Insight into excitation-related luminescence properties of carbon dots: synergistic effect from photoluminescence centers in the carbon core and on the surface

Abstract

Excitation-dependent luminescence (EDE) or excitation-independent luminescence (EIE) property of carbon dots (CDs) has attracted considerable attention. For the first time, we found that nitrogen doped CDs possess adjustable EDE and EIE properties by changing the corresponding environmental pH values for photoluminescence (PL) properties. Structural characterizations and property tests demonstrate that the unique photoluminescence properties of CDs can be attributed to the synergistic effect from the PL centers in the carbon core and on the surface. Doping nitrogen into the carbon core improves the emission efficiency of PL centers; these enhanced PL sites can even dominate fluorescence emission. Passivated surfaces of CDs modified with amino groups not only further boost the emission efficiency, but also make energy levels of PL centers more uniform. In addition, the mechanism of synergistic effect is investigated, which opens up the possibility of designing CDs with desirable PL characteristics. Such nitrogen-doped CDs with adjustable excitation-related luminescence properties have potential applications in detecting acidity or alkalinity of a certain physiological environment by intuitively observing the change in the wavelength.

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1. Introduction

Since the discovery of carbon-based fluorescence nanostructures in 2004, carbon dots (CDs), as a new class of nanomaterials, have been attracting enormous attention due to their amazing optical properties.¹ Unlike other fluorescent materials, such as semiconductor quantum dots (SQDs), organic dyes and rare earth metal NPs, CDs are advantageous in their green synthesis due to their unique features (good aqueous solubility, high photostability, easy functionalization, low toxicity and excellent biocompatibility).^2–8 To date, CDs have been explored and applied in bioimaging, sensing, etc.^9–13 Efforts have been made, such as exploring synthetic methods, changing starting materials, doping of other elements and functionalizing the surface of CDs, to prepare high quality CDs with excellent photoluminescence quantum yield (PLQY) and controllable luminescence.^14–18 However, only a few researches show that the obtained CDs exhibit high PLQY or desired optical properties because the PL origin of CDs is yet unclear.

As is well known, the reason why CDs have either excitation-dependent luminescence (EDE) or excitation-independent luminescence (EIE) property is ambiguous up to now. It seems that most of the CDs modified by a surface passivation and/or doping with other elements (nitrogen and/or sulfur) have emissions in EIE features.^19–21 Recently, some investigations have revealed that the EIE property of the CDs appears only when the amino groups fully cover the surface. Li et al.²⁰ have prepared N-CDs with different surface density of amino-groups by controlling reaction temperature and found that the excitation property depends on the surface states related to the presence or absence of surface groups such as carboxyl, hydroxyl and amino-groups. Yuan et al. fabricated a series of CDs with EDE and EIE properties by controlling the content of nitrogen, proving that the characteristic optical properties of CDs stem from their energy levels.²¹ The research further demonstrates that the surface states of CDs could be completely passivated by amino groups, leading to a single energy level and high luminescence. At present, studies about controllable luminescence of CDs are still insufficient; therefore, numerous efforts should be devoted to reveal the PL emission mechanism of CDs.

Consequently, it is imperative to probe into the PL origin of CDs for a variety of practical applications with desired characteristics. In this study, numerous CDs were synthesized with citric acid (CA) and ethylenediamine (EDA) using hydrothermal processes. CA acts as a carbon source and EDA as the nitrogen source. We report the original observation of the adjustable EDE and EIE features depending on the pH values of N doped CDs for the first time. Structural characterizations and property tests demonstrate that the unique photoluminescence properties of CDs can be attributed to the synergistic effect from PL centers in the carbon core and on the surface. Nitrogen mainly appears in the carbon core with low content of EDA. As EDA increases, the surface of CDs is passivated by amino groups further. Doping nitrogen into the carbon core improves the emission efficiency of PL centers; these enhanced PL sites can even dominate fluorescence emission when fluorescence originated from the surface PL centers quenches. Passivated surfaces of CDs modified with amino groups not only boost the emission efficiency, but also make the energy levels of PL centers relatively uniform. Moreover, the mechanism of synergistic effect is shed light on, which opens up the possibility of designing CDs with desirable PL characteristics.

