A green route for lignin-derived graphene electrodes: A disposable platform for electrochemical biosensors

The tremendous growth of disposable electrode-based portable devices for point-of-care testing requires mass production of disposable electrodes in a low-cost and sustainable manner. Here, we demonstrate a green route for the conversion of biomass lignin, patterning, and reduction of the lignin-derived graphene electrodes by sequential laser lithography, water lift-off and sodium borohydride (NaBH₄) treatment, and their use for electrochemical lactate biosensors. Energy-saving and localized laser lithography converted the aromatic ring-rich lignin into porous laser-induced graphene (LIG). The conductivity and attachment of the LIG to the substrate were optimized in a factorial experiment with laser power and scan speed as variables. Characterization results revealed the conversion of partial heteroatoms (e.g., Na, S, O) into granular inorganic compounds on the LIG surface under laser treatment. Water was used as an eco-friendly solvent for the patterning of the LIG (P-LIG) by a lift-off process, where the inorganic residues and un-reacted lignin were dissolved, exposing the macro-/micro-pores in the P-LIG. NaBH₄ induced a reduction of the P-LIG (P-rLIG) resulting in improved electrochemical kinetics with lower charge transfer resistance (27.3 Ω) compared to the LIG (248.1 Ω) and the P-LIG (61.4 Ω). The porous P-rLIG served as a 3D electrode for the deposition of Prussian blue and lactate oxidase for disposable electrochemical lactate biosensors, delivering a good analytical performance towards lactate detection with a linear range up to 16 mM and a high sensitivity (1.21 μA mM^-1). These lignin-derived disposable electrodes, utilizing renewable resources together with low-energy consumption fabrication and patterning, may contribute to the sustainable manufacturing of biosensors for point-of-care and point-of-use applications.