Ultrasonication-assisted carbonization of Gracilaria fisheri: effects on physicochemical, and structural properties, and bioactivities.

Conventional hydrothermal carbonization of biomass generally produces carbon dots (CD) with low yield and variable quality, mainly due to incomplete hydrolysis. This study evaluated how ultrasonication pretreatment and sequential pre/post-treatment influenced hydrothermal carbonization of Gracilaria fisheri and the properties of the resulting CD. Probe ultrasonication (Sonics VC750, 20 kHz, 13 mm probe) was applied to 50 mL suspensions at 20, 50, and 80% amplitude for cumulative ultrasonication-on time of 5 or 15 min, using 5 s on/5 s off pulses and ice-bath cooling. Hydrothermal carbonization was performed at 200 °C for 8 h. Recovered-solid yield and physicochemical parameters were used to select optimal conditions. PreCD80%/5min, which showed the highest yield and zeta-potential among PreCD was selected for subsequent post-treatment. Pre/PostCD further increased the recovered-solids yield to 90.47% (Pre/PostCD80%/5min). FTIR spectra confirmed the retention of oxygen-rich functional groups, while TEM images showed smaller, more dispersed particles following sequential treatment. Pre/PostCD80%/5min exhibitedstronger UV-C attenuation and excitation-dependent photoluminescence, whereas PreCD80%/5min showedgreater photostability under 254 nm irradiation. Pre/PostCD80%/5min exhibited the highest antioxidant activity, including an ABTS radical-scavenging activity of 68.03 mmol Trolox equivalent/mg and a metal-chelating ability of 0.13 mmol ethylenediaminetetraacetic acid equivalent/mg.All tested CDs exhibited minimum inhibitory concentrations of 3.50 mg/mL against Staphylococcus aureus, Listeria monocytogenes, Vibrio parahaemolyticus, and Pseudomonas aeruginosa. These findings demonstrate that the sonication sequence influenced the recovered-solids yield and bioactivities. The effects of ultrasonication on antioxidant and antibacterial activities warrant further investigation to clarify the associated changes in material properties and functionality. The produced CD requires validation in food applications and a safety assessment.