A novel highly selective "turn-on" fluorescence sensor for silver ions based on Schiff base

Article abstract of DOI:10.1002/cjoc.201400601

A novel optical chemical sensor L was designed and synthesized for the determination of silver ions. The sensor L was derived from 1-naphthaldehyde and 3,4,5-tris(hexadecyloxy)benzohydrazide. The sensor L shows high sensitivity and selectivity for Ag+ det

Full text of DOI:10.1002/cjoc.201400601

FULL PAPER
DOI: 10.1002/cjoc.201400601
A Novel Highly Selective “Turn-On” Fluorescence Sensor
for Silver Ions Based on Schiff Base
Qi Lin,* Qingping Yang, Bin Sun, Taibao Wei,* and Youming Zhang*
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China;
Key Laboratory of Polymer Materials of Gansu Province; College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, China
A novel optical chemical sensor L was designed and synthesized for the determination of silver ions. The sensor
L was derived from 1-naphthaldehyde and 3,4,5-tris(hexadecyloxy)benzohydrazide. The sensor L shows high sen-
＋
＋
＋
＋
＋
＋
sitivity and selectivity for Ag＋ detection in comparison to other metal cations (Mg2 , Ca2 , Al3 , Cr3 , Fe3 , Co2 ,
＋
＋
＋
＋
＋
＋
Ni2 , Cu2 , Zn2 , Cd2 , Hg2 , Pb2 ) and has no significant response to other common metal cations. Upon addition
of Ag＋, the fluorescent emission of the sensor L was enhanced dramatically but other metal cations had no same
response. The detection limit for Ag＋ was 1.2×10−7 mol/L. Therefore, the sensor L is useful for Ag＋ detection
with some advantages including sensitivity, selectivity, simplicity and low-cost.
Keywords silver ions detection, fluorescent sensor, high selectivity, coordination mechanism, high sensitivity
Introduction
which can recognize silver ions from amongst calcium,
magnesium, nickel, zinc, copper, mercury, lead, cobalt,
cadmium, iron, and chromium. In addition, fluorescence
methods would be also suitable for silver ions detection
because they have some advantages, such as relative
sensitivity, convenience and cheapness.[26-30] The
mechanisms of the detection methods are to date mostly
based on complexation of silver ions with sensor.[31]
And the detection limit of sensor L for Ag＋ was
1.2×10−7 mol/L.
Silver ion, like other heavy metals cadmium, chro-
mium, copper and mercury etc, is one of indispensible
high toxic heavy metals, and has been used extensively
in photography, batteries and semiconductor industry.[1,2]
Thousands of tons of silver ions and its compounds are
released into the environment from industrial wastes
and emissions annually.[3,4] However, excessive intake
of silver ions and long-term accumulation of silver ions
can lead to insoluble precipitates in skin and eyes[5].
Moreover, it is important to detect some special
ions.[6-17] For example, recent information about the in-
teraction of silver with essential nutrients, especially Se,
Cu, Vitamin E and Vitamin B12, has focused attention on
its potential toxicity.[18] Thus, the development of sensi-
tive methods for the determination of trace amounts of
silver ions is of considerable importance for environ-
mental protection and human health.[19] Previously,
much effort has been devoted to designing various fluo-
rescent sensors for the detection of silver ions.[20-23]
Though most reported chemosensors for heavy/transi-
tion metal ions are based upon quenching of fluores-
cence intensity (‘turn off’),[24] only a few are based
upon enhancement of fluorescence intensity (‘turn
on’).[25] Thus, highly selective and sensitive “Turn-On”
fluorescent sensor for determination of silver ions is of
increasing interest.
Experimental
Materials and instruments
All reagents and starting materials were obtained
from commercial suppliers and used as received unless
otherwise noted. All cations were used as the
perchlorate salts, which were purchased from Alfa Aesar
and used as received. Fresh double distilled ethanol was
used throughout the experiment. Nuclear magnetic
resonance (NMR) spectra were recorded on Varian
Mercury 400 and Varian Inova 600 instruments. Mass
spectra were recorded on a Bruker Esquire 6000 MS
instrument. Fluorescence spectra were recorded on a
Shimadzu RF-5301PC spectrofluorophotometer.
Synthesis and characterization of sensor L
Synthesis of sensor L Compound 3,4,5-tris(hexa-
decyloxy)benzohydrazide was synthesized according to
literatures methods.[32] As shown in Scheme 1, sensor L
Here, we report a novel acylhydrazone-based
‘turn-on’ type fluorescence chemosensor L (Scheme S1),
*
E-mail: weitaibao@126.com
Received September 2, 2014; accepted October 28, 2014; published online November 7, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400601 or from the author.
