Introduction

Recently nanofabrication processes to develop new multifunctional constructs, such as carbon dots (CDs), are gaining high interest to further understand, develop and propose new interface platform procedures that are green, sustainable, and industrially relevant to applied perceptions. Thereof, the term nanofabrication is being taken as a collection of techniques that produces artifacts with nano-domain scale, i.e., patterns (materials) with sizes of 100 nm or less in at least one dimension1. Among various nanofabricated materials, CDs contemporarily benefit from photophysics and quantum chemistry domains2. For example, CDs offer characteristic features, such as excellent photoluminescence (PL), aqueous solubility, various size ranges, photoinduced electron transfer properties, up-conversion features, high stability, conductivity, photocatalytic potential, tunable surface functionalities, and facile synthesis using green precursors, etc. all make CDs as robust candidates for multipurpose applications3,4,5,6.

Until now, numerous approaches, e.g., microwave, laser ablation, hydrothermal, solvothermal, and other synthetic processes, have been projected and exploited to devise CDs with requisite features. However, due to chemical-based synthesis issues, green synthesis procedures, such as CDs from agro-industrial products, are getting popular due to their overall cost-effectiveness, chemical-free, facile process, and ease of deployment7. Therefore, various types of renewable carbon feedstocks, e.g., peel extracts, have been used to develop CDs. However, they need preprocessing and/or pretreatment prior to performing the hydrothermal or carbonization process to generate CDs. To further avoid the above-mentioned preparatory limitations, the utilization of pulp-free juice extracts is considered more convenient as “ready-to-use” green CDs precursors6,7,8. Additionally, green synthesis yields highly fluorescent CDs as new materials of interest to replace conventional fabrication procedures. In addition, green synthesis approaches also limit the utilization of heavy metals, which can lead to toxicity concerns and environmental problems9. Highly stable, fluorescent nitrogen-doped carbon dots (N-CDs) have been effectively synthesized from readily available citrus lemon juice, as demonstrated in studies by Tadesse et al.10 and Zhang et al.11. The selection of lemon juice as a precursor is primarily justified by its composition; it is rich in various phytochemicals such as flavones, flavanones, flavonoids, limonoids, and triterpenoids. These compounds are reported to play a vital role in producing ultra-small carbon dots and facilitate their stable formation through green synthesis methods12. However, despite these advantages, green synthesis methods also exhibit inherent limitations. The natural variability in the composition of bio-based precursors such as lemon juice can affect the reproducibility and consistency of the resulting N-CDs. Furthermore, achieving precise control over particle size distribution, surface chemistry, and uniformity remains more challenging compared to conventional synthetic routes. These factors must be carefully optimized to ensure reproducibility and functional performance, particularly in applications requiring scalability or industrial implementation13. In this study, N-CDs were produced using a hydrothermal method, showcasing an innovative approach to creating carbon-based materials with tunable properties that broaden their application potential. A particularly promising area of interest is their use in addressing pressing environmental remediation challenges.

Environmental pollution issues caused by various pollutants, including antibiotic compounds, are of supreme interest that demand suitable solutions. The direct or indirect release of antibiotic-based emerging contaminants into different environmental matrices, such as water and soil, causes significant environmental and human-health-related consequences5,14. In addition, the long-term and/or misuse of antibiotics provokes a lack of response against infectivity being treated with regular antibiotics. Specifically, in agriculture, antibiotics are used to shield from bacterial spoilage and, at the same time, to ensure crop growth. However, its excessive use adversely affects soil fertility characteristics. Other contamination sources include the leaching from discarded medications and waste discharge from hospitals and industries15. Among different antibiotics, tylosin (TYL) has been widely exploited and subject to the mobility rate in soil16, TYL and its residual metabolites can cause various consequences. As per available literature reports, several techniques or processes, such as chemical-based oxidation methods, advanced oxidation processes, adsorption using different solid adsorbents, biological methods, or electrochemical techniques16,17,18. Carbon-based materials have emerged as a particularly promising approach for removing tylosin (TYL) from various environmental matrices, exhibiting synergistic adsorptive and photocatalytic properties19. Furthermore, nitrogen-doped carbon dots (N-CDs) are gaining traction for their potential in environmental sensing. Recent research highlights their remarkable photoluminescent features, high sensitivity, and selectivity in detecting metal ions. These nanomaterials have demonstrated encouraging outcomes in identifying environmentally and biologically significant ions, such as iron (Fe3+), magnesium (Mg2⁺), and copper (Cu2⁺), making them valuable for both environmental monitoring and biomedical applications20,21,22.

