Isatinphenylsemicarbazones as efficient colorimetric sensors for fluoride and acetate anions - Anions induce tautomerism

Article abstract of DOI:10.1016/j.saa.2013.12.073

The anion induced tautomerism of isatin-3-4-phenyl(semicarbazone) derivatives is studied herein. The interaction of F^-, AcO ^-, H₂PO4-, Br^- or HSO4- anions with E and Z isomers of isatin-3-4-phenyl(semicarbazone) and N-methylisatin-3-4- phenyl(semicarbazone) as sensors influences the tautomeric equilibrium of these sensors in the liquid phase. This tautomeric equilibrium is affected by (1) the inter- and intra-molecular interactions' modulation of isatinphenylsemicarbazone molecules due to the anion induced change in the solvation shell of receptor molecules and (2) the sensor-anion interaction with the urea hydrogens. The acid-base properties of anions and the difference in sensor structure influence the equilibrium ratio of the individual tautomeric forms. Here, the tautomeric equilibrium changes were indicated by naked-eye experiment, UV-VIS spectral and ¹H NMR titration, resulting in confirmation that appropriate selection of experimental conditions leads to a high degree of sensor selectivity for some investigated anions. Sensors' E and Z isomers differ in sensitivity, selectivity and sensing mechanism. Detection of F^- or CH₃COO^- anions at high weakly basic anions' excess is possible.

Full text of DOI:10.1016/j.saa.2013.12.073

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Isatinphenylsemicarbazones as efficient colorimetric sensors for fluoride
and acetate anions – Anions induce tautomerism
a
a
b
b
ˇ a,
´
⇑
Klaudia Jakusová , Jana Donovalová , Marek Cigán , Martin Gáplovsky , Vladimír Garaj ,
a
´
Anton Gáplovsky
a Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina CH-2, SK-842 15 Bratislava, Slovakia
b Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, Odbojárov 10, SK-832 32 Bratislava, Slovakia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Anion induced tautomerism of
isatinphenylsemicarbazone sensors
was investigated.
Acid–base properties of anions
influence the equilibrium ratio of the
individual tautomeric forms.
Sensor E and Z isomers differs in
sensitivity, selectivity and sensing
mechanism.
Sensors provide an excellent signal to
noise ratio and also wide detection
range.
Detection of strongly basic anions in
both organic and semi-aqueous
media is possible.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 August 2013
Received in revised form 6 December 2013
Accepted 15 December 2013
Available online 21 December 2013
The anion induced tautomerism of isatin-ꢁ3-4-phenyl(semicarbazone) derivatives is studied herein. The
interaction of Fꢁ, AcOꢁ, H2PO4ꢁ, Brꢁ or HSO4 anions with E and Z isomers of isatin-3-4-phenyl(semicarba-
zone) and N-methylisatin-3-4-phenyl(semicarbazone) as sensors influences the tautomeric equilibrium
of these sensors in the liquid phase. This tautomeric equilibrium is affected by (1) the inter- and intra-
molecular interactions’ modulation of isatinphenylsemicarbazone molecules due to the anion induced
change in the solvation shell of receptor molecules and (2) the sensor–anion interaction with the urea
hydrogens. The acid–base properties of anions and the difference in sensor structure influence the equi-
librium ratio of the individual tautomeric forms. Here, the tautomeric equilibrium changes were indi-
cated by ‘‘naked-eye’’ experiment, UV–VIS spectral and 1H NMR titration, resulting in confirmation
that appropriate selection of experimental conditions leads to a high degree of sensor selectivity for some
investigated anions. Sensors’ E and Z isomers differ in sensitivity, selectivity and sensing mechanism.
Detection of Fꢁ or CH3COOꢁ anions at high weakly basic anions’ excess is possible.
Keywords:
Isatinphenylsemicarbazones
Self-association
Anion induced tautomerism
Anion basicity
Colorimetric sensors
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
‘‘binding site’’ responsible for the interaction with the detected
ion or neutral molecule and (2) the ‘‘signalling subunit’’, which de-
Ion and neutral molecule recognition and signalling is currently
one of the most intense emerging areas of supramolecular chemis-
try. The receptor molecule normally consists of two parts: (1) the
fines the measured signal type. While anions play a very important
role in many chemical and biological processes, their identification
and quantification is often complicated; especially in biological
systems. Since one way to solve this problem is to utilize new opti-
cal receptors specifically designed for individual anions, the focus
of supramolecular chemistry over the last twenty years has centred
⇑
Corresponding author. Tel.: +421 2 60296306; fax: +421 2 60296342.
ˇ
E-mail address: cigan@fns.uniba.sk (M. Cigán).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.12.073

422
K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
on the development of selective colorimetric or fluorescent anion
sensors [1–4]. Research in this area continues today in the work
of Wenzel, Chudzinski, Bergamaschi, Jiménez and others [5–8]. In-
creased attention now focuses especially on fluorescent receptors
due to their high sensitivity and simple application. The typical an-
ion-receptor molecule contains usually a pyrrole, amide, indolocar-
bazole, guanidine, imidazole or urea/thiourea structural fragment
[9–11]. Anions interact with receptors particularly through hydro-
gen bonding interactions [4,10–13]. These interactions affect
mainly the charge redistribution in the receptor resulting in a
change in the measured signal. This change also occurs, but less of-
ten, when the anion-receptor interaction influences the tautomeric
or other equilibrium between specific receptor structural forms
[14,15].
tetrabutylammonium salts purchased from Sigma–Aldrich (USA),
and used without further purification. The N,N-dimethylformam-
ide (DMF) solvent was of UV-spectroscopy grade (UvasolÒ, Merck,
Germany), and DMF was dried with CaH2 and distilled under re-
duced pressure.
General method
All titration experiments were carried out at 298.16 K. The Ia,
Ib, IIa and IIb receptor solutions were titrated withꢁthe corre-
sponding anion solutions (Fꢁ, AcOꢁ, H2PO4ꢁ, Brꢁ or HSO4 ) to obtain
a 1 ꢃ 10ꢁ4 mol dmꢁ3 receptor concentration of the resultant solu-
tion. The titration process in the anion concentration range of
1 ꢃ 10ꢁ5 mol dmꢁ3 to 1 ꢃ 10ꢁ2 mol dmꢁ3 was monitored by
UV–VIS spectroscopy (in a 1 cm cuvette; 10 cm cuvette was used
to determine the detection limits).
