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Lanthanoid-Doped Phosphate/Vanadate Mixed Hollow Particles as Ratiometric Luminescent Sensors Paulo Cesar de Sousa Filho, Eric Larquet, Diana Dragoe, Osvaldo Antonio Serra, and Thierry Gacoin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14837 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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ACS Applied Materials & Interfaces

Lanthanoid-Doped Phosphate/Vanadate Mixed Hollow Particles as Ratiometric Luminescent Sensors

Paulo C. de Sousa Filho,†‡* Eric Larquet,‡ Diana Dragoë, § Osvaldo A. Serra,† Thierry Gacoin‡

†

Rare Earth Laboratory; Department of Chemistry; Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto; University of São Paulo. Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto, SP, Brazil. ‡

Solid State Chemistry Group/Laboratoire de Physique de la Matière Condensée; Ecole Polytechnique. Route de Saclay, 91128 Palaiseau Cedex, France.

§

Institute de Chimie Moleculaire et des Matériaux d’Orsay. Université Paris-Sud11/Université Paris-Saclay. Rue du Doyen Georges Poitou, 91400 Orsay, France.

*

Email: [email protected] 1 ACS Paragon Plus Environment


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ABSTRACT: Rare earth (RE) phosphates and vanadates are structurally similar compositions that display distinct but complementary luminescent properties. The properties of these phosphors can be combined in REPO4-REVO4 heterostructures during the development of new sensing technologies for biological applications. This work presents the synthesis of hollow RE phosphate/vanadate colloidal particles and evaluates their applicability as luminescent markers. Hydrothermal treatments of RE hydroxycarbonate particles in the presence of the PO43- and VO43- precursors afforded the final REPO4-REVO4 solids in a twostep template synthesis. We converted precursor hydroxycarbonate particles into the final heterostructures and characterized their structure and morphology. According to our detailed study into the spectroscopic properties of Eu3+-doped particles and their luminescence response to several species, the presence of the phosphate and vanadate phases in a single particle provided different chemical environments and enabled the design of a ratiometric approach to detect H2O2. These results open new perspectives for the development of new intracellular luminescent markers. KEYWORDS: phosphates; vanadates; rare earth; H2O2 detection; luminescence.

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INTRODUCTION Rare earth (RE)-based particles display special optical, magnetic, and redox properties1-3 that allow their application in various fields such as lighting,4,5 luminescent labeling,6 energy conversion,7,8 and environmental catalysis,9,10 among others.2,11 Luminescent markers that involve emissions from trivalent lanthanoids (Ln3+) offer many advantages, especially regarding their use in biological systems.6,11,12 For instance, line-type emissions, high shifts between excitation and emission, long-lived excited states, and wide excitability13,14 make Ln3+-based markers more suitable than semiconductor quantum dots or organic fluorophores for a broad range of bio-labeling methodologies.12,15 RE vanadates are a particularly interesting class of RE phosphors: they present wellknown luminescent properties including very high emission efficiencies under ultraviolet (UV)16,17 or near-infrared (NIR)18,19 radiation. In addition, the high chemical stability of RE vanadates enables the use of liquid-phase synthesis methods to control their morphological properties. Consequently, one can tailor the composition, particle shape, density, and crystallinity of the final structures to the intended application.20-22 Such characteristics have successfully aided the development of new luminescent biolabeling strategies. For example, Eu3+-doped yttrium vanadate can afford a timeresolved luminescent marker of hydrogen peroxide in biological systems,23 whereas control of the final RE3+ compositions gives multimodal magnetic/luminescent sensors via inclusion of paramagnetic Gd3+ ions.24 Luminescent markers like (Y,Eu)VO4 and (Gd,Eu)VO4 have attracted growing attention from the scientific community because they are potentially applicable in the detection of hydrogen peroxide and other reactive oxygen species (ROS) with spatial and temporal resolution. This kind of measurement can contribute valuable information 3 ACS Paragon Plus Environment


