β-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water

Article abstract of DOI:10.1016/j.saa.2014.09.111

A novel sensing system based on the β-carboline core has been designed and synthesized for Hg²⁺ detection in water. We have demonstrated that a straight forward methodology can provide rapid, sensitive and selective recognition (cross-contamination experiments) for Hg²⁺ over a wide pH range. The vivid fluorescence change from blue to colorless can be clearly discriminated by the naked eye. Furthermore, there is a good negative correlation between the fluorescent intensity and the concentration of Hg²⁺ in the range 1.0 × 10^-6 M-7.0 × 10^-6 M. β-Carboline as a fluorophore synthesized via this route also provides a new strategy for the design of novel fluorescence probes and fluorochromes.

Full text of DOI:10.1016/j.saa.2014.09.111

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
2+
b-Carboline-functionalized dithioacetal as Hg -selective fluorescence
probe in water
1
1
⇑
Na Li , Jiang-Kun Dai , Hong-Tao Du, Mao-Sen Yuan, Ji-Wen Zhang, Jun-Ru Wang
College of Science, Northwest A&F University, Yangling 712100, Shaanxi, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
A water-soluble fluorescence probe
has been designed and synthesized.
The probe was highly effective for the
2
+
detection and discrimination of Hg
b-Carboline as a fluorophore is
significant for the design of novel
probes.
.
a r t i c l e i n f o
a b s t r a c t
A novel sensing system based on the b-carboline core has been designed and synthesized for Hg2+ detec-
Article history:
Received 15 August 2014
Received in revised form 24 September 2014
Accepted 25 September 2014
Available online xxxx
tion in water. We have demonstrated that a straight forward methodology can provide rapid, sensitive
2+
and selective recognition (cross-contamination experiments) for Hg over a wide pH range. The vivid
fluorescence change from blue to colorless can be clearly discriminated by the naked eye. Furthermore,
2+
there is a good negative correlation between the fluorescent intensity and the concentration of Hg in
À6
À6
the range 1.0 Â 10 M–7.0 Â 10 M. b-Carboline as a fluorophore synthesized via this route also pro-
vides a new strategy for the design of novel fluorescence probes and fluorochromes.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
b-Carboline
Dithioacetal
Fluorescent probe
Mercury ion
Aqueous solution
Introduction
of diseases [1–3]. The mercury ion Hg2+, one of the more severe
environmental pollutants, is very harmful to the human body [4].
2
+
Today, with increasingly rapid industrialization in many parts
of the world, more and more heavy metal ions are being discharged
into ecosystem. Bioaccumulation through the food chain has
become a serious threat to human life and health. Chronic expo-
sure to high levels of these heavy metal ions can lead to a series
When absorbed, Hg damages the brain, kidneys, and endocrine
2+
system [5]. Therefore, the rapid and selective detection of Hg is
very important. Atomic fluorescence spectrometry [6,7], cold vapor
atomic fluorescence spectrometry [8], and gas or liquid chromatog-
raphy [9,10] are usually used to quantitatively analyze Hg2+ con-
tent. However, all of these methods have complicated
procedures, high costs, or low mobility. Significant efforts have
been devoted to discovering effective fluorescence probes for
⇑
Corresponding author. Tel./fax: +86 29 8709 2829.
E-mail address: wangjunru@nwsuaf.edu.cn (J.-R. Wang).
First two authors contributed equally to this work.
2
+
1
Hg , owing to the fluorescent probe method to detect analytes,
http://dx.doi.org/10.1016/j.saa.2014.09.111
386-1425/Ó 2014 Elsevier B.V. All rights reserved.
1
Please cite this article in press as: N. Li et al., b-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.111

2
N. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
is of outstanding sensitivity, neither invasive nor damaging, and
has a short response time [11–19]. In recent years, many excellent
sensors have been developed for the detection of Hg2 [20–27].
However, most usually require complicated syntheses, expensive
instruments and are insoluble in aqueous solutions [28,29]. The
design and synthesis of new fluorescent probes that display a sen-
sitive response to Hg in water remain important goals.
b-Carboline, with the common tricyclic pyrido[3,4-b]indole ring
structure, has been widely used in the study of antitumor drugs
DMSO-d
122.17, 120.98, 119.04, 114.48, 111.00. EI-MS: m/z = 169.06 (calcu-
lated 168.07 for C11 ).
