A simple, reversible, colorimetric and water-soluble fluorescent chemosensor for the naked-eye detection of Cu2?+ in ~?100% aqueous media and application to real samples

Article abstract of DOI:10.1016/j.saa.2017.07.038

A simple, reversible, colorimetric and water-soluble fluorescent chemosensor ADA for the naked-eye detection of Cu^2?+ was developed. Sensor ADA showed high selectivity and sensitivity toward Cu^2?+ in ~?100% aqueous media over wide pH range. Sensor ADA exhibited a red-shift in the absorption spectra from 466 to 480?nm that is accompanied by significant color change from light yellow to yellowish brown instantaneously. The Cu^2?+ recognition is based on the chelation–enhanced fluorescence quenching (CHEQ) effect of the paramagnetic nature. The lowest detection limit is determined to be 15.8?nM, which is much lower than the allowable level of Cu^2?+ in drinking water set by U.S. Environmental Protection Agency (~?20?μM) and the World Health Organization (~?30?μM). The 1:1 binding process was confirmed by fluorescence measurements, IR analysis and DFT studies. Moreover, sensor ADA was successfully applied for determination of trace level of Cu^2?+ with 4 reuse cycles in various water samples, which affords promising potential in ion-detection field.

Full text of DOI:10.1016/j.saa.2017.07.038

Accepted Manuscript
A simple, reversible, colorimetric and water-soluble fluorescent
chemosensor for the naked-eye detection of Cu2+ in ~100%
aqueous media and application to real samples
Tao Sun, Qingfen Niu, Tianduo Li, Zongrang Guo, Haixia Liu
PII:
S1386-1425(17)30601-7
doi: 10.1016/j.saa.2017.07.038
SAA 15325
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
25 April 2017
30 June 2017
20 July 2017
Please cite this article as: Tao Sun, Qingfen Niu, Tianduo Li, Zongrang Guo, Haixia Liu
A simple, reversible, colorimetric and water-soluble fluorescent chemosensor for the
,
naked-eye detection of Cu2+ in ~100% aqueous media and application to real samples,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi:
1
0.1016/j.saa.2017.07.038
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ACCEPTED MANUSCRIPT
A simple, reversible, colorimetric and water-soluble fluorescent
2
+
chemosensor for the naked-eye detection of Cu in ~100% aqueous
media and application to real samples
Tao Sun, Qingfen Niu,* Tianduo Li, Zongrang Guo, Haixia Liu
Shandong Provincial Key Laboratory of Fine Chemicals, School of Chemistry and
Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, People’s
Republic of China
Abstract
A simple, reversible, colorimetric and water-soluble fluorescent chemosensor ADA
2
+
for the naked-eye detection of Cu was developed. Sensor ADA showed high
2
+
selectivity and sensitivity toward Cu in ~100% aqueous media over wide pH range.
Sensor ADA exhibited a red-shift in the absorption spectra from 466 to 480 nm that is
accompanied by significant color change from light yellow to yellowish brown
2
+
instantaneously. The Cu recognition is based on the chelation–enhanced fluorescence
quenching (CHEQ) effect of the paramagnetic nature. The lowest detection limit is
2
+
determined to be 15.8 nM, which is much lower than the allowable level of Cu in
drinking water set by U.S. Environmental Protection Agency (~20 μM) and the World
Health Organization (~30 μM). The 1:1 binding process was confirmed by fluorescence
measurements, IR analysis and DFT studies. Moreover, sensor ADA was successfully
2
+
applied for determination of trace level of Cu with 4 reuse cycles in various water
samples, which affords promising potential in ion-detection field.
Keywords: Colorimetric; water-soluble fluorescent chemosensor; reversibility; naked-
2
+
eye detection; Cu ; chelation–enhanced fluorescence quenching.
*
To whom the correspondence should be addressed
E-mail: qf_niu1216@qlu.edu.cn
Telephone: +86(531)89631760
Fax number: +86(531)89631208
1

