Dual off-on and on-off fluorescent detection of Zn2+/Cd 2+ ions based on carbazolone substituted 2-aminobenzamides

Article abstract of DOI:10.1039/c3ra45717f

Two new 2-aminobenzamide structural isomers, 4-isoACOBA and 5-isoACOBA, as fluorescent probes for Cd²⁺ and Zn²⁺ were fabricated with carbazolone as fluorophore and N,N-bis(2-pyridylmethyl)ethylenediamine (BPEA) as chelator. With Cd²⁺/Zn²⁺ as input, the two probes are characteristic of the transformation from "off-on" to "on-off" molecular switch by interchanging the substitution position of the fluorophore from C-4 to C-5 at the benzene ring. 4-IsoACOBA is a Cd ²⁺-specific turn-on fluorescent probe exhibiting good discrimination between Cd²⁺ and Zn²⁺ with F_Cd2+/F _Zn2+ = 2.48, while 5-isoACOBA is a Zn²⁺-specific turn-off probe with F_Cd2+/F_Zn2+ = 4.50. The binding behaviours of 4-isoACOBA-Cd(ii) and 5-isoACOBA-Zn(ii) were deeply investigated by UV and fluorescence titration, ESI-MS analysis, and DFT study. The results indicate that both the electron donating/withdrawing ability and the substituted position of the fluorophore have remarkable influences on the probe sensing properties and selectivity.

