A benzo-15-crown-5-modifying ratiometric-absorption and fluorescent OFF-ON chemosensor for Cu2 +

Article abstract of DOI:10.1016/j.saa.2016.02.041

One new benzo-15-crown-5-modifying fluorene Schiff base (FBC), together with the C=N-linked fluorene-3,4-dimethoxybenzene (FBDMO) and fluorene-benzene (FB) references, has been designed and facilely synthesized. The binding of Cu^{2 +} with nitrogen atom of C=N moiety in these three compounds can inhibit the photo-induced electronic transition process and induce the ratiometric-absorption and fluorescent OFF-ON response to Cu^{2 +}. Whereas the employment of benzo-15-crown-5 moiety in FBC as additional binding platform for Cu^{2 +} not only amplifies the fluorescent enhancement of FBC via preventing the isomerization of C=N moiety, but also endows this compound high selectivity and rapid response towards Cu^{2 +} over the references FB and FBDMO. These results render FBC highly sensitive ratiometric-absorption and fluorescent OFF-ON detecting potential for Cu^{2 +} with the detection limit of 3.91 × 10^{- 6} M.

Full text of DOI:10.1016/j.saa.2016.02.041

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 161 (2016) 144–149
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A benzo-15-crown-5-modifying ratiometric-absorption and fluorescent
OFF–ON chemosensor for Cu2+
a
b,
a
Yuting Chen a, , Xinxin Wang , Kaili Wang , Xiuling Zhang
⁎
⁎
a
Key Laboratory of Coordination Chemistry and Functional Materials in Universities of Shandong, Department of Chemistry Dezhou University, Dezhou 253023, China
Beijing Municipal Research Institute of Environmental Protection, Beijing 100037, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
One new benzo-15-crown-5-modifying fluorene Schiff base (FBC), together with the C_N-linked fluorene-3,4-
dimethoxybenzene (FBDMO) and fluorene-benzene (FB) references, has been designed and facilely synthesized.
The binding of Cu2+ with nitrogen atom of C_N moiety in these three compounds can inhibit the photo-induced
Received 17 September 2015
Received in revised form 15 February 2016
Accepted 28 February 2016
Available online 3 March 2016
electronic transition process and induce the ratiometric-absorption and fluorescent OFF–ON response to Cu2+
.
Whereas the employment of benzo-15-crown-5 moiety in FBC as additional binding platform for Cu2+ not
only amplifies the fluorescent enhancement of FBC via preventing the isomerization of C_N moiety, but also en-
dows this compound high selectivity and rapid response towards Cu2+ over the references FB and FBDMO. These
results render FBC highly sensitive ratiometric-absorption and fluorescent OFF–ON detecting potential for Cu2+
with the detection limit of 3.91 × 10−6 M.
Keywords:
Chemosensor
Benzo-15-crown-5
Ratiometric-absorption
Isomerization
© 2016 Elsevier B.V. All rights reserved.
Fluorescence enhancement
1. Introduction
Taking into account the fluorescence quenching effect of the C_N
isomerization and the photo-induced electron transfer (PET) process
Copper, as an essential trace element in the human body, plays an
important role in a variety of fundamental biological processes. Howev-
er, many neurodegenerative diseases including Alzheimer's, Menke's
and Wilson's diseases have been revealed to result from excessive or
deficient amount of copper ion [1–3]. Moreover, excessive copper ion
is also harmful to the environment. Thus, it is extremely imperative to
develop highly efficient analytical method for detecting and monitoring
copper concentration. Fluorescent sensing method has attracted
increasing attention in recent years due to its high sensitivity, rapid
responsiveness, and low-detection-limit [4–13]. As a consequence, a
variety of copper-detecting fluorescence sensors have been developed
on the basis of different fluorescent chromophores, such as Bodipy
associated with the electron lone pairs on N atom in some Schiff
derivatives [30–36], herein, we designed and synthesized a new and
easy-to-make Schiff base fluorescent molecule, in which the fluorene
fluorophore is linked with the benzo-15-crown-5 (BC) moiety as the
additional binding scaffold via a C_N bridge (FBC). In addition, two ref-
erence compounds, namely the C_N-linked fluorene-benzene (FB) and
fluorene-3,4-dimethoxybenzene (FBDMO), were prepared to evaluate
the binding function of benzo-15-crown-5 in FBC, Scheme 1. Investiga-
tion over the sensing properties of these three compounds for different
metal ions revealed that the unique nitrogen/BC-binding model be-
tween FBC and Cu2+ not only gives FBC the high selectivity and excel-
lent binding affinity to Cu2+ over other tested metal ions, but also
induces the highly sensitive ratiometric-absorption and fluorescent
OFF–ON (54-fold enhancement) response to Cu2+ through inhibiting
both PET and C_N isomerization process, which endows FBC the dual
and porphyrin, [14–19]. Most of the reports on the detection of Cu2+
,
however, were designed on the sensing mechanism of fluorescence
quenching (ON–OFF) for Cu2+ owing to its intrinsic paramagnetic
quenching nature [20–26]. Nevertheless, fluorescence ON–OFF signal
is usually less sensitive and offers limited spatial resolution [27–28].
