Synthesis of Lanthanide-Functionalized Carbon Quantum Dots for Chemical Sensing and Photocatalytic Application

: Tunable photoluminescent-functionalized carbon quantum dots CQDs@Ln (TFA) 3 (Ln = Eu, Tb; TFA: triﬂuoroacetylacetone) were designed and synthesized by introducing lanthanide complexes into the modiﬁed CQDs surface through the carboxyl group. The as-prepared CQDs@Ln (TFA) 3 emit strong blue–green light with the peak at 435 nm and simultaneously show the characteristic emission of Ln 3 + under irradiation of 365 nm light in aqueous solution. Moreover, these functionalized CQDs exhibit excellent photoluminescence properties. In addition, a white luminescent solution CQDs@Eu / Tb (TFA) 3 was obtained by adjusting the ratio of Eu 3 + / Tb 3 + and the excitation wavelengths. Moreover, CQDs@Tb (TFA) 3 can be utilized as a ﬂuorescent probe for the sensitive and selective detection of MnO 4 − without interference from other ions in aqueous solution. These results provide the meaningful data for the multicomponent assembly and the photoluminescent-functionalized materials based on the modiﬁed CQDs and lanthanide, which can be expected to have potential application in photocatalytic or sensors.


Materials
Citric acid, urea, NaOH and HCl all the other reagents purchased from Aladdin Industrial Corporation (Shanghai, China), were all used directly in the as-received condition without further purification.

Characterization
Photoluminescence (PL) emission measurements were performed using RF-5301 PC spectrophotometer with 450 W xenon lamp as the excitation source. The FTIR spectra were recorded on a Nexus 912 AO446 spectrophotometer. The morphology and microstructure of the CQDs were examined by high-resolution transmission electron microscopy (HRTEM) on a Philips Tecnai G2 F30 microscope (Philips, The Netherlands) with an accelerating voltage of 200 kV. The UV absorption of the samples was measured with UV−vis spectrophotometer Lambda 750 from PerkinElmer.The interaction between the elements of the CQDs were determined with PHI-5000 X-ray photoelectric spectrometry (XPS). 1 H NMR spectra of the samples were measured with Ascend 400 spectrometer with D2O and deuterated chloroform as solvent.

Preparation and Structural Characterization of CQDs and CQDs@Ln (TFA) 3 (Ln = Eu,Tb)
CQDs were successful prepared by a hydrothermal method and confirmed by the TEM images (Figure 1a), which revealed that the CQDs were uniform nanoparticles with an average size about 2.5 nm with a standard deviation of ±0.45 nm. Furthermore, HRTEM image ( Figure 1b) shows that the CQDs have certain crystal structure with average lattices spacing about 0.23 nm and there is a uniform dispersion without apparent aggregation.
The fluorescence spectrum displays the emission of CQDs at different excitation wavelengths ranging from 340 to 500 nm and the observation reveals that the CQDs show typical wavelength-dependent photoluminescence behavior in aqueous solution (Figure 2a). It can be obviously observed the changes in emission wavelength and intensity with respect to the excitation wavelength due to the different surface states and size dispersion of the carbon nanoparticles. The strongest fluorescence emission band, located at 448 nm is also observed under 365 nm excitation, which shows blue-green fluorescence under UV light (Figure 2b). In addition, the aqueous solution of CQDs presents bright yellow photoluminescence centered at 540 nm in the emission spectrum under the excitation wavelength of 450 nm in Figure S1.  The fluorescence spectrum displays the emission of CQDs at different excitation wavelengths ranging from 340 to 500 nm and the observation reveals that the CQDs show typical wavelengthdependent photoluminescence behavior in aqueous solution (Figure 2a). It can be obviously observed the changes in emission wavelength and intensity with respect to the excitation wavelength due to the different surface states and size dispersion of the carbon nanoparticles. The strongest fluorescence emission band, located at 448 nm is also observed under 365 nm excitation, which shows blue-green fluorescence under UV light (Figure 2b). In addition, the aqueous solution of CQDs presents bright yellow photoluminescence centered at 540 nm in the emission spectrum under the excitation wavelength of 450 nm in Figure S1. CQDs@Ln (TFA)3 were successfully synthesized by introducing the unsaturated lanthanide complex Ln (TFA)3 (Ln = Tb, Eu) into the surface of CQDs via coordinated band (Scheme 1). The characteristic peaks of FTIR spectroscopy ( Figure 3) disappeared at about 1643 cm −1 (-C=O), indicating the CQDs had combined with the Ln (TFA)3 (Ln = Tb, Eu). Compared with CQDs@Tb (TFA)3, it can be clearly seen that the blue-shift from 315 nm to 343 nm, attributing to n-π* transition of-C=O. The blue-shift increases the π* energy of the whole system due to the coordination between CQDs and lanthanide complexes. (Figure. CQDs@Ln (TFA) 3 were successfully synthesized by introducing the unsaturated lanthanide complex Ln (TFA) 3 (Ln = Tb, Eu) into the surface of CQDs via coordinated band (Scheme 1). The characteristic peaks of FTIR spectroscopy ( Figure 3) disappeared at about 1643 cm −1 (-C=O), indicating the CQDs had combined with the Ln (TFA) 3 (Ln = Tb, Eu). Compared with CQDs@Tb (TFA) 3, it can be clearly seen that the blue-shift from 315 nm to 343 nm, attributing to n-π* transition of-C=O. The blue-shift increases the π* energy of the whole system due to the coordination between CQDs and lanthanide complexes. (Figure 4) In addition, the characteristic peaks of Eu3d (1135.68 ev) and Tb3d (1242.46 ev) can be observed in the XPS spectrum. (Figure S2a). Compared to the pure CQDs (531.5 ev), the O1 s ( Figure S2b) of CQDs@Tb (TFA) 3 composite (532.4 ev) has a higher binding energy, which can be attributed to the formation of ground state complexes between carboxyl groups on the surface of CQDs and rare earth ions, resulting in a decrease in the electron density of O atoms [46]. In order to further confirm the crystal structure of the composite material, we made Raman spectra of CQDs and CQDs@Tb (TFA) 3 ( Figure S3). It can also be observed that the D peak disappeared in CQDs@Tb (TFA) 3 and the G peak becomes sharp after the carboxyl coordination, indicating CQDs@Tb (TFA) 3 still has a crystalline structure. All the above experimental results confirmed the successful synthesis of CQD@Ln (TFA) 3 , and the surface functionalization did not change the crystal structure of CQDs.
Catalysts 2020, 10, 833 5 of 14 spectra of CQDs and CQDs@Tb (TFA)3 ( Figure S3). It can also be observed that the D peak disappeared in CQDs@Tb (TFA)3 and the G peak becomes sharp after the carboxyl coordination, indicating CQDs@Tb (TFA)3 still has a crystalline structure. All the above experimental results confirmed the successful synthesis of CQD@Ln (TFA)3, and the surface functionalization did not change the crystal structure of CQDs.   confirmed the successful synthesis of CQD@Ln (TFA)3, and the surface functionalization did not change the crystal structure of CQDs.

