Novel rod-Y2O3:Eu3+@0.01YVO4:Eu3+ with open core/shell nanostructure and “off-and-on” fluorescent performance

College of Chemistry & Environment, South Laboratory of Materials for Energy Conversi 510006, China. E-mail: tiesl@scnu.edu.cn College of Information & Optoelectronic Normal University, Guangdong Provinci Functional Materials and Devices, Guangzh edu.cn † Electronic supplementary information image and PL spectra of Y2O3:Eu @0. pure Y2O3:Eu , pure YVO4:Eu , Y2O3: 0.4YVO4:Eu 3+ in the presence of increasi response of rod-Y2O3:Eu @0.01YVO4:E concentration of Cu; uorescen 0.01YVO4:Eu –Cu (10 8 mol L ) in th of glutamic acid and leucine. See DOI: 10 Cite this: RSC Adv., 2017, 7, 52955


Introduction
2][3][4][5][6][7] They are highly popular among researchers and are gaining increasing importance in our daily lives.Y 2 O 3 :Eu 3+ and YVO 4 :Eu 3+ are well-known red-emitting phosphors.Y 2 O 3 :Eu 3+ shows excellent luminescent performance with high brightness, color purity, and stability. 5,8YVO 4 :Eu 3+ also shows prominent luminescent performance owing to the strong 5 D 0 -7 F 2 transition due to the efficient energy transfer from VO 4 3À to Eu 3+ , which provides a high quantum yield of photoluminescence (PL q z 70%). 9,10Rare-earth-doped nanocomposites with core/shell structures have been investigated by many researchers in attempts to obtain outstanding luminescent performance: examples with close and continuous shells on their cores include Y 2 O 3 :Eu 3+ @SiO 2 , 11 CePO 4 :Tb/LaPO 4 , 12 SiO 2 @YVO 4 :Dy 3+ /Sm 3+ , 13 g-Fe 2 O 3 @YVO 4 :Eu 3+ , 14 and Y 2 O 3 :Eu 3+ @SiO 2 @YVO 4 :Eu 3+ . 15owever, these reported nanostructures had fully coated shells, and therefore showed independent shell-only or coreonly PL properties on energy transfer from the shell (or core) to the core (or shell).It is reasonable to assume that their lifetimes in both the core and shell depended on the 5 D 0 -7 F 2 transitions in the Eu 3+ ions, giving different results for each system. 15The preparation processes for obtaining these coreshell nanostructures were complex; furthermore, their uorescences were not signicantly affected by trace surrounding molecules and ions, as shown by the limited variation in the before-and-aer uorescence intensities.Therefore, these nanomaterials generally cannot be used as uorescent probes.
Recently, quantum dots (QDs) have drawn great attention because of their unique physical and chemical properties and potential applications. 16These nanoparticles have been employed in diverse applications such as light-absorbing components of solar cells [17][18][19] and light-emitting components of LEDs and lasers, 20 especially as uorescent probes for trace detection of some molecules and ions. 21"Off-and-on" uorescent probes are generally composed of many kinds of QDs and have attracted great attention because of their sensitive response to some trace ions and molecules.For example, diethyldithiocarbamate-functionalized CdSe/CdS QDs have been used as a uorescent probe for copper ion detection, 22 manganese oxide QDs have been applied as a uorescent probe for the detection of metal ions in aqueous media, 23 and carbon dots have been used as uorescent probes for "off-on" detection of Cu 2+ and L-cysteine in aqueous solutions. 24However, agglomeration of these QDs can easily occur during their preparation and preservation as well as application because of their high surface energies, making them unstable.In addition, their PLs are easily affected by many external factors.As a result, their uorescence properties change very easily.
In this study, we develop a luminescent nanomaterial with wide-wavelength response and good stability for use as a PL probe in aqueous solutions.Specically, a novel open core-shell nanostructure, rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ , was designed and prepared, in which the core was partially coated with a quantum shell.In the open nanostructure proposed in this paper, the quantum shell plays a key role in the sensitivity of the material to trace ions.

