Determination limit of fluorescence turn-on probes for the acetate anion

Article abstract of DOI:10.1002/hlca.201200282

Three fluorescent turn-on probes containing 3,6-dichloro-9H-carbazole as carbazyl part have been designed and synthesized. Among studied anions F ^-, AcO^-, H₂PO₄^-, Cl ^-, Br^- and I^-, AcO^- showed the strongest binding ability with all probes. The strong basic anions, such as AcO^-, H₂PO₄^-, and F^-, induced a significant red-shift in absorption and a concomitant increase in fluorescent emission of the probes caused by photoinduced electron transfer (PET). The determination limit of probe 3 (Scheme 1) toward AcO^- is 3.0×10^-7 M. ¹H-NMR Titration experiments shed light on the nature of the interaction between the probes and the anions. Theoretical investigation further illustrated the possible binding mode in these host-guest interactions and the roles of molecular frontier orbitals in molecular interplay. Copyright

Full text of DOI:10.1002/hlca.201200282

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Determination Limit of Fluorescence Turn-On Probes for the Acetate Anion
by Xuefang Shang*a), Jianmei Yuana), Zhimin Dua), Yingling Wanga), Shenyu Jiab), Jie Hanb),
Yue Li b), Jinlian Zhangb), and Xiufang Xuc)
a) Department of Chemistry, Xinxiang Medical University, Jinsui Road 601, Xinxiang 453003,
P. R. China (phone: þ 86-373-3029128; fax: þ 86-373-3029959; e-mail: xuefangshang@126.com)
b) School of Pharmacy, Xinxiang Medical University, Jinsui Road 601, Xinxiang 453003, P. R. China
c) Department of Chemistry, Nankai University, Tianjin 300071, P. R. China
Three fluorescent turn-on probes containing 3,6-dichloro-9H-carbazole as carbazyl part have been
designed and synthesized. Among studied anions Fꢀ, AcOꢀ, H2PO4ꢀ, Clꢀ, Brꢀ and Iꢀ, AcOꢀ showed the
strongest binding ability with all probes. The strong basic anions, such as AcOꢀ, H2PO4ꢀ, and Fꢀ, induced a
significant red-shift in absorption and a concomitant increase in fluorescent emission of the probes
caused by photoinduced electron transfer (PET). The determination limit of probe 3 (Scheme 1) toward
AcOꢀ is 3.0 ꢁ 10ꢀ7 m. 1H-NMR Titration experiments shed light on the nature of the interaction between
the probes and the anions. Theoretical investigation further illustrated the possible binding mode in these
hostꢀguest interactions and the roles of molecular frontier orbitals in molecular interplay.
Introduction. – Selective anion sensing by artificial receptors are particularly
attractive due to its important role in a wide range of environmental, clinical, chemical,
and biological applications [1 – 10]. Among biologically important anions, AcOꢀ has
attracted growing attention for its established role in enzymes and antibodies.
Moreover, AcOꢀ is also a significant component of numerous metabolic processes.
Therefore, the recognition and sensing of AcOꢀ becomes increasingly important in
comparison to other biologically functional anions [11 – 13].
Currently, a fluorescence probe has attracted increased attention being constructed
according to the receptor-chromophore principle, which involves binding sites and a
fluorophore, and which can translate the receptorꢀanion binding into an optical signal
[14][15]. Among these signaling mechanisms [16 – 19], the photoinduced electron
transfer (PET) mechanism is often exploited for probes, showing the fluorescence
response to the guest binding. Such fluorescent host molecules based on PETare mostly
structurally simple [20].
Various types of artificial receptors have been developed, which employ H-bonding
due to specific binding sites, such as those of urea or thiourea [21], amides [22],
thioamides [23], pyrroles [24][25], or of azamacrocycles [26] heading for binding of
anions with size and shape selectivity in various media. Phenol groups are among the
most frequently used fragments to design neutral receptors for anion recognition by
directional H-bonding interaction of anions even in aqueous solutions [27][28].
Although anion receptors containing phenol groups as binding sites have been
investigated, recognition mechanism based on the synergism of phenol and 9H-
carbazole is scarcely reported [29][30].
ꢀ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Despite the high sensitivity of modern analytical instruments and methods, some
components of detection such as the level of trace concentration have been left aside
(no signal). The minimum concentration in environment and organisms is essential for
the judgment of the pollutant content of anions and the causes of disease. Therefore,
research on the determination limit of anion recognition is still important. However,
literature on the determination limit of anion recognition is rather scarce.
Herein, we report the synthesis of three fluorescent anion probes, 1 – 3, based on
3,6-dichloro-9H-carbazole derivatives (Scheme 1) providing phenol OH and carbazole
NH groups as binding sites, and together with the PET signaling mechanism and the
determination limit with their three ligands. We reason that the OH and NH groups
incorporated in the conjugated system with the carbazole moiety (fluorophore) will
render the probes capable of sensing anions optically in organic media. As expected,
the probes 1 – 3 showed fluorescence turn-on upon the addition of anions in organic
solvents.
Scheme 1. General Synthetic Route to the Target Compounds 1 – 3
Results and Discussion. – UV/VIS Titration. The anion-binding ability of 1 – 3 were
investigated in detail by UV/VIS spectroscopy in DMSO. Fig. 1,a, shows the changes in
UV/VIS spectra of 1 (2.0 ꢁ 10ꢀ5 m) upon the titration with AcOꢀ in DMSO solution.
The absorption spectrum of free 1 exhibited two broad absorption bands centered at
330 and 400 nm, which could be assigned to the excitation of p electrons in the
carbazole system and the intramolecular charge-transfer (ICT) transitions within the
whole structure of 1, respectively [19]. With the stepwise addition of AcOꢀ ion to the
solution of 1 (2.6 ꢁ 10ꢀ5 m), the absorbance values at 330 and 400 nm were reduced in
intensity, and a new absorbance band at 500 nm developed gradually. The red-shift
phenomenon occurred, induced by the addition of AcOꢀ, in the absorption spectrum of
1. The reason could be the enhancement of the conjugative hostꢀguest effect after
H-bonding between 1 and AcOꢀ [31][32]. However, a clear isosbestic point at 430 nm
was observed, indicating stable complex formation between 1 and AcOꢀ [33].
Particularly, the presence of Fꢀ and H2POꢀ4 induced changes in the UV/VIS spectra of 1
similar to those of AcOꢀ. However, the additions of excess equiv. of Clꢀ, Brꢀ, and Iꢀ
resulted only in slight changes in the UV/VIS spectra of 1 (Fig. 2,a), which indicated

