Influence of Tail Groups during Functionalization of ZnO Nanoparticles

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The influence of tail groups during functionalization of ZnO nanoparticles on binding enthalpies and photoluminescence Wei Lin, Jochen Schmidt, Michael Mahler, Torben Schindler, Tobias Unruh, Bernd Meyer, Wolfgang Peukert, and Doris Segets Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03079 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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The influence of tail groups during functionalization of ZnO nanoparticles on binding enthalpies and photoluminescence ‖

Wei Lin†‡, Jochen Schmidt†, Michael Mahler†, Torben Schindler§, Tobias Unruh§, Bernd Meyer , Wolfgang Peukert†‡, Doris Segets†‡* † Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstraße 4, 91058 Erlangen, Germany ‡Interdisciplinary Center for Functional Particle Systems (FPS), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Haberstraße 9a, 91058 Erlangen, Germany §Chair of Crystallography and Structural Physics, Friedrich-Alexander-Universität ErlangenNürnberg (FAU), Staudtstraße 3, 91058 Erlangen, Germany

ǁ Interdisciplinary Center for Molecular Materials (ICMM) and Computer-Chemistry-Center (CCC), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstraße 25, 91052 Erlangen, Germany

ABSTRACT: We report on the tailoring of ZnO nanoparticle (NP) surfaces by catechol derivatives (CAT) with different functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The influence of electron-donating/-withdrawing properties on enthalpy of ligand binding (∆H) was resolved, and subsequently linked with optical properties. First, as confirmed by

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ultraviolet/visible (UV/Vis) and Fourier transform infrared (FT-IR) spectroscopy results, all CAT molecules chemisorbed to ZnO NPs, independent of the distinct functionality. Interestingly, the ζ-potentials of ZnO after functionalization shifted to more negative values. Then, isothermal titration calorimetry (ITC) and a mass-based method were applied to resolve the heat release during ligand binding and the adsorption isotherm, respectively. However, both heat- and massbased approaches alone did not fully resolve the binding enthalpy of each molecule adsorbing to the ZnO surface. This is mainly due to the fact that the Langmuir model oversimplifies the underlying adsorption mechanism, at least for some of the tested CAT molecules. Therefore, a new, fitting free approach was developed to directly access the adsorption enthalpy per molecule during functionalization by dividing the heat release measured via ITC by the amount of bound molecules determined from the adsorption isotherm. Finally, the efficiency of quenching the visible emission caused by ligand binding was investigated by photoluminescence (PL) spectroscopy which turned out to follow the same trend as the binding enthalpy. Thus, the functionality of ligand molecules governs the binding enthalpy to the particle surface which in turn, at least in the current case of ZnO, is an important parameter for the quenching of visible emission. We believe that establishing such correlations is an important step towards a more general way of selecting and designing ligand molecules for surface functionalization. This allows developing strategies for tailored colloidal surfaces beyond empirically driven formulation on a case by case basis.


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Introduction At the nanoscale, large surface-to-volume ratio is the dominant player in many physical and chemical properties of materials. Thus, controlled interfaces are highly relevant for various applications ranging from particle-polymer composites, pigments, bio-imaging, light emitting diodes (LEDs) to energy harvesting and storage.1–4 Therefore, surface modification of nanoparticles (NPs) is of great importance when aiming for high performance particle-based applications. However, the interplay between surface functionality, binding behavior, and optical properties is complex and challenging to access. It is well-known in organic chemistry and polymer synthesis that a rational design of building block groups with different electron-donating/-withdrawing properties can lead to specific application properties.5–10 For colloidal NPs much attention has been paid to the ligand anchor groups which can be divided into L, X, Z type of ligands depending on the electronic structure of particle-ligand binding motifs.11–36 This concept of classification of covalent bonds for metal coordination complexes was adapted to NPs by Owen et al.37 In brief, the L-type ligands have a lone electron pair that coordinates to the particle surface while the X-type ligands have only one electron and thus need one electron from the particle surface to form the binding. The Z-type ligands bind through the metal atom on the particle surface as electron acceptor. There are some case studies trying to resolve the influence of the tail substituent on physical and chemical properties of particles as well as ligand binding behaviors. Different catecholate-type ligands were found to alter the optical band gap energy of TiO238 and coordination of phenyldithiocarbamate ligands were proven to decrease the optical band gap of CdSe quantum dots.39 It was also found that alkyl thiols lead to reduced quantum yield for the ZnO NPs.40 For Au NPs, the influence of different alkyl chain length on the binding strength as well as solubility


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was systematically studied.41,42 For ITO NPs, it was found that an increase of the chain length of alkyl amine leads to a decrease of the interaction between ITO NPs and the ligands.43 Thus, the two common ways to investigate particle-ligand interactions are: i) varying the anchor group of the ligands, ii) varying the alkyl chain length of the ligands. Meanwhile, the studies by using substituents with different electron-donating/-withdrawing properties focus only on the optical properties. To the best of our knowledge, the interplay between the substituents with different electron-donating/-withdrawing properties, quantitative thermodynamic findings on binding enthalpy, and photoluminescence (PL) quenching is not yet reported. Such approach, however, provides a rational way to select and design ligands for tailored colloidal surfaces. In this study, we functionalized ZnO NPs with catechol (CAT) derivatives with the same anchor group but different tail groups on the benzene ring, namely 4-tert-butylcatechol (tertCAT), pyrocatechol (pyroCAT), 2,3-dihydroxynaphthalene (naphCAT), ethyl 3,4-dihydroxybenzoate (esterCAT), and 4-nitrocatechol (nitroCAT). The molecular structures can be found in Figure 1A. In our previous work, a widely-applicable approach to study the thermodynamics and kinetics of ligand binding to colloidal ZnO NPs was demonstrated for esterCAT.44,45 Herein, we further develop this concept to resolve the binding of CAT molecules with different electron-donating/withdrawing properties induced by the variations in their chemical structures. First, ultraviolet/visible (UV/Vis) and Fourier transform infrared (FT-IR) spectroscopy were applied to characterize the tailored ZnO NPs and to confirm that functionalization was successful. Interestingly, chemisorption of different CAT species was always accompanied by a strong shift of the ζ-potentials of ZnO to more negative values.


