The invention features a method of attaching a ligand that has a free carboxyl group to a solid support by adding an amino group to the ligand to form a ligand-amino derivative, converting the ligand amino derivative to a ligand sulfhydryl derivative, attaching the ligand sulfhydryl derivative to a protein to form a ligand-linker-protein conjugate, and applying the ligand-linker-protein conjugate to the solid support. The method is particularly useful for immobilizing small molecule ligands having a free carboxyl group, such as cloxicillin, to a lateral-flow test strip, in order to make a detection zone on the test strip that exhibits a clear signal and enhanced sensitivity.