Lanthanide-Doped Energy Cascade Nanoparticles: Full Spectrum

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Lanthanide-Doped Energy Cascade Nanoparticles: Full Spectrum Emission by Single Wavelength Excitation Dengfeng Peng, Qiang Ju, Xian Chen, Ronghua Ma, Bing Chen, Gongxun Bai, Jianhua Hao, Xvsheng Qiao, Xianping Fan, and Feng Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00775 • Publication Date (Web): 27 Mar 2015 Downloaded from http://pubs.acs.org on March 29, 2015

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Chemistry of Materials

Lanthanide-Doped Energy Cascade Nanoparticles: Full Spectrum Emission by Single Wavelength Excitation Dengfeng Peng,† Qiang Ju,† Xian Chen,† Ronghua Ma,‡ Bing Chen,‡ Gongxun Bai,§ Jianhua Hao,§ Xvsheng Qiao,‡ Xianping Fan,‡ and Feng Wang*,†,# †

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, China ‡

State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China §

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Hum, Hong Kong SAR, China #

City Universities of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China

ABSTRACT: We describe the use of a layer-by-layer hierarchical nanostructure to exploit the synergy of different lanthanide ions for converting single wavelength excitation into emissions spanning the whole spectral region. By lining up a set of lanthanide ions with matched energy levels in a core−shell nanostructure, we demonstrate well-defined cascades of energy transfer that gives access to optical emissions from a large collection of lanthanide ions (Tb3+, Eu3+, Dy3+, Sm3+, Nd3+, Yb3+, and Er3+) after excitation into a common sensitizer of Ce3+ featuring a broad absorption. Through optimization of the nanoparticle structure and surface coating, high quantum yields of up to 90 % are achieved. Our results highlight that the controlled energy cascades at nanometer scale provide new opportunities for applications such as fighting against counterfeiting and sensing small molecules.

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INTRODUCTION Modifying photon energy with small luminescent materials has provided exciting opportunities for applications in photonics, photovoltaics, diagnostics, and therapeutics.1-4 Organic dyes were originally proposed to serve most of the applications due to their low cost, high quantum yield, and vastly available dye molecules.5 Beginning in the 1980s, the development of nanoparticle research has added semiconductor quantum dots (QDs) featuring size-tunable emissions into the library of small luminescent materials to overcome the drawbacks of organic dyes such as poor photochemical stability.6-9 Lanthanide-doped nanoparticles emerged in the late 1990s represent another class of luminescent nanomaterials being developed as a promising alternative to organic dyes and QDs. These nanoparticles are characterized by large Stokes shift, sharp emission bandwidth, as well as high resistance to optical blinking and photobleaching.10-16 As the most prominent feature, a single nanoparticle can be incorporated with a combination of lanthanide dopant ions that produce excitons of distinct energies, thereby giving rise to highly designable emission profiles.17-21 Despite the enormous research efforts on multiple lanthanide-doped nanoparticles, excitation of a large collection of dopant ions by a single-wavelength light remains an arduous task, owing to the narrow and characteristic absorption bands of different lanthanide ions.22-24 Energy transfer from a sensitizer (e.g., [VO4]3- host lattice or Ce3+ co-dopant) to the activators can lead to effective excitation. But a general energy donor is lacking for sensitizing a broad range of lanthanide activators due to mismatched resonant frequencies or luminescence quenching between particular donor/acceptor pairs.19,

20, 25, 26

Although partitioning dopant ions in

core−shell nanostructures can lead to eliminated luminescence quenching,27-29 the problem of energy-level mismatch remains unresolved.


