A novel colorimetric chemosensor for Cu2+ with high selectivity and sensitivity based on rhodamine B

Article abstract of DOI:10.1007/s11164-015-2024-3

9-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H, 5H-benzo[ij]-quinolizine rhodamine B hydrazone (FHQRH), a novel colorimetric chemosensor for Cu²⁺, was synthesized by a three-step synthetic route. It was found that FHQRH showed a high selectivity for Cu²⁺ ions and excellent anti-interference capability toward other metal ions. The FHQRH solution exhibited a visual color change after the addition of Cu²⁺, owing to the open spirocyclic structure via coordination with Cu²⁺. The complexation coefficient of FHQRH toward Cu²⁺ was measured to be 1:1. Furthermore, this Cu²⁺ chemosensor has a remarkable low detection limit of 0.45 μM, which is 2 % of the toxic level (20 μM) in drinking water as defined by the US Environment Protection Agency (EPA).

Full text of DOI:10.1007/s11164-015-2024-3

Res Chem Intermed
DOI 10.1007/s11164-015-2024-3
2
+
A novel colorimetric chemosensor for Cu with high
selectivity and sensitivity based on Rhodamine B
1
,2
2
2
•
Changjiang Li
Yuchuan Zheng
•
Kaiqiang Xiang
2
Baozhu Tian
•
Yunchang Liu
2
Jinlong Zhang
1
•
•
Received: 2 March 2015 / Accepted: 17 March 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H,
5H-benzo[ij]-quinolizine
?
2
rhodamine B hydrazone (FHQRH), a novel colorimetric chemosensor for Cu , was
synthesized by a three-step synthetic route. It was found that FHQRH showed a high
2
?
selectivity for Cu ions and excellent anti-interference capability toward other
metal ions. The FHQRH solution exhibited a visual color change after the addition
2
of Cu , owing to the open spirocyclic structure via coordination with Cu . The
?
2?
2
complexation coefficient of FHQRH toward Cu was measured to be 1:1. Fur-
?
2
?
thermore, this Cu chemosensor has a remarkable low detection limit of 0.45 lM,
which is 2 % of the toxic level (20 lM) in drinking water as defined by the US
Environment Protection Agency (EPA).
Keywords Rhodamine B Á 8-Hydroxyjulolidine Á Chemosensor Á High selectivity Á
2
?
Cu ions
Introduction
As a first-row transition metal elements, copper plays a pivotal role in biology for
many cellular processes that are essential for life [1]. Most studies have shown that a
variety of enzymes need copper to serve as an indispensable cofactor in respiration,
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2024-
3) contains supplementary material, which is available to authorized users.
&
&
Baozhu Tian
Baozhutian@ecust.edu.cn
Jinlong Zhang
jlzhang@ecust.edu.cn
1
2
Department of Chemistry, Huangshan University, Huangshan 245041, China
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
of Science and Technology, Shanghai 200237, China
123

