A mechanosynthesized, sequential, cyclic fluorescent probe for mercury and iodide ions in aqueous solutions

Article abstract of DOI:10.1016/j.saa.2014.05.067

A fluorescent Hg²⁺-selective chemosensor, 2,5- dimethoxybenzaldehyde thiosemicarbazone (1), was quantitatively prepared by grinding 2,5-dimethoxybenzaldehyde and thiosemicarbazide together in a ball mill for 15 min. The excitation and emission maxima of compound 1 are 347 and 450 nm, respectively. The reaction of this ligand with Hg²⁺ was investigated by FT-IR, ¹H NMR, and fluorescence titration. Results show that the composition of the resulting Hg complex 1-Hg is 2:1 1:Hg, and that the S and imino N atoms serve as the binding sites of the ligand to the Hg ²⁺ ions. Coordination-assisted fluorescence quenching results show that compound 1 exhibits a highly selective fluorescence response to trace amounts of Hg²⁺ in water. More importantly, the resulting complex 1-Hg can be used as a turn-on fluorescence probe for I^- at a detection limit of 8.4 × 10^-8 M. Thus, compound 1 is a relatively stable, sequential, cyclic fluorescent probe for Hg²⁺ and I^-.

Full text of DOI:10.1016/j.saa.2014.05.067

Accepted Manuscript
A mechanosynthesized, sequential, cyclic fluorescent probe for mercury and
iodide ions in aqueous solutions
Shangwen Chen, Pipi Wang, Chunmei Jia, Qiang Lin, Wenbing Yuan
PII:
S1386-1425(14)00858-0
http://dx.doi.org/10.1016/j.saa.2014.05.067
SAA 12229
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
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Received Date:
Revised Date:
Accepted Date:
20 November 2013
15 March 2014
16 May 2014
Please cite this article as: S. Chen, P. Wang, C. Jia, Q. Lin, W. Yuan, A mechanosynthesized, sequential, cyclic
fluorescent probe for mercury and iodide ions in aqueous solutions, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.05.067
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A mechanosynthesized, sequential, cyclic fluorescent probe for mercury
and iodide ions in aqueous solutions
Shangwen Chen,a Pipi Wang,a Chunmei Jia,a Qiang Lin*b and Wenbing Yuan*a, b
aKey Laboratory of Tropical Biological Resources of the Ministry of Education, Department of Chemical
Engineering, Hainan University, 58 Renmin Road, Haikou, 570228, China. E-mail: wbyuan@hainu.edu.cn.
bSchool of Chemistry and Chemical Engineering, Hainan Normal University, 99 Longkun South Road,
Haikou 571158, China. E-mail: linqiang@hainnu.edu.cn
Abstract
A fluorescent Hg2+-selective chemosensor, 2,5-dimethoxybenzaldehyde thiosemicarbazone (1), was
quantitatively prepared by grinding 2,5-dimethoxybenzaldehyde and thiosemicarbazide together in a ball mill
for 15 min. The excitation and emission maxima of compound 1 are 347 and 450 nm, respectively. The
1
reaction of this ligand with Hg2+ was investigated by FT-IR, H NMR, and fluorescence titration. Results
show that the composition of the resulting Hg complex 1-Hg is 2:1 1:Hg, and that the S and imino N atoms
serve as the binding sites of the ligand to the Hg2+ ions. Coordination-assisted fluorescence quenching results
show that compound 1 exhibits a highly selective fluorescence response to trace amounts of Hg2+ in water.
More importantly, the resulting complex 1-Hg can be used as a turn-on fluorescence probe for I- at a
detection limit of 8.4×10-8 M. Thus, compound 1 is a relatively stable, sequential, cyclic fluorescent probe
for Hg2+ and I-.
Keywords: Fluorescence probe, mercury, iodide, mechanochemical reaction
1. Introduction
Mercury is highly toxic to humans and to the environment [1, 2]. By contrast, iodide (I-) is essential in
biological activities such as neurological and thyroid functions [3–6]. Thus, the design and synthesis of
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optical chemosensors with high selectivity for and sensitivity to Hg2+ and I- have attracted considerable
attention during the past decades. Most fluorescent probes for Hg2+ mainly consist of fluorophores, such as
fluorescein [7], coumarin [8–10], and rhodamine [11, 12], and receptors, such as macrocycles [13, 14] or
sulfur-containing compounds [15–17].
Meanwhile, comparatively few fluorescent probes for I- are currently available [5, 6, 18–22]. Turn-on I-
probes are particularly scarce [20–22]. On the one hand, finding an efficient acceptor that can specifically
bind to I- remains a major challenge because of anion hydration, particularly in aqueous solutions. On the
other hand, the heavy-atom effect of I- generally causes fluorescence quenching, which reduces the
sensitivity of I- detection. Metal coordination compounds are potential alternative detector materials that can
address the aforementioned difficulties. In these coordination complexes, I- replaces the original fluorescent
ligands, which are released into the solution. To date, copper [20, 23], silver [24], and mercury [22]
complexes have been used to detect I- via a turn-on fluorescence response.
In this study, we developed
a
new fluorescent chemosensor, 2,5-dimethoxybenzaldehyde
thiosemicarbazone (1), to detect Hg2+ in aqueous solutions. The resultant Hg2+ complex of this chemosensor
shows high sensitivity for I- detection. Interestingly, compound 1 exhibits high stability for Hg2+ detection
