Optical Monitoring of the Capillary Filling Dynamics Variation in Nanoporous Anodic Alumina toward Sensing Applications

Fluid imbibition-coupled laser interferometry (FICLI) is a technique in which the kinetics of a fluid infiltrating a nanoporous anodic alumina (NAA) membrane is monitored by the interference of a laser beam at the membrane top and bottom surfaces. Further processing of the measured data results in an estimate of the pore radius. In this work, we study the accuracy of FICLI in the detection of small changes in pore radius, and we evaluate the possibility of using such detection as a sensing paradigm. The accuracy is estimated by measuring samples with increasing pore radius, obtained by successive wet etching steps, and repeatability is evaluated by using different liquids. For decreasing pore radius, samples obtained by the successive deposition of polyelectrolyte double layers are used. With the aim of evaluating the possibility of the FICLI method to sense biological binding events, BSA attachment detection is demonstrated by applying FICLI to samples before and after immobilization of the protein. Results show that the technique permits an accurate estimation of the pore radius, the pore-etching rate (with a radius variation of r(etch,DI) = 1.05 nm/min +/- 0.11 nm/min), and the polyelectrolyte double layer thickness (with a radius variation of r(PAH/PSS) = 3.2 nm +/- 0.2 nm per polyelectrolyte double layer). Furthermore, the pore radius reduction measured after BSA immobilization (d(BSA) = 4.9 nm +/- 1.1 nm) is in good agreement with the protein size, as reported in the literature. With these results, we provide a sound basis for the applicability of FICLI as a sensitive technique for the characterization of NAA pore radius modifications.