Highly selective and sensitive coumarin–triazole-based fluorometric ‘turn-off’ sensor for detection of Pb2+ ions

Article abstract of DOI:10.1002/bio.3468

Exposure to even very low concentrations of Pb²⁺ is known to cause cardiovascular, neurological, developmental, and reproductive disorders, and affects children in particular more severely. Consequently, much effort has been dedicated to the de

Full text of DOI:10.1002/bio.3468

Received: 16 August 2017
DOI: 10.1002/bio.3468
Revised: 10 December 2017
Accepted: 18 January 2018
R E S E A R C H A R T I C L E
Highly selective and sensitive coumarin–triazole‐based
2
+
fluorometric ‘turn‐off’ sensor for detection of Pb ions
1,2
2
1
1
|
|
|
Shaily
Ajay Kumar
Iram Parveen
Naseem Ahmed
1
Department of Chemistry, Indian Institute of
Technology, Roorkee, India
Abstract
2+
2
Exposure to even very low concentrations of Pb is known to cause cardiovascular, neurological,
developmental, and reproductive disorders, and affects children in particular more severely. Con-
sequently, much effort has been dedicated to the development of colorimetric and fluorescent
Department of Chemistry, D.B.S. (P.G.)
College Dehradun, India
Correspondence
Shaily and Naseem Ahmed, Department of
Chemistry, Indian Institute of Technology,
Roorkee 247667, India.
2+
sensors that can selectively detect Pb ions. Here, we describe the development of a triazole‐
2+
based fluorescent sensor L5 for Pb ion detection. The fluorescence intensity of chemosensor
2+
Email: shailydbs@gmail.com; nasemfcy@iitr.ac.in
L5 was selectively quenched by Pb ions and a clear color change from colorless to yellow could
Funding information
be observed by the naked eye. Chemosensor L5 exhibited high sensitivity and selectivity towards
2+
Ministry of Human Resource and Development
2
Pb ions in phosphate‐buffered solution [20 mM, 1:9 DMSO/H O (v/v), pH 8.0] with a 1:1 bind-
(
MHRD), New Delhi, India, Grant/Award Num-
6
−1
ing stoichiometry, a detection limit of 1.9 nM and a 6.76 × 10 M binding constant. Additionally,
ber: MHR01‐23‐200‐428
low‐cost and easy‐to‐prepare test strips impregnated with chemosensor L5 were also produced
2+
for efficient of Pb detection and proved the practical use of this test.
KEYWORDS
2+
coumarin, fluorescence, Pb ion, sensor, triazole
1
|
INTRODUCTION
irritability, neurological, cardiovascular, and developmental disorders,
hypertension, mental retardation and slowed motor responses. Lead
even affects human soft tissues and organs causing cancer and acts syn-
Heavy‐metal pollution in the environment due to industrialization and
urbanization is a serious issue worldwide especially in developing coun-
tries, and has attracted growing attention from researchers and scien-
tists. Among the heavy‐metal ions cadmium, lead and mercury, lead is
the most abundant and toxic element. Around 300 million tonnes of
mined lead are still circulating in aquatic systems (i.e. in ground water,
surface water and aquifers) and soil, such that these polluted aquatic
systems and soils have become a serious threat to the environment,
[
4,5]
ergistically with other carcinogens.
Low concentrations of lead in the
human body act as a powerful neurotoxin that can trigger behavioural
problems, interfere with brain development and slow nerve conduction
[
6–8]
speeds.
The most common molecular targets of lead include zinc‐
and calcium‐binding proteins that control gene expression and cell sig-
[
9]
nalling, respectively, and lead to protein disfunction. Therefore, due
to their hazardous nature, heavy metals, especially lead, cadmium and
mercury, are banned by the European Union's Restriction of the Use
of Certain Hazardous Substances (RoHS) in Electrical and Electronic
[1,2]
and animal and human health.
Toxic lead enters the environment
through its use in waste plastic, dyes, heavy fuel oils, batteries, gasoline
and blast furnaces, etc. In addition lead, due to its non‐degradable
nature, is an environmental pollutant and difficult to detoxify by chem-
ical or biological means, thereby resulting in contaminated portable
[
10]
Equipment Directive.
