Polyhydroquinoline nanoaggregates: A dual fluorescent probe for detection of 2,4,6-trinitrophenol and chromium (VI)

Article abstract of DOI:10.1016/j.saa.2020.118087

Fluorescent polyhydroquinoline (PHQ) derivative was fabricated utilizing one-pot engineered course. The PHQ derivative indicated aggregation induced emission enhancement (AIEE) with arrangement of nanoaggregates of size 11–13 nm in 95% watery DMF medium. The fluorescence emission of PHQ nanoaggregates was extinguished by including TNP and Cr (VI). They indicated prevalent fluorescence quenching towards both TNP and Cr (VI) over other meddling nitro-compounds and metal particles. In light of results got we presumed that both photo-induced fluorescence quenching of PHQ nanoaggregates by TNP, while Inner Filter Effect (IFE) was in charge of fluorescence quenching of PHQ nanoaggregates by Cr (VI). The PHQ nanoaggregates empowered identification of TNP and Cr (VI) down to 0.66 μM (TNP) and 0.28 μM (Cr (VI)). The use of PHQ nanoaggregates were reached out for location of TNP and Cr (VI) in genuine water tests.

Full text of DOI:10.1016/j.saa.2020.118087

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118087
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Polyhydroquinoline nanoaggregates: A dual fluorescent probe for
detection of 2,4,6-trinitrophenol and chromium (VI)
Jigyasa a, Deepak Kumar a, Priya Arora a, Harminder Singh b, Jaspreet Kaur Rajput a,
⁎
a
Department of Chemistry, Dr. B R Ambedkar National Institute of Technology, Jalandhar 144011, Punjab, India
b
DAV University, Jalandhar 144001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorescent polyhydroquinoline (PHQ) derivative was fabricated utilizing one-pot engineered course. The PHQ
derivative indicated aggregation induced emission enhancement (AIEE) with arrangement of nanoaggregates
of size 11–13 nm in 95% watery DMF medium. The fluorescence emission of PHQ nanoaggregates was
extinguished by including TNP and Cr (VI). They indicated prevalent fluorescence quenching towards both
TNP and Cr (VI) over other meddling nitro-compounds and metal particles. In light of results got we presumed
that both photo-induced fluorescence quenching of PHQ nanoaggregates by TNP, while Inner Filter Effect (IFE)
was in charge of fluorescence quenching of PHQ nanoaggregates by Cr (VI). The PHQ nanoaggregates empowered
identification of TNP and Cr (VI) down to 0.66 μM (TNP) and 0.28 μM (Cr (VI)). The use of PHQ nanoaggregates
were reached out for location of TNP and Cr (VI) in genuine water tests.
Received 11 October 2019
Received in revised form 7 January 2020
Accepted 20 January 2020
Available online 21 January 2020
Keywords:
Fluorescent nano-aggregates
Polyhydroquinolines
Test-strips
Dual detection probe
AIEE
© 2020 Published by Elsevier B.V.
1. Introduction
conditions of chromium, Cr (III) and Cr (VI), Cr (VI) is known to have
cancer-causing nature and mutagenicity towards living life forms [10].
Ecological security concerns have arrived at alarmingly abnormal
states inferable from nonstop unreasonable human exercises. The un-
regulated use and disposal of hazardous compounds have brought
about negative impacts on the earth. Substantial metal particles and
nitroaromatic compounds (NACs) fall into the classification of hazard-
ous compounds that posture potential danger to human wellbeing
and environment [1]. NACs, for example, trinitrotoluene (TNT), 2,4,6-
trinitrophenol (TNP), 2,4-dinitrotoluene (2,4-DNT), 2,4-dinitrophenol
(2,4-DNP), 4-nitrotoluene (4-NT), nitrobenzene (NB), and so on are ar-
ranged under mixes with prospective danger of fear based oppression
and natural security in various nations [2]. Among different NACs, TNP
otherwise called picric acid, is viewed as the most threatening as it
shows high explosive velocity [3] and low wellbeing coefficient [4].
Likewise, TNP has higher water dissolvability than TNT [5], along these
lines genuinely contaminating ground water, soil and air [1]. TNP is
broadly utilized in rocket powers, firecrackers, dyes [6], matches, phar-
maceutical, and leather businesses [2]. The persistent ingestion of TNP
may cause skin/eye bothering, sickness, dazedness, spewing, and stom-
ach torment [4,7]. There are reports where TNP has been utilized by ter-
rorist groups for blasts in several areas [8].
