Light modulation (vis-NIR region) based on lanthanide complex-functionalized carbon dots

Abstract

A new kind of carbon dots with an average diameter of approximately 3–5 nm were synthesized using L-lysine. Subsequently, a series of lanthanide complex-functionalized carbon dots were designed and synthesized, denoted as Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd). In addition, by changing the ratio of Eu complexes and carbon dots, four kinds of Eu complex-functionalized carbon dots were also obtained (Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, Eu-CDs-4). The derived nanomaterials were characterized by Fourier-transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and fluorescence spectroscopy. Upon visible-light excitation, these lanthanide complex-functionalized carbon dots show multicolor visible (Eu; with red, orange, grey and blue colors, respectively) and near-infrared (Sm, Er, Nd, Yb) luminescence (emission covered from 400 nm to 1700 nm spectral region).

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1. Introduction

Carbon nanodots (CDs) are a new class of carbon-based nanomaterials with sizes below 10 nm, which have unique photoluminescence properties. They were first obtained during purification of single-walled carbon nanotubes through preparative electrophoresis in 2004.¹ CDs are generally spherical, with amorphous carbon in the center and functional groups such as carboxyl and amino around the outside. Under visible or ultraviolet irradiation, the outer carbons are passivated by functional groups and cannot deliver energy to inner carbons, and energy is released by emitting visible light.² In comparison with heavy-metal based quantum dots (Q-dots), CDs have advantages in tunable photoluminescence, easy functionalization, low toxicity, good biocompatibility, and the absence of blinking.^3–5 In contrast to molecular fluorescent dyes, the CDs show broader excitation spectra and low photobleaching.⁶ Therefore, the CDs have drawn great attention due to their excellent properties,^7,8 and have been widely used for various purposes such as imaging, biosensing,^9,10 and biomedical research.¹¹

Lanthanides are becoming vital to many kinds of advanced optical materials and technologies based on a wide range of emission spectra covering the ultraviolet (UV)-visible-near-infrared (NIR) region, and those involving NIR luminescence have received growing interest in view of exciting applications in telecommunications and associated lasers and LED/OLED devices, as well as in bio-sciences, etc.^12,13 Among the trivalent lanthanide ions, Nd³⁺, Yb³⁺, Er³⁺, and Ho³⁺ show NIR emissions, while Eu³⁺ ions (red light) and Tb³⁺ ions (green light) can provide visible emission, which can be seen by naked eyes.¹⁴ The emission bands of Sm³⁺ are located in both visible and NIR regions, which makes it have more applications like biological diagnostics and fluorescent labels.

However, lanthanide ions suffer from low luminescence quantum efficiency due to their low molar absorptivity (ε ≈ 1–10 M⁻¹ cm⁻¹) and narrow absorption band.^15,16 One of the most useful strategies that has been employed to overcome this drawback is the sensitization of the lanthanide emission through the coordination with organic ligands, the so-called “antenna effect”.¹⁷ In this approach, the organic ligands possessing a high absorption co-efficient are first excited followed by transfer of the excited energy to the Ln³⁺ ions via the triplet state of the organic molecules,^18,19 such as, diketones, quinolines, phenanthrolines, cryptands, etc.^20–22 Especially, the ligands with visible light absorption have been paid to considerable attention in bio-application field due to the advantageous of excitation with visible light over ultraviolet-excitation.²³

Considering their relatively low chemical, optical, and thermal stabilities, lanthanide complexes have been excluded so far from practical applications. They were usually applied by introducing into a host matrix, including silica-based materials,¹² polymers,²⁴ liquid crystals,²⁵ and polyhedral oligomeric silsesquioxane (POSS)²⁶ to form “organic–inorganic hybrid materials”. Since there are lots of modified groups on the surface of CDs, such as carboxyl and amino group, they could combine with lanthanide complexes. In addition, the water soluble CDs can supplement the limitation of the hydrophobicity of lanthanide complexes, which is advantageous for optical bioimaging. However, there are rare reports on constructing a system based on CDs and lanthanide complexes, which offers and regulates different emission covering from visible to NIR spectral region.^27–29

In this paper, a new kind of carbon dots with an average diameter of approximately 3–5 nm were synthesized by using L-lysine. Then a series of lanthanide complexes-functionalized carbon dots were designed and synthesized based on the covalently linking between each other, denoted as Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd). Upon visible-light excitation, these lanthanide complexes-functionalized carbon dots display multicolor visible (Eu: with red, orange, grey and blue colors, respectively) and NIR (Sm, Er, Nd, Yb) luminescence (light-regulation covered from 400 nm to 1700 nm spectral region), which is of particular interest for organic light-emitting diodes (OLED) devices, biological analytical sensors, lasers, and optical amplification applications.

