A biosensor based on surface plasmon resonance（SPR）was used to detect antibiotic residue in milk quickly and specifically.The monoclonal antibody was immobilized on the gold surface of SPR sensor with the self-assembled-monolayer technique.The sensor possesses high sensitivity due to SPR technology and high specificity due to the affinity between antigen and antibody.In addition to be binded to antibiotic

antibody was also bounded to other components in milk

which is called non-specific binding（NSB）

and will introduce great measurement error.So

the milk was defatted and diluted firstly.The experiment shows that the NSB of milk can be avoided after defatted and diluted 4 times

closing to deionized water.Furthermore

the processed milk samples with different ampicillin concentrations were measured and the work curve was built up.The limit of detection（LOD）of used sensor is 1.56 μg/L with 7.8 % CV.The LOD is lower than the maximum residue limit（MRL） of ampicillin.