6
) d: 143.07, 141.14, 137.42, 136.30, 129.22, 127.19,
+
8 2
H N
Synthesis of 4-(3-bromopropoxy)benzaldehyde(4) [36]
2
+
A mixture of p-hydroxybenzaldehyde (610 mg, 5.0 mmol), 1,3-
dibromopropane (1.00 mL, 10.0 mmol), K
tetrabutylammonium hydrogen sulfate (34 mg, 0.1 mmol), and KI
83 mg, 0.5 mmol) in acetonitrile (20 mL) was refluxed for 8 h.
2 3
CO (690 mg, 5.0 mmol),
[
30–33]. It also has another merit, that of being a good luminophor,
(
which derives from its aromatic planar configuration; however,
this property has been largely ignored. To the best of our knowl-
edge, the study of b-carboline derivatives as luminous molecules
The filtrate was collected and concentrated under reduced pres-
sure to afford a residue, which was purified by column chromatog-
raphy over silica gel eluting with petroleum ether/ethyl acetate
2+
has not yet been reported. Recently, dithioacetal as an Hg -recog-
nizing group has been developed, and exhibited high sensitivity
(
10:1, v/v; 6:1, v/v) to give 982 mg (81%) of compound 4 as a white
1
solid. CAS Registry Number: 17954-81-3. H NMR (500 MHz,
CDCl ) d: 9.94 (s, 1H), 8.11 (d, J = 5 Hz, 1H) 7.89 (d, J = 5 Hz, 1H),
.03 (d, J = 5 Hz, 2H), 4.25 (t, J = 5 Hz, 2H), 3.66 (t, J = 5 Hz, 2H),
2
+
and selectivity for Hg [25]. However, the analyses were con-
ducted only in organic solutions. Herein, we have designed a
3
7
2
1
2+
water-soluble chemosensor for Hg by incorporating a b-carboline
fragment with a dithioacetal that contains a hydrophilic carboxylic
group. This compound exhibited a rapid and sensitive response to
13
.37–2.42 (m, 2H). C NMR (125 MHz, CDCl
32.45, 130.06, 114.87, 114.25, 121.81, 65.69, 32.16, 29.62. EI-
11BrO ).
3
) d: 190.80, 132.42,
MS: m/z = 241.98, 243.99 (calculated 243.10 for C10
H
2
2
+
Hg in aqueous solution, accompanied by a distinct color and fluo-
rescence change.
Synthesis of 4-(3-(9H-pyrido[3,4-b]indol-9-yl)propoxy)benzaldehyde
5) [37]
(
Experimental
A mixture of compound 2 (336 mg, 2.0 mmol), NaH (144 mg,
All solvents and reagents were obtained from commercial
6.0 mmol) and dry N,N-dimethylformamide (DMF, 20 mL) was stir-
red in an ice bath for 30 min, and then compound 4 (532 mg,
2.2 mmol) was added to the solution. After reacting for 24 h at
RT and extraction with dichloromethane (DCM), the organic phase
was dried over Na SO . The solvent was removed, and the residue
sources without further purification. HgCl
2
, FeCl
O, Ba(CH COO)
Á8H O, and FeSO
3
Á6H
2
O, CuSO
Á7H O,
Á8H
4
,
CH
3
COOAg, MgSO
4
Á7H
2
O, SrCl
2
Á6H
2
3
2
, ZnSO
4
2
3
Al(NO )
3
Á7H O, (CH
2
3
COO)
2
PbÁ3H
2
O, CdSO
4
2
4
2
O
were purchased from the Aladdin Chemical Reagent Co. The silica
gel and GF254 silica gel used in the analytical thin-layer chromatog-
raphy (TLC) and preparative thin-layer chromatography (PTLC)
were produced by the Qingdao Haiyang Chemical Co., Ltd, which
we utilized during the experiment procedure. Electron ionisation
mass spectroscopy (EI-MS) was undertaken with a Thermo Fisher
spectrometry instrument. 1H nuclear magnetic resonance (NMR)
spectra were recorded at 25 °C on a Bruker Avance 500 MHz spec-
2
4
was purified by PTLC using DCM/ethyl acetate/triethylamine
(20:1.5:0.2, v/v/v) as the eluent, to give 541 mg (82%) of compound
1
5 as a yellow solid. Melting point: 107–108 °C. H NMR (500 MHz,
CDCl ) d: 9.90 (s, 1H), 8.94 (s, 1H), 8.49 (d, J = 5 Hz, 1H), 8.16 (d,
3
J = 8 Hz, 1H), 7.98 (d, J = 5 Hz, 1H), 7.84 (d, J = 8.5 Hz, 2H), 7.48–
7.54 (m, 2H), 7.30 (t, J = 5 Hz, 1H), 6.97 (d, J = 5 Hz, 2H), 4.67 (t,
J = 5 Hz, 2H), 4.00 (t, J = 5 Hz, 2H), 2.43–2.48 (m, 2H). 13CC NMR
1
3
trometer. C NMR spectra were recorded at 25 °C on a Bruker
Avance 125 MHz spectrometer. Tetramethylsilane (TMS) in CDCl
and dimethyl sulfoxide-d (DMSO-d ) were investigated as inter-
nal references for H and C NMR spectra, respectively. Ultraviolet
UV)–visible absorption spectra were determined on a Hitachi U-
310 spectrophotometer. The fluorescence spectra were measured
with Hitachi F-4500 spectrophotometer. High-performance
(125 MHz, CDCl ) d: 190.78, 163.39, 141.20, 139.20, 136.44,
3
3
132.05, 131.91, 130.25, 128.52, 121.98, 121.17, 119.88, 114.73,
6
13
6
114.63, 109.32, 64.70, 39.65, 28.66. EI-MS: m/z = 331.15 (calcu-
lated 330.14 for C21H18N O ).
