Carbamodithioate-based fluorescent chemosensor for Hg(II): a staged response approach and investigation into the sensing mechanism

Article abstract of DOI:10.1002/bio.3205

Carbamodithioate-based compound C1 was designed and synthesized as a new fluorescent probe for Hg^{2 +} ions. Upon the addition of Hg^{2 +} ions, it displayed a rare staged response: the emission spectra of C1 first showed an apparent red-shift, followed by a dramatic decrease. To investigate the sensing mechanism, control compounds C2 with the same phenanthroimidazole unit and C3 with the same carbamodithioate functionality were synthesized. On comparison, the first step sensing process was ascribed to decreasing photoinduced electron transfer on the coordination of Hg^{2 +} with the lone pair electrons of the nitrogen atom on the phenanthroimidazole ring. The affinity of Hg^{2 +} and the carbamodithioate unit with four sulfur atoms then induced changes in intramolecular charge transfer efficiency and the second step fluorescent response.

Full text of DOI:10.1002/bio.3205

Received: 10 May 2016
DOI 10.1002/bio.3205
Revised: 25 July 2016
Accepted: 29 July 2016
R E S E A R C H A R T I C L E
Carbamodithioate‐based fluorescent chemosensor for Hg(II): a
staged response approach and investigation into the sensing
mechanism
Xiaohong Cheng1
Zhicheng Zhong1 Tingting Ye2 Bingjie Zhang2
*
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1 Hubei Key Laboratory of Low Dimensional
Optoelectronic Materials and Devices, Hubei
University of Arts and Science, Xiangyang,
People’s Republic of China
Abstract
Carbamodithioate‐based compound C1 was designed and synthesized as a new fluorescent
probe for Hg2+ ions. Upon the addition of Hg2+ ions, it displayed a rare staged response: the
emission spectra of C1 first showed an apparent red‐shift, followed by a dramatic decrease. To
investigate the sensing mechanism, control compounds C2 with the same phenanthroimidazole
unit and C3 with the same carbamodithioate functionality were synthesized. On comparison,
the first step sensing process was ascribed to decreasing photoinduced electron transfer on
the coordination of Hg2+ with the lone pair electrons of the nitrogen atom on the
phenanthroimidazole ring. The affinity of Hg2+ and the carbamodithioate unit with four sulfur
atoms then induced changes in intramolecular charge transfer efficiency and the second step
fluorescent response.
2 College of Chemical Engineering and Food
Science, Hubei University of Arts and Science,
Xiangyang, People’s Republic of China
Correspondence
Xiaohong Cheng, Hubei Key Laboratory of Low
Dimensional Optoelectronic Materials and
Devices, Hubei University of Arts and Science,
Xiangyang 441053, Hubei Province, People’s
Republic of China.
Email: chengxiaohong0807@126.com
KEYWORDS
Fluorescent probe, intramolecular charge transfer, mercury, photoinduced electron transfer, stage
response approach
The detection of some analytes, including metal ions, anions, explo-
sives, proteins, DNA and RNA in certain samples is very important in
medical diagnosis, combating terrorism, environmental monitoring,
etc. For example, mercuric ions (Hg2+), one of the more severe environ-
mental pollutants, are very harmful to humans. Specifically, methylmer-
cury, a product of the microbial biomethylation of Hg2+, is known to
cause brain damage and other chronic diseases.[1–6] Therefore, moni-
toring of Hg2+, one of the most common and stable forms of mercury
pollution, is increasingly required. The ability to recognize and sense
biologically and environmentally important metal ions using a fluores-
cence technique has become significant in chemical sensing in recent
years. Among the various methods available for the detection of Hg2+
ions, techniques based on fluorescence sensors have many number of
advantages in terms of their high sensitivity, specificity, simplicity of
implementation and fast response times.[7,8] Thanks to the enthusiastic
efforts of scientists, many good chemosensors for Hg2+ ions have been
reported, and some design rules have been summarized.[9–22]
to create upper‐rim‐substituted calix [4]arenes as selective
extractants.[24] However, it has rarely been utilized for metal‐sensing
purposes. According to Pearson’s hard and soft acids and bases theory,
Hg2+ (soft acid) can preferentially interact with the sulfur atom (soft
base).[25] Therefore, it is expected that the carbamodithioate moiety
containing four sulfur atoms has exceptionally strong affinity towards
Hg2+. To further our interest in fluorescent sensor development for
Hg2+ detection,[26–29] we herein reported an acceptor–spacer‐donor
system C1, comprising
a phenanthroimidazole moiety as the
fluorophore and carbamodithioate functionalities as ligating groups.
