INTRODUCTION

Recently, various technologies for the synthesis of carbon nanoparticles (CNPs) of various types have been created by the efforts of many scientists, and both their process and fundamental characteristic features have been sufficiently studied [1–14]. The huge research interest in the CNP materials is associated with their unique luminescent, optical, physical, chemical, electrical, biomedical, and other important properties. The CNP materials synthesized by many researchers reliably demonstrated in practice that they can be potentially successfully used for the creation of new types of sensing devices [5, 15, 16], quantum generators [17], solar cells [3, 18], optical probes for the visualization of biological objects [19–21], electrochemical supercondensers and batteries [22–25], and catalysts [26, 27], as well as light-emitting, optoelectronic, and other instruments. The high biocompatibility, environmental friendliness, and nontoxicity as well as the excellent luminescent properties and high quantum yield of emission of the CNP materials make them indispensable for wide application in modern medicine and bioengineering [2–4, 19, 20, 28, 29].

In view of the specific structure and small linear dimensions of the particles, the key properties and parameters of the CNP materials except geometric dimensions and shape of the particles are fundamentally associated with the presence of various types of functional groups on the surface of the particles and doping elements in the bulk [9, 29]. On one hand, these characteristic features of carbon nanoparticles make it possible to flexibly and in a wide range control the key parameters of the particles during their synthesis. On the other hand, each specific application of them requires additional technologies of separation and modification of the functional groups after the synthesis of the particles to obtain carbon nanoparticles with the required parameters.

The application of the CNP materials in practice requires deep and comprehensive studies of the dependences of the key parameters of the particles on the individual properties of different types of liquid and solid media. Since the vast majority of applications of carbon nanoparticles provides for the use of the particles in various liquid and liquid-containing media, one of the key tasks is the investigation of the effect of pH of solutions on the absorption and photoluminescence spectra and value of the quantum yield of emission of carbon nanoparticles. The degree of the effect of pH of the medium on the specified parameters is closely related to the types and properties of the molecular structures and concentrations of the functional groups on the surface of the particles as well as the shape, dimensions, and elemental composition of the carbon cores of the nanoparticles [9, 29–38]. According to the published data, the pH of the medium most vividly and differently affects the absorption and photoluminescence spectra and value of the quantum yield of emission of those types of particles to the luminescence of which functional groups containing O, N, and S heteroatoms and H atoms make a great contribution [9, 30–32].

In practice, varying the pH of the medium of carbon nanoparticles often leads to a bathochromic or hypsochromic shift as well as an increase or decrease in the intensities of the absorption and photoluminescence bands, which is, in turn, accompanied by a change in the quantum yield of emission. The directions and values of the shifts of the absorption and photoluminescence bands are associated with both the structural characteristic features of the particles and parameters of their energy levels and the properties of the solvents. Here, the photoluminescence spectra of carbon nanoparticles often demonstrate higher sensitivity to pH of the medium than the absorption spectra. For example, according to the results of [9], an increase in pH of an aqueous solution from 2 to 10 induces a weak change in the intensity of the absorption band of the particles doped with N and S associated with the n–π* transition of the –CO functional groups, while the maximum of the band shifts by 10 nm (from 307 to 297 nm). As opposed to absorption, the intensity of the photoluminescence band increases more than fourfold, and its maximum shifts by 23 nm (from 482 to 505 nm). Here, the shift of the absorption band is hypsochromic, and that of the photoluminescence band is bathochromic.

A strong change in the intensities and positions of the maxima of the absorption and photoluminescence bands is observed in a range of pH of 3–12 of an aqueous solution of the particles containing –COOH and –OH functional groups with a high density [31]. With a growth in pH of the solution in a range of pH of 3–12, the absorption band intensifies 2.2-fold and shifts from 375 to 410 nm, while the photoluminescence band weakens more than 18.5-fold and shifts from 448 to 510 nm. The growth in the absorption and decrease in the intensity of photoluminescence of this type of particles upon the transition from an acid medium to the region of higher pH is accompanied by a strong decrease in the quantum yield of emission. In these types of particles, the main mechanism of the solvatochromic effect is the process of protonation and deprotonation of the –COOH and –OH functional groups of the particles in an acidic and alkaline medium, respectively. Similar phenomena are observed in the aqueous solutions of carbon nanoparticles containing –NH2, –NHR, or –COOH groups [33]. Here, the processes of protonation in an acidic medium and deprotonation in an alkaline medium of the specified groups of particles differently affect the parameters of the photoluminescence band. The protonation and deprotonation of the particles with –NH2 groups do not generate noticeable shifts of the photoluminescence band. The photoluminescence band of the carbon nanoparticles with –NHR groups gives a hypsochromic shift upon protonation, while the maximum of the band slightly shifts upon deprotonation. The photoluminescence band of the particles with –COOH groups has a bathochromic shift upon deprotonation and remains unchanged upon protonation.

Both strong [30, 34–36] and extremely weak [29, 37–39] dependences of the quantum yield of emission on pH of the solutions are observed in a wide range of pH of the aqueous solutions of different types of carbon nanoparticles. A typical example of the dependence of the quantum yield of emission on pH of an aqueous solution is the dependence for the particles synthesized from folic acid by the hydrothermal method. The value of the quantum yield of emission of an aqueous solution of the specified particles is about 15% at a value of pH of 1, and the quantum yield grows with the growth in pH and acquires the maximum value (94.5%) at pH of 5. With further growth in pH, the quantum yield of emission monotonically decreases and is about 8% at a value of pH of 13 [30]. According to the results of [36], the intensity of photoluminescence of carbon nanoparticles with jointly doped N and S atoms has a small value in a range of pH of 2–4, sharply and strongly grows in a range of pH of 4–6, and then slightly changes up to pH of 13. As opposed to [36], it has been demonstrated in [38, 39] that the dependence of the quantum yield of emission of the particles on pH of the solution weakens with the increase in the concentration of N or N and S atoms in the carbon core of the nanoparticles.

A potent effect of the processes of modification and reduction of the surface groups of carbon nanoparticles on the value and dependence of the quantum yield of emission on pH of the solution of particles has been shown in [37]. The modification and reduction of carbon nanoparticles lead to lower values of the quantum yield of emission in comparison with the initial particles, but the values of the quantum yields of emission of the modified and reduced particles in a wide range of pH of 1–14 remain almost unchanged. This vividly demonstrates the connection of the dependence of the quantum yield of emission on pH of the solutions with the surface functional groups of the nanoparticles. This assumption is confirmed by the results of [29], where, first, particles with the value of the quantum yield of emission in an aqueous solution of about 100% have been synthesized and, second, the luminescence of the carbon nanoparticles is determined only by the transitions of the electron levels of the carbon cores of the particles. Here, the value of the quantum yield of emission of the particles in the range of pH of 1–12 slightly depends on pH of the aqueous solution.