Photoluminescence Properties and Chemical Sensing Performance of CQDs@Ln (TFA) 3
The luminescence properties of CQDs@Ln (TFA) 3 were measured at room temperature in detail. The emission spectra of CQDs@Eu (TFA) 3 (a) and CQDs@Tb (TFA) 3 (b) are mainly obtained to display the corresponding characteristic peaks in Figure 5. In the CQDs@Eu (TFA) 3 (Figure 5a), the emission spectrum shows four sharp peaks located at 580, 591, 613 and 652 nm, which can be assigned to 5 [47,48]. As can be clearly seen that the emission spectra of CQDs@Eu (TFA) 3 are dominated by the intense 5 D 0 → 7 F 2 transition at 613 nm and the peak centered at 544 nm comes from 5 D 4 → 7 F 5 of Tb 3+ in CQDs@Tb (TFA) 3 . The peak in the range of 380-500 nm originated from the CQDs and the emission band of CQDs in the CQDs@Ln (TFA) 3 is centered at about 435 nm.The dual-emitting fluorescent composite is not only a change from "one" to "two", but simplifies the design of CQDs proportional sensing and expands corresponding applications.
Catalysts 2020, 10, 833 6 of 14 of CQDs@Eu (TFA)3 are dominated by the intense 5 D0→ 7 F2 transition at 613 nm and the peak centered at 544 nm comes from 5 D4→ 7 F5 of Tb 3+ in CQDs@Tb (TFA)3. The peak in the range of 380-500 nm originated from the CQDs and the emission band of CQDs in the CQDs@Ln (TFA)3 is centered at about 435 nm.The dual-emitting fluorescent composite is not only a change from "one" to "two", but simplifies the design of CQDs proportional sensing and expands corresponding applications. MnO4 − is commonly used as a disinfectant in agricultural and plays an important role in medicine and health. However, MnO4 − with high concentration will increase the ecological risk [49]. Therefore, identifications of these solution and ions are of great importance in the life. To identify the selective fluorescence sensing of anion ions by the CQDs@Tb (TFA)3, the quenching performance was carried out in the presence of different anion ions with a concentration of 10 −3 M at 365 nm excitation wavelength. Figure 6 shows PL emission spectra of CQDs@Tb (TFA)3 in the presence of various anion ions including CO3 2− , NO3 − , S2O8 2− , SO4 2− , I − , Br − , Cl − , CrO4 2− , Cr2O7 2− and MnO4 − under λex = 365 nm and the corresponding anion bar graph (ICQDs/ITb) are shown in illustration. The luminescent intensity of the characterization of Tb 3+ can clarify the effect of the chemical-sensing. The peak centered at 545 nm is ascribed to the 5 D4→ 7 F5 of Tb 3+ ions and the green emission is weakened with different anions ( Figure 6) Furthermore, it is obviously observed that MnO4 − quenches the intensity notably. Comparing the fluorescence intensity ratio (ICQDs/ITb) of the double emission peaks of each anion, it was observed that the value of ICQDs/ITb was the lowest when MnO4 − present. Therefore, it can be vividly demonstrated that the CQDs@Tb (TFA)3 fluorescent sensor was highly selective towards MnO4 − over the other anion ions.  Table 1. It can be seen that the LOD of this method is comparable or lower than some other reported methods [50,51]. The LOD of CrO 4 2− , Cr 2 O 7 2− and MnO 4 − can be calculated as follows: where σ is the standard deviation of the repeated detection blank solution (N = 30); I 1 is the dual fluorescence intensity ratio (I CQDs /I Tb ) of CQDs@Tb (TFA) 3 in water; I 0 is the average value of I CQDs /I Tb ; S is slope of the linear curve [49]. Moreover, CQDs@Eu (TFA) 3 has the same fluorescence properties as CQDs@Tb (TFA) 3 . As shown in Figure S5, CQDs@Eu (TFA) 3      There are two possible reasons for the quenching mechanism of CQDs@Ln (TFA)3 (Ln = Eu, Tb) by MnO4 − : First, there is overlap between the excitation of the composite material and the absorption of MnO4 − . That is, the excitation and adsorption energy competition between Tb 3+ and MnO4 − may cause fluorescence quenching, and CQDs accelerate the energy competition process [49] and further lead to the fluorescence quenching of CQDs@Ln (TFA)3. Second, we suspect that MnO4 − initially reacted with CQDs, resulting in the formation of MnO2, and Mn element adhered to the surface of the composite to form a ground state complex, which accelerated the interaction between MnO4 − and Ln 3+ , which ultimately led to fluorescence quenched of CQDs@Ln (TFA)3. This result will provide a new possibility to be applied in the photocatalysis field.

