A simple pyridine-based colorimetric chemosensor for highly sensitive and selective mercury(II) detection with the naked eye

Article abstract of DOI:10.1515/chempap-2015-0061

Two easily-prepared pyridine-based derivatives of (Z)-2-(4-amino-phenyl)-3-(pyridine-4-yl)acrylonitrile (I) and (Z)-2-phenyl-3-(pyridin-4-yl)acrylonitrile (II ) were designed, synthesised and characterised. Due to the formation of a complex with Hg²⁺

Full text of DOI:10.1515/chempap-2015-0061

Chemical Papers 69 (4) 527–535 (2015)
DOI: 10.1515/chempap-2015-0061
ORIGINAL PAPER
A simple pyridine-based colorimetric chemosensor for highly
sensitive and selective mercury(II) detection with the naked eye
a
a
a
b
aJian-Ting Pan, Fei Zhu, Lin Kong, Long-Mei Yang, Xu-Tang Tao,
aYu-Peng Tian, Hong-Bo Lu, a,b,cJia-Xiang Yang*
c
aDepartment of Chemistry, Key Laboratory of Functional Inorganic Materials of Anhui Province, Anhui University,
Hefei 230039, China
bState Key Laboratory Materials, Shandong University, Jinan 502100, China
cAcademy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230026, China
Received 4 June 2014; Revised 17 August 2014; Accepted 17 September 2014
Two easily-prepared pyridine-based derivatives of (Z)-2-(4-amino-phenyl)-3-(pyridine-4-yl)acrylo-
nitrile (I ) and (Z)-2-phenyl-3-(pyridin-4-yl)acrylonitrile (II ) were designed, synthesised and char-
acterised. Due to the formation of a complex with Hg2+, hence leading to an enhanced ICT effect,
I exhibits a visible colour change from light yellow to orange, rendering it suitable for use as a
naked-eye sensor for rapid detection of Hg2+ in an aqueous ethanol solution. When mixed with
Hg2+, I interacts with Hg2+ in a 2 : 1 (Y1–Hg2+) stoichiometry via a coordination bond with an
association constant of 7.7 × 108 M−2 (R2 = 0.96). The present probe I exhibits excellent repro-
ducibility, reversibility, sensitivity and selectivity with the presence of low concentration of Hg2+
(1.74 × 10−10 M).
c
ꢀ 2014 Institute of Chemistry, Slovak Academy of Sciences
Keywords: mercury ion sensor, pyridine, colorimetric, specific selectivity, easy-to-prepare, charge
transfer
Introduction
et al., 2011; Tian & Ihmels, 2011; Ren et al., 2012;
Chen et al., 2013; Goswami et al., 2013; Lu et al.,
2013; Madhu et al., 2013; Shafeekh et al., 2013; Bera
et al., 2014; Carter et al., 2014; Huang et al., 2014;
Li et al., 2014; Thirupathi et al., 2014). For example,
Thirupathi et al. (2014) reported a ratiometric fluo-
rescence chemosensor based on tyrosine derivatives for
monitoring mercury ions in aqueous solutions. Li et al.
(2014) reported cyclometallated ruthenium complex-
modified upconversion nanophosphorous derivatives
for the selective detection of Hg2+ ions in water to
improve the selectivity and sensitivity of a measure-
ment, by using ratiometric measurements. The ratio-
metric method can overcome the limitations of inten-
sity measurements due to a built-in correction for envi-
ronmental effects, and makes possible signal-rationing,
thereby increasing the dynamic range. The perceived
colour change would be useful not only for the ra-
Contamination by heavy metal ions, in particular
the mercury ion (Hg2+), has caused serious harm to
the environment and human health. Hg2+ can eas-
ily penetrate biological membranes and cause serious
damage to the brain, central nervous system and kid-
neys, in addition to the respiratory system, skin, blood
and eyes. (Azevedo-Pereira & Soares, 2010; Gundacker
et al., 2010; Hansen et al., 2011; Chemnasiri & Her-
nandez, 2012; Farhadi et al., 2012; Jenssen et al.,
2012; Koenig et al., 2013; Xing et al., 2013; Zhao et
al., 2014; Zhong et al., 2014). Hence, it is imperative
to screen suitable Hg2+ detection systems with high
selectivity, sensitivity and reliability. Many methods
have been developed specifically for Hg2+ ion detec-
tion (Coronado et al., 2005; Kim et al., 2010, 2012;
Misra & Shahid, 2010; Cheng et al., 2011; Dalapati
*Corresponding author, e-mail: jxyang@ahu.edu.cn
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J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
Fig. 1. Synthetic routes for I and II.
