Renewable, cheap and green biomass resources could meet the urgent need for producing graphene quantum dots (GQDs) on a large scale if high quality can be obtained. Here we report the gram-scale synthesis of single-crystalline GQDs derived from lignin biomass via a two-step method. The synthetic processes involve the oxidized cleavage step followed by the aromatic refusion step of alkali lignin molecules. Notably, this approach successively converts the biomass commonly considered as waste into a high-valued nanoscale product. The as-prepared single-crystalline GQDs, presenting a hexagonal honeycomb graphene network, are 1–3 atomic layers thick. Due to the bright fluorescence, up-conversion properties, long-term photostability, water solubility and biocompatibility, these high quality GQDs have potential to be excellent nanoprobes for multicolour bioimaging. The utilization of renewable biomass resources paves the way for green, low-cost and large-scale production of high quality GQDs and allows for the development of sustainable applications.

A lignosulfonate-modified graphene hydrogel (LS-GH) for Pb(II) adsorption was synthesized through a one-step method. The three-dimensional (3D) porous architecture of the graphene hydrogel was functionalized by using lignosulfonate with diverse oxygen containing groups. Benefiting from a large specific surface area, multiple porosity and sufficient active sites, the LS-GH adsorbent exhibited ultrahigh adsorption capacity (1210 mg g⁻¹) for Pb(II) removal, which was among the highest of previously reported Pb(II) adsorbents. Importantly, the free-standing and flexible LS-GH can be used as a column-packed device, providing an efficient pathway for the fast removal of Pb(II) with an ultrahigh adsorption capacity of 1308 mg g⁻¹ within 40 min. The high capacity LS-GH adsorbent is low cost, eco-friendly and recyclable, which could be an attractive adsorbent for the purification of wastewater on a large scale.

Emerging flexible supercapacitors have motivated tremendous research interest in portable energy devices. However, challenges still exist in the pursuit of cheap and renewable electrode materials. Herein, a metal-free and flexible supercapacitor was fabricated based on lignosulfonate functionalized graphene hydrogels (LS-GHs). The supercapacitor shows comparable or even higher performance than reported transition metal based pseudocapacitive supercapacitors, which can be attributed to the reversible redox charge transfer of quinone groups in lignin. It presents an impressive specific capacitance of 432 F g⁻¹ in an aqueous electrolyte, which is nearly 2 times higher than that of a pure graphene hydrogel (238 F g⁻¹). Moreover, the device exhibits high rate capability (81.0% capacitance retention at 20 A g⁻¹) and cycling stability (90.0% capacitance retention over 10 000 cycles). The resulting LS-GH electrodes are further fabricated into a flexible solid-state supercapacitor using H₂SO₄–polyvinyl alcohol (PVA) gel as the electrolyte. The integrated flexible device not only maintains high capacitive performances (408 F g⁻¹ at 1 A g⁻¹, 75.4% capacitance retention at 20 A g⁻¹ and 84.0% capacitance retention over 10 000 cycles), but also exhibits excellent mechanical flexibility. This work paves the way to develop flexible energy devices based on metal-free, renewable and low-cost biomass resources.