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1
. Introduction
The design and synthesis of fluorescence chemosensors for the
efficient detection of trace metal ions is one of the most important research
topics in biochemistry, toxicology and environmental science [1–5].
Among heavy and transition metal ions, it is well-known that copper ion is
the third largest essential trace element of body, and it also plays a very
important role in various biological processes such as gene expression,
structural enhancement of proteins as well as metalloenzymatic reactions
2
+
[
1,6,7]. However, the alteration of intracellular Cu homeostasis may
cause some severe diseases such as coronary heart disease, liver, anaemia
as well as neurodegenerative disease [8,9]. Hence, developing a simple and
2
+
efficient method for Cu detection with high sensitivity and selectivity has
been an attractive research topic of enormous significance.
Fluorescence technique as a mean in directly detecting analytes,
which has real-time, sensitive and selective characteristics for specific
chemical species detection, has been developed rapidly [10–16]. In recent
2
+
years, a number of chemosensors for detection of Cu ions have been
reported [17–25]. However, some of them exhibit poor water solubility and
the higher detection limit, therefore limiting their practical application in
ion-detection field.
Among all types of fluorescence sensors, the sensors based on Schiff
bases were widely used due to their flexible design and low cost. It is well-
2

ACCEPTED MANUSCRIPT
known that the Schiff bases can be synthesized easily by aldehyde and
amine derivatives, and have forceful complexing capacities with many
metal ions, resulting in various optical properties [26–28]. In view of these
important facts, we developed a simple fluorescent molecule ADA as the
target sensor that could strongly bind the metal ions through Schiff base
moiety.
Herein, in our work, we reported a low-cost Schiff base derivative
2
+
ADA as a colorimetric and fluorescent chemosensor to sensing Cu in
~100% aqueous media via a turn-off fluorescence response. Sensor ADA
2
+
exhibits high sensitivity and selectivity to Cu ions over other metal ions.
Through Job-plot, FT-IR spectra and DFT studies, the bonding mode of
2
+
sensor ADA with Cu was further investigated. This sensor ADA greatly
2+
enhanced the ability of the system to complex with Cu , with a
7
−1
complexation constant above 10 M .The lowest detection limit (15.8 nM)
2
+
of Cu achieved nanomolar levels in ~100% aqueous media with an
instantaneous response time. Importantly, this simple and water-soluble
sensor ADA is renewable and thus can be used practically as a rapid,
2
+
selective and reversible sensor for the trace level of Cu analysis in
environmental water samples over wide pH range.
2
. Experimental
2
.1. Materials and solvent
3

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Materials and solvents used in experiments were of analytical grade
without further purification. Deionized water was used thoughout the
experiments. The solution of metal ions were prepared from nitrate salts
+
+
2+
3+
2+
2+
2+
2+
+
2+
2+
3+
2+
(
Na , K , Ca , Fe , Co , Ni , Cu , Zn , Ag , Cd , Hg , Al , Pb ,
2
+
3+
Fe and Cr ) with 1.0 mM in deionised water to evaluate the cations
binding interaction with ADA. ADA was dissolved in DMSO to obtain the
stock solution of ADA (1.0 mM) and then diluted with deionized water to
prepare the analytical solution (5.0 µM). The wide pH range solutions were
prepared by adjustment of HAc–NaAc (50 mM, pH 4.76) solution with 1
mM HCl or1 mM NaOH.
2
.2. Instrument
1
Spectra of H NMR (TMS as internal standard) were measured on a
Bruker–AVANCE 400 NMR Spectrometer in deuterated solvents (DMSO–
d6). FT-IR spectra was performed using a Bruker ALPHA FT–IR
−1
spectrometer with the KBr pellet technique over the 4000–400 cm range.
High resolution mass spectra (HRMS) were carried out on a Agilent 6510
Accurate–Mass Q–TOF LC/MS system. Fluorescence spectra were
obtained using a Hitachi F–4600 spectrofluorometer with a scan rate at
1
200 nm/min at room temperature. The fluorescence spectra were obtained
with excitation at 466 nm, and excitation and emission slit widths were 5
and 5 nm, respectively.
2
.3. Synthesis of chemosensor ADA
4

