Concentration quenching in cerium oxide dispersions via a Förster resonance energy transfer mechanism facilitates the identification of fatty acids

Abstract

The energy exchange phenomena of cerium oxide based nanoparticles in a medium have been studied by means of a meticulous approach. A concentration dependent non-radiative pathway has been revealed for the particles due to the close proximity between them which causes the extinction of fluorescence. The calibration plot, according to the Stern–Volmer equation, showed a good linear relationship within the acceptable error limit, and the value of Q, denoting the exchange interaction, was close to 6, implying dipolar coupling between particles. Theoretical analysis of spectroscopic data showed that Förster resonance energy transfer (FRET) is the dominant mechanism responsible for the interparticle excitation transfer and the Förster radius (R₀) calculated was 68.6 A°. The distance dependence of FRET has been utilized to analyse the conformation and chain length of fatty acids by interrupting the energy transfer efficiency among the particles, and thus a simple analytical tool based on FRET for the qualitative as well as quantitative assessment of fatty acids has been projected.

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Introduction

The physical dimensions of semiconductor nanocrystals often serve as a resource of innovative photochemical properties due to quantum confinement when their dimension is less than their corresponding Bohr exciton radius, as well as Debye length, which usually falls in the nanometer range. These ‘artificial atoms’ are called quantum dots, and their zero dimension restricts the number of electrons, which causes quantization of energy levels in the density of states (DOS).¹ The most fascinating outcome of this phenomenon is the widening of the band gap and its resultant blue shift. The influence of nano dimensions on the band gap leads to the tunability of the optical properties of nanoparticles and therefore a wide range of applications is possible due to the size dependent properties.²

Cerium dioxide is a well known semiconductor and is extremely useful for its luminescence, non-toxicity, high refractive index, chemical and thermal stability etc.^3–7 On account of its properties, cerium oxide have been employed as an oxygen reservoir, catalyst, gas sensor, abrasive etc.⁸ In fact, when the Bohr radius and Debye length of ceria are taken into account, which are ∼7 nm and 3 nm, respectively, ceria based nano-structures with appropriate dimensions are proficient in exhibiting innovative optical properties.^9,10 A few research groups have reported a blue colored emission from ceria nanoparticles when their aspect ratio shrinks below 3 nm.⁶ With reference to the studies by Tanigucchi et al., a luminescence quantum yield of 59% was achieved by ceria with a sheet like morphology.¹¹ In the light of the scarcity of outstanding luminescent materials, the control and improvement of the properties of existing nano-phosphors has always been a major aspiration in the world of research.¹² In the aspect of designing efficient phosphor materials, phenomena like luminescence enhancement, quenching, delay in emission, decay rate etc. have vital significance. Among these, quenching of luminescence is a prevailing phenomenon which is highly undesirable due to the resulting reduction in overall quantum yield, which in turn affects the luminescence efficiency. But even adverse phenomena need to be probed for many functional applications like molecular sensing, imaging, drug release profiling, DNA detection etc. if the underlying quenching mechanism is to be thoroughly understood.^13,14

Different pathways have been proposed so far for clearing up the mystery behind fluorescence quenching, from simple collisional energy exchange to nanoparticle surface energy exchange (NSET).^15,16 In the present study, various spectroscopic tools have been employed to investigate the existence of a non-radiative energy transfer mechanism between ceria nanoparticles, based on the principle of Förster Resonance Energy transfer (FRET). FRET is a distant energy transfer process making use of dipolar pairing between donor and acceptor molecules.¹⁷ Due to its dependence on distance, FRET has now emerged as a convenient technology at the single molecular detection limit, and is found to be suitable for studying the distance between two molecules or two neighboring sites on a specific macromolecule, during protein conformational change, protein interaction or enzyme activity.^18–24 Since FRET physically originates from the weak electromagnetic coupling of two dipoles, introducing additional dipole like metal nanoparticles provides more coupling interactions and thus FRET efficiency can be tuned.²⁵ There have been many recent efforts for the development of fluorescence assays based on this principle for applications like DNA detection. Mirkin et al. developed a method for the analysis of DNA which is based on gold nanoparticles.²⁶

