A Thiourea-Containing Fluorescent Chemosensor for Detecting Ga3+

Article abstract of DOI:10.1007/s10895-020-02624-w

A thiourea-based fluorescent chemosensor NADA, (E)-2-(4-(diethylamino)-2-hydroxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, has been designed and synthesized. NADA could detect Ga³⁺ through a fluorescent turn-on with a low detec

Full text of DOI:10.1007/s10895-020-02624-w

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02624-w
ORIGINAL ARTICLE
A Thiourea-Containing Fluorescent Chemosensor for Detecting Ga3+
Soyoung Park1 & Hangyul Lee1 & Ju Byeong Chae1 & Cheal Kim1
Received: 5 August 2020 /Accepted: 15 September 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A thiourea-based fluorescent chemosensor NADA, (E)-2-(4-(diethylamino)-2-hydroxybenzylidene)-N-(naphthalen-1-
yl)hydrazine-1-carbothioamide, has been designed and synthesized. NADA could detect Ga3+ through a fluorescent turn-on
with a low detection limit (0.29 μM). Importantly, NADA could effectively discriminate Ga3+ from Al3+ and In3+. The binding
mechanism of NADA with Ga3+ was identified by ESI-mass, NMR titration, and DFT calculations.
Keywords Ga3 + Thiourea Chemosensor Calculations Fluorescence
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Introduction
Some approaches have been developed to detect gallium
ions including titrimetric method, absorption, ion exchange
and atomic spectrometry [13, 14]. However, these methods
demand high-cost instruments or complex preparation proce-
dures [15, 16]. Therefore, it is imperative to develop new, low-
cost and convenient methods for detecting gallium ions [17,
18]. Fluorescent methods have attracted lots of attention be-
cause they have easy operational advantage, fast response
times, convenience, high sensitivity and selectivity [19–21].
Some fluorescent chemosensors for detecting Ga3+ have been
reported, but they are less effective due to the inhibition of
Al3+ or In3+ [22–25].
The naphthyl moiety, one of the fluorophores, can act as an
ideal signal part because of high fluorescence quantum yield
and good sensitivity [26, 27] In addition, another effective
fluorophore, N,N-diethylaminophenol group may enhance
the ICT (intramolecular charge transfer), resulting in a large
bathochromic shift in emission [28–30]. On the other hand,
the thiourea moiety can easily chelate metal ions with two
nitrogen atoms and one sulfur atom, and be used as a linker
[31–34]. Therefore, a thiourea-based sensor with naphthyl
moiety and diethylaminophenol group can efficiently bind to
transition metal ions with a unique and strong fluorescence.
Herein, we report a thiourea-based fluorescent sensor
NADA with the diethylaminophenol and naphthyl groups.
NADA selectively detected gallium ions with a low detection
limit. The binding mechanism of gallium ion to NADA was
explained by fluorescent and UV-vis experiments, Job plot
analysis, ESI-mass, 1H NMR titration, DFT calculation.
Gallium, one of the group 13 elements, is uncommon element
found in little amounts in the soil. Nevertheless, it has an im-
perative impact on our daily life [1, 2]. Due to its unique optical
and electrical properties, gallium and its derivatives are broadly
applied to energy storage of solar cell, catalysis and the manu-
facture of microelectronic product such as LEDs [3, 4]. As the
scale of the electronics industry is growing, the consumption of
gallium is getting increased. Therefore, we are easily exposed
to gallium ions. Gallium ions are known to be carcinogenic and
highly noxious to humans and animals [5–7]. For example,
visual, auditory toxicities and microcytic anemia may material-
ize in patients medicated with gallium nitrate. In addition, Ga-
based substances that are absorbed in the body through food
cause severe diseases including asthenia, vertigo, anemia and
skin cancer [8, 9]. Therefore, dependable methods are required
for the detection of gallium ion [10]. However, it has been
challenging to distinguish Ga3+ from Al3+ and In3+ due to the
similar properties of group 13 elements [11, 12].
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-020-02624-w) contains supplementary
material, which is available to authorized users.
* Cheal Kim
chealkim@snut.ac.kr
1
Department of Fine Chem, Seoul National Univ. of Sci. and Tech.
(SNUT), Seoul 01811, South Korea

