A method of increasing a density of carboxylic acids on a surface of a carbon nanoparticle is disclosed. The method includes contacting an oxygen-containing functional group on a surface of a carbon nanoparticle with a reducing agent to provide a hydroxyl group; reacting the hydroxyl group with a diazoacetate ester in the presence of a transition metal catalyst to provide an ester, the diazoacetate ester having the structure wherein R is a C1-8 hydrocarbyl, preferably tert-butyl, methyl, ethyl, isopropyl, allyl, benzyl, pentafluorophenyl, or N-succinimidyl; and cleaving the ester to provide a carboxylic acid group. Surface-functionalized carbon nanoparticles made by the method are also disclosed.