Green synthesis of carbon quantum dots and the preparation of bio-nanocomposites based on chitosan with modified refractive indices and optoelectronic properties

Abstract

Carbon quantum dots (CQDs) were synthesized via a green hydrothermal route from Nerium oleander flower dye. XRD and FTIR spectroscopic investigations were used to identify the CQD particles. The CQD particles were embedded into a chitosan host biopolymer to produce polymer composites with enhanced optical properties. FTIR results verified –OH/–COOH/–NH surface functionalities on the CQDs and their hydrogen-bonding interactions with the CS host polymer. XRD results showed the principal CS diffraction peaks moving from 20.56° (CS) to ∼21.0° (CSCD20) and 21.42° (CSCD40), evidencing matrix rearrangement and crystallinity suppression upon CQD doping. UV-vis-NIR spectra revealed new absorption features at ∼240, ∼301, and ∼402 nm with a pronounced redshift of the edge. Compared with pristine CS (high UV transmittance), the doped films exhibited stronger UV absorption while preserving high transmittance toward the near-IR region. Optical band-gap analyses (Tauc/ASF) demonstrated a substantial narrowing from ∼4.95 eV (CS) to ∼2.45 eV (CSCD20) and ∼2.22 eV (CSCD40), indicating enhanced visible-light harvesting. Concomitantly, the refractive index increased and remained comparatively flat across the UV-visible range (up to ∼2.7), and both the real and imaginary parts of the dielectric function rose at low photon energies, consistent with higher electronic polarizability and charge-transfer capacity. Drude/Wemple–DiDomenico evaluations indicated an increased carrier density-to-effective-mass ratio and reduced optical resistivity with CQD content, while sheet resistance and thermal emissivity decreased across 190–1100 nm. Thermal analyses (TGA/DSC) showed improved robustness: peak decomposition temperatures rose from ∼240 °C (CS) to ∼270 °C/295 °C (CSCD20/CSCD40) and T_g from ∼95 °C to ∼110 °C/120 °C. Overall, Nerium-derived CQDs provide an eco-friendly, effective route to tailor the optical constants and stability of CS films for photonic coatings, optical sensing, and sustainable bio-optoelectronic components.

Carbon quantum dots (CQDs) were synthesized via a green hydrothermal route from Nerium oleander flower dye.

1. Introduction

The increasing environmental burden caused by synthetic plastics, including their non-biodegradability and petrochemical dependence, has accelerated the global search for sustainable alternatives. Biopolymers have, therefore, emerged as an eco-friendly class of materials derived from renewable natural resources, such as plants, animals, and microorganisms, because they exhibit excellent biodegradability, biocompatibility, and non-toxicity, making them particularly attractive for biomedical, environmental, and optoelectronic applications.^1–3 Among these natural macromolecules, chitosan (CS) stands out as one of the most versatile and extensively investigated biopolymers; it is also a linear polysaccharide obtained through the deacetylation of chitin, which is abundantly present in the exoskeletons of crustaceans, such as crabs and shrimps.^4,5 Owing to the presence of reactive amino (–NH₂) and hydroxyl (–OH) groups along its backbone, chitosan demonstrates remarkable chelating ability, film-forming capability, and compatibility with various inorganic and organic fillers.^6,7 These properties allow chitosan to serve as a functional platform for the development of biopolymer-nanocomposite^8,9 materials with tunable physical, optical, and biological characteristics. In parallel, natural dyes have recently gained significant scientific attention as sustainable and non-toxic alternatives to synthetic colorants, while conventional artificial dyes—although widely used in textiles, plastics, and cosmetics—are often associated with hazardous byproducts, heavy-metal residues, and environmental persistence, leading to bioaccumulation and toxicity.^10,11 In contrast, dyes extracted from natural resources, such as flowers, leaves, bark, and fruits, offer high biodegradability, low allergenicity, and good environmental safety.^12,13 Their chromophoric structures, typically rich in π-conjugated systems and functional groups, like –OH, –COOH, and –NH₂, facilitate strong light absorption and redox reactivity, making them valuable candidates for optical and photocatalytic applications.¹⁴ The fabrication techniques for CQDs may be classified into two approaches: “top-down” and “bottom-up”. The top-down methodology employs physical or chemical techniques to transform extensive sp² carbon domains into diminutive CQDs, primarily through chemical oxidation, laser ablation, arc discharge, and electrochemical synthesis. The bottom-up approach employs small molecules as precursors for carbon sources, generating sp² carbon domains via the integration of hydrothermal, microwave-assisted, and pyrolysis techniques, leading to CQDs with adjustable dimensions and shapes.¹⁵

Among the known methods, the bottom-up approach is the most common route to produce carbon quantum dots (CQDs) because of its straightforward, affordable, and controllable nature. For example, green synthesis using natural biomaterials has become a particularly attractive alternative to conventional chemical processes, as it does not need hazardous reagents, reduces energy consumption, and minimizes environmental impact. Plants are good precursors for CQD production since they are abundant, renewable, inexpensive, and rich in carbon-containing biomolecules. Different plant-based resources, including fruits, peels, seeds, roots, flowers, and leaves, have been successfully employed as eco-friendly carbon precursors for the synthesis of CQDs. In the present work, CQDs have been synthesized using a green hydrothermal technique from plant leaf extract, an environmentally friendly, low-cost, and scalable method for the synthesis of CQDs with multiple surface functional groups and excellent optical properties for potential applications in polymer nanocomposites and optoelectronic devices.^16,17 Mohammed et al.¹⁸ prepared CQDs from coffee extract using a green hydrothermal method and embedded them in a polymer matrix. The incorporation of CQDs enhanced the structural and optical properties of the host polymer. Hussein et al.¹⁹ prepared water-soluble CQDs from the natural dye derived from Bougainvillea flowers by a green hydrothermal approach. The synthesized CQDs showed good dispersibility in water and good optical characteristics, which suggested that plant-derived precursors might be a suitable source for the sustainable production of CQDs. Among various natural dye sources, Nerium oleander has emerged as a promising renewable pigment source due to its abundance and rich phytochemical content. Nerium oleander is a perennial evergreen plant bearing vibrant red or pink flowers that contain bioactive constituents such as flavonoids, alkaloids, glycosides, tannins, and phenolic compounds.²⁰ These molecules possess extended π-orbitals and abundant polar groups that can act as carbon precursors for carbon quantum dots (CQDs) through green synthesis routes; moreover, the use of Nerium oleander extracts not only valorizes an underutilized floral biomass but also aligns with the principles of sustainable chemistry by avoiding toxic reagents and minimizing chemical waste.²¹ Carbon quantum dots (CQDs) are nanoscale (often <10 nm) carbon-based materials with unusual optical features, including intense photoluminescence, adjustable optical band gaps, and size-dependent fluorescence. They also have good chemical stability, high water solubility, easy surface functionalization, rich surface functional groups, and low toxicity. Thanks to their excellent physicochemical features, CQDs have received great interest for applications in biosensing, bioimaging, photocatalysis, drug delivery, photovoltaic devices, light-emitting diodes (LEDs), photodetectors, sensors, and other sophisticated optoelectronic technologies.^22–24 Carbon quantum dots (CQDs) are mostly composed of a core–shell structure, where the carbon core has a diameter of 2–3 nm and the lattice spacing is around 0.2 nm. The carbon core may be composed of either graphitic (sp² hybridized) or amorphous (mixed sp²/sp³ hybridized) structures. The core is coated with a self-passivating shell comprising oxygen- and/or nitrogen-rich functional groups such as hydroxyl (–OH), carbonyl (C O), carboxyl (–COOH), and amino (–NH₂) groups. These surface functional groups improve the dispersibility, surface reactivity, and optical characteristics of the CQDs and their interaction with polymer matrices. Moreover, the structural features and physicochemical properties of CQDs are substantially affected by the precursor material, synthesis pathway, and processing factors such as pH, solvent type, reaction temperature, and synthesis time.^25–27 When synthesized from natural dye precursors, such as Nerium oleander, CQDs inherit heteroatom dopants (N, O, or S) and functional surface moieties that improve solubility, charge transfer, and interfacial bonding with polymer matrices.²⁸ The surface of the CQDs encompasses modified or connected various chemical groups, such as polymer chains, oxygen-containing groups, and nitrogen-based amino-containing groups.²⁹ Thus, incorporating such eco-derived CQDs into a chitosan biopolymer matrix generates hybrid films that combine the mechanical flexibility and biocompatibility of chitosan with the superior optical and electronic performance of CQDs. These synergistic effects enhance absorption in the UV-visible region, reduce the optical bandgap, and reinforce structural and thermal stability, thereby paving the way for sustainable materials with applications in optoelectronics, sensors, photocatalysis, and bio-packaging.³⁰

The chitosan (CS) polymer matrix shows enhancements in optical properties with the addition of carbon quantum dots (CQDs), as reported by Mohammed et al.³¹ The addition of CQDs leads to improved optical properties, a reduction in the refractive index, and a shift in the absorption edge toward longer wavelengths (red shift). Mamand et al.³² prepared CQDs using a hydrothermal process and added them to the CS matrix. The results showed that the absorption edge shifted from 4.97 to 2.35 eV.

