Ixora coccinea flower-derived green luminescent carbon quantum dots for Fe³⁺ recognition and preparation of Pd nanoparticles for the Suzuki–Miyaura coupling and cyanation process

Abstract

The solvothermal method of producing spherically shaped and highly fluorescent tiny (∼3 nm) nitrogen-doped carbon quantum dots (NCQDs) with high quantum yield (43%) from Ixora coccinea flowers and o-phenylenediamine has been approached sustainably. Since these NCQDs exhibit excitation-dependent fluorescence activity, they were used to detect the Fe³⁺ ion selectively in real-time in human blood serum, because it is a component of human blood. The limit of detection (LOD) of the NCQDs was found to be 60 nM. We also synthesized palladium nanoparticles utilizing the NCQDs without additional reducing or stabilizing agents because these quantum dots serve as both. Numerous characterization techniques were used to characterize the nanoparticles, and all the characterizations showed the presence of NCQDs, which stabilize the nanoparticles. Palladium nanoparticles’ small size (4 nm) and large surface area (35.77 m² g⁻¹) enable them to exhibit strong catalytic activity in Suzuki–Miyaura coupling and cyanation processes. It is obvious that synthetic palladium nanoparticles and NCQDs have a variety of uses in the fields of catalysis and sensing respectively.

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1. Introduction

Because of its high quantum yield, photoluminescence, low toxicity, and biocompatibility, carbon quantum dots have garnered significant attention among all the nanomaterials as an alternative to fluorescent organic molecules for a variety of applications, including bioimaging, biocatalysis, and sensing.¹ When purifying single-walled carbon nanotubes in 2004, Xu et al. unintentionally found carbon dots. However, in 2006, Sun et al. effectively proved the synthetic process of fluorescence emissive carbon dots with surface passivation and gave the term “carbon quantum dots” (CQDs).^2,3 These CQDs are quasispherical, zero-dimensional nanocrystals smaller than 10 nm.⁴ There are several ways to synthesize CQD, including bottom-up methods like hydrothermal,⁵ solvothermal,⁶ pyrolysis,⁷ and microwave irradiation,⁸ and top-down methods like arc discharge,⁹ laser ablation,¹⁰ electrochemical exfoliation,¹¹ plasma treatment,¹² and chemical oxidation.^13,14 Because bottom-up synthesis is inexpensive, easy to carry out, and environmentally friendly, it is classified as a green synthetic approach.¹⁵ The common use of this CQD in the field of sensors due to its sensing ability of various analytes such as Ni²⁺, Pd²⁺, Fe³⁺, Fe²⁺, Mg²⁺, K⁺, Cu²⁺, and many more via quenching methods has been reported. This sensing of various analytes by CQDs is also due to different functional groups present on the surface of the quantum dots. Functional groups like –OH, –NH₂, and –COOH, etc. transfer their electrons to the metals and cause quenching in the fluorescence intensities.⁹ In the year 2022, Nagaraj et al. synthesized CQDs from the endosperm of Borassus flabellifer for the detection of Fe³⁺ ions.¹¹ In 2016, A Tyagi et al. synthesized CQDs from lemon peel extract as a photocatalyst and for the sensing of Cr⁶⁺ ions.¹² Various heteroatom-doped quantum dots have also been used in the field of sensing and are giving high quantum yields due to the functional groups present on its surface after doping with heteroatoms.¹⁰ In 2022, G. Ge et al. synthesized nitrogen-doped CQDs using a hydrothermal method for the selective detection of Fe³⁺ ions.¹³

These quantum dots are famous for their electron-donating as well as electron-withdrawing capabilities. They can donate electrons to the electron-lacking species and accept electrons from electron-rich species.¹⁶ As we know, the surface of the CQDs is covered by various functional groups that can be used as a source for the preparation of metal nanoparticles. These CQDs act as reducing as well as stabilizing agents and the metal nanoparticles have numerous applications in the field of catalysis.^17,18

