A fluorescence-switchable luminogen in the solid state: A sensitive and selective sensor for the fast "turn-on" detection of primary amine gas

Article abstract of DOI:10.1039/c3cc41414k

The emission of pyrrole-substituted benzoic acid can be repeatedly switched between the dark and bright states in the solid state by chemical fuming and heating processes, enabling it to work as a rapid sensitive fluorescent sensor for primary amine detec

Full text of DOI:10.1039/c3cc41414k

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A fluorescence-switchable luminogen in the solid
state: a sensitive and selective sensor for the fast
‘‘turn-on’’ detection of primary amine gas†
Tianyu Han,ab Jacky W. Y. Lam,a Na Zhao,a Meng Gao,a Zhiyong Yang,a
Engui Zhao,a Yuping Dong*b and Ben Zhong Tang*acd
Cite this: Chem. Commun., 2013,
49, 4848
Received 24th February 2013,
Accepted 30th March 2013
DOI: 10.1039/c3cc41414k
www.rsc.org/chemcomm
The emission of pyrrole-substituted benzoic acid can be repeatedly Thus, development of sensitive detection systems for amines has
switched between the dark and bright states in the solid state by both academic value and technological application. Much work has
chemical fuming and heating processes, enabling it to work as a been done in the area of developing sensors for amine detection in
rapid sensitive fluorescent sensor for primary amine detection.
solution.6 For practical application, it is preferable to perform the
detection on a solid support because it requires no complex and
Development of sensitive and selective sensory systems for the expensive spectrofluorometer and it is thus simple, quick, green
detection of chemical and biological agents has attracted much and convenient.7 Unfortunately, such sensing systems are rather
attention because they play important roles in industrial, environ- limited.8 In this work, we investigated the photophysical properties
mental and biological processes and systems.1 Among them, the of a pyrrole derivative, 4-(2,5-diphenyl-1-pyrrolyl)benzoic acid (TPPA,
fluorescence (FL)-based techniques are superior to other analytical Fig. 1A), and found that its FL can be repeatedly switched between
methods and enjoy advantages such as high sensitivity, low back- dark and bright states by fuming with primary amine gas and
ground noise and a wide working range.2 Thanks to the enthusiastic heating processes, making it promising as a rapid, sensitive and
efforts of scientists and technologists, a large variety of fluorescent selective FL chemosensor for amine detection.
sensors have been developed. Many of them operate in a ‘‘turn-off’’
TPPA was prepared according to the synthetic route shown
mode: an initially highly emissive dye becomes non-fluorescent in Scheme S1 in the ESI.† Detailed procedures can be found in
when it interacts with a quenching species through a mechanism our previous publications.9 Its structure was well characterized
of FL resonance energy transfer.3 Few of the sensing systems using IR, NMR and MS spectroscopies with satisfactory analysis
function in a ‘‘turn-on’’ mode though they are more sensitive, results (Fig. S1 and S2, ESI†). TPPA absorbs at 292 nm in a
less likely to generate false-positive signals and compatible with dilute THF solution (10 mM) (Fig. 1A). When UV irradiated at
continuous monitoring of biological processes and events.4
290 nm, it emits weakly at 464 nm. The FL quantum yield (FFL)
Amines are derivatives of ammonia and are widely used in the estimated using quinine sulfate is merely 2.7%, presumably
food and dyeing industries, gas treatment plants and in medical due to the active rotation of its peripheral phenyl rings, which
diagnosis.5 Low molecular weight amines, however, are toxic effectively consumes the energy of the excitons via nonradiative
and easily absorbed through the skin. Many high molecular relaxation channels. The FFL value drops to 0.25% when 90% of
weight amines, on the other hand, are biologically highly active. water is added to the THF solution. Since TPPA is insoluble in
a Department of Chemistry, Institute for Advanced Study, State Key Laboratory of
Molecular Neuroscience, Institute of Molecular Functional Materials and Division
of Biomedical Engineering, The Hong Kong University of Science & Technology
(HKUST), Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
b College of Materials Science and Engineering, Beijing Institute of Technology,
5 South Zhongguancun Street, Beijing, 100081, China.
E-mail: chdongyp@bit.edu.cn; Fax: +86-10-6894-8982
c Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory,
State Key Laboratory of Luminescent Materials and Device, South China University
of Technology (SCUT), Guangzhou 510640, China
d HKUST Shenzhen Research Institute, No. 9 Yuexing 1st RD, South Area,
Hi-tech Park, Nanshan, Shenzhen, China 518057
Fig. 1 (A) Absorption and emission spectra of TPPA in solution (THF; 10 mM) and
aggregated states (THF–water mixture 1 : 9 v/v; 10 mM). Excitation wavelength:
290 nm. (B) Molecular orbital amplitude plots of HOMO and LUMO of TPPA.
