Green synthesis of fluorescent N,O-chelating hydrazone Schiff base for multi-analyte sensing in Cu2+, F? and CN? ions

Article abstract of DOI:10.1016/j.jphotochem.2018.03.018

A colorimetric and fluorometric hydrazone Schiff's base derived from dehydroacetic acid by three-steps and high-yield syntheses under green approach employing ethanol as a solvent has been prepared. Multi-analyte sensing for both metal cation (Cu²⁺) and anions (F^? and CN^?), with high sensitivity and competitive selectivity, was encountered. The sensing mechanism of anion detection found to be deprotonation of N–H and O–H moieties in the presence of ions. However, metal cation like copper(II) ions chelation with sensor, leads to diminish intra-molecular charge transfer (ICT) with chelation induced quenching of fluorescence (CHQF). Contrary, anionic interaction ensued in heightened ICT as well as photo-induced electron transfer (PET) processes. The Job's plots interpretation rendered stoichiometry of 2:1 with Cu²⁺/F^? and 1:1 with CN^?. Moreover, the detection limits of 0.962 ppm (Cu²⁺), 0.023 ppm (F^?) and 0.073 ppm (CN^?) were much lower than WHO guidelines. Further, either water or methanol was employed to differentiate F^?/CN^? ions with sensor in THF, with prominent visible naked eye and fluorometric responses.

Full text of DOI:10.1016/j.jphotochem.2018.03.018

Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Green synthesis of fluorescent N,O-chelating hydrazone Schiff base for
multi-analyte sensing in Cu2+, Fꢀ and CNꢀ ions
Neeraj Sainia, Nicha Prigyaia, Chidchanok Wannasiria, Vuthichai Ervithayasuporna,
,
*
Suda Kiatkamjornwongb,c
a
Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Center for Inorganic and Materials Chemistry, Faculty of Science,
Mahidol University, Rama VI Road, Ratchathewi District, Bangkok 10400, Thailand
b
Faculty of Science, Chulalongkorn University, Phayathai Road, Bangkok 10330, Thailand
FRST, Division of Science, The Royal Society of Thailand, Sanam Suepa, Dusit, Bangkok 10300, Thailand
c
A R T I C L E I N F O
A B S T R A C T
Article history:
A colorimetric and fluorometric hydrazone Schiff’s base derived from dehydroacetic acid by three-steps
and high-yield syntheses under green approach employing ethanol as a solvent has been prepared. Multi-
analyte sensing for both metal cation (Cu2+) and anions (Fꢀ and CNꢀ), with high sensitivity and
competitive selectivity, was encountered. The sensing mechanism of anion detection found to be
deprotonation of NꢀꢀH and OꢀꢀH moieties in the presence of ions. However, metal cation like copper(II)
ions chelationwith sensor, leads to diminish intra-molecular charge transfer (ICT) with chelation induced
quenching of fluorescence (CHQF). Contrary, anionic interaction ensued in heightened ICT as well as
photo-induced electron transfer (PET) processes. The Job’s plots interpretation rendered stoichiometry of
2:1 with Cu2+/Fꢀ and 1:1 with CNꢀ. Moreover, the detection limits of 0.962 ppm (Cu2+), 0.023 ppm (Fꢀ)
and 0.073 ppm (CNꢀ) were much lower than WHO guidelines. Further, either water or methanol was
employed to differentiate Fꢀ/CNꢀ ions with sensor in THF, with prominent visible naked eye and
fluorometric responses.
Received 31 January 2018
Received in revised form 1 March 2018
Accepted 12 March 2018
Available online 13 March 2018
Keywords:
Dehydroacetic acid
1,8-Naphthalimide
Fluorescence
Copper
Fluoride
Cyanide
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
based sensing probes display higher selectivity with stable optical
responses [16,17]. As an example, Pu et al. [18] accounted various
Constructing dual responsive chemosensors for the efficient
and selective recognition of metal ions as well as anions
simultaneously are still a challenging task for the scientific
community [1–3]. Nowadays, research scenario has been focused
more on developing optical signaling probes due to high
competitive selectivity, rapid naked eye detection in ambient
light and economical testing, over conventional detection methods
[4,5]. Various receptor molecules interacting through unidirec-
tional hydrogen bonding [6,7], ions induced Si-O bond cleavage
reactions [8], molecular encapsulation [9], chelation [10] etc.
displaying either colorimetric or fluorometric responses, have
been contrived and tested toward ions [11]. But the majority of
them exhibited severe limitations such as non-discrimination
between fluoride and cyanide ions, interference from other
competing ions specially acetate ions, long time to response
and/or irreversibility [12–15]. On the other side, deprotonation
diarylethene based chemosensors for fluoride and copper ions in
acetonitrile. However, Wan and co-workers described a phenol
derived indicator system with dual optical responses toward
fluoride and risky cyanide ions. Even though, these systems were
excellent examples of chemosensors but effective only in pure
organic environments limiting their application to off-site detec-
tion. In addition, aqueous analysis suffers from solvent competi-
tion and probe insolubility, ensuing sensing of ions an uphill battle
[19,20]. Probes that can effectively detect multiple ions in aqueous
medium are highly desirable but still quite rare [21,22]. Further,
synthetic methodologies followed in the previous reports involve
the use of hazardous chemicals such as pyridines, dioxanes,
nitrobenzene, etc. with high temperature and tedious extraction
processes [1,15,18]. This puts a negative impact on the develop-
ment of economical sensing probes as well as on human health.
It has been well demonstrated that both metal ions as well as
anions are crucial constituents in various biological, manufacturing
and metallurgical processes [23–26]. Inparticular, copper ions act as
cofactorandactivesiteconstituentinvariousmetalloenzymesdueto
its versatile redox and binding properties[27–29]. Also, fluorideions
* Corresponding author.
E-mail address: vuthichai.erv@mahidol.ac.th (V. Ervithayasuporn).
https://doi.org/10.1016/j.jphotochem.2018.03.018
1010-6030/© 2018 Elsevier B.V. All rights reserved.

216
N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
have been a crucial constituent in toothpastes and drinking water.
