Chemically Grafted Polymeric Filters for Chemical Sensors: Hyperbranched Poly(acrylic acid) Films Incorporating β-Cyclodextrin Receptors and Amine-Functionalized Filter Layers

We report a new “molecular-filter” approach for enhancing the selectivity of chemical sensors. Specifically, we describe electrochemical sensors prepared from Au electrodes coated with β-cyclodextrin-functionalized, hyperbranched poly(acrylic acid) (PAA) films capped with a chemically grafted, ultrathin polyamine layer. The hyperbranched PAA film is a highly functionalized framework for covalently binding the β-cyclodextrin molecular receptors. The thin, grafted polyamine overlayer acts as a pH-sensitive “molecular filter” that selectively passes suitably charged analytes. Poly(amidoamine) dendrimers or poly- d -lysine is used as 10−15-nm-thick filter layers. The results show that at low pH, when the polyamines are fully protonated, positively charged redox probe molecules, such as benzyl viologen (BV), do not permeate the filter layer. However, at high pH, when the filter layer is uncharged, BV penetrates the filter layer and is reduced at the electrode. The opposite pH dependence is observed for negatively charged redox molecules such as anthraquinone-2-sulfonate (AQS). Both BV and AQS specifically interact with the β-cyclodextrin receptors underlying the polyamine filter layers.