Synthesis process
All the chemical reagents were analytical grade.Aqueous solutions of the precursors, Y(NO 3 ) 3 and Eu(NO 3 ) 3 , were obtained by dissolving 1.061 g of Y 2 O 3 and 0.0105 g of Eu 2 O 3 (99.99%) in dilute HNO 3 , and the resulting mixtures were then heated to remove excess HNO 3 solution.Next, 2 mol L À1 of NaOH solutions were used to adjust the pH of the above mixtures to 9. Finally, 0.0110 g of NH 4 VO 3 (molar ratio n V : n Y ¼ 0.01) was dispersed into the above suspensions, which were then subjected to ultrasonication for 15 min.The prepared precursor solutions were then transferred to a 100 mL autoclave and heated at 180 C for 24 h.Aer cooling, the sample was collected, washed several times with water and ethanol, dried at 60 C, and then annealed at 800 C for 3 h.The nal sample was identied as the partially coated core@shell rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ .

Fabrication and measurement of Cu 2+ (off) and amino acid (on) sensor
Rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ (5 mL, 0.03 mg mL À1 ) was placed in a centrifugal tube.Then, Cu(NO 3 ) 2 was also added to the container, giving a nal concentration of Cu 2+ ranging from 10 À4 to 10 À10 mol L À1 .Aer shaking well, the uorescence spectra of the solutions were monitored under excitation at 280 nm.

Characterization
The X-ray diffraction (XRD) patterns of the prepared samples were examined on an X-ray powder diffractometer (AXSD8-Advance, Bruker Company, Germany) with Cu Ka radiation (l ¼ 0.15418 nm, 40 kV, 30 mA).The Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet (Impact 410) infrared spectrophotometer using KBr pellets over the range of 400-4000 cm À1 .The morphologies of the samples were observed using scanning electron microscopy (SEM) (ZEISS ULTRA 55, Carl Zeiss NTS GmbH, Germany) and highresolution transmission electron microscopy (HR-TEM, JEOL JEM-2100HR, Japan).The excitation and emission spectra were taken on Hitachi F-4600 spectrouorometer equipped with a 150 W xenon lamp as the excitation source.All the measurements were performed at room temperature.FT-IR.The FTIR spectra of pure Y 2 O 3 :Eu 3+ and rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ are exhibited in Fig. 2. As shown in Fig. 2a, absorption peaks at 3441, 1520, 1410, and 560 cm À1 can be clearly observed in the spectrum of Y 2 O 3 :Eu 3+ .A broad peak can also be seen at 3441 cm À1 due to the stretching mode of the hydroxyl groups. 25,26Meanwhile, the typical absorption peak 3À tetrahedron. 28The FT-IR spectra reveal clearly that Y 2 O 3 :Eu 3+ and YVO 4 :Eu 3+ co-existed in the prepared nanocomposite.SEM, EDS, and HR-TEM.The morphology of rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ is exhibited in Fig. 3.The particles were uniform and monodispersed rods, with an average diameter of $50 nm and a length of $400 nm.In order to determine the chemical composition of the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ , EDS measurement was performed on the area of the material SEM in Fig. 4. The EDS analysis indicated that the main composition of the nanorods is yttrium and oxygen.Small amount of vanadium and europium were also detected.These results indicated that the chemical composition of the rod-Y 2 -O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ is uniform.

