Advances in Materials Physics and Chemistry, 2013, 3, 314-319
Published Online December 2013 (http://www.scirp.org/journal/ampc)
http://dx.doi.org/10.4236/ampc.2013.38043
Open Access AMPC
A New Heptamethine Cyanine-Based Near-Infrared
Fluorescent Probe for Divalent Copper Ions with High
Selectivity
Zhixiang Han1,3*, Qingqing Yang1, Lihui Liang2*, Xiaobing Zhang3
1School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, China
2Hunan Provincial People’s Hospital, Changsha, China
3State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry & Chemical Engineering,
Hunan University, Changsha, China
Received November 4, 2013; revised December 5, 2013; accepted December 16, 2013
Copyright © 2013 Zhixiang Han et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accor-
dance of the Creative Commons Attribution License all Copyrights © 2013 are reserved for SCIRP and the owner of the intellectual
property Zhixiang Han et al. All Copyright © 2013 are guarded by law and by SCIRP as a guardian.
ABSTRACT
A new near-infrared fluorophore 2-(2-Aminoethyl) pyridine-tricarbocyanine (1) was rationally designed and synthe-
sized as a fluorescent probe for detection of Cu2+ with high selectivity. The response of Probe 1 is based on the fluores-
cence quenching upon binding to Cu2+. The sensing performance of the proposed Cu2+-sensitive Probe 1 was then in-
vestigated. The probe can be applied to the quantification detection of Cu2+ with a linear concentration range covering
from 4.8 × 10−7 to 1.6 × 10−4 mol/L and a detection limit of 9.3 × 10−8 mol/L. The experimental results showed that the
response of 1 to Cu2+ was independent of pH in medium condition (pH 6.0 - 8.0), and exhibited excellent selectivity
towards Cu2+ over other common metal cations.
Keywords: Near-Infrared Dye; Fluorescent Probe; Cu2+ Ions
1. Introduction
The design and synthesis of fluorescent probes for se-
lective and sensitive detection of metal ions have at-
tracted wide-spread interests of chemists, biologists,
clinical biochemists and environmentalists in recent
years [1]. Copper is an essential trace element in both
plants and animals, including humans. Among the essen-
tial heavy metals, the abundance of copper ranks the
third in human body. It participates in many biological
processes, such as haemoglobin synthesis (in utilization
of Fe and regeneration of Hb), development of connec-
tive tissue, normal functions of the central nervous sys-
tem, and oxidative phosphorylation [2-4]. Nevertheless,
copper of high concentration is highly toxic to some or-
ganisms such as many bacteria and viruses [5]. Owing to
its toxicity for bacteria, elevated concentrations of copper
would ham- per the self-purification capability of the sea
or rivers, and destroy the biological reprocessing systems
in water. Copper is also found to be harmful to human at
high concentration and has been suspected to cause the
damage of infant liver in recent years. Accordingly,
searching for efficient and reproducible analytical meth-
ods for the copper assay is of great importance for envi-
ronment and human health.
Many analytical methods for detection of copper, in-
cluding atomic absorption spectrometry (AAS) [6], in-
ductively coupled plasma-mass spectroscopy (ICP-MS)
[7], inductively coupled plasma-atomic emission spec-
trometry (ICP-OES) [8], spectrophotometry [9], voltam-
metry [10] and fluorescence spectroscopy [11], have
been developed so far. Among these methods, fluores-
cence spectroscopy offers significant advantages due to
its nondestructive character, high sensitivity and speci-
ficity, and the availability of a wide range of indicative
dyes. Several fluorophores have been used to design flu-
orescent probes for divalent copper ions including cal-
cein [12], rhodamine [13], naphthalimide [14,15], pyrene
[16,17], tris(2,2’-bipyridine)-ruthenium(II) [18], ben-
*Corresponding author.

