Dual Naked-Eye Optical Sensor Based on Imidazolium Cation and Napthalamide for Specific Detection of Fluoride

Article abstract of DOI:10.1007/s10895-020-02494-2

A novel naked eye fluorescence sensor (ANI) based on naphthalimide and imidazolium moieties for fluoride detection has been designed and synthesized by multiple step synthesis. The fluorescence response of ANI sensor was significantly quenched in the pres

Full text of DOI:10.1007/s10895-020-02494-2

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02494-2
ORIGINAL ARTICLE
Dual Naked-Eye Optical Sensor Based on Imidazolium Cation
and Napthalamide for Specific Detection of Fluoride
Jaturong Kongwutthivech1 & Thawatchai Tuntulani1 & Vinich Promarak2 & Boosayarat Tomapatanaget1
Received: 23 October 2019 /Accepted: 20 January 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A novel naked eye fluorescence sensor (ANI) based on naphthalimide and imidazolium moieties for fluoride detection has been
designed and synthesized by multiple step synthesis. The fluorescence response of ANI sensor was significantly quenched in the
presence of fluoride ion upon the interaction between an acidic amide proton and acidic C2 proton (-C(2)H-) of imidazolium
compound. The binding behavior of ANI and F− ion was extensively explored by using NMR titration. Comparison of binding
ability of ANI and AN sensors addressed the dominant electrostatic and hydrogen bonding interaction with F− ion. Consequently,
ANI sensor highlights a strong binding with F− ions with a high selectivity over AN. Interestingly, ANI demonstrated a naked-
eye response with colorimetric and fluorometric assay.
.
.
.
Keywords Anion recognition Fluorescence Fluoride anion Dual optical sensor
Introduction
compounds, metal ions and various anions because of their
high selectivity, high sensitivity and fast response [7–10].
Generally, the anion recognition unit usually utilizes several
binding sites such as pyrrole, urea, thiourea and amide due to
hydrogen bonding interactions between NH position and an-
ions [11–14]. Moreover, electrostatic interactions would pro-
mote the anion complexation because the strength of positive
charge moiety could attract anions. Interestingly, imidazole
compound is a heterocyclic aromatic compound which can
be protonated to form imidazolium moiety [15]. In recent
years, imidazolium salts were very popular for anion detec-
tion because it can attract various anions by using their pos-
itive charge and hydrogen bonding moiety [16]. In addition,
naphthalimide is a popular dye because it was easily modified
with a variety of functional groups and offered strong absorp-
tion and fluorescence response [17–19]. In this work, we
have designed and synthesized a novel naked-eye fluores-
cence sensor (methylated imidazolium, ANI) based on
naphthalimide and imidazolium moieties acting as the signal-
ing and binding sites, respectively. Owing to the
photophysical properties of sensor, the ANI sensor exhibited
the color and fluorescence changes in a particular of fluoride
ion. As anticipated, to further develop the effectively comple-
mentary interaction for the promoting a high binding affinity
by taking a benefit of positive charge and strong acidic proton
on imidazolium moiety (C2), the binding ability of (non-
methylated imidazole, AN) without positive charge in its
structure were clearly compared to ANI.
Anion recognition and sensing has been growing fast and very
attractive to supramolecular chemists due to their important
roles in many fields such as biological, environmental, clinical
and chemical applications [1–3]. Among several biologically
related anions, fluoride is very interesting anion because fluo-
ride plays the essential role in dental care and osteoporosis [4].
However, the high concentration of fluoride in our bodies can
cause dental and skeletal fluorosis and inhibit the biosynthesis
of neurotransmitters in the fetuses [5, 6]. Therefore, research
which is involving the development of sensitive chemosensors
for fluoride detection becomes more challenge.
Fluorescence chemosensors have been widely used for
molecular and ion recognition including biological
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-020-02494-2) contains supplementary
material, which is available to authorized users.
