Turn-on fluorescent probe for Zn2+ ions based on thiazolidine derivative

Article abstract of DOI:10.1002/aoc.5624

In this study, simple on–off fluorescent/UV–visible (UV–Vis) probes were easily prepared using 2-(2-hydroxyphenyl)thiazolidine-4-carboxylic acid (Sen-1) and/or 2-(2-hydroxy-5-nitrophenyl)thiazolidine-4-carboxylic acid (Sen-2) for fast detection of Zn

Full text of DOI:10.1002/aoc.5624

Received: 25 November 2019
DOI: 10.1002/aoc.5624
Revised: 11 February 2020
Accepted: 15 February 2020
F U L L P A P E R
Turn-on fluorescent probe for Zn2+ ions based on
thiazolidine derivative
1
Hayriye Genç Bilgiçli1
| Ahmet T. Bilgiçli1
| Armagan Günsel
|
ꢀ
Burak Tüzün2 | Derya Ergön1 | M. Nilüfer Yarasir1
| Mustafa Zengin1
1Department of Chemistry, Sakarya
University, 54050Sakarya, Turkey
2Department of Chemistry, Cumhuriyet
University, Sivas, Turkey
In this study, simple on–off fluorescent/UV–visible (UV–Vis) probes were
easily prepared using 2-(2-hydroxyphenyl)thiazolidine-4-carboxylic acid
(Sen-1) and/or 2-(2-hydroxy-5-nitrophenyl)thiazolidine-4-carboxylic acid
(Sen-2) for fast detection of Zn2+ ions. Their sensing properties towards
common metal ions were investigated using UV–Vis and fluorescence spec-
troscopies. Sen-1 and Sen-2 displayed a significant change with the addi-
tion of Zn2+ ions in the UV–Vis spectra. The addition of Zn2+ ions induced
a 104 nm bathochromic shift for Sen-1. The binding ratio towards Zn2+
metal ions was determined to be 1:1 by using Job plot analysis and fluores-
cence spectroscopy. The association constant and free energy (ΔG) of Sen-1
and Sen-2 towards Zn2+ ions were calculated by the Benesi–Hildebrand
equation. The limit of detection of Sen-1 towards Zn2+ ions is
3.73 × 10−8 M, which is about 1/100 of the value recommended by the
World Health Organization for drinking water. Sen-1 was successfully
applied to detect Zn2+ ions in water samples and the fluorescence test strip
was prepared for visual detection of Zn2+ ions. Finally, the quantum chemi-
cal parameters of Sen-1 and Sen-2, such as highest occupied molecular
orbital, lowest unoccupied molecular orbital, and chemical hardness, were
investigated by the Becke, three-parameter, Lee–Yang–Parr, Hartree–Fock,
and M062x methods.
Correspondence
Ahmet T. Bilgiçli, Sakarya University,
Department of Chemistry, 54050. Sakarya,
Turkey.
Email: abilgicli@sakarya.edu.tr
Funding information
Sakarya University, Grant/Award
Number: 2018-3-12-195
K E Y W O R D S
quantum chemical calculation, test kit, thiazodiline, zinc fluorescence sensor
1 | INTRODUCTION
divalent zinc, Zn2+, is a trace nutrient critical for physiologi-
cal function.[9–11] Zinc deficiency is a major health risk. The
The synthesis of fluorescent chemosensors has received
considerable attention in recent years.[1–3] In particular, the
investigation of specific sensors for ionic species is of great
interest due to their significance for the environment and
health.[4–8] It is well known that metal ions are related to
disease and human health, for example zinc, which is an
essential trace element for all organisms. Zinc is the second
most abundant metal ion in the human body and its
daily zinc requirement is ~10–15 mg. Although zinc ions
are very useful for human health, it is known that abnormal
levels of free zinc ion concentration in cells is triggered by
diseases such as Parkinson's disease,[12] diabetes,[13] prostate
cancer,[14,15] and Alzheimer's disease.[16] It is therefore
important to develop sensitive and selective compounds for
fast detection of zinc ions.[17,18] Some sensitive compounds
can be used for the detection of zinc ions with UV-Vis and
Appl Organometal Chem. 2020;e5624.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5624

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GENÇ BILGIÇLI ET AL.
fluorescence spectroscopy technics. Thus, some chemical
compounds have been designed and synthesized for the
detection of zinc ions in recent years. The heterocyclic com-
pounds can be given as an example due to their usability in
chemistry, biochemistry and medicine, etc. Among these
heterocyclic compounds, especially thiazolidine and thia-
zole derivatives have been extensively investigated by scien-
tists over the last decade .[19]
and metal salts of Ag+, Na+, K+, Cu2+, Zn2+, Pd2+, Cd2+
,
Fe2+, Hg2+, Pb2+, and Ni2+ were purchased from Sigma-
Aldrich and Merck. FT-IR spectra were recorded on a
Perkin Elmer Spectrum Two spectrophotometer. UV–Vis
studies were performed on an Agilent Model 8453 diode
array spectrophotometer. The fluorescent properties were
recorded on a Hitachi F-7000 fluorescence spectropho-
1
tometer. H and 13C NMR spectra were recorded on a
In this study, we synthesized the thiazolidine
derivatives 2-(2-hydroxyphenyl) thiazolidine-4-carboxylic
Bruker 300 MHz spectrometer.
acid
(Sen-1)
and
2-(2-hydroxy-5-nitrophenyl)
thiazolidine-4-carboxylic acid (Sen-2) as simple on–off
fluorescent/UV–Vis probes for fast detection of Zn2+
ions. Sen-1 and Sen-2 showed high selectivity and
sensitivity towards Zn2+ ions in both the absence and
presence of competitive metal ions. Sen-1 in particular
demonstrated very fast response and excellent sensitivity
to zinc ions in fluorescent measurements. The binding
ratio of Sen-1 and Sen-2 towards Zn2+ metal ions was
determined to be 1:1 using Job plot analysis. The associa-
tion constant and free energy (ΔG) of Sen-1 and
Sen-2 towards Zn2+ ions were calculated by the Benesi–
Hildebrand equation. The association constant is
3.59 × 105 M−1 for Sen-1 and 4.00 × 105 M−1 for Sen-2.
The free energy (ΔG) for Zn2+ ions was found to be
−7.57 kcal M−1 for Sen-1 and −7.63 kcal M−1 for Sen-2.
The binding constant value for Zn2+ ions was determined
by fluorescence spectroscopy as 2.0 × 105 M−1 for Sen-1.
The limit of detection (LOD) of Sen-1 for Zn2+ ions is
3.73 × 10−8 M, which is about 1/100 of the value rec-
ommended by the World Health Organization (WHO) for
drinking water. The synthesized thiazolidine derivate
Sen-1 was successfully applied in the detection of Zn2+
ions in water samples and a fluorescence test strip was
prepared for visual detection of Zn2+ ions. Finally, Sen-1
and Sen-2 were investigated by quantum chemical calcu-
lations. Their quantum chemical parameters, such as
highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO), and chemical
hardness were calculated by the Becke, three-parameter,
Lee–Yang–Parr (B3LYP), Hartree–Fock (HF), and M062x
methods. The fourier transform infrared (FT-IR) spectros-
copy, nuclear magnetic resonance (NMR), and UV–Vis
spectra of Sen-1 and Sen-2 were further confirmed by
density functional theory calculations.
2.2 | General synthesis of thiazolidine-
4-carboxylic acid derivatives[20]
The benzaldehyde derivative (10 mmol) was dissolved in
EtOH (10 ml). L-cysteine hydrochloride (1.57 g, 10 mmol)
and NaOAc (0.98 g, 12 mmol) in water (10 ml) were
added to this prepared solution. The reaction mixture
was stirred for 24 hr at room temperature. The precipitate
was then separated by filtration and washed several times
with ethanol. The obtained thiazolidine-4-carboxylic acid
derivatives were soluble in water, ethanol (hot), DMF,
and DMSO (Scheme 1).
2.2.1 | (4R)-2-(2-hydroxyphenyl)
thiazolidine-4-carboxylic acid (Sen-1)
1H NMR (300 MHz, DMSO-d6) δ: 3.05–2.94 (m, 2H), 3.21
(dd, 1H, J = 6.8, 10.2 Hz), 3.34 (dd, 1H, J = 6.9, 9.4 Hz),
3.83 (dd, 1H, J = 6.9, 8.9 Hz), 4.21 (dd, 1H, J = 5.2,
6.8 Hz), 5.65 (s, 1H), 5.84 (s, 1H), 6.73–6.83 (m, 4H), 7.06
(td, 1H, J = 7.6, 1.7 Hz), 7.13 (td,1H, J = 7.7, 1.7 Hz), 7.30
(d, 1 H, J = 7.7 Hz), 7.34 (dd, 1 H, J = 1.7, 7.7 Hz) 9.83 (s,
2H, –OH). 13C NMR (75 MHz, DMSO) δ: 173.6, 173.1,
155.8, 155.2, 129.7, 128.8, 128.6, 128.3, 126.7, 124.9, 119.7,
119.4, 116.3, 115.7, 68.3, 66.3, 65.9, 65.5, 38.8, 37.8.
2.2.2 | (4R)-2-(2-hydroxy-4-nitrophenyl)
thiazolidine-4-carboxylic acid (Sen-2)
1H NMR (300 MHz, DMSO-d6) δ: 8.41 (d, 1H, J = 2.7 Hz),
8.18 (d, 1H, J = 2.7 Hz), 8.10–7.96 (m, 1H,), 7.03–6.87 (m,
2 | EXPERIMENTAL
2.1 | Materials
SCHEME 1 Synthesis of (4R)-2-(2-hydroxyphenyl)
thiazolidine-4-carboxylic acid (Sen-1) and (4R)-2-(2-hydroxy-
4-nitrophenyl)thiazolidine-4-carboxylic acid (Sen-2)
All solvents (dimethyl sulfoxide [DMSO], tetrahydrofu-
ran [THF], ethanol [EtOH], methanol [MeOH], acetone)

