pubs.acs.org/acssensors
Review
Schiff Base Aggregation-Induced Emission Luminogens for Sensing
Applications: A Review
Jingfei Wang, Qingye Meng, Yongyan Yang, Shuangling Zhong, Ruiting Zhang, Yuhang Fang, Yan Gao,
and Xuejun Cui*
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ABSTRACT: Fluorescence sensing can not only identify a target
substrate qualitatively but also achieve the purpose of quantitative
detection through the change of the fluorescence signal. It has the
advantages of immense sensitivity, rapid response, and excellent
selectivity. The proposed aggregation-induced emission (AIE)
concept solves the problem of the fluorescence of traditional
fluorescent molecules becoming weak or quenched in high
concentration or aggregated state conditions. Schiff base
fluorescent probes have the advantages of simple synthesis, low
toxicity, and easy design. They are often used for the detection of
various substances. In this review we cover late developments in
Schiff base compounds with AIE characteristics working as
fluorescence sensors.
KEYWORDS: chemical sensors, Schiff base, fluorescent probe, sensor, ions, pH response, bacteria, aggregation-induced emission
W
significantly enhanced, which can be clearly observed by the
naked eye, and the detection sensitivity is high. (b) The
TURN-OFF type is a fluorescence quenching chemical sensor.
This type of fluorescent chemical sensor can usually emit
strong fluorescence. When the detected substance is added, the
fluorescence emission is greatly weakened or even quenched.
Due to the relatively high background fluorescence signal,
when the concentration of the detected substance is close to
the detection limit, it is difficult to observe the fluorescence
signal change. (c) Ratiometric fluorescent chemical sensors
typically emit dual fluorescence. After identifying the substance
to be detected, the intensity of one fluorescence emission
gradually decreases, while the intensity of the other
fluorescence emission is gradually boosted. The purpose of
ratio detection is achieved by the different changes of the
intensity of the dual fluorescence.34−40
A series of changes will occur in the optical properties of the
fluorophore because of the interaction between the fluorescent
chemical sensor and the recognition groups of the samples,
including the fluorescence properties such as fluorescence
ith the development of industrial production and the
improvement of human living standards, metal ions and
various toxic anions have come to pose a certain threat to the
well-being of humans. It is significant to test these
contaminants specifically.1−7 There are numerous traditional
bioanalytical techniques which can effectively detect very low
concentrations of analytes, such as UV−vis spectroscopy,8−12
liquid chromatography−mass spectrometry,13−16 atomic absorption spectroscopy,17−20 and other techniques. However,
high prices and complicated and time-consuming operations
limit these instruments in practical applications for simple,
rapid, and reliable bioassays and various toxicity assays.
Fluorescence sensing technology shows various advantages
like low cost, easy synthesis, quick selectivity, high sensitivity,
and real-time detection. Because of these facts, it brings
extensive attention in environmental chemistry, biology, and
clinical analysis.21−27 The established nominal instrument
support of small organic molecular probes for sensing shows
obviously high sensitivity and easy operation. Fluorescence
changes produced by fluorescent probes can be analyzed
easily.28−33
According to the change of the fluorescence before and after
the reaction, fluorescent chemical sensors can be divided into
three categories: (a) The TURN-ON type is a fluorescenceenhanced chemical sensor. This type of fluorescent chemical
sensor has no fluorescence or very weak fluorescence itself.
When the substrate is identified, the fluorescence is
© 2022 American Chemical Society
Received: July 19, 2022
Accepted: August 17, 2022
Published: September 1, 2022
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lifetime and intensity, emission wavelength, etc. Common
fluorescent chemical sensing mechanisms include excited-state
intramolecular proton transfer (ESIPT), fluorescence resonance energy transfer (FRET), intermolecular charge transfer
(ICT), ligand-to-metal charge transfer (LMCT), photoinduced
electron transfer (PET), and so on.41−51
Unfortunately, most luminescent materials tend to be deeply
emissive in the solution state, whereas very frail or no emission
is recognized in the solid or aggregated state. The
phenomenon in which fluorescent materials turn nonemissive
in aggregates is called aggregation-caused quenching (ACQ).
The ACQ phenomenon is harmful for the detection of
chemical substances. In 2001, Tang’s team came up with a new
anti-ACQ concept known as aggregation-induced emission
(AIE).52−55 Restriction of intramolecular motions (RIM) is
the center of the AIE mechanism. RIM can be explained as the
restriction of intramolecular rotations (RIR) and restriction of
intramolecular vibrations (RIV).56 In recent years, AIE
materials have been widely used as probes in many
fields.29,57−61 Traditional aggregation-induced emission luminogens (AIEgens) usually have a conjugated structure like
benzene. Different from traditional luminescent materials
containing significant conjugates, novel nontraditional luminophores usually carry electron-rich moieties. More and more
unconventional systems have been reported with AIE properties in recent years.62−66 For example, there are some polymers
with nonconjugated structures of electron-rich atoms (N, O, S,
P) or groups (−C� O, −C�N−, −COOH). The clusteringtriggered emission mechanism (CTE) has been indicated to
clarify those atypical luminophores grounded in the AIE
mechanism. The CTE mechanism believes that the electron
clouds of electron-rich atoms or groups overlap during
aggregation, and the movement within the molecule is
restricted by the aggregation, resulting in transition emission.67−72
The Schiff base reaction was proposed by Hugo Schiff in
1864.73 The Schiff bases’ independently conjugated moieties,
C�N double bonds, endow them with fluorescence emission
and bonding capability simultaneously. Schiff base materials
have been extensively examined in disparate applications such
as fluorescent probes, metal complexes, photovoltaic devices,
pharmaceuticals, hydrogels, liquid crystals, fluorescence imaging, catalytic actions, and so on.74−77 Moreover, Schiff base
compounds simultaneously stabilize metal ions in multiple
oxidation states, usually as metal ligands. The Schiff base
structure exhibits excellent performance in the detection of
metal ions. Not limited to only metal ions, plenty of Schiff base
compounds show response and selectivity for other chemical
substances. Up to now, there are many types of Schiff bases
that have been explored as chemical sensors.78−82 C�N
isomerization is an extremely strong excited-state decay
process, so compounds with C�N bonds have weaker
emission. When the isomerization process is inhibited, a
great fluorescence enhancement phenomenon is exhibited.
