Figure 3. (A) HRTEM image of 80 μM C8-TPE-C4TAB. (B) CL signals of the luminol-Co2+-H2O2system upon adding CTAB and C8-TPE-C4TAB. (C) CL signals of the luminol-Co2+-H2O2system upon adding CTAB with the different concentrations. (D) CL signals of the luminol-Co2+-H2O2system upon adding conventional cationic surfactants.
Mechanism of C8-TPE-C4TAB-mediated intensive and long-lasting CL emissions. As indicated above, the addition of C8-TPE-C4TAB endowed more effective CL reaction than conventional cationic surfactants. It is reported that cationic micelle can adsorb anionic reactants and benefit the CL reaction.39 At pH 7.4, the C8-TPE-C4TAB micelle was positively charged (+11.0 mV) because of the outside ternary ammonium cation (Figure S5), while luminol was negatively charged (–4.8 mV). The electrostatic attraction between positively charged C8-TPE-C4TAB micelle and negatively charged luminol dianion/peroxide anion easily leads to surface adsorption and enrichment of reactants.40-42 Such an electrostatic attraction-assisted CL emission was also provided by SDS (sodium dodecyl sulfonate, a kind of typical anionic surfactant). As shown in Figure S6, the CL intensity of luminol-Co2+-H2O2system distinctly decreased upon adding SDS. This is because the outside sulfonic acid groups make SDS micelle negatively charged, which possesses strong electrostatic repulsion toward luminol dianion and peroxide anion.20
To further understand the mechanism of C8-TPE-C4TAB-enhanced CL intensity and long-lasting emission, a few characterizations were performed. The generation of reactive oxygen species (ROS) is important to the CL signals. We tested the CL inhibition with the addition of different ROS scavengers. As displayed in Figure S7, p -benzoquinone (HQ) and thiourea caused visible CL suppression, while NaN3induced slight CL decrease, suggesting the primary ROS were superoxide radical (O2•−) and hydroxyl raidical (OH).43,44 The electron paramagnetic resonance results also demonstrated the continuous generation of O2•− andOH during the CL reaction (Figure 4). It is reported that many ROS possess longer lifetime in hydrophobic environment.45 When inserting TPE into the backbone of surfactant in this work, the hydrophobicity of alkyl chain increased, this might benefit the stabilization of ROS and promote the CL reaction.