Construction of a hypoxia responsive upconversion nanosensor for tumor imaging by fluorescence resonance energy transfer from carbon dots to ruthenium complexElectronic supplementary information (ESI) available: Supplementary figures. See DOI: 10.1039/c8tb00278a

In this work, we synthesized a hypoxia responsive upconversion nanosensor by the conjugation of oxygen insensitive upconversion carbon dots (CDs) with an oxygen sensitive ruthenium( ii ) complex (Rud 2 b). The CD-Rud 2 b conjugate was then PEGylated to mediate the formation of nano-sized assemblies of the resultant polymer probe (CD-Ru-mPEG) in aqueous solution. The strategy allows the oxygen sensitive probe (Rud 2 b) to be excited by NIR irradiation and achieve upconverted emission through the Förster resonance energy transfer process from the CDs. The oxygen sensitivity of the ruthenium moiety was well-preserved in the polymeric aggregates because the PEG chains could prohibit the occurrence of self-quenching that may occur due to the close packing of the probes. Such an upconversion photoluminescence property makes the CD-Ru-mPEG nanosensor work as a biocompatible imaging system to monitor the oxygen level in cells and the hypoxic condition in vivo . Hypoxia responsive upconversion nano-aggregates are synthesized which can be excited by NIR light to give oxygen dependent phosphorescence emission via the FRET process.