A sensitive and highly selective fluorescent sensor for In3+

Article abstract of DOI:10.1039/c0ob00002g

A simple neutral fluorescent sensor based on pyrazolo[1,5-a]pyrimidine exhibited a unique selectivity for indium(iii) ion (In³⁺) over various other metal ions with dramatic fluorescence response in acetonitrile. The Royal Society of Chemistry 2

Full text of DOI:10.1039/c0ob00002g

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3
+
A sensitive and highly selective fluorescent sensor for In †
Yan-Chao Wu,*a,b Hui-Jing Li and Hua-Zheng Yang
b
a
Received 14th April 2010, Accepted 2nd June 2010
First published as an Advance Article on the web 16th June 2010
DOI: 10.1039/c0ob00002g
A simple neutral fluorescent sensor based on pyrazolo[1,5-
a]pyrimidine exhibited a unique selectivity for indium(III) ion
3+
(
In ) over various other metal ions with dramatic fluores-
cence response in acetonitrile.
Indium is an important element identified by its brilliant indigo
blue flame coloration. It has significant specific applications, most
notably as a component of III–V semiconductor devices, gas
1
sensors, and transparent electrically conductive films. Indium(III)
salts have attracted great attentions in chemistry and biological
sciences due to their water-tolerant and air-stable properties, as
2
3+
well as their “low toxicities”. However, nowadays, In is believed
3
+
to interfere with Fe metabolism from the sites of absorption,
transportation, utilization, and storage in the cells. In was
found to be toxic to the liver and kidneys. In is more reactive
toward biological membranes than Cd and Hg . Moreover,
Scheme 1 Synthesis of pyrazolo[1,5-a]pyrimidine 1
3
3+
4
3+
13
The interaction of pyrazolo[1,5-a]pyrimidine 1 with In(OTf)3
2
+
2+ 5
was investigated by spectrofluorimetric titration experiments in
acetonitrile (Fig. 1). The maximum excitation wavelength was
selected at 309 nm according to its UV–vis spectra absorption
in acetonitrile.
3
+
6
In is extremely toxic to the lungs and is a suspected carcinogen.
3
+
Therefore, recognition and selective sensing of In becomes a
high priority. Since fluorescent sensors are widely appreciated in
the field of chemistry, biology, and environmental sciences because
of their simplicity, high sensitivity and low detection limits, the
development of an In -selective fluorescent sensor would find
7
3
+
8
3
+
applications in the study of cellular In ion as well as in dealing
9
with its environmental assay.
During our ongoing projects aiming at the development of
10
various functional heterocycles, we found that pyrazolo[1,5-
11
a]pyrimidines exhibit interesting fluorescent spectral properties.
Thus, we further examined the correlation between their substi-
12
tution patterns and their fluorescent spectroscopic properties.
Complementary to these efforts, herein we report the appli-
cation of pyrazolo[1,5-a]pyrimidine 1 (Scheme 1) as a simple,
sensitive and surprisingly highly selective fluorescent sensor for
3
+
In . Pyrazolo[1,5-a]pyrimidine 1 was smoothly synthesized from
commercially available ethyl cyanoacetate (2, Scheme 1). Thus,
successive deprotonation of 2 with potassium hydroxide, coupling
with carbon disulfide and methylation with dimethyl sulfate
afforded thioacetal 3 in 93% yield. Treatment of 3 with hydrazine
afforded aminopyrazole 4 in 80% yield, which was in turn
converted to pyrazolo[1,5-a]pyrimidine 1 (98%) by condensation
with pentane-2,4-dione (5).
Fig. 1 Fluorescence emission spectra of pyrazolo[1,5-a]pyrimidine 1 (1 ¥
-5
3+
10 M) in the presence of In(OTf)
3 3
in CH CN. The concentration of In :
-5
0
, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 ¥ 10 M. lex = 309 nm.
As shown in Fig. 1, pyrazolo[1,5-a]pyrimidine 1 shows a
relatively stable emission band around 412 nm. When the con-
3
+
centration of In was increased up to 1.0 molar equivalent (1 ¥
-
5
1
0 M), 95% quenching of the initial fluorescence of pyrazolo[1,5-
a]pyrimidine 1 was observed. From the fluorescence titration
a
State Key Laboratory of Element-Organic Chemistry, Institute of
experiments, the plotting of the change of fluorescence intensity
Element-Organic Chemistry, Nankai University, Tianjin 300071, China.