In this work, carbon quantum dots (CQDs) obtained from Nerium oleander flower extract were synthesized using a green one-pot hydrothermal process and then mixed into a chitosan (CS) matrix to make chitosan/CQD (CSCD) nanocomposite films with varied CQD concentrations. The main purpose of the present work is to study the effect of CQD incorporation on the structural, morphological, thermal, and optical characteristics of the CS matrix and to correlate the CQD concentration with changes in the properties. The originality of the current work is the use of a naturally generated flower extract as a sustainable, low-cost, non-toxic, and eco-friendly carbon precursor for the synthesis of CQDs and their use as multifunctional nanofillers in biodegradable polymer matrices. The synthesized CQDs feature numerous surface functional groups, such as –OH, –NH₂, and C O groups, that may operate as electron-donating and electron-accepting sites. These functional groups engage with the hydroxyl and amino groups of the chitosan chains via strong intermolecular hydrogen bonding, which facilitates better polymer chain rearrangement and structural compatibility and enhances the amorphous nature of the nanocomposites. Moreover, the highly π-conjugated carbon core of the CQDs is beneficial for the delocalization of π-electrons and the construction of localized electronic states in the forbidden band gap, and is therefore advantageous for the excitation of electrons from the valence band to the conduction band. Thus, the refractive index rises, the absorption edge moves to longer wavelengths, and the optical band gap is significantly lowered from 4.90 to 2.22 eV. These advances indicate that the proposed CS/CQD nanocomposites are attractive candidates for advanced optoelectronic and photonic applications and for environmentally friendly applications.

2. Materials and methods

2.1. Dye extraction from Nerium oleander flowers and CQD fabrication

Nerium oleander flowers were harvested from Nerium oleander plants cultivated in Sulaymaniyah, Iraq, and desiccated in the dark at ambient temperature for approximately one week. The dried bracts were pulverized into a fine powder using a coffee grinder, then fifteen grams of the dried powder were dissolved in 225 milliliters of distilled water. The extraction temperature was maintained between 40 °C and 60 °C for 60 min using a solid-to-liquid ratio of 7 : 20 g mL⁻¹ to maximize the extraction yield of phytochemicals from Nerium oleander flowers. After extraction, the suspension was centrifuged at 5000 rpm for 10 min to remove coarse plant residues and other insoluble materials. The resulting supernatant was then filtered through Whatman no. 1 filter paper to eliminate any remaining macroscopic particulates.

Carbon quantum dots (CQDs) were synthesized by a one-pot hydrothermal method. A total of 100 mL of the filtered flower extract was transferred into a Teflon-lined stainless-steel autoclave and heated at 180 °C for 24 h. The reaction temperature (180 °C) and duration (24 h) were adopted from previous literature demonstrating that these conditions are effective for the hydrothermal carbonization of plant-derived precursors and the formation of fluorescent CQDs.^33,34 After natural cooling to room temperature, the resulting brownish suspension was centrifuged at 5000 rpm for 20 min to sediment larger carbonaceous aggregates and incompletely carbonized particles. The clear supernatant, containing the CQDs, was carefully collected and filtered to remove any residual coarse particulates before further characterization. The purified solution exhibited strong fluorescence under ultraviolet light, confirming the successful formation of CQDs, as shown in Fig. 1.

Fig. 1. Dye extraction from the Nerium oleander flowers.

2.2. Chitosan solution preparation and composite film fabrication

Chitosan powder (degree of deacetylation: 75%) was used to prepare the polymer solution. Briefly, 2.5 g of chitosan was dissolved in 250 mL of 1% (v/v) acetic acid under continuous magnetic stirring for 12 h to obtain a homogeneous solution. Subsequently, 0.625 g of glycerol was added as a plasticizer and stirred until completely dissolved. The incorporation of glycerol reduced the intermolecular interactions between chitosan chains, thereby enhancing the flexibility and mechanical integrity of the resulting films.

Three film formulations were prepared. The first film, designated CSCD0, consisted of the chitosan/glycerol solution without the addition of CQDs and served as the control sample. The CQD solution, synthesized as described in the previous section, was obtained as an aqueous dispersion and used directly without further dilution. To prepare the composite films, 20 mL and 40 mL of the CQD aqueous solution were separately added to identical chitosan solutions to produce CSCD20 and CSCD40, respectively. Each mixture was magnetically stirred until a homogeneous dispersion was achieved. The resulting solutions were cast into clean Petri dishes and dried at room temperature for approximately 120 h to obtain the chitosan/CQD composite films. The overall preparation procedure is illustrated in Fig. 2.

Fig. 2. Schematic of the CQD fabrication and CQD-doped CS composite film fabrication.

2.3. Characterization techniques

The functional groups in the BQD and their possible interactions with the polymer composites were analyzed using an FTIR spectrophotometer (Thermo Scientific, Nicolet iS10) operating over a wavenumber range of 600 to 4000 cm⁻¹, with a resolution of 2 cm⁻¹. UV-vis absorption spectra were acquired using a high-performance Jasco V-570 UV-vis-NIR spectrophotometer, offering insights into the optical characteristics of the BQD, including their absorption, emissions, and the band gaps of the polymer, BQD, and BQD-doped polymer composite. XRD diffraction for the polymers and BQD-doped polymers was conducted using a Philips PW1730 X-ray diffractometer, which utilizes Cu Kα radiation (wavelength λ = 1.15 Å) as its X-ray source; the 2θ scanning range was 10–80°. This was used to assess the crystallinity of the polymers and BQD-doped polymers.

2.4. Quantum yields (QY)

Fluorescein served as the standard material. The QY was calculated using the following formula:

1

where Φ, A, and I are the QY, absorbance, and integral emission fluorescence, respectively, Φ_R, A_R, and I_R are fluorescein QY, absorbance, and integral emission FL, respectively, and η is the refractive index of the solvent.

3. Results and discussion

3.1. Fluorescence (FL) properties

The optical characteristics of the carbon quantum dots (CDs) derived from Nerium oleander flowers were systematically analyzed through their fluorescence (FL) and absorption spectra; the fluorescence emission behavior of the CDs under various excitation wavelengths is presented in Fig. 3. The CDs displayed a pronounced excitation-dependent emission, where increasing the excitation wavelength from 300 nm to 400 nm resulted in a gradual red shift (bathochromic shift) of the emission maxima; this phenomenon is a hallmark of many carbon-based nanostructures and is typically attributed to the heterogeneous distribution of emissive sites, surface defect states, and particle size variations within the CDs.^35,36 Such excitation dependence suggests that multiple energy levels, associated with different surface functionalities and oxygen/nitrogen-containing groups, participate in the emission process.³⁷ The highest fluorescence intensity was recorded at 360 nm excitation, corresponding to a maximum emission peak centered at 433 nm, which indicates efficient radiative recombination of excited-state electrons. The shift toward longer wavelengths with increasing excitation energy implies that different emissive traps are selectively activated depending on the excitation wavelength, a common feature in CDs synthesized via green or biomolecular precursors.³⁸ The excitation spectrum monitored at 433 nm emission (Fig. 4) further confirmed that 360 nm is the optimal excitation wavelength, as both the excitation and emission bands exhibit strong overlap, suggesting an efficient electronic transition between the ground and excited states. This excitation–emission behavior can be attributed to the combined effects of π–π* transitions in sp²-hybridized carbon domains and n–π* transitions from surface oxygenated functional groups such as C O and C–OH.³⁹ Furthermore, the quantum yield (QY) of the synthesized CDs was calculated to be 2.96%, using fluorescein (QY = 95%) as the standard reference under identical conditions. Although the QY is relatively moderate compared with synthetic CDs prepared through high-temperature solvothermal routes, it is consistent with values typically reported for green-synthesized CDs derived from plant-based sources.⁴⁰ The moderate QY can be attributed to the presence of nonradiative pathways introduced by surface defects or incomplete carbonization, which partially quench photoluminescence; nevertheless, the emission stability, broad excitation response, and visible-range fluorescence confirm the suitability of these CDs for advanced photonic and environmental applications, including bioimaging, photocatalysis, and sensing.⁴¹

Fig. 3. FL spectra of the CDs at various excitation wavelengths.

Fig. 4. Excitation spectrum and emission spectrum of the CDs.

