Self-Assembled Monolayers Based Upon a Zirconium Phosphate

Article pubs.acs.org/cm

Self-Assembled Monolayers Based Upon a Zirconium Phosphate Platform§ Agustín Díaz,† Brian M. Mosby,† Vladimir I. Bakhmutov,† Angel A. Martí,‡ James D. Batteas,† and Abraham Clearfield*,† †

Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States Department of Chemistry and Department of Bioengineering, Rice University, 6100 S. Main Street, Houston, Texas 77005, United States

‡

S Supporting Information *

ABSTRACT: Organically surface-modified α-zirconium phosphate was obtained by reacting the surface P−O−H groups of α-zirconium phosphate nanoparticles (α-ZrP) with octadecyltrichlorosilane (OTS). Surface functionalization of α-ZrP with OTS was accomplished using a one-step synthesis producing highly hydrophobic nanoparticles. The formation of P−O−Si bonds arising from nucleophilic attack of POH to the silane was confirmed by solid-state NMR experiments. The surface coverage of the organic modifier was characterized by TGA, AFM, and FTIR. In addition, we show the applicability of this system with a photoinduced electron-transfer reaction in a nonpolar solvent. Using an organically surface-modified α-ZrP previously loaded with tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+), the quenching of the luminescence of Ru(bpy)32+ in the presence of pbenzoquinone was monitored; a static quenching constant (Ks) value of 8.82 × 102 M−1 and a dynamic quenching constant (KD) value of 6.99 × 102 M−1 were obtained. KEYWORDS: surface chemistry, surface functionalization, layered compounds, silanes, tetravalent phosphonates, photoinduced electron transfer

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paper, we are exploring the surface chemistry of the α phase of ZrP (α-ZrP). ZrP surfaces may be functionalized to obtain SAMs that are different in structure from the well-known varieties. The surface modification of ZrP is based on the reaction of the hydroxyphosphate on the surface of ZrP with silanes. During the last 50 years, extensive research has evolved around the chemistry of ZrP materials.15−20 The tunability of their size, morphology, and aspect ratios has opened a wide gamut of applications,21 such as biosensors,19 fuel cells,22,23 catalysis,24 photoinduced charge separation,25,26 Pickering emulsions,27 and drug delivery,15,16,28 among many others. These applications can be greatly improved if the surface of the ZrP particle could be designed to fulfill a specific purpose. For example, modifying the surface of ZrP-based catalysts with specific groups can enhance their solubility in organic solvents, maximizing the reactant−catalyst interaction, and, therefore, the reaction effectiveness of the material in any solvent. On the other hand, ZrP materials have an important role in polymeric nanocomposites. ZrP’s serve as nanofillers in the polymeric interface where their presence profoundly affects the chemical, mechanical, and thermal properties of the nanocomposite.29−38 Each of these properties can be tuned if the surface of the ZrP

INTRODUCTION Self-assembled monolayers (SAMs) have been around for more than 30 years with works such as those proposed by Sagiv.1 Silanes were found to react with wet glass through the hydroxyl groups on the surface. Since then, a plethora of SAMs have been produced on silica and silicon surfaces.2−8 In addition, SAMs may be functionalized by surface reactions or by prefunctionalization prior to preparing the monolayer. As a result, a wide range of applications for SAMs have emerged. The functionalization of silica and silicon surfaces allows changes to the surface properties of the obtained materials, such as friction, wettability, and adhesion, among others.9 Furthermore, the end groups of the silanol may be changed to suit many applications, such as chemical sensors, biosensors, microelectronics, thin-film technology, cell adhesion photolithography, and a variety of important protective coatings, composites, and chemical reactions.10 A second type of SAM involves thiols interacting with metal surfaces, such as gold, silver, or copper. As with silanes, there has been a similar outpouring of studies.3,11 Zirconium phosphates (ZrP’s) are acidic, inorganic cationexchange materials and usually present a layered structure. There are various phases of zirconium phosphate that differ in their interlamellar spaces and their crystalline structure. Among all the ZrP phases, the most widely used are the α phase (Zr(O3POH)·H2O) and the γ phase, whose crystal structures were elucidated by Clearfield and co-workers.12−14 In this © 2013 American Chemical Society

