A Citric Acid-Derived Ligand for Modular Functionalization of Metal

Article pubs.acs.org/Langmuir

A Citric Acid-Derived Ligand for Modular Functionalization of Metal Oxide Surfaces via “Click” Chemistry Lee M. Bishop,†,‡ Joseph C. Yeager,† Xin Chen,† Jamie N. Wheeler,† Marco D. Torelli,† Michelle C. Benson,† Steven D. Burke,† Joel A. Pedersen,‡ and Robert J. Hamers*,† †

Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States Environmental Chemistry and Technology, University of WisconsinMadison, 1525 Observatory Drive, Madison, Wisconsin 53706, United States

‡

S Supporting Information *

ABSTRACT: Citric acid is a widely used surface-modifying ligand for growth and processing of a variety of nanoparticles; however, the inability to easily prepare derivatives of this molecule has restricted the development of versatile chemistries for nanoparticle surface functionalization. Here, we report the design and synthesis of a citric acid derivative bearing an alkyne group and demonstrate that this molecule provides the ability to achieve stable, multidentate carboxylate binding to metal oxide nanoparticles, while also enabling subsequent multistep chemistry via the Cu(I)-catalyzed azide− alkyne cycloaddition (CuAAC) reaction. The broad utility of this strategy for the modular functionalization of metal oxide surfaces was demonstrated by its application in the CuAAC modification of ZnO, Fe2O3, TiO2, and WO3 nanoparticles.

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INTRODUCTION Metal oxides are of increasing interest for application in solidstate lighting, hybrid organic−inorganic light-emitting diodes, and dye-sensitized solar cells.1−3 Especially when used in nanostructured form, functionalization of the oxide surface is often necessary to control and/or impart specific chemical and physical properties, such as interfacial charge transfer and wetchemical processability. Previously used approaches included the use of carboxylic4−11 and phosphonic acids11−17 as surfacebinding groups. A disadvantage to the use of phosphonic acids is the need for extensive heating to enable formation of a nonlabile substrate-surface bond.16,17 Additionally, the binding of individual (monodentate) carboxylic acid groups is relatively weak, so that the ligands are readily removed.6,10 However, the use of multidentate binding domains is generally recognized to dramatically enhance the stability of the ligand−oxide linkage.18 Citric acid, a tridentate carboxylic acid, has been widely used as a stabilizing ligand for many types of nanoparticles because the citrate ion provides good surface binding, imparts water solubility to the resulting nanoparticles and reduces aggregation.19,20 Indeed, citrate has been shown to be effective with many materials including gold19,20 and other metals,21 numerous metal oxides including iron oxides,22−24 ZnO,25,26 and TiO2,27,28 and silicates and aluminosilicates.29 Despite this, there are very few reports of the synthesis of citric acid derivatives,30 likely due in part to harsh reaction conditions and competitive elimination side reactions in the direct functionalization of the free hydroxyl group of citric acid.31−33 This has restricted the development of more versatile multidentate ligands, and to our knowledge no functionalized derivative of © 2011 American Chemical Society

citric acid has been studied in the context of nanoparticle functionalization. The use of modular chemistries based on dependable irreversible reactions, sometimes referred to as “click” chemistry, is a particularly attractive way to functionalize nanoparticles because it provides substantial versatility in the types of functional groups presented by the nanoparticle to its environment. Previous reports of the “click” functionalization of nanoparticles either make use of monodentate (and therefore more labile) surface linkages or have only been proven with a single metal oxide.34−36 Here, we report the design and synthesis of a citric acid derivative bearing an alkyne group and demonstrate that this combination provides the ability to form stable, multidentate carboxylate linkages to a variety of metal oxide nanoparticles, while also enabling subsequent multistep chemistry via the Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) “click” reaction.34−40 Our results with ZnO, Fe2O3, TiO2, and WO3 indicate that the chemistry outlined here represents a versatile, modular approach to functionalizing the surfaces of a wide variety of metal oxide nanoparticles.

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RESULTS AND DISCUSSION Synthesis of Citric Acid Derivative Bearing a Pendant Alkyne Group. Our synthesis began with the alkylation of trimethyl citrate using propargyl trifluoromethanesulfonate as shown in Scheme 1 to yield O-propargyl trimethyl citrate (1). Received: October 22, 2011 Revised: November 30, 2011 Published: December 6, 2011 1322

dx.doi.org/10.1021/la204145t | Langmuir 2012, 28, 1322−1329

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Scheme 1. Synthesis of O-Propargylcitric Acid (2)a

a

In addition, surface-bound 2 exhibits a free acid stretch centered from 1736 to 1714 cm−1 that is weak relative to the zinc carboxylate stretches. We conclude from these results that 2 forms either one bidentate inner-sphere complex or a mixture of tridentate and bidentate inner-sphere complexes with the ZnO surface. These results are consistent with those reported in the literature for the adsorption of citric acid onto TiO2 surfaces.41 Stability of Surface-Bound Ligand. The stability of the interaction of O-propargyl citric acid (2) with the ZnO surface was compared with that of the monodentate carboxylic acid 4(trifluoromethoxy)phenylacetic acid (3). Spectra are shown in Figure 2. IR analysis of the 3-modified ZnO surface (prepared

TfO = trifluoromethanesulfonate.

