A large Stokes shift, sequential, colorimetric fluorescent probe for sensing Cu2+/S2- and its applications

Article abstract of DOI:10.1016/j.jphotochem.2019.01.006

Copper ions (Cu²⁺) and sulfide (S^2?) are important markers in many physiologies and pathological processes. In this work, a new near-infrared fluorescent probe 1 for colorimetric and sequential detection of Cu²⁺/S^2?

Full text of DOI:10.1016/j.jphotochem.2019.01.006

JournalofPhotochemistry&PhotobiologyA:Chemistry373(2019)146–153
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A large Stokes shift, sequential, colorimetric fluorescent probe for sensing
Cu2+/S2- and its applications
⁎
Ji-Zhen Lia, Tao-Hua Lengb, , Zhi-Qiang Wangc, Li Zhoua, Xue-Qing Gongc, Yong-Jia Shena,
⁎
Cheng-Yun Wanga,
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, PR China
b National Food Quality Supervision and Inspection Center (Shanghai), Shanghai Institute of Quality Inspection and Technical Research, Shanghai 200233, PR China
c Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular
Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Copper ions (Cu2+) and sulfide (S2−) are important markers in many physiologies and pathological processes. In
this work, a new near-infrared fluorescent probe 1 for colorimetric and sequential detection of Cu2+/S2− was
designed and developed. The probe showed a rapid (less than 1 min), highly selective and sensitive response
toward copper ions. Notably, the probe could also be applied to detect S2− through reversible formation-se-
paration of complex 1-Cu2+ and CuS with a large Stokes shift of 234 nm. The detection limit for Cu2+ and S2−
was found to be 1.8 × 10-8 M and 1.5 × 10-8 M, respectively. Furthermore, the binding stoichiometry between 1
and Cu2+ was found to be 1:1, the binding mode was also demonstrated using density functional theory (DFT)
calculations and contrastive compound research. In addition, the probe was successfully applied in real water
samples assay for the detection of Cu2+, and the strip papers experiments also showed that probe 1 can be used
Copper ions
Sulfide
Large Stokes shift
Colorimetric
Near-infrared
DFT calculations
to detect Cu2+ and S2−
.
1. Introduction
S2− is mainly generated in natural gas purification, petroleum refining,
sewage treatment and paper-making [17]. Sulfide anions can also
combine with H+ under acidic conditions, generating more toxic H2S or
HS-, which not only pollutes the atmosphere but also causes organ’s
damage when exposing to high concentration of H2S [18,19]. Until
now, a number of fluorescence probes for its detection have been de-
signed and developed. [20–22]. However, most of these probes are
mainly based on the organic reaction like the cleavage of sulfonate or
dinitrophenyl ester [23], Michael addition [24,25], and thiolysis of the
7-nitro-1,2,3-benzoxadiazole (NBD) amine or ester bond [26,27]. These
reactions are plagued by some serious limitations like synthesis requires
strict conditions and relatively time-consuming procedures, which may
limit their applications in analytical and biological conditions.
Copper, as the third most abundant transition metal ions, is an
important trace mineral element in organisms [1], plays key roles in
biological and environmental fields [2]. However, unregulated amounts
of copper may destroy the metabolic balance of physiological systems,
and induce many health issues, such as Parkinson’s [3] and Wilson’s [4]
diseases. In addition, copper is also a significant pollutant due to its
widespread use in industry and agriculture. Therefore, the development
of efficient methods for copper (II) detection may be very meaningful
for the human health and environment. Compared with other tradi-
tional methods [5,6], fluorescent probes have attracted increasing at-
tention on account of their high sensitivity and selectivity [7], real-time
detection with rapid respond time [8] and potential biological appli-
cations [9]. To date, many fluorescence probes for copper detection
have been reported over the several years [10,11].
Herein, based on the previous works reported by our group [28], we
synthesized
a new dicyanoisophorone-based derivative (probe 1,
Scheme 1). The dicyanoisophorone moiety was chosen as a fluorophore
because of its large π-conjugation system and long emission wavelength
in NIR region [29], and the salicyladehyde azine was introduced as
schiff base unit due to its high affinity for cations and excited state
intra-molecular proton transfer phenomenon (ESIPT) characteristics
More interestingly, among these probes, some reactive sulfur spe-
cies, such as glutathione (GSH), cysthine (Cys), homocysteine (Hcy) and
sulfide anion (S2−) could also be monitored via the probe-copper
complex [12–16]. As a toxic substance and environmental pollutant,
⁎ Corresponding authors.
