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Article

One-Pot, Optimized Microwave-Assisted Synthesis of Difunctionalized and B–N Co-Doped Carbon Dots: Structural Characterization

by
Hector Daniel Ibarra-Prieto
1,2,
Alejandra Garcia-Garcia
1,2,*,
Faustino Aguilera-Granja
3,
Diana Carolina Navarro-Ibarra
4 and
Ignacio Rivero-Espejel
5,*
1
Centro de Investigación en Materiales Avanzados, S.C. (CIMAV), Subsede Monterrey, Av. Alianza Norte 202, Parque PIIT, Apodaca 66628, Nuevo León, Mexico
2
Grupo de Síntesis y Modificación de Nanoestructuras y Materiales Bidimensionales-CIMAV, Subsede Monterrey, Monterrey 66628, Nuevo León, Mexico
3
Instituto de Física “Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosí, Álvaro Obregón 64, San Luis Potosí 78000, San Luis Potosí, Mexico
4
Tecnológico Nacional de Mexico, Instituto Tecnológico del Valle de Etla, Abasolo S/N, Barrio del Agua Buena, Santiago Suchilquitongo 68230, Oaxaca, Mexico
5
Centro de Graduados e Investigación, Instituto Tecnológico de Tijuana, Tijuana 22000, Baja California, Mexico
*
Authors to whom correspondence should be addressed.
Nanomaterials 2023, 13(20), 2753; https://doi.org/10.3390/nano13202753
Submission received: 17 August 2023 / Revised: 7 October 2023 / Accepted: 10 October 2023 / Published: 12 October 2023
(This article belongs to the Section 2D and Carbon Nanomaterials)
Browse Figures
Versions Notes

Abstract

:
In this work, we employed a novel microwave-assisted synthesis method to produce nitrogen and boron co-doped carbon dots (B–N co-doped CDs). To achieve optimal synthesis, we conducted a comprehensive parameter modulation approach, combining various synthesis temperatures, times, and precursor concentrations, while keeping the power constant at 150 W and pH 5. Using maximum fluorescence emission as our response variable, the best conditions were identified as 120 °C, 3 min, and a precursor concentration of 1 mg/mL. Characterization using field emission scanning electron microscopy revealed these CDs to have a spherical morphology with an average size of 10.9 ± 3.38 nm. Further high-resolution transmission electron microscopy showed an interplanar distance of 0.23 nm, which is in line with prior findings of CDs that present a 0.21 nm distance corresponding to the (100) plane of graphite. Optical properties were ascertained through UV–vis absorption, identifying distinct π–π* and n–π* transitions. Fluorescence spectroscopy highlighted an emission peak at 375 nm when excited at 295 nm, achieving a quantum yield of 56.7%. Fourier-transform infrared spectroscopy and Raman spectroscopy analyses confirmed the boronic acid and amine groups’ presence, underscoring the graphitic nature of the core and the co-doping of boron and nitrogen. These empirical observations were compared with theoretical investigations through simulated Raman spectra, proposing a potential structure for the CDs. X-ray photoelectron spectroscopy further endorsed the co-doping of nitrogen and boron, along with the detection of the specified functional groups. All these characteristics could lend this nanomaterial to different types of applications such as fluorescent probes for a broad range of analytes and for fluorescent cell imaging.

Graphical Abstract

1. Introduction

Carbon dots (CDs) have recently emerged as a promising class of zero-dimensional nanomaterials. These materials are composed of sp2 and sp3 carbon atoms arranged in graphitic-like architectures or amorphous polymeric chains confined into spheric particles [1,2,3,4]. CDs have garnered significant attention due to their unique optical and electronic properties, biocompatibility, and potential for various applications in fields such as biomedicine [5], sensing [6], and energy conversion [7].
The optical properties of CDs, such as their high photoluminescence quantum yield, tunable fluorescence emission, and excitation-dependent behavior, have led to their use as fluorescent sensors for different analytes [8,9,10,11,12,13,14,15] and in vitro and in vivo fluorescent cellular imaging probes [9,16,17,18]. Furthermore, CDs have shown potential in controlled drug delivery [19,20], anticancer therapy [18,21,22,23], solar cells [24,25,26], and, recently, as endoplasmic reticulum in situ antioxidants [27]. Several methods have been developed for the synthesis of CDs, including arc discharge [28], electrophoresis [29], hydrothermal synthesis [2,30,31,32], electrochemical methods [33,34], ultrasonic-assisted synthesis [35], and microwave-assisted synthesis [36,37,38].
For example, Mansour et al. reported the microwave-assisted synthesis of CDs using a high-concentrated L-cysteine solution in a multimodal microwave reactor for 60 s at 800 W [37]. While their method was successful in producing CDs, a lack of control over the parameters of synthesis led to different size distributions and forms of the nanoparticles and the formation of large precipitates even after dialysis was performed.
Researchers have turned to innovative approaches such as monomodal microwave reactors to overcome the limitations of traditional synthesis methods. This method offers several advantages, such as rapid heating and cooling, improved reaction efficiency, and precise control over reaction conditions. Microwave reactors have been demonstrated to be highly effective in synthesizing high-quality CDs with precise control over their size, morphology, and physicochemical properties [39,40].
A doping method with heteroatoms is used to enhance the physicochemical properties of the CDs. This method involves the introduction of non-carbon atoms, such as nitrogen, sulfur, or phosphorus, into the carbon lattice of CDs [41]. This process can modify the electronic structure and bandgap of the CDs, resulting in enhanced absorption and emission properties, increased conductivity, and improved stability [14]. Specifically, the addition of nitrogen and boron as dopants brings extra benefits. Firstly, nitrogen as a dopant adds free electrons to the carbon network, enhancing the conductivity properties due to its n-type dopant nature [42]. Otherwise, boron, due to its lower valence than carbon, acts like a p-type dopant in the network, which can lead to reactive sites for catalytic chemical sensing [43].
The addition of both nitrogen and boron to CDs has been studied recently. Typically, hydrothermal methods have been used to develop B and N co-doped CDs with different types of precursors, such as 3-aminophenylboronic acid [44] and boric acid/tieguanyin tea residues [45], with treatments of 8 h at 160 °C and 6 h at 220 °C, respectively. Microwave-assisted techniques have been used to achieve the preparation of B and N co-doped CDs with softer conditions. These previous works were developed using citric acid, urea, or ethylenediamine as a nitrogen source and boric acid as a boron source. All of the studies that applied household multimodal microwave ovens with treatment times from 4 to 60 min, at 160 °C to 200 °C, and using 720 to 900 W of power, led to the development of B and N co-doped CDs with QY from 7 to 66.5% [46,47,48,49].
In this work, we present a one-pot, highly controlled, fast, and reproducible microwave-assisted synthesis of difunctionalized and B–N co-doped CDs utilizing a sole precursor as the carbon and functional group source and water as a solvent in a monomodal microwave reactor. In addition, we discuss the parameter modulation of the method, varying the time, temperature, power, and concentration of synthesis to obtain the optimal parameters to accomplish reproducibility. Finally, the structural and optical properties of CDs were studied.

2. Experimental Section

2.1. Microwave-Assisted Synthesis of the B–N Co-Doped CDs

The difunctionalized, B–N co-doped CDs were prepared by the adaptation and modification of the hydrothermal method described by Shen and co-workers [32]. A variable amount of 3-aminophenylboronic acid monohydrate (3APBA) was mixed in ultrapure water (18.2 MΩ·cm−1) to obtain concentrations of 1, 3, 5, and 10 mg/mL, and the pH was modulated to 5. The solution was sonicated until complete dispersion and then placed on the microwave reactor (CEM Discover 2.0).

