π-Conjugated molecules identified for reversible and visual detection of F? in aqueous: Effect of heterocycle unit on sensing performance

Article abstract of DOI:10.1016/j.saa.2019.117451

To illustrate the impact of molecular structure, especially heterocycle unit, on the sensing performance, two kinds of π-conjugated molecules containing aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2) were identified and compared each other. Even with similar structures, they possessed quite different spectral and colorimetric responses to F^?, Cl^?, Br^?, I^?, NO₃ ^?, HSO₄ ^?, H₂PO₄ ^?, ClO₄ ^? and CH₃COO^?, etc. The reason might result from the difference in withdraw-electron ability of aromatic and heterocyclic rings, which would lead to different acidity of active H in the target π-conjugated molecules. In acidic aqueous, Dye 1 expressed a reversible ratiometric-colorimetric response to F^?, accompanying with a visual color change from bright yellow to purple, a nice linear range of 2.0–35.0 × 10^?6 mol/L and a low detection limit of 1.60 × 10^?7 mol/L. While Dye 2 did not react with any anion due to its weak acidity of active hydrogen. Under the optimized conditions, Dye 1 was successfully applied for colorimetric or naked-eye detection of F^? in environmental water, tea and toothpaste samples with RSD ≤ 3.1%. The recognition mechanism for Dye 1 to F^? was confirmed to be deprotonation one with a 1:1 binding stoichiometry.

Full text of DOI:10.1016/j.saa.2019.117451

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
π-Conjugated molecules identified for reversible and visual detection of
F− in aqueous: Effect of heterocycle unit on sensing performance
⁎
⁎
Zhengquan Yan , Cong Wang, Yulian Tang, Yanjie Zhu, Qianying Cao, Tianran Yang, Lei Hu
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2019
Received in revised form 26 July 2019
Accepted 3 August 2019
Available online 05 August 2019
To illustrate the impact of molecular structure, especially heterocycle unit, on the sensing performance, two kinds
of π-conjugated molecules containing aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2) were identified
and compared each other. Even with similar structures, they possessed quite different spectral and colorimetric
responses to F−, Cl−, Br−, I−, NO3−, HSO−4 , H2PO−4 , ClO4− and CH3COO−, etc. The reason might result from the dif-
ference in withdraw-electron ability of aromatic and heterocyclic rings, which would lead to different acidity of
active H in the target π-conjugated molecules. In acidic aqueous, Dye 1 expressed a reversible ratiometric-
colorimetric response to F−, accompanying with a visual color change from bright yellow to purple, a nice linear
range of 2.0–35.0 × 10−6 mol/L and a low detection limit of 1.60 × 10−7 mol/L. While Dye 2 did not react with any
anion due to its weak acidity of active hydrogen. Under the optimized conditions, Dye 1 was successfully applied
for colorimetric or naked-eye detection of F− in environmental water, tea and toothpaste samples with RSD
≤ 3.1%. The recognition mechanism for Dye 1 to F− was confirmed to be deprotonation one with a 1:1 binding
stoichiometry.
Keywords:
Heterocycle π-conjugated system
Reversible and visual sensing
F−
Relationship between structure and perfor-
mance
Recognition mechanism
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
color change from greenish yellow to blue in the presence of F−. Our
group [38] also developed a D-A-D typed conjugated chromophore
To efficiently construct optical sensors with obvious colorimetric
change and revisable sensing property has been attracting more and
more attention for on-site monitoring environmental pollution, due to
their simple and convenient sensing processes [1–6]. For this, a plethora
of multifunctional organic molecules with different structures and tun-
able optical properties have been developed to monitor various
analytes, such as heavy metal ions [7–9], anions [10–15], organic pollu-
tions [16–20] and toxic gases [15,21–23]. Among all the analytes,
fluorion (F−) is often be ignored in our daily life for it is widely utilized.
People, who assimilate the excess or deficient F−, will suffer from the
diseases such as kidney failure, collagen breakdown, bone disorders, im-
mune system disruption, gastric and kidney disorders, etc. [24–29]
Therefore, it is urgent for scientific workers nowadays to develop
some effective probes to monitor F− on-site [30–36]. For example,
based on a diarylethene combining to a 1,8-naphthalimide Schiff base,
Fu, et al. [37], developed a typical colorimetric and fluorescent dual-
mode sensor for F−, which exhibited high selective and prominent
(PBHPI) and its functionalized testing-paper strips for on-site F− detec-
tion with obvious color change from light yellow to dark red and a low
detection limit of 7.8 × 10−7 mol/L. However, the proposed PBHPI could
only be applied in pure organic phase, which will limit it to be applied in
practice.
