Reliable Paper Surface Treatments for the Development of Inkjet-Printed Electrochemical Sensors

Abstract

The need for disposable, simpler, and accurate paper-based analytical devices represents an open research field that is focused on simpler fabrication methods. To achieve this, four feasible methodologies are proposed for the direct printing of an electrochemical sensor on a biodegradable paper substrate using commercial gold and silver inks, which are compatible with inkjet printing technology. Four substrate treatment strategies are evaluated: printing the active elements directly on the hydrophilic bare paper, a hydrophobic gas-phase coating over all the substrate, a hydrophobic silane ink that is selectively printed on the paper, and a hydrophobic coating that is selectively printed and blocks the surface porosity. Thanks to the ability of the inks to conform the cellulose fibers, the resulting working electrodes tune their electrochemical active area, showing higher active electrochemical areas when the roughness is increased. The most planar consideration is achieved blocking the paper porosity with SU-8 and the direct printing on bare paper maximizes the electrochemical response with the smallest geometric area, with current values 2.7-times higher than the theoretical one. Although all methods allow a functional electrochemical sensor, the highest reproducible results are accomplished with the blocked paper, consequently allowing a higher controlled and robust manufacturing approach.