Convenient and highly effective fluorescence sensing for Hg2+ in aqueous solution and thin film

Article abstract of DOI:10.1016/j.bmc.2009.04.018

A quinolinocyclodextrin, that is, MQAS-β-cyclodextrin (MQAS = N-(2-methyl-8-amino- quinolyl)-p-aminobenzene sulfonamide) was synthesized. Further experiments showed that it could form very stable stoichiometric 2:1 complex with Zn²⁺ in water. Significantly, the resultant quinolinocyclodextrin/Zn²⁺ complex showed the specific fluorescence response to Hg²⁺ over other metal ions, which could be readily distinguished in either aqueous solution or the PVA-based thin film. This finding would enable Zn·3₂ complex as a convenient and highly efficient fluorescence sensor for the detection of Hg²⁺.

Full text of DOI:10.1016/j.bmc.2009.04.018

Bioorganic & Medicinal Chemistry 17 (2009) 3887–3891
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Convenient and highly effective fluorescence sensing for Hg2+ in aqueous
solution and thin film
*
Yu Liu , Miao Yu, Yong Chen, Ning Zhang
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A quinolinocyclodextrin, that is, MQAS-b-cyclodextrin (MQAS = N-(2-methyl-8-amino- quinolyl)-p-ami-
nobenzene sulfonamide) was synthesized. Further experiments showed that it could form very stable
stoichiometric 2:1 complex with Zn2+ in water. Significantly, the resultant quinolinocyclodextrin/Zn2+
complex showed the specific fluorescence response to Hg2+ over other metal ions, which could be readily
distinguished in either aqueous solution or the PVA-based thin film. This finding would enable Znꢀ32
Received 24 February 2009
Revised 13 April 2009
Accepted 14 April 2009
Available online 17 April 2009
complex as a convenient and highly efficient fluorescence sensor for the detection of Hg2+
.
Keywords:
Cyclodextrin
Complex
Hg2+
Ó 2009 Elsevier Ltd. All rights reserved.
Fluorescence
Sensor
1. Introduction
resultantZnꢀ32 complexexhibitedthespecificfluorescenceresponse
to Hg2+, which could be readily distinguished in either aqueous solu-
tion or the PVA-based thin film. This finding would enable Znꢀ32
complex as a convenient and highly efficient fluorescence sensor
Mercury is a highly toxic and widespread global pollutant, which
could easily pass through biological membranes such as skin, respi-
ratory, andgastrointestinal tissues.1 Accumulation of mercuryin hu-
man body is reported able to cause many serious adverse health
effects, such as damages in liver, kidney, immune system, nervous
system and other organs.2 This alarming situation makes a lot of ef-
forts greatly contributing to detecting the distribution and content
of mercury and preventing its pollution. Towards this goal, a variety
of methods have been developed to monitor mercury contamination
inwater, food, soil, andsoon. Amongthevariousmethodsto monitor
mercury ions under the toxicological and environmental conditions,
fluorescence chemosensors are widely regarded as one of the most
effective ways. So far, a number of fluorescence Hg2+ sensors, such
as small molecules,3 gold nanoparticles,4 DNAzymes,5 protein,6 oli-
gonucleotide7 and polymers,8 have been reported. However, most
of these sensors suffer from the poor water solubility and the non-
specific interference from Cu2+, Ni2+ and other competing metal ions.
Recently, we reported that several quinolino-modified b-cyclodex-
trinscouldbe usedasfluorescenceZn2+ probes, showingthesatisfac-
for the detection of Hg2+
.
2. Results and discussion
The coordination behavior of 3 with Zn2+ was investigated by
the UV–vis titration at 298 K in aqueous solution. Figure 1a illus-
trates the typical UV–vis curves of 3 with the gradual addition of
Zn2+. As can be seen in Figure 1a, a new absorption peak appeared
at 352 nm in the UV–vis spectrum of 3, and its intensity gradually
increased with the stepwise addition of Zn2+. This absorption was
expected to correspond to the formation of a five-membered che-
late ring between two nitrogen atoms in MQAS unit of 3 and Zn2+
,
which extended the conjugated system and thus resulted in the
appearance of the new absorption in the long wavelength region.
