Carbon Dots Derived from Soft and Hard Biomass via Hydrothermal and Probe-Type Sonochemical Routes: A Comparative Study

Abstract

Carbon dots (CDs) were synthesized from nettle leaves and waste walnut shells using hydrothermal and probe-type sonochemical methods. The samples were comprehensively characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), zeta potential measurements, and elemental analysis. The results revealed that all samples consisted predominantly of amorphous and defective carbon structures with limited sp² domains. Among the synthesized samples, hydrothermally prepared nettle-derived CDs (NLHtc-CDs) exhibited the highest fluorescence quantum yield, which can be attributed to the relatively higher nitrogen content of the nettle biomass. Morphological analyses indicated that the probe-type sonochemical route favored the formation of more homogeneous carbon nanostructures for lignin-rich, hard biomass such as walnut shells (WSScc-CDs), whereas the hydrothermal method resulted in more uniform CDs for softer biomass such as nettle leaves. Under UV irradiation, the dispersions displayed distinct photoluminescence emissions: orange emission for hydrothermally synthesized nettle- and walnut-derived CDs (NLHtc-CDs and WSHtc-CDs), green emission for sonochemically synthesized nettle-derived CDs, and yellow emission for sonochemically synthesized walnut-shell-derived CDs. These observations highlight the dominant influence of precursor composition and surface chemistry on the optical properties of the CDs. Overall, the findings suggest that the precursor biomass composition may play an important role in determining the structural and optical characteristics of biomass-derived carbon dots and may have a stronger influence than the synthesis route under the investigated conditions. © 2026 Elsevier B.V.