2. Experiment

2.1 Materials and instruments

Citric acid monohydrate (CA) was purchased from Guangfu Reagent Company (Tianjin, China). Ethylenediamine (EDA) was obtained from Aladdin Reagent (Shanghai, China). All reagents were of analytical grade and used as received without further purification. Deionized water (18.2 MΩ cm) by Milli-Q was used in all the experiments.

Transmission electron microscopy (TEM) was performed by a JEM-2100 transmission electron microscope at an acceleration voltage of 120 kV. XRD measurements were carried out by an X-ray diffractometer (Philips X'Pert, Holland) with Cu Kα radiation (λ = 1.54059 Å), with operating voltage and current at 40 kV and 35 mA, respectively. X-ray photoelectron spectra (XPS) were acquired by a PHI 550 photoelectron spectrometer equipped with a Mg Kα (hν = 1253.6 eV). The X-ray gun was operated at 15 kV and 20 mA. The background pressure of the residual gases during the measurements was lower than 10⁻⁶ Pa. Fourier transform infrared (FTIR) spectra were obtained using a Nicolet 360 FTIR spectrometer with the KBr pellet technique. UV-vis absorption spectra were obtained by an Agilent UV Cary5000 spectrophotometer. FLS920 spectrofluorometer was used to obtain the steady-state emission spectra, fluorescence lifetimes and quantum yields. Quantum yields were determined by a relative method using quinine sulfate as the fluorescence standard.

2.2 Preparation of CDs

In a typical synthesis, 0.21 g of CA and different amount of EDA 0 μL, 20 μL, 100 μL, 500 μL, and 1000 μL were dissolved in 10 mL of deionized water, and 1 mL of EDA was added into 10 mL of water as the control. The precursor solution was transferred to a 20 mL Teflon-lined stainless steel autoclave. After sealing, the autoclave was heated and the temperature was maintained at 160 °C for 6 h. Then, the reactors were cooled down to room temperature naturally. Afterwards, the yellowish/brown/yellow transparent solution was dried to remove water and excess EDA molecules by a rotary evaporator at 90 °C. Then, the samples were dissolved in 10 mL of deionized water again and dialyzed for 24 h with a dialysis membrane (MWCO 1000 Da from Sigma-Aldrich) to further remove the excess reagents. Finally, the samples were dried by vacuum freeze drying. The samples were denoted as CA-CDs, N-CDs1 and N-CDs2, respectively, when EDA was 0 μL, 20 μL and 1000 μL.

2.3 PLQY measurement

The quantum yield of the carbon dots was determined by a comparative method. Quinine sulfate in 0.1 M H₂SO₄ with a known QY 0.54 was introduced as a standard sample. The samples were dissolved in deionized water at different concentrations. The UV-vis absorbance spectra were obtained. The absorbance of the solutions at the excitation wavelength was also obtained. Emission spectra of the same solutions were obtained at an excitation wavelength of 360 nm, and the integrated PL intensity from 380 to 650 nm was calculated accordingly. Then, a graph was plotted using the integrated fluorescence intensity versus absorbance. A trend line was then added for each curve with an intercept at zero. Absolute values of QY were obtained according to the following simplified eqn (1):where subscripts ST and X denote standard and test, respectively. ϕ is the fluorescence quantum yield, G represents the gradient from the plot of integrated fluorescence intensity versus absorbance, and η stands for the refractive index of the solvent. To minimize reabsorption effects, the optical densities in the 20 mm fluorescence cuvettes were kept under 0.1 at the excitation wavelength. The excitation slit width of 2.5 nm and emission slit width of 2.5 nm were consistent.

3. Results and discussion

Three types of CDs were successfully prepared through hydrothermal method with fixed amount of CA and different amounts of EDA, as shown in Scheme 1. The resultant CD solutions were very homogeneous without any perceptible change even after being placed at room temperature for several months.

