ZnS Nanoparticles Sensitize Luminescence of Capping-Ligand-Bound

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ZnS Nanoparticles Sensitize Luminescence of Capping-Ligand-Bound Lanthanide Ions Rodney A. Tigaa, Gary J. Lucas, and Ana de Bettencourt-Dias* Department of Chemistry, University of Nevada, Reno, Nevada 89557, United States S Supporting Information *

ABSTRACT: 2,6-Bis(diethylamide)-4-oxo(3-thiopropane)pyridine (BDP) and 2,6-bis(methyl ester)-4-oxo(3-thiopropane)pyridine (BMP) were synthesized. These compounds chelated LnIII ions and sensitized their emission. The 3:1 complexes of BDP displayed efficiencies of 18% and 12% for EuIII and TbIII, respectively. The analogous complexes of BMP had efficiencies of 23% and 18% for EuIII and TbIII, respectively. Both BDP and BMP were used to cap ZnS nanoparticles (NPs) in a one-pot synthesis, and then LnIII ions were added, resulting in systems with metal ions at the surface of the capped NPs. Photoexcitation of the EuIII and TbIII systems through NPs capped with these two ligands, with the carboxylato derivative of BMP [dicarboxylato4-oxo(3-thiopropane)pyridine] and the nonchromophore 3-mercaptopropionate, resulted in sensitized LnIII-centered emission. The EuIII-containing systems displayed higher efficiencies in the range 0.04−0.23% than the corresponding TbIII-containing systems with efficiencies in the range 0.01−0.15%. The NPs capped with BDP were the exception; in this case, efficiencies of 0.36% and 0.79% for EuIII and TbIII, respectively, were observed.

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INTRODUCTION Light emission of lanthanide (LnIII) ions has applications in sensing1 and imaging,2−4 among others.5 Each ion has a unique emission color, and the core nature of the 4f orbitals leads to narrow emission bands, while the f−f transitions, which are responsible for the emission, are parity-forbidden; thus, the ions have low molar absorptivity (ε ≤ 1 M −1 cm −1 ). 6,7 Consequently, population of the emissive f state is often achieved through an antenna, a coordinated ligand that sensitizes the metal ion through energy transfer from a ligand-based excited state.6 Efficient energy transfer can be achieved by fine-tuning the excited states of the antenna to best match the emissive f excited state.6−8 To date, this fine-tuning has been accomplished through more or less extensive synthetic procedures to functionalize ligands. However, the use of nanoparticles (NPs) as sensitizers is advantageous because their band-gap energies can be tuned through NP size control.9−11 Semiconducting NPs doped with LnIII ions were shown to display LnIII-centered emission, such as doped YVO4 NPs described by Riwotzki and Haase12 or II−VI NPs described by Petoud and co-workers,13 which were doped with LnIII either during NP growth or after NP formation. Similarly, Vela and co-workers14 demonstrated that doping NPs with EuIII results in a simultaneous enhancement of LnIII-centered emission and © 2017 American Chemical Society

a protection of the emission from vibrational quenching in protic solvents. However, these NPs often lose their structural integrity because of the high temperatures used during their synthesis.13 Other attempts to improve luminescence led to Eudoped LaF3 NPs,15−17 where surface capping ligands transfer energy to the lattice, which then sensitizes the EuIII-centered emission.16 In a reverse process, Hildebrandt and co-workers18 described that energy transfer from LnIII ions bound to surface capping ligands with subsequent NP-centered emission was seen. Sensitized luminescence could also be achieved by chelating LnIII ions to antennas that are grafted onto nanocomposites,19 which show increased emission compared to the nongrafted systems.20,21 In addition, emissive biotinylated complexes of GdIII and TbIII conjugated to NPs for multimodal imaging were recently described by Biju and coworkers.22 However, to date, only one instance of energy transfer from a NP to a LnIII ion bound to a capping chromophore and subsequent metal-centered emission has been reported.23 Instead, Gallagher and co-workers24 observed that quenching of the emission of aqueous thioglycolic acid capped CdTe quantum dots via electron transfer to bound EuIII Received: October 29, 2016 Published: February 27, 2017 3260