Z. X. HAN ET AL. 315
zoxazole [19], porphyrin [20,21], spiropyran [22,23],
BODIPY [24], and so on. Unfortunately, limitations of
the currently available probes for Cu2+ include low sensi-
tivity, and/or excitation profiles in the ultraviolet or visi-
ble region, which can damage living samples and cause
interfering autofluorescence from native cells.
The light in the near-infrared region (NIR) around
650 - 900 nm can penetrate more deeply into tissues,
which is of importance to study on living organism im-
aging. Moreover, it has a further advantage that auto-
fluorescence is not observed upon NIR excitation. Hep-
tamethine cyanine dyes [25], one of the important kinds
of NIR dyes, has been widely used in various fields, and
been employed as fluorescent labels in the studies of
fluorescence imaging with biological mechanisms. And a
few probes based on heptamethine cyanine dyes have
been employed to detect metal ions or small molecules
[26]. However, to the best of our knowledge, only few
NIR fluorescent probes based on cyanine dyes have been
reported for divalent copper ion assay detection [27].
Searching for new NIR probe for copper detection with
high selectivity is still an active field as well as a chal-
lenge for the analytical chemistry research.
Herein, we report the synthesis and properties of a
novel NIR fluorescent probe 2-(2-Aminoethyl) pyridine-
tricarbocyanine (1) for the detection of Cu2+ with good
selectivity and high sensitivity. Tricarbocyanine and
2-(2-aminoethyl) pyridine were selected as the reporter
and cheleator, respectively. The probe exhibited stable
response towards Cu2+ over the concentration range from
4.8 × 10−7 to 1.6 × 10−4 mol/L with a working pH range
from pH 6.0 to 8.0.
2. Experimental
2.1. Reagents
Before being used, N, N’-dimethylformamide (DMF)
was subjected to simple distillation from K2CO3. IR-780
iodide was purchased from Sigma-Aldrich. 2-(2-Amino-
ethyl) pyridine was purchased from Alfa Aesar. All other
chemicals were of analytical reagent grade, purchased
from Shanghai chemical Reagent Corporation (Shanghai,
China), and used without further purification. Twice dis-
tilled water was used throughout all experiments. Thin
layer chromatography (TLC) was carried out using silica
gel 60 F254, and column chromatography was conducted
over silica gel (100 - 200 mesh), both of which were ob-
tained from the Qingdao Ocean Chemicals (Qingdao,
China).
2.2. Synthesis of Compound 1
Synthetic route for Compound 1 was depicted in Scheme
1. Briefly, IR-780 iodide (11.3 mg, 0.0167 mmol) and
2-(2-Amino-ethyl) pyridine (20.4 mg, 0.167 mmol) were
Scheme 1. Synthetic pathway of Compound 1.
dissolved in anhydrous DMF (3 mL) in a 25 mL round
bottom flask. The mixture was stirred at 80˚C for 4 h
under an argon atmosphere. The solvent was removed
under reduced pressure, then purified on silica gel chro-
matography eluted with CH2Cl2/ethanol (100:1, V/V) to
afford the desired product as a blue solid (7.1 mg, yield
56%). 1H NMR (400 MHz, CDCl3): δ 0.93 (t, 6H, J = 7.4
Hz), 1.55(s, 12H), 1.63 - 1.73(m, 6H), 2.45 (t, 4H, J = 6.0
Hz), 3.21(t, 2H, J = 6.4 Hz), 3.92 (t, 4H, J = 6.8 Hz),
4.14(d, 2H, J = 6.4 Hz), 5.76 (m, 2H), 7.05(t, 2H, J = 6.8
Hz), 7.15(d, 2H, J = 8.0 Hz), 7.27 - 7.33(m, 4H), 7.43(m,
2H), 7.60(d, 2H, J = 12.8 Hz), 7.75(m, 1H), 8.50(d, 1H, J
= 5.2 Hz), 8.68(br, 1H). ESI-MS: [M-I−] = 625.3,
calculated: [M]+ = 625.9.
2.3. Apparatus
1H NMR spectra were recorded on a INOVE-400 (Varian)
spectrometer operating at 400 MHz. All chemical shifts
are reported in the standard δ notation of parts per mil-
lion. LC-MS analyses were performed using an Agilent
1100 HPLC/MSD spectrometer; UV-Vis absorption
spectra were recorded with a Shimadzu MultiSpec-1501
spectrophotometer. All fluorescence measurements were
carried out on a HITACHI F4500 (Japan) with excitation
slit set at 10.0 nm and emission at 20.0 nm. The pH
measurements were carried out on Mettler-Toledo Delta