These Schiff base compounds exhibit typical AIE characteristics. The fluorescence of the Schiff base molecules in which
there is no conventional chromophore can be explained by the
CTE mechanism.
Several review articles have been broadcast that discuss
Schiff base molecules used in ion detection. However, there is
no review on Schiff base molecules with AIE characteristics
used as chemical sensors.76,83 In this review, we first summarize
the common types of Schiff base fluorescent probes and
Review
sensing mechanisms. We then focus on the latest progress of
AIE probes as sensors for various ions, pH values, thiols,
nitrogen-containing compounds, and bacteria (Figure 1). The
Figure 1. Schematic illustration of Schiff base AIEgens in chemical
sensing.
latest design concept of a Schiff base AIE probe that is suitable
for practical applications is proposed by summarizing the work
of previous researchers. Looking forward, the future is
considered to depend on the advancement of modern analysis.
■
DETECTION OF CATIONS
Detection of Aluminum. As the third most generous
metallic element on earth, aluminum is universally used in
many fields. Al3+ is also present in natural water and can enter
the human body through food and drinking water. Excess Al3+
can affect the lung system and can cause serious health hazards
such as Alzheimer’s and Parkinson’s diseases, kidney failure,
and osteomalacia. It is meaningful to find elementary, rapid,
sensitive, and selective methods to detect excess Al3+ in
environmental and biological systems.84−86
Wang’s team synthesized AIE Schiff base probe L1 through
the condensation reaction in ethanol of nicotinic acid
hydrazide and a modified aldehyde derivative of tetraphenylethylene. With the addition of water, the photoluminescence
(PL) intensity of the L1 solution (MeOH/water) gradually
increased. Probe L1 is insoluble in water; the free rotation of
the aromatic ring is prohibited, and the nonradiative transition
of the excited-state energy is covered up, resulting in enhanced
fluorescence. Probe L1 belongs to the TURN-ON type of
chemical sensors. The limit of detection (LOD) of probe L1 is
16.5 μM for Al3+. The PL intensity of L1 displays an acceptable
linear relationship with the concentration of Al3+ in the range
of 0−18 μM at 521 nm. Furthermore, probe L1 has good pH
stability. The combination of recognition groups like phenol
hydroxyl O, aldehyde O, and imine N in L1 with Al3+ forbids
the PET process in L1, which results in the emission
enhancement. AIE Schiff base probe L1 shows application
prospects in the detection of Al3+.84
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Figure 2. (A) PL spectra of L2 in MeOH/water solution with various water fractions. (B) PL spectra of L2 in the presence of increasing
concentrations of Al3+. (C) Structure of L2. (D) PL spectra of L2 in different pH buffer solutions. (E) The possible mechanism for fluorometric
concerting of L2 with solutions with different pH values. Photos of L2 under 365 nm UV light in (F) the absence and presence of different ions and
(G) different pH buffer solutions (concentration, 100 μM; λex = 365 nm). (Reprinted with permission from ref 87. Copyright 2018 Royal Society
of Chemistry.)
Figure 3. (A) Structure of L3 (left) and L4 (right). (B) PL spectra of L3 (left) and L4 (right) in THF/HEPES buffer solution (pH = 7.4) with
different water fractions; λex = 438 nm. (C) Possible mechanism of Cu2+ detection by L2 [R is −CH2(ph) and −CH(ph)2]. (D) PL spectra of L3
and L4 with different ions. Inset: photos of L3 and L4 with added Cu2+ under 365 nm UV light; λex = 435 nm. (E) Possible species formed in the
detection of Zn2+ [R is −CH2(ph) and −CH(ph)2]. (F) PL spectra of L3 (λex = 400 nm) and L4 (λex = 410 nm) with different ions. Inset: photos
of L3 and L4 with added Zn2+ under 365 nm UV light (EtOH/HEPES, v/v = 9/1, pH = 7.40). (Reprinted with permission from ref 91. Copyright
2021 Royal Society of Chemistry.)
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the potential to sense metal cations. Zn2+ can form a chelated
coordination with L5 as the ligand.92
Ni’s team designed AIE Schiff base L6 based on 2-hydroxy5-(1,2,2-triphenylvinyl)benzaldehyde and o-phenylenediamine.
