A new rhodamine-based colorimetric chemosensor for naked-eye detection of Cu2 + in aqueous solution

Article abstract of DOI:10.1016/j.saa.2015.12.031

A new colorimetric probe 1 based on rhodamine B lactam was developed for naked-eye detection of Cu^{2 +}. The optical feature of 1 for Cu^{2 +} was investigated by UV-vis absorption spectroscopy. Upon the addition of Cu^{2 +}, the 1 displayed a distinct color change from colorless to pink, which can be directly detected by the naked eye. The stoichiometry of 1 to Cu^{2 +} complex was found to be 1:1 and the naked-eye detection limit was determined as low as 2 μM. The results suggest that the probe 1 may provide a convenient method for visual detection of Cu^{2 +} with high sensitivity.

Full text of DOI:10.1016/j.saa.2015.12.031

A new rhodamine-based colorimetric chemosensor for naked-eye detection of
Cu2 + in aqueous solution
Yang Hu, Jing Zhang, Yuan-Zheng Lv, Xiao-Huan Huang, Sheng-li
Hu
PII:
DOI:
Reference:
S1386-1425(15)30355-3
doi: 10.1016/j.saa.2015.12.031
SAA 14209
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Revised date:
Accepted date:
4 October 2015
18 December 2015
26 December 2015
Please cite this article as: Yang Hu, Jing Zhang, Yuan-Zheng Lv, Xiao-Huan Huang,
Sheng-li Hu, A new rhodamine-based colorimetric chemosensor for naked-eye detection
of Cu2 + in aqueous solution, (2015), doi: 10.1016/j.saa.2015.12.031
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ACCEPTED MANUSCRIPT
A new rhodamine-based colorimetric chemosensor for
naked-eye detection of Cu2+ in aqueous solution
Yang Hua, Jing Zhangb, Yuan-Zheng Lva, Xiao-Huan Huanga, Sheng-li Hu a,*
a Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Department of
Chemical and Enviromental Engineering, Hubei Normal University, Huangshi
435002, PR China
b Faculty of materials science & engineering, Hubei University,Wuhan 430062, PR
China
Abstract
A new colormetric probe 1 based on rhodamine B lactam was developed for
naked-eye detection of Cu2+. The optical feature of 1 for Cu2+ was investigated by
UV-vis absorption spectroscopy. Upon the addition of Cu2+, the 1 displayed a distinct
color change from colorless to pink, which can be directly detected by the naked eye.
The stoichiometry of 1 to Cu2+ complex was found to be 1:1 and the naked-eye
detection limit was determined as low as 2 μM. The results suggest that the probe 1
may provide a conveniently method for visual detection of Cu2+ with high sensitivity.
Key words
Rhodamine B, Colormetric probe, Cu2+, Spirolactam
1. Introdution
*Corresponding author: E-mail：hushengli168@126.com Tel.: +86 0714 6515602 Fax: +86 0714
6515602
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The development of chemosensors for the detection of metal ions has received
considerable attention because of their important roles in medicine, living systems and
the environment.1,2 Among the methods available for chemosensors, a colorimetric
technique appeared to be the most attractive, which could be widely used owing to the
low cost and lack of equipment required, and a color change that can easily be
observed by the naked eyes, even at very low analyte concentration. On the other
hand, Cu2+ is not only a significant metallic pollutant, but also an essential element for
living organisms.3-6 So far, many progress has been achieved in the creation of a
chemosensor for Cu2+,7-18 however there is still room to develop easy-to-make,
simple-to-use and observable colorimetric sensors for Cu2+ in aqueous solution. Work
related to this area is of great challenge and increasing interest.
Rhodamine is an ideal platforms for development of colormetric and fluorescent
probe owing to its excellent photophysical properties.19,20 Specifically, it is based on
spirolactam (colorless and nonfluorescent) to ring-opened amide (color and
fluorescent) process utilized for the detection of metal ions.21-31 Based on this
mechanism, our ambition was to design and synthesize a new colormetric probe for
detection of Cu2+ in aqueous media. Herein, we introduce a novel and simple method
to prepare rhodamine B-based chemosensor by two steps reaction. The sensor 1
showed “off–on” type chromogenic behavior toward Cu2+ over other interfering metal
ions.
2. Experimental
2.1. Reagents