This paper focuses on study of the interactions between E and Z
isomers of isatin-3-4-phenyl(semicarbazone) and N-methylisatin-
3-4-phenyl(semicarbazone) (Scheme 1) with the following anions:
1H NMR titration experiments were carried out in DMF-d7 solu-
tion using tetramethylsilane as the internal standard.
Fꢁ, AcOꢁ, H2POꢁ, Brꢁ or HSOꢁ4 . The receptor dimethylformamide
4
(DMF) solution was titrated with the standard DMF solution of
the particular anion containing tetrabutylammonium (TBA) cation
as the counter-ion, and the titration effects were monitored by UV–
VIS and 1H NMR spectroscopy.
Association constant determinations
The anion-receptor association constants Kass of the studied
isatinphenylsemicarbazones with the particular anions (in the case
of 1:1 complex stoichiometry) were determined using the well-
known relation describing the complex anion concentration [17]:
ꢀ
Experimental
Synthesis
Alim ꢁ A0
ꢁ
A ¼ A0 þ
c0 þ cA þ 1=Kass
2c0
Synthesis of the studied compounds was published by Jakusová
et al. [16]. Studied E and Z isomers of I and II were synthesized
from 1H-indole-2,3-dione (isatin) or 3-methylindol-2,3(1H)-dione
(N-methylisatin), respectively, and 3-amino-1-phenylurea (phe-
nylsemicarbazide). A solution of phenylsemicarbazide in hot abso-
lute ethanol was added to isatin or N-methylisatin in hot absolute
ethanol. The reaction mixture was refluxed for 2 h and the formed
precipitate (Ia; E isomer) was filtered off from the hot solution,
washed with ethanol and dried. Z isomer Ib was isolated and puri-
fied with column chromatography using silica gel as stationary
phase and hexol (the mixture of methylpentanes and n-hexane;
ur ꢂ 2:1)/ethyl acetate (ur ꢂ 1:1) as mobile phase. Isomers IIa
and IIb were isolated from the evaporated reaction mixture and
purified using the same method as for Ib.
ꢁ
h
i
1=2
2
ꢁ
ꢁ
ꢁ ðc0 þ cA þ 1=KassÞ ꢁ 4c0cA
;
where A0 is the absorbance of free isatinphenylsemicarbazone, A is
the isatinphenylsemicarbazone absorbance measured after anion
addition, Alim is the isatinphenylsemicarbazone absorbance mea-
sured with excess amount of the particular anion, c0 is the overall
ꢁ
concentration of isatinphenylsemicarbazone and cA is the overall
concentration of the added anion Aꢁ. For the nonlinear fitting, this
equation was rewritten to the following form:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
A ¼ A0 þ c1 ꢄ ðP1 ꢁ A0Þ ꢄ ðc0 þ x þ 1=P2 ꢁ ðc0 þ x þ 1=P2Þ^2 ꢁ 4 ꢄ c0 ꢄ xÞ;
where c0 = 1 ꢃ 10ꢁ4
,
c1 = 1/2c0 = 5 ꢃ 103, parameter P1 = Alim
,
ꢁ
parameter P2 = Kass and x ¼ cA . The A0 value was fixed to the absor-
bance value A for x = 0. The large difference in constants c0, c1 and
the x-data range leads us to rescaling the used nonlinear function
(i.e. x and fitted parameter 1/P2 by a factor of 104). After this mod-
ification, we used a standard non-linear least-squares approach of
the Nelder-Mead minimization method to determine the fitting
parameters P1 and P2 [18].
Spectroscopic measurements
Electronic absorption spectra were obtained on a HP 8452A
(Hewlett Packard, USA) diode array spectrophotometer, and 1H
NMR spectra were recorded at 300 MHz and 600 MHz on VNMRS
(Varian, USA).
Where Job’s plot indicated other than 1:1 stoichiometry, the
Kass value and the stoichiometry of the complex were determined
using the Double-logarithm equation [19,20]:
Materials
ꢂ
ꢃ
All anions in the titration experiments, with the exception
of the sodium NaH2PO4 salt, were added in the form of
n
ꢁ
A0 ꢁ A
cA
Kass
¼
;
A ꢁ A
1
where parameter n gives the binding sites number (if n = a, then the
complex stoichiometry is 1:a).
Theoretical calculations
The structural geometries of Ia and Ib isomers, CH3COONa, NaBr
and NaF salts and their complexes were optimized using quantum-
chemical calculations at the B3LYP 6-31+G(d,p) level. Na+
counter-ions were used instead of tetrabutylammonium to reduce
computational time and resources. Stationary points were charac-
terized as minima by harmonic vibration frequency computations
Scheme 1. Molecular structure of the studied isatin-phenylsemicarbazones.

K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
423
at the same theoretical level as the geometric optimization. Single
point energies were calculated at the MP2(full)6-311++G(d,p) le-
vel, and zero-point vibration energies and thermal corrections to
the free energies were determined using unscaled B3LYP 6-
31+G(d,p) frequencies. All calculations were performed by the
Gaussian 09 program package [21]. The association energy be-
tween the studied isatinphenylsemicarbazones and the particular
anions was determined by the equation:
increasing anion concentration. No further shift in this absorption
band is observed at Fꢁ concentration of 1 ꢃ 10ꢁ2 mol dmꢁ3, and
the newly formed absorption band has its maximum at 391 nm.
The dependence of Ia absorbance at 328 nm and 430 nm on Fꢁ con-
centration is shown in the inset of Fig. 1. The decrease in the absor-
bance at 328 nm corresponds to the decrease in the concentration
of the starting compound (Ia) with increasing Fꢁ concentration.
Conversely, the change in the absorbance at 430 nm indicates the
change in the concentration of a newly formed compound, as a con-
sequence of the Fꢁ interaction with Ia. Fig. 1 clearly demonstrates
that within the Fꢁ concentration range of 0–5 ꢃ 10ꢁ3 mol dmꢁ3
the absorbance sharply increases (430 nm) or decreases (328 nm)
with increasing Fꢁ concentration.
