Comparison of two rhodamine-based polystyrene solid-phase fluorescent sensors for mercury(II) determination

Article abstract of DOI:10.1177/1747519820904854

Two novel rhodamine-based polystyrene solid-phase fluorescent sensors PS-AC-I and PS-AC-II with different coordination atoms (O or S) are synthesized and shown to be able to detect Hg(II) ions. They are characterized by Fourier-transform infrared spectroscopy and by scanning electron microscopy analysis. Their fluorescent properties, including response time, pH effects, fluorescence titrations, metal ion competition and recycling, are investigated and compared. Sensor PS-AC-II displayed higher selectivity and sensitivity to Hg(II), with a lower detection limit of 0.032 μM, which was 15 times better than PS-AC-I. A detection mechanism involving the Hg(II) chelation-induced ring-opening of the rhodamine spirolactam is proposed with the aid of theoretical calculations.

Full text of DOI:10.1177/1747519820904854

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research-arti
0cle2020 4854CHL0010.1177/1747519820904854Journal of Chemical ResearchLiu et al.
Research Paper
Journal of Chemical Research
–8
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Comparison of two rhodamine-based
polystyrene solid-phase fluorescent sensors
for mercury(II) determination
© The Author(s) 2020
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2
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1
Yuanyuan Liu , Yi Li , Linghui Zong and Jingyi Zhang
Abstract
Two novel rhodamine-based polystyrene solid-phase fluorescent sensors PS-AC-I and PS-AC-II with different
coordination atoms (O or S) are synthesized and shown to be able to detect Hg(II) ions. They are characterized by
Fourier-transform infrared spectroscopy and by scanning electron microscopy analysis. Their fluorescent properties,
including response time, pH effects, fluorescence titrations, metal ion competition and recycling, are investigated and
compared. Sensor PS-AC-II displayed higher selectivity and sensitivity to Hg(II), with a lower detection limit of 0.032µM,
which was 15 times better than PS-AC-I. A detection mechanism involving the Hg(II) chelation-induced ring-opening of
the rhodamine spirolactam is proposed with the aid of theoretical calculations.
Keywords
fluorescent properties, Hg(II) detection mechanism, metal ion competition, rhodamine-based polystyrene
microspheres, solid-phase fluorescent sensors
Date received: 30 August 2019; accepted: 16 January 2020
Chloroacetylated polystyrene microspheres, which are pre-
Introduction
pared from polystyrene microspheres by Friedel-Crafts
Recently, more and more chemical fluorescent sensors have
acylation, exhibit regular shapes and high chemical resist-
been developed for metal ion detection due to their high sen-
ance, and possess a large number of chloroacetyl groups on
1
–4
sitivity, short detection times, and simple operation.
16,17
the surface to further conduct chemical modifications.
Unfortunately, many of them cannot be reused on a regular
basis. Hence, the design of reusable solid-phase fluores-
cence sensors has become a popular research topic. In gen-
eral, these solid-phase sensors consist of a small-molecule
fluorescent probe and carriers. Rhodamine is such a probe
These unique properties make these microspheres ideal
scaffolds for heavy metal ion sensors.
1School of Pharmaceutical and Chemical Engineering, Chengxian
College, Southeast University, Nanjing, P.R. China
5
–7
possessing excellent photophysical properties and exhib-
its chelation-enhanced fluorescence (CHEF),
2
College of Food Science and Light Industry, Nanjing Tech University,
Nanjing, P.R. China
8–12
whereas
polystyrene microspheres as carriers have been widely used
in enzyme immobilization, chromatographic separation, and
biological sensing because of their large surface area, stable
Corresponding author:
Yi Li, College of Food Science and Light Industry, Nanjing Tech
University, Nanjing 211816, P.R. China.
Email: liynj2012@njtech.edu.cn
13–15
material properties, and surface reaction abilities.