E-mail addresses: length@sqi.org.cn (T.-H. Leng), cywang@ecust.edu.cn (C.-Y. Wang).
https://doi.org/10.1016/j.jphotochem.2019.01.006
Received 22 August 2018; Received in revised form 5 January 2019; Accepted 6 January 2019
Availableonline11January2019
1010-6030/©2019ElsevierB.V.Allrightsreserved.

J.-Z. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry373(2019)146–153
Scheme 1. The synthetic route of probe 1 and compound 6.
[30]. We envisioned that the probe could induce a large Stokes shift
while bonding with metal ions. As expected, probe 1 displayed high
sensitivity and selectivity toward Cu2+. What’s more, the complex 1-
Cu2+ also exhibited good selectivity and sensitivity for S2− with a
distinct fluorescence enhancement and large Stokes shift of 234 nm,
which was much longer than reported previously (Table S1).
2.3. Theoretical calculation
The density functional theory (DFT) calculations were performed by
the B3LYP method with the 6–31 G(d) basis set using the Gaussian 09
package. During the calculations, no geometry constraint or symmetry
was imposed. To guarantee that all the structures studied were genuine
minima on the potential energy surface, frequency calculations were
performed at the same theoretical level.
2. Experiments section
2.1. Materials and instruments
3. Results and discussion
All reagents and solvents were purchased from commercial sup-
pliers and analytical grade. The UV–vis spectra were recorded on a
Varian Cary 500 spectrophotometer using a 1 cm path length quartz
cell, and the fluorescence spectra were recorded on CARY Eclipse
Spectrophotometer. 1H (400 MHz) and 13C (100 MHz) NMR spectra
were recorded on a Bruker AM-400 spectrometer in DMSO-d6 and
CDCl3 with tetramethylsilane (TMS) as an internal standard. High re-
solution mass spectrometry data were recorded on a Waters LCT
Permier XE spectrometer. The measurements of pH were done using a
pH-10C digital pH meter. Melting points were obtained in melting-point
tubes using an SGWX-4 apparatus.
3.1. Design and synthesis of probe
Since schiff-base fluorescent probes have been widely used in metal
ions detection owing to their high selectivity, sensitivity, reliability and
special complexation sites toward metal ions accompanied with obvious
optical signal changes [33,34], the well-designed probe 1 was synthe-
sized by the condensation of salicyladehyde azine (4) and a dicyanoi-
sophorone derivative (3), and the intermediate products 3 and 4 were
synthesized in good yields by the methods reported in the literatures
[31,32]. To explore the influence of molecular structure on the re-
cognition of metal ions, we synthesized a contrastive compound 6. All
the detailed characterization data determined by 1H and 13C NMR,
HRMS were presented in supporting information (S1-S14).
2.2. Synthesis of probe 1
Synthetic route of probe 1 is shown in Scheme 1. Compound 3 and
compound 4 were synthesized according to the previous works [31,32].
Compound
3
(100 mg, 0.3 mmol) and compound
4
(41 mg,
3.2. Solvent effect on the probe 1
0.3 mmol) were dissolved in 10 mL EtOH, and a drop of acetic acid was
added as catalyst. Then the mixture was heated to 80℃ and reacted for
12 h under N2 atmosphere. After cooling, the solution was poured into
ice water (50 mL), the crude precipitate was collected by filtration and
washed with water for three times. After being dried under vacuum, the
crude product was further purified by silica column chromatography
(petroleum ether/dichloromethane = 1/1, v/v) to give the pure pro-
duct as a yellow solid (105 mg, 80% yield). Mp: 242.2–243.5 °C. 1H
NMR (d6-DMSO, 600 MHz) δ: 11.46 (s, 1H, -OH), 11.09 (s, 1H, -OH),
9.03 (s, 1H, -CH = N-), 8.97 (s, 1H, -N = CH-), 8.07 (s, 1H, Ar-H), 7.80
(d, J = 8.8 Hz, 1H, Ar-H), 7.79 (d, J = 7.6 Hz, 1H, Ar-H), 7.43 (d,
J = 8.4 Hz, 1H, Ar-H), 7.39-7.30 (m, 2H, −CH = CH-), 7.04 (d,
J = 8.8 Hz, 1H, Ar-H), 7.02-6.96 (m, 2H, Ar-H), 6.86 (s, 1H, −CH=),
2.62 (s, 2H, −CH2-), 2.56 (s, 2H, −CH2-), 1.03 (s, 6H, -2CH3). ESI-
HRMS: Calc. Mass for [C27H24N4O2+H]+: 437.1978; found 437.1976.