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To a stirred solution of 2-hydroxy-1-naphthaldehyde (1.72 g, 10.0
mmol), hydrazine (0.16 g, 5.0 mmol) in 30 mL ethanol was added and
continuously stirred. After refluxing for 3 h, the mixture was cooled to
room temperature. A yellow precipitate isolated from the reaction mixture
by filtration, then washed with EtOH for three times and dried. The desired
compound ADA was obtained as a light yellow solid (1.60 g, 4.70 mmol,
1
9
2
4% yield). H NMR (400 MHz, DMSO–d ) δ = 12.89 (s, 2H), 10.01 (s,
6
H), 8.67 (d, J = 12.0 Hz, 2H), 8.06 (d, J = 12.0 Hz, 2H), 7.94 (d, J = 8.0
Hz, 2H), 7.63 (t, J = 12.0 Hz, 2H), 7.45 (t, J = 12.0 Hz, 2H), 7.31 (d, J =
1
3
8
1
.0 Hz, 2H). C NMR (100 MHz, DMSO–d ) δ = 151.73, 148.60, 145.97,
6
43.09, 131.07, 130.62, 130.02, 122.46, 117.99, 112.63, 110.69; FT–IR
–
1
(
KBr, cm ) ν = 3409 (O-H), 1614 (C=N), 1579 (C=C); HRMS (ESI): m/z
+
[
M+H] calcd for: C H N O : 341.1212, found 341.1254.
22
16
2
2
2
.4. Calculation of the DL and Ka
The detection limits of ADA for Cu was accurately estimated using
the following equation:
2
+
DL =K×σ/s.
where the value of K is 3, σ is the standard deviation of the blank signal,
and S is the slope of the linear calibration [29–31].
Binding constant K was calculated based on the Benesi–Hildebrand
a
Eq. (1) [32,33].
1
1
1


2
F F {K 

F F



Cu

}

F F

0
a
max
0
max
0
(1)
5

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Herein, F , F, and F
are the emission intensities (at 547 nm) of the
0
max
organic moiety considered in the absence of metal ion, at an intermediate
metal ion concentration, and at a concentration of complete interaction,
respectively, and K is the binding constant concentration.
2+
2
.5. Detection of Cu in water-samples by ADA
The water samples were used to prepare the required samples. The
river water was filtered to remove the insoluble materials. All the water
2
+
samples (2 mL) were spiked with different Cu concentrations (0–5.0
µM). Then ADA (5.0 µM) was added and the experiments were run after
equilibration for 1 min.
3
. Results and discussion
3
.1. Synthesis and structural characteristics of ADA
The general synthetic procedure for ADA is listed in Scheme 1. The
fluorescent chemosensor ADA was synthesized by the reaction of 2-
hydroxy-1-naphthaldehyde and hydrazine in EtOH under reflux for 3 h,
1
which was well characterized by H NMR, HRMS and FT-IR spectra (Figs.
S1–S3).
Scheme 1
3
.2. Colorimetric study
Colorimetric “naked-eye” sensing is the simplest and safest way to
6

ACCEPTED MANUSCRIPT
observe the selectivity of a designed chemosenor to various analysts. As
shown in Fig. 1, the photographs of ADA in ~100% aqueous solution upon
+
+
+
2+
2+
2+
2+
addition of various metal ions such as Na , K , Ag , Ca , Cu , Ba , Co ,
2
+
2+
2+
2+
2+
3+
3+
Ni , Pb , Hg , Zn , Cd , Fe and Al . Interestingly, the investigated
2
+
results exhibited that only Cu induced rapid and obvious naked-eye color
change from light yellow to yellowish brown, while other tested metal ions
remained unable to produce the same effect. This obvious color change can
2
+
be easily used for the naked-eye detection of Cu in ~100% aqueous
solution.
Fig. 1
3
.3. Absorption spectral response of ADA to Cu2+
Based on the above colorimetric results, the UV–vis absorption
2
+
response of ADA to Cu was investigated in ~100% aqueous solution. As
shown in Fig. 2, the solution of free ADA (5.0 µM) displayed three distinct
absorption band at ca. 405, 431 and 466 nm, respectively, which could be
attributed to the typical absorption of naphthalene moiety (405 and 431 nm)
2
+
and the π-π* transition (466 nm) [34]. Upon addition of 1.0 equiv of Cu ,
the band at 466 nm of ADA was shifted to 480 nm, which can be explained
2
+
by the formation of ADA–Cu complex. However, other tested metal ions
7

