Assessing oxygen carrying properties of MPs using RuDPP
In our previous studies, PFC conjugated chitosan hydrogels were tested for oxygen transport properties using commercial phosphorescent dots[23] and needle sensors.[24]Our investigations showed these hydrogels enhanced oxygen transport in aqueous conditions and allowed oxygen tensions to reach higher equilibrium states compared to controls not integrating PFCs. In this study, we were interested in understanding the impact of our PFC-MPs on oxygen tensions using RuDPP in finer detail and at a cellular level. RuDPP has a long fluorescence lifetime and a long Stokes-shift. It quenches in the presence of oxygen and thus is a suitable probe dye for determining dissolved oxygen.[25] A mixture of MP solution and RuDPP in PBS was injected into a sealed, custom-built chamber and exposed to gases (Figure 2A) . We measured the fluorescence intensity after flowing pure N2 and pure O2 gasses to create oxygen partial pressures at the extremes of 0% oxygen and 100% oxygen in solution. A solution without any MPs was used as a control. As shown in Figure 2B , when switching from pure N2 to pure O2, the fluorescent intensity decreased due to oxygen quenching of RuDPP. We also observed a decrease in fluorescence intensity in the presence of MPs which can be attributed to increased dissolved oxygen concentration. The sensitivity of the optical oxygen sensor with (I) and without (I 0) a quencher (here oxygen) was quantified in term of the ratio I 0/I 100, as shown in Figure 2C at equilibrium. Our results indicated that the solution containing PFC-MPs exhibited the highest sensitivity to oxygen with the maximum quenching ratio ofI 0/I 100 =1.1. Following saturation, we then stopped oxygen flow and studied the release kinetics. We observed that the control group showed complete RUDPP recovery within 3 min whereas MPs slowed the RuDPP recovery process to 10 min (Figure 2D) . This can be explained by MPs’ ability to uptake oxygen and then release it gradually into a low oxygen tension aqueous environment. These results confirm that PFC-MPs can act as both an oxygen absorbing material as well as a reservoir to release oxygen gradually in an aqueous environment. This finding is in agreement with our earlier work with PFC-modified hydrogels where we examined their ability to deliver and sustain biological levels oxygen for enhancing cellular functions essential in wound healing.[26,27] Most recent evidence from another research group shows the potential of PFC nanoemulsions as artificial oxygen carriers.[28] In this study, PFC nanoemulsions exhibited five times higher dissolved O2 concentration compared to water, as measured by a submerged O2 probe.