Static secondary ion mass spectrometry for the surface characterisation of individual nanofibres of polycaprolactone functionalised with an antibacterial additive

Electrospinning (ES) of polymer solutions generates non-woven webs of nanofibres. The fibre diameter ranges between 10 nm and 1 μm depending on the operating conditions. Surface functionalisation can be performed by the use of suitable additives. Detailed characterisation of the molecular composition at the fibre surface is a key issue. Biodegradable nanowebs with potential antibacterial activity have been prepared by ES of solutions containing polycaprolactone (PCL) and a functionalising additive with PCL segments and hexyldimethylammonium groups (PCLhexaq). Static secondary ion mass spectrometry with Bi₃⁺ projectiles has been applied to individual nanofibres. The positive ion mass spectra contain several signals with high structural specificity allowing the presence of PCLhexaq to be traced back in spite of its low concentration (0.16–1.4% w/w relative to PCL) and its structural similarity to the PCL fibre matrix. Imaging of structural ions visualises the homogeneous distribution of PCLhexaq over the fibre surface. Quantifying the surface concentration of PCLhexaq relative to that of PCL reveals electric field-driven surface enrichment of the additive during ES. Finally, nanofibres subjected to leaching in water for up to 72 h have been analysed. The PCLhexaq surface concentration decreases almost linearly with time at a rate of 0.6% h⁻¹.