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Inorganic Chemistry

Table 1. Radiative (τrad) and Observed (τobs) Emission Lifetimes and Intrinsic (ΦLn Ln), Sensitized (Φsen), and Overall Emission (Φ) Efficiencies of the 3:1 Ligand-to-LnIII Complexes as Well as the Singlet, 1S, and Triplet, 3T, Energies of BDP and BMP in Acetonitrile and DCP in a Tris/HCl (pH ∼ 7.4)-Buffered Dimethyl Sulfoxide/H2O (1:19, v/v) Solution at 25.0 (±0.1) °C complex III

Eu -BDP TbIII-BDP EuIII-BMP TbIII-BMP EuIII-DCP TbIII-DCP a

S [cm−1]a

1

3

T [cm−1]a

30100 ± 280

24200 ± 10

30500 ± 100

22900 ± 190

31000 ± 100

26000 ± 100

τobs [ms]

τrad [%]

ΦLn Ln[%]

Φsen [%]

± ± ± ± ± ±

2.75 ± 0.11

39.7 ± 1.6

44.7 ± 1.8

4.32 ± 0.03

33.6 ± 0.3

68.6 ± 0.5

3.29 ± 0.12

46.9 ± 1.8

29.8 ± 0.4

1.09 1.13 1.45 1.92 1.54 1.52

0.03 0.01 0.01 0.05 0.01 0.02

Φ [%] 17.7 11.6 23.0 18.1 14.4 3.7

± ± ± ± ± ±

0.3 0.9 1.6 0.8 0.2 0.7

Measured as the GdIII complex.32

Figure 1. UV−vis absorption, excitation, and emission spectra of 3:1 complexes of EuIII (top) and TbIII (bottom) with BDP and BMP in acetonitrile and DCP in a Tris/HCl (pH ∼ 7.4)-buffered dimethyl sulfoxide/H2O (1:19, v/v) solution.

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RESULTS AND DISCUSSION Three new ligands, BDP, BMP, and DCP, that are capable of capping the NPs on one end and chelating LnIII on the other end were prepared using reported methods for molecules with similar functional groups29−31 and fully characterized by NMR spectroscopy and mass spectrometry (Scheme S1 and Figures S1−S19). To assess their efficiency as antennas, their photophysical properties were evaluated. BDP, BMP, and DCP showed emission maxima at 349, 346, and 406 nm when excited at 267, 271, and 276 nm, respectively (Figure S20). The compounds display singlet- and triplet-state energies (Table 1 and Figures S21 and S22) adequate for energy transfer to EuIII and TbIII. Upon coordination of BDP, BMP, and DCP to the metal ions in a 3:1 ligand-to-metal stoichiometry, the characteristic 5D0 → 7FJ (J = 1−4) transitions of EuIII and 5 D4 → 7FJ (J = 6−2) of TbIII were observed (Figure 1). The moderate emission efficiencies (Table 1) of 18% and 12% for the EuIII and TbIII complexes of BDP, respectively, suggest incomplete energy transfer. Similar EuIII and TbIII emission spectra were observed for the analogous BMP spectra, with emission efficiencies of 23% and 18%, respectively (Table 1 and Figure 1). The characteristic transitions were also observed when the DCP complexes were excited at 273 nm

ions was reported, without metal-centered emission. To use NPs, with their size-tunable excited-state energies, as sensitizers for LnIII emission with the metal ions appended to and not within the NP, we became interested in ZnS NPs, which are chemically stable and have lower toxicity than the more commonly studied CdSe NPs.25,26 The bulk ZnS band gap of 3.68 eV results in absorption in the UV, and thus sensitization of the visible-emitting LnIII is possible. Surface-modified semiconducting NPs are usually synthesized through ligand exchange, which often leads to etching of the surfaces and degrades the optical properties.27 However, Vela and co-workers27 demonstrated that it is possible to synthesize surface-modified CdS NPs in the presence of the desired capping ligands in a one-pot precipitation method without ligand exchange. We used this method to cap ZnS NPs with the newly isolated ligands 2,6-bis(diethylamide)-4-oxo(3thiopropane)pyridine (BDP) and 2,6-bis(methyl ester)-4-oxo(3-thiopropane)pyridine (BMP). The BMP ligand on the surface of the NPs was saponified to yield 2,6-dicarboxylato-4oxo(3-thiopropane)pyridine (DCP)-capped NPs. 3-Mercaptopropionate (3-MPA) was also used as a capping ligand for comparison because it is not a chromophore and thus is not directly involved in sensitizing the LnIII emission.28 3261

