Fluorescent primary sensor for zinc and resultant complex as secondary sensor towards phosphorylated biomolecules: INHIBIT logic gate

Article abstract of DOI:10.1016/j.ica.2012.12.030

Imine linked "on-off" multi-responsive and selective chemosensor has been synthesized and evaluated for cation recognition properties. The receptor shows high sensitivity and selectivity for Zn²⁺ through changes in fluorescence intensity based

Full text of DOI:10.1016/j.ica.2012.12.030

Inorganica Chimica Acta 399 (2013) 1–5
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Fluorescent primary sensor for zinc and resultant complex as secondary
sensor towards phosphorylated biomolecules: INHIBIT logic gate
Kamalpreet Kaur a, Vimal K. Bhardwaj a, Navneet Kaur b, Narinder Singh a,
⇑
a Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
b Centre for Nanoscience & Nanotechnology, Panjab University, Chandigarh, Punjab 160014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2012
Received in revised form 7 December 2012
Accepted 13 December 2012
Available online 7 January 2013
Imine linked ‘‘on–off’’ multi-responsive and selective chemosensor has been synthesized and evaluated
for cation recognition properties. The receptor shows high sensitivity and selectivity for Zn2+ through
changes in fluorescence intensity based on ESIPT and charge transfer (CT) mechanism. The Zn2+ complex
of the receptor can be used for phosphate quantification and for recognition of phosphorylated biomol-
ecules through cation displacement approach.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Chemosensor
Fluorescence
Schiff base
ESIPT
Zinc complex
Displacement approach
1. Introduction
of molecular logic gates [9,10]. The increasing role of such systems
in supramolecular devices, as well as constant need to pursue
In recent years, the use of organic compounds for chemosensor
development has made remarkable progress [1]. These chemosen-
sors relay through change in magnetic, electronic or optical prop-
erties when it binds to any specific analyte. The fluorescent
chemosensors have attracted considerable attention due to high
sensitivity [2], convenient use and real application in biological
systems [3]. In order to achieve practical applications, the future
of chemosensors is dependent upon various types of fluorescence
mechanisms [4–6]. Recently, the title of ‘‘Organic compounds as
sensor for metal ion and resultant metal complex as sensor for an-
ion’’ is gaining popularity. The system works through cation dis-
placement assay [7]: the receptor presents a unique cavity and
on binding with the metal ion shows a significant change in the
fluorescence profile of the receptor. The resultant metal ion com-
plex acts as an ideal candidate for the recognition of anions
through the liberation of the sensor from the metal complex due
to the strong affinity of the anion towards the metal ion; conse-
quently bringing a considerable change in the fluorescence profile
of the receptor–metal complex. The recognition of anions with me-
tal complexes [8] has advantage as it validates anion detection
even in semi-aqueous medium. In addition to the analytical appli-
cations, the concept has direct implementation in the development
superior technologies, will raise a wide interest of cation displace-
ment assay.
2. Experimental
2.1. Materials and methods
Chemicals were purchased from Sigma Aldrich and were used
without further purification. The NMR spectra were recorded on
a Avance-II (Bruker) instrument, which operated at 400 MHz for
1H NMR and 13C NMR in DMSO-d6. IR spectra were recorded on a
Bruker Tensor 27 spectrometer for the compounds in the solid
state as KBr discs. The absorption spectra were recorded on a Spe-
cord 250 Plus Analytikjena spectrometer spectrophotometer. The
fluorescence measurements were performed on a Perkin Elmer
L55 Fluorescence spectrophotometer. The mass spectrum of
1ꢀZn2+ complex was recorded on Waters Micromass Q–T of Micro
Instrument.
2.2. Synthesis of compound 1
This compound was prepared by stirring 4,40-diaminodiphenyl-
methane (198 mg, 1.0 mmol) along with 4-(diethylamino)-
2-hydroxybenzaldehyde (193 mg, 2.5 mmol) in methanol. The
reaction mixture was stirred for 12 h at room temperature. The
⇑
Corresponding author. Tel.: +91 1881242176; fax: +91 1881223395.
