Communication pubs.acs.org/cm
Gas-Responsive Photoluminescence of YVO4:Eu3+ Nanoparticles Dispersed in an Ultralight, Three-Dimensional Nanofiber Network Satoru Takeshita,* Yoshihiro Takebayashi, Hiroyuki Nakamura, and Satoshi Yoda Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan S Supporting Information *
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>90% porosity, and related oxide aerogels are suitable for supporting many nanoparticles,19,20 but a thick skeleton of particle-neck-particle structure interferes with gas accessibility to nanoparticle surfaces. Their extremely fragile nature also limits their practical uses. Graphene-based porous carbons have been extensively studied for battery and catalysis applications;21 however, the plate-like structures of graphitic carbons hinder gas permeation. They also cannot be used with luminescent materials due to their black color. All-inorganic luminescent aerogels have also been reported,22 but they lack mechanical toughness because of the absence of an elastic component. We propose that fixing the nanoparticles inside a threedimensional ultrafine nanofiber network is the best way to resemble “free-standing nanoparticles in the air” without losing mechanical toughness. The main issue of this approach is to control the network homogeneity at the nanoscale to attain transparency, i.e., low light scattering. Among the variety of possible nanofibrous polymers,23−25 only few have been succeeded used to construct a transparent monolith of threedimensional nanofiber network.26 One successful example is the cross-linked chitosan aerogel recently reported from our group.27 Chitosan is a biobased polysaccharide feasible for diverse applications due to its low cost and environmentally friendliness. In this paper, we describe the preparation of an ultralight nanocomposite aerogel of chitosan nanofibers/ YVO4:Eu3+ nanophosphor and demonstrate its interactive PL behavior with formaldehyde vapor. The key point to achieve a homogeneous distribution of nanoparticles in the nanofiber network is to prevent the aggregation of nanoparticles and/or chitosan chains during preparation. Simply mixing chitosan solution and nanoparticle colloid leads to rapid formation of a white precipitate of aggregates through multisite adsorption of chitosan chains onto nanoparticle surfaces. Thus, we developed a sequential soaking method as shown in Scheme 1. First, cross-linked chitosan hydrogel was prepared by mixing chitosan and formaldehyde solutions, and the hydrogel was soaked in Na3VO4 solution with a certain concentration, Cyv (0.005 ≤ Cyv ≤ 0.1 mol L−1). The preformed gel matrix provides NH2 (partly NH3+) groups, which attract VO43− anions and become nucleation sites of the nanoparticles. Then the VO43−-containing hydrogel was soaked in (Y,Eu) (NO3)3 solution at the concentration Cyv, resulting in in situ formation of YVO4:Eu3+ nanoparticles in the gel matrix.
anophosphors, inorganic luminescent nanoparticles, have attracted much attention in recent decades for their potential industrial uses and fundamental scientific interest.1,2 Nanosizing the phosphor particles induces some interesting features that are not observed in bulk materials, such as optical transparency, quantum effects, and surface activities due to their high surface areas.3−5 In particular, dynamic physicochemical phenomena occurring at nanophosphor surfaces lead to environmentally responsive photoluminescence (PL), e.g., PL enhancement and quenching by the surrounding media,6,7 fluorescent resonance energy transfer (known as FRET) with surface species,8−10 and PL switching by surface redox reactions.11,12 These interactive PL behaviors have opened up new methodologies for imaging and sensing.13,14 However, most of the previous studies have focused on biomedical applications because they mainly used colloidal nanophosphors, which interact efficiently with solute species in vitro and in vivo but not with gases in a dry environment. To realize an efficient interaction with gas molecules, nanophosphors need to be homogeneously dispersed and fixed in a transparent and highly gas permeable matrix without losing surface activities, as if they were “free-standing in the air.” In this study, we have succeeded in synthesizing a structurally new monolithic nanocomposite close to this imaginary structure by fixing Eu3+-doped YVO4 (YVO4:Eu3+) nanoparticles in an ultralight three-dimensional network of chitosan nanofibers. To fabricate a free-standing monolith containing functional nanoparticles, a supporting