A highly sensitive pyridine-dicarbohydrazide based chemosensor for colorimetric recognition of Cu2+, AMP2-, F- and AcO- ions

Article abstract of DOI:10.1039/c8nj00918j

A new pyridine-dicarbohydrazide based colorimetric chemosensor L1 is designed and synthesized for the detection of anions and cations. The chemosensor allows the 'naked-eye' recognition of toxic cation Cu²⁺ and biologically relevant anions AMP^2-, F^- and AcO^- in DMSO/H₂O (8/2, v/v) solution with a fast response time (～5 s). The selectivity, sensitivity and binding stoichiometry of L1 for the aforementioned ions are investigated using colorimetric, UV-Vis spectroscopic and ¹H NMR studies. The binding mode is analyzed by UV-Vis and ¹H NMR studies, which are supported by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The optimized geometries indicate that the carbohydrazide moiety provides the binding site for the cation and anions. The HOMO-LUMO energy gap for the complex of AMP^2- with L1 is least among all the complexes, possibly due to the charge-transfer effect. The detection limits of L1 for Cu²⁺ and AMP^2- were determined to be 0.12 μM and 0.08 μM, respectively. Furthermore, the potential application of the sensor for the detection of fluoride ion in commercially available toothpaste was demonstrated successfully.

Full text of DOI:10.1039/c8nj00918j

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A highly sensitive pyridineꢀdicarbohydrazide based chemosensor for colorimetric
recognition of Cu2+, AMP2ꢀ, Fꢀ and AcOꢀ ions
Rakesh Kumar*a, Harshita Jaina, Parveen Gahlyan,a Ankita Joshib and C. N. Ramachandranb
aDepartment of Chemistry, University of Delhi, Delhiꢀ110007, India
bDepartment of Chemistry, Indian Institute of Technology Roorkee, Uttarakhand ꢀ 247667, India
*Corresponding address: rakeshkp@email.com
Abstract
A new pyridineꢀdicarbohydrazide based colorimetric chemosensor L1 is designed and
synthesized for the detection of anions and cations. The chemosensor allows the ‘nakedꢀeye’
recognition of toxic cation Cu2+ and biologically relevant anions AMP2ꢀ, Fꢀ and AcOꢀ in
DMSO/H2O (8/2, v/v) solution with fast response time (~5 s). The selectivity, sensitivity and
binding stoichiometry of L1 for aforementioned ions are investigated through colorimetric, UVꢀ
1
Vis spectroscopic and 1H NMR studies. The binding mode is analyzed by UVꢀVis and H NMR
studies, which is supported by density functional theoretical (DFT) and timeꢀdependent density
functional theoretical (TDDFT) calculations. The optimized geometries indicate that
carbohydrazide moiety provides the binding site for cation and anions. The HOMOꢀLUMO
energy gap for the complex of AMP2ꢀ with L1 is least among all the complexes, possibly due to
chargeꢀtransfer effect. The detection limits of L1 for Cu2+ and AMP2ꢀ were determined to be 0.12
ꢁM and 0.08 ꢁM, respectively. Furthermore, the potential application of sensor for detection of
fluoride ion in commercially available toothpaste was demonstrated successfully.
Key words: Pyridineꢀdicarbohydrazide, Colorimetric sensor, Cation, Anions, DFT
1

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1. Introduction
The development of colorimetric sensors to recognize cations, anions and neutral species has
attracted much interest of researchers due to their significant role in physiological and
environmental applications. In the human body, Cu2+ is one of the most vital and third most
abundant transition metal ions. It plays indispensible role in various biological processes by
acting as cofactor in the functioning of enzymes such as superoxide dismutase, cytochrome c
oxidase and tyrosinase.1 However, excess intake can cause adverse effects to human health such
as vomiting, lethargy, increased blood pressure, Alzheimer’s, Menke’s, Parkinson’s and
Wilson’s diseases.2ꢀ4 Cu2+ is also used in various industries worldwide. Consequently, wide use
of copper causes contamination to water resources. Therefore, large number of fluorescent and
colorimetric sensors has been developed for detection of Cu2+ ion selectively.5ꢀ10
Among various anions, recognition of biologically relevant phosphate, fluoride and acetate
anions has been the topic of intensive research due to their physiological effects. Fluoride ion is
essential in dental health and osteoporosis treatment.11 However, excessive intake of fluoride can
lead to various diseases such as gastric problems, kidney problems, fluorosis and urolithiasis.12
Acetate ion is involved in the field of biochemistry, environment and pharmaceutical science.
