Tailoring porosity and surface functionalization in nanoporous anodic alumina rugate filters for enhanced interferometric biosensing.

The influence of porosity and surface functionalization strategies on the biosensing performance of nanoporous anodic alumina rugate filters (NAA-RFs) is systematically investigated using reflectometric interference spectroscopy (RIfS). Highly ordered NAA-RF structures were fabricated by sinusoidal anodization and subsequently tuned through controlled pore-widening processes to achieve different porosity levels. Structural and optical characterization confirmed the formation of modulated nanoporous architectures exhibiting well-defined Fabry-Pérot interference patterns and characteristic stop bands, whose spectral position depends on the effective refractive index of the structure. The sensing performance was evaluated using two distinct surface functionalization strategies. A sequential electrostatic approach based on protein A (SpA) and anti-IgG was employed for IgG detection, while a covalent functionalization using APTES and glutaraldehyde (GTA) enabled the detection of lactate dehydrogenase (LDH). The results demonstrate that increasing porosity significantly enhances the effective optical thickness (EOT) response due to the larger internal surface area available for molecular binding. Kinetic analysis further revealed that higher porosity leads to not only increased saturation amplitudes but also longer characteristic response times, indicating diffusion limitations within the nanoporous network. Overall, the results highlight the importance of simultaneously tailoring structural parameters and surface chemistry to optimize the performance of NAA-based interferometric biosensors, demonstrating the potential of modulated nanoporous architectures for sensitive and selective biomolecule detection.