Alkylation using less potent electrophiles such as propargyl bromide was unsuccessful under a variety of conditions. Subjecting 1 to various basic ester saponification conditions resulted in competitive elimination of propargyl alcohol to give aconitic acid (not shown). This result differs from the report by Meissner and Jutzi of the base-catalyzed hydrolysis of the methyl esters of a variety of O-alkyltrimethyl citrate derivatives in 71−80% yield,30 a difference that is likely due to the better leaving-group ability of the propargyl alkoxide of 1 used here relative to that of the straight-chain alkyl alkoxides (ΔpKa ≈ 2) investigated by them. Methyl ester hydrolysis of 1 under acidic conditions yielded O-propargyl citric acid (2), which was found to be stable to decomposition at room temperature over the course of a week in both highly acidic (pH = 1) and basic (pH = 14) aqueous solution. Binding of Ligand to ZnO Nanoparticles. To characterize the surface-binding properties of the ligand, we began by confirming successful binding of 2 to ZnO nanoparticles by infrared (IR) spectroscopy. The free acid 2 exhibits a strong sharp carbonyl peak at 1721 cm−1 and a weaker broad peak centered at 1408 cm−1 that we attribute to a combination of the C−O stretching and O−H deformation vibrations as well as the CH2CO deformation vibration (Figure 1). After treatment with

Figure 2. (a) Carboxylate region of the IR spectra of alkyne-modified ZnO nanoparticles immediately after functionalization (top) and after immersion in 80 °C MeCN for 10 s (middle) and for 6 days (bottom). (b) Carboxylate region of the IR spectra of 3-modified ZnO nanoparticles immediately after functionalization (top) and after a 10 s treatment with 80 °C acetonitrile (bottom). A spectrum of the bare ZnO nanoparticulate film prior to treatment with 2 or 3 was used as the reference samples. Spectra were offset vertically for clarity.

in an identical manner as the alkyne-modified surface, see above) revealed absorption bands centered at 1570 and 1403 cm−1, characteristic of the asymmetric and symmetric carbonyl stretches of a zinc carboxylate species. In addition, a set of strong absorption bands diagnostic of the CF3 group of 3 are observed at 1274, 1230, and 1188 cm−1 (Figure 2b and Figure S2). Stability studies were conducted by submerging 3- and alkyne-modified ZnO nanoparticulate thin films in MeCN at 80 °C and assessing ligand desorption by IR analysis. Integration under the relevant IR absorption bands allows quantification of the amount of ligand remaining on the ZnO surface. Figure 2a shows IR spectra of the ZnO surface immediately after grafting the citric acid derivative 2 and after the grafted sample was immersed in MeCN at 80 °C for 10 s and for 6 days. The nearly constant IR absorbance in the carboxylate region

Figure 1. Carboxylate region of the IR spectra of 2 on the surface of ZnO nanoparticles (top) and that of neat 2 (bottom). A spectrum of the bare ZnO nanoparticulate film prior to treatment with 2 was used as the background for the top spectrum. The spectrum of neat 2 was scaled arbitrarily for comparison with that of the functionalized ZnO surface. Spectra were offset vertically for clarity. For full spectra see Figure S1 (Supporting Information).

a 10 mM solution of 2 in acetonitrile (MeCN), ligand binding was evidenced by characteristic changes in the frequency of the vibrational modes of the carbonyl groups. The 2-modified ZnO surface exhibits a strong broad peak centered at 1603 cm−1 and a weaker broad peak centered at 1425 cm−1, consistent with the asymmetric and symmetric stretches of a zinc carboxylate species (Figure 1).6−11 The latter peak at 1425 cm−1 likely overlaps with the above-mentioned peak of 2 at 1408 cm−1. 1323


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is absent on the ZnO surface (Figure 3b, top), indicating the azide group was consumed in the formation of a triazole ring. Finally, the citrate carbonyl stretches (see above) remain unchanged on the ZnO surface, indicating the complex between citrate ligand 2 and the ZnO surface remains intact after CuAAC functionalization, in agreement with the expected stable nature of that complex. Further confirmation of reaction success comes from XPS data. Figure 4 shows XPS data of a sample exposed to the full

indicates that more than 90% of the citric acid derivative 2 remains on the surface after 6 days under these conditions. Similar measurements were performed on the monodentate acetic acid derivative 3; in this case, however, the IR intensities of the carboxylate peaks and C−F peaks were reduced to ∼25% of their initial value within 10 s (Figure 2b). Thus, we conclude that the tridentate citric acid derivative synthesized here provides substantially increased stability compared to monodentate ligands, in agreement with previous studies.18,41 Copper-Catalyzed Azide−Alkyne Cycloaddition (CuAAC) Reaction on ZnO Surfaces. To examine the possibility of conducting multistep functionalization of the ZnO surfaces, the alkyne-modified ZnO nanoparticles were then subjected to CuAAC reaction conditions using 4-azido-1trifluoromethoxybenzene (4), a molecule whose CF3 group is a good marker for verification of reaction success by IR and XPS. Nonaqueous conditions using CuOAc catalyst in tetrahydrofuran solvent were selected to avoid the possibility of etching the ZnO surface in the presence of water.42 IR spectra show evidence for covalent bonding of azide 4 to the surface, as a set of peaks diagnostic of the CF3 group are observed at 1263, 1229, and 1159 cm−1 when the nanoparticles were subjected to the full CuAAC conditions (Figure 3b, top) but are not

