Figure 4. Time-dependent EPR spectra of the luminol-Co2+-H2O2-C8-TPE-C4TAB system.
As mentioned in CTAB concentration study, the formation of micelle plays important role in enhancing the CL emission. Thus, the total micelle number would affect the CL reaction. That is, less micelle leads to the high concentrations of reactants on the surface of micelle, and promotes the CL emission. In contrast, the average surface concentrations of reactants decreased with the increasing total micelle number. In this case, the CL reaction was inhibited and only weak CL emission could be observed. For C8-TPE-C4TAB and CTAB improved luminol-Co2+-H2O2systems, the final concentration of C8-TPE-C4TAB and CTAB were 80 μM and 2.0 mM, respectively. The aggregation number of micelle was calculated to be 64 in CTAB micelle, while it became 36 in C8-TPE-C4TAB micelle.46 The total micelle numbers of CTAB micelle and C8-TPE-C4TAB micelle were determined to be 3.8×1015 and 2.3×1014, respectively. The CTAB micelle number was 16-folds higher than that of C8-TPE-C4TAB micelle. Obviously, a single C8-TPE-C4TAB micelle surface exist more luminol dianion and peroxide anion, which contributes the higher CL signals.
In addition to high enrichment effect, CRET is also needed be considered in the CL enhancement of C8-TPE-C4TAB. The TPE fluorophore of C8-TPE-C4TAB could act as the CRET acceptor, and its AIE characters could avoid the luminescence quenching during the formation of micelle.47,48 In contrast, CTAB in the absence of intrinsic fluorophore could not act as the CRET acceptor. Then, the CL emission spectra of luminol-Co2+-H2O2system with the addition of CTAB or C8-TPE-C4TAB were obtained. As shown in Figure 5A, the emission maximum of luminol-Co2+-H2O2system located at 425 nm upon adding CTAB. The 425 nm light belongs to the excited luminol radicals.49 However, the emission maximum shifted to 486 nm in the presence of C8-TPE-C4TAB (Figure 5B) and the CL emission spectrum was overlapped with the fluorescence emission profile of C8-TPE-C4TAB. The shifted emission maximum and characterized CL spectrum implied the occurrence of CRET in luminol-Co2+-H2O2-C8-TPE-C4TAB system.50,51 To determine the CRET efficiency (φ), the integral areas of emission of TPE acceptor (denoted as IA) and total system (denoted as IT) were acquired by peak fitting analysis. Based on the equation: φ = IA / IT, the φ was calculated to be 94.0%. As a result, the high enrichment and CRET contributed the intensive CL of luminol-Co2+-H2O2-C8-TPE-C4TAB system. The schematic illustration of C8-TPE-C4TAB micelle-amplified CL was displayed in Figure 6.