Selective ratiometric fluorescence detection of acetate based on a novel Schiff base derivative

Article abstract of DOI:10.1007/s10895-011-0972-6

A ratiometric fluorescence and colorimetric probe based on 5,5′-methylene-bis-salicylaldehyde-pnitrophenylhydrazone (1), which was characterized by 1H NMR, mass spectrometer and elemental analysis, was prepared through Schiff base reaction. The probe exhibited visible color changes from yellow to purple upon interacting with acetate ion. Particularly, the compound showed ratiometric fluorescence response to acetate with significant blue shift about 150 nm from 410 nm to 560 nm. Thence, the probe 1 was a selective ratiometric fluorescence probe for acetate ion in DMSO solution.

Full text of DOI:10.1007/s10895-011-0972-6

J Fluoresc (2012) 22:397–401
DOI 10.1007/s10895-011-0972-6
ORIGINAL PAPER
Selective Ratiometric Fluorescence Detection of Acetate
Based on a Novel Schiff Base Derivative
Ge Liu & Jie Shao
Received: 10 April 2011 /Accepted: 6 September 2011 /Published online: 16 September 2011
#
Springer Science+Business Media, LLC 2011
Abstract A ratiometric fluorescence and colorimetric
probe based on 5,5′-methylene-bis-salicylaldehyde-p-
nitrophenylhydrazone (1), which was characterized by
1H NMR, mass spectrometer and elemental analysis, was
prepared through Schiff base reaction. The probe
exhibited visible color changes from yellow to purple
upon interacting with acetate ion. Particularly, the com-
pound showed ratiometric fluorescence response to acetate
with significant blue shift about 150 nm from 410 nm to
560 nm. Thence, the probe 1 was a selective ratiometric
fluorescence probe for acetate ion in DMSO solution.
detecting trace amounts of AcO− is important not only for
biological process, but also for environmental applications.
One of the tools for detecting AcO− is to utilize a
fluorescent chemosensor [6, 7]. However, most artificial
fluorescent chemosensors give rise to quenching or en-
hancement of signals upon AcO− binding [6–8]. Only a few
examples were reported to be ratiometric fluorescence
sensors toward acetate ion [9]. The ratiometric fluorescence
signaling, which involves the measurement of changes in
the ratio of the fluorescence intensities at two different
wavelengths, is superior to the conventional method of
monitoring the fluorescence intensity at a single wavelength
[10]. Unfortunately, the ratiometric fluorescence sensors for
anions were rarely reported in the literature. Accordingly,
development of ratiometric fluorescence probe for acetate is
very necessary.
.
.
.
Keywords Anion probe Colorimetric Ratiometric
.
Fluorescence Schiff base
Introduction
To date, a variety of signaling mechanisms have been
proposed and utilized for optical detection of different
species, including photo-induced electron transfer (PET)
[11, 12], photo-induced charge transfer (PCT) [13, 14],
excimer/exciplex formation [15], and excited-state intra-/
intermolecular proton transfer (ESIPT) [16]. However, two
signaling mechanisms used frequently in development of
fluorescence anion sensor are PET and PCT. In general, the
hydrogen-bonded complex with anion activated the PET
process more efficiently and showed up a greater fluores-
cence quenching. Accordingly, fluorescence of the anion
PET chemosensor is ‘switch-off’ rather than ‘switch-on’
upon ion sensing unlike most PET sensors for cations [17–
19]. In addition, fluorescence anion sensor based PCT
contains at least two parts: the fluorophore and the anion
binding site. Furthermore, the anion binding site as an
electron-donating group is conjugated to a fluorophore
group with an electron-withdrawing unit. When the host–
guest system forms, it undergoes photo-induced charge
The selective recognition and sensing of anions is an
important research topic due to their importance in
chemistry and biotechnology [1–3]. Among various anions,
acetate (AcO−) has attracted the interest of chemists due to
that acetate is a critical component of numerous metabolic
processes [4] and the rate of AcO− production and
oxidation has been frequently used as an indicator of
organic decomposition in marine sediments [5]. Thus,
G. Liu
Department of Chemistry, Chifeng University,
Chifeng 024000, China
J. Shao (*)
Department of Chemistry and Materials Science,
Nanjing Forestry University,
Nanjing 210037, China
e-mail: njshao@live.cn