The aim of this work is the investigation in a wide range of pH of the characteristic features of the effect of pH of the medium on the spectral parameters of optical absorption and photoluminescence as well as on the value of the quantum yield of emission of carbon nanoparticles of different types synthesized from lentil flour both for the optimization and improvement of the emission and optical properties and for the broadening of the fields of application of the particles.

EXPERIMENTAL

Materials and Technology of Preparation of the Samples of Carbon Nanoparticles

The samples of carbon nanoparticles were synthesized from lentil flour by thermal carbonization in an inert atmosphere followed by the treatment of the obtained carbon material in concentrated nitric acid [40]. Weighed amounts of 20 g of a dry powder of the lentil flour were carbonized in a quartz reactor at 400 (CNPs-400) and 500°C (CNPs-500) in a nitrogen flow for 3 h. The synthesized CNPs-400 and CNPs-500 solid carbon materials were mechanically ground to an average particle size of 150 μm. A powder of CNPs-400 in the amount of 3.0 g was exposed in 45 mL of concentrated HNO3 (extra-pure grade, 50%) for 40 h at room temperature. The obtained mixture was treated at 150°C in an air atmosphere to a constant dry weight (CNPs-400NA). A sample of CNPs-500NA was obtained in a similar manner from a powder of CNPs-500.

Since the samples of the powders of CNPs-400NA and CNPs-500NA contain particles that are soluble and insoluble in pure water [40], the soluble (CNPs-400NA/S, CNPs-500NA/S) and insoluble (CNPs-400NA/NS, CNPs-500NA/NS) particles of the samples were separated and individually studied. To separate the soluble and insoluble particles, 50 mg of a dry powder of CNPs-400NA was transferred into a beaker with distilled water with a volume of 200 mL, and the mixture was dispersed by sonication (125 W, 2 min) for the purpose of maximum dissolution and exposed at room temperature for 72 h. During the specified period of exposure, the insoluble particles almost fully precipitated onto the bottom of the beaker. The solution of CNPs-400NA/NS was collected from the beaker by careful canting, and the insoluble residue was multiply rinsed with distilled water for a long time to maximally remove the residual soluble particles. Then the insoluble particles of CNPs-400NA/NS were extracted from the aqueous solution of CNPs-400NA/S by centrifugation, and the soluble particles were extracted by slow evaporation of water at 80°C. The particles of CNPs-500NA/S and CNPs-500NA/NS were separated in a similar manner.

A sample of CNPs-400NA/S/AM was synthesized by the hydrothermal treatment of an aqueous solution of CNPs-400NA/S with the concentration of the particles of 0.5 mg/mL in the presence of ammonia. Fifty milliliters of a solution of CNPs-400NA/S and 10 mL (25%) of an extra-pure grade ammonia solution were transferred inside an externally heated autoclave. The pressurized autoclave was heated in the working chamber of a UT 4654 laboratory furnace up to 200°C and exposed at this temperature for 4 h. After the completion of the hydrothermal treatment, the autoclave was allowed to cool down to room temperature inside the furnace in the pressurized state. Unreacted ammonia from the solution of CNPs-400NA/S/AM after the hydrothermal treatment was extracted by slow evaporation at 80°C to a fourfold decrease in the volume of the solution. Then the formed insoluble and relatively large carbon particles were extracted from the solution of CNPs-400NA/S/AM by centrifugation.

A sample of CNPs-500NA/S/HP was prepared by a special treatment of 250 mg of a powder of CNPs-500NA/S in 10 mL of a solution of hydrogen peroxide (25%) at 100°C. The treatment was carried out to a constant dry weight. Then the dry powder of CNPs-500NA/S/HP was dissolved in distilled water. The insoluble and relatively large particles from the solution of CNPs-500NA/S/HP were extracted by centrifugation.

Solutions of CNPs-400NA/S and CNPs-400NA/ S/AM with different values of pH were prepared using high-purity KOH and HCl. The concentration of the particles of CNPs-400NA/S and CNPs-400NA/S/AM in the initial solutions remained unchanged (0.07 mg/mL) at all the values of pH. Solutions of CNPs-400NA/S and CNPs-400NA/S/AM with the values of pH of 12.88, 11.95, 10.0, 8.0, 7.0, 6.0, 5.0, 3.01, and 0.13 were prepared for the study. The solutions of CNPs-400NA/S and CNPs-400NA/S/AM with different values of pH were exposed at room temperature for 10 h after preparation for the stabilization of their parameters.

A solution of CNPs-400NA/NS was prepared by the transfer of 10 mg of a dry powder of CNPs-400NA/NS to 200 mL of a solution of KOH with a value of pH of 12.0. The powder quickly and completely dissolved immediately after the transfer. Solutions of CNPs-400NA/NS with the values of pH of 13.0, 12.0, 7.0, 3.0, 1.0, and 0.2 and concentration of particles of 0.05 mg/mL were prepared for the study. To accelerate and stabilize the main processes of action of KOH on the carbon particles, the solution of CNPs-400NA/NS was exposed at 80°C for 1 h (without losses of the solvent).

When measuring the optical density, the initial solutions of the carbon nanoparticles were diluted with alkaline (acidic) solutions with the values of pH of the initial solutions. This made it possible to preserve the initial values of pH of the diluted solutions. The optical density spectra of all the solutions under study were measured relative to the spectrum of distilled water.

All the solutions under study were centrifuged on an OPN-8 laboratory centrifuge with the separation factor FS = 6600 for 20 min.

The Method for Measuring the Optical and Luminescent Parameters of the Samples

The studies of the spectra and measurement of the value of the optical density D at the wavelength of excitation radiation of the solutions of carbon nanoparticles were performed on a Lambda 35 spectrophotometer (PerkinElmer). QS-10 quartz cells with a thickness of 10 mm were used in the studies of the optical and luminescent properties. The optical spectra of all the solutions of nanoparticles were measured relative to the absorption of distilled water in a range of wavelengths of 190–950 nm.

