Figure 6. Schematic illustration of CTAB micelle (left) and C8-TPE-C4TAB micelle (right) amplified CL of the luminol-Co2+-H2O2system.
To understand whether the insert location affected the CL emission, the CL signals of luminol-Co2+-H2O2system upon adding C4-TPE-C8TAB (8th cationic headgroup insert), and TPE-C12TAB (12th cationic headgroup insert) were recorded. As shown in Figure S8A, the CL intensity decreased as the extended insert location. A possible reason was due to the decreased CRET efficiency because the distance between excited luminol and TPE increases with the extended insert location (Figure S8B). The rasied distance increased the energy loss and then decreased the CRET efficiency. Accordingly, all subsequent CL experiments were conducted with C8-TPE-C4TAB.
It is reported that the pH distribution in CTAB micelle solution is not uniform because of the adsorption of hydroxyl anions around the outside ternary ammonium cation layer.20 Therefore, the local pH around micelle surface is slightly higher than that in solution. As is known, luminol could emit the strong CL under alkaline environment.52 To understand whether the surface adsorption hydroxyl anions affect the CL reaction, the CL signals of luminol-Co2+-H2O2-C8-TPE-C4TAB system without and with the introduction of NaBr or NaCl were investigated. As shown in Figure S9, the CL intensity decreased upon adding 1.0 mM Br or Cl. The addition of Br or Cl would squeeze out the adsorbed hydroxyl anions, decreasing the pH around micelle surface and inhibiting the CL reaction. Such a result was also observed in CTAB-modified montmorillonite improved CL system, the removal of halide counterions of CTAB layer had a positive effect on the CL amplification.53
It should be noticed that the acidity of H2O2 is very weak (pK a ~11.7), that means the molar fraction of peroxide anion form increases with an increase in pH. However, under pH 7.4, the molar fraction of peroxide anion form was calculated to be 5.0×10‒5, suggesting most H2O2 exists as neutral molecule form. This is why luminol-Co2+-H2O2system only produces ultraweak CL emission. After the addition of C8-TPE-C4TAB, the adsorbed peroxide anions increased the local pH, which benefits the following CL reaction, and the consumed peroxide anions would be supplied by aqueous H2O2. The pH-dependent molar fractions of peroxide species were calculated and listed in Table S1. In a ward, the non-uniform pH distribution may impel the diffusion of H2O2 from solution to micelle surface. However, the reaction rate was still slow due to the low molar fraction of peroxide anions. Interestingly, this process endows slow CL kinetics and long-lasting CL emission characters of luminol-Co2+-H2O2system. On the other hand, the addition of surfactant also increased the viscosity of solution. The increased viscosity inhibited the diffusion of peroxide anions and limited the following CL reaction. To verify the viscosity-mediated CL performances, the CL signals of luminol-Co2+-H2O2system were investigated in the absence and presence of glycerol. The CL kinetics became slow as the introduction of glycerol, as shown in Figure S10. These results demonstrated that C8-TPE-C4TAB-induced long-lasting CL emission might be attributed to the increased viscosity and the decreased fusion of reactants. Taken together, C8-TPE-C4TAB-mediated intensive and long-lasting CL emission is due to the synergistic effect of micelle-improved enrichment, the diffusion control of reactants and high-efficient CRET.
Long-lasting performances of the proposed CL system. To perform the long-lasting emission more clearly, the CL images were taken in real time by camera. For acquiring effective images, the concentrations of luminol, Co2+, H2O2, and C8-TPE-C4TAB were 60 μM, 10 nM, 2.5 mM, and 80 μM, respectively. The image was acquired with an interval of 4 min. Since the reaction occurs quickly, the 0 min point was not obtained; we herein provided the 0.5 min image as the first picture. As shown in Figure 7, bright blue emission was easily observed after the mixing of all CL substrates. Interestingly, there was no visible light attenuation even after 16 min reaction (Figure S11), suggesting this luminol-Co2+-H2O2-C8-TPE-C4TAB system was stable. Despite of the gradually decreased emission with the increasing reaction time, the CL emission was still observed after 60 min reaction. These results further verified the feasibility of the enhancement of intensive and long-lasting luminol CL by adding C8-TPE-C4TAB. The bright and long-lasting CL emission makes the luminol-Co2+-H2O2-C8-TPE-C4TAB system possible to construct a facile cold light for practical applications.