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Harnessing the synergy between upconverting nanoparticles and lanthanide complexes in a multi-wavelength responsive hybrid system Riccardo Marin, Ilias Halimi, Dylan Errulat, Yacine Mazouzi, Giacomo Lucchini, Adolfo Speghini, Muralee Murugesu, and Eva Hemmer ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01381 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Riccardo Marin,a Ilias Halimi,a Dylan Errulat,a Yacine Mazouzi,a† Giacomo Lucchinib, Adolfo Speghini,b Muralee Murugesua* and Eva Hemmera,c* a

University of Ottawa, Department of Chemistry and Biomolecular Sciences, 10 Marie Curie, Ottawa, Ontario, K1N 6N5, Canada.

b

Nanomaterials Research Group, Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, I-37314 Verona, Italy. c

Centre for Advanced Materials Research (CAMaR), University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada.

†

Current address: Sorbonne Université, Laboratoire de Réactivité de Surface (LRS) UMR7197 CNRS, 4 Place Jussieu - 75005, Paris, France.

KEYWORDS upconverting nanoparticles, lanthanide, complex, hybrid system, hyperspectral imaging, energy transfer, films.

ABSTRACT: We prepared a hybrid system composed of a continuous film of dinuclear lanthanide complex [Ln2bpm(tfaa)6] (Ln = Tb or Eu) and upconverting nanoparticles (UCNPs) using a straightforward drop-cast methodology. The system displayed visible emission under nearinfrared (NIR) excitation, simultaneously stemming from sub-10-nm UCNPs and [Ln2] complexes, the latter species being otherwise directly excitable only using UV-blue radiation. In light of the results of steady-state – including power-dependent – and time-resolved optical measurements, we identified the radiative, primarily ligand-mediated nature of the energy transfer from Tm3+ ions in the UCNPs-to-Ln3+ ions in the complexes. Hyperspectral mapping and electron microscopy observations of the surface of the hybrid system confirmed the continuous and concomitant distribution of UCNPs and lanthanide complexes over the extensive composite films. Key features of the hybrid system are the simultaneous UV-blue and NIR light harvesting capabilities and their ease of preparation. These traits render the presented hybrid system a formidable candidate for the development of photoactivated devices capable to operate under multiple excitation wavelength and to transduce the absorbed light into narrow, well-defined spectral regions.

“The whole is greater than the sum of its parts” is a quote by the Greek philosopher Aristotle that elegantly condenses the idea that a harmonious interaction of elements produces a total effect that is greater than the sum of the individual entities. The translation of this concept to materials science is straightforward due to the availability of innumerous materials that feature intriguing properties and can be paired to benefit from their synergy. To that end, amalgamation of different optically active materials is a lively field due to the number of applications for which such composites can be exploited. Indeed, the interaction between semiconductors, metallic nanoparticles, inorganic phosphors and/or organic fluorophores joined in composed systems has led to the preparation of devices with superior capabilities in terms of light harvesting,1-2 molecule or ion sensing,3-5 temperature probing.6-7, photoluminescence color 8-9 and lifetime tuning.10-11

case benefiting from energy transfer (ET) between the UCNPs and the molecules.

Inspired by those results, herein we present a unique solid-state hybrid system composed of films of dinuclear fluorescent lanthanide (Ln3+) complexes amalgamated with small upconverting nanoparticles (UCNPs) (Scheme 1). This hybrid system has augmented optical properties since the emission of the complex can be obtained exciting with both UV and near-infrared (NIR) light, in the latter

Scheme 1. Operation scheme of the proposed hybrid system. UCNPs are valuable species in the frame of optical systems based on ET mechanisms12 since they (i) allow for the control of the interaction and relative placement between the moieties of choice at the nanometer scale, (ii) feature high surface-to-volume ratio, favoring sur-

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face interactions while minimizing bulk effects,13-15 and (iii) showcase NIR operational capabilities, a paramount asset in contexts like light harvesting 16-19 and biomedical applications.20-26 Recently proposed solid-state systems based on the interaction between UCNPs with other species make use of plasmonic materials (e.g. gold27-29 and CuS nanoparticles30), organic molecules,31-32 and semiconductors.14, 33-37 Surprisingly, very limited attention has been paid to Ln3+ complexes in this context.16 These complexes are usually efficient fluorophores, exhibiting a strong broadband absorption - which stems from the organic ligands - and discrete emission lines - characteristic of Ln3+ ions - that are weakly influenced by the chemical environment.38-39 The invariability of the emission lines position of Ln3+based fluorophores makes these systems of tremendous interest for sensing20, 40-42 and light conversion.43-45 The route usually adopted to combine UCNPs and Ln3+ complexes is anchoring molecules of the latter to the surface of the former, yielding independent entities applicable for sensing purposes.20 Herein, we detach from this elaborate approach. Specifically, we propose a straightforward drop-cast method to prepare an “inverse” hybrid system starting from a mixed solution of the two moieties, thus eliminating tedious synthetic processes for surface modification and tethering. This system assembles in extensive [Tb2] or [Eu2] continuous films decorated with UCNPs. The so-prepared system features light harvesting capabilities both in the UV and NIR wavelength range and allows to concentrate the emitted energy in spectrally narrow regions. The obtained composed material represents an uncommon inverse hybrid system using UCNPs and Ln3+ complexes that does not rely on surface modification procedures, yet features efficient ET among the moieties. The reported proof-of-concept represents a blueprint, providing ground for the development of, for instance, photoactive and opto-electronic devices.

