A novel jointly colorimetric and fluorescent sensor for Cu2+ recognition and its complex for sensing S2? by a Cu2+ displacement approach in aqueous media

Article abstract of DOI:10.1016/j.saa.2018.06.068

In this work, a simple and easily synthesized Schiff-based derivative colorimetric and fluorescent sensor (1), 4-dimethylamino-benzoic acid (2-imidazole formaldehyde)-hydrazide, was obtained for the detection of Cu²⁺ and S^2?. The compound 1 exhibited dual spectral responses to Cu²⁺, that is, vivid color change and fluorescence enhancement in the presence of Cu²⁺. The detection limits were valued as 0.46 μM and 15 nM according to absorption and fluorescent response, respectively. Both of them are below the World Health Organization (WHO) guidelines for drinking water (31.5 μM). In addition, the ensemble (1-Cu²⁺) selectively and sensitively detected a low concentration of S^2?. As the addition of S^2? instantly removed Cu²⁺ from the ensemble (1-Cu²⁺) resulting in a color change from yellow to colorless and a “turn-off” fluorescent response. The detection limit for S^2? was estimated as 0.12 μM (from fluorescent method) and 0.68 μM (from absorption method), respectively, each of which was also lower than the maximum allowable level of S^2? (15 μM) in drinking water defined by the WHO. The binding process was confirmed via UV–vis absorption, fluorescence measurements, ¹H NMR, mass spectroscopy and density functional theory calculation. What's more, successful practical application of test paper is used to inspect the S^2? which means the convenient and rapid assay in real samples can be achieved.

Full text of DOI:10.1016/j.saa.2018.06.068

Accepted Manuscript
A novel jointly colorimetric and fluorescent sensor for Cu2+
recognition and its complex for sensing S2− by a Cu2+
displacement approach in aqueous media
Hao Fang, Peng-Cheng Huang, Fang-Ying Wu
PII:
DOI:
Reference:
S1386-1425(18)30610-3
doi:10.1016/j.saa.2018.06.068
SAA 16227
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 15 March 2018
Accepted
18 June 2018
date:
Please cite this article as: Hao Fang, Peng-Cheng Huang, Fang-Ying Wu , A novel
jointly colorimetric and fluorescent sensor for Cu2+ recognition and its complex for
sensing S2− by a Cu2+ displacement approach in aqueous media. Saa (2018), doi:10.1016/
j.saa.2018.06.068
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ACCEPTED MANUSCRIPT
A novel jointly colorimetric and fluorescent sensor for Cu2+ recognition and its
complex for sensing S2- by a Cu2+ displacement approach in aqueous media
Hao Fang, Peng-Cheng Huang, Fang-Ying Wu
College of Chemistry, Nanchang University, Nanchang 330031, China
Abstract: In this work, a simple and easily synthesized Schiff-based derivative colorimetric and
fluorescent sensor (1), 4-dimethylamino-benzoic acid (2-imidazole formaldehyde)-hydrazide, was
obtained for the detection of Cu2+ and S2-. The compound 1 exhibited dual spectral responses to
Cu2+, that is, vivid color change and fluorescence enhancement in the presence of Cu2+. The
detection limits were valued as 0.46 μM and 15 nM according to absorption and fluorescent
response, respectively. Both of them are below the World Health Organization (WHO) guidelines
for drinking water (31.5 μM). In addition, the ensemble (1-Cu2+) selectively and sensitively
detected a low concentration of S2-. As the addition of S2- instantly removed Cu2+ from the
ensemble (1-Cu2+) resulting in a color change from yellow to colorless and a “turn-off”
fluorescent response. The detection limit for S2- was estimated as 0.12 μM (from fluorescent
method) and 0.68 μM (from absorption method), respectively, each of which was also lower than
the maximum allowable level of S2- (15 μM) in drinking water defined by the WHO. The binding
1
process was confirmed via UV-vis absorption, fluorescence measurements, H NMR, mass
spectroscopy and density functional theory calculation. What's more, successful practical
application of test paper is used to inspect the S2- which means the convenient and rapid assay in
real samples can be achieved.
