From peels and coffee grounds to tunable carbon nanodots: waste-derived biocarbon meets ultrafast laser ablation

Water-soluble, photoluminescent carbon nanodots (CNDs) were produced in minutes by picosecond-laser ablation of biocarbons obtained from orange peel, avocado peel, and spent coffee grounds. Mild pyrolysis at 350 °C first transformed the wastes into ash-free biocarbon with distinct degrees of aromatic ordering. Subsequent irradiation with 1064 nm, 150 ps laser pulses in water generated stable brown colloids whose optical and structural characteristics could be traced back to the precursor composition. Avocado-derived dots were the most abundant and smallest (2.2 ± 0.3 nm), displaying amorphous structure and bright blue-green emission centred at 430 nm under 330 nm excitation. In contrast, orange- and coffee-derived dots were larger (5-40 nm), partially graphitic, and markedly less emissive. Thermogravimetric and Raman analyses linked the superior yield and fluorescence of the avocado system to its high lignin content and oxygen-rich surface, favoring efficient laser fragmentation and creating abundant emissive surface traps. Excitation-dependent PL arises from these functional groups rather than size-quantisation, as corroborated by FTIR, XPS, and multi-peak PL deconvolution. The entire process avoids harsh chemicals, produces intrinsically water-dispersible nanodots, and valorises low-value biomass, offering a scalable, environmentally benign alternative to conventional hydrothermal or acid-oxidative syntheses. This work provides both mechanistic insight and practical guidelines for tuning CND performance through intelligent waste selection, opening new avenues for sustainable bio-imaging, sensing, and optoelectronic applications. This work establishes a direct feedstock-to-function relationship and positions ultrafast laser processing as a versatile, green platform for tailoring CND performance through rational waste selection.