Aqueous monosaccharide solutions including glucose or fructose have been hydrothermally treated in a closed system to form colloidal carbon spheres. In-situ Raman and C-13 NMR have been used to quantify the intramolecular dehydration moiety, HMF, as an intermediate. An aqueous glucose solution forms a carbon sphere via an intermolecular dehydration route without forming HMF during initial hydrothermal treatment and followed by carbonization at 170-180 degrees C. However, an aqueous fructose solution initially forms HMF by intramolecular dehydration at 120-140 degrees C. Upon subsequent polymerization, microscopic carbon-containing spheres assemble to larger spheres, thereby generating a grain-like surface morphology. The carbon sphere contains a dense hydrophobic carbon core and a hydrophilic shell.