 Herein, we have elaborately designed a long
wavelength emission, two channel (colorimetric and fluores-
cent) dihydrogen phosphate chemosensor (S2–Cd) according to
the twisted intramolecular charge transfer (TICT) mechanism
based on the 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd2+
complex (S2, Scheme 1), in which the phenazine group acts as a
fluorophore, imidazole and pyridine groups are in the same
sensor molecule, to enhance the coordination capacity required
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education of China, Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: zhangnwnu@126.com;
Tel: +086 9317973191
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj00578j
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2.3. General procedure for UV-vis experiments
UV-vis spectroscopy was carried out after the addition of tetra-
butylammonium salts in DMSO, while keeping the ligand
concentration constant (2.65 Â 10À5 M), on a Shimadzu
UV-2550 spectrometer. The solutions of the anions were prepared
from the tetrabutylammonium salts of H2PO4À, FÀ, ClÀ, BrÀ, IÀ,
AcOÀ, HSO4À, ClO4À, CNÀ, and the Cd2+ ion solution was
prepared from its perchlorate salt.
Scheme 1 Synthesis of compound S2.
2.4. General procedure for fluorescence spectra experiments
to coordinate a cadmium ion. The S2–Cd sensor showed both
colorimetric and fluorescent selectivity for H2PO4 in DMSO–
Fluorescence spectroscopy was carried out after the addition of
tetrabutylammonium salts in DMSO, while keeping the ligand
concentration constant (2.65 Â 10À5 M), on a Shimadzu
RF-5301 fluorescence spectrometer. The excitation wavelength
was 400 nm. The solutions of anions were prepared from the
tetrabutylammonium salts of H2PO4À, FÀ, ClÀ, BrÀ, IÀ, AcOÀ,
HSO4À, ClO4À, CNÀ, and the Cd2+ ion solution was prepared
from its perchlorate salt.
À
H2O (8 : 2, v/v) HEPES buffer solutions over other common
physiologically important anions. To the best of our knowledge,
this is the first time that a long wavelength emission fluores-
cent cÀadmium-based chemosensor with high selectivity for
H2PO4 was designed.
1
2. Experimental section
2.5. General procedure for H NMR experiments
1
2.1. Materials and physical methods
The H NMR titration experiment was investigated by addition
of increasing concentrations of Cd(ClO4)2 in DMSO-d6 to the
solution of S2 (DMSO-d6) in an NMR tube. Then further experi-
ment was carried out by addition of increasing concentrations
of TBAH2PO4 in DMSO-d6 solution. The spectra were recorded
after mixing and the temperature of the NMR probe was kept
constant at 298 K.
Fresh double distilled water was used throughout the experi-
ment. All other reagents and solvents were commercially avail-
able at analytical grade and were used without further
1
purification. H NMR and 13C NMR spectra were recorded on
a Mercury-400BB spectrometer at 400 MHz. Chemical shifts
are reported in ppm downfield from tetramethylsilane (TMS,
d scale with solvent resonances as internal standards) and
UV–vis spectra were recorded on a Shimadzu UV-2550 spectro-
meter. Photoluminescence spectra were recorded on a Shimadzu
RF-5301 fluorescence spectrophotometer. Melting points were
measured on an X-4 digital melting-point apparatus (uncor-
rected). Infrared spectra were recorded on a Digilab FTS-3000
FT-IR spectrophotometer.