The carbon–carbon (C–C) coupling reaction is one of the most fascinating areas of study in organic chemistry. The Suzuki–Miyaura coupling reaction is currently the most widely used technique for creating C–C bonds.¹⁹ Numerous industries including pharmaceuticals, plastics, natural goods, etc., use the biaryl compounds that result from C–C coupling processes.²⁰ Palladium complexes with ligands (such as phosphene, imidazole, and N-heterocyclic carbenes) acting as catalysts can be used to create these molecules.^21–23 There are additional uses for aryl nitriles in polymers, natural goods, and pharmaceuticals.^24,25 However, several hazardous cyanide sources, including potassium cyanide,²⁶ sodium cyanide,²⁷ and zinc cyanide,²⁸ are employed in the production of these aryl cyanide compounds using a palladium catalyst.²⁹ However, K₄[Fe(CN)₆]·3H₂O is a relatively inexpensive and non-toxic cyanide source from which T. Schareina et al. produced aryl cyanide in 2004.³⁰ However, it is tedious to synthesize such compounds because these palladium catalysts are more expensive and the reactions are time-consuming. To overcome these situations, various nanoparticles have been used to synthesize these compounds. There are various palladium nanoparticle syntheses such as green methods^31,32 and from quantum dots.^33–35 They are used for aryl cyanations^31,36 and C–C bond formation.^33,34,37

In this work, we present the production of nitrogen-doped carbon quantum dots (NCQDs) using o-phenylenediamine and Ixora coccinea flowers. These NCQDs serve as a reducing and stabilizing agent during the synthesis of palladium nanoparticles (PdNPs) and as a fluorescent probe for the selective detection of the Fe³⁺ ion. These PdNPs produced from NCQDs have been employed as catalysts in aryl cyanation processes and Suzuki–Miyaura coupling reactions.

2. Experimental

A summary of the dual applications of nitrogen-doped carbon quantum dots (NCQDs), namely fluorescence detection of Fe³⁺, and Pd nanoparticle (PdNP) synthesis from the prepared NCQDs, is given in the Experimental section. Additionally, how the nanoparticles will be useful in Suzuki–Miyaura coupling and cyanation reactions is also described briefly.

2.1 Material

Ixora coccinea flowers were collected from VIT Vellore. All necessary chemicals and solvents were purchased from chemical suppliers such as Avra, Sigma Aldrich, and TCI chemicals. Using a Bruker Ascent 400 Hz spectrometer, the chemical structure of the produced molecule was examined using ¹H (100 MHz) spectra. Tetramethylsilane or TMS was used as the internal standard and CDCl₃ and DMSO are used as the solvents when measuring the chemical shift. To record FTIR data, a JASCO 4100 spectrometer was utilized. The absorption studies from UV-Visible spectra were measured on a Hitachi (model 2910) and the FL emission studies were recorded on a Hitachi model (F-7000). Bruker, D8-Advance P-XRD was used for the powder XRD analysis of the PdNPs.

2.2 Preparation of NCQDs

Ixora coccinea flowers, also known as West Indian jasmine flowers, were gathered, cleaned of debris with double distilled water, and then allowed to air dry for two days, after which the flowers were ground into a fine powder using a mortar and pestle. One gram of this powder was extracted using ethanol for 2 hours. Then 40 mL of the extract was mixed with 200 mg of o-phenylenediamine and placed in an autoclave set at 180 °C for six hours. Then, bright green emissive NCQDs were formed as shown in Fig. 1. The obtained quantum dots were allowed to cool to ambient temperature and were filtered through Whatman filter paper and then centrifuged at 6000 rpm for 15 min. After that, pure NCQDs were obtained by dialysis using a cellulose membrane in a glass container and refrigerated for further use to detect Fe³⁺ and for preparation of Pd nanoparticles. The quantum yield of this NCQD was found to be 43.5% (S1).

Fig. 1 Preparation of NCQDs from Ixora coccinea flowers.

2.3 Fluorescence detection of Fe³⁺

Using double distilled water, stock solutions of different metals were prepared with a concentration of 1 mM from their corresponding salts. Each of the solutions was diluted to 100 μM concentration. Already prepared NCQDs were diluted 50 times. A cuvette was filled with 2 mL of this NCQD solution, and its fluorescence spectrum was measured at the excitation wavelength of 400 nm. Then, 100 μL of each metal salt solution (100 μM) was introduced one at a time to the cuvette, and the fluorescence was measured. This experiment shows that out of all metal salts, only Fe³⁺ ions change the fluorescence intensity, which makes it clear that this NCQD is sensitive towards Fe³⁺ ions only.

