The sustainable green synthesis of fluorescent carbon quantum dots from red Aloe cameronii: antioxidant, anticancer and bioimaging applications†

Lokesh Bheemayya^a, Ravindra R. Kamble*^a, Vishwa B. Nadoni^a, Manojna R. Nayak^a, Mallika S. Wali^a, Arun K. Shettar^b and Joy H. Hoskeri^c
aDepartment of Chemistry, Karnatak University, Dharwad-580003, India. E-mail: ravichem@kud.ac.in
^bDepartment of Preclinical Research and Drug Development, Cytxon Biosolutions Pvt Ltd, Hubli-580031, India
^cDepartment of Bioinformatics and Biotechnology, Karnataka State Akkamahadevi Women's University, Vijayapura-586108, India

First published on 29th July 2025

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1. Introduction

The novel class of spherical or quasi-spherical fluorescent nanostructured materials referred to as carbon quantum dots (CQDs) has sparked a lot of interest due to their unique physicochemical properties, which include photobleaching resistance, high fluorescence, low cost, low water solubility, cell penetrability, and biocompatibility.^1,2 They have become a feasible and safe alternative to metal-based semiconductor quantum dots, which are also used for bioimaging, sensing, catalysis,³ optoelectronics, and nanomedicine.⁴ Over the last few years, significant progress has been made in the synthesis, characterization, and applications of carbon-based quantum dots, photoluminescent CQDs, and organic dyes due to their superior aqueous solubility, chemical inertness, ease of modification, and resistance to photobleaching.^5,6 CQDs have exhibited remarkable electronic capabilities as electron donors and acceptors, exhibiting chemiluminescence and electrochemical luminescence, which gives them a wide range of applications, such as in optronics, catalysis, sensing,⁷ bioimaging,⁸ nonlinear optical processes, cancer treatment,⁹ surface treatment, and environmental remediation.¹⁰ CQDs have been prepared using various synthetic approaches, including bottom-up and top-down routes.¹¹ Furthermore, many functional groups from bioinspired green precursors have enabled the production of self-passivated CQDs, and these are well known to have progressive effects on the material properties.^12,13 These advantages have spurred significant attempts to further expand this intriguing field by investigating a variety of green resources. Several attempts have been undertaken to enhance emission efficiency and synthesis productivity. A number of researchers have investigated the sensing and bioimaging capabilities of CQDs produced from bioinspired precursors. Various metal-ion sensors have been developed based on changes in the fluorescence intensity of CQD induced by energy transfer mechanisms.¹⁴ CQDs have excellent selectivity for toxic metal ions, such as mercury and lead, as well as for ions involved in biological activity, such as iron and carbon, and this has been reported in both aqueous solutions and actual samples. Furthermore, their nontoxicity toward cells, even at high concentrations, makes them ideal for biological applications.¹⁵ Several investigations have demonstrated their cell internalization and potential application as fluorescent probes in vitro. ROS play a crucial role in the damage of biological structures, degradation of chemical products and polymers, food spoilage, etc. Therefore, antioxidants and free-radical scavengers are crucial for applications in sectors as diverse as health, food, packaging, cosmetics, corrosion protection, etc.¹⁶ The emergence of CQDs has sparked a surge of interest, primarily due to their captivating optical properties, remarkable biocompatibility, and their potential for use as versatile theranostic agents, particularly the application of CQDs in cancer treatment.¹⁷ One such area of interest is the investigation of the anticancer activity of CQDs toward the lung cancer A549 cell line.^18,19 Lung cancer remains a leading cause of cancer-related deaths worldwide, underscoring the urgent need for the development of novel and effective therapeutic strategies. In view of the above discussion, the present findings of our investigation deal with the impact of varying CQD concentrations on the bioimaging of the lung cancer A549 cell line.

2. Experimental section

2.1. Chemicals

Sigma Aldrich provided all the analytical-grade chemicals and media components used in this experiment. The red aloe vera plant was purchased from a local market in Dharwad. Infrared spectra in the range of 4000–5000 cm⁻¹ were obtained on a PerkinElmer Spectrum Version 10.5.4. A UV-vis spectrophotometer (Hitachi, U-3310) and spectrofluorometer (Hitachi, F-7000) were utilized. An X-ray diffractometer with accessories (model: Smart Lab SE, Rigaku Corporation) was used, and atomic force microscopy and transmission electron microscopy images were obtained using a TALOS F200S G2 (200 kV, FEG, CMOS camera 4k × 4k) TEM at 200 kV.

