A Phenylamine-Oligothiophene-Based Fluorescent Chemosensor for Selective Detection of Hg(II)

Article abstract of DOI:10.1007/s10895-016-1793-4

A phenylamine-oligothiophene-based fluorescent chemosensor I3TEA was reported. This sensor exhibited highly selective and fast detection of Hg²⁺ ion in MeCN/H₂O (8/2, v/v) solution through fluorescence quenching. The detection was unaffected by the other competitive metal cations. The low detection limit was found to be 5.92?×?10^?7?M. In addition, the recognition process is reversible and confirmed by EDTA experiment.

Full text of DOI:10.1007/s10895-016-1793-4

J Fluoresc (2016) 26:1053–1058
DOI 10.1007/s10895-016-1793-4
ORIGINAL ARTICLE
A Phenylamine-Oligothiophene-Based Fluorescent Chemosensor
for Selective Detection of Hg(II)
Qingfen Niu1 & Xingxing Wu1 & Tianduo Li1 & Yuezhi Cui1
Shanshan Zhang1 & Qiuchen Su1
&
Received: 19 November 2015 /Accepted: 27 March 2016 /Published online: 7 April 2016
#
Springer Science+Business Media New York 2016
Abstract A phenylamine-oligothiophene-based fluorescent
chemosensor I3TEA was reported. This sensor exhibited
highly selective and fast detection of Hg2+ ion in MeCN/H2O
(8/2, v/v) solution through fluorescence quenching. The detec-
tion was unaffected by the other competitive metal cations.
The low detection limit was found to be 5.92 × 10−7 M. In
addition, the recognition process is reversible and confirmed
by EDTA experiment.
systems. Mercury is considered a great health threat because
both elemental and ionic mercury can be converted into highly
toxic organo-mercury compounds by bacteria in the environ-
ment, which subsequently bio-accumulates through the food
chain [1, 2]. Mercury can cause many serious health issues
since it can easily and quickly pass through the outer skin,
respiratory and cell membranes, leading to DNA damage, mi-
tosis impairment, and permanent damage to the central ner-
vous system [3, 4]. Therefore, the development of more effi-
cient chemosensors which can monitor Hg2+ ions is very
important.
.
.
Keywords Fluorescence chemosensor Mercury ion
.
Phenylamine-oligothiophene Quenching
In recent years, fluorescent sensors are very useful
tools to monitor HTM ions because of their superior
sensitivity and selectivity, low cost, facile operation,
non-destructive analysis, instant response, local observa-
tion and the wide spread availability of equipment for
analysis [5–7]. To date, many fluorescent chemosensors
for selective detection of Hg2+ and other HTM ions
have been reported [8–28], but some practical applica-
tions are limited. Therefore, the development of more
efficient fluorescent chemosensors for detecting HTM ions
is still a challenge.
Introduction
The development of sensitive imaging instruments capable of
detecting heavy and transition metal (HTM) ions has attracted
much attention due to the wide use of these metal ions and
their subsequent impact on the environmental and biological
Highlights
Our previous studies reported the synthesis, electrochemi-
cal and photophysical properties of a phenylamine-
oligothiophene-based derivative I3TEA [29] (Scheme 1).
Today, in order to further expand our interest to the
fluorescent chemosensor for metal-ion screening studies,
we continue to investigate its cation-sensing properties.
In this paper, the experimental results demonstrated that
I3TEA exhibited an obvious fluorescence turn-off re-
sponse to Hg2+ with high selectivity and sensitivity over
other metal cations such as Mn2+, Cr6+, Na+, K+, Ag+,
Ca2+, Al3+, Co2+, Cu2+, Ni2+, Zn2+, Pb2+, Cd2+, Fe2+
and Cr3+ in MeCN/H2O (8/2, v/v) solution. In addition, this
sensor showed low detection limit and rapid response time.
• A phenylamine-oligothiophene-based fluorescent chemosensor I3TEA
was reported.
• The sensor exhibited highly selective and sensitive detection of Hg2+
.
• This sensor could rapidly detect Hg2+ ion in real time.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-016-1793-4) contains supplementary material,
which is available to authorized users.
* Qingfen Niu
qf_niu1216@qlu.edu.cn
1
Shandong Provincial Key Laboratory of Fine Chemicals, Qilu
University of Technology, Jinan 250353, People’s Republic of China