2
Journal of Chemical Research 00(0)
Figure 1. Design strategy for rhodamine-based polystyrene fluorescent sensors.
Scheme 1. Preparation of sensors PS-AC-I and PS-AC-II.
In this paper, two novel rhodamine-based polystyrene polystyrene microspheres PS-AC to I or II in tetrahydrofuran
sensors, PS-AC-I and PS-AC-II, with different coordina- using sodium bicarbonate (NaHCO ) as an acid binding
3
tion atoms (O or S) are designed, prepared, and character- agent. The most satisfactory result was obtained when the
ized (Figure 1). Their fluorescent properties, including reaction was performed in hot tetrahydrofuran for 24h. The
response time, pH effects, fluorescence titrations, metal ion crude products were washed with methanol and water several
competition and recycling, are investigated and compared. times, and then dried under vacuum at 30°C for 24h to give
The detection mechanism is discussed in detail with the aid PS-AC-I and PS-AC-II. By comparison of their FTIR spec-
1
8,19
of theoretical calculations.
tra (Figure 2), the C-Cl stretching vibration of PS-AC
at
-1
9
1
94cm was replaced by C-N stretching vibrations at
013cm− and 1015cm , respectively, indicating that some
1
−1
Results and discussion
chloroacetyl groups on the surface of microspheres were sub-
stituted. The corresponding C=O stretching vibrations and
aromatic C=C skeletal vibrations were observed around
Synthesis and characterization
Compound I was synthesized by the condensation of rhoda-
mine B with ethylenediamine under refluxing conditions,
which then afforded II via substitution with Lawesson’s rea-
gent in toluene (Scheme 1). The structures were confirmed
−1
−1
−1
1
695cm , 1608cm , and 1502cm , respectively, along
with significant enhancement in intensity. Moreover, the
color of the microspheres changed from white to orange, fur-
ther confirming that I and II were successfully immobilized
on the surface of PS-AC.
1
1
by H NMR and mass spectrometry. In the H NMR spectra,
as a result of the deshielding effect of the N-atom, the CH of
2
the N-ethyl side chain appeared as quartet at 3.13–3.51ppm.
Scanning electron micrography (SEM)
The CH of the N-ethyl side chain appeared as a triplet at
3
1.08ppm, with a coupling constant of around 7.0Hz. All the In order to illustrate that I and II had been successfully
aromatic protons resonated in the range of 6–8ppm. The [M immobilized on the surface of PS-AC, a comparison of the
+
+
5
H] peak for I and II appeared at m/z 485.29 and m/z SEMs of PS-AC and PS-AC-I was undertaken. SEM allows
01.49, respectively, which is in good agreement with the imaging directly on the sample surface. As shown in Figure
calculated values (485.29 for I and 501.27 for II). 3, the SEM for PS-AC-I was quite different from PS-AC.
The preparation of sensors PS-AC-I and PS-AC-II was From the two images (×150) in Figure 3(a) and (b), all
carried out using a 1:1 molar ratio of chloroacetylated the microspheres demonstrated good sphericity and

Liu et al.
3
monodispersity, with no significant changes in morphology. monitoring. Moreover, the detection of metal ions is easily
However, from the two images (×1000) in Figure 3(c) and interfered by protons, so it is necessary to explore the fluo-
3(d), the diameter of PS-AC-I increased compared with that rescence intensity of these sensors at different pHs. As
of PS-AC, and the surface had many recesses. The obvious shown in Figure 4, free sensors PS-AC-I and PS-AC-II
changes in surface morphology indicated that the immobili- produced negligible fluorescence with changes at pH values
zation process was successful.
of 6.0–9.0. However, obvious fluorescence enhancement
was detected when the pH was less than 6, which might be
because of protonation causing the spirocycle to open. For
further spectral analysis, the pH should be adjusted to 7.4.
Response time and pH dependence
Fluorescence analysis indicated that the recognition between
PS-AC-I and Hg(II) was complete in 60 min, whereas
PS-AC-II responded more quickly to Hg(II) and its fluores-
cence intensity was stable 10 min after adding Hg(II). Thus,
both sensors had potential utility in Hg(II) real-time
Fluorescence titration of Hg(II)
To explore the sensing properties, fluorescence titrations of
PS-AC-I and PS-AC-II with Hg(II) were conducted in
acetonitrile. Upon incremental addition of Hg(II), enhanced
fluorescence emission was observed at 580nm (Figure 5).
For PS-AC-I, Hg(II) expressed a linear concentration range
from 0 to 8µM with a correlation coefficient of 0.9958. The
detection limit was determined to be 0.483µM based on
3
s/k, where s is the standard deviation and k is the slope of
2
0,21
thecalibrationplot.
ComparedwithPS-AC-I, PS-AC-II
had a much higher fluorescence response to Hg(II), which
showed a linear concentration range from 0.05 to 0.35µM
with a correlation coefficient of 0.9835 and a lower detec-
tion limit of 0.032µM. Moreover, the microsphere surface
of PS-AC-II showed a color change from orange to rosy
red after the addition of Hg(II) (Figure 5(b)), which illus-
trated that PS-AC-II could perform a fluorescence “off-on”
response, allowing visual recognition of Hg(II) detection.
Selectivity to various metal ions
To evaluate the selectivity of PS-AC-I and PS-AC-II, com-
Figure 2. FTIR spectra of PS-AC, PS-AC-I and PS-AC-II.
mon metal ions including alkali, alkaline earth, and
Figure 3. SEM images of PS-AC (a, c) and PS-AC-I (b, d).