L6 exhibits faint red emission in pure THF solution. However,
the emission increased about 10 times when the fraction of
water reached 90%. The restriction of the rotation of the
benzene contributed to the AIE effect of L6. The L6 molecule
can form an intermolecular hydrogen bond with hydrone,
which hampers its PET effect. More energy is dissipated in the
form of emission, enhancing the PL intensity. Zn2+ can induce
the fluorescence color change of L6 in a TURN-ON way. The
LOD of Zn2+ is 1 × 10−3 μM. The emission intensity of L6
increases linearly with increasing ion concentration from 0 to
10 μM. The fluorescence spectrum change is supposed to
result from the coordination between L6 and Zn2+ ions. The
chelation limits the PET effect in L6, preventing ESIPT and
nonradioactive processes, thereby inducing fluorescence
enhancement and emission wavelength changes. Furthermore,
L6 has low cytotoxicity. In conclusion, it shows wide prospects
in many applications.90
Detection of Copper. Copper is widely present in the
human body. It is the third most abundant transition metal in
the human body after iron and zinc. Although copper is an
essential element of biological systems and is considered one of
the important nutrients for plants and animals, at higher
concentrations copper has toxic effects on organisms and is
associated with several diseases, such as Alzheimer’s,
Parkinson’s, Menkes, and Wilson’s disease. Because of the
paramagnetic nature of Cu2+, most of the Cu2+-sensitive AIE
Schiff base probes belong to the TURN-OFF type.93−95
Schiff bases L3 and L4, synthesized by Jin’s team, can not
only detect Zn2+ but also be sensitive to Cu2+. Both L3 and L4
show fluorescence quenching after the addition of Cu2+. As a
TURN-OFF sensor, the LODs of L3 and L4 are 77.4 and 1.35
μM for Cu2+, respectively, and the linear ranges of Cu2+
detection by L3 and L4 are 0−4 and 0−7 μM, respectively.
The nitrogen atoms from C�N, the benzothiazole moiety,
and oxygen from −OH all participate in the coordination with
Cu2+. The fluorescence quenching of L3 and L4 can be
attributed to the paramagnetic nature of Cu2+ or the LMCT
process.91
Probe L7 was synthesized by the reaction of 2-hydroxy-1naphthaldehyde and 5-amino-3-phenylpyrazole. L7 shows high
solubility in many organic solvents. Water is a poor solvent for
L7. In THF/water mixtures, the emission spectra reached the
top when the water fraction was 90%, indicating the AIE
characteristic of L7. The aggregation of L7 will be triggered
when the water fraction increases. The RIM of aromatic rings
in L7 can inhibit the C�N bond through the intermolecular
and intramolecular hydrogen bonds. The nonradiative pathway
was blocked to enhance the emission. Probe L7 belongs to the
TURN-OFF type of chemical sensors. The LOD is 3.1 μM for
Cu2+. The PL intensity is linearly related to the concentration
of Cu2+ from 0 to 8 μM. The paramagnetic nature of Cu2+
leads to the metal-to-ligand charge-transfer reaction between
Cu2+ and L7. The fluorescence quenching is called chelationenhanced quenching (CHEQ).78
Zhao’s team synthesized L8 based on coumarin with an AIE
effect. L8 also works as a TURN-OFF Cu2+ sensor. In THF/
water mixtures, the fluorescence intensity is enhanced with the
addition of water. Once the PL intensity increases, a red shift
can be observed in the maximum emission wavelength. The
Probe L2 was synthesized from salicylaldehyde and 2aminobenzothiazole by Laskar’s team (Figure 2). In methanol,
L2 is nonemissive, and when the water fraction is increased
from 0% to 90%, the PL spectra is increased 20-fold higher.
According to the crystal structure, the rotation of intramolecular hydrogen bonds in the molecule is restricted. That is
the reason why L2 shows AIE properties. Probe L2 belongs to
the TURN-ON type of chemical sensors; the LOD is 1.2 ×
10−5 μM for Al3+. The linear range of Al3+ detection by L2 is
200−380 μM. L2 chelates with Al3+-chelated bidentate
salicylaldehyde to form a tricomplex. The combination breaks
the imine bond in L2, and the molecular motion is further
restricted, which leads to the emission enhancement.87
Detection of Zinc. The detection of biologically essential
metal ions is very important in biological research and
environmental inspection. Zinc is one of the essential trace
elements and participates in many significant physiological
processes in the human body. Human growth and development, reproductive genetics, immunity, the endocrine system,
and other processes are inseparable from the participation of
zinc.88−90
Jin’s team obtained three types of AIE-active Schiff bases
based on 2-(2′-hydroxyphenyl)benzothiazole by an aldamine
condensation reaction (Figure 3). The hydroxyl groups are
close to the nitrogen atoms of C�N; thus, they tend to form
stable six-membered rings through intramolecular hydrogen
bonding, exhibiting ESIPT behavior. Meanwhile, due to the
existence of coordination sites and oxygen and nitrogen atoms,
they can serve as recognition moieties for binding metal ions.
When the fraction of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer is enhanced, the molecules in
THF/HEPES (HEPES buffer solution, pH = 7.4) mixtures are
aggregated. The PL spectra show the AIE phenomenon. As a
protic solvent, water can destroy intramolecular hydrogen
bonds and block the ESIPT progress. Under this situation, the
emission is from enol*. When the fraction of water reaches
70%, aggregates form in the mixed solution, leading to
enhanced emission from keto*. Both L3 and L4 show a
fluorescence enhancement after the addition of Zn2+. As a
TURN-ON sensor, the LODs of L3 and L4 are 91.1 and 1.01
μM for Zn2+, respectively. The linear ranges of Zn2+ detection
by L3 and L4 are both 0−5 μM. The oxygen atom and
nitrogen atom from the Schiff base C�N coordinate with Zn2+
forming the structure in Figure 2. The ligands exist in the enol
form. After coordination with Zn2+, the ESIPT process is
inhibited. As a consequence, the coordination strengthens the
emission from the excited state of enol*.91
Laskar’s team used salicylaldehyde as the raw material to
synthesize Schiff base L5. In comparison with the emission in
polar solutions, the λex of L5 has a blue shift in nonpolar
solutions. The results indicate that the emission of L5 comes
from an intramolecular charge-transfer/twisted intramolecular
charge-transfer (ICT/TICT) transition. In MeOH/water
mixtures, the maximum of the PL intensity occurs at f water =
50% (the fraction of water). From f water = 0% to f water = 50%
there is a red shift of λex. L5 is in an amorphous state in
aqueous solution, which is not conducive to emission. But
when it comes to MeOH/PEG mixtures, the PL intensity does
not reduce with the increase of poly(ethylene glycol) (PEG).
