Introduction

Among various carbon resource materials for the synthesis of Carbon Quantum Dots (CQDs), natural resources are increasingly favored for their environmental friendly properties1. The mahua flower, scientifically known as Madhuca longifolia, as the carbon source for CQD synthesis. The Mahua tree, a large deciduous tree thriving in dry tropical and subtropical climates, is economically significantt for many people in eastern India states such as Andhra Pradesh, Gujarat, Madhya Pradesh, Odisha, Chhattisgarh, Jharkhand, Bihar, and Uttar Pradesh Mahua flowers are traditionally used as a food supplement in tribal regions and serve various medicinal purposes, including as a tonic, analgesic, cooling agent, astringent, demulcent, and diuretic2,3. Inspired by the economic importance of plant, we chose mahua flowers for CQD synthesis due to their rich sugar content, including sucrose, glucose, fructose, arabinose, maltose, and rhamnose4. Among different approaches, bottom-up synthesis was the best method for synthesizing CQDs from Mahua flowers without any chemical additives or doping for additional functionality. CQDs are particularly intriguing when synthesized at a particle size smaller than 10 nanometers as they exhibit strong and tunable fluorescent properties due to the quantum confinement effect5. These properties enable their applications in areas such as drug delivery, bioimaging, sensors in medical diagnostics, and chemical sensing6. The origin of these fluorescent behaviors lies in their structure-function relationship. Typically, zero-dimensional (0D) CQDs have a core rich in sp2-carbon, resembling a graphite structure where the carbon atoms are arranged in a hexagonal lattice. Surrounding this core is an amorphous shell, which varies significantly regarding chemical functionalities and their distribution on the surface. The chemical groups on the shell have unevenly distributed charges, allowing them to interact differently with various solvents and foreign molecules. This structural arrangement allows CQDs to combine the beneficial properties of both graphitic carbon and functional groups, significantly enhancing their utility in the field of sensing, bioimaging, and drug delivery7. Recently, CQDs have garnered significant attention as antimicrobial agents driven by the need for novel antibiotics to combat multidrug resistance in bacterial infections. Despite the effectiveness of antibiotics, concerns remain about their potential toxicity and the likelihood of infections developing resistance to these formulations8. To address these challenges, the development of nanomaterials exhibiting robust antimicrobial capabilities has opened up new possibilities for restraining the proliferation of microorganisms9. However, the long-term biological safety and potential toxicity of these nanomaterials pose significant challenges, necessitating a comprehensive understanding of their chemical structure, surface functionalities, and application for antibacterial actions10.

On the other hand, metal ions are integral to numerous physiological processes in living organisms, making their selective and sensitive detection a critical research area. Due to their environmental stability, ferric ions (Fe3+), pose significant risks to human health, ecosystems, plants, and animals. Fe3+ ions are crucial in biological processes such as DNA and RNA synthesis, oxygen metabolism, and haemoglobin function11. Consequently, the development of sensitive and selective detection approaches in biological systems is crucial for deeper exploration of the physiological and pathophysiological roles of Fe3+ in living organisms12. Furthermore, the widespread use of these metal ions has driven scientific efforts to develop simple and cost-effective methods for their detection in aqueous environments. Additionally, the precise emission properties of CQDs have garnered significant interest for their potential in chemo-sensing applications, particularly for detecting heavy and transition metal ions in aqueous environments13. In this regard, the fluorescence-based identification of metal ions using carbon quantum dots presents a novel opportunity for the selective and sensitive detection of these ions. The presence of a conjugated π-electron system enhances the likelihood of CQDs forming complexes with various metal ions14.

In this report, detailed characterizations of the Mahua flower-derived CQDs are provided with comprehensive insights into the structural and optical properties. In addition to the characterization, we explored the functional aspects of the synthesized CQDs by assessing their antifungal, antibacterial, and cytotoxic activities. Furthermore, the fluorescence-based titration of CQDs was performed to evaluate their reactivity with different metal ions (Ni2+, Cu2+, Co2+, Fe3+, Na+, and K+). This multi-faceted investigation aimed to elucidate the potential applications of Mahua flower-derived CQDs in various domains, aligning with the plant’s traditional significance and economic importance in the specified region.

