Studying oxygen levels in MPs
We used the ruthenium complex Tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) dichloride (Santa Cruz, cat#sc-216023B), or RuDPP, as an oxygen sensing molecule. RuDPP shows a large Stokes shift by emitting orange light (610 nm peak) when excited with blue light (455 nm peak). We used a custom-made chamber with two inlets for sample and gas(Figure S1) , a CCD camera (Hamamatsu ORCA-R2/model: C10600-10B), and HCImage Live software (version 4.8.3.0, Hamamatsu Corporation, NJ) to measure light intensity. 1 mg of RuDPP was dispersed in 1X phosphate buffered saline (PBS) solution (pH=7.4) with or without PFC-MPs (10 mg.ml-1) and sonicated thoroughly before getting injected into the channel. A light emitting diode (LED, 4µW.cm-2) was used as the excitation source (peak wavelength 455nm) and a long wave pass filter (Thorlabs, FMP1, cutoff wavelength 600 nm) was used to minimize unwanted wavelengths. We compared the oxygen level within the chamber using two gases including nitrogen (N2) and oxygen (O2) to represent 0% and 100% oxygen, respectively. N2 gas was initially injected into the channel followed by imaging and then the gas source was switched to O2 to present a saturated state. For each condition, 8 frames per second were obtained for up to 60s. The fluorescent intensity of unquenched images (I0) was divided by quenched images (I) on a frame-by-frame basis to obtain a Stern-Volmer image.