2.4 Synthesis of PdNPs

Pd nanoparticles (PdNPs) were synthesized using the previously prepared NCQDs. This procedure involved filling a round-bottom flask with 100 mL of 10 Mmol of Pd(OAc)₂ in water, adding 20 mL of NCQD gradually, and then keeping the mixture in a stirred condition in an oil bath at 80 °C. It was noted that after two hours, the compound solution colour changed from light brown to black, indicating that Pd(II) reduced to Pd(0), and the formation of Pd nanoparticles³¹ and schematic representations are shown in Fig. 2. Subsequently, the mixture was repeatedly centrifuged using water, ethanol, and acetone to eliminate any water-soluble components. The resultant nanoparticles were oven-dried and then confirmed using several characterization techniques. After confirmation, these nanoparticles were used for Suzuki–Miyaura coupling and cyanation reaction and excellent reaction yields were obtained.

Fig. 2 Preparation of PdNPs from NCQDs.

2.5 General protocols for Suzuki–Miyaura-coupling reaction catalyzed by PdNPs

The catalytic activity of PdNPs was utilized for the Suzuki–Miyaura coupling process. In this specific process, a combination of solvents, namely EtOH : H₂O (1 : 1 v/v), was used to dissolve aryl halide (1 equivalent), boronic acid (1.15 equivalent), potassium carbonate (2 equivalents), and PdNPs (0.3 mol%). After that, an inert atmosphere of nitrogen was used to screen the reaction mixture, and it was stirred under the optimized conditions (Table S2, ESI†). Thin layer chromatography (TLC) was used to monitor the reaction, and after the completion of the reaction, filtering was used to recover the catalyst. After that, the reaction mixture was extracted using a brine solution and ethyl acetate, and the organic layer was collected before being dried against anhydrous Na₂SO₄. Next, the product was purified in column chromatography using n-hexane and ethyl acetate as the eluent. Using H¹ NMR spectroscopy, the final products were characterized (the recyclability of the catalyst after 3 cycles has been shown in the ESI† (Fig. S6)).

2.6 General protocols for cyanation reaction catalyzed by PdNPs

We used these specific PdNPs for the cyanation reaction after obtaining excellent yields from the earlier reactions. In a 50 mL round-bottom flask, aryl halide (1 equivalent), K₄[Fe(CN)₆]·3H₂O (0.18 equivalent), Na₂CO₃ (1.5 equivalent), PdNPs (0.5 mol%), and DMF as a solvent have been mixed and the optimized reaction conditions (Table S3, ESI†) were found. For the cyanation reactions, K₄[Fe(CN)₆]·3H₂O was employed as a cyanide source. TLC was used to track the reaction, and the catalyst was removed after the completion of the reaction using a simple filtration method. Additionally, ethyl acetate and brine solution were used to extract the product from the reaction mixture. The ethyl acetate layer was then collected and dried using Na₂SO₄. After that, n-hexane and ethyl acetate were used as an eluent to purify the products in column chromatography. Then the purified product was confirmed using H¹ NMR (the recyclability of the catalyst after 3 cycles has been shown in the ESI† (Fig. S6)).

3. Results and discussion

3.1 Characterization of the NCQDs and PdNPs

3.1.1 Characterization and optical behavior of the NCQDs. FTIR characterization was used to obtain the evidence of NCQD's functional groups, as shown in Fig. 3. The broad peak at 3345 cm⁻¹ displays the asymmetric stretching vibration of the –OH and –NH groups.³⁸ The aliphatic stretching vibrations C–H, C=N, C=O, C=C, –CH₂, and –OH of carboxylic acid, and –C–O of ether are represented by the peaks at 2922, 2129, 1721, 1617, 1505, 1450, 1273, and 1043 cm⁻¹ in the synthesized NCQDs, respectively. Using HRTEM analysis, the size and shape of the NCQDs were examined. The average particle size of the quantum dots was determined to be approximately ∼3 nm, and they were present in a spherical shape, well distributed in the solution, as depicted in Fig. S1 (ESI†).

Fig. 3 The FTIR spectrum of NCQDs.