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Herein, we systematically investigate lanthanide interactions in layer-by-layer hierarchical nanostructures. We demonstrate that the electronic transitions between a series of spectrally divergent lanthanide ions can be rationally cascaded to offer efficient energy transfer across multiple core−shell interfaces. This capability offers desirable energy exchange effects between virtually any ion pairs. By tuning the energy cascade through control of core−shell combinations, we achieve efficient sensitization of lanthanide luminescence spanning from ultraviolet (UV) to near-infrared (NIR) spectral regions by a common energy donor. RESULTS AND DISCUSSION We chose to exploit the effect of energy cascade by employing a core−shell NaGdF4 host lattice, which can accommodate the whole group of lanthanide dopants, in addition to demonstrated ability to render high luminescence efficiencies. Ce3+ featuring a broad UV absorption band due to an allowed 4f-5d transition was selected as the model sensitizer. A series of activator ions (Tb3+, Eu3+, Dy3+, and Sm3+) were involved to produce UV and visible emissions at different wavelengths. For Tb3+ and Eu3+ activators that display high resistance to concentration quenching, we used a basic core−shell structure that comprises an activator-doped core covered by a Ce3+-doped shell (Figure 1, a and b). For Dy3+ and Sm3+ activators whose emissions are readily quenched at elevated dopant concentrations (> 3 mol%),28 a multilayered structure was used to spread a high content of activators over separate layers for reducing the local dopant concentrations (Figure 1, c and d). As Dy3+ and Sm3+ ions are susceptible to highenergy surface oscillations, a protection shell layer of NaYF4 is also applied.


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Figure 1. Schemes of the core−shell design (the core and shell layers are highlighted with different background colors), TEM images of the as-synthesized nanoparticles at different stages of the preparation, and excitation (blue line) and emission (red line) spectra of the resultant nanoparticles after removal of oleate ligands for (a) Tb3+, (b) Eu3+, (c) Dy3+, and (d) Sm3+ activators, respectively. Insets of the luminescence spectra are luminescence photographs of the corresponding nanoparticle colloids under irradiation of a hand-held UV lamp (254 nm). Scale bars for TEM images are 100 nm. The dopant concentrations of Ce3+, Tb3+, Eu3+, Dy3+, and Sm3+ in the corresponding layers are 15 mol%, 15 mol%, 15 mol%, 2.5 mol% and 2.5 mol%, respectively.

All nanoparticles were synthesized following an epitaxial deposition process that has been extensively investigated by several independent groups.6, 28-36 TEM images taking at different stages of the preparation show a gradual increase in nanoparticle size along with the coating process, in consistence with the epitaxial growth process. The TEM observations, together with enhanced luminescence in comparison with the homogeneously doped nanoparticle counterparts (Fig S1, Supporting Information), supports the formation of core−shell nanostructures. The poor


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luminescence in the homogeneous nanoparticles is ascribed to redox reaction (e.g.; Ce3+ + Eu3+ → Ce4+ + Eu2+) or cross-relaxation (e.g.; Sm3+: 4G5/2 + 6H5/2 → 6F9/2 + 6F9/2 and Dy3+: 4F9/2 + 6

H15/2 → 6F3/2 + 6H9/2) among the dopant ions. Notably, the core−shell nanoparticles display

dominant emission peaks of the activator ions ranging from UV to visible after excitation into Ce3+ at 250 nm, suggesting efficient sensitization processes. Furthermore, high quantum yields of 90%, 75%, 50%, and 45% were respectively achieved for the Tb3+, Eu3+, Dy3+, and Sm3+ activated samples, validating largely eliminated nonradiative relaxations.

Figure 2. (a) A comparison of emission spectra for the NaGdF4:Eu@NaGdF4:Ce nanoparticles with various surface coverages. Inset: luminescence photographs of the corresponding nanoparticle colloids. (b, c) Absorption and excitation spectra of the ligand-free and oleate-capped NaGdF4:Eu@NaGdF4:Ce nanoparticles, respectively. The excitation spectra are normalized to 7F0 → 5L6 transition of Eu3+ at ~395 nm. Oleate ligands add absorption to the nanoparticles but subtract their excitation intensity.