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about the physiological significance of ROS25 – ROS activate several cellular signaling pathways

during

inflammation,

differentiation,

proliferation,

migration,

and

apoptosis.26 Luminescence measurements provides responses in a time scale that spans from milliseconds to hours, so it is possible to infer detailed mechanisms of cell activity by adequately detecting the luminescence of ROS.23,24,27 Other sensors based on non-luminescent inorganic nanoparticles have also been recently designed.28 The luminescent response of Eu3+-doped vanadate particles to peroxide could involve an Eu2+/Eu3+ redox couple.23,24 In this case, suitable reduction of Eu3+ to divalent europium and further oxidation of Eu2+ by the peroxide should produce luminescence from trivalent europium that is directly proportional to the concentration of the oxidizing species. More recently, Duée et al. demonstrated that Eu3+-doped REVO4 particles elicited a luminescence response to H2O2 that dismissed the need for previous reduction of Eu3+ to Eu2+ because the peroxide quenched the Eu3+ luminescence.29 However, both these approaches required that absolute luminescence intensities be measured to determine the concentrations of H2O2/ROS, which called for complex optical equipment and sample preparation procedures. To improve and simplify the aforementioned H2O2 detection methodologies, the REVO4 particles should exhibit a reference luminescence signal that enables ratiometric measurements.30 Here we propose the use of mixed RE phosphate-vanadate colloidal sub-microparticles as luminescent sensors for ROS, particularly H2O2. Compared to the RE vanadate phase, the RE phosphate phase provides high chemical stability and a chemical environment with distinct characteristics, resulting in Eu3+ luminescence that is less susceptible to ROS. This approach shall allow the design of a ratiometric methodology to detect H2O2 via emissions from Eu3+ in REPO4 and REVO4.


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EXPERIMENTAL Chemicals. Rare earth nitrate hydrates (Y(NO3)3.6H2O, Eu(NO3)3.5H2O, 99.99% purity) were acquired from Aldrich and used without further purification. Urea (CO(NH2)2, 99% Merck), ammonium metavanadate (NH4VO3, 99% Aldrich), and ammonium hydrogen phosphate ((NH4)2HPO4 99%+, Aldrich) were also used as received. For luminescence activity tests and characterizations, hydrogen peroxide (H2O2, 30% m/v, Aldrich), sodium hypochlorite (NaClO, 10-15% aqueous solution, Aldrich), sodium nitrite (NaNO2, >97% Aldrich), peroxyacetic acid (HOOAc, 39% in acetic acid, Fluka), and tert-butyl hydroperoxide (C4H9OOH, TBHP, 70% solution in water, Aldrich) were employed. Ammonia (NH3(conc)., 25% m/v) and 80 mmol L-1 poly(ammonium acrylate) solution (PAA, prepared by dissolution of poly(acrylic acid), ((CH2CHCOOH)n, MW 1800 g mol-1, with concentrated ammonia, final pH = 8.5) were also used in different steps. Hydroxycarbonate Precursors. Rare earth hydroxycarbonate particles [RECO3OH.xH2O, where RE = (Y0.80Eu0.20)] were prepared by conventional homogeneous precipitation with urea.31-33 To this end, 10 mmol L-1 RE(NO3)3 solutions were prepared by dissolution of solid nitrates in deionized water, followed by stirring at room temperature (for homogenization) and addition of solid urea at a final concentration of 15 g L-1. The solution was kept under vigorous stirring for 30 min and heated at 95 °C for 3 h. The final suspension was centrifuged (15500 rpm, 20 min) and washed with deionized water until a final conductivity of ~100 µS cm-1 was attained. Mixed REPO4-REVO4 Particles. Precursor hydroxycarbonate particles were converted into mixed phosphate/vanadate structures via a two step hydrothermal treatment, adapted from the methodology described by Jia et al.33 First, a 0.20 mol L-1 RECO3OH.xH2O colloid was ultrasonicated (40 kHz, 5 min). Then, 0.20 mol L-1 (NH4)2HPO4 solution was added dropwise, so that a final 2/1 RE3+/PO43- ratio (mol/mol) was achieved. The mixture was stirred at 70 °C 5 ACS Paragon Plus Environment