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J Fluoresc (2012) 22:397–401
Scheme 1 Synthesis route for
the compound 1
NH2
NH
OHC
HO
CHO
N
OH
OH
N
CH3CH2OH
refluxing
+
NH
NH
OH
NO2
1
NO2
NO2
transfer (PCT) from the donor to the acceptor upon
excitation by light. The consequent change in dipole
moment results in ratiometric fluorescence change [20].
In this paper, we reported a ratiometric fluorescence and
colorimetric anion probe 1 based on a simple Schiff base
derivative. In DMSO solution, the probe 1 exhibited photo-
induced charge transfer (PCT) processes upon binding
anions in the excited state. As a result, the chemosensor 1
showed ratiometric fluorescence responses towards anions
tested with strong basicity such as AcO− and F− ions.
F96 Spectrophtometer made by Shanghai Lengguang Tech-
nology Co.,LTD.. The width of the slits is 5 nm.
All reagents for synthesis were obtained commercially and
used without further purification. In the titration experiments,
all the anions were added in the form of tetra-butyl-
ammonium (TBA) salts, which were purchased from Alfa
Aesar Chemical, stored in a vacuum desiccator containing
self-indicating silica and dried fully before using. DMSO was
dried with CaH2 and the distilled at reduced pressure.
Synthesis of 5,5′-methylene-bis-salicylaldehyde-p-
nitrophenylhydrazone (1)
Experimental
The synthesis route of probe 1 was shown in Scheme 1.
5,5′-methylene-bis-salicylaldehyde [1] (0.256 g, 1 mmol)
in ethanol (10 ml) was added dropwise a solution of p-
nitrophenylhydrazine (0.306 g, 2 mmol) in ethanol
(25 ml). Then the mixture was heated to reflux under
magnetic stirring for 2 h. During the reaction a yellow
precipitate appeared which was collected by filtration,
washed with ethanol and the pure product (0.391 g) was
Apparatus and Chemicals
1H NMR spectra were obtained on a Varian UNITY Plus-
300 MHz Spectrometer. ESI-MS was performed with a
MARINER apparatus. C, H, N elemental analyses were
made on an elementar vario EL. UV–vis spectra were
recorded on a TU-1810 Spectrophotometer made by Beijing
Puxi Tongyong apparatus company with quartz cuvette (path
length = 1 nm), and fluorescence spectra were recorded on a
1
obtained. Yield = 74%. H NMR (DMSO-d6,300 MHz) δ:
11.28 (s, 2H, –OH), 9.98 (s, 2H, –NH), 8.34 (s, 2H, Ar–
H), 8.11 (d, J=5.4 Hz, 4H, Ar–H), 7.63 (d, J=0.6 Hz, 2H,
Ar–H), 7.09 (t, J=8.4 Hz, 6H, Ar–H), 6.84 (d, J=4.8 Hz,
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
−
2H, Ar–H), 3.84 (s, 2H, –CH2 ); ESI-MS: m/z calcd. for
C27H22N6O6 [M]−: 526.16, found: 525.10; elemental
analysis calcd. For C27H22N6O6: C, 61.16; H, 4.82, N,
12.84; found: C, 61.39; H, 4.89; N, 12.48.
Results and Discussion
UV–vis Spectral Titrations and Naked-eye Experiments
The anion-binding properties of compound 1 were
investigated by UV–vis spectroscopy in dry DMSO
solution. Figure 1 shows the changes in the absorption
spectrum of 1 as a function of AcO− concentration at
room temperature. The free 1 showed a main absorbance
band centered at 427 nm, which could be assigned to the
300
400
500
600
700
Wavelength/nm
Fig. 1 UV–vis spectral changes of the probe 1 (2×10−5 M) in
absence and presence of acetate ion in DMSO