Compound C1 could serve as a new fluorescent chemosensor for
Hg2+ and displayed a rare staged response: the emission spectra of
C1 first showed an apparent red‐shift, followed by a dramatic
decrease. Moreover, control compounds C2 and C3 were synthesized
to investigate the sensing mechanism systematically (Scheme 1). We
described the new fluorescent chemosensor for Hg2+ ions in detail.
In terms of sensor design, acyclic receptors are generally more
popular than cyclic structures because synthesis of a cyclic structure
is usually expensive.[23] Among these acyclic receptors, the
carbamodithioate moiety was first used by Roundhill and co‐workers
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1
EXPERIMENTAL
Tetrahydrofuran (THF) was dried over and distilled from K–Na alloy
under an atmosphere of dry nitrogen. All other reagents were of
Luminescence 2016; 1–8
wileyonlinelibrary.com/journal/bio
Copyright © 2016 John Wiley & Sons, Ltd.
1

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CHENG ET AL.
SCHEME 1 Structural comparison of probe C1 and references C2 and C3
analytical reagent grade and used without further purification. Double‐
distilled water was used in all experiments.
The 1H and 13C NMR spectra were measured on Varian Mer-
cury300 spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as
internal standard. The electrospray ionization mass spectra (ESI‐MS)
were measured on a Finnigan LCQ advantage mass spectrometer. Ele-
added dropwise to the solution. After reacting overnight at room tem-
perature, the solvent was removed under reduced pressure to give the
crude product. The crude product was purified by silica gel column
chromatography using dichloromethane: petroleum ether (1:1, v/v) as
the eluent to give compound C3 as a yellow solid (177 mg, 27%).1H
NMR (300 MHz, CDCl3): δ=3.39 (s, 6H), 3.51–3.58 (m, 10H), 3.69–
3.71 (m, 4H), 6.91 (s, 1H), 6.94–6.95 (m, 2H), 6.98 (s, 2H), 7.01–7.04
(m, 5H), 7.08–7.10 (m, 4H), 7.31 (s, 1H), 7.34–7.36 (d, J = 3.0, 2H),
7.38 (s, 1H), 7.41–7.44 (d, J = 4.5, 2H). 13C NMR (75 MHz, CDCl3):
δ=34.2, 42.0, 45.8, 50.4, 112.7, 115.1, 123.6, 124.1, 124.7, 125.5,
127.3, 128.1, 129.7, 132.5, 133.6, 146.2, 147.2, 147.5, 196.8 ppm.
MS (ESI), m/z [M + H]+: 656.1, calcd, 656.2. C36H40N4S4 (EA) (%,
found/calcd): C, 66.08/65.81; H, 5.91/6.14; N, 8.31/8.53.
mental analyses were performed using
a CARLOERBA‐1106
microelemental analyzer. Photoluminescence spectra were performed
on a Hitachi F‐4500 fluorescence spectrophotometer.
