Molecular organization at surfaces of amphiphilic polymers with cationic pyridinium backbones and non-polar alkyl side chains was studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. These surfaces were bactericidal in nature. Bacterial assays, involving the determination of cell viability after contact with the surfaces, had shown that the antibacterial activity was influenced by surface hydrophobicity [1]. NEXAFS spectroscopy was used to probe the top 1–3 nm of the polyelectrolyte surfaces, and assess the correlation between molecular structure and antibacterial property.

Thin films of the pyridinium polymers were exposed to X-ray photons and the intensity of emitted Auger electrons was measured using a channeltron electron detector. To excite both C 1s and N 1s resonances, the X-ray energy was varied from 270 eV to 440 eV. Compositional depth profiling was achieved by varying the relative position of the detector, and hence the path length of Auger electrons within the polymer film. The resulting changes in the intensities of characteristic resonances were analyzed to obtain the composition profile. Depth profiling was also achieved by varying the negative bias on a metal grid in the electron path toward the detector. By progressively increasing the retarding bias on the grid, increasingly thinner surface layers could be probed.

NEXAFS analysis revealed that the non-polar alkyl groups preferentially segregated to the polymer surface, thereby covering the higher surface-energy pyridinium rings. From this analysis, the surface organization of quaternized pyridinium polymers with different lengths of alkyl side chains will be discussed. Preliminary evidence suggests that surfaces that are densely covered by alkyl groups show lower antibacterial activity, possibly due to hindrance of interactions between the bacterial cell envelopes and the cationic pyridinium rings.

[1] Krishnan, S.; Ward, R. J.; Hexemer, A.; Sohn, K. E.; Lee, K. L.; Angert, E. R.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Langmuir 2006, 22, 11255-11266.