Full text of DOI:10.1039/c3ra45717f

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Dual off–on and on–off fluorescent detection of
2
+
2+
Zn /Cd ions based on carbazolone substituted
-aminobenzamides†
Cite this: RSC Adv., 2014, 4, 3591
2
a
a
a
a
ab
Qin-Chao Xu, Xue-Hui Zhu, Can Jin, Guo-Wen Xing* and Yuan Zhang*
Two new 2-aminobenzamide structural isomers, 4-isoACOBA and 5-isoACOBA, as fluorescent probes for
2+
2+
Cd
and Zn
were fabricated with carbazolone as fluorophore and N,N-bis(2-pyridylmethyl)
2+
2+
ethylenediamine (BPEA) as chelator. With Cd /Zn as input, the two probes are characteristic of the
transformation from “off–on” to “on–off” molecular switch by interchanging the substitution position of
2+
the fluorophore from C-4 to C-5 at the benzene ring. 4-IsoACOBA is a Cd -specific turn-on
2+
2+
fluorescent probe exhibiting good discrimination between Cd and Zn with FCd2+/FZn2+ ¼ 2.48, while
2+
5
-isoACOBA is a Zn -specific turn-off probe with FCd2+/FZn2+ ¼ 4.50. The binding behaviours of 4-
Received 10th October 2013
Accepted 2nd December 2013
isoACOBA–Cd(II) and 5-isoACOBA–Zn(II) were deeply investigated by UV and fluorescence titration, ESI-
MS analysis, and DFT study. The results indicate that both the electron donating/withdrawing ability and
the substituted position of the fluorophore have remarkable influences on the probe sensing properties
and selectivity.
DOI: 10.1039/c3ra45717f
www.rsc.org/advances
5
–7
have been extensively investigated, it is still challenging to
Introduction
2
+
develop efficient uorescent sensors which can distinguish Zn
2
+
Zinc and cadmium are both in the IIB group of the periodic and Cd with high selectivity and sensitivity, since zinc and
table. However, they exert distinct inuence on animals and cadmium are usually coordinated with uorescent sensors with
human beings. For one thing, cadmium plays an essential role similar uorescent signal output.
in many processes including electroplating, metallurgy, agri-
In recent years, 2-aminobenzamide analogues have been
1
8
culture and war industry, etc.; for another, it is a toxic heavy concerned as heat shock protein 90 inhibitors. Soon aerwards
metal element and considered as one of 126 priority pollutants we developed a novel 2-aminobenzamide analogue BJ-B11 (ref. 9)
as pronounced by the U.S. Environmental Protection Agency. and elucidated its inspiring antitumor and anti-HSV activities.
The long-term cadmium intake will affect human hematopoi- Meanwhile, some attractive uorescent properties of these
etic, neural, kidney, and other organ function, bringing great compounds were observed in the study. Inspired by the obser-
2
harm to human health, especially to children. By contrast, zinc, vations, we successfully designed a series of Zn(II) and Cd(II)-
0
an essential element for human, is widely distributed in almost specic off–on uorescent probes ZnABA/ZnABA and CdABA/
all tissues and organs. It is the component of a variety of
enzymes, directly involved in the synthesis of the nucleic acid
3
and protein and stimulating lymphocytes into the division.
Fluorescence is a highlighted method to recognize and sense
ions and small molecules due to its operational simplicity
effectiveness, high sensitivity and low detection limit. The
design and application of various uorogenic probes have made
4
great progress in recent years. Even though the uorescent
recognition and sensing systems for zinc and cadmium ions
a
Department of Chemistry, Beijing Normal University, Beijing 100875, China. E-mail:
gwxing@bnu.edu.cn; yuanzhang@bnu.edu.cn
b
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of
Education, College of Chemistry, Beijing Normal University, Beijing 100875, China
†
Electronic supplementary information (ESI) available: Detailed synthetic
procedures and characterization of 4- and 5-isoACOBA. Fig. S1–S5 and copies of
Scheme
1 Design route of carbazolone substituted 2-amino-
NMR spectra for all the new compounds. See DOI: 10.1039/c3ra45717f
benzamide fluorescent probes for Zn(II)/Cd(II) ions.
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0
10
CdABA (Scheme 1), with tetrahydroindazolone as uorophore study, we selected carbazolone as the uorophore, which
and N,N-bis(2-pyridylmethyl)ethylenediamine (BPEA) as chelator. possesses the moderate electron-withdrawing ability between
More recently, carbazole was introduced into the sensing system indazolone and carbazole, and expect the designed probe 4- and
11
to generate 4 and 5-isoACBA (Scheme 1), which exhibit an 5-isoACOBA could have different uorescence sensing activities to
2
+
2+
10,11
excellent uorescence on–off–on response for Zn(II) and pyro- Cd or Zn compared with the former probes
phosphate (PPi) ions. Based on these results, herein we focus on Carbazolone 3 was prepared from 2-nitroiodobenzene (1)
carbazolone, an alternative uorophore, to fabricate new uo- and dimedone in two steps according to the literature method
reported by us.
12
rescent probes 4 and 5-isoACOBA for Zn(II)/Cd(II) ions (Scheme 1). (Scheme 2). Then, 3 was attached to the benzene ring by
aromatic nucleophilic substitution in the presence of K CO
Cs CO to afford 5/9 in 74–86% yield. Subsequently, the BPEA
2
3
/
2
3
Results and discussion
moiety was incorporated into the aromatic ring under Buch-
wald–Hartwig coupling conditions to provide 7/10 in 63–69%
yield. Aer hydration of 7/10 with KOH and H , the desired
13
Indeed, uorescent sensor, usually consist of a signal label and
a receptor. The exquisite combination of them usually leads to
excellent recognition for the target analytes. We have synthe-
sized a series of 2-aminobenzamide analogues with indazolone
and carbazole as uorophore respectively which exhibit high
2 2
O
probe 4-/5-isoACOBA was accomplished in good yield.
The metal ion selectivity of 4-/5-isoACOBA was investigated
by adding various metal salts in 25 mM HEPES buffer con-
taining 15% ethanol at pH 7.4. 4-IsoACOBA exhibited weak
10,11
selectivity and sensitivity for Zn(II), Cd(II) or PPi ions.
It is

uorescence emission (Fig. 1). Interestingly, most heavy and
inspiring to explore the relationship between the nature of the
3
+
2+
3+
2+
2+
transition metal ions, such as Cr , Mn , Fe , Sm , Co and

uorophore and the photophysical properties of the sensor. We
2
+
2+
10,11
Cu had no turn-on response to the sensor, except that Cd and
found two important facts from the studies.
2
+
Zn induced 8.3-fold and 3.4-fold uorescence enhancement
respectively. Moreover, the uorescence of 4-isoACOBA showed
First, when the electron donating/withdrawing ability of the