tiometric method of detection but also for rapid vi-
sual sensing. Although many fluorometric ratiomet-
ric probes for Hg2+ have been developed (A¨ıt-Haddou
et al., 2001; Chen & Chen, 2005; Gunnlaugsson et
al., 2005; Liang et al., 2007; Guo et al., 2010; Hu &
Chen, 2011; Wen et al., 2011), the number of selec-
tive and sensitive colorimetric sensors based on the
mechanism of internal charge transfer (ICT) available
for the detection of Hg2+ ions in aqueous media re-
mains limited (Sheng et al., 2008; Mei et al., 2012;
Shellaiah et al., 2013; Wang et al., 2013). Moreover,
many existing colorimetric probes for Hg2+ are sub-
ject to some disadvantages such as complicated syn-
thetic routes, low sensitivity, high cost and lengthy
response time. Accordingly, there remains a need for
new simple and easily-prepared colorimetric probes in
an aqueous medium for the rapid, sensitive, selective
visual detection of Hg2+ over other relevant metal ions
(Lee et al., 2007).
The current study details the synthesis of two
simple structural compounds I and II (Fig. 1) and
the photochemical elucidation of their selective colour
changes in the presence of Hg2+ ions and the possible
signalling mechanism. I is a donor-π-acceptor (D-π-
A)-type structural compound, in which the two func-
tional groups aminophenyl and pyridine ring are con-
nected through a double bond, with the amino group
moiety acting as the electron donor and the pyridinyl
group as the electron acceptor. The design of this com-
pound seeks to make use of the coordination ability of
the well-known pyridine structure unit and the strong
electron-donating ability and hydrophilic characteris-
tic of the amine group which can act not only as a
signal group but also enhance the water solubility.
Probe I acts as a naked-eye sensor for the Hg2+ ion
with high selectivity and sensitivity as well as good
reversibility in an aqueous ethanol solution, accompa-
nied by a colour change from light yellow to orange
due to its ICT increase. In order to elucidate the role
of the signal group to the colorimetric-sensing abili-
ties for Hg2+, compound II which was without the
amino-group was also synthesised. This study high-
lights a new concept and design of a sensor using a
simple synthetic method, avoiding complicated struc-
tures and varied detection methods.
Experimental
All commercially available chemicals were of rea-
gent grade and used without further purification. Prior
to use, all solvents were purified using the conventional
methods. All of the starting materials are readily
available at low cost. The metal salts tested included
Hg(ClO4)2 · 3H2O, CuSO4 · 5H2O, CdSO4 ·8H2O,
MnSO4 ·H2O, ZnSO4 · 7H2O, MgSO4, K2SO4, AgNO3,
Pb(NO3)2, CoCl2 · 6H2O, NiCl2 ·6H2O, CaCl2 and
BaCl2 · 2H2O purchased from Aldrich (USA) and used
as received.