The luminescence of the solutions of carbon nanoparticles was studied on a specialized scanning microphotoluminescence unit (MPL) with parabolic mirror for the focusing of the excitation radiation and collection of luminescent emission. Luminescence was excited perpendicularly to the output window of the cell by modulated (537 Hz) laser radiation with a wavelength of 406 nm with an average power of 10 mW. The diameter of the excitation radiation beam in the focal plane of the parabolic mirror was ~30 μm. When measuring the photoluminescence, the internal surface of the output mirror of the cell (the cell window–solution of CNPs boundary) was located in the focal plane of the parabolic mirror of the MPL unit.

Quinine sulfate dihydrate in a 0.1 M solution of sulfuric acid (QY ~ 54.0%) and rhodamine PH-40 in ethanol (QY ~ 95.0%) were used as the standards for the measurement of the values of the quantum yield of emission (QY) of the solutions of carbon nanoparticles. The values of the quantum yield of the samples of solutions under study QYsp were calculated by the formula specified in [40]:

$$Q{{Y}_{{{\text{sp}}}}} = Q{{Y}_{{{\text{st}}}}}\frac{{R_{{{\text{PL}}}}^{{{\text{sp}}}}n_{{{\text{sp}}}}^{2}}}{{R_{{{\text{PL}}}}^{{{\text{st}}}}n_{{{\text{st}}}}^{2}}},$$
(1)

where QYst is the quantum yield of emission of the standard, \(R_{{{\text{PL}}}}^{{{\text{sp}}}}\) and \(R_{{{\text{PL}}}}^{{{\text{st}}}}\) are the integral value of the luminescent capacity RPL(λ) [40] in the specified range of wavelengths upon excitation by the radiation of 406 nm, and nsp and nst are the refractive indices of the samples of the solutions of the carbon nanoparticles and standard, respectively.

RESULTS AND DISCUSSION

The optical density spectra of the solutions of CNPs-400NA/S at different values of pH are presented in Fig. 1a. As is seen, two absorption bands with the maxima at 194 and 350 nm are clearly seen in the spectrum of a neutral solution (pH of 7.0). The intense band at 194 nm is associated with the absorption of the π–π* transitions of the C=C and/or N=O bonds of the functional groups, and the band at 350 nm is associated with the absorption of the n–π* transitions of the –COOH groups [40]. Note that the strong growth in absorption of the alkaline and acidic solutions of the carbon nanoparticles in a region of 190–230 nm at the values of pH of 13, 3, and 0.13 is associated with the presence of hydrated OH– and Cl– ions with a high concentration in the solutions [41]. While the intensity of the band at 350 nm changes insignificantly in the acidic solutions of CNPs-400NA/S in a range of pH of 0.13–7, the intensity of this band strongly decreases in the spectra of the alkaline solutions. Here, a new wide absorption band with the maximum at about 407 nm is formed in the alkaline solutions. This means that, apparently, the centers responsible for the band at 350 nm demonstrate high stability in an acidic medium and low stability in an alkaline medium.

Fig. 1.
figure 1

Optical density spectrum of the solutions of (a) CNPs-400NA/S, (b) CNPs-400NA/NS, (c) CNPs-400NA/S/AM, and (d) CNPs-500NA/S/HP at different values of pH. Insets: (a) the optical density spectra of solutions of CNPs-400NA/S with the values of pH of 13 and 0.13 relative to a neutral solution of CNPs-400NA/S and (d) the optical density spectra of the neutral solutions of CNPs-500NA/S and CNPs-500NA/S/HP.

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In an acidic solution of CNPs-400NA/S at a low value of pH (0.13), the contour of the band at 350 nm splits, becomes distorted, and shifts to the red region of the spectrum by 24 nm (Fig. 1a, inset). As is seen from Fig. 1a (inset), the band at 407 nm also has a significant nonuniform broadening from the side of long waves. The half-width of the bands at 374 and 407 nm is 878 and 897 meV, respectively. Here, let us note that the optical centers responsible for the band at 407 nm are stable under the condition pH ≥ 9. With the decrease in pH to 9, the intensity and half-width of the band at 407 nm monotonically decrease, and the position of the peak changes slightly, and the band completely disappears at pH of 8. The detailed analysis of the absorption spectra of the particles of CNPs-400NA/S in acidic and alkaline media in a wide range of pH makes it possible to draw an important conclusion. First, the band at 407 nm appears upon the deprotonation, and the band at 374 nm appears upon the protonation of the –COOH functional groups of nanoparticles. Second, the band at 374 nm (3.316 eV) is formed by the superposition of two, 3.303 eV (375.4 nm) and 2.983 eV (415.7 nm), and the band at 407 nm (3.047 eV) is formed by the superposition of three, 3.285 eV (377.5 nm), 3.053 eV (406.2 nm), and 3.005 eV (412.6 nm), symmetrical absorption bands with a Gaussian contour.

The optical density spectra of the particles of CNPs-400NA/NS in an aqueous solution at different values of pH are presented in Fig. 1b. As opposed to the spectrum of CNPs-400NA/S, the spectrum of the particles of CNPs-400NA/NS in a neutral medium consists only of a wide (200–700 nm) and intense absorption shoulder. A barely visible band with the maximum at about 273 nm is observed against the background of the strong absorption shoulder. A decrease in pH of the solution from 13 to 12 slightly affects the parameters of absorption of the particles of CNPs-400NA/NS. Further decrease in pH from 12 to 7 induces a decrease in the absorption which is nonuniform over the spectrum. Here, the band at 273 nm disappears from the spectrum.

In an acidic solution, up to a value of pH of 3, the absorption spectrum of CNPs-400NA/NS is little different from the spectrum of particles in a neutral solution (Fig. 1b). However, as is seen from Fig. 1b, the absorption spectrum sharply changes at a high concentration of protons in the solution of CNPs-400NA/NS, and a new weak band with a diffuse maximum at ~264 nm appears in the spectrum. It should be noted that a sharp decrease in the value of absorption at a value of pH ≤ 1 is associated with the precipitation of a significant part of CNPs-400NA/NS as flakes. It is useful to note that such a sharp decrease in the solubility in an acidic medium is observed in aqueous solutions of graphite oxides at a value of pH ≤ 2 [42]. Because of this, to increase the accuracy of the optical and luminescent parameters being measured, the insoluble particles were extracted from the solutions by centrifugation in these cases.