In the control experiment, the UV–vis spectrum of Zn2+ within
the appropriate concentration range displayed no appreciable
absorption between 200 and 450 nm under comparable experi-
mental conditions. These indicated that the Zn2+ was coordinated
to 3. Moreover, the molar ratio method using the UV–vis spectrom-
etry gave the coordination stoichiometry between 3 and Zn2+. As
tory water solubility and the good sensing ability to Zn2+ 9
. In this
work, we synthesized another quinolinocyclodextrin, that is,
MQAS-b-CD (3, MQAS = N-(2-methyl-8-amino-quinolyl)-p-amino-
benzene sulfonamide), and found that it could form very stable stoi-
chiometric 2:1 complex with Zn2+ in water. Significantly, the
seen in Figure 1b, the curve of
D
A3=Zn2þ
(
D
A3þZn2þ ¼ A3þZn2þ ꢁ A3, A3
was defined as the absorption intensity of 3 at 352 nm) versus
Zn2+/3 molar ratio showed an inflexion point at a molar ratio of
0.5, which corresponded to a 2:1 coordination stoichiometry be-
tween 3 and Zn2+. Based on these observations, we deduced the
* Corresponding author. Tel./fax: +86 022 23503625.
E-mail address: yuliu@nankai.edu.cn (Y. Liu).
0968-0896/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2009.04.018

3888
Y. Liu et al. / Bioorg. Med. Chem. 17 (2009) 3887–3891
bance still changed with the increase of Zn2+ concentration up to
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.06 (b)
(a)
1:1 Zn2+/3 molar ratio. Through a simple calculation based on the
apparent stability constant of Znꢀ32 complex and the concentra-
tions of Zn2+ and 3, there should existed 5.5% free 3 in solution
at a molar ratio of 0.5. With the increase of Zn2+ concentration
up to 1:1, 4.2% free 3 would further convert to Znꢀ32 complex,
which consequently resulted in the further increase of the absor-
bance upon the addition of Zn2+ between 0.5 and 1 Zn2+/3 molar ra-
tio. Considering the concentrations of 3 and Zn2+ employed in our
experiments, this strong binding ability of 3 to Zn2+ indicated that
most of 3 was converted to Znꢀ32 in a 2:1 3/Zn2+ mixture. There-
fore, Znꢀ32 could be prepared in situ by mixing 3 and Zn(OAc)2 with
a molar ratio of 2:1 in our experiments. Like many reported sulfo-
namidoquinoline/Zn2+ complexes,10 Znꢀ32 exhibited a satisfactory
fluorescence emission (quantum yield 0.312), giving an emission
maximum at 500 nm.
0.04
0.02
0.00
0.0
0.2
0.4
0.6
0.8
1.0
[ Zn2+] /L
k
a
250
300
350
400
450
Wavelength (λ//nm)
Significantly, Znꢀ32 showed the good fluorescence sensing abil-
ity to Hg2+. As can be seen in Figure 2, the fluorescence intensity of
Znꢀ32 complex at 500 nm gradually decreased with the stepwise
addition of Hg2+. After 1 equiv of Hg2+ was added, ca. 50% of the
emission intensity of Znꢀ32 complex was quenched, and this value
would further increase to 88% when adding 4 equiv of Hg2+. A pos-
sible reason for the quenched fluorescence may be the competition
of Hg2+ and Zn2+ in coordinating with 3. Using the same methods,
the stoichiometry and the apparent stability constant (log K0,
K0 = [Hgꢀ32]/[Hg2+][3]2) for the coordination of 3 with Hg2+ were
measured to be 2:1 and 13.3, respectively. On the other hand,
the apparent stability constants (log K, K = [Mꢀ32]/[M2+][3]2) of
other metal ions, such as Cu2+, Ni2+, Cd2+, Cr2+, etc, with 3 were
determined to be less than 10. This indicated that, with the addi-
tion of Hg2+ to Znꢀ32, a part of Znꢀ32 would convert to fluorescent
inert Hgꢀ32, which consequently resulted in the quenched fluores-
0.06
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(d)
(c)
0.04
0.02
0.00
0.0
0.2
0.4
0.6
[Hg2+]/L
0.8
1.0
k
a
cence. In addition, the detection limit (3r
/K) of this probe for Hg2+
300
400
500
was determined to be 3.0 ꢂ 10ꢁ7 mol Lꢁ1
.