1H NMR and 13C NMR spectra were recorded
on a Bruker Avance 400 MHz NMR spectrometer
(Bruker, Switzerland) at ambient temperature using
DMSO-d6 as solvent; chemical shifts are given in ppm
(δ) values (internal standards TMS for 1H and 13C
NMR spectra). The splitting patterns were described
as singlet (s), doublet (d), triplet (t), quartet (q) or
multiplet (m). Elemental analysis was recorded on a
Vario ELIII. The high-resolution mass spectra (HR-
MS) were recorded on a Thermo Fisher LTQ-Orbitrap
XL mass spectrometer (Thermo Fisher, USA) using an
electrospray ion source (ESI).The FT-IR spectra were
recorded using a Nicolet 380 FT-IR spectrometer in-
strument (KBr discs) in the range of 400–4000 cm−1
(Thermo Fisher, USA). The UV-VIS absorption spec-
tra were recorded on the UV-265 spectrophotometer
(Shimadzu, Japan) with a quartz cuvette (path length
of 1 cm) and studies were performed in AR grade
ethanol and distilled water. The one-photon excited
fluorescence (OPEF) spectra measurements were per-
formed using the Hitachi F-7000 fluorescence spec-
trophotometer (Tianmei,China).
(Z)-2-(4-Nitrophenyl)-3-(pyridin-4-yl)acrylonitrile
(III ): a mixture of 4-pyridinecarboxaldehyde (0.214 g,
2 mmol) and 4-nitrophenylacetonitrile (0.324 g,
2 mmol) in 20 mL of ethanol were refluxed for approx-
imately 1 h after adding piperidine drop-wise. The re-
sulting precipitate was cooled to ambient temperature,
washed with ethanol and recrystallised from ethanol
to afford III as a white solid (0.452 g, 90 % yield). FT-
IR (KBr, cm−1) selected bands: 3108 (w), 3071 (w),
3041 (w), 2220 (m), 1597 (s), 1514 (s), 1344 (s), 856
(s). 1H NMR (DMSO-d6, 400 MHz) δ: 7.85 (d, J = 6.0
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J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
529
Fig. 2. UV-VIS spectra of I (50.0 M) on addition of metal ions: K+, Pb2+, Ni2+, Hg2+, Mn2+, Zn2+, Cu2+, Cd2+, Ag+, Mg2+
Ca2+, Ba2+ and Co2+ (5.0 eq.) in ethanol : H2O (9 : 1, vol.) solvent (a). Naked-eye image of I–M complexes (b).
,
Hz, 2H), 8.09 (d, J = 8.8 Hz, 2H), 8.35 (s, 1H), 8.39
(d, J = 8.8 Hz, 2H), 8.81 (m, J = 6.0 Hz, 2H). 13C
NMR (DMSO-d6, 100 MHz) δ: 112.93, 116.37, 122.72,
124.32, 127.47, 139.01, 140.19, 143.86, 147.85, 150.54.
(Z)-2-(4-Aminophenyl)-3-(pyridine-4-yl)acryloni-
trile (I ): a mixture of compound III (0.251 g, 1 mmol)
and SnCl2 (0.949 g, 5 mmol) in 20 mL of ethanol were
stirred at 80◦C for 2 h. The resulting solution was
cooled to ambient temperature and the pH adjusted
to 8, then extracted with a large amount of ethyl ac-
etate and the volatiles removed under vacuum. The
residue was purified by flash silica gel chromatogra-
phy (ethyl acetate : petroleum ether = 1 : 5 vol.) to
afford I as an orange-red solid (0.166 g, 75 % yield).
FT-IR (KBr, cm−1) selected bands: 3390 (m), 3317
(m), 3032 (w), 2923 (w), 2849 (w), 2214 (m), 1584
(s), 1515 (s), 1415 (s), 832 (s). 1H NMR (DMSO-
d6, 400 MHz) δ: 5.79 (s, 2H), 6.66 (d, J = 8.8 Hz,
2H), 7.49 (d, J = 8.4 Hz, 2H), 7.70 (s, 1H), 7.73
(d, J = 6.0 Hz, 2H), 8.68 (d, J = 6.4 Hz, 2H). 13C
NMR (DMSO-d6, 100 MHz) δ: 113.78, 115.01, 117.43,
119.79, 122.27, 127.34, 132.90, 141.50, 150.23, 150.95.
HR-MS (ESI-MS): m/z = 222.1021 ([M + H]+), calc.
for [C14H12N3]+ = 222.1031.