Moreover, permissible concentration limits of
lead ions in drinking water have been rigorously defined by the United
States Environmental Protection Agency (USEPA) and the World Health
−1
[
3]
water and food. Exposure to lead ions may cause serious health prob-
lems in humans and especially in children such as muscle paralysis,
Organization (WHO) as 16 ppb (16 μg L ) and 10 ppb (48 nM) respec-
[
11,12]
tively.
Likewise, an action level of 2.5 μM for lead in products
anticipated for use by infants and children has been set by the United
[13]
Abbreviation used: AAS, atomic absorption spectroscopy; DFT, density
functional theory; DMSO, dimethyl sulphoxide; ESI, electro spray‐ionization;
FT‐IR, Fourier transform infra‐red; HOMO, highest energy occupied molecular
orbital; HRMS, high‐resolution mass spectrum; LMCT, ligand‐to‐metal charge
transfer; LUMO, lowest unoccupied molecular orbital; NMR, nuclear magnetic
resonance; TLC, thin layer chromatography; UV, ultraviolet; WHO, World
Health Organization.
States Food and Drug Administration.
Therefore, determination
and monitoring of lead levels are very important to ensure that heavy‐
metal lead concentrations remain below the defined toxic level. Several
sophisticated analytical techniques such as inductively coupled plasma
atomic emission spectroscopy (ICP‐MS), anodic stripping voltammetry
Luminescence. 2018;1–9.
wileyonlinelibrary.com/journal/bio
Copyright © 2018 John Wiley & Sons, Ltd.
1

2
SHAILY ET AL.
and atomic absorption spectroscopy (AAS) are currently used for detec-
quartz cell. Ultraviolet (UV)–vis spectra were measured using a
Shimadzu UV‐2450 spectrophotometer. The pH of the solution was
measured using a MRFS Toshniwal pH meter. Melting points of the
compounds were measured using an Optimelt automated melting
point apparatus. Fourier transform infra‐red (FT‐IR) spectra were
recorded using an Alpha FT‐IR spectrometer (Bruker) with the sam-
ple prepared in KBr pellets. Fluorescence quantum yields and lifetime
spectra were measured using a fluorescence spectrometer FLS 980
(Edinburgh Instruments) and a Horiba Jobin Yvon Fluorocube lifetime
system, respectively.
2+
[14–16]
tion of Pb in the environment.
However, most methods have
several disadvantages such as cell impermeability, induced background
expression and poor metal selectivity, plus these standard techniques
can only measure total lead content. So, there is still a strong need to
explore the use of rapid, easy to use, economical, highly sensitive and
2+
real‐time monitoring techniques to detect trace levels of Pb . In recent
years, to meet this requirement, several research groups have devel-
oped highly sensitive fluorescent probes that selectively respond to
[
17–25]
lead ions.
Coumarin and its derivatives are an important class of
fluorescent compounds that have been used to construct various
fluorescent chemosensors due to their excellent photophysical prop-
2
2
.1
|
Synthesis and characterization of 4‐((1‐(2‐oxo‐
H‐chromen‐4‐yl)‐1H‐1,2,3‐triazol‐5‐yl)methoxy)‐2H‐
chromen‐2‐one (L5)
[26–29]
erties of small size and high quantum yield.
In an earlier
publication we reported the synthesis of various functionalized cou-
[30–33]
For the synthesis of title compound L5, first we synthesized precursors
marin‐derived scaffolds as a cation–anion sensor
, the current
1
1
and 3 from commercially available 4‐hydroxycoumarin. Synthesis of
involved a single‐step substitution reaction of 4‐hydroxycoumarin
study focusses mainly on the design and development of efficient
organic scaffolds that sensitively and selectively detect metal ions
and anions in aqueous–organic media. Here we describe the devel-
opment of a coumarin–triazole‐based receptor 4‐((1‐(2‐oxo‐2H‐chro-
men‐4‐yl)‐1H‐1,2,3‐triazol‐5‐yl)methoxy)‐2H‐chromen‐2‐one L5 for
with propargyl bromide. The two‐step synthesis of 3 involved
tosylation of 4‐hydroxycoumarin followed by substitution of tosyl with
azide. The detailed experimental procedure for the synthesis of the
starting compounds 1, 3 and the final compound L5 are described
below and in Scheme 1 (ESI, Figures S1‐S5).
