Multiphysics Analysis of Hydrogel-Functionalized Plasmonic Sensors

Polyelectrolyte gels hold great promise for application as transducer material in plasmonic sensors to in-situ determine the concentration of the constituents in unknown fluids. In the present work, an ionic-sensitive hydrogel, which is deposited on a rigid sensor substrate with gold-coated nanopillars, is modeled and numerically investigated. A coupled multiphase continuum model is established based on the Theory of Porous Media, with the primary field quantities displacement, fluid pressure, ion concentrations, and electric potential. The respective boundary conditions are determined according to electro-chemical equilibrium requirements between the hydrogel and the external solution bath. The model incorporates electrostatics, fluid, and ion fluxes driven by pressure gradients, diffusion, advection and electromigration, and the solid polymer deformation is governed by a modified Neo - Hooke material model in the framework of finite deformations. The numerical investigation of the hydrogel is performed for representative unit cells in both a transient two-dimensional and a stationary three-dimensional setting, revealing that the presence of the nanopillar structure significantly changes the spatial distribution of the volume fraction of the fluid phase and of other field variables. This might significantly influence the performance of the overall sensor system and should be considered in the further development of plasmonic sensors with polyelectrolyte gels as transducer material.