Phosphate anions (both inorganic and organic) such as inorganic phosphates and ATP, ADP,
AMP also possess relevance in various biological processes. Adenosineꢀ5ꢀtriphosphate (ATP) is
a multifunctional nucleotide, plays pivotal role as molecular energy currency in all living
organisms and is involved in DNA duplication and trancription.13 Hydrolysis of ATP produce
adenosine diphosphate, adenosine monophosphate, and inorganic phosphates under cellular
condition. Adenosine monophosphate (AMP) is involved in bioenergetics, regulation of
carbohydrate metabolism including glycogen phosphorylase and phosphofructokinase.14,15 The
2

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phosphorylation of proteins induces structural modifications. As a result, activity of enzymes
switches on or off. Thus, the activity of enzymes that catalyze metabolic processes can be
measured by recognizing AMP selectively. During metabolic processes, AMP is produced from
ATP. Thus, sensors that distinguish AMP from ATP give useful information on the metabolic
processes. Although considerable efforts have been devoted for developing sensors for
recognition of Fꢀ and AcOꢀ16,17 but there are few reports on detection of AMP2ꢀ.18,19 Therefore,
development of sensors for recognition of Fꢀ, AcOꢀ and AMP2ꢀ holds great significance in anion
recognition chemistry.
A number of analytical methods have been reported for the sensing of cations and anions such as
ion selective electrodes,20 voltammetry,21 potentiometry,22 ion exchange resins23 and PVC
membrane sensor24 but these methods have demerits of difficult sample preparation, low
sensitivity, expensive instrumentation and complicate handling. While colorimetric chemosensor
is considered to be most attractive and advantageous due to simple, costꢀeffective, convenient,
sensitive, selective and ‘nakedꢀeye’ detection ability of target ions. Therefore, there is high
demand of colorimetric sensors capable of detection in visible wavelength region. Various
chemosensors have been reported in literature that utilize functional groups such as ureas,25
thioureas,26 callixpyrroles,27 amides and benzimidazoles28 etc. to recognize biologically relevant
anions via hydrogen bonding interactions or deprotonation. Also, various Schiff’s base and
amide based sensors have been reported for the detection of cations. In recent years, there has
been development of large number of colorimetric sensors but rigorous multiꢀstep synthesis,
slow response time and recognition of only cations or anions confine their applicability. Lately,
N Guchhait et al.29 and V K Bhardwaj et al.30 reported pyridineꢀcarbohydrazide based
chemosensor for sensing of only cations. These findings prompted us to develop a fast
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responsive colorimetric chemosensor L1 by simple synthetic methodology that can act as sensor
for multiple targets. Herein, we report the synthesis and application of L1 for simultaneous
detection of toxic Cu2+ cation and biologically relevant AMP2ꢀ, AcOꢀ, Fꢀ anions, where pyridine
unit is attached to benzothiazole unit via carbohydrazide linkage. Nitrogens of pyridine unit and
carbohydrazide linkage were expected to provide metal binding as well as hydrogen bonding site
for anions. Benzothiazole unit was attached to pyridine unit as it acts as electron donor and was
expected to recognize multiple targets. To the best of our knowledge, this is the first example for
the detection of Cu2+ and AMP2ꢀ ions by a single sensor.
2. Experimental
2.1 Materials and methods
All solvents and reagents used for synthesis and UVꢀVis studies were analytical and
spectroscopic grade. Pyridineꢀ2,6ꢀdicarboxylic acid, 2ꢀmethyl benzothiazole, hydrazine hydrate
were purchased from Spectrochem Pvt. Ltd. and used as purchased without further purification.
The solutions of metal ions were prepared from their chloride salts (Sigma Aldrich and Sisco
Research Laboratories Pvt. Ltd) and solutions of anions were prepared from tetraꢀnꢀbutyl
ammonium salts or sodium salts (Spectrochem Pvt. Ltd.). FTꢀIR spectra were recorded on Perkin
Elmer Fourierꢀtransform infrared spectrometer using KBr disks. Melting points of the
1
compounds were recorded on Buchi Mꢀ560 instrument. H and 13C NMR were recorded at room
temperature on JNMꢀEXCP 400 (JEOL, USA) spectrometer using tetramethylsilane (TMS) as
internal standard and chemical shift values are reported in parts per million (ppm). HRMS was
recorded on Agilent technologies, G6530AA (LCꢀHRMSꢀQꢀTOF) spectrometer. UVꢀVis
measurements were performed on PerkinꢀElmer Lambda 35 scanning double beam Spectrometer
equipped with a 50 mm integrating sphere using 1 cm quartz cell.
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2.2 Synthesis of Chemosensor L1
The synthesis of chemosensor (E)ꢀbisꢀN’ꢀ((benzo[d]thiazolꢀ2ꢀylmethylene)ꢀpyridineꢀ2,6ꢀ
dicarbohydrazide (L1) was accomplished in three steps (Scheme1).