Scheme 1 Schematic of CD preparation with different structural features. Dosages of EDA (0 μL, 20 μL and 1000 μL) strongly influence the structural features and PL properties of the resultant CDs. (1) CA-CDs solution prepared without EDA is yellowish and emits weak blue fluorescent light under a 365 nm UV lamp. No nitrogen and nitrogen-containing groups were present in the graphitic carbon core and on the surface. (2) N-CDs1 solution prepared with 20 μL EDA is brown and emits strong blue green fluorescent light under 365 nm UV lamp excitation. Nitrogen is doped into the graphitic carbon core and few amino groups exist on the surface, as exhibited in N-CDs1 structure with red color. (3) N-CDs2 solution prepared with 1000 μL EDA is yellow and emits strong blue fluorescent light under 365 nm UV lamp excitation. Nitrogen is doped into the graphitic carbon core and the surface could be completely passivated by amino groups, as shown in N-CDs2 with red color.

3.1 Formation and structural features of CDs

The morphologies and crystallinity of CA-CDs, N-CDs1 and N-CDs2 were studied by TEM and XRD. Fig. 1A shows that CA-CDs are nearly spherical and possess rather uniform particle size with an average diameter of about 1.62 nm (the inset of Fig. 1A). The N-CDs1 and N-CDs2 particles have broad sizes (the inset of Fig. 1B and C). The average diameters are about 1.87 and 1.44 nm, respectively. XRD pattern (Fig. S1, ESI†) exhibits a typical broad diffraction peak at about 23° associated with the graphitic structure.²² FT-IR spectra were obtained to identify the functional groups of CA-CDs, N-CDs1 and N-CDs2, as shown in Fig. 1D. The main absorption bands of CA-CDs and N-CDs1 are very similar. Broad absorption bands at 3100–3700 cm⁻¹ and 2800–3100 cm⁻¹ can be ascribed to the stretching vibration of O–H and C–H, respectively.¹¹ The intense absorption peaks at about 1720 cm⁻¹ and 1402 cm⁻¹ can be assigned to C=O and COOH, respectively.²³ The peaks at 1187 cm⁻¹ and 1043 cm⁻¹ can be attributed to the stretching vibrations of C–O and symmetric stretching vibration of C–O–C, respectively. These typical absorption bands suggest that both CA-CDs and N-CDs1 contain plenty of –COOH groups.²⁴ Note that the N-CDs1 show a weak absorption at 1558 cm⁻¹ because of the bending vibration of N–H. Absorption peak of C=O stretching vibration shifts to a lower wavenumber as compared to that of CA–CDs.²¹ The distinction indicates that N–CDs1 contain little amide groups. The surfaces of CA–CDs and N–CDs1 contain diversity of oxygen-containing groups, such as, C–O, C=O and COOH, which makes the surfaces unpassivated. However, the absorption peaks of N-CDs2 are quite different from those of CA-CDs and N-CDs1. Peaks at 3200–3400 cm⁻¹ and 1587 cm⁻¹ can be assigned to the –NH groups and those at 1658 cm⁻¹ and 1389 cm⁻¹ can be attributed to C=O and C–N stretching vibrations, respectively.²¹ Carboxyl group (–COOH) obviously disappears and the N-related bonds become strong. Amidation can take place between –COOH and –NH₂. The surface of N-CDs2 could be completely passivated by the amino groups forming passivated surfaces. The gradual changes in the surface groups of CDs with the proportion of raw materials are shown in Fig. S2.