Highly conductive copper-doped nano biochar derived from black liquor lignin for rubber-based strain and liquid sensors

The emerging field of flexible electronics often relies on expensive conductive materials for sensing applications. To address this, industrially available biobased lignin was transformed into sustainable highly conductive copper-doped nano biochar (Cu@LNB) through hydrothermal coordination and carbonization in this study. Spectral and morphology analyses revealed that Cu²⁺ ions served not only as morphology-controlling agents but also as a metal source to enhance the conductivity of Cu@LNB. With the doping of Cu²⁺ ions and carbonization treatment, the structure and morphology of Cu@LNB changed significantly from irregular-shaped micron-scaled agglomerates to uniform spherical nanoparticles with an average size of 69 nm. Furthermore, Cu²⁺ ions were reduced into copper nanoparticles embedded within the carbon frameworks of Cu@LNB. After carbonization at 800℃, the conductivity of Cu@LNB reached as high as 30.2 S/m at 5 MPa. Consequently, the highly conductive Cu@LNB was integrated into a carboxy nitrile rubber matrix using latex compounding technology, constructing a 3D segregated conductive network for strain and liquid sensing. The resulting rubber-based strain sensor exhibited two linear response regions in the strain range of 0–6 % and 6–80 % with gauge factor of 172 and 1693, respectively. Moreover, the strain sensor possessed rapid response/recovery speed and high cyclic stability for human movements detection. For liquid sensing, the rubber-based sensor demonstrated distinguishable detection capabilities towards various organic liquids. Therefore, this study opens a new avenue for developing renewable and sustainable conductive nano biochar for flexible sensing applications.