S
S
N
SeO2
CHO
CH3
Dioxane:H2O
N
9:1
Reflux
(1)
(1)
(i) EtOH
EtOH,12h,80oC
CH3COOH
(ii) NH2NH2.H2O
Reflux
O
N
O
N
O
O
O
O
N
N
N
NH
HN
OH
OH
NH
HN
H2N
NH2
S
S
(2)
(L1)
N
N
Scheme. 1 Synthetic route to Chemosensor L1
2.2.1 Synthesis of Benzo[d]thiazoleꢀ2ꢀcarbaldehyde (1)
A slightly modified procedure31a was used to synthesize benzo[d]thiazoleꢀ2ꢀcarbaldehyde (1).
2ꢀMethylbenzothiazole (3 g, 1 mmol) was added to the solution of selenium dioxide (4.00 g,
1.5 mmol) in 1, 4ꢀ dioxane/H2O (9:1, v/v). The reaction mixture was refluxed for 12 h. After
the completion of reaction as monitored by TLC, the reaction mixture was filtered. The filtrate
obtained was evaporated under reduced pressure to obtain crude product which was purified by
column chromatography using eluting solvent (hexane: ethyl acetate, 9: 1, v/v) to achieve 1 as
1HNMR(400MHz,DMSOꢀd6)δppm9.36(1H,s,C
H
O),8.14(1H, d, J =7.9
white solid in 70% yield.
Hz,Ar
H),8.05
(1H, d, J= 8.0 Hz, Ar
H), 7.54 ꢀ 7.42 (2H, m, ArH) (Fig. S1 ESI†).
2.2.2 Synthesis of Pyridineꢀ2, 6ꢀdicarbohydrazide (2)
Pyridineꢀ2,6ꢀdicarboxylic acid was first converted to diethyl pyridineꢀ2,6ꢀdicaboxylate by
refluxing in ethanol for 4ꢀ5 h. The product obtained was then converted to pyridineꢀ2,6ꢀ
dicarbohydrazide (2) by refluxing in presence of hydrazine hydrate in ethanol as reported in
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1HNMR(400MHz,DMSOꢀd6)δppm10.60(2H,s,N
H),8.09(3H,
s, Ar
literature.31b
H), 4.58 (4H, s, NHꢀ
NH) (Fig. S2 ESI).
2.2.3 Synthesis of (E)ꢀbisꢀN’ꢀ(benzo[d]thiazolꢀ2ꢀylmethylene)ꢀpyridineꢀ2, 6ꢀcarbohydrazide
(L1)
Pyridineꢀ2,6ꢀdicarbohydrazide (2, 0.1g, 1 mmol) was dissolved in absolute ethanol in presence of
acetic acid (1ꢀ2 mL). Benzo[d]thiazoleꢀ2ꢀcarbaldehyde (1, 0.167g, 1.2 mmol) was added to the
reaction mixture and refluxed for 12 h. A white precipitate obtained was filtered, washed with
1
ethanol and dried in vacuum to obtain white solid L1 (0.23g, 90%); m.p. 220oC; H NMR
(400MHz, DMSOꢀd6) δ ppm 12.73 (2H, s, NH), 9.16 (2H, s, C=NH), 8.40 (1H, d, J = 7.9Hz,
ArH), 8.36ꢀ8.29 (1H, m, ArH), 8.16 (1H, d, J = 7.7Hz, ArH), 8.08 (1H, d, J = 8.1Hz, ArH), 7.59ꢀ
7.49 (2H, m, ArH) (Fig. S3 ESI†); 13C NMR (101 MHz, DMSOꢀd6) δ ppm 165.35, 160.40,
153.75, 148.23, 145.01, 140.97, 134.65, 127.33, 124.00, 123.47, 123.14 (Fig. S4 ESI†); IR (KBr,
ν (cmꢀ1)): 3167 (NH), 3022, 1671 (CO), 1551 and 1334, 1166, 922, 747, 667 (Fig. S5 ESI†);
ESIꢀMS (CH3OH), m/z calcd. for C23H15N7O2S2 (M + H)+ , 486.0801, found 486.0812 (Fig. S6
ESI†).