It is broadly utilized in metallurgy, leather tanning, dying and catalysis
industry [11]. The most extreme degree of Cr (VI) in drinking water, al-
lowable by World Health Organization is 1 μM [12]. In any case, shock-
ingly because of unregulated mechanical waste release into the inland
surface water, the substantial metal contamination is viewed as genuine
risk in numerous pieces of world, particularly the developing nations
[13]. Globally the defilement of drinking water supply by Cr (VI) is of ex-
traordinary concern [14]. Ingestion of Cr (VI) polluted water can cause
queasiness, liver and kidney harm [15] and hemolysis [16].
There are different very much established scientific procedures for
location of Cr (VI) that includes electrochemistry [17], atomic absorp-
tion spectroscopy [18], inductively coupled plasma mass spectrum
(ICP-MS) [19,20], and high performance liquid chromatography
(HPLC) [21,22]. Similarly numerous analytical methods have been re-
ported for monitoring TNP content in common matrix, including ion
chromatography [23], electrochemical techniques [23,24], ion-
mobility spectrometry [25], Raman spectroscopy [26], and so forth
have been reported. Despite the fact that these strategies offer high af-
fectability however experience the ill effects of complicated instrumen-
tation, tedious and advanced pretreatment, subsequently restricting
their utilization for on location identification [27,28]. Among every
one of these strategies the fluorescence based chemosensors are the
one that have advanced a ton inferable from on location appropriate-
ness, high affectability, cost viability and accessible instrument prereq-
uisite [10,29]. Considering the advantages of fluorescent-based touchy
Then again, chromium (VI) is a famous lethal substantial metal that
aggravates the biological system [9]. Among the two watery stable
⁎
Corresponding author.
E-mail address: rajputj@nitj.ac.in (J.K. Rajput).
https://doi.org/10.1016/j.saa.2020.118087
1386-1425/© 2020 Published by Elsevier B.V.

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Jigyasa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118087
sensors, a marketed sensor FidoXT have been made [30]. It involves am-
plifying fluorescence polymers that empowered location of military-
grade mixes utilized in hand crafted explosives and IED's, incorporating
PETN in parts per quadrillion (ppq) [31,32]. There are different fluores-
cent based sensors like nanosheets [33,34], nanoaggregates [35], conju-
gated polymers [36], quantum dots[37], carbon dots [38], metal organic
frameworks [39,40] answered to identify and measure different metal
particles and NACs. There are various fluorescent sensors that permit lo-
cation of metal particles or NACs in non-aqueous medium; anyway their
appropriateness is interfered with when connected in watery medium
[41]. This is on the grounds that most fluorescent mixes show aggrega-
tion caused quenching (ACQ) in aqueous medium. It's a phenomenon
wherein a fluorophore show debilitated fluorescence in aqueous me-
dium attributable to solid collection actuated π-π stacking by hydropho-
bic aromatic backbone [35]. Over the previous decades there are rich
reports of accumulation induced emission (AIE) or aggregation induced
emission enhancement (AIEE) materials, which demonstrate no or frail
fluorescence in arrangement yet show solid luminescence in the aggre-
gated state [42]. Organic molecules such as pentacenequinone [43],
naphthalimide [44], α-cyanostilbene [42], tetraphenylethylene [45],
siloles, arylethene derivatives that emit strongly in their aggregated or
solid state have been reported [43].
In this, we have synthesized a polyhydroquinoline subordinate by an
effortless one-pot course which framed fluorescent nanoaggregates in
blended fluid media. Polyhyroquinoline are significant structural motif
in view of biological as well as medicinal applications. It acts like
donor-acceptor framework where the heading of charge movement is
from the benzene ring towards the quinoline ring [46]. As far as we
could possibly know it's the primary report where fluorescent
nanoaggregates of polyhydroquinolines are utilized for double identifi-
cation of Cr (VI) and TNP.
the fluorescent nanoaggregates was studied using high resolution trans-
mission electron microscope (HR-TEM) of make Jeol/JEM 2100. The UV
spectra were recorded on Agilent Technologies Cary arrangement UV–
vis spectrophotometer with a quartz cuvette (way length, 1 cm). The
fluorescence spectra were recorded on Agilent Cary Eclipse spectrofluo-
rimeter. The cut width for fluorescence test was at 5 nm (excitation)
and 5 nm (emission) for all fluorescence considers. DELTADP-auto Dip
coater was utilized for getting ready visual test strips. Time-Resolved
fluorescence studies were performed on Horiba Jobin Yvon Time-
Resolved Fluorescence Spectrometer instrument utilizing time-
correlated single photon counting (TPSPC) strategy. The cyclic volt-
ammetry was performed utilizing CH instruments (USA), CHI-600E.
