Eu 3+ and Tb 3+ doped LaPO 4 nanorods, modi ﬁ ed with a luminescent organic compound, exhibiting tunable multicolour emission †

a Co-precipitation reaction followed by hydrothermal treatment were used to synthesise Eu 3+ or Tb 3+ doped LaPO 4 nanorods, of 5 – 10 nm in width and 50 – 100 nm in length. Surface modi ﬁ cation of the as-prepared nanoparticles with a selected luminescent organic compound resulted in formation of hybrid inorganic – organic nanomaterials. The products obtained exhibited tunable multicolour luminescence, dependent on the surface modi ﬁ cation and applied excitation wavelength. The colour of their emission can be altered from red-orange to yellow-green. Powder X-ray di ﬀ raction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) con ﬁ rmed the structure and morphology of the products synthesized. Successful surface modi ﬁ cation of the nanophosphors was evidenced by analytical and spectroscopic techniques such as dynamic light scattering (DLS) – providing size distribution histograms and zeta potentials of the nanoparticles; IR spectroscopy and elemental analysis which proved the presence of an organic phase in the structure; spectro ﬂ uorometry (excitation/emission spectra and luminescence decay curves) which con ﬁ rmed the formation of hybrid, surface modi ﬁ ed nanomaterials revealing tunable multicolour emission.


Introduction
Luminescent nanomaterials based on lanthanide ions (Ln 3+ ) have fascinated researchers for over the last two decades. [1][2][3][4][5] The spectroscopic properties of Ln 3+ ions are unique among the other elements, which result from their electronic conguration [Xe]4f n (n ¼ 0- 14). 4f orbitals of lanthanide ions are shielded by 5p and 6s shells, which makes the 4f-4f electronic transitions nearly insensitive to the coordination environment of Ln 3+ ion, and provides narrow spectral width of emission or absorption bands as well as long-lived luminescence. 6 The parity-forbidden character of the 4f-4f transitions results in a very low molar absorption coefficients and also low efficiencies of Ln 3+ emission, when the direct excitation of 4f-4f absorption bands is applied. However, some Ln 3+ ions, like Tb 3+ and Ce 3+ , can be effectively excited via the allowed 4f-5d transitions, which strongly enhances the luminescence efficiency. 7 Also the other, partially allowed processes like charge transfer (CT) observed in O 2À containing host materials and energy transfer (ET), can increase absorption of the excitation light and therefore luminescence intensity. 8,9 These specic properties of Ln 3+ ions allow the design of nanomaterials revealing effective luminescence by applying ET and CT phenomena (indirect excitation). Much effort has been made to study ET between, e.g.: Ce 3+ and Tb 3+ , Gd 3+ and Eu 3+ or in up-converting systems: Yb 3+ and Er 3+ , Yb 3+ and Tb 3+ . [10][11][12][13] Also ET from the host or ligand to Ln 3+ ion has been extensively investigated. 14 The emission bands of Ln 3+ ions can be observed in the ultraviolet, visible and near infrared ranges.
