Characterization of PFC- MPs
It has been well established that oxygen carrying properties of PFC-MPs
stem from the PFC substitutions, [18,19] therefore
in this report we focused on studying the composition of microfluidic
generated MPs while confirming the presence of PFC molecules(Figure 1). Figure 1B shows the TGA spectrum of crosslinked
non-PFC-MPs and PFC-MPs. The weight loss in both spectra begins at
approximately 90°C and continues until 130°C which is likely associated
with loss of absorbed water in the crosslinked MPs. Based on these data
both MPs were able to absorb water correlating to ~25%
via hydrophilic moieties in chitosan.[20] Another
major mass loss stage is observed between 280-400°C, which is related to
thermal decomposition of chitosan.[21] Overall,
around 43.1% w/w of PFC-MPs and 52.82% w/w of non-PFC-MPs were lost
when heated above 400°C, suggesting higher thermal stability of PFC-MPs
compared to non-PFC-MPs. Carbon–fluorine (C-F) bonds in PFC molecules
are strongly polarized, which accounts for their thermal
stability.[22] Next, XPS was used to provide a
quantitative analysis from the outer 10 nm of MP surfaces. XPS spectra
showed carbon (C1s at 284.7 eV) and oxygen (O1s at 531.5 eV) as well as
fluorine (F1s at 688.6 eV) on surface of PFC-MPs (Figure 1C),suggesting the presence of PFC molecules on the surface of MPs. This is
expected since the particles were crosslinked while still in the oil
phase. We tried Fourier-transform infrared spectroscopy (FTIR) as well(Figure S2) ; however, due to PFC abundance being lower than the
detection threshold (2 wt%), we could not detect C-F bonds.