While considerable research has focused on tylosin (TYL) remediation in water, direct removal from soil samples remains under-explored. Addressing this gap, this work presents a facile nanofabrication method for nitrogen-doped carbon dots (N-CDs), employing pulp-free lemon juice as a sustainable, green precursor.

The primary objectives of this study are: (1) to develop a green and cost-effective approach for synthesizing N-CDs using pulp-free lemon juice as a sustainable carbon source; and (2) to investigate the influence of key synthesis parameters, such as temperature and reaction time, on their resulting physicochemical properties. The newly fabricated N-CDs, synthesized under varying temperature and reaction time conditions, were thoroughly characterized using a comprehensive suite of techniques, including FTIR, Zeta Potential analysis, DLS, UV–Vis spectroscopy, SEM, TEM, and EDS. Ultimately, this study aims to assess the potential of these fabricated N-CDs for the remediation of TYL from contaminated soil samples.

Experimental

Chemicals/reagents

Fresh Meyer lemons (Citrus × limon) were purchased from local markets in Riyadh, Saudi Arabia. The chemical reagents used were analytical grade: urea (97% purity), acetone (≥ 99.5%), and methanol (100%), all used without further purification. Distilled water (DI) was used throughout the experimental procedures. Pharmaceutical grade tylosin tartrate was obtained from the Arab Company for Medical Applications. Soil samples were collected from: (1) an industrial zone in Riyadh (latitude 24° 33ʹ 04.1ʹʹ N, longitude 46° 52ʹ 48.4ʹʹ E) and (2) horse stables in Medina (latitude 24° 39ʹ 09.0ʹʹ N, longitude 39° 32′ 28.4′′ E).

Biosynthesis of green fluorescent N-CDs

The experimental methodology in this study, as adapted from Cárdenas-Alcaide et al.23, details a comprehensive methodology for the synthesis and purification of N-CDs sourced from lemon juice (see Fig. 1 for schematic overview).

Fig. 1
figure 1

Schematic illustration for the synthesis N-CDs from lemon juice.

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First, 210 mL of lemon juice was filtered through 20 μm pore paper. 21 g of urea was added to the as-prepared pulp-free lemon juice. The solution was stirred at 800 rpm at room temperature until urea was completely dissolved. The mixture was then transferred into porcelain crucibles, and these were kept in a furnace at 160, 210, and 260 °C for 2 and 4 h. The resulting black powder was cooled down to room temperature and then dispersed in acetone. The solution was centrifuged at 5,500 rpm for 15 min to separate the precipitate from the supernatant. The supernatant was discarded, and the N-CDs were purified by washing three times with acetone and a 10/90 (v/v) methanol/acetone solution. Finally, the washed N-CDs were dried at 60 °C for 24 h, weighed gravimetrically, and standardized to 10 μg/mL by dispersing 10 μg in 10 mL of acetone with 15 min sonication. The resulting N-CDs suspensions were stored for further characterization and application.

Instrumental and analytical evaluations of N-CDs

Surface chemistry was studied with a Spectrum 100 Fourier Transform Infrared (FTIR) Spectrometer (Perkin-Elmer, USA). The measurements were performed in the transmission range of 400 cm−1 to 4000 cm−1 with 64 scans. For zeta (ζ) potential and dynamic light scattering (DLS), four measurements per sample were performed with the Zetasizer Ultra (Malvern Panalytical, UK). The optical properties were analyzed with an Evolution 201 UV–Visible spectrophotometer (Thermo Fisher Scientific, USA). To analyze the morphology, structure, and elemental composition of the N-CDs, a scanning electron microscope (SEM) (JSM-IT500HR, JEOL, Japan) equipped with an X-ray energy-dispersive spectroscope (EDX) (STD-PC80, JEOL, Japan) operated at 15 kV and a transmission electron microscope (TEM) (JEM-1400 flash, JEOL, Japan) were utilized.

