English

• Carbon dots (CDs) are amorphous, quasi-spherical carbon nanomaterials with sp² hybrid carbon structures. They have a size of 1-10 nm^[1]. Its special properties such as high stability, biocompatibility, low photobleaching, and low toxicity have attracted wide attention^[2]. However, due to its low quantum yield, its development is also limited. Therefore, it has been reported that heteroatom doping^[3] and surface passivation^[4] can significantly improve the performance of CDs. Doping with heteroatoms is a low-cost and simple method that has become increasingly popular in recent years for improving the photoluminescence (PL) emission performance and regulating the electronic and optical properties of CDs. The overlapping atomic orbitals of heteroatoms and carbon atoms^[5], as well as the push-pull electronic effect of heteroatoms^[6], lead to changes in their electronic structure, nanostructure, and chemical composition. The "collaborative coupling" effect^[7-9] of multielement doping can even be used to better change the active site on the surface of CDs, thereby improving the quantum yield of CDs. They have significant impact in various fields, including fluorescence sensing^[10], bioimaging^[11], drug delivery^[12], catalysis^[13], and photothermal therapy^[14], etc. However, metal ions have more electrons and unoccupied orbitals, as well as larger atomic radii than non-metal atoms. Metal ion-doped CDs can significantly alter their electron density distribution and energy gaps, allowing for the tuning of their physical and chemical properties. For instance, Li's team^[15] developed high fluorescence quantum yield manganesedoped CDs (Mn-CDs) using a straightforward approach. The prepared CDs have a fluorescence lifetime of up to 8.56 ns and can be used to detect heavy metal Hg²⁺ at the detection limit level of nmol·L^-1. Han's research group^[16] constructed bimetallic co-doped (Ag & Cu) CDs for synergistic tumor therapeutic diagnostics using L-arginine as a precursor. The study found that Ag doping improved the photoluminescence intensity, while Cu²⁺ functionalization modulated the size of the CDs, increasing their drug-carrying capacity. At the same time, CDs doped with Mg and N were found to have higher quantum yield^[17]. It was demonstrated that not only the addition of the N element passivated the surface of CDs but also Mg and citric acid could introduce Mg into CDs through chelation. Both Mg and N elements significantly enhanced the photoluminescence of CDs, and their combined effect had a synergistic impact on the luminescence of CDs. Currently, there are fewer reports on the co-doping of CDs with metallic and non-metallic elements as fluorescent sensors.

  This study aims to better understand the effects of hydrothermal co-doped metals and nonmetals on BCDs. Using natural product chamomile as a carbon source, the effects of N and Pr co-doping on the structure, physical properties, and optical properties of BCDs were studied, and the role of N and Pr elements in surface defect passivation of BCDs was discussed. At the same time, a sensitive sensor of Pr/N-BCDs for organic pollutants 2, 4-dinitrophenylhydrazide (2, 4-DNPH) was constructed. The 2, 4-DNPH is an environmental pollutant with high toxicity and poor biodegradability. Its main sources are untreated industrial waste-water and rocket fuel exhaust emissions of rocket engines. Its slow degradation and high biotoxicity can cause serious health hazards^[18-21]. At present, there are also some technologies to detect it^[22-24]. The sensing mechanism in this study is based on the internal filtration effect (IFE) and dynamic quenching between 2, 4-DNPH and Pr/N-BCDs, which causes Pr/N-BCDs fluorescence quenching (Fig. 1). The method has good selectivity and has certain practicability.

  Figure 1

  

  Figure 1.  Diagram of the fluorescence quenching mechanism of Pr/N-BCDs

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  1.   Experimental

  1.1   Materials

  Chamomile, tyrosine, praseodymium nitrate, 2, 4-DNPH, citric acid, sodium citrate, p-nitrophenylhydrazine (p-NPH), o-nitrobenzaldehyde (o-NBA), p-nitrobenzoic acid (p-NBA), o-nitroaniline (o-NA), 2, 4, 6-trinitrophenylhydrazine (TNPH), p-nitrophenol (p-NP), 2, 4, 6-trinitrophenol (TNP), m-nitroaniline (m-NA), o-nitrophenol (o-NP), phenol (NP), and nitrobenzene (NB) were all analytically pure reagents and did not undergo any purification steps. At the same time, all standard solutions were prepared by dissolving standard reagents with ultra-pure water.