3.2. UV-vis absorption analysis of CDs

The UV-vis absorption spectrum of the Nerium oleander-derived carbon quantum dots (CDs) exhibits three distinct absorption bands at approximately 209 nm, 276 nm, and 328 nm, followed by a continuous absorption tail extending into the visible region (Fig. 5); these absorption characteristics are typical of carbon-based nanodots composed of conjugated π-domains and oxygen-containing surface groups. The strong absorption peak at 209 nm is assigned to the π → π* transition of C C bonds, originating from aromatic domains and conjugated sp²-hybridized carbon centers;^37,42,43 this high-energy band reflects the presence of small conjugation segments within the CDs, suggesting a predominance of localized π-electron systems rather than extended graphitic networks. The short conjugation length is consistent with the mild synthesis temperature and oxygen-rich nature of the biomolecular precursor, which limits carbonization and induces a blue shift in the π → π* transition.⁴⁴ The second absorption feature at 276 nm corresponds to the π → π* transition associated with the graphitic sp² core of the CDs;⁴⁵ this band is characteristic of the intrinsic π-electron delocalization within nanocrystalline graphitic domains, reflecting partial graphitization achieved during the carbonization of the Nerium oleander extract. The relative intensity of this peak compared with that at 209 nm indicates that the CDs contain a mixture of amorphous and crystalline carbon regions, where small sp² clusters are embedded within an amorphous carbon matrix.⁴⁶ The presence of this absorption feature is widely reported for green-synthesized CDs, in which the graphitic core contributes to stable luminescence and charge delocalization across the conjugated network. The third absorption band observed at 328 nm is attributed to the n → π* transition of carbonyl (C O) functional groups located on the surface of the CDs;⁴⁷ this transition originates from the excitation of nonbonding electrons (n) on oxygen atoms into the π* antibonding orbitals of C O bonds. The appearance of this band provides clear evidence of oxygenated surface functionalities, including carboxyl, amide, and hydroxyl moieties, introduced during the hydrothermal carbonization of the floral extract;⁴⁸ these surface groups not only stabilize the CDs in aqueous media but also introduce emissive surface states that govern the photoluminescence behavior and excitation dependence observed in Fig. 3 and 4. The broad and slowly decaying absorption tail extending into the visible region is typically attributed to defect-state absorption or sub-bandgap transitions, indicating the presence of localized energy states within the optical bandgap.⁴⁹ Such a tail is a characteristic feature of CDs containing multiple surface traps and heterogeneously distributed oxygen/nitrogen functional groups, so this optical behavior supports a core–shell structural model, where a π-conjugated sp² carbon core (responsible for π → π* transitions) is surrounded by a shell rich in oxygenated groups (contributing to n → π* transitions and surface trap states). The coexistence of these features provides a rational explanation for the excitation-dependent fluorescence emission: as the excitation wavelength increases, electrons are promoted to different trap levels associated with surface or edge states, resulting in a bathochromic shift in the emission maxima. Furthermore, the relatively weak absorption in the visible range and the absence of any strong band above 350 nm confirm that the CDs consist predominantly of small conjugated domains rather than large graphitic sheets, so this structural feature aligns well with the observed blue photoluminescence (λ_em = 433 nm) and a moderate quantum yield of 2.96%, typical of green-synthesized CDs where surface defects act as both radiative and nonradiative recombination centers.⁵⁰ Moreover, the combined UV-vis and photoluminescence characteristics suggest that the synthesized CDs possess a highly tunable optical response, making them suitable for environmental and photonic applications, such as fluorescence sensing, photocatalysis, and optoelectronic devices, where surface-state engineering plays a crucial role.⁵¹

Fig. 5. UV-vis absorption spectrum of the Nerium oleander-derived carbon quantum dots (CQDs) showing the characteristic π → π* and n → π* transitions.

3.3. FTIR studies

Fourier-transform infrared (FTIR) spectroscopy was employed to elucidate the structural features of the synthesized carbon quantum dots (CQDs), pristine chitosan (CSCD0), and the CQD-doped chitosan films (CSCD20 and CSCD40). In light of these techniques, the FTIR spectrum of the prepared CQDs (Fig. 6a) revealed characteristic peaks at 3275 cm⁻¹ (O–H stretching), 2917 and 2845 cm⁻¹ (C–H stretching), 1701 cm⁻¹ (C O stretching), 1576 cm⁻¹ (N–H bending or C C aromatic stretching), and 1065 cm⁻¹ (C–O stretching), confirming the presence of hydroxyl, carboxyl, and amine functionalities on the CQD surface.^52,53 These polar groups are consistent with oxygen and nitrogen doping, contributing to the solubility and functional reactivity of the CQDs. The FTIR spectrum of pristine chitosan (Fig. 6b) showed a broad band at 3281 cm⁻¹ corresponding to O–H and N–H stretching vibrations due to extensive hydrogen bonding, along with bands at 2914 and 2867 cm⁻¹ attributed to C–H stretching of aliphatic chains; also, the amide I and II bands appeared at 1642 and 1554 cm⁻¹, respectively, indicating the presence of residual N-acetyl groups and free amine functionalities.⁵⁴ The peak at 1084 cm⁻¹ was assigned to C–O–C stretching vibrations from glycosidic linkages, while the band at 846 cm⁻¹ was associated with skeletal vibrations of the pyranose ring.⁵⁵ Upon doping chitosan with CQDs, spectral shifts and intensity changes were observed, as shown in Fig. 7, reflecting interactions between the CQDs and the chitosan matrix. In the CSCD20 film (Fig. 7a), the O–H/N–H stretching band appeared at 3256 cm⁻¹, slightly red-shifted, suggesting the formation of new hydrogen bonds. Also, the C–H stretching bands remained at 2920 and 2867 cm⁻¹, while the amide I and II bands were observed at 1639 and 1554 cm⁻¹, respectively, showing minor shifts indicative of interaction without disruption of the chitosan backbone. The C–O stretching shifted to 1015 cm⁻¹, and the pyranose ring band appeared at 852 cm⁻¹. For the CSCD40 film, further red shifts were noted, with the O–H/N–H band at 3250 cm⁻¹ and C–H bands at 2924 and 2855 cm⁻¹. The amide I band persisted at 1639 cm⁻¹, while the amide II band slightly shifted to 1558 cm⁻¹. The C–O–C band shifted further to 1018 cm⁻¹, and the pyranose ring band was detected at 849 cm⁻¹. These systematic shifts and variations in peak intensities with increasing CQD concentration confirm the successful incorporation of the CQDs into the chitosan matrix and indicate the presence of specific interactions, primarily hydrogen bonding and possibly electrostatic interactions, between the functional groups of the CQDs and chitosan (Fig. 8). Such interactions may influence the overall structural integrity, hydrophilicity, and potential functional applications of the resulting nanocomposite films.

Fig. 6. FTIR spectra of the (a) synthesized carbon quantum dots (CQDs), showing the characteristic functional groups, and (b) pristine chitosan film (CSCD0), confirming the typical chitosan structural features.

Fig. 7. FTIR spectra of the CQD-doped chitosan films: (a) CSCD20 (doped with 20 mL CQDs) and (b) CSCD40 (doped with 40 mL CQDs), illustrating the peak shifts and intensity changes and indicating the interactions between chitosan and CQDs.

Fig. 8. Schematic of the molecular interactions between the chitosan polymer chains and surface functional groups of the carbon quantum dots (CQDs).

3.4. XRD analysis

Fig. 9 illustrates the XRD pattern obtained for CQDs, pure CS, and CS-doped films. Especially for detecting crystallinity, phase composition, and lattice changes owing to doping, X-ray diffraction (XRD) is an essential method for investigating the structural characteristics of materials; also, the naturally occurring biopolymer chitosan has a semi-crystalline form shaped by molecular arrangement and hydrogen bonding. Structural alterations resulting from doped Nerium oleander quantum dots (QDs) at different concentrations are visible via variations in diffraction angles and crystallinity.

Fig. 9. XRD patterns of CQDs, pure chitosan and CS doped with 20 and 40 mL of CQDs.

The XRD pattern of the synthesized NOQDs first confirms the successful formation of nanocarbon material. A characteristic broad diffraction peak centered at approximately 20–25° together with the graphite-related (002) reflection at around 26.5° and a weak diffraction peak in the 40–45° region are observed. The broad hump reflects the amorphous/disordered carbon framework and nanoscale dimensions of the quantum dots, whereas the peak near 26.5° is attributed to graphitic carbon domains, confirming the partial graphitic structure of the synthesized NOQDs.

The XRD patterns of pure chitosan (CSCD0) and the NOQD-doped chitosan films (CSCD20 and CSCD40) further reveal the influence of NOQDs on the polymer structure. Pure chitosan exhibits a broad diffraction peak at 20.56°, characteristic of its semicrystalline nature. After incorporating NOQDs, this peak shifts to 21.00° for CSCD20 and 21.42° for CSCD40, indicating interactions between the oxygen-containing functional groups of the NOQDs and the hydroxyl and amino groups of chitosan. These interactions promote structural rearrangement within the polymer matrix, resulting in modified crystallinity and improved structural organization. The observed peak shift confirms the successful incorporation of NOQDs into the chitosan matrix and suggests enhanced interfacial compatibility, which is beneficial for the development of chitosan-based nanocomposites for biomedical, sensing, and nanotechnology applications.