3. Results and discussion
To evaluate the binding ability of compound S2 towards Cd2+
ions, we carried out UV-vis and fluorescence experiments in
DMSO–H2O (8 : 2, v/v) buffered with HEPES (pH 7.24, 10 mM) by
adding aliquots of Cd2+ metal ions as its perchlorate salt. The
absorption spectrum of compound S2 (26.5 mM) in DMSO–H2O
(8 : 2, v/v) HEPES buffer (pH 7.24) solution exhibits two absorp-
2.2. Synthesis of sensor S2
2,3-Diamino-phenazine was prepared following the reported tion bands at 400 and 264 nm corresponding to phenazine and
procedure.12,13 pyridine, respectively (ESI,† Fig. S1). Upon addition of Cd2+ ions
Synthesis of compound S2. 2,3-Diamino-phenazine (0.42 g, (0–27.64 equiv.), the band at 400 nm is red-shifted to 416 nm
2 mmol), 2-pyridylaldehyde (0.28 g, 2.5 mmol) and a catalytic and the band at 264 nm is red-shifted to 275 nm. Seven
amount of acetic acid (AcOH) were combined in hot absolute isosbestic points are observed at 268, 311, 328, 351, 405, 432
DMF (20 mL). The solution was stirred under reflux conditions and 475 nm, indicating formation of the S2–Cd complex (Fig. 1).
for 8 hours; after cooling to room temperature, the brown In the fluorescence spectrum, the emission of S2 appeared at
precipitate was filtrated, washed with hot absolute ethanol the maximum emission wavelength was 540 nm in DMSO–H2O
three times, then recrystallized with DMF–H2O to get a brown (8 : 2, v/v) HEPES buffer (pH 7.24) solution when excited at
powdery product S2 (0.9 mmol) in 45% yield (m.p. > 300 1C), lex = 416 nm (ESI,† Fig. S2). This is due to twisted intra-
1H NMR (DMSO-d6, 400 MHz) d 13.57 (s 1H, NH), d 8.88 (d 1H, molecular charge transfer (TICT) from pyridine to the phena-
ArH), 8.59–8.28 (m 2H, ArH) 8.23–8.13 (m 4H, ArH) 7.91–7.88 zine moiety. Upon addition of increasing amounts of Cd2+ ions
(m 2H, ArH) 7.67 (m 1H, ArH). 13C-NMR (DMSO-d6, 150 MHz) (0–23.24 equiv.) to the solution of S2 in DMSO–H2O (8 : 2, v/v)
d 158.76, 149.93, 148.84, 147.20, 142.10, 141.78, 140.28, 140.13, HEPES buffer (pH 7.24), the fluorescence emission band at
139.71, 137.98, 130.12, 129.75, 129.10, 128.97, 126.49, 123.18, 540 nm is red-shifted to 600 nm, directly leading to a strong red
115.70, 106.88. IR (KBr, cmÀ1) v: 3137.21 (NH), 1658.68(CQN). emission (Fig. 2). This fluorescence emission band shift is
ESI-MS m/z: (M + H)+ Calcd for C18H11N5H 298.3; found 298.3. attributed to the formation of the S2–Cd complex due to
Anal. calcd for C18H11N5: C, 72.73; H, 3.70; N, 23.57%; found C interaction between Cd2+ ions and imidazole nitrogen and
72.81, H 3.65, N 23.61%.
the nitrogen atom of the pyridine moiety as a result of which
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Fig. 1 (a) Absorbance spectra of S2 (26.5 mM) at different concentrations of Cd2+ (0–27.64 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2). (b) A plot of absorbance
depending on the concentration of Cd2+ in the range from 0 to 27.64 equivalents. The detection wavelength was 416 nm.
Fig. 2 (a) Fluorescence spectra of S2 (26.5 mM) at different concentrations of Cd2+ in DMSO–H2O (pH = 7.24, v/v = 8 : 2). (b) A plot of fluorescence intensity depending
on the concentration of Cd2+ in the range from 0 to 23.14 equivalents. The detection wavelength was 600 nm.
the TICT from the pyridine to the phenazine moiety is sup- spectroscopy and fluorescence spectroscopy in DMSO–H2O
pressed resulting in a fluorescence emission band shift. A Job (8 : 2, v/v) HEPES buffer (pH 7.24). When 40 equivalents of
plot for the binding between S2 and Cd2+ shows a 2 : 1 stoichio- H2PO4À were added to the DMSO–H2O (8 : 2, v/v) HEPES buffer
metry (Fig. 3), and the association constant of the S2–Cd (pH 7.24) solutions of the sensor S2–Cd (15 equiv.), S2–Cd
complex was determined to be 5.5 Â 106 MÀ2 (ESI,† Fig. S3). responded with a dramatic color change, from pink to yellow.