2+
the identification of Pb ions in a combined organic and aqueous
medium.
[
34]
Step 1: Synthesis of 4‐(prop‐2‐ynyloxy)‐2H‐chromen‐2‐one (1)
A solution of propargyl bromide (237.92 mg, 2.0 mmol) in acetone
was added to stirred suspension of 4‐hydroxycoumarin
324.24 mg, 2.0 mmol) and K CO (276.41 mg, 2.0 mmol) in
a
2
|
EXPERIMENTAL
(
2
3
acetone, and the resulting reaction mixture was refluxed at 60°C
for 12 h. After completion of the reaction, the solution was allowed
to cool at room temperature and then poured into ice‐cold water
for precipitation. The precipitate was filtered and washed subse-
quently with water and acetone. The resulting precipitate was dried
Metal salts were purchased from HiMedia (Mumbai, India) and Loba
Chemie (Mumbai, India); 4‐hydroxycoumarin was purchased from
Sigma Aldrich. All other general solvents and reagents were of ana-
lytical grade and used without further purification. For electronic
and fluorescence spectral studies, spectroscopic grade DMSO and
water were used. Thin layer chromatography was carried out on alu-
under vacuum and a pure product was obtained as a brown solid with
1
1
13
a 95% (380 mg) yield. M.P. = 132°C; H‐NMR (CDCl
3
, 400 MHz) δ =
mina‐backed plates coated with Merck F254. H and C nuclear
2
.68 (t, J= 2.20 Hz, 1H), 4.87 (d, J = 2.20 Hz, 2H), 5.83 (s, 1H), 7.26–
magnetic resonance (NMR) spectra of compounds were measured
13
7.30 (m, 2H), 7.62–7.64 (m, 1H), 7.82 (d, J = 7.32 Hz, 1H); C‐NMR
on
3 6
a JEOL ECX‐400‐II spectrometer in CDCl and DMSO‐d ,
(
CDCl
3
, 100 MHz) δ = 65.8, 73.5, 77.0, 87.3, 118.6, 119.6, 126.5,
chemical shifts were reported on the δ scale in ppm (parts per mil-
lion) with tetramethylsilane as an internal reference. Analysis of
high‐resolution mass spectra (HRMS) was performed on a Bruker
microToF‐Q‐II electrospray‐ionization (ESI) mass spectrometer. Fluo-
rescence emission spectra were recorded between 400–600 nm
−1
1
2
1
28.2, 129.3, 149.0, 158.3, 160.0; IR; νmax(cm ) 3281, 3231, 3077,
124, 1717, 1618, 1562, 1489, 1450, 1410, 1362, 1250, 1191,
106, 987, 934, 841, 755, 650, 517, 476.
Step
2:
Synthesis
of
2‐oxo‐2H‐chromen‐4‐yl
4‐
[
35]
(λex = 390 nm) on a Fluoromax‐4 spectrofluorometer using a 3‐cm
methylbenzenesulfonate (2)
SCHEME 1 Synthesis route of chemosensor L5

SHAILY ET AL.
3
FIGURE 1 (a) Colorimetric (b) Fluorometric naked‐eye detection of L5 in absence and presence of various metal ions (20 equiv.) in phosphate‐
buffered solution [20 mM, 1:9 DMSO/H O (v/v), pH 8.0]
2
FIGURE 2 (a) UV–vis absorbance spectra of
L5 in the absence or presence of some metal
ions. (b) UV–vis absorbance titration of L5
2+
with Pb , concentrations ranging from 0 to
0 equiv.
2

4
SHAILY ET AL.
Next, 6.0 mL of dry pyridine and 4‐hydroxycoumarin (324.28 mg,
.0 mmol) were added to a 25 mL round bottom flask, followed
1002, 936, 834, 752, 643, 528, 499. HRMS (ESI+) m/z for L5 calcd.
+
2
C21H13N O [M+Na] : 410.0753 found 410.0784.
