Technical Abstract: Toxins in foodstuffs are always a threat to food safety Among many toxins related to food, ricin (category B toxin) from castor beans has been mentioned in some poisoning cases happened. Atomic Force Microscopy (AFM) is a widely used nanotechnology to detect biospecies in vitro and in situ. The AFM recognition imaging and force spectroscopy have the advantage to probe topography images and molecular recognition simultaneously and can be used to detect biomolecules in a single-molecule level. We used modified AFM tip with aptamer to detect single ricin protein and investigate the binding affinity of aptamer to ricin. Aptamers are single-stranded oligonucleotides generated from in vitro selection and have high affinities to their targets, which can be small molecules, proteins, virus, and cells. The affinity between the aptamer and its target species is based on the structure-function relationship of these biospecies. Therefore, we also used molecular modeling methods to investigate the binding conformations of aptamer-ricin as well as antibody-ricin interactions. The results of AFM images and computer simulation model were used to compare aptamer with antibody for their different binding affinities to ricin. Since aptamers are increasingly used for the biosensor research, our goal is to develop novel biosensors for detecting various food toxins based on DNA aptamers. We found that, in single-molecule level, the aptamer showed slightly higher affinity to ricin when compared with antibody. We developed the methods for surface immobilizations of ricin and aptamer, and tested the affinity and specificity. Our study proved that the aptamers can be effectively used for the future biosensor devices.