FLUORESCENT PROBE FOR ZN2+IONS
3 of 21
1
1
1
1
2H), 5.83 (s, 1H), 5.68 (s, 1H), 4.10 (td, 1H, J = 6.4,
1.8 Hz), 3.90 (ddd, 1H, J = 8.7, 6.5, 2.0 Hz), 3.29 (1H, ddd,
J = 8.5, 6.7, 1.7 Hz), 3.20 (ddd, 1H, J = 8.2, 6.5, 1.7 Hz),
3.04–2.96 (m, 1H), 2.96–2.84 (m, 1H). 13C NMR (75 MHz,
DMSO) δ: 173.39, 172.94, 162.42, 161.86, 140.74, 140.04,
131.06, 127.43, 125.94, 125.27, 124.72, 122.25, 116.43,
116.00, 66.17, 66.08, 65.53, 64.88, 37.98, 37.69.
=
+
×
ð2Þ
ðF −F0Þ ðFmax −F0Þ Ka½Cꢀ ðFmax −F0Þ
where F0 is the emission intensity of Sen-1 in the absence
of Zn2+ ions, F is the emission intensity of Sen-1 at the
intermediate Zn2+ ion concentration, Fmax is the
emission intensity of Sen-1 at the concentration of com-
plete saturation, Ka is the binding constant, and [C] is the
concentration of Zn2+ ions.
2.3 | Job plot measurements
The binding ratio of Sen-1 and Sen-2 towards Zn2+ metal
ions was determined using Job plot analysis. Sen-1 and
2.5 | Determination of limit of detection
(LOD) of Sen-1 towards Zn2+ ions by
fluorescence method
Sen-2 (1 × 10−3 M) and metal ion solutions (1.0 × 10−3
M
Zn2+ ions) were prepared in a 95% (v/v) water–DMSO
mixture. First, solutions of Sen-1 or Sen-2 (100, 90, 80,
70, 60, 50, 40, 30, 20, 10, and 0 μl) were prepared. Then
solutions of Zn2+ ions (0, 10, 20, 30, 40, 50, 60, 70, 80, 90,
and 100 μl) were added to the previously prepared solu-
tions of Sen-1 or Sen-2, respectively. These mixtures were
diluted to a total volume of 3.0 ml using 95% (v/v) water–
DMSO solvent. The obtained mixture were recorded at
room temperature by using UV–Vis spectroscopy.
The LOD of Sen-1 was calculated based on fluorescence
emission during titration with Zn2+ ions. The change in
fluorescence emission at λmax = 455 nm was plotted
against the concentration of Zn2+ ions and the LOD was
calculated using the equation:
LOD = 3σ=k
where LOD, σ and k are limit of detection, the standard
deviation of the response and theslope of the calibration
curve respectively.
2.4 | Determination of the association
constants of Sen-1 and Sen-2 towards Zn2+
ions
2.4.1 | UV–Vis method
2.6 | Fluorescence quantum yield of
Sen-1 towards Zn2+ ions
To determine the association constant (Ka) and free
energy change (ΔG) due to the interaction between
Sen-1/Sen-2 and Zn2+ ions, the absorption spectra were
further analyzed using the Benesi–Hildebrand relation.
The association constants of Sen-1 and Sen-2 towards
Zn2+ ions were calculated according to the titration curve
using the following nonlinear least squares fitting:
The fluorescence quantum yield (ΦF) of the interaction of
Sen-1 with Zn2+ ions (Sen-1 + Zn) was determined at
room temperature by a comparative method using the
equation
Astd F n2
D
ΦF = ΦFðstdÞ
ð3Þ
A Fstd n2std Dstd
Ax −A0=Amax −A0
2 × Ka × Csen × ð1−ðAx −A0=Amax −A0ÞÞ
Cmetal
=
2
ð1Þ
ðAx −A0=Amax −A0Þ × Ka
where F and Fstd are the areas under the fluorescence
emission curves of the sample and the standard,
respectively. A and Astd are the respective absorbances
of the sample and standard at the excitation
+
2
where Ka is the association constant, Csen is the
concentration of the sensor probe, and Cmetal is the
concentration of metal ions.
wavelengths, respectively. n2 and n2 are the refractive
std
indices of the solvents used for the sample and standard.
D and Dstd are the dilution factors of the sample and
reference, respectively. 9-anthracene carboxylic acid was
used as the standard owing to its appropriate
emission peak area. The quantum yield of 9-anthracene
carboxylic acid is 0.86 at 3.0 × 10−4 M concentration in
ethanol.[21]
2.4.2 | Fluorescence method
The binding constant value for Sen-1 with Zn2+ ions was
calculated by fluorescence spectroscopy according to the
modified Benesi–Hildebrand equation:

4 of 21
GENÇ BILGIÇLI ET AL.
ω = μ2=2η = χ2=2η
ð11Þ
2.7 | Calculation method
Theoretical studies are one of the most important tools
used to compare the activities of molecules. Many pro-
grams and methods are used to make theoretical calcula-
tions. These programs can work in great harmony
with each other. The GaussView 5.0.8, Gaussian09
AS64L-G09RevD.01, ChemDraw Professional 15.1, and
Chemcraft V1.8 packages were used.[22–26] The studied
molecules were analyzed using the HF[26] and
B3LYP,[26,27] M06-2X[28] methods with the 3-21G, 6-31G,
and sdd basis sets. The HOMO and LUMO values of the
molecules were used to explain the molecular activity. The
chemical reactivity parameters of the molecules, such as
Energy of the Highest Occupied Molecular Orbital
(EHOMO), Energy of the Lowest Unoccupied Molecular
Orbital (ELUMO), ΔE (HOMO – LUMO energy gap), elec-
tronegativity (χ), chemical potential (μ), chemical hard-
ness (η), electrophilicity (ω), nucleophilicity (ε), global
softness (σ), and proton affinity (PA) were used to obtain
Electrophilicity and nucleophilicity are used to
predict organic and inorganic reaction mechanisms.
Nucleophilicity (ε) is defined as the inverse of
electrophilicity:
ε = 1=ω
ð12Þ
3 | RESULTS AND DISCUSSIONS
3.1 | Absorption studies
The UV–Vis properties of Sen-1 and Sen-2 were investi-
gated by experimental and calculated methods. The absor-
bance and molar absorptivity values of Sen-1 and Sen-2
are given in table S1 in the absence/presence of metal
ions. The calculated and experimental data of Sen-1 and
Sen-2 agree with each other. A main band at 280 nm and a
small shoulder peak at around 328 nm were observed for
Sen-1 in the experimental absorption spectrum in 95%
(v/v) water–DMSO mixture (Figure 1). Although the
experimental and computational UV–Vis spectra are simi-
lar, the experimental results are closer to the visible region
by ~100 nm. Similarly, two bands were observed at 249 nm
and 327 nm for Sen-2 in the UV–Vis spectrum (Figure 2).
information about the activities of the molecules[29–38]
:
ꢀ
ꢁ
∂E
∂N
μ = −χ =
ð4Þ
ð5Þ
υðrÞ
ꢀ
ꢁ
ꢀ
ꢁ
1
2
∂2E
∂N2
1
∂μ
η =
=
2 ∂N
υðrÞ
The ionization energy (I) and electron affinity (A) of the
Sen-1, Sen-2 and their zinc complexes were calculated with
HOMO and LUMO energies. I and A values are related to
electronegativity, global softness and chemical hardness,
3.1.1 | Metal ion binding titration
studies
which were obtained using the following equations[39]
:
The metal ion sensing properties of Sen-1 and Sen-2
were investigated by UV–Vis spectroscopy in the pres-
ꢀ
ꢁ
I + A
2
χ = −μ =
ð6Þ
ð7Þ
ence of metal ions K+, Ag+, Ca2+, Mg2+, Mn2+, Mg2+
,
Pb2+, Zn2+, Fe2+, Hg2+, Pd2+ Ni2+, and Cu2+. The con-
centration of the metal salts was chosen to be 10−3 mol/L
to eliminate absorption decreases due to dilution. Before
the titration experiment, the aggregation behaviors of
Sen-1 and Sen-2 were investigated. UV–Vis electronic
absorption spectra of thiazolidine derivatives were
recorded at different concentrations. The UV–Vis spectra
for Sen-1 and Sen-2 at different concentrations in a 95%
(v/v) water–DMSO mixture are shown in Supporting
Information Figures S1 and S2, respectively. As the con-
centrations of Sen-1 and Sen-2 increased, the main band
intensities at 280 nm for Sen-1 and at 327 nm for Sen-2
also increased. New band formation due to aggregation
was not observed. These results show that there are
monomeric species in the solution in the absence of
metal ions. UV–Vis electronic absorption changes for
Sen-1 and Sen-2 were recorded during the titration in
I −A
2
η =
It is well known that global softness is defined as the
inverse of chemical hardness:
σ = 1=η
ð8Þ
ð9Þ
ꢀ
ꢁ
−EHOMO −ELUMO
χ = −μ =
2
ꢀ
ꢁ
ELUMO −EHOMO
η =
ð10Þ
2
The global electrophilicity index[40] (ω), investigated
by Parr et al., is the inverse of nucleophilicity:
the presence of metal ions K+, Ag+, Ca2+, Mg2+, Mn2+
,

FLUORESCENT PROBE FOR ZN2+IONS
5 of 21
FIGURE 1 Experimental (in DMSO) and
calculated UV–Vis spectra of Sen-1
FIGURE 2 Experimental (in DMSO) and
calculated UV–Vis spectra of Sen-2
Mg2+, Pb2+, Zn2+, Fe2+, Hg2+, Pd2+ Ni2+, and Cu2+at
room temperature. Sen-1 and Sen-2 are optically sensi-
tive to Pd2+, Cu2+, and Zn2+ ions, but they are not opti-
cally sensitive to the other metal ions used in this study.
The titration of thiazolidine derivatives (Sen-1 and Sen-
at 268 and 377 nm due to interaction with the Pd2+ ions
(Supporting Information Figure S4). The Cu2+ ion is
optically sensitive to Sen-1 and Sen-2. Similar titration
experiments were made for Cu2+ions. During the titra-
tion of Sen-1 with Cu2+ ions, the main band at 279 nm
slightly increased and a new peak at 360 nm appeared
due to the interaction of Sen-1 with Cu2+ ions
(Supporting Information Figure S5). During the titration
of Sen-2 with Cu2+ ions, the main band at 327 nm in the
electronic spectrum did not change, but new bands were
observed at 261 and 370 nm due to interaction with the
Cu2+ ions, similar to the titration with Pd2+ ions
(Supporting Information Figure S6). The significant
changes in the absorption spectra demonstrated the selec-
tivity of Sen-1 and Sen-2 against Pd2+ and Cu2+ ions. In
2) with the other metal ions (K+, Ag+, Ca2+, Mg2+
,
Mn2+, Mg2+, Pb2+, Fe2+, Hg2+, and Ni2+) did not cause
any significant change in the UV–Vis spectra.
During the titration of Sen-1 with Pd2+ ions, the main
band at 279 nm in the electronic spectrum slightly
increased and a new band was observed at 321 nm due to
the interaction of Sen-1 with Pd2+ ions (Supporting
Information Figure S3). During the titration of Sen-2
with Pd2+ ions, the main band at 327 nm in the elec-
tronic spectrum did not change, but new bands appeared

