Colorimetric detection of acid-base in organic solvents, water, and food products using polydiacetylene/Zn²⁺/ZnO nanocomposites with tunable sensitivity

Colorimetric sensors based on polydiacetylene (PDA) materials have been developed for detecting acid-base. Since PDAs often change color upon exposure to organic solvents, most of previous studies used water as a medium. This presents a problem for detecting organic acids and bases with limited solubility in water. In this contribution, we explore the applications of PDA/Zn²⁺/ZnO nanocomposites as colorimetric sensors for detecting organic acid-base in various organic solvents and food products. Owning to its high color stability, this class of PDA materials can be dispersed in tetrahydrofuran, dimethyl sulfoxide, and ethanol while the blue phase remains. We have found that the sensitivity of PDA/Zn²⁺/ZnO nanocomposites to acid-base varies with the solvent properties. The sensors prepared by dispersing in different solvents can detect acid-base at different concentrations. Raman spectroscopy carried out directly in the liquid suspensions reveals the mechanisms of color transition in each system. Fine-tuning of the sensitivity can also be achieved by varying the ZnO ratios within nanocomposites. The ability to control the sensitivity allows the utilization of PDA/Zn²⁺/ZnO nanocomposites for semi-quantitative analysis of acid-base in food products. Solid-state sensors fabricated by embedding in nylon filters can determine the concentration of sorbic acid in fruit juices. The freshness of milk can be indicated via naked-eye detection as well. Our study expands the applications of PDA materials as colorimetric sensors for food industries.