Detoxification of Toxic Organic Dye by Heteroatom-Doped Fluorescent Carbon Dots Prepared by Green Hydrothermal Method Using Garcinia mangostana Extract

Abstract

In this work, nitrogen- and oxygen-doped fluorescent carbon dots (NOFCDs) were prepared using a simple green hydrothermal carbonization technique. Aqueous ammonia and the plant extract of Garcinia mangostana were used as a source of nitrogen and carbon, respectively. The prepared NOFCDs were examined using a variety of microscopic and spectroscopic techniques for physicochemical characterization. Fluorescence and UV–visible spectroscopy measurements were used to analyze the NOFCDs’ excitation-dependent fluorescent emission and absorption nature. It is observed that fluorescence intensity was excited at 320 nm and the strongest emission peak absorbed at 405 nm. The HRTEM imaging revealed NOFCDs to be spherical in shape with particle size at 3.58 nm. Moreover, the amorphous nature of NOFCDs was verified by X-ray diffraction, Raman spectroscopy, and SAED pattern techniques. Through FTIR, XPS, and EDS investigations, the doping of nitrogen and oxygen over the surface of the NOFCDs was evaluated and validated. The presence of carboxylic, amine, hydroxyl, and carbonyl functional groups on NOFCDs’ surface was shown by XPS and FTIR investigations. By using NaBH₄, the obtained NOFCDs were demonstrated to possess excellent catalytic activity in the detoxification of methylene blue. Overall, this study revealed that the NOFCDs synthesized from green sources could be a promising material for catalytic and environmental remediation applications.

1. Introduction

Nanoscience and nanotechnology have brought rapid advancements in various fields, including drug delivery, catalysis, sensors, energy storage devices [1,2,3]. Among various nanomaterials, quantum dots (QDs) have drawn increasing interest due to their distinct electrical and luminous capabilities. By definition, the QDs are semiconductor nanoparticles with ultra-small size (2–10 nm) and are made of 100–10,000 atoms possessing the capability to transport electrons [4]. Despite having unique qualities, the presence of heavy metals makes them exceedingly hazardous, which consequently led to the discovery of brand-new, benign carbon nanoparticles known as “carbon dots” (CDs) [5]. The CDs refer to a recently discovered class of carbon compounds with a diameter <10 nm possessing minimal cytotoxicity [6]. Moreover, the high biocompatibility of CDs with fluorescent properties has led to their application in bioimaging and drug delivery. Since their discovery, CDs have been the subject of intensive study for potential applications in biosensing, catalysis, and biomedicine [5,7]. Compared to QDs, the CDs exhibit superior light-bleaching resistance, good chemical stability, minimum cytotoxicity, and excellent biocompatibility, as each CD possesses its own typical characteristics [6].

The incorporation of a dopant such as metal or heteroatom such as N, B, P, and S during manufacture can enhance the electrical, optical, and catalytic properties of CDs [7,8]. More specifically, nitrogen is preferable for doping with CDs because of its pentavalent nature and size-similarity with carbon. Such enhancement, especially in fluorescence and catalytic property, is caused by improvement in the electrical environment due to dopants’ injection of electrons into CDs. Nitrogen-doped CDs (NOFCDs) are promising candidates for use as fluorescent probes in bioimaging, biomedicine, and related fields due to their distinctive optical properties. The NOFCDs are fabricated using nitrogen precursors such as nitric acid, ammonia, folic acid, ethylene diamine, and ammonium citrate [9]. The highly energetic electrons and holes (excitons) in the CDs could be stabilized via single-atom doping, allowing more effective passivation of surface-active sites. This phenomenon has the potential to vastly enhance the fluorescence emission characteristics due to an increase in quantum yield following a rise in the percentage of heteroatoms in CDs.