J Fluoresc (2012) 22:397–401
399
Fig. 2 Color changes of the probe 1 (2×10−5 M) in absence and presence of 30 equiv various anions tested (from left to right: 1 only; 1 + F−; 1 +
AcO−; 1 + H2PO4−; 1 + Cl−, Br− or I−) in DMSO, respectively
intramolecular charge transfer (ICT) nature of p-nitro-
nitrobenzene substituent [21]. There were gradual in-
crease in 272 nm and decrease in 427 nm upon addition of
acetate ions. Synchronously, a new red-shifted absor-
bance band at 565 nm (about 138 nm red shift) was
observed, which responsible for naked-eye detection of
acetate with color changes from yellow to deep purple
(Fig. 2). The probe 1 demonstrated color changes from
yellow to light purple and brown upon exposure to
fluoride and dihydrogen phosphate, respectively. Howev-
er, no visible color changes were observed when the other
anions tested were added. The new band and color changes
resulted from the host bond by anion and deprotonation of
anion binding sites, which could be further validated by 1H
NMR titrations of 1 with fluoride ion. In addition, there
were two well-defined spectral isobestic points at 318 nm
and 473 nm, respectively, suggesting the formation of a 1/
AcO− complex.
Florescence Titrations
Next, fluorescence responses to various anions of 1 (2×
10−5 M) were studied in DMSO solution. When a solution of
1 was excited at 365 nm, the compound 1 exhibited main
emission around 560 nm with shoulder emission around
410 nm. Just as Fig. 4 showed, the presence of acetate ion
induced disappearance of emission at 560 nm and appear-
ance of fluorescence enhancement at 410 nm (a 150-nm
emission band shift), which was rarely seen in the ever-
reported anion probes. Accordingly, ratiometric fluorescence
detection of AcO− could be achieved in virtue of the probe 1.
Such ratiometric changes of fluorescence could be rational-
ized on basis of photo-induced charge transfer (PCT)
mechanism [20]. In compound 1, the anion binding site (–
OH, electron-donating group) is conjugated to the fluoro-
phore group (5,5′-methylene-bis-salicylaldehyde). When 1
interacted with acetate, the consequent changes in dipole
moment were obtained and therefore ratiometric fluorescence
changes were observed. Importantly, the compound 1 could
act as a high selective ratiometric fluorescence probe for
acetate ion, which could be seen in Fig. 5.
In comparison to acetate ion, a similar optical
response is observed when the probe is treated with
F− and H2PO4 (see Figure 3). However, addition of the
−
other anions with weak basicity such as Cl−, Br− and I−
did not result in any obvious spectral changes and color
changes, which were demonstrated in Figs. 2 and 3. The
results indicated that the probe 1 could selective and
1H NMR Titrations
−
naked-eye detection anions such as AcO−, F− and H2PO4
To shed light on the nature of the interactions between the
probe 1 and the anions, H NMR spectral changes upon
with strong basicity from the anions such as Cl−, Br− and
I− with weak basicity.
1
60
50
40
30
20
10
0
1.2
1.0
1 + AcO-
0.8
1 + F-
0.6
1 only, 1 + Cl-, Br- or I-
0.4
1 + H2PO4-
0.2
0.0
300
400
500
600
700
400
450
500
550
600
650
700
750
800
Wavelength/nm
Wavelength/nm
Fig. 4 Fluorescence changes of the probe 1 (2×10−5 M) in absence
and presence of acetate ion in DMSO (λex=365 nm)
Fig. 3 UV–vis spectral changes of the probe 1 (2×10−5 M) in
absence and presence of 30 equiv various anions tested in DMSO

