A rhodamine B-based "turn-on" fluorescent sensor for detecting Cu2+ and sulfur anions in aqueous media

Article abstract of DOI:10.1039/c3ra45931d

A novel selective fluorescent chemosensor L based on rhodamine B derivative has been designed and synthesized to optically detect Cu²⁺ and S^2- in aqueous buffer solution. The fluorescence of L was excited by Cu²⁺ with a 1:1 binding ratio, and could be used as a "turn-on" type sensor. Otherwise, the emission band would undergo a slight red-shift. Once binding with Cu²⁺, the copper complex can exhibit high selectivity for S^2-. Upon addition of Cu²⁺, the color of the chemosensor L changes from colorless to pink. Additionally, the resulting pink solution changes to colorless immediately upon the addition of S^2-. However, no phenomenon change could be observed in the presence of other anions. Conclusively, the color changes suggest that sensor L could serve as a "naked-eye" sensor for Cu²⁺ and S^2-.

Full text of DOI:10.1039/c3ra45931d

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PAPER
A rhodamine B-based “turn-on” fluorescent sensor
for detecting Cu2+ and sulfur anions in aqueous
media†
Cite this: RSC Adv., 2014, 4, 5718
a
a
a
a
Dan Guo,ab Zhengping Dong, Chao Luo, Wenyan Zan, Shiqiang Yan
*
and Xiaojun Yaoa
A novel selective fluorescent chemosensor L based on rhodamine B derivative has been designed and
synthesized to optically detect Cu2+ and S2ꢀ in aqueous buffer solution. The fluorescence of L was
excited by Cu2+ with a 1 : 1 binding ratio, and could be used as a “turn-on” type sensor. Otherwise, the
emission band would undergo a slight red-shift. Once binding with Cu2+, the copper complex can
exhibit high selectivity for S2ꢀ. Upon addition of Cu2+, the color of the chemosensor L changes from
colorless to pink. Additionally, the resulting pink solution changes to colorless immediately upon the
addition of S2ꢀ. However, no phenomenon change could be observed in the presence of other anions.
Received 18th October 2013
Accepted 11th December 2013
DOI: 10.1039/c3ra45931d
www.rsc.org/advances
Conclusively, the color changes suggest that sensor L could serve as a “naked-eye” sensor for Cu2+ and S2ꢀ
.
function like nitric oxide (NO), carbon monoxide (CO) in animal
systems.27,28 The sulfur anion is also an important anion in
biological and environmental samples. Most environmental
suldes are released by industries which are involved in
conversion of sulfur, such as preparation of sulfuric acid and
dyes, cosmetic manufacturing, and production of wood pulp.
On the other hand, sulde anions can also be generated from
microbial reduction of sulfate by anaerobic bacteria or from the
sulfur-containing amino acids in meat protein.29 The proton-
ated forms of sulfur, such as HSꢀ or H2S are more toxic than
sulde. If mammal drinks the water contaminated with sulde,
it will damage mucous membranes and cause respiratory
problem. For example, low concentration of hydrogen sulde
has been proven to produce dizziness, while higher concentra-
tion can cause loss of consciousness, permanent damage of
brain tissues, or even suffocation.30,31 However, current major
methods for H2S detection, such as inductively coupled plasma
atomic emission spectroscopy and electrochemical determina-
tion, titration and ion chromatography, oen require compli-
cated sample processing, which would lead to predicament for
accurate analysis of this important molecule.32–36 Therefore,
development of a quick and sensitive method for immediate
sulde detection in aqueous media is of high interest.
Introduction
Transition-metal ions are toxic, causing serious environmental
and health issues.1,2 As one of most common transition-metal
ions, Cu2+ is one of the highly toxic and global widespread
pollutants. As the third most abundant transition metal in the
human body, Cu2+ plays a vital role in many biological
processes.3–5 Some reports showed that excessive levels of Cu2+
accumulated could induce severe neurodegenerative diseases,
such as Menkes and Wilson's diseases, familial amyotrophic
lateral sclerosis, prion disease and Alzheimer's disease.6–11 Hence,
there remains a signicant challenge for development of rapid
detection and removal of copper from pollutants. Currently, many
techniques have been reported for detecting of Cu2+ ion,
including atomic absorption spectrometry, inductively coupled
plasma atomic emission spectroscopy.12–16 On the other hand,
uorescent chemosensors have attracted considerable attention
due to its simplicity, low-cost and sensitivity. In past few years,
many chemosensors with “turn-off” signal upon binding of Cu2+
have been reported.17–19 However, it is not as sensitive as uo-
rescene enhancement response.20–26 Therefore, it is also necessary
to design and synthesize uorescent probes of “turn-on” type in
aqueous systems, especially for the response enhancement.