3
5
by the step‐by‐step addition of 1.0 equiv. of p‐toluenesulfonyl
chloride (380 mg, 2.0 mmol) at room temperature, the resulting
reaction was refluxed and stirred continuously for 4 h. After com-
pletion of the reaction, the resulting mixture was poured into a
beaker with ice‐cold water to obtain a precipitate. The resulting
precipitate was filtered, washed subsequently with cold water
and dried under vacuum to get the pure product as a white solid
2
.2
|
UV–vis and fluorescence spectral
measurements
−
3
A stock solution of chemosensor L5 (1 × 10 M) was prepared in
DMSO, followed by preparation of a sample solution of L5 (10 μM)
1
2
in phosphate‐buffered solution (20 mM, DMSO:H O 1:9, v/v,
with a 66% yield (417 mg) yield. M.P. = 112°C; H‐NMR
pH 8.0) in a quartz cuvette (1 cm × 1 cm), and final addition of 10 μl
(
400 MHz, CDCl
3
, ppm): δ 2.45 (s, 3H), 6.29 (s, 1H), 7.22–7.32
−
3
+
+
2+
3+
of aqueous stock solution (1 × 10 M) of metal salt (K , Na , Ba
,
,
(
m, 2H), 7.38 (d, J = 8.8 Hz, 2H), 7.51–7.59 (m, 1H), 7.63 (dd, J =
2+
2+
3+
2+
3+
2+
2+
2+
2+
2+
+
13
Ca , Mg , Cr , Mn , Fe , Co , Ni , Cu , Zn , Pb , Ag , Al
2
.0, 8.4 Hz, 1H), 7.88 (d, J = 8.4 Hz, 2H); C‐NMR (100 MHz,
2+
2+
+
2+
Cd , Hg , Hg and Sn ). Fluorescence spectra of the chemosensor
CDCl
3
, ppm): δ 21.7, 103.6, 115.0, 116.9, 123.2, 124.4, 128.4,
−
1
L5 were recorded with a slit width of 3 nm.
1
3
1
7
30.4, 131.7, 133.2, 146.8, 153.5, 157.8, 160.7; IR; νmax (cm )
066, 2919, 2863, 1930, 1862, 1809, 1723, 1619, 1566, 1486,
450, 1374, 1286, 1222, 1176, 1132, 1069, 932, 871, 820, 812,
50, 699, 585, 573, 538, 503.
3
3
|
RESULTS AND DISCUSSION
[36]
Step 3: Synthesis of 4‐azido‐2H‐chromen‐2‐one (3)
.1
|
Colorimetric and fluorimetric responses of L5 to
To the solution of compound 2 (316.32 mg, 1.0 mmol) in
methanol, 5 equiv. of sodium azide were added and resulting
reaction mixture was continuously stirred for 3 h at room tem-
perature. Then the reaction mixture was poured into a beaker
of ice‐cold water, and filtered to get the resulting precipitate,
which was then washed with methanol and water. Finally the
precipitate was dried under vacuum to get the pure product as
2+
Pb ion
The optical properties of chemosensor L5 (10 μM) were studied using
absorption and fluorescence emission spectroscopy at room tempera-
2
ture in 1:9 v/v DMSO:H O using phosphate‐buffered solution at
pH 8.0. The chemosensor L5 displayed a strong absorption band
located at 390 nm which was attributed to the π–π* transition. Upon
addition of 20 equiv. of various metal ions of environmental relevance
a light brown powder in 82% (153 mg) yield. M.P. = 159°C;
+
+
+
2+
2+
3+
2+
3+
2+
2+
2+
1
such as Na , K , Ag , Ca , Ba , Cr , Mn , Fe , Co , Ni , Zn ,
H‐NMR (400 MHz, CDCl
3
, ppm): δ 6.13 (s, 1H), 7.27–7.32 (m,
2+
2+
2+
2+
2+
3+
+
Cu , Pb , Cd , Hg , Sn , Al and Hg as nitrate salts to the solu-
1
H), 7.32–7.39 (m, 1H), 7.54–7.63 (m, 1H), 7.67–7.76 (m, 1H);
2+
tion of chemosensor L5, only Pb ion showed high affinity with the
development of a new band at 410 nm. The position changes in bands
were associated with a perceived color generation from colorless to
yellow that could easily be detect by the naked eye in contrast with
other metal ions, for which no significance changes were observed
13
3
C‐NMR (100 MHz, CDCl , ppm): δ 100.3, 114.9, 117.0,
−1
1
2
1
1
23.5, 124.4, 133.2, 153.5, 153.7, 160.9; IR; νmax (cm ) 3052,
990, 2928, 2852, 2369, 2280, 2219, 2151, 2126, 1812, 1726,
609, 1561, 1486, 1450, 1386, 1330, 1274, 1253, 1181, 1144,
021, 938, 853, 761, 714, 641, 508.