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J Fluoresc (2016) 26:1053–1058
Scheme 1 The synthetic route
for the sensor I3TEA
Experimental
Reagents
Results and Discussion
To explore the sensing ability of I3TEA, 17 metal cations in-
cluding Mn2+, Cr6+, Na+, K+, Ca2+, Co2+, Ni2+, Cu2+, Zn2+,
Fe3+, Pb2+, Ag+, Cd2+, Hg2+, Al3+, Cr3+ and Fe2+ has been
studied using both UV–Vis and fluorescence techniques. The
UV-Vis absorption spectrum of sensor I3TEA (5 μM) was in-
vestigated in the presence of metals cations (10 μM) in MeCN/
H2O (8:2, v/v) solution. As is shown in Fig. 1, sensor I3TEA
exhibited strong absorption in the range 300–450 nm, however,
almost no remarkable changes were observed in absorption
spectra upon the addition of various metal cations such as
Mn2+, Cr6+, Na+, K+, Ca2+, Co2+, Ni2+, Zn2+, Fe3+, Pb2+,
Cd2+, Al3+, Cr3+ and Fe2+ ions. Surprisingly, the strong absorp-
tion intensity of peak of sensor I3TEA highly quenched upon
addition of Hg2+ to the solution. These obvious observations
indicate that sensor I3TEA shows high selectivity towards Hg2+
in MeCN/H2O (8:2, v/v) solution.
To further research the availability of I3TEA as a highly
selective sensor for Hg2+, the selectivity studies were performed
by recording the fluorescence spectra of I3TEA after the addi-
tion of representative metal cations in MeCN/H2O (8:2, v/v)
solution. All the results indicated that the fluorescence intensity
of I3TEA strongly quenched in the presence of the Hg2+ ion (2.0
equiv.) (Fig. 2), accompanied by a green-yellow color fluores-
cence turn-off response (Fig. 2, inset), while, other metal cations
induced no or small significant change in the fluorescence emis-
sion spectra relative to the free I3TEA. These dramatical obser-
vations also indicate that sensor I3TEA displays considerably
high selectivity toward Hg2+ in MeCN/H2O (8:2, v/v) solution
over other competitive metal cations.
Unless otherwise stated, solvents and reagents were of analyt-
ical grade from commercial suppliers, and were used without
further purification. Metal salts were purchased from Sigma–
Aldrich and used as received. MeCN was spectrometric grade
and purchased from Qingdao Yage Chemical Reagent
Company. Water was deionized with a Milli-QSP reagent wa-
ter system (Millipore) to a specific resistivity of 18.4 MΩcm.
MeCN and deionized water were used in all of the experi-
ments. All other reagents are analytical grade and also from
Beijing Chemical works. The salts used in stock solutions of
metal ions were MnCl2·4H2O, K2Cr2O7, NaNO3,
Ca(NO3)2·4H2O, Al(NO3)3·9H2O, Pb(NO3)2, Cu(NO3)2·3H2O,
AgNO3, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, Fe(NO3)3·9H2O,
Co(NO3)2·6H2O, Ni(NO3)2·6H2O, KNO3, Hg(NO3)2·H2O,
FeCl2·4H2O and CrCl3·6H2O. I3TEA was prepared as
reported previously [29].
Apparatus
All UV-visible absorption spectra were recorded with a
Shimadzu UV-2600 spectrophotometer at room temperature.
All fluorescence measurements were carried out on a Hitachi
F-4600 fluorescence spectrophotometer with a scan rate at
1200 nm/min. The excitation wavelength was set at 320 nm.
The slits for excitation and emission were set at 5 nm/5 nm,
respectively.
General Procedures for Spectral Determination
0.8
0.6
0.4
0.2
0.0
+
2+ 3+
+
2+
I3TEA, Ag , Ni , Al , Na , Pb
,
3+ 2+ 2+ 2+ 2+
+
Fe , Ca ,Co ,Cu , K ,Zn
All tests described in this paper were carried out at room tem-
perature. All the metal salts of Mn2+, Cr6+, Na+, K+, Ag+, Ca2+,
Fe3+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Hg2+, Fe2+ and
Cr3+ were dissolved in water to prepare the stock solution with
the concentration of 1.0 × 10−3 M. I3TEA was dissolved in
MeCN to give the stock solution (5.0 × 10−3 M) and diluted
with a mixed solution of MeCN/H2O to prepare the analytical
solution (5.0 × 10−6 M) in MeCN/H2O (8/2, v/v) solution. The
stock solution of the metal cations and I3TEAwas used directly
in the spectroscopic measurement. For the sensitivity measure-
ment, different concentrations of Hg2+ ions were added to the
assay solution, and the fluorescence spectra were recorded. The
selectivity was checked by addition of Mn2+, Cr6+, Na+, K+,
Ag+, Ca2+, Fe3+, Al3+, Co2+, Ni2+, Cd2+, Zn2+, Pb2+, Cu2+, Fe2+
and Cr3+ in to the stock solution.
3+
2+
2+
6+
2+
Cr , Fe , Cd , Cr , Mn
2+
Hg
300
350
400
450
500
550
Wavelength (nm)
Fig. 1 Fluorescence spectra of I3TEA (5.0 × 10−6 M) in aqueous
solution (MeCN/H2O, 8/2, v/v) in the presence of various cations (2.0
equiv). Inset: Photos of I3TEA without and with addition of Hg2+ under
the irradiation of UV light at 365 nm