Keywords: 4-Dimethylamino-benzoic acid hydrazide derivative, colorimetry, fluorescence, Cu2+,
S2-
Corresponding author: Fang-Ying Wu, Tel: + 86 7913969882(O). E-mail address: fywu@ncu.edu.cn
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1. Introduction
Biological important ions, such as metal ions and anions, play indispensable roles in cell
biology. Keeping the integrity and homeostasis of these species would be very significant for
physiological and pathological processes [1]. Among various metal ions, copper is an essential
trace element and plays important roles in many biological systems [2]. As the third plentiful
essential trace element found in the human physiology, copper is indispensable for carrying out
several necessary processes and plays diverse parts in human physiopathology [3]. However, excess
of copper causes oxidative stress and related symptoms, which may lead to diabetes and many
neurodegenerative disorders such as Parkinson's [4], Alzheimer's [5], Wilson's [6], and Menke's
diseases [7]. Therefore, the development of ideal chemosensors for the monitoring of Cu2+ with
high selectivity, low detection limit, and rapid response is highly needed [8, 9].
As a toxic traditional pollutant, sulfide can be released during industrial processes and also
formed in biological systems due to the formation of the sulfur-containing amino acids in meat
proteins or microbial reduction of sulfate by anaerobic bacteria [10, 11]. The protonated forms,
HS- and H2S, are even more toxic than sulfide itself [12]. Sulfide can damage the human
respiratory and nerve systems, causing people to lose consciousness or even die at a very low
concentration (ppm level) [13]. On the other hand, some studies have shown that sulfide
participates in many physiological processes, such as vasodilation, angiogenesis, neuromodulation
regulation of inflammation and apoptosis [14]. Nevertheless, abnormal sulfide production is linked
to human diseases such as hypertension, Down's syndrome, Alzheimer's disease, as well as liver
cirrhosis [15]. So, the quantitative detection of S2- is of great importance for both environmental
and biological systems.
Owing to distinct advantages of simplicity, sensitivity, selectivity, and low cost [16-18], as
well as the applicability in rapid tracking of targets in toxicological, environmental and biological
samples [19-21], optical chemosensors like fluorescent and colorimetric ones, have received
considerable attention for the detection of Cu2+ or S2-. Nevertheless, the development of the
chemosensors capable of sequential detection both of Cu2+ and S2- is still in high demand. More
importantly, probes with colorimetric and fluorescent feature to accurately monitor the two targets
both by the naked eye and fluorescent microscopy are rarely reported.
In recent years, the displacement methods using the Cu2+-based chemosensing ensembles
have been widely used for highly selective detection of anions just like S2- [22, 23]. In these
displacement methods, the sensor itself displayed a specific recognition toward Cu2+ with a
change in the color or in the fluorescent intensity by forming a stable ensemble with Cu2+. And
then Cu2+ will be removed from the ensemble due to the formation of more stable complex
[Cu(S)x]n- with the addition of S2-. However, most of the reported displacement methods using the
Cu2+-based ensembles showed limitations to some aspects, including time-consuming synthetic
-
procedures [24], and the interference from the other sulfur-containing anions like HSO3 [25].
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Thus, it is still challenging to develop appropriate jointly colorimetric and fluorescent
chemosensors for Cu2+ recognition and S2- sensing with Cu2+-based ensemble.
In the present work, as shown in Scheme 1, a p-dimethylaminobenzamide derivative 1 was
used for colorimetric and fluorescent detection of Cu2+ and subsequently, its Cu2+-based ensemble
can selectively detection S2-. Generally, p-dimethylaminobenzamide and its derivatives have a
wide range of applications including medicine [26], anticorrosion [27] and sensing [28]. In order
to meet the demand of the recognition of different ions and testing, we can use various kinds of
aldehydes and ketones as raw materials and synthesize different types of Schiff-base compound.
The chemosensing ensemble we prepared showed several promising sensing properties: (1)
The sensor for detection of Cu2+ was based on fluorescence enhancement with the almost zero
background, which is very different existing reports based on paramagnetic nature of Cu2+; (2)
Highly selective and sensitive detection of Cu2+ and S2- over the other anions in aqueous solutions
were achieved by colorimetric and fluorescence detection methods; (3) The Schiff-based derivative
as the probe was synthesized easily with a high yield. To the best of our knowledge, this is an
appropriate example of a Schiff-based system for sequential detection both of Cu2+ and S2- in
aqueous solutions.
