Identification of a novel click-derived 1,2,3-triazole as selective Hg2+ ion detector: computational and experimental investigations

Article abstract of DOI:10.1007/s11696-021-01804-7

A new triazole-substituted compound (7), namely {5-((2-methyl-4-(3-methyl-4-(prop-2-ynyloxy) benzyl) phenoxy) methyl)-1-(3-(nitrophenoxy) propyl)-1h-1, 2, 3-triazole}, was synthesized through the coupling of azido and propargylated precursors via click ap

Full text of DOI:10.1007/s11696-021-01804-7

Chemical Papers
https://doi.org/10.1007/s11696-021-01804-7
ORIGINAL PAPER
Identifcation oꢀ a novel click‑derived 1,2,3‑triazole as selective Hg2+
ion detector: computational and experimental investigations
1
2
3
4
5
6
Rabail Ujan · Nasima Arshad · Fouzia Perveen · Pervaiz Ali Channar · Bhajan Lal · Mumtaz Hussain ·
6
4
7
Zahid Hussain · Aamer Saeed · Syeda Aaliya Shehzadi
Received: 26 April 2021 / Accepted: 26 July 2021
©
Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
A new triazole-substituted compound (7), namely {5-((2-methyl-4-(3-methyl-4-(prop-2-ynyloxy) benzyl) phenoxy) methyl)-
-(3-(nitrophenoxy) propyl)-1h-1, 2, 3-triazole}, was synthesized through the coupling of azido and propargylated precur-
1
sors via click approach using 1,3-dipolar cycloaddition reaction. This compound was further explored for its selectivity
2
+
and sensitivity toward mercury (Hg ) ions detection. Quantum chemical DFT calculations were performed to examine the
adsorption of Hg(OOCCH ) on the surface of compound 7. The charge distributions before and after adsorption showed
3
2
+2
charge transfer from 7 to Hg(OOCCH ) which indicated that 7 was sensitive to Hg(OOCCH ) molecule for Hg detection.
3
2
3 2
The lowing in energy gap (ΔE), higher electrical conductivity and increased density of states (DOS) after Hg(OOCCH )
3
2
+
2
adsorption further depicted the potential of 7 as a chemical sensor for Hg sensing. The photophysical potential of com-
2
+
2+
2+
2+
+
2+
2+
+
4+
2+
pound (7) was examined by employing a range of cations (Ba , Ca , Co , Hg , K , Mg , Mn , Na , NH and Pd ).
2
+
A signiꢀcant hyperchromic shift in compound 7 spectrum (UV/ and ꢁuorescence) upon equimolar addition of Hg ions
2+
indicated that the triazole-based compound (7) has exhibited selective interaction with Hg ion in preference to other cations.
2
+
The maximum chelation of compound (7) with Hg was observed at pH 5.1 (slightly acidic medium). The compound (7)
2
+
2+
capability to recognize Hg was observed even at 0.1 µM detectable limit, indicating greater sensitivity of 7 toward Hg .
2
+
No signiꢀcant eꢂect of competitive metal ions on 7 – Hg complex further authenticated robust selectivity and sensitivity
2
+
2+
of 7 toward sensing Hg ions. Binding mechanism indicated the formation of 2:1 complex of 7 - Hg . DFT and spectral
ndings complimented each other and proving the promising chemosensor candidacy of the compound 7 for Hg2 ions.
+
ꢀ
Graphic abstract
2
+
Keywords Click-derived 1,2,3-triazole · Chemosensor · Selective Hg detection · Computational DFT studies ·
Spectroscopic studies
*
Nasima Arshad
nasimaa2006@yahoo.com; nasima.arshad@aiou.edu.pk
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

Chemical Papers
Introduction
ions and anions (Chang et al. 2007; Hung et al. 2009a,
b; Shehzadi et al. 2018). 1,2,3-Triazole-modiꢀed calix[4]
crowns as binding sites for alkali/transition metal ions
(Zhan et al. 2009), 1,2,3-triazole ring-appended chem-
osensor joined by anthraquinone and bis(2-azidoethoxy)
ethane for the selective optical/electrochemical detection
Heavy metals, especially mercury, lead, cadmium and
arsenic (Hg, Pb, Cd, As), are very toxic to human health
as well as the foremost threat to both terrestrial and aquatic
life and hence always remain major concern in environ-
mental and chemistry ꢀelds (Ali et al. 2019). Among all
heavy metals, Hg is considered the most harmful and
its accumulation within human and animal bodies may
severely damage heart, brain, kidney and other important
organs (Li et al. 2012). Methylmercury, the metabolized
3
+
of Al ions (Kim et al. 2010), hexapodal triazole, linked
to a cyclophosphazene core rhodamine-based chemosensor
2
+
for the selective determination of Hg ions (Ozay et al.