(Z)-2-Phenyl-3-(pyridin-4-yl)acrylonitrile (II ): a
mixture of 4-pyridinecarboxaldehyde (0.550 g, 5 mmol)
and phenylacetonitrile (0.585 g, 5 mmol) in 30 mL of
H2O, 6 mL of 5 mass % aqueous NaOH solution was
added drop-wise. The mixture was stirred at ambi-
ent temperature for 12 h to afford a white precipi-
tate. The precipitate was filtered, washed with wa-
ter and dried to afford II as a white solid (0.978 g,
94 % yield). FT-IR (KBr, cm−1) selected bands: 3061
(w), 3033 (w), 2927 (w), 2214 (m), 1590 (s), 1543
(s), 1498 (s), 758 (s), 683 (s). 1H NMR (DMSO-d6,
400 MHz) δ: 7.50–7.58 (m, 3H), 7.82 (d, J = 6.4 Hz,
4H), 8.11 (s, 1H), 8.77 (d, J = 5.6 Hz, 2H). 13C
NMR (DMSO-d6, 100 MHz) δ: 114.70, 116.97, 122.62,
126.13, 129.27, 130.07, 132.93, 140.20, 140.74, 150.43.
HR-MS (ESI-MS): m/z = 207.0918 ([M+H]+), calc.
for [C14H11N2]+ = 207.0922.
Results and discussion
I and II were synthesised by simple procedures and
1
characterised by H and 13C NMR, FT-IR and MS.
The original spectra are available in Supplementary
Information.
The chromophore of chemosensor I was based on a
D-π-A system, which could exhibit an obvious colour
change, thereby making the chemosensor colorimet-
ric. The selective and sensitive signal response of I to
Hg2+ ions was embodied in absorption. All titration
experiments were carried out in ethanol : H2O (9 : 1,
vol.) as a solvent.
Fig. 2 shows that K+, Pb2+, Ni2+, Mn2+, Zn2+
,
Cu2+, Cd2+, Mg2+, Ca2+, Ba2+ and Co2+ ions do not
cause any significant change in the I spectrum; only
Ag+ ions result in a 9 nm red-shift, which could proba-
bly be attributed to their low affinity to the receptor I .
However, the effect of Ag+ ions is negligible compared
with the marked change in the UV-VIS spectrum af-
ter adding Hg2+. The absorption spectrum changes a
little after the combination with different metal ions,
demonstrating that the receptor I possesses specific
colorimetric selectivity to Hg2+ in ethanol : H2O (9 : 1,
vol.) binary solutions. Spectral data were recorded at
1 min after adding Hg2+ ions, there was a marked
change in the absorption spectra of compound I , with
a new band appearing at 437 nm. The image clearly
shows, a colour change after adding mercury ion to
the solution which is significantly different from oth-
ers, changing from light yellow to orange, suggesting
that chemosensor I can serve as a “naked-eye” indi-
cator of Hg2+ ions. Moreover, the wavelength of max-
imum excitation peak also red-shifts from 384 nm to
437 nm. This result demonstrates the unique selectiv-
ity of I towards Hg2+. Hence, it may be predicted that
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J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
Fig. 3. Representation of donor and acceptor part of receptor
I.
Fig. 4. HOMO and LUMO orbital of receptors I and II calcu-
lated by B3LYP method with the 6-31G* basis-set.
the addition of the Hg2+ ions increases the ICT effect.
The colorimetric sensor I is a conjugated form of two
subunits: the aminophenyl and pyridine ring and the
existing ICT (internal charge transfer) itself. When I
bonded with Hg2+, to some extent, most of the elec-
tron contributed to Hg2+, which is responsible for the
red shift of the UV-VIS spectra.
The design of receptor I makes it a fitting candi-
date to serve as an ICT probe. The pyridine ring is
presumed to act as the electron acceptor while the re-
maining amine group is presumed to act as the donor
(Fig. 3). The result of receptor I as an effective ICT
probe was represented in terms of the major change
in the absorption pattern as well as the visual appear-
ance (from light yellow to orange) of the receptor I
upon addition of Hg2+. The receptor II was weak-
ened by the missing NH2 group. In order to state the
importance of the role of amino groups in the donor
in the colour change, compound II was tested under
the same conditions. It does not exhibit any spectral
changes upon the addition of metal ions (Fig. S1),
showing that the amino groups act as a single group,
and plays a vital role in the colour changes induced in
the mercury, which can be observed by the naked eye.