The high potential of Ln 3+ -doped nanomaterials is used in many different applications like lighting, phosphors production, organic light emitting diodes (OLEDs and LEDs), lasers, optical ampliers or waveguides and such areas like medicine and biology. 4,[15][16][17][18][19] Some of these applications result from the strong interest in Ln 3+ -doped hybrid inorganic-organic materials. [20][21][22] In general, hybrid materials have altered properties that can be tailored according to the needs, e.g. they can show increased mechanical resistance, thermal stability, luminescence efficiency etc. [23][24][25] Also the multifunctionality and the possibility to modulate their properties are important factors increasing development of such hybrid materials. [26][27][28][29][30][31] Such sophisticated bi-or multifunctional hybrid/composite nanomaterials can exhibit simultaneously different desired properties like luminescence and magnetism, which are crucial for development of advanced bioimaging, drug delivery, luminescence tracing, document protection etc. 28,29,32,33 One of the areas most recently studied is surface modication of nanocrystals (NCs). Signicantly increased stability of colloids, lowered cytotoxicity of NCs and their higher biocompatibility, possibility of NCs use as drug carriers, production of biological sensors and detectors are only a few examples of the numerous applications that result from the inorganic-organic characteristics of hybrid nanomaterials. 20,[34][35][36] In this study we report the preparation of hybrid inorganicorganic nanomaterials based on LaPO 4 nanorods doped with Tb 3+ or Eu 3+ ions, having organically modied surface. Our aim was to synthesize the nanomaterials exhibiting tunable luminescence dependent on the excitation wavelength used and study their photophysical properties. Such nanocomposites exhibit bright, multicolour luminescence originating from inorganic and organic components. They can be applied as advanced phosphors, luminescence tracers, biomarkers, etc. What is more, nanomaterials based on lanthanide phosphates reveal low cytotoxicity, 24 and the use of functional organic compound as a surface modier can alter the surface properties of the nanostructures modied and extend the range of their potential applications.  3 and La(NO 3 ) 3 aqueous solutions, respectively. Ammonium phosphate monobasic NH 4 H 2 PO 4 (Sigma-Aldrich, ReagentPlus®, $98.5%) was used as a source of phosphate ions. Polyethylene glycol (PEG) 6000 (Alfa Aesar, 98%) was used as a surfactant. The synthetic procedure leading to 2,3-di(3,4-dioctyloxyphenyl)-quinoxaline-6-carboxylic acid starts with the preparation of the appropriate ketone -3,3,4,4-tetraoctyloxydibenzoyl, which has already been described in literature. 37 From this ketone and 3,4-diaminobenzoic acid, nal compound was prepared according to the method described by E. J. Foster 38,39 and H.-J. Chen. 40 In all experiments, ultra-pure distilled water and absolute ethanol were used.

Synthesis of nanophosphors -
The typical synthesis was carried out to get 1.5 g of the nal product. The co-precipitation reaction was performed in ambient conditions, using a hot-plate magnetic stirrer. The initial pH of the prepared RE(NO 3 ) 3 solutions was around 5. Solution A: La(NO 3 ) 3 (11.62 mL, 0.494 M) and Eu(NO 3 ) 3 (1.52 mL, 0.419 M) were mixed together at 9/1 molar ratio, and lled with water up to 75 mL. 50 mL of ethanol was added to the asprepared solution. Subsequently, 0.75 g of PEG was dissolved in this solution. Solution B: 25% molar excess of NH 4 H 2 PO 4 was dissolved in the same solvent systems, with addition of the same amount of PEG. The as-prepared solution B was added dropwise to solution A during 20 minutes. Aerwards the obtained white precipitate was centrifuged and washed with water and ethanol several times. To get well-crystallised nanomaterial, the as-prepared product was dispersed in 70 mL of water, transferred into a Teon lined vessel and treated under hydrothermal conditions for 120 minutes at 200 C and 40 bar (microwave autoclave -ERTEC, Magnum II, 600 W). When the reaction was complete, the purication procedure was repeated. The nal product was dried under vacuum overnight. Tb 3+ doped product was prepared in the same way, using Tb(NO 3 ) 3 instead of Eu(NO 3 ) 3 .

Surface modication of nanorods
The following organic compound was selected as an exemplary surface modier because of its complex character, presence of functional groups allowing further chemical modication, potential liquid crystal properties and intensive luminescence. 41 5 mg of 2,3-di(3,4-dioctyloxyphenyl)-quinoxaline-6-carboxylic acid (C 53 H 78 N 2 O 6for the reader's convenience the simple abbreviation the "organic" will be used throughout the article) dissolved in 10 mL of THF was added slowly to a stirred suspension of nanorods (50 mg) in 25 mL of THF. The reaction mixture was stirred at room temperature for further 72 h and the obtained yellow precipitate was then centrifuged (5 min, 13 000 rpm). In the next step, pure THF was added to the nanomaterial. Aerwards, the mixture was sonicated for 60 s and then centrifuged once more (5 min, 13 000 rpm). The procedure was repeated until no trace of excess of the organic compound was found as determined by TLC. The nal, hybrid nanomaterials (yellow powders), revealed tunable multicolour luminescence originating from both inorganic and organic components. Here is worth noting, that there was no observed release of the organic compound from the nanoparticles surface, aer washing the product in water. Only a long sonication and washing with THF or toluene caused a slow release of the organic compound. Fig. 1 illustrates a scheme of the nanomaterials surface modication.