uorophore changed, the uorescence response is switched
+
+
2+
2+
no changes upon adding 100-fold excess of Na , K , Mg , Ca
into the solution. The results indicate that 4-isoACOBA is a
from on–off to off–on sensing action. With indazolone, a rela-
tive electron-decient aromatic substituent, as the uorophore,
2
+
0
0
Cd -sepecic off–on uorescent probe with FCd2+/FZn2+ ¼ 2.48.
all the four metal probes (ZnABA/ZnABA , CdABA/CdABA )
10
possess off–on uorescent functionality. In sharp contrast,
when carbazole, a relative electron-rich aromatic substituent,
was employed as the uorophore, the two probes (4 and 5-iso-
ACBA) exhibit on–off type uorescent response to Zn(II)/Cd(II).
Second, the substituted position of uorophore on the
benzamide ring is crucial to the probe for preference of Cd or
Zn . Upon changing the indazolone group at the aromatic ring
from 4- to 5-position, the structures of ZnABA/ZnABA are con-
verted into CdABA/CdABA . Correspondingly, the metal ions
1
1
2
+
2
+
0
0
0
2+
selectivity of CdABA/CdABA was switched to discriminate Cd
2
+
10b
from Zn with FCd2+/FZn2+ ¼ 2.27–2.36.
In general, both the electron donating/withdrawing ability and
the substituted position of the uorophore have remarkable
inuence on the probe sensing behaviour and selectivity. In this
Fig. 1 Fluorescence spectra of (a) 4-isoACOBA and (b) 5-isoACOBA
on the addition of various metal ions. Experimental conditions: 4- or 5-
isoACOBA (10 mM, 25 mM HEPES buffer containing 15% ethanol, 0.1 M
+
+
+
+
2+
2+
2+
2+
3+
4
NaClO , pH 7.4), 10 mM Li , K , Rb , Cs , Ca , Mg , Ba , Sr , Cr ,
2+
3+
2+
2+
2+
2+
2+
Scheme 2 Synthetic routes for 4- and 5-isoACOBA.
Mn , Fe , Sm , Co , Cu , Cd and Zn , lex ¼ 318 nm.
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Table 1 Spectroscopic data for the probes used in this study
a
ꢁ1
ꢁ1
b
c
d
ꢁ1
Compound
labs,max (nm)
3 (M cm
)
l
em (nm)
F
a
K (M )
4
4
4
5
5
5
-IsoACOBA
245, 312, 360
245, 312
243, 312
263, 312, 365
263, 312
262, 312
12 744
12 744
12 294
23 237
18 740
17 392
430
434
448
422
464
443
0.032
0.103
0.037
0.074
0.039
0.083
—
2
+
12
8
-IsoACOBA + Cd2
-IsoACOBA + Zn
-IsoACOBA
-IsoACOBA + Zn
-IsoACOBA + Cd
1.89 ꢂ 10
9.70 ꢂ 10
—
1.62 ꢂ 10
1.02 ꢂ 10
+
2+
9
2+
12
a
b
c
Data were examined at labs 312 nm. The excitation wavelength was 318 nm. Quantum yields were determined by using quinine sulfate (in 0.05
d
M H
2
SO
4
, F ¼ 0.55) as the reference. The association constant (K
a
) was determined according to the methods reported in ref. 6a, 7g, j and 14.
To our surprise, 5-isoACOBA itself displays much stronger properties of 4-/5-isoACOBA. As indicated in Fig. 2a, UV spectra of
uorescent emission at 422 nm than 4-isoACOBA (Fig. 1). When 4-isoACOBA have three obvious absorption peaks at 245, 312 and