* Boosayarat Tomapatanaget
tboosayarat@gmail.com
1
Supramolecular Chemistry Research Unit, Department of Chemistry,
Faculty of Science, Chulalongkorn University, Phayathai Road,
Patumwan, Bangkok 10330, Thailand
2
School of Molecular Science and Engineering, Vidyasirimedhi
Institute of Science and Technology (VISTEC),
Wangchan, Rayong 21210, Thailand

J Fluoresc
Experimental
was collected and dissolved with methanol and recrystallized
with dichloromethane to obtain compound 2 as yellow solid
(0.56 g, 30% yield).
General Information
1H NMR (400 MHz, d6-DMSO): δ (in ppm) = 8.75 (d, J =
8.8 Hz, 1H, ArH), 8.41 (d, J = 7.2 Hz, 1H, ArH), 8.24 (d, J =
8.4 Hz, 1H, ArH), 7.70 (t, J = 8.0 Hz, 1H, ArH), 7.49 (d, J =
3.6 Hz, 2H, ArH), 7.41 (d, J = 7.2 Hz, 1H, ArH), 7.28 (d, J =
6.8 Hz, 2H, ArH), 3.14 (t, 2H, CH2), 2.86 (t, J = 6.6 Hz, 2H,
CH2).
1H NMR and 13C NMR were recorded on a Varian Mercury
Plus 400 NMR spectrometer and Bruker Avance 400 NMR.
MALDI-TOF mass spectra were recorded on Bruker
Doltonic. HRMS mass spectra were recorded on Bruker
Compact. Absorption spectra were recorded on a Varian
Cary 50 UV-Vis spectrophotometer. All fluorescence spectra
were measured by Cary Eclipse Varian fluorescence spectro-
photometer. All naked-eye fluorescence photos were recorded
by Samsung galaxy S5. All reagents were purchased from
commercial suppliers and they were used without further pu-
rification. All commercial grade solvents were purified by
distillation before used. Thin-layer chromatography (TLC)
was performed on silica gel plates (Kieselgel 60 F254,
1 mm, Merck). Spectrophotometric grade dimethylsulfoxide
(DMSO) used in UV-Visible and fluorescence measurement
was provided from Merck.
13C NMR (100 MHz, d6-DMSO): δ (in ppm) = 163.99,
163.14, 150.97, 136.81, 134.25, 130.74, 129.86, 128.99,
128.86, 128.75, 128.70, 127.79, 124.19, 122.19, 120.21,
107.79, 103.88, 46.35.
Synthesis of 2-Chloro-N-(2-((1,3-Dioxo-2-Phenyl-2,3
-Dihydro-1H-Benzo[de]Isoquinolin-6-yl)Amino)Ethyl)Acet
amide (3)
The compound 2 (1.00 g, 3.02 mmol) and Na2CO3 (1.06 g,
10 mmol) were suspended in 20 mL CH2Cl2 and stirred at
0 °C for 5 min under nitrogen atmosphere. Chloroacetyl chlo-
ride (0.5 mL, 6.28 mmol) was added to the mixture solution
by using the syringe and then the mixture solution was stirred
overnight under nitrogen atmosphere. The reaction mixture
was extracted with DI water and the organic phase was
collected. The solvent was evaporated to give a crude product.
The crude product was dissolved with methanol and recrys-
tallized with dichloromethane to obtain the orange powder
(0.62 g, 50% yield).
Typical Procedure for the Synthesis of ANI and AN
Synthesis of 6-Bromo-2-Phenyl-1H-Benzo[de]
Isoquinoline-1,3(2H)-Dione (1)
4-Bromo-1,8-naphthalic anhydride (5 g, 18 mmol) and aniline
(2 mL, 21.9 mmol) were dissolved in dried methanol. The
mixture was stirred for 5 min. The catalytic amount of pyri-
dine was added into the mixture and it was refluxed overnight
under nitrogen atmosphere. Subsequently, the brownish crude
product was purified by washing with methanol and dichloro-
methane 3 times, and recrystallized with methanol to obtain
compound 1 as white solid (6.02 g, 95% yield).
