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Optical interferometer biosensors based on porous silicon (PSi) are being studied for chemical and biological sensor applications. In particular, single PSi is a promising sensing platform for biomolecules and based on monitoring changes in the optical thickness (2nL) from Fabry-Pérot fringes due to magnetic particle-labels covering PSi surfaces. These methods offer a fast, and one-step method for immunoassaying by combining nano-sized superparamagnetic beads (SPBs) with interferometer PSi platforms. Furthermore, SPBs covered with biomolecules have a higher reflective index than the biomolecules alone, which results in larger shifts of the optical thickness (2nL) by the penetration of SPBs inside pore walls of PSi. In this work, we immobilized protein A onto macropore PSi and used optical reflection to detect human IgG immobilized onto nano-sized SPBs by measuring changes of optical thickness (2nL). Furthermore, the optical thickness (2nL) was proportional to mass of the biomolecules, thus the Δ2nL corresponded to the mass fraction of active IgG with SPBs inside PSi pores. Therefore, we quantified the changes of optical thickness (2nL) to enable the detection of SPBs functionalized human IgG based on protein-A modified macropore sized PSi platform.