1,3,5-Trinitrotoluene sensor based on silole nanoaggregates

Article abstract of DOI:10.1166/jnn.2019.15945

Bis(methyltetraphenyl)silole and bis(methyltetraphenyl)silole siloxane nanoaggregates for the detection of TNT were developed by using aggregation-induced emission property. Their absolute quantum yields and critical water concentration for onset of aggre

Full text of DOI:10.1166/jnn.2019.15945

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Nanoscience and Nanotechnology
Vol. 19, 991–995, 2019
Copyright © 2019 American Scientific Publishers
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1,3,5-Trinitrotoluene Sensor Based on
Silole Nanoaggregates
Bomina Shin and Honglae Sohn∗
Department of Chemistry, Chosun University, Gwangju 501-759, Korea
Bis(methyltetraphenyl)silole and bis(methyltetraphenyl)silole siloxane nanoaggregates for the detec-
tion of TNT were developed by using aggregation-induced emission property. Their absolute
quantum yields and critical water concentration for onset of aggregation were measured. Average
particle size for both nanoaggregates were measured and tuned by controlling the water fraction by
volume. Absolute quantum yield of both nanoaggregates in 90% water volume fraction increased
by more than 40 times. Detection of TNT was achieved from the quenching PL measurement of
both nanoaggregates by adding the TNT. A linear Stern–Volmer relationship was observed for the
detection of TNT.
Keywords: Sensor, Explosive, Silole, Aggregate, Quenching.
IP: 93.179.90.84 On: Tue, 25 Dec 2018 07:17:55
1. INTRODUCTION
electrical phenomenon could be applied in various fields,
Copyright: American Scientific Publishers
such as electron transporting materials,9 light-emitting
Materials with high emission are very intriguing and
highly demanded in optical and electronic devices.
Previously developed materials with known fluorescent
molecules exhibited high fluorescence in dilute solution
exhibit low or no emission in the solid state, because
the strong intermolecular ꢀ–ꢀ stacking interaction causes
an aggregation-caused emission quenching (ACEQ).1 The
ACEQ is generally attributed to a nonradiative deacti-
vation process, such as excitonic coupling, excimer for-
mation, and excitation energy migration to the impurity
traps.2ꢁ3 Since the most of the fluorescent molecules are
donor–acceptor type or flat ꢀ-conjugated molecules, they
are more prone to molecular aggregation and fluorescence
quenching in the solid state.4ꢁ5
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diodes (LEDs),10–12 chemical sensors,13ꢁ14 and flexible
memory storage.15 Siloles is structured with ꢀ-electron
systems containing Si atom as a part of cyclic 5-membered
ring that shows an AIEE.16 These non-emissive molecules
are induced to be efficiently emissive by aggregate for-
mation. The above effect of aggregation shows opposite
behavior from the one observed in common luminophore
systems, where aggregate formation usually cause decrease
in PL efficiency. On the other hand, nanoaggregate has
a surprising increase in PL efficiency when it nanoag-
gregates in water.17–19 AIEE has been attributed by the
restricted rotation of the phenyl rings, which limits the
nonradiative decay pathways.20
Decrease of emission efficiency in the solid state has
been a major problem in optical device applications.
Many attempts through chemical, physical, and engineer-
ing approaches have been made to prevent the formation
of less emissive species. To overcome this problem, an
aggregation-induced emission enhancement (AIEE) is an
alternative approach is to control the fluorescence prop-
erties in the solid state. Recently, many aromatic AIEE
compounds have been reported.6
Sensors offering new approaches to the rapid detection
of ultra-trace anaytes from explosives, have attracted atten-
tion because explosives are important chemical species to
detect in mine fields, forensic investigations, and home-
land security applications. Silole have also been shown to
have potential as chemo-sensors for detecting explosives.14
Because explosives such as 1,3,5-trinitrotoluene (TNT) are
electron deficient compounds due to nitro moiety in a
molecule, photoluminescence (PL) of silole decreases via
electron transfer quenching.
Siloles have recently attracted much attention due to
its unique electronic properties.7ꢁ8 The unique optical and
Herein, we report synthesis and optical characteriza-
tion of two different types of silole fluorophore nanoag-
gregates such as silole nanoaggregate and silole-siloxane
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 2
1533-4880/2019/19/991/005
doi:10.1166/jnn.2019.15945
991