† These transformations illustrate that the surface passivation can be realized by increasing the amount of EDA in the hydrothermal reaction. Surface passivation is performed gradually. Along with the increase in EDA, the surface of CDs is covered with more amino-groups. It can be measured by FTIR, and the disappearance of the characteristic peaks of oxygen-containing groups and appearance of the N-related characteristic peaks indicate more amino-groups on the surface of CDs. The substantial differences between the passivated CDs and the unpassivated CDs are the surface groups. Passivated CDs are CDs with one pure group on their surface. Chemical compositions and bonding manners of the three types of CDs were examined by XPS, as shown in Fig. 2. Fig. 2A presents the XPS survey scan spectra of CA-CDs, N-CDs1 and N-CDs2. Peaks centered at about 285 eV, 400 eV, and 531 eV correspond to C 1s, N 1s, and O 1s, respectively. Nitrogen is not detected in CA-CDs. The XPS results indicate that both N-CDs1 and N-CDs2 are mainly composed of carbon, nitrogen, and oxygen. The component ratio for each type of CDs are listed in Table S1,† in which oxygen proportion decreases from CA-CDs, N-CDs1 to N-CDs2. N-CDs2 has nitrogen component as high as 17.45%, about 7% higher than that of N-CDs1. The higher carbon contents and lower oxygen contents in N-CDs1 and N-CDs2 than those of the CA-CDs indicates enhanced carbonization degrees with EDA.^21,22 High-resolution spectra of C 1s, N 1s and O 1s of prepared CDs were deconvolved according to a combination of 80% Gaussian-20% Lorentzian line shape (Fig. 2B–F and S3†). The component ratios are listed in Table S2.† The deconvolved C 1s spectra illustrate that C–C/C=C (284.7 eV), C–OH (286.3 eV) and O–C=O (288.9 eV) exist in CA-CDs. Five types of carbon C–C/C=C (284.5 eV), C–N (285.5 eV), C–OH (286.3 eV), C–O–C/C=N (287.3 eV) and O–C=O (288.4 eV) are present in N-CDs1.^23,26,27 The types of carbon bond in N-CDs2 are similar to N-CDs1. High-resolution N 1s spectra reveal that N exists in the forms of pyrrolic N (400.3 eV) and graphite N (401.3 eV) both in N-CDs1 and N-CDs2. However, two different types of N peaks at 398.8 eV and 399.5 eV in N-CDs2 can be assigned to pyridinic N and –NH₂ groups, respectively.^28–30 To conclude, N-CDs1 maintains N mainly in the carbon core. N contributes to constructing the carbon core and acts as functional passivated groups (amino groups) on the N-CDs2 surface. The high-resolution O 1s spectra of N-CDs1 (Fig. S3†) could be decomposed into four peaks at 530.32 eV, 531.26 eV, 532.24 eV and 533.25 eV, indicating the presence of –OH, *O=C–O, C–O, and O=C–O*, respectively.¹⁷ The decrease in the content of O=C–O* from CA-CDs to N-CDs2 implies a decline of –COOH group on the N-CDs2 surface. These results from XPS analysis are in good agreement with those from FT-IR analysis, which suggests that the carbon core structure and functional groups on the surface of the three CDs differentiate significantly.

