Photoluminescent carbogenic nanoparticles for potential healthcare applications

Abstract

Carbon dots (C-dots) are zero-dimensional fluorescent nanomaterials typically measuring below 10 nm. They have gained attention for their unique structure and properties, including cost-effective fabrication, easy chemical functionalization, high photoluminescence, tunable optical properties, resistance to photobleaching, thermal stability, and biocompatibility. Their growing use in biomedical applications is driven by their impressive potential as highly responsive optical sensors, monitoring probes, antimicrobial agents, and trackable carriers for drug delivery. This thesis explores two distinct methods for modifying the structure, and therefore their properties, of C-dots, each based on different precursor materials: one involves electrochemical surface functionalization, while the other employs an oxidizing agent to modify structural properties. The study examines how these approaches uniquely impact the optical and structural properties of each C-dot system, as well as their potential applications in the biomedical field.

Firstly, a novel electrochemical mechanism is presented, that significantly transforms the structural and optical features of C-dots that were synthesized via pyrolysis of citric acid and ethanolamine at 300°C. This method uses electrogenerated hypochlorite for electrochemical etching, which causes intense oxidation and breaks carbon-carbon bonds on the nanoparticle surface. As a result, the particle size is progressively reduced while the quantum yield increases dramatically, reaching an enhancement of up to 640%. This research is the first to provide concrete evidence of this highly efficient reshaping mechanism in C-dots, enabling precise control over particle size and photoluminescence emission properties.

Secondly, this chapter investigates the structural, optical, and biological properties of C-dots (CU100D) synthesized via pyrolytic treatment of citric acid and urea at 230°C, particularly after oxidation treatment with NaClO. This treatment reduced the particle size from 4.3 nm to 2.9 nm, enhancing the surface area-to-volume ratio and altering the physicochemical properties, while FTIR analysis showed improved dispersibility. Optical properties underwent notable transformations, as NaClO treatment decreased UV intensity and increased transparency, while boosting the quantum yield by up to 350%, addressing the common challenge of low quantum yield compared to commercial graphene quantum dots. The irreversible destruction of the 2-(2'-hydroxyphenyl) benzothiazole (HTTP) fluorophore, a byproduct formed during synthesis, upon interaction with NaClO, suggests potential for novel functionalities beyond fluorescence imaging. This cost-effective and scalable method yielded nanoparticles with high cell viability and strong antifungal activity, demonstrating their excellent performance for biomedical applications.

Finally, the citric acid-urea-derived C-dots, following NaClO treatment, were compared to commercially available surface-modified graphene quantum dots functionalized with amine groups and imidazole. This comparison underscored the CU100D nanoparticles' cost efficiency, superior optical properties—such as a markedly higher quantum yield—and enhanced cell viability, showcasing their benefits over functionalized graphene quantum dots.

In conclusion, this thesis presents simple, green, cost and time-efficient, and scalable strategies for generating multifunctional C-dots with improved optical properties and versatile chemical compositions. Both approaches allowed the fine-tuning of the structural characteristics, and thereby the optical properties of C-dots, with significant quantum yield improvements. Treatment with NaClO enhances the C-dots' optical properties, biocompatibility, and biological activity. Overall, these findings highlight the potential of C-dots as a promising, cost-effective alternative to conventional fluorescent materials in various applications.