2014), a reversible turn-oꢂ ꢁuorescence chemosensor
based on 2,7-bis(1,2,3-triazol-4-yl)methoxy)naphthalene
2
+
3+
form of Hg by aquatic microbes, has been reported to
cause serious problems for human health and ecology
for selective detection of Fe ions (Singh et al. 2018), a
1,2,3-triazole-based thiosemicarbazide as a ꢁuorescence
2
+
(
EPA Fact Sheet 2001). In many developing countries,
chemodosimeter for Hg ion detection (Lin et al. 2015)
are the examples from some reported literature on triazole-
derived chemsensors for metal ion detection. There are
many other reports on selective metal ion detections using
1,2,3-triazole-based chemosensors (Lau et al. 2011; Hri-
shikesan et al. 2011; Shi et al. (2013; Hung et al. 2009a,
b). The structures of some representative bis 1,2,3-tria-
zole chemosensors for detection of metal ions are given
in Fig. 1 (Jabeen et al. 2016).
including Pakistan, mercury concentration in drinking
water has been detected above the permissible limit as
recommended by US-EPA (Arshad and Imran 2017).
Development of chemical sensors is comparatively
more recent, but it has proven to be most eꢃcient and
eꢂective among other modern sensor technologies. Due
to substantial sensitivity, selectivity, small size, dynamic
range, easy measurement and low cost, chemical sensors
have gained parallel attention to be used in environmental,
industrial and medicinal monitoring and in defense/ pub-
lic security as well (Wen 2016). As for as environmental
issues are concerned, sensing and detection of heavy met-
als and other environmental toxins are one of the emerging
strategies to explore new synthetic compounds as chemical
sensors (Lvova 2020; Shabir et al. 2017). Spectroscopic,
electrochemical, HPLC (high-performance liquid chroma-
tography) and ICP-MS (inductively coupled plasma mass
spectrometry) are the most employed and rapid techniques
for chemical sensing and quantiꢀcation of toxic metal ions
including Hg ions (Suvarapu and Baek 2017).
In the current study, we report the successful click syn-
thesis of a new 1,2,3-triazole-substituted compound. This
compound was further investigated computationally and
2+
experimentally for its sensitivity and selectivity toward Hg
ion detection.
Experimental
Apparatus, reagents and chemicals
Analytical grade chemicals, regents and solvents were used
in experimental work, and all commercial products were
purchased from Sigma-Aldrich. Compounds’ synthesis was
carried out by following the literature (Zhang et al. 2005).
Column chromatography was performed with silica gel 60
from Fluka (0.043–0.06 mm) while thin layer chromatog-
raphy (TLC) was carried out on 0.25 mm pre-coated glass
plates (silica gel 60 F254, Merck AG, Darmstadt, Germany).
Metal solutions were prepared from their respective salts.
Deionized water (0.05 micro siemens) was obtained from
HEJ analytical laboratory and used throughout the experi-
Click chemistry is a newer approach of synthesis and
has been frequently employed to disclose click-derived
chemosensors. These chemosensors are categorized by
their detection mechanism along with structural features.
In click chemistry, Cu(I)-catalyzed azide–alkyne cycload-
dition is the most eꢃcient and reliable reaction of binding
two molecular building blocks. A wide range of chemical
sensors contain click-derived triazoles, and their sensing
ability is attributed to the recognition of both cations and
anions (Lau et al. 2011).
1
Among triazole family, 1,2,3-triazole moiety has pro-
vided an upfront molecular linking approach implemented
in various synthetic applications. 1,2,3-Triazole-based
chemosensors, being simpler in synthesis with high sen-
sitivity, have become the most important structural motif
among various click-derived triazoles in chemical sens-
ing ꢀeld. The 1,2,3-triazole framework obtained by the
click reaction has been utilized as binding sites for metal
ments. H NMR was recorded on a Bruker Avance 300 MHz
instrument. Chemical shifts (δ) in ppm and coupling con-
stant (J) values in Hertz (Hz) were calculated. EI-MS spec-
tra were observed on JEOL JMS600 mass spectrometer.
FT-IR spectra of compounds were recorded on VECTOR
22 (Bruker) while absorption and emission spectra were
recorded on UV-visible (Shimadzu 1800) and ꢁuorescence
(F-7000; Model FL2133-007) spectrophotometers, respec-
tively, and the quartz cells used in these experiments were
1
3

Chemical Papers
Fig. 1 Some representative bis
1
,2,3-triazole chemosensors for
detection of metal ions (Jabeen
et al. 2016)
of 1 cm path length. In ꢁuorescence spectrophotometer, the
instrumental parameters used were wavelength scan, emis-
sion scan mode, 2400 nm/s scan speed, 0.0 s time delay,
completion of reaction, DMF was removed by freeze drying
and 1-(3-azidopropoxy)-3-nitrobenzene (5) was puriꢀed by
column chromatography using hexane and dichloromethane
(8:2, v/v) as eluent.
