Facilely prepared carbon dots and rare earth ion doped hybrid composites for ratio-metric pH sensing and white-light emission

Abstract

A facile method was developed to synthesize fluorescent carbon-dot–Eu³⁺ hybrid composites (CD–Eu–HCs) by one-pot hydrothermal methods. The prepared composites demonstrate unique dual fluorescence which originates from the blue emission of the CDs and intrinsic photoluminescence of the Eu³⁺ ions, respectively. Moreover, such dual fluorescent characteristics show quite different responses for different pH value environments and have been developed in a ratiometric pH sensor. Lastly, they can realize white light emission by co-doping of Tb³⁺ and the color temperature becomes tunable by adjusting the relative proportion of Eu³⁺.

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Carbon dots (CDs), which have attracted great attention in recent years, represent a new type of photoluminescent (PL) nanomaterial due to their unique properties, including less toxicity, good compatibility, excellent stability and relatively easy surface modification.^1–3 In addition, the precursor materials are cheap, various and abundant, varying from small organic molecules to all kinds of environmentally friendly biomass.^4–7 To date, many methods have been developed to prepare various CDs, such as electrochemical synthesis,^8,9 hydrothermal method,^10,11 microwave,^12,13 laser ablation,^14,15 plasma treatment,¹⁶ etc.^17,18 CDs demonstrate molecular like optical properties which depend strongly on the surface groups and the fabrication conditions and most of them show a kind of similarity, the excitation-dependent emission behaviour in which the blue-green emission is dominant.^19–21 Only a few shows yellow and red even tunable emission.^22,23 In addition, the photoluminescence of most solid-state CDs could be quenched owing to its strong self-absorption.²⁴ These shortcomings limit their applications in many aspects, like daily used white light emitting materials, which are usually composed of three primary colors, blue, green, and red. Also, the bioimaging and biosensing usually require the red or near infrared emission materials to avoid the disturbance from the luminescence of biological tissues under ultraviolet-blue light excitation.²⁵

On the other hand, rare earth (RE) ions show unique PL properties, like intense and sharp line emissions, long lifetime and high quantum efficiency. Among all lanthanide ions, Tb³⁺ and Eu³⁺ are the best for luminescent green and red emitters. Although some lanthanide ions show tunable optical properties, the strong RE-based emitter is still rare. It is expected that the tunable emission can be realized by combining CDs with lanthanide ions. Some lanthanide complex has been developed to functionalize CDs, however, the method used for preparation needs relative complex treatment and procedure.^26,27

In this work, a new type of CDs–Eu–HCs was synthesized by one-pot hydrothermal method as illustrated in Scheme 1. In a typical synthesis, 1 mmol ethylenediaminetetraacetic acid tetrasodium salt (EDTA·4Na) and 0.25 mmol europium nitrate hexahydrate (Eu(NO₃)·6H₂O) was dissolved in deionized water and stirred to transparent solutions, then transferred into Teflon and heated to 200 °C for 5 hours. After cooling down to room temperature, the obtained faint yellow solution was centrifuged by adding moderate isopropanol at 10 000 rpm, then the precipitate was redispersed into deionized water. This procedure was repeated three times to remove excess unreacted materials. The homogeneous solution was very stable, which could be maintained at room temperature for several months. The control CDs were prepared in the same condition without adding the europium nitrate salts.

Scheme 1 Schematic illustration about the formation process of CDs–Eu–HCs.

The formation of the CDs–Eu–HCs was confirmed by transmission electron microscope (TEM) image, as shown in Fig. 1(a)–(d). The resulting products appear as spherical particles with good monodispersity (see 1(a)). The detailed structure can be clearly seen in Fig. (1b). There are many small dark black dots packaged by organic materials in the bigger nanospheres, which can be easily observed after electron beam bombarding several minutes. The dark dots can be demonstrated to be the CDs by the high resolution TEM (HRTEM) measurements, because well-resolved lattice fringes with interplanar spacings of 0.25 nm and 0.34 nm were observed, corresponding to the (100) and (020) diffraction facets of graphite.^10,20,28 The mean size of the CDs–Eu–HCs was determined to be 45 ± 12 nm, as shown in Fig. 1(d). The typical XRD pattern of CD–Eu–HCs show main peak centring around 25°, corresponding to the interlayer spacing in graphite (0.34 nm), as shown in Fig. S1.† The pattern is similar to the control CDs, except the main peak shows a little shift toward large angle and the reason may come from the amorphous structures formed by the surrounded organic materials.^5,29

Fig. 1 (a) The TEM graph of prepared CDs–Eu–HCs. (b) Local structure details after electron beam bombarding. (c) HRTEM graph of one composite particles, the red circle shows the packaged CDs. (d) Size distribution of prepared CDs–Eu–HCs.

