Sustainable hydrothermal synthesis of carbon dots from cyanobacterial and microalgal biomass residues

We report a sustainable and straightforward hydrothermal synthesis of carbon dots (CDs) derived from biomass residues of the microalga Chlorella vulgaris and the cyanobacterium Leptolyngbya boryana . Three feedstocks were investigated: C. vulgaris post-lipid extraction (CVR), untreated L. boryana biomass (LBB), and its methanol-extracted residue (LBR). The resulting CDs formed stable aqueous colloids with negative zeta potentials and exhibited significant variations in size and optical properties, depending on the precursor nature. Dynamic Light Scattering (DLS) revealed average hydrodynamic diameters of ∼2 nm for LBB and LBR, and ∼16 nm for CVR-derived CDs. Photoluminescence analysis indicated that the emission behavior is predominantly governed by surface-state transitions rather than size-dependent band-to-band transitions with quantum confinement, as evidenced by similar emission maxima (400–445 nm) despite pronounced size differences. All CDs samples exhibited high photostability under continuous excitation, while their photoluminescence quantum yields remained in the moderate range (3.22% to 4.27%). This work highlights the potential of phytoplankton biomass waste as a low-cost, renewable carbon source to produce photoluminescent nanomaterials, underscoring the principles of green chemistry and a circular economy.