and ABM-B were designed for the preparation of mAbs by introducing spacer arms at the 4'-OH and 5'-OH of ABM, respectively, to couple with KLH (Fig. , Supplementary Fig. , and Supplementary Information). After the extensive screening and pairing of mAbs, 3A9 from hapten ABM-A and 2C5 from hapten ABM-B were combined to establish a sandwich immunoassay for ABM, affording a LOD of 1012.1 nM (Fig. ). The low sensitivity of the sandwich immunoassay for ABM was possibly due to steric hindrance caused by the overlap of the antibody binding sites, reducing the binding ability of each mAb to the ABM. The results of ABM demonstrated the need for careful hapten design and extensive mAb screening to establish highly sensitive sandwich immunoassays. It is worth noting that the study had not always witnessed the dominant performance of sandwich immunoassays for the detection of small molecules, which is heavily dependent on the structure of the target analyte and optimum antibody pairs. These results using a real analyte proved that small molecules with certain epitope distances can be detected by sandwich immunoassay.