Experimental section

Plant material collection and taxonomic authentication

Fresh Mahua flowers (Madhuca longifolia) were harvested from trees in the local forest of Baunsapokhari, located at coordinates 20°38’05.1"N 85°10’19.4"E, Hindol, Dhenkanal, Odisha, India. The plant was identified and authenticated by Prof. Rojalin Sahu, School of applied sciences, KIIT Deemed to be University, Bhubaneswar, Odisha, India. The permission was taken from National Biodiversity Authority, India (NBA). Madhuca longifolia var. latifolia Syn. Madhuca indica is publicly available at digital herbarium of Regional Plant Resource Centre, Bhubaneswar, India with Acc. No. 290.

Chemicals

Ethanol was procured from Merck. All other reagents were used without further purification. Dendritic cells (DCs) were grown in a complete Iscove’s Modified Dulbecco’s Medium (IMDM) supplemented with 10% Foetal Bovine Serum (FBS) and antibiotic solution (Gibco). The cells were maintained at 37° C in a humidified incubator with 5% CO2. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was purchased from Invitrogen, Thermofisher Scientific.

Synthesis of CQDs

CQDs were synthesized from mahua flower by hydrothermal process with slight modification to the literature15. Initially, the petals of freshly plucked flowers were churned to get 60 ml of fresh juice, utterly devoid of pulp. This juice was introduced into a Teflon-lined stainless-steel autoclave with 100 ml capacity. The hermetically sealed autoclave was placed in a hot air oven, which maintained a constant temperature of 150 °C. After 24 h, the autoclave was unsealed, and dark brown product was obtained. The product was carefully filtered, and centrifuged at 10,000 rpm for 20 min to eliminate any residual organic moieties. The residual solids were thrown away, and the dark brownish solution was kept at 4 °C. The entire solution was filtered at a regular interval of 4 days over 2 weeks to remove any settled particles. Subsequently, the resulting dark brown CQDs were collected and subjected to characterization and evaluated their optical properties. Figure 1a illustrates the detailed synthesis protocol.

Flow cytometry analysis

Cells were dissociated from the plates after the experimental incubation period and subsequently washed with FACS buffer (3% FCS in PBS). Following the washing step, cells in the FACS buffer were collected using an LSRII flow cytometer (BD Biosciences). The obtained data were then analysed using Flow Jo-X software (Treestar).

Cytotoxicity assay

Cytotoxicity of CQDs against Dendritic Cells (DC) was determined by MTT assay and flow cytometry analysis after incubating different concentrations of CQDs with Green Fluorescent Protein (GFP) expressed DCs. After that, the percentage of viable cells was measured by determining Fluorescein isothiocyanate (FITC) positive cells. The cytotoxicity of CQDs in vitro was investigated using the MTT assay in flat-bottomed microtiter plates with 96 wells. Each well was seeded with a volume of 200 µL of complete culture medium containing 1 × 104 cells on the first day of the experiment and then incubated for 24 h to allow cell attachment to the wells. The next day, fresh medium containing varying percentages (2.5%, 5%, and 10%) of the CQDs sample was added to replace the old medium, and the cells were then incubated at the optimal temperature of 37 ºC with 5% CO2 for 72 h. To assess the cytotoxicity, 10µL of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was added, and the plates were subsequently incubated for 4 h at 37 °C in the dark. After this incubation period, the medium was removed, and 100µL of dimethyl sulfoxide (DMSO) was added to each well, followed by measuring the absorbance at 570 nm.

Characterization

The freshly synthesized CQDs were thoroughly characterized using advanced analytical instruments. The crystallinity of CQD was analysed by Bruker D8 Advance X-ray diffractometer instrument for using Cu-K\(\:\alpha\:\) as incident ray. The Zeiss Gemini FESEM model SEM 450 was chosen to visualize the structural morphology of the carbon quantum dots. High-resolution transmission electron microscope (HR-TEM) enabling high-resolution imaging. Also, the roughness and particle size were measured using Atomic Force Microscopy (AFM) technique. The functional groups present in the CQDs were identified through Fourier Transform Infrared Spectroscopy (FTIR), conducted using a Thermo Scientific Nicolet iS5 FT-IR spectrometer. The binding energy of functional groups is calculated by X-ray Photoelectron Spectroscopy (XPS). The optical absorption spectrum was recorded using a JASCO V 770 model UV-Vis spectrophotometer, providing insights into the linear optical absorption properties of the CQDs. The photoluminescence (PL) spectra of the samples were measured with a 315 nm excitation wavelength using an Edinburgh FLS 1000 photoluminescence spectrophotometer, providing valuable insights into the fluorescence behaviour of the CQDs. The measurement is conducted under room temperature by using Xe lamp as excitation source. These advanced instruments and techniques collectively contributed to a comprehensive understanding of the structural, optical, and chemical characteristics of the synthesized CQDs.