2. Experimental section

2.1. Materials

All the chemicals were used as received without further purification. EuCl₃·6H₂O (99.99%), ErCl₃·6H₂O (99.99%), YbCl₃·6H₂O (99.99%), SmCl₃·6H₂O (99.99%) and NdCl₃·6H₂O (99.99%) were purchased from Sigma Aldrich. 1-(2-Thenoyl)-3,3,3-trifluoroacetone (Htta, 99%), L-lysine (C₆H₁₄N₂O₂·H₂O, AR), ethanol, sodium hydroxide (NaOH, AR) were obtained from Aladdin.

2.2. Syntheses of Ln(tta)₃(H₂O)₂ (Ln = Eu, Sm, Er, Nd, Yb)

Ln(tta)₃(H₂O)₂ complexes (Ln = Eu, Sm, Er, Nd, Yb) were synthesized according to a reported method.¹⁴ An appropriate amount of 1.0 M sodium hydroxide solution was added dropwise to the Htta ethanol solution under stirring to adjust the pH value to approximately 7. Then the LnCl₃ (Ln = Eu, Sm, Er, Nd, Yb) ethanol solution was added into this mixture under stirring with the molar ratio of Ln³⁺ : Htta = 1 : 3. An appropriate amount of water was added, and the mixture was heated at 85 °C and reflux for 5 h, and then cooled using ice water. The precipitates were collected by filtration, washed with water and ethanol three times and dried at 50 °C.

2.3. Synthesis of pure carbon nanodots (CDs)

2.33 g L-lysine was dissolved into 25 g deionized water under stirring, and the solution was then added to a stainless steel autoclave (50 mL), which was placed in a drying furnace followed by hydrothermal treatment at 200 °C for 4 h. The obtained solution was centrifuged at 10 000 rpm for 10 min to remove the black precipitate. Then, the solution containing CDs was lyophilized for further use.

2.4. Syntheses of Eu complexes functionalized CDs with different ratio of Eu complexes and CDs (Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4)

0.82 g Eu(tta)₃(H₂O)₂ was dissolved in 10 mL ethanol. Then different amount (0.11 g, 0.17 g, 0.22 g, 0.27 g, respectively) of CDs was added into the above solution and kept at 80 °C under reflux for 5 h. To remove the excess lanthanide complexes, the product was dialyzed in the membrane tubing with a molecular weight cut-off of 3500 against ethanol for 48 h until the dialysate shows no visible luminescence under 405 nm LED illumination. After removing the ethanol, four Eu complexes functionalized CDs, Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4, were obtained, respectively.

2.5. Syntheses of lanthanide complexes functionalized CDs (Ln-CDs, Ln = Sm, Er, Nd, Yb)

The synthesis procedure for Ln-CDs (Ln = Sm, Er, Nd, Yb) was similar to that of Eu-CDs except that the amounts of Sm(tta)₃(H₂O)₂, Nd(tta)₃(H₂O)₂, Er(tta)₃(H₂O)₂, and Yb(tta)₃(H₂O)₂ are 0.84 mmol, 0.91 mmol, 0.98 mmol, and 0.97 mmol, respectively, and the amount of CDs is 0.22 g.

2.6. Characterization

Fourier-transform infrared (FT-IR) spectra were collected on KBr pellets in the 4000–400 cm⁻¹ region on an AVATAR370 FTIR infrared spectrophotometer. High resolution transmission electron microscopy (HR-TEM) recorded using a JEOL-2010F provided further insight into the structure and crystallinity of the products. The elemental analysis was performed with energy dispersive X-ray spectroscopy (EDX, Oxford). The excitation and emission spectra were recorded by an Edinburgh FLS920 fluorescence spectrometer equipped with a 450 W Xe-lamp as an excitation source. The X-ray diffraction (XRD) patterns were recorded with a DLMAX-2550 diffractometer equipped with a rotating anode and with a Cu Kα radiation source (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Escalab 250 spectrometer with monochromatized Al Kα excitation.