The above design principles of receptors I and II
were further elucidated by density functional theory
(DFT) calculations carried out with the Gaussian 09
program. The value of I absorption peak was calcu-
lated at 380 nm and that of II at 300 nm; both were
in good agreement with the experimental values of
384 nm and 304 nm, respectively, within an error range
of 4 nm. The HOMO–LUMO orbitals of the energy-
minimised structures of receptors I and II are shown
in Fig. 4. Due to the existence of NH2, I exhibited a
better ICT than II , rendering it a better ICT probe.
To evaluate the sensing behaviour of I towards
Hg2+, the absorption spectral titration of I with Hg2+
in ethanol : H2O (9 : 1, vol.) was carried out. Fig. 5
shows the changes in the absorption spectrum of I as
a function of Hg2+ concentration at ambient temper-
ature. In the absence of Hg2+ ions, the spectrum of
probe I is characterised by an absorption band cen-
tred at 384 nm (ε = 1.2 × 104 M−1 cm−1). A new
sharp absorption band at 437 nm appeared with the
prior absorption band at 384 nm declining and ab-
Fig. 5. UV-VIS titration spectra of I (50.0 M) upon addition
of various amounts of Hg(ClO4)2 (0–5.0 eq.; with step
of 0.1 eq.) in ethanol : H2O (9 : 1, vol.) solvent. Inset:
colour change of I, I + Hg(II).
sorbance increased with the increasing Hg2+ concen-
tration (0–5.0 eq. of Hg2+); when the Hg2+ concentra-
tion increased to 25.0 M, the absorbance of I itself
was slightly larger than of the new peak. On adding
Hg2+ ions to the solution of I , the absorbance of the
new peak did not change, due to the saturation effect.
The limit of detection of the sensor I for Hg2+ was
found to be 17.4 nM, using the equation for limit of
detection = 3σ/K, where σ is the standard deviation
of blank measurements and K is the slope between
the absorbance and the Hg2+ concentration (Fig. S2)
(Wei et al., 2013).
From the UV-VIS titration spectra, a definite ra-
tiometry was noted, with one isosbestic point at
405 nm and a 2 : 1 reaction stoichiometry (I : Hg2+),
which shows that forming / indicates the formation
of a new complex between the receptor I and Hg2+
;
the formation of the new complex is responsible for
generating the orange colour following the addition of
Hg(ClO4)2 to the solution of the receptor I (Fig. 5).
To corroborate the 2 : 1 ratio between I and Hg2+
ions, Job’s plot analysis was also performed (Goswami
et al., 2013). From the Job’s plot, it can be observed
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J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
531
Fig. 6. Job’s plot for 2 : 1 complexation of I with Hg(II) ion
Fig. 7. UV-VIS titration spectra of I–Hg(II) complex upon
EDTA addition in ethanol : H2O (9 : 1, vol.) solvent.
(Inset: colour change of I, I + Hg(II) and I + Hg(II)
+ EDTA).
in ethanol : H2O (9 : 1, vol.) solvent.
that the absorbance variation measured (∆A = Aobs
– Ai) achieved a maximum value at a molar fraction
of (x(I ) = [I ]/([ I ] + [Hg2+])) at approximately 0.64,
confirming that a 2 : 1 stoichiometry was most prob-
able for the binding mode of Hg2+ and I (Fig. 6).
Based on the 2 : 1 stoichiometry, the association con-
stant was determined by a non-linear least squares fit
of data with the following equation (Fig. S3) as the ref-
erence method; the association constant of I for Hg2+
was calculated as 7.7 × 108 M−2 (R2 = 0.96) (Yang
et al., 2011).