1
2
2
(
3
Synthesis of 2-(((4-(3-(9H-pyrido[3,4-b]indol-9-yl)propoxy)phenyl)
(2-methoxy-2-oxoethyl)thio) methyl)thio) acetic acid(6) [38]
a
(
liquid chromatography (HPLC) analysis were carried out with a
Shimadzu LC-15C instrument.
Boron trifluoride diethyl etherate (380
to a DCM solution of 5 (660 mg, 2.0 mmol) and methyl thioglyco-
late (268 L, 3.0 mmol). After stirring in an ice bath for 8 h, the
lL, 3.0 mmol) was added
Synthesis of 9H-pyrido[3,4-b]indole (2) [34,35]
l
reaction was quenched with water. Then, the suspension was con-
centrated under reduced pressure to afford a yellow oily residue. A
mixture of the residue and aqueous NaOH solution (20 mL, 0.1 M)
in methanol/tetrahydrofuran (THF) (1:1, v/v, 20 mL) was stirred at
RT for 24 h. The solution was extracted with ethyl acetate, and then
the pH of the aqueous phase was adjusted to 3–4 by the addition of
aqueous HCl (0.1 M). The solution was cooled in a refrigerator, and
a white floc precipitate appeared. Purified by PTLC using DCM/
methanol/formic acid (15:1:0.15, v/v/v) as the eluent, 377 mg
L
-tryptophan (5.00 g, 24.5 mmol) and distilled water (100 mL)
were added to a 250 mL single-necked, round-bottomed flask
equipped with a magnetic stirrer. Dilute sulfuric acid (0.1 M) was
added drop wise to the solution until the L-tryptophan dissolved
completely, and then formaldehyde (37%, 7.27 mL, 73.5 mmol)
was added to the solution. After reacting for 6 h at room tempera-
ture (RT), the mixture was filtered, and the filter cake (a pale-yel-
low powder) was obtained. A mixture of the dry pale-yellow
powder, SeO
2
(5.44 g, 49.0 mmol), and acetic acid (150 mL) were
(37%) of compound 6 was obtained as a white solid. Melting point:
1
refluxed for 30 h, then the reaction mixture was cooled to RT to
afford a red-brown oil, which was purified by flash column chro-
matography using petroleum ether/ethyl acetate (1:1, v/v) as the
eluent to give 1.398 g (34%) of compound 2 as a pale-yellow solid.
6
177–178 °C. H NMR (500 MHz, DMSO-d ) d: 8.31–8.36 (m, 3H),
8.13 (s, 1H), 7.82 (s, 1H), 7.70 (s, 1H), 7.55 (s, 1H), 7.26 (s, 2H),
7.03 (s, 1H), 6.85 (s, 1H), 5.23 (s, 1H), 4.68 (s, 2H), 4.03 (s, 3H),
3.41–3.56 (m, 2H), 3.20–3.25 (m, 2H), 2.31 (d, 2H), 0.84 (s, 2H).