It precludes dental caries [30]. However, over intake of fluoride
ions is deuced for many medical ailments such as fluorosis,
osteoporosis or even urolithiasis [31,32]. Similarly, excess
concentration of cyanide ions poses hazardous impingement
on the living systems [33–35]. For these reasons, an impetus
always exists for the development of economical optical probes
that assist in maintaining a periodic check and in-turn regulate
ion’s concentration in the ecosystem [36,37].
salts and metal perchlorates were used for anion and cation
studies, respectively.
2.2. Preparations
2.2.1. Synthesis of 6-bromo-2-butyl-benzoisoquinoline-1,3-dione (2)
4-Bromo-1,8-naphthalic anhydride (0.277 g, 1 mmol) was dis-
solved in ethanol (20 mL). Later, n-butylamine (0.073 g, 1 mmol)
was dropwise added to the ethanolic solution. The reaction
mixture was refluxed with gentle stirring for 12 h, in an oil bath.
The resulting mixture was allowed to cool to room temperature
and concentrated under vacuum. The crude product was filtered
down and washed 3–4 times by ethanol. Recrystallization of 2 in
ethanol obtained a white crystalline solid. m.p. 103–104 ꢁC. yield:
Majorly, 1,8-naphthalimide moiety has been exploited as
fluorophore unit in different sensing systems due to eminent
quantum yields, proficient photo-stability and prominent Stokes’
shift [38,39]. Moreover, pyran ring systems have evinced bio-
compatibility, scuttling unexampled dimensions of sensor appli-
cation in biological system. Additionally, a pyrone derivative such
as dehydroacetic acid based receptor system puts forth excellent
chelation based sensing of metal ions [40,41]. Literature survey
revealed that there is still paucity of dual responsive sensors that
can selectively detect copper ions in addition to differentiating
fluoride and cyanide ions within same measurement method. To
work on this site, a cost efficient dual responsive chemosensor,
based on 1,8-naphthalimide and hydroxylpyrones ring system
with a hydrazone linker, has been synthesized in ethanol under
ambient temperature conditions. Apparently, due to paramag-
netic behavior, Cu2+ ions N,O-chelation with 4, resulted in
diminished ICT process and complete quenching of emission
intensity. Besides, anions induced deprotonation elicit the internal
charge transfer (ICT) and PET processes. In the present report, NMR
titration experiments delineated the deprotonation based recog-
nition mechanism. Sensor 4 loaded paper strip experimentation
revealed the practical applicability via portable sensor kit for on-
site detection of multi-analytes in aqueous medium. Additionally,
it has been demonstrated that both deionized water and methanol
assisted in selective discrimination of Fꢀ and CNꢀ ions with 4, in
THF. Indeed, the fluorescent N,O-chelation successfully demon-
strated that an efficient sensing probe has been synthesized under
a green solvent like ethanol that can effectively detect multiple
analytes generating instantaneous optical responses.
0.295 g (89%). 1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS)
d (ppm): 8.63–
8.61 (dd, 1H, ArH), 8.55–8.52 (dd, 1H, ArH), 8.39–8.37 (d, 1H, ArH),
8.02–8.00 (d,1H, ArH), 7.83–7.79 (t,1H, ArH), 4.16–4.12 (t, 2H, CH2),
1.72–1.64 (m, 2H, CH2), 1.46–1.36 (m, 2H, CH2), 0.96–0.92 (t, 3H,
CH3) as shown in Fig. S1.13C{1H} NMR (400 MHz, CDCl3, 25 ꢁC, TMS)
d
(ppm): 163.68 (C12), 163.66 (C1), 133.25 (C9/C3), 132.08 (C5),
131.27 (C8), 130.66 (C7), 130.26 (C4), 129.03 (C6), 128.17 (C2),
123.24 (C10), 122.38 (C11), 40.54 (C13), 30.32 (C14), 20.54 (C15),
14.01 (C16) as depicted in Fig. S2.
2.2.2. Synthesis of 2-butyl-6-hydrazinyl-benzoisoquinoline-1,3-dione
(3)
Compound2(0.332 g,1 mmol) was dissolved in 25 mL of ethanol
followed by the addition of 80% hydrazine hydrate (0.3 mL,
3.3 mmol). The resulting solution was refluxed with stirring for
3 h in an oil bath. Finally, the precipitated orange colored crude
product was collected under vacuum suction and washed with
ethanol. Recrystallization of 3 was carried out in acetonitrile to
obtain an orange crystalline solid. m.p. 220–223 ꢁC. yield: 0.241 g
(85%).1H NMR (400 MHz, CDCl3, 25 ꢁC, TMS)
d (ppm): 9.10 (s, 1H,
NH), 8.59–8.57 (d,1H, ArH), 8.40–8.38 (d,1H, ArH), 8.27–8.25 (d,1H,
ArH), 7.63–7.59 (t,1H, ArH), 7.23–7.21 (d,1H, ArH), 4.65 (s, 2H, NH2),
4.01–3.97 (t, 2H, CH2), 1.60–1.53 (m, 2H, CH2), 1.36–1.27 (m, 2H,
CH2), 0.92–0.88 (t, 3H, CH3) as shown in Fig. S3. 13C{1H} NMR
(400 MHz, CDCl3, 25 ꢁC, TMS)
d (ppm): 163.79 (C12), 162.94 (C1),
2. Experimental
153.19 (C7),134.22 (C9),130.58 (C3),129.30 (C5),128.23 (C4),124.13
(C2),121.75 (C11),118.45 (C6),107.40 (C10),104.01 (C8), 38.93 (C13),
29.86 (C14), 19.87 (C15), 13.77 (C16) as depicted in Fig. S4.
2.1. Materials and instrumentations
4-Bromo-1,8-naphthalic anhydride, n-butylamine, 2-methox-
yethanol, 3-acetyl-4-hydroxy-6-methyl-pyran-2-one hydrazine
hydrate (80%), DMSO-d6 and chloroform-d1 were incurred from
Sigma Aldrich. Tetrabutylammonium salts of different anions and
metal perchlorates were bought from TCI chemicals and dried
prior to use. Ethanol and THF employed for the synthesis or
analysis were used as supplied without further purification. Water
utilized in the analysis processes was obtained from Millipore
system and deionized prior to use. Commercially available
toothpaste (Colgate) and mineral water (Nestle) were purchased
and analyzed for ions. All the metal ions and anions were incurred
by the dissolution of their respective TBA salts in deionized water.