Fig. 1 TEM images of CA-CDs (A), N-CDs1 (B), N-CDs2 (C) and size distribution in the insets, FT-IR (D) spectra of CA-CDs, N-CDs1 and N-CDs2.

Fig. 2 XPS survey scan spectra of CA-CDs, N-CDs1 and N-CDs2 (A), C 1s high-resolution XPS spectra of CA-CDs (B), N-CDs1 (C), N-CDs2 (D) and N 1s high-resolution XPS spectra of N-CDs1 (E), N-CDs2 (F).

3.2 Optical properties of CDs and dependence on pH

The different functional groups on the CDs surfaces give rise to the change of UV-vis absorption spectra. UV-vis absorption of CA-CDs, N-CDs1 and N-CDs2 in a water solution at pH values of 3, 7 and 11 are quantified, respectively. CA-CDs only have a weak absorption band attributed to n → π* transition of C=O at about 335 nm; it is slightly influenced by the pH change (Fig. S4A†).²³ Two typical absorption bands can be clearly observed in the UV-vis absorption spectra of N-CDs1 and N-CDs2 (Fig. 3A and S4B†). One peak at ∼240 nm can be attributed to the π → π* transition of C=C/C=N bonds, whereas the strong absorption band at ∼340 nm could be associated with the n → π* transition.^19,31 It is worth noting that compared with the blank CA-CDs, absorptions improve and red shifts occur in the N-CDs1 and N-CDs2 due to presence of electron donating groups (–NH₂, –NHR, etc.) through the auxochromic effect.²⁵ Moreover, the red shift phenomenon becomes obvious as the pH decreases for N-CDs1 and N-CDs2, which can be understood in terms of protonation–deprotonation of –COOH, –NH₂ and –NHR.^19,32,33 To further locate the origin of EIE features of CDs, the PL properties of the three types CDs were measured under different pH values. As expected, the emission spectra of CDs with EDE feature have a broader band than that of EIE, revealing the presence of various surface trapping sites.³⁴ Fig. S5† shows that the emission spectra of CA-CDs strongly depend on the excitation wavelengths and lead to red shift. When the excitation wavelength varies from 250 nm to 360 nm, the changes in the emission wavelengths are about 55 nm, 81 nm and 92 nm for pH values of 3, 7 and 11, respectively. N-CDs1 and N-CDs2 show weaker excitation-dependent emissions under a pH of 3 with a red shift at about 19 nm and 11 nm. However, they exhibit excitation-independent emission at pH 7 and 11 (Fig. 3 and S6†) with the same excitation wavelengths (from 250 to 360 nm). The excitation wavelengths of the three CDs were obtained at two different emission wavelengths (430 nm and 470 nm) with pH values of 3, 7 and 11, as shown in Fig. S7–S9.† The excitation spectra at different emission wavelengths follow the same trend for CDs with EIE features, whereas CDs with EDE features exhibit different excitation spectra. These outcomes suggest that different emission states exist in the carbon core and the surface wherein strong coupling emerge.³⁵ The PL properties of N doped CDs with different degrees of passivation by the amino groups are similar to those of N-CDs1 and N-CDs2 (Fig. S10 and S11†). However, the optical properties of CDs prepared only with EDA are significantly different from the other CDs prepared with CA and EDA, as shown in Fig. 4. The reason for the difference may be attributed to the lack of oxygen-containing groups.³⁶ To the best of our knowledge, this is the original finding that unpassivated surfaces (contain different surface groups) of the CDs doped by nitrogen possess adjustable excitation-dependent luminescence (EDE) and EIE properties, and Fig. 5 shows the changes in the EDE and EIE properties for all the prepared CDs.

Fig. 3 UV-vis absorption spectra of N-CDs1 (A) in water solution at pH values of 3, 7, and 11, the inset shows the samples under day light and UV lamp excitation (the excitation wavelength is 365 nm), and fluorescence spectra of N-CDs1 at pH 3 (B), 7 (C) and 11 (D) with excitation wavelengths from 250 nm to 360 nm.

Fig. 4 The optical properties of CDs prepared only with 1 mL of EDA in 10 mL of water (A) UV-vis absorption spectra of CDs in a water solution at pH values of 3, 7, and 11, the inset shows the samples under day light and UV lamp excitation (the excitation wavelength is 365 nm), and fluorescence spectra at pH 3 (B), 7 (C) and 11 (D) with excitation wavelengths from 300 nm to 360 nm.

Fig. 5 The changes of emission peak wavelength with excitation wavelength of all the prepared CDs at different pH (A) 3, (B) 7, and (C) 11.