and for I- monitoring after numerous repeated use (Scheme 1).
Scheme 1. Design principle of the cyclic fluorescence probe 2,5-dimethoxybenzaldehyde thiosemicarbazone (1) for Hg2+ and I-
2. Experimental
2.1. Materials and Equipment
All reagents and solvents (reagent-grade) were purchased from Aladdin Reagent Inc. (Shanghai, China) and
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used as received without any further purification. Doubly distilled water was used throughout the
experiments unless otherwise noted. All fluorescence measurements were conducted on an F-280
spectrofluorometer (Tianjin Gangdong Sci. & Tech. Development. Co., Ltd., China) equipped with a xenon
lamp source and a 1.0 cm quartz cell. The scan rate was set to 1200 nm/min. All pH measurements were
performed on a pHS-3C digital pH-meter (Shanghai REX Instrument Factory, China) equipped with a
combined glass-calomel electrode. 1H NMR measurements were conducted using a Bruker AV 40.
2,5-Dimethoxybenzaldehyde and thiosemicarbazide were ground for 15 min at 20 Hz in a QM-3B shaking
mill (Nanjing Nanda Instrument Plant, China).
2.2. Mechanochemical synthesis of 1
2,5-Dimethoxybenzaldehyde (0.831 g, 5 mmol) and thiosemicarbazide (0.456 g, 5 mmol) were solvent-free
ground for 15 min at 20 Hz to obtain compound 1, which is a loose yellow powder, at 100% yield.
Comparatively, the previous solution-based synthesis of 1 [25] required 30 mL of ethanol (5 mmol of the
1
starting materials) and 5 h of reflux to achieve an 80% yield. H NMR (MeOH-d4, δ): 11.40 (s, 1H, =C–H),
8.38 (s, 1H, N–H), 8.17 (s, 1H, N–H), 8.06 (s, 1H, N–H), 7.64 (s, 1H, Ph–H), 6.98 (d, 2H, Ph–H), 3.78 (s, 6H,
–CH3). IR (KBr, cm-1): 3432, 3315, 3179, 3015, 2965, 2942, 2838, 1645, 1600, 1526, 1495, 1467, 1433,
1365, 1285, 1262, 1223, 1081, 1047, 1024, 947, 836, 709, 560.
2.3. Synthesis of complex 1-Hg
Hg(NO3)2·2H2O (0.180 g, 0.5 mmol) was slowly added to a stirred, hot ethanol solution of compound 1
(0.239 g, 1 mmol). Light-yellow precipitates were obtained. After filtration and washing with fresh hot
ethanol, the product was obtained as a yellowish solid, which was dried at room temperature under vacuum
for 2 d (0.076 g, 20%). IR (KBr, cm-1): 3457, 2999, 2933, 2834, 1668, 1626, 1600, 1576, 1493, 1383, 1283,
1220, 1179, 1041, 945, 809, 715. HRMS (Fig. S8): [Hg(1)2-H]+ = 679.1122, [Hg(1)2]2+ = 340.0592.
Different Hg(NO3)2·2H2O concentrations, namely, 0, 0.25, 0.50, and 0.75 equiv, were added to four
deuterated methanol solutions (0.5 mL each) of compound 1 (0.10 mmol). 1H NMR spectra (MeOH-d4, δ) of
1 + 0.25Hg2+, 1 + 0.5Hg2+ and 1 + 0.75Hg2+: 1 + 0.25Hg2+, 11.72 (s, 1H), 8.52 (s, 1H, NH2), 8.42 (s, 2H),
7.64 (s, 1H), 6.98 (t, 2H), 3.78 (s, 6H); 1 + 0.5Hg2+, 12.11 (s, 1H), 8.95 (s, 1H), 8.83 (s, 1H), 8.46 (s, 1H),
7.64 (s, 1H), 7.00 (t, 2H), 3.75 (s, 6H); 1 + 0.75Hg2+, 12.11 (s, 1H), 8.95 (s, 1H), 8.83 (s, 1H), 8.46 (s, 1H),
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7.64 (s, 1H), 7.00 (t, 2H), 3.75 (s, 6H).
2.4. General fluorescence measurements
A stock solution of 1.0 mM compound 1 was prepared by dissolving a specific mass of compound 1 in an
appropriate volume of ethanol. The concentration of each metal nitrate in water was 1.0×10-3 M.
Appropriate amounts of each metal salt solution were transferred into a series of 10 mL volumetric flasks.
Doubly distilled water was then added to dilute the solutions. Exactly 100 µL of 1.0 mM compound 1 was
added into each metal-salt stock solution using a micropipette, and the resulting solutions were thoroughly
mixed. To ensure that the reaction was complete, the mixtures were incubated at room temperature for 3 min.
For the fluorescence determination, the excitation and emission wavelengths were set at 347 and 450 nm,
respectively, and the corresponding slit width was 2 nm.
3. Results and discussion
3.1. Mechanosynthesis of 1
We used planetary and shaking ball mills to mechanosynthesize compound 1. Initially, we attempted to
perform the reaction in a planetary ball mill, but the starting materials remained unreacted even after 1 h of
grinding. The starting materials were also ground using the planetary ball mill via liquid- (or ion)-assisted
grinding in the presence of EtOH (75 µL), MeOH (75 µL), DMF (50 µL), or anhydrous Na2SO4 (50 mg).
However, complete reactions were not achieved, as indicated by the unreacted starting materials that
remained. Of the two types of ball mill, the shaking type provides higher mechanical energy for driving a
complete reaction. As expected, pure product 1 was obtained when the shaking ball mill was used. This
method did not require any solvents or salts. Mechanochemical synthesis is a greener and more efficient
approach and is thus preferable over solution-based synthesis because it does not require the use of bulk
solvents, heating, or long reaction times [26, 27].
3.2. Compound 1 fluorescence probe for Hg2+
We then determined the fluorescence of compound 1 and found that it showed strong fluorescence upon
excitation with a UV lamp (Scheme 1). We also investigated the stability of 1 in aqueous solutions with
different pH (2.0 to 12.0) under UV illumination. The results show that compound 1 is insensitive to the pH
4 / 15