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The achieving of substantially higher quantum yields with respect to previous reports37, 38 is also attributed to an optimization of surface coating. We noticed that the as-synthesized nanoparticles comprising a capping layer of oleate ligands show very poor luminescence (Figure 2a). By contrast, a significant emission enhancement was observed for the ligand-free nanoparticle counterparts (Figure 2a). The phenomena can hardly be explained by the surface quenching effect associated with the oscillation of oleic molecules because an inert layer of NaYF4 did not lead to appreciable improvement in emission intensity (Figure 2a and Figs. S2 and S3a, Supporting Information). By comparative investigation of the absorption and excitation spectra for both samples (Figure 2, b and c), a screen effect of the capping ligands is proposed to account for the suppressed luminescence in the oleate-capped nanoparticles. Oleate ligands absorb UV light but fail to transfer their energy to the lanthanide dopants, thereby subtracting the energy of the incident light to excite the nanoparticles. The broad emission bands observed in the oleate-coated samples are attributed to the ligand fluorescence. After oleate ligands were added back to the ligand-free nanoparticles, the broad emission band recovered along with a decrease in overall emission intensity, which validates the screen effect of the capping ligands (Fig S3b, Supporting Information). To eliminate the screen effect for efficient luminescence, all the nanoparticles were subject to a ligand removal treatment prior to optical characterizations. The energy cascade through the core−shell interface is dominated by the network of the Gd3+ sublattice. When the Gd3+ ions were replaced by inactive Y3+ ions, the sensitized emission diminished markedly because direct interaction of the donor and the acceptor in separate layers is very limited (Figs. S5 and S6, Supporting Information).36 By using the Gd3+ sublattice as a conduit, a low concentration of activator ions are able to capture most of the excitation energy, leading to sensitized emission from higher-lying energy levels (e.g., 5D3, 5D2, and 5D1 levels of


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Eu3+ and 5D3 level of Tb3+) due to eliminated cross-relaxations (Figs. S5 and S6, Supporting Information).

Figure 3. Schemes of the core−shell design (the core and shell layers are highlighted with different background colors), TEM images of the as-synthesized nanoparticles at different stages of the preparation, and emission spectra of the resultant nanoparticles after removal of oleate ligands for (a) Nd3+, (b) Yb3+, and (c) Er3+ activators, respectively. Scale bars for TEM images are 100 nm. The dopant concentrations of Ce3+, Nd3+, Yb3+, and Er3+ in the corresponding layers are 15 mol%, 15 mol%, 50 mol%, 10 mol%, respectively.

In a further set of experiments, we investigated the effect of energy cascade on sensitizing NIR luminescent lanthanides. Strikingly, we observed a very efficient energy transfer from Ce3+ to Nd3+ through the network of Gd3+ ions (Figure 3a), as evidenced by the dominant emission peaks of the Nd3+ over that of the Ce3+ (Fig. S7, Supporting Information). High efficiency of the energy transfer process was further validated by a large preservation of the sensitized Nd3+ emissions at a lower activator concentration, which appear in UV spectral region due to eliminated cross-relaxations (Fig. S8, Supporting Information). The findings offered a tantalizing


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possibility of sensitizing Yb3+ by Ce3+ through a Ce3+ → Gd3+ → Nd3+ → Yb3+ energy cascade (Figure 3b). By further cascading Er3+ to the energy chain via Yb3+,39 the emission spectrum was extended beyond 1500 nm (Figure 3c).

Figure 4. (a) Quasi continuous emission spectrum of a three-layered nanostructure in the UV to visible spectral region. (b) Emission spectrum of a five-layered nanostructure activated with multiple emitters, showing ultra-broad band emission in the UV to NIR part of the electromagnetic spectrum. The emission spectrum was compiled from two parts recorded with Hamamatsu R928P (UV to visible) and R5509-72 (visible to NIR) PMTs, respectively. The Eu3+ emission were used as a reference for the compilation. (c) Emission spectra of a series of nanoparticles composed of two emission layers of varying composition. (d) Compiled luminescence photographs of the nanoparticles in (c) dispersed in water (1 mg mL-1) under irradiation of a hand-held UV lamp (254 nm). (e) Schematic illustration of an anti-counterfeiting pattern printed on a paper substrate with both normal gray ink and nanoparticle ink. (f) Photograph of the pattern showing invisibility of the nanoparticle label in ambient light. (g) Photograph of the pattern in (f) after UV illumination is added, showing multicolor label created by the nanoparticles.

