Fluorescent Sensing of both Fe(III) and pH Based on 4-Phenyl-2-(2-Pyridyl)Thiazole and Construction of OR Logic Function

Article abstract of DOI:10.1007/s10895-016-1855-7

In the presented paper we investigated a 2-pyridylthiazole derivative, 4-phenyl-2-(2-pyridyl)thiazole (2-PTP), as the molecular fluorescent switches. It was firstly found that 2-PTP could perform a “turn-on” fluorescent sensing for Fe(III) with selectivity and reversibility. A 2:1 stoichiometry between 2-PTP and Fe(III) was determined according to the molar ratio method. The binding constant was evaluated as (1.90?±?0.05)?×?10⁵ (L/mol)². The detection limit was found as 2.2?×?10^?7?M (S/N?=?3). Secondly, 2-PTP also exhibited a pH-dependent dual-emission. The pK_a(2-PTP-H⁺/2-PTP) value was then estimated as 2.0. To explain the identical emission at 479?nm of both the Fe(III) coordinated form and the protonated form of the ligand, we proposed a “locked” conformation. Finally, combining the two external stimuli as inputs, an OR logic gate was constructed using the fluorescent emission at 479?nm as the output channel.

Full text of DOI:10.1007/s10895-016-1855-7

J Fluoresc
DOI 10.1007/s10895-016-1855-7
ORIGINAL ARTICLE
Fluorescent Sensing of both Fe(III) and pH Based
on 4-Phenyl-2-(2-Pyridyl)Thiazole and Construction of OR
Logic Function
Ming-Yang Yang1 & Xiao-Long Zhao2 & Ming-Hua Zheng1,3 & Yue Wang3 & Jing-Yi Jin2,3
Received: 25 January 2016 /Accepted: 14 June 2016
#
Springer Science+Business Media New York 2016
Abstract In the presented paper we investigated a 2-
pyridylthiazole derivative, 4-phenyl-2-(2-pyridyl)thiazole (2-
PTP), as the molecular fluorescent switches. It was firstly
found that 2-PTP could perform a Bturn-on^ fluorescent sens-
ing for Fe(III) with selectivity and reversibility. A 2:1 stoichi-
ometry between 2-PTP and Fe(III) was determined according
to the molar ratio method. The binding constant was evaluated
as (1.90 0.05) × 105 (L/mol)2. The detection limit was found
as 2.2 × 10−7 M (S/N = 3). Secondly, 2-PTP also exhibited a
pH-dependent dual-emission. The pKa(2-PTP-H+/2-PTP) val-
ue was then estimated as 2.0. To explain the identical emission
at 479 nm of both the Fe(III) coordinated form and the pro-
tonated form of the ligand, we proposed a Blocked^ confor-
mation. Finally, combining the two external stimuli as inputs,
an OR logic gate was constructed using the fluorescent emis-
sion at 479 nm as the output channel.
Introduction
Molecular fluorescent switches have been developing rapidly
in recent because of their wide applications in chemosensors
for various species of biological and/or environmental signif-
icance and in construction of logic gates in molecular level
[1–4]. Great efforts have been devoted to the development of
different fluorescent sensors based on many fluorophores [5,
6]. For example, being the most abundant transition metal
species, iron plays important roles in various enzyme cataly-
sis, cellular metabolism, and DNA, RNA synthesis [7–11].
Imbalance of iron could lead to cell damage and death [12,
13]. Detection of Fe(III) is thus of great importance and has
been intensively pursed based on the fluorescent sensors with
high selectivity [14]. Several rhodamine or BODIPY deriva-
tives have been reported as chemosensors for Fe(III) based on
the switch-on fluorescence [15–20], which is more appreciat-
ed due to the improved sensitivity compared to the quenching
mechanism. However, the reported probes were generally in-
volved in relatively complicated structures and time-
consuming synthesis. In our opinion, it remains as a challenge
to perform the turn-on sensing of Fe(III) with selectivity and
reversibility on the platform based on a fluorophore with
structural simplicity.
.
.
.
.
Keywords Fluorescence Fe(III) pH 2-pyridylthiazole
OR logic gate
* Ming-Hua Zheng
zhengmh@ybu.edu.cn
Pyridylthiazole unit (PT) has been applied in the develop-
ment of various fluorescent probes [21–24]. Here we hope to
report a PT derivative with structural simplicity, 4-phe-
nyl-2-(2-pyridyl)thiazole (2-PTP), as a Bturn-on^ fluores-
cent sensor for Fe(III) with high selectivity and revers-
ibility (Scheme 1). We also investigated the protonation
effects on both absorption and emission of the titled com-
pound. An OR logic gate was then constructed based on the
fluorescent responses of 2-PTP to the two external stimuli,
Fe(III) and protons.
* Jing-Yi Jin
jyjin-chuppem@ybu.edu.cn
1
Department of Chemistry, Yanbian University, Yanji city,
Jilin, Province 133002, China
2
Key Laboratory of Natural Resources of Changbai Mountain &
Functional Molecules, Ministry of Education, Yanbian University,
Yanji city, Jilin, Province 133002, China
3
Research Centre of Chemical Biology, Yanbian University, Yanji
city, Jilin, Province 133002, China

