Highly selective visual detection of Cu(II) utilizing intramolecular hydrogen bond-stabilized merocyanine in aqueous buffer solution

Article abstract of DOI:10.1021/ol100381g

A Cu²⁺-specific colorimetric sensor 1, which is stabilized by an intramolecular hydrogen bonding, was designed and developed. The color of 1 changes from purple to blue on addition of 1.0 μM Cu²⁺ in aqueous buffer solution, which can be detected by the naked eye. The analytical detection limit for Cu²⁺ by the naked eye is as low as 1.0 μM. The stoichiometry for 1 and Cu²⁺ in complex is 2:1 in aqueous solution.

Full text of DOI:10.1021/ol100381g

ORGANIC
LETTERS
2010
Vol. 12, No. 10
2202-2205
Highly Selective Visual Detection of
Cu(II) Utilizing Intramolecular Hydrogen
Bond-Stabilized Merocyanine in
Aqueous Buffer Solution
Zhao-Qi Guo,†,‡ Wei-Qiang Chen,*,† and Xuan-Ming Duan*,†
Laboratory of Organic NanoPhotonics and Key Laboratory of Photochemical
ConVersion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, No. 2, Zhongguancunbeiyitiao, Haidian
District, Beijing 100190, China
chenwq7315@mail.ipc.ac.cn; xmduan@mail.ipc.ac.cn
Received March 1, 2010
ABSTRACT
A Cu2+-specific colorimetric sensor 1, which is stabilized by an intramolecular hydrogen bonding, was designed and developed. The color of
1 changes from purple to blue on addition of 1.0 µM Cu2+ in aqueous buffer solution, which can be detected by the naked eye. The analytical
detection limit for Cu2+ by the naked eye is as low as 1.0 µM. The stoichiometry for 1 and Cu2+ in complex is 2:1 in aqueous solution.
The development of highly selective sensors for metal ions
is particularly important, since metal ions can have detri-
mental effects on humans and the environment.1 Copper is
the third most abundant essential trace element in the human
body and is commonly found as Cu2+ in natural water.2
However, free Cu2+ is potentially very toxic to aquatic life,
both acutely and chronically. For example, microorganisms
are affected by even submicromolar concentrations of Cu2+.3
Excessive uptake of copper causes liver or kidney damage.4
Several analytical methods5 and a number of probes6,7 have
been developed for sensitive, selective, and accurate detection
of Cu2+. Colorimetric methods, in particular, are extremely
attractive because they can be easily read with the naked
eye. Most of the reported colorimetric sensors7 for Cu2+ are
used in mixed solvent (water/organic solvent), but only a
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† Technical Institute of Physics and Chemistry.
‡ Graduate School of Chinese Academy of Sciences.
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10.1021/ol100381g  2010 American Chemical Society
Published on Web 04/19/2010