Also, there are studies involving quantum dots as FRET pairs on account of their high photo stability, great emission intensity and photo-bleaching resistance. Their broad absorption and narrow emission spectra allow single-wavelength excitation of multiple donors and can avoid crosstalk with acceptor fluorophores. They can also be coupled to multiple acceptor fluorophores for higher efficiency in energy transfer, and can act as the support structure for biomolecules for imaging purposes or to simplify assay design.¹⁴ Leong et al. have developed a single-step quantum dot-mediated FRET system to investigate the structural composition and in vitro dynamic behaviour of a plasmid DNA hybrid nanostructure.²⁷ Song et al. designed a positively-charged, compact QD-DNA complex for the detection of nucleic acids.²⁸

Herein we have incorporated the principle of FRET with quantum dots based on ceria which serve as a facile analytic model for the identification of fatty acids. Monodisperse ceria nanoparticles of average size ∼2 nm were synthesised by adopting a thermal decomposition strategy. The size induced optical properties, e.g. fluorescence, exhibited by the ceria crystals while approaching nano size have already been reported.²⁹ In the present study, an attempt has been made to evaluate the mechanism behind the concentration dependent quenching of the fluorescence. Despite the significant research activity in the field of nano cerium dioxide in recent years, this manuscript is the first to report a FRET based energy transfer mechanism in the cerium dioxide nano-system. Also, efforts have been made to use the quenching mechanism as an effective tool for the identification of fatty acids.

Experimental section

Materials and synthesis

All the chemicals were used as received without further purification. Cerium acetate (99.9%), stearic acid (90%) and lauric acid (90%) were purchased from Merck, India. Diphenyl ether (99%), decanoic acid (90%) and oleyl amine (70%) were procured from Sigma Aldrich and oleic acid (90%) from Alfa Aesar, UK. Commercial olive oil (extra virgin olive oil) has been procured from Jindal Retail (India) Pvt. Ltd. Common solvents such as acetone, ethyl alcohol, cyclohexane and toluene (analytical grade) were procured from Merck, India.

The precursor employed in the present synthetic strategy was cerium acetate which is decomposed upon the supply of heat in diphenyl ether solvent to form ceria. In a typical synthesis, 0.005 moles of cerium acetate was dissolved in 100 ml diphenyl ether in a round bottom flask. Oleic acid was utilized to functionalize the surface of the synthesized nanoparticles. About 0.02 mole oleic acid and 0.023 mole oleyl amine were added to the reaction mixture which was refluxed at its natural boiling point (∼260 °C) for 1 h. Oleic acid, in the presence of oleyl amine, is expected to undergo ionization to form the corresponding oleate ion which is capable of coordinating with the positive core of the nanoparticles formed. As the reaction proceeded, the solution turned brown, indicating the formation of ceria nanocrystals. After the reaction time, the mixture was allowed to cool to room temperature. Subsequently, acetone was added to the reaction mixture to precipitate the oleic acid coated nanoparticles. The precipitate obtained was separated by centrifugation and washed thoroughly with acetone several times to get rid of excess oleic acid. Finally, after washing, the precipitate was dried in an air oven to a slightly brownish powder. The particles could easily be dispersed in nonpolar solvents, e.g. hexane, toluene etc., indicating the successful surface modification by oleic acid.

A parent dispersion of ceria nanoparticles in toluene was prepared by suspending 0.01 g dried nanoparticles in 25 ml toluene (0.002 M) and sonicating for about 10 min. Ceria dispersions of different concentration were prepared from this parent suspension upon dilution. Dispersions having concentrations in the range 0.0001–0.0018 M were prepared by dilution followed by sonication for 10 min. For the estimation of oleic acid in a real sample, 2 ml of commercial olive oil was added to a 0.00018 M ceria dispersion and the photoluminescence spectrum was collected.