(
Figures 1a and 2a).
Step 4:
triazol‐5‐yl)methoxy)‐2H‐chromen‐2‐one, L5
To the solution of compound 1 (200.19 mg, 1.0 mmol) and 3
187.15 mg, 1.0 mmol) in 25 mL of THF:H O (2:1), 10 mol%
sodium ascorbate (19.81 mg), 3 mol% CuSO ·5H O (7.49 mg)
Synthesis of 4‐((1‐(2‐oxo‐2H‐chromen‐4‐yl)‐1H‐1,2,3‐
To validate the selectivity of chemosensor L5, the absorption titra-
[37]
tion experiment was also performed. Upon addition of 0–20 equiv. of
2+
Pb to the solution of chemosensor L5, the absorbance at 390 nm
(
2
4
2
and DIPEA (258.5 mg, 2.0 mmol) were added sequentially. The
resulting reaction mixture was refluxed for 3 h. After completion
of the reaction, the mixture was poured into ice‐cold water, the
resulting compound was filtered and precipitated with EtOH,
followed by recrystallization in ethanol to furnish the pure title
compound L5 as a brown solid with 88% purity (340 mg) yield,
1
M.P. = 229°C; H‐NMR (400 MHz, CDCl
3
, ppm): δ 5.59 (s, 2H),
6
2
.24 (s, 1H), 7.04 (s, 1H), 7.35 (t, J = 8.0 Hz, 1H), 7.40–7.49 (m,
H), 7.59 (d, J = 8.0 Hz, 1H), 7.67 (dt, J = 0.8, 8.8 Hz, 1H), 7.78
(
dt, J = 0.8, 8.8 Hz, 1H), 7.86 (td, J = 0.8, 8.0 Hz, 2H), 9.09 (s,
1
3
1
1
1
3
H); C‐NMR (100 MHz, CDCl , ppm): δ 55.9, 113.7, 116.5,
17.0, 117.9, 123.0, 127.6, 127.7, 127.9, 128.0, 128.9, 129.0,
29.6, 137.3, 137.6, 143.4, 143.5, 143.6, 145.0, 150.1, 151.5; IR;
−
1
ν
max (cm ) 3134, 3084, 2963, 2922, 2845, 1730, 1618, 1564,
488, 1447, 1400, 1363, 1326, 1273, 1243, 1184, 1102, 1035,
2+
FIGURE 3 Job's plot of the [L5 + Pb ] complex in phosphate‐
1
buffered solution [20 mM 1:9 DMSO/H O (v/v), pH 8.0]
2

SHAILY ET AL.
5
2+
FIGURE 4 Selectivity and competition studies of L5 (10 μM) with Pb in presence of other metal ions in phosphate‐buffered solution [20 mM, 1:9
2+
2
DMSO/H O (v/v), pH 8.0]. Grey and black bars represent the absorbance of the [L5 + metal ions] system and the L5+Pb system in the presence
of 20 equiv. of some metal ion respectively
FIGURE 5 (a) Fluorescence responses of
chemosensor L5 (1.0 μM) in phosphate‐
buffered solution [20 mM, 1:9 DMSO/H O
2
(
v/v), pH 8.0] to various metal ions (20 μM).
b) Fluorescence responses of chemosensor
(
L5 (1.0 μM) in phosphate‐buffered solution
20 mM, 1:9 DMSO/H O (v/v), pH 8.0] to
[
2
2+
various concentrations of Pb (0–20 μM).
