Article Chemistry of Materials New Synthesis is published by the American Chemical Strategies for Society. 1155 Sixteenth Street Subscriber access provided by N.W., NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
Luminescent YVO4:Eu and EuVO4 Chemistry of Materials is published by the American Chemical Nanoparticles with Society. 1155 Sixteenth Street Subscriber access provided by N.W., NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
H2O2 Selective Sensing Properties Chemistry of Materials is published by the
Natacha Duée, Chrystel American Chemical Society. 1155 Ambard, Franck Pereira, Sixteenth Street N.W.,
Subscriber access provided by NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
David Portehault, Bruno Viana, Karine Valle, Chemistry of Materials Denis Autissier, and is published by the Chemical Clement American Sanchez Society. 1155
Sixteenth Street Subscriber access provided by N.W., NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/ acs.chemmater.5b01083 Chemistry of Materials is published • Publication Date by the American Chemical (Web): 29Society. Jun 2015 1155 Sixteenth Street Subscriber access provided by N.W., NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
Downloaded from http:// pubs.acs.org on July 6, 2015 Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street Subscriber access provided by N.W., NEW Just Accepted Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
“Just Accepted” manuscripts have been online prior to technical Chemistry of editing, Materials formatt is published the Society provides “Just byAccepted” as American Chemical dissemination of scientific material as s Society. 1155 appear in full in PDF format accompanie Sixteenth Street N.W., Subscriber access provided by NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
fully peer reviewed, but should not be co readers and citable by the Digital Objec Chemistry of Materials to authors. Therefore, the “Just Accept is published by the in the journal. After a manuscript is tec American Chemical 1155 published as a Accepted” WebSociety. site and Street Subscriber provided by N.W., NEW changes toaccess theSixteenth manuscript Washington, DC text and/or YORK UNIV
20036 Published by American Chemical Society. Copyright ©
and ethical guidelines that apply to th or consequences arising from the use Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street Subscriber access provided by N.W., NEW Washington, DC YORK UNIV 20036 Published by American Chemical Society. Copyright ©
Page 1 of Chemistry 10 of Materials
1 2 3 4 5 6 7 8
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 10
New Synthesis Strategies for Luminescent YVO4:Eu and EuVO4 Nanoparticles with H2O2 Selective Sensing Properties Natacha Duée1,2, Chrystel Ambard1, Franck Pereira1, David Portehault2, Bruno Viana3, Karine Vallé1, Denis Autissier1, Clément Sanchez2,* 1
CEA, DAM, France
2
Sorbonne Universités UPMC Univ Paris 06 - CNRS - Collège de France, UMR 7574 Chimie de la Matière Condensée de Paris Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France
3
PSL Research University, Institut de Recherche de Chimie Paris, CNRS – Chimie ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France
ABSTRACT: Nowadays, because of safety demands, the controlled design of efficient and selective sensors for hydrogen peroxide is paramount. Therefore we develop herein new strategies of aqueous synthesis of crystalline YVO4:Eu and EuVO4 nanoparticles based on a rigorous control of the pH and of the nucleation step via microwave heating. These routes allow a precise control of composition, nanostructure and surface states of the resulting luminescent nanoparticles that are structurally and optically characterized via a large set of modern techniques. Moreover, these nanoparticles exhibit reproducible optical responses that are highly selective to hydrogen peroxide and present excellent detection thresholds as low as 0.05 ppm in solution. These remarkable performances allow the design of a new family of H2O2 sensors, which surpass state-of-the-art inorganic optical sensors in liquid phase detection tests.
