1. Introduction
As a classical cold light, chemiluminescence (CL) has attracted wide
research interest in last decades due to its advantages of free light
source and low background, which makes CL technique essential in the
exploration of lighting systems for various applications, e.g. ,
sensing, bioimaging and rescue
operations after disasters.1-5 To achieve effective
and long-period lighting, a long-lasting emission system is usually
required.6-8 However, most existing CL systems show
flash-type light emission, which hinders their
applications.9-11 In contrast, CL systems with strong
intensity and glow-type emission are more charming to the development of
cold light sources. To meet this concern, few CL systems with
sustained-release of reactants have been developed viaintroducing hydrogel or tablet.12,13 For example, Liuet al . reported intensive and long-lasting CL hydrogels by
integrating chitosan, N -(4-aminobutyl)-N -ethylisoluminol
and Co2+.12 With the combination of
luminol tablet and PVP–H2O2 complex
solution, our group constructed a long-lasting (~13 h)
CL light using Tris–Co2+ complex as the
catalyst.13 Nevertheless, the above fabricated systems
were complicated, limiting their wide use. Therefore, it is highly
necessary to explore a simple and long-lasting CL system.
It is reported that typical
luminol–H2O2–Co2+system possesses relatively slow CL kinetics under neutral pH
conditions.14 Unfortunately, the ultraweak CL
intensity of
luminol–H2O2–Co2+system at neutral condition hinders its further application. It is
generally accepted that the introduction of surfactant could enhance CL
intensity of most CL systems.15-17 For instance, Xieet al . found that the CL intensity of peroxyoxalate
derivativel–H2O2 system can be greatly
enhanced upon introducing cetyltrimethylammonium bromide (CTAB, a kind
of typical cationic surfactant).18 However, the
previous reports demonstrated that the CL promotion was only implemented
with high surfactant concentration, usually above critical micelle
concentration (CMC), and CMCs of conventional surfactants are generally
in mM level.19,20 The requirement of high surfactant
concentration is conflicting to the green chemistry and environment
friendly. Therefore, the development of surfactant with low CMC is
required in order to decrease its usage and side effect to environment.
CL resonance energy transfer (CRET) is usually utilized to further
improve CL signals. In this case, additional fluorophores as energy
acceptors are required in conventional surfactant-enhanced CL systems.
For example, Song et al . reported the construction of intensive
and long-lasting multicolor CL systems by introducing fluorescein or
rhodamine B as the energy acceptor.7 Nevertheless, the
multicomponent involvement results in system complexity and weak
stability. Accordingly, a direct linking of fluorophore into surfactant
would make CL system simple. Hence, the introduction of novel surfactant
with ultralow CMC and CRET acceptor is appealing for a simple and
long-lasting CL system.
Stepanek et al . proposed fluorescent block copolymer surfactant
by linking p -vinyl-9,10-diphenylanthracene (DPA) into
polystyrene-block -poly(methacrylic acid).21Unfortunately, the aggregation-caused quenching (ACQ) properties of DPA
led to weak fluorescence upon forming micelle. The ACQ features of the
fabricated fluorescent block copolymer surfactant would restrict its
potential applications in CL as the CRET acceptor. In contrast,
aggregation-induced emission (AIE) fluorophores show bright emission
after aggregates.22-25 It is well-known that Tang’s
group has explored many AIE fluorophres based on tetraphenylethylene
(TPE).26-28 By inserting TPE into the backbone of
surfactant, our group has reported a
few novel fluorescent surfactants
with ultralow CMC due to the increased hydrophobicity of alkyl chain
(<60
μM).29-33 Herein, we described our attempts to apply
the developed AIE surfactants into long-lasting
luminol–H2O2–Co2+ CL
system.34 In a case study, we selected the
TPE inserted
dodecyltrimethylammonium bromide (4th cationic headgroup, denoted as
C8-TPE-C4TAB) for the CL signal
enhancement at pH 7.4. The CL intensity revealed 20-fold increment in
the presence of only 80 μM C8-TPE-C4TAB,
and glow-type emission appeared. The enhanced CL and long-lasting
emission were due to the integration of micelle-mediated reactant
enrichment and CRET between luminol and TPE (Figure 1). In other word,
the CL reactants were enriched through electrostatic attraction, which
subsequently benefits the CL reaction and the CRET process. The
long-lasting CL was visible to naked eyes even after 60 min. The
proposed fluorescent surfactant-assisted long-lasting emission showed
advantages of convenient operation, low surfactant consumption and
environment-friendly. The development of simple and green long-lasting
CL systems for analytical sensing or lighting is possible by introducing
novel fluorescent surfactants with ultralow CMCs.