2.2. Determination of the Quantum Yield

The quantum yields (QYs) of the B–N co-doped CDs were obtained using the slope method [50]. Four different solutions of 4-methylumberipherone QY = 22% were prepared in ethanol and their absorbances were less than 0.1 at 300 nm. The QY was calculated using the following equation:
ϕ n = ϕ s G r a d n G r a d s ( η n ) 2 ( η s ) 2
where ϕ is the quantum yield, Grad is the slope of the correlation between the absorbance and the integrated fluorescence emission, and η is the refractive index of the solvent. The subscript “n” refers to the sample and “s” to the standard.

2.3. Modulation of Synthesis Parameters

Different iterations of the experiment were conducted to determine the optimum synthesis parameters for the B–N co-doped CDs. Firstly, with previous standard parameters (160 °C, 7 min, 150 W) four amounts of concentration were used to determine maximum PL intensity. Afterward, Table 1 shows a correlation between different values of time and temperature. The power, concentration, and pH remained constant.
An experiment design was developed with the PL of the nanomaterial as the response variable, and the decision was made after the maximum PL intensity of all the samples to optimize the parameters for the reaction.

2.4. Purification Process of the B–N Co-Doped CDs

After cooling to room temperature, the dispersion was passed through a cotton-filled column to remove non-reacted precursor and then centrifuged at 10,000 rpm to remove large precipitates. Further, the dispersion was dialyzed in 500–1000 Da bag tubes for 2 cycles of 30 min each at 6000 rpm in a centrifuge. Finally, the B–N co-doped CDs were preserved at 4 °C.

2.5. Characterization

After the obtention of the optimal parameters, all the characterizations were conducted with the sample obtained using the following conditions: 3 min, 120 °C, 150 W, and 1 mg/mL. Ultraviolet–visible spectroscopy was performed in a Cary 50 (Varian, Palo Alto, CA, USA). The photoluminescence (PL) studies were conducted on a Cary Eclipse Spectrophotometer (Varian, Palo Alto, CA, USA). Fourier-transform infrared (FTIR) was measured by letting a droplet of the dispersion dry on a PE-983 FTIR Spectrophotometer (Perkin Elmer Inc. Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) spectra were collected from an ESCALAB 250 Xi XPS (Thermo Fischer Scientific inc. Waltham, MA, USA). High-resolution transmission electron microscopy (HRTEM) measurements were performed using a JEM-2200FS (JEOL Ltd. Akishima Tokyo, Japan) and transmission electron microscopy (TEM) was conducted using a Hitachi 7700 (Hitachi Ltd. Hitachi, Ibaraki, Japan); both procedures were conducted by depositing a drop on a copper grid and evaporating the solvent. Size distribution was obtained with different TEM images with a sample of over 130 particles. Raman measurements were carried out using a LabRAM HR Evolution Raman spectrometer with a 325 nm laser (HORIBA Ltd. Minami-ku, Tokyo, Japan).

2.6. Theoretical Calculations

Theoretical calculations were performed by employing density functional theory (DFT) analyses focused on the B–N co-doped CDs, leveraging the computational capabilities of the GAUSSIAN09 software suite [51]. The study was primarily directed towards extracting Raman spectra and characterizing electronic and structural properties.
Different structural arrays, constructed based on previous spectroscopical results with 10 nitrogen atoms, 7–8 boron atoms, 4 oxygen atoms (considered the boronic acid groups), 44 carbon atoms, and 26 hydrogen atoms, for a total of 91–92 atoms, were subjected to optimization processes allowing the derivation of equilibrium configurations associated with electronic and structural properties. Two of the most representative arrays were B–N-LALN2DZ and Pyrrole-LALN2DZ, which were tailored to fit the chemical composition of the structures with vacancies, B–N bonds, pyrrolic nitrogen, and functional groups in the surroundings of the particle. The insights gleaned from these optimized motifs were subsequently compared with experimental data of the nanomaterial. The rest of the structural motifs will be analyzed in depth in a further vibrational study of B–N co-doped CDs.
The different structural motifs are minimized at the DFT level of the Kohn–Sham equations. The LANL2DZ basis set was utilized to solve these equations, originally proposed by Hay and Wadt [52], in conjunction with the Perdew–Burke–Ernzerhof (PBE) formulation for the exchange–correlation functional [53]. This combination delivered a desirable balance between computational efficiency and precision, particularly during the execution of fully unrestricted spin-polarized structural optimizations. The selection of the PBE functional was substantiated by consulting the relevant references in the current literature, which demonstrate its effectiveness in analyzing interactions among graphene sheets, boron, and nitrogen atoms. Zhang et al. [54] further validated the method’s competence in accurately rendering these atoms’ electronic and magnetic properties.
Upon acquiring the Raman spectra, a detailed comparison was made with the experimental results. The resulting stick spectra were broadened using a Gaussian line shape function, implementing a full width at half the maximum height of 10 cm−1. This process enhanced the clarity of the spectra visualization.

3. Results and Discussion

3.1. Field Emission Scanning Electron Microscopy and High-Resolution Transmission Electron Microscopy

The TEM micrograph depicted in Figure 1a illustrates a homogeneous distribution of quasi-spherical particles with a mean diameter of 10.9 ± 3.38 nm (Figure 1b). This quasi-spherical form is attributed to the tendency of nanostructures to minimize their surface and interfacial energies, resulting in agglomerates with round-like shapes [55]. Nevertheless, graphitic nanostructures are not perfectly round-shaped, and this could be attributed to the functional groups and the heteroatomic co-doping within the sample, which induces modification in the particle’s crystalline structure and, overall, in its morphology as explained in the TEM section [56].
Moving forward to the HRTEM image portrayed in Figure 1c, it represents a single N–B co-doped CD. The diameter of the displayed particle measures 9.2 nm. Moreover, a meticulous examination reveals four distinct lattice orientations with an inner-lattice spacing of 0.24 nm.
Conventional studies [57] often report a 0.21 nm inter-lattice spacing corresponding to the graphite orientation (100). Nonetheless, the slight disparity in our case can be attributed to the incorporation of nitrogen and boron atoms within the lattice structure, thereby provoking electrostatic repulsion, which is consistent with previous works of doped CDs. Further inspection of the surroundings exposes amorphous carbon, which is synonymous with the functional groups bonded to the particle. All these characteristics are consistent with the evidence reported by Endo and coworkers [56], where they explain the co-existence of three different types of stacking in graphitic-like carbon nanostructures. Graphitic stacking, glide plane stacking, and rotational turbostratic stacking can be found in this kind of nanomaterial, leading to a single perfect graphitic crystalline structure. In the case of CDs, the presence of boron and nitrogen as co-dopants leads to rotational turbostratic stacking, functional groups such as boronic acids and amine promote glided plane stacking. This leads to a single graphitic crystalline structure (0.24 nm (100) plane).
A detailed size profile was performed, which is illustrated in Figure 1c inset. Here, measurements of the distance between the lattices were undertaken. The observed consistent inter-lattice spacing of 0.24 nm reaffirms the data from the HRTEM analysis. The profile’s depth indicates the particle’s spherical form, which serves as a differential factor between bidimensional graphene quantum dots and carbon dots. A spherical form, as demonstrated in our study, is suggestive of carbon dots. This aspect helps distinguish CDs from their graphene counterparts, which typically assume a planar or slightly curved structure. Figure 1d presents the proposed schematic representation of the particle’s growth process.

3.2. UV–Vis Spectroscopy and Fluorescence

The N–B co-doped CDs were subjected to spectroscopic analysis due to their remarkable optical properties. Initially, a UV–vis spectroscopy scan was conducted, revealing three bands at 205 nm, 232 nm, and 295 nm. These bands were attributed to π–π*, π–π*, and n–π* transitions, respectively (Figure 2a, blue, yellow, and green boxes, respectively) [58].
The interaction between the aromatic groups within the CD network explains the first band. At the same time, the second is attributed to extended conjugation (–C=C) within the carbon network of the CDs. The third band is due to the flow of electrons from the amine groups that functionalize the CD surfaces towards the carbon structure with extended conjugation within the network. The n–π* transition corresponded with the excitation band obtained through fluorescence spectroscopy. When excited at this wavelength (295 nm), the material emits a band at 375 nm (Figure 2a, black and red lines) with a QY of 57.6% [59].