On the basis of the results of the study, after the treatment of the particles of CNPs-400NA/S and CNPs-400NA/NS in an alkaline medium, the solubility of the particles of CNPs-400NA/S slightly changes in a wide range of pH, while the particles of CNPs-400NA/NS which are insoluble in pure water acquire solubility. The treatment of the particles of CNPs-400NA/NS in an alkaline medium imparts them with the capacity for high solubility not only in an alkaline but also in acidic and neutral media. Moreover, after the treatment of the particles of CNPs-400NA/NS and CNPs-500NA/NS in an alkaline medium, a certain fraction of nanoparticles retain good solubility in an acidic medium down to very low values of pH. The use of KOH, LiOH, NaOH, Na2S, and NH4OH as the base and the acids HCl, H2SO4, and HNO3 for changing pH of the solutions shows that both the solubility of the particles of CNPs-400NA/NS and CNPs-500NA/NS and the parameters of their absorption spectra in a range of pH of 0.2–13.0 only slightly depend on the types of alkalis and acids. The specified characteristic features give the grounds to state that, first, the solubility and change in the absorption spectra of the particles of CNPs-400NA/NS and CNPs-500NA/NS in acidic and alkaline solutions are based on the processes of protonation and deprotonation of the functional groups of carbon nanoparticles. Second, the effect of strong dependence of the solubility of carbon nanoparticles on pH of the solution may become a good way for the separation of nanoparticles with different properties.

According to the results of the analysis of the optical density D spectrum of the particles of CNPs-400NA/S/AM in a neutral solution (Fig. 1c), two weak bands of NH1 (274 nm) and NH2 (365 nm) associated with the electron transitions of the pyridine nitrogen [40] and a band at 407 nm are present against the background of an intense and wide absorption shoulder. In an alkaline medium, with the growth in pH from 7 to 13, the intensities of the bands of NH1 and NH2 weaken, and the band at 407 nm intensifies. In an acidic medium, with the decrease in pH from 7 to 0.13, the intensities of the bands of NH1 and NH2 grow insignificantly, and the band at 407 nm gradually weakens and completely disappears at pH < 5. The appearance of the band at 407 nm in an alkaline medium shows that the –COOH groups are removed and/or partially modified after the hydrothermal treatment of particles of CNPs-400NA/S in the presence of ammonia. Here, the band at 350 nm associated with the –COOH group is completely camouflaged by the high intensity of the wide absorption shoulder in the spectrum of a solution of particles of CNPs-400NA/S/AM at pH of 7.

The processes of protonation and deprotonation of the –COOH groups more vividly and pronouncedly manifest themselves in the particles of CNPs-500NA/S/HP (Fig. 1d). As follows from Fig. 1d, the treatment of the particles of CNPs-500NA/S in a solution of H2O2 substantially changes their absorption spectrum (inset). The short-wavelength absorption band strongly narrows, intensifies, and shifts to the red region of the spectrum by 8 nm. The band at 350 nm in the spectrum of the particles of CNPs-500NA/S/HP remains and intensifies. This means that the density of the –COOH groups of particles of CNPs-500NA/S/HP is higher than that of the initial particles of CNPs-500NA/S, which shows the higher intensity of the band at 407 nm of the particles of CNPs-500NA/S/HP at a value of pH of 13. Here, in an acidic medium, the intensity of the band at 350 nm of the particles of CNPs-500NA/S/HP is slightly dependent on pH of the solution. Upon the treatment of the particles of CNPs-500NA/S, the increase in the H2O2/CNPs weight ratio in the solution increases the intensity of the band at 350 nm of the particles of CNPs-500NA/S/HP in neutral and acidic media as well as of the band at 407 nm in an alkaline medium.

The luminescence of all the samples of the solutions of carbon nanoparticles was studied under the same conditions of excitation (λexc = 406 nm, Pexc = 10 mW) and collection of the photoluminescence emission and sizes of the cells, as well as at similar values of the concentrations of the carbon nanoparticles (0.07, 0.05 mg/mL) in the solutions. This approach makes it possible to minimize the errors of the measurement of the photoluminescence spectra associated with the hardware function of the MPL unit and concentration effects of the nanoparticles in the solutions. Figure 2 shows the photoluminescence spectra of the solutions of CNPs-400NA/S, CNPs-400NA/NS, CNPs-400NA/S/AM, and CNPs-500NA/S/HP with different values of pH. The photoluminescence spectra of the solutions of CNPs-400NA/S and CNPs-400NA/NS at a value of pH of 7.0 consist of the main absorption band with the maximum at 530 and 528 nm, respectively, and a weak band at 754 nm. The main photoluminescence bands of the carbon nanoparticles under study have an asymmetric shape of the contour and are formed by the superposition of six individual photoluminescence bands with the symmetrical contours of Gaussian shape. The short-wavelength component with the maximum at about 461 nm is determined by the π–π* transition of the carbon core of the particles, and the other components are associated with the electronic transitions of the functional groups and lattice defects of the carbon nanoparticles [43].

Fig. 2.
figure 2

Normalized photoluminescence intensity spectra of the solutions of (a) CNPs-400NA/S, (b) CNPs-400NA/NS, (c) CNPs-400NA/S/AM, and (d) CNPs-500NA/S/HP with different values of pH upon excitation by laser radiation of 406 nm. Inset: (d) spectrum in the vicinity of the maximum of the photoluminescence band of the solution of CNPs-500NA/S/HP at values of pH 13, 7, and 0.13.

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The position of the maxima, half-width (Δ1/2), asymmetry components (δ1, δ2), and intensity of the main photoluminescence bands of the solutions of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM depend on the value of pH of the medium (Figs. 3, 4). As is seen from Fig. 3, the dependences of the intensities and shift of the peaks Δλmax = λmax – \(\lambda _{{\max }}^{{{\text{pH7}}}}\) of the photoluminescence bands of the samples of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM on pH are substantially different. Here, the peaks of the photoluminescence bands of the samples of CNPs-400NA/NS and CNPs-400NA/S/AM in an alkaline medium shift to the red region of the spectrum with the growth in pH, while in acidic medium, they shift to the blue region of the spectrum with the decrease in pH. The peak of the photoluminescence band of the sample of CNPs-400NA/S in both alkaline and acidic media insignificantly shifts to the blue region of the spectrum. Note that the maxima of the photoluminescence bands of the particles of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM in a neutral solution have a value of 530, 528, and 510 nm, respectively.

Fig. 3.
figure 3

Dependence of the shift of the maxima of the photoluminescence bands on pH of the solutions of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM. Inset: dependence of the photoluminescence intensity on pH of the solutions of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM.

