Deep and dip: Immobilization on paper substrate using Deep Eutectic solvent to fabricate reusable dip immersion colorimetric sensor arrays

A novel method for immobilization of hydrophilic indicators on the surface of the paper-based strips, for using in aqueous solution, has been developed. For this aim, a hydrophobic deep eutectic solvent (DES) was used to immobilize indicators on a paper without hydrophobic barriers. The proposed user-friendly colorimetric strip presented homogeneous color, reversibility, and significant leaching stability over months. For designing the pH strip as a model analyte, two ethanolic pH indicators were dissolved in DES and spotted on a Benchkote paper. The fabricated strip coupled with multivariate analysis exhibited excellent performance in semi-quantitative and quantitative analysis of pH value in the range of 2.0 - 12.0 with 0.5 unit increments based on their color variations. The principal component analysis of different buffer solutions in whole pH range demonstrates 100% classification accuracy. Also, partial least square (PLS) regression method was used to estimate the pH values of different samples through image analysis. A root mean square error for calibration and prediction of the pH value in the whole range of universal buffer was obtained as 0.22 and 0.21, respectively. The applicability of the prepared pH strip confirmed by fast and easy detection of pH in seven different real matrices, including food (dairy and juice products), biological (bacteria and cell culture media), and tap water samples.

Automated summary

A novel method has been developed for immobilizing hydrophilic indicators on the surface of paper-based strips in aqueous solution. The method utilizes a hydrophobic deep eutectic solvent (DES) to immobilize the indicators without the need for hydrophobic barriers. The resulting colorimetric strip demonstrates homogeneous color, reversibility, and significant leaching stability over months.