Scheme 1 The synthesis process of compound 1 and binding model among 1, Cu2+ and S2-.
2. Experimental
2.1. Materials and instrumentation
All reagents and solvents were used without purification. Imidazole-2-carboxaldehyde,
4-dimethylamino-benzoic acid ethyl ester and hydrazine hydrate were bought from Aladdin
Reagent Holding Limited Liability Company (Shanghai, China). Inorganic salts including AgNO3,
AlCl3, Ba(NO3)2, Cu(NO3)2, Ca(NO3)2 CdCl2, CrCl3, CoCl2, FeCl3, HgCl2, KNO3, Mg(NO3)2,
NaCl , NiCl2, Pb(NO3)2 and ZnCl2 were purchased from Qing Xi Technology Limited Liability
−
Company (Shanghai, China). Other anionic (Ac−, Br−, Cl−, F−, HCO3 , HPO42−, I−, S2O32−, SCN−)
and some compounds (EDTA, CH4N2S, GSH, Hcy, Cys) were purchased from Elaboration Institute
of Technology (Shanghai, China). All aqueous solutions were prepared using ultrapure water from
a Milli-Q system.
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Absorption measurements were carried out on Shimadzu-2550 UV-vis spectrophotometer
(Shimadzu, Japan) using a 1.0 cm quartz cell. Fluorescence spectra were recorded on F-4600
spectrofluorimeter (Hitachi, Japan) equipped with a xenon lamp source and a 1.0 cm quartz cell,
the scan speed was 12000 nm min-1 and the band pass of excitation and emission was set as 5 and
2.5 nm, respectively, and voltage was 700 V. ESI-MS data were recorded on a Waters
ZQ4000/2695 mass spectrometer (Waters, America). NMR spectra were recorded on an Agilent
Technologies 400/54 Annual Refill (Bruker BioSpin, America) with DMSO-d6 as the solvent and
tetramethylsilane (TMS) as internal standard. Elemental analyses (C, H and N) were performed on
an Elementary Vario EL analyzer (Elementar, Germany).
2.2 Synthesis of compound 1
As shown in Scheme 1, the reflux of 4-dimethylamino-benzoic acid ethyl ester (0.3584 g, 2
mmol) and hydrazine hydrate (85%,0.5 ml) in 25 mL of ethanol solution were removed until the
complete consumption of imidazole-2-formaldehyde, which was monitored by TLC (about 16 h).
4-Dimethylamino-benzoic acid hydrazide (0.1791 g, 1.0 mmol) and imidazole-2-formaldehyde
(0.096 g, 1.0 mmol) were solved in 25 mL ethanol and kept refluxing until complete consumption
of imidazole-2-formaldehyde, which was monitored by TLC (about 12 h). The reaction mixture was
cooled to room temperature and a white precipitate was obtained through recrystallization. The
white precipitate was collected by filtration and washed three times with methanol to get purified
precipitate, and the total yield was up to 73%. Ultimately, a white solid of compound 1 was obtained
by the vacuum dryer and was characterized by NMR, Elemental analyses and ESI mass data,
which were consistent with the proposed formulation.
Compound 1: 1H NMR (400 MHz, DMSO-d6): δ (ppm) 12.73 (s, 1H), 8.30 (s, 1H), 7.78(d,
J=12Hz, 2H), 7.10(s, 1H), 6.96(d, J=8Hz, 2H), 6.73(d, J=8Hz, 2H), 2.98(s, 6H); 13C NMR (400
MHz, DMSO-d6) δ (ppm): 167.94, 152.59, 143.49, 132.93, 129.55, 123.84, 119.60, 111.30, 39.91;
ESI-MS m/z [M-H]+: calcd, 258.13 found, 258.14; Element Anal.: calcd. for C13H15N5O: C, 60.69;
H, 5.88; N, 27.22%, found: C, 61.28; H, 5.62; N, 26.65%.