matrix is practically essential as a physical scaffold. Most of the nanoparticle composites reported so far are composed of inorganic nanoparticles embedded in a dense polymer resin, which provides not only mechanical toughness and processability, but also physicochemical stability.15 In other words, the dynamic chemical activities of nanoparticle surfaces are blocked by the surrounding resin. For example, we previously discovered photobleaching behavior of a YVO4:Bi3+,Eu3+ nanophosphor/polyurethane composite: The nanophosphor surface exhibited photoredox reactions with the resin, resulting in oxidative decomposition of the resin accompanied by reductive photobleaching of the nanophosphor.16−18 This effect can be applied to interactive PL devices, e.g., a photooxidative cleaner for pollutant gases combined with a sensing function, but only if we can utilize the nanoparticle surfaces without them being blocked by the surrounding resin. Ultralight porous materials have been focused as matrices for functional nanoparticles because they make efficient use of nanoparticle surfaces. Silica aerogel, i.e., highly porous silica of © XXXX American Chemical Society
Received: September 29, 2016 Revised: November 11, 2016
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DOI: 10.1021/acs.chemmater.6b04160 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
linkings (CN species),28 which does not affect the red PL of YVO4:Eu3+. As shown in Figure 2, YVO4:Eu3+ nanoparticles are distributed in the three-dimensional network of chitosan
Scheme 1. Preparation of Chitosan/YVO4:Eu3+ Composite Aerogels
The gel matrix prevents aggregation of the nanoparticles by fixing them on the nanofibers. The obtained hydrogel was soaked in methanol to exchange the solvent and then dried with supercritical CO2 to give the composite aerogel. XRD and IR analyses (Figures S1−S3) confirm the formation of crystalline YVO4 with the tetragonal zircon-type phase in the amorphous chitosan matrix. Figure 1 shows photographs of chitosan/YVO 4 :Eu 3+ composite organogels and aerogels. According to thermog-
Figure 2. SEM images of (a) pure chitosan and (b−e) composite aerogels prepared with (b,d) Cyv = 0.01 and (c,e) 0.1 mol L−1. (d,e) Low resolution images.
nanofibers of 5−10 nm in diameter. Low resolution images and elemental analysis (Figures S5 and S6) confirm that the distribution of the nanoparticles is homogeneous at least over the micrometer scale. TEM observation (Figure S7) shows that the nanoparticles are elliptical in shape and ∼8 nm in length. This is consistent with the crystallite sizes calculated from XRD, 6−7 nm (Table S1), showing that the nanoparticles are single crystalline. The specific surface areas calculated from nitrogen sorption isotherms (Figure S8) are 540 and 660 m2 g−1 for composite (Cyv = 0.01 mol L−1) and pure chitosan aerogels, respectively. The small surface area of the former implies that rough surfaces of the nanofiber network are covered with the nanoparticles. The composite aerogel shows a larger mean pore size, 90 nm, with a broader distribution compared to that of the pure chitosan aerogel, 76 nm (see bottom plots of Figure S8). This is consistent with the larger cross-sectional pore areas in the composite aerogels as observed in the SEM images. This causes light scattering and decreases the transparency of the monoliths with higher Cyv, as evident in the UV−visible transmission spectra (Figure S9). The nanoparticles probably prevent the shrinkage of the chitosan gel matrix during the drying process (see also Table S1 for details). According to the compression test, both composite and pure chitosan aerogels were compressed homogeneously up to ∼95% strain without forming cracks (Figures S10 and S11). Their compression curves are similar to those of flexible fibrous foams, indicating that the mechanical toughness of the nanofiber network remains in the composite aerogels. The elastic moduli are 0.13 and 0.11 MPa for composite (Cyv = 0.01 mol L−1) and pure chitosan aerogels, respectively. The composite aerogels show characteristic PL peaks of YVO4:Eu3+ (Figure 3),29 and their intensities roughly increase with increasing Cyv. The excitation spectra consist of a broad band at ∼300 nm corresponding to charge transfer transition from O2− to V5+. The emission spectra consist of sharp peaks from 4f−4f transitions of Eu3+. Because the microstructure of the aerogel shows well-exposed nanoparticle surfaces, we
Figure 1. Photographs of composite organogels (top) and aerogels (bottom) under white and 302 nm UV light.