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for 30 min and transferred to a Teflon-lined stainless steel autoclave at 200 °C. After 6 h, the autoclave was cooled in ice, and a 0.20 mol L-1 NH4VO3 solution was added dropwise under stirring until a 2:1 RE3+/VO3- ratio (mol/mol) was obtained. The mixture was kept at 200 °C for additional 12 h. After cooling to room temperature, the final suspension was dialyzed against deionized water for 48 h (final conductivity of ~80 µS cm-1). Characterizations. X-ray diffraction patterns were acquired on a PANalytical (Philips) X’Pert diffractometer operating with Cu-Kα radiation (1.5418 Å, Kα1+Kα2). Thermal analyses (thermogravimetry, TGA, and differential thermal analysis, DTG) were performed on a Netzsch Luxx STA 409 PC analyzer under N2/O2 (50%/50%) at a 0.5 mL min-1 flux and heating rate of 5 °C min-1. N2 adsorption/desorption isotherms were acquired on an ASAP 2010 H Micromeritics instrument at 77 K. Infrared spectra (FTIR) were measured on a Bruker Equinox 55 spectrometer in KBr pellets. X-ray photoelectron spectra (XPS) were acquired at ~3 x 10-7 mbar on a ThermoFisher K-Alpha Surface Analysis spectrometer; monochromatic Al Kα (1486.7 eV) X-ray source at an angle of 56° (source/analyzer) with solids deposited into 400−µm spots. Morphology and structure of the prepared particles were evaluated οn a JEOL 2010F transmission electron microscope (TEM) operating at 200 kV and on a Hitachi 4100 Scanning Electron Microscope (SEM) at 2 kV. Dynamic light scattering data and ζpotentials were obtained οn a Malvern Zetazizer Nano SZ equipment. Luminescence spectra were measured οn Hitachi F4500 and οn Horiba-JobinYvon Fluoromax spectrofluorometers, both equipped with Xe lamps. Quantum yields were measured with a Horiba Quanta-Phi integrating sphere. Details concerning the luminescence response to H2O2 and other species are described in the Supporting Information.


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RESULTS AND DISCUSSION Homogeneous precipitation processes involving urea afford monodisperse spherical RE hydroxycarbonate particles. These particles are widely used as precursors of RE oxides via thermal decomposition.31,32 Jia et al.33 have recently shown that RE hydroxycarbonates can be converted into more thermodynamically stable structures in liquid phase if adequately exposed to solutions containing precursor counter anions such as fluorides, phosphates, or vanadates. Hence, hydroxycarbonates can serve as templates to form readily dispersible inorganic complex structures by wet chemical methods. Such methods are advantageous: they allow for accurate control of particle size and distribution through tailoring of the morphologies of the precursor. These methods involve Kirkendall-type growth mechanisms,34 where the difference between the diffusion coefficients of RE3+ cations and counter anions in the solid-solid interface formed between the precursor and the final compositions limits the conversion rates.35 Hollow particles arise because RE3+ cations diffuse from the RECO3OH.xH2O phase to the outer surface of the particle at a higher rate than anions diffuse from the solid-liquid interface to the particle core.36 The scientific literature contains only a few reports on the combination of distinct phosphate and vanadate phases in a single particle,17,37,38 so we decided to employ this approach to synthesize mixed REPO4-REVO4 particles. We treated conventional amorphous hydroxycarbonate particles measuring between 300 and 400 nm in two steps (Figures 1a and 2a) [RECO3OH.xH2O, x≈1.1 (thermal analysis); IR: 3500-2700 cm-1 (νOH), 1650+1550+1400 (δHOH+νasCO), 1081 cm-1 (νsCO), 843 cm-1 (δsOCO), 750-700 cm-1 (δasOCO); Figures S1-S3].



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hydroxycarbonate

particles

((Y,Eu)CO3OH.xH2O), (b) intermediate particles arising after formation of the first phosphate layer, and (c-f) final (Y,Eu)PO4-(Y,Eu)VO4 mixed particles, with (f) their respective EDX elemental mappings. (Insets in (a), (b), and (c): DLS profiles of aqueous particle suspensions; inset in (d): SEM micrograph of a (Y,Eu)PO4-(Y,Eu)VO4 final particle).