2.3 UV-vis and fluorescence titration measurement
A stock solution of compound 1 at a concentration of 1.0 mM was prepared in acetonitrile and
stored in a cold and dark place. Absorption and fluorescence experiments were carried after
appropriate dilution. All absorbance and fluorescence measurements were carried out in
CH3CN/H2O (v/v = 3:2) pH 7.4 Tris-HCl solution.
2.4 Calculation methods
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The density functional theory using Becke’s three parameterized Lee-Yang-Parr (B3LYP)
exchange functional with 6-311G basis sets, in Gaussian-09 programs has been employed to
obtain optimized structure of 1 and 1-Cu2+ complex in gaseous state.
3. Results and discussion
3.1 Colorimetric and fluorescent responses of compound 1 to Cu2+ in aqueous buffered solutions
As the free amino active hydrogen plays an important role in the tight binding with Cu2+ [29,
30], we expect compound 1 could combine with copper ions directly and would induce the change
of the absorption and/or the fluorescence spectra. The spectroscopic experiments were carried out
in CH3CN/H2O (v/v = 3:2) pH 7.4 Tris-HCl solution. The optimal condition was obtained from
experimental results shown in Fig.S1 and Fig.S2. The absorption spectrum of compound 1 was
measured in the presence of an increasing concentration of Cu2+. As shown in Fig.1a, compound 1
showed a strong absorption band at 342 nm. Upon gradual addition of Cu2+, the absorbance peaked
at 342 nm gradually decreased and a new absorption band peaked at 406 nm appeared along with
color change from colorless to yellow. Moreover, two isosbestic points at 286 and 365 nm were
observed, demonstrating that only one compound between compound 1 and Cu2+ was formed.
Similarly, fluorescent response to Cu2+ was investigated in detail. As shown in Fig.1b,
compound 1 emitted very weak fluorescence. The fluorescence intensity at 470 nm enhanced as the
concentration of Cu2+ increased. As shown in Fig.S3, the linear equations for UV and fluorescence
followed as y=0.040x+0.0035 (R=0.9988) and y=106.8x-13.08 (R=0.9997), respectively. The
detect limit for Cu2+ were estimated as 0.46 μM and 15 nM according to absorption and fluorescent
response using the basis of 3σ/k, where σ is the standard deviation of the blank measurements and k
is the slope of the intensity as a function of the concentration of Cu2+. The binding stoichiometry
between compound 1 and Cu2+ was investigated by Job’s plot analysis (Fig.S4). When the mole
fraction of Cu2+ was increased to 0.5, the fluorescence intensity variation at 470 nm reached the
maximum. This result show that a 1:1 complex formation between compound 1 and Cu2+. The
binding constant of 1-Cu2+ complex was calculated to be 4.3×107 M-1 and 5.4×107 M-1 (Fig.S5).
That result indicated that compound 1 had a steady binding affinity for Cu2+.
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Fig.1 (a) Absorption spectral change of 1 (10 μM). Inset: the absorbance at 406 nm versus the
concentration of Cu2+ added. (b) Fluorescence spectral change of 1 (1 μM) with increasing
concentration of Cu2+. Inset: fluorescence intensity at 470 nm versus the concentration of Cu2-
added.
The colorimetric and fluorescent responses of compound 1 to various metal ions (Ag+, Al3+,
Ba2+, Cu2+, Cr3+, Ca2+, Cd2+, Co2+, Fe3+, Hg2+, K+, Mg2+, Ni2+, Na+, Pb2+ and Zn2+) were investigated
in aqueous solutions (Fig.2). Among the tested metal ions, only Cu2+ induced change of the
absorption and the huge increase of fluorescence emission sensitively. In addition, the color and
emission responses to Cu2+ were not considerably interfered by other metal ions like 1000 times the
concentration of Na+, K+ Al3+, Cd2+ and Co2+; 500 times of Ca2+, Ag+ and Pb2+, 100 times of Cr3+,
Mg2+, Zn2+, Ni2+, Mn2+ and Hg2+ (Fig. 3). The overall results indicate that 1 possesses a more steady
binding affinity for Cu2+ than any other metal ions. To verify the feasibility of this method, using
the standard addition method of copper ion in tap water, lake water and spring water were tested and
obtained satisfactory results (Table S1). Recovery and the relative standard deviation are
99%-103% and 0.4%-2.1%, respectively.