INTRODUCTION Over the past years, there has been a growing interest in the detection and quantification of hydrogen peroxide (H2O2). This molecule is indeed involved in a broad range of applications, including food-processing1, pharmaceutical, cosmetic2 and chemical industries,3 as well as biology4-5 and environment.6 H2O2 is also a decomposition product of triacetone triperoxide (TATP), which is an explosive increasingly used in terrorist activities.7 For this reason, detecting H2O2 is a paramount challenge for safety applications.8-9 Analytical methods are used nowadays in order to measure the concentration of H2O2 in various media.10-11 These techniques are efficient but can seldom be miniaturized and require a specific training. These drawbacks prevent them from an on-site use. Therefore, developing sensitive, selective, inexpensive and portable technologies for the detection of H2O2 is a really challenging and important goal. Fluorescent chemical sensors have been widely exploited for the detection and quantification of explosives traces.12-13 Their performances are mainly dependent on the sensitive material. Fluorescent organic materials are efficient in terms of sensitivity, selectivity and response time14 but their quick degradation is a major drawback15. Sol-gel materials offer a much longer life span, especially
in the case of inorganic sensitive coatings.16-17 Nanoparticles provide an even bigger interest thanks to their important surface-to-volume ratio that enhances the interaction between the target and the sensitive material. Rare earth ion-doped yttrium orthovanadates such as YVO4:Eu have been the subject of many researches, notably because of display applications.18-21 They have also attracted recent interest as sensors. Especially, Casanova et al.22 demonstrated that YVO4:Eu nanoparticles enable detection of H2O2 in a range of about 0.2 to 200 ppm. However, this efficient method suffers from the requirement of a preliminary treatment of the sensor: the nanoparticles must be photobleached to ensure sensing. Because surface states are of prime importance to control sensing properties, the development of a reliable synthetic pathway toward calibrated doped yttrium vanadate nanoparticles is a prerequisite to tune and enhance sensing properties, while strongly binding surface ligands used to stabilize nanoparticle surfaces23 should be avoided to ensure fast sensing. Riwotzki and Haase18 reported the preparation of colloidal solutions of nanocrystalline lanthanide doped YVO4 by a hydrothermal method at 200 °C. Huignard et al.24 described an optimized synthesis of colloidal YVO4:Eu nanoparticles through classical precipitation reactions. The same team25 reported another ap-
1
ACS Paragon Plus Environment
Page 3 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
proach to yttrium orthovanadate nanocrystals by direct precipitation from Y3+ and VO43- salts in water, using citrate complexing agents to limit the size of particles and to increase their stability. The process leads to stable and highly concentrated transparent colloidal solutions in water. Herein we present new approaches for the synthesis of efficient hydrogen peroxide sensors based on YVO4:Eu nanoparticles. Our first aim is to provide a comparative study of different preparation methods of YVO4:Eu and EuVO4 nanoparticles, in order to select the best route to design a reliable and efficient sensitive material. We develop new methods combining pH control over the whole precipitation process together with microwave treatment. This approach is discussed in view of two reference syntheses reported by Huignard et al.24-25 The second goal is to optimize the material for the application, especially by finding the best europium rate to provide highly sensitive H2O2 detection in liquid phase. EXPERIMENTAL SECTION Syntheses The titrator used to ensure constant pH in methods a and b is a Titroline alpha Schott apparatus. The microwave oven used in method a is an Anton Paar Synthos 3000 multimode oven equipped with an immersion temperature probe. The temperature is risen to 150 °C in 3 min and maintained during 15 min. Method a. YVO4:Eu nanoparticles are synthesized by precipitation. 50 mL of a solution of Y(NO3)3,6H2O and Eu(NO3)3,5H2O with a total concentration of metal cations reaching 0.1 mol·L-1 is added dropwise to 37.5 mL of a 0.1 mol·L-1 Na3VO4 solution at pH 12.5. A white precipitate is observed. pH decreases during the addition. When the pH reaches a chosen value (9, 10 or 11), a titration device is used to maintain a constant pH value, by adding aliquots of a 0.2 mol·L-1 solution of tetramethylammonium hydroxide. After the addition is completed, the suspension is treated in a microwave oven during 15 min at 150 °C (4.7 bar). After cooling, the suspension is dialyzed against water over three days, during which water is renewed two times a day. The molecular weight cut-off of the dialysis membrane was 3500 Da. Powders are collected by drying of the colloidal solutions at ambient temperature. In addition, the obtained opaque suspension can be either diluted by water to a concentration of 10-3 mol·L-1, or dialyzed a second time against ethanol for solvent exchange. Method b. The procedure is similar as method a, except that the thermal treatment under microwave irradiation is replaced by conventional heating at 60 °C for 30 min. Purification and powders recovery are performed as in method a.