In addition, the reversible research using ethylene
diamine tetraacetic acid (EDTA) indicated the current
response to be completely reversible (Fig. 7). It is of
great interest that compound I could be regenerated
by adding an excess of EDTA to the solution con-
taining I –Hg2+, then the mercury ions were captured
by EDTA, as a result of the resurgence of I , which re-
sulted in recovery of the original colour. This indicates
the decomplexation of I –Hg2+, as EDTA strips Hg2+
away from the binding side. EDTA forms a chelate
with the mercury ion which is adequately stable. This
stability results from the multiple sites within the lig-
and that give rise to a cage-like structure in which the
Hg2+ is effectively surrounded and isolated from the
solvent molecules (Chen et al., 2013; Shafeekh et al.,
2013; Wang et al., 2013).
Fig. 8. Fluorescence emission spectra of
I (50.0 M) in
ethanol : H2O (9 : 1, vol.) solvent (λex = 405 nm,
Slit: 10 × 10) in the presence of increasing amount of
Hg(ClO4)2 (0–5.0 eq.; with step of 0.1eq.) pre-dissolved
in deionised water. (Inset: fluorescence intensity of so-
lution of I containing different content of Hg2+ at λem
= 530 nm).
dition of different metal ions (5 eq.) to the solution
of I in a particular order. As portrayed in Fig. 9, the
detection of Hg2+ in the presence of other metal ions
is not impeded, i.e., the interference in the detection
of the Hg2+ is not observed. Hence, I can be used
as a selective and sensitive colorimetric sensor for the
Hg2+ ion by the naked eye.
The effect of pH influence on the UV-VIS response
of I to Hg2+ was examined (Wu et al., 2010; Zhou
et al., 2013). It is apparent from Fig. 10a that, over
a pH range of 5–14, the visible absorption band cen-
tred at 354 nm was unchanged. A decrease in pH from
2 to 1 engendered a shift in the maximal absorption
wavelength to 314 nm; this 40 nm shift was due to pro-
tonation of the pyridine ring and the amine groups to
form the A-π-A structure. When the pH values were
changed from 3 to 4, this engendered a shift in the
maximal absorption wavelength to 403 nm; this 49 nm
The fluorescence titration of I (50.0 M) in the
presence of different Hg2+ concentrations was also per-
formed. Fig. 8 shows that sensor I exhibited a flu-
orescence emission at 530 nm. Upon increasing the
concentration of Hg2+ to 25 × 10−5 M, the emission
intensity at 530 nm gradually reduced, which can be
explained by compound I itself having the intramolec-
ular charge transfer process from amino group moiety
to the pyridinyl group. When combined with Hg2+
,
to some extent increasing the ICT effect, most of the
electrons then contributed to the Hg2+. (Zhu et al.,
2008).
In order to understand the selectivity of I to Hg2+
ions, an interference study was carried out by the ad-
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532
J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
Fig. 9. Metal ion selectivity profile of receptor I (50.0 M):
change in absorbance intensity of receptor + 5.0 eq.
Mn+ (black bars); change in absorbance intensity of
receptor + 5.0 eq. Mn+, followed by 5.0 eq. Hg2+ (red
bars).
Fig. 11. Partial 1H NMR spectra (400 MHz, DMSO-d6) of
probe I at 25◦C without metal cations (a) and in the
presence of 0.5 eq. of Hg2+ (b).
shift was due to protonation of the pyridine ring only,
thus enhancing the electron-withdrawing effect of the
pyridine ring, inducing a red shift. This was further
confirmed by adding Hg2+ (0–5.0 eq.) directly into the
solutions of pH 1 and pH 4 (Figs. S4 and S5), with no
obvious changes to each peak.
To gain an insight into the sensing mechanism and
binding mode of I with Hg2+, the FT-IR, MS, 1H
NMR and elemental analysis were investigated. The
coordination compound formed between I and Hg2+
was found to be 2 : 1 in stoichiometry, which was con-
firmed from the Job’s plot by UV-VIS spectrometry.