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Tm2O3 (1.5 mg, 0.00625 mmol) in a 50-mL three necked round bottom flask. 10 mL of a 1:1 TFA:H2O mixture was added and the slurry was refluxed at 90 °C until it became clear. Then, the stirring was stopped and the temperature lowered to 60 °C to allow the precursors to dry overnight. Subsequently, sodium trifluoroacetate (340 mg, 2.5 mmol) was added in the flask along with 10 mL of octadecene, 5 mL of oleic acid, and 5 mL of oleylamine. The mixture was degassed under vacuum increasing the temperature to 120 °C. After 30 min, 10 mL of the mixture was poured into a 35 mL microwave reaction vessel filled with nitrogen, which was placed in a CEM microwave reactor (Discovery CEM). The reaction mixture was subjected to 300 W MW irradiation to reach 300 °C, immediately followed by heating at 295 °C for 10 min. After cooling to room temperature, UCNPs were precipitated adding a 1:5 hexane:ethanol mixture and centrifuged at 10,300 rcf for 20 min. The product was further washed with a 1:3 mixture of hexane:ethanol and centrifuged three times. After purification, oleate-coated UCNPs were stored in 5 mL of toluene for further use. Synthesis of Tb3+ and Eu3+ complexes ([Tb2] and [Eu2]). The 2,2-bipyrimidine was synthesized following an established synthetic procedure,47 while the synthesis of the complexes was performed following a slightly modified protocol reported previously.4849 In brief, a saturated ammonia solution (1.2 mL, 1.62 mmol) was added to a stirring solution of tfaa (0.197 mL, 1.62 mmol) in 5 mL of ethanol. The resulting mixture was allowed to stir for 15 min, followed by the addition of a 5 mL of ethanolic solution of bpm (42.7 mg, 0.27 mmol) and TbCl3·6H2O (202 mg, 0.54 mmol) or EuCl3·6H2O (198 mg, 0.54 mmol) in 5 mL of ethanol for [Tb2] and [Eu2], respectively. The resulting solution was allowed to stir for 1 h, after which the mixture was filtered and allowed to evaporate in air. Both complexes were obtained as colorless solids. Amalgamation of UNCPs and [Ln2] complexes into hybrid systems.

Experimental section Chemicals. Gadolinium oxide (Gd2O3, 99.99%), ytterbium oxide (Yb2O3, 99.99%), thulium oxide (Tm2O3, 99.99%), erbium oxide (Er2O3, 99.99%) were purchased from Alfa Aesar. Sodium trifluoroacetate (98%), trifluoroacetic acid (TFA, 98%), 1-octadecene (98%), oleic acid (99%), oleylamine (70%), 1,1,1 trifluoroacetylacetonate (tfaa, 98%), triphenylphosphine (99%), nickel chloride hexahydrate (NiCl2·6H2O, 99.9%), europium chloride hexahydrate (EuCl3·6H2O, 99.9%), terbium chloride hexahydrate (TbCl3·6H2O, 99.9%), ammonium hydroxide (NH4OH, 28% in water) were purchased from Sigma-Aldrich. 2-chloropyrimidine (99%) was purchased from AK Scientific. Zinc powder (99.9%) was purchased from AnalaR. Reagent grade dimethylformamide (dried over 4 Å molecular sieves prior to use), dichloromethane, chloroform, hexane, and ethanol (99%) were purchased from Alfa Aesar and Sigma Aldrich and used as received unless otherwise specified. Microwave-assisted synthesis of NaGdF4: 20 % Yb3+,0.5% Tm3+ upconverting nanoparticles (UCNPs). The synthesis of NaGdF4 nanoparticles doped with 20 % Yb3+ and 0.5 % Tm3+ (hereafter UCNPs) was performed via a microwave (MW)-assisted thermal decomposition procedure.46 For the synthesis of UCNPs, Ln3+-trifluoracetate precursors were prepared mixing Gd2O3 (361.1 mg, 0.99375 mmol), Yb2O3 (98.5 mg, 0.25 mmol) and

In a typical procedure, a solution of the complex of choice was prepared in chloroform at a concentration of 40 mg mL-1. Meanwhile, a predetermined amount of UCNPs was precipitated from toluene upon addition of acetone and centrifugation. The supernatant was discarded and the particles were re-dispersed in the chloroform solution containing the [Ln2] to obtain a UCNPs:complex mass ratio of 1:1 (2:1, 1:2, and 1:4 ratios were also tested). The so-prepared dispersion was sonicated for 10 min and diluted to the desired concentration (40, 20, 10 or 2 5 mg mL-1). Afterwards, 20 μL of the mixed sol was deposited on a glass microscope slide preventively washed with ethanol. The solvent was dried off overnight obtaining the correspondent hybrid system as a combination of UCNPs and [Ln2] complexes. Preparation of mixed sols for energy transfer study in solution. A dispersion of UCNPs with a concentration of 1 mg mL-1 and a concentrated (16 mg mL-1) solution of [Tb2] were prepared in chloroform. The ET between the moieties was investigated upon titration of the UCNP dispersion with aliquots of the [Ln2] solution, recording the emission upon 980 nm excitation. Characterization techniques. The crystalline structure of the UCNPs was assessed by means of Xray powder diffraction (XRPD) using a Rigaku Ultima IV diffractometer working in a θ-2θ Bragg-Brentano geometry. The diffraction pattern was recorded using Cu Kα filtered radiation