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Fig. 1. Changes in the absorption spectra of probes 1 – 3 (20 mm) in absence and presence of AcOꢀ in
DMSO. Arrows indicate the increase of anion concentration.
that 1 showed different binding ability with Fꢀ, H2PO4ꢀ, and almost no or very weak
binding ability with Clꢀ, Brꢀ, and Iꢀ.
Similar effects are observed with 2. As the concentration of AcOꢀ increased, the
absorption intensity at 315 and 390 nm decreased. At the same time, a new absorbance
at 500 nm formed, and the intensity increased gradually. The appearance of an
isosbestic point at 425 nm indicated formation of a stable complex between 2 and
anions. Moreover, addition of Fꢀ or H2PO4ꢀ induced similar changes in the spectra of 2,
in comparison to AcOꢀ (Fig. 2,b). However, the additions of Clꢀ, Brꢀ, and Iꢀ to 2
virtually led to almost no spectral changes, indicating that the interaction between 2,
and Clꢀ, Brꢀ, and Iꢀ was very weak and could be neglected.
For free 3, strong absorptions at 340 and 400 nm appeared. With the addition of
AcOꢀ ion, the absorption intensity decreased, and a new absorption peak at 430 nm
developed gradually (Fig. 1,c). A clear red-shift phenomenon occurred with 3 and
AcOꢀ. In addition, one isosbestic point at 425 nm appeared, indicating formation of a
stable complex [31]. The presence of H2POꢀ4 and Fꢀ induced similar changes in UV/
VIS spectra of 3, but the additions of excess equiv. of Clꢀ, Brꢀ, and Iꢀ resulted only in