4
00 V PMT voltage, 0.5 s response time,5.0 nm each EX-slit
and EM slit, 200.0 nm EM start wavelength and 900 nm EM
end wavelength.
Characterization data
Synthesis of 1‑(3‑Azidopropoxy)‑3‑Nitrobenzene (5)
2
Yield: 60%; R Value: 0.8 cm; 3187 (SP C-H stretching),
f
3
2
959 (SP C–H stretching), 2105 (C–N), 1570, 1434 (C=C
+
.
aromatic), 1153 (C–O); EI.MS: m/z 224 (M :C H N O ,
9
10
4
3
calc. 222), m/z 180 (C H NO ), m/z 152 (C H NO ), m/z
9
10
3
7
3
6
3
1
1
1
39 (C H NO ); H NMR (300 MHz, CDCl ): δ 7.81 (d,
6
4
3
H, J=8.1 Hz, Ar–H), 7.71 (t, 1H, J =4 Hz, Ar–H), 7.41
(
t, 1H, J = 7.438–7.384 Hz, Ar–H), 7.21 (dd, 1H, J = 2.1,
1
.8 Hz, Ar–H), 4.19 (t, 2H, J = 7.2 Hz, PhOCH CH ),
2
2
3
.59 (t, 2H, J=6.3 Hz, PhOCH CH CH Br), 2.37 (m, 2H,
2
2
2
PhOCH CH CH Br).
2
2
2
Compound 2 was synthesized by dissolving of 3-nitrophenol
(
100 mg, 0.64 mmol) (1) in 10 ml acetone. After formation
Synthesis of Bis(3‑Methyl‑4‑(Prop‑2‑Yn‑1‑Yloxy)
Phenyl)Methane (6)
of clear solution, potassium carbonate (134 mg, 1.5 mmol)
was added and stirred for 1 h at 55 °C. Then 1,3-dibromo-
propane (0.13 ml, 2 mmol) was added followed by heating
to reꢁux for 5 h. After completion of reaction, 20 ml water
and dichloromethane were added to separate the organic and
aqueous phase. The organic phase was collected, dried and
ꢀ
ltered, and solvent was removed through rotary evaporator
to give 1-(3-bromopropoxy)-3-nitrobenzene (2) as transpar-
100 mg (0.44 mmol) 4,4’-methylene bis(2-methylphenol) (4)
was dissolved in 10 ml of ethanol and addition of 181 mg
(3 mmol) K CO After stirring for 45 min at 60 ℃, 0.15 ml
ent liquid. Without further puriꢀcation 2 was treated with
excess of NaN in DMF for 2 h at room temperature. After
3
2
3.
1
3

Chemical Papers
1
(
4 mmol) of propargyl bromide (5) was added. The reaction
(C H O ), m/z 227 (C H O ); H NMR (300 MHz,
1
8
17
3
2
15 14
2
mixture was heated to reꢁux for six hours. The reaction was
monitored with TLC using hexane: DCM (2: 8, v/v)). The
workup of ꢀnal product was carried out in the mixture of
dichloromethaneand water (1:1, v/v). Dichloromethane was
evaporated through rotary, followed by silica gel column
chromatography to give 6 as white solid.
CDCl ): δ 8.23 (s, 1H), 7.80 (d, 1H), 7.68 (s, 1H), 7.50
(t, 1H, J = 8.2 Hz), 7.37 (dd, 1H J = 2.1 Hz),7.00 (m, 6H),
5.08 (s, 2H), 4.73(d, 2H), 4.57 (t, 2H, J = 6.6 Hz), 4.10 (t,
2H, J = 5.7 Hz), 3.71 (s, 2H), 3.50 (s, 1H), 2.33 (m, 2H),
2.08 (d, 6H, J = 12.9 Hz).
