Detection principle of dose response curves. A constant bias U = +200 mV is applied and current I is detected. Hereby illumination is switched on and off with a shutter. During the periods without illumination no current can flow. The base line current I0 is detected. After 2 rinsing steps analyte is added (in this case 4AP dissolved in 25% methanol and 75% phosphate buffer at pH 5, geometry So) and the respective photocurrent I is recorded in phosphate buffer with final pH = 7.8. This process is repeated while successively adding more analyte (in the present example 4AP concentration was increased from 25 μM to 4.55 mM). The respective oxidation current response ΔI(c) for each analyte concentration c is derived by subtracting the base line I0 from the detected photocurrent I(c). The dose response curve for the present example is displayed in Figure 4a.