A multi-responsive thiosemicarbazone-based probe for detection and discrimination of group 12 metal ions and its application in logic gates

Article abstract of DOI:10.1039/c8nj02011f

A new simple 3-in-1 multi-response thiosemicarbazone-based chemosensor has been synthesized and characterized. The probe not only exhibited high sensitivity towards the most familiar and abundant group 12 metal ions, viz., Zn²⁺, Cd²⁺ and Hg²⁺, in MeCN-H₂O (1 : 1, v/v) medium but also can efficiently distinguish them through significant changes in their absorption and emission spectral behavior. The selectivity response was found to follow the order Hg²⁺, Cd²⁺, Zn²⁺ due to the different degrees of stability of their respective complexes, which was further established by TDDFT calculations and interference studies. The binding affinities of the probe towards these metal ions were investigated by absorption, fluorescence emission, fluorescence lifetime, mass spectral and ¹H NMR spectral measurements. The effects of solvent polarity on the probe molecule were also examined. Due to the observation of different binding affinities and the ensuing significant changes in absorbance at different wavelengths by a combination of different inputs, L can be judiciously applied for the construction of some basic logic gates (AND, OR, NOT, IMPLICATION and INHIBIT).

Full text of DOI:10.1039/c8nj02011f

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A multi-responsive thiosemicarbazone-based
probe for detection and discrimination of group
Cite this:DOI: 10.1039/c8nj02011f
12 metal ions and its application in logic gates†
Soma Sarkar, Tapashree Mondal, Swapnadip Roy, Rajnarayan Saha,
Ashish Kumar Ghosh‡ and Sujit S. Panja
*
A new simple 3-in-1 multi-response thiosemicarbazone-based chemosensor has been synthesized and
characterized. The probe not only exhibited high sensitivity towards the most familiar and abundant
group 12 metal ions, viz., Zn2+, Cd2+ and Hg2+, in MeCN–H2O (1 : 1, v/v) medium but also can efficiently
distinguish them through significant changes in their absorption and emission spectral behavior. The
selectivity response was found to follow the order Hg2+, Cd2+, Zn2+ due to the different degrees of
stability of their respective complexes, which was further established by TDDFT calculations and inter-
ference studies. The binding affinities of the probe towards these metal ions were investigated by
absorption, fluorescence emission, fluorescence lifetime, mass spectral and 1H NMR spectral measure-
ments. The effects of solvent polarity on the probe molecule were also examined. Due to the observa-
tion of different binding affinities and the ensuing significant changes in absorbance at different
wavelengths by a combination of different inputs, L can be judiciously applied for the construction of
some basic logic gates (AND, OR, NOT, IMPLICATION and INHIBIT).
Received 25th April 2018,
Accepted 3rd August 2018
DOI: 10.1039/c8nj02011f
rsc.li/njc
numerous health problems, such as superfacial skin disease,
diabetes, prostate cancer, Alzheimer’s disease, and Parkinson’s
Introduction
With the increasing growth of mining and industry over the disease.5 Cadmium is a highly toxic and carcinogenic metal. In
past decades, heavy metal ion pollution has become a major the list of top twenty hazardous substances reported by the
global problem because many of these metal ions are highly Agency for Toxic Substances and Disease Registry and USEPA,
toxic and adversely affect human health and the environment. cadmium ranks seventh.6 It is extensively used in industry in
Hence, over the last few decades, selective and sensitive recog- Ni–Cd batteries, electroplated steel, and ceramic enamels and is
nition of heavy transition metal ions has drawn intense attention used in agriculture in phosphate fertilizers.7 Cadmium can cause
due to increasing concerns about human health and environ- acute and chronic toxicity to kidneys, lungs, bone and the
mental safety.1 Additionally, some heavy and transition metal nervous system by inducing calcium metabolism disorders and
ions are highly crucial to the lives of organisms.2 In biological renal dysfunction and can cause several other diseases, such as
systems, zinc, cadmium and mercury play distinct roles, pneumonitis and pulmonary edema; it also increases the possi-
although they belong to the same group of the periodic table bility of certain types of cancer.8 Moreover, mercury is one of the
(group 12) and thus exhibit similar electronic configurations and most harmful and extremely hazardous metals. Both the ionic
chemical properties. In the human body, zinc is the second most and elemental forms of mercury can be converted by microorgan-
abundant micronutrient behind iron. Zinc ion (Zn2+) serves as isms into organic methylmercury, which is mostly absorbed into
an essential ingredient for various enzymes and is essential in the human body through a daily diet of fish.9,10 Exposure to Hg
neural signal transmission, gene transcription and apoptosis.3 causes a wide array of diseases, viz., arrhythmia, cardiomyopathy,
Its deficiency causes the genetic autosomal disorder entero- neurological disorders, Minamata disease, prenatal brain
pathica;4 moreover, imbalance of zinc metabolism may lead to damage and gastric disorders.11 Among various important transi-
tion metal ions, Zn2+, Cd2+ and Hg2+ all are relatively significant
environmental pollutants. In biosystems, the accumulation of
Department of Chemistry, National Institute of Technology Durgapur, WB, 713209,
these ions in excess amounts can cause a series of severe diseases
India. E-mail: sujit.panja@gmail.com; Tel: +91-9474914282
and can ultimately to lethal consequences.12 Therefore, the
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
development of fluorescent sensors for selective and sensitive
recognition of these ions is of crucial importance.
c8nj02011f
‡ Central Institute of Mining and Fuel Research, Dhanbad 828 108, India.