Fig. 1
figure 1

(a) Schematic diagram of the synthesis of CQDs from Mahua flower. (b) PXRD pattern of CQD. (c) FESEM images and EDAX colour mapping showing the elemental composition. (d) TEM image at 50 nm, (e) SAED diffraction pattern, (f) Particle size distribution graph, (g) HR-TEM image at 5 nm, (h) Interplanar spacing showing d = 0.384 nm, and (j) AFM profile image of CQDs.

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Results and discussion

Structural and morphological properties

PXRD analysis

A noteworthy observation in the XRD pattern was the noticeable presence of a broad peak at 2θ = 20.76°, suggesting the existence of amorphous carbon within the structure of the CQDs. Moreover, as shown in Fig. 1b, distinct and sharp peaks occurring at 2θ values of 28.25°, 40.51°, 50.24° and 58.59° corresponding to the diffractions of (002), (101), (102), and (103) planes were respectively identified16. The interplanar spacing of the (002) plane was calculated by Bragg’s formula (\(\:d=\frac{n\lambda\:}{2sin\theta\:}\)) as 0.314 nm. These well-defined peaks suggest the coexistence of graphite carbon phases within the structure of the CQDs17. The elucidation of these diffraction patterns provides valuable insights into the crystalline nature and structural composition of the synthesized CQDs. Although the amorphous carbon presence is indicated by the broad peak, the distinct diffraction patterns provide essential insights into the structural composition of the synthesized CQDs.

Morphology analysis

The microscopic examination of the synthesized carbon dots provides an insightful exploration of their structural characteristics and nanoscale features. Field Emission Scanning Electron Microscopy (FESEM) was used to characterize the morphology of the carbon quantum dots (CQDs). A conducting silver film was sputter-coated onto the sample to ensure charge balance during imaging. The FESEM images revealed the carbon dots, which were discernible and distributed across the material’s surface. The scale bar in the images represents 500 nm and 1 μm. Additionally, Energy Dispersive X-ray Spectroscopy (EDAX) was performed to analyse the elemental composition of the CQDs. The analysis identified two main elements: carbon and oxygen, with their respective atomic weight percentages of 66.7% and 33.3%, and atomic percentages of 72.7% and 27.3%, as illustrated in the Fig. 1c.

Employing HR-TEM for this analysis ensures a thorough inspection of the material. The images from Fig. 1d reveal the distinct spherical shape of the CQDs. Figure 1g shows the High-Resolution TEM image at 5 nm. The Selected Area Electron Diffraction (SAED) pattern Fig. 1e provided insights into characteristics indicative of a combined amorphous and crystalline structure of the synthesized CQDs18. The interplanar spacing (d) calculated from the ImageJ software was found to be 0.384 nm(Fig. 1h). The interplanar spacing is approximately 0.384 nm which corresponds to the graphite-like layers19. The corresponding histogram in Fig. 1f, created using Normal distribution analysis in Origin software, illustrates this size distribution with each having an average diameter of about 5.77 nm elegantly. This measurement was determined by analyzing 100 particles using ImageJ software. The HR-TEM image showcases a collection of distinct and individual CQDs, characterized by uniform spherical shapes devoid of aggregation.

Atomic Force Microscopy (AFM) topographical images were utilized to measure the size and roughness of the synthesized carbon quantum dots (CQDs)20. The dispersive sample was deposited on a glass substrate using a drop-cast technique and subsequently dried for 15 min at 70 °C. During analysis, the dried sample was scanned at a rate of 0.5–1 line per second. The AFM image depicted in Fig. 1i, along with its height profile, confirmed that the size of the CQDs falls within the quantum region (< 10 nm), and surface roughness is found to be 6.5 nm.