Hydrothermal synthesis of carbon dots is one of the finest synthesis methods among all synthesis methods. After the precursor solution for carbon dot synthesis has been prepared, it is sealed inside a Teflon vial and then heated in an autoclave for around 12 h at a temperature of 180 °C. The resulting solution is filtered to remove big particles and the target CDs are then dispersed in the filtrate solution [10,11,12]. In this way, photoluminescent CDs can be synthesized in a rapid, economical, and environmentally friendly way. Carbon dots can be synthesized using a variety of chemicals such as citric acid, L- cysteine, ethylenediamine, o-phenylenediamine, etc., but their synthesis is complicated by multiple steps as well as the use of strong acids and high temperatures [6]. On the contrary, green synthesis methods are uncomplicated, cost-effective, and environmentally benign. Numerous studies have reported the synthesis of CDs by green methods through hydrothermal carbonization of natural sources including bamboo leaves [13], sweet red pepper [14], milk [15], Hylocereusundatus [16], Punicagrantum [17], Phyllanthus acidus [18], Prunus mume [19], and Prunus persica [20]. The phytochemicals in G. mangostana biomass were used as a sustainable source for the fabrication of NOFCDs through hydrothermal treatment. Obviously, synthesis by a green approach involves reducing or eliminating the use of hazardous chemicals.

Garcinia mangostana, a tropical evergreen tree also known as the purple mangosteen (Queen of Fruits), grows in tropical regions around the world and bears edible fruits with a sweet taste. It is cultivated in the humid forests of several Southeast Asian countries, including Indonesia, Malaysia, Sri Lanka, Philippines, and Thailand [21,22]. The pericarp (peel, rind, hull, or ripe) of G. mangostana has been widely used in traditional medicine to treat chronic ulcers, diarrhea, dysentery, and stomach pain. G. mangostana extracts have been shown to possess antioxidant, anticancer, antiallergic, anti-inflammatory, antibacterial, and antiviral properties, which are associated with the presence of several bioactive compounds including xanthones and anthocyanins [23]. Moreover, studies have shown that its consumption can provide the daily dose of vitamins A, C, and B complex [24,25] along with a large amount of sucrose and essential mineral components such as magnesium, iron, zinc, calcium, and phosphorus [26]. Most importantly, the total polyphenolic content in outer pericarp, inner pericarp, and pulp was shown to be 2930, 3404, and 133 mg gallic acid equivalent (GAE)/100 g [26], which should be beneficial for the use of phytochemicals in G. mangostana extract as a carbon source in the green synthesis of NOFCDs in this study. A total of 70% by weight of biomass is discarded as waste in G. mangostana fruit [27]. A comprehensive review on growth, cultivation, climate condition, production volume and cost, nutritional composition, and health benefits are all well-documented elsewhere [22,23,26,27].

In this study, we sought to create a straightforward hydrothermal carbonization procedure for the generation of nitrogen and oxygen-doped fluorescent carbon dots (NOFCDs) using G. mangostana extract and aqueous ammonia. In addition, both UV–Visible (UV-Vis) and Fluorescence spectroscopy were used to characterize the as-synthesized NOFCDs for a wide range of physicochemical characteristics including high-resolution transmission electron microscopy (HRTEM), electron dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. The synthesized nitrogen-passivated carbon dots were used as a promising catalyst envisaging their effectiveness in sunlight and UV light-driven chemical processes. They possess the capacity to absorb light across a broad spectrum, exhibiting photosensitizing, upconversion photoluminescence, chemical inertness, and redox characteristics. The NOFCDs can act as a hybrid for the photodegradation of pollutants entering aquatic systems. Additionally, the large surface area and adsorption capacity of NOFCDs can facilitate the effective removal of impurities in aqueous systems. Furthermore, the selection of G. mangostana as a precursor for synthesis can enhance the surface passivation of nitrogen functional groups on CDs.

2. Materials and Methods

2.1. Materials

Samples of G. mangostana were obtained from the highland regions of Kodaikanal in Tamil Nadu, India. Chemicals such as MB, aqueous ammonia, and sodium borohydride were procured from Sigma-Aldrich (St. Louis, MO, USA). In this experiment, all the solutions made with double-distilled water only.

2.2. Preparation of G. mangostana Extract

G. mangostana fruits were washed using running water and cut into small pieces by segmentation and a weighed amount of 50–60 g was finely ground in a mixer grinder by adding a small amount of double-distilled water. The resulting extract was filtered once through cotton to remove the larger particles and then through Whatman No. 40 filter paper (Sigma-Aldrich) to obtain a crystal-clear extract which was used as a carbon source for synthesis of NOFCDs.