Figure 4. Fluorine (1s) region of XPS spectra of the alkyne- and citric acid-modified ZnO nanoparticles after being subjected to CuAAC conditions (top and bottom, respectively) and that of alkyne-modified ZnO nanoparticles after being subjected to copper-free CuAAC conditions (middle). Spectra were offset vertically for clarity.

CuAAC conditions and a control experiment exposing the sample to the full procedure omitting the addition of the Cu catalyst. Fluorine atoms are observed on the ZnO nanoparticles subjected to the CuAAC conditions (Figure 4, top) but are not present on those subjected to copper-free CuAAC conditions (Figure 4, middle). One challenge in applying the CuAAC reaction to metal oxides is that copper can potentially adsorb to the metal oxide surfaces, complicating the surface chemistry. In our experiments minor amounts of copper were sometimes observed by XPS on the ZnO surface under the CuAAC conditions described above. Therefore, to determine whether the adsorption of azide 4 to the alkyne-modified ZnO surface proceeds through the CuAAC reaction or some other copper-mediated process, we performed two further control experiments. First, alkyne-modified ZnO nanoparticles were subjected to CuAAC conditions in the presence of trifluoromethoxybenzene, a non-azide-bearing analogue of 4, which resulted in no detectable CF3 signal by IR and XPS analysis. These results indicate that the azide moiety of 4 is critical for its copper-mediated adsorption to alkyne-modified ZnO surfaces. The second additional control experiment was performed by exposing nanoparticles coated with citric acid (instead of the alkyne-bearing derivative 2) to CuAAC conditions in the presence of azide 4. These experiments resulted in nanoparticles exhibiting barely detectable levels of surface fluorine atoms by XPS analysis (Figure 4, bottom). These results show that the alkyne group of citrate derivative 2 is necessary in order for azide 4 to bind to the citrate-functionalized ZnO surface in the presence of Cu. Taken together, these results clearly support the formation of a

Figure 3. (a) Reaction of azide 4 with alkyne-modified surface to produce CuAAC surface adduct. (b) IR spectra of alkyne-modified ZnO nanoparticles after being subjected to full CuAAC conditions (top) and copper-free CuAAC conditions (middle) in addition to a spectrum of pure azide reactant 4 (bottom). A spectrum of the bare ZnO nanoparticulate film prior to treatment with 2 was used as the reference sample for the top two spectra. The spectrum of azide 4 was scaled arbitrarily to facilitate comparison with that of the functionalized ZnO surface. Spectra were offset vertically for clarity. For full spectra see Figure S3 (Supporting Information).

observed in the control CuAAC reaction omitting addition of the Cu catalyst (Figure 3b, middle).43 In addition, the diagnostic azide stretch of 4 at 2114 cm−1 (Figure 3b, bottom) 1324


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covalent triazole linkage between azide 4 and the alkynemodified ZnO surface under CuAAC conditions. Nanoparticle Thin Film Stability. To verify that the ZnO nanoparticle thin film retained its overall morphology through the above procedures, we examined the film by SEM. Figure 5

Figure 5. Representative cross-sectional SEM images of bare, alkynemodified, and CuAAC-modified ZnO nanoparticulate thin films.

Figure 6. UV/vis absorbance spectra of dye 5 in MeCN acquired in transmission mode (bottom) and those of alkyne-modified ZnO nanoparticles after being subjected to 5 under CuAAC conditions (middle) and copper-free CuAAC conditions (top) in diffuse reflectance mode using a sample of Teflon as a reference. Spectra were offset vertically for clarity.