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(λ = 1.5401 Å) over the 20-60° angle range with a step-size of 2θ = 0.02°. The morphology of the UCNPs was assessed using a transmission electron microscope (TEM, FEI TecnaiTM Spirit) operated at 150 kV. The specimen was prepared depositing a drop of the dispersion in toluene on a formvar/carbon film supported on a 300-mesh copper grid. The morphology of the hybrid system was investigated using aforementioned TEM as well as a scanning electron microscope (SEM, JEOL JSM-7500F FESEM) operated at 120 kV. SEM observations were conducted on the hybrid system prepared on a glass slide (as above described) after gold-sputtering (3 nm gold layer thickness) in a vacuum coater (Leica EM ACE200). Energy dispersive X-ray (EDX) measurements were performed on a Zeiss GeminiSEM 500 equipped with a Bruker EDX detector. Profilometry measurements were performed with a Dektak profilometer using a stylus with 2 µm tip and an applied force of 2 mg. Optical absorption spectra were recorded on a Cary Varian 5000 in the 300-1200 nm range with a scan step of 0.5 nm and a scan speed of 600 nm min1 . For recording diffuse reflectance spectra (DRS) an accessory for solid-state measurements was used. Excitation spectra were acquired on a Varian Cary Eclipse spectrofluorimeter. Solid-state emission spectra under UV light excitation and hyperspectral data were obtained on hybrid system prepared on glass slides with a custom-built hyperspectral microscope (IMA UpconversionTM by PhotonEtc, Montreal, Canada) equipped with an inverted optical microscope (Nikon Eclipse Ti-U), a broadband camera for color imaging, a set of galvanometer mirrors, a Princeton Instruments SP-2360 monochromator/spectrograph, a Princeton Instruments ProEM EMCCD camera for detection of visible emission, a Nikon Halogen Lamp (IntensiLight 100 W) with a single band DAPI filter cube for 390 nm light excitation, and a 980 nm laser diode (power at the sample of approximately 4.0 × 105 W cm-2 with a spot size of approximately 1. 5 µm in diameter) collecting the emission epifluorescently. Upconversion spectra in solution were acquired on the same setup in a 1 cm optical path glass cuvette and collecting the emission in a 90° configuration. For luminescence lifetime measurements and steadystate upconversion spectroscopy, a 980 nm diode laser in current modulated pulsed mode was used as the radiation source (CNI Optoelectronics Tech). The beam was focused on the sample and the emission gathered with a 40x objective and a beam splitter (Thorlabs, BSS10), in backscattering mode. The steady-state upconversion spectra were recorded using a half-meter monochromator (Andor, Shamrock 500i) equipped with a 1200 lines mm-1 grating and CCD detector (Andor, iDus420-BVF). The decay curves were recorded with a half-meter monochromator (Andor, Shamrock 500i) equipped with a 1200 lines mm-1 grating and a Hamamatsu R928 photomultiplier tube connected to a 500 MHz digital oscilloscope (LeCroy, WaveRunner LT342). Effective decay times were obtained upon integrating the emission decay curves. Data analysis and plotting were performed with the hyperspectral imager’s PHySpecV2 software as well as OriginPro®.

Results and discussion Characterization of the UCNPs and [Tb2] and [Eu2] complexes. NaGdF4 doped with 20 % Yb3+ and 0.5 % Tm3+ was the preferred material for the synthesis of UCNPs due to the good upconverting performance of this host-lattice/dopant combination.50-51 Moreover,

Figure 1. TEM micrograph (A), particle size distribution centered at 7.3 nm obtained counting 200 nanoparticles (B), XRD pattern with the reference pattern (PDF #00-27-0697) for cubic -NaGdF4 (C), and upconversion spectrum under 980 nm excitation (D) of UCNPs. The relatively low intensity of the NIR Tm3+ emission follows from the instrumental response of the utilized setup, optimized for the visible optical range. Molecular structure (E), along with normalized diffuse reflectance and emission spectra of [Tb2] and [Eu2] (F). Emission spectra (solid lines) were obtained under 390 nm excitation.

