The invention pertains to methods of increasing the density of carboxylic acids on the surface of a carbon nanoparticle that can be functionalized with biologically relevant molecules, such as antibodies or peptides, for biomedical applications. Advantageously, the method could increase functionalization of a nanoparticle by at least about 1x107 functional groups/g of nanoparticle. 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 and then cleaving the ester to provide a carboxylic acid group. The carboxylic acid can further be secondarily functionalized to an acyl chloride, an amide, pegylated, a biotinylate, a folate, a thiol, a maleimide, an active ester, an amine, a chelated gadolinium, an azide, an alkyne, a protein tag, or a dendrimer. Examples of notable nanoparticles that can be derivatized using this method include carbon nanoparticles such as carbon nanotubes, fullerenes, graphenes, graphene oxides, and nanodiamonds; with or without fluorescent properties. Fluorescent nanoparticles are of particular interest for functionalization as they are applicable to both research and diagnostic applications and can be visualized through microscopy.