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Currently, developing fluorescent sensors of heavy and chemosensor using pyrene as a fluorophore and pyridine
transition metal ions remains an active research topic in the thiosemicarbazone as an ionophore to construct a small
environmental, biology and medicinal chemistry fields due to organic Schiff base-type molecule (L) for multi-ion analysis.
the prospective applications of these sensors in molecular Furthermore, Schiff bases can be easily prepared and can be
catalysis, biological fluorescence imaging and environmental applied to optical sensors due to their excellent spectroscopic
detection.13 A fluorescence chemosensor is a device which responses.22 Here, we report the synthesis, characterization and
senses analytes with fluorescent signal transduction. Com- fluorimetric and colorimetric recognition properties of a probe
pared to fluorescent sensors, colorimetric sensors formed by containing N2S donor atoms to selectively bind the most
the combination of a chromophore and receptor unit have familiar and abundant group 12 metal ions, viz., Zn2+, Cd2+
attracted much interest because naked-eye detection is a very and Hg2+, at trace levels. To the best of our knowledge, this is
simple, rapid and inexpensive process to obtain qualitative the first pyrene-based chemosensor that can detect these metal
information.14
ions simultaneously. The selectivity response was found to
Moreover, the design of a versatile sensor with multiple follow the order of Hg2+, Cd2+, Zn2+. Based on its absorption
binding sites that exhibits different colorimetric or fluores- spectral responses and reversible and reproducible characteris-
cence responses for various analytes can be applied to detect tics, the unique receptor L can mimic composite logic operations
more than one analyte simultaneously. In this regard, multi- (OR, AND, NOT, INHIBIT and IMPLICATION) by influencing a
analyte sensors have been developed.15 Recently, some thiose- specific set of inputs in a systematic way. Computational studies
micarbazone compounds have been constructed as multi-analyte employing time-dependent density functional theory (TD-DFT)
sensors due to their properties of chelate formation with the were performed to further elucidate the experimental results.
analytes.16 Also, thiosemicarbazide compounds and their transi-
tion metal complexes have aroused significant interest due to
Experimental
Materials
their bioinorganic and pharmacological activities. When the
thiosemicarbazide is connected to an N-heterocyclic compound,
the ring N atom becomes a strong electron pair donor to form
The starting materials (chemical reagents and solvents) were of
coordination compounds with transition metals.17 In the neutral
analytical grade and were obtained from commercial sources.
form, thiosemicarbazones behave as N–N–S tridentate chelators
Pyrene-1-carboxaldehyde, pyridine-2-carboxaldehyde and thio-
towards transition metal ions; these N2S chelated ligand–metal
semicarbazide were purchased from Sigma-Aldrich Chemical
complexes have received considerable interest.18 Again, for
Company. Solvents such as ethanol, acetonitrile, and diethyl
sensing, the system must contain a fluorophore as the signaling
ether were of HPLC grade and were used without further
moiety. Among various fluorophores, pyrene is an effective fluor-
purification. The metal salts were purchased from Merck (India).
escent probe due to its chemical stability, high fluorescence
Deionized water (distilled) was used throughout the experiments.
quantum yield, long fluorescence lifetime and monomer–excimer
dual emission properties.19 A variety of multi-ion sensors have
Apparatus
been constructed; however, no pyrene derivative has been reported
as a sensor for Zn2+, Cd2+ and Hg2+ ions simultaneously. Therefore,
Absorbance spectra were measured with a Shimadzu UV-1800
Spectrophotometer. All fluorescence measurements were per-
it is essential to develop a pyrene-based visual sensor and fluor-
formed with a Hitachi F-2500 Spectrofluorimeter using a 1 cm
escent sensor for sensitive detection of these group 12 elements.
quartz cuvette with a slit width of 2.5 nm and a scan speed of
In the field of information technology, molecular logic gates
5 nm sꢀ1. The PMT voltage of the fluorimeter was maintained
have gained much attention for molecular-level information
at 700 V. pH measurements were conducted with a Eutech
processing due to their ability to provide information at the
molecular scale.20 In this field, to execute a useful estimation,
Instruments digital pH meter (model pH2100). Proton nuclear
magnetic resonance (1H NMR) spectra of the probe and its
the switching process should be a reversible and reproducible
Hg-complex were recorded on a Bruker DPX 300 MHz spectro-
chemical process. A truth table has been constructed by following
the principles of binary logic function using a few basic logic
operations; AND, OR, NOT, XOR, NAND, INHIBIT, etc. INHIBIT
mass Q-TOF microTM instrument under ESI mode. The infrared
logic systems are receiving the most interest due to their non-
commutative and nonassociative behaviour, i.e., a single input
can stop the entire system. A receptor that has different affinities
resolved fluorescence lifetimes were measured by time-correlated
and is associated with different optical outputs in terms of
meter in CDCl3 solution at room temperature with TMS as the
internal standard. The mass spectra were acquired on a Micro-
spectrum was recorded on a Thermo Nicolet iS10 spectrophoto-
meter in the range of 4000 to 400 cmꢀ1 using KBr pellets. Time-
single photon counting (TCSPC) using a Horiba DeltaFlext
wavelength and intensity toward various metal ions due to multi-
instrument. The structures of the probe and its metal complexes
ple binding modes is predicted to carry out simple and integrated
logic operations. Hence, molecules containing different coordina-
were optimized by TD-DFT using Gaussian 09 software.
tion sites for a combination of ions or a specific ion are being
designed for use as potential molecular operators.21
Synthesis and characterization
Combining all these concepts in our consideration, we have The Schiff base structure of probe L was obtained by a 2 step
designed and synthesized a simple 3-in-1 multi-responsive condensation reaction in satisfactory yield (Scheme 1). In the
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Scheme 1 Synthesis of receptor L.
first step, 2 mmol solid thiosemicarbazide (0.18226 gm) was
added to an ethanolic solution of pyridine-2-aldehyde (0.2142 gm,
2 mmol). The mixture was refluxed for 5 h, then cooled. After
evaporating the solvent under reduced pressure, a yellowish white
precipitate appeared; it was washed several times with ethanol
and dried under vacuum to afford the pure solid of pyridine
thiosemicarbazone. The yield was 76.75%. Then, in the second
step, 1 mmol pyridine thiosemicarbazone was added to a hot
ethanolic solution of pyrene-1-carboxaldehyde (1 mmol). The
resulting bright yellowish solution was refluxed for 6 h, then
cooled to room temperature. After evaporating the solvent
under vacuum, a light yellow solid was precipitated. The solid
was washed with ether several times and dried; the final
product (L) was obtained in 71.8% yield. FTIR/cmꢀ1 (KBr) n:
1610 (CQN), 3160 (N–H) (ESI,† Fig. S1). 1H NMR (300 MHz,
CHCl3-d): 9.138 (s, 1H, NH), 8.646 (s, 1H, CHQN), 8.421 (s, 1H,
CHQN), 7.98–7.94 (m, 2H, pyrenyl-H), 7.87–7.84 (m, 4H,
pyrenyl-H), 7.78–7.74 (m, 3H, pyrenyl-H), 7.34–7.31 (m, 4H,
pyridyl-H) (ESI,† Fig. S2). ESI MS: 392.5 (L) and 393.5 (L-H+)
(ESI,† Fig. S3).