The AIE effect of L5 can be ascribed to RIR. L5 is a TURNON sensor for Zn2+; the LOD is 0.1565 μM. The range of the
linear relationship between the concentration of Zn2+ and the
PL intensity is 0−12 μM. The O and N coordinating sites have
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Figure 4. (A) Structure of L10. (B) PL spectra of L10 in THF/water mixtures with different fractions of THF. PL spectra of L10 solutions with the
addition of different concentration of (C) Cu2+, (D) Fe3+, (E) E. coli, and (F) S. aureus. (G) Growth of E. coli and S. aureus at a temperature of 37
°C (concentration, 2 mg/mL; λex = 360 nm). (Reprinted with permission from ref 98. Copyright 2022 Elsevier B.V.)
phenomenon can be attributed to the RIR mechanism. The
aggregation of molecules limits the rotation of the benzene ring
and C�N isomerization. The RIM process blocks the
attenuation of nonradiative energy and enhances the
fluorescence and red shift. After Cu2+ was put into THF/
H2O (10/90, v/v), fluorescence quenching could be clearly
observed. Cu2+ binds to L8 through the N and O in the imine
and hydroxyl groups, which are directly connected to the
aromatic ring, interfering with the intramolecular hydrogen
bond between the N atom and H atom of the imine. The LOD
is 0.36 μM for Cu2+. The emission intensity of L8 at 565 nm
shows a clear linear relationship with the concentration of Cu2+
when the concentration is less than 1.2 μM.96
Chen’s team designed and synthesized an IRMOF-3
framework L9 with an AIE effect, which was obtained by the
reaction of Schiff bases with Zn2+. The precursor ligand has no
emission in THF solution, and the fluorescence constantly
rises with the increase of water. The ground-state highest
occupied molecular orbital (HOMO) electrons are mainly
discovered in the benzene ring with the amino group, and
others are located on the imine moiety, whereas the lowest
unoccupied molecular orbital (LUMO) electrons spread in the
imine moiety and phenolic ring. Migration of electrons
between the ground and excited states creates conditions for
ESIPT. The special topologies of the IRMOF-3 framework
restricted the intramolecular motions, rotations, and vibrations
of rigid skeletons. The restriction contributed to the precursor
AIE ligands donating to the framework a boosted fluorescence
emission. The electronic effect was changed by changing the
functional group on the benzene ring of the precursor L9. L9
belongs to the TURN-OFF type of chemical sensors. The
LOD is 1.35 × 10−4 μM for Cu2+. Cu2+ destroyed the flower
structure of the IRMOF-3 framework. Besides, because of the
paramagnetic nature of Cu2+, the electron−hole pair is
separated under excitation. The electron transfers to Cu2+
with the energy transfer, thus engendering the fluorescence
quenching of L9.97
Cui’s group obtained L10 by modifying chitosan with
salicylaldehyde (Figure 4). As a chitosan derivative, L10
exhibits conspicuous AIE performance and detection of Cu2+,
Fe3+, Gram-positive bacteria, and Gram-negative bacteria.
Different from other Schiff base compounds, L10 shows
amazing water solubility. At f THF = 0%, L10 solutions exhibit
weak fluorescence. As the percentage of THF increases, the
aggregates of L10 formed, which resulted in the enhancement
of fluorescence. In concentrated L10 solution, the N, O, and H
atoms in L10 molecules can achieve contact between the
entanglement of the intramolecular chain. The AIE phenomenon is achieved through conformational rigidity and electron
cloud stacking. Cu2+ can form complexes with L10 and break
the intramolecular hydrogen bonds in L10, and then quench
the fluorescence. L10 belongs to the TURN-OFF type of
sensor for Cu2+. The LOD is 5.45 × 10−2 μM. The emission
intensity of L10 increases linearly with increasing ion
concentration from 5 to 80 and 160−800 μM.98
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However, too much F− can cause fluorosis, leading to an
increase in bone density.102−104
Schiff base L14 was synthesized by Laskar’s team. The
compound showed an AIE property. Additionally, it can also
be used for highly sensitive detection of F−. In MeOH/water
mixtures, L14 molecules gathered in its poor solvent. At f water =
70%, the PL intensity approached the peak level. The
maximum is 30 times higher than the PL intensity in pure
MeOH solvent. Due to the ESIPT effect of L14, there was a
red shift and reduction at f water = 90%. The aggregated
molecules restrict the movement of the molecules, and the enol
conformation is more favorable for the formation of the
ground state owing to the action of intramolecular hydrogen
bonds. Upon excitation, the enol form isomerizes to the ketone
form, resulting in bright emission. For F−, L14 belongs to the
TURN-ON type of sensor. The LOD is 1.4 × 10−5 μM. The
F− concentration was linearly proportional to the emission
intensity in the range of 10−30 μM. The deprotonation of OH
in the phenol exchange is the photochemical property of L14.
