The green synthesis of carbon quantum dots (CQDs) and characterization of polycaprolactone (PCL/CQDs) films

The synthesis of photoluminescent carbon quantum dots (CQD) from natural products is one of the most remarkable types of carbon-based nanomaterials due to their small size, high water solubility, nontoxicity, high stability, and excellent biocompatibility. New RC-CQDs have been prepared from natural material (rosehip fruit) via a hydrothermal method. CA-CQDs samples were synthesized from a synthetic carbon source (citric acid) by the same method for comparison with RC-CQDs. The maximum quantum yields of the as-prepared RC-CQDs and CA-CQDs samples were 23.8% and 9.6%, respectively. High-resolution transmission electron microscopy (RTEM) of RC-CQDs revealed a sphere shape with an approximate size of 5 nm, CA-CQDs were greater than 5 nm. Biodegradable polycaprolactone (PCL), which is used in bioengineering applications thanks to its mechanical and biodegradable properties, was used as a polymer matrix in this study. The PCL/RC-CQDs and PCL/CA-CQDs nanocomposite films were produced via liquid phase ultrasonic mixing at different amounts of RC-CQDs and CA-CQDs in PCL matrix (0.3-; 0.5-; 1.2-; 2.0% and 4.0% by weight), while unfilled PCL was also produced for comparison. The structural, mechanical, and biodegradable behaviors of PCL/RC-CQDs nanocomposite films have been investigated to improve the mechanical properties of PCL and increase its potential usability in load-bearing applications. PCL/RC-CQDs-2.0 film gave the maximum tensile strength value (17.65 MPa) with an increment of 27.7% and the highest value of the yield strength 9.02 MPa (an increase of 40.9%) and the highest value of microhardness 4.80 HV0,025 (an increase of 14.8%) were obtained using PCL/RC-CQDs-4.0 film compared with the PCL. The tensile strength and % elongation at break values of the PCL/CA-CQDs-0.3 film decreased by 14.8% and 133.1% compared with the PCL, respectively. The microhardness of the PCL/CA-CQDs-0.3 film was 5.48 HV0,025, increasing by 31.1% in comparison with 4.18 HV0,025, for the PCL. Biodegradability test results showed that films containing RC-CQDs exhibited lower weight loss than the PCL and films containing CA-CQDs. This result was attributed to the amorphous structure, low surface roughness and high contact angle of the RC-CQDs containing films. And also, CA-CQDs made PCL films more hydrophilic according to low contact angle results.