MMP9 and neurodegenerative disease: Streptavidin-modified polystyrene nanoparticles (PSNPs) are proposed as carrier agents, applied to the detection of matrix-metalloproteinase 9 (MMP-9), a potential biomarker overexpressed in different diseases, including cancer (breast, ovarian, cell lung, colorectal cancer) [14], neurodegenerative diseases [15, 16], or chronic wounds [17].