We initially investigated the optical behaviour of probe 1 in
common organic solvents including toluene, dichlormethane, ethyl
acetate, tetrahydrofuran, ethanol, acetonitrile, DMF and DMSO. The
absorption and fluorescence spectra of probe 1 were shown in Fig. S15
and the corresponding data were summarized in Table S2. As depicted,
the absorption maxima showed a small range of movement with the
change of solvent polarity. However, fluorescence spectra showed
pronounced solvent-polarity dependent changes in the difference sol-
vents. In the toluene, there was very weak fluorescence emission could
be observed. It was then found that a large fluorescence enhancement
in DMSO-water (3:1, v/v) solvent (Fig. S15B). The changes of the
fluorescence spectrum of probe 1 in different ratios of DMSO-water
were also monitored (Fig. S15D). As a result, DMSO-water (2:1, v/v)
was selected as the ideal solvent media for this work.
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Fig. 1. (A) UV–vis spectral changes of probe 1
(10 μm) in the presence of different concentra-
tions of Cu2+ (0–2.5 equiv). Inset: colour
changes of probe (Left) and probe-Cu2+ (Right)
under daylight; (B) UV–vis spectral changes of
probe 1 (10 μm) in the presence of various other
metal ions (10 equiv) and Cu2+ (2.5 equiv). All
the tests were carried out in DMSO-HEPES
buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
3.3. Spectral studies of the probe 1 with Cu2+
Additionally, 1:1 bonding mode was confirmed by a Job’s plot analysis
(Fig. S18).
First, the absorption and fluorescence properties of this probe with
Cu2+ were investigated in DMSO-HEPES buffer (2:1, v/v, 20 mM
HEPES, pH = 7.4). As shown in Fig. 1A, free probe 1 exhibited a strong
absorption band at 425 nm (ε = 34856 M−1 cm-1) which was corre-
sponding to the S0-S4 type of transition (see TD-DFT calculation results,
table S3). In continuous addition of Cu2+(0–2.5 equiv), the maximum
peak at 425 nm decreased gradually. At the same time, a new absorp-
tion band formed at 475 (ε = 32467 M−1 cm-1) nm and an isosbestic
point appeared at 440 nm, which indicated a new stable complex be-
tween probe 1 and Cu2+ was formed. Besides, a distinct colour change
from light yellow to brown could be observed by naked eye (Fig. 1A
insert). Considering the coordination properties of schiff-base to metal
ions such as zinc, aluminum, iron, and mercury, we performed the se-
To examine the selectivity of probe 1 toward Cu2+, we performed
the fluorescence studies of probe 1 in the presence of various other
cations. As shown in Fig. 2B, no obvious changes could be observed
except for Cu2+, which displayed a distinct quenching of fluorescence
intensity with probe 1. Moreover, the anti-interference experiments
have also been conducted by adding Cu2+ (2.5 equiv) to the solution of
probe 1 containing interfering metal ions (2.5 equiv) (Fig. S19). In the
presence of these competitive species, the fluorescence intensity of
probe 1 could be also completely quenched by Cu2+, which demon-
strated that coexistence metal ions did not cause substantial inter-
ference in Cu2+ detection.