Recently, the hydrogen sulde (H2S) has proved as a novel
gasotransmitter, and exhibited important physiological
Recently, rhodamine B-based dyes have been widely used as
uorescence reporting groups due to their excellent spectro-
scopic properties, such as large molar extinction coefficient,
high uorescence quantum yields, long absorption and emis-
sion wavelength.37 The sensing mechanisms of these chemo-
sensors probes are mainly based on the structure changes of
spirocyclic towards its open-cycle forms.38–45 On the grounds of
these strategies, we designed a novel rhodamine B moiety of L
as a “turn-on” uorescent probe for Cu2+ sensing in aqueous
aCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, PR China. E-mail: yansq@lzu.edu.cn; Fax: +86 931 8912582; Tel: +86 931
8912582
bHuayin Ordnance Test Center, Huayin 714200, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45931d
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spectra were determined on a Varian UV-Cary100 UV-vis spec-
trophotometer. Fluorescence spectra measurements were per-
formed on a Hitachi F-4500 uorescence spectrophotometer.
Mass spectra were obtained on a Bruker Daltonics-esquire 6000
mass spectrometer. All pH values were recorded on a PHS-3C
digital pH meter (Shanghai, China). All spectra were recorded at
room temperature.
Synthesis of compound 2
Compound 2 was synthesized based on the reported procedures
with minor changes.46 The process is the following steps: p-
cresol (33 g, 0.305 mol) were completely dissolved in sodium
hydroxide solution (10 mol Lꢀ1, 50 ml). Following full devel-
opment of a gold color, paraformaldehyde (18 g, 0.6 mol) was
added to the solution. The resulting solution was heated to
323 K for 1 hour with a magnetic stirrer bar, and then cooled to
room temperature. The yellow solid was collected and dissolved
again in hot water. The pH value was adjusted to 3 with HCl (2
mol Lꢀ1). The yellow precipitate was collected by ltration and
dried overnight in a vacuum oven at 323 K, which afforded a
creamy yellow solid compound 1. Then, compound 1 (10 g, 0.06
mol) was added to 300 ml of CHCl3 in a 500 ml round bottom
ask, aerwards, activated MnO2 (21.7 g, 0.25 mol) was added
to the solution with mechanically stirring and reuxing over-
night, aer which the reaction was cooled to room temperature
and ltered, washed with CHCl3 thoroughly until the ltrate
became colorless. Aer the solvent was evaporated under
reduced pressure, the crude product was puried by ash
column chromatography on silica gel (petroleum ether–ethyl
acetate ¼ 2 : 1, v/v), affording compound 2 as yellow solid
(6.27 g, yields: 62.89%). 1HNMR (CDCl3, 400 MHz): d 2.22 (s, 3H,
Ar–CH3), 3. 01 (s, H, CH2–OH), 4.60 (s, 2H, Ar–CH2), 7.15, (s, 1H,
Ar–H), 7.30 (s, 1H, Ar–H), 9.72 (s, 1H, N]C–H), 11.03 (s, 1H, Ar–
OH) ppm. 13C NMR (CDCl3, 100 MHz): d 20.09, 59.96, 119.86,
128.88, 128.97, 132.38, 136.66, 157.00, 196.59 ppm.
Scheme 1 Synthesis of the fluorescent sensor L.
solution (Scheme 1). In the absent of Cu2+ ions, the L exists in
spirocyclic form, which is colorless with weak uorescent
intensity. Upon addition of Cu2+ ion into L, the spirocycle
structure opened and the solution showed clear pink color,
which also generated a uorescence enhancement response.
Additionally, high selectivity toward Cu2+ ion over other metal
ions was clearly observed. Meanwhile, we also found L–Cu2+
complex can be employed to detect sulde. By adding sulde to
the solution of L–Cu2+ complex, the color of the aqueous change
from pink to colorless.
