A spectroscopic and thermal investigation into the relationship between composition, secondary structure and physical characteristics of electrospun zein nanofibers

Electrospun zein nanofibers have attracted interest as drug delivery systems due to their propensity for controlled drug release, flexible structure and low toxicity. However, comparatively little is known regarding the relationship between production method and fiber characteristics, both in terms of fiber architecture and protein structure. Here we use a range of imaging and spectroscopic techniques to elucidate the effects of solvent composition on zein secondary structure, fiber diameter and fiber integrity, plus we utilize the new technique of transition temperature microscopy to examine the thermal properties of the fibers. Zein nanofibers were prepared using ethanol, acetic acid and water mixes as solvents, alone and with plasticizers (polyethylene glycol, glycerol) and casein. Electrospinning was performed under controlled conditions and the products characterized using scanning electron microscopy (SEM), attenuated total reflection Fourier Transform infrared spectrometry (ATR - FTIR) and transition temperature microscopy (TTM). The choice of solvent, concentration and voltage, alongside the presence of additives (plasticizers and casein) were noted to influence both the diameter of the fibers and the tendency for bead formation. A relationship was noted between protein secondary structure and fiber architecture, with an enhanced β-sheet content, enhanced by the inclusion of casein, being associated with higher beading. In addition, thermal imaging of electrospun zein fiber mats was successfully achieved using TTM via two dimensional mapping of the softening temperatures across the spun samples, in particular demonstrating the plasticizing effects of the polyethylene glycol and glycerol.