À
Á
D
Eass ¼ ꢁ Ecomplex ꢁ ðEsalt þ EisomerÞ :
Finally, energies were calculated using counterpoise corrections
[22].
The absorbance value at 430 nm begins to decrease at Fꢁ con-
centration above 5 ꢃ 10ꢁ3 mol dmꢁ3, and this decrease becomes
Results and discussions
very progressive at Fꢁ concentration above 9 ꢃ 10ꢁ3 mol dmꢁ3
.
UV, VIS spectral titrations
In the presence of tetrabutylammonium acetate (CH3COOꢁ), the
absorbance changes in the UV–VIS spectrum of E isomer Ia at
410 nm and 328 nm are similar to those in the presence of Fꢁ.
However, in contrast to Fꢁ anions, the absorbance at both wave-
lengths at high CH3COOꢁ concentration reaches the limit value
and remains almost unchanged with further increase in CH3COOꢁ
concentration (Fig. 2).
The polarity of the solvent and concentration of isatin-3-4-
phenyl(semicarbazones) have a major impact on their UV–VIS
spectra [16,23]. Depending on the experimental conditions, the
UV–VIS spectra of the E isomers of these compounds may show
one or two main absorption maxima at 328 nm and 410 nm. The
corresponding Z isomers have only one maximum at approxi-
mately 340 nm, and the longwavelength absorption band at
410 nm was not observed in their UV–VIS spectra.
Brꢁ, HSOꢁ4 and H2POꢁ anions have an opposite effect on the UV–
4
VIS spectrum of isatinphenylsemicarbazone Ia to that of Fꢁ and
CH3COOꢁ anions (Fig. 3).
An increase in Brꢁ, HSO4ꢁ and H2POꢁ concentration results in de-
4
Anion interaction with (E)-isatin-3-4-phenyl(semicarbazone) (Ia) and
(E)-N-methylisatin-3–4-phenyl(semicarbazone) (IIa)
creased intensity in the absorption band localized at 410 nm and
increased absorption band intensity with its maximum at 328 nm.
The opposite anion influence on the absorption spectra behav-
iour of Ia during UV–VIS titrations can be rationalized by the
chemical equilibrium between various tautomeric forms of E iso-
mer Ia. As concluded in our previous work [23], the mutual ratio
of the Ia absorbance intensity at 410 nm and 328 nm in DMF de-
pends on the concentration of Ia. This concentration dependence
results from the existing hydrazide-hydrazonol tautomeric equilib-
rium of Ia in solution (Scheme 2 – forms A and B).
In the UV–VIS spectrum, tautomer B has an absorption band
with maximum at 410 nm. This absorption band is in the shape
and position of absorption maxima identical with the absorption
band of E isomer Ia after addition of Fꢁ and CH3COOꢁ anions in
the concentration range of 0–1 equivalent. Performed experiments
with anions show that the ratio of tautomeric forms A and B
A very intense absorption band at 328 nm plus weak absorption
at 410 nm was observed in the UV–VIS spectrum of Ia in DMF, at Ia
concentration of 1 ꢃ 10ꢁ4 mol dmꢁ3 and in the absence of added
anions. The presence of tetrabutylammonium fluoride (Fꢁ) in the
Ia solution in DMF leads to a decrease in absorption intensity at
328 nm and increased absorption at 410 nm (Fig. 1). Surprisingly,
Fꢁ concentration above 5 ꢃ 10ꢁ3 mol dmꢁ3 results in the gradual
hypsochromic shift of the longwavelegth maximum at 410 nm with
Fig. 1. Evolution of the UV–VIS spectrum of isatinphenylsemicarbazone E isomer Ia
in DMF during titration of the Ia solution by TBA+Fꢁ (cIa = 1 ꢃ 10ꢁ4 mol dmꢁ3; cF
=
-
Fig. 2. Evolution of the UV–VIS spectrum of isatinphenylsemicarbazone E isomer Ia
in DMF during titration of the Ia solution by TBA+CH3COOꢁ (cIa = 1 ꢃ 10ꢁ4 mol dmꢁ3
;
0 mol dmꢁ3
,
1 ꢃ 10ꢁ5 mol dmꢁ3
,
2.5 ꢃ 10ꢁ5 mol dmꢁ3
,
3.5 ꢃ 10ꢁ5 mol dmꢁ3
5 ꢃ
,
cCH3COO- = 0 mol dmꢁ3
,
1 ꢃ 10ꢁ5 mol dmꢁ3
,
2.5 ꢃ 10ꢁ5 mol dmꢁ3
,
3.5 ꢃ 10ꢁ5
-
10ꢁ5 mol dmꢁ3
,
5.5 ꢃ 10ꢁ5 mol dmꢁ3
,
7.5 ꢃ 10ꢁ5 mol dmꢁ3
,
1 ꢃ 10ꢁ4 mol dmꢁ3
,
mol dmꢁ3, 5 ꢃ 10ꢁ5 mol dmꢁ3 and 1 ꢃ 10ꢁ4 mol dmꢁ3). Inset: Corresponding depen-
dence of absorbance values at 328 nm and 410 nm for receptor Ia.
1 ꢃ 10ꢁ3 mol dmꢁ3, 5 ꢃ 10ꢁ3 mol dmꢁ3 and 1 ꢃ 10ꢁ2 mol dmꢁ3). Inset: Correspond-
ing dependence of absorbance values at 328 nm and 430 nm for receptor Ia.