Computational analysis
Characterization data
Computational studies were performed using DFT by
SCM-ADF modeling suite 2018. Optimization, molecu-
lar orbital calculations, density of state calculations and
Yield: 90%; melting point: 55 °C; R Value: 0.73; IR (KBr):
f
−
1
−1
(
(
PhOCH CCH=3295 cm ), (PhOCH CCH=2117 cm ),
2
2
−1 +
PhOCH CCH = 1128 cm ). EI.MS: m/z 304 (M
2
adsorption of Hg(OOCCH ) on compound 7 were inves-
C H O , calc. 304.15), m/z 289 (C H O ), m/z 265
3 2
2
1
20
2
20 17
2
1
tigated using (LDA-GGA) due to Becke (Becke 1988) and
Perdew (Perdew 1986). The double zeta (DZ) basis sets
demonstrated the electron density consisting at good level
of accuracy by keeping frozen core of nitrogen, carbon,
oxygen and Hg atoms small. Scalar relativistic correc-
tions were incorporated through the zeroth-order regular
approximation (Nosheen et al. 2020). Presently, PW91
functional was implemented to determine energetic and
electron transfer due to adsorption of Hg(OOCCH ) on
(
C H O ), m/z 237 (C H O ), m/z 225 (C H O ) H
1
8
17
2
16 13
2
15 13
2
NMR (300 MHz, CDCl ): δ 6.95 (dd, 2H J = 9, 6.5 Hz,
3
Ar–H), 6.8 (d, 2H J=6.5 Hz, Ar–H), 4.65 (d, 4H J=2.1 Hz
PhOCH CCH), 3.78 (s, 2H PhCH Ph), 2.46 (t, 2H
2
2
J=2.1 Hz, PhOCH CCH), 2.21 (s, 6H, PhCH ).
2
3
Synthesis of Target Compound
5
‑((2‑Methyl‑4‑(3‑Methyl‑4‑(Prop‑2‑Ynyloxy) Benzyl)
Phenoxy) Methyl)‑1‑(3‑(Nitrophenoxy) Propyl)‑1H‑1,
, 3‑Triazole (7)
3
2
7
. Optimized geometries and adsorption energies were
2
based on equilibrium geometry calculations. The adsorp-
tion energies were calculated by following equation.
ꢀ
ꢁ
ΔEads = Ecomplex− E + E
1
2
Here, E
refers to the energy of the Hg(OOCCH )
3 2
2
complex
on 7, and E and E refer to the energy of optimized com-
1
pound 7 and Hg(OOCCH ) , respectively. ΔE refers to
3
2
ads
adsorption energy of the system.
1
00 mg (0.33 mmol) of bis (3-methyl-4-(prop-2-ynyloxy)
phenyl)methane was dissolved in 10 ml DMF. After forma-
tion of clear solution, 60 mg (0.7 mmol) 1-(3-azidopropoxy)-
General procedure for spectroscopic measurements
3
-nitrobenzene was added, with the addition of azide, cop-
per sulfate pentahydrate and sodium ascorbate were added
into the solution. The reaction was heated to reꢁux at 60 °C
for12 hours. Color of reaction changed from green to brown.
Reaction was monitored through TLC analysis. DMF was
evaporated through freeze drying and ꢀltered through ꢀlter
paper. Dichloromethane is added in the reaction mixture,
and ꢀltration was performed. Solvent was evaporated and
separated by silica gel column chromatography using hexane
and ethyl acetate (4: 6, v/v).
Initially, one millimolar stock solution of 7 was prepared in
DMSO and DMF, separately, and spectrum was recorded,
individually. The target compound 7 showed good result in
DMF than DMSO; hence further work was proceeded with
the solution of 7 in DMF. Stock solutions of all metals in
one millimolar concentration were prepared in deionized
water. The absorption and emission spectra of compound
7
in the presence of metal ions were recorded using equi-
molar concentrations (0.5 µM each) in DMF/H O (1:1, v/v
2
ratio). The mixing of 7 and metal ions was done within the
Characterization data
cuvette, and each spectrum was recorded after 5 min. For
2
+
2+
Hg ion detection, 0.1–1000 µM concentrations of Hg
Physical state: gummy solid; yield: 35%; R value:
f
were added, separately, into 0.5 µM solution of 7, stirred,
stay for ꢀve minutes, and then analyzed through spectro-
photometer. The rest of the other titrations and experi-
ments were performed according to the same conditions.
−
1
0
.7 cm; IR (KBr): (PhOCH CCH = 3297 cm ),
2
−1 −1
(
(
PhOCH CCH=2117 cm ), (PhOCH CCH=1130 cm ),
−1 +
2
2
ArCH CN = 2453 cm ); EI.MS: m/z 526 (M
2
C H N O , calc. 528), m/z 509 (C H N O ), m/z 266
3
0
30
4
5
30 28
4
5
1
3

Chemical Papers
Results and discussion
Chemistry
quite intense molecular ion peak at 526 Da (Fig. S6). The
successful ring closure to form 1,2,3-triazole was conꢀrmed
1
by H-NMR spectrum showing a singlet of CH of triazole
ring in the downꢀeld region at 8.2 ppm. The other aromatic
protons appeared in the region of 7.8–6.8 ppm (Fig. S7).