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Fig. 4.
figure 4

Dependence of the half-width of photoluminescence bands on pH of the solutions of CNPs-400NA and CNPs-400NA/NS.

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As opposed to the shift Δλmax, the values of Δ1/2, δ1, and δ2 of the main photoluminescence band of the particles of CNPs-400NA/S quite strongly depend on pH of the solution (Fig. 4). The values of Δ1/2 and δ1 monotonically decrease with the growth in pH in a range from 0.13 to 10 but the value of δ2 slightly changes in this range. The parameters Δ1/2, δ1, and δ2 especially strongly depend on pH in a range of 10–13 where the photoluminescence band sharply narrows with the growth in pH, and its symmetry increases. Note that the value of Δ1/2 of the photoluminescence band of a neutral solution of CNPs-400NA/S is 551.1 meV, while it is 554.5 and 494.6 meV at a value of pH of 0.13 and 13, respectively.

The dependences of the parameters Δ1/2, δ1, and δ2 of the particles of CNPs-400NA/NS and CNPs-400NA/S on pH of the medium have a substantial difference (Fig. 4). As is seen from Fig. 4, with the increase in pH of the solution of CNPs-400NA/NS from 0.2 to 13, the value of δ2 constantly grows, while δ1 first grows and then remains unchanged and then decreases in a range of 8–13. The change in the value of Δ1/2 of the photoluminescence band is small throughout the entire range of pH. In other words, while an increase in pH of acidic and alkaline media is accompanied by a decrease in Δ1/2 and an increase in the symmetry of the main photoluminescence band of the particles of CNPs-400NA/S, on the contrary, the value of Δ1/2 of the photoluminescence band of the particles of CNPs-400NA/NS in the specified media grows with the growth in pH. Here, the symmetry of the photoluminescence band gradually increases with the growth in pH in a range of 0.2–11.6 and then sharply decreases.

A detailed analysis of the results of decomposition of the photoluminescence spectra of the samples of CNPs-400NA/S and CNPs-400NA/NS into components shows that the position of the peaks of the bands at 461 and 754 nm changes slightly in a wide range of pH. The peaks and widths of four other photoluminescence bands have a weak dependence on pH of the solutions, but their intensities substantially change with the change in pH. Here, the intensity of the band at 461 nm of the samples of CNPs-400NA/S and CNPs-400NA/NS is always higher in acidic than in alkaline media.

The dependences of the quantum yield of emission and optical density of the solution at an excitation wavelength of 406 nm (D406) on pH of the solutions of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM are presented in Fig. 5. It is seen that, first, the characters of the dependences of the quantum yield of emission and optical density of the particles of CNPs-400NA/S and CNPs-400NA/NS are substantially different in a wide range of pH. Second, in both types of samples of nanoparticles, the value of the quantum yield of emission does not correlate with the value of D406 of the particles. The sharp decrease in the value of D406 and increase in the quantum yield of emission of the particles of CNPs-400NA/NS at a value of pH ≤ 3 are associated with the precipitation of some nanoparticles in the solution. The precipitation of the carbon nanoparticles decreases the value of absorption in the long-wavelength region of the spectrum more than at the excitation wavelength λexc (Fig. 1). This decreases the effect of photoluminescence self-absorption in the solution, which, in turn, induces a growth in the quantum yield of emission QY. It is clearly seen from the dependence presented in Fig. 6 that the particles of CNPs-400NA/NS have stronger absorption in the spectral region of the photoluminescence band (450–700 nm) and at the excitation wavelength (406 nm) than the particles of CNPs-400NA/S. This fact is the main reason for the decrease in the quantum yield of emission of the particles of CNPs-400NA/NS by self-absorption. Note that the value of QY of the solution of CNPs-400NA/S in a range of pH of 7–13 exceeds QY of the solution of CNPs-400NA/NS more than twofold (Fig. 5).

Fig. 5.
figure 5

Dependences of the quantum yield of emission QY and optical density D on pH of the solutions of (a) CNPs-400NA/S, (b) CNPs-400NA/NS, and (c) CNPs-400NA/S/AM.

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Fig. 6.
figure 6

Ratio of the optical density D spectra of the solutions of CNPs-400NA/NS and CNPs-400NA/S with the concentrations of 0.05 mg/mL at a value of pH of 12.

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The dependences of QY and D406 on pH of the solutions of the particles of CNPs-400NA/S and CNPs-400NA/NS (Figs. 5a, 5b) have a significant difference from the corresponding dependences of the particles of CNPs-400NA/S/AM (Fig. 5c). The absorption of the particles of CNPs-400NA/S/AM in the ranges of pH of 0.13–6 and 7–13 quite linearly depends on the value of pH of the solution (Fig. 5c), but in an acidic medium, the average rate of growth in the absorption is 5.7-fold higher than in an alkaline medium. The value of the quantum yield of emission of the specified type of particles monotonically grows up to pH of 7 with the growth in pH in an acidic medium and decreases in an alkaline medium. The value of QY of the particles of CNPs-400NA/S/AM reaches the maximum value (10.28%) in a neutral solution. It has been shown in [40] that increasing the temperature THT and duration tHT of the hydrothermal treatment increases the quantum yield of emission of the particles of CNPs-400NA/S/AM. At the values of THT of 200°C and tHT of 250 h, the value of the quantum yield of emission of the particles of CNPs-400NA/S/AM increases to 58.3% (upon excitation by a laser radiation of 406 nm), and despite a substantial growth in QY, the character of the dependence of QY on pH of the medium is retained.

The treatment of the initial particles of CNPs-500NA/S in an aqueous solution of hydrogen peroxide substantially affects the parameters of the photoluminescence spectrum of the particles in addition to the absorption spectrum. The peak of the photoluminescence band of the particles shifts from 528 to 532 nm in a neutral solution, and its half-width narrows from 536.6 to 431.7 meV (Fig. 2d). A small (4 nm) redshift and a significant narrowing of the photoluminescence band are determined by the sharp change in the types and density of the functional groups of the carbon nanoparticles. Despite the narrowing of the photoluminescence band, the value of the quantum yield of emission of the particles of CNPs-500NA/S/HP increases up to 9.65% (QY ~ 1.35% for the particles of CNPs-500NA/S). An increase in pH of the neutral solution of the particles of CNPs-500NA/S/HP to 13 and a decrease to 0.13 slightly affect the width of the photoluminescence band. Here, it should be noted that two close maxima at 532 and 539 nm are distinguished on the peak of the photoluminescence band of the particles of CNPs-500NA/S/HP in a neutral medium (Fig. 2d, inset). The position of the maximum at 532 nm does not depend on pH of the medium, and the maximum at 539 nm shifts to the red (by 5 nm) and blue (by 4 nm) regions of the spectrum at pH of 13 and 0.13, respectively.

