New rhodamine derivative as OFF-ON fluorescent chemosensor for detection of Cu2+

Article abstract of DOI:10.1007/s11426-011-4367-y

A novel fluorescence probe for the detection of Cu²⁺ has been designed and synthesized, which is based on a ring-opening reaction of spirolactam triggered by Cu²⁺. This probe exhibits excellent Cu ²⁺ ion selectivity as wel

Full text of DOI:10.1007/s11426-011-4367-y

SCIENCE CHINA
Chemistry
October 2011 Vol.54 No.10: 1635–1639
doi: 10.1007/s11426-011-4367-y
• ARTICLES •
New rhodamine derivative as OFF-ON fluorescent chemosensor
for detection of Cu2+
MA Xing1,2, LIU Jun1, DU YuGuo1,2, WEI GuoHua1* & WEI DongBin1*
1State Key Laboratory of Environmental Chemistry and Ecotoxicology; Research Center for Eco-Environmental Sciences, Chinese Academy of
Sciences, Beijing 100085, China
2College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Received March 20, 2011; accepted June 28, 2011; published online August 5, 2011
A novel fluorescence probe for the detection of Cu2+ has been designed and synthesized, which is based on a ring-opening re-
action of spirolactam triggered by Cu2+. This probe exhibits excellent Cu2+ ion selectivity as well as significant color changes
visible to the naked eye at the concentration of 0.03 mM (ca. 2.0 mg/L), the WHO (World Health Organization) recommended
level (2.0 mg/L) of Cu2+ ions in drinking water. This novel rhodamine-based fluorescence probe can be directly used to detect
Cu2+ with satisfactory sensitivity and selectivity in aqueous solution.
copper, fluorescent chemosensor, rhodamine, naked-eye detection
sor [12–16]. Rhodamine derivatives are nonfluorescent and
1 Introduction
colorless, whereas addition of targeted metal cation leading
to ring-opening of the corresponding spirolactam gives rise
to strong fluorescence emission and a pink color. Based on
the understanding of the sensing mechanism of rhoda-
mine-based molecular sensors, herein, we disclose a design
and synthesis of a new rhodamine-based chemosensor 1,
with rhodamine B as the fluorophore and 4-N,N-bis-(2-
acetoxyethyl) aminobenzaldehyde, a Cu2+-selective ligand,
as the ion acceptor. Chemosensor 1 shows a selective, sen-
sitive fluorescence enhancement response to Cu2+ in a
mixed aqueous environment. The improvement in the color
response, rapid and accurate recognition of Cu2+ with na-
ked-eye will make this approach a very promising one for
the detection of Cu2+ in water media.
Copper is the third-most abundant transition metal in the
body [1]. Cu2+ plays a critical role as a catalytic cofactor for
a variety of metalloenzymes and transcriptional events [2].
However, under overloading conditions, copper exhibits
toxicity in that it causes Alzheimer’s and Wilson’s diseases
probably by its involvement in the production of reactive
oxygen species [3, 4]. Therefore, a fast, convenient, and
reliable method for Cu2+ detection is required, particularly
in drinking water monitoring. Sensors based on the ion-
induced changes in fluorescence appear to be particularly
attractive because of the simplicity and high detection limit
of the fluorescence [5]. Recently, rhodamine derivatives
have been utilized as useful fluorophores due to their excel-
lent spectroscopic properties of large molar extinction coef-
ficient and high fluorescence quantum yield. The fluores-
cent chemosensors based on the rhodamine derivatives have
been applied as Pb2+ sensor [6], Hg2+ sensor [7, 8], Fe3+
sensor [9], Cr3+ sensor [10], Ag+ sensor [11] and Cu2+ sen-
2 Experimental
2.1 General experimental
Fluorescence spectra were obtained by using a Hitachi
F-7000 fluorescence spectrometer equipped with an xenon
*Corresponding author (email: wgh@rcees.ac.cn; weidb@rcees.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