The inset in (b) shows a Stern–Volmer (SV)
plot λex = 390 nm, λem = 462 nm

6
SHAILY ET AL.
gradually decreased and, accordingly, one new lower‐energy band
of the chemosensor L5. The maxima was evidently observed at 0.5
2+
emerged around 410 nm with a well defined isosbestic point at 417 nm
and indicated that the L5 and Pb a formed 1:1 complex (Figure 6a).
2+
(
Figure 2b). Saturation in the absorption spectra was produced by the
The binding constant for chemosensor L5 with Pb was evaluated
2
+
6
−1
addition of 20 equiv. of Pb relative to L5, implying that the efficient
cation reception event was consummate. Moreover, level of color
change was not influenced by the addition of >20 equiv. of metal
by SV plot, with an association constant 6.76 × 10
M
(Figure 5b
inset) and detection limit of 1.9 nM (Figure S8). The limit of detection
2+
for Pb , obtained in the present study was compared with the other
methods reported in the literature and is given in Table 1. From the
Table 1 it is evident that the limit of detection obtained in our method
2+
Pb cations. The appearance of a clear isosbestic points at 417 nm
indicated the formation of a strong complexation between the
2+
2+
chemosensor L5 and Pb . Job's method observed by absorption
is the lowest for Pb ion.
spectra was applied to obtain the binding stoichiometry of L5 with
We measured the fluorescence lifetime (τ) of bare chemosensor
L5 to be 7.66 nsec using 390 nm as the excitation wavelength. The life-
2+
2+
Pb in the complex. Absorbance at 390 nm was plotted against a mole
2+
fraction of L5 at a constant total concentration of L5 and Pb , which
time of L5 was shortened to 6.36 nsec after the addition of Pb ions.
2+
indicated the 1:1 binding stoichiometry of L5 with Pb in the complex
The change in L5 lifetime indicates the strong interaction of receptor
[
38]
(
Figure 3).
The association constant of L5 with Pb was calculated using an
2+
6
−1
SV plot, and was found to be 4.59 × 10
M
(Figure 2b inset), with
2+
a detection limit of 3.16 nM (Figure S7), implying that Pb had high
[
39–41]
2+
affinity for L5.
To explore further the efficacy of L5 as a Pb
ion‐selective chemosensor, competition experiments were performed
in which L5 was first incubated with various metal cations, and then
2+
Pb was added. The competition‐based absorbance profiles for these
metal ions are shown in Figure 4. The distinct responses of selectivity
and interference studies revealed that L5 could be used to distinguish
2+
Pb ions from many other metal ions.
To demonstrate the ability of chemosensor L5 to function as a
2+
fluorescent sensor for Pb , we performed a fluorescence emission
experiment in 20 mM 1:9 v/v DMSO:H O phosphate‐buffered
2
solution at pH 8.0. The chemosensor L5 display a high emission band
at 463 nm with an excitation wavelength of 390 nm and absolute
f
quantum yield (Φ = 0.76). Upon interaction with different metal ions
2+
(
20 equiv.), only Pb showed quenching with absolute fluorescence
f
quantum yield (ϕ = 0.054) (Figure 5a). To understand the binding abil-
ity of chemosensor L5, the corresponding emission titration experi-
2+
2+
ment was conducted using Pb . By adding Pb , the emission
intensity at 463 nm corresponding to chemosensor L5 gradually
2+
decreased, and red‐shifted to 480 nm when 20 equiv. of Pb was
added to the system (Figure 5b), which may be attributed to the forma-
tion of the receptor–metal complex (Scheme 2) via heavy atom
[
42]
[43]
spin‐orbit coupling , energy or electron transfer.
These results
suggested that L5 can act as a ‘switched on‐off’ fluorescence sensor
2+
2+
for Pb ions particularly. Additionally, the addition of Pb made the
fluorescence color clearly change from colorless to blue (Figure 1b).
To explain this complex formation further, the binding stoichiom-
2+
FIGURE 6 (a) Job's plot of the L5 + Pb complex in phosphate‐
buffered solution [20 mM, 1:9 DMSO/H O (v/v), pH 8.0]. (b)
2+
etry of the chemosensor L5‐Pb system was determined by way of
continuous‐variation method (Job plots), wherein the variation of the
emission intensity at 462 nm was plotted against the molar fraction
2
2+
Lifetime fluorescence spectra of L5 in the absence or presence of Pb
ion with excitation at 390 nm
SCHEME 2 Proposed sensing mechanism of
2+
compound L5 for Pb

SHAILY ET AL.