TNT Sensor Based on Silole Nanoaggregates
Shin and Sohn
O
Si
Si
Si
Si
Figure 1. Chemical structure of bis(methyltetraphenyl)silole (left) and bis(methyltetraphenyl)silole siloxane (right).
nanoaggregate. Evidently, those nanoaggregates showing
high PL efficiency through AIEE, would have the high effi-
ciency in sensing explosives. An AIEE of silole and silole-
siloxane nanoaggregates and their Stern–Volmer analyses
for the comparison of the detection efficiencies of explo-
sives are reported.
2.3. Preparation of Bis(methyltetraphenyl)
Silole Siloxane
To a solution of 1-chloro-1-methyl-2,3,4,5-tetraphenyl-1-
silacyclopentadiene (4.58 g, 10 mmol) in THF (200 mL)
was added H2O (0.18 g, 10 mmol) and triethlyamime
(1.01 g, 10 mmol). After stirred at room temperature
for 24 hrs. After evaporating THF from the mixture, the
remaining solid was treated with water and diethylether.
The organic layer was separated and dried (MgSO4ꢂ. The
solvent was removed in vacuo, and the solid was washed
with ethanol for purification.24
2. EXPERIMENTAL DETAILS
2.1. General
All synthetic manipulations were carried out under an
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ꢀ
1
atmosphere of dry argon gas using standard vacuum-line
Mp ≥250 C. H NMR (300 MHz, CDCl3 (ꢃ 7.26)):
ꢃ 6.7–6.8 and 6.9–7.2 (m, 40H, Ph), 0.38 (s, 6H). 13C
NMR (100 MHz, CD2Cl3 (ꢃ 77.00)): ꢃ 154.6, 139.07,
138.81, 137.34, 130.16, 129.53, 128.54, 128.08, 126.99,
126.46, −2.7.
Copyright: American Scientific Publishers
Schlenk technique. All solvents were purified and degassed
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before use according to standard literature methods:
diethylether, tetrahydrofuran (THF), n-Hexane, ethanol
were purchased from Aldrich Chemical Co. Inc. and dis-
tilled from sodium/benzophenone ketyl. All other reagents
(Aldrich) such as diphenylacetylene, lithium wire, and
methyltrichlorosilane were purchased and used without
any purification. Spectroscopic grade THF from Fisher
Scientific was used for the fluorescence measurements.
The concentration of silole nanoaggregates for the fluores-
cence measurement was 10 mg/100 mL.
2.4. Instruments and Data Acquisitions
Fluorescence emission spectrum were recorded with the
use of a Perkin-Elmer luminescence spectrometer LS 50B.
The solvents were determined to be free of emitting impu-
rities prior to use. To avoid changing the concentrations
of the emitting compound, the experiments were carried
out with solution by preparing the quencher in the same
solution of the emitting compound. Absolute PL quan-
tum yield (QY) was obtained by using a Quantaurus-
QY Absolute PL quantum yield spectrometer C11347-11
2.2. Preparation of Bis(methyltetraphenyl)Silole
The preparation method of bis(methyltetraphenyl)silole
was previously reported.21
Restriction of
Intramolecular
rotation (RIR)
O
Si
Si
Aggregation-induced
emission (AIE)
Figure 2. Schematic diagram for the preparation of silole nanoaggregates.
992
J. Nanosci. Nanotechnol. 19, 991–995, 2019