The quantum yield of emission of the particles of CNPs-500NA/S/HP depends on pH of the medium and conditions of treatment of the initial particles in a solution of hydrogen peroxide. For example, the value of the quantum yield of emission of the particles of CNPs-500NA/S/HP is 9.65% in a neutral solution, while the quantum yield of emission decreases to 4.71 and 3.67% at the values of pH of 13 and 0.13, respectively. Let us also note that an increase in the H2O2/CNPs weight ratio from 11 to 22 during the treatment of the particles of CNPs-500NA/S increases the quantum yield of emission in a neutral solution from 9.65 to 17.34%.

Therefore, the dependences of the quantum yield of emission and optical absorption on pH of the solutions of CNPs-400NA/S and CNPs-400NA/NS clearly demonstrate a strong and complex effect of pH of the solution on various parameters of the functional groups of carbon particles and particles as a whole. At the values of pH < 3 and pH > 10, respectively, the protons and hydroxyl ions of the solution induce a strong rearrangement of the molecular structures, symmetry, elemental compositions, and charge density and also substantially change the density of the functional groups of particles of CNPs-400NA/S and CNPs-400NA/NS. The strongest effect of OH– ions on the quantum yield of emission and absorption of the particles of CNPs-400NA/S is observed in a range of pH of 10–13, where the quantum yield of emission decreases substantially, and the absorption grows (Fig. 5a). A strong effect of H+ ions on the quantum yield of emission and absorption of the particles of CNPs-400NA/NS occurs in a range of pH of 0.1–3. In this range of pH, the value of the quantum yield of emission sharply grows, while the value of absorption in the region of excitation and the luminescence band decreases (Fig. 5b). The specified characteristic features provide sufficient grounds to assume that the deprotonation of the functional groups of particles of CNPs-400NA/S creates electron levels localized on the functional groups which participate in the process of absorption in the region of excitation wavelengths and photoluminescence and increase the rate of nonradiative recombination. On the contrary, the protonation of the functional groups of particles of CNPs-400NA/NS decreases the rate of nonradiative recombination and reduces the effect of photoluminescence self-absorption.

The phenomenon of the change in the intensity of photoluminescence of carbon nanoparticles upon the exposure to excitation is often observed in practice, and it is determined by the interaction of electromagnetic radiation with the functional groups of particles [43]. Certain types of functional groups are modified under the action of excitation radiation. In the case where the modified functional groups participate in the processes of absorption, excitation, and/or emission, the exposure to excitation is often accompanied by a change in the intensity of photoluminescence of carbon nanoparticles. Note that, in some types of samples, this effect also appears under the action of the intrinsic photoluminescence radiation of the particles in addition to the excitation radiation. The efficiency of the photostimulated modification of carbon nanoparticles depends both on the individual properties (sizes, dispersity, elemental composition, defectiveness of the lattice of the carbon core, type and density of the functional groups) of the carbon particles themselves and on the wavelengths of the irradiating radiation and properties of the medium surrounding the particles. From the practical standpoint, this property of the particles generates a unique possibility for controlling the parameters of carbon nanoparticles [44].

The exposure of the samples of the solutions of carbon nanoparticles with different values of pH to exciting radiation shows the presence of photostimulated processes and the degree of stability of the emission of the nanoparticles. At all the values of pH, directly after switching on the excitation, the luminescence of the solutions of various types of particles possesses the decrease, growth, and stabilization of the intensity of photoluminescence characteristic of carbon nanoparticles [10, 40, 43] (Fig. 7). It is seen from Fig. 7a that, at the initial stage of exposure of the solution of CNPs-400NA/S for 10–50 s, the intensity of photoluminescence rapidly decreases at the values of pH of 13, 12, 7, and 5, while almost no fast decrease is observed at pH of 0.13, 3, 6, 8, and 10. As opposed to the solution of CNPs-400NA/S, the initial fast decline in the photoluminescence in the solution of CNPs-400NA/NS is observed only at the values of pH ≥ 12 (Fig. 7b).

Fig. 7.
figure 7

Temporal dependence of the intensity of the photoluminescence band of the solutions of (a) CNPs-400/NA/S, (b) CNPs-400NA/NS, (c) CNPs-400NA/S/AM, and (d) CNPs-500NA/S/HP with different values of pH upon exposure to laser radiation with a wavelength of 406 nm.

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After the fast decline in the photoluminescence of the solutions of carbon nanoparticles, the rate of decline decreases with the time of exposure and then stabilizes, or grows and then stabilizes (Fig. 7). The duration of the process of slow decline in photoluminescence is 500–700 s depending on the value of pH of the solutions. The stabilization of the intensity of photoluminescence of the studied samples of CNPs-400NA/S and CNPs-400NA/NS at different values of pH was achieved after a ~15 min exposure to excitation (Figs. 7a, 7b). This time is longer in the samples of CNPs-400NA/AM and CNPs-500NA/S/HP (Figs. 7c, 7d). Because of this, to increase the accuracy, the photoluminescence spectra of all the studied samples were measured only after the maximum stabilization of the intensity of photoluminescence of the latter. It is easy to note from the obtained results that the maximum decline in the photoluminescence of all the particle types of carbon nanoparticles is observed in an alkaline medium at a high concentration of OH– (Fig. 7). For example, a 30 min exposure of the solution of CNPs-500NA/S/HP at a value of pH of 13 decreases the intensity of photoluminescence fivefold. It is apparent from the character of the decline in the quantum yield of emission in an alkaline medium (Fig. 5) that the deprotonation of the functional groups of carbon nanoparticles intensifies the interaction of the particles with the electromagnetic radiation and decreases the stability of their photoluminescence.

Therefore, the investigation of the aqueous solution of the particles of CNPs-400NA with different values of pH confirms the presence of particles soluble (CNPs-400NA/S) and insoluble (CNPs-400NA/NS) in pure water in this material [40]. The obtained results make it possible to assume that the solubility of the particles of CNPs-400NA in pure water is associated with the presence of a hydrogen bond between the functional groups containing O, N, and H atoms and its concentration. On the basis of the results of these studies as well as published data [42], the strong hydrogen bond between the particles of CNPs-400NA/NS is mainly determined by the presence of a high concentration of carboxyl groups in the carbon nanoparticles. In the presence of hydroxyl ions, the ‒COOH, –OH, and –NH2 groups are partially or fully deprotonated, which imparts carbon nanoparticles with good solubility in an alkaline medium.