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All the reagents were purchased from commercial suppliers and used without
further purification. The salts used in stock solutions of metal ions were CoCl2·6H2O,
ZnCl2, MnCl2·4H2O, KCl, NaCl, CuCl2·2H2O, NiCl2·6H2O, CdCl2·2H2O, HgCl2,
FeCl3·6H2O, MgCl2·6H2O, CrCl3·6H2O, Pb(NO3)2.
2.2. Apparatus
NMR spectra were measured on a Varian Mercury 300 spectrometer operating at
300 MHz for 1H and 75 MHz for 13C relative to tetramethylsilane as internal standard.
MS spectra were obtained on a Finnigan Trace MS spectrometer. Elemental analyses
were performed with a Vario EL-III instrument (Germany). IR spectra were recorded on a
Perkin-Elmer PE-983 infrared spectrometer as KBr pellets with absorption reported in
cm-1. Absorption spectra were determined on UV-2501 PC spectrophotometer.
2.3. Synthesis of rhodamine derivate (1)
The synthetic route of the compound 1 is shown in Scheme 1. Rhodamine B
hydrazide (2)32 was synthesized according to the previously reported procedure. To a
100mL flask, rhodamine B hydrazide(2) (0.91 g, 2.0 mmol) was dissolved in 50 mL
ethanol. After the addition of acetylacetone (1 mL, excess), the stirred mixture was
heated to reflux for 24 h. The residue was recrystallized from ethanol and afforded
compound 1 as a white solid, yield: 85% (Scheme 1). The colorless crystals were
obtained by slow evaporation of a solution of 1 in CHCl3-CH3OH (20:1, v/v) mixture
at room temperature. M.p.184–185oC, IR(νmax, KBr, cm-1): 3446, 2972, 2364, 1714,
1620, 1576, 1513, 1429, 1352, 1272, 1221, 1115, 1084, 968, 865, 818, 756; 1H NMR
(300 MHz, CDCl3): δ 11.12(s, 1H), 7.18-7.98 (m, 4H), 6.29-6.49(m,6H), 4.96(s,1H),
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3.34 (q,8H), 1.93(s,3H), 1.54 (s,3H),1.16(t,12H, J=6.9 Hz); 13CNMR (75 MHz,
CDCl3): δ 195.6, 165.5, 163.0, 153.6, 150.2, 148.6, 132.8, 129.2, 128.1, 128.0, 123.9,
122.9, 107.4, 103.8, 97.5, 96.6, 66.2, 43.9, 28.5, 17.6, 12.1. ESI-mass: m/z 539.37
([M+H]+). Elemental analysis: C33H38N4O3, Calcd: C, 73.58; H, 7.11; N, 10.40. Found:
C, 73.42; H, 7.03; N, 10.29. Crystal data for C33H38N4O3: crystal size: 0.20×0.08×
0.05 mm, monoclinic, space group P21/c, a = 17.4047(17) Å, b = 15.0879(19) Å, c =
11.5212(11)Å, V = 1367.1(2) Å3, Z = 4, T = 293(2) K, θ max = 50.04o, Dc = 1.214 g
cm-3, 14012 reflections measured, 5309 unique (Rint = 0.0767). Final R indices
[I>2σ(I)]: R1 = 0.0697, wR2 = 0.1277 and GOF =1.025.
(Scheme 1)
2.4. X-ray crystallography of compound 1
Suitable single crystals of 1 for X-ray structural analysis were obtained by slow
evaporation of a solution of 1 in CHCl3-CH3OH (20:1, v/v) mixture at room
temperature. The diffraction data was collected with a Bruker SMART CCD
diffractometer using a graphite monochromated MoKα radiation (λ = 0.71073Å) at
296(2) K. The structures were solved by direct methods with SHELXS-97 program
and refinements on F2 were performed with SHELXL-97 program by full-matrix
least-squares techniques with anisotropic thermal parameters for the non-hydrogen
atoms. All H atoms were initially located in a difference Fourier map. All H atoms
were placed in geometrically idealized positions and constrained to ride on their
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parent atoms, with C–H = 0.93Å and Uiso(H) = 1.2Ueq(C).
2.5. Analytical procedure
The stock solutions of 1(1.0×10-5 M) were prepared by dissolving 1 in EtOH/
water (9:1, v/v) containing HEPES buffer(10 mM, pH =7.0). The cationic stocks were
all in H2O with a concentration of 3.0×10-3 M for UV–vis absorption spectra analysis.
For metal ion absorption titration experiments, each time 3 mL solution of 1 filled in a
quartz cell of 1 cm optical path length, and we increased concentrations of metal ions
by stepwise addition of different equivalents using a micro-syringe. After each
addition of Cu2+ ion, the solution was stirred for 5 min. The volume of cationic stock
solution added was less than 100 μL with the purpose of keeping the total volume of
testing solution without obvious change.
3. Results and discussion
3.1. Synthesis and structural characteristics of 1
The compound 1 was obtained by the reaction of rhodamine B hydrazide (6) with
excess acetylacetone in EtOH under reflux. The yield of 1 was 85%. The structures of
1 were identified by using 1H NMR, 13C NMR, ESI-MS (Fig. S1-3), and further