1H NMR (400 MHz, d6-DMSO): δ (in ppm) = 8.65 (d, J =
8.4 Hz, 1H, ArH), 8.50 (s, 1 H, ArH), 8.42 (d, J = 7.2 Hz, 1H,
ArH), 8.24 (d, J = 8.4 Hz, 1H, ArH), 7.87 (s, 1H, ArH), 7.71
(t, J = 7.6 Hz, 1H, ArH), 7.48 (d, J = 7.2 Hz, 2H, ArH), 7.42
(d, J = 7.2 Hz, 1H, ArH), 7.29 (d, J = 7.6 Hz, 2H, ArH), 6.88
(d, J = 8.4 Hz, 1H, ArH), 4.10 (s, 2H, CH2), 3.47 (t, J =
5.6 Hz, 2H, CH2), 3.45 (t, J = 6.0 Hz, 2H, CH2).
1H NMR (400 MHz, d6-DMSO): δ (in ppm) = 8.57 (t, J =
7.4 Hz, 2H, ArH), 8.32 (d, J = 8.0 Hz, 1H, ArH), 8.23 (d, J =
8.0 Hz, 1H, ArH), 8.01 (t, J = 7.8 Hz, 1H, ArH), 7.51 (d, J =
7.4 Hz, 2H, ArH), 7.45 (d, J = 7.2 Hz, 1H, ArH), 7.37 (d, J =
6.8 Hz, 2H, ArH).
13C NMR (100 MHz, d6-DMSO): δ (in ppm) = 163.12,
150.62, 136.50, 134.15, 131.37, 131.06, 130.77, 130.32,
129.84, 129.18, 128.80, 128.69, 128.59, 128.48, 128.10,
127.80, 127.15, 126.54, 124.38, 119.37, 103.78, 53.71,
43.70, 42.61.
13C NMR (100 MHz, d6-DMSO): δ (in ppm) = 163.14,
163.08, 135.75, 132.69, 131.57, 131.36, 130.93, 129.92,
129.15, 129.01, 128.85, 128.81, 128.75, 128.26, 123.34,
122.57.
Synthesis
of N-(2-((1,3-Dioxo-2-Phenyl-2,3-Dihydro-1H-Benzo[de]
Isoquinolin-6-yl) Amino) Ethyl)-2-(1H-Imidazol-1-yl)
Acetamide (AN)
Synthesis of 6-((2-Aminoethyl)Amino)-2-Phenyl-1H-Benzo[de]
Isoquinoline-1,3(2H)-Dione (2)
Compound 1 (2.00 g, 5.68 mmol) was dissolved in 5 mL
ethylene glycol monomethyl ether (EGME) and
ethylenediamine (1 mL, 14.9 mmol) was added into the solu-
tion. The mixture was refluxed for 4 h under nitrogen atmo-
sphere. After the reaction was completed, the product was
precipitated by adding ice into the mixture. The sediment
Imidazole (0.20 g, 2.94 mmol) and NaH powder in cata-
lytic amount were dissolved in 10 mL THF at 0 °C and
stirred for 15 min. The compound 3 (0.50 g, 1.22 mmol)
in 15 mL of DMF/THF (1:99) mixing solvent was slightly
added into the mixture solution. The reaction was stirred
overnight under nitrogen atmosphere. The solvent was

J Fluoresc
removed by a rotary evaporator to give a crude product
which then was extracted with CH2Cl2/H2O 3 times. The
organic phase was collected and evaporated under vacuum
to yield crude product. Then, the crude product was recrys-
tallized with methanol to obtained the yellow solid (0.36 g,
67% yield).
138.19, 136.97, 134.71, 131.37, 130.38, 129.68, 129.23,
128.36, 124.96, 124.23, 123.71, 122.89, 120.84, 108.90,
104.28, 50.75, 42.75, 38.19, 36.34.
MALDI-TOF mass: Anal. calcd. For [C26H23N5O3]+
m/z = 454.19, Found m/z = 453.742. ESI-HRMS: Anal.
Calcd for [C26H24IN5O3 + K]+ m/z = 620.06, Found m/z =
619.5627.