Wavelength (λ//nm)
It is noteworthy that Znꢀ32 maintained its good fluorescence
sensing ability to Hg2+ over a wide pH range. As can be seen in Fig-
ure 3, Znꢀ32 barely fluoresced at a pH value below 4 either with or
without Hg2+, which may be due to the competition of H+ at low pH
values leading to a weak coordination ability of metal ion with 3.9a
However, when the pH value increased to the 5–10 range, Znꢀ32
showed the satisfactory fluorescence sensing ability to Hg2+ with
Figure 1. (a) UV–vis spectra of 3 upon the addition of Zn2+ in buffer solution (pH
7.2, I = 0.1 M NaNO3) at 25 °C. ([3] = 40 l lM from a to k); (b) the
M, [Zn2+] = 0–40
absorption changes of 3 at 352 nm upon the addition of Zn2+; (c) UV–vis spectra of 3
upon the addition of Hg2+ in buffer solution (pH 7.2, I = 0.1 M NaNO3) at 25 °C.
([3] = 40
l lM from a to k); (d) the absorption changes of 3 at
M, [Hg2+] = 0–40
352 nm upon the addition of Hg2+
.
the FZnꢀ3
=F
value varied in a range of 0.15–0.74. This avail-
Znꢀ32
2þHg2þ
able pH scope of Znꢀ32 for Hg2+ was practicable to the drinking
water, food and soil.
possible coordination mode of Znꢀ32 as shown in Scheme 1. More-
over, the binding ability of 3 to Zn2+ was quantitatively determined
by a competitive binding method9a using fluorescence titration,
and the apparent stability constant (log K, K = [Znꢀ32]/[Zn2+][3]2)
of Znꢀ32 complex was observed to be equal to 12.4. It should be
noted that, when the Zn2+/3 molar ratio exceeded 0.5, the absor-
As an effective Hg2+ sensor, Znꢀ32 exhibited good fluorescence
sensing selectivity to Hg2+. Figure 4 illustrates the fluorescence
intensities of Znꢀ32 at 500 nm in the presence of various metal ions,
especially possible competing ions when Znꢀ32 was used as one
N
N
N
N
Zn
NH
N
N
O2S
SO2
O2S
O
S N
O
H
H2N
O
O
HN
O
O
O
O
N
H
N
H
1
NH
NH
HN
O
N
O
NH S
O
O
NH
OH
.
Zn 32
2
3
Scheme 1. The chemical structures of 1, 2, 3 and Znꢀ32.

Y. Liu et al. / Bioorg. Med. Chem. 17 (2009) 3887–3891
3889
Hg2+ sensor in the environment. As can be seen in Figure 4, the
fluorescence intensity of Znꢀ32 only showed slight changes
(1.06 > F/F0 > 0.95) in the presence of a large excess of (500 equiv)
400
300
200
100
0
of alkali metal ions (Na+, K+) and alkali earth metal ions (Ca2+
Mg2+) or the equivalent amount of transition metal ions (Fe2+
,
,
a
u
Co2+, Ni2+, Cu2+, Mn2+, Cd2+), and these changes in no case ap-
proached the corresponding quenching effect observed for Hg2+
(F/F0 = 0.5), which unambiguously demonstrated the high sensing
selectivity of Znꢀ32 to Hg2+ over other ions.
Besides in water, Znꢀ32 also exhibited the good fluorescence
sensing ability to Hg2+ in organic thin film, which could be readily
distinguished under the UV light. Herein, the organic thin film was
prepared by casting the aqueous poly(vinyl alcohol) (PVA) solution
of Znꢀ32 on glass slips and then dried in air. As can be seen in Figure
5, the fluorescence intensity of Znꢀ32—contained film obviously
quenched when Hg2+ was dripped on, but quite slightly or hardly
responded to other metal ions, such as Cu2+, Ni2+, Cr2+, which
was basically consistent with the sensing selectivity of Znꢀ32 in
water. In the control experiment, Zn/2 or Zn/1 system, which
was poorly soluble in water, was found to difficult to achieve a
homogeneous dispersion in the PVA film.
400
500
600
Wavelength (λ//nm)
Figure 2. Fluorescence spectral changes of Znꢀ32 (10
l
M) complex with addition of
Hg2+ (0–40
l
M
from
a
to u) in aqueous buffer solution (pH 7.20) at 298 K
(kex = 352 nm).