ravimetry analysis (Figure S4), the amount of YVO4:Eu3+ nanoparticles in the aerogels increased from 2.7 to 12 wt % with increasing Cyv from 0.005 to 0.02 mol L−1, and saturated at ∼12 wt % for higher Cyv. The apparent densities and porosities of the aerogels are 0.04−0.05 g cm−3 and 97−98%, respectively, regardless of the nanoparticle amount (Table S1). The samples prepared with Cyv ≤ 0.02 mol L−1 are homogeneously translucent, whereas those with Cyv > 0.02 mol L−1 sometimes show a white ring-like pattern at the monolith surface. This can be explained by local precipitation of the nanoparticles along the spots where the gel touches the container during the soaking process. The yellow color of the aerogels is attributed to impurities of chitosan cross-linked with aldehydes, such as Maillard reaction compounds originating from imperfect crossB
DOI: 10.1021/acs.chemmater.6b04160 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
fitting curves in Figures S13−S20 and parameters summarized in Figure 4b,c and Table S2). The fraction of the fast component (red triangles in Figure 4b) of τ1 ∼ 200−300 s increases with increasing formaldehyde concentration, while that of the slow component (blue triangles in Figure 4b) of τ2 ∼ 1000−2500 s does not show a clear concentration dependence. As the former is the predominant component, the combined PL intensity (black circles in Figure 4b) decreases with increasing formaldehyde concentration. For comparison purposes, we also measured photobleaching curves of chitosan/YVO4:Eu3+ composite xerogel (blue curves in Figure 4a) prepared by drying the organogel in the ambient pressure. The ambient drying caused a drastic shrinkage of the gel due to surface tensions and resulted in nanoparticles embedded in a dense chitosan matrix (Figures S21−S24). The xerogel shows monoexponential curves with a slow time constant of >8000 s and does not show clear formaldehyde concentration dependence. Judging from these results, the fast and gas-concentration depending photobleach of the aerogel is probably attributable to a direct access of the gas to exposed nanoparticle surfaces, whereas the slow photobleach of the xerogel would depend on the migration of formaldehyde in the dense chitosan matrix. We note that the photobleaching curve for the aerogel with 0.5 mg L−1 formaldehyde shows a recovering component instead of the slow component. In our previous study, the recovering of photobleached YVO4:Bi3+,Eu3+ occurred through reoxidation of V4+ to V5+ by oxygen in the air after the surrounding organic compounds were completely decomposed.16 This means the composite aerogel has potential as not only a sensor but also a decomposer for air cleaning similar to photocatalysts.29 Further work is needed to unveil the complicated photoredox reactions that actually occur in the system. In conclusion, we have developed a structurally new luminescent nanocomposite designed to resemble “free-standing nanoparticles in the air” by fixing YVO4:Eu3+ nanoparticles in an ultralight chitosan nanofiber network. The nanocomposite has the mechanical toughness of the nanofiber network, red photoluminescence from the nanoparticles, and a translucent appearance due to low light scattering. The highly open nanostructure of ≥97% porosity leads to an interactive photobleaching behavior responding to formaldehyde vapor in the air. This new nanostructure enables us to exploit dynamic chemical functions of nanoparticle surfaces and opens up a new application field of luminescent nanomaterials.
Figure 3. PL (a) excitation and (b) emission spectra of composite aerogels (see the assignment details in the Supporting Information).
investigated its gas-responsive PL in the presence of reducing gas. We chose formaldehyde vapor, which normally needs a complicated derivatization with hydrazine compounds to measure its concentration, as a model gas. Figure 4a shows
Figure 4. (a) Change in PL intensity with UV irradiation duration for composite aerogel (black) and xerogel (blue) in different formaldehyde concentrations. Cyv = 0.01 mol L−1. (b,c) Changes in fitting parameters of composite aerogels with formaldehyde concentration.
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changes in PL intensity with UV irradiation time with different formaldehyde concentrations. The PL intensity does not change in the ambient air, whereas it decreases drastically with increasing irradiation time in the formaldehyde atmosphere. In our previous work, photobleaching of YVO4:Bi3+,Eu3+ originated from a photoreduction of V5+ in YVO4 to V4+ by surface-attached organic species,17 which was confirmed by electron spin resonance spectroscopy. Because the formation of V4+ was also confirmed in this study (Figure S12), formaldehyde acts as a reductant and causes the photobleaching. The normalized PL intensity, I(t), at the irradiation time t can be well fitted with the biexponential equation: I(t ) = I0 + I1exp( −t /τ1) + I2exp(−t /τ2)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04160. Experimental details, PL assignments, XRD, IR, UV−vis, and ESR spectra, SEM and TEM images, nitrogen sorption isotherms, compression profiles, and photobleaching curves (PDF)
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(1)
AUTHOR INFORMATION
Corresponding Author
*S. Takeshita. E-mail:
[email protected].