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Inorganic Chemistry Scheme 1. Synthesis of Ligand-Capped ZnS NPs

(Figure 1). The overlap of the absorption and excitation spectra in each case indicates that energy was transferred from the ligands to the Ln III ions. However, the weak bands corresponding to the direct f−f excitation seen in the excitation spectrum of the EuIII-BDP complex indicate that, in this case, direct excitation is competitive with sensitization (Figure 1). The capped ZnS NPs were prepared as shown in Scheme 1 using procedures described by Shamsipur and co-workers33 and Vela and co-workers27 (details are given in the Supporting Information) to yield NPs capped with 3-MPA, BDP, BMP, and DCP. They were purified by centrifugation to facilitate precipitation and repeated washings with isopropyl alcohol; the binding of the ligands as capping ligands and the absence of free ligand in solution were confirmed by the absence of the S− H stretching vibration (2550 cm−1) in the Fourier transform infrared (FT-IR) spectra (Figure S23a−c) and the presence of the symmetric and asymmetric carboxylate stretching vibrations at 1396 and 1561 cm−1 in the FT-IR spectra of NPs functionalized with 3-MPA (Figure S23a) and DCP (Figure S23d). The latter indicate the successful saponification of BMP. The capping of the NPs is further corroborated by 1H NMR spectroscopy; the spectra show the ligand resonances broadened (Figures S24−S27), consistent with the lack of free rotation due to binding to the surface of the nanocrystals.16 Compositional analysis by energy-dispersive X-ray (EDX) spectroscopy showed significant amounts of C, N, O, and S due to the presence of the ligands on the surface of the NPs (Table S1), with all Zn/S ratios close to 1:1. LnIII-containing systems of the capped NPs were prepared by mixing adequate amounts of LnIII salts and NPs as determined from titration experiments of the H2O-soluble 3-MPA-capped NPs (details are given in the Supporting Information). To isolate the capped NPs with LnIII ions complexed onto the capping ligands, the systems were centrifuged to facilitate precipitation and the powders dried under vacuum. Binding of the lanthanides was confirmed by IR spectroscopy because the carbonyl and carboxylato stretching vibrations shifted (Table 2 and Figure S28a−d) after LnIII salts were added. The difference between the asymmetric and symmetric vibrational frequencies, Δν, indicates a mix of bidentate chelate and bridging coordination modes for 3-MPA and a mix of bidentate bridging and monodentate coordination modes for DCP (Table 2).28,34 Compositional analysis by EDX showed significant amounts of EuIII and TbIII (Table S1). Transmission electron microscopy (TEM) images of the capped ZnS NPs show monodisperse particles with average

Table 2. Vibrational Frequencies of All Capped ZnS NPs and Their LnIII Systemsa NP system

ν(CO) [cm−1]

BDP Eu-BDP Tb-BDP BMP Eu-BMP Tb-BMP 3-MPA Eu-3MPA Tb-3MPA DCP Eu-DCP Tb-DCP

1588 1591 1591 1725 1652 1652

νas(COO−) [cm−1]

νs(COO−) [cm−1]

Δν(COO−) [cm−1]