2.3 Sample preparation and UVꢀVis studies
Stock solutions of chloride salts (1 mM) of metal ions (Cr3+, Mn2+, Fe3+, Hg2+, Ag+, Ca2+, Cu2+,
ꢀ
Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Al3+), sodium or potassium salts (1 mM) of anions (AcOꢀ, H2PO4 ,
2ꢀ
ꢀ
2ꢀ
ꢀ
HPO4 , HSO4 , SO4 , NO2 , AMP2ꢀ, ADP2ꢀ, ATP2ꢀ), tetra butyl ammonium salts (1 × 10ꢀ3 mol Lꢀ
1) of anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ) were prepared in distilled water and stock solution of chemosensor
L1 (10 ꢁM) was prepared in DMSO/H2O (8:2, v/v) solution. Cation and anion binding studies
were performed by adding 30 ꢁL of stock solutions of cations and anions into 2.97 mL of L1
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stock solution in a quartz cuvette using a micropipette and recording the changes in absorption
spectrum of L1. Selectivity experiments were performed by adding 15 ꢁL of Cu2+ to 2.97 mL of
sensor L1 in a quartz cuvette and then adding 15 ꢁL of each metal ion into this solution. UVꢀVis
studies were performed at room temperature. Same procedure was followed for AMP2ꢀ, AcOꢀ and
Fꢀ anions.
2.4 Evaluation of binding constant and detection limit
Binding constant was determined for the formation of cation (Cu2+) and anion (AMP2ꢀ, AcOꢀ and
Fꢀ) complexes with L1 using BenesiꢀHildebrand (BꢀH) plot (Eqn. 1).
1/(AꢀAo) =1/{Ka (AmaxꢀAo)C} + 1/(AmaxꢀAo)
(Eqn. 1)
where, Ao and A are the maximum absorbance of L1 in absence and presence of cation or anions
respectively. C is the concentration of ions added during the titration. Chemosensor L1 with
effective concentration of 10ꢀ5 M was used for titrations with the ions (cations and anions). The
effective concentration of ions changed from 0 to 10×10ꢀ5 M. The binding constant (Ka) was
determined from the slope of BꢀH plot. The detection limit was evaluated on the basis of UVꢀVis
titration studies using Eqn. 2. Standard deviation was obtained by carrying out the UVꢀVis
absorption spectra of L1 ten times.
Detection limit = 3σ/Ka
(Eqn. 2)
where, σ is the standard deviation and Ka is the slope obtained from BꢀH plot.
2.5 Job’s plot measurement
The stoichiometry of the complexes was determined using Job’s Plot. Stock solutions of L1 (10
ꢁM) and Cu2+ ions (10 ꢁM) were prepared in DMSO/H2O (8:2, v/v). Then, 2.7, 2.4, 2.1, 1.8, 1.5,
1.2, 0.9, 0.6, 0.3 mL of L1 and 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7 mL of Cu2+, respectively
were added in the quartz cuvette in order to make the total volume to 3 mL. Then, absorbance of
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the solutions was recorded through UVꢀVis titration at room temperature. Same procedure was
followed for AMP2ꢀ, AcOꢀ and Fꢀ anions. Mole fraction of L1 (ᵡH) was plotted against ∆A/Ao× ᵡH.
The binding stoichiometry was determined by the maximum concentration of the complex as
obtained from the Job’s Plot.
3. Results and discussion
Synthesis
The chemosensor L1 was synthesized via condensation reaction between benzo[d]thiazoleꢀ2ꢀ
carbaldehyde (1) and pyridineꢀ2, 6ꢀdicarbohydrazide (2) in good yield as shown in the Scheme
1. The structure of L1 was elucidated by FTꢀIR, HRMS, 1H and 13C NMR studies.
3.1 Cation sensing
The chemosensor L1 was investigated for sensing of different environmentally and biologically
important metal ions through nakedꢀeye colorimetric experiments. The colorimetric selectivity of
L1 was studied on addition of 1equiv. of various metal ions into the solution of L1 (1×10ꢀ5 mol
Lꢀ1). The solution of L1 exhibit an instantaneous color change from colorless to pale yellow only
upon addition of Cu2+ (1 equiv.) while no color change was observed on the addition of other
metal ions (Cr3+, Mn2+, Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Al3+) as shown
in Fig. 1(a). This result indicated that L1 could serve as a potential candidate of a ‘nakedꢀeye’
chemosensor for Cu2+. Further investigation to examine the sensing ability of L1 was conducted
through UVꢀVis titration studies. The UVꢀVis absorption spectra of L1 showed absorption bands
at 260 nm and 332 nm. The band at 332 nm might be due to internal charge transfer (ICT) and
band at 260 nm due to π → π* transition. Upon excitation of L1 at 270 nm and 340 nm, the
compound was found to be only slightly emissive (Fig. S7 ESI†). Thus we focused our
investigation on the changes observed in the ground state of L1 rather than its excited state. The
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absorbance spectrum indicated either no or minimal changes relative to sensor L1 in case of all
other metal ions except for remarkable changes in case of Cu2+ ion (Fig. 1(b)).