where I1 and τ1 are the fraction and time constant of a fast bleaching component, I2 and τ2 are those of a slow component, and I0 is the fraction of a constant component, respectively (see
ORCID
Satoru Takeshita: 0000-0002-5340-8953 C
DOI: 10.1021/acs.chemmater.6b04160 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
Properties of YVO4:Bi3+,Eu3+ Nanophosphor. J. Electrochem. Soc. 2010, 157, J74−J80. (18) Takeshita, S.; Watanabe, T.; Isobe, T.; Sawayama, T.; Niikura, S. Improvement of the Photostability for YVO4:Bi3+,Eu3+ Nanoparticles Synthesized by the Citrate Route. Opt. Mater. 2011, 33, 323−326. (19) Hund, J. F.; Bertino, M. F.; Zhang, G.; Sotiriou-Leventis, C.; Leventis, N.; Tokuhiro, A. T.; Farmer, J. Formation and Entrapment of Noble Metal Clusters in Silica Aerogel Monoliths by γ-Radiolysis. J. Phys. Chem. B 2003, 107, 465−469. (20) Aghajamali, M.; Iqbal, M.; Purkait, T. K.; Hadidi, L.; Sinelnikov, R.; Veinot, J. G. C. Synthesis and Properties of Luminescent Silicon Nanocrystal/Silica Aerogel Hybrid Materials. Chem. Mater. 2016, 28, 3877−3886. (21) Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y. Highly Efficient Nonprecious Metal Catalysts Towards Oxygen Reduction Reaction Based on Three-Dimensional Porous Carbon Nanostructures. Chem. Soc. Rev. 2016, 45, 517−531. (22) Sayevich, V.; Cai, B.; Benad, A.; Haubold, D.; Sonntag, L.; Gaponik, N.; Lesnyak, V.; Eychmüller, A. 3D Assembly of AllInorganic Colloidal Nanocrystals into Gels and Aerogels. Angew. Chem., Int. Ed. 2016, 55, 6334−6338. (23) Primo, A.; Quignard, F. Chitosan as Efficient Porous Support for Dispersion of Highly Active Gold Nanoparticles: Design of Hybrid Catalyst for Carbon−Carbon Bond Formation. Chem. Commun. 2010, 46, 5593−5595. (24) El Kadib, A.; Bousmina, M. Chitosan Bio-Based Organic− Inorganic Hybrid Aerogel Microspheres. Chem. - Eur. J. 2012, 18, 8264−8277. (25) Wang, H.; Shao, Z.; Bacher, M.; Liebner, F.; Rosenau, T. Fluorescent Cellulose Aerogels Containing Covalently Immobilized (ZnS)x(CuInS2)1‑x/ZnS (Core/Shell) Quantum Dots. Cellulose 2013, 20, 3007−3024. (26) Kobayashi, Y.; Saito, T.; Isogai, A. Aerogels with 3D Ordered Nanofiber Skeletons of Liquid-Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators. Angew. Chem., Int. Ed. 2014, 53, 10394−10397. (27) Takeshita, S.; Yoda, S. Chitosan Aerogels: Transparent, Flexible Thermal Insulators. Chem. Mater. 2015, 27, 7569−7572. (28) Wei, W.; Wang, L.-Y.; Yuan, L.; Wei, Q.; Yang, X.-D.; Su, Z.-G.; Ma, G.-H. Preparation and Application of Novel Microspheres Possessing Autofluorescent Properties. Adv. Funct. Mater. 2007, 17, 3153−3158. (29) Shiraishi, Y.; Takeshita, S.; Isobe, T. Two Photoenergy Conversion Modes of YVO4:Eu3+ Nanoparticles: Photoluminescence and Photocatalytic Activity. J. Phys. Chem. C 2015, 119, 13502−13508.
Funding
This work was supported by JSPS KAKENHI Grant Number JP16K17491. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. T. Kodaira, Dr. N. Hiyoshi, and Dr. A. Takahashi at AIST for the ESR, TEM, and nitrogen sorption measurements.
