to detect β-gal and other enzymatic activities. The core 2-arylbenzothiazole
derivatives were highly fluorescent, and this had been rationalized by ESIPT
process. When combined with the enzymic substrates, the ESIPT effect of
2-arylbenzothiazole derivatives was blocked because there were no more hydroxyl
protons at the ortho-position. However, enzyme-induced hydrolysis of weakly
fluorescent enzyme substrate derivatives respectively liberated either fluorescent
compounds. In addition, the coliform bacteria had also been detected because they
could grow in the presence of a β-gal substrate which was transformed by β-gal into a
fluorescent product that could readily be detected. Moreover, Wei et al. [23] successfully reported a series of ESIPT-based 2-(benzothiazol-2
0 -yl)-phenol fluorogenic
substrates for β-gal detection in 2017. They developed a novel efficient method for
the synthesis of important indoxyl glycoside substrates by using 1-acetylindol-3ones as intermediates and synthesized new precipitating fluorogenic substrates for
β-gal detection based on 2-(benzothiazol-2
0 -yl)-phenols and the ESIPT effect. And
they have also assessed the application of the fluorogenic substrates in the detection
of foodborne pathogenic bacteria.
Recently, although some fluorescent probes for β-gal have been developed, most
of the reported sensors were fabricated with traditional fluorophores, which suffered
from ACQ effect [17]. It causes the fluorescence in the aggregate state to be weaker
than with that in solution. Therefore, it is still highly demanded to explore β-gal
fluorescent probes, which could accumulate in living cells or tumor tissue imaging
without ACQ effect. In 2001, Tang’s group firstly found and reported the phenomenon of “aggregation-induced emission (AIE)” [24]. The compounds with AIE
effect are almost nonluminescent when they are dissolved in a good solvent but
emit intensely in a poor solvent. Meanwhile, the compounds with AIE effect have
significant advantages, such as bright luminescent in the aggregate state and large
Stokes shift [25–27]; thus, they have been greatly developed for detection of β-gal
[28, 29]. In 2015, Peng et al. [28] reported a salicylaldehyde azine derivative
SA-β-gal (11) for light-up detection of β-gal activity in living cells based on both
AIE and ESIPT effects. When the hydroxyl groups at the ortho-position on the
benzene ring of salicylaldehyde azine were substituted by β-galactopyranoside,
ESIPT process was blocked. Upon the addition of β-gal, the β-galactopyranoside
group on SA-β-gal was cleaved, and the restored hydroxyl group occurred in ESIPT
to regain the AIE characteristics, resulting in bright fluorescence. Contrary to
traditional β-gal fluorescent sensors, SA-β-gal could emit strongly in the aggregation
state, which can well avoid the ACQ phenomenon. The probe showed a large Stokes
shift, a high light-up ratio, and a high sensitivity (0.014 UmL
À1 ) toward β-gal.
Moreover, it was worth mentioning that the probe could also be well retrained in
living cells emitting strong fluorescence (Fig. 4).
In 2017, Jiang et al. [29] designed and synthesized a tetraphenylethylene-based
turn-on probe TPE-Gal (12) for β-gal detection with AIE effect in aqueous samples
and in living cells. TPE-Gal was designed to bear a positively charged pyridinium
pendant. And a substrate of β-galactosidase-D-galactose residue was conjugated to
the terminal of the pyridinium pendant. In the presence of β-gal, the D-galactose
residue was cleaved and resulted in a phenolate intermediate. Then the intermediate
192
A. Bi et al.
derivatives were highly fluorescent, and this had been rationalized by ESIPT
process. When combined with the enzymic substrates, the ESIPT effect of
2-arylbenzothiazole derivatives was blocked because there were no more hydroxyl
protons at the ortho-position. However, enzyme-induced hydrolysis of weakly
fluorescent enzyme substrate derivatives respectively liberated either fluorescent
compounds. In addition, the coliform bacteria had also been detected because they
could grow in the presence of a β-gal substrate which was transformed by β-gal into a
fluorescent product that could readily be detected. Moreover, Wei et al. [23] successfully reported a series of ESIPT-based 2-(benzothiazol-2
0 -yl)-phenol fluorogenic
substrates for β-gal detection in 2017. They developed a novel efficient method for
the synthesis of important indoxyl glycoside substrates by using 1-acetylindol-3ones as intermediates and synthesized new precipitating fluorogenic substrates for
β-gal detection based on 2-(benzothiazol-2
0 -yl)-phenols and the ESIPT effect. And
they have also assessed the application of the fluorogenic substrates in the detection
of foodborne pathogenic bacteria.
Recently, although some fluorescent probes for β-gal have been developed, most
of the reported sensors were fabricated with traditional fluorophores, which suffered
from ACQ effect [17]. It causes the fluorescence in the aggregate state to be weaker
than with that in solution. Therefore, it is still highly demanded to explore β-gal
fluorescent probes, which could accumulate in living cells or tumor tissue imaging
without ACQ effect. In 2001, Tang’s group firstly found and reported the phenomenon of “aggregation-induced emission (AIE)” [24]. The compounds with AIE
effect are almost nonluminescent when they are dissolved in a good solvent but
emit intensely in a poor solvent. Meanwhile, the compounds with AIE effect have
significant advantages, such as bright luminescent in the aggregate state and large
Stokes shift [25–27]; thus, they have been greatly developed for detection of β-gal
[28, 29]. In 2015, Peng et al. [28] reported a salicylaldehyde azine derivative
SA-β-gal (11) for light-up detection of β-gal activity in living cells based on both
AIE and ESIPT effects. When the hydroxyl groups at the ortho-position on the
benzene ring of salicylaldehyde azine were substituted by β-galactopyranoside,
ESIPT process was blocked. Upon the addition of β-gal, the β-galactopyranoside
group on SA-β-gal was cleaved, and the restored hydroxyl group occurred in ESIPT
to regain the AIE characteristics, resulting in bright fluorescence. Contrary to
traditional β-gal fluorescent sensors, SA-β-gal could emit strongly in the aggregation
state, which can well avoid the ACQ phenomenon. The probe showed a large Stokes
shift, a high light-up ratio, and a high sensitivity (0.014 UmL
À1 ) toward β-gal.
Moreover, it was worth mentioning that the probe could also be well retrained in
living cells emitting strong fluorescence (Fig. 4).
In 2017, Jiang et al. [29] designed and synthesized a tetraphenylethylene-based
turn-on probe TPE-Gal (12) for β-gal detection with AIE effect in aqueous samples
and in living cells. TPE-Gal was designed to bear a positively charged pyridinium
pendant. And a substrate of β-galactosidase-D-galactose residue was conjugated to
the terminal of the pyridinium pendant. In the presence of β-gal, the D-galactose
residue was cleaved and resulted in a phenolate intermediate. Then the intermediate
192
A. Bi et al.