Functional groups and Surface Chemistry

FTIR Spectroscopy

FTIR spectra analysis of the synthesized CQDs obtained from the Mahua flower provides valuable insights into the molecular composition and functional groups present in the CQDs. As shown in Fig. 2(a), CQDs exhibited a sharp peak centered at 1662 cm− 1 assigned to the aromatic C = C stretching vibration, which is the elementary unit of the CQDs. Additionally, CQDs showed a broad band between 3300 and 3500 cm− 1, indicating O − H stretching vibration. The peaks observed at 769 cm⁻¹ and 809 cm⁻¹ can be attributed to C–H out-of-plane bending vibrations. Sharp peaks at 1521, 1399, and 1013 cm− 1 are assigned to C = O, C = C stretching vibrations, and C-O stretching vibrations, indicating the presence of carboxylate. The presence of the C = C, C = O, and C-O peaks indicates that the carbon quantum dots (C-QDs) possess a graphitic structure. Additionally, the OH, C = O, and C-H bending peaks suggest the presence of hydroxyl, carbonyl, and alkyl groups on the surface of the C-QDs16,21,22. UV-Vis absorption spectroscopy also provides qualitative insights into the presence of abundant C = C and C = O conjugated structures in CQDs. The carboxyl groups located at the edges allow CQDs to have negative charges due to the ionization of –COOH, facilitating the excellent dispersion of CQDs in water23. The presence of such functional groups ensures their solubility in water, and thus no further chemical modification is desired to stabilize these CQDs.

XPS analysis

The X-ray photoelectron spectroscopy (XPS) analysis of the synthesized carbon quantum dots (C-QDs) confirms their high purity, with a composition of 70.83% carbon and 29.17% oxygen and no detectable impurities. The full XPS spectrum shows two peaks at the survey scan of the C-QDs presented in Fig. 2(b). The C 1s spectrum (Fig. 2(c)) reveals distinct peaks at 283.08 eV (C-C bonds), 284.40 eV (C = C bonds), and 286 eV (C = O groups), indicating a mix of sp2 and sp3 hybridized carbon atoms along with oxygen-containing groups. Similarly, the O 1s spectrum (Fig. 2(d)) highlights peaks at 530.66 eV (C-O bonds), 531.82 eV (C = O groups), and 532.91 eV (C-OH or C-O-C groups), showing the presence of oxygen-rich functional groups that may enhance the material’s reactivity and hydrophilicity24.

Zeta potential study

Zeta potential studies are conducted to determine the surface charge of a compound. In the analysis of carbon quantum dots (CQDs), the mean zeta potential was found to be -20.88 mV, as illustrated in the Fig. 2e. The single peak observed at -20.88 mV, with a peak width of 11 mV, indicates the presence of negatively charged surface moieties22. These moieties contribute to the excellent dispersion of CQDs in water-based solvents. Additionally, the CQDs demonstrate high colloidal stability, as there was no turbidity observed even after one month of storage at room temperature. This stability highlights their suitability for long-term use.

Fig. 2
figure 2

(a) FTIR spectra of CQDs, (b) survey XPS spectra of CQDs, (c) XPS of C1s, (d) XPS of O1s, and (e) Zeta potential peak.

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Optical properties

UV-Vis spectral analysis

The collected absorption spectra of the synthesized compounds were obtained through the use of unpolarized light within the range of 200–800 nm. In the preparation of samples for analysis, 1 mL of the synthesized product was carefully diluted 200 times with ethanol serving as a stabilizing solvent. Ethanol was employed as a blank for the spectral measurements. The resulting absorption spectra are depicted in Fig. 3a. CQDs, synthesized from various carbon sources or through different synthesized methods, consistently show diverse absorption characteristics. Despite this variation, they commonly exhibit strong absorption in the ultraviolet (UV) region (200–400 nm), often with a tail extending into the visible range. The absorbance peaks show at 282 nm and 340 nm. These absorption bands are typically attributed to the n − π* transition of the C = O and n − π* transition of C = C bonds16. The dark brown solution was placed in the UV chamber to confirm the possible optical transformation of carbon-based quantum dots, and UV light (354 nm) was passed on the tube containing the sample (Inset of Fig. 3a).