C. Li et al.
photosynthesis, and scavenging reactive oxygen species [2, 3]. Consequently,
copper ingestion must be severely controlled to maintain intracellular copper
homeostasis [4]. Both intracellular copper deficiency and copper overload can cause
organ dysfunction, leading to various diseases such as jaundice, hemolysis, and
neurodegenerative diseases including Alzheimer’s disease (AD) and Parkinson’s
disease (PD) [5, 6]. Therefore, extensive efforts have been made to develop efficient
methods for analyzing the concentration of copper ions in cells and organisms.
Conventionally, the concentrations of metal ions are determined by using the
following methods: atomic absorption spectroscopy (AAS), inductively coupled
plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission
spectrometry (ICP-AES), spectrophotometry, and cyclic voltammetry (CV) [7].
However, all these approaches are very complicated, time consuming, and
expensive. Compared to the conventional methods mentioned above, chemosensors
have been shown to sense analytes in real-time and real-space with high selectivity
and sensitivity. Consequently, much effort have been devoted to exploiting
chemosensors based on fluorescence and colorimetry in recent years.
Rhodamine dyes, a xanthene class dye, have been used in many fields [8]. Herein,
we synthesized a new rhodamine dye, 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-
1
H,5H-benzo[ij]-quinolizine rhodamine B hydrazone (FHQRH), synthesized by the
reaction between Rhodamine B hydrazide and 9-formyl-8-hydroxy-2,3,6,7-tetrahy-
dro-1H,5H-benzo[ij]-quinolizine. It was found that FHQRH showed very high
2
sensitivity and selectivity for sensing Cu ions. Benesi–Hildebrand plot and Job’s
?
2
?
plot analyses indicated that FHQRH forms a 1:1 stoichiometric complex with Cu
.
Experimental
Chemicals and instruments
4
-(2-hydroxyethyl)-1-piperazineethanesulfonicaci (HEPES), Rhodamine B and
ethylenediaminetetraacetic acid (EDTA) were purchased from Aladdin Chemical
Co., Ltd. (Shanghai, China), 8-hydroxyjulolidine was purchased from TCI, Inc.,
(
solutions of Cu , Na , Al , K , Ca , Mn , Fe , Co , Ni , Zn , Hg
Shanghai, China). All chemicals were analytical reagent grade. The aqueous
3?
2
?
?
3?
?
2?
2?
2?
2?
2?
2?
,
2
?
2?
?
3?
Pb , Ba were from their chloride salts, while Ag and Cr are from their nitrate
salts. N, N-dimethylformamide (DMF) was dried before use, while other chemicals
were employed without any further treatment. Deionized water was used throughout
the work. All the measurements were performed at room temperature (RT).
Reactions were monitored by thin layer chromatography (TLC). Column chro-
matography separation was carried out using silica gel (200–300 mesh). Nuclear
magnetic resonance (NMR) spectra were recorded on a Brucker AM-400 MHz
instrument with tetramethylsilane as internal standard. Mass spectrometry analyses
were performed using a Waters LCT Premier XE spectrometer. UV–Vis absorption
spectra were carried out on a SHIMADZU UV-2450 spectrophotometer.
1
23

2?
A novel colorimetric chemosensor for Cu with high…
Synthesis of 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H, 5H-benzo[ij]-
quinolizine
8
-hydroxyjulolidine (5.0 g, 26.4 mmol) was dissolved in DMF (5 mL). Subse-
quently, the solution was added dropwise to a precooled solution containing DMF
10 mL) and POCl (4.6 g, 30 mmol). The reaction was stirred at room temperature
(
3
for 30 min, and then heated to reflux for another 30 min under N atmosphere.
2
Then, the reaction solution was cooled to room temperature and the pH was adjusted
to neutral with a saturated sodium bicarbonate solution. Finally, the blue-green
precipitate was collected by filtration, washed with water, and dried in a vacuum
drying oven overnight [9–11]. The crude product was purified by silica gel
chromatography (EtOAc/PE = 1:10, V/V), and further recrystallized from ethanol
1
to get yellowish crystals (3.82 g, 67 %). H NMR (400 MHz, CDCl , TMS) d:11.81
3
(
1H, s), 9.36 (1H, s), 6.83 (1H, s), 3.29–3.25 (4H, dd), 2.68–2.65 (4H, t), 1.98–1.80
1
3
(
4H, m). C NMR (100 MHz, CDCl , TMS) d: 191.49, 159.26, 149.46, 131.1,
3
1
13.57, 110.58, 105.16, 50.24, 49.88, 27.17, 21.60, 20.52, 19.60 ppm.
Synthesis of Rhodamine B hydrazide
Rhodamine B hydrazide was synthesized by the condensation reaction of
Rhodamine B and excess hydrazinium hydrate [12–14]. Rhodamine B (4.79 g,
1
0 mmol) and hydrazinium hydrate (12 mL) were dissolved in ethanol (20 mL) and
refluxed at 120 °C for 8 h. After the pink color disappeared, the solution was cooled
to room temperature and its pH was adjusted to neutral with a 0.5 M NaOH
solution. The off-white powder obtained by filtration was purified by silica gel
chromatography (EtOAc), and further recrystallized with petroleum ether to get
1
lavender crystals (3.9 g, 85 %). H NMR (400 MHz, CDCl , TMS) d: 7.95–7.93
3
(
s), 3.61(2H, s), 3.37–3.31(8H, q), 1.18–1.14 (12H, t). C NMR (100 MHz, CDCl3,
1H, m), 7.46–7.44 (2H, m), 7.12–7.10 (1H, m), 6.47–6.42 (4H, dd), 6.31–6.28 (2H,
1
3
TMS) d:166.16, 153.86, 151.57, 148.89, 132.53, 130.03, 128.11, 128.12, 123.84,
1
23, 108.04, 104.54, 97.96, 65.93, 44.38, 12.62 ppm.
Synthesis of 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H,5H-benzo[ij]-
quinolizine rhodamine B hydrazone (FHQRH)
FHQRH was synthesized by the reaction of 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-
1
H, 5H-benzo[ij]-quinolizine, and Rhodamine B hydrazide [15–17]. Rhodamine
hydrazide (2.28 g, 5 mmol) and 9-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H, 5H-
benzo[ij]-quinolizine (1.08 g, 5 mmol) were dissolved in ethanol (20 mL) and
refluxed 8 h. After the mixture was cooled to room temperature, the obtained
precipitate was collected by filtration and dried in vacuum. The crude product was
purified by silica gel chromatography (EtOAc:PE = 1:2, V/V), and further
1
recrystallized using methanol to get FHQRH (1.77 g, 54 %). H NMR (400 MHz,
CDCl , TMS) d: 10.99 (1H, s), 8.76 (1H, s), 7.95–7.93 (1H, m), 7.46–7.44 (2H, m),
3
7.11–7.09 (1H, m), 6.52–6.50 (2H, d), 6.47 (1H, s), 6.44–6.43 (2H, d), 6.24 (2H,
dd), 3.34–3.28 (8H, q), 3.13–3.08 (4H, dd), 2.64–2.56 (4H, dt), 1.88–1.82 (4H, m),
123

