Spectroscopic and computational studies on the development of simple colorimetric and fluorescent sensors for bioactive anions

Article abstract of DOI:10.1080/10610278.2012.752092

Spectroscopic techniques (UV-vis and fluorescence) and computational (HF/6-31G(d,p)) methods were adopted to investigate the anion recognition ability of two novel Schiff base receptors (L ¹ and L ²) synthesised from the condensation

Full text of DOI:10.1080/10610278.2012.752092

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Spectroscopic and computational studies on the
development of simple colorimetric and fluorescent
sensors for bioactive anions
Darshna Sharma a , Ankit R. Mistry a , Rati Kanta Bera b & Suban K. Sahoo a
a Department of Applied Chemistry , SV National Institute of Technology (SVNIT) , Surat ,
395007 , Gujrat , India
b Department of Chemistry , Sant Longowal Institute of Engineering and Technology ,
Longowal , 148106 , Punjab , India
Published online: 19 Dec 2012.
To cite this article: Darshna Sharma , Ankit R. Mistry , Rati Kanta Bera & Suban K. Sahoo (2013) Spectroscopic and
computational studies on the development of simple colorimetric and fluorescent sensors for bioactive anions,
Supramolecular Chemistry, 25:4, 212-220, DOI: 10.1080/10610278.2012.752092
To link to this article: http://dx.doi.org/10.1080/10610278.2012.752092
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Supramolecular Chemistry, 2013
Vol. 25, No. 4, 212–220, http://dx.doi.org/10.1080/10610278.2012.752092
Spectroscopic and computational studies on the development of simple colorimetric and
fluorescent sensors for bioactive anions
Darshna Sharmaa, Ankit R. Mistrya, Rati Kanta Berab and Suban K. Sahooa*
aDepartment of Applied Chemistry, SV National Institute of Technology (SVNIT), Surat 395007, Gujrat, India; bDepartment of Chemistry,
Sant Longowal Institute of Engineering and Technology, Longowal 148106, Punjab, India
(Received 6 July 2012; final version received 19 November 2012)
Spectroscopic techniques (UV–vis and fluorescence) and computational (HF/6-31G(d,p)) methods were adopted to
investigate the anion recognition ability of two novel Schiff base receptors (L1 and L2) synthesised from the condensation
of 5-nitrosalicylaldehyde and 2-aminophenol or 2-aminothiophenol. Both the receptors portrayed a visually detectable
colour change upon addition of fluoride, acetate and dihydrogen phosphate anions from light yellow (L1) or colourless (L2)
to yellow due to the formation of hydrogen bonding complexes in 1:1 stoichiometry and/or deprotonation between these
anions and the receptors. No significant colour change was observed upon addition of other anions such as chloride,
bromide, iodide and hydrogen sulphate.
Keywords: anion recognition; colorimetric and fluorescent sensors; Schiff bases; HF/6-31G(d,p)
1. Introduction
investigators. Many examples are available about the
acetate-selective sensor molecules in the literature (9–15),
but most of the sensors have complicated structures that are
synthesised by multi-step procedures and also interfered
due to the presence of competitive anions. As an alternative,
Schiff bases are widely investigated in the recent times
for detecting anions by both colorimetric and fluorimetric
methods because of their simple one-step synthetic
procedure (18–28). Herein, we have investigated the
anion recognition ability of two novel Schiff bases (Figure
1) 2-((2-hydroxyphenylimino)methyl)-4-nitrophenol (L1)
and 2-((2-mercaptophenylimino)methyl)-4-nitrophenol
(L2) by experimental (UV–vis and fluorescence) and
computational (HF/6-31G(d,p)) methods.
There is bourgeoning interest in the design and synthesis of
suitable receptor for the selective encapsulation and sensing
anions such as acetate, fluoride and dihydrogen phosphate
(DHP) due to their important roles in biomedicinal,
environmental and chemical processes (1–5). Among the
various types of sensing techniques, the development of
anion-selective sensors through the naked eye (colori-
metric) and fluorescent responses has attracted consider-
able attention because of their ability to provide a simple,
sensitive, selective, precise and economical method for
online monitoring up to very low concentrations of target
anions. In designing such anion sensors, the receptor unit
should have the potential to bind or interact with the target
anion (analyte) selectively and efficiently through either
electrostatically or hydrogen bonding, and also the binding
unit must be connected suitably to a light-emitting group
(fluorophore unit) that produces a distinct signal during the
anion recognition procession (6–8). With regard to the
binding sites, the functional groups such as amine, urea,
thiourea, amide, imidazolium, pyrroles and phenolic-OH
are numerously used owing to their ability to perform as
hydrogen donors (9–15).
2. Results and discussion
2.1 Design of anion sensors
The anion recognition behaviour of the receptor L1
containing two phenolic-OH as anion-binding units was
investigated recently towards anions such as F2, Cl2, Br2
and OH2 (29). It was reported that the receptor L1 showed
colorimetric and ‘turn-off’ fluorescent responses in the
presence of high electronegative and small-size anions F2
and OH2 because of their ability to form intermolecular
hydrogen bonding and/or deprotonating phenolic-OH
Among the various bioactive anionic analytes, acetate
is one of the biochemically important anions because it acts
as a critical component in numerous metabolic processes
(16). Also, the rate of acetate production and oxidation has
been frequently used as an indicator of organic decompo-
sition in marine sediments (17). Therefore, the design of
acetate-selective receptor is particularly challenging for
1
proton. The H NMR spectral data supported the anion
recognition with the disappearance of the OH proton peak
attached to the nitro-substituted aromatic ring and the
shifting of other OH and imine (CHvN) proton peaks.
*Corresponding author. Email: suban_sahoo@rediffmail.com
q 2013 Taylor & Francis