ACCEPTED MANUSCRIPT
2
+
2+
2+
2+
2+
2+
3+
2+
3+
3+
2+
including Pb , Ba , Cd , Ni , Co , Fe , Fe , Zn , Al , Cr , Ca ,
2
+
+
+
Hg , Na and K (1.0 equiv.) did not induce significant change in UV–vis
absorption. These outstanding observations indicate that sensor ADA has
2
+
2+
the strongest binding ability with Cu and exhibits highly selective to Cu
over other tested metal ions.
Fig. 2
3
.4. Fuorescence spectral response of ADA to Cu2+
The fluorescent spectral response of ADA (5.0 µM) to Cu was
2
+
investigated in ~100% aqueous media. As displayed in Fig. 3, the free
sensor ADA showed strong fluorescence emission intensity at 547 nm with
bright green fluorescence when excited at 466 nm. Upon addition of Cu2+
(1.0 equiv.) to the solution of ADA, the fluorescence intensity of the sensor
ADA was almost completely quenched accompanied by an obvious color
change from green to colorless. The quenching phenomena may be due to
the chelation–enhanced fluorescence quenching (CHEQ) effect of metal-
to-ligand charge transfer (MLCT) between sensor ADA and paramagnetic
2
+
Cu ion[35,36]. However, other metal ions caused no significant emission
peak at 547 nm. These observed changes further confirmed the high
2
+
selectivity of sensor ADA to Cu .
Fig. 3
8

ACCEPTED MANUSCRIPT
2
+
Next, the fluorescent competitive experiment for Cu detection was
performed in the presence of other tested 14 metal ions. As shown in Fig.
2
+
S4, the fluorescence intensity of ADA in the presence of Cu was
unaffected by the addition of other competing metal ions, demonstrating
the good anti-interference ability of sensor ADA.
2
+
To ascertain the complex form between ADA and Cu , Job’s plot
analysis was conducted by maintaining the 10 mM total concentration of
2
+
ADA and Cu ions in ~100% aqueous media. As shown in Fig. 4, the
2
+
2+
change in Job’s plot happened at [Cu ]/([Cu ]+[ADA]) = 0.5, which
2
+
meant that Cu and ADA formed a complex with a ratio of 1:1.
Fig. 4
In order to further gain insight into the binding interaction between
2
+
ADA with Cu , we carried out fluorescence titration experiments in detail
2
+
of ADA with Cu in ~100% aqueous media. As displayed in Fig. 5, the
fluorescence spectra of ADA (5.0 µM) had a strong peak at 547 nm, while
2
+
as Cu ions (0–2 equiv) were gradually added, significant decrease in
2
+
fluorescence intensity occurs. When the amount of Cu reached 1.0 equiv,
the fluorescence intensity achieves its minimum, and then almost remained
2
+
constant with increasing Cu ions, indicating that the sensor ADA and
2
+
Cu had coordinated with 1:1 stoichiometry. From the results of
9

ACCEPTED MANUSCRIPT
2
+
fluorescence titration, the association constant of the ADA–Cu complex
7
–1
was determined as 1.24 × 10 M on the basis of Benesi–Hildebrand
3
12
analysis (Fig. S5). This binding value is within the range (10 –10 ) of
2
+
those reported chemosensors for Cu [37–40]. For practical applications,
the detection limit (DL) of ADA was also estimated from the titration data
–8
and the DL was calculated to be as low as 1.58 × 10 M by using the Eq.
1) (Fig. S6), which is better than most of the previous reports for Cu2+
(
sensing (Table 1) [41–49]. As shown in Table 1, sensor ADA can be
applied to the testing environment in ~100% aqueous media. The relative
lower detection limit was 15.8 nM, which shows that senor ADA can serve
2
+
as a colorimetric chemosensor for the detection of Cu with high
sensitivity compared with other chemosensors. Most importantly, it was
2
+
significantly lower than the allowable level of Cu in drinking water set
by U.S. Environmental Protection Agency (~20 μM) and the World Health
Organization (~30 μM) [50–52]. Therefore, the sensor ADA can be a
2
+
powerful tool for the detection of Cu ions in drinking water.
Fig. 5
Table 1
10