J Fluoresc
Scheme 1 Synthesis of 4-
phenyl-2-(2-pyridyl)-5-(2-
thiophenyl)thiazole
Experimental
with 200 mL EtOAc followed by washing with brine. The
organic layer was dried over anhydrous Na2SO4 and evapo-
rated under reduced pressure. Separation through flash col-
umn chromatography using a mixture of n-hexane and
EtOAc as eluents provided a white solid as the product.
Yield: 81.0 %. Mp: 72–73 °C. IR (film, cm−1): 3111, 3057,
1576, 1499, 1462, 1461, 1273, 1240, 1065, 993, 781, 740,
Materials and Methods
All chemicals were purchased from Acros and used as re-
ceived without further purification. All reactions were per-
formed under argon atmosphere using purified solvents by
standard methods. For all spectrometric measurements,
HPLC grade solvents were used. Flash chromatography was
performed with 100–200 mesh silica gel (Qingdao, China)
and thin-layer chromatography (TLC) was carried out on sil-
ica coated glass sheets (Qingdao silica gel 60 F-254).
UV-vis absorption spectra were measured with a Hitachi
UV-3010 spectrophotometer and the fluorescence spectra were
recorded on a Hitachi F-4500 fluorescence spectrophotometer.
All spectra measurements were recorded at 20 °C. Melting
points were taken on a Thomas-Hoover capillary melting point
1
683. H NMR (CDCl3, 300 MHz, ppm) δ = 7.37–7.39 (m,
2H), 7.46–7.51 (m, 2H), 7.63 (s, 1H), 7.83–7.88 (m, 1H), 8.03
(d, 2H, J = 7.0 Hz), 8.36 (d, 1H, J = 7.8 Hz), 8.58 (d, 1H,
J = 4.6 Hz).13C NMR (CDCl3, 75 MHz, ppm) δ = 115.3,
119.9, 124.5, 126.4, 128.2, 128.8, 134.5, 137.1, 149.5,
151.5, 156.7, 168.8. Anal. Calcd. (%) for C14H10N2S: C
70.56, H 4.23, N 11.76. Found: C 70.48, H 4.31, N 12.01.
Results and Discussion
1
apparatus and uncorrected. H nuclear magnetic resonance
(NMR) and 13C NMR spectra were recorded with a Burker
AV300 (300 MHz) instrument using tetramethylsilane as the
internal standard. IR spectra were recorded on a Perkin-
Elmer 1200 FT-IR spectrometer. Elemental analysis were per-
formed at Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun, China.
As shown in Scheme 1, preparation of 2-PTP was very simple
and the starting materials could be obtained commercially
with cheapness. Although the titled compound had been re-
ported about 60 years ago [26], the application in
chemosensor remains unknown. Considering that the PT unit
is a structural analogous to 2,2′-bipyridyl, its possible chela-
tion to metal ions could be expected. We then examined ex-
perimentally the effects of various metal ions (Li(I), Na(I),
K(I), Ag(I), Mg(II), Ca(II), Mn(II), Fe(II), Co(II), Ni(II),
Cu(II), Zn(II), Cd(II), Hg(II), Pb(II), and Fe(III)) on both ab-
sorbance and emission of the ligand.
Synthesis of 4-Phenyl-2-(2-Pyridyl)Thiazole (2-PTP) [25]
To 25 mL N,N-dimethyl formamide (DMF) containing α-
b r o m o a c e t o p h e n o n e ( 1 . 1 g , 5 . 5 m m o l ) , 2 -
pyridinecarbothioamide (760 mg, 5.5 mmol) was added.
After kept stirring for 24 h, the reaction mixture was diluted
Figure 1 depicted the changes of UV-Vis absorption of 2-
PTP after addition of 5 eq. metal ions, which could be divided
Fig. 1 UV-Vis absorption of 2-PTP (10 μM in MeCN) with the presence
Fig. 2 Fluorescence spectra of 2-PTP (1 μM in MeCN, excited at
of various screened metal ions (10 eq.) at 25 °C
365 nm) with the presence of various screened metal ions (10 eq) at 25 °C

