Figure 2. CL signals of the
luminol-Co2+-H2O2system in the absence (A) and the presence (B) of
C8-TPE-C4TAB.
CL enhancement in the presence of
C8-TPE-C4TAB or conventional cationic
surfactant. As referred in previous work,34 the CMC
of C8-TPE-C4TAB was measured to be 46
μM. In this case, C8-TPE-C4TAB existed
as the micelle but not monomer or dimer with 80 μM concentration.
According to HRTEM measurement (Figure 3A), the formation of
C8-TPE-C4TAB micelle
(~6.2 nm, 100 counts) was observed. These results
demonstrated that the enhanced CL was attributed to the
C8-TPE-C4TAB micelle-promoted CL
reaction. To understand the differences of
C8-TPE-C4TAB micelle and typical ternary
ammonium micelle promoted CL, the CL signals of
luminol-Co2+-H2O2system upon adding C8-TPE-C4TAB and
cetyltrimethylammonium bromide (CTAB) were measured. The CMC of CTAB is
reported to be 1.0 mM, and thus 2.0 mM CTAB was added to ensure the
formation of CTAB micelle.37 The HRTEM results
indicated the formation of micelle with 2.0 mM CTAB (Figure S4). As
shown in Figure 3B, the addition of CTAB largely enhanced the CL
signals, indicating the universality of micelle-promoted CL. However,
the CTAB-induced CL enhancement was lower than that of
C8-TPE-C4TAB. Since the CTAB
concentration is much higher than that of
C8-TPE-C4TAB, the CTAB-induced CL
enhancement may depend on its concentration. To understand the
concentration effect, the CL signal of
luminol-Co2+-H2O2system were recorded upon adding CTAB with the different concentrations.
As manifested in Figure 3C, the CL signals gradually increased with an
increase in CTAB concentration, and showed a sudden increment around
CMC. Further higher concentrations didn’t show dramatic increase of CL.
Therefore, the CTAB concentration with 2-folds of CMC would be enough to
afford the stable micelle conditions.
In order to further figure out the possible reasons of the different CL
enhancement characters, the other conventional cationic surfactants were
tested, including tetradecyltrimethylammonium bromide (TTAB),
dodecyltrimethylammonium bromide (DTAB) and decyltrimethylammonium
bromide (DeTAB). To guarantee the formation of corresponding micelle,
the concentrations of these surfactants were set to be 2-folds of their
CMCs.38 The concentrations of TTAB, DTAB, and DeTAB
were 8, 32, and 114 mM, respectively. It was seen that the
micelle-improved CL signals increased with the increasing alkyl chain
length of surfactants (Figure 3D). Upon the addition of cationic
surfactants with short alkyl chain length, even with ultrahigh
concentrations (>30 mM), the CL signals were much lower
than that by adding C8-TPE-C4TAB (80
μM). These results suggested that
C8-TPE-C4TAB was superior to
conventional cationic surfactants on the CL improvement toward
luminol-Co2+-H2O2system.