The development of three-dimensional (3D) porous graphitic structures is of great interest for electrochemical sensing applications as they can support fast charge transfer and mass transport through their extended, large surface area networks. In this work, we present the facile fabrication of conductive and porous graphitic electrodes by direct laser writing techniques. Irradiation of commercial polyimide sheets (Kapton tape) was performed using a low-cost laser engraving machine with visible excitation wavelength (405 nm) at low power (500 mW), leading to formation of 3D laser-induced graphene (LIG) structures. Systematic correlation between applied laser dwell time per pixel (“dwell time”) and morphological/structural properties of fabricated electrodes showed that conductive and highly 3D porous structures with spectral signatures of nanocrystalline graphitic carbon materials were obtained at laser dwell times between 20 and 110 ms/pix, with graphenelike carbon produced at 50 ms/pix dwell time, with comparable properties to LIG obtained with high cost CO2 lasers. Electrochemical characterization with inner and outer sphere mediators showed fast electron transfer rates, comparable to previously reported 2D/3D graphene-based materials and other graphitic carbon electrodes. This work opens the way to the facile fabrication of low-cost, disposable electrochemical sensor platforms for decentralized assays.

Interdigitated and square laser-induced graphene (LIG) electrodes were successfully fabricated by direct laser writing of common natural cork bottle stoppers. The laser graphitization process was performed with a low-cost hobbyist visible laser in a simple, fast, and one-step process under ambient conditions. The formation of LIG material was revealed by extensive characterization using Raman, attenuated total reflection-Fourier transform infrared (ATR-FTIR), and X-ray photoelectron (XPS) spectroscopies. Electron microscopy investigation showed that the formed LIG structure maintained the hierarchical alveolar structure of the pristine cork but displayed increased surface area, disorder, and electrical conductivity, promising for electrochemical applications. Open planar and sandwich supercapacitors, assembled from fabricated electrodes using poly(vinyl alcohol) PVA/H + as an electrolyte, exhibited a maximum areal capacitance of 1.56 mF/cm 2 and 3.77 mF/cm 2 at a current density 0.1 mA/cm 2 , respectively. Upon treatment with boric acid (H 3 BO 3 ), the areal capacitance of the resulting boron-doped LIG devices increased by ca. three times, reaching 4.67 mF/cm 2 and 11.24 mF/cm 2 at 0.1 mA/cm 2 current density for planar and sandwich configurations, respectively. Supercapacitor devices showed excellent stability over time with only a 14% loss after >10 000 charge/discharge cycles. The easy, fast, scalable, and energy-efficient method of fabrication illustrated in this work, combined with the use of natural and abundant materials, opens avenues for future large-scale production of "green" supercapacitor devices.

Porous graphitic carbon electrodes were fabricated by laser scribing of commercial polyimide tape. The process was performed by a simple one-step procedure using visible wavelength laser irradiation from a low-cost hobbyist laser cutter. The obtained electrodes displayed a highly porous morphology, rich in three-dimensional (3D) interconnected networks and edge planes, suitable for electrochemical sensing applications. Spectral characterization by Raman and X-ray photoelectron spectroscopy (XPS) techniques revealed a crystalline graphitic carbon structure with a high percentage of sp2 carbon bonds. Extensive electrochemical characterization performed with outer-sphere [Ru­(NH3)6]3+ and inner-sphere [Fe­(CN)6]4–, Fe2+/3+, and dopamine redox mediators showed quasi-reversible electron transfer on the graphitic carbon surface, mainly dominated by a mass diffusion process. Fast heterogeneous electron-transfer rates, higher than similar carbon-based materials and higher than other graphitic carbon electrodes produced by either visible or infrared laser irradiation, were obtained for these electrodes. Thin-layer transport mechanisms occurring in parallel to the main diffusion-limited mechanism were taken into consideration, but overall, the observed enhanced electron-transfer rate effects were ascribed to the large specific surface area of the extended 3D porous network, rich in defects and edge planes. The superior electrocatalytic properties of the fabricated electrodes allowed electrochemical differentiation between the biomarkers ascorbic acid, dopamine, and uric acid in solution. The compatibility of fabricated electrodes with lightweight portable and handheld instrumentation makes such electrodes highly promising for the realization of low-cost disposable sensing platforms for point-of-care applications.

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Ultra-sensitive and responsive humidity sensors were fabricated by deposition of graphene oxide (GO) on laser-induced graphene (LIG) electrodes fabricated by a low-cost visible laser scribing tool. The effects of GO layer thickness and electrode geometry were investigated. Sensors comprising 0.33 mg/mL GO drop-deposited on spiral LIG electrodes exhibited high sensitivity up to 1800 pF/% RH at 22 °C, which is higher than previously reported LIG/GO sensors. The high performance was ascribed to the high density of the hydroxyl groups of GO, promoted by post-synthesis sonication treatment, resulting in high water physisorption rates. As a result, the sensors also displayed good stability and short response/recovery times across a wide tested range of 0–97% RH. The fabricated sensors were benchmarked against commercial humidity sensors and displayed comparable performance and stability. Finally, the sensors were integrated with a near-field communication tag to function as a wireless, battery-less humidity sensor platform for easy read-out of environmental humidity values using smartphones.