Results and discussion
Furthermore, the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ was examined by TEM and HR-TEM in order to verify its partially coated structure, as displayed in Fig. 5.The composite structure of the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ was investigated with a HRTEM, some breaking areas could be found on the rod and it exhibited a discontinuous interface.As shown in Fig. 5b1, the measured lattice spacing of the nanorod was 0.35 nm, indicating a (200) crystal growth direction and a SAED pattern recorded (le) from the same area, which conrmed that the nanorod is single crystalline and indicated that the growth direction is (200) of YVO 4 :Eu 3+ . 9Therefore, a lattice spacing of the crystalline can be identied 0.29 nm (222) crystal growth direction of Y 2 O 3 :Eu 3+ in Fig. 5b2. 29Obviously, the partially coated structure had been successfully synthesized, while the YVO 4 :Eu 3+ shell partially covered the Y 2 O 3 :Eu 3+ core with a size less than 5 nm, similar emission property expected to that of a typical quantum dot.
PL.The excitation spectra of pure Y 2 O 3 :Eu 3+ (a), pure YVO 4 :Eu 3+ (b), and rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ (c) are shown in Fig. 6 (le).Only one peak appeared for pure Y 2 O 3 :Eu 3+ , at 254 nm, which corresponded to the charge-transfer band (CTB) related to an electronic transition from the 2p orbital of O 2À to the 4f orbital of Eu 3+ . 30Pure YVO 4 :Eu 3+ also only possessed one excitation band, at 308 nm, which can be regarded as a charge transfer from the O atoms of the ligands to the central V atom   inside the VO 4 3À group. 31However, the spectrum of rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ contained a broader excitation band in the range of 200-350 nm, which can be seen as two peaks centered at $254 nm and $280 nm, similar to that of a $5 nm YVO 4 :Eu 3+ quantum dot layer, albeit slightly blue-shied. 32The former peak was likely related to the CTB in Y 2 O 3 :Eu 3+ , while the latter corresponded to a vanadate band in the open quantum shell of YVO 4 :Eu 3+ .This special optical property is only observed in partially coated core/shell structures, and cannot be replicated in fully coated core/shell materials.In order to show this, a sample was prepared with more NH 4 VO 3 , corresponding to a V/Y molar ratio of 0. Cu 2+ (off) and amino acid (on) sensing characteristics of rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ nanoparticles.The uorescence quenching that occurs upon the addition of Cu 2+ can be observed in Fig. 7, which shows the uorescence intensity of the partially coated rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ decreasing   gradually as the Cu 2+ concentration was increased.Meanwhile, pure Y 2 O 3 :Eu 3+ (Fig. S3 †), pure YVO 4 :Eu 3+ (Fig. S4 †), Y 2 O 3 :Eu 3+ coated with 0.01YVO 4 :Eu 3+ by physical blending (Fig. S5 †), and fully coated Y 2 O 3 :Eu 3+ @0.4YVO 4 :Eu 3+ (Fig. S6 †) were exposed to the same conditions to illustrate the excellent properties of the partially coated structure.Fig. 8 shows that the partially coated rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ had a linear relationship with the Cu 2+ concentration (linear regression equation: y ¼ 0.0396x + 0.5655, R 2 ¼ 0.9962), with the uorescence intensity decreasing with increasing concentration of Cu 2+ .However, there were no obvious linear relationships between the concentration of Cu 2+ and the other four materials.The partially coated nanostructures were also tested in the presence of 2, 4, 6, and 8 Â 10 À8 mol L À1 (Fig. S7 †) and 2, 4, 6, and 8 Â 10 À10 mol L À1 (Fig. S8 †) of Cu 2+ .Good linear relationships were also observed for these Cu(II) concentrations.In Fig. 9, I and I 0 are the uorescence intensities of Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ in the presence and absence of Cu 2+ , respectively.Linear regression equations can be obtained for I/I 0 under these conditions, which are y ¼ À1.4175x + 85.365, R 2 ¼ 0.9947 ((a): 2, 4, 6, 8 Â 10 À8 mol L À1 grads) and y ¼ À1.076x + 93.51, R 2 ¼ 0.9954 ((b): 2, 4, 6, 8 Â 10 À10 mol L À1 grads).
This shows that the partially coated rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ nanostructures possess excellent properties that make them suitable for use as a uorescent probe for the detection of Cu(II) ions.Quantum dots are already frequently used for the detection of minute quantities of elements.The partial YVO 4 :Eu 3+ shells are as thin as quantum dots are small, providing outstanding properties that enable the detection of minute quantities of elements via use as a uorescent probe.The emission intensity of the rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ decreased as the concentration of Cu(II) ions increased.Thus, a uorescently quenched probe (off state) was obtained.
The quenching (off) of the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ -Cu 2+ (10 À8 mol L À1 ) system can be explored as a new uorescent probe for the detection of amino acids based on the competition mechanism between amino acids, Cu 2+ , and rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ .Glutamic acid and leucine were employed to illustrate this principle.The uorescence spectra of rod-Y 2 -O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ -Cu 2+ (10 À8 mol L À1 ) with glutamic acid (Fig. S9    The mechanism of the "off-and-on" detection of Cu 2+ and amino acids using anisotropic rods of Y 2 O 3 :Eu 3+ that were partially coated with 0.01YVO 4 :Eu 3+ as uorescent probes is shown in Scheme 1.There were many -OH groups on the surface of the YVO 4 :Eu 3+ quantum shell in the rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ composite nanostructure while dispersed in aqueous solution, which can easily combine with Cu 2+ to form [Cu(OH) 4 ] À .As is well known, [Cu(OH) 4 ] À ions appear blue in aqueous solutions due to their absorption of red light, while the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ nanocomposite is a red phosphor when excited with UV light.Therefore, the uorescence intensity of the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ nanocomposite was reduced or even quenched in the presence of trace Cu 2+ , allowing this OCSNS to be used as a probe for the detection of Cu 2+ .On other hand, the uorescence intensity of the rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ -Cu 2+ (10 À8 mol L À1 ) system was increased in the presence of trace amino acids.The amino acids possess a strong binding preference toward Cu 2+ , which removed Cu 2+ from the surface of the partially coated nanocomposite, thus decreasing the concentration of [Cu(OH) 4 ] À and allowing the uorescence of the system to recover.The opposite uorescent responses in the presence of Cu 2+ and amino acids give this rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ OCSNS the potential to be used to detect trace Cu 2+ and amino acids, exhibiting "off-and-on" uorescence properties.