E-mail address: nsingh@iitrpr.ac.in (N. Singh).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.12.030

2
K. Kaur et al. / Inorganica Chimica Acta 399 (2013) 1–5
O
OH
+
N
N
Methanol
Stirr
H2N
NH2
OH
HO
N
3
N
N
1
2
Scheme 1.
Fig. 1. The optimized structure of receptor 1: (a) enol form and (b) keto form, calculated at the B3LYP/6-31G⁄ level. The red, blue and gray spheres refer to O, N and C atoms
respectively.
yellow colored precipitates were formed during the course of reac-
tion and these precipitates were washed with MeOH. The solid was
recrystallized from MeOH, giving a solid with 90% yield and was
characterized with IR and NMR spectroscopy. IR (KBr) 3550,
the changes in fluorescence spectrum of sensor upon addition
of that metal salt (100 M). The fluorescence spectrum of 1
was recorded with excitation wavelengths of 400 nm. For titra-
l
tions, volumetric flasks were taken each containing standard
1688, 1101 cmꢁ1
.
1H NMR (400 MHz, DMSO-d6) d 1.13(t, 12H, –
solution of sensor 1 (10
lM) along with varied amounts of a zinc
CH3), 3.38(q, 8H, –N–CH2–), 3.95(s, 2H, –CH2), 6.06(d, 2H, Ar),
6.31(d, 2H, Ar), 7.20(d, 2H, Ar), 7.28(d, 2H, Ar), 7.31(s, 1H, Ar),
8.66 (s, 2H, –CH@N–), 13.65 (s, 2H, –OH) 13C (100 MHz, DMSO-
d6) d 12.50, 43.71, 43.88, 96.81, 103.79, 107.20, 108.11, 120.72,
129.62, 134.10, 138.10, 160.50, 161.42, 163.46.
nitrate salt (0–100 M) in HEPES buffered DMF/H2O (7:3, v/v). In
l
order to determine the stoichiometry of the complex formed
from receptor 1 and Zn2+, solutions of 1 and Zn2+ were prepared
as1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 and 9:1. These solutions
were kept for 1 h, and were shaken occasionally. The plot of
[HG] versus [H]/([H] + [G]) was used to determine the stoichiom-
etry of the complex formed. The fluorescence intensity at
462 nm was used for calculations. The concentration of [HG]
2.3. Cation recognition properties of 1
was calculated by the equation [HG] =
any possible interference due to different cations for the estima-
tion of Zn2+, solutions were prepared containing 1 (10
M) and
Zn2+ (50
M) along with both without and with other interfering
metal ions (50 M) in DMF/H2O (7:3, v/v) HEPES buffered solu-
DI/Iox [H]. To evaluate
The cation recognition studies were performed at 25 1 °C
and before recording any spectrum a sufficient time was give
to ensure the uniformity of the solution. The cation binding abil-
l
l
ity of 1 (10
of 1 (10 M) along with fixed amounts of particular metal ni-
trate salt (100 M) in HEPES buffered DMF/H2O (7:3, v/v). The
cation recognition behavior of 1 (10 M) was evaluated from
lM) was determined by preparing standard solutions
l
l
tion (pH = 7.0 0.1). The fluorescence intensity of each solution
was recorded at 462 nm.
l
l

K. Kaur et al. / Inorganica Chimica Acta 399 (2013) 1–5
3
2.4. Synthesis of 1ꢀZn2+ complex
lies far apart from the nitrogen of –CH@N to exhibit ESIPT. The
thermodynamic criteria to show ESIPT process is that the energy
of the ground state keto form should be lower than that of the enol
state i.e. [Eketo(S1) < EEnol(S1)]. The molecular orbital diagrams show
that energy gap between HOMO–LUMO of the keto form of 1 is les-
ser (E = 0.68409 eV) than the energy gap between HOMO–LUMO of
the enol form (1.57174 eV) (Fig. S4).