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3
.5. Binding mode studies
To determine the nature of the complexation between ADA and
2
+
Cu , FT-IR spectra of ADA were recorded in the absence and presence of
2
+
2+
Cu ion (Fig. S7). Upon addition of Cu (1.0 equiv.), the hydroxyl O–H
stretching band at 3409 cm−1 in ADA was observed to disappear
2
+
completely. The IR-data indicated that the Cu ion coordinated to two
deprotonated phenolic oxygen atoms. The obvious color change observed
2
+
in the ligand solution of ADA upon binding to Cu ion is attributed to the
deprotanation of phenolic –OH which allows the charge transfer from
ligand to metal ion. The charge transfer from ligand to metal is also
responsible for fluorescence quenching behavior of sensor ADA upon Cu2+
complexation. The characteristic frequency of the imine C=N band
−1
decreased from 1624 to 1612 cm . These outstanding results strongly
indicate that the hydroxyl oxygen atom and the imine nitrogen atom are
2
+
involved in the Cu ion coordination. From the above observed results, a
2
+
proposed fluorescence sensing mechanism associated with Cu binding to
ADA is displayed in Scheme 2. Sensor ADA exhibits a high selectivity
2
+
and sensitivity to Cu in terms of absorption and fluorescence. The
2
+
fluorescence quenching of ADA upon titration with Cu was due to its
paramagnetic effect, which quenched excited state of fluorophore by
electron transfer.
11

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Scheme 2
3
.6. Theoretical calculation studies
To better understand the fluorescence character of ADA and the effect
of metal ion titration, density functional theory (DFT) calculations at the
B3LYP/6–31G level were performed for ADA and its 1:1 complex ADA–
2
+
Cu [53]. The frontier molecular orbitals (HOMO and LUMO) of the ADA
2
+
monomer and ADA–Cu complex are shown in Fig. 6. In ADA monomer
(
Fig. 6a), the HOMO (–3.54 eV) resided at the imine and hydroxyl units
wheres LUMO (–2.04 eV) located at the all fragment. However, the π
electrons of HOMO (–6.02 eV) and LUMO (–5.49 eV) of the complex
2
+
ADA–Cu mainly situated at its coordination center (Fig. 6b). The
calculated energy gap (Δ) between HOMO and LUMO for ADA is found
2
+
to be 1.50 eV. Similarly, for ADA–Cu complex, the calculated energy gap
(Δ) is found to be 0.53 eV. The calculated energy gap between HOMO and
2
+
LUMO (0.53 eV) of ADA-Cu is much lower than that of free ADA (1.50
2
+
eV). Therefore, the complexation between ADA and Cu is energetically
favorable. These results obtained from the DFT studies suggesting that the
2
+
interaction of Cu to ADA effectively reduces the HOMO–LUMO energy
2
+
gap of ADA–Cu complex and stabilizes the system [54]. In addition, the
decrease of energy gap can explain the red shift of the absorption peak of
2
+
sensor ADA upon addition of Cu ion. On the other hand, according to the
12