Instrumental techniques

The X-ray diffraction (XRD) patterns of the dried and powdered specimens were obtained using a Philips X’PERT PRO diffractometer with Ni-filtered Cu Kα₁ radiation (λ = 1.5406 Å) using a 30 mA current at 40 kV. Powder samples were scanned in the continuous scan angle range 5–100 degree (2θ) at a scanning speed of 2 degree per min with a step size of 0.04°. The morphology and average size of the nanocrystals were investigated by high resolution transmission electron microscopy (HR-TEM) using a FEI Tecnai 30 G2 S-Twin microscope operated at 300 kV and equipped with a Gatan CCD camera. A small amount of the nanoparticles was dispersed in toluene and ultrasonicated to get a stable suspension. Samples for TEM study were prepared by dropping a microdroplet of the suspension onto a 400 mesh copper grid and drying the excess solvent naturally. Size measurements for the cerium oxide in suspension were carried out at 25 °C by photon correlation spectroscopy (PCS) on a Zetasizer 3000 HSA, Malvern Instruments, Worcestershire, UK, using a 60 mW He–Ne laser producing a 633 nm wavelength with a General Purpose algorithm and Dispersion Technology Software (v. 1.61) at a 90° detection angle. Fourier transform infrared (FTIR) spectra of the as-prepared products were recorded at room temperature using a KBr (Sigma Aldrich, 99%) pellet method on a Nicolet Magna IR-560 spectrometer in the 400 to 4000 cm⁻¹ range. The absorption spectra of the samples were obtained using a UV-visible 2401 PC spectrophotometer (Shimadzu, Japan) in the wavelength range 200–800 nm. The PL spectrum of the surfactant coated nanoparticle suspension in toluene was taken at room temperature using a Cary Eclipse spectrofluorometer (Varian, Australia). Fluorescence lifetimes were measured using a IBH (FluoroCube) time-correlated picosecond single photon counting (TCSPC) system. The nanoparticle dispersions were excited with a pulsed diode laser (<100 ps pulse duration) at a wavelength of 400 nm (NanoLED-11) with a repetition rate of 1 MHz. The detection system consisted of a microchannel plate photomultiplier (5000U-09B, Hamamatsu) with a 38.6 ps response time coupled to a monochromator (5000 M) and TCSPC electronics (data station hub including Hub-NL, NanoLED controller and preinstalled Fluorescence Measurement and Analysis Studio (FMAS) software).

Results and discussion

Preliminary characterization

The synthetic approach employed for the present study is based on the one-step thermal decomposition of cerium acetate in diphenyl ether reported earlier.²⁹ The mechanism behind the nucleation of nanoparticles by this approach, which involves the formation of free radical intermediates, is also a well established phenomenon.³⁰ According to the published literature, oleic acid in the presence of oleyl amine undergoes dissociation to form oleate ions and its negatively charged carboxylate head chemisorbs onto the positively charged nanoparticle core. In the FT-IR spectrum (please see Fig. S5 in the ESI†), two peaks at 1433 and 1515 cm⁻¹, due to the stretching vibration of the carboxylate group, confirm the presence of the oleate group chemisorbed on the nanoparticle surface.³¹ Therefore, the nanoparticles formed will comprise a monolayer of surfactant on the particle surface. The X-ray diffractograms of the particles synthesized in this report and their TEM images are shown in Fig. 1 (please see Fig. S1 in the ESI† also). The XRD patterns fitted well with the cubic fluorite phase of ceria (according to JCPDS card no. 34-0394) and the nanocrystalline nature of the particles was evidenced by the broadening of the peaks.^32,33 An anomalous small peak observed at a 2θ value of approximately 20° is attributed to the presence of oleic acid surfactant.³⁴ The TEM images confirm the formation of ultra fine and fairly uniform sized ceria nanocrystals with spherical morphology. The high resolution images indicate that the nanoparticles are single crystals and a selected area diffraction (SAED) pattern shows (111), (200), (220), and (311) Debye–Scherrer rings with corresponding interplanar spacings of 0.31, 0.27, 0.19, and 0.16 nm (Fig. S2†).

Fig. 1 (a) XRD patterns and (b) HR-TEM image of the as-synthesized ceria nanoparticles.

The average particle size derived from multiple TEM images was 2 ± 0.17 nm. After surface modification, the alkyl tail of the fatty acid protrudes out of the nanoparticle surface. The steric repulsion imparted by the fatty acid chain prevents the nanoparticles coming closer and the particles will remain well dispersed as evidenced by TEM. The particles were dispersible in non-polar solvents owing to the hydrophobic surface endowed by the oleic acid surfactant. The photon correlation spectroscopy data for the nanoparticles in toluene dispersion (Fig. S3 of the ESI†) gives the hydrodynamic size as 3.9 nm, which matches well with the TEM observation when the chain length of the surfactant is considered.^35,36