To investigate on element composition and function groups, energy dispersive X-ray spectra (EDX), Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were conducted. From the EDX integral elements distribution (Fig. S2†), the prepared CDs–Eu–HCs contain elements C, N, O and Eu, indicating the formation of multiple-elements composites. The FTIR spectra of precursor EDTA·4Na, control CDs and CDs–Eu–HCs are shown in Fig. 2. The spectra of control CDs and CDs–Eu–HCs are very similar but quite different from the precursor. In the spectrum of the precursor, the peak at 3523 cm⁻¹ is the stretching O–H vibrations coming from the EDTA·4Na crystal water, vibration at 3225 cm⁻¹ corresponds to C–N vibration, 2800–3000 cm⁻¹ to the C–H stretching bands. The peaks at 1587, 1600 and 1415 cm⁻¹ can be attributed to the stretching vibration of COO⁻, respectively.⁴ The band 1225–1324 cm⁻¹ is assigned to aliphatic C–H vibrations, and the sharp peak at 1120 cm⁻¹ is the asymmetric stretching vibrations of C–N–C.^{10,17,30–32} After hydrothermal treatment, a new peak at 3420 cm⁻¹ emerges, corresponding to the stretching vibrations C–OH from hydroxyl groups, indicating the formation of CDs. And more, there are two weak peaks around 3420 cm⁻¹, coming from the newly formed amino groups. C–H stretching band, C–N–C stretching band and C=O band are broadened and weakened in CDs, which indicates multiple structures of corresponding groups on the nanoparticle surfaces. As for CDs–Eu–HCs, there are some difference comparing to the CDs. The peaks at about 3420 cm⁻¹, 1600 cm⁻¹ and 1405 cm⁻¹ are strengthened and broadened, indicating the existence of bridged EDTA in the composites and interaction among Eu³⁺ with the surface groups including C=O, –OH and –NH₂ groups.²⁶

Fig. 2 FTIR spectra of (a) EDTA·4Na, (b) CDs and (c) CDs–Eu–HCs.

The surface groups are also investigated by XPS analysis (Fig. S3†). The deconvolution of the C 1s spectrum of the CDs–Eu–HCs indicates the presence of three types of carbon bonds: sp² C=C/C–C (284.5 eV), C–O/C–N (286.4 eV) and C=O (288.1 eV). The deconvolution of the N 1s spectrum indicates that new amino groups (–NH₂) is formed in the composites. The high resolution O 1s spectra can be deconvoluted into C–OH (530.8 eV), O–C=O* (531.5 eV) and O*–C=O (532.4 eV), indicating the existence of oxygen-rich groups. The Eu 3d spectrum shows a weak peak at 1135.1 eV, belonging to the Eu³⁺.³³ These results are consistent with previous EDX and FTIR results, and indicate that the composites have various elements and abundant surface groups, especially carboxyl, hydroxyl and amino groups, which can chelate rare earth ions easily. Here we propose the possible formation procedure of composites: in the hydrothermal treatments reaction, the formed CDs chelate with Eu³⁺ and connect some unreacted EDTA, both of them can bridge other CDs to form lager hybrid composites, as illustrated in Scheme 1.

The optical properties were investigated by UV-vis absorption spectra and PL spectra. The Fig. 3 represents the absorption spectra of the control CDs and the CDs–Eu–HCs. Both of them show obvious absorption bands in the UV to blue region (<450 nm), which contain the π–π*, n–π* and surface states transitions. The π–π* transition (around 230 nm) originates mainly from aromatic sp² domains, and the n–π* transition (around 320 nm) and the surface trap states relates to the C=O, –NH₂ and other surface groups.^20,21,32 The hollow around 300 nm is an artifact from the spectrometry. The slight difference of CDs and composites measured at higher concentration are shown in right inset and several weak narrow peaks can be observed in the absorption of composites, which comes from the intrinsic 4f–4f absorption of the Eu³⁺ ions.³⁴ The top-left inserted digital photos from left to right are the images of the CDs, CDs under UV light, CDs–Eu–HCs and CDs–Eu–HCs under UV light, respectively.