Recently, Sharma, et al. [39], reported a selective colorimetric sensor
(Dye 2), combining 2-hydroxy-1-naphthaldehyde with 2-aminophenol,
for F− detection in mixed solvent of 1% H2O: 99% CH3CN. However, the
characterized absorption peak for F− was just a weak shoulder one,
which led to not high sensitivity. By changing 2-aminophenol into 2-
amino-3-hydroxypyridine (Dye 1), Ding, et al. [40], developed selective
fluorescent sensor for Al3+ in DMSO. However, the colorimetric re-
sponse of Dye 2 to anions in aqueous has not been investigated and re-
ported. What's more, the impact of the molecular structure such as
introduce of heterocycle unit on the sensing property (Dye 1 and Dye
2), as far as we known, has not been investigated and compared so far.
With this aim in mind, two kinds of π-conjugated molecules con-
taining aromatic heterocyclic (Dye 1) and aromatic ring (Dye 2,
Scheme 1) were identified in this work, whose UV–vis absorption spec-
tral property, especially colorimetric responses to different anions were
⁎
Corresponding authors.
E-mail addresses: yanzhq2008@163.com (Z. Yan), kaixinguolei333@163.com (L. Hu).
https://doi.org/10.1016/j.saa.2019.117451
1386-1425/© 2019 Elsevier B.V. All rights reserved.

2
Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
Scheme 1. The synthetic route of the target dyes (Dye 1 and Dye 2) and the proposed chelating mode with F−
.
investigated and compared in detail. It is expected the resulting rela-
tionship between molecular structure and sensing performance will
provide a new insight for designing colorimetric sensors with desirable
property in the future.
(TMS) as the reference and DMSO d6 as solvent. Mass spectral analyses
were recorded on a GCMS-QP2010 mass spectrometer (Shimadzu Bio-
tech, Japan). Concentrations of total F− were also determined using an
Inductively Coupled Plasma Mass Spectrometer (ICPMS) (Perkin-
Elmer, Elan DRC Plus). UV–vis spectra were conducted on a Lambda
35 UV/Vis spectrometer using a 1-cm square quartz cell.
2. Experimental
2.1. Chemicals and apparatuses
2.2. Syntheses of Dye 1 and Dye 2
All tetrabutylammonium salts of AR grade were purchased from
Acros Chemical Reagent Co. Ltd., and the other analytical reagents uti-
lized in the work were gotten from Sinopharm Chemical Reagent Co.
Ltd. All the organic solvents were redistilled and dried before use.
Water used throughout was doubly deionized one.
Dye 1 was prepared using 2-hydroxyl-1-naphthaldehyde and 2-
amino-3-hydroxyl pyridine as raw materials. In brief, after 0.688 g
(4.0 mmol) 2-hydroxyl-1- naphthaldehyde was dissolved in 50.0 mL
absolute ethanol and acidified with 1.6 mL glacial acetic at room tem-
perature, 0.444 g (4.0 mmol) 2-amino-3-hydroxyl pyridine in 30.0 mL
ethanol was dropped into. The mixture was then refluxed at 85 °C for
another 4.0 h under stirring conditions. After it was cooled to room tem-
perature, the resultant precipitate was filtered and purified by recrystal-
lizing three times from ethanol to provide Dye 1 in the yield of 92.5%. 1H
NMR (400 MHz, DMSO d6, δ): 15.72 (s, 1H, H-C=N), 10.28 (s, 1H, -OH),
9.48 (s, 1H, -OH), 8.39 (d, J = 7.7 Hz, 1H, Ar\\H), 7.94 (d, J = 7.7 Hz, 1H,
Ar\\H), 7.81 (d, J = 7.8 Hz, 1H, Ar\\H), 7.69 (d, J = 7.8 Hz, 1H, Ar\\H),
7.49 (t, J = 7.6 Hz, 1H, Ar\\H), 7.27 (t, J = 7.6 Hz, 1H, Ar\\H), 7.11 (t, J
= 7.5 Hz, 1H, Ar\\H), 7.01 (t, J = 7.7 Hz, 1H, Ar\\H), 6.96 (t, J = 7.5 Hz,
1.0 mmol/L tetrabutylammonium salt standard solutions were pre-
pared freshly with CH3CN at room temperature and diluted to the de-
sired concentration when used. The present dyes, Dye 1 and Dye 2
were synthesized in our lab, whose stock solutions were prepared in
DMSO with 2.0×10–4 mol/L and stored at 4 °C. NaOH and HCl standard
aqueous solutions were freshly prepared in the same concentration of
0.01 mol/L.
13C NMR and 1H NMR spectra were recorded using a Bruker AVANCE
III HD spectrometer operating at 500 MHz, using tetramethyl-silane
Fig. 1. UV–vis spectral and visible response of a, c) Dye 1 and b, d) Dye 2 toward different anions (Cl−, Br−, I−, HSO−4 , NO−3 , ClO−4 , H2PO−4 , CH3COO– or F−).

Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
3
Fig. 2. Effect of water-content (v/v%) on the absorption intensity of a) Dye 1 at 530 nm and b) Dye 2 at 525 nm with or without F−, CH3COO– or H2PO4−
.