The synthesis of target compound 7 was achieved according
to Scheme 1, (Jabeen et al. 2016). Initially, 3-nitrophenol
2
+
(
1) was reacted in reꢁuxing acetone with 1,3-dibromopro-
Computational studies on compound 7 for Hg ion
detection
pane in the presence of K CO as base to achieve com-
2
3
pound 2 which on further treatment with NaN produced
3
1
-(3-azidopropoxy)-3-nitrobenzene (5). In another reaction,
Firstly, single-point energy (SPE) was assessed to ꢀnd the
correct spin multiplicities (SMs) for the compound 7 and
Hg(OOCCH ) . After that, 7 and Hg(OOCCH ) molecule
compound (4), namely 4,4’-methylenebis(2-methylphenol),
was treated with propargyl bromide in reꢁuxing acetone to
prepare compound (6). In ꢀnal step, the Cu(I)-catalyzed
cycloaddition of (5) with bis(3-methyl-4-(prop-2-yn-1-
yloxy)phenyl)methane (6) yielded the target compound 7 in
3
2
3 2
were optimized geometrically using equilibrium geometry
calculations. The total charge of Hg(OOCCH ) molecule
3
2
was taken as zero to assign equilibrium geometry that has
a singlet multiplicity (SM). In order to determine the sta-
ble conꢀguration, the Hg(OOCCH ) molecule was placed
8
6% yield after column chromatographic puriꢀcation.
The structures of synthesized starting compounds 5, 6
3
2
and target compound 7 were conꢀrmed via spectroscopic
techniques. The EI-MS of compound 2 showed the molecu-
above the 7. To understand the energetic behavior of the
Hg(OOCCH ) adsorption, adsorption energies were calcu-
3
2
1
lar ion peak at 261 Da (Fig. S2), while H-NMR spectrum
lated and are presented in Table 1. The adsorption energy,
showed three peaks in aromatic region comprising four
aromatic protons. The two triplets at 3.5 ppm and 4.1 ppm
were assigned to N CH and –OCH protons, respectively
(ΔE ), for 7 - Hg(OOCCH ) complex was computed as
ads
3 2
− 6.09 eV which revealed that adsorption Hg(OOCCH )
3
2
on 7 was exothermic and indicated eꢂective sensing of
3
2
2
(
Fig. S3). The molecular ion peak of bis (5-methyl-2-(prop-
Hg(OOCCH ) molecule. The O−Hg and C=O bond
3
2
2
-ynyloxy) phenyl) (6) was observed at 304 Da in the EI-
lengths of adsorbed Hg(OOCCH ) molecule were calcu-
3 2
mass spectrum (Fig. S4). Further structural conꢀrmation was
lated to be 2.75 Å and 1.63 Å when adsorbed on 7 structure,
these values being larger than respective bond lengths of
1
observed by H-NMR spectrum where a doublet at 4.6 ppm
was assigned for CH protons of propargyl group. The CH
free Hg(OOCCH ) molecule revealing that mercury acetate
2
2
3 2
group between two phenyl rings resonated at 3.7 ppm as
has been eꢂectively sensed and detected by 7. The distances
singlet, while acetylenic proton appeared at 2.4 as triplet
between O−Hg and C=O for Hg(OOCCH ) molecule were
3
2
due to coupling with propargylic CH group (Fig. S5). The
computed as 2.21 Å and 1.25 Å, respectively, before adsorp-
tion. Similarly, plane of encircled ring of compounds 7 was
tilted by an angle 78.910 after adsorption of Hg(OOCCH )
2
structural conꢀrmation of ꢀnal compound 7 was also per-
formed by both MS and NMR. The molecular mass of the
compound 7 was deduced from EI-MS spectrum showing a
3
2
which showed eꢂective sensing of mercury by 7.