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Fig. 2. Changes in the absoprtion spectra of probes 1 – 3 (20 mm) induced by addition of 20 equiv. of tested
anions
slight changes in the UV/VIS spectra of 3, which indicated very weak interactions
which could be neglected (Fig. 2,c).
Fluorescence Titration. The photophysical responses of 1 – 3 toward the addition of
the anions mentioned above were also investigated. As shown in Fig. 3,a, the
fluorescence (FL) spectra of 1 exhibited very weak response. Upon the addition of
AcOꢀ (0 – 10 equiv.) to the solution of 1, the fluorescence centered at 471 nm (excited
at 427 nm) became enhanced. To account for such FL enhancement, the PET
mechanism was exploited. For 1, the excited state of the fluorophore was not, or only to
a minor extent, enhanced by electron transfer (ET) from the receptor to the
fluorophore and the receptorꢀanion interaction. The addition of Fꢀ induced a similar
fluorescence enhancement. Nevertheless, FL emission of 1 was insensitive to the
additions of excess equiv. of H2PO4ꢀ, Clꢀ, Brꢀ, and Iꢀ ions (Fig. 4,a), indicating very
weak, hence negligible interactions.
In the FL titration spectra of 2, the intensity centered at 500 nm enhanced
significantly with the increase of AcOꢀ concentration (Fig. 3,b), which indicated that 2
interacted with AcOꢀ. The addition of Fꢀ induced similar FL enhancement. However,

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Fig. 3. Changes in the emission spectra of probes 1 – 3 (26 mm) in absence and presence of AcOꢀ in DMSO.
a) 1; lex 427 nm; b) 2; lex 422 nm; c) 3; lex 435 nm.
the additions of H2POꢀ4 , Clꢀ, Brꢀ, and Iꢀ caused very weak FL enhancements, so that
these hostꢀguest interactions could be neglected (Fig. 4,b). As for 3, the additions of
AcOꢀ, Fꢀ, H2POꢀ4 induced strong FL enhancements. With the addition of anions, the
intensity of emission band at 480 nm with a shoulder centered at 520 nm gradually
enhanced. There were almost no changes upon the additions of Clꢀ, as well as Brꢀ and
Iꢀ (Fig. 4,c), suggesting very weak, hence ignorable interactions.
Binding Constants. The above analyses indicated that the spectral changes could be
ascribed to the formation of 1:1 hostꢀguest complexes. The binding constants obtained
by using the method of nonlinear least-squares calculation according to the UV/VIS
and FL data are compiled in the Table [34 – 36]. The trend of binding of 1 – 3 to anions
are as follows: AcOꢀ > Fꢀ > H2PO4ꢀ >> Clꢀ ꢂ Brꢀ ꢂ Iꢀ. The selectivity for specific
anions could be rationalized on the basis of the anion basicity, and the interactions
between the host and the anionic guests. However, multiple H-bonding interactions are
also necessary in high-affinity anion-binding sites [37]. As expected from their basicity,
AcOꢀ, Fꢀ, and H2POꢀ4 bound more strongly than the other anions studied; in addition,
the triangle structure of AcOꢀ may well match with 1 – 3 in terms of shape and could
form multiple H-bonding interactions. Consequently, AcOꢀ can be selectively differ-
entiated from other anions based on affinity constant. Especially, the selectivity was

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Fig. 4. Changes in intensity of the emission band in the presence of 10 equiv. of tested anions. a) 1; 471 nm;
b) 2; 490 nm; c) 3; 480 nm.
Table. Binding Constants of Compounds 1 – 3 with Various Anions
Aniona)
Binding constant (Ks)
1
2
3
AcOꢀ
Absorption
Emission
(1.25 ꢃ 0.71) ꢁ 105
(1.38 ꢃ 0.31) ꢁ 104
(2.12 ꢃ 0.49) ꢁ 105
(3.81 ꢃ 0.13) ꢁ 105
(3.51 ꢃ 0.73) ꢁ 104
(5.08 ꢃ 0.81) ꢁ 105
Fꢀ
Absorption
Emission
(2.36 ꢃ 0.54) ꢁ 104
(9.04 ꢃ 0.25) ꢁ 103
< 10
< 10
(6.63 ꢃ 0.17) ꢁ 103
(5.04 ꢃ 0.26) ꢁ 103
< 10
78 ꢃ 16
(8.65 ꢃ 0.32) ꢁ 103
(9.18 ꢃ 0.24) ꢁ 103
H2POꢀ4
Absorption
Emission
(1.41 ꢃ 0.52) ꢁ 103
(7.42 ꢃ 0.06) ꢁ 103
Clꢀ (Brꢀ or Iꢀ )
NDb)
ND
ND
a) All anions were added in form of Bu4Nþ salts. b) The binding constant could not be determined.
more apparent for 3. For the same anion, the binding ability is: 3 > 1 > 2. The reason
may be that the electron-withdrawing groups in the phenolic part could enhance the
acidity of the probe, and the binding ability increased. As a result, the anion-binding
ability of 3 was higher than that of 2, whereas that of 1 was medium. Moreover, the
binding constants obtained by UV/VIS data were in the same order of magnitude with

