Multi-folding paper-based fluidic device for bimodal quantum dot-linked duplex immunoassay of cancer biomarkers

In this work, we report the development of an integrated, multi-folding paper-based device for the quantum-dot (QD)-based simultaneous detection of two cancer biomarkers—carcinoembryonic antigen (CEA) and cancer antigen 125 (CA125)—in serum using dual-mode fluorescence–voltammetric sensing. The device comprises a fluidic conduit with a single sample zone connected to two spatially separated assay zones, positioned adjacent to a circular three-electrode electrochemical cell. Two sandwich immunoassay formats are implemented in the assay zones using PbS- and CdS-based QD–labeled reporter antibodies as barcode probes. Fluorescence readout is performed in situ under UV illumination with smartphone-based image acquisition. For electrochemical readout, the immunocomplexes are dissolved to release Pb(II) and Cd(II) ions; the assay zones are subsequently folded onto the adjacent electrochemical cell, enabling vertical elution of the metal ions, which are detected by anodic stripping voltammetry. By synergistically integrating the origami-inspired folding with vertical elution, the dual optical and electrochemical activity of QD barcode labels, and a bismuth-modified transducer, this work establishes the first bimodal quantum dot–linked duplex paper-based immunosensing platform. The device offers clear advantages over existing paper-based immunosensors, underscoring its potential for point-of-care (PoC) diagnostic applications. • The first bimodal quantum-dot linked duplex paper-based immunosensor for CEA and CA 125 in serum is developed • The paper device consists of a fluidic conduit adjacent to a circular 3-electrode electrochemical cell. • Two sandwich immunoassay protocols are implemented in the two spatially separated assay zones • Fluorescence detection is performed in situ by UV illumination and smartphone image capture. • For electrochemical detection, the device is folded, the quantum dots are dissolved and the metals are measured by stripping voltammetry