within the pH range of 2 to 9 (Fig. S1, Supporting Information). When 1 was placed in solutions with pH > 9,
the fluorescence intensity significantly decreased, possibly because of the decomposition of the –C=N–
group.
Fig. 1. Changes in the fluorescence intensity of 2,5-dimethoxybenzaldehyde thiosemicarbazone (1,
1.0×10-5 M) at 450 nm and at an excitation wavelength of 347 nm in the presence of (a) 0.5 equiv Hg2+ or
10.0 equiv of other metal ions, and (b) Hg2+ and competing metal ions in water
After determining the stability of compound 1 in aqueous solutions, we investigated its fluorescence
properties in the presence of various metallic ions (Hg2+, Na+, K+, Mg2+, Ca2+, Cu2+, Co2+, Ni2+, Mn2+, Zn2+,
Cd2+, Pb2+, Fe3+, and Cr3+). The fluorescence spectra (λex = 347 nm) of 1 (1.0×10-5 M) were obtained in water
containing different metal cations [10 equiv of all metal ions, except for Hg2+ (0.5 equiv)] [Fig. 1(a)]. In the
absence of other metal ions, 1 showed strong fluorescence at 450 nm. However, when the stock solution of 1
was added to an aqueous solution containing 0.5 equiv Hg2+, the fluorescence changed, and the intensity
gradually decreased. By contrast, other metal ions did not induce significant changes in the fluorescence
intensity; Cu2+ and Fe3+ caused only slight fluorescence quenching. Competitive experiments involving Hg2+
and other metal ions [Fig. 1(b)] were also performed. Even in the presence of excess amounts of other metals
(10 equiv), Hg2+ (0.5 equiv) still significantly reduced the fluorescence of 1. These observations demonstrate
that 1 has high sensitivity to and selectivity for Hg2+.
5 / 15