The 2 : 1 (S2 : Cd2+) stoichiometry was further confirmed by The apparent color change from pink to yellow could be
the ESI-MS spectrum of the S2–Cd complex where a molecular distinguished by the naked eye. The absorption spectrum of
ion peak at 705.3 was observed, which corresponds to the the sensor S2–Cd (15 equiv.) in DMSO–H2O (8 : 2, v/v) HEPES
[S2–Cd–H]+ complex (ESI,† Fig. S6).
buffer (pH 7.24) solution exhibits two absorption bands at 416
The recognition profiles of the chemosensor S2–Cd and 275 nm. When 40 equivalents of H2PO4À were added to the
(15 equiv.) towards various anions, H2PO4À, FÀ, ClÀ, BrÀ, IÀ, DMSO–H2O (8 : 2, v/v) HEPES buffer (pH 7.24) solution of the
AcOÀ, HSO4À, ClO4À, CNÀ, were primarily investigated by UV-vis sensor S2–Cd (15 equiv.), the band at 416 nm is blue-shifted to
400 nm and the band at 275 nm is blue-shifted to 264 nm
(Fig. 4). To validate the selectivity of the sensor S2–Cd (15 equivÀ.),
the same tests were applied using FÀ, ClÀ, BrÀ, IÀ, AcOÀ, HSO4
,
ClO4À, CNÀ anions, and these anions induced little changes in
the UV-vis spectrum of the sensor (Fig. 5). Meanwhile, the
fluorescent measurements for the sensor S2–Cd (15 equiv.) were
performed in DMSO–H2O (8: 2, v/v) HEPES buffer (pH 7.26)
solution. As shown in Fig. 6, in the fluorescence spectrum, the
emission of S2–Cd (15 equiv.) appeared at the maximum emis-
sion wavelength was 600 nm in DMSO–H2O (8: 2, v/v) HEPES
buffer (pH 7.24) solution when excited at lex = 400 nm. When
40 equivalents of H2PO4À were added to the DMSO–H2O (8:2, v/v)
HEPES buffer (pH 7.24) solutions of the sensor S2–Cd (15 equiv.),
the fluorescence emission band at 600 nm is blue-shifted to
540 nm. The apparent color change from red to yellow could be
distinguished by the naked eye. To validate the selectivity of the
Fig. 3 Job’s plot examined between Cd2+ and S2, indicating the 2 : 1 stoichio-
metry for S2–Cd clearly.
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Fig. 4 Absorbance spectra of S2–Cd (15 equiv.) in DMSO–H2O (pH = 7.24,
À
v/v = 8 : 2) in the presence of H2PO4 (40 equiv.). Inset: the photograph from
Fig. 7 Fluorescence emission data for a mixture of S2–Cd and each of the
various anions, as the tetrabutylammonium salts, in the DMSO–H2O (pH = 7.24,
v/v = 8 : 2) solution (excitation wavelength = 400 nm). Inset: visual fluorescence
emissions of S2–Cd (26.5 mM) upon the addition of H2PO4À, FÀ, ClÀ, BrÀ, IÀ, AcOÀ,
HSO4À, ClO4À, CNÀ (40 equiv., respectively) in DMSO–H2O (pH = 7.24, v/v = 8 : 2)
on excitation at 365 nm using a UV lamp at rt.
left to right shows the change in the absorbance of S2–Cd (15 equiv.), S2–Cd
À
(15 equiv.) + H2PO4 (40 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
Fig. 5 Absorbance emission data for a mixture of S2–Cd (15 equiv.) and each
of the various anions as the tetrabutylammonium salts, in the DMSO–H2O
(pH = 7.24, v/v = 8 : 2) solution. Inset: color changes observed for S2–Cd (15 equiv.)