3.3. Parameter Modulation

During the synthesis of boron and nitrogen co-doped carbon dots (N–B co-doped CDs), it is essential to optimize various parameters to obtain the desired optical properties. One such parameter is the amount of sole precursor used, which was varied in the experiments conducted to optimize CD synthesis. The fluorescence spectra of the CDs were obtained using different concentrations of 3APBA, namely, 1, 3, 5, and 10 mg/mL. The results showed that the fluorescence intensity decreased as the initial solution concentration increased (Figure 3b) and can be attributed to the possibility of obtaining larger particle sizes and aggregation, combined with an amount of unreacted precursor, resulting in a decrease in the optical property.
On the other hand, lower concentrations can provide better control over the size of the N–B co-doped CDs, promote particle dispersion in the solvent, and avoid waste of unreacted precursors, as evidenced by increased fluorescence intensity. Thus, the optimum precursor concentration to obtain the desired optical properties of CDs is 1 mg/mL.
In addition to the precursor concentration, temperature and reaction time are other parameters that need to be optimized in the synthesis of N–B co-doped CDs. A study was conducted to determine the effects of temperature and reaction time throughout the microwave-assisted synthesis of CDs. These experiments were carried out using a standard pH of 5 and a concentration of 1 mg/mL of 3APBA.
Fluorescence spectra were obtained at constant time and varying temperatures of 120 °C, 140 °C, 160 °C, and 180 °C, and at constant temperature and varying times of 3, 5, 7, and 10 min for each case (Figure 3a). The results showed that the material’s optimal reaction time and temperature are 9 min and 140 °C. However, since the conditions of 3 min and 120 °C yielded similar results, it was decided to use these parameters to synthesize all batches used in this project. The decision to use these parameters was also made since the lower temperature and shorter reaction time would be more energy-efficient and cost-effective (Figure 3b). Overall, the optimum conditions for synthesizing N–B co-doped CDs are 3 min at 120 °C, pH 5, power 150 W, and a 1 mg/mL precursor concentration. These conditions should be used as a starting point for further optimization and experimentation to obtain the desired optical properties of CDs for specific applications.
Table 2 shows a comparison of the preparation of B and N co-doped CDs. Xiao and co-workers developed a solvothermal methodology using the conditions of 480 min at 160 °C in an autoclave, with 1 mg/mL of 3APBA as a precursor leading to a QY of 7% at 430 nm [44]. Following this, a household microwave oven was utilized as the first approach to obtain this type of co-doped nanomaterial. Tran and co-workers utilized boric acid and passion fruit juice in low concentrations to obtain CDs in 20 min at a temperature of 170 °C with 800 W of power, leading to a QY of 50% at 440 nm [46]. High concentrations of citric acid and boric acid in the presence of urea were used by Hsieh and co-workers [47] and Bian and co-workers [48] for time periods of 15 min and 4 min, respectively. For the first case, 180 °C with 720 W of power was used, leading to a QY of 30.2% at 450 nm; meanwhile, in the second case, researchers only controlled the approximately 900 W of power (given by a household microwave oven) and no QY was reported emitting at 460 nm. Finally, a variation in this procedure was published by Tian et. al, using ethylenediamine as a nitrogen source instead of urea. After 60 min of radiation at a temperature of 200 °C and 800 W of power, a QY of 66.5% was achieved at 446 nm [49]. In the case of this work, a similar QY was obtained at 375 nm, with the conditions of 3 min, 130 °C, and 150 W, using a monomodal microwave reactor because it enables full parameter control.

3.4. Fourier-Transform Infrared Spectroscopy and Raman Spectroscopy

FTIR was used to investigate the functional groups in the internal structures and surfaces of the N–B co-doped CDs. The FTIR spectrum (Figure 4a) showed a N–H scissoring mode at 702 cm−1, indicating the presence of a primary amine [60]. This signal was supported by the peaks observed at 1337 cm−1, 1624 cm−1, 3503 cm−1, and 3524 cm−1, which corresponded to the stretching modes of C–N [60,61], bending modes of N–H [60], and two stretching modes of N–H for primary amines, respectively (see embedded graph in Figure 4a). Additionally, the signal at 2588 cm−1 was attributed to an overtone derived from the presence of stretching modes of primary amines [60], confirming the presence of this functional group in the nanostructure. A signal located at 2155 cm−1 corresponded to a stretching mode of C≡N. The peaks observed at 1030 cm−1, 1091 cm−1, 1201 cm−1, and 1341 cm−1 were associated with the vibrational deformation modes of B–O, stretching modes of C–B, bending modes of B–O–H, and stretching modes of B–O, respectively [32]. The peak observed at 3350 cm−1 corresponding to the stretching mode of O–H demonstrated the presence of the boronic acid group in the nanomaterial structure. The peaks observed at 1441 cm−1, 1491 cm−1, and 1580 cm−1 were characteristic of aromatic groups and corresponded to the bending modes of symmetric C=C, asymmetric C=C, and aromatic C=C, respectively. Therefore, the presence of an aromatic motif in the nanostructure can be confirmed. The findings from Raman and XPS spectroscopies, which are presented below, support this theory. In addition to corresponding to the stretching mode of C–N, the peak observed at 1337 cm−1 may also correspond to the stretching mode of B–N. This suggests that the material may be doped with a substitution in the graphene network in addition to having functional groups on the surface.
Raman spectroscopy was employed to investigate the structural composition and functionalization of B–N co-doped CDs functionalized with boronic acids and amines. The Raman spectra revealed significant multisignals, shedding light on the unique properties of the material. In the low-wavenumber region (below 1200 cm−1), various signals related to the co-doping of nitrogen and boron were observed. These signals predominantly correspond to breathing vibration modes. The D peak of nitrogen appeared at approximately 1193 cm−1, indicating the presence of nitrogen species within the N–B co-doped CDs [60]. Additionally, Raman shifts associated with pyridinic or pyrrolic nitrogen that modify the breathing vibration mode of the aromatic rings were observed around ~1000–1050 cm−1 [62]. The signals related to boron doping exhibited characteristics of sp2-hybridized boron at ~800–1000 cm−1 [62]. These multisignals collectively provide insights into the incorporation of nitrogen and boron species into the carbon matrix, with each peak corresponding to distinct vibrational modes within the breathing mode region [62]. Moving to the mid-wavenumber range (1200–1800 cm−1), the Raman spectra exhibited notable signals related to functional groups and carbon–carbon bonding. These signals primarily fall into the kekulé vibrational modes. The D band at 1369 cm−1 and the G band at 1618 cm−1 indicated the presence of disorder and sp2-hybridized carbon atoms within the N–B co-doped CDs. The intensity ratio of the D and G bands (ID/IG) was found to be approximately 0.439, indicating the presence of more sp2 than sp3 carbon atoms in the lattice. Furthermore, a signal associated with boronic acid stretching vibrations was observed at 1453 cm−1, while amine stretching vibrations appeared at 1512 cm−1. These grouped signals within the kekulé mode region suggest the successful functionalization of the N–B co-doped CDs with boronic acids and amines. In the high-wavenumber region (1800–4000 cm−1), a Raman signal at 1821 cm−1 indicated the presence of a C≡N bond within the N–B co-doped CDs. This signal falls into stretching vibration modes, which are characterized by stronger stretching of chemical bonds. The existence of the C≡N bond was validated by a stretching vibration peak at 1254 cm−1 in the FTIR spectra. Finally, multisignals corresponding to the region of the 2D band were found at 2632 cm−1. According to Maciel, the shape and the shift to the lowest frequencies of the signal is due to the combination of n-type and p-type doping, which is consistent with the presence of boron (electron-deficient) and nitrogen (electron-rich) [63].
The Raman spectra of the carbon dots were analyzed in depth using a deconvolution technique to identify and assign nine prominent peaks inside the 800–1800 cm−1 region: SL, S, DS, D, A1, A2, GG, D′, and C [62]. The SL peak at 1188.14 cm−1 corresponds to the fundamental breathing mode of smaller polycyclic aromatic hydrocarbons (PAHs) and the secondary breathing mode of naphthalene. This suggests a multilayer PAH structure with defects and amorphous carbon on the surface. The S peak at 1267.84 cm−1 indicates the frustrated breathing mode of larger PAHs containing graphitic nitrogen and the secondary breathing mode of a seven-or-more-membered ring connected to graphitic nitrogen. This confirms the presence of aromatic rings and graphitic nitrogen within the multilayer PAH structure. The Ds peak at 1358.39 cm−1 is associated with disorder-induced Raman scattering resulting from structural defects, while the D peak at 1448.36 cm−1 represents the breathing mode of sp2 carbon atoms in disordered carbon materials. These findings suggest the presence of defects, amorphous carbon, or the multilayer PAH structure within the carbon dots. The A1 and A2 peaks at 1534.63 cm−1 and 1584.72 cm−1, respectively, are overtone and combination bands of the D peak, providing information about the degree of disorder, the extent of conjugation, and the presence of surface functional groups. The GG peak at 1612.48 cm−1 is assigned to the G-band associated with graphene-like materials, indicating the presence of sp2 carbon atoms in the multilayer PAH structure. The D′ peak at 1634.64 cm−1 represents the breathing mode of sp3 carbon atoms in disordered carbon materials, potentially originating from heteroatomic dopants, amorphous carbon, or the multilayer PAH structure. The C peak at 1832.57 cm−1 indicates the presence of carbon–oxygen (C=O) bonds, which may result from the incorporation of carbonyl groups on the surface, consistent with the information obtained via XPS [64].

