Terbium–Aspartic Acid Nanocrystals with Chirality-Dependent

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Terbium−Aspartic Acid Nanocrystals with Chirality-Dependent Tunable Fluorescent Properties Baojin Ma,†,⊥ Yu Wu,‡,⊥ Shan Zhang,† Shicai Wang,† Jichuan Qiu,† Lili Zhao,† Daidong Guo,† Jiazhi Duan,† Yuanhua Sang,† Linlin Li,§ Huaidong Jiang,*,∥ and Hong Liu*,† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Department of Obstetrics and Gynecology, Peking University Third Hospital, Beijing 100191, China § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences National Center for Nanoscience and Technology (NCNST), Beijing 100083, China ∥ School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China ‡

S Supporting Information *

ABSTRACT: Terbium−aspartic acid (Tb−Asp) nanocrystals with chirality-dependent tunable fluorescent properties can be synthesized through a facile synthesis method through the coordination between Tb and Asp. Asp with different chirality (dextrorotation/D and levogyration/L) changes the stability of the coordination center following fluorescent absorption/emission ability differences. Compared with L-Asp, DAsp can coordinate Tb to form a more stable center, following the higher quantum yield and longer fluorescence life. Fluorescence intensity of Tb−Asp linearly increases with increase ratio of D-Asp in the mixed chirality Tb−Asp system, and the fluorescent properties of Tb−Asp nanocrystals can be tuned by adjusting the chirality ratio. Tb− Asp nanocrystals possess many advantage, such as high biocompatibility, without any color in visible light irradiation, monodispersion with very small size, and long fluorescent life. Those characteristics will give them great potential in many application fields, such as low-cost antifake markers and advertisements using inkjet printers or for molds when dispersed in polydimethylsiloxane. In addition, europium can also be used to synthesize Eu−Asp nanoparticles. Importantly, the facile, low-cost, high-yield, mass-productive “green” process provides enormous advantages for synthesis and application of fluorescent nanocrystals, which will have great impact in nanomaterial technology. KEYWORDS: tunable fluorescence, chirality, print, stereo visualization ecent research progress in fluorescent nanomaterials has demonstrated that biosafety plays an important role in their applications. The great demand for fluorescent nanomaterials requires that the fluorescent nanomaterials possess a high fluorescence efficiency, long lifetime, low synthesis cost, and mass production feasibility. Generally, fluorescent nanomaterials can be divided into two categories: inorganic and organic.1 The inorganic nanomaterials mainly include quantum dots (e.g., CdS, InP, ZnS)2−4 and lanthanide (Ln)-doped nanoparticles.5−9 Quantum dots are usually prepared via a hydrothermal method, which has a high manufacturing cost and is difficult to scale up for mass production. In addition, most types of quantum dots are toxic to humans to varying extents. Ln-doped nanoparticles are commonly prepared by doping Ln ions into an inorganic or organic matrix to endow fluorescent properties to the nanoparticles. However, Ln-doped fluorescent nanoparticles usually have relatively low fluorescence efficiency due to

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quenching at a high concentration of Ln ions in the matrix. Organic fluorescent materials are constructed from organic molecules in solution or as aggregates that possess good fluorescent properties. Because most organic fluorescent molecules are aromatic compounds, they usually are highly toxic to the human body. Moreover, organic molecule fluorescent materials normally require long synthetic routes, which makes them difficult to mass-manufacture and leads to high prices. Further, organic fluorescent molecules undergo fluorescence quenching, which limits their applications in many cases. The ideal fluorescent materials should possess a low toxicity, high fluorescence intensity, long fluorescent lifetime, high-yield production, and should be amenable to mass Received: December 5, 2016 Accepted: February 1, 2017 Published: February 1, 2017 1973