Chin. J. Chem. 2014, 32, 1255—1258
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Lin et al.
FULL PAPER
was synthesized as follows: 1-naphthaldehyde (1 mmol),
3,4,5-tris(hexadecyloxy)benzohydrazide (1 mmol) and
acetic acid (0.1 mL, as a catalyst) were added to ethanol
(20 mL), then the reaction mixture was stirred under
refluxed conditions for 24 h. After removing the solvent,
the precipitate of sensor L was yielded. Recrystalliza-
tion with CHCl3C2H5OH afforded solid of sensor L.
Characterization of sensor L Yield: 75%, m.p.
Results and Discussion
The synthesis procedure of L is shown in Scheme
S1 in SI. The selective recognition of the target ion from
competitive systems is an essential parameter for the
practical application of chemosensors. To investigate the
ability of recognition of the sensor L for Ag＋, the fluo-
rescent spectral changes were investigated upon addi-
＋
＋
tion of 50 μL various metal perchlorates (Mg2 , Ca2 ,
1
89－91 ℃; H NMR (Figure S1) (CDCl3, 400 MHz) δ:
＋
＋
＋
＋
＋
＋
＋
＋
＋
Cr3 , Fe3 , Co2 , Ni2 , Cu2 , Zn2 , Cd2 , Hg2 , Pb2 )
and Al(NO3)3 2×10−3 mol/L in aqueous solution to
ethanol solutions of L (2.0×10−5 mol/L), respectively.
As shown in Figure 1 and Figure S5, no significant
spectral changes were observed in the fluorescence
spectra except Ag＋, the experimental results suggest
that L shows a notable selectivity to Ag＋. Moreover, to
investigate the ability of recognition of the sensor L for
Ag ＋ in different solvent, the fluorescent spectral
changes were investigated upon addition of various
9.77 (s, 1H, NH), 9.04 (s, 1H, N＝CH), 8.65 (d, J＝6.4
Hz, 1H, ArH), 7.99 (s, 1H, ArH), 7.87 (t, J＝7.8 Hz, 2H,
ArH), 7.51－7.45 (m, 3H, ArH), 7.11 (s, 2H, ArH), 3.98
(t, J ＝ 8 Hz, 6H, OCH2), 1.77 (t, J ＝ 8 Hz, 6H,
OCH2CH2CH2), 1.43－1.25 (m, 72H, CH2), 0.88 (t,
J＝6 Hz, 9H, CH3); 13C NMR (Figure S2) (CDCl3, 600
MHz) δ: 166.95, 152.78, 142.26, 141.34, 107.87, 133.80,
131.10, 130.71, 129.06, 128.82, 127.74, 127.37, 127.00,
126.19, 125.26, 124.60, 107.87, 105.91, 105.12, 73.56,
73.45, 69.39, 69.11, 68.92, 31.93, 30.34, 30.23, 29.73,
29.67, 29.38, 29.28, 29.17, 26.09, 22.70, 14.13; IR
(Figure S3) (KBr) v: 3450 (N－H), 1717 (C＝O), 1649
(C ＝ N) cm−1; MS-ESI (Figure S4) calcd for
C66H111N2O4 [L＋H]＋: 995.8544; found 995.8096.
＋
＋
＋
＋
＋
＋
metal perchlorates (Mg2 , Ca2 , Cr3 , Fe3 , Co2 , Ni2 ,
＋
＋
＋
＋
＋
Cu2 , Zn2 , Cd2 , Hg2 , Pb2 ) and Al(NO3)3 to ethanol-
H2O (Figure S6) solutions of L and DMSO (Figure S7)
respectively. As shown in Figure S6 and Figure S7, no
significant spectral changes were observed in the fluo-
rescence spectra, the experimental results suggest that L
shows a notable selectivity to Ag＋ only in ethanol sol-
vent.
Scheme 1 The synthetic route to sensor L
To obtain insight into the binding properties of L
toward Ag＋, a fluorescence titration experiment was
conducted. Figure 2 shows the fluorescence spectra of
sensor L in the presence of different amounts of Ag＋ in
ethanol. Free sensor L shows rather weak emission at
448 nm. However, after addition of silver ions, the
emission band at 448 nm emerged and its intensity in-
creased gradually. Moreover, as shown in the Figure S8,
the fluorescence intensity of L at 448 nm increases li-
nearly with the concentration of Ag＋, where a nearly
linear plot of I485nm versus the amounts of Ag＋ in the
range of 0－2.0 equiv. was displayed (linear depend-
ency coefficient R＝0.994). The detection limit of silver
ions assay can reach 1.2×10−7 mol/L,[33] which sug-
gests that L shows a notable sensitivity to silver ions.
To investigate the ability of anti-interference of the
sensor L for Ag＋, the experiment of anti-interference of
sensor L solution in the presence of different metal ions
was carried out and the results are shown in Figure 3. It
O
O
O
O
C
16H33Br
K2CO3 , KI, acetone, reflux 72 h
C
16H33
O
OC16H33
OC16H33
HO
OH
OH
H
O
N
NH2
.
NH2NH2 H2O (85%)
C2H5OH, reflux 24 h
C16H33
O
OC16H33
OC16H33
O
H
O
N
NH2
＋
＋
should be mentioned that the addition of Mg2 , Ca2 ,
C2H5OH, reflux 24 h
glacial acetic acid
＋
＋
＋
＋
＋
＋
＋
＋
＋
Al3 , Cr3 , Fe3 , Co2 , Ni2 , Cu2 , Zn2 , Cd2 , Hg2
C16H33
O
OC16H33
＋
and Pb2 enhanced slightly the fluorescence of L in the
presence of Ag＋, which did not interfere the analysis of
Ag＋. The results indicated that L can be used as a po-
tential sensor for Ag＋ with high selectivity in the pres-
ence of other competitive species. Several factors might
cooperate to achieve the unique selectivity and high
sensitivity of the sensor L for the Ag＋ detection, such
as the suitable radius of Ag＋, the structural rigidity and
the binding ability of the sensor L with Ag＋.
OC16H33
OC16H33
OC16H33
OC16H33
N
NH
O
L
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 1255—1258

