Visual colorimetric and fluorescence turn-on probe for Cu(II) ion based on coordination and catalyzed oxidative cyclization of ortho amino azobenzene

Article abstract of DOI:10.1016/j.ica.2018.12.023

2,2′-diaminoazobenzene (L1), a visual colorimetric and fluorescence turn-on detection probe for Cu²⁺ ion, has been discovered successfully. The yellow L1 solution changed into light-purple solution rapidly with dropping of Cu²⁺ ion observed by the naked eyes, for the formation of 1:1 complex of L1 and Cu²⁺ ion calculated by Job-plot method. Non-emissive L1 transformed into 2-(2-aminophenyl) benzotriazole (L1-BTA) with strong fluorescence at the peak around 475 nm, owing to the Cu²⁺ catalyzed oxidative cyclization of L1, accompanied by the fluorescence intensity gradually increasing within 20 min. And L1 was applied for quantitative detection of Cu²⁺ with good linear relationship between its UV absorbance and Cu²⁺ concentration, and exhibited high fluorescence selectivity toward Cu²⁺ over other metal ions (Fe³⁺, Zn²⁺, Cd²⁺, Co²⁺, Ni²⁺, Mn²⁺, Na⁺, Al³⁺ and Ca²⁺) in EtOH solution.

Full text of DOI:10.1016/j.ica.2018.12.023

InorganicaChimicaActa487(2019)234–239
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Inorganica Chimica Acta
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Research paper
Visual colorimetric and fluorescence turn-on probe for Cu(II) ion based on
coordination and catalyzed oxidative cyclization of ortho amino azobenzene
Ren Hong⁎, Wu Ping, Li Fei, Jin Li, Lou Dawei
School of Chemistry and Pharmaceutical Engineering, Jilin Institute of Chemical Technology, Jilin City 132022, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
2,2′-diaminoazobenzene (L1), a visual colorimetric and fluorescence turn-on detection probe for Cu2+ ion, has
been discovered successfully. The yellow L1 solution changed into light-purple solution rapidly with dropping of
Cu2+ ion observed by the naked eyes, for the formation of 1:1 complex of L1 and Cu2+ ion calculated by Job-
plot method. Non-emissive L1 transformed into 2-(2-aminophenyl) benzotriazole (L1-BTA) with strong fluor-
escence at the peak around 475 nm, owing to the Cu2+ catalyzed oxidative cyclization of L1, accompanied by
the fluorescence intensity gradually increasing within 20 min. And L1 was applied for quantitative detection of
Cu2+ with good linear relationship between its UV absorbance and Cu2+ concentration, and exhibited high
fluorescence selectivity toward Cu2+ over other metal ions (Fe3+, Zn2+, Cd2+, Co2+, Ni2+, Mn2+, Na+, Al3+
and Ca2+) in EtOH solution.
Fluorescence probe
Amino azobenzene
Coordination
Oxidative cyclization
Cu(II) ion
1. Introduction
the recognition mechanism [8–11]. The “on–off” fluorescent chemo-
sensor was relatively easy to achieve due to the intrinsic paramagnetic
Copper is the third largest essential trace element of human body
only less than zinc and iron, being required by many living organisms
for normal physiological processes. However, excessive levels of Cu2+
are highly toxic to organisms [1], and Cu2+ is a significant environ-
mental pollutant as well throughout the world due to its widely ap-
plications in industry, agriculture, and the daily life of men [2]. Thus,
WHO has set regulations to guarantee the health of people that limit for
Cu2+ in drinking water is 30 μM [3]. For the detection of trace amounts
of Cu2+ in biological and environmental implications, it is very im-
portant to develop more sensitive, convenient and reliable methods.
The fluorescent probes possess the merits of sensitivity, low price,
simple operation, good reproducibility and non-destruction, which has
more advantages than the traditional methods of inductively coupled
plasma mass spectroscopy, high-performance liquid chromatography
mass spectrometry, atomic absorption spectroscopy, and electro-
chemical methods [4]. The design and synthesis of small molecule
fluorescent probes for the efficient detection of Cu2+ ions have at-
tracted considerable attention in the fields of biochemistry, toxicology,
environmental science and so on [5–7].
nature of Cu2+, exhibiting fluorescence quenching to a certain extent
[12]. But the “on–off” fluorescence quenching might occur in some
particular environment as illusions and could offer false positive results
[8]. In contrast to the “on–off” fluorescent Cu2+ chemosensor, the
“off–on” probes can interact with Cu2+ ions to give the enhancement of
fluorescence intensity, which are more attractive than “on–off” ones.