2
+
2+
adding 1.0 equiv. of Zn , the emission intensity presented an 360 nm. With increasing the amounts of Cd from 0 to 1.0
2
+
almost complete quenching ($94%), while Cd only led to a equiv., the absorbance gradually decreased at 245 and 360 nm,
moderate quenching (ꢀ73%). Additionally, other metal ions and simultaneously a slight increase emerged in the absorbance
+
+
+
+
2+
2+
2+
2+
3+
2+
including Li , K , Rb , Cs , Ca , Mg , Ba , Sr , Cr , Mn , at 312 nm with two typical isobestic points arising at 296 and 318
3
+
and Fe gave no obvious uorescent changes, indicating that 5- nm, suggesting that the 4-isoACOBA–Cd(II) complex could be
isoACOBA is an on–off uorescent probe for Zn and Cd , formed. Meanwhile, UV spectra of 5-isoACOBA also exhibited
2
+
2+
2
+
especially for Zn with FCd2+/FZn2+ ¼ 4.50.
The detailed binding behaviours of 4-isoACOBA and 5-iso- interestingly all the absorption peaks decreased gradually with
ACOBA with Cd /Zn were examined by absorption and emis- the addition of Zn (0–1.0 equiv.) and no obvious isobestic point
sion titrations in HEPES buffer containing 15% ethanol under was observed. In addition, similar absorption titration results
physiological conditions. Table 1 shows the photophysical
three absorption peaks at 263, 312 and 365 nm (Fig. 2b). But
2
+
2+
2+
2
+
Fig. 3 Fluorescence spectra of (a) 4-isoACOBA upon addition of Cd
2+
Fig. 2 UV absorption spectra of (a) 4-isoACOBA (10 mM) upon addition (0–2.0 equiv.) and (b) 5-isoACOBA upon addition of Zn (0–1.5
of Cd (0–1.0 equiv.) and (b) 5-isoACOBA (10 mM) upon addition of equiv.). Experimental conditions: 4- or 5-isoACOBA (10 mM, 25 mM
2+
2+
Zn (0–1.0 equiv.) in 25 mM HEPES buffer containing 15% ethanol (0.1 HEPES buffer containing 15% ethanol, 0.1 M NaClO
M NaClO 318 nm.
, pH ¼ 7.4).
4
, pH ¼ 7.4), lex ¼
4
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2
+
were obtained for 4-isoACOBA–Zn(II) and 5-isoACOBA–Cd(II)
complexes respectively (Fig. S3†).
For 5-isoACOBA, the addition of Zn caused a large bath-
ochromic shi from 422 nm to 464 nm (Table 1). It is reported
2
+
As the uorescence titration indicated (Fig. 3 and S4†), in the that the coordination of probes with Zn usually accompanies
2
+
7d,10
absence of Cd , 4-isoACOBA showed a weak uorescence deprotonation. As shown in ESI-MS spectrum (Fig. S2†), the
emission peak at 430 nm with low quantum yield of 0.032 major peak at 635.5 corresponding to [5-isoACOBA + Zn–H] was
Table 1). Upon addition of Cd from 0 to 2 equiv., the intensity observed, suggesting that 5-isoACOBA binds Zn in a 1 : 1
+
2
+
2+
(
of the emission band gradually increased to reach a plateau at stoichiometry. In addition, the 1 : 1 binding ratio of 5-iso-
2
+
2+
Cd
$1.0 equiv. Meanwhile, the maximum uorescence ACOBA with Zn
was further conrmed by Job's plot
2
+
emission peak was redshied to 434 nm with improved (Fig. S1b†). The association constant of 5-isoACOBA for Zn
9
ꢁ1
quantum yield of 0.103. The Job's plot further conrmed that was calculated to be 1.62 ꢂ 10 M by uorescence spectros-
2+
2
+
1
(
: 1 complex between 4-isoACOBA and Cd was formed copy in metal–ligand-buffered solutions with different Zn
2
+
6a,14
Fig. S1a†). The binding affinity of 4-isoACOBA towards Cd
concentrations.
Density functional theory (DFT) calculations were utilized to
aminoethyl ether)-N,N,N ,N -tetraacetic acid) buffer as described rationalize the dual uorescence off–on and on–off sensing
was determined using cadmium–EGTA (ethylene glycol bis(2-
0
0
7j
7g
2+
2+
by Jiang and Guo, and the association constant (K
isoACOBA–Cd(II) was evaluated to be 1.89 ꢂ 10
a
) of 4- process for Cd /Zn with 4- and 5-isoACOBA as probes.
1
2
ꢁ1
M . We carried out DFT calculations with B3LYP method using 6-
3
1G(d) as basis sets to obtain two optimized structures of 4-
isoACOBA–Zn(II), which are amide tautomer and imidic acid
tautomer respectively (Fig. 4). Interestingly, the total electronic
energies of the two tautomers were very close to each other,
suggesting that the amide tautomer and imidic acid tautomer of
4-isoACOBA–Zn(II) should have the same stabilities. When we
performed the geometry optimization of 4-isoACOBA–Cd(II), its
amide tautomer structure calculation can not be converged.
However, the calculation nally provided the optimized struc-
ture of the imidic acid tautomer of 4-isoACOBA–Cd(II) (Fig. 4).
Similar results were also obtained for the geometry optimiza-
tions of 5-isoACOBA–Zn(II) and 5-isoACOBA–Cd(II) (Fig. S5†).
Based on above ndings, the structures of imidic acid tautomer
2
+
2+
of 4- or 5-isoACOBA with Cd /Zn ions are further investigated
in the following study to rationalize the uorescence off–on and
on–off switch effects.
Tables 2 and 3 show the calculated molecular orbital (MO)
2
+
2+
surfaces of 4-/5-isoACOBA and its complex with Cd /Zn
respectively. For free 4-isoACOBA, the HOMO electron density
resides on benzamide and the lone pairs of ethylenediamine
Fig. 4 The optimized geometry structures of 4-isoACOBA–Zn(II) and group. By contrast, the LUMO electron density is located on
-isoACOBA–Cd(II).
benzamide and the hetero atoms (N and O) of carbazolone.
4
Table 2 Surfaces of the MOs of 4-isoACOBA, 4-isoACOBA–Zn(II) and 4-isoACOBA–Cd(II)
-IsoACOBA 4-IsoACOBA–Zn(II)
4
4-IsoACOBA–Cd(II)
LUMO
HOMO
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Table 3 Surfaces of the MOs of 5-isoACOBA, 5-isoACOBA–Zn(II) and 5-isoACOBA–Cd(II)
5-IsoACOBA
5-IsoACOBA–Zn(II)
5-IsoACOBA–Cd(II)
LUMO
HOMO
Since the p* / n radiative pathway is inhibited during the substitution position of carbazolone moiety from C-4 to C-5 at
excited state, only benzamide moiety of 4-isoACOBA generates the benzene ring, the structure of 4-isoACOBA is converted into
very weak uorescence emission (off state). In the case of 5- 5-isoACOBA. It is worth noting that the 4- and 5-isoACOBA show
isoACOBA, the HOMO electron density is equally distributed on remarkably different uorescent characteristics for preference
2
+
2+
2+
benzamide and carbazolone moieties, but the LUMO is mainly of Cd or Zn . 4-IsoACOBA is a Cd -specic turn-on uores-
2
+
located on the benzamide. During the emissive state of 5-iso- cent probe exhibiting good discrimination between Cd and
2
+
2+
ACOBA, the electron density between HOMO and LUMO is Zn with FCd2+/FZn2+ ¼ 2.48; while 5-isoACOBA is a Zn -specic
signicantly redistributed. Therefore, an ICT (intramolecular turn-off probe with FCd2+/FZn2+ ¼ 4.50.
charge transfer) process takes place to cause the strong uo-
In the previous study, indazolone-substituted 2-amino-
rescence emission of 5-isoACOBA (on state).
Upon complexation with Zn , interestingly, 4- and 5-iso- Zn
ACOBA–Zn(II) have almost the same localizations of HOMO and benzamides sensors are on–off molecular switchs with Zn or
benzamides probes show off–on uorescent response for both
2
+
2+
2+
and Cd ; however, carbazole-substituted 2-amino-
2
+
2
+
LUMO, in which HOMO resides on carbazolone and LUMO is Cd as input. All of the observations enlighten us that both the
mainly located on the BPEA chelating fragment. Under the electron donating/withdrawing ability and the relative position
conditions, either PET or ICT process cannot be carried out. As a of the uorophore moiety have signicant impacts on the probe
result, 4-isoACOBA–Zn(II) still shows low uorescence emission, sensing property and selectivity. The results illustrated in the
however, 5-isoACOBA–Zn(II) exhibits a signicant uorescence study not only open up a new route for the design of sensors for
quenching (off state). The results are well consistent with the metal ions based on the electron effect of the uorophore, but
2
+
spectral studies in which 5-isoACOBA is a Zn -specic turn-off also provide a simple and useful 2-aminobenzamide platform to