Moreover, some reported copper-detective sensors are often obtained
though relatively tedious multi-step synthetic procedure [16,29]. Thus,
developing easy-to-synthesize and Cu2+-selective sensor based on the
signal transduction of fluorescence enhancement (OFF–ON) is highly
desirable.
optical detecting potential towards Cu2+
.
2. Results and discussion
2.1. The synthesis of fluorine-based Schiff base derivatives
As shown in Scheme 1, the synthesis of the aimed fluorene Schiff
sensor FBC involved the prior generation of p-benzo-15-crown-5-benz-
aldehyde according to the literature procedures [37], followed by the re-
action with 2-aminofluorene in EtOH under refluxing condition. These
⁎
Corresponding authors.
E-mail addresses: ch_yt@126.com (Y. Chen), wangkaili@cee.cn (K. Wang).
http://dx.doi.org/10.1016/j.saa.2016.02.041
1386-1425/© 2016 Elsevier B.V. All rights reserved.

Y. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 161 (2016) 144–149
145
Scheme 1. Synthesis of FBC as well as the references FBDMO and FB. (i) NaOH, 1,11-dichloro-3,6,9-trioxaundecane, butyl alcohol; (ii) POCl3, DMF; (iii) Acetic acid, EtOH.
compounds were characterized by 1H NMR and mass spectroscopies in
addition to elemental analysis.
being close to 4.85 × 104 M−1 [39–41]. This is further evidenced by
the fluorescence Job's plot, together with the absorption titration exper-
iments of FBC with Cu2+, Fig. 2 [42,43]. The detection limit of this com-
pound for Cu2+ ion was determined to be 3.91 × 10−6 M under the
2.2. Spectral characteristics of fluorine-based Schiff base derivatives
To investigate the selectivity of FBC for metal ion, the photophysical
properties of this compound upon addition of different metal ions such
as Li+, Na+, K+, Mg2+, Ba2+, Ca2+, Cd2+, Ni2+, Mn2+, Fe2+, Fe3+
,
Zn2+, Co2+, Hg2+ or Cu2+ (10 equiv.) were investigated by UV absorp-
tion and fluorescent measurements in MeOH with the concentration of
2 × 10−5 M. As shown in Fig. 1A, the free FBC exhibits a strong absorp-
tion at 346 nm and an obviously weak shoulder band at 283 nm
assigned to the n–π* and π–π* electronic transition, respectively [38].
This is also true for the reference FBDMO. Nevertheless, the two corre-
sponding absorption bands of the reference FB are respectively blue-
shifted to 340 and 275 nm with an almost equal absorbancy. These
results suggest that the strong absorption of FBC at 346 nm may be
ascribed to the electronic transition of lone pairs from both N atom of
C_N unit and the hydroxybenzene O atom in benzo-15-crown-5 moie-
ty. Meanwhile, the weak fluorescence emission (OFF state) is observed
for these three compounds, with the maximum emission centered at
380 nm for FBC as well as FBDMO, and 373 nm for FB, respectively,
Fig. 1B.