J Fluoresc (2016) 26:1053–1058
1055
2000
1.0
0.8
0.6
0.4
0.2
0.0
+
2+ 3+
+
3+
+
Ag ,Ni , Al , Na , Fe , K ,
Intercept = - 0.01988
Slope = 0.18193
R = 0.99651
2+ 2+ 2+ 3+ 2+
I3TEA
1600
Ca ,Co ,Cu , Cr , Fe
,
2+
2+
2+ 6+
2+
Cd , Pb , Zn ,Cr ,Mn
1200
800
400
0
Hg2+
2+
Hg
400
450
500
550
600
650
Wavelength (nm)
0
1
2
3
4
5
Fig. 2 UV–vis absorption spectra of I3TEA (5 μM) in the presence of
2.0 equiv. of different metal ions
2+
[Hg ] /µM
Fig. 4 Calibration curve of I3TEA–Hg2+ in MeCN/H2O (8/2, v/v)
solution
The binding stoichiometry between the sensor and metal
cation is very important in relation to the sensing properties.
The fluorescence titration of I3TEA (5 μM) with Hg2+ (0–2.0
equiv) was therefore carried out in MeCN/H2O (8/2, v/v) so-
lution shown in Fig. 3. Upon addition of Hg2+ ion, the fluo-
rescence intensity decreased gradually at 498 nm and then
tended to be saturated with 1.0 equiv. of Hg2+. However, when
more Hg2+ solution was titrated, the fluorescence intensity
showed negligible changes and the curve (Fig. 3, inset)
remained relatively constant, indicating the formation of a
1:1 binding between I3TEA and Hg2+. From the fluorescence
titration experimental results (Fig. 4), a good liner relationship
between the fluorescence intensity and the Hg2+ concentration
was found in the concentration range (0–5 μM) with corre-
sponding correlation coefficient (R) of 0.99651. The “R” value
demonstrated a 1:1 stoichiometric ratio between I3TEA and
the binding metal cation Hg2+. In addition, the fluorometric
titration data was used to obtain the detection limit of I3TEA
for Hg2+. The detection limit was calculated to be
5.92 × 10−7 M for Hg2+ ion based on the equation
DL = K × SD/S, where K = 3, SD is the standard deviation
of the blank solution, and S is the slope of the calibration curve
[30, 31]. The DL was sufficiently low to detect submillimolar
concentration of the Hg2+ ion, which belongs to the range
found in many chemical and biological systems. To further
determine the stoichiometry of the I3TEA–Hg2+ complex,
the fluorescence intensity of sensor I3TEA at 498 nm were
plotted as a function of their molar fraction under a
constant total concentration (5.0 μM). Figure 5 shows
the result of Job’s plot analysis, the maximum emission
intensity was reached at a molar fraction of 0.5, which
further gave solid evidence for the formation of a 1:1
complex of I3TEA-Hg2+.
Sensor I3TEA likely chelates metal ion via the amine N
atom of the aniline moiety, as other reported [32–34]. Mercury
nitrate has structure in which covalent bonding is important
400
350
300
250
200
150
100
50
2000
2000
1500
0
1500
1000
2+
Hg
500
10 µM
1000
0
0
1
2
3 4 5 6 7 8 9 10
2+
[Hg ] µM
500
0
450
500
550
600
650
0
Wavelength (nm)
0.25
0.50
0.75
1.00
Fig. 3 Fluorescence emission spectra of I3TEA (5 μM) was titrated with
Hg2+ (0–2.0 equiv) in aqueous solution (MeCN/H2O, 8/2, v/v). Inset: Plot
of the fluorescence intensity at 498 nm as a function of Hg2+
concentration
2+
2+
X = [Hg
]
/{[Hg
]
+[I3TEA]}
Fig. 5 Job’s plot for determining the stoichiometry for I3TEA and Hg2+
in MeCN/H2O (8/2, v/v). The total concentration was 5 μM