Received: November 7, 2012 Revised: February 5, 2013 Published: February 9, 2013 723

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nanoparticle is modified with specific molecules to maximize, or minimize, certain interactions within the polymeric interface. Moreover, the bioapplicability of ZrP will be profoundly impacted by the surface modification of the loaded nanoparticles. Properties, such as the mucous adhesion of the particle to different organs, could be tuned with different surface modifiers in order to improve the particle−organ interaction, maximizing the absorption of the loaded drug into the specific target. Furthermore, the potential of ZrP as a drug carrier for cancer treatment can be significantly improved if the surface of the drug-loaded particle is modified with specific tumor vectors to target cancerous cells, while avoiding healthy tissues, reducing some of the side effects associated with chemotherapy. Schematic representations of α-ZrP are provided in Scheme 1. The layers are composed of phosphate tetrahedra enclosing

with OTS and performed a photoinduced electron-transfer reaction using benzoquinone (BQ) as the electron acceptor in a nonpolar solvent.

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Synthesis. The α-ZrP nanoparticles were synthesized following the reflux method reported by Sun and co-workers.21 The typical procedure consists of the dropwise addition of 200 mL of a 0.05 M ZrOCl2·8H2O aqueous solution to a 200 mL solution of H3PO4 (6 M). The phosphoric acid solution was preheated in an oil bath at 94 °C in a 500 mL round-bottom flask before the addition of the zirconyl chloride. The resulting solution was refluxed with constant stirring at 94 °C for 2 days. The product was filtered and washed several times with water and dried in an oven at 70 °C. The dried α-ZrP was ground with a mortar and pestle into fine powders. This material, as characterized by XRPD, showed an intense peak at low angles (2θ = 11.6°) corresponding to a distance of 7.6 Å. The surface modification of the α-ZrP nanoparticles was carried out by suspending the nanoparticles in hot toluene at 100 °C for 1 h under constant stirring and switching between vacuum and N2 atmosphere in a Schlenk line, to eliminate all the water from the surface of the α-ZrP. A solution of octadecyltrichlorosilane (OTS) dissolved in toluene was then added (1:10 OTS/ZrP molar ratio) and reacted with the “dry” αZrP suspended in the toluene for 12 h. The product was washed several times by centrifugation with dry hexanes, and then with a mixture of hexanes and ethanol. The final product (OTS/α-ZrP) was dried at 70 °C for 24 h. The intercalation of tris(2,2′-bipyridyl)ruthenium(II) into α-ZrP is reported elsewhere.11,24 The surface functionalization of Ru(bpy)32+@ ZrP with OTS was carried out following exactly the same procedure used for the pristine α-ZrP (vide supra). Chemicals and Materials. All chemicals were purchased from Sigma-Aldrich Chemical Co. and used as received with the exception of phosphoric acid, 85% (v/v) (H3PO4), which was purchased from Fisher and the octadecyltrichlorosilane, which was purchased from Gelest, Inc. Instrumentation. The complete characterization of the materials was performed using several analytical methods. XRPD experiments were performed from 2 to 40° (2θ angle) using a Siemens D8 X-ray diffractometer system with a copper anode source (Kα1, λ = 1.5406 Å) with a filtered flat LiF secondary beam monochromator. The divergence, receiver, and detector slit widths were 2 mm; the scatter slit width was 0.6 mm. The interlayer distances were determined using the Bragg’s Law for the (002) diffraction plane of the diffraction pattern for α-ZrP. Thermogravimetry experiments were carried out on a TGA Q500 TA Instrument. The temperature was ramped at 5 °C min−1 under a flow of N2 up to 1000 °C. The first weight loss (below 110 °C) was attributed to absorbed water on the surface. The following weight losses were assigned knowing the thermodecomposition of the material. The 29Si{1H}, 31P{1H}, 13C{1H}, and 1H MAS NMR experiments were performed with a Bruker Avance-400 spectrometer (400 MHz for 1H) using a standard 4 mm MAS probe head. Standard one pulse (direct nuclear excitation) and/or CP pulse sequences were applied in these experiments at relaxation delays necessary for a quantitative analysis of the spectra. The MAS NMR spectra have been recorded at variation in spinning rates between 5 and 14 kHz. The external standards used for 29Si, 1H, 13C, and 31P NMR experiments were TMS and H3PO4 solutions, respectively. The 1 H T1 measurements were performed with a standard inversion− recovery pulse sequence where RF pulses were carefully calibrated. The 1H relaxation data were treated with a standard program. UV− visible spectra were collected using a Shimadzu UV−vis spectrophotometer (model No. UV- 2450) in quartz cuvettes with a 10 mm path length. Photoluminescence characterization was done using a photoluminescence spectrometer (HORIBA Jobin Yvon Fluorolog 3) with a xenon lamp as the source of excitation. The spectra obtained were not corrected for detector or instrumental deficiencies. Timeresolved photoluminescence (TRPL) was recorded using a timecorrelated single-photon counting technique with an Edinburgh

