A new 1,8-naphthalimide-based colorimetric and turn-on fluorescent Hg2+ sensor

Article abstract of DOI:10.1016/j.saa.2012.11.113

A new fluorescent molecule 4-(bis(2-(ethylthio)ethyl)amino)-N-n-butyl-1,8- naphthalimide (BTABN) was reported. Its UV-vis absorption and fluorescence spectra were almost not dependent on the pH, but remarkably and selectively affected by Hg²⁺. Upon the addition of Hg²⁺ in EtOH/H ₂O (1/2, v/v), the absorption wavelength maxima were blueshifted from 436 nm to 376 nm with a naked-eye observed color change, and the fluorescence was greatly enhanced with a 14 nm blueshift within 1 min. Na⁺, K ⁺, Ca²⁺, Mg²⁺, Cu²⁺, Zn ²⁺, Cr³⁺, Pb²⁺, Ni²⁺, Fe ²⁺, Mn²⁺, Co²⁺, Cd²⁺ showed no obvious interferences with the detection. The binding stoichiometry indicates that a 1:1 complex is formed between Hg²⁺ and BTABN. The response of the UV-vis absorption and the fluorescence spectra to Hg²⁺ can be attributed to the joint contribution of the PET and ICT process.

Full text of DOI:10.1016/j.saa.2012.11.113

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new 1,8-naphthalimide-based colorimetric and ‘‘turn-on’’ fluorescent Hg2+ sensor
a
a
Zhiyuan Zhang a, Yuhua Chen b, Dongmei Xu a, , Liang Yang , Aifeng Liu
⇑
a College of Chemistry, Chemical Engineering and Materials Science, National Engineering Laboratory for Modern Silk, Key Laboratory of Organic Synthesis of Jiangsu Province,
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Soochow University, Suzhou, Jiangsu 215123, China
b College of Pre-clinical Medical and Biological Science, Soochow University, Suzhou 215123, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
A new molecule for optically sensing
Hg2+ in aqueous solutions is
reported.
"
Its UV–vis absorption and
fluorescence spectra were carefully
studied.
"
"
"
The spectra are almost insensitive to
H+ within a wide range of pH.
The spectra are highly selective and
sensitive to Hg2+
.
The spectra changes are the
coefficient results of PET and ICT.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2012
Received in revised form 20 November 2012
Accepted 30 November 2012
Available online 20 December 2012
A new fluorescent molecule 4-(bis(2-(ethylthio)ethyl)amino)-N-n-butyl-1,8-naphthalimide (BTABN) was
reported. Its UV–vis absorption and fluorescence spectra were almost not dependent on the pH, but
remarkably and selectively affected by Hg2+. Upon the addition of Hg2+ in EtOH/H2O (1/2, v/v), the
absorption wavelength maxima were blueshifted from 436 nm to 376 nm with a naked-eye observed
color change, and the fluorescence was greatly enhanced with a 14 nm blueshift within 1 min. Na+, K+,
Ca2+, Mg2+, Cu2+, Zn2+, Cr3+, Pb2+, Ni2+, Fe2+, Mn2+, Co2+, Cd2+ showed no obvious interferences with the
detection. The binding stoichiometry indicates that a 1:1 complex is formed between Hg2+ and BTABN.
The response of the UV–vis absorption and the fluorescence spectra to Hg2+ can be attributed to the joint
contribution of the PET and ICT process.
Keywords:
1,8-Naphthalimide
Colorimetric
Fluorescent sensor
Fluorescence sensor
Hg2+
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
reproductive development system damage [1,2]. As one of the
most toxic heavy metal ions, its detection is of great importance.