Shin and Sohn
TNT Sensor Based on Silole Nanoaggregates
(Hamamatsu, Japan) with of 150 W xenon monochro-
matic light source, multichannel Czerny-Turner type spec-
troscope, and 3.3 inch spectral on integrating sphere.
Dynamic light scattering (DLS) measurement was con-
ducted by employing a DLS-8000HL (Otsuka Electronics,
Japan) with 10 mW He–Ne Laser (Max 30 mW), pho-
tomultiplier tube detector, and silicon photodiode monitor
detector.
Water:silole in THF
(A)
9:1
8:2
7:3
6:4
5:5
4:6
3:7
2:8
1:9
0:10
3. RESULTS AND DISCUSSION
Siloles have attracted attention because of their unusual
electronic and optical properties caused from extensively
delocalization through silicon atom of five membered ring
at LUMO level. Silole have also been shown to have
potential as chemo-sensors for detecting explosives.14 One
of the characteristic optical property of silole is AIEE.
AIEE is result from the steric effect of the peripheral
phenyl rings caused nonplanar conformations, which pre-
vents them from packing via ꢀ–ꢀ stacking interactions
and quenching by excimers. The restriction of intramolec-
ular rotations in the aggregates blocks the nonradia-
tive decay, hence changing the weak emitters to strong
emitters.
400
450
500
550
600
650
700
Wavelength (nm)
Water:silole in THF
(B)
9:1
8:2
7:3
6:4
5:5
4:6
3:7
2:8
1:9
Two different types of nanoaggregates containing
silole molecules, silole nanoaggregate and silole-siloxane
nanoaggregate were provided and examined for their
optical properties in nanoaggregates. Figure 1 shows
0:10
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400
500
550
600
650
700
the molecular structures and intermolecular rotations.
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Wavelength (nm)
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In case of bis(methyltetraphenyl)silole, two silole units
are directly connected through Si–Si bond which could
cause more strict intramolecular rotation and conjuga-
tion of silicon ꢄ bond. However, two silole units in
bis(methyltetraphenyl)silole siloxane are connected by
oxygen atom, which allows more flexible intramolecular
rotation.
50
40
30
20
10
0
(C)
A
B
Bis(methyltetraphenyl)silole and bis(methyltetraphenyl)
silole siloxane show emission near 490 and 480 nm,
when excited at 360 nm, respectively. Absolute quan-
tum yield (QY) of bis(methyltetraphenyl)silole and
bis(methyltetraphenyl)silole siloxane are measured in THF
solution and exhibits 0.8% and 0.4%, respectively. This
result might be due to the short conjugation of silole unit
through Si–Si bond. To prepare the nanoaggregates, the
mother solution was prepared by dissolving 10 mg of silole
in 100 mL THF. The silole nanoaggregates were prepared
by rapid injection of 1 mL silole solution into a differ-
ent proportions ratios of THF and distilled water such as
THF:H2O = 0:9, 1:8, 2:7, and so forth. Schematic diagram
for the preparation of silole nanoaggregates is shown in
Figure 2.
0
10 20 30 40 50 60 70 80 90 100
Water fraction/Vol (%)
Figure 3. PL spectra of bis(methyltetraphenyl)silole (A) and
bis(methyltetraphenyl)silole siloxane (B) nanoaggregates in water-
THF mixtures (% water from top; 90%, 80%, 70%, 60%,
50%, 40%, 30%, 20%, 10%, 0%). Plot of the relative inten-
sity of bis(methyltetraphenyl)silole nanoaggregate (red) and
bis(methyltetraphenyl)silole siloxane nanoaggregate (blue) versus
water by volume (C).
%
Figure 3(A) shows the relative PL spectra of
bis(methyltetraphenyl)silole nanoaggregate in water-
THF mixtures (% water from top; 90%, 80%, 70%,
60%, 50%, 40%, 30%, 20%, 10%, 0%). Figure 3(B)
shows the relative PL spectra of bis(methyltetraphenyl)
silole siloxane nanoaggregate in water-THF mix-
tures. Figure 3(C) shows the PL efficiency of
bis(methyltetraphenyl)silole nanoaggregate versus % water
by volume. Bis(methyltetraphenyl)silole nanoaggregate
J. Nanosci. Nanotechnol. 19, 991–995, 2019
993