comfirmed by X-ray diffraction analysis. The molecular view of 1 is shown in Fig. 1.
The crystal structure of 1 clearly reveals the unique spirolactam ring formation, the
ketone carbonyl oxygen (O2) and nitrogen atom (N4) form the intramolecular
hydrogen bond. Notably, a hexatomic ring (C9-C11-C12-N4-H-O2) is formed with
the aid of the hydrogen binding.
（Fig. 1）
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3.2. Spectral characteristics
To examine the binding properties of 1 with metal ions, the UV/vis titration
absorption spectra of 1(10μM) in EtOH/water (9:1, v/v) containing HEPES
buffer(10mM, pH =7.0) was first explored in the presence of 2 equivalent of different
metal ions and the results were depicted in Fig. 2. A solution of 1 in EtOH/ water (9:1,
v/v) containing HEPES buffer (10mM, pH = 7.0) is colorless. The sensor 1 exhibited
very weak absorption in the visible range. Upon binding of metal ions (K+, Na+, Ni2+,
Mg2+, Mn2+, Cr3+, Pb2+, Zn2+, Cd2+, Cu2+, Co2+, Fe3+ and Hg2+)(as their chloride salts),
it was found that only the Cu2+ ion causes the absorption peak intensity obvious
change at 552 nm. At the same time, the solution changed from colorless to pink
which could easily be detected by the naked-eye. Other metal ions did not cause this
colour change under the same conditions (Fig. 3). Accordingly, 1 was sensitive to
Cu2+ ions in the visible light region.
(Fig. 2 )
(Fig. 3 )
To further investigate the sensing properties of compound 1, UV/Vis titration of
compound 1 with Cu2+ was performed. With the addition of increasing amounts of
Cu2+ to a solution of 1 (10 μM) in EtOH/water (9:1, v/v) containing HEPES buffer
(10mM, pH =7.0), the maximum absorbance at 552 nm increased dramatically with
each addition of Cu2+ (Fig. 4), which can be attributed to the delocalized xanthene
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moiety of the ring-open amide form of 1.
(Fig. 4 )
To investigate the binding stoichiometry between 1 and the Cu2+ ion, the Job’s
plot experiment33 was carried out by keeping the total concentration of 1 and Cu2+
ions at 10μM and changing the molar ration of Cu2+([Cu2+]/[1+Cu2+]) from 0 to 1. As
shown in Fig.5, the result shows that a maximum at a molar fraction of 0.5, indicating
the formation of 1:1complex of 1 and Cu2+. The 1:1 complexation is also supported
from ESI-MS spectra analyses (Fig. S4), which show a prominent peak at 600.18
(calculated value, 600.22) due to [1+Cu2+–H)]+. The stability constants Ka of 1:1
1-Cu2+ complexe was determined by the Benesi–Hilderbrand equation:34,12
1/∆A = 1/∆Asat +1/(∆Asat Ka[Cu2+])
(1)
where ∆A is the absorbance difference at 552 nm and ∆Asat is the maximum
absorbance difference at 552 nm. The association constant Ka was evaluated
graphically by plotting1/∆A against 1/[Cu2+]. Typical plots (1/∆absorbance vs. 1/[Cu2+]
are shown in Fig.6. Data were linearly fitted according to eqn 1 and the Ka value was
obtained from the slope and intercept of the line. The Ka value of 1-Cu2+complexe
was 6.06× 104 M-1. The analytical detection limit of Cu2+ by the naked eye35 is as low
as 2.0 μM (Fig. S5), lower than the limit of copper in drinking water (～20 μM).
These results suggest that compound 1 has a high selectivity to Cu2+, and could be
exploited as a colorimetric sensor for Cu2+.
(Fig. 5)
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(Fig. 6)
To test the practical applicability of our visual chemosensor for Cu2+, the
competition experiments of Cu2+ mixed with other metal ions were also carried out
from UV-vis absorption spectra and the results were shown in Fig. 7. When 1(10 μM)
was treated with 2 equiv of Cu2+ in the presence of the same concentration of other
metal ions, the Cu2+-induced UV/vis response of 1 was almost unaffected by the
presence of the other metal ions.
(Fig. 7)
Additionally, the effects of anionic counterions on the sensing behavior of
compound 1 to Cu2+ were also investigated. The separate concomitant additions of
different copper salts, such as Cu(AcO)2, CuCl2, Cu(NO3)2, and CuSO4 to the
compound 1, gave rise to the same absorption profiles (Fig. 8) and visible colour
changes in their response to Cu2+ (Fig. S6) indicating a negligible effect of the counter
anions on the recognition ability and photophysical properties of compound 1.
(Fig. 8)