424
K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
tautomer A molecule self-association (due to solvent and E isomer
concentration influences), there is less tautomer B in the system,
and this results in lower intensity of the absorption band at
410 nm in the UV–VIS spectrum. The presence of anions affects
the degree of the hydrazide A self-association in the solution
through intermolecular interactions. Basicity/acidity and the rela-
tive base/acid character of the Ia NH hydrogens play a crucial role
here. Basic anions influence the solvation shell of the self-associate
and this interaction leads to the non-associated form of hydrazide
A formation. Concurrently, basic anions mainly interact with the
most acidic NH hydrogen of hydrazide A resulting in hydrazonol
B formation. A gradual increase in Fꢁ or CH3COOꢁ concentration
leads to the situation where the most acidic hydrogen in all A mol-
ecules in the solution interacts with the anion, and the Ia E isomer
is present in the solution only in its hydrazol tautomeric form B. In
the UV–VIS spectrum of Ia, this state formation corresponds to the
absorption band intensity maximum at 410 nm. Further increase in
the Fꢁ concentration, up to 10ꢁ2 mol dmꢁ3, results in a hypsochro-
mic shift of the absorption band with maximum at 410 nm. We as-
sume that the intermolecular interaction of Fꢁ with NH hydrogens
of the isatin fragment and with the second hydrogen in the urea
part of the semicarbazone Ia skeleton (Scheme 3 – form C) are
responsible for this change.
Fig. 3. Evolution of the UV–VIS spectrum of isatinphenylsemicarbazone E isomer Ia
in DMF during titration of the Ia solution by TBA+H2PO4ꢁ (cIa = 1 ꢃ 10ꢁ4 mol dmꢁ3
;
2 POꢁ
cH
= 0 mol dmꢁ3, 2.5 ꢃ 10ꢁ5 mol dmꢁ3, 5 ꢃ 10ꢁ5 mol dmꢁ3, 1 ꢃ 10ꢁ4 mol dmꢁ3
,
4
2 ꢃ 10ꢁ4 mol dmꢁ3
,
9 ꢃ 10ꢁ4 mol dmꢁ3). Inset: Corresponding dependence of
absorbance values at 328 nm and 410 nm for receptor Ia.
The 1H NMR spectra of Ia recorded at different Fꢁ concentrations
support our previous conclusions (Fig. 4). Surprisingly, the signal
for the NH hydrogen adjacent to the imine group present in the ori-
ginal 1H NMR spectrum of Ia (Fig. 4 – N(2)–H) completely vanishes
at Fꢁ concentration equal to 10ꢁ5 mol dmꢁ3 (0.1 equiv.). However,
in the UV–VIS spectra of Ia, the addition of Fꢁ anions in this concen-
tration results in only a very small increase in the absorption band
intensity at 410 nm. Therefore, we assume that the basicity of Fꢁ
anions at this Fꢁ concentration is sufficient to change the solvation
of the self-associate, leading to decreased stability of the associated
AS form (Scheme 3). Further increases in the Fꢁ concentration to
less than 1 equivalent causes the dissociation of the associated form
AS and simultaneous formation of hydrazonol B (Scheme 3).
In 1H NMR spectrum, increase in the Fꢁ concentration up to 1
equivalent changes the chemical shifts of almost all H-signals to
a higher or lower field. In the UV–VIS spectrum, an increase in Fꢁ
concentration above 0.1 equivalent leads to an increase in the
intensity of the absorption band with maximum at 410 nm which
attains a limit value at Fꢁ concentration of 1 equivalent of Ia.
Fꢁ anion concentrations over 1 equivalent lead to weakening of
the intramolecular hydrogen bond in E isomer Ia and simultaneous
1H NMR signal broadening for the second hydrogen in the Ia urea
skeleton (Scheme 3 – form C; Fig. 4 – N(3)–H) accompanied by
reduction in N(3)–H signal intensity. This intramolecular hydrogen
bond weakening is responsible for the hypsochromic shift of the
longwavelength absorption band with maximum at 410 nm. In-
crease in Fꢁ concentration to 10ꢁ2 mol dmꢁ3 results in signal loss
for the isatin NH hydrogen (Fig. 4 – N(isatin)–H). As confirmed
by Job’s dependence (Fig. 5), the interaction of Ia with Fꢁ anions
is a complex process, and it is clearly evident from this dependence
that the studied reaction system consists of multiple equilibrium
processes, as depicted in Fig. 1. The reaction stage represented in
Fig. 5 by straight lines a and b corresponds to the gradual change
of the associated form AS of E isomer Ia to the hydrazonol form
B. The stoichiometric ratio of the reactants (Ia:Fꢁ) at this stage of
the reaction is 1:1. The portion of Job’s plot characterized by line
c delineates the process of Fꢁ anion intermolecular interaction
with NH hydrogens of the isatin fragment and the second hydrogen
in the urea moiety of Ia (Scheme 3 – Form C). At this reaction stage,
the Ia:Fꢁ stoichiometric ratio changes to 1:2. As mentioned above,
interaction with the second NH hydrogen of the urea skeleton is
confirmed by 1H NMR signal shift at increasing Fꢁ concentration
(Fig. 4 – signal N(3)–H).
Scheme 2. Hydrazide–hydrazonol chemical equilibrium of studied isatin-
phenylsemicarbazones.
depends on anion concentration in solution. In accordance with an-
ion basicity (or acidity), the equilibrium state with a smaller or a
larger amount of hydrazonol B is attained. The interaction of an-
ions with Ia does not lead to the appearance of a new absorption
band, but it shifts the hydrazide–hydrazonol tautomeric equilib-
rium to the left or to the right, depending on the anion acid–base
properties. This equilibrium shift is accompanied by an increase
or decrease in the intensity of the absorption band with maximum
at 410 nm. As stated in the above mentioned publication of our
group [23], the ratio of tautomers in DMF depends on the form
of A self-asociation. With increasingly favourable conditions for

K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
425
Scheme 3. Presumed non-covalent intermolecular interactions of derivative Ia with Fꢁ anions during titration of the Ia solution by TBA+Fꢁ.
CH3COONa salts: by 92.38 and 83.45 kJ molꢁ1
,
respectively.
However, in contrast to Ia-Fꢁinteraction, further increase in CH3-
COOꢁ concentration up to 10ꢁ2 mol dmꢁ3 does not lead to a
hypsochromic shift of the Ia absorption band at 410 nm (Fig. 2).
Although the UV–VIS spectral changes in Ia due to increased
CH3COOꢁ concentration can be described by Scheme 3 steps for
Fꢁ anions, structure C is not formed in the presence of CH3COOꢁ
anions. The intramolecular hydrogen bond in the hydrazonol B
structure is so strong that interaction with CH3COOꢁ is unable to
disrupt this bond even at high CH3COOꢁ concentrations. In contrast
to Fꢁ titration, the hypsochromic shift of the absorption maximum
in the UV–VIS spectrum of Ia at high CH3COOꢁ concentrations was
therefore not observed.