† Electronic supplementary information (ESI) available: Synthesis and character-
ization, emission, IR and UV spectra and X-ray diffractograms. See DOI: 10.1039/
c3cc41414k
c
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water and a level-off tail is observed at the longer wavelength region integrating sphere is 4.0%, it jumps to 52.4% after being fumed
of the UV spectrum, its molecules must have been aggregated in the with butylamine gas, showing a B13-fold enhancement. On the
aqueous mixture with such high water content. Clearly, the FL of other hand, secondary and tertiary amine gases exert no effect
TPPA is quenched by aggregate formation, which is exactly opposite on the FL process of TPPA. In addition to visual observation, we
to that observed in its sodium salt form and ester derivative.9 In the also checked the FL response of the paper strip to various
aggregated state, the rotation of the phenyl rings is restricted due to organic amine gases using a spectrofluorometer. Although the
the physical constraint, which blocks the nonradiative pathways and peak maximum varies slightly with the type of amine, the
hence enhances the FL. On the other hand, as suggested by the overall result is consistent with that from the visual observation
appearance of the carbonyl stretching vibration at 1681 cmÀ1 in the and demonstrates the high selectivity of TPPA towards primary
IR spectrum, the TPPA molecules dimerize presumably via hydrogen amines. Among them, those with a low molecular mass such
bonding between the carboxylic acid groups (Fig. S3A, ESI†).10 as ethylamine and butylamine exhibit a stronger emission
When one molecule is photoexcited, an excimer-like species will ‘‘turn-on’’ effect, probably due to their higher volatility and
be formed, whose nonradiative relaxation quenches the light smaller size, which can help them to easily diffuse into the
emission of the luminophore. The competition between the TPPA lattice without suffering severe steric hindrance.
constructive and destructive effects may consequently render
TPPA non-emissive in the aggregated state.
To have a quantitative picture, we dipped TPPA drop-casting
films into aqueous solutions of butylamine with different
To better understand the photophysical properties of TPPA, concentrations and measured their FL. As depicted in Fig. 2B,
a theoretical calculation was carried out. The optimized molecular the FL spectrum was intensified upon increasing the amine
structure and HOMO and LUMO plots are shown in Fig. 1B. The concentration. The plot of emission intensity versus amine
HOMO of TPPA is dominated by the orbitals from the central pyrrole concentration gives a straight line with a detection limit of
ring and the peripheral phenyl rings, while the orbitals from the 3 mM and a correlation coefficient of 0.9877. The fumigation
benzoic acid moiety contribute mainly to the LUMO level. The process caused no FL enhancement in the sodium salt and
energy band gap is calculated by subtracting the LUMO energy level ester of TPPA. It also exerts no change in the mass spectrum of
from the HOMO energy level and is found to be equal to 3.49 eV.
TPPA. These suggest that the carboxylic acid group plays an
The above discussion shows that dimer formation is mainly important role in the amine sensing and rule out the possibility
responsible for the emission quenching of TPPA in the aggregated of formation of amide from TPPA and amine vapor as its
state. If such formation is disrupted by external stimuli, its FL may formation requires the use of a dehydrating agent and catalysts,
be restored, making the dye molecule to serve as a ‘‘turn-on’’ which is unlikely, in most cases, to be triggered by such a
fluorescent sensor for that analyte. Amine is capable of forming a simple chemical fuming process. The formation of a hydrogen
hydrogen bond with the carboxylic acid group. With regard to this, bond between the amine molecule and the carboxylic acid
we utilized TPPA adsorbed on a filter paper as a chemosensor for group of TPPA changes the packing arrangement of the dye,
amine detection. As shown in Fig. 2A, the dye-loaded paper strip as revealed by the spectral difference in its X-ray diffractograms
stuck on the inner wall of a quartz cell is nonemissive under before and after gas fumigation (Fig. S4, ESI†). The absorption
350 nm UV irradiation. When a few drops of primary amine liquid of the TPPA is also altered in the presence of amine gases, with
was added at the bottom of the cell, intense blue light was the spectrum of the fumed film being red-shifted from that of
recorded, which could be readily observed by the naked eye. While the untreated one (Fig. S5, ESI†). Observation using transmission
the FFL value of TPPA solid powder determined by a calibrated electron microscopy (TEM) shows that nanofibres are formed by
slow evaporation of the THF solution of TPPA at room temperature,
which disintegrate into shorter and thicker cables upon exposure to
butylamine gas for 40 s (Fig. 3). Analysis using IR spectroscopy
depicts that the carbonyl absorption peak shifts to 1625 cmÀ1 after
being fumed with butylamine gas, while no obvious peak shift was
observed when treated with triethylamine gas (Fig. S3, ESI†).