Photoluminescence behaviour

In recent times, there have been a surge in published results on the observation of PL emissions in QDs which is considered as one of the most prominent properties of these materials25,26. Additionally, PL behaviors are an important parameter in evaluating the photocatalytic capability of QDs and other types of photocatalysts27,28. From the various excitation and emission wavelength scans, the maximum excitation (λex) occurred at 393 nm and the maximum emission (λem) peak was observed at 500 nm and was shown in Fig. 3b. The PL emission spectra of CQDs were measured in various excitation wavelengths. In recent years, there has been significant interest in studies that compare PL emission to Stokes-type emission patterns. Typically, it is expected that the emission wavelength of the PL beam is longer than the excitation wavelength of the laser due to the loss of some energy and matches the PL beam to the Stokes-type emission20,29. Notably, the relative intensities of emission/excitation at different wavelengths are compared to confirm this phenomenon. As can be seen from Fig. 3c, when the excitation wavelength increases from 330 to 500 nm, there is a red shift in the emission peaks ranging from 478.3 nm to 576.49 nm. We can also find that all chromaticity coordinates of QDs were located in an aqueous color change from light blue to green and again to yellow with increasing wavelength of excitation Fig. 3d. The size effect on the band gap energies and the polydispersity nature of QDs account for these properties. Smaller QDs typically have larger band gaps and emit light at shorter wavelengths (higher energy), while larger QDs have smaller band gaps and emit at longer wavelengths (lower energy). This relationship is governed by quantum confinement effects, where the size of the QD affects the energy levels available to electrons and holes within the material. In a polydisperse sample, QDs of various sizes are present, each emitting light at different wavelengths due to their distinct band gap energies. This can lead to a broadening of the emission spectrum compared to a monodisperse sample.

Fig. 3
figure 3

(a) UV-Vis spectra of CQDs with an inset of visual changes due to UV and Visible light, (b) Photoluminescence excitation-emission spectra, (c) emission spectra at different excitation wavelengths (d) Chromaticity diagram showing the transformation of colour from light blue to yellow in CIE 1931 at different excitation, (e) peak shift of emission spectra, (f) peak shift of intensity due to different excitation wavelength; Stability of CQDs (g) in different pH and (h) at different storage periods.

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Figure 3e depicts the relationship between the excitation wavelength and the peak position of emission. The linear behaviour observed in this figure for the as-prepared QD suspension is attributed to the quantum size effect, indicating that the bandgap energy depends on the size of the QD. The consistent emission wavelengths further provide evidence of the quantum confinement effect. As shown in Fig. 3f, the luminescent intensity exhibits a continuous increase as the excitation wavelength undergoes a redshift, commencing from 330 nm and peaking at a maximum signal of 522.78 nm when excited at 410 nm. Thereafter, the emission intensity of PL spectra decreases gradually upon redshift of the excitation wavelength.

The excitation-dependent luminescent behaviour may be due to another mechanism called Förster resonant energy transfer (FRET), in which reabsorption occurs among neighbouring QDs which account for the redshift of the emission spectra30. The non-uniform distribution observed in the absorption spectrum corresponds to elevated radiative rates at higher energy and reduced radiative rates at lower energy, as illustrated in Fig. 3c. This aligns with the trend depicted in Fig. 3e, where the PL intensity shows an increase at shorter wavelengths and a decrease beyond a specific wavelength towards longer wavelengths. An observable Stokes shift of 0.68 eV is noted for an excitation wavelength of 393 nm and an emission wavelength of 500 nm. This substantial value can be attributed to the quantum confinement effect, resulting from interactions between discrete electron and hole levels. The spatial overlaps of electron and hole wave functions experience a significant increase when the material sizes decrease to the QD scale31.

Figure 3g depicts the fluorescence spectra acquired at various pH levels using an excitation wavelength of 393 nm. The data clearly illustrate the pH sensitivity of the fluorescence intensity exhibited by our synthesized carbon dots. Notably, it is crucial to highlight that despite variations in pH, the fluorescence peak remained constant at 500 nm. Upon excitation at 393 nm, the fluorescence intensity experienced a decline with the transition from acidic to basic conditions. The alteration in pH from acidic to alkaline environments potentially induces the protonation and deprotonation of carboxyl groups, leading to the electrostatic charging of the CQDs, ultimately resulting in a shift in the Fermi level.

In Fig. 3h, the PL emission spectrum of the CQDs over different storage durations is depicted, revealing a minimal decrease in intensity. This observation confirms the remarkable stability of the synthesized CQDs when stored at room temperature under neutral pH conditions which makes them highly suitable for practical applications.

Quantum yield

The Quantum Yield (QY) of the synthesized carbon quantum dots (CQDs) was determined through sample dilution in deionized water. Quinine sulphate with a concentration of 0.1 M [H2SO4] served as a standard reference with a known QY of 0.5432. The calculation of QY was performed using the equation:

$$\:QY=Q{Y}_{ref}\frac{{\eta\:}^{2}}{{\eta\:}_{ref}^{2}}\frac{I}{{I}_{ref}}\frac{{A}_{ref}}{A}$$
(1)

where QYref represents the QY of the reference material (0.54 for quinine sulphate), η is the refractive index of the solvent, ηref is the refractive index of quinine sulphate, A is the absorption at the given wavelength, and I is the integrated fluorescence emission intensity. The fluorescence QY of the CQDs at λex = 410 nm was determined to be 7.18%, and this value was compared to the integrated luminescence intensity of standard quinine sulphate. Table 1 details a QY of CQDs synthesized in various sources.