C. Li et al.
1
3
1
1
1
2
6
.16–1.13 (12H, t). C NMR (100 MHz, CDCl , TMS) d: 163.64, 155.52, 154.60,
3
53.30, 151.31, 148.44, 145.40, 132.79, 130.03, 129.18, 128.13, 123.73, 123.03,
12.43, 108.08, 107.16, 106.94, 105.80, 98.03, 66.01, 50.04, 49.65, 44.35, 27.06,
?
2.08, 21.23, 20.47, 12.64 ppm. ESI-MS m/z (M ? H) : calcd, 656.3601; found,
56.3606.
Ion sensing measurements
Various ion solutions (3 mM) were obtained by dissolving corresponding salts in
deionized water. The solution of FHQRH (0.3 mM) was dissolved in acetonitrile
solution, and then diluted to 10 lM with CH CN/H O (1:4, V/V) and HEPES buffer
3
2
(
1 mM, pH 7.3). For each titration experiment, 3 mL of the above solution was
taken out, and the appropriate amount of the ion stock solution was added into the
system using a micro-pipette. The UV–Vis spectrum was recorded 1 min after
reaction. All the measurements were carried out at room temperature.
Job’s plot measurement
2
?
The concentrations of FHQRH acetonitrile solution and Cu aqueous solution were
.3 mM. The reactions were carried out in 11 test tubes containing CH CN/aqueous
0
3
2
?
(
1:4, V/V) HEPES buffer (1 mM, pH 7.3), FHQRH acetonitrile solution and Cu
aqueous solution. For each test, 2.9 mL buffer, FHQRH acetonitrile solution and
2
Cu aqueous solution were added into the test tube in turn. The total concentration
?
2
?
of FHQRH and Cu was kept at 10 lM. The absorbance values of these mixed
solutions were recorded at k = 558 nm.
CHO
POCl /DMF
3
N
OH
N
OH
N
O
N
N
O
N
NH NH2
2
O
N NH2
O
EtOH, reflux
O
N
O
N
CHO
OH
N
O
N
+
N
EtOH, reflux
N
N
N NH
2
O
HO
O
N
Scheme 1 Synthesis of FHQRH
1
23

