(79c) Photothermal Conversion Efficiency of Multi-Color Emissive Carbon Dots: A Chemical and Thermal Analysis

A series of highly stable, cost-effective, and environmental-friendly multi-color emissive carbon dots with systematically tailored chemical and physical properties were synthesized through a modified acid reagent engineered hydrothermal synthesis protocol. Detailed optical and thermal characterizations together with computational modeling revealed their remarkable photothermal (light-to-heat) conversion performance and the underlying mechanisms governing this observation were studied and discussed. Results showed a significantly higher light-to-heat conversion in red color emitting carbon dots compared to their blue, cyan, green, and yellow counterparts. Apparently, more acid treatment during the fabrication of carbon dots enhances the aromatization and introduces stronger electron donating functional groups (such as C-O-C, and C═O groups) into the lattice which gives rise to a more conjugated and oxidated carbon dot structure. In return, the HUMO and LUMO band gaps will get smaller which destabilizes the electrons in π-π* orbitals and significantly increases the frequency of electron-phonon coupling in carbon dots. This phenomenon (similar to localized surface plasmon resonance) initiates a thermal vibration in the carbon dot lattice (the phonon-phonon coupling) which eventually dissipates heat into the surrounding media that we interpret as temperature increase in macroscopic scale. The highest thermal gradient observed was 11 °C/s in red color emitting carbon dots when illuminated with a blue laser (350 mW, continuous beam). Finally, these nanocarbons were employed as efficient photothermal nanomaterials to achieve ultrafast thermocycling for quantitative real-time PCR owing to their potential of implementing 30 cycles of 95 °C to 60 °C in less than 10 minutes. This work will set the first stage for bandgap engineering to utilization of carbon dots as efficient and effective nanomaterials for forward-looking photothermal applications and studies.