shows that the overall films maintained their morphology and thickness. Average estimated film thicknesses were 2.4 ± 0.6 μm (28 measurements) for bare ZnO film, 2.3 ± 0.4 μm (13 measurements) for alkyne-modified ZnO, and 2.5 ± 0.6 μm (13 measurements) for the CuAAC-modified sample. Highresolution images did not reveal any detectable difference between the individual nanoparticles but are at the limit of resolution of the SEM. Modification of ZnO Surfaces with an Organic Dye Bearing a Pendant Azide. To explore the modular utility of these alkyne-modified ZnO surfaces, this approach was applied to several other organic azides. We focused our attention on azide-bearing organic dye 5 (Figure S4, Supporting Information) because the functionalization of ZnO with visible lightabsorbing dyes is of great interest for dye-sensitized solar cell applications.1−3 Additionally, while the experiments above used the nonpolar aryl azide 4, the dye molecule 5 is an ionic alkyl azide bearing a wide variety of functional groups. Therefore, successful application of the CuAAC reaction with this molecule would provide support for the applicability of this strategy to a wide variety of organic azides. The successful functionalization of alkyne-terminated ZnO nanoparticles with 5 under CuAAC conditions was readily verified by diffuse reflectance UV−vis spectrometry measurements. Spectra of the dye-modified ZnO nanoparticles were compared with similar data from a control experiment in which the sample was subjected to CuAAC conditions in the absence of the Cu catalyst. Figure 6 shows the diffuse reflectance UV− vis spectra of these films in addition to the UV−vis absorbance spectra of dye 5. The diagnostic absorbance peaks of 5 are strong and shifted by ∼10 nm from the parent compound; these same peaks are barely detectable in the control sample. Ligand Binding and Subsequent CuAAC Reaction on Other Metal Oxides. Given the ability of citric acid to bind to a variety of surfaces,19−29 we anticipate this functionalization strategy to have broad utility. Indeed, we have prepared nanoparticulate thin films from metal oxides bearing divalent metal atoms (ZnO, see section above) as well as those bearing trivalent (Fe2O3), tetravalent (TiO2), and hexavalent (WO3) metal atoms and demonstrated successful binding of citric acidderived ligand 2 to those surfaces. Figure 7 shows the IR spectra of the Fe2O3-, TiO2-, and WO3-bound adducts along with those of the ZnO-bound adduct and free 2 for comparison. Because the nanoparticle films from different nanoparticles do not have the same exposed surface area (due to differences in size and shape of nanoparticles of different composition), the spectra shown here were scaled individually

Figure 7. Carboxylate region of the IR spectra 2 on the surface of ZnO, WO3, Fe2O3, and TiO2 nanoparticles along with that of neat 2. Spectra of the bare nanoparticulate films prior to treatment with 2 were used as the backgrounds for the metal oxide spectra. Because of differences in the exposed surface area of the different nanoparticle films, the spectra were scaled individually to achieve comparable heights for the carboxylate peaks. Spectra were offset vertically for clarity.

and cannot be compared quantitatively. However, the spectrum on the WO3 film was notably weaker than on the other oxides. The TiO2- and Fe2O3-bound adducts both exhibit features similar to those of the ZnO-bound adduct described above, namely free acid stretches around 1725 cm−1 and carboxylate stretches around 1600 and 1400 cm−1, suggesting that 2 binds to all these metal oxide surfaces by forming similar inner-sphere complexes. In contrast, the strongest IR absorbance band of the WO3-bound adduct is that of the free acid at 1736 cm−1, with extremely weak carboxylate stretches, suggesting that ligand 2 binds to WO3 primarily through formation of an outer-sphere complex wherein the majority of the carboxylic acid groups remain in their free acid form. This difference is likely due to the extremely low isoelectric point of WO3 relative to those of the other metal oxides studied.44 This would decrease the stability of deprotonated carboxylic acid groups of 2 near the 1325


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CONCLUSION We have developed a novel alkyne-modified citric acid derivative, provided evidence that it forms a thermally stable inner-sphere complex with ZnO nanoparticles and shown that the CuAAC reaction can be used to further modify the resulting alkyne-modified ZnO nanoparticles. Through the use of a CF3bearing aryl azide we have provided evidence for the formation of a covalent linkage between the alkyne-modified surface and the organic azide. The ability to employ different organic azides was demonstrated by the use of an ionic alkyl azide dye bearing a wide variety of functional groups. The broad utility of this strategy for the modular functionalization of metal oxide surfaces was demonstrated by its application in the modification of Fe2O3, TiO2, and WO3 nanoparticles.

WO3 surface, inhibiting the formation of the inner-sphere metal carboxylate bonds observed on the other metal oxide surfaces. Finally, the exact position of the free acid stretch on all four metal oxide surface shifts over a range of 24 cm−1, a feature we attribute to varying levels of hydrogen bonding of the free acid. In addition, we have subjected the alkyne-modified Fe2O3, TiO2, and WO3 nanoparticles to full CuAAC conditions as well as copper-free CuAAC conditions using CF3-bearing aryl azide 4. Figure 8 shows the fluorine (1s) region of the XPS spectra of