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In a subsequent step, the heat release was recorded by using isothermal titration calorimetry (ITC) which is an important technique to characterize the thermodynamics of ligand adsorption (whereby functionalities are ranging from small molecules to biomolecules) to colloidal particles.46–50 However, for three CAT molecules of the current study (nitroCAT, pyroCAT, and tertCAT), no plateau of heat release was found. Similarly, mass-based method was also not able to fully resolve the association constant based on Langmuir fitting for all ligand molecules under investigation as the particle size distribution (PSD) of ZnO changes upon adding small amounts of nitroCAT. The reason is that the Langmuir model is too rough to describe the underlying physics of liquid-borne NPs with heterogeneous surfaces. There are also some cases reported in literature where the Langmuir model is not able to fit the experimental data, i.e. protein adsorption,51 or adsorption of binary solutes of different sizes.52 To overcome this limitation, we propose an extended, new approach of data analysis which combines both, heat- and mass-based methods to directly determine the binding enthalpy per bound molecules without any fitting model. This is realized by dividing the heat release measured via ITC by the amount of bound molecules obtained from the mass-based method. Noteworthy, this allows the unambiguous ordering of a wide range of functionalities in terms of binding enthalpy. As main outcome of these studies, it was found that due to enhanced dissociation, the electron-donating/-withdrawing properties of the tail group strongly affect the binding enthalpy. Finally, the influence of different CAT molecules on the visible emission of ZnO was analyzed by PL spectroscopy. The results showed i) that particle-particle interactions at high ZnO concentration can lead to PL quenching and thus lower the normalized PL intensity and ii) that CAT molecules quench the visible emission of ZnO NPs effectively in the same order as the binding enthalpy.


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Thus, our study sheds light on the correlations between electron-donating/-withdrawing properties, ligand binding enthalpy analyzed by quantitative thermodynamic data, and PL quenching. Noteworthy, the new approach of combining heat- and mass-based methods does not require any predefined fitting models. It can now be applied to systematically categorize molecules in terms of different aspects such as electron-donating/-withdrawing properties, redox potential, and hydrophobicity. We believe that such correlations will become an important contribution en route towards tailored colloidal interfaces as well as knowledge-based design of functional ligands for NP-based applications.

Experimental Part Materials. All chemicals were used as received: zinc acetate dihydrate (ACS Grade, 98 %, VWR, Germany), lithium hydroxide (98 %, VWR, Germany), pyrocatechol (99 % SigmaAldrich, referred to pyroCAT in the following), 4-tert-butylcatechol (99 %, Sigma-Aldrich, referred to tertCAT in the following), 2,3-dihydroxynaphthalene (98 %, Alfa Aesar, referred to naphCAT in the following), ethyl 3,4-dihydroxybenzoate (98 %, VWR, referred to esterCAT in the following), 4-nitrocatechol (97 %, Sigma-Aldrich, referred to nitroCAT in the following), hydrochloric acid (2 M, Sigma-Aldrich, Germany), ethanol (99.98 %, VWR, Germany), and nheptane (99 % HPLC, ROTH, Germany). Instruments. Optical properties of the samples were determined from UV/Vis absorbance spectra recorded using a Cary 100 Scan UV/Vis spectrophotometer (Varian Deutschland GmbH, Germany) with a plastic cuvette of 10 mm optical path length (Brand GmbH). The spectral resolution of all UV/Vis measurements was 1 nm. FT-IR spectra were obtained with a Digilab


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FTS3100 FT-IR spectrometer with a spectral resolution of 2 cm-1. PL spectra were recorded using quartz cuvettes with a path length of 10 mm in a Jobin-Yvon photoluminescence spectrometer (Horiba, Japan). All ζ-potentials were derived from electrophoretic mobilities based on the Smoluchowski model measured on ZnO EtOH suspension using a Malvern Nano ZS Instrument (Malvern Instruments GmbH, Germany). After exposure to a certain amount of CAT molecules, all samples were washed once and redispersed in EtOH. No background electrolyte was added for the measurements to avoid uncontrolled interference of the ions with the particle surface. CAT properties. To quantify the chemical modification of the CAT derivatives by the different electron-donating/-withdrawing strength of the functional tail groups we calculated the Mulliken electronegativity (EN) using density-functional theory (DFT). The calculations were performed with the ORCA code using the B3LYP functional and the def2-TZVP basis set.53,54 The Mulliken electronegativity is the negative of the electronic chemical potential and is given by the arithmetic mean of the vertical ionization potential (IP) and electron affinitiy (EA).55 The ionization potential was calculated as total energy difference between the cation and the neutral molecules at the neutral geometry. For the electron affinity we used the approach of Tozer and De Proft,55 which gives also reliable results in case of negative electron affinities. As shown in Table 1, the calculated Mulliken electronegativity nicely follows the expected order of tertCAT < pyroCAT < naphCAT < esterCAT < nitroCAT.