6 of 21
GENÇ BILGIÇLI ET AL.
addition, the color change observed by even the naked
eye from colorless to yellow/green also may be indicative
of the interaction of Sen-1 and Sen-2 with Pd2+ and
Cu2+ ions during the titration.
Zn2+ions, Sen-1 and Sen-2 reacted immediately and
were selective to these ions (Figure 5 was given as an
example). After the titration of Sen-1 and Sen-2 with
Pd2+, Zn2+ions were added to the solution. After
addition of Zn2+ions, new bands were observed at
~370 nm for Sen-1 and 385 nm for Sen-2. These results
indicate that the sensor compounds prefer Zn2+ ions to
Pd2+ ions.
The effect of pH on UV–Vis sensing properties was
also investigated. After compounds Sen-1 and Sen-2
were saturated with Zn2+, Pd2+, or Cu2+ ions, the pH
was adjusted to an acidic value of 5 or 6. When the pH
fell below 6, Sen-1 and Sen-2 did not show sensor behav-
ior towards Zn2+, Pd2+, or Cu2+ ions. When the pH was
adjusted to 6, the peak of 380 nm for the interaction of
Zn2+ with Sen-1 started to decrease. When the pH
reached 5, the spectrum reverted to its original state
(Figure 6). The same results were obtained for Sen-2 with
a small difference: when the pH was 6, the spectrum
reverted to its original state (Figure 7). In summary,
when the pH is greater than 5, Sen-1 and Sen-2 act as
optical sensors towards Zn2+, Pd2+, or Cu2+ ions.
Sen-1 and Sen-2 are highly sensitive to Zn2+ ions.
During the titrations of Sen-1 and Sen-2 with Zn2+ ions,
new bands were observed at 370 and 385 nm, respectively
(Figure 3 for Sen-1 and Figure 4 for Sen-2). This red shift
obtained by titration implies that Sen-1 and Sen-2 are
optically sensitive to Zn2+ ions. Some experiments were
carried out to determine whether Sen-1 and Sen-2 were
more selective against Pd2+, Cu2+, and Zn2+ ions. After
Sen-1 and Sen-2 were saturated with Pd2+ or Cu2+ ions,
Zn2+ions were added to the solution. On the addition of
The stoichiometry and association constants of Sen-1
and Sen-2 towards Zn2+ ions were calculated from the
UV–Vis spectral data. The binding ratio between Sen-1
or Sen-2 and Zn2+ metal ions was determined by using
Job plot analysis. As shown in Figure 8 for Sen-1 and
Figure 9 for Sen-2, the Job plots for Sen-1 and Sen-2
with Zn2+ metal ions exhibited 1:1 stoichiometry. A pic-
ture of Sen-1 was taken under a UV lamp after the addi-
tion of Zn2+ ion solution (0, 10, 20, 30, 40, 50, 60, 70,
80, 90, and 100 μl) (Figure 8 inset). With increasing con-
centration of Zn2+ ions, the color change under the UV
lamp (seen by the naked eye) may be indicative of the 1:1
binding ratio. The association constant of Sen-1 and Sen-
2 towards Zn2+ ions was calculated from the titration
data by using the Benesi–Hildebrand equation. The asso-
ciation constants of Sen-1 and Sen-2 were found to be
3.59 × 106 M−1 (Supporting Information Figure S7) and
4.0 × 105 M−1 (Supporting Information Figure S8),
respectively.
FIGURE 3 UV–Vis spectra of Sen-1 during titration with
Zn2+ ions. Inset: plot of absorbance versus added Zn2+ ions
3.2 | NMR titration studies
The interaction properties of Zn2+ ions and the
thiazolidine compounds Sen-1 and Sen-2 were investi-
1
gated by H NMR titration experiments. After taking the
1H NMR of the pure molecule in DMSO-d6, Zn2+ ions in
DMSO-d6 solution were added into the tube and the
1
changes in the H MR spectra after the addition of Zn2+
ions were observed (Figures 10 and 11). It was expected
FIGURE 4 UV–Vis spectra of Sen-2 during titration with
Zn2+ ions. Inset: plot of absorbance versus added Zn2+ ions
that the phenolic OH group would disappear as the Zn2+
_

FLUORESCENT PROBE FOR ZN2+IONS
7 of 21
FIGURE 5 The effect of Zn2+ions on absorption spectra of Sen-1 (a) and Sen-2 (b) after saturation with Pd2+ ions
FIGURE 6 The effect of pH on absorption spectra of Sen-1
after saturation with Zn2+ ions
FIGURE 7 The effect of pH on absorption spectra of Sen-2
after saturation with Zn2+ ions
ratio increased and therefore the oxygen of hydroxyl
group must be interacting with Zn2+. While this effect is
seen in the hydroxyphenyl derivative, it is not seen in the
2-hydroxy-5-nitrophenyl derivative because the hydroxyl
peak shrinks as a result of the electron-withdrawing
effect of the nitro group. The hydrogen atom peaks for
the second carbon in the heterocyclic ring are shifted to
5.80 ppm (for (2R,4R)-(2-hydroxyphenyl)thiazolidine-
4-carboxylic acid) and 5.62 ppm (for (2S,4R)-
(2-hydroxyphenyl)thiazolidine-4-carboxylic acid). Since
the proton in the (2S,4R) isomer is in the same plane as
the carboxylic acid group, it is shifted further downfield
(5.80 ppm). Similarly, in the (2R,4R) isomer, the proton
and carboxylic acid groups are on different planes and
their magnetic fields are not affected by each other.
Hence, this proton is shifted more upfield (5.62 ppm). As
Zn2+ is added, the proton peak in the (2S,4R) isomer
becomes broader and lies to the more upfield. This effect,
which is not seen in the other (2R,4R) isomer, is a result
of the configurations of the stereoisomer described above.
Since the groups interacting with Zn2+ are in the reverse
plane, this proton remains under the influence of the
magnetic field of Zn2+ (Scheme 2). This result is also seen
1
in the H NMR spectra of the 2-hydroxy-5-nitrophenyl
derivative. For the proton affected by the magnetic field
of the metal, the relaxation time is extended and the
peaks become broader. As the amount of Zn2+ increases,
it can be seen that the splits in the aromatic ring peaks
lose their sharpness because the presence of the metal
disrupts the 1H NMR resolution.[41,42] Because
a
constant resolution cannot be achieved, the cleavage
intensity of the peaks changes and the cleavages lose
their clarity.

8 of 21
GENÇ BILGIÇLI ET AL.
rhodamine are less than 25 nm. Stokes shifts less than
25 nm may cause self-quenching and measurement
errors.[43,44] The low Stokes shifts are a disadvantage for
quantitative analysis and bioimaging applications. The
obtained Stokes shift values for Sen-1 and Sen-2 are
promising for quantitative analysis and bioimaging
applications.
Because Sen-1 and Sen-2 have high selectivity to zinc
ions, the emission spectra and Stokes shifts of Sen-1 and
Sen-2 were compared before and after interaction with
Zn2+ ions. The emission peaks of Sen-1 + Zn and Sen-
2 + Zn appeared at 455 and ~488 nm, respectively, after
interaction with Zn2+. The interaction of Sen-1 with
Zn2+ caused a 104 nm red shift in addition to a 71 nm
Stokes shift in the fluorescence emission spectrum
(Supporting Information Table S1). Recently, many
fluorescent sensors have been developed to detect zinc
ions. Some of them cause fluorescence enhancement in
emission intensity and some cause fluorescence
quenching in emission intensity after interaction with
zinc ions. In this work, Sen-1 showed a significant
increase in fluorescence emission intensity with 104 nm
red shift when compared to other Zn2+ fluorescence sen-
sors in the literature.
FIGURE 8 Job plot diagram of Sen-1 for Zn2+ ions. Inset:
photograph of the mixtures under UV light
3.3.2 | The effect of metal ions on
fluorescence properties
The metal ion sensing behaviors of the synthesized
thiazolidine derivatives Sen-1 and Sen-2 were investi-
gated by fluorescence emission spectroscopy in the pres-
ence of the metal ions K+, Ag+, Ca2+, Mg2+, Mn2+, Mg2+
,
Pb2+, Zn2+, Fe2+, Hg2+, Pd2+ Ni2+, and Cu2+. The fluo-
rescence emission spectra of Sen-1 and Sen-2 with the
metal ions in 95% (v/v) water–DMSO mixture were
recorded at room temperature. Fluorescence emission
changes of Sen-1 and Sen-2 were investigated with the
addition of metal ions at the increasing concentrations.
According to the results, Zn2+ ions caused only signifi-
cant fluorescence enhancement for Sen-1. The other
metal ions did not cause any fluorescence enhancement.
On the contrary, some of them caused fluorescence
quenching, while some showed no effect. Figures 14 and
15 show the color changes in Sen-1 and Sen-2 under
daylight and UV light in the presence of different
metal ions. Sen-1 showed fluorescence enhancement
towards Zn2+ ions under a UV lamp. This observed fluo-
rescence enhancement can be easily detected by the
naked eye.
FIGURE 9 Job plot diagram of Sen-2 for Zn2+ ions. Inset:
photograph of the mixtures under UV light
3.3 | Fluorescence studies
3.3.1 | Fluorescence spectra of the
synthesized thiazolidine derivatives
The fluorescence behaviors of Sen-1 and Sen-2 were
investigated in a 95% (v/v) water–DMSO mixture at room
temperature. Sen-1 and Sen-2 exhibited emission peaks
at 351 and 369 nm, respectively, when excited at 279 nm
(Figure 12 for Sen-1 and Figure 13 for Sen-2). The
fluorescence spectra were found to be mirror images of
the absorption spectra for Sen-1 and Sen-2. Before
interaction with Zn2+ ions, the Stokes shift values of
Sen-1 and Sen-2 were 71 and 42 nm, respectively
(Supporting Information Table S1). Generally, Stokes
shifts of some common dyes such as fluorescein and
Supporting Information Figure S9 shows the fluores-
cence changes for Sen-1 during titration with Fe2+, Ni2+
,
Cu2+, Ag+, Pd2+, and Pb2+ ions. The Sen-1 fluorescence