1
13
CAS: 244-63-3. Melting point: 199–200 °C. H NMR (500 M Hz,
6
C NMR (125 MHz, DMSO-d ) d: 191.80, 170.50, 163.69, 141.15,
DMSO-d
6
) d: 11.61 (s, 1H), 8.90 (s, 1H), 8.33 (d, J = 5 Hz, 1H), 8.09
138.94, 136.52, 133.00, 132.24, 130.12, 129.23, 128.76, 127.75,
122.45, 119.98, 115.04, 114.81, 110.51, 65.58, 65.00, 52.54, 33.91,
13
(
d, J = 5 Hz, 1H), 7.56 (m, 3H), 7.23 (m, 1H). C NMR (125 MHz,
Please cite this article in press as: N. Li et al., b-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.111

N. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
3
Scheme 1. Synthesis of compound 6. (a) H
tetrabutylammonium hydrogen sulfate, reflux, 8 h, 81%; (d) NaH, DMF, 0 °C 30 min, RT, 24 h, 82%; (e) HSCH
.1 M NaOH, RT, 24 h (37% yield over two steps).
2
O, HCHO, H
2
SO
4
, RT, 6 h; (b) SeO
2
, CH
3
COOH, reflux, 30 h (34% yield over two steps); (c) 1,3-dibromopropane, CH
3
CN, K
2
CO
3
, KI,
2 3
COOCH , BF O, CH Cl , 0 °C, 8 h; (f) CH OH/THF (1/1, v/v),
3
3
ÁEt
2
2
2
0
À5
Fig. 1. (a) Fluorescent intensity curve of probe 6 (1.0 Â 10 M) at different pH values (pH adjusted by HCl and NaOH solutions) at 293 K. (b) Fluorescent intensity curve of
À5
2+
À5
probe 6 (1.0 Â 10 M) in the absence (blue) and presence (red) of Hg (1.0 Â 10 M) at different pH values (pH adjusted by HCl and NaOH solutions) at 293 K. The
excitation wavelength was 360 nm, and the emission intensity was measured at 462 nm. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Fig. 2. (a) Fluorescence titration of compound 6 (1.0 Â 10 M) with Hg2+ in a pH 6.8 PBS solution at 293 K. [Hg2+] (1 Â 10 M): 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3,
À5
À5
À4
4
, 5, 10, and 15. The excitation wavelength was 360 nm. The emission intensity was measured at 462 nm. (b) Absorbance intensity titration of probe 6 (1.0 Â 10 M) with
2+
2+
À4
Hg in a pH 6.8 PBS solution at 293 K. [Hg ] (1 Â 10 M): 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0.
Please cite this article in press as: N. Li et al., b-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water, Spectrochimica Acta
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4
N. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
À5
Fig. 3. Fluorescent intensity change of compound 6 (1.0 Â 10 M) in a pH 6.8 PBS solution at 293 K in the presence of 10 equivalents of different metal ions. Inset: compound
in the presence of different metal ions. A: 6 (1.0 Â 10À5 M). BÀM: 6 + Hg , Cu , Fe , Ag , Mg , Sr , Ba , Zn , Al , Pb , Cd , and Fe (1.0 Â 10 M).
2+
2+
3+
+
2+
2+
2+
2+
3+
2+
2+
2+
À4
6
2
9.47, 28.67, 28.47. EI-MS: m/z = 510.94 (calculated 510.63 for
C
26
26 2 5 2
H N O S ).
Results and discussion
Synthesis
The synthetic approach to the title compound is outlined in
Scheme 1. Compound 1 was used as the starting material to obtain
b-carboline. After reacting with formaldehyde and selenium diox-
ide, respectively, compound 1 was cyclized and converted to b-
carboline. In the presence of K
allowed to react with 1,3-dibromopropane, to produce compound
, which can combine with compound 2 to produce compound 5.
The title compound 6 can be obtained via reacting compound 5
with methyl thioglycolate with the help of a Lewis acid, BF O,
2 3
CO and KI, compound 3 was
4
ÁEt
3 2
and subsequent hydrolysis. The carboxylic group facilitated the
good solubility of compound 6 in methanol and water. At the same
time, an ester group was retained, to keep its stability in air.
2
+
Fluorescence and UV study with Hg
To evaluate the applicability of compound 6 as an Hg2+ fluores-
cence probe in different environments, its photophysical proper-
ties were studied in aqueous solution at various pH values. The
results showed that the fluorescence intensity remained high at
pH values between 5.05 and 10.77 in water (Fig. 1), which sug-
gested that compound 6 could function over a wide range of pH
for detection. Phosphate-buffered solution (PBS) at pH 6.8 was
selected as the detecting environment because the pH of drinking
water is about 6.8.
The titration reaction curve of 6 toward Hg2+ was investigated,
2
+
and compound 6 was found to exhibit high sensitivity toward Hg
.