Fig. 1 (a) Absorption spectra and (b) colour changes of probe L (2.5 ꢁ
10ꢀ5 M) with various metal ions (4 equiv. each) in Tris buffered MeCN/H2O
(1 : 1, v/v, pH 7) solution.
was treated with various metal ions, a remarkable color change
from colorless to yellow was noted for Hg2+ only (Fig. 1b). The
addition of several other metal ions (e.g., Fe3+, Cr3+, Ag+, Zn2+,
Cd2+, Mn2+, Ni2+, Co2+, Cu2+) did not exhibit any detectable
color change. This observation can easily be elucidated by the
presence of the extended tail in the visible region in the
electromagnetic spectrum, as seen in the absorption spectrum
for Hg2+. This result indicates that L can be used as a highly
selective naked-eye sensor for Hg2+ ion. This colorimetric recogni-
tion toward various metal ions was further demonstrated by
UV-Visible and fluorimetric methods.
Measurement procedures
Because the receptor L is not soluble in water, all the experi-
ments were carried out in a MeCN/H2O (1 : 1, v/v) solvent
mixture. First, a 25 ꢁ 10ꢀ5 M stock solution of L was prepared
in the solvent mixture. The stock solutions of Ag+, Co2+, Pb2+,
Cr3+, Zn2+, Ca2+, Na+, Cu2+, Mn2+, Ni2+, Cd2+, Hg2+ and Fe3+
(1 mM) were obtained by dissolving their metal chloride salts in
the solvent mixture (MeCN/H2O, 1 : 1). Before each absorption
or fluorescence measurement, the solutions were allowed to
stand for 10 min to complete the formation of the metal–ligand
complex. A wide pH range of Tris–HCl buffered solutions of L
(25 mM) in the absence and presence of metal ions, viz., Zn2+
(2.5 ꢁ 10ꢀ4 M), Cd2+ (2.5 ꢁ 10ꢀ4 M) and Hg2+ (2.5 ꢁ 10ꢀ4 M),
was obtained by adjusting the pH with the addition of requisite
amounts of HCl or NaOH solution.
Absorption studies
In order to investigate the sensing properties of chemosensor L
toward various metal ions, it was necessary to select a suitable
solvent. Hence, the absorption spectra of L were recorded in
different solvents (THF, DMF, DMSO, MeCN, EtOH and MeOH).
As shown in the ESI,† Fig. S7, among the various solvents, the
probe displayed the maximum absorbance in MeCN solution.
In DMSO, the absorbance of L was also high; however, due to
the intrinsic coordination and oxidizing properties of DMSO,
we chose MeCN medium for the analysis. Further, in 50%
aqueous–MeCN solution, appreciable spectra were obtained
during the titration with metal ions. For this reason, MeCN/
H2O (1 : 1, v/v) medium was selected for all the spectroscopic
studies. The UV-Vis absorption spectrum of L revealed a broad
absorption band at around 316 nm in MeCN/aqueous (1 : 1, v/v)
Tris–HCl buffer (10 mM, pH 7) medium, which can be ascribed
to the presence of the enlarged conjugated system of the pyrene
Results and discussion
Visual colorimetric analysis
In the absence of metal ions, the MeCN/aqueous (1 : 1, v/v) chromophore unit in the probe. Selectivity is an important and
solution of the ligand (L) was colorless. When a solution of L essential requirement for an excellent chemosensor. Therefore,
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the selectivity of L was tested by the addition of chloride salts of Similarly, upon incremental addition of Hg2+ ion, disappear-
a variety of metal ions: Ag+, Cu2+, Fe3+, Cr3+, Mn2+, Hg2+, Ni2+, ance of the absorption peak at 316 nm was also seen, with the
Co2+, Na+, Ca2+, Pb2+, Al3+, Zn2+ and Cd2+ (Fig. 1a). Addition of appearance of a new peak at 394 nm and a small peak at
only Zn2+, Cd2+ and Hg2+ ions to L produced distinct spectral 286 nm along with two clear isosbestic points at 340 nm and
changes, whereas other metal ions did not significantly influ- 292 nm (Fig. 2c). In each case, the presence of two well-defined
ence the spectral properties of L. The intensity of the absorp- isosbestic points indicates the formation of a stable complex of
tion maximum at 316 nm exhibited a significant decrease L with the respective metal ions as well as the occurrence of two
upon addition of Zn2+, Cd2+ and Hg2+ along with bathochromic types of simultaneous electronic transitions originating from
shifts.