few of them are used in neat aqueous solution.7f Conse-
quently, the development of nake-eyed sensors for Cu2+ in
neat aqueous solution has the important feature that they can
be used to evaluate the analyte concentration rapidly.
The conversion of spiropyran to merocyanine8 was used
for metal ion sensing by phenolate oxygen in the merocya-
nine via cooperative ligation of other chelating functionality
attached at the 8-position or N-position (Scheme 1).6a,7g,9
However, the use of organic solvents and light-black
protection in the measurement limits their application.
Therefore, thermo-/photostable merocyanine from spiropyran
is desirable. Herein, we introduce naphthalen-1,8-diol moiety
into spiropyran 1a, which is likely to convert to merocyanine
our design. Compound 1 can be obtained by condensation
of 1,8-dihydroxy-2-naphthaldehyde 9 with 1,2,3,3-tetram-
ethyl-3H-indolium iodide in ethanol (shown in Scheme 2).8
Scheme 2. Synthesis of 1 and 2
Scheme 1. Chemical Structure of the Spironaphthanopyran 1
and the Interconversion of Spiropyran to Merocyanine
Compound 9 was obtained in three steps starting from com-
mercially available naphthalen-1,8-diol 3.11 In the same pro-
cedure, compound 2, without an additional chelating hydroxyl
group, was also synthesized in control. The detailed experi-
mental procedures can be found in the Supporting Information.
Compound 1 exists mainly as merocyanine in solution.
In its 1H NMR spectrum in CDCl3, a singlet at 14.58 ppm is
assigned to the proton of the hydroxyl group involved in a
strong intramolecular hydrogen bonding; two doublets (6.18
and 8.66 ppm) with a coupling constant of 14.2 Hz are
assigned to the protons of trans-CHdCH bond and a singlet
at 3.48 ppm is assigned to the proton of N-CH3. In contrast,
compound 2 exists as the spiro-form in chloroform due to
chemical shifts at 2.73 ppm for N-CH3 and a coupling
constant of 10.2 Hz for cis-CHdCH. The results demonstrate
the intramolecular hydrogen bonding in molecule 1 makes
the equilibrium shift to merocyanine and stabilizes the
merocyanine. About 20% intramolecular proton tranfer
tautormer existing in CDCl3 and CD3CN vanished when
CD3OD and DMSO-d6 were used as solvent (Figure 1).
When an equivalent of Cu(OAc)2 was added to a solution
of 1 in DMSO-d6, the naphtholate together with hydroxy
group bind to Cu2+, which results in the peaks broadening
1b due to the formation of a six-membered-ring intramo-
lecular hydrogen bond between hydroxyl group and carbonyl
(or naphthanolate) group (Scheme 1). The formed hydrogen
bonding will stabilize the zwitterion form 1c and increase
its solubility in polar solvent. Furthermore, the hydroxyl
group together with naphthanolate could bind to the metal
ion and act as a sensor. In this context, we present the
synthesis and the spectroscopic evaluation of colorimetric
chemosensor 1, which is developed for the highly selective
visual detection of Cu2+ in aqueous buffer solution.
Theoretical calculations are performed to evaluate the sum
of electronic and zero-point energies of spiro-form 1a and
mero-form 1b on the isolated gas phase by using the
Gaussian 03 program (b3lyp/6-31+G(d,p)).10 The result
shows the energy of mero-form 1b is 18.4 kcal/mol lower
than that of spiro-form 1a. This demonstrates 1 exists mainly
in mero-form 1b in the equilibrium and is in accordance with
1
and shifting in the H NMR spectrum.
The intramolecular hydrogen bond-stabilized merocyanine
exhibits a strong absorption band in the range of 500-650
nm in selected solvents. (Figure 2). The absorption maxi-
mium of 1 in methanol appears at 603 nm with a extinction
coefficient (ε) of 7.4 × 104 M-1 cm-1, which is assigned to
the transition to the intramolecular charge transfer state from
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Org. Lett., Vol. 12, No. 10, 2010
2203