2.3. Synthesis of chemosensor (PHQ)
The compound (PHQ) was synthesized through a one-pot multi-
component reaction as depicted in Scheme 1.
A blend of 3-
methoxybenzaldehyde (1 mmol), ethyl acetoacetate (1 mmol),
dimedone or 5,5-dimethyl-1,3-cyclohexanedione (1 mmol), ammo-
nium acetate (1 mmol) was warmed in on a preheated water bath for
30 min. The advancement of the reaction was monitored by TLC. After
completion of the reaction, the mixture was dissolved in hot EtOH. It
was later cooled to room temperature. The solid product so acquired
was filtered, followed by washing with distilled water. The pure product
was then acquired through recrystallization of impure product from
EtOH. The formation of product was affirmed by FT-IR, 1H NMR, ESI–
MS and 13C NMR (Supporting Information Figs. ES1–ES3).
2.4.
Compound
PHQ:
ethyl-1,4,7,8-tetrahydro-2,7,7-4-(3-
methoxyphenyl)-5-(6H)-oxoquinolin-3-carboxylate
Light yellow solid; yield: 85.03%; M.p. 188–190 °C; FT-IR (cm−1):
2. Experimental section
3306, 3217, 3083, 2948, 2107, 1696, 1607, 1472, 1383, 1267, 1204,
1142, 1026, 864, 766, 685, 578, and 507 cm−1 1H NMR (400 MHz,
;
2.1. Chemicals and reagents
CDCl3, δ ppm) 0.91 (s, 3H, CH3), 1.06 (s, 3H, CH3), 1.21 (t, 3H,
OCH2CH3), 2.18–2.29 (m, 4H, 2xCH2), 2.36 (s, 3H, _C-CH3), 3.74 (s,
3H, OCH3), 4.06 (q, 2H, OCH2CH3), 5.04 (s, 1H, Ar\\C\\H,), 5.93 (s, 1H,
N\\H), 6.65 (d, Ar\\H), 6.86–7.08 (m, 2H, Ar\\H), 7.26 (s, 1H, Ar\\H),
Dimedone or 5,5-dimethyl-1,3-cyclohexanedione (99%), ethyl
acetoacetate (98%), N,N-dimethylformamide from Loba Chemie; 3-
methoxybenzaldehyde (98%) and ammonium acetate (98%) from Avra
Synthesis. Milli-Q water was utilized all through the experimental
procedure.
and 7.26 (s, 1H, Ar\\H). ESI MS (m/z): calculated for C22H27NO4
=
369.19; found 370.21 [M + 1]. Anal. calcd. for C22H27NO4: C, 71.52; H,
7.37; and N, 3.79. Found: C, 71.62; H, 7.99; and N, 3.54.
2.2. Instrumentation
2.5. Synthesis of fluorescent nanoaggregates
Fourier Transform IR (FTIR) spectra were recorded on Agilent Cary
600 in the range 400–4000 cm−1. 1H Nuclear Magnetic Resonance spec-
tra were recorded at 400 MHz with the guide of Advance 400 spectro-
photometer, utilizing tetramethylsilane as the internal standard and
CDCl3 as a dissolvable. The following abbreviations were utilized to de-
pict peak parting designs: s = singlet, d = doublet, t = triplet, br =
broad, and m = multiplet. ESI–MS spectra were recorded on a XEVO
G2-XS QTOF spectrometer. Melting points were recorded by
Gallenkamp apparatus and were uncorrected. The nanomorphology of
The nanoaggregates of PHQ were set up by re-precipitation tech-
nique. In a typical procedure, fitting measure of DMF and water was uti-
lized to get blends with water part of 0–99%. The stock arrangement of
PHQ was set up in DMF (10−3 M). An aliquot of this stock arrangement
was moved to the ceaselessly mixed DMF:water blends (0–99%) to get
30 μM of PHQ in DMF:water blends 0–99%. The so formed
nanoaggregates were permitted to remain at room temperature for
30 min. The subsequent blends were characterized through spectral
investigation.