320 pH meter (Shanghai, China).
2.4. Measurement Procedures
4-(2-Hydroxyethyl) piperazine-1-ethanesulfonic acid
(HEPES) buffer {0.1 mol/L, pH = 7.4, I = 0.1 (NaNO3)}
were prepared by dissolving appropriate HEPES and
NaNO3 in water, adjusting to pH = 7.4 by 1.0 mol/L
NaOH with the volume to 1000 ml in a volumetric flask.
A 2.0 × 10−5 mol/L stock solution of 1 was prepared
by dissolving 1 in CH3CN. A stock standard solution of
Cu2+ (0.01 mol/L) was prepared by dissolving an appro-
priate amount copper in water and adjusting the volume
to 100 ml, then further diluted to 1 × 10−3 - 1 × 10−7
mol/L stepwise. The buffered solutions of wide pH range
were obtained by adjustment of 0.1 mol/L HEPES solu-
tion with HCl or NaOH solution. The complex solution
of Cu2+/1 was prepared by adding 5.0 mL the stock solu-
tion of 1 and 1.0 mL mentioned above solution of Cu2+ in
a 10 mL volumetric flask. Then the mixture was diluted
to 10mL with pH 7.4 HEPES buffer solution. In the ob-
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Z. X. HAN ET AL. 316
tained solution, the concentrations were 1 × 10−5 mol/L
of 1 and 1 × 10−3 - 1 × 10−8 mol/L of Cu2+. Blank solu-
tion of 1 was prepared under the same conditions without
Cu2+. All the solution above were protected from light
and kept at 4˚C for further use.
3. Results and Discussion
3.1. Spectral Characteristics
Figure 1 showed the fluorescence spectra of 1 in HEPES
buffer solutions with different concentrations of Cu2+,
which recorded at excitation wavelength of 640 nm and
emission wavelength of 670 - 800 nm. The spectrum of
free 1 exhibited very strong fluorescence emission in a
buffer solution. Addition of Cu2+ to a solution of 1, fluo-
rescence signal exhibited a remarkable quenching. The
fluorescence intensity of 1 was gradually decreased with
increasing Cu2+ concentration. These results provided a
proof for the formation of an inclusion complex of 1 with
Cu2+, which constituted the basis for the determination of
Cu2+ concentration with 1. It is worthy to note that the
fluorescence intensity of Probe 1 can be recovered upon
addition of the coordinating reagents ethylenediamine-
tetraacetic acid (EDTA).
In order to better understand the variation of fluores-
cence intensity with the concentrations of Cu2+, the ab-
sorption spectra of 1 in the absence and presence of Cu2+
were recorded (Figure 2). In the absorption spectrum of
1, the results showed a strong absorption band at 643nm
in the absence of Cu2+, while the addition of Cu2+ ions
decreased with no obvious shift in absorbance at 643 nm.
From the fluorescence spectra and UV-is spectra, it is
indicated that the fluorescence changes of 1 were more
likely to be caused by the change of quantum yield rather
than spectral shifts. Similar results were reported by
Tang et al. [28].
In addition, Probe 1 works well and no detectable
change in the linear range, detection limit or other ana-
lytical performance is found after it has been stored for
several weeks in the dark at 4˚C, which implies that the
NIR fluorescent probe used is stable.
3.2. Principle of Operation
The complexation equilibrium of 1 (A) with Cu2+ (B)
with an association constant K can be expressed by the
following equation:
K
nm
mBnAA B (1)
where Cu2+ (B) and 1 (A) is established by formation of a
complex with a complexing ratio of m:n. According to
the modified Stern–Volmer equation [29], the relation-
ships for the changes of fluorescence intensities, the
concentration of Cu2+ [B] in solution and the concentra-
tion of 1 [A] in solution can be expressed as follows:
Figure 1. Fluorescence emission spectrum of 1 (10 µmol/L)
with different concentration of Cu2+ (From top to bottom: 0,
0.5, 1.0, 2.5, 5.0, 10, 20, 50, 100, 200 µmol/L) in CH3CN/H2O
(1:1, v/v) solution. Inset shows the linear responses with
divalent copper ions concentrations.
Figure 2. Chages in the UV-vis spectra of 1 (10 µmol/L)
(black line) upon the addition of Cu2+ (10 µmol/L) (red line).
 