The selective interaction of sensor 1 with Cu2+ in methanol
encourage us to further investigate its binding property in
aqueous solution. Fortuanately, compound 1 can be dissolved
in basic solution due to the probable deprotonation. The
changes in the absorption spectra of 1 upon addition of Cu2+
in PBS solution (pH 9.18) are shown in Figure 3a. The
Figure 1.
1H NMR spectra of 1 in selected solvent in the range of
6.0-8.8 ppm: (a) CDCl3; (b) CD3CN; (c) CD3OD; (d) DMSO-d6;
and (e) DMSO-d6 with 1.0 equiv of Cu(OAc)2
the ground state. The absorption maximium shifts to 548
nm with ε ) 3.1 × 104 M-1 cm-1 in phosphate-buffered
saline (PBS, pH 9.18) solution, and 547 nm with ε ) 5.1 ×
104 M-1 cm-1 in hexane. In contrast, there was no absorption
at 500-650 nm for 2 in hexane, but the absorption intensity
of the band increased as the solvent polarity increased (Figure
S1, Supporting Information). The results indicate compound
1 shows strong solvatochromism due to the solute-solvent
interaction, and for compound 2, solvatochromism was
governed by the shift of the equilibrium.8
The metal ions binding properties and selectivity of
compound 1 were first investigated in methanol solutions
with use of metal acetate salts. Chemosensor 1 response to
Cu2+, Fe2+, and Fe3+ lead to absorption variation (Figure
S2, Supporting Information). The absorption peak at 565 nm
vanished for 1 in methanol gradually with the addition of
Cu2+, while a new absorption band appeared at 636 nm. The
color of the solution changed from blue to cyan (Figure S3,
Supporting Information). The stoichiometry for 1 and Cu2+
is 1:1 in methanol determined by a Job’s plot12 (Figure S4,
Supporting Information). Ascribed to the equilibrium, 1c
existing in methanol, two oxygen atoms were both partially
negatively charged and easily chelated to Cu2+.
Figure 3
. (a) Absorption spectra of 1 (10 µM) in pH 9.18 PBS
buffer solution in the presence of different concentrations of Cu2+
.
Inset: The absorption change of 1 at 670 nm upon the initial
concentration of [Cu2+]. (b) Job’s plot for determing the stoichi-
ometry for 1 and Cu2+ in PBS buffer solution. Total concentration
) 1.0 × 10-5 mol/L.
intensity of the absorption maximium at 548 nm was
gradually reduced with the addition of Cu2+, while a new
absorption band appeared at 670 nm with a red shift of 122
nm. The absorption change at 670 nm gradually increased
with the concentration of Cu2+ increasing (inset in Figure
3a). The analytical detection limit of Cu2+ by the naked eye
is as low as 1.0 × 10-6 M. The association constant is (1.0
( 0.15) × 1010 M-2 (Supporting Information). The stoichi-
ometry of Cu2+ and 1 is 1:2 determined by a Job’s plot
(Figure 3b).12
Furthermore, we investigated the metal ions binding
property and selectivity of compound 1 in PBS solutions (pH
9.18). Figure 4a (the black bar portion) illustated the
absorption resopnse of 1 to metal ions Na+, K+, Ag+, Mg2+,
Cu2+, Ca2+, Cd2+, Co2+, Hg2+, Ni2+, Pb2+, Zn2+, Fe2+, and
Fe3+ in aqueous solution. No significant absorption changes
were observed with common interferences, the purple color
of 1 showed no change when other metal ions were added
Figure 2. Absorption spectra of 1 (10 µM) in selected solvents.
(12) Job, P. Ann. Chim. 1928, 9, 113.
Org. Lett., Vol. 12, No. 10, 2010
2204

(50 µM) was added to 10 µM 1 followed by addition of
Cu2+ (10 µM). The results are also shown in Figure 4a (the
shaded bar portion). Our sensor showed a small change
response to Cu2+ with or without addition of other interfering
metal ions. These experimental results showed that the
response of the sensor to Cu2+ is little affected by the
presence of the other possible contaminating metal ions.
In conclusion, a highly sensitive and selective merocya-
nine-based colorimetric chemosensor 1 was developed for
the detection of Cu2+ in PBS solution (pH 9.18). The
recognition of Cu2+ gave rise to color changes from purple
to blue that were clearly visible to the naked eye. Such Cu2+
selective colorimetric chemosensors could meet the sensitive
and selective requirements for biomedical and environmental
application and be of great importance for real-time and
instrument free detection.
Figure 4.
(a) Absorption responses of 1 (10 µM) to 10 µM Cu2+
or 50 µM other metal ions (the black bar portion) and to the mixture
50 µM other metal ions with 10 µM Cu2+ (the shaded bar portion).
(b) Photograph of sensor 1 containing 50 µM metal ions in PBS
buffer solutions.
Acknowledgment. This research was supported by the
National Natural Science Foundation of China (Grant Nos.
50773091 and 20702056) and the National Basic Research
Program of China (2010CB934103).
except for Cu2+. However, with the addition of 1 equiv of
Cu2+, the color changed from purple to blue (Figure 4b).
These results indicated that our proposed sensor exhibited
high selectivity to Cu2+ over other metal ions. In river water
and seawater, the concentrations of some other contaminating
metal ions, such as Zn2+, Ni2+, and Pb2+, are significantly
higher than that of Cu2+; selective detection of Cu2+ in the
presence of these metal ions with high concentration is a
challenge to the application of most common sensors. To
test the practical applicability of our visual chemosensor for
Cu2+, competition experiments were also carried out. Five
times the concentration of the above-mentioned metal ions
Note Added after ASAP Publication. In the version
published ASAP April 19, 2010 two doublet values were
incorrect in the paragraph describing Figure 1; the correct
version published on the web May 14, 2010.
Supporting Information Available: Experimental pro-
cedures and characterization data of compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL100381G
Org. Lett., Vol. 12, No. 10, 2010
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