Table 1 CQDs obtained from various natural resources.
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Applications

Metal sensing

Fe3+ sensing study

In a standard procedure to study selectivity for different metal ions, stock solutions of various metal salt cations were prepared at a concentration of 100µM using distilled water. These metal ions include Cu+ 2, Co+ 2, K+, Na+, Ni+ 2, and Fe+ 3. The maximum fluorescence intensity of CQDs is denoted as F0 by utilizing 2 mL of CQDs diluted 50 times in a cuvette. Each metal ion solution (1.5 mL) was sequentially added to 0.5 mL of the CQDs solution to prepare a homogeneous mixture. The maximum fluorescence intensity of CQDs with different metal ions was recorded as F at an excitation wavelength of 350 nm. All experimental procedures were conducted at room temperature. The selectivity of the CQDs prepared for Fe+ 3 metal ions was assessed by analyzing the measured fluorescence spectrum. The fluorescence spectrum depicted in Fig. 4a illustrates varying quenching and shifts in fluorescence intensity of the CQDs in response to different metal ions. Specifically, it is evident from the spectrum that CQDs interacting with Fe+ 3 ions exhibit reduced fluorescence emission intensity compared to other metal ions. This suggests the presence of energy exchange between Fe+ 3 ions and the CQDs49. From the histogram plot depicted in Fig. 4b, notable alterations in the F/F0 values are observed both in the absence and presence of various metal ions within the CQDs solution. Specifically, Fe+ 3 ions display the lowest F/F0 value. This observation confirms that, relative to other metal ions, Fe+ 3 ions exert a strong quenching effect on the fluorescence of the CQDs. The selectivity for Fe (III) ions is due to the ion-selective properties of the oxygenated functional groups on carbon quantum dots (CQDs) and the increased charge-transfer effects. When Fe (III) ions bind to these oxygenated groups, complexes are formed, altering the energy state distribution. This change enhances the nonradiative recombination of charge carriers, resulting in photoluminescence (PL) quenching50,51. The fluorescent response of the CQD sensing probe was further evaluated to determine its selectivity for Fe³⁺ ions. This evaluation involved examining the effect of various interfering metal ions, each at a concentration of 100µM, including Ni2+, Co2+, Cu2+, K+, and Na+, in a solution containing CQDs and Fe3+. A homogeneous solution was prepared by adding 100µM of each interfering metal ion. Subsequently, photoluminescence measurements were conducted using the specified parameters. The results, illustrated in Fig. 4c, indicated that the presence of interfering metal ions did not significantly affect the detection of Fe3+ ions. Figure 4d shows the sensing of Fe+ 3 with CQDs – based fluorescent probe.

Fig. 4
figure 4

(a) Sensing of different cations. (b) Selectivity of Fe3+ sensing and (c) Sensing of Fe3+ in the presence of different interfering cations. (d) Schematic Representation of the Sensing of Fe+ 3 with CQDs-based Fluorescent Probe.

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Biological activity study

Antifungal activity of CQDs

In the laboratory setting, in-vitro experimentation was carried out on two distinct fungal species: Aspergillus Niger and Fusarium Oxysporum, cultivated on Potato Dextrose Agar (PDA) medium. For each experimental trial, a total volume of 5 ml was employed, comprising 3 ml of PDA and 2 ml of CQDs at varying concentrations (specifically, 4× and 10× dilution). This mixture was inoculated onto both Aspergillus Niger and Fusarium Oxysporum, which were centrally placed on the agar plates. The entire setup was then incubated at room temperature for 7 days, facilitating the in-vitro assay to observe the impact of CQD concentrations on the growth and development of the fungi.