Scheme 1. Schematic Representation of α-ZrP Viewed along the b Axis Showing Its Unit Cell and the Formed Layered Structure (A) and along the c Axis (B) Showing the Surface of the Layersa

a

EXPERIMENTAL SECTION

The hydrogen atoms were omitted for clarity.

ZrO6 octahedra (Scheme 1A). Three of the phosphate oxygen atoms bond to three Zr ions, leaving P−OH groups protruding into the interlayer space. Scheme 1B is a representation of the α-ZrP surface. Early use of the silanes to modify the structure of α-ZrP was directed toward obtaining porous pillared products.39,40 The silanes utilized were either triethoxy- or methoxyaminopropyl silanes where the amino group allowed direct intercalation of the silane between the layers. In this case, there is a direct competition between the grafting on the surface and the intercalation of the silane through an acid−base reaction between the amino group and the phosphate on the layers. Extensive NMR studies showed that hydrolysis of the ethoxy groups to hydroxyl groups occurred, accompanied by polymerization of the silane. Roziere et al.39 also attempted prepolymerizing the silane into octa(3-aminopropylsilasquioxane) before incorporation into the ZrP, but neither effort produced a porous product. Our own research41 showed that a porous product could be achieved (∼200 m2/g) if the αZrP is first exfoliated and then allowed to encapsulate the added silane. Similar results were obtained by Takei et al.,42 who spread the layers of α-ZrP apart by intercalating octylamine and then adding 1,2-bis(dimethyl-chlorosilyl)ethane, dichloromethylvinylsilane, and dichlorodimethylsilane in toluene. The product contained both silanes and undisplaced octylamine. Here, we present the surface modification of α-ZrP nanoparticles with octadecyltrichlorosilane (OTS). In addition, we surface-functionalized ZrP nanoparticles fully loaded with tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+@ZrP)26,43,44 724