Furthermore, the EDTA-adding experiment was conducted to analyze the
chemical reversibility behavior of the binding of 1 and Cu2+ (Fig. 9). When excess
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EDTA was added into the mixture solution of 1 and Cu2+, the absorbance intensity at
552 nm decreased immediately. Thus, the sensing process was considered to be
reversible rather than an ion-catalyzed reaction.
(Fig. 9)
An NMR study for complexation was not available due to the paramagnetic
property of Cu2+.[36] Instead, IR of 1 and 1-Cu2+ complex was measured to study the
binding mechanism (Fig. 10). The IR spectra was primarily characterized by band in
the double-bond region. Two bands, 1714cm-1 and 1620 cm-1, were associated with
the ketone carbonyl and amide carbonyl absorption in chemosensor 1, respectively.
upon addition of Cu2+, two carbonyl groups IR peak at 1714 cm-1 and 1620 cm-1
dispeared, a new IR peak appeared at 1591 cm-1. This suggests that the ketone and
amide carbonyl O of 1 is actually involved in the coordination with Cu2+.
(Fig. 10)
According to the above experiments, a proposed binding mechanism of Cu2+ with
1 was shown in Scheme 2. The oxygen on the enolic hydroxyl group, oxygen on the
amide carbonyl group, as well as nitrogen on the hydrazone moiety can cooperatively
participate in the binding with Cu2+. The clorimetric change from colorless to pink
can be attributed to the spirolactam ring-opening, which was induced by the
complexation of Cu2+. Moreover, this process is reversible, which has been proved by
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the test using EDTA/Cu2+.
4. Conclusion
In summary, a novel and simple structure rhodamine-base colorimetric
chemosensor 1 was developed for the detection of Cu2+ in neutral ethanol aqueous
solution. The recognition of Cu2+ gave rise to color changes from colorless to pink
that was clearly visible to the naked eye. The response of the sensor to Cu2+ is
unaffected by the presence of other commonmetal ions. Its selectivity is excellent, and
the naked eyes detection of Cu2+ at 2 μM level is possible. Such Cu2+ selective
colorimetric chemosensor could meet the sensitive and selective requirements for
biomedical and environmental application and be of great importance for real-time
and instrument free detection.
Supplementary material
The crystallographic data (excluding structure factors) of 1 have been deposited
with the Cambridge Crystallographic Center as supplementary publication no.
1406446. Copy of this information may be obtained free of charge viawww: http://
www.ccdc.cam.ac.uk or from The Director, CCDC, 12 Union Road, Cambridge
CB221EZ, UK (fax: t44 1223/336 033; email: deposite@ccdc.cam.ac.uk). Structural
factors are available on request from the authors.
Acknowledgments
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We thank the Science Technology Foundation for Creative Research Group of
HBDE (Grant No. T201311), and the National Natural Science Foundation of China
(Grant No. 21501055) for financial support.
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O
O
O
O
O
H
COOH
N
N
N2H4 H2O
EtOH
N NH2
Cl
EtOH
N
O
N
N
O
N
N
O
N
Rhodamine B
1
2
Scheme 1
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Fig. 1
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1.2
1.0
0.8
0.6
0.4
0.2
0.0
1+Cu2+
1 only, K+, Na+,Ni2+, Mg2+, Mn2+,
Cr3+,Pb2+,Zn2+,Cd2+,Co2+,Fe3+,Hg2+
400
450
500
550
600
Wavelength (nm)
Fig. 2
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Fig. 3
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1.0
0.8
0.6
0.4
0.2
0.0
20
Cu2+
0
400
450
500
550
600
Wavelength (nm)
Fig. 4
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0.25
0.20
0.15
0.10
0.05
0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
[Cu2+]/([1]+[Cu2+])
Fig. 5
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80
70
Y=7.233x-4.386
R= 0.9913
K= 6.06x104M-1
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
11
1/[Cu2+] x105M-1
Fig. 6
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1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.06
1.00 1.01 1.06 1.10 0.92 0.99 1.10 1.05 1.02 0.98 1.06 1.05 1.08
+
+
2+
2+
2+
3+
2+
2+
2+
2+
3+
2+
3+
2+
a
i
r
g
e
d
o
n
-K
-N
2+
-N
-Al
-C
-F
-H
Cu
-Z
-Mg
-C
-Pb
-Mn
-C
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Fig. 7
23