Supramolecular Chemistry
213
Figure 1. Designing of anion sensors based on receptors L1 and L2.
However, as the sensing ability of an artificial neutral
receptor towards anions depends on factors such as
conformational complementarity and anion basicity, we
thought the triangular (Y-shaped) shaped acetate anion
might be fittest to make multiple hydrogen bonds with the
receptor L1 with higher selectivity and efficiency among
the various bioactive anions including fluoride. This
assumption was well supported by the results reported
recently with a positional isomer of L1, which showed
acetate-selective colorimetric response in aqueous aceto-
nitrile (30). Therefore, we wish to examine again the anion
sensing ability of L1 towards anions such as AcO2,
H2PO24 , HSO24 , F2, Cl2, Br2 and I2. Also, for
comparison, we have investigated a new receptor L2
containing one phenolic-OH and another thiophenol-SH as
anion recognition units.
the presence of ketoenamine form of L1 in solution
because of the electron-withdrawing NO2 group that
enhances the acidity of the phenolic-OH group to favour
the tautomeric transformation from enolimine to ketoena-
mine form. In contrast to L1, L2 exhibited a colourless
solution due to a weak absorption at ,430 nm that also
indicate predominantly the enolimine form of L2 in
solution. On addition of equivalent amount of various
anions, obvious hyperchromic spectral shift at 440 nm for
L1 and 430 nm for L2 was observed in the order AcO2
.
F2 . H2PO24 and AcO2 < F2 . H2PO24 , respectively.
The variations in the absorbance bands are due to the
recognition of the receptors with the hydrogen bond-
forming anions, which reduce the OZH bond strength and
facilitate deprotonation to increase charge delocalisation.
Alternatively, addition of anionic species favoured the
formation of ketoenamine form that intensified the yellow
Firstly, the colorimetric response of the Schiff base
receptors L1 and L2 (5.0 £ 1025 M) in acetonitrile has
been investigated before and after the addition of
equivalent amount of each anion to evaluate the anion
recognition properties. As shown in Figures 2(a) and 3(a),
we observed a detectable colour change from light yellow
(L1) or colourless (L2) to yellow in the presence of AcO2,
F2 and H2PO24 anions. However, no obvious colour
change was observed in the presence of HSO24 , Cl2, Br2
and I2 in both the receptors, even the anions were
excessive presumably due to weaker interactions with the
receptors that failed to alter any structural changes. In
addition, it was observed that the intensity of colour
decreases reversibly on addition of small amount of water
(Figure S1 of the Supplementary Information, available
online). The presence of protic solvents such as water can
compete with anions for binding sites and disturb the H-
bond interactions between the host and the anionic guest
that lead to a reversal of the visual colour (21).
Secondly, the UV–vis experiments of the receptors L1
and L2 in acetonitrile were carried out in the absence and
presence of anions such as AcO2, H2PO42, HSO42, F2,
Cl2, Br2 and I2 (Figures 2(b) and 3(b)). Receptor L1
exhibited a light yellow colour in solution because of the
Figure 2. Colour changes (a) and UV–vis spectra (b) of the
receptor L1 (5.0 £ 1025 M, vial was mentioned as Lig) in the
absence and presence of 1 equivalent of different anions (Colour
Online).
absorption band with maxima at ,440 nm due to n ! p
*
transition. The appearance of peak above 400 nm indicates