FLUORESCENT PROBE FOR ZN2+IONS
9 of 21
FIGURE 10 1H NMR spectra of Sen-1 during the titration with Zn2+ ions
FIGURE 11 1H NMR spectra of Sen-2 during the titration with Zn2+ ions
spectra did not show any significant change during the
addition of these metal ions. Although the titration of
Sen-1 with Fe2+ ions gives a fluorescence peak at around
450 nm, its intensity is negligible. The color changes
under UV light were not seen in the presence of Fe2+
ions. Supporting Information Figure S10 shows

10 of 21
GENÇ BILGIÇLI ET AL.
The addition of Zn2+ ions caused a new fluorescence
emission band at 459 nm for Sen-1. This indicates strong
complex formation of Zn2+ ions with Sen-1. In addition,
the observed color change under UV light from colorless
to bright turquoise may be indicative of interaction with
Zn2+ ions for Sen-1 (Figure 16). Similarly, the addition
of Zn2+ ions caused a new broad fluorescence emission
band at ~500 nm for Sen-2. However, it did not show
strong fluorescence, unlike the Sen-1 band, and there
was no visible change under UV light because the pres-
ence of a nitro group in a benzene ring is known to lead
to fluorescence quenching (Supporting Information Fig-
ure S11).[45–47]
SCHEME 2 Estimated interaction of thiazolidine-4-carboxylic
acid diastereomers with Zn (R = H or NO2)
Selectivity is an important parameter when testing
the performance of fluorescent probes. Thus, the fluores-
cence behavior of Sen-1 in the presence of metal ions
(Fe2+, Cu2+, Hg2+, Ca2+, Pb2+, Pd2+, Zn2+, Mg2+, Cu2+
,
Cd2+, Al3+, K+, Na+, Mn2+, Ag+) was also investigated in
95% (v/v) water–DMSO mixture at room temperature. As
shown in Figure 17, the addition of Zn2+ ions in the pres-
ence of other metal ions caused significant fluorescence
enhancement at 459 nm for Sen-1. This indicates that
Sen-1 has a high selectivity for Zn2+ ions in the presence
of other metal ions.
Reversibility studies of Sen-1 and Sen-2 towards Zn2+
ions were also performed. After interaction of Sen-1 and
Sen-2 with Zn2+ ions, ethylene diamine tetra acetic acid
(EDTA) was added to the solution. The addition of EDTA
caused a reduction in fluorescence intensity at 459 nm
for Sen-1 and ~500 nm for Sen-2. At the same time, the
main fluorescence intensities at ~351 nm for Sen-1 and
~369 nm for Sen-2 were observed at the original intensi-
ties (Figure 18 for Sen-1 and Figure 19 for Sen-2).
The binding constant value of Sen-1 with Zn2+ ions
was also calculated by fluorescence spectroscopy
according to the modified Benesi–Hildebrand equation.
FIGURE 12 Absorption and emission spectra of Sen-1 and
Sen-1 + Zn complex:
, absorption (Sen-1);
, emission
(Sen-1); , emission (Sen-1 + Zn)
fluorescence changes for Sen-2 during titration with
Pd2+, Fe2+, Ni2+, and Cu2+ ions. The addition of these
metal ions caused fluorescence quenching in Sen-2. A
new emission band due to fluorescence did not appear.
The Sen-2 fluorescence spectra did not show any signifi-
cant change during the addition of the other metal ions.
FIGURE 13 Absorption and emission
spectra of Sen-2 and Sen-2 + Zn complex:
, absorption (Sen-2);
, emission
(Sen-2); , emission (Sen-2 + Zn)

FLUORESCENT PROBE FOR ZN2+IONS
11 of 21
FIGURE 14 Color change of Sen-1 under daylight (a) and UV light (b) in the presence of different metal ions
FIGURE 15 Color change of Sen-2 under daylight (a) and UV light (b) in the presence of different metal ions
The association constant for Sen-1 towards Zn2+ ions is
Ka = 2.0 × 105 M−1 according to the fluorescence
experiment (Supporting Information Figure S12). The
detection limit of the sensor is a significant parameter for
real sample applications. Thus, the LOD of Sen-1 was
calculated based on the fluorescence emission during
titration with Zn2+ ions (Supporting Information Figure
S13). The LOD of Sen-1 towards Zn2+ ions calculated to
be 0.37 nM, which is about 1/100 of the value rec-
ommended by the WHO for drinking water. Sen-1, with
LOD = 0.37 nM, can be used as a selective sensor for the
analysis of Zn2+ ions in real environmental water
samples.
Figure 20. The addition of Zn2+ ions caused a fluores-
cence enhancement for Sen-1, and the addition of EDTA
to this solution caused fluorescence quenching. This pro-
cedure was repeated five times and the same results were
obtained each time. The turn on–off fluorescence behav-
ior of Sen-1 after interaction with Zn2+ ions might be
explained by considering the strong binding affinity of
EDTA to Zn2+ ions. According to these results, Sen-1
and Sen-2 may be recyclable through treatment with
suitable reagents such as EDTA.
The fluorescence is electron transfer or energy transfer
between the sensor probes and metal ions, therefore the
metal ion sensing behavior of the sensor probe can lead to
a change in the electronic structure. The electron transfer
or energy transfer between the sensor probes and metal
The turn on–off fluorescence behavior of Sen-1 on
sequential addition of Zn2+ions and EDTA is shown in

12 of 21
GENÇ BILGIÇLI ET AL.
FIGURE 16 Fluorescence spectra
of Sen-1 during titration with Zn2+ions
FIGURE 17 Fluorescence
spectra of Sen-1 in the presence
of metal ions
ions can result in fluorescence enhancement or
quenching. In the emission studies, Sen-1 and Sen-2
exhibited weak fluorescence at 351 and 369 nm, respec-
tively, when excited at 279 nm. This observed weak fluo-
rescence for Sen-1 and Sen-2 is due to the free electrons of
the –NH group in the thiazolidine (Scheme 3). After the
addition of Zn2+ ions, Sen-1 displayed a significant
enhancement in fluorescence emission intensity. The
interaction of metal ions with oxygen atoms in
the benzene ring and nitrogen atom in the thiazolidine
group changes their electron-withdrawing properties. The
possible reasons may be the formation of deprotonated
Sen-1 and also prevention of photo-induced electron
transfer processing by the addition of Zn2+ ions.[48–50]
The fluorescence quantum yield (ΦF) of Sen-1 after
saturation with Zn2+ ions was determined at room tem-
perature by the comparative method in water
(Supporting Information Figure S14). The ΦF value of

