A selective and sensitive turn-on fluorescent chemosensor for recognition of Hg2+ ions in water

Article abstract of DOI:10.1002/chem.201201604

Based upon highly selective and irreversible Hg²⁺-promoted deprotection of the dithioacetal reaction, a new water-soluble turn-on fluorescent chemosensor (1) was prepared and exhibited high selectivity and sensitivity towards the Hg²⁺ ion over other heavy and transition-metal ions in pure water by transforming a weakly fluorescent precursor (colorless) to a highly fluorescent aldehyde (yellow-green; see figure) with a 155-fold increase in fluorescent intensity. Copyright

Full text of DOI:10.1002/chem.201201604

COMMUNICATION
DOI: 10.1002/chem.201201604
A Selective and Sensitive “Turn-on” Fluorescent Chemosensor for
Recognition of Hg2+ Ion in Water
Huiling Dai, Yuanyuan Yan, Yong Guo, Lingling Fan, Zhiping Che, and Hui Xu*[a]
Heavy and transition-metal (HTM) ions usually play im-
portant roles in various biological systems or have an ex-
tremely toxic impact on the environment.[1] In particular,
mercury ions are considered to be very dangerous environ-
mental pollutants by bioaccumulating through the food
chain when they are ingested or inhaled by human beings,[2]
consequently, the development of fluorescent chemosensors
for the detection of the Hg2+ ion over other HTM ions with
excellent selectivity and sensitivity is still a challenging task.
Recently, a lot of sensors for the recognition of Hg2+ ion
have been developed with good performance.[3] Further-
more, many attractive probes for the detection of mercury
ions based on the specific mercury-promoted desulfurization
reaction have also been reported.[4] More recently, another
series of new interesting sensors, based on the mercury-pro-
moted deprotection of dithioacetals groups, have been de-
veloped.[5] However, these probes for the recognition of the
Hg2+ ion were conducted in organic solutions.[5] Meanwhile,
1,8-naphthalimides have been frequently used as the fluoro-
phores to prepare fluorescent chemosensors for metal cat-
ions and protons in recent years.[6] Encouraged by the
above-mentioned results, herein, we have designed a simple
chemosensor 1 (Scheme 1) by incorporating a 1,8-naphthali-
mide fragment with a dithioacetal that contained two hydro-
Scheme 1. Synthetic approach for chemosensor 1.
philic carboxylic groups, into a single molecular framework.
We envisaged that the two carboxylic groups at the dithioa-
cetal group would make 1 operate in aqueous solution, and
therefore enable us to disclose a new highly sensitive and se-
lective probe for the Hg2+ ion over other HTM ions in pure
water without the requirement for additional organic sol-
vent.
As described in Scheme 1, 4-bromo-1,8-naphthalimide re-
acted with piperidine to give 4-piperidine-1,8-naphthalimide
(2, 86%),[7] which was further allowed to react with ethanol-
amine to afford 3 in a 79% yield.[8] Then 4 was obtained in
a 68% yield by the reaction of 3 with phosphorus tribro-
mide.[9] In the presence of K2CO3 and KI, subsequently, in-
termediate 4 was allowed to react with 4-hydroxybenzalde-
hyde to produce 5 in a 56% yield,[10] which smoothly reacted
with methyl thioglycolate to give 6 (96%).[5a] Finally, the
target compound 1 was obtained in a 53% yield by hydroly-
sis of 6 with potassium hydroxide in a water–methanol mix-
ture.[11] The chemical structures of compounds 1–6 were well
characterized by IR spectrometry, 1H and 13C NMR spec-
troscopy, and HRMS (see the Supporting Information).
The pKa’ of 1 was about 3.3, and the fluorescent intensity
was almost a constant minimal value at pH values >4.24 in
water (Figure S1, see the Supporting Information). It sug-
gested that 1 could function over a wide range of pH values
for detection. Figure 1 shows the fluorescent intensity ratio
(I/I0) of sensor 1 in the presence of Hg2+, Ag+, Zn2+, Fe3+
,
[a] Dr. H. Dai, Y. Yan, Y. Guo, L. Fan, Z. Che, Prof. Dr. H. Xu
Laboratory of Pharmaceutical Design & Synthesis
College of Sciences, Northwest A&F University
Yangling 712100 (P.R. China)
Cd2+, Pb2+, Ca2+, Cu2+, Mg2+, and Ba2+ ions in water. The
fluorescent behavior of sensor 1 was very weak. Interesting-
ly, only when Hg2+ ion was added to the pure water solution
of 1, a 155-fold increase of the fluorescent intensity was ob-
served at 535 nm. This was because the dithioacetal of
sensor 1 was deprotected by Hg2+ ion to give aldehyde 5,
Fax : (+86)29-87091952
E-mail: orgxuhui@nwsuaf.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201604.
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tration of the added Hg2+ ion in water, and the titration re-
action curve showed a steady increase until a plateau was
reached when 1 equiv of Hg2+ ion was added.
To investigate the utility of 1 as an ion-selective fluores-
cent chemosensor for Hg2+ ions, cross-contamination experi-
ments were conducted in the presence of Hg2+ at a concen-
tration of 1ꢁ10À5 m mixed with other metal ions, such as
Ag+, Zn2+, Fe3+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+ and Cu2+
,
