Fluorescent nano-Assembly of organic conjugate molecules with benzoxazole moiety and its application in sensor

Article abstract of DOI:10.1166/jnn.2019.15928

Fluorescence labeling or sensing is a very useful analytical technique for investigating the structure of living cells or for detecting ionic metals. Numerous materials including inorganic quantum dots and organic fluorescent dyes have been used and inves

Full text of DOI:10.1166/jnn.2019.15928

Article
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Nanoscience and Nanotechnology
Vol. 19, 1052–1055, 2019
Copyright © 2019 American Scientific Publishers
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Fluorescent Nano-Assembly of Organic Conjugate
Molecules with Benzoxazole Moiety and
Its Application in Sensor
Hyong-Jun Kim
Department of Chemical Engineering, Kongju National University, Cheonan 31080, Korea
Fluorescence labeling or sensing is a very useful analytical technique for investigating the structure
of living cells or for detecting ionic metals. Numerous materials including inorganic quantum dots and
organic fluorescent dyes have been used and investigated for further development so far. However,
they are inherited natural discrepancies of cyto-toxicity or easy degradation. Thus, developing highly
emissive, biocompatible, and chemically readily modifiable luminescent materials is strongly desired.
Here, we report the enhanced photoluminescence of an oxazole derivative for possible use in the
field of fluorescent sensor.
Keywords: Fluorescence Sensor, Nano Aggregate, ESIPT, Benzoxazole, Zinc.
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1. INTRODUCTION
1H nuclear magnetic resonance (NMR) and 13C NMR
Copyright: American Scientific Publishers
spectra were obtained in CDCl3 at 400 MHz and 100 MHz
Nitrogen-containing heterocycles have been known to have
high electron mobility and ability to support a number of
charge carriers.1 Among those heterocyclic molecules, the
compounds having the hydroxyphenyl-benzoxazole (HBO)
unit have an interesting behavior of the excited state intra-
molecular proton transfer (ESIPT) upon electromagnetic
wave excitation.2–4 ESIPT has received considerable atten-
tion due to the characteristic four-level photo-physical state
transition between the two different tautomers. Based on
this unique property, ESIPT molecules are gaining interest
for potential application in organic light emitting diodes,
chemosensor, photostabilizer, and etc.
Detection and quantification of transition metal, espe-
cially, Zn2+ is crucial to human health. Zinc is very
important in biological processes. Failure to maintain zinc
level causes in a number of severe neurological diseases
such as Alzheimer’s disease, hypoxia ischemia, and cer-
tain Parkinson’s disease.5ꢀ6 Therefore, highly selective and
sensitive detection of Zn2+ is of toxicological and environ-
mental concern.
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on a Bruker Avance 500. High resolution mass spectra
were obtained with HP5890 series II. Elemental analysis
was carried with Thermo Electron Flash EA 1112. UV/Vis
absorption spectra were recorded on a Varian Cary50 spec-
trometer. Luminescence spectra were collected on a Pho-
ton Technology International QM4 spectrometer with a
xenon lamp as a light source.
2.2. Synthesis
2.2.1. 2-(benzoxazole-2-yl)-4-Bromophenol (1)
2-Aminophenol (23.0 mmol) and 5-bromosalicylic acid
(23.0 mmol) were added in 30 ml of poly-phosphoric acid
ꢀ
and the mixture was heated to 130 C and stirred for 4 h.
After cooling to room temperature, the reaction mixture
was precipitated in ice-water, filtered and washed with
DI water. The product was recrystallized from acetic acid
and dried in vacuo. 400 MHz H NMR (CDCl3ꢁ: ꢂ 7.06
(d, 1H), 7.44 (m, 2H), 7.53 (d, 1H), 7.65 (m, 1H), 7.77
(m, 1H), 8.18 (d, 1H), 11.50 (s, 1H).
1
Here, we report a new fluorescent molecule and demon-
strate fluorescence sensor application.
2.2.2. Tert-Butyl-2-(benzoxazole-2-yl)-4-Bromoph-Enyl
Carbonate (2)
2. EXPERIMENTAL DETAILS
2.1. Materials and Characterization
All chemical materials were purchased from Sigma
Aldrich and used without further purification.
1 (2.76 mmol) is dissolved in dry, distilled tetrahy-
drofuran (THF) with di-tert-butyldicarbon-ate and
(4-dimethylamino)pyridine (5 mol%). When the reaction
appeared complete by TLC, the solution was concentrated
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J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 2
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doi:10.1166/jnn.2019.15928