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1.1
Synthesis of compound C1
A mixture of 9,10‐phenanthroquinone (208 mg, 1 mmol), compound 1
(623 mg, 1.5 mmol) and ammonium acetate (1.23 g, 16 mmol) in ace-
tate acid (AcOH) (10 ml) was heated to 100°C for 30 min. The above
hot solution was then cooled to room temperature, and the resulting
yellow solid was collected by filtration and washed successively with
acetate acid (AcOH), dilute sodium hydrogen carbonate solution and
water. The crude product was further dried under reduced vacuum,
and then purified by silica gel column chromatography using acetone
as the eluent to afford compound C1 as a yellow solid (259.2 mg,
43%). 1H NMR (300 MHz, d6‐Acetone): δ=3.30 (m, 10H), 3.57 (m, 6H),
3.71 (s, 4H), 7.61–7.76 (m, 4H), 8.05–8.07 (d, J = 6.0 Hz, 2H),
8.49–8.61 (m, 4H), 8.82–8.88 (m, 2H). MS (ESI), m/z [M + H]+: 604.0,
calcd, 604.2. C31H33N5S4 (EA) (%, found/calcd): C, 61.69/61.57; H,
5.96/5.57; N, 11.29/11.60.
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1.4
Preparation of solutions of metal ions
A 1 mmol quantity of each inorganic salt [MgSO4, MnSO4·2H2O, Zn
(NO3)2·6H2O, Ni(NO3)2·6H2O, Pb(NO3)2, Ba(NO3)2, Ca(NO3)2·4H2O,
CoCl2·6H2O, Fe(NO3)3·9H2O, Al(NO3)3·9H2O, Cu(NO3)2·3H2O,
CdSO4·8H2O, Cr(NO3)3·9H2O, (NH4)2Fe(SO4)2·6H2O, LiCl, NaNO3,
KNO3, AgNO3 or Hg(ClO4)2·3H2O] was dissolved in distilled water
(10 ml) to afford a 1 × 10−1 mol/l aqueous solution. The stock solutions
were diluted to the desired concentrations with water when needed.
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1.5
Fluorescence titration of C1, C2 or C3 with Hg2+
ions
A solution of C1, C2 or C3 (1 × 10−5 mol/l) was prepared in THF. The
solution of Hg2+ (1 × 10−3 mol/l) was prepared in distilled water. A
solution of C1, C2 or C3 was placed in a quartz cell (10.0 mm wide)
and the fluorescence spectrum was recorded. Aliquots of the Hg2+
ion solution were introduced and changes in the fluorescence intensity
were recorded at room temperature each time.
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1.2
Synthesis of compound C2
Compound C2 was obtained by following
a procedure similar
to that for C1, using 9,10‐phenanthroquinone (208 mg, 1 mmol), 4‐
(dimethylamino)benzaldehyde (223.6 mg, 1.5 mmol) and ammonium
acetate (1.23 g, 16 mmol) in glacial AcOH (10 ml), with acetone: petro-
leum ether (1:1, v/v) as the eluent to afford compound C2 as a yellow
solid (63 mg, 31.5%). 1H NMR (300 MHz, d6‐Acetone): δ=2.98–3.10
(m, 6H), 6.88–6.90 (d, J = 6.0, 2H), 7.60–7.66 (m, 4H), 8.16–8.18 (m,
2H), 8.56 (s, 2H), 8.82–8.85 (d, J = 9.0, 2H). MS (ESI), m/z [M + H]+:
338.3, calcd, 338.2. C23H19N3 (EA) (%, found/calcd): C, 81.67/81.87;
H, 5.86/5.68; N, 12.58/12.45.
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1.6
Fluorescence titration of C1 + Hg2+ with S2−
anions
A solution of C1 (1 × 10−5 mol/l) was prepared in THF. The solution of
Hg2+ with the concentration of 25 μM was added to the above solu-
tion. The solution of NaS (1 × 10−3 mol/l) was prepared in distilled
water. A solution of C1 + Hg2+ was placed in a quartz cell (10.0 mm
wide) and the fluorescence spectrum was recorded. Aliquots of the
S2− ion solution were introduced to the above C1 + Hg2+ solution
and changes in the fluorescence intensity were recorded at room tem-
perature each time.
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1.3
Synthesis of compound C3
Under an argon atmosphere, compound 1 (416 mg, 1 mmol) and
diethyl 4‐(diphenylamino)benzylphosphonate (727 mg, 2 mmol) were
dissolved in 30 ml THF, and NaH (0.2 g, 3.7 mmol) in 10 ml THF was

CHENG ET AL.
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(UV/vis) spectra, and gave satisfactory spectral data (Figures S1–S8).