FLUORESCENT PROBE FOR ZN2+IONS
13 of 21
FIGURE 20 Reversible fluorescence intensity at 455 nm of
Sen-1 on sequential addition of Zn2+ions and EDTA
FIGURE 18 Changes in the fluorescence spectrum of Sen-1
with addition of Zn2+ ions and EDTA. Inset: fluorescence changes
from 300 to 400 nm
fluorescence spectroscopy. During the titration of Sen-1
and Sen-2 with Cd2+ ions, no significant changes in the
UV–Vis spectra were observed, but the addition of Zn2+
ions after saturation with Cd2+ ions caused a significant
optical change in the UV–Vis spectra (Figure 21 for Sen-
1 and Supporting Information Figure S15 for Sen-2).
Contrary to the situation with Cd2+ ions, the titration of
Fe3+ ions caused some spectral changes in the UV–Vis
spectra, but as soon as Zn2+ ions were added, Sen-1 and
Sen-2 released Fe3+ ions and showed optical sensitivity
towards the Zn2+ ions (Figure 22 for Sen-1 and
Supporting Information Figure S16 for Sen-2).
The interference effect of Cd2+ and Fe3+ ions on Sen-
1 was also investigated by fluorescence spectroscopy. The
addition of Fe3+ ions caused fluorescence quenching,
while not significant changes were observed during the
titration Cd2+ ions. The addition of Zn2+ ions to these
saturated solutions of Cd2+ or Fe3+ ions caused high fluo-
rescence enhancement (Figures 23 and 24). From the
UV–Vis and florescence results, Sen-1 may be suitable
for use as a fluorescence sensor for the determination of
Zn2+ ions.
FIGURE 19 Changes in the fluorescence spectrum of Sen-2
with addition of Zn2+ ions and EDTA. Inset: fluorescence changes
from 300 to 400 nm
Sen-1 + Zn was found to be 0.52 (Supporting Informa-
tion Table S1).
3.5 | Application
3.4 | Interference studies
3.5.1 | Determination of Zn2+ions in
water samples
Many fluorescent sensors have been developed to detect
Zn2+ ions,[51–53] but most of them suffer from interfer-
ence from other transition metal ions. In particular, it is
difficult for fluorescent sensors to distinguish Zn2+ and
Cd2+ ions due to their similar chemical properties. In this
study, the interference effect of Cd2+ and Fe3+ ions on
Sen-1 and Sen-2 was investigated by UV–Vis and
Zinc is one of the most important elements for humans.
The deficiency and excess of Zn2+ ions have many nega-
tive effects on health, therefore, according to the WHO,
the concentration of zinc in drinking water should be
below 3 μg/m,[54] and a higher rate of zinc than this is
undesirable. Thus, it is important to know the amount of

14 of 21
GENÇ BILGIÇLI ET AL.
SCHEME 3 Plausible mechanism of a new
probe for the fluorescent detection of Zn2+ ions
FIGURE 21 The interference effect of
Cd2+ions on the absorption spectra of Sen-1
zinc in drinking water. Sen-1 was successfully used to
detect Zn2+ ions in water samples (Sapanca Lake water
and laboratory tap water). The fluorescence intensity of
Sen-1 was recorded before and after the addition of 10 μl
of Zn2+ ion solution. We examined the effect of deionized
water on the fluorescence change and found no enhanc-
ing effect (Supporting Information Figure S17). After
addition of 10 μl of Zn2+ ion solution, the fluorescence
intensity of Sen-1 was significantly enhanced. A fluores-
cent peak appeared around 455 nm when tap water or
lake water was used instead of deionized water
(Supporting Information Figure S18). The experiments
were repeated five times under the same conditions and
the results are shown in Table 1. The average amounts of
Zn2+ ions in the five samples were 0.15 and 0.11 μg/ml in
lake water and tap water, respectively. Sen-1 can
therefore be successfully used to detect Zn2+ ions in
water samples without any interference from other
metal ions.
3.5.2 | Fluorescence test strip for visual
detection of Zn2+ ions
A fluorescence test strip for visual detection of Zn2+ ions
was prepared. A solution of Sen-1 was prepared in a 95%
(v/v) water–DMSO mixture and immobilized onto a silica
gel plate. Solutions containing zinc at different concen-
trations were added as spots onto the silica gel plate. The
picture of plate was taken under the UV lamp. The

FLUORESCENT PROBE FOR ZN2+IONS
15 of 21
FIGURE 22 The interference effect of
Fe3+ions on the absorption spectra of Sen-1
FIGURE 23 The fluorescence spectra of Sen-1 during the
addition of Zn2+ions following saturation with Cd2+ ions
FIGURE 24 The fluorescence spectra of Sen-1 during the
addition of Zn2+ions following saturation with Fe3+ ions
initially blue fluorescence spots changed to bright tur-
quoise as the concentration Zn2+ ions increased
(Figure 25).
The most important parameter in comparing the
activities of molecules is the HOMO energy parameter.
The HOMO energy parameter is the Highest Occupied
Molecular Orbital (HOMO) indicating the electron donat-
ing ability of studied molecules. The HOMO structures of
the molecules are shown in Figure 26. It can be seen
from this figure where the molecules will give electrons
to metal atoms.[55] The activity of a molecule increases
when HOMO energy value increases. In the light of pre-
vious explanations, the activity of the studied molecules
were found to be Sen-1 > Sen-2 in all basis set in
Table 2.
3.6 | Calculation results
A good way to learn about the activities of molecules
and the thermodynamics of the reactions is to use quan-
tum chemical calculations to determine parameters
related to molecules. The parameters obtained from the
quantum chemical calculations give informations about
molecules. Quantum chemical parameters such as
EHOMO, ELUMO, and ΔE (HOMO – LUMO energy
gap) can be used to compare the activities of
molecules.[30–32]
On the other hand, the LUMO energy parameter indi-
cates that electron accepting abilities of molecules from
metal atoms. The LUMO structures of Sen-1 and Sen-2
are shown in Figure 26. It can be seen from this figure

16 of 21
GENÇ BILGIÇLI ET AL.
TABLE 1 The determination of Zn2+ ions in water samples
Sample
Added
–
Found value of Zn2+ ions
–
Total sample number
SSD
–
RSD (%)
–
Deionized water
5
10 μl
–
0.21 μg/ml
0.15 μg/ml
0.39 μg/ml
0.15 μg/ml
0.35 μg/ml
0.014
0.019
0.026
0.015
0.022
7.00
Lake water
Tap water
5
5
12.65
6.50
10 μl
–
10.54
6.39
10 μl
Note. RSD, relative standard deviation; SSD, sample standard deviation.
Because the energy gap value shows the electron transfer
values between the transition metal atoms and the ligand
molecules. A molecule with a low energy gap is more
active.
It is thought that Sen-1 and Sen-2 form two
different complexes with Zn metal in 95% (v/v)
water–DMSO solution. First, the molecule forms a
complex with DMSO and Zn metal (Supporting Infor-
mation Figures S19 and S20). Second, the molecule
forms a complex with the water molecule (H2O) and
Zn metal (Supporting Information Figure S19 and
S20). These two complexes were studied in three dif-
ferent phases and the effect of dielectric constant in
the complexes was investigated. The values obtained
from the theoretical studies were compared with the
experimental results obtained in DMSO media. The
complexes were optimized to comment about them.
The Gibbs free energy values were calculated and
compared using the energies of these optimized
structures.
FIGURE 25 Images of Sen-1 without the addition of zinc
solution under UV light (a) and after the addition of zinc solution
at different concentrations under UV light (b)
where the molecules will accept electrons from metal
atoms.[56]
The energy gap (ΔE) is also important parameter
when comparing the activities of molecules. If a molecule
has a small energy gap, it is the most active molecule.[57]
The Gibbs free energy (ΔG) was calculated using
equation:
FIGURE 26 The HOMO, LUMO, and electrostatic potentials (ESPs) of Sen-1 and Sen-2 molecules

