Various methods for altering surface characteristics of a microsphere are provided. One method includes coupling an enolic acid to the microsphere to modify the surface characteristics of the microsphere. The surface characteristics may include charge density and/or pKa. A reagent can be coupled to the microsphere via the enolic acid. The reagent may include a biomolecule. The modified surface characteristics may increase a stability of the reagent when the reagent is coupled to the microsphere. The modified surface characteristics may also improve performance of an assay carried out with the microsphere. Another embodiment relates to a microsphere that includes an enolic acid coupled to a polymer core of the microsphere such that the enolic acid modifies surface characteristics of the microsphere. A reagent can be coupled to the microsphere via the enolic acid.