The strong interaction of receptor I with the Hg2+
ions was supported by FT-IR measurements. The re-
ceptor I –Hg2+ complex was centrifuged, washed with
water, and dried in vacuum. The obvious infrared
absorption peak at 1115 cm−1 was ascribed to the
characteristic stretching vibration band of the group
of Hg(II) perchlorate trihydrate (Hg(ClO4)2 ·3H2O)
(Fig. S6. In addition, the FT-IR shows the existence
of the CN absorption and the band shift of NH2
(from 3323 cm−1 and 3385 cm−1 to 3379 cm−1 and
3468 cm−1) and the weakening of the pyridinyl group.
The result was also confirmed by MS analysis. The ad-
dition of 1 mM Hg2+ solution to a 2 mM solution of I
in ethanol led to the formation of a precipitate. The
precipitate was isolated by centrifugation and puri-
fied by repeated washings with ethanol. Mass spectral
analysis of the precipitated product showed a mass ion
peak at m/z 679.5115 (calc. 679.1745), corresponding
to [2I + Hg2+ + ClO−4 + 2H2O – HClO4)]+, indicat-
ing a 2 : 1 adduct of I and Hg2+. Moreover, the ele-
mental analysis results of the metal complex showed
N 10.05 %, C 41.14 %, H 2.81 % (calculated: N 9.46 %,
Fig. 10. Effect of pH on interaction of I in water (a); scheme of molecular level interactions involved (b).
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J. T. Pan et al./Chemical Papers 69 (4) 527–535 (2015)
533
Fig. 12. Proposed binding mechanism of Hg2+ by I.
C 40.57 %, H 3.18 %), providing further evidence of a
2 : 1 complex formation. Additionally, 1H NMR titra-
tion experiments were conducted to gain an insight
vation Foundation of Anhui Province (no. 2006KJ007TD).
Supplementary data
into the essence of the coordination of Y1 by Hg2+
.
The partial 1H NMR spectra of both I and the I –Hg2+
adduct are shown in Fig. 11 (Xie et al., 2013). Upon
the addition of 0.5 equiv. of Hg2+ ions to the solution
of I , the chemical shift of the all proton (Fig. 11a)
showed a downfield shift, as anticipated (Fig. 11b).
Due to the decrease in electron density in the pyridine
ring moiety, the protons on the pyridine ring moiety of
I (H1 and H2 in Figs. 11a and 11b, respectively) ex-
hibited a significant downfield shift, which suggested
that the nitrogen atom of the pyridine ring moiety of
The supplementary data associated with this arti-
cle (A simple pyridine-based colorimetric chemosensor
for highly sensitive and selective mercury(II) detection
with the naked eye) can be found in the online version
of this paper (DOI: 10.1515/chempap-2015-0061).
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I was participating in the complexation with Hg2+
.
This coordination led to the changes in the absorp-
tion. Moreover, with increasing the concentration of
Hg2+, the maximal absorption wavelength of probe I
exhibited a red shift from 490 nm to 570 nm in DMSO
(Fig. S7). On the basis of the above experimental re-
sults, a plausible mode of binding I with Hg2+ in a
ratio of 2 : 1 is proposed, as shown in Fig. 12.
Conclusions
A readily-prepared and simple structural colori-
metric sensor I for Hg2+ was designed and synthe-
sised. Comparison with II indicated that the strong
donor group (NH2) acted as a signal group and played
an important role in the process of colorimetric recog-
nition. Upon the reaction of I with Hg2+ in an aque-
ous ethanol solution, a distinct colour change from
light yellow to orange occurred immediately, render-
ing the “naked-eye” detection of Hg2+ ions possible.
Significantly, the obvious colour change exhibited a
high selectivity and sensitivity to Hg2+ between the
other common metal ions, forming a stable 2 : 1 I –
Hg2+ complex. This work extends the range of possi-
ble methods to detect Hg2+ ions in biological, chemical
and environmental samples.
Acknowledgements. This work was financially supported
by the National Natural Science Foundation of China (nos.
21101001, 50873001 and 61107014), the Educational Commis-
sion of Anhui Province of China (no. KJ20142D02),the 211
Project of Anhui University, and the Team for Scientific Inno-
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