Fig. 1(a) The color changes of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) upon addition of 1 equiv.
of metal ions (Cr3+, Mn2+, Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Al3+) and
(b) UVꢀVis spectral changes of chemosensor L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) solution
with 1 equiv. of chloride salts of metal ions (inset shows the color change of sensor L1 in
the absence and the presence of Cu2+ under UVꢀVis light).
The binding affinity of L1 towards Cu2+ was studied by UVꢀVis titration experiment. The
successive addition of Cu2+ ion (0 ꢀ 10 x 10ꢀ5 M) displayed gradual increase in the absorption
band at 260 nm along with the concomitant decrease in the absorption band at 330 nm and
appearance of new band at 404 nm. Two isosbestic points were also obtained at 308 nm and 362
nm (Fig. 2(a)) suggesting formation of stable product between sensor L1 and Cu2+ ion. The
binding constant and binding stoichiometries of the complexes were determined through BꢀH
plot (Fig. 2(b)) and Job’s plot (Fig. 2(c)).
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Fig. 2(a) UVꢀVis spectral changes of chemosensor L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) on
addition of (0.5 to 10 equiv.) Cu2+ ions (inset shows increase in absorbance intensity as the
function of equivalents of copper), (b) BꢀH plot for UVꢀVis titration of L1 with Cu2+ (0ꢀ
10×10ꢀ5M) showing 1:1 binding stoichiometry and (c) Job’s Plot showing 1:1 binding
stoichiometry between L1 and Cu2+.
The plot of 1/(AꢀAo) v/s 1/[Cu2+] showed linearity which means 1:1 binding between L1 and
Cu2+. Binding constant for L1 and Cu2+ was determined as 9.08 x104 Mꢀ1 from the BꢀH plot. The
detection limit was calculated to be 0.12 ꢁΜ which is much lower than the maximum
permissible limit of 31.5 ꢁM for Cu2+ in the drinking water according to World Health
Organization guidelines (1993).32 The detection limit is significant as compared to the sensors
reported in literature (Table S1 ESI†). In Job’s Plot, maxima was achieved at 0.5 mole ratio of
sensor which also confirmed 1:1 binding stoichiometry between sensor L1 and Cu2+. To
investigate the selectivity of L1 as a chemosensor for Cu2+ ions, competitive experiments were
executed by the addition of metal ions (1 equiv.) to the solution of L1 containing Cu2+ ions
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(1equiv.). There was no significant interference of other metal ions in the detection of Cu2+ metal
ion (Fig. 3).
Fig. 3 Absorption response of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) at the wavelength of 404
nm in presence of cations.
Thus, L1 proved to be an excellent colorimetric sensor for Cu2+ in the presence of other
competing metal ions. The response time of L1 towards Cu2+ was also determined. In
DMSO/H2O (8:2, v/v, 10 ꢁM) solution, the absorbance of L1 at 404 nm changed immediately (5
s) upon the addition of 1 eq. Cu2+ and remained constant thereafter (Fig. 4). This ultrafast
sensing by L1 made it suitable for realꢀtime detection of Cu2+ ion. To the best of our knowledge,
such a fast response time have not been reported earlier by any colorimetric sensor (Table S2
ESI†).
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Fig. 4 Absorption response of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) at the wavelength of 404
nm in presence of Cu2+ (1 equiv.) with respect to time.
1
The binding mode of L1 with Cu2+ was further investigated through H NMR titrations. Upon
the addition of Cu2+ (1 equiv.) to the solution of L1 in DMSO, ꢀNH protons were shifted
downfield from 12.73 to 13.01 and ꢀN=CH protons from 9.15 to 9.3. Both the peaks became
broad with decreased intensity. Other peaks were shifted only slightly (approx 0.02ppm) upfield
(Fig. 5). These observations suggested that Cu2+ was bound to L1 through nitrogen atoms of
carbohydrazide moiety of the chemosensor L1. Consequently, we proposed 1:1 binding between
L1 and Cu2+ on the basis of BꢀH plot, Job’s plot and 1H NMR experiments.
O
N
O
N
N
NH
HN
Cu
Cl
S
S
N
N
1
Fig. 5 Stack plot of partial H NMR spectra of L1 in the presence of (0ꢀ2.0 equiv.) of
Cu2+ ions along with the possible structure of L1 and Cu2+ complex.
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3.2 Anion sensing
The binding of biologically and environmentally relevant anions by chemosensor L1 was
investigated for naked ion detection. Upon the addition of various anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcO−,
ꢀ
2ꢀ
ꢀ
2ꢀ
ꢀ
H2PO4 , HPO4 , HSO4 , SO4 , NO2 , AMP2ꢀ, ADP2ꢀ and ATP2ꢀ to L1 (1.0 × 10ꢀ5 M), only AMP2ꢀ
exhibit dramatic color change from colorless to intense yellow. Fꢀ and AcOꢀ exhibit color change
from colorless to light yellow while other anions did not exhibit any change in color (Fig. 6(a)).
