Bio-inspired ultrasonic microreactor for efficient synthesis of indigo-emitting carbon dots with tunable morphology and enhanced optical properties

Carbon dots (CDs) are promising fluorescent nanomaterials with broad applications in optoelectronics, healthcare, and artificial photosynthesis. However, synthesizing indigo-emitting CDs with optimal properties remains challenging due to the inefficiencies and complexity of conventional methods. This study introduces a high-performance ultrasonic microreactor inspired by biomimetic leaf vein structures to enhance CD synthesis efficiency. We developed a COMSOL Multiphysics-based optimization framework to improve flow field uniformity and examine transport dynamics within the microchannel. This framework identified key parameters, including leaf vein contours, fractal angles, depth-to-width ratios, and inlet configurations, that govern flow characteristics. Additionally, we optimized ultrasonic energy transfer by directly coupling the transducer with the microreactor, determining the optimal frequency (21 kHz) and power (100 W) for maximum reactor performance. Visualization experiments revealed how ultrasound regulates bubble dynamics, enhancing interfacial area and stabilizing suspension behavior. Using this optimized system, we synthesized indigo-emitting CDs with a maximum PLQY of 27.5 % and a narrow FWHM of ∼78 nm under 365 nm excitation. Multivariate experiments revealed how flow velocity, reaction temperature, and ultrasonic modulation influence the optical properties of the CDs. This work underscores the synergistic combination of ultrasonic energy, biomimetic design, and simulation-guided optimization, providing a solid foundation for scalable synthesis of carbon-based nanomaterials with applications in optoelectronics, healthcare, and beyond.