Plasticizer-Free Thin-Film Sodium-Selective Optodes Inkjet-Printed on Transparent Plastic for Sweat Analysis

Abstract

A novel strategy to functionalize transparent flexible plastic films with an optical ion-sensing layer using an inkjet-printing technology is described. The hydrophobic sensing chemicals that include a sodium ionophore, a lipophilic proton chromoionophore, and a lipophilic ion-exchanger are co-deposited onto substrates such as transparent polyester film sheets in the absence of any plasticizer and/or hydrophobic polymer matrix. The inkjet-printing process enables the formation of optode films with nanoscale thickness/roughness that readily facilitate interfacing with aqueous samples. Using a smartphone detector, the colorimetric response of the optodes is shown to reach 95% of equilibrium values within 100 s in response to different concentrations of sodium ions, which is more rapid than traditional ion-selective optodes based on plasticized PVC films as the sensing layer. The new optodes also exhibit high selectivity to Na+ over interfering ions including K+, Ca2+, and Mg2+. Chemical leaching experiments show that the highly hydrophobic optode components remain in place on the plastic substrate surface. Hence, excellent sensor stability and fully reversible optical responses are obtained, which is essential for potential continuous monitoring applications. Further testing of the sensors with undiluted human sweat samples is shown to yield accurate values for sodium concentrations. Therefore, the use of plasticizer-free ion-selective optode nanolayers that enable highly selective ion sensing on a clear plastic support is likely to expand the range of available chemical sensors suited for preparing wearable real-time sweat analysis devices.

Smart Materials Enabled with Artificial Intelligence for Healthcare Wearables

“…Section 2.1.2) have been used in many wearable devices. [133][134][135][136][137][138] Color changes can be detected by the Figure 6. Electrochemical sensors for wearable applications.…”

“…[139] Colorimetric sensors can be fabricated on different wearable substrates, such as paper, [140] hydrogel, [141] fiber, [142] fabric, [143] and yarn. [133,134] Wearable colorimetric devices include onskin platform, [135,144,145] smart band, [143] wristband, [146] contact lenses, [138] T-shirt, [133] etc., and are usually used to detect human sweat, [136,137,143] tears, [138] VOCs, [134] UV, [147] etc. Kim et al [134] demonstrated an eye-readable and wearable H 2 S sensing platform based on colorimetric dye-loaded nanofiber yarn, which could be incorporated into a worker's garment as a visual indicator; and was adequate for inexpensive mass-production (Figure 7A).…”

Unintended Changes of Ion-Selective Membranes Composition—Origin and Effect on Analytical Performance

“…An ISM as understood here is a system as typically used in potentiometric sensors: a membrane containing ionophore—a ligand able to preferentially bind analyte ions (called primary ions) in the lipophilic medium [ 1 , 2 ]. Due to the presence of a highly selective ionophore, the ISM allows determination of contents of free ions of interest, in the presence of other chemical forms of analyte, in complex matrices, including blood, serum or environmental samples [ 3 , 4 , 5 , 6 , 7 , 8 ]. This makes ISM-based sensors attractive tools for many applications.…”

Digital printing of selective and reversible ion optodes on fabrics: toward smart clothes for epidermal chemical sensing

“…We recently discovered that these hydrophobic sensing chemicals readily adsorb on hydrophilic substrates such as cellulose paper, enabling ion sensing in the absence of a plasticized polymer matrix. [12][13][14][15] The optode cocktail without the plasticized polymer becomes inkjet-printable because of its ideal viscosity and surface tension. As a result, ISOs can be fabricated in an unprecedentedly precise and reproducible fashion, making batch calibration possible.…”

“…This optode is also reversible and has a 90% response time of <2.5 min. When a commonly used ionophore, sodium ionophore VI, 12,15 is printed along with Chromoionophore I and KTFPB to create Na + optodes, the reversibility is unsatisfactory (Fig. 4A).…”