2.3. Spectral properties of FBC upon addition Cu2+
Notably, upon addition of Cu2+ (10 equiv.), the strong absorption of
FBC centered at 346 nm from n–π* electronic transition is significantly
decreased, whereas the weak one at 283 nm is slightly increased with
a concomitant blue-shift of ca. 10 nm, resulting in the varying ratio of
I273/I346 from 0.54 to 16.93, Fig. 1A. Meanwhile, a remarkable change
also occurs in the fluorescence emission of FBC after adding Cu2+: the
weak fluorescence emission from FBC centered at 380 nm is enhanced
by an approximate 54-fold, simultaneously accompanied by a blue-
shift of ca. 9 nm Fig. 1B. The quantitative fluorescent titration experi-
ments for FBC with Cu2+ show that the fluorescent enhancements of
FBC were mainly observed along with the increasing Cu2+ from 1 to ap-
proximate 3 equiv. Subsequently the fluorescent emission spectrum
keeps almost unchanged upon further increase in the Cu2+ amount
added even to 10 equiv., Fig. 2A. Moreover, the emission intensity (F-
Fmin) of FBC in the range of 334–475 nm increases linearly with the
added Cu2+ amount (0–3 equiv.) based on the Benesi–Hildebrand
equation, Fig. 2B, indicating the possible 1:1 binding stoichiometry be-
tween FBC and Cu2+ with the approximate association constant (Ka)
Fig. 1. The UV absorption (A) and fluorescence emission spectra (B) of FBC as well as the
references FB and FBDMO (2 × 10−5 M) before and after adding Cu2+ (10 equiv.) in
MeOH, with the excitation wavelength of 290 nm.

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Y. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 161 (2016) 144–149
Fig. 2. The fluorescence emission (A, B) and UV absorption spectra (C, D) of FBC upon addition of increasing Cu2+ amount (0, 0.2, 0.4, 0.6, 0.8, 0.9 1, 1.2, 1.4, 1.5, 1.6, 1.8, 2, 2.5, 3, 4, 5, 6, 7, 8, 9
and 10 equiv., respectively) at the concentration of 2 × 10−5 M in MeOH; The inset in B is the fluorescence Job's plot showing the 1:1 binding stoichiometry between FBC and Cu2+ in FBC–
Cu2+ system.
present condition [44], which can be compared with some recently re-
ported fluorescence sensors for Cu2+ [16]. These results show that
FBC can be utilized as a potential ratiometric-absorption and fluores-
by approximate 16-fold, but with almost no-shift in its maximum emis-
sion wavelength under acid condition, which is evidently different from
that of FBC–Cu2+ system as before-mentioned. This can be due to the
protonation of N atom in the C_N moiety of FBC under acid condition,
Figs. 3 and S1 (Supporting Information), again showing that the signifi-
cant fluorescence enhancement of FBC upon addition of Cu2+ may be
attributed to the inhibition of both PET and C_N isomerization process
[49].
cence OFF–ON sensor for Cu2+
.
2.4. Spectral sensing mechanism of FBC to Cu2+
To understand the sensing mechanism of FBC with Cu2+, the control
experiment with the reference FB was carried out under the same ex-
perimental condition as mentioned above. As shown in Fig. 1, the addi-
tion of Cu2+ (10 equiv.) into FB also results in an evident decrease in the
absorption of FB at 340 nm ascribed to the n–π* electronic transition
and an obvious enhancement in its fluorescence intensity, suggesting
that the binding of Cu2+ with the N atom of C_N moiety in FB can in-
hibit the photo-induced electronic transition (PET) and lead to the fluo-
rescence enhancement (ON state), which should be true for FBC [45,46].