at a concentration of 1ꢁ10À4 m. As indicated in Figure 3, it
clearly suggested that the selectivity of 1 towards Hg2+ was
almost completely unaffected by other competitive ions
(10 equiv).
Figure 1. Fluorescent intensity ratio (I/I0) of 1 in distilled water at 293 K
in the presence of 10.0 equiv of different metal ions. Inset: Photograph of
1 in the presence of different metal ions. A: 5 (1.0ꢁ10À5 m); B: 1 (1.0ꢁ
10À5 m); C–L: 1+Hg2+, Ag+, Zn2+, Fe3+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+
and Cu2+ (1.0ꢁ10À4 m).
,
accompanied by the transformation of weak fluorescence
(colorless, inset of Figure 1) to strong fluorescence (yellow–
green). However, the addition of other metal ions, namely,
Ag+, Zn2+, Fe3+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+, and Cu2+
,
did not lead to a clear enhancement of the fluorescent inten-
sity of 1. The reaction time profile of 1 (1ꢁ10À5 m) towards
Hg2+ (1ꢁ10À5 m) in water was examined in Figure S3 (in the
Supporting Information), and the time for the reaction time
curve to reach a stable plateau was 3 min after Hg2+ ion ad-
dition.
To obtain more insight into the fluorescent properties, the
titration reaction curve of 1 toward the Hg2+ ion was inves-
tigated as shown in Figure 2. The fluorescent intensity of
sensor 1 increased in response to the increase in the concen-
Figure 3. Results of the competition experiments between Hg2+ and se-
lected metal ions in distilled water at 293 K; the concentration of Hg2+
was 1ꢁ10À5 m and that of each competing metal ions was 1ꢁ10À4 m; the
excitation wavelength was 416 nm and the emission intensity was mea-
sured at 535 nm.
To examine the sensitivity of 1 towards Hg2+, its detection
limit was evaluated. As shown in Figure 4, the fluorescence
titration profile of 1 (10À6 m) with Hg2+ demonstrated that
Hg2+ could be detected at least down to 3ꢁ10À8 m, and the
fluorescent intensity of 1 increased linearly with the concen-
tration of Hg2+ (0–1.00)ꢁ10À6 m (R2 =0.9951).
1
As depicted in the partial H NMR spectra in Figure 5a
(sensor 1), b (1+Hg2+), and c (5, for comparison), when
1 was treated with 1.1 equiv of Hg2+, the resonance of the
aldehyde proton appeared similar to that in Figure 5c.
1
Based upon the above H NMR data, the proposed sensing
mechanism was described as depicted in Figure 5d.
In summary, we have developed a new water-soluble
“turn on” chemosensor with high selectivity and sensitivity
for recognition of Hg2+ ion over other HTM ions in pure
water. The sensor was designed based upon a unique and ir-
reversible Hg2+-promoted deprotection reaction of the di-
thioacetal by transforming a weakly fluorescent precursor
Figure 2. Fluorescence titration of 1 (1ꢁ10À5 m) with Hg
ACHTUNTRGNEUNG(ClO4)2 in dis-
tilled water at 293 K. [Hg2+]: 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 5.0, 10ꢁ
10À5 m. The excitation wavelength was 416 nm and the emission intensity
was measured at 535 nm.
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Fluorescent Chemosensor for Recognition of Hg2+
COMMUNICATION
(colorless) to a highly fluorescent aldehyde product (yellow–
green) with a 155-fold increase in fluorescent intensity.
Moreover, 1 could behave as a fluorescent sensor over
a wide range of pH values for the detection of Hg2+. Based
on its good water solubility, and highly selective and sensi-
tive response to Hg2+, this probe could be used to deter-
mine Hg2+ ion concentrations in aqueous environments.
These results will encourage us to design more chemosen-
sors for other specific ions in water.
Acknowledgements
The present research was partly supported by the National Natural Sci-
ence Foundation of China (Nos.: 31071737, 31171896), the Fok Ying
Tong Education Foundation for Young Talents (No.: 121032), and the
Special Funds of Central Colleges Basic Scientific Research Operating
Expenses (QN2009045). We also thank to Prof. H. L. Zhang for the
NMR experiments.
Figure 4. Fluorescence titration of 1 (1ꢁ10À6 m) with Hg
ACHTUNTRGNEUNG(ClO4)2 in dis-
tilled water at 293 K. [Hg2+]: 0, 0.03, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7,
0.8, 1.0ꢁ10À6 m.
Keywords: dithioacetals · fluorescence · mercury · sensors
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Published online: && &&, 0000
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Fluorescent Chemosensor for Recognition of Hg2+
COMMUNICATION
Fluorescence
H. Dai, Y. Yan, Y. Guo, L. Fan,
Z. Che, H. Xu* ............... &&&& — &&&&
A Selective and Sensitive “Turn-on”
Fluorescent Chemosensor for Recogni-
tion of Hg2+ Ion in Water
Based upon highly selective and irre-
versible Hg2+-promoted deprotection
of the dithioacetal reaction, a new
water-soluble “turn on” fluorescent
chemosensor 1 was prepared and
exhibited high selectivity and sensitiv-
ity towards the Hg2+ ion over other
heavy and transition-metal ions in
pure water by transforming a weakly
fluorescent precursor (colorless) to
a highly fluorescent aldehyde (yellow–
green; see figure) with a 155-fold
increase in fluorescent intensity.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!