OCH3
O
O
O
O
O
O
O
Solvent-free
Water bath
NH4 OAc
O
CH3
O
H3CO
H3C
N
H
CH3
CH3
2
3
4
1
PHQ
Scheme 1. One-pot route for the synthesis of compound PHQ.

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3
2.6. Fluorescence quenching titrations
medium. The PHQ nanoaggregates indicated considerable fluorescence
quantum yield of 0.042.
A 2.5 ml solution of PHQ nanoaggregates (30 μm in 95% aqueous
DMF) was titrated with increasing concentrations of TNP and Cr (VI)
(0–170 μM). The change in emission spectra of PHQ nanoaggregates
was analyzed after incubation of 10 min (TNP) and 15 min (Cr (VI)).
The emission spectra of each sample were estimated upon excitation
3.2. Photophysical properties of fluorescent nanoaggregates
The UV–Vis absorption spectrum of PHQ show absorption bands at
293 and 358 nm in DMF that could be expected to π-π* changes. So as
to think about the wonders of AIEE appeared by PHQ derivative, UV–
Vis and fluorescence studies were performed [46]. The UV–Vis and fluo-
rescent spectrum of PHQ derivative were recorded with various volume
part of water (fw), while keeping its concentration at 30 μM. The UV–Vis
spectral pattern of PHQ remained ideal up to 60% fw, with λmax. abs. at
364 nm (Fig. 1a). Further addition of over 60% fw in DMF arrangement
at 370 nm. For selectivity investigations of NACs solution of 10−3
M
of: phenol, 2-nitrotoluene, 1,3-dinitrobenzene, nitromethane, nitroben-
zene and TNP were prepared and diluted to required concentrations.
Similarly aqueous solution of metal ions: Fe2+, Hg2+, Cu2+, Cs+, Cd2+
,
Zn2+, Pb2+, Sn2+, Co2+, NO3−, SCN−, N−3 , AcO−and Cr6+ were set up
for selectivity examines.
of PHQ absorbance band indicated stamped changes.
A fast
2.7. Fluorescence quantum yield
bathochromic move is seen alongside phantom widening and level off
tailing. The emerging of level-off tail in UV–Vis range (Fig. 1b) estimated
the presence of Mie scattering brought about by aggregated particles
[50,51]. The increase in polarity of the medium results in feeble π-π
stacking among nanoaggregates which further prompts extension of vi-
able conjugation in them.
The fluorescent profile of PHQ in differed DMF:water portions were
likewise researched. In unadulterated DMF, PHQ showed frail fluores-
cence discharge centered at 423 nm on excitation at 370 nm. Strangely
on expanding the fw to 60% in DMF checked elevate in fluorescence
emission alongside bathochromic move of 23 nm was observed
(Fig. 1c). Later on expanding the fw from 60% to 95% a little increase in
fluorescence power was watched. Further expansion of 99% fw to DMF
arrangement of PHQ brought about abatement in fluorescence power
(Fig. 1d). This perception is generally seen in atoms indicating AIEE
properties. Wherein after arrangement of aggregates, atoms appended
distinctly to the outside of collected particles discharge light and in
this manner add to overall drop in original fluorescence of the complete
aggregated structure.
To better comprehend the marvels of AIEE, fluorescence lifetime
study was likewise explored. The augmentation in fluorescence lifetime
of nanoaggregates of organic compound in contrast with that of unadul-
terated organic compound affirmed the development of
nanoaggregates (Fig. ES4). A deferred fluorescence decay time uncov-
ered that the non-radiative procedures of the electronically energized
state of PHQ are diminished attributable to generation of molecular ag-
gregation [52].
Fluorescence quantum yield (ϕ) of PHQ was calculated with respect
to quinine sulphate (QS) in 0.1 M H2SO4 as per following formulae:
Is ꢀ AR ꢀ ηs2
ϕs ¼ ϕR
ꢀ
ð1Þ
2
IR ꢀ AS ꢀ ηR
where ϕS = quantum yield of sample (PHQ); ϕR = quantum yield of
reference (0.54); IS = Integrated fluorescence intensity of sample;
IR = Integrated fluorescence intensity of reference; AR = absorbance
of the reference; AS = absorbance of the sample; ηS = refractive
index of sample; ηR = refractive index of reference.