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Table 1. DFT-calculated Mulliken electronegativity (EN), ionization potential (IP) and electron affinity (EA) of the different CAT molecules molecule

EN (eV) IP (eV) EA (eV)

tertCAT

2.99

7.70

-1.73

pyroCAT

3.07

8.02

-1.89

naphCAT

3.41

7.54

-0.72

esterCAT

3.68

8.14

-0.77

nitroCAT

4.60

8.74

0.45

ITC measurements. Heat flow measurements were performed at 25 °C using a TAMIII thermostat (TA Instruments) equipped with a nanocalorimeter (TA Instruments). For each measurement, 16 aliquots with a volume of 25 µL CAT solution were injected from a 500 µL Hamilton glass syringe into 1 ml ZnO suspension. The concentration of ZnO was 38.5 mM (with respect to Zn2+, hereafter the same) and the concentration of CAT solution was 27.8 mM to cover the whole range of chemisorption. The reference vial was filled with 1.2 mL of absolute ethanol to ensure similar heat capacity on both sides. The titration vial was stirred at 80 RPM to mix the molecule solution with the ZnO suspension. The time interval between consecutive injections was set in each experiment to 60 min in order to reach an equilibrium state for each titration step. Contributions from dilution effects were subtracted using a suitable reference measurement by titrating CAT molecules into absolute EtOH only. Sample preparation of ZnO NPs. ZnO NPs with a balanced mean x1,3 of 3.7 nm were synthesized and washed once by flocculation with threefold volume of heptane according to the literature.56,57 Noteworthy, in agreement with our previously reported work,58 a trimodal PSD of ZnO was found in current study (see more details in Figure S1 in Supporting Information


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(SI) S1). The PSD and the corresponding amount of esterCAT needed to form 1 monolayer chemisorbed on the available particle surface were calculated according to our previous studies (see reference 44, SI page 1-2). In the current case, 2.78 µmol of esterCAT molecule is needed to form one monolayer for 1 mL of 38.5 mM ZnO suspension, whereby one monolayer is the amount of esterCAT molecules needed to totally cover all of the available ZnO surface area under consideration of a dihedral binding angle of 75.1 ° (see reference 44, SI page 1-2). However, as in the current study we are investigating the influence of the tail group with fixed binding motif, the applied anchor groups were kept constant for all CAT molecules. Noteworthy, the anchor group is the main influencing factor for the binding of CAT molecules on the ZnO surface. Moreover, as the derived binding enthalpy (with the unit of kJ mol-1) by the new, fitting free approach developed herein is normalized per molecule, it is more straightforward to use the molar amount as reference. Based on these facts, identical amounts of anchor groups were applied for the ZnO surface throughout the whole study for different CAT molecules. However, to set our results in perspective and make them comparable to the findings of others on different material systems/anchor groups, the molar concentrations of all CAT molecules were set the same as in the case of esterCAT and referenced to the monolayers in the case of esterCAT. For UV/Vis, ζ-potential, and titration-UV measurements (for details of the latter see our previous study, reference 44), the ligand exchange reaction was done by providing different amounts of CAT molecules dissolved in ethanol. After equilibration for 30 min, the samples were washed once more to remove the unbound species with threefold volume of heptane and redispersed in ethanol. The samples for FT-IR measurements were prepared by drying the pellets after washing under vacuum at room temperature overnight, followed by compression of the dry powders with KBr to disks. For PL measurements, 34.3 nmol of esterCAT is needed to form one


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monolayer for 2.5 ml of 0.19 mM ZnO NP suspensions. Thus, PL measurements were done by adding different CAT molecules up to 34.3 nmol (corresponding to one monolayer of esterCAT) directly into 2.5 ml of 0.19 mM ZnO NP suspensions.

Results and discussions Successful functionalization of ZnO with different CAT molecules. From our previous work it is expected that when the CAT molecules successfully bind to the ZnO surface (molecular structures are shown in Figure 1A), the absorbance of ZnO changes because of the strong particle-ligand interactions. As shown in Figures 1B to 1F, the characteristic peaks of different CAT molecules after exposure of ZnO NP suspensions (1 ml of 38.5 mM) to different CAT molecules (4.17 µmol, corresponding to 1.5 monolayers of esterCAT) were found in all samples after washing, which means that all the CAT molecules successfully bind to the ZnO surface. In Figure 1B and 1C the π π* transition of the aromatic ring of pure ligands with electron-donating properties, i.e. tertCAT and pyroCAT (open symbols) was found at 281 and 278 nm in the spectra of pure molecules, respectively.59 These characteristic peaks were retrieved also from the ZnO NP suspensions after exposure to tertCAT and pyroCAT after washing. Interestingly, the absorbance features of ZnO did not change that much due to functionalization as especially in the region from 300 to 355 nm where both, tertCAT and pyroCAT have no absorbance features, more or less the same exciton peak as prior functionalization was recorded. On the contrary, as shown in Figures 1D to 1F, the absorbance of ZnO with naphCAT, (medium electron-withdrawing properties), and in particular that of ZnO with esterCAT and


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nitroCAT (both with high electron-withdrawing properties) are quite different from the features of ZnO prior to functionalization. Meanwhile, the typical peaks of the three CAT molecules can be clearly found in the spectra as well. It is noteworthy that the absorbance onset of ZnO NP suspensions changes after exposure to nitroCAT from around 350 to 500 nm and from around 425 to 500 nm when compared with that of as-prepared ZnO and pure nitroCAT, respectively. Although for quantitative interpretation of the spectra input from theoretical calculations would be required, based on these results, it can already be deduced that i) the binding of different CAT molecules is successful independent of the tail group on the aromatic ring and ii) naphCAT, and especially esterCAT and nitroCAT seem to have a stronger interaction with the ZnO surface because the absorbance of ZnO changes dramatically after functionalization.