2?
A novel colorimetric chemosensor for Cu with high…
Results and discussion
Design and synthesis of FHQRH
FHQRH was synthesized by three steps. As shown in Scheme 1, 9-formyl-8-
hydroxy-2,3,6,7-tetrahydro-1H, 5H-benzo[ij]-quinolizine, and rhodamine B hy-
drazide were synthesized according to the previous report. Then, the two
compounds were connected by condensation reaction under certain conditions.
1
13
The results of H NMR, C NMR, and electrospray ionization mass spectrometry
ESI-MS) analyses indicated that FHQRH was successfully synthesized (Supporting
Information, Fig. S1–Fig. S4).
(
2
1
Spectroscopic studies of FHQRH toward Cu
Figure 1A shows the UV–Vis spectra of FHQRH in the presence of different metal
2
ions. In the absence of Cu ions, a strong absorption band (350–450 nm) was
?
Fig. 1 A UV–Vis absorption spectra and B Photograph of 10 lM FHQRH solutions in the presence of
?
3?
?
2?
2?
3?
2?
different metal ions (50 lM): a without metal ions, b Na , c Al , d K , e Ca , f Mn , g Fe , h Cu
i Co , j Ni , k Zn , l Hg , m Pb , n Ba
,
2?
2?
2?
2?
2?
2?
123

C. Li et al.
2
?
found, which can be attributed to 8-hydroxyjulolidine group. Upon addition of Cu
ions, the peak at 440 nm exhibited a slight decrease and a strong new absorption at
3
58 nm appeared. A weak new peak was found after adding Fe ions while no new
?
5
peak was seen in the presence of other metal ions. Additionally, a visible color
2?
Fig. 2 A UV–Vis spectra of 10 lM FHQRH under various concentrations of Cu ions (0–120 lM).
B Linear regression plot of the maximum absorption at 558 nm against increasing concentrations of Cu
2
?
(
0–50 lM). The inset shows the absorption curves of 10 lM FHQRH under various concentrations of
2?
Cu (0–50 lM)
1
23

2?
A novel colorimetric chemosensor for Cu with high…
2
?
change from canary to pink was observed with the naked-eye after addition of Cu
ions (Fig. 1B). These results indicated that the FHQRH chemosensor has a high
2
?
selectivity to Cu ions.
To investigate the sensitivity of FHQRH for Cu ions, UV–Vis spectra were
2
?
2
?
recorded upon addition of different concentrations of Cu ions (0–120 lM). In
Fig. 2A, an obvious change of UV–Vis absorption spectra was found upon
2
gradually increasing the concentration of Cu ions. When the concentrations of
?
2
?
Cu ions were in the range of 0–50 lM, a good linear relationship between the
2
?
absorbance values and the concentrations of Cu ions was observed (Fig. 2B).
Limit of detection (LOD) was calculated by using the equation:
LOD ¼ 3r=k
where r is the standard deviation of blank measurements while k stands for the slope
of absorption versus Cu concentration [18]. The result showed that FHQRH for
2
?
2
?
Cu in acetonitrile has a LOD of 0.45 lM, which is 2 % of the toxic level (20 lM)
in drinking water as defined by the US EPA [19].
At the same time, we studied the Benesi–Hildebrand plot [20], which is the
2
?
absorbance value of 1/(A - A ) at 558 nm changed with the variation of 1/[Cu ].
0
2
?
As shown in Fig. 3, 1/(A - A ) and 1/[Cu ] exhibit a very good linear relationship
0
(
Cu is 1:1 (Fig. 3).
r = 0.99925), demonstrating that the complexation coefficient of FHQRH toward
2
?
2
?
0
Fig. 3 Benesi–Hildebrand plot of 1/(A - A ) versus 1/[Cu ]
123

C. Li et al.
2
1
Time-response of FHQRH to Cu
2
?
To investigate the response speed of FHQRH to Cu ions, time-response UV–Vis
spectra of the systems were studied. Figure 4 shows the variation of absorbance
2
values as a function of reaction time under three different Cu concentrations (50,
?
2
?
7
After 5 min, the absorbance at 558 nm no longer changes. Therefore, FHQRH can
0, 100 lM). Every response time of FHQRH with Cu ions was less than 40 s.
2
?
be used for real-time detection of Cu . Moreover, all measurements were
performed after a 5-min incubation except for a specific instance.
Selectivity of FHQRH toward metal ions
Selectivity is a very important performance characteristic for an ion-selective probe.
To investigate the selectivity of FHQRH toward different metal ions, we measured
?
the UV–Vis spectra in the presence of 15 different metal ions, including Na , Ba
2?
,
2
?
3?
2?
3?
2?
2?
?
2?
2?
3?
2?
2?
?
Pb , Al , Zn , Fe , Cu , Hg , Ag , Ca , Ni , Cr , Mn , Co , and K ,
respectively. As shown in Fig. 5a (the front row), FHQRH has excellent selectivity
2
toward Cu when compared with other metal ions. No significant absorbance
?
3
change could be observed in the presence of other ions except for Fe , which
?
2
?
caused a slight decrease. To test the effect of coexisting ions on Cu sensing, the
absorbance and color variations of FHQRH in the presence of copper and other
metal ions are shown in Fig. 5a (the back row) and b, respectively. The results
2
?
indicated that FHQRH has an excellent selectivity for Cu
interference ability.
ions and anti-
2
?
Fig. 4 Time-dependent absorbance of 10 lM FHQRH with 50, 70, 100 lM Cu
1
23