Excessive quantum dots (QDs) at higher doping levels (CSCD40) have the potential to disrupt the intermolecular hydrogen-bonding network in chitosan, which might result in a decrease in crystallinity despite the peak shift and a compromise of long-range order. Moreover, the crystalline phase was altered, as demonstrated by the observed shift in peak position, which indicates that there were intermolecular interactions between chitosan and QDs; these interactions most likely occurred via hydrogen bonding or coordination effects. Nerium oleander QDs doped into chitosan cause structural changes; first, ordered chain alignment decreases the crystallinity, then probably leads to disturbance at greater doping levels. These structural modifications affect the mechanical, optical, and thermal characteristics, thereby rendering the material appropriate for use in nanotechnology and biomedicine. Pure chitosan generally has stronger intermolecular hydrogen bonds between amines and hydroxyl groups than NOQD-doped chitosan. When the doping amount was 20 mL, the diffraction angle increased and further increased with increasing NOQD concentration, indicating a decrease in crystallinity and intermolecular hydrogen bonding. Acting as foreign additions, QDs disturb the normal packing of chitosan chains when they mix with the matrix. The decreased peak intensity implies a drop in the proportion of crystalline domains, i.e., the chitosan matrix has a higher number of amorphous phase regions. For medical and nanocomposite uses, this may enhance the optical and mechanical characteristics as well as the flexibility of the material.

3.5. Morphology investigations

One of the most important methods for studying surface morphology is using field-emission scanning electron microscopy (FESEM), which is widely used. As shown in Fig. 10A, chitosan is homogeneous. In FESEM, a homogeneous sample is one in which the surface does not show signs of significant surface irregularities, such as fractures, large pores, or phase separation. The fact that there is little to no agglomeration of polymer chains or contaminants in the chitosan matrix or film is an attribute of its good formation. At the nanoscale, the homogeneity of chitosan may be achieved by the formation of strong hydrogen bonds between the chains of chitosan and the use of a manufacturing technology that is well-regulated.⁵⁶

Fig. 10. Morphological appearance of (A) pure CS, (B) CS doped with 20 mL of CQDs and (C) CS doped with 40 mL of CQDs.

Carbon, hydrogen, nitrogen, and oxygen are the four elements that make up CNDs. The composition of CNDs might change depending on the precursor source and the method of production. Carbon and oxygen are the most prevalent components, and they are typically accompanied by a large number of carboxyl (COOH) groups throughout the composition. A significant portion of the characteristics and potential applications of CNDs are determined by the specific synthesis technique and the precursor that is used.⁵⁷ The FESEM findings indicate that CNDs generated with 20 mL of Nerium oleander extract resulted in partial aggregation, due to the formation of both large and small lumps instead of a uniformly dispersed phase, as shown in Fig. 10B. These functional groups, in conjunction with other chemical groups, enhance water solubility and surface modification, which in turn makes it possible to achieve greater levels of functionalization. These functional groups on the CNDs have the capacity to promote electrostatic interactions and hydrogen bonding, both of which have the potential to result in particle clustering rather than dispersion of the particles in a uniform manner.⁵⁸ In spite of the fact that the oxygen-containing groups make carbon nanodots (CNDs) more soluble in water, certain particles may have inadequate surface functionalization, which would result in partial agglomeration. Phase separation and localized aggregation arise if quantum dots are inadequately dispersed at low concentrations (20 mL). The interaction between the quantum dots and the chitosan matrix intensifies at higher concentrations, resulting in improved film dispersion. A more uniform structure is attained by raising the quantity of quantum dots, hence enhancing electrostatic interactions with the CS chains, as shown in Fig. 10C. Another reason is the increased interactions between chitosan and the quantum dots. Because the quantum dots have higher amounts of OH, –COOH, and –NH₂ at high concentrations, this enables more hydrogen bonds to form with the CS. These are the main reasons for the improved film quality and increased compatibility. As mentioned in the XRD section, the material with 40 mL quantum dot concentration is higher in crystallinity than the 20 mL counterpart. These functional groups, when present in sufficient quantities, provide electrostatic repulsion, which keeps the particles from clustering together. However, particle clustering occurs at lower concentrations because the surface charge is not enough to overcome the van der Waals forces. The most important evidence is that no phase separation can be seen, which reveals strong interaction between the added CQDs and chitosan molecules.

3.6. Thermal behavior via TGA-DSC analysis

The thermal behavior of the pristine chitosan film (CSCD0) and its nanocomposite counterparts doped with 20 mL and 40 mL of carbon quantum dots (CSCD20 and CSCD40, respectively) was investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The corresponding thermal profiles are depicted in Fig. 11a–c, and key thermokinetic parameters are summarized in Table 1. TGA analysis revealed that all samples underwent three main stages of degradation, typical of polysaccharide-based materials, so the initial minor mass loss below ∼180 °C was attributed to the release of moisture and volatile impurities.⁵⁹ The primary thermal degradation of the chitosan matrix occurred in the range of ∼190–330 °C, with increasing onset and peak temperatures observed upon CQD incorporation. CSCD0 showed a peak decomposition temperature at 240 °C with a total mass loss of 34%, while CSCD20 and CSCD40 showed peaks at 270 °C and 295 °C, with reduced mass losses of 30% and 26%, respectively. These trends indicate that CQD doping suppresses thermal decomposition, likely due to restricted chain mobility and the heat-shielding effect imparted by the carbonaceous nanophase.⁶⁰ The activation energy (E_a) calculated by the Coats–Redfern method confirmed this trend, rising from 13.96 kJ mol⁻¹ in CSCD0 to 16.08 kJ mol⁻¹ in CSCD20 and further to 18.74 kJ mol⁻¹ in CSCD40; this progressive enhancement suggests that the thermal degradation pathway requires more energy as CQDs are introduced, consistent with stronger polymer-nanoparticle interactions and enhanced network rigidity.⁶¹ DSC analysis provided further insights into the phase transitions within the materials. CSCD0 exhibited a sharp endothermic peak at 255 °C, indicative of the melting of semi-crystalline domains; this peak shifted to 275 °C and 295 °C in CSCD20 and CSCD40, respectively, reinforcing the conclusion that CQD incorporation stabilizes the chitosan matrix thermally. Additionally, the glass transition temperature (T_g), inferred from the baseline shift in heat flow, rose with CQD content: from ∼95 °C in CSCD0 to ∼110 °C in CSCD20 and ∼120 °C in CSCD40. This T_g elevation confirms that CQDs reduce polymer chain mobility by creating a more interconnected or constrained network structure (R4). Moreover, the combined TGA and DSC results underscore the thermally reinforcing role of carbon quantum dots in chitosan films. CQD doping not only delays the onset and peak of decomposition but also increases the energy required for degradation and raises key thermal transition temperatures (T_g and T_m). These enhancements make CQD-doped chitosan films more suitable for applications where thermal endurance and structural stability are essential, such as in packaging, biomedical scaffolds, and sensor matrices.^62,63

Fig. 11. TGA and DSC plots of the (a) CSCD0, (b) CSCD20 and (c) CSCD40 films.

Table 1. Thermal degradation summary of the CSCD0-40 films.

Sample	Main decomposition stage (°C)	Peak DTG temp. (°C)	Initial mass (%)	Final mass (%)	Total mass loss (%)	Activation energy (Ea, kJ mol−1)	Glass transition temp. (Tg, °C)	Melting temp. (Tm, °C)
CSCD0	∼190–300	240	100	−67	34	13.96	95	255
CSCD20	∼200–320	270	100	−71	30	16.08	110	275
CSCD40	∼210–330	295	100	−75	26	18.74	120	295

3.7. Optical properties

3.7.1. Absorbance, transmittance and reflectance measurements

Certain CS optical characteristics and the influence of CQDs on CS's optical behavior can be identified from the UV-vis spectrum. To this end, we measured UV light between 200 and 1100 nm. Stronger intermolecular interactions such as van der Waals forces and hydrogen bonding might provide wider absorption bands at longer wavelengths, therefore lowering the energy gap between the ground and the excited states. Trace quantities of C O and C N conjugation are responsible for the π → π* transition seen by the peak at 200 nm. Either residual acetyl groups already present in chitosan or oxidative degradation might cause this change. The n → π* transition is attributed to the peak seen at a wavelength of 300 nm, which most likely results from the interaction between conjugated bonds within the structure and lone-pair electrons on oxygen or nitrogen.