Scheme 1 Synthesis of target
compound 7 by azide-alkyne
cycloaddition
1
3

Chemical Papers
Table 1 Geometric and electronic parameters of the optimized compound 7 and its complex with (Hg(OOCCH ) calculated by using DFT at
3
2
PW91 functional and DZ basis set
Compound 7
–
Hg(OOCCH3)2
Complex [7 - Hg(OOCCH3)2]
Bond lengths
B.L
O–Hg = 2.21 Å C=O = O–Hg = 2.75 Å C=O = 1.63 Å
1
.25 Å
Energies/eV
Adsorption energies /eV
MO’s
E
ΔEads
EHOMO
EHOMO-1
ELUMO
ELUMO+1
ΔEHOMO-LUMO
ΔEHOMO-1-LUMO+1
E1=−400.91
–
−6.02
−6.23
−5.25
−3.70
0.77
E2 =−189.0
–
Ecomplex= −596.00
−6.09
−1.04
−2.74
−0.62
−0.30
0.417
2.44
2.53
Additionally, the structural variation and Mullikan
charges analysis of atoms of 7 before and after adsorption
of Hg(OOCCH ) are shown in Fig. 2. Flipping of circled
after adsorption of Hg(OOCCH ) , HOMO isodensities
3 2
were shifted to nitrobenzene of compound 7 and LUMO
isodensities were shifted on Hg(OOCCH ) depictive of
3
2
3 2
moiety at angle of 45°, while the charge distribution, i.e.,
the fact that 7 acted as donor and Hg(OOCCH ) per-
3
2
−
0.186 on O atom before adsorption and − 0.144 on this
formed as acceptor for electric charge. For HOMO-1, a
decrease was observed in orbital densities distribution on
7, whereas LUMO + 1 was concentrated on same position
before and after adsorption. Hence, according to HOMO,
LUMO, HOMO-1 and LUMO + 1 isodensities distribu-
atom after adsorption, indicated that charge transfer from
7
to Hg(OOCCH ) took place during process of chemical
3 2
sensing, as shown in Fig. 2.
The molecular orbital (HOMO and LUMO) representa-
tions for 7 before and after Hg(OOCCH ) adsorption are
tions compound 7 acted as donor and Hg(OOCCH ) was
3
2
3 2
represented in Fig. 3. It is apparent from the data in Table 1
and from Fig. 3 that the HOMO–LUMO gap decreased from
considered to be acceptor.
Another important electronic descriptor calculated was
the density of states (DOS) which is fundamentally the
number of diꢂerent electronic states at a particular energy
level that electrons are allowed to occupy (Yong et al.
2016). The density of states (DOS) plots for optimized
structure of 7 before and after Hg(OOCCH3)2 adsorption
are represented in Fig. 4. The most signiꢀcant application
of the DOS plots is to prepare a demonstration of molec-
ular orbitals, the bonding, anti-bonding or non-bonding
nature of the orbitals and their interactions (Yong et al.
0
.77 eV to 0.417 eV after adsorption. The decrease in ΔE
value after adsorption of Hg(OOCCH ) on compound 7
3
2
revealed enhanced electrical conductivity according to fol-
lowing equation (Bhuvaneswari et al. 2018).
ꢀ
ꢁ
−
Eg
3
∕2
ꢀ
= AT exp
2
kT
where σ is the electrical conductivity, A is the constant,
2
016). The region of the curves on lower negative side
ΔE is energy gap, k is the Boltzmann’s constant and T is
the temperature. The increased electrical conductivity of the
structure after adsorption is demonstrated that compound 7
could be considered a potential sensor for Hg(OOCCH )
indicated LUMOs and on greater negative side showed
HOMOs, as shown in Fig. 4.
Comparison of the graphs before and after adsorption
showed that intensity of DOS on LUMOs side increased
due to electron density transfer from HOMO of 7 to the
LUMO of complex of 7 with Hg(OOCCH ) ; as a result,
3
2
detection. The distribution of isodensities on 7 also changed
after adsorption of Hg(OOCCH ) , as obvious from Fig. 3.
3
2
3
2
For HOMO and LUMO orbitals, isodensities on compound 7
population of LUMOs was increased. Figure 4 demon-
were shifted close to Hg(OOCCH ) highlighting the regions
3
2
strates that there was appearance of one more state at
of compound 7 involved in sensing mercury acetate. Shifting
4
.7 eV due to which HOMO-LUMO was decreased. This
of isodensities toward Hg(OOCCH ) revealed that aniline
3
2
decrease was responsible for increase in electrical con-
ductivity. After adsorption, smoothness in the DOS peaks
indicated that electron transfer process became gradual
which played important role in electrical conductivity
change.
part of compound 7 is involved in sensing Hg(OOCCH ) .