1H, Ar\\H), 6.77 (d, J = 7.5 Hz, 1H, Ar\\H). 13C NMR (400 MHz,
DMSO d6) δ: 109.1, 115.7, 118.1, 119.5, 120.7, 122.5, 123.4, 127.6,
128.3, 128.8, 129.3, 133.8, 134.7, 143.1, 149.7, 155.3, 176.1. MS: m/z
[M + H]+ 264.512 (theoretical value: 265.261).
Dye 2 was synthesized using 2-hydroxyl-1-naphthaldehyde and 2-
amino phenol as raw materials instead with the yield of 89.4%.
1H NMR (400 MHz, DMSO d6, δ) 15.17 (s, 1H, H-C=N), 10.90 (s, 1H,
-OH), 9.48 (s, 1H, –OH), 8.11 (d, J = 7.8 Hz, 1H, Ar\\H), 7.98 (d, J =
7.8 Hz, 1H, Ar\\H), 7.82 (d, J = 7.8 Hz, 1H, Ar\\H), 7.68 (d, J = 7.8 Hz,
1H, Ar\\H), 7.51 (t, J = 8.0 Hz, 1H, Ar\\H), 7.37 (d, J = 7.5 Hz, 1H,
Ar\\H), 7.28 (t, J = 7.5 Hz, 1H, Ar\\H), 7.15 (t, J = 7.7 Hz, 1H, Ar\\H),
6.73 (d, J = 7.7 Hz, 1H, Ar\\H).MS: m/z [M + H]+ 264.112 (theoretical
value: 263.325).
For two kinds of tea aqueous samples, after 0.10 g dry tea sample
was grinded into powder, 10.0 mL H2O was added and refluxed for
10.0 min under stirring condition. The resultant mixture was filtered
by 0.45 μ microporous membrane and the filtrate was collected directly
for subsequent UV–vis spectral or colorimetric analyses.
3. Results and discussion
3.1. Colorimetric response of Dye 1 and Dye 2 to different anions
UV–vis spectral responses of Dye 1 and Dye 2 toward different an-
ions, such as F−, Cl−, Br−, I−, CH3COO−, H2PO−4 , HSO−4 , NO3− and ClO−4
in the form of tetrabutylammonium salts were first investigated in
CH3CN-DMSO (V/V, 1/9) respectively. For Dye 1 in CH3CN-DMSO (V/V,
1/9), there is a strong double absorption peak at 448 nm and 473 nm
with ε = 2.217 × 104 L·mol−1·cm−1 and 2.359 × 104 L·mol−1·cm−1
2.3. Preparation of toothpaste and tea aqueous samples
(Fig. 1a). Upon addition of Cl−, Br−, I−, HSO−4 , NO−3 , ClO4−, H2PO−4
,
,
Two kinds of toothpaste samples purchased in a local market near
Qufu Normal University, whose aqueous samples were prepared as
below: Take 1.00 g toothpaste sample and dissolve it in 10.0 mL H2O
under ultrasonic condition. The resultant dispersion was filtered by
0.45 μ microporous membrane for subsequent UV–vis spectral or color-
imetric analyses.
CH3COO– or F−, three kinds of anions, i.e., F−, CH3COO– and H2PO−4
would make its UV–vis absorption intensity at 448 nm decreased
greatly, accompanying with a new absorption peak emerging at
530 nm. Especially, the color of Dye 1 in CH3CN-DMSO (V/V, 1/9)
changes obviously from yellow to brown or purple in the presence of
F−, CH3COO– and H2PO−4 (Fig. 1c), which could be discriminated by
naked eyes. While for Dye 2 (Fig. 1b), there is a double absorption
peak at 449 nm and 472 nm with the molar absorption coefficient (ε)
of 1.842 × 104 L·mol−1·cm−1 and 1.905 × 104 L·mol−1·cm−1 in
CH3CN-DMSO (V/V, 1/9). Only F− and CH3COO– could make its UV–vis
Fig. 3. Effect of HCl content on a) the absorption intensity at 530 nm; b) the color of Dye 1
in the presence of different anions with addition of 15.0 μL 0.01 mol/L HCl.
Fig. 4. Time response curve of Dye 1 to F− measured at 530 nm in 5.0% H2O-10% CH3CN-
85% DMSO (cDye 1 = 2.0 × 10−5 mol/L, c−F = 1.0 × 10−5 mol/L, 15.0 μL HCl).

4
Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
0, 1.0%, 2.0%, 3.0%, 5.0% to 10%. As shown in Fig. 2a, with increasing ad-
dition of H2O even to 10% v/v, there is still an obvious absorption at
530 nm for Dye 1 in the presence of either F− or CH3COO−. Addition
of H2O could not differentiate F− from CH3COO−, owing to the fact
that the acidity of active hydrogen atoms in Dye 1 are quite strong,
which will react with F− or CH3COO– preferentially against H2O. For
Dye 2, the weak peak at 525 nm in the presence of F− or CH3COO– de-
creases to nearly disappear upon addition of 5.0% v/v H2O. Accordingly,
5% v/v H2O was selected for all the subsequent experiments.