The sensing behavior of L1 towards anions was further studied through UVꢀVis titrations. The
UVꢀVis absorption spectrum of chemosensor L1 showed absorption bands at 260 nm and 332
nm. On addition of anions into the stock solution of sensor L1, major changes were observed in
case of biologically relevant anions AMP2ꢀ, Fꢀ and AcOꢀ with the appearance of new band at 405
nm in the UVꢀVis absorption spectrum (Fig. 6(b)).
Fig. 6(a) The color changes of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) solution upon addition of
various anions (1equiv.), (b) UVꢀVis spectral changes of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v)
solution with (10 equiv.) sodium, potassium or tetra butyl ammonium salt of various
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ꢀ
2ꢀ
ꢀ
2ꢀ
ꢀ
anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcO−, H2PO4 , HPO4 , HSO4 , SO4 , NO2 , AMP2ꢀ, ADP2ꢀ and ATP2ꢀ
).
On gradual addition of AMP2ꢀ ion to the chemosensor L1, intensity of new band at 405 nm
increased with concomitant decrease in the absorption intensity of band at 332 nm and increase
in the absorption intensity of band at 260 nm. Two isosbestic points were observed at 287 nm
and 362 nm (Fig. 7(a)). These spectral changes demonstrated the formation of stable complex
between AMP2ꢀ anion and L1. Similar changes were observed in the absorption spectra of L1
using Fꢀ (Fig. 7(b)) and AcOꢀ (Fig. 7(c))
Fig. 7 UVꢀVis absorption spectra of L1 (10 ꢁM, DMSO/H2O, 8:2,v/v) in presence of (a)
Na2AMP (10 ꢁM to 100 ꢁM), (b) TBAF (10 ꢁM to 500 ꢁM) and (c) NaAc (10 ꢁM to 500
ꢁM).
The binding stoichiometry of L1 with AMP2ꢀ, AcOꢀ and Fꢀ ions was evaluated to be 1:1 as
confirmed by Job’s plot (Fig. 8(a) and S8 ESI†). Binding constants and limit of detection for
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binding of L1 with AMP2ꢀ, AcOꢀ and Fꢀ were determined through BꢀH Plot (Fig. 8(b) and S9
ESI†).
Fig. 8(a) BenesiꢀHildebrand Plot and (b) Job’s Plot for UVꢀVis titration of L1 with AMP2ꢀ
showing 1:1 stoichiometry.
The values of binding constant of anions were found to be in the order AMP2ꢀ>AcOꢀ>Fꢀ while
detection limit was found in the reverse order i.e., Fꢀ = AcOꢀ>AMP2ꢀ as summarized in the Table
1.
Table. 1 Binding constant (Ka) values for Chemosensor L1.
Anions
Fꢀ
G:H
1:1
1:1
1:1
1:1
Binding Constant Ka(Mꢀ1)
8.0 × 103
Detection Limit (ꢁM)
1.4
1.4
AcOꢀ
AMP2ꢀ
Cu2+
8.18 × 104
1.47 × 105
0.08
0.12
9.08 × 104
The detection limit of L1 for AMP2ꢀ, AcOꢀ and Fꢀ was found to be higher than many of the
reported sensors (Table S3 ESI†). Detection limit of Fꢀ is 1.4 ꢁΜ which is much lower than
maximum permissible limit of fluoride i.e., 1.5 ppm in drinking water according to WHO
1984 guideline and Indian standard drinking water specification 1991.33 The selectivity of
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sensor L1 towards AMP2ꢀ, Fꢀ and AcOꢀ ions over other anions was investigated through UVꢀ
Vis competitive studies (Fig. 9 and S10 ESI†). From the UVꢀVis spectrum for competitive
experiment, only minor interference was observed in intensity of the absorption band at 405
nm, which revealed that L1 is highly selective for the recognition of AMP2ꢀ ion.
Fig. 9 Absorption response of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) at the wavelength of 405
nm in presence of various anions.
1H NMR titration experiment
1H NMR titrations were performed in order to determine the mechanism of interaction of L1
with AMP2ꢀ, AcOꢀ and Fꢀ ions. With the addition of TBAF (0.5 to 1equiv.) to L1 (1equiv.) in
DMSOꢀd6, sharp –NH peak at 12.73 ppm was broadened and shifted downfield to 15.00 ppm.