3.4. pH effect studies
lective experiments of probe 1 towards other metal ions, including K+
,
,
Na+, Ag+, Zn2+, Ni2+, Pb2+, Cd2+, Mg2+, Hg2+, Fe2+, Ba2+, Ca2+
The effect of pH ranging from 2.0 to 10.0 on the fluorescence re-
sponse of probe 1 to Cu2+ was investigated in DMSO-HEPES buffer
(2:1, v/v, 20 mM HEPES). As shown in Fig. 3 and Fig. S20, probe 1
displayed stable emission intensity in pH range 2.0-8.0. However, the
fluorescence intensity of probe 1 could not be quenched by Cu2+ in the
acid region (pH ≤ 5.0), which might be due to the protonation of the
two nitrogen atoms and hydroxyl groups of probe 1, hence the ability to
chelate with copper decreases. Again, the decreasing fluorescence in-
tensity of probe 1 in the basic region (pH > 8.0) might due to the
deprotonation of the two hydroxyl groups. Therefore, probe 1 can be
used as a reliable copper sensor in pH 6-8.
Co2+, Cr3+, Fe3+, and Al3+ (Fig. 1B). Upon the addition of 10 equiv of
each cation, the probe 1 showed almost no or slightly changes in ab-
sorbance spectra. In contrast, only Cu2+ induced a remarkable red shift
of absorbance spectra, indicating that probe 1 featured an excellent
selectivity to Cu2+
.
To further study the Cu2+-responsive sensitivity of probe 1, we
performed the fluorescence titration experiments. As shown in Fig. 2A,
the probe displayed a long wavelength emission band at 659 nm, the
addition of Cu2+ (0–25 μm) leads to a gradually decreased in fluores-
cence intensity. This change might be attributed to the chelation-en-
hanced fluorescence quenching (CHEQ) effect [35] and the para-
magnetic property of Cu2+. What’s more, we also found that the
fluorescence intensity of probe 1 showed a good linearity (R = 0.9946)
to Cu2+ concentrations in the range of 0–10 μm. The detection limit
(3σ/K) of probe 1 for Cu2+ was calculated to be 1.8 × 10−8 M (Fig.
S16), which is much lower than 1.3 μM allowed by environmental
protection agency (EPA), and the binding constant (K) was calculated as
1.71 × 103 M-1 by using Benesi-Hildebrand equation (Fig. S17).
3.5. DFT calculations
To further investigate the photophysical properties of probe 1 and
1-Cu2+, DFT calculations were performed by using the Gaussian 09
program. The HOMO and LUMO orbitals of probe 1 and 1-Cu2+ were
listed in Fig. 4 and Table S4. In the probe, the energy gaps between
HOMO and LUMO orbitals were measured to be 2.97 eV. Besides, the α
Fig. 2. (A) Fluorescence spectra changes of
probe
1
(10 μm) upon addition of Cu2+
(0–25 μm). Inset: photograph of probe 1 (Left)
and probe-Cu2+ (Right) on excitation 365 nm
using a UV lamp; (B) Fluorescence spectra of
probe 1 (10 μm) with different metal ions (Cu2+
25 μm, other metal ions 100 μm). λex
= 425 nm. All the tests were carried out in
DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES,
pH = 7.4).
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Fig. 3. The structure changes of probe 1 in the acidic and basic pH.
and β orbital of 1-Cu2+ were respectively 2.66 eV and 1.79 eV. These
HOMO and LUMO energy gaps decrease in the 1-Cu2+ indicated that
the energy required for the excitation of electron reduced as compared
to the free probe, and caused a red shift in the absorption wavelength,
which was consistent with the experimental results.
added, the UV–vis absorbance and fluorescence of 1-Cu2+ could be
largely restored. These results demonstrated that the binding of 1-Cu2+
and S2− was chemically reversible.
The fluorescence emission spectral titration experiments of 1-Cu2+
with S2− were subsequently studied. As shown in Fig. 5B, upon addi-
tion of S2− to the solution of 1-Cu2+, the emission intensity at 659 nm
exhibited a remarkable increase. Fig.S22 showed a good linear re-
lationship (R2 = 0.9953) and the detection limit was calculated as
1.5 × 10-8 M. All the spectra data were recorded within one minute
after S2- was added, and the intensity became stable over an extended
period of time (Fig. S23), indicating that complex 1-Cu2+ could act as a
real-time and stable probe for S2- detection.