Synthesis of rhodamine sensor (compound L)
Experimental
RBH was synthesized from rhodamine B according to the
literature method.47
Reagents
All reagents and solvents were purchased from commercial
suppliers. All chemicals used in this work were of analytical
reagent grade and used without further purication. The solutions
of the metal ions were prepared from NaNO3, Mg(NO3)2$6H2O,
RBH (0.30 g, 0.66 mmol) was dissolved in 15 ml absolute
ethanol. Excess compound 2 (0.16 g, 0.99 mmol) was added to
the reaction mixture, then the mixture was stirred and reux
under N2 atmosphere for 6 hour. Then the solvent was removed
under reduced pressure, and the residue was puried by
recrystallization with ethanol as yellow solid (0.29 g, yields:
Al(NO3)3$9H2O,
KNO3,
Ca(NO3)2$4H2O,
Cr(NO3)3$9H2O,
MnCl2$4H2O, Fe(NO3)3$9H2O, FeCl2$4H2O, Co(NO3)2$6H2O,
Ni(NO3)2$6H2O, Cu(NO3)2$3H2O, Zn(NO3)2$6H2O, AgNO3,
Cd(NO3)2$4H2O, Ba(NO3)2, Hg(NO3)2$0.5H2O, Pb(NO3)2. The solu-
tions of anions were prepared from NaNO2, NaNO3, KCN, KSCN,
Na2SO4, Na2SO3, Na3PO4$12H2O, Na2HPO4, KH2PO4, NaAc, NaCl,
NaBr, KI, Na2S. Tris–HCl buffer solutions (1 ꢁ 10ꢀ2 mol Lꢀ1, pH
7.2) and all other solutions were prepared in deionized water.
1
72.7%). HNMR (400 MHz, CDCl3): d 1.19–1.13 (t, J ¼ 7.0 Hz,
12H, NCH2CH3), 2.19 (s, 3H, ben–CH3), 2.75 (s, 1H, ben–CH2–
OH), 3.37–3.29 (q, J ¼ 7.0 Hz, 8H, NCH2CH3), 4.62–4.64 (d, J ¼
5.5 Hz, 2H, ben–CH2–OH), 6.25–6.52 (m, 6H, xanthene–H),
6.82–6.79 (d, J ¼ 1.1 Hz, 1H, ben–H), 6.98 (s, 1H, ben–H), 7.11–
7.19 (m, 1H, Ar–H), 7.47–7.54 (td, J1 ¼ 6.5, J2 ¼ 1.2 Hz, 2H, Ar–
H), 7.93–8.04 (m, 1H, Ar–H), 8.91 (s, 1H, N]C–H), 11.17 (s, 1H,
ben–OH); 13C NMR (101 MHz, CDCl3): d 12.60, 20.23, 44.38,
Instruments
1H and 13C NMR spectra were taken on a Varian Mercury 300 62.09, 66.16, 97.99, 105.13, 108.27, 118.00, 123.37, 124.04,
MHz NMR spectrometer in CDCl3 solutions, with tetrame- 127.86, 128.09, 128.23, 128.53, 129.37, 130.81, 131.24, 133.56,
thylsilane (TMS, 0.00 ppm) as an internal standard. Absorption 149.10, 151.30, 151.83, 153.32, 154.46, 164.33. ESI-MS
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spectrometry showed a peak with m/z 605.6 [M + H]+, calculate
for C37H40N4O4 ¼ 604.74. Anal. calcd for C37H40N4O4 (604.74): C
73.42, H 6.61, N 9.26, O 10.71 found: C 73.21, H 6.65, N 9.06, O
11.08%.
Results and discussion
UV-vis spectroscopic studies of L in presence of Cu2+
The spectra responses of chemosensor L in the absence or
presence of Cu2+ in different pH values were evaluated rst. As
shown in Fig. 1, the absorption of free L was very weak between
pH 4 and 10. When the pH value was lower than 4, it showed
increased absorption. It was attributed to the ring opening of
rhodamine occurred at acid conditions (pH < 4) for strong
protonation (Scheme S1†). However, the absorbance intensity
had signicant enhancement aer the addition of Cu2+ ion
between pH 4 and 9. It was due to the formation of ring-opened
L–Cu2+ complex (Scheme 2). Then under basic conditions, the
absorbance intensity decreased due to the formation of
Cu(OH)2. The data indicated that L has good absorption
response for Cu2+ under physiological pH conditions. There-
fore, further UV-vis studies were carried out in CH3CN/Tris–HCl
solution (1 : 1 v/v, pH ¼ 7.2).