Anaqueous solution of zinc nitrate (0.2975 g, 1 mmol) was
added to a solution of 1 (0.548 g, 1 mmol) in THF. The solution
was refluxed for three hours. On completion of the reaction, the
solution was filtered and the solvent evaporated, a yellowish-
brown compound was obtained, washed with methanol and dried
under vacuum. The dark-brown colored powder was obtained on
recrystallization from methanol. Yield 67%. mp = 205 °C. Selected
3.3. Metal recognition properties
IR (KBr, cmꢁ1): 1709 (s)
mC@N. UV–Vis absorbance:kmax. (nm),
e
(Mꢁ1 cmꢁ1) in DMF/H2O (7:3, v/v)400 (38432). ESI-MS (m/z):
In order to evaluate the cation recognition behavior of 1, the
changes in fluorescence spectrum of receptor 1 were recorded
upon addition of a particular metal salt (Fig. 2A). Upon addition
372.9 [M+H]+, where M = [1 + Zn2+ + NO3ꢁ + THF].
of 100 l lM solution of 1, a new band
M solution of Zn2+ to the 10
2.5. Anion recognition properties of 1ꢀZn2+
was emerged at kmax = 462 nm. Under the same conditions (as used
for Zn2+), the fluorescence response of 1 was checked for other me-
The studies were performed similar to those of cation recogni-
tion properties 1, except that tetrabutyl ammonium salts of anions
were used and detailed concentrations are mentioned in the text of
manuscript.
tal ions such as Na+, K+, Mg2+, Ca2+, Ba2+, Sr2+, Al3+, Cr3+, Fe3+, Mn2+
,
Cd2+, Co2+, Ni2+, Cu2+, Ag+, Hg2+ and Pb2+ (Fig. 2 A). The addition of
Ba2+, Cd2+ and Pb2+ resulted in precipitation; however, no signifi-
cant fluorescence change of 1 was detected in the presence of other
tested metal ions. The observed band for Zn2+ complex of 1 is ex-
plained as follows: receptor 1 is exhibiting keto–enol tautomerism
with equilibrium in the two tautomeric forms as also projected
from density functional theory (DFT) results by having approxi-
mately equal energies for enol (ꢁ1726.75 a.u.) and keto
(ꢁ1726.81 a.u.). Ideally, dual channel emission is exhibited by
the receptor if keto and enol forms exist in equilibrium with each
other. Upon addition of Zn2+, it seems that enol form is binding
with Zn2+, thus the enol form is eliminated from the equilibrium;
consequently shifting the equilibrium towards enol form and fluo-
rescent enhancement is observed. Moreover the enhancement is
not observed exactly at 457 nm. The binding of Zn2+ close to fluo-
3. Result and discussion
3.1. Synthesis of receptor
Receptor 1 was synthesized by stirring 4,40-diaminodiphenyl-
methane (198 mg, 1.0 mmol) along with 4-(diethylamino)-2-
hydroxybenzaldehyde (193 mg, 2.5 mmol) in methanol as shown
in Scheme 1 and was characterized with IR and NMR spectroscopy.
The IR spectrum of 1 shows a band at 1688 cmꢁ1 corresponding to
mC@
N stretching (Fig. S1), 1H NMR spectrum of 1 is showing signals
at d 8.66 (s, 2H, –CH@N–), 13.65 (s, 2H, –OH) and 13C NMR spec-
trum of 1 is showing signals at d 161.42 and 163.46.
