Here, we report for the first time a facile and fast one-step strategy to fabricate monodisperse gold nanocubes (GNCs) by spin-coating a gold precursor-loaded PMMA dispersion on N-doped silicon. A vapor induced phase separation allows PMMA self-assembly into a nanoporous film including the GNCs in its holes. Control of size and shape until obtaining monodisperse GNCs was achieved by tailoring the concentration of the gold precursor. Upon increasing this concentration, the optical properties of the GNCs such as the surface plasmon resonance has been red-shifted from 540 to 650 nm, in relation to the strong interaction between closely separated GNCs, increase in their number and PMMA thickness on the substrate surface. Therefore, the high density of hotspots in the gaps between closely spaced GNCs led to the sensitive detection of pyridine with an unprecedented SERS threshold of detection of 10−12 M. SERS experiments revealed the high chemical enhancement and allowed few molecule detection without any surface modification of the GNCs.