Energy transfer in diiodoBodipy-grafted upconversion nanohybrids

Abstract

Steady-state and time-resolved emission studies on nanohybrids consisting of NaYF₄:Yb,Er and a diiodo-substituted Bodipy (UCNP–IBDP) show that the Yb³⁺ metastable state, formed after absorption of a near-infrared (NIR) photon, can decay via two competitive energy transfer processes: sensitization of IBDP after absorption of a second NIR photon and population of Er³⁺ excited states.

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Understanding resonant energy transfer (RET) processes between emissive nanoparticle donors and organic dye acceptors is of paramount importance for both fundamental photophysics and application oriented studies. Detailed studies on the RET phenomenon can be found in the literature mostly regarding Quantum Dot (QD) donors.¹ In contrast, only a few examples of time-resolved studies have been reported for upconversion nanoparticles (UCNPs) acting as RET donors.^2–5

Whereas QDs undergo down-conversion after excitation, the UCNPs emit in the visible spectral range upon near infrared (NIR) excitation due to multiple intra-configurational 4fⁿ electron transitions.⁶ This up-conversion process paves the way to initiate a photodynamic (PDT) reaction with deeply penetrating NIR light and shall enable more efficient PDT at larger tissue volumes.

There are however two challenges to make this possible. Since the PDT may occur only upon efficient ET from UCNPs to photosensitisers, anchoring photosensitisers (PSs) directly to UCNPs and finding PSs of an appropriate excitation spectrum (i.e. overlapping the luminescence of up-converting lanthanides) are of critical importance.⁷ In particular, ytterbium and erbium co-doped sodium yttrium fluoride (NaYF₄:Er³⁺,Yb³⁺) nanoparticles have been used as energy donors for different PSs, such as Ru(bpy)₃²⁺,⁸ zinc phthalocyanine,^5,9 merocyanine 540,¹⁰ hypericin,¹¹ methylene blue,^12,13 rose bengal,³ pyropheophorbide a,¹⁴ chlorin e6,¹⁵ and a diiodo-Bodipy compound.¹⁶ These UCNPs have also been proved to be successful energy donors to QDs¹⁷ and other fluorescent dyes¹⁸ such as rhodamine 6G.¹⁹ In all these systems, the spectral overlap of the UCNP emission spectrum with the PS absorption spectrum is a prerequisite for efficient energy transfer (ET).

Moreover, for organic/inorganic fluorophores the ET yield (η) depends on the donor-to-acceptor separation distance (r_DA) with an inverse 6^th power law due to the dipole–dipole coupling mechanism following a simple relation η = [1 + (r_DA/R₀)⁶]⁻¹, where R₀ is the Förster distance (R₀ < 10 nm).²

For upconverting nanoparticles, there are numerous donors within a single nanoparticle, which display a whole range of effective distances to acceptors anchored at the surface, and the photophysics behind UC-RET is a more complex phenomenon.

Different strategies have been used to anchor the PS molecules as dense and as close to the surface as possible, by grafting or adsorption to the surface, or by covalent linkage to the nanoparticle functional capping.

In this context, we have recently designed a nanohybrid made of NaYF₄:Yb³⁺,Er³⁺ UCNPs and a diiodo-substituted Bodipy derivative (namely, 3-(2′,6′-diiodo-1′,3′,5′,7′-tetramethyl-4′,4′-difluoro-4′-bora-3′a,4′a-diaza-s-indacen-8′yl)propanoic acid; IBDP), which is anchored to the nanoparticle surface directly and embedded in the organic capping (a polyethyleneglycol, PEG, derivative).¹⁶ This nanohybrid was dispersible in water due to its polymeric coating.

Now, to determine the efficiency of the energy transfer process between IBDP and the UCNP core in UCNP–IBDP nanohybrids, both steady-state and time-resolved emission measurements were carried out with water- or organic-dispersible nanohybrids (Fig. 1). Nanohybrids were prepared from the same batch of oleate-capped UCNPs (UCNP@OA), and their photophysical properties were compared with those of the UCNP@OA and naked UCNPs (UCNP_naked), which also originated from the same UCNP@OA batch.

