TITLE: Green Synthesis of Fluorescent Carbon Dots from Sweet Basil (Ocimum basilicum) Leaves via Hydrothermal Method
AUTHORS: Hugo Monreal-Contreras, Manoj-Kumar Arthikala, Ravichandran Manisekaran

[DESCRIPTION]
Carbon-based materials play significant roles in the development of material science. Carbon dots (CDs), a new rising star in the carbon family. Carbon dots (C-dots) have gained more attention among researchers due to their outstanding fluorescent property which can be tuned based on several factors like high photostability, excellent water solubility, tunable fluorescence and optical properties, high quantum yield (QY), low toxicity, good biocompatibility, and environmental friendliness. One among them is based on a carbon source, so scientists have widely investigated several synthetic and natural materials to produce C-dots for diverse applications. Among these approaches, hydrothermal/carbonization treatment is frequently applied for the preparation CDs because of the outstanding advantages, such as high yield, simple manipulation, easy control, uniform products, lower air pollution, low energy consumption and so on. In this protocol we intend to use the sweet basil, scientifically known as Ocimum basilicum leaves as a precursor to produce C-dots using water as a solvent by adopting the hydrothermal methodology. The green synthesis methods by means of green precursors of synthesis involves the usage of inexpensive or recycled materials, while the chemical synthesis methods involve toxic chemical reagents or organic solvents as precursors. The synthesized C-dots have been confirmed by their fluorescent image and UV-Visible spectrometer. 

[STEPS]
SECTION: Step 1: Preparation of basil leaves
1. Freshly collected   should be sorted first to ensure only healthy leaves are chosen for the C-dots synthesis.

 

SECTION: Step 1: Preparation of basil leaves
2. Then it is washed thoroughly and rinsed with distilled water to remove the debris, dust or any other unwanted substance attached on to the leaf surface.
SECTION: Step 1: Preparation of basil leaves
3. Excess water is decanted.
SECTION: Application of C-dots:
26. The synthesized can be used for wide range of applications and are broadly classified into: 

1. Biomedical Applications
    a.      Bioimaging
    b.      Drug delivery
    c.      Photodynamic therapy

2. Optoelectronic Applications
    a.      Light-emitting diodes (LEDs)
    b.      Photodetectors
    c.      Photovoltaic devices

3. Sensing Applications
    a.      Biosensors
    b.      Chemosensors
    c.      Environmental monitoring

4. Security and Anti-Counterfeiting
    a.      Security inks
    b.      Security labels
    c.      Anti-counterfeiting coatings

5. Agricultural and Environmental Applications
    a.      Plant growth promotion
    b.      Crop yield enhancement
    c.      Soil remediation
SECTION: Step 1: Preparation of basil leaves
6. The resulting powder can be stored at 4 °C for less than a week. 
 

SECTION: Step 1: Preparation of basil leaves
5. Dried leaves are crushed to fine powder using pestle and mortar.
 

SECTION: Step 1: Preparation of basil leaves
None. 
SECTION: Step 1: Preparation of basil leaves
None. 
SECTION: Step 2: Preparation of precursor solution
7. To prepare the precursor solution, deionized water can be used as a solvent.
SECTION: Step 2: Preparation of precursor solution
8. 1 gof powder is mixed with 10 mL of water and dispersed under magnetic stirring or sonication process. 
 

 

SECTION: Step 3: Synthesis process
9. As this methodology uses hydrothermal process, a non-corrosive Polytetrafluoroethylene (PTFE) lined stainless steel autoclave reactor must be used. 


 


 

SECTION: Step 3: Synthesis process
10. The characteristic of the reactor – it should withstand a temperature of 250 °Cand an applicable pressure is  .

SECTION: Step 3: Synthesis process
11. Depends on the synthesis volume the reactor should be filled only 60-70 % capacity.
SECTION: Step 3: Synthesis process
12. Then the precursor solution is added to the reactor completely.
SECTION: Step 3: Synthesis process
13. Finally, the reactor should be tightened to its maximum limit. 

 

SECTION: Step 3: Synthesis process
14. The whole setup is placed inside the oven which is pre-heated to 180 °C for a period of 600 min without any external disturbances.


 

SECTION: Step 3: Synthesis process
15. After the process, the reactor is allowed to be cooled down naturally inside the oven.
SECTION: Step 4: C-dots purification
16. After completing the step 3, the resulting black turbid solution is transferred to the centrifuge tubes and C-dots are collected.
SECTION: Step 4: C-dots purification
17. The solution is purified by centrifugation with 4000 rpmfor 30 min .

SECTION: Step 4: C-dots purification
18. Discard the pellet which is burnt residue of leaf powder, and the brownish solution is collected without disturbing the pellet at the bottom.
SECTION: Step 4: C-dots purification
19. Later, to obtain pure C-dots the solution is centrifuged at 13000 rpm for 10 minat 8 °C.

SECTION: Step 4: C-dots purification
20. The solution collected from (step 4-19) is passed through 0.22 µm pore size nylon sterile syringe filters.
SECTION: Step 4: C-dots purification
21. The final solution is preserved at 4 °C for further use. 
 


SECTION: Step 5: C-dots formation confirmation
22. Qualitative testing 1 - The formation can be checked primarily by exposing the solution under the UV light to visualize the blue fluorescence. 

 

SECTION: Step 5: C-dots formation confirmation
23. The fluorescence of C-dots is shown in Figure 5 at 1:3 dilution.

 

SECTION: Step 5: C-dots formation confirmation
24. Qualitative testing 2 - The C-dot solution can be diluted for example 500 µLwith 3 mLof water in a quartz cuvette and measured in UV-Visible spectroscopy without any background noise.

SECTION: Step 5: C-dots formation confirmation
25. The UV-visible spectroscopy data is shown is Figure 6.



 



SECTION: Step 1: Preparation of basil leaves
4. The leaves are allowed to dry at 60 °C for a duration of 1440 minin a heating oven with a temperature increment of 10 °C/ 1 min.