According to the mechanism deduced above, pH effect was investi-
gated on the interaction between Dye 1 and F− or CH3COO– by addition
of 0.01 mol/L HCl aqueous solution accompanying with the color
changes (Fig. 3). As shown in Fig. 3, with the addition of HCl, the absorp-
tion peak at 530 nm will decrease in the presence of either F− or
CH3COO−. When the volume of 0.01 mol/L HCl aqueous solution
reaches 15.0 μL, the absorption at 530 nm disappears with the color re-
covery to bright yellow (Fig. 3b) in the presence of CH3COO−, whose ab-
sorption intensity is b5% relative to that in the presence of F−. The
reason might be the high bonding energy between free H+ and
CH3COO−. Therefore Dye 1 will be a promising colorimetric sensor for
selectively sensing F− in aqueous solution in the weak acidic conditions
(15.0 μL 0.01 mol/L HCl diluted into 10 mL).
Fig. 5. The UV–vis spectra of Dye 1-F− complex upon periodical addition of 0.01 mol/L
NaOH or 0.01 mol/L HCl from 0 to 1.0 equiv.
absorption intensity decreased with a weak peak emerging at 525 nm
and a weak color change from yellow to brown or light brown (Fig. 1d).
The results suggest that there is a stronger interaction between Dye
1 and F−, CH3COO– or H2PO4− than that between Dye 2 and F−, CH3COO–
or H2PO−4 . With the introduction of electron-withdrawing pyridine het-
erocycle unit, the ability of push-pull electrons increases from Dye 2 to
Dye 1, which results in the corresponding ε increasing from 1.905 × 10-
4 L·mol−1·cm−1 for Dye 2 to 2.359 × 104 L·mol−1·cm−1 for Dye 1. Also,
the introduction of electron-withdrawing pyridine heterocycle unit will
lead to stronger acidity of active hydrogen atoms in Dye 1 than that in
Dye 2, which are easier to chelate with anions such as F−, CH3COO– or
H2PO−4 . Accordingly, introduction of electron-withdrawing heterocycle
unit will help to improve the UV–vis spectral and visible response ability
to different anions.
The response rate and stability of Dye 1 to F− was illustrated by re-
cording the absorption intensity at 530 nm within 1 h upon addition
of F− in 5.0% H2O-10% CH3CN-85% DMSO (Fig. 4). From Fig. 4, it is
easy to find that the present Dye 1 takes on a fast response of
b4.0 min to F−, which could keep no changed any more in the following
tested time, suggesting that the sensing process of Dye 1 to F− is quick
and reliable.
3.2. Response reversibility
To express the reversibility for Dye 1 to sense F−, the UV–vis absorp-
tion spectra of Dye 1-F− complex were recorded with periodical addi-
tion of 0.01 mol/L NaOH or 0.01 mol/L HCl from 0 to 1.0 equiv. As
shown in Fig. 5, the absorption intensity at 530 nm for Dye 1-F−
To improve the selectivity and the practical applicability, the effect of
water content was investigated by changing H2O volume fraction from
Fig. 6. a) The absorption spectra of Dye 1 upon addition of F− with different concentrations (From up to down: 0, 1, 2, 3, 5, 7.5, 10, 12.5, 15, 20, 25, 30, 35 × 10−6 mol·L−1); b) The linear
relationship between ln(A530)/ln(A448) and c−F ; c) The corresponding color change of Dye 1 in presence of F− with different concentrations; d) The contrasting color of Dye 1 before and
after addition of commercial toothpaste of its aqueous qualitatively.

Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
5
Table 1
Determination results for environmental samples (n = 5).
Samples
c−F in sample (10−6 mol/L)
Spiked (10−6 mol/L)
Found (c−F , 10−6 M)
c−F by ICP- MS(10−6 M)
Recovery (%)
R.S.D. (%)
Water sample 1
Water sample 1
Tea 1
Tea 2
Toothpaste 1
Toothpaste 2
0.00
0.00
2.66
2.37
7.80
1.69
10.00
10.00
10.00
10.00
10.00
10.00
9.85
9.98
13.12
12.15
18.92
12.24
10.00
10.02
13.28
12.66
18.08
13.15
98.5
99.8
103.6
98.2
106.3
104.7
1.1
1.5
2.6
3.1
1.9
2.3
complex decreases gradually with increasing addition of H+, accompa-
nying with the original double absorption at 447 nm and 473 nm in-
creasing until they recover completely. On the contrary, upon addition
of OH−, the absorption at 530 nm for Dye 1-F− complex will come
into being again. All the phenomena indicate that the colorimetric re-
sponse of Dye 1 to F− processes excellent reversibility.
samples directly. As shown in Fig. 6d and Fig. S1, upon addition of fluo-
rinated samples, even original solid toothpaste, the color of Dye 1 solu-
tion will change into brown or purple from bright yellow, i.e., Dye 1
could be applied for qualitative analysis of F− by naked eyes.
