Synthesis of Novel Rhodamine-Derived Carbon Quantum Dots to Create Natural Fluorescent Silk Fibroin

Fluorescent silk fibroin, produced by modifying the diet of Bombyx mori, has recently emerged as a promising material for a wide range of applications, including smart textiles, optoelectronics, environmental sensing, and biomedical technologies. However, a key challenge remains in developing nontoxic fluorescent materials to produce fluorescent silk. In this study, three nontoxic carbon quantum dots (CQDs) were synthesized from Rhodamine B using a hydrothermal method. Transmission Electron Microscopy (TEM) confirmed successful synthesis, revealing CQDs with diameters of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{2 - 3 ~ n m}$</tex> that exhibited strong fluorescence under UV light. Photoluminescence spectroscopy showed strong emission peaks at approximately <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$470 ~\text{nm}, 550 ~\text{nm}$</tex>, and 600 nm for B-CQDs, G-CQDs, and O-CQDs under excitation wavelengths of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$370 ~\text{nm}, 370 ~\text{nm}$</tex>, and 400 nm, respectively. To evaluate the performance of CQDs in comparison with traditional Rhodamine-based approaches, six fluorescent materials were used in this study: O-CQD, G-CQD, B-CQD, Rhodamine B (RhB), Rhodamine 101 (Rh101), and Rhodamine 110 (Rh110). These materials were incorporated into silkworm chow at a concentration of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.1 \text{wt} \%$</tex> and fed to silkworms to produce fluorescent silk with enhanced mechanical and optical properties. From Scanning Electron Microscopy (SEM) imaging, it was found that CQDs produced silk that was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 3.48 ~\text{nm}$</tex> thicker than Rhodamine derivatives. The CQDs were shown to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 130 \%$</tex> mechanically stronger compared to control group silk (silk from silkworms fed a non-fluorescent diet), whereas silk derived from Rhodamine-fed silkworms demonstrated only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 62.9 \%$</tex> of the control group's strength. In addition, the fluorescent silk was processed into a silk fibroin solution, which was subsequently fabricated into various forms tailored for biomedical applications.