  1.2   Apparatus

  The fluorescence spectrum was obtained using a fluorescence spectrophotometer (Perkelmer LS55, USA). UV-Vis spectra were obtained using a UV-visible spectrophotometer (UV-2550, Japan). Fourier transform infrared (FTIR) spectroscopy was obtained from a spectrometer (IR Prestige-21, Japan) using the potassium bromide compression method. The X-ray photoelectron spectroscopy (XPS) spectral data was obtained on an X-ray photoelectron spectrometer (Thermo ESCALAB 250, USA). The transmission electron microscope (TEM) (JEOL JEM 2800, Japan, 20 kV) was used to observe the morphology and size of the samples. The structures of the prepared CDs were determined with powder X-ray diffraction (PXRD, Shimadzu XRD-7000) (Cu Kα irradiation, λ =0.154 18 nm, U=40 kV, I=30 mA, 2θ=10°-80°). The fluorescence lifetime of the samples was measured and the mechanism was explored using the steady-state transient fluorescence spectrometer (FLSP920, UK). The hydrothermal preparation of CDs was performed in a constant temperature drying oven (DHG-9145AF, China).

  1.3   Synthesis of the samples

  Chamomile powder (1.0 g), praseodymium nitrate (0.3 g), and tyrosine (0.3 g) were dissolved in ultrapure water (25 mL) and sonicated for 30 min to make the solution evenly mixed. The mixture was transferred to a 50 mL Teflon-lined autoclave and subjected to a hydrothermal reaction at 200 ℃ for 18 h. After the solution had cooled to room temperature, a yellowish Pr/N-BCDs solution was obtained. The solution was centrifuged at 8 000 r·min^-1 for 10 min. The centrifuge was filtered by a 0.22 μm filter membrane, and the filtrate was dried at a constant temperature of 60 ℃ to obtain brown solid powder, which was used for subsequent experiments and characterization. BCDs, Pr-BCDs, and N-BCDs were prepared under the optimal reaction conditions for comparison.

  1.4   Quantum yield determination

  Taking quinine sulfate standard material as a reference, the absorption values and fluorescence peak integral areas of BCDs, Pr-BCDs, N-BCDs, Pr/N-BCDs, and quinine sulfate at the optimal excitation wavelength were determined, and the fluorescence quantum yields of each substance were calculated according to Eq.1^[25]. In this formula, Y is the fluorescence quantum yield, S is the integrated area of the fluorescence peak, and A is the absorbance. 1 and 2 indicate the substance to be measured and the reference substance, respectively.

  $ Y_1=Y_2 \frac{S_1}{S_2} \cdot \frac{A_2}{A_1} $

  (1)

  1.5   Detection of 2, 4-DNPH

  For the quantitative detection of 2, 4-DNPH, 1.40 mL Pr/N-BCDs solution (diluted 20 times), 0.80 mL HAc-NaAc buffer solution (pH=5.00), and an appropriate amount of 2, 4-DNPH standard solution were added to the 10.00 mL system, diluted to the mark with ultrapure water and shake well, and kept for 5 min before testing. The fluorescence of Pr/N-BCDs was measured when the excitation wavelength was 320 nm, the emission wavelength was 400 nm, and the slit of excitation light and emission light were 5 nm. The change in fluorescence intensity (ΔF=F₀-F) had a good linear relationship with the concentration of 2, 4-DNPH and could be used for the quantitative detection of 2, 4-DNPH.

  1.6   Actual sample testing

  Quantitative sewage was collected from local sewage treatment plants, filtered by centrifugation. 2, 4-DNPH was tested in wastewater with the above detection method, and the standards were added and recycled.