the metal complex on both the higher and lower energy side
To better assess the interactions between L and representa- with respect to the transition originating from the ligand itself
tive metal ions (Zn2+, Cd2+ and Hg2+), spectrophotometric (at 316 nm). The absorbance at the blue end may be ascribed to
titration experiments were performed independently for each the transition from the ground electronic level to the excited
metal ion in aqueous MeCN medium (Fig. 2). Progressive electronic state of the ligand in the presence of metal, whereas
addition of Zn2+ to L resulted in a small band at 280 nm and the absorbance observed at the red end indicates a charge
a new red shifted absorption signal at 373 nm; these signals transfer mechanism between the ligand and metal ion. The
were gradually enhanced with increasing amount of Zn2+ ions, sigmoidal curves shown in the ESI,† Fig. S8–S10, were obtained
while the intensity of the absorption at 316 nm decreased corre- by plotting the changes in the maximum absorption intensity
spondingly (Fig. 2a). Two distinct isosbestic points appeared at as a function of the Zn2+, Cd2+ and Hg2+ concentration. The
286 nm and 340 nm. After the addition of more than 1 equiv. of most important feature was the shift in the absorbance maxi-
Zn2+, no further change was noted. On successive addition of mum of L from 316 nm to 394 nm (Dl = 78 nm) after addition of
Cd2+ to L, a similar trend was observed. There was a gradual Hg2+; this shift was 57 nm for Zn2+ and 41 nm for Cd2+. This
decrease of the absorbance at 316 nm and a systematic growth result demonstrates that the probe not only detects three
of the absorbance maxima at 357 nm and around 284 nm along elements in group 12 of the periodic table, but also can easily
with two distinct isosbestic points at 339 nm and 289 nm (Fig. 2b). distinguish Zn2+, Cd2+ and Hg2+ from other metal ions.
Fig. 2 Spectrophotometric titration spectra of probe L (2.5 ꢁ 10ꢀ5 M) upon addition of (a) Zn2+ ion ([Zn2+] = 0, 2.5, 5, 7.5, 10, 12.5, 15, 20, 25, 37.5 mM),
(b) Cd2+ ion ([Cd2+] = 0, 1.25, 2.5, 5, 7.5, 10, 12.5, 15, 20, 25 mM) and (c) Hg2+ ion ([Hg2+] = 0, 1.25, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25 mM) in Tris
buffered MeCN/H2O (1 : 1, v/v, pH 7) solutions.
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Emission studies
A linear Stern–Volmer plot is generally indicative of a single class
of fluorophore; here, the quenching mechanism was predicted
to be static in nature, which was further ascertained from the
absorption spectra of L in the presence of these metal ions, as
static quenching is a phenomenon of ground state complex
formation.13a The quenching mechanism was also confirmed
from time-resolved fluorescence measurements, which will be
discussed later.
The changes in the emission properties of the Schiff base
ligand (L) were also investigated in the presence of these metal
ions in MeCN/aqueous (1 : 1, v/v) Tris–HCl buffer (10 mM, pH 7)
medium. Upon excitation at 350 nm, the probe L displayed a
broad emission peak with a maximum intensity at around
452 nm, which is an important characteristic of the pyrene
moiety. Fascinatingly, upon treatment of receptor L with Zn2+
ion, fluorescence quenching was observed. With increasing
concentration of Zn2+ up to 1 equiv., the intensity of the emis-
sion at 452 nm reached a minimum value (Fig. 3a), indicating
the formation of a weakly fluorescent [L–Zn2+] complex. The
quantum yield of the Zn-complex was enumerated to be 0.047,
slightly lower (3.2-fold) than that of the isolated ligand (Ff = 0.149).
A similar trend was noted in the fluorescence emission spectra
of L when excited at 350 nm and 385 nm, respectively, during
titration of L with Cd2+ and Hg2+; the emission band at 452 nm
linearly decreased up to 1 equiv. of Cd2+ ions and 2 equiv. of
Hg2+ ions (Fig. 3b and c). The calculated quantum yields of
the [L–Cd2+] and [L–Hg2+] complexes were 0.038 and 0.024,
respectively, which are 4-fold and 6.2-fold lower than that of
receptor L. It is known that among the relevant metal ions,
Zn2+, Cd2+ and Hg2+ show high thermodynamic affinities for
ligands with N or S donor atoms. Hence, the decrease of
emission intensity was ascribed to the introduction of Zn2+,
Cd2+ or Hg2+ and, accordingly, the occurrence of strong
complexation with L through N and S heteroatoms followed
by separation of the intermolecular p-stacked pyrene moieties.
The decrease in the excimer intensity was also attributed to the
heavy metal effect of these metal ions.
Effect of pH
To examine the pH sensitivity of L in the absence and presence
of Zn2+, Cd2+ and Hg2+ ion, absorption studies were conducted
in a pH range from 3 to 12 (Fig. 4). The absorbance at 316 nm
of the metal free sensor L was slightly weak at low pH but
possessed a good response in the pH range of 5 to 12. However,
on addition of 1 equivalent of Zn2+, the absorbance at 373 nm
remained relatively constant in the entire pH range; this indi-
cated insensitivity to pH and also indicated that the sensor L can
detect Zn2+ over a wide range of pH values from 3 to 12. In the
presence of Cd2+ and Hg2+ ion, L did not show any significant
change in absorbance at 357 nm and 394 nm, respectively, in the
pH range of 6 to 12; this indicates that the probe L can be used in
the common environmentally and physiologically relevant range
of pH 6.0 to 8.0. Hence, all the spectral studies of L were
performed at pH 7.
Specificity confirmation
The selectivity of a sensor towards a guest in the presence of
other competitive species is an important property. Hence, in
order to examine the effects of other metal ions on the binding
behaviour of L with Zn2+, Cd2+ and Hg2+, competitive experi-
ments were conducted in MeCN/aqueous medium and moni-
tored by UV-Vis. Consequently, the absorption intensities for
Zn2+, Cd2+ and Hg2+ were investigated at 373 nm, 357 nm and
394 nm, respectively, in the presence of several other metal ions
(Mn2+, Cr3+, Ni2+, Co2+, Fe3+, Cu2+, Ag+, Na+, Ba2+, Pb2+,
The fluorescence quenching efficiency can be determined
by the Stern–Volmer relation:23 I0/I = 1 + KSV [Q]; from this
equation, the quenching constant can be calculated. I0 and I are
the emission intensities in absence of quencher (Q) and the
presence of quencher (Q), respectively, KSV is the Stern–Volmer
quenching constant and [Q] is the concentration of the quencher.