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MATERIALS AND METHODS

Materials. Dichloromethane, diethyl ether (unstabilized), and tetrahydrofuran (unstabilized) were purified by passage through a column of activated alumina under N2 pressure.45 Butvar B-98 resin, terpineol (anhydrous, mixture of isomers), propargyl alcohol, NaH, 4(trifluoromethoxy)aniline, CaH2, NaNO2, NaN3, concentrated aqueous HCl, Na2CO3 (anhydrous), 4-(trifluoromethoxy)phenylacetic acid, anhydrous citric acid, sodium L-ascorbate, Cu(BF4)·xH2O, and ethyl cellulose 46080 and 46070 were obtained from Sigma-Aldrich; anhydrous Na2SO4 was obtained from EM Science; anhydrous MgSO4 and CuSO4·5H2O were obtained from Mallinckrodt; trimethyl citrate, ethyl acetate, MeCN, MeOH, and hexanes were obtained from Fisher Scientific; silica gel (SiliaFlash P60, 40−62 μm, 230−400 mesh) was obtained from Silicycle; ZnO nanoparticles (20 nm average particle size) and WO3 nanoparticles (30−70 nm average particle size) were obtained from Nanostructured & Amorphous Materials; Fe2O3 nanoparticles (alpha, 30 nm average particle size) were obtained from MTI Corp.; EtOH (100%) was obtained from Decon Laboratories; glass slides (2 mm thick) with fluorine-doped SnO2 (300 nm thick) on the top surface (FTO) were obtained from Hartford Glass; boron-doped silicon was obtained from Addison Engineering, Inc.; CuOAc was obtained from Strem Chemicals and was stored in an inert-atmosphere glovebox; Chromeo 546 azide dye 5 was obtained from Active Motif; these materials were used without further purification. Trifluoromethanesulfonic anhydride was obtained from Sigma-Aldrich and distilled prior to use. Pyridine was obtained from Sigma-Aldrich and distilled from CaH2 prior to use. Water was distilled and passed through a Barnsted Nanopure Infinity purification system prior to use. Tris(3-hydroxypropyltriazolylmethyl)amine was synthesized from 3-azido-propan-1-ol and tripropargylamine according to literature procedures.42 Measurement. 1H NMR and 13C NMR spectra were recorded on Varian Mercury Plus-300 (300 MHz) or Varian Unity-500 (500 MHz) spectrometers as indicated. Chemical shifts (δ) are reported in parts per million (ppm) relative to residual protiated solvent (CDCl3 = 7.24 ppm [1H], 77.23 ppm [13C]; CD3OD = 3.31 ppm [1H], 49.00 ppm [13C]). NMR data are reported in the following format: s = singlet, d = doublet, t = triplet, q = quartet; coupling constant; integration. Mass spectrometry data were recorded at the Mass Spectrometry Facility of the Chemistry Instrument Center of the University of WisconsinMadison. Electrospray ionization (ES) time-of-flight (TOF) mass spectra were recorded on a Waters (Micromass) LCT spectrometer with a sample cone voltage of 20 V. Electron impact (EI) mass spectra were recorded on a Waters (Micromass) Autospec spectrometer by inserting the sample on a piece of glass capillary into the source, and the resulting vapors are bombarded with 70 eV electrons; perfluorokerosene is used for calibration. IR spectra for organic chemicals were collected on a Bruker Vertex 70 FTIR spectrometer in transmission mode using NaCl plates. IR reflection−absorption spectra of metal oxide thin films were collected on the same spectrometer using a VeeMaxII variable angle specular reflectance accessory with a wire grid polarizer. All spectra were collected with a resolution of 4 cm−1 with p-polarized light and an

Figure 8. Fluorine (1s) region of XPS spectra of the alkyne-modified WO3 (bottom 2), Fe2O3 (middle 2), and TiO2 (top 2) nanoparticles after being subjected to CuAAC conditions (upper spectra) and those of identical particles after being subjected to copper-free CuAAC conditions (lower spectra). Spectra were offset vertically for clarity.

the resulting nanoparticles. The presence of fluorine atoms on the nanoparticles exposed to full CuAAC conditions and their absence on nanoparticles exposed to copper-free CuAAC conditions confirms the successful covalent bond formation with all three alkyne-modified nanoparticles in the presence of the copper catalyst. Minor amounts of copper were observed by XPS on the Fe2O3, TiO2, and WO3 surfaces. IR analysis of the same nanoparticles provides further confirmation of successful CuAAC reaction, as peaks diagnostic of the CF3 group of covalently surface-bound 4 are observed in the spectra of the experimental but not control samples (Figure S5, Supporting Information). These observations are consistent with those of the analogous ZnO experiments (see section above). From measurement of the XPS peak intensities of the fluorine (1s) and metal atoms, the molecular coverages on the different oxides can be roughly estimated, yielding coverages of ∼8 × 1014 molecules/cm2 on ZnO and Fe2O3, ∼3 × 1014 molecules/cm2 on TiO2, and ∼5 × 1013 molecules/cm2 on WO3. Because determining absolute coverages from XPS data requires knowledge of the inelastic mean free paths of electrons within the oxide that are often poorly known and can be affected by the size, shape, and surface composition of the particles, these values should be used as only a rough approximation to the true coverages. Nevertheless, the values are consistent with high molecular coverages on ZnO, Fe2O3, and TiO2 and somewhat lower coverage on WO3. This is in agreement with the FTIR measurements where binding of the ligand appears to be weaker than on the other oxides investigated. 1326


