A novel, surface immobilization electrochemical assay allows for rapid, accurate and highly sensitive detection of microorganisms and biological molecules. Known surface immobilization methods are utilized to bind an analyte to a surface. A binding material with a covalently attached electroactive complex generates electrical current in the presence of analyte. An electrode is used to detect the current, that is directly related to the concentration of analyte. The invention is especially suitable for detection of Cryptospiridium parvum.