Following the incubation of fungi in media containing CQD, the radial growth of mycelium was meticulously observed. The inhibition rate (%) was calculated using the formula:

$$\:Inhibition\:rate\:\left(\%\right)=\:\frac{\left(R-r\right)100}{R}$$
(2)

where R represents the radial growth of fungal mycelia on the control plate, and r represents the radial growth of fungal mycelia on the plate treated with CQD. The results revealed a notable inhibitory effect on both fungal species in the presence of CQD. Particularly, at a 4× dilution, both species exhibited significant inhibition, with Fusarium Oxysporum demonstrating a more pronounced effect compared to Aspergillus Niger. This observation suggests that the concentration of CQDs in the solution plays a crucial role, and the higher concentration at 4× dilution contributes to a more potent inhibitory effect on fungal growth, as depicted in Fig. 5a,b.

Fig. 5
figure 5

Antifungal study of CQDs against (a) Aspergillus Niger and (b) Fusarium oxysporum (c) Photograph of inhibition zone containing S. aureus treated with CQDs, distilled water, and chloramphenicol and (d) Zones of inhibition (in mm) generated by CQDs sample, distilled water, and chloramphenicol against S. aureus. (e) Cytotoxicity study with different concentrations of CQDs (f) Cytotoxic assay of CQDs against dendritic cells by MTT assay and (g) Cytotoxic assay of CQDs against U87-MG cells by MTT assay.

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Antibacterial study

The antibacterial activity of CQDs was explored with S. aureus as the model pathogens of Gram-positive bacteria. Different treatment volumes of CQDs were applied to the bacterial cell plates, chloramphenicol discs were used as positive control, and distilled water-loaded discs were used as negative controls for the CQDs and were kept for incubation at 30 ºC. The next day, the area of inhibited bacterial growth was evaluated by measuring the diameter of the inhibition zone (IZ) around the discs Fig. 5c. The assay was repeated thrice. Antibacterial activity was expressed as the mean zone of inhibition diameters (mm) produced by the CQD extract Fig. 5d. The results revealed that the applying 25 µl of CQDs resulted in an inhibition zone diameter of 11.66 mm, whereas 50 µl of CQDs led to more significant inhibition zone with a diameter of 15.66 mm. These findings suggest a dose-dependent relationship between CQD concentration and antibacterial efficacy against S. aureus. The increase in the inhibition zone diameter with higher CQD volumes indicates enhanced bacterial growth inhibition, highlighting the potential of CQDs as effective antibacterial agents.

Cytotoxicity application of CQDs

Initially, we assessed the cytotoxic impact of CQDs on DCs through the MTT assay, which depends on the premise that metabolically active cells reduce MTT to purple formazan. Initially, DCs were exposed to various concentrations of CQDs for 24 h, and the formation of formazan was gauged at 570 nm. As depicted in Fig. 5e, roughly a 37% decline in cell viability was noted at a concentration of 10 µl, whereas with treatment using a 20 µl CQDs concentration, about 62% cell demise was observed. No noteworthy variance in cell viability was detected up to a 5 µl concentration. The identical experiment was authenticated through flow cytometry analysis by incubating the equivalent concentration of CQDs with GFP-expressing DCs. As portrayed in Fig. 5f, increased cell mortality was noted at the 10 and 20 µl treatment levels. For the in-vitro cytotoxicity assessment, the IC50 values were determined via nonliner regression analysis, and the IC50 value was ascertained to be 3.1 (Fig. 5g).

Conclusion

In summary, the carbon-based quantum dots are successfully synthesized from Mahua flowers via a simple hydrothermal method. The green CQDs exhibit remarkable optical properties, including strong fluorescence and excellent stability over different pH ranges as well as storage durations. Comprehensive characterization using various techniques confirms the structural and chemical composition of the CQDs. Furthermore, our investigation into biomedical applications reveal their potential for biomedical and sensing applications. The antifungal assays demonstrate significant inhibitory effects on Aspergillus Niger and Fusarium oxysporum, which are helpful in combating fungal infections. Cytotoxicity studies on dendritic cells reveal a dose-dependent cytotoxic effect of CQDs. Moreover, the antibacterial assays against Staphylococcus aureus highlight the antibacterial efficacy of CQDs. Metal sensing study highlights the high selectivity and sensitivity of the CQDs towards Fe2+ ions, as evidenced by the significant fluorescence quenching. The interference from other metal ions was minimal, indicating the robust selectivity of the CQDs for Fe3+ detection. Overall, our findings suggest that Mahua flower-derived CQDs hold promise as environmentally friendly and versatile materials with potential applications in various domains, including biomedical, environmental, and analytical fields. Further research and development on these CQDs could lead to the practical utilization of Mahua plants to address social and economic growth in the eastern and central Indian peninsula.