Hierarchical ZnO/Polyamide‑6 Nanofibers with Enhanced Antibacterial Activity Fabricated by Atomic Layer Deposition and Hydrothermal Growth

Background

Organic‑inorganic hierarchical nanostructures leverage the high surface‑to‑volume ratio of biomimetic designs, enabling applications in catalysis, superhydrophobic surfaces, optoelectronics, piezoelectronics, and antibacterial materials. Flexible fibers, in particular, offer high aspect ratio, lightweight, and tensile strength, making them attractive substrates for advanced nanostructures. Electrospinning is a cost‑effective, continuous route to produce nanofiber mats that can be further functionalized.

Atomic layer deposition provides conformal, monolayer‑controlled coatings on high‑aspect‑ratio substrates, making it ideal for uniformly decorating nanofibers. The combination of electrospinning and ALD has yielded core‑shell architectures such as PA‑6/ZnO, ZnO/TiO₂, TiO₂/ZnO, and others, often followed by hydrothermal growth to create ZnO nanorods (NRs) and other morphologies.

In this work, PA‑6 NFs are seeded with ZnO via ALD and then subjected to hydrothermal treatment, resulting in two distinct hierarchical morphologies. These structures are evaluated for antibacterial performance, highlighting their potential in protective textiles and biomedical applications.

Experimental Part

PA‑6 nanofibers were spun from a 15 wt % PA‑6 solution in formic acid using a 12 kV applied voltage and a 10 cm tip‑collector distance. The mats were vacuum‑dried at 60 °C for 12 h to remove residual solvent. ALD of ZnO was performed at 110 °C in a custom chamber using diethyl zinc and water, with 50, 100, or 150 cycles to create seed layers. The hydrothermal step involved dipping the seeded mats in a 0.025 M hexamethylenetetramine/0.025 M zinc nitrate solution for 1, 3, or 6 h at 90 °C. After growth, the samples were rinsed and air‑dried.

Morphology was examined by FE‑SEM (Hitachi S4800, 1 kV) and HRTEM (JEM‑2100F, 200 kV). Elemental mapping was performed with STEM–EDX. Crystallinity was assessed by XRD (Bruker D8 ADVANCE, Cu Kα). XPS (Kratos Axis Ultra, Al Kα) characterized surface chemistry. Antibacterial activity was quantified by measuring inhibition zones against S. aureus on 3 mm thick membranes using a vernier caliper.

Results and Discussion

ALD ZnO Coating NFs

FE‑SEM images (Fig. 1a–d) show uniform PA‑6 NFs with diameters of 125 ± 75 nm and 30 ± 16 nm. After 150 ALD cycles, a continuous ZnO shell of ~14.6 nm thickness is evident (Fig. 1e), confirming conformal coverage. XPS spectra (Fig. 2) reveal increasing Zn 2p signals with cycle number and a shift in O 1s peaks toward lower binding energies, indicating denser ZnO coverage.

PA 6‑ZnO Hierarchical NFs

Hydrothermal growth transforms the ZnO seeds into nanorods whose morphology depends on both ALD cycles and growth time. After 3 h, 50‑cycle samples form water lily‑like clusters, whereas 100‑ and 150‑cycle samples develop caterpillar‑like structures (Fig. 3f–h). Extending the hydrothermal period to 6 h reduces nanorod density on bare PA‑6 NFs but preserves the hierarchical forms on seeded samples (Fig. 3i–l). TEM (Fig. 4a) confirms that many NR tips detach from the fiber, explaining the observed morphology variations.

XRD (Fig. 4c) shows that longer hydrothermal exposure promotes the α‑phase of ZnO and reorients PA‑6 crystallites, while XPS (Fig. 5) indicates a shift of the O 1s peak to lower binding energy with time, reflecting changes in ZnO surface chemistry.

Antibacterial Activity

Inhibition zone diameters increase with hydrothermal time for 150‑cycle samples, reaching 1.5 mm for caterpillar‑like structures versus 1.03 mm for water lily‑like ones (Fig. 6). The superior performance of caterpillar‑like NRs is attributed to their higher aspect ratio and possibly altered ZnO surface chemistry.

Conclusions

We successfully fabricated ZnO‑coated PA‑6 nanofibers with controllable hierarchical morphologies using ALD seed layers followed by hydrothermal growth. The number of ALD cycles determines whether the ZnO seed layer is continuous or discontinuous, which in turn dictates whether water lily‑ or caterpillar‑like nanorods form during hydrothermal processing. Hydrothermal time further tunes the crystal orientation and surface chemistry of ZnO. Antibacterial testing against S. aureus shows that caterpillar‑like structures provide the strongest inhibition, suggesting that both morphology and chemical composition are critical for antimicrobial efficacy.