Fig. 2 (a) UV-vis absorption of L (2 ꢁ 10ꢀ5 mol Lꢀ1) in the presence of
different metal ions (1 ꢁ 10ꢀ4 mol Lꢀ1) in CH3CN/Tris–HCl solution
(1 : 1 v/v, pH ¼ 7.2). (b) Photographs of color changes of 2 ꢁ 10ꢀ5 mol
L
ꢀ1 L after the addition of 1 ꢁ 10ꢀ4 mol Lꢀ1 various metal ions (from left
to right: ion-free sensor L, Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+
,
Fe3+, Co2+, Ni2+, Zn2+, Ag2+, Cd2+, Ba2+, Hg2+, Pb2+, Cu2+).
an obvious color change from colorless to pink. A slight increase
at 555 nm aer the addition of Fe2+ was also observed at the
same concentration. We infer that Fe2+ ion have lower binging
affinity to L compared with Cu2+ in aqueous media. Under the
same conditions, no obvious response could be observed upon
the addition of other ions. Therefore, L can serve as a “naked-
eye” indicator for Cu2+ ion. Even in the presence of miscella-
neous competitive metal ions, the addition of Cu2+ ion still
resulted in a large absorption change at 556 nm (Fig. 3). This
indicates that the selectivity of L to Cu2+ ion is excellent in the
presence of other competitive cations in acetonitrile–water
medium.
The solution of L in CH3CN/Tris–HCl solution (1 : 1 v/v, pH
¼ 7.2) is colorless. As shown in Fig. 2, the absorption spectra of
L alone (2 ꢁ 10ꢀ5 mol Lꢀ1) exhibited no band in the region
beyond 500 nm, which indicated that L was of the spirolactam
form. The addition of Cu2+ into the solution immediately
resulted in a strong absorption band centered at 556 nm, with
Titration experiment showed that the absorption spectra of L
to Cu2+ ion also gradually increased at 401 nm and 556 nm, with
higher Cu2+ concentration (0–10 ꢁ 10ꢀ5 mol Lꢀ1) (Fig. 4). The
Fig. 1 UV-vis absorption of free L (2 ꢁ 10ꢀ5 mol Lꢀ1) and L + 5 equiv.
of Cu2+ ion in CH3CN/Tris–HCl solution (1 : 1 v/v) with different pH
conditions. The absorption wavelength is 556 nm.
Fig. 3 Absorption spectra of L to various metal ions in CH3CN/Tris–
HCl solution (1 : 1 v/v, pH ¼ 7.2). The black bar represent the spectra of
L (2 ꢁ 10ꢀ5 mol Lꢀ1) obtained with (1 ꢁ 10ꢀ4 mol Lꢀ1) of metal ions.
The red bar represents the spectra that occur upon the subsequent
addition of (1 ꢁ 10ꢀ4 mol Lꢀ1) of Cu2+ to the above mentioned solu-
tions. The absorption wavelength is 556 nm.
Scheme 2 Proposed binding mode of probe L toward Cu2+
.
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absorbance of L in the presence of excess amount of Cu2+. [Cu2+
]
is the concentration of Cu2+ ion added. Plotting of 1/(A ꢀ A0)
versus 1/[Cu2+] showed a linear relationship (Fig. 6), which
indicates that L bound with Cu2+ in a 1 : 1 binding stoichiom-
etry, and the association constant Ka is determined from the
slope as 6.47 ꢁ 104 Mꢀ1. The ESI-mass spectra of L–Cu2+ also
showed a 1 : 1 stoichiometry. The unique peak at m/z ¼ 666.18,
(calcd ¼ 666.27) corresponding to [CuL]+ was clearly observed
when Cu2+ was added to L (Fig. S6†).
Fluorescence spectroscopic studies of L in presence of Cu2+
The effects of pH on the sensor L was evaluated rst. Fig. 7
shows that for ion-free sensor L, the uorescence intensity was
strong at pH < 6. It was due to the ring opening of rhodamine for
the strong protonation. The uorescence intensities of ion-free
sensor L gradually decreased with the increase of pH value.49 In
the pH range from 6 to 10, the uorescence signal of L without
Cu2+ ion was weak. Upon the addition of Cu2+ ion, there was an
obvious uorescence emission at 581 nm in pH from 6 to 9. This
Fig. 4 UV-vis spectra change of L (2 ꢁ 10ꢀ5 mol Lꢀ1) upon the addi-
tion of increasing amounts of Cu2+ ion in CH3CN/Tris–HCl solution
(1 : 1 v/v, pH ¼ 7.2) (from bottom to top: [Cu2+] ¼ 0, 2, 4, ., 60, 70, 85,
100 ꢁ 10ꢀ6 mol Lꢀ1). Inset: absorbance at 556 nm of L as a function of
Cu2+ concentration.
