X-ray photoelectron spectroscopy (XPS) analysis was conducted for the different modification steps on a silicon wafer with the fabricated SiNPs presented in Figure . For the clean silicon wafer, only silicon and oxygen elements from the native oxide layer are observed. Once the APDMES is covalently bonded, a rise in carbon and nitrogen atomic concentration is detected. Furthermore, a significant increase in carbon and nitrogen atomic concentration occurs once the glutaraldehyde and IgG antibody molecules are bonded to the surface. The concentration of silicon and oxygen is reduced in every modification step, as the silicon substrate surface is covered and screened by the introduced organic compounds ad-layers. The XPS measurements show that the antibody modification process to the sensing device was successful. Full XPS spectra are shown in Figure .