Probing Inclusion Complexation Mechanisms in Polycyclodextrin Nanofibers via ESR Spectroscopy

The molecular-level mechanism of inclusion complexation in polycyclodextrin nanofibers (poly-CD NF) has remained unexplored despite their emerging potential as size-selective sorbents and functional interfaces for molecular separations. In this study, we present the first systematic investigation of host-guest interactions in poly-CD NF using spin-labeled electron spin resonance (ESR) spectroscopy. Two nitroxide probes, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and di-tert-butyl nitroxide (DTBN), were employed to evaluate how CD cavity size affects inclusion complexation of guest molecules. The outer hyperfine splitting parameter (2A_zz) at low temperature was used as a quantitative measure of the local hydrophobicity of the nitroxide radicals, while the line shape of the ESR spectrum provided further insights into spin-spin interactions and radical proximity within the polymer network. Comparative analyses of NF and powder form of CD complexes showed consistent spectral signatures, confirming similar inclusion environments in both systems. The combined spectral parameters revealed distinct affinities and dynamic constraints of TEMPO and DTBN toward hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and hydroxypropyl-γ-cyclodextrin (HP-γ-CD) based NF, correlating with their cavity sizes. Poly-HP-β-CD NF and pristine HP-β-CD powder were found to best fit both TEMPO and DTBN, showing a higher uptake of nitroxide radicals, followed by poly-HP-γ-CD NF and pristine HP-γ-CD powder, and then poly-HP-α-CD NF and pristine HP-α-CD powder, which contained negligible amounts of TEMPO and relatively low amounts of DTBN. Further analysis of ESR spectra showed that poly-HP-β-CD NF and pristine HP-β-CD powder share similar microenvironments, with cavity size influencing binding affinity and retention. Spectral simulations allowed for meaningful insights into molecular dynamics of the two nitroxides in the CD cavities. These findings establish ESR spectroscopy as a powerful tool for probing nanoscale heterogeneity and inclusion complexation dynamics in poly-CD-based NF adsorbents, providing fundamental insights crucial for designing next-generation filtration materials for water purification and molecular recognition applications.