We present an optical biosensor in this study which is based on the LC orientation as a function of the peptide concentration to detect amyloid-beta-42 (Aβ42) antigen-antibody binding events. These binding of Aβ42 peptide to an Aβ42 antibody which is on a Dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) occurs on an immobilized surface forming an immunocomplex. The disturbed orientation of LCs as a result of the binding of the formed immunocomplex observed using the polarized optical microscope. The microscopic and optical response of the LC-immunocomplex was determined as a function of decreasing Aβ42 peptide concentration. Aβ42 peptide concentrations as of 1000 pg/ml, 500 pg/ml, 200 pg/ml, 100 pg/ml, 50 pg/ml, 25 pg/ml, 20 pg/ml, 15 pg/ml, 10 pg/ml, 5 pg/ml, and 1 pg/ml, respectively were interacted with the optimum Aβ42 antibody concentration as of 25 µg/ml. There was a remarkable change in the reflection spectra of the samples with the decreasing Aβ42 peptide concentration. The concentration as low as 1 pg/ml of Aβ42 peptide was able to successfully detected in our system.