DOI: 10.1021/acsnano.6b08140 ACS Nano 2017, 11, 1973−1981

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Figure 1. Characterization of Tb−Asp nanocrystals with different chirality. (a,b) SEM images of Tb−D-Asp nanocrystals at different magnification. (c) TEM, HRTEM, and fast Fourier transform (FFT) images of Tb−D-Asp nanocrystals. (d) SEM image of Tb−L-Asp. (e) TEM, HRTEM, and FFT images of Tb−L-Asp. (f) X-ray diffraction spectra of D-Asp and Tb−D-Asp nanocrystals. (g) Fourier transform infrared spectra of D-Asp and Tb−D-Asp nanocrystals. (h,i) X-ray photoelectron spectra of Tb−D-Asp and Tb−L-Asp nanocrystals.

toxicity and good fluorescent properties and to have great applications in many fields. Meanwhile, the chirality of a molecule usually has important influence on the properties of materials.39−44 Therefore, the chirality of Asp may make Tb− Asp nanocrystals possess different fluorescence properties. Herein, we report the synthesis of terbium−aspartic acid (Tb−Asp) nanocrystals via facile chemical coordination at room temperature and ambient pressure using aqueous/organic interfacial coordination over a short time. The Tb−Asp nanocrystals can be quickly obtained in a large quantity in a precipitation process by mixing a stoichiometric ratio of Tb ions and Asp molecules in an aqueous/alcohol system. The resulting Tb−Asp nanoparticles are well crystallized in the nanoscale and possess high fluorescence without any quenching effects. Importantly, the fluorescent properties of Tb−Asp nanocrystals can be tuned by the chirality of Asp. Adjusting the ratio of dextrorotation/D and levogyration/L in Tb−Asp can change the fluorescence properties of the nanocrystals, which should be used to check the chirality of the Asp product. To test the coordination polymerization between a rare earth metal and amino acid to obtain nontoxic fluorescent nanocrystals, europium (Eu)−Asp or Tb/Eu−Asp nanoparticles were also successfully synthesized using the same method.

production. Although many efforts have been directed toward these materials, there are very few existing fluorescent materials that match the above requirements. Aspartic acid (Asp) is a naturally occurring organic molecule that is chiral and can be mass-synthesized and has been manufactured commercially. As a basic unit of proteins, Asp is nontoxic and can be absorbed in vivo, which has led to its application in many fields. Therefore, if Asp molecules can be used as building blocks for fluorescent nanomaterials, the fluorescent nanomaterials should have great applications in many fields. Recently, organometallic and coordination chemistry have been used widely to synthesize metal−organic nanoscale structures.10−13 A few studies have shown that the coordination polymerization of metal ions using amino acid ligands followed by precipitation can produce metal coordination polymer structures. In particular, two groups have demonstrated that Asp molecules can be coordinated with Cu2+ and Zn2+ ions to easily produce Cu/Zn−Asp nanofibers due to the unique molecular structure of Asp.14,15 These results provide us with a starting point to synthesize stable fluorescent nanomaterials by coordinating rare earth ions with Asp molecules. Terbium (Tb) is a very popular rare earth element for construction of fluorescent materials,16−18 and because its outmost electronic structure is similar to Zn and Cu ions, Tb should have the same ability to coordinate with Asp to form fluorescent nanoparticles. The mechanism of Tb−Asp nanoparticles synthesis relies on the fact that Asp can generate coordination polymers with transition metal ions by forming coordinative bonds through the amino and carboxyl groups on Asp.19,20 Based on this idea, we proposed to synthesize Tb−Asp nanostructures via coordinative polymerization for fluorescence applications. The proposed fluorescent material is expected to possess low

RESULTS AND DISCUSSION Both Tb−dextrorotary Asp (Tb−D-Asp) and levorotatory Asp (Tb−L-Asp) were synthesized. The morphology of the Tb−DAsp and Tb−L-Asp nanocrystals was characterized using a scanning electron microscope (SEM) and transmission electron microscope (TEM) (Figure 1a−e). The Tb−D-Asp nanocrystals were uniform with sheet morphology and were approximately 15 nm in diameter (Figure 1a−c). The Tb−D-Asp nanocrystals possessed a hexagonal lattice structure, and the 1974

