Smart MDD—A portable optical device for rapid, automated, and ultra-sensitive detection of malathion in liquid samples

Pesticides are often used in agriculture to reduce post-harvest losses due to contamination and to increase productivity. Long-term exposure to these pesticides in food leads to serious health issues in humans and animals. Advanced sensing techniques are crucial for detecting pesticide traces in agricultural products present in low amounts. This study demonstrates an aptamer-based colorimetric assay for detecting organophosphorus pesticides, namely, malathion. In the absence of malathion, the aptamer binds with cationic polymer PDDA, preventing its aggregation with gold nanoparticles. Upon binding with malathion, the PDDA is left free, forming aggregation with AuNPs, resulting in a color change from red to blue. This assay is integrated into a smart optical detection device with a built-in display for standalone operations. The optical absorbance ratio (518/633 nm) was utilized as a marker to detect malathion traces in water, achieving a limit of detection of 248.36 pM within the quantification range (100-1000 pM) and a sensitivity of 0.0015 a.u./pM. Polynomial regression models were applied to compare the performance of the spectrophotometer and the device, yielding R2 values of 0.9468 and 0.9489, demonstrating a strong correlation between the intensity ratio and malathion concentration. A predictive model developed using polynomial regression to estimate malathion concentration based on the device's measured intensity ratio achieved a root mean square error of 9.85%. These findings highlight the potential of the developed device for accurate and reliable pesticide detection. The portability and cost-effectiveness promise its use for on-site monitoring in environmental and precision agriculture settings.