Characterization
Electron microscopy measurements were carried out using transmission electron microscope-TEM Zeiss LIBRA 200FE, operating at 200 kV. Powder XRD (X-ray diffractograms) were recorded on a Bruker AXS D8 Advance diffractometer, using Cu Ka radiation (l ¼ 1.5406Å). The elemental analysis of the products was performed using an Elementar Analyser Vario EL III. Setaram Setsys 1200 device, was used for simultaneous thermogravimetric-differential thermal analysis (TG-DTA), with a heating rate of 5 C min À1 in air. IR spectra were recorded on FT-IR spectrophotometer, JASCO 4200. The IR spectra were measured in transmission mode, the samples were mixed with KBr, ground and pressed forming transparent discs. The particle size distribution (hydrodynamic diameter) and zeta (z) potential of the nanomaterials synthesized were recorded on Malvern Zetasizer Nano ZS, equipped with dynamic light scattering (DLS) module (He-Ne laser 633 nm, max 4 mW). Before measurements each product was dispersed in MiliQ quality water, forming stable aqueous colloid (0.1 mg mL À1 ). The excitation/emission spectra and luminescence decay curves of the dried products were measured in ambient conditions, using a Hitachi F-7000 spectrouorometer. All spectra were appropriately corrected for the apparatus response.

Structure and morphology
The recorded powder XRD patterns of LaPO 4 :Tb 3+ 10% and LaPO 4 :Eu 3+ 10% were compared with the pattern from ICDD (International Centre for Diffraction Data) standards database (Fig. 2). Both diffractograms are similar and t well to that of the hexagonal, hydrated lanthanum phosphate, LaPO 4 $0.5H 2 O (ICDD 000-046-1439). However, the diffractograms reveal also some very small reexes (overlapping with the background noise), which are probably related to the monoclinic phase (monazite). The obtained XRD patterns exhibit broadened reexes, which indicate the nanocrystallinity of the products synthesized, and at some extent, it can be due to the superposition of reexes of hexagonal and monoclinic phase. Surface modication. Surface modication of the prepared inorganic nanophosphors by coating with luminescent organic modier was carried out to get hybrid nanomaterials, exhibiting tunable multicolour luminescence, dependent on the excitation   wavelength. The morphology and structure of the nanoparticles obtained, were not affected by the organic surface modication. The presence of the organic modier molecules on the nanoparticles surface was conrmed and examined by elemental analysis, FT-IR spectroscopy, DLS studies (z-potential and particle size distribution) and luminescence spectroscopy.
Additionally, TG-DTA measurements were performed to investigate the stability of the organic surface layer and conrm the composition of the nanomaterials synthesized. The results obtained agree well with the elemental analysis data, and they are presented in full in ESI (Fig. S1 †). Fig. 4 illustrates the IR spectra of LaPO 4 :Eu 3+ 10%, LaPO 4 :-Tb 3+ 10%, LaPO 4 :Eu 3+ 10%@organic, LaPO 4 :Tb 3+ 10% @organic, and pure organic compound. All spectra recorded reveal broad absorption peaks around 3400 cm À1 and 1640 cm À1 , corresponding to the O-H stretching (n) and deformation (s) vibrations, respectively. The observed O-H bonds correspond to water molecules adsorbed on the nanorods surface and structural water molecules (hydration of phosphates). The absorption peaks around 2924 and 2865 cm À1 are related to nC-H vibrations of -CH 2 groups of organic compound molecules (adsorbed on the surface of phosphate nanorods). The spectra of the pure and modied phosphates exhibit very intensive and broad peaks assigned to the vibrations of phosphate groups. The peaks around 1050, 950 cm À1 were assigned to the stretching vibrations within PO 4 groups, and around 615, 542 cm À1 to the bending vibrations within these groups. 42,43 What is more, the quite intensive band observed at $960 cm À1 conrms the coexistence of monoclinic phosphate (minor phase) together with the hexagonal lanthanum phosphate (major phase), which was mentioned during discussion of the XRD patterns. The peaks below 1700 cm À1 in the spectrum of the pure organic compound, correspond to the vibrations of numerous bonds in the very complex structure of this compound. The presence of peaks around 2900 and 1200 cm À1 in the spectra of the organically modied phosphates (LaPO 4 :-Eu 3+ 10%@organic and LaPO 4 :Tb 3+ 10%@organic), conrms their successful modication with the organic compound used.