2.2. Preparation of fluorescent carbon quantum dots from red Aloe cameronii

Initially, red aloe vera leaves were washed, cleaned, and chopped into pieces. The pieces of leaves were dried in sunlight for one day. Then, the dry leaves were ground and used as a precursor for synthesizing CQDs. Red Aloe cameronii powder (2.0 g) was added to Milli-Q water (80 ml) and ethanol (20 ml). The mixture was transferred to a Teflon-lined autoclave. The hydrothermal autoclave was kept in a muffle furnace for 12 h at 180 °C. After cooling the contents of the hydrothermal autoclave, centrifugation was carried out for 15 min at 20000 rpm, and the liquid portion was separated (Scheme 1). The resultant brown solution was filtered via a 0.22 μm membrane. To make the CQDs more usable, they were dialyzed using a dialysis membrane and freeze-dried.

2.3. DPPH free-radical-scavenging ability assay

The radical scavenging activity of carbon quantum dot samples was studied using DPPH radicals as a reagent, by following the reported procedure.²⁰ In this method, the required volume of DPPH radical solution in ethanol (60 μM) was mixed with different concentrations (20 μg ml⁻¹, 40 μg ml⁻¹, 60 μg ml⁻¹, 80 μg ml⁻¹ and 100 μg ml⁻¹) of testing samples. The reaction was incubated for 30 min under dark conditions at room temperature. After the incubation period, the absorbance of the reaction mixture was measured at 517 nm using a UV-vis spectrophotometer. For the positive control, ascorbic acid was used as a reference standard. The DPPH scavenging activity of each sample was calculated using the following equation:

where A_c is the absorbance of the control reaction (100 μl of ethanol with 100 μl of the DPPH solution), and A_t is the absorbance of the test sample. Experiments were done in triplicate. The IC₅₀ value was calculated for all the samples. Lower absorbance by the reaction mixture indicated higher free-radical activity.

2.4. Anticancer activity based on the MTT cell proliferation assay

The effect of test samples of CQDs on the viability of lung cancer A549 cells was assessed though the standard procedure of the MTT [2-(4,5-dimethylthiazol-2-yl)-3,5-diphenyl-2H-tetrazol-3-ium bromide] calorimetric assay. At first, the selected cell-line monolayer culture was subjected to trypsinisation to get a cell suspension, and then the cell count was adjusted to 1.0 × 10⁵ cells per ml using DMEM (Dulbecco's modified Eagle's medium) containing 10% FBS and seeded in 96-well microtiter plates for culturing (Falcon, Becton – Dickinson, Franklin Lakes, NJ, USA). After 24 h of incubation, the used media were drained and fresh media along with different concentrations (20, 40, 60, 80, and 100 μg ml⁻¹) of test samples were added. Further, the MTT assay was terminated after 24 h of treatment. The medium was removed and 200 μl of DMSO was added, and the amount of formazan formed was measured at 595 nm on a Model 680 microplate reader (Bio-Rad Laboratories, Inc., Hercules, CA, USA). The percentage growth inhibition was calculated using the following formula:

% inhibition = OD of test sample ÷ OD of control × 100.

The concentration of test drug needed to inhibit cell growth by 50% (IC₅₀) is generated from the dose–response curve for each cell line.²¹

2.5. In vitro imaging applications of carbon quantum dots

A549 cells were used for in vitro cell-imaging studies. After the cells were seeded in 35-mm dishes for 24 h, 100 μl of different concentrations (25 μg, 50 μg, and 100 μg) of aqueous solutions of CQDs were added into the dishes. Cells without added carbon were considered to be control groups. After incubation with CQDs for 2 h, the cells were washed with 1× phosphate buffered saline (PBS) three times. A confocal laser scanning microscope (CLSM; Olympus FV1000) was used for the imaging of the cells.²²

2.6. Antibacterial activity

In the present study, the prepared CQDs were subjected to an antimicrobial study using the agar well method.²³ In the case of the antibacterial study, the test drugs were tested against Gram-positive S. cereus bacteria and Gram-negative E. coli. The test samples were used to treat the 24-h microbial culture at 30 μg, 60 μg, 90 μg, and 120 μg concentrations. The results were compared with the standard antibacterial drug ciprofloxacin. The results are represented in terms of the zone of inhibition, which was measured after 24 h of incubation at 37 °C.