The NCQDs showed a light-yellow colour when viewed with the naked eye and they exhibited a bright green emission under a UV lamp. To investigate the optical properties of the synthesized NCQDs, UV-visible and fluorescence spectra were recorded. In Fig. 4, the absorption spectrum shows 3 different bands at 280, 370, and 440 nm where the absorption peak at 280 nm is attributed to the π–π* transition of C=C and C=N, which arises during the synthesis process. Peaks at 370 and 440 nm can be attributed to the n–π* transition of C=O and N=C (which comes due to the nitrogen doping). When excited at 440 nm, the NCQDs exhibit green emission at 500 nm, which suggests that it has a typical luminescent property. Like other carbon dots, this NCQD exhibits excitation-dependent photoluminescence (PL) behaviour (Fig. S2(a), ESI†); this could be because of surface and NCQD size imperfections. Using quinine sulphate as a reference, the quantum yield of the NCQDs was found to be 43.6%. The photostability of the NCQDs was demonstrated by the fact that, following an hour of irradiation at 440 nm, there was no discernible change in fluorescence intensity.

Fig. 4 UV-visible analysis of NCQDs.

The pH effect of the NCQDs with different pH values (from 4 to 12) was investigated, which is shown in (Fig. S2(b), ESI†). 0.1 M HCl and NaOH were used and adjusted for the preparation of pH solutions using a pH meter as per the required pH. At 500 nm, the emission showed lower intensity at pH 4 than at neutral and basic pH, which can be attributed to surface coverage of the NCQDs with –OH, –NH₂, and –COOH functional groups. However, in neutral pH, deprotonation of the respective functional groups³⁹ occurred and did not show any significant change, confirming the stability of the NCQDs at neutral pH.

3.1.2 Selective and sensitive detection of Fe³⁺. The prepared NCQDs were applied as a fluorescent probe to detect Fe³⁺ ions selectively. Several cations (K⁺, Ca²⁺, Mg²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zr⁴⁺, Sr⁴⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Pb²⁺, Li⁺, and Hg²⁺) of the corresponding nitrate salts were taken for the selectivity test. In this specific experiment, 2 mL of 50 times diluted NCQDs were mixed with 100 μm of each metal cation, and the resulting fluorescence emission occurred at 500 nm with the excitation wavelength at 440 nm. There is a noticeable shift in the fluorescence intensity following the addition of the Fe³⁺ ion. Apart from Fe³⁺, all the metal ions showed no or negligible change in their fluorescence intensity, as shown in Fig. 5(a). Hence, it is obvious that the NCQDs are selective towards Fe³⁺ ions only. When Fe³⁺ ions were added to the NCQD solutions, the green emissive solution changed to a non-emissive solution under UV light or generally quenched upon the addition of Fe³⁺, as shown in Fig. 5(b). This selectivity was brought about by the complex that is formed between the hydroxyl, the carboxylic group found in the NCQD, and the Fe³⁺ ion.⁴⁰

Fig. 5 (a) Selectivity study of NCQDs with different cations, and (b) NCQDs after the addition of Fe³⁺ along with other cations.

Additionally, the NCQDs were tested for selectivity to Fe³⁺ in the presence of other ions. 100 μm of the most common interfering cations: K⁺, Mg²⁺, Cr³⁺, Fe²⁺, Co²⁺, Ni²⁺, Zr⁴⁺, Sr⁴⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Pb²⁺, Al³⁺ and Hg²⁺ along with 100 μm of Fe³⁺ were added to NCQDs, as illustrated in Fig. 6(a). The results indicate that the addition of various metal ions in addition to Fe³⁺ did not significantly alter the detection of Fe³⁺ ions. As shown in Fig. 6(b), upon adding Fe³⁺ to the NCQDs, the fluorescence intensity of the NCQDs gradually decreased. Increasing the Fe³⁺ concentration caused a decline in the fluorescence intensity of the NCQDs. The titration was carried out until the addition of 100 μm Fe³⁺ to the NCQD solution. The emission intensity of the NCQDs decreased after the addition of Fe³⁺ within 1 second, as given in Fig. S3(a) (ESI†).

Fig. 6 (a) Interference studies of Fe³⁺ with different cations and (b) titration of different concentrations of Fe³⁺ ion with NCQDs.

For the calculation of the limit of detection (LoD) of Fe³⁺, the relative fluorescence response of the NCQDs (F₀/F) versus the concentration of Fe³⁺ (in μm) is given in Fig. S3(b) (ESI†). Where F₀ represents the fluorescence intensity before Fe³⁺ addition and F represents after addition. The correlation coefficient was found to be 0.98271 with a linear range of 1 to 8 μm, which indicates a good linearity relationship. The LOD was found to be 0.06 μm, calculated from the formula,

LOD = 3σ/slope

where σ represents standard deviation. The LOD obtained for this one is comparably low with the others. The comparison table for different carbon quantum dots is given in the table given in Table S1 (ESI†).