Sample extraction procedure

The soil samples (0–10 cm depth) were collected from the industrial region in Riyadh city (24° 33ʹ 04.1ʹʹ N 46° 52ʹ 48.4ʹʹ E), and from horse stables in Medina region (24° 39ʹ 09.0ʹʹ N 39° 32ʹ 28.4ʹʹ E). The soil samples were ground through a 2 mm sieve, soil was spiked with Tyl by first dissolving 20 mg of it in 10 mL of DI. 10 g of sandy soil (SS) and 10 g of silt soil (Si) were then treated with this TYL solution. 1 g of each treated soil (in triplicate) was dried to remove the water and then thoroughly mixed. These soil samples were then placed in 50 mL Falcon tubes. Subsequently, 40 mg of N-CDs was dissolved in 1 mL of DI in 1.5 mL Eppendorf tubes and sonicated for 25 min. This solution was then transferred to the Falcon tubes containing the soil samples. Finally, the 12 tubes, including the control group, were placed on a shaker and agitated for 20 h at 150 rpm, at room temperature. The control group, in contrast, was treated only with 1 mL DI.

Antibiotic extraction method

Following a 20-h shaking period, 0.8 g of NaCl was added to the soil samples in the Falcon tubes, which were then vortexed at 2800 rpm for 1 min to ensure sample homogenization. Next, 8.0 mL of acetonitrile was added to each sample, and they were again vortexed at 2800 rpm, for 5 min. The samples were then stored in a freezer at − 20 °C for over 16 h (overnight). Afterwards, a 2 mL portion of the supernatant was taken and centrifuged at 5410 rpm for 10 min. After centrifugation, 1 mL of the supernatant was collected and transferred to vials for subsequent analysis. The entire extraction process was performed in triplicate. The extraction and analysis methods were adapted based on the research conducted by Paranhos et al.24.

Equipment and chromatographic conditions

The results of the remediation of soil were conducted on an LC–MS/MS system consisting of an Agilent 1290 Series LC system (Agilent Technologies Inc., USA) combined with a Triple Quad LC–MS 6460 detector. All sample solutions were filtered through a 0.45 μm micropore filter to eliminate particulate matter before injection into the LC–MS system. The chromatographic separation was performed on an Agilent Zorbax Eclipse Plus C18 RRHD (2.1 × 50 mm, 1.8 µm) column, with a mobile phase consisting of 50% Acetonitrile/formic acid (0.1%) and 50% Water/formic acid (0.1%), at a flow rate of 0.2 mL per minute and oven temperature at 35 °C. About a 1.0 μL solution was injected into the LC–MS system for analysis. The desolvation line (DL) and heat block (HB) temperatures were maintained at 250 °C and 400 °C, respectively. The mass spectrometry parameters included a precursor ion [M+ H]+ at 916.40 m/z and a product ion at 173.65 m/z. The retention time for tylosin was 0.679 min, and the total run time was 3 min. For quantification, the external standard calibration method was employed, with a stock solution concentration of 1 mg/L−1. Calibration curves were constructed within the concentration range of 5 to 30.0 mg L−1, yielding a high coefficient of determination (r2 = 0.999).