When the doping rate is low (20 mL) in the chitosan polymer, as shown in Fig. 12A, a broad band is observed around 20.1 nm, indicating an π → π* electronic transition. However, when the doping concentration is increased to 40 mL, three distinct peaks are formed. The first is located at 240.1 nm, and the second peak around 301.2 nm. The enhancement resulting from the interactions between quantum dots and chitosan is likely due to n → π* transitions of non-bonding electrons on nitrogen (NH₂ groups) or oxygen (OH groups). The appearance of a third peak centered at 402.12 nm is associated with the presence of surface plasmon resonance generated by the quantum dots (QDs), which greatly increases the material's absorption of radiation. This is also accompanied by a red shift and a change in the energy of the absorbed photons from the ultraviolet (UV) to the visible region. Moreover, this transition is correlated with an increase in the energy state's width, which is probably a result of NOQD aggregation. The results show that the electronic structure of the material can be changed radically by the increase in QD concentration, which enhances the potential applications of the material for optoelectronic, photocatalytic, and sensing devices. As far as the optical transparency is concerned, pure chitosan has a transmittance of 90% at 300 nm, which is a region where neither electronic transitions nor any significant absorption of UV radiation is observed. Contrastingly, the transmittance of QD-doped materials is very low, around 40%, over a wide range of wavelengths from 200 to 380 nm, as shown in Fig. 12B. The significant reduction of transmittance confirms that a significant amount of UV photons are absorbed, which is a desired effect that stems from the quantum confinement effect and resulting energy band changes in the QDs. Furthermore, the QDs cause additional electronic shifts and light scattering, which together enhance the UV protectiveness of the material. In the spectrum outside the UV region, the transmittance increases gradually from 380 nm to 800 nm, and then remains constant at 99% above 800 nm. This transparency to visible and near-infrared light is very interesting for applications in optical coatings or transparent films.

Fig. 12. (A) UV-vis absorption spectra of CS and CQD-doped CS films at various concentrations; the inset shows photographs of the CS : CQD film under UV irradiation and visible light. (B) Transmittance and reflectivity spectra of CS and CQD-doped CS films as a function of incident wavelength.

Strong UV absorption of NOQDs causes a significant increase in reflectance in the UV range (200–400 nm) when they are doped into chitosan, thereby increasing light scattering and re-emission and thus surface reflectance. The addition of QDs changes the refractive index of chitosan, thereby reflecting more light rather than letting it pass through. The reflectivity in relation to QD concentration (400–700 nm) and light scattering by QDs causes the reflectance to change depending on the QD concentration beyond 400 nm, as shown in Fig. 12B. Increased light scattering brought about by higher QD concentrations influences the visible spectrum reflectance. Higher doping levels allow QDs to participate in many electronic transitions, hence changing the reflectivity behavior at several wavelengths. Chitosan, QDs, and incoming light interact to produce interference events affecting reflectance up to 700 nm.

Our study shows better optical performance in the UV-vis region when using Nerium oleander-derived quantum dots doped into chitosan, as compared to the literature study that used a green colloidal chemical process to functionalize ZnS quantum dots (QDs) with chitosan in water at room temperature.⁶⁴ In particular, the composites of Nerium oleander QD and CS show increased absorption intensity, suggesting better photon capture. Also, the improved surface functionalization and quantum dot dispersion inside the chitosan matrix were likely caused by the natural phytochemical substances found in Nerium oleander (such as flavonoids, phenolic acids, tannins, terpenoids, saponins, and cardiac glycosides),⁶⁵ which also lead to increased reflectivity and transmittance. Based on these findings, our bio-derived quantum dots seem to provide a better platform for light interaction, which might lead to enhanced photocatalytic activity when exposed to sunlight. Generally, the addition of carbon quantum dots (CQDs) into the polymer matrix results in a red shift of the absorption edge and a lowering of the optical transmittance. The reason for this is the abundant π-conjugated domains and surface functional groups of the CQDs that allow the delocalization of π-electrons and create localized electronic states in the polymer band gap. Therefore, the possibility of electronic transitions and photon absorption is increased, and the nanocomposite may absorb more incoming light, particularly in the visible range. The optical transmittance becomes smaller, and the absorption edge shifts to longer wavelengths.

3.7.2. Absorption coefficient and refractive index study

The optical absorption spectra of materials provide critical insights into their band gap energies and structure. Also, a key parameter derived from these spectra is the absorption coefficient (α), which measures the light absorbed per unit thickness of a material.⁶⁶Fig. 13 presents the absorption coefficient for pure and NOQD-doped CS films; these data were calculated directly using the equation below:⁶⁷

α = 2.303A/t, 2

where A is the absorbance, and t represents the optical path length (film thickness). It is observed that the absorption coefficient (α) increases with increasing concentration of NOQD, as shown in Fig. 13, which is probably correlated to the structural changes in NOQD. These structural changes enable the efficient transfer of charges from the host polymer to the embedded nanoparticle system. The energy required for electronic transitions is rather high for pure chitosan (4.95 eV). The sheer amount of NOQDs added to the polymer, however, brings the energy requirement down to 2.45 eV and 2.22 eV with 20 mL and 40 mL of NOQDs, respectively, allowing this material to absorb lower-energy photons close to the visible light region. This change is due to the fact that the organization of the polymer structure is destroyed upon the incorporation of the quantum dots, leading to a reduction in molecular packing and an overall increase in disorder. This increase in the amorphous (less-crystalline) region, therefore, creates defect levels within the electronic band structure. These localized states and additional electronic states introduced directly by the quantum dots create a decrease in the energy barrier for photon absorption and a widening of the energy range of the absorption.

Fig. 13. Absorption coefficient variation with the incident photon energy of CS and doped NQQDs.

The development of novel materials is often guided by two most significant considerations: optical dielectric constant and refractive index. The refractive index (n) for the CS film as well as the CS films doped with NOQDs was ascertained using the Fresnel formula.⁶⁸ Thus, the reflectance (R) and extinction coefficient (k = αλ/4π) values were measured. The development of optoelectronic devices depends on the accuracy of data on the refractive index characteristic. Theoretically, the refractive index of the medium connects its density and mean polarizability with a particular temperature and pressure. As such, the refractive index determines the optical efficiency.⁶⁹ The refractive index also determines the reflectance as well as absorption. The complex refractive index of the films was obtained from eqn (5) (ref. 67) as follows:

3

n × λ = n(λ) + k(λ), 4

5

As shown in Fig. 14A, the extinction coefficient increased with increasing wavelength when the quantum dots were doped into the chitosan matrix. This is due to several factors, including increased light absorption, which indicates an increased response of the material in the visible range. As a result of the decreased crystallinity and bandgap energy, the electronic structure of the chitosan changed. As shown in Fig. 14B, the refractive index increased when the chitosan matrix interacted with NOQDs, remained unchanged at the lowest wavelength from 200 nm to 400 nm, and decreased thereafter. When the amount of NOQDs increased, the refractive index increased, invariably up to 520 nm. This results from increased optical density and increased polarizability for electromagnetic waves. Including QDs into the CS matrix promotes material compactness and lowers free volume. Since light encounters greater resistance in a denser substance, its refractive index increases. The higher refractive index in the UV spectrum means materials absorb/scatter UV light with greater effectiveness, so they are valuable in optical filters, UV-protective coatings, optical coatings, and photonic equipment. The observed refractive index in the research on chitosan (1 − x): CuO_x (4 ≤ x ≤ 12) based polymer nanocomposites is almost 2.6. By contrast, our present work shows a higher refractive index of 2.7, which stays very consistent from the UV to visible wavelengths. This stability and higher refractive index value point to better optical homogeneity and stronger light interaction, which qualifies our material for applications that require constant refractive behavior across a wide spectrum.⁷⁰ The refractive index values obtained in the current investigation are higher than those reported in earlier studies,³² which indicates the enhanced optical performance of the produced CS/CQD nanocomposites. The enhanced refractive index is ascribed to the strong interfacial interactions between the CQDs and the chitosan matrix, the increased delocalization of the π-electrons and the formation of localized electronic states, which together enhance polarizability and light–matter interaction in the nanocomposite, rendering the developed nanocomposites promising candidates for optoelectronic applications.

Fig. 14. (A) Extinction coefficient and (B) refractive index variations of CS and the doped Nerium oleander quantum dots.