4
Journal of Chemical Research 00(0)
transition-metal ions were used in competitive experiments
(
Figure 6). Almost no fluorescence intensity enhancement
+
2+
2+
2+
+
2+
was detected after adding Ag , Ca , Cd , Co , K , Mg ,
Na , Ni , Pb , Zn , Fe , Al , Cr , and Co ions except
+
2+
2+
2+
3+
3+
3+
3+
2+
for PS-AC-I which showed a weak response to Cu (black
bars). However, when Hg(II) was added, significant varia-
tion was observed (red bars). Sensor PS-AC-II had a much
higher fluorescence response than PS-AC-I, along with a
significant color change from orange to rosy red (Figure
5(b)), whereas with other metal ions it remained yellow. The
above results indicate that PS-AC-I and PS-AC-II are highly
selective to Hg(II) compared with other common metal ions.
Detection mechanism for Hg(II)
According to the theory of hard-soft acids and bases, hard-
to-hard or soft-to-soft bonds form faster, leads to the forma-
22–24
tion of more stable compounds.
In PS-AC-I and
PS-AC-II, the O, S and N atoms acted as suppliers of elec-
trons in the coordination process with metal ions. Hg(II) is a
soft acid, whereas the donor S is a typical soft base, so S has
high selectivity for Hg(II). According to the fluorescence
properties, Hg(II) can coordinate with the N and S atoms on
the rhodamine ring, and then quickly form stable coordina-
tion bonds. However, the donor O is a hard base, which
leads to a longer response time for Hg(II). As shown in
Figure 7, before the addition of Hg(II), rhodamine forms a
closed lactam spirocycle. Hg(II) ions can chelate via the O,
S, and N atoms, leading to spirolactam ring-opening along
with a significant fluorescence enhancement at 580nm.
Figure 4. Fluorescence responses of PS-AC-I (a) and PS-
AC-II (b) at different pH values.
Figure 5. Fluorescence titration spectra of PS-AC-I (a) and PS-AC-II (b) in acetonitrile upon the incremental addition of Hg(II).