FT-IR (ATR) νmax cm−1: 3369 (m, N-H stretching); 3291
(m, N-H stretching); 2919–2849 (m, C-H stretching); 1640 (s,
C=O stretching); 1361 (s, C-H stretching); 1236 (s, aromatic);
1149 (m, C-N stretching,).
Study on Complexed Properties
1H NMR (400 MHz, d6-DMSO): δ (in ppm) = 8.65 (d, J =
8.4 Hz, 1H, ArH), 8.42 (d, J = 7.2 Hz, 1H, ArH), 8.38 (s, 1H,
NH), 8.25 (d, J = 8.4 Hz, 1H, ArH), 7.88 (s, 1H, CONH), 7.71
(t, J = 8.0 Hz, 1H, ArH), 7.56 (s, 1H, ArH), 7.47 (t, J = 3.6 Hz,
2H, ArH), 7.42 (t, J = 7.2 Hz, 1H, ArH), 7.28 (d, J = 6.8 Hz,
2H, ArH), 7.07 (s, 1H, ArH), 6.87 (d, J = 8.4 Hz, 1H, ArH),
6.85 (s, 2H, ArH), 4.67 (s, 2H, CH2), 3.48 (t, J = 2.6 Hz, 2H,
CH2), 3.43 (t, J = 5.4 Hz, 2H, CH2). 13C NMR (100 MHz, d6-
DMSO): δ (in ppm) = 167.55, 163.12, 150.65, 138.10,
136.50, 134.18, 130.81, 129.88, 129.18, 128.69, 128.06,
127.80, 124.40, 122.35, 120.41, 120.31, 103.77, 48.70,
42.36, 37.50.
The test solution was contained 10 μM of AN or ANI sensors
and 100 μM of tetrabutyl ammonium salt of various anions
including F−, Cl−, Br−, I−, CN−, OH−, PO43−, AcO− and BzO−
in DMSO solution. The reaction was stirred for 5 min prior
measurement. Fluorescence spectra were recorded under the
excitation at 448 nm.
Binding Studies
The 1 × 10−3 M of ANI solution and 1 × 10−1 M stock solution
of tetrabutyl ammonium fluoride was prepared in spectropho-
tometric grade DMSO. The test solution contained 1 × 10−5
M
MALDI-TOF mass: Anal. calcd. For [C25H21N5O3]+
m/z = 439.16, Found m/z = 439.678.
of ANI sensors and various amount of tetrabutyl ammonium
fluoride in DMSO solution in the range of 0–5000 equiv. The
20 μL of ANI stock solution was pipetted into a 1-cm quartz
cell and the volume of the stock solution of tetrabutyl ammo-
nium fluoride was various added. After that, the final volume
was adjusted to be 2 mL by DMSO. The reaction was stirred
for 5 min prior to measurement. Fluorescence spectra were
recorded under the excitation at 448 nm.
HRMS (ESI): Anal. Calcd for [C25H21N5O3 + H]+ m/z =
440.18, Found m/z = 440.1364.
Synthesis
of 1-(2-((2-((1,3-Dioxo-2-Phenyl-2,3-Dihydro-1H-Benzo[de]
Isoquinolin-6-yl)Amino)Ethyl)Amino)-2-Oxoethyl)
-3-Methyl-1H-Imidazol-3-Ium (ANI)
NMR Titration Studies
Compound AN (0.581 g, 1.0 mmol) was dissolved in
methanol. Next, methyl iodide (0.1 mL, 1.60 mmol) was
slowly added into the methanol solution and then refluxed
overnight at 60 °C under nitrogen atmosphere. The solu-
tion was evaporated by a rotary evaporator to give a crude
product which was purified by washing with methanol 2–3
times and recrystallized with dichloromethane. The orange
powder was obtained (0.18 g, 50% yield).
The 1 × 10−2 M of ANI solution and 1 × 10−2 M stock so-
lution of tetrabutyl ammonium fluoride was prepared in d6-
DMSO. Various amounts of F− stock solution was added
into the NMR tube by micro syringe from 0 to 4 equiv. and
NMR spectra were collected after each addition of F− ion.
