A smartphone-based semiquantitative system for chlorpyrifos detection via digital image analysis of enzymatic paper microfluidic cards: proof of concept for organophosphate screening

OBJECTIVES: The need for rapid, point-of-need screening of pesticide residues in resource-limited settings requires moving beyond subjective visual interpretation of colorimetric tests. Organophosphate pesticides pose significant public health risks, especially in low- and middle-income countries, where laboratory-based detection methods are often inaccessible. METHODS: We developed an integrated system combining paper-based microfluidics, smartphone imaging, and chemometric modeling. A three-zone paper microfluidic card was engineered to incorporate an acetylcholinesterase inhibition assay, along with internal positive and negative controls. A 3D-printed imaging enclosure with controlled LED lighting standardizes smartphone image capture. Images were processed via an automated pipeline to extract CIELAB color features, and a partial least squares regression (PLSR) model was calibrated to predict chlorpyrifos concentration. RESULTS: The system achieved a limit of detection of 0.35 mg/L and a limit of quantification of 1.06 mg/L. Validation on an independent set yielded a root-mean-square error of prediction of 0.82 mg/L and a mean absolute error of 12.7%. Recovery rates in spiked lettuce extract ranged from 89% to 110%. The a* channel in CIELAB space showed the strongest correlation with concentration, and the PLSR model performed robustly for semiquantitative screening. CONCLUSIONS: This smartphone-based system transforms a qualitative enzymatic assay into a reliable, semiquantitative screening of organophosphate (chlorpyrifos) residues in resource-limited settings such as Sierra Leone. As a laboratory-validated prototype, it requires future field testing before operational deployment and validated system performance in terms of detection limits, accuracy, precision, and robustness against matrix effects in a representative vegetable matrix (lettuce extract).