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Fig.2 (a) Absorption and (b) fluorescence spectra (λex = 350nm, slit 10 nm / 2.5 nm), visible color
change under ambient light (c), and emission color change under UV light (λex = 365 nm) (d) of
compound 1 in the presence of various metal ions (50 equiv). (left to right: only 1, Cu2+, Ag+, Al3+,
Ba2+, Cr3+, Ca2+, Cd2+, Co2+, Fe3+, Hg2+, K+, Mg2+, Ni2+, Na+, Pb2+ and Zn2+).
Fig.3 (a) Absorption and (b) fluorescence responses of 1 towards Cu2+ and other competitive ions
and compounds in CH3CN/H2O (v/v = 3:2, pH 7.4) Tris-HCl solution.
3.2 Colorimetric and fluorescent responses of the ensemble (1-Cu2+) to S2- in aqueous buffered solutions
We prepared the ensemble (1-Cu2+) by adding 1 equiv. of Cu2+ to the aqueous solutions
containing compound 1 (10 μM) and investigated the responses to S2- in CH3CN/H2O (v/v = 3:2)
pH 7.4 Tris-HCl solution. As shown in Fig. 4a, a gradual decrease of absorbance (406 nm) was
observed by increasing the concentration of S2-. Meanwhile, the fluorescence intensity at 470 nm
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decreased gradually upon the addition of S2- and reached the saturation point at about 10 equiv. of
S2-, as shown in Fig. 4b. The absorption and fluorescence spectra of the ensemble in the presence of
S2- returned to the original spectra of compound 1. The change of the fluorescent intensity at 470
nm induced by S2- as a function of time was measured and the response time of the ensemble to S2-
needed 2 minutes. This result indicated that the ensemble showed rapid response which is good for
application in real samples.
Fig.4 (a) Absorption spectra of 1-Cu2+ (10 μM) (λ = 406 nm). Inset: absorbance at 406 nm versus
the concentration of S2- added. (b) Fluorescence spectra of 1-Cu2+ (1.0 μM) with increasing
concentration of S2- (λex = 350 nm). Inset: fluorescence intensity at 470 nm versus the
concentration of S2- added.
In order to evaluate the selectivity of the ensemble sensing for S2- in aqueous solutions, the
absorption and emission spectra of the ensemble (1-Cu2+) were measured in the presence of various
−
anions (Ac−, Br−, Cl−, F−, HCO3 , HPO42−, I−, S2O32−, SCN−) and some compounds (EDTA,
CH4N2S, Cys, Hcy, GSH). As shown in Fig.5, only S2- induced the change of the absorption
spectrum and the turn-off response of the fluorescence, whereas the other anions (including
sulfur-containing ones) did not induce any significant change in the absorption and emission spectra.
Fig.5c presents a visible color change of the ensemble under ambient light, and Fig.5d presents a
visible emission color change of the ensemble under UV light. The ensemble solution containing S2-
displayed colorless and emitted weak fluorescence. In contrast, the ensemble solution displayed a
yellow color as well as emitted bright blue fluorescence in the absence or presence of other
competing anions.
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Fig.5 (a) Absorption and (b) fluorescence spectra (λex = 350nm, slit 10 nm/2.5 nm), (c) visible
color change under ambient light, and (d) emission color change under UV light (λex = 365 nm) of
1-Cu2+(1 μM) in the presence of various anions (10 equiv). From left to right was None, S2-, Ac−,
−
Br−, Cl−, F−, HCO3 , HPO42−, I−, S2O32−, SCN−, EDTA, CH4N2S, Cys, Hcy, GSH)
Further, the effect of coexisting substance on the detection of S2- was investigated. The
presence of the following amounts of foreign species compared with the concentration of S2-
2-
-
resulted in less than ±10% error: 1000 equiv. of F-, Cl-, Br-, Ac-, HPO4 , 500 equiv. of I-, HCO3 ,
2-
S2O3 , Cys, Hcy, GSH, EDTA, 200 equiv of CH4N2S, SCN- (Fig. 6). In addition, the fluorescent
response to S2- was not affected by the presence of the other anions. The results indicated that the
method possessed high selectivity for detection S2- via colorimetric as well as fluorescent responses.