Fig. 6 shows the decay curves of CA-CDs, N-CDs1 and N-CDs2 at different pH values. Table S3† gives the best fitted PL lifetimes of CDs according to the plots. Three decay curves of CA-CDs could be fitted with a triple-exponential decay. N-CDs1 at pH value of 3 displays a similar behavior. The decay curve of N-CDs2 at pH 3 can be fitted with a double-exponential one, indicating that different energy levels lead to the formation of various radiative transitions. This can be attributed to the involvements of different emission trap sites present in the carbon core and on the surface.²⁶ Although decay curves of N-CDs1 and N-CDs2 both at pH values of 7 and 11 fit with a double-exponential, longer lifetimes more than 90% dominates the fluorescence emission. Alternatively, the time-resolved emission of N-CDs2 can also be expressed by a mono-exponential decay function. Pure state (100%) with one lifetime component suggests a relatively uniform PL radiative process from the carbon core and surface.^37,38 Photoluminescence quantum yields (PLQY) of the prepared CDs are shown in Fig. S12 and Table S3.† Even though CDs doped with or without N own –COOH and –OH groups on their surfaces, N-CDs1 exhibit higher relative PLQY than CA-CDs. The significant difference of PLQY indicates that doping N atoms into the carbon core generates new emission traps.³⁹ Relative PLQY of the N-CDs2 reaches 67.89% at a pH of 7, which is higher than that of the N-CDs1 (29.92%) due to surface passivation by the amide groups.^40,41

Fig. 6 Decay curves of CA-CDs (A), N-CDs1 and N-CDs2 (B) at pH 3, 7 and 11.

3.3 The mechanism of pH-relied EDE and EIE properties

Based on the above results, the PL radiative processes of CDs are illustrated in Scheme 2. Carbon dots consist of aromatic structures in the carbon core and functional groups on the surface (Scheme 1), which result in a series of emission traps. The PL properties of the emission traps on the surface are sensitive to the external pH environment. For CA-CDs, aromatic structures and chemical groups are carbon backbone-type and –COOH, respectively.

Scheme 2 Representation for the FL mechanism of CA-CDs, N-CDs1, and N-CDs2. (a) Electrons excited from the ground state and trapped by the surface states; (b) excited electrons return to the ground state via a non-radiative route; (c) excited electrons return to the ground state via a radiative route.

PL intensities of the surface states could be quenched by surface charges owing to the deprotonation of –COO⁻ groups in neutral and alkaline systems. Consequently, CA-CDs show a lower PLQY and strong EDE property. For N-CDs1, the developed N-containing pyrrolic and graphite in the carbon core and amino groups on the surfaces (few) cooperatively enhance PLQY. At different pH environments, –COOH and N-containing groups can be protonized or deprotonized. For example, when pH equals to 3, N-containing groups are protonized (–NH³⁺), which decreases the PL of the nitrogen state (the nitrogen-doping introduces a new type of state), whereas –COOH groups remain and exhibit a relatively high PL of oxygen state (oxygen in CDs have different types of states corresponding to a relatively wide distribution of different energy levels to generate broad and excitation-dependent emission spectra). Nitrogen PL mainly from the carbon core and oxygen PL primarily from the surface affect interactively giving rise to the EDE property. At pH 7 or 11, –COOH can be deprotonized (–COO⁻), which declines the PL of oxygen state, whereas N-containing groups maintain a high PL in carbon core. This behavior leads to the EIE property. For N-CDs2, carbon core and the surface are greatly doped by the N-containing groups than that of N-CDs1. Nitrogen PL dominates and shows EIE capability at pH of 3, 7 and 11.

4. Conclusions

In summary, we report the original observation of the adjustable EDE and EIE features of CDs doped with nitrogen at different pH values for the first time. Unpassivated CD surfaces doped with a few nitrogens can exhibit EDE at a pH 3 and EIE feature at pH values of 7 and 11; completely passivated CD surfaces doped with sufficient nitrogen only display EIE feature at acidic, neutral and basic environments. N-Containing and O-containing structures/groups from carbon cores and the surfaces are influenced by solution pH values. Owing to the dominant behavior of the N-containing structures and groups, EDE and EIE properties of CDs can be determined by nitrogen and oxygen photoluminescence from the carbon core and the surface with a synergistic effect. Such nitrogen-doped CDs with adjustable excitation-related luminescence properties have potential applications in detecting acidity or alkalinity of a certain physiological environment by intuitively observing the wavelength change.

Acknowledgements

This study was supported by the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant no. J1103307). The authors would like to thank the Natural Science Foundation of China (no. 21271094).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20562c

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