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fluorescence data, which indicated that the results of binding constants obtained by
UV/VIS data were confirmed by fluorescence.
Determination Limit. For cation recognition, determination limits were sufficiently
reported [38][39], whereas reports on the determination limits for anions are almost
nonexistent. Thus, the determination limit in case of 3 with AcOꢀ was studied (Fig. 5).
The interaction of 3 with AcOꢀ could be detected down to a concentration of 3.0 ꢁ
10ꢀ7 m; at this point [AcOꢀ] is 4.7 ꢁ 10ꢀ7 m, with a 1.3 times enhanced absorption
intensity. Under optimal conditions, the FL intensity of 3 and the concentration of
added AcOꢀ displayed a good linear relationship (0 – 2 mm), indicating the quantitative
detection of AcOꢀ in the above concentration range. These results revealed that 3 could
potentially be used as a probe for monitoring AcOꢀ levels in physiological and
environmental systems.
Fig. 5. Fluorescence intensity (482 nm) of 3 (0.30 mm) in DMSO as a function of concentrations of AcOꢀ
(0 – 1.8 mm) with an excitation wavelength of 435 nm
1H-NMR Titration. To further shed light on the nature of the interactions of 1 – 3
and anions, 1H-NMR spectral changes upon addition of AcOꢀ with Bu4Nþ as
counterion to the (D6)DMSO solution of 2 and 3 (1 ꢁ 10ꢀ2 m) were investigated. As
shown in Fig. 6, the signal at d(H) 11.84 (NH), which was assigned to the NH H-atom,
broadened and exhibited a downfield shift to d(H) 12.28 upon addition of increasing
equiv. of AcOꢀ, thus indicating strong H-bonding interactions [40]. The OH H-signal at
d(H) 12.24 became weaker and finally disappeared with the increase of AcOꢀ, which
indicated deprotonation of the OH groups. At the same time, the signal at d(H) 9.05
(CH in the Schiff base parts) was shifted downfield, indicating a potential H-bonding.
The signals of the H-atoms of the carbazole part (d(H) 7.47 (Hd), 8.25 (He)) and the
phenol parts (d(H) 7.35 (Hc), 7.17 (Ha), 6.94 (Hb)) exhibited slight upfield shifts,
evidencing the increase of the electron density on there moieties due to through-bond
effects. The reason may be as follows: in compound 2, the OH groups form
intramolecular H-bonds with NH, a finding established by theoretical investigations.
With the addition of AcOꢀ, intramolecular H-bonds are cleaved, and H-bonds are

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formed between the interacting sites (NH and OH) and AcOꢀ. With further addition of
AcOꢀ, OH moieties are deprotonated, and the potential CH ··· OAc bonds develop
[41]. The proposed binding mode of 2 with anions in solution is outlined in Scheme 2.
Scheme 2. Proposed HostꢀGuest Binding Modes in Solution
For 3, the signals of OH (d(H) 11.95) and NH (d(H) 11.93) overlapped. The signal
of OH disappeared, and the signal of NH was shifted downfield after addition of AcOꢀ,
which indicates that the H-bonds between carbazole NH in 3 and AcOꢀ were formed
and deprotonation took place in the OH moieties. The carbazole H-signals (d(H) 8.04
(Hd) and 8.25 (He)) and the H-signals of the phenol part (d(H) 7.44 (Hb), 7.58 (Hc), and
7.00 (Ha)) exhibited slight upfield shifts, evidencing the increase of the electron density