The removal efficacy of TYL was calculated by using the following formula:

$$Removal\, Efficacy (\%)=\frac{Initial\, Concentration-Final\, Concentration}{Initial \,Concentration}\times 100.$$
(1)

Results and discussion

Characterization of N-CDs

The thermal carbonization of pulp-free lemon juice with urea led to distinct visual changes reflecting structural evolution. At 160 °C, the initially pale-yellow solution gradually darkened into a brown viscous residue, while higher temperatures (210 °C and 260 °C) accelerated the reaction, yielding dry black solids within 2–4 h due to enhanced carbonization and aromatic domain formation. Upon redispersion in water (1 mg/mL), the N-CDs formed homogeneous brownish-yellow colloidal solutions under ambient light (Fig. 2a). Under UV illumination (365 nm), all samples exhibited green photoluminescence (Fig. 2b), with the highest intensity observed for N-CDs synthesized at 160 °C for 4 h. The lighter coloration and weaker emission at 160 °C for 2 h reflect incomplete sp2-domain formation, while the diminished fluorescence and color at 260 °C for 4 h suggest thermal degradation and loss of radiative surface groups. These observations offer initial insight into the structural and optical evolution of the N-CDs, guiding subsequent detailed analyses.

Fig. 2
figure 2

Photograph of the different N-CDs solutions (1 mg/mL) under (a) ambient light, and (b) UV light at 365 nm.

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FTIR spectroscopy was used to determine the characteristic functional groups on the surface of the N-CDs, and the results obtained are shown in Fig. 3. The FTIR spectra of different N-CDs were shown to be similar at different temperatures and times. Specifically, the N-CDs that were synthesized at temperatures of 160 °C, 210 °C, and 260 °C have corresponding emissive peaks at 3345, 2120, and 1636 cm−1. A distinct peak with a wide and powerful shape at approximately 3345 cm−1. This peak is associated with the vibration of O–H stretching, which suggests that the N-CDs samples contain hydroxyl groups. These hydroxyl groups are responsible for the hydrophilic properties of the samples. The peak observed at 2120 cm−1 indicates the bending vibration of C=C in an aromatic structure25. However, the absorption peak seen at 1636 cm−1 is associated with the existence of nitro (NO2)26. According to the FTIR analysis results, it was found that the absorption of the functional group remained constant when exposed to temperatures ranging from 160, 210 and 260 °C for 2 to 4 h. Therefore, the FTIR analysis demonstrates the presence of distinct functional groups, such as alcohols, amines, aromatic rings, and carbonyls on the surface, along with a core containing sp2/sp3 carbon in the N-CDs derived from citrus lemon juice. The diverse range of surface functional groups significantly enhances the water solubility of these N-CDs10.

Fig. 3
figure 3

FTIR spectrum of the N-CDs synthesized at different durations and temperatures.

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The optical characteristics of the as-prepared N-CDs dispersed in water (1 mg/mL) were evaluated by UV–Vis spectrophotometer (Fig. 4). The UV–visible spectra of the N-CDs prepared at 160 °C for 2 h show only two absorption peaks at 288 and 333 nm. Whereas all other five N-CDs samples (i.e., prepared at: 160 °C for 4 h; 210 °C for 2 h; 210 °C for 4 h; 260 °C for 2 h; and 260 °C for 4 h) display highly coincident absorption peaks of 288, 333, and 407 nm, thus indicating that after the first 2 h of synthesis, the structure of the N-CDs does not remarkably change over this temperature range. The absorption peaks at 288 nm and 333 nm could be attributed to the π-π* transition of the aromatic –C=C– bonds in the sp2 hybrid orbitals27,28. The peak at 407 nm could be assigned to the n–π* transition of –C=O, C–N, or –C–OH bonds in the sp3 hybrid orbitals, which could be from hydroxyl (−COOH) or amine (−NH2) group on the surface of the N-CDs10,28. At 407 nm, the absorption intensity decreased with the temperature. In this peak, the sample at 160 °C for 4 h revealed the strongest absorbance among the samples obtained, indicating optimal surface group retention. In general, the temperature and synthesis duration had a significant influence on the absorbance of the N-CDs, indicating that these variables have a major impact on their structure and intrinsic properties. In addition, the results from this analysis are consistent with the spectral features observed in the FTIR spectroscopy.

Fig. 4
figure 4

UV–Vis absorption spectra of the N-CDs synthesized at different durations and temperatures.