Fig. 2. Fluorescence titration of 2,5-dimethoxybenzaldehyde thiosemicarbazone (1, 1.0×10-5 M) by adding an
aqueous Hg2+ solution (λex = 347 nm).
The fluorescence responses of 1 to selected metal ions were also observable by the naked eye using a UV
lamp (λem = 365 nm, Scheme 1; Fig. S2, Supporting Information). The reduction in fluorescence was
prominently observed only for Hg2+ (0.5 equiv). Most other metal ions (10.0 equiv) did not cause any
observable fluorescence changes and exhibited intensities similar to that of 1. The fluorescence titration of 1
(1.0×10-5 M) with Hg2+ was conducted in water. The presence of Hg2+ in an aqueous solution of 1 efficiently
reduced the fluorescence. Fig. 2 shows the fluorescence spectral changes of 1 with increasing amounts of
Hg2+. The presence of 0.5 equiv Hg2+ resulted in significant quenching of the fluorescence compared with
that of the metal-free solution but had no effect on the spectral shape. The quenched fluorescence intensity of
the system shows a highly linear relationship with lower Hg2+ concentrations (10-7 to 5.0×10-6 M, linearly
dependent coefficient: R2 = 0.999; Figs. 2 and S3, Supporting Information).
6 / 15

Fig. 3 Job’s plot for [1] + [Hg2+] = 1.0×10-5 M in water
The Job’s plot shown in Fig. 3 was prepared from the fluorescence spectra of 1 and Hg2+ at a total
concentration of 1.0×10-5 M. The plot reveals a 2:1 binding mode for 1 and Hg2+. The fluorescence
quenching can be explained by the formation of an Hg2+ complex of compound 1 as a result of the
MLCT-based heavy-metal ion effect [28]. Moreover, a previous study [29] concluded that the added Hg2+
forms a coordination complex with the S and N atoms of compound 1.
Fig. 4. 1H NMR spectra of 2,5-dimethoxybenzaldehyde thiosemicarbazone (1) (in MeOH-d4) in the presence
of different concentrations of Hg2+
3.3. 1H NMR and FT-IR studies
1
To provide further evidence for the interaction between 1 and Hg2+, we recorded the H NMR spectra (in
MeOH-d4, Fig. 4) of 1 in the presence of different Hg2+ concentrations. A set of peaks shifted as the Hg2+
concentrations increased from 0 equiv to 0.75 equiv. No further change was observed after the Hg2+
concentration exceeded 0.5 equiv. This result confirms that the mercury complex of compound 1 follows a
2:1 stoichiometry. Fig. 5 shows that upon the addition of Hg2+, the peaks assigned to Ha and Hb exhibit
observable downfield shifts (0.78 and 0.77 ppm, respectively). These shifts are attributed to the deshielding
effect caused by the reduced electron density, which resulted from the coordination of S to Hg2+. Meanwhile,
the Hd protons exhibit a dramatically downfield shift (0.71 ppm), which indicates that Hg2+ bonded to the N
7 / 15

