Detection principle and redox schemes. a) Sketch of the detection scheme. A bias voltage U is applied to a Au electrode versus an Ag/AgCl reference electrode in the bath solution. The Au electrode is coated with CdS QDs which are attached via a BDT layer. p APP is in solution degraded by ALP to 4AP. Upon illumination of the QDs electron hole pairs are generated. This leads to oxidation of 4AP to 4QI on the QD surface, whereby electrons are transferred to the QD. Electrons are passed to the Au electrode and can be detected as oxidation current I. b) Without QDs as redox mediator oxidation of 4AP can't happen in case the bias potential U is not positive enough. Energy levels E are shown. For oxidation the Fermi level EF of the Au electrode would need to be lower than the energy level at which electrons upon oxidation of 4AP are released. c) Illuminated QDs can act as redox mediator. Defect states (DS) at the QD surface (which are energetically above the valance band VB) prevent light generated electron hole pairs from immediate recombination. In this way electrons resulting from the oxidation of 4AP to 4QI can be transferred to the DS of the QD. In turn electrons from the conduction band (CB) can be drained via the BDT layer to the gold electrode, which is detected as oxidation/photocurrent.