The 80 μL of ANI solution and varied volume of the stock
solution of tetrabutyl ammonium fluoride at 80 μL, 160 μL,
240 μL and 320 μL, respectively, were prepared in the
NMR tube. After that, the final volume was adjusted to be
400 μL by d6-DMSO.
FT-IR (ATR) νmax cm−1: 3297 (m, N-H stretching), 2920–
2849 (m, C-H stretching), 1639 (s, C=O stretching), 1358 (s,
C-H stretching), 1232 (m, aromatic), 1145 (m, C-N
stretching).
1H NMR (400 MHz, d6-DMSO): δ (in ppm) = 9.05 (s, 1H,
-C(2)H-), 8.65 (d, J = 7.2 Hz, 1H, ArH), 8.44 (d, J = 6.8 Hz,
1H, ArH), 8.26 (d, J = 8.4 Hz, 1H, ArH), 7.84 (s, 1H, CONH),
7.72 (t, J = 8.0 Hz, 1H, ArH), 7.68 (s, 1H, ArH), 7.65 (s, 1H,
ArH), 7.48 (t, J = 3.6 Hz, 2H, ArH), 7.41 (t, J = 7.2 Hz, 1H,
ArH), 7.28 (d, J = 6.8 Hz, 2H, ArH), 6.87 (d, J = 8.4 Hz, 1H,
ArH), 4.98 (s, 1H, CH2), 3.87 (s, 3H, CH3), 3.49 (t, J = 5.2 Hz,
2H, CH2), 3.47 (t, J = 4.4 Hz, 2H, CH2). 13C NMR (100 MHz,
d6-DMSO): δ (in ppm) = 166.22, 164.45, 163.63, 151.10,
Naked-Eye Detection
Typically, 10 μM of ANI solution was prepared into a vial.
Various amount of anion stock solution was added by a mi-
cropipette. The solution was stirred for 5 min. Finally, the
solution was adjusted to be 2 mL by 100% DMSO solution.
Subsequently, the solution was taken a photo with Nikon
D550 camera for colorimetric assay. Moreover, the solution

J Fluoresc
was excited under UV lamp at 265 nm and taken a photo with
Samsung Galaxy S5 smartphone for fluorometric assay.
13C-NMR spectra of compounds 1, 2, 3, AN and ANI,
MALDI-TOF and ESI-HRMS data of AN and ANI sen-
sors were shown in Supporting Information.
Results and Discussion
Selectivity Studies of AN and ANI Sensors
Synthesis of AN and ANI Sensors
The selectivity studies of AN and ANI sensors against various
anions including F−, Cl−, Br−, I−, CN−, OH−, PO43−, AcO− and
BzO− were carried out in 100% DMSO solution. As shown in
Fig. 1a, the fluorescence spectra of sensor ANI at 530 nm
were largely quenched in the presence of 10 equiv. of F− ion
under the PET process. For the other anions, the fluorescence
spectra were also slightly changed. Based on a strong base of
F−, it is possibly due to the stronger hydrogen bonding inter-
action between protons at amide group and –C(2)H– signals
and a electrostatic force towards F− ion. These strong interac-
tions induced the large fluorescence quenching.