A
3.2. Photophysical properties of receptor 1 and molecular model
calculations
The photo-physical properties of 1 were evaluated in HEPES
buffered DMF/H2O (7:3, v/v) solvent system using UV–Vis absorp-
tion and fluorescence spectra. The UV–Vis absorption spectrum of
1 (10
lM) showed a band at 400 nm (Fig. S2) in HEPES buffered
DMF/H2O (7:3, v/v) solvent system. The band is because of intra-
molecular charge transfer with imine linkage. The fluorescence
spectra of receptor 1 upon excitation at kmax = 400 nm (at room
temperature) showed emission bands at kmax = 457 and 509 nm
in HEPES buffered DMF/H2O (7:3 v/v) solvent system. The dual
channel emission is explained on the basis of excited state intra-
molecular proton transfer (ESIPT) process involving keto and enol
tautomers. The modified Jablonski diagram exhibiting the ESIPT
phenomenon is shown in Fig. S3. The ESIPT process involves the
quick transfer of a hydroxyl proton to imine nitrogen through a
six- or five membered ring hydrogen-bonding configuration. The
enol So ground state is photo excited and it transforms into the
enol S1 excited state which then changes into the more stable keto
S1[11], photo excited tautomer, which in turns decays to the
ground state in a radiative way. The longer wavelength band no-
ticed at 509 nm is due to keto form and the emission band found
at 457 nm is assigned as fluorescence emission from the excited
enol state. The density functional theory using Becke’s three
parameterized Lee–Yang–Parr (B3LYP) exchange functional with
6-31G⁄ basis sets, on Gaussian-09 programs [12] revealed the opti-
mized structures (Fig. 1). The optimized structures of enol form
and the keto form (Fig. 1A and B) showed that the structures are
unsymmetrical. One part of the structures has the geometry suit-
able for the proton transfer through ESIPT, the second part of the
same molecule has the geometrical restriction, where –OH group
B
Fig. 2. (A) Fluorescence spectra of 1 (10
metal nitrate salts in HEPES buffered DMF/H2O (7:3, v/v) solvent system; (B)
Fluorescence emission spectra of 1 (10
M) upon successive addition of Zn2+ (0–
100 M) in HEPES buffered DMF/H2O (7:3, v/v); excited at 400 nm.
lM) upon addition of 100 lM of different
l
l

4
K. Kaur et al. / Inorganica Chimica Acta 399 (2013) 1–5
rophore may have influenced the ring currents and thus resulted in
the modulation of the charge transfer (CT) band. Thus the observed
band is the consequence of two phenomena i.e. ESIPT and modula-
tion of CT band [13].
ence of Zn2+ ion. The stoichiometry of the complex formed was
determined by Job’s plot [16] at 462 nm (Fig. S6) and it was found
to be 1:1. In order to evaluate the reaction time for zinc sensing,
the time-dependence of fluorescence intensity of receptor 1 was
checked and results were compared to other metal ions (Fig. S7).
Receptor 1 reached equilibrium immediately upon mixing with
In order to investigate the properties of 1 as a sensor forZn2+, a
fluorescence titration was carried out by adding aliquots of Zn2+
(0–100
v/v) (Figure 2B). The addition of incremental amounts of Zn2+ (0–
100 M) to the solution of compound 1 resulted in emergence of
lM) to the solution of 1 in HEPES buffered DMF/H2O (7:3,
the solution of Zn2+
.
l
3.4. Competitive metal binding test
anew band and seven fold enhancement of fluorescence intensity
at 462 nm, which is exactly same as observed in metal binding test
shown in Fig. 2A. The detection limit of 1 [14] as a fluorescent sen-
sor for the analysis of Zn2+ was determined from a plot of normal-
ized fluorescence intensity as a function of the concentration of the
added metal ions and it was found that 1 has a detection limit of
To check the practical applicability of 1 as selective fluorescence
sensor for Zn2+; a competitive experiment was performed for the
estimation of Zn2+ (5 equiv.) in the presence of any of Na+, K+,
Mg2+, Ca2+, Ba2+, Sr2+, Al3+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cd2+, Cu2+
,
Ag+, Hg2+ or Pb2+(5 equiv.). No significant difference in the intensity
was observed by comparing the intensity with and without other
metal ions (Fig. S8). The distinctive binding of Zn2+ suggests an
interesting panorama to develop receptor 1 as a sensor for Zn2+
even in the presence of other metal ions.