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Ma X, et al. Sci China Chem October (2011) Vol.54 No.10
1
lamp, 1.0 cm quartz cells, and slits of 1.0/1.0 nm. The H
and 13C NMR spectra were recorded on ARX 400 spec-
trometers for solutions in CDCl3 or CD3OD. Chemical
shifts are given in ppm downfield from internal Me4Si.
High-resolution mass spectrometry was conducted in a pos-
itive mode using ESI-source. Thin layer chromatography
(TLC) was performed on silica gel HF254 with detection by
charring with 30% (v/v) H2SO4 in MeOH or in some cases
by a UV detector.
2.4 Fluorescence analysis
1.00 mL of probe 1 (1.0 mM) was blended with 0.10 mL of
Cu2+ solution (10.0 mM) in HEPES buffer (20.0 mM, pH
7.4) in a 10 mL colorimetric tube. After addition of 20.0
mM of HEPES buffer to bring the volume up to 10.00 mL,
the mixture was equilibrated for 10 min and then the fluo-
rescence intensity was recorded at ex/em = 550/590 nm
alongside a reagent blank. The excitation and emission slits
were both set to 1.0/1.0 nm.
2.2 Synthesis
3 Results and discussion
Rhodamine B and 4-N,N-bis-(2-hydroxyethyl)aminobenza-
ldehyde were purchased from Alfa Aesar. All chemicals
were of reagent grade and used without further purification.
Ultrapure water with a millipore purification system (Mil-
li-Q water) was used throughout the analytical experiments.
Compound 1: Rhodamine hydrazine (2, 0.46 g, 1.0 mmol)
was dissolved in absolute ethanol (20 mL). An excess of
4-N,N-bis-(2-acetoxyethyl)aminobenzaldehyde (3, 1.17 g,
4.0 mmol) was added and the mixture was heated at reflux
for 4 h. The precipitate produced was filtered and washed
with cold ethanol. The crude product was purified by re-
crystallization from ethanol to afford 1 as yellow solid (0.58
3.1 Designation and synthesis of probe 1
Rhodamine hydrazide 2 was synthesized according to the
literature procedure [17]. Chemosensor 1 was synthesized
by treating rhodamine hydrazide 2 with 4-N,N-bis-2-ace-
toxyethyl) aminobenzaldehyde 3 [18] with a high yield
(Scheme 1). The molecular structure was confirmed by
NMR and MS.
3.2 Selectivity of probe 1
Probe 1 can dissolve in a co-solvent of CH3CN and HEPES
solution (20.0 mM, pH 7.4, 1/1, V/V), and the solution itself
is non-fluorescent and colorless. A variation of pH from 5.0
to 10.0 does not cause significant changes in fluorescence
intensity, which implies that probe 1 itself is relatively sta-
ble and of potential use in aqueous media. When Cu2+ is
added into the solution, it can chelate with 4-N,N-bis-(2-
acetoxyethyl) aminobenzaldehyde, induce ring-opening at
C–N bond in spirolactam and generate a rhodamine-type
intermediate with a strong fluorescence signal and a blue
purple color visible to the naked eye (Figure 1). The
three-dimensional excitation emission matrix (3D-EEM)
fluorescence spectroscopic analysis on probe 1 solution and
mixture solution of probe 1 and Cu2+ was performed at
25℃ in 20.0 mM HEPES buffer (pH 7.4), and the corre-
sponding spectra are shown in Figure 2. It can be seen that
there was no significant fluorescence signal for probe 1 so-
lution, while a strong fluorescence peak rapidly appeared
after adding Cu2+ to the probe 1 solution (Figure 3). The
maximum excitation wavelength of test solution was at 550
nm, and the maximum emission wavelength at 590 nm. The
selectivity of probe 1 for Cu2+ against other individual cati-
1
g, 79%). H NMR (400 MHz, CDCl3), δ (TMS, ppm): 1.15
(t, 12H, J = 16 Hz), 2.02 (s, 6H), 3.31 (m, 8H), 3.68 (t, 4H,
J = 6.2 Hz), 4.21 (t, 4H, J = 6.2 Hz), 6.26 (d, 2H, J = 8.1
Hz), 6.45 (m, 4H), 6.65 (d, 2H, J = 8.6 Hz), 7.07 (d, 1H, J =
8.1 Hz), 7.45 (m, 1H), 7.51 (t, 2H, J = 16 Hz), 7.70 (d, 2H,
J = 8.8 Hz), 8.01 (d, 1H, J = 8.1 Hz). 13CNMR (CDCl3,
100 MHz) δ (TMS, ppm): 12.5 (4C), 20.5 (2C), 44.3 (4C),
55.5 (2C), 61.6 (2C), 67.2, 98.3 (2C), 103.5, 108.1, 111.7,
112.5, 114.8 (2C), 123.8, 126.7, 127.4, 128.3 (2C), 128.6,
129.1, 129.5, 132.5, 134.6, 141.2, 146.5, 149.2 (2C), 151.9,
152.6, 152.9, 165.9, 170.1, 170.3. ESI(+)-HRMS (m/z): [M
+ H]+ calcd. for C43H50N5O6: 732.3761, found 732.3768.
2.3 Sample preparation
Stock solution of probe 1 (1.0 mM) was prepared in a
co-solvent of CH3CN and HEPES (4-(2-hydroxyethyl)-1-
piperazine-ethanesulfonic acid) buffer (20.0 mM, pH 7.4,
1/1, V/V), and the stock solutions of Cu2+ and other cation
ions (10.0 mM) were prepared by dissolving corresponding
chloride salts in HEPES buffer (20.0 mM, pH 7.4).
Scheme 1 Synthesis of rhodamine-based chemosensor 1.