absorbance at 556 nm had an approximate 643-fold enhance- result suggested that L could act as a uorescent probe for Cu2+
ment. The Job's plot was conducted to determine the binding
stoichiometry of the L–Cu2+ complex, wherein the total
concentration of L and Cu2+ ion is 4 ꢁ 10ꢀ5 mol Lꢀ1 and the
mole fraction of Cu2+ ion is in the range from 0 to 1. As shown in
Fig. 5, we can observe that the absorbance went through a
maximum peak at a molar fraction of about 0.5, indicating a
1 : 1 binding stoichiometry between Cu2+ and L. To achieve this
stoichiometry, carbonyl O, imino N, and phenol O atoms of L
are the most likely binding sites for Cu2+. The proposed struc-
ture of L–Cu2+ is illustrated in Scheme 2.
Assuming a 1 : 1 association between L and Cu2+, the asso-
ciation constant (Ka) of L–Cu2+ was determined using the Ben-
esi–Hildebrand equation as follows.48
1
1
1
¼
þ
(1)
KaðAmax ꢀ A0Þ½Cu2þꢂ
A ꢀ A0
Amax ꢀ A0
Fig. 6 Benesi–Hildebrand plot (absorbance at 556 nm) of L using eqn
(1), assuming 1 : 1 stoichiometry for association between L and Cu2+
A and A0 represent the absorbance of L solution in the
presence and absence of Cu2+ ion, and Amax is the saturated
.
Fig. 7 Fluorescence intensity of free L (2 ꢁ 10ꢀ5 mol Lꢀ1) and L + 5
Fig. 5 Job's plot of L and Cu2+ in CH3CN/Tris–HCl solution (1 : 1 v/v, equiv. of Cu2+ ion in CH3CN/Tris–HCl solution (1 : 1 v/v) with different
pH ¼ 7.2). The total concentration of L and Cu2+ ions is 4 ꢁ 10ꢀ5 mol pH conditions. The excitation wavelength is 530 nm. The emission
L
ꢀ1. The absorbance was collected at 556 nm.
wavelength is 581 nm.
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under physiological conditions (i.e. pH 7.2). The experiments recognition of Cu2+ by L was hardly inuenced by other co-
have also been conducted in Tris–HCl solutions, wherein the existing metal ions.
selectivity of sensor L towards Cu2+ is also good. However, the
From the uorescence titration experiments (Fig. 10), upon
uorescence intensity in Tris–HCl solutions is not as strong as addition of Cu2+ into CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼
that in mixed solvents of acetonitrile and Tris–HCl buffer. 7.2) of L, a new emission band centered at 572 nm (the excita-
Therefore, further uorescent studies were also carried out in tion wavelength is 530 nm) was developed. Similar to the
CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2).
absorption response, the uorescence intensity increased
L in CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2) showed a gradually with higher Cu2+ concentration. Meanwhile, the
very weak uorescence in the absence of metal ions when emission band underwent a slightly red-shied from 572 to 581
excited at 530 nm. However, the addition of Cu2+ ion resulted in nm aer 2 equiv. of Cu2+ were added. The uorescence intensity
remarkably enhanced uorescence intensity. Under the same at 581 nm had an approximate 7-fold enhancement. The red-
condition, additions of other metal ions including Na+, Mg2+, shi of the emission peak can be ascribed to the recombination
Al3+, K+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Zn2+, Ag+, Cd2+, Ba2+, of the orbitals aer the formation of ring-opened L–Cu2+
Hg2+ and Pb2+ did not cause any discernible changes. On the complex. The emission intensity of ion-free sensor L was
other hand, the Fe2+–ligand complex, in contrast to the rest of measured 10 times and the standard deviation of blank
the ions in study, exhibited lower uorescence intensity than measurements was determined. The uorescence intensity of L
the free ligand (Fig. 8). Similarly, parallel experiments in the (2 ꢁ 10ꢀ5 mol Lꢀ1) at 581 nm was found to increase linearly with
presence of potentially competitive metal ions were also carried the concentration of Cu2+ in range of 0–0.8 ꢁ 10ꢀ5 mol Lꢀ1 (R2
out (Fig. 9) and the results suggested that the uorescent ¼ 0.9910) (Fig. S7†). The detection limit was then calculated
with the eqn (2):31
Detection limit ¼ 3s/k
(2)
where s is the standard deviation of blank measurement, and k
is the slope of the intensity versus Cu2+ concentration. The
detection limit of Cu2+ in CH3CN/Tris–HCl solution (1 : 1 v/v,
pH ¼ 7.2) was measured to be 2.43 ꢁ 10ꢀ8 mol Lꢀ1
.