The series of “off–on” fluorescent probes for copper testing have been
synthesized, containing rhodamine [13], coumarin [14], boron di-
pyrromethene [15], fluorescein [16], 8-hydroxyquinoline [17], and
Schiff base derivatives [18] and so on. In addition to the above com-
pounds, azobenzene has emerged as signaling unit in fluorescence
probes, employing by various coordinating groups which give rise to
different spectral responses depending upon their electronic properties
[19]. For example, Lee et al. have synthesized a series of azo anilines
with different electron- donating units, which can give application in
the detection of Cu2+ with “turn-on” fluorescence due to the Cu2+
-
catalyzed cyclization reaction [20]. In 2018, a fluorescence probe,
named 3,3′-[biphenyl-4,4′-diyldi(E) diazene-2,1-diyl] bis (4-amino-
naphtha-lene) (BDA), which can be used as a selective fast, and re-
versible probe for detect Cu2+ through fluorescent enhancement at
407 nm [21]. The BDA has shown potential applications in water
sample monitoring recovery (%) closed to 100% and low RSD (below
3%), exhibiting prospect of application in environmental monitoring
Up to now, a great number of fluorescent chemosensors with kinds
of molecular structures and different recognition mechanisms have
been reported for the detection of copper ions, which can be classified
into two mainly categories: “on–off”, “off–on” chemosensors based on
⁎ Corresponding author.
E-mail address: renhong_jl@163.com (H. Ren).
https://doi.org/10.1016/j.ica.2018.12.023
Received 15 September 2018; Received in revised form 5 December 2018; Accepted 11 December 2018
Availableonline12December2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

H. Ren et al.
InorganicaChimicaActa487(2019)234–239
[21]. Generally speaking, the interaction of azobenzene and its deri-
vatives with Cu2+ ions is often accompanied by obvious color changes,
making visual inspection more ideal [22–25].
ability of L1 to Cu2+, the addition of EDTA to L1 and Cu2+ system
showed that the process of titrating L1 with EDTA was reversible at
several times (Fig. 2b), reflecting the chelation Cu2+ with L1. Con-
taining two coordination groups located at the ortho position of azo
bonds as L1, the azo fluorescein recognized Zn2+ metal center with a
1:1 binding mode [27] and fluoresced through the photoinduced elec-
tron transfer mechanism. However, in the presence of Cu2+, no marked
enhanced fluorescence change of L1 can be observed due to the para-
magnetic nature of Cu2+. Although no fluorescence of L1 solution was
observed after adding Cu2+, the purple solution faded to colorless after
tens of minutes, and showed strong fluorescence after 10 h (Fig. S2).
Due to the inspiration sparked by relative studies and strong re-
quirements for Cu2+ ion detection, we developed a fluorescence probe
for Cu2+ detection based on a typical azo compound, 2,2′-diaminoa-
zobenzene (L1). A “turn-on” fluorescence and good selectivity were
envisioned via the coordination process involving the chromophore and
metal ions, and the Cu2+-promoted oxidative cyclization reaction. The
performance for Cu2+ detection and the reaction mechanism of probe
L1 were investigated in detail.
Interestingly, these changes in the UV spectra were not found in Fe3+
,
Zn2+, Cd2+, Co2+, Mn2+, Na+, Al3+ and Ca2+ ions (Fig. S3). But in
the case of Ni2+, the formation of 1:1 complex of L1 and Ni2+ has
appeared, which is also accompanied by the visual color change (from
yellow to grey green) (Fig. S4). However, the solution of Ni2+ with L1
has been placed overnight, exhibiting no fluorescence to a certain de-
gree of surprising.
2. Results and discussion
The probe L1 was gained through the oxidation of o-diamino-
benzene by potassium superoxide as reported procedure [26]. The UV
absorbance spectrum of L1 was measured in EtOH solution at a final
concentration of 10–100 µM, revealing the strong band with two ab-
sorption peaks at 453 nm and 312 nm, a linear relationship was ob-
served between the absorption intensity and concentration of com-
pound L1 with ε ≈ 1.35 × 104 L·mol−1·cm−1 (Fig. S1). The big value of
ε can make chromogenic reaction between L1 and Cu2+ more sensitive.