3.7.3. Optical dielectric function study

Figuring out the optical responses of materials depends greatly on the optical dielectric constant, which consists of imaginary (ε_i) and real (ε_r) components. It is a basic characteristic that controls the interaction of a material with electromagnetic waves, therefore affecting characteristics, like light absorption, reflection, and transmission, whereas ε_i relates to energy dissipation resulting from absorption losses, and ε_r indicates the capacity of the material to store electric energy. Both parameters have been calculated using the following two equations:⁷¹

ε_r = n² − k², 6

ε_i = 2nk. 7

The addition of quantum dots to polymer matrix materials like chitosan yields a huge modulation of the composite material's optical dielectric function. This tunability, which is accomplished through the variation of the bandgap, the delocalization of charges, and the resonance of surface plasmons, is very beneficial for applications in solar cells, sensors, optical coatings and nonlinear optics. This study focuses on elucidating the mechanism of the increased real dielectric constant (ε_r) and its wavelength dependence and thus is related to the structural changes, dopant interactions and electronic transitions that determine the macroscopic optical properties. In Fig. 15A, ε_r is enhanced at low photon energy, and the enhancement is directly proportional to the concentration of quantum dots. These dopant-matrix interactions result in separate peaks in the real and imaginary spectra of the dielectric function. Most importantly, a larger dielectric constant makes the material more polarizable in the presence of an electric field, which directly promotes highly efficient charge separation in the material. Based on the existing methods, we calculated the bandgap energy and type of electronic transitions using the imaginary part of the dielectric function. As shown in Fig. 15B, the results show an inverse relationship: the higher the NOQD concentration, the lower the bandgap energy. These enhancements of the optical dielectric properties at lower photon energies arise from a synergistic interplay of various factors: a gain in electronic polarization, localized defect states (as demonstrated by XRD analysis), an increase in the charge carrier density provided by the NOQDs, and strong quantum effects. Moreover, this dielectric response can be concentration-dependent, mainly caused by the incorporation of highly polarizable units, which, if present, will significantly change the electronic environment of the composite and render it more charge delocalized. Finally, our NOQD-doped system showed better optical dielectric properties than the traditional CuO-doped PVA/CS nanocomposites in the low-energy regime, which is attributed to the considerable increase in the polarizability and light–matter interaction. Effective dopant integration and dispersion within the CS matrix help to explain the improvement in charge distribution and dielectric response over the CuO-doped PVA/chitosan system.⁷²

Fig. 15. (A) Real and (B) imaginary parts of the optical dielectric constant of CS and 20 mL- and 40 mL-loaded NOQDs.

3.7.4. Optical band gap analysis

Through the process of electrons transitioning from the lower energy state or ground state to higher energy levels in σ, π, and n-orbitals, polymer compounds are able to absorb ultraviolet energy emanating from light, a phenomenon known as UV-vis absorption.⁷³

The optical band gap (E_g) is the energy difference between the maximum of the valence band (VB) and the minimum of the conduction band (CB), which is the energy area where no electronic states can exist. The energy of the electron must be greater than or equal to E_g to transition from the valence band to the conduction band. The size of the band gap is an important factor in determining the optical and electrical characteristics of a material. Often, broad band-gap materials demonstrate poor absorption in the visible region, restricting their applicability in many optoelectronic devices. The reduction of the band gap increases the absorption of visible light and raises their potential for optoelectronic devices.⁷⁴ One method that may be used for the investigation of optically induced transitions is the optical absorption approach. This section provides information on the band structure and energy gap in crystalline as well as amorphous materials.⁷⁵ The process of absorption plays a crucial role in establishing the optical properties of polymers based on their composition. The following equation is referred to as the Tauc model and was used to obtain the absorption coefficient based on the UV-vis spectra:

α(v) = B(hv − E_g)^x/hv. 8

Different linear connections were found: one for x = 1/2 (direct transition), and a second for x = 2 (indirect transition). Both of these relationships were independent of one another. It is possible to assume that factor B remains constant over the whole range of optical frequencies.⁷⁶ This is because factor B is dependent on the transition probability. In this way, we calculated the bandgap energy as shown in Fig. 16A and B.

Fig. 16. Bandgap energy calculations of CS and CQD-doped CS films at different CQD concentrations using the Tauc and ASF methods: (A) Tauc plots for direct allowed transitions, (B) Tauc plots for indirect allowed transitions, (C) ASF plots for direct allowed transitions, and (D) ASF plots for indirect allowed transitions.

Another accurate method is to use ASF to calculate the bandgap energy, which uses a mathematical model to directly match the UV-vis absorbance spectra as follows:

9

By using the wavelengths λ_g, h, and c, respectively, the absorption coefficient, which is denoted by the symbol α(λ), is linked to the optical band gap, Planck's constant, and the speed of light. As a consequence of Beer–Lambert law, eqn (10) will be altered or reorganized in the following manner:⁷⁷

10

Taking into consideration the magnitude of D₁ = [B(hc)^x−1d/2.303] in conjunction with the reflection that influenced the constant D₂. By fitting the absorbance spectra to eqn (10), it is feasible to determine the optical band gap, which is not dependent on the thickness of the film. By substituting the variable λ_g with the expression E^ASF_gap = 1239.83/λ_g, it is possible to derive the band gap quantity in electron volts (eV). To put it more simply, if we take the curve represented by vs. (1/λ) and extend its linear section at the point where is located, we may be able to obtain the value of E_g. Fig. 16C and D display the current sample plotted against (1/(λ)) for the values As shown by the simplest squares method, m is equal to 1/2 and 2.

Inherently, pure chitosan (CS) has a wide band gap, but adding Nerium oleander-derived carbon quantum dots (CQDs) to chitosan material is a good way to narrow the band gap. This reduction in the band gap is mainly attributed to the reduction in crystallinity of the polymer at low CQD concentration (as confirmed by XRD analysis). The resulting structural modification generates localized sub-bandgap energy states, which allow for easier electron transitions. Moreover, FESEM images show that, at these lower concentrations, the particles might be aggregated within the CS matrix, which can also help reduce the band gap. For higher CQD concentrations, on the contrary, the band gap still decreases, but the crystallinity recovers (but not as much as in the case of pure CS). Is there any reason for this? This effect is primarily due to surface functional groups (–OH and –COOH) present on the quantum dots that provide new intermediate energy levels. Moreover, the increased doping level leads to better dispersion of the particles and to better intermolecular interactions. These interactions lead to delocalization of charge carriers (electrons and holes) and reduce the energy needed for an electron transition. From the methodological side, the calculated values of the band gaps with the Tauc and absorption spectrum fitting (ASF) methods show very good agreement, especially for the direct optical transitions, which are in excellent agreement with the imaginary dielectric constant (ε_i) data. Finally, the optical properties of our Nerium oleander-based QD/chitosan nanocomposites are better than those of the available alternatives. Our system resulted in a minimum bandgap of ∼2.0 eV, which is much narrower than the ∼4.0 eV reported for the conventional chitosan-functionalized ZnS quantum dots as presented in Table 2. This effective band gap narrowing directly corresponds to boosted visible-light absorption, which leads to high efficiency of the photocatalysis under solar light. In addition to the decrease in band gap, the synergistic effect of chitosan biopolymer and the phytochemical-enriched plant extract results in greater UV-vis absorption intensity, transmittance and reflectivity. The results in this study indicate that our synthesized nanocomposite is a very efficient light-harvesting system that is more effective than the traditional systems based on ZnS/chitosan.⁶⁴ In another report, the film consisted of glycol chitosan-capped CdS quantum dots with a bandgap energy of 2.73 eV, showing good consistency, stability, and fluorescence.⁷⁸

Table 2. Bandgap energy calculations of the CS and doped-CSCDs based on the Tauc, ASF and ε_i methods.

Sample	E Tauc (eV)	E Tauc (eV)	E ASF (eV)	E ASF (eV)	E g ε i (eV)
CSCD0	5.14	4.65	5.16	4.63	5
CSCD20	2.91	2.1	2.94	2.11	2.48
CSCD40	2.5	1.99	2.51	1.98	2.28

The optical band gap values obtained from the optical dielectric loss spectra were compared with those predicted using the Tauc and absorption spectrum fitting (ASF) techniques to determine the most likely electronic transition, as described before.^79–81 The contrast between the band gap values obtained from the ε_i spectra and those obtained from the Tauc and ASF models is shown in Fig. 15 and 16 and summarized in Table 2. The results show that the values of the band gap of the CSCD0 and CSCD40 samples obtained from the ε_i spectra are more consistent with the results calculated by the Tauc model for a direct allowed transition (x = 1/2), which suggests that the direct transition is the dominant optical transition in these samples. For the CSCD20 sample, on the other hand, the band gap value derived from the (ε_i) spectrum is closer to that calculated by the ASF model for an indirect permitted transition (x = 2), showing that the indirect transition is more suited for this composition. The optical band gap was calculated using the ASF and Tauc methods. The ASF technique allows a direct evaluation of the absorption spectrum without knowing the thickness of the layer. On the other hand, the Tauc technique allows the absorption coefficient to be calculated from the measured absorbance and the thickness of the film by using eqn (2). The optical band gap is frequently better estimated using the Tauc method since it incorporates the experimental thickness and the absorption coefficient.

