Inorganica Chimica Acta 467 (2017) 325–329

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Inorganica Chimica Acta

journal homepage: www.elsevier.com/locate/ica

Research paper

A colorimetric probe for dual sensing of Hg2+ and Cu2+ ions in water
Yi-Wun Sie a, Chia-Lin Li a, Chin-Feng Wan b, Jie-Hong Chen a, Ching-Han Hu a, Hongbin Yan c, An-Tai Wu a,⇑
a
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
b
Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
c
Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON L2S 3A1, Canada

a r t i c l e i n f o a b s t r a c t

Article history: A colorimetric probe (L) was prepared and its metal ion sensing properties were investigated. Probe L
Received 12 May 2017 exhibited obvious colorimetric response toward Hg2+ and Cu2+ with different color changes in water.
Received in revised form 31 July 2017 The association constant for probe L-Hg2+ and probe L-Cu2+ in water containing 0.5% DMSO (v/v) was
Accepted 17 August 2017
determined as 5.62  107 M 1 and 1.26  107 M 1, respectively. The detection limit of probe L for the
Available online 19 August 2017
analysis of Hg2+ and Cu2+ was determined as 0.19 ppm and 0.39 ppm, respectively. Moreover, probe L
can be used for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples.
Keywords:
Ó 2017 Elsevier B.V. All rights reserved.
Chemosensor
Water soluble
Colorimetric

Development of new fluorescent or colorimetric probes for the synthesis, low water solubility and lack of sensitivity. Therefore,
detection of different analytes simultaneously is emerging as an for practical applications, a water soluble probe for the detection
area of great interest [1–4], since such systems would lead to faster of Hg2+ and Cu2+ metal ions is still in great demands.
analytical processing and potential cost reductions. However, sin- Multidentate ligands have been widely used as a metal-binding
gle probes for differential detection of multi-metal ions are still reagent in most aspects of coordination chemistry[26–30]. Com-
rare [5]. Since the recognition units of the sensors are usually quite plexes of the hexadentate ligand (probe L) have been prepared
different in their interaction with cations or anions, developing [31] as a new class of coordination chemistry-based antitumor
such sensors with the capability to recognize multiple analytes is agents [29]. However, to the best of our knowledge, utilizing probe
a challenging task. It is worth noting that multifunctional sensors (L) as colorimetric probes for detection of cation has not been
that produce different fluorescence or color responses to different reported. Therefore, we report the colorimetric probe (L), which
metal ions are highly desirable in environmental applications due could selectively detect Hg2+ and Cu2+ among a series of ions based
to the presence of multiple analytes of interest in these samples. on the distinct color changes that are visible to naked eyes. Probe L
As a result, intense research has been focused on the development can be readily prepared by a simple Schiff-base reaction between
of sensitive and selective receptors for the qualitative and quanti- 2-hydrazinylpyridine and 1,10-phenanthroline-2,9-dicarbalde-
tative recognition of multi metal ions. Among various important hyde, as shown in Scheme 1. The structure of probe L was con-
metal ions, the Cu2+ is a significant environmental pollutant and firmed by 1H and 13C NMR and mass spectroscopy (Figs. S1 and S2).
also plays a critical role in various biological processes [6–8]. The The absorption response of probe L was investigated in water
accumulation of excess amounts of Cu2+ or their misregulation containing 0.5% DMSO (v/v) in the presence of various metal ions
can cause a series of severe diseases such as Alzheimer’s and as their perchlorate salts: Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+,
Parkinson’s diseases [9]. On the other hand, Hg2+ and its deriva- Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+. The solution of probe L
tives are significant environmental pollutants with inherent high showed an absorption band at 360 nm. In the presence of Hg2+,
toxicity. In the marine environment, Hg2+ ion is converted by bac- the absorption spectra of probe L were red-shifted, with two major
teria into toxic methylmercury, a highly potent neurotoxin, and is bands at 400 and 550 nm, as shown in Fig. 1. The solution of probe
passed up the food chain to accumulate in humans [10]. L in the presence of Hg2+ showed a dramatic color change from
To the best of our knowledge, while a few probes for dual detec- light yellow to red, as shown in Fig. 2. In the presence of Cu2+,
tion of Hg2+ and Cu2+ have been reported [11–25], there are a num- the absorption spectra of probe L also showed a red shift, with a
ber of limitations in these probes, such as their complicated dramatic color change from light yellow to orange which could
easily be seen by the naked-eye, as shown in Fig. 2. These results
⇑ Corresponding author. indicate that probe L can function as a ‘‘naked-eye’’ colorimetric
E-mail address: antai@cc.ncue.edu.tw (A.-T. Wu). sensor capable of differentiating Hg2+ and Cu2+.

http://dx.doi.org/10.1016/j.ica.2017.08.035
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
326 Y.-W. Sie et al. / Inorganica Chimica Acta 467 (2017) 325–329

N N
N NHNH2
HC CH
N N
OHC CHO MeOH N N
HN NH

N N

Scheme 1. Synthesis of probe L.