Fig. 1 Descriptive image illustrating the up-conversion nanoparticles with different coatings and solubility selected for this study.

This made it possible to compare the influence of the nanoparticle capping and the solvent itself on the optical properties of the nanohybrids.

Upconversion nanoparticles (NaYF₄:Yb³⁺(16%),Er³⁺(4%)) capped with oleate (UCNP@OA) were synthesised by following a protocol described in the literature with some modifications (see ESI Fig. S1–S3†).²⁰ Transmission electron microscopy (TEM and high resolution TEM, HRTEM) showed that the UCNPs are uniform hexagonal prisms (38.4 ± 1.7 and 13.6 ± 1.4 nm, height and side, respectively, Fig. S1†). Then, naked UCNPs (UCNP_naked) and UCNP@PEG²¹ were synthesised by removing the oleate ligand of UCNP@OA by acidification with HCl,²² whereas UCNP@PEG were synthesised by exchanging the oleate with HS-PEG-NH₂ ²¹ (see Fig. S4 and ESI† for further details).

We have previously reported that the ligand exchange reaction of UCNP@PEG with IBDP led to water-dispersible nanohybrids (UCNP–IBDP@PEG),¹⁶ see Fig. S5† and experimental details in the ESI.† Similar methods were applied to UCNP@OA nanoparticles in order to produce lipophilic hybrid (UCNP–IBDP@OA) nanoparticles. Briefly, a mixture of IBDP and the UCNP@OA was sonicated in water in the presence of triethylamine for 15 min to ensure the deprotonation of the IBDP carboxylic group and, consequently, its grafting to the UCNP surface.²² Then, the mixture was stirred for 24 h in an orbital shaker at room temperature. Then, the nanoparticles were centrifuged and washed five times in acetonitrile in order to remove the excess of IBDP. Finally, the UCNP–IBDP@OA nanohybrid (pink powder) was re-suspended in toluene (see experimental details in the ESI†).

The UV-Vis absorption spectra (Fig. 2) and TGA analyses (not shown) of UCNP–IBDP@OA and UCNP–IBDP@PEG revealed an IBDP load of 40% and 10%, respectively. Fig. 2 shows the comparison between the emission spectra of the water-dispersible nanoparticles, i.e., UCNP_naked, UCNP@PEG, and UCNP–IBDP@PEG on the left panel, while right panel compares the spectra of the toluene-dispersible nanoparticles, i.e., UCNP@OA and UCNP–IBDP@OA, after 975 nm excitation.

Fig. 2 (Left) UV-Visible absorption spectrum of IBDP in UCNP–IBDP@PEG (blue line); emission spectra of water solutions of UCNP naked (black line), UCNP@PEG (red line), and UCNP–IBDP@PEG (green line). (Right) UV-Visible absorption spectrum of IBDP in UCNP–IBDP@OA in toluene (blue line); emission spectra of toluene solutions of UCNP@OA (black line) and UCNP–IBDP@OA (red line). The spectra were recorded using a front face set-up after excitation at 975 nm. The green to red emission intensity ratio was found to be 3.2 (UCNP@OA), 1.6 (UCNP@IBDP), 2.5 (naked) and 2.5 (UCNP@PEG), and 1.8 (UCNP@IBDP@PEG), respectively. The UCNP–IBDP@PEG and UCNP–IBDP@OA solutions contained 1 mg of the nanohybrid and the IBDP concentration was 0.7 μM and 1.7 μM, respectively.

The UV-Visible absorption spectrum of IBDP has been included in both solvents to compare the overlap between the absorption spectrum of IBDP (the energy acceptor) and the emission of the UCNP (energy donor) in both water and toluene, as such overlap is a prerequisite for RET to occur.^3,5 The overlap is quantified with the spectral overlap integral (J) calculated as J = Σσ_D(λ)σ_D(λ)λ⁴δλ, where σ_D is the normalized donor emission spectrum, the σ_A is the acceptor molar extinction coefficient, and λ is the wavelength of light.