The described Ia-CH3COOꢁ interaction mechanism is also sup-
ported by 1H NMR spectra. Here, the addition of 1 equivalent CH3-
COOꢁ anions to the Ia solution leads to the disappearance of the
imine NH hydrogen signal. However, signals of the second hydro-
gen in the semicarbazone fragment and isatin NH hydrogen can
still be observed even at CH3COOꢁ concentration equal to 10ꢁ2
-
Fig. 4. Dependence of the 1H NMR signals for N(2)–H, N(3)–H and N(isatin)–H
nitrogens of the E isomer Ia on the Fꢁ concentration during titration of the Ia
solution by TBA+Fꢁ (measured in DMF-d7; cIa = 1 ꢃ 10ꢁ4 mol dmꢁ3).
mol dmꢁ3. Rather surprisingly, the Ia:CH3COOꢁ stoichiometric ra-
tio resulting from the Job’s plot is approximately 2:1 (Fig. S1),
and we therefore presume that this particular Job’s plot does not
correspond to the real reactant’s stoichiometric ratio. The mea-
sured increase in absorbance at 410 nm in the Ia UV–VIS spectrum
after CH3COOꢁ addition is most likely the result of two processes.
The absorbance of E isomer Ia at 410 nm is influenced not only by
direct intermolecular interaction of CH3COOꢁ anions with a partic-
ular Ia hydrogen, but most likely also by the shift in the hydrazide–
hydrazonol equilibrium resulting from Ia solvation changes in the
presence of the CH3COOꢁ anions in the solution. Employment of
this latter process depends specifically on both the anion and the
isatinfenylsemicarbazone structures. The Ia:CH3COOꢁ stoichiome-
tric ratio of 2:1 is also supported by the calculated parameter n = 2
in the Double-logarithm equation, where n defines the number of
binding sites in the equation [19,20]. Moreover, the Benesi–Hilde-
brand equation for the Ia-CH3COOꢁ interaction gave rather the
exponential grow curve as a straight line. The stoichiometric ratio
for all other reactants determined from the Job’s plot was 1:1 (ex-
cept for the above mentioned anomaly of the Ia:Fꢁ stoichiometric
ratio of 1:2 at high Fꢁ concentration).
The interaction of Brꢁ, HSO4ꢁ and H2POꢁ anions with compound
4
Fig. 5. Stoichiometric analysis of Fꢁ anion binding to receptor Ia in DMF – Job’s plot.
Ia have an opposite effect on the intensity of the absorption band
with maximum at 410 nm in the UV–VIS spectrum of Ia compared
to the interaction with Fꢁ and AcOꢁ anions. ꢁThe intensity of this
At this stage of the Ia-Fꢁ interaction, the 1H NMR signal of the
NH hydrogen in the vicinity of the phenyl group shifts with
increasing anion concentration towards lower values.
band decreases in the presence of Brꢁ, HSO4 and H2POꢁ anions
4
(Fig. 3). We assume that the lower basicity of Brꢁ, HSO4ꢁ and
The effect of CH3COOꢁ anions on the UV–VIS spectrum of E iso-
mer Ia in the concentration range 0–1 equivalent is the same as
that described for Fꢁ anions. Quantum chemical calculations pre-
dict that the Ia form B is significantly stabilised by both NaF and
H2POꢁ anions compared to Fꢁ and AcOꢁ shifts the equilibrium de-
4
scribed in Scheme 2 to the left, to the more stabilized hydrazide
form A. This is also supported by the quantum-chemical calcula-
tions (Table 1) which predict the association energy of Ia form B

426
K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
with NaBr salt is negative (ꢁ33.81 kJ molꢁ1) in contrast to the asso-
ciation energies of Ia form B with NaF and CH3COONa salts. Only a
small amount of hydrazonol form B is present in the solution at Ia
concentration of 10ꢁ4 mol dmꢁ3 in DMF. The above mentioned
equilibrium is thus significantly shifted to the left. The sensitivity
that weakening or decay of the intramolecular hydrogen bond be-
tween isatin NH hydrogen and carbonyl of the isatin heterocycle in
molecule Ib occurs at this reaction stage due to solvation sphere
change (Scheme 4 – Structure E). Increase in Fꢁ concentration
above 2 ꢃ 10ꢁ3 mol dmꢁ3 (up to 10ꢁ2 mol dmꢁ3) results in new
absorption band formation with maximum at 395 nm (Fig. 6).
Again, the position of this absorption band is very similar to the
absorption band of the corresponding E isomer Ia.
The new absorption band at 395 nm which is formed by the
superposition of two absorption bands and which has a relatively
large half-width is assigned to tautomeric structure F (Scheme 4).
The superposition of the absorption bands and resultant large
half-width highlight the existence of a second intra-molecular
hydrogen bond between the NH hydrogen in the vicinity of the
phenyl group and the imine nitrogen. In contrast to the interaction
with Fꢁ anion, this absorption band is not formed during the Ib
interaction with CH3COOꢁ anions, even at high CH3COOꢁ concen-
trations (Fig. S6). At CH3COOꢁ concentration between 0 and
10ꢁ2 mol dmꢁ3, the changes in the UV–VIS spectrum of semicarba-
zone Ib are the same as those observed during titration with Fꢁ
anion concentrations below 2 ꢃ 10ꢁ3 mol dmꢁ3. The product of
this equilibrium is form E of Z isomer Ib (Scheme 4). Calculations
of Ia as a sensor for Brꢁ, HSO4ꢁ and H2POꢁ anions is not optimal
4
at this concentration. A decrease in the Ia concentration below
10ꢁ5 mol dmꢁ3 in the absence of anions shifts the equilibrium in
Scheme 2 to the right, in favour of hydrazonol B, and thus increases
the sensitivity of Ia for Brꢁ, HSO4ꢁ and H2POꢁ anion detection.
