Alginate-based hydrogels embedded with ZnO nanoparticles as highly responsive colorimetric oxygen indicators

Abstract

A hydrogel-based colorimetric oxygen indicator that exhibits high photoreduction efficiency and fast oxygen response was produced by integrating ZnO nanoparticles and a redox dye into an alginate hydrogel matrix. We found that varying the ZnO and glycerol [external sacrificial electron donor (SED)] content significantly affected the swelling behavior and strength of the hydrogels. We also found that retained water molecules and glycerol in the hydrogel serve as good electron donors, whereas ZnO nanoparticles are effective photocatalysts that affect the decoloration. By irradiating the hydrogel with UV light for 20 min, the UV intensity can induce the acceleration of the ZnO nanoparticles, creating photogenerated pores that are captured by the SEDs. Consequently, the photogenerated electrons rapidly reduced methylene blue to leuco-methylene blue, causing a color change from blue to colorless. Utilizing their porous network structures that facilitate oxygen diffusion, the hydrogel-based oxygen indicators exhibit fast recoloration (∼3 min) under a continuous ambient air (20 vol% O₂ in nitrogen gas) flow rate of 500 mL min⁻¹. The repeatability performance was improved by glycerol, illustrating the role of the external SED in our hydrogel-based oxygen indicator. We believe that the current study highlights the potential for the development of stimuli-responsive hydrogels as colorimetric oxygen-sensing materials.