Mercury and its compounds are widely used in appliances and
instruments, metallurgy, chemical industry, medicine, and mili-
tary, and easily enter the environment, including air, soil and
water. Hg2+ can be bioaccumulated in the brain and kidney
through food chain and atmospheric circulation and seriously
threaten people’s health by resulting in the central nervous and
Among plenty of methods for detecting Hg2+, fluorescent sensor
becomes the most valuable one because of its high selectivity
and sensitivity, easy operation, fast response, low cost and real
time monitoring [3,4]. A large number of articles regarding fluores-
cence detection of Hg2+ have been published [5,6]. Most of the sen-
sors are based on fluorescein [7], benzoxadiazole [8], coumarin [9],
rhodamine [10–15], cyanine [16–18] and 1,8-naphthalimide [19–
21]. However, many of these sensors have shortcomings such as
cross-sensitivity toward other metal ions, short emission wave-
lengths, sensitivity to pH and low water solubility. Therefore, the
design of better sensors for rapid detection of Hg2+ in the environ-
ment and biological systems is very meaningful.
⇑
Corresponding author. Address: College of Chemistry, Chemical Engineering
and Materials Science, Soochow University, No. 199 Ren-ai Road, Suzhou Industrial
Park, Suzhou, Jiangsu 215123, China. Tel.: +86 512 65882027; fax: +86 512
65880089.
E-mail address: xdm.sd@163.com (D. Xu).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.113

Z. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
9
1,8-Naphthalimide is a favorite reporter for fluorescent sensors
owning to its high absorption coefficient, high fluorescence and
quantum yield, large Stokes shift, good photostability and easy to
modification. N, S and O, especially S atom, are commonly used po-
tential binding sites for Hg2+ [5,6]. Therefore, in this work, we de-
signed and synthesized a novel fluorescent sensor for Hg2+ by
attaching a functional group containing one N and two S atoms
to the C-4 position of 1,8-naphthalimide. The effects of proton
and metal ions on the UV–vis absorption and fluorescence spectra
of the sensor were carefully studied, and the binding mode and the
sensing mechanism were investigated.
10 mM stock solutions. When examining the fluorescence response
of BTABN upon pH, 50 L stock solution of BTABN was taken into
10 mL volumetric flasks, volatilized CH2Cl2 and diluted to 10
with ethanol (EtOH) and Britton–Robinson buffer at different pH
in a volume ratio of 1/2 (1/2, v/v). When studying the fluorescence
l
lM
sensing behaviors of BTABN to metal ions, 50
BTABN was put into a 10 mL volumetric flask, volatilized CH2Cl2,
mixed with 100 L one of the stock solutions of the metal salts,
and diluted with EtOH/H2O (1/2, v/v) in sequence. The concentra-
tion of BTABN and metal ions was 10 M and 100 M, respectively.
In the UV–vis absorption and fluorescence titration, the mixed
stock solutions of Hg2+ salt varied from 0 to 500
L and the concen-
tration of Hg2+ was 0–500
M.
lL stock solution of
l
l
l
l
l
Experimental
When the fluorescent quantum yield (Us) of BTABN was evalu-
ated, the absorbance of the solutions was controlled less than 0.05
in order to make the testing results reliable. Us was estimated from
the absorption and fluorescence spectra of BTABN according to Eq.
(1), where the subscript s and r stand for the sample and reference
Materials, instruments and methods
Materials
4-Bromo-1,8-naphthalic anhydride (BNA) (98%) were supplied
by Anshan HIFI Chemical Co., Ltd. n-Butylamine (n-BA), diethanol-
amine (DEA), SOCl2 of analytical grade were bought from Sinop-
harm Chemical Reagent Co., Ltd. NaH (55–65% in kerosene),
bis(2-chloroethyl)amine hydrochloride and ethanethiol (EtSH)
(99%) were purchased from J&K Chemical Co., Ltd. NaCl, KCl, MgCl2,
CaCl2, CrCl3ꢀ6H2O, MnSO4ꢀH2O, FeCl3ꢀ6H2O, FeCl2ꢀ7H2O, CoCl2ꢀ6H2
O, Ni(NO3)2ꢀ6H2O, CuCl2ꢀ2H2O, Zn(NO3)2ꢀ6H2O, CdCl2ꢀ2.5H2O,
HgCl2 and Pb(NO3)2 were the metal cations sources, which were
provided by Sinopharm Chemical Reagent Co., Ltd. All the reagents
were used as received. The solvents used in synthesis were of ana-
lytical grade, others were spectroscopy grade. They were used
without special treatment.
