A novel chemosensor based on rhodamine derivative for colorimetric and fluorometric detection of Cu2+ in aqueous solution

Article abstract of DOI:10.1007/s10895-012-1040-6

A new rhodamine-based reversible chemosensor (2) was synthesized, which exhibits high sensitivity and selectivity for Cu²⁺ but no significant response toward other competitive metal ions in aqueous solution. Upon the addition of Cu²⁺, the spirolactam ring of 2 was opened and the solution color changed from colorless to red. Strangely, an unexpected fluorescence quenching was observed, which is contrary to the fluorescence turn-on of the most rhodaminebased chemosensors. The likely novel sensing mechanism has been proposed. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10895-012-1040-6

J Fluoresc (2012) 22:799–805
DOI 10.1007/s10895-012-1040-6
SHORT COMMUNICATION
A Novel Chemosensor Based on Rhodamine Derivative
for Colorimetric and Fluorometric Detection of Cu2+
in Aqueous Solution
Yu Wang & Hai-qiang Wu & Jian-hua Sun & Xiao-ya Liu &
Jing Luo & Ming-qing Chen
Received: 22 October 2011 /Accepted: 5 March 2012 /Published online: 24 March 2012
#
Springer Science+Business Media, LLC 2012
Abstract A new rhodamine-based reversible chemosensor
(2) was synthesized, which exhibits high sensitivity and
selectivity for Cu2+ but no significant response toward other
competitive metal ions in aqueous solution. Upon the addi-
tion of Cu2+, the spirolactam ring of 2 was opened and the
solution color changed from colorless to red. Strangely, an
unexpected fluorescence quenching was observed, which is
contrary to the fluorescence turn-on of the most rhodamine-
based chemosensors. The likely novel sensing mechanism
has been proposed.
disease [3–5]. Besides, copper is a significant metal pollut-
ant due to its widespread use. Considering the important
roles of Cu2+ in biological and environmental systems
[6–9], a considerable effort has been devoted to the devel-
opment of Cu2+-selective fluorescent chemosensors in the
past few years [10]. However, in most cases fluorescence
changes can only be observed in non-aqueous solvent
[11–13], which greatly limits their analytical application in
real samples. Therefore, the development of highly sensitive
and selective fluorescent chemosensor for Cu2+ in aqueous
solution is imperative.
Rhodamine-based dyes possess excellent spectroscopic
properties such as a large molar extinction coefficient, high
fluorescence quantum yield, and visible light excitation as
well as long wavelength emission [14, 15]. Besides, rhoda-
mine derivatives are well-known for their spirolactam (fluo-
rescence “off”) to ring-opened amide (fluorescence “on”)
equilibrium, during which significant fluorescence enhance-
ment as well as color changes will take place, and based on
this mechanism, a variety of rhodamine-based spirolactam
derivatives have been designed as probes for the recognition
of different metal ions such as Pb2+ [16, 17], Hg2+ [18–20],
Cu2+ [21–24], Fe3+ [25–27], Cr3+ [28, 29].
2+
.
.
.
Keywords Rhodamine 6G Chemosensor Cu
Fluorescence
Introduction
In recent years, the design and synthesis of selective and
sensitive chemosensors for the detection of transition metal
ions have attracted considerable attention [1, 2]. Among the
various transition metal ions, Cu2+ plays a critical role as a
catalytic cofactor for a variety of metalloenzymes, including
superoxide dismutase, cytochrome c oxidase and tyrosinase.
However, if unregulated, copper can cause oxidative stress
and disorders associated with neurodegenerative diseases,
such as Alzheimer’s disease, Wilson’s disease, and Menke’s
Herein, we report a new rhodamine-based chemosen-
sor 2, which has been demonstrated to be a reversible,
sensitive and selective chemosensor for Cu2+ in aqueous
solution. Chemosensor 2 was readily synthesized in two
steps (Scheme 1). Rhodamine 6G reacted with hydrazine
hydrate to give the intermediate 1, which was further
treated with 4-(diethylamino) salicylaldehyde to afford
chemosensor 2 in 66 % overall yield. All the compounds
:
:
:
:
:
Y. Wang H.-q. Wu J.-h. Sun X.-y. Liu (*) J. Luo (*)
M.-q. Chen
School of Chemical and Material Engineering,
Jiangnan University,
214122, Wuxi, People’s Republic of China
e-mail: lxyjiangnan@126.com
1
were confirmed by H NMR, 13C NMR spectra and MS
data.
e-mail: jingluo19801007@126.com