3.1.3 Real-time applications. To determine the practical relevance and real-time application of NCQDs with Fe³⁺, different water samples and human blood serum were tested for Fe³⁺ content. The tap water and lake water samples were collected from the Vellore Institute of Technology, Vellore, for the analysis. Both the water samples were spiked with a known concentration of Fe³⁺, but in the case of blood serum, Fe³⁺ was measured directly. After that, the fluorescence intensities were analyzed for each sample of different concentrations. The recovery of Fe³⁺ varied from 97.2% to 99.3% (Tables 1 and 2).

Table 1 Determination of Fe³⁺ ions spiked in environmental water samples

Sample	Spiked Fe^3+ (μM)	Fe^3+ found (μM)	Recovery (%)	RSD (n = 3) (%)
Tap water	25	24.3	97.2	1.28
	50	48.7	97.4	1.27
	75	74.2	98.9	1.29
Lake water	25	24.5	98	0.98
	50	48.9	97.8	0.97
	75	74.5	99.3	0.98

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Table 2 Determination of Fe³⁺ ions in human blood serum

Samples	Added (μL)	Found (μm)
Blood serum	25	24.8
	50	48.6
	75	74.5

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3.1.4 Sensing mechanism of NCQDs with Fe³⁺. The sensing mechanism of NCQDs selectively towards Fe³⁺ is given in Fig. S4 (ESI†), which is explained as follows. The NCQDs show bright green emissive fluorescence without the addition of Fe³⁺. This may be due to the fact that when the light falls into the NCQDs, it captures the energy, and the electron moves from the ground state (HOMO) to the excited state (LUMO). When the relaxation occurs, the molecule releases the photons and causes the fluorescence emission.⁴¹

The surface of the NCQDs was covered with –OH, –COOH, and –NH₂ groups, as proven by FTIR analysis. Due to the half-filled orbital and distinct coordination of Fe³⁺, the NCQDs exhibited a relatively high degree of selectivity towards Fe³⁺. Fe³⁺ may interact with the functional group-covered NCQD surface. When the five half-filled orbitals of Fe³⁺ combined with those groups on the surface of the NCQDs, they absorbed the excited electrons to form a non-radiative combination that quenched the fluorescence.⁴² The schematic representation is illustrated in Fig. S4 (ESI†).

3.2 Characterization of palladium nanoparticles (PdNPs)

The synthesized palladium nanoparticles (PdNPs) from the NCQDs were characterized using various characterization techniques, such as Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (p-XRD), field emission scanning electron microscope (FESEM), energy dispersivee X-ray (EDS), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET), thermogravimetric analysis (TGA), and induced coupled plasma-optical emission spectroscopy (ICP-OES). The FT-IR characterization was performed for the PdNPs, which is given in Fig. 7(a). The peaks at 3369, 2929, 2122, 1520, 1415, and 1033 cm⁻¹ are attributed to –OH & –NH, aliphatic and aromatic –CH, C=N, aliphatic –CH₂, –OH of carboxylic acid, and –C–O of ether stretching vibrations, respectively. This is due to the absorption of phytochemicals present in the NCQDs during the reduction and formation process of the PdNPs. The peaks are slightly shifted due to the interaction of the PdNPs and the NCQDs.

Fig. 7 (a) FTIR analysis of NCQDs and PdNPs. (b) BET analysis of PdNPs.

To determine the surface area as well as the porosity of the PdNPs, BET analysis was employed. From the study, it was found that it had a surface area of 35.77 m² g⁻¹. The N₂ adsorption–desorption isotherm of the PdNPs is shown in Fig. 7(b). It exhibits a type II isotherm with a hysteresis loop, Fig. 7(b), which indicates the characteristics of a mesoporous material. Due to its larger surface area, it acts as an excellent nanocatalyst for the reactions.