UV spectroscopic property of probe L1 (83.3 µM) and its response to Cu
(II) ion were investigated at room temperature. Upon the addition of
Cu2+, an apparent color change from yellow to light-purple could be
easily observed by the naked-eye (inset, Fig. 1a), which indicated a
good response of L1 towards Cu2+. Simultaneously, the UV–Vis ab-
sorption titration experiment of L1 with various concentrations of Cu2+
in EtOH solution was performed. As shown in Fig. 1a, with increasing
Cu2+ concentration, the absorption peaks at 312 and 453 nm gradually
decreased and the new absorption peaks at 350 and 516 nm gradually
enhanced with isosbestic points at about 326, 383 and 487 nm. A plot of
absorbance at 516 nm of L1 vs Cu2+ concentration (16.7–83.3 µM)
showed a linear relation, hence the quantitative detection of Cu2+ via
UV–Vis absorption is available (Fig. 1b). The limit of detection (LOD) in
UV–Vis is close to 16.7 µM for Cu2+ ion (A = 0.253), the absorption (A)
was just greater than 0.2 to ensure the accuracy of measurement. The
absorption spectra remained unchanged when more than 1.0 eq of
Cu2+ was added, indicating the formation of a 1:1 (M:L) complex in
solution [27]. The mode of coordination of the L1 (83.3 µM) with the
Cu(II) ions was further investigated by absorption spectrophotometric
titration, the 1:1 stoichiometry of the complexation L1 to Cu2+ was also
supported by Job-plot method (Fig. 2a). To further prove the response
As following, the fluorescence spectra of L1 was measured at a
concentration of 1.0 µM with excitation wavenumber of 369 nm,
showing no obvious emission peaks in the 370–600 nm region. In the
fluorescence titration experiment, L1 was subjected to excitation at
369 nm and monitored after each stepwise addition of Cu2+ ions to the
solution. A gradual enhancement of the fluorescence intensity was
observed at 473 nm with the equiv of Cu2+ ion to L1 from 1.0 to 25.0,
and the mixture of L1 and Cu(II) ion showed fluorescently blue-green,
which has distinction of L1 solution (Fig. 3). The limit of detection
(LOD) for Cu2+ ion is 0.648 µM calculated by the equation LOD = (3.3
σ/k), in which σ is the standard deviation of the y-intercepts of the
regression lines and k is the slope of the calibration graph. The reaction
mechanism of fluorescence probes for Cu2+ has been explained by two
main types of probes, one is ‘‘complexation” probes and the other is
Cu2+-promoted ‘‘reactive” probes. The ‘‘complexation” probes con-
taining N, O, and S atoms in their molecular structures to coordinate
with metal center, are usually reversible with the addition of Cu2+
complexing ligands, such as EDTA and S22− [28]. The ‘‘reactive” probes
are noted for their high selectivity, including Cu2+-promoted reaction
like hydrolysis, oxidation and reduction and other relative reaction
processes. According to the reported literatures, azo aniline is a typical
recognition group for Cu2+ detection [29,30]. In this study, the Cu2+
-
catalyzed oxidative cyclization mechanism was also proposed for L1
(Fig. 4). L1 contains a N]N bond which can reversibly isomerize be-
tween trans and cis configurations at the cost of consuming energy of
Fig. 1. UV spectrophotometric titrations of L1 (16.7 µM) with various numbers of equivalents of Cu(II) ion in EtOH solution. (Insert a: colour change of L1 and with
Cu2+; b: The linear relationship of Abs vs different concentrations of the Cu2+ ion at 516 nm). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
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H. Ren et al.
InorganicaChimicaActa487(2019)234–239
Fig. 2. (a) The UV titration profiles for Job’s method to confirm the 1:1 equivalent ratio of the M:L binding stoichiometry of the complex in EtOH under 516 nm; (b)
UV adsorption switching cycles of L1 (83.3 µM) controlled by alternating addition of EDTA and Cu2+ in EtOH solution.
the molecule, resulting in no fluorescent emission. In order to confirm
the sensing mechanism of L1, 2-(2-aminophenyl) benzotriazole (L1-
BTA) was synthesized with Cu(II) as an oxidant, which has shown the
emission peak at 475 nm (Fig. 5). Compared with the fluorescence
spectra of “L1 + Cu2+” and L1-BTA, the position of the peak is similar
though a bit red shift about 2 nm which can afford demonstration of
Cu2+-catalyzed oxidative cyclization mechanism. Moreover, we in-
duced EDTA equivalent to Cu2+ to the fluorescent solution
(L1 + Cu2+), and fluorescence intensity was still unchanged (Fig. S5a),
reflecting that the reaction preferred catalysis rather than chelation in
fluorescence spectra [31]. The product of the reaction between L1 and
Cu2+ was analyzed by High performance liquid chromatography
(HPLC) (Fig. S5b). Under the pretreatment and chromatographic con-
ditions, the retention time of L1 and L1-BTA was 8.59 and 12.92 min
respectively, and most of L1 has been transformed into L1-BTA. On the
other hand, the solution (L1 + Ni2+) has no fluorescence with the ex-
citation wavelength at 369 nm, indicating the emission of solution
(L1 + Cu2+) is dominated by Cu2+-catalyzed. Upon being exposed to
Cu2+, N]N isomerization was suppressed by selective catalytic cycli-
zation to triazole which are also confirmed by the same type of probes
[1]. The EPR spectroscopy was introduced to get light into the me-
chanism (Fig. 6). The g = 2.171 of red line can be attributed to Cu2+
,
Fig. 4. Proposed mechanism of the probe L1 to Cu2+ ion.