Figure 1. A) The molecular structure of different CAT molecules used in this work sorted by their electron-donating/-withdrawing properties from electron-donating to electron-withdrawing. B-F) UV/Vis spectra of as-prepared ZnO (grey star), ZnO NP suspensions (1ml of 38.5 mM)


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after exposure to tertCAT (black dot), pyroCAT (blue square), naphCAT (green triangle), esterCAT (magenta inverted triangle), and nitroCAT (red diamond) molecules (4.17 µmol, corresponding to 1.5 monolayers of esterCAT) after washing (closed symbols), together with the corresponding spectra of the different pure CAT molecules (open symbols). All spectra were normalized to 1 at the wavelength with the highest absorbance.

Figure 2A shows the FT-IR results of as-prepared ZnO and ZnO NP suspensions after exposure to different CAT molecules (1ml of 38.5 mM ZnO mixed with 4.17 µmol CAT molecules, corresponding to 1.5 monolayers of esterCAT) after washing and drying. For all the samples, the peak of ZnO stretching can be found at 459 cm-1 as well as the characteristic peaks of the -COO asymmetric and symmetric stretching vibration at 1583 and 1413 cm−1. This indicates that there are still some acetate molecules present at the ZnO surface. Figure 2B is the enlargement of Figure 2A in the region of 1300 to 1750 cm-1. The peak of the tert-butyl group was found at 1363 cm-1 for ZnO with tertCAT.60 The aromatic ring skeleton vibration peak was found to be at 1490 and 1470 cm-1 for pyroCAT and naphCAT, respectively, while the aromatic ring skeleton vibration peak around 1500 cm-1 was assigned to tertCAT, esterCAT and nitroCAT. In addition, the band at 1497 cm-1 and the shoulder at 1593 cm-1 were assigned to the naphthalene group in the case of ZnO with naphCAT.61 The stretching vibration of the C=O group on the aromatic ring appeared at 1683 cm-1 for ZnO with esterCAT,44 while the peak at 1325 cm-1 in case of ZnO with nitroCAT is believed to be the NO2 group.62 Therefore, from FTIR studies performed on the powders in the solid state, the successful binding of all different CAT molecules to the ZnO surface was confirmed as well.


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A

B ZnO

COO-

Intensity (-)

as-prepared ZnO

Intensity (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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tertCAT pyroCAT naphCAT esterCAT

4000

tertCAT

C=C naphCAT esterCAT

nitroCAT

3000 2000 1000 Wavenumber (cm-1)

t-Bu

phenyl

pyroCAT

C=O

nitroCAT

COO-

NO2

1650 1500 1350 Wavenumber (cm-1)

Figure 2. A) FT-IR spectra of as-prepared ZnO and ZnO NP suspensions after exposure to different CAT molecules (1ml of 38.5 mM ZnO mixed with 4.17 µmol CAT molecules, corresponding to 1.5 monolayers of esterCAT) after washing and drying, the break is within the range of CO2 bands from 2200 to 2500 cm-1. B) Enlargement of the fingerprint region from 1300 to 1750 cm-1.

Influence of different CAT molecules on the ZnO surface. Due to the successful binding of all CAT molecules to the ZnO surface, surface properties will be different from as-prepared ZnO NPs. Figure 3 demonstrates the results of ζ-potential measurements of ZnO NP suspensions (1 ml of 38.5 mM) exposed to varying amounts of different CAT molecules after washing and redispersion in 1 ml EtOH. All samples show a sharp increase of ζ-potential (shift to more negative values) in the region below 2.78 µmol of added CAT molecules. However, the maximum magnitude where the ζ-potential fully levels off is not the same for the different ligands. It was found that the final ζ-potentials of ZnO with tertCAT and pyroCAT level off at around -31 mV while those of ZnO with naphCAT and esterCAT reach a final value around -


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50 mV. The highest increase in the magnitude of ζ-potential is found for ZnO with nitroCAT on the surface, which goes up to -56 mV.

0 Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-15 -30

tertCAT pyroCAT naphCAT esterCAT nitroCAT

-45 -60 0 3 6 9 12 Added CAT molecuels (µmol)

Figure 3. ζ-potential of ZnO NP suspensions (1 ml of 38.5 mM) after exposure to different amounts of CAT molecules after washing. 2.78 µmol of CAT molecule in 1 ml of 38.5 mM ZnO suspension corresponds to one monolayer in the case of esterCAT.

Based on these results, it becomes clear that the tail group of the CAT molecules plays an important role and has to be considered to achieve ZnO NPs with tailored surface properties. The CAT molecules could be roughly divided into three groups according to the estimation of the interactions between molecules and ZnO surface: i) tertCAT and pyroCAT as relatively weak binding ligands because features of CAT molecules cannot be clearly seen in the absorbance spectra and final ζ-potentials only level off at around -31 mV; ii) naphCAT and esterCAT showing strong influence on the ZnO NPs surface as features of CAT molecules are found in the absorbance spectra and the final ζ-potentials change from -16.5 to -50 mV; iii) nitroCAT having the strongest interaction with dramatic changes in the absorbance spectra and a final ζ-potential


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of up to -56 mV. These observations are in agreement with the electron-donating/-withdrawing properties of the tail groups (tertCAT ~ pyroCAT < naphCAT ~ esterCAT < nitroCAT). However, the unambiguous ordering of different CAT molecules according to their binding enthalpy can only be realized based on quantitative thermodynamic insights. This will be discussed in the following sections.

