The localized surface plasmon resonance (LSPR) nanosensor based on the optical properties of Ag nanotriangles is shown to aid in the understanding of the interaction between amyloid β-derived diffusible ligands (ADDL) and the anti-ADDL antibody, molecules possibly involved in the development of Alzheimer's disease. By varying the concentration of anti-ADDL antibody, a surface confined binding constant of 3.0 × 107 M-1 for the interaction of ADDLs and anti-ADDLs was measured. Influences of Cr, the nanoparticle adhesion layer, will be shown to be the limiting factor in the sensitivity of this assay. This is the first nonmodel application of the LSPR nanosensor.