1
0n
q
FF m
K
AB
F

 (2)
Assuming 0
F
FF

, one can obtain:
  
loglog1 loglog
q
F
K
nAm
F


 

 B (3)
Here F0 and F denote the fluorescence intensities of 1
in the absence and presence of Cu2+, respectively. Kq is
fluorescence quenching constant. The calibration curve
was constructed by recording the fluorescence intensity
values of 1 in the presence of different Cu2+ concentra-
tion. In the range between 1.0 × 10−7 and 5.0 × 10−5
mol/L Cu2+, the fluorescence intensity is linearly de-
pendent on the Cu2+ concentration. The dependency can
be described by the following equation:
2
log0.9343log Cu4.80
F
F


 

 
 (4)
It is obvious from Equation (4) that m is the slope of
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Z. X. HAN ET AL. 317
log(ΔF/F) versus log[Cu2+], which was calculated to be 1
approximately. Quenching constant (Kq) is 6.3 × 104.
The relative fluorescence intensity α is defined as the ra-
tio of free A [A]f, to the total amount of A [A]t, in the so-
lution. It can be experimentally determined by measur-
ing the fluorescence intensity of 1 in the solution:



ft
bt
t
A
F
F
A
FF



(5)
Here Fb is the fluorescence intensity of 1 in the blank
buffer solution and Ft represents the fluorescence inten-
sity of 1 in the solution when 1 is completely complexed
with Cu2+. F is the fluorescence intensity of 1 actually
measured when in contact with Cu2+ solutions of a given
concentration. The relationship between the α and Cu2+
concentration [B] can be represented as:
 
