A rhodamine-based dual chemosensor for the visual detection of copper and the ratiometric fluorescent detection of vanadium

Article abstract of DOI:10.1016/j.dyepig.2009.11.007

The optical properties of a novel, rhodamine-based derivative, synthesized by reacting rhodamine hydrazide and 5-chlorosalicylaldehyde in ethanol, were investigated in methanol:HEPES solution. The novel sensor displayed selectivity for Cu²⁺, as evidenced by a colourless to dark red colour change, which was characterized using UV-visible spectroscopy and which also allowed visual detection of Cu²⁺. In contrast, selectivity towards VO²⁺ was determined from changes in the emission spectra in the nanomolar range. This represents the first reported rhodamine-based sensor capable of detecting both Cu²⁺ and VO²⁺ using two different modes. Crown Copyright

Full text of DOI:10.1016/j.dyepig.2009.11.007

Dyes and Pigments 86 (2010) 50e55
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A rhodamine-based dual chemosensor for the visual detection of copper
and the ratiometric fluorescent detection of vanadium
,
a
c
Fang-Jun Huo a, Jing Su b, Yuan-Qiang Sun a, Cai-Xia Yin b , Hong-Bo Tong , Zong-Xiu Nie
*
a Research Institute of Applied Chemistry, Shanxi University, Taiyuan, 030006, China
b Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China
c Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The optical properties of a novel, rhodamine-based derivative, synthesized by reacting rhodamine
hydrazide and 5-chlorosalicylaldehyde in ethanol, were investigated in methanol:HEPES solution. The
novel sensor displayed selectivity for Cu2þ, as evidenced by a colourless to dark red colour change, which
was characterized using UVevisible spectroscopy and which also allowed visual detection of Cu2þ. In
contrast, selectivity towards VO2þ was determined from changes in the emission spectra in the nano-
molar range. This represents the first reported rhodamine-based sensor capable of detecting both Cu2þ
and VO2þ using two different modes.
Received 16 October 2009
Received in revised form
24 November 2009
Accepted 25 November 2009
Available online 24 December 2009
Keywords:
Rhodamine derivative
Cu2þ
Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
VO2þ
UVevisible
Naked-eye
Fluorescent
1. Introduction
organisms. Copper has been shown to be a biochemically essential
metal, such as, copperezinc superoxide dismutase and its role in
Rhodamine derivatives are excellent fluorophores and chro-
mophores and have attracted considerable interest due to their
very good photophysical properties [1e3], such as long absorption
the enzymatic defense against oxygen toxicity. The detection of
Cu2þ has important implications in the areas of environmental and
biological analysis [6e19]. Vanadium is an essential trace element
due to its significant roles in the environment, industry and phys-
iological systems [49]. Laboratory and epidemiological evidence
suggests that vanadium may also play a beneficial role in the
prevention of heart-disease, despite its toxicity at mlꢀ1 levels [50].
Vanadium remains a relatively unknown trace element, as it is still
being targeted for use in various clinical applications worldwide.
The development of an analysis method for vanadium is also same
important [51].
Herein, we reported a novel dual chemosensor for detecting
Cu2þ by UVevisible spectroscopy and VO2þ using fluorescence
spectra. This chemosensor employs a rhodamine-based derivative
(CSR), which was prepared by the reaction of excessive 5-chlor-
osalicylaldehyde (CS) and rhodamine hydrazide.
and emission wavelength, high fluorescence quantum yield (
F),
large extinction coefficient ( ), and high light stability. Rhodamine
3
derivatives with a spirolactam-ring moiety, which is non-fluores-
cent and colourless, can be converted to the open-ring form in the
presence of a proton or metal ion and display characteristically
strong fluorescence emission and red colour [4]. Rhodamine-based
spirolactams are considered to offer promise as molecular scaffolds
and often are employed in OFFeON fluorescent or colourimetric
chemosensors [4,5]. Recently, several rhodamine-modified che-
mosensors have been developed for heavy- and transition-metal
ions such as Cu2þ [6e19], Hg2þ [20e36], Fe3þ [36e40], Cr3þ
[40e43], Cd2þ [44], Pb2þ [45] and Agþ [46] because of the wide use
of these metal ions and their subsequent impact on the environ-
ment and human health [47,48]. Copper is the third most abundant
essential trace element in the human body after iron and zinc, and
is important in many fundamental physiological processes in
2. Experimental section
2.1. General
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES;
caution: stable; combustible; incompatible with strong oxidizing
* Corresponding author. Tel./fax: þ86 351 7011022.