7
TABLE 1 A comparison of detection limit obtained in the present study with other reported fluorometric methods
Type of probe
LOD
Merits of probe
Mode of assay
Reference
8
[17]
Imidazopyridine‐ferrocene dyad
1.32 × 10–
M
M
Colorimetric and fluorometric
Colorimetric and fluorometric
Colorimetric and fluorometric
Fluorometric
‘Turn‐on’
‘Turn‐on’
‘Turn‐on’
‘Turn‐on’
Quenching
‘Turn‐on’
Quenching
−
9
[18]
[26]
[44]
[45]
[46]
Triazole‐tethered ferrocene−anthracene conjugates
Triazole substituted 8‐hydroxyquinoline (8‐HQ)
2.0 × 10
M
8
–
3.36 × 10
−
7
1
,3,6‐trihhdroxy Xanthone
1.8 × 10
M
8
9
9
BSA–Cu NCs
1.0 × 10–
8.0 × 10–
1.9 × 10–
M
M
M
Fluorometric
Azino bis‐Schiff base
Coumarin‐ triazole
Colorimetric and fluorometric
Colorimetric and fluorometric
This work
FIGURE 7 Selectivity of L5 (10 μM) in phosphate‐buffered solution [20 mM, 1:9 DMSO/H
2
O (v/v), pH 8.0]. Black bars and grey bars represent the
fluorescence intensity of [L5 + metal ions] system and the L5+Pb system in the presence of 20 equiv. of different metals respectively. λex = 390 nm,
em = 462 nm
2+
λ
2
+
2+
L5 with Pb
ions (Figure 6b). The preferential sensitivity of
participating in interaction with Pb . Therefore, the observed FT‐IR
2+
2+
chemosensor L5 for selective detection of Pb ion was performed
data clearly suggested that chemosensor L5 can chelate Pb through
by competition experiment in presence of other tested metal ions
interaction with the carbonyl group (C=O) oxygen and triazole nitrogen
2+
(
20 equiv.). As shown in Figure 7, no interference was observed while
and form the L5+Pb complex. Moreover, HRMS (ESI+) data for L5
+
performing the competition experiment. Therefore, receptor L5 was
calcd. C21
13 3
H N O
5
[M+Na] : 410.0753 found 410.0784 and for
2+ +
2+
2+
employed as an excellent chemosensor for the detection of Pb ions.
L5+Pb
21 13 3 5
calcd. C H N PbO [M+Na+Pb ] : 618.0508, found
618.0935, suggested that there is a 1:1 complexation between L5
2+
and Pb (ESI, Figures S6 and S9).
2
+
3
.2
|
Nature of binding interactions of L5 with Pb
2+
To further understand the binding behaviour of L5 with Pb
,
2+
The interaction of chemosensor L5 and the Pb ion was also investi-
density functional theory (DFT) calculations were performed for L5
2+
gated by FT‐IR and HRMS spectroscopy. We measured the FT‐IR
and L5+Pb (Figure 8). Structural optimization and computational
calculations were carried out using the Gaussian09 quantum chemis-
try package and results were viewed with GaussView 5. In the
present scenario, DFT supported B3LYP/6‐31G model chemistry
was implemented particularly for L5, whereas for metal complexes
LANL2DZ (Los Alamos National Laboratory 2‐double‐z–Acronyms)
was used as a basis set to optimize the geometry of the complex.
2
+
spectra of chemosensor L5 and its complex with Pb to examine their
binding sites (Figure S10). The discrete vibrations located at 3134,
−1
−1
3
084 cm and ~1730 cm in the IR spectrum of L5 were respectively
assigned to the Csp2–H stretching vibration of the benzene ring and
the carbonyl group (C=O) stretching vibration of the lactone on the
2+
coumarin moiety. When binding with Pb ion, a new significant vibra-
−
1
2+
tion located at 1547 cm appeared which was attributed to the N=N
The optimized structure of L5 and the L5+Pb complex showed
2+
stretching vibration of the triazole structure, because N=N was fixed
that the Pb ion binds to L5 at nitrogen atoms of the triazole and
carbonyl groups of lactone on the coumarin moiety, with Pb−N
2+
and showed a strong IR signal upon binding with Pb ions. The vibra-
tion wavelength corresponding to the C=O stretching vibration slightly
and Pb−O bond lengths 2.368
Å and 2.032 Å, respectively.