Dye 1 was also applied for quantitatively sensing F− in some envi-
ronmental water samples, tea samples and toothpaste samples by stan-
dard spiked method under the optimized conditions. To obtain the
percent recovery, a known concentration of F− (10.0 × 10−6 mol/L)
was added into and the resulting total F− concentration was deduced
from the linear regression equation, ln(A530)/ln(A448) = 0.0516c +
0.1880 and ICP-MS as well (Table 1). From Table 1, we could find that
all the recoveries of the spiked F− in enviromental water, toothpaste
and tea samples are between 98.2% and 106.3% with a relative standard
deviation (R.S.D.) of b3.1%. Especially, the total F− contents deduced
from the linear regression equation are quite consistent to those from
ICP-MS, confirming its practical applicability for sensing F− in some nat-
ural samples.
3.3. Analytical parameters
A series of UV–vis spectral titrations accompanying with color
changes were recorded in 5.0% H2O-10% CH3CN-85% DMSO by increas-
ing c−F from 2.0 × 10−6 mol/L to 3.5 × 10−5 mol/L under the optimized
conditions above.
As shown in Fig. 6a, the absorption intensity of Dye 1 at 448 nm de-
creases accompanying with the one at 530 nm increasing upon addition
of F−. The linear calibration curve, the limit of detection (LOD) and the
detection precision for F− were deduced from Fig. 6b. The linear regres-
sion equation is ln(A530)/ln(A448) = 0.0516c + 0.1880 in the range of cF−
1.0–35.0 × 10−6 mol/L with a correlation coefficient (R2) of 0.9912,
where A530 and A448 are the absorption intensity at 530 nm and
448 nm respectively. According to the definition of LOD, three times of
average deviation of ln(A530)/ln(A448) in 20 blank samples without F−
used here [41,42], the LOD for F− detection is up to 1.60
× 10−7 mol/L, far below the permissible limit of 7.89 × 10−5 mol/L F−
in drinking water definitized by the World Health Organization
(WHO) [34,43]. Also from Fig. 6c, we could find that the color of Dye 1
will change gradually from bright yellow to brown, and further to pur-
ple with increasing F− concentration, suggesting that Dye 1 would be
a potential colorimetric sensor for both visual and spectral detection
of F− in aqueous.
3.5. Recognition mechanism
To quantify the binding stoichiometry of Dye 1 with F−, Job's plot
was drawn by recording the absorption intensity against F− mole frac-
tion. For this, series of solution with changing F− mole fraction from 0
to 1 were prepared while maintaining the total concentration of Dye 1
and F− at 3.0 × 10−5 mol/L. The absorption intensity at 530 nm was re-
corded, which was compared with F− mole fraction (Fig. 7). From Fig. 7,
a 1:1 stoichiometry could be concluded between Dye 1 and F− with a
proposal chelating mode seeing in Scheme 1.
Further, 1H NMR titration experiments were carried out for Dye 1
upon addition of different equiv. of F− in DMSO d6. As shown in Fig. 8,
there are two sharp signals at 15.71 and 10.29 ppm, attributed to the hy-
drogen atoms in HC_N and HO- in pyridine ring, respectively. The
3.4. Application in practice
To illustrate its potential applicability, Dye 1 was first applied for
naked-eye determining F− in commercial toothpaste and its aqueous
Fig. 7. Job's plots for Dye 1 and F− (The total concentration kept at 3.0 × 10−5 mol/L).
Fig. 8. 1H NMR spectral titration of Dye 1 with different equiv. of F− in DMSO d6.

6
Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
Fig. 9. The optimized structures, total dipole moments, energy levels and electronic cloud density of Dye 1 in the absence/presence of F−
.
relative intensities of both the signals become weakened to vanish with
addition of F− from 0.0 to 1.0 equiv. There is no obvious change ob-
served for other peaks, suggesting Dye 1 chelates F− by two binding
sites, the hydrogen atoms in HC_N and HO- in pyridine ring with a
1:1 stoichiometry as proposed in Scheme 1.
applicability in aqueous owing to the increasing acidity of active hydro-
gen atoms. Under the optimized conditions (5.0% H2O-10% CH3CN-85%
DMSO, cDye1 = 2.0 × 10−5 mol/L and 15.0 μL HCl), Dye 1 exhibited a
highly reversible colorimetric and visible response toward F− with ob-
vious color change from bright yellow to purple and a low detection
limit of 1.60 × 10−7 mol/L, far below the permissible limit of 7.89
× 10−5 mol/L F− in drinking water definitized by WHO. When applied
for detection of F− in real environmental water, tea and toothpaste sam-
ples, all the relative standard deviations (R.S.D.) were b3.1%. By 1H NMR
titration, Job's plot, and theoretical calculation, the deprotonation recog-
nition mechanism was confirmed with a 1:1 binding mode between
Dye 1 and F−. The work will provide a good in-sight for effectively con-
structing optical sensors with improved sensing property in the future.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2019.117451.
