Development of miniaturised ion-selective sensors using thin-film nanostructured materials

Ion-selective electrodes represent a fundamental technology in a wide range of fields including healthcare, analytical chemistry or water quality assessment, among others. The basic structure of ion selective electrodes can be divided in three different layers, the sensing element, typically an ionophore, the polymeric matrix where such ionophores are embedded and the ion transducer, which serves as an interlayer between the sensing film and the electrode. The present thesis represents a comprehensive and in-depth study of these 3 elements of current ion sensors from a materials science perspective, using different nanomaterials and determining their impact on the electrochemical signals. Within the present work, a new deposition method based on the low-cost and non-vacuum aerosol assisted chemical vapour deposition (AACVD) technique was optimised for the synthesis of highly homogeneous thin films, with a roughness in the nanometre range, and a controllable thickness. This method represented an improvement compared to the current technology based on drop casting, which typically leads to rough an relatively thick films. To further improve the sensing capabilities of these sensors, the ion-to-electron transducer intermediate film was optimised by testing different nanomaterials such as nanocarbons and metallic nanoparticles. These nanomaterials could reduce the electrochemical noise while maintaining a Nernst sensitivity of 59 mV Log[C]-1, which is the current standard of sensitivity of ion-selective electrodes. After the integration of a suitable ion-to-electron transducer, non-equilibrium techniques such as pulse voltammetry were adapted to the process, increasing the sensitivity beyond the standard Nernst limit, and allowing a multiplexed detection of ions. Finally, the limitations of current ion sensors in terms of the thickness and the lack of suitable ionophores for the selective detection of certain ions were tackled by adapting ionically imprinted nanomaterials. These nanomaterials offered the possibility of synthetizing a highly stable and miniaturisable devices with tailored selectivity