In the present work, we report a novel, non-toxic, and low-cost method for the production of carbon quantum dots (CQDs) from Nerium oleander flower extract using a green hydrothermal process and their incorporation into a chitosan (CS) polymer matrix. The produced CS/CQD nanocomposites show a significant decrease in optical band gap compared to earlier investigations (Table 3). The as-synthesized CQDs, with a rich abundance of functional groups (e.g., –OH, –NH₂, and C O groups) and heteroatom-containing surface defects, may induce strong intermolecular hydrogen-bonding interactions with the hydroxyl and amino groups of the CS polymer, which resulted in the observed improvement. These interactions improve the structural compatibility of the CQDs with the polymer matrix and increase the amorphous character of the composite. Moreover, the strongly π-conjugated carbon core of the CQDs allows delocalization of the π-electrons and the formation of local electronic states in the forbidden band gap, facilitating electron excitation from the valence band to the conduction band. Therefore, the optical band gap is greatly lowered, and the prepared CS/CQD nanocomposites are attractive candidates for optoelectronic applications.

Table 3. Comparison of the bandgap energy (E_g) of the prepared CS : CQD film with previously reported CS-based nanocomposite films.

Sample	E g (eV)	Reference
CS: Cu nanoparticle (NPs)	5.2–3.53	82
CS: ZnO NPs	4.66–3.26	83
CS: MgO NPs	3.7–3.4	84
CS: Fe2O3 NPs	3.16–2.11	85
CS: CQDs	5.25–2.55	31
CS: CD	5–2.28	Present work

3.7.5. W–D model study

Studying optical materials' dispersion characteristics is essential since these characteristics are important for optical communications performance and integrated optical system design. Using the single-oscillator model given by Wemple and DiDomenico, we assessed the energy dispersion of the optical conditions to find out if the dispersion parameters are correlated with the optical constants measured earlier in this study. The suggested model characterizes the dispersion of the refractive index using the following formula:⁸⁶

11

12

A material's average energy gap may be expressed as E_o, the energy of the single oscillator, and the average intensity of electronic transitions as E_d, the energy of dispersion. Finding the values of the oscillator's parameters is as simple as graphing the square of the photon energy (hv)² on the x-axis and the refractive index factor (n² − 1)⁻¹ on the y-axis, as shown in eqn (12). In this graph, we can see that the y-intercept is given by the equation (E_dE_o), and that the line's slope equals the expression's reciprocal, as shown in Fig. 17. One may determine the values of n_o, the static refractive index, and ε, the zero-frequency dielectric constant, by using the E_d and E_o values. With (hv) = 0, we may use DiDomenico's eqn (12) to find them. Next, we may determine the oscillator strength, f = E_dE_o, for the model with a single oscillator. The oscillator strength f describes the capacity of the initial and final states of the electrons to absorb photon energy⁸⁷ as follows:

13

Fig. 17. Plots of (n² − 1)⁻¹ against (hv)² for the CS and doped-CQD samples.

The polarizability level may be determined or understood by examining the relationship between the material's polarization distribution and vibrational strength, as well as the moments of the optical spectrum M₋₁ and M₋₃⁸⁸ as follows

14

15

Due to changes in the electronic structure of the CS, the performance is enhanced in the NOCQDs. This is due to several reasons. Central quantum dots (CQDs) improve the electrical polarization in a polymer matrix by introducing localized states. Photons and electrons interact more strongly as a result, leading to an increase in E_d. However, on the contrary, E_g ∼ E_o, which is due to the decreased crystallinity compared to pure CS, as well as the formation of particle aggregation and interactions between them. Due to the decrease in band gap energy, because the localized electronic state has increased and the charge-carrier transition has increased, M₋₁ and M₋₃ increased, as shown in Table 4. Higher dispersion moments are the result of the dielectric response being altered and the oscillator strength being amplified by these extra states. Using green tea as a ligand doped into CS, the behavior of our Zn-MC system exhibits behavior very comparable to that reported in previous investigations, therefore demonstrating consistency and dependability in the optical and structural features attained.⁸⁹

Table 4. Optical dispersion parameters of the CS doped CQDs.

Sample	E d (eV)	E o (eV)	M −1 (dimensionless)	M −3 (eV−2)	n o (dimensionless)	f (eV2)
CSCD0	1.327	4.683	0.283	0.012	1.132	6.219
CSCD20	1.546	2.872	0.538	0.065	1.240	4.440
CSCD40	2.358	2.745	0.859	0.114	1.363	6.475

3.7.6. Optical dielectric parameters calculations

The real dielectric constant, denoted by the symbol ε_r, is tied to several factors, including the high-frequency dielectric ε_∞, the carrier concentration N_C, and the plasma resonance frequency ω_p. It would be beneficial to identify these parameters to gain a comprehensive understanding of the optical properties of the films that were investigated for use in photoelectric and optoelectronic applications. To ascertain these parameters, the equations shown below were used. These parameters were acquired by determining the slope and intercept of the straight lines that are generated from the plot that illustrates the relationship between (ε_r ∼ n²) and λ² (see Fig. 18A for clarification)⁶⁷ as follows:

16

17

Fig. 18. Plots of (A) ε_rversus λ² and (B) ε_iversus λ³ of CS doped with CQDs.

In this equation, e represents the electronic charge, while c represents the speed of light in free space. A representation of the derived values for these variables can be seen in Table 5. It is possible that these levels will rise with the amount of doped CQDs; this might be due to the fact that the refractive indices will also increase. To provide further clarification, the concentration of charge carriers in the system under investigation, as well as the amount of doped CQDs that is exposed to incident photons, both grow. One of the reasons for the increase in ε_∞ with increasing doped CQDs is that the refractive index rises. Frequently, CQDs have functional groups that contain O or N atoms, which enhance charge transfer. These surface states serve as concentrations of attraction for extra charge carriers, which results in an increase in As can be seen from eqn (17), w_p changes vertically at the root of so w_p has increased.

Table 5. Optical dielectric parameters of the CS and 20 mL- and 40 mL-loaded CQDs.

Sample	ε ∞ (dimensionless)	N/m* (kg−1 m−3)	τ × 10−19 (s)	µ opt × 107 (m2 V−1 s−1)	N C × 1026 (m−3)	ρ opt (Ω m)	ω p × 1015 (rad s−1)
CSCD0	1.38	1.836 × 1055	23.8	9.51	0.736	0.8918	0.230
CSCD20	3.03	1.118 × 1057	2.84	1.13	4.48	0.122	1.79
CSCD40	4.22	3.07 × 1057	3.6	1.43	12.32	0.035	2.98

After obtaining the ratio (N/m*) from eqn (16) and (17), the relaxation time, denoted by τ, is established by the following equation. This equation, which represents the ε_i, may be found in Fig. 18B (ref. 90) as follows:

18

The values of the relaxation time, τ, which are taken from the slope of the straight lines drawn by graphing ε_ivs. λ³ (referred to in Fig. 18B), are shown in Table 5. It is evident from the table that the relaxation time, τ, for the investigated film increases with increasing concentration of QDs in the CS matrix. Now that we have established this link, we can consider how the optical electron mobility, µ_opt, and optical resistivity, ρ_opt, are influenced by the QDs in the CS matrix⁹¹ as follows:

19

20

The FESEM results show that charge carriers undergo substantial scattering at lower concentrations of CQDs because of the massive aggregations, which reduces µ_opt. While there is an improvement in film homogeneity with increasing CQD doping, localized charge traps are still created, which decrease the relaxation period (τ) and further reduce it from 2.3 × 10⁻¹⁸ s to 3.6 × 10⁻¹⁹ s (Table 5). Carriers become more localized and optical mobility decreases when the CQD concentrations rise due to an increase in the dielectric constant and polarization. In light of the findings from this study, the optical resistivity decreased from 0.89 to 0.035 Ω m. Large aggregations, caused by defects and charge trapping, occur at low concentrations of CQDs. A smoother film surface, with less grain boundary scattering and better charge transport, is the result of increasing CQD concentrations. The presence of surface functional groups (–OH, –COOH, and –NH₂) and π-conjugated structures in the CQDs causes them to release more charge carriers. Through electrostatic interactions or hydrogen bonds, these functional groups enable charge transfer with the CS matrix. The most obvious and important cause for these changes in electronic structure is that, as the concentration of CQDs rises, there are more charge carriers accessible, which raises N, which in turn increases ω_p from 2.30 × 10¹⁴ to 2.98 × 10¹⁵ and decreases relaxation time. The optical mobility values obtained in the current investigation are better than the values reported in earlier studies⁸⁰ showing enhanced light–matter interaction in the fabricated CS/CQD nanocomposites. The poor optical mobility is attributed to the elevated refractive index of the nanocomposites, which is related to the slow propagation velocity of the light through the material. This behavior is attributed to the enhancement of the electronic polarizability added by the integration of the CQDs at a microscopic level. The CQDs include several π-conjugated domains and surface functional groups, which lead to more delocalization of π-electrons and hence higher interaction with the input electromagnetic field. This facilitates the polarisation of the electron cloud, which improves the dielectric constant and optical performance; hence, the nanocomposites are promising candidates for advanced optoelectronic applications.