Nevertheless, the maximum absorption of FB at 275 nm from π–π* elec-
tronic transition is found to be slightly red-shifted (6 nm) upon addition
of Cu2+, just an opposite shift relative to that of FBC, implying that
hydroxybenzene O atom of benzo-15-crown-5 moiety in FBC may be
coordinated with Cu2+ in FBC–Cu2+ system. Moreover, the fluores-
cence enhancement of FB exhibits an obviously weak extent in compar-
ison with that of FBC upon addition of Cu2+ (12-fold for FB vs 54-fold
for FBC). This may be attributed to nitrogen/oxygen-binding model of
FBC for Cu2+, which can partly suppress the isomerization of C_N moi-
ety and cause the fluorescence amplification of FBC [47,48]. In addition,
pH-dependent optical properties of FBC along with decreasing pH value
from 6.58 to 2.74 in MeOH/H2O (8:2) are highly similar to the titration
experiments of FB with Cu2+: The absorption of FBC at 284 nm from π-
π* electronic transition is red-shifted by ca. 6 nm, and the fluorescence
emission of FBC is found to be increased only in fluorescence intensity
To further evaluate the binding function of benzo-15-crown-5 in
FBC with Cu2+, the optical properties of the reference FBDMO upon ad-
dition of Cu2+ (10 equiv.) were investigated under before-mentioned
condition. Upon addition of Cu2+ (10 equiv.), FBDMO displays a
ratiometric-absorption (I273/I346) change in UV absorption spectrum
similar to FBC, Fig. 1. Meanwhile the fluorescence emission of this com-
pound is enhanced, but with an obviously weak degree in comparison
with that of FBC (29-fold vs 54-fold), Fig. 1. Moreover, the time-
dependent fluorescent experiments show that the saturation time for
FBC–Cu2+ system is less than FBDMO–Cu2+ complex, suggesting
more rapid chelating response of FBC to Cu2+, Fig. S2. These results
show that not only the hydroxybenzene O atom but the benzo-15-
crown-5 moiety in FBC as a binding platform is linked with Cu2+ in
FBC–Cu2+ system. Additionally, the absorption of FBC centered at
202 nm ascribed to the phenol unit is found to be red-shifted by
15 nm along with the increasing Cu2+ concentration (0–10 equiv.),
while the corresponding one for FBDMO keeps almost unchanged,
Fig. S3, which may be attributed to the parallel transition dipoles be-
tween hydroxybenzene moieties in the former, suggesting the proposed
sandwich complex between FBC and Cu2+, Fig. 4, [50,51], which is fur-
ther proved by the mass spectrum of FBC–Cu2+ system with the peak at
1186.14 for [2FBC + 2CuCl2 + H+]+, Fig. S4. This rigid sandwich struc-
ture between FBC and Cu2+, combined with the strong chelating ability

Y. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 161 (2016) 144–149
147
Fig. 3. The UV absorption and fluorescence emission spectra of FBC along with the pH change from 6.85 to 2.74 in MeOH/H2O (8:2), with the excitation of 290 nm.
to Cu2+, results in more fluorescence enhancement for FBC than for
FBDMO upon addition of Cu2+, Fig. 1B.
model between FBC with Cu2+ not only increases the chelating affinity
and selectivity of this compound to Cu2+ over all other tested metal
ions, but also induces high sensitive ratiometric-absorption and fluores-
cence OFF–ON response to Cu2+ via suppressing the PET and C_N
isomerization process in FBC, rendering this compound the dual-
2.5. Metal ion competitive experiments
It is worth noting that the UV absorption of FBC keep almost un-
channel detecting potential towards Cu2+
.
