Quantum dots are semiconducting nanoparticles that can be prepared with interesting optical properties. The fluorescent properties of quantum dots are one of the key advantages for their use as optical labels for biorecognition events and biocatalytic processes. We have prepared semiconductor quantum dots conjugated with Nile Blue (NB), and demonstrate that NB-functionalized quantum dots can act as versatile probes to analyze different biocatalyzed transformations, and can be used for the quantitative detection of NADPH as well as NADH. This approach provides a new path for the optical detection of NAD(P)H and for the quantitative analysis of NAD(P)(+)-dependent biotransformations.