E-mail: ycwu@iccas.ac.cn
3
+
I
0
/(I - I) against 1/[In ] afforded a line with a good linear
0
b
correlation coefficient (R = 0.9988, see ESI†), indicating the
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key
14
Laboratory of Molecular Recognition and Function, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
formation of a possible 1 : 1 complex. The detection limit of 1 as a
3+
fluorescent sensor for the analysis of In was determined from the
†
Electronic supplementary information (ESI) available: Experimental
plot of the fluorescence intensity as a function of the concentration
1
1
3
procedures, product characterization, copies of H NMR and
NMR spectra, fluorescence data, and fluorescence spectra. See DOI:
0.1039/c0ob00002g
C
of added metal ions. It was found that pyrazolo[1,5-a]pyrimidine
-
7
-1
3+
1
1 has a detection limit of 1.9 ¥ 10 mol L for In , which is low
3
394 | Org. Biomol. Chem., 2010, 8, 3394–3397
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3
+
enough for the detection of the submillimolar concentration range
In with atom N-8 would lead to little downshift of protons H-
d because the electronic effect (more than 3 bonds) is little and
the conjugation of N(8)–C(7)=C(6)–C(5)–C(d) (single-double-
3
+
of In .
3
+
In order to determine the stoichiometry of 1-In complex,
the method of continuous variations (Job’s method) was also
used (Fig. 2). As expected, the result obtained from the Job plot
unambiguously indicates the formation of a 1 : 1 complex between
single-single bonds) is inhibited. Therefore, the complexation
of In with atom N-4 should belong to the possible binding
3
+
18
mechanism, which results in the downshifts of the protons (H-
6, H-d and H-e) through the s–p hyperconjugation and p–p
conjugation (Fig. 4). The similar chelating ring can be found in
a reported literature, where In ion, superior to other selected
metal ions, simultaneous chelates with a sp2 nitrogen and an ester
oxygen to result in a five-member chelating ring. We previously
believed that the possible six-member chelating ring in our case
3
+
pyrazolo[1,5-a]pyrimidine 1 and In .
19
3+
(
Fig. 4) decrease the electron density at the pyrimidine part
of pyrazolo[1,5-a]pyrimidine 1, and thereby result in quenching
of the fluorescence, which matches our previous observations
that pyrazolo[1,5-a]pyrimidines with the pyrimidine units bearing
12
strong electron-withdrawing groups show no fluorescence. If
this were indeed the case, simple protonation would equally well
quench the fluorescence of pyrazolo[1,5-a]pyrimidine 1. However,
no significant spectral changes of pyrazolo[1,5-a]pyrimidine 1
n
occurred in the presence of 100 equivalents of acidic Bu
4
NHSO
4
3
+
Fig. 2 Job plot between pyrazolo[1,5-a]pyrimidine 1 and In in CH
3
CN.
in acetonitrile (Fig. 5). Although we are not able at this time to
accurately explain the underlying mechanism of this fluorescence
quenching. For these ligand donor atoms that are essentially
decoupled electronically from the HOMO/LUMO residing in
the chromophoric unit, one might invoke PET-type quenching.
Alternatively, an inversion in the energy level of n–pi* vs. pi–
pi* transitions and rapid ISC might explain this binding-induced
quenching.
3+
-5
[1] + [In ] = 2 ¥ 10 M. lex = 309 nm. I
A
represents the fluorescence
intensity of pyrazolo[1,5-a]pyrimidine 1 in the presence of In . I
represents the fluorescence intensity of free 1.
3+
B
The complexation of pyrazolo[1,5-a]pyrimidine 1 with In(OTf)
was also demonstrated by NMR titration experiments in CD CN
see ESI†). Partial H NMR spectra of 1 in the absence and
3
3
1
(
3
+
presence of In (1.0 molar equivalent) are shown in Fig. 3.
3+
Fig. 4 Possible binding mechanisms for 1 with In .
1
Fig. 3 (a) Partial H NMR spectra of free pyrazolo[1,5-a]pyrimidine 1.
1
(
b) Partial H NMR spectra of 1 in the presence of In(OTf)
3
(1 molar
equiv.).
Protons H-b adjacent to atom O-12 experience downfield shift
3
+
3+
of 0.20 ppm upon addition of In , which indicates that In binds
15
with atom O-12 or O-11. However, protons H-a adjacent to
atom S-13 show no shift, which implies that In has no binding
with atom S-13 or N-1. The binding of In with atom N-1
3
+
3
+
Fig. 5 Fluorescence responses of 1 in CH
3
CN (1 mM) upon ad-
should induce the downfield shift of protons H-a by the n–p
n
n
n
ditions of various anions (100 mM of Bu
4
NF, Bu
4
NCl, Bu
4
NBr,
16,17
conjugation of S(13)–C(2)=N(1) bonds.