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Instruments single-photon counting spectrometer (model No. OD470) equipped with a 443.6 nm picosecond laser diode. UV−vis spectrophotometric and steady-state luminescence measurements were performed suspending a determined amount of the probe-exchanged ZrP in water to make a 0.008% (w/v) suspension. The transmission electron micrographs (TEM) of the samples were acquired using a JEOL 2010 transmission electron microscope at an acceleration voltage of 200 kV. Samples were prepared using copper grids from Ted Pella. Scanning electron microscopy (SEM) images were acquired on a JEOL JSM-7500F (FE-SEM). The atomic force microscopy (AFM) images for the ZrP nanoplatelets were taken with an Agilent 5500, in contact mode. The ZrP nonmodified nanoplatelets were deposited on a Si-APTES (3-aminopropyltriethoxysilane) substrate by immersion of the freshly cleaned Si-APTES wafer in an ethanol suspension of the nanoparticles (0.008% w/v) for 6 h. The ZrP surface-modified nanoplatelets were deposited on a Si-OTS (octadecyltrichlorosilane) substrate by immersion of the freshly cleaned Si-OTS wafer in a hexane suspension of the nanoparticles (0.008% w/v) for 6 h. The FTIR were performed using a Bruker Tensor 27 spectrometer in an ATR (attenuated total reflection) mode with a diamond ATR Prism Model Helios. XPS data were acquired with a Kratos Axis ULTRA X-ray photoelectron spectrometer equipped with a 165 mm hemispherical electron energy analyzer. The incident radiation was the monochromated Al Kα X-ray line (1486.7 eV) with a source power of 120 W (12 kV, 10 mA). Survey scans of up to 1400 eV were carried out at an analyzer pass energy of 160 eV with 1.0 eV steps and a dwell time of 300 ms. Multiplexed high-resolution scans for Zr(3d), P(2p), Si(2p), O(1s), and C(1s) were taken at a pass energy of 40 eV with 0.1 eV steps and a dwell time of 150 ms. The survey and high-resolution spectra were obtained with averages of five scans. The C(1s) peak at 284.8 eV was set as a reference for all XPS peak positions to compensate for energy shifts due to the spectrometer work function.

for the nanoplatelets (Figure 1C,D). On the other hand, the AFM friction images show a marked contrast between both nanoparticles, showing the change of polarity on the surface of the materials (Figure SI-1, Supporting Information). This phenomenon can be also noted in the topographical images shown in Figure 1C,D, where the topographical features of the polar nanoplatelet and surface are clearly highlighted by the polar tip (SiO2), whereas these features are missing for the nonpolar nanoparticle over the nonpolar surface. Figure 2A shows the FTIR of α-ZrP before and after the surface modification (OTS/α-ZrP). α-ZrP has two character-

Figure 2. (A) FTIR of α-ZrP before (red) and after (blue) the surface modification with OTS. (B) TGA of α-ZrP before and after the surface modification with OTS.

istic IR bands that correspond to the splitting of the crystallization water band at 3592 and 3509 cm −1 , ν as (OH),45 which remains in the spectrum of the surface-modified material. This confirms the XRD results showing that there was no intercalation or displacement of the intercalated water molecule. In addition, the FTIR spectrum of OTS/α-ZrP shows the strong characteristic bands associated with the asymmetric and symmetric stretching of the C−H, between 2900 and 3000 cm−1 and bending at ca. 1450 cm−1. The presence of the silane on the surface-modified material was also confirmed by XPS (Figure SI-2, Supporting Information). The TGA of OTS/α-ZrP shows an increase in weight loss compared to the pristine α-ZrP (Figure 2B). The weight loss for α-ZrP is ca. 12%, where two water molecules are vaporized per mole of α-ZrP. The first weight loss comes from the intercalated water molecules, at ca. 120 °C, and the second from the condensation of the phosphate at ca. 540 °C, producing ZrP2O7 as the final pyrolysis product. The TGA for the OTS/α-ZrP shows a 16.6% weight loss, where two new weight losses, in addition to the typical weight losses for α-ZrP, can be appreciated. The first weight loss for the OTS/α-ZrP material is ca. 2% at 78 °C and is attributed to the intercalation of ethanol within the aliphatic chains on the surface-modified nanoplatelets during the cleaning procedure. The second weight loss takes place at 135 °C, with a ca. 3% of weight loss, corresponding to the vaporization of the intercalated water molecule. It is important to note the shift of this specific weight loss, by 15 °C, consistent with the encapsulation of the nanoplatelets by the aliphatic groups, making it harder for the water molecules to escape from the interlayer region. The third weight loss takes place at 300 °C with a weight loss of ca. 5% due to the thermodecomposition of the aliphatic chain on the surface of ZrP. The final weight loss takes place at 525 °C and is mainly attributed to the condensation of the phosphate groups of ZrP. This last weight loss is decreased by 15 °C when compared to that of the pristine α-ZrP, probably because of the pyrolysis of the aliphatic chains causing damage to the crystal