3. Results and discussion

The synthesis procedure of Ln-CDs is illustrated in Scheme 1. For the preparation of pure carbon nanodots (CDs), the L-lysine was used, which is very environmentally-friendly, and the synthesis method is very simple. The obtained pure CDs are spherical, with amorphous carbon in the center and functional groups such as carboxyl surround. The 1-(2-thenoyl)-3,3,3-trifluoroacetone (tta) ligand was employed to sensitize the lanthanide ions as the first ligand, and the carboxy groups on the surface of CDs can coordinate with the lanthanide ions via exchanging two water molecules from the first coordination sphere of the Ln(tta)₃(H₂O)₂ complexes introduced. In this way, the Ln-CDs are formed and the Ln(tta)₃ complexes are bonded to the CDs. The coordination structures of Ln-CDs nanomaterials are proposed and also charted in Scheme 1. The morphology of the CDs and Ln-CDs was investigated by high resolution transmission electron microscopy (HR-TEM). As shown in Fig. 1A, it can be observed that the synthesized pure CDs possess an amorphous globular structure with an average diameter of approximately 3–5 nm. After the functionalization with lanthanide complexes, the morphologies of Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd) can be substantially conserved in comparison with that of the pure CDs, as shown in Fig. 1B, the HR-TEM of representative Eu-CDs. The composition of the Eu-CDs was analyzed by energy dispersive X-ray (EDX) spectroscopy (Fig. 1C), and the presence of Eu element suggests the successful functionalization of Eu complex in Eu-CDs. In Fig. 1D, the typical XRD data of Eu-CDs show a wide peak centered at around 22.9 degree with an interlayer spacing of 0.388 nm (by MDI Jade based on Bragg diffraction equation).

Scheme 1 The synthesis procedure of Ln-CDs (Ln = Eu, Sm, Er, Nd, Yb), and their emission from visible region to near-infrared (NIR) region.

Fig. 1 (A) HR-TEM image of pure carbon dots (CDs), the white circle shows the CDs; (B) HR-TEM image of Eu-CDs-2; (C) the energy dispersive X-ray (EDX) spectrum of Eu-CDs-2; (D) the XRD pattern of Eu-CDs-2.

FT-IR spectra of the pure CDs, Ln(tta)₃(H₂O)₂ complex and Ln-CDs were measured and analyzed. Since the FT-IR spectra of Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd) are very similar, the representative FT-IR spectra of Eu(tta)₃(H₂O)₂ complex and Eu-CDs, as well as pure CDs are shown in Fig. S1.† The spectrum of Eu-CDs shows a strong absorption band at 1668 cm⁻¹, attributed to the carboxylic acid groups of CDs. The intense peaks at 1139 and 1204 cm⁻¹ in Fig. S1A† are due to C–O stretching, which are consistent with 1141 and 1195 cm⁻¹ in the spectrum of Eu(tta)₃(H₂O)₂ (Fig. S1B†), suggesting the functionalization of CDs with europium complex. The results above suggest that the lanthanide complexes have been conjugated on the surface of the CDs.

The surface compositions and elemental analysis for the obtained Eu-CDs were investigated by X-ray photoelectron spectroscopy (XPS) (Fig. 2). As shown in Fig. 2A, the full range XPS analysis of Eu-CDs clearly shows three peaks at 284.7, 532.9 and 1141.6 eV, which are attributed to C 1s, O 1s and Eu 3d, respectively. In Fig. 2B, the high-resolution XPS spectrum of C 1s confirms the presence of C–C (sp³, 284.7 eV), C–O (sp³, 286 eV), and C=O (sp², 288.2 eV) bonds, respectively. The three fitted peaks at 531.8, 532.9 and 535.2 eV in the high-resolution O 1s spectrum are assigned to C=O, C–O–C/C–OH and O=C–OH groups, respectively (Fig. 2C). It indicates that the Eu-CDs have plenty of carboxyl groups on the surfaces, which is consistent with the results of FT-IR spectra. The high-resolution Eu 3d spectrum was shown in Fig. 2D and there are two peaks in the Eu³⁺ 3d region. The peak around 1141.6 eV corresponds to Eu³⁺ 3d_5/2, whereas the peak at 1171.8 eV is a satellite peak.³⁰ The data indicates that the europium element does exist in the Eu-CDs nanomaterials.

Fig. 2 The X-ray photoelectron spectroscopy (XPS) spectra of Eu-CDs-2 nanomaterial (A), and its corresponding high-resolution XPS spectra of C 1s (B), O 1s (C), and Eu 3d (D).

The excitation and emission spectra of pure CDs are shown in Fig. S2† and 3, respectively. The excitation spectrum was obtained by monitoring the strongest emission at 485 nm and recorded at room temperature. As shown in Fig. S2,† the excitation spectrum of CDs contains a broad band between 325 and 480 nm with maximum peak at 396 nm, which covers a large part of visible region. With excitation wavelength increasing from 360 nm to 460 nm, the corresponding emission spectra were obtained and excitation wavelength-dependent. As shown in Fig. 3, the strongest emission band shifts from 432 to 584 nm as the excitation wavelength moves from 360 to 460 nm. Upon blue LED illumination (peak wavelength = 405 nm), the pure CDs displays blue luminescence in water, which can be easily detected by naked eyes (inset in Fig. 3).