In the first step, RECO3OH.xH2O solids treated with PO43- ions formed particles that contained an initial phosphate shell over the partially consumed hydroxycarbonate phase. The phosphates grew as ~50-nm thick rod-like structures emerging from a spherical shell (Figures 1b and S4); a highly crystalline rhabdophane-type (P6222) REPO4.xH2O phase coexisted with amorphous RECO3OH.xH2O (Figure 2a) [IR: 3530+3160 cm-1 (νOH), 1656+1555+1382 (δHOH +νasCO+νasNO), 1120+1006 cm-1 (νasPO(A+2B)), 965 cm-1 (νsPO43-(A)), 843 cm-1 (δsOCO), 742+700 cm-1 (δasOCO), 621+538cm-1 (δasOPO(A+2B)), Figure S2]. Although the production of RECO3OH-REVO4 structures is also possible, we opted for producing phosphate-containing intermediates (i.e. RECO3OH-REPO4) in order to 8 ACS Paragon Plus Environment

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minimize hindrance of the vanadate phase by the phosphate shell, whereas preserving the spherical morphology of precursor particles.33 Thus, in the second hydrothermal step, the remaining hydroxycarbonate phase in RECO3OH-REPO4 reacted completely, and the tetragonal (I41/amd) REVO4 phase emerged on the surface of particles, as evidenced in Figures 1c-f, 2a and S5. TEM images revealed the formation of hollow ~500-nm structures with irregular surface. Morphological distinction between the REPO4 and REVO4 phases was not possible. No preferential concentration of phosphates and vanadates could be detected at the available scales, as depicted by EDX mappings (Figure 1f) [REPO4-REVO4, P/V≈ 1.7 mol/mol, (Supporting Information); IR: 3441+3070 cm-1 (νOH), 1629 cm-1 (δHOH), 1513 cm-1 (νasCO, adsorbed carbonates) +1352 (νasNO, adsorbed nitrates), 1077+1002 cm-1 (νasPO(A+2B)), 885-784 cm-1 (νasVO(A+2B)), 619+581+541cm-1 (δasOPO(A+2B)), 449 cm-1 (δasOVO(A+2B), Figure S2].



(a) Final REPO4-REVO4

Relative Intensity

(RE=Y,Eu)

*

*

*

Intermediate

Precursor YVO4 (JCPDS 00-017-0341) YPO4.0.8H2O (JCPDS 00-042-0082)

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Precursor Intermediate Final REPO4-REVO4

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20 0

-20 -40 -60 2

4

6

8

10

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pH

Figure 2. (a) XRD patterns and (b) ζ potentials vs pH curves of the precursor (Y,Eu)CO3OH.xH2O (blue), the intermediate (Y,Eu)CO3OH.xH2O-(Y,Eu)PO4 (red), and the final (Y,Eu)PO4-(Y,Eu)VO4 (black) particles. (In (a), standard reflections of YVO4 and YPO4.0.8H2O are represented as magenta and green histograms, respectively, and red asterisks denote peaks of tetragonal yttrium phosphate; dotted lines in (b) correspond to sigmoidal fits of experimental data).

Compared to the structures of the precursors, the colloidal stability of the final REPO4REVO4 particles was lower because the hydroxycarbonates displayed a more easily protonable surface (Figure 2b). Anyway, the final REPO4-REVO4 presented moderate colloidal stability at pH < 6.5 (ζ =10-20 mV at 0.001 mol L-1 ion strength) and highly charged surfaces (|ζ| > 40 mV) at basic pH. The surface alterations that phosphate and vanadate growth introduced agreed with the displacement of isoelectric points to lower pH values. 10 ACS Paragon Plus Environment

