Fully inkjet-printed poly(N-isopropylacrylamide)/graphene electrodes for electroanalytical sensing with pH switchable selectivity

Fully inkjet-printed electrochemical sensors, based on a graphene electrode coated with cross-linked poly(Nisopropylacrylamide) (PNIPAAm), are presented. The PNIPAAm coating is realized by combined inkjet printing and simultaneous photopolymerization using an inkjet printer with integrated UV light source. The inkjet ink composition and printing parameters are both optimized to polymerize the PNIPAAm pre-cursors as rapid as inkjet printing, while guaranteeing at the same time the fulfilment of rheologic ink requirements, such as surface tension and viscosity. In order to achieve reproducible and homogeneous PNIPAAm coverage, the surface of the graphene electrode is modified by introducing cavities of an inkjet-printed dielectric grid. The electrochemical performance of the PNIPAAm/graphene electrodes is tested with standard redox couples of positive, neutral and negative charges. A filtering effect of negatively charged redox species in near neutral pH electrolyte solutions is obtained. PNIPAAm is a well-known thermo-responsive polymer, but the variation of solution and sensor temperature does not influence the filtering properties of the PNIPAAm hydrogel obtained herein by inkjet printing. A switching of the filter characteristics of the material is observed at pH 2, at which the PNIPAAm hydrogel becomes permeable for negatively charged and impermeable for positively charged redox species. Finally, the filtering characteristics of the PNIPAAm/graphene electrode are evaluated for the electrochemical detection of the positively charged analyte dopamine in the presence of the negatively charged interferent ascorbic acid.