A Novel Highly Selective “Turn-On” Fluorescence Sensor for Silver Ions Based on Schiff Base
ing ability of L could be ascribed to the specific
complexation behavior of L with Ag＋.[31] In order to get
more information for complexation between sensor L
and Ag＋, the mass spectrum was studied in the presence
of Ag＋, as shown in Figure S9. Moreover, as shown in
Figure S10, we also carried out the Job’s plot to confirm
the binding stoichiometry between sensor L and Ag＋.
The results show that sensor L forms a 1∶1 complex
with Ag＋ in ethanol. To confirm the complexation
＋
1
process of L with Ag , the H NMR titration experi-
ments (Figure S11) in CDCl3 were studied in the ab-
sence and presence of different concentrations of Ag＋.
As shown in Figure S11 (a－c), in the concentration
dependent 1H NMR of L, the NH (Ha) and N＝CH (Hb)
resonance signals showed obvious downfield shifts with
the concentration of L. However, after addition of Ag＋
to the solution of L, and with the amounts of Ag＋ in-
creasing, the NH (Ha) and N＝CH (Hb) showed signifi-
cant downfield shifts (Figure S10 d), and all of the
phenyl protons shifted upfield. Meanwhile, FT-IR spec-
tra were also studied in the absence and presence of Ag＋.
As shown in Figure S12, in the IR spectra of L, peaks at
1713 and 1642 cm−1 are attributed to the C＝O and CH
＝N groups, respectively. However, after addition of
Ag＋, these peaks showed obvious red-shift, indicating L
binds the Ag＋ from the C＝O and CH＝N groups. Ac-
cording to these results, we proposed a possible recog-
nition mechanism of L for Ag+ as shown in Scheme 2.
Figure 1 Variation of the fluorescence intensity at 448 nm
(λex＝332 nm) of compound L in ethanol in the presence of 10.0
equiv. of the respective metal ions. (Insert) Fluorescence changes
＋
＋
(under UV light) of L in the presence of 10 equiv. of Mg2 , Ca2 ,
＋
＋
＋
＋
＋
＋
＋
＋
＋
Cr3 , Fe3 , Co2 , Ni2 , Cu2 , Zn2 , Ag＋, Cd2 , Hg2 , Pb2 .
Scheme 2 Structure of L and proposed mechanism for the rec-
ognition of the sensor L with Ag＋
Hd
Hc
OC16H33
OC16H33
Hh
Ha
Hb
N
N
Ag+
UV
Figure 2 Fluorescence spectra of the sensor L in ethanol upon
the addition of 0－2 equiv. silver ions (λex＝332 nm).
O
OC16H33
Hg
He
Hf
L
Fluorescence OFF
OC16H33
OC16H33
H
N
N
O
OC16H33
Ag
Fluorescence ON
Conclusions
In conclusion, a novel sensor L was prepared suc-
cessfully. Sensor L demonstrates an Ag ＋ -specific,
highly sensitive and selective turn-on sensing behavior
based on the coordination mechanism. The spectro-
scopic properties and the detection limit of sensor L for
Ag＋ (1.2×10−7 mol•L−1) indicate that it is a potential
powerful sensor which is likely to image free silver ions
at very low concentration.
Figure 3 Fluorescence intensity at 448 nm (λex＝332 nm) of L
in the presence of Ag＋ after addition of other metal ions.
The high sensitivity and selectivity of the Ag＋ sens-
Chin. J. Chem. 2014, 32, 1255—1258
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Lin et al.
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Acknowledgement
This work was supported by the National Natural
Science Foundation of China (Nos. 21064006,
21262032 and 21161018), the Program for Changjiang
Scholars and Innovative Research Team in University of
Ministry of Education of China (No. IRT1177), the
Natural Science Foundation of Gansu Province (No.
1010RJZA018), the Youth Foundation of Gansu Prov-
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