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The reaction mixture was heated to reflux for 16 h and concentrated in vacuo to yield the product as a brown oil (648 mg, >99% yield). 1H NMR (300 MHz, CD3OD: δ 4.31 (d, J = 2.7 Hz, 2H), 3.07 (AB q, ΔνAB = 24.1 Hz, JAB = 16.0 Hz, 4H) 2.78 (t, J = 2.4 Hz, 1H). 13C NMR (125 MHz, CD3OD): δ 173.7, 173.2, 80.7, 79.8, 75.5, 53.9, 470.3. IR: 3287, 2940, 2125, 1721, 1408, 1211, 1087, 1028, 929 cm−1. HRMS (TOF MS ES+) exact mass calcd for C9H10NaO7 [M + Na]+: 253.0324; found 253.0320. 4-Azido-1-trifluoromethoxybenzene (4). A round-bottom flask was charged with 4-(trifluoromethoxy)aniline (1.30 g, 7.34 mmol), H2O (32 mL), and hydrochloric acid (1.25 mL, 37% in H2O, 15.2 mmol). The reaction mixture was then cooled to 0 °C after which NaNO2 (8 mL, 0.92 M in H2O, 7.36 mmol) was added, and the resulting yellow solution was stirred a further 10 min. A solution of NaN3 (8 mL, 1.10 M in H2O) was then added dropwise with concomitant gas evolution. The reaction mixture was stirred for 45 min, during which a yellow oil separated out of solution. After warming to room temperature and stirring for 1 h further, the reaction mixture was extracted with ethyl acetate (3 × 20 mL). The organic extracts were washed with brine, dried over MgSO4, filtered, and concentrated in vacuo to yield product (1.36 g, 91%) as a brown oil that was used without further purification. 1 H NMR (300 MHz, CDCl3): δ 7.19 (d, J = 8.7 Hz, 2H), 7.02 (d, J = 8.7 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 146.2 (apparent d, J = 1.8 Hz), 139.0, 122.9 (apparent t, J = 1.5 Hz), 120.7 (q, J = 256 Hz), 120.4. IR: 2426, 2114 (strong), 1608, 1502 (strong), 1259 (strong), 1207 (strong), 1167 (strong), 1130, 1109, 849, 837, 825 cm−1. HRMS (EI+) exact mass calcd for C7H4F3NO [M−N2]+: 175.0245; found 175.0240. Nanoparticle Preparation, Functionalization, and Ligand Binding Studies. ZnO and Fe2O3 Nanoparticulate Thin-Film Preparation. Films were prepared in a procedure analogous to published methods.48,49 A centrifuge tube was charged with ZnO or Fe2O3 nanoparticles (2.00 g), Butvar B-98 resin (10.0 g, 10 wt % in EtOH), terpineol (8.00 g), and EtOH (10 mL). The resulting mixture was mixed on a vortex mixer for 10 s and subjected to sonication for 1 h using a Sonics Vibra-Cell probe sonicator. Sonication took place in on−off cycles to avoid excessive heat buildup, typically 30 s on and 10−30 s off. The resulting paste was transferred to a round-bottom flask and concentrated on a rotary evaporator at 20 mbar using a bath temperature of 80 °C. The resulting thickened paste was printed in 5 mm diameter circles onto FTO slides (for ZnO nanoparticles) or boron-doped silicon(111) wafers (for Fe2O3 nanoparticles) using a Systematic Automation MC-1 screen printer. After printing, the films were annealed for 5 min at 120 °C and then 30−90 min at 500 °C. TiO2 Nanoparticulate Thin-Film Preparation. Films were prepared as described previously,46 with the exception that only two layers of nanocrystalline TiO2 paste were applied to a clean FTO slide that had not been treated with TiCl4. WO3 Nanoparticulate Thin-Film Preparation. A centrifuge tube was charged with WO3 nanoparticles (2.85 g), ethyl cellulose 46080 (6.00 g, 10 wt % in EtOH), ethyl cellulose 46070 (500 mg, 10 wt % in EtOH), terpineol (6.00 g), and EtOH (8 mL). The mixture was warmed to 40 °C, mixed on a vortex mixer for 30 s, and subjected to sonication for 2 h using a Sonics Vibra-Cell probe sonicator. Sonication took place in on−off cycles to avoid excessive heat buildup, typically 30 s on and 10−30 s off. The resulting paste was transferred to an Erlenmeyer flask and concentration on a rotary evaporator. The thickened paste was printed in 5/8 in. diameter circles on boron-doped silicon(100) slides using a Systematic Automation MC-1 screen printer. The films were heated for 5 min at 120 °C, after which they were subjected to a second printing pass. The films were then annealed for 15 h at 300 °C. Adsorption of O-Propargylcitric Acid (2), 4-(Trifluoromethoxy)phenylacetic Acid (3), or Citric Acid to Metal Oxide Nanoparticles. The FTO- or silicon-supported metal oxide nanoparticulate thin film circle was heated to 150 °C for 10 min on a hot plate, after which it was removed from the hot plate and placed immediately into a solution of O-propargylcitric acid (2), 4-(trifluoromethoxy)phenylacetic acid (3), or anhydrous citric acid (2.5 mL, 10 mM in MeCN). After 12 h (for ZnO, Fe2O3, and TiO2) or 5 h (for WO3) the

