A quantum dot-electrode system was developed which allows the sensitive detection of NADH (nicotinamide adenine dinucleotide). The colloidal semiconductive CdSe/ZnS nanocrystals (quantum dots) are attached to gold by chemisorption via a dithiol compound. The current signal call be triggered by illumination of the quantum-dot-modified electrode surface. Because of photoexcitation, electron-hole pairs are generated in the quantum dots, which can be detected as anodic or cathodic photocurrent. The immobilization of the nanocrystals is verified by amperometric photocurrent and quartz crystal microbalance (QCM) measurements. This study shows that CdSe/ZnS quantum dot-modified electrodes allow concentration dependent NADH detection in the range of 20 mu M to 2 mM already at rather low potentials (around 0 V vs. Ag/AgCl, 1 M KCl). Therefore such electrodes can be used in combination with NADH-producing enzyme reactions for the light-triggered analysis of the respective substrates of the biocatalyst. It can be shown that glucose detection is feasible with such an electrode system and photocurrent measurements.