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J Fluoresc (2012) 22:799–805
Scheme 1 Synthetic route of
chemosensor 2
Experimental Section
Procedures of Metal Ion Sensing
Reagents
A stock solution of 2 (1.0 mM) was prepared in absolute
CH3CN. Tris–HCl buffer solution (10 mM, pH07.0) was
prepared under adjustment by a pH meter. To a 10-mL volu-
metric tube containing 2.9 mL of acetonitrile, 100 μL of
1.0 mM 2, different concentration of Cu2+ (or other metal ions)
was added and the reaction mixture was diluted to 10 mL with
the buffer. The reaction solution was kept at room temperature
for 2 min. Then, 3.0 mL of the solution was transferred to a
1 cm cell for the absorption or fluorescence measurement.
General experimental procedure for the synthesis of the
chemosensor 2
Rhodamine 6G, Hydrazine hydrate, 4-(Diethylamino) salicy-
laldehyde and Tris(hydroxymethyl)aminomethane (Tris) were
purchased from commercial suppliers and used as received.
All the solvents were of analytic grade, and double distilled
water was used throughout the experiment. The solutions of
metal ions were prepared from their nitrate salts (Ag+, Al3+,
Ba2+, Ca2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mg2+, Mn2+,
Ni2+, Pb2+, Zn2+) in distilled water except for CdCl2.
The synthetic route to chemosensor 2 is shown in
Scheme 1.
Apparatus
Absorption spectra were recorded on a Shimadzu UV-2550
spectrophotometer (Tokyo, Japan). Fluorescence spectra
were performed on Perkin-Elmer LS 50B fluorescence spec-
trometer (both excitation and emission slit widths: 3 nm). 1H
NMR and 13C NMR spectra were measured on a BrukerAV-
300 spectrometer with TMS as an internal standard and
CDCl3 as solvent. Mass spectra were obtained from a HP
1100 LC–MS spectrometer. All pH measurements were
made with a Sartorius basic pH-MeterPB-10.
Synthesis of Rhodamine 6G Hydrazide (1)
Rhodamine 6G hydrazide (1) was prepared according to a
literature method [14]. To a 250 mL flask, rhodamine-6G
(3.83 g, 8 mmol) was dissolved in 90 mL ethanol. 12.0 mL
(excess) hydrazine hydrate (85 %) was then added dropwise
with vigorous stirring at room temperature. After the addition,
the stirred mixture was heated to reflux for 10 h, during which
Fig. 1 UV–vis absorption
spectra of 2 (10 μM) upon
addition of (a) different metal
ions (10 μM for all cations) and
(b) Cu2+ (0∼5 equiv.) in
CH3CN–water (3:7, v/v) at pH
7.0 aqueous solution. Inset (b)
absorbance at 527 nm as a
function of Cu2+ concentration