E-mail address: yincx@sxu.edu.cn (C.-X. Yin).
0143-7208/$ e see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.11.007

F.-J. Huo et al. / Dyes and Pigments 86 (2010) 50e55
51
agents; protect from moisture) was purchased from Sigma-
eAldrich. CSR was synthesized according to literature. HEPES
solutions were adjusted to pH 7.0 by adding NaOH (0.1 M) to
aqueous HEPES (10 mM). Cation salts were purchased from
Shanghai city of China. All the common chemicals were of analyt-
ical grade.
A Beckman
F50 pH meter was used to determined pH. UVevis
spectra were recorded on an HP8453 spectrophotometer. A PO-120
quartz cuvette (10 mm) was purchased from Shanghai city of China.
1H NMR and 13C NMR spectra were recorded on a Bruker DRX-
300 MHz NMR spectrometer. Fluorescence spectra were measured
on Cary Eclipse fluorescence spectrophotometer. Electrospray
ionization (ESI) mass spectra were measured with an LCeMS 2010A
(Shimadzu) instrument. A yellow single crystal of CSR was mounted
on a glass fiber for data collection. Cell constants and an orientation
matrix for data collection were obtained by least-squares refine-
ment of diffraction data from reflections with 3.7e25.0ꢁ for CSR
using a Bruker SMART APEX CCD automatic diffractometer. Data
were collected at 223 K using Mo K
a
radiation (
l
¼ 0.710713 Å) and
the -scan technique and corrected for Lorentz and polarization
u
effect (SADABS) [52]. The structures were solved by direct methods
(SHELX97) [53], and subsequent difference Fourier map and then
refined in F2 using a full-matrix least-squares procedure and
anisotropic displacement parameters.
Fig. 2. Crystal structure of CSR, all hydrogen atoms were omitted for clarity.
2.3. Description of the structure of CSR
2.2. Preparation of CSR
An intramolecular O3dH3.N2 hydrogen bond is observed in
the molecular structure with the O3.N2 distance of 2.620(7) Å and
the O3dH3.N2 angle of 146ꢁ, which is closed the six-membered
pseudo-ring N2eC29eC30eC31eO3eH3. The dihedral angle
between the spirolactam-ring system and xanthene ring plane is
88.53(2)ꢁ. Weak intermolecular C6dH6a.O3 hydrogen bonds also
stabilize the crystal structure, forming one-dimensional infinite
molecular chains along the c axis with the C6.O3 distance of 3.401
(9) Å and the C6dH6a.O3 angle of 159ꢁ.
CSR was prepared in more high yield by reacting rhodamine
hydrazide with excessive 5-chlorosalicylaldehyde (Fig. 1). Rhoda-
mine hydrazide (0.46 g, 1 mmol; caution: toxic; contact with water
or acids liberates toxic gas; can become highly flammable in use;
danger of cumulative effects) was dissolved in 20 mL of absolute
ethanol. An excessive of 5-chlorosalicylaldehyde (CS, 4 mmol) was
added and the mixture was refluxed for 8 h. The ensuing solution
was cooled, concentrated to 10 mL, and allowed to stand at 0 ꢁC
overnight in the refrigeratory. The precipitate was filtered and
washed three times with 20 mL of cold ethanol. After drying under
reduced pressure, the reaction afforded 0.43 g (76%) as a white
2.4. General UVevis and fluorescence spectra measurements
solid. CSR compound. 1H NMR (CDCl3),
d
(ppm): 1.15 (t, J ¼ 6.78 Hz,
Since the chemosensor was not fully soluble in 100% aqueous
media, methanol was used as a solubilizing medium. CSR stock
solutions were prepared in methanol. The UVevis and fluorescence
spectra were obtained in mixed methanol/HEPES aqueous buffer
(1:1, v/v, 10 mM, pH 7.0) solutions. Aqueous metal ion solutions
were also prepared. Fluorescence measurements were carried out
with a slit width of 10 nm.