2.3. Synthesis of NOFCDs Using G. mangostana Extract

The synthesis of NOFCDs was executed by a hydrothermal carbonization process using G. mangostana extract as a precursor. Initially, about 29 mL of fruit extract was carefully mixed with 1 mL of aqueous ammonia and then transferred into a 50-mL stainless steel autoclave lined with Teflon. A hot-air oven was used to maintain the mixture at 180 °C for 12 h. After completion of the reaction, the autoclave was opened and allowed to cool to ambient temperature to obtain a dark brown-colored solution indicating the formation of NOFCDs. Finally, the prepared NOFCDs were filtered through a Whatman No. 40 filter paper and centrifuged at a speed of 10,000 rpm for 30 min to remove the bigger particles. The NOFCDs were stored at 10 °C until further characterization and application [28,29].

2.4. Characterization Method

The optical properties of the synthesized NOFCDs were measured using both UV-Vis (Perkin Elmer Lambda 850+, Beaconsfield, United Kingdom) and fluorescence (Jasco FP 8500, Tokyo, Japan) spectrophotometers. Wavelengths of stimulation ranging from 270 nm to 400 nm were tested to find the sweet spot for maximum luminescence. HRTEM images of the microstructure and morphology of NOFCDs were obtained by dispersing them in water and depositing them on a copper grid coated with carbon. After complete drying at room temperature, the specimen was placed in a JEOL JEM 2100 electron microscope (Tokyo, Japan) and images were captured using a 200 kV accelerating voltage. The NOFCDs and the dried G. mangostana extract were analyzed by FTIR spectroscopy using a Jasco FTIR 460 Plus spectrophotometer (Tokyo, Japan). The XRD patterns were analyzed using CuK radiation (1.54 Å) at 40 kV and 30 mA in a PANalytical X’Pert Diffractometer (Malvern, United Kingdom).

2.5. Evaluation of Catalytic Detoxification of Methylene Blue by NOFCDs

With the assistance of a Perkin Elmer Lambda UV-Vis spectrophotometer, the catalytic activity of the newly synthesized NOFCDs was examined by taking the MB solution in a 1-cm quartz cuvette, followed by initially stirring a mixture containing 1 mL of 0.1 M solution of MB, 1 mL of water, and 0.5 mL of freshly-prepared ice-cold NaBH₄. Then, 100 μL of NOFCDs was added to the above mixture and the absorption spectra were obtained at predetermined intervals of time.

3. Results and Discussion

The synthesis of luminous NOFCDs was accomplished by hydrothermal carbonization at a temperature of 180 °C and time duration of 12 h using the extract of G. mangostana and aqueous ammonia as a source of carbon and nitrogen, respectively. The transformation of the color of G. mangostana extract from dirty white to a dark brown color provided conclusive evidence of the formation of carbon dots. The as-synthesized luminous NOFCDs were kept in the refrigerator at 10 °C and utilized for further studies.

3.1. UV Spectroscopy for G. mangostana Extract and NOFCDs

UV-Vis spectroscopy is an excellent technique for investigating CDs’ optical properties. In Figure 1a, the UV-Vis spectra of G. mangostana extract (carbon precursor) and as-synthesized NOFCDs in water are shown. Unlike the G. mangostana extract, which showed an absorption peak in the UV region at the wavelength 279 nm, the NOFCDs showed a significant peak at the wavelength 284 nm. In addition, NOFCDs showed a typical peak at 325 nm in the fluorescence spectra illustrated in Figure 1b, which is consistent with the absorption spectra results realized in Figure 1a. All these peaks represent the carbon-passivating n-π*transition of non-bonding electrons in the nitrogen moiety or carbonyl group [30,31]. Carbon compounds are rendered inert due to the π–π* transition of C=C (sp²) bonds.