3. Results and discussion

3.1. Characterisation of fluorescent carbon quantum dots

Red aloe vera is an important tree that generates a huge amount of waste biomass. Interestingly, the biomass of red aloe vera leaves is composed of cellulose, lignin, and hemicellulose. Recent studies demonstrated that naturally available biomass can be exploited to fabricate carbon materials. Hence, in this study we used red aloe vera leaves as the bio-precursor for the fabrication of CQDs through a hydrothermal approach. The leaves are primarily composed of carbohydrates and proteins. During the hydrothermal treatment, the organic substances of the leaves are easily carbonized to CQDs. Aloe vera leaves were carbonized at 180 °C for 12 h. After the carbonization process, the obtained mixture was centrifuged and filtered to separate the CQDs. Similarly, previous research found that several types of biomass could be used as a sustainable precursor for CQD manufacture. Red Aloe cameronii, a carbon source, undergoes intricate thermal and chemical changes to produce CQDs. Hydrothermal techniques are usually used for synthesizing CQDs and the information about the precise process by which red Aloe cameronii forms CQDs, the functional groups generated during carbonization, and possible chemical modifications has been obtained.²⁴ The production of CQDs from red Aloe cameronii likely involves nucleation and thermochemical breakdown. Polysaccharides, phenolic compounds, and organic acids are involved in the production of CQDs.²⁵

Further, the fluorescence properties are improved by oxidation processes, which add oxygen-containing functional groups (–OH, –COOH, –CO) to the CQD surface. The functional groups that are formed during the carbonization process are crucial in defining the physicochemical properties of CQDs. Table 1 indicates various plant sources, methods of formation and applications of CQDs that have been reported.

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The prepared CQDs appear brown in daylight, but when exposed to UV light, they emit blue light. The results were strongly consistent with previous publications, in which several types of sustainable precursors were used for CQDs.³⁰ Fig. 1A shows the XRD pattern with a peak with Miller indices of (002) at around 24.5°. To determine the crystal structure and phase purity of CQDs, XRD analysis was performed. The semi-crystalline nature of CQDs is reflected in their periodic lattice structure, which enables them to diffract X-rays and produce characteristic diffraction patterns. A broad and intense peak at 2θ ≈ 24.5°, corresponding to the (002) diffraction plane, was observed. This peak is indicative of carbon layers and confirms the presence of a carbon-based structure. The broadness of the peak signifies the size and partially crystalline nature of the CQDs.³¹ This structure is often due to the presence of oxygen-containing functional groups, which disrupt the perfect crystallinity, resulting in a semi-crystalline structure. Fourier-transform infrared (FT-IR) spectroscopy was used to investigate the surface structure, functional groups, and bonding characteristics of the synthesized CQDs. The FT-IR spectrum, shown in Fig. 1B, reveals a broad and intense absorption band at around 3420 cm⁻¹, corresponding to O–H stretching vibrations and likely resulting from hydrolysis during reduction.³² An absorption band near 2884 cm⁻¹ is attributed to C–H stretching vibrations of aromatic groups. A distinct peak at approximately 1489 cm⁻¹ is associated with aromatic CC stretching, possibly overlapping with CO stretching from oxygen-containing groups. Strong peaks at around 1133 cm⁻¹ are indicative of asymmetric and symmetric stretching vibrations of C–O–C and C–O bonds, respectively. These features confirm the presence of abundant carboxyl (–COOH) groups, enhancing the hydrophilicity of the CQDs. Overall, the FT-IR analysis confirms the functionalization with hydroxyl (–OH), carbonyl (CO), and alkyl/aryl C–groups, which contribute to improved solubility and surface reactivity.