Ma X, et al. Sci China Chem October (2011) Vol.54 No.10
1637
no detectable increase of both fluorescence signal and color
changes to the naked eye.
3.3 Sensitivity of probe 1
Figure 5 shows the emission fluorescence spectra of probe 1
on addition of Cu2+ with various concentrations. When ex-
cited at 550 nm, the emission fluorescence intensity of
probe 1 at 590 nm increased more than 26-fold on increas-
ing the concentration of Cu2+ from 0 to 0.10 mM. A satis-
factory linear relationship between fluorescence intensity
and Cu2+ concentration was observed with the correlation
coefficient as high as 0.9936. Similarly, as shown in Figure 6,
the UV absorption spectra of probe 1 had a significant
change as addition of Cu2+, and the absorbance at 705 nm
enhanced from 0.003 to 0.200 on increasing concentration
of Cu2+ from 0 to 0.10 mM.
Figure 1 “Naked-eye” color changes of probe 1 (0.10 mM in 20.0 mM of
HEPES buffer system, pH 7.4) before and after adding Cu2+ (1.0 mM).
on ions was explored by adding probe 1 (0.10 mM) to each
aqueous solution of Ag+, Ca2+, Cu2+, Ni2+, Mg2+, Fe2+, Hg2+,
Pb2+, Co2+, Mn2+, Cd2+, and Zn2+ (1.0 mM), respectively. As
shown in Figure 4, only Cu2+ induced a significant fluores-
cence signal, and appeared a blue purple color from the col-
orless test solution (probe 1 as blank). Addition of other
cation ions up to a concentration as high as 1.0 mM caused
In order to check the effect of ion strength on the fluo-
Figure 2 Fluorescence characteristics of probe 1 (0.10 mM) and mixture of probe 1 (0.10 mM) and Cu2+ (0.05 mM) in HEPES buffer system (20.0 mM,
pH 7.4). (a) 3D-EEM spectra of probe 1 in HEPES buffer system; (b) emission scanning spectra of probe 1 when fixing excitation wavelength at 550 nm; (c)
excitation scanning spectra of probe 1 when fixing emission wavelength at 590 nm; (d) 3D-EEM spectra of Cu2+ and probe 1; (e) emission scanning spectra of
Cu2+ and probe 1 when fixing excitation wavelength at 550 nm, (f) excitation scanning spectra of Cu2+ and probe 1 when fixing emission wavelength at 590 nm.

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Figure 3 Time-dependant fluorescence scanning on the mixture of probe
1 (0.10 mM) and Cu2+ (0.05 mM) in HEPES buffer system (20.0 mM, pH
7.4). The excitation wavelength was set at 550 nm, and the emission wave-
length at 590 nm, with slit widths of 1.0 nm.
Figure 5 (a) Fluorescence emission changes of probe 1 (0.10 mM) rec-
orded 10 min after reaction with various concentrations of Cu2+ (0–0.10
mM) in 20.0 mM HEPES buffer system (pH 7.4) at 25 °C. Excitation
wavelength was at 550 nm, with silt widths of 1.0 nm; (b) dependence of
fluorescence intensity at 590 nm with respect to concentrations of Cu2+
ions (0–0.10 mM) in 20.0 mM HEPES buffer system (pH 7.4) at 25 °C.
Excitation wavelength was at 550 nm, with silt widths of 1.0 nm.
Figure 4 Cation selectivity of probe 1. Each cation (Ag+, Ca2+, Cu2+, Ni2+,
Mg2+, Fe2+, Hg2+, Pb2+, Co2+, Mn2+, Cd2+, and Zn2+) was added at a concen-
tration of 1.0 mM to a 20.0 mM HEPES buffer system (pH 7.4) of probe 1
(0.1 mM). (a) Fluorescence spectra of cation ions with probe 1; (b) fluo-
rescence intensity of each cation ion and probe 1, the excitation wavelength
was set at 550 nm, and the emission wavelength at 590 nm, with slit widths
of 1.0 nm.
Figure 6 UV absorption changes of probe 1 (0.10 mM) recorded 10 min
after reaction with various concentrations of Cu2+ (0–0.10 mM) in 20.0
mM HEPES buffer system (pH 7.4) at 25 °C.
and probe 1 (0.1 mM) in 20.0 mM HEPES buffer (pH 7.4)
at 25 °C. It was found that the addition of NaNO3 did not
change the fluorescence intensity at all, the mean fluores-
cence intensity of 4 runs was 239.53 and 244.12 a.u. for
rescence characteristics of probe 1, a solution of 1.0 M
NaNO3 was added to the reaction system of Cu2+ (1.0 mM)

Ma X, et al. Sci China Chem October (2011) Vol.54 No.10
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color changes could be exclusively confirmed with na-
ked-eye when the Cu2+ concentration increased to 0.03 mM
(ca. 2.0 mg/L), the recommended standard value for Cu2+ in
drinking water by WHO, EU and Australia. This observa-
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check if the Cu2+ concentration of the sample meets the
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In summary, we have designed, synthesized, and investi-
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detection based on the ring-opening of spirolactam. Fluo-
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Figure 7 “Naked-eye” color changes of probe 1 (0.10 mM in 20.0 mM
HEPES buffer, pH 7.4) upon addition of different concentration of Cu2+
(From left: 0, 0.03 and 0.07 mM).
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This work was supported in partial by the National Natural Science Foun-
dation of China (20921063 & 20877090) and the National High Technol-
ogy Research and Development Program (863 Program, 2007AA06A407).