Enriching the surface density of immobilized capture antibodies enhances the detection signal of antibody sandwich microarrays. In this study, we improved the detection sensitivity of our previously developed P-Si (porous silicon) antibody microarray by optimizing concentrations of the capturing antibody. We investigated immunoassays using a P-Si microarray at three different capture antibody (PSA – prostate specific antigen) concentrations, analyzing the influence of the antibody density on the assay detection sensitivity. The LOD (limit of detection) for PSA was 2.5ngmL −1 , 80pgmL −1 , and 800fgmL −1 when arraying the PSA antibody, H117 at the concentration 15μgmL −1 , 35μgmL −1 , and 154μgmL −1 , respectively. We further investigated PSA spiked into human female serum in the range of 800fgmL −1 to 500ngmL −1 . The microarray showed a LOD of 800fgmL −1 and a dynamic range of 800fgmL −1 to 80ngmL −1 in serum spiked samples.