Melting point was measured with the open capillary tube method
using a GALLENKAMP variable heater melting point apparatus.
ESI-Mass (negative mode) was calculated employing a microTOF-Q
III instrument. UV–vis spectra were measured using a slit width of
1.0 nm and matched quartz cells on a Shimadzu UV-2600 UV–vis
spectrophotometer. Fluorescence spectra and quantum yield (filter
glass 2.5, dark offset) were recorded with a HORIBA Fluoro Max
Plus Spectrofluorometer. 1H and 13C{1H} NMR spectra were
incurred on a Bruker 500 MHz spectrometry using TMS as an
internal reference. NMR chemical shifts were reported in ppm and
referenced to residual protonated solvent. Tetrabutylammonium
2.2.3. Synthesis of 2-butyl-6-(2-(1-(4-hydroxy-6-methyl-2-oxo-
pyran-3-yl)-ethylidene)-hydrazinyl)-benzoisoquinoline-1,3-dione (4)
Compound 3 (0.283 g, 1 mmol) and 3-acetyl-4-hydroxy-6-
methyl-pyran-2-one (0.168 g, 1 mmol) were added in ethanol
(25 mL). The mixture was refluxed with continuous stirring at
100 ꢁC. After refluxing for 12 h, the reaction mixture was left to cool
at room temperature and concentrated under vacuum. The crude
product was further purified by recrystallization with acetonitrile-
hexane mixture (95:5) to give a novel compound 4 (0.345 g) as
yellow crystalline solid. m.p. 253–254 ꢁC. yield: 0.345 g (80%). 1H
NMR (400 MHz, CDCl3, 25 ꢁC, TMS)
d (ppm): 15.52 (s, 1H, OH),
8.62–8.60 (d, 1H, ArH), 8.50–8.48 (d, 1H, ArH), 8.19–8.17 (d, 1H,
ArH), 7.99 (s, 1H, NH), 7.74–7.70 (t, 1H, ArH), 7.13–7.11 (d, 1H, ArH),
5.89 (s, 1H, ArH), 4.15–4.11 (t, 2H, CH2), 2.77 (s, 3H, –CH3), 2.20 (s,
3H, –CH3), 1.72–1.64 (m, 2H, CH2), 1.46–1.36 (m, 2H, CH2), 0.96–
0.92 (t, 3H, –CH3) as shown in Fig. S5. 13C{1H} NMR (400 MHz,
CDCl3, 25 ꢁC, TMS)
d (ppm): 178.62 (C17), 166.38 (C15), 164.39
(C14), 163.89 (C1/C12), 163.04 (C18), 144.78 (C7), 133.57 (C9),
131.68 (C3), 129.55 (C5), 126.40 (C4), 125.53 (C2), 123.74 (C11),
119.91 (C6), 115.59 (C10), 107.35 (C8), 104.30 (C16), 97.09 (C13),
40.42 (C19), 30.44 (C20), 20.61 (C23), 20.27 (C21), 15.99 (C22),
14.06 (C24) as depicted in Fig. S6. HRMS (ESI): [M - H]ꢀ calcd for
[C24H22N3O5]ꢀ, m/z 432.1559; found m/z 432.1583 (Fig. S7).

N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
217
2.3. General spectroscopic procedures
2.3.5. Competitive experiments
Stock solution of 4 (2 ꢂ 10ꢀ5 M) in THF (10 mL) & perchlorate
salts of metal ions (4 ꢂ10ꢀ4 M)/TBA salts of anions (4 ꢂ10ꢀ4 M) in
deionized water (5 mL) were prepared. Fluorometric competitive
2.3.1. UV–vis and fluorometric measurements of 4 with Cu2+
Stock solution of 4 (2 ꢂ 10ꢀ5 M) in THF (10 mL) & perchlorate
salts of metal ions (4 ꢂ10ꢀ4 M) in deionized water (5 mL) were
prepared. UV–vis spectrometric study involved sequential addition
study involved initial addition of 100
of metal ions to the solution of 4 (2 mL) in a cuvette, followed by
the addition of 100
of Fꢀ or CNꢀ/200 of Cu2+ ions,
mL of relevant anions/200 mL
of 20–300
On the other side, fluorometric study involved the gradual addition
of 20–500
L of Cu2+ ions into the solution of 4 (2 mL) in a cuvette.
m
L of Cu2+ ions into the solution of 4 (2 mL) in a cuvette.
m
L
mL
respectively. After stirring the solutions for 1 min, emission
spectra were recorded employing a slit width of 5/5 nm.
m
The obtained solutions were stirred for 1 min and UV–vis/
fluorometric spectra were collected at room temperature by
adjusting slit width parameters to 2/2 nm and 5/5 nm, respectively.
3. Result and discussions
3.1. Synthesis methodology of 4
2.3.2. Job plot measurements
Stock solutions of sensor 4 (10ꢀ4 M) in THF (10 mL) & Cu(ClO4)2
(10ꢀ4 M) in deionized H2O (10 mL) were prepared. A solution of
sensor 4 in varying amounts i.e. 0.5, 0.45, 0.40, 0.35, 0.30, 0.25,
0.20, 0.15, 0.10, 0.05 and 0 mL were transferred to individual
volumetric flasks (5 mL). Later, Cu2+ solution, 0, 0.05, 0.10, 0.15,
0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.5 mL, was added to a solution
of sensor 4 separately. Finally, each flask was made up to 5 mL with
THF. Then, individual flask solution was immixed for 1 min and
transferred to fluorescence cuvette (2 mL). All the fluorescence
spectra were recorded at room temperature.
Initially, two reactive precursor compounds i.e. 6-bromo-2-
butyl-benzoisoquinoline-1,3-dione (2) and 2-butyl-6-hydrazinyl-
benzoisoquinoline-1,3-dione (3), have been consecutively synthe-
sized employing ethanol as reaction medium [42–45]. Later
condensation reaction between compound 3 and 3-acetyl-4-
hydroxy-6-methyl-pyran-2-one in 1:1 ratio in ethanol for 12 h
results in the formation of deep yellow solution of novel hydrazone
Schiff’s base product (80% yield) i.e. 2-butyl-6-(2-(1-(4-hydroxy-6-
methyl-2-oxo-pyran-3-yl)ethylidene)hydrazinyl)-benzoisoquino-
line-1,3-dione (4), as shown in Scheme 1. Ambient temperature
conditions were used to conclude the condensation reaction.
