<p>The widely accepted application of polylactic acid (PLA) films as agricultural mulch films has led to the accumulation of microplastics (MPs) in soil ecosystems, causing severe white pollution. Developing a convenient method to detect PLA in the environment is essential for pollution control. In this study, single-atom nickel-doped nitrogen on carbon nanosheets (Ni–N-C) combined with gold–palladium nanoparticles (AuPdNPs) enabled highly sensitive lactic acid detection. Through oxidation catalyzed by the single-atom metal, the detection range for lactic acid was 50 μmol·L⁻¹ to 50 mmol·L⁻¹, with a detection limit of 10 μmol·L⁻¹. This approach indirectly quantified PLA in MPs via lactic acid detection on Ni–N-C + AuPdNPs. This is the first electrochemical method to indirectly determine depolymerized polylactic acid for environmental PLA quantification. The detection current showed minimal fluctuation over 7 days and remained unaffected by common soil interferents. It demonstrates strong applicability in real environments by indirectly assessing MP residues through their depolymerization products, avoiding complex separation processes. High PLA recovery rates in water and soil highlight its potential for environmental monitoring. A stable electrochemical detection of microplastic monomers was provided for such detection approaches.</p> Graphical Abstract <p></p>