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The ζ potential values were measured to obtain the surface charge of N-CDs. All samples exhibited negative zeta potentials (Table 1), confirming the presence of negatively charged functional groups on their surfaces. The measured values ranged from − 43.86 ± 3.63 mV to − 51.8 ± 3.37 mV, with the most negative potential (− 51.8 ± 3.37 mV) observed for N-CDs synthesized at 210 °C for 4 h. This significant surface charge is attributed to abundant carboxyl (–COOH) and hydroxyl (–OH) groups, as confirmed by FTIR analysis.

Table 1 Zeta potential values of N-CDs synthesized under different temperature and time conditions.
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Notably, the most negative charge (− 51.8 ± 3.37 mV) occurred at 210 °C for 4 h, suggesting optimal nitrogen doping and surface group formation at this condition. While samples at 160 °C showed minimal time-dependent variation (± 0.48 mV between 2 and 4 h), higher temperatures induced greater charge variability, with 260 °C samples exhibiting reduced negativity at longer durations.

DLS analysis precisely determined the N-CDs particle sizes, revealing a clear correlation between synthesis time and particle size across all temperatures. The mean hydrodynamic diameter increased from 147.6 nm (2 h) to 303.9 nm (4 h) at 160 °C. This growth pattern persisted at higher temperatures: 1274 nm (2 h) to 3134 nm (4 h) at 210 °C, and 1143 nm (2 h) to 1344 nm (4 h) at 260 °C. These results demonstrate that extended reaction times consistently promote particle agglomeration across all synthesis temperatures.

The surface morphology and structure of the N-CDs were analyzed using SEM, TEM, and Image J software. The particle size distribution corresponding to the number of particles in the field of view of the TEM micrograph was determined and presented as a relevant histogram. As observed in Fig. 5, the N-CDs at 160 °C for 2 h showed an average particle size of 20.04 ± 0.04 nm. The SEM micrograph (Fig. 5a) with a magnification of ×20,000 displays a big chunk with smaller pieces on the surface. However, the TEM micrograph (Fig. 5b) indicates that the N-CDs are uniformly distributed with a narrow size distribution. Furthermore, the histogram presented in Fig. 5c provides a representation of particle diameter estimation through statistical analysis based on the data from a total of 140 particles selected from the TEM image.

Fig. 5
figure 5

(a) SEM and (b) TEM micrographs of the N-CDs at 160 °C for 2 h. (c) Particle size distribution determined from the TEM micrograph.

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The SEM image for the N-CDs synthesized at 160°C for 4 h (Fig. 6a) with a magnification of 10,000 × illustrates the irregular surface topography. Figure 6b also demonstrates that these N-CDs are uniform and agglomerative. According to the histogram shown in Fig. 6c, the average size of the N-CDs at 160 °C for 4 h could be estimated to be 15.27 ± 0.92 nm considering 140 nanoparticles.

Fig. 6
figure 6

(a) SEM and (b) TEM micrographs of the N-CDs at 160 °C for 4 h. (c) Particle size distribution determined from the TEM micrograph.

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The SEM micrograph with a × 5000 magnification for the sample produced at 210 °C for 2 h (Fig. 7a), displays asymmetrical particles with sharp and well-defined edges. The TEM image in Fig. 7b confirms the production of agglomerated products as well as mono-dispersed particles, with N-CDs of 16.09 ± 0.35 nm in size (Fig. 7c), considering a total number of 200 particles in the field of view of the TEM micrograph.

Fig. 7
figure 7

(a) SEM and (b) TEM micrographs of the N-CDs at 210 °C for 2 h. (c) Particle size distribution determined from the TEM micrograph.

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In the SEM micrograph of N-CDs synthesized at 210 ºC for 4 h (Fig. 8a) with a magnification of × 1000. The sample has a smooth base with some cracks and big chunks on the surface. In addition, the TEM micrograph (Fig. 8b) illustrates the arrangement of the produced N-CDs. The corresponding particle size distribution histogram of the N-CDs at 210 °C for 4 h is displayed in Fig. 8c, indicating a particle size of 12.48 ± 0.23 nm considering 140 nanoparticles.