Heat release of different CAT molecules during binding to ZnO NPs. In order to get quantitative thermodynamic data, the heat release of different CAT molecules while binding to ZnO NPs was monitored by ITC. Figure 4A shows the cumulative heat that is released during titration of different CAT molecules into the ZnO NP suspensions (1 ml of 38.5 mM). In the first instance, it can be clearly seen that the heat release of nitroCAT is much higher than that of all other samples and increases continuously. However, we get some hints from small angle X-ray scattering (SAXS) measurements that in case of nitroCAT the PSD of ZnO is shifted towards finer particles, even when only small amounts of ligand are added into ZnO NP suspensions. In contrast, the PSD is rather constant for esterCAT and tertCAT molecules, at least for the investigated concentrations in this work (see SI Figure S1 for more details). Therefore, the ongoing change of the available surface area that may lead to the continuous heat increase in the case of nitroCAT will be investigated in detail by future studies. It can be seen more clearly in Figure 4B that the heat release of esterCAT levels off at around 58 mJ until the equilibrium state is observed while the heat release of naphCAT is a bit lower and reaches a final value of 41 mJ. Moreover, both tertCAT and pyroCAT have a fast increase in the region within 3.1 mM (which corresponds to 1.25 monolayers of esterCAT) where chemisorption is expected to be the dominant effect, which goes up to 17 and 20 mJ for tertCAT


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and pyroCAT, respectively. However, for both molecules, the heat release does not level off but keeps slowly increasing without reaching any plateau. For tertCAT this could carefully be ascribed to ongoing weak coordination due to the fact that the larger tertCAT has the lowest surface coverage because of steric effects (the data will be shown in the next section) which results in a stronger driving force to approach the ZnO surface in the physisorption regime. For pyroCAT there could be some oxidation of pyroCAT molecules going on due to the long ITC measurement time.59 It could be concluded that the cumulative heat release during titration of different CAT molecules

into

ZnO

NP

suspensions

follows

the

order

of

tertCAT < pyroCAT < naphCAT < esterCAT < nitroCAT. However, due to the missing final plateau of heat release in case of nitroCAT, tertCAT and pyroCAT, fitting of ITC data based on a Langmuir model was only possible for esterCAT and pyroCAT (see SI S2 Figure S2 and Table S1 for details and fitting).

A

B nitroCAT esterCAT naphCAT pyroCAT tertCAT

75 -Q (mJ)

0.2 -Q (J)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

0.0

50

esterCAT naphCAT pyroCAT tertCAT

25 0

0 2 4 6 8 Added CAT molecules (mM)

0 2 4 6 8 Added CAT molecules (mM)


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Figure 4. A) Cumulative heat release during titration of different CAT molecules into ZnO NP suspensions (1 ml of 38.5 mM) measured by ITC. B) Zoom in of the cumulative heat release in Figure 4A without the data of nitroCAT.

Adsorption isotherm by mass-based method. Without successful fitting, ITC measurements alone only give the cumulative heat release during ligand binding. Thus, unambiguous ordering of the CAT molecules according to the extracted binding enthalpy is not possible. Therefore titration-UV as mass-based method was employed to shed light on the actual amount of CAT molecules binding to the ZnO surface when the particles are exposed to a certain CAT concentration.44 In brief, titration-UV is a combination of titration and UV/Vis spectroscopy in which hydrochloric acid is used to dissolve all ZnO NPs and the applied amount is used to calculate the solid concentration of ZnO. The same sample was measured by UV/Vis spectroscopy to determine the amount of CAT molecules released from the surface after dissolution of ZnO NPs with the calibration line for different CAT molecules (see SI S3 Figure S3 and Table S2). Noteworthy, although the typically required amount of acid solution is below 20 µl (0.5 M) for 1 ml of ZnO suspension to dissolve all the ZnO NPs, the dilution effect was taken into consideration when calculating the amount of bound CAT molecules. As shown in Figure 5, nitroCAT and esterCAT have the highest amount of bound molecules when the ZnO NPs are exposed to a fixed molar ligand concentration. Next, napthCAT with medium electron-donating properties is slightly lower than nitroCAT and esterCAT. Noteworthy, although pyroCAT is the smallest molecule, it only shows the second lowest amount of bound molecules. This is ascribed to the low binding affinity of pyroCAT to the ZnO surface. In the case of tertCAT with the lowest amount of bound molecules, this effect is even more pronounced


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as tertCAT has the electron-donating tert-butyl group in addition to some superimposed steric effects induced by the tert-butyl group. Interestingly, as it can be seen from the grey circle in Figure 5, in case of CAT molecules with higher surface coverage (nitroCAT, esterCAT, and naphCAT), after a plateau like state, an ongoing secondary, relatively slow increase of bound molecules is observed. In line with our previous observations, this could be due to physisorption,44 or the aforementioned strong change of the underlying PSD of ZnO NPs in the presence of CAT molecules, or both. Due to the fact that the surface area of ZnO changes upon adding nitroCAT (see SI S1, Figure S1E), and the aforementioned additional effects discovered along ITC analysis, again the data could not be fitted for all molecules (see details on the Langmuir fitting in SI S4, Figure S4 and Table S3).