Method c. The synthesis is the same as in method b, without any control of the pH, according to the protocol previously described by Huignard et al.24 Method d. YVO4:Eu nanoparticles are synthesized by precipitation according to another work of Huignard et al.25 using citrate ions to limit the growth of the nanoparticles and to stabilize the colloidal solution. Briefly, 30 mL of a 0.1 mol·L-1 solution of sodium citrate is added to 40 mL of a solution of Y(NO3)3,6H2O and Eu(NO3)3,5H2O with a total concentration of metal cations reaching 0.1 mol·L-1. 30 mL of a 0.1 mol·L-1 Na3VO4 solution at pH 12.5 is then added to the mixture. The solution is aged during 30 min at 60°C. Purification and powders recovery are performed as in method a. Characterizations The powders compositions have been determined by ICPAES with a Horiba Jobin Yvon apparatus, Model Activa. Dynamic light scattering measurements were made on a Malvern Zetasizer Nano-ZS apparatus. Powder X-ray diffraction (XRD) studies were performed on a Panalytical X’Pert Pro and a d8 Brucker Advance diffractometers with CuKα radiation. Peak identification was performed by comparison with reference ICCD cards 00-015-0809 and 04-008-2103 for tetragonal EuVO4 and YVO4, respectively. Coherence lengths were determined using Scherrer’s formula by measuring the full width at half-maximum of the diffraction peaks. Specific surface areas were estimated by nitrogen sorption at 77 K according to the BET method thanks to a Micromeritics Tristar 3000 model. Transmission electron microscopy (TEM) was performed using a Tecnai Spirit G2 microscope operating at 120 kV, and high resolution TEM (HRTEM) was performed at Institut des Matériaux de Paris Centre, UPMC Univ Paris 6, on a 200 kV JEOL JEM 2011 microscope. The samples were prepared by depositing a drop of the colloidal suspensions on a carbon grid. Absorption spectra were recorded on a Lambda 900 Perkin Elmer spectrometer. Excitation and emission spectra, as well as photobleaching curves, were recorded on a Horiba Jobin Yvon FluoromaxP spectrophotometer. Photobleaching curves were obtained by acquiring the evolution of the 617 nm emission intensity under either a continuous excitation at 270 nm or under a limited excitation when using the antiphotobleaching option of the spectrophotometer. In this case, a shutter closes the excitation slit except during the data acquisition every 10 seconds. Luminescence decay curves were measured under excitation coming from a tunable optical parametric oscillator excitation source Ekspla NT 342B-SH covering UV, visible and NIR range. Decay profiles were analyzed with a Jobin Yvon HR250 monochromator coupled to a Roper ICCD camera.
2
ACS Paragon Plus Environment
Chemistry of Materials
The quantum yields were calculated using the following equation, valid for an optical density ranging between 0.01 and 0.1.