it displays a spectral distribution of the emission lines that is particularly favorable for the developed hybrid system (vide infra). MWassisted synthesis allowed for the straightforward preparation of monodisperse, quasi-spherical UCNPs crystallized in the cubic NaGdF4 polymorph (α-phase, Fm-3m - Figure 1A-C). The prepared sub-10-nm UCNPs displayed upconverted light emission under 980 nm laser irradiation, characterized by the presence of Tm3+specific lines (Figure 1D). [Tb2] and [Eu2] complexes used in this study (Figure 1E) were purposely chosen due to their chemical and optical stability,48 which allows for the preparation of robust hybrid systems. Their thorough structural characterization can be found elsewhere.49 Both [Tb2] and [Eu2] show optical absorption capabilities in the UV-blue wavelength range, enabled by the bpm and tfaa ligands. These complexes featured a strong visible emission (Figure 1F) under UV (390 nm) light excitation. It is of interest to note that the energy stabilization of the ligand scaffold passing from solution to solid-state resulted in a considerable red-shift of the absorption spectra of the complexes (Figure S1). This well-known phenomenon is of particular value for this study, since it led to an appreciable overlap between the complexes’ absorption and UCNPs emission ( vide infra). Indeed, the choice of Yb3+/Tm3+ as the pair of dopant elements


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for the preparation of UCNPs was dictated by the superior capability of this combination of dopants to provide high energy (i.e. UV and blue) upconverted light compared to other Ln3+ ions combinations.51-53 Formation of the hybrid systems. The choice of sub-10-nm UCNPs for the preparation of the hybrid system derives from the need to maximize the surface effects that promote the interaction between the complexes and the UCNPs, favoring their close proximity. Indeed, the surface of the UCNPs is decorated by oleate molecules, which renders them highly hydrophobic.54 This oleate hydrophobic layer prompts the interaction with the tfaa moieties of the [Ln2] complexes. The validity of this assumption was confirmed by alternatively using UCNPs featuring no organic molecules on their surface, i.e. ligand-free (Figure S2). Indeed, the absence of hydrophobic ligands on the particle’s surface hindered the formation of a film. Using a concentrated starting mixture of UCNPs and complex (40 mg mL-1 for each moiety), a hybrid system was formed where [Tb2] or [Eu2] were arranged in a continuous, dense film as demonstrated by optical microscopy and SEM observations (Figure 2 and S3). The thickness of this film was determined to be approximately 7 ± 2 μm by means of profilometry (Figure S3).

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SEM micrographs (Figure 2A-iii, B-iii) show that in both cases the morphology of this film was characterized by the presence of needlelike crystals embedded in a compact matrix overall featuring a relatively homogeneous emission. Decreasing the concentration of both complex and UCNPs to 5 mg mL-1, the film became less dense, with the appearance of more isolated needle-like crystals. These well-defined structures were covered by a much thinner layer of [Tb 2] or [Eu2] molecules that did not organize in a distinct morphology. Direct observations of UCNPs in these hybrid systems via SEM was challenging, since the solid-state systems were coated with a gold layer to prevent charging phenomena. Although expected to be thin (approximately 3 nm), this layer was of the same order of magnitude of the UCNPs, thus it rendered impossible to discern the location of the UCNPs. However, direct evidence of the concomitant presence of the moieties was obtained via EDS analysis performed on a film prepared from 40 mg mL-1 concentrated dispersion of UCNPs and [Tb2] (Figure S4). A more systematic study of the effect that the initial solution concentration and the UCNPs-to-[Ln2] ratio have on the film morphology and optical properties is reported in the Supporting Information (Figure S5, S6, S7, S8, and S9). The results from these tests further corroborate the evidence that UCNPs and [Ln2] species interaction is fundamental to obtain a continuous film, whereas a decreased amount of UCNPs in the starting solution led to the formation of isolated islands of material on the glass substrate. To further investigate the interaction between the moieties, a sample was prepared casting a mixture of UCNPs and [Tb2] directly on a carbon-coated copper grid. SEM observations on this specimen showed an arrangement of the UCNPs decorating the surface of the crystals of the complex (Figure S2). This disposition of UCNPs is expected to be analogous to the one in the hybrid system prepared on glass slides, where the preferential accumulation of the particles in correspondence of the surface of [Tb2] and [Eu2] crystals is also supported by hyperspectral imaging observations (vide infra). Energy transfer mechanism in the hybrid systems.

Figure 2. Micrographs of hybrid systems containing UCNPs and [Tb2] (A) or [Eu2] (B) acquired on an optical microscope (i, ii, iv, v) and SEM (iii, vi) highlight a similar morphology of the two systems. A continuous distribution of complex molecules across the film (i, ii) or scattered needle-like crystals (iv, v) are observed at high and low concentration of the starting solution of UCNPs and complex, respectively. Scale bars are 20 μm for photomicrographs and 10 μm for electron micrographs. Photomicrographs are presented in real colors.