To intuitively understand the bonding nature between Dye 1 and F−,
theoretical calculation was implemented in the Gaussian 03 suite of
program using the B3LYP/6-31G level of theory (Fig. 9). [44] From
Fig. 9, it is easy to find that the bonding distance between F− and H in
HC_N or HO- in pyridine ring is 1.67 Å or 2.01 Å, hinting there are
strong hydrogen bonding interactions among these atoms in the opti-
mized structure of Dye 1. What's more, upon addition of F−, the molec-
ular dipole moment increases from 2.9976 D to 4.6021 D, accompanying
with a decreasing HOMO-LUMO energy gap (ΔE) from 3.6548 eV to
3.2841 eV, which well accounts for the long-wavelength shift from
448 nm to 530 nm in absorption spectra in the presence of F−.
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
4. Conclusion
In conclusion, two kinds of π-conjugated molecules with and with-
out aromatic heterocycle unit (Dye 1 and Dye 2) were synthesized,
whose colorimetric and UV–vis spectral response to different anions
were investigated and compared in detail. With introduce of aromatic
The authors gratefully acknowledge the financial supports from Na-
tional Natural Science Foundation of China (Nos: 21806089, 21277103)
and Shandong Provincial Natural Science Foundation (Nos:
ZR2018MB038, ZR2016BQ13).
heterocycle, Dye 1 possessed more excellent sensing property to F−
,
such as improved selectivity, sensitivity and reversibility, especially

Z. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117451
7
[23] Q. Wang, W.J. Wang, S.Q. Li, J. Jiang, D.X. Li, Y. Feng, H.T. Sheng, X.M. Meng, M.Z. Zhu,
X. Wang, A mitochondria-targeted colorimetric and two-photon fluorescent probe
for biological SO2 derivatives in living cells, Dyes Pigments 134 (2016) 297–305.
[24] Y.P. Li, S.L. Wang, Z.R. Nan, F. Zang, H.L. Sun, Q. Zhang, W. Huang, L.L. Bao, Accumu-
lation, fractionation and health risk assessment of fluoride and heavy metals in soil-
crop systems in Northwest China, Sci. Total Environ. 663 (2019) 307–314.
[25] L.W. Su, D.J. Lin, J.Y. Uan, Novel dental resin composites containing LiAl-F layered
double hydroxide (LDH) filler: fluoride release/recharge, mechanical properties,
color change, and cytotoxicity, Dental Mater 35 (2019) 663–672.
References
[1] T. Rasheed, C.L. Li, Y.L. Zhang, F. Nabeel, J.X. Peng, J. Qi, L.D. Gong, C.Y. Yu,
Rhodamine-based multianalyte colorimetric probe with potentialities as on-site
assay kit and in biological systems, Sensors Actuators B Chem. 258 (2018) 115–124.
[2] Y.J. Kim, H.S. Kim, J.W. Chon, D.H. Kim, J.Y. Hyeon, K.H. Seo, New colorimetric
aptasensor for rapid on-site detection of Campylobacter jejuni and Campylobacter
coli in chicken carcass samples, Anal. Chim. Acta 1029 (2018) 78–85.
[3] X.C. Li, F. Yang, J.X.H. Wong, H.Z. Yu, Integrated smartphone-app-chip system for on-
site parts-per-billion-level colorimetric quantitation of aflatoxins, Anal. Chem. 89
(2017) 8908–8916.
[26] D.J. Kang, X.L. Yu, S.R. Tong, M.F. Ge, J.C. Zuo, C.Y. Cao, W.G. Song, Performance and
mechanism of Mg/Fe layered double hydroxides for fluoride and arsenate removal
from aqueous solution, Chem. Eng. J. 228 (2013) 731–740.
[4] P. Joshi, R. Painuli, D. Kumar, Label-free colorimetric nanosensor for the selective on-
site detection of aqueous Al3+, ACS Sustain. Chem. Eng. 5 (2017) 4552–4562.
[5] L. Hu, B.H. Zhu, L. Zhang, H. Yuan, Q. Zhao, Z.Q. Yan, Chitosan-gold nanocomposite
and its functionalized paper strips for reversible visual sensing and removal of
trace Hg2+ in practice, Analyst 144 (2019) 474–480.
[27] Y. Zhou, J.F. Zhang, J. Yoon, Fluorescence and colorimetric chemosensors for
fluoride-ion detection, Chem. Rev. 114 (2014) 5511–5571.
[28] L. Chen, K.S. Zhang, J.Y. He, W.H. Xu, X.J. Huang, J.H. Liu, Enhanced fluoride removal
from water by sulfate-doped hydroxyapatite hierarchical hollow microspheres,
Chem. Eng. J. 285 (2016) 616–624.