atoms of the complex and induced a reduction in the electron density of Hd. Other protons of 1 exhibit
negligible responses. These changes in the proton chemical shifts indicate the binding mode of Hg2+ to 1, as
shown in Scheme 1.
We also determined the coordination modes of 1 with Hg2+ by comparing the IR spectra of 1 and 1-Hg.
The FT-IR spectra of the two compounds show significant differences, thus confirming that Hg2+ directly
coordinate to the N and S atoms of 1. The FT-IR spectra (Fig. S4, Supporting Information) show that after
the coordination of 1 with Hg2+, the vibrational mode of –C=N– in 1 shifted from 1645 cm-1 to 1627 cm-1.
This shift indicates direct binding between the N atom and Hg2+. In the mercury complexes, the shifting of
the IR band at 1224 cm-1, which corresponds to the C=S group of 1, to approximately 1220 cm-1 shows that
the S atom was coordinated to Hg2+ [25]. Furthermore, the strong peak that appeared at 1383 cm-1 in the IR
-
spectrum of 1-Hg is assigned to uncoordinated NO3 , which balanced the charge of Hg2+ and explains the
neutrality of the ligand. All these results indicate that 1-Hg has a clear coordination structure (Scheme 1),
1
which is consistent with the findings obtained from HRMS (Fig. S5, Supporting Information), Job’s plot, H
NMR and IR.
3.4. 1-Hg fluorescent probe for I-
Compound 1 shows strong fluorescence under UV irradiation. When Hg2+ was added into a solution of 1, a
1-Hg complex was obtained because of the coordination bond that formed between 1 and Hg2+. Moreover, 1
is a potential turn-off-type chemosensor for Hg2+. I- can coordinate to Hg2+ to form an insoluble salt, HgI2,
which has an extremely low solubility constant (Ksp = 3.2×10–29) [30]. We wanted to determine whether I-
would react with the Hg2+ of complex 1-Hg and simultaneously liberate the fluorescent molecule 1. The
occurrence of this reaction would indicate a linear relationship between the I- concentration and the
fluorescence changes of the resultant 1-Hg solution. This relationship could then be used in designing a
chemosensor for I- based on the precipitation of HgI2.
8 / 15

-
2-
Fig. 5. Fluorescence spectra of 1-Hg (5.0×10-6 M) in the presence of 2.0×10-5 M F-, Cl-, Br-, NO3 , SO4
,
-
-
H2PO4 , PO43-, CH3CO2 and I-
The fluorescence changes of the 1-Hg complex in the presence of a number of anions were systematically
examined to determine the performance of the complex in anion sensing. Fig. 6 shows the changes in the
fluorescence emissions of complex 1-Hg upon the addition of a series of anions (2 equiv of I-; 20 equiv of F-,
-
-
-
Cl-, Br-, NO3 , SO42-, H2PO4 , PO43- and CH3CO2 ) in aqueous solutions. Among the anions (Fig. 6), I- caused
a significant increase in fluorescence. This result indicates that the selectivity of the 1-Hg complex for I- is
high compared its selectivity for other anions. Br- caused an increase in fluorescence, but the enhancement is
considerably lower than that induced by I-.
In addition, we also investigated the effect of S2- on 1-Hg. As shown in Fig. S6 (Supporting Information),
sulfide ions caused a significant increase in fluorescence, which seriously interferes with the identification of
iodine ions. Meanwhile, Fig. S7 (Supporting Information) shows that S2- induces the fluorescence of 1-Hg
increasing faster than I-. Despite the presence of sulfide ions will interfere with the determination of iodide,
this does not prevent 1-Hg showing to be a good fluorescence probe for I- when sulfide ions don’t exist.
The effects of fluorescent 1 on the mixture of Hg2+-Cl-, Hg2+-Br-, Hg2+-I- and Hg2+-SCN- have also been
carried out. As depicted by Fig. S8 (Supporting Information), with the addition of compound 1, these four
mixtures reveal similar performances. Initially, four mercury mixtures quench the fluorescence intensity of 1,
however, with continued addition of 1, their fluorescence intensities start to increase. This is may be because
a trace amount of free Hg2+ coming from Hg2+-Cl-, Hg2+-Br-, Hg2+-I- and Hg2+-SCN- coordinate with 1,
which quenches the fluorescence of 1. Accordingly, when the concentration of the added compound 1
9 / 15