White-Light Tuning of CQDs@Eu/Tb (TFA)3
Considering that the blue-green light of CQDs, red light of Eu 3+ and green light of Tb 3+ as light emission centers, respectively, the tunable white emission in CQDs@Eu/Tb (TFA)3 can be obtained by adjusting the proportion of Eu 3+ /Tb 3+ . The emission spectrum of CQDs@Eu/Tb (TFA)3 and the corresponding CIE chromaticity diagram under the excitation wavelengths (340-365 nm)was given in Figure 10. The photoluminescence color of CQDs@Eu/Tb (TFA)3(Eu 3+ :Tb 3+ = 1:1) can be tuned from  There are two possible reasons for the quenching mechanism of CQDs@Ln (TFA) 3 (Ln = Eu, Tb) by MnO 4 − : First, there is overlap between the excitation of the composite material and the absorption of MnO 4 − . That is, the excitation and adsorption energy competition between Tb 3+ and MnO 4 − may cause fluorescence quenching, and CQDs accelerate the energy competition process [49] and further lead to the fluorescence quenching of CQDs@Ln (TFA) 3 . Second, we suspect that MnO 4 − initially reacted with CQDs, resulting in the formation of MnO 2 , and Mn element adhered to the surface of the composite to form a ground state complex, which accelerated the interaction between MnO 4 − and Ln 3+ , which ultimately led to fluorescence quenched of CQDs@Ln (TFA) 3 . This result will provide a new possibility to be applied in the photocatalysis field.

White-Light Tuning of CQDs@Eu/Tb (TFA) 3
Considering that the blue-green light of CQDs, red light of Eu 3+ and green light of Tb 3+ as light emission centers, respectively, the tunable white emission in CQDs@Eu/Tb (TFA) 3 can be obtained by adjusting the proportion of Eu 3+ /Tb 3+ . The emission spectrum of CQDs@Eu/Tb (TFA) 3 and the corresponding CIE chromaticity diagram under the excitation wavelengths (340-365 nm)was given in Figure 10. The photoluminescence color of CQDs@Eu/Tb (TFA) 3 (Eu 3+ :Tb 3+ = 1:1) can be tuned from white to blue by changing the excitation wavelength from 340 nm to 365 nm. In addition, Figure 11 shows the emission spectrum of CQDs@Eu/Tb (TFA) 3 in different ratios of Eu(TFA) 3 :Tb (TFA) 3 at the same excitation wavelength of 350 nm. It is interesting that the white emission is located at CIE region (x = 0.32 and y = 0.27) when excited with the same 350 nm laser. In summary, we can obtain white luminescence by adjusting the fluorescence emission spectra of CQDs@Eu/Tb (TFA) 3 at different excitation wavelengths when the ratio of lanthanide ions fixed and when the ratio of lanthanide ions different at the same excitation wavelength.