Kim
Fluorescent Nano-Assembly of Organic Conjugate Molecules with Benzoxazole Moiety and Its Application in Sensor
tBOC
HO
O
N
O
N
O
Br
Br
2
1
C6H13
C6H13
HO
B
HO
OH
B
OH
O
N
C6H13
C6H13
OH
HO
Figure 2. UV/vis absorption (left) and fluorescence intensity (right)
spectrum of DBO 10−5 M THF solution (black lines), DBO dispersion
(blue broken lines), and its solid film (red dots).
N
O
DBO
in Figure 2, having slight hypsochomic shift from that
of the monomer 1, whose absorption maxima is 340 nm
attributed to the syn-enol structure.3 Halogen atom of
1 extends conjugation length of benzoxazole to yield a
slight red-shift by about 8 nm.7 The photoluminescence
spectrum of DBO solution has emission at the wave-
length of 527 nm. This large Stokes’ shift over 190 nm
is caused by the ESIPT phenomena of the enol-keto
tautomerization.2ꢀ3ꢀ8 The relative quantum efficiency (ꢃ)
Figure 1. Synthetic scheme of DBO.
and precipitated in DI water, washed with ethanol. The
product was dried in vacuo. 400 MHz H NMR (CDCl3ꢁ:
ꢂ 1.63 (s, 9H), 7.04 (d, 1H), 7.42 (m, 2H), 7.51 (d, 1H),
7.62 (m, 1H), 7.75 (m, 1H), 8.16 (d, 1H).
1
2.2.3. 4,4ꢀ-(9,9-dihexyl-9H-fluorene-2,7-diyl)
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Bis(2-(benzo[d]oxazol-2-yl)phenol) (DBO)
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−5
2 (0.8 mmol) and 9,9-dihexylfluorene-2,7-di-boronic acid
(0.4 mmol), palladium catalyst (5 mol%) are placed in a
two-necked round bottom flask charged with 7 ml of THF
under argon. 1 M Na2CO3 solution 4 ml is added and
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of DBO 10 M THF solution was 0.08.
It is worth to point out that the photo-luminescence
of the fluorene is suppressed, whose usual photophysi-
cal emission at ∼410 nm.11 One possible explanation is
the radiationless decay pathway increase due from the
energy loss via vibrational or rotational motions of the
molecules.2–4ꢀ10 The other culprit would be that the excited
spin density is popular not on the center fluorene, but
on the benzoxazole pendant groups. From the density
functional theory calculation using B3LYP/6-311G+(d),11
excited state dipoles have been moved toward the pendant
benzoxazole groups to excite enol moieties rather than to
excite the fluorene group.
ꢀ
stirred for 48 h at 80 C. After cooling, the reaction mix-
ture was poured into methanol. The precipitate is isolated
by filtration and washed with DI water and methanol and
dried under vacuo.
Above synthesized oligomer is dissolved in chloroform
(3 ml) and trifluoroacetic acid (2 ml) was added. After
stirring for 12 h, solvents was removed by evaporation
and dried under vacuo. The compound, DBO, was purified
by chromatography and recrystallization to yield yellow
needle shape crystals (yield 81%). 1H NMR(400 MHz,
CDCl3ꢁ: ꢂ 0.80 (m, 10H), 1.13 (m, 12H), 2.13 (m, 4H),
7.26 (d, 4H), 7.41–7.48 (m, 4H), 7.72–7.60 (m, 6H),
7.77–86 (m, 6H), 8.36 (s, 2H); 13C NMR (100 MHz,
CDCl3) ꢂ 14.1, 22.8, 24.5, 30, 31.8, 44, 53.7, 110.4, 123.8,
124.7, 127.3, 127.4, 127.9, 128.9, 129, 129.1, 130.6, 136.6,
139.5, 141.2, 141.8, 148.6, 150.5, 162.9; Anal. Calcd. for:
C51H48N2O4: C, 81.35; H, 6.43; N, 3.72 Found C, 81.31;
H, 6.41; N, 3.70. HRMS (FAB): calculated m/z 752.36;
observed m/z 752.39.
The torsional angle of ca ∼39ꢀ between the main fluo-
rene and the benzoxozole play a key role to reduce ꢄ–ꢄ
interaction and thus, widens the radiative decay pathway.
As a result, huge increase in the fluorescence is observed
for the solid film and dispersion of DBO by more than
4 times from its solution (black solid lines) as shown in
Figure 2 (red dots and blue dashes) with relative quantum
efficiency of 0.42. Moreover, DBO solid film has another
small Stokes’ shift at ∼430 nm, since the vibration restric-
tion of the phenyl rings and the hydrocharbon side chains
give birth to radiative decay of the center fluorene groups
leading structural vibrations. This emission color change is
further utilized to detect ions and further application onto
biosensor has been performed.