Fig. 3 Absorption spectra of prepared CDs and CDs–Eu–HCs. The top left inset shows the digital photo of, from left to right, CDs, CDs under UV light (365 nm), CDs–Eu–HCs and CDs–Eu–HCs under UV light, respectively. The right inset shows the different details of absorption spectra of CDs and composites.

The PL behaviour of CDs–Eu–HCs with different excitation wavelength is as shown in Fig. 4. The emissions consist of a broad band from 360 to 550 nm and a number of discrete lines from 550 to 750 nm. The first one shows a typical excitation wavelength dependent emission like most emission behaviour of the CDs as previous reported.^35,36 According to previous structure analysis, the first band should be assigned to embedded CDs in the hybrid composites. The other emission band can be sure to be the intrinsic emission of the Eu³⁺ transition between ⁵D₀–⁷F_J, (J = 0–4), according to the unique line shape and characteristic wavelength of Eu³⁺ ions.³⁷ The quantum yield of CDs–Eu–HCs is 20% under 360 nm exciting.

Fig. 4 PL spectra of prepared CDs–Eu–HCs excited by different wavelength from 320 nm to 420 nm.

The unique dual fluorescence character of prepared CDs–Eu–HCs shows different response for different pH value. The emission band of CDs is more stable than that of Eu³⁺ as pH value varying from 2 to 10, as shown in Fig. 5(a). The fluorescence intensity of the CDs shows smaller change, but the luminescence intensity of Eu³⁺ increases greatly with rising of the pH value under 365 nm excitation. The reason may be the protonation effect of the surface groups in the acidic condition and result in the combination force between Eu³⁺ ions and the surface groups is weakened.³⁸ Such characteristic of CDs–Eu–HCs can be developed to nanoparticle-based pH sensors in single excitation mode for achieving ratiometric fluorescence detection. As shown in Fig. 5(b), the ratio between PL intensity of CDs band (I_CDs) and Eu³⁺ (I_Eu) shows good linearly change in the larger range from pH = 2 to pH = 10. Compared to the traditional pH sensors with one reference, this system with two references can give more reliable and accurate measurements in a larger range pH value and shows great potential for application in monitoring chemical reactions or diagnosis of cancers.^30,39

Fig. 5 (a) PL spectra of CDs–Eu–HCs in different pH value conditions. (b) The linear relationship between pH value and the ratio of PL intensity between CDs and Eu³⁺ ions.

Finally, the CDs–Eu–HCs can realize the white light emission by doping the Tb³⁺ ions. By fixing the proportion of precursor EDTA·4Na and Tb³⁺ to 1 : 0.1, the doped composite samples with the relative proportion of Eu³⁺ used 0.1, 0.2 and 0.3 are prepared and, for simplicity, named sample A, B and C, respectively. The PL spectra of three samples are shown in Fig. 6. The inset demonstrates the CIE (Commission Internationale Ed I'eclairage) coordinates. The coordinates of sample A is (0.26, 0.26), and the correlated color temperature (CCT) is 17 000 K giving pale blue white light. With increasing proportion of Eu³⁺, the coordinates of sample B and C shift to (0.34, 0.30), and (0.36, 0.30), the CCT is 5245 K and 4210 K, respectively. Both of sample B and C show warm white light (Fig. S4†), which indicate that the light color is tunable and red-shifted with increasing Eu³⁺ content.

Fig. 6 PL spectra of Tb³⁺ doped CDs–Eu–HCs with 0.1, 0.2 and 0.3 proportion of Eu³⁺ and marked with A, B, and C, respectively. The inset at the top right corner is the CIE coordinates of three different composites on the CIE 1931 chromaticity chart.

In conclusion, a new type of composites contain CDs and rare earth ions was fabricated successfully by simple one-step hydrothermal way. The final product is spherical nanoparticles and shows good monodispersity and stability. Based on structure characterization, it is found that the composites are consisted of CDs and packaged organic materials. Further study indicates that both the composites and pure CDs show abundant surface groups, and these groups can chelate with Eu³⁺. Several CDs connect each other, forming bigger composite nanoparticles. The composites show unique dual photoluminescence and this characteristic has been developed to a ratiometric pH sensors which shows good accuracy and linear relationship in the range of pH = 2–10. Lastly, by co-doping with Tb³⁺, the composites can realize white light emission, and the color can be tuned by varying the proportion of Eu³⁺.

Acknowledgements

This work was supported by the Major State Basic Research Development Program of China (973 Program) (no. 2014CB643506), the National Natural Science Foundation of China (Grant no. 51202019, 21403084), the Program for Chang Jiang Scholars and Innovative Research Team in University (no. IRT13018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11386a

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