1
1
1
n
n
t
nK AB




m
(6)
The response of 1 for different concentrations of Cu2+
was shown in Figure 3. Three curves are calculated us-
ing Equation (6) with different K and ratios of Cu2+ and 1.
It can be seen that the best curve was 1:1 complex ratio
and an appropriate K of 1.09 × 105 fits form the experi-
mental data. The curve can serve as the calibration curve
for the detection of Cu2+ concentration. A practically
usable range for quantitative determination covered from
4.8 × 10−7 to 1.6 × 10−4 mol/L (0.05 ≤ α ≤ 0.95) [29]. The
detection limit was 9.3 × 10−8 mol/L (defined as three
times standard deviation of blank solution).
3.3. Effect or pH
The effects of pH on the fluorescence intensity of 1 in the
presence of Cu2+ were carried out at a pH range from 5.0
to 9.0 with fixed the Cu2+ concentration at 5 μmol/L
(Figure 4). In lower pH value, the fluorescence intensity
of 1 decreased with decreasing pH value, which might be
caused by the protonation of Compound 1 without bind-
ing with the metal ion. On the other hand, too high pH
would lead to form the precipitation of Cu(OH)2, and
reduce its complexation with 1. In a wide range of pH
from 6.0 to 8.0, acidity did not affect the determination
of Cu2+ with Compound 1. In other words, the response
behavior of Compound 1 is independent of pH in me-
dium condition, which is convenient for practical appli-
cations of the proposed probe in determination of Cu2+.
3.4. Selectivity
Under the same conditions, the ability of 1 to recognize
Cu2+ was further investigated by mixture 100 μmol/L
Cu2+ with the other background anions and metal ions.
The experiments were carried out by recording the
changes of the fluorescence intensity before and after
adding the interferants into the pH 7.4 HEPES buffer
solution. As shown from Figure 5, one can see that the
proposed probe exhibited a relatively high selectivity for
Cu2+ ions over a large number of mono-, bi-, and triva-
lent cations. Fortunately, normal interferents like Hg2+ do
not interfere, which is better than that of the probe re-
ported in literatures.
3.5. Preliminary Analytical Application
The proposed probe was applied to the determination of
copper ions in water samples of Xiang River. The river
water samples were simply filtrated and showed that no
Cu2+ was present in them. All the water samples were
spiked with standard Cu2+ solutions at different concen-
tration levels, and then analyzed their concentrations
with proposed Probe 1. Results are shown in Table 1.
One can see that the recovery study of spiked Cu2+
Figure 3. Relative fluorescence intensity α of 1 as a function
of log[Cu2+] in CH3CN/H2O (1:1, v/v) solution. The curves
fitting the experimental data were calculated from Equation
(6). (1) m:n = 1:1, K= 1.09 × 105; (2) m:n = 1:2, K= 1.09 ×
1010; (3) m:n = 1:3, K= 1.46 × 1015. (●) data points experi-
mentally obtained.
Figure 4. Effect of pH on the emission of 1 (10 μmol/L) with
5 μmol/L Cu2+ at 715 nm in CH3CN/H2O (1:1, v/v) solution.
Excitation was provided at 640 nm.
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Z. X. HAN ET AL. 318
Figure 5. Emission change of 1 at 715 nm upon addition of
each cation in CH3CN/H2O (1:1, v/v) solution at pH 7.40
{0.1 mol/L HEPES, I = 0.1 (NaNO3)}: 1, none; 2, Zn2+; 3,
Cd2+; 4, Mn2+; 5, Ca2+; 6, Mg2+; 7, K+; 8, Na+; 9, Li+; 10,
Al3+; 11, Co2+; 12, Ni2+; 13, Pb2+; 14, Hg2+; 15, Fe3+; 16, Ag+
17, Cu2+. With the exception of Cu2+, Hg2+ and Al3+ where
10−4 mol/L of cation were added, each solution contained
10−3 mol/L of interest. The concentration of 1 was 10
μmol/L and excitation was provided at 640 nm.
Table 1. Recovery study of spiked determination of copper
in Xiang River water with proposed Probe 1.
Sample Cu2+ spiked
(mol/L)
Cu2+ recovered
(mol/L) Recovery (%)
1 0 0 –
2 1.0 × 10−5 (1.02a ± 0.03b) × 10−5 102.0
3 5.0 × 10−6 (4.93a ± 0.04b) × 10−6 98.6
aAverage were calculated with n = 3, bStandard deviations.
determined by the 1-based probe showed satisfactory re-
sults. The present probe is useful for the determination of
Cu2+ in real samples.
4. Conclusion
In summary, a new near-infrared fluorescent probe was
designed and synthesized for the detection of Cu2+ based
on quenching the fluorescence of tricarbocyanine chro-
mophore with 1-Cu2+ complexation. Compared to re-
ported fluorescent probes, 1-based probe showed high
selectivity and large stokes shift over existing reagents
and methods for the fluorescence determination of Cu2+
in neutral medium. And the proposed method can be
used for the determination of Cu2+ in real samples.
5. Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (Grant 20505008, 21105038),
“973” National Key Basic Research Program of China
(2007CB310500), Ministry of Education of China
(NCET-07-0272), and Hunan Natural Science Founda-
tion (06JJ4010, 07JJ3025).
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