(rhodamine B, Ur = 0.97 in ethanol), respectively.
U is the quantum
yields, A represents the absorbance at the excitation wavelength, S
refers to the integrated emission band areas and nD is the solvent
refractive index.
Ss Ar n2Ds
Sr As
Us
¼
Ur
ð1Þ
n2Dr
Synthetic procedures and characterization data
BTABN was synthesized from 4-bromo-1,8-naphthalic anhy-
dride (BNA), n-butylamine (n-BA), diethanolamine (DEA), SOCl2
and ethanethiol (EtSH) by four steps, as shown in Scheme 1.
Instruments
IR was recorded on a Nicolet Magan-550 spectrometer (Nicolet
Co., USA). LC–MS was collected on an Agilent 1200/6220 spectrom-
eter (Agilent Co., USA). 1H NMR and 13C NMR spectra were carried
out on a 400 and 100 MHz Varian Unity Inova spectrometer (Var-
ian Co., USA) respectively. The elementary analysis was measured
on a Carlo-Erba EA1110 CHNO-S (Carlo-Erba Co., Italy). UV–vis
spectra were obtained on a U-3900 spectrophotometer (Perkin–El-
mer Co., USA). Fluorescence spectra were taken on a Fluoromax-4
spectrofluorometer (Slit width: 5 nm). pH values were measured
on a Mettler-Toledo FE20 pH meter (Mettler-Toledo Co., USA).
Melting point was determined on an X-6 Microscopic melting
point tester (Beijing Tech Instrument Co., Ltd., China). All the pH
values and the spectral measurements were performed at 25 °C.
Intermediate
Intermediate 1 was synthesized by the reaction between BNA
and n-BA, according to Ref. [22]. Its melting point was 104–
105 °C, as reported in the reference. Intermediate 1 reacted with
DEA to afford intermediate 2, as described in Ref. [23]. 1H NMR
(CDCl3, 400 MHz) d (ppm): 0.95 (3H, t, J = 8.0 Hz), 1.41 (2H, m),
1.66 (2H, m), 3.60 (4H, t, J = 5.0 Hz), 3.86 (4H, t, J = 5.0 Hz), 4.08
(2H, t, J = 8.0 Hz), 7.33 (1H, d, J = 8.0 Hz), 7.58 (1H, t, J = 8.0 Hz),
8.38 (1H, d, J = 8.0 Hz), 8.41 (1H, d, J = 8.0 Hz), 8.84 (1H, d,
J = 8.0 Hz). LC–MS [M+H]+: m/z, found. 357.1812. Calcd. 357.1800.
The characterization data were in accordance with that reported
in the reference. Intermediate 2 (0.4 g, 1.12 mM) was dissolved in
toluene (PhMe) (10 mL) and added SOCl2 (2 mL, 27.5 mM). After
reflux for 7 h, the solvent was evaporated under reduced pressure,
and the residue was purified by column chromatography using
CH2Cl2 as eluent to afford intermediate 3 (0.35 g, yield: 81.5%). IR
(KBr pellet, cmꢁ1): 2960, 2929, 2869, 1694, 1656, 1592, 1466,
Methods
BTABN was dissolved in dichloromethane to form 2 mM stock
solutions. Metal salts were dissolved in deionized water to get
OH
O
O
O
O
N
O
O
Br
EtOH,
Br
n-BA
reflux
N
reflux
CH3OC2H4OH
DEA,
C4H9
C4H9
N
O
OH
1
2
Cl
Cl
S
S
O
O
SOCl2
reflux
N
EtSH,NaH
THF,reflux
N
N
O
C4H9
C4H9
N
O
PhMe,
3
BTABN
Scheme 1. Synthetic route of BTABN.

10
Z. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
1396, 1356, 785, 759. 1H NMR (CDCl3, 400 MHz) d (ppm): 0.98 (3H,
t, J = 7.5 Hz), 1.45 (2H, m), 1.73 (2H, m), 3.59 (4H, t, J = 6.5 Hz), 3.80
(4H, t, J = 6.5 Hz), 4.18 (2H, t, J = 7.5 Hz), 7.46 (1H, d, J = 8.0 Hz),
7.75 (1H, t, J = 7.5 Hz), 8.55 (2H, t, J = 8.5 Hz), 8.61 (1H, d,
J = 7.5 Hz). LC–MS [M+H]+: m/z, found. 393.1130. Calcd. 393.1100.
