Journal of Materials Science and Nanomaterials
Open Access

Carbon quantum dots; Tunable emission; Bioimaging applications; Photonic materials; Fluorescent nanomaterials; Emission wavelength control; Surface passivation; Quantum confinement; Photoluminescence behavior; Biocompatibility; Biomedical imaging; Optical properties; Carbon nanomaterials; Green synthesis; Solvothermal method; Excitation-dependent emission; Energy band engineering; Fluorescent probes; Nano-optics; Multiphoton imaging

Introduction

Carbon quantum dots (CQDs), a class of fluorescent carbon-based nanomaterials, have garnered significant attention in recent years due to their remarkable optical properties, chemical stability, biocompatibility, and low toxicity. Unlike traditional semiconductor quantum dots that often rely on heavy metals, CQDs offer an environmentally benign alternative, especially suitable for biomedical and photonic applications. Among their most fascinating features is the tunable photoluminescence, which enables control over emission wavelengths through manipulation of size, surface states, and chemical composition. This tunability is crucial for applications in bioimaging, where emission in specific spectral windows—particularly the near-infrared (NIR) region—is desirable for deep tissue imaging. In photonics, tunable emission opens doors to flexible optical devices, light-emitting diodes (LEDs), lasers, and sensors [1-5].

The emission properties of CQDs are influenced by a combination of quantum confinement effects, surface functional groups, and the degree of graphitization. Synthesis methods such as hydrothermal, solvothermal, microwave-assisted, and electrochemical routes have enabled precise control over these parameters. The ability to engineer emission behavior based on excitation wavelength or doping elements (e.g., nitrogen, sulfur, phosphorus) expands the applicability of CQDs in complex environments. Additionally, their water solubility, low cytotoxicity, and strong fluorescence make CQDs ideal for live-cell imaging and in vivo tracking.

This study explores the mechanisms behind tunable emission in carbon quantum dots and examines how these characteristics can be optimized for targeted bioimaging and photonic applications. The work also reviews recent advances in synthesis approaches that enable emission customization and discusses how doping, surface passivation, and hybridization affect luminescent performance. The convergence of material science, photophysics, and biomedicine is essential to harness the full potential of CQDs in next-generation optical and diagnostic technologies [6-10].

Discussion

The unique luminescence behavior of carbon quantum dots stems from a synergy between quantum confinement and surface defect emissions. Unlike inorganic quantum dots, where bandgap tuning is predominantly size-dependent, CQDs exhibit a more complex mechanism involving both intrinsic (graphitic core) and extrinsic (surface traps and functional groups) contributions to photoluminescence. This dual-mode emission allows for broad tunability, often seen as excitation-dependent fluorescence, which can be fine-tuned by altering synthesis parameters such as temperature, pH, precursor materials, and reaction time.

Synthesis methods play a pivotal role in defining the structure and emission characteristics of CQDs. Top-down approaches like arc-discharge, laser ablation, or electrochemical oxidation and bottom-up strategies such as pyrolysis, hydrothermal treatment, and microwave synthesis allow for scalable production with controllable properties. Green synthesis routes using natural precursors—such as citric acid, fruit extracts, or biomass waste—offer sustainable pathways to produce biocompatible CQDs suitable for in vivo applications. Among these, hydrothermal methods have proven particularly effective for tuning emission by controlling the degree of carbonization and functional group composition on the CQD surface.

Surface passivation significantly enhances quantum yield and enables emission tuning by modifying the electronic states of CQDs. Passivating agents like polyethylene glycol (PEG), ethylenediamine, and various amines can introduce nitrogen-containing functional groups that shift the emission to longer wavelengths. Doping with heteroatoms (N, S, B, P) further tailors the electronic structure and optical output, improving brightness and photostability. Such surface engineering techniques are central to adapting CQDs for specific bioimaging tasks, where strong signal-to-noise ratio and spectral selectivity are essential.

In biomedical imaging, CQDs have demonstrated utility in both fluorescence and multiphoton imaging, owing to their excitation flexibility and resistance to photobleaching. CQDs emitting in the NIR region are especially valuable for non-invasive imaging, offering deeper tissue penetration and minimal background interference. Their small size facilitates easy cellular uptake, and functionalization with targeting ligands (e.g., folic acid, antibodies) enables specific labeling of cancer cells or organs.