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The formation of hollow structures increased the surface area of the final particles as compared to the structure of the precursor RECO3OH.xH2O (Figure 3a). The initial hydroxycarbonate particles exhibited a practically reversible adsorption/desorption curve as a result of their smooth surface and dense structure; the BET specific surface area was 11.3 m2 g-1. The hydrothermal treatment steps increased the surface areas due to improved particle porosities, as evidenced by the pronounced hysteresis loops in the type IIb adsorption/desorption isotherms (Figure 3a).39 Such characteristics agreed with disordered micro-sized pores with non-uniform shape and were a consequence of the Kirkendall-type formation mechanism.34-36 As a result of the improved crystallinity and the reduced degree of porosity obtained after the final hydrothermal step, the final REPO4-REVO4 structures had smaller specific surface area (18.5 cm2 g-1) as compared to the partially consumed hydroxycarbonate intermediate particles (27.6 cm2 g-1). Therefore, production of REPO4REVO4 particles by the proposed methodologies effectively increased the surface area of the final product and yielded low-density structures with improved permeability to the dispersing medium as compared to compact particles of similar size. Comparison of the luminescence quantum yields (i.e., the ratio between the photons that the material absorbed and emitted during the luminescence processes under excitation at a specific λ wavelength, qexpλ)13 of the prepared particles in suspension and exposed to different media also attested to the synthetic advantages of the proposed method (Figure S8-S10). The non-radiative deactivation of emitting states makes Eu3+-doped compounds sensitive to the composition of the solvent (D2O or H2O) – replacement of high-energy O-H oscillators with O-D groups improves the intensities and quantum yields.3-5,13 On the other hand, hydrogen peroxide can29 quench Eu3+-activated vanadates because a higher amount of O-H groups can coordinate to the emitting species. In both cases, the degree of such effects is probably related to the amount of Eu3+ ions that are effectively exposed to the liquid phase. Figure 3b shows that the relative changes in the luminescence quantum yields under excitation at the 5L6 level 11 ACS Paragon Plus Environment


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(394 nm) were higher in the samples with larger surface areas. Although quantum yields were low for all the samples (qexp394 < 14%) because the absorptivity at 394 nm was also low, in D2O the quantum yield enhanced by 70-80% for the intermediate and final particles as compared to the quantum yield obtained in H2O suspensions. In contrast, the quantum yields of the initial hydroxycarbonates increased by only 10% (Figure 3b). Conversely, the decrease in the quantum yield of the initial hydroxycarbonate particles in the presence of H2O2 was less pronounced, but the intermediate and final particles were more strongly affected by hydrogen peroxide, which revealed improved surface susceptibility to the dispersing medium. The quantum yield of the final mixed particles was the most sensitive to H2O2 because the vanadate phase had higher affinity for peroxide groups and formed peroxocomplexes more easily as compared to phosphates or carbonates.40,41 Hence, mixed particles exposed to H2O2 should bear a higher amount of surface hydroxyl groups, to culminate in more effective luminescence quenching.


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(a)

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Adsorbed Volume (cm g )

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Quantum Yield (%)

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Intermediate

Precursor

(Y,Eu)PO4-(Y,Eu)VO4

(Y,Eu)CO3OH-(Y,Eu)PO4

(Y,Eu)CO3OH.xH2O

Figure 3. (a) N2 adsorption/desorption isotherms at 77 K of the precursor hydroxycarbonate (YCO3OH.xH2O, blue), the intermediate (YCO3OH-YPO4, red), and the final phosphatevanadate (YPO4-YVO4, black) particles. (b) Quantum yields of 1 mmol L-1 colloids of the (Y0.80,Eu0.20) precursor, intermediate, and final particles dispersed in H2O (cyan), D2O (orange) or 1 mol L-1 H2O2(aq.) (magenta) under λexc = 394 nm. (Insets in (a): TEM images illustrating the evolution of the characteristics of the particles on going from the precursor to the final solids).