3.2. Fluorescence Spectroscopy for G. mangostana NOFCDs

Fluorescence spectra of synthesized NOFCDs at different excitation wavelengths are shown in Figure 2. Evidently, an increase in excitation wavelength from 285 nm to 325 nm resulted in an elevation of fluorescence intensity, however, an increase in excitation wavelength from 325 nm to 375 nm reduced the fluorescence intensity, which may be caused by the n–π* transition [28,31]. Additionally, the π–π*transitions in the graphitic carbon at the center of the molecule may also contribute to this tendency. However, the highest intensity was seen at an excitation wavelength of 325 nm (405 nm).

It is assumed that the fluorescence of NOFCDs is caused by the radiative recombination of excited nonbonding electrons from the carbonyl and heteroatom (nitrogen) portion that is present over the NOFCDs [32,33]. The anti-Stokes fluorescence involving adsorption of two or more photons at the same time and emission as visible light may be responsible for the distinctive photoluminescent quality of NOFCDs [34].

3.3. HRTEM Analysis of Biogenic NOFCDs

The HRTEM picture of the biogenic NOFCDs captured in the image scale of 20 nm and 5 nm are shown in Figure 3a,b, respectively. Apparently, the images revealed that the NOFCDs were spherical in shape and the particle size distribution histogram developed by determining the size of particles directly from TEM image using ImageJ software showed that the particle size ranged from 1 to 8 nm with an average diameter of 3.58 nm (Figure 3c). Moreover, the amorphous nature of NOFCDs is depicted in Figure 3d [28,31]. Based on HRTEM image in Figure 3, the d-spacing was estimated to be 0.36 nm.

3.4. EDX Analysis

EDX is a supplementary technique that was carried out during the HRTEM analysis using the JEOL JEM 2100 electron microscope. Figure 4 shows the EDX spectra of NOFCDs which provides clear information about the presence of elements and their relative composition. Based on the peak area of carbon, oxygen, and nitrogen, the as-synthesized NOFCDs were estimated to contain carbon in the largest proportion (75.4%), followed by oxygen (15.1%), and nitrogen (9.5%), implying the successful doping of nitrogen on the surface of CDs in NOFCDs [16,28].

3.5. FTIR Spectra of G. mangostana Extract and NOFCDs

In order to examine the surface chemistry of NOFCDs as well as the functional group makeup of these compounds, FTIR spectroscopy was utilized. In Figure 5, the FTIR spectra of G. mangostana extract and NOFCDs are shown stacked on one another. The FTIR spectrum of the extract reveals absorption bands at 3446, 2923, and 1749 cm⁻¹. These absorption bands, which may be assigned to −OH, −CH, and −C=O functional groups, respectively, can be found in the spectrum.

The FTIR spectra of NOFCDs, on the other hand, displayed absorption bands at 3419, 2919, 1708, 1606, 1372, and 1113 cm⁻¹. These absorption bands are attributed to −OH, −CH, −C=O, −C=C, and −C−O−C, respectively. In greater detail, the confirmation of the doping of nitrogen and oxygen on the surface of CDs in NOFCDs was provided by stretching vibrations at 1372 cm⁻¹ (−C−N) and 1708 cm⁻¹ (−C=O). As a whole, the as-synthesized NOFCDs exhibited the presence of functional groups such as acid, amine, hydroxyl, and carbonyl [19,35].

3.6. XRD Spectra for Synthesized NOFCDs

The XRD pattern of NFOCDs is shown in Figure 6 depicting the presence of an amorphous carbon phase in CDs. Strong bands appeared at 28.20°, 43.77°, and 44.75° which correlated well with the diffractions of (002), (100), and (101) [36]. This finding suggested the existence of turbostratic graphitic carbon in the structure of NOFCDs. It is worth pointing out that turbostratic graphic carbon is a unique class of carbon having structural ordering in between that of amorphous carbon phase and crystalline graphite phase. In addition, the XRD diffraction peaks corresponding to high potassium impurity in NOFCDs occurred at 35°, 38°, and 51° whose presence may be traced back to the carbon source. More specifically, G. mangostana fruit used as a carbon source for the preparation of NOFCDs was reported to contain the highest proportion of potassium (48 mg/100 g) compared to other minerals [26]. Owing to the small size of NOFCDs, the XRD peaks can be explained only in part. Based on Bragg’s equation, the d-spacing (interlayer distance) of as-synthesized NOFCDs was estimated to be 0.37 nm and 0.21 nm for (002) and (100) planes of carbon, which corroborated well with that obtained by HRTEM (0.36 nm).