Fig. 8
figure 8

(a) SEM and (b) TEM micrographs of the N-CDs at 210 °C for 4 h. (c) Particle size distribution determined from the TEM micrograph.

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Figure 9a reveals the SEM image of N-CDs synthesized at 260 °C for 2 h captured at × 10,000 magnification, showing cubic-like structures with smaller particles on the surface. The TEM image (Fig. 9b) clearly illustrates quasi-spherical N-CDs in the previously noted array. The histogram data (Fig. 9c) estimates an average size of 8.74 ± 0.36 nm based on examining 60 individual nanoparticles.

Fig. 9
figure 9

(a) SEM and (b) TEM micrographs of the N-CDs at 260 °C for 2 h. (c) Particle size distribution determined from the TEM micrograph.

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At a magnification of × 30,000, the SEM micrograph of the N-CDs synthesized at 260 °C for 4 h (Fig. 10a) shows large structures with a molten-like appearance. In the micrograph depicted in Fig. 10b, the accumulation of the smallest N-CDs is also observable. The particle size distribution histogram in Fig. 10c demonstrates a very small average diameter of 6.63 ± 0.07 nm, determined from 80 selected N-CDs particles in the field of view of the TEM image.

Fig. 10
figure 10

(a) SEM and (b) TEM micrographs of the N-CDs at 260 °C for 4 h, and (c) particle size distribution determined from the TEM micrograph.

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Overall, the samples acquired a mixture of cubic and spherical structures. Relevant SEM analysis, agglomeration was also observed since carbon dots tend to stay in one location and therefore grow larger due to their tendency to form new, larger structures29. In addition, at elevated temperatures, a substantial portion of the smallest particles agglomerate, forming larger particles characterized by inhomogeneous sizes30.

It is noteworthy that the obtained hydrodynamic diameters measured by DLS were considerably inconsistent with the particle sizes calculated through image analysis using ImageJ software. In each instance, the DLS-assisted measured diameter is significantly larger than what was observed in the TEM micrographs. This fluctuation can be attributed to the presence of an aqueous environment during the DLS assay, in contrast to the dry sample conditions during TEM analysis31. In this context, the particle size seemed to decrease significantly as the synthesizing temperatures increased, as evident from the size calculated through TEM image analysis.

Furthermore, the morphology remains similar when synthesized at the same temperature, indicating the influence of this parameter on the particle’s shape and structure. For example, the samples in Figs. 5 and 6 prepared at 160 °C exhibited irregular surface topography. The N-CDs in Figs. 7 and 8, when synthesized at 210 °C appear to have distinctive features with well-defined edges and a more cubic-like morphology. In Figs. 9 and 10 the N-CDs prepared at 260 °C seem to present a propensity for agglomeration, resulting in the formation of larger, molten-like structures alongside smaller agglomerations present on the surface.

The elemental composition of the biosynthesized N-CDs was analyzed by EDX. The EDX scanning performance was done with a voltage range of 15 kV. The N-CDs at 160 °C for 2 h (Fig. 11a,b) were composed of two main elements, carbon at the level of 84.05% (Fig. 11c) and oxygen at the level of 15.95% (Fig. 11d).

Fig. 11
figure 11

(a) SEM micrograph (mapping area), (b) EDX spectrum, and EDS elemental mapping for (c) carbon and (d) oxygen in the N-CDs at 160 °C for 2 h at a magnification of × 18,000.

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The EDS analysis (Fig. 12a,b) revealed the elemental composition of carbon, oxygen, and potassium in the mass percentages of 70.26%, 28.70%, 1.04% (Fig. 12c–e), respectively, in the N-CDs at 160 °C for 4 h. In the EDX spectra (Fig. 12b), the presence of C, O, and K in the N-CDs structure is confirmed.

Fig. 12
figure 12

(a) SEM micrograph (mapping area), (b) EDX spectrum and elemental mapping for (c) carbon, (d) oxygen, and (e) potassium in the N-CDs at 160 °C for 4 h at a magnification of × 15,000.