Bound CAT molecules (mM)

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3 2 1 0

nitroCAT esterCAT naphCAT pyroCAT tertCAT

0 1 2 3 4 Free CAT molecules (mM)

Figure 5. Amount of CAT molecules bound to the ZnO surface determined by titration-UV procedure. The grey circle indicates occurrence of additional phenomena besides chemisorption.

Combined information from heat- and mass-based investigation. From the complications that arose during fitting of ITC and titration-UV data discussed in the last two sections, it is concluded that the Langmuir model oversimplifies the underlying binding mechanism and is thus


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not always applicable (see SI S2 and S4). Hence, a new approach of combined data evaluation for ordering functionalities according to their binding enthalpy will be proposed in the following. When heat release and amount of bound CAT molecules are independently derived by ITC and titration-UV measurements, respectively, the enthalpy of all CAT molecules binding to the ZnO NPs can be directly calculated without any fitting models: ∆ =

(1)

,

where ∆H is the enthalpy of CAT molecules binding to ZnO (J mol-1), QITC is the cumulative heat release of CAT molecules binding to ZnO NPs measured by ITC (J), and nboundCAT, mass based is the amount of bound CAT molecules derived from the mass-based method (µmol). Figure 6 shows the enthalpies of different CAT molecules binding to ZnO NP suspensions calculated from Eq. 1 without assuming any binding model. First of all, it is recognized that in case of nitroCAT (red diamonds) the binding enthalpy keeps increasing instead of decreasing like it is observed for all other CAT molecules. As mentioned (see above and SI S1), this is ascribed to the fact that the PSD of ZnO changes even when adding small amounts of nitroCAT (threshold is at about 5.56 µmol of nitroCAT in 1 ml of 38.5 mM ZnO NP suspensions which corresponds to 2 monolayers of esterCAT). Therefore, the ongoing change of the available surface area may lead to the continuous increase of the binding enthalpy when nitroCAT is exposed to ZnO NPs. However, the first two values of the enthalpy in case of nitroCAT (closed red diamonds in Figure 6) can be used for direct comparison with the other CAT molecules as in this region the PSD of the ZnO NPs is nearly unchanged (< 5 %) (see SI S1 Figure S1E for more details).


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Noteworthy, the enthalpies of all other CAT molecules decrease when more CAT molecules are provided. This is due to the fact that surface sites of high affinity will be occupied in the beginning at lower ligand concentrations prior CAT molecules bind to the sites of medium and low affinity. Thus, the surface of the ZnO NPs is not homogeneous as the surfaces of such liquid-borne particles can include various heterogeneities like defects, acetate occupied surface, EtOH occupied surface, hydroxyl occupied surface, different crystal facets, and surface occupied with cations (e.g. small amounts of Li+ could be present from the synthesis). They all can affect the binding affinity which makes the in-depth characterization of NP interfaces tedious and highly complex and clearly restricts the applicability of the Langmuir model. It turned out that the enthalpies of all CAT molecules follow the order of tertCAT < pyroCAT < naphCAT < esterCAT < nitroCAT. This is perfectly in agreement with the electron-donating/-withdrawing properties of tail groups which affect the dissociation of hydroxyl groups during ligand binding, leading to stronger interaction between surface cations and deprotonated oxygen species.63

60

−∆H (kJ mol-1)

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45

nitroCAT nitroCAT >1.39 µmol esterCAT naphCAT pyroCAT tertCAT

30 15 0 2 4 6 Added CAT molecules (µmol)

Figure 6. Enthalpies of different CAT molecules binding to ZnO NP suspensions (1 ml of 38.5 mM) obtained by dividing the heat release from ITC results by the amount of bound CAT


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molecules from titration-UV data. 2.78 µmol of CAT molecule in 1 ml of 38.5 mM ZnO suspension corresponds to one monolayer in the case of esterCAT.

Influence of CAT molecules on the optical properties of ZnO. After sorting the different CAT molecules in terms of their binding enthalpies, it is also important to look into the influence of ligand molecules on optical properties, like PL. Before measuring the emission of CATfunctionalized ZnO NPs, the concentration of ZnO needs to be adjusted in a way that selfquenching due to particle-particle interaction is prevented. Therefore, the PL intensity at the defect emission (517 nm)64, 65 with the excitation wavelength at 330 nm was normalized to the concentration of ZnO. From Figure 7A, we can see that at concentrations lower than 0.19 mM the normalized PL intensity is more or less constant. The slight decrease is due to the signal to noise ratio at very low concentration. Therefore, the PL of ZnO with CAT molecules was measured at a ZnO concentration of 0.19 mM to avoid self-quenching. As shown in Figure 7B, the PL intensity of ZnO NP suspensions (see SI S5 Figure S5 for PL spectra of ZnO with different amounts of CAT molecules) decreases dramatically during functionalization for all the CAT molecules, even when only small quantities (6.86 nmol, corresponding to 0.2 monolayer of esterCAT), clearly below the required concentration to reach a monolayer, are added. However, beyond this general trend, tertCAT and pyroCAT with low binding enthalpy show a much slower decrease than naphCAT, esterCAT and nitroCAT with higher binding enthalpy. Noteworthy, the PL quenching for all molecules follows the same order as the binding enthalpy (Figure 6) as well as the electron-donating/-withdrawing properties of the tail group.


