A new thiosemicarbazone fluorescent probe based on 9,9′-bianthracene for Hg2+ and Ag+

Article abstract of DOI:10.1016/j.saa.2019.02.036

A new 9,9′-bianthracene-based thiosemicarbazone (D1) has been successfully synthesized and utilized for chemosensors. The properties of D1 were systematically investigated by UV–Vis, fluorescence titration and theoretical calculations. As a result, D1 exhibits a characteristic fluorescence quenching phenomenon in the presence of Hg²⁺ or Ag⁺ compared to other metal cations (Na⁺, K⁺, Mg²⁺, Ba²⁺, Al³⁺, Zn²⁺, Fe²⁺, Pb²⁺, Cu²⁺, Co²⁺, Cd²⁺, Ni²⁺ and Mn²⁺). The detection limits of Hg²⁺ and Ag⁺ reach 6.62 × 10^?7 M and 1.99 × 10^?5 M, respectively. This is mainly attributed to the Hg²⁺ (or Ag⁺) forms a stable five-membered ring with the N atom in Schiff base C[dbnd]N and the S atom in thiourea. The results suggest that the probe D1 is a promising candidate for chemosensors in aqueous media due to its highly selectivity for Hg²⁺ and Ag⁺.

Full text of DOI:10.1016/j.saa.2019.02.036

Accepted Manuscript
A new thiosemicarbazone fluorescent probe based on 9,9′-
bianthracene for Hg2+ and Ag+
Zhen-E Chen, Hai Zhang, Zafar Iqbal
PII:
DOI:
Reference:
S1386-1425(19)30155-6
https://doi.org/10.1016/j.saa.2019.02.036
SAA 16812
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Received date: 22 July 2018
Revised date:
Accepted
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21 January 2019
16 February 2019
Please cite this article as: Z.-E. Chen, H. Zhang and Z. Iqbal, A new thiosemicarbazone
fluorescent probe based on 9,9′-bianthracene for Hg2+ and Ag+, Spectrochimica Acta Part
A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.02.036
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ACCEPTED MANUSCRIPT
A new thiosemicarbazone fluorescent probe based on 9,9-
bianthracene for Hg2+ and Ag+
Zhen-E Chena,*, Hai Zhanga, Zafar Iqbalb
a School of Chemistry and Chemical Engineering, Qianbei Featured Resources Applied Research Key Laboratory,
Zunyi Normal College, Guizhou Zunyi 563006, China
bApplied Chemistry Research Centre, PCSIR Laboratories Complex, Feroze pur Road Lahore 54000, Pakistan
*Corresponding Author: Tel.: +86 0851 28924799. Fax: +86 0851 28924799. Email:
zhenechen@vip.sina.com
ABSTRACT: A new 9,9-bianthracene-based thiosemicarbazone (D1) has been successfully
synthesized and utilized for chemosensors. The properties of D1 were systematically investigated
by UV–Vis, fluorescence titration and theoretical calculations. As a result, D1 exhibits a
characteristic fluorescence quenching phenomenon in the presence of Hg2+ or Ag+ compared to
other metal cations (Na+, K+, Mg2+, Ba2+, Al3+, Zn2+, Fe2+, Pb2+, Cu2+, Co2+, Cd2+, Ni2+ and Mn2+).
The detection limits of Hg2+ and Ag+ reach 6.62 × 10−7 M and 1.99 × 10−5 M, respectively. This is
mainly attributed to the Hg2+ (or Ag+) forms a stable five-membered ring with the N atom in Schiff
base C=N and the S atom in thiourea. The results suggest that the probe D1 is a promising candidate
for chemosensors in aqueous media due to its highly selectivity for Hg2+ and Ag+.
Keywords: Fluorescent probe, Thiosemicarbazone, 9,9'-Bianthracene, Hg2+ and Ag+
1. Introduction
As important industrial raw materials, mercury and silver have an irreplaceable role in the modern
chemical industry [1-3], especially in the fields of chloralkali industry, goldmining, pharmaceutical
industry, photographic imaging technology, catalytic technology, semiconductors and so on [4-6].
