Plant polymer as sensing material: Exploring environmental sensitivity of dielectric properties using interdigital capacitors at ultra high frequency

Abstract

An interdigital capacitor (IDC) system was designed in order to provide a controlled high exposure (high surface/thickness ratio) to the environmental atmosphere of a sensing layer of which the dielectric properties were identified by a finite element method simulation and validated using a common parallel plate capacitor technique. The dielectric properties of one of the most sensitive and widely available plant polymers in nature: wheat gluten (WG) proteins were determined at ultra high frequency (500MHz–1000MHz) at 25°C and at two different values of relative humidity (RH). Increasing relative humidity from 20% RH to 80% RH increased the dielectric loss and permittivity of wheat gluten from 0.39±0.01 to 0.84±0.02 and from 5.01±0.06 to 7.07±0.18, respectively. This effect was discussed in the light of wheat gluten composition (constituting amino acids), structure (high molecular weight, proteins unfolding and mobility) and water content (adsorbed water-bonding state). In addition to RH, two other analytes known as food quality markers, carbon dioxide (CO2) and ethanol were studied in terms of sensitivity. The sensitivities of 10.0±0.4 fF/%RH, 31.38±0.06 fF/%CO2 and 25.50±0.05 pF/%ethanol obtained should pave the way for the development of innovative green radio frequency identification (RFID) tags using renewable, cheap and biodegradable plant polymers as gas and vapor sensors. The sensors are intended to be interfaced to low-cost, ultra high frequency, passive, RFID tags for monitoring food quality and freshness volatile markers in packaging headspace.