2.2 Carbon/cation ratio. The carbon-cation ratio was calculated as follows. According to our XPS results showing the C/O atomic ratio is 2.97 (Figure ), we could estimate the number of moles of carbon in the 0.5 mg of GOM is 12 𝑚𝑔/𝑚𝑚𝑜𝑙 × 𝑛(𝐶) + 16 𝑚𝑔/𝑚𝑚𝑜𝑙 × 𝑛(𝑂) ≈ 0.5 𝑚𝑔