From Forest to Electronics—Green Graphene for Biosensor and Applications

Laser-Induced Graphene (LIG) has recently established itself as a very attractive material for the fabrication of electrodes for multiple applications, including bioelectronics. Sparked by the discovery of graphene, a 2D material with outstanding electrical and mechanical properties, alternative synthesis techniques, that can complement traditional ones such as chemical vapor deposition (CVD) or wet-chemistry approaches, have been investigated, with LIG showing the possibility for less cumbersome production and patterning of graphenic structures. For LIG synthesis, a laser source, more commonly but not exclusively, a CO₂ laser, is focused on a carbon-based, polymeric substrate, inducing a photothermal conversion of the carbon bonds to a 3D, porous graphenic structured film of sp² hybridized carbon.<br/>Conventionally, a polyimide (PI) film is used for laser irradiation, allowing for a green, mask-free, one-step approach for both the synthesis of the material and fabrication of active structures. This decreases the burden of synthesis optimization, when compared to conventional techniques, while increasing the flexibility of patterning and fabrication of electronic elements, opening the applicability of LIG for a myriad of applications, ranging from energy harvesting and storage, mechanical actuation and sensing, electronics and more specifically, electrochemical sensor production. Other natural, carbon-based polymers, such as cellulosic materials, can also be used for LIG synthesis, and by harnessing the potentialities of this laser processing technology, naturally sourced LIG, with tuneable compositions, morphologies and conductive properties can be produced and explored for bioelectronic applications. More specifically, common, widely available paper substrate can be successfully applied for the fabrication of LIG electrodes, bringing added value to these mundane substrates and increasing the efficiency, accessibility and flexibility of production for these active electronic elements.<br/>Here, we present the adaptation of the laser irradiation process for LIG production toward the fabrication of paper-based, planar electrodes, targeting electrochemical sensor development. Through an appropriate fire-retardant chemical pre-treatment, LIG was successfully synthesized from both chromatography and office paper substrates, being fully characterized in terms of morphological, chemical and conductive properties, showing sheet resistances as low as 56 Ω sq^-1, comparable to LIG films produced from PI. Subsequently, it was successfully applied for patterning of planar electrodes, to produce on-chip, three-electrode electrochemical cells for electrochemical sensing purposes, which were fully characterized in terms of their electrochemical properties by cyclic voltammetry and electrochemical impedance spectroscopy. Electrochemical characterization indicated the applicability of these paper-based LIG cells for tracking of electrochemical reactions, showing an electrochemical active area more than doubling the geometric area, and charge transfer kinetics comparable to LIG electrodes produced from polyimide, with Heterogeneous Electron Transfer (HET) rate constant k₀ as high as 7.15 x 10^-4cm s^-1. These on-chip cells were subsequently applied for the development of bi-enzymatic glucose sensors, showing good analytical performance for these simple, on-chip glucometers. This paves the way for the development of more sustainable, accessible and alternative electronic devices and elements, with potential application for wearable bioelectronics and flexible electrochemical monitoring systems.