It is known that, at the values of pH > 10, the acidic carboxyl groups (–COOH) of carbon nanoparticles are to a significant extent deprotonated (–COO–) [33]. In other words, the CNPs–COOH particles are transformed to the CNPs–COO– state in an alkaline solution. This, first, increases the electron density on the oxygen atoms of the carboxyl groups of the particles, which often gives a redshift of the position of the maximum of the photoluminescence band of carbon nanoparticles (Fig. 3). Second, the loss of the proton of the –COOH group breaks the hydrogen bond between the carboxyl groups of the neighboring particles, which sharply increases the solubility of the carbon nanoparticles in water. In the acidic solutions of CNPs–COOH (to a value of pH ~ 1), the –COOH groups are not noticeably protonated in the least. Because of this, the blueshift of the maximum of the photoluminescence band associated with the –COOH groups is insignificant in the spectra of acidic solutions.

Upon decreasing pH < 3 of the alkaline solutions of CNPs-400NA/NS where the particles predominantly possess the CNPs–COO– structure and have high solubility, the –COO– groups are protonated. The initial CNPs–COOH structure and, hence, the hydrogen bond between the –COOH groups of the neighboring carbon particles are recovered as a result. Then the particles enlarge, lose their solubility, and precipitate in the solution. This means that the protonation and deprotonation of the particles of the CNPs-400NA/NS type are well reversible processes. Here, protonation leads to the blueshift, and deprotonation, vice versa, leads to the redshift of the maximum of the photoluminescence band (Fig. 3). At the same time, the results of the study of the absorption and quantum yield of emission (Fig. 5b) show that the particles of CNPs–COO– possess a smaller value of QY than the particles of CNPs–COOH in aqueous solutions. This means that the deprotonation of the carboxyl groups of carbon nanoparticles decreases the quantum yield of photoluminescence of the particles of CNPs-400NA/NS.

The processes of protonation and deprotonation of the amine groups (–NH2) which are often present on the surfaces of carbon nanoparticles after their synthesis and play a very important role in the formation of the luminescent properties of most types of carbon nanoparticles are different from the corresponding processes for the CNPs–COOH particles. Note that the hydrothermal treatment of carbon nanoparticles in the presence of ammonia or urea is widely used for the improvement of their luminescent properties. As is seen from Fig. 5, the photoluminescence spectrum of the particles of CNPs-400NA/S/AM is substantially different from the spectra of CNPs-400NA/S and CNPs-400NA/NS. The blueshift (by 20 nm) of the photoluminescence band and increase in the quantum yield of emission of the solution of CNPs-400NA/S/AM after the hydrothermal treatment are determined by the formation of the particles with the CNPs–NH2 structure in the solution. It is known that donating –NH2 groups are insignificantly deprotonated in alkaline solutions in a wide range of pH but are quite efficiently protonated (\( - {\text{NH}}_{3}^{ + }\)) in acidic solutions, which decreases the charge density of the π–π* system of the aromatic domains [45] of carbon nanoparticles. Therefore, if a –NH2 group substantially affects the parameters of the photoluminescence band that is associated with the π–π* transition of the aromatic domain of carbon nanoparticles, a decrease in the electron density in the π-system of the domain and a weak blueshift of the photoluminescence band of the CNPs–NH2 particles are expected in an acidic medium [46, 47]. On the other hand, a growth in the electron density in the π-system of the domain and a strong redshift of the photoluminescence band are expected in an alkaline medium, which is in good agreement with the obtained experimental results (Fig. 3).

As opposed to the photoluminescence bands of the particles of CNPs-400NA/NS, CNPs-400NA/S, and CNPs-400NA/S/AM, the position of the main peak of the photoluminescence band (532 nm) of the particles of CNPs-500NA/S/HP in a wide range (0.13–13) is almost independent of pH of the solution (Fig. 2, inset). As was shown earlier, the second maximum (539 nm) has a blueshift (by 4 nm) in an acidic medium and a redshift (by 5 nm) in an alkaline medium. The independence of the main peak of the photoluminescence band of the particles of CNPs-500NA/S/HP from pH of the medium confirms that the green luminescence of these particles is closely related to the carbonyl groups that are hardly protonated in an acidic medium. This means that particles of the CNPs-500NA/S/HP type mainly have the CNPs–CO structure. The behavior of the photoluminescence curves at the maximum at 539 nm is apparently associated with the effect of protonation and deprotonation of the small amount of –COOH and/or –OH groups present on the CNPs–CO particles.

Therefore, on one hand, the studies of the characteristic features of the photoluminescence spectra and values of the quantum yield of emission of different types of carbon nanoparticles in aqueous solutions in a wide range of pH confirm the predominant effect of the functional groups on the main parameters of these spectra [19, 32, 37]. On the other hand, they show a strong effect of the functional groups on the parameters of carbon nanoparticles containing N and O heteroatoms. The main mechanism of the effect is associated with the complex interactions of the electrons of the unshared pairs of N and O with the π-system of the aromatic rings of carbon nanoparticles. Despite the higher value of the electronegativity of an oxygen atom in comparison with a nitrogen atom, the effect of the uncoupled electrons of N of the CNPs–NH2 particles on the parameters of the π-system of the carbon core is greater than the effect of the uncoupled electrons of O of the particles of the CNPs–COOH, CNPs–OH, and CNPs–CO types. This fact is mainly determined by the shorter distance between the nearest C atoms of the carbon core and N atom of the –NH2 and \( - {\text{NH}}_{3}^{ + }\) groups in comparison with the distance between the O atoms of the –COOH, –COO–, –CO, and –OH groups and the C atoms of the carbon core of the nanoparticle. Because of this, the parameters of the absorption and photoluminescence spectra as well as the value and stability of the quantum yield of emission of various types of carbon particles with nitrogen-containing groups exhibit high sensitivity to various external actions.