The hydrogen bond is followed by proton transfer from the
OH in phenol to F−. This process generated phenoxide ion
and led to an enhanced “push−pull” effect. With increasing
concentration of F−, the imine N and OH in the hydrogen
bonds were disrupted; the deprotonation of phenolic OH
enables the ESIPT process.44
Mathavan’s team designed Schiff base L15 based on a
julolidine derivative. L15 revealed better solubility and brighter
emission in polar solvents. Since rotor C�N double bonds
exist in the compound, it could form aggregates in THF/water
mixtures upon introducing the poor solvent, water. When the
fraction of water changed from 10% to 90%, the fluorescence
wavelength red-shifted from 485 to 535 nm. The fluorescence
intensity of L15 increased 13-fold. The π−π stacking
interactions induced by the aromatic molecules of L15 lead
to spectral changes, resulting in the weakening of vibrational
motion. Intramolecular rotation of the C�N group of L15 was
restricted when the molecule gathered, preventing energy loss
from nonradiative decay, resulting in fluorescence enhancement. L15 exhibited sensitive and selective sensing toward F−
with the color changing from yellow to orange. This indicates
that L15 can be a TURN-ON sensor for F−. The LOD is 0.23
μM. A plot of the fluorescence intensity of L15 shows a linear
relationship with the concentration of F− in the range of 0−20
and 30−60 μM. The ICT process occurs after the protons of
the amino or phenolic groups interact with acceptor fluoride
ions. Owing to the ICT mechanism, oxide anions can be
generated by deprotonation, and the gradual addition of
fluoride ions can enhance their ICT activity from oxygen
atoms to aromatic rings. Furthermore, L15 can not only detect
F− but also show sensitivity to lysozyme.105
AIE Schiff base probe L16 shows a selective fluorescence as
a TURN-ON sensor for F− and H2PO4− anions. When the
water content was increased from 0% to 90%, aggregates
formed in the mixed solvents and generated a 700-fold increase
in PL intensity. The AIE properties of L16 in THF/water
mixtures are capable of being naturally recognized by the
naked eye under UV light. The increased water content
isomerizes C�N, leading to enhanced fluorescence emission.
This restriction blocks the nonradiative channel, enhancing the
fluorescence intensity. The naphthalene part of L16 formed
hydrogen bonds between C�N and OH in the naphthalene
group. When it come to the pyrene part, the aggregation causes
the restriction of C�N. The LOD for F− is 72.3 μM. There
Detection of Ferric Ion. Iron is a fundamental nutrient for
life and is the most abundant trace element in the human body.
It plays an essential role in various physiological processes such
as human cell metabolism, enzyme catalysis, muscle contraction, osmotic pressure, DNA and RNA synthesis, nerve
conduction, and regulation of intracellular acid−base balance.
It works as an oxygen carrier in hemoglobin, and both iron
excess (hyperferremia) and iron deficiency (hypoferremia) can
result in serious health problems. Therefore, the use of new
analytical methods to detect iron species is of extreme
significance for the monitoring of iron concentrations in
various biochemical processes.99,100
L10 can not only detect Cu2+ but also shows sensitivity to
Fe3+. Fe3+ has a high positive charge density and strong
electron-trapping properties, which may lead to quenching. As
a TURN-OFF sensor, the LOD for Fe3+ is 3.45 × 10−2 μM.
This meets the Fe3+ concentration range of 14−32 μM in
normal human serum. In the range of 5−200 μM, the change
of the fluorescence intensity has a linear relation with the
concentration of Fe 3+ . L10 show great prospects in
bioimaging.98
L11 and L12 were synthesized by Harathi and Thenmozhi.
The AIE characteristics of L11 and L12 were investigated in
mixed THF/water solutions. Upon increasing f water from 10%
to 99.5%, the fluorescence intensity gradually increased and
reached a maximum at f water = 90% for L11 and 60% for L12.
The increased water suppresses the C�N isomerization
process by forming intramolecular hydrogen bonds with L11
and L12, showing the phenomenon of fluorescence enhancement. L12 shows lower solubility than L11 in water. When
f water is beyond 60%, aggregates precipitate out, resulting in a
dramatic drop in fluorescence intensity. C�N and −OH in
the compounds can effectively combine with metal ions. The
solutions have a visible discoloration in the presence of Fe3+,
Fe2+, Cu2+, and Co2+, indicating the wide application of L11
and L12 in naked eye sensors. Spectroscopic studies show that
both L11 and L12 exhibit excellent selectivity for Fe3+ in mixed
THF/water solution. They can work as TURN-OFF sensors.
The LODs are 0.1317 and 0.293 μM, respectively. A plot of PL
intensity against the concentration of Fe3+ shows a linear
relation; the range of L11 is 0−40 μM, and for L12 it is 0−25
μM. LMCT complexes formed by Fe3+ and compounds lead to
fluorescence quenching due to the PET process.101
Zhang’s group designed a novel AIE coumarin Schiff base
derivative L13, which contains imidazole and imine groups as
hydrophilic groups and coordination sites to form complexes.