while two peaks of blue line owing to the reaction of Cu2+ with L1
(g = 2.089) and the decreasing concentration of Cu2+ (g = 2.241),
respectively.
Fig. 3. (a) Fluorescence spectra of L1 with addition Cu2+ in EtOH solution. Insert: color change of L1 and with Cu2+ under UV lamp. (b) Fluorescence spectra
changes of L1 (1.0 µM) in the presence of different concentrations of Cu2+ in EtOH solution (0.0–25.0 equiv). (Insert b: The linear relationship of relative fluor-
escence intensity vs different concentrations of the Cu2+ ion at 473 nm).
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InorganicaChimicaActa487(2019)234–239
Fig. 7. Time-dependent changes in the fluorescence intensity at λ = 473 nm
(λexc = 369 nm) observed for the reaction between L1 (30.0 µM) and Cu2+
(2.7 × 10−3 mol·L−1) in EtOH solution at room temperature.
Fig. 5. Black solid line: fluorescence spectra of L1 with addition Cu2+ in EtOH
solution; Red dotted line: the curve of benzotriazoles product of L1.
Fig. 8. Fluorescence intensity changes of the L1 (1.0 µM) to Cu2+ (3.3 µM) in
the presence of various teat cations (10 equiv) in EtOH solution at λ = 473 nm
(λexc = 369 nm).
Fig. 6. ESR spectra of CuSO4·5H2O (red line) and reaction product of
CuSO4·5H2O and L1 solid powders by grinding (blue line). (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)
One of the most indispensable qualities of a fluorescent probe is the
high selectivity toward the specific analyte over other relevant species.
To evaluate the selectivity of L1 (1.0 μM) for Cu2+ (3.3 μM), a variety of
metal ions (10.0 μM) including Fe3+, Zn2+, Cd2+, Co2+, Ni2+, Mn2+
Na+, Al3+ and Ca2+ were examined under the same condition as Cu2+
,
.
Besides, the reaction kinetic behavior between L1 and Cu2+ was
investigated through time-dependent fluorescence spectra. As shown in
Fig. 7, the fluorescent intensity at 473 nm of the tested solution in-
creased quickly within 10 min. Therefore, probe L1 could be applied in
real-time determination of Cu2+ in environmental and biological con-
ditions. The enhancement rate slowed down and finally remained al-
most unchanged after 20 min. Therefore, the detection of Cu2+ in EtOH
solution through Cu2+-catalyzed cyclization reaction could be accom-
plished within 20 min at room temperature. The kinetic studies as re-
ported [20] have established that the reactivity rate of cyclization re-
action is corresponding to the number of amine group introduced to the
benzene ring. The diamino groups of L1 has accelerated the oxidative
cyclization reaction in EtOH compared with the monoamine azo-
benzene. Very recently, a rhodamine-triazole (RHT) fluorescent chemo
dosimeter for Cu2+ detection has been published, the binding RHT and
Cu2+ finished within 60 min, which is much longer than L1 [32]. It is
concluded that the reaction rate of transforming azo anilines to ben-
zotriazoles has the advantages than RHT derivatives for Cu(II) ions in
specific conditions.
As shown in Fig. 8, only addition of Cu2+ resulted in a dramatic
fluorescence enhancement of the solution of L1, while other metal ions
did not lead to obvious fluorescence. Furthermore, the performance of
L1 in Cu2+ detection in the presence of other competitive species was
also tested. The competitive experiments showed that these metal ions
displayed no evident interference in the fluorescence increase caused by
Cu2+. The above results indicate that the selectivity of L1 for Cu2+ in
EtOH system is reasonably high.