214
D. Sharma et al.
Figure 3. Colour changes (a) and UV–vis spectra (b) of the receptor L2 (5.0 £ 1025 M, vial was mentioned as L) in the absence and
presence of one equivalent of different anions (Colour Online).
colouration. However, the appearance of new peak at
375 nm on addition of anions indicates the participation
of OZH groups of the receptors L1 and L2 in anion
recognition process. In contrast, upon addition of other
anions, no noticeable spectral changes were observed.
In order to elucidate the possible structure of the
receptors L1 and L2, the geometries of the receptors and
their tautomeric forms were optimised using the HF/6-
31G(d,p) method in both gas phase and acetonitrile. From
the optimised structure of receptors L1 and L2 (Figure 4),
it can be observed that the two possible binding sites (i.e.
phenolic-OH/thiophenol-SH groups) are present in the
same side, and therefore no apparent conformational
changes in the receptors are required for the encapsulation
of the incoming anions. The calculated results on relative
energy and the energy gap of ELUMO, EHOMO of the
receptors L1 and L2 and their tautomers are listed in
Table 1. From Table 1, it can be observed that the relative
energy between the enolimine and ketoenamine forms for
L2 is higher than that of L1 in both gas phase and
acetonitrile medium. In case of L1, the relative energy in
gas phase (22.88 kJ mol21) decreases to 3.73 kJ mol21 in
acetonitrile, which indicates that the tautomeric equili-
brium (enolimine $ ketoenamine) was moved from left
towards the right in acetonitrile and therefore both
ketoenamine and enolimine forms could be present.
However, the relative energy for L2 in both gas phase
(33.42 kJ mol21) and acetonitrile (15.44 kJ mol21) is
higher for tautomeric transformation. Furthermore, the
calculated band gap DE (DE ¼ ELUMO 2 EHOMO) of
enolimine form is larger than that of its ketoenamine
tautomer. Thus, the ketoenamine form of the receptors
with lower band gap may be responsible for the light
yellow colouration and absorbance at longer wavelength.
2.2 UV–vis titrations
The spectrophotometric titrations of the receptors L1 and
L2 with the anions (AcO2, F2 and H2PO42) have been
performed in acetonitrile to calculate the binding constant
(K) for the respective anions. Upon successive addition of
anions to a fixed concentration of the receptors, L1 and L2