Optical properties and fluorescence quenching

The absorbance and emission spectra, photoluminescence measurements at different concentrations and the dependence on the concentration of the nanoparticle dispersion in toluene are shown in Fig. 2. One of the notable features of the absorbance spectrum is its broadness over almost the whole visible region. The emission spectrum (λ_ex = 400 nm) was also fairly broad, with an emission maximum at about 510 nm. The broadness in the optical spectra has been attributed to the creation of defects in the system during the process of the nanoscaling of the crystals.²⁹

Fig. 2 (a) Absorption and emission spectra of the nanoparticles; the shaded portion showing the spectral overlap and the inset showing green coloured emission, (b) emission spectra of the dispersion at different concentrations (0.0001–0.0013 M) and (c) variation of emission intensity with concentration. The inset shows the linearly fitted portion of the quenching region alone.

The emission in the green region showed an initial rise in intensity with concentration, and after reaching a maximum, emission intensity seemed to decrease linearly. The inset of Fig. 2c shows the linearly fitted portion of the graph which shows quenching. Fluorescence quenching is generally described by the Stern–Volmer equation (eqn (1))

where I₀ and I are the fluorescence intensity of the fluorophore in the absence and presence of quencher, respectively, [Q] indicates the concentration of quencher and K_SV is the Stern–Volmer quenching constant.³⁷ As there is no external quencher in the present system, the Stern–Volmer equation has been modified and plotted as I₀/I against [Q] as shown in Fig. 3a. Here I₀ is the emission intensity of the parent dispersion whose concentration does not fall in the quenching range (0.000639 M) and [Q] is the concentration of the dispersion added to a fixed volume of parent suspension. The variation in intensity against solid concentration is almost linear as indicated by near perfect linear regression with a R² value of 0.9902, which indicates effective quenching.

Fig. 3 (a) Stern–Volmer plot for the concentration quenching of the dispersions and (b) fluorescence lifetime measured for the dispersion at concentrations (i) 0.000868 M and (ii) 0.0013 M.

The lifetime of the excited state measured for dispersions at two different concentrations (0.000868 and 0.0013 M) yielded different values of 10 and 3 ns, respectively, as depicted by Fig. 3b. The emission intensity can be related to lifetime according to the equation I = Ae^−(E/kt), where A is a constant, k is the decay constant and t is the lifetime.³⁸ This is accounted for by the closeness of the nanoparticles with increasing concentration leading to a faster energy relaxation mechanism without radiative emission. The particles could transfer the excess energy within 3 ns by a non-radiative pathway which resulted in the quenching of fluorescence.

Quenching mechanism based on FRET

Many mechanisms like complex formation, collisional processes and other energy transfer methods have been proposed so far for the quenching effect.^15,39 As in the present system, the nanoparticles are sterically well spaced by bulky oleic acid groups, the former two mechanisms can be ruled out. Moreover, the fast fluorescence decay for concentrated dispersions within the first 3 ns is a clear signature of non-radiative energy transfer.⁴⁰ The absorbance and emission spectra of the nanoparticles show considerable spectral overlap indicated by the shaded region in Fig. 2a, which indicates a FRET mechanism for the quenching effect. FRET is a distance dependent energy transfer process in which energy is transferred from an excited donor to an acceptor molecule through a dipole–dipole interaction. An efficient FRET demands certain conditions to be satisfied like (i) the spectral overlap of donor emission and acceptor absorbance spectra, (ii) a desirable distance between the donor and acceptor and (iii) the possibility of dipole–dipole interaction.^{13,14,17,41,42}

The significance of spectral overlap lies in the fact that a ground state acceptor can be energetically transferred to a higher level at the expense of an excited donor. Fig. 2 shows that there is considerable spectral overlap between the absorption and emission of nanoparticles from about 450 to 750 nm (panel a). The absorption shoulder (λ_max) is at 400 nm and the maximum emission peak is at ∼510 nm. Both the spectra are fairly broad, extending over almost the whole visible range. It is noteworthy that the effective overlap of the spectra has been enabled by this extensive spread over the whole wavelength under consideration. As the spectral broadening originates from the defects associated with the miniaturization of the crystals, the root of the quenching effect lies with the nano dimensions of the system.^29,43 So, in the present system, the optical spectra of the nanoparticles are in favour of energy transfer between themselves through a FRET mechanism.