with: R referring to the reference fluorophore (rhodamine 6G), Q the quantum yield, m the slope of straight line (area of the fluorescence peak) = f (absorbance at 280 nm), n the refractive index of the solvent. Luminescence measures were performed using an excitation wavelength of 280 nm at 21 °C. Sensing properties were evaluated through the Stern– Volmer constant KSV calculated using the following equation:
With: I0 the initial fluorescence intensity, I the fluorescence intensity after the addition of H2O2, [H2O2] the concentration of the H2O2 solution. RESULTS AND DISCUSSIONS Syntheses Different methods of aqueous precipitation have been explored for the synthesis of Y1-xEuxVO4 nanoparticles. Briefly, methods a and b consist in adding Y, Eu sources to vanadates at constant pH values (9, 10, 11) along the mixing procedure. Methods c and d are reference syntheses. Method c is similar to method b except that the pH is not controlled. Method d consists in adding vanadates to Y, Eu precursors in the presence of citrates to limit particle growth. Microwave-assisted hydrothermal posttreatment was performed at 150°C in method a while methods b, c and d correspond to common aging at 60°C. In the case of methods b and c, after aging of the colloidal solution, the diffusing suspensions recovered after purification have Y and Eu concentration of about 0.1 mol·L-1 and show poor colloidal stability. Indeed, the particles quickly flocculate into 200 nm aggregates as detected by dynamic light scattering. Enhancement of the dispersion (140 nm aggregates) by microwave treatment (method a) could originate from higher crystallinity of the particles, resulting in a modification of the surface density of oxo-hydroxo groups and then a change in surface charges. The colloidal solution obtained by method d is transparent and stable for several years. After dilution to a concentration of about 10-3 mol·L-1, stable colloidal suspensions can be conserved for all methods during several weeks without stirring (100 nm aggregates). x values in Y1-xEuxVO4 were varied from 0 to 1 for the different methods by adding corresponding amounts of Y and Eu precursors. In all cases, elemental analyses show that the actual composition corresponds to the ratio introduced. XRD (Figure 1) indicates that all methods lead
to the same pure compounds. Patterns of EuVO4 (x = 1) powders are consistent with the corresponding tetragonal structure, while Y0.95Eu0.05VO4 samples crystallize into the tetragonal zircon phase of bulk YVO4. The europium ion substitutes yttrium in the YVO4 lattice.18 For methods a, b, and c, peak narrowing indicates that the crystallite size increases with a decrease of the europium content. Broad peaks for method d show that the corresponding samples are poorly crystalline. EuVO4 and Y0.95Eu0.05VO4 were especially scrutinized. For EuVO4, the microwave-assisted treatment in method a (pH 9) leads to 17 nm coherence lengths, while methods b (pH 9) and c result in smaller coherence lengths of 15 nm. The coherence length is estimated at 4 nm for method d, which yields much wider XRD peaks. This small value originates from the small size of the particles obtained by using citrates as surface stabilizing ligands. For Y0.95Eu0.05VO4, method a results in 15 nm coherence length while method c leads to higher coherence lengths of 18 nm. Noteworthy, pH in method b influences the crystallite size, reaching 14, 17 and 19 nm at pH 9, 10 and 11, respectively. TEM pictures show that the powders are made of nanoparticles (Figure 2). Electron microscopy is consistent with analyses of XRD patterns. Indeed, methods a (pH 9), b (pH 9) and c lead to particles ranging from about 20 to 40 nm. Particles resulting from method d are smaller than 5 nm. Maintaining pH at higher values (10 and 11) in method b (Figure 2–d) results in a large polydispersity. In all cases the particles seem to be the result of the aggregation of primary nanoscale units. This observation is particularly clear from the sample obtained at pH 11 (Figure 2–d), with “grapefruit”-like nanoparticles. Lattice fringes evidenced in Figure 2–b with corrugated surfaces suggest that microwave-heated samples also originate from the aggregation of primary particles.
d - EuVO4
c - EuVO 4
Intensity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 10
b - pH 9 - EuVO4
b - pH 9 - Y 0. 95Eu 0.05VO4
a - pH 9 - EuVO4 15
20
25
30
35
40
45
50
55
60
65
70
75
2 theta (°)
Figure 1. XRD data of EuVO4 and Y0.95Eu0.05VO4 powders obtained by methods a, b, c and d, reference pattern (ICDD 00-015-0809) for tetragonal EuVO4.
