In Situ Study of Layer-by-Layer Polyelectrolyte Deposition in Nanopores of Anodic Aluminum Oxide by Reflectometric Interference Spectroscopy

Abstract

The modification of cylindrical anodic aluminum oxide (AAO) nanopores by alternating layer-by-layer (LBL) deposition of poly(sodium-4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) was studied in situ by reflectometric interference spectroscopy (RIfS). In particular, the kinetics of polyelectrolyte deposition inside the pores with a diameter of 37 ± 3 nm and a length of 3.7 ± 0.3 μm were unraveled, and potential differences in the LBL multilayer growth compared to flat silicon substrates as well as the effect of different ionic strengths and different types of ions were investigated. RIfS measures the effective optical thicknesses, which is-for a constant pore length-proportional to the effective refractive index of the AAO sample, from which, in turn, the deposited mass of the polymer or the corresponding layer thickness can be estimated. Compared to the multilayer growth by the LBL deposition on the flat aminosilane-primed silicon wafers, which was assessed by spectroscopic ellipsometry, the thickness increment per deposited bilayer, as well as the dependence of this increment on the ionic strength (0.01-0.15) and the counterion type (Na+ vs Ca2+) inside the aminosilane-primed nanopores, was for the first bilayers to within the experimental error identical. For thicker multilayers, the pore diameter became smaller, which led to reduced thickness increments and eventually virtually completely filled the pores. The observed kinetics is consistent with the mass-transport-limited adsorption of the polyelectrolyte to the charged surface according to a Langmuir isotherm with a negligible desorption rate. In addition to fundamental insights into the buildup of polyelectrolyte multilayers inside the AAO nanopores, our results highlight the sensitivity of RIfS and its use as an analytical tool for probing processes inside the nanopores and for the development of biosensors.