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Figure 2. Fluorescence difference between Tb−D-Asp and Tb−L-Asp nanoparticles. (a) Excitation spectrum at 543 nm; (b) emission spectrum at 378 nm excitation. Inset: Image of Tb−D-Asp and Tb−L-Asp under UV light. (c) emission spectrum at 488 nm excitation; (d) fluorescence lifetime of Tb−D-Asp and Tb−L-Asp under 378 nm.

characterization results of Tb−Asp, we can find that Tb−Asp nanocrystals have the same HRTEM and fast Fourier transform (FFT) results with hexagonal graphene network, and the location of XRD diffraction peak of Tb−Asp is also similar to the hexagonal graphene network.45−49 Based on the lattice images of HRTEM and FFT patterns and the results from TEM observation, we can deduce that Tb−Asp nanocrystals are disk-like with hexagonal graphene-like crystal structures. Both Tb−D-Asp and Tb−L-Asp nanocrystals grow along the (100) plane (shown in Figure 1c,e) to form round disk-like nanoparticles, and no preferential growth direction can be observed with the growth plane. Further, a full elemental analysis on an elemental analyzer (Vario EL CUBE, Elementar, Germany) revealed that the ratio of C/N is approximately 3.95 and 4.02 in Tb−D-Asp and Tb−L-Asp, respectively, which is close to the ratio of C/N in pure Asp (4.0). This result confirms that the Asp unit participates in a coordination reaction with Tb ions as the holonomic molecule. Fourier transform infrared (FTIR) spectroscopy was performed on Asp and Tb−Asp to investigate the change in the structure of Asp before and after Tb coordination (Figure 1g). The FTIR spectrum of Asp is consistent with those found in previous works.23−25 The blue dashed line shows the location of the β-COOH groups’ stretching vibration (1688 cm−1); the green dashed line shows the location of the αCOOH groups’ stretching vibration (1643 cm−1), and the purple dashed line shows the location of the COOH groups’ bending vibration (656 cm−1) (the insets in Figure 1g). After chemical coordination, the peaks of the COOH groups’ stretching vibration disappeared, and only the bending vibration peak remained. This means that the COOH groups have participated in the reaction. The red and yellow dashed lines show the NH2 groups’ scissoring vibration (1595 cm−1) and rocking vibration (1507 cm−1), respectively (the insets in Figure 1g). After chemical coordination, the NH2 groups’ scissoring vibration remained, but the rocking vibration

lattice distance was approximately 0.33 nm from the TEM and HRTEM results (Figure 1c). Although there was no obvious morphology difference between Tb−D-Asp and Tb−L-Asp, Tb−L-Asp nanocrystals were larger in size (∼30 nm, inset in Figure 1d,e) than the Tb−D-Asp nanocrystals. Tb−L-Asp nanocrystals also had a hexagonal lattice structure, but the lattice distance of the nanocrystals changed to 0.35 nm when the D-Asp was replaced by L-Asp (inset in Figure 1e). Meanwhile, reactant concentration was used to control the size of Tb−Asp. As shown in Figure S3, the size of Tb−Asp increases with the increased reactant concentration. The morphology of the as-purchased D-Asp and L-Asp chemicals was observed on SEM (Figure S1) to check the morphological difference between pure Asp and Tb−Asp coordinated nanocrystals. Both D-Asp and L-Asp are large platelike particles with good crystallization and are very different from the Tb−D-Asp and Tb−L-Asp nanocrystals. To illustrate the crystallization difference between pure Asp crystals and Tb−Asp nanoparticles, the X-ray diffraction (XRD) patterns of D-Asp and Tb−D-Asp were normalized and are shown in Figure 1f. The non-normalized XRD patterns of D-Asp and Tb−D-Asp, L-Asp, and Tb−L-Asp are shown in Figure S2. The XRD patterns of pure Asp show the typical peaks with high intensity, which are consistent with those determined in previous reports (D-Asp, JCPDS #23-1519, and L-Asp, JCPDS #39-1523).21,22 This means that pure D-Asp and L-Asp have high crystallinity. However, no strong diffraction peak can be seen on the XRD pattern for the Tb−Asp nanocrystals (Figure S2), which indicates that the samples lose crystallinity after chemical coordination by Tb ions. Meanwhile, the broad peak in the normalized XRD pattern of Tb−Asp is located at approximately 2θ = 21.5°, indicating that the size of Tb−Asp is very small, which is consistent with the results of SEM and TEM. The above results all demonstrate that the morphology and crystal structure of Tb−Asp nanocrystals are very different from those of Asp before coordination with Tb ions. Based on the 1975