Supramolecular Chemistry
215
1
Figure 4. Optimised structure of L (a) and L (c) and their ketoenamine form (b and d) with selected bond lengths (A) in gas phase by
2
˚
HF/6-31G(d,p) method.
resulted in hyperchromic shift above 400 nm with the
formation of isosbectic points, which indicate the
formation of single complex species between the receptors
and the anions added (Figure 5 and Figure S2 of the
Supplementary Information, available online). The spec-
tral data were used to calculate the binding constant (K)
between the receptors and the anions by applying the
Benesi–Hildebrand equation (31):
be AcO2 . F2 . H2PO42 for L1 and AcO2 < F2
.
H2PO24 for L2. The trend can be explained with the
following principles: (i) the ability to hold the multiple
hydrogen-bonding interactions between the hosts and the
anionic guests, (ii) the shape complementarity between the
receptor and the anions and (iii) alkalescence of interacting
anions and nature of the binding sites of receptor.
The higher acetate recognition ability of receptor L1 was
presumably due to the shape complementarity of L1 with
the two phenolic-OH binding sites forming multiple
hydrogen bonds with acetate anions. Also, the alkales-
cence of acetate anion was higher than the other anions. In
contrast to L1, L2 with both OH and SH binding sites
showed similar affinity towards fluoride and acetate.
However, both the receptors showed lower binding ability
for the tetrahedrally shaped H2PO24 .
ꢀ
ꢁ
1
1
1
¼
þ 1 ;
ð1Þ
A 2 A0 Amax 2 A0 K½X0ꢀ
where A is the absorbance measured with different
concentrations of the anions, A0 is the absorbance of the
free receptor, Amax is the maximum absorbance of receptor
and anions, X0 is the concentration of the anions added and
K is the association constant. Then, the graphs were plotted
between 1/(A 2 A0) against 1/[X0], and the binding
constant (K) was determined from the ratio of intercept/
slope. The calculated binding constants (K) for the
receptors L1 and L2 towards AcO2, F2 and H2PO24
anions are summarised in Table 2.
The selectivity of the host for a specific anion of interest
could be rationalised on the basis of not only by the by
guest basicity but also by the complementary shape and
stoichiometry between the host and the anion guest. Hence,
the stoichiometry analysis of the receptors L1 and L2 with
the anions was done using Job’s plot method. The Job’s
plots (Figure S3 of the Supplementary Information,
Obviously from Figures 2(b), 3(b) and Table 2, the
trends for the anion-binding abilities were determined to
Table 1. Calculated relative energy [Erelative ¼ ET(ketoenamine)–ET(enolimine)] and energy gap DE (DE ¼ ELUMO–EHOMO) of L1, L2
and their tautomer obtained at HF/6-31G(d,p).
Gas (acetonitrile)
Parameters
L1
Tautomer of L1
L2
Tautomer of L2
Erelative (kJ mol21
ELUMO (a.u.)
EHOMO (a.u.)
)
0 (0)
22.88 (3.73)
0.04463 (0.04859)
20.29476 (20.29749)
891.07 (908.63)
0 (0)
33.42 (15.44)
0.04307 (0.04843)
20.30195 (20.30282)
905.85 (922.21)
0.06327 (0.05998)
20.30888 (20.30439)
977.08 (956.65)
0.05684 (0.05812)
20.30967 (20.30568)
962.27 (955.15)
DE (kJ mol21
)
Note: 1 Ha ¼ 2625.5 kJ mol21
.

216
D. Sharma et al.
Figure 5. UV–vis spectra of the receptors (1.0 £ 1025 M) L1 (a) and L2 (b) with different equivalents of AcO2. Inset shows the
absorbance at (a) 440 nm and (b) 430 nm as a function of AcO2 concentrations.
available online) for the receptors with AcO2, F2 and
H2PO24 inferred that all the anions bind with the receptor in
the ratio 1:1. Furthermore, the variation in the absorption
Table 2. Binding constants (K) of the receptors L1 and L2 with
spectra (Figure 5) and the 1H NMR spectrum (Figure S4 of
the corresponding anions.
the Supplementary Information, available online) obtained
Anions
L1 (M21
)
L2 (M21
)
for the acetate–L1 complex indicates similar modes of
complexation as reported for fluoride ion with L1 (29). The
two phenolic-OH groups of L1 interact with acetate anions
to form the 1:1 acetate–L1 complex. In addition, the imine
proton peak of free L1 at 9.3 ppm was downfield shifted to
9.2 ppm, and the variations in the peaks of both the
Acetate
2.85 £ 103
2.22 £ 103
1.35 £ 103
2.62 £ 104
2.12 £ 104
1.58 £ 103
ND
Fluoride
DHP
Other anions
ND
Note: ND, binding constant could not be determined.

