Nanomolar Detection Limits of Cd²⁺, Ag⁺, and K⁺ Using Paper-Strip Ion-Selective Electrodes

Abstract

Paper-based ion-selective electrodes (ISEs) are simple, flexible, and cost-efficient in comparison to conventional solid-contact ISEs. Yet, paper-based ISEs have poor limits of detection (in the micromolar range) relative to conventional solid-contact ISEs. Here we describe the construction and optimization of ISEs based on commercially available filter paper modified with single-walled carbon nanotubes (SWCNTs), sputtered gold, and conductive polymer poly(3-octylthiophene) to support an ion-selective membrane. The ion-selective membrane presented here is based on the copolymer methyl methacrylate-decyl methacrylate (MMA-DMA). The copolymer MMA-DMA is highly water-repellent and has a low coefficient of diffusion, which makes it particularly suitable for the creation of sensors with high performance in reaching low limits of detection. Three different configurations of the electrodes have been characterized by using contact angle surface analysis, oxygen influence, and testing for the presence of a water layer. Paper-strip ISEs for cadmium, silver, and potassium ions were developed with groundbreaking limits of detection of 1.2, 25.1, and 11.0 nM, respectively. In addition to such low limits of detection, paper-strip ISEs display high selectivity for their ion of interest and high reproducibility.

Single strip solid contact ion selective electrodes on a pencil-drawn electrode substrate

“…7,16 Interestingly, other reports indicate that presence of sputtered gold and conductive polymer such as poly(3-octylthiophene) is necessary to produce stable paper based ISEs. 5 This discrepancy between reported data could perhaps arise from differences in the thickness of CNTs layer deposited onto the paper surface, the concentration of surfactant (unwashed surfactant may reduce contact angle between water droplet and the substrate, thus increasing surface wetting) 17 and purity of used materials. However, to produce functional sensing devices, CNTs have to be typically dispersed in a liquid [18][19][20] (often water and surfactant solution) via ultrasonication and are later deposited onto the paper/electrode by drop-casting, 21,22 ink-jet printing, [23][24][25] dip-coating [26][27][28] or spin coating.…”

“…[1][2][3][4][5][6][7][8][9][10] This versatile character can be attributed to their excellent electrical conductivity, high tensile strength and ability to function simultaneously as recognition element and transducer (transduction via the formation of electrical double layer).…”

Paper-Based Potentiometric Ion Sensing

“…11,12 Mensah et al further reported nanomolar detection limits of another type of paper-strip ISE constructed from carbon-nanotube-modified filter paper, sputtered gold, conductive polymer, and an ISM. 13 In another report, Bobacka and co-workers performed potentiometric measurements by pressing solid-contact electrodes against sample-impregnated filter paper. 14 Neither of these approaches used wax-imprinted paper to define a more fully integrated paper-based potentiometric cell.…”

Solid-contact Ca2+-selective electrodes based on two-dimensional black phosphorus as ion-to-electron transducers

“…[1][2][3][4][5] With a view to miniaturization, mass fabrication and measurements in small volumes, solid-contact ion-selective electrodes (SC-ISEs) with a solid inner contact as the ion-to-electron transducer, have been receiving much attention.…”