The LOD of the ensemble (1-Cu2+) for S2- in aqueous solutions was investigated by a linear
response of 1-Cu2+ as a function of S2- concentration (Fig. S6). The linear equations for UV and
fluorescence are y=0.384-0.039x (R = 0.9968) and y=998.8-108.58x (R = 0.9942), respectively.
The LOD of the ensemble (1-Cu2+) for S2- were calculated to be 0.68 μM (from absorption index)
and 0.12 μM (from fluorescence index). The detection limit for S2- was lower than the maximum
allowable level of S2- (15 μM) in drinking water set by the WHO. This result demonstrated that the
assay was suitable to monitoring S2- concentration in environment water.
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Fig.6 (a) Absorbance and (b) fluorescence responses of 1-Cu2+ toward S2- and various competitive
anions and compounds.
To further investigate practical application of the ensemble (1-Cu2+), the test papers were
immersed into the 1-Cu2+ aqueous solution for 2 days and then air-dried to detect S2- in real samples.
As depicted in Fig.7, the yellow color of the test strips was faded gradually with the increasing of
Cu2+ concentration. Also, it can be seen that the blue emission color weakened gradually. This
observation suggested that the test paper immersed 1-Cu2+ can be used conveniently to sense S2- by
the naked-eyes.
Fig.7 (a) Visible color change (from yellow to white) of the test strips after the addition of S2-. (b)
The corresponding emission color change (from light blue to colorless) of the test strips under a
hand-held UV lamp.
For analysis of S2- in real sample, the samples (tap water, lake water and blood samples) were
spiked with 2.0, 4.0 and 6.0 μM of S2-, and analyzed by proposed fluorescent method. A good
agreement was obtained between the spiked and measured S2- amounts. The recovery percentage
was found in the range of 97.69-102.7%. All the measurements were performed three times. The
results are listed in Table 1, which showed satisfactory recovery and RSD values for the sample.
The recovery value for S2- was greater than 97.69 %, and the relative error was lower than 2.27%
for S2-. These values were quantitative and it suggests that the presented procedure could be
applied for the detection of S2- in real samples.
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Table 1 Determination of S2- in samples*
Sample
Added
Found (μM)
Average
RSD (%) Recovery
(μM)
(μM)
(n=3)
(%)
Tap water
2.0
4.0
6.0
2.0
4.0
6.0
2.0
4.0
6.0
2.037
4.041
6.015
2.065
4.095
6.099
1.883
3.904
5.879
2.082
4.048
6.017
2.002
4.011
6.072
1.892
3.910
5.899
2.036
4.039
6.036
2.093
4.046
6.015
1.899
3.909
5.863
2.052±0.021
4.043±0.005
6.023±0.012
2.053±0.047
4.050±0.042
6.062±0.043
1.991±0.008
3.908±0.003
5.880±0.018
1.045
0.117
0.192
2.270
1.042
0.707
0.403
0.082
0.302
102.6
101.1
101.0
102.7
101.3
101.0
99.57
97.69
98.00
Lake
water
Blood
*No measured ions were detected in the tested samples
3.3 Reversible detection
As the presence of S2- alone has no influence on the emission of probe 1, the decrease in the
fluorescence intensity is due to the interaction between Cu2+ and S2- apparently. The fluorescence
is recovered following the addition of further Cu2+ (1.0 equiv.). The reversible cycles, as shown in
Fig. 8, fluorescence on and off, could be repeated for 5 times under the same condition. This
alteration showed the capability of sensor 1 as a good reversible and reusable sensor.
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Fig.8 Reversible changes in the emission intensity of 1 at 470 nm (ex = 350 nm) upon sequential
addition of Cu2+ and S2- solution for 5 cycles.
3.4 The proposed reaction mechanism
The binding mode between 1 and Cu2+ was analyzed by mass spectrometry (MS). As shown
in Fig. S7, peaks at m/z 258.1358 and 320.0673 belonged to [1+H+] and [1+Cu2+] respectively,
which further proved the 1:1 stoichiometry ratio of between 1 and Cu2+.