[29] B.S. Zhu, Y. Jia, Z. Jin, B. Sun, T. Luo, X.Y. Yu, L.T. Kong, X.J. Huang, J.H. Liu, Controlled
synthesis of natroalunite microtubes and spheres with excellent fluoride removal
performance, Chem. Eng. J. 271 (2015) 240–251.
[30] Y.M. Shen, X.Y. Zhang, Y.Y. Zhang, H.T. Li, Y.D. Chen, An ICT-modulated strategy to
construct colorimetric and ratiometric fluorescent sensor for mitochondria-
targeted fluoride ion in cell living, Sensors Actuators B Chem. 258 (2018) 544–549.
[6] Z.Q. Yan, L. Hu, J.M. You, Sensing materials developed and applied for bio-active Fe3
+ recognition in water environment, Anal. Methods 8 (2016) 5738–5754.
[7] L. Xing, X.Y. Zheng, W.Y. Sun, H. Yuan, L. Hu, Z.Q. Yan, UV-vis spectral property of a
multi-hydroxyl Schiff-base derivative and its colorimetric response to some special
metal ions, Spectrochim. Acta A 203 (2018) 455–460.
[8] Z.Q. Yan, Q. Zhao, M.J. Wen, L. Hu, X.Z. Zhang, J.M. You, A novel polydentate ligand
chromophore for simultaneously colorimetric detection of trace Ag+ and Fe3+
,
[31] A.J. Beneto, A. Siva,
A phenanthroimidazole based effective colorimetric
Spectrochim. Acta A 186 (2017) 17–22.
chemosensor for copper(II) and fluoride ions, Sensors Actuators B Chem. 247
(2017) 526–531.
[32] V.K. Harikrishnan, S.M. Basheer, N. Joseph, A. Sreekanth, Colorimetric and fluorimet-
ric response of salicylaldehyde dithiosemicarbazone towards fluoride, cyanide and
copper ions: spectroscopic and TD-DFT studies, Spectrochim. Acta A 182 (2017)
160–167.
[33] M.K. Salomon-Flores, I.J. Bazany-Rodriguez, D. Martinez-Otero, M.A. Garcia-Eleno, J.J.
Guerra-Garcia, D. Morales-Morales, A. Dorazco-Gonzalez, Bifunctional colorimetric
chemosensing of fluoride and cyanide ions by nickel-POCOP pincer receptors, Dal-
ton Trans. 46 (2017) 4950–4959.
[34] P. Jayasudha, R. Manivannan, K.P. Elango, Highly selective colorimetric receptors for
detection of fluoride ion in aqueous solution based on quinone-imidazole ensemble
influence of hydroxyl group, Sensors Actuators B Chem. 237 (2016) 230–238.
[35] H.Y. Kim, H.G. Im, S.K. Chang, Colorimetric and fluorogenic signaling of fluoride ions
by thiophosphinated dichlorofluorescein, Dyes Pigments 112 (2015) 170–175.
[36] X.J. Zheng, W.C. Zhu, D. Liu, H. Ai, Y. Huang, Z.Y. Lu, Highly selective colorimetric/flu-
orometric dual-channel fluoride ion probe, and its capability of differentiating can-
cer cells, ACS Appl. Mater. Interfaces 6 (2014) 7996–8000.
[9] L. Hu, L.W. Yin, F. Wang, D.H. Yu, C.S. Wang, M.W. Hui, L.Y. Chu, X. Zhu, Z.Q. Yan, Mo-
lecular spectra of a D-p-A typed polydentate ligand chromophore and its simulta-
neous response to trace Cu2+ and Co2+, Spectrochim.Acta A 220 (2019), 117130.
[10] S.N.K. Elmas, F. Ozen, K. Koran, A.O. Gorgulu, G. Sadi, I. Yilmaz, S. Erdemir, Selective
and sensitive fluorescent and colorimetric chemosensor for detection of CO23− an-
ions in aqueous solution and living cells, Talanta 188 (2018) 614–622.
[11] X.X. Jin, H.L. Chen, W.X. Zhang, B.Y. Wang, W. Shen, H.F. Lu, A novel purine
derivative-based colorimetric chemosensor for sequential detection of copper ion
and sulfide anion, Appl. Organomet. Chem. 32 (2018) e4577.
[12] L. Nie, Q. Zhang, L. Hu, Y.M. Liu, Z.Q. Yan, Modified hydrazone derivatives for
ratiometric and colorimetric precognition: relationship between architectures and
performances, Sensors Actuators B Chem. 245 (2017) 314–320.
[13] A. Singh, S.K. Sahoo, D.R. Trivedi, Colorimetric anion sensors based on positional ef-
fect of nitro group for recognition of biologically relevant anions in organic and
aqueous medium, insight real-life application and DFT studies, Spectrochim. Acta
A 188 (2018) 596–610.