The energy transfer from UCNPs to the complex in the hybrid system enabled [Tb2] and [Eu2] emission under NIR (980 nm) excitation along with characteristic Tm3+ lines. To better understand the interplay between UCNPs and the complexes, it is convenient to briefly lay out the mechanisms governing the optical properties of the complexes. [Tb2] and [Eu2] emission follows from the absorption of UV-blue photons that fosters the promotion of an electron to the S1 level of the bpm and/or tfaa– ligands (Figure 3A, C). Through intersystem crossing and ET processes, the triplet state (T1) of tfaagets populated. From there, the energy is transferred to the Ln3+ ions, where electron radiative de-excitation events take place, ultimately leading to the characteristic Tb3+ or Eu3+ emission (Figure 1F).38 The bright ligand-sensitized emission of the investigated complexes proceeds from the favorable energy difference between the β-diketonate T1 and the emitting level of Tb3+ (5D4) and Eu3+ (5D0) of 2680 and 5850 cm-1, respectively, which limits back-ET from Ln3+ to the ligands (Figure 3A – [Tb2], C – [Eu2]). Notably, the investigated complexes did not show emission under continuous-wave NIR (980 nm) excitation. However, when the hybrid systems (obtained from 40 mg mL-1 concentrated starting solution) were irradiated with light of mentioned wavelength, indirect excitation of the [Ln2] molecules was achieved thanks to the ET from UCNPs to [Tb 2] or [Eu2]. The emission spectrum of both hybrid systems featured Tm3+ characteristic lines


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Figure 3. Energy level schemes of UCNPs amalgamated with [Tb2] (A) or [Eu2] (C) yielding the corresponding hybrid systems. Absorption and emission processes (solid upward and downward arrows), non-radiative processes (grey dashed arrows), ET mechanisms within each moiety (grey dotted arrows) and between UCNPs and the complex (orange solid arrows) are depicted. DRS and solid-state excitation spectra, UV-triggered [Ln2] (green and red dashed lines) and NIR-triggered upconversion (blue lines) emission spectra are reported in B and D. The emission spectra of the UCNPs alone (dashed blue lines) and those of the hybrid systems (solid blue lines) are normalized to the maximum of the 1G4 → 3F4 emission (centered at 650 nm). Hence, the spectra perfectly overlap in this wavelength range.

alongside with the contributions of Tb3+ (547 nm) or Eu3+ (612 nm) sensitized emission (Figure 3B, D). This was accompanied by the marked quenching of Tm3+ emission bands that overlap with the absorption spectrum of the complexes (centered at 362, 450 and 480 nm), evidencing the ET taking place. The extent to which each band experienced quenching (Figure 3B, D) followed the superimposition between the specific Tm3+ emission and the excitation spectrum of the complex. These quenched bands arise from the 1D2 → 3H6 (362 nm), 1D2 → 3F4 (450 nm), and 1G4 → 3H6 (480 nm) electronic transitions of Tm3+, the latter being a three-photon order transition, whilst the other two are four-photon order processes.55 The transition’s photon order (i.e. the number of photons needed to populate the emitting level) is given by the slope of the emission intensity vs excitation power double logarithmic plot. For UCNPs alone, the obtained values for 1D2 → 3F4, 1G4 → 3H6, and 1G4 → 3F4 (650 nm) transitions were 2.56, 2.19, and 2.17 respectively. These values were lower than the theoretically expected values of 4, 3, and 3, but in line with reported values for similar fluoride-based Yb3+/Tm3+-doped UCNPs (Figure 4A).55 Discrepancies between theoretical and experimental values are common for nano-sized systems and the power density range spanned in this study likely falls outside the unsaturated regime.56-57 The observed values remained unchanged for UCNPs in both hybrid systems, and the indirectly triggered [Tb2] or [Eu2] emissions showcased a photon-order (2.89 and 2.77) in line

Figure 4. Integrated 1D2 → 3F4, 1G4 → 3H6, and 1G4 → 3F4 Tm3+ emissions vs 980 nm excitation power density for UCNPs alone (A) and amalgamated with [Tb2] (B) and [Eu2] (C), as well as NIR triggered Tb3+ and Eu3+ emissions. Decay curves obtained under 980 nm excitation of the UCNPs alone and the two hybrid systems monitoring Tm3+ 1 D2 → 3F4 (D) and 1G4 → 3H6 (E) transitions and the emission of the indirectly excited complexes (F).


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with the highest photon-order transition of Tm3+ investigated (1D2 → 3 F4 – 2.66 and 2.44 for the two systems) (Figure 4B, C). The slightly higher values found for the complex emissions could indicate a partial involvement of Tm3+: 1I6 level in the ET mechanism. This provides evidence that the ET takes place preferentially from the higher energy level of Tm3+ to the ligand scaffold of the complex (Figure 3A, C). Indeed, if a direct ET from the Ln3+ ions in UCNP to the Ln3+ ions in the complex would occur it would take place prominently from the more easily populated Tm3+: 1G4 level to Tb3+: 5D4 or Eu3+: 5D1,0 levels. This would result in an observed slope for [Tb2] or [Eu2] emission closer to the one of Tm3+: 1G4 → 3H6 transition. The predominant role played by the ligands in the ET mechanism is also confirmed by studying the behavior of a mixture of UCNPs and [Tb2] in a chloroform dispersion (Figure S10). There, the quenching of the UCNP emission and the energy transferred to the complex (hence the intensity of its sensitized emission) was limited as a consequence of the blue-shifted absorption of the complex in solution. This shift led to a reduced overlap between the emission and the absorption of the moieties involved in the ET mechanism, ultimately yielding a less efficient energy transfer.