4
Similar changes in absorbance behaviour to those in the unme-
thylated derivative Ia were observed for 3-methylisatinfenylse-
micarbazone IIa (Fig. S2). However, in contrast to Ia, the increase
in the Fꢁ concentration over 1 equivalent has only small effect
on the UV–VIS spectrum of IIa. Fꢁ anions are unable to disrupt
the intramolecular hydrogen bond in the hydrazonol B structure
and shift the equilibrium to the hydrazonol form C (Scheme 3),
even at the high Fꢁ concentration of 10ꢁ2 mol dmꢁ3. Similar to
the result observed in the Ia-CH3COOꢁ interaction, there was no
hypsochromic shift here in the absorption band with maximum
at 410 nm.
Hydrazonol C of the methyl derivative IIa is not formed in the
presence of Fꢁ anions. As mentioned above, the reactant’s stoichi-
ometric ratio for the IIa-Fꢁ interaction determined from Job’s plot
is 1:1 in the entire Fꢁ concentration range (Fig. S3).
in Table
by approximately 28 kJ molꢁ1 more in the complex with Fꢁ
anion
ass = 55.63 kJ molꢁ1 than with the CH3COOꢁ anion
1 show that form E of isomer Ib is stabilised
(
D
E
)
The titration of the IIa solution with CH3COOꢁ anions has the
same effect on the UV–VIS spectrum of IIa as titration with Fꢁ
(Fig. S4). Increased CH3COOꢁ concentration increases the hydrazo-
nol B concentration and, similar to the Ia-CH3COOꢁ interaction,
this does not shift the equilibrium from hydrazonol form B to hyd-
razonol form C. The basicity of the acetate anions is insufficient to
disrupt intramolecular hydrogen bonding in the hydrazonol B form
of semicarbazone IIa.
(DE
ass = 27.67 kJ molꢁ1).
Brꢁ, HSOꢁ4 and H2POꢁ anions do not affect the UV–VIS spectrum
4
of Z isomer Ib in the 0 to 10ꢁ2 mol dmꢁ3 concentration range. The
performed quantum-chemical calculations in Table 1 support this
finding, as no local energy minimum corresponding to the E Ib iso-
mer form was identified for the Brꢁ anion.
The Fꢁ and CH3COOꢁ effect on the UV–VIS spectrum of Z isomer
IIb differs from the effect of these anions on the unmethylated Z
isomer Ib spectrum, mainly at low anion concentrations up to
1 ꢃ 10ꢁ4 mol dmꢁ3. Only a small increase in the UV–VIS spectrum
for IIb at 410 nm is observed with Fꢁ or CH3COOꢁ concentration in-
crease up to 1 equivalent (Figs. S7 and 7).
Brꢁ, HSOꢁ4 and H2POꢁ anions affect the UV–VIS spectrum of 3-
4
methylisatinphenylsemicarbazone IIa in the same manner as in
isatinphenylsemicarbazone Ia (Fig. S5).
Interaction of the anions with (Z)-isatin-3-4-phenyl(semicarbazone)
(Ib) and (Z)-N-methylisatin-3-4-phenyl(semicarbazone)
The different Fꢁ and CH3COOꢁ effects on Ib compared to IIb at
concentration up to one equivalent results from the different
charge density on the isatin carbonyl oxygen. The methyl substitu-
ent in position 1 on the isatin fragment increases the charge density
on oxygen, and thus increases the strength of the intramolecular
hydrogen bond. The IIb solvation change in the presence of Fꢁ
and CH3COOꢁ anions in this anion concentration range is unable
to disrupt the hydrogen bonding. The tautomeric form F originates
almost in one step with the disappearance of the intramolecular
hydrogen bond with isatin oxygen at anion concentrations above
1 equivalent (Scheme 4). The decrease in the absorbance intensity
at 350 nm and its increase at 410 nm in the UV–VIS spectra high-
light form F formation in the solution (Scheme 4). At this interac-
tion stage, the effect of Fꢁ anions on the equilibrium is greater
than that of CH3COOꢁ anions. Almost all the hydrazide D form of
Z isomer IIb is transformed into the F form at Fꢁ concentration of
5 ꢃ 10ꢁ3 mol dmꢁ3. In contrast to Fꢁ anions, the presence of
CH3COOꢁ anions in a 10ꢁ2 mol dmꢁ3 CH3COOꢁ solution concentra-
tion results in approximately 25% conversion of the Z isomer IIb
form D into the hydrazonol form F (Fig. 7). Further increase in the
CH3COOꢁ concentration leads to additional form D ? form F
conversion. Increased Fꢁ concentration to 7 ꢃ 10ꢁ3 mol dmꢁ3 leads
to a relatively large decrease in the half-width of the absorption
band maximum at 410 nm. Although the position of the absorption
maximum remains unchanged, the shoulder on the batochromic
side of the absorption band disappears (Fig. S7). This absorption
band half-width decrease is most likely connected with intramolec-
ular hydrogen bond loss in the hydrazonol form F, followed by the
formation of hydrazonol form G (Scheme 4).
Compound Ib does not form associates in solution and does not
possess the absorption band at 410 nm characteristic for corre-
sponding E isomers [16,23]. DMF, Ib has the absorption maximum
at 350 nm (Scheme 4 – Structure D). The intensity of this absorp-
tion band at 350 nm varies dependent on Fꢁ anion concentration.
At Fꢁ concentration between 0 and 2 ꢃ 10ꢁ3 mol dmꢁ3, there is de-
creased intensity and simultaneous hypsochromic shift of approx-
imately 25 nm in this band (Fig. 6). The position of this newly
formed absorption band at 325 nm is very close to the absorption
maximum position of the unmethylated E isomer Ia. It is assumed
Table 1
Calculated [B3LYP 6-31+G(d,p)/MP2/6-311++G(d,p) level] association energies
(DE , 298.15 K) for complexes of Ia (form B) and Ib (form E) with
ass kJ mol–1
CH3COONa, NaBr and NaF salts.
Structure
D
Eass (kJ mol–1
)
Ia
B-CH3COONa
B-NaBr
B-NaF
83,45
-33,81
92,38
Ib
E-CH3COONa
E-NaBr
E-NaF
27,67
–
55,63
Energy minimum corresponding to structure E anion interaction with N-hydrogen
was not established.

