Thermoresponsive PNIPAM Gel-Coated PCL Nanofibers: Characterization, Vapor-Phase Release of Rosemary Essential Oil, and Activity Against Aspergillus flavus

Rosemary essential oil (REO) was incorporated into electrospun poly(ε-caprolactone) (PCL) nanofibers, which were subsequently modified with a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM)-gel coating using an immersion-based approach. The coated fibers exhibited temperature-dependent surface wetting behavior below and above the lower critical solution temperature (LCST) of PNIPAM. PNIPAM coating reduced water vapor transmission at 25 °C and increased the water vapor transmission ratio (WVTR40/WVTR25) from 1.65 to 2.31, whereas thickness-normalized water vapor permeability did not show a distinct LCST-dependent transition. Headspace analysis by HS-SPME-GC-MS showed increasing accumulation of REO volatiles with incubation time and temperature. After 48 h, PNIPAM coating reduced the total headspace amount of the selected volatile markers by approximately 7.2-fold at 25 °C and 4.9-fold at 40 °C. A significant coating × temperature interaction (p < 0.05) indicated that the coating modified the temperature dependence of volatile transfer, with the 40/25 °C ratio increasing from 1.65 in uncoated fibers to 2.41 in coated fibers. In non-contact antifungal assays against toxigenic Aspergillus flavus, REO-loaded fibers produced transient growth inhibition at 25 °C, whereas inhibition persisted throughout the 10-day incubation at 40 °C and was greater for the uncoated REO-loaded fibers. None of the fiber systems significantly affected AFB1 accumulation at 25 °C, while AFB1 was undetected in all samples, including the untreated control, at 40 °C. Overall, the PNIPAM-gel coating provided substantial retention of REO volatiles while introducing temperature-dependent modulation of their transfer to the gas phase, although the response did not represent a simple LCST-triggered release switch.