The maximum of the PL intensity appeared when the fraction
of water was 90%. After the addition of Fe3+, the color of the
L13 solution (DMSO/water, v/v = 1:9) changed from
colorless to evident yellow. The addition of other ion metals
did not change the selectivity of L13 to Fe3+. The combination
between L13 and Fe3+ makes the associated occupied
molecular orbitals, HOMO and LUMO, transfer from a
certain moiety to the whole complex. L13 works as a TURNON-type probe for Fe3+. The LOD is 48.8 μM. Moreover, the
detection of Fe3+ by L13 is not affected by the pH value. L13
performed broad applications in industry and life.80
■
DETECTION OF ANIONS
Detection of Fluoride. Among various anions, fluoride is
the most common influencing factors for dental health. It is
very useful in osteoporosis and orthodontic treatments.
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Probe L2 can not only detect Al3+ but is also capable of
sensing the pH value. Under acidic conditions (pH = 1−5),
the absorption spectrum of L2 is blue-shifted compared to the
absorption in acid-free methanol. The protonation of the imine
nitrogen atom leads to an increase in the distance between the
phenolic group and the imine N (O−H, N). Under neutral
conditions, the salicylaldehyde moiety part is more distorted.
When excited by UV light, it is a nonradiative transition. That
is why there is no fluorescence in neutral conditions. At
physiological (pH = 6−8), L2 forms aggregates due to
interactions (hydrogen bonds and short contacts). Increasing
the pH from 9 to 14 increases the proton extraction, creating a
negative charge, leading to the formation of resonance
structures, resulting in a red shift in the absorption spectrum.87
Guo’s group designed Schiff bases L19, L20, and L21 with
different substituent groups. All of them exhibit a different
emission. All of them show PL intensity being enhanced with
the extension of water in THF/water mixtures. From the
theoretical calculation, the small energy difference between the
HOMO and LUMO makes the electronic transition, which
results in a red-shifted fluorescence emission. From pH = 7 to
pH = 12, the fluorescence intensity constantly increased. The
three Schiff base compounds show a good linear relationship
between the PL intensity and pH value (L19, pH range of 9−
11; L20, pH range of 10−12; L21, pH range of 8−11.)
Furthermore, all of them show excellent photostability and
reversibility. The −OH groups in L19, L20, and L21 ionize
protons to form negative oxygen ions under alkaline
conditions. The strong bases will isomerize C�N to form
C−N groups. Because oxygen ions are stronger electron
donors than hydroxyl groups, the ionization of hydroxyl groups
boosts intramolecular charge transfer and enhances solution
fluorescence.110
was a linear relationship between the fluorescence intensity
and concentration of F− in the range of 0−52 μM. L16 can
form complexes with anions. Interaction of intermolecular
hydrogen bonds with C�N groups in the presence of F− ions
induces a deshielding effect.106
Detection of Cyanide. As a toxic substance, the presence
of excess cyanide can cause great damage to the environment
and human health. Cyanide destroys the central nervous
system by reducing the oxidative metabolism of oxygen and
damaging the electron-transport chain of mitochondria.
However, apart from cyanide release from natural products
(cassava, bitter almonds, and potatoes), cyanide is broadly
used in chemical production processes such as gold extraction,
electroplating, and polymer manufacturing.24,107,108
Qiao’s team designed and synthesized probe L17 base on
the ESIPT and AIE strategy. L17 exhibited different optical
properties in solvents of different polarities and produced an
obvious Stokes shift, which indicates the ESIPT property of
L17. In DMSO/water mixtures, the PL intensity reached a
maximum at f water = 70%. Because of the ESIPT phenomenon,
L17 molecules form intermolecular hydrogen bonds with water
molecules. The intramolecular motions are less restricted,
which leads to the decline of the PL intensity. The actual
intramolecular hydrogen bonds and coplanar functional groups
of L17 cooperate with the intramolecular proton transfer and
light-induced structural transformation of L17, which helps to
stiffen the molecular conformation, and then inhibit the
rotation of the biphenyl moieties and C�N. The restriction
lessens the energy loss during the electron transition. It is very
beneficial to the existence of the AIE effect. The addition of
CN− evidently enhanced the fluorescence. L17 belongs to the
TURN-ON type of sensor. The LOD is 13.2 μM for CN−.
After CN− was added, the C atom in the CN− and the O in
L17 formed a new carbonyl group. The C�N in L17 was
destroyed and formed −NH. L17 has expansive operation
prospects in food and biological detection.82
L18 was designed and synthesized by Dong’s team. It is a
colorimetric and fluorescence TURN-ON sensor for cyanide.
L18 was totally dispersed in pure THF solvent; the addition of
water makes the free rotation of the single bond between the
naphthalene ring and C atoms restricted. In THF/water
mixtures, L18 belongs to a J-aggregate. When the water
content exceeded 40%, L18 formed a J-aggregate in solvents
leading to a TICT process occurring in rigid light-emitting
molecules. The π−π stacking between adjacent naphthalene
rings forms L18. Before and after adding CN−, the color of the
L18 solution transfers from colorless to yellow, visible to the
naked eye. The LOD is 0.216 μM. The fluorescence intensity
rises with the increase of concentration of CN− in the range of
5−35 μM. Furthermore, the sensor is reversible with the
addition of HCl. The proton interaction between C�N and
hydrogen atoms on L18 leads to a change in the fluorescence
color.109
■
DETECTION OF THIOLS
Glutathione (GSH) is considered the most abundant biothiol.