10 l
M for Zn2+. The association constant Ka of 1 for zinc was calcu-
lated on the basis of the Benesi–Hildebrand plot [15] (Fig. S5) and it
was found to be 2.2 ꢂ 103 Mꢁ1. The observed selectivity of Zn2+ can
be explained in terms of hard-hard interactions between Zn2+ and
donor sites, as compare to soft-hard interactions between other
metal ions (with d10 configuration) and donor sites. On addition
of Zn2+ to the solution of receptor, the coordination of Zn2+ occurs
through hard donors, imine nitrogens and is further facilitated by
the binding from a polar ꢁOH group. It has been seen that the
availability of a pseudo-cavity provided by imine N of the ligands,
which is supported by chelation through –OH group and size dif-
ference of zinc as compare to other metal ions has an important
role in cation binding and selectivity. Hence although both Ag+/
Hg2+ and Zn2+ ions have similar electronic configurations, however
change of florescence intensity of receptor was seen only in pres-
3.5. Anion recognition studies of zinc complex of receptor 1
The zinc complex of receptor 1 was synthesized by refluxing the
aqueous solution of zinc nitrate with a solution of 1 in THF. The
1ꢀZn2+ complex has been characterized by ESI Mass, IR and com-
pared through fluorescence spectroscopy. The mass spectrum
showed m/z at 372.9, which correspond to [M+1]+ where M is
[1 + Zn2+ + THF + NO3ꢁ] (Fig. S9). The IR spectra of 1ꢀZn2+ complex
indicates that the mC@N stretching band of 1 on complexation with
Zn2+ was shifted to a higher frequency by 21 cmꢁ1confirming that
A
B
C
Fig. 3. (A) Changes in fluorescence intensity of 1ꢀZn2+ (10
l
M) upon addition of 20
l
l
M of tetrabutyl ammonium salts of different anions in DMF/H2O (7:3, v/v) solvent system;
(B) Fluorescence emission spectra of 1ꢀZn2+(10
l
M) upon addition of PO4 (0–20
M) in DMF/H2O (7:3, v/v); (C) Changes in fluorescence intensity of 1ꢀZn2+ (10
lM) upon
3ꢁ
addition of 50 lM of phosphorylated species in DMF/H2O (7:3, v/v) solvent system.

K. Kaur et al. / Inorganica Chimica Acta 399 (2013) 1–5
5
imine linkages of receptor 1 are coordinating with Zn2+ (Fig. S1).
Appendix A. Supplementary material
The effect of anions on the fluorescence profile of 1ꢀZn2+ was exam-
ined in HEPES buffered DMF/H2O (7:3, v/v) solvent system. The
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.12.030.
fluorescence profile of 1ꢀZn2+ complex showed no considerable
ꢁ
change in the presence of any of Fꢁ, Clꢁ, Brꢁ, Iꢁ, CNꢁ, ClO4
,
References
NO3ꢁ, CH3COOꢁ and HSO4ꢁ. However, addition of phosphate to
the 1ꢀZn2+ complex leads to quench the band at 462 nm. Out of
the tested anions, maximum quenching was observed in case of
phosphate (Fig. 3A). The selective binding of phosphate ion to
1ꢀZn2+ complex can be explained due to the total anionic charge
density of all the O–P oxygen atoms that participate in the complex
formation between phosphate ion and zinc ion. In contrast to this,
all other anions have lesser charge density in comparison to phos-
phate ion. Hence a significant quenching is observed upon addition
of phosphate to the 1ꢀZn2+ complex [17]. The time-dependence of
fluorescence intensity [18] of 1ꢀZn2+ complex was measured and
the outcome of the result was compared with other anions
(Fig. S10). The 1ꢀZn2+ complex attained equilibrium immediately
upon after mixing with phosphate.
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The competitive anion binding test was carried out with 1ꢀZn2+
complex in the presence of 2 equiv. of one anion out of Fꢁ, Clꢁ, Bꢁrꢁ3,
Iꢁ, NO3ꢁ, CNꢁ, AcOꢁ, ClO4ꢁ and HSO4ꢁ alongꢁw3 ith 2equiv of PO4
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l
M for PO4ꢁ3. Thus
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4. Conclusion
In conclusion, we have synthesized an ‘‘on–off’’ multi-respon-
sive and selective sensor 1 as a probe to monitor the Zn2+ concen-
tration through changes in fluorescence intensity based on ESIPT
and charge transfer (CT) mechanism. The receptor showed high
sensitivity and selectivity for Zn2+ detection at concentrations
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ranging from 0 to 100 lM with detection limit of 10 lM. The
changes in the fluorescence signature of the 1ꢀZn2+ complex in
the presence of phosphate anion are significantly promising.
Therefore, the Zn2+ complex of the receptor can be used for phos-
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Authors are thankful to CSIR (Project No. 01(2417)/10/EMR-II)
for research funding. V.K.B. is thankful to the DST for the DST-IN-
SPIRE Faculty fellowship and research grant. We are also thankful
to SAIF Chandigarh for NMR and Mass Spectra.