J Fluoresc
Fig. 3 UV-Vis absorption of 2-PTP (10 μM in MeCN) upon continuous
addition of 0–3 eq. of Fe(III) at 25 °C
Fig. 5 Fluorescence spectra of 2-PTP (1 μM) in the presence of different
concentrations of Fe(III) in MeCN at 25 °C
to three classes. Only Fe(III) could result in a significant red-
shift of the absorption maximum from 320 nm to 365 nm
while the three ions (Cu(II), Cd(II), and Hg(II)) led to the
bathochromic shift to some extent, respectively. In the other
hand, the other metal ions had little effects on the absorption
of 2-PTP. We further investigated the fluorescent responses of
the ligand to the metal ions in screen. In the experimental
conditions ([2-PTP] = 1 μM in MeCN, excited at 365 nm),
the free ligand exhibited a weak emission. Only addition of
Fe(III) could significantly enhance the emission intensity at
479 nm whereas other metal ions had little effects on the
emission (Fig. 2). According to the observed selectivity and
the change of fluorescence, 2-PTP should be suggested as a
Bturn-on^ fluorescent sensor for Fe(III) with selectivity.
We described the enhanced fluorescent emission at 479 nm
to the coordinative complex of 2-PTP to Fe(III). Formation of
the stable Fe(III) complex could be judged from UV-Vis titra-
tion experiment (Fig. 3), where it could be found that a gradual
decrease of absorbance at 320 nm of 2-PTP with addition of
Fe(III), accompanied with a gradual increase at 365 nm. After
addition of 1.5 eq. Fe(III), the spectral difference could be
neglected. The clear and fixed isobestic point at 343 nm in
the evolution of UV-Vis absorption indicated the presence of a
coordination equilibrium between the ligand and Fe(III). The
stoichiometry between Fe(III) and 2-PTP was then determined
as 1:2 with mole ratio method (Fig. 4).
To evaluate the binding constant Kas, [(F-Fmin)/(Fmax
-
Fmin)] was plotted against log[Fe(III)] according to the fluo-
rescence titration experiment (Fig. 5) [24], where Fmin and
Fmax are the fluorescence intensity at 479 nm in absence of
Fe(III) and in presence of the saturated concentration of Fe(III)
(1.5 eq.), respectively. Here the fraction, (F-Fmin)/(Fmax-Fmin),
is named after α, which could be regarded as the representa-
tive of the formed Fe(III) complex in the system. The binding
constant Kas could be rewritten as below (Eq. 1).
h
ꢀ.ꢁ i
.ꢀ
. ꢁ
2
2
Kas ¼ FeðIIIÞ−L2
L
½FeðIIIފ ¼ α 1−α
2
½FeðIIIފ ð1Þ
Fig. 6 Relative fluorescence ratio, α, as a function of log[Fe(III)] (M) in
MeCN. The curve fitting for experimental data was calculated as
mentioned in text (■, experimentally observed data points)
Fig. 4 Mole ratio method for determination of the binding stoichiometry
of 2-PTP with Fe(III) obtained from variations in absorption at 320 nm