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RESULTS AND DISCUSSION Figure 1A shows the X-ray powder diffraction (XRD) patterns for the pristine α-ZrP and the surface-modified material. The

Figure 1. (A) X-ray powder diffraction of α-ZrP before (red) and after (blue) the surface modification. (B) SEM image of the surfacemodified nanoplatelets (OTS/α-ZrP). (C) AFM image of an α-ZrP nanoplatelet over an APTES surface-modified SiO2 wafer. (D) AFM image of a surface-modified OTS/α-ZrP nanoplatelet over an OTS surface-modified SiO2 wafer.

XRD patterns show no significant difference between the two materials, which eliminates the possibility of the intercalation of OTS into α-ZrP due to the lack of diffraction peaks at lower angles. Both materials show the characteristic diffraction peak at 7.6 Å for the 002 planes of α-ZrP. Moreover, SEM and AFM of both materials show similar nanocrystal structures and morphologies between them (Figure 1B−D). Nevertheless, the AFM topography images showed a slight change in height 725

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nonexponential process. At the same time, the relaxation curves are well-described with a stretched exponential47

structure of the ZrP particles, allowing a faster thermodecomposition. Figure 3A shows the single-pulse magic-angle spinning (MAS) solid-state 1H NMR spectrum (top) recorded for the

β

I = I0(1 − 2e−(τ / T1) )

(1)

where the β parameter takes the values between 0.72 and 0.62. Such a relaxation is often observed in solids when 1H relaxation is not dominated by spin diffusion to paramagnetic centers.48 Under these conditions, the different 1H resonances show different T1 times. For the sharp 1H resonances, the T1 times are determined as 0.62−0.64 s versus 1.0 and 0.79 s obtained for P−OH and water signals, respectively. It is remarkable that similar T1 values have been reported for a water resonance in crystalline zirconium phosphates47,48 where the 1H T1 data have been discussed in terms of motional modulation of interproton dipole−dipole interactions due to H2O 180° flips, for example. The Si−O−Alk groups in the ZrP sample show the resonances at 30.3, 28.1, 27.5, 20.9, 15.9, and 9.9 ppm in the 13 C{1H} CP MAS NMR spectrum recorded at a spinning rate of 10 kHz (Figure 3B). The lines are broadened as it is generally observed in amorphous solids.48 At the same time, the two sharp lines with values of 60.1 and 14.3 ppm can be attributed to CH3CH2OH, in accordance with the 1H MAS NMR spectrum and TGA results. The 31P MAS NMR experiments, performed for the ZrP sample spinning at various rates with the single-pulse protondecoupled and cross-polarization pulse sequences, have provided reliable signal assignments. Figure 3C shows the 31 1 P{ H} MAS NMR spectrum where intense lines at −17.3, −19.0, and −20.8 ppm can be well-attributed to the HPO4− moieties in Zr(HPO4)2 layers.46,49,50 A ratio for these moieties with different phosphorus environments is calculated after a deconvolution procedure as 1/5.1/1.9, respectively. In addition, the 31P{1H} MAS NMR spectrum exhibits a low-intense line centered at −31.2 ppm. This resonance can be well-assigned to PO4− groups.17,41 Since the HPO4−/PO4− ratio is determined by integration as ∼30 to 1, the resonance at −31.2 ppm can be attributed to O3P−O−Si−Alk situated at the surface of the ZrP nanoplatelet.41 This is in a good accordance with the 1H MAS NMR data. The 29Si{1H} CP MAS NMR spectrum recorded at a spinning rate of 10 kHz is shown in Figure 3D. It should be noted that, despite using cross-polarization, the experiment required a significant number of scans. In some sense, this correlates with a low content of organic ligand on the surface of Zr phosphate, as it has been noted above on the basis of 1H MAS NMR. The 29Si{1H} CP MAS NMR spectrum exhibits two resonances with similar integral intensities that are detected at −64.1 and −74.1 ppm. We assign these signals to O−P− (O3Si−Alk) and O−P−(O2(HO)Si−Alk) groups, respectively, because the intensity of the high-field resonance decreases strongly in the 29Si{1H} MAS NMR spectrum with direct excitation of 29Si nuclei. Our next step was to show that this new synthetic pathway can be used to improve, or diversify, the many applications already known for α-ZrP-based materials. Brunet and coworkers have shown efficient electron-transfer reactions upon photoactive species intercalated within ZrP layers.51−53 Moreover, Colón and co-workers have also shown that electrontransfer reactions can take place between suspended nanoparticles of ZrP fully loaded with phototoactive electron donors and the electron acceptor in solution.25,26,44 We surface-