The UV-vis absorption spectrum of the derived CQDs provides valuable information about their electronic structure, surface states, and chemical composition. The UV-vis absorption spectrum of the derived CQDs (Fig. 1C) shows a peak at around 313 nm due to the π → π* transition of the sp² carbon network, while absorption peaks at shorter wavelengths are usually due to surface passivation.³³ For the fluorescence of the CQDs dispersed in ethanol, the emission spectrum was independent of the excitation wavelength and had a broad asymmetric peak at around 335 nm. This longer-wavelength peak is usually induced by the surface passivation of CQDs, as depicted in Fig. 1D showing the fluorescence emission peak detected at 335 nm. The EDX technique is useful for examining the chemical composition of CQDs and revealing information about their purity.³⁴ The elemental makeup of the CQDs was ascertained using the EDX data, and the weights and atomic ratios of carbon and oxygen were determined. Fig. 2 illustrates the composition of the CQDs, determined using energy-dispersive X-ray (EDX) analysis, and we observed the carbon and oxygen atomic percentages to be 67.54% and 32.46%, respectively, as shown in Table 2.

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The TEM-based particle size histogram provides a visual representation of the size distribution of the synthesized fluorescent CQDs. This analysis confirms that the CQDs exhibit a highly monodispersed and uniform spherical morphology, indicating that the synthesis process was well controlled. The histogram reveals the size distribution, with particle diameters predominantly ranging from 3 to 5.5 nm, placing them firmly in the quasi-zero-dimensional regime, a typical characteristic of CQDs.³⁵ Measurements of several individual CQD particles yielded an average particle size of 4.765 nm. These quantitative results demonstrate the minimal variation in particle size, which shows the high-quality, reproducible synthesis. The standard deviation further confirms the precision of the fabrication process; many CQD critical properties are size-dependent for applications in optoelectronics and bioimaging, and this quantitative insight into the degree of monodispersity reinforces the conclusion that the synthesized CQDs are uniformly and consistently produced.³⁶ The CQDs are revealed to be homogeneously dispersed with spherical morphology and a size distribution of 10–20 nm. TEM images show a group of CQDs with well-resolved lattice fringes. The crystal lattice found in the high-resolution TEM (HRTEM) image (Fig. 3) reveals the crystallinity of the as-manufactured CQDs.

The surface charge and colloidal stability of the CQDs in suspension are reflected by the zeta potential, which is an essential characteristic property. Variables such as surface functional groups and synthesis methods can influence the zeta potential. It was found that the zeta potential of the synthesized CQDs was +35.8 mV, indicating their stability in an aqueous environment, as depicted in Fig. 4.

4. Pharmacological evaluations

4.1. Antioxidant activity

In recent years, CQDs have garnered significant attention from the scientific community due to their unique properties and versatile applications.³⁷ These carbon-based nanomaterials possess exceptional characteristics, such as good biocompatibility, low toxicity, stable photoluminescence, and excellent optical properties.³⁸ One of the promising applications of CQDs is as potential antioxidants. Antioxidants play a crucial role in neutralizing the harmful effects of free radicals and ROS, which are known to contribute to various diseases and cellular damage.^39,40 CQDs have demonstrated potent antioxidant activity, making them promising candidates for various biomedical applications. The antioxidant property is largely attributed to their unique structure and surface chemistry. The core of CQDs is composed of a carbon-based matrix, which can be functionalized with various heteroatoms and functional groups, such as hydroxyl, carboxyl, and amino groups, which can act as effective free-radical scavengers to neutralize oxidative stress. Further, the small size and high surface-area-to-volume ratio of CQDs facilitate efficient interaction with biological systems, enhancing their antioxidant efficacy.⁴¹ The ability of CQDs to quench ROS and inhibit lipid peroxidation has been extensively studied, establishing their potential as powerful antioxidant agents. The antioxidant properties of CQDs have been explored in biomedical contexts, including bioimaging, drug delivery, and cancer therapy.⁴² For example, incorporating CQDs into drug delivery systems can enhance the bioavailability and targeting capabilities of antioxidant drugs, thereby enhancing therapeutic efficacy.⁴³ In conclusion, the antioxidant capabilities of CQDs offer a promising approach for addressing various health challenges. Their unique properties, combined with their antioxidant potential, make them valuable in the field of biomedical research and development. Moreover, antioxidant activity is a key area of research for CQDs due to implications in both biological and environmental systems.⁴⁴ The 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay is a widely employed technique for evaluating the antioxidant potential of various compounds and materials, including CQDs.⁴⁵ The DPPH assay relies on the ability of antioxidants to quench the stable DPPH radical, a process that leads to a decrease in the absorption of the DPPH radical at 517 nm. By measuring the extent of this decrease, researchers can quantify the antioxidant activity of the material under investigation. In the case of CQDs, several studies have reported the successful application of the DPPH assay to evaluate their antioxidant properties.⁴⁶ The results have shown that the antioxidant activity of CQDs can be modulated by factors, such as their size, surface functionalization, and the presence of specific functional groups, and a study which synthesized CQDs from citric acid and ethylenediamine exhibited their remarkable free-radical scavenging activity, as evidenced by their ability to effectively quench DPPH radicals.⁴⁷ Understanding the DPPH-based antioxidant activity of CQDsis crucial for their potential applications in various fields, including biomedicine where their ability to mitigate oxidative stress could be highly beneficial. In this present study the prepared CQDs at different concentrations (20–100 μg), along with standard drug ascorbic acid, were subjected to antioxidant activity testing using the DPPH assay. The results revealed that both the tested samples and standard drug showed dose-dependent activity, i.e., with an increase in the concentration there was an increase in the percentage of inhibition. For a concentration of 20 μg, the sample CQDs and ascorbic acid showed inhibition percentages of around 24.09 ± 0.0133% and 44.76 ± 0.0051%, respectively. At a higher concentration of 100 μg, the percentages of inhibition increased to 79.40 ± 0.0109% and 90.92 ± 0.0053%. The IC₅₀ values of the test sample of CQDs and the standard drug were found to be 57.52 μg ml⁻¹ and 30.53 μg ml⁻¹, respectively. The overall antioxidant results are shown in Table 3 and Fig. 5.