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For the N-CDs at 210 °C for 2 h (Fig. 13a,b), the elemental analysis revealed the following composition: carbon (C) at 68.69% (Fig. 13c), oxygen (O) at 27.68% (Fig. 13d), potassium (K) at 1.87% (Fig. 13e), and calcium (Ca) at 1.76% (Fig. 13f).

Fig. 13
figure 13

(a) SEM micrograph (mapping area), (b) EDX spectrum and elemental mapping for (c) carbon, (d) oxygen, (e) potassium, and (f) calcium in the N-CDs at 210 °C for 2 h at a magnification of × 19,000.

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The elements present in the N-CDs at 210 °C for 4 h (Fig. 14a,b) included the following elements and their respective mass percentages: carbon (C) at 70.47% (Fig. 14c), oxygen (O) at 22.68% (Fig. 14d), potassium (K) at 3.22% (Fig. 14e), calcium (Ca) at 1.48% (Fig. 14f), and zirconium (Zr) at 2.15%.

Fig. 14
figure 14

(a) SEM micrograph (mapping area), (b) EDX spectrum and elemental mapping for (c) carbon, (d) oxygen, (e) potassium, and (f) calcium in the N-CDs at 210 °C for 4 h at a magnification of × 23,000.

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The N-CDs synthesized at 260 °C for 2 h (Fig. 15a,b) exhibited the following elemental composition: carbon (C) at 63.70% (Fig. 15c), oxygen (O) at 32.75% (Fig. 15d), silicon (Si) at 0.49% (Fig. 15e), and potassium (K) at 3.05% (Fig. 15f).

Fig. 15
figure 15

(a) SEM micrograph (mapping area), (b) EDX spectrum and elemental mapping for (c) carbon, (d) oxygen, (e) silicon, and (f) potassium in the N-CDs at 260 °C for 2 h at a magnification of × 17,000.

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Finally, the elemental composition of the N-CDs at 260 °C for 4 h (Fig. 16a,b), in terms of mass percentages, is as follows: carbon (C) at 65.93% (Fig. 16c), oxygen (O) at 23.67% (Fig. 16d), aluminum (Al) at 1.96% (Fig. 16e), silicon (Si) at 5.83% (Fig. 16f), potassium (K) at 2.07% (Fig. 16g), and calcium (Ca) at 0.55%.

Fig. 16
figure 16

(a) SEM micrograph (mapping area), (b) EDX spectrum and elemental mapping for (c) carbon, (d) oxygen, (e) aluminum, (f) silicon, and (g) potassium in the N-CDs at 260 °C for 4 h at a magnification of × 13,000.

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It is worth noting that the N-CDs synthesized at 260 °C for 2 and 4 h contain silicon in their elemental composition (see Figs. 15e, 16f). The presence of this element could be ascribed to the hygroscopic properties of the samples, which interact with a silicone substrate for the procedure of examination. The existence of specific functional groups on the surface and edges of N-CDs may be the cause of this phenomenon32. However, the results of the EDS analysis confirm that carbon is the major element in all the samples.

Tylosin mitigation potential of N-CDs

The results of the analysis, as shown in Table 2, indicate that the concentrations of TYL decreased in both SS and SI soils when treated with N-CDs prepared at 160 °C for 4 h, compared to the untreated control soil. This decline is primarily due to the incorporation of soil with green, fluorescent N-CDs material. The removal efficiency of N-CDs was 12.3% in SS soil and 39.2% in SI soil, with SS soil demonstrating a lower removal rate. This reduced adsorption capacity may be caused by the aggregation or competitive adsorption of organic matter onto the surfaces of N-CDs, as noted by Gil-Díaz et al.33. Furthermore, the author highlighted that the success of nanoremediation heavily depends on soil properties, even when using the same nanoparticles. In this study, SS soil contains a higher organic matter content than SI soil, which likely contributed to its diminished adsorption efficiency.