The second method is one step less than the first method, but
the reaction system is complicated and the purification of the tar-
get product is difficult, which may be result from the instability of
DMA.
Sensitivity of BTABN to pH
BTABN
To a solution of NaH (0.15 g, 6.25 mM) and EtSH (3 mL,
40.4 mM) in dry tetrahydrofuran (THF) (15 mL), intermediate 3
(0.2 g, 0.5 mM) in dry THF (5 mL) was added dropwise over
10 min under stirring. The mixture was refluxed for 48 h, then
the solvent was evaporated under reduced pressure and the resi-
due was purified by column chromatography using ethyl acetate/
cyclohexane (1/10, v/v) as eluent to afford BTABN (0.12 g, yield:
54.1%). IR (KBr pellet, cmꢁ1): 2959, 2928, 2869, 1695, 1655,
1591, 1465, 1397, 1356, 784, 760. 1H NMR (CDCl3, 300 MHz) d
(ppm): 0.902 (3H, t, J = 7.2 Hz), 1.215 (6H, t, J = 7.5 Hz), 1.448
(2H, m), 1.703 (2H, m), 2.474 (4H, m), 2.703 (4H, t, J = 7.5 Hz),
3.611 (4H, t, J = 7.2 Hz), 4.177 (2H, t, J = 7.5 Hz), 7.327 (1H, d,
J = 8.1 Hz), 7.691 (1H, t, J = 8.1 Hz), 8.494 (2H, d, J = 7.8 Hz), 8.575
(1H, d, J = 7.2 Hz). 13C NMR (CDCl3, 100 MHz) d (ppm): 13.90,
20.08, 30.03, 31.36, 32.15, 38.09, 39.97, 41.00, 53.61, 55.25,
117.76, 118.55, 123.10, 126.07, 127.70, 129.83, 130.07, 131.46,
152.38, 163.90. LC–MS [M + H]+: m/z, found. 445.1980. Calcd.
445.1900. Elementary analysis: C24H32N2O2S2 (444.1900); Calcd.
(%): C 64.14, H 7.29, N 6.21, found (%): C 64.83, H 7.25, N 6.30.
In order to examine the disturbance of proton to the detection
of Hg2+, the UV–vis absorption and fluorescence spectra of BTABN
in EtOH/Britton–Robinson buffer (1/2, v/v) solutions with different
pH values were determined (Figs. 1 and 2). The spectra did not
show obvious change from pH 2.11 to 10.93, which well matches
the above design idea. This pH insensitivity of BTABN is beneficial
to the detection of Hg2+ in different media.
Sensing behaviors of BTABN to Hg2+
The response of UV–vis absorption spectra to Hg2+
The response of BTABN to different metal ions, such as Na+, K+,
Mg2+, Ca2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
and Pb2+ were investigated first by UV–vis absorption spectra.
The EtOH/H2O (1/2, v/v) solution of BTABN (10
low in color without cations. Its absorption wavelength maximum
was 436 nm and the molar extinction coefficient was 13,600 Mꢁ1
cmꢁ1. Upon addition of the above cations (100
M), the maximal
lM) is bright yel-
-
l
weakening and broadening (toward short wavelength) of the
Results and discussion
Design and synthesis of BTABN
0.4
0.4
The photoinduced electron transfer (PET) system is one of the
most popular approaches to the design of ‘‘turn-on’’ fluorescent
sensors. However, the PET-based sensors are usually pH sensitive,
which interferes in the detection of other cations. Intramolecular
charge transfer (ICT) is another important mechanism for the de-
sign of optical sensors. ICT-based sensors have obvious blueshift
or redshift in the UV–vis absorption and fluorescence spectra,
accompanied by naked eye color changes. However, they generally
do not exhibit remarkable fluorescence enhancement. It can be
rationally imagined that highly selective and sensitive colorimetric
and ‘‘turn-on’’ optical sensors can be achieved if the advantages of
the PET and ICT-based sensors are integrated. 1,8-Naphthalimide is
one of the most suitable fluorophores for building PET and ICT-
based sensors [19–21,24–26]. The functional groups containing
N, S and O, especially S atom, are frequently-used ligand for Hg2+
0.2
0.0
4
8
12
0.2
pH
0.0
300
400
500
600
Wavelength /nm
Fig. 1. UV–vis absorption response of BTABN upon different pH values. Solvent:
EtOH/Britton–Robinson buffer (1/2, v/v); c: 10 M. From top to bottom, pH: 9.18,
2.98, 8.60, 4.42, 2.11, 6.92, 6.12, 3.94, 5.38, 10.93, 10.18 and 7.92. Inset: plots of
l
maximal absorbance (A) depending on the pH values.
