(F-226) Development of a Carbon Dot Synthesis and Characterization Protocol for Undergraduate Bioengineering Courses

Briefly, CDs were synthesized using a protocol by Long et al. [1]. Sucrose was used as the carbon source, and was thoroughly mixed with phosphoric acid oxidant and water. 1 mL of phosphoric acid and 3 mL of deionized water were used per gram of sucrose. The solution was then incubated for 0.8 hours in an 80 °C water bath, and a color change to yellow or orange was observed. After incubation, ammonium hydroxide (NH4OH) passivator was added to induce CD fluorescence. After NH4OH addition the solution color darkened to deep orange or brown, and the solution continued to darken over time. Different amounts of NH4OH were added to investigate the impact of reagent ratio on CD spectral properties, which were monitored using fluorescence spectrophotometry at a 405 nm excitation wavelength and visually observed using an ultraviolet (UV) flashlight.

When different volumes of NH4OH were added relative to sucrose and phosphoric acid, CDs with visually distinct fluorescence emission under UV light were observed (Figure 1A). Samples A, B, and C used 3, 1, and 0.2 mL of NH4OH per gram of sucrose, respectively. The corresponding emission wavelength appeared to shift from green to red as the NH4OH ratio was reduced, and was accompanied by a decrease in fluorescence intensity (Figure 1B). Additional NH4OH ratios tested also followed this trend, with 2.5, 2, and 1.5 mL NH4OH CDs showing emission spectra between 3 mL and 1 mL samples (Figure 2A). When these samples were allowed to incubate with NH4OH at room temperature for an additional 48 hr, however, their fluorescence spectra appeared to normalize. This suggests that while NH4OH ratio may impact CD optical properties, rapid purification of the CDs may be required to maintain their distinct properties.   

This work confirms that CDs can be synthesized on a relevant time scale for classroom use, requiring only a few minutes of setup and less than an hour of incubation. Synthesis can be completed with accessible equipment including a stir plate, balance, and a water bath, and CD fluorescence can be observed visually with only a UV flashlight. Additional characterization using a UV-Vis spectrophotometer, however, reveals interesting impacts of synthesis conditions on CD optical properties. Future work will explore the impact of additional synthesis conditions including temperature and reaction time on CD properties, and establish CD characterization protocols using x-ray diffraction, UV-Vis, or transmission electron microscopy.