The z-potential and average particle size distribution of the nanomaterials synthesized were measured by the DLS method. All of the measurement were carried out at pH ¼ 7. The z-potential recorded for LaPO 4 :Eu 3+ 10% was +29.3 mV, and for LaPO 4 :Tb 3+ 10% it was +34.6 mV. The z-potential recorded for the surface modied LaPO 4 :Eu 3+ 10%@organic and LaPO 4 :Tb 3+ 10%@organic nanorods decreased to +17.8 mV and +27.3 mV, respectively. The decreased z-potential values conrmed surface modication of the nanomaterials synthesized. What is more, all of the nanomaterials exhibited relatively high surface charge, additionally conrming their stability at neutral pH, which is important in potential bioapplications. The average particle size distribution of the products synthesized is presented in Fig. 5. The approximate sizes of LaPO 4 :Eu 3+ 10% (a) and LaPO 4 :Tb 3+ 10% (b) particles are about 300 nm, and increase aer surface modication for LaPO 4 :Eu 3+ 10%@organic (c) and LaPO 4 :Tb 3+ 10%@organic (d) to about 500 nm. This fact clearly conrms surface alterations of the nanomaterials modied, manifested by increased hydrodynamic diameter of the nanoparticles. However, the recorded DLS curves revealed the polydispersity of the nanomaterials obtained, which was caused by particles agglomeration and their "sticking" to bigger clusters aer surface modication. The hydrodynamic radius/diameter of the particles analysed is usually larger in comparison to the real particle sizes from TEM data, since the DLS method takes into account surface solvation of the particles, and their agglomeration/aggregation in the colloidal solution. One must remember, that the data presented are onlya rough approximation of the nanoparticles sizes because of their highly anisotropic shape (DLS size measurements assume the spherical shape of the analysed objects).  modied products, namely LaPO 4 :Eu 3+ 10%@organic and LaPO 4 :Tb 3+ 10%@organic. All spectra were recorded for solid products (dried powders). Fig. 6a presents three excitation spectra of LaPO 4 :Eu 3+ 10% (l em ¼ 620 nm) and LaPO 4 :Eu 3+ 10% @organic (l em ¼ 500, 620 nm). For bare LaPO 4 :Eu 3+ 10% (l em ¼ 620 nmthe maximum of 5 D 0 / 7 F 2 transition in Eu 3+ ion), a dominant broad band centred at 256 nm related to O 2À / Eu 3+ charge transfer (CT) transition can be observed. This band is the  most intensive one in the whole spectrum because the mentioned CT transition is allowed by selection rules, in contrast to the forbidden 4f-4f transitions within Eu 3+ ion (the series of narrow bands observed at 300-400 nm). For the surface modied LaPO 4 :Eu 3+ 10%@organic nanomaterial, the same dominant band can be observed (l em ¼ 620 nm). The series of intrinsic 4f-4f transitions is hardly visible in this spectrum, because they overlap with the appearing absorption band of the organic surface modier. The third plot obtained for the same modied nanomaterial, whose excitation spectrum was recorded at l em ¼ 500 nm (the maximum of emission band for the organic compound) does not reveal the characteristic bands related to the transitions in Eu 3+ ions. In this spectrum only a very broad absorption band centred at 282 nm corresponding to the surface organic compound can be observed in the whole presented wavelength range. Fig. 6b shows three excitation spectra of LaPO 4 :Tb 3+ 10% (l em ¼ 543 nm) and LaPO 4 :Tb 3+ 10%@organic (l em ¼ 500, 543 nm). The spectrum of LaPO 4 :Tb 3+ 10% (l em ¼ 543 nm is the maximum of the most intensive in the emission spectrum 5 D 4 / 7 F 5 transition, within Tb 3+ ions), shows a dominant broad band centred at 213 nm