Figure 3. (A) The single-pulse 1H NMR spectrum (top) and the partially relaxed 1H MAS NMR spectrum (bottom, delay time between 180° and 90° pulses is 0.45 s) recorded for the ZrP sample spinning at a rate of 14 kHz. (B) The 13C{1H} CP MAS NMR spectrum of the ZrP sample recorded at a spinning rate of 10 kHz. (C) The 31P{1H} MAS NMR spectrum of the ZrP sample recorded at a spinning rate of 8 kHz (top). The same spectrum scaled with a coefficient of 4 (bottom). (D) The 29Si{1H} CP MAS NMR spectrum recorded for the ZrP sample at a spinning rate of 10 kHz.

sample at a spinning rate of 14 kHz, where the three broad resonances are centered at 8.2, 6.5, and 1−2 ppm and accompanied by intense sidebands. The spectrum illustrates strong proton−proton dipolar interactions typical for relatively rigid solids. The low-field lines at 8.2 and 6.5 ppm can be well assigned to HPO4− and H-bonded water, respectively.17,46 A superposition of the broad line centered at 1−2 ppm and sharp resonances with δ of 1.5, 1.3, and 0.87 ppm can be attributed to alkyl protons of the organic ligand with less mobility (these alkyl groups are located closer to the Zr phosphate surface) and protons of more mobile alkyl groups remote from the surface. In accord with this assignment, the broad component is accompanied by intense sidebands while the sharp lines show sidebands with very low intensity (Figure 3A). Moreover, the wide and narrow components are wellobserved in the partially relaxed 1H MAS NMR spectrum recorded by the inversion−recovery experiments performed at a delay time of 0.45 s (Figure 3A, bottom). On the basis of the TGA and the particle dimension, the O− Si−Alk/P−OH ratio for a ZrP nanoplatelet completely modified is ca. 1:3.4 on the surface. However, the integration of the 1H signals leads to an O−Si−Alk/P−OH ratio of 1:26 for the whole particle. Nevertheless, taking into account an average particle thickness of 8 nm (Figure 1C,D), there should be an average of ca. 10 nanosheets of ZrP per nanoplatelet, resulting in an O−Si−Alk/P−OH ratio of ca. 1 to 2.6 on the surface of ZrP, in fair agreement with that calculated by TGA. Finally, a very weak and sharp resonance at 4 ppm obviously belongs to residual ethanol, as confirmed by TGA. The room-temperature inversion−recovery experiments performed for the ZrP sample spinning at 14 kHz have shown spin−lattice relaxation of all the protons as a 726

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with the phosphate groups on the surface of the nanoplates, and no intercalation took place during the reaction. Moreover, the surface modification of ZrP fully loaded with Ru(bpy)32+ was also successful and no leaching occurs during the reaction. A successful electron-transfer reaction was possible in a nonpolar solvent. These reactions now open a wide gamut of new applications for the well-known and versatile zirconium phosphate family. More experiments on surface modification on ZrP are promised.