ACCEPTED MANUSCRIPT
fluorescence emission mechanism, as the frontier molecular orbitals are
spreading over one naphthalene plane, the imine and hydroxyl units in the
2
+
ADA–Cu complex, the excited electron of the imine and hydroxyl units
is back to the ground state, and thus leads to the quenching of fluorescence
by the effect of LMCT. The theoretical calculation results confirmed
intramolecular charge transfer. Furthermore, the theoretical calculation
2
+
results also confirmed the proposed binding mode of ADA with Cu .
Fig. 6
3
.7. Binding reversibility of the sensor ADA
The reversibility is one of the most important factors for fluorescent
chemosensors in practical applications. As can be seen from Fig. 7, the
2
+
2–
alternate additions of constant concentration of Cu and S to the sensor
ADA solution resulted in the changes in the fluorescence intensity and
color. Obviously, the ADA-based colorimetric chemosensor showed good
stability with a little emission signal decay for several successive cycles,
2
+
indicating the decomposition of ADA–Cu complex formed a highly
−36
stable species CuS (Ksp, CuS = 1.27 × 10 ) [55]. Such a reversible cycle is
2
+
2–
repeated four times by the alternate change of Cu and S addition. The
observed results clearly demonstrate that these processes are fully
reversible. Therefore, ADA can be used as a reversible colorimetric
2
+
chemosensor for Cu ion in pure aqueous solution.
13

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Fig. 7
3
.8. The effect of pH
The fluorescence intensity of the sensor ADA and ADA–Cu over a
wide range of pH was studied in ~100% aqueous medium, as shown in Fig.
. Almost no obvious change in the fluorescence intensity of ADA at 547
nm was observed in the pH range from 4.0 to 12.0, revealing that the free
2
+
8
2
+
ADA is stable over the wide pH range. Upon addition of Cu ions, the
fluorescence intensity at 547 nm decreased, illustrating the formation of
2
+
2+
ADA-Cu complex. The fluorescence intensity of the ADA-Cu complex
at 547 nm was observed to be constant and relatively unaffected in the pH
range of 6.0 to 12.0. Thus, the sensor ADA has the maximal sensing
response over a wide pH range, indicating that sensor ADA is promising
for the analysis of environmental samples.
Fig. 8
3
.9. Effect of response time
Besides high sensitivity and excellent selectivity, a fast response time
is another important factor for a sensor to detect analytes in real-time.
2
+
Therefore, the response time of sensor ADA to Cu ions (2.0 equiv) in
100% aqueous solution was determined by recording the change of the
fluorescence intensity (Fig. S8). The investigated results showed that the
~
14

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fluorescence intensity of the reaction ADA-Cu2+ system at 547
nm instantaneously and completely quenched and reached the minimum
within 30 s, and then remained unchanged over an extended period of time
due to the fast CHEQ process, which confirmed the high-speed response
2
+
of the sensor ADA for Cu ions. Thus, the sensor ADA can be applied to
2
+
detect Cu ions in real-time.
3
.10. Application to real water samples
To evaluate the potential use of the sensor ADA to determine Cu2+
2
+
ions in real water samples, which was made to determine Cu ions in tap
water, distilled water and river water. For the analysis of samples, the
standard addition method was adopted. From Table 2, the recoveries and
2
+
the relative errors of Cu in the three real samples were in the range of 98–
02% and 1–2.5%, respectively. The DLs in this method are quite low in
1
–7
–7
the range of 1.92 × 10 ~ 2.15 × 10 M, which are much lower than those
of the reported methods [56–60]. From these analytical results, a good
agreement was obtained between the added and measured (high recoveries
and low relative errors) of spiked samples, demonstrating the good
2
+
capability of the method for the determination of Cu in ~100% aqueous
solution.
Table 2
4
. Conclusions
15

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In summary, we reported a simple, reversible, colorimetric and water-
soluble fluorescent chemosensor ADA for the naked-eye detection of Cu2+
in ~100% aqueous solution with an instantaneous response time. Sensor
2
+
ADAexhibited obvious fluorescence quenching behavior toward Cu with
high sensitivity and selectivity over other metal ions. The higher binding
7
–1
constant (1.24 × 10 M ) and lower detection limit (15.8 nM) makes it
2
+
promising to sense Cu ion at nanomolar concentration levels in ~100%
aqueous solution over a wide pH range. In addition, DFT calculations
effectively verify the experimental observations supporting a 1:1 binding
2
+
mode of sensor ADA with Cu ion. Furthermore, this simple and water-
soluble sensor ADA is renewable so, and thus can be used practically as a
rapid, selective and reversible sensor for trace Cu2+ analysis in
environmental water samples.
Acknowledgment
The authors are grateful for the financial support from the National
Natural Science Foundation of China No. 21376125/51402157 and
Program for Scientific Research Innovation Team in Colleges and
Universities of Shandong Province.
16