3
2
It is apparent from Fig. 3 that before adsorption HOMO
densities were concentrated on six-membered ben-
zene ring structure along ether linkage and LUMO isoden-
sities were concentrated on nitrobenzene ring. However,
1
3

Chemical Papers
Fig. 2 Optimized structure and Mullikan charge distributions of (top) compound 7 and (bottom) Hg(OOCCH ) adsorbed on 7 calculated at
3
2
LDA-GAA level theory and PW91 functional
2
+
Selective detection of Hg Ion by UV
and ꢀuorescence spectroscopy
metal ions, separately, and shown in Fig. 5 (only shown
2
+
for 7 - Hg complex). A signiꢀcant rise in the absorp-
tion peak intensity of 7 (0.5 µM) was observed in the
2
+
Selective recognition property of receptor 7 toward vari-
presence of equimolar concentration of Hg ions. The
percent hyperchromism was evaluated to be 50% which
3
+
+
3+
+
2+
2+
ous metals ions such as Bi , K , Yb , Na , Sn , Ba ,
2
+
2+
2+
+2
Rb , Zn and Hg has been studied. Initially, absorption
spectrum of 7 was recorded and a broadband with max-
ima at ≈ 275–285 nm was observed, as shown in Fig. 5.
indicated interaction of compound 7 toward Hg . Similar
experiments were performed with other metals, but no sig-
niꢀcant changes in absorption intensity were observed in
*
+2
The broader peak may be attributed to various n – π and
compound 7 spectrum as observed with Hg ions. These
*
π – π transitions (Shabir et al. 2017). Then absorption
results pointed toward selectivity of compound 7 toward
+
2
spectra of 7 have been recorded in the presence of each
Hg ions.
1
3

Chemical Papers
Fig. 3 Schematic molecular orbital (MO) diagram of (Left) compound 7 (Right) compound 7 with adsorbed Hg(OOCCH ) showing HOMO,
3
2
LUMO and HOMO–LUMO energy gap (ΔE)
Fig. 4 The density of states (DOS) a for optimized structure of (Left) compound 7 (Right) compound 7 with adsorbed Hg(OOCCH ) calculated
3
2
at LDA-GGA level of theory
To further authenticate the DFT and UV ꢀndings for
500 nm and maximum emission was observed for the com-
pound 7 at wavelength 275 nm with excitation wavelength
of 250 nm. By using the selected excitation wavelength, then
emission spectrum was taken in the presence and absence of
various metal ions, as shown in Fig. 6.
+
2
the selectivity of targeted compound (7) toward Hg ions,
ꢁ
7
uorescence study was conducted because the compound
showed ꢁuorescence in UV region. The excitation spec-
trum was conducted by scanning the wavelength from 200 to
1
3

Chemical Papers
Fig. 7 Fluorescence spectra of receptor 7 with increasing concentra-
2
+
tion of Hg ion
Fig. 5 Absorption spectra of compound 7 only and 7 - Hg2+
Fig. 6 Fluorescence spectra of receptor 7 without and in the presence
Fig. 8 Eꢂect of pH on emission spectra of compound 7
of metal ions
Upon addition of one equivalent of each metal ions into
by varying the concentration of mercury from 0.1 µM to
1000 µM; meanwhile, the concentration of compound 7 was
kept constant, as shown in Fig. 7. It is clear from Fig. 7 that
2
+
the receptor 7, separately, only Hg displayed a signiꢀcant
enhancement in excitation intensity at 275 nm, whereas other
tested metal ions did not induce such substantial changes in
the ꢁuorescence intensity of sensor 7. The changes in emis-
sion spectrum of 7 in the presence of other metal ions are
less signiꢀcant and indicated that receptor 7 exhibits a higher
2+
even at lowest Hg concentration (0.1 µM) the ꢁuorescence
intensity of 7 has signiꢀcant value. However, an increase
in ꢁuorescence intensity could be observed with increas-
2
+
ing Hg concentrations. These results clearly demonstrate
that receptor 7 shows higher selective and sensitive binding
2
+
selective response toward the Hg ion over the other tested
metal ions. The reason for metal ion chelation of compound
2+
capability toward Hg as detectable changes in the intensity
2
+
7
could be attributed to the presence of lone pair of electrons
have been observed even with 0.1 µM concentration of Hg
on the nitrogen atom of triazole group. Nitrogen, being its
high electronegativity, may drag the electrons toward itself
and enhances the electron density on the ring and hence may
resulted in covalent bond formation with metal ions.
ions. The limit of detection was calculated to be 0.06 µM
by using slope of the straight-line equation by applying the
formula: LOD=3.3×S.D/ slope.
Eꢁect of pH
Concentration study
2
+
The eꢂect of pH on binding ability of ligand 7 with Hg
+
2
Furthermore, the recognition limit of compound 7 for Hg
ion was investigated in DMF, as shown in Fig. 8. The vari-
ations in pH were induced by addition of hydrochloric acid
and sodium hydroxide while the concentration of compound
7 was kept constant at 0.5 µM. An increase in ꢁuorescence
ion was explored through concentration study. The ꢁuores-
2
+
cence titration of receptor 7 with Hg was carried out, and
spectra were taken under parallel experimental conditions
1
3

Chemical Papers
intensity was observed when pH was increased from 1.7
to 5.1. Maximum intensity was observed at pH 5.1, while,
by increasing pH above 8.0, a pronounced drop in the peak
intensity was observed. Maximum rise in fluorescence
acidic or basic medium the increases in absorption intensity
are lower as compared to neural pH medium. This change in
2
+
absorption intensity of compound 7-Hg complex is pos-
sibly due to protonation and deprotonation process which
destabilized the complex formation.