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Fig. 6. 1H-NMR Spectra of compounds 2 and 3 in (D6)DMSO (10 mm) upon the addition of molar equiv.
of AcOꢀ. From bottom to top: addition of 0, 0.5, 1, 2, and 5 equiv. of AcOꢀ.
on these moieties due to through-bond effects. However, the signal at d(H) 9.02 (CH in
Schiff base part) almost did not change, evidencing that the corresponding H-atom
does not participate the hostꢀguest interaction. The proposed binding mode of
hostꢀguest in solution is depicted in Scheme 2.
1
According to the results from UV/VIS, FL, and H-NMR titrations, the proposed
hostꢀguest binding modes in solution can be seen as shown in Scheme 2. In the complex
structure, an anion interacts with 1 – 3 via NꢀH and OꢀH H-bonds. With further anion
addition, OH undergoes deprotonation, and the H-bonding still exists between the NH
of carbazole and the anion.
Theoretical Investigations. The geometries of 1 – 3 were optimized (Fig. 7) using
density functional theory at B3LYP/3 – 21G level with Gaussian03 program [42]. Fig. 7
shows that the intramolecular H-bonds are indeed possible between NH of carbazole
and OH of the benzene ring of compounds 1 – 3. In compound 1, one OH group
underwent a prototropic 1,5-shift to the Schiff base N-atom. Compounds 2 and 3 both
exist in their bis-phenolic form. In view of geometry, the anion-binding ability of
compound 2 was weaker than that of compounds 1 and 3 due to specific hinderance of

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Fig. 7. Optimized geometries of compounds 1 – 3
the MeO group. The distance of the two H-atoms of the OH groups in compound 3 was
2.485 ꢂ, which was close to the distance of the two O-atoms in AcOꢀ (2.309 ꢂ).
Therefore, compound 3 showed the strongest binding ability for AcOꢀ among the
anions tested.
Selected frontier orbitals for 1 – 3 are shown in Fig. 8. We looked at the molecular
frontier orbitals to explain the UV/VIS absorption spectra for the hostꢀguest
interaction by electron transition of frontier orbitals. The highest HOMO density in
1 – 3 was mainly localized on the carbazole and phenol moieties. In contrast, the highest
LUMO density was mainly localized on one of the phenol moieties, indicating that the
electron transition of the highest LUMO causes the red shift phenomenon in the UV/
VIS spectra of the ligand (1 – 3)ꢀanion complexes.
Conclusions. – In conclusion, three fluorescent anion probes have been designed
and synthesized. The probes are based on PET FL enhancement (turn-on), which

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Fig. 8. Molecular orbital levels of compounds 1 – 3
offers the potential for high sensitivity. The results indicate that these probes show the
strongest binding ability for AcOꢀ among the anions tested. Theoretical investigation
indicates that it is the electron transition of the highest LUMO that causes the red-shift
phenomenon in UV/VIS spectra of probeꢀanion complexes. The determination limit of
3 for AcOꢀ is 3.0 ꢁ 10ꢀ7 m, whereby the concentration of AcOꢀ is 4.7 ꢁ 10ꢀ7 m. Under
optimal conditions, the FL intensity of 3 and the concentration of added AcOꢀ display
a good linear relationship (0 – 2 mm) indicating the quantitative detection of AcOꢀ in
the above concentration range.
Experimental Part
General. Most of the starting materials were obtained commercially, and all reagents and solvents
used were of anal. grade. All anions, in the form of Bu4Nþ salts such as Bu4NF, Bu4NCl, Bu4NBr, Bu4NI,
Bu4NOAc, and Bu4NH2PO4 were purchased from Sigma-Aldrich Chemical Co., stored in a desiccator
under vacuum, and used without any further purification. The Bu4N salts were dried for 24 h in vacuo
with P2O5 at 333 K. DMSO was distilled in vacuo after drying on CaH2. UV/VIS Titration experiments
were conducted on a Shimadzu UV2550 spectrophotometer at 298 K. The affinity constant, Ks, was
obtained by nonlinear least-squares calculation method for data fitting. 1H-NMR Spectra were recorded
on a Varian UNITY Plus-400-MHz spectrometer; d in ppm rel. to Me4Si as internal standard, J in Hz.
ESI-MS was performed with a MARINER apparatus; in m/z. Elemental analyses (C, H, N) were
performed on a Vanio-EL instrument. Compounds 1 – 3 were synthesized according to the route outlined
in Scheme 1.
3,6-Dichloro-1,8-dinitro-9H-carbazole was synthesized as described in [43]. Yield: 86%. M.p. 282 –
1
2838 ([43]: 2848). H-NMR ((D6)DMSO): 11.30 (s, NH); 8.98 (s, 2 H); 8.48 (d, J ¼ 4.0, 2 H). ESI-MS:

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324.30 ([M ꢀ H]ꢀ). Anal. for C12H5Cl2N3O4 (326.09): C 44.20, H 1.55, N 12.89; found: C 44.36, H 1.61, N
12.85.
3,6-Dichloro-9H-carbazole-1,8-diamine (4). In a three-neck, 250-ml round-bottomed flask, 3,6-
dichloro-1,8-dinitro-9H-carbazole (5 mmol) and Pd/C were suspended in EtOH (100 ml). The soln. was
heated at reflux, and NH2NH2 · H2O (10 ml) dissolved in EtOH (30 ml) was added dropwise. After the
addition was completed, the mixture was heated at reflux for 12 h until the soln. became colorless. Then,
the hot suspension was filtered, and the filter was washed with EtOH. The EtOH was removed in a rotary
1
evaporator, resulting in a gray-white solid which was recrystallized from EtOH. Yield: 59%. H-NMR
((D6)DMSO): 10.67 (s, NH); 8.91 (s, 2 H); 8.36 (s, 2 H); 5.42 (s, 2 NH2). Anal. for C12H9Cl2N3 (266.13): C
54.16, H 3.41, N 15.79; found: C 54.28, H 3.66, N 15.65.
2,2’-[(3,6-Dichloro-9H-carbazole-1,8-diyl)bis(nitrilomethylylidene)]diphenol (1). Diamine
4
(1 mmol, 265 mg) and salicylaldehyde (¼2-hydroxybenzaldehyde; 2 mmol, 122 mg) were suspended in
35 ml of EtOH. The mixture was heated under reflux for 4 h. The yellow precipitate was separated by
1
filtration. The solid was washed with Et2O and dried under vacuum. H-NMR ((D6)DMSO): 12.17 (s,
2 OH); 11.88 (s, NH); 9.07 (s, 2 H); 8.26 (d, J ¼ 4.0, 2 H); 7.77 (d, J ¼ 4.0, 2 H); 7.47 (d, J ¼ 4.0, 4 H); 7.01
(d, J ¼ 8.0, 4 H). ESI-MS: 472.2 ([M ꢀ H]ꢀ). Anal. for C26H17Cl2N3O2 (474.34): C 65.83, H 3.61, N 8.86;
found: C 65.65, H 3.12, N 8.59.
2,2’-[(3,6-Dichloro-9H-carbazole-1,8-diyl)bis(nitrilomethylylidene)]bis(6-methoxyphenol) (2). The
synthesis procedure was similar to that of compound 1. 1H-NMR ((D6)DMSO): 12.24 (s, 2 OH); 11.84 (s,
NH); 9.05 (s, 2 H); 8.25 (s, 2 H); 7.46 (d, J ¼ 4.0, 2 H); 7.33 (d, J ¼ 8.0, 2 H); 7.16 (d, J ¼ 8.0, 2 H); 6.94 (t,
J ¼ 16.0, 2 H); 3.84 (s, 6 H). ESI-MS: 532.3 ([M ꢀ H]ꢀ). Anal. for C28H21Cl2N3O4 (534.39): C 62.93, H
3.96, N 13.27; found: C 63.03, H 3.94, N 13.67.
2,2’-[(3,6-Dichloro-9H-carbazole-1,8-diyl)bis(nitrilomethylylidene)]bis(6-bromophenol) (3). The
synthesis procedure was similar to that of compound 1. 1H-NMR ((D6)DMSO): 11.91 (d, J ¼ 16.0,
2 H); 9.02 (s, 2 CH); 8.26 (d, J ¼ 4.0, 2 H); 8.03 (d, J ¼ 4.0, 2 H); 7.58 (dd, J ¼ 12.0, 2 H); 7.43 (d, J ¼ 4.0,
2 H); 6.98 (d, J ¼ 8.0, 2 H). ESI-MS: 627.7 ([M ꢀ H]ꢀ). Anal. for C26H15Br2Cl2N3O2 (632.13): C 49.40, H
2.39, N 6.65; found: C 49.77, H 2.36, N 6.82.
This work was supported by the Natural Science Fund of Henan Province (2010B150024,
112300410104), the Young Teacher Fund of Henan Province (2011GGJS-126), and the Doctorial
Starting Fund of Xinxiang Medical University.
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Received May 30, 2012