Anomalous Perturbation of the O2 Sensitivity of Poly(aromatic) Hydrocarbons by Magnetic Quantum Dots

Molecular oxygen is known to be an efficient quencher of the excited states of organic molecules. This fact has been exploited to develop fluorescent O2 sensors, which is topical for cancer screening and many other applications. In this regard, our group and others have reported the development of robust, ratiometrically reporting chemosensors by conjugating O2-sensitive organic chromophores to inorganic quantum dots. Recently, an attempt was made to prepare a multifunctional sensor system by attaching the emissive poly(aromatic) hydrocarbons pyrene and perylene to magnetic nanomaterials, specifically CdSe/CdMnZnS and ZnSe/ZnMnS/ZnS quantum dots. However, the fluorescence of both dyes became invariant to oxygen levels, even if the solutions were fully saturated. We have ruled out any material dependency beyond the presence of Mn2+ ions by studying control samples, while molecular dynamics simulations negated any possibility of spatial sequestration of O2 by the magnetic fields. In the case of pyrene, the proximity of the magnetic dots induces significant singlet-triplet mixing; however, this does not explain this curious observation. As such, we believe that the exchange interaction between O2 and the dyes is perturbed by both the microenvironment of the system and the inhomogeneous magnetic fields such that quenching is not observed.