  2.   Results and discussion

  2.1   Optimisation of the synthesis conditions

  Pr/N-BCDs with strong fluorescence and high quantum yield were produced by optimising the amount of chamomile, tyrosine and praseodymium nitrate as well as the reaction temperature and time. The study found that the fluorescence intensity of BCDs was the highest when the reaction temperature was 180 ℃ (Fig.S1a, Supporting information), the reaction time was 12 h (Fig.S1b), and the dosage of chamomile was 1.0 g (Fig.S1c). Hereon, Pr doping, N doping, and Pr and N co-doping of BCDs were studied separately. It was found that the best reaction temperature of doped BCDs was 200 ℃ (Fig. S1a), the best dosage of nitrogen source tyrosine was 0.3 g (Fig. S1d), and the best dosage of metal source praseodymium nitrate was 0.3 g (Fig.S1e). However, compared with Pr and N doping alone (12 h), Pr and N co-doping took a longer time (18 h) (Fig. S1b). Moreover, the fluorescence of Pr/N-BCDs prepared under this condition was stronger than that of N-BCDs and Pr-BCDs. This might be because the co-doping of Pr and N produced a certain synergistic effect in BCDs, and the long-term reaction could help the formation and stability of this synergistic effect, also minimize the surface defects of BCDs, and improve the filling state and the emissivity, resulting in stronger fluorescence intensity^[26]. At the same time, each carbon dot showed good water solubility. It was probably because the large amount of carbon and oxygen elements contained in the carbon source itself enhanced its hydrophilicity^[27]. The fluorescence characteristics and the subsequent detection of the Pr/N-BCDs were studied under optimal synthesis conditions.

  2.2   Characterization

  The morphology and size of BCDs, Pr-BCDs, N-BCDs, and Pr/N-BCDs had been characterized by TEM. It was found that each carbon dot was nearly spherical and had good dispersion (Fig. 2 and S2). The average particle size of BCDs was approximately 2.0 nm (Fig. 2a). Upon doping with N, the particle size of N-BCDs increased to roughly 2.6 nm (Fig.S2a). Conversely, doping with Pr led to a reduction in the particle size of Pr-BCDs to approximately 1.7 nm (Fig.S2c). However, when Pr and N were co-doped, the particle size of Pr/N-BCDs was approximately 2.4 nm (Fig. 2b), falling between the sizes observed for individual doping with Pr-BCDs and N-BCDs. This observation could be attributed to the combined effects of the two elements. The particle size of BCDs varies after doping with different elements primarily due to alterations in their lattice structure, stemming from the interactions between the doped elements and C atoms^[16]. The high-resolution TEM (HRTEM) images of N-BCDs and Pr/N-BCDs showed that they both had distinct lattice fringes with N-BCDs of 0.213 2 nm (Fig.S2b) and Pr/N-BCDs of 0.217 6 nm (Fig. 2c), which corresponded to the (100) plane of graphene^[28]. The results show that the chamomile is carbonized successfully and formed graphitized BCDs.

  Figure 2

  

  Figure 2.  TEM images and particle size distributions (Inset) of (a) BCDs and (b) Pr/N-BCDs; (c) Lattice fringes shown in HRTEM image at 5.0 and 2.0 nm scales of Pr/N-BCDs

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  The XRD patterns of BCDs, N-BCDs, Pr-BCDs, and Pr/N-BCDs are shown in Fig. 3a. BCDs had a wide diffraction peak at 30°. The diffraction peak of N-BCDs was about 23°, which corresponds to the characteristic peak (002) face of graphene. Pr-BCDs showed a diffraction peak at 43°, corresponding to the (100) plane of graphene. With the addition of Pr and N elements, the intensity of XRD diffraction peaks gradually decreased and became wider. This indicates that the grain size and crystallinity of doped BCDs were reduced^[29]. Pr/N-BCDs combined the diffraction peak properties of Pr-BCDs and N-BCDs. In general, the diffraction intensity of these BCDs was low, which means that the sample belongs to the amorphous phase.