FLUORESCENT PROBE FOR ZN2+IONS
17 of 21
TABLE 2 The calculated quantum chemical parameters in different basis sets
_
PI
EHOMO
ELUMO
I
A
ΔE
η
σ
χ
ω
ε
Dipol Energy
B3LYP/3-21g level
Sen-1
Sen-2
−6.306 −0.781
−6.588 −2.869
6.306
6.588
0.781
2.869
5.525 2.762 0.362 3.543 −3.543 2.273 0.440 6.032
3.719 1.860 0.538 4.728 −4.728 6.011 0.166 7.255
−28857.733
−34391.244
B3LYP/6-31g level
Sen-1
Sen-2
−6.237 −0.724
−6.584 −3.075
6.237
6.584
0.724
3.075
5.513 2.756 0.363 3.481 −3.481 2.198 0.455 6.941
3.509 1.754 0.570 4.829 −4.829 6.646 0.150 5.382
−29005.219
−34567.836
B3LYP/sdd level
Sen-1
Sen-2
−6.336 −1.026
−6.671 −3.423
6.336
6.671
1.026
3.423
5.310 2.655 0.377 3.681 −3.681 2.552 0.392 7.266
3.248 1.624 0.616 5.047 −5.047 7.841 0.128 4.933
−29008.117
−34572.194
HF/3-21g level
Sen-1
Sen-2
−8.952
−9.748
3.151
0.951
8.952 −3.151 12.104 6.052 0.165 2.900 −2.900 0.695 1.439 6.663
9.748 −0.951 10.699 5.350 0.187 4.398 −4.398 1.808 0.553 8.694
−28730.297
−34235.050
HF/6-31g level
Sen-1
Sen-2
−9.205
−9.832
3.089
0.633
9.205 −3.089 12.294 6.147 0.163 3.058 −3.058 0.761 1.315 7.623
9.832 −0.633 10.465 5.233 0.191 4.600 −4.600 2.022 0.495 7.891
−28875.483
−34408.973
HF/sdd level
Sen-1
−9.333
2.655
0.411
9.333 −2.655 11.988 5.994 0.167 3.339 −3.339 0.930 1.075 7.772
10.019 −0.411 10.430 5.215 0.192 4.804 −4.804 2.213 0.452 5.775
−28878.056
−34412.950
Sen-2 −10.019
M062X/3-21g level
Sen-1
Sen-2
−7.706
0.212
7.706 −0.212
8.189 1.838
7.918 3.959 0.253 3.747 −3.747 1.773 0.564 8.434
6.352 3.176 0.315 5.013 −5.013 3.957 0.253 7.699
−28849.513
−34380.379
−8.189 −1.838
M062X/6-31g level
Sen-1
Sen-2
−7.774
0.084
7.774 −0.084
7.857 3.929 0.255 3.845 −3.845 1.882 0.531 8.670
6.237 3.118 0.321 5.179 −5.179 4.301 0.233 7.921
−28997.008
−34557.372
−8.298 −2.061
8.298
2.061
M062X/sdd level
Sen-1
Sen-2
−7.913 −0.221
7.913
0.221
2.434
7.692 3.846 0.260 4.067 −4.067 2.150 0.465 9.019
5.994 2.997 0.334 5.430 −5.430 4.920 0.203 8.275
−29000.331
−34562.174
−8.427 −2.434
8427
ꢂ
ꢃ
X
TABLE 3 Energy values of studied molecules (eV)
2 +
ΔG = E
− EM
+
Eligand + Ewater or DMSO
M−Lcomplex
Gas phase
Aqueous phase DMSO phase
ð13Þ
Sen-1
−28875.4832 −28876.2263
−34408.9727 −34410.0156
−15048.6752 −15049.1016
−28876.2183
−34410.0053
−15049.0969
−2078.7628
−17775782
Sen-2
where EM−L
is the sum of the electronic and thermal
free energies of the metal complex, Eligand is the sum of
the electronic and thermal free energies of the ligand,
complex
DMSO
Water
Zn metal
−2078.5102
−1759.0174
−2078.7651
−1777.7412
2 +
and EM is the sum of the electronic and thermal free
energies of the metal ion.[56–61]
When we look at the calculations in Table 3, we can
see that four different complexes were obtained for Sen-1
and Sen-2 in three different phases. The Sen-2 complexes
are more stable than the Sen-1 complexes in all three
phases. When looking at the four different complexes, it
was found that the Gibbs free energy values of the
Zn-Sen-2-DMSO were higher (Table 4). If we examine
the complexes formed in more detail, it can be seen that
the complexes formed with DMSO are more stable
than the complexes formed with water (H2O) or in the
gas phase.
The energy difference between the HOMO and
LUMO is termed the HOMO–LUMO gap. HOMO and
LUMO are sometimes called frontier orbitals. Thus, the
frontier molecular orbital diagrams of the complexes
formed by Sen-1 and Sen-2 molecules with Zn2+ metal
were investigated. The frontier molecular orbital

18 of 21
GENÇ BILGIÇLI ET AL.
TABLE 4 Energy values of studied molecules (eV)
Gas phase
Aqueous phase
−32853.8143
−45826.3425
−38418.0364
−51388.6363
−121.0816
DMSO phase
−32853.8042
−45826.3318
−38417.9955
−51388.5635
−121.2450
Zn-Sen-1-water
−32853.1067
−45825.3443
−38416.0500
−51388.0285
−140.0959
Zn-Sen-1-DMSO
Zn-Sen-2-water
Zn-Sen-2-DMSO
ΔG for Sen-1-water
ΔG for Sen-1-DMSO
ΔG for Sen-2-water
ΔG for Sen-2-DMSO
−142.1685
−123.2734
−123.4385
−169.5497
−151.5144
−151.6493
−171.3632
−151.7779
−151.8832
TABLE 5 Calculated 13C NMR and 1H NMR results for Sen-1 and Sen-2
Sen-1
Sen-2
Theoretical
Gas
Experimental
DMSO
124.9
128.3
119.4
155.2
115.7
128.8
65.5
Theoretical
Gas
Experimental
DMSO
140.74
131.06
125.27
162.42
116.43
125.94
65.53
37.98
66.17
173.39
8.18
DMSO
118.97
DMSO
136.14
C1
119.39
132.47
122.11
153.71
111.21
132.76
62.88
36.51
57.71
173.13
7.30
C1
136.34
127.48
130.59
158.60
109.31
131.60
57.89
33.91
58.93
173.13
9.04
C2
131.70
121.22
153.80
112.71
133.63
64.34
36.65
58.29
177.65
7.42
C2
128.86
129.47
160.67
110.82
133.07
58.77
34.13
59.48
178.32
9.22
C3
C3
C4
C4
C5
C5
C6
C6
C13
C15
C16
C23
H7
C12
C14
C15
C22
H7
37.8
66.3
173.1
6.83
H8
8.41
8.38
7.13
H8
6.76
7.21
7.03
H9
6.94
7.35
6.83
H9
9.06
9.20
8.10
H10
H12
H14
H17
H18
H19
H21
H25
7.74
7.96
7.34
H11
H13
H16
H17
H18
H20
H24
4.88
5.81
–
4.26
5.06
9.83
5.69
5.86
5.83
5.38
5.41
5.84
3.27
3.11
3.29
3.71
3.50
3.34
2.86
3.18
3.04
2.98
3.33
3.00
3.92
4.32
4.10
3.56
4.06
4.21
1.07
1.82
–
1.47
2.12
–
9.27
10.12
–
9.80
10.37
9.83
diagrams of the zinc complexes formed with Sen-1 or
Sen-2 are shown in Supporting Information Figures S21
and S22, respectively. In the HF/6-31g basis set, HOMO
and LUMO shapes were calculated for the ground and
the excited states of the Sen-1 and Sen-2 Zn complexes.
The HOMO energy value of the Sen-1 and Sen-2 mole-
cules is lower than the HOMO energy value of the Zn
complex. However, the LUMO energy value of the Sen-1
and Sen-2 molecules is higher than the HOMO energy
value of the Zn complex. Consequently, the energy gap
values of the Zn complexes are lower than those of the
Sen-1 and Sen-2 molecules. The most important reason
is that the rearrangement of the electron distiribution of
Sen-1 and Sen-2 after the interaction with Zn2+ ions.
Because of changes in the energy level causing by elec-
tron redistribution, the interaction of Sen-1 and Sen-2
with zinc ions is expected to lead to fluorescence
enhancement. When the experimental results and