2.8. Cyclic voltammetry (CV) experiments
The cyclic voltammetry was performed using tetrabutylammonium
fluoride as a supporting electrolyte, glassy carbon electrode as a work-
ing electrode, platinum as a counter electrode and Ag/AgCl as a refer-
ence electrode. The scan rate was 50 mV/s. The LUMO energy level
was calculated by applying the equation: ELUMO = −(4.8 − E1/2Fc,
+ + Ered onset), where Ered onset is onset reduction potential relative
Fc
to Ag/AgCl reference electrode. The HOMO energy level was calculated
by applying the equation: EHOMO = ELUMO − Eg, optical, where Eg, optical
is optical band gap, estimated from the absorption edge (Eg,
(eV) = 1240 / λonset) [47–49].
optical
2.9. Visual detection of TNP using luminescent test strips
The consequences of UV–Vis absorption, fluorescence emission and
time-resolved fluorescence study unarguably supported the presence
of AIEE wonders in PHQ derivative. The aggregation of molecules totally
restricted the intramolecular rotation (RIR). There are two sorts of ag-
gregates: H-type and J-type depending on blue and red shift in absorp-
tion/emission, respectively. In this on increasing fw in DMF, PHQ
derivatives showed J-type aggregation in which absorption and emis-
sion bands are red-shifted which are intense and narrow contrasted
with monomer [53,54]. The HR-TEM examination further affirmed the
arrangement of requested nanoaggregates. The nanoaggregates of size
of 11–13 nm were obtained. The nanoparticles within the aggregate
were spherical in morphology (Fig. 2).
The optical stability of the nanoaggregates was additionally assessed
by setting it under a constant 365 nm UV light brightening. The change
in fluorescence emission was determined at interim of 20 min inside re-
stricted light time of 60 min. The fluorescence of the chemosensor didn't
weaken much during consistent light, which demonstrates positive op-
tical stability of the PHQ nanoaggregates (Fig. ES6).
TNP can pollute compartments, subjects, attire and different mate-
rials by leaving its follows during assembling of fireworks, transporta-
tion and handling. Cr (VI), other notorious sully water bodies through
leakage from tanning and metallurgy industry. Henceforth their on loca-
tion identification is highly desirable. To empower versatility and mini-
mal effort on location identification, fluorescent test strips were
manufactured. The Whatman filter paper was covered in PHQ
nanoaggregates arrangement (95% watery DMF) through the guide of
DELTADP-auto Dip coater. The coater was worked at a down speed of
150 mm, delay time 2 s, tank time 15 s, and number of cycles 2. The
test strips were later dried in vacuum oven. TNP and Cr (VI) solutions
of varied concentrations were spotted on dried test strips. The strips
were then seen under 365 nm UV light and pictures were taken utilizing
advanced camera.
3. Results and discussion
3.1. Design and synthesis of chemosensor
3.3. Fluorescent detection of TNP by PHQ nanoaggregates
The compound PHQ was synthesized through an advantageous one-
pot course. The so obtained chemosensor was well described by FTIR, 1H
NMR, Fluorescence and UV examination. PHQ derivative filled in as pro-
ficient chemosensor for identification of TNP and Cr (VI) in 95% fluid
To investigate the use of PHQ nanoaggregates for recognition of TNP,
fluorescence quenching titrations were carried out. The gradual addi-
tion of TNP was done and change in emission profile was analyzed.
The fluorescence emission of PHQ was steadily extinguished up to

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Jigyasa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118087
Fig. 1. Changes in UV–Visible (a & b) and fluorescence profile (c & d) of PHQ in DMF-water mixtures with different water volume fractions.
addition of 170 μM of TNP alongside diminishing of blue emission of
fluorescent nanoaggregates. The adjustment in fluorescence profile
was analyzed utilizing Stern-Volmer plot:
KSV is Stern-Volmer constant. The S-V plot showed linear relationship
with the TNP concentration in the range of 0–70 μM (Fig. 3c). As
shown in Fig. 3b the S-V plot deviated from linearity bending in upward
direction at high concentration of TNP. This linearity at low concentra-
tion of TNP is ascribed to static quenching whereas the upward bending
at high concentration of TNP can be presumably due to dynamic
quenching. The regression equation was F0/F = 0.016x(TNP) + 1.123
with the correlation coefficient (r) of 0.987 (Fig. 3(c)), and the detection
F0=F ¼ KSV ½Aꢁ þ 1
ð2Þ
where F0 and F are fluorescence intensity of PHQ nanoaggregates in
presence and absence of TNP, [A] is the concentration of analyte and
Fig. 2. HR-TEM images of PHQ nanoaggregates.