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It is known that ZnO NPs show visible emission because of defects which are usually considered to be on the particle surface.66 Thus, it could be deduced that the CAT molecules preferentially start to bind to the defect sites at the ZnO NPs surface during early stages of the functionalization process, which leads to the PL quenching. Noteworthy, although emission pathways for NPs are complex and getting the full picture requires advanced spectroscopy like e.g. lifetime measurements,67 such a comparatively simple correlation already gives hints how to systematically direct future studies. Generally, this gives further support for our approach to order molecules with the same binding motif according to features like electron-donating/withdrawing properties and to systematically investigate the influence of functionalities on the binding enthalpy. As the interaction between ligands and particles needs to be investigated for applications like solar cells where the PL of particles is supposed to be as high as possible, such understanding is required to come to a more rational design of particle-based devices.

A ×108 8 6

B 100 PL loss (%)

Norm. PL intensity (a.u. M-1)

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4 2

80 60 40


20 0

0.1 1 10 Concentration of ZnO (mM)

0 10 20 30 Added CAT molecules (nmol)

Figure 7. A) Normalized PL intensity at 517 nm of ZnO NPs at different concentrations for an excitation wavelength λexc of 330 nm and slit of 2 nm; B) PL loss of ZnO NP suspensions (2.5 ml of 0.19 mM) after adding different amounts of CAT molecules for an excitation wavelength λexc of 330 nm and slit of 3 nm (the latter was chosen in order to get higher signal to see the


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quenching more clearly). 34.3 nmol of CAT molecule in 2.5 ml of 0.19 mM ZnO suspension corresponds to one monolayer in the case of esterCAT.


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Conclusions In conclusion, we shed light on the tailoring of ZnO nanoparticles (NPs) by catechol derivatives (CAT) with different building block groups or functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The connections between electron-donating/-withdrawing properties, the thermodynamic quantities (∆H) describing ligand adsorption onto ZnO NPs, and the photoluminescence (PL) quenching of ZnO NPs during functionalization were unraveled. Ultraviolet/visible (UV/Vis) and Fourier transform infrared (FT-IR) spectroscopy results showed that the typical features of different building block groups were monitored after functionalization and washing. This means that different CAT molecules can bind to ZnO NPs independent of the distinct functionalities. Noteworthy, the ζ-potential of ZnO NPs modified by tertCAT and pyroCAT reached around -31 mV while that of naphCAT and esterCAT levelled off at around 50 mV. ZnO capped by nitroCAT showed the highest increase up to -56 mV. However, to unambiguously order different CAT molecules along their binding enthalpy, a new, fitting free approach which combines isothermal titration calorimetry (ITC) as heat-based technique with a mass-based method was developed to determine the binding enthalpy. This was necessary as both, heat- and mass-based approaches alone did not fully resolve the binding enthalpy of each molecule to the ZnO surface. The reason was that the Langmuir model oversimplifies the complex situation of ligand binding to highly heterogeneous substrates as it is the case for liquid-borne NPs. Noteworthy, as with the herein developed combined approach no fit was necessary and thus no adsorption model had to be assumed, the surface heterogeneity of ZnO NPs could be experimentally accessed and the ligands could be unambiguously ordered according to their binding enthalpies to the particle surface. The binding enthalpies of the CAT


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molecules follow the order of tertCAT < pyroCAT < naphCAT < esterCAT < nitroCAT, which is in agreement with the electron-donating/-withdrawing properties of the tail groups. Finally, the efficiency of quenching visible emission of ZnO due to the binding of different CAT molecules was investigated by PL spectroscopy which resulted in the same order as found for the binding enthalpy and the electron-donating/-withdrawing properties. Hence, our approach allows for categorizing a wide range of molecules - in addition to anchor groups/binding motif and chain length - in terms of different aspects such as electron-donating/-withdrawing properties, redox potential, and hydrophobicity. We believe that our work is an important contribution towards knowledge-based selection and design of ligands for tailoring colloidal surfaces in various particle-based applications.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. PSD of ZnO measured by SAXS, fitting of ITC data, calibration lines of different CAT molecules, fitting of adsorption isotherm by Langmuir model, PL spectra of ZnO with different CAT molecules, and heat flow profile of esterCAT and nitroCAT recorded by ITC can be found in the supporting Information. AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence “Engineering of Advanced Materials” (project EXC 315) (Bridge Funding). Moreover, we would like to thank Dr. Alexandra Burger (FAU) for helpful discussion.