Meanwhile, the selectivity of AN sensor against various
anions was also determined by using fluorescence spectrosco-
py. The results showed that no changes of fluorescent re-
sponses were observed in the presence of any anions as shown
in Fig. 1b. It suggested that AN sensor is unable to bind with
any anions through hydrogen bonding although there are am-
ide group in its structure. This result suggested that the posi-
tive charge and strong acid proton on C2 played a crucial role
for anion binding affinity, especially F− ion. To easily under-
stand, the normalized fluorescence intensities (I/I0) of ANI
and AN sensors with various anions were compared in the
Fig. 1c. The black bar and the red bar, defined as I/I0 of ANI
and AN sensors, respectively, exhibited the large difference of
AN and ANI sensors were synthesized following to
Scheme 1. They started from 4-bromo-1, 8-naphthalic
anhydride to give compound 1 by using amide conden-
1
sation [20, 21]. The H-NMR spectrum showed the char-
acteristic aromatic region around 7.20–7.50 ppm
assigned to aromatic protons of aniline ring. Next, com-
pound 1 was coupled with ethylenediamine in ethylene
glycol monomethyl ether (EGME) as a solvent to obtain
compound 2 [22–25]. Moreover, the ethylene protons
were observed at 3.14 and 2.86 ppm which confirmed
the structure of compound 2. Compound 3 was prepared
from compound 2 and chloroacetyl chloride under low
1
temperature [26]. H-NMR spectrum showed the charac-
teristic peaks of amide proton and aliphatic protons at
7.80 ppm and 4.10 ppm, respectively. Subsequently,
compound 3 was attacked by imidazole compound under
1
a catalytic amount of NaH to give AN sensor [15]. H-
NMR spectrum showed the new signals of imidazole
moiety in the range of aromatic region. Finally, the AN
sensor was methylated with methyl iodide (CH3I) to
yield ANI sensor [27]. 1H-NMR spectrum showed the
1
new peaks of methyl proton at 3.87 ppm. H-NMR and
Scheme 1 The synthesis pathway of AN and ANI sensors: (i) pyridine, CH3OH, reflux 60 °C, (ii) EGME, reflux 120 °C, (iii) Na2CO3, CH2Cl2, 0 °C,
(iv) NaH, THF, DMF, 0 °C, (v) CH3OH, reflux 60 °C

J Fluoresc
Fig. 1 Fluorescence spectra of a) ANI sensor (10 μM) b) AN sensor 10 μM) and c) intensity bars of host (ANI and AN) upon addition of various anions
including F-, Cl-, Br-, I-, CN-, OH-, PO43-, AcO- and BzO- (10 equiv) in 100% DMSO solution (λex = 448 nm)
black and red bars in case of F− anion implying that only
sensor ANI preferentially bound with F− ion through electro-
static interactions of the positive charge of imidazolium moi-
ety and the additional hydrogen bonding of -C(2)H- and F−
ion.
investigated by Job’s plot analysis as shown in Fig. S15. The
plot of (I0-I)(1-x) versus mole fraction of F− ion showed the
highest point of the graph at 0.5 mol of F− anion implying that
the binding mode of ANI sensor and F− ion was 1:1
stoichiometry.
1
Deeply understanding the binding mechanism, the H
Binding Affinities Studies
NMR titration in d6-DMSO was employed to verify the inter-
action between ANI sensor and F− ion. Figure 3 displayed the
changes of chemical shift of ANI sensor in the presence of F−
ion between 0 to 4.0 equiv. It showed the disappearance of the
acidic amide proton (red dot) at δ = 7.84 ppm and downfield
shift of -C(2)H- position (green dot) at δ = 9.05 ppm in the
presence of F− ion.
Moreover, the new triplet peak at δ = 16.00 ppm was de-
veloped corresponding to the H2F+ [28–30]. This species was
possibly generated by the excess amount of F− ion which
To investigate the binding properties, the binding constant of
ANI sensor and F− ion was examined by fluorescence titration
method. The fluorescence spectra between ANI sensor with
various amounts of F− ion were shown in Fig. 2. The fluores-
cence intensity of ANI sensor was gradually decreased upon
the increment of F− ion from 0 to 5000 equiv. The log Ks value
of ANI sensor toward F− ion was 3.05. Moreover, the stoi-
chiometric ratio between ANI sensor and F− ion was further

J Fluoresc
Fig. 2 a) Fluorescence spectral titration of ANI sensor (10 μM) in the presence of different amounts of F- ion (0 – 5000 equiv) in 100% DMSO solution.
b) Fluorescent titration curves of ANI sensor in the presence of F- ion
deprotonated the acidic protons at amide position. Definitely,
the observation of the downfield shift of these signals and
further deprotonation upon adding excess F− ion could con-
firmed that the amide proton and the acidic proton of C2
enabled to interact with F− anion.