As shown in Fig. 2a, on addition of Hg2 to a PBS solution of com-
pound 6, a gradual decrease in the intensity of the 462 nm emis-
sion band was observed. The fluorescence intensity of compound
+
2
+
6
declined by 70% after the addition of 1.0 equivalent of Hg to
the PBS solution. The quantum yields before and after combining
2
+
with Hg had been determined, and their values are 0.289 and
0
.066 (Table S1, in the Supporting Information), respectively.
Similar to the fluorescence spectra, the absorption spectra also
showed a visible response toward Hg2 . On addition of Hg to
the PBS solution of compound 6, the absorbance intensity
increased smoothly (Fig. 2b). The linear response range of com-
+
2+
2
+
pound 6 for Hg was measured under optimal experimental con-
Fig. 4. Chromatographic conditions: C18 reversed-phase column, column temper-
ature of 40 °C, detection wavelength of 360 nm, eluted with water:methanol (1:1)
over 10 min at a flow rate of 0.5 mL/min.
ditions. As shown in Fig. 2a, the fluorescent intensity of compound
6
decreased with an increasing concentration of Hg2+. Although a
Please cite this article in press as: N. Li et al., b-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.111

N. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
5
Fig. 5. Proposed sensing mechanism for complex (6 + Hg2+).
good Stern–Volmer relationship could not be obtained, there is a
2
of three alkyl carbons (two CH and one CH) to a dithioacetal,
good negative correlation between the fluorescent intensity (y)
and the concentration of Hg ion in the range 1.0 Â 10 M–
which results in the pendant arm being sufficiently flexible to lead
to an interaction with Hg .
2+
À6
2+
–
6
7
.0 Â 10 M.
Fig. 3 illustrates the fluorescent intensity changes of compound
Conclusions
2
+
2+
3+
+
2+
2+
2+
2+
3+
6
in the presence of Hg , Cu , Fe , Ag , Mg , Sr , Ba , Zn , Al ,
2
+
2+
2+
Pb , Cd and Fe in pH 6.8 PBS solution. Compound 6 exhibited a
In summary, we have developed a water-soluble fluorescence
probe by incorporating a b-carboline fragment with a dithioacetal.
This probe exhibits a rapid response and high selectivity and sen-
2
+
2+
high selectivity toward Hg , and only Hg could quench the fluo-
rescence of compound 6. The vivid fluorescence changes from blue
to colorless could be clearly discriminated by the naked eye. The
fluorescence intensity of compound 6 changed very little in the
presence of the other metal ions, because compound 6 can com-
2+
sitivity to Hg over other metal ions in water. Moreover, this probe
can function over a wide pH range (from 5.05 to 10.77) for the
2+
2+
detection of Hg ion. Based on a complex reaction, Hg can bind
with this probe to form a complex, resulting in obvious fluores-
cence quenching. Significantly, b-carboline as a fluorophore has
potential for the design of novel fluorescence probes.
2
+
bine with Hg ion to give complex whereas the other metal ions
cannot. To gain more insight into the ion selectivity of compound
2
+
6
for Hg , cross-contamination experiments were carried out in
2
+
2+
the presence of Hg mixed with other metal ions, namely Cu
,
Fe , Ag , Mg , Sr2+, Ba , Zn , Al , Pb , Cd and Fe . As illus-
trated in Fig. S1 (in the Supporting Information), it is evident that
the selectivity of compound 6 towards Hg2 is almost unaffected
by the competing ions.
3
+
+
2+
2+
2+
3+
2+
2+
2+
Acknowledgments
+
This research was financially supported by the National Natural
Science Foundation of China (Grant Nos. 31270388 and 21202132)
and Fundamental Research Funds for the Central Universities (Nos.
QN2011066 and QN2011033).
It is well known that reaction time is an important indicator
when evaluating a probe: a fast-responding probe has more prac-
2
+
tical value. The reaction time of compound 6 towards Hg in pH
.8 PBS solution was investigated, and the results are shown in
6
Appendix A. Supplementary material
Fig. S2 (in the Supporting Information). From the reaction time
curve, it can be seen that compound 6 has a rapid response, and
reaches a relatively stable plateau less than 0.5 min after the addi-
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.09.111.
2
+
tion of Hg , which indicates that compound 6 can efficiently
detect Hg +.
2
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(
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9
2
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2
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1
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Please cite this article in press as: N. Li et al., b-Carboline-functionalized dithioacetal as Hg2+-selective fluorescence probe in water, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.111