Unfortunately, we failed to obtain the 13C NMR spectra of compound
C1 and C2 because of the poor solubility of the phenanthroimidazole
moiety in them.
1.7
Fluorescence intensity changes of C1 with
different metal ions
A solution of C1 (1 × 10−5 mol/l) was prepared in THF. The solutions of
metal ions (1 × 10−1 mol/l) were prepared in distilled water. A solution
of C1 (3.0 ml) was placed in a quartz cell (10.0 mm wide) and the fluo-
rescence spectrum was recorded. Different ion solutions were intro-
duced and changes in the fluorescence intensity were recorded at
room temperature each time.
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2.2
Sensing properties
We tried to add Hg2+ ions to the diluted solution of compound C1,
and investigate the sensing behavior of C1 towards mercury ions. As
shown in Figure 1, on increasing the concentration of Hg2+, the fluo-
rescent spectrum of compound C1 displayed apparent changes. First,
as the concentration of Hg2+ ions increased from 0 to 10 μM, the fluo-
rescent intensity increased and the emission maximum gradually
shifted from 414 to 422 nm. In fact, the emission spectrum changed
immediately at Hg2+ concentration as low as 1 μM. To see the results
more visually, we summarized the changes in intensity at 422 nm as a
function of mercury concentrations. As shown in Figure S9, over a
range of 0–10 μM, there was a good linear relationship between the
intensity change and the concentration of Hg2+. A linear regression
curve could be simulated, and the point at which this line crossed
the abscissa was taken as the limit of detection and equaled
~2.5 × 10−7 mol/l.[30] It suggested that compound C1 could detect
the presence of Hg2+ ions quantitatively. A Job plot was used to deter-
mine the binding stoichiometry of C1 with Hg2+ ions. The total con-
centration of chemosensor C1 and Hg2+ was held constant while the
mole fraction of Hg2+ ions was altered; the increasing fluorescence
value, ΔI (ΔI = I – I0, where I0 and I are the fluorescence intensities
at 422 nm in the absence and presence of Hg2+, respectively) was
plotted against the mole fraction. The maximum fluorescence increas-
ing occurred at a mole fraction of 0.5, indicating the formation of a 1:1
complex (Figure S10).
Fluorescence titration of C1 with Ag+ ions
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1.8
A solution of C1 (1 × 10−5 mol/l) was prepared in THF. The solution of
Hg2+ (1 × 10−3 mol/l) was prepared in distilled water. A solution of C1
was placed in a quartz cell (10.0 mm wide) and the fluorescence spec-
trum was recorded. Aliquots of the Ag+ ion solution were introduced
and changes in the fluorescence intensity were recorded at room tem-
perature each time.
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2
RESULTS AND DISCUSSION
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2.1
Synthesis and structural characterization
Probe C1 was readily synthesized in one step by coupling 9,10‐
phenanthroquinone with aldehyde 1 according to a reported proce-
dure (Scheme 2),[23] and the reference compound C2 was prepared
using the same procedure by coupling 9,10‐phenanthroquinone with
4‐(dimethylamino)benzaldehyde. The Wittig reaction of aldehyde 1
with diethyl (4‐(diphenylamino)benzyl) triphenyl)‐phosphonate gave
reference compound C3. The whole synthesis route was simple and
the purification was easy. Their structures were well characterized
by 1H, 13C NMR, ESI‐MS, elemental analysis and ultraviolet/visible
SCHEME 2 Synthetic route for probe C1 and the reference compounds C2 and C3

4
CHENG ET AL.