3
ACS Paragon Plus Environment
Page 5 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
26
Fleury et al. suggested recently that YVO4:Eu particles originate from the coalescence of amorphous spheroidal primary units, which crystallize in an oriented manner by lattice matching between neighboring building blocks, to ensure lowered strain energy. Amorphous area in particles from method b at pH 9 and 11 (Figure 3a, b) might be reminiscence of this amorphous initial state. These poorly crystalline parts are distinguishable between nanorods with a diameter of 5 nm (Figure 3b), which seem to aggregate within secondary structures. Further heat treatment exacerbates anisotropy, as evidenced in microwave-treated Y0.95 Eu0.05VO4 (Figure 3c, method a). Such oriented growth is unlikely to originate from the aggregation of amorphous spheroidal particles. Indeed, deeper examination by HRTEM together with Fourier filtering to highlight the (020) fringes (Figure 3d) demon
Figure 2. (HR)TEM pictures of (a) Y0.95 Eu0.05VO4 (method a, pH 9); (b) Y0.95 Eu0.05VO4 (method a, pH 9); (c) Y0.95 Eu0.05VO4 (method b, pH 9); (d) Y0.95 Eu0.05VO4 (method b, pH 11); (e) Y0.95 Eu0.05VO4 (method c); (f) EuVO4 (method d).
strates the single crystal orientation of the onedimensional (1D) assembly, although a few defectuous areas are still observed. Growth occurs preferentially along the direction perpendicular to the (-102) plane.
Figure 3. HRTEM pictures of Y0.95 Eu0.05VO4 nanoparticles obtained by method b at (a) pH 9 and (b) pH 11. Black arrows highlight amorphous area within the nanoparticles. (c-f) HRTEM pictures and corresponding (020) Fourier filtered images of Y0.95 Eu0.05VO4 nanoparticles obtained by method a (microwave heating at 150°C) at pH 9. Arrows in picture e highlight internal angles at the junction between primary particles. Slight smoothing of these angles occurs as an artifact upon image processing by Fourier filtering in f.
4
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Single crystal behavior for such anisotropic assemblies was only observed through epitaxial self-assembly of primary crystalline nanoparticles by an oriented attachment mechanism.27-28 Further demonstration of ordered assembly can be obtained from the particular internal angles at the junction between two primary particles (Figure 3e, f). Since the 1D assemblies are only encountered for samples heat-treated above 100°C (by microwave or conventional heating (not shown)), we hypothesize that oriented attachment occurs in a second step. First, amorphous aggregates are formed according to Fleury et al., and then secondary growth into anisotropic shapes occurs by oriented attachment of the crystallized nanoparticles. The specific surface area is another crucial information when studying sensors and must be maximized to ensure optimized interaction between the target and the sensitive material. SBET values (Table 1) show that pH control in method b provides an original parameter to control the surface area. Indeed, an increase in pH results in a decrease of the specific surface area, in agreement with the increase in crystallite size. This evolution suggests higher aggregation at higher pH. Actually, larger amounts of base are necessary to maintain pH 11 instead of pH 9. Therefore, ionic strength increases and could screen more efficiently electrostatic repulsions between primary particles, resulting in extended aggregation, as exemplified by big aggregates observed by TEM (Figure 2d). At pH 9, the pore size distribution evaluated by the Barret-Joyner-Halenda (BJH) calculation shows a pore diameter between 10 and 20 nm. Overall, in view of their high surface area values and low crystallite and particle sizes, methods a (pH 9), b (pH 9), c and d are promising to produce efficient powders for detection.