ACCEPTED MANUSCRIPT
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1+Cu(AcO)2
1+CuCl2
1+Cu(NO3)2
1+CuSO4
1 only
400
450
500
550
600
Wavelength (nm)
Fig. 8
24

ACCEPTED MANUSCRIPT
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b) 1+Cu2+
(d) 1+Cu2++ EDTA+ Cu2+
(a) only 1
(c) 1+Cu2++ EDTA
400
450
500
550
600
Wavelength (nm)
Fig. 9
25

ACCEPTED MANUSCRIPT
1
1+Cu2+
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 10
26

ACCEPTED MANUSCRIPT
O
O
O
O
Cu2+
N
H
N
N
N
EDTA
N
O
N
N
O
N
1-Cu2+
1
M/Z= 539.37 ([M+H]+)
M/Z= 600.18 ([M+Cu2+-H]+)
colorful
colorless
Scheme 2
27

ACCEPTED MANUSCRIPT
Captions:
Schem 1. Synthesis of rhodamine derivate 1
Fig. 1. Crystal structure of the compound 1 (30% probability level for the thermal
ellipsoids).
Fig. 2. UV–vis spectral changes of compound 1 (10 μM) in EtOH/water(9:1, v/v)
containing HEPES buffer(10mM, pH =7.0) upon additions of various metal ions (20
μM ).
Fig. 3. Visual color change of 1 (10 μM) in EtOH/water (9:1, v/v) containing HEPES
buffer (10mM, pH=7.0) upon addition of 2.0 eqiv of various metal ions as their
chloride salts.
Fig. 4. UV–vis absorption spectra of 1(10 μM) in EtOH/water (9:1, v/v) containing
HEPES buffer (10mM, pH = 7.0) upon the addition of Cu2+ (0–20μM).
Fig. 5. Job’ plot of 1 and Cu2+. The total concentration of 1 and Cu2+ was kept at a
fixed 10 μM. The absorbance is monitored at 552 nm.
Fig. 6. Benesi–Hildebrand plot of chemosensor 1 with Cu2+.
Fig. 6. Competitive experiments in the 1 + Cu2+ system with interfering metal ions. [1]
= 10 μM, [Cu2+] = 20 μM, and [Mn+] = 20 μM.
Fig. 8. Absorption spectra of 1 (10 μM) in EtOH/water(9:1, v/v) containing HEPES
buffer(10 mM, pH =7.0) in the presence of different copper salts (20 μM).
Fig. 9. Absorption response in EtOH/water(9:1, v/v) containing HEPES buffer
(10mM, pH =7.0). a: Only 1 (10 μM); b: 1(10 μM) with Cu2+ (20 μM); c: 1 (10μM)
28

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with Cu2+ (20 μM) and then addition of EDTA (30 μM); d: 1 (10 μM) with Cu2+ (20
μM) and EDTA (30 μM) then addition of Cu2+ (35 μM).
Fig. 10. Infrared spectra (KBr) of free 1 (a) and 1-Cu2+ complex (b) at room
temperature.
Scheme 2. Proposed binding mechanism for 1 with Cu2+.
29