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Figure and table captions:
Scheme 1. The synthetic route of sensor ADA.
Fig. 1. Corresponding naked-eye colorimetric readout response upon
addition of 1.0 equiv. of different metal ions into ADA (5.0 μM) in ~100%
aqueous media.
Fig. 2. The UV–vis absorption spectra of ADA (5.0 µM) in the presence
+
+
2+
3+
2+
2+
2+
2+
+
of metal ions including Na , K , Ca , Fe , Co , Ni , Cu , Zn , Ag ,
2
+
2+
3+
2+
2+
3+
Cd , Hg , Al , Pb , Fe and Cr ions (1.0 equiv.) in ~100% aqueous
media.
Fig. 3. (a) The fluorescence spectra of free ADA (5.0 µM) and in the
presence of 15 kinds of cations (1.0 equiv) in ~100% aqueous media at
5
47nm. (b) The color changes of ADA in the absence and presence of
various cations under the 365nm UV lamp.
Fig. 4. Job's plot for determining the binding stoichiometry of sensor
2
+
ADA and Cu in ~100% aqueous media. The total concentration of sensor
2
+
ADA and Cu was 5.0 μM.
Fig. 5. Fluorescence spectra of ADA (5.0 μM) in the presence of 0 ~ 2.0
2
+
equiv. of Cu in ~100% aqueous media at 547 nm. Inset: Fluorescence
2
+
intensity at 547 nm upon gradual addition of Cu . λ = 466 nm, slits: 5
ex
nm/5 nm.
2
+
Scheme. 2. Proposed complexation model of ADA with Cu and the
observed colorimetric change.
22

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2
+
Fig. 6. The HOMO–LUMO gap of (a) ADA and (b) ADA-Cu calculated
at the B3LYP/6-31G level, respectively.
Fig. 7. (a) Reversible color changes of ADA in ~100% aqueous solution
2+
2–
(
5.0 μM) upon alternate addition of Cu and S ions; (b) reversible
changes in fluorescence intensity of ADA (5.0 μM) at 547 nm upon
2
+
2–
alternate addition of Cu and S ions.
Fig. 8. Effect of pH on the fluorescence intensity of ADA (5.0 μM) in ~100%
2
+
aqueous media in the absence and presence of Cu ions (1.0 equiv.).
2
+
Table 1. Comparison of different fluorescence approaches for Cu sensing.
2
+
Table 2. The analytical results of Cu sensing in real water samples.
23

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Scheme 1.
24

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Fig. 1.
25

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Fig. 2.
0
0
0
0
0
0
0
0
0
0
.50
.45
.40
.35
.30
.25
.20
.15
.10
.05
2+
+
3+
2+
2+
2+
3+
ADA, Ag ,Al , Ca ,Cd , Co Cr , Hg
3+
2+
2+
2
+
+
+
2+
Fe , Fe , K ， Na , Ni , Zn , Pb
2+
Cu
4
00
500
600
700
800
wavelength/nm
26

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Fig. 3(a).
3
3
2
2
1
1
500
000
500
000
500
000
ADA
Ag+ ,Al3+ , Ca 2+ ,Cd2+
Co 2+ , Cr 3+ , Fe 2+
Hg 2+ , K + ,Na+ , Ni 2+
Zn2+,Fe3+
,
,
,
2
+
Cu
5
00
0
4
50
500
550
600
650
700
750
800
wavelength/nm
(
b)
27

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Fig. 4.
6
4
2
00
00
00
0
0
.0
0.2
0.4
0.6
0.8
1.0
2+
2+
[
Cu ]/([ADA]+[Cu ])
28

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Fig. 5.
3500
3
3
2
2
1
1
500
000
500
000
500
000
3000
0
2
2
500
000
2
+
1500
[
Cu ]
1
000
500
0
0
2
4
6
8
10
2.0 equiv
2+
-6
[
Cu ]/10
M
5
00
0
4
50
500
550
600
650
700
750
800
Wavelength/nm
29