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Experimental Section
Materials and Equipment
4-(Diethylamino)-2-hydroxybenzaldehyde (98%), 1-naphthyl
isothiocyanate (98%), hydrazine (80%) and gallium(III) ni-
trate hydrate (Ga(NO3)3·xH2O, 99.9%) were acquired com-
mercially from TCI (located in Tokyo, Japan) and Sigma-
Aldrich (located in St. Louis, USA). A Varian 400 MHz spec-
1
trometer was used to afford 13C and H NMR spectra. With
Perkin Elmer spectrometers (Lambda 365 for UV-vis and LS
45 for fluorescence), absorption and emission data were pro-
vided. A Thermo Finnigan LCQTM Advantage MAX quad-
rupole ion trap instrument was employed to collect ESI-MS
spectra.
Fig. 1 Fluorescent changes of NADA (2 × 10−6 M) with varied cations
(12 equiv.; Co2+, K+, Cu2+, Pb2+, Ca2+, Zn2+, Na+, Al3+, Mn2+, Fe2+
,
Cr3+, Ni2+, Fe3+, Mg2+, Cd2+ and In3+ and Ga3+) (λex = 400 nm)
Synthesis of Sensor NADA
1
Compound NH (N-(naphthalen-1-yl) hydrazine
carbothioamide) was synthesized through the nucleophilic ad-
dition of 1-naphthyl isothiocyanate with hydrazine. 4 mmol of
1-naphthyl isothiocyanate and 5 mmol of hydrazine
monohydrate were dissolved in 15 mL of ethanol
(EtOH). The reaction mixture was stirred until a
white-colored powder was formed, and the resultant
powder was filtered and washed with ether. 1H NMR (deuter-
ated DMSO) δ (ppm): 9.2 (s, 1H), 7.9 (m, 2H), 7.7 (d, 1H, J =
8 Hz), 7.6 (s, 1H), 7.5 (m, 4H) 4.9 (s, 2H).
(Fig. S1). H NMR (DMF-d7): δ 11.54 (s, 1H), 10.18 (s,
1H), 9.79 (s, 1H), 8.62 (s, 1H), 8.0 (m, 2H) 7.91 (d, 1H),
7.72 (m, 2H), 7.55 (t, 3H), 6.33 (d, 1H), 6.25 (s, 1H), 3.40
(m, 4H). 1.15 (t, 6H). 13C NMR (deuterated DMSO): δ (ppm):
12.5 (3C), 43.8 (3C), 97.3 (1C), 104.00 (1C), 107.5 (1C),
123.3 (1C), 125.4 (1C) 125.9 (3C), 126.1 (1C), 126.4 (1C),
127.8 (1C), 130.5 (1C), 133.7 (1C), 142.4 (1C), 150.2 (1C),
158.2 (1C). ESI-mass: calcd (calculated) for C22H25N4OS +
H+ ([NADA + H+])+: 393.17 found 393.33.
NADA was synthesized by the reaction of NH (217.29 mg,
1 mmol) and 4-(diethylamino)-2-hydroxybenzaldehyde
(231.9 mg, 1.2 mmol) in EtOH (5 mL). The solution was
stirred for 16 h at 24 °C. Pale yellow powder was formed,
filtered and washed with ether. NADA was soluble in
DMSO, DMF and THF, and stable for 6 h in the solvents
Fluorescent and UV-Visible Titrations
Compound NADA (0.39 mg, 1 × 10−3 mmol) was dissolved
in 1.0 mL of DMSO. 6 μL (1 × 10−3 M) of the NADA was
diluted in 2.994 mL MeOH to make 2 × 10−6 M. 0.3–3.6 μL
(or 0.3–3.3 μL for UV-vis titration) of Ga(NO3)3 (2 × 10−2 M)
Scheme 1 Synthesis of NADA