Further isolation of the pure compounds were achieved through
recrystallization in pure ethanol rather than tedious column
chromatography. The detailed experimental procedures and the
characterizations of 4 are described in above.
2.3.3. UV–vis and fluorometric measurements of 4 with Fꢀ/CNꢀ
Stock solution of 4 (2 ꢂ 10ꢀ5 M) in THF (10 mL) & TBA salts of
anions (4 ꢂ10ꢀ4 M) in deionized water (5 mL) were prepared. UV–
vis spectrometric study involves sequential addition of 5–120 mL of
Fꢀ/20–400
On the other side, fluorometric study involve the gradual addition
of 5–150
L of Fꢀ/10–400 L of CNꢀ ions into the solution of 4
m
L of CNꢀ ions into the solution of 4 (2 mL) in a cuvette.
3.2. Colorimetric responses of 4 with Cu2+
m
m
(2 mL) in a cuvette. The incurred solutions were stirred for 1 min,
prior to recording UV–vis/fluorometric spectra at room tempera-
ture, using slit width of 2/2 nm and 5/5 nm respectively.
The changes observed in the absorption spectral pattern of 4
with the addition of different metal ions in THF/H2O are shown in
Fig.1. It is inferred from the absorbance plots that with the addition
of 2 equiv. of different metal ions (except for Cu2+), the absorption
band at 412 nm undergoes slightly red shift (8 nm), with a minor
change in the absorption intensity. Contrary to this, with the
addition of Cu2+ (2 equiv.) to the solution of 4, the ICT absorption
band at 412 nm was completely vanished and solution color
changed from yellow to colorless (Fig. 1 Inset). This in turn pointed
out that chelation of Cu2+ ions with 4 ensued in reduced electron
density, leading to diminished ICT process [46]. Hence, sensor 4
showed high selectivity towards Cu2+ ions over other common
metal ions.
2.3.4. Job plot measurements
Stock solutions of sensor 4 (10ꢀ4 M) in THF (10 mL) & TBA-X (X-
Fꢀ and CNꢀ) salts (10ꢀ4 M) in deionized H2O (10 mL) were
prepared. Different volumes of sensor 4 i.e. 0.5, 0.45, 0.40, 0.35,
0.30, 0.25, 0.20, 0.15, 0.10, 0.05 and 0 mL were measured and
transferred to volumetric flasks of 5 mL capacity. Additionally, 0,
0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.5 mL of the Fꢀ/
CNꢀ solution were added to above solution separately. Finally,
every flask was filled up to 5 mL mark with THF. Then, individual
solutions were mixed thoroughly for 1 min and later pipette out
(2 mL) to fluorescence cuvette. Fluorescence spectra were obtained
at room temperature.
To obtain further insight into the binding characteristics of Cu2+
with 4, spectrophotometric titration experiments were conducted
with Cu2+ ions in aqueous-organic solution. Upon sequential
Scheme 1. Synthetic approach to prepare the hydrazone Schiff base of chemosensor 4 derived from 1,8-naphthalimide and dehydroacetic acid.

218
N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
Fig. 3. Absorbance spectra (a) of 4 (2 ꢂ 10ꢀ5 M) with the addition of 1 equiv. of
different anions in THF/H2O; (b) Inset: color change noticed (bottom) in sensor 4
(2 ꢂ 10ꢀ5 M) solution with anions (1 equiv.).
CO32ꢀ) of various anions, (Fig. 3). Visual inspection of 4 solution,
before and after the addition of anions displayed noticeable color
changes only with Fꢀ and CNꢀ ions, which confirmed existence of
strong interaction exist between 4 and Fꢀ/CNꢀ ions. Additionally,
comportment of the OH group in vicinity of hydrazone group
offered additional interaction site for Fꢀ and CNꢀ ions. On the other
hand, the addition of other anions to the solution of 4 did not bring
on any appreciable color changes. Also, absorption spectrum of 4
showed absorbance maxima at 412 nm due to intra-molecular
Fig. 1. Absorbance spectra (a) of 4 (2 ꢂ 10ꢀ5 M) with the comportment of 2 equiv. of
different metal ions in THF/H2O; (b) Inset: changes in the color observed (bottom)
with the addition of different metal ions (2 equiv.) to the solution of 4 (2 ꢂ 10ꢀ5 M)
solution in ambient light.
accession of Cu2+ to a solution of 4, the ICT absorption band at
412 nm undergoes a slight bathochromic shift (3 nm) with a
simultaneous uninterrupted decrease in the absorbance intensity,
that halts at 3 equiv. of Cu2+ ions (Fig. 2). A clear isosbestic point
was indicated at 355 nm ascertaining the existence of equilibrium
between the two species i.e. 4 and 4 + Cu2+, during the titration
experiment. As a result, yellow solution of 4 slowly changed to
colorless solution (Fig. 2 Inset).
charge transfer phenomenon (ICT) i.e. n !
p* transitions [15].
Addition of 1 equiv. of various anions (except for Fꢀ and CNꢀ ions),
the absorption spectra of 4 did not display any substantial change
with minor fluctuation in absorbance intensity at 412 nm.
However, with Fꢀ ions (1 equiv.), the ICT absorption band at
412 nm (1.6 ꢂ 104 L molꢀ1 cmꢀ1) underwent a drastic decrease and
ultimately split up into two adjacent bands positioned at 408 nm
and 428 nm. Simultaneously, two new absorption maxima at 580
and 620 nm, with prominent bathochromic shift (172 nm and
192 nm respectively), were observed as depicted in Fig. 3. This
leads to visible naked eye color change of solution 4 from yellow to
blue in ambient light; Fig. 3 Inset. Similarly, addition of CNꢀ ions (1
equiv.) to the solution 4, ensued in two new bands at 581 nm and
622 nm with diminished 412 nm band. Herein, the solution color
shows perceptible changes from yellow to violet. These remarked
changes in the absorption spectra with Fꢀ/CNꢀ ions were due to the
deprotonation and subsequent ameliorated electron donor pro-
pensity of N-/O- moiety, within the receptor molecule, resulting in
heightened ICT process [46,47]. Thus, 4 emerged as a relevant
probe for naked-eye recognition of Fꢀ and CNꢀ ions, with
noticeably chromogenic changes, in aqueous-organic medium.
