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Detection and discrimination of nanoparticles is a vital step in several analytical and diagnostic procedures. Towards this, the authors present in the current study, for the first time, an all poly(methyl-methacrylate) (PMMA) polymer membrane-based solid-state sensor capable of detecting single silica nanoparticles. The sensor is based on a single cylindrical submicron pore of 450 nm in diameter and 1 m in length, patterned by electron beam lithography in a PMMA membrane. It was subsequently integrated into a PMMA-based electrophoretic flow detector system containing two electrolyte reservoirs. Silica nanoparticles of 100 nm in diameter were dispersed in an electrolyte and detected as they temporarily block the current flow during translocation through the submicron pore, driven by an electric field. The submicron pore was highly stable, and able to not only detect but also discriminate between silica nanoparticles of different dimensions recognised by different amounts of current blockade produced as they translocated through the pore. The translocations of individual 100 and 150 nm diameter silica nanoparticles through the single submicron pore, and thus the amounts of current blockade they produce, were shown in very close agreement with the results evaluated mathematically using the model presented in this study.