In summary, we have successfully reported and characterized a vi-
sual colorimetric and fluorescence turn-on probe L1, which affords an
example of application on known amino azobenzene. The L1 is capable
of recognizing Cu2+ over other metal ions (Fe3+, Zn2+, Cd2+, Co2+
,
Ni2+, Mn2+, Na+, Al3+ and Ca2+), and rapid colorimetric in UV and
fluorescence turn-on detection can be achieved, showing a visible color
change form yellow to light-purple and a significant fluorescence en-
hancement, respectively. The L1 can coordinate to Cu2+ as UV spec-
trum, forming a 1:1 complex conformed by Job-plot analyses and
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InorganicaChimicaActa487(2019)234–239
relative chemical studies. It is worthy that both the coordination and
catalytic reaction can be observed within the interaction of L1 to metal
ions, the “turn-on” chemosensor of L1 for Cu2+ are based on oxidative
cyclization of azo aromatics. To further study for fluorescence probe
needs relative research such as the preparation of amino azobenzene
derivatives and the much subsequent work is undergoing in our lab.
5 × 10−3 mol·L−1. During the UV procedure to obtain C-A relationship,
the original L1 solution were added 5, 10, 15, 20, 25, 30, 35, 40, 45,
50 μL per time in 2.5 mL alcohol solution. UV spectral titration of azo
compound solution with metal salts: Initial solution is composed of
0.2 mL 1.25 × 10−3 mol·L−1 L1 in 3.0 mL alcohol solution. Then the
initial solution was gradually added by salt solution (5 × 10−3
mol·L−1) by 10 μL per time. The formation of a 1:1 complex in solution
can be achieved by mixing the Cu(II) salt and L1 solution with same
concentration (1.25 × 10−3 mol·L−1), the volume is from 0 μL to
150 μL (λ = 516 nm). UV absorption switching cycles of L1 (50.0 µM)
controlled by alternating addition of EDTA and Cu(II) salt solution
12.5 μL per time with same concentration (0.02 mol·L−1). The 2.0 mL
“Cu2+ + L1” solution (molar ratio of Cu2+:L1 = 1:1, c0 = 83.3 µM) are
placed overnight to exhibit fluorescence emission.
3. Experimental section
3.1. Materials and methods
All the chemicals were of analytic grade and used as received. Water
used was distilled. NMR spectra were collected on a Bruker-400 spec-
trometer with chemical shifts reported as parts per million (ppm; in
DMSO‑d6, TMS as internal standard). Mass spectra were determined
with a Thermo mass spectrometer. IR spectra were recorded on a
Nicolet Impact 410 spectrometer between 400 and 4000 cm−1, using
the KBr pellet method. UV–vis spectra were obtained with a Cary 60
spectrophotometer of Agilent Technologies. The fluorescence data were
recorded on a Perkin-Elmer LS 45 luminescence spectrometer. For all
luminescence measurement, excitation and emission slit widths of
10 nm were used. The wave number of excited light for UV lamp is
365 nm. Electron Spin Resonance Measurement. Electron paramagnetic
resonance (EPR) measurements were performed at RT using A300-10/
12 X-band spectrometer (Bruker) operating at 9.84 GHz. The solid
sample was placed into a quartz tube (Wilmad Lab Glass), and the
quartz tube was put in the resonator cavity. The gyromagnetic factor (g)
was calculated by the relationship hν0 = gβH, where h is the Planck
constant (h = 6.626 × 102−34 J s), ν0 is the microwave frequency, β is
the Bohr magnetron (β = 9.262 × 10−24 J/T), H is the magnetic field
strength. High performance liquid chromatography (HPLC) analysis
was carried out on a Shimadzu LC-20AB system (Kyoto, Japan)
equipped with a CTO-10A Scolumn oven and a SPD-M20A detector. The
chromatographic separation of analysts was performed on a VP-ODS
C18 column (4.6 mm × 250 mm, 5 μm). The mobile phase used was
methanol/water (70: 30). Its flow rate was 1.0 mL/min and the detec-
tion wavelength was monitored at 280 and 311 nm for L1 and L1-BTA
respectively. The injection volume was 20 μL.
The fluorescence spectroscopy of azo compound were recorded
upon the addition of metal salts while keeping the L1 concentration
constant (1.0 µM) in sensing experiments. The solution of benzotriazole
products composed of 1.0 µM L1-BTA within 2.0 mL alcohol to undergo
the florescence testing.
Acknowledgments
This work is supported by the scientific research fund of Jilin
Institute of Chemical Technology and the National Nature Science
Foundation of China (No. 21375046, 21405058). The authors are also
thankful to Pr. LU Dayong, Key Laboratory for Special Functional
Materials in Jilin Provincial Universities, Jilin Institute of Chemical
Technology for measurement and discussion on EPR.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2018.12.023.
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