Tuning the sensitivity of fluoroionophore-based K⁺ sensors via variation of polymer matrix: A comparative study

Optical sensors were prepared via immobilization of K⁺ fluoroionophore (K⁺FI) into different hydrophilic polymers and their properties (luminescence brightness, stability, sensitivity, cross talk to sodium, dynamic range, response time) were investigated. Depending on the character of the used polymeric matrix, the fluorescence quantum yields for the K⁺-fluoroionophore complex (500 mM K⁺) span from 44 % in commercial polyurethane hydrogel D1 to 18 % in poly(2-hydroxyethyl methacrylate). Dynamic intensity measurements show reversibility of the sensors and response times (t₉₀) below 1 min for most of the materials. Blending of poly(2-hydroxyethyl methacrylate) with polyurethane hydrogels is necessary to ensure compatibility of the indicator and the matrix. Comparison of dissociation constants (K_d) reveals moderate sensitivity enhancement in more hydrophilic polyurethane hydrogel D1 (K_d 51.8 mM) and much stronger enhancement in poly(2-hydroxyethyl methacrylate) (K_d 15.4 mM) compared to previously used hydrogel D4 (K_d 60.4 mM). Drastic increase in the sensitivity upon addition of negatively-charged aliphatic sulfonates is observed (K_d < 3 mM). In case of the latter, the enhancement in the sensitivity is significantly lower in presence of sodium ions. Modification of a commercially available polyurethane hydrogel with sulfonate groups enables preparation of stable sensors with improved binding ability (K_d 11.1 mM). This study demonstrates the fundamental role of the polymer matrix in characteristics of the solid-state optical potassium sensors and provides the guidelines for general design of ion sensors with improved characteristics.