Page 6 of 10
band extending from 250 to 300 nm, peaking at 277 nm. The agreement between absorption and excitation spectra reveals the good energy transfer from the excited vanadate to the europium ions. This emission process has been widely investigated in the literature.18, 29-30 One deviation from the luminescence spectrum reported for YVO4:Eu nanoparticles29 concerns the hypersensitive (∆J=+/-2) transition 5D0-7F2 (Figure 4–b) at about 620 nm. The luminescence patterns of samples originating from different methods differ between 613 and 617 nm. While the 613 nm emission is predominant with methods a and b, the 617 nm emission is more intense when methods c and d are used. Some variations are also observed in the 5 D0-7F4 transition at about 700 nm. Therefore, we can assume that the pH control affects the europium ions environment. It is well known that the Eu3+ ion can probe its crystal environment.31 The symmetry of the Eu3+ site in the crystal lattice leads to a characteristic intensity pattern of the luminescence peaks. Reisfeld et al.32 have shown that the ratio I(D0-F2)/I(D0-F1) gives an estimation of the Eu3+ site symmetry. When the ratio is inferior to 1 the site is totally symmetric. A ratio of 10 is attributed to very low symmetry sites. In our case, the ratio is around 5, as observed in the bulk material.
Optical properties Neither the UV-visible absorption nor the excitation spectra (Figure 4–a) show differences between the four preparation methods. The absorption spectrum shows a broad band peaking at 278 nm that is typical of the charge transfer from the oxygen ligands to the central cation of VO43ions. The excitation spectrum exhibits a strong Sample
Preparation method
Specific surface (m2.g-1)
EuVO4
a (pH 9)
93 ± 5
Y0.95 Eu0.05 VO4
a (pH 9)
108± 5
EuVO4
b (pH 9)
114 ± 5
Y0.95 Eu0.05 VO4
b (pH 9)
116 ± 5
Y0.95 Eu0.05 VO4
b (pH 10)
92 ± 5
Y0.95 Eu0.05VO4
b(pH 11)
66 ± 4
EuVO4
c
168 ± 5
Y0.95 Eu0.05 VO4
c
140 ± 5
EuVO4
d
130 ± 5
Figure 4. (a) Absorption and excitation (λem = 617 nm) spectra of EuVO4 colloids obtained by method a at pH 9; (b) Emission (λexc = 270 nm) spectra of EuVO4 colloids obtained by methods a (pH 9), b (pH 9), c and d. (c): focus on the 5D07F1,2 and (d) 5D07F4.
Table 1. Specific surface areas SBET of YVO4:Eu powders.
5
ACS Paragon Plus Environment
Page 7 of 10
Quantum yields (see Table 2) were found slightly smaller than reported in the literature.29 They are also divided by five compared to the bulk material. A first explanation of this difference in yields from bulk to nanomaterials is the quenching of the europium luminescence on the surface of the particles by water molecules.29 A second reason was reported by Riwotzki and Haase33. Energy transfer to europium could only take place from vanadate first neighbors. Therefore, transfers from distant vanadates groups to surface sites are highly probable in nanoparticles, leading to de-excitation of the vanadate species. Consequently, a low quantum yield of 8.5% is observed. For EuVO4 and Y0.5Eu0.5VO4 nanoparticles, because of the high amount of europium ions, an additional quenching path takes place from energy transfer between neighboring europium ions, leading to a major drop of the quantum yield.29 The microwave treatment of method a improves efficiently the quantum yield compared to method b-pH 9. This shows that the energy losses are reduced by the microwave treatment. All in all, the
Sample
Preparation method
Quantum yield (%)
EuVO4
a (pH 9)
7.8 ± 0.1
EuVO4
b (pH 9)
3 ± 0.1
Y0.5 Eu0.5 VO4
b (pH 9)
5.5 ± 0.1
Y0.75 Eu0.25 VO4
b (pH 9)
12.3 ± 0.2
Y0.95 Eu0.05 VO4
b (pH 9)
8.5 ± 0.1
EuVO4
c
3.2 ± 0.1
EuVO4
d
5.5 ± 0.1
Table 2. Quantum yields of YVO4:Eu colloids. 120
b (antiphotobleaching) 100