Naked Eye Detection
For Extensive works, the use of ANI sensor for visual detec-
tion was examined in DMSO. Firstly, ANI sensor (10 μM)
was titrated with various anions including F−, Cl−, Br−, I−,
Fig. 3 NMR titration spectra of ANI sensor (100 mM) upon addition of various concentration of F- ion including 0 equiv, 0.5 equiv, 1.0 equiv, 2.0 equiv
and 4.0 equiv

J Fluoresc
Fig. 4 Colorimetric assay of ANI sensor (10 μM) responses upon addition of vary concentration (0-900 equiv) of various anions including F-, Cl-, Br-, I-,
CN-, OH-, PO43-, AcO- and BzO-
CN−, OH−, PO43−, AcO− and BzO− in the concentration range
from 0 to 9.0 mM. Interestingly, the colour of ANI sensor was
changed in the presence of F− or OH− ion. In the case of F−
ion, the colour of ANI sensor was gradually changed from
green to purple at the concentration of F− ion over 200 equiv
(2 mM). This phenomenon can be explained that the depro-
tonation of the acidic proton of ANI sensor by F− ion provided
the unstable intermediate species. In similar feature, the case
of OH− ion induced the gradually colour change from green to
purple at the concentration of OH− ion over 600 equiv
(6 mM). All of other anions did not induce the colour change
of ANI sensor as shown in Fig. 4. Definitely, ANI sensor can
be served as naked-eye detection for F− ion at 200 equivalents
(631.02 ppm) without the interference of hydroxyl anion.
Apart from the fluorescent properties of naphthalimide unit
in ANI which performed the characteristic of a strong green
brightness, the visual fluorometric assay with various anion
was also investigated in DMSO.
Fig. 5 Fluorometric assay of sensor ANI (10 μM) responses upon the addition of vary concentration (0-900 equiv) of various anions including F-, Cl-,
Br-, I-, CN-, OH-, PO43-, AcO- and BzO- (λexc = 265 nm)

J Fluoresc
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Upon adding the various anions in concentration range
from 0 to 900 equiv (0–9.0 mM.) in the ANI sensor, and
exposed under UV lamp at 265 nm in the black box, the
results showed that the green brightness of luminescence of
ANI sensor was gradually quenched upon the addition of F−
ion at 200 equiv. (2 mM). Possibly, PET process occurred
upon the formation of ANI⊂F− complex. Fortunately, ANI
sensor offered the benefit of the fluorometric assay of F− ion
determination in the range of 200–900 equiv. (2.0–9.0 mM).
On the other hand, the other anions did not exhibit the bright-
ness changes of ANI sensor except OH− ion as shown in
Fig. 5. The brightness of ANI sensor was largely decreased
after adding 600 equiv. (6.0 mM) of OH− ion. The excess OH−
ion might deprotonate the acidic proton of ANI sensor to
provide the unstable complex. Therefore, ANI sensor
highlighted an effectively promising selectivity for F− without
the interference of OH− in the range of 2–5 mM by dual
detection of colorimetric and fluorometric assays.
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Conclusion
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New fluorescence sensors based on imidazole unit including
AN and ANI sensors have been successfully synthesized. The
fluorescence spectra of ANI sensor showed a large decrease
upon binding with F− ion. The ANI sensor forms complex
with F− ion in a 1:1 stoichiometry and the log K value of this
complex is 3.05. With quantitative analysis, the limit of detec-
tion and limit of quantification of ANI sensor toward F−. were
0.18 mM and 0.60 mM, respectively. According to the NMR
spectrum of ANI sensor, the excess F− ion exhibited the grad-
ual downfield shift and disappearance of amide position and
the proton of H2F+ appeared at 16 ppm. This result confirmed
that interactions of ANI sensor and F− through the electrostat-
ic and hydrogen bonding interactions of C2 acidic proton in
the ANI sensor base. Particular in advantage of ANI sensor
provided the visual determination of F− ion. ANI is the alter-
native candidate for detection of F− in organic solvent and
further development for aqueous system is continue.
Acknowledgments This work is financial supported from Thailand
Research Fund (RSA6080012 and RTA6080005) and J.K. gratefully ac-
knowledges the scholarship from the Graduate School, Chulalongkorn
University to commemorate 72nd anniversary of his Majesty King
Bhumibala Aduladeja.
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