To evaluate such surface effects, we acquired XPS spectra of pure Eu3+ compounds (i.e., EuPO4-EuVO4, Figure 4 and Figure S11). The spectra evidenced that the phosphate phase emerged at the surface of the precursor hydroxycarbonate: a P 2p signal arose at 133.3 eV for the intermediate particles. In addition, a spin-orbit split (∆=7.6 eV) V2p doublet appeared at 525.2 eV (2p1/2) and 517.6 eV (2p3/2), which agreed with formation of the EuVO4 phase after the second hydrothermal treatment. The appearance of this doublet was related to the reduced 13 ACS Paragon Plus Environment


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relative intensity of the P2p signal at 133.3 eV for the final particles, which was a consequence of the lower surface abundance of phosphates in the final particles as compared to the intermediate particles. The Eu 3d and 4d cores had essentially the same signals in hydroxycarbonate and in the intermediate and final particles (Figure 4a). The Eu3+ 3d3/2 and 3d5/2 peaks appeared at 1165 and 1135 eV, respectively, and the Eu3+ 4d3/2 and 5d5/2 peaks arose at 143.0 and 136.8 eV, respectively. However, the Eu 3d signals exhibited side peaks (1157, 1143, and 1125 eV) ascribed to divalent europium ions.42 After exposure to H2O2 solution, the signals related to divalent europium in final particles became less intense, which qualitatively indicated that a Eu2+/Eu3+ oxidation process took place. This suggested that the europium ions in the final EuPO4-EuVO4 particles were redox-active toward H2O2, which should allow their use as sensors via photo/chemical reduction by previously described methodologies.23,24 Figure 4b also suggested that vanadium ions were redox sensitive to hydrogen peroxide because the V 2p1/2 (525.2 eV) and V 2p3/2 (517.6 eV) signals broadened toward higher binding energies after exposure of the final particles to H2O2. Given that the high binding energy components of V 2p cores are usually ascribed to vanadium(V) ions,43 broadening of the signals toward higher binding energies after exposure to H2O2 could be associated with the oxidation of vanadium(IV) defects initially present in the vanadate phase of the EuPO4-EuVO4 particles. Additionally, the formation of peroxocomplexes and the hydroxylation of surface vanadate groups could broaden the V 2p signals. Moreover, the O 1s signal (~532 eV) became broader and more asymmetric when the final particles were exposed to a peroxide solution (Figure 3b). Because this is a characteristic feature of increased contribution from hydroxylated surfaces as compared to lattice oxygen,43-45 the amount of surface OH groups also increased significantly when the final REPO4-REVO4 particles were placed in H2O2 solutions. Given the high similarity between XPS signals and that the observed effects (i.e., Eu2+ and V4+ oxidation) should not decrease the luminescence quantum yields, the results indicated that the sensitivity of the luminescence of final REPO4-REVO4 14 ACS Paragon Plus Environment

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solids to H2O2 arose mainly from the degree of hydroxylation of the particle surfaces, which culminated in effective quenching of the Eu3+ luminescence in the presence of peroxide in solution.

(a) Eu3+ 3d3/2

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516

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510

150 145 140 135 130 125

Binding Energy (eV) Figure 4. XPS spectra of the precursor EuCO3OH.xH2O (blue), the intermediate EuCO3OH.xH2O-EuPO4 (red), and the final EuPO4-EuVO4 particles before (black) and after (green) exposure to ~10 mol L-1 H2O2(aq.): (a) europium 3d core, (b) oxygen 1s and vanadium 2p cores, and (c) europium 4d and phosphorus 2p cores. (Inset in (b): amplification of the V 2p core signals).

Since the surface state of the final REPO4-REVO4 particles was sensitive to H2O2, the synthesized systems were able to provide a luminescent response that depended on the concentration of peroxide (Supporting Information). Figure 5 illustrates the luminescence profiles of (Y,Eu)PO4-(Y,Eu)VO4 (20% Eu) colloids at different concentrations of H2O2. The excitation signals (Figure 5a), which consisted of a broad band centered at 270 nm due to VO43-→Eu3+ and O2-→Eu3+ energy transfers and low-intensity 4f-4f Eu3+ absorptions between 350 and 500 nm, decreased with increasing concentrations of H2O2. There were no significant changes in the shape or position of the excitation bands. The emission spectra (Figure 5c,d) 15 ACS Paragon Plus Environment