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4.2. Anticancer activity of carbon quantum dots against the lung cancer A549 cell line

The emergence of carbon dots, a novel class of carbon-based nanomaterials, has sparked a surge of interest within the biomedical research community, primarily due to their captivating optical properties, remarkable biocompatibility, and the potential for their use as versatile theranostic agents; in particular, the application of The IC₅₀ values of the test sample of CQDs in cancer treatment and diagnosis has garnered significant attention, with numerous studies exploring their potential in various cancer models.⁴⁸

One such area of interest is the investigation of the anticancer activity of CQDs toward the lung cancer A549 cell line. Lung cancer remains one of the leading causes of cancer-related deaths worldwide, underscoring the urgent need for the development of novel and effective therapeutic strategies.⁴⁹ Carbon-based nanomaterials, such as CQDs, have been identified as promising candidates for targeted cancer therapy due to their unique physicochemical properties, including high surface area, tunable surface chemistry, and the ability to incorporate various anticancer agents. Considering this background, the present study investigated the potential of CQDs in terms of their MTT-based anticancer activity toward the A549 lung cancer cell line.⁵⁰ CQDs have been explored as carriers for drug delivery and other biomedical applications due to their high variability, chemical stability, and unique characteristics, such as highly tailorable surface chemistry and high carrier capacity, as well as the feasibility of incorporating a diverse range of therapeutic agents, including anticancer drugs, into their structure.⁵¹ To evaluate the anticancer activity of CQDs on the A549 lung cancer cell line, different concentrations of CQDs (20–100 μg) were used for the MTT (3-(4,5-dimethylthiazol-2-yl))-based assay. This colorimetric technique, which measures the reduction of the tetrazolium dye MTT by metabolically active cells, allows for the assessment of cell viability and proliferation, and the results are shown in Table 4.

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The results showed that CQDs have shown significant anticancer activity, inhibiting the cell growth as the concentration increased. At an initial concentration of 20 μg, the percentage of cell viability was seen to be 80.76 ± 0.0099%. Further, at a higher concentration (100 μg), the percentage of cell viability decreased to 21.91 ± 0.0077%. The IC₅₀ value for the CQD sample was found to be 62.75 μg ml⁻¹. The standard drug cisplatin showed a cell viability percentage of around 13.75 ± 0.0063%. Furthermore, morphological studies revealed the effects of CQDS on the treated cells. With an increase in the concentration, there was a decrease in the cell numbers with the appearance of apoptosis features, such as cell elongation, cell shrinkage, cell turgidity and the formation of apoptotic bodies. The current study demonstrated the potential of CQDs to exhibit anticancer activity against the A549 lung cancer cell line. Further investigation into the underlying mechanisms, optimization of the carbon dot properties, and in vivo validation will be crucial for establishing the clinical relevance of this novel nanomaterial for cancer therapy. The results of cell viability studies are shown in Table 4 and Fig. 6. The microscopic morphological effects of CQDs on lung cancer A549 cells are shown in Fig. 7.