corresponding to 4f 8 / 4f 7 5d 1 (f-d) allowed transition. The bands at higher wavelengths are assigned to the forbidden 4f-4f transitions in Tb 3+ ion. In the spectrum of LaPO 4 :Tb 3+ 10%@organic recorded at the same l em ¼ 543 nm, the slightly shied f-d transition is also dominant. The other 4f-4f transitions are not visible. The last excitation spectrum of this modied nanomaterial (l em ¼ 500 nm) also reveals the dominant band related to f-d transition in Tb 3+ ion. However, the very broad absorption band (with maximum ranging from 250 to 280 nm) assigned to the surface organic modier, can be observed in the whole spectrum, as well. Fig. 6c presents three emission spectra of LaPO 4 :Eu 3+ 10% (l ex ¼ 250 nm) and LaPO 4 :Eu 3+ 10%@organic (l ex ¼ 250, 300 nm). In the rst spectrum of LaPO 4 :Eu 3+ 10% recorded at l ex ¼ 250 nm (the position of the most intensive CT transition in the excitation spectrum), six narrow, split bands corresponding to the 5 D 0 / 7 F J (J ¼ 0-5) transitions can be observed. The hypersensitive electric dipole 5 D 0 / 7 F 2 transition is sensitive to the site symmetry alterations. 44,45 The ratio between integrated areas of the 5 D 0 / 7 F 2 and 5 D 0 / 7 F 1 transition bands is informative about the presence of symmetry centre in the site occupied by the Eu 3+ ions. The values calculated both for LaPO 4 :Eu 3+ 10% and LaPO 4 :Eu 3+ 10%@organic are close to 1 (1.04 and 1.01 respectively). The ratio higher than 1 indicates that the Eu 3+ ions are situated at sites without inversion symmetry. However, in the LaPO 4 :Eu 3+ material the Eu 3+ ions occupy sites with D 2 symmetry in the LaPO 4 structure which is non-centrosymmetric and the presence of an inversion centre cannot be assumed. [46][47][48] The product exhibits an intense, bright red luminescence. When the surface modied nanomaterial was excited at the same wavelength, the intensity of the mentioned transitions decreased, and a new broad band appeared in the range of 500-550 nm. This band corresponds to the emission of the organic compound. As a consequence of these alternations in the spectrum shape, the observed luminescence of the product was tuned to yellowish emission. Upon exciting the organic modi-ed nanomaterial at 300 nm (absorption range of the organic compound), the characteristic bands of Eu 3+ ions can hardly be observed, in contrast to the very high intensity and broad emission band of the organic compound. The resulting emission of the product is green. Fig. 6d shows three emission spectra of LaPO 4 :Tb 3+ 10% (l ex ¼ 210 nm) and LaPO 4 :Tb 3+ 10%@organic (l ex ¼ 210, 300 nm). The spectrum of LaPO 4 :Tb 3+ 10%, recorded at l ex ¼ 210 nm (the position of the most intensive transition in the excitation spectrum), presents four narrow bands assigned to the 5 D 4 / 7 F J (J ¼ 6-3) transitions, characteristic of Tb 3+ ions. 7 The product exhibits bright green luminescence. The spectrum of the LaPO 4 :Tb 3+ 10%@organic nanophosphor excited at the same wavelength reveals four bands typical of Tb 3+ ions, as well.  However, the intensity of these bands decreased in comparison to that in the spectrum of the unmodied product. Besides these bands, less intensive bands around 500-550 nm, corresponding to the organic modier can also be observed. The colour of the product emission is still green, however shied towards blue. The spectrum of the modied nanomaterial excited at 300 nm reveals only the intensive, broad band assigned to the organic compound. Its luminescence is greenish, namely the colour of emission is slightly altered when compared to that of the unmodied nanophosphor.

