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.