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lifetime observed for Tb3+: 5D4 level (Figure 4F and Table 1). Therefore, no lifetime is reported for this decay curve, since it does not provide meaningful information to interpret the ET mechanism. Lastly, the fact that 1G4 → 3H6 (480 nm) and 1G4 → 3F4 (650 nm) transitions are not equally quenched in the presence of the complex (Figure 3B, D) further supports a radiative nature of the ET. This is because those transitions stem from the same energy level, thus, in case of resonant ET, it is plausible that they would be quenched to the same extent. Overall, these results suggest three main characteristics of the investigated ET occurring under 980 nm light excitation from UCNPs to [Tb2] or [Eu2]. First, the ET from UCNPs to the complexes leads to the NIR-sensitized emission of the [Ln2], which is otherwise not achievable directly exciting them with 980 nm light (Scheme 1). Second, the appearance of this NIR-sensitized emission of [Tb2] and [Eu2] in the hybrid system is accompanied by a quenching of Tm3+ emission bands that depends on their overlap with the excitation spectra of the complexes. Last, the ET proceeds through re-absorption of the upconverted light from [Ln2] predominantly via the organic ligand scaffold. Although the close proximity of the moieties (favored by the small UCNP size) theoretically supports a non-radiative ET process via dipole-dipole interaction, the lack of significant lifetime shortening suggests that this type of contribution – albeit not being unquestionably excluded – is largely outweighed by radiative processes.

In the context of ET, non-radiative processes are more strongly influenced by the distance between the energy donor and energy acceptor than radiative ones, due to their dipole-dipole interaction nature.58 Non-radiative ET is unambiguously distinguished from radiative processes since it induces a shortening of the lifetime of the donor energy level involved in the ET. Nonetheless, it has to be mentioned that in the case of UCNPs, the steady-state emission spectrum quenching is considerably more pronounced than the observed lifetime shortening.59-60 Also, the rich energy level scheme of Ln3+ ions results in innumerable activation and de-activation pathways. This, in conjunction with the different accessibility of Ln3+ ions in the core or surface of the particles, makes it challenging to predict the effect of the ET on the kinetics of the relaxation processes in UCNPs.59-60 For the case under study, a non-radiative ET could only occur from the Tm3+: 1D2 energy level since, as opposed to 1G4, this is the only one that simultaneously shows (i) energy resonance with the ligands’ energy levels and (ii) spectral overlap of the emission stemming from it and the DRS of [Ln2] complexes. To elucidate the nature of the ET in the hybrid systems, lifetime measurements were performed exciting the film with 980 nm light and monitoring the 1 D2 → 3F4 and 1G4 → 3H6 emission lines of Tm3+ (Figure 4D, E). The lifetime of both 1D2 and 1G4 levels did not show any appreciable shortening upon amalgamation with the complexes, indicating that the quenching of the emission occurs mainly due to radiative ET processes (Table 1). The appearance of a second, long-lived decay component for the 1G4 → 3H6 transition in the presence of [Tb2] was a consequence of the partial overlap between Tb3+: 5D4 → 7F5 and Tm3+: 1G4 → 3H6 emission bands, and is congruent with the longer

Spatial distribution of the spectral features. In view of its foreseeable applications, it is pivotal that the developed hybrid system responds equally throughout the whole film. This homogeneous optical response is achieved in case of a simultaneous and continuous presence of the two moieties composing the hybrid system. In order to probe this, films obtained from UCNPs amalgamated with [Tb2] and [Eu2] were spectrally investigated by means of hyperspectral imaging (Figure 5). The photomicrographs under white and UV light recorded on the films obtained from a 40 mg mL1 mixture of UCNPs and [Ln2] display films of complex over an extended surface (here approximately 100 x 100 μm2). Hyperspectral mapping was performed over a selected region of interest (ROI) considering three distinct spectral ranges, where two transitions of Tm3+ (1G4 → 3F4 and 1G4 → 3H6) and the emission from the complex (either Tb3+: 5D4 → 7F5 or Eu3+: 5D0 → 7F2) are centered. Notably, the features of the film that can be observed under UV illumination in the ROI (Figure 5A, E) are also distinguished under NIR excitation (Figure 5B, F) with an excellent overlap between the spatial distribution of the UCNP emission and

Table 1. Effective lifetime values extracted from the decay curves recorded exciting at 980 nm and monitoring the emissions from Tm3+ 1D2 → 3 F4 and 1G4 → 3H6 transitions and the triggered emissions of Tb3+ and Eu3+ in the hybrid systems. The values are averages of at least three separate measurements performed on the specific samples. Tm3+: 1D2 → 3F4 τ, ms 0.165 ± 0.020

Tm3+: 1G4 → 3H6 τ, ms 0.226 ± 0.020

Tb3+: 5D4 → 7F5 τ, ms -

Eu3+: 5D0 → 7F2 τ, ms -

UCNPs + [Tb2]

0.160 ± 0.018

-

1.730 ± 0.010

-

UCNPs + [Eu2]