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Figure 3. Formation schematic of Tb−Asp nanocrystals and the structural change between Tb−D-Asp (a) and Tb−L-Asp (b).

light irradiation, the fluorescence intensity of Tb−D-Asp is much higher than that of Tb−L-Asp. Hence, Tb−D-Asp nanocrystals are feasible to use as a fluorescence nanomaterial because of their nanoscale size and strong fluorescence. The intensity of the emission peak can be affected by many experimental conditions, so it is not strong evidence to support the emission difference between Tb−D-Asp and Tb−L-Asp. To study the differences in the fluorescent emission of the two samples quantitatively, the fluorescence life and quantum yield of the two samples were measured. As shown in Figure 2d, the fluorescent life of Tb−D-Asp (14 ms) was distinctly longer than that of Tb−L-Asp (10 ms). Accordingly, the measured quantum yield of Tb−D-Asp (26.67%) was 1.75-fold higher than Tb−LAsp (15.21%). To measure the durability of Tb−Asp under UV light, the Tb−Asp powder was continuously exposed to UV light, and the fluorescence intensity at different time points was measured (Figure S5). No obvious intensity decrease could be detected, even after irradiation for 8 days. Therefore, Tb−Asp possesses high fluorescence durability. Based on the structure characterization and fluorescence property measurements, we proposed a coordination polymerization mechanism and suggested a hexagonal crystal structural model for the Tb−Asp nanocrystals following a previous study on Zn−aspartic acid.15 The structural models of Tb−D-Asp and Tb−L-Asp are shown in Figure 3. For Tb−D-Asp (Figure 3a), each D-Asp molecule possesses two different carboxyl groups (α and β, inset in Figure 1g) and one amino group. After deprotonation with OH−, carboxyl groups have the ability to coordinate transition metal ions. To achieve the lowest energy in the coordination system, three D-Asp molecules coordinate Tb ions and form a structural unit. At the coordination center, Tb ions function as the nuclei to coordinate with carboxyl and amino groups to form a tetrahedron structure, with three carboxyl groups and one amino group located at the four vertexes of the tetrahedron. The three D-Asp molecules coordinate other Tb ions and form another tetrahedron, and six Tb−D-Asp tetrahedron units connect together to form a hexagonal ring-like structure. These hexagonal ring-like structures connect together repeatedly to form a hexagonal lattice crystal. This is why the morphology of Tb−Asp nanocrystals are plate-like nanosheets. For Tb−L-Asp nano-