3.6. Spectral studies of complex 1-Cu2+ with S2−
On the basis of Cu2+ has strongly binding affinities to sulfide, we
envisioned that the complex 1-Cu2+ could be used as an excellent turn-
on probe to further detect S2−. To confirm this assumption, we ex-
amined the absorbance and fluorescence response of the complex 1-
Cu2+ towards S2−. As shown in Fig. 5A, 1-Cu2+ exhibited an absorp-
tion band at 475 nm. After adding 3 equiv S2−, the maximum peak was
gradually blue-shifted, and the spectra eventually matched the original
probe 1. Besides, the colour of solution 1-Cu2+ turned into the initial
yellow, indicating the recovery of probe 1 from the 1- Cu2+ complex
through the chelation of S2- with copper. The reversible behavior of S2−
complexation with 1-Cu2+ was subsequently confirmed by EDTA ti-
tration. As shown in Fig. S21, when 1 equiv of EDTA solution was
To determine the selectivity of complex 1-Cu2+, we examined the
UV–vis absorption and fluorescence spectra toward various other an-
ions such as F−, Cl−, Br−, I−, CN−, AcO−, PO43-, HPO42-, H2PO4
,
−
SCN−, SO42-, SO32-, HSO4−, HSO3−, NO3−. As shown in Fig. S24, when
complex 1-Cu2+ was treated with the S2- (3 equiv), dual responses of
absorbance spectra and fluorescence intensity could be obtained. Upon
adding other anions (3 equiv), no significant change was observed ex-
cept CN−, which caused slightly increase of fluorescence and blue shift
Fig. 4. Frontier molecular orbital profiles of probe 1 and 1-Cu2+ based on DFT calculations.
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Fig. 5. (A) UV–vis spectral changes of 1-Cu2+ (10 μm) in the presence of different concentrations of S2− (0–30 μm.); (B) Fluorescence spectra changes of 1-Cu2+
(10 μm) upon addition of S2- (0–30 μm). λex = 425 nm. All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
of absorbance. Furthermore, in the presence of these anions, the red
emission of probe 1 could be largely restored by subsequent addition of
S2- (Fig. S25). These results obviously showed that the complex 1-Cu2+
can selectively detect S2-.
on these three coordination bonds, Mayer bond order analysis of the
probe-Cu2+ compound was conducted through the Multiwfn program
[36]. The values involving Cu2+ are listed in Table S5. As is known to
all, the value size of the Mayer bond order is positively correlated with
the strength of the bond. If the value approximates zero, it means there
is no or almost no bonding between two atoms. From the above data,
we can clearly see that the Mayer bond orders of O38-Cu56, O55-Cu56,
N54-Cu56 are much larger than those of N40-Cu56 and C39-Cu56
which are close to zero. Therefore, it can be concluded that these two
CueO coordinate bonds and one CueN coordinate bond are the
The effect of pH ranging from 2.0 to 10.0 on the fluorescence re-
sponse of complex 1-Cu2+ with S2− was also investigated (Fig. S26).
The emission intensity was significantly enhanced when pH was above
5, which indicated that the complex 1-Cu2+ could be used for the de-
tection of S2− in neutral and alkaline conditions.
dominant bonds of the probe bonding to Cu2+
.
To further confirm the binding of Cu2+ through these three atoms of
probe 1, we synthesized a contrastive compound 6 to mimic the binding
sites of probe 1. Compound 6 has the similar structure as probe 1 in
most parts except one hydroxyl group at the ortho-position of the schiff
base unit was removed. As a consequence, compound 6 showed nearly
no emission even in pure DMSO solution when excited at 420 nm
(ε = 68,685 M−1 cm−1). In contrast, probe 1 showed a strong fluor-
escence at 659 nm with excitation at 425 nm. Then, we carried out the
3.7. Mechanism studies
With the fact that binding ratio between 1 and Cu2+ was 1:1 stoi-
chiometry, we explained how the probe binds the Cu2+ through the
DFT calculations. The optimized geometries of the probe 1 and 1-Cu2+
complex were shown in Fig. 6. As can be seen, there were two in-
tramolecular hydrogen bonds (HBs) between the phenolic hydroxyls
and the nearby nitrogen atom. In the 1-Cu2+ complex, the phenol group
rotated and it combined with Cu2+ through two obvious Cu-O co-
ordinate bonds and one Cu-N coordinate bond.
titration and selectivity experiments of compound 6 toward Cu2+
.
However, the fluorescence and absorbance spectra of compound 6
showed no change upon addition of Cu2+ ions (Fig. S27). From these
studies, it is clearly that the two deprotonated phenolate groups and
In order to determine whether the binding of Cu2+ mainly depends
one nitrogen atom of probe 1 binding with the Cu2+
.