1561 1546

1395 1388

166 158

1547

1388

159

1586 1635 1633

1420 1451 1463

166 184 170

νas and νs denote the asymmetric and symmetric vibrations of the carboxylates. a

sizes of 4.1 ± 0.7 nm for 3-MPA, 2.8 ± 0.5 nm for BDP, 3.1 ± 0.3 nm for BMP, and 3.4 ± 0.8 nm for DCP-capped ZnS NPs (Figure 2 and Table S2), consistent with reported NPs stabilized by thiol-capping ligands.35 The capped NPs are crystalline with interplanar spacings (0.30−0.32 nm) that correspond to the (111) plane of cubic ZnS (insets in Figure 2).36,37 After the metal ions are added, the NPs remain monodisperse with unaffected average sizes and interplanar spacings and thus similar band gaps (Figures 2 and 3 and Table S2). The particle sizes that we report here are similar to the sizes seen for LnIII-doped ZnS NPs capped with stearate and trioctylphosphine oxide.34 Powder X-ray diffraction (PXRD) of the capped ZnS NPs confirms the crystallinity of the systems corresponding to cubic zinc blende (JCPDS 05-0566).38 The diffraction peaks are broadened because of the small size of the NPs (Figure S29).38 The crystallinity and morphology of all of the samples remain unchanged after LnIII complexation (Figure S30). The 3-MPA-, BDP-, BMP-, and DCP-capped NPs show absorption maxima at 274, 267, 305, and 288 nm, respectively (Figure 4a−d). As expected, the NP absorption is blue-shifted compared to bulk ZnS (345 nm).39 The band gaps of the NPs determined from the UV−vis absorption spectra correspond to energies of 4.28, 3.97, 3.80, and 3.83 eV for 3-MPA-, BDP-, BMP-, and DCP-capped NPs, respectively. While the size 3262

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Inorganic Chemistry

Figure 2. TEM images of (a) 3-MPA-, (b) BDP-, (c) BMP-, and (d) DCP-capped ZnS NPs. The insets show the interplanar spacings of the ligandcapped ZnS NPs.

that 7.6 and 8.8 H2O molecules, respectively, were coordinated to the metal ions, which indicates that only one carboxylate is present per ion. The presence of these H2O molecules influences the low emission efficiencies and short emission lifetimes observed for the 3-MPA-capped NP systems. While BMP, DCP, and BDP are chromophores and therefore able to sensitize LnIII emission, 3-MPA− is not a chromophore and unable to sensitize LnIII emission (Figure S31).28 In these excitation spectra of the 3-MPA complexes with LnIII, primarily bands corresponding to f−f transitions of the LnIII ions were observed, which indicates that direct excitation is the main pathway to luminescence. As seen in Figure 4a, the excitation spectra have the NP characteristic profile, which therefore indicates that, for the NPs capped with 3-MPA, energy transfer occurs directly from the NP to sensitize LnIII-centered luminescence. Residual NP fluorescence was observed in all of the systems, which, in turn, suggests incomplete energy transfer, as reflected in the measured emission quantum yields, especially for the NPs capped with 3-MPA− and DCP (Table 4). The emission efficiencies of the NPs capped with BDP and BMP were higher than those with 3-MPA (Table 4) because of the ability of the capping chromophores to act as additional sensitizers. Similarly, the determined EuIII sensitization efficiencies of the BDP and BMP NPs are significantly higher than those of the 3-MPA and DCP NPs (Table 4). Unexpectedly, the overall emission and sensitization efficiencies of the DCP NPs are lower than those of the BMP NPs but comparable to those of the 3-MPA NPs (Table 4). This can be attributed to the less favorable position

changes are small, as seen by TEM, the variation in the calculated band gaps is influenced by the presence of capping ligands and their own spectroscopic signature. The capped NPs can be excited in the range 267−305 nm, leading to NP-based emission maxima at 323, 324, 381, and 434 nm, with tailing up to 600 nm (Figure 4a−d). Upon coordination of the LnIII salts, no appreciable change of the excitation spectra is seen (Figure 4a−d), and the red and green emissions of EuIII and TbIII are observed, as evidenced by the presence of the characteristic bands (Figure 4a−d) in the emission spectra. The unchanged excitation and characteristic LnIII emission spectra show that energy was transferred from the capped NPs to the LnIII ions. Of all of the NP systems studied, only the 3-MPA-capped ZnS NPs are soluble in H2O. Their emission decay curves were fitted to second-order exponentials corresponding to emission lifetimes of 17.0 ± 0.2 and 148.2 ± 3.3 ns in H2O and 18.0 ± 0.6 and 151.7 ± 0.5 ns in D2O, respectively. For comparison, Saha and co-workers40 reported an average emission lifetime of 35 ns for 3-mercaptoethanol-capped ZnS NPs in H2O with decay curves that were fitted to third-order exponentials. Our 3MPA-capped ZnS NPs had an emission quantum yield of 0.065 ± 0.003 compared to a value of 0.024 for the reported 3mercaptoethanol-capped NPs.40 Upon the addition of Ln ions, the NP-based emission was too weak to allow for determination of the lifetimes. Because this system is H2O-soluble, through lifetime studies (Table 3) and using the Horrocks equation (see the Supporting Information),41 after EuIII and TbIII were added, it was found 3263

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Figure 3. TEM images of the EuIII (left) and TbIII (right) systems of (a) 3-MPA-, (b) BDP-, (c) BMP-, and (d) DCP-capped ZnS NPs. The insets show the interplanar spacings of the LnIII NP systems.