Supramolecular Chemistry
217
aromatic ring protons indicate possible conformational
changes in L1 during the encapsulation of acetate anion.
Further to confirm the binding modes between the receptors
and the anions, we performed the absorption titrations of L1
and L2 with tetrabutylammonium hydroxide (TBAOH),
under identical conditions to those described for other
anions. The results from these anion titrations are shown in
Figure 6 and Figure S5 of the Supplementary Information,
available online. The similar spectral and colorimetric
changes observed for both AcO2 and OH2 indicate the
formation of the same species in solution. Also, the results
clearly demonstrate that both anions are functioning here as
a base, giving rise to the deprotonation upon interaction
with the receptors L1 and L2.
In order to predict the possible structure of the anion–
receptor complex, structural optimisation was performed
at HF/6-31G(d,p) level by focusing on the proposed
binding modes between the receptors (L1 and L2) and
the anions. The optimised global minimum structure
of anion–receptor complexes clearly indicates that the
Figure 6. UV–vis spectra of the receptors (5.0 £ 1025 M) (a) L1 and (b) L2 in the absence and presence of equivalent amounts of AcO2
and OH2. Inset shows the colour changes of the receptors upon addition of anions (Colour online).

218
D. Sharma et al.
1
Figure 7. Optimised structure with selected bond lengths (A) of anion–receptor complexes (L : a–c and L : d–f) in gas phase at HF/6-
2
˚
31G(d,p) method.
binding modes of receptors with fluoride, acetate and
phosphate anions are almost similar. The phenolic-OH and
thiophenol-SH moieties and the imine (CHvN) proton of
the receptors interact with the anions to form a 1:1 anion–
receptor complex. However, the optimised parameters
shown in Figure 7 inferred that AcO2 is forming two
similar and stronger hydrogen bonds than DHP. This is
mainly due to the Y-shaped structure of the acetate anion
that facilitates the formation of multiple hydrogen-bonded
complex with the receptors than the tetrahedral-shaped
DHP. However, due to higher electronegativity and small
size, fluoride binds strongly only with OH group of 5-
nitrosalicyladehyde but weakly with another OH/SH
group. The lack of conformational complementary shape
of fluoride anion to interact with L1 and L2 resulted
weaker binding than acetate. Thus, proper orientation of
the receptor for acetate anion recognition through
hydrogen bonding and predicted stoichiometry obtained
from the theoretical structural optimisation corroborates
well with the experimental results.
online). The fluorescence quenching is occurred due to the
blocking of ESIPT process on interaction of OH groups of
L1 with acetate ion through hydrogen bonding followed by
the intermolecular charge transfer. However, under similar
condition, fluoride ions need 2 equivalents to quench the
emission intensity of L1. Importantly, the quenching of L1
by acetate anion was not affected in the presence of other
competitive anions. In contrast to L1, receptor L2 showed
emission at 360 nm when excited at 275 nm. The emission
of L2 was not affected noticeably on addition of acetate and
fluoride anions (Figure S7 of the Supplementary Infor-
mation, available online).
3. Conclusions
Two simple anion-selective chemosensors L1 and L2 were
reported. Colorimetric and spectral titrations in aceto-
nitrile inferred that (i) both the sensors showed visually
detectable colorimetric responses on recognition with
acetate, fluoride and DHP anions in 1:1 stoichiometry from
light yellow (or colourless) to intense yellow, (ii) the
detection limit (3s/S) approximated from the spectral
titrations for acetate anions with L1 and L2 is down to
4.57 £ 1027 and 1.72 £ 1027 M, respectively, (iii) the
titrations with TBAOH demonstrate that the deprotonation
event has a significant effect on the anion recognition
process, (iv) the effect of anion was diminished by adding
protic solvent and (v) the fluorescence of L1 was quenched
whereas the fluorescence of L2 remains unchanged in the
presence of AcO2 and F2.
2.3 Fluorescence titrations
The anion-binding behaviour of the sensor L1 was also
investigated by fluorescence titrations in acetonitrile.
The sensor L1 showed a strong emission band centred
at 505 nm (lex ¼ 450 m) presumably due to the excited-
state intramolecular proton transfer (ESIPT) process.
The ESIPT is a well-known process in intramolecularly
hydrogen-bonded Schiff bases, where a fast tautomeric
transformation from enolimine to ketoenamine occurs in
the excited states (32). Upon addition of acetate, there was a
significant decrease in the emission intensity of L1 (Figure
8). Only 1 equivalent of acetate is sufficient to quench the
emission intensity of L1. Addition of excess amount of
acetate resulted no change in the emission spectrum of L1
(Figure S6 of the Supplementary Information, available
4. Experimental
4.1 Materials and methods
All the starting reagents and solvents used for the
experiments were purchased commercially in the purest
form and were used without further purification. All the

