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Formalin has been used as the preservative of fishes in the concentration range of 15–25 mgL−1. However, there have been a high frequency of violations in the optimum use of formalin levels. The consumption of fishes treated with excessive formalin levels leads to nasopharynx, leukaemia and sinonasal cancer and there is a huge demand for the development of formalin sensor. Conventional formalin sensors such as chromogenic and mass balance sensors fall short in real-time analysis due to the lack of specificity and sensitivity in the interference medium. In this context, it has been emphasized to develop a non-enzymatic electrochemical biosensor with microwave synthesized CdS nanoparticles as a nanointerface owing to its surface limited kinetics. NaCl of 1 mM was considered as an electrolyte solution in the present study. Dynamic sensing characteristics with varying formalin levels of 5–50 mgL−1 was studied in three different concentration ranges as 5–15 mgL−1 (concentration of formalin < NaCl; conversion of formalin to formic acid), 20–30 mgL−1 (concentration of formalin ∼ NaCl; equilibrium between the oxidative and reductive product), 35–50 mgL−1 (concentration of formalin > NaCl; complete oxidation of formic acid to CO 2). Hence, with the exhibition of such a dynamic sensitivity based on electrolyte, the developed biosensor acts as an electrochemical comparator showcasing a switch-like behaviour in detecting formalin levels. The threshold concentration of formalin required for the comparator effect was found to be 14.845 mgL−1. The developed biosensor, most essentially, exhibited a versatility in quantifying formalin levels in real-time fish samples. Image 1 • Electrochemical detection of formalin using CdS nanoparticles as nanointerface. • Dynamic sensitivity based on electrolyte concentration showcasing a switch-like behaviour. • A novel electrochemical comparator effect is reported. • Heterogeneous rate constant and surface coverage are consistent with comparator effect. • Threshold concentration of formalin to exhibit comparator effect is 14.845 mgL−1. [ABSTRACT FROM AUTHOR]