12H, CH3), 3.34 (q, J ¼ 6.78 Hz, 8H, CH2), 6.27 (dd, J ¼ 8.29 Hz, 2H),
6.49(m, 4H), 6.81 (d, J ¼ 8.76 Hz, 2H,), 7.06 (s, 1H), 7.10e7.13 (d,
J ¼ 8.75 Hz, 1H), 7.18e7.20 (d, J ¼ 6.48 Hz, 1H), 7.56 (m, 2H), 8.0 (d,
J ¼ 6.27 Hz, 1H), 9.00 (bs, 1H), 10.96 (bs, 1H). 13C NMR (CDCl3)
d
(ppm): 12.4, 44.2, 66.2, 97.8, 104.8, 108.0, 118.2, 119.1, 123.2, 124.0,
127.9, 128.5, 129.3, 130.1, 130.6, 133.2, 149.0, 150.1, 150.7, 153.2,
156.9, 164.2. ESIeMS: m/z 595.3 (55%, [CSR þ H]þ), 617.3 (23%,
CSR þ Na]þ); [CSR] calculated 594.2. Crystal data for C35H35ClN4O3:
crystal size: 0.5 ꢂ 0.5 ꢂ 0.4 mm, monoclinic, space group Pca21 (No.
29). a ¼ 21.467(6) Å, b ¼ 11.827(3) Å, c ¼ 12.344(3) Å, V ¼ 3134.04
(3) Å3, Z ¼ 4, T ¼ 223 K, qmax ¼ 25.0ꢁ, 5104 reflections measured,
4447 unique (Rint ¼ 0.0747). Final residual for 388 parameters and
2.5. Detection range
The UVevis spectrum was characterized by a main band centred
at 557 nm. The low detection threshold for Cu2þ was in the order of
10ꢀ6e10ꢀ5 M and at this level the colour change was very obvious.
The fluorescence emission was measured for each sample by
exciting at 530 nm and spectra were measured from 540 to 700 nm.
The sensitivity range for VO2þ was 10ꢀ7e10ꢀ6 M.
5104 reflections with I > 2
GOF ¼ 1.27. The crystal structure is shown in Fig. 2.
s
(I): R1 ¼ 0.1231, wR2 ¼ 0.2485 and
3. Results and discussion
As a rhodamine derivative with a spirolactam group, CSR is non-
fluorescent and colourless. Ring opening of the corresponding
spirolactam gives rise to strong fluorescence emission and a pink
colour. In a similar way, CSR as a ligand on a spirolactam-ring can
induce colour change and a fluorescence change upon the addition
of metal ions. The chemosensing behavior of CSR was investigated
using UVevis and fluorescence measurements.
Fig. 1. Synthesis of CSR chemosensor.

52
F.-J. Huo et al. / Dyes and Pigments 86 (2010) 50e55
The pH-titration experiments showed that the proper pH range
for the determination of the Cu2þ ions was pH 6.0e10.0 in UVevis
spectra and for the detection of the VO2þ ions being pH 7.0e8.0 in
fluorescence spectrum. These suggest that the chemosensor is
suitable candidate for potential use in biological systems.
