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This article describes the device design and fabrication of two different configurations (flow-through and stopped-flow) of a sequential fluid delivery platform on a microfluidic paper-based device. The developed device is capable of storing and transporting reagents sequentially to the detection channel without the need for external power. The device comprises two components: an origami folding paper (oPAD) and a movable reagent-stored pad (rPAD). This 3D capillary-driven device eliminates the undesirable procedure of multiple-step reagent manipulation in a complex assay. To demonstrate the scope of this approach, the device is used for electrochemical detection of biological species. Using a flow-through configuration, a self-calibration plot plus real sample analysis using a single buffer introduction are established for ascorbic acid detection. We further broaden the effectiveness of the device to a complex assay using a stopped-flow configuration. Unlike other electrochemical paper-based sensors in which the user is required to cut off the device inlet or rest for the whole channel saturation before measurement, herein a stopped-flow device is carefully designed to exclude the disturbance from the convective mass transport. As a proof of concept, multiple procedures for electrode modification and voltammetric determination of serotonin are illustrated. In addition, the research includes an impedimetric label-free immunosensor for α-fetoprotein using the modified stopped-flow device. The beneficial advantages of simplicity, low sample volume (1 μL), and ability to perform a complex assay qualify this innovative device for use with diverse applications.

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A multistep paper-based analytical device (mPAD) was designed and applied to the voltammetric determination of total inorganic arsenic. The electrodeposition of gold nanoparticles on a boron-doped diamond (AuNP/BDD) electrode and the determination of total inorganic arsenic is accomplished with a single device. Total inorganic arsenic can be determined by first reducing As(V) to As(III) using thiosulfate in 1.0 mol L HCl. As(III) is then deposited on the electrode surface, and total inorganic arsenic is quantified as As(III) by square-wave anodic stripping voltammetry the potential range between -0.25 V and 0.35 V (vs. Ag/AgCl), best at around 0.05 V. Under optimal conditions, the voltammetric response for As(III) detection is linear in the range from 0.1 to 1.5 μg mL and the limit of detection (3SD/slope) is 20 ng mL. The relative standard deviation at 0.3, 0.7 and 1.0 μg mL of As(III) are 3.6, 4.3 and 3.3, respectively (10 different electrodes). The results show that the assay has high precision, a rather low working potential, and excellent sensor-to-sensor reproducibility. The method was employed to the determination of total inorganic arsenic in rice samples. Results agreed well with those obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Graphical abstract A multistep paper-based analytical device (mPAD) is described that integrates a AuNP/BDD electrode preparation step and a detection step into a single device. The AuNPs are easily deposited on the BDD electrode by applying electrodeposition potential. The total inorganic arsenic concentration in rice samples was determined by using square-wave anodic stripping voltammetry.