The integration of CQDs for bioimaging applications allows for enhanced visualization of cancer cells, particularly in studies involving the A549 cell line, where their multicolor fluorescence can aid in identifying cellular structures and monitoring therapeutic responses.⁵² Moreover, facile methods for preparing CQDs, which often utilize environmentally friendly reagents, further contribute to their appeal as viable alternatives to traditional CQDs, thereby promoting their practical application in cellular imaging and targeted therapies for lung cancer. The core structure of carbon dots is primarily composed of carbon, surrounded by or embedded with heteroatoms and various functional groups, which can be tailored to optimize the optical properties and enhance the solubility in aqueous environments.⁵³ This adaptability not only facilitates their use in bioimaging applications but also supports their functionalization for targeted delivery in therapeutic contexts, positioning CQDs as promising candidates for innovative approaches in cancer diagnostics and treatment.⁵⁴ Indeed, the versatility of carbon quantum dot synthesis has enabled the development of luminescent materials with precisely tuned emission profiles, designed to effectively operate in the red and near-infrared regions. This capability is particularly advantageous for bioimaging in deep-tissue settings, where penetration depth is crucial, thereby overcoming some limitations associated with conventional fluorescent probes. Furthermore, the increasing focus on designing CQDs s with long-wavelength emissions enhances their utility in biomedical applications, particularly in bioimaging, as these materials can significantly improve visualization in complex biological environments, ensuring that the details of cellular processes within the A549 cell line are clearly captured. The inherent biocompatibility and low toxicity of CQDs have also been well documented, making them an attractive choice for in vitro studies involving the A549 lung cancer cell line.^55,56 These properties not only promote their safety for biological applications but also facilitate prolonged exposure during imaging studies, thereby allowing researchers to effectively monitor cancer progression and therapeutic efficacy over extended periods without compromising cell health. Additionally, the unique optical properties of CQDs, including their resistance to photobleaching and tunable fluorescence, enable consistent performance in dynamic imaging scenarios, further solidifying their position as superior alternatives to traditional organic dyes and CQDs in the pursuit of effective lung cancer bioimaging strategies.⁵⁷ The efficacy of CQDs for bioimaging applications has been shown to vary significantly with concentration, influencing the degree of cellular uptake and fluorescence intensity, which are crucial for enhancing the visibility of cancerous tissue during imaging procedures.⁵⁸ Furthermore, studies have indicated that low concentrations of CQDs may facilitate optimal cellular internalization while mitigating potential cytotoxicity, thus promoting a favourable environment for imaging, whereas higher concentrations may lead to saturation or resultant cellular stress, ultimately affecting the imaging resolution and accuracy.⁵⁹

4.3. Bioimaging studies

In the current study, we present the findings of our investigation into the impact of varying carbon quantum dot concentrations on the bioimaging of the lung cancer A549 cell line. The A549 cell line, a widely used in vitro model for lung cancer research, was selected due to its clinical relevance, and the pressing need for improved imaging techniques to enable the early diagnosis and targeted treatment of this lethal disease.⁶⁰ Our experiments revealed that A549 cells exhibit markedly different fluorescence profiles when treated with CQDs at varied concentrations, with optimal imaging performance observed at a medium concentration that balanced effective cellular uptake and minimized toxicity, thereby supporting the hypothesis that concentration modulation is critical for enhancing the efficacy of the CQD-based bioimaging of lung cancer. Using a non-fluorescence filter, the cells were seen with no emission, whereas without the addition of CQDs, the cells appeared dark and exhibited no luminesence.⁶¹