In conclusion, we successfully prepare a water soluble quinolin-
ocyclodextrin/Zn2+ complex in situ as a fluorescence sensor for
Hg2+. This sensor system presents the obvious fluorescence re-
sponses, which can be readily monitored by both eyes and fluores-
cence spectroscopy, to Hg2+ in either water solution or organic thin
film, and this fluorescence sensing was not obviously affected by a
variety of cations. Considering its satisfactory water solubility, con-
venience in Hg2+-detection, and high sensing specificity for Hg2+
over other competing cations, this complex is expected to have po-
tential application to meet the detection requirements of an Hg2+
assay in environmental fields.
500
400
300
200
100
Zn32
Zn32 + Hg2+
3. Experimental
3.1. Materials
0
0
2
4
6
8
10
12
All chemicals used were reagent grade unless noted. 2-Methyl-8-
amino-quinoline was obtained from J & K Chemical LTD. Acetanilide,
pH
Figure 3. Fluorescence intensities of Znꢀ32 (10
lM) in the absence and presence of
M) at various pH values in aqueous solution at 298 K (kex = 352 nm,
Hg2+ (10
l
kem = 500 nm).
1.0
0.8
0.6
0.4
0.2
0.0
Figure 4. Fluorescence intensities of Znꢀ32 (10
metal ions. ([K+] = [Na+] = [Ca2+] = [Mg2+] = 5 mM, [Fe2+] = [Co2+] = [Ni2+] = [Cu2+] =
[Mn2+] = [Cd2+] = [Hg2+] = 10
M). All data were obtained in aqueous buffer solution
lM) in the presence of various
l
(pH 7.2) at 298 K (kex = 352 nm, kem = 500 nm) and normalized with respect to the
emission intensity of free Znꢀ32 (F0).
Figure 5. Images of Znꢀ32—contained PVA film in the absence (a) and (b) presence
of metal ions under the low intensity UV light (k = 360 nm).

3890
Y. Liu et al. / Bioorg. Med. Chem. 17 (2009) 3887–3891
sulfurochloridic acid, and succinic anhydrides were all obtained
from Aldrich. Poly(vinyl alcohol) (PVA, Mw = (7.79 0.2) ꢂ 104)
was provided by the Chemical & Engineering Department of Tianjin
University and used as a polymer matrices. b-Cyclodextrin (b-CD) of
reagent grade was recrystallized twice from water and dried in va-
cuo at 95 °C for 24 h prior to use. N,N-Dimethyl-formamide (DMF)
was dried over CaH2 for 2–3 days and then distilled under reduced
pressure prior to use. Pyridine was dried over CaH2 for 2–3 days
and then distilled prior to use. 6-Amino-b-CD was prepared accord-
ing to the procedure described in the literature.11 4-Acetamidoben-
zene-1-sulfonyl chloride was prepared by the reaction of acetanilide
with HSO3Cl.
C, 61.32; H, 4.82; N, 13.41; S, 10.23. Found: C, 61.08; H, 4.90; N,
13.57; S, 10.17.
3.3.3. N-MQAS-4-amino-4-oxobutanoic acid (2)
To 200 mL of toluene was added 1 (3.0 g, 9.6 mmol) and succi-
nic anhydrides (9.6 g, 96 mmol). The mixture was stirred at 150 °C
for 5 h. After cooled to room temperature, the precipitate was col-
lected by filtration, washed with 100 mL of hot water and dried in
vacuo to give a slight yellow solid, which was recrystallized from
EtOH twice and dried in vacuo to afford 2 (2.4 g, 61% yield) as a
slight green crystal. MS (ESI): m/z 412.20 ([MꢁH]ꢁ), 447.98
([M+Cl]ꢁ), 824.92 ([2MꢁH]ꢁ). 1H NMR (DMSO-d6, 300 MHz, TMS,
ppm): d 2.504 (s, 4H, –CH2–CH2– overlapped with the peak of
DMSO), 2.674 (s, 3H, quinoline-CH3), 7.434 (m, 2H, quinoline),
7.617 (m, 3H, quinoline), 7.837 (d, J = 8.7 Hz, 2H, Ph), 8.212 (d,
J = 8.4 Hz, 2H, Ph), 9.603 (s, 1H, Ph–NH–), 10.292 (s, 1H, quino-
3.2. Measurements
Elemental analyses were performed on a Perkin–Elmer-2400C
instrument. 1H NMR spectra were recorded on a Varian Mercury
VX300 spectrometer. UV–vis spectra were recorded in a 10 mm
quartz cell on a Shimadzu UV-3600 UV–vis-NIR spectrophotometer
(SHIMADZU corporation) equipped with a TCC-240 thermoelectri-
cally temperature controlled cell holder to keep the temperature at
298 K. Fluorescence spectra were recorded in a conventional
quartz cell (10 ꢂ 10 ꢂ 45 mm) at 298 K on a JASCO FP-750 fluores-
cence spectrometer (xenon lamp photosource) equipped with a
PTC-348WI temperature controller to keep the temperature at
298 K. The measurement of pH value in water was performed on
a PHS-3C pH-meter (Shanghai Rex Instruments Factory). Fluores-
cence images were performed on a ZF-7B three-used ultraviolet
analysis instrument (Shanghai Kanghua Biochemistry instrument
Co., LTD) and a Kodak Z885 zoom digital camera (Eastman Kodak
Company) for photo collection. Tris-HCl buffer solution (pH 7.2)
was used as solvent in all spectral measurements.