Since photo-induced electron (PET) is generally accepted as a me-
chanism for the “turn-on’’ fluorescent probe. To verify whether the
mechanism for the detection of S2− has undergone a PET process, we
calculated the HOMO–LUMOs energies of accepter and donor. As illu-
strated in Fig. 7 and Scheme 2, in the presence of Cu2+, the HOMO
(donor, E=−5.813 eV) has higher energy than of the acceptor (E=
−6.355 eV), therefore, upon excitation of complex 1-Cu2+, the electron
can be transferred from the HOMO (donor) to the low-energy HOMO of
the acceptor and quenches the fluorescence. Upon the addition of S2−
,
the Cu2+ ions are captured by S2- to produce stable CuS and release free
probe 1. Meanwhile, the energy of the receptor HOMO is lowered to
−5.957 eV, inhibiting the electron transfer from the HOMO donor (E=
−5.957 eV) to acceptor HOMO (E= −5.303 eV), and it leads to an
enhancement of the fluorescence. These results agree with the PET
mechanism reported for metal complexes [37–39].
3.8. Reversibility of the probe 1 for Cu2+ and S2−
For many practical applications, a good recyclability and reusability
is necessary. As shown in Fig. S28, the reversible experiments of probe
1 was repeated 5 times by the modulation of Cu2+ and S2− addition, no
obvious emission intensity attenuation at 659 nm could be observed,
indicating that probe 1 could be used as a reversible ON-OFF-ON sensor
for Cu2+ and S2- detection.
Fig. 6. The optimized structures of the probe and the probe-Cu2+ compound
(C: grey, H: white, N: blue, O: red, Cu: pink).
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Fig. 7. S2− recognition mechanism based on photo-induced electron transfer (PET).
Considering the toxicity of Cu2+ in the environment, we examined
the ability of probe 1 to quantitatively detect Cu2+ in real water sam-
ples. All the water samples were collected from laboratory tap water
and river water (Qingchun River, East China University of Science and
Technology, Shanghai, China), and filtered through a membrane before
used. Then a given volume (5 μM, 10 μM, 15 μM, 20 μM, 25 μM) of
Cu2+ aqueous solutions were added into the test solution (DMSO:
H2O = 2:1, v/v). We tested the fluorescent response of probe a to all
these water samples at 659 nm and used distilled water as a control
experiment. As shown in Fig. 9, the results displayed a good consistency
between water samples and distilled water, and the recovery of Cu2+
was between 82˜104% (Table 1). These results suggested that probe 1
has potential application for quantitative detection of Cu2+ in real
water samples.
4. Conclusions
Scheme 2. Proposed sensing mechanism of probe 1 for Cu2+ and S2−
.
In summary, we have designed and synthesized a highly selective
and sensitive fluorescent probe 1 that could detect Cu2+ and H2S se-
quentially and reversibly. The probe exhibited remarkable colour
change from light yellow to brown and significant fluorescence
quenching in the presence of Cu2+ with a detection limit of 1.8 × 10−8
M. In addition, the binding mode was also demonstrated using density
functional theory (DFT) calculations and contrastive compound re-
search. Notably, based on a complex 1-Cu2+ formation-separation
process and CuS formation, the complex of 1-Cu2+ ensemble could be
used as a turn-on type probe for H2S detection with a large Stokes shift
of 234 nm, and the detection limit reached as low as 1.5 × 10−8 M.
3.9. Applications in real samples
To examine the practical applications of probe 1, the test strips were
prepared by immersing filter paper into the solutions of probe 1 and 1-
Cu2+, and then drying in air. As shown in Fig. 8, the filter papers written
with Cu2+ and S2− solutions displayed significant changes in the colour
and fluorescence intensity. What’s more, the intensity of the coloured
writings also changed with the different concentrations of Cu2+
.
Fig. 8. The colour and fluorescence change of the strip papers in the presence of Cu2+ (0, 10 μm, 20 μm) and S2− (0, 20 μm) under daylight (left) and UV lamp
(right).
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Fig. 9. Fluorometric determination of Cu2+ in three water samples by probe 1 (10 μM) in DMSO/H2O solution (2:1, v/v). λex = 425 nm.
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