functionalized ZrP nanoparticles fully loaded with tris(2,2′bipyridyl)ruthenium(II) (Ru(bpy)32+@ZrP)26,44 with OTS, following exactly the same procedure that we used for the pristine α-ZrP. It is important to mention that, during the synthesis, there was no significant leaching of Ru(bpy)32+ into the solution and the principal photophysical properties of Ru(bpy)32+@ZrP remained unaltered and are consistent with the previous reported material by Marti ́ et al. (Figure 4 and

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ASSOCIATED CONTENT

S Supporting Information *

Additional characterization of these materials, including the AFM topography and friction images of OTS/α-ZrP over a polar and nonpolar surface, the XPS spectra of α-ZrP before and after the surface modification (OTS/α-ZrP), the UV−vis spectra of Ru(bpy)3@ZrP before and after the surface modification, and the fluorescence lifetime measurements at different concentrations of BQ. 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: clearfi[email protected]. Author Contributions

Figure 4. Photoluminescent spectra of OTS surface-modified Ru(bpy)32+@ZrP with different concentrations of BQ in 1,2dichlorobenzene. Inset: Stern−Volmer plot for the quenching with BQ using steady-state fluorescence intensity (spheres) and fluorescence lifetime (squares). λex = 445 nm.

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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.

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Figure SI-3, Supporting Information).11,18 The resulting material is extremely hydrophobic, making it impossible to suspend in water and easy to suspend in any nonpolar solvent. Because of the hydrophobicity of the synthesized nanoparticles, we decided to perform a photoinduced electron-transfer reaction with a hydrophobic electron acceptor, 1,2-benzoquinone (BQ).54 Figure 4 shows the photoluminescence spectra of surfacemodified Ru(bpy)32+@ZrP suspended in 1,2-dichlorobenzene (0.008% w/w) with different concentrations of BQ in the solution (from 0 to 728 μM). The steady-state photoluminescence spectra of the surface-modified Ru(bpy)32+@ ZrP suspended in 1,2-dichlorobenzene resemble the spectrum of Ru(bpy)32+ in aqueous solution (λmax = 597 nm) and the non-surface-modified material in water reported by Marti ́ and Colón (λmax = 592 nm),26 with a λmax of 595 nm. This result shows that the microenvironment of the intercalated Ru(bpy)32+ is intact in the interlayer region and was not affected by the surface modification reaction. The inset in Figure 4 also shows the Stern−Volmer plot for the quenching of the surfacemodified Ru(bpy)32+@ZrP by BQ (inset), indicating a combined dynamic and static quenching mechanism, due to the lack of overlapping Stern−Volmer plots for the steady-state and photoluminescence lifetime experiments and the upward curvature of the fractional photoluminescence.55 On the basis of the Stern−Volmer plots, we obtained a static quenching constant (KS) value of 8.82 × 102 M−1 and a dynamic quenching constant (KD) value of 6.99 × 102 M−1.

ACKNOWLEDGMENTS We acknowledge the TAMU Microscopy and Imaging Center for the TEM facilities and the TAMU X-ray powder diffraction facilities. We thank the Ford Foundation for a Postdoctoral Fellowship to A.D. and the R. A. Welch Foundation, Grant No. A-0673. A.A.M. thanks the Welch Foundation, Grant No. C1743, for financial support. Also, we want to acknowledge Carrie Carpenter and Jessica Spears for their help with AFM images.

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ABBREVIATIONS OTS, octadecyltrichlorosilane; ZrP, zirconium phosphate; Ru(bpy)32+, (2,2′-bipyridyl)ruthenium(II); BQ, p-benzoquinone

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REFERENCES

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CONCLUSION In conclusion, the surface functionalization of α-ZrP with octadecyltrichlorosilane was successful. The silane was reacted 727

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