Soap film as a rapidly renewable and low-cost sensor for detecting ammonia in water and saliva

In this work, a novel, low-cost soap-film sensor for detecting ammonia in environmental water and saliva was developed. A 3D printed circular frame with two stainless steel electrodes was implemented as a carrier for a soap film. The soap film was manually generated by pulling the 3D printed frame from a mixture solution containing 20.7 g/L gelatin, 241.4 mL/L Glycerol, 34.5 mL/L Triton X-100 and 100 mM boric acid . The conductivity of the soap film was measured with a lab-made detector. For liquid sample analysis, sample and NaOH solution were mixed in a reagent bottle as the gas-diffusion vessel (GDV). The released ammonia from the reaction was then diffused into the headspace of the GDV and absorbed by the soap film. The conductivity increasing rate of the soap film was used for quantifying the ammonium concentration in sample. The analytical performance parameters were demonstrated, including a linear range of 0–500 μM, a relative standard deviation of 3.2 % ( n  = 10), and a limit of detection of 14 μM (0.2 mg N/L). This method showed a 90–110 % recovery rate for saliva and water samples. Compared to a reference indophenol blue method, the relative deviations of two method were less than 7.4 % ( n  = 33). The proposed method is easy to operate, and the device itself is compact and low cost. The soap-film sensor enables rapid renewal to avoid contamination.