changed upon addition of other metal ion such as Li+, Na+, K+, Mg2+
,
,
Ba2+, Ca2+, Cd2+, Ni2+, Mn2+, Fe2+, Fe3+, Co2+, Hg2+, or Zn2+
4. Experimental section
Fig. 5A. This is also true for the fluorescent emission spectra of FBC,
except for Co2+ and Hg2+ cations which cause some fluorescence
enhancement of FBC but with an observably weak degree (2.1- and
1.9-fold for Co2+ and Hg2+, respectively) in comparison with that of
FBC–Cu2+ complex, Fig. 5B. Furthermore, addition of other tested
metal ions into FBC–Cu2+ system results in very minor and unremark-
able changes in both the UV absorption and fluorescent emission spec-
tra of this system, Figs. S5 and S6 (Supporting Information), suggesting
the excellent selectivity of FBC to Cu2+. In contrast, the electronic ab-
sorption and fluorescent emission spectra of FBDMO is found to be
closely dependent on metal ion added, in particular Co2+ and Fe3+
with significant fluorescence enhancement (5- and 24-fold, respective-
ly), Fig. S7 (Supporting Information). Moreover, the optical properties of
FBDMO–Cu2+ system were easily affected by other metal ion, Fig. S8
(Supporting Information). This is also true for FB, Figs. S9 and S10
(Supporting Information). These results show that the employment of
benzo-15-crown-5 moiety in FBC as additional binding platform for
Cu2+, associated with the N binding site of C_N moiety, not only am-
plifies the fluorescent enhancement of FBC via restraining the C_N
isomerization as mentioned above, but also increases the selectivity
and binding affinity of FBC to Cu2+ in comparison with the references
FBDMO and FB.
4.1. Material and methods
Column chromatography was carried out on silica gel (Merck,
Kieselgel 60, 70–230 mesh) with the indicated eluents. The solvent of
MeOH used for spectral experiments was purified via the standard
methods. Other reagents and solvents were used as received. 1H and
13C NMR spectra (1H—400 MHz and 13C—100 MHz) were recorded on
a Bruker DPX 400 MHz spectrometer in CDCl3 with shifts referenced to
SiMe4 (0.00 ppm). MALDI-TOF mass spectrum was taken on a Bruker
BIFLEX III ultra-high resolution Fourier transform ion cyclotron resonance
(FT-ICR) mass spectrometer with alpha-cyano-4-hydroxycinnamic acid
as matrix. Elemental analysis was performed on an Elementar Vavio El
Ш. UV absorption spectra were recorded on a U-4100 spectrophotome-
ter. Steady-state fluorescence spectroscopic studies were performed on
an F 4500 (Hitachi). The slit width was 5 nm for emission, and the photon
multiplier voltage was 500 V. Detection limit (DL) is defined as the con-
centration corresponding to a signal 3 times the noise level of the back-
ground. DL = (0.03 × RSDB) / (xA / c0), where RSDB is both the relative
standard deviation of the background expressed as a percent and the sen-
sitivity (the slope of the calibration curve of intensity versus composi-
tion), xA is the net analyte signal (i.e., the signal above background), and
c0 is the composition of the element in the sample.
3. Conclusion
4.2. Spectroscopic response experiments
Briefly summarizing above, one new benzo-15-crown-5-modified
fluorine derivative with a C_N bridge (FBC), together with two refer-
ences FBDMO and FB, has been developed. The nitrogen/BC-binding
Stock MeOH solutions (1 mM) of Li+, Na+, K+, Ba2+, Mg2+, Ca2+
,
Mn2+, Cd2+, Zn2+, Cu2+ and Hg2+ were prepared from the chloride
salts, of Co2+, Fe3+, and Ni2+ from the nitrate salt, and of Fe2+ from fer-
rous ammonium sulfate and used immediately. FBC as well as FBDMO
and FBC (2 mM) in MeOH was also prepared. Selectivity test solutions
were prepared by placing 0.1 mL of the probe's stock solution into a
test tube (10 mL), adding an appropriate aliquot of each metal stock so-
lution and then diluting the obtained mixture with MeOH to give the
final concentration. The spectral titration experiments were accom-
plished through a stepwise addition of Cu2+ (from 0 to 10 equiv.) to
the probe's stock solution (2 × 10−5 M) in MeOH in 10 mL volumetric
flask. Job's plot was plotted using equimolar concentration of FBC and
Cu2+, with the total concentration of FBC and Cu2+ being a constant
of 2.0 × 10−5 M. Competitive experiment solutions were obtained by
Fig. 4. The proposed sandwich complexing model between FBC and Cu2+
.