The successful sensitization of various lanthanide ions by a common sensitizer suggests that it is possible to create a multitude of emission processes by single wavelength excitation. For example, we have assembled a three-layered nanoparticle to integrate the emission of Ce3+, Gd3+,


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Tb3+, and Eu3+ ions (Fig. S9, Supporting Information). In response to deep UV excitation, the nanoparticles are able to emit light that covers almost the entire UV and visible parts of the electromagnetic spectrum (Figure 4a). The intense light emissions of Ce3+ and Gd3+ in the UV spectral region largely result from the low activator contents, which avoid complete consumption of the sensitizer energy. An inert shell layer of NaYF4 also promotes the quasi full spectrum emission by protecting Tb3+ activators and by suppressing dissipation of Gd3+ energy due to surface quenchers.29 By including additional layers activated with Nd3+ and Yb3+/Er3+ (Fig. S10, Supporting Information), the emission spectrum can be readily extended to the NIR region (Figure 4b) with dual mode luminescence capabilities (Fig. S11, Supporting Information), demonstrating versatility of the layer-by-layer structure for flexible spectral conversion. Furthermore, the emission intensity balance of different layers can be precisely adjusted through control of dopant concentration (Figure 4c). The effect gives access to a broad range of color output with high quantum yields (Figure 4d), providing a new class of multicolor security ink for fighting against counterfeiting (Figure 4, e-g). In contrast to anti-counterfeiting techniques that take advantage of fluorescent dyes or upconversion nanoparticles,40-42 this method avoids the use of lasers to develop the anti-counterfeiting labels and offers high stability against photobleaching and chemical degradation (Fig. S12, Supporting Information). We found that efficient energy cascade can also be realized across two sets of nanoparticles having different dopant ions (Fig. S13, Supporting Information). The effect promoted the construction of a novel sensor system fully composed of lanthanide-doped nanoparticles for probing small molecules. As a proof of concept, NaGdF4:Ce (15 %) and NaGdF4:Eu (15 %) nanoparticles were prepared and surface modified with biotin to detect avidins (Figure 5a). After UV irradiation, a dilute water dispersion comprising the binary nanoparticle mixture (0.1 mg mL-


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each) displays a steadily enhanced Eu3+ emission with the addition of increasing amount of

avidin (Figure 5b), which can be ascribed to the formation of avidin-biotin complex that brings the nanoparticles into close proximity (Figure 5a). In addition, a linear relationship between the Eu3+ emission intensity and the avidin concentration was recorded in a concentration range of 80−400 nM (Figure 5c), demonstrating availability of the sensor system for quantitative assay.

Figure 5. (a) Basic design for sensing avidin with the lanthanide-doped nanoparticles. (b) Emission spectra of the sensor in the presence of different concentration of avidin. Inset: photograph of the corresponding colloidal dispersion under irradiation of a hand-held UV lamp (254 nm). (c) Emission intensity of Eu3+ at ~615 nm as a function of avidin concentration.

CONCLUSIONS We have demonstrated rational energy cascades in multishell nanoparticles for efficient excitation of diverse lanthanide activators via a common sensitizer. By tuning the composition and topological arrangement of the core and shell layers, the energy cascade process can be deliberately engineered to convert single wavelength excitation into full spectrum emission