Upon excitation at 975 nm (where Yb³⁺ absorbs), the emission spectra of all the UCNPs exhibited three bands owing to Er³⁺ emission: an intense band at 540 nm (⁴S_3/2 → ⁴I_15/2), another close emission at 520 nm (²H_11/2 → ⁴I_15/2), and a red band at longer wavelengths (at ca. 670 nm, ⁴F_3/2 → ⁴I_15/2).²³ Power dependence measurements showed that emission was proportional to the 2^nd power of excitation intensity, which is consistent with previously reported values (Fig. S8†).²⁴ As expected, all the bands of UCNP_naked were less intense than those of the PEG-capped UCNP, which is consistent with the capacity of PEG to passivate the UCNP surface (Fig. 1, left).

A comparison between the emission spectrum of each nanohybrid, UCNP–IBDP@PEG and UCNP–IBDP@OA, and its precursor, i.e., UCNP@PEG and UCNP@OA, respectively, revealed that the IBDP induced quenching of the green UCNP emission band, where IBDP absorption was present. The efficiency of the RET was calculated by using simple formula based on either emission intensity or emission lifetimes, η = (I_D − I_DA)/I_D = 1 − τ_DA/τ_D, where I_DA and I_D are the integrated emission of the D–A nanohybrid (UCNP–IBDP@PEG and UCNP–IBDP@OA) and the respective nanohybrid precursor emission (UCNP@PEG and UCNP@OA), while τ_DA and τ_D are the luminescence lifetimes of D–A and D alone species, at the D emission wavelength. The intensities in the 513–560 nm range decreased by ca. 50% and 30% for UCNP–IBDP@OA and UCNP–IBDP@PEG, respectively (similar results were obtained when the intensities at 546 nm were compared). In quantitative terms, the J overlap of UCNP–IBDP@OA in toluene was larger than that of UCNP–IBDP@PEG in water (9.948 × 10¹⁵ nm⁴ M⁻¹ cm⁻¹ and 1.226 × 10¹³ nm⁴ M⁻¹ cm⁻¹, respectively), but, as mentioned above, the loading of IBDP was also larger in the former. No emission of energy acceptor molecules appeared in the nanohybrids, which is consistent with the negligible fluorescence quantum yield of IBDP (Φ_f = 0.02 in methanol).²⁵

As a control experiment, small volumes of a toluene solution of a diiodoBodipy lacking the carboxylic acid group (specifically, 4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene), IBDPnc, 5 × 10⁻³ M) were added to an oleate-capped UCNP dispersed in toluene (1 mg ml⁻¹) up to a final IBDPnc concentration of 0.45 mM. Fig. S9† shows the decrease of the green emission with increasing concentrations of IBDPnc. It was expected that this substance would not anchor the UCNP surface and, consequently, the quenching of the UCNP emission was presumed to be due to filter effects.

To gain insight into the nature of the intermolecular quenching process in the nanohybrids and corroborate the filter effects in the UCNP@OA/IBDPnc mixtures, we carried out time-resolved measurements of all the UCNPs. The emission of UCNP@OA, recorded at 546 and 654 nm, showed no time-dependence in the presence of increasing IBDPnc concentrations up to the highest concentrations tested. This confirmed that the energy transfer in the intermolecular system occurred via a trivial emission–reabsorption mechanism.

The kinetic traces at λ_exc = 975 nm excitation for the UCNP_naked, UCNP@PEG, UCNP@OA, UCNP–IBDP@PEG, and UCNP–IBDP@OA showed characteristic rise and decay phases (Fig. 3, Table 1, and ESI†).²⁶ The rise and decay lifetimes (τ_rise and τ_decay, respectively) were determined by fitting the data and these two components correspond to the sensitisation and subsequent decay of the Er³⁺ excited state. All the fittings exhibited good quality match and the τ_rise and τ_decay decays were estimated in triplicate.

Fig. 3 Kinetic profiles at 654 nm of (A) UCNP_naked (black trace), UCNP@PEG (red trace), and UCNP–IBDP@PEG (green trace) in water and (B) UCNP@OA (black trace) and UCNP–IBDP@OA (red trace) in toluene; amplification on the 0–0.2 ms time scale. Note that intensities are not real.