[5,6]. Therefore,
a thioether-rich bis[2-(ethylthio)ethyl]amino
group is chosen as the receptor and attached to the C-4 position
of the 1,8-naphthalimide. The binding ability of the S atom to pro-
ton is weaker than that of the N atom, leading to the possibility of
eliminating the PET-caused pH sensitivity, while the thiophilic nat-
ure of mercury can arouse PET-caused fluorescence enhancement.
The extreme affinity of Hg2+ to sulfur over other metal ions can im-
prove the specificity of the sensor [27]. In addition, the N atom
directly linked to the C-4 position of the naphthalene ring can in-
6.0x105
8.0x105
4.0x105
3.0x105
0.0
4
8
12
pH
duce ICT in the sensor molecule upon coordinating with Hg2+
.
The constructed sensor (BTABN) is therefore expected to meet
the designed demand.
0.0
BTABN was tried to be synthesized by two routes. One is a four-
step reaction process, as shown in Scheme 1. Intermediate 2, 3 and
BTABN can be easily purified by column chromatography, and
BTABN can be obtained successfully with 53.2% yield. In the other
method, 3,9-dithia-6-monoazaundecane (DMA) is first prepared by
bis(2-chloroethyl)amine hydrochloride and EtSH according to Ref.
[28], then, reacted with N-n-butyl-4-bromo-1,8-naphthalimide.
500
600
Wavelength /nm
700
Fig. 2. Fluorescence response of BTABN upon different pH values. Solvent: EtOH/
Britton–Robinson buffer (1/2, v/v); c: 10 M; kex: 430 nm; slit width: 5 nm. From top
to bottom, pH: 7.92, 2.11, 2.98, 9.18, 8.60, 6.92, 10.16, 6.12, 4.42, 10.93, 5.42 and
l
3.94. Inset: plots of fluorescence intensity maxima (I) depending on the pH values.

Z. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
11
absorption peak at 436 nm were observed for Hg2+ among all the
cations (Fig. 3).
When the concentrations of Hg2+ increase from 0 to 500
lM, the
UV–vis absorption peak centered at 436 nm gradually reduced and
disappeared along with a concomitant new peak centered at
376 nm appeared and gradually grew, and a 60 nm blueshift led
to the solution changing from bright yellow to colorless, as shown
in Fig. 4. The results suggest that BTABN has high selectivity to
Hg2+ and can be used as a colorimetric and macroscopic sensor
for Hg2+ in EtOH/H2O (1/2, v/v). A clear observed isosbestic point
at 400 nm indicates the formation of BTABN/Hg2+ complex [29].
Furthermore, the addition of Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe3+
Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ (100
M) did not show signif-
icant effect on the UV–vis spectrum of the EtOH/H2O (1/2, v/v)
,
l
solution of BTABN (10 l lM), as presented in
M) and Hg2+ (100
Fig. 4. UV–vis absorption spectra of BTABN with various concentrations of Hg2+
.
Fig. 5. This result indicates that the system BTABN/Hg2+ is hardly
affected by these coexistent cations. According to the above data,
BTABN is a reliable highly selective colorimetric sensor for Hg2+
in EtOH/H2O (1/2, v/v).