Singlet for ꢀN=CH was also shifted downfield from 9.16 ppm to 9.25 ppm while benzothiazole
protons were shifted only slightly towards the upfield (Fig. 10(a)). The broadening of the ꢀNH
peak and downfield shifting of –NH and –N=CH peaks indicate that the hydrogen atoms of ꢀNH
group might be experiencing hydrogen bonding interactions with Fꢀ ion. This is evident from
absence of triplet at 16 ppm (due to formation of [HF2]ꢀ type of species) as reported in
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literature.34 On the addition of acetate ions (0.25 to 1 equiv.) to L1, peak for ꢀNH at 12.73 ppm
was disappeared, sharp –N=CH peak broadened and shifted slightly downfield. Peaks for
pyridine protons and benzothiazole protons also shifted slightly upfield, (Fig. S11 ESI†) as
electron density increases due to deprotonation.35 These changes in chemical shift values
revealed that carbohydrazide moiety is involved in binding acetate ion. On the addition of AMP2ꢀ
(1equiv.) to L1 (1equiv.) in DMSOꢀd6, peak for ꢀNH at 12.73 ppm was disappeared, sharp –
N=CH peak was broadened and shifted upfield from 9.158 to 9.07 (approx 0.09 ppm) while
peaks for pyridine protons and benzothiazole protons became broad and shifted upfield by
approx. 0.05 ppm (Fig. 10(b)). Although AMP2ꢀ and HPO42ꢀ have same charge but only AMP2ꢀ
binds with L1 as observed through UVꢀVis spectra. These results indicate that there exist some
interactions between L1 and adenosine or sugar unit of adenosine monophosphate also.
Therefore, 1H NMR experiments revealed that complex formation between sensor L1 and anions
(Fꢀ, AcOꢀ and AMP2ꢀ) occurs through NꢀH… Fꢀ and NꢀH… Oꢀ hydrogen bonds.
O
N
O
N
N
N
N
H
O
H
O
O
N
O
N
O
N
S
S
P
N
F-
N
H
N
OH
N
S
S
N
NH2
N
N
=
N
O
N
N
OH
HO
Fig. 10 Stack plot of partial 1H NMR spectra of L1 in presence of (0ꢀ1.0 equiv.) of (a) TBAF
and (b) Na2AMP recorded in DMSOꢀd6.
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DFT calculations
To elucidate the electronic structure of chemosensor L1 and its complexes with anions (Fꢀ,
CH3COOꢀ and AMP2ꢀ) and cation (Cu2+) ions, density function theoretical (DFT) calculations
were performed using Gaussian 09 software.36 The structures of L1 and its complexes with the
anions Fꢀ, CH3COOꢀ and AMP2ꢀ were optimized by using the hybrid functional Beckeꢀ3ꢀLeeꢀ
37
YangꢀParr (B3LYP) in conjugation with 6ꢀ311g** basis set. For Cu2+, LANL2DZ basis set
was used.38ꢀ39 The presence of positive frequencies for all the structures ensure minimum in the
potential energy surface. The timeꢀdependent density functional (TDꢀDFT)40 calculations were
carried out at the same level of theory in dimethylsulfoxide (DMSO) by employing the
polarizable continuum model (PCM).41 The geometries of the above systems were plotted using
the MOLDEN software.42 The UVꢀVis spectra of free ligand as well as its complexes with the
anions (Fꢀ, CH3COOꢀ and AMP2ꢀ) and cation (Cu2+) were determined and depicted along with
their ground state geometries in Fig. 11. It can be clearly seen from the figure that for free ligand,
maximum absorption occurs at 350 nm that matches well with the experimental absorption
spectrum. In presence of anions, the absorption maximum shifted towards longer wavelength in
agreement with experimental observation. For instance, the maximum absorption peak of Fꢀ and
CH3COOꢀ based complexes appear at ~ 420 nm and that of AMP2ꢀ based complex lies at 407 nm.
Such a change in the maximum absorption wavelength is probably due to the acidic nature of the
NꢀH bond present in the ligand as reflected from the absence of one of the NꢀH bonds in the
optimized geometries of the complexes. In presence of Cu2+ ions, there occurs decrease in the
intensity of absorption band at 350 nm and appearance of new band at ~ 410 nm which
corroborates very well with experimental observation. The optimized structure of complex of L1
with Cu2+ shows that pyridine and imine nitrogens bind with Cu2+. In addition, the experimental
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1H NMR spectrum shows downfield shifting of hydrogens of carbohydrazide moiety. This is due
to binding of Cu2+ ion with imine hydrogens. Therefore, the optimized structures corroborate
1
well with the experimental H NMR spectrum. The HOMOꢀLUMO energy gap (∆EHꢀL) of the
free ligand as well as its complexes was calculated and is listed in Table S4 ESI†. From the table,
it can be seen that ꢂEHꢀL of complexes is reduced compared to that of L1. Among all the
complexes, the smallest value of ꢂEHꢀL is obtained for the complex with AMP2ꢀ ion. This can be
possibly due to charge transfer from donor AMP2ꢀ ion to acceptor ligand which is in agreement
with its highest binding constant compared to other complexes.