J Fluoresc
Fig. 9 The two-input OR logic gate and its truth table. Addition of 2 eq.
of Fe(III) and 0.1 M TFAwas set as the two external inputs, respectively,
while the emission at 479 nm of 2-PTP was monitored as the output
channel. All operations were carried out in MeCN ([L] = 1 μM, excited
at 365 nm) at 25 °C
Fig. 7 Evolution of absorption and fluorescence of 2-PTP excited at
350 nm with pH in MeCN:H2O = 1:9 at 25 °C. Solid line: absorption;
dashed line: fluorescence
logic gate was then constructed regarding the emission at
479 nm as the output. Either or both addition of the two spe-
cies could trigger on the fluorescence. Missing either of the
two inputs would make the output silent (Fig. 9). The changes
induced by the input signals, 2.0 eq Fe(III) and 0.1 M
trifluoroacetic acid (TFA), could be compensated by introduc-
tion of EDTA and triethylamine as the corresponding reset
operations, respectively.
According to the 1:2 stoichiometry and Eq. 1, Kas was fitted
as (1.90 0.05) × 105 (L/mol)2 with R2 = 0.988 (Fig. 6) [27, 28].
Accordingly, the detection limit was found as 2.2 × 10−7
M
(S/N = 3) [29]. We also noticed that the Fe(III) complex could
be destroyed by addition of excess of EDTA, and the emission at
479 nm was thus quenched as before used.
We also investigated the effects of protonation on both
absorption and emission of 2-PTP. When pH was higher than
4.0, both spectra have little changes compared to the free
ligand. With decrease of pH from 4.0 to 0.5, the absorption
maxima was red-shifted from 320 nm to 355 nm while a new
emission at 479 nm appeared (Fig. 7). A clear and fixed
isosbestic point as observed in pH-dependent evolution of
both spectra indicated the presence of protonation equilibrium
of 2-PTP. The red-shifted spectra were then reasonably as-
cribed to the protonated form of 2-PTP (2-PTP-H+). The
pKa(2-PTP-H+/2-PTP) value was then estimated as 2.0 from
the UV-Vis absorption titration experiment (Fig. 7).
It was a interesting finding that both Fe(III) coordinated
form and the protonated form of 2-PTP had the same emis-
sion. As proposed in our previous reports of 2-PT units as pH
sensors [22], we believed that a possible hydrogen bond
should result in the proton-locked conformation of 2-PTP-
H+. Accordingly, a chelation of the ligand to Fe(III) should
induce the similar form, which may be the reason that both
forms had the identical emission properties (Fig. 8).
Conclusions
In summary, we investigated the possible applications of 2-
PTP as fluorescent sensors in detail. Both introduction of
Fe(III) and protons could red-shift the emission maxima to
479 nm. The turn-on fluorescent sensing for Fe(III) was firstly
explored. According to the experimental, a stoichiometry of
1:2 was determined and the binding constant was fitted as
(1.90 0.05) × 105 (L/mol)2 with R2 = 0.988. The pKa (2-
PTP-H+ / 2-PTP) value was estimated as 2.0. An OR logic
function was then fulfilled based on the fluorescent responses
at 479 nm to Fe(III) and protons. A possible mechanism was
also suggested to explain the appearance of the similar
bathochromic emission based on the locked conformation of
the ligand.
Combining the turn-on fluorescent responses of 2-PTP to
the two external stimuli (Fe(III) and protons), a two-input OR
Acknowledgments We thanks the financial supports from the National
Natural Science Foundation of China (NSFC 21062023).
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