disappeared, which indicates that the NH2 groups participated in the coordination reaction during the synthesis of Tb−Asp nanocrystals. To study the effect of chirality of Asp on the coordination center structure, X-ray photoelectron spectroscopy (XPS) was performed on Tb−D-Asp and Tb−L-Asp, as shown in Figure 1h,i. The location of peaks due to O 1s, N 1s, and C 1s are consistent with previous reports.26 The binding energy of the Tb 4d is similar to the value in other Tb-containing structures.27,28 The Tb 4d binding energy is different between Tb−D-Asp (150.05 eV) and Tb−L-Asp (149.60 eV), which suggests that the Tb ions possess different lattice fields and indicates that the crystal structure of the Tb-centered coordinated compound is different for Tb−D-Asp and Tb−LAsp. Typically, for the same chemical connection, a higher binding energy value means that the connection is more stable. Therefore, Tb−D-Asp has the more stable coordination center. To make the chirality of Tb−D-Asp and Tb−L-Asp intuitive and clear, the circular dichroism (CD) spectroscopy was performed by a CD spectrometer (J-810, JASCO). As Figure S4 shows, the absorption peaks of Tb−D-Asp and Tb−L-Asp have the opposite absorption around 215 nm, which can be attributed to the center coordination of Tb and chirality of Asp. Therefore, the properties of Tb−D-Asp and Tb−L-Asp should be different owing to the cellular response of nanomaterials to chirality following the chirality-dependent characteristic.29,30 To assess whether the chirality of Asp can affect the fluorescent properties of Tb−Asp nanocrystals, excitation and emission spectra of Tb−D-Asp and Tb−L-Asp nanocrystals were recorded and are shown in Figure 2a−c. As shown in Figure 2a, both Tb−D-Asp and Tb−L-Asp have a very similar absorption spectrum, which is consistent with the absorption spectra of Tb ions doped in other matrices.31 Unexpectedly, all of the absorption peaks of Tb−D-Asp are approximately 2.5fold higher than those of Tb−L-Asp (Figure 2a). Correspondingly, the fluorescent emission intensity of Tb−D-Asp is almost 3-fold higher than that of Tb−L-Asp, regardless of whether it is excited at 378 nm (Figure 2b) or 488 nm (Figure 2c), but the peaks are consistent with previous reports in different matrices.32,33 Fluorescent images of both types of powders are shown in the inset in Figure 2b. It is obvious that under UV 1976

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Figure 4. Relation between fluorescence intensity and the D/L-Asp ratio. (a) Fluorescence intensity changes under the different D-Asp content with a large gradient (from 0 to 100%). (b) Relation curve of (a). (c) Fluorescence intensity change under different D-Asp contents with a small gradient (from 75 to 100%). (d) Relation curve of (c).

fluorescence of Tb−Asp. Based on the above results, we proposed that Tb−Asp may have a tunable fluorescent property by coordinating Tb ions with differing ratios of D-Asp and LAsp. To confirm the ability of chirality to tune the fluorescence properties, we synthesized different samples of Tb−Asp with different ratios of D-Asp/(D-Asp + L-Asp). Emission spectra and fluorescent images under UV light are shown in Figure 4. The intensity of the peak in the Tb−Asp fluorescent spectrum increased with an increase in the ratio of D-Asp from 0 to 100% (Figure 4a). The digital picture of the emission of the samples also shows a brightness increase with an increase in the ratio of D-Asp (inset in Figure 4a). Figure 4b shows the relation between the fluorescence intensity and chirality ratio (D-Asp/ (D-Asp + L-Asp), and it is clear that the relation possesses good linearity at a high gradient from 0 to 100%. To further check the linearity of the fluorescence properties versus the D/L-Asp ratio, Tb−Asp with the ratio of D-Asp/(DAsp + L-Asp) from 70 to 100% was measured, and the result is shown in Figure 4c,d. The relationship curve still has a good linearity. These results confirmed that the ratio of D-Asp and LAsp can control the fluorescence properties of Tb−Asp. We can also use the linear relationship between the chirality ratio of D/ L-Asp and fluorescence intensity at 543 nm under irradiation at 378 nm to measure the ratio of D-Asp/L-Asp in the D/L-Asp mixture. Because the preparation of Tb−Asp coordination is facile and low cost and because the fluorescent measurement is reliable, the proposed method provides a convenient way to check the chirality ratio of an Asp product. Europium, which is another popular rare earth ion, was used to synthesize rare earth−Asp nanocrystals to further verify the coordination between rare earth ions and Asp. As expected, Eu−D-Asp and Eu−L-Asp nanoparticles could be synthesized using similar synthesis process (Figure S6a,b) and possessed

