Capacitive Touchscreen Sensors for the Measurement of Ions in Electrolytes

This work proposes and explores the use of mutual capacitive touchscreens as sensing tools for ion concentrations in an electrolyte, measuring directly from drops placed on the screen surface. This is explored using a standard, commercially available touchscreen made up of a conductive indium tin oxide on glass, coated with a parylene C layer. Simulations using COMSOL Multiphysics show that sample polarisation close to the surface dominates the sensor capacitance. The model quantifies the signal change for samples placed off the touchscreen electrodes and experiments confirm an optimal drop volume of 200 µL for measurements with the touchscreen prototype. Thin polymer coatings turn out essential for measuring electrolytes via the projected mutual capacitance. The touchscreen senses a tight semi-log correlation of its capacitance with the conductivity of electrolytes below 100 μS/cm. This correlation is dependent on anion concentration and nearly independent of the measured cations for a positive electrode. Furthermore, a new approach is developed to fabricate simplified, affordable touchscreen-inspired ITO sensors using standard laser-cutter equipment. The fabricated devices show more than double the sensitivity for measuring aqueous samples compared to the commercial touchscreen. Interdigital comb structures increase the sensitivity further from 4.0 to 8.8 compared to a simple line structure. Simulations help guide the design of sensing structures concerning sensitivity and spatial robustness. Finally, the challenge of selectivity is addressed through ion-selective membranes. Initial results on the formulation and fabrication of polymeric membranes show a deterioration of the sensitivity to 1.3 for poly(n-butyl) acrylate and 2.4 for poly vinyl chloride. While reproducible polymer membranes can successfully be fabricated on an ITO glass sensor, the research identifies a need for additional work on ion-selective films. This work demonstrates that liquid measurements on a touchscreen do not require direct contact between the sample and the electrodes for conductivity determination. In the future, polymer layers may prevent electrode degradation and have the potential to be translated in the future towards use with existing mobile technologies. A sub-micromolar detection limit for ions and a linear detection window until 500 µM is shown. The presented devices are suitable for trace metal contaminant detection in drinking water and the presence of soil nutrients such as phosphate, nitrate, phosphate, and potassium.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2022

Author dc:creator

• Horstmann, Sebastian

Advisors dc:contributor.advisor

• Daly, Ronan
• Hall, Elisabeth

• capacitive sensors
• electrolytes
• touchscreens
• environmental sensing
• laser patterning
• COMSOL Multiphysics
• indium tin oxide
• low-cost fabrication

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.90423

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/343012