highest luminescence quantum yield is obtained for a europium concentration of 25%. The photobleaching of the EuVO4 particles is very fast under UV irradiation (Figure 5). The decrease in the luminescence intensity is higher for methods c and d than for methods a and b. Photobleaching in citratederived vanadates (method d) has also been reported by Takeshita et al.34, who showed that the surfacecomplexing citrate anions are involved in the photobleaching process. Our results show that the structure and surface state are also involved in photobleaching. Furthermore, bleaching is negligible when the amount of europium in the material is low (3% intensity loss in 30 min for Y0.95Eu0.05VO4 preparation method b). A stationary mode is reached and the luminescence intensity becomes insensitive to the UV irradiation. The intensity of the stationary mode differs from one sample to another, showing that the optical properties are really sensitive to small changes in the particles. Furthermore bleaching can be eliminated by limiting the UV exposition time with the antiphotobleaching option of the spectrometer (see Figure 5). The EuVO4 5D0 lifetimes obtained by methods a and b are higher than for methods c and d (Table 3). The lifetime measured for Y0.95Eu0.05VO4 synthesized by method b at pH 9 (460 µs) is lower to the 740 µs value published33 for Y0.95 Eu0.05 VO4 nanoparticles and indeed much lower than the 1100 µs published by Huignard et al.25 (for samples prepared by method d) and close to the lifetime of the bulk material (525 µs). This could be due to transfers by energy diffusion toward surface defects and nonresonant mechanisms between rare-earth ions that depend strongly on the nanocrystal preparation and quenching phenomenon.35 Difference between Y0.95 Eu0.05 VO4 and EuVO4 can be easily explained by the large extent of energy migration and losses at high europium percentage. H2O2 sensor properties
b
Time (min)
The luminescence response upon exposure to liquid hydrogen peroxide is investigated for the nanoparticles dispersed in water. When some H2O2 is added to the YVO4:Eu colloids the luminescence intensity is reduced (Figure 6) in a few seconds. All transitions are concerned by this intensity decrease. Notice that figure 6 represents the integrated intensity which is the most important parameter for sensor. The phenomenon can also be observed to the naked eye under UV rays as an extinction of the red luminescence when the amount of H2O2 added is sufficiently high.
Figure 5. Photobleaching curves (λexc = 270 nm, λem = 617 nm) of EuVO4 obtained by the preparation methods a, b, c and d: top, measurement made using an “antiphotobleaching” option of the spectrometer limiting the exposure time to UV to the minimum duration; bottom, measurements made under continuous excitation.
The europium percentage was shown above (Tables 2 and 3) as a paramount parameter to control the optical properties of vanadates. Its role in the sensing properties was then studied. Aliquots of H2O2 are added to YVO4:Eu colloids containing varying percentages of europium. Luminescence spectra are acquired before and
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
80
a 60
c
40
d
20
0 0
10
20
30
40
50
60
6
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
after the addition. The 5D0 intensity decrease is then measured (Table 4). The higher the europium percentage in the particles, the better the sensitivity in H2O2 detection (Table 4). Indeed, EuVO4 particles show the highest sensitivity. The reason for such a dependence on the Eu content may be linked to the defects concentration in the structure or could be related to the energy migration toward surface Eu3+ species in the nanoparticles. Interestingly, Casanova and coworkers22 observed an opposite trend during detection with YVO4:Eu particles: after photobleaching, H2O2 led to an increase in the luminescence intensity and suggest a different sensing mechanism. Again, this observation points at the importance of defects and changes in morphology, crystallinity in the detection from our low temperature-derived material. The decay profiles (Supplementary information) have been measured for a better understanding of the detection mechanism. For different excitation wavelengths, corresponding to vanadates or Eu3+ excitation, the fluorescence intensity decreases, thus showing that the detection does not involve the energy transfer from vanadate groups to europium species, but that H2O2 solely impacts the europium emission lifetime.
