A novel dual-parameter proximity and touch sensor using SiO2 nanoparticles and NaCl with commercial acrylic-based encapsulation

Abstract

This study shows the development and analysis of a novel capacity proximity sensor (CPS) based on a sensing layer made up of a mixture of silicon dioxide nanoparticles (SiO₂) and sodium chloride (NaCl), and an encapsulation layer based on a commercial acrylic-based varnish. The encapsulated and non-encapsulated proximity sensors were characterised using impedance spectroscopy (IS), revealing that the resulting impedimetric and capacitance responses exhibit different sensitivities and working sensing ranges. The non-encapsulated sensor presents impedimetric and maximum capacitive sensitivities of 0.0775 cm⁻¹ and -0.9831 cm⁻¹, respectively, within a 2–14 cm sensing range. In contrast, the encapsulated CPS shows maximum impedimetric and capacitive sensitivities of 0.3447 cm⁻¹ and −3.349 cm⁻¹, respectively, and an operation sensing range of 0–3 cm. The results show a 75% decrease in the total sensing range that could be attributed to: (i) a reduction of the effective sensing area due to a reduction of the roughness as demonstrated by SEM analysis, (ii) insulation effects limiting the impact of the material under test (MUT) on the charge carriers distribution, and (iii) decreased charge carrier density involved in the sensing process. Despite the reduced operational range, the encapsulation layer maintains the dual-parameter sensing capabilities, preserves the integrity of the sensing layer, and enables its dual functionality as a proximity and touch sensor. The reported comparison between the encapsulated and non-encapsulated CPSs highlights the effects of the encapsulation layer. The encapsulated version introduces a simple, fast, and cost-effective novel approach for developing CPSs that outperforms some reported CPSs in terms of reliability due to its dual-parameter sensing capability and sensitivity.