3.5. X-ray Photoelectron Spectroscopy

Figure 5 shows the high-resolution XPS spectra for boron, carbon, nitrogen, and oxygen, respectively. The binding energies of 191 eV, 191.6 eV, and 192.3 eV were assigned to B–C [64], Graphitic C [65,66], and B–O/B–N bonds [67,68], respectively, indicating the presence of boronic acid groups on the surface of the nanomaterial and evidence of substitutional doping within the graphitic network (Figure 5a). The binding energies of 284.6 eV, 284.8 eV, 285.7 eV, and 286.7 eV (Figure 5b) were assigned to C–B/Graphitic B [68], C–C/C=C [65,69], C–N/C–Graphitic N [70,71], and C–O bonds [70], respectively. These results suggest the presence of an ordered graphitic-like carbon structure. Furthermore, evidence of the formation of the primary amine and boronic acid (C–B) bonds, as well as substitutional doping with nitrogen and boron, was observed. The binding energies of nitrogen were consistent with those obtained for carbon and boron, indicating substitutional doping in the graphitic network. The binding energies of 399.1 eV for N–B [67] and 400.1 eV for pyrrolic N–C [65] further support this conclusion, as does the binding energy of 401.6 eV assigned to primary amine N–C [65,71] (Figure 5c). The binding energy of 402.2 eV assigned to N=O [71] suggests the formation of oxides on the surface of the nanomaterial. Finally, the binding energies for oxygen (Figure 5d) confirm the presence of boronic acid groups with the signal of 532.2 eV for O–B [67], as well as evidence of oxide formation on the surface of the nanomaterial with the signal of 532.8 eV for O=C/O=N [72]. The binding energy of 533.8 eV was assigned to the hydroxyl groups surrounding the CDs [72].

3.6. Theoretical Calculations and Raman Spectra Correlation

Figure 6a,b display the low-energy configurations of two proposed structural arrays of B–N co-doped CDs, B–N-LALN2DZ and Pyrrole-LALN2DZ, respectively, based on empirical data. B–N-LALN2DZ (Figure 6a) was formulated considering the presence of amino and boronic acids near the particle, with boron and nitrogen atoms substituting carbon atoms, along with a B–N bond. Pyrrole-LALN2DZ (Figure 6b) extends the previous array by incorporating a vacancy and a pyrrole group.
The theoretical Raman spectra of B–N-LALN2DZ (Figure 6c) exhibit seven representative signals corresponding to those detected in the experimental data. The frequencies at 1833 cm−1 (C assignment, carbonyl stretching mode) and 1612 cm−1 (GG assignment, stretching modes of PAHs) from the experimental data are not found in the theoretical spectrum due to the lack of carbonyl groups and symmetry loss from boron and nitrogen doping.
The theoretical Raman spectra of Pyrrole-LALN2DZ (Figure 6e) illustrate eight peaks that align with the experimental signals. However, the C assignment associated with the carbonyl stretching mode is absent in the spectrum due to the exclusion of C=O bonds in the theoretical structural array.
Table 3 compiles the data from theoretical and experimental Raman spectra, comparing them with a nine-peak model of doped PAHs proposed by Sabin et al. [62]. It includes the range of assignments, peak names, and Raman shifts for each signal in the experimental spectrum. Both theoretical models present shifts due to the influence of heteroatomic doping on the electronic nature of the array. For example, the boron atom’s less electronegative nature can increase electron density in adjacent bonds, strengthening them and causing a blue shift. In contrast, nitrogen’s higher electronegativity can redistribute electron density, weakening nearby bonds and resulting in a redshift.
These models, as first approximations, offer valuable insights into the structure of B–N co-doped CDs. However, their finite nature might fail to account for some vibrational modes observed in the experimental Raman spectra, like interlayer interactions. Moreover, edge effects in these models, where atoms on the particle’s periphery could behave differently compared with those in a larger and more realistic model, might impact the electronic structure and vibrational modes, affecting the calculated Raman spectra. Nonetheless, these models provide a reasonably good fit with the experimental data. Pyrrole-LALN2DZ appears to provide a more accurate representation of the system due to its structural array, impurities, vacancies, specific bonds, functional groups, and the alignment of peaks with experimental data.

4. Conclusions

In this work, we developed nitrogen and boron co-doped and functionalized carbon dots using a monomodal microwave-assisted synthesis approach. The uniform size distribution of these B–N co-doped CDs has a mean diameter of 10.90 ± 3.39 nm and was confirmed using TEM analysis. The analysis of HRTEM micrographs revealed a 0.24 nm interplanar distance, which is higher than the (100) crystalline distance of 0.21 nm of graphite. This discrepancy is attributed to the presence of heteroatomic doping. The distinct optical properties of B–N co-doped CDs were revealed using UV–vis absorption and fluorescence spectroscopy, where π–π* and n–π* transitions were identified, and an emission peak at 375 nm with excitation at 295 nm was observed with a QY of 56.7%. Precise control of synthesis parameters led to these optimal properties, with a reaction time of 3 min, a temperature of 120 °C, a microwave power of 150 W, a pH of 5, and a precursor concentration of 1 mg/mL, resulting in the highest fluorescence emission intensity. Advanced spectroscopic methods further verified the successful doping and functionalization of these quantum dots. The FTIR, Raman, and XPS spectra methods confirmed the presence of boronic acid and amine functional groups, an ordered graphitic-like carbon structure, and the successful incorporation of nitrogen and boron as dopants. Theoretical Raman spectra calculations were compared with the deconvoluted experimental data and it was found that both models present similarities with the experimental Raman spectra. The Pyrrole-LALN2DZ model seems to fit more with the complexity of the experimental results. Nevertheless, further investigations will be conducted to elucidate the in-depth vibrational behavior of the B–N co-doped CDs. This significant accomplishment provides a promising basis for the potential utilization of these N-B co-doped CDs in different applications, such as fluorescent probes for cellular lines. Moreover, N-B co-doped CDs have the capacity to be applied in fluorescent recognition methods for a broad range of analytes due to the capacity of this material to 1: change its surface area with pH changes and 2: be covalently functionalized due to the presence of amine and boronic acid groups surrounding the particles. This allows us to add other molecules to make specific recognitions via covalent bonds or weak interactions. Also, boronic acid groups are well known to interact covalently with 1,3-diols in water pollutants.