EuIII-based systems, except in the case of BDP, which suggests that the position of the energy levels, determined by absorption

of the individual energy levels of all of the system components. Overall, the measured emission efficiencies were higher in the 3264

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Figure 4. Normalized UV−vis absorption, excitation, and emission spectra of (a) 3-MPA-, (b) BDP-, (c) BMP-, and (d) DCP-capped ZnS NPs only and their LnIII systems in acetonitrile.

the NP systems, except the BDP NPs, were best fitted as double exponentials (Table 4), most likely due to the different binding modes of the LnIII ions with the carboxylates.42 We attribute the observed moderate emission efficiencies to unoptimized particle sizes and thus band gap locations, as well as ligand energy levels, compared to the emissive states of the LnIII ions. In fact, the 5D0 and 5D4 levels of EuIII and TbIII, respectively, might be too low for energy transfer from the capped NPs to the LnIII.23 Because the band gaps of the NPs are influenced by the presence of the capping ligands and their

Table 3. Lifetimes, τ, and Coordinated H2O Molecules, q, of the 3-MPA-Capped NPs With LnIII in H2O and D2O at 25.0 ± 0.1 °C system

τH2O [ms]

τD2O [ms]

q

EuIII TbIII

0.13 ± 0.02 0.40 ± 0.01

1.91 ± 0.25 2.32 ± 0.01

7.6 ± 1.1 8.8 ± 0.2

and phosphorescence spectroscopy (vide supra), are better suited for EuIII sensitization. The lifetime decay curves of all of 3265

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Inorganic Chemistry

Table 4. Radiative (τrad) and Observed (τobs) Emission Lifetimes and Intrinsic (ΦLn Ln), Sensitized (Φsen), and Overall Emission (Φ) Efficiencies of the Capped ZnS NPs in Acetonitrile at 25.0 ± 0.1 °C system III

Eu -3-MPA

τrad [ms] 1.95 ± 0.05

TbIII-3-MPA EuIII-BMP

2.05 ± 0.08

TbIII-BMP EuIII-DCP

2.89 ± 0.18

TbIII-DCP EuIII-BDP TbIII-BDP

2.89 ± 0.02

τobs [ms]

ΦLn Ln [%]

Φsen × 10−2 [%]

± ± ± ± ± ± ± ± ± ± ± ± ± ±

57.9 ± 2.3

6.1 ± 0.3

56.6 ± 5.0

41.9 ± 4.5

39.1 ± 0.5

8.9 ± 0.2

49.5 ± 2.0

73.9 ± 2.9

0.19 1.13 0.19 1.19 0.33 1.16 0.43 1.16 0.43 1.13 0.33 1.10 1.43 1.47

0.05 0.05 0.04 0.06 0.10 0.13 0.10 0.06 0.04 0.02 0.06 0.07 0.06 0.10

3.5 3.5 1.1 1.1 23.4 23.4 14.6 14.6 3.5 3.5 1.8 1.8 36.4 78.9

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9 0.9 0.3 0.3 2.5 2.5 0.7 0.7 0.1 0.1 0.4 0.4 6.2 10.4

solvent absorption. The emission and excitation spectra were measured on either a PerkinElmer Lambda LS-55 or a Horiba Jobin Yvon Nanolog fluorimeter (Horiba FL3-22-iHR550) equipped with an ozone-free xenon lamp of 450 W (Ushio). For the PerkinElmer fluorimeter, the data were collected in phosphorescence mode with 0 ms delay, 10 ms gate time, and 16 ms cycle time. The excitation and emission slit widths were set at 15 and 5 nm, and a scan rate of 250 nm min−1 was used. For the measurements recorded on the Horiba fluorimeter, the excitation and emission slit widths were set at 3/1 and 1 nm, and an integration time of 0.1 s at 0.5/1.0 nm increments was used. All of the excitation and emission measurements were conducted at 25.0 ± 0.1 °C, unless otherwise stated. Electron Microscopy. The capped ZnS NPs were prepared for TEM and EDX analyses by dispersing a small number of the dots in nbutanol followed by sonication. Afterward, a drop of the sample was deposited onto a 400 mesh copper carbon grid (SPI Supplies). The grid was allowed to dry before analysis was conducted on a JEOL2100F field-emission transmission electron microscope, equipped with an Oxford energy-dispersive spectrometer. PXRD. The NPs were analyzed by PXRD using a Phillips diffractometer with a Cu Kα radiation source (λ = 0.154 Å). Each sample was measured at 0.02° intervals from 2θ = 10 to 90°.