  Figure 3

  

  Figure 3.  (a) XRD patterns and (b) FTIR spectra of BCDs, N-BCDs, Pr-BCDs, and Pr/N-BCDs

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  The composition and surface functional groups of BCDs, N-BCDs, Pr-BCDs, and Pr/N-BCDs were investigated using FTIR (Fig. 3b). The BCDs themselves were found to have double peaks in the 3 100-3 600 cm^-1 region, which are inferred to be O—H and N—H stretching vibrations. The peak at 1 645 cm^-1 might be C=O, C=C, C=N, and N=O stretching vibrations. The peak at 1 380 cm^-1 was the in-plane bending vibration of O—H and C—H, and the stretching vibration of C—O and C—N at 1 120 cm^-1. It indicates that the surface of BCDs is rich in functional groups such as hydroxyl, amino, and carboxyl groups. After doping the BCDs with Pr metal, it was found that the infrared structure maps of Pr-BCDs did not change significantly, and it was inferred that the doping of Pr metal does not significantly change the molecular structure of hydroxyl and amino groups, which might mean that the doped Pr metal content is low or well dispersed. When the BCDs were N-doped, the double peaks in the 3 100-3 600 cm^-1 region converged to one broad peak, and the peaks between 2 950 and 2 825 cm^-1 were obvious relative to those before doping. Moreover, in the range of 1 500-1 000 cm^-1, in addition to the original absorption peaks at 1 380 and 1 120 cm^-1, there was an additional peak near 1 245 cm^-1. The new peak is probably because of the doped nitrogen linking to the aromatic ring and forming a C—N stretching vibration. However, when the BCDs were doped with Pr and N, the infrared structure changed from the double peaks in the region of 3 100-3 600 cm^-1 before doping to a large broad peak. It is probably because the doping of praseodymium and nitrogen alters the local environment or interaction of the vibration of the hydroxyl group and amino group, or it is due to the formation of broad peaks by hydroxyl group bonding^[22]. The sharp peaks at 1 645, 1 380, and 1 120 cm^-1 all became broad peaks, which might mean that the number, state, or local environment of C=O, C—N, and C—O bonds or functional groups have been changed by the synergistic effect of praseodymium and nitrogen. Meanwhile, a new sharp peak appeared at 831 cm^-1, which might be related to the vibrational modes of Pr—O, Pr—N, or N—H, considering the doping of praseodymium and nitrogen. However, oxygen-containing functional groups such as carboxyl and hydroxyl groups were formed on the surface of BCDs both before and after doping, which are hydrophilic and make them highly dispersible in water.

  The XPS spectra of BCDs (Fig. 4a), N-BCDs (Fig. S3a), Pr-BCDs (Fig.S3b), and Pr/N-BCDs (Fig. 4b) show that N and Pr doping of BCDs is successful, and the doping of different elements can adjust the chemical composition and properties of BCDs. Pr-BCDs (Fig. S3c) and Pr/N-BCDs (Fig. 4c) show four peaks at 284.8, 407.0, 532.4, and 933.83 eV, indicating that they contained 0.48% and 0.28% Pr in addition to C, N and O, respectively. The 3d_5/2 peak was fitted as two peaks at 935.50 and 933.83 eV, belonging to the Pr⁴⁺ and Pr³⁺ respectively^[30]. The 3d_3/2 peak was fitted into three peaks at 952.72, 954.50, and 958.08 eV, corresponding to the Pr³⁺ and Pr⁴⁺ oxidation states respectively. By calculation^[31], the Pr³⁺ content on the surfaces of Pr-BCDs and Pr/N-BCDs was 71.55% and 68.45%, respectively, while the Pr⁴⁺ content was 28.45% and 31.55%, respectively. The Pr⁴⁺ content of Pr/N-BCDs gradually increased with the addition of the N element. High-resolution C1s spectra showed that BCDs (Fig. S3d), N-BCDs (Fig.S3e), Pr-BCDs (Fig.S3f), and Pr/N-BCDs (Fig.S3g) all contained C—C bonds (284.80 eV), C—O—C bonds (286.17 eV) and O—C=O bonds (288.10 eV). High-resolution O1s spectra showed that CQDs (Fig.S3h), N-CQDs (Fig.S3i), Pr-BCDs (Fig.S3j), and Pr/N-BCDs (Fig. S3k) all contained O—H bonds (530.74 eV), C—O bonds (531.93 eV) and C=O bonds (533.00 eV). With the introduction of Pr and N elements, the proportion of O—H and C=O bonds gradually decreased. High-resolution N1s spectra showed that BCDs (Fig. S3l), N-BCDs (Fig. S3m), and Pr/N-BCDs (Fig.S3o) all contained C=N (398.90 eV), N—C (399.50 eV), and N—H (400.50 eV), while Pr-BCDs (Fig. S3n) did not contain C=N and N—C has moved from the original 399.50 to 401.50 eV. Thus, successful doping with Pr and N is demonstrated. This is consistent with the results of infrared spectrum analysis.

