Raman. Raman spectroscopy was carried out on a Renishaw InVia, using a 532 nm laser, 1200 mm -1 grating, and 50x lens. Samples were prepared by vacuum filtering 6 mL of a 1 mg ml -1 IPA, water GNP dispersion onto a hydrophobic PVDF membrane support. The StreamHR function was used to map 1024 points over a 100 µm 2 area .