2
+
intensity of 7 - Hg adduct till slightly acidic pH (5.1)
and then quenching in the ꢁuorescence intensity in basic
pH (8.0–11) showed that compound can display maximum
binding in acidic media compared to basic one. Supramo-
lecular complex formation between ligand and metal ions
is highly dependent on the pH of the medium. The maxi-
mum enhancement in absorption intensity of sensor 7 upon
Study of Interference of Other Metals on 7 – Hg2+
Complex
The selectivity of a chemosensor for a particular analyte
is an important parameter in practical applications. To fur-
ther verify the high selectivity of receptor 7 as a responsive
2
+
interaction with Hg was recorded at pH 5.1 while in high
2
+
sensor for Hg , competitive experiments were performed
by screening the addition of various interfering metal ions
+
2
+3
+2
+2
+2
+2
+2
+1
(
such as Ba , Bi , Zn , Yb , Co , Cu , Fe , K ,
+1
+1
+2
+2
+3
+2
2+
Na , NH , Ni , Pb , Sb and Sn ) to 7- Hg solution
4
The spectra have shown no signiꢀcant eꢂect on ꢁuorescence
2
+
emission of 7-Hg complex in the presence of competitive
metal ions, as shown in Fig. 9. These responses illustrate that
receptor 7 displays robust selectivity and sensitivity toward
2
+
sensing Hg ions.
Binding mechanism
1
Further H-NMR spectrum of 7 was recorded after the addi-
2
+
1
tion of Hg ion and compared with individual H-NMR
spectrum of 7 to see the binding site and the binding mecha-
nism of ligand to metal, as shown in Fig. 10. No signiꢀcant
shift in ppm value of protons was observed. The prominent
peak of triazole was observed at 8.229 ppm without mercury,
Fig. 9 Fluorescence emission response of 7-Hg complex (0.5 μM)
in the presence of one equivalent of various metal ions in 0.05 M in
DMF solution at pH 5.1
Fig. 10 1H-NMR spectrum of 7 and 7 - Hg2+ complex
1
3

Chemical Papers
and very small change has been observed with mercury in
BL and SAS contributed to photophysical studies; and AS contributed
to design synthesis.
1
case of H-NMR peak at 7.682 ppm. It revealed that might
be middle nitrogen of triazole involved in the binding to
Funding None.
the metal. However, when 0.5 equivalent of Hg(OAc) was
2
added, the absorption band was decreased signiꢀcantly and
Declarations
hypochromic shift of the absorption spectra was observed
(
Fig. 10), showing the formation of dimer of ligand in the
Conꢀlict oꢀ interest Authors declare that there is no conꢁict of interest.
ground state. Based on this observation, a 2:1 complex for-
mation was proposed with binding site being nitrogen of
triazole ring and oxygen of 3-nitrophenyl ring.
Ethical approval We want to publish our manuscript having no pub-
lication charges.
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3
+
+
3+
+
2+
2+
2+
2+
3+
2+
(
Bi , K , Yb , Na , Sn , Ba , Rb , Zn , Fe , Ni ,
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+
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Authors and Aꢀliations
1
2
3
4
5
6
Rabail Ujan · Nasima Arshad · Fouzia Perveen · Pervaiz Ali Channar · Bhajan Lal · Mumtaz Hussain ·
6
4
7
Zahid Hussain · Aamer Saeed · Syeda Aaliya Shehzadi
1
5
6
7
M. A. Kazi Institute of Chemistry, University of Sindh,
Institute of Chemistry, Shah Abdul Latif University,
Khairpur 66020, Pakistan
Jamshoro 76080, Pakistan
2
Department of Chemistry, Allama Iqbal Open University,
Department of Chemistry, University of Karachi,
Karachi 75270, Pakistan
Islamabad 44000, Pakistan
3
Research Center for Modeling and Simulations, National
Sulaiman Bin Abdullah Aba Al-Khail-Centre
University of Sciences and Technology (NUST), Islamabad,
Pakistan
for Interdisciplinary Research in Basic Sciences (SA-CIRBS),
International Islamic University, Islamabad 44000, Pakistan
4
Department of Chemistry, Quaid-I-Azam University,
Islamabad 45320, Pakistan
1
3