−1
2+
shifted to 1726 cm after binding with Pb ions, which indicated that
the C=O group of the coumarin moiety attached to an alkoxy oxygen
Moreover, in the highest energy occupied molecular orbital (HOMO)
of L5, electron density is mainly located on the coumarin moiety

8
SHAILY ET AL.
2+
FIGURE 8 Optimized structure, calculated HOMO and LUMO of L5 and L5+Pb
which is attached with alkoxy oxygen atom whereas in the lowest
unoccupied molecular orbital (LUMO) electron density is located on
the coumarin moiety which is attached to triazole ring. However,
4
4
|
APPLICATION
.1
|
Visual color changes on TLC plates
2+
for L5+Pb , in the HOMO the electron density is mainly localized
on the coumarin moiety which is attached to a triazole ring, while
To investigate the potential application of L5 as a metal ion sensor in the
solid state, we have prepared a test kit using TLC plates coated with a
solution of L5 (10 μM) followed by drying in air. The color of the TLC
plate changed to bright yellow as well as becoming non‐fluorescent
2+
in the LUMO the electron density is localized on the Pb ion,
indicating a strong ligand‐to‐metal charge transfer (LMCT) process.
The corresponding energy values indicated that the energy gap
2+
2+
between the HOMO and LUMO orbitals of the L5+Pb complex
under irradiation with long UV light only for Pb ions, supporting the
was lower than L5, which might be responsible for the bathochromic
practical applicability of receptor L5. This experiment demonstrates
2+
2+
shift of L5 upon complexation with Pb in the UV–vis absorption
that compound L5 has the potential to detect Pb ions in the solid state
spectra.
(Figure 9).
2+
FIGURE 9 Photographs of (a) colorimetric, and (b) fluorometric test kits: thin layer chromatography (TLC) coated with L5 (1 mM) for detecting Pb
2
in phosphate‐buffered solution [20 mM, 1: 9 DMSO/H O (v/v), pH 8.0]

SHAILY ET AL.
9
5
|
CONCLUSION
[
[
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In conclusion, we have successfully developed a coumarin–triazole
2
+
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scaffold as the fluorescent sensor L5 for Pb detection when com-
+
+
+
2+
pared with other competitive metal ions, e.g. K , Na , Ag , Ca
,
2+
2+
3+
2+
3+
2+
2+
2+
2+
2+
2+
2+
Ba , Cr , Mn , Fe , Ni , Co , Zn , Cu , Pb , Cd , Hg , Sn
3
+
+
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Al and Hg . The chemosensor works as both as a colorimetric and
2+
‘
turn‐off’ fluorescence probe for Pb
. Significant fluorescence
[
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N. Tagmatarchis, Chem. – Eur. J. 2010, 16, 11969.
2
+
Pb ions, while the presence of other metal ions caused no change
[
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in fluorescence intensity. This sensor has the capability to detect
2
+
Pb in the nano‐molar range with detection limits of 1.9 nM. Changes
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00696.
1
in the chemosensor can be seen by the naked eye and can be used as
2
+
[27] A. P. W. Arachchilage, F. Wang, V. Feyer, O. Plekan, R. G. Acres, K. C.
an optical sensor for Pb determination with significant color changes
Prince, J. Phys. Chem. A 2016, 120, 7080.
on TLC plates. This easy, economical, and reliable method has advan-
[
28] R. Nazir, A. J. Stasyuk, D. T. Gryko, J. Org. Chem. 2016, 81, 11104.
2+
tages over other reported methods for the detection of Pb ions.
[
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ACKNOWLEDGMENTS
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30] Shaily, A. Kumar, N. Ahmed, Supramol. Chem. 2017, 29, 146.
We thank the Ministry of Human Resource and Development (MHRD),
New Delhi, India (grant number MHR01‐23‐200‐428) for financial
support.
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