Table 1 Luminescence lifetimes for the UCNPs excited at 975 nm^a

UCNP	^4S3/2 → ^4I15/2^b green emission		^4F9/2 → ^4I15/2^c red emission
	τ rise, μs	τ decay, μs	τ rise, μs	τ decay, μs
a Average value of three fittings. b λ em = 546 nm. c λ em = 654 nm.
Water dispersible
UCNPnaked	45 ± 7	78 ± 3	70 ± 10	277 ± 6
UCNP@PEG	44 ± 5	82 ± 2	69 ± 7	281 ± 4
UCNP–IBDP@PEG	54 ± 7	67 ± 2	63 ± 7	252 ± 4
Toluene dispersible
UCNP@OA	94 ± 4	119 ± 1	122 ± 18	304 ± 8
UCNP–IBDP@OA	58 ± 10	96 ± 6	61 ± 6	317 ± 4

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The decay lifetimes of the UCNP emission were in the tens to hundredths μs range and those for the red emission were longer (ca. 250–320 μs) than those of the green (65–125 μs), see Table 1. The fitted kinetics for each of the UCNPs is shown in the ESI (Fig. S10–S20†).

In this study, we used the same batch of UCNP@OA for the preparation of the other four samples and no change in the crystalline phase, size, or shape of the UCNP core was detected upon hybrid formation.

Therefore, the observed differences in the decay lifetimes shall be attributed to non-radiative processes due to the interaction of the UCNPs with the solvent, the capping ligand, and, in the case of the nanohybrids, the diiodoBodipy acceptor molecules.

As expected, taking into account that O–H vibrations deactivate the lanthanide excited states due to multiphonon non-radiative interactions,²⁷ the decay of the green and red emissions was longer for the UCNPs dispersed in toluene than in water (Table 1). This impact of the solvent was less pronounced for the red emission (ca. 300 vs. 280 μs) as compared to green (ca. 80 vs. 120 μs), which suggests that the population of higher energy levels and non-radiative deactivation of these levels are more susceptible to the presence of O–H vibrations.² Larger discrepancies can be observed for the luminescence rise-times, where the toluene dispersed UCNP@OA exhibited almost twice as long values (94 and 122 μs for green and red, respectively) as compared to water soluble UCNP@PEG (44 and 69 μs for green and red, respectively).

This observation demonstrates the fast saturation of intermediate excited state levels of Er³⁺ ions in water dispersible UCNP@PEG NPs and most probably supports the hypothesis of the critical role of the Yb³⁺ energy migration network in releasing the absorbed energy through the surface Yb³⁺ ions.

In addition, the presence of IBDP in UCNP–IBDP@PEG led to shortening of τ_decay (from 82 μs to 67 μs, η = 1 − τ_DA/τ_D = 18%). Similar behaviour was observed for the green emission of the lipophilic nanohybrid (τ_decay of 119 μs and 96 μs, η = 19%, for UCNP@OA and UCNP–IBDP@OA, respectively, Table 1).

The presence of IBDP in both UCNP–IBDP@PEG and UCNP–IBDP@OA demonstrated faster rise-times of the red emission band kinetics, in comparison with non-IBDP NPs. This behavior indicates that the IBDP plays a role in the up-conversion and changes the balance between ETU and depopulation phenomena during the intermediate steps despite evident energy mismatch between the excited states of Er³⁺ donors and energy levels of IBDP acceptors.

Moreover the red emission decay in UCNP–IBDP@PEG was ca. 10% shorter than that of UCNP@PEG, while the red decay in UCNP–IBDP@OA was slightly longer than that in UCNP@OA. The interpretation of such relations is not straightforward and requires further investigations.

Regarding the rise-times (τ_rise) of the green and red emission donor hybrids, again the hydrophilic UCNP@PEG NPs exhibited shorter values than UCNP@OA. Remarkably, the τ_rise of the UCNP–IBDP@OA green emission decreased almost two-fold as compared to that of UCNP@OA, and similar shortening was also detected for the τ_rise of the red emission. Such consistent behaviour was not observed for PEG coated NPs.