Theoretical calculations
In order to further verify the conguration of L–Cu2+, we carried
out density functional theory (DFT) calculations with the B3LYP
exchange functions using Gaussian 09 package, and introduce
the LANL2DZ effective core potential (ECP) to represent the core
electrons of the Cu atom. The valence electrons are described by
LANL2DZ basis set, while all the other atoms are described
commonly by 6-31G(d) basis set. All the thermodynamic data
are obtained at this computational level. The molecule forms a
Fig. 8 Fluorescence of L (2 ꢁ 10ꢀ5 mol Lꢀ1) in the presence of
different metal ions (1 ꢁ 10ꢀ4 mol Lꢀ1) in CH3CN/Tris–HCl solution
(1 : 1 v/v, pH ¼ 7.2). The excitation wavelength is 530 nm.
Fig. 9 Fluorescence spectra of L to various metal ions in CH3CN/Tris–
HCl solution (1 : 1 v/v, pH ¼ 7.2), the black bar represent the spectra of
L (2 ꢁ 10ꢀ5 mol Lꢀ1) obtained with (1 ꢁ 10ꢀ4 mol Lꢀ1) of metal ions. Fig. 10 Fluorescence emission spectra changes of L (2 ꢁ 10ꢀ5 mol
The red bar represents the spectra that occur upon the subsequent
L
ꢀ1) upon the addition of increasing amounts of Cu2+ ion in CH3CN/
addition of (1 ꢁ 10ꢀ4 mol Lꢀ1) of Cu2+ to the above mentioned solu- Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2) (from bottom to top: [Cu2+] ¼ 0,
tions. The excitation wavelength is 530 nm. The emission wavelength 2, 4, ., 40, 50, 60, 70, 80, 90, 100 ꢁ 10ꢀ6 mol Lꢀ1). Inset: fluorescence
is 581 nm.
titration profile of L at 581 nm in increasing of Cu2+ concentration.
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planar structure, as displayed in Fig. 11. The Cu–N bond length that only by adding S2ꢀ into the solution of L–Cu2+ could cause
˚
is 1.975 A, and the distances of Cu–O1, Cu–O2, Cu–O3 are 1.992, this change, whereas other anions failed to produce any
˚
1.912, 1.978 A, respectively. The optimized conguration shows discernible spectral change (Fig. 12). For further investigation, a
that Cu2+ ions occupy the acylhydrazone coordination centers of solution of L in CH3CN/Tris–HCl buffer (1 : 1 v/v, pH ¼ 7.2)
L at the same time.
containing 2 equiv. of Cu2+ was titrated by the solution of Na2S.
The UV-vis spectral pattern of the titration experiment was
similar but in reverse direction to the titration curve obtained
with Cu2+ (Fig. 13). Upon the addition of S2ꢀ to L–Cu2+ complex,
black precipitate was formed. This phenomenon showed that
Cu2+ released from complex L–Cu2+ was captured by sulde
anion to produce CuS. The formation of CuS was also ascer-
tained by the XRD measurement. The sensing capability of L–
Cu2+ complex was further tested in the presence of other anions,
which may interfere the estimation of copper and sulde
UV-vis spectroscopic studies of L–Cu2+ complex in presence of
S2ꢀ
We have further studied the inuence of different anions on UV-
vis absorbance of L–Cu2+ complex. Upon addition of Na2S to L (2
ꢁ 10ꢀ5 mol Lꢀ1) and Cu2+ (4 ꢁ 10ꢀ5 mol Lꢀ1) complex in
CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2), the UV-vis
absorption was decreased and the color of the L–Cu2+ complex
changed from pink to colorless. The optical properties of the L–
Cu2+ complex were also studied in the presence of different
anions such as CNꢀ, SCNꢀ, SO32ꢀ, SO42ꢀ, PO43ꢀ, HPO4
,
2ꢀ
H2PO4ꢀ, Clꢀ, Brꢀ, Iꢀ, NO2ꢀ, NO3ꢀ, and AcOꢀ. It is worth noted
Fig. 13 UV-vis titration spectra of L (2 ꢁ 10ꢀ5 mol Lꢀ1) with 4 ꢁ 10ꢀ5
mol Lꢀ1 of Cu2+ upon addition of sodium sulfide (4 ꢁ 10ꢀ5 mol Lꢀ1) in
CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2) (from top to bottom:
[S2ꢀ] ¼ 0, 2, 4, ., 40, 50, 60, 70 ꢁ 10ꢀ6 mol Lꢀ1). Inset: changes in the
absorbance at 556 nm with incremental addition of S2ꢀ
.