K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
427
Scheme 4. Presumed non-covalent intermolecular interactions of derivative Ib with Fꢁ anions during titration of the Ia solution by TBA+Fꢁ.
Fig. 6. Evolution of the UV–VIS spectrum of isatinphenylsemicarbazone Z isomer Ib
in DMF during titration of the Ib solution by TBA+Fꢁ (cIb = 1 ꢃ 10ꢁ4 mol dmꢁ3
;
cF- = 0 mol dmꢁ3
,
1.5 ꢃ 10ꢁ5 mol dmꢁ3
,
2.5 ꢃ 10ꢁ5 mol dmꢁ3
,
5 ꢃ 10ꢁ5 mol dmꢁ3
,
,
,
1 ꢃ 10ꢁ4 mol dmꢁ3, 2.5 ꢃ 10ꢁ4 mol dmꢁ3, 3 ꢃ 10ꢁ4 mol dmꢁ3, 4 ꢃ 10ꢁ4 mol dmꢁ3
Fig. 7. Evolution of the UV–VIS spectrum of isatinphenylsemicarbazone Z isomer
5 ꢃ 10ꢁ4 mol dmꢁ3
,
1 ꢃ 10ꢁ3 mol dmꢁ3
,
2 ꢃ 10ꢁ3 mol dmꢁ3
,
3 ꢃ 10ꢁ3 mol dmꢁ3
IIb in DMF during titration of the IIb solution by TBA+CH3COOꢁ (cIIb = 1 ꢃ 10ꢁ4
-
,
5 ꢃ 10ꢁ3 mol dmꢁ3
,
7.5 ꢃ 10ꢁ3 mol dmꢁ3 and 1 ꢃ 10ꢁ2 mol dmꢁ3). Inset: corre-
mol dmꢁ3
;
cCH3COOꢁ = 0 mol dmꢁ3
,
1 ꢃ 10ꢁ4 mol dmꢁ3
,
5 ꢃ 10ꢁ4 mol dmꢁ3
1 ꢃ 10ꢁ3 mol dmꢁ3, 1.5 ꢃ 10ꢁ3 mol dmꢁ3, 2.5 ꢃ 10ꢁ3 mol dmꢁ3, 5 ꢃ 10ꢁ3 mol dmꢁ3
and 1 ꢃ 10ꢁ2 mol dmꢁ3). Inset: Corresponding dependence of the absorbance
values at 344 nm and 410 nm for receptor IIb.
sponding dependence of absorbance values at 325 nm, 364 nm and 420 nm for
receptor Ib.
Similar to the result for Z isomer Ib, Brꢁ, HSO4ꢁ and H2POꢁ
4
anions do not affect the Z isomer IIb UV–VIS spectrum. We assume
that, contrary to E isomers’ receptors Ia and IIa, the receptor-weak
basic anion interaction cannot compete with the relatively strong
intramolecular hydrogen bond between the isatin NH hydrogen
and the isatin carbonyl in molecules Ib and IIb [16], and therefore
it does not affect the hydrazide–hydrazonol equilibrium, even at
Although the Kass association constants of the studied Ia and IIa
E isomers with various anions have quite close values for both an-
ion groups (Table 2), the selectivity of these isomers for a specific
anion in the mixed anion sample is sometimes higher than one
would expected from simple Kass value comparison. For example,
the titration of the Ia and Fꢁ anion solution at 1 ꢃ 10ꢁ4 mol dmꢁ3
final concentration for both components, with the tetrabutylam-
monium bromide (Brꢁ) solution did not change the UV–VIS
spectrum even at high Brꢁ concentration (cBrꢁ = 100 equiv. cFꢁ).
Br- anions cannot compete with the Fꢁ anions during Fꢁ interaction
with Ia. Based on this result we assume that specific anion-
receptor interaction complementarity is the key component for
selectivity of isatinphenylsemicarbazone receptors Ia and IIa.
Because the Clꢁ anion is weaker basic than Brꢁ anion, the Fꢁ or
CH3COOꢁ anions’ determination is possible in biological samples
at high Clꢁ anions’ excess. On the other hand, addition of 1 equiv-
high Brꢁ, HSOꢁ4 or H2POꢁ concentrations.
4
Analytical applications
Receptor selectivity depends on the applied experimental con-
ditions. As shown in Figs. 1–3 and Figs. S2–S5 and in corresponding
parts of the text, the Ia–IIb receptors divide the guest anions into
two groups: relatively strong and relatively weak basic anions.
The anion basicity thus influences the specific type of anion-
receptor interaction. Strong basic anions interact gradually with
the particular urea hydrogens in isatinphenylsemicarbazone
receptors Ia and IIa and shift the hydrazide–hydrazonol equilib-
rium to the hydrazonol side. In contrast, receptor-weak basic anion
interaction accompanied by corresponding change in the receptor
solvation shell leads to a shift in the hydrazide–hydrazonol
equilibrium to the hydrazide side.
alent of CH3COOꢁ anions to the Ia + Fꢁ solution mixture (cFꢁfinal
=
cCH3COOꢁfinal = 1 ꢃ 10ꢁ4 mol dmꢁ3) leads approximately to receptor
signal doubling with increased absorbance at 410 nm. Therefore,
the simultaneous presence of both Fꢁ and CH3COOꢁ anions in a
sample does not allow discrete quantitative determination of the
single anion, and only the sum of these anions can be assessed.

428
K. Jakusová et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 421–429
In the presence of relatively strongly basic anions (Fꢁ and CH3-
COOꢁ), the E isomers transform into the tautomeric hydrazonol
form B with UV–VIS absorption maximum at 410 nm. The increase
in Fꢁ concentration above 1 equivalent leads to loss of the intramo-
lecular hydrogen bond in the E isomers’ structure and simulta-
neous hydrazonol C formation. This structural change is reflected
in the UV–VIS absorption spectra of E isomers Ia and IIa by hypso-
chromic shift of the hydrazonol B absorption band at 410 nm.
Table 2
The association constants Kass (molꢁ1 dm3) of the studied
E
and
Z
isomers of
isatinphenylsemicarbazones I and II with anions in DMF at 298.16 K.