  Figure 4

  

  Figure 4.  XPS spectra of (a) BCDs and (b) Pr/N-BCDs; (c) High-resolution Pr3d spectrum of Pr/N-BCDs

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  2.3   Optical properties of the samples

  UV-Vis spectroscopy and fluorescence spectroscopy were used to investigate the optical properties of BCDs, Pr-BCDs, N-BCDs, and Pr/N-BCDs. In the UV-Vis absorption spectrum of the BCDs (Fig. 5a), the absorption peak at 282 nm belongs to the π-π* transition of C=C, and the absorption peak at 322 nm belongs to the n-p* transition of C=O. After doping with N and Pr elements respectively, new absorption peaks were generated at 274 nm, which might be due to the influence of the two elements on their surface structure. When Pr and N were co-doped, in addition to the new absorption peak at 274 nm, the absorption peak at 322 nm was still retained, but its intensity was significantly reduced. This might be because the doping of Pr and N makes the conjugated system in the precursor compound smaller, which leads to the structural change of Pr/N-BCDs^[31]. The strong absorption band of Pr/N-BCDs at 322 nm was close to the optimal excitation wavelength of 320 nm, which is the fluorescence emission center of Pr/N-BCDs. From the fluorescence spectrum (Fig. 5b), it could be seen that under the optimum excitation of 365 nm, the BCDs produce maximum emission at 440 nm. The excitation and emission peak positions of the BCDs were gradually blue shift and the fluorescence intensity increased significantly with the doping of Pr and N elements. When Pr and N were co-doped, their fluorescence intensity might reach its optimum due to synergistic effects, with the maximum excitation wavelength blue shifting to 320 nm and the emission wavelength blue shifting to 400 nm. The quantum size effect of BCDs might be caused by the synergistic effect of Pr and N^[32], or the negative induction effect of graphite N reduces the π electron density, thereby promoting the radiative recombination luminescence of electron-hole pairs on the surface of BCDs^[33]. The fluorescence spectra of Pr/N-BCDs (Fig. 5c) showed that the fluorescence spectra of Pr/N-BCDs are excitation-dependent. As the excitation wavelength of Pr/N-BCDs increased, their fluorescence intensity reached its maximum value at the excitation wavelength of 320 nm and then gradually decreased. The position of the emission wavelength changed as the excitation wavelength changed. With the doping of Pr and N, the fluorescence quantum yield of BCDs gradually increased from 8% to 20%, and the quantum yield of Pr/N-BCDs was higher than that of BCDs doped with Pr and N alone. The results are shown in Table S1.

  Figure 5

  

  Figure 5.  (a) UV-Vis spectra of the samples; (b) Fluorescence spectra of the samples; (c) Excitation wavelength dependence of Pr/N-BCDs

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  2.4   Fluorescence quenching mechanism