3.7. Catalytic Activity of NOFCDs in the Reduction Reaction of Methylene Blue Using NaBH₄

Methylene blue (MB) is an anionic dye used extensively in the production of a variety of materials in textiles, paper, plastics, and even rubber. The reduction reaction of MB was followed in real-time at room temperature using UV-Vis spectroscopy in the range from 200 to 800 nm. Absorption bands were found at 613 nm and 664 nm for the aqueous solution of MB, which corresponded to π–π* and n–π* transition in the azo group [37]. Figure 7a shows the UV-Vis spectra of the reduction in MB by NaBH₄in the absence of NOFCDs. It was found that the intensity of absorbance for MB reduced from 2.45 to 1.15 arbitrary units upon increasing the reaction time from 0 to 2 h. However, in the presence of added as-synthesized NOFCDs (10 μL), a relatively higher reduction in absorbance intensity was observed for MB at all time intervals (Figure 7b), with the absorbance value reducing from 2.50 to 0.25 arbitrary units on increasing the reaction time from 0 at 16 min. As shown in Figure 7c, this detoxifying reaction followed pseudo-first-order kinetics and the straight plot showed a high coefficient of determination (R²) of 0.9971 with the rate constant being 0.284 min⁻¹. The observed tendency revealed the high catalytic efficiency of NOFCDs synthesized by a green method using G. mangostana extract.

The observed catalytic efficiency of the synthesized NOFCDs for detoxifying MB with sodium borohydride (NaBH₄) can be understood using the Langmuir–Hinshelwood mechanism as shown in Figure 8a [30], with the as-synthesized NOFCDs acting as a go-between for the electron transfer between the donor (BH₄⁻) and the acceptor (MB). More elaborately, the detoxication process involves the transfer of electrons from the donor to the acceptor, which lowers the energy barrier between the reactant and the product and speeds up the dye reduction rate [37,38]. Figure 8b shows a schematic illustration of the structural modification in MB during reduction by NaBH₄ after addition of NFOCDs to turn from a blue color to colorless (leuco MB). The results discussed before of this study suggested that NOFCDs can function as a reductive catalytic agent for the detoxification of hazardous organic dyes and thus may be used as a promising nanocatalyst in a wide range of future applications aimed at environmental remediation. Table 1 shows that the performance comparison of NOFCDs used in this study was better than the other reported materials for detoxifying MB dye by catalytic reduction [39,40,41,42,43,44,45,46,47].

NOFCDs have the potential to act as electron transfer mediators by collecting the photogenerated electrons and reducing significantly the amount of electron–hole recombination. The catalytic degradation of organic dyes was accelerated due to the nature of the catalytic material. NOFCDs play an important role in catalytic degradation because nitrogen doping on CDs affects their band gaps. This has enabled the development of a novel valence band and conduction band-energy levels. As a result, distinct electron transport channels emerged at the band gap areas of the synthesized NOFCDs. Furthermore, the functionalization of CDs could be regulated by the addition of a nitrogen atom to their surface. This allows the efficient suppression of the excited charge-carrier recombination and the induction of charge delocalization on a broad scale. NOFCDs may be able to transport electrons more efficiently than other materials due to their high quantum yield and strong photoluminescent characteristics. Moreover, the conduction and valence bands generate electron–hole pairs when exposed to light, resulting in drastic changes in the kinetics of the degradation reaction. One of the major advantages of utilizing NOFCDs as catalysts is that they improve the separation of electron–hole pairs and inhibit the recombination of these pairs. When the dye and NOFCDs were mixed and exposed to light, the electrons were driven from the valence band to the conduction band of NOFCDs, resulting in the creation of electron–hole pairs. The introduction of NOFCDs could boost the band gap energy significantly and create free electrons (e⁻) along with holes (h⁺), which initiate the catalytic breakdown of organic pigments. Due to the presence of positive holes (h⁺), the oxygen molecules are oxidized to produce superoxide (O₂^•) radicals, and the water molecules are oxidized to produce hydroxyl (OH^•) radicals. These highly reactive radicals damage the color molecules, causing dye molecules to undergo a change in color. Moreover, the efficient degradation of MB dye demonstrated in this study is achieved by using NOFCDs as green catalysts prepared with G. mangostana extract.

