Fig. 1. Scanning electron microscopy image of the ionic liquid-graphene-titania-Nafion composite film. Scale bar: 500 nm.
Fig. 2. Energy-dispersive X-ray spectroscopy mapping image of the ionic liquid-graphene-titania-Nafion composite. (A) Fluorine (B) Titanium.
Fig. 3. Nyquist plots for the impedance measurements in the presence of 5.0 mM K3Fe(CN)6/K4Fe(CN)6 in 0.05 M phosphate buffer (pH 7.0) at a bare GCE (□), titania-Nafion (○), ionic liquid-titania-Nafion (◆), ionic liquid-graphenetitania-Nafion (▲) composite modified glassy carbon electrode.
Fig. 4. Linear sweep voltammograms of 0.5 μM capsaicin at a titania-Nafion (a), ionic liquid-titania-Nafion (b), graphene-titania-Nafion (c) and ionic liquid-graphenetitania-Nafion (d) composite modified glassy carbon electrode in 0.04 M Britton-Robinson buffer (pH 1.0) at a scan rate of 100 mV/s.
Fig. 5. Oxidation current of 10 μM capsaicin obtained at glassy carbon electrodes modified with titania-Nafion composites incorporated with 1-butyl-3-methylimidazolium (A), 1-hexyl-3-methylimidazolium (B), 1-ethyl-3-methylimidazolium (C), 1,3-dimethoxy-2-methylimidazolium (D) and 1-benzyl-3-methylimidazolium (E) hexafluorophosphate in 0.05 M pH 7.0 phosphate buffer at a scan rate was 100 mV/s.
Scheme 1. Reaction mechanism of the oxidation of capsaicin, and the chemical structures of the ionic liquids used: 1-butyl-3-methylimidazolium (A), 1-hexyl-3-methylimidazolium (B), 1-ethyl-3-methylimidazolium (C), 1,3-dimethoxy-2-methylimidazolium (D) and 1-benzyl-3-methylimidazolium (E) hexafluorophosphate.
Fig. 6. Effect of the accumulation time on the oxidation peak current of 0.1 μM capsaicin at the graphene-titania-Nafion (dashed line) and ionic liquid-graphene-titania-Nafion (solid line) composite-modified electrode in 0.04 M Britton- Robinson buffer (pH 1.0) at a scan rate of 100 mV/s.
Fig. 7. Effect of concentration of ionic liquid on the oxidation peak current of 100 μM capsaicin at the composite-modified electrode in 0.04 M Britton-Robinson buffer (pH 1.0) at a scan rate of 100 mV/s.
Fig. 8. Effect of amount of graphene on the oxidation peak current of 100 μM capsaicin at the composite-modified electrode in 0.04 M Britton-Robinson buffer (pH 1.0) at a scan rate of 100 mV/s.
Fig. 9. Calibration curves for capsaicin obtained at the composite-modified electrode in the concentration of 10 μM, 1 μM, 0.5 μM, 0.1 μM, 0.05 μM, and 0.03 μM. Insert (A): calibration curves for capsaicin obtained at ionic liquidgraphene-titania-Nafion (solid line) and graphene-titania-Nafion (dashed line) composite-modified electrode in the concentration range from 0.03 μM to 1.0 μM. (B): Linear sweep voltammograms of buffer (solid line) and 0.03 μM capsaicin (dotted line) at ionic liquid-graphene-titania-Nafion composite.
Table 1. Comparison of the present capsaicin sensor with different reported methods.
Table 2. Recovery of capsaicin spiked in Korean hot pepper (Chungyang pepper) solution.