The plot of I0/I vs. quencher concentration exhibited a linear
relationship24 (ESI,† Fig. S11–S13); from these plots, the values
of KSV for the Zn2+, Cd2+ and Hg2+ complexes with L were found to
be 8.5 ꢁ 104 Mꢀ1, 9.3 ꢁ 104 Mꢀ1 and 9.7 ꢁ 104 Mꢀ1, respectively.
Fe2+, Al3+) (Fig. 5). As shown in Fig. 5a, in presence of Zn2+
,
most of the metal ions showed negligible effects on the detec-
tion of Zn2+ except for Cd2+ and Hg2+. After addition of Cd2+ to a
solution bearing probe L and Zn2+, the absorption maximum at
Fig. 3 Fluorescence spectral changes of L (2.5 ꢁ 10ꢀ5 M) at different concentrations of (a) Zn2+ (top to bottom [Zn2+] = 0, 1.25, 2.5, 5, 7.5, 10, 12.5, 15, 20,
25, 50 mM) (lex= 350 nm) (b) Cd2+ (top to bottom [Cd2+] = 0, 2.5, 5, 7.5, 10, 12.5, 15, 20, 25, 50 mM) (lex= 350 nm) (c) Hg2+ (top to bottom [Hg2+] = 0, 2.5, 5,
7.5, 10, 12.5, 15, 17.5, 20, 25, 50 mM) (lex= 385 nm) in Tris buffered MeCN/H2O (1 : 1, v/v, pH 7) solutions.
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competitive experiment for the selectivity of Cd2+ verified that
the sensing ability of L towards Cd2+ was not greatly influenced
by the presence of most other metal ions but that only Hg2+
interfered with the detection of Cd2+ by L (Fig. 5b). The addition
of Hg2+ to a solution of [L–Cd2+] adduct initially led to a slight
decrease and red-shift in the absorbance centered at 357 nm
caused by Cd2+; then, upon gradually adding Hg2+, a new, signi-
ficantly red-shifted absorption signal centered at 394 nm was
observed, which is a characteristic feature of the absorbance
caused by Hg2+, as shown in the ESI,† Fig. S15. In contrast, in
Fig. 5c, it can be observed that the absorbance exhibited by the
mixture of Hg2+ with most other metal ions, including Zn2+ and
Cd2+, was very similar to that caused by Hg2+ only, suggesting
that the sensing ability of L towards Hg2+ was not perturbed by
other metal ions. These results obviously demonstrate that L
has stronger binding ability with Hg2+ than with Zn2+ or Cd2+
under the same experimental conditions. The results of the
competitive experiments also showed that the sensor L can be
successfully used for selective detection of Zn2+, Cd2+ and Hg2+
Fig. 4 Effects of pH on the absorbance of L at lmax = 316 nm, L + Zn2+
complex at lmax = 373 nm, L + Cd2+ complex at lmax = 357 nm and
L + Hg2+ complex at lmax = 394 nm.
373 nm caused by Zn2+ immediately decreased and readily blue in the presence of the most relevant competing metal ions. It is
shifted to 357 nm, which is a significant feature of the absor- also noteworthy that Hg2+ can remove Zn2+ and Cd2+ from their
bance caused by Cd2+. Upon addition of Hg2+ even at very low respective metal–ligand complexes, [L–Zn2+] and [L–Cd2+]. Again,
concentrations (0.2 equiv.) to a solution of [L–Zn2+] adduct, the Cd2+ could remove only Zn2+ from the [L–Zn2+] complex.25 Very
absorption maximum at 373 nm caused by Zn2+ also decreased interestingly, this result indicated the stability order of the
and red shifted; then, upon gradually adding Hg2+, an absorp- corresponding metal–ligand complexes to be [L–Hg2+] 4
tion band at 394 nm appeared, which is characteristic of [L–Cd2+] 4 [L–Zn2+]. This conclusion was further verified from
the absorbance caused by Hg2+ (ESI,† Fig. S14). However, the the binding constant values and theoretical experiments,
Fig. 5 Variation of the maximum absorbance of L (25 mM) with 10 equiv. of other competitive metal ions followed by the addition of 2 equiv. of (a) Zn2+
(b) Cd2+ and (c) Hg2+ in Tris buffered MeCN/H2O (1 : 1, v/v, pH 7) solution. (lmax = 373 nm for Zn2+, lmax = 357 nm for Cd2+ and lmax = 394 nm for Hg2+.)
,
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Scheme 2 Probable sensing mechanism.
as presented later. The probable competitive sensing abilities of complex; this is compatible with the results of the competitive
L towards Zn2+, Cd2+ and Hg2+ ions has been nicely shown in a studies. The detection limits (DL) were calculated from the
schematic representation (Scheme 2).
absorption titration profiles of L with Zn2+, Cd2+ and Hg2+ ions
on the basis of 3s/K, where s is the standard deviation of the
blank solution and K is the absolute magnitude of the slope of
the calibration curve. The detection limits for Zn2+, Cd2+ and
Hg2+ ions were found to be 1.94 ꢁ 10ꢀ7 M, 1.57 ꢁ 10ꢀ7 M and
1.52 ꢁ 10ꢀ7 M (ESI,† Fig. S20–S22), respectively. The results are
summarized in Table 1.