0.5

0.4
Host
Absorbance (AU)

0.3

0.2 2+
Hg

0.1
2+
Cu

0.0
300 400 500 600 700
Wavelength (nm)

Fig. 1. UV/vis spectra of probe L (30 lM) recorded in H2O (0.5% DMSO) after
addition of 5 equiv of various metal ions.

In the UV–vis titration profiles (Fig. 3a) of Hg2+, the UV–vis Fig. 3. Fluorescence spectra of probe L (30 lM) in H2O (0.5% DMSO) upon addition
absorbance of probe L gradually increased, accompanied with of increasing concentrations (a) Hg2+ (top); (b) Cu2+ (bottom).
a red-shift to 550 nm. This observation suggests the formation of
a new species. Meanwhile, the solution of probe L showed a dra-
matic color change from yellow to red which could easily be seen probe L for the analysis of Hg2+ ion was determined as 0.19 ppm
by the naked-eye. On the other hand, with the addition of increas- and for Cu2+ it was determined as 0.39 ppm.
ing amounts of Cu2+ to a solution of probe L (Fig. 3b), the major For the standpoint of practical applications, real-time determi-
peak at 360 nm decreased gradually, accompanied by a shoulder nation of analytes is highly desirable. In this respect, the absor-
band at 470 nm. Taken together, these titration results indicate bance of probe L in water containing 0.5% DMSO at 550 nm upon
that probe L can be used for selective sensing of Hg2+ and Cu2+ addition of 5.0 equiv of Hg2+ and 480 nm upon addition of Cu2+
based on the distinct color changes in environmental analysis. was monitored over time. As shown in Fig. 4, the absorbance pla-
From the UV–vis titration profiles, the association constants for teaued within one min after addition of the Hg2+ and Cu2+. The
probe L-Hg2+ and L-Cu2+ were determined as 5.62  107 M 1 and effect of pH on the absorption bands of the probe L–Hg2+ or probe
1.26  107 M 1, respectively, by the Stern-Volmer plot. A Job plot L–Cu2+ complex in H2O (containing 0.5% DMSO) solution was also
indicated a 1:1 stoichiometric complexation of probe L with Hg2+ examined. It was found that the absorbance intensity of probe
and Cu2+ ions (Figs. S3 and S4). In addition, the formation of 1:1 L–Hg2+ or probe L–Cu2+ in H2O (containing 0.5% DMSO) solution
complex between probe L and Hg2+ was further confirmed by the showed a significant absorption band in the pH range 4–9
appearance of a peak at m/z 638, assignable to [probe L + Hg2+ + (Fig. S7), suggesting that probe L could be applied in the analysis
H2O] (Fig. S5). Similarly, the peak at m/z 533 was assignable to of environmental aqueous samples with a relatively wide pH
[probe L + Cu2+ + 3H2O 2H+] (Fig. S6). The detection limit of range.

Fig. 2. The color changes observed by naked eye of probe L (30 lM) upon addition of 5 equiv. of various metal ions in H2O (0.5% DMSO).
 Y.-W. Sie et al. / Inorganica Chimica Acta 467 (2017) 325–329 327

550 nm The practical application of probe L for selective sensing of Hg2+

0.25 or Cu2+ in different source of water such as DI, tap, drink, ditch, lake
and ground water was also been demonstrated. Hg(ClO4)2 and Cu
(ClO4)2 were first dissolved in water of the above source, followed
0.20
by the addition of probe L into each water sample. All the water
samples containing Hg2+ showed clear color change from bright
Absorbance (AU)

0.15 yellow to red (Fig. 5a). On the other hand, addition of probe L into
each water sample with Cu2+ clearly led to color change from
0.10
bright yellow to orange, which is readily observed by the

0.05

0.00

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Time (min)

0.14 480 nm

0.12
Fig. 6. Photographs of test paper for detecting Hg2+ and Cu2+ in water.
0.10
Absorbance (AU)

0.08
550 nm
0.06
0.2

0.04

0.02
A-A0

0.1
0.00
-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Time (min)

Fig. 4. Time dependent experiments of (a) Hg2+ (Top); (b) Cu2+ ion (Bottom).
0.0
H )+C ly

g( Co )
H )+C )

H +C )
)

H +P )

g( +N )
H )+N )

H )+M )

M )

H )+L )
)

H )+F )

Fe )
)
n
)o

n(

g(
u(

(
d(

a(

i(

i(
l(

a(

(
e(

(
b(

(
g( Zn
g( )+K
g( +A

)+
+

)+
)