As pointed out earlier, the energy transfer by FRET is accomplished through a dipole–dipole interaction between the transition dipole moments of the acceptor and donor.¹⁷ Therefore the detailed investigation of the interaction between the nanoparticles during emission cannot be dismissed. The alignment of dipole moments is a prevalent phenomenon in cerium dioxide owing to the electronegativity of oxygen. Therefore, the interaction of crystals on account of the dipole moment is highly probable in ceria. In fact, there are many reports which support the existence of dipole–dipole interactions in ceria during the course of investigation of crystal growth and morphology evolution.^44,45

According to Dexter’s theory of energy transfer, the source of the interaction which results in concentration quenching can be estimated using eqn (2)

in which I/X is the emission intensity per quencher concentration, Q is the exchange interaction and K and β are constants for a given lattice.⁴⁶ The value of Q is the significant factor which reveals the type of interaction. Q = 6, 8 or 10 for electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q), or electric quadrupole–quadrupole (Q–Q) interactions, respectively.^12,38,46 The slope of the linear fit of the log(I/X) versus log X data gives the value of Q. The Q value calculated for the present system from Fig. 4 is 6.36 (slope = −2.12, Q = −3 × slope) which is closer to 6, indicating a dipole–dipole interaction.

Fig. 4 Plot of log(I/X) against log X fitted linearly to show a dipole–dipole interaction among the particles.

The prerequisite of close proximity in FRET limits the distance between the donor and acceptor to the order of 100 Å owing to the dipole–dipole interaction. For distance estimation, we considered the Förster radius (R₀) which is the critical distance between the donor and acceptor at which the energy transfer rate is 50%.⁴² R₀ can be calculated according to eqn (3)

where [A]_1/2 is the half-quenching concentration, the concentration at which emission intensity is reduced to half.⁴⁷ According to Förster theory, the energy transfer rate at different distances can be calculated asusing which the energy transfer rates of the dispersions at the concentrations under study can be estimated.⁴⁸ But there is also another equation relating R₀ and E which is given aswhere r refers to the distance between the particles corresponding to the energy transfer rate.⁴⁸ Therefore, it is possible to estimate the distance between particles at concentrations which fall in the quenching region by knowing the corresponding value of emission intensity.

The [A]_1/2 calculated from the linear regression (inset of Fig. 2c) is 0.001229 M and the value obtained for R₀ using eqn (3) is 68.6 Å. The r value calculated at different concentrations using eqn (3)–(5) was plotted against its corresponding concentration and is shown graphically in Fig. 5. It was observed that the distance calculated among the particles for concentrations in the quenching range is of the order of 100 Å which supports the proposed mechanism. The inset of Fig. 5 shows the enhancement in energy transfer efficiency when the separation among the particles is decreasing. As the particles approach closer, the dipoles are freed to interact more effectively which amplifies the energy transfer rate. The quenching phenomenon was not observed for dilute dispersions, likely due to the absence of a proximal acceptor particle within the desired distance range of FRET. Scheme 1 represents a summary of the mechanism.

Fig. 5 Variation of distance among particles with concentration of dispersion. The inset shows the enhancement in energy transfer efficiency with a decrease in interparticle distance.

Scheme 1 Mechanism proposed based on FRET for the concentration quenching of ceria dispersions.

Effect of spacers on quenching

As FRET is a quenching mechanism highly sensitive to distance between the fluorophores, the presence of any moiety in the dispersion capable of changing the distance among the particles can cause a divergence in quenching rate. The impact can be visualised in the emission spectra of the corresponding dispersion and it can act as the source of structural identification of the moiety. In the present study, such a moiety which may come in between the particles, other than solvent, is denoted as a ‘spacer’. Fatty acids differing in chain length as well as structures such as decanoic acid (C₁₀H₂₀O₂), lauric acid (C₁₂H₂₄O₂), stearic acid (C₁₈H₃₆O₂) and oleic acid (C₁₈H₃₄O₂) have been used as spacers. To a fixed volume of concentrated dispersion (0.0018 M), a fixed volume (3.5 ml) of individual spacer was added and photoluminescence spectra were collected.