J Fluoresc
dissolved in MeOH were added to sensor NADA (2 × 10−6 M,
3.0 mL). After mixing them in a minute, fluorescence spectra
(or UV-visible spectra) were taken.
0.12
0.09
0.06
0.03
Quantum Yields
With quinine as a standard fluorophore (ФF = 0.54 in 1 × 10−1
M H2SO4), quantum yield (Ф) was provided. The equation is
as follows [35]:
2
ΦFðxÞ ¼ ΦFðsÞ ðAS FX=AX FÞ ðnx=nsÞ
300
400
500
600
700
(ФF: fluorescent quantum yield; A: absorbance; F: area of
fluorescence emission curve; s: standard; x: unknown; n: re-
fractive index of the solvent).
λ (nm)
Fig. 3 UV-vis changes of probe NADA (2 × 10−6 M) with different
concentrations of Ga3+ (0–11 equiv)
Job Plot
equiv. 5.25 μL (2 × 10−2 M) of Ga3+ ion was added into the
solutions to afford 3.5 equiv. 6 μL of NADA (1 × 10−3 M) was
added into the blended solutions. The solutions were well
blended, and their fluorescence spectra were taken.
A stock solution of sensor NADA (1 × 10−3 M) was given in
1.0 mL of DMSO. Ga3+ solution (1 × 10−3 M) was provided
with its nitrate salt in MeOH (1.0 mL). 0.27–0.03 mL of the
NADA was transferred to several quartz cuvettes. 0.03–
0.27 mL of Ga3+ solution was added to diluted NADA.
Each quartz cuvette was filled with 3 ml of MeOH.
The solutions were well blended, and their fluorescence
spectra were taken.
1H NMR Titration
Four NMR tubes of NADA (3.9 mg, 1 × 10−2 mmol) dis-
solved in deuterated dimethylformamide (0.7 mL) were
afforded, and diverse equivalents (0, 0.5, 1.0 and 2.0 equiv)
of Ga(NO3)3 dissolved in dimethylformamide (0.3 mL) were
Competition Experiments
1
transferred separately to the solutions of NADA. H NMR
Sensor NADA (0.39 mg, 1 × 10−3 mmol) was dissolved in
DMSO (100 μL). Stock solutions (0.6 mmol) of
Ga(NO3)3, KNO3, Cr(NO3)3, Fe(NO3)3, Cd(NO3)2,
Al(NO3)3, Ni(NO3)2, Ca(NO3)2, NaNO3, Co(NO3)2,
Mn(NO3)2, In(NO3)3, Cu(NO3)2, Fe(NO3)2, Pb(NO3)2, and
Mg(NO3)2, were dissolved in 3.0 mL MeOH. 3.6 μL of each
metal (2 × 10−2 M) was added into 3.0 mL MeOH to make 12
data were gained after blending the solutions for 30 s.
Calculations
All theoretical calculations for NADA and NADA-Ga3+ were
made using the gaussian 16 W program [36]. DFT calcula-
tions for geometry optimization were performed by applying
the basis of B3LYP/6-311G to C, O, N, H and S atoms, and
800
600
400
200
0
800
600
400
200
0
450
500
550
0.0
0.2
0.4
0.6
1.0
[NADA]/([NADA]+[Ga03.+8])
λ (nm)
Fig. 2 Fluorescent spectra of NADA (2 × 10−6 M) with the variation of
concentrations of Ga3+ (0–12 equiv)
Fig. 4 Job plot for the binding of NADA with Ga3+

J Fluoresc
Scheme 2 Proper structure of
NADA-Ga3+
LANL2DZ to Ga3+ [37–43]. The integral equation formalism
polarizable continuum model was employed to consider the
effect of the solvent, methanol [44, 45]. The energy level and
transition state of NADA and NADA-Ga3+ were analyzed
through time-dependence DFT calculation.
To check out the fluorescent behavior of NADA, the fluo-
rescent emission change was measured with varied cations
(Co2+, K+, Cu2+, Pb2+, Ca2+, Zn2+, Na+, Al3+, Mn2+, Fe2+,
Cr3+, Ni2+, Fe3+, Mg2+, Cd2+ and In3+) in MeOH. As exhibited
in Fig. 1, NADA (Ф = 0.01) and NADA with most cations
displayed little fluorescence band at 457 nm (λex = 400 nm).
Al3+, In3+ and Pb2+ increased slightly the band. However, the
addition of Ga3+ showed a dramatic increment of fluorescence
at 457 nm (Ф = 0.134) within 2 min (Fig. S2). Therefore,
NADA may be employed as a sensor for the selective fluores-
cent detection of Ga3+. Importantly, NADA could effectively
discriminate Ga3+ from In3+ and Al3+.
Results and Discussion
The intermediate compound NH was produced via the
nucleophilic addition of 1-naphthyl isothiocyanate with
hydrazine. Sensor NADA was produced via the conden-
sation reaction of NH with 4-(diethylamino)-2-
hydroxybenzaldehyde (Scheme 1), and verified by 13C
NMR, 1H NMR and ESI-mass.
To know the photophysical character of NADA to Ga3+, fluo-
rescent and UV-vis titrations were achieved (Figs. 2 and 3). In the
fluorescent titration, the fluorescence emission of NADA at
Fig. 5 Energy-minimized structures of (a) NADA and (b) NADA-Ga3+

