Preparation and characterization of carbon quantum dot- based composite thin films

Abstract

Carbon quantum dots (CQDs) are zero-dimensional, carbon-based nanoparticles with fluorescence in the 2–10 nm range, offering advantages such as facile synthesis, surface functionalization, water solubility, biocompatibility, low toxicity, and simple preparation methods. Common synthesis techniques include hydrothermal, solvothermal, and microwave-assisted methods; CQDs produced via hydrothermal treatment of precursor materials exhibit quantum yields of 60–80 %. CQDs find applications across a wide spectrum, including biomedical imaging, biosensors, drug- delivery systems, photoelectrocatalysis, optoelectronics, and energy storage. In particular, their low toxicity in both in vivo and in vitro biocompatibility assays makes them suitable as fluorescent markers for diverse biomedical functions, from cancer imaging to tracking administered drug particles. Their photoluminescence emissions can be tuned to produce strong blue light around 450 nm under 350 nm excitation, enabling high-contrast cellular imaging. With quantum yields reaching up to 80 %, CQDs are versatile nanomaterials whose electronic and optical properties can be tailored for specific applications through surface modifications. Characterized by UV–Vis spectroscopy, CQDs emit intense blue light near 450 nm when excited at 350 nm. These features render them promising for smart packaging, anti-counterfeiting labels, and biosensor applications. In biosensors, CQDs leverage their surface functional groups to selectively detect analytes such as metal ions, pH changes, and glucose. The presence of metal cations, anions, or biomolecules induces measurable shifts in their photoluminescence spectra, enabling both environmental pollutant monitoring in water and visualization of ionic balance in cellular environments. In energy technologies, CQDs enhance photoelectric conversion and energy storage in devices ranging from solar cells to supercapacitors. By adjusting their size and surface chemistry to tune bandgaps, CQDs serve as photosensitizers in dye-sensitized solar cells, significantly boosting power conversion efficiencies in both organic and perovskite systems. In optoelectronic devices, CQD-based light-emitting diodes (LEDs) and fluorescent dyes offer compact, low-cost lighting solutions for flexible display technologies and smart packaging. For example, CQD-LEDs have been developed to deliver tunable colors and broad emission spectra. When integrated into polymer matrices, CQDs block approximately 70–90 % of UV radiation, thereby providing UV protection and extending the lifespan of packaging and coating materials. Additionally, their bright blue fluorescence under UV illumination generates directly readable signals for anti-counterfeiting tags and smart- packaging sensors. In food packaging and security labelling, CQD-functionalized surfaces emit vibrant blue fluorescence under UV light, serving both as shelf-life indicators and as easily readable anti-counterfeiting markers. In the field of food packaging, CQDs play critical roles in active packaging strategies through UV protection, strong moisture-barrier properties, antibacterial activity, and antioxidant functions. Antimicrobial agents immobilized on CQD surfaces inhibit microbial growth within packages, thereby extending product shelf life. These functionalities are widely researched in nanocomposite film technologies to ensure consumer safety and product quality. Facing the environmental pollution and microplastic threats posed by petroleum-based plastics, biodegradable polymers have become the focus of sustainable packaging solutions. Alongside PLA, PHA, PBS, and PBAT, starch-based materials offer cost- effectiveness and biodegradability. Key parameters for biodegradable polymers include mechanical strength, oxygen and water-vapor barrier performance, thermal stability, and cost balance. Active and smart packaging increasingly incorporates antimicrobial or antioxidant additives and sensor functionalities to manage food safety and shelf life. Starch, an abundant and low-cost polysaccharide, is prominent in biodegradable film production. However, pure starch films suffer from brittleness and high water-vapor permeability, which limit their use in food-packaging applications. To overcome these weaknesses, composite films have been developed by incorporating plasticizers such as glycerol and nano-fillers like ZnO, cellulose derivatives, or CQDs into the starch matrix. In CQD-reinforced starch films uniformly dispersed via ultrasonication, water- vapor permeability decreases by 20–30 %, while water-contact angles increase significantly. Optically, these composites maintain over 85 % transparency in the visible region and exceed 90 % UV-blocking capacity. Photoluminescence measurements under 350 nm excitation reveal strong blue emissions around 450 nm, offering the potential for smart packaging and anti-counterfeiting applications. Although CQD additions did not enhance tensile strength compared to pure starch films, they increased elongation at break, improving flexibility. Consequently, these films—insufficient for single-layer packaging—are recommended as functional interlayers in multilayer packaging systems. In this study, blue-fluorescent CQDs with approximately 70 % quantum yield were synthesized via the hydrothermal treatment of a citric acid and ethylenediamine mixture. These CQDs were then incorporated at loadings of 0.1–1 wt % into an optimized (36.4 wt % glycerol) starch–glycerol matrix, producing homogeneous thin biocomposite films via solution casting. The chosen base starch film (Film C) exhibited the most balanced hydration and visual characteristics. FT-IR analyses revealed intensity decreases at 925 cm⁻¹ and 1721 cm⁻¹ bands, indicating hydrogen- bond interactions between CQDs and starch chains. Photoluminescence studies showed strong blue emission at 445 nm with an approximate quantum yield of 70 %. Barrier testing demonstrated that CQD-reinforced films significantly reduced water- vapor permeability and moisture uptake as CQD content increased. For instance, C- CQD-0.5 and C-CQD-1 samples exhibited 35-40% and 55-70% lower water-vapor permeability compared to the pristine starch film, respectively. Water-vapor sorption tests at 58 % and 99 % relative humidity confirmed consistently lower moisture uptake across all CQD-filled films. Water-contact-angle measurements showed marked increases in surface hydrophobicity for films with 0.5–1 wt % CQD, minimizing water–surface interaction. Mechanical characterization, including tensile strength and elongation at break, revealed that CQD addition did not significantly improve tensile strength but increased elongation by 20–35 %, indicating enhanced flexibility. These findings suggest that CQD-reinforced films, while flexible, lack sufficient strength for standalone use and are therefore suited as intermediate layers in multilayer packaging systems. Optical analyses confirmed that the composites maintain over 85 % transparency in the visible range and block 70–90 % of UV-A, UV-B, and UV-C radiation, thus preserving product visibility and preventing photodegradation. The photoluminescence performance provides direct visual cues for smart-packaging applications. In moisture-uptake, solubility, and long-term durability tests, the C-CQD-0.5 formulation stood out: its 24-hour solubility was measured at 36.6 ± 0.6 %, and at 28 days it remained at 30.8 ± 0.8 %, compared to 42.8 ± 1.6 % and 37.0 ± 2.4 % for the pure film. These results indicate that CQDs densify the matrix, reducing water interaction and preserving structural integrity in humid environments. In conclusion, starch–glycerol films containing 0.5–1 wt % CQDs exhibit the most balanced barrier, optical, and photoluminescent performance. The data demonstrate that these formulations are strong candidates for eco-friendly, biodegradable, and multifunctional food packaging materials. Future work should involve shelf-life studies incorporating antimicrobial and antioxidant additives, scaling up production, and conducting cost–sustainability analyses to enable the commercial deployment of ecological and functional food-packaging alternatives to petroleum-based plastics.