J Fluoresc (2012) 22:799–805
801
Fig. 2 Time course of the response of 2 (10 μM) to 5 equiv Cu2+ in
CH3CN–water (3:7, v/v) aqueous solution
Fig. 4 The plot of absorption spectra of 2 at 527 nm: [1/(A-A0)] vs 1/[Cu2+]
ethanol (5×15 mL) and dried in vacuo to afford 2 (1.0 g,
the dark purple solution disappeared and pink precipitate
appeared. The resulting precipitate was filtered, washed 4
times with 20 mL CH3CH2OH–H2O (1:1, v/v) and dried in
vacuo to afford rhodamine 6G hydrazide (2.75 g, yield 80 %)
1
yield 83 %) as yellow solid. H NMR (300 MHz, CDCl3),
δ (ppm): 11.01 (s, 1H), 9.10 (s, 1H), 7.99 (dd, 1H), 7.51 (dd,
2H), 7.13 (dd, 1H), 6.92 (d, 1H), 6.44 (s, 2H), 6.32 (s, 2H),
6.13 (b, 2H), 3.49 (s, 2H), 3.32 (q, 4H), 3.23 (q, 4H), 1.90
(s, 6H), 1.33 (t, 6H), 1.13 (t, 6H). 13C NMR (CDCl3) δ (ppm):
163.7, 160.6, 154.1, 151.9, 151.0, 150.4, 147.5, 133.0, 132.7,
130.5, 128.4, 127.9, 124.0, 123.0, 117.9, 107.5, 106.1, 103.3,
98.3, 96.8, 66.5, 44.5, 38.3, 16.8, 14.8, 12.6. ESI-MS: m/z
604.1 for [2+H]+, cal. For C37H41N5O3, 2, 603.76.
1
as light pink solid. H NMR (300 MHz, CDCl3), δ (ppm):
7.98 (dd, 1H, Ar–H), 7.48 (dd, 2H, Ar–H), 7.08 (dd, 1H,
Ar–H), 6.41 (s, 2H, xanthene–H), 6.28 (s, 2H, xanthene–H),
3.49–3.68 (b, 4H, N–NH2, NHCH2CH3), 3.24 (q, 4H,
NHCH2CH3), 1.94 (s, 6H, xanthene–CH3), 1.35 (t, 6H,
NHCH2CH3). 13C NMR (CDCl3) δ (ppm): 166.2, 152.2,
151.8, 147.5, 132.6, 129.9, 128.1, 127.7, 123.8, 123.0,
118.0, 104.9, 96.8, 66.1, 38.4, 16.7, 14.8. ESI-MS: m/z
429.1 for [1+H]+, cal. For C26H28N4O2, 1, 428.53.
Results and Discussion
Synthesis of Target Compound 2
UV Absorption Studies
Rhodamine 6G hydrazide (1) (2.0 mmol, 0.857 g) and 4-
(diethylamino) salicylaldehyde (4.0 mmol, 0.774 g) were
mixed in 2:1 (v/v) methanol/dichloromethane (90 mL) with
0.10 mL of acetic acid and refluxed for 20 h (if precipitation
appeared, then added 5∼10 mL dichloromethane). Then the
solvent was removed in vacuo. The residue was washed by
Figure 1a shows the representative chromogenic behavior of
the dye towards various competitive metal ions in CH3CN–
water (3:7, v/v) at pH 7.0. Absorption spectra of free 2
(10 μM) in CH3CN–water (3:7, v/v) at pH 7.0 showed an
absorbance at 388 nm and no absorption band above 500 nm,
indicating that spirolactam form exists predominantly. The
Fig. 3 The UV–vis absorbance response of 2 (10 μM) upon the
addition of various metal ions in CH3CN–water (3:7, v/v) aqueous
solution. Black bars represent the absorbance of 2 to the metal ions
of interest (10 μM for Cu2+; 20 μM for all other metal ions). Gray bars
represent the changes that occur upon subsequent addition of Cu2+
(10 μM) to the solution. The absorbance was measured at 527 nm
Fig. 5 Job’s plot of the complexation between 2 and Cu2+. The total
concentration of 2 and Cu2+ was kept at a fixed 20 μM. The absor-
bance was measured at 527 nm

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J Fluoresc (2012) 22:799–805
Fig. 6 Fluorescence titration of
2 (10 μM) by excitation at (a)
λex0386 nm; b λex0512 nm in
CH3CN–water (3:7, v/v) at pH
7.0 aqueous solution
characteristic peak at 66.5 ppm (spiro carbon) in the 13C NMR
spectrum of 2 also supports this consideration [30]. The addi-
tion of various metal ions such as Ag+, Al3+, Ba2+, Ca2+, Cd2+,
Co2+, Cr3+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+
have little change to the UV–vis spectrum of chemosensor 2
and the color of the solutions containing these ions remained
relatively unchanged. In contrast, Cu2+ resulted in a prominent
change. Upon the addition of Cu2+, the absorption was signif-
icantly enhanced with a new absorption band centered at
527 nm, clearly suggesting spirolactam bond cleavage fol-
lowed by the formation of a delocalized xanthenes moiety of
the rhodamine group as a result of Cu2+ binding. Accordingly,
the solution exhibited an obvious and characteristic color
change from colorless to pink, which was clearly visible to
the naked eye. Moreover, the change in the solution color was
very fast and observed within 1 min upon addition of 5 equiv.
Cu2+ to the solution of 2 (10 μM) (Fig. 2). These results
indicated that compound 2 exhibited a high selectivity for
Cu2+ over various other metal ions and thus, could serve as
a good selective naked-eye chemosensor for Cu2+ in CH3CN
aqueous media. The excellent selectivity was further demon-
strated by the competition experiment, which revealed that the
Cu2+ induced absorbance enhancement was not significantly
interfered in the presence of various coexistent ions (2 equiv.)
(Fig. 3).
with a total concentration of 20 μM. As shown in Fig. 5, the
Job’s plot shows that a maximum absorption was observed
when the molar fraction reached 0.5, which is indicative of a
1:1 stoichiometric complexation between 2 and Cu2+.
Fluorescence Properties
The fluorescence responses of chemosensor 2 towards Cu2+
and other metal ions were also investigated. As shown in
Fig. 6, fluorescence spectra (λex0386 nm) of free 2 (10 μM)
in CH3CN–water (3:7, v/v) at pH 7.0 displayed an emission
band centered at 512 nm and a strong green fluorescence
was observed, which is quite different from most of the
reported rhodamine-based chemosensors with no obvious
fluorescence signal before the complexation of ions due to
the predominant spirocyclic form. The green fluorescence of
2 is possibly due to contribution from Schiff base of salicy-
laldehyde. More interestingly, upon the addition of Cu2+, the
original emission band at 512 nm decreased significantly
accompanied by a blue-shift to 500 nm, and concomitantly a
Stoichiometry of Metal Complexation
To understand the coordination behavior between 2 and
Cu2+, the UV–vis absorption spectral titration experiment
of 2 (10 μM) with Cu2+ (0 to 50 μM) in aqueous solution
(pH07.0) was carried out. As shown in Fig. 1b, the intensity
of the new 527 nm absorption band increased with increas-
ing Cu2+ concentration. According to the linear Benesi-
Hildebrand expression [31, 32], the association constant of
2 with Cu2+ ion was found to be 4.877×104 M−1, and the
measured absorbance [1/(A-A0)] at 527 nm varied as a
function of 1/[Cu2+] in a linear relationship (R00.99603),
indicating the 1:1 stoichiometry between the Cu2+ ion and 2
(Fig. 4). This binding mode was further supported by a Job’s
plot evaluated from the absorption spectra of 2 and Cu2+
Fig. 7 a Visible and b fluorescence (excitation at 365 nm) color
change of 2 (10 μM) in the presence of different concentrations of
Cu2+ in CH3CN–water (3:7, v/v) aqueous solution