3. RESULTS AND DISCUSSION
UV/Vis absorption spectrum of 10−5 M DBO in THF solu-
tion has its maximum absorption at 332 nm as shown
J. Nanosci. Nanotechnol. 19, 1052–1055, 2019
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Fluorescent Nano-Assembly of Organic Conjugate Molecules with Benzoxazole Moiety and Its Application in Sensor
Kim
Figure 4. (A) Illustration of selective fluorescence labeling with DBO
nanoparticles. (B) Fluorescence microscopy image after selective binding
of biotinylated DBO-PCDA nanoparticles on the patterned avidin surface
(diameters are 500 ꢅm). (C) A fluorescence image of DBO THF solution,
DBO-PCDA nanoparticle dispersion, and DBO solid film. (D) A SEM
image of DBO-PCDA nano-assembly diameter of ∼100 nm.
Figure 3. UV/vis absorption (left) and PL (right) spectra of 10−5
DBO THF solution (black solid lines) upon addition of 10 equiv. metal
ions of Zn2+(red dots), Fe2+ (black broken lines), and Cu2+. (Blue dash-
dot-dash).
M
is injected to the pentacosadiynoic acid (PCDA) vesi-
cles of diameter of ∼100 nm with green emission as
seen on Figure 4(C).3 As expected, synthesized DBO
nano-assembly has the fascinating optical property. Gen-
erally, organic nanoparticles are poor fluorescent materials
because of self-quenching between absorption and their
emission. These highly emissive and highly photo-stable
organic nanoparticles have the possible application in the
immunofluorescence labeling as shown in Figure 4.
Spectroscopic analysis was performed by adding
10 equivalent amounts of cations onto 10−5 M DBO
THF solutions. Absorption changes after addition of metal
cations onto DBO solution are not significant. Cu2+ or
Fe2+ ions show just a small hump at 400 nm in Figure 3.
The solution turned to pale-yellow due to a new absorption
band at ∼400 nm.12 Meanwhile, their photo-luminescence
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intensity decreased, but no spectral change is observed.
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The lack of a spectral change indicates a fluorescence tran-
4. CONCLUSION
sition is from an identical excited state regardless of the
presence of metal cations. Thus, the formation of new
absorption species prohibits radiation decay pathway, such
as intermolecular packing or ꢄ–ꢄ∗ quenching.
Highly emissive photo-stable organic materials have
many applications in the bio-imaging. New benzoxazole
molecule was synthesized and its photophysical behav-
ior has been studied. DBO shows peculiar ESIPT mech-
anism and fluorescence enhancement upon aggregation,
whose solid state quantum yield of 42%. Further fluores-
cent labeling and metal ion, especially Zn2+ cation sensing
have been demonstrated with the nanoparticles assembled
with functionalized diacetylene molecules for application
in bio-imaging.
To the contrary, added Zn2+ ions move the absorp-
tion spectrum by 7 nm to longer wavelength, suggesting
extended conjugation length. Fluorescence spectrum shows
significant increase with a spectral blue-shift. The photo-
luminescence response of DBO is different from the usual
ESIPT process and displays ratiometric responses.13ꢀ14 The
weak fluorescence of DBO in the absence of Zn2+ might
be attributed to irradiative channels from the n–ꢄ∗ state.15
In the presence of Zn2+ that coordinates with the lone
pair of the oxygen, the energy of the n-ꢄ∗ state would be
raised so that the ꢄ–ꢄ∗ state becomes the lowest excited
state, leading a substantial increase in the fluorescence
intensity.16ꢀ17 In addition, Zn2+ chelating to DBO could
not only induce a conformation restriction18 but also block
photoinduced electron transfer quenching of the singlet
excited state of benzoxazole moiety, thus, a big fluores-
cence enhancement could be observed.19
Acknowledgment: This work was supported by the
research grant of the Kongju National University in 2016.
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Received: 17 November 2017. Accepted: 13 February 2018.
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