Stronger emission induction was observed when the TPPA-
embedded filter paper strips were directly dipped into the
amine solutions due to the better contact of the solid papers
with liquid (Fig. S6, ESI†). The ‘‘turn-on’’ process is very fast. As
shown in Fig. 4A, the relative emission intensity of the TPPA
Fig. 2 (A) Fluorescent photographs and emission spectra of TPPA adsorbed on
filter papers fumed by different amines. Abbreviation: ethylamine (EA), butylamine
(BA), hexylamine (HA), cyclohexylamine (CHA), heptylamine (HeA), ethylenediamine
(EDA), propanediamine (PDA), pyridine (PD), dimethylamine (DMA), diethylamine
(DEA), dipropylamine (DPA), N,N-diisopropylamine (DIPA), decylamine (DA),
di-n-octylamine (DOA) and triethylamine (TEA). The photos were taken under
365 nm UV irradiation from a hand-held UV lamp. (B) Emission spectra of TPPA
drop-casting films after being dipped into aqueous solutions of butylamine with
different concentrations. Inset: change in FL intensity with butylamine concen-
tration. Excitation wavelength: 350 nm.
Fig. 3 TEM images of TPPA drop-casting films (A and B) before and (C and D)
after being fumed with butylamine gas.
c
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the Guangdong Innovative Research Team Program of China
(201101C0105067115).
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film becomes progressively higher with an increase in the
exposure time, and reaches its maximum value within a short
time down to 30 s. A short video was made to demonstrate the
fast response of the device to butylamine gas (see ESI†). In
addition to being rapid, the detection method also enjoys the
advantages of reversibility and reproducibility. The FL of the
fumed TPPA film can be ‘‘turned-off’’ easily by heating the film
at 100 1C for 60 s. The ‘‘dark’’ and ‘‘bright’’ states can be
interconverted to each other many times without fatigue and
loss in the FL intensity, as these stimuli are nondestructive in
nature (Fig. 4B).
In summary, a convenient, selective and sensitive fluores-
cent chemosensor has been developed. TPPA is nonemissive in
both solution and solid states. It transforms into a strong
fluorophore in the presence of amine, enabling it to work as
a ‘‘turn-on’’ sensor for amine gas detection. Thus, the TPPA-
loaded filter paper strip becomes emissive when fumed with
primary amine gases, but still remains nonemissive when exposed
to other amines. The detection is rapid and strong, and can be
readily observed by the naked eye due to the ‘‘lighting up’’ nature
of the sensory process. The emission of TPPA can be switched
between the dark and bright states repeatedly by fuming and
heating processes, demonstrating a prototype device using TPPA
for detecting primary amines in real-world applications. Since
ammonia is generated during food degradation, we would like to
utilize TPPA for such detection. This is currently under active
investigation in our laboratory.
This work was partially supported by the National Basic
Research Program of China (973 Program; 2013CB834701),
National Science Foundation of China (20974028, 51073026,
51061160500 and 21074011), the RPC and SRFI Grants of
HKUST (RPC11SC09 and SRFI11SC03PG), the Research Grants
Council of Hong Kong (HKUST2/CRF/10 and N_HKUST620/11),
the Innovation and Technology Commission (ITCPD/17-9), the
University Grants Committee of Hong Kong (AoE/P-03/08), the
Specialized Research Fund for the Doctoral Program of Higher
Education (Grant No. 20091101110031) and the Major Project
Seed Research Program of Beijing Institute of Technology
(Grant No. 2012CX01008). B. Z. Tang thanks the support of
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4850 Chem. Commun., 2013, 49, 4848--4850
This journal is The Royal Society of Chemistry 2013