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(46) Williams, E. S., Major, K. J., Tobias, A., Woodall, D., Morales, V., Lippincott, C., Moyer, P. J., Jones, M. Characterizing the Influence of TOPO on Exciton Recombination Dynamics in Colloidal CdSe Quantum Dots. J. Phys. Chem. C 2013, 117, 4227–4237. (47) Chakraborty, S., Joshi, P., Shanker, V., Ansari, Z. A., Singh, S. P., Chakrabarti, P. Contrasting Effect of Gold Nanoparticles and Nanorods with Different Surface Modifications on the Structure and Activity of Bovine Serum Albumin. Langmuir 2011, 27, 7722–7731. (48) Chakraborti, S., Joshi, P., Chakravarty, D., Shanker, V., Ansari, Z. A., Singh, S. P., Chakrabarti, P. Interaction of Polyethyleneimine-Functionalized ZnO Nanoparticles with Bovine Serum Albumin. Langmuir 2012, 28, 11142–11152. (49) Limo, M. J., Perry, C. C. Thermodynamic Study of Interactions Between ZnO and ZnO Binding Peptides Using Isothermal Titration Calorimetry. Langmuir 2015, 31, 6814–6822. (50) Freire, E.; Mayorga, O. L.; Straume, M. Isothermal titration calorimetry. Anal. Chem. 1990, 62, 950A−959A. (51) Yang, H., Etzel, M. R., Evaluation of Three Kinetic Equations in Models of Protein Purification Using Ion-Exchange Membranes. Ind. Eng. Chem. Res. 2003, 42, 890-896 (52) Franses, E., Siddiqui, F. A., Ahn, D. J., Chang, C., Wang, N. L., Thermodynamically Consistent Equilibrium Adsorption Isotherms for Mixtures of Different-Size Molecules. Langmuir 1996,11, 3177-3183 (53) Neese, F., ORCA - An Ab Initio, DFT and Semiempirical electronic structure package, Version 3.0.1, https://orcaforum.cec.mpg.de (54) Weigend, F., Ahlrichs, R., Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305 (55) Tozer, D.J., De Proft, F., Computation of the Hardness and the Problem of Negative Electron Affinities in Density Functional Theory. J. Phys. Chem. A 2005, 109, 8923-8929 (56) Spanhel, L., Anderson, M. A., Semiconductor clusters in the sol-gel process: quantized aggregation, gelation, and crystal growth in concentrated zinc oxide colloids. J. Am. Chem. Soc., 1991, 113, 2826–2833. (57) Segets, D., Gradl, J., Taylor, R. K., Vassilev, V., Peukert, W. Analysis of Optical Absorbance Spectra for the Determination of ZnO Nanoparticle Size Distribution, Solubility, and Surface Energy. ACS Nano 2009, 3, 1703–1710. (58) Schindler, T., Walter, J., Peukert, W., Segets, D., Unruh, T., In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities. J. Phys. Chem. B 2015, 119, 15370−15380 (59) Pillar, E. A., Zhou, R., Guzman, M. I. Heterogeneous Oxidation of Catechol. J. Phys. Chem. A 2015, 119, 10349–10359. (60) Coates, J. Interpretation of Infrared Spectra, A Practical Approach. Encyclopedia of Analytical Chemistry, John Wiley & Sons, Ltd. 2006. DOI: 10.1002/9780470027318.a5606 (61) Srivastava, A., Singh, V. B., Theoretical and experimental studies of vibrational spectra of naphthalene and its cation. Indian Journal of Pure & Applied Physics 2007, 45, 717-720 (62) Bixner, O., Lassenberger, A., Baurecht, D., Reimhult, E. Complete Exchange of the Hydrophobic Dispersant Shell on Monodisperse Superparamagnetic Iron Oxide Nanoparticles. Langmuir 2015, 31, 9198–9204.


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TOC


100

PL loss (%)

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∆H

80 60 40 20 0

electronegativity

0

5

10

15

Added CAT molecules (nmol)


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Figure 1. A) The molecular structure of different CAT molecules used in this work sorted by their electronegativity from weak to strong. B-F) UV/Vis spectra of as-prepared ZnO (grey star), ZnO NP suspensions (1ml of 38.5 mM) after exposure to tertCAT (black dot), pyroCAT (blue square), naphCAT (green triangle), esterCAT (magenta inverted triangle), and nitroCAT (red diamond) molecules (4.17 µmol, corresponding to 1.5 monolayers of esterCAT) after washing (closed symbols), together with the corresponding spectra of the different pure CAT molecules (open symbols). All spectra were normalized to 1 at the wavelength with the highest absorbance. 37x21mm (300 x 300 DPI)


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Figure 2. A) FT-IR spectra of as-prepared ZnO and ZnO NP suspensions after exposure to different CAT molecules (1ml of 38.5 mM ZnO mixed with 4.17 µmol CAT molecules, corresponding to 1.5 monolayers of esterCAT) after washing and drying, the break is within the range of CO2 bands from 2200 to 2500 cm-1. 63x52mm (300 x 300 DPI)


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Figure 2. B) Enlargement of the fingerprint region from 1300 to 1750 cm-1. 63x52mm (300 x 300 DPI)


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Figure 3. ζ-potential of ZnO NP suspensions (1 ml of 38.5 mM) after exposure to different amounts of CAT molecules after washing. 63x52mm (300 x 300 DPI)


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Figure 4. A) Cumulative heat release during titration of different CAT molecules into ZnO NP suspensions (1 ml of 38.5 mM) measured by ITC. 63x52mm (300 x 300 DPI)


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Figure 4. B) Zoom in of the cumulative heat release in Figure 4A without nitroCAT. 63x52mm (300 x 300 DPI)


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Figure 5. Amount of CAT molecules bound to the ZnO surface determined by titration-UV procedure. The grey circle indicates occurrence of additional phenomena besides chemisorption. 63x52mm (300 x 300 DPI)


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Figure 6. Enthalpies of different CAT molecules binding to ZnO NP suspensions (1 ml of 38.5 mM) obtained by dividing the heat release from ITC results by the amount of bound CAT molecules from titration-UV data. 63x52mm (300 x 300 DPI)


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Figure 7. A) Normalized PL intensity at 517 nm of ZnO NPs at different concentrations for an excitation wavelength λexc of 330 nm and slit of 2 nm; 63x52mm (300 x 300 DPI)


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Figure 7. B) PL loss of ZnO NP suspensions (2.5 ml of 0.19 mM) after adding different amounts of CAT molecules for an excitation wavelength λexc of 330 nm and slit of 3 nm (the latter was chosen in order to get higher signal to see the quenching more clearly). 63x52mm (300 x 300 DPI)


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Table of Content 12x6mm (300 x 300 DPI)


Influence of Tail Groups during Functionalization of ZnO Nanoparticles

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