The UVevis absorption spectrum of CSR in 50% CH3OH solution
(methanol:HEPES ¼ 1:1, v/v, pH 7.0) had no absorption band from
400 to 800 nm. The absorption spectrum did not change signifi-
cantly in the presence of 10ꢀ3 M concentrations of representative
lanthanide ions (Eu3þ, Yb3þ etc), alkaline earth metal ions (Mg2þ
Ba2þ etc), or transition-metal ions (such as Fe3þ, Co2þ, Ni2þ, Zn2þ
,
,
Hg2þ, Pb2þ etc) and the colour of the solutions containing these
ions remained relatively unchanged. However, upon interaction
with 10ꢀ5 M Cu2þ ions a new strong absorption peak for CSR
appeared at 557 nm (Fig. 3a). When changes at 557 nm were
monitored, good selectivity was observed for Cu2þ ion with a more
than 30-fold increase in absorbance intensity (Fig. 3b). The effect
caused by Cu2þ was so substantial that it could be detected by the
naked-eye with a distinct solution colour change from colourless to
dark red (Fig. 4). A titration was conducted with the CH3OHeHEPES
Fig. 4. Photograph of CSR (20
except Cu2þ that is 20
M) in methanoleHEPES (1:1, v/v, pH 7.0) solution. From left to
right, then from top to bottom: CSR (20
Cr3þ, Al3þ, Eu3þ, Ru3þ, Sm3þ, Sn4þ, Ni2þ, Co2þ, Yb3þ, Hg2þ, Ga3þ, Zr4þ, Er3þ, Ho3þ, Fe3þ
Zn2þ, Mg2þ, Mn2þ, La3þ, Nd3þ
mM) in the presence of various metal ions (1000 mM
m
system containing 20
and the end point was reached with a 1:1 molar ratio of CSR and
Cu2þ (20 L Cu2þ) (Fig. 5), This correlates to a quantitative detection
of Cu2þ with CSR at micromolar levels (Fig. 5, inset). (Fig. 5, inset).
m
L CSR ([CSR] ¼ 20
m
M), and Cu2þ (20
mM),
m
M), and CSR with Cu2þ, VO2þ, Bi3þ, Tb3þ, Pb2þ
,
,
.
m
This detection mode should be consistent with that reported
previous by Xiang [6,7,13] and Zhou [19]. In the UVevis spectra, the
absorption at 557 nm for VO2þ-CSR was only 20% of that for Cu2þ
-
CSR (Fig. 3). The VO2þ-CSR solution was lighter red than the
equivalent Cu2þ-CSR solution (Fig. 4).
The detailed fluorescence spectrum of CSR was plane when
excited at 530 nm, (Fig. 6a) with no fluorescence intensity changes
were observed in emission spectra with a wide range of environ-
mentally and physiologically important metal ions: Bi3þ, Tb3þ
Pb2þ, Cr3þ, Al3þ, Eu3þ, Ru3þ, Sm3þ, Sn4þ, Ni2þ, Co2þ, Yb3þ, Hg2þ
,
,
Ga3þ, Zr4þ, Er3þ, Ho3þ, Fe3þ, Zn2þ, Mg2þ, Mn2þ, La3þ, Nd3þ, Naþ, and
Kþ (up to 500 equiv). However, under identical conditions, VO2þ
significantly enhanced the fluorescence intensity. Nanomolar
concentrations of VO2þ significantly enhanced the peak at 583 nm.
The emission peak of CSR at 583 nm (lex ¼ 530 nm) increased
dramatically upon addition of VO2þ (Fig. 6b). When 10 equiv VO2þ
was added to the solution of CSR, a more than 40-fold increase in
fluorescence intensity at 583 nm was observed, whereas 10 equiv
Cu2þ induced only a 3.6-fold enhancement in emission at 577 nm.