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REFERENCES
(1) Feldmann, C. Luminescent Nanomaterials. Nanoscale 2011, 3, 1947−1948. (2) Liu, Y.; Tu, D.; Zhu, H.; Chen, X. Lanthanide-Doped Luminescent Nanoprobes: Controlled Synthesis, Optical Spectroscopy, and Bioapplications. Chem. Soc. Rev. 2013, 42, 6924−6958. (3) Trindade, T.; O’Brien, P.; Pickett, N. L. Nanocrystalline Semiconductors: Synthesis, Properties, and Perspectives. Chem. Mater. 2001, 13, 3843−3858. (4) Lim, X. The Nanolight Revolution is Coming. Nature 2016, 531, 26−28. (5) Purcell-Milton, F.; Gun’ko, Y. K. Quantum Dots for Luminescent Solar Concentrators. J. Mater. Chem. 2012, 22, 16687−16697. (6) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media. Chem. Mater. 2003, 15, 4604−4616. (7) Huignard, A.; Buissette, V.; Franville, A.-C.; Gacoin, T.; Boilot, J.P. Emission Processes in YVO4:Eu Nanoparticles. J. Phys. Chem. B 2003, 107, 6754−6759. (8) Medintz, I. L.; Clapp, A. R.; Mattoussi, H.; Goldman, E. R.; Fisher, B.; Mauro, J. M. Self-Assembled Nanoscale Biosensors Based on Quantum Dot FRET Donors. Nat. Mater. 2003, 2, 630−638. (9) Rowland, C. E.; Fedin, I.; Zhang, H.; Gray, S. K.; Govorov, A. O.; Talapin, D. V.; Schaller, R. D. Picosecond Energy Transfer and Multiexciton Transfer Outpaces Auger Recombination in Binary CdSe Nanoplatelet Solids. Nat. Mater. 2015, 14, 484−489. (10) Diaz, S. A.; Gillanders, F.; Jares-Erijman, E. A.; Jovin, T. M. Photoswitchable Semiconductor Nanocrystals with Self-Regulating Photochromic Förster Resonance Energy Transfer Acceptors. Nat. Commun. 2015, 6, 6036. (11) Casanova, D.; Bouzigues, C.; Nguyên, T.-L.; Ramodiharilafy, R. O.; Bouzhir-Sima, L.; Gacoin, T.; Boilot, J.-P.; Tharaux, P.-L.; Alexandrou, A. Single Europium-Doped Nanoparticles Measure Temporal Pattern of Reactive Oxygen Species. Nat. Nanotechnol. 2009, 4, 581−585. (12) Abdesselem, M.; Schoeffel, M.; Maurin, I.; Ramodiharilafy, R.; Autret, G.; Clément, O.; Tharaux, P.-L.; Boilot, J.-P.; Gacoin, T.; Bouzigues, C.; Alexandrou, A. Multifunctional Rare-Earth Vanadate Nanoparticles: Luminescent Labels, Oxidant Sensors, and MRI Contrast Agents. ACS Nano 2014, 8, 11126−11137. (13) Giepmans, B. N. G.; Adams, S. R.; Ellisman, M. H.; Tsien, R. Y. The Fluorescent Toolbox for Assessing Protein Location and Function. Science 2006, 312, 217−224. (14) Sun, L.-D.; Wang, Y.-F.; Yan, C.-H. Paradigms and Challenges for Bioapplication of Rare Earth Upconversion Luminescent Nanoparticles: Small Size and Tunable Emission/Excitation Spectra. Acc. Chem. Res. 2014, 47, 1001−1009. (15) Paul, D. R.; Robeson, L. M. Polymer Nanotechnology: Nanocomposites. Polymer 2008, 49, 3187−3204. (16) Hara, H.; Takeshita, S.; Isobe, T.; Sawayama, T.; Niikura, S. A Unique Photofunction of YVO4:Bi3+,Eu3+ Nanophosphor: Photoluminescent Indication for Photochemical Decomposition of Polyurethane. Mater. Sci. Eng., B 2013, 178, 311−315. (17) Takeshita, S.; Ogata, H.; Isobe, T.; Sawayama, T.; Niikura, S. Effects of Citrate Additive on Transparency and Photostability D
DOI: 10.1021/acs.chemmater.6b04160 Chem. Mater. XXXX, XXX, XXX−XXX