  It could be seen from the fluorescence spectrum of Fig. 6a that 2, 4-DNPH has a certain fluorescence quenching effect on Pr/N-BCDs. Therefore, a Pr/N-BCDs fluorescence probe was constructed for the direct determination of 2, 4-DNPH. However, the luminescence mechanisms for CDs fluorescence quenching generally include static quenching, dynamic quenching, Förster resonance energy transfer (FRET), IFE, etc^[34]. We studied the fluorescence quenching mechanism of Pr/N-BCDs by 2, 4-DNPH through the effects of UV-Vis absorption and fluorescence spectra, fluorescence lifetime, and temperature^[35]. It could be seen from their UV-Vis absorption spectra that the addition of quencher 2, 4-DNPH did not change the original absorption spectra of Pr/N-BCDs but produced the superposition of the UV-Vis absorption spectra of Pr/N-BCDs and 2, 4-DNPH. In addition, the absorption spectra of 2, 4-DNPH overlapped well with the excitation spectra and emission spectra of Pr/N-BCDs. Meanwhile, the fluorescence lifetime of Pr/N-BCDs before and after the addition of 2, 4-DNPH was measured (Fig. 6b), and it was found that the fluorescence lifetime of Pr/N-BCDs decreased from 6.29 to 5.80 ns (Table S2), but the change was not significant. Therefore, it is speculated that the mechanism of action is caused by fluorescence IFE, which is consistent with the mechanism that Cr(Ⅵ) could quench nitrogen and sulfur co-doped carbon dots (N, S-CDs) studied by Chen et al^[36]. According to the Stern-Volmer equation, F₀/F=1+ K_svc_Q (F₀ and F: fluorescence intensity in the presence and absence of the quencher respectively, c_Q: the concentration of the quencher), the plots of F₀/F vs c_Q obtained showed that the slope of high temperature was greater than that of low temperature (Fig. 6c), which is also consistent with the mechanism that the increase of temperature leads to the enhancement of the dynamic quenching effect^[37]. The above analysis shows that the fluorescence quenching of Pr/N-BCDs by 2, 4-DNPH in this system may be caused by the IFE and dynamic quenching.

  Figure 6

  

  Figure 6.  (a) UV-Vis and fluorescence spectra of Pr/N-BCDs and Pr/N-BCDs+2, 4-DNPH, (b) Fluorescence attenuation curves of Pr/N-BCDs and Pr/N-BCDs+2, 4-DNPH; (c) Plots of F₀/F vs c_Q at different temperatures

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  2.5   Selective and interference determination

  To explore the feasibility of Pr/N-BCDs detection of 2, 4-DNPH, 10 μmol·L^-1 different nitro organic contaminants were taken in 1 mL solution separately, and their effects on the emission intensity of Pr/N-BCDs were analyzed according to experimental methods (Fig. 7a). Compared with p-NPH, o-NBA, p-NBA, o-NA, TNPH, p-NP, TNP, m-NA, o-NP, NP, and NB, 2, 4-DNPH has the best fluorescence quenching effect on Pr/N-BCDs. On this basis, a fluorescent probe for the detection of 2, 4-DNPH was constructed using Pr/N-BCDs. In addition, the influence of possible co-existing substances and other ions in the detection of 2, 4-DNPH (10 μmol·L^-1) was also investigated (Fig. 7b). The results showed that 400-fold K⁺ and Pb²⁺, 100-fold p-NPH, o-NBA, o-NP, NB, Cu²⁺, Co²⁺, Ca²⁺, and Mg²⁺, 50-fold Cr(Ⅵ), p-NBA, TNPH, TNP, and NP, 20-fold o-NA, p-NP, and m-NA and 10-fold Fe³⁺ interfered with the system within ±5%. Therefore, the prepared Pr/N-BCDs have good selectivity and anti-interference for the determination of 2, 4-DNPH.

  Figure 7

  

  Figure 7.  (a) Effects of different organic pollutants on fluorescence quenching intensity of Pr/N-BCDs; (b) Influence of possible coexisting interfering substances on the sensing of 2, 4-DNPH by Pr/N-BCDs

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  2.6   System condition optimization