crystals, the formation process is similar to the Tb−D-Asp nanocrystals. However, the lattice spacing is approximately 0.70 nm, which is larger than the lattice spacing of Tb−D-Asp (0.66 nm) because of the coordination center change when D-Asp is replaced by L-Asp. In this hexagonal structure, the lattice spacing is calculated to be approximately 0.67 nm according to the Asp molecule space length from The Cambridge Crystallographic Data Centre (DDCC number: D-Asp/663722 and L-Asp/652520), which is similar to the measured lattice spacing (0.66 and 0.70 nm for Tb−D-Asp and Tb−L-Asp, respectively) from HRTEM images. The structure characterization results indicate that Asp participates in the coordination reaction and forms Tb−Asp nanocrystals, which leads to the loss of its original crystal structure and crystallinity. After coordination, the carboxyl groups lose their stretching vibration, and the amino groups lose their rocking vibration. All inferences from the proposed hexagonal structure are consistent with the corresponding FTIR experimental results. Therefore, the proposed hexagonal structure is efficient and reasonable. The structural models of Tb−D-Asp and Tb−L-Asp nanocrystals can be used to explain the difference in fluorescent properties between Tb−L-Asp and Tb−D-Asp nanocrystals. As shown in Figure 3a, three Asp molecules coordinate with one Tb ion and form a regular tetrahedron. However, when D-Asp molecules were substituted by L-Asp (Figure 3b), the Tbcentered D-Asp tetrahedron was distorted. The structure distortion decreased the stability and enlarged the lattice spacing from 0.66 to 0.70 nm. According to previous reports,34−36 the structure of the coordination center is more stable; the quantum yield is higher, and the fluorescence life is longer. This is why Tb−D-Asp nanocrystals possess higher fluorescence than Tb−L-Asp nanocrystals. These results suggest that chirality can affect the crystal structure of Tb−Asp and the 1977

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ACS Nano fluorescent properties similar to those of Tb−Asp nanocrystals. The main emission light of Eu−Asp is red light, which is consistent with other Eu-doped fluorescent materials.37 When Tb and Eu ions are both coordinated with Asp, the color of Tb/Eu−Asp can be tuned by adjusting the molar ratio of Tb/ Eu. When the percentage of Eu in Eu/Eu + Tb is 10%, a yellow color can be obtained under UV light irradiation (Figure S6c,d). To confirm that Tb−Asp nanocrystals have high biosafety, human preosteoblast (MC3T3-E1) cells were chosen to measure the cytocompatibility of the fluorescent materials using the cell counting kit (CCK-8). As shown in Figure S7, the cytoactivity is very high when the concentration is under 400 μg/mL. Further, propidium iodide (PI), which only stains dead cells (Figure S8), was used at different Tb−Asp concentrations. There were few dead cells at the 600 μg/mL concentration. Therefore, it is clear that the Tb−Asp nanocrystals have high security as a fluorescent material. As a mass-produced fluorescent nanomaterial, Tb−Asp can be used in fluorescent markers or for low-cost advertisement. Tb−Asp can function as a good fluorescent ink when dispersed in water, which is applicable in many fields, especially for food packaging with an anticounterfeit effect. When the “Tb−Asp” was written on paper using a paintbrush, it could not be recognized by naked eyes under normal room illumination. However, it could be clearly observed under UV light irradiation (Figure 5a). More impressively, the Chinese

To improve the environmental stability for the practical application of Tb−Asp, nanocrystals were dispersed in polydimethylsiloxane (PDMS) to obtain a round plate using a casting method.38 The Tb−Asp nanocrystals can be easily dispersed in PDMS to construct fluorescent thin film or stereoscopic structure because of their very small size and monodispersion characteristics. As shown in Figure 6a, Tb− Asp can be mixed in PDMS uniformly and can thus form transparent films. Although the Tb−Asp concentration can lower the transparency of the Tb−Asp@PDMS system, the films maintain good transparency when the doping concentration is below 200 mg/mL. When the doping concentration is 400 mg/mL, the film becomes semitransparent (Figure 6a). When the concentration is 100 mg/mL, the Tb−Asp@PDMS system (marked as Tb−Asp100@PDMS) possesses good fluorescence intensity (Figure 6b) with high transparency. Further, with an increasing Tb−Asp concentration, the emission becomes brighter. Figure 6c shows the light range and transparency of Tb− Asp100@PDMS under UV light. The circle presents the range of green light, and the figure under the Tb−Asp100@PDMS film can be observed clearly. Further, Tb−Asp100@PDMS still possesses high flexibility (Figure 6d). The stereoscopic chicken shape (Figure 6f) was prepared by a mold method using the MakerBot Software (MAKERBOT DESKTOP 3.10.0, MakerBot, USA). The stereoscopic chicken shape possesses high fluorescence intensity and clearly shows the shape under the UV light. The durability was also studied (Figure 6e). Under corrosive conditions, the Tb−Asp100@PDMS system can maintain high fluorescence intensity, even when it is soaked at pH 0 or 13 for 10 days. Therefore, Tb−Asp100@PDMS has high corrosion resistance with stable fluorescence intensity and good transparency.

