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.