Liu et al.
5
Figure 6. Fluorescence responses of PS-AC-I (a) and PS-AC-II (b) to various metal ions.
Theoretical calculations also indicated that chelation with Conclusion
Hg(II) made the total energy of PS-AC-I and PS-AC-II
Two rhodamine-based fluorescent probes were immobi-
lized on chloroacetylated polystyrene microspheres PS-
AC by chemical bonding to form solid-phase fluorescence
sensors PS-AC-I and PS-AC-II. Their structures were
identified by FTIR and SEM analysis. Both sensors exhib-
ited good metal ion selectivity, instantaneous responses to
Hg(II), and environmental friendliness through recycling.
Compared with PS-AC-I, PS-AC-II had a much higher
and more rapid fluorescence response to Hg(II), which
was attributed to the thiophilicity of Hg(II). A sensing
mechanism involving the Hg(II) chelation-induced ring-
opening of the rhodamine spirolactam was proposed with
the aid of theoretical calculation. The present work indi-
cated that PS-AC-I and PS-AC-II could be used as poten-
tial solid-phase fluorescent sensors for further monitoring
Hg(II) in real samples.
decrease, from 11.07kJ/mol and 5.11kJ/mol to −202.86kJ/
mol and −213.83kJ/mol, respectively. The exothermic reac-
tion and lower energy (−213.83kJ/mol) meant relatively
better stability, which made PS-AC-II give a higher fluores-
cence response to Hg(II). Moreover, PS-AC-II with an eth-
ylenediamine chain could serve as a hypothetical aza-crown
ether, and the better electron-donating ability of the S-atom
led to this sensor having a stronger chelating ability to
Hg(II) than PS-AC-I.
To verify the ability to regenerate the solid fluorescent
sensors, sodium sulfide (Na S) titrations were preformed.
2
The addition of Na S, which had stronger chelating ability
2
to Hg(II), caused the disappearance of fluorescence and
reformed the free sensors. The most interesting observation
was that the sensing behavior was revived by adding Hg(II)
again. As shown in Figure 8, sensors PS-AC-I and
PS-AC-II could be reused more than five times in acetoni-
trile. The slight decrease in their fluorescence intensity
might be due to the loss of some the rhodamine derivative
Experimental
on the surface of the polystyrene microspheres. In addition, All reagents used were of analytical grade. Melting points
the surface color of PS-AC-II constantly changed in the were measured on an X-4 microscope electrothermal appa-
process of reuse, indicating that the rhodamine derivatives ratus (Taike China) and are uncorrected. Nuclear magnetic
immobilized on microspheres presented an “off-on” state. resonance spectra (NMR) were recorded on a Bruker
This reversibility was attributed to the Hg(II) chelation- AV-400 spectrometer with CDCl as the solvent and tetra-
3
induced ring-opening of the rhodamine spirolactam. Cost methylsilane (TMS) as the internal standard. Infrared (IR)
savings, environmental friendliness by recycling, and high spectra were recorded as KBr disks using a Thermo Nicolet
selectivity for Hg(II) should make sensors PS-AC-I and 380 FT-IR spectrophotometer. Mass spectra were obtained
PS-AC-II useful in the detection of real samples.
using a Thermo TSQ Quantum Access MAX mass

6
Journal of Chemical Research 00(0)
Figure 7. Detection mechanisms of PS-AC-I and PS-AC-II with Hg(II).
spectrometer. Scanning electron micrographs were obtained extracted with CH Cl (3×100mL) and the combined
2
2
using a JEOL JSM 5900 LV scanning electron microscope. organic layer was dried over Na SO and evaporated. The
2
4
Fluorescence emission spectra were recorded on a residue was dissolved in hydrochloric acid (1mol/L,
Shimadzu RF-5301PC spectrofluorometer using cuvettes 100mL), then sodium hydroxide solution (1mol/L) was
(
1cm path length) at r.t. The slit widths were set to 5nm. added until a pink solid formed. Pure compound I (0.97g)
Chloroacetylated polystyrene microspheres (PS-AC, 200– was obtained following filtration, washing with water and
00μm) were purchased from Nanjing WANQING then drying.
3
Chemical Glassware Instrument Co., Ltd.
2-( 2-A m inoe thy l)- 3’, 6’- bi s(die thy lami no)
spiro[isoindoline-1,9’-xanthen]-3-one (I). Pink solid, yield
2
5 1
8
0%, m.p. 213–214°C (lit. m.p. 217–219 °C) ; H NMR
Synthesis of compound I
(
400 MHz, CDCl ): δ=7.85–7.81 (m, 1H, ArH), 7.39–7.35
3
Compound I was synthesized according to the reported (m, 2H, ArH), 7.03–7.01 (m, 1H, ArH), 6.36 (d, J = 8.8 Hz,
25,26
method.
Rhodamine B (1.2g, 2.5mmol) was dissolved in 2H, ArH), 6.30 (d, J = 2.4 Hz, 2H, ArH), 6.19 (dd, J = 2.4
ethanol (50mL) and ethylenediamine (3.4mL) was added Hz, 8.8 Hz, 2H, ArH), 3.26 (q, J = 7.2 Hz, 8H, NCH ), 3.13
2
dropwise with vigorous stirring. The reaction mixture was (t, J = 6.8 Hz, 2H, NCH ), 2.37 (t, J = 6.8 Hz, 2H, NH CH ),
2
2
2
refluxed for 24h. The solvent was removed under reduced 1.08 (t, J = 7.0 Hz, 12H, CH ); ESI-MS: m/z calcd for
3
+
pressure, then water (100mL) was added. The mixture was C H N O [M+H] : 485.29; found: 485.29.
3
0
36
4
2

