Dielectric Permittivity and Surface Charge Density in Layer-by-Layer Poly(diallyldimethylammonium chloride)/Poly(styrenesulfonate) Nanostructured Films: Implications for Biosensing

The development of sensors based on nanostructures is an important task in fundamental and applied sciences to increase their sensitivity and selectivity. A better comprehension of materials aggregation at the nanoscale can provide insights and innovations for sensing and electronic appliances. We analyzed the electrical properties of polyelectrolytes assembled in a multilayered architecture with nanometric thickness control. We have fabricated layer-by-layer (LbL) thin films alternating positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged poly(styrenesulfonate) (PSS) in a PDDA/PSS architecture. A homemade setup tracked the nanostructure growth by measuring the capacitance after each deposited layer onto gold interdigitated electrodes (IDEs). The capacitance increase linearly after each adsorbed bilayer, which is associated with the dielectric layer accumulated onto the IDEs. We interpreted such behavior in terms of the electrostatic potential on the IDEs, resulting in a dielectric constant of 21 ± 3 for the PDDA/PSS film. Furthermore, we observed the capacitance changing in a zigzag-like behavior according to the outermost deposited layer due to the charge reversal process. Polycation surface charge density was estimated as (5.8 ± 0.2) × 10–20 C/μm2. We also analyzed the strong and weak character of PDDA and poly(allylamine hydrochloride) (PAH). The methodology proposed here allows for a better material choice that best suits a particular LbL application. Besides, the quantitative evaluation of the outermost layer charge density should enhance the adsorption of molecules that are routinely employed as active layers in (bio)sensing applications, contributing to improvements in (bio)sensor engineering.