Fig. 3 The emission spectrum of pure CDs in water (λ_ex = 360, 380, 396, 420, 440 and 460 nm, respectively). Inset: the photograph of the CDs in water under excitation with 405 nm LED light.

Fig. S3† shows the excitation spectra of Eu(tta)₃(H₂O)₂ and Eu-CDs-2 (monitored at the maximum emission of Eu³⁺ ions at 613 nm) with a broad band in the visible region from around 400 to 470 nm, which is attributed to the absorption of the ligands (tta and CDs). It can be observed that the excitation spectrum of the Eu-CDs-2 nanomaterial is different from that of Eu(tta)₃(H₂O)₂. The excitation spectrum of Eu-CDs-2 becomes broader than that of Eu(tta)₃(H₂O)₂ complex. Compared with the excitation spectrum of Eu(tta)₃(H₂O)₂ complex, the maximum excitation spectrum of Eu-CDs-2 shifts from 397 nm to 403 nm. It may be due to the decreased electron density of europium after its coordination with oxygen atoms of CDs, resulting the red-shift of the excitation spectrum of Eu-CDs-2. Fig. 4 shows the emission spectrum and photographs of samples (Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4) that by rational combination of Eu(tta)₃(H₂O)₂ complex with CDs at different concentrations (under excitation with 405 nm), as well as their corresponding Commission Internationale de l'Eclairage (CIE) chromaticity coordinates. As shown in Fig. 4A, for Eu-CDs-1, under excitation with visible light, the emission spectrum mainly shows the characteristic emission of Eu³⁺ ion with the strongest emission peak at 613 nm, but shows no emission of CDs. Along with the CDs contents increasing in Eu-CDs, the emission spectra of Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4 show both emissions of Eu³⁺ ion and CDs, producing the Eu-CDs with tunable visible colors, which can also be observed from their color change displayed in the photographs and CIE chromaticity coordinates (Fig. 4B and C). Therefore, the light modulation in visible region can be achieved by rational ratio of Eu complex and carbon dots. The Eu-CDs in this case provide access to many potential applications, such as OLED devices, biological analytical sensors, lasers, and optical amplification applications.

Fig. 4 (A) The emission spectra of Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4 (λ_ex = 405 nm); (B) photographs of Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4 in water under illumination with 405 nm LED light; (C) Corresponding Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the multicolour emissions from the samples of Eu-CDs-1, Eu-CDs-2, Eu-CDs-3, and Eu-CDs-4.

The optical properties of the Eu-CDs were further studied and it shows that the Eu-CDs have good temperature-sensitive properties. The emission spectra of representative Eu-CDs-2 were systematically investigated at different temperature and the effect of temperature (from 293 to 333 K) on the emission spectra is shown in Fig. 5A. As the temperature increases, the emission intensities of Eu³⁺ ion and CDs both decrease, in which the decreasing is rapid for Eu³⁺ ion and slow for CDs. Fig. 5B shows the temperature dependence of the ratio between the intensity of 613 nm (Eu³⁺) to 500 nm (CDs) emission (I₆₁₃/I₅₀₀). The linear relationship can be fitted as a function of I₆₁₃/I₅₀₀ = 18.9 − 0.0512T with a correlation coefficient (R²) of 0.992, where T is the ambient temperature of Eu-CDs. It can be observed that the intensity ratio of 613 and 500 nm emissions decreases monotonously with temperature increasing, which shows that the Eu-CDs are temperature-sensitive in the range of 293–333 K (covering the physiological range, 25–45 °C). As is known that the emission intensity of fluorophores decreases with increasing temperature due to thermal deactivation.^31,32 Therefore, the data included in the results indicate that the Eu-CDs could be potential candidates used as nanothermometers in the range of 293–333 K.

Fig. 5 Temperature-dependent emission spectra of Eu-CDs-2 (A). The intensity ratio of Eu³⁺ luminescence (at 613 nm) to CDs luminescence (at 500 nm) as a function of temperature (B).

