Fig. 2
Comparison of the X-ray diffraction (XRD) patterns (a) and Raman spectra (b) of pristine GO (I) and the GOs treated with different components of the simulated human lung fluids (II−VII), showing that the organic acids present in the simulated human lung fluids were the primary constituents responsible for the bioreduction of GO. XRD patterns: the incomplete reduction of GO in the absence of sodium citrate (II) indicates the important role of citrate, whereas the fact that significant reduction of GO by citrate was not observed at acidic pH (III) but was observed at neutral pH (IV) indicates that the effectiveness of citrate is pH dependent. In comparison, significant GO reduction occurred in the absence of sodium acetate (V), indicating that even though acetate was capable of reducing GO (VI), its overall contribution was small. The significant reduction of GO by ALF in the absence of both citrate and citric acid (VII) indicates that other organic acids also made substantial contributions to GO reduction. Raman spectra: the increase in the ID/IG ratio (indicative of the formation of new small-sized graphitic domains) for the GOs treated with different lung fluid components is generally consistent with the roles of the organic acids determined from the XRD patterns