Luminescent properties
For more detailed specication of the luminescence colours one can refer to the included chromaticity diagram (CIE 1964 10 deg observer), presented in Fig. 7. The photographs of the products synthesized, taken in daylight and under UV light, showing their multicolour emission, are included in Fig. 7, as well. Fig. 8 presents the luminescence decay curves and calculated radiative lifetimes for the 5 D 0 / 7 F 2 and 5 D 4 / 7 F 5 transitions of Eu 3+ and Tb 3+ ions, respectively. All data were recorded at 293 K; l em ¼ 620 nm, l ex ¼ 250 nm for the Eu 3+ doped compounds and l em ¼ 543 nm and l ex ¼ 210 nm, for the Tb 3+ doped compounds. In hexagonal lanthanum phosphate, all lanthanide ions should be at the sites of the same type (coordination environment). 24 However, the experimental proles were successfully tted to the biexponential function of decay, namely y ¼ A 1 exp(Àx/s 1 ) + A 2 exp(Àx/s 2 ) + y0. The nanomaterials synthesized exhibit high surface-to-volume ratio. Therefore a large part of ions forming the material is placed on or near the surface of nanocrystals. Hence, aer surface modi-cation with the organic compound used, a signicant number of the surface/near surface ions were localised in a new coordination environment (altered local site symmetry). The reason for this phenomenon was a strong coordination/binding of the organic molecules to the nanoparticles surface. This is why, the nanophosphors obtained exhibit a shorter second component of luminescence decay. Here is worth noting, that the presence of a small amount of monoclinic LaPO 4 in the sample can affect the lifetime components and disturb their decay proles. However, because of the large contribution (z30%) of the second lifetime components and their signicant shortening aer the surface modication, we assume that the discussed biexponential character of the luminescence decay is predominantly caused by the differently emitting surface ions. The calculated luminescence lifetimes for the products synthesized are in the range of 4.25-4.96 ms (s 1 ) and 1.08-2.70 ms (s 2 ). The detailed values are presented in Fig. 8. Such relatively long radiative lifetimes are in line with literature data for lanthanide doped inorganic phosphors. 49,50 The observed lifetimes for Eu 3+ and Tb 3+ doped products are generally similar, however the lifetimes of Eu 3+ ions are slightly shorter when compared to those of Tb 3+ ions. Analysis of the decay proles leads to a conclusion that the modied nanomaterials exhibit a shorter average lifetime, in comparison to their unmodied analogues. The lifetime shortening is particularly pronounced in the second lifetime component (s 2 ), assigned to the surface ions (shortening from 1.75 to 1.08 ms and from 2.70 to 2.24 ms for Eu 3+ and Tb 3+ ions, respectively). This phenomenon can be explained by a strong interaction between surface ions and organic molecules attached to the nanoparticles surface, resulting in enhanced luminescence quenching. The results obtained are in agreement with the data on the emission decrease of the modied nanorods (see the emission spectra in Fig. 6a and b).

Conclusions
The highly luminescent, crystalline nanomaterials doped with Eu 3+ and Tb 3+ ions were synthesized via the co-precipitation approach followed by hydrothermal treatment. The nanomaterials formed were in the form of elongated nanorods (5-10 nm in width and 50-100 nm in length) composed of hexagonal LaPO 4 $0.5H 2 O. Subsequently, the products obtained were modied with a luminescent organic compound. The surface modication resulted in a formation of hybrid inorganicorganic nanomaterials, which exhibited tunable and multimodal luminescence. The products emission could be tuned from red-orange to yellow-green luminescence. Successful modication of the surface of nanocrystals was checked by DLS, IR spectroscopy, elemental analysis, TG-DTA and spectrouorometry. These novel, functional nanomaterials can be applied in luminescence tracing, detection techniques, multicolour imaging, as novel light sources and in many other special applications requiring sophisticated, hybrid nanomaterials exhibiting tunable emission. The products synthesized can be also used in biomedical applications requiring multifunctionality of nanomaterials.