Ultrafast synthesis of carbon quantum dots from fenugreek seeds using microwave plasma enhanced decomposition: application of C-QDs to grow fluorescent protein crystals

Herein, we present the rapid synthesis of mono-dispersed carbon quantum dots (C-QDs) via a single-step microwave plasma-enhanced decomposition (MPED) process. Highly-crystalline C-QDs were synthesized in a matter of 5 min using the fenugreek seeds as a sustainable carbon source. It is the first report, to the best of our knowledge, where C-QDs were synthesized using MPED via natural carbon precursor. Synthesis of C-QDs requires no external temperature other than hydrogen (H2) plasma. Plasma containing the high-energy electrons and activated hydrogen ions predominantly provide the required energy directly into the reaction volume, thus maximizing the atom economy. C-QDs shows excellent Photoluminescence (PL) activity along with the dual-mode of excitation-dependent PL emission (blue and redshift). We investigate the reason behind the dual-mode of excitation-dependent PL. To prove the efficacy of the MPED process, C-QDs were also derived from fenugreek seeds using the traditional synthesis process, highlighting their respective size-distribution, crystallinity, quantum yield, and PL. Notably, C-QDs synthesis via MPED was 97.2% faster than the traditional thermal decomposition process. To the best of our knowledge, the present methodology to synthesize C-QDs via natural source employing MPED is three times faster and far more energy-efficient than reported so far. Additionally, the application of C-QDs to produce the florescent lysozyme protein crystals “hybrid bio-nano crystals” is also discussed. Such a guest–host strategy can be exploited to develop diverse and complex "bio-nano systems". The florescent lysozyme protein crystals could provide a platform for the development of novel next-generation polychrome luminescent crystals.


1.
Synthesis of C PY -QD using thermal decomposition.

2.
Optical emission spectra (OES) of the excited hydrogen plasma.       Table: Quantitative Analysis of XPS analysis of as-synthesized C PE -QDs. Table S1-Wide scan of as-synthesized C PE -QDs. Table S2-Peak fitting results obtained after deconvolution of carbon peak. Table S3-Peak fitting results obtained after deconvolution of oxygen peak. Table S4-Peak fitting results obtained after deconvolution of nitrogen peak. 9. Deconvolution of nitrogen peak N 1s (XPS analysis) of as-synthesized C PE -QDs.    12. XPS analysis of Fenugreek-seeds and as-synthesized C PY -QDs.     17. TEM images of multifaceted shape of C PY -QDs synthesized by thermal decomposition method. 18. PL emission spectra of C PY -QDs excited at various energies. 19. Comparison of PL spectra of as synthesized C PE -QDs and C PY -QDs. 20. FTIR spectra of as synthesized C PY -QDs.  22. Summery of the various synthesis techniques using natural carbon sources. Table S13 23. References

Synthesis of C PY -QD using thermal decomposition
Carbon Quantum dots were synthesized by thermal decomposition (pyrolysis) method. As received fenugreek seeds were crushed using a mixer grinder (Tiger mixer grinder, Japan).
Ground fenugreek powder (0.2 g) was transferred to the crucible cup (AS ONE, Japan) and was heated using a heat plate (AS ONE, Japan) at a constant temperature of 500 °C for 3 hours. Subsequently, the crucible was allowed to cool down to room temperature.
Carbonization of the fenugreek powder turned into a dark-gray product, and it was dissolved in deionized water followed by the sonication for 5 minutes. The black color suspension was centrifuged at 15000 rpm for 10 minutes to remove the large un-dissolved particles. The supernatant was filtered using 100 nm pore size filter (PALL ACRO DISC, Japan) and subjected to the dialysis using dialysis kit (Float-A-Lyzer G2 Dialysis, Japan) for further purification. The purified C PY -QDs was transferred to the glass vial and stored for further characterization 1 .

Optical emission spectra (OES) of the excited hydrogen plasma.
Fig. S1: The OES spectra show peaks at 658.2 486.9, 434.7 and 463.8nm that assigned to H α , H β, H γ and secondary hydrogen, respectively.

Fig. S3:
Higher resolution TEM image of C PE -QDs shows that C PE -QDs have crystalline structure and the distance between the lattices fringes is 0.21 nm assigned to (100) plane. 7. XPS analysis of as-synthesized C PE -QDs. 8. Table: Quantitative Analysis of XPS analysis of as-synthesized C PE -QDs.

PL emission spectra of C PY -QDs excited at various energies.
Fig. S14: PL emission spectra of C PY -QDs excited at various energies (260, 280, 300, 320, 340, 360 and 380nm, C PY -QDs were found to be nearly independent on the excitation energy.

19.
Comparison of PL spectra of as synthesized C PE -QDs and C PY -QDs.

FTIR spectra of as synthesized C PY -QDs
Fig. S16: FTIR spectra of as-synthesized C PY -QDs, Carbon quantum dots have mainly the C=O, OH and C-O peaks, respectively.

Effect of pH on PL of as synthesized C PY -QDs (Environmental stability)
Fig. S17: PL emission spectra of C PY -QDs dispersed in water at various pH (acidic to basic) (excitation wavelength 320 nm). C PY -QDs were found to be independent of the excitation energy.