FLUORESCENT PROBE FOR ZN2+IONS
19 of 21
TABLE 6 Selected frequencies of ligand at the HF/6-31g level
atoms of Sen-1 and Sen-2 have calculated chemical shifts
between 111 and 173 ppm, and the aliphatic carbon
atoms have calculated chemical shifts between 33 and
62 ppm. The aliphatic hydrogen atoms of these molecules
have calculated chemical shifts between 1 and 5 ppm,
and the aromatic hydrogen atoms have calculated chemi-
cal shifts between 5 and 9 ppm.[56–61]
The calculated chemical shift values can be compared
to the experimental values. The calculated chemical shift
values were investigated by drawing correlation graphs
(Supporting Information Figures S25 and S26). Com-
pared with experimental observations, the correlation
values were found to be very close to 1. .[58–61] Theoretical
calculations were done in two phases: the gas phase and
DMSO. Experimental procedures were performed in
DMSO solvent. For both phases, the correlation values
obtained were 0.99, which shows that the calculations
were correct.
Frequency
Compounds Band (cm−1
)
Vibration modea
STRE (aromatic CH)
STRE (aliphatic CH)
STRE (C=O)
Sen-1
1
2
3406
3318
1946
1794
1687
1458
1418
1208
882
3
4
STRE (C=C)
5
BEND (H-C=C)
BEND (H-O-C)
BEND (H-C-S)
BEND (C-C-C)
TORS (H-C=C=C)
TORS (H-O-C-C)
STRE (S-C)
6
7
8
9
10
11
1
818
698
Sen-2
3438
3331
1948
1797
1631
1563
1444
1321
1203
974
STRE (aromatic CH)
STRE (aliphatic CH)
STRE (C=O)
2
3
4
STRE (C=C)
3.6.2 | FT-IR spectra
5
BEND (H-C=C)
STRE (O-N)
6
The FT-IR spectra used to identify functional groups and
molecular structure are calculated at the HF/6-31g(d,p)
level. A detailed study of the optimized structures of the
ligands was calculated in the VEDA program.[57–61]
The FT-IR spectra of Sen-1 and Sen-2 are shown in
Supporting Information Figures S27 and S28. The main
peaks in the obtained spectra are listed in Table 6, which
shows the numerical value of the theoretical frequencies
of the functional groups in the ligands. In general, when
the FT-IR spectra of Sen-1 and Sen-2 are examined, the
aliphatic carbons in Sen-1 and Sen-2 vibrate in the range
3331–3318 cm−1, while the aromatic carbons vibrate
between 3406 and 3438 cm−1. The vibration frequency of
the C=O bond is ~1950 cm−1 and that of the C–S bond is
~695 cm.[57–61]
7
STRE (C-C)
8
BEND (H-O-C)
BEND (H-O-C)
TORS (H-C=C=C)
TORS (H-O-C-C)
STRE (S-C)
9
10
11
12
818
695
_
_
Note. BENDING (BEND), ; STRENGTH (STRE), ; TORSION (TORS).
theoretical calculations were evaluated together, the
interaction of Sen-1 with Zn2+ ions caused fluorescence
enhancement, but the same effect was not seen for Sen-2
due to the fluorescence quenching properties of the nitro
groups in benzen ring.
3.6.1 | NMR spectra
3.6.3 | UV–Vis spectra
Chemical shift values are important to determine the
structure of molecules. The chemical shift values for the
hydrogen and carbon atoms of Sen-1 and Sen-2 were cal-
culated at the HF/6-31++g level by using the gauge
including atomic orbital method. The chemical shift
values were calculated by subtracting the theoretical
values of tetramethylsilane.[57]
UV/Vis spectroscopy is used in chemistry for
thequantitative/qualitative analysis of many different
compounds, such astransition metal ions, conjugated
organic compounds, complexes and biological macromol-
ecules, which can absorb UV or visible wavelengths. An
absorption spectrum consists of several absorption bands
that show the structure of the molecule. This spectrum
provides information about the electronic transitions that
may occur in the molecule. The UV–Vis spectra of the
Sen-1 and Sen-2 were calculated basis set in chloroform
(ε = 4.711), methanol (ε = 32.613), DMSO (ε = 46.826),
water (ε = 78.355), and n-methyl formamide mixture
1
The calculated 13C NMR and H NMR chemical shift
values for Sen-1 and Sen-2 are shown in Table 5. In
NMR spectra, the atomic labeling of the optimized struc-
tures for Sen-1 and Sen-2 are shown in Figures S23 and
S24. From Table 5, we can see that the aromatic carbon

20 of 21
GENÇ BILGIÇLI ET AL.
(ε = 181.56) at HF/6-31++g.[62] The UV–Vis spectra of
Zn complexes of Sen-1 and Sen-2 were calculated at
HF/6-31++g in the gas, water and DMSO phases. The
results of the calculations show that there is one basic
peak for Sen-1 (Supporting Information Figure S29),
which shows that an electronic transition has occurred.
This electronic transition is called the n ! π* transition.
When UV–Vis spectrum of Sen-2 is examined, two basic
peaks are seen, which shows there are two electronic
transitions (Supporting Information Figure S30). These
electronic transitions are the n ! π* and π ! π*
transitions.[62]
The UV–Vis spectra of the complexes obtained from
experimental procedures were compared with the theo-
retical results in three estimated forms containing water,
DMSO and in the gas phase. When the spectra were
examined, the maximum peaks for the Sen-1 complexes
were seen at 172, 186, and 162 nm and the maximum
peaks for the Sen-2 complexes were at 170, 178, and
17 nm.
Sen-1 and Sen-2 was determined on sequential addition
of Zn2+ions and EDTA. Sen-1 was successfully used for
detection of Zn2+ ions in water samples and a fluores-
cence test strip was successfully prepared for visual detec-
tion of Zn2+ ions. The results indicate that Sen-1 can be
used in a simple and effective method for visual and fast
detection of Zn2+ ions.
ACKNOWLEDGMENTS
This research was made possible by Tubitak Ulakbim,
High Performance and Grid Computing Center (TR-Grid
e-Infrastructure) and supported by the Research Fund of
Sakarya University (Project Number: 2018-3-12-195).
ORCID
Hayriye Genç Bilgiçli https://orcid.org/0000-0001-6909-
316X
Ahmet T. Bilgiçli https://orcid.org/0000-0002-4144-7357
ꢀ
Armagan Günsel https://orcid.org/0000-0003-1965-
1017
Burak Tüzün https://orcid.org/0000-0002-0420-2043
M. Nilüfer Yarasir https://orcid.org/0000-0002-7327-
7137
4 | CONCLUSION
Mustafa Zengin https://orcid.org/0000-0002-0243-1432
The thiazolidine derivatives Sen-1 and Sen-2 were
synthesized and characterized by common spectroscopic
techniques. The experimental results and the theoretical
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Genç Bilgiçli H,
Bilgiçli AT, Günsel A, et al. Turn-on fluorescent
probe for Zn2+ ions based on thiazolidine
derivative. Appl Organometal Chem. 2020;e5624.
https://doi.org/10.1002/aoc.5624
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