line-NH–),
12.103
(s,
1H,
COOH).
Anal.
Calcd
for
C20H19N3O5Sꢀ0.25H2O: C, 56.86; H, 5.73; N, 9.95; S, 7.59. Found:
C, 56.90; H, 5.70; N, 10.00; S, 7.60.
3.3.4. MQAS-b-CD (3)
To 20 mL of anhydrate DMF was added 2 (220 mg, 0.53 mmol)
and HOBt (81 mg, 0.6 mmol). After the mixture was stirred at
0 °C for 2 h, DCC (124 mg, 0.6 mmol) was added. The mixture
was stirred at 0 °C for 3 h, then 6-amino-b-CD (600 mg, 0.53 mmol)
was added. The reaction mixture was stirred for 6 h in an ice bath,
then at room temperature for 2 days. The precipitate was filtrated
off, and the filtrate was poured into 300 mL of acetone and stirred
for 3 h. The precipitate was collected by filtration, dried in vacuo,
and purified by column chromatography on a Sephadex CM-25 col-
umn with 1 M NH3 aqueous solution (500 mL) as the eluent to give
3 as a brown solid (200 mg, 25% yield). MS (ESI): m/z 1527.54
([MꢁH]ꢁ), 1563.43 ([M+Cl]ꢁ). 1H NMR (D2O, 300 MHz, TMS,
ppm): d 2.565 (d, J = 8.1 Hz, 2H, –CH2–), 2.677–2.884 (5H, CO–
CH2 and quinoline-CH3), 2.888–4.273 (42H, H2ꢃH6 of b-CD),
5.029 (d, J = 16.5 Hz, 7H, H1 of b-CD), 7.373 (s, 1H, H7 of quinoline),
7.454–7.642 (4H, H3, H5 of Ph and H3, H5 of quinoline), 7.775 (d,
J = 7.2 Hz, 1H, H6 of quinoline), 7.919 (d, J = 8.1 Hz, 2H, H2, H6 of
Ph), 8.406 (d, J = 8.4 Hz, 1H, H4 of quinoline). Anal. Calcd for
C62H88N4O38SꢀH2O: C, 48.69; H, 5.80; N, 3.66; S, 2.10. Found: C,
48.50; H, 5.90; N, 3.64; S, 2.04.
3.3. Synthesis
3.3.1. N-(2-Methyl-8-amino-quinolyl)-p-acetylamidobenzene-
sulfonamide (MQAAS)
To the solution of 2-methyl-8-amino-quinoline (840 mg,
5.4 mmol) in 3 mL of anhydrate pyridine was added 4-acetamido-
benzene-1-sulfonyl chloride (1.42 g, 6 mmol) in potions. The mix-
ture was stirred for 1 h at room temperature, then at 60 °C for 3–
4 h. The reaction mixture was poured onto 20 mL of ice. At the
same time the glass stick was used to stir the mixture and rub
the wall of the glass beaker to produce a brown precipitate. The
precipitate was collected by filtration and dried in vacuo to afford
the crude product of MQAAS (800 mg, 43% yield) as brown needles.
Mp: 200 °C. The crude product was used directly without further
purification.
Acknowledgements
We thank 973 Program (2006CB932900), NNSFC (20772062),
and Key Project of Chinese Ministry of Education (No. 107026)
for financial support.
References and notes
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