0.161 ± 0.010

0.217 ± 0.021

-

0.560 ± 0.040

UCNPs


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Figure 5. Photomicrographs for the hybrid systems obtained from a highly concentrated [Ln2] solution under white and UV light illumination along with the region of interest (ROI) (A – [Tb2], E – [Eu2]) over which spectral maps under 980 nm light irradiation were obtained (B, F). Tm 3+ and indirectly triggered Tb3+ or Eu3+ emissions were monitored over an area of approximately 20 x 20 μm2. The absolute intensity of the emission bands fluctuated throughout the hybrid system (C, G) indicating some variability in the total amount of material distributed over the surface. However, the constancy of the ratio between the integrated emission of the complex vs Tm3+: 1G4 → 3H6 transition (squares) and Tm3+:1G4 → 3F4 vs Tm3+:1G4 → 3 H6 (circles) confirmed the simultaneous presence of the two moieties throughout the hybrid system and the interaction between them (D, H). Scale bars are 20 μm in the photomicrographs and 5 μm in ROIs and spectral maps. Photomicrographs are presented in real colors.

the indirectly (NIR) triggered emission of the complexes. To quantitatively confirm this visual evidence, spectra obtained under NIR excitation were extracted from the hyperspectral maps at four different spots (Figure 5C, G). Although some fluctuations in the absolute intensity of the signals were noticeable, the ratio between the

integrated intensity of the three signals did not exhibit significant fluctuations throughout the film surface (Figure 5D, H). Hyperspectral imaging performed over three distinct ROIs on a second hybrid film – prepared under identical conditions – evidences the reliability of the optical properties over larger samples areas and from film-to-


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film (Figure S11). Similar behavior is also shown by the hybrid system obtained at lower concentration (Figure S12 and S13). This experimental evidence – in hand with SEM and TEM observations (Figure S2) – suggests that, upon [Tb2] and [Eu2] crystallization, the UCNPs organize in close proximity of the complexes, hence fostering the ET process upon NIR excitation. Therefore, albeit only preliminary optimization with respect to concentration, ratio and nature of the moieties was carried out, the results demonstrate that the preparation of a multiwavelength-responsive hybrid systems composed of UCNPs and [Ln2] can be promptly achieved. This is accomplished by taking advantage of the hydrophobic interaction between both moieties, which prompts their assembly in structures with augmented optical properties. These results open opportunities for the preparation of similar systems based on UCNPs and lanthanide complexes, yet leaving space for a case-by-case optimization in terms of amalgamated moieties and film morphology.

Conclusions We have here reported a hybrid system composed of a film amalgamating two lanthanide complexes [Tb2] and [Eu2] with Tm3+doped upconverting nanoparticles (UCNPs). This hybrid system was prepared via a straightforward drop-cast approach, exploiting the interaction between the complex and nanoparticular moieties. The choice of NaGdF4: 20 % Yb3+, 0.5% Tm3+ as the UCNP material ensured good spectral overlap of the near-infrared (NIR) triggered upconverted emission with the excitation spectra of the lanthanide complexes. The proximity of the two species, along with mentioned spectral overlap, promoted effective energy transfer from the UCNPs to the lanthanide complexes. By virtue of this energy transfer, while retaining their strong emission under UV illumination, the complexes were indirectly excitable using NIR light, thus resulting in augmented light-harvesting capability. The combination of power-dependent and time-resolved spectroscopic studies revealed the predominantly radiative, ligand-mediated nature of the energy transfer, setting the ground for further understanding of these amalgamated systems. Finally, the use of hyperspectral imaging enabled to assess the concomitant presence of UCNPs and lanthanide complexes and the occurrence of the energy transfer mechanism between them throughout the hybrid system. Overall, these results are expected to serve as a blueprint for the design of a new class of all lanthanide-based composite materials where the synergy of the separate entities results in bettered properties, making this hybrid system of interest for opto-electronic or color tuning applications.

Supporting Information. Comparison of [Tb2] and [Eu2] absorption and excitation spectra in solution and in solid-state; TEM micrographs of systems obtained from oleate-coated and ligand-free UCNPs; SEM micrographs of a hybrid system on a TEM grid; profilometry measurements; EDX analysis; characterizations of films obtained at different moieties concentration and relative ratio; study of the energy transfer in chloroform; hyperspectral mapping on additional films of hybrid system. This material is available free of charge via the Internet at http://pubs.acs.org.

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Prof. Eva Hemmer [email protected] Prof. Muralee Murugesu [email protected] ACKNOWLEDGMENT R. M., I. H., D. E., Y. M. M. M. and E. H. gratefully acknowledge the financial support provided by the University of Ottawa, the Canadian Foundation for Innovation (CFI), and the Natural Sciences and Engineering Research Council of Canada (NSERC). A. S. and G. L. gratefully acknowledge University of Verona, in the framework of the “Ricerca di base 2015” project, for financial support. Y. M. is grateful to Labex Michem and Sorbonne Universités for the financial support. We would also thank Prof. H. Alper for his support.