Sample
Preparation method
EuVO4
a (pH 9)
34 µs
EuVO4
b (pH 9)
34 µs
EuVO4
c
4 µs
EuVO4
d
3 µs
Y0.95 Eu0.05 VO4
b (pH 9)
460 µs
5
D0 lifetime
Table 3. 5D0 lifetime of YVO4:Eu colloids. A good repeatability of the detection results is essential in order to obtain a reliable sensor. The repeatability of the detection results was tested on four 0.1 mol·L-1 EuVO4 solutions with the addition of 12 ppm of H2O2. The results show an average diminution of 72% of the luminescence intensity and a standard deviation of 3% for the preparation method b. On the contrary, the standard deviation obtained by methods c and d is high (superior to 10%) and probably not sufficient to design a reliable device. Furthermore, the detection results are extremely dependent on the colloid concentration. The latter is optimized in order to obtain the highest Stern–Volmer constant KSV possible. In optimized conditions corresponding to highly diluted colloids of EuVO4 with 10-3 mol·L-1 concentration (Figure 6), KSV = 2880 L·g-1. As a result, very high sensitivity is obtained, since the detection threshold corresponds to a H2O2 concentration as low as 0.05 ppm. EuVO4 particles therefore surpass stateof-the-art inorganic optical sensors.36 In addition, a linear relationship is observed between the decrease in lumines-
Page 8 of 10
cence and the amount of H2O2 added, in the 0-0.25 ppm range. The sensor is not only highly sensitive, it is also quantitative for H2O2 concentrations below 0.25 ppm. Furthermore, the detection selectivity was tested with 1 ppm of toluene, ethylene glycol, ethanol and acetone. Their addition to EuVO4 (method b, pH 9) suspensions leads to negligible modifications of the luminescence intensity (smaller than 5%), whereas the addition of 1 ppm of H2O2 leads to a 60% decrease. This reveals the good selectivity of the sensor. These detection tests clearly show that the YVO4:Eu, and more precisely EuVO4 nanoparticles are promising for the detection of hydrogen peroxide. DISCUSSION All the syntheses described in this work lead to YVO4:Eu nanoparticles with adjustable Eu content showing interesting properties. They notably all allow to detect a few ppm of hydrogen peroxide within a few seconds, while EuVO4 nanomaterials show the highest sensitivity. According to decay profiles (SI), the mechanism is related to quenching of the Eu3+ emission. Increasing the Eu content enhances energy transfer through the particles to the surface. Therefore, the detection is most likely associated to surface modifications through H2O2 exposure. One can suggest that such an effect might be related to changes in the europium coordination polyhedra, via the formation of europium-peroxo complexes, as already reported in the litterature.38
Sample
5 D0 luminescence intensity Preparation decrease after the addition method of 12 ppm of H2O2 (%)
EuVO4
a (pH 9)
49 ± 6
EuVO4
b (pH 9)
70 ± 6
EuVO4
c
48 ± 24
EuVO4
d
42 ± 22
Y0.5 Eu0.5 VO4
b (pH 9)
55 ± 6
Y0.75 Eu0.25 VO4
b (pH 9)
20 ± 6
Y0.95 Eu0.05 VO4
b (pH 9)
7±6
5
Table 4. D0 luminescence intensity decrease after the addition of 12 ppm of H2O2 in solution. Because of the targeted applications, for instance as explosive sensors, a perfect control of the reproducibility of the synthesis and the performances of the material is a crucial parameter. The optical properties of the nanoparticles are clearly highly sensitive to changes in the morphology, structure, crystallinity, specific surface and so forth. If the specifically high proportion of surface atoms in nanostructured materials is a clear advantage for lowered detection threshold, it is also a source of non-
7
ACS Paragon Plus Environment
Page 9 of 10
reproducibility. Indeed, small changes in the particles bring major modifications to the detection tests results. This drawback was specifically encountered with nanoparticles originating from previously described protocols (methods c and d). The main issue to be addressed in the aqueous precipitation technique is the control of the pH. Actually, Huignard et al.25 showed that it is a critical parameter that must be well controlled to ensure the yttrium orthovanadate phase formation. Indeed, the acidification of a vanadate solution leads to a spontaneous condensation.37 The species obtained at pH