2?
A novel colorimetric chemosensor for Cu with high…
2
?
Fig. 5 Effects of different metal ions (50 lM) on Cu ion sensing: A column-graph of FHQRH under
various metal ions (front row) and in the presence of coexistent metal ions (back row). None FHQRH,
?
2?
2?
3?
2?
3?
2?
2?
?
2?
2?
a Na , b Ba , c Pb , d Al , e Zn , f Fe , g Cu , h Hg , i Ag , j Ca , k Ni , l Cr , m Mn
3?
2?
,
2
n Co , o K . B Photograph of FHQRH in the presence of coexistent metal ions
?
?
2?
2?
Fig. 6 Job’s plot of FHQRH with Cu . [FHQRH] ? [Cu ] was kept constant at 10 lM. The
absorbance was measured at 558 nm
Job’s plot analysis
2
?
In order to determine the cooperation of FHQRH and Cu , a Job’s plot was used.
As shown in Fig. 6, every point in the schematics means that the system was
123

C. Li et al.
2
?
performed with [FHQRH] ? [Cu ] = 10 lM. The maximum absorbance intensity
2
?
was appeared at 0.5 lM, implying that the complex coefficient of FHQRH to Cu
was 1:1.
Analysis of interaction mechanism
2
?
Generally, the exclusive selectivity of the chemosensor for Cu might be due to a
particularly high thermodynamic affinity for typical N, O-chelate ligands and fast
metal-to-ligand binding kinetics [21, 22]. Because EDTA has a stronger binding
2
?
2?
ability towards Cu than other chelating agents, if this Cu probe works following
?
2
the principle of chelation, the color can turn back to the state without Cu ions
after addition of EDTA [23]. When we put EDTA solution into the system of
2
?
FHQRH containing Cu , the color of the solution became straw yellow and the
UV–Vis spectra at 558 nm disappeared at the same time (Fig. 7). This process can
be reversed. Therefore, we can speculate that the mechanism of the reaction was the
same as a few probes based on rhodamine [24–26]. FHQRH has spirocyclic and
open-cycle forms, in which the spirocyclic form is colorless while open-cycle form
2
?
2?
is colored. When Cu
coordination, corresponding to the color change (Scheme 2).
ions are added, the cycle will be opened via Cu
Conclusion
In summary, we successfully synthesized a novel colorimetric chemosensor,
-formyl-8-hydroxy-2,3,6,7-tetrahydro-1H,5H-benzo[ij]-quinolizine rhodamine B
hydrazone (FHQRH), using Rhodamine B and 8-hydroxyjulolidine as source
9
2
?
Fig. 7 Influence of EDTA (0.5 mM) on complexation of 10 lM FHQRH and 50 lM Cu
1
23

2?
A novel colorimetric chemosensor for Cu with high…
N
O
N
N
O
N
Cu2+
H
N
N
N
EDTA
N
N
O
HO
N
O
HO
Cu2+
Scheme 2 The proposed reaction mechanism
materials. The results showed that FHQRH has a high selectivity and sensitivity
2?
2
toward Cu ions. The LOD and complexation coefficient of FHQRH toward Cu
?
are 0.45 lM and 1:1, respectively. Meanwhile, this chemosensor has good anti-
interference capability toward other metal ions and highly visible color change upon
2
addition of Cu . Thus, FHQRH has potential applications in detecting trace
?
2
amounts of Cu ions in environment.
?
Acknowledgments This work was supported by Anhui Provincial Higher Educational Institutions
Natural Science Foundation of China (KJ2011B168); the Natural Science Foundation of China (No.
21401065).
Conflict of interest The authors declare that they have no conflict of interest.
Ethical standard The research described in this paper did not involve human participants and/or
animals.
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