Furthermore, at lower concentrations there was more fluorescence and numerous cells were seen. As the concentration increased, there was a decrease in the intensity of fluorescence, and only a small number of cells were visible.⁶² Moreover, a systemic investigation demonstrated that enhanced fluorescence intensity was correlated with increased cellular viability and functionality, which suggests a direct relationship between the concentration of CQDs and their imaging capabilities, as well as highlighting the need for further studies to explore the underlying mechanisms governing this phenomenon.⁶³ In addition, our analysis facilitated a better understanding of how CQDs can be optimized for therapeutic applications in conjunction with imaging techniques, corroborating the prevailing notion that their unique properties, such as tunable fluorescence and reduced toxicity, position them as viable candidates for improving the diagnosis and treatment of cancer.⁶⁴ Furthermore, the results emphasize the importance of a thorough assessment of the synthesis methods and surface modification of CQDs to achieve desirable optical characteristics and biocompatibility, which are essential for their successful application in cancer diagnostics and therapeutics.⁶⁵ In conclusion, the findings from our study underscore the versatility and potential of CQDs as effective bioimaging agents, illustrating that careful concentration management is imperative for maximizing their imaging efficacy while ensuring cellular safety and minimizing adverse effects, thereby paving the way for their broader adoption in the field of cancer theranostics.⁶⁶ The results are depicted in Fig. 8.

4.4. Antibacterial activity of carbon quantum dots

CQDs, a novel class of carbon-based nanomaterials, have garnered significant attention in recent years due to their unique physicochemical properties, including their small size, biocompatibility, ease of synthesis, and tunable surface functionality, rendering them promising candidates for a wide range of applications, including in biomedicine, bioimaging, drug delivery, and as antibacterial agents.⁶⁷ The antibacterial activity of CQDs has been extensively investigated against various bacterial strains, including Escherichia coli and Staphylococcus aureus, two pathogens responsible for a wide array of human infections.⁶⁸ These carbon nanostructures have demonstrated potent antibacterial activities against a broad spectrum of pathogens, positioning them as potential alternative antibacterial tools.⁶⁹ The growing prevalence of antibiotic-resistant bacterial strains has spurred the development of novel antibacterial materials, highlighting the importance of exploring the antibacterial properties of carbon nanomaterials.⁷⁰ In the present study, the synthesized CQDs were tested on E. coli and S. aureus, with the standard drug ciprofloxacin used as a positive control group. Different concentrations of CQDs (30–120 μg) were tested against the selected pathogens.

The results revealed that all concentrations of CQDs exhibited significant effects against both tested pathogens, with an increase in the maximum zone of inhibition as the concentration increased. These results were comparable to those observed with the standard drug, as shown in Fig. 9. The CQDs have shown significant results, e.g. at a concentration of 120 μg, they showed zones of inhibition of around 52 mm and 39 mm for S. aureus and E. coli, respectively, which were comparable with the results from the standard drug ciprofloxacin. The results are shown in Table 5.

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5. Conclusions

In this study, we effectively illustrated an environmentally responsible and sustainable method for producing fluorescent CQDs from red Aloe cameronii and investigated their applications. In addition to producing highly luminous and biocompatible CQDs with exceptional physicochemical features, the green synthesis technique is consistent with sustainability principles. With their powerful antioxidant capabilities, these CQDs efficiently scavenge free radicals. Furthermore, the CQDs show potential as promising candidates for targeted cancer therapy by selectively preventing the proliferation of cancer cells while not affecting the healthy cells. Their successful application in imaging the A549 cell line, where they showed bright fluorescence, effective cellular uptake, and good bioimaging capabilities, further confirmed their efficacious potential as fluorescent probes for bioimaging. Overall, this research provides a significant contribution to the development of multifunctional nanomaterials, i.e., CQDs, leveraging green synthesis for applications in therapeutics, diagnostics, and bioimaging. The results pave the way for the future exploration of plant-based CQDs for sustainable and advanced biomedical technologies.

Author contributions

Lokesh Bheemayya: conceptualization; data curation; formal analysis; investigation; methodology; software; and visualization. Ravindra R. Kamble: formal analysis; investigation; project administration; resources; and supervision. Vishwa B. Nadoni: formal analysis; validation. Manojna R. Nayak: formal analysis; validation. Mallika S. Wali: formal analysis; validation. Arun K. Shettar: data curation; formal analysis; validation. Joy H. Hoskeri: data curation; formal analysis; validation.

Data availability

The data supporting this article is contained within the main manuscript.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank DST, New Delhi (SAIF program), DST-PURSE-II Phase and the University Scientific Instrumentation Centre (USIC), Karnatak University, Dharwad, India for providing NMR, UV-visible, fluorescence, powder X-ray, and EDX-SEM data. One of the authors (LB) profusely thanks University Grants Commission (UGC), New Delhi for providing the Senior Research Fellowship (SRF).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj00308c

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