To get the morphology of the PdNPs, FESEM analysis was done. From the analysis, we observed that the nanoparticles exhibited a spherical morphology Fig. 8(a)–(c). In their morphology, it is clearly visible that the quantum dots are nicely coated with the nanoparticles. For the confirmation of elements present in the PdNPs, EDAX analysis was carried out. This confirmed the palladium, carbon, nitrogen, and oxygen present in the nanoparticles and they are uniformly distributed, which is due to the interaction of the NCQDs and PdNPs as shown in Fig. 8(d). ICP-OES analysis confirmed that the PdNPs have a palladium content of 50.28% w/w.

Fig. 8 (a)–(c) FESEM Image of PdNPs, (d) EDAX image of PdNPs, and (e)–(g) HRTEM image of PdNPs.

For the confirmation of the magnified morphology and size of the PdNPs, HRTEM analysis was carried out. From the HRTEM analysis, it was confirmed that the shape of the PdNPs exhibited a spherical morphology, and the size ranged from 4 to 5 nm, which is shown in Fig. 8(e)–(g).

Thermogravimetric analysis was performed to determine the thermal stability of the PdNPs and the absorbed chemicals of NCQDs on the PdNPs, Fig. 9(a). The TG analysis was carried out at a heating rate of 10 °C per minute in a nitrogen atmosphere from 40 to 800 °C. The initial weight loss of 5–6%, which occurred up to 150 °C temperature, was due to the moisture absorbed by the PdNPs. Almost 18% sharp weight loss occurred at 400 °C due to the removal of volatile organic molecules that are present in the NCQDs. A total of around 30% weight loss at around 800 °C is due to the phytochemicals that are present in the NCQD absorbed in the PdNPs.³¹

Fig. 9 (a) TGA image and (b) XRD image of PdNPs.

To determine the precise composition and crystallinity of the prepared PdNPs, a powder X-ray diffraction analysis was carried out (Fig. 9(b)). The large peak at about 2θ = 23° shows the existence of carbon dots; furthermore, there are three notable peaks at 2θ = 40.03°, 46.51° and 68.03° corresponding to the diffractive indices as (111), (200) and (220) which validates the PdNP nanoparticle's formation from NCQDs. The size of the Pd nanoparticles is 4.4 nm. This was calculated using the formula D = (0.94λ)/β cos θ, which has a good acceptance with the HRTEM result. It is confirmed by the observed evidence that Pd nanoparticles have been produced from the quantum dots, and their small size makes it a clear choice for good catalytic conversions in coupling processes. XRD data after 3 times recycled PdNPs after Suzuki–Miyaura coupling and cyanation reactions have been given in Fig. S6 (ESI†).

3.2.1 Application of PdNPs in Suzuki–Miyaura coupling reaction. The PdNPs were first tested for the Suzuki–Miyaura coupling reactions. To optimize the reaction conditions, phenylboronic acid of substituted benzene and bromobenzene were selected as model reactions. EtOH:H₂O was a convenient solvent system for the optimized reactions. As shown in Table S2 (ESI†), a different set of reactions was carried out to get the optimized reaction conditions. The reaction did not proceed without a base and Pd catalyst (Table S1, Sl. no. 11 and 12, ESI†). During the optimization reactions, a variety of bases, including K₂CO₃, Na₂CO₃, NaOH, and KOH, as well as a variety of solvents, including EtOH, H₂O, DMF, THF, acetonitrile, acetone, and 1,4-dioxane, were utilized at varying temperatures. However, the reaction proceeded smoothly under the ideal reaction conditions, which were the EtOH : H₂O (1 : 1) solvent system, K₂CO₃ (2 equivalents) as the base, and PdNPs (0.3 mol%) at 40 °C (Table S2, Sl. no. 17, ESI†). The yield did not rise further even with increases in temperature, catalyst loading, and time (Table S2, entries 16 and 18, ESI†). Thus, aryl halide (1 equivalent), phenylboronic acid (1.15 equivalent), PdNPs (0.3 mol%), and K₂CO₃ (2 equivalent) at 40 °C for 1.5 hours are the ideal reaction conditions for the Suzuki–Miyaura reaction. The plausible mechanism for the Suzuki–Miyaura coupling reaction with PdNPs has been given in Fig. S5(a) (ESI†).

The substrate scope in the Suzuki–Miyaura coupling reaction with PdNPs with the optimized conditions was analyzed with various substituted groups of aryl halide as well as phenylboronic acid and has been given in Table 3. The substituted aryl halide and phenylboronic acid gave excellent yields in the optimized conditions. The products were purified by column chromatography and confirmed by H¹ NMR. As compared to aryl chloride, aryl bromide gave excellent yields in these particular reaction conditions.

