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.