Visual and label-free detection of urea based on amorphous photonic films with non-iridescent structural colors

Background: Photonic actuator materials that change their color in response to stimuli are uniquely suited for analyte sensing and visual detection. However, the changing iridescence produced by conventional photonic crystal structures in hydrogel materials can seriously confuse optical readouts, thus limiting their widespread application. Results: Here, we report novel visual urea sensors with angle-independent structural colors. The urease-modified hydrogels were constructed into amorphous photonic structures. Based on the urease catalytic action induced charge variation within the hydrogel matrix, it leads to the volume change and corresponding diffraction wavelength shift of the photonic hydrogel biosensors, thereby enabling the stable and repeatable detection of urea. The response range of the sensors to urea solution ranged from 0 to 80 mM, with a linear range of 1-10 mM and a detection limit of 0.36 mM.