Supramolecular Chemistry
219
Figure 8. Changes in the emission spectrum of L1 (1.0 £ 1025 M) upon addition of AcO2 from 0 to 1.0 £ 1025 M in acetonitrile.
anions were used in the form of tetrabutyl ammonium
salts [(n-C4H9)4NF, (n-C4H9)4NCl, (n-C4H9)4NBr, (n-
C4H9)4NI, (n-C4H9)4NHSO4 and (n-C4H9)4NH2PO4] and
were purchased from Spectrochem Pvt. Ltd, Mumbai,
India. The synthesis of the receptor L1 was done by
following the available literature (29).
give yellow crystals. Yield: 90%; mp 1818C; IR (KBr
pellete, ymax, cm21): 3394.5, 3067.1, 2923.3, 2852.4,
2750.0, 1692.8, 1617.3, 1584.8, 1520.3, 1489.3, 1439.0,
1385.4, 1337.5, 1280.4, 1215.9, 1104.5, 987.9, 903.7,
748.5, 723.7, 708.4, 637.9; 1H NMR (DMSO-d6, d, ppm):
4.40 (s, 1H) (SH), 7.02 (d, 1H), 7.25 (d, 1H), 7.47 (t, 1H),
7.57 (t,1H), 8.14 (m, 2H), 8.27 (s, 1H), 9.10 (s, 1H)
(CHvN), 11.35 (s, 1H) (OH).
The absorption and fluorescence spectra were recorded
in acetonitrile on a Cary 50 Varian UV–vis and
PerkinElmer LS55 luminescence spectrophotometer,
respectively, at room temperature by adopting a similar
procedure. Stock solutions of the receptors (1.0 £ 1023 M)
and anions (1.0 £ 1023 M) were prepared in acetonitrile.
These solutions were used for all spectroscopic studies
after appropriate dilution. Required amount of the receptor
and anions was taken directly into cuvette by using
micropipette for spectroscopic titrations. The sample for
1H NMR study was prepared by mixing both anion and
receptor in an appropriate ratio. Then, the mixture was
made soluble in DMSO-d6, and spectrum was recorded on
a Bruker Avance II 400 spectrometer by keeping
tetramethylsilane (TMS) as an internal standard.
4.3 Computational methods
In order to investigate the anion-binding behaviours of the
receptors, theoretical calculations were carried out with
the Gaussian 09 W computer program (33). Optimisations
of the receptors and anions have been carried out without
symmetry constraints by applying HF/6-31G(d,p) method.
The structural optimisation of the receptor was also
examined in acetonitrile media by using the conductor-like
polarised continuum model (34). The harmonic vibrational
frequency calculations using the same methods as for
the geometry optimisations were used to ascertain the
presence of a local minimum.
4.2 Synthesis of L2
Supporting data
Supplementary material is available online.
Receptor L2 was prepared by the Schiff base condensation
reaction of 5-nitrosalicaldehyde (0.5 g, 0.0029 mol) with
2-aminothiophenol (0.37 g, 0.0029 mol) in methanolic
medium (30 ml) at room temperature. The reaction
mixture was refluxed for 1 h to obtain yellow-coloured
precipitate which was washed with cold ethanol and ether.
The solid obtained was recrystallised from ethanol to
Acknowledgements
The authors thank Director, SV National Institute of Technology,
Surat, for encouragement and for providing necessary research
facilities. This work was made possible by a grant from the DST,
New Delhi, under Fast Track Young Scientist Scheme.

220
D. Sharma et al.
(21) Mahapatra, A.K.; Manna, S.K.; Sahoo, P. Talanta 2011, 85,
2673–2680.
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