The excitation spectra of the Ln-CDs (Ln = Sm, Er, Yb, Nd) are very similar to that of Eu-CDs (Fig. S2†), also show broad bands assigned to the absorption of the ligands. The excitation wavelength was set at 405 nm that extends to the visible region, and the emission spectra of Ln-CDs (Ln = Sm, Er, Yb, Nd) are shown in Fig. 6 and display characteristic NIR emissions of the corresponding lanthanide ion, which are very similar with those of the corresponding Ln(tta)₃(H₂O)₂ complexes (Fig. S4†). As shown in Fig. 6A, the Sm-CDs display three characteristic NIR luminescence of Sm³⁺ ion, which all come from the ⁴G_5/2 excited state, and the strongest emission at 941 nm is assigned to ⁴G_5/2 → ⁶F_5/2 transition. The emission spectrum of Yb-CDs is displayed in Fig. 6B, and the spectrum exhibits a broad band of 900–1110 nm with a sharp peak at 980 nm (upon excitation with 405 nm) assigned to the ²F_5/2 → ²F_7/2 transition. The materials with characteristic Yb³⁺ luminescence has been proven very useful in lasers and optical amplifiers.³³ As shown in Fig. 6C, upon excitation with visible light, the Nd-CDs show the characteristic Nd³⁺ ion emissions with band at 895, 1059, and 1326 nm, attributed to the f–f transitions of ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2, and ⁴F_3/2 → ⁴I_13/2, respectively. It is worthy to point out that the Nd-CDs show two significant emissions in the NIR region, one at 1.06 μm often used in laser system, and the other at 1.3 μm with potential applications for optical amplification, an important spectral window for optical telecommunication. Fig. 6D shows the emission spectrum of Er-CDs (λ_ex = 405 nm). A broad band from 1425 nm to 1655 nm was obtained with the strongest emission at 1534 nm, which is attributed to the transition from the excited state (⁴I_13/2) to the ground state (⁴I_15/2) of Er³⁺ ion. As is well known, the emission at 1.53 μm is located in the third telecommunication window and can be used in telecommunication area according to the low attenuation of this energy transition.³⁴

Fig. 6 The near-infrared emission spectra of Sm-CDs (A), Yb-CDs (B), Nd-CDs (C), and Er-CDs (D) nanomaterials (λ_ex = 405 nm).

As discussed above, the visible and NIR-luminescence was obtained in the corresponding Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd) upon excitation with visible absorption of the ligands (tta and CDs), respectively. That is, the emissions from the Ln-CDs do arise by sensitizing the lanthanide ions from the ligands moiety, because no absorption of the lanthanide ions occurs at the excitation wavelength (405 nm). Thus, it is obvious that the intramolecular energy transfer does happen between the ligands and the lanthanide ions in the corresponding Ln-CDs. Therefore, it can be concluded that the lanthanide complexes are coordinated with the CDs in the corresponding Ln-CDs.

4. Conclusion

In summary, we have successfully introduced lanthanide complexes into CDs via covalent bonding. The obtained Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd) nanomaterials preserve the original morphology of CDs. Fluorescence studies show the lanthanide complexes are coordinated with the CDs in the corresponding Ln-CDs, and sensitized by visible light the Ln-CDs (Ln = Eu, Sm, Er, Yb, Nd) display characteristic visible (Eu³⁺) as well as near-infrared (Sm³⁺, Er³⁺, Yb³⁺, Nd³⁺) emission (emission covered from 400 nm to 1700 nm spectral region). In addition, the light modulation in visible region can be achieved by rational ratio of Eu complex and carbon dots. Therefore, this work may open up new perspectives for developing nanomaterials with potential applications in biotechnology, OLED devices, lasers, and optical amplification applications.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 21571125 and 21231004), Innovation Program of Shanghai Municipal Education Commission (13ZZ073), the Science and Technology Commission of Shanghai Municipality (13NM1401101, 14520722200, 13DZ2292100), Shanghai Rising-Star Program (14QA1401800), and the project from State Key Laboratory of Rare Earth Resource Utilization (RERU2016013). We are also grateful to Instrumental Analysis & Research Center of Shanghai University.

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Footnotes

† Electronic supplementary information (ESI) available: The FT-IR spectra of Eu-CDs-2, Eu(tta)₃(H₂O)₂, and pure CDs. Excitation spectrum of pure CDs in water. Excitation spectra of Eu(tta)₃(H₂O)₂ and Eu-CDs-2. The near-infrared emission spectra of Sm(tta)₃(H₂O)₂, Yb(tta)₃(H₂O)₂, Nd(tta)₃(H₂O)₂ and Er(tta)₃(H₂O)₂ complexes. See DOI: 10.1039/c6ra06709c

‡ Jinghua Liu and Xiaoqian Ge equally contributed to this work.

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