Acoustic Wave Catalyzed Urea Detection Utilizing a Pulsatile Microdroplet Sensor

We observe variations in the electrical resistance across a conducting water microdroplet when it was placed on a glass substrate before being mechanically vibrated at natural frequency with the help of an acoustic source. The reduction in the resistance across the droplet was magnified owing to the formation of vortices in the matrix when the periodic oscillation of the surface was increased. The variation in the resistance could be tuned with the frequency of the sound source, which was found to be maximum when a 10 μL droplet was vibrated at ∼320 Hz. Interestingly, the variation in resistance across the oscillating droplet could follow and distinguish the musical notes in the octaves (“sur”) or rhythmic cycles (“taal”) originating from musical instruments such as flute, harmonium, whistle, and tabla. Further, when a suspension of urease-stabilized gold–cadmium–sulfide nanocomposite was suspended inside the droplet and mixed with an analyte containing urea solution, the change in the resistance during the operational time period was found to monotonically vary with the concentration of urea in the analyte. The enzymatic reaction between urea and urease was found to follow a faster first-order chemical kinetics than the commonly observed Michaelis–Menten pathway owing to the presence of the moving nanocomposites and mixing vortices under the optimal acoustic excitations. The specific lock-and-key enzymatic reaction helped in extending these experimental results to estimate the unknown levels of urea in human blood serum samples.