This indicated that except for Cu2þ, common coexisting metal ions
did not interfere with the measurement of VO2þ. The stoichiometry
Fig. 3. (a) UVevis spectra of CSR (20
methanol-HEPES (1:1, v/v, pH 7.0) solution; (b) Optical density of the probe CSR
(20 M) at 557 nm upon addition various metal ions.
mM) in the presence of various metal ions in
Fig. 5. Plot of absorbance intensity of CSR (20
20, 40, 60, 80 M in methanoleHEPES (1:1, v/v, pH 7.0) solution.
m
M) at 557 nm against [Cu2þ] 0, 5, 10, 15,
m
m

F.-J. Huo et al. / Dyes and Pigments 86 (2010) 50e55
53
Fig. 8. The proposed mechanism for fluorescence response of CSR to VO2þ basis of ESI-
MS data which afforded a peak at m/z 678.3 assigned for [CSR þ VO þ H2OeH]þ ion
(calcd. 678.2).
of CSR binding with VO2þ ions was determined by fluorescence
titration with VO2þ in the same solvent (Fig. 7). As the concentra-
tion of VO2þ ions increased, the CSR fluorescence increased
significantly until at 10 equiv VO2þ the fluorescence intensity did
not change. The significant fluorescence enhancement may be due
to the V]O double bond, which increased the degree of conjuga-
tion for the whole rhodamine molecule (Fig. 8). Electrospray ioni-
zation mass (ESI-MS) provided direct evidence for the above
proposed mechanism by a peak at m/z 678.3 (calcd for 678.2,
[CSR þ VO þ H2OeH]þ), which indicated the formation of the
complex of CSR and VO2þ
.
Interestingly, both the CSR-Cu2þ and the CSR-VO2þ ensembles
exhibited the excellent selectivity towards pyrophosphate anion
(P2O47ꢀ, PPi) in UVevis spectra (Fig. 9) and in fluorescence spectrum
Fig. 6. (a) Fluorescence spectra of CSR (200 nM) with various metal ions (100
except VO2þ that is 2
M) in methanoleHEPES (1:1, v/v, pH 7.0) solution (lex ¼ 530 nm,
lem ¼ 583 nm, slit: 10 nm/10 nm); (b) The fluorescence intensity at 583 nm of CSR
(200 nM) upon addition of various metal ions (100 M). Each
M except VO2þ that is 2
spectrum is acquired 10 min after metal ions addition.
mM
m
m
m
Fig. 9. (a) UVevis spectra of the [CSR-Cu2þ] ensemble (20
(1:1, v/v, pH 7.0)) was added with various foreign anions (1000
m
M in methanoleHEPES
M) including Fꢀ, Clꢀ
m
,
Fig. 7. Fluorescence spectra of CSR (200 nM) in the presence of different concentra-
tions of VO2þ (0, 0.2, 0.4, 0.8, 1.2, 1.6, 2.0, 4.0, 6.0, 8.0
M) in methanoleHEPES (1:1, v/v,
Brꢀ, Iꢀ, AcOꢀ, SCNꢀ, NOꢀ3 , CO32ꢀ, S2ꢀ, SO24ꢀ, H2PO4ꢀ, HPO24ꢀ, PO43ꢀ and P2O47ꢀ (PPi,
m
400
m
M); (b) UVevis spectra: with addition of [P2O47ꢀ] ¼ 0, 40, 80, 160, 200, 240, 280,
pH 7.0) solution (lex ¼ 530 nm, lem ¼ 583 nm, slit: 10 nm/10 nm). Each spectrum is
360, 400 mM to methanoleHEPES (10 mM, 1:1, v/v, pH 7.0) buffer, the [CSR-
acquired 10 min after VO2þ addition.
Cu2þ] ¼ 20
mM.

54
F.-J. Huo et al. / Dyes and Pigments 86 (2010) 50e55
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A new rhodamine-based derivative (CSR) was prepared and
characterized by 1H NMR and 13C NMR and X-ray crystallography.
CSR exhibited different selectivity at micromolar levels for Cu2þ in
UVevisible spectroscopy and at nanomolar levels for VO2þ in
fluorescence spectra. This is, to the best of our knowledge, the first
reported example of a dual detecting rhodamine derivative capable
of detecting both Cu2þ and VO2þ. CSR displayed Cu2þ-selective
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No. 20801032), Shanxi Provincial Natural
Science Foundation (No. 2009021006-2), the Shanxi Province
Foundation for Returneess (200815), the Shanxi Provincial Foun-
dation for Leaders of Disciplines in Science.
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