UV-vis titration experiments were carried out to determine the
sensitivities of 4 toward Fꢀ and CNꢀ ions. As shown in Fig. S8(a),
following spectroscopic changes observed when a solution of 4 in
THF were treated with increasing quantities of TBAF solution in
deionized water. Upon sequential addition of TBAF to 4, the band at
412 nm progressively decreased, whereas two new bands with
peak positions at 580 nm and 620 nm displayed a gradual increase
and attained its maximum absorbance intensity at 1.2 equiv. of Fꢀ
ions. The yellow solution of 4 slowly changed to blue color solution,
Fig. S8(a) Inset. An isosbestic point at 480 nm further ensured that
equilibrium exists between 4 and 4 + Fꢀ species, during the course
3.3. Colorimetric responses of 4 with Fꢀ and CNꢀ
To investigate the colorimetric response patterns of
4
(2 ꢂ 10ꢀ5 M in THF) with anions, the UV–vis absorbance experi-
ments were conducted with tetrabutylammonium salts (TBA–X:
X = Fꢀ, Clꢀ, Brꢀ, Iꢀ, CNꢀ, SCNꢀ, AcOꢀ, ClO4ꢀ, HSO4ꢀ, H2PO4ꢀ, NO2
,
ꢀ
Fig. 2. Absorbance titration spectra of 4 (2 ꢂ 10ꢀ5 M) with various concentrations of
Cu2+ (0–3 equiv.) in THF/H2O; Inset: the color of 4 (left) and 4 + Cu2+ (right) upon the
addition of 3 equiv. of Cu2+
.

N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
219
of the titration. As demonstrated in Fig. S8(b), addition of TBACN to
4, also displayed a similar pattern as that of Fꢀ with a decrease in
band at 412 nm and simultaneously increases in two new bands
positioned at 581 nm and 622 nm. Herein, an isosbestic point was
observed at 482 nm affirming existence of equilibrium between 4
and 4 + CNꢀ species. The solution color slowly changes from yellow
to violet with the incremental addition of 0–3.5 equiv. of CNꢀ,
Fig. S8(b) Inset. Therefore, it was reasoned out that Fꢀ or CNꢀ ions,
induced deprotonation of OꢀꢀH and NꢀꢀH moieties, resulting in
the prolonged conjugation in 4. Due to meliorated electronic
delocalization, the energy required for p–p transitions scale down,
ensuing two new absorption bands at around 580 nm and 620 nm.
3.4. Fluorometric responses of 4 with Cu2+
The emission intensity changes observed in the spectra of 4
induced by cations, such as Cu2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+
,
Hg2+ are shown in Fig. 4. Addition of Cu2+ ions to the solution of 4
ensued a sudden sharp decrease in the emission intensity at
520 nm compared to other competitive metal ions. The fluores-
cence color changed from bright green to dull blue with Cu2+ ions
only (Fig. 4 Inset). It was interpreted that complete quenching of
emission intensity was observed as a consequence of paramagnetic
behavior of Cu2+ ions [46].
To work out the detection range and correlation between
fluorescence responses of sensor 4 at 520 nm and Cu2+ ion
concentration, fluorescence titration experiments were per-
formed. It was remarked that incremental addition of Cu2+ (0–5
equiv.) to a solution of 4, results in consistent decreases in emission
intensity at 520 nm with a slightly bathochromic shift (10 nm), as
depicted in Fig. 5. The solution color changed from yellow to
colorless in Cu2+, with the fluorescence color change from bright
green to dull blue (Fig. 5 Inset). Further, detection limits were
calculated based on fluorometric titration results observed at
Fig. 5. Emission titration spectrum of 4 (2 ꢂ 10ꢀ5 M) in the presence of incremental
concentrations of Cu2+ ions (0ꢀ5 equiv.) in THF/H2O; Inset: the color of 4 (left) and
4 + Cu2+ (right) with 5 equiv. of Cu2+ ions.
maximum wavelength, employing formulation 3s/C; where, s
corresponds to the standard deviation of the blank and C is the
slope of each titration curve and found to be 0.962 ppm.
Further, competitive experiments were carried out to establish
the selectivity of 4 toward Cu2+ ions. As shown in Fig. 6, sensor 4
was treated 2 equiv. of Cu2+ in the presence of other metal ions
(Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+) of the same concentration.
The fluorescence quenching pattern obtained evidently ensued
that other common competing metal ions have relatively negligi-
ble interference.
Job’s plot was delineated based on the fluorescence intensity of
4 at 530 nm, against the molar fraction of 4, maintaining the
constant total concentration, to obtain the binding ratio between 4
and Cu2+. The maxima was attained when the molar fraction of (4)/
[(4) + (Cu2+)], was 0.67, indicating that the stoichiometric ratio of 4
with Cu2+ to be 2:1 Fig. S9(a). As interaction of 4 with Cu2+ ions,
resulted in drastic change in the emission intensity therefore
emission titration experiments were employed to calculate the
association constant and quenching constant. Association constant
were calculated according to the Benesi–Hildebrand equation as
follows:
1/(I ꢀ Io) = 1/{K(Imax ꢀ Io)[Ion]n} + 1/[Imax ꢀ Io]
Fig. 4. Emission spectral pattern (a) of 4 (2 ꢂ 10ꢀ5 M) obtained with the addition of
2 equiv. of different metal ions in THF/H2O; (b) Inset: color change observed
(bottom) upon the addition of different metal ions (2 equiv.) to the sensor 4
(2 ꢂ 10ꢀ5 M) solution, under the UV-lamp (365 nm).
Fig. 6. Relative emission intensity bar graph showing competitive selectivity of 4
(2 ꢂ 10ꢀ5 M) toward Cu2+ (2 equiv.) in the presence of other competing metal ions (2
equiv.) in THF/H2O.