All the fatty acids caused an enhanced emission with respect to the parent dispersion as shown in Fig. 6. The change in PL intensity of the CeO₂ dispersions as a function of different fatty acids and their volumes added are shown in the inset. Whereas oleic acid gave the maximum intensity spectrum, the emission intensity showed an inverse relation with chain length for the rest. Though the contribution of the spacer towards the overall dipole moment can enhance the energy transfer efficiency to a great extent, it is improbable for the present system. This is because as electronic effects are not felt beyond three or four carbons, the difference in chain length alone has a low expectation of causing any marked influence to the dipole moment.⁴⁹ Also, the presence of a double bond in the alkyl chain of the oleic acid favours non-polar character in the chain which may in turn reduce the dipolar nature, in contrast to the observed result.⁵⁰ Therefore, an improvement in emission bestowed by the spacers is likely due to the separation induced by them in between the particles.

Fig. 6 Emission spectra of the dispersion after the addition of different spacers: (a) oleic acid, (b) decanoic acid, (c) lauric acid, (d) stearic acid and (e) parent CeO₂ NP dispersion without addition of spacer. The inset shows the variation in emission intensity with increase in the concentration of spacers.

The manner in which surfactant like molecules align in a medium strongly depends on certain parameters like chain length compatibility, chain cohesion, molecular interactions etc. which contribute to the intermolecular distance.⁴⁹ As chain length increases, the van der Waals interaction among the chains of adjacent molecules increases and vice versa.⁴⁹

In a medium, the extent of solvation depends on this attractive force, which in turn, affects the intermolecular separation.⁵¹ For shorter fatty acids, as the attractive interaction is weak, molecules are farther from each other in the medium. For oleic acid, this intermolecular attraction is again weak due to the kink induced by the cis-bond at the C9 position.⁵¹ Due to the inefficiency of remaining as closely packed as its straight chain colleagues, a solvent can significantly separate these molecules. So the distance created by the spacers between the nanoparticles will be highly influenced by this intermolecular attraction which decides the order of enhancement provided by them in the emission, as depicted in Scheme 2. Thus each spacer will leave its own signature in the emission spectra depending upon its structure and chain length. The linear increase in the emission intensity with the increase in the concentration of the spacers (inset for Fig. 6) also substantiates the proposed rationale based on the structure of the spacer.

Scheme 2 Enhancement in emission intensity provided by the spacers by altering the distance between particles.

The linear intensity versus concentration profiles shown as the inset of Fig. 6 allow the quantitative estimation of fatty acids as well. An attempt has been made to estimate the amount of oleic acid present in a commercially available olive oil. To a fixed volume of a concentrated dispersion of CeO₂ (0.0018 M), 2 ml of commercial olive oil was added and photoluminescence spectra were collected. It was observed that the photoluminescence spectra of pure and commercial oleic acids showed little difference in PL intensity (Fig. S6†) which gave a net oleic acid content in the commercial olive oil of ∼85% (1.7 ml in 2 ml) which is close to the certified content of 86.5%. This data demonstrates the practical applicability of the proposed technique for the quantification of fatty acids.

Conclusions

A spectroscopic investigation of the concentration quenching behaviour of ceria nanoparticles in dispersion has been carried out. The particles exhibited linear Stern–Volmer characteristics and the inter-particle dipole interaction has been established by graphical interrogation. The distance calculated between two particles according to Förster theory falls within the FRET limits which, along with other theoretical explorations, indicated a mechanism based on Förster resonance for the transfer of electronic excitation. The hypersensitivity of FRET to distance ultimately served as a yardstick to distinguish different fatty acids based on their structure and conformation. The present study supplies a model for the structural elucidation of molecules by means of a simple analytical approach.

Acknowledgements

The authors are thankful to the Director, CSIR-NIIST for providing the necessary facilities for the work and the CSIR-Central Glass & Ceramic Research Institute for continuing the same. Authors thank the Department of Science & Technology and CSIR, India for providing the HR-TEM facility to NIIST. Authors also acknowledge Mr M. Kiran for the HR-TEM imaging and analysis. Authors gratefully acknowledge Mr Nandajan and Mr Akhil of NIIST for lifetime measurements. Authors AK and TSS acknowledge CSIR for the CSIR Fellowships. This work was partly funded by the Indian Rare Earths Limited Technology Development Council (IRELTDC), DAE, India.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17326k

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