The growth of supermassive black holes is often linked to galaxy interactions, which drive gas toward the nucleus and fuel AGN activity. However, nuclear emission is also observed in galaxies considered isolated, suggesting alternative triggering mechanisms. These galaxies, free from strong environ mental effects, serve as a reference to study ionisation processes in the absence of interactions. We aim to identify the ionising sources in a sample of nearby isolated late-type edge-on galaxies, focus ing on whether the ionisation responsible for extraplanar diffuse ionised gas (eDIG), when present, is driven by star formation or nuclear activity. We analyse long-slit optical spectra, obtained along the major axis of 12 galaxies from the Catalogue of Isolated Galaxies (CIG), for which no previous spectroscopic data were available. The sample includes systems both exhibiting and lacking eDIG features identified in Fabry-Perot Hα data. Emission-line ratios are measured and used in BPT and WHAN diagrams to classify the dominant ionisation mechanisms. Most galaxies show line ratios consistent with star formation, but some fall in LINER or Seyfert regions, suggesting the presence of low-luminosity AGNs. Our results show that nuclear activity can arise in isolated environments, and support the use of such galaxies to study AGN triggering in the absence of external interactions.