A naphthyl thiourea-based effective chemosensor for fluorescence detection of Ag+ and Zn2+

Article abstract of DOI:10.1002/bio.4114

A naphthyl thiourea-based effective chemosensor HNC, (E)-2-(2-hydroxy-3-methoxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, was synthesized. HNC showed quick responses toward Ag⁺ and Zn²⁺ through marked fluorescence turn-on in different solvent conditions, respectively. Binding proportions of HNC to Ag⁺ and Zn²⁺ were found to be 2:1 and 1:1, respectively. Detection limits of HNC for Ag⁺ and Zn²⁺ were calculated as 3.82 and 0.21 μM. Binding processes of HNC for Ag⁺ and Zn²⁺ were represented using Job’s plot, DFT, ¹H NMR titration, and ESI-MS.

Full text of DOI:10.1002/bio.4114

Received: 11 April 2021
DOI: 10.1002/bio.4114
Revised: 22 June 2021
Accepted: 27 June 2021
R E S E A R C H A R T I C L E
A naphthyl thiourea-based effective chemosensor for
+
2+
fluorescence detection of Ag and Zn
Yuna Seo | Soyoung Park | Gyeongjin Kim | Minji Lee | Cheal Kim
Department of Fine Chemistry, Seoul National
Abstract
University of Science and Technology (SNUT),
Seoul, South Korea
A naphthyl thiourea-based effective chemosensor HNC, (E)-2-(2-hydroxy-3-
methoxybenzylidene)-N-(naphthalen-1-yl)hydrazine-1-carbothioamide, was synthesized.
Correspondence
Soyoung Park and Minji Lee, Cheal Kim,
Department of Fine Chemistry, Seoul National
University of Science and Technology (SNUT).
Seoul 01811, South Korea.
+
2+
HNC showed quick responses toward Ag and Zn through marked fluorescence
turn-on in different solvent conditions, respectively. Binding proportions of HNC to
+
2+
Ag and Zn were found to be 2:1 and 1:1, respectively. Detection limits of HNC
Email: soyp19@gmail.com;
+
2+
for Ag and Zn were calculated as 3.82 and 0.21 μM. Binding processes of HNC
chealkim@snut.ac.kr
+
2+
1
for Ag and Zn were represented using Job’s plot, DFT, H NMR titration, and
Funding information
ESI-MS.
The National Research Foundation of Korea,
Grant/Award Numbers: 2018R1A2B6001686,
NRF-2020R1A6A1A03042742
K E Y W O R D S
+
2+
Ag , calculations, chemosensor, fluorescent, thiourea, Zn
1
|
INTRODUCTION
The naphthyl group, one of the fluorophores, can be a valuable sig-
[
33,34]
nal in part due to its excellent sensitivity and high quantum yield.
The thiourea moiety with N and S atoms can act as a linker and chelate
+
Ag is a significant valuable metal ion and broadly used in industry,
[
35–37]
such as in the manufacturing of fungicides, in electronic and electrical
selectively soft metal ions via the hard–soft acid–base theory.
In
[
1–5]
applications, and in photographic production.
These broad appli-
[6,7]
addition, O-vanillin has unique spectroscopic properties as a chromo-
+
[38–40]
cations increase the amounts of Ag in environmental systems.
phore.
Therefore, we expected that a naphthyl thiourea-based
+
[8]
Therefore, Ag could cause serious problems for humans. For
sensor with an O-vanillin moiety can effectively detect soft metal ions
+
+
2+
instance, Ag can inactivate sulfur-containing enzymes and bind to
such as Ag and Zn with a strong and unique emission.
2
+
imidazole, carboxyl, and amine moieties of diverse metabolites. Zn
Here, we describe a naphthyl thiourea-based effective sensor
[9,10]
+
2+
is an abundant, essential metal ion in biology.
For several decades,
HNC that could detect Ag and Zn using different fluorescent col-
2
+
+
2+
Zn has been well recognized as an essential element in fundamental
biological metabolisms such as growth and reproduction of living
ours, while chemosensors capable of detecting both Ag and Zn are
very rare. In particular, HNC showed the lowest detection limit
[
11–13]
2+
+
organisms and gene transcription.
Therefore, an imbalance in
[14,15]
(0.21 μM) for Zn among chemosensors for detecting both Ag and
2
+
2+
+
2+
Zn levels may be related to pathological troubles.
Particularly,
Zn . HNC reacted with Ag and Zn at the ratios of 2:1 and 1:1,
2
+
excessive and deficiency levels of Zn result in neurodegenerative
respectively. Considering Job’s plot, ESI-MS, fluorescence, and
1
problems such as iron deficiency, peripheral neuropathy, and
UV–visible light titrations, calculations and H NMR titration, detection
[
16]
+
2+
+
2+
Parkinson's disease.
Therefore, effective sensing of Ag and Zn
processes of Ag -2ꢀHNC and Zn -HNC were illustrated.
[17–19]
has constantly attracted intensive attention in diverse areas.
+
2+
To detect the transition metals such as Ag and Zn , several
approaches such as electrochemistry, surfaces plasmon resonance
spectroscopy and atomic absorption spectroscopy (AAS) have been
2
|
EXPERIMENTAL
| General information
2.1
[
20–25]
developed.
cated instruments and disciplined engineers.
rescence methods can readily and easily detect target ions with fast
However, they demand time-consuming, sophisti-
[
26–28]
Conversely, fluo-
Chemicals (analytical and spectroscopic grade) were commercially pro-
1
13
vided from Sigma-Aldrich. H and C NMR measurements were exe-
cuted on a Varian 400 MHz machine. ESI-MS was provided with a
[29–32]
response and are easy to use.
Luminescence. 2021;1–8.
wileyonlinelibrary.com/journal/bio
© 2021 John Wiley & Sons, Ltd.
1

2
SEO ET AL.
Thermo MAX machine. UV–vis and fluorescence measurements were
2.5
|
Job’s plot
carried out using Perkin Elmer spectrometers (Lambda 25 and LS45).
+
ꢂ5
For Ag , HNC (1 ꢁ 10 mol) was dissolved in 1000 μl DMSO. 150 μl
ꢂ2
of the HNC (1 ꢁ 10 M) was diluted with 29.85 ml DMSO solution
to provide 50 μM. Next, 0.3–2.7 ml of the diluted HNC was put into
2
.2
|
Synthesis of HNC
ꢂ5
cuvettes. Ag(NO
3
) (2 ꢁ 10 mol) was dissolved in 1000 μl of DMSO;
+
ꢂ2
2
.2.1
|
Synthesis of NH
75 μl of the Ag (2 ꢁ 10 M) was diluted with 29.925 ml DMSO
solution to make 50 μM; 0.3–2.7 ml of the diluted Ag was put into
+
Hydrazine (4 ꢁ 10ꢂ3 mol) was added to 1-naphthyl isothiocyanate
each HNC. After 30 s, fluorescence spectra were measured.
ꢂ3
2+
ꢂ5
(
3 ꢁ 10 mol) in CH
3
CH
2
OH (10 ml). After stirring the solution for
For Zn , HNC (1 ꢁ 10 mol) was dissolved in 1000 μl DMSO;
ꢂ2
1
5 h, a white powder was obtained and rinsed with ethyl alcohol and
150 μl of the HNC (1 ꢁ 10 M) was diluted with 29.85 ml buffer/
MeCN solution (7:3) to make 50 μM; 0.3–2.7 ml of the diluted HNC
1
ether. H NMR (dimethyl sulphoxide (DMSO)-d
6
): 9.21 (s, 1H), 7.90
2+
(2 ꢁ 10ꢂ2 M) was
(
m, 2H), 7.81 (d, 1H, J = 8.1 Hz), 7.73 (s, 1H), 7.50 (m, 4H) 4.90 (s, 2H).
was put into cuvettes; 75 μl of the Zn
diluted with 29.925 ml buffer/MeCN solution (7:3) to make 50 μM.
2
+
0.3–2.7 ml of the diluted Zn was put into each HNC. After 30 s,
2
.2.2
|
Synthesis of HNC
fluorescence spectra were measured.
O-Vanillin (0.076 g, 5 ꢁ 10ꢂ4 mol) was added to NH (0.108 g,
ꢂ4
5
ꢁ 10 mol) in CH
3
CH
2
OH (10 ml). The solution was shaken for
2.6
|
Competition experiments
ꢃ
1
3 h at 25 C. A white-coloured powder was filtered and rinsed with
1
HNC (1 ꢁ 10ꢂ5 mol) was dissolved in 1000 μl DMSO. Next, 20 μmol
ether (510 mg; 30%). H NMR (DMSO-d
6
): 11.88 (s, 1H), 10.31
(
s, 1H), 9.26 (s, 1H), 8.58 (s, 1H), 7.98 (m, 1H), 7.89 (m, 2H), 7.76 (m, 1H),
of various metals (Ni(NO
3
)
2
, Pb(NO
KNO Zn(NO
, Hg(NO , Co(NO
3
)
2
, Fe(NO
3
)
3
, In(NO
Ga(NO
, Ca(NO
3
)
3
, NaNO
3
,
1
3
7
.54 (m, 4H), 6.99 (m, 1H), 6.77 (m, 1H), 3.35 (s, 3H). C NMR
): δ 176.57, 147.82, 146.04, 139.67, 135.70, 133.61,
Mn(NO
Cu(NO
Al(NO
3
)
2
,
Cr(NO
3
)
3
,
3
,
3
)
2
,
3
)
3
,
Cd(NO
3
)
2
,
(
DMSO-d
6
3
)
2
, Mg(NO
3
)
2
3
)
2
3
)
2
3
)
2
, Fe(NO
3
)
2
and
1
1
C
30.52, 127.97, 127.83, 126.83, 126.68, 126.33, 125.91, 125.33,
3 3
) ) were also dissolved in 1000 μl DMSO.
+
ꢂ2
+
23.40, 120.74, 118.78, 112.86, 55.91 ppm. ESI-mass: cald for
For Ag , 36.0 μl of each cation (2 ꢁ 10 M) and 36.0 μl of Ag
+
+
+
ꢂ2
19
H
17
N
3
O
2
S + H ([HNC + H ]) : 576.18 found 576.34.
(2 ꢁ 10
(
M) was put into 3000 μl DMSO; 6 μl of HNC solution
2+
ꢂ2
1 ꢁ 10 M) was put into the mixed solutions. For Zn , 2.4 μl of
2+
each cation (2 ꢁ 10 M) and 2.4 μl of Zn (2 ꢁ 10ꢂ2 M) were trans-
ferred into 3000 μl of buffer/MeCN solution (7:3). After 30 s, fluores-
cence spectra were measured.
ꢂ2
2
.3
|
Fluorescence and UV–visible light titrations
+
ꢂ5
For Ag , HNC (1 ꢁ 10 mol) was dissolved in 1000 μl DMSO, and 6.0 μl
ꢂ2
of the HNC (1 ꢁ 10 M) was diluted with 2.994 ml DMSO solution to
provide 20 μM. Ag(NO
) (2 ꢁ 10ꢂ5 mol) was dissolved in 1000 μl of
2.7
|
1
H NMR titration
3
DMSO. Next, 3–24 μl of the Ag(NO
3
) stock (2 ꢁ 10ꢂ2 M) was put into
HNC (20 μM). After 30 s, UV–visible and fluorescence spectra were taken.
Three NMR tubes of HNC (7.0 mg, 2 ꢁ 10ꢂ5 mol) dissolved in
For Zn , HNC (1 ꢁ 10ꢂ5 mol) was dissolved in 1000 μl DMSO,
2
+
DMSO-d
(1000 μl) were made, and 0–1.0 equivalents of AgNO
6
3
dis-
ꢂ2
1
and 6.0 μl of the HNC (1 ꢁ 10
buffer/MeCN solution (7:3, v/v, 0.1 M bis-tris, pH 7.0) to provide
M) was diluted with 2.994 ml
solved in DMSO-d
6
(1000 μl) were added to the solution of HNC.
H
NMR titration data were obtained after mixing the solution for 20 s.
2
0
0 μM. Zn(NO
3
)
2
(2 ꢁ 10ꢂ5 mol) was dissolved in MeCN (1 ml).
For Zn2+, two NMR tubes of HNC (7.0 mg, 2 ꢁ 10ꢂ5 mol) dis-
.3–3.0 μl of the Zn(NO
3
)
2
stock (2 ꢁ 10ꢂ2 M) was put into HNC
solved in DMSO-d
6
(200 μl) and CD CN (500 μl) were made, and 0–1
3
(
20 μM). After 30 s, UV–visible and fluorescence spectra were taken.
equivalents of Zn(NO
3
)
2
dissolved in DMSO-d
1
6
(1000 μl) were added
to the solution of HNC. H NMR titration data were obtained after
mixing the solution for 20 s.
2
.4
|
Quantum yields
Quantum yield (Ф) was derived from the following equation with qui-
2.8
|
Calculations
[41]
F 2 4
nine as standard: (Ф = 0.53 in 100 mM H SO )
+
2+
All calculations for HNC, Ag -2ꢀHNC and Zn -HNC were performed
[43–46]
2
[42]
ΦFðxÞ ¼ ΦFðsÞ ðASFX=AXFÞðnx=nsÞ
with the Gaussian 16 W program.
The B3LYP/6-31G (d,p)
basis set was used for HNC and the LANL2DZ basis was applied for
+
2+ [47–49]
where F: area of fluorescence emission curve; s: standard; A: absor-
bance; n: refractive index of the solvent; x: unknown.
Ag and Zn
.
Imaginary frequencies were not discovered in
2+
+
HNC, Ag -2ꢀHNC and Zn -HNC, signifying that these structures

SEO ET AL.
3
+
2+
2+
+
3+
2+
2+
+
2+
2+
defined local minima. The integral equation formalism variant
Ag , Pb , Co , Na , Al , Ni , Zn , K , Mn and Fe in DMSO
(
IEFPCM) was used to consider the solvent effects of dimethyl sulfox-
(Figure 1). On addition of the cations (12 equiv) into HNC
[
50,51]
+
ide and acetonitrile.
The transition states and energy levels of
(Ф = 0.0051), only Ag exhibited the striking fluorescence increment
+
2+
HNC, Ag -2ꢀHNC and Zn -HNC were obtained through TD-DFT.
(Ф = 0.0214) at 340 nm, whereas other metals showed no or slight
variations. These results indicated that sensor HNC was clearly able
+
to distinguish Ag .
3
|
RESULTS AND DISCUSSION
To analyze the photophysical characteristic of HNC, the fluores-
+
cence and UV–vis titrations of HNC with Ag were conducted
+
HNC was provided with the imine formation of O-vanillin with NH in
(Figures 2 and 3). The incremental addition of Ag to HNC caused a
1
13
ethyl alcohol (Scheme 1) and affirmed by H and C NMR and
striking emission increment at 436 nm with saturation at 12 equiv of
+
2+
+
+
ESI-MS. The sensing abilities of HNC to Ag and Zn were examined
with varied analytical tools such as UV–vis spectroscopy, fluorescence
Ag . Upon the addition of Ag (0–12 equiv) to HNC for UV–vis titra-
tion (Figure 3), the absorption at 330 nm was diminished, and two
definite isosbestic points at 302 and 361 nm appeared. The detection
1
spectroscopy, ESI-MS, H NMR titration, DFT calculation.
+
limit of HNC for Ag was provided to be 3.82 μM (3σ/k) with fluores-
[
52]
cence titration (Figure S1).
+
3
.1
|
Fluorescence sensing for Ag
Job’s plot test was performed to determine the association pro-
+
portion of HNC with Ag (Figure S2). A maximal intensity showed up
+
The fluorescence sensing variation of HNC was studied toward varied
at the molar fraction of 0.7, which means that one Ag may combine
2
+
3+
3+
2+
3+
2+
2+
3+
2+
[53]
+
cations such as Ca , Fe , In , Cd , Cr , Cu , Mg , Ga , Hg
,
with two HNC.
The structure of the Ag -2ꢀHNC complex was
SCHEME 1 Synthesis of HNC
FIGURE 1 Fluorescence intensity changes of
HNC (20 μM) with 12 equiv of metal ions in
+
DMSO. Inset: Photographs of HNC and Ag -
+
2
2
ꢀHNC (blue line = HNC and red line = Ag -
ꢀHNC)

4
SEO ET AL.
FIGURE 2 Fluorescence variations of HNC
+
(
20 μM) with increment of Ag . Inset:
Fluorescence intensity (at 436 nm) as the amount
+
+
of Ag (blue line = HNC and red line = Ag -
ꢀHNC)
2
+
SCHEME 2 Structure of Ag -2ꢀHNC
containing varied cations (12 equiv), most of them did not interfere
+
3+
2+
2+
with the detection of Ag using HNC, while for Fe , Fe and Hg
2
+
inhibited c. 30–50% and Cu inhibited completely. Additionally, we
+
checked the reversibility of HNC to Ag , but it was not successful.
FIGURE 3 Absorption variations of HNC (20 μM) with increment
+
+
of Ag (blue line = HNC and red line = Ag -2ꢀHNC)
2
+
3
.2
|
Spectroscopic response of HNC to Zn
2+
3+
+
proved with ESI-MS (Figure S3). The intensity of 809.25 (m/z)
The fluorescence spectra of HNC with varied cations (Co , Ga , K ,
+
+
2+
2+
2+
3+
+
3+
2+
2+
3+
2+
3+
2
was consistent with the [2ꢀHNC + Ag ] [calcd, 809.11]. Moreover,
Cu , Pb , Ca , Ga , Na , Al , Mn , Fe , Cr , Ni , Fe , Mg
1
+
2+
+
3+
2+
the H NMR titration demonstrated a clear interaction of HNC and
, Cd , Ag , In and Zn ) were measured in buffer/MeCN solution
+
+
Ag (Figure S4). On addition of Ag into HNC, the protons, H
, H
1 6
,
(7:3, v/v). As shown in Figure 4, HNC (Ф = 0.0137) and HNC with most
cations exhibited a small emission at 480 nm (λex = 380 nm). Alterna-
H
7, and H , were greatly shifted down field, while the protons
8
2+
attached to the benzene and the naphthyl moieties were moved
tively, HNC with Zn exhibited a marked greenish-blue fluorescence
+
slightly down field. These outcomes led us to propose that Ag may
response (Ф = 0.0864). Therefore, HNC could work as an effective
2
+
coordinate with the S of C=S and the N of imine. Job’s plot, ESI-MS
fluorescence sensor for detecting Zn . Conversely, at this stage it is
1
+
+
2+
analysis and H NMR titration suggested that Ag would react with
not clear why HNC detects only Ag in DMSO and only Zn in buffer/
two HNC (Scheme 2).
MeCN solution (7:3, v/v), respectively.
+
2+
Binding constant of HNC with Ag was provided to be
To analyze the chemosensing properties of HNC to Zn , fluores-
8
ꢂ1
0
[54]
5
ꢁ 10 M with Li s equation (Figure S5),
which is in the scope
cence and UV–visible light titrations were carried out (Figures 5
and 6). The emission of HNC at 480 nm increased up to the 0.8 equiv
2+
2
9
+
of those (10 to 10 ) formerly reported for Ag sensors. The practica-
+
ble sensing ability of HNC for Ag was executed with a competition
of Zn (Figure 5). In Figure 6, the absorbance peak at 330 nm con-
+
2+
test (Figure S6). On addition of HNC to Ag solution (12 equiv)
stantly decreased up to 0.8 equiv of Zn . Two absolute isosbestic

SEO ET AL.
5
FIGURE 4 Fluorescence intensity changes of
HNC (20 μM) with cations (0.8 equiv) in buffer/
MeCN solution (7:3, v/v). Inset: Photographs of
2
+
HNC and Zn -HNC (blue line = HNC and red
2+
line = Zn -HNC)
FIGURE 5 Fluorescence variations of HNC
2
+
(
20 μM) with increment of Zn . Inset:
Fluorescence emission (at 480 nm) as the amount
2
+
2+
of Zn (blue line = HNC and red line = Zn
HNC (0.8 equiv))
-
2+
points appeared at 269 and 353 nm, indicating binding of HNC with
(Figure S10). On addition of Zn into HNC, the protons, H
1
6
, H , H7,
2
+
2+
Zn . Using fluorescence titrations, the detection limit of Zn with
and H
8
, were slightly shifted down filed. These outcomes led us to
2+
[55]
HNC was obtained as 0.21 μM (3σ/k) (Figure S7).
Interestingly,
propose that Zn may coordinate with the O molecule of the phenol
2+
HNC has the lowest detection limit for Zn among chemosensors for
group, the S of C=S and the N of the imine. The analysis of Job plot,
+
2+
1
detecting both Ag and Zn (Table S1).
ESI-mass and H NMR titration proposed the appropriate binding for-
2+
2+
To determine the complexation ratio of HNC and Zn , Job’s plot
test was performed (Figure S8). The highest value appeared at molar
fraction of 0.5, which meant the formation of a 1:1 complex of HNC
mation of Zn -HNC (Scheme 3).
2+
The binding constant of HNC with Zn
was obtained as
[56]
4
ꢂ1
5.3 ꢁ 10 M with the Benesi–Hildebrand equation (Figure S11).
2+
2+
with Zn . This was verified with ESI-MS analysis (Figure S9). The
To determine whether HNC effectively detects to Zn in solution
+
intensity of 492.08 (m/z) was proposed to be [HNC-H + DMSO +
including other metal ions, competition experiments were conducted
2
+ +
]
1
Zn
(calcd, 492.04) in the positive-ion peak. Moreover, the
H
(Figure S12). Most metal ions did not inhibit the detection of HNC
2
+
2+
2+
2+
3+
NMR titration demonstrated a clear interaction of HNC and Zn
with Zn
,
while Cu
,
Fe
and Fe
showed fluorescence

6
SEO ET AL.
2
+
quenching of 30–50%. The pH response of Zn -HNC was obtained
at various pH ranges (6.0–9.0) (Figure S13). The result exhibited a
large fluorescence increment at pH 7.0–9.0, indicating that HNC
2
+
could detect Zn
over the pH range 7.0–9.0. Additionally, we
2+
checked the reversibility of HNC to Zn , but this was not successful.
+
3
.3
|
Theoretical calculations for Ag
+
To further understand the sensing mechanism of HNC to Ag , calcula-
tions were carried out. The energy-optimized patterns of HNC and
+
Ag -2ꢀHNC are shown in Figure 7. HNC had a dihedral angle of 2.40
+
(
1 N, 2 N, 3C, 4S), whereas Ag -2ꢀHNC had a dihedral angle of ꢂ6.76
+
(
1 N, 2 N, 3C, 4S). The interaction of HNC and Ag induced a distor-
tion in the structure of HNC.
FIGURE 6 Absorption variations of HNC (20 μM) with increment
of Zn (blue line = HNC and red line = Zn -HNC)
Based on the energy-optimized patterns, TD-DFT calculations
were implemented to investigate molecular orbital transitions and
2
+
2+
+
electronic transition states of HNC and Ag -2ꢀHNC. For HNC, the
main absorption was identified as the second excited state, which
originated from the HOMO!LUMO transition (341.51 nm), showing
the intramolecular charge transfer (ICT) characteristic (Figures S14
+
and S15). For Ag -2ꢀHNC, the main molecular orbital contribution of
the 1st excited state was determined for the HOMO!LUMO
transition (367.10 nm), showing the ICT characteristic (Figures S15
and S16). Considering the similar ICT characteristics and molecular
2+
SCHEME 3 Structure of Zn -HNC
+
FIGURE 7 Energy-optimized patterns of (a) HNC and (b) Ag -2ꢀHNC

SEO ET AL.
7
2+
FIGURE 8 Energy-optimized patterns of (a) HNC and (b) Zn -HNC
+
orbital transitions of the two compounds, the chelation of Ag
(Figures S18 and S19). Due to similar orbital characteristics in two
2
+
with HNC induced a decrease in the HOMO–LUMO energy gap of
compounds and the formation of the rigid form after binding to Zn
,
+
Ag -2ꢀHNC compared with that of HNC, which was consistent with
the fluorescence change process can be inferred by CHEF effect. The
restriction of nonradiative transition and improvement of radiative
transition occurred during the formation of the metal complex.
Furthermore, the change in the HOMO–LUMO energy gap
corresponded to the bathochromic movement in UV–visible
experiments. Taking into consideration Job’s plots, calculations,
the red shift in the UV–visible spectra. The decrease in the energy
gap could be the cause of the fluorescence enhancement. Therefore,
+
the fluorescence response of the Ag -2ꢀHNC complex could be dem-
onstrated by the chelation-enhanced fluorescence (CHEF) effect. Inte-
1
grating all the results from the Job’s plot, calculations, H NMR
+
1
titration, and ESI-MS, we suggested a sensing mechanism for Ag -
H NMR titration, and ESI-MS, we suggested a conceivable detection
2
+
2
ꢀHNC (Scheme 2).
mechanism for Zn -HNC (Scheme 3).
2+
3
.4
|
Theoretical calculations for Zn
4
|
CONCLUSION
2
+
To illustrate the sensing process of HNC for Zn , calculations were
We demonstrated an effective naphthyl thiourea-based fluorescence
2
+
made. The energy-optimized patterns of HNC and Zn -HNC are
shown in Figure 8. HNC with a distorted structure displayed a dihedral
angle of 177.64 (1O, 2 N, 3 N, 4S), which resulted from the rotation of
sensor HNC that showed a selective fluorescence turn-on mechanism
+
2+
for Ag and Zn over the varied metal ions. The complexation ratio
+
2+
of HNC with Ag and Zn were calculated to be 2:1 and 1:1. HNC
2
+
2+
+
the naphthyl group. Zn -HNC with a more flatted structure had a
dihedral angle of ꢂ0.97 (1 N, 2 N, 3C, 4S), which resulted from a small
rotation of the naphthyl group in the formation of the complex.
To examine molecular orbitals and transition energies, TD-DFT
calculations were conducted through energy-optimized patterns of
could be applied to sense Zn and Ag at low detection limits (0.21
2
and 3.82 μM). Importantly, HNC has the lowest detection limit for Zn
+
+
2+
among chemosensors for detecting both Ag and Zn . Sensing
+
2+
processes of HNC to Ag and Zn were elucidated using fluores-
1
cence titration, UV–vis, ESI-MS, DFT, and H NMR titration.
2
+
HNC and Zn -HNC. For HNC, the main absorption appeared from
the HOMO-1!LUMO transition (335.73 nm; Figures S17 and S18),
indicating ICT characteristics. As indicated from the most relevant
ACKNOWLEDGEMENTS
The National Research Foundation of Korea (NRF-2020R
2+
colour change in Zn -HNC, the excited state (386.18 nm)
mainly originated from HOMO!LUMO with ICT characteristics
1A6A1A03042742
acknowledged.
and
2018R1A2B6001686)
is
gratefully

8
SEO ET AL.
ORCID
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