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However, with the development of industry, their pollution problems have become increasingly
prominent, which poses a serious threat to human health and the earth's ecosystem. Excessive
exposure to mercury and its derivatives can cause serious heart diseases, as well as disorders of the
brain, kidney and central nervous system [7-13]. In addition, silver ions can also inactivate the
sulphur-containing enzymes in the human body and interact with active substances such as amines,
imidazoles, and carboxylic acids [14], resulting in various diseases. What is even more worrying is
that they can also be enriched in living organisms through the food chain, which will seriously
threaten human health [6, 15-17]. For these reasons, it is highly imperative to develop methods for
rapid detection of Hg2+ and Ag+.
So far, many methods such as atomic absorption spectrometry [18], inductively coupled plasma
mass spectrometry [19], atomic emission spectroscopy [20], cyclic voltammetry [21], extraction
spectrophotometry and fluorescence probes [22, 23] have been used for rapid detection of Hg2+ and
Ag+. Among these reported methods, fluorescence probes have drawn increasing attention owing to
their advantages of high sensitivity and excellent selectivity, low detection limit, convenience,
cheapness and quickness etc. Although many fluorescence probes for Hg2+ [5, 12, 24-31] and Ag+
[23, 32-42] detection have been reported, there are still few studies on the simultaneous detection
of these ions by a single probe. Due to the suitable bandwidth, high fluorescence quantum efficiency
and good thermal stability, 9,9'-bianthracene has been widely used in blue light materials. However,
as far as we know, fluorescent probes based on 9,9'-bianthracene for Hg2+ or Ag+ detection have not
been reported. Considering the special spatial structure where the two anthracene rings are
perpendicular to each other, this helps to reduce the aggregation of molecules derived from the
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intermolecular ππ interactions as well as to maintain the stability of fluorescence, so the 9,9'-
bianthracene was selected as the fluorophore group. Besides, based on the excellent properties
exhibited in the detection of Hg2+ [43-52] and Ag+ [53-55], thiosemicarbazone was selected as the
recognition group. Finally, we designed and synthesized a new 9,9'-bianthracene based fluorescent
probe (D1) for the selective detection of Hg2+ and Ag+. Its absorption spectra, fluorescence emission
spectrum and sensing behavior have been investigated in detail.
2. Experimental section
2.1. Apparatus
1H and 13C NMR spectra were recorded on a Bruker Avance 600 MHz spectrometer in CDCl3 or
DMSO with tetramethylsilane as inner reference. The absorption and emission spectra in N,N-
dimethylformamide solution were measured at room temperature on a SHIMADZU UV-3600 Plus
spectrophotometer and Hitachi F-4600 fluorescence spectrophotometer. HRMS data were recorded
on a Finnigan MAT 95 XP mass spectrometer (Thermo Electron Corporation).
2.2. Reagents
THF was distilled under an argon atmosphere over sodium metal for complete dryness. Other
reagents were purchased from commercial sources and used without further purification. All
inorganic salts solutions of KNO3, Na(OAc)2, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, ZnSO4·7H2O,
FeSO4, Pb(NO3)2, Cu(NO3)2·3H2O, HgCl2, AgNO3, Co(NO3)2·6H2O, Cd(NO3)2·4H2O,
NiSO4·6H2O, MnSO4·H2O and Ba(NO3)2 were dissolved in distilled water and respective solutions
−
2−
were prepared. The solutions of anions (Cl−, Br−, I−, CO32−, HCO3 , C2O42−, CH3COO−, SCN−, SO4
,
−
SO32−, PO43− and NO3 ) were prepared from their Na+ or K+ salts. The solution of D1 was prepared
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in DMF.