Author Contributions

H.D.I.-P.: Synthesis of methodology, characterization, results discussions, and writing the original draft. A.G.-G.: characterization of methods, results discussions, supervision, and funding acquisition. F.A.-G.: theoretical calculations and results discussions. D.C.N.-I.: theoretical calculations and results discussions. I.R.-E.: conceptualization, funding acquisition, results discussions, and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by CIMAV grant number 25003 (internal project).

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the CONAHCyT Ph.D. fellowship #744216 and J. Alejandro Arizpe Zapata, Lilia Magdalena Bautista Carrillo, and L. Gerardo Silva Vidaurri from CIMAV for technical support. Finally, we wish to acknowledge the computational resources and technical support offered by the ACARUS supercomputer center from the Universidad de Sonora, México.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Wang, Y.; Hu, A. Carbon Quantum Dots: Synthesis, Properties and Applications. J. Mater. Chem. C 2014, 2, 6921–6939. [Google Scholar] [CrossRef]
  2. Serhan, M.; Sprowls, M.; Jackemeyer, D.; Long, M.; Perez, I.D.; Maret, W.; Tao, N.; Forzani, E. Carbon Dots Obtained Using Hydrothermal Treatment of Formaldehyde. Cell Imaging in-Vitro. Nanoscale 2014, 6, 9071–9077. [Google Scholar]
  3. Miao, P.; Han, K.; Tang, Y.; Wang, B.; Lin, T.; Cheng, W. Recent Advances in Carbon Nanodots: Synthesis, Properties and Biomedical Applications. Nanoscale 2015, 7, 1586–1595. [Google Scholar] [CrossRef] [PubMed]
  4. Farshbaf, M.; Davaran, S.; Rahimi, F.; Annabi, N.; Salehi, R.; Akbarzadeh, A. Carbon Quantum Dots: Recent Progresses on Synthesis, Surface Modification and Applications. Artif. Cells Nanomed. Biotechnol. 2018, 46, 1331–1348. [Google Scholar] [CrossRef] [PubMed]
  5. Su, W.; Wu, H.; Xu, H.; Zhang, Y.; Li, Y.; Li, X.; Fan, L. Carbon Dots: A Booming Material for Biomedical Applications. Mater. Chem. Front. 2020, 4, 821–836. [Google Scholar] [CrossRef]
  6. Gao, X.; Du, C.; Zhuang, Z.; Chen, W. Carbon Quantum Dot-Based Nanoprobes for Metal Ion Detection. J. Mater. Chem. C 2016, 4, 6927–6945. [Google Scholar] [CrossRef]
  7. Li, X.; Rui, M.; Song, J.; Shen, Z.; Zeng, H. Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 4929–4947. [Google Scholar] [CrossRef]
  8. Hou, Y.; Lu, Q.; Deng, J.; Li, H.; Zhang, Y. One-Pot Electrochemical Synthesis of Functionalized Fluorescent Carbon Dots and Their Selective Sensing for Mercury Ion. Anal. Chim. Acta 2015, 866, 69–74. [Google Scholar] [CrossRef]
  9. Cheng, C.; Xing, M.; Wu, Q. A Universal Facile Synthesis of Nitrogen and Sulfur Co-Doped Carbon Dots from Cellulose-Based Biowaste for Fluorescent Detection of Fe3+ Ions and Intracellular Bioimaging. Mater. Sci. Eng. C 2019, 99, 611–619. [Google Scholar] [CrossRef]
  10. Zhao, L.; Wang, Y.; Zhao, X.; Deng, Y.; Xia, Y. Facile Synthesis of Nitrogen-Doped Carbon Quantum Dots with Chitosan for Fluorescent Detection of Fe3+. Polymers 2019, 11, 1731. [Google Scholar] [CrossRef]
  11. Fan, Q.; Li, J.; Zhu, Y.; Yang, Z.; Shen, T.; Guo, Y.; Wang, L.; Mei, T.; Wang, J.; Wang, X. Functional Carbon Quantum Dots for Highly Sensitive Graphene Transistors for Cu2+ Ion Detection. ACS Appl. Mater. Interfaces 2020, 12, 4797–4803. [Google Scholar] [CrossRef] [PubMed]
  12. Liang, Y.; Xu, L.; Tang, K.; Guan, Y.; Wang, T.; Wang, H.; Yu, W.W. Nitrogen-Doped Carbon Dots Used as an “on–off–on” Fluorescent Sensor for Fe3+ and Glutathione Detection. Dye. Pigment. 2020, 178, 108358. [Google Scholar] [CrossRef]
  13. Latief, U.; ul Islam, S.; Khan, Z.M.S.H.; Khan, M.S. A Facile Green Synthesis of Functionalized Carbon Quantum Dots as Fluorescent Probes for a Highly Selective and Sensitive Detection of Fe3+ Ions. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2021, 262, 120132. [Google Scholar] [CrossRef] [PubMed]
  14. Ye, S.; Zhang, M.; Guo, J.; Song, J.; Zeng, P.; Qu, J.; Chen, Y.; Li, H. Facile Synthesis of Green Fluorescent Carbon Dots and Their Application to Fe3+ Detection in Aqueous Solutions. Nanomaterials 2022, 12, 1487. [Google Scholar] [CrossRef]
  15. An, Q.; Lin, Q.; Huang, X.; Zhou, R.; Guo, X.; Xu, W.; Wang, S.; Xu, D.; Chang, H.T. Electrochemical Synthesis of Carbon Dots with a Stokes Shift of 309 Nm for Sensing of Fe3+ and Ascorbic Acid. Dye. Pigment. 2021, 185, 108878. [Google Scholar] [CrossRef]
  16. Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem. Int. Ed. 2013, 52, 3953–3957. [Google Scholar] [CrossRef]
  17. Lu, W.; Gong, X.; Yang, Z.; Zhang, Y.; Hu, Q.; Shuang, S.; Dong, C.; Choi, M.M.F. High-Quality Water-Soluble Luminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. RSC Adv. 2015, 5, 16972–16979. [Google Scholar] [CrossRef]
  18. Thara, R.C.; Korah, B.K.; John, B.K.; Mathew, B. One-Pot Synthesized Multifunctional Carbon Nitride Dots for Fluorescent Sensing, Bioimaging, and Selective Cytotoxic Effect on Cancer Cells. J. Mol. Liq. 2022, 368, 120809. [Google Scholar] [CrossRef]
  19. Zhao, C.; Song, X.; Liu, Y.; Fu, Y.; Ye, L.; Wang, N.; Wang, F.; Li, L.; Mohammadniaei, M.; Zhang, M.; et al. Synthesis of Graphene Quantum Dots and Their Applications in Drug Delivery. J. Nanobiotechnol. 2020, 18, 142. [Google Scholar] [CrossRef]
  20. D’souza, S.L.; Chettiar, S.S.; Koduru, J.R.; Kailasa, S.K. Synthesis of Fluorescent Carbon Dots Using Daucus Carota Subsp. Sativus Roots for Mitomycin Drug Delivery. Optik 2018, 158, 893–900. [Google Scholar] [CrossRef]
  21. Pirsaheb, M.; Mohammadi, S.; Salimi, A.; Payandeh, M. Functionalized Fluorescent Carbon Nanostructures for Targeted Imaging of Cancer Cells: A Review. Microchim. Acta 2019, 186, 231. [Google Scholar] [CrossRef] [PubMed]
  22. Huang, C.L.; Huang, C.C.; Der Mai, F.; Yen, C.L.; Tzing, S.H.; Hsieh, H.T.; Ling, Y.C.; Chang, J.Y. Application of Paramagnetic Graphene Quantum Dots as a Platform for Simultaneous Dual-Modality Bioimaging and Tumor-Targeted Drug Delivery. J. Mater. Chem. B 2015, 3, 651–664. [Google Scholar] [CrossRef] [PubMed]
  23. Ge, J.; Jia, Q.; Liu, W.; Guo, L.; Liu, Q.; Lan, M.; Zhang, H.; Meng, X.; Wang, P. Red-Emissive Carbon Dots for Fluorescent, Photoacoustic, and Thermal Theranostics in Living Mice. Adv. Mater. 2015, 27, 4169–4177. [Google Scholar] [CrossRef]