  The effect of the pH environment on the performance of the sensor was investigated. It was found that the fluorescence of the system tended to be stable in the pH range of 2.0-6.0 (Fig. 8a). At the same time, different buffer solution systems with pH=5.00 were studied experimentally (Fig. S4a). When 0.80 mL of HAc-NaAc buffer solution with pH=5.00 was selected, the sensor had the best detection performance (Fig. S4b). This indicates that the functional groups or charge states on the surfaces of 2, 4-DNPH and Pr/N-BCDs in this pH environment are in the best state conducive to interaction. To obtain the most sensitive detection of this sensor for 2, 4-DNPH. Further study showed that when the dosage of Pr/N-CQDs (diluted 20 times) was 1.40 mL (Fig. 8b), the fluorescence quenching effect of the system was the best and the fluorescence quenching effect could remain stable for at least 6 h (Fig. 8c) after reaction for 5 min at room temperature (Fig. 8d). As a result, the sensor's response conditions are mild and the response is fast.

  Figure 8

  

  Figure 8.  Effect of (a) pH, (b) amount of Pr/N-BCDs, (c) reaction time, and (d) temperature on the sensing of 2, 4-DNPH by Pr/N-BCDs

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  2.7   Standard curve

  Under optimal conditions, the sensing effect of Pr/N-BCDs as fluorescent probes was investigated. As shown in Fig. 9, with the addition of 2, 4-DNPH, the fluorescence intensity of Pr/N-BCDs at 400 nm gradually decreased, and the quenching degree (ΔF=F₀-F) showed a good linear relationship (standard curve) with the concentration of 2, 4-DNPH, with a linear range of 0.50-20 μmol·L^-1 (Inset of Fig. 9). The regression equation was ΔF=10.63c+10.48 (R²=0.996 7), and the detection limit was 2.1 nmol·L^-1 according to the formula: LOD=3σ/k, where LOD is the limit of detection, σ is standard deviation, and k is slope.

  Figure 9

  

  Figure 9.  Fluorescence spectra of Pr/N-BCDs with different concentrations of 2, 4-DNPH added

  c_{2, 4-DNPH}=0, 0.50, 0.70, 0.80, 1.0, 4.0, 5.0, 6.0, 7.0, 8.0, 10, 15, 20 μmol·L^-1, respectively; Inset: the standard curve for the sensing of 2, 4-DNPH by Pr/N-BCDs.

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  Data from other 2, 4-DNPH sensors were compared with the linear range and detection limit obtained (Table 1). The results indicate that the detection method in this work has a wider linear range and lower detection limit compared to other literature in Table 1.

  Table 1

  

  Table 1.  Comparison of methods for the detection of 2, 4-DNPH

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  Test method	Detection range / (μmol·L-1)	LOD / (μmol·L-1)	Reference
  Fluorescence method (cellulose/test paper)	0.0-1.0	0.16	[38]
  Electrochemical method (composite materials)	0.50-90	0.080	[39]
  Electrochemical method ((Cu4P4W30/PDDA-GO)7)	1.0-40	0.036	[23]
  Fluorescence method (Zn-MOF)	0.10-5.0×102	0.10	[40]
  Fluorescence method (CTS-NA-OAc)	0.0-40	0.49	[41]
  Fluorescence method (carbon dots)	0.50-20	0.002 1	This work

  2.8   Actual sample determination

  To evaluate the precision and accuracy of the developed fluorescent probe, 2, 4-DNPH detection was performed in wastewater. Different concentrations of 2, 4-DNPH standard solutions were added to sewage samples for spiking recovery experiments. In summary (Table 2), the recovery rate of 2, 4-DNPH ranged from 95.00% to 101.0%. The relative standard deviation (RSD) of the method was less than 5%. The results indicate that the constructed method can be used for the detection of 2, 4-DNPH in wastewater. Compared with other literature, the method developed in this study is fast, environmentally friendly, has a wide linear range, and is cost-effective.