Binding stoichiometry, binding constant and detection limit
In order to determine the stoichiometric ratio of L with metal
ions, a series of solutions of L, Zn2+, Cd2+ and Hg2+ were
prepared in Tris HCl containing a MeCN/H2O (1 : 1, v/v) solvent
mixture. The total concentrations of the resulting solutions
remained constant (25 mM). The mole fractions of the externally
added metal ions were varied from 0.1 to 1.0. The absorbances
Aggregation studies of L
at 373 nm for Zn2+, 357 nm for Cd2+ and 394 nm for Hg2+ were The emission properties of pyrene are very sensitive in polar
plotted against the mole fractions of the respective metal ions. solvents due to the p–p interactions between pyrenyl groups.27
In the presence of metal ions, the absorbance spectra attained Therefore, the emission spectra of L were recorded in MeCN
maxima at a mole fraction of 0.5. Job plot analysis (ESI,† only and MeCN–water binary mixtures containing varied amounts
Fig. S16) reveals that the binding between the host receptor L of water (Fig. 6). The fluorescence spectrum of L (30 mM) in pure
and the guest metal ions exhibited 1 : 1 binding stoichiometry, MeCN showed a very poor emission maximum at 413 nm. On
which was confirmed by ESI-mass spectrometric analysis (ESI,† increasing the water fraction to 80%, the fluorescence intensity
Fig. S4–S6). Based on the Benesi–Hildebrand equation,26 the of the excimer sharply increased (B65 times) associated with a
binding constants of L with Zn2+, Cd2+ and Hg2+ were estimated large red-shift (D = 50 nm), as shown in the inset (a) of Fig. 6.
to be 3.125 ꢁ 105 Mꢀ1, 3.81 ꢁ 105 Mꢀ1 and 5.58 ꢁ 105 Mꢀ1
,
However, on addition of further water content (up to 90%), the
respectively (ESI,† Fig. S17–S19), indicating that [L–Hg2+] was fluorescence intensity decreased bathochromically due to the
the most stable complex and [L–Zn2+] was the least stable solubility effect.19f Correspondingly, the quantum yield (F) of L
Table 1 Spectroscopic and photophysical characteristics of L in the absence and presence of Zn2+, Cd2+ and Hg2+ ions in Tris buffered MeCN/H2O
(1 : 1, v/v, pH 7) solution
Compound
labs (nm)
lem (nm)
Isosbestic points (nm)
KSV (Mꢀ1
)
K (Mꢀ1
)
DL (M)
L
316
452
452
452
452
—
—
—
—
[L–Zn2+] complex
[L–Cd2+] complex
[L–Hg2+] complex
280, 373
284, 357
286, 394
286, 340
289, 339
292, 340
8.5 ꢁ 104
9.3 ꢁ 104
9.7 ꢁ 104
3.125 ꢁ 105
3.81 ꢁ 105
5.58 ꢁ 105
1.94 ꢁ 10ꢀ7
1.57 ꢁ 10ꢀ7
1.52 ꢁ 10ꢀ7
KSV is the Stern–Volmer quenching constant, K is the binding constant and DL is the limit of detection.
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The proton of the NH group exhibited slight downfield shifts for
Zn2+ and Cd2+ and a downfield shift of 0.06 ppm for Hg2+. Both
these shifts indicated strong complexation of Zn2+/Cd2+/Hg2+
with the azomethine nitrogen. Further, downfield shifting was
observed for the pyridyl ring protons, which suggests the
construction of a five-membered ring through the pyridyl N
and azomethine N with Zn2+/Cd2+/Hg2+ ion. Meanwhile, most of
the pyrene protons displayed upfield shifts compared to that of
free L. These metal ions are soft in nature and prefer to bind
soft donors such as S; hence, it is expected that another five-
membered ring may be formed through the azomethine N
connected to the pyridyl ring and the S of the CQS group with
Zn2+/Cd2+/Hg2+. Upon addition of Hg2+, the shifting of the
protons was greater compared to that when adding Zn2+ and
Cd2+, indicating the stronger binding ability of L with Hg2+; this
is consistent with the results of the competitive studies.
Fig. 6 Effects of water ratio on the fluorescence emission intensity of L
(30 mM) (lex = 385 nm, slit: 2.5 nm). Inset (a) changes in the wavelength
maximum and inset (b) changes in the quantum yield of L with varying
water fractions (0% to 90%).
Time-dependent density functional theory (TDDFT)
calculations
To determine the theoretical aspects of the observed selective
responses of L towards Zn2+, Cd2+ and Hg2+ ions, time-
dependent density functional theory (TDDFT) calculations were
performed using the Gaussian 09 program. The structures of
the compounds were optimized using Becke’s three parameters
with the Lee–Yang–Parr functional (B3LYP) and a lanl2dz basis
set. The energy-optimized stable structures and the probable
binding modes of L with Zn2+, Cd2+ and Hg2+ ions are presented
in Fig. 8. The contributions of the orbital transitions and their
corresponding oscillator strengths are shown in Tables S1 and
S2 (ESI†). The HOMO–LUMO diagram exhibits that the electron
density in the HOMO of L is located mainly on the pyridine-
thiosemicarbazone moiety, which provides the most binding
influence towards metal ions, and that the electron density in
the LUMO of L is positioned mostly on the pyrene moiety.
However, the orbital positions of the HOMO and LUMO in the
L–Zn2+, L–Cd2+ and L–Hg2+ adducts changed due to complexa-
tion. The calculated structures show that the corresponding
metal ions were perfectly encapsulated into the cavity of L by
strong electrostatic interactions through the imine-N, pyridyl-N
and thiopheno-S atoms of L. The corresponding energy values
of the individual HOMOs and LUMOs were lower for the com-
plexes compared to that of the free sensor L (Table 2), which
may be responsible for the red shifts in the UV-Vis absorption
spectra upon complexation. The total energies of the complexes
were lower in comparison to that of free L, indicating higher
stability of the complexes. The descending order of the stabili-
zation energy of the complexes is L–Zn2+ 4 L–Cd2+ 4 L–Hg2+;
hence, the ascending stability order is L–Zn2+ o L–Cd2+ o L–Hg2+.
Therefore, the theoretical studies are in good agreement with the
experimental observations.
was also rapidly enhanced (B57.5 times) with increasing water
content (0% to 80%) and then decreased, which is shown in the
inset (b) of Fig. 6. This sharp enhancement in fluorescence
intensity along with a bathochromic shift in the MeCN–water
binary mixtures reveals aggregation of L, which is ascribed to the
aggregation-induced enhanced emission (AIEE) phenomenon28
and the transformation of the excited state from n–p* to p–p*.23
1H NMR studies
To understand the interaction of L with Zn2+, Cd2+ and Hg2+,
1H NMR studies were performed independently by adding these
metal ions to a CHCl3-d solution of L (Fig. 7). Upon addition of
2 equivalents of Zn2+, Cd2+ and Hg2+ to L, significant spectral
changes were noted. The peak at 8.421 ppm for the azomethine
proton connected with the pyridyl ring was shifted upfield toward
8.389 ppm for Zn2+, 8.361 ppm for Cd2+ and 8.332 ppm for Hg2+.