Table 3 Substrates of Suzuki–Miyaura coupling reactions with different aryl halides and phenylboronic acids

Sl. no	Aryl halide	Product	Time (h)	Yield (%)
Reaction conditions: aryl halide (1 eq.), boronic acid (1.15 eq.), base (2 eq.), PdNPs (0.3 mol% respect to the aryl halide), solvent (EtOH : H2O (1 : 1) (5 mL)) in a nitrogen atmosphere, yield (%) – isolated yield after separation by column chromatography.
1			1.5	94
2			1.5	78
3			1.5	91
4			2	64
5			1.5	95
6			1.5	92
7			1.5	90
8			2	62
9			1.5	88
10			1.5	89
11			1.5	80
12			1.5	85

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3.2.2 Application of PdNPs in cyanation reaction. This catalyst was used for the aryl halide cyanation process after demonstrating good catalytic activity of PdNPs in the Suzuki–Miyaura coupling reactions. K₄[Fe(CN)₆]·3H₂O was selected as the cyanide source for this specific reaction due to its non-toxic nature and environmental friendliness. For this particular reaction, bromobenzene was selected as a model substrate, similar to the Suzuki–Miyaura reactions.

For the optimization of the reactions, various bases like K₂CO₃, Na₂CO₃, KOH, NaOH and various solvents like EtOH, H₂O, THF, acetonitrile, and DMF, were used with different temperatures with or without the presence of Pd catalyst. But with Na₂CO₃ (1.5 equivalents), DMF (6 mL) solvent at 110 °C with PdNPs (0.5 mol%) and K₄[Fe(CN)₆]·3H₂O (0.18 equivalent as source of cyanide) for 3.5 h, it turned out to be the best condition (Table S4 Sl. no. 16, ESI†). The plausible mechanism is given in the ESI† in Fig. S5(b).

The substrate scope in the aryl halide cyanation reaction with PdNPs with the optimized conditions was analyzed with various substituents on the aryl halide and the data are given in Table 3. The substituted aryl halide and K₄[Fe(CN)₆] 3H₂O give excellent yields in the optimized conditions. The products were purified by column chromatography and confirmed by H¹ NMR. As compared to aryl chloride, aryl bromide gave excellent yields in these particular reaction conditions (Table 4).

Table 4 Aryl cyanation reactions with different aryl halides and K₄[Fe(CN)₆]·3H₂O

Sl. no	Aryl halide	Product	Time (h)	Yield (%)
Reaction conditions: aryl halide (1 eq.), K4[Fe(CN)6]·3H2O (0.18 eq.), Na2CO3 (1.5 eq.), PdNPs (0.5 mol% respect to the aryl halide), solvent (DMF (6 mL)) in a nitrogen atmosphere, yield (%) – isolated yield after separation by column chromatography.
1			3.5	93
2			5	71
3			10	89
4			10	58
5			6	79
6			6	90
7			7	87
8			12	91
9			12	62

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4. Conclusion

The current work demonstrated the sustainable synthesis of nitrogen-doped carbon quantum dots (NCQD) from Ixora coccinea flowers and o-phenylenediamine using a solvothermal technique. The synthetic NCQDs have a surface coated with many functional groups and measure ∼3 nanometres in size. They produced fluorescence with a bright green colour. These NCQDs were discovered to have a quantum yield of 43.5%. The research reports that these NCQDs can detect Fe³⁺ ions and may find practical usage in real-time applications and in human serum. In addition, these NCQDs serve as a stabilizing and reducing agent when palladium nanoparticles (PdNPs) are being prepared. The functional groups in the NCQDs stabilize the nanoparticles and allow them to reduce from Pd(II) to Pd(0). Furthermore, these PdNPs catalyze the aryl cyanation and Suzuki–Miyaura coupling reactions. Due to their tiny size and large surface area, these nanoparticles, which are derived from NCQDs, provide excellent yields for both processes. In comparison to other reported catalysts, the yields are comparably good.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There is no conflict of declaration.

Acknowledgements

Namrata P. Hota expresses gratitude to Vellore Institute of Technology for its Research Associateship, which enables her to get financial support. The DST-FIST NMR facility at VIT University is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj02217c

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