Eco-Friendly Colorimetric Nanofiber Design: Halochromic Sensors with Tunable pH-Sensing Regime Based on 2-Ethyl-2-Oxazoline and 2-n-Butyl-2-Oxazoline Statistical Copolymers Functionalized with Alizarin Yellow R

Colorimetric nanofibers provide visual, easy-to-interpret sensors for personal use as well as advanced applications. The potential of 2-n-butyl-2-oxazoline (B) and 2-ethyl-2-oxazoline (E) statistical copolymers as a universal, versatile support platform for nanofibrous halochromic sensor design is demonstrated. These polymers are electrospinnable from eco-friendly solvent systems, while wettability, moist adsorption capacity, and water-solubility of the membranes can be easily tuned by changing the B/E monomer ratio, ensuring wide applicability. The halochromic sensing functionality is introduced by incorporating the alizarin yellow R (AYR) chromophore, which is covalently modified with an ethyl ester-group or a short poly(2-n-butyl-2-oxazoline) chain, which is demonstrated to simultaneously prevent dye-leaching and allows tuning of the halochromic pH-sensing window. The colorimetric nanofibrous sensors reversibly respond toward aqueous solutions of different pH, (hydrochloric) acid and alkaline (ammonia) vapors, and several biogenic amines with detection limits as low as 5 ppb. Tunability of sensor responsivity, sensitivity, and pKa via manipulation of dye-polymer interactions, determined by support polymer structure and semi-crystallinity, as well as the chain length of the AYR-modified polymer, are discussed. Preliminary proof-of-principle studies indicate the potential of the developed sensors for sub-ppm biogenic amine vapor detection, which may serve as the basis for future applications in food packaging or breath analysis.

Automated summary

Colorimetric nanofibers with halochromic sensing functionality were developed using 2-n-butyl-2-oxazoline (B) and 2-ethyl-2-oxazoline (E) statistical copolymers as support platform. The sensors can detect aqueous solutions of different pH, (hydrochloric) acid and alkaline (ammonia) vapors, and biogenic amines with low detection limits. Tunability of sensor responsivity, sensitivity, and pKa can be achieved through manipulation of dye-polymer interactions and polymer structure.