Nanoporous Carbon\nMaterials as Solid Contacts for\nMicroneedle Ion-Selective Sensors

Continuous sensing of biomarkers, such as potassium ions\nor pH,\nin wearable patches requires miniaturization of ion-selective sensor\nelectrodes. Such miniaturization can be achieved by using nanostructured\ncarbon materials as solid contacts in microneedle-based ion-selective\nand reference electrodes. Here we compare three carbon materials as\nsolid contacts: colloid-imprinted mesoporous (CIM) carbon microparticles\nwith ∼24–28 nm mesopores, mesoporous carbon nanospheres\nwith 3–9 nm mesopores, and Super P carbon black nanoparticles\nwithout internal porosity but with textural mesoporosity in particle\naggregates. We compare the effects of carbon architecture and composition\non specific capacitance of the material, on the ability to incorporate\nion-selective membrane components in the pores, and on sensor performance.\nFunctioning K<sup>+</sup> and H<sup>+</sup> ion-selective electrodes\nand reference electrodes were obtained with gold-coated stainless-steel\nmicroneedles using all three types of carbon. The sensors gave near-Nernstian\nresponses in clinically relevant concentration ranges, were free of\npotentially detrimental water layers, and showed no response to O<sub>2</sub>. They all exhibited sufficiently low long-term potential\ndrift values to permit calibration-free, continuous operation for\nclose to 1 day. In spite of the different specific capacitances and\npore architecture of the three types of carbon, no significant difference\nin potential stability for K<sup>+</sup> ion sensing was observed\nbetween electrodes that used each material. In the observed drift\nvalues, factors other than the carbon solid contact are likely to\nplay a role, too. However, for pH sensing, electrodes with CIM as\na carbon solid contact, which had the highest specific capacitance\nand best access to the pores, exhibited better long-term stability\nthan electrodes with the other carbon materials.