upon the addition of H2PO4À, FÀ, ClÀ, BrÀ, IÀ, AcOÀ, HSO4À, ClO4À, CNÀ (40 equiv.,
respectively) in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
À
Fig. 8 The proposed structures of S2 for Cd2+ and S2–Cd for H2PO4 anions.
with the evidence (ESI,†Fig. S8). Thus, the TICT states of the
rotors caused the yellow emission of S2. Upon the addition of
Cd2+, the interaction between Cd2+ and S2 decreased the twisted
intramolecular charge transfer ability and the formation of the
S2–Cd complex decreased the dihedral angle between the two
units, further decreasing their electron transfer.20,21 Thus, the
binding of Cd2+ prohibited the TICT formation of S2 and
can enhance the coplanarity of the conjugated system22 (ESI,†
Fig. S8), largely promoting the strÀong red fluorescence emission.
And upon the addition of H2PO4 to the S2–Cd system, because
of the interactions between H2PO4À and Cd2+, the S2–Cd complex
was decomposed and the pyridine and phenazine units were
nearly perpendicular via the conformational change. Thus, the
Fig. 6 Fluorescence spectra of S2–Cd (15 equiv.) upon an excitation at 400 nm in
À
binding of H2PO4 prohibited the ICT formation of free S2 and
À
DMSO–H2O (pH = 7.24, v/v = 8 : 2) in the presence of H2PO4 (40 equiv.). Inset:
can decrease the coplanarity of the conjugated system, largely
promoting the yellow fluorescence emission.
the photograph from left to right shows the change in the fluorescence of S2–Cd
À
(15 equiv.), S2–Cd + H2PO4 (40 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
The interaction and binding behavior between S2–Cd
À
1
(15 equiv.) and H2PO4 were investigated with their H NMR,
sensor S2–Cd (15 equiv.), the same tests were also applied using IR spectra and ESI-MS spectrum. As shown in Fig. 9, after
FÀ, ClÀ, BrÀ, IÀ, AcOÀ, HSO4À, ClO4À, CNÀ anions, and these adding 0.1–0.5 equiv. of Cd2+, the NH (Ha) peak at 13.57 ppm
anions induced little changes in the fluorescent spectrum of the disappeared. Meanwhile, the signal of the hydrogen atoms in
sensor (Fig. 7).
the benzene and pyridine rings (8.87 ppm and 8.54 ppm)
As shown in Fig. 8, between the pyridine and phenazine showed a downfield shift, indicating a charge transfer from
units, there was a dihedral angle in S2.11,19 DFT studies agree the phenazine and pyridine groups to the Cd2+ ion.23 However,
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Fig. 9 1H NMR spectra (400 MHz, DMSO-d6) of free S2 and in the presence of Cd2+, H2PO4À, respectively.
as increasing concentrations of TBAH2PO4 were added to the producing the MS signals at m/z 705.3. After TBAH2PO4 was
solution of S2–Cd, it was noted that the signal of the hydrogen added to the solution of S2–Cd, the S2–Cd complex was decom-
atoms in the benzene and pyridine rings (8.76 ppm and 8.46 ppm) posed and inorganic cadmium phosphate was formed.24
showed an upfield shift. Thus, the results of the 1H NMR
In addition, the interaction and binding mechanism were
titration suggested that the conformational change between further confirmed by the UV-vis spectroscopy and Fluorescence
À
the phenazine and pyridine was attained during the ion recogni- spectroscopy. When 40 equivalents of H2PO4 were added to
tion process. To elucidate the binding mode of S2 with cadmium the DMSO–H2O (8 : 2, v/v) HEPES buffer (pH 7.24) solutions of
perchlorate, the IR spectrum of the S2–Cd complex was sensor S2–Cd (15 equiv.), the optical absorbance and fluores-
recorded (ESI,† Fig. S5). The important change was observed cence intensity returned to the levels observed for the free
in the absorption band corresponding to the CQN group in the compound S2 (Fig. 10).