In order to verify the mechanism between Cu2+ and sensor 1 for the changes of fluorescence
intensity and the proposed interaction, electronic properties of ground state and excited state of 1
and 1-Cu2+ complex were applied by molecular orbital calculation. The calculate results showed
that the lowest singlet/doublet electronic transition for sensor 1 and 1-Cu2+ was HOMO–LUMO
transition. As shown in Fig. 9, it was noticed that the fluorescence enhancement by Cu2+ could be
explained in terms of the occupancy of the frontier orbitals. And the molecular orbitals were
relevant to the excitations and the contributions of orbital transitions for 1 and 1-Cu2+ complex. In
sensor 1, the electron densities of HOMO were only distributed around the imidazole moiety,
while those of LUMO were distributed in the 4-dimethylamino-benzoyl moiety. Upon excitation
of the sensor 1, an electron cloud would be transferred from the imidazole to the
4-dimethylamino-benzoyl, resulting in the quenching of sensor 1. Thus, a PET mechanism was
confirmed. While for 1-Cu2+ complex, due to the absorption of copper ion electronic effect, the p
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electrons cloud in the HOMO of the 1-Cu2+ complex were located around the Cu2+ center. The
results show that, HOMO localized on the imidazole group. So the PET process was inhibited, and
there was no electron transfer upon excitation. So the fluorescence was enhanced compared with
that of sensor 1.
On the other hand, the orbital energies and spatial distributions of the HOMO and LUMO of
1 and the 1-Cu2+ complex were also generated. The calculated energy gap between the HOMO and
LUMO of the 1-Cu2+ complex was 1.78 eV, which is significantly lower than that of 1 (3.46 eV).
The results showed that the HOMO and LUMO of the 1-Cu2+ complex were more stable than
those of sensor 1, also proving that the binding of Cu2+ to the sensor 1 stabilizes the system. These
were in full agreement with experimental observations.
Fig.9 HOMO/LUMO and band gap energies (-ΔE = EHOMO-ELUMO) of 1 and 1-Cu2+ complex by
DFT calculation.
To further explore the working mechanism and binding mode of the sensor 1 to Cu2+ and
(1-Cu2+) to S2-, the nuclear magnetic titration experiments were performed. As shown in Fig. 10,
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when Cu2+ was added to sensor 1, the H near the benzene ring has a certain movement and the
signal of Ha and Hb disappeared. While adding S2- sequentially, signals of Ha and Hb reappeared
again. So, we guess the complex formation occurred between Cu2+ and 1, and then the complex
1-Cu2+ disassembled upon addition of S2- as shown in Scheme 2. The N and O complexation
between compound 1 and Cu2+ is formed through two five membered rings, which enhanced the
planar rigid structure of compound 1 resulting in fluorescent enhancement. These results supported
the proposed mechanism.
Fig.10 1H NMR spectral changes of 1、1-Cu2+and 1-Cu2+upon addition of S2- in (CD3)2SO.
Scheme 2 Illustration of the proposed mechanism of 1 for the sequential detection of Cu2+ and S2-.
4. Conclusion
In summary, a new colorimetric and fluorescent sensor 1 has been developed, which could
selectively detect Cu2+ over other metal ions; what is more, the ensemble (1-Cu2+) could detect a
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low concentration of S2- by a colorimetric change as well as a fluorescent change. Its application in
quantitative detection of S2- using simple paper-made test kits was performed. Sensor 1
demonstrated a highly selective ‘‘off-on-off’’ type fluorescent switching property. The combining
response mechanism was confirmed via UV-vis absorption, fluorescence measurements, 1H NMR,
mass spectroscopy and density functional theory calculation.
Author contributions
The first two authors contributed equally to this work.
Acknowledgements
This work is financially supported by Natural Science Foundation of China (No. 21365014,
21765014, and 21505067).
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Graphical Abstract (for review)
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Highlights
1. A colorimetric and fluorescent assay for Cu2+ and S2- in aqueous solution based on
4-Dimethylamino-benzoic acid hydrazide was presented.
2. The sensor for detection of Cu2+ was based on fluorescence enhancement and the background
was almost zero.
3. The colorimetric and fluorescent assay shows excellent selectivity over other ions by the
naked eye and spectra.
4. The assay was applied to detect S2- on the test strips.
5. The experiment has a low detection limit for Cu2+ and S2-
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