FIGURE
1
Fluorescent emission spectra of
C1 (10 μM, in THF) in the presence of differ-
ent concentrations of Hg2+ (excited at 335 nm)
Surprisingly, with the continued addition of Hg2+ ions, the emis-
displayed similar changes as in the case of probe C1. Moreover, the
emission spectra of compound C2 reached a plateau at a Hg2+ ion
concentration of 10 μM (namely, 1.0 equiv. of Hg2+ ions) and there
was no change in the fluorescence intensity on the subsequent addition
of Hg2+ ions. Accordingly, we speculated that there was a 1:1
stoichiometry between compound C2 and Hg2+ ions,[31] which led to
the red‐shift in the emission spectra at mercury ion concentrations of
as 0–1.0 equiv. In compound C2, the N,N‐dimethyl moiety was passive
and there was almost no response with Hg2+ ions. Therefore, it was
reasonable that both in compound C1 and C2, the phenanthroimidazole
moiety played the dominant role in the interaction with Hg2+ and led to
the changes on emission spectra.
sion intensity centered at 422 nm declined dramatically. When 2.5
equiv. of mercury ions were added, the emission intensity at 422 nm
reached a minimum with a 60‐fold decrease compared with the inten-
sity at a 10 μM Hg2+. However, when the concentration reached
25 μM, no big change could be observed, even on further increasing
the concentration of Hg2+ ions. With the aid of normal UV lamp it
can be seen that the fluorescent color changes on addition of Hg2+
ions (Figure 1, inset).
The above experimental results showed that probe C1 had a staged
fluorescent response towards Hg2+ ions: first a red‐shift and increase
and subsequent decrease. This phenomenon was very rare and
unanticipated. We then designed the reference compound C2, which
possessed the same phenanthroimidazole moiety, and reference com-
pound C3, which possessed the same carbamodithioate moiety, in
order to explore the mutual interaction between probe C1 and Hg2+
ions (Scheme 1). As demonstrated in Figure 2, on the addition of mer-
cury ions from 0 to 10 μM, the emission spectra of reference C2
In order to explore the possible sensing mechanism of probe C1
for Hg2+ ions from another aspect, reference C3 was synthesized,
which contained the same carbamodithioate moiety as compound
C1, but not the phenanthroimidazole moiety. As a control, we added
Hg2+ ions into a dilute solution of reference C3 and investigated its
response towards Hg2+ ions in THF in detail (Figure 3). The fluorescent
FIGURE 2 Fluorescent emission spectra of C2 (10 μM, in THF) in the
FIGURE 3 Fluorescent emission spectra of C3 (10 μM, in THF) in the
presence of different concentrations of Hg2+ (excited at 335 nm)
presence of different concentrations of Hg2+ (excited at 345 nm)

CHENG ET AL.
5
spectra of compound C3 showed a monotonic decrease with increas-
ing concentrations of Hg2+ ions: the emission intensity centered at
425 nm decreased as the concentration of Hg2+ ions changing from
0 to 11 μM. In compound C3, the triphenylamine moiety was passive
towards Hg2+ ions and we speculated that the decrease in the emission
spectra could be ascribed to the affinity of Hg2+ and the
carbamodithioate unit in both compound C1 and C3 according to
Pearson’s hard and soft acids and bases theory.
affinity between Hg2+ and the carbamodithioate unit containing four
sulfur atoms, which would induce changes in the intramolecular charge
transfer (ICT) efficiency and the concomitant emission spectra.[38] The
above results were unexpected, but indicated that compound C1 could
act as a bifunctional probe towards Hg2+ ions, because both the
fluorophore unit (phenanthroimidazole moiety) and the recognition
(carbamodithioate moiety) had a sensing response to Hg2+
.