imidazole ring, which shifts from 1658.68 to 1619.83 cmÀ1. And
To further investigate the interaction between S2–Cd
the absorption band of NH at 3137.21 disappeared. In order (15 equiv.) and H2PO4À, UV-vis absorption spectral variation
to explore the interaction and binding mechanism of the of the sensor S2–Cd (15 equiv.) in DMSO–H2O (8 : 2, v/v) HEPES
present system, the products S2 with Cd2+ and S2–Cd with buffer (pH 7.24) was monÀitored during titration with different
H2PO4À were subjected to mass spectral analyses. The ion peaks concentrations of H2PO4 from 0 to 14.59 equivalents. As
were detected at m/z 705.3 (ESI,† Fig. S6), which correspond to shown in Fig. 11, seven isosbestic points are clearly observed
[S2–Cd–H]+. The peak at m/z 298.3 demonstrated the presence of at 268, 311, 328, 351, 405, 432 and 475 nm with increasing
À
[M
+
H]+ (ESI,† Fig. S7). The
M
was indicated S2 as concentrations of H2PO4
.
a final product after TBAH2PO4 was added to the solution of
At the same time, fluorescence emission spectral variation
S2–Cd. Based on the above findings, we propose that the reaction of sensor S2–Cd (15 equiv.) in DMSO–H2O (8 : 2, v/v) HEPES
mechanism in this system may proceed through the route buffer (pH 7.24) was monitored during titration with different
À
depicted in Fig. 8: the presence of Cd2+ leads to the formation concentrations of H2PO4 from 0 to 16.31 equivalents, as
of S2–Cd, which is then converted to a conformational change, shown in Fig. 12. The selectivity of S2–Cd (15 equiv.) for
Fig. 10 (a) Fluorescence spectra of S2, S2–Cd (15 equiv.) and S2–Cd + H2PO4À (40 equiv.) upon excitation at 400 nm in DMSO–H2O (pH = 7.24, v/v = 8 : 2). Inset: the
À
photograph from left to right shows the change in the fluorescence of S2, S2–Cd (15 equiv.), S2–Cd + H2PO4 (40 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
(b) Absorbance spectra of S2, S2–Cd (15 equiv.) and S2–Cd + H2PO4À (40 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2). Inset: the photograph from left to right shows the
À
color change in S2, S2–Cd (15 equiv.), S2–Cd + H2PO4 (40 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
c
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À
Fig. 11 (a) Absorbance spectra of S2–Cd (15 equiv.) at different concentrations of H2PO4 (0-14.59 equiv.) in DMSO–H2O (pH = 7.24, v/v = 8 : 2). (b) A plot of
À
absorbance depending on the concentration of H2PO4 in the range from 0 to 14.59 equivalents. The detection wavelength was 400 nm.
Fig. 12 (a) Fluorescence spectra of S2–Cd (15 equiv.) at different concentrations of H2PO4À in DMSO–H2O (pH = 7.24, v/v = 8 : 2). (b) A plot of fluorescence intensity
À
depending on the concentration of H2PO4 in the range from 0 to 16.31 equivalents. The detection wavelength was 540 nm.
À
H2PO4 over other anions has been examined. Results of our using Ca2+, Cu2+, Pd2+, Zn2+, Cr3+, Mg2+ ions, Results of our
studies have revealed that when 40 equivalents of other anions studies have revealed that when 40 equivalents of other metal
were added to the DMSO–H2O (8 : 2, v/v) HEPES buffer (pH 7.24) ions were added to the DMSO–H2O (8 : 2, v/v) HEPES buffer
solutions of S2–Cd and H2PO4À, all potentially competitive (pH 7.24) solutions of S2–Cd and H2PO4À, these ions exerted no
anions exerted no or little influeÀnce on the absorbance and or little inÀfluence on the absorbance and fluorescence detection
fluorescence detection of H2PO4 in DMSO–H2O (8 : 2, v/v) of H2PO4 in DMSO–H2O (8 : 2, v/v) HEPES buffer (pH 7.24)
HEPES buffer (pH 7.24), as shown in Fig. 13.
except the Cu2+ ions (ESI,† Fig. S9).