2.3. Syntheses
2.3.1. Synthesis of 4,4'-([9,9'-bianthracene]-10,10'-diyl)dibenzaldehyde (4):
To a mixture of 10,10'-dibromo-9,9'-bianthracene (0.948 g, 1.85 mmol), 4-formylphenylboronic
acid (0.696 g, 4.64 mmol), and tetrakis(triphenylphosphine)palladium (0) (0.214 g, 0.185 mmol) in
THF (50 mL), 2 M potassium carbonate aqueous solution (10 mL, 20 mmol) was added. The mixture
was stirred and heated to reflux under argon for 24 h. After the mixture was cooled to ambient
temperature, the reaction mixture was poured into water and extracted with DCM for three times.
The combined extracts were washed with brine, dried over anhydrous MgSO4, and then filtered.
Evaporation of the solvent was carried out under reduced pressure. Crude product was purified by
chromatography on silica gel using petroleum ether/dichloromethane/ethyl acetate (40/20/1, v/v) as
eluent and dried under vacuum to afford a yellow solid in 90% yield (936 mg). 1H NMR (600 MHz,
CDCl3) δ ppm 10.27 (s, 2H, CHO), 8.23 (d, J = 7.8 Hz, 4H, ArH), 7.85 (d, J = 7.8 Hz, 4H, ArH),
7.75 (d, J = 9 Hz, 4H, ArH), 7.397.36 (m, 4H, ArH), 7.26 (d, J = 8.4 Hz, 4H, ArH), 7.217.19 (m,
4H, ArH). 13C NMR (150 MHz, CDCl3) δ ppm 192.17, 146.10, 136.37, 135.96, 134.12, 132.46,
131.39, 130.06, 129.78, 127.28, 126.78, 125.94. HRMS (ESI, m/z): [M + H]+ calcd for [C42H27O2]+:
563.2006, found: 563.2053.
2.3.2. Synthesis of (2E,2'E)-2,2'-(([9,9'-bianthracene]-10,10'-diylbis(4,1-phenylene))bis(methanyl-
ylidene))bis(hydrazine-1-carbothioamide) (D1):
4,4-([9,9-Bianthracene]-10,10-diyl)dibenzaldehyde (4) (0.281 g, 0.5 mmol), thiosemicarbazide
(0.091 g, 1.0 mmol) and 25 mL ethanol were added to round-bottom flask. Then glacial acetic acid
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(0.5 mL) was added to the flask. The reaction mixture was stirred at room temperature for 48 h. The
precipitates were collected by centrifugation. The pure product was obtained by repeatedly washing
with dichloromethane and centrifugation, as a yellow solid in 74.3% yield (263 mg). The solubility
of D1 was DMF > DMSO > THF > CH3CN in different solvents. 1H NMR (600 MHz, DMSO) δ
ppm 11.55 (s, 2H, NNHC=S), 8.26 (s, 4H, NH2), 8.138.12 (m, 6H, CH=N and ArH), 7.717.63
(m, 8H, ArH), 7.397.37 (m, 4H, ArH), 7.227.20 (m, 4H, ArH), 7.04 (d, J = 8.4 Hz, 4H, ArH).
13C NMR (150 MHz, DMSO) δ ppm 178.10, 141.94, 139.64, 137.03, 133.87, 132.69, 131.47,
130.65, 129.33, 127.60, 126.72, 126.37, 126.10, 125.84. HRMS (ESI, m/z): [M + H]+ calcd for
[C44H33N6S2]+: 709.2208, found: 709.2180.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis route of (2E,2E)-2,2'-(([9,9-bianthracene]-10,10-diylbis(4,1-phenylene))-
bis(met-hanylylidene))bis(hydrazine-1-carbothioamide) (D1) has been exhibited in Scheme 1. The
intermediate compound 10,10-dibromo-9,9-bianthracene (3) was synthesized from compound
anthraquinone according to the same procedure as reported literature [56]. The fluorescence probe
D1 was synthesized by the condensation reaction of 4,4-([9,9'-bianthracene]-10,10-diyl)
13
dibenzaldehyde (4) and thiosemicarbazide. All structures were confirmed by 1H NMR, C NMR
and HRMS analyses (Fig. S1–S5).
