Fig. 3: Illustration of urea detection by the pH-responsive superwetting surface.

a Schematic representation of urea detection based on the OH⁻ released from the urease–catalyzed reaction. The surface could be reused after washing and drying. (b) Sensitivity test with different concentrations of urea (100, 200, 400, 600, 800 nM and 1, 10, 100, 200, 400, 600, 800, 1, 2, and 3 mM). c Linear relationship between the CA and urea concentrations ranging from 100 nM to 1 µM. d Wetting states of droplets with different concentrations of urea within the linear range. e Relationship between the change in the CA (∆CA = | CA − CA₀ |, where CA and CA₀ are the CAs of the experimental group and the blank control, respectively) and interfering agents (Na⁺, K⁺, ascorbic acid, glucose, and glycine). All substances were tested at 1 mM with 10 U mL⁻¹ of urease at 37 °C for 10 min of incubation. The initial pH was 3. The volume of the test droplet was 2 µL. Error bars refer to the standard deviation of three replicates