  24. Stepanidenko, E.A.; Ushakova, E.V.; Fedorov, A.V.; Rogach, A.L. Applications of Carbon Dots in Optoelectronics. Nanomaterials 2021, 11, 364. [Google Scholar] [CrossRef] [PubMed]
  25. Yang, J.; Tang, Q.; Meng, Q.; Zhang, Z.; Li, J.; He, B.; Yang, P. Photoelectric Conversion beyond Sunny Days: All-Weather Carbon Quantum Dot Solar Cells. J. Mater. Chem. A 2017, 5, 2143–2150. [Google Scholar] [CrossRef]
  26. Molaei, M.J. The Optical Properties and Solar Energy Conversion Applications of Carbon Quantum Dots: A Review. Sol. Energy 2020, 196, 549–566. [Google Scholar] [CrossRef]
  27. Yang, L.; Huang, H.; Wang, T.; Zhou, D.; Chen, Q.; Li, D.; Chen, S.; Lin, P. Endoplasmic Reticulum-Targetable Selenium-Doped Carbon Nanodots with Redox-Responsive Fluorescence for in Situ Free-Radical Scavenging in Cells and Mice. Arab. J. Chem. 2023, 16, 105036. [Google Scholar] [CrossRef]
  28. Dey, S.; Govindaraj, A.; Biswas, K.; Rao, C.N.R. Luminescence Properties of Boron and Nitrogen Doped Graphene Quantum Dots Prepared from Arc-Discharge-Generated Doped Graphene Samples. Chem. Phys. Lett. 2014, 596, 203–208. [Google Scholar] [CrossRef]
  29. Xu, X.; Ray, R.; Gu, Y.; Ploehn, H.J.; Gearheart, L.; Raker, K.; Scrivens, W.A. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736–12737. [Google Scholar] [CrossRef]
  30. Li, L.; Wang, X.; Fu, Z.; Cui, F. One-Step Hydrothermal Synthesis of Nitrogen- and Sulfur-Co-Doped Carbon Dots from Ginkgo Leaves and Application in Biology. Mater. Lett. 2017, 196, 300–303. [Google Scholar] [CrossRef]
  31. Shen, T.Y.; Jia, P.Y.; Chen, D.S.; Wang, L.N. Hydrothermal Synthesis of N-Doped Carbon Quantum Dots and Their Application in Ion-Detection and Cell-Imaging. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2021, 248, 119282. [Google Scholar] [CrossRef] [PubMed]
  32. Shen, P.; Xia, Y. Synthesis-Modification Integration: One-Step Fabrication of Boronic Acid Functionalized Carbon Dots for Fluorescent Blood Sugar Sensing. Anal. Chem. 2014, 86, 5323–5329. [Google Scholar] [CrossRef]
  33. Ming, H.; Ma, Z.; Liu, Y.; Pan, K.; Yu, H.; Wang, F.; Kang, Z. Large Scale Electrochemical Synthesis of High Quality Carbon Nanodots and Their Photocatalytic Property. Dalt. Trans. 2012, 41, 9526–9531. [Google Scholar] [CrossRef] [PubMed]
  34. Bao, L.; Zhang, Z.L.; Tian, Z.Q.; Zhang, L.; Liu, C.; Lin, Y.; Qi, B.; Pang, D.W. Electrochemical Tuning of Luminescent Carbon Nanodots: From Preparation to Luminescence Mechanism. Adv. Mater. 2011, 23, 5801–5806. [Google Scholar] [CrossRef] [PubMed]
  35. Huang, H.; Li, S.; Chen, B.; Wang, Y.; Shen, Z.; Qiu, M.; Pan, H.; Wang, W.; Wang, Y.; Li, X. Endoplasmic Reticulum-Targeted Polymer Dots Encapsulated with Ultrasonic Synthesized near-Infrared Carbon Nanodots and Their Application for in Vivo Monitoring of Cu2+. J. Colloid Interface Sci. 2022, 627, 705–715. [Google Scholar] [CrossRef]
  36. Wang, Q.; Zheng, H.; Long, Y.; Zhang, L.; Gao, M.; Bai, W. Microwave-Hydrothermal Synthesis of Fluorescent Carbon Dots from Graphite Oxide. Carbon 2011, 49, 3134–3140. [Google Scholar] [CrossRef]
  37. Ahmed Abdel Hamid, M.; Elagamy, S.H.; Gamal, A.; Mansour, F.R. Microwave Prepared Nitrogen and Sulfur Co-Doped Carbon Quantum Dots for Rapid Determination of Ascorbic Acid through a Turn off–on Strategy. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2023, 293, 122440. [Google Scholar] [CrossRef]
  38. De Medeiros, T.V.; Manioudakis, J.; Noun, F.; Macairan, J.R.; Victoria, F.; Naccache, R. Microwave-Assisted Synthesis of Carbon Dots and Their Applications. J. Mater. Chem. C 2019, 7, 7175–7195. [Google Scholar] [CrossRef]
  39. Lin, Z.; Xue, W.; Chen, H.; Lin, J. Peroxynitrous-Acid-Induced Chemiluminescence of Fluorescent. Anal. Chem. 2011, 83, 8245–8251. [Google Scholar] [CrossRef]
  40. Philippidis, A.; Stefanakis, D.; Anglos, D.; Ghanotakis, D. Microwave Heating of Arginine Yields Highly Fluorescent Nanoparticles. J. Nanoparticle Res. 2013, 15, 1414. [Google Scholar] [CrossRef]
  41. Garcia, A.G.; Baltazar, S.E.; Castro, A.H.R.; Robles, J.F.P.; Rubio, A. Influence of S and P Doping in a Graphene Sheet. J. Comput. Theor. Nanosci. 2008, 5, 2221–2229. [Google Scholar] [CrossRef]
  42. Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752–1758. [Google Scholar] [CrossRef] [PubMed]
  43. Shokri, R.; Amjadi, M. Boron and Nitrogen Co-Doped Carbon Dots as a Chemiluminescence Probe for Sensitive Assay of Rifampicin. J. Photochem. Photobiol. A Chem. 2022, 425, 113694. [Google Scholar] [CrossRef]
  44. Xiao, N.; Liu, S.G.; Mo, S.; Li, N.; Ju, Y.J.; Ling, Y.; Li, N.B.; Luo, H.Q. Highly Selective Detection of P-Nitrophenol Using Fluorescence Assay Based on Boron, Nitrogen Co-Doped Carbon Dots. Talanta 2018, 184, 184–192. [Google Scholar] [CrossRef]
  45. Xu, J.; Guo, Y.; Qin, L.; Yue, X.; Zhang, Q.; Wang, L. Green One-Step Synthesis of Boron and Nitrogen Co-Doped Carbon Dots Based on Inner Filter Effect as Fluorescent Nanosensors for Determination of Fe3+. Ceram. Int. 2023, 49, 7546–7555. [Google Scholar] [CrossRef]
  46. Tran, H.L.; Darmanto, W.; Doong, R.A. Ultrasensitive Detection of Tetracycline Using Boron and Nitrogen Co-Doped Graphene Quantum Dots from Natural Carbon Source as the Paper-Based Nanosensing Probe in Difference Matrices. Nanomaterials 2020, 10, 1883. [Google Scholar] [CrossRef]
  47. Te Hsieh, C.; Sung, P.Y.; Gandomi, Y.A.; Khoo, K.S.; Chang, J.K. Microwave Synthesis of Boron- and Nitrogen-Codoped Graphene Quantum Dots and Their Detection to Pesticides and Metal Ions. Chemosphere 2023, 318, 137926. [Google Scholar] [CrossRef]
  48. Bian, W.; Wang, X.; Wang, Y.; Yang, H.; Huang, J.; Cai, Z.; Choi, M.M.F. Boron and Nitrogen Co-Doped Carbon Dots as a Sensitive Fluorescent Probe for the Detection of Curcumin. Luminescence 2018, 33, 174–180. [Google Scholar] [CrossRef]
  49. Tian, B.; Fu, T.; Wan, Y.; Ma, Y.; Wang, Y.; Feng, Z.; Jiang, Z. B- and N-Doped Carbon Dots by One-Step Microwave Hydrothermal Synthesis: Tracking Yeast Status and Imaging Mechanism. J. Nanobiotechnol. 2021, 19, 456. [Google Scholar] [CrossRef]