exceeds the concentration of free Hg2+, there is obvious fluorescence enhancement appeared.
Fig. 6. Fluorescence spectra of 1-Hg (5.0×10-6 M) in the presence of I-. The inset figure shows the changes in
the fluorescence spectra of 1-Hg (5.0×10-6 M) upon the addition of I- (0-1.0×10-6 M) in an aqueous solution.
The excitation and emission maxima are 347 and 450 nm, respectively.
Table 1 The limit detection of I- for the known chemsensors
Compound
Mechanism
A limit of detection (mol/L) Reference
8.4×10-8
Thiosemicarbazone-Hg2+ Formation of HgI2
This work
DQAg-Ag+
F-L-Cu2+
Formation of HgI2
Redox reaction
7.16× 10-6
1.07× 10-7
24
20
Tripodal
benzimidazole-based
compound
Receptor
2.1× 10-6
6
Potassium iodate
Redox reaction
7.4 4 × 10−7
33
The 1-Hg complex shows high specificity for I-. Thus, we further investigated the sensitivity of the 1-Hg
complex to I- in aqueous solutions. A fluorescence titration experiment was performed to determine the
interaction between I− and the 1-Hg complex. Fig. 7 shows that the fluorescence intensities gradually
increased as the I- concentration was increased from 1.0×10−7 M to 5.0×10-5 M. In addition, at higher I-
concentrations (8.0×10-6 M to 4.0×10-5 M), the changes in the relative fluorescence intensities show a curved
trend. The I- concentration at the turning point was 4.0×10−5 M, which is indicative of a 13-fold increase in
the fluorescence intensity. The enhanced fluorescence intensity of the system shows a highly linear
relationship with lower I- concentrations (10-7 M to 8.0×10-6 M, linearly dependent coefficient: R2 = 0.993,
Fig. 7). Moreover, the detection limit is 8.4×10-8 M (Fig. 7, inset). This extremely low detection limit (10-8) is
10 / 15

rarely achieved by currently available chemosensors (Table 1) [6, 20, 21, 24, 31–33].
All these results show that compound 1 is a highly accurate and highly selective ratiometric sensor for
Hg2+ in aqueous solutions. Moreover, its resultant complex 1-Hg has proven to be a unique fluorescence
probe for I- with an extremely low detection limit. Interestingly, the resultant mixture can be used to detect
Hg2+ and I- repeatedly (Scheme 1). The recyclability of 1 upon the addition of Hg2+ and subsequent I- (Fig. 8)
shows that 1 has a relatively high sensor stability in aqueous solutions. Moreover, as a fluorescent probe for
I-, compound 1-Hg is superior to the carbazole–thiosemicarbazone–Hg designed by Mahapatra [22] in terms
of the detection limit and of the linear relationship between the changes in the fluorescence intensity and the
I- concentration.
Fig. 7. Recyclability of 1 (1.0×10−6 M) as indicated by the fluorescence spectral changes induced by the
addition of Hg2+ (1.0×10−6 M) and subsequent I- (2.0×10−6 M) in aqueous solutions.
4. Conclusions
We presented a new thiourea-based fluorescent probe 1, which is readily obtained via ball milling.
Compound 1 shows higher selectivity for Hg2+ over other competitive ions in aqueous solutions, and its
fluorescence intensity responding to Hg2+ linearly in the micromolar level concentration range. The detection
limit of the probe is 1.85×10−7 M, which indicates that compound 1 can be used as an efficient Hg2+-selective
turn-off fluorescent chemosensor. The resultant complex 1-Hg was then designed for the selective detection
of I- in aqueous solutions. The results show that 1-Hg has high selectivity for and sensitivity to I- based on
the extremely low detection limit of 8.4×10-8 M. Therefore, compound 1 is an excellent bifunctional, cyclic
fluorescence probe for Hg2+ and I- that can be used in routine analyses.
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Acknowledgements
We would like to acknowledge the NSFC (Grant Nos. 21061005 and 21241008), the
International Joint Project of Hainan Province (Grant No. Hjkj2008-23), and Hainan University Foundation
(Grant No. hd12xm02) for their financial support.
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Highlights
A mechanosynthesized, sequential, cyclic fluorescent probe for mercury
and iodide ions in aqueous solutions
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A fluorescent Hg2+ chemosensor (1) is quantitatively prepared by mechanochemistry.
1 shows a highly selective fluorescent response to trace amounts of Hg2+ in water.
The resulting complex 1-Hg is a highly selective fluorescence probe for I-.
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Graphical abstract
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