À
To further validate the selectivity of S2–Cd (15 equiv.) for
The Job plot between S2–Cd (15 equiv.) and H2PO4
H2PO4À over other metal ions, the same tests were also applied was implemented, demonstrating a 1/2 stoichiometry for the
Fig. 13 (a) Fluorescence spectra of S2–Cd (15 equiv.) in response to H2PO4À (40 equiv.) in the presence of various anion species in DMSO–H2O (pH = 7.24, v/v = 8 : 2).
À
The detection wavelength was 540 nm. (b) Absorbance spectra of S2–Cd (15 equiv.) in response to H2PO4 (40 equiv.) in the presence of various anion species in
DMSO–H2O (pH = 7.24, v/v = 8 : 2). The detection wavelength was 416 nm.
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DMSO solution of S2–Cd (0.1 M). The test strips containing
À
S2–Cd were utilized to sense H2PO4 and other anions. As
À
shown in Fig. 16, when H2PO4 and the other anions were
added on the test kits respectively, the obvious color change
À
was observed only with H2PO4 solution under the 365 nm UV
lamp. And potentially competitive anions exerted no influence
on the detection of H2PO4À by the test strips. Therefore, the test
À
strips could conveniently detect H2PO4 in solutions.
4. Conclusion
In conclusion, we have presented a new long wavelength emis-
sion dihydrogen phosphate chemosensor (S2–Cd) according to
the twisted intramolecular charge transfer (TICT) mechanism
based on the 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd2+
complex. The sensor S2–Cd showed both fluorescent and colori-
À
Fig. 14 Job’s plot examined between H2PO4 and S2–Cd, indicating the 1 : 2
stoichiometry clearly.
À
metric selectivity for H2PO4 in DMSO–H2O (8 : 2, v/v) HEPES
buffer solutions. To the best of our knowledge, this is the first
time that a long wavelength emiÀssion fluorescent chemosensor
with high selectivity for H2PO4 was designed based on the
2-pyridine-1H-imidazo[4,5-b]phenazine and Cd2+ complex. Thus,
it is believed that this receptor will have a role to play in the
sensing, detection, and recognition of H2PO4À ions. In addition,
test strips based on S2–Cd were fabricated, which also exhibit a
À
good selectivity to H2PO4 as in solution. We beÀlieve the test
strips could act as convenient and efficient H2PO4 test kits.
Acknowledgements
À
Fig. 15 Influence of pH on the fluorescence of S2–Cd–H2PO4 in DMSO–H2O
(pH = 7.24, v/v = 8 : 2). Excitation wavelength is 400 nm.
This work was supported by the National Natural Science
Foundation of China (No. 21064006, 21262032 and 21161018),
the Program for Changjiang Scholars and Innovative Research
Team in the University of Ministry of Education of China
(No. IRT1177), the Natural Science Foundation of Gansu Pro-
vince (No. 1010RJZA018), the Youth Foundation of Gansu
Province (No. 2011GS04735) and NWNU-LKQN-11-32. We thank
Dr Ran Fang (Lanzhou University) for DFT studies. The
high performance computing facility at the Gansu Computing
Center is also acknowledged.
S2–Cd and H2PO4À, as shown in Fig. 14. The association
constant was determined to be 4.15 Â 108 MÀ2 (ESI,† Fig. S4)
À
and the detection limit of the sensor towards H2PO4 is 2.8 Â
10À6 M, which indicates that this sensor could potentially be
À
used as a probe for H2PO4 monitoring.
The selectivity of S2–Cd (15 equiv.) to H2PO4À was examined
over a wide range of pH values (Fig. 15). No apparent changes
in the fluÀorescence spectra were observed, and the detection
of H2PO4 can work well in the range of pH 2.0–11.0. To
investigate the practical application of the sensor S2–Cd,
test trips were prepared by immersing a filter paper into a
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