)
H (

)
)

)
g

g(
H

g(
g(

g(

g(
g(

g(

g(

g(

g(

H
H
H

Metal ion

0.16

500 nm
0.14

0.12

0.10

0.08
A-A0

0.06

0.04

0.02

0.00
C )+H ly

C )+C )

C )+C )

C +C )
)

C )+N )

C )+M )

M )

C )+L )
)

C )+F )

Fe )
)
n
)o

n(

g(
g(

o(

d(

a(

i(

i(
l(

a(
u( Pb(

(
e(

(
(
u( Zn
u( )+K
N
u( +A

)+

)+
)+

)+
)

)
C (

)
u

u(
C

u(
u(

u(
u(

u(

u(

u(

u(

u(

u(

C
C

C
C

Metal ion

Fig. 5. The color changes observed by naked eye in different source of water Fig. 7. Competition experiment of probe L towards (a) Hg2+ (top); (b) Cu2+ (bottom)
containing (a) Hg2+ (top) (b) Cu2+ (bottom) respectively. in the presence of 5.0 equiv. of other metal ions.
328 Y.-W. Sie et al. / Inorganica Chimica Acta 467 (2017) 325–329

naked-eye. These results indicated that probe L can be used for of the aromatic protons displayed downfield shift compared to
selective sensing of Hg2+ and Cu2+ based on the distinct color those of probe L alone.
changes in real samples. To understand the binding modes for the interaction of receptor
In practical applications of analyte-sensing, an ability to adopt L with Hg2+, we have applied density functional theory (DFT) to
test paper-based platforms is quite often highly desirable, as it pro- verify our proposals. We used the B3LYP [32,33] functional, along
vides an inexpensive approach to detect the presence of an analyte. with the 6 31 + G (d) basis set in our DFT computations. The
Toward this goal, filter paper was first coated with probe L by soak- computations were performed with the Gaussian09 program series
ing in an aqueous solution of probe L followed by air drying. When [34]. The relative energies of two proposed complexes, the opti-
a drop of aqueous solution containing Hg2+ or Cu2+ was introduced mized structure and selected bond distances of the lowest energy
to this probe L-coated test paper, the color of the paper changed probe L-Hg2+ complex were illustrated in Fig. 9. It is observed that
from light yellow to light orange or brown instantly, respectively, the Hg2+ binds to these six nitrogens in probe L with reasonable
as shown in Fig. 6. This indicated that the change in color on the bond length. From the DFT result indicated that the presence of
test paper was caused by the interaction of probe L and Hg2+ and two pyridine groups of probe L with partially overlapping confor-
Cu2+, implying that the visually test paper showed specific recogni- mation was relatively stabilized through the formation of probe
tion to Hg2+ and Cu2+. L-Hg2+. The closed form provides more binding sites resulting more
The selectivity of probe L toward Hg2+ and Cu2+ was further stable complex than the open form.
ascertained by competition experiments. As shown in Fig. 7, when In summary, we have successfully designed and synthesized a
probe L was treated with Hg2+ or Cu2+ in the presence of other simple colorimetric probe L, capable of recognizing Hg2+ and Cu2+.
metal ions of the same concentration, relatively low interference Probe L exhibited an excellent selectivity and sensitivity towards
was observed for the detection of Hg2+ in the presence of other Hg2+ and Cu2+ with distinct color change in water. Moreover, probe
ions. For the detection of Cu2+, probe L shows good selectivity in
the presence of most metal ions, except in the presence of Hg2+
where enhancement in the absorbance was observed. This
enhancement is most probably due to the displacement of Cu2+
by Hg2+ from probe L-Cu2+. Thus, probe L can be used as a selective
colorimetric sensor toward Hg2+ and Cu2+ in the presence of most
competing ions.
In order to better understand the complexation behavior of
probe L with Hg2+, 1H NMR experiments were carried out in
DMSO-d6. As shown in Fig. 8, upon addition of 0.25 M equivalent
of Hg2+, three new peaks appeared at 6.5 ppm. Meanwhile, the aro-
matic proton signals become complex. In addition, the NH signal at
11.56 ppm split to two peaks. These observations obviously indi-
cated the presence of asymmetry structure of probe L in the pres-
ence of Hg2+. Upon the addition of 1.0 equivalent of Hg2+, however,
the three new peaks at 6.5 ppm disappeared, accompanied by the
appearance of a new peak at 8.8 ppm. In addition, the NH at
11.56 ppm was shifted downfield to 13.2 ppm. Meanwhile, most Fig. 9. The DFT computations for the possible binding modes of probe L-Hg2+.

Fig. 8. 1H NMR titration plots of probe L with Hg+2 in DMSO.

 Y.-W. Sie et al. / Inorganica Chimica Acta 467 (2017) 325–329 329

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