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presented the characteristic set of Eu3+ 5D0→7FJ transitions, which were also affected by different concentrations of H2O2. In these spectra, the integrated intensities correlated with the concentration of H2O2 in a very broad range (Figure 5e,f, right axis and Table S2). In general, the absolute luminescence intensities displayed two ranges of roughly linear dependence upon H2O2 concentrations for both excitation wavelengths, as evaluated by correlation coefficients (R2) and slopes (b) obtained from linear fits of experimental data (i.e., I = a − b log([ H 2 O 2 ]) , where I is the normalized emission intensity and a is the intercept, Table S3). Under 270 nm excitation, the two correlation regions were from 0 to [H2O2]=1 mmol L-1 (R2=0.77 and b=0.062) and from 10 to 400 mmol L-1 (R2=0.86 and b=0.21). For excitation at 394 nm, higher correlations and steepest slopes were attained for [H2O2]>50 mmol L-1 (R2=0.95, b=0.30), whereas concentrations from 0 to 10 mmol L-1 yielded a b=0.078 slope with R2=0.75. Such observations agreed with the behavior recently reported for (Y,Eu)VO4 and EuVO4 nanoparticles29 – the absolute luminescence intensities provided a direct indication of the concentration of H2O2. In addition, a similar surface state-dependent activity of Eu3+doped compounds has recently been reported, and the degree of surface hydroxylation in YPO4:Eu3+ nanorods has been shown to be directly related to the enhanced or quenched luminescence induced by the presence of arsenates in solution.46 The REPO4-REVO4 mixed systems described here offered an additional advantage: not only the absolute intensities, but also the ratios between the intensities could be used to correlate Eu3+ luminescence with the concentration of H2O2 (Figure 5e,f, left axis). The ratio between the hypersensitive 5D0→7F2 and the magnetic dipole-allowed 5D0→7F1 Eu3+ transitions (I02/I01) has been extensively used as a descriptive parameter of the chemical environment occupied by lanthanoid ions.13,14 In the (Y,Eu)PO4-(Y,Eu)VO4 system, the luminescence spectra involve contributions from Eu3+ emissions in both the phosphate and vanadate phases. Although the phosphate and vanadate phases are structurally similar, they provide remarkably different chemical environments, to result in highly different I02/I01 ratios for each phase.16 16 ACS Paragon Plus Environment

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Moreover, the vanadate phase provides more covalent and polarizable sites, so the hypersensitivity of ∆J = 2,4 transitions to alterations in the environment is expected to be more pronounced in REVO4 than in the phosphate phase. Consequently, the hypersensitive Eu3+ 5D0→7F2 transition should be affected to a larger extent in the vanadate phase of the prepared (Y,Eu)PO4-(Y,Eu)VO4 particles. In addition, given that hydrogen peroxide tends to preferentially coordinate to vanadate groups, more pronounced quenching should occur in the REVO4 phase, which is more directly affected by surface hydroxylation. Indeed, because quenching of Eu3+ emissions happens to different extents in phosphate and vanadate phases, a non-constant intensity ratio is observed when (Y,Eu)PO4-(Y,Eu)VO4 particles are exposed to different concentrations of H2O2. This effect is more clear in emission spectra with normalized intensities (insets in Figure 5c,d and Figure S12). Because individual (Y,Eu)PO4 and (Y,Eu)VO4 particles do not display significantly different I02/I01 ratios with different concentrations of H2O2, this ratiometric approach is possible only when the Eu3+ emissions are combined in two phases (although absolute intensities are reduced, Figures S13-S14).



(a)

7.0

5

7

L6← F0

0.8 0.6 0.4 380

390

400

410

420

λ (nm)

0.2

1.0 6.8

[H2O2]=0

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6.6 I02/I01 ratio

[H2O2]=0

Relative Intensity (a.u.)