Time-resolved fluorescence studies
Time-resolved fluorescence studies are very important to eluci-
date the decay processes of the ligand and the complexes.
Consequently, the excited state fluorescence lifetimes were
measured using 290 nm, 375 nm, 340 nm and 375 nm LED
Fig. 7 1H NMR spectra of L in absence and presence of Zn2+, Cd2+ and
Hg2+ in CDCl3.
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Fig. 8 Frontier molecular orbitals and energy level diagrams of L and the [L–Zn2+], [L–Cd2+] and [L–Hg2+] adducts.
Table 2 Comparison of the theoretical energies of L in the absence and
calculated from the equations: Kr = Ff/t and t = 1/(Kr + Knr).
Biexponential fitting was obtained for all the decay processes,
and the w2 values indicated that this was the best fit for all the
decay behaviors. The biexponential decay pattern of the free
ligand reflects the presence of a short-lived minor component
(0.598 ns, 18.57%) and a long-lived major component (3.79 ns,
81.43%). After complexation of L with Zn2+, Cd2+ and Hg2+ ions,
the faster decay components (b1) of L were found to increase
with a decrease in the values of the longer decay components
(b2), as shown in Table 3. The average fluorescence lifetime
values of L and the L–Zn2+, L–Cd2+ and L–Hg2+ complexes were
estimated to be 1.9, 1.95, 1.99 and 1.96 ns, respectively. The
apparent linear nature of the Stern–Volmer plots alone is not
sufficient to reach an unambiguous conclusion regarding
whether the mechanism of quenching is dynamic or static in
nature. Most of the time, in graphs, the concentration of
quencher is taken in an oversimplified way without discrimi-
nating the concentrations of free and complexed quencher.
Measurement of the fluorescence lifetime is the most definitive
method to differentiate between the two mechanisms because
we know that for static quenching, t0/t = 1, whereas for dynamic
quenching, F0/F = t0/t. The fluorescence lifetime of L was found
to remain almost identical upon addition of Zn2+, Cd2+ and
Hg2+ ions, which further established the static nature of the
quenching mechanism.24a Notably, the values of Kr and Ff
of the complexes were lower and the Knr values were higher
compared to that of the free ligand. The greater Knr values of
the complexes revealed the greater extent of the activities of the
nonradiative pathways, resulting in higher quenching of the
emission intensity.
presence of Zn2+, Cd2+ and Hg2+ ions
Compound
Energy of HOMO (eV) Energy of LUMO (eV) DE (eV)
L
ꢀ0.12574
ꢀ0.08610
ꢀ0.11126
ꢀ0.12545
ꢀ0.15406
0.03964
0.03314
0.0306
L–Zn2+ complex ꢀ0.14440
L–Cd2+ complex ꢀ0.15605
L–Hg2+ complex ꢀ0.17824
0.02418
sources for the L, L–Zn2+, L–Cd2+ and L–Hg2+ systems, respec-
tively, while monitoring the emission at 452 nm. The fluores-
cence lifetime decay plots of L and its metal complexes are
presented in Fig. 9. The fluorescence lifetime values, quantum
yields, and radiative (Kr) and non-radiative (Knr) decay constants
of L and its corresponding metal complexes are tabulated in
Table 3. The radiative and non-radiative decay constants were
Counter ion effect and reusability of L
In addition to sensitivity and selectivity, recyclability is a prerequi-
site to develop a good sensor for practical applications because it
determines the cost-effectiveness of real-sample measurements.
Fig. 9 Changes in the time-resolved fluorescence decay of (a) L (lex
=
290 nm) and the (b) L–Zn2+ (lex = 375 nm), (c) L–Cd2+ (lex = 340 nm) and
(d) L–Hg2+ (lex = 375 nm) complexes at lem = 452 nm.
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Table 3 Fluorescence lifetime decay properties of L in the absence and presence of Zn2+, Cd2+ and Hg2+ ions at lem = 452 nm
Compound
t (ns)
b (%)
hti (ns)
w2
Ff
Kr (109 sꢀ1
)
Knr (109 sꢀ1
)
L
t1 = 0.598
t2 = 3.79
t1 = 0.794
t2 = 4.32
t1 = 0.628
t2 = 3.81
t1 = 1.05
t2 =4.5
b1 = 18.57
b2 = 81.43
b1 = 31
b2 = 61.84
b1 = 28.04
b2 = 70.96
b1 = 39.22
b2 = 60.78
1.9
1.02
0.149
0.0784
0.448
L–Zn2+ complex
L–Cd2+ complex
L–Hg2+ complex
1.95
1.99
1.96
1.09
0.98
0.98
0.047
0.038
0.024
0.0241
0.0191
0.0122
0.4887
0.4834
0.4979
We found that the ‘‘turn-off’’ band of L by Zn2+ at 316 nm was
reversed by the addition of Fꢀ and S2ꢀ, although these anions
did not cause any effects on the absorbance of free L. The
ensembles L–Cd2+ and L–Hg2+ immediately entered a ‘‘turn-on’’
2ꢀ
state by the addition of S2ꢀ only. However, other anions (SO4
,
OAcꢀ, NO3ꢀ, Fꢀ, Brꢀ) did not elicit any remarkable response in
the absorbance of these complexes. As shown in ESI,† Fig. S23,
upon treatment with 2 equiv. of S2ꢀ, the reappearance of the
absorption band at 316 nm implies the formation of [MYx]
(M = Zn2+/Cd2+/Hg2+ and Y = S2ꢀ) from the L-M2+ ensembles,
freeing the ligand.29 This ‘‘ON–OFF–ON’’ reversible cycle can be
repeated several times (ESI,† Fig. S23a–c). Thus, the sensor can
be chemically reset by S2ꢀ, representing the reversibility.
