Integration of an Aerosol-Assisted Deposition Technique for the Deposition of Functional Biomaterials Applied to the Fabrication of Miniaturised Ion Sensors

Abstract

Ion-selective electrodes are at the forefront of research nowadays, with applications in healthcare, agriculture and water quality analysis among others. Despite multiple attempts of miniaturization of these polyvinyl chloride (PVC) gel-based ion sensors, no ion-sensing devices with a thickness below the micrometer range, and operating using open circuit potential, have been developed so far. This work reports the causes of this thickness limitation in potassium-selective sensors. Highly homogeneous ion-sensing films were fabricated by a method based on aerosol assisted chemical vapour deposition, leading to smooth surfaces with 27 ± 11 nm of roughness. Such homogeneity allowed the systematic study of the performance and ionic diffusion properties of the sensing films at sub-micrometer scales. Sensitivities below the Nernst response were found at low thicknesses. The nature of this reduction in sensitivity was studied, and a difference in the superficial and bulk compositions of the films was measured. An optimal configuration was found at 15 µm, with a good selectivity against Na+ (KK+, Na+ = −1.8) a limit of detection in the range of 10−4 M and esponse time below 40 s. The stability of sensors was improved by the deposition of protective layers, which expanded the lifespan of the ion sensors up to 5 weeks while preserving the Nernst sensitivity.

In Vivo Sensing of pH in Tomato Plants Using a Low-Cost and Open-Source Device for Precision Agriculture

“…This coating could also improve the long-term stability of the sensors which proved crucial when incorporated during in vivo studies, as shown in the next section. These improvements in the drift and noise rates were likely a consequence of the lowest adsorption of biomolecules, as described in previous reported work [ 25 ]. The use of ruthenium oxide also led to a higher selectivity when compared to other metal oxide-based sensors, representing a promising approach for the development of low-cost devices for the in vivo monitoring of pH.…”

“…To improve the performance of the sensing films by reducing the electrochemical noise, a cellulose-based coating containing PDMS elastomer at a ratio of 60:40 was developed. This coating composition was selected following previous reported work that indicated an improvement in sensing stability of ion-selective electrodes using elastomer coatings [ 34 ] and a potential decrease in biomolecule adsorption when PDMS is combined with ethyl cellulose [ 25 ]. As such, it could be applied to improve the stability of the pH-sensing films, which could monitor biomarkers in biological solutions.…”

“…All film precursors had previously been dissolved in this carrying solvent, allowing the deposition of the films onto the copper conductive electrode. Deposition temperature is crucial during aerosol deposition, since it can greatly influence the roughness of the deposited films, thereby having an impact on the sensing performance [ 25 ]. In this case, the temperature of the substrate was controlled using an analogue temperature sensor physically attached to the aluminium substrate where the sensing device was placed.…”

“…To improve the performance of the sensing films, a cellulose: PDMS film was deposited onto the RuO x film. This deposition was carried out by using a custom-made aerosol deposition method as reported previously [ 25 ]. The deposition system consisted of an ultrasonic atomiser operating at a frequency of 110 kHz, and a low-cost air pump.…”

Physically Tailoring Ion Fluxes by Introducing Foamlike Structures into Polymeric Membranes of Solid Contact Ion-Selective Electrodes

“…These were also found to have little effect, and the equivalent circuit for the bare SC layer closely matches that of a pure capacitor . Valinomycin in a plasticized PVC ISM also shows no thermally induced changes below 100 °C; hence, the maximum temperature of 60 °C was found to have little effect.…”