Nanoscale upconversion for oxygen sensing.

Optical oxygen sensors have many promising qualities but rely on excitation by violet or blue wavelengths that suffer from high levels of scattering and absorption in biological tissues. Here we demonstrate an alternative method using 980nm near-infrared light to initially stimulate ceramic upconverting nanoparticles (UCNPs) contained within a novel form, electrospun core-shell fibers. The emission of the UCNPs excites a molecular optical oxygen sensor, the subsequent phosphorescent emission being dynamically quenched by the presence of molecular oxygen. The potential for use of such an energy transfer within electrospun fibers widely used in biological applications is promising. However, current knowledge of such 'handshake' interactions is limited. Fiber-based carriers enabling such optical conversions provide unique opportunities for biosensing as they recapitulate the topography of the extracellular matrix. This creates a wide array of potential theranostic, fiber-based applications in disease diagnosis/imaging, drug delivery and monitoring of therapeutic response. Using a fiber-based vehicle, we observed gaseous oxygen sensing capabilities and a linear Stern-Volmer response allowing highly accurate calibration. Configurational aspects were also studied to determine how to maximize the efficiency of this 'handshake' interaction.