Solvent: EtOH/H2O (1/2, v/v); c: 10 lM. From top to bottom at 436 nm, the equiv. of
Hg2+: 0, 0.3, 0.5, 0.7, 1, 3, 5, 7, 8, 9, 10, 12, 14, 16, 18, 20, 30 and 50. Inset: color of the
solutions before (yellow) and after (colorless) addition of Hg2+. (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
The response of fluorescence spectra to Hg2+
Hg2++Fe3+
To exam the sensing behavior of BTABN to Hg2+ in EtOH/H2O (1/
2, v/v) further, the response of fluorescence spectra to various
Hg2++Fe2+
0.2
Hg2++K+
Hg2++Pb2+
Hg2++Cr3+
Hg2++Zn2+
Hg2++Mn2+
metal ions was investigated. The BTABN solution (10 lM) in the
absence of Hg2+ emitted very weak fluorescence and the maximal
fluorescence wavelength was 530 nm with the fluorescence quan-
Hg2++cd2+
tum yield 0.101. Upon addition of Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+
,
0.1
Hg2++Na+
Hg2++Mg2+
Hg2++Ca2+
Hg2++Cu2+
Hg2++Ni2+
Hg2++Co2+
Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ (100
lM) to the
solution of BTABN (10
l
M), only Hg2+ caused a marked fluores-
cence enhancement with a fluorescence quantum yield 0.384 and
a 14 nm blueshift, as shown in Fig. 6, and the response time was
less than 1 min (Fig. 7). Fe3+ also aroused some fluorescence
enhancement, but the fluorescence spectra did not show obvious
blueshift or redshift. Accordingly, BTABN has high selectivity to
Hg2+ and can be used as a ‘‘turn-on’’ fluorescent sensor for Hg2+
in EtOH/H2O (1/2, v/v).
Hg2+
0.0
300
400
500
600
Wavelength /nm
Fig. 5. Effects of coexisting ions on the UV–vis absorption spectra of BTABN and
Hg2+. Solvent: EtOH/H2O (1/2, v/v); c: 10
M for BTABN and 100 M for metal ions.
l
l
Furthermore, it was found that the fluorescence of BTABN was
gradually enhanced with the addition of Hg2+ from 0
l
M to
M (Fig. 8). The maximal fluorescence intensity (I) and the
concentration of Hg2+ ([Hg2+]) showed good linear relationship be-
tween Hg2+ concentration 10 M ([Hg2+]/[BTABN]
M and 100
500
l
l
l
from 1 to 10 in the insets in Fig. 8), based on which the detection
limit of BTABN was 1.27 ꢂ 10ꢁ6 M, and the association constant
was 1.12 ꢂ 104 Mꢁ1
.
In order to determine the influence of other cations on the fluo-
rescence detection of Hg2+ in EtOH/H2O (1/2, v/v), Na+, K+, Mg2+
,
Ca2+, Cr3+, Mn2+, Fe3+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+
Fe3+
Fe2+
Co2+
Ni2+
Zn2+
Fig. 6. Fluorescent spectra of BTABN with different metal ions. Solvent: EtOH/H2O
(1/2, v/v); c: 10 M for BTABN and 100 M for metal ions; kex: 436 nm; slit width:
0.2
Mn2+
Cr3+
K+
l
l
5 nm. From top to bottom: Hg2+, Fe3+, Zn2+, Cr3+, Co2+, Cu2+, none, Ca2+, Cd2+, Ni2+
,
Mg2+, Fe2+, Pb2+, K+, Mn2+, Na+. Inset: fluorescence of the solutions before (very
weak) and after (green) addition of Hg2+. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Na+
Ca2+
0.1
Pb2+
Cd2+
(100
(100
l
l
M) were added to the solution of BTABN (10 l
M) and Hg2+
None
Mg2+
Cu2+
M). All the cations showed negligible disturbance except
for Fe3+ quenching the fluorescence slightly (Fig. 9).
