Nanomaterials are a family of materials with distinct qualities and promising properties, with applications like bioimaging to drug delivery in medical, biosensing in technology, and energy conversion and storage in environmental sciences[1]. Some of its key characteristics are its nanostructured scale (> 100 nm in size) and essential optical, electrical, and conductive capabilities such as sensing, photocatalysis, electrocatalysis, and so on. The merging of nanotechnology and several areas of research has increased scientists’ interest in developing superior nanoscale materials. The use of photoluminescent carbon-based nanoparticles has increased substantially in past 10 years[2]. These particles are mostly amorphous nanocarbon variants with a high heteroatom concentration[3–6]. Various carbon nanomaterials, including individual carbon nanotubes, graphene nanosheets, carbon nanodots (CNDs), nanodiamonds, carbon nanoribbons/nanorods, and fluorescent polymer dots, are garnering attention due to their wide-ranging applications, spanning from high-performance electrochemistry to biomaterials[7, 8]. CNDs are nanoparticles characterized by their zero-dimensional nature, presenting as amorphous quasispherical particles with cores ranging from amorphous to nanocrystalline. These cores predominantly consist of graphitic or turbostratic carbon-based material and typically measure less than 10 nm. They exhibit a hybridized carbon structure comprising both sp3 and sp2 carbon bonding. CNDs derived from natural biomass hold greater appeal compared to chemically synthesized counterparts due to their customizable photoluminescence (PL) features, diverse emission colors, water solubility, outstanding long-term photostability (lack of photobleaching), cost-effectiveness, simplicity of preparation, and amenability to functionalization [9–12]. There are two types of manufacturing techniques used: TOP-DOWN and BOTTOM-UP, with different thermal and chemical strategies. Some of them are extremely cost-effective, environmentally friendly, as well as simple methods of synthesising CNDs from different sources of carbon. The hydrothermal method is a widely used thermal method with no loss of material and a closed, pressurised reaction performed at higher temperatures without exposing any environmental hazards. CNDs with properties like size and PL can be procured by employing various synthetic techniques [13, 14]. The photoluminescence (PL) exhibited by CNDs possessing particle dimensions below 10 nm is attributed to quantum confinement effects and the composition of their surface functional groups. These factors contribute to alterations in energy band gaps, leading to modifiable emission characteristics. Catalysis, drug carrier, imaging, sensing, solar-energy conversion, and lightning, are just a few of the areas that show tremendous promise[15], and much more research is underway like optical sensors[16] and information encryption[17]. In biomedicine, CNDs have been used as antibacterial and anticancer agents, as well as for wound healing and bioimaging[4, 14, 18, 19]. CNDs made of carbon materials have a lower toxicity toward the environment and living organisms than metal nanoparticles (NPs)[20]. Major role of water solubility performed by CNDs has been proven by Hydrophilic CNDs.
For example, hydrophilic CNDs linked to drugs serve as effective carriers due to their excellent dispersion in water, facilitated by their ability to engage in multiple hydrogen-bonding interactions. As demonstrated by Wang et al., they synthesized CNDs using commercial beer and then attached them to doxorubicin hydrochloride, thereby creating a promising anticancer treatment approach[21]. Quang et al. synthesized carbon nanodots (CNDs) from waste wine cork via the hydrothermal method, yielding CDs with an average diameter of ~ 6.2 ± 2.7 nm. The characterized optical properties revealed excitation-dependent photoluminescence associated with surface functional groups, achieving a quantum yield of 1.54%. Successfully applied in bioimaging, these CDs showcase promise for fluorescence imaging applications [22]. Nevertheless, the quantum yield (QY) of the produced CNDs remains relatively low, limiting their potential applications. Therefore, it remains essential and imperative to synthesize CNDs with high fluorescence. Heteroatom doping represents an effective strategy for enhancing the surface activity and electrical properties of CNDs, thereby boosting their QY and fluorescence characteristics
[23–25]. To facilitate doping, various heteroatom sources can be employed, such as urea, thiourea, and nitrogen-rich compounds like amino acids, including Alanie, Glutamine, Tyrosine, and Arginine[23]. In our study, we utilized L-Arginine as the nitrogen source, known for being non-toxic and abundant in nitrogen. This choice of nitrogen source not only enhanced the fluorescence but also had a significant impact on the quantum yield of the produced carbon nanodots (CNDs). Specifically, CNDs derived from cinnamon bark wood powder were doped with L-Arginine amino acid to augment their fluorescence.
The surge in popularity of medicinal herbs aligns with the integration of innovative materials like cinnamon bark powder and carbon nanodots (CNDs). Historically, these herbs have served as effective home remedies for various ailments such as coughs, digestive issues, and skin allergies. Cinnamon, renowned for its high antioxidant content and more than 30 compounds combating oxidative stress, seamlessly fits into this traditional healing practice[26]. When considering the use of these materials together, both possessing medical applications, a notable potential outcome emerges. Utilizing a medically oriented carbon source for synthesizing CNDs, cinnamon bark powder proves particularly advantageous in this regard.
In this study, we employed a green synthesis approach for the preparation of highly fluorescent nitrogen-doped carbon nanodots (CNDs). Cinnamon bark served as the carbon source in a hydrothermal reaction, and L-Arginine was used for doping, enhancing both fluorescence and quantum yield (QY). The structural and optical properties of the synthesized CNDs and nitrogen-doped CNDs were comprehensively analysed using various techniques. UV-Vis spectroscopy provided insights into the absorption characteristics, while fluorescence spectroscopy shed light on the emission properties. Transmission electron microscopy (TEM) allowed for a detailed examination of the particle morphology. Additional structural analysis was carried out using Fourier-transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). Furthermore, excitation and emission spectra were investigated to identify the optimal excitation wavelength for fluorescence imaging. The quantum yield was calculated to quantify the efficiency of fluorescence. To validate the practical application, we conducted fluorescence microscopy on yeast cells treated with both CNDs and nitrogen-doped CNDs. This analysis confirmed the successful synthesis and fluorescence enhancement of the carbon nanodots, providing visual evidence of their effectiveness in biological settings.