Conclusions
In conclusion, partially coated core-shell nanostructures, rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ , were rst fabricated by a facile hydrothermal approach.The crystal structure, morphology, and optical properties were characterized in detail by XRD, FTIR, SEM, TEM, and PL spectroscopy.This revealed that a quantum shell of YVO 4 :Eu 3+ had been partially coated onto the surfaces of the Y 2 O 3 :Eu 3+ core.The excitation spectra of this special nanostructure can be deconstructed into two components centered at $254 and $280 nm, covering a much wider excitation period of UV wavelengths than is obtained for either material alone.Other OCSNSs composed of an anisotropic core or substrate and an open shell with a thickness of less than 5 nm can also be fabricated in the same way, and the photoluminescent performances of these materials in the presence of trace ions or molecules can be examined.As an example, super sensitive probes consisting of rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ with "off-and-on" properties can be employed to detect minute amounts of Cu(II) ions (off), followed by reuse as a uorescent probe for various amino acids (on).Far more examples of this kind of OCSNS, their probing properties, and their selectivity performances will be examined in the future for the purpose of inventing novel uorescent probes.

3. 1
Structural and morphological properties of rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ nanoparticles XRD.The XRD pattern of the partially coated rod-Y 2 O 3 :-Eu 3+ @0.01YVO 4 :Eu 3+ nanostructure was collected, as shown in Fig.1, and all of the XRD diffraction peaks can be readily indexed to the cubic structure of Y 2 O 3 and the tetragonal structure of YVO 4 , in accordance with the JCPDS cards 41-1105 and 17-0341.No additional diffraction peaks were observed, showing that there were no other impurities in this sample.

Fig. 4 A
Fig. 4 A SEM image of as-prepared nanorods (up) with collection area marked, and EDX signals (down, EDS spectrum).

4 .
SEM and HR-TEM images of the nanocomposite prepared with 0.436 g of NH 4 VO 3 (molar ratio n V : n Y ¼ 0.4) are shown in Fig.S1(see ESI †).The HR-TEM image shows that a continuous interface can be clearly observed, meaning that the shell of YVO 4 :Eu 3+ had totally covered the surface of the core Y 2 O 3 :Eu 3+ .The excitation spectrum of this material is shown in Fig.S2.† Surprisingly, the full coated sample showed only one excitation band at 308 nm, corresponding to that of YVO 4 :Eu 3+ .The continuous shell fully encompassed the core, enhancing the transition competition between the shell and the core.It can be concluded that efficient energy transfer from Y 2 O 3 :Eu 3+ to YVO 4 :Eu 3+ occurred following the excitation into the CTB of Y 2 O 3 :Eu 3+ .33Therefore, the novel nanostructure rod-Y 2 O 3 :Eu 3+ @0.01YVO 4 :Eu 3+ , in which YVO 4 :Eu 3+ was only partially coated on the Y 2 O 3 :Eu 3+ core, acted as a smart phosphor which exhibited PL under wide-band excitation covering the range of 200-350 nm, as displayed in Fig.6.This wide-band response phosphor emitted strong red light, and may be a candidate as a uorescent probe for use in chemical and biological detection.

Fig. 8 I
Fig. 8 I/I 0 relationships of five different samples with increasing Cu 2+ concentration.

Fig. 11
Fig. 11 Linear relationship of the fluorescence intensity versus the concentration of leucine over the range of 0-1.0 Â 10 À8 mol L À1 .The error bars represent the standard deviations of three measurements.