uorescent probe.
As for Cd sensing process, 4- and 5-isoACOBA–Cd(II) also
further design other uorescent probes for important analytes.
2
+
have the similar surfaces of HOMO and LUMO. The HOMO is
on carbazolone moiety, and LUMO mainly resides on benza-
Acknowledgements
mide unit. Both 4-isoACOBA–Cd(II) and 5-isoACOBA–Cd(II) The project was nancially supported by Beijing National
display uorescence enhancement (on state), which is charac- Natural Science Foundation (2122031), the National Natural
terized with typical ICT emission. The calculations rationalize Science Foundation of China (20772013, 21272027), and Beijing
2
+
that 4-isoACOBA is an efficient Cd -specic turn-on probe.
Municipal Commission of Education. Y. Zhang thanks the grant
support from the Fundamental Research Funds for the Central
Universities.
Conclusion
In summary, we have successfully constructed two new uo-
rescent chemosensors 4- and 5-isoACOBA based on 2-amino-
benzamide scaffold with carbazolone as uorophore and N,N-
bis(2-pyridylmethyl)ethylenediamine as chelator. 4- and 5-iso-
ACOBA are structural isomers with each other, and exhibit
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2
+
2+
unique properties to detect Zn /Cd by dual off–on and on–off
uorescent sensing process. When interchanging the

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3596 | RSC Adv., 2014, 4, 3591–3596
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