A likely interpretation for the lipophilic UCNP–IBDP@OA could be that the presence of IBDP makes a competitive decay possible for the metastable state of Yb³⁺ (Yb³⁺*). Specifically, Yb³⁺* could be involved not only in the population of Er³⁺ excited states but also in that of IBDP (IBDP*) after the absorption of a second photon (Fig. 4). As a consequence, the rise-times of both red and green emissions would reflect the decrease of the contribution of Yb³⁺* to populate the Er³⁺ excited states involved in those emissions.

Fig. 4 Schematic picture showing the competitive energy transfer from Yb³⁺ excited state (Yb³⁺*) to Er³⁺ and IBDP.

To corroborate this hypothesis we prepared the lipophilic Yb³⁺(IBDP)₃ complex (see ESI† for experimental details) which was excited at 975 nm. Remarkably, the anti-Stokes IBDP fluorescence (Fig. 5 left, in blue) was detected in spite of the considerably low absorption cross section of the Yb³⁺ ions (as compared to Stokes excitation of IBDP) at the NIR and the low fluorescence of the diiodo-Bodipy.

Fig. 5 Left: comparison between the emission spectra of IBDP (black) and of Yb³⁺(IBDP)₃ complex in toluene under NIR 975 nm excitation. The inset demonstrates the power dependence – the anti-Stokes emission intensity, which is proportional to the 2^nd power of the excitation intensity. Right: kinetic time profile of the anti-Stokes IBDP emission in Yb³⁺(IBDP)₃ under 975 nm laser excitation.

This suggested that the Yb³⁺(IBDP)₃ complex can up-convert following the proposed scheme

Yb³+–IBDP + hν₁ → Yb³⁺*–IBDP

Yb³⁺*–IBDP + hν₁ → Yb³⁺–IBDP*

Yb³⁺–IBDP* → Yb³⁺–IBDP + hν₂(hν₂ > hν₁)

It is known that Bodipy compounds are capable of undergoing two-photon absorption processes.²⁸ Therefore, control experiments were carried out to confirm the involvement of the lanthanide in the emission of IBDP in the Yb³⁺(IBDP)₃ when excited at 975 nm. Fig. 5 left shows the much more efficient emission of the Bodipy in the complex. Moreover, time-resolved measurements evidenced that the emission lifetime of Yb³⁺–IBDP* was ca. 5 μs (Fig. 5 right), a great deal longer than that of IBDP*, whose emission lifetime was not higher than a few ns. Finally, power-dependence measurements corroborated that the IBDP emission after NIR excitation of Yb³⁺(IBDP)₃ was proportional to the 2^nd power of the 975 nm excitation intensity (inset of Fig. 5 left). The latter observation evidenced the need to involve two low energy 975 nm photons, to obtain a visible emission of IBDP.

Conclusions

We have prepared hydrophobic and hydrophilic UCNP–IBDP nanohybrids, which showed a decrease in their green to red emission compared to their precursors. Time-resolved experiments demonstrated that the quenching of the green emission was due to the energy transfer from the UCNP to the anchored IBDP. Furthermore, the decrease in the efficiency of green and red emissions in UCNP–IBDP was attributed to the competitive decay of Yb³⁺* via energy transfer to IBDP after absorption of a second photon. This process was more competitive in an organic solvent. Therefore, a comparison between the emission rise-times, as well as between the emission decay lifetimes, of the nanohybrids and those of UCNP_naked, UCNP@PEG, and UCNP@OA gave valuable information about the excited states involved in UCNP emission kinetics.

Acknowledgements

We thank the Spanish Ministry of Economy and Competitiveness (Projects CTQ2014-60174; M. G. B.: Ramón y Cajal contract and L. F. S.: F.P.U. grant). The work was partially supported by the grant DEC-2012/05/E/ST5/03901 from Narodowe Centrum Nauki, Poland.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM pictures and their corresponding histograms, UV-visible, power dependence plot and decay fittings. See DOI: 10.1039/c5nr07229h

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