[14] Q.Q. Zou, F.R. Tao, H.T. Wu, W.W. Yu, T.D. Li, Y.Z. Cui, A new carbazole-based color-
imetric and fluorescent sensor with aggregation induced emission for detection of
cyanide anion, Dyes Pigments 164 (2019) 165–173.
[15] W. Zhang, K.X. Xu, L.X. Yue, Z.L. Shao, Y. Feng, M. Fang, Two-dimensional carbazole-
based derivatives as versatile chemosensors for colorimetric detection of cyanide
and two-photon fluorescence imaging of viscosity in vitro, Dyes Pigments 137
(2017) 560–568.
[16] D.M. Chen, C.X. Sun, Y. Peng, N.N. Zhang, H.H. Si, C.S. Liu, M. Du, Ratiometric fluores-
cence sensing and colorimetric decoding methanol by a bimetallic lanthanide-
organic framework, Sensors Actuators B Chem. 265 (2018) 104–109.
[17] J. Dong, G.Y. Chen, W. Wang, X. Huang, H.P. Peng, Q.L. Pu, F. Du, X. Cui, Y. Deng, Z.
Tang, Colorimetric PCR-based microRNA detection method based on small organic
dye and single enzyme, Anal. Chem. 90 (2018) 7107–7111.
[18] I.I. Ivanov, N.I. Klyui, V.A. Skryshevsky, Colorimetric analysis of optical reflection
from thin porous silicon for detection of organic liquids, Sensors Actuators B
Chem. 280 (2019) 102–108.
[37] Y.L. Fu, C.B. Fan, G. Liu, S.Z. Pu, A colorimetric and fluorescent sensor for Cu2+ and F-
based on a diarylethene with a 1,8-naphthalimide Schiff base unit, Sensors Actua-
tors B Chem. 239 (2017) 295–303.
[38] L. Hu, J. Xu, C. Wang, Y.J. Zhu, Y.L. Tang, L.W. Yin, Z.Q. Yan, A π-conjugated chromo-
phore dye and its functional paper strips for visually on-site sensing F- and its reac-
tion mechanism, ChemistrySelect 4 (2019) 4118–4124.
[39] D. Sharma, S.K. Sahoo, R.K. Bera, R. Kamal, Spectroscopic and computational study of
a naphthalene derivative as colorimetric and fluorescent sensor for bioactive anions,
J. Fluoresc. 23 (2013) 387–392.
[40] W.H. Ding, C. Wei, X.J. Zheng, D.C. Fang, W.T. Wong, L.P. Jin, A highly selective fluo-
rescent chemosensor for Al-III ion and fluorescent species formed in the solution,
Inorg. Chem. 52 (2013) 7320–7322.
[41] Q. Zhao, H. Yuan, X.M. Xu, L. Hu, P.W. Gong, Z.Q. Yan, A D-pi-A-pi-D organic conju-
gated molecule with multiple chelating points: spectral property and its reversible
visual sensing Cu2+, Dyes Pigments 165 (2019) 217–222.
[42] G. Zhao, F.F. Song, G. Wei, R.L. Wu, Z. Yan, F.Y. Zhang, S.Y. Guang, H.Y. Xu, Molecular
design for novel sensing materials with self-screening interference effect (SSIE): re-
versible recognizing Cu2+ in aqueous and biologic samples, Sensors Actuators B
Chem. 286 (2019) 163–172.
[43] S.Y. Chen, H. Yu, C. Zhao, R. Hu, J. Zhu, L. Li, Indolo[3,2-b]carbazole derivative as a
fluorescent probe for fluoride ion and carbon dioxide detections, Sensors Actuators
B Chem. 250 (2017) 591–600.
[19] M.H. Lan, W.M. Liu, Y. Wang, J.C. Ge, J.S. Wu, H.Y. Zhang, J.H. Chen, W.J. Zhang, P.F.
Wang, Copolythiophene-derived colorimetric and fluorometric sensor for
lysophosphatidic acid based on multipoint interactions, ACS Appl. Mater. Interfaces
5 (2013) 2283–2288.
[20] M.H. Lan, J.S. Wu, W.M. Liu, W.J. Zhang, J.C. Ge, H.Y. Zhang, J.Y. Sun, W.W. Zhao, P.F.
Wang, Copolythiophene-derived colorimetric and fluorometric sensor for visually
supersensitive determination of lipopolysaccharide, J. Am. Chem. Soc. 134 (2012)
6685–6694.
[44] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, et al.,
[21] L. Feng, C.J. Musto, J.W. Kemling, S.H. Lim, K.S. Suslick, A colorimetric sensor array for
identification of toxic gases below permissible exposure limits, Chem. Comm. 46
(2010) 2037–2039.
Gaussian 03.Revision a.1, Gaussian, Inc, Pittsburgh, PA, 2004.
[22] Y.N. Guo, S.F. Xue, M.M. Dirtu, Y. Garcia, A versatile iron(II)-based colorimetric sen-
sor for the vapor-phase detection of alcohols and toxic gases, J. Mater. Chem. C 6
(2018) 3895–3900.