  50. Levitus, M. Tutorial: Measurement of Fluorescence Spectra and Determination of Relative Fluorescence Quantum Yields of Transparent Samples. Methods Appl. Fluoresc. 2020, 8, 033001. [Google Scholar] [CrossRef]
  51. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Petersson, G.A.; Nakatsuji, H.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, USA, 2009; pp. 1–23. [Google Scholar]
  52. Hay, P.J.; Wadt, W.R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283. [Google Scholar] [CrossRef]
  53. Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. [Google Scholar] [CrossRef] [PubMed]
  54. Zhang, C.; Cao, Y.; Dai, X.; Ding, X.Y.; Chen, L.; Li, B.S.; Wang, D.Q. Ab-Initio Study of the Electronic and Magnetic Properties of Boron-and Nitrogen-Doped Penta-Graphene. Nanomaterials 2020, 10, 816. [Google Scholar] [CrossRef] [PubMed]
  55. Liu, Y.; Tan, J.; Thomas, A.; Ou-Yang, D.; Muzykantov, V.R. The Shape of Things to Come: Importance of Design in Nanotechnology for Drug Delivery. Ther. Deliv. 2012, 3, 181–194. [Google Scholar] [CrossRef] [PubMed]
  56. Endo, M.; Kobori, K.; Takeuchi, K.; Dresselhaus, M.S. Evidence for Glide and Rotation Defects Observed in Well-Ordered Graphite Fibers. J. Mater. Res. 1995, 10, 1461–1468. [Google Scholar] [CrossRef]
  57. Li, M.; Zhang, S.X.A. Carbon Dots with Continuously Tunable Full-Color Emission and Their Application in Ratiometric PH Sensing. Chem. Mater. 2014, 26, 6084. [Google Scholar] [CrossRef]
  58. Shejale, K.P.; Jaiswal, A.; Kumar, A.; Saxena, S.; Shukla, S. Nitrogen Doped Carbon Quantum Dots as Co-Active Materials for Highly Efficient Dye Sensitized Solar Cells. Carbon 2021, 183, 169–175. [Google Scholar] [CrossRef]
  59. Habiba, K.; Makarov, V.I.; Avalos, J.; Guinel, M.J.F.; Weiner, B.R.; Morell, G. Luminescent Graphene Quantum Dots Fabricated by Pulsed Laser Synthesis. Carbon 2013, 64, 341–350. [Google Scholar] [CrossRef]
  60. IR: Amines. Available online: https://www.orgchemboulder.com/Spectroscopy/irtutor/aminesir.shtml (accessed on 9 January 2022).
  61. Gerry, M.C.L.; Merer, A.J.; Westwood, N.P.C. The Infrared Spectrum of Gaseous Aminoborane, H2N = BH2: Location of the Fundamentals and Rotational Structure in the 401; Band (BN Stretching Vibration at 1337 cm−1). J. Mol. Spectrosc. 1985, 110, 153–163. [Google Scholar] [CrossRef]
  62. Ayiania, M.; Weiss-Hortala, E.; Smith, M.; McEwen, J.S.; Garcia-Perez, M. Microstructural Analysis of Nitrogen-Doped Char by Raman Spectroscopy: Raman Shift Analysis from First Principles. Carbon 2020, 167, 559–574. [Google Scholar] [CrossRef]
  63. MacIel, I.O.; Anderson, N.; Pimenta, M.A.; Hartschuh, A.; Qian, H.; Terrones, M.; Terrones, H.; Campos-Delgado, J.; Rao, A.M.; Novotny, L.; et al. Electron and Phonon Renormalization near Charged Defects in Carbon Nanotubes. Nat. Mater. 2008, 7, 878–883. [Google Scholar] [CrossRef] [PubMed]
  64. Qu, D.; Zheng, M.; Zhang, L.; Zhao, H.; Xie, Z.; Jing, X.; Haddad, R.E.; Fan, H.; Sun, Z. Formation Mechanism and Optimization of Highly Luminescent N-Doped Graphene Quantum Dots. Sci. Rep. 2014, 4, 5294. [Google Scholar] [CrossRef] [PubMed]
  65. Thaweesak, S.; Wang, S.; Lyu, M.; Xiao, M.; Peerakiatkhajohn, P.; Wang, L. Boron-Doped Graphitic Carbon Nitride Nanosheets for Enhanced Visible Light Photocatalytic Water Splitting. Dalt. Trans. 2017, 46, 10714–10720. [Google Scholar] [CrossRef] [PubMed]
  66. Panchakarla, L.S.; Govindaraj, A.; Rao, C.N.R. Nitrogen- and Boron-Doped Double- Walled Carbon Nanotubes. ACS Nano 2007, 1, 494–500. [Google Scholar] [CrossRef] [PubMed]
  67. Bhimanapati, G.R.; Kozuch, D.; Robinson, J.A. Large-Scale Synthesis and Functionalization of Hexagonal Boron Nitride Nanosheets. Nanoscale 2014, 6, 11671–11675. [Google Scholar] [CrossRef]
  68. Kawaguchi, M.; Kawashima, T.; Nakajima, T. Syntheses and Structures of New Graphite-like Materials of Composition BCN(H) and BC3N(H). Chem. Mater. 1996, 8, 1197–1201. [Google Scholar] [CrossRef]
  69. Hu, H.; Sun, X.; Chen, W.; Wang, J.; Li, X.; Huang, Y.; Wei, C.; Liang, G. Electrochemical Properties of Supercapacitors Using Boron Nitrogen Double-Doped Carbon Nanotubes as Conductive Additive. Nano 2019, 14, 1950080. [Google Scholar] [CrossRef]
  70. Das, D.; Kim, D.M.; Park, D.S.; Shim, Y.B. A Glucose Sensor Based on an Aminophenyl Boronic Acid Bonded Conducting Polymer. Electroanalysis 2011, 23, 2036–2041. [Google Scholar] [CrossRef]
  71. María, L.; Betancur, A.F.; Zarate, D.G.; Torres, D.; Hoyos, L.; García, A.; Piit, P.; De Síntesis, L.; Nanoestructuras, D.; Bidimensionales, M.; et al. Diamond & Related Materials Simultaneous N Doping and Reduction of GO: Compositional, Structural Characterization and Its Effects in Negative Electrostatic Charges Repulsion. Diam. Relat. Mater. 2019, 97, 107447. [Google Scholar]
  72. Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R.D.; Stankovich, S.; Jung, I.; Field, D.A.; Ventrice, C.A.; et al. Chemical Analysis of Graphene Oxide Films after Heat and Chemical Treatments by X-Ray Photoelectron and Micro-Raman Spectroscopy. Carbon 2009, 47, 145–152. [Google Scholar] [CrossRef]
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Figure 1. (a) TEM micrography of the B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL; (b) size distribution of the B–N co-doped CDs; (c) HRTEM image showing the 0.24 nm planar distance in three different orientations of B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL; (c) inset: particle profile plot showing the distance between planes and the spherical-like nature of the particle; and (d) proposed scheme of the arrangement of the layers within the B–N co-doped CDs.
Figure 1. (a) TEM micrography of the B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL; (b) size distribution of the B–N co-doped CDs; (c) HRTEM image showing the 0.24 nm planar distance in three different orientations of B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL; (c) inset: particle profile plot showing the distance between planes and the spherical-like nature of the particle; and (d) proposed scheme of the arrangement of the layers within the B–N co-doped CDs.
