An assay system is provided in which gossypol is used as a biological marker to detect evolved resistance of insects to Bt cotton. Detection of gossypol using a monoclonal antibody ELISA-based protocol enables at risk populations of insects to be evaluated for evolved resistance to Bt present in a genetically modified cotton.The specificity of the monoclonal antibody to gossypol also enables the production of nanoparticles having a conjugated monoclonal antibody which retains the ability to selectively bind gossypol. Accordingly, nanoparticles can be provided with additional target ligands, such as antibodies, so as to specifically attach to tumors or cancer cells thereby delivering the gossypol to the target cells.