Table 2 Tylosin concentration (mg L−1) at SS and SI soils before and after treatment with N-CDs.
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Additionally, SI soil has a significantly higher calcium carbonate (CaCO₃) content than SS soil. Elevated levels of CaCO₃ are known to increase soil alkalinity, as reported by Ab-Manan and Ab-Aziz34. An increase in pH in sandy loam soil has been shown to enhance TYL sorption, as noted by Zhou et al.35. Consequently, higher pH may reduce TYL mobility and promote its interaction with N-CDs, thereby improving removal efficiency.

In this study, the maximum removal efficacy of TYL achieved using N-CDs was 39.2%. Similarly, Hu et al.36 prepared nitrogen-doped mesoporous carbon nanospheres (NMCN) by the hydrothermal method. The removal efficacy of TYL from wastewater by this material was 24%. However, combining N-CDs with other catalytic materials has shown higher removal efficiencies in previous studies, particularly from aqueous environments, which are less complex than soil systems. Cao et al.37 prepared nitrogen-infused carbon with cadmium sulfide semiconductor particles (CdS/NC–T), which effectively degraded tetracycline in water, achieving 83% reduction efficiency in just one hour. Furthermore, Ai et al.38 suggested that N-CDs in a composite form can offer more active sites, potentially enhancing their removal efficacy.

A study by Vijeata et al.39 demonstrated the remarkable photocatalytic efficiency of CDs synthesized from Azadirachta indica (neem) leaves. These CDs achieved degradation rates of 90.73% for methylene blue (MB), 98.25% for malachite green, 52% for rhodamine 6G, and 6.13% for methyl orange. These results highlight the significant potential of green synthesis methods to produce highly effective nanomaterials for degrading a diverse range of organic pollutants in environmental remediation efforts.

The main mechanism for TYL sorption by N-CDs could be surface complexation40. Specifically, H-bonding due to the hydrogen atoms of the hydroxyl groups of tylosin being drawn towards the unshared electron pairs on the oxygen atoms in the hydroxyl groups of the N-CDs41. This H-bonding interaction has previously been reported when TYL was mitigated from soil by goethite material42. The existence of these hydroxyl groups on N-CDs surface has been verified by FTIR analysis. Additionally, according to Luo et al.41 the ketone group on TYL could offer strong electron-withdrawing ability to form π-π EDA interaction with the π-conjugated groups of N-CDs. Furthermore, electrostatic interactions may contribute to the adsorption, given the negative surface charge of the N-CDs. These mechanisms are consistent with established sorption behavior of carbon nanomaterials in aqueous environments43 and have been similarly observed in green carbon-based materials used for pollutant remediation44,45.

The current study is considered as the first report on the utilization of raw, non-metallic nitrogen-doped carbon dots for the remediation of tylosin in soil systems. The results indicate that N-CDs synthesized from citrus-derived pulp-free lemon juice exhibit moderate adsorption capacity for tylosin across both sandy and silt soil matrices. It is expected that increasing the N-CDs concentration would further improve removal efficiency by enhancing the density of active surface sites available for sorption.

Conclusions

This study successfully demonstrated the potential of pulp-free lemon juice as a sustainable and readily available carbon precursor for the facile synthesis of multifunctional nitrogen-doped carbon dots (N-CDs) via a simple carbonization process. The resulting N-CDs exhibited desirable green-light emission, tunable physicochemical properties depending on the synthesis parameters, and promising catalytic activity in the degradation of Tylosin from contaminated soil. Notably, the optimal N-CDs, synthesized at 160 °C for 4 h, achieved a significant Tylosin removal efficacy of 39.2%, as confirmed by LC–MS/MS, positioning them as a viable candidate for cost-effective environmental remediation. In conclusion, this work offers valuable insights into the valorization of bio-waste for the production of highly fluorescent and photocatalytic nanomaterials with significant applied potential. To fully leverage these promising findings, future research should prioritize a comprehensive investigation into the precise catalytic mechanisms of these N-CDs, explore strategies for further enhancing their degradation efficiency for a broader spectrum of environmental contaminants under diverse and environmentally relevant conditions, and assess the feasibility of scaling up the synthesis process for practical application.