Convective Flow-Assisted Catalytic Propulsion of Polymer Beads Coated with Pd Nanoparticles for the Detection of Enzyme Inhibitors and Industrial Effluents

Ultrasensitive detection of enzyme inhibitors and heavy metal contaminants in effluents constitutes a major interdisciplinary research area. Multiple methods based on colorimetry, conductance measurement, reactivity estimation, etc., have been developed in this direction, which often involve the use of sophisticated equipment and complex methodologies. Herein, we present a simple, table-top technique for the detection of enzyme inhibitors and industrial effluents using convective flow-assisted propulsion of catalytic palladium nanoparticle (NP)-coated polymer motors in solution. The motors display buoyancy-driven self-propulsion, which is further enhanced in the presence of secondary catalysts like catalase and horseradish peroxidase, generating auxiliary convection in the experimental chamber. The convection and the motor propulsion were sensitive toward molecules that inhibited the enzyme activity, thereby offering a platform for using NP-coated catalytic motors as dynamic sensing probes for harmful chemicals in liquids. Initially, effective sensing of trace amounts of Cu²⁺, Ag⁺, and Cd²⁺ ions was performed based on the reduced propulsion of the catalytic motors through the fuel solution. The detection limit of Cu²⁺ in the aqueous environment was found to be less than the permissible limit suggested by the Central Ground Water Board (CGWB), Government of India. Experiments were also conducted to demonstrate the generic applicability of motor-based detection of different enzyme inhibitors (in the micromolar concentration range), such as a flavonoid (hydroquinone) and an aromatic dipeptide (diphenylamine). Finally, it was demonstrated that the catalytic NP-coated polymer motors could also be used effectively to detect industrial effluents using samples containing different compounds that acted as enzyme inhibitors. A simple model connecting the enzyme inhibition and motor propulsion speed is presented to explain the experimental observations.