incident illumination angle of 50° off normal. The background used in calculating the absorbance was the single beam spectrum of the relevant bare film prior to functionalization. Sloping baselines were removed by subtracting polynomial splines to improve the clarity of the spectra. Absorption bands were integrated by fitting the data to Voigt functions. X-ray photoelectron spectroscopy (XPS) was performed using an Al Kα source with a quartz crystal monochromator (nominally 1486.6 eV photon energy) with an analyzer resolution between 0.1 and 0.2 eV and an electron takeoff angle of 45°. All spectra shown were baselinecorrected. The same number of sweeps were acquired for the experimental and control samples of each metal oxide substrate. Approximate molecular coverages were calculated using the fluorineto-metal atomic ratio in analogy to published reports.46 Scanning electron microscopy (SEM) of the films was carried out using a Leo Supra 55VP FE-SEM microscope. All images were taken with a 4 or 5 keV electron accelerating voltage using the chamber Everhart-Thornley secondary electron detector. SEM imaging was performed in both top-down and cross-sectional geometries. For the latter, samples were scored on the reverse side and broken through the center of the printed dots to expose the film cross sections. Samples were mounted on conductive carbon tape or in copper clips with a top contact to provide grounding to the electrically conductive FTO layer on the upper surface. UV/vis spectroscopy measurements in transmission mode were performed on a Shimadzu UV-2401PC spectrometer, and those in diffuse reflectance mode were performed on a Jasco V-570 UV/vis/ NIR spectrometer with an ISN-470 integrating sphere attachment using a sample of Teflon for the background spectra. Organic Synthesis. Propargyl Trifluoromethanesulfonate. This material was synthesized in a procedure analogous to that developed by Vedejs et al.,47 with the following changes. Upon reaction completion the reaction mixture was filtered through a plug of Celite, and the filtrate was concentrated in vacuo. The concentrated filtrate was extracted with hexanes and filtered through a plug of Celite and anhydrous Na2SO4 (∼1:1), and the filtrate was concentrated in vacuo and used without further purification. O-Propargyltrimethyl Citrate (1). This material was synthesized in a procedure analogous to that developed by Meissner and Jutzi.30 A nitrogen-purged flame-dried round-bottom flask submerged in a water bath at room temperature was charged with NaH (1.67 g, 60% dispersion in mineral oil, 41.8 mmol) and Et2O (20 mL). To this stirred suspension trimethyl citrate (10.3 g, 43.9 mmol) was added slowly in three portions over the course of 5 min with hydrogen gas evolution, after which the reaction mixture was stirred for 20 min. Crude propargyl trifluoromethanesulfonate (13.6 g, 48.2 mmol) in a nitrogen-purged round-bottom flask was dissolved in Et2O (45 mL) and added over the course of 2 min to the reaction mixture. The flask that had contained the propargyl trifluoromethanesulfonate was rinsed with Et2O (15 mL), which was then added to the reaction mixture. A thick, darkly colored semisolid separated out of solution and further solidified while the reaction mixture was stirred for 14 h, after which the supernatant was decanted. The solid was extracted twice with Et2O (15 mL each). The solid was then dissolved in HCl (20 mL, 0.5 M in H2O), and the resulting solution was extracted twice with Et2O (15 mL each). The supernatant and organic extracts were combined and washed with HCl (5 mL, 0.5 M in H2O) and then brine, dried over anhydrous MgSO4, filtered, concentrated in vacuo, and purified by silica gel chromatography (33% ethyl acetate in hexanes) to yield the product as a yellow oil (3.41 g, 29% yield). 1H NMR (300 MHz, CDCl3): δ 4.25 (d, J = 2.4 Hz, 2H), 3.75 (s, 3H), 3.65 (s, 6H), 3.08 (AB q, ΔνAB = 31.3 Hz, JAB = 15.9 Hz, 4H), 2.38 (t, J = 2.4 Hz, 1H). 13 C NMR (75 MHz, CDCl3): δ 170.9, 170.1, 79.7, 78.8, 74.6, 53.5, 52.9, 52.1, 39.5. IR: 3278, 3003, 2956, 1734 (strong), 1438, 1360, 1286, 1200, 1173, 1065, 1028, 1002 cm−1. HRMS (TOF MS ES+) exact mass calcd for C12H16NaO7 [M + Na]+: 295.0794; found 295.0799. O-Propargylcitric Acid (2). A round-bottom flask equipped with a reflux condenser was charged with trimethyl O-propargyltrimethyl citrate (1) (767 mg, 2.82 mmol) and HCl (56 mL, 2.4 M in H2O). 1327


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TiO2, and WO3 subjected to full CuAAC conditions and copper-free CuAAC conditions can be found in Figure S5.