  Table 2

  

  Table 2.  Determination of 2, 4-DNPH in sewage samples through the spiked recovery method

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  Concentration / (μmol·L-1)			Recovery / %	RSD/%
  Original found	Added	Detected
  —	1.00	1.01	101.0	3.5
  —	5.00	5.04	100.8	1.9
  —	10.0	9.50	95.00	3.4

  3.   Conclusions

  In summary, we prepared Pr/N-BCDs by co-doping of Pr and N elements using biomass chamomile as carbon source. The prepared Pr/N-BCDs had uniform size and good water solubility. After doping, the grain size and crystallinity of BCDs decreased. Due to the synergistic effect of Pr and N elements, the fluorescence quantum yield of doped BCDs was increased from 8% to 20%. Finally, based on Pr/N-BCDs with good selectivity for 2, 4-DNPH, a fluorescence probe with wide linear range and low detection limit was successfully constructed. The quenching mechanism was investigated and it was speculated that the quenching might be caused by energy transfer and dynamic quenching. The method had been successfully applied to the determination of 2, 4-DNPH in sewage with a good recovery rate. It has great potential in the detection of 2, 4-DNPH.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 22063010), the Natural Science Foundation of Shaanxi Province (Grant No. 2022QFY07-05), and Yan'an Science and Technology Plan Project (Grants No.2022SLJBZ-002, 2023-CYL-193). Conflicts of interest: The authors declare no competing financial interest.
  
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  Figure 1  Diagram of the fluorescence quenching mechanism of Pr/N-BCDs

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  Figure 2  TEM images and particle size distributions (Inset) of (a) BCDs and (b) Pr/N-BCDs; (c) Lattice fringes shown in HRTEM image at 5.0 and 2.0 nm scales of Pr/N-BCDs

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  Figure 3  (a) XRD patterns and (b) FTIR spectra of BCDs, N-BCDs, Pr-BCDs, and Pr/N-BCDs

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  Figure 4  XPS spectra of (a) BCDs and (b) Pr/N-BCDs; (c) High-resolution Pr3d spectrum of Pr/N-BCDs

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  Figure 5  (a) UV-Vis spectra of the samples; (b) Fluorescence spectra of the samples; (c) Excitation wavelength dependence of Pr/N-BCDs

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  Figure 6  (a) UV-Vis and fluorescence spectra of Pr/N-BCDs and Pr/N-BCDs+2, 4-DNPH, (b) Fluorescence attenuation curves of Pr/N-BCDs and Pr/N-BCDs+2, 4-DNPH; (c) Plots of F₀/F vs c_Q at different temperatures

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  Figure 7  (a) Effects of different organic pollutants on fluorescence quenching intensity of Pr/N-BCDs; (b) Influence of possible coexisting interfering substances on the sensing of 2, 4-DNPH by Pr/N-BCDs

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  Figure 8  Effect of (a) pH, (b) amount of Pr/N-BCDs, (c) reaction time, and (d) temperature on the sensing of 2, 4-DNPH by Pr/N-BCDs

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  Figure 9  Fluorescence spectra of Pr/N-BCDs with different concentrations of 2, 4-DNPH added

  c_{2, 4-DNPH}=0, 0.50, 0.70, 0.80, 1.0, 4.0, 5.0, 6.0, 7.0, 8.0, 10, 15, 20 μmol·L^-1, respectively; Inset: the standard curve for the sensing of 2, 4-DNPH by Pr/N-BCDs.

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  Table 1.  Comparison of methods for the detection of 2, 4-DNPH

  Test method	Detection range / (μmol·L-1)	LOD / (μmol·L-1)	Reference
  Fluorescence method (cellulose/test paper)	0.0-1.0	0.16	[38]
  Electrochemical method (composite materials)	0.50-90	0.080	[39]
  Electrochemical method ((Cu4P4W30/PDDA-GO)7)	1.0-40	0.036	[23]
  Fluorescence method (Zn-MOF)	0.10-5.0×102	0.10	[40]
  Fluorescence method (CTS-NA-OAc)	0.0-40	0.49	[41]
  Fluorescence method (carbon dots)	0.50-20	0.002 1	This work

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  Table 2.  Determination of 2, 4-DNPH in sewage samples through the spiked recovery method

  Concentration / (μmol·L-1)			Recovery / %	RSD/%
  Original found	Added	Detected
  —	1.00	1.01	101.0	3.5
  —	5.00	5.04	100.8	1.9
  —	10.0	9.50	95.00	3.4

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