Ratiometry-based Two-Photon O2 Sensing using QD-Ru-amPEI Nanoprobes

Abstract

Hypoxia is one of the major characteristics of solid tumors and is linked with increasing resistance to radiotherapy and drug treatment. For quantification and monitoring of pO2 in biological samples, QD-Ru-amPEI nanoprobes have been constructed by co-wrapping O2-sensitive ruthenium (Ru) with O2-insensitive quantum dots (QD) in amphiphilic polymer vesicles to enable photoluminescence-based ratiometric sensing. In this study, we have examined the feasibility of QD-Ru-amPEI-based ratiometric detection of pO2 by two-photon microscopy by conducting two-photon ratiometric detection of the nanoprobes in aqueous media, live cells, spheroid, and in vivo mouse model. Two-photon Ru/QD intensity ratio measurements from QD-Ru-amPEI solution exposed to physiologically relevant pO2 levels from 0 to 20% demonstrated responsiveness according to the two-component Stern-Volmer relationship. Two-photon ratio measurements of QD-Ru-amPEI-probed HCT116 cells incubated in normoxic (20% pO2) and hypoxic (0% pO2) conditions corresponded with those from the solution calibration experiment. Spheroids were used as tumor models due to their well-studied oxygen distribution profile and ability to model solid tumors. Two-photon ratiometric imaging of QD-Ru-amPEI-probed HCT116 spheroids was capable of visualizing a hypoxic gradient from the proliferative rim towards the necrotic core. Immunofluorescence detection via staining of hypoxic markers pimonidazole and CA9 confirmed the presence of these layers in these spheroids. Lastly, two-photon ratiometric imaging conducted on subcutaneously injected QD-Ru-amPEI in mice while alternating administration of anesthesia with 10% and 100% oxygen yielded significant differences. These results demonstrate that ratiometry-based two-photon O2 sensing of QD-Ru-amPEI is capable of detecting O2 partial pressure and distribution in biological tissue. Such raise possibilities for the detection of hypoxic tumors in vivo as well as the monitoring of their pO2 levels.