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J Fluoresc (2016) 26:1053–1058
2000
1600
1200
800
400
0
and easily form amine-mercury(II) salt π-complexes with
amine ligand, specially primary amine I3TEA. The proposed
binding model between I3TEA and Hg2+ was suggested in
Scheme S1. The IR spectra of I3TEA was recorded in the
presence of Hg2+ ion (Fig. S1). IR spectra of I3TEA and
I3TEA-Hg2+ complex exhibited that the peak at 3332 and
3411 cm−1 corresponding to the characteristic NH2 group
stretching vibration absorption of I3TEA was shifted to a
higher wavenumber (3534 and 3589 cm−1) upon addi-
tion of Hg2+ (1.0 equiv). A substantial blue shift upon
coordination of the N-donor atom of the amine group
indicates electron transfer from oligothiophene to
phenylamine after coordination to the mercury center
in the case of I3TEA. Our experimental results present-
ed are in good agreement with the reported literature.
Thus, the changes above can be explained by the amine
group of I3TEA participated in the formation of amine-
mercury(II) salt π-complex.
I3TEA
I3TEA+Hg2++ETDA
I3TEA+Hg2+
400
450
500
550
600
650
Wavelength (nm)
Fig. 7 Fluorescence spectra of I3TEA in the absence and presence of
Hg2+ and EDTA. [Hg2+] = 10 μM, [I3TEA] = 5 μM, [EDTA] = 50 μM
toward Hg2+ ion in MeCN/H2O (8/2, v/v) solution as shown in
Fig. 7, the solution changed from colorless to green-yellow
when EDTA (5.0 equiv) was added to the solution of I3TEA/
Hg2+, and the fluorescence was turned on in several seconds.
These obvious results indicate that the coordination process is
reversible and thus I3TEA is a reversible chemosensor for
Hg2+ ion in aqueous solutions.
Reaction time is an important factor for sensors, thus
the effect of the reaction time on the binding process of
Hg2+ ion to I3TEA was investigated (Fig. 8). Following
the addition of Hg2+ ion (10 μM) to I3TEA (5 μM), the
fluorescence intensity of I3TEA was quenched rapidly,
reaching a stable value within 3 min and then remaining
constant from 3 to 10 min. Thus the rapid, stable complexation
of Hg2+ ion by I3TEA and the resulting fast response profile
are important features for robust, real time detection of Hg2+
ion by portable device in field.
High selectivity for the analyte of interest over a complex
background of various competitive metal cations is an impor-
tant feature for an effective sensor. To further gauge selectivity
of the sensor I3TEA toward Hg2+, the competition experi-
ments were carried out by recording the fluorescence spectra
of I3TEA (5 μM) with Hg2+ (2.0 equiv.) in the presence of
other competing metal cations (2.0 equiv.) shown in Fig. 6. No
significant variation in the fluorescence was observed in the
presence and absence of other metal cations, and the relative
error was less than 5 %. These results above indicate that the
recognition of Hg2+ by I3TEA is almost uninfluenced by other
coexisting metal cations.
The reversibility of the chemosensor is particularly attrac-
tive properties for practical application. The EDTA experi-
ments were conducted to examine the reversibility of I3TEA
2000
1600
1200
800
400
0
metal+Hg
metal
700
600
500
400
300
200
100
0
Fig. 6 Fluorescence intensity of I3TEA (5 μM) compared to various
metal cations in MeCN/H2O (8/2, v/v) solution. The red bars represent
the emission of the MeCN in the presence of 2.0 equiv. of metal ions; the
black bars represent the emission of the above solution upon the addition
of 2.0 equiv. of Hg2+
0
2
4
6
8
10
Time /min
Fig. 8 Fluorescence quenching profile of addition Hg2+ (10 μM) to
I3TEA (5 μM) in MeCN/H2O (8/2, v/v) solution from 1 to 10 min

J Fluoresc (2016) 26:1053–1058
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Conclusions
In summary, a phenylamine-oligothiophene-based fluorescent
sensor I3TEA was reported, and its sensing ability toward
metal cations was investigated. I3TEA exhibited highly selec-
tive, sensitive and rapid fluorescence turn-off response to
Hg2+ ion in aqueous solution (MeCN/H2O, 8/2, v/v). The
quite low detection limit was found to be 5.92 × 10−7 M.
The coordination process of sensor I3TEA and Hg2+ was
chemically reversible with EDTA. In addition, this sensor
could detect Hg2+ ion in real time, permitting its incorporation
into a portable mercury detection kit in aqueous environment.
Acknowledgment We gratefully acknowledge the support by NSF
China Nos. 21376125/21276149 and Program for Scientific Research
Innovation Team in Colleges and Universities of Shandong Province.
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