Furthermore, considering that mercury ions were able to form a
very stable complex with sulfide anion (HgS, Ksp = 4.0 × 10−53),[39] we
added an aqueous solution of sodium sulfide to the C1–Hg2+ complex
to investigate the reversibility of the sensing system. We fixed the con-
centration of C1 at 10 μM and that of the added Hg2+ ions at 25 μM
(Figure 4, blue line) considering that the addition of 25 μM of Hg2+ ions
led to the minimum emission intensity during the fluorescent titration
We compared the chemical structures and sensing behavior of
probe C1 and references C2 and C3. When we added Hg2+ ions to
the solution of compound C1, there was a two‐step change in the fluo-
rescent spectra: red‐shift and increase firstly and the consequence
decrease. In compound C2, which contained the same
phenanthroimidazole moiety as compound C1, the addition of Hg2+
ions only induced the red‐shift and an increase in the emission spectra;
however, with reference compound C3, which possessed the same
carbamodithioate moiety as compound C1, the addition of Hg2+ ions
induced a monotonic decrease in the emission intensity. Accordingly,
we speculated the possible sensing mechanism (Scheme 3) between
probe C1 and Hg2+ ions: first, Hg2+ ions had strong affinity with the
nitrogen atoms of the phenanthroimidazole moieties in C1, and Com-
plex 1 between the phenanthroimidazole moieties and Hg2+ ions was
spontaneously formed, in a manner similar to previous cases of chela-
tion‐enhanced fluorescence (CHEF).[32–36] The CHEF effect depended
directly on the photoinduced electron transfer (PET) mechanism. Dur-
ing the PET process, the lone pair electrons of the nitrogen atom of the
imidazole ring were induced in the free ligand under the exciting radi-
ation, quenching its fluorescence. While coordinating with Hg2+ ions,
the same lone pair electrons became involved in metal–ligand bond
formation, decreasing the PET quenching effect and leading to the
CHEF effect. Correspondingly, the fluorescence intensity increased
with a shift in the spectra from 414 to 422 nm. Afterwards, the contin-
ued addition of Hg2+ to the system induced a sharp decrease in the
emission spectra at 422 nm. The association constant of Complex 1
for Hg2+ was calculated to be ~1.7 × 105 M−1 using the equation in
Scheme S1.[37] We speculated that the interaction between compound
C1 and Hg2+ ions was ascribed to formation of Complex 2 due to the
FIGURE 4 Fluorescent emission spectra of C1 + Hg2+ (Hg2+: 25 μM,
blue line, excited at 335 nm) in the presence of different concentra-
tions of S2− and C1 + Hg2+ (Hg2+: 10 μM, red line, excited at 335 nm)
for comparison
SCHEME 3 Postulated sensing process of C1 towards Hg2+

6
CHENG ET AL.
experiment. Upon addition of the sulfide anion, the fluorescent inten-
sity at 422 nm increased as expected. As shown in Figure 4, the emis-
sion spectra became closer to that of C1 + Hg2+ (Hg2+: 10 μM,
Figure 4, red line) with increasing concentrations of S2− anion. How-
ever, the emission spectra reached a plateau when the concentration
of S2− anions was 60 μM. These experimental results indicated that
addition of the sulfide anion could preferentially snatch mercury ion
in Complex 2 to form stable HgS species, as we reported previously.[40]
As a result, the liberated carbamodithioate moiety recovered its elec-
tron‐donating ability during the ICT process with recovery of the fluo-
rescence. Therefore, to summarize: the transformation from Complex
1 to Complex 2 was reversible on addition of S2− anions. However,
the transformation from C1 to Complex 1 was irreversible because no
blue‐shift in the emission spectra could be observed, even on further
increasing the concentration of S2− anions.
To confirm, the reaction mixture of C1 with Hg2+ was character-
ized by ESI‐MS spectrometry. The ESI‐MS spectrum of C1 in
Figure 5 (left) revealed a main peak at 604.0 nm before the addition
of mercury ions, corresponding to [C1 + H]+ (m/zcalcd = 604.2). After
addition of a small amount of Hg2+ (Hg2+: C1 < 1:1), a relatively weak
peak at ~ 804.1 appeared, coinciding exactly with the adduct species
of [C1 + Hg]2+, namely, Complex 1 (m/zcalcd = 804.5). We then added
Hg2+ ions (Hg2+: C1 > 1:1) into the above solution continually and
observed that the previous peak at 804.1 disappeared, and another
apparent peak at 1004.8 emerged, corresponding to the adduct spe-
cies [C1 + 2Hg]4+, namely, Complex 2 (m/zcalcd = 1005.1). The changes
in the ESI‐MS spectrum of C1 with and without the presence of Hg2+
ions provided some evidence of a ‘two‐step’ interaction between
probe C1 and Hg2+ ions, and both the fluorophore unit
(phenanthroimidazole
moiety)
and
the
recognition
unit
(carbamodithioate moiety) had a sensing response towards Hg2+
.
