A 3D-microfluidic paper-based analytical device for the kinetic-catalytic colorimetric determination of vanadium with smartphone-based readout

Metal ion detection at trace levels remains challenging for paper-based analytical devices, as they often require preconcentration steps to reach low-ppb detection limits due to their limited sensitivity. This constraint reduces the overall simplicity and portability of paper-based devices, narrowing their suitability for truly filed-deployable analysis. A colorimetric kinetic-catalytic method in a three-dimensional origami microfluidic paper-based analytical device (3D-μPAD) is reported herein for the sensitive determination of vanadium. The method relies on the catalytic activity of vanadium in the oxidation of gallic acid by bromate in acidic media with formation of a colored product in the detection areas of the 3D-μPAD and smartphone-based readout. The proposed method was successfully employed using two different calibration modes (fixed-time and initial-rate), yielding limits of detection and quantification in the range 3.1-3.6 μg/L and 10.3-12.0 μg/L, respectively, with a repeatability and intermediate precision lower than 7.2% and 7.5%, respectively (expressed as relative standard deviation). The fixed-time and initial-rate methods showed sensitivities of 0.18 L/μg and 4.1·10^-4 L/μg·s, respectively, working ranges up to 500 μg/L, and a sample throughput in the range of 48-96 h^-1. The method was validated against six certified reference materials of diverse matrices (seawater, sediment, soil and industrial sludge), showing good agreement with the certified values at the 95% confidence level using both the fixed-time and the initial-rate methods (t-test; p < 0.05). The implementation of kinetic-catalytic reactions in μPADs is expected to enable further advances toward sensitive detection of metals.