Hg2+
0.0
300
400
500
600
Wavelength /nm
Binding stoichiometry and sensing mechanism
To explore the possible binding mode of Hg2+ and BTABN, Job’s
plot analysis was carried out. The difference of the absorbance
Fig. 3. UV–vis absorption spectra of BTABN with different metal ions. Solvent:
EtOH/H2O (1/2, v/v); c: 10 M.
l

12
Z. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
9.0x105
6.0x105
3.0x105
0.04
0.03
0.02
0.01
0.0
0.4
0.8
0
10
Time /min
20
30
[Hg2+]/[BTABN+Hg2+]
Fig. 10. Job’s plot for Hg2+ versus BTABN. Solvent: EtOH/H2O (1/2, v/v); total
Fig. 7. Time response of BTABN to Hg2+. Solvent: EtOH/H2O (1:2, v/v); c: 10
lM for
concentration ([BTABN + Hg2+]): 50
lM; A0 and A: Absorbance before and after
BTABN and 100
l
M for Hg2+; kex: 436 nm; slit width: 5 nm.
addition of Hg2+ at 436 nm, respectively.
1.2x106
1.2x106
8.0x105
4.0x105
6.0x105
0.0
0
20
40
[Hg2+]/[BTABN]
500
600
700
Wavelength /nm
Fig. 8. Fluorescence spectra of BTABN with various concentrations of Hg2+. Solvent:
EtOH/H2O (1/2, v/v); c: 10 lM for BTABN and 0–500 l
M for Hg2+; kex: 436 nm; slit
width: 5 nm. From bottom to top, the equiv. of Hg2+: 0, 0.1, 0.3, 0.5, 0.7, 1, 3, 5, 7, 8,
9, 10, 12, 14, 16, 18, 20, 30, and 50. Inset: the relationship between the maximal
fluorescence intensity (I) and the concentration of Hg2+
.
before (A0) and after (A) addition of Hg2+ at 436 nm exhibited a
maximum when [Hg2+]/[BTABN + Hg2+] was 0.5 (Fig. 10), which
indicates the 1:1 stoichiometry between Hg2+ and BTABN.
Fig. 11. 1H NMR spectra of BTABN before (1) and after (2) addition of Hg2+
.
To further seek the detailed information on the complex of Hg2+
and BTABN, 1H NMR spectra of BTABN before and after addition of
Hg2+ were tested (Fig. 11). As the results of addition of Hg2+, the sig-
nals of the protons in the CH2 linked to the S atom remarkably
shifted downfield 0.64 (3.32–2.68) ppm (c), and 0.54 (2.99–
2.45) ppm (b), respectively, the peak of the proton (a) in the
terminal CH3 in SCH2CH3 moved from 1.17 to 1.30 ppm, and the
S
O
Hg2+
S
N
N
O
Scheme 2. Proposed complexation mechanism of BTABN with Hg2+
.
peak corresponding to the proton (d) in the CH2 attached to the N
atom in C-4 position of BTABN moved from 3.58 to 3.63 ppm. In
addition, the peaks of the aryl protons neighboring to the N atom
(e, f and h) shifted from 7.31, 8.62 and 7.68 to 7.51, 8.73 and
7.88 ppm respectively, and the peak of the aryl protons (g) splitted
and shifted from 8.49 to 8.58 and 8.66 ppm. These data indicate that
the S and N atoms participate in the coordination of BTABN and
Hg2+ and BTABN binds Hg2+ via a tridentate chelation. Conse-
quently, the proposed complex mechanism of BTABN with Hg2+ is
given as Scheme 2.
8.0x105
4.0x105
0.0
Conclusions
In summary, new fast responsive colorimetric and ‘‘turn on’’
fluorescent Hg2+ sensor 4-(bis(2-(ethylthio)ethyl)amino)-N-n-bu-
tyl-1,8-naphthalimide (BTABN) was successively designed and
Fig. 9. Effects of coexist ions on the fluorescence maxima of BTABN and Hg2+
Solvent: EtOH/H2O (1/2, v/v); c: 10 lM for BTABN and 100 lM for metal ions; kex:
.
436 nm; slit width: 5 nm.

Z. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 8–13
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The project supported by the National Natural Science Founda-
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National Engineering Laboratory for Modern Silk, Soochow Univer-
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