TNT Sensor Based on Silole Nanoaggregates
Shin and Sohn
exhibits a nearly identical emission band, however the
emission band of bis(methyltetraphenyl)silole siloxane
nanoaggregates red-shifts by 20 nm in nanoaggregates.
For bis(methyltetraphenyl)silole solution in pure THF,
the PL is very weak near 490 nm. In solutions between
0% and 50% water by volume, the PL efficiency of
bis(methyltetraphenyl)silole nanoaggregates does not
increased. However, the PL efficiency increases over 50%
water by volume, which indicates the onset of aggre-
gation. In case of bis(methyltetraphenyl)silole siloxane
nanoaggregate, the PL efficiency increases over 70% water
by volume. As the water fraction is increased, the PL
efficiency increases dramatically. Critical water concentra-
tion for the formation of bissilole and bissilole siloxane
nanoaggregate are a minimum volume-fraction of 50%
and 70% water, respectively. Since AIEE is due to result
from the steric effect and the restriction of intramolecular
rotations in the aggregates, more strict intramolecular
rotation of bissilole exhibits lower % water of the onset
of aggregation. Absolute quantum yield of bissilole and
bissilole siloxane nanoaggregates in 90% water volume
fraction were 32.4% and 18.7%, respectively. The PL
efficiencies for both nanoaggregates increased more than
40 times.
Figure 4, obtained by dynamic light scattering mea-
surements shows the size of nanoaggregate and size
distribution in different water fractions. Particle diam-
eter decreases as water fraction increases, but the
dispersity of diameter for given any suspension is
about 3%. Both nanoaggregates exhibit a minimum
in size at 90% water. Average particle diameters for
(A)
400 450 500 550 600 650 700
Wavelength (nm)
(B) 50
(A) 35
30
(B)
40
25
20
15
10
5
30
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20
10
0
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0
28 34 41 50 61 74
Particle size (nm)
51
62
76
92
112
Particle size (nm)
(C) 50
(D) 35
30
40
25
20
15
10
5
400
450
500
550
600
650
700
30
20
10
0
Wavelength (nm)
0.25
0.2
(C)
A
B
0
102 124 152 186 227 277
Particle size (nm)
194 226 265 310 362
Particle size (nm)
0.15
0.1
35
(E) 50
(F)
30
25
20
15
10
5
40
30
20
10
0
0.05
0
0
2
4
6
8
10
0
Molarity (10–6
)
35 43 52 64 77 94
Particle size (nm)
113 136 163 195 234
Particle size (nm)
Figure 5. Quenching PL spectra of increasing fraction by 500 part per
billion (ppb) of each TNT concentration in bis(methyltetraphenyl)silole
nanoaggregates solution (A) and bis(methyltetraphenyl)silole siloxane
nanoaggregates solution (B). Stern–Volmer plot showing sensing effi-
ciency for both nanoaggregates (C).
Figure 4. DLS data showing an increase in size when the water frac-
tion ratio decreases to 90% (A), 80% (B), 70% (C), and 60% (D) for
bis(methyltetraphenyl)silole nanoaggregate and 90% (E) and 80% (F) for
bis(methyltetraphenyl)silole siloxane nanoaggregate.
994
J. Nanosci. Nanotechnol. 19, 991–995, 2019

Shin and Sohn
TNT Sensor Based on Silole Nanoaggregates
the bis(methyltetraphenyl)silole nanoaggregate are 35, 60,
110, 260 nm as the water volume fraction decreases
90 to 60%, respectively. (See Figs. 4(A to D)). Aver-
age particle diameters for the bis(methyltetraphenyl)silole
siloxane nanoaggregate are 39 and 130 nm as the water
volume fraction decreases 90 to 80%, respectively. (See
Figs. 4(E to F)) Decrease of particle size at higher water
concentrations could be explained that the hydrophobic
organic molecules are prone to aggregate to a higher extent
in the hydrophilic environment.
the water fraction was increased to 90%. Both silole
nanoaggregates used for the detection of TNT and showed
greater sensitivity compared to previously reported value.
Acknowledgment: This research was financially sup-
ported by the Agency for Defense Development
and the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (NRF-
2016R1D1A1B03933216).
The ability of the silole nanoaggregate with a volume-
fraction of 90% water to detect TNT through PL quench-
ing was investigated by adding successive aliquots of
an identical volume-fraction of 90% water stock solu-
tion of TNT to the nanoaggregate. Figures 5(A) and (B)
show quenching PL spectra of increasing fraction by
500 part per billion (ppb) of each TNT concentration
in both silole nanoaggregates solution. The decrease in
PL was monitored as a function of added TNT. The
responses to quenching between both silole nanoaggre-
gates were analyzed using the Stern–Volmer equation:22
I0/I = KSV ꢅQꢆ+1. In this equation, I is the fluorescence
intensity at quencher concentration, [Q], I0 is the inten-
sity at ꢅQꢆ = 0, and KSV is the Stern–Volmer constant.
Linear Stern–Volmer relationships are observed for both
nanoaggregates.
References and Notes
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Copyright: American Scientific Publishers
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4,350 M−1 14
constant of oligo(tetraphenyl)silole nanoaggregate with a
volume-fraction of 90% water was 4,100 M−1 23
Thus,
.
Trogler et al reported that the Stern–Volmer
.
the detection efficiency of bis(methyltetraphenyl)silole
nanoaggregates in the water fraction of 90% for TNT
increased by about 4 to 5 times compared to that
of previous reported value. Higher detection efficiency
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4. CONCLUSION
Aggregation-induced emission properties of conjugated
silole nanoaggregates and non-conjugated silole siloxane
nanoaggregates and their successful sensing of explosive
were reported. Both silole nanoaggregates exhibited an
aggregation-induced emission enhancement of more than
40 times increase to normal photoluminescence when
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Received: 21 December 2017. Accepted: 18 January 2018.
J. Nanosci. Nanotechnol. 19, 991–995, 2019
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