In photonic applications, the stable, tunable emission of CQDs supports their integration into LED technologies, optical sensors, and wavelength-tunable lasers. Hybrid systems combining CQDs with polymers or metal nanostructures have led to enhanced optical responses and broadened functional applications. However, despite the impressive advances, CQDs still face limitations such as relatively lower quantum yields compared to their metal-based counterparts, batch variability, and incomplete understanding of emission mechanisms. These challenges call for further exploration using advanced spectroscopic and computational methods to unravel the interplay between structure, composition, and emission behavior.

Conclusion

Carbon quantum dots offer a powerful platform for both bioimaging and photonic applications, driven by their tunable emission properties, environmental safety, and functional versatility. Through careful synthesis and surface modification, their photoluminescence can be adjusted across a broad spectral range, enabling tailored solutions for diverse imaging and optical device needs. The inherent biocompatibility and excitation-dependent fluorescence of CQDs make them ideal candidates for next-generation fluorescent probes in biomedical contexts, while their stability and tunability support innovation in photonic systems.

Synthesis techniques such as hydrothermal and green chemistry approaches allow for scalable, low-cost production with controllable properties. The strategic use of dopants and passivating agents improves quantum yields and expands emission capabilities. Applications in live-cell imaging, targeted diagnostics, LED technologies, and optical sensors highlight the multifunctionality of CQDs and their potential to bridge gaps between nanomaterials and practical technologies.

Future research should aim at enhancing quantum efficiency, achieving emission stability under physiological conditions, and understanding the photophysical mechanisms in greater depth. Integration with other nanomaterials and exploration of hybrid nanostructures could further expand their utility. As the field progresses, carbon quantum dots stand out as a cornerstone of safe, tunable, and efficient luminescent materials for both scientific exploration and real-world deployment.

References

1. Jang KL, Livesley WJ, Angleitner A, Reimann R, Vernon PA (2002) Genetic and environmental influences on the covariance of facets defining the domains of the five-factor model of personality. Pers Individ Dif 33: 83-101.

   Indexed at, Google Scholar, Crossref

2. DeYoung CG, Quilty LC, Peterson JB (2007) Between facets and domain: 10 aspects of the Big Five. J Pers Soc Psychol 93: 880-896.

   Indexed at, Google Scholar, Crossref

3. Gosling SD, Vazire S, Srivastava S, John OP (2004) Should we trust web-based studies? A comparative analysis of six preconceptions about internet questionnaires. Am Psychol 59: 93-104.

   Indexed at, Google Scholar, Crossref

4. Jha A, Kumar A (2019) Biobased technologies for the efficient extraction of biopolymers from waste biomass. Bioprocess Biosyst Eng 42: 1893-1901.

   Indexed at, Crossref

5. Martău GA, Mihai M, Vodnar DC (2019) The Use of Chitosan, Alginate, and Pectin in the Biomedical and Food Sector-Biocompatibility, Bioadhesiveness, and Biodegradability. Polymers 11: 1837.

   Indexed at, Crossref

6. Adhikari BB, Chae M, Bressler DC (2018) Utilization of slaughterhouse waste in value-added applications: Recent advances in the development of wood adhesives. Polymers 10: 176.

   Indexed at, Crossref

7. Fang Y, Guo S, Phillips GO (2014) Soy proteins: A review on composition, aggregation and emulsification. Food Hydrocoll 39: 301-318.

   Crossref

8. Benítez JJ., Castillo PM, del Río JC, León-Camacho M., Domínguez E et al.(2018) Valorization of Tomato Processing by-Products: Fatty Acid Extraction and Production of Bio-Based Materials. Materials 11: 2211.

   Indexed at, Crossref

9. Tran D-T, Lee HR, Jung S, Park MS, Yang J-W ( 2018) Lipid-extracted algal biomass based biocomposites fabrication with poly(vinyl alcohol) . Algal Res 31: 525-533.

   Crossref

10. Damm T, Commandeur U, Fischer R, Usadel B, Klose H (2016) Improving the utilization of lignocellulosic biomass by polysaccharide modification. Process Biochem 51: 288–296.

    Crossref