J Fluoresc (2012) 22:799–805
803
Scheme 2 Proposed mechanism and binging mode between 2 and Cu2+. (The colored starlike patterns represent for fluorescence of fluorophores at
different conditions by excited 2 at 386 nm)
new emission band centered at 548 nm appeared which also
decreased with increasing Cu2+ concentration. And when
the Cu2+ concentration increased up to 5 equiv. of chemo-
sensor 2, its fluorescence was completely quenched
(Fig. 6a). The visible and fluorescence color change of 2
in the presence of different concentrations of Cu2+ are
shown in Fig. 7. It is well known that rhodamine-based
chemosensors always show a fluorescence enhancement
response (turn on) upon the addition of analytes. As far as
we know, there is only one reported example showing the
similar phenomenon that has been described above [33].
Interestingly, when 2 was excited by 512 nm, the inten-
sity of the fluorescent peak at 548 nm gradually increased
with increasing amount of Cu2+ as usual rhodamine-based
chemosensors act (Fig. 6b), which suggests that the new
emission band at 548 nm should be attributed to the ring-
opening of the spirocycle and the formation of a delocalized
xanthenes moiety of the rhodamine group. Based on the
finding from the afore-mentioned experiments and the
chemistry regarding the Cu2+-mediated decylization of rho-
damine [34], we proposed the likely mechanism of the
Cu2+-induced fluorescence turn-off response as shown in
Scheme 2. Chemosensor 2 is most likely to chelate with
Cu2+ via its carbonyl O, imino N, and phenol O atoms to
induce the ring opening of rhodamine spirocycle [35]. As a
consequence, a new absorption band at 527 nm characteristic
of the opened rhodamine group appeared. We conjecture that,
when excited by 386 nm, the salicylaldehyde Schiff base part
of 2 was excited and gave green fluorescence at 512 nm which
overlap with the absorption band of the newly formed opened
rhodamine fluorophore. As a result, the rhodamine fluoro-
phore was excited to give a corresponding new emission band
at 548 nm when Cu2+ was added. However, Cu2+ might act as
a quencher toward green fluorescence of Shiff base of salicy-
laldehyde, thus, both of the two emission band (512 nm and
548 nm) decreased with gradual addition of Cu2+.
As shown in Fig. 8, the addition of other metal ions such as
Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Fe3+, Hg2+, K+, Mg2+,
Mn2+, Ni2+, Pb2+, Zn2+ did not induce obvious change in the
fluorescence spectra (λex0386 nm) except for a small decrease
in the emission intensity, indicating the good selectivity for
Cu2+. This result also suggests that the new emission band at
548 nm should be attributed to the Cu2+-induced opening of the
rhodamine spirocycle.
Fig. 8 Fluorescence emission (λex0386 nm) changes of 2 (10 μM)
upon addition of different metal ions (10 μM for all cations) in
CH3CN–water (3:7, v/v) at pH 7.0 aqueous solution

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J Fluoresc (2012) 22:799–805
the other species in biological systems and contribute to the
development of fluorescent sensors for transition-metal ions
based on the rhodamine platform.
Acknowledgments This work was supported by the Natural Science
Foundation of China (Grant 20974041, 21174056).
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