Scheme 1.
3.2. Optical properties of D1
3.2.1. Optimization of experimental conditions
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In order to get the best selectivity of fluorescence probe D1, further experiments must be carried
out in order to establish detection environmental conditions. According to the solubility trend of D1,
DMF was selected as the solvent. Fig. S6 displays optical properties of D1 with different ratio of
DMF and water. With the fraction of DMF in mixtures less than 90%, the fluorescence intensity of
D1 decreases significantly. Therefore, the mixture of DMF/H2O (9/1, v/v) was selected as the
optimized solvent systems for further detection experiments. Optical responses of D1 (5.0 × 10−6
M) toward multifarious metal ions and anions were investigated in the medium. D1 has a strong
characteristic fluorescence emission peak at 454 nm originated from 9,9-bianthracene. Obvious
fluorescence quenching of D1 can be observed in the presence of Cu2+, Hg2+, Ag+, Cd2+, and the
quenching degrees caused by Ag+, Hg2+ and Cu2+ are much higher than that brought by Cd2+ (Fig.
S7a). All of anions don’t bring obvious influence to the emission of D1 (Fig. S7b). Interestingly, the
fluorescence quenching by Cu2+ can be blocked by adjusting the pH value of probing medium to
3.0 (through the introduction of HNO3), but fluorescence quenching by Hg2+ and Ag+ do not seem
to be influenced by the alteration of pH (Fig. S8). We believe that this may be due to the strong
interaction between D1 and Hg2+ or Ag+ which is not easily affected by the protonation of amino
groups, and can form a stable chelate structure. It is interesting that the case of Cu2+ is just the
opposite. The higher pH value in the D1 solution, the better deprotonation of the amino group and
therefore promoting the binding of D1 to Cu2+ which resulting in the fluorescence quenching.
Correspondingly, as the pH decreases, the N atom of C=N and the NH2 become more and more
difficult to deprotonate. It makes the binding of D1 and Cu2+ gradually weakened and eventually
leads to the fluorescence quenching not occurring. Based on these results, D1 can be used as a
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fluorescent probe for Hg2+ and Ag+ as well as DMF/H2O (V/V, 9/1) (pH = 3.0) is selected as the
testing medium for the further experiments.
Fig. 1.
Fig. 2.
3.2.2. Selectivity over metal ions
Selectivity of D1 to multifarious metal ions have been investigated by UV–Vis absorption and
fluorescence emission spectroscopy at room temperature in DMF/H2O (9/1, v/v, pH = 3.0). The
addition of Hg2+ and Ag+ (10 eq.) make obvious changes in the UV–Vis absorption spectra and the
fluorescence spectra. Meanwhile, other metal cations such as K+, Na+, Mg2+, Al3+, Zn2+, Fe2+, Pb2+,
Cu2+, Co2+, Cd2+, Ni2+, Mn2+ and Ba2+ do not show significant impact (see Fig.1 and 2). The
absorption spectra of D1 shows characteristic absorption of the anthracene group (λmax = 360, 381
and 403 nm), respectively, which can be ascribed to the superposition of the absorption generated
by the ππ* transition and the aromatic ring vibration transition. With the addition of Hg2+ or Ag+,
the absorption peaks at 328 nm attributed to the ππ* transition of C=N exhibit a blue shift and a
significant decrease, indicating that Hg2+ or Ag+ reacts with the thiosemicarbazide group to form a
stable complex.
3.3. Titration experiment of D1 toward Hg2+
In order to further understand the binding affinity of D1 and Hg2+, titration experiment was carried
out (Fig.3). With the addition of incremental Hg2+ (0–10 eq.) to the solution of D1, the absorption
spectra at 328 nm and fluorescence intensity of D1–Hg2+ solution at 456 nm decreased gradually
(Fig. 3a-3b). Corresponding calibration curve of the fluorescence is presented in Fig.3c as a function
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of the Hg2+ concentration. The detection limit (LOD) of D1 for Hg2+ is calculated to be as low as
6.62 × 10−7 M based on 3σ criteria [57].