CONCLUSION In summary, Tb−Asp coordination complex nanocrystals with chirality-tunable fluorescent properties can be synthesized via a facile synthesis method using Tb nitrite and Asp as reactants. The formation of plate-like Tb−Asp nanocrystals is based on a coordination polymerization mechanism by connecting a Tb center with Asp to form Tb−Asp tetrahedrons with a planar hexagonal network structure. In this coordination process, the amino and carboxyl groups play an important role not only in the formation of tetrahedrons but also in the connection of the tetrahedrons to form networked nanocrystals. Using Asp with different chirality (dextrorotatory and levorotatory) changes the stability of the coordination center and the absorption/ emission ability. Compared with L-Asp, D-Asp coordinates Tb to form a more stable center, leading to a higher quantum yield and longer fluorescence life. The fluorescence intensity of Tb− Asp increases linearly with an increase in the ratio of D-Asp in the mixed chirality Tb−Asp system, and the fluorescent properties of Tb−Asp nanocrystals can be tuned by adjusting the ratio. Tb−Asp nanocrystals possess many advantage compared with many other fluorescent nanoparticles, such as relatively high biocompatibility following low toxicity, no color during visible light irradiation, small, monodispersed nanoparticles, and long fluorescent life, which suggests that they will have great potential in many applications, such as low-cost anticounterfeit markers and advertisements by inkjet printers and fluorescent stereovisualization. In addition, other rare earth ions, such as Eu, can be used to synthesize Eu−Asp nanoparticles and Eu/Tb−Asp nanoparticles, and the color of

Figure 5. Applications of Tb−Asp as fluorescent ink. (a) Tb−Asp as the fluorescent labeling to show “Tb−Asp”. (b,c) Inkjet printing using Tb−Asp through an inkjet printer: (b) Chinese character of “university” and (c) leaves.

characters of “university” (Figure 5b) and some leaves (Figure 5c) were successfully printed on a commercial office jet printer. Similarly, the printed characters and leaves were invisible under room light, but they could be seen clearly under UV irradiation. This provides information for the use of Tb−Asp in paper confidentiality and extends its applications. The Eu−Asp and Tb/Eu−Asp nanoparticles can also be used for this application. Eu−Asp with pink emission and Tb/Eu− Asp nanocrystals containing 90% of Tb and 10% of Eu with yellow emission as ink for checking the fluorescent property are shown in Figure S9. The word “Hello” was visible in yellow and pink colors under UV light. This demonstrates that the different rare earth elements and Asp coordinated polymer nanocrystals can be used for ink in writing and printing fluorescent applications. 1978

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Figure 6. Tb−Asp@PDMS with bright and durable fluorescence. (a) Transmittance at different Tb−Asp nanoparticle concentration under bright field (b) Samples under UV light showing the brightness change. (c) Transmittance of Tb−Asp100@PDMS under UV light. (d) Flexibility of Tb−Asp100@PDMS. (e) Durability and corrosion resistance of Tb−Asp100@PDMS at different pH and different time. (f) (Top) Chicken shape mold made by MakerBot Solfware. Inset: Mold under bright light. (Bottom) Fluorescent stereoscopic chicken shape under UV light. PI staining were obtained by an inverted microscope (Olympus IX71, Japan) with a 543 nm laser source.