The level of GSH concentration is directly related to many
diseases like liver injury, cancer, AIDS, and cardiovascular
disease. Monitoring changes in GSH concentration is
necessary to understand local cell problems.113−115
The Schiff base nanoaggregate L22 was designed by Cao’s
team with teraphenylethene (TPE) and diketopyrrolopyrrole
(DPP). In CH2Cl2, L22 exhibited weak emission. The PL
intensity increased upon the addition of hexane solvent. The
same phenomenon appeared in DMSO/glycerol systems with
disparate viscosities, indicating that L22 is a ordinary AIEactive compound. Poor solvents like hexane and THF restrict
the intramolecular rotation process of the TPE units, and then
enhance the emission. At room temperature, L22 in THF/
water (4/6, v/v) showed a response to low concentrations of
GSH (1−0.1 mM), and the emission was enhanced with
increasing GSH. When the concentration of GSH is in the
range of 0−1 mM, the concentration rises linearly with
emission intensity, indicating that L22 belongs to the TURNON type of probe. The LOD is 0.05 μM. But when the
concentration of GSH increased, L22 showed a ratiometric
mode for the fluorescence signal. Glutathione is a tripeptide
containing sulfhydryl groups bound by glutamic acid, cysteine,
and glycine. Under acidic conditions (pH = 2.12), very little
GSH promotes the hydrolysis of C�N. The amino group on
GSH attaches to the imine nitrogen of L22, and then the water
molecule attacks the imine carbon, which finally leads to the
breaking of the imine bond to generate aldehyde in DPP and
■
RESPONSE TO PH
The luminescent properties of organic Schiff base molecules
are mostly influenced by protonation and deprotonation active
sites in the molecule. Suitable pH values make humans remain
healthy. Variation of pH in the environment may affect
people’s living condition. Therefore, it is vital to investigate the
spectral properties of compounds in different acids and
bases.110−112
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Table 1. Various Schiff Base AIEgens in Chemical Sensors
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Table 1. continued
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Table 1. continued
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Table 1. continued
amine in TPE. L22 works as a TURN-ON fluorescence sensor
to detect GSH. In contrast, when the concentration of GSH is
too high in an alkaline environment (pH = 8.75), nucleophilic
reaction will occur in the GSH−L22 system and destroy the
conjugation structures. Fluorescence energy will transfer from
the TPE part to the DPP part (FRET) to achieve a yield ratio
fluorescence mode.116
The ammonia electro-oxidation reaction (AOR) is extremely
essential for wastewater remediation, nitrogen cycling, and the
hydrogen economy. Hydrazine (N2H4) is one of the necessary
intermediates in the AOR process, and understanding the
content of the intermediates is of great significance for
designing efficient electrocatalysts for the AOR. Shao’s team
designed L24 based on the AIE-active molecule TPE for
detection of N2H4 via the Schiff base reaction. The
combination of N2H4 leads to fluorescence quenching of
L24. When −CHO was replaced by C�N, the molecular
motion constraints were lifted, resulting in fluorescence
quenching. L24 can act as sensor to detect N2H4 in AORs
which use commercial Pt/C as an electrocatalyst.118
Compared to hydrazine, hydrazine ion has been gradually
used in the chemical industry. Hydrazine ion is poisonous to
the human body, and exposure will result in cancer and dermal
corrosion to mutagenesis. L25 was synthesized by Yang’s group
based on thiophene−cyanodistyrene. The position of the
emission peak at the maximum fluorescence wavelength is
affected by solvent polarity. In THF/water (poor solvent/good
solvent) mixtures, due to the greater polarity of water, the
maximum fluorescence wavelength also exhibits a significant
red shift with the increase of water components. The maximum
of the PL intensity occurred at f THF = 90%. L25 shows an
obvious PET phenomenon in polar solvents. The addition of
THF broke the PET in L25 and resulted in the fluorescence
enhancement. L25 belongs to the TURN-ON type of
fluorescent probe for N2H62+ when the pH is <5. The
detection limit (DL) is 10.5 μM. An obvious linear relationship
was determined between the fluorescence intensities and the
concentration of N2H62+ in the range of 0−0.08 M. The
binding of N2H62+ to the thiophene site on L25 destroys the
PET process and strengthens the emission.119
■
DETECTION OF NITROGEN-CONTAINING
COMPOUNDS
Ammonia levels can diagnose diseases such as liver
insufficiency and diabetes and also have an effect on the
atmosphere. Most fluorescence sensors for ammonia are
mainly based on acid−base interactions or pH and show little
selectivity for ammonia in the presence of organic amines.
Dong’s group synthesized L23 by self-assembly. The molecular
assemblies can act as a fluorescence sensor for NH3 gas. The
classic ICT process occurs in L23 owing to the existence of
diethylamino and carboxylic acid. The former works as an
electron donor and the latter is an electron acceptor. With
increasing solvent polarity, the maximum emission wavelength
endures a strong red shift. Additionally, in L23 there is not
only intermolecular hydrogen bonding but also the weak
interactions in C�O···O, C−H···O, and C�O···O. Furthermore, the conformation of L23 is twisted. The emission is
enhanced by the restriction of molecular motion and the
reduction of nonradiative energy loss. Ammonia can react with
the carboxyl group on L23 through a proton-transfer process.
After exposure to ammonia gas, the chemical structure of L23
changes, and the luminescence also changes. With increasing
ammonia concentration, the PL spectrum blue-shifts with
decreasing intensity. The sensor can also be recycled, and heat
treatment can desorb the ammonia gas adsorbed in the sensing
device. When heated at 100 °C for 5 min, the fluorescence of
the gas-phase device can be switched back to the original
state.117
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DETECTION OF BACTERIA
Bacterial infection poses enormous threats to human health.