Fig. 11 DFT optimized structures and the calculated absorption spectra for (a) L1 and its
complexes with (b) Fꢀ, (c) CH3COOꢀ, (d) AMP2ꢀ and (e) Cu2+
.
4. Application of L1
Detection of fluoride in toothpaste using chemosensor L1
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Fluoride is an essential constituent of the commercially available toothpaste. Thus, we determine
the potential of chemosensor L1 towards the detection of fluoride ion in toothpaste. A solution
was prepared by adding 20 mg of the commercially available toothpaste to 5 mL of distilled
water and sonicated for 15 minutes. The mixture was then filtered. On addition of 20 µL of the
toothpaste solution into 10 µM solution of L1, the color of the solution changed from colorless to
yellow which is comparable to the color of chemosensor L1 in presence of fluoride ion. Further
investigation was executed through the UVꢀVis spectrum. The UVꢀVis spectrum of the
chemosensor L1 (10 µM) in the presence of toothpaste solution shows a broad band at 405 nm
similar to that observed for L1 in presence of bare fluoride (Fig. 12(a)). On gradual addition of
toothpaste solution, there occurs concomitant increase in band at 405 nm as shown in Fig. 12(b).
Although chemosensor L1 could detect acetate and fluoride, but most of the toothpastes contains
only fluoride. Thus the chemosensor L1 can successfully detect F− in toothpaste.
Fig. 12(a) UVꢀVis spectral changes of L1 (10 ꢁM, DMSO/H2O, 8:2, v/v) solution in presence
of TBAF and toothpaste and (b) UVꢀVis absorption spectra of L1 (10 ꢁM, DMSO/H2O, 8:2,
v/v) in presence of fluoride extract from toothpaste.
5. Conclusions
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In conclusion, pyridine dicarbohydrazide based sensor (L1) was developed via facile and
expedient methodology that possesses the ability to sense both cations and anions
colorimetrically. For cation recognition study, chemosensor L1 exhibited colorimetric response
towards Cu2+ selectively. It shows high sensitivity for Cu2+ with detection limit of 0.12 ꢁM.
Anion recognition study revealed that sensor also possessed ability to bind AMP2ꢀ, Fꢀ and AcOꢀ.
However, the chemosensor had highest binding ability for AMP2ꢀ ion among all other anions as
determined by the Ka value (1.47 × 105 Mꢀ1) and limit of detection (0.08 ꢁM). From DFT
calculations, it was determined that copper ion binds with carbohydrazide linkage and pyridine
unit of chemosensor. Anions (Fꢀ and AcOꢀ) bind with one ꢀNH group and AMP2ꢀ with both ꢀNH
groups of chemosensor through hydrogen bonding. It colorimetrically distinguished AMP2ꢀ from
ADP2ꢀ and ATP2ꢀ. Since, there are limited colorimetric sensors available for recognition of
AMP2ꢀ, chemosensor L1 is a significant addition to the available chemosensor. The practical
utility of our chemosensor L1 was also exploited to determine the presence of fluoride ion in
commercially available toothpaste. It is also noteworthy that a single chemosensor for the
detection of both Cu2+ and AMP2ꢀ has not been reported till date. Thus a single chemosensor L1
acts as potential candidate for detection of Cu2+ metal ion and biologically important AMP2ꢀ, Fꢀ
and AcOꢀ anions.
1
13
† Electronic Supplementary Information (ESI) available: The H NMR, C NMR, FTIR of
compound L1 in DMSOꢀd6. Fluorescence spectra, Job’s plot and BꢀH plot of L1 with AcOꢀ and
Fꢀ. Stack plot of partial 1H NMR of L1 with AcOꢀ. Table for sensing abilities of various receptors
reported in literature for Cu2+ and AMP2ꢀ, Fꢀ and AcOꢀ. Frontier molecular orbitals and HOMOꢀ
LUMO energy gaps of L1 and its complexes with Cu2+, AMP2ꢀ, Fꢀ and AcOꢀ anions.
Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
The authors thank University Grants Commission (UGC) for providing the fellowship. The
authors would also like to acknowledge University science instrument center (USIC), University
of Delhi for providing instrumentation facility.
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Graphical Abstract
Single colorimetric chemosensor for detection of both Cu2+ and AMP2ꢀ ions has been developed
for the first time.
26