1.0

1.0


(e)

(c)

0.6 590

600

610

0.2

620

630

λ (nm)

0.4 [H2O2]=400 mmol L

300

350

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0.6 6.0 0.5 5.8 0.4 5.6

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[H2O2]=400 mmol L-1

0.0

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600

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700

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10-2

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D2O

D2O [H2O2]=0

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-3

[H2O2]=400 mmol L

-1

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Time (ms)

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3.6

0.8 590 600 610 620 630

0.6 0.4

1.1

3.8

I02/I01 Ratio


10-1

[H2O2]=0

1.0

λ (nm)

[H2O2]=400 mmol L

-1

0.9 3.4

2.8

0.0

2.6

600

650

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3.0

0.2

550

0.8

3.2

10-3

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10-2

10-1

100

101

102

0.5 103

[H2O2] (mmol L-1)

Figure 5. (a) Excitation spectra (λem=615 nm; inset shows an amplification of the 5L6→7F0 absorption), (b) normalized 5D0 decay curves monitoring the Eu3+ 5D0→7F2 transition (λexc=270 nm), and (c,d) emission spectra at (c) λexc=270 nm and (d) λexc=394 nm of 1 mmol L-1 (Y,Eu)PO4-(Y,Eu)VO4 (20% Eu) colloids at concentrations of H2O2 from 0 to 400 mmol L-1. (e,f) Dependence of the I02/I01 ratios (left axis, blue squares) and of the normalized intensities (right axis, red circles) of (Y,Eu)PO4-(Y,Eu)VO4 particles upon the concentration of H2O2 under (e) λexc=270 nm and (f) λexc=394 nm excitation. [In (b), × denotes the luminescence decay curve of 1 mmol L-1 particles in D2O; insets in (b) and (c) show normalized emission spectra in the 5D0→7F1 and 5D0→7F2 transitions range. Open symbols in (e) and (f) denote [H2O2] = 0 ].

Results in Figure 5e,f suggested that the I02/I01 ratio and the concentration of peroxide also correlated in two distinct ranges to give roughly linear monologarithmic relations within a ~10% relative error (i.e., (I 02 / I 01 ) = c − s log([ H 2 O 2 ]) , Table S3, where c is the intercept and s is the slope). Excitation at 270 nm provides low slopes and correlations at small H2O2 concentrations (R2=0.20, s=0.024), but better results were observed for [H2O2]>10 mmol L-1 (R2=0.79, s=0.58). Concerning the use of I02/I01 ratios, excitation at 394 nm provides the best results in this case as further discussed, with correlation ranges at 0-10 mmol L-1 (R2=0.54, s=0.12) and at 10-450 mmol L-1 (R2=0.86, s=0.24). 18 ACS Paragon Plus Environment

Normalized Intensity (a.u.)


4.0

100

0.3 103

[H2O2] (mmol L-1)

(d)

(b)


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Hydrogen peroxide caused a luminescence quenching effect that could also be assessed by analysis of the luminescence decays (Figure 5b and Table S2) because increasing concentrations of H2O2 progressively reduced the 5D0 excited state lifetimes. Hence, the presence of H2O2 should provide effective non-radiative deactivation pathways to depopulate the Eu3+ excited states and consequently reduce the luminescence intensities and lifetimes. Figure 5b also evidenced a progressive deviation from the monoexponential behavior with increasing concentrations of H2O2. This occurred because luminescence quenching resulted in Eu3+ emissions of comparable intensities in the phosphate and vanadate phases, making the occupation of different sites more clearly measurable by luminescence lifetimes. Using 5D0 luminescence lifetime results, we constructed a Stern-Volmer plot in order to confirm the correlation between quenching parameters and peroxide concentrations (Figure 6a). In this case, experimental data were fitted by a (τ 0 / τ EXP ) = 1 + k qτ 0 [ H 2 O2 ] [ Eu 3+ ] relation, where τ0 is the 5D0 luminescence lifetime for particles in pure water, τEXP is the lifetime at different concentrations of H2O2 and kq is the Stern-Volmer quenching constant for this system. The results evidence a very good correlation (R2=0.98) between luminescence lifetimes and the amount of quencher species at low peroxide concentrations (0

Vanadate Mixed ... - ACS Publications

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