film was removed and gently rinsed under a stream of MeCN, yielding the modified metal oxide nanoparticulate thin film. Diagnostic IR absorption bands used for determining binding of the ligands to the ZnO surface were observed centered from/at the following values: 1736−1714, 1620−1580, and 1440−1410 cm−1 for 2 on ZnO; 1570, 1510, 1403, 1274, 1230, and 1188 cm−1 for 3 on ZnO; 1725−1695, 1610−1565, and 1430−1390 cm−1 for citric acid on ZnO. Representative IR spectra of 2 and 3 bound to the ZnO nanoparticulate thin film can be found in Figures S1 and S2, respectively (Supporting Information). Diagnostic IR absorption bands used for determining binding of 2 to the Fe2O3 surface were observed centered from/at the following values: 1722−1700, 1614− 1525, 1441−1349 cm−1. Diagnostic IR absorption bands used for determining binding of 2 to the TiO2 surface were observed centered from/at the following values: 1743−1724, 1662−1567, and 1471− 1363 cm−1. Diagnostic IR absorption bands used for determining binding of 2 to the WO3 surface were observed centered from/at the following values: 1736, 1604−1541, and 1458−1362 cm−1. Representative IR spectra of 2 bound to the Fe2O3, TiO2, and WO3 nanoparticulate thin films can be found in Figure 7. Ligand Surface-Binding Stability Studies. The relevant FTO slide containing the alkyne- or 3-modified ZnO nanoparticulate thin film was placed in a 24 mL vial containing MeCN (20 mL) at 80 °C for a measured amount of time. The FTO slide was then removed from the vial and gently rinsed under a stream of MeCN, after which an IR spectrum of the ZnO film was recorded. This procedure was then repeated, replacing the FTO slide in the same 80 °C MeCN solution as before. The absorption bands integrated were centered from 1736 to 1714, 1620−1580, and 1440−1410 cm−1 for the alkyne-modified surface and at 1274, 1230, and 1188 cm−1 for the 3-modified surface. Representative IR spectra for these experiments can be found in Figure 2. CuAAC Reaction on Alkyne-Modified ZnO Nanoparticles. In an inert-atmosphere glovebox, an FTO slide containing the alkynemodified ZnO nanoparticulate thin film was placed in a 100 mL vial equipped with a magnetic stir bar. The vial was then charged with tris(3-hydroxypropyltriazolylmethyl)amine (6.5 mg, 0.015 mmol) and CuOAc (1.8 mg, 0.015 mmol), followed by a solution of the relevant azide (0.10 mL, 250 mM in tetrahydrofuran) and tetrahydrofuran (2.4 mL). The resulting mixture was then stirred gently for 24 h, taking care to place the stir bar on the opposite side of the vial as the FTO slide. Upon reaction completion, the vials were removed from the glovebox, and the FTO slides were removed and rinsed gently under streams of MeCN in H2O (50% v/v), MeOH, and tetrahydrofuran. Diagnostic IR absorption bands used for determining success of the CuAAC reaction with azide 4 were observed at 1263, 1229, and 1159 cm−1, as can be seen in Figure 3 and Figure S3. Diagnostic UV/vis absorption peaks used for determining success of the CuAAC reaction with 5 were observed at 524 and 559 nm, as can be seen in Figure 6. CuAAC Reaction on Alkyne-Modified Fe2O3, TiO2, and WO3 Nanoparticles. The FTO- or silicon-supported alkyne-modified metal oxide nanoparticulate thin film was placed in a 100 mL vial equipped with a magnetic stir bar. The vial was then charged with H2O (0.95 mL), MeOH (0.95 mL), a solution of 4-azido-1-trifluoromethoxybenzene (0.10 mL, 250 mM in MeOH), a solution of tris(benzyltriazolylmethyl)amine and CuSO4·5H2O (for Fe2O3 and TiO2) or Cu(BF4)2·xH2O (for WO3) (0.40 mL, 4.31 mM copper, 8.62 mM amine, in 50% v/v MeOH in H2O), and a solution of sodium Lascorbate (0.10 mL, 108 mM in H2O). The resulting mixture was then stirred gently for 24 h (for Fe2O3, and TiO2) or 17 h (for WO3), taking care to place the stir bar on the opposite side of the vial as the sample. Upon reaction completion the FTO slides were removed and rinsed gently under streams of H2O, MeOH in H2O (50% v/v), and MeOH. Diagnostic IR absorption bands used for determining success of the CuAAC reaction with azide 4 on alkyne-modified Fe2O3 were observed at 1497, 1455, 1259, and 1221 cm−1. Those for alkynemodified TiO2 were observed at 1498, 1456, 1261, and 1225 cm−1. Those for alkyne-modified WO3 were observed at 1497, 1456, 1257, and 1223 cm−1. Representative IR spectra of alkyne-modified Fe2O3,

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ASSOCIATED CONTENT

* Supporting Information S

IR spectra of ZnO nanoparticulate thin film after exposure to 2 and subsequent reaction with 4 as well as that of the corresponding no-Cu control; IR spectra of alkyne-modified Fe2O3, TiO2, and WO3 nanoparticulate thin films after reaction with 4 and those of the corresponding no-Cu controls; NMR spectra of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS L.M.B. thanks Ryan Franking and Kacie Louis for helpful discussions. This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences Contract DE-FG0209ER16122. Tests of the environmental stability of the functionalized nanoparticles were supported by the National Science Foundation Grant DMR-0832760. Dr. Martha M. Vestling is acknowledged for recording mass spectral data.

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REFERENCES

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A Citric Acid-Derived Ligand for Modular Functionalization of Metal

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