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Y. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 161 (2016) 144–149
Fig. 5. The UV absorption (A) and fluorescence emission spectra (B) of FBC in MeOH upon addition of Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Cd2+, Ni2+, Mn2+, Fe2+, Co2+, Hg2+, Fe3+, Cu2+, or
Zn2+, respectively, with the excitation of 290 nm.
successive addition of Cu2+ and other texted metal ion into the FBC
solution (2 × 10−5 M) with the ratio being 10:10:1 in MeOH. After
completely mixing for 1 h, measurements of UV–vis absorption and
fluorescent emission were carried out at the above mentioned spectro-
photometers with a 1 cm standard quartz cell.
2H, J = 14.8 Hz), 7.65 (s, 1H), 7.55 (d, 1H, J = 3.6 Hz), 7.42 (s, 1H),
7.40 (t, 1H, J = 14.2 Hz), 7.34(d, 1H, J = 8 Hz), 7.31 (d, 1H, J = 4 Hz),
7.29 (d, 2H, J = 12 Hz), 4.01 (s, 3H); 3.96 (s, 3H); 3.93 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ159.5, 152.4, 152.0, 149.1, 144.8, 143.6,
141.7, 138.9, 130.1, 127.1, 125.6, 125.3, 124.9, 120.5, 120.4, 120.1,
116.9, 113.7, 112.6, 57.3, 37.3; MS (MALDI-TOF): an isotopic cluster
peaking at m/z 329.24, [Calcd. for (MH)+329.19]; Anal. Calcd for
C22H19NO2: C, 88.22; H, 5.81; N, 4.25. Found: C, 88.05; H, 5.42; N, 4.34.
4.3. Preparation of benzo-15-crown-5-modified fluorine Schiff-base (FBC)
The compound of p-benzo-15-crown-5-benzaldehyde was prepared
according to the literature procedures [37]. Then new prepared p-
benzo-15-crown-5-benzaldehyde (60 mg, 0.2 mmol) was dissolved in
EtOH (20 mL), followed by the addition of 2-aminofluorene (36 mg,
0.2 mmol) and one drop of glacial acetic acid. The resulting reaction
mixture was refluxed for 6 h under N2 and then cooled to room temper-
ature. The white precipitate was filtrated and washed by EtOH. Recrys-
tallization from CHCl3 and MeOH yielded pure sample of FBC as white
powder, 71 mg, 78%. 1H NMR (CDCl3, 400 MHz) δ 8.44 (s, 1H), 7.80 (t,
2H, J = 16 Hz), 7.61 (s, 1H), 7.55 (d, 1H, J = 8 Hz), 7. 40 (t, 2H, J =
12 Hz), 7.33 (t, 2H, J = 16 Hz), 7.25 (d, 1H, J = 8 Hz), 6.93 (d, 1H, J =
8 Hz), 4.26 (d, 4H, J = 24 Hz), 3.95 (s, 6H), 3.79 (s, 8H); 13C NMR
(100 MHz, CDCl3) δ159.5, 152.4, 151.5, 149.8, 144.8, 143.7, 141.8,
139.9, 130.3, 127.2, 126.7, 125.3, 124.9, 120.7, 120.5, 120.0, 117.9,
112.9, 111.7, 71.6, 71.5, 70.8, 70.7, 69.8, 69.7, 69.2, 69.1, 37.3; MS
(MALDI-TOF): an isotopic cluster peaking at m/z 459.18, [Calcd. for
(MH)+459.13]; Anal. Calcd for C28H29NO5: C, 72.18; H, 6.36; N, 3.05.
Found: C, 72.55; H, 6.72; N, 2.94.
Acknowledgment
Financial support from the Natural Science Foundation of China
(21371028), the Scientific and Technological Project of Shandong Prov-
ince (2014GGH217002), Science Foundation of Beijing Municipal Re-
search Institute of Environment Protection (2014-A-09) is gratefully
acknowledged.
Appendix A. Supplementary data
Synthesis and characteristics of FBC as well as FBDMO and FB; the
UV absorption and fluorescence emission spectra of FBC upon addition
of HCl; the UV absorption and fluorescence emission spectra of FBC–
Cu2+ system, as well as FBDMO and FB upon addition of different
metal ions in MeOH. This materials can be found in the online version,
at http://dx.doi.org/10.1016/j.saa.2016.02.041
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