4. Conclusions

The NOFCDs were successfully synthesized using the one-pot hydrothermal carbonization procedure, with G. mangostana serving as the green carbon precursor and aqueous ammonia as the nitrogen precursor. They were characterized to be spherical in shape with an average particle size of 3.58 nm based on the HRTEM image, while the XRD pattern confirmed that NOFCDs are graphitic and amorphous in nature, FTIR and EDX spectroscopy verified the nitrogen and oxygen doping on NOFCDs, and UV-Vis spectroscopy facilitated the examination of their optical properties. As the removal of hazardous dyes from contaminated effluents is a top priority, this study’s findings that biogenic NOFCDs can catalyze the detoxification of methylene blue by sodium hydroxide are particularly noteworthy. The reduction in MB by NaBH₄ in the presence of NOFCDs was complete within 16 min implying a remarkable catalytic efficiency. Furthermore, the use of natural resources such as G. mangostana as a precursor for the preparation of NOFCDs can pave the way for environmental remediation in a green way.

Author Contributions

Conceptualization, V.A. and B.S.I.; Methodology, V.A., N.S., and K.R.; Software, N.S. and J.V.K.; Validation, K.R., J.V.K., N.A., and B.S.I.; Formal analysis, V.A., N.S., K.R., and J.V.K.; Investigation, V.A., N.S., and K.R.; Resources, N.S., J.V.K., and N.A.; Data curation, V.A., K.R., J.V.K., and B.S.I.; Writing—original draft, V.A., K.R., N.S., and N.A.; Writing—review and editing, N.S., J.V.K., and B.S.I.; Visualization, K.R., J.V.K., N.A., and B.S.I.; Supervision, V.A., N.S., N.A., and B.S.I. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All data generated or analyzed during this study are included in this article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. UV-Vis spectra of G. mangostana extract and NOFCDs (a) and the excitation spectra of NOFCDs (b).

Figure 2. Fluorescence spectra of synthesized NOFCDs at different excitation wavelengths from 285 to 325 nm (a) and 325 to 375 nm (b).

Figure 3. HRTEM image of NOFCDs captured at 20 nm scale (a) and 5 nm scale (b) along with the particle distribution histogram (c) and SAED pattern (d) of NOFCDs.

Figure 4. EDX spectra of as-synthesized NOFCDs. C, carbon; N, nitrogen; O, oxygen.

Figure 5. FTIR spectra of G. mangostana extract and NOFCDs.

Figure 6. XRD pattern of as-synthesized NOFCDs.

Figure 7. UV–Visible spectra of the reduction in MB using NaBH4 in the (a) absence of NOFCDs; (b) presence of NOFCDs; and (c) pseudo-first-order kinetics plot for the reduction in MB with NFOCDs.

Figure 8. Graphical representation of the Langmuir–Hinshelwood mechanism for detoxification of MB using NFOCDs (a) and a schematic illustration of structural modification in MB during reduction by NaBH4 after addition of NFOCDs to turn from a blue color to colorless (leuco-MB) (b).

Table 1

Performance comparison of NOFCDs used in this study with other reported materials for detoxifying MB dye by catalytic reduction.

Serial No.	Catalyst Material/Source	MB Reduction Time (min)	References
1	ZnONPs/Plant extract	150 min	[39]
2	AgNPs/Fruit extract	30 min	[40]
3	AgNPs/Seed extract	30 min	[41]
4	AgNPs/Bark extract	30 min	[42]
5	AgNPs/Leaf extract	20 min	[43]
6	Ag andAuNPs/GK kashayam1	20 min	[44]
7	Ag andAuNPs/Coconut oil	20 min	[45]
8	AgNPs/Seed extract	19 min	[46]
9	Ag/PSNM nanocomposite2	18 min	[47]
10	NOFCDs	16 min	This study

¹ GK kashayam, Ayurvedic Guggulutiktham kashayam; ² PSNM, poly(styrene-N-isopropylacrylamide-methacrylic acid.

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