Conclusions
In summary, we have successfully synthesized a novel lanthanide functional carbon quantum dots dual emission fluorescent composite CQDs@Ln (TFA)3 (Ln = Eu, Tb) by introducing the lanthanide complex Ln (TFA)3 into the surface of CQDs with the carboxyl group. The as-synthesized

Conclusions
In summary, we have successfully synthesized a novel lanthanide functional carbon quantum dots dual emission fluorescent composite CQDs@Ln (TFA)3 (Ln = Eu, Tb) by introducing the lanthanide complex Ln (TFA)3 into the surface of CQDs with the carboxyl group. The as-synthesized

Materials
Citric acid, urea, NaOH and HCl all the other reagents purchased from Aladdin Industrial Corporation (Shanghai, China), were all used directly in the as-received condition without further purification.

Characterization
Photoluminescence (PL) emission measurements were performed using RF-5301 PC spectrophotometer with 450 W xenon lamp as the excitation source. The FTIR spectra were recorded on a Nexus 912 AO446 spectrophotometer. The morphology and microstructure of the CQDs were examined by high-resolution transmission electron microscopy (HRTEM) on a Philips Tecnai G2 F30 microscope (Philips, The Netherlands) with an accelerating voltage of 200 kV. The UV absorption of the samples was measured with UV−vis spectrophotometer Lambda 750 from PerkinElmer. The interaction between the elements of the CQDs were determined with PHI-5000 X-ray photoelectric spectrometry (XPS). 1 H NMR spectra of the samples were measured with Ascend 400 spectrometer with D 2 O and deuterated chloroform as solvent.

Synthesis of CQDs and CQDs@Ln (TFA) 3
The CQDs are synthesized according to the reported procedure [12,33] as follows: citric acid (1.0 g) and urea (2.0 g) is reacted at 160 • C for 6 h in DMF (10 mL). NaOH aqueous solution (50 mg·mL −1 , 20 mL) is mixed with the obtained solution when cooling to room temperature, and then centrifuged for 15 min. Immediately after the precipitate is dissolved in dilute HCl aqueous solution (5%, 20 mL) and stirred for 10 min, and then centrifuged again at 16,000 r min −1 for 15 min. The precipitate is collected, dissolved in water and centrifuged (16,000 r min −1 , 15 min) twice.
CQDs@Ln (TFA) 3 is obtained based on the combination of covalent bonds in Scheme 1. CQDs powder 50 mg obtained above, trifluoroacetylacetone (TFA) (1.5 mmol) and NaOH (1.5 mmol) were dissolved in 15 mL of absolute ethanol, reacted at 60 • C for 2 h. Then added LnCl 3 ·6H 2 O (Ln = Eu, Tb) 0.5 mmol to continue the reaction for 3 h. The mixture solution is cooled with ice water and suction filtered, and then the product was dried in an oven at 50 • C for 12 h. (TFA) 3 Typically, white light source is composed of red, green and blue primary colors. Herein, CQDs@Eu/Tb (TFA) 3 hybrid material with three luminous centers was successfully prepared via introducing Ln (TFA) 3 (Eu 3+ :Tb 3+ = 1:1) into the aqueous solution of CQDs. Subsequently, a mixed solution with white light emission is obtained by adjusting the emission spectrum of the hybrid material at different excitation wavelengths. Moreover, white light can be obtained by adjusting the ratio of different lanthanide complexes at the same excitation wavelength.

Conclusions
In summary, we have successfully synthesized a novel lanthanide functional carbon quantum dots dual emission fluorescent composite CQDs@Ln (TFA) 3 (Ln = Eu, Tb) by introducing the lanthanide complex Ln (TFA) 3 into the surface of CQDs with the carboxyl group. The as-synthesized hybrids showed excellent photoluminescence properties with the application in the chemical-sensing and photocatalytic field. The intense white emission can be obtained with the excitation wavelength at 350 nm. Moreover, CQDs@Tb (TFA) 3 can be used to selectively detect MnO 4 − in aqueous solution with the linear range from 0-50 µM. It can be drawn a conclusion that the lanthanide functionalized CQDs have potential application in photocatalytic water decomposition and further detect the residue of photocatalytic materials in water due to the special photoluminescence properties and sensing performance.