own spectroscopic signature, it is, in addition, possible that back energy transfer occurs from the capping ligands to the NPs, leading to a decreased overall efficiency (Tables 1 and 4). In summary, we synthesized new thiol-based ligands that cap ZnS NPs and bind LnIII ions and successfully adopted a one-pot synthesis method to isolate new capped NPs. These new monodisperse crystalline cubic ZnS NPs with sizes in the range 3−4 nm were decorated with LnIII ions. We showed that they sensitize LnIII-centered emission with moderate efficiency and that the energy levels of these systems are better suited to sensitize EuIII than TbIII, except in the case of BDP-capped NPs. While three of the capping agents are chromophores and thus potentially involved in the energy transfer, one is not, and thus we have shown here the first example of surface LnIII sensitization through NPs capped with a nonchromophore. Our findings provide a new framework of nanocrystal− lanthanide interaction and energy transfer that has potential applications in imaging and sensing.

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Φ × 10−2 [%]

EXPERIMENTAL SECTION

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All chemicals were reagent grade and were used as received. All of the solvents, except ethanol and methanol, were distilled before use. Millipore water (18.2 MΩ) was used for the NP synthesis. The LnIII salts were dried under reduced pressure and heating. The stock solutions were titrated against ethylenediaminetetraacetic acid in 30% hexamine (w/v) using xylenol orange as an indicator, followed by storage in a glovebox at a controlled atmosphere (O2 < 0.5 ppm and H2O < 2 ppm). The detailed synthetic procedures of the ligands and their LnIII complexes, nanoparticles and their LnIII systems are provided in the Supporting Information. NMR Spectroscopy. The 1H and 13C NMR spectra were collected using Varian NMR spectrometers operating at either 400 or 500 MHz with chemical shifts, δ, reported in ppm against tetramethylsilane [Si(CH3)4] or 2,2-dimethyl-2-silapentane-5-sulfonate and deuterated chloroform (CDCl3) or deuterium oxide (D2O) used as the solvent. Mass Spectrometry. Electrospray ionization mass spectrometry data were acquired using a Waters Micromass ZQ quadrupole in positive and low-resolution modes. IR Spectroscopy. All of the FT-IR spectra were measured on a Nicolet 6700 FT-IR in ATIR mode. The IR data for each sample were collected in the range 4000−590 cm−1, with 32 scans at 4 cm−1 resolution per spectrum, and a background correction for CO2 and H2O was conducted. UV−Vis Absorption and Photoluminescence. The UV−vis absorption spectra were taken on a PerkinElmer Lambda 35 UV−vis spectrometer with slit widths set at 0.5 nm and a scan rate of 480 nm min−1. The scan range was appropriately set for each sample to avoid

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02638. Detailed synthetic procedures of the ligands and their LnIII complexes as well as NPs and their LnIII systems, 1H and 13C NMR and FT-IR spectra, mass spectra of the intermediates and ligands, UV−vis absorption, fluorescence, and phosphorescence data of the ligands and their LnIII complexes as well as the NPs and their LnIII systems, and EDX, TEM, and PXRD data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (775) 682-8421. ORCID

Ana de Bettencourt-Dias: 0000-0001-5162-2393 Notes

The authors declare no competing financial interest. 3266

DOI: 10.1021/acs.inorgchem.6b02638 Inorg. Chem. 2017, 56, 3260−3268

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Inorganic Chemistry

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ACKNOWLEDGMENTS Financial support of this work through the National Science Foundation is gratefully acknowledged (Grant CHE-1363325).

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