the emission can be tuned by changing the ratio of Eu/Tb, which will lead to the use of such materials in broader applications. Most importantly, the facile, low-cost, high-yield, and mass-productive “green” process provides enormous advantage for the synthesis and application of the fluorescent nanomaterials, which will have a great impact on nanomaterial technology.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08140. Figure S1: SEM image of original D-Asp and L-Asp. Figure S2: XRD spectra of Tb−Asp and Asp. Figure S3: Size of Tb−D-Asp under the different reactant concentration. Figure S4: CD spectra of Tb−D-Asp and Tb−LAsp. Figure S5: Fluorescence intensity of Tb−Asp under UV light at different times. Figure S6: Fluorescent spectra of Eu−D-Asp and Eu-L-Asp nanoparticles. Figure S7: Security of Tb−Asp nanocrystals. Figure S8: PI staining at the different Tb−Asp nanocrystal concentrations. Figure S9: Tb−Asp, Tb-Eu−Asp, and Eu−Asp materials as fluorescent ink (PDF)

EXPERIMENTAL SECTION Tb−Asp Nanocrystal Synthesis. First, 1 mmol NaOH and 1 mmol Asp were added to 6 mL of alcohol and stirred for 10 min. Next, 2/3 mmol Tb(NO3)3 was dissolved in 1 mL of deionized water. Then, Tb(NO3)3 aqueous solution was added to the mixture of NaOH and Asp dropwise and stirred for 20 min. After the reaction, the precipitate was collected by centrifugation (6000 rpm for 6 min) and washed with alcohol three times. One part of the powder was dispersed in alcohol for morphology characterization, and another part of the powder was dried for structure characterization, cell experiments, and fluorescence measurement. Eu−Asp or Tb/Eu−Asp samples were prepared according the same method. Characterization of Tb−Asp Nanocrystals. The morphology of Tb−Asp nanocrystals was observed by scanning electron microscopy (SEM S-4800, Hitachi, Japan) and transmission electron microscopy (TEM JEM-2100, JEOL, Japan). X-ray diffraction patterns of Tb−Asp nanocrystals and original Asp were recorded on a Bruker D8 advance powder diffractometer equipped with a sealed tube with Cu Kα X-ray source (D8Advance, Bruker, Germany). Fluorescence spectra, quantum yield, and fluorescence lifetime of Tb−Asp nanocrystals were obtained on a fluorescence spectrophotometer (FLS980, Edinburgh Instruments, Britain). FTIR spectra were obtained on a spectrometer (Nicolet Nexus 670, Thermo Electron Corporation, USA). XPS spectra were measured by X-ray photoelectron spectroscopy (ESCALAB 250, ThermoFisher SCIENTIFIC, USA). Cell Experiments. MC3T3-E1 cells were purchased from the Cell Bank of the Chinese Academy of Sciences (Shanghai, China) and cultured in 1640 medium supplemented with 10% fetal bovine serum and 1% penicillin and streptomycin. All cells were maintained in a humidified atmosphere of 5% CO2 at 37 °C, and the culture medium was changed every 2 days. Cell viability was assayed by a microplate reader (Multiscan MK3, Thermo, USA). Cell fluorescence images of

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Linlin Li: 0000-0003-1041-4533 Hong Liu: 0000-0003-1640-9620 Author Contributions ⊥

B.M. and Y.W. contributed equally to this work and should be regarded as co-first authors.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are thankful for the National Natural Science Foundation of China (Grant Nos. 51372142 and 51402063), the Fundamental Research Funds of Shandong University 1979

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(2014QY003-09, 2014JC019), and the Program of Introducing Talents of Discipline to Universities in China (111 Program No. b06015).

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DOI: 10.1021/acsnano.6b08140 ACS Nano 2017, 11, 1973−1981

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1981

DOI: 10.1021/acsnano.6b08140 ACS Nano 2017, 11, 1973−1981