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spanning from UV to NIR. The approach can be readily applied to a broad range of core/shell combinations, implying an innovative route for the construction of luminescent materials that expand the range of applications for lanthanide-doped nanoparticles. EXPERIMENTAL METHODS Reagents: Gadolinium(III) acetate hydrate (99.9%), cerium(III) acetate hydrate (99.9%), terbium(III) acetate hydrate (99.9%), europium acetate hydrate (99.9%), dysprosium(III) acetate hydrate (99.9%), samarium(III) acetate hydrate (99.9%), neodymium(III) acetate hydrate (99.9%), ytterbium(III) acetate hydrate (99.9%), erbium(III) acetate hydrate (99.9%), yttrium(III) acetate hydrate (99.9%), sodium hydroxide (NaOH, >98%), ammonium fluoride (NH4F, >98%), 1-octadecene (90%), oleic acid (90%), poly(acrylic acid) (PAA, MW≈1800), 1-ethyl-3-(3dimethly-aminopropyl) carboxylate (EDC), N-hydroxysuccinimide (NHS), biotin hydrazide, avidin, and phosphate buffered saline (PBS) were all purchased from Sigma-Aldrich and used as received. Physical Measurements: Transmission electron microscopy (TEM) measurements were carried out on a Philips CM-20 transmission electron microscope operating at an acceleration voltage of 200 kV. Ultraviolet-visible absorption spectra were acquired on a SHIMADZU UV-1700 PharmaSpec UV-Vis spectrophotometer. Photoluminescence spectra in the UV and visible range were obtained from water dispersion of the nanoparticles (0.1 wt %) on an F-4600 spectrophotometer (Hitachi). Emission spectra in the NIR region were recorded with solid-state nanoparticles by using an Edinburgh FLSP920 spectrophotometer equipped with a nitrogencooled NIR photomultiplier tube (Hamamatsu R5509-72). Luminescence quantum yields were measured on the FLSP920 spectrometer coupled with an integrating sphere. Luminescence


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digital photographs were taken with a Nikon D5100 camera. Unless otherwise stated, all measurements were carried out at room temperature. Synthesis of Nanoparticles: We synthesized the lanthanide-doped nanoparticles through coprecipitation in a binary solvent mixture of oleic acid and 1-octadecene.35 In a typical procedure, lanthanide acetates were heated in the solvent at 150 oC to form the lanthanide-oleate precursor solution. Thereafter, a mixture of NaOH and NH4F in methanol was added and the resultant solution was heated to 290 oC under argon for 1 h to promote the formation of nanoparticles. The core−shell nanoparticles were synthesized following the same protocol except that preformed core nanoparticles were included in the precursor solution. The detailed experimental settings for nanoparticles of varying compositions are summarized in Table S1 of the supporting information. Synthesis of Ligand-Free Nanoparticles: The as-prepared nanoparticles were extracted from the cyclohexane dispersion and re-dispersed in equal volume of HCl solution (0.1 M in deionized water). The slurry was then sonicated at 45 oC for 1 h to remove the surface oleate ligands. After the reaction, the nanoparticles were collected via centrifugation at 14000 rpm for 30 min and washed with ethanol and deionized water several times, and re-dispersed in deionized water. Bio-Conjugation of the Nanoparticles: 12 mL of diethylene glycol solution containing 0.4 g of PAA was heated to 110 oC with vigorous stirring under argon flow, to which 8 mL of cyclohexane solution containing 0.4 mmol as-synthesized nanoparticles was injected into the solution. The mixture was heated to 240 °C and kept at this temperature for 30 min under argon flow. Upon cooling to room temperature, the PAA-capped nanoparticles were collected by centrifugation and washed several times with deionized water. The PAA-capped nanoparticles


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solution was then mixed with 110 mg NHS and 180 mg EDC to form a transparent solution, followed by the addition of 70 mg biotin hydrazide and magnetic stirring for 24 hour at room temperature. After washing and redispersion, the biotinylated nanoparticles were dispersed in 10 mL PBS and stored at 4 °C. Avidin Detection: 90 mL of biotinylated NaGdF4:Ce and 90mL of NaGdF4:Eu nanoparticles were added to the wells of a 96-well microplate, followed by the addition of 20 mL deionized water comprising varying amount of avidin. A BioTeK Synergy multi-mode microplate reader was used to analyze the optical response under an excitation wavelength of 260 nm. The delay time and gate time were set at 100 µs and 1.5 ms, respectively. When compared to steady-state detection, the time-resolved detection technique was found to offer higher signal-to-noise ratio, owing to elimination of interference of autofluorescence from the target protein molecules. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Feng Wang, E-mail: [email protected] Funding Sources This work was supported by the Research Grants Council of Hong Kong (CityU 109413, Project Code: 9041978), the National Natural Science Foundation of China (Nos. 21303149 and 51332008), and the Science Technology and Innovation-Committee of Shenzhen Municipality


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Table of Contents Graphic


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Lanthanide-Doped Energy Cascade Nanoparticles: Full Spectrum

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