Application in logic functions
The reversible and reusable switching process of L can be
represented by a molecular logic gate by constructing two-
input and two-output combinatorial logic circuits.30 The logic
circuit is planned with dual stimulant inputs of Hg2+ (In1) and
S2ꢀ (In2) and with the absorbances at 316 nm (OUT1) and
394 nm (OUT2) as the output modes. The threshold values of
absorbance were set to be 1.01 and 0.32, respectively, for
316 nm and 394 nm. Absorbance lower than the threshold
Fig. 10 (a) Output signals of the logic gate in the presence of different
inputs, (b) corresponding truth table of the logic gate, (c) corresponding
bar diagram of the absorbance outputs of L at lmax = 316 nm (OUT1, blue
bars) and lmax = 394 nm (OUT2, red bars) in the presence of two inputs
(Hg2+ and s2ꢀ), (d) representation of the IMP/INH two-input and two-
output-based logic circuit. Dotted lines indicate the threshold levels of the
absorbance outputs.
values have been assigned as ‘‘0’’, and absorbances higher than Consequently, both the complementary IMP/INH logic func-
the threshold values are considered as ‘‘1’’, corresponding to tions can be performed in parallel at one time by the sensor L
the ‘‘off’’ and ‘‘on’’ states, respectively. There are four possible (Fig. 10d).
input combinations, (0, 0), (1, 0), (0, 1) and (1, 1), as shown in
the truth table in Fig. 10b. In the absence of ions (considering
the input signal ‘‘0, 0’’, or no input) or in the presence of only
S2ꢀ (In2 is ‘1’ and In1 is ‘0’), OUT1 at 316 nm is switched ‘‘on’’
(output signal ‘‘1’’). Upon interaction with only Hg2+, i.e., In1 is
‘1’ and In2 is ‘0’, the absorbance at OUT1 enters the ‘‘off’’ state
(‘‘0’’). When both the inputs are present together (input signal
‘‘1–1’’), the system regained the initial absorbance at 316 nm
implying the ‘‘on’’ state (‘‘1’’). These combinations led to the
construction of an ‘‘IMPLICATION’’ logic gate. However, in the
absence of both inputs (Hg2+ or S2ꢀ) and in the presence of only
S2ꢀ (In2), the absorbance at 394 nm (OUT2) was very low, which
indicates the ‘‘off’’ state (output signal ‘‘0’’) of the system.
However, in the presence of only Hg2+ (In1), a significant
enhancement of the absorbance was observed, implying that
OUT2 enters the ‘‘on’’ state (‘‘1’’). These combinations represent
the construction of an ‘‘INHIBIT’’ logic gate. The IMPLICATION
Fig. 11 Logic gate diagram for simultaneous monitoring of Zn2+, Cd2+
logic function output is just complementary to an INHIBIT
logic function and is closely related to the ‘‘if-then’’ phrase.31 and Hg2+ using L.
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Table 4 Truth table for all plausible strings of input data and the corresponding output digits
Input
Output
In1 (Zn2+
)
In2 (Cd2+
)
In3 (Hg2+
)
OUT1 lmax = 373 nm
OUT2 lmax = 357 nm
OUT3 lmax = 394 nm
1
0
0
1
1
0
1
0
1
0
1
0
1
1
0
0
1
0
1
1
1
1
0
0
0
0
0
0
0
1
0
1
0
0
0
0
0
1
0
1
1
1
Furthermore, the set-reset of L was also explained by Zn2+ absorbances as outputs. All the results clearly elucidated that
and S2ꢀ and also by Cd2+ and S2ꢀ. In the case of the inputs Zn2+ the chemosensor L provides a new approach for the detection of
and S2ꢀ, the absorbances at 316 nm and 373 nm were used as the most familiar and abundant group 12 metal ions simulta-
OUT1 and OUT2, respectively (ESI,† Fig. S24). Again, when Cd2+ neously or separately.
and S2ꢀ were chosen as inputs, OUT1 and OUT2 were at 316 nm
and 357 nm, respectively (ESI,† Fig. S25). These outputs also
Conflicts of interest
generated combinatorial IMP/INH logic circuits.
Further, the selectivity of probe L between Zn2+, Cd2+ and
There are no conflicts to declare.
Hg2+ was verified via a sequential logic circuit with three inputs,
viz., Zn2+ (In1), Cd2+ (In2), and Hg2+ (In3), and three ‘‘turn-on’’
output signals of absorbance at lmax = 373 nm (OUT1 due
Acknowledgements
to Zn2+), 357 nm (OUT2 due to Cd2+) and 394 nm (OUT3 due
to Hg2+) (Fig. 11). All the input and output data are clearly
The authors thankfully acknowledge DST-FIST (SR/FST/CSI-
represented in Table 4. A combination of AND, OR and NOT
267/2015 (C)) and NIT Durgapur for creating and providing
infrastructural facilities for research. The authors are thankful
logic functions was used. When any one of the three inputs
1
(Zn2+/Cd2+/Hg2+) was present, the corresponding output signal
to Visva-Bharati University for the H NMR spectra and IACS,
Kolkata for the ESI-mass spectra.
(at lmax = 373 nm, 357 nm or 394 nm) became high (‘‘1’’),
indicating the ‘‘ON’’ state. Thus, an OR logic gate could be
generated. Again, when all three inputs were high (‘‘1’’), only
Notes and references
the output signal at 394 nm (OUT3) due to Hg2+ was observed
and the signals for Zn2+ or Cd2+ were not observed, resulting in
a NOT logic gate. This observation can be explained by the
preference of L for complexation with Hg2+ even in the presence
of Zn2+ and Cd2+. All these results clearly elucidate that this
sensor can simultaneously or separately detect multiple ions by
using appropriate outputs.
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