Fig. 11 Calculated energy-minimized structure of L with Cu2+
.
Fig. 14 UV-vis responses of L–Cu2+ to various anions in CH3CN/Tris–
HCl solution (1 : 1 v/v, pH ¼ 7.2). The black bars represent the absor-
bance responses of L (2 ꢁ 10ꢀ5 mol Lꢀ1) and Cu2+ ion (4 ꢁ 10ꢀ5 mol Lꢀ1
)
in the presence of anions (4 ꢁ 10ꢀ5 mol Lꢀ1) of interest. The red bars
represent the change of the absorbance that occurs upon the subse-
Fig. 12 UV-vis titration spectra of L (2 ꢁ 10ꢀ5 mol Lꢀ1) with 4 ꢁ 10ꢀ5 quent addition of S2ꢀ (4 ꢁ 10ꢀ5 mol Lꢀ1) to the above solution. The
mol Lꢀ1 of Cu2+ upon addition of different anions (4 ꢁ 10ꢀ5 mol Lꢀ1) in intensities were recorded at 556 nm. (1) L–Cu2+ (2) CNꢀ (3) SCNꢀ (4)
CH3CN/Tris–HCl solution (1 : 1 v/v, pH ¼ 7.2) (inset, A: L + Cu2+. B: L + SO32ꢀ (5) SO42ꢀ (6) PO43ꢀ (7) HPO42ꢀ (8) H2PO4ꢀ (9) Clꢀ (10) Brꢀ (11) Iꢀ
Cu2++ S2ꢀ).
(12) NO2ꢀ (13) NO3ꢀ (14) AcOꢀ (15) S2ꢀ
.
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(Fig. 14). The receptor L–Cu2+ complex is well selective in 15 A. Vaisanen, R. Suontamo, J. Silvonen and J. Rintala, Anal.
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¨
presence of S2ꢀ. ESI-MS of the above media displayed a 17 N. Shao, Y. Zhang, S. M. Cheung, R. H. Yang, W. H. Chan,
molecular peak [L + H+] at m/z 605.6 and a molecular-ion peak [L
T. Mo, et al., Anal. Chem., 2005, 77, 7294–7303.
+ Na+] at m/z 627.5 which conrmed the identity of free L 18 S. H. Kim, J. S. Kim, S. M. Park and S. K. Chang, Org. Lett.,
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Conclusions
In summary, a rhodamine-based uorimetric probe L was
designed and synthesized. Studies showed that L exhibited 21 J. Zhang, C. W. Yu, S. Y. Qian, G. Lu and J. L. Chen, Dyes
highly selective binding with Cu2+ over other metal ions with a
Pigm., 2012, 92, 1370–1375.
uorescence turn-on effect. The chemosensor L displayed a one- 22 Z. P. Dong, X. Tian, Y. Z. Chen, J. R. Hou and J. T. Ma, RSC
to-one complex formation with Cu2+ ions in a broad pH range.
Adv., 2013, 3, 2227–2233.
Obvious increases in colorimetric changes were observed upon 23 C. W. Yu, L. X. Chen, J. Zhang, J. H. Li, P. Liu, W. H. Wang,
the addition of Cu2+ into the CH3CN/Tris–HCl solution (1 : 1 v/v,
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and Cu2+ is dissociable only in the presence of sulde anion and
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Acknowledgements
The authors acknowledge nancial support from the NSFC
(Grants 21301082), the Natural Science Foundation of Gansu 28 R. Wang, Antioxidants and Redox Signaling, 2003, vol. 5, pp.
(no. 1308RJYA028) and Huayin Ordnance Test Center. We thank
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29 Hydrogen Sulde, World Health Organization, Geneva, 1981,
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