B. Arvas, B. Ucar, T. Acar et al.
Tetrahedron 88 (2021) 132127
applications to detect different cations and anions in various me-
dium [29e31]. Coumarin ring can easily be functionalized in
different positions by different florophore and chromophore
groups to improve its sensitivity and selectivity in chemosensor
applications [32]. In this context, imine C]N functional group in
Schiff base is known to be a good ligand for metal ions and useful
for chemosensor properties [33]. Coumarin ring with hydroxy and
imine functional groups have nitrogen and oxygen atoms to coor-
dinate with metal ions, and especially with good fluorescence
embedded, they can be used successfully as fluorescence chemo-
sensors for metal ions [34e36]. In this study, a new coumarin based
Schiff base chemosensor for fluorescence detection of Zn2þ was
designed and synthesized and its photophysical properties were
investigated. This coumarin nucleus was synthesized and charac-
terized and used for the first time as a fluorescence probe for Zn2þ
determination.
potential interference impact of metals each other, the fluorescence
quenching effects of binary metals and all metal mixture were
studied by adding Zn2þ and other metals to the FS3 solution and the
results were given in Fig. 1C. Herein, Agþ metal ion was excluded
from the binary metal measurements, due to the precipitation of
the metal ion in the form of AgCl salt. The interference effect of Fe3þ
was seen, but the change in maximum emission wavelength as a
result of interference showed that we can eliminate this effect in
real case studies. In addition, EDTA shows specificity for different
metal ions at different pHs. In order to eliminate the interfering
effect of Fe3þ ions, it is removed from the real sample by adjusting
the pH to 1 for EDTA-Fe3þ complexation as stated in the literature
[37]. The fluorescence quenching effect of the other binary metals
remained very low. In this way, FS3 is specific for Zn2þ detection
among the other metals that have been tried.
To further elucidate the Zn2þ-sensible properties of FS3, fluo-
rescence titrations of various concentrations of Zn2þ were carried
out. As presented in Fig. 1D, in the presence of varied amount of the
quencher, emission intensity of FS3 was decreased from F0 to F.
Also, the emission data were used by creating a graph between F0/F
and [Zn2þ]. The Stern-Volmer graph given in Fig.1D demonstrated a
linear correlation between the emission intensity of FS3 and the
2. Results and discussion
2.1. Synthesis of fluorescence sensor (FS3)
Methyl
2-((7-hydroxy-4-methyl-2-oxo-2H-chromen-8-yl)
addition of Zn2þ in the range of 0e100
mM with a correlation co-
methyleneamino)-3-phenyl- propanoate (FS3) was prepared by the
condensation reaction of 8-formyl-7-hydroxy-4-methylcoumarin
(2) with phenylalanine methyl ester (Scheme 1). The structure of
the compound was obtained from its spectral data (Fig. S1-S5) and
they are in accordance with the structure of FS3.
efficient of 0.9943. The Stern-Volmer constant (KSV) was found as
6.999. The linearity of the graph pointed out that the quenching
was static for the FS3eZn2þ system and from the linearity of the
above mentioned graph, LOD for the determination of Zn2þ was
found to be 3.43 ꢀ 10ꢁ9 M. In the literature, in the studies on the
detection of zinc in different samples, the LOD value for Zn2þ varies
between 5.98 ꢀ 10ꢁ6 - 4.98 ꢀ 10ꢁ9 M [38]. In the present study, the
LOD value for determining the Zn2þ by FS3 was found to be lower
than the aforementioned studies. Additionally, this value was
consistent with the data given by the WHO (World Health Orga-
nization) for the detection of Zn2þ ion in biological and environ-
mental products [39]. The Zn2þ ion in the solution interacted with
FS3 in a very short time (about half a minute), causing fluorescence
quenching and this was a very fast detection time compared to the
studies in the literature [40]. So, there was a significant improve-
ment in LOD value and a decrease in detection time.
The binding constants and stoichiometry of the interaction be-
tween metal ions and ligand molecules can be found theoretically
with data obtained from the spectroscopic measurements and us-
ing the Benesi-Hildebrand method [41,42]. As shown in Fig. 2, when
the mole fraction of [Zn2þ]/(([Zn2þ]þ[FS3]) was about 0.5, the
emission intensity of Zn2þ reached the maximum, indicating that
FS3 and Zn2þ in acetonitrile solution were combined with stoichi-
ometry at 1:1. Meanwhile, the association constant of FS3 with
Zn2þ was calculated to be 2.39 ꢀ 104 Mꢁ1 according to the Benesi-
Hildebrand method. The 1:1 stoichiometry between FS3 and Zn2þ
was further verified by the ESI-MS analysis. As shown in Fig. S6,
2.2. Fluorescence sensing of Zn2þ with FS3
One of the most important points in fluorescence spectroscopy
studies is to determine the suitable concentration of the molecule
to be measured. Fluorescence measurements at different concen-
trations of the FS3 were taken to determine the optimal dilution
rate without concentration-dependent damping. As shown in
Fig. 1A, the optimal dilution ratio without fluorescence quenching
was determined to be 1:30. In the measurements taken without
dilution, it was determined that the fluorescence intensity of the
molecule was quenching due to the high concentration.
The binding behavior of FS3 with various metal ions (Agþ, Naþ,
Kþ, Mg2þ, Ca2þ, Sn2þ, Cd2þ, Ni2þ, Ba2þ, Co2þ, Liþ, Cu2þ, Hg2þ, Sb3þ
,
Cr3þ, Fe3þ, Pb2þ, Al3þ, Mn2þ, Zn2þ) was studied by fluorescence
spectroscopy. The effect of various metal ions on the fluorescence
intensity of FS3 was shown in Fig. 1B. A notable quenching was
determined in the fluorescence intensity of FS3 with the addition of
all metals but the addition of Zn2þ to FS3 solution formed about 60-
fold more fluorescence quenching. Here, while all other metals
caused partial fluorescence quenching, the quenching effect of Zn2þ
ion to FS3 was close to 100%.
Furthermore, to investigate the specificity of FS3 and the
Scheme 1. Synthesis scheme of fluorescence sensor (FS3).
2