CONCLUSIONS

The investigation of the absorption spectra of aqueous solutions of various types of carbon nanoparticles in a wide range of pH has shown that, first, both the intensity and shape of the absorption spectra of the particles substantially depend on the value of pH of the solution and individual properties of the particles themselves. Second, the functional groups of the particles make the main contribution to the change in the absorption spectra of carbon nanoparticles depending on pH of the solutions. With a change in pH of the aqueous solutions, phenomena such as the growth and decline in absorption; appearance, disappearance, and change in the symmetries and widths of the contours of the absorption bands associated with the functional groups of carbon nanoparticles; and bathochromic and hypsochromic shifts of the absorption bands are observed in the absorption spectra of the studied samples of particles. Depending on the types of the functional groups of carbon nanoparticles, the strongest change in the absorption spectra occurs in the ranges of pH of 9–13 and 0.1–3 in alkaline and acidic media, respectively (samples of CNPs-400NA/S, CNPs-400NA/NS, CNPs-400NA/S/AM, and CNPs-500NA/S/HP).

It has been found that the processes of protonation and deprotonation of the groups such as –COOH, ‒OH, and –NH2 play the predominant role in the change in the absorption spectra of the studied carbon nanoparticles upon a transition from an acidic medium to an alkaline medium. For example, the protonation and deprotonation of the –COOH group of the particles of CNPs-400NA/S leads to the formation of the adsorption bands at 374 and 407 nm, respectively. In addition, a strong dependence of the solubility of carbon nanoparticles on the processes of protonation and deprotonation and concentration of the N and O atoms in the composition of the particles has been shown. It has been demonstrated that, in the process of hydrothermal treatment of the particles of CNPs-400NA/S in the presence of ammonia, the –COOH groups are removed and/or partially modified.

It has been found on the basis of the results of the luminescence studies that the position of the peaks, half-widths, symmetry, and intensities of the main photoluminescence bands of the solutions of the studied samples of carbon nanoparticles depend on the value of pH of the medium. The degree and character of manifestation of the specified parameters of the studied particles also depend on the individual properties of the particles in addition to the value of pH. For example, the peaks of the photoluminescence bands of the samples of CNPs-400NA/NS and CNPs-400NA/S/AM shift to the red region of the spectrum with the growth in the density of the hydroxyl ions in an alkaline medium and to the blue region of the spectrum with the growth in the protons in an acidic medium. The strongest change in the widths and symmetries of the photoluminescence bands of all the studied samples of particles is observed in an alkaline medium in a range of pH of 9–13. These characteristic features give evidence of the presence of stronger and more complex interactions of the solvate shell with the functional groups of the particles in an alkaline medium.

The photoluminescence spectra of the solutions of the particles of CNPs-400NA/S and CNPs-400NA/NS at a value of pH of 7 consist of the main bands at 530 and 528 nm, respectively, and a weak band at 754 nm. It has been shown by the method of decomposition of the spectra that the main photoluminescence bands have an asymmetric shape of the contour and are formed by the superposition of six individual bands with Gaussian shape. The short-wavelength band with the maximum at 461 nm is associated with the carbon core of the particles, while other, longer wavelength, bands are associated with the functional groups and lattice defects of the carbon nanoparticles. The peaks of the photoluminescence bands at 461 and 754 nm are almost independent of pH of the medium in a wide range. The peaks and widths of other bands weakly depend on pH, but their intensities strongly change with the change in pH of the medium. The intensity of the band at 461 nm of the samples of CNPs-400NA/S and CNPs-400NA/NS is always higher in acidic media than in alkaline media.

The process of modification of carbon nanoparticles in a solution of hydrogen peroxide has been studied and analyzed in detail. In particular, it has been found that the green luminescence of the sample of CNPs-500NA/S/HP is associated with the formation of the particles with the CNPs–CO structure and partial removal of the –COOH group from the surface of the particles.

Both the parameters of the absorption and photoluminescence bands and the value of the quantum yield of emission of the solutions of the particles of CNPs-400NA/S, CNPs-400NA/NS, and CNPs-400NA/S/AM demonstrate different character of the dependence on pH of the medium. Here, the value of the quantum yield of emission does not correlate with the value of the optical density of the particles. It has been shown that the value of the quantum yield of emission increases from 10.28 to 58.3% in the case of an increase in the duration of the hydrothermal treatment of the particles of CNPs-400NA/S/AM from 4 to 250 h, but, here, the character of the dependence of the quantum yield of emission on pH of the medium is retained. The value of the quantum yield of emission of the particles of CNPs-500NA/S/HP also depends on the condition of the treatment of the initial particles in a solution of hydrogen peroxide in addition to pH of the medium. Increasing the H2O2/CNPs weight ratio from 11 to 22 increases the quantum yield of emission from 9.65 to 17.34%.

The presence of a strong and complex effect of pH of the solution on the various parameters of the functional groups of carbon nanoparticles has been shown. At the values of pH < 3 and pH > 10, respectively, the protons and hydroxyl ions of the medium induce the rearrangement of the molecular structures, symmetry, elemental compositions, charge density, and concentration of the functional groups of carbon nanoparticles. It has been noted that the strongest effect of the OH– ions on the quantum yield of emission and absorption spectrum of the particles of CNPs-400NA/S occurs in a range of pH of 10–13, and that of the H+ ions occurs in a range of 0.1–3.

The photostimulated processes and stability of emission of various types of carbon nanoparticles have been studied by the method of exposure to exciting radiation in a wide range of pH. It has been shown that the exposure of all the studied samples of carbon nanoparticles to excitation leads to a decrease in the intensity of photoluminescence; here, the value and character of the decrease in the emission are associated both with the properties of the particles and the value of pH of the medium. The maximum decline in the photoluminescence (up to fivefold) from all the types of carbon particles is observed in an alkaline medium at a high concentration of OH–. It has been shown that the deprotonation of the functional groups intensifies the interaction of the particles with the electromagnetic radiation and decreases the stability of their emission.

The investigation and analysis of the photoluminescence spectra and characteristic features of the quantum yield of emission of different types of carbon nanoparticles in aqueous solutions in a wide range of pH make it possible to determine the predominant effect of the functional groups on the main parameters of the spectra of the particles under consideration. The specified parameters of the nanoparticles are also substantially affected by the N and O heteroatoms present in the composition of the particles. The main mechanism of the effect of N and O on the photoluminescence and quantum yield of emission is associated with the complex interactions of the electrons of the unshared pairs of the nitrogen and oxygen atoms with the π-system of the aromatic rings of the carbon nanoparticles. It has been shown that the uncoupled electrons of N of the CNPs–NH2 particles more strongly affect the parameters of the π-system of the carbon core than the uncoupled electrons of O of the particles of the CNPs–COOH, CNPs–OH, and CNPs–CO types.