Fluorescence competition experiments in D1 solution were conducted in the presence of 10 eq.
of various other metal ions and Hg2+ (Fig. 4). The results suggest that the presence of other metal
ions brings insignificant interference to the detection of Hg2+, which means that the fluorescence
probe D1 has an excellent selectivity towards Hg2+.
Fig. 3.
Fig. 4.
3.4. Titration experiment of D1 toward Ag+
Ag+ titration experiment of fluorescence probe D1 was carried out similarly to Hg2+ (Fig.5). The
results are similar to Hg2+ (Fig. 5a-5b). The detection limit (LOD) of D1 for Ag+ is calculated from
the plot of emission intensity change (Fig. 5c) and it is calculated as 1.99 × 10−5 M based on 3σ
criteria. It can be seen that the detection sensitivity of D1 to Ag+ is slightly lower than that of D1 to
Hg2+.
Furtherly, competitive experiments with Ag+ and other ions have also been completed. Similarly,
the presence of other metal ions does not significantly interfere with the detection of Ag+ (Fig. 6).
Schiff base D1 displayed an excellent selectivity towards Ag+, the emission at 456 nm decreases
and an intense peak at 332 nm (Fig. 7) is formed, which may be attributed to the coordination of
Ag+ with heteroatoms resulting in a weaker electron donating ability and intramolecular charge
transfer capability.
Fig. 5.
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Fig. 6.
Fig. 7.
3.5. Recognition mechanism
In order to confirm the complexation of the Schiff base D1 with Hg2+ (or Ag+), the job curve
of the probe D1 and Hg2+ (or Ag+) coordination was made in the experiment. The total concentration
of probe D1 and Hg2+ (or Ag+) is fixed to 100 μmol/L, and the fluorescence intensities were recorded
by varying the mole ratio of Hg2+ (or Ag+). The result indicates the stoichiometry ratio between D1
and Hg2+ (or Ag+) is 1:2 (Fig. S9), which can be further verified by the experimental data from
fluorescent titration, namely, by the extended Benesi-Hildebrand equation [58-60] based on 1:2 and
a linear correlation coefficient of 0.99 can be obtained (Fig. S10). The complexing constants
between D1–Hg2+and D1–Ag+ have also been calculated to be 2.29 × 1010 M−2 and 5.36 × 108 M−2
according
to
the
extended
Benesi–Hildebrand
equation
is
shown
below:
1
1
1
.
I0  Imin



n 2

I0  I
K I  I
M
a 
min 
0

In the formula, I0 and I represent the fluorescence intensity of the absence and presence of Hg2+
(or Ag +), Imin represents the minimum of the fluorescence intensity, Ka is the complexing constant.
Energy level of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) play an important role in studying the photoelectric properties of organic
conjugated molecules. To insight into the geometrical and electronic distribution of D1, density
functional theory (DFT) calculations were performed with Gaussian 09 program using B3LYP/6-
31G(d) level. As shown in Fig.8, the HOMO is distributed mainly on 9,9-bianthracene, the LUMO
spreads around the linkage between thiosemicarbazide and bianthracene. HOMO–LUMO excitation
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partly moves the electron density distribution from 9,9-bianthracene unit to the thiosemicarbazide
unit, thus leading to weak fluorescence. The energy gap between HOMO and LUMO is calculated
to be 3.39 eV.
Fig. 8.
Based on the above experimental results, we believe that the fluorescence quenching phenomenon
is mainly due to the fact that Hg2+ (or Ag+) forms a stable five-membered ring with the S atom and
the N atom of C=N in D1. The corresponding possible mechanism is graphically depicted in Scheme
2.
Scheme 2.