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I0 relates to the fluorescence of 4 without any ion, I is the
fluorescence recorded with ion, Imax corresponds to the fluores-
cence in the presence of added [Ion]max and K is the association
constant. which was calculated from the slope of the straight line
of the B-H plot of 1/(I ꢀ I0) against 1/[Ion]n. The association
constant value of 4 with Cu2+ was determined from the emission
intensity plots measured at 530 nm. The association constant (Ka)
of 4 with Cu2+ were calculated to be 1.26 ꢂ 105 L molꢀ1, Fig. S10(a).
Moreover, to elucidate the quenching phenomenon in 4 with Cu2+
ions, the Stern-Volmer (S-V) plot, a graph was plotted between
intensity ratio, Io/I and concentration of Cu2+ ions as shown in
Fig. S11(a). The slope of the obtained straight line (R2 0.9765 for Cu2
+) rendered the quenching constant, KS-V 2.04 ꢂ105 mol Lꢀ1 for Cu2
+. This deciphered that quenching of emission intensity in the
interactive ground state 4 + Cu2+, was the entirely active process
[47,48]. Additionally, noticeable decrease in the quantum yield
value of 4 from 4.24 to 1.54; in Fig. S12(a) was obtained. Thus,
detectable fluorescence quenching was observed only with Cu2+
ions compared to all other metal ions.
deep blue in Fꢀ with the fluorescence color change from bright
green to dark, Fig. S13(a) Inset. The detection limit was found to be
0.023 ppm. These results were further ascertained through
decreased quantum yield value from 4.24 to 1.41 with Fꢀ (1
equiv.) as shown in Fig. S12(b). Further, 4 solution was titrated with
CNꢀ ions and it was observed that the sequential addition of 0–3.5
equiv. of CNꢀ ions, resulted in consistent decrement in the
emission intensity at 519 nm with a slightly red shift of 8 nm, with
fluorescence color changes from bright green to dull green, Fig. S13
(b) & Inset. The detection limit was calculated to be 0.073 ppm.
Quantum yield value of 4 dropped down to 1.59 with CNꢀ (1
equiv.); Fig. S12(c).
In order to ascertain the selectivity of 4 toward recognition of
Fꢀ/CNꢀ ions, competitive experiments with 1 equiv. of Fꢀ and 1
equiv. of other common competitive anions (Clꢀ, Brꢀ, Iꢀ, SCNꢀ,
AcOꢀ, ClO4ꢀ, HSO4ꢀ, H2PO4ꢀ, NO2ꢀ, CO32ꢀ) were performed. It was
observed that the fluorescence intensity of 4 with Fꢀ was not
significantly interfered even in the presence of competing anions
Fig. 8(a). The fluorescence quenching observed with Fꢀ was not
affected with all of the competing anions together. Similar
fluorescence experiments were carried out to confirm the selective
nature of 4 toward CNꢀ ions in the presence of other anions.
Addition of 1 equiv. of CNꢀ to a solution of 4, in the presence of
relevant anions (1 equiv.), still quenched the fluorescence
affirming that competitive anions have negligible interference in
the recognition of CNꢀ ions; Fig. 8(b). These results revealed the
high selectivity of 4 for the recognition of Fꢀ/CNꢀ ions.
3.5. Fluorometric responses of 4 with Fꢀ and CNꢀ
Next, to further examine the dual-mode recognition behavior of
4 toward anions, the fluorometric experiments were performed at
an excitation wavelength of 412 nm, under ambient conditions.
Initially, the fluorescence spectra were recorded in the presence of
various anions (Fꢀ, Clꢀ Brꢀ, Iꢀ, CNꢀ, SCNꢀ, AcOꢀ, ClO4ꢀ, HSO4
,
ꢀ
H2PO4ꢀ, NO2ꢀ, CO32ꢀ) as shown in Fig. 7 and the results show that
the fluorescence of 4 was quenched significantly only with Fꢀ and
CNꢀ ions, with a minor bathochromic shift (5 nm). Anion induced
deprotonation of NꢀꢀH and OꢀꢀH groups at the receptor site,
facilitate lone pair of nitrogen/oxygen for heightened PET process
[46], leading to quenching of emission intensity.
Continuous variation methodology was followed to determine
the fluorogenic quantitative sensing ability of 4 toward Fꢀ/CNꢀ
ions. As deduced from the Job’s plot of 4 with Fꢀ and CNꢀ anions,
the maxima was achieved when the molar fractions of (4)/
Fluorometric titration experiments were also conducted to
investigate parameters such as detection limit and the results
obtained are shown in Fig. S13(a). A consistent decline with a
slightly bathochromic shift (5 nm) in the emission intensity at
515 nm was observed for 4, when the Fꢀ concentration increased
from 0ꢀ1.5 equiv. The solution color changed from bright yellow to
Fig. 7. Emission spectra (a) of 4 (2 ꢂ 10ꢀ5 M) with the addition of 1 equiv. of various
anions in THF/H2O solution; (b) Inset: color change observed (bottom) upon the
addition of various anions (1 equiv.) to the sensor 4 (2 ꢂ 10ꢀ5 M) solution, under UV-
lamp (365 nm).
Fig. 8. Relative emission intensity bar graphs showing competitive selectivity of 4
(2 ꢂ 10ꢀ5 M) toward (a) Fꢀ (1 equiv.) and (b) CNꢀ (1 equiv.) in the presence of
competing anions (1 equiv.).

N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
221
[(4) + (Fꢀ)] and (4)/[(4) + (CNꢀ)], were 0.69 and 0.5 respectively,
indicating that the ratios of 4 with Fꢀ (2:1) and CNꢀ (1:1) were
achieved in Fig. S9(b) and (c). The association constant (Ka) of 4
with Fꢀ and CNꢀ was calculated to be 1.5 ꢂ105 L molꢀ1 and
changes observed in the 1H NMR spectra were depicted in Fig. 9.