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(57) Tian, B.; Fernandez-Bravo, A.; Najafiaghdam, H.; Torquato, N. A.; Altoe, M. V. P.; Teitelboim, A.; Tajon, C. A.; Tian, Y.; Borys, N. J.; Barnard, E. S.; Anwar, M.; Chan, E. M.; Schuck, P. J.; Cohen, B. E. Low irradiance multiphoton imaging with alloyed lanthanide nanocrystals. Nat. Commun. 2018, 9, 3082. (58) Medintz, I.; Hildebrandt, N. FRET - Förster Resonance Energy Transfer, 2013. (59) Bhuckory, S.; Hemmer, E.; Wu, Y.-T.; Yahia-Ammar, A.; Vetrone, F.; Hildebrandt, N. Core or Shell? Er3+ FRET Donors in Upconversion Nanoparticles. Eur. J. Inorg. Chem. 2017, 2017, 5186-5195. (60) Rojas-Gutierrez, P. A.; Bhuckory, S.; Mingoes, C.; Hildebrandt, N.; DeWolf, C.; Capobianco, J. A. A Route to Triggered Delivery via Photocontrol of Lipid Bilayer Properties Using Lanthanide Upconversion Nanoparticles. ACS Appl. Nano Mater. 2018, 1, 5345-5354.


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A hybrid system was prepared from upconverting nanoparticles amalgamated with a lanthanide complex film, which can be excited using both UV and near-infrared by virtue of the energy transfer between the optically active moieties.

Table of Contents artwork

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Scheme 1. Operation scheme of the proposed hybrid system. 481x292mm (150 x 150 DPI)


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Figure 1. TEM micrograph (A), particle size distribution centered at 7.3 nm obtained counting 200 nanoparticles (B), XRD pattern with the reference pattern (PDF #00-27-0697) for cubic α-NaGdF4 (C), and

upconversion spectrum under 980 nm excitation (D) of UCNPs. The relatively low intensity of the NIR Tm3+ emission follows from the instrumental response of the utilized setup, optimized for the visible optical range. Molecular structure (E) and normalized diffuse reflectance and emission spectra of [Tb2] and [Eu2] (F). Emission spectra (solid lines) were obtained under 320 nm excitation. 247x338mm (150 x 150 DPI)


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Figure 2. Micrographs of hybrid systems containing UCNPs and [Tb2] (A) or [Eu2] (B) acquired on an optical microscope (i, ii, iv, v) and SEM (iii, vi) highlight a similar morphology of the two systems. A continuous distribution of complex molecules across the film (i, ii) or scattered needle-like crystals (iv, v) are observed at high and low concentration of the starting solution of UCNPs and complex, respectively. Scale bars are 20 μm for photomicrographs and 10 μm for electron micrographs. Photomicrographs are presented in real colors. 304x398mm (150 x 150 DPI)


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Figure 3. Energy level schemes of UCNPs amalgamated with [Tb2] (A) or [Eu2] (C) yielding the corresponding hybrid systems. Absorption and emission processes (solid upward and downward arrows), non-radiative processes (grey dashed arrows), ET mechanisms within each moiety (grey dotted arrows) and between UCNPs and the complex (orange solid arrows) are depicted. DRS and solid-state excitation spectra, UV-triggered [Ln2] (green and red dashed lines) and NIR-triggered upconversion (blue lines) emission spectra are reported in B and D. The emission spectra of the UCNPs alone (dashed blue lines) and those of the hybrid systems (solid blue lines) are normalized to the maximum of the 1G4 → 3F4 emission (centered at 650 nm). Hence, the spectra perfectly overlap in this wavelength range. 452x314mm (150 x 150 DPI)


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Figure 4. Integrated 1D2 → 3F4, 1G4 → 3H6, and 1G4 → 3F4 Tm3+ emissions vs 980 nm excitation power density for UCNPs alone (A) and amalgamated with [Tb2] (B) and [Eu2] (C), as well as NIR triggered Tb3+

and Eu3+ emissions. Decay curves obtained under 980 nm excitation of the UCNPs alone and the two hybrid systems monitoring Tm3+ 1D2 → 3F4 (D) and 1G4 → 3H6 (E) transitions and the emission of the indirectly excited complexes (F). 263x239mm (150 x 150 DPI)


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ACS Photonics

Figure 5. Photomicrographs for the hybrid systems obtained from a highly concentrated [Ln2] solution under white and UV light illumination along with the region of interest (ROI) (A – [Tb2], E – [Eu2]) over which

spectral maps under 980 nm light irradiation were obtained (B, F). Tm3+ and indirectly triggered Tb3+ or Eu3+ emissions were monitored over an area of approximately 20 x 20 μm2. The absolute intensity of the emission bands fluctuates throughout the hybrid system (C, G) indicating some variability in the total amount of material distributed over the surface. However, the constancy of the ratio between the integrated emission of the complex vs Tm3+: 1G4 → 3H6 transition (squares) and Tm3+:1G4 → 3F4 vs Tm3+:1G4 → 3H6 (circles) confirms the simultaneous presence of the two moieties through the hybrid system and the interaction between them (D, H). Scale bars are 20 μm in the photomicrographs and 5 μm in ROIs and spectral maps. 292x367mm (150 x 150 DPI)


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Harnessing the Synergy between Upconverting ... - ACS Publications

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