400
J Fluoresc (2012) 22:397–401
20
15
10
5
2 equiv
1 equiv
0.5 equiv
0 equiv
0
-
-
-
-
-
-
I
AcO
F
H2PO4 Cl
Br
1 only
Fig. 5 Ratiometric fluorescence changes (I410/I560) of the probe 1 (2×
10−5 M) in absence and presence of 30 equiv various anions tested in
DMSO
addition of AcO− to the DMSO-d6 solution of 1 (1×
10−2 M) were investigated. Figure 6 showed 1H NMR
spectral changes of the receptor 1 (0.01 M) in DMSO-d6 in
the absence and presence of different equiv of acetate ions.
Chemical shifts of the compound resulting from the
formation of hydrogen bond between the binding sites and
the anion can be attributed to two effects [22]: (1) through-
bond effects, which increase the electron density of the
benzene ring and promote upfield shifts in 1H NMR
spectrum, and (2) through-space effects, which polarize
C–H bond in proximity to hydrogen bond, create the partial
positive charge on the proton and cause downfield shifts.
Obviously, it was found that the proton signals (δ: 11.28
and 9.98) underwent a slim downfield shift and were
broaden (Fig. 6), with increasing AcO− concentration from
0 equiv to 2 equiv indicating the formation of a host–guest
hydrogen-bonding complex [30]. The phenyl protons
especially for the protons (7.09 and 6.84 ppm) clearly
12
11
10
9
8
7
6
5
4
ppm
Fig. 6 1H NMR titrations of the probe 1 (1.0×10−2 M, DMSO-d6)
with different equiv of acetate
shifted upfield, indicative of the increase in the electron
density of the phenyl ring owing to the through-bond
effects. As mentioned above, the proposed binding mode in
solution was demonstrated in Scheme 2.
Association Constants
Affinity constants of probe 1 for anionic species in
DMSO, which were shown in Table 1, were determined
by non-linear fitting analyses of the titration curves
according to the Eq. 1 derived from the 1:1 host–guest
complexation [23].
o
1=2
2
ðYlim ꢀ Y0ÞfcH þ cG þ 1=Kass ꢀ ½ðcH þ cG þ 1=KassÞ ꢀ 4cH cGꢁ
2cH
Y ¼ Y0 þ
ð1Þ
where cG and cH are the concentration of guest and host,
respectively and Y is the absorption of UV–vis or intensity
of emission at certain concentration of host and guest. Y0 is
the absorption of UV–vis or intensity of emission of host
only and Ylim is the maximum absorption of UV–vis or
intensity of fluorescence of host when guest was added.
Kass is the affinity constant of host–guest complexation.
Table 1 Association constants Kass (mol−1•L) between receptor 1 and
anions in DMSO
AcO-
−
Anions
AcO−
F−
H2PO4
Cl−
Br−
I−
N
OH
OH
N
N
OH
OH
N
NH
NH
NH
NH
Kass (L·mol−1
Kass (L·mol−1
)
)
5,556a
12,500b
1,639
8,333
817
NDc
ND
ND
ND
ND
ND
-
O
O
3,333
a The Association constants were determined by the UV–vis spectra
b The Association constants were determined by the fluorescence
spectra
NO2
NO2
NO2
NO2
Scheme 2 The proposed host–guest interaction mode in the solution
c ND—The association constant could not be determined

J Fluoresc (2012) 22:397–401
401
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Obviously, the trends of binding ability of the probe 1
−
with anions tested were AcO−>F−>H2PO4 ∼Cl−∼Br−∼I−.
The selectivity for special analyte of the host molecule
could be rationalized on the basis of not only the guest
basicity but also shape complementarity between the host
and the anionic guests [24]. Obviously seen from Scheme 2,
the plane triangle acetate anion might be the fittest for the
hydrogen atoms on binding sites of the probe among the
anions. Interestingly, the association constants of the probe
1 with the same anion, which are determined by the UV–vis
spectra and the fluorescence spectra, are very different. The
acidity of the –OH moiety will be greatly enhanced upon
excitation which has been proven in the literature [29]. As
we all know, the stronger hydrogen bond interactions will
form between the group with strong acidity and anions.
Therefore, the association constants of the probe 1 with the
same anion determined by the fluorescence spectra are
bigger than those determined by the UV–vis spectra except
fluoride ion.
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In summary, a selective ratiometric fluorescence probe for
acetate ion was designed and synthesized. In this probe, a
hydrazone unit and a phenol unit are anion binding sites,
and 5,5′-methylene-bis-salicylaldehyde group is a fluoro-
phore. Presence of anions such as acetate resulted in
ratiometric fluorescence changes of the probe with
150 nm blue shift from 410 nm to 560 nm. Such ratiometric
fluorescence changes could be rationalized on basis of
photo-induced charge transfer (PCT) mechanism.
18. Jung HS, Ko KC, Lee JH, Kim SH, Bhuniya S, Lee JY, Kim Y,
Kim SJ, Kim JS (2010) Rationally designed fluorescence turn-on
sensors: a new design strategy based on orbital control. Inorg
Chem 49:8552–8557
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simple but effective dual redox and fluorescent ion pair
receptor based on a ferrocene–imidazopyrene dyad. Org Lett
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Acknowledgements This work was supported by Natural Science
Foundation of Universities of Inner Mongolia Autonomous Region
(No: NG09168) and the Start-Up Fund of the Nanjing Forestry
University (No: 163101026).
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