4. Conclusion
In summary, a new fluorescence probe (D1) employing 9,9-bianthracene and thiosemicarbazone
as skeletons has been designed and synthesized successfully. The results show that Hg2+ or Ag+ can
cause fluorescence quenching of D1 in DMF/H2O (9/1, v/v) (pH = 3.0) solution, which indicates
that D1 can be used to qualitatively detect the presence of these two metal ions, especially, D1 has
high selectivity and sensitivity towards to Hg2+. When Ag+ is absent, the Hg2+ content can be
quantitatively detected, and the detection limit is as low as 6.62 × 10-7 M; when Ag+ is present, Hg2+
can also be determined by adding a masking agent. This work clearly demonstrates that the
presented probe D1 possess the unique optical response to Hg2+ and Ag+, which provides an
alternative direction for the development of fluorescent probes of high selectivity and stability to
Hg2+ and Ag+ in aqueous phase.
Acknowledgements
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We are grateful to the National Natural Science Foundation of China (21462054), the Science
and Technology Cooperation Project of Guizhou Province, China (LH[2015]7048), the
Academician Workstation of Zunyi Normal College of Guizhou Province, China (LH[2015]4003),
the Science and Technology Cooperation Project of Zunyi City, China ([2016]3) for the financial
support.
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Figure Captions
Scheme 1. The synthesis route of fluorescent probe D1
Fig. 1. UV–Vis absorption spectra of D1 (~5 × 10−6 M) upon addition of 10 eq. of various metal
ions in DMF/H2O (9/1, V/V, pH = 3.0).
Fig. 2. Fluorescence emission spectra of D1 (~5 × 10−6 M) upon addition of 10 eq. of various metal
ions in DMF/H2O (9/1, V/V, pH = 3.0) (λex: 330 nm). Inset: fluorescence color change of the solution
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of D1 and D1 induced by Hg2+ or Ag+ under a portable UV lamp (365 nm).
Fig. 3. UV–Vis (a) and fluorescence (b) titration spectra of D1 (~5 × 10−6 M) in DMF/H2O (9/1,
V/V, pH = 3.0) with various concentrations of Hg2+, and the linear relationship between I0/I and
[Hg2+] (c) (Inset is the corresponding enlarged region with [Hg2+] from 5 × 10−7 to 20 × 10−6 M)
(λex: 330 nm).
Fig. 4. Fluorescent intensity ratio (I/I0) of the competition experiments between Hg2+ and other
metal ions in DMF/H2O (9/1, V/V, pH = 3.0) solution with an emission of 456 nm (λex: 330 nm).
Fig. 5. UV–Vis (a) and fluorescence (b) titration spectra of D1 (~5 × 10−6 M) in DMF/H2O (9/1,
V/V, pH = 3.0) with various concentrations of Ag+, and the linear relationship between I0/I and [Ag+]
(c) (Inset is the corresponding enlarged region with [Ag+] from 3 × 10−5 to 5 × 10−5 M) (λex: 330
nm).
Fig. 6. Fluorescent intensity ratio (I/I0) of the competition experiments between Ag+ and other metal
ions in DMF/H2O (9/1, V/V, pH = 3.0) solution with an emission of 456 nm (λex: 330 nm).
Fig. 7. Fluorescence emission spectra of D1 (~5 × 10−6 M) in DMF/H2O (9/1, V/V, pH = 3.0) upon
addition of 10 eq. of Hg2+ and Ag+.
Fig. 8. Isodensity surface plots of the HOMO-1, HOMO, LUMO and LUMO+1 of D1 (isodensity
value = 0.02).
Scheme 2. The proposed mechanism of D1 toward Hg2+ (or Ag+).
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Highlights
1、 A 9,9'-bianthracene-based fluorescent probe (D1) with special spatial structure has
been designed and synthesized.
2、 The probe D1 is highly selective in sensing Hg2+ and Ag+ with minimal influence from
other common metals.
3、 “Turn-off” fluorescence alterations of D1 are observed upon the addition of Hg2+ or Ag+
in the aqueous medium.
4、The detection limits of Hg2+ and Ag+ reach ∼ 0.66 μM and 19.9 μM, respectively.
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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8