In-Series Production of Integrated Bi-Doped Laser-Induced Graphene Sensors for Simultaneous Detection of Zn(II), Cd(II), and Pb(II)

Laser-induced graphene (LIG) is a breakthrough material for electrochemical sensing, and its sensing ability can be tuned through targeted modifications. Herein, a xurography/laser approach is proposed to fabricate in-series, self-contained LIG sensors incorporating bismuth (Bi-LIG), conceived for the simultaneous detection of Zn(II), Cd(II), and Pb(II). The Bi-LIG sensors were assembled using poly(vinyl alcohol)-assisted Bi doping and xurography, without the need for polyimide chemical pretreatment. The manufacturing process relies on forming a PVA layer incorporating Bi3+ on a polyimide foil via stencil printing, followed by laser-induced formation of the Bi-doped LIG sensing layer, the counter electrode, and contact tracks; the sensors were completed by stencil printing to form reference electrodes and by thermal lamination to insulate. The formation of the Bi-LIG sensing layer was carefully studied, examining the effects of laser power and Bi content. After determining the optimal laser power to form Bi-LIG, Bi precursor concentration revealed a nonmonotonic effect on electrochemical performance, with electrochemical impedance spectroscopy and Raman analysis showing an optimal concentration of 20 mM, characterized by lower charge-transfer resistance and reduced Raman-active disorder. Morphochemical analyses (SEM, EDX, XPS) confirmed the presence of highly dispersed, predominantly oxidized Bi species, with no visible nanoparticle formation. Bi-LIG sensors enabled simultaneous, reproducible detection of Zn(II), Cd(II), and Pb(II) via square-wave anodic stripping voltammetry (RSD ≤ 10%; n = 3), yielding submicromolar LODs (≤0.1 μM) and well-resolved peaks. These sensors achieved quantitative recoveries of the three metals in artificial sweat (Rec 95–112%; RSD ≤ 12%; n = 3).