Construction of a hypoxia responsive upconversion nanosensor for tumor imaging by fluorescence resonance energy transfer from carbon dots to ruthenium complex

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 Forster 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.