Initially, Fꢀ ions (0–1 equiv.) were gradually added to 4 and
corresponding spectra were analyzed. It was noticed that with the
addition of 0.2 equiv. of Fꢀ to 4, the resonance signal related to
0.62 ꢂ 105 L molꢀ1
,
respectively, using the Benesi–Hildebrand
OꢀꢀH and NꢀꢀH proton at
d 15.51 ppm/7.99 ppm vanished
equation, Fig. S10(b) and (c). From the association constant values,
it is interpreted that the Fꢀ has the higher fluorescence binding
affinity compared with CNꢀ towards 4. The detection limit of 4 was
found to be 1.2 ꢂ 10ꢀ6 M for Fꢀ and 2.79 ꢂ 10ꢀ6 M for CNꢀ. An
almost straight line (R2 0.9879/0.9864 for Fꢀ/CNꢀ) with quenching
constants, KS-V: 3.54 ꢂ105 molLꢀ1/2.41 ꢂ105 molLꢀ1 for Fꢀ/CNꢀ,
respectively; Fig. S11(b) and (c), further support the enhanced PET
process.
completely; Fig. 9(a). Thus, it was concluded that Fꢀ ions
interacted with 4 through deprotonation of NꢀꢀH and OꢀꢀH
moieties [50,52,53]. Beside this, other aromatic ring protons also
displayed prominent up-field chemical shifts. These chemical
shifts were encountered as a consequence of deprotonation, which
resulted in increased electron density leading to enhanced ICT
process within the receptor molecule [54,55]. Later, with the
addition of 1 equiv. of Fꢀ ions, a very weak signal appeared at
16.51 ppm, which affirmed the existence of [FHF]ꢀ dimer species
[56]. This dimer formation further supports that deprotonation
took place upon interaction of 4 with Fꢀ ions. These results
confirmed that Fꢀ ions interact with hydrazone as well as hydroxyl
group protons, as shown in Fig. 9(a) Inset.
3.6. Mechanism of interaction of 4 with ions
In order to elucidate the bonding interaction of Cu2+ ions with 4,
NMR titration experiments were performed, in DMSO-d6 as shown
in Fig. S14. Initially, sequential addition of Cu2+ ions (0ꢀ5 equiv.) to
Moreover, similar interaction mechanism between 4 and CNꢀ
ions was concluded from the NMR titration experiment. Different
equivalents (0–1 equiv.) of TBACN were sequentially added to the
solution of 4 in CDCl3 and the observed 1H NMR spectra were
shown in Fig. 9(b). When 0.2 equiv. CNꢀ was added, the signal
existing for the proton of OꢀꢀH and NꢀꢀH groups at 15.51 ppm/
7.99 ppm disappeared immediately, which evidently showed that
deprotonation occur instantaneously [54,55]. The other aromatic
protons resonance signals undergoes a gradual upfield shift due to
the shielding effect. Based on the analysis and interpretation of 1H
NMR titration spectra, we proposed a potential mechanism for the
interaction between receptor 4 and CNꢀ as depicted in Fig. 9(b)
Inset. Thus, 4 behaves as a highly sensitive chemosensor for both Fꢀ
and CNꢀ ions in aqueous-organic medium.
the 4, resulted into complete disappearance of OꢀꢀH (
d 15.51 ppm)
proton signal. Additionally, aliphatic protons in vicinity of
hydrazone group and other aromatic ring protons undergoing
upfield shift, a redistribution of electric charge on the naphtha-
limide rings was encountered upon interaction with Cu2+ ion
[49,50]. These observations clearly indicated that two sensor
molecules encircled the Cu2+ ions copper ions in a square planar
geometry, with N/O atoms of hydrazone/hydroxy groups, respec-
tively, as interacting sites [46,51,52] Interpretation of the experi-
ment data affirmed the proposed mechanism and related 2:1
stoichiometry.
Further to gain insight into the sensing mode of 4 toward Fꢀ/
CNꢀ ions, NMR titration experiments were performed in CDCl3 and
Fig. 9. 1H NMR titration plot of 4 with (a) Fꢀ (0–1 equiv.) and (b) CNꢀ (0–1 equiv.) in CDCl3. Inset: Deprotonation phenomenon in 4, with Fꢀ/CNꢀ ions.

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N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
Fig. 10. Photographs of 4 (10ꢀ3 M) with Fꢀ and CNꢀ ions (4 ꢂ10ꢀ4 M) in THF solution, under ambient light/UV lamp with (a)/(b) H2O (0–5 equiv.) and (c)/(d) MeOH (0–5
equiv.), respectively.
Fig. 11. Photographs of paper strips of 4 (10ꢀ3 M) with Cu2+, Fꢀ and CNꢀ ions (4 ꢂ10ꢀ4 M) in THF/H2O solution under (a) ambient (b) UV radiation.
3.7. Discrimination with deionized water/methanol
Both colorimetric and fluorometric spectral patterns undergo a
significant change with the addition of deionized water to the
4 + Fꢀ/4 + CNꢀ THF solution. During the colorimetric experiments,
successive addition of deionized water (5 equiv.) to the 4 + Fꢀ
solution, ensued in visible naked eye color change from blue to
orange Fig. 10(a). As plotted in Fig. S15(a), following colorimetric
changes were observed as a result of drastic decrease in the
absorption intensity of bands at 580/581 nm and 620/622 nm. In
contrast only, slight change in the intensity of blue color was
observed in 4 + CNꢀ solution. However, during the fluorometric
Fig. 12. Photographs of visible naked eye color of commercial toothpaste (NaF,
experimentation, Fig. S16(a), addition of Fꢀ ions (1 equiv.) to 4
20 mg/kg); white and with 4 (10ꢀ3 M in THF); blue, under ambient light. (For
interpretation of the references to colour in this figure legend, the reader is referred
solution, resulted in complete quenching of the emission intensity,
but gradual addition of deionized water (0–5 equiv.) to this
to the web version of this article.)

N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
223
Fig. 13. Photographs of fluoridated mineral water (Fꢀ, 0.5 mg/L) with 4 (2 ꢂ 10ꢀ5 M) in the presence of Cu2+, Fꢀ and CNꢀ ions (1.5, 0.07 (less than 0.08) and 2 mg/L, WHO
permissible limits) under (a) ambient (b) UV radiation.
solution, again leads to the enhancement in emission intensity. On
the other side, minimum emission intensity was observed with the
addition of CNꢀ (3 equiv.) to a solution of 4. Later, sequential
addition of deionized water (0–5 equiv.) to the 4 + CNꢀ solution,
resulted in thorough quenching of the emission intensity Fig.10(b).
Similar colorimetric and fluorometric experiments shown in
Figs. S15(b) and S16(b), were performed with methanol. Addition
of methanol (0–5 equiv.) to 4 + Fꢀ solution, resulted in visible
naked eye color change from blue to grey as shown in Fig. 10(c).