J Fluoresc
457 nm increased constantly until the amount of Ga3+ reached up
to 12 equiv. On addition of Ga3+ into NADA for UV-vis titration
(Fig. 3), the absorbance of 380 nm constantly decreased, and the
absorbance of 400 nm increased consistently until the amount of
Ga3+ reached up to 11 equiv. An explicit isosbestic point
displayed at 390 nm suggested that the reaction of NADA with
Ga3+ afforded one species. The detection limit of Ga3+ with
NADA was calculated to be 0.287 μM (3σ/k) through fluores-
cence titration (R2 = 0.9941) (Fig. S3) [46]. Interestingly, NADA
has the lowest detection limit compared to other naphthyl deriv-
ative sensors (Table S1).
major absorptions of NADA and NADA-Ga3+ were ICT (Fig.
S10). Given the similar orbital movement in two molecules
and the formation of a more rigid structure in the complex, the
fluorescence turn-on process for NADA-Ga3+ could be CHEF
(chelation enhanced fluorescence) effect. Non-radiative tran-
sitions like rotation and vibration were limited during complex
formation, while radiative transition like fluorescence emis-
sion increased. The reduced HOMO-LUMO energy gap was
consistent with bathochromic shift in UV-vis spectra.
Considering various experimental results and theoretical cal-
culations, plausible detection mechanism for NADA-Ga3+
was suggested in Scheme 2.
For the purpose of studying the complexation ratio of
NADA with Ga3+, Job plot experiment was accomplished
and resulted in the highest value at a molar fraction of 0.5
(Fig. 4). It indicated that one Ga3+ combined with one
NADA. It was further supported by ESI-mass analysis (Fig.
S4). The peak of m/z: 537.16 was proposed to be [NADA(-
2H+) + Ga3+ + DMSO]+ (calcd, 537.09) in positive-ion spec-
trum. Moreover, the proton NMR titration was implemented
to analyze how to bind NADA with Ga3+ (Fig. S5). With the
addition of diverse concentrations of Ga3+ (0.5, 1.0 and 2.0
equiv) into NADA, the proton H9 of -NH and proton H2 of -
OH disappeared. The proton H8 of -NH and proton H1 of the
imine were shifted to down field (0.082 and 0.195 ppm, re-
spectively). These results led us to assume that Ga3+ would
coordinate to two nitrogen atoms and one oxygen one of
NADA. Summarizing the results of Job plot, ESI-mass and
1H NMR titration, the appropriate structure of NADA-Ga3+
was proposed (Scheme 2).
Conclusion
We represented the thiourea-based fluorescent chemosensor
NADA that can selectively detect Ga3+ by a fluorescence turn-
on mechanism. NADA had very low detection limit
(0.287 μM) and high binding constant (1.2 × 105 M−1) for
Ga3+. Importantly, NADA showed the lowest detection limit
compared to other naphthyl derivative sensors. In addition,
NADA could effectively discriminate Ga3+ from Al3+ and
In3+. Binding mechanism of NADA to Ga3+ could be ex-
plained by ESI-mass, NMR titration, UV-vis and fluorescent
titration.
Acknowledgements The National Research Foundation of Korea (NRF)
(2018R1A2B6001686) is greatly acknowledged.
The binding constant of NADA-Ga3+ complex was given
to be 1.2 × 105 M−1 from Benesi-Hildebrand equation (Fig.
S6) [47]. Competition experiment was accomplished to know
if NADA could effectively bind to Ga3+ with the coexisting
other cations (Fig. S7). Most cations did not inhibit the bind-
ing of NADA with Ga3+. However, Cu2+ showed fluorescence
quenching of NADA-Ga3+, and In3+, Fe2+ and Ag+ did some
fluorescence quenching (30–60%).
To get insights into detection mechanism of NADA toward
Ga3+, theoretical calculations were conducted. The energy-
minimized patterns of NADA and NADA-Ga3+ were exhibit-
ed in Fig. 5. The structure of NADA was slightly distorted by
the rotation of the naphthyl ring (1O, 2 N, 3 N, 4 N = 129.61°),
whereas NADA-Ga3+ showed a more flattened structure (1O,
2 N, 3 N, 4 N = −0.43°) with restricted rotation of the naphthyl
ring in the complex formation process.
To understand molecular orbital transitions and possible
transition states, TD-DFT calculations were executed. The
main absorption of NADA was observed at 373.98 nm, affect-
ed by HOMO → LUMO and HOMO-1 → LUMO transitions
(Fig. S8). The major absorption of NADA-Ga3+ appeared at
418.48 nm through HOMO → LUMO and HOMO-1 →
LUMO+1 transitions (Fig. S9). Because of showing the ap-
parent transition of the molecular orbital, the characteristics of
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