Association constants (Kass
)
Anion
Compd
Fꢁ
CH3COOꢁ
Brꢁ
H2POꢁ4
Ia
1.1 ꢃ 105a
1.0 ꢃ 102b
4.0 ꢃ 105
7.8 ꢃ 103d
1.0 ꢃ 102e
3.6 ꢃ 103
3.5 ꢃ 104c
1.2 ꢃ 103
1.6 ꢃ 103
1.0 ꢃ 102
3.7 ꢃ 104
3.8 ꢃ 104
1.9 ꢃ 103
Weakly basic anions, such as Brꢁ, HSO4ꢁ and H2POꢁ, shift the tauto-
IIa
Ib
4
meric equilibrium for both E isomers Ia and IIa to the hydrazide
form A. However, due to the specific anion-receptor interaction
complementarity, the detection of Fꢁ or CH3COOꢁ anions at high
weakly basic anions’ excess is possible.
IIb
1.9 ꢃ 102
a
b
c
CFꢁ1 = 0–1 equiv. Ia.
CFꢁ1 > 1 equiv. Ia.
Complex 1:2 – Kass in molꢁ2 dm6.
CFꢁ1 = 0–20 equiv. Ib.
The intramolecular hydrogen bond between the NH hydrogen
of the urea moiety beside the imine nitrogen and the isatin car-
bonyl oxygen in Z isomer Ib is lost in the presence of Fꢁ and
CH3COOꢁ anions, and a new structure very similar to the corre-
sponding E isomers is formed. The absorption maximum of this Z
isomer shifts hypsochromically to 325 nm. The increased Fꢁ cre-
ates conditions required for unmethylated Z isomer Ib tautomeric
equilibrium shift to hydrazonol F formation. CH3COOꢁ basicity is
insufficient to shift the tautomeric equilibrium towards hydrazo-
nol F form and therefore it is impossible to observe the absorption
band maximum at 395 nm in the UV–VIS spectrum of Ib, even in a
large excess of CH3COOꢁ anions. The methylated Z isomer IIb spec-
trum changes very little with increased Fꢁ or CH3COOꢁ concentra-
tion up to 1 equivalent. However, at high anion concentrations
above 1 equivalent, the tautomeric form F of Z isomer IIb is formed
in almost one step with loss of the hydrogen bond with isatin car-
bonyl oxygen. These changes are reflected in the UV–VIS spectrum
of IIb by simultaneous decrease in the absorption intensity at
350 nm and increase at 410 nm. The weakly basic Brꢁ, HSO4ꢁ or
d
e
CFꢁ1 > 20 equiv. Ib
Detection of anions in aqueous environments is currently the
most interesting target in the chemosensor field [24]. Water plays
an important role in analysis of real industrial samples and an
aqueous environment is required to detect anions in biological sys-
tems. Due to excellent Ia and IIa E isomer sensitivity in organic
media, where receptor signal saturation occurs at 1 equivalent
anion concentration, these isomers can be used for Fꢁ or CH3COOꢁ
sensing in semi-aqueous media. Although the sensitivity in
semi-aqueous media decreases with increasing water content,
due to water competition, higher amounts of anions can be deter-
mined before receptor saturation (Fig. S8). Because the addition of
strongly basic OH- anions has the same effect as addition of Fꢁ or
CH3COOꢁ anions, care should be taken when interpreting data in
semi-aqueous media from aqueous samples with pH higher than
10 (Fig. S9).
H2POꢁ anions do not affect the Z isomers’ UV–VIS spectra.
Compꢁared to E isomers Ia and IIa, the relatively weak basic
4
Brꢁ, HSO4 and H2POꢁ anions do not affect the Ib and IIb Z isomers’
4
Sensors Ia–IIb provide an excellent signal to noise ratio, a rela-
tively wide detection range and, based on anion concetration
range, they can be useful in the detection of environmentally or
biologically important anions (Fꢁ or CH3COOꢁ) in both organic
and semi-aqueous media. Moreover, their sensitivity can be in-
creased by incorporation of the fluorogenic unit into the receptor
structure.
UV–VIS spectrum. Hence, detection of Fꢁ or CH3COOꢁ anions is not
affected by interference from weak basic anions. Moreover, the
smaller sensitivity of Z isomers’ receptors compared to E isomers
allows identification of strong basic anion between 1 and 100
equivalent of Ib or IIb in organic media. Due to the absence of
aggregate/hydrazide/hydrazonol equilibrium and its shift to hyd-
razonol side with dilution, the Z isomer IIb exhibits low detection
limit of 1 ꢃ 10ꢁ6 mol dmꢁ3 and 5 ꢃ 10ꢁ6 mol dmꢁ3 toward Fꢁ and
CH3COOꢁ anions, respectively, in organic media (10 cm path
length). These detection limits are very similar to those for E
isomers Ia and IIa (1 ꢃ 10ꢁ6 mol dmꢁ3 toward both Fꢁ and
CH3COOꢁ anions; 10 cm path length).
Acknowledgments
This publication is the result of the projects’ implementation:
(1) The Competence Center for Intelligent Technologies to Enable
Electronization and Informatization of Systems and Services, ITMS
26240220072; and (2) Development of Competence center for
research and development in molecular medicine, ITMS:
26240220071; supported by the Research & Development Opera-
tional Programme funded by the ERDF.
Although the studied Ia–IIb isatinphenylsemicarbazone recep-
tors’ Kass values are very similar to those of other colorimetric sen-
sors [14,24], these Ia–IIb receptors provide an excellent signal to
noise ratio reflecting absorbance changes up to 2 absorbance units,
and also wide detection range up to receptor 100 equivalent. More-
over, sensitivity can be improved by incorporation of the fluoro-
genic unit into the receptor’s molecular structure. Our
comprehensive study of spectral properties and anion binding of
Ia–IIb receptors’ fluorescent derivatives with various fluorogenic
units in the phenyl side of the urea moiety is now in progress.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.saa.
2013.12.073.
Conclusion
References
E and Z isomers of isatin-3-4-phenyl(semicarbazone) I and N-
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Ia, IIa and Z isomers Ib, IIb differ in sensitivity, selectivity and
sensing mechanism. Behaviour of the isatinphenylsemicarbazones
in the presence of anions depends on the sensor structure and on
anion acidic/basic properties.
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