3.7.7. Optoelectronic parameters study

The important factors affecting the optical response of a film are the wavelength of the photons impinging on the film at different angles throughout the optical spectrum and the wavelength of the photons hitting the film surface at right angles. On encountering the surface of the film, these photons excite the surface electrons, resulting in two significant parameters: thermal emissivity (ε_th) and sheet resistance (R_S). The first of these parameters establishes a measure of the film's radiative response, while the second establishes a measure of the film's electrical response; they can both be mathematically described by Kirchhoff's laws of radiation and conduction, respectively,⁹² as follows:

21

22

In this context, c is the speed of light, n is the refractive index of the sample, and T is the transmittance spectrum spanning 190 to 1100 nm. Fig. 19A displays the wavelength dependence of the two parameters. The rate of heat emission from a film is proportional to the photon's energy or wavelength when it strikes its surface. A number of factors affect thermal emission, the most important of which are temperature, viewing angle, and surface characteristics.

Fig. 19. (A) Sheet resistance (R_S) and (B) thermal emissivity (ε_th) variations with the incident wavelength of pure CS and doped QDs.

As the wavelength increases, the sheet resistance and thermal emissivity both increase (Fig. 19B). The UV-vis-NIR range is fairly transparent because the plasma frequency, i.e., the collective oscillation of free electrons, falls in the infrared part of the electromagnetic spectrum. At higher frequencies, the electrons are unable to follow the oscillating field, and very little is absorbed or reflected from the light. The relatively high surface resistance and the relatively high thermal emissivity of the pristine CS polymer can be attributed to its insulating and semi-crystalline nature and the low free charge carrier density.⁹³ The thermal emission can, so to speak, be modified. As shown in Fig. 19A and B, both parameters drop significantly when the material is doped with NOCQDs, indicating that the increased availability of mobile charge carriers and the changes in the electronic structure of the material towards a more conductive intermediate state as a result of CQD incorporation are responsible for the drop in both parameters. The introduction of CQDs therefore reduces the possibility of charge localization and enhances electron transport in the polymer matrix; moreover, the addition of the weak interaction between the vibrational mode and free electrons increases the thermal emissivity.⁹⁴ The structural and functional stability of optical components are important factors to consider when it comes to thermal stress, especially in high-power applications where too high a temperature can affect performance. Furthermore, low ε_th is important in this aspect since it will reduce the heat losses from the surface of the material, which will improve thermal management and the efficiency of operation of devices like solar cells.⁹⁵ With reduced emissivity, solar cells can hold more of the energy absorbed, enhancing the energy conversion efficiency as well as the transport of charge carriers. In addition, the R_S of the film has an inverse relationship with both the refractive index and reflectivity, and a direct relationship with the optical transmittance. In addition, the CQDs in the CS matrix will increase the reflectivity of the films and at the same time reduce the sheet resistance, which is a desirable combination for optoelectronic applications and will also help to increase the reflectivity of the films in certain layers of a solar device, thus creating a conducive environment for the redirection of photons to the active region. This effect that will increase the internal light trapping and maximise the photo-conversion efficiency.^56,96

3.7.8. Non-linear optical parameters

It is stated that a material has non-linear optical (NLO) properties if it responds in a manner that is not linear to variations in the intensity of the electromagnetic radiation that is falling on it. This feature is highly valued in a variety of technological fields, including communications, frequency transformation, and optical switching, amongst others. Considering that polymer complexes have the potential to be used in optical electronics, there has been a growing interest in research into their NLO features over the course of the last few years.⁹⁷ When a polymer based on chitosan is linked with QDs, they exhibit improved NLO properties, and new possibilities for practical applications emerge.⁹⁸

P = χ⁽¹⁾E + χ⁽²⁾E² + χ⁽³⁾E³ + … 23

The material's polarization is denoted by P, the linear susceptibility by χ⁽¹⁾, the second-order nonlinear optical susceptibility by χ⁽²⁾, which correlates to second-harmonic generation, and the third-order nonlinear optical susceptibility by χ⁽³⁾.⁸⁸ Therefore, through the application of the following relationships (eqn (24) and (25)), the values of χ⁽¹⁾ and χ⁽³⁾ may be determined as follows:

24

25

26

The NLO refractive index is denoted by n₂, while the linear refractive index will be denoted by n_o. The process of determining the numerical value of the NLO susceptibility (n) in order to calculate the NLO refractive index (n₂) is as follows. When the concentrations of the QD dye increased, the values of χ⁽¹⁾ and χ⁽³⁾ also increased, which is a fortunate development. CQDs enhance the polarizability of electronic signals by including delocalized electrons and π-conjugated systems. This leads to a rise in the χ⁽¹⁾, which is directly accountable for the optical reaction of the material of interest. At low concentrations, due to the significant decrease in crystallinity and the formation of aggregation of NOCQDs particles, χ⁽¹⁾ and χ⁽³⁾, and n₂ increase. However, with increasing QD concentration in the CS matrix, the crystallinity increased compared to the initial concentration, but the sample became more homogeneous, and the charge carrier ability increased. This increased the interaction of the film with the photon energy and further enhanced the material response, as shown in Table 6. Strong multi-photon absorption and greater non-linear optical coefficients are made possible by the declining optical bandgap as the CQD concentration rises. Quantum confinement shown by CQDs results in a larger density of electronic states close to the Fermi level. This amplifies higher-order electron transitions, thereby increasing the χ⁽³⁾. When it comes to fiber-optic communication, materials that have strong non-linear responses make it possible to convert energy entirely to optical signals, which reduces the need for electronic components and ultimately results in quicker data transfer. Our system, where Mn-MC is doped into CS, has a maximum third-order sensitivity (χ⁽³⁾) of 1.3 × 10⁻¹¹ esu, a χ⁽¹⁾ of 0.16, and a n₂ of 2.8 × 10⁻¹² esu, demonstrating its nonlinear optical (NLO) properties. The strong nonlinear optical response in our produced material is shown by these values being much higher than those achieved for the CS: Mn-MC system reported in the literature. Effective interaction between Mn-MC and the chitosan matrix is responsible for this augmentation as it increases the electronic polarization and optical nonlinearity.⁹⁹

Table 6. Non-linear optical parameters of CS and CS loaded with 20 and 40 mL of CQDs.

Sample	n average (dimensionless)	χ (1) (dimensionless)	χ (3) (esu)	n 2 (esu)
CSCD0	1.269	0.048	9.524 × 10−16	2.827 × 10−14
CSCD20	1.680	0.145	7.574 × 10−14	1.698 × 10−12
CSCD40	1.796	0.177	1.684 × 10−13	3.531 × 10−12

4. Conclusions

The current study successfully shows how to improve the optical and structural properties of chitosan (CS) biopolymer films by incorporating Nerium oleander-derived carbon quantum dots (CQDs). The excitation-dependent emission of the CQD particles observed in the FL study, indicating a gradual red shift (bathochromic shift) with increasing excitation wavelength from 300–400 nm, implied that multiple energy levels, corresponding to various surface functional groups and/or oxygen/nitrogen-containing groups, are involved in the emission process. The two effects are attributed to π–π* transitions from the sp²-hybridized carbon domains and n–π* transitions of the surface oxygenated functional groups (C O and C–OH), and are confirmed by the UV-vis spectra. The green-synthesized CQDs containing oxygen and nitrogen groups were efficiently bound with the chitosan matrix, resulting in more uniform and better interfacial bonding. The moderate shift of the XRD peaks and the increased thermal resistance indicated structural reinforcement and a reduction in chain mobility. The morphology suggested that good dispersion of CQDs into the host polymer was achieved and no phase separation occurred. Thermal properties determined by DSC and TGA indicated stable and improved thermal characteristics of the CS-doped films. Optically, the incorporation of CQDs resulted in a significant reduction in the bandgap energy, higher refractive index and marked redshifts, which suggest that the films formed by the incorporation of CQDs had improved light absorption and enhanced electronic transitions. The improvement in the optical dielectric response and NLO parameters revealed the potential of the films for optoelectronic, UV shielding and photonic device applications. In summary, this green strategy offers an efficient pathway towards engineered chitosan-based nanocomposites by light processing with tunable optical and thermal properties for sustainable applications in the fields of sensors, coatings and energy-related devices.

Ethics approval and consent to participate

The study involved natural flower material only and did not involve human participants or animals. Nerium oleander (NO) flowers were collected in the summer season. The flower material was taxonomically identified by Dr Karzan Qurbani (Department of Biology, College of Science, University of Raparin). The collection complied with local environmental guidelines. No protected species were involved, and no specific permit was required for the collection of fallen leaves. A voucher specimen (Accession no. RAP BIO-NO-2026-023) has been deposited in the Herbarium of the University of Raparin.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be available from the corresponding author upon reasonable request.