To assess the specificity of the sensing behavior of C1, various
ions were examined in parallel under the same conditions. Surpris-
ingly, upon the addition of other ions such as Mg2+, Mn2+, Zn2+
,
Ni2+, Pb2+, Ba2+, Ca2+, Co2+, Fe3+, Al3+, Cu2+, Cd2+, Cr3+, Fe2+, Li+,
Na+ and K+, there was hardly any change in the emission spectra
(Figure S11). Therefore, it could be concluded that C1 displayed
extremely good selectivity for Hg2+, rather than of the other ions
examined. Moreover, to test the reproducibility of the sensing
behavior of compound C1 towards Hg2+ ions, we studied the fluo-
rescent properties of C1 in the presence and absence of Hg2+ ions
in acetone. As shown in Figures S12 and S12, C1 displayed both
high sensitivity and good selectivity towards Hg2+ ions as in THF.
Meanwhile, considering that Hg2+ and Ag+ ions possessed some sim-
ilarities, for instance, both are soft acid ions and have special affinity
towards sulfur,[41,42] and sometimes Hg2+ chemosensors gave a
response to trace silver ions, we also investigated the sensing behav-
ior of C1 towards silver ions. As shown in Figure 6, upon the
FIGURE 5 ESI‐MS spectra of C1 and C1 + Hg2+

CHENG ET AL.
7
C2 and C3 with C1 indicated that compound C1 could act as a bifunc-
tional probe towards Hg2+ ions, because both the fluorophore unit and
the recognition unit had a sensing response to Hg2+. The results
reported here might provide some useful information for the design
of new probes for metal ions, namely that subtle adjustment to the
chemical structure dramatically affected the sensing property in the
present system. Further study on the design of probes for toxic metal
ions with better performance is still in progress in our laboratory.
ACKNOWLEDGMENTS
This work was financially supported by the Science and Technology
Research Project of Department of Education of Hubei Province
(B2015147).
FUNDING INFORMATION
This work was financially supported by the Science and Technology
Research Project of Department of Education of Hubei Province
(B2015147).
FIGURE 6 Fluorescent emission spectra of C1 (10 μM, in THF) in the
presence of different concentrations of Ag+ (excited at 335 nm)
ABBREVIATIONS USED
AcOH
CHEF
Acetic acid
addition of silver ions, the sensing system exhibited different
changes from those seen on the addition of Hg2+ ions: the fluores-
cent intensity centered at ~414 nm declined gradually with the
increasing amounts of Ag+. In order to investigate the sensing mech-
anism between C1 and Ag+ ions, we inspected the responses of ref-
erence compounds C2 and C3 towards Ag+ (Figures S14 and S15).
Although we added an excess of silver ions, there were hardly any
changes in the emission spectra of reference C2. However, the
changes to compound C3 displayed a similar trend to that of C1:
the fluorescent intensity declined gradually with increasing amounts
of Ag+ ions. Therefore, it was obvious that both compounds C1
and C3, which contained the carbamodithioate moiety, had a fluores-
cent response towards Ag+. By contrast, reference C2 showed virtu-
ally no change upon the addition of silver ions due to the lack of a
carbamodithioate unit. Based on these results, we conjectured that
the sensing response of C1 towards Ag+ ions could be ascribed to
interaction of the carbamodithioate moiety and Ag+ ions. The
obtained experimental results were unexpected but quite important.
On the one hand, compound C1 could also be regarded as a good
probe towards Ag+ ions, since it was also necessary to probe Ag+ ions
due to its strong toxicity to the human and our environment.[43,44] On
the other hand, compound C1 could discriminate Hg2+ and Ag+ ions
through different fluorescent response.
Chelation‐enhanced fluorescence
ESI‐MS Electrospray ionization mass spectra
ICT
Intramolecular charge transfer
Photoinduced electron transfer
Tetrahydrofuran
PET
THF
TMS
UV/vis
Tetramethylsilane
Ultraviolet/visible
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SUPPORTING INFORMATION
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