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.