Biochar as an alternative sustainable platform for sensing applications: A review

Biomasses are emerging as alternatives to traditional synthetic carbon nanostructures to produce greener, carbon based, sensing platforms. More recently, biochar, a porous carbonaceous material produced by direct (i.e. solvent free) pyrolysis of biomasses, was also investigated, leading to a further decrease in the fabrication environmental footprint. Moreover, valuable byproducts (biogas and bio-oil) are also produced, in line with the atom economy principle of green chemistry. Here we review more than 100 papers published in the last five years describing biochar-based sensing platforms. The fabrication processes, tailoring and analytical applications of biochar-based sensors are presented and discussed aiming at shedding light on biochar potentialities in replacing less sustainable traditional materials. Electrochemical platforms were mainly developed (70% of the published papers), but also optical and other sensing mechanisms (electrogenerated luminescence and resistivity) were exploited. Analytical performances for biochar-based sensors are often reported as comparable to those of the best carbon-based sensing platforms, mainly thanks to the very high specific surface area and the presence of reactive surface functional groups (self-doping). Biochar decoration with metal/metal oxide nanoparticles and enzymes is also employed to further improve its sensing properties. Notwithstanding biochar applications were only recently introduced and mechanisms behind their selectivity and sensitivity are still under investigation, their analytical performances and greenness foster biochar employment as sensing platform.