A composition for use as an assay reagent comprises a solid support comprising a member of a signal producing system and a coating of a synthetic copolymer. The synthetic copolymer comprises a first polymerized monomer comprising a pendant moiety comprising a reactive functionality or a derivative of a reactive functionality and a second polymerized monomer comprising a pendant moiety comprising at least 1 carbon atoms and at least 2 heteroatoms. In some embodiments the copolymer comprises a polyethylenic backbone comprising the pendant moiety comprising a reactive functionality or a derivative of a reactive functionality and the pendant moiety comprising at least 1 carbon atom and at least 2 heteroatoms.