Gram-positive and Gram-negative bacteria may produce
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exotoxins that then lead to pathological changes. In clinics and
hospitals, intrinsic mutations and horizontal gene transfer
make bacteria become “superbugs”, which endows them with
resistance to current antibiotics. It is vital to explore fast and
noninvasive techniques for bacterial susceptibility assessment.
Chitosan-based Schiff base AIE material L10 shows great
biocompatibility and photostability. L10 binds to the bacterial
membranes through positive and negative charge interactions.
It is appropriate for L10 to work as TURN-ON sensors to
detect Gram-positive and Gram-negative bacteria with negative
charge. The electrostatic interaction between L10 and bacteria
affects the fluorescence of L10. Both Gram-negative bacteria
(Escherichia coli) and Gram-positive bacteria (Staphylococcus
aureus) enhance the PL intensity of L10. Moreover, it can
inhibit the growth of E. coli and S. aureus. It shows excellent
prospects in antimicrobial applications.98
Designed by Fang et al., L26 also shows selectivity to Grampositive bacteria (Bacillus subtilis and S. aureus) and Gramnegative bacteria (E. coli). The AIE behavior of L26 was
researched by differing the water content in organic solvent/
water mixtures. L26 exhibits different aggregation behaviors in
diverse solvents. The addition of the poor solvent, water,
causes fluorescence enhancement. These results verified the
AIE effect of L26. The addition of water isomerized C�N in
L26 and inhibited the PET process between the pyrene
molecules and aldimine N. The suppressed PET process
reduces nonradiative energy transitions and enables fluorescence enhancement. The L26 nanoparticles have a positively
charged surface, which can bind with the phospholipid bilayer
membrane. After the combination, the bacterial membrane was
physically disrupted, leading to the effusion of the intracellular
components from the bacteria cells. L26 can not only work as
fungicide for Gram-positive bacteria and Gram-negative
bacteria, but also show TURN-OFF fluorescence sensing for
bacteria.120
Review
Many Schiff base AIEgens have shown the advantages of being
easy to synthesize and having low cost, low detection limit, and
good selectivity. Despite that there have been enormous
advances in Schiff base AIEgen sensors, the need for
improvement still cannot be ignored. There is still a need to
be able to detect more disparate varieties of chemical
substances. Furthermore, imine bonds are susceptible to
hydrolysis in aqueous media. Many existing systems still
need to be performed in a mixed solution of organic solvent
and water, which is not conducive to in vivo imaging
applications. Especially, AIEgens with metal responsiveness
to important biological metal ions like K+, Ca2+, Na+, and Mg2+
demand further research to support the understanding of the
intracellular functions of these metal ions. With the development of organic room-temperature phosphorescence (RTP)
materials, it is still a challenge to explore Schiff base AIEgens
with RTP to expand the applications.
■
AUTHOR INFORMATION
Corresponding Author
Xuejun Cui − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China; Weihai
Institute for Bionics-Jilin University, Weihai 264400, People’s
Republic of China; orcid.org/0000-0003-2125-1733;
Email: cui_xj@jlu.edu.cn
Authors
Jingfei Wang − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
Qingye Meng − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
Yongyan Yang − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
Shuangling Zhong − College of Resources and Environment,
Jilin Agricultural University, Changchun 130118, People’s
Republic of China
Ruiting Zhang − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
Yuhang Fang − College of Chemistry, Jilin University,
Changchun 130012, People’s Republic of China
Yan Gao − College of Chemistry, Jilin University, Changchun
130012, People’s Republic of China; State Key Laboratory of
Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, People’s
Republic of China; Weihai Institute for Bionics-Jilin
University, Weihai 264400, People’s Republic of China;
orcid.org/0000-0001-5248-8907
■
CONCLUSION AND PERSPECTIVES
In this review, we have encapsulated and discussed Schiff base
compounds with AIE effects used as sensors to detect different
ions (Table 1). Schiff base compounds are generally used as
ligands for many metal ions. Al3+ can combine with the
nitrogen atom in imines, phenolic hydroxyl oxygen atoms, and
aldehyde oxygen atoms of the probes. The combination
inhibits the PET process of the probe and enhances the
emission. On the contrary, the Schiff base AIEgens for Cu2+
almost all belong to the TURN-OFF type of sensor. This can
be ascribed to the paramagnetic nature of Cu2+. The
intramolecular hydrogen bonds of the luminophore are broken,
which brings about the fluorescence. When it comes to Zn2+,
the mechanism is similar to that for Al3+. Zn2+ can form a
chelated coordination complex with a Schiff base as the ligand.
For Fe3+, the combination of the Schiff base and Fe3+ can
change the PET process and result in different variations of
fluorescence. Both F− and CN− achieve fluorescence enhancement by changing the way electrons are transferred. When it
comes to pH values, thiols, and nitrogen-containing compounds, the addition may affect the structure of the sensors
and lead to spectral changes or quenching. Moreover, the
positively charged Schiff base can interact with negatively
charged bacterial membranes selectively, and then achieve
applications in antibacterial sensing.
In conclusion, the key to the use of Schiff base compounds
for sensing lies in the changeable electron cloud distribution.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acssensors.2c01550
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors are thankful for the funding from the
Pharmaceutical Health Industry Development Special Project
of the Science and Technology Department of Jilin Province,
People’s Republic of China (no. 20210401172YY) and the
Interdisciplinary Research Funding Program for Doctoral
Students of Jilin University (no. 101832020DJX026).
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