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Figure 2. (a) UV–vis/fluorescence spectra of the B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL. The absorbance results show the three characteristic absorbance bands (green light), the excitation band corresponding to the n–π* band of absorbance (black line), and the emission band. (b) Fluorescence emission at different synthesis concentrations of 3APBA.
Figure 2. (a) UV–vis/fluorescence spectra of the B–N co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL. The absorbance results show the three characteristic absorbance bands (green light), the excitation band corresponding to the n–π* band of absorbance (black line), and the emission band. (b) Fluorescence emission at different synthesis concentrations of 3APBA.
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Figure 3. (a) Fluorescence emission at different times of 3 min, 5 min, 7 min, and 9 min and temperatures of 120 °C, 140 °C, 160 °C, and 180 °C, and (b) heat map of the maximum fluorescence emission at different times and temperatures.
Figure 3. (a) Fluorescence emission at different times of 3 min, 5 min, 7 min, and 9 min and temperatures of 120 °C, 140 °C, 160 °C, and 180 °C, and (b) heat map of the maximum fluorescence emission at different times and temperatures.
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Figure 4. (a) FTIR spectra of the N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL. Inset (a) shows FTIR spectra from 3450 cm−1 to 3550 cm−1 where the two vibrational modes of the primary amine are found. (b) The Raman spectra obtained with a 325 nm laser of N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL, and (c) Raman spectra from 800 cm−1 to 2000 cm−1 with a deconvolution of nine signals of N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL.
Figure 4. (a) FTIR spectra of the N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL. Inset (a) shows FTIR spectra from 3450 cm−1 to 3550 cm−1 where the two vibrational modes of the primary amine are found. (b) The Raman spectra obtained with a 325 nm laser of N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL, and (c) Raman spectra from 800 cm−1 to 2000 cm−1 with a deconvolution of nine signals of N–B co-doped CDs synthetized using the following conditions: 120 °C, 3 min, 150 W, and 1 mg/mL.
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Figure 5. High-resolution XPS spectra of (a) B1s, (b) C1s, (c) N1s, and (d) O1s of N–B co-doped CDs synthetized with 120 °C, 3 min, 150 W, and 1 mg/mL.
Figure 5. High-resolution XPS spectra of (a) B1s, (b) C1s, (c) N1s, and (d) O1s of N–B co-doped CDs synthetized with 120 °C, 3 min, 150 W, and 1 mg/mL.
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Figure 6. Theoretical structural arrays (a) B–N-LALN2DZ and (b) Pyrrole-LALN2DZ. Calculated Raman spectra of the theoretical structural arrays (c) B–N-LALN2DZ and (e) Pyrrole-LALN2DZ, and (d) Raman Spectra of the N–B co-doped CDs with nine deconvoluted signals.
Figure 6. Theoretical structural arrays (a) B–N-LALN2DZ and (b) Pyrrole-LALN2DZ. Calculated Raman spectra of the theoretical structural arrays (c) B–N-LALN2DZ and (e) Pyrrole-LALN2DZ, and (d) Raman Spectra of the N–B co-doped CDs with nine deconvoluted signals.
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Table 1. Parameters of the microwave-assisted synthesis of the B–N co-doped CDs.
Table 1. Parameters of the microwave-assisted synthesis of the B–N co-doped CDs.
TimeT (°C)TimeT (°C)TimeT (°C)TimeT (°C)pHC (mg/mL)Power
312031403160318051150 W
512051405160518053150 W
712071407160718055150 W
9120914091609180510150 W
Table 2. Comparison of N and B co-doped CDs using hydrothermal and microwave (MW) techniques.
Table 2. Comparison of N and B co-doped CDs using hydrothermal and microwave (MW) techniques.
PrecursorsConcentrationType of ReactorTime
(min)		Temperature
(°C)		Power
(W)		Ex/Em (nm)QY%References
3-aminophenylboronic acid1 mg/mLSolvothermal480160N/A350/4307[44]
boric acid/Passion fruit juice150 mg/10 mLMW-Multimodal20170800360/44050[46]
Citric acid/urea/boric acid10 g, 10 g, 5 gMW-Multimodal15180720360/45030.2[47]
Citric acid/urea/boric acid1.093 g, 1 g, 1gMW-Multimodal4N/A~900360/460N/A[48]
Citric acid/ethylenediamine/boric acidN/AMW-Multimodal60200800350/44666.5[49]
3-aminophenylboronic acid1 mg/mLMW-Monomodal3130150295/37556.7This Work
Table 3. Peak assignment of the B–N co-doped CDs and the theoretical structural arrays, B–N-LALN2DZ and Pyrrole-LALN2DZ, and the shifts of each of the signals.
Table 3. Peak assignment of the B–N co-doped CDs and the theoretical structural arrays, B–N-LALN2DZ and Pyrrole-LALN2DZ, and the shifts of each of the signals.
Reference
Assignment
(cm−1) [62]		Peak NameExperimental
Assignment
(cm−1)		Theoretical
Assignment
(cm−1)
(B–N-LALN2DZ)		Theoretical
Assignment
(cm−1)
(Pyrrole-LALN2DZ)		Shift
B–N-LALN2DZ (cm−1)		Shift
Pyrrole-LALN2DZ
(cm−1)
900–1075SL118811891219+1+31
1150–1200S126812911285+23+17
1250–1300DS135813511345−7−13
1340–1380D144314561432+13−11
1400–1460A1153515431525−8−10
1480–1550A2158515911573+6−12
1570–1600GG1612-1594-−18
1605–1650D’163516391642+4+7
1750–1800C1833----
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Ibarra-Prieto, H.D.; Garcia-Garcia, A.; Aguilera-Granja, F.; Navarro-Ibarra, D.C.; Rivero-Espejel, I. One-Pot, Optimized Microwave-Assisted Synthesis of Difunctionalized and B–N Co-Doped Carbon Dots: Structural Characterization. Nanomaterials 2023, 13, 2753. https://doi.org/10.3390/nano13202753

AMA Style

Ibarra-Prieto HD, Garcia-Garcia A, Aguilera-Granja F, Navarro-Ibarra DC, Rivero-Espejel I. One-Pot, Optimized Microwave-Assisted Synthesis of Difunctionalized and B–N Co-Doped Carbon Dots: Structural Characterization. Nanomaterials. 2023; 13(20):2753. https://doi.org/10.3390/nano13202753

Chicago/Turabian Style

Ibarra-Prieto, Hector Daniel, Alejandra Garcia-Garcia, Faustino Aguilera-Granja, Diana Carolina Navarro-Ibarra, and Ignacio Rivero-Espejel. 2023. "One-Pot, Optimized Microwave-Assisted Synthesis of Difunctionalized and B–N Co-Doped Carbon Dots: Structural Characterization" Nanomaterials 13, no. 20: 2753. https://doi.org/10.3390/nano13202753

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