On the other hand,
n
n
n
n
n
n
Bu
4
NI, Bu
4
NOAc, Bu
4
NHSO
4
, Bu
4
NH
2
PO
4
, Bu
4
NNO
3
, Bu
4
NCN,
3
+
the formation of 1-In complex results in large shifts to low
field of proton H-6 (Dd = 0.46 ppm), protons H-d (Dd = 0.34
ppm) and protons H-e (Dd = 0.36 ppm), which indicates that
n
Bu
4
NOTf). lex = 309 nm.
To obviate the possibility that the dramatic fluorescence re-
sponse of pyrazolo[1,5-a]pyrimidine 1 in the presence of In(OTf)
3
+
In complexes with the pyrimidine part. The complexation of
3
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2
0
results from the formation of 1-OTf complex. We tested the
fluorescent responses of pyrazolo[1,5-a]pyrimidine 1 to various
anions including OTf (Fig. 5). No significant spectral changes
were carried on (Fig. 6). Thus, pyrazolo[1,5-a]pyrimidine 1 (1
3
+
mM) was treated with 2 mM of In in the presence of background
metal ions (20 mM), respectively. These background metal ions
have no significant interference with the detection of In , which
suggests that pyrazolo[1,5-a]pyrimidine 1 has potential to be used
as an In -selective fluorescent chemosensor.
-
3
+
of pyrazolo[1,5-a]pyrimidine 1 occurred in the presence of 100
-
-
-
-
-
-
-
-
-
equivalents of F , Cl , Br , I , AcO , HSO
4
, H
2
PO
4
, NO
3
, CN ,
-
3+
OTf , which indicates that pyrazolo[1,5-a]pyrimidine 1 shows little
sensitivity towards these tested anions and the formation of 1-In
3
+
In summary, we present pyrazolo[1,5-a]pyrimidine 1 as a
promising fluorescent sensor for the fluorimetric In detection,
3
+
complex leads to the corresponding fluorescence response. On the
other hand, these anions, possessing different values of acidity,
cause little fluorescent response of 1, illustrating its promising
3
+
which shows high sensitivity and selectivity toward In over a
wide range of metal ions in acetonitile. Its binding properties have
been extensively studied mainly by spectrofluorimetric titration
experiments, Job’s method, NMR titration experiments and
competition experiments. These experiments were performed in
anhydrous acetonitrile. When aqueous solutions of In salts were
added to the acetonitrile solution of pyrazolo[1,5-a]pyrimidine
1, desired results could also be obtained by drying the system
over anhydrous sodium sulfate. Studies on making new derivatives
of the heterocycle suitable for aqueous use are in progress. We
application potential. In deed, InCl
neutral salt of In(OTf) , give the similar fluorescent responses
of pyrazolo[1,5-a]pyrimidine 1. In(OTf) was selected here as a
representative of indium(III) salts just because most of metal salts
3
and InBr
3
, similar as the
3
3
3
+
used in our experiments are triflate salts and In(OTf)
for good comparisons.
3
was used
With the above results in hands, we further tested the fluorescent
responses of pyrazolo[1,5-a]pyrimidine 1 to various metal ions
including common biologically available ions (see ESI†). As
shown in Fig. 6 (selected representative results), no significant
spectral changes of 1 occurred in the presence of 100 equiv-
3
+
envision that this kind of In -selective fluorescent sensors could
find practical applications in the future.
+
2+
3+
4+
3+
alents of Na (Ia), Mg (IIa), Sc (IIIb), Hf (IVb), Nb (Vb),
Acknowledgements
3
+
2+
3+
2+
2+
Cr (VIb), Mn (VIIb), Fe (VIIIb), Co (VIIIb), Ni (VIIIb),
2
+
2+
3+
3+
2+
3+
Cu (Ib), Zn (IIb), Al (IIIa), Ga (IIIa), Pb (IVa), Bi (Va)].
We thank the National Natural Science Foundations of China,
Ministry of Science and Technology and the Chinese Academy of
Sciences for the financial supports.
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The white bars represent the response to the addition of 2 mM of In to
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6
98–705.
3
+
good selectivity towards In (IIIa) over various other tested metal
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3
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3
+
1
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2
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+
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3
+
(
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3
+
17 In ion can complex with some dithiocarboxylate anions, see: (a) S.
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+3
8
9
Although numerous fluorescent sensors for a variety of metal ions have
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3+
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-1
3+
The occupational exposure limit (OEL) of indium is 0.10 ugL in USA
18 In ion can easily complex with neutral sp2 nitrogen atoms, see: J. Lu
-1
and UK, which is lower than that of toxic lead (0.15 ugL ) in these two
countries.
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