However, no detectable color change was observed in 4 + CNꢀ
solution. Interpretation of fluorometric results revealed that of
4 + Fꢀ solution showed slight increase in emission intensity from
completely quenched state. Contrary to this, residual fluorescence
of 4 + CNꢀ solution undergoes complete quenching, Fig. 10(d).
Since Fꢀ ions exhibits high hydration energy and strong
electronegativity [19], as a result significant solvent competitive-
ness exists, ensuing in reduced interaction between 4 and Fꢀ ions.
As a consequence, in the presence of deionized water/methanol,
intra-molecular charge transfer (ICT) process gets diminished,
contributing to visible naked eye color changes. However, PET
process become more favorable leading to slight restoration of
fluorescence [46]. Hence, deionized water or methanol could be
used for the colorimetric as well as fluorometric discrimination of
the two anions in organic medium.
4.2. Real samples analysis
As established earlier, sensor 4 responds selectively for fluoride
ions in solid state as well. In order to validate solid state
applicability in practical samples, commercially available products
such as toothpaste were analyzed for fluoride ions contents. A
gram of toothpaste was treated with 4 (10ꢀ3 M, 400
mL), sample
color changes immediately from white to blue, ascertaining the
presence of fluoride ions (20 ppm) as shown in Fig. 12.
Further, Cu2+ as well as Fꢀ/CNꢀ ions were analyzed in
fluoridated mineral water to corroborate competitive selectivity.
Commercially available fluoridated mineral water (Fꢀ, 0.5 mg/L)
was supplemented with Cu2+ (2 mg/L), Fꢀ (1.5 mg/L) and CNꢀ
(0.07 mg/L) ions. Detectable ions solutions (1 mL) were treated
separately with 4 (1 equiv. for Cu2+ and Fꢀ, 15 equiv. for CNꢀ) in the
presence of major constituent of mineral water such as Na+, Mg2+
Ca2+ salts. Prominent visible naked eye color changes from yellow
to colorless (Cu2+) and blue (Fꢀ/CNꢀ) was observed as shown in
Fig. 13(a). Additionally, emission intensity undergoes completely
quenching, depicted in Fig. 13(b). Thus, it has been successfully
established that 4 displayed high competitive selectivity for ions in
commercial commodities within WHO defined limits.
5. Conclusions
4. Practical applications
In ratiocination, a novel dehydroacetic acid based hydrazone
Schiff’s base sensor from three-step and high-yield syntheses
under green approach like ethanol solvent has been prepared that
exhibited high sensitivity and selectivity for Cu2+, Fꢀ and CNꢀ ions,
within WHO accentuated detection limits. The presence of two
receptor sites in a molecular sensor revealed instantaneous optical
responses with ions. Sensor possessing N,O chelation with Cu2+
ions accounted for reduced ICT process, resulting in disappearance
of absorption band at 412 nm. This brought on yellow color
solution to colorless. However, with Fꢀ and CNꢀ ions, two new
absorption bands at 580/620 nm and 581/622 nm, imparting blue/
violet color to the solution, respectively, were observed. As anions
induced deprotonation resulted in enhanced ICT within the
molecules. Emission intensity was substantially quenched in the
presence of detectable metal ions as well as anions, due to inherent
paramagnetism and enhanced PET process, respectively. Competi-
tive experiments result further uncovered that common compet-
ing ions had negligible effect on the selective sensing of Cu2+, Fꢀ
and CNꢀ ions, under ambient conditions. Deprotonation phenom-
enon of NꢀꢀH and OꢀꢀH moieties in the presence of ions was
ascertained through 1H NMR titration experiments. Job’s plots
interpretation worked out 2:1 (Cu2+/Fꢀ) and 1:1 (CNꢀ) stoichiom-
etry with sensor molecule. Sensor displayed strong binding affinity
for Cu2+/Fꢀ compared to CNꢀ, as deduced from the B–H plots.
4.1. Paper strips experiments
To ascertain the solid-state sensing ability of 4, sensor loaded
paper strips (Whatman filter paper no.1) were developed for the
real-time analysis of Cu2+, Fꢀ and CNꢀ ions in aqueous medium.
Initially, filter papers (3 ꢂ 1 cm2) were plunged with 4 (10ꢀ3 M)
solution for about 5 mins and then dried off completely. Later,
aqueous solution of Cu2+ ions were added on the loaded paper
strips, the color of the paper strip changed immediately from
yellow to colorless under ambient light as shown in Fig. 11(a).
Similarly, with Fꢀ and CNꢀ ions in water solution, the color changed
from yellow to dark blue/gray, respectively. Under UV lamp
(lex = 412 nm), an apparent color change of 4 from bright green to
dark occurred immediately, Fig. 11(b). The observed color changes
were due to the deprotonation phenomenon observed in 4 with
Cu2+/Fꢀ/CNꢀ ions. Thus, sensor 4 has demonstrated both naked eye
color and fluorometric recognition efficiency for metal ion as well
as anions in the solid phase. Competitive metal ions and anions
have not posed any interference during the paper strip experi-
mentation. Hence, sensor 4 loaded paper strips can be conve-
niently used for the recognition of Cu2+/Fꢀ/CNꢀ ions, without any
additional equipmentation.

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N. Saini et al. / Journal of Photochemistry and Photobiology A: Chemistry 358 (2018) 215–225
Additionally, deionized water or methanol, effectively discriminate
the Fꢀ ions with noticeable color variation from blue to orange/
grey, respectively, in THF. Moreover, slight restoration of fluores-
cence was also observed. Contrary to this, with CNꢀ ions, complete
quenching of emission intensity, with no visible naked eye color
change, were encountered. Paper strip experimentation further
established the practical applicability for on-site detection of ions
in aqueous medium. Analysis of ions in the commercial
commodities further established the competitive selectivity of
sensor. Thus, sensor emerged out as an economical alternative for
the aqueous analysis of Cu2+, Fꢀ and CNꢀ ions.
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The research work was supported by the Mahidol University for
postdoctoral fellowship and financially supported by the Thailand
Research Fund (RSA5980018 and DPG6080001), the Center of
Excellence for Innovation in Chemistry (PERCH-CIC), Office of the
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jphotochem.2018.
03.018.
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