Liu et al.
7
phosphate buffer was added. The fluorescence intensity
was determined with an excitation wavelength of 500nm
after 60min and 10min, respectively.
Fluorescence titrations with Hg(II)
Sensor PS-AC-I (0.01g) was swollen in acetonitrile
(200μL), then Hg(II) (0–16μM) was added. The fluores-
cence intensity was determined with an excitation wave-
length of 500nm after 60min.
Sensor PS-AC-II (0.012g) was swollen in acetonitrile
(200μL), then Hg(II) (0.05–0.65μM) was added. The fluo-
rescence intensity was determined with an excitation wave-
length of 500nm after 10min.
Selectivity to various metal ions
Figure 8. Fluorescence intensity changes for PS-AC-I and PS-
Sensor PS-AC-I (0.01g) or PS-AC-II (0.012g) was swol-
len in acetonitrile (200μL), then 1.0 mL of Hg(II) (10μM
for PS-AC-I and 5μM for PS-AC-II) was added. The fluo-
rescence intensity was determined with an excitation wave-
AC-II after alternate treatment with Hg(II) and Na S solution.
2
Synthesis of compound II
+
2+
2+
2+
2+
length of 500nm after adding Ag , Ca , Cd , Co , Cu ,
Compound II was synthesized according to the reported
+
2+
+
2+
2+
2+
3+
3+
3+
3+
2
7
K , Mg , Na , Ni , Pb , Zn , Fe , Al , Cr , or Co .
method. Compound I (0.48g, 1mmol) and Lawesson’s
reagent were dissolved in toluene (40mL) and the reaction
mixture was refluxed for 12h. The solvent was removed
under reduced pressure, and the residue was purified via
flash column chromatography on silica gel (CH Cl /
Theoretical calculations
The 3D structures of PS-AC-I and PS-AC-II with Hg were
created using ChemBio3D and converted to a mol2 file.
Theoretical calculations were performed using the SYBYL
2
2
CH OH, 35: 1 v/v) to afford II (0.085g).
3
2
- ( 2-A m in oet hy l) - 3’ , 6’- bis( di ethy lamino)
2
8
X 2.0 software. Geometry optimization was carried out
using the standard Tripos force-field with a distance-
dependent dielectric function and an energy gradient of
spiro[isoindoline-1,9’-xanthene]-3-thione (II). Pink solid,
1
yield 17%, m.p. 96-97 ºC; H NMR (400 MHz, CDCl ):
3
δ=8.07–8.05 (m, 1H, ArH), 7.42-7.38 (m, 2H, ArH),
−1
−1
0
.001kcal·mol ·Å . The lowest energy conformer was
7
6
2
.03–7.01 (m, 1H, ArH), 6.32 (d, J = 2.4 Hz, 2H, ArH),
.26 (d, J = 8.8 Hz, 2H, ArH), 6.18 (dd, J = 2.4 Hz, 8.8 Hz,
selected and minimized using the Powell method until a
−
1
−1
root-mean-square (rms) deviation of 0.001kcal·mol ·Å
was achieved.
H, ArH), 3.51 (t, J = 7.0 Hz, 2H, NCH ), 3.26 (q, J = 7.0
2
Hz, 8H, NCH ), 2.44 (t, J = 7.0 Hz, 2H, NH CH ), 1.08 (t,
2
2
2
J = 6.8 Hz, 12H, CH ). ESI-MS: m/z calcd for C H N OS
3
30 36
4
+
[
M+H] : 501.27; found: 501.49.
Recycling test
Sensor PS-AC-I (0.01 g) or PS-AC-II (0.012 g) was swol-
len in acetonitrile (200μL), then 220μL of Hg(II) (10μM
for PS-AC-I and 5μM for PS-AC-II) was added. The fluo-
rescence intensity was determined with an excitation wave-
Preparation of PS-AC-I and PS-AC-II
Chloroacetylated polystyrene microspheres PS-AC
(170mg, 1mmol) were swollen in dry tetrahydrofuran
length of 500nm, then Na S (0.1mM) was added. The
(
30mL) overnight, then compound I (484mg, 1mmol) or
2
fluorescence intensity was tested again, then the micro-
spheres were filtered, washed with water and acetonitrile
several times, and dried under vacuum at 30°C for 24h to
recover the free sensors. More than four replicates of each
test were carried out.
II (501mg, 1mmol) and sodium bicarbonate (84mg,
1
2
mmol) were added. The reaction mixture was refluxed for
4h and then filtered. The microspheres were washed with
water and methanol several times and then dried under vac-
uum at 30°C for 24h to give PS-AC-I and PS-AC-II.
Declaration of conflicting interests
Response times and pH dependence of PS-
AC-I and PS-AC-II
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
Sensor PS-AC-I (0.01g) or PS-AC-II (0.012g) was swol- article.
len in acetonitrile (200μL), then Hg(II) (10μM, 200μL)
was added. The fluorescence intensity was determined with Funding
an excitation wavelength of 500nm every 1min.
Sensor PS-AC-I (0.01g) or PS-AC-II (0.012g) was for the research, authorship, and/or publication of this article: This
swollen in acetonitrile (200μL), then NaH PO -Na HPO
work was supported by the Natural Science Foundation of Jiangsu
The author(s) disclosed receipt of the following financial support
2
4
2
4

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Journal of Chemical Research 00(0)
Province (BK20161542), the Natural Science Foundation of the 11. Rasheed T, Li C, Nabeel F, et al. Chem Eng J 2019; 358: 101.
HigherEducationInstitutionsofJiangsuProvince(17KJB150006), 12. Adhikari S, Mandal S, Ghosh A, et al. Sensor Actuat B Chem
and the Jiangsu Overseas Visiting Scholar Program for University
Prominent Young & Middle-aged Teachers and Presidents (2017). 13. Qin L, He X, Zhang W, et al. J Chromatogr A 2009; 1216:
07.
2016; 234: 222.
8
1
1
1
1
1
4. Zhang D, Zhou W, Li J, et al. Polymers 2016; 8: 142/1.
5. Kumar P, Kumar V and Gupta R. RSC Adv 2017; 7: 7734.
6. Leng L, Li Y, Liu Y, et al. Anal Lett 2017; 50: 2944.
7. Qu J, Xu Y, Liu J, et al. Polym Eng Sci 2016; 56: 1407.
8. Guo Y, Yang Y, Zhang L, et al. Macromol Res 2011; 19:
1202.
ORCID iD
Yi Li
https://orcid.org/0000-0002-0906-2799
References
1
2
3
4
5
6
. Rasheed T, Bilal M, Nabeel F, et al. Sci Total Environ 2018;
15: 476.
. Rasheed T, Li C, Bilal M, et al. Sci Total Environ 2018; 640: 20. Neupane LN, Kim JM, Lohani CR, et al. J Mater Chem
74.
2012; 22: 4003.
. Anitha I, Sheela Gopal M and Thomas D. J Fluorescence 21. Hu B, Hu L, Chen M, et al. Biosens Bioelectron 2013; 49:
016; 26: 725.
499.
. Zhang W, Wu L, Han X, et al. J Hazard Mater 2019; 378: 22. Li Y, Washino Y, Hyakutake T, et al. Anal Sci 2017; 33: 599.
6
19. Lu C, Gao B, Liu Q, et al. Colloid Polym Sci 2008; 286: 553.
1
2
1
20674.
. Rasheed T, Li C, Zhang Y, et al. Sensor Actuat B Chem
018; 258: 115.
. Ni J, Li B, Zhang L, et al. Sensor Actuat B Chem 2015; 215:
74.
23. Malinina EA, Korolenko SE, Goeva LV, et al. Russ J Inorg
Chem 2018; 63: 1552.
24. Zhang J, Wang C, Ji L, et al. Chem Res Toxicol 2016; 29:
841.
25. Soh JH, Swamy KMK, Kim SK, et al. Tetrahedron Lett
2007; 48: 5966.
26. Bordini J, Calandreli I, Silva GO, et al. Inorg Chem Commun
2013; 35: 255.
2
1
7
8
9
. Rasheed T, Nabeel F, Li C, et al. J Lumin 2019; 208: 519.
. Lan T, Wang F, Xi X, et al. Anal Sci 2016; 32: 1223.
. Rasheed T, Li C, Nabeel F, et al. New J Chem 2018; 42:
1
0940.
27. Liu M, Liu T, Li Y, et al. Talanta 2015; 143: 442.
1
0. Jeong J, Rao BA and Son YA. Sensor Actuat B Chem 2015; 28. Clark M, Cramer RD and Opdenbosh NV. J Comput Chem
20: 1254.
1989; 10: 982.
2