Variation in the responses of carbon quantum dots (CQDs) synthesized from native coconut husk and coconut husk-derived charcoal

We compared the autoclave method for the synthesis of carbon quantum dots (CQDs) from plain coconut husks and husk-derived charcoal in the current study. Although several publications reported the synthesis of CQDs from the two types of husks, no prior reports were available for direct comparison of the two CQDs. The filtrated CQDs were characterized using Excitation-Emission Matrix (EEM) spectroscopy. In comparison to plain husk, the charcoal material was effective for obtaining high concentrations of CQDs because larger amounts of the charcoal husk could be packed into a vessel for autoclave treatment. EEM provided a thorough evaluation of the correlations between the excitation and emission spectra. CQDs synthesized from charcoal and plain exhibited markedly different EEM profiles. Between the two CQDs, we observed an opposite response of fluorescent (FL) intensity and peak wavelengths to pH variation in the aqueous solvent. In contrast, there was no difference in terms of temperature dependence. As an illustration of biological applications, the quenching of papain enzyme self-FL was also investigated. When papain molecules were combined with CQDs derived from plain husk, significant quenching of the papain self-FL was observed, but this phenomenon was not observed when CQDs derived from charcoal husk were used. Our research clarified the differences of CQDs from two types of coconut husks. The knowledge is helpful to apply the CQDs for various biological applications.