This study represents the development of a biosensor which is based on the liquid crystal (LC) orientation as a function of the peptide concentration to detect an amyloid-beta-42 (A beta 42) antibody-antigen binding events. The A beta 42 peptide binds to the A beta 42 antibody forming an immunocomplex which is immobilized on the Dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) coated surface. The disturbed orientation of LCs as a result of the binding of the formed immunocomplex was observed using the polarized optical microscope (POM) as a function of decreasing A beta 42 peptide concentration from 1000 to 1 pg/ml. The concentration, as low as 1 pg/ml of A beta 42 peptide was able to be successfully detected in our system. Apolipoprotein E4 (ApoE4